Chan, HLW

Choy, CL

Kwok, KW

Lau, ST

PolyU Institutional Repository

PZT/P(VDFfT'rFE)  Nanocomposite  Hydrophones for Ultrasonic  Measurements H.L.W.  Chan, S.T. Lau, K.W. Kwok, and C.L.  Choy Department of Applied  Physics and Materials Research Centre, The  Hong  Kong Polytechnic University, Hunghom, Kowloon, Hong Kong. calibration of medical  ultrasonic  transducers  have Abstract - Needle-type  hydrophones  for  the been  fabricated  using  lead zircon te titanateivinylidene [PZTiP(VDF-TrFE)] 0-3 nanocom-posite  films.  The fluoride-trifluoroethylene 0-3  nanocomposite  film  has been  prepared  by incorporating nanosized PZT powder (derived from a sol-gel process) in a P(VDF-TrFE) matrix. Since the piezoelectric coefficients of PZT and P(VDF-TrFE) have  opposite  signs,  the  PZT  and  P(VDF-TrFE) opposite  directions  in  order  to  give  enhanced phases in the  nanocomposites  have  been  poled  in with  0.2  volume  fraction  f  PZT  has been piezoelectric  activity.  A  nanocomposite  hydrophone characterized and found to be more sensitive than a P(VDF-TrFE)  hydrophone of similar  structure. INTRODUCTION measuring  theacoustic  outputs  from medical Piezoelectric  hydrophones  are  widely  used  for imagingitherapeutic  ultrasonic Knowledge of the  acoustic  output  from  edical equipment. ultrasonic equipment is important in the optimization of treatment  and  he improvement  ofimaging effectiveness,  a   well as fornsuring  safety. Polpinylidene fluoride  (PVDF)  and  vinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) copolymers have  good  mechanical  flexibility  and  acoustic impedances  close  to  those of water  and  human  tissues and give a uniform response over a wide frequency range when used as sensing element in hydrophones. element sizes in  the  range  of 0.2 - 0.5 mm are [l-31 Single element  PVDF hydrophones with available commercially but as the PVDF element of this size has small capacitance, their sensitivities are greatly  affected  by  cable  shunting. piezoelectrically active ceramic particles dispersed in Piezoelectric 0-3 composites  consist  of a  3-dimensionally  connected  polymer  atrix. Although  non-piezoelectric  polymer  matrices  have mostly been used in previous works, 0-3 composites piezoelectric have also attracted some interest. [4-61 in which both the ceramic and polymer phases are When  aP(VDF-TrFE)  copolymer  is used as the matrix,  as  the  piezoelectric  coefficients  of  the  ceramic and copolymer have opposite signs, it is necessary to pole the two phases in opposite directions in order  to obtain reinforced piezoelectric responses. To produce a  homogeneous  hydrophone  lement  with  asmall thickness,  it is imperative  to  use  ceramic  powder  with size in the nanometer range. The nanocomposite has an  advantage over  the  copolymer  P(VDF-TrFE) in  that it has  higher  permittivity,  hence  circumventing  the cable  shunting  problem  encountered in copolymer hydrophone. FABRICATION OF PZTiP(VDF-TrFE) 0-3 NANOCOMPOSITES The  procedure  for  preparing  PZT  powder  by  the sol- gel method has been described in our previous report. at SOOOC and has an average crystallite diameter of [7] The powder used in this study has been annealed about 80 nm.  The  P(VDF-TrFE)  70/30  mol% copolymer  supplied  byPiezotech  has   Curie temperature of 102°C  upon  heating  and  a melting temperature of 150°C as determined by differential scanning  calorimetry (DSc). To prepare 0-3 nanocomposite thin films, 2 g of the copolymer was dissolved in 10 ml of methyl-ethyl-ketone (MEK) and PZT  nanocrystalline  powder  was  then  blended  into  the solution  to  form a mixture.  The  ceramic  volume fraction  of the  composite is 0.2. To  fabricate  acapacitor  structure  for  poling,  an  aluminum  lower electrode was thermally evaporated on a glass slide. Then  the  composite  was  spin-coated  onto  the  Al/glass substrate using a photoresist spinner. To produce a film  of  thickness  about 6 pm, a  rotation  speed  of 1600 RPM and  duration of I min  were  used.  After  the  spin- coating  procedure,  the  film  was  kept  overnight at room  temperature  and  then  annealed  at  120'C  for 0-7803-4095-7/98/$10.00 0 1998 IEEE 1998 IEEE ULTRASONICS SYMPOSIUM - 611 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 3, 2008 at 22:10 from IEEE Xplore.  Restrictions apply.2 h to remove the solvent. An  AI top electrode was then  thermally  evaporated on the  surface of the composite  film  to  produce  the  desired  capacitor structure. A P(VDF-TrFE) film of similar thickness and  structure  was  also  prepared. To pole the ceramic inside the composite, the sample was heated in an  oven to a temperature of 115OC, copolymer. A DC voltage of 30 kV/mm was applied which was above the Curie temperature (T,) of the across  the  electrodes  for 60 min.,  then  the  electric  field was  switched  off  and  the  sample  was  cooled  slowly to off when the copolymer was in a paraelectric phase, room temperature. As the electric field was switched only  the  ceramic  inside  the  composite  was  poled.  The copolymer  phase  was  then  poled in a  direction applying an electric field of - 60 kV/mm at room opposite to that  of  the  ceramic at room temperature  by temperature. For comparison, another P(VDF-TrFE) temperature by applying  an  electric  field of - 60 film  was  prepared  and  was  also  poled at room kVlmm.  After  poling,  both  the  nanocomposite  and copolymer  films  were  peeled off from  the  glass  slides and  the  AI electrodes  were  etched  away in KOH. Circular  elements  of  diameter - 0.7mm were cut out from  these films for  subsequent  hydrophone fabrication. CONSTRUCTION  OF  HYDROPHONES shown schematically in Fig. I .  The center wire in the The construction of a needle-type hydrophone is stainless-steel tubing was held in position by epoxy. polished to expose the tip of  the  wire.  A  circular After the epoxy had set, the tip of the needle was element prepared as described in the last section was glued to the tip of the exposed wire by silver-filled hydrophone  elements  were  used:  (a)  P(VDF-TrFE), conductive  epoxy  (Ablestik).  Three  types  of (b) PZTiP(VDF-TrFE) nanocomposite with only the ceramic  phase  poled,  (c)  P(VDF-TrFE)  with  the Copolymer or sensing  element EPOXY  nanocompositeSteel  tubing \ /  / / Copper  wire Chromium-gold electrode Figure I (a)  Schematic  diagram  of  the  nanocomposite needle-type  hydrophone. Figure I (b) Photograph of the  hydrophones ceramic and copolymer poled in opposite directions. A chromium-gold electrode was evaporated onto the needle tip which  connects  the  top  surface  of  the film to the steel tubing, served as the ground electrode. The cable  length of the  hydrophone  was  about 0.3 m,  which was the shortest practical ength required to connect  the  ydrophone  output  signal  into  anamplifier. PERFORMANCE  OF  THE  HYDROPHONES Testing of the hydrophones was carried out in a water  tank.  Two  plane  transducers,  with  centre frequency  of 5 MHz  and 7.5 MHz  and  of  diameter  d = water.  The  hydrophone  was  positioned  at  he  near 12.4 mm were  used to generate acoustic waves in field  -far  field  transition  point  T (T = d2/4h where 1 is A  bilaminar  membrane  hydrophone  with  element  size the acoustic wavelength in water) of the transducers. 0.5 mm  ( anufactured  by  GEC-Marconi  a d calibrated  at  National  Physical  Laboratory,  United Kingdom)  together  with  an  amplifier  was  used  to measured  the  acoustic  pressure P at  position  T  from 1 to 10 MHz.  Then  the  needle  hydrophone  to be characterized was placed at the same field point. The end-of-cable  loaded  sensitivity of the  hydrophone ML can  be  calculated  from: M L = ( V L N ~ M ~  (1) where V, and  V, are  the  measured  voltage  output  from the needle and membrane hydrophone, respectively, and M, is the end-of-cable loaded sensitivity of the standard  membrane  hydrophone.  If  an  amplifier  is used,  the  sensitivity  (M,) at the  output  of an amplifier which  is  connected to  the  hydrophone  can be related  to the  end-of  <able  loaded  sensitivity of the  hydrophone (ML) as  follows: 612 - 1998 IEEE ULTRASONICS SYMPOSIUM Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 3, 2008 at 22:10 from IEEE Xplore.  Restrictions apply.M,=GML ( 2 )  where G is the  gain  of  the  amplifier.  The  input 7 pFN IMR. As  the impedance of both  e impedance  of  the  HPS4504A  digitizing  oscilloscope is hydrophone  and  the  load  can  be  assumed  to be capacitive, hence, using the end-of-cable capacitance C of the  hydrophone  measured  at  different  frequencies (Fig. 2), and  the  load  capacitance C,, = 7 pF,  the  end- of-cable open-circuit sensitivity M, can be calculated from: .- IO $ 0  2 1 2 3 4 5 6 7 8 9 1 0  U Frequency ( M H z  ) Figure 2 End-of-cable  capacitance  C of  hehydrophones  with  different  sensing  elements  as  afunction of frequency. a  P(VDF-TrFE) , b PZTP(VDF-TrFE) with only the ceramic phase c  PZT/P(VDF-TrFE)  with  the  ceramic  and  the poled,  and copolymer  phase  poled in opposite  directions. EXPERIMENTAL  RESULTS Fig. 2 shows  the  end-of-cable  capacitance  C  of  the three different types of hydrophones as a function of frequency and it can be seen that the nanocomposite hydrophone  with  only  the  ceramic  phase  poled  has  the opposite  directions, the  capacitance  of  the  hydrophone highest  capacitance. After both phases were poled in decreases.  This is mainly  due to  the  decrease in relative  permittivity  in  the  copolymer  phase  caused  by better  dipole  alignment.  The  end-of-cable  oaded sensitivity  (with  the  amplifier) M, ofthe hydrophones as a function of frequency is shown in Fig. 3. After correcting  for  amplifier  gain  using  eq. (2). the  end-of- cable  open-circuit  sensitivities M, of the  three hydrophones are shown in Fig. 4. Both Figs. 3 & 4 show  that  he  hydrophone  with  the  nanocomposite element  containing  ceramic  and  copolymer  phases poled in opposite  directions  has  the  highest  sensitivity. A A A A A  0 9 0.5 . *  . * .  g 0.0 5 l 2 3 4 5 6 7 8 9 1 0  Frequency (MHz) Figure 3 End-of-cable  loaded  sensitivity  (with  the amplifier) M, of  the  hydrophones  with  different sensing  elements. P(VDF-TrFE) , m PZT/P(VDF-TrFE) with only the ceramic phase poled,  and A PZT/P(VDF-TrFE)  with  the  ceramic  and  the copolymer  phases  poled in opposite  directions. 0.05  0.06 E 0.00 . l . . . . . . . .  I 2 3 4 5 6 7 8 9 1 0  Frequency(MHz) Figure 4 End-of-cable open-circuit sensitivity M, of the  hydrophones  with  different  sensing  elements. P(VDF-TrFE) , m PZT/P(VDF-TrFE) with only the ceramic phase A PZT/P(VDF-TrFE)  with  the  ceramic  and  the poled,  and copolymer  phases  poled in opposite  directions CONCLUSION  AND  DISCUSSION copolymer  and  PZT/P(VDF-TrFE) 0-3 Needle-type  hydrophones  with  P(VDF-TrFE) nanocomposites  asensing  elements  have  b en the  ceramic  and  the  copolymer  phases  poled in fabricated  and  characterized.  The  hydrophone  with opposite  directions  is  found  to have  the  highest 1998 IEEE ULTRASONICS SYMPOSIUM - 613 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 3, 2008 at 22:10 from IEEE Xplore.  Restrictions apply.sensitivity.  Work  is in progress to  fabricate hydrophones  by  dip-coating  a  nanocomposite  on  a  fine pin tip and  then  poling  the  sensing  element  by  corona discharge.  The  results  will be  reported in due  course. 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. PolyU5159198P)  and  also supported  by  the  PolyU  Centre  for  Smart  Materials. REFERENCES [l] M.C.  Ziskin  and P.A. Lewin  ed., Ultrasonic [2]  P.A.  Lewin,  “Miniature  Piezoelectric  Polymer Exposimelry, Ann  Arbor,  CRC  Press,  1993. Hydrophones  in  Biom dical  Ultrasonics”, Ferroelecirics, vol. 60,  pp.127-139,  (1984). [3] G.R.  Harris,  “Hydrophone  Measurements in Diagnostic  Ultrasound  Fields”,  IEEE Trans. Ultrasonics,  Ferroelectrics and Frequency Conrrol,UFFCvol. 35,  pp. 87-101, 1988. [4] D.P.  Skinner,  R.E.  Newnham  and  L.E.  Cross, Bull.. vol. 13, pp.599-607, 1978. “Flexible Composite Transducers”, Mafer. Res. [ 5 ]  T. Furukawa, K. Ishida  and  E.Fukada, “Piezoelectric  properties in the  composite Phys., vol. 50,  pp  4904-4912,  1979. systems  of  polymers  and  PZT  ceramics”, J.  Appi. [6] D.K. Das-Gupta ed., Ferroelecbic Polymers and Ceramic Polymer Composites, Aedermannsdorf, [7]  Q.F. Zbou,  H.L.W.  Chan  d  C.L.  Choy, Trans  Tech.  Pub.,  1994. “Nanocrystalline  Powder  and  Fibres of Lead Zirconate  Titanate  Prepared  by the  Sol-Gel Process”, .I Materials Processing Technology, vol. 63(1-3), pp. 281-285, 1997. 614 - 1998 IEEE ULTRASONICS SYMPOSIUM Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 3, 2008 at 22:10 from IEEE Xplore.  Restrictions apply.

PZT/P(VDF-TrFE) nanocomposite hydrophones for ultrasonic measurements

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