IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-17, NO. 2, JUNE 1968

Pyroelectric Transducers for the Measurement of HeatTransfer Under Conditions of Hypersonic Flow

TRAVIS G. HICKMAN, MEMBER, IEEE, AND GLENN A. BURDICK, SENIOR MEMBER, IEEE

Abstract-Pyroelectric detectors have been fabricated frompolycrystalline samples of barium titanate. These detectors weremounted in a model for tests in a hypersonic wind tunnel and cali-brated. The method of calibration is discussed and tables of calibra-tion data are included. Thermocouples were mounted in the model toevaluate the detectors. The detectors were tested and results of thetests are discussed. Advantages of pyroelectric detectors are pre-sented and typical output traces from both the pyroelectric detectorsand the thermocouples are shown.

INTRODUCTIONiD UE TO THE growing emphasis on missiles,

rockets, and reentry vehicles, modern daytechnology is making greater use of hypersonic

wind tunnels than it has in the past. These tunnelssimulate conditions of reentry making it possible tostudy the behavior of various aerodynamic models undersuch conditions.The Langley Hot Shot Tunnel [1] is such a tunnel

and was constructed primarily to obtain force measure-ments, pressure and flow distributions, and heat transferdata for hypersonic configurations.

It is the purpose of this paper to discuss the use ofpyroelectric sensors, to measure heat transfer in hyper-sonic flows, and to present evidence that pyroelectric de-tectors are superior, in certain aspects, to existing in-strumentation techniques.The normal technique for heat transfer measurements

at the Langley Hot Shot Tunnel is a thermocouple sys-tem. These thermocouples are bonded to the inside of atest model and the output from each thermocouple issampled throughout the run. These discrete data are re-corded as a function of time and reduced by computer.The computer program must solve the necessary heat-transfer equations for the given thermocouple locationson the model. Obviously, any uncertainties in thethermal characteristics of the model limit the accuracyof the system as do any approximations in the heat-transfer equations.By comparison, the output of pyroelectric detectors is

directly proportional to the heat-transfer rate, and thedetectors can be mounted on the surface of the model,thus eliminating heat-transfer calculations altogether.

Manuscript received August 4, 1966; revised January 16, 1968.This work was carried out while the authors were at Sperry Micro-wave Electronics Company, Division of Sperry Rand Corporation,Clearwater, Fla., and was supported by NASA under ContractNASW-974.

T. G. Hickman is now with Scientific-Atlanta, Inc., Atlanta, Ga.G. A. Burdick is now with the College of Engineering, University

of South Florida, Tampa, Fla.

Detector output is a continuous function as opposed toa sampling of discrete data points.

Theory of OperationThe pyroelectric effect may be defined as the char-

acteristic of ferroelectric materials to possess an elec-trical polarization whose magnitude P8 is temperaturedependent. A detector utilizing this effect can be madeby fabricating a thin slab of such a material, attachingelectrodes on both sides, and completing the electricalcircuit through an external resistor. When the tem-perature is changed, the change in polarization gives riseto a current which when passed through the externalresistor may be expressed as

dq dPI dPs dT= A -= A -dt dt dT dt

where A is the area of the electrode, T is temperature,P8 is the spontaneous polarization, dP,IdT is essen-tially constant for most materials over a limited tem-perature range and can be considered as such for thisdiscussion. This can be seen from Fig. 1, which resultsfrom laboratory data taken on a sample of bariumtitanate. Fig. 2 is the equivalent circuit for a detector.(R. is its resistance and C, is its capacitance.)

Since these detectors give an output directly propor-tional to the rate of change of temperature, it is onlylogical to consider them for the measurement of heattransfer. It can be shown that the output voltage devel-oped across an external resistor is directly proportionalto the incident energy density.Of the numerous pyroelectric materials examined,

barium titanate and triglycine sulfate had the highestsensitivities (signal per unit heat transfer). Ceramic de-tectors of barium titanate were chosen for these par-ticular tests since they are easily fabricated and haveadequate sensitivity that is highly repeatable.

Since the sensitivity of an element increases with de-creasing thickness [2], it is desirable to fabricate detec-tors as thin as is practical. The elements were ground toa thickness of 7 mils with no lapping. This produces ele-ments free of cracks and of sufficient sensitivity for thetests. Thinner crack-free samples were achieved onlywith considerable additional effort. It was felt that thisadditional effort was unwarranted for the tests to beconducted inasmuch as heat-transfer rates on the orderof 100 W/cm2 would be encountered.

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HICKMAN AND BURDICK: PYROELECTRIC TRANSDUCERS FOR HEAT TRANSFER MEASUREMENT

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Fig. 1. Spontaneous polarization versus temperaturefor barium titanate.

I t( ) CX | R,,Fig. 2. Equivalent circuit for a pyroelectric detector.

TABLE I

288o:BlI.0 POlI1N ILE O[X I EC lo. _ 2I 11 5"

0881 86081

212 2 .LO. SC.E.-CEIE IllUCE

Fig. 3. Cutaway drawing of detector mounted in model.

120

Fig. 4. Nylon screw which supports the detector as viewedfrom back side of model.

Detector* Responsivity Detector Output(V/W) (mV/W.cm2)1 0.880 4.402 0.865 4.324 0.840 4.205 0.840 4.20

* Ceramic detector 3 was shorted out in attempting to pole it.

Prior to mounting detector elements in the model pro-vided by the Langley Research Center, four elements, allcircular in cross section with a diameter of 100 milsand a thickness of 7 mils, were prepared and measuredunder equivalent conditions. The sensitivities measuredfor these samples are tabulated in Table I.

Mounting of SensorsThe model used for pyroelectric heat-transfer mea-

surements was constructed with a thin outer skin sothat thermocouple data could be taken for the purposesof comparison. The holes were drilled and tapped in thisskin and nylon plugs screwed in from the exterior sur-face leaving only sufficient hole depth above the stud toprovide a flush surface after the sensor was installed.The sensors were held in place by an epoxy. Aluminumwas previously evaporated onto each side of the detectorto serve as electrodes. While other materials were triedas electrodes, aluminum was the most tenacious. One ofthese surfaces was connected to the center conductor in

Fig. 5. Close-up view of completed model showing positions of eachdetector. The numbers beside each letter give the average powerdensity (W/cm2) as measured by the detector in the test.

the nylon stud while the other was connected to the sur-face of the model by a conductive paint which served asthe ground connection. This configuration can be seen inFig. 3. A close-up picture of one of the nylon screwsmounted in the model is shown in Fig. 4. Note the fourcircular holes where the thermocouples were later in-stalled by NASA personnel. The 2-mil stainless-steelskin forms the bottom of these holes. The position of thedetectors, the black spots on the surface of the model,can be seen in Fig. 5. The letters designate each detec-tor. One detector J is on the underside of the model andis not visible in the photograph.

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, JUNE 1968

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Fig. 6. Total output power of the GE 1200/1CL/HT heatlamp as a function of applied voltage.

CalibrationThe surface of the detectors was coated with black

paint to obtain greater absorption of radiant energy.A General Electric 1200/1CL/HT heat lamp with an

elliptical reflector was used to obtain radiant energiesintense enough to duplicate tunnel convective heatingconditions. The total output power of this lamp wasplotted against applied voltage. The resulting curve isshown in Fig. 6. It was assumed that the power receivedfrom this lamp at a given point in space is proportional,not to the total input power, but to the total outputpower after the absorption region of the quartz is passed.A detector was placed at the focal point of the ellip-

tical reflector with a spring-loaded shutter placed be-tween the detector and the source. A single pulse wasallowed to hit the detector and the magnitude of thepeak voltage response was recorded. This measurementwas taken as a function of total output power from theheat lamp and the results are shown in Fig. 7(a). Theline is extrapolated on the lower end and does not gothrough the origin. This is due to the relatively highabsorption at the long wavelengths in the quartz anddoes not imply that a signal would exist for zero energy.The true response in this absorptive region is shown bythe dotted line at the lower end of the curve. The de-tector response as a function of incident power for lowpowers could be measured quantitatively using a blackbody radiation source.The results of these measurements are given in Fig.

7(b), and these results are transferred to Fig. 7(a). The

0 100 200 300 400 500 600 700 800 900 1000 1100TOTAL OUTPUT POWER FROM LAMP (WATTS)

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(b)Fig. 7. (a) Detector output as a function of the output power

(see Fig. 8) of the heat lamp used for calibration. (b) Detector out-put as a function of the inverse square of the distance from theblack body radiation source.

TABLE II

Area Responsivity Detector Outputecor (cm2XlO-2) (V/W) (mV/W.cm2)A 20 0.186 38.0B 5.1 0.156 7.8C 5.1 0.169 8.5D 5.1 0.163 8.2E 5.1 0.149 7.5F 5.1 0.136 6.8H 5.1 0.095 4.8J 5.1 0.163 8.2K 5.1 0.116 5.8L 5.1 0.149 7.5

* Responsivity and Detector Output columns are the calibrationsfor an amplifier with a 1-MQ input impedance.

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HICKMAN AND BURDICK: PYROELECTRIC TRANSDUCERS FOR HEAT TRANSFER MEASUREMENT

scale of Fig. 7 (b) was related to that of 7 (a) by requiringthe extrapolated line from the black body data to passthrough the point relating to highest power for the GEheat lamp. It is seen that the correlation between thesetwo sets of data is quite good and illustrates the linearitybetween signal and incident powers into the region of100 W/cm2.The detectors, just prior to going into the tunnel, were

recalibrated with a black body radiation source andtheir response during the recalibration is given inTable II.

Experimental DataA total of five tunnel runs were made. During the first

three runs six pyroelectric sensors produced usable re-sults; on the fourth run eight, and on the fifth run ninesensors gave results. Comparison data from the thermo-couples are available on only the first two runs due to adata system malfunction on runs three through five.A typical response from pyroelectric sensor B during a

tunnel run is shown in Fig. 8(a). The slow risetime re-sults from the recording system which has a 5-MQ inputimpedance and dc to 100-Hz response. It should be notedthat the detectors have a 3- to 5-ms risetime when usedwith a 1-1VIQ input impedance amplifier and a dc to 20-kHz recorder.

For comparison purposes the signal from a thermo-couple sensor adjacent to the pyroelectric sensor isshoown in Fig. 8(b). This signal and the one shown inFig. 8(a) have been reduced to heat-transfer rate andplotted in Fig. 9.

It will be noted that there is excellent agreement be-tween the two sensors not only at individual points butin the trend of the data.A typical response from a sensor on the leading edge,

in this case sensor H, is shown in Fig. 10(a). The re-sponse of an adjacent thermocouple sensor is shown inFig. 10(b). These data have been reduced to the heat-transfer rate using the techniques mentioned earlier andare plotted in Fig. 11. The data in Fig. 10(a) is fromtest run number four and that in Fig. 10(b) is from runnumber one. (Experience has shown that conditions arereproducible to the extent that meaningful comparisonscan be made between different runs.)

It is apparent that the agreement between the twosensors is not as good as for the case shown in Fig. 9.There are two factors which provide a possible explana-tion for this situation. Sensor H is located on the leadingedge of the model and therefore subjected to a moresevere environment than sensor B. Particle impact, forexample, could cause a shift in calibration. The sharpupward trend in the heat-transfer rate as measuredat longer test times by sensor H can possibly be ex-plained by referring to Fig. 1 and the discussion ontheory. It was pointed out that the signal is proportionalto dPs/dT. As the sensor is heated up this quantity in-creases in a negative direction. Although this situation

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(b)Fig. 8. (a) Response from pyroelectric detector B during a tunnlel ruln.

(b) Response from thermocouple adjacenit to detector B durinig atunnel run.

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Fig. 9. Heat-tranisfer rates reduced from Fig. 8(a) and (b).

would lead to a greater output per unit of lheat-transferrate, it did not exist at sensor B since the lheat-transferrate was lower and was not heated enough to signifi-cantly change dPs/dT.

All pyroelectric sensors, except K, were operating atthe end of the tests and were apparently undamaged.Sensor K was destroyed during the second run by adirect hit from a segment of the Mylar diaphragm usedin starting the tunnel.Although the sensors were made of a ferroelectric ma-

terial, there was no evidence of spurious output due tomodel acceleration or vibration.

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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, JUNE 1968

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(b)Fig. 10. (a) Response from pyroelectric detector H during a tunnel

run. (b) Response from thermocouple adjacent to detector H dur-ing a tunnel run.

TI ME (MILLI SECONDS)

Fig. 11. Heat-transfer rates reduced from Fig. 10(a) and (b).

CONCLUSIONThe results further corroborate work by Mattes and

Perls [3] suggesting that pyroelectric detectors arecompetitive with thermocouples for heat-transfer mea-surements in impulse facilities. The results obtained bythe pyroelectric and thin-skin thermocouple sensors arein close agreement. The chief advantage of the pyro-electric sensor is the ease of data reduction. Addition-aliy, the pyroelectric sensor is low in cost, can be easilyinstalled and replaced in the model, and is quite sensi-tive and durable.

REFERENCES[1] F. M. Smith et al., "Description and initial calibration of the

Langley Hotshot Tunnel with some real-gas charts for nitrogen,"NASA Tech. Note D-2023.

[2] "Pyroelectric detection techniques and materials," NASA CR-44,April 1964.

[3] B. L. Mattes and T. A. Perls, "A transducer for the measurementof thermal power," Rev. Sci. Instrum., vol. 32, p. 332, March 1961.

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