Poling of ferroelectric 0-3 composites

Abstract

Poling of ferroelectric lead titanate / polyvinylidene fluoride - trifluoroethylene (PT/P(VDF-TrFE)) 0-3 composites has been studied. Successful poling of various 0-3 composites like PT, PZT & PLZT dispersed in polymer matrices has been reported in the literature. However, the poling recipes / techniques are still quite empirical because a firm understanding of the physical processes involved in poling has not been established. The present work aims at a systematic study of the physical processes relevant to the poling process. Understanding of the physical processes behind poling will give an idea on the materials selection and the design of poling parameters for tailored applications. Thick composite films with lead titanate (PT) ceramic volume fractions from 0.05 to 0.3 were fabricated. The PT powder was prepared by the metal alkoxide sol-gel method. The gel was sintered at 600 C for one hour. The average diameter and standard deviation of the PT particles were determined as 300 nm and 170 nm respectively by a particle size analyzer, and the crystalline structure was characterized by X-ray diffractometry. 30 um thick composite films were prepared by solution casting followed by compression molding. Uniform dispersion of the ceramic particles in the copolymer matrix was confirmed by examining fracture surfaces of the samples under a scanning electron microscope. Gold electrodes were deposited on both surfaces of the composite samples. The dielectric properties of the composite samples were studied from -20 C to 120 C in the frequency range of 100 Hz to 1 MHz. The capacitance and loss factor were measured as a function of both temperature and frequency. The results were compared with a prediction by the Bruggeman model and good agreement is found. Curie's transitions were identified at 106 C and 65 C in the heating and cooling curve respectively. The polarization development in the composite film under poling conditions was extensively studied. Composite films were polarized at an elevated temperature of 120 C under a high d.c. electric field of 50 MV/m for different poling times to polarize the ceramic phase only. Unpoled samples were measured for reference: the pyroelectric and piezoelectric coefficients, the pyroelectric profiles across the film thickness and the XRD patterns showed null polarization consistently. The samples were then subjected to the poling process for different poling times. The pyroelectric and piezoelectric coefficients were measured at various poling times and the pyroelectric profile along the sample thickness direction was measured by the laser intensity modulation method (LIMM). Pyroelectric profiles also revealed the composite with 30 vol-% PT can be uniformly polarized along the thickness direction in 103 s. The XRD patterns of the unpoled and poled samples were compared. Peak intensity changes in (002) and (200) were observed and the poling ratio of the inclusions could therefore be estimated. The result shows that higher poling ratio under the same poling condition can be obtained in the composites with higher ceramic volume fraction. Both measurements, LIMM and XRD show that the inclusions can be polarized to a saturated state in 103 s, which is found to be several times shorter than the time scale reported previously. Theoretical modeling has been developed to study the effective polarization response of the composite systems under an arbitrary applied electric field. The model intends to explain the physical features related to the poling process. The electric fields acting in the constituent phases, remanent polarization in the ceramic particles and interfacial compensating charges as a function of poling time have been evaluated. Both experimental observation and theoretical simulation indicate that the PT ceramic inclusions can be fully polarized fairly quickly at an elevated temperature where the conductivity of the polymer matrix is high enough to provide a desirable condition for the build-up of high electric field acting in the ceramic phase. Model predictions on the piezoelectric and pyroelectric activities of the 0-3 composites based on the calculated polarization behaviour of the inclusions as a function of poling time were also discussed.