IEEEANDTRANSACTIONS ULTRASONICS VOL SU-32 NO 4 JULY 1985SONICSON 499

Piezoelectric Composite Materials for Ultrasonic Transducer Applications Part 11 Evaluation of

Ultrasonic Medical Applications T R GURURAJA WALTER A SCHULZELESLIE E CROSS AND ROBERT E NEWNHAM

Abstmct-The electro-acoustic properties of Lead zirconate titanate (PZT)rod-polymercomposites relevantforultrasonictransducer ap-plications are reported Acoustic impedance of the composite materials was measured by three different techniques in the frequency range 03- 35 MHz Dependence of the acoustic impedance as a function of vol-umefractionofPZTandfrequencywasalsomodeled theoretically Time-delay spectrometry was employedto calibrate the free-field trans- mitting and receiving voltage responses of the composite materials The acousticimpedanceof thecomposite materialswas inthe rangeof 3-10 M rayl The figure of merit in the receiving mode of composite materials was three times that of PZT The figure of merit for a 20-percentPZTcomposite (2 = 73 M rayl)was further enhanced by 50 percent using a single-layer impedance transformer of lucite (2 =33 M rayl)Thesecompositematerialsweremolded into curvedshapes by simple thermal process to fabricate focused transducers The axial and lateral beam profiles of focused composite transducers are presented

A I INTRODUCTIOK

DETAILED STUDY on the resonant modesof vibra-tion of lead zirconatetitanate(PZT) rod-polymer

composites was already reported in Part I [l] The results showed that although the polymer is piezoelectrically in-active it plays a substantial role in the composite from the acousticpoint of view Moreover it was shown thatthe mechanical properties of the Spurrs epoxy markedly influ- ence the piezoelectric response of the composite Larger ultrasonic displacement amplitude of the epoxy as com- pared to that of the PZT at the thickness-mode resonance was evidence of the efficient coupling of the acoustic en-ergy from PZT to theepoxy Since the acoustic impedance of the epoxy is relatively close to that of the human body an effective coupling of acousticenergyfromthetrans-ducertothehumanbody is ensured Thepresent paper deals with the evaluation of PZT rod-polymer composites with 1-3 connectivity as an ultrasonic transducer for med-ical diagnostic applications Since the acoustic impedance of the human body is very close to thatof water the trans- ducer evaluation was carried out with water asthe loading medium

ManuscriptreceivedNovember 26 1984 This work was supported by the North American Philips Laboratories

T R Gururaja L E Cross and R E Newnhan are with the Materials Research Laboratory The Pennsylvania State University University Park PA 16802 USA

W A Schulze was with the Materials Research Laboratory The Penn-sylvania State University He is now with the NewYork StateCollege 01 Ceramics Alfred Univeristy Alfred NY 14802 USA

Initially the composite transducers were tested in the pulse-echo mode (Section 11) In general understanding the behavior of a transducer solely from thepulse-echo measurement is extremely difficultbecausethosemea-surements combine both transmitting and receiving char-acteristics of the transducer The results are also modified by the characteristicimpedance of thetransduceras a functionoffrequencyThereforetheseparametersare measuredindividuallySection I11 describes theexperi-mentalresultsandtheoreticalmodeling of the acoustic impedance of the composites as a function of frequency and volume percent PZT The transmitting and receiving voltageresponse of the compositetransducersaredealt with in Section IV and V respectively These results to-gether with a knowledge of the piezoelectric response of thecompositefromPart I of this work will be usedto account for the performance of the composite as an utra-sonic transducer The effect of a single quarter-wavelength matching layer on the performanceof the composite trans-ducer is considered inSectionVI It will be shownthat thesecompositematerialscanbe molded intocurved shapes by a simple thermalprocesstofabricatefocused transducer The axial and lateral beam profile of thus fo-cused composite transducers are presented in Section VII and a concluding discussion is given in Section VIII

11 PULSE-ECHOMEASUREMENTS

The pulse-echo response of the airbackedcomposite transducers were determined by the tone-burst pulse-echo method described by Erikson (21 A schematic diagram of the experimental set up is shown in Fig 1 The composite tranducer was mounted in a water tank attheend of a stainlesssteeltubeusingsiliconerubber as a glueThe transducer mounting had independent angular adjustment in two orthogonal planes The tone-burst signal of an ap-propriate frequency from a function generator (Interstate-F74) was fed into a power amplifier (ENI-411A) The am-plified tone-burst (10 V peak) of 15 to 20 cycles with a repetition rate of approximately1KHz was used to ex- cite the transducer A finely polished ( 1 pm) steel block (10 X 10 X 25 cm) was used as a reflector The steel reflector was placed at the a2 hdistance from the trans-ducer where a is the radius of the transducer and h is the

0018-95371850700-0499$0100 8 1985 IEEE

- - -

500 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-32 NO 4 JULY 1985

Frequency

lHP5315B)

Fig 1 Block diagram of tone-burst pulse-echo method

TABLE I PULSE-ECHORESPONSEOF COMPOSITETRANSDUCERS

Volume Percent PZT

PZT Rod Echo Signal Diameter (50 Q Load) (mm) (V5(volt))

6-dB BandwidthFigure (BW (MHz)) Q

of Merit (V50BW)

Echo Signal (diode isolation)

(VD (VOW) 6-dB BandwidthFigure

(BW (MHz)) of Merit

(VDBW)

10 045 08 04 5 6 032 075 15 050 5 108 325 045 I 46 45 120

24 045 20 045 30 05 045 24

1 o 085 085028 035 056

07025 045

32 10 21 045 095 20 028 125 5

Commercial Transducer

22539

Center frequencyf = 225-MHz

wavelength of the center frequency of the transducer The distance a2Xcorresponds to the transition from nearfield to far field At this distance the transducer beam is a col- limated plane wave with a smoothly varying amplitude and phase profile [2]

Theamplitude of the echo signalfor a constant IO-V peak input was measured as a functionof frequency around the peak response The following parameters were deter-mined

I ) Frequency of the maximum pulse-echo amplitude(fo) (also referred to here as center frequency) together with the corresponding value of peak amplitude

2) Frequencies c f l and f2)at which the pulse-echo am-plitude is one half (- 6 dB) of the maximum pulse-echo response The product of the maximum pulse-echo am-plitude and the 6-dB bandwidth cf2 -fi)was used as the principal figure of merit for assessing the performance of the composite transducer The quality factor Q defined as cfof2 - f l ) was used to characterize the broadband nature of the transducer

The electric impedance of some composite transducer atresonance was relatively high (gtgtW Q ) compared to the 504 internal impedance of the amplifier which is in parallel with the transducer Therefore the received echo signal was loaded by the lower impedance of the amplifier Since the electrical impedance of the composite depends

12 3 0 121 3

on the volume percent PZT and thickness the echosignals were loaded to different extents Thus a comparisonof the pulse-echoresponse of different composite transducers was difficult The electrical loading effects were corrected by using back-to-back zener diodes which were inserted between the amplifier and the transducer The diodes al-low voltage to be applied to the transducer during excita- tion while acting as a high impedance during reception The excitation and the reflected signalswere detected using a 1OX oscilloscope probe The total capacitance of the probe cables and diodes was about 15 PF

Thecompositetransducers fabricated to operate in thickness-moderesonanceatafrequency of 225 MHz This frequency often used in ultrasonic imaging systems was originally chosen for demonstrating the performance of the composite transducers

The data presented in Table I refer to composites of thickness 06 mm with acenterfrequencyaround 225 MHz With 50-Qtermination the highest echo signal am- plitude (24 V) was received from the composites with 20-and 30-volume-percent PZT rods of diameter 045 mm These samples had comparable voltage-bandwidth prod-uct The highest pulse-echo amplitude (325 V) with the back-to-backzenerdiodeterminationcamefromthe composite sample with 20-percent PZT rods of diameter 045 mm

1 O-(Rohe 5601)

GURURAJAMATERIALS-PARTel ul PIEZOELECTRIC I1COMPOSITE 501

Both types of ten-percent composites had higher elec-trical impedancesandwereseverely loaded by the 504 terminationThehigh-impedancetermination using the zener diodes increased the pulse-echo voltage amplitude by a factor of twoHowever this was significantly below the values recorded for 20- and 30-percent PZT as shown in Table I

Thepulse-echoresponses of composites werecom-pared with a commercial transducer (Rohe 5601 with the transducer diameter = 13 mm) This transducer was sup- posedly provided with backing and matching layers for the optimumperformanceThecompositetransducerswith 20- and 30-percent PZT compared favorably with that of

ducer in the receiving mode appears more like a current source than a voltage source The problem is even more serious for composites with five- and ten-percent PZT be- cause the value of the impedance minimum at resonance frequencyincreasesasthevolumepercentPZTisde-creased

2) As the testing frequency decreases the sound wave- length increases hence the a2X axial distance at which the steel reflectorispositionedbecomescloser For a transducer of diameter 19 cm resonating at 350 KHz the a2Xdistance is approximately21 cm (assuming C = 1480 cms in water at 20degC) The travelling time for sound in water from the transducer to the reflector and backfor this

the commercial transducer However the bandwidth of thedistanceisapproximately28 P S To establish a steady- composite transducer was narrower (Table I) It is inter- esting to note that the pulse-echo figure of merit of sev- eralcompositetransduceriscomparabletothat of an optimized commercial piezoelectric transducer

As it emerges from the study of the thickness-mode res-onance of the compositessamples [ l ] thin composites resonating at approximately 225 MHz do not vibrate as collectiveunit ThePZT rods vibrate quite freely with only a lightdampingfromthe epoxy matrix Thecom-posite transducer canbe pictured as acting similarto a PZT disc with a light backing material As the frequency of the thickness-mode resonance is decreased by increas- ing the thickness of the sample the lateral interaction be-tween the rods through the epoxy becomes stronger and

state condition the transducerwas excited for a minimum of15 cycles At 350 KHz the transducer excitation cor- respondsto43 p s which is largerthanthepulse-echo round-trip time This means that the echoes begin return-ing before the electrical excitation has stopped thus pos-ing a serious problem

A possible solution was to use thespectrumanalyzer method [2] The transducer was excited by an electrical impulse (250 V peak) of duration less than100ns supplied by an ultrasonic transducer analyzer (UTA-3KB-Aero-tech) A time-delayed gate is triggered withthis excitation pulse and it is adjusted in time delay and gate length to pass only the reflected echo Theprocessed signalfrom theanalyzer was displayed onahigh-frequencyoscillo-

the composite acts more like a homogeneous material Thescope(Gould-OS3300)todetermineringdowntime of strong lateral interaction in thick samples was found to be a result of the long wavelength of the transverse wave com- pared to the periodicity of the latticeThis was clearly seen from the observation of vibration pattern of the com-positesampleresonating in thickness mode at 270 kHz [l] Since the stress transfer between the PZT and poly-mer is betterwhen thisrequirementissatisfiedanim-provement was expected in the pulse-echo figure of merit in composites operating at lower frequencies

To experimentally verifytheabovepropositioncom-positeswith arange of thickness from 06 to 515 mm fabricated to study their performance as a function of fre- quency at peak response The results indicated some im-provement in the pulse-echo response compared to mea- surements at 225 MHzForexamplea relatively thick composite(thickness = 317 mm) withthicknessmode resonance at 450 KHz displayed a loop gain of 10 (echo amplitude of 10 V for 10 V excitation) and Q of 35

However it should be noted here that the evaluation of thick composites resonating in the range of a few hundred KHz by the tone-burst pulse-echomethodwascomplex

followingthedue acoustic fromto reason impedance

the transducer The signal from the UTA-3 was displayed on a spectrum analyzer (HP 3585A) toobtain a plot of amplitudeversusfrequency of the reflected echo Since the excitation pulse was very narrow the problem of in- terference of the echo signal with the excitation pulse en- countered in the tone-burst method was partlysolved However the methodwas not free of other limitations Since the transducer is not matched electrically to the pul-ser not all the pulse energy is delivered to the transducer The receiver has a terminating impedance of 50 a thus it has the serious loading effect on the received signal

Asexplainedearlier it was very difficulttoevaluate composite transducers and realize their advantages solely by pulse-echo measurementsTherefore thecomposite materials were characterized separately for their acoustic impedanceandtransmittingandreceivingvoltagere-sponses which are reported in subsequent sections

111 CHARACTERISTIC IMPEDANCEACOUSTIC

Thecharacteristic acoustic impedance 2 (referred to as here of the is an on) transducer

1) The electrical impedance at resonance frequency of important property because it determines the effectiveness the composite samples increased linearly with increase inof the coupling of ultrasonic energy from the transducer thickness A 20-percent PZT composite with a thickness to the load The acoustic impedance is the product of den-of 4 mm resonating at 350 KHz had a minimum imped- sity p and velocity of sound C in the material In a single ance of about 400 Q At this relatively high impedancephasematerialmeasurement of these properties is rela- leveleven the diode isolation cannot providetheneces- tively easy and well described [3]However it is not so sarydecouplingfromthe 504 terminationThetrans- forcompositematerialsCompositesusually exhibit anis-

502 IEEE TRANSACTIONSULTRASONICSON SONICS VOL SU-32 NO 4 JULY 1985AND

otropic behavior The properties of the constituent phases must be averaged properly In the PZT rod-polymer com-posites considered here the interaction between polymer and PZT and among neighboring PZT rods is a function of frequency Thereforedependence of acoustic imped-ance on frequency also needsto be considered Three tech-niquesnamelyreflectiontransmissionandresonance wereusedtodeterminetheacousticimpedanceand its frequencydependenceTheresultsarereported in the following subsections Theacousticimpedance of the composites were also theoretically modeled and compared with the experimental data

A Rejection Technique

In the reflection technique the acoustic impedance of a composite was determined by comparing the amplitude of reflection of a plane acoustic wave from the sample with thatfromastandard material of known acoustic imped-ance eg stainless steel The experimental setup was the same as that used for the tone-burst pulse-echo technique (Fig 1 ) The reflector was placed at the a2X distance A commercial ultrasonic transducer was used to measure the pulse-echo signal from both the sample and the standard reflector The transmitting voltage response and receiving voltage sensitivity of the transducer need not be taken into account as the same transducer is used for measuring re-flections from boththestandardmaterialand thespeci-men

The tone-burst pulse-echo experiment was carried out as a function of frequency with a stainless steel block (10 cm X I O cm X 254) reflector The driving voltage of the transducer was kept constant at 10 V It can be shown [4] that the pulse-echo amplitude X is proportional to the re-flectioncoefficient atthewater-reflector interface That is [4]

z - zX a ___

2 + z where Zl and Z2 are the acoustic impedance of the steel reflector and water respectively

Thestainless-steel blockwas thenreplaced by a com- posite sample and the pulse-echo amplitude Y was noted If 2 is the acoustic impedance of the composite sample

z - zYCY z + Zz

From (1) and (2) the acoustic impedance of the com-posite sample (Z) can be expressed in terms of X Y ZI and Z Since it is knownthat Z = 454 X lo6 ray1 and Z2 = 15 X lo6 rayl [S] theexpressionfortheacoustic impedance of the composite sample is given by

X lo6 rayl (3)

By substituting the values of X and Y at a specified fre-quency the acoustic impedance can be calculated from(3)

=Px 7 f i

0 05 Io l 5 Frequency (MHz)

(a)

l 2 3 4 Frequency (MHz)

( b)

Fig 2 Acousticimpedance of composites as a function of frequency (a) 0-15 MHz (b) 1-35 MHz

The validity of the technique was tested by determining the acoustic impedance of a few standard materials such as lucite fused silica and aluminum with known acoustic impedance of 31 13 and 17 M rayl respectively The measured values of Z for these materials were 32 126 and171 M rayl which are in excellent agreement The experiments were also conducted with transducers of dif- ferent type (focused and nonfocused) and with the reflec-torplaced at different distances (at a2X distance at the focal length of the transducer etc) No substantial differ-ence in the measured impedances was observed

Composite samples of thickness 2-3 cm with 5 10 20 and 30 volume percentPZTrods of diameter 045 mm were fabricated The samples were not poled because such thicksamplesrequireexcessively large poling voltages (40-60 kV) Fig 2(a) shows a plot of the acoustiz imped-ance of several different composites in the frequency range from 03 to 13 MHz and Fig 2(b) extends the plot from 1 to 35 MHz Low-frequency data were obtained using a lead metaniobate transducer (Ultran Lab Inc diameter= 25 cms) with a center frequency of 07 MHz and a very broadbandwidth (Q = 1) Thedistancebetweenthe transducer and the reflector was 75 cm corresponding to the a2X distanceat 07 MHz Measurements in the fre-quency range from 1 to 35 MHz were carried out using a

503 GURURAJA et 01 PIEZOELECTRIC COMPOSITE MATERIALS-PART I 1

focused transducer (Rohe 5616 diameter = 19 cm focal length = 9 cm) with a center frequency of 225 MHz The separation between the transducer and reflectorin this case was 9 cm which is the focal length of the transducer

The following observations were madeby inspecting the acousticimpedanceplots of Fig 2 As expected the acoustic impedance increased with increase in the volume percentPZTMoreovertheacousticimpedance was stronglyfrequencydependentThecompositesamples showed a series of minima in 2 at low frequency Above 25 MHz the variations are minimized and the value of 2 reached a saturation The position of the first minimum in Z occurred at a higher frequency for largervolume percent PZTTheacousticimpedanceincreasedsteadily at fre- quencies below the first minimum Theacousticimped-ance of single phase epoxy measured by the same proce-dure is also shown in Fig 2 for comparison There was no variation of 2 of epoxy in the frequency range consid-ered in this work The results were analyzed carefully to find an explanation for the observed dispersion in 2

The minima for 5 10 20 and 30 percent PZT compos-ites occur at 04506250825and 1125 MHz respec-tively The frequencies have an inverserelationship with the lateral periodicity of the corresponding composite The product of the frequency at the acoustic impedance min-ima and the lateral periodicity of the composite was found to be a constant of value 800 ms It is interesting to note that the frequency corresponding to the minimain 2 in each composite matches with the resonance frequency f [ I ] The resonance frequency f corresponds to the standing wave pattern of the transverse waves with the wavelength equal to the lateral periodicity of the lattice It should be pointed out here that the value of the acoustic impedance at the minima was approximately the same for all volume percent PZT (16 M rayl) which is surprisingly lower than that of the epoxy (23 M rayl)

There is a small discrepancy in the values of acoustic impedance around 1 MHz measured using the two differ- ent transducers This is possibly because the transducers were operating far away from their center frequencies and hencewerenotverysensitiveThus themeasurements performed in the transition region from one transducer to the other are not very precise

All of these phenomena may be explained as follows The acoustic wave incident on the composite sets it into vibration Due to the difference in compliancebetween the two component phases namely PZT and Spurrs epoxy a transverse wave originating at the interface and propagat-ing in a direction perpendicular to the rod axis is launched into the epoxy matrix At the frequency where the trans-verse wavelength is equal to the periodicity resonant scat-tering of waves by the vertical planes of the PZT rods re- sults in two-dimensionala standing wave patternas described in Part I [l] Measurements of the surface dis-

Fig 3 Echo 4gnal from compoite samples ( a ) At B frequency far away from the minima in Z ( h ) At a frequency corresponding to minima i n 2 (c) At a frequency corresponding to minima i n 2 (expanded time scale)

established after a few cycles of the incident wave The acoustic impedance minima at this frequency are caused by the cancellation effects between the incident wave and thelargeamplitudevibration of the epoxy with a phase difference of 180

Support for the above explanation is provided in Fig 3 where tone-burst pulse-echo signals reflected from a com-posite sample are shown Fig 3(a) depicts a typical signal measured at frequencies far away from the minima in the acoustic impedance The signalbuilds up to a saturation

placements of the composite at this frequency showed thatvalue in one ortwocyclesdependingonthe Q of the the epoxy at the center of theuni t cell of the periodic lat- transducerandremainsconstantTheamplitude is af-tice vibrates 180 out of phase with PZT and with a much fected only by the reflections from the rear surface of the larger amplitude This standing wave pattern is most likely composite sample as indicatedby an increase in amplitude

504

65

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-32 NO 4 JULY 1985

TABLE I1 ACOUSTICIMPEDANCE (2)OF COMPOSITES

~~ ~~ ~ ~ ~

Region 1 Region 3 (Frequency gt 2 MHz) (Frequency lt 04 MHz)

Volume Percent Measured Z Measured Z PZT Calculated Z at 35 MHz Calculated Z at 03 MHz

~~ ~

5 26 23 - 33 10 2 9 2 8 41 35

60 49 2030 30

36 45 31 82

in the last fewcycles The reflected signalfrom acom-posite at the acoustic impedance minima is shown in Fig 3(b) The amplitude approaches a saturationvalue but the signal declines after the first two cycles and reaches a min-imum value Interference of the signal with the reflected

For unpoled PZT rods the elastic modulus is E = l ~ ~= 6098 X 10 Nm2 [7] For the epoxy the elastic modulus was found tobe E = 47 X lo9 Nm2 [l] Fromthecal-culated values of El using (4) and the mean densityiilisted in Table I1 of Part I[l] the acoustic impedance of the com-

signal from the rearsurface of the composite sample makesposites was evaluated from the expression 2 = ( PEl) it difficult to analyze the wave pattern after a few cycles Figure 3(c) presentssimilardatawithanexpandedab-scissa Thus the acousticimpedanceminimaobserved arises from the destructive interference between the in-cident signal and the reflections from the composite sam-ple Therefore the minima in acoustic impedance are only an apparent phenomena becauseof the complex resonance in the composites

Resonance experiments [ l ] indicated another strong lat- eralresonancemodereferred to as J 2 Thisresonance was associated with standing waves along the unit cell di-agonal For a five-percent sample fr2 was approximately 07 MHz and a corresponding minimum was seen in the acoustic impedance plot as expected (Fig 2(a)) Acoustic impedance for the ten-percent sample is also seen to ap-proachaminimumaround 100 MHz(Fig2(a)) which corresponds to fr2 for the ten-percent composite In addi-tion to the acoustic impedance minima just described ad-ditional minima were observed up to about 2 MHz pos- sibly due to more complex lateral modes

B Modeling of Acoustic Impedance The acoustic impedance of the composites was modeled

toexplain theobservedfrequency dependence discussed in the previous section

Above 2 MHz the acoustic impedance was found to be relatively independent of frequency At this frequency ex-perimental evidenceshowed that thePZTrodswerevi-brating without an appreciable lateral interaction There-fore this situation can be modeled using Reuss averaging [6] which assumes that the constituent phases experience the same stress Accordingto the Reuss averagingscheme the modulus of elasticity parallel to the length of the rod El is given by

where u I and u2 are volume fractions and E l and E2 are the appropriate elastic moduli of the constituent phases

Both the measuredandcalculatedvaluesarecompared inTable 11 The frequencyrangeabove2MHzwhere the wavelength of the transverse waves is much smaller thantheperiodicity of the lattice is calledregion 1 for the purpose of discussion There is a good agreement be tweenthetheoryandtheexperimentsupportingthe ex-planation of the mode of vibration outlined above for fre- quencies about 2 MHz

At frequencies below the minima in 2 corresponding to J the lateral interaction increases gradually and the two phases vibrate in a cooperative mode The displacements of PZT and epoxy were found not only to be in phase but also of almost equalamplitude [l] Thus the Voigt av- eraging scheme [6] which assumes constant strain on the constituentphasescan be used to estimate the effective longitudinal modulus El of the composite The composite modulus according to the Voigt model is given by

El = ~ 1 E l+ ~ 2 E 2 ( 5 )

Using the given values of the elastic moduli for the PZT and epoxy the effective modulus and acoustic impedance of the composites were calculated and they are compared with the measured values of 2 at 03 MHz in Table 11 This frequency range where thewaveiength of transverse waves is larger than the periodicity is called region 3 in this pa-per The theoretically predicted increase in the acoustic impedance at frequencies below the first acoustic imped-ance minima was seen in all the composites However the measured values were on the average20 percent lower than the estimated value The discrepancy between the exper- iment and theory is possibly due to the diffraction losses Because of the relatively small size of the composite sam-ple (diameter = 19 cm) compared to the size of the steel reflector (10 X 10 cm) it is conceivable that not all of the acousticenergywasreflectedbackfromthecomposite sample to the transducer

A correction to account for the diffraction losses in the measurements of acoustic impedance was applied experi-mentally by using the standard reflectorof exactly the same

505 GURURAJA et al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE 111 Doto m m c t e d fw IOY Compor(t0 difhoclion lorus

iOO 05 10

Frequency (MHz)

Fig 4 Correction on the value of acoustic impedance due to diffraction losses

ACOUSTICIMPEDANCEBY TRANSMISSIONTECHNIQUE

Measured LongitudinalMeasured Z Calculated Z

Volume Percent Velocity lo6(atrayl lo6 ray1 (from PZT W s ) 05 MHz) Table 11)

20 2682 i 8 61 60 30 82 2780 It 14 85

along the PZT rod axis The method chosenfor measuring the longitudinal velocity was similar to the standing-wave

dimension as the composite sample Then the experimen-tal setup for the two reflections to be compared becomes identical and hence the uncertainty in the measurement because of the diffraction lossesis minimized Fig 4 com-pares the earlier data with the new data corrected for dif-fraction losses for a ten-percent composite sample As can be seenthecorrected value of the acousticimpedance approaches 41 M rayl at 03 MHz which is in excellent

strainmodel (Table 11) Based on the experimentaland theoretical results the acoustic impedance- as a function of frequency can be classified into three regions

In region 1 corresponding to frequencies above 2 MHz thetransverse wavelength is muchsmallerthanthe rod spacing d andthetwophasesappear to bedecoupled There is not much influence of epoxy on the vibration of thePZTrods A significantimprovement in the pulse-echo response in terms of the figure of merit cannot be

agreement with the predicted values based on the constant Thelongitudinal velocity was determined for 20- and

expected in this region if the composite is used as a single elementtransducerOntheotherhandthesituation is desirableforalinear- or phased-arraytransducercon-structionwheretheacousticalcross-couplingbetween elements should be as low as possible

In region 3 corresponding to the frequency far below thefirstminimum in the acousticimpedancethecom-posite acts as a homogeneous material Maximum stress is transferred from PZT to polymer and vice versa The composite is expected to perform better as a singleele-ment transducer with an improvedpulse-echofigure of merit

In region 2 the periodicity of the lattice and the wave-length of the transverse waves are comparableThis re-gionishelpful in understanding thecomplexvibrational modes in two dimensionallyperiodicstructures In this region the composite can possibly be used as a resonant sound absorber

C Transmission Technique

The acoustic impedance of the composites was also de-termined by measuring the velocity of longitudinal waves

method described by McSkimin [8] An acoustic wave of a particular frequency is made to impinge on the sample The wave transmitted to thespecimenisreflectedback and forth giving rise to a series of transmitted pulses At discrete frequencies the emerging wave trains will be in phaseleadingtoaconstructiveinterferenceThecon-structive interference occuri at a frequency for which the path length (twice the specimen thickness) is equal to an integralnumberof wavelengths in thespecimenatthat frequency The principle of the method is to determine the value of the integer at oneof the discrete frequencieswhen the in-phase condition is satisfied The experiment is de-scribed in detail elsewhere [9]

30-percent PZT composites around05 MHz which is be-low the frequencycorrespondingtotheacousticimped-ance minima (Fig 2(a)) The velocity values were mutli-plied by the average density to obtain theacousticim-pedance The resulting data shownin Table 111 are in ex-cellent agreement with the values estimated theoretically for region 3

D Resonance Technique The velocity of longitudinal waves determined by the

resonance technique described in Part I [ l] was also used to determine the acoustic impedance of composite sam-ples The velocitites for different composities are listed in thelastcolumn ofTable V of Part I [ l ] Valuesof the acoustic impedance evaluated from the measured velocity anddensity of composities are listed inTable IV Theo- retically calculated acoustic impedance are also tabulated for comparison In the piezoelectricresonanceexperi-ments PZT rods in the composite are electrically poled Hence for calculating the elastic modulus of the compos-itethemodulus of the PZT rods E = = 105 X 10 Nm2 (7) was used

As it emerges from Table IV in region 1 ( gt2 MHz) a pooragreement betweenthe estimatedandmeasured acoustic impedancewas foundThis was anticipated be-cause the rods are vibrating relatively freely and the res-onancetechniquemeasuredthe velocity of PZT phase only Hence the product of this velocity with the average density of the sample gives a much higher estimate of the acoustic impedance

In region 3 (lt05 MHz) theagreementbetweenthe

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

GURURAJAMATERIALS-PARTel ul PIEZOELECTRIC I1COMPOSITE 501

Both types of ten-percent composites had higher elec-trical impedancesandwereseverely loaded by the 504 terminationThehigh-impedancetermination using the zener diodes increased the pulse-echo voltage amplitude by a factor of twoHowever this was significantly below the values recorded for 20- and 30-percent PZT as shown in Table I

Thepulse-echoresponses of composites werecom-pared with a commercial transducer (Rohe 5601 with the transducer diameter = 13 mm) This transducer was sup- posedly provided with backing and matching layers for the optimumperformanceThecompositetransducerswith 20- and 30-percent PZT compared favorably with that of

ducer in the receiving mode appears more like a current source than a voltage source The problem is even more serious for composites with five- and ten-percent PZT be- cause the value of the impedance minimum at resonance frequencyincreasesasthevolumepercentPZTisde-creased

2) As the testing frequency decreases the sound wave- length increases hence the a2X axial distance at which the steel reflectorispositionedbecomescloser For a transducer of diameter 19 cm resonating at 350 KHz the a2Xdistance is approximately21 cm (assuming C = 1480 cms in water at 20degC) The travelling time for sound in water from the transducer to the reflector and backfor this

the commercial transducer However the bandwidth of thedistanceisapproximately28 P S To establish a steady- composite transducer was narrower (Table I) It is inter- esting to note that the pulse-echo figure of merit of sev- eralcompositetransduceriscomparabletothat of an optimized commercial piezoelectric transducer

As it emerges from the study of the thickness-mode res-onance of the compositessamples [ l ] thin composites resonating at approximately 225 MHz do not vibrate as collectiveunit ThePZT rods vibrate quite freely with only a lightdampingfromthe epoxy matrix Thecom-posite transducer canbe pictured as acting similarto a PZT disc with a light backing material As the frequency of the thickness-mode resonance is decreased by increas- ing the thickness of the sample the lateral interaction be-tween the rods through the epoxy becomes stronger and

state condition the transducerwas excited for a minimum of15 cycles At 350 KHz the transducer excitation cor- respondsto43 p s which is largerthanthepulse-echo round-trip time This means that the echoes begin return-ing before the electrical excitation has stopped thus pos-ing a serious problem

A possible solution was to use thespectrumanalyzer method [2] The transducer was excited by an electrical impulse (250 V peak) of duration less than100ns supplied by an ultrasonic transducer analyzer (UTA-3KB-Aero-tech) A time-delayed gate is triggered withthis excitation pulse and it is adjusted in time delay and gate length to pass only the reflected echo Theprocessed signalfrom theanalyzer was displayed onahigh-frequencyoscillo-

the composite acts more like a homogeneous material Thescope(Gould-OS3300)todetermineringdowntime of strong lateral interaction in thick samples was found to be a result of the long wavelength of the transverse wave com- pared to the periodicity of the latticeThis was clearly seen from the observation of vibration pattern of the com-positesampleresonating in thickness mode at 270 kHz [l] Since the stress transfer between the PZT and poly-mer is betterwhen thisrequirementissatisfiedanim-provement was expected in the pulse-echo figure of merit in composites operating at lower frequencies

To experimentally verifytheabovepropositioncom-positeswith arange of thickness from 06 to 515 mm fabricated to study their performance as a function of fre- quency at peak response The results indicated some im-provement in the pulse-echo response compared to mea- surements at 225 MHzForexamplea relatively thick composite(thickness = 317 mm) withthicknessmode resonance at 450 KHz displayed a loop gain of 10 (echo amplitude of 10 V for 10 V excitation) and Q of 35

However it should be noted here that the evaluation of thick composites resonating in the range of a few hundred KHz by the tone-burst pulse-echomethodwascomplex

followingthedue acoustic fromto reason impedance

the transducer The signal from the UTA-3 was displayed on a spectrum analyzer (HP 3585A) toobtain a plot of amplitudeversusfrequency of the reflected echo Since the excitation pulse was very narrow the problem of in- terference of the echo signal with the excitation pulse en- countered in the tone-burst method was partlysolved However the methodwas not free of other limitations Since the transducer is not matched electrically to the pul-ser not all the pulse energy is delivered to the transducer The receiver has a terminating impedance of 50 a thus it has the serious loading effect on the received signal

Asexplainedearlier it was very difficulttoevaluate composite transducers and realize their advantages solely by pulse-echo measurementsTherefore thecomposite materials were characterized separately for their acoustic impedanceandtransmittingandreceivingvoltagere-sponses which are reported in subsequent sections

111 CHARACTERISTIC IMPEDANCEACOUSTIC

Thecharacteristic acoustic impedance 2 (referred to as here of the is an on) transducer

1) The electrical impedance at resonance frequency of important property because it determines the effectiveness the composite samples increased linearly with increase inof the coupling of ultrasonic energy from the transducer thickness A 20-percent PZT composite with a thickness to the load The acoustic impedance is the product of den-of 4 mm resonating at 350 KHz had a minimum imped- sity p and velocity of sound C in the material In a single ance of about 400 Q At this relatively high impedancephasematerialmeasurement of these properties is rela- leveleven the diode isolation cannot providetheneces- tively easy and well described [3]However it is not so sarydecouplingfromthe 504 terminationThetrans- forcompositematerialsCompositesusually exhibit anis-

502 IEEE TRANSACTIONSULTRASONICSON SONICS VOL SU-32 NO 4 JULY 1985AND

otropic behavior The properties of the constituent phases must be averaged properly In the PZT rod-polymer com-posites considered here the interaction between polymer and PZT and among neighboring PZT rods is a function of frequency Thereforedependence of acoustic imped-ance on frequency also needsto be considered Three tech-niquesnamelyreflectiontransmissionandresonance wereusedtodeterminetheacousticimpedanceand its frequencydependenceTheresultsarereported in the following subsections Theacousticimpedance of the composites were also theoretically modeled and compared with the experimental data

A Rejection Technique

In the reflection technique the acoustic impedance of a composite was determined by comparing the amplitude of reflection of a plane acoustic wave from the sample with thatfromastandard material of known acoustic imped-ance eg stainless steel The experimental setup was the same as that used for the tone-burst pulse-echo technique (Fig 1 ) The reflector was placed at the a2X distance A commercial ultrasonic transducer was used to measure the pulse-echo signal from both the sample and the standard reflector The transmitting voltage response and receiving voltage sensitivity of the transducer need not be taken into account as the same transducer is used for measuring re-flections from boththestandardmaterialand thespeci-men

The tone-burst pulse-echo experiment was carried out as a function of frequency with a stainless steel block (10 cm X I O cm X 254) reflector The driving voltage of the transducer was kept constant at 10 V It can be shown [4] that the pulse-echo amplitude X is proportional to the re-flectioncoefficient atthewater-reflector interface That is [4]

z - zX a ___

2 + z where Zl and Z2 are the acoustic impedance of the steel reflector and water respectively

Thestainless-steel blockwas thenreplaced by a com- posite sample and the pulse-echo amplitude Y was noted If 2 is the acoustic impedance of the composite sample

z - zYCY z + Zz

From (1) and (2) the acoustic impedance of the com-posite sample (Z) can be expressed in terms of X Y ZI and Z Since it is knownthat Z = 454 X lo6 ray1 and Z2 = 15 X lo6 rayl [S] theexpressionfortheacoustic impedance of the composite sample is given by

X lo6 rayl (3)

By substituting the values of X and Y at a specified fre-quency the acoustic impedance can be calculated from(3)

=Px 7 f i

0 05 Io l 5 Frequency (MHz)

(a)

l 2 3 4 Frequency (MHz)

( b)

Fig 2 Acousticimpedance of composites as a function of frequency (a) 0-15 MHz (b) 1-35 MHz

The validity of the technique was tested by determining the acoustic impedance of a few standard materials such as lucite fused silica and aluminum with known acoustic impedance of 31 13 and 17 M rayl respectively The measured values of Z for these materials were 32 126 and171 M rayl which are in excellent agreement The experiments were also conducted with transducers of dif- ferent type (focused and nonfocused) and with the reflec-torplaced at different distances (at a2X distance at the focal length of the transducer etc) No substantial differ-ence in the measured impedances was observed

Composite samples of thickness 2-3 cm with 5 10 20 and 30 volume percentPZTrods of diameter 045 mm were fabricated The samples were not poled because such thicksamplesrequireexcessively large poling voltages (40-60 kV) Fig 2(a) shows a plot of the acoustiz imped-ance of several different composites in the frequency range from 03 to 13 MHz and Fig 2(b) extends the plot from 1 to 35 MHz Low-frequency data were obtained using a lead metaniobate transducer (Ultran Lab Inc diameter= 25 cms) with a center frequency of 07 MHz and a very broadbandwidth (Q = 1) Thedistancebetweenthe transducer and the reflector was 75 cm corresponding to the a2X distanceat 07 MHz Measurements in the fre-quency range from 1 to 35 MHz were carried out using a

503 GURURAJA et 01 PIEZOELECTRIC COMPOSITE MATERIALS-PART I 1

focused transducer (Rohe 5616 diameter = 19 cm focal length = 9 cm) with a center frequency of 225 MHz The separation between the transducer and reflectorin this case was 9 cm which is the focal length of the transducer

The following observations were madeby inspecting the acousticimpedanceplots of Fig 2 As expected the acoustic impedance increased with increase in the volume percentPZTMoreovertheacousticimpedance was stronglyfrequencydependentThecompositesamples showed a series of minima in 2 at low frequency Above 25 MHz the variations are minimized and the value of 2 reached a saturation The position of the first minimum in Z occurred at a higher frequency for largervolume percent PZTTheacousticimpedanceincreasedsteadily at fre- quencies below the first minimum Theacousticimped-ance of single phase epoxy measured by the same proce-dure is also shown in Fig 2 for comparison There was no variation of 2 of epoxy in the frequency range consid-ered in this work The results were analyzed carefully to find an explanation for the observed dispersion in 2

The minima for 5 10 20 and 30 percent PZT compos-ites occur at 04506250825and 1125 MHz respec-tively The frequencies have an inverserelationship with the lateral periodicity of the corresponding composite The product of the frequency at the acoustic impedance min-ima and the lateral periodicity of the composite was found to be a constant of value 800 ms It is interesting to note that the frequency corresponding to the minimain 2 in each composite matches with the resonance frequency f [ I ] The resonance frequency f corresponds to the standing wave pattern of the transverse waves with the wavelength equal to the lateral periodicity of the lattice It should be pointed out here that the value of the acoustic impedance at the minima was approximately the same for all volume percent PZT (16 M rayl) which is surprisingly lower than that of the epoxy (23 M rayl)

There is a small discrepancy in the values of acoustic impedance around 1 MHz measured using the two differ- ent transducers This is possibly because the transducers were operating far away from their center frequencies and hencewerenotverysensitiveThus themeasurements performed in the transition region from one transducer to the other are not very precise

All of these phenomena may be explained as follows The acoustic wave incident on the composite sets it into vibration Due to the difference in compliancebetween the two component phases namely PZT and Spurrs epoxy a transverse wave originating at the interface and propagat-ing in a direction perpendicular to the rod axis is launched into the epoxy matrix At the frequency where the trans-verse wavelength is equal to the periodicity resonant scat-tering of waves by the vertical planes of the PZT rods re- sults in two-dimensionala standing wave patternas described in Part I [l] Measurements of the surface dis-

Fig 3 Echo 4gnal from compoite samples ( a ) At B frequency far away from the minima in Z ( h ) At a frequency corresponding to minima i n 2 (c) At a frequency corresponding to minima i n 2 (expanded time scale)

established after a few cycles of the incident wave The acoustic impedance minima at this frequency are caused by the cancellation effects between the incident wave and thelargeamplitudevibration of the epoxy with a phase difference of 180

Support for the above explanation is provided in Fig 3 where tone-burst pulse-echo signals reflected from a com-posite sample are shown Fig 3(a) depicts a typical signal measured at frequencies far away from the minima in the acoustic impedance The signalbuilds up to a saturation

placements of the composite at this frequency showed thatvalue in one ortwocyclesdependingonthe Q of the the epoxy at the center of theuni t cell of the periodic lat- transducerandremainsconstantTheamplitude is af-tice vibrates 180 out of phase with PZT and with a much fected only by the reflections from the rear surface of the larger amplitude This standing wave pattern is most likely composite sample as indicatedby an increase in amplitude

504

65

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-32 NO 4 JULY 1985

TABLE I1 ACOUSTICIMPEDANCE (2)OF COMPOSITES

~~ ~~ ~ ~ ~

Region 1 Region 3 (Frequency gt 2 MHz) (Frequency lt 04 MHz)

Volume Percent Measured Z Measured Z PZT Calculated Z at 35 MHz Calculated Z at 03 MHz

~~ ~

5 26 23 - 33 10 2 9 2 8 41 35

60 49 2030 30

36 45 31 82

in the last fewcycles The reflected signalfrom acom-posite at the acoustic impedance minima is shown in Fig 3(b) The amplitude approaches a saturationvalue but the signal declines after the first two cycles and reaches a min-imum value Interference of the signal with the reflected

For unpoled PZT rods the elastic modulus is E = l ~ ~= 6098 X 10 Nm2 [7] For the epoxy the elastic modulus was found tobe E = 47 X lo9 Nm2 [l] Fromthecal-culated values of El using (4) and the mean densityiilisted in Table I1 of Part I[l] the acoustic impedance of the com-

signal from the rearsurface of the composite sample makesposites was evaluated from the expression 2 = ( PEl) it difficult to analyze the wave pattern after a few cycles Figure 3(c) presentssimilardatawithanexpandedab-scissa Thus the acousticimpedanceminimaobserved arises from the destructive interference between the in-cident signal and the reflections from the composite sam-ple Therefore the minima in acoustic impedance are only an apparent phenomena becauseof the complex resonance in the composites

Resonance experiments [ l ] indicated another strong lat- eralresonancemodereferred to as J 2 Thisresonance was associated with standing waves along the unit cell di-agonal For a five-percent sample fr2 was approximately 07 MHz and a corresponding minimum was seen in the acoustic impedance plot as expected (Fig 2(a)) Acoustic impedance for the ten-percent sample is also seen to ap-proachaminimumaround 100 MHz(Fig2(a)) which corresponds to fr2 for the ten-percent composite In addi-tion to the acoustic impedance minima just described ad-ditional minima were observed up to about 2 MHz pos- sibly due to more complex lateral modes

B Modeling of Acoustic Impedance The acoustic impedance of the composites was modeled

toexplain theobservedfrequency dependence discussed in the previous section

Above 2 MHz the acoustic impedance was found to be relatively independent of frequency At this frequency ex-perimental evidenceshowed that thePZTrodswerevi-brating without an appreciable lateral interaction There-fore this situation can be modeled using Reuss averaging [6] which assumes that the constituent phases experience the same stress Accordingto the Reuss averagingscheme the modulus of elasticity parallel to the length of the rod El is given by

where u I and u2 are volume fractions and E l and E2 are the appropriate elastic moduli of the constituent phases

Both the measuredandcalculatedvaluesarecompared inTable 11 The frequencyrangeabove2MHzwhere the wavelength of the transverse waves is much smaller thantheperiodicity of the lattice is calledregion 1 for the purpose of discussion There is a good agreement be tweenthetheoryandtheexperimentsupportingthe ex-planation of the mode of vibration outlined above for fre- quencies about 2 MHz

At frequencies below the minima in 2 corresponding to J the lateral interaction increases gradually and the two phases vibrate in a cooperative mode The displacements of PZT and epoxy were found not only to be in phase but also of almost equalamplitude [l] Thus the Voigt av- eraging scheme [6] which assumes constant strain on the constituentphasescan be used to estimate the effective longitudinal modulus El of the composite The composite modulus according to the Voigt model is given by

El = ~ 1 E l+ ~ 2 E 2 ( 5 )

Using the given values of the elastic moduli for the PZT and epoxy the effective modulus and acoustic impedance of the composites were calculated and they are compared with the measured values of 2 at 03 MHz in Table 11 This frequency range where thewaveiength of transverse waves is larger than the periodicity is called region 3 in this pa-per The theoretically predicted increase in the acoustic impedance at frequencies below the first acoustic imped-ance minima was seen in all the composites However the measured values were on the average20 percent lower than the estimated value The discrepancy between the exper- iment and theory is possibly due to the diffraction losses Because of the relatively small size of the composite sam-ple (diameter = 19 cm) compared to the size of the steel reflector (10 X 10 cm) it is conceivable that not all of the acousticenergywasreflectedbackfromthecomposite sample to the transducer

A correction to account for the diffraction losses in the measurements of acoustic impedance was applied experi-mentally by using the standard reflectorof exactly the same

505 GURURAJA et al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE 111 Doto m m c t e d fw IOY Compor(t0 difhoclion lorus

iOO 05 10

Frequency (MHz)

Fig 4 Correction on the value of acoustic impedance due to diffraction losses

ACOUSTICIMPEDANCEBY TRANSMISSIONTECHNIQUE

Measured LongitudinalMeasured Z Calculated Z

Volume Percent Velocity lo6(atrayl lo6 ray1 (from PZT W s ) 05 MHz) Table 11)

20 2682 i 8 61 60 30 82 2780 It 14 85

along the PZT rod axis The method chosenfor measuring the longitudinal velocity was similar to the standing-wave

dimension as the composite sample Then the experimen-tal setup for the two reflections to be compared becomes identical and hence the uncertainty in the measurement because of the diffraction lossesis minimized Fig 4 com-pares the earlier data with the new data corrected for dif-fraction losses for a ten-percent composite sample As can be seenthecorrected value of the acousticimpedance approaches 41 M rayl at 03 MHz which is in excellent

strainmodel (Table 11) Based on the experimentaland theoretical results the acoustic impedance- as a function of frequency can be classified into three regions

In region 1 corresponding to frequencies above 2 MHz thetransverse wavelength is muchsmallerthanthe rod spacing d andthetwophasesappear to bedecoupled There is not much influence of epoxy on the vibration of thePZTrods A significantimprovement in the pulse-echo response in terms of the figure of merit cannot be

agreement with the predicted values based on the constant Thelongitudinal velocity was determined for 20- and

expected in this region if the composite is used as a single elementtransducerOntheotherhandthesituation is desirableforalinear- or phased-arraytransducercon-structionwheretheacousticalcross-couplingbetween elements should be as low as possible

In region 3 corresponding to the frequency far below thefirstminimum in the acousticimpedancethecom-posite acts as a homogeneous material Maximum stress is transferred from PZT to polymer and vice versa The composite is expected to perform better as a singleele-ment transducer with an improvedpulse-echofigure of merit

In region 2 the periodicity of the lattice and the wave-length of the transverse waves are comparableThis re-gionishelpful in understanding thecomplexvibrational modes in two dimensionallyperiodicstructures In this region the composite can possibly be used as a resonant sound absorber

C Transmission Technique

The acoustic impedance of the composites was also de-termined by measuring the velocity of longitudinal waves

method described by McSkimin [8] An acoustic wave of a particular frequency is made to impinge on the sample The wave transmitted to thespecimenisreflectedback and forth giving rise to a series of transmitted pulses At discrete frequencies the emerging wave trains will be in phaseleadingtoaconstructiveinterferenceThecon-structive interference occuri at a frequency for which the path length (twice the specimen thickness) is equal to an integralnumberof wavelengths in thespecimenatthat frequency The principle of the method is to determine the value of the integer at oneof the discrete frequencieswhen the in-phase condition is satisfied The experiment is de-scribed in detail elsewhere [9]

30-percent PZT composites around05 MHz which is be-low the frequencycorrespondingtotheacousticimped-ance minima (Fig 2(a)) The velocity values were mutli-plied by the average density to obtain theacousticim-pedance The resulting data shownin Table 111 are in ex-cellent agreement with the values estimated theoretically for region 3

D Resonance Technique The velocity of longitudinal waves determined by the

resonance technique described in Part I [ l] was also used to determine the acoustic impedance of composite sam-ples The velocitites for different composities are listed in thelastcolumn ofTable V of Part I [ l ] Valuesof the acoustic impedance evaluated from the measured velocity anddensity of composities are listed inTable IV Theo- retically calculated acoustic impedance are also tabulated for comparison In the piezoelectricresonanceexperi-ments PZT rods in the composite are electrically poled Hence for calculating the elastic modulus of the compos-itethemodulus of the PZT rods E = = 105 X 10 Nm2 (7) was used

As it emerges from Table IV in region 1 ( gt2 MHz) a pooragreement betweenthe estimatedandmeasured acoustic impedancewas foundThis was anticipated be-cause the rods are vibrating relatively freely and the res-onancetechniquemeasuredthe velocity of PZT phase only Hence the product of this velocity with the average density of the sample gives a much higher estimate of the acoustic impedance

In region 3 (lt05 MHz) theagreementbetweenthe

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

502 IEEE TRANSACTIONSULTRASONICSON SONICS VOL SU-32 NO 4 JULY 1985AND

otropic behavior The properties of the constituent phases must be averaged properly In the PZT rod-polymer com-posites considered here the interaction between polymer and PZT and among neighboring PZT rods is a function of frequency Thereforedependence of acoustic imped-ance on frequency also needsto be considered Three tech-niquesnamelyreflectiontransmissionandresonance wereusedtodeterminetheacousticimpedanceand its frequencydependenceTheresultsarereported in the following subsections Theacousticimpedance of the composites were also theoretically modeled and compared with the experimental data

A Rejection Technique

In the reflection technique the acoustic impedance of a composite was determined by comparing the amplitude of reflection of a plane acoustic wave from the sample with thatfromastandard material of known acoustic imped-ance eg stainless steel The experimental setup was the same as that used for the tone-burst pulse-echo technique (Fig 1 ) The reflector was placed at the a2X distance A commercial ultrasonic transducer was used to measure the pulse-echo signal from both the sample and the standard reflector The transmitting voltage response and receiving voltage sensitivity of the transducer need not be taken into account as the same transducer is used for measuring re-flections from boththestandardmaterialand thespeci-men

The tone-burst pulse-echo experiment was carried out as a function of frequency with a stainless steel block (10 cm X I O cm X 254) reflector The driving voltage of the transducer was kept constant at 10 V It can be shown [4] that the pulse-echo amplitude X is proportional to the re-flectioncoefficient atthewater-reflector interface That is [4]

z - zX a ___

2 + z where Zl and Z2 are the acoustic impedance of the steel reflector and water respectively

Thestainless-steel blockwas thenreplaced by a com- posite sample and the pulse-echo amplitude Y was noted If 2 is the acoustic impedance of the composite sample

z - zYCY z + Zz

From (1) and (2) the acoustic impedance of the com-posite sample (Z) can be expressed in terms of X Y ZI and Z Since it is knownthat Z = 454 X lo6 ray1 and Z2 = 15 X lo6 rayl [S] theexpressionfortheacoustic impedance of the composite sample is given by

X lo6 rayl (3)

By substituting the values of X and Y at a specified fre-quency the acoustic impedance can be calculated from(3)

=Px 7 f i

0 05 Io l 5 Frequency (MHz)

(a)

l 2 3 4 Frequency (MHz)

( b)

Fig 2 Acousticimpedance of composites as a function of frequency (a) 0-15 MHz (b) 1-35 MHz

The validity of the technique was tested by determining the acoustic impedance of a few standard materials such as lucite fused silica and aluminum with known acoustic impedance of 31 13 and 17 M rayl respectively The measured values of Z for these materials were 32 126 and171 M rayl which are in excellent agreement The experiments were also conducted with transducers of dif- ferent type (focused and nonfocused) and with the reflec-torplaced at different distances (at a2X distance at the focal length of the transducer etc) No substantial differ-ence in the measured impedances was observed

Composite samples of thickness 2-3 cm with 5 10 20 and 30 volume percentPZTrods of diameter 045 mm were fabricated The samples were not poled because such thicksamplesrequireexcessively large poling voltages (40-60 kV) Fig 2(a) shows a plot of the acoustiz imped-ance of several different composites in the frequency range from 03 to 13 MHz and Fig 2(b) extends the plot from 1 to 35 MHz Low-frequency data were obtained using a lead metaniobate transducer (Ultran Lab Inc diameter= 25 cms) with a center frequency of 07 MHz and a very broadbandwidth (Q = 1) Thedistancebetweenthe transducer and the reflector was 75 cm corresponding to the a2X distanceat 07 MHz Measurements in the fre-quency range from 1 to 35 MHz were carried out using a

503 GURURAJA et 01 PIEZOELECTRIC COMPOSITE MATERIALS-PART I 1

focused transducer (Rohe 5616 diameter = 19 cm focal length = 9 cm) with a center frequency of 225 MHz The separation between the transducer and reflectorin this case was 9 cm which is the focal length of the transducer

The following observations were madeby inspecting the acousticimpedanceplots of Fig 2 As expected the acoustic impedance increased with increase in the volume percentPZTMoreovertheacousticimpedance was stronglyfrequencydependentThecompositesamples showed a series of minima in 2 at low frequency Above 25 MHz the variations are minimized and the value of 2 reached a saturation The position of the first minimum in Z occurred at a higher frequency for largervolume percent PZTTheacousticimpedanceincreasedsteadily at fre- quencies below the first minimum Theacousticimped-ance of single phase epoxy measured by the same proce-dure is also shown in Fig 2 for comparison There was no variation of 2 of epoxy in the frequency range consid-ered in this work The results were analyzed carefully to find an explanation for the observed dispersion in 2

The minima for 5 10 20 and 30 percent PZT compos-ites occur at 04506250825and 1125 MHz respec-tively The frequencies have an inverserelationship with the lateral periodicity of the corresponding composite The product of the frequency at the acoustic impedance min-ima and the lateral periodicity of the composite was found to be a constant of value 800 ms It is interesting to note that the frequency corresponding to the minimain 2 in each composite matches with the resonance frequency f [ I ] The resonance frequency f corresponds to the standing wave pattern of the transverse waves with the wavelength equal to the lateral periodicity of the lattice It should be pointed out here that the value of the acoustic impedance at the minima was approximately the same for all volume percent PZT (16 M rayl) which is surprisingly lower than that of the epoxy (23 M rayl)

There is a small discrepancy in the values of acoustic impedance around 1 MHz measured using the two differ- ent transducers This is possibly because the transducers were operating far away from their center frequencies and hencewerenotverysensitiveThus themeasurements performed in the transition region from one transducer to the other are not very precise

All of these phenomena may be explained as follows The acoustic wave incident on the composite sets it into vibration Due to the difference in compliancebetween the two component phases namely PZT and Spurrs epoxy a transverse wave originating at the interface and propagat-ing in a direction perpendicular to the rod axis is launched into the epoxy matrix At the frequency where the trans-verse wavelength is equal to the periodicity resonant scat-tering of waves by the vertical planes of the PZT rods re- sults in two-dimensionala standing wave patternas described in Part I [l] Measurements of the surface dis-

Fig 3 Echo 4gnal from compoite samples ( a ) At B frequency far away from the minima in Z ( h ) At a frequency corresponding to minima i n 2 (c) At a frequency corresponding to minima i n 2 (expanded time scale)

established after a few cycles of the incident wave The acoustic impedance minima at this frequency are caused by the cancellation effects between the incident wave and thelargeamplitudevibration of the epoxy with a phase difference of 180

Support for the above explanation is provided in Fig 3 where tone-burst pulse-echo signals reflected from a com-posite sample are shown Fig 3(a) depicts a typical signal measured at frequencies far away from the minima in the acoustic impedance The signalbuilds up to a saturation

placements of the composite at this frequency showed thatvalue in one ortwocyclesdependingonthe Q of the the epoxy at the center of theuni t cell of the periodic lat- transducerandremainsconstantTheamplitude is af-tice vibrates 180 out of phase with PZT and with a much fected only by the reflections from the rear surface of the larger amplitude This standing wave pattern is most likely composite sample as indicatedby an increase in amplitude

504

65

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-32 NO 4 JULY 1985

TABLE I1 ACOUSTICIMPEDANCE (2)OF COMPOSITES

~~ ~~ ~ ~ ~

Region 1 Region 3 (Frequency gt 2 MHz) (Frequency lt 04 MHz)

Volume Percent Measured Z Measured Z PZT Calculated Z at 35 MHz Calculated Z at 03 MHz

~~ ~

5 26 23 - 33 10 2 9 2 8 41 35

60 49 2030 30

36 45 31 82

in the last fewcycles The reflected signalfrom acom-posite at the acoustic impedance minima is shown in Fig 3(b) The amplitude approaches a saturationvalue but the signal declines after the first two cycles and reaches a min-imum value Interference of the signal with the reflected

For unpoled PZT rods the elastic modulus is E = l ~ ~= 6098 X 10 Nm2 [7] For the epoxy the elastic modulus was found tobe E = 47 X lo9 Nm2 [l] Fromthecal-culated values of El using (4) and the mean densityiilisted in Table I1 of Part I[l] the acoustic impedance of the com-

signal from the rearsurface of the composite sample makesposites was evaluated from the expression 2 = ( PEl) it difficult to analyze the wave pattern after a few cycles Figure 3(c) presentssimilardatawithanexpandedab-scissa Thus the acousticimpedanceminimaobserved arises from the destructive interference between the in-cident signal and the reflections from the composite sam-ple Therefore the minima in acoustic impedance are only an apparent phenomena becauseof the complex resonance in the composites

Resonance experiments [ l ] indicated another strong lat- eralresonancemodereferred to as J 2 Thisresonance was associated with standing waves along the unit cell di-agonal For a five-percent sample fr2 was approximately 07 MHz and a corresponding minimum was seen in the acoustic impedance plot as expected (Fig 2(a)) Acoustic impedance for the ten-percent sample is also seen to ap-proachaminimumaround 100 MHz(Fig2(a)) which corresponds to fr2 for the ten-percent composite In addi-tion to the acoustic impedance minima just described ad-ditional minima were observed up to about 2 MHz pos- sibly due to more complex lateral modes

B Modeling of Acoustic Impedance The acoustic impedance of the composites was modeled

toexplain theobservedfrequency dependence discussed in the previous section

Above 2 MHz the acoustic impedance was found to be relatively independent of frequency At this frequency ex-perimental evidenceshowed that thePZTrodswerevi-brating without an appreciable lateral interaction There-fore this situation can be modeled using Reuss averaging [6] which assumes that the constituent phases experience the same stress Accordingto the Reuss averagingscheme the modulus of elasticity parallel to the length of the rod El is given by

where u I and u2 are volume fractions and E l and E2 are the appropriate elastic moduli of the constituent phases

Both the measuredandcalculatedvaluesarecompared inTable 11 The frequencyrangeabove2MHzwhere the wavelength of the transverse waves is much smaller thantheperiodicity of the lattice is calledregion 1 for the purpose of discussion There is a good agreement be tweenthetheoryandtheexperimentsupportingthe ex-planation of the mode of vibration outlined above for fre- quencies about 2 MHz

At frequencies below the minima in 2 corresponding to J the lateral interaction increases gradually and the two phases vibrate in a cooperative mode The displacements of PZT and epoxy were found not only to be in phase but also of almost equalamplitude [l] Thus the Voigt av- eraging scheme [6] which assumes constant strain on the constituentphasescan be used to estimate the effective longitudinal modulus El of the composite The composite modulus according to the Voigt model is given by

El = ~ 1 E l+ ~ 2 E 2 ( 5 )

Using the given values of the elastic moduli for the PZT and epoxy the effective modulus and acoustic impedance of the composites were calculated and they are compared with the measured values of 2 at 03 MHz in Table 11 This frequency range where thewaveiength of transverse waves is larger than the periodicity is called region 3 in this pa-per The theoretically predicted increase in the acoustic impedance at frequencies below the first acoustic imped-ance minima was seen in all the composites However the measured values were on the average20 percent lower than the estimated value The discrepancy between the exper- iment and theory is possibly due to the diffraction losses Because of the relatively small size of the composite sam-ple (diameter = 19 cm) compared to the size of the steel reflector (10 X 10 cm) it is conceivable that not all of the acousticenergywasreflectedbackfromthecomposite sample to the transducer

A correction to account for the diffraction losses in the measurements of acoustic impedance was applied experi-mentally by using the standard reflectorof exactly the same

505 GURURAJA et al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE 111 Doto m m c t e d fw IOY Compor(t0 difhoclion lorus

iOO 05 10

Frequency (MHz)

Fig 4 Correction on the value of acoustic impedance due to diffraction losses

ACOUSTICIMPEDANCEBY TRANSMISSIONTECHNIQUE

Measured LongitudinalMeasured Z Calculated Z

Volume Percent Velocity lo6(atrayl lo6 ray1 (from PZT W s ) 05 MHz) Table 11)

20 2682 i 8 61 60 30 82 2780 It 14 85

along the PZT rod axis The method chosenfor measuring the longitudinal velocity was similar to the standing-wave

dimension as the composite sample Then the experimen-tal setup for the two reflections to be compared becomes identical and hence the uncertainty in the measurement because of the diffraction lossesis minimized Fig 4 com-pares the earlier data with the new data corrected for dif-fraction losses for a ten-percent composite sample As can be seenthecorrected value of the acousticimpedance approaches 41 M rayl at 03 MHz which is in excellent

strainmodel (Table 11) Based on the experimentaland theoretical results the acoustic impedance- as a function of frequency can be classified into three regions

In region 1 corresponding to frequencies above 2 MHz thetransverse wavelength is muchsmallerthanthe rod spacing d andthetwophasesappear to bedecoupled There is not much influence of epoxy on the vibration of thePZTrods A significantimprovement in the pulse-echo response in terms of the figure of merit cannot be

agreement with the predicted values based on the constant Thelongitudinal velocity was determined for 20- and

expected in this region if the composite is used as a single elementtransducerOntheotherhandthesituation is desirableforalinear- or phased-arraytransducercon-structionwheretheacousticalcross-couplingbetween elements should be as low as possible

In region 3 corresponding to the frequency far below thefirstminimum in the acousticimpedancethecom-posite acts as a homogeneous material Maximum stress is transferred from PZT to polymer and vice versa The composite is expected to perform better as a singleele-ment transducer with an improvedpulse-echofigure of merit

In region 2 the periodicity of the lattice and the wave-length of the transverse waves are comparableThis re-gionishelpful in understanding thecomplexvibrational modes in two dimensionallyperiodicstructures In this region the composite can possibly be used as a resonant sound absorber

C Transmission Technique

The acoustic impedance of the composites was also de-termined by measuring the velocity of longitudinal waves

method described by McSkimin [8] An acoustic wave of a particular frequency is made to impinge on the sample The wave transmitted to thespecimenisreflectedback and forth giving rise to a series of transmitted pulses At discrete frequencies the emerging wave trains will be in phaseleadingtoaconstructiveinterferenceThecon-structive interference occuri at a frequency for which the path length (twice the specimen thickness) is equal to an integralnumberof wavelengths in thespecimenatthat frequency The principle of the method is to determine the value of the integer at oneof the discrete frequencieswhen the in-phase condition is satisfied The experiment is de-scribed in detail elsewhere [9]

30-percent PZT composites around05 MHz which is be-low the frequencycorrespondingtotheacousticimped-ance minima (Fig 2(a)) The velocity values were mutli-plied by the average density to obtain theacousticim-pedance The resulting data shownin Table 111 are in ex-cellent agreement with the values estimated theoretically for region 3

D Resonance Technique The velocity of longitudinal waves determined by the

resonance technique described in Part I [ l] was also used to determine the acoustic impedance of composite sam-ples The velocitites for different composities are listed in thelastcolumn ofTable V of Part I [ l ] Valuesof the acoustic impedance evaluated from the measured velocity anddensity of composities are listed inTable IV Theo- retically calculated acoustic impedance are also tabulated for comparison In the piezoelectricresonanceexperi-ments PZT rods in the composite are electrically poled Hence for calculating the elastic modulus of the compos-itethemodulus of the PZT rods E = = 105 X 10 Nm2 (7) was used

As it emerges from Table IV in region 1 ( gt2 MHz) a pooragreement betweenthe estimatedandmeasured acoustic impedancewas foundThis was anticipated be-cause the rods are vibrating relatively freely and the res-onancetechniquemeasuredthe velocity of PZT phase only Hence the product of this velocity with the average density of the sample gives a much higher estimate of the acoustic impedance

In region 3 (lt05 MHz) theagreementbetweenthe

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

503 GURURAJA et 01 PIEZOELECTRIC COMPOSITE MATERIALS-PART I 1

focused transducer (Rohe 5616 diameter = 19 cm focal length = 9 cm) with a center frequency of 225 MHz The separation between the transducer and reflectorin this case was 9 cm which is the focal length of the transducer

The following observations were madeby inspecting the acousticimpedanceplots of Fig 2 As expected the acoustic impedance increased with increase in the volume percentPZTMoreovertheacousticimpedance was stronglyfrequencydependentThecompositesamples showed a series of minima in 2 at low frequency Above 25 MHz the variations are minimized and the value of 2 reached a saturation The position of the first minimum in Z occurred at a higher frequency for largervolume percent PZTTheacousticimpedanceincreasedsteadily at fre- quencies below the first minimum Theacousticimped-ance of single phase epoxy measured by the same proce-dure is also shown in Fig 2 for comparison There was no variation of 2 of epoxy in the frequency range consid-ered in this work The results were analyzed carefully to find an explanation for the observed dispersion in 2

The minima for 5 10 20 and 30 percent PZT compos-ites occur at 04506250825and 1125 MHz respec-tively The frequencies have an inverserelationship with the lateral periodicity of the corresponding composite The product of the frequency at the acoustic impedance min-ima and the lateral periodicity of the composite was found to be a constant of value 800 ms It is interesting to note that the frequency corresponding to the minimain 2 in each composite matches with the resonance frequency f [ I ] The resonance frequency f corresponds to the standing wave pattern of the transverse waves with the wavelength equal to the lateral periodicity of the lattice It should be pointed out here that the value of the acoustic impedance at the minima was approximately the same for all volume percent PZT (16 M rayl) which is surprisingly lower than that of the epoxy (23 M rayl)

There is a small discrepancy in the values of acoustic impedance around 1 MHz measured using the two differ- ent transducers This is possibly because the transducers were operating far away from their center frequencies and hencewerenotverysensitiveThus themeasurements performed in the transition region from one transducer to the other are not very precise

All of these phenomena may be explained as follows The acoustic wave incident on the composite sets it into vibration Due to the difference in compliancebetween the two component phases namely PZT and Spurrs epoxy a transverse wave originating at the interface and propagat-ing in a direction perpendicular to the rod axis is launched into the epoxy matrix At the frequency where the trans-verse wavelength is equal to the periodicity resonant scat-tering of waves by the vertical planes of the PZT rods re- sults in two-dimensionala standing wave patternas described in Part I [l] Measurements of the surface dis-

Fig 3 Echo 4gnal from compoite samples ( a ) At B frequency far away from the minima in Z ( h ) At a frequency corresponding to minima i n 2 (c) At a frequency corresponding to minima i n 2 (expanded time scale)

established after a few cycles of the incident wave The acoustic impedance minima at this frequency are caused by the cancellation effects between the incident wave and thelargeamplitudevibration of the epoxy with a phase difference of 180

Support for the above explanation is provided in Fig 3 where tone-burst pulse-echo signals reflected from a com-posite sample are shown Fig 3(a) depicts a typical signal measured at frequencies far away from the minima in the acoustic impedance The signalbuilds up to a saturation

placements of the composite at this frequency showed thatvalue in one ortwocyclesdependingonthe Q of the the epoxy at the center of theuni t cell of the periodic lat- transducerandremainsconstantTheamplitude is af-tice vibrates 180 out of phase with PZT and with a much fected only by the reflections from the rear surface of the larger amplitude This standing wave pattern is most likely composite sample as indicatedby an increase in amplitude

504

65

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-32 NO 4 JULY 1985

TABLE I1 ACOUSTICIMPEDANCE (2)OF COMPOSITES

~~ ~~ ~ ~ ~

Region 1 Region 3 (Frequency gt 2 MHz) (Frequency lt 04 MHz)

Volume Percent Measured Z Measured Z PZT Calculated Z at 35 MHz Calculated Z at 03 MHz

~~ ~

5 26 23 - 33 10 2 9 2 8 41 35

60 49 2030 30

36 45 31 82

in the last fewcycles The reflected signalfrom acom-posite at the acoustic impedance minima is shown in Fig 3(b) The amplitude approaches a saturationvalue but the signal declines after the first two cycles and reaches a min-imum value Interference of the signal with the reflected

For unpoled PZT rods the elastic modulus is E = l ~ ~= 6098 X 10 Nm2 [7] For the epoxy the elastic modulus was found tobe E = 47 X lo9 Nm2 [l] Fromthecal-culated values of El using (4) and the mean densityiilisted in Table I1 of Part I[l] the acoustic impedance of the com-

signal from the rearsurface of the composite sample makesposites was evaluated from the expression 2 = ( PEl) it difficult to analyze the wave pattern after a few cycles Figure 3(c) presentssimilardatawithanexpandedab-scissa Thus the acousticimpedanceminimaobserved arises from the destructive interference between the in-cident signal and the reflections from the composite sam-ple Therefore the minima in acoustic impedance are only an apparent phenomena becauseof the complex resonance in the composites

Resonance experiments [ l ] indicated another strong lat- eralresonancemodereferred to as J 2 Thisresonance was associated with standing waves along the unit cell di-agonal For a five-percent sample fr2 was approximately 07 MHz and a corresponding minimum was seen in the acoustic impedance plot as expected (Fig 2(a)) Acoustic impedance for the ten-percent sample is also seen to ap-proachaminimumaround 100 MHz(Fig2(a)) which corresponds to fr2 for the ten-percent composite In addi-tion to the acoustic impedance minima just described ad-ditional minima were observed up to about 2 MHz pos- sibly due to more complex lateral modes

B Modeling of Acoustic Impedance The acoustic impedance of the composites was modeled

toexplain theobservedfrequency dependence discussed in the previous section

Above 2 MHz the acoustic impedance was found to be relatively independent of frequency At this frequency ex-perimental evidenceshowed that thePZTrodswerevi-brating without an appreciable lateral interaction There-fore this situation can be modeled using Reuss averaging [6] which assumes that the constituent phases experience the same stress Accordingto the Reuss averagingscheme the modulus of elasticity parallel to the length of the rod El is given by

where u I and u2 are volume fractions and E l and E2 are the appropriate elastic moduli of the constituent phases

Both the measuredandcalculatedvaluesarecompared inTable 11 The frequencyrangeabove2MHzwhere the wavelength of the transverse waves is much smaller thantheperiodicity of the lattice is calledregion 1 for the purpose of discussion There is a good agreement be tweenthetheoryandtheexperimentsupportingthe ex-planation of the mode of vibration outlined above for fre- quencies about 2 MHz

At frequencies below the minima in 2 corresponding to J the lateral interaction increases gradually and the two phases vibrate in a cooperative mode The displacements of PZT and epoxy were found not only to be in phase but also of almost equalamplitude [l] Thus the Voigt av- eraging scheme [6] which assumes constant strain on the constituentphasescan be used to estimate the effective longitudinal modulus El of the composite The composite modulus according to the Voigt model is given by

El = ~ 1 E l+ ~ 2 E 2 ( 5 )

Using the given values of the elastic moduli for the PZT and epoxy the effective modulus and acoustic impedance of the composites were calculated and they are compared with the measured values of 2 at 03 MHz in Table 11 This frequency range where thewaveiength of transverse waves is larger than the periodicity is called region 3 in this pa-per The theoretically predicted increase in the acoustic impedance at frequencies below the first acoustic imped-ance minima was seen in all the composites However the measured values were on the average20 percent lower than the estimated value The discrepancy between the exper- iment and theory is possibly due to the diffraction losses Because of the relatively small size of the composite sam-ple (diameter = 19 cm) compared to the size of the steel reflector (10 X 10 cm) it is conceivable that not all of the acousticenergywasreflectedbackfromthecomposite sample to the transducer

A correction to account for the diffraction losses in the measurements of acoustic impedance was applied experi-mentally by using the standard reflectorof exactly the same

505 GURURAJA et al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE 111 Doto m m c t e d fw IOY Compor(t0 difhoclion lorus

iOO 05 10

Frequency (MHz)

Fig 4 Correction on the value of acoustic impedance due to diffraction losses

ACOUSTICIMPEDANCEBY TRANSMISSIONTECHNIQUE

Measured LongitudinalMeasured Z Calculated Z

Volume Percent Velocity lo6(atrayl lo6 ray1 (from PZT W s ) 05 MHz) Table 11)

20 2682 i 8 61 60 30 82 2780 It 14 85

along the PZT rod axis The method chosenfor measuring the longitudinal velocity was similar to the standing-wave

dimension as the composite sample Then the experimen-tal setup for the two reflections to be compared becomes identical and hence the uncertainty in the measurement because of the diffraction lossesis minimized Fig 4 com-pares the earlier data with the new data corrected for dif-fraction losses for a ten-percent composite sample As can be seenthecorrected value of the acousticimpedance approaches 41 M rayl at 03 MHz which is in excellent

strainmodel (Table 11) Based on the experimentaland theoretical results the acoustic impedance- as a function of frequency can be classified into three regions

In region 1 corresponding to frequencies above 2 MHz thetransverse wavelength is muchsmallerthanthe rod spacing d andthetwophasesappear to bedecoupled There is not much influence of epoxy on the vibration of thePZTrods A significantimprovement in the pulse-echo response in terms of the figure of merit cannot be

agreement with the predicted values based on the constant Thelongitudinal velocity was determined for 20- and

expected in this region if the composite is used as a single elementtransducerOntheotherhandthesituation is desirableforalinear- or phased-arraytransducercon-structionwheretheacousticalcross-couplingbetween elements should be as low as possible

In region 3 corresponding to the frequency far below thefirstminimum in the acousticimpedancethecom-posite acts as a homogeneous material Maximum stress is transferred from PZT to polymer and vice versa The composite is expected to perform better as a singleele-ment transducer with an improvedpulse-echofigure of merit

In region 2 the periodicity of the lattice and the wave-length of the transverse waves are comparableThis re-gionishelpful in understanding thecomplexvibrational modes in two dimensionallyperiodicstructures In this region the composite can possibly be used as a resonant sound absorber

C Transmission Technique

The acoustic impedance of the composites was also de-termined by measuring the velocity of longitudinal waves

method described by McSkimin [8] An acoustic wave of a particular frequency is made to impinge on the sample The wave transmitted to thespecimenisreflectedback and forth giving rise to a series of transmitted pulses At discrete frequencies the emerging wave trains will be in phaseleadingtoaconstructiveinterferenceThecon-structive interference occuri at a frequency for which the path length (twice the specimen thickness) is equal to an integralnumberof wavelengths in thespecimenatthat frequency The principle of the method is to determine the value of the integer at oneof the discrete frequencieswhen the in-phase condition is satisfied The experiment is de-scribed in detail elsewhere [9]

30-percent PZT composites around05 MHz which is be-low the frequencycorrespondingtotheacousticimped-ance minima (Fig 2(a)) The velocity values were mutli-plied by the average density to obtain theacousticim-pedance The resulting data shownin Table 111 are in ex-cellent agreement with the values estimated theoretically for region 3

D Resonance Technique The velocity of longitudinal waves determined by the

resonance technique described in Part I [ l] was also used to determine the acoustic impedance of composite sam-ples The velocitites for different composities are listed in thelastcolumn ofTable V of Part I [ l ] Valuesof the acoustic impedance evaluated from the measured velocity anddensity of composities are listed inTable IV Theo- retically calculated acoustic impedance are also tabulated for comparison In the piezoelectricresonanceexperi-ments PZT rods in the composite are electrically poled Hence for calculating the elastic modulus of the compos-itethemodulus of the PZT rods E = = 105 X 10 Nm2 (7) was used

As it emerges from Table IV in region 1 ( gt2 MHz) a pooragreement betweenthe estimatedandmeasured acoustic impedancewas foundThis was anticipated be-cause the rods are vibrating relatively freely and the res-onancetechniquemeasuredthe velocity of PZT phase only Hence the product of this velocity with the average density of the sample gives a much higher estimate of the acoustic impedance

In region 3 (lt05 MHz) theagreementbetweenthe

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

504

65

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOLSU-32 NO 4 JULY 1985

TABLE I1 ACOUSTICIMPEDANCE (2)OF COMPOSITES

~~ ~~ ~ ~ ~

Region 1 Region 3 (Frequency gt 2 MHz) (Frequency lt 04 MHz)

Volume Percent Measured Z Measured Z PZT Calculated Z at 35 MHz Calculated Z at 03 MHz

~~ ~

5 26 23 - 33 10 2 9 2 8 41 35

60 49 2030 30

36 45 31 82

in the last fewcycles The reflected signalfrom acom-posite at the acoustic impedance minima is shown in Fig 3(b) The amplitude approaches a saturationvalue but the signal declines after the first two cycles and reaches a min-imum value Interference of the signal with the reflected

For unpoled PZT rods the elastic modulus is E = l ~ ~= 6098 X 10 Nm2 [7] For the epoxy the elastic modulus was found tobe E = 47 X lo9 Nm2 [l] Fromthecal-culated values of El using (4) and the mean densityiilisted in Table I1 of Part I[l] the acoustic impedance of the com-

signal from the rearsurface of the composite sample makesposites was evaluated from the expression 2 = ( PEl) it difficult to analyze the wave pattern after a few cycles Figure 3(c) presentssimilardatawithanexpandedab-scissa Thus the acousticimpedanceminimaobserved arises from the destructive interference between the in-cident signal and the reflections from the composite sam-ple Therefore the minima in acoustic impedance are only an apparent phenomena becauseof the complex resonance in the composites

Resonance experiments [ l ] indicated another strong lat- eralresonancemodereferred to as J 2 Thisresonance was associated with standing waves along the unit cell di-agonal For a five-percent sample fr2 was approximately 07 MHz and a corresponding minimum was seen in the acoustic impedance plot as expected (Fig 2(a)) Acoustic impedance for the ten-percent sample is also seen to ap-proachaminimumaround 100 MHz(Fig2(a)) which corresponds to fr2 for the ten-percent composite In addi-tion to the acoustic impedance minima just described ad-ditional minima were observed up to about 2 MHz pos- sibly due to more complex lateral modes

B Modeling of Acoustic Impedance The acoustic impedance of the composites was modeled

toexplain theobservedfrequency dependence discussed in the previous section

Above 2 MHz the acoustic impedance was found to be relatively independent of frequency At this frequency ex-perimental evidenceshowed that thePZTrodswerevi-brating without an appreciable lateral interaction There-fore this situation can be modeled using Reuss averaging [6] which assumes that the constituent phases experience the same stress Accordingto the Reuss averagingscheme the modulus of elasticity parallel to the length of the rod El is given by

where u I and u2 are volume fractions and E l and E2 are the appropriate elastic moduli of the constituent phases

Both the measuredandcalculatedvaluesarecompared inTable 11 The frequencyrangeabove2MHzwhere the wavelength of the transverse waves is much smaller thantheperiodicity of the lattice is calledregion 1 for the purpose of discussion There is a good agreement be tweenthetheoryandtheexperimentsupportingthe ex-planation of the mode of vibration outlined above for fre- quencies about 2 MHz

At frequencies below the minima in 2 corresponding to J the lateral interaction increases gradually and the two phases vibrate in a cooperative mode The displacements of PZT and epoxy were found not only to be in phase but also of almost equalamplitude [l] Thus the Voigt av- eraging scheme [6] which assumes constant strain on the constituentphasescan be used to estimate the effective longitudinal modulus El of the composite The composite modulus according to the Voigt model is given by

El = ~ 1 E l+ ~ 2 E 2 ( 5 )

Using the given values of the elastic moduli for the PZT and epoxy the effective modulus and acoustic impedance of the composites were calculated and they are compared with the measured values of 2 at 03 MHz in Table 11 This frequency range where thewaveiength of transverse waves is larger than the periodicity is called region 3 in this pa-per The theoretically predicted increase in the acoustic impedance at frequencies below the first acoustic imped-ance minima was seen in all the composites However the measured values were on the average20 percent lower than the estimated value The discrepancy between the exper- iment and theory is possibly due to the diffraction losses Because of the relatively small size of the composite sam-ple (diameter = 19 cm) compared to the size of the steel reflector (10 X 10 cm) it is conceivable that not all of the acousticenergywasreflectedbackfromthecomposite sample to the transducer

A correction to account for the diffraction losses in the measurements of acoustic impedance was applied experi-mentally by using the standard reflectorof exactly the same

505 GURURAJA et al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE 111 Doto m m c t e d fw IOY Compor(t0 difhoclion lorus

iOO 05 10

Frequency (MHz)

Fig 4 Correction on the value of acoustic impedance due to diffraction losses

ACOUSTICIMPEDANCEBY TRANSMISSIONTECHNIQUE

Measured LongitudinalMeasured Z Calculated Z

Volume Percent Velocity lo6(atrayl lo6 ray1 (from PZT W s ) 05 MHz) Table 11)

20 2682 i 8 61 60 30 82 2780 It 14 85

along the PZT rod axis The method chosenfor measuring the longitudinal velocity was similar to the standing-wave

dimension as the composite sample Then the experimen-tal setup for the two reflections to be compared becomes identical and hence the uncertainty in the measurement because of the diffraction lossesis minimized Fig 4 com-pares the earlier data with the new data corrected for dif-fraction losses for a ten-percent composite sample As can be seenthecorrected value of the acousticimpedance approaches 41 M rayl at 03 MHz which is in excellent

strainmodel (Table 11) Based on the experimentaland theoretical results the acoustic impedance- as a function of frequency can be classified into three regions

In region 1 corresponding to frequencies above 2 MHz thetransverse wavelength is muchsmallerthanthe rod spacing d andthetwophasesappear to bedecoupled There is not much influence of epoxy on the vibration of thePZTrods A significantimprovement in the pulse-echo response in terms of the figure of merit cannot be

agreement with the predicted values based on the constant Thelongitudinal velocity was determined for 20- and

expected in this region if the composite is used as a single elementtransducerOntheotherhandthesituation is desirableforalinear- or phased-arraytransducercon-structionwheretheacousticalcross-couplingbetween elements should be as low as possible

In region 3 corresponding to the frequency far below thefirstminimum in the acousticimpedancethecom-posite acts as a homogeneous material Maximum stress is transferred from PZT to polymer and vice versa The composite is expected to perform better as a singleele-ment transducer with an improvedpulse-echofigure of merit

In region 2 the periodicity of the lattice and the wave-length of the transverse waves are comparableThis re-gionishelpful in understanding thecomplexvibrational modes in two dimensionallyperiodicstructures In this region the composite can possibly be used as a resonant sound absorber

C Transmission Technique

The acoustic impedance of the composites was also de-termined by measuring the velocity of longitudinal waves

method described by McSkimin [8] An acoustic wave of a particular frequency is made to impinge on the sample The wave transmitted to thespecimenisreflectedback and forth giving rise to a series of transmitted pulses At discrete frequencies the emerging wave trains will be in phaseleadingtoaconstructiveinterferenceThecon-structive interference occuri at a frequency for which the path length (twice the specimen thickness) is equal to an integralnumberof wavelengths in thespecimenatthat frequency The principle of the method is to determine the value of the integer at oneof the discrete frequencieswhen the in-phase condition is satisfied The experiment is de-scribed in detail elsewhere [9]

30-percent PZT composites around05 MHz which is be-low the frequencycorrespondingtotheacousticimped-ance minima (Fig 2(a)) The velocity values were mutli-plied by the average density to obtain theacousticim-pedance The resulting data shownin Table 111 are in ex-cellent agreement with the values estimated theoretically for region 3

D Resonance Technique The velocity of longitudinal waves determined by the

resonance technique described in Part I [ l] was also used to determine the acoustic impedance of composite sam-ples The velocitites for different composities are listed in thelastcolumn ofTable V of Part I [ l ] Valuesof the acoustic impedance evaluated from the measured velocity anddensity of composities are listed inTable IV Theo- retically calculated acoustic impedance are also tabulated for comparison In the piezoelectricresonanceexperi-ments PZT rods in the composite are electrically poled Hence for calculating the elastic modulus of the compos-itethemodulus of the PZT rods E = = 105 X 10 Nm2 (7) was used

As it emerges from Table IV in region 1 ( gt2 MHz) a pooragreement betweenthe estimatedandmeasured acoustic impedancewas foundThis was anticipated be-cause the rods are vibrating relatively freely and the res-onancetechniquemeasuredthe velocity of PZT phase only Hence the product of this velocity with the average density of the sample gives a much higher estimate of the acoustic impedance

In region 3 (lt05 MHz) theagreementbetweenthe

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

505 GURURAJA et al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE 111 Doto m m c t e d fw IOY Compor(t0 difhoclion lorus

iOO 05 10

Frequency (MHz)

Fig 4 Correction on the value of acoustic impedance due to diffraction losses

ACOUSTICIMPEDANCEBY TRANSMISSIONTECHNIQUE

Measured LongitudinalMeasured Z Calculated Z

Volume Percent Velocity lo6(atrayl lo6 ray1 (from PZT W s ) 05 MHz) Table 11)

20 2682 i 8 61 60 30 82 2780 It 14 85

along the PZT rod axis The method chosenfor measuring the longitudinal velocity was similar to the standing-wave

dimension as the composite sample Then the experimen-tal setup for the two reflections to be compared becomes identical and hence the uncertainty in the measurement because of the diffraction lossesis minimized Fig 4 com-pares the earlier data with the new data corrected for dif-fraction losses for a ten-percent composite sample As can be seenthecorrected value of the acousticimpedance approaches 41 M rayl at 03 MHz which is in excellent

strainmodel (Table 11) Based on the experimentaland theoretical results the acoustic impedance- as a function of frequency can be classified into three regions

In region 1 corresponding to frequencies above 2 MHz thetransverse wavelength is muchsmallerthanthe rod spacing d andthetwophasesappear to bedecoupled There is not much influence of epoxy on the vibration of thePZTrods A significantimprovement in the pulse-echo response in terms of the figure of merit cannot be

agreement with the predicted values based on the constant Thelongitudinal velocity was determined for 20- and

expected in this region if the composite is used as a single elementtransducerOntheotherhandthesituation is desirableforalinear- or phased-arraytransducercon-structionwheretheacousticalcross-couplingbetween elements should be as low as possible

In region 3 corresponding to the frequency far below thefirstminimum in the acousticimpedancethecom-posite acts as a homogeneous material Maximum stress is transferred from PZT to polymer and vice versa The composite is expected to perform better as a singleele-ment transducer with an improvedpulse-echofigure of merit

In region 2 the periodicity of the lattice and the wave-length of the transverse waves are comparableThis re-gionishelpful in understanding thecomplexvibrational modes in two dimensionallyperiodicstructures In this region the composite can possibly be used as a resonant sound absorber

C Transmission Technique

The acoustic impedance of the composites was also de-termined by measuring the velocity of longitudinal waves

method described by McSkimin [8] An acoustic wave of a particular frequency is made to impinge on the sample The wave transmitted to thespecimenisreflectedback and forth giving rise to a series of transmitted pulses At discrete frequencies the emerging wave trains will be in phaseleadingtoaconstructiveinterferenceThecon-structive interference occuri at a frequency for which the path length (twice the specimen thickness) is equal to an integralnumberof wavelengths in thespecimenatthat frequency The principle of the method is to determine the value of the integer at oneof the discrete frequencieswhen the in-phase condition is satisfied The experiment is de-scribed in detail elsewhere [9]

30-percent PZT composites around05 MHz which is be-low the frequencycorrespondingtotheacousticimped-ance minima (Fig 2(a)) The velocity values were mutli-plied by the average density to obtain theacousticim-pedance The resulting data shownin Table 111 are in ex-cellent agreement with the values estimated theoretically for region 3

D Resonance Technique The velocity of longitudinal waves determined by the

resonance technique described in Part I [ l] was also used to determine the acoustic impedance of composite sam-ples The velocitites for different composities are listed in thelastcolumn ofTable V of Part I [ l ] Valuesof the acoustic impedance evaluated from the measured velocity anddensity of composities are listed inTable IV Theo- retically calculated acoustic impedance are also tabulated for comparison In the piezoelectricresonanceexperi-ments PZT rods in the composite are electrically poled Hence for calculating the elastic modulus of the compos-itethemodulus of the PZT rods E = = 105 X 10 Nm2 (7) was used

As it emerges from Table IV in region 1 ( gt2 MHz) a pooragreement betweenthe estimatedandmeasured acoustic impedancewas foundThis was anticipated be-cause the rods are vibrating relatively freely and the res-onancetechniquemeasuredthe velocity of PZT phase only Hence the product of this velocity with the average density of the sample gives a much higher estimate of the acoustic impedance

In region 3 (lt05 MHz) theagreementbetweenthe

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

506 IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985

TABLE IV ACOUSTIC B Y RESOKANCE TECHNIQUEIMPEDANCE V A L U E S AS DETERMINED

Region 1 ( gt 2 MHz) Region 3 ( 2 0 4 MHz) Wavelength ltlt Periodicity Wavelength gtgt Periodicity

Volume Percent CalculatedPZT 2 Measured 2 Calculated 2 Measured Z

5 26 3 7 51 3 7

2 0 3 6 10 29 59 49 49

-103 7 3 84 7 5

30 45 103

theoryandtheexperimentisexcellent (Table IV) Itis interesting to note that the three completely independent techniques give extremely consistent data on the acoustic impedance in region 3

IV CALIBRATION TRANSDUCEROF COMPOSITE AS A

RECEIVER WAVESOF ULTRASONIC

Thissectiondeals withtheevaluation of composite transducers as an ultrasonic receiver The receiving sen-sitivity is considered to be of greater importance than the transmitting response especially in biomedical-diagnos- ticapplicationsbecause only a limitedacousticenergy level can be applied to the human body This limit has to be sufficiently low so bioeffects of ultrasound i f any are minimizedHence if the receivingsensitivity is aug- mented the body can be iterrogated at lowerultrasonic levels Therefore initial attention was focused on the re-ceiving properties of the composite transduceres

The receiving characteristics of a transducer were as-sessed by its free-field voltage sensitivity (MO)The free field sensitivity is the ratio of the output open circuit volt- age to thefree-fieldsoundpressure in the undisturbed plane progressive wave [ I l l For an ultrasonic transducer in addition to a highest possible voltage sensitivity a fast pulse echo time and low ringing are required in order to achieve good axial resolution In an air backed transducer these parameters are mainly determined by the piezoelec-triccouplingcoefficient k andacousticimpedance 2 of the transducer relative to that of the human body In com- posite material since both k and 2 are dependent on fre- quency the reception characteristics are also expected to be a function of frequency

The time-delay spectrometry (TDS)technique [12] which allows the determination ofthefree-fieldvoltage sensitivity as a continuous function of frequencywas used for calibrating the composite transducers The TDS con-cept is based on converting a propagation time from trans-mitter to receiver into a certain frequency shift by keeping a constant frequency sweep rateso the time and frequency are linked togetherConsequentlyselectivity in time is proportional to selectivity in frequency with the sweep rate as conversion factor (eg 0-20 MHz sweep in one second corresponds to 20 Hz per microsecond) Only one direct signal will be detected if a band-pass filter receiving the electrical signal from the ultrasonic hydrophone is swept

I h I

t- I Tronrmittsr

H P W A Temp=23C

Fig 5 Experimental set up for calibration of ultrasonic transducers

with a suitable delay in relation to the transmitter driving signalandhas an appropriate (narrow) bandwidth Hence

the TDS technique virtually eliminates the effects of mul- tiple transmission paths standing waves and other inter-ference caused by reflected signals

The experimental arrangement employed for calibration (Fig 5 ) utilized a spectrum analyzer (HP 3585A) with a built-in frequency offset unit The sine swept signal from the tracking generator drives a specially designed trans-ducer (transmitter) via a power amplifier (EN1411A) The ultrasonic signal in the far field was detected with a needle-like (1 mmdiameter)calibrated polyvinylidenefluoride (PVF) polymer probe [131 and fed into the spectrum ana-lyzer input ( l MQ termination) The detected signal was compared with the calibration curve of the probe (Fig 6 ) to determine the absolute pressure at the probe as a func-tion of frequency The unknown receiver (composite) was then positioned exactly in the probe loction and its voltage response was recorded on the spectrum analyzer Ratio of the voltage response of the compositetransducertothe sound pressure as determined by the polymer probe gives the free-field voltage sensitivity of the receiver The result is expressed in volts per micropascal or in decibels [dB = 20 log (MO voltip Pa)]

In thisexperimentthespectrumanalyzer was inter- faced with a computer (HP 9825A) to record the data at each step and calculate the free-field voltage sensitivity as a function of frequency The result was transferred back and displayed on the spectrum analyzer in decibelsrelative to l VpPa

The calibration uncertainty of the polymer probe (Fig 6 ) in frequency range from1 to 10 MHz is reportedly k15 dB and amp2 dB from 01 to l MHz [141 The probe exhib- itedgoodfrequency characteristics up to 10 MHzwith

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

507 GURURAJA et al PIEZOEL

End 01 (

Voltage ( d B ra

+fJdB

relatively high sensitivityand gave highly reproducible calibration of the composite transducers

A number of commercial and custom-made transducers were used as transmitters for calibrating the composites as receivers The center frequency of the transducer var- ied from 03 to 35 MHz The diameter of the transducers ranged from 05 to 25 cm Such a wide variety of trans-mitters were used to make sure that the receiver was al-ways in the far-field of the transmitter thedistance be-tween the transmitter and receiver was maintained at 22 cms the transmitter chosen in a particular case was such thatthisdistance was greater than ~ ( a + ampX where a and a2 are the radii of the transmitter and receiver re-spectively and X is the wavelength Under such a condi-tion it was estimated from Sabinrsquos calculations [l51 that the error introduced due to marginal test distance is less than 1 dB

Using Masonrsquos model [161 Shaulov and Smith[ 171have derived an expression for the maximum open-circuit volt- age response (IVImax)and the 3-dB bandwidth (Ahde)of a homogeneous transducer without backing or matching layers

The expressions are given by

and

In these expressions g is the piezoelectric voltage coef-ficient P is the incident pressure t is the thickness of the transducer C is the velocity of sound in the piezoelectric medium and Z1and Z are the acoustic impedanceof the loading medium and the transducer respectively

The gain bandwidth product G given by the product of the maximum voltage sensitivity and the 3-dB bandwidth is

For PZT 501A g33= 26 X VmlN C = 3800 mis and G = 40 V HzPa

The gain bandwidth product G was used in this work as the figure of merit in analyzing the receiving response of the composite transducer The experimentally determined value of G for the composites was compared with the cal-culated value of G for PZT Any improvement in the gain bandwidth product was consequently attributed to a more effective coupling of the ultrasonic energy dueto the epoxy phase

Composite samples with 10 20 and 30 volume percent PZT and thickness ranging from 06 to 515 mm were cal-ibrated for theirreceivingvoltageresponseandthere-sults are summarized in Table V The maximum voltage sensitivity and the 3-dB bandwidth were the two main pa-rametersmeasured in each case From these measure-ments the gain bandwidth product G was calculated

The measurements on each sample were carried out at least twice to check the reproducibility Most of the com-posites were calibrated using more than one transmitter The reproducibility in thevoltagesensitivity wasalways within 05 dB From the consistency in the measurements performed a maximum error in the figure of merit was estimated to be ten percent

From the velocity of the transverse waves in the Spurrs epoxy (1150 ms) thetransverse wavelengthwas calcu-lated at the frequencyof the maximum reception sensitiv-ity As discussed earlier the interaction between the PZT and the epoxy increases when the wavelength of the trans-

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

045 460 09 0352 326 362

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32NO 4 JULY 1985508

TABLE V RECEIVINGVOLTAGE RESPONSE( M o ) OF COMPOSITE TRANSDUCERS

Transverse

050 055 144 252 3623 184 204 53 1 154 818 221 245 750 133 998 286 317 933 I15 1073

1047 109 1141 425 1128 110 1241383

8051 6 4

09 20 09 20

Maximum Volume Rod Periodicity Maximum at Maximum Sensitivity Bandwidth of Merit

Frequency at Wavelength 3-dB Figure

PercentSample Thickness Response h A f 3 d B MIAfDiameter (d) Sensitivity Y= MONo PZT (mm) t (mm) (mm) (MHz) (mm) X (PVIPa) (KHz)(VIHzlPa)

1 0 0 10 045 066 127 225 050 039 I30 288 374 101 10 045 193 127 088 129 101 200 105 210 102 10 045 364 127 044 260 204 496 168 832 103 10 045 515 127 030 383 301 955 120 1146

225066200045 0625254 04520 I 0525305 0400395

09 20 202045 045 204 20 203

205045

25 l028

30 1073

09 20

09 5150300

260

20

0 6 0625 184 306 6 84 133 909

224140 045

20

30244 575

to that of free PZT rods is a further confirmation of the inferred vibration pattern in the composite when the pe-riodicity is larger than the transverse wavelength As the wavelength approaches the periodicity laterally resonant modes are set up andthe interference between the two modes results in a reduced figure of merit (sample 101) The figure of merit G gradually increases for larger ratio y = Xd asexpectedThe figure of merit for y 4 is increased three folds over that of PZT Samples 102 201 and 301 are made of 10 20 and 30 volume percent PZT and have approximately thesame value of the ratio y ( ~ 2 ) It is interesting to note that the figure of merit of these samples is close to 80 (VHzPa) Similar agreement can be found between samples 103 and 203 with y = 3

The observed behavior indicated that the performance of a composite is directly related to the ratio between the wavelength of transverse waves and the periodicity of the lattice regardless of volume percent of PZT This experi-mental observation suggested that one of the ways of in-creasing the efficiency of the composite for operation at highfrequency(2to 5 MHz) is to scaledownthecom-posite structure

V CALIBRATION AS AOF COMPOSITE TRANSDUCERS TRANSMITTEROF ULTRASONICWAVES

Thetransmittingcharacteristics of atransducer for medical imagingcanbecharacterized by its voltagere-sponse (S) and Q The transmitting voltage response is the ratio of the sound pressure apparent at a distance of onemeter in a specified direction from theeffective acoustic center of the transducer tothevoltageapplied across the electrical input terminals [ 111 In this study the transmitting voltage response of the compositetrans-ducers was determined at the near-field far-field transition region (a2Xdistance) along the axis of the circular trans-ducer The Q of the transducer is the ratio of frequency at peak response to 3-dB bandwidth

- - I g 0 l

Frequency (MHz)

Fig7 Receiving voltage sensitivity of a composite transducer (fo = 225 MHz)

verse wave becomes larger compared to the periodicity Thus the data were analyzed with respect to the ratio y of the wavelength of the transverse waves to the periodicity of the lattice An improvement in the figure of merit was expected as this ratio increased

A typical plot of thereceivingvoltagesensitivity as a function of frequency foracompositewithcenterfre-quency fo = 225 MHz is shown in Fig 7 It is very in- teresting to note that the minimaseen in the receiving voltage sensitivities at frequencies below 25 MHz corre-spond to frequencies of lateral resonant modesJl J2 etc At these frequencies the acoustic energy is dissipated in setting up complex lateral resonances in the composite

AS shown in Table V the figure of merit G for both ten-and 20-percent PZT composite with fo = 225 MHz ap-pears to be independent of the volume fraction of PZT and is very close to that of the calculated value of 40 (VHz Pa)forsingle-phasePZT Theresonance dataandthe acousticimpedanceresultsbothindicatedthatthePZT rods in the composite atfo = 225 MHz vibrate relatively freely The figure of merit G of the composite being close

07

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Wavelength Maximum Maximum Response

509 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

TABLE VI TRANSMITTINGVOLTAGE RESPONSE (so) OF COMPOSITE TRANSDUCERS

Transverse at Frequency Volume

ThicknessPercent Sample No (mm)PZTResponse

100 10 I02 10 103 10

200 20 20 1 20 202 20 203 20 204 20

30 1 30

at Maximum So X in decibels

(h) ( L mm Periodicity d (relative to 1 pPalV)

066 225 050 039 1958 364 0425 270 213 1850 515 0287 400 315 1815

066 225 050 055 2019 254 0540 213 236 1940 305 0480 239 266 1915 395 0370 311 345 1887 460 0312 368 409 1866

244 0597 193 263 1964

The same experimental setup shown in Fig 5 was used for the calibration of the composites as a transmitter The compositetransducertobecalibratedisexcited by the tracking generator of the spectrum analyzer via the power amplifier The ultrasonic signal at the a21X distancede-tected by the calibratedneedle-likepolymerprobe[l31 was fed into the spectrum analyzer input (1 MQ) The de-tected signal was adjusted for the calibration of the probe to calculate the pressure at a2X distance The excitation voltage as a function of frequency was also measured by feeding the output of the power amplifier to the input (1 Ma) of the spectrum analyzer Quotient of the detected pressuretotheexcitation voltagegivesthetransmitting voltage response In this experimentalsothespectrum analyzer was interfaced with a computer (HP 9285A) to record the data at each step to calculate So in dB (relative to 1 pPaV)The testdistancebetweenthetwotrans-ducers being only a2 h an error is introduced in the mea-surements [151 Since the radii of the composite transmit-ter andtheprobewerekeptconstantandallthe measurements were performed at a2X distance from the projector the correction factor is a constant The correc-tion factor was estimated to be approximately equal to35 dB which should be added to the measured So in all the experiments

Composites of different thickness and volume fraction were calibrated for their transmitting voltage response at their thickness mode resonance frequency The results are summarizedin Table VI Fig 8 showsatypical plot of transmitting voltage sensitivity of a composite transducer withf = 225 MHz

It was observed that for transducers operating around 225 MHz the transmitting voltage response (S) depends on the volume percent PZT There was a 6-dB reduction in Soobserved going from the 20-percent PZT composite to the ten-percent PZT composite A further reduction of 8-10 dB was observed for the five-percent composite The observationsuggeststhatthetransmittingresponse of a composite transducer is proportional to the volume frac- tion of the piezoelectrically active material present in the compositeIfthishypothesis is correct it plausibly ex-

0 25 5 0

Frequency (MHz1

Fig8Transmitting voltage response of acompositetransducer ( f o = 225 MHz)

Q = - h A h d B

5 to 6 -38

67 48 51 43 48

50

plains why the 20-percent composites gave about twice the pulse-echo signal of the ten-percent composite (Table 1)

Thecompositetransducersresonating below 1 MHz were calibrated to examine the effect of the cooperative interactionbetweenPZTandepoxyonthetransmitting voltage response However a frequency independent fig-ure of merit could not be defined to ascertainthefre-quency dependence of the transmitting voltage response because of the following reasons

1) At low frequencies where the dimensions of the pro-jectoraresmallincomparison withthe wavelength in water the transmitting characteristics are well described [111 The piezoelectric material being stiffness controlled aconstantvoltageapplied to the transducerresults in a constant displacement Furthermore since acoustic pres-suregenerated by the transmitter is proportional tothe accelerationthetransmittingvoltageresponseincreases at a rate of 12 dBoctave [So CY ( f req~ency)~] At high fre-quencies where the transducer dimensions are compara-ble to the wavelength the simple source concept cannot be applied because of resonance effect 2) The transmitting voltage response also dependson the directivity pattern of the transducer [181 When the circumference of the cir-cular transducer is less than one half wavelength that is

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

o 30 PZT Compaite 20Y PZT CMPo8itr m o 10 PZT Compo8ites

Frequency (MHz)

Fig 9 Plot of maximumtransmitting voltage response (S) as a function of frequency at maximum S

ka = 27ralX lt 05 the piston behaves like a point source and when ka exceeds three the piston is more directional

In this study the following assumptions were made to evaluatethefrequencydependence of the transmitting voltage response of the composite Since all the compos-ites are operated at half-wavelength thickness resonance the thickness t of the sample bears a constant relationwith wavelength at the operating frequency Furthermore since the amplitude of excitation signal is kept constant a con-stant displacement can still be assumed in comparing the transmitting voltage response at different frequencies Un- der this assumption the transmitting voltage response is expected to increase at a rate of12 dBoctave as the fre- quency is increased 1111 Because the radius of the trans-duceriskeptconstantanincrease in the operatingfre-quency makes the ultrasonic beam from the tranducer less divergent resulting in an increased pressure along the axis [ 181 Fromthe aboveargumentitappearsthatthefre-quency dependence of the transmitting voltage response should be greater than 12 dBoctave

Fig 9 shows the plot of maximum transmitting voltage response as a function of frequency The transmittingvolt-age response for 10 and 20 percent PZT composites were 1958 and 2019 dB (1 pPaV) respectively With the pre-viously stated assumptions areduction of 24 dB in the transmitting voltage response was expected for the mea-surementsperformedat 05 MHz However a reduction of only H)dB was actually measured Also it is observed from Fig 9 that at low frequencies (around 05 MHz) the rate of increase of S with frequency fits a straight lineof slope 8-10 dBoctave

These results confirm that the transmitting voltage re-sponse of composite transducers is as anticipateda func-tion of frequency Atfrequencieswherethe transverse wavelength is large compared to the periodicity there ap-pears to be an increase ( 5 14 dB) in the transmitting volt- age response The enhancement can be attributed to the improvedinteractionbetweenPZTandepoxyat lower frequencies ( lt05MHz)The improvedperformance should however be extended to a higher frequency range (2-5 MHz) which is typical for ultrasonic diagnostic ap-plicationsThiswillrequire havinga largeratio of the

transverse wavelength to the periodicity at the operating frequency

The Q of the composite transducers operating at differ-ent frequencies are compared in the last column of Table VI For composites resonant around 225 MHz Q is ap-proximately six and is comparable to the values of Q mea-sured by the resonance technique in air [l] Coupling the transducer to a water load has not altered the bandwidth characteristics which is an indication of poor coupling of ultrasonic energy into water Composites resonant around 05 MHz had relatively large Q in air (20-30) but by water loadingthe Q wasreduced toabout fourThis is most likely due to a better matching of ultrasonic energy be-tween the composite and water

The acoustic impedance at 225 MHz for a given vol-ume percent PZT composite is much lower than the value at about 05 MHz The data on acoustic impedance sug-gest that the transfer of acoustic energy from water to the composite or vice versa should be better at high frequen-cies The results on the transmitting and receivingvoltage responses indicate that at higher frequency the energy is not being properly coupled to the piezoelectric phase al-though the transfer of energy between the two media may be more effective The observed behavior is again an in-dication of pooracousticcouplingbetweenPZTand polymer at frequencies (225 MHz) where the transverse wavelength ismuch smaller than the periodicity Thus if the composites prepared in the study are operated around 05 MHz there appears to beefficient transfer of acoustic energy from PZT to water via the epoxy phase As a re-sult substantial improvement in the receiving and trans-mitting voltage responses was observed

VI EFFECTOF THE MATCHING LAYERON THE

PERFORMANCEOF COMPOSITE TRANSDUCERS

Thecompositetransducers showedasubstantialim-provement in receivingandtransmittingvoltagere-sponses when operated at resonance in the low frequency range where the transverse wavelength is large compared to the periodicity The Q of the composite transducer was reducedfromabout25 toabout fourwhencoupledwith water load The measured value of Q is relatively large for medical diagnostic applications where aQ of 2-25 is usu- ally recommended [ 191 for a good axial resolution Larger Q in the composite transducers was also apparent from the slow pulse-rise time and prolonged ringing in the pulse- echo response

Relatively large Q valuesobserved in the composite transducers was attributed to the acoustic impedance mis-match between the transducer and the load The acoustic impedance of a 20 percent PZT composite in the low-fre- quencyrangeis 73 M ray1 (Table IV) which results in a pressure reflectivity of 65 percent with water load To im- prove the impedance matching a matching layer between the transducer and the load was considered For optimum transmission the matching layer has to be a quarterwave-length in thickness and of characteristic impedance equal

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

511 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

to the geometric mean of those of the transducer and of the loading medium [20] In the present situationZ of the matchinglayer was calculated tobe33M rayl Several polymer systems have acoustic impedance in this range

A 20-percent PZT composite 307 mm in thickness was chosen to study the effect of a quarter-wave matching layer Sincelucite (Z = 32M rayl) was readily available and had anacousticimpedanceveryclosetotherequired value a quarter-wave matching layer of lucite material was pressure-bonded to the transducer using an epoxy (Tracon 2115) The thickness of the bonding layer was less than 25 pm The frequency ( f o ) at maximum pulse-echo response was 054 MHzThiscomposite withthematchinglayer was characterized for its receiving and transmitting volt-age response

The gain bandwidth product G in the receiving mode of thematchedtransducer was180 (VHzlPa) which is 45 times higher than that of a corresponding PZT rod The transducer has a very broad band response with Q of 18 The transmitting voltage sensitivity was 1865 dB relative to 1 pPalV andQ in the transmission mode was also about 18 Comparing the results of the matched transducer with those of unmatched20-percentPZTcompositessuchas 201 202 251 (Tables VandVI)operatingaround 05 MHz it can be seen that major contributions for the en-hanced response came from the broadening of the band-width This is quite evident from the pusle-echo response of the matchedtransducershown in Fig 10 The pulse- rise time is very fastandtheringing is low (Fig 10(a)) as exemplified in the broad nature of the frequency spec-trum(Fig10(b))Theseexperimentsdemonstrate that compositetransducers withasinglematchinglayercan have excellent performance in medical diagnosticappli-cations

As mentionedpreviouslytheimprovedperformance should however be extendedtothehigherfrequency range(2-5MHz)typicalforultrasonicdiagnosticappli-cations From the experimental results the substantial im-provement can clearly be attributed to the strong interac-tionbetween thePZTand epoxywherethetransverse wavelengthwas largecompared to the periodicity This observation suggests that one of the ways of increasing the efficiency of the composite for operation at high frequency is to scale down the composite structure This means that the rod diameter and the periodicity of the lattice in the composite structureshouldbescaleddown by at least a factor of five to operate at approximately 2 MHz

VII FOCUSEDCOMPOSITETRANSDUCER

This section deals with shaping a composite transducer to focus a sonic beam to a narrow beamwidth Composite materialscanbemolded intocurvedshapes by a simple thermal process as explained in the following

Thin composites ( = 06 mm) of 10 and 20-percent PZT wereprepared in a flat shape by the usual procedure A spherical mold with a curvature of 95 cm was heated in a small oven to 80degC which is just above the glass transition

+ f r e q u e n c y

(b)

Fig 10 Pulse-echoresponse of a compositetransducer with asingle matching layer (spectrum analyzer method) (a) Waveform of the echo single (b) Frequency content of the echo signal

temperature of the Spurrs epoxy (Tg = 70degC at 100 Hz) At this temperature epoxy was quite soft and flexible The composite sample was placed on the mold and allowed to reachthermalequilibriumAslightdeformationunder gravity was noticed The backing mold was placed on top to force the composite to conform to the spherical curva-ture of the mold The mold set was kept at 80degC for about an hour and cooled slowly ( = i o per minute) to room tem- perature

To measure the axial beam profile the focused compos-ite transducer was driven by continuous wave excitation at the frequency of maximum response The calibrated min-iaturehydrophoneprobe(SectionIV)[l31 wasused to measure the transmitting voltage response along the axis of the transducer Fig 11 shows the axial beam profile of a 20-percent PZT composite transducer (fo = 225 MHz) before and after focusing The peak response for nonfo- cused transducer was 200 dB relative to 1 pPalV at 121 cm which corresponds to the expecteda 2 hdistance After focusing the response peaked at an axial distance of 76

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

IEEE TRANSACTIONS ON SONICS AND ULTRASONICS VOL SU-32 NO 4 JULY 1985512

Axial Dirtoncr (cm1

Fig 11 Axial beam profile of a nonfocused and focused composite trans ducer

~~

Maximum So in Volume Frequency at decibels Beamwidth Percent Sample Maximum S D = a2X Distance T (relative to l i e criteria Pulse-Echo PZT No (MHz) Focused Peak at Voltage(cm) S (cm)lpPaiV) Vat T

1 1082 Yes - 66 201 E 051 1 1

lOOb 10 202 No 121 116 1949 1 oo 13

I OOa 205 No 123 1968 1510

Yes - 063 71 2012 1 1

20 200a 201 No 121 121 200 083 I 9 Yes 2085 - 76 058 24

2019 1 oo 2 620 200b 201 No121 Yes

121 066 2066- 76 22

Radius of curvature = 95 cm Beamwidth measured at a pressure level l l e = 0368 of the peak

cm with a gain of 85 dB in the voltage response Table VI1 summarizes the results of the focusing studies on both ten- and20-percentPZTcompositetransducersThe maximum response in all the cases occurred at a distance slightly farther than the expected 6 cm whch was based on the theoretical calculations by ONeil 1211 Thisdis-crepancy is probably due to the fact that the theory was developed for a single-phase material and may not be fully valid for the composite transducer

Thelateralbeam profile at the distance of maximum response was measured using the polymer probeAn x-y-z micromanipulator was used to move the hydrophone probe away from the axisof the transducer in steps of 063 mm Thelateralbeamwidthwascalculatedfromthepoints where theeffective relativepressurewas lle = 0368 of thepeak 1221 The lateralbeamwidths at the peakre-sponse are given in Table VII There was nominally a 30-to 40-percent reduction in the beam width after focusing The beamwidths are slightly larger than the theoretically calculated value of 46 mm [22]

Thetone-burstpulse-echoresponse with the stainless steelreflector at the distance corresponding to the peak

response was measured forboth thenonfocusedand fo-cused transducers (Table VIX) Since the transmitting volt- age response was increased by a factor of two (6 dB) on focusing a considerableimprovement in the pulse-echo amplitude was also expected Surprisingly it was noticed that in most cases there was actually a small decrease in the pulse-echb amplitude It was suggested that the spher-ical curvature on the transducer affects the receiving volt- age sensitivity and is the cause for the decreased pulse-echo signal A further support for this is presented in the following

The degree of the concavity h of the transducer is com-parable to the wavelength X in water at 225 MHz (hlX = 07) and hence modifies substantially the phase profile of thereflectedbeamat the transducer faceSince the re-ceiving voltage sensitivity is dependent on the phase pro-file of the pressure at the transducer face [15] focusing was expected to affect the receiving sensitivity As antic-ipated the average gain in bandwidth productof a focused transduceroperating at 225 MHz was 26 MHz VPa compared to the measured value of 36 HzVlPa for a flat transducer (Table V)

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

513 GURURAJA er al PIEZOELECTRICCOMPOSITEMATERIALS-PART I1

VIII CONCLUSION The composite transducers were initially characterized

by the pulse-echo method To evaluate thecomposites thoroughlytheacousticimpedanceand the transmitting and receiving voltage responses were measured separately as a function of frequency Frequency dependence of the acousticimpedance was modeled by calculatingthe averageelasticmodulus attwoextremeconditionsThe acousticimpedancemodeledusingthe Voigt constant strain model showedexcellent agreementwith the mea-suredvalues at 03 MHzMeasuredvalues ofacoustic impedance at 35 MHz were in excellent agreement with the values modeled by the constant stress model

The figure of merit in the receiving mode for a com-positewith fo = 225 MHz was close tothecalculated valueforasingle-phase PZTThis resultindicatedvir-tually no contribution from the epoxy phase becauseof the weak interaction between PZT and epoxy As the operat-ing frequency was reduced below 05 MHz the figure of merit G was increased by a factorof three The substantial improvement in G providedadditionalevidence for the stronginteraction between PZT and epoxy at low fre- quencies A similar improvement was observed in the transmitting voltage responseforcompositesoperated around 05 MHz

The performance of the composite transducer was fur- ther improved by the use of a quarter wavelength matching layer of lucite material between the transducer and load The figure of merit G in the receiving mode of the com-posite transducer was increased by about 50 percent when the matching layer was used The Q of the matched trans-ducer was less than two which is advantageous in achiev- ing good axial resolution

The results showthat the 1-3 compositematerialsare excellent candidates for medical diagnostic transducer ap-plications However it should be noted that the improved performance of the composite transducer- should be ex- tended to higherfrequencies (2-5 MHz) typical forthe ultrasonicdiagnosticapplicationsIt was demonstrated that the composite transducers can be focused by a rela-tively simple technique As aresult of focusingit was possible to generate a concentrated sonic beam with nar-rower beamwidth

ACKNOWLEDGMENT The authors are gratefulto Dr W A Smith and Dr A

Shaulov of North American Philips Laboratories and Dr P A Lewin of Drexel University (Dept of Electrical and Communication Engineering and Biomedical Engineering and Science Institute) for their comments and discussions MrsLindaWebster for preparing samplesand Mr P Moses for writing software for computer interfacing

REFERENCES [ I ] T R Gururaja W A Schulze L E Cross R E Newnham B A

Auld and J Wang ldquoPiezoelectric composite materials for ultrasonic

transducer applications Part I Resonant modes of PZT rod-polymer compositesrdquo IEEE Trans Sonics Ultrason vol SU-32 no 4 pp 481-498

[2] KR Erikson ldquoTone-burst testing of pulse-echo transducerrdquo IEEE Trans Sonics Ultrason volSU-26 no 1pp 7-14 1979

[3] H J McSkimin ldquoUltrasonic methods for measuring the mechanical properties of liquids and solidsrdquo in Physical Acoustics WP Mason Ed NewYork Academic 1964 vol 1 part A chap 4

[4] LE Kinslerand A R Frey Fundamentals of Acoustics 2nd ed New York John Wiley 1962

(51 J L Rose and B B Goldberg BasicPhysics in DiagnosricUltra-sound New York John Wiley 1979

[6] S Lees and C L Davidson ldquoUltrasonic measurement of some min-eral filled plasticsrdquo IEEE Trans Sonics Ultrason vol SU-24 pp 222-2251917

[7] DA Berlincourt D RCurranand H JaffeldquoPiezoelectricand piezomagnetic materials and their function in transducersrdquo Physical Acoustics vol 1 W P MasonEd New York Academic 1964 part A chap 4

[8] H J McSkimin ldquoMeasurements of elastic constants at low temper-atures by means of ultrasonic waves-Data for silicon and germanium single crystals and for fused silicardquo J Appl Phys vol 24 pp 988- 997 1953

[9] T R Gururaja W A Schulzeand LE CrossldquoBulkmechanical properties of the Spurrs epoxy and eccogel polymersrdquo submitted to Ultrasonics

1101 W D OrsquoBrien JrldquoSafety of ultrasoundrdquo Handbook of Clinical Ultrasound New York John Wiley 1978

[ 1 l ] R J Bobber Underwater Electroacoustic Measurements Naval Re- search Laboratory Washington DC 1970

[121 PA Lewin ldquoCalibration and performance of evaluation of miniature ultrasonic hydrophones using time delay spectrometryrdquo in Proc I981 IEEE Ultrason Symp 1981 pp 660-664

[13] - ldquoMiniature polymer hydrophonepiezoelectric ultrasonic probesrdquo Ultrasonics pp 213-216 1981

[l41 - privatecommunication 1151 G A Sabin ldquoCalibration of piston transducers at marginal test dis-

tancesrdquo J Acoust Soc Amer vol 36 pp 168-173 1964 [l61 W Mason Transducerand Wave Filters P Electromechanical

Princeton NJ Van Nostrand 1942 [l71 A Shaulov add W A Smith to be published [l81BeraneckLL Acoustics New York McGraw-Hill 1954 [l91 R ABenjavicandJ AZagzebskirsquoUltrasonicpulse-echobeam

width and axial tesponse Approximations for clinical broadband fo-cu$edtransducerrsquo Ultrasound in Med and Biol vol 7 pp 63-71 1981

[20] J H Goll lsquoThe design and broadband fluid-loaded ultrasonic trans-ducersrsquo IEEE Trans Sonics Ultrason vol SU-26 pp 385-393 1979

[21] HT OrsquoNeil lsquoTheory of focusing radiatorsrdquo J Acous Soc Amer vol 21 pp 516-526 1949

1221 S LeesldquoUsefulcriteria in describing the field pattern of focusing transducersrdquo Ultrasonics vol 16 pp 229-232 1978

TR Gururaja for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Walter A Schulze foraphotographand biography seepage497 of the July 1985 issue of this TRANSACTIONS

Leslie E Cross for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS

Robert E Newnharn for a photograph and biography please see page 497 of the July 1985 issue of this TRANSACTIONS