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ELECTRICAL CONDUCTIVITY AND PYRO-ELECTRIC MEASUREMENTS ON PZT : PVDF COMPOSITES

Anita TRIPATHI, A.R.TRIPATH1 and P.K.C.PILLA1 Department Of Physics, I.I.T.Delhi, New Delhi-110016, India.

Abstract

PZT :PVDF composites were prepared in different weight ratios. The electrical conductivity of these samples was measured as a function of temperature an field. It was found thaElthe conduct'vity increases from lO-"(ohm cm)-' to 10- ( ohm cm)-' as the percentage of PZT in the composites was increased from 70% to 90% by weight. The pyro - electric current and dielectric constant of these composites were also studied. The Pyro electric coefficient increased from 40 to 76 n Coul/cd/°C as PZT was increased from 70% to 90% by weight in the composites at 50 OC.

1 Introduction - Polymer / ferroelectric ceramic composites have received much attention as new piezoelectric materials for applications in hydrophones and medical ultrasound [l -41. If the composites are formulated and fabricated correctly , they offer a combination of properties and a diversity of application unobtainable with ceramics or polymers alone. Electrical c o n d u c t i v i t y i s o n e o f t h e e s s e n t i a l e l e c t r i c a l characteristics of a material and the elucidation of the c o n d u c t i v i t y m e c h a n i s m is o f p r i m a r y i m p o r t a n c e in determining its practical applications.

In the present paper , the temperature dependence of the pyroelectric coefficients and the conduction mechanisms of PVDF - PZT composite in 0-3 connectivity pattern are discussed.

- 2 Experimental PVDF ( Janssen Chimica , 9elgium) and PZT (Central Electronics , India) were grounded together in required proportions using acetone as thinner. The pellets were made with a hydraulic press under a load of 10 tones on a lcm diameter die. The load was applied after the die attained a temperature of 15OoC. The die was cooled to room temperature under the applied load. On cooling the load was released and the pellet was taken out and its surfaces were polished and cleaned with propanol. Pure PVDF films has been made by dissolving the powder in Dimethylacetamide (DMA) at 7OoC. The solvent cast films were prepared by pouring the solution on to glass substrates . Films (100 u n ) were allowed to dry at 6OoC for 24 to 27 hours in an oven to remove traces of the solvent. The film was cut in to rectangular pieces. Quick drying silver paint was used for the electrodes.

CH3029-6/91/0000-0501 $01 .OO Copyright 1991 IEEE

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The electrical conductivity measurements were carried out as a function of temperature at constant electric field. The voltage was drawn from a high voltage regulated power supply and current was measured by a Keithley Electrometer model no 5 l c ) C . pyro electric coefficient was measured after poling these samples at 8OoC using a poling field of 10 kV\cm. Dielectric measurements were done with a Bewlett- Packard Impedance Analyzer model no 4192 A.

3 Results And Discussions -- - In order to understand the conduction mechanism prevailing in P Z T - P V D F c o m p o s i t e s , the c u r r e n t v o l t a g e ( I- V ) characteristics of PVDF, PZT and the composites with 70wt% PZT (PZ7) , 80wt% PZT ( PZ8) and 90wt% PZT ( PZ9) were plotted up to an electric field of 4OkV\cm. The typical plots at 4OoC &lOO°C are shown in Figs 1&2. The slope values of the current voltage plots at 4OoC for the composites was found to be around 1 indicating that they follow Ohm’s law even at high fields, while pure PVDF follows Ohm’s law at lower fields only. The different slope values in I vs V curves of pure PZT indicates that Ohm’s law is not applicable here.

At 1 0 0 ° C the slope value of I vs V curves are less than 1 for the composites. The sublinear dependence of current on applied field seems to indicate that there is a decrease either in the number of charge carriers and\ or their mobility with increasing field and temperature . This sublinear behaviour may be due to the fact that these samples are biphasic and have very high density of traps out of which some of them may be filled while others may be empty under equilibrium conditions. On application of the voltage some of the carriers will get trapped an2 cause a fall in the number of thermal free carriers available for conduction. The density of traps increases with the ceramic contents. This seems plausible, since the thermal expansion coefficient of P Z T and PVDF differ considerably, therefore, when the composites are heated it is possible that more voi,r?s and other defects are created. These would give rise to greater number of trapping sites and corresponding fall in the conductivity [5-71.

The log of ,J vsrE plot for PZT, DVDF, pZ7, PZ8 and P Z 9 at 4OoC are shown in fig3. These plots show a linear dependence at high fields only. This indicate the possibility of Schottky - Richardson or Pool- Frenkel type of conduction in this range. The slight curvature observed at lower fields may be attributed to space charges and surface inhomogeinity. The slope value of the straight line portion of log J vs r E curves were calculated , slong with the theoretically calculated values of PSR and ,$ PF . The values of dielectric constant e’ used in the calculation of p are experimentally m e a s u r e d high f r e q u e n c y d i e l e c t r i c c o n s t a n t o f the composites. It has been found t h a t 3 is excessive compared to JPF for all the samples. TCTB behaviour shows

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the presence of ionic conduction in the samples . Oas Gupta eta1 [8] have also observed the presence of ionic conduction in PVDF , where the carriers hop through the defect sites along the chains. The results of the present investigation show that the presence of PZT in the composite progressively increases the ionic conduction.

Fig4 shows the log r vsl\T plot for samples PZT,PZ9 and PZ8 at constant field of 10kV\cm. The activation energies for PZT ( 0.38eV), PZ9(.55 eV), PZ8 ( .75 eV) , PZ7 (.45eV) and for PVDF (.39eV) were calculated. naximum value has been observed for the composite containing 804 PZT.

Fig 5 shows the variation of pyroelectric coefficient versus temperature for different samples. The pyroelectric coefficient was maximum for PZT ( 95n coul\cm\°C ) , while the value observed for PVDF is much higher than that reported elsewhere [SI. The value of pyro-electric coefficient for the composites lie in between these two values.

In conclusion , it is suggested that the steady state conduction behaviour in the polymer ceramic composite may originate from an ionic hopping mechanism. The pyro electric coefficient of the composite has been observed to be significantly higher than that of PVDF depending upon ‘.he composition.

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Figl. Variation of log1 vs logv plot at AOOC f o r 1) PVDF 2) P Z ~ 3 ) PZ8 4) PZ9 5 ) PZT.

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