Depolarization currents, dielectric

Dielectric Polarization under electric Hied Temperature step or scan Depolarization current... 

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. 

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. 

The global t.s.c. is a convoluted spectrum of all dielectrically active relaxations excited between Tp and T0. The relaxation time r (T) is related to the measured depolarization current iT by ... 

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. 

As shown in Figure 11.1, to obtain a complex TSC spectrum, a static electric field E is applied to the sample at a polarization temperature labeled Tp for a time tp, which is necessary to obtain polarization saturation, i.e., the equilibrium polarization. Afterward, the sample is cooled down to a temperature T0 in such a way that the dielectric relaxation proceeds extremely slowly, so that after removal of the field the sample retains a frozen-in polarization. The depolarization current, Id, caused by the return to equilibrium of dipolar units, is then recorded by increasing the temperature at a constant rate from T0 up to the final temperature Tf, where Tf > Tp. The plot of Id as a function of temperature is a complex TSC spectrum. 

In the work described here, TSC spectroscopy experiments have been performed using a TSC and Relaxation Map Analysis (RMA) dielectric spectrometer developed in our laboratory and available from Settaram. The TSC cell is shown in Figure 11.3. Powdered compounds (some tens of milligrams) were compressed into flat disks of 0.8-cm diameter and approximately 1 mm thick. The sample was placed between stainless steel disks, which ensures electrical contact, thus allowing polarization and current measurement. The maximum applied voltage was 500 V, with an accuracy of 0.1 V. The depolarization current was recorded using a very... 

While in lumped-circuit methods the dielectric response is measured in the frequency domain, following the ajpplication of a sinusoidal alternating electrical field, for frequencies below 10 Hz it is advantageous to cany out the measurements in the time domain because it is less time consuming. The polarization or depolarization current following the application of a step-like electrical field is measured as a function of time. 

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), 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. 

The actuation principle of EAPap actuators was investigated in terms of ion migration and piezoelectric effects. To physically investigate the actuation mechanism, several tests were performed. TSC measurement showed a linear relationship of depolarized current with the applied electric field, indicating dipolar orientation. By comparing XRD spectra before and after electrical activation, the possibility of recrystallization in the cellulose material was observed. Dielectric property measurement indicated a dependence of the dielectric constant on fiber direction. Thus, we conclude that the combination of piezoelectric effect and ionic migration effect might be the actuation principle. 

Characterization of Polymer Blends by Dielectric Spectroscopy and Thermally Simulated Depolarization Current... 

In series of publications [25,27,29,35-40] several methods were used for eharaeterization of the microphase structure of the semi-IPNs studied. Small-angle X-ray seattering (SAXS), differential scanning calorimetry (DSC) [27, 35-37], dynamic mechanical thermal analysis (DMTA) [27, 30-32], dielectric relaxation spectroseopy (DRS), and thermally stimulated depolarization currents (TSDC) [25, 39, 40] measurements have shown that pure PCN is characterized by a typical homogeneous structure, but for segmented LPU the microphase separation on the level of hard and soft domains due to their thermodynamic immiscibUity was denoted. As for semi-IPNs, the destruction of the microphase separated morphology of LPU was observed and the microphase separation between PCN and LPU phases, expected from the difference of solubility parameters, was not found. 

An alternative method to observe dielectric properties is termed thermal stimulated currents (TSC). This method involves polarization of a sample at high temperature (relative to Tg) and quenching to a temperature where depolarization is kineticaUy prevented in the time scale of the experiment. The temperature is then increased and the depolarization current is measured, yielding peak values associated with polymer transitions analogous to t", E" and tan S values obtained by conventional dielectric and dynamic mechanical measurements. The TSC spectra can reveal secondary relaxations, glass transitions and liquid or crystalline phase transitions and hquid crystalhne phase transitions. TSC has been applied to PBT/PC and PA6/ABS blends to study the intermixing of the components of the respective blends [58]. The TSC method is described in several references [59-61]. 

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