JOURNAL OF MATERIALS SCIENCE LETTERS 7 (1988) 167-170 Dielectric and pyroelectric properties of polymer/ceramic composites D. K. DAS-GUPTA, M. J. ABDULLAH School of Electronic Engineering Science, University College of North Wales, Dean Street, Bangor, Gwynedd LL57 1UT, UK Ferroelectric ceramics such as Lead Zirconate Tita- 10 + nate (PZT) and BaTiO3 have high dielectric permittiv- ity, high piezo and pyroelectric coefficients and high ZeAl electromechanical coupling which make them very 10-6 useful in numerous applications, notably in high energy storage capacitors, acoustic emission detection, vidi- gon targets, medical imaging systems etc. However, their poor mechanical strength and relatively high value of acoustic impedance have restricted their use in some applications. On the other hand, piezoelectric polymers such Polyvinylidene Fluoride (PVDF) and its copolymer with trifluorethylene (TrFE) have rela- tively low acoustic impedance which could provide a good acoustic matching to water or tissues. Moreover, its good mechanical strength makes it a very attractive material, although its piezo and pyroelectric coef- ficient are relatively low as compared to ceramics. Therefore, a ferroelectric composite may be designed to combine the superior electro-active properties of the ceramics and the mechanical properties of polymers. ~fhe composites can be prepared by several methods, such as by embedding a piezoelectric rod ceramic in a polymer matrix, by a replamineform process and by mixing ceramic and polymer phases together at cer- tain temperatures [1-3]. The latter method is con- venient as the composite could be prepared to any desired size and composition. The present work reports 10_s the result of a study of absorption currents as well as the dielectric and pyroelectric properties of PZT/PVDF composites. Composite samples were prepared from PZT 10_+ ceramic powder (obtained from Unilator, UK) mixed up with PVDF (grade Solef 11010, supplied by Laporte Trad., UK) at 443 K using a hot roller machine. The film was then pressed to approximately 200/~m and an 10-~ aluminium electrode of 2cm x 2cm was vacuum evaporated on both sides of the film. The samples were ZlA> then thermally treated in an evaucated measurement chamber (< 10 6torr) at 373K for 24 hours, before 10-8 current absorption measurements were performed. The dielectric dispersion measurements were made using a General Radio Bridge (type 1621) or using a system comprising of a Solartron frequency response 10-' analyser and BBC microcomputers which have recently been developed at UNCW, Bangor. The pyroelectric currents were measured using a direct method [4] by applying a linear heating rate of approximately loCmin t to the samples which have been poled appropriately. Absorption currents and dielectric measurements were also made with PZT discs and n ~ca D~ A rl D~D 10-7 °'~e~ ~'~x X "~-- X ~.,,. O~ °~e .... " ~A ~ ~....._. A 10-8 I 0 I E + 1 10z 103 10~ time (sec) Figure l Charging currentsin PZT/PVDF(50/50)at different fields at 363K. (x) 1.75 x 105Vm i, (e) 3.5 x 105Vm -I, (A) 1.75 x 106Vm -1, (D) 7 x 106Vm -I, piezel (a composite of PZT and PVDF copolymer, marketed by Daikin Industries Ltd, Japan). Fig. 1 shows a set of charging currents at different fields at 363K in a PZT/PVDF composite with 50 vol % PZT. It may be observed that the current decreases progressively in time; however, at longer time and higher fields the rate of decrease is reduced possibly due to the tendency to reach a steady state level. The observed broad peak at low charging fields, X~. x X X~ • X'~-X ~. X •\. \ "\ \ ~X~. X ~X """ X 50% PZT ~'~X ~e ~ X~X10*/*PZT 50% PZT ~ 10 °,'o PZT 10 -10 ~ I I i 10 10 2 103 10~ time {sec) Figure 2 Charging(x) and discharging (@) currentsin PZT/PVDF at different compositions at 363K withpolingfield7 × 106Vm ~. 0261-8028/88 $03.00 + .12 © 1988 Chapman and Hall Ltd. 1 67