Thermally stimulated depolarization

A detailed discussion of the statistical thermodynamic aspects of thermally stimulated dielectric relaxation is not provided here. It should suffice to state that kinetics of most of the processes are again complicated and that the phenomenological kinetic theories used to described thermally stimulated currents make use of assumptions that, being necessary to simplify the formalism, may not always be justified. Just as in the general case, TSL and TSC, the spectroscopic information may in principle be available from the measurement of thermally stimulated depolarization current (TSDC). However, it is frequently impossible to extract it unambiguously from such experiments. 

For most experiments on nonisothermal TSR, simple cooling of the sample to the desired initial temperature and a linear increase in T after excitation are sufficient to obtain TSC and TSL glow curves. Some techniques require more elaborate heating cycles, the details of which depend on the relaxation mechanism under study and on whether it is necessary to discriminate between simnltaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated conductivity (see Chapter 2). 

Thermally Stimulated Depolarization Currents in Amorphous Chalcogenides... 

The po and Pi ratio in equation (2.3) determines which of two factors—namely, equilibrium or nonequilibrium (due to emission from traps) carriers—dominate in the relaxation process. That is, the depolarization current contains two maximum one is related to release of carriers from trap the origin of the other lies in the change of conductivity with temperature [14-18]. Although only one of the peaks mentioned contains information about trap parameters, it is possible to discriminate between simultaneously occurring processes, e.g., thermally stimulated depolarization and thermally stimulated dielectric relaxation. 

FIGURE 18.11 Thermally stimulated depolarization currents of PVP K30 demonstrating two different global relaxation peaksPi is the (5-relaxation peak (representing molecular motion belfry, and P2 is the a-relaxation peak (representing mobility ). 

Shmeis, R. A., Z. V fong, and S. L. Krill. 2004. A mechanistic investigation of an amorphous pharmaceutical and its solid dispersion, Part I Acornparative analysis by thermally stimulated depolarization current and differential scanning calorimetl harm Re 1 2025-2030. 

The dielectric behavior of PMCHI was studied by Diaz Calleja et al. [210] at variable frequency in the audio zone and second, by thermal stimulated depolarization. Because of the high conductivity of the samples, there is a hidden dielectric relaxation that can be detected by using the macroscopic dynamic polarizability a defined in terms of the dielectric complex permittivity e by means of the equation ... 

G. Gallego Ferrer, M. Monleon Pradas, J. L. Gomez Ribelles, and P. Pissis, Swelling and thermally stimulated depolarization currents in hydrogels formed by interpenetrating polymer networks, J. Non-Cryst. Solids 235-237, 692-696 (1998). 

Thermally stimulated depolarization currents are detected in a sample first cooled to low temperature in a capacitor with shorted electrodes, then warmed slowly with the electrodes connected to a sensitive d.c. electrometer. In this way the dielectric relaxation processes occurring in the sample are displayed separately, according to their activation energies and barrier heights, during the scan over temperature. 

Abbreviations DEA, dielectric analysis >OC. degree of crystallinity DSC, di erential scanning calorimetry LM, local mobility (secondary relaxations) SR, structural relaxation 7g, determination of glass transition temperature TSDC. thermally stimulated depolarization current spectroscopy XRD, X-ray difTractometry. Source Adapted from Ref. 15. 

TSDCT thermally stimulated depolarization current technique... 

The principal characteristics of the triboelectret state in polymers recorded experimentally are i) the efficient surface charge density (ESCD) value and ii) the thermally stimulated depolarization (TSD) current spectrum, i.e. the discharge current dependence of the electret on its heating temperature. The analysis of TSD spectra helped to estimate the parameters of the triboelectret state, including the homo- to heterocharge relation in a dielectric, activation energy of the charge relaxation processes, relaxation time and others. 

Figure 21. Schematic diagram of an apparatus for thermally stimulated depolarization current (TSDC) experiments.

The electrical properties, i.e, volume resistivity, dielectric permittivity and dielectric loss factor, as well as thermally stimulated depolarization current, were measured on polypropylene-polycarbonate (PP-PC) blends. The results confirm the existence of some interactions between the non--compatible components of PP-PC blends. 

Thermally stimulated depolarization (TSD) current was also measured using an Ekco type 616B electrometer. The samples were polarized at the following conditions polarization temperature 150 0, polarization time 2 min, electric field 8 kV/cm. The heating rate was 1°C/min. 

Bisphenol-A carbonate has been widely studied by dielectric [8-26], dynamic mechanical [27 31] and thermally stimulated depolarization (TSD) [10- 13 32 35] techniques. However, differences in the compositions of the materials studied, and in their thermal history and pretreatment, have led to apparently conflicting results being reported in the literature, as discussed in detail in a recent paper [6]. In the present study contour maps of complex relative permittivity for both basic and u.v.-resistant grades of LEXAN have been obtained over an extended range of experimental conditions using a single apparatus, with each grade of material subject to the same thermal history. 

Thermally stimulated depolarization (TSD), e.g. [101]. TSD is applicable to dielectric polymers that are electrets. Depolarization currents are monitored and these currents are related to the relaxation of electric charges in the sample as a function of the number of events that take place in the sample at the molecular and supermolecular level. 

Fig. 9.8. Thermally stimulated depolarization (TSDp) curves of Mg(OH)2, using H2-saturated Pd electrodes, at different polarization temperatures, Tp ,. The TSDp minimum at 118 K is caused by H charge carriers, i.e. protons in the conduction band, reaching a density of 0.9 x 10 mol in good agreement with the theoretical values of 1.5 x 10 mol" (Eqn 9.14a), assuming E = 2 eV (with kind permission of Verlag Chimie).

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