Chan, HLW

Choy, CL

Ng, PKL

Applied Physics Letters

PolyU Institutional Repository

APPLIED PHYSICS LETTERS VOLUME 74, NUMBER 20 17 MAY 1999Effect of poling procedure on the properties of lead zirconate titanate/vinylidene fluoride-trifluoroethylene compositesH. L. W. Chan,a) P. K. L. Ng, and C. L. ChoyDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University,Hunghom, Kowloon, Hong Kong~Received 20 January 1999; accepted for publication 16 March 1999!Lead zirconate titanate/vinylidene fluoride-trifluoroethylene @PZT/P~VDF-TrFE!# 0–3 compositesfor pyroelectric sensor and piezoelectric transducer applications have been fabricated byincorporating PZT powder into a P~VDF-TrFE! copolymer matrix. The properties of thesecomposites can be tailored to suit designated applications by varying the ceramic volume fractionand by using different poling procedures. As both phases in the composite are ferroelectric, and thepiezoelectric coefficients of the ceramic and copolymer phases have opposite signs while thepyroelectric coefficients have like signs, special ways can be used to produce three groups ofsamples with ~1! only the ceramic phase poled, ~2! two phases poled in the same direction to achievereinforced pyroelectric activity and reduced piezoelectric activity, and ~3! two phases poled inopposite directions to obtain reinforced piezoelectric activity and reduced pyroelectric activity. Inthis work, original experimental results on the properties of PZT/P~VDF-TrFE! composites poledunder different conditions are presented and possible reasons behind the reinforcement andcancellation of piezoelectric and pyroelectric properties are discussed. © 1999 American Instituteof Physics. @S0003-6951~99!01120-1#The P~VDF-TrFE! 70/30 mol % copolymer ~supplied byPiezotech! used in the present study has a Curie temperatureTch at 103 °C upon heating and Tcc at 70 °C upon cooling asmeasured by differential scanning calorimetry. The lead zi-conate titanate ~PZT! powder PKI 502 is supplied by Ultra-sonic Powders Ltd.. The copolymer pellets are dissolved inmethyl-ethyl-ketone and suitable amounts of PZT powderare blended into the P~VDF-TrFE! solution to form compos-ites with different volume fractions of ceramic f. Afterevaporation of the solvent, the composites were compressionmolded into disk samples 15 mm in diameter and about 0.4mm thick. The poling procedure for the three groups ofsamples is as follows:Group 1: Samples are heated to 120 °C ~above Tch! andan electric field E0 of ;35 kV/mm is applied to the compos-ite for 1 h. The field E0 is then switched off before cooling toroom temperature. As E0 is switched off above Tch while thecopolymer is still in a paraelectric state, only the ceramicphase is poled. To verify this, the same procedure is appliedto a copolymer sample. The copolymer sample has no detect-able piezoelectric and pyroelectric activities, thus showingthat it is not polarized.Group 2: The poling procedure is similar to that appliedto group 1 samples except that E0 is maintained while cool-ing to room temperature. As the sample cools through Tchand Tcc with E0 still applied, the copolymer phase is poled inthe same direction as the ceramic phase.Group 3: The poling procedure is similar to that appliedto group 2 samples to produce composites with both phasespoled in the same direction. In order to polarize the ceramicphase in an opposite direction relative to the copolymer,these samples are reheated to 50 °C and then a poling field ofa!Electronic mail: apahlcha@polyu.edu.hk3020003-6951/99/74(20)/3029/3/$15.00 Copyright ©2001. A10 kV/mm is applied in the reverse direction for 0.5 h. Thesamples are then cooled to room temperature with the re-verse poling field kept on. As the applied field is much lowerthan the coercive field of P~VDF-TrFE!, the polarization inthe copolymer phase is not affected but the polarization inthe PZT phase is reversed.The piezoelectric d33 coefficient of the samples wasmeasured using a Pennebaker model 8000 piezo d33 meterfrom American Piezo-Ceramics Inc.. The pyroelectric coef-ficient p of the samples was measured using the digital inte-gration technique.1Below the Curie temperature ~;350 °C!, PZT has a te-tragonal ABO3 unit cell structure2 which gives it a spontane-ous polarization Ps as its center of positive charge does notcoincide with its negative charge center ~Fig. 1!. Polingcauses Ps to align in the thickness direction. When subjectedFIG. 1. Piezoelectric effect in a ceramic with tetragonal ABO3 structure~e.g., PZT!, ~a! before tension, ~b! under tension. The polarization increasesas the separation of the positive and the negative charge centers increases.9 © 1999 American Institute of Physicsll Rights Reserved.3030 Appl. Phys. Lett., Vol. 74, No. 20, 17 May 1999 Chan, Ng, and Choyto a tensile stress, the separation between the two chargecenters increases thereby resulting in a longer dipole momentarm and hence a higher Ps . As Ps increases, positivecharges will be generated on the anode ~the electrode sub-jected to positive voltage during poling! and hence PZT hasa positive piezoelectric d33 coefficient. P~VDF-TrFE! has asemicrystalline structure; the dipoles exist in the rigid crystallamellas3 and can be aligned along the thickness direction bythe application of an external poling field. After being poledand then subjected to a tensile stress, the thickness increasearises largely from the deformation of the amorphousregion,4 so the rigid crystal lamellas are dispersed in a largervolume ~Fig. 2!, thereby causing a decrease in Ps (Ps is pro-portional to the dipole moment per unit volume! and hence adecrease of the positive charges on the anode. Hence,P~VDF-TrFE! copolymer has a negative piezoelectric d33 co-efficient due to this dimensional effect.4 As the piezoelectriccoefficients in the ceramic and the copolymer have oppositesigns, hence when the ceramic and copolymer phases in thePZT/P~VDF-TrFE! composites are poled in the same direc-tion ~group 2!, their piezoelectric activities partially canceland when they are poled in opposite directions ~group 3!,their piezoelectric activities are reinforced ~Fig. 3!. Sampleswith only the ceramic phase poled ~group 1! have piezoelec-tric coefficients in between those of group 2 and group 3samples ~Fig. 3!.For most pyroelectric ceramics, their pyroelectric coeffi-cients are negative because the spontaneous polarization de-creases as the temperature increases. This is because as thetemperature increases, the increase in thermal energy causesthe disordering of dipoles which results in a decrease of posi-tive charges on the anode. For pyroelectric polymers, theirpyroelectric coefficients are also negative because the disor-dering of crystalline dipoles increases with increasing tem-perature which causes a decrease in spontaneouspolarization.5 Hence, when the ceramic and the copolymerphases in the PZT/P~VDF-TrFE! composites are poled in thesame direction ~group 2!, their pyroelectric activities are re-inforced and when they are poled in opposite directions~group 3!, the pyroelectric activities partially cancel ~Fig. 4!.Group 1 samples have pyroelectric coefficients in betweenthat of group 2 and group 3 samples ~Fig. 4!.In summary, we have prepared PZT/P~VDF-TrFE! 0–3FIG. 2. Piezoelectric effect in a P~VDF-TrFE! copolymer, ~a! before ten-sion, ~b! under tension. The spontaneous polarization decreases as the vol-ume increases.Copyright ©2001. Acomposites with the ceramic and copolymer phases poled inspecial ways. The piezoelectric and pyroelectric properties ofthese composites vary greatly with the poling procedures.Composites with the ceramic and copolymer phases poled inthe same direction can be used as sensing elements in pyro-electric sensors6 as they have enhanced pyroelectric activitybut reduced piezoelectric activity, thereby minimizing thevibration induced electrical noise. Composites with the ce-ramic and copolymer phases poled in opposite directions canbe used to fabricate ultrasonic transducers7 as they have re-inforced piezoelectric property but reduced sensitivity totemperature fluctuation.FIG. 3. Piezoelectric d33 coefficient of the composites as a function ofvolume fraction of ceramic f. When the ceramic and copolymer phases arepoled in the same direction ~m!, partial cancellation occurs. When the twophases are poled in opposite directions ~3!, reinforcement is obtained. ~j!represents d33 of composites with only the ceramic phase poled.FIG. 4. Pyroelectric coefficient p of the composites as a function of volumefraction of ceramic f. When the ceramic and copolymer phases are poled inthe same direction ~m!, reinforcement occurs. When the two phases arepoled in opposite directions ~3!, partial cancellation is obtained. ~d! repre-sents p of composites with only the ceramic phase poled.ll Rights Reserved.3031Appl. Phys. Lett., Vol. 74, No. 20, 17 May 1999 Chan, Ng, and ChoyThe work described in this paper was substantially sup-ported by a grant from the Research Grants Council of theHong Kong Special Administrative Region ~Project No.PolyU 5159/98P!.1 Y. Xu, Ferroelectric Materials ~Elsevier, North–Holland, Amsterdam,1991!.2 L. E. Cross, in Ferroelectric Ceramics: Tailoring Properties for SpecificApplications in Ferroelectric Ceramic-Tutorial Review, Theory, Process-Copyright ©2001. Aing and Applications, edited by N. Setter and E. L. Colla ~Brikhauser,Verlag Basel, 1992!, pp. 1–86.3 M. G. Broadhurst and G. T. Davis, Ferroelectrics 60, 3 ~1984!.4 M. G. Broadhurst, G. T. Davis, J. E. McKinney, and R. E. Collins, J.Appl. Phys. 49, 4992 ~1978!.5 T. Furukawa, J. X. Wen, K. Suzuki, Y. Tashina, and M. Date, J. Appl.Phys. 56, 829 ~1984!.6 Y. Chen, H. L. W. Chan, N. M. Hui, Y. W. Wong, and C. L. Choy, Sens.Actuators A 69, 156 ~1998!.7 H. L. W. Chan, S. T. Lau, K. W. Kwok, and C. L. Choy, in Proceedingsof 1998 IEEE Ultrasonics Symposium, Sendai, Japan, October 1998.ll Rights Reserved.

Effect of poling procedure on the properties of lead zirconate titanate/vinylidene fluoride-trifluoroethylene composites

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