Simulation of field-induced thermally stimulated currents in dielectrics. Application to the dipolar case in polymeric systems

Abstract

Numerical calculations of thermally stimulated depolarization currents (TSDC) and thermally stimulated polarization currents (TSPC) have been carried out by using the bistable model of Fröhlich and taking into account the temperature dependence of equilibrium polarization according to a Langevin function. For dipolar processes characterized by a single relaxation time as well as by a spectrum of relaxation times obeying classical distribution functions (Wagner, Cole-Cole, Fuoss-Kirkwood, Davidson-Cole, Havriliak-Negami), the following predictions have been obtained: (1) A TSDC peak must be always of larger magnitude than the corresponding TSPC peak as a result of an increase in polarization taking place during the cooling step; (2) For the same reason, the magnitude and area of a TSDC peak must be dependent on the cooling rate adopted in the poling process; it also follows that, unless an instantaneous cooling is postulated, such a peak can no more be considered as depicting the relaxation properties of the material at the temperature of polarization; (3) A TSPC peak must be followed by a significant current reversal as a necessary result of the tangential convergence of the transient polarization with its decreasing saturation limit in the high-temperature range. These predictions have been tested in a polymeric system selected for its well resolved relaxation spectrum (styrene-butadiene block copolymer). Full qualitative agreement between experiment and theory has been found as far as the behavior of the α relaxation (glass transition) is concerned. These results show that the variations of equilibrium polarization occuring during the nonisothermal steps of TSDC and TSPC experiments cannot be a priori neglected, as is still commonly done for the determination of dipolar relaxation parameters.