Development of Miniature Needle-Type Hydrophone with Lead Zirconate Titanate Polycrystalline Film Deposited by Hydrothermal Method

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1 Japanese Journal of Applied Physics Vol. 45, No. 5B, 26, pp #26 The Japan Society of Applied Physics Development of Miniature Needle-Type Hydrophone with Lead Zirconate Titanate Polycrystalline Film Deposited by Hydrothermal Method Hiroshi KITSUNAI, Norimichi KAWASHIMA, Shinichi TAKEUCHI, Mutsuo ISHIKAWA 1, Minoru KUROSAWA 1 and Etsuzo ODAIRA 2 Graduate School of Biomedical Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba-ku, Yokohama , Japan 1 Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama , Japan 2 Department of Electronics and Communications, Faculty of Engineering, Musashi Institute of Technology, , Tamazutsumi, Setagaya-ku, Tokyo , Japan (Received November 3, 25; revised February 15, 26; accepted February 21, 26; published online May 25, 26) A needle-type miniature hydrophone was developed using hydrothermally deposited lead zirconate titanate (PZT) polycrystalline films on a titanium (Ti) wire with a diameter of.3 mm in this study. Two types of hydrophones were fabricated as a trial for this paper. One is a hydrophone with hydrothermally deposited PZT polycrystalline films on the end surface and side surface near the end of the Ti wire. The other one is a hydrophone with hydrothermally deposited PZT polycrystalline films only on the end surface of the Ti wire. We measured the frequency characteristics of the relative receiving sensitivity and directivity of the fabricated hydrophones. The frequency characteristics of the relative receiving sensitivities of the fabricated hydrophone type A or B were measured in the frequency range from 1 to 2 MHz. Our hydrophone with deposited PZT films on the end and side faces of the Ti wire had problems in the directivity of relative receiving sensitivity. The relative sensitivity of the side direction was higher than that of the front direction. The directivity of our hydrophone with hydrothermally deposited PZT films only on the end surface of the Ti wire was improved remarkably. [DOI: /JJAP ] KEYWORDS: hydrothermal method, hydrophone, PZT polycrystalline film, Ti wire, frequency characteristics, directivity 1. Introduction We have been studying the hydrothermally synthesizing technology of piezoelectric polycrystalline films for the fabrication of piezoelectric actuators or ultrasound sensors. 1,2) The hydrothermally synthesizing technology is suitable for the fabrication of a minute ultrasound sensor. Needle-type miniature hydrophones were developed for the estimation of sound fields or sound intensities using medical ultrasound diagnostic equipments. We have developed a needle-type miniature hydrophone with hydrothermally synthesized lead zirconate titanate (PZT) polycrystalline films on the end of a titanium wire. 3) Polyvinylidene difluoride (PVDF) piezoelectric polymer films or PZT piezoelectric ceramics are used for the tiny receiving element in most conventional needle-type miniature hydrophones. 4,5) This tiny receiving element was bonded on a thin metal wire by adhesion process in conventional miniature hydrophones. This adhesion process is a difficult fabrication process. Therefore, there were problems in terms of performances like the frequency characteristics and directivity of receiving sensibility were unstable. The needle-type miniature hydrophone with a very tiny receiving element can be fabricated easily by depositing of PZT polycrystalline films directly on the end surface of a thin Ti wire by the hydrothermal method. 6 9) Furthermore, it can be expected that hydrophones fabricated by the hydrothermal method have a more stable performance than hydrophones fabricated by conventional fabrication techniques. The estimated performances of our fabricated hydrophone by the hydrothermal method are reported in this paper. 2. Experimental Methods 2.1 Deposition of PZT polycrystalline films on Ti wire PZT polycrystalline films were deposited hydrothermally 4688 Table I. Hydrothermally synthesizing conditions of PZT polycrystalline films on Ti wires for fabrication of needle-type miniature hydrophones. Concentration Quantity Stirring (mol/l) Time speed (h) ZrOCl ml (rpm) TiO g Pb(NO 3 ) ml KOH 4. 3 ml on the end of a Ti wire with a diameter of.3 mm for the fabrication of hydrophones. The crystal nucleation process was accomplished at 14 C and the crystal growth process was accomplished at 16 C. 1) One cycle of each process as accomplished respectively. Other conditions except temperature were the same in both processes of the hydrothermal method. The deposition conditions of PZT polycrystalline films on the Ti wires for the fabrication of the hydrophones are shown in Table I. In this study, two types of hydrophone were fabricated for the comparison of the frequency characteristics and directivities of receiving sensibility. The PZT polycrystalline film was deposited hydrothermally on the end and side surfaces of the Ti wire without any pretreatment for the fabrication of one type of hydrophone (we call the as hydrophone type A below). The PZT polycrystalline film was deposited only on the end of the Ti wire by masking treatment to avoid the deposition of PZT on the side surface of the Ti wire for the fabrication of another type of hydrophone (we call this as hydrophone type B below). Our hydrothermal synthesis of PZT is performed under the conditions of a high temperature, a high pressure, and in a strong alkaline solution. Therefore, the masking material should not be dissolved under such conditions. Fluorocarbon polymer (Neofuron FEP; Daikin Industries) was used as the masking material in

2 Type A Type B Fig. 1. SEM images of hydrothermally synthesized PZT films on titanium wire with diameter of.3 mm for fabrication of needle-type miniature hydrophone. this study. Figure 1 shows the scanning electronic microscope (SEM) images of the Ti wires with deposited PZT polycrystalline film for type A and type B hydrophones. 2.2 Assembly of hydrophones The Ti wire with hydrothermally deposited PZT film is connected to the inner conductive line (signal line) of a coaxial cable, and an insulating layer of acrylic resin is coated to the side surface of the Ti wire and the inner conductive line. After that, the conductive resin is coated all over the surface of the hydrophone to shield and ground it, and conductive resin is connected with the outer conductive line (GND line) of a coaxial cable. Figures 2 and 3 show the schematic diagram and image of the fabricated hydrophone, respectively. 2.3 Performance estimation of fabricated hydrophone To estimate the received ultrasound waveform, the frequency characteristics and directivity of receiving sensitivity were measured for the estimation of our fabricated hydrophone of type A or type B. The frequency characteristics and directivity of receiving sensibility were measured by using the measurement system shown in Fig. 4. The burst wave outputted from the function generator (HP81116A, Hewlett Packard) is amplified by a power amplifier (A3-1381, ENI) with a gain of 55 db, and is applied to a transmitting ultrasound probe. The received waveform by the hydrophone is amplified using a preamplifier in a pulsar/receiver (PR-58, Panametrics), and is 4689 Fig. 2. Schematic diagram of needle-type miniature hydrophone (type A or type B) with hydrothermally deposited PZT films on Ti wire by hydrothermal method. displayed on an oscilloscope (5463B, Hewlett Packard). The applied voltage to the transmitting ultrasound probe was approximately 34 V. The measurements were performed in the frequency range from 1 to 2 MHz. Five commercial ultrasound probes with nominal frequencies of 2.25, 3.5, 5, 7.5, and 1 MHz (I3-18-R, Staveley Sensors) were employed as the transmitters. The aperture of a commercial ultrasound probe is 12.7 mm. The ultrasound irradiated to water in a water tank using the commercial

3 Fig. 3. Images of fabricated needle-type miniature hydrophone with hydrothermally synthesized PZT polycrystalline film on Ti wire with diameter of.3 mm. Fig. 4. Measurement system for frequency characteristics and directivity of receiving relative sensitivity of needle-type miniature hydrophone fabricated by hydrothermal method. ultrasound probes was received by the fabricated hydrophone at various frequencies. Essentially, the sensitivity calibration of the hydrophone should be performed in an acoustic far-field. For far-field measurements, the distances between the sound source and the fabricated hydrophone should be 26.9 mm for the measurement frequency of 1 MHz, and 538 mm for 2 MHz. Because our commercial ultrasound probes have low transmitting sensitivity, it is difficult to measure the received voltage from the fabricated hydrophone with a sufficient signal to noise ratio at a distance of 538 mm and a frequency of 2 MHz. The measurements were performed at a distance of 2 mm between the sound source and the fabricated hydrophone. Consequently, the measurements were performed in the near-field. Accordingly, the frequency characteristics of receiving sensitivities were evaluated by the normalization of receiving voltage using the maximum 469 value of the received voltage. We call this type of receiving sensibility as relative receiving sensitivity in this paper. The directivity of our fabricated hydrophone was measured at a frequency of 5 MHz using the same measurement system shown in Fig. 4. The directivity measurement of our hydrophone is performed by fixing the location and direction of the transmitting ultrasound probe, and by rotating under to test our fabricated hydrophone around its acoustic receiving surface as the center of rotation. 3. Results and Discussion Figure 5 shows the ultrasound waveform received by the fabricated hydrophone (type A) at 1 MHz. The received ultrasound waveform could be observed at approximately 1 ms after the rise time of trigger for transmission. The measured frequency characteristics of the relative receiving sensitivity of the fabricated hydrophone (type A or B) are

4 Ttansmitted pulse Trigger for transmission 1MHz Received pulse Time/div : 2µs Fig. 5. Example of received ultrasound waveform by our fabricated hydrophone as trial. Relative receiving sensitivity [db] Type B (φ.3mm) Type A (φ.3mm) Frequency [MHz] Fig. 6. Frequency characteristics of receiving relative sensitivity of fabricated hydrophone with deposited PZT films by hydrothermal method B Type A Type MHz +6 hydrophone type A. The hydrophone type A has a larger acoustically active area with hydrothermally deposited PZT polycrystalline films on the side surface of the Ti wire than on the end surface. The directivity of the hydrophone type B was improved. Relative receiving sensitivity in the front direction was higher than that in the side directions in the directivity of our fabricated hydrophone type B. The relative receiving sensitivities in the side directions were suppressed remarkably, because PZT polycrystalline film was not deposited hydrothermally on the side surface of the Ti wire in our hydrophone type B. 4. Conclusions Two types of needle-type miniature hydrophones were fabricated using hydrothermally deposited PZT polycrystalline films on a Ti wire with a diameter of.3 mm. PZT polycrystalline films were deposited not only on the end surface but also on the side surface of the Ti wire for one type of hydrophone named as type A. PZT polycrystalline films were deposited only on the end surface of the Ti wire for another type of hydrophone named as type B. The masking technique using a fluorocarbon polymer (Neofuron FEP) was employed to avoid the deposition of PZT polycrystals on the side surface of the Ti wire for the hydrophone type B. We measured the frequency characteristics and directivities of relative receiving sensitivity for the performance estimation of the fabricated hydrophones. The frequency characteristics of the relative receiving sensitivities of the fabricated hydrophone type A or B were measured in the frequency range from 1 to 2 MHz. The hydrophone type A has an undesirable directivity in which relative receiving sensitivity in the side direction is higher than that in the front direction. Directivity was improved in the hydrophone type B by avoiding the hydrothermal deposition of PZT polycrystals on the side surface of the Ti wire using the masking technique. We will fabricate a hydrophone using a Ti wire with a smaller diameter in our future work. The effects of PZT synthesized on the side surface of a Ti wire in terms of the frequency characteristics and directivity of receiving sensitivity will be considered. Furthermore, because new calibration methods for the sensitivity of a hydrophone were proposed, 11,12) we will consider the proposals for calibration for the fabrication of a hydrophone using the hydrothermal method. Fig. 7. Directivity of trial fabricated hydrophone with PZT films deposited by hydrothermal method. shown in Fig. 6. Figure 7 shows the measured directivities of the relative receiving sensibility of the fabricated hydrophone of types A or B. The relative sensitivity of the side direction was higher than that of the front direction in our hydrophone type A. Such directivity is an undesirable characteristic of the hydrophone. It was thought that the undesirable directivity was due to the structure of the ) M. Ishikawa, M. K. Kurosawa, A. Endo and S. Takeuchi: Jpn. J. Appl. Phys. 44 (25) ) N. Katsura, M. Ishikawa, T. Sato, M. Takeuchi, N. Kawashima, S. Takeuchi and M. Kurosawa: IEEE Ultrasonic Symp. Proc., Honolulu, 23, p. 1. 3) H. Kitsunai, T. Suzuki, M. Ishikawa, N. Kawashima, E. Ohdaira, M. Kurosawa and S. Takeuchi: presented at 24 IEEE Ultrasonic, Ferroelectrics and Frequency Control Joint 5th Anniversary, Montreal, 24. 4) M. Ide and E. Ohdaira: Proc. 1st Symp. Ultrasonic Electronics, Tokyo, 198, Jpn. J. Appl. Phys. 2 (1981) Suppl. 2-3, p ) M. Ide and E. Ohdaira: IEEE Trans. Ultrason. Ferroelectr. Freq. Control 35 (1988) ) K. Shimomura, T. Tsurumi, Y. Ohba and M. Daimon: Jpn. J. Appl. Phys. 3 (1991) 2174.

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