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The Seventh Western Pacific Regional Acoustics Conference

Kumamoto, Japan, 3-5 October 2000

HYDROTHERMALLY DEPOSITED PZT FILM AND ITS APPLICATION TO BENDING VIBRATION DEVICES

Hidehiko YASUI*, Minoru K. KUROSAWA** and Toshiro HIGUCHI* *The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan Phone: +81-3-5841-6466, Fax: +81-3-5800-6968 E-mail: hide@intellect.pe.u-tokyo.ac.jp **Tokyo Institute of Technology

ABSTRACT We examined the performance of PZT thin film deposited with hydrothermal method on high intensity operation. We report the result and some PZT devices fabricated with hydrothermal method. Hydrothermally deposited PZT film was examined on large electric field at resonance frequency. Thickness of the PZT film was about 12 microns. It indicated prominent performance about the vibration velocity and the neat frequency response around the resonance. At this point, the PZT film was superior to bulk PZT materials. The maximum vibration velocity of 1.3 m/sec was obtained and the frequency response was continuous and little distorted. In addition to those results, we will present the possibility of the PZT thin film devices by hydrothermal method. The devices are bending vibrators, an earphone for hearing appliances and an actuator for fluid using progressive wave. The maximum SPL of the earphone was 75 dB and we can observe the acoustic streaming by progressive wave by using hydrothermal PZT film transducer. KEYWORDS: PZT film, Ultrasonic vibrator, Bending vibrator, Progressive

wave, Earphone

INTRODUCTION

We have reported studies about PZT thin film deposited with hydrothermal method and some applications to ultrasonic transducers for sensors and actuators [1]-[3]. In addition, we improved the depositing process by controlling the posture of base material in the solution. And the thin film was examined on high intensity operation. It is very important to examine the characteristics of intensely driven thin film, because a lot of piezoelectric transducers are used at large vibration amplitude range. We measured the limit of vibration velocity. The results showed us the hydrothermally deposited film has prominent performance on vibration velocity

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characteristics. Further, we report some PZT devices fabricated by using the improved hydrothermal

method. The fabricated devices were earphones for hearing appliances and actuators for fluid. Diaphragms of the earphone had bimorph type structure. A fluid actuator moved water by acoustic streaming with its progressive wave. Then, we present the possibility of the hydrothermal PZT film devices.

HYDROTHERMALLY DEPOSITTED FILM

In hydrothermal method, the deposit process is carried out on a titanium base material in aqueous solution of melted ions, Pb2+, Zr4+ and Ti4+ containing KOH. This process has two repetitional reactions, namely nucleation and crystal growth. By the first reaction, nuclear crystals of PZT are deposited on the titanium base at 160 degree Celsius. By the next reaction, nuclear crystals are grown up at 120 degree Celsius as the reaction times. We repeated the second react four times and obtained 12µm thick on a side of titanium base material. The deposited film is poly-crystal structure as shown in Fig. 1. Because of this depositing system, we can deposit PZT film on optional flat and round surface. Other merits of hydrothermal method are thick deposited PZT film, automatic polarization and unnecessary of annealing process. We improved the depositing process by controlling the posture of base materials. Base materials are supported by titanium wire basket in an autoclave so as to be kept standing parallel to the solution flow. This flow control by the wire basket promotes the reaction of the solution around the base materials. Then it results uniform deposition and performance improvement of the PZT film.

CHARACTERISTCS ON HIGH INTENTION DRIVE The disk transducer for the experiment was driven at the 0th mode flexural vibration supported at the center. The structure and appearance of the transducer are shown in Fig. 2. The measurements of the vibration velocity were carried out with a laser Doppler vibrometer, and the measured point was the edge of the flexural disk. We measured the vibration velocity at each resonance frequency with changing driving voltage. The maximum vibration velocity of 1.3

Fig. 2 Transducer disk for measurement

(diameter: 9.8mm, thickness: 0.2mm)

Fig. 1 SEM photograph of the hydrothermal

PZT film surface.

Electrode Cross Section

Disk

Ti base PZT film 'LVN

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m/sec was obtained as shown in Fig. 3. The results indicate the vibration velocity linearly increased and did not saturate, but the devices brake down due to high electric field at 2.5kV/mm. At this moment, the maximum internal stress was calculated to be 18.2MPa. Even in case of the best PZT material, the maximum vibration velocity is about 1 m/sec. hence the limitation of the vibration velocity of the film material is almost same as the bulk material. But the internal stress of the bulk PZT is more than 30 MPa. It’s almost two times of the film material. We think it causes the sparseness of hydrothermal PZT. The frequency response that was measured with changing driving frequency was indicated in Fig. 4. In the case of bulk materials, it is well known that the response curves are distorted and jumping phenomenon are observed. However, in the case of hydrothermal PZT film devices, the response curves have little distortion. There was no jumping and dropping phenomenon on response curves that is often observed in the case of bulk PZT vibrator.

APPLICATION DEVICES

Hearing Appliance. Now, electromagnetic type is used for an earphone of a hearing appliance. If we can replace it by a piezoelectric earphone, the battery will run a few times as long as the former. We fabricated the bimorph construction disk as the transducer. As shown in Fig. 5, the diameter of the disk was 6mm, the thickness was 5 ,10 or 20µm. The electrodes were formed with vacuum evaporator on the both sides of the disk. The transducer with supported at the center was driven by the driving voltage, 1Vrms. One example of frequency response of the hydrothermal diaphragm disk is indicated in Fig. 6. The resonance frequency of the fabricated hydrothermal diaphragm was about 3-4kHz. This is good for the earphone of hearing appliances. The maximum sound pressure level of one 5 µm disk at 1kHz was 75dB. Other elements reproduced 60 to 70 dB. About 110dB sound pressure level is required for practical use. That is, it was not enough to use practically. But yet there are a lot of structure problems, to say, how to support, how to seal and how to wiring and so on. After all we indicated the possibility of piezoelectric transducer for hearing appliance. Actuating Fluid. Acoustic streaming in fluid is generated by vibration of a transducer. We made use of the hydrothermal flexural transducer to generate progressive wave and acoustic streaming to the circumference direction. The transducer disk was the same size as the hydrothermal PZT evaluation. We made the progressive wave on the transducer disk. By using the progressive wave, we can handle more freely because of actuating the fluid parallel with the transducer disk. When

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0 10 20 30 40 50

Vel

ocity

[m

/s]

Driving Voltage [Vp-p

]

Fig. 3 Vibration velocity at resonance frequency

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11.2 11.4 11.6 11.8 12 12.2

Vel

ocity

[m

/s]

Frequency [kHz]

Fig. 4 Frequency response intensely driven

30Vp-p

Driving Voltage

48Vp-p

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we drove the transducer on fluid containing dyed particles, and investigated the effect to the fluid. W we could observe the circumference direction flow by progressive wave.

SUMMARY We improved the hydrothermal PZT and investigated the characteristics of the new PZT film on high intensity operation. The maximum vibration velocity was 1.3m/s, and the internal stress was 18.2MPa at the condition. The frequency response has no distortion and the linearity of frequency response around the resonance was excellent. That was superior to bulk material up to high intensity operation. As a result, we found the new hydrothermal PZT film is suitable for the devices driven within high intensity range. That is, hydrothermal PZT film is good for some sensors and actuators. In addition, we fabricated an earphone and actuator for fluid by using the hydrothermal PZT. The earphone could perform 75 dB sound pressure level. And we observe the preference flow actuated by the hydrothermal transducer. After all, we indicated the possibility of those hydrothermal PZT film devices.

ACKNOWLEDGMENTS This work was supported by the Grant-in-aid for general scientific research of the Ministry of Education, Science, Sports and Culture, and by the Proposal-Based New Industry Creative Type Technology R&D Promotion Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors would like to thank Mr. T. Iwakura and Mr. T. Ohtani of Rion Co., Ltd., for their cooperation in valuable advices and technical support about hearing appliances.

REFERENCES

1. T. Morita, M. Kurosawa and T. Higuchi: “A cylindrical shaped micro ultrasonic motor utilizing PZT thin Film,” in Proc. of Transducers ’99, Sendai, June (1999) 1744-1747.

2. T. Kanda, T. Morita, M. K. Kurosawa and T. Higuchi: “A flat type touch probe sensor using PZT thin film vibrator,” in Proc. of Transducers ’99, Sendai, June (1999) 1580-1511.

3. H. Yasui, M. Kurosawa, T. Kanda and T. Higuchi: “Piezoelectric Transducer for Hearing Aid Using PZT Thin Film,” in Proc. of 16th Int. Cong. on Acoustics, Seattle, (1998) 2201-2202.

4. M. K. Kurosawa, H. Yasui, T. Kanda and T. Higuchi: “Performance of Hydrothermal PZT Film on High Intensity Operation,” In Proc. of IEEE Workshop on MEMS, pp. 56-61, Jan. 23-27 (2000).

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Fig. 6 Frequency response of sound pressure level

VÕ

Sealed Space connected to ear

Plastic Film

HydrothermalPZT Film Electrode

Fig. 5 Structure of the hearing appliance Frequency [kHz]

0 40 1 2 3 4 5

60

80

100

120