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Piezoelectric composite hydrophone array
Sensors and Actuators A 96 (2002) 14±20
Piezoelectric composite hydrophone array S.T. Lau*, K.W. Kwok, H.L.W. Chan, C.L. Choy Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hunghom, Hong Kong, China Accepted 9 October 2001
Abstract The 0±3 composite thin ®lms, consisting of lead titanate (PT) powder embedded in a poly(vinylidene ¯uoride-tri¯uoroethylene) (P(VDFTrFE)) copolymer matrix were prepared by a spin-coating method. The ceramic and copolymer phases of the composite ®lm were poled in opposite directions. The piezoelectric and pyroelectric coef®cients of the composite ®lm were measured. Since the piezoelectric coef®cients of PT and P(VDF-TrFE) have opposite signs while the pyroelectric coef®cients have the same sign, the composite ®lm has a higher piezoelectric but lower pyroelectric coef®cient than P(VDF-TrFE). This composite ®lm was then used to fabricate an eight-element hydrophone array and the performance of the hydrophone array was evaluated in water. The hydrophone array has good receiving sensitivity in the frequency range of 2±8 MHz. The angular responses of the array element were characterized and the experimental results agreed well with the theoretical predictions. The acoustical and electrical cross-coupling between the array elements were discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PT; P(VDF-TrFE); Composite; Hydrophone array
1. Introduction The increasing sophistication of ultrasonic techniques for non-destructive testing of various materials, including human body, has brought an increasing concern for precise measurements of the ultrasonic ®elds in the megahertz frequency range. Considerable effort has been put into the development of high performance hydrophones [1] and hydrophone arrays [2]. For a system using hydrophone arrays, the mechanical scanning process is not needed and the spatial distribution of an ultrasonic ®eld can be obtained in a matter of seconds. Piezoelectric ceramics, such as lead zirconate titanate (PZT) have been used as the sensing elements of hydrophones since 1950s. They have large piezoelectric coef®cient d33 and large dielectric permittivity e. However, performance of the ceramic hydrophones is limited by the large acoustic impedance mismatch between the ceramic and water or human tissues. Recently, poly(vinylidene ¯uoride) (PVDF) and (vinylidene ¯uoride-tri¯uoroethylene) copolymers (P(VDF-TrFE)) have been widely used as the sensing elements of hydrophones [1±3] since they have high degree of ¯exibility and low acoustic impedance (4 Mrayl) *
Corresponding author. Tel.: 86-852-2333-5702; fax: 86-852-2333-7629. E-mail address: [email protected] (S.T. Lau).
which facilitates impedance matching with water and human tissues. However, d33 and e values of these polymers are usually low. The small e value is not practically desirable in sensor applications since the output signal will be greatly reduced by the stray capacitance associated with the long hydrophone cables. Piezoelectric 0±3 composites with piezoelectric ceramic powder embedded in a passive polymer matrix have been extensively studied for sensor applications since they combine the good piezoelectric properties of the ceramic with the good mechanical properties of the polymer [4±6]. Recently, 0±3 composites with piezoelectric ceramic powder embedded in a piezoelectric polymer matrix have also received increasing attention [7,8]. The piezoelectric coef®cients of the ceramic and polymer have opposite signs, while their pyroelectric coef®cients have the same sign. Our previous works [9,10] have shown that, by poling the two phases in opposite directions, the piezoelectric activity of the composite can be enhanced, while the pyroelectric activity is reduced. Therefore, it is of great interest to use this composite material for fabricating hydrophones with good receiving sensitivity, but low response to temperature ¯uctuation. In the present work, we have prepared PT/P(VDF-TrFE) 0±3 composite ®lms with the two phases poled in opposite directions, and the performance of an eight-element hydrophone array made from this composite ®lm has been evaluated.
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 5 7 - 9
S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
2. Preparation of 0±3 PT/P(VDF-TrFE) composite films The lead titanate (PT) powder was prepared by a sol±gel method [11]. The average crystallite diameter of the powder was about 60 nm. The (P(VDF-TrFE), 70/30 mol%), supplied by Piezotech Co., has a Curie temperature of 105 8C upon heating and a melting temperature of 149 8C. To prepare 0±3 composite ®lms, the copolymer pellets were ®rst dissolved in methyl-ethyl-ketone (MEK). Then a suitable amount of PT powder was blended into the solution and dispersed uniformly by ultrasonic agitation. The resulting mixture was spin-coated on an aluminum/glass substrate to give a ®lm of thickness about 6 mm. After dried at room temperature, the composite ®lm was annealed at 120 8C for 2 h to remove the solvent and to increase the crystallinity of the copolymer phase. An aluminum top electrode was evaporated on the composite ®lm to form a capacitor structure. The ceramic volume fraction of the composite ®lm was about 0.2. A P(VDF-TrFE) ®lm of a similar thickness was also prepared. To prepare composite ®lms with the ceramic and copolymer phases poled in opposite directions, the two phases must be poled separately. The composite ®lm was ®rst poled under a dc ®eld of 65 MV/m at 120 8C for 1 h. Since the dc ®eld was applied at a temperature above the Curie temperature of the copolymer, only the ceramic phase was poled. The composite ®lm was then re-poled under an ac ®eld of amplitude 70 MV/m and frequency 10 Hz at 70 8C. Since the dielectric relaxation time of P(VDF-TrFE) is much longer than 1 s, the copolymer phase of the composite ®lm can be poled by the application of several cycles of the ac ®eld while the polarization state of the ceramic phase is not altered signi®cantly [9]. The direction of the resultant polarization (remanent polarization) in the copolymer phase is determined by the electric ®eld direction in the last-half cycle of the ac voltage. Accordingly, the copolymer phase of the composite ®lm was poled in a direction opposite to the polarization direction of the ceramic phase. For comparison, a copolymer ®lm was also poled under an ac ®eld of amplitude 70 MV/m and frequency 10 Hz at 70 8C. 3. Piezoelectric and pyroelectric coefficients of the composite films The experimental details of the measurements of piezoelectric and pyroelectric coef®cients have been reported in previous publications [12,13]. The piezoelectric coef®cient d33 (strain/applied electric ®eld) was determined by measuring the surface displacement induced in the sample by the application of an ac ®eld at 15 kHz using a heterodyne laser interferometer [12]. The pyroelectric coef®cient p was measured using a dynamic method [13]. The sample temperature was sinusoidally modulated at room temperature at a frequency of 5 mHz and an amplitude of 1 K using a Peltier element. The 908 out of phase component of the resulting pyroelectric current with respect to the temperature modulation was measured using a lock-in ampli®er.
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Table 1 Piezoelectric and pyroelectric coefficients of the composite and copolymer films Samples
d33 (pm/V)
p (mC/m2 K)
PT/P(VDF-TrFE) 0±3 composite P(VDF-TrFE) copolymer
14 10.5
14 22
The observed values of d33 and p for the composite and copolymer ®lms are given in Table 1. It should be noted that the ®lm is laterally clamped by the glass substrate, so the observed d33 value is smaller than that of a free-standing sample [14]. As shown in Table 1, the d33 value of the composite ®lm is about 30% higher than that of the copolymer ®lm, showing the enhancement of piezoelectric activity by the oppositely poled ceramic phase. On the other hand, resulting from the partial cancellation of pyroelectric response by the oppositely poled ceramic phase, the p value of the composite ®lm is about 40% lower than that of the copolymer ®lm. These results indicate that a hydrophone array with the composite ®lms as the sensing elements will have a higher receiving sensitivity and will be less affected by temperature ¯uctuation (because of the smaller p value) as compared to a hydrophone array with the copolymer ®lms as the sensing elements. 4. Construction of the hydrophone array The poled composite ®lms were peeled off from the glass substrate and the Al electrodes were etched away in potassium hydroxide solution (KOH). Circular disks of diameter about 0.6 mm were cut out from the ®lms and used as the sensing elements of the hydrophone array. The construction of the eight-element hydrophone array is shown schematically in Fig. 1. Copper wires of diameter 1 mm were held by epoxy in the center of the holes in a polyacetal (POM) housing which was ®xed in a stainless steel tubing of diameter 4 mm and length 160 mm. After the epoxy had set, the tips of the copper wires were polished to obtain ¯at surfaces. The circular elements were glued to the tips of exposed wires using silver-®lled conductive epoxy (Ablestik) and the edge of the elements was insulated by epoxy. A chromium/gold layer of thickness 0.15 mm was evaporated onto the top surfaces of elements to make contact with the steel tubing and served as the ground electrode. The copper wires were connected to an impedance matching circuit at the end of the stainless steel tubing. The center-to-center separation between the elements was 2.5 mm. 5. Characterization of the hydrophone array 5.1. Receiving sensitivity Testing of the hydrophone array was carried out in a water tank. A transducer (6.35 mm diameter, Panametrics) was
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S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
Fig. 1. Schematic diagram of the eight-element hydrophone array.
used to generate acoustic waves. The array was placed at a distance in the far-®eld region of the acoustic waves (distance > a2 =l, where a is the transducer radius and l is the wavelength of the acoustic wave) to receive the waves. The voltage VL generated by the array element was measured using an oscilloscope. For comparison, the voltage VS generated by a bilaminar PVDF membrane hydrophone (GEC-Marconi, Type Y-34-3598) located at the same position was measured. The end-of-cable loaded sensitivity (ML) of the array element is given by VL (1) ML MS VS where MS is the end-of-cable loaded sensitivity of the membrane hydrophone. Since the impedances of the array
element and electrical load (i.e. oscilloscope) can be assumed to be capacitive, the end-of-cable open-circuit sensitivity Mo of the array elements is calculated as C Cel (2) Mo ML C where C is the end-of-cable capacitance of the array element and Cel is the capacitance of the load (8 pF). Fig. 2 shows the end-of-cable capacitance of the array elements as a function of frequency, measured using an impedance analyzer (HP 4194A). It can be seen that the capacitance of each array element is almost unchanged with frequency in the range of 2±8 MHz, and the difference in capacitance between the eight elements is about 10%. The dependence of the receiving sensitivity Mo on frequency for
Fig. 2. End-of-cable capacitance of the array elements as a function of frequency: first element (counted from the end of the array) (&); second element (); third element (); fourth element (5); fifth element (*); sixth element (~); seventh element (~); and eighth element (^).
S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
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Fig. 3. End-of-cable open-circuit sensitivity Mo of the eight elements as a function of frequency: first element (counted from the end of the array) (&); second element (); third element (); fourth element (5); fifth element (*); sixth element (~); seventh element (~); and eighth element (^).
each array element is shown in Fig. 3. The sensitivities of all the eight elements are very close to each other at each frequency. For each element, the sensitivity decreases slightly with increasing frequency. As the frequency increases from 2 to 8 MHz, the sensitivity decreases by less than 30% (3 dB relative to 1 V/mPa). 5.2. Angular response of the array elements and crosscoupling evaluations Acoustical and electrical cross-coupling between the array elements can produce undesirable artifacts in the frequency response of an individual element. In the present measurement, a transducer was used to generate a short burst
of ultrasonic wave at its center frequency in water. Three transducers (6.35 mm diameter, Panametrics) of frequencies 2.25, 5 and 10 MHz were used. The array element was positioned on the acoustic axis of the transducer and at a point well into the far-®eld region. To measure the angular response of an array element, the hydrophone array was rotated about an axis through the element in two orthogonal directions. The ®rst direction, in which the axis of rotation was parallel to the tube axis (Fig. 4a), measured the angular response of the element in the plane perpendicular to the tube axis. The second direction, in which the axis of rotation was perpendicular to the tube axis (Fig. 4b), measured the angular response in the plane parallel to the tube axis. The angular response curve was plotted as the output voltage of
Fig. 4. Schematic diagram of the rotation directions for the measurement of the angular response of the array elements (a) in the plane perpendicular to the tube axis; (b) in the plane parallel to the tube axis.
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S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
the element versus the angular position y of the element (i.e. the incident angle of the acoustic wave). Fig. 5 shows the angular response in the plane perpendicular to the tube axis at different frequencies for the ®rst
(counted from the end of the array) and the ®fth array elements. It is seen that the two array elements at different positions show similar angular response at each frequency. At 2.25 MHz, the output voltage of the element decreases slowly with the increasing incident angle. At this frequency, the wavelength l of the acoustic wave (0.7 mm) is comparable to the diameter of the array elements. As frequency increases (l decreases), the output voltage decreases more faster with increasing incident angle and becomes zero at certain angles, beyond which there are small side lobes. The number of side lobes increases with increasing frequency. The angular response of a hydrophone with a single circular element can be calculated using the diffraction theory and an unbaf¯ed piston model [15]. In this model, the angular response of the hydrophone is given by 2J1 kr sin y 1 cos y V y Vo (3) kr sin y 2 where Vo is the output voltage when the acoustic wave is normally incident on the element, k (2p/l) is the wave number, r is the radius of the element and J1 is the Bessel function of the first-order. As shown in Fig. 5, the theoretical values calculated using Eq. (3) agree quite well with the experimental values at different frequencies for different array elements. This implies that the acoustical cross-coupling from the adjacent vibrating elements to the element under test is not severe. Since the phases of the acoustic (plane) waves incident on all the elements are the same, the effect of electrical cross-coupling on the angular response of the element under test is not significant. The angular response in the plane parallel to the tube axis at different frequencies for the ®rst and the ®fth array elements are shown in Figs. 6 and 7, respectively. Different from the response curves in the plane perpendicular to the tube axis (Fig. 5), there are several ripples on the main response curves, showing the effect of electrical crosscoupling from the adjacent elements. In this case, because of different traveling distances, the phases of the acoustic waves incident on each element are different, thus resulting in voltages of different phases generated at different elements. Through electrical cross-coupling, voltages of different phases superimpose and interference occurs. Since the ®rst array element only has neighbor elements on one side, the interference effect is weaker and hence the ripples are smaller (Fig. 6). If the ef®ciency of electrical cross-coupling is characterized by a coef®cient a, the resultant voltage is given by [16] " # N X n F y V y 1 2 a cos nkL sin y (4) n1
Fig. 5. Angular response of the array elements in the plane perpendicular to the tube axis at (a) 2.25 MHz; (b) 5 MHz; and (c) 10 MHz. Experimental values: first element (counted from the end of the array) (&); fifth element (5); and theoretical values (Ð).
where V(y) is the uncoupled angular response of a single circular element, N is the number of neighboring element pairs, L is the center-to-center separation between elements, Lsin y is hence the path difference between the waves
S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
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Fig. 6. Angular response of the first element (counted from the end of the array) in the plane parallel to the tube axis at (a) 2.25 MHz; (b) 5 MHz; and (c) 10 MHz. Experimental values (&); and theoretical values (Ð).
Fig. 7. Angular response of the fifth element (counted from the end of the array) in the plane parallel to the tube axis at (a) 2.25 MHz; (b) 5 MHz; and (c) 10 MHz. Experimental values (5); and theoretical values (Ð).
incident on two adjacent elements. The unbaffled model (Eq. (3)) is used to calculate the uncoupled angular response V(y) in this work. As shown in Figs. 6 and 7, if a is taken to be 0.11, good agreements are obtained between the theoretical values calculated using Eq. (4) and the experimental
values at different frequencies for the first and the fifth array elements. It should be noted the factor of 2 in Eq. (4) was replaced by 1 in calculating the angular response for the first element, since it only has neighbor elements on one side.
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S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
6. Conclusions An eight-element hydrophone array has been fabricated using 0±3 PT/P(VDF-TrFE) composite ®lms with the two phases poled in opposite directions as the sensing elements, and its performance has been evaluated. The hydrophone array shows good receiving sensitivity over several MHz. The array elements also exhibit good angular response in the plane perpendicular to the tube axis. However, because of electrical cross-coupling, several ripples are observed in the angular response curves of the elements in the plane parallel to the tube axis. With the use of an eight-channel multiplexer, the hydrophone array will be useful for acquiring more information at different locations in a much shorter time by measuring the output voltage of each element in turn. Acknowledgements Financial support from the Research Grants Council of the Hong Kong Special Administrative Region (Project no. PolyU 5159/98P) and the Centre for Smart Materials of The Hong Kong Polytechnic University are acknowledged. References [1] P.A. Lewin, Ultrason. 19 (1981) 213±216. [2] R.C. Preston, Trans. IEEE Ultrason., Ferroelectrics and Freq. Control 35 (1988) 122±139. [3] H.L.W. Chan, D.C. Price, Rev. Sci. Instrum. 62 (1991) 203±207. [4] R.Y. Ting, Ferroelectrics 67 (1986) 143±157. [5] H. Banno, K. Ogura, Jpn. J. Appl. Phys. 30 (1991) 2250±2252. [6] R.P. Tandon, R. Singh, Polymers & Polymer Composites 2 (1994) 287±292. [7] C.J. Dias, D. K. Das-Gupta, Piezo- and pyroelectricity in ferroelectric ceramic-polymer composites, in: D.K. Das-Gupta (Ed.), Ferroelectric Polymers and Ceramic/Polymer Composite, Tran Tech Publications Ltd., Switzerland, 1994, pp. 217±249. [8] H.L.W. Chan, P.K.L. Ng, Y. Chen, C.L. Choy, Ferroelectrics 201 (1997) 225±234. [9] B. Ploss, B. Ploss, F.G. Shin, H.L.W. Chan, C.L. Choy, Appl. Phys. Lett. 76 (2000) 2776±2778.
[10] S.T. Lau, K.W. Kowk, H.L.W. Chan, C.L. Choy, Ferroelectrics, 2001, in press. [11] Y. Chen, H.L.W. Chan, C.L. Choy, J. Am. Ceram. Soc. 81 (1998) 1231±1236. [12] C.M. Leung, H.L.W. Chan, C. Surya, C.L. Choy, J. Appl. Phys. 88 (2000) 5360±5363. [13] C. Dias, M. simon, R. Quad, D.K. Das-Gupta, J. Phys. D: Appl. Phys. 26 (1993) 106±110. [14] D. Royer, V. Kmetik, Electron. Lett. 28 (1992) 1828±1830. [15] B. Delannoy, H. Lasota, C. Bruneel, R. Torduet, E. Bridoux, J. Appl. Phys. 50 (1979) 5189±5195. [16] C.S. Kino, C.S. DeSilets, Ultrason. Imag. 1 (1979) 189±209.
Biographies S.T. Lau was born in Hong Kong in 1975. She received the BSc and MPhil degrees in physics from the Hong Kong Polytechnic University in 1998 and 2001, respectively. Currently, she is a PhD student in applied physics at the Hong Kong Polytechnic University. Her research interest focuses on the preparation and characterization of PMN-PT single crystals and PMNPT/polymer 1±3 composites for device applications. K.W. Kwok was born in Hong Kong in 1963. He received the BSc and MPhil degrees in physics from the Chinese University of Hong Kong in 1987 and 1990, respectively, and the PhD degree in physics from the Hong Kong Polytechnic University in 1997. Dr. Kwok is currently a Lecturer in the Department of Applied Physics at the Hong Kong Polytechnic University. H.L.W. Chan was born in Hong Kong in 1948. She received the BSc and MPhil degrees in physics from the Chinese University of Hong Kong in 1970 and 1974, respectively, and the PhD degree in Material Science from Macquarie University, Australia in 1987. From 1987 to 1991, Dr. Chan worked as a Research Scientist at CSIRO Division of Applied Physics in Sydney, NSW, Australia. She was responsible to set up the standards for medical ultrasound in Australia. She then worked at GEC-Marconi Australia for 1 year as a senior acoustic designer before she returned to Hong Kong in 1992. Dr. Chan is presently a Professor in the Department of Applied Physics at the Hong Kong Polytechnic University. C.L. Choy was born in Malaysia in 1938. He received the PhD in physics from Rensselaer Polytechnic Institute, USA in 1968, and then worked as a research associate for 1 year at Cornell University. He was a visiting scientist at the University of Leeds and the University of Massachusetts in 1974 and 1981, respectively. His main research interest is in the structure and physical properties of polymers and composites. Dr. Choy is presently a Chair Professor and head of the Department of Applied Physics at the Hong Kong Polytechnic University.
Piezoelectric composite hydrophone array S.T. Lau*, K.W. Kwok, H.L.W. Chan, C.L. Choy Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hunghom, Hong Kong, China Accepted 9 October 2001
Abstract The 0±3 composite thin ®lms, consisting of lead titanate (PT) powder embedded in a poly(vinylidene ¯uoride-tri¯uoroethylene) (P(VDFTrFE)) copolymer matrix were prepared by a spin-coating method. The ceramic and copolymer phases of the composite ®lm were poled in opposite directions. The piezoelectric and pyroelectric coef®cients of the composite ®lm were measured. Since the piezoelectric coef®cients of PT and P(VDF-TrFE) have opposite signs while the pyroelectric coef®cients have the same sign, the composite ®lm has a higher piezoelectric but lower pyroelectric coef®cient than P(VDF-TrFE). This composite ®lm was then used to fabricate an eight-element hydrophone array and the performance of the hydrophone array was evaluated in water. The hydrophone array has good receiving sensitivity in the frequency range of 2±8 MHz. The angular responses of the array element were characterized and the experimental results agreed well with the theoretical predictions. The acoustical and electrical cross-coupling between the array elements were discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: PT; P(VDF-TrFE); Composite; Hydrophone array
1. Introduction The increasing sophistication of ultrasonic techniques for non-destructive testing of various materials, including human body, has brought an increasing concern for precise measurements of the ultrasonic ®elds in the megahertz frequency range. Considerable effort has been put into the development of high performance hydrophones [1] and hydrophone arrays [2]. For a system using hydrophone arrays, the mechanical scanning process is not needed and the spatial distribution of an ultrasonic ®eld can be obtained in a matter of seconds. Piezoelectric ceramics, such as lead zirconate titanate (PZT) have been used as the sensing elements of hydrophones since 1950s. They have large piezoelectric coef®cient d33 and large dielectric permittivity e. However, performance of the ceramic hydrophones is limited by the large acoustic impedance mismatch between the ceramic and water or human tissues. Recently, poly(vinylidene ¯uoride) (PVDF) and (vinylidene ¯uoride-tri¯uoroethylene) copolymers (P(VDF-TrFE)) have been widely used as the sensing elements of hydrophones [1±3] since they have high degree of ¯exibility and low acoustic impedance (4 Mrayl) *
Corresponding author. Tel.: 86-852-2333-5702; fax: 86-852-2333-7629. E-mail address: [email protected] (S.T. Lau).
which facilitates impedance matching with water and human tissues. However, d33 and e values of these polymers are usually low. The small e value is not practically desirable in sensor applications since the output signal will be greatly reduced by the stray capacitance associated with the long hydrophone cables. Piezoelectric 0±3 composites with piezoelectric ceramic powder embedded in a passive polymer matrix have been extensively studied for sensor applications since they combine the good piezoelectric properties of the ceramic with the good mechanical properties of the polymer [4±6]. Recently, 0±3 composites with piezoelectric ceramic powder embedded in a piezoelectric polymer matrix have also received increasing attention [7,8]. The piezoelectric coef®cients of the ceramic and polymer have opposite signs, while their pyroelectric coef®cients have the same sign. Our previous works [9,10] have shown that, by poling the two phases in opposite directions, the piezoelectric activity of the composite can be enhanced, while the pyroelectric activity is reduced. Therefore, it is of great interest to use this composite material for fabricating hydrophones with good receiving sensitivity, but low response to temperature ¯uctuation. In the present work, we have prepared PT/P(VDF-TrFE) 0±3 composite ®lms with the two phases poled in opposite directions, and the performance of an eight-element hydrophone array made from this composite ®lm has been evaluated.
0924-4247/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 5 7 - 9
S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
2. Preparation of 0±3 PT/P(VDF-TrFE) composite films The lead titanate (PT) powder was prepared by a sol±gel method [11]. The average crystallite diameter of the powder was about 60 nm. The (P(VDF-TrFE), 70/30 mol%), supplied by Piezotech Co., has a Curie temperature of 105 8C upon heating and a melting temperature of 149 8C. To prepare 0±3 composite ®lms, the copolymer pellets were ®rst dissolved in methyl-ethyl-ketone (MEK). Then a suitable amount of PT powder was blended into the solution and dispersed uniformly by ultrasonic agitation. The resulting mixture was spin-coated on an aluminum/glass substrate to give a ®lm of thickness about 6 mm. After dried at room temperature, the composite ®lm was annealed at 120 8C for 2 h to remove the solvent and to increase the crystallinity of the copolymer phase. An aluminum top electrode was evaporated on the composite ®lm to form a capacitor structure. The ceramic volume fraction of the composite ®lm was about 0.2. A P(VDF-TrFE) ®lm of a similar thickness was also prepared. To prepare composite ®lms with the ceramic and copolymer phases poled in opposite directions, the two phases must be poled separately. The composite ®lm was ®rst poled under a dc ®eld of 65 MV/m at 120 8C for 1 h. Since the dc ®eld was applied at a temperature above the Curie temperature of the copolymer, only the ceramic phase was poled. The composite ®lm was then re-poled under an ac ®eld of amplitude 70 MV/m and frequency 10 Hz at 70 8C. Since the dielectric relaxation time of P(VDF-TrFE) is much longer than 1 s, the copolymer phase of the composite ®lm can be poled by the application of several cycles of the ac ®eld while the polarization state of the ceramic phase is not altered signi®cantly [9]. The direction of the resultant polarization (remanent polarization) in the copolymer phase is determined by the electric ®eld direction in the last-half cycle of the ac voltage. Accordingly, the copolymer phase of the composite ®lm was poled in a direction opposite to the polarization direction of the ceramic phase. For comparison, a copolymer ®lm was also poled under an ac ®eld of amplitude 70 MV/m and frequency 10 Hz at 70 8C. 3. Piezoelectric and pyroelectric coefficients of the composite films The experimental details of the measurements of piezoelectric and pyroelectric coef®cients have been reported in previous publications [12,13]. The piezoelectric coef®cient d33 (strain/applied electric ®eld) was determined by measuring the surface displacement induced in the sample by the application of an ac ®eld at 15 kHz using a heterodyne laser interferometer [12]. The pyroelectric coef®cient p was measured using a dynamic method [13]. The sample temperature was sinusoidally modulated at room temperature at a frequency of 5 mHz and an amplitude of 1 K using a Peltier element. The 908 out of phase component of the resulting pyroelectric current with respect to the temperature modulation was measured using a lock-in ampli®er.
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Table 1 Piezoelectric and pyroelectric coefficients of the composite and copolymer films Samples
d33 (pm/V)
p (mC/m2 K)
PT/P(VDF-TrFE) 0±3 composite P(VDF-TrFE) copolymer
14 10.5
14 22
The observed values of d33 and p for the composite and copolymer ®lms are given in Table 1. It should be noted that the ®lm is laterally clamped by the glass substrate, so the observed d33 value is smaller than that of a free-standing sample [14]. As shown in Table 1, the d33 value of the composite ®lm is about 30% higher than that of the copolymer ®lm, showing the enhancement of piezoelectric activity by the oppositely poled ceramic phase. On the other hand, resulting from the partial cancellation of pyroelectric response by the oppositely poled ceramic phase, the p value of the composite ®lm is about 40% lower than that of the copolymer ®lm. These results indicate that a hydrophone array with the composite ®lms as the sensing elements will have a higher receiving sensitivity and will be less affected by temperature ¯uctuation (because of the smaller p value) as compared to a hydrophone array with the copolymer ®lms as the sensing elements. 4. Construction of the hydrophone array The poled composite ®lms were peeled off from the glass substrate and the Al electrodes were etched away in potassium hydroxide solution (KOH). Circular disks of diameter about 0.6 mm were cut out from the ®lms and used as the sensing elements of the hydrophone array. The construction of the eight-element hydrophone array is shown schematically in Fig. 1. Copper wires of diameter 1 mm were held by epoxy in the center of the holes in a polyacetal (POM) housing which was ®xed in a stainless steel tubing of diameter 4 mm and length 160 mm. After the epoxy had set, the tips of the copper wires were polished to obtain ¯at surfaces. The circular elements were glued to the tips of exposed wires using silver-®lled conductive epoxy (Ablestik) and the edge of the elements was insulated by epoxy. A chromium/gold layer of thickness 0.15 mm was evaporated onto the top surfaces of elements to make contact with the steel tubing and served as the ground electrode. The copper wires were connected to an impedance matching circuit at the end of the stainless steel tubing. The center-to-center separation between the elements was 2.5 mm. 5. Characterization of the hydrophone array 5.1. Receiving sensitivity Testing of the hydrophone array was carried out in a water tank. A transducer (6.35 mm diameter, Panametrics) was
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S.T. Lau et al. / Sensors and Actuators A 96 (2002) 14±20
Fig. 1. Schematic diagram of the eight-element hydrophone array.
used to generate acoustic waves. The array was placed at a distance in the far-®eld region of the acoustic waves (distance > a2 =l, where a is the transducer radius and l is the wavelength of the acoustic wave) to receive the waves. The voltage VL generated by the array element was measured using an oscilloscope. For comparison, the voltage VS generated by a bilaminar PVDF membrane hydrophone (GEC-Marconi, Type Y-34-3598) located at the same position was measured. The end-of-cable loaded sensitivity (ML) of the array element is given by VL (1) ML MS VS where MS is the end-of-cable loaded sensitivity of the membrane hydrophone. Since the impedances of the array
element and electrical load (i.e. oscilloscope) can be assumed to be capacitive, the end-of-cable open-circuit sensitivity Mo of the array elements is calculated as C Cel (2) Mo ML C where C is the end-of-cable capacitance of the array element and Cel is the capacitance of the load (8 pF). Fig. 2 shows the end-of-cable capacitance of the array elements as a function of frequency, measured using an impedance analyzer (HP 4194A). It can be seen that the capacitance of each array element is almost unchanged with frequency in the range of 2±8 MHz, and the difference in capacitance between the eight elements is about 10%. The dependence of the receiving sensitivity Mo on frequency for
Fig. 2. End-of-cable capacitance of the array elements as a function of frequency: first element (counted from the end of the array) (&); second element (); third element (); fourth element (5); fifth element (*); sixth element (~); seventh element (~); and eighth element (^).
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Fig. 3. End-of-cable open-circuit sensitivity Mo of the eight elements as a function of frequency: first element (counted from the end of the array) (&); second element (); third element (); fourth element (5); fifth element (*); sixth element (~); seventh element (~); and eighth element (^).
each array element is shown in Fig. 3. The sensitivities of all the eight elements are very close to each other at each frequency. For each element, the sensitivity decreases slightly with increasing frequency. As the frequency increases from 2 to 8 MHz, the sensitivity decreases by less than 30% (3 dB relative to 1 V/mPa). 5.2. Angular response of the array elements and crosscoupling evaluations Acoustical and electrical cross-coupling between the array elements can produce undesirable artifacts in the frequency response of an individual element. In the present measurement, a transducer was used to generate a short burst
of ultrasonic wave at its center frequency in water. Three transducers (6.35 mm diameter, Panametrics) of frequencies 2.25, 5 and 10 MHz were used. The array element was positioned on the acoustic axis of the transducer and at a point well into the far-®eld region. To measure the angular response of an array element, the hydrophone array was rotated about an axis through the element in two orthogonal directions. The ®rst direction, in which the axis of rotation was parallel to the tube axis (Fig. 4a), measured the angular response of the element in the plane perpendicular to the tube axis. The second direction, in which the axis of rotation was perpendicular to the tube axis (Fig. 4b), measured the angular response in the plane parallel to the tube axis. The angular response curve was plotted as the output voltage of
Fig. 4. Schematic diagram of the rotation directions for the measurement of the angular response of the array elements (a) in the plane perpendicular to the tube axis; (b) in the plane parallel to the tube axis.
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the element versus the angular position y of the element (i.e. the incident angle of the acoustic wave). Fig. 5 shows the angular response in the plane perpendicular to the tube axis at different frequencies for the ®rst
(counted from the end of the array) and the ®fth array elements. It is seen that the two array elements at different positions show similar angular response at each frequency. At 2.25 MHz, the output voltage of the element decreases slowly with the increasing incident angle. At this frequency, the wavelength l of the acoustic wave (0.7 mm) is comparable to the diameter of the array elements. As frequency increases (l decreases), the output voltage decreases more faster with increasing incident angle and becomes zero at certain angles, beyond which there are small side lobes. The number of side lobes increases with increasing frequency. The angular response of a hydrophone with a single circular element can be calculated using the diffraction theory and an unbaf¯ed piston model [15]. In this model, the angular response of the hydrophone is given by 2J1 kr sin y 1 cos y V y Vo (3) kr sin y 2 where Vo is the output voltage when the acoustic wave is normally incident on the element, k (2p/l) is the wave number, r is the radius of the element and J1 is the Bessel function of the first-order. As shown in Fig. 5, the theoretical values calculated using Eq. (3) agree quite well with the experimental values at different frequencies for different array elements. This implies that the acoustical cross-coupling from the adjacent vibrating elements to the element under test is not severe. Since the phases of the acoustic (plane) waves incident on all the elements are the same, the effect of electrical cross-coupling on the angular response of the element under test is not significant. The angular response in the plane parallel to the tube axis at different frequencies for the ®rst and the ®fth array elements are shown in Figs. 6 and 7, respectively. Different from the response curves in the plane perpendicular to the tube axis (Fig. 5), there are several ripples on the main response curves, showing the effect of electrical crosscoupling from the adjacent elements. In this case, because of different traveling distances, the phases of the acoustic waves incident on each element are different, thus resulting in voltages of different phases generated at different elements. Through electrical cross-coupling, voltages of different phases superimpose and interference occurs. Since the ®rst array element only has neighbor elements on one side, the interference effect is weaker and hence the ripples are smaller (Fig. 6). If the ef®ciency of electrical cross-coupling is characterized by a coef®cient a, the resultant voltage is given by [16] " # N X n F y V y 1 2 a cos nkL sin y (4) n1
Fig. 5. Angular response of the array elements in the plane perpendicular to the tube axis at (a) 2.25 MHz; (b) 5 MHz; and (c) 10 MHz. Experimental values: first element (counted from the end of the array) (&); fifth element (5); and theoretical values (Ð).
where V(y) is the uncoupled angular response of a single circular element, N is the number of neighboring element pairs, L is the center-to-center separation between elements, Lsin y is hence the path difference between the waves
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Fig. 6. Angular response of the first element (counted from the end of the array) in the plane parallel to the tube axis at (a) 2.25 MHz; (b) 5 MHz; and (c) 10 MHz. Experimental values (&); and theoretical values (Ð).
Fig. 7. Angular response of the fifth element (counted from the end of the array) in the plane parallel to the tube axis at (a) 2.25 MHz; (b) 5 MHz; and (c) 10 MHz. Experimental values (5); and theoretical values (Ð).
incident on two adjacent elements. The unbaffled model (Eq. (3)) is used to calculate the uncoupled angular response V(y) in this work. As shown in Figs. 6 and 7, if a is taken to be 0.11, good agreements are obtained between the theoretical values calculated using Eq. (4) and the experimental
values at different frequencies for the first and the fifth array elements. It should be noted the factor of 2 in Eq. (4) was replaced by 1 in calculating the angular response for the first element, since it only has neighbor elements on one side.
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6. Conclusions An eight-element hydrophone array has been fabricated using 0±3 PT/P(VDF-TrFE) composite ®lms with the two phases poled in opposite directions as the sensing elements, and its performance has been evaluated. The hydrophone array shows good receiving sensitivity over several MHz. The array elements also exhibit good angular response in the plane perpendicular to the tube axis. However, because of electrical cross-coupling, several ripples are observed in the angular response curves of the elements in the plane parallel to the tube axis. With the use of an eight-channel multiplexer, the hydrophone array will be useful for acquiring more information at different locations in a much shorter time by measuring the output voltage of each element in turn. Acknowledgements Financial support from the Research Grants Council of the Hong Kong Special Administrative Region (Project no. PolyU 5159/98P) and the Centre for Smart Materials of The Hong Kong Polytechnic University are acknowledged. References [1] P.A. Lewin, Ultrason. 19 (1981) 213±216. [2] R.C. Preston, Trans. IEEE Ultrason., Ferroelectrics and Freq. Control 35 (1988) 122±139. [3] H.L.W. Chan, D.C. Price, Rev. Sci. Instrum. 62 (1991) 203±207. [4] R.Y. Ting, Ferroelectrics 67 (1986) 143±157. [5] H. Banno, K. Ogura, Jpn. J. Appl. Phys. 30 (1991) 2250±2252. [6] R.P. Tandon, R. Singh, Polymers & Polymer Composites 2 (1994) 287±292. [7] C.J. Dias, D. K. Das-Gupta, Piezo- and pyroelectricity in ferroelectric ceramic-polymer composites, in: D.K. Das-Gupta (Ed.), Ferroelectric Polymers and Ceramic/Polymer Composite, Tran Tech Publications Ltd., Switzerland, 1994, pp. 217±249. [8] H.L.W. Chan, P.K.L. Ng, Y. Chen, C.L. Choy, Ferroelectrics 201 (1997) 225±234. [9] B. Ploss, B. Ploss, F.G. Shin, H.L.W. Chan, C.L. Choy, Appl. Phys. Lett. 76 (2000) 2776±2778.
[10] S.T. Lau, K.W. Kowk, H.L.W. Chan, C.L. Choy, Ferroelectrics, 2001, in press. [11] Y. Chen, H.L.W. Chan, C.L. Choy, J. Am. Ceram. Soc. 81 (1998) 1231±1236. [12] C.M. Leung, H.L.W. Chan, C. Surya, C.L. Choy, J. Appl. Phys. 88 (2000) 5360±5363. [13] C. Dias, M. simon, R. Quad, D.K. Das-Gupta, J. Phys. D: Appl. Phys. 26 (1993) 106±110. [14] D. Royer, V. Kmetik, Electron. Lett. 28 (1992) 1828±1830. [15] B. Delannoy, H. Lasota, C. Bruneel, R. Torduet, E. Bridoux, J. Appl. Phys. 50 (1979) 5189±5195. [16] C.S. Kino, C.S. DeSilets, Ultrason. Imag. 1 (1979) 189±209.
Biographies S.T. Lau was born in Hong Kong in 1975. She received the BSc and MPhil degrees in physics from the Hong Kong Polytechnic University in 1998 and 2001, respectively. Currently, she is a PhD student in applied physics at the Hong Kong Polytechnic University. Her research interest focuses on the preparation and characterization of PMN-PT single crystals and PMNPT/polymer 1±3 composites for device applications. K.W. Kwok was born in Hong Kong in 1963. He received the BSc and MPhil degrees in physics from the Chinese University of Hong Kong in 1987 and 1990, respectively, and the PhD degree in physics from the Hong Kong Polytechnic University in 1997. Dr. Kwok is currently a Lecturer in the Department of Applied Physics at the Hong Kong Polytechnic University. H.L.W. Chan was born in Hong Kong in 1948. She received the BSc and MPhil degrees in physics from the Chinese University of Hong Kong in 1970 and 1974, respectively, and the PhD degree in Material Science from Macquarie University, Australia in 1987. From 1987 to 1991, Dr. Chan worked as a Research Scientist at CSIRO Division of Applied Physics in Sydney, NSW, Australia. She was responsible to set up the standards for medical ultrasound in Australia. She then worked at GEC-Marconi Australia for 1 year as a senior acoustic designer before she returned to Hong Kong in 1992. Dr. Chan is presently a Professor in the Department of Applied Physics at the Hong Kong Polytechnic University. C.L. Choy was born in Malaysia in 1938. He received the PhD in physics from Rensselaer Polytechnic Institute, USA in 1968, and then worked as a research associate for 1 year at Cornell University. He was a visiting scientist at the University of Leeds and the University of Massachusetts in 1974 and 1981, respectively. His main research interest is in the structure and physical properties of polymers and composites. Dr. Choy is presently a Chair Professor and head of the Department of Applied Physics at the Hong Kong Polytechnic University.