Chan, HLW

Choy, CL

Kwok, KW

Lau, ST

Zhou, Q

PolyU Institutional Repository

PZT/P(VDF-TrFE) Nanocomposites for Ultrasonic Hydrophone Application S.T. Lau, Q.F. Zhou, K.W. Kwok, H.L.W. Chan and C.L.Choy Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hunghom, Kowloon, Hong Kong. Abstract 0-3 composites with nanosized lead zirconate titanate (PZT) powder imbedded in a vinylidene fluoride- trifluoroethylene copolymer (P(VDF-TrFE)) matrix have been fabricated. Since the piezoelectric coefficients of PZT and P(VDF-TrFE) have opposite signs, the PZT and P(VDF-TrFE) phases in the nanocomposites were poled in opposite directions in order to enhance the piezoelectric activity. The PZT powder, prepared by a sol-gel process, has a tetragonal structure. The ratio of the (002) and (200) X-ray diffraction peaks was used as a measure of the degree of poling in the ceramic phase. The piezoelectric coefficient of the nanocomposite film was measured by a laser interferometry technique. A needle type hydrophone for the calibration of medical ultrasonic transducers was fabricated using the nanocomposite as the sensing element and its performance was evaluated. 1. Introduction Piezoelectric ceramic/polymer 0-3 composites consisting of piezoelectric ceramic particles imbedded in a polymer matrix have been extensively studied over the past decade for the use as piezoelectric sensors and actuators [ 1,2]. They have relatively high hydrostatic coefficient (dh) and low acoustic impedance as compared with the constituent ceramic. Recently, 0-3 composites with both phases piezoelectrically active have also attracted some interest [3-51. Previous works [6,7] show that the two phases of a piezoelectric ceramic/piezoelectric polymer 0-3 composite can be poled either in the same or opposite directions. As the piezoelectric coefficients of the ceramic and the copolymer are opposite in sign, the composite with both phases poled in opposite directions will have piezoelectric activities of the two phases reinforced and pyroelectric activities partially canceled each other. As a result, the composite will have high piezoelectric but low pyroelectric activities, thereby reducing the electric noise due to temperature fluctuation. In the present work, we aim at fabricating PZT/P(VDF-TrFE) 0-3 nanocomposite thin films with both phases poled in opposite directions, and studying their piezoelectric properties and hydrophone performances, 2. Sample Preparation The procedure for preparing nanosized lead zirconate titanate (PZT) ceramic powder by the sol-gel method has been described in our previous report [SI. The powder used in this study has a Zr/Ti ratio of 48/52 and has a tetragonal structure. It has been annealed at 900°C for 1 h, and has an average crystallite diameter of about 80 nm. The vinylidene fluroide-trifluoroethylene copolymer (P(VDF-TrFE)) was obtained in pellet form from Piezotech in France. It contains -30 mol% of TrFE as confirmed by differential scanning calorimeter (DSC) measurements. The Curie temperature of the copolymer upon heating was 102°C and the melting temperature was 150°C. In the preparation of PZT/P(VDF-TrFE) 0-3 composite films, the copolymer was first dissolved in methyl-ethyl-ketone (MEK) at room temperature and an appropriate amount of PZT powder was gradually introduced into the solution with constant stirring. The composite solution was then spin-coated on an Al/glass substrate of which the aluminum layer serving as bottom electrode was thermally evaporated. The film was kept overnight at 40°C and then annealed at 120°C for 2 h to remove the solvent and to increase the crystallinity of the copolymer phase. Another thin layer of aluminum was thermally IPh International Symposium on Electrets, 1999 0-7803-5025-1/99/$10.00 01999 IEEE Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 02:36 from IEEE Xplore.  Restrictions apply.75 6 evaporated on the film to serve as a top electrode. The ceramic volume fraction of the composite film is about 0.2, and the thickness is about 6 pm. 3. Sample Poling In the present study, three types of sample were prepared: sample A (only the PZT phase poled), sample B -  (both phases poled in the same direction), and sample C (both phases poled in opposite, directions). To prepare sample A, the composite film was heated to 12OOC and a DC electric field of 35 W / m  was.applied across the electrodes for 3 h. Then the electric field was switched off and the sample was cooled slowly to room temperature. As the electric field is applied only at temperature higher than the Curie temperature of the copolymer, only the dipoles in PZT were aligned. To prepare samples B and C, the poled composite film (sample A) was re-poled at room temperature under an AC electric field of 50 W / m  in order to align the dipoles in the copolymer. Sample B was re-poled such that the copolymer phase was polarized in a direction parallel to the polarization of the PZT phase, while the copolymer phase in sample C was polarized in a direction opposite to that of the PZT phase. The intensities 4002) and 4 2 0 ~ )  of the (002) and (200) peaks in the X-ray diffraction pattern were used to evaluate the degree of poling of the PZT in the composites. The polarization ratio a is defined as [9] where I'(oo2) and I'(200) are the intensities after poling, and 4002) and I(200) are the intensities before poling. The a values of all samples are very close, about 0.26, indicating that the pre-existed polarization in the PZT phase has not been disrupted by the AC electric field in polarizing the copolymer phase. As compared with the a value of a fully poled PZT sample (0.55), the smaller a values of the composites indicate that the PZT particles inside the composites are not fully polarized. Attempts have been made, but a cannot be improved significantly. 4. Piezoelectric Coefficient d33 A Mach-Zehnder type heterodyne interferometer (SH-129 from B.M. Industries, France) was used to measure the surface displacement of the composite film (Fig. 1). A linearly polarized laser beam, L (frequency fL,  h = 632.8 nm for a He-Ne Laser) is split into a reference beam, R, and a probe beam, P. R is directed through a Dove prism and a polarizing beam splitter into a photodetector. The frequency of P is shifted by a frequency fB (70 MHz) in a Bragg cell, and then this beam (now labeled S) is phase modulated by the surface displacement of the film sample, x = U cos(2.nfUt) (vibration frequency f,, displacement amplitude U). For small vibration displacement, only the components of the detected signal at fB and fB+fu are significant, and the ratio of their amplitudes R gives the displacement amplitude U [lo]: where J, and J1 are the Bessel function of the zeroth and the first order, respectively. The ratio R is measured using a spectrum analyzer (Hp 3589A). The piezoelectric coefficient d33 (straidapplied field) of the film can then be calculated as where V is the voltage applied across the film. The measured d33 values for the composite samples are given in Table 1. As mentioned in section 3, the PZT particles inside the composites are not fully polarized, so the d33 value of sample A is small, about 1.82 pmN. The d33 value of sample C is about 40 % larger than that of sample B, indicating that the piezoelectric activity of the composite is enhanced by the oppositely oriented dipoles in the two phases. It should be noted that as R = J1(47tu/h)/JO(4~u/h) - 27cdh = ~ / 1 0 0 7  (2) d33 = (u/t)/(V/t) = UN (3 1 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 02:36 from IEEE Xplore.  Restrictions apply.75 7 the film is laterally clamped on the glass substrate, the measured d33 value is smaller than the value of the Sample A (only the PZT ceramic phase poled) B (both phases poled in the same direction) C. (both phases poled in opposite directions) bulk sample [ 1 13. ....................................................................................................................................................................................................................... d33 (~d) 1.82 5.2 8.0 Dove Prism Polarizing Heterodyne Interferometer ..................................................................................................................................... I Spectrum Analyzer I Sample I I Oscilloscope I f" Figure 1. A Mach-Zehnder type heterodyne interferometer. Table 1 : Piezoelectric coefficients of the samples 5. Hydrophone Performance Evaluation Sample C, in the form of circular disk of diameter lmm, was fabricated into a needle type hydrophone (Fig. 2). To evaluate the performance of the hydrophone, its receiving sensitivity was measured as a function of frequency. A commercial transducer (10 MHz, 0.25 inch diameter, Panametrics V312) is used to generate acoustic waves in water. The acoustic waves are received by the needle type hydrophone placed at the far- field near-field transition point T (T = a2/4h where a is the radius of the transducer and h is the acoustic wavelength in water) for the transducer, and the signal from the hydrophone is measured by a digitizing oscilloscope (HP 54504A). For comparison, a standard PVDF bilaminar shielded membrane hydrophone with an active element of diameter 0.5 mm (GEC-Marconi, Type Y-34-3598) is also used to measure the acoustic waves at the same field point T. The end-of-cable loaded receiving sensitivity (ML) of the needle type hydrophone is then calculated from: (4) VL M, =-Ms v s  where VL and Vs are the measured voltage output using the needle-type and membrane hydrophones, respectively, and Ms is the end-of-cable loaded receiving sensitivity of the membrane hydrophone. Fig. 3 shows the variation of the receiving sensitivity'of the hydrophones with frequency. It can be seen that the composite hydrophone has better sensitivity than the standard PVDF bilaminar shielded membrane hydrophone. Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 02:36 from IEEE Xplore.  Restrictions apply.75 8 -220 3 240 bpoxy h nanocomposite 3 9 E 260 2. -280 ? 8 - e -300 v) electrode Copper wire -320 - 1 - - A A z : : l m 4  - - I I I I I I I I  Figure 2. Schematic diagram of the nanocomposite needle. Figure 3. End-of-cable loaded sensitivity of the hydrophones: 0-3 composite with both phases poled in opposite directions, .; standard PVDF membrane A. 6. Conclusion PZT/P(VDF-TrFE) 0-3 nanocomposite thin films with 0.2 ceramic volume fraction have been prepared and characterized. The piezoelectric coefficient of the composite with both phases poled in opposite directions is enhanced. Needle-type hydrophone with the composite as the sensing element has been fabricated and shown to have good receiving sensitivity. 7. Acknowledgements The work described in this paper was substantially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region (Project No. PolyU5 159/98P) and also supported by the PolyU Centre for Smart Materials. 8. References [ l ]  C.E. Murphy, T. Richardson, G.G. Roberts, Ferroelectrics, 189, 134, 1992. [2]. C.  Dias, M. Wenger, P. Blanas, R.J. Shuford, Y. Hinton and D.K. Das-Gupta, in Proc. 2nd Zntl. Conf: on Znteiligent Materials [3] T. Furukawa, K. Ishida and E. Fukada, J. Appl. Phys., 50, 4904, 1979. [4] C. Muralidhar and P.K.C. Pillai, J. Mater. Sci., 23, 1071, 1976. [5] H.L.W. Chan, Y. Chen and C.L. Choy, IEEE Trans. on Dielectrics and Electric. Insul., 3, 800, 1996. [6] H.L.W. Chan, Y. Chen and C.L. Choy, IntegratedFerroelectrics, 9, 207, 1995. [7] H.L.W. Chan, W.K. Chan, Y. Zhang and C.L. Choy, IEEE Trans. on Dielectrics and Electric. Insul., 5, 505, 1998. [8] Q. F. Zhou, H.L. W. Chan and C.L. Choy, J. Materials Processing Technology, 63, 281, 1997. [9] H. Yamazaki and T. Kitayama, Feroelectrics, 33, 147, 1981. , [IO] Z. Zhao, H.L.W. Chan and C.L. Choy, Ferroelectrics, 195, 35, 1997. [ I  I ]  R. Steinhausen, T. Hauke, W. Seifert, V. Mueller, H. Beige, S. Seifert, P. Loebmann and D. Sporn, in Proc. 4th Intl. Conz on (rcrM 94), 437, 1994. Intelligent Materials (KIM 98), 1998. Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 02:36 from IEEE Xplore.  Restrictions apply.

PZT/P(VDF-TrFE) nanocomposites for ultrasonic hydrophone applications

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