Thermal depolarization

Celaschi and Mascarenhas (1977) studied nearly dry lysozyme by elec-tret thermal depolarization, thermal-stimulated pressure, isothermal polarization decay, and thermogravimetry. For a change in temperature of the sample from 250 K to room temperature, desorption of water dipoles was the main process responsible for electrical depolarization. 

Depolarization thermal studies have been reported by Leveque et al. (1981) for partially hydrated keratin, by Bridelli et al. (1985) for lysozyme, and by Anagnostopoulou-Konsta and Pissis (1987) for casein. These studies reveal a rich and complex thermal depolarization spectrum, shown in Fig. 16. It is difficult to explain the spectrum with a model based on the reorientation of noninteracting dipoles with a few distinct relaxation times. Pissis and colleagues (personal communication... 

In thermal optical analysis (TOA), the conversion of plane-polarized light to ellipti-cally polarized light is measured in semi-crystalline polymers. The intensity of the depolarized light transmitted through a sample is a function of the level of crystallinity. Melting and recrystallization phenomena can be analyzed the technique does not appear to be sensitive to glass transitions [88]. The TOA technique is also referred to as thermal depolarization analysis (TDA) and depolarized light intensity method (DLI). 

Fig. 8 First-, second-, and third-order permittivity of 56/44 mol-% P(VDF-TrFE) as a function of temperature. The nonlinearities were measured after poling the sample with E ox = 130 V/pm ( ), after thermal depolarization (O), and after poling with poi = —130 V/mm (A). Bottom right the remanent polarization reconstructed from the first- and second-order permittivity (Reprinted with permission from Heiler and Ploss (1994))...

On the experimental side the observed thermally activated transition from Mu to Mu (Holzschuh et al., 1982 Odermatt et al., 1988) is important in two respects. First, it demonstrates conclusively that, at least in diamond, Mu is metastable with Mu being the stable configuration. Figure 10 shows the lifetime broadening of the Mu signal (depolarization... 

Another unusual feature of CuCl and CuBr is the presence of two Mu centers with nearly identical isotropic hyperfine parameters. One of the centers, Mu7, occurs preferentially at low temperatures but is metastable as evidenced by a thermally activated transition to the second center, Mu77 (see Fig. 13). As the temperature increases, the effects of this transition first appear as an increse of the Mu7 depolarization rate (lifetime broadening). At higher temperatures the transition becomes fast enough so that... 

Typically, nonisothermal relaxation is effectively employed in the studies of thermally stimulated luminescence (TSL), condnctivity (TSC), polarization, and depolarization. 

It is also necessary to note that the success of TSR techniques to obtain information on trapping states in the gap depends on whether or not the experiment can be performed under conditions that justify equation (1.2) to be reduced to simple expressions for the kinetic process. Usually, the kinetic theory of TSR phenomena in bulk semiconductors—such as thermoluminescence, thermally stimulated current, polarization, and depolarization— has been interpreted by simple kinetic equations that were arrived at for reasons of mathematical simplicity only and that had no justified physical basis. The hope was to determine the most important parameters of traps— namely, the activation energies, thermal release probabilities, and capture cross section— by fitting experimental cnrves to those oversimplified kinetic descriptions. The success of such an approach seems to be only marginal. This situation changed after it was reahzed that TSR experiments can indeed be performed under conditions that justify the use of simple theoretical approaches for the determination of trapping parameters ... 

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. 

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