Chan, HLW

Chen, Y

Choy, CL

PolyU Institutional Repository

Study  on  PZT4NF,NF, Piezoelectric 0-3 Composites H.L.W. Chan, Y. Chen  and C.L. Choy Dept. of Applied Physics and Materials  Research Center, Hong Kong Polytechnic, Hung  Horn, Kowloon, Hong Kong ABSTRACT PZT4NFzNF, 0-3 composites with different volume percent of ceramic  have been fabricated The dielectric constants and electrical resistivities of the composites were measured as fbnctions of temperature in order to find the optimum poling conditions. The electromechanical properties of the piezoelectric composites  were also measured and  compared to  model calculations. iN1'RODUCTION Piezoelectric  ceramicipiezoelectric polymer 0-3 composites  represent  a  class of piezoelectric materials having potential to replace  conventional  piezoceramics  and piezopolymers. M e r  combining the two phases in a 0-3 connectivity, the  composite must be poled in a high electric field in order  to elicit piezoelectric response. One important parameter we need to consider in the poling of a composite is the ratio of the resistivities p of the constituent phases since it governs  the effective poling field across each  phase. (Maxwell-Wagner effect [l])  Another  parameter is the relaxation time T of the composite, which determines the poling time Both p 2nd T are hnctions of temperature. In order to find the optimum poling temperature and poling time, the dielectric constants and resistivities of the PZT-4 ceramic, VF,/VF, copolymer  and 0-3 composites with different volume fraction of ceramic were measured as fbnctions of  temperature. COMPOSITE  PREPERATION The VF,/VF, copolymer supplied by Atochem North America Inc. has a nominal W, content of 30%. The DSC (Differential Scanning Calorimetry) endotherms of the as- supplied pellets was measured at a heating rate of 10°C/min using a Perkin Elmer D S c 7  Thermal Analyzer. The Curie transition temperature fGr the first heating (TTc) of  the copolymer was 105OC and the melting temperature Tm was 1550C This indicated hat  the VF, content of  the copolymer was several percent higher than 30. [ 2 ]  The PZT powder supplied by the Shanghai Institute of Ceramics has a grain size between 1 and 3 pm and has properties similar to the Vernitron (Morgan Matroc Ltd.) PZT-4  composition. The copolymer pellets were first dissolved in Methyl-Ethyl-Ketone (MEK), and then a suitable amount of ceramic powder was blended into the solution. The mixture was then poured onto a petri-dish and  after evaporation of the solvent, the mixture was triturated.  Then the  composite  was moulded into disc shape by hot pressing at 37.7 kgimmz and 200°C in a  compression  machine. Composite discs with different volume percent of ceramic formed using the method described above  have  a  diameter of 12.5 mm and thickness ranging from 0.5 to 0.9 mm. RESISTIVITY h&ASUREM€NTS Polymers  tend to have higher resistivities while ceramics have somewhat lower resistivities. The resistivities of  both  the ceramic and polymer decrease as the  temperature increases but usually the gradient is higher for the polymer. Experimental  measurements  of  the  ceramic and the copolymer  resistivities pc and p, were  carried out in order  to find a  suitable  poling temperature for the composite. The  constant voltage  method [3] was used to measure the resistance of the unpoled samples, a IOOV d.c  voltage was applied to  the sample using the built-in voltage  source  of Keithley 617  Electrometer, and the resistance of the sample was measured using the same electrometer. Resistivities of the samples are shown in Fig.1. A sharp decrease of the copolymer resistivity was observed  at  bout  the  Curie temperature (105OC). This indicated that if the composites were poled above IOOOC, the resistivity ratio pdp, would be about 10, which is close to  the ratio of  the coercive field of the copolymer  and  ceramic at high temperature. loG CeramcLdurne percent 4- 0% 4- 11% 4- 21% -.- 32% --t 42% --t 52% -X- 61% -e PI6 77% -_ .I -1-lWh 40 ' $0 ' JiI ' lba ' l& ' l40 ' 1' Tmperature, TCC) Fig. 1 Resistivities of  the 0-3 composites vs temperature. D CH3416-5 0-7803-1847-1/95/$4.0001995IEEE 198 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 26, 2008 at 04:11 from IEEE Xplore.  Restrictions apply.DIELECTRIC ONSTANT h&ASUREMENTS The capacitance of  the unpoled disc samples at 1 kHz was measured using an HP4194A Impedance Analyzer and the resulting dielectric constants  at different temperatures  are shown in Fig. 2. A relaxation peak was observed near the Curie temperature of the copolymer [4]. This peak is prominent for the pure copolymer but becomes broader as the volume  percent of ceramic increased which is different from the previously reported value of  8.5 for  the  PZT-PVDF 0-3 composites [ 5 ] ,  the Furukawa model gave values slightly lower  than the experimental measurements. However. as the temperature increased, for high volume  percent of ceramic, the Furukawa model agreed better  with  the experimental results (Fig.  3b)  than the Yamada model  with n=4.3. If n=3 was  used in eqn. (l), the predicted  values from the Yamada model and  Furukawa model  were identical. + 0 .  P ) .  E c Ceramxvolume percen! .- n ,oo - 3% - 11% I - 21% I32% 1 --t 42% - 52% . - 61% -X-- 72% 20 4 0  60 80 100 120 Temperature, T('C) F ig2  Dielectric constants of the composites vs temperature. Values of the measured dielectric constants were compared to various  models, namely, the Voigt (parallel) model, Reuss (series) mcdel , Yamada model [eqn. ( l ) ]  and Furukawa model [eqn.(2)] for 0-3 composites [5-71. In the Yamada  model: E = EP{ 1 4- [~~(E,-Ep)]/[nEp+(E,-Ep)( 1 -($)l}. . . . . .. ... . . .  .. ..... where E, E, and E~ are  the dielectric constants of the composite,  ceramic and copolymer respectively, 4 is the volume  fraction of  the ceramic and n is a parameter describing the shape of' the ellipsoidal particles [S.  In the Furukawa model for dispersed spherical particles, the dielectric constant of a composite  was given as: E = Ep[2Ep -l- E, - 24(Ep - 2,)]/[2Ep -+ E, f $(Ep-E,)] ..... ........ .(2) It  was noted that at a lower temperature (e.g. 3@C, as  shown in Fig. 3a), the experimental results agreed very well with the Yamada  model  with a shape  parameter n of 4.3, 0 10 20 30 40 50 60 70 80 90 100 Ceramic volume percent, (YO) 0 10 20 30 40 50 60 70 80 9 0 1 M )  Cerarric volume percent, (YO) Fig.3 Dielectric constant of the composites as a hnction of volume percent of ceramics (a)  at 30°C, (b) at 12OoC. In previous  reports [5-81, 0-3 composites were prepared by mixing the copolymer and ceramic powder in a hot rolling mill. As the copolymer was still highly viscous in its molten state, clustcring of the  ceramic powder might become a problem. In the present work, since the ceramic powder  was dispersed in a very dilute  copolymer solution. it is very likely that each ceramic particle would be coated with a layer of copolymer.  This is consistent with the assumptions of the Yamada and Furukawa models [5,6] and hence a close agreement can be obtained even for high volume percent of ceramic. 199 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 26, 2008 at 04:11 from IEEE Xplore.  Restrictions apply.RELAXATON TIME OF THE 0-3 COMPOSITES If the composite disc samples were modeled as a parallel plate  capacitor having the measured  values of resistivity p and dielectric constant E, the RC constant and hence  the relaxation time T = I/(RC) = PE can be evaluated It is seen from Fig.  4 that  the relaxation time T dropped from several  hundred seconds  to several seconds  as the temperature increased from 30 to 120oC. However, during poling, a very high d.c. electric field was applied to the sample. Since the resistivity of a dielectric increases in an electric field [9], the value of p (and hence T.) during poling may be much higher than p (and T.) measured  under normal condition. Hence much longer poling time t, (typically two hours)  was used to  ensure that t, >> T. 103 4- 0% 4- 11% \ - -A- 21% - 32% *\ A 42% --t 52% - 61% -C+- 72% -- 77% -100% 20 40 60 80 100 120 Temperature. TeC) Fig.4 Relaxation  time of  the composites  vs temperature POLING AND PIEZOELECTRlC PROPERTIES OF THE SAMFLES DC thermal poling was used to pole the samples in an oil bath at 12OOC. and the typical poling conditions were tabulated in Table 1. Values of  the thickness electromechanical  coupling coefficient k, were  valuated using the method proposed by Sherrit et al [ IO]  taking into account the high loss in the copolymer. The charge  constants djj was measured using a Pennebaker model 8000 piezo d,, tester  from American Piezo-Ceramics, Inc..  The volume fraction of ceramic 4 was calculated  using  the  measured density. TABLE I THE PROPERTIES OF PZT4/P(VDF/TrFE) 0-3 COMPOSITES. From Table 1 ,  it can be seen that  the d,, constants of the composites decreased from -30.9 pC/N at 4 = 0.11 to a small value  of -14.9 pC/N at 4 = 0.52, then changed sign to +13.2 pC/N at 4 = 0.61,  and remain positive for higher value of $. This strongly suggested that the copolymer phase also contributed to the d,, value of the composite As a first estimate  of  the contribution of both  phases to the d,, constant, assume d,, = a$Gd,,,+ ( l-4)d33p ............................... .(3) where a is the ceramic poling ratio, d33c and d,,, are the charge constants of the  ceramic and copolymer  respectively, and G represents the loca! field coefficient [5], G = nE/[nE+(E, -:)l... ....................................... (4) Using n = 4 3 and a = 0.6, d,, of the composites were calculated and shown in Fig. 5 together with the measured values. It is clear that equations ( 3 )  and (4) give the right trend indicating that the djj contribution of the copolymer tended to  cancel the contribution  of the ceramic. The thickness electromechanical coupling coefficient k, of the composites  was smaller than that of  the ceramic or the copolymer, especially around I$ = 0.5  to 0 66.  The 200 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 26, 2008 at 04:11 from IEEE Xplore.  Restrictions apply.thickness mode resonance peaks,  although  visible,  were  very weak. ACKNOWLEDGEMENTS ? . -x- Theoretical / / Experimental Ceramic volume percent, (%) Fig.5 Measured d;; values  compared to the theoretical  values calculated using eqn (3) CONCLlJSlON AND DISCUSSION 0-3 composites of PZT4NF2IVF, copolymer  with different volume percent of ceramic were fabricated using solution cast and hot-press method. The dielectric strength of these composites is lower than that prepared using hot two-roll mill and  hence breaks down  more  easily  under  high poling  field. However, using a dilute  solution  of  the copolymer  can  help to circumvent  powder  clustering problem and the ceramic particles are likely to be surrounded by copolymer  layers as modeled  by previous  workers,  hence  the dielectric constants of the composites agreed  very  well to the model predictions even for high volume  percent of ceramic. The poling temperature was chosen to be 120°C, at which the copolymer resistivity was rather low. hence the resisitivity ratio of the two phases was about 10, which is close to the ratio of  their coercive fields.  However,  since  the Curie temperature of the copolymer used in this  experiment was 1OS0C, in order to pole the copolymer,  the  electric field must  be  kept  on  when the composite was cooled  through its Curie temperature T, (T, for the composites was  also  around The contribution to the piezoelectric  activities by the two piezoelectric  phases  seemed to cancel  ach other resulting in a variation of d,, from negative to positive as the ceramic content increased. k, was also fairly  small  In transducer applications, it is  not  desirable to have a cancellation effect , hence, we are trying to pole the two phases  oppositely so that reinforcement will occur  Details of this poling  study will  be reported later [ 1 l ]  1 OOOC). The authors would  like to thank Prof. Q. Yin of the Shanghai Institute of Ceramics for supplying the ceramic powders. The  first  and the second author would  also  like to thank Dr. P.W. Chan for his encouragement and support Financial support from the Research Grants Council (RGC) is also acknowledged. REFERENCES [ l ]  A.R. Von  Hippel, "Dielectrics and Waves", John Wiley and Sons, 1954, pp.228. [2] P.E. Bloomfield  and S. Preis, "Piezoelectric and Dielectric Properties of Heat  Treated  and  Polarized VF,NF, Copolymers", IEEE Trans. Electrical InsulL, Vol. EI-21, ~ ~ 5 3 3 - 5 3 7 ,  1986. [3] User Manual,  Keithley 617 Electrometer. [4] T. Furukawas, M. Ohuchi, A Chiba  nd M. Date, "Dielectric  Relaxation and Molecular Motions in Homopolymers and  Copolymers of Vinylidene  Fluoride and Trifluoroethylene", Mncrontolecules, Vol. 17, [5] T. Yamada, T Ueda and T. Kitayama, "Piezoelectricity of a hig-content lead  ziconate titanatdpolymer composite", J.  Appf. Phys., Vol 5 3 ,  pp. 4328-4332, June, 1982. [6] T. Furukawa, K. Ishida and E. Fukada, "Piezoelectric properties in the composite systems of polymers and PZT ceramics", J.  Appl. Phys., Vol. 50, pp.4904-4912, July, 1979. [7] C J. Dias and D.K. Das-Gupta, "Piezo- and Pyroelectricity in Ferrroelectric Ceramic-Polymer Composites", Chap. 8 in "Ferroelecfric Polynlers and Ceramic-Polymer Conposites", ed.  D.K. Das-Gupta, Trans Tech Pub., Switzerland, pp. 217-248, 1994. [S] H Yamazaki and T. Kitayama, "Pyroelectric Properties of Polymer-Ferroelectric Composites", Ferroelectrrcs. Vol. 33, pp. 147-153, 1981 [9] J Malecki  and B. Hilczer,  "Dielectric  Behaviour of Polymers and Composites", Chap 7 in "Ferroelectric Polynters and Ceramic-Polymer Composiles", ed. D.K Das-Gupta, Trans Tech Pub., Switzerland, pp. 18 1-2 16, 1994. [lo] S .  Sherrit, H.D. Wiederick and B.K. Mukherjee, "Non- iterative Evaluaticrl of the Real  and  Imaginary  Material Constants of Piezoelectric Resonantors", Ferroelectrics, Vo1.134, pp. 111-119, 1992. [ l  l] H.L W. Chan, Y. Chen  and C.L. Choy, "A poling  study of PZTP(VDF-TrFE) copolymer  0-3 composites",in proceeding  of the 2nd  Pacific-Rim  Conference  on Ferroelectric Applications Melbourne, Australia, Nov . 1994 pp.1384-1390, 1984. 201 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on November 26, 2008 at 04:11 from IEEE Xplore.  Restrictions apply.

Study on PZT4/VF₂/VF₃ piezoelectric 0-3 composites

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