INTERRELATION BETWEEN PYROELECTRIC PROPERTIES AND RESIDUAL POLARIZATION IN POLYVINYLIDE FLUORIDE INTENDED FOR MAKING PYROELECTRIC SENSORS

Pyroelectricity and fractional thermally stimulated depolarization currents (TSDC) were experimentally studied in polyvinylidene fluoride (PVDF) films poled by application of high DC field at room temperature. The obtained polarization is found to be less thermally stable than the correlated compensating charge. It is demonstrated by investigation of the charge balance and the pyroelectric coefficient measured during cooling in a fractional experiment, that trapped polarization-compensating charge is responsible for a partial realignment of polarization during cooling. The polarization and the volume charges in PVDF samples poled by application of a high field at room temperature are not thermally frozen. All processes of their buildup and relaxation are interrelated. If there is any polarization, it is necessarily compensated by trapped charges. The intention of the paper is to investigate in more depth the nature of the residual polarization in PVDF by comparing TSDC during successive heating of the poled samples to higher and higher temperatures (fractional TSDC) with pyroelectric currents obtained during the subsequent cooling the sample in each thermal cycle. It has been found that the poled PVDF samples should be conditioned by heating to 80 oC and cooling down for obtaining the stable pyrocoefficient.