Journal of the Korean Physical Society, Vol. 57, No. 4, October 2010, pp. 1122∼1126

Ultrasonic Transducer for the Hydrothermal Method

Peter Bornmann∗ and Tobias Hemsel

Mechatronics and Dynamics, University of Paderborn, Fuerstenallee 11, 33102 Paderborn, Germany

Walter Littmann

ATHENA Technologie Beratung GmbH, Weikenweg 24, 33106 Paderborn, Germany

Ryo Ageba, Yoishi Kadota and Takeshi Morita

Graduate School of Frontier Sciences, University of Tokyo, Kashiwano-ha 5-1-5, Kashiwa 277-8563, Japan

(Received 23 November 2009)

Direct ultrasound irradiation is advantageous for increasing the efficiency of the hydrothermalmethod, which can be used to produce piezoelectric thin films and lead-free piezoelectric ceramics.To apply ultrasound directly to the process, transducer prototypes were developed regarding theboundary conditions of the hydrothermal method. LiNbO3 and PIC 181 were proven to be feasiblematerials for high-temperature-resistant transducers (≥200 ◦C). The resistance of the transducer’shorn against a corrosive mineralizer was achieved by using Hastelloy C-22. The efficiency of theultrasound-assisted hydrothermal method depends on the generated sound field.The impedance andthe sound field measurements have shown that the sound field depends on the filling level and onthe position and design of the transducer.

PACS numbers: 85.50.-n, 77.55.H-, 77.55.hj, 81.20.KaKeywords: High-temperature transducer, Hydrothermal method, Lithium-niobate transducerDOI: 10.3938/jkps.57.1122

I. INTRODUCTION

Lead-free production processes and materials are ofgrowing interest due to legislation. These new materi-als require new production processes. For the produc-tion of piezoelectric thin films and lead-free piezoelectricceramics, the hydrothermal method has several advan-tages. It is a rather simple method with inexpensivefeedstock and comparatively low process temperatures.Furthermore, the controllability of the particle size, mor-phology and degree of agglomeration are advantageous.Using the hydrothermal method, piezoelectric thin filmscan be formed on tiny and complex-shaped substrates.Due to the process temperatures being below the Curie-temperature, no poling of the thin films is required. Thedrawbacks of the conventional hydrothermal method areits rather long reaction times and the suboptimal qualityof the resulting materials [1,2].

Direct ultrasound irradiation enhances the hydrother-mal method and improves its efficiency. The appliedacoustic energy shortens the reaction times. Further-more, ultrasound assistance results in fine-grained pow-ders for the production of lead-free piezoelectric materi-als and smoothens the surfaces of the piezoelectric thin

∗E-mail: peter.bornmann@upb.de; Fax: +49-5251-60-62-78

Fig. 1. (Color online) Principal setup for the ultrasound-assisted hydrothermal method.

films. Altogether, the quality of the resulting materialscan be improved while the reaction time is shortened.

The aim of this project is to build a system for theultrasound-assisted hydrothermal method. For direct ul-trasound irradiation, an ultrasound transducer that isapplicable within the temperature and the chemical con-ditions of the hydrothermal method is required. Theprincipal setup of this system is shown in Fig.1. Theultrasound transducer is integrated in a pressure vesseland irradiates the fluid inside.

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II. DEVELOPMENT OF THE ULTRASOUNDTRANSDUCER

1. Requirements for the Ultrasound Transducer

The hydrothermal method is carried out at pressuresup to 20 atm generated by heating the pressure vesseland its contents up to temperatures of more than 200 ◦

C. The required transducer should be able to work effi-ciently under these conditions without an external cool-ing system. Therefore, piezoelectric materials that areapplicable at this temperature range must be chosen.Also, the pre-stress in the piezoelectric elements must bemaintained to avoid damage due to tensile stress duringvibration.

In order to forward the acoustic energy to the fluid,the tip of the transducer will be dipped into the aqueoussolution and, hence, must be resistant to it. Finally,the geometry and the position of the transducer shouldbe optimized to create an optimal sound field for thehydrothermal synthesis.

2. Transducer Design Approach

Heating up a conventional bolted lagevin-type trans-ducer can induce a loss of pre-stress due to the differ-ent coefficients of thermal expansion of the components.Decreasing pre-stress increases the risk of damaging thepiezoelectric discs due to tensile stress and can also de-crease the performance of the transducer. To avoid this,aluminium washers were integrated in parallel to thepiezoelectric discs. Aluminium has a higher coefficient ofthermal expansion than the steel bolt (or Hastelloy C-22respectively). Depending on the amount and the mate-rial of the piezoelectric discs, aluminium washers withappropriate thicknesses can compensate for the loss ofpre-stress or even increase the pre-stress.

The high-temperature application necessitates suit-able piezoelectric materials. Two possible materials werechosen for the transducer prototypes. Those two mate-rials were PIC 181 and LiNbO3. PIC 181 was chosenbecause it is designed for high-power acoustic applica-tions, has a comparatively high Curie temperature of330◦ and is available in the desired shapes. LiNbO3 waschosen because of its extremely high Curie temperatureof 1210◦ and because it is lead-free.

As mentioned before, corrosive mineralizers (e.g.,KOH) are used for the hydrothermal method. For directirradiation, the transducer must be resistant against thecorrosive solution. Therefore, the horn of the transducerwas made from Hastelloy C-22, which is a special alloydesigned for applications in corrosive environments.

One of the prototypes built is shown in Fig. 2. Thistransducer was manufactured from Hastelloy C-22 andcontained ten LiNbO3 discs. Two steel washers were in-tegrated into the design to preserve the pre-stress. The

Fig. 2. (Color online) Ultrasound transducer made fromHastelloy C-22 with ten 36◦ Y-cut LiNbO3 discs and alu-minium compensation washers.

horn of the transducer was designed to maximize the ve-locity amplitudes; therefore, its diameter tapered over ashort distance. The tip of the horn had a bigger diameterto enlarge the irradiating cross-sectional area.

III. EXPERIMENTAL RESULTS

1. Temperature Dependence of the TransducerPrototypes

A serious problem of the first simple transducer pro-totype without amplitude magnification was a dramaticperformance loss at a temperature of about 140◦C. Es-pecially, for the PIC 181 ceramics, the process tempera-tures are a challenging requirement. To analyze the in-fluence of the temperature, we carried out a comparisonof two transducers in a constant temperature oven. Thecompared transducers only differed in the material of thecompensation washers. One was built with aluminiumwashers; the other with steel washers. Steel washers werechosen to achieve less pre-stress compensation comparedto the aluminium washers and, thus, to compare the ef-fects. Both transducers were constructed with PIC 181ceramics. Figure 3 shows the resonant frequencies ofthese transducers versus temperature. The resonant fre-quencies of both transducers decrease linearly with risingtemperature. There is no significant difference betweenthe aluminium and the steel washers. The slope is about

-9Hz◦C

. The main reason for the decreasing resonant fre-quency is suspected to be the decreasing dynamic stiff-ness of the transducer with rising temperature. This issuspected because measurements on passive aluminiumand steel rods have also shown a linear dependence.

The changing resonant normalized frequency can beovercome by applying a resonance control. More impor-tant for the process is the performance of the transducerat the process temperature. In Fig. 4, the dependence ofthe velocity amplitudes at free vibration on the temper-ature is shown. The amplitudes vary in a range of lessthan 10%. At temperatures above 120 ◦C the ampli-tudes of the transducer with aluminium washers seem toincrease while the amplitudes of the transducer with steelwashers slightly decrease. Anyway, it cannot be proven

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Table 1. Comparison between the PIC 181 and the LiNbO3 transducers.

Velocity [m/s] Frequency [kHz] Power [W] Voltage [VRMS ] Current [ARMS ]

PIC 181 3.6 30.9 6.3 18.7 336

LiNbO3 3.4 31.0 8.6 93.8 96

Fig. 3. (Color online) Resonant frequencies versus temper-ature of two aluminium transducers with PIC 181 rings anddifferent washer materials and of a transducer made fromhastelloy with LiNbO3 rings.

Fig. 4. (Color online) Velocity amplitudes versus temper-ature for two aluminium transducers with PIC 181 rings anddifferent washer materials and for a transducer made fromhastelloy with LiNbO3 rings.

that this effect results only from the compensation wash-ers. The hydrothermal synthesis takes up to 24 hours.To analyze the long-time stability of the performance,one transducer was driven in resonance for several hoursat 210 ◦C while the velocity amplitudes were monitored.During the whole time the amplitude was stable.

The results show that PIC 181 is suitable for the tar-get temperature range and, thus, can be used for thisapplication. A positive effect of the aluminium washerscompared to the steel washers can be assumed from thevelocity amplitude measurements. Anyhow, the differ-ence in the performance is not too big, so both trans-ducers are suitable for the hydrothermal method.

As mentioned before, LiNbO3 was also chosen as a

suitable material for high-temperature applications. Tocompare the performance of a LiNbO3 transducer to atransducer with hard PZT, we built two transducers ofthe same shape that differed in the piezoelectric material.The PZT transducer uses 4 PIC 181 discs (thickness:2.5 mm); the other one contains ten 36◦ Y-cut LiNbO3

discs (thickness: 1.27 mm). Both transducers were madefrom Hastelloy C-22 (cf. Fig. 2). The temperature depen-dences of the LiNbO3 transducer are also shown in Fig.3 and Fig. 4. The resonant frequency of this transducerdecreases linearly with rising temperature, but the slopeis lower compared to the other transducers. The sus-pected reason is the different temperature dependencesof aluminium and hastelloy. The velocity amplitude ofthe LiNbO3 transducer increases considerably at highertemperatures. This unexpected result will be investi-gated in a future work.

Table 1 shows a comparison of the performances of thePIC 181 transducer and the LiNbO3 transducer at roomtemperature. The results were achieved by driving thetransducers in resonance and measuring their character-istics. The transducer with LiNbO3 discs requires about30% more input power to achieve the same velocity am-plitude. Furthermore, the distributions of the currentand the voltage are different. The PIC 181 transducerrequires a lower input voltage but a higher current; theLiNbO3 transducer behaves the other way around. Thedifferences can be explained by the different material pa-rameters of PIC 181 and LiNbO3 (e.g., d33 and ε33).

Anyway, the results show that LiNbO3 is a suitablematerial for high-power ultrasonic transducers in high-temperature applications. Higher process temperaturescan be achieved with it than with PIC 181 due to itshigher Curie temperature, and the performance is com-parable to that of the PIC 181 transducer. Improvementsare required in the manufacturing process because pre-stressing LiNbO3 discs is delicate due to their fragility.The stress distribution should be homogenous. Further-more, tolerances for plane parallelism of the componentsshould be limited.

2. Load Dependence and Sound Field

Besides being applicable in corrosive and high-temperature environments the transducer should gener-ate a sound field that supports the hydrothermal methodin an optimal way. To maximize the efficiency of theprocess, the acoustic field inside the pressure vessel isthe deciding factor. The influence of the filling level and

Ultrasonic Transducer for the Hydrothermal Method – Peter Bornmann et al. -1125-

Fig. 5. Depth of immersion versus impedance at resonancefor an ultrasound transducer dipped in water.

the arrangement of the transducer were investigated byimpedance and sound field measurements.

The fluid inside the pressure vessel is excited with thedriving frequency of the ultrasound transducer. The fill-ing level of the vessel decides the response of the fluid.If a standing wave is to be generated, the filling levelcan be estimated from the wavelength, which dependson the excitation frequency and the velocity of sound inthe fluid. Assuming the boundary conditions for the fluidinside the vessel to be fixed at the bottom and free atthe fluid surface, the possible filling levels for generatinga standing wave can be calculated from Eq. (1) [3] whereH is the filling level and λ is the wavelength in the fluid:

H = (2 n + 1)λ

4, with n = 0, 1, 2, · · · , . (1)

Obviously there are several possibilities for the fillinglevel, depending on how many nodal points (n) arewanted in the water column. In addition to the fillinglevel, the location of the transducer plays a decisive rolein the forwarded acoustic power. Suitable locations canbe indicated by the impedance of the transducer. Atpoints of maximum impedance, the forwarded energy isalso maximal. Figure 5 shows the dependence of theimpedance in resonance on the location of the transduc-ers tip. The measurements were carried out in a water-filled basin. The maximum impedance occurs at a 15 mmdepth of immersion, so this point indicates an optimumfor power propagation to the fluid.

The resulting sound field in the water basin with thetransducer located at the point of maximum impedanceis shown in Fig. 6. This sound field measurement wascarried out using the refracto-vibrometry method afterZipser [4–6]. The dark and the light areas indicate ar-eas of high sound pressure and the different colors showthe different signs of the pressure amplitudes. The dark

Fig. 6. (Color online) Sound field (standing wave) in awater basin at 25 kHz.

Fig. 7. (Color online) Sound field in air with reflections.

lines between those areas are minima of the sound pres-sure. These nodal lines remain stationary, which showsthat a standing wave is created. The point of maximumimpedance corresponds to the point of maximum soundpressure and, hence, to a nodal point of the vibrationvelocity in the fluid. Due to the boundary conditions,the sound pressure has a minimum at the water’s sur-face and a maximum at the bottom of the vessel. Whena standing wave is desired and the filling level is chosenas proposed in Eq. (1), the point of maximum impedanceis located one fourth of a wavelength beneath the water’ssurface at the point of maximum sound pressure.

If a standing wave is not desired or if a reflector isplaced in the sound field, the reflected sound waves in-terfere with the emitted sound field. Depending on theshape and the location of the reflector, traveling wavesand, thus, streaming effects can be achieved. Figure 7shows a sound field with reflections measured in air. Thereflections were forced by placing a steel reflector in thesound field. Systematic reflections can be useful for cre-ating special sound fields and, hence, special reactionconditions.

The impedance measurements were carried out withdifferent types of transducers. Due to the strong ampli-tude magnification of the Hastelloy transducer (cf. Fig.2), this transducer showed a high load sensitivity. Thisresults in strong damping when the transducer is dippedin water, which makes it complex to control. In con-trast, transducers without amplitude magnification arenot that sensitive to changes in the load conditions, butthe achievable velocity amplitudes are smaller. Regard-

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ing the process, an optimum for the velocity amplitudesshould be found.

IV. CONCLUSIONS

To improve the hydrothermal method for the produc-tion of lead-free piezoelectric materials and piezoelectricthin films, we designed transducer prototypes regard-ing the boundary conditions of the process. For cor-rosion resistance, Hastelloy C-22 was chosen. For high-temperature-resistant ultrasound transducers, PIC 181and LiNbO3 were proven to be suitable materials.

The advantages of ultrasound assistance for improv-ing the hydrothermal method have already been shownin first tests [1]. The efficiency of the process depends onthe created sound field. The sound field inside the pres-sure vessel and the transmittable power depend on thefilling level and on the position and the geometry of theultrasound transducer. By adjusting these parametersin a certain manner, standing waves or streaming effectscan be achieved. Hence, one of the next steps in thisongoing research is to determine the correlation betweenthe sound field and the resulting materials.

ACKNOWLEDGMENTS

The research activities at the University of Paderbornand the University of Tokyo were supported by the NewEnergy and Industrial Technology Development Organi-zation (NEDO).

REFERENCES

[1] Y. Kadota, M. Ishikawa, H. Hosaka and T. Morita, IEEETrans. Ultrason. Ferroelectr. Freq. Control 56, 1, (2009).

[2] Y. J. Ma, J. H. Cho, Y. H. Lee and B. I. Kim, Mater.Chem. Phys. 98, 5 (2006).

[3] R. D. Blevins, Formulas For Natural Frequency And ModeShape (Krieger Publishing Company, Florida, 1979).

[4] L. Zipser and H. Franke, Refracto-Vibrometry for Visual-izing Ultrasound in Gases, Fluids and Condensed Matter(IEEE Ultrasonics Symposium, New York, 2007), p. 28.

[5] L. Zipser, R. Behrend and S. Lindner, DE 100 57 922 C2.[6] L. Zipser and S. Lindner, DE 100 57 924 C2.