A number of methods exist for deriving the activation energy EA of properties of a system from the results of thermally stimulated relaxation processes, such as thermally stimulated currents and thermoluminescence. The standard analysis of such processes assumes that the isothermal relaxation of the system decays exponentially with time. An extension of this analysis, based on the use of the system's natural time, is presented for systems possessing an arbitrary time dependence of the isothermal relaxation, provided that this relaxation at different temperatures can be described by a suitably scaled master function. In particular, it is shown that for systems in which the isothermal relaxation decays with the time t as exp(-(t/ tau )alpha ), the parameter determined by most of the methods is not the activation energy EA of the scaling time but rather the product alpha EA.
While thermally stimulated depolarization currents, sometimes in conjunction with the thermal slicing technique, are often used to determine the activation energies and relaxation times of the processes occurring in a material, little attention has been paid to the question of whether the results do represent the true properties of the system. In this paper, a theoretical analysis and the results of calculations are presented for systems whose isothermal behaviour can be described in terms of a distribution of relaxation times (DRT). It is found that for a DRT with a common activation energy E0, the activation energy EBO derived from the Bucci or BFG relaxation time is often less than E0, white if the DRT is associated with a distribution of activation energies the prefactor of the BFG relaxation time often differs from its true value. Simple criteria are proposed for establishing whether the parameters derived from any given set of measurements do in fact correspond to those of the system, and for distinguishing between a DRT with a distribution of activation energies and one with a single activation energy.
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