Thermally stimulated polarization

A variation on TSC is thermally stimulated polarization current (TSPC) spectroscopy. In the case of TSPC the sample is mounted as in TSC. However, the sample is immediately quenched to Tq without... 

Surface conditions influence partial discharge behavior as well. Thermally stimulated polarization (TSP) and depolarization (TSD) properties of PP/CaC03/nonionic surfactant systems [12] have been studied. TSP curve of a simple PP/CaC03 composite, containing no surfactant, stored under ambient... 

TSC is a technique for detecting the transitions that depend on changes in the mobility of molecular scale dipolar structural units. TSC is based on the ability of polar molecules to be moved by an electrostatic field. Two types of current are generated thermally stimulated polarization current (TSPC) and thermally stimulated depolarization current (TSDC). TSPC is generated when dipolar structures orient in a static electric field with increasing in the temperature. TSDC is generated because of the relaxation of the previously polarized molecules as reported in Fig. 5. 

Keywords thermally stimulated ciurent (TSC), thermally stimulated polarization (TSP), thermally stimulated depolarization (TSD), Williams-Landel-Ferry (WLF) equation, relaxation, theimorheology, activation energy, ionic mobility, interfadal relaxation. 

Short temperature windows of polarization steps Thermally stimulated polarization current (TSPC)... 

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 ... 

Thermally stimulated dielectric relaxation of a solid in a polarized (electret) state usually was selected for the following reasons ... 

The method of field-induced thermally stimulated current (FITSC) consists of measuring— according to a definite (usually linear) heating scheme—the currents generated by the buildup and release of a polarized state in a high-resistivity solid... 

Equation (2) expresses the model of the crystal polarization used in the molecular modeling of PVDF reported in this chapter, where is the dipole of each repeat unit of the single chain in vacuum, Ap is the change in dipole moment of the repeat unit of the chain in going from the vacuum environment to the environment of the packed crystal and (cos tp) is the attenuation of the dipole moment of the repeat unit along the fe-axis due to thermally stimulated oscillations about thec-axis. Ap is directly related to the local electric field (Eioc, V/m) through the repeat unit polarizability (ot, m ) ... 

When the film is short-circuited and heated to high temperatures at which the molecules attain a sufficiently high mobility, a current is observed in the external circuit. This phenomenon is called pyroelectric effect, thermally stimulated current, or, when the film has been polarized by a static field prior to measurement, depolarization current. The conventional definition of pyroelectricity is the temperature dependence of spontaneous polarization Ps, and the pyroelectric constant is defined as dPJdd (6 = temperature). In this review, however, the term will be used in a broader definition than usual. The pyroelectric current results from the motion of true charge and/or polarization charge in the film. Since the piezoelectricity of a polymer film is in some cases caused by these charges, the relation between piezoelectricity and pyroelectricity is an important clue to the origin of piezoelectricity. 

Thermally stimulated methods are based on the fact that a sample frozen in a deformed or polarized state gradually returns to equilibrium as various types of molecular motions set in with increasing temperature. 

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. 

Previous findings on the actual microemulsion are given in references (9 17) where the system s phase map vs.concentration in the temperature interval (-20°C + 80°C), viscosity measurements,dielectric analysis of liquid samples against both concentration and frequency, the thermally stimulated dielectric polarization release (TSD), electro-optical phenomena, light scattering, Raman spectroscopy and sound propagation investigations are reported. 

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. 

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).

Thermally stimulated current (TSC) spectroscopy is used to characterize the relaxation processes and structural transitions occurring in samples that have been polarized at a temperature greater than the temperature where molecular motion in the sample is enhanced and subsequently quenched so that the high mobility state is frozen. On heating the sample at a controlled rate, depolarization of the polymer electrets (molecular or ionic dipoles, trapped electrons, mobile ions) occurs and the oriented dipoles, frozen in the quenched sample, relax to a state of thermal equilibrium. This relaxation process is observed as a depolarization current, which is typically of the order of picoamperes, and is referred to as the thermally stimulated current. 

Thermally stimulated current analysis (TSCA) is another member of the TEA group of techniques. In TSCA, the sample is subjected to a constant electric field and the current that flows through the sample is measured as a function of temperature. The mode of operation usually involves heating the sample to a high temperature under the applied field followed by quenching to a low temperature. This process aligns dipoles within the sample. The polarization field is then switched off and the sample is reheated and the current flow resulting from the relaxation of the induced dipoles back to the disordered state is monitored. 

For the determination of the characteristic parameters of the process i.e. the activation energy and the relaxation time factor, the initial rise method is mostly used. This method is based on the fact that for restricted conditions [ 15], the current density during a thermal stimulated measurement can be expressed by Arrhenius equation. From TSDC curves in Fig. 12, under polarization temperature of 0 °C, the... 

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