Analysis of thermally stimulated currents

Abstract

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.

TSDC spectroscopy of relaxational and interfacial phenomena

“…In addition, such materials often obey the time/temperature superposition principle, and so the response of the system can be analysed in terms of a reduced time [51][52][53]. It was reported that the commonly observed power-law behaviour in frequency imposes a quite general type of response on experiments performed in nonisothermal conditions [53].…”

“…It was reported that the commonly observed power-law behaviour in frequency imposes a quite general type of response on experiments performed in nonisothermal conditions [53]. In a measurement of TSDC the reduced time ξ scales linearly with the inverse of heating rate b [51][52][53] dn…”

A simple model of entropy relaxation for explaining effective activation energy behavior below the glass transition temperature

“…TSDC has high sensitivity and the ability to selectively probe the relaxation kinetics during heating in a very restricted range of temperatures, by the TS method, 32 in which the polarization is applied in a narrow window ⌬T Ϸ 5 K. Note that in TS the Bucci method 33 is applied for the calculation of the relaxation time, so that each peak is considered, to a first approximation, as correspondent to a Debye process, characterized by a single value of the relaxation time parameters. The TS method in polymers therefore consists on fitting the depolarization peaks to Arrhenius-or Eyring-type equation and provides a value of E act associated to the chosen polarization temperature T p .…”

Interpretation of the TSDC fractional polarization experiments on the ?-relaxation of polymers

“…2 In spite of the advantages of TSDC for characterizing the relaxation processes of polymers, the interpretation of the TSDC spectra is still controversial. 9,10 The TSDC peaks are commonly interpreted on the basis of a distribution of relaxation times with an Arrhenius temperature dependence. 3,11,13 In order to "isolate" each of the relaxation components, several techniques involving partial polarization of the sample are used.…”