Thermally stimulated current peaks

For observing the various thermally stimulated current peaks, the mobile units of the sample are oriented by a static electrical field (E). When the polarization... 

The electrical current of a coplanar interdigilal gold/LPPP/gold device is space charge limited due to p-type charge earner traps localized in the bandgap [28]. This can be inferred from the field dependence of the dark current at room temperature. The thermally stimulated current spectrum exhibits two peaks, corresponding to two distinct trap levels ,1 and ,", which can be calculated from the rise in current, /, below the peak temperature ... 

As to be noticed in Fig. 5, the 5°C peak shows considerable tailing to the lower temperature region. This means the presence of some amount of shallower trapps in the polymer. As shown by Fig. 7, the thermally stimulated current from these traps becomes somewhat clearer with slower heating rate. However, the density of these traps is too small to analyze. Pai and Patora show also the presence of such kind of traps [3), (4). 

Figure 5. Thermally stimulated current of poly-N-vinylcarbazole showing clear peak at 5°C. Peaks increase with the increase of UV illumination of the total light of the Hg lamp of Figure 3, at —150°C in vacuo 5—40 min. Collecting voltage 30V (Au +) heating rate 3.3°C/min. Films were cleaned prior to the measurement.

Figure 6. Typical experimental curve of the thermally stimulated current in poly-N-vinylcarbazole near 5°C peak...

As already stated and shown by Fig. 8, when the film is irradiated at room temperature in air by UV-light prior to the measurement of thermally stimulated current, the current peak at 5°C increased considerably, i.e. the population of the 0.56 eV traps increased by UV-irradiation in air. The photocarriers... 

Sanda et al. (1988) measured thermally stimulated currents in DEH doped PC. A peak in the spectrum was observed at 0.50 eV. The peak was attributed to the thermal excitation of holes from DEH donor states. The activation energy is in good agreement with values later derived from conventional photocurrent transient measurements by Schein and Mack (1988), Mack et al. (1989), and Kitamura and Yokoyama (1991). A secondary peak was observed at higher energies and attributed to trap states. 

Another way of investigating the depth in energy to which charges are trapped (a measure of the stability towards thermal decay) may be obtained by thermally stimulated currents (TSC). In this technique, electrodes on the two sides of the electret are connected via a sensitive current meter and the specimen is then heated at a constant, slow rate (1 Cmin-1, say). Discrete current peaks are observed as a function of temperature as successively more deeply trapped charges are released (Fig. 7.22). Dipolar relaxation may also give peaks in the TSC spectrum (van Turnhout, 1975). 

Poling and heating profiles used to measure thermally stimulated current. (1) Standard measurement, (11) partial heating method or peak cleaning method (111) thermal sampling method... 

Figure 7.34 Thermally stimulated current expenment performed in the PL-DETA cell. Strong interfacial and poling temperature peaks are present as well as those predicted from dielectric...

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]. 

Thermally Stimulated Current (TSC) studies allow us to investigate the transition spectra of amorphous polymers. The relaxation modes observed around and above the glass transition T have common features (1) The TSC peak isolated around Tg corresponds to a distribution of relaxation times following an Arrhenius equation. The width of the distribution characterizes the distribution of the order parameter. (2) The TSC peak observed some 50° above Tg is well described by a Fiilcher-Vogel equation. This mode, which can also be distributed, has been associated with the dielectric manifestation of the liquid-liquid transition (Ty). 

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 ). 

Moura Ramos J, Correia Nl, Taveira-Marques R, Collins G (2002) The activation eneigy at Tg and the fragility index of indomethacin, predicted from the influence of the heating rate on the temperature position and on the intensity of thermally stimulated depolarization current peak. Pharm Res 19 1879-1884... 

Some secondary relaxations of the components in thermoplastic AIPNs have been investigated by thermally stimulated depolarization current (TSDC) techniques and thermally stimulated conductivity (TSC) measurements [10,11]. It was found that upon addition of S-co-AA to CPU, the secondary and 3 CPU peaks (at ca. -140 °C and ca. -100 °C, respectively) shift slightly to lower temperatures, i.e., the corresponding relaxations become faster, these shifts being more pronounced at low S-co-AA contents. The shifts can be related to physical interactions between the IPN components and to their partial miscibility. Rizos et al. [15] have shown that as a result of such interactions, changes in the local free volume may occur, affecting the secondary relaxation times. The same changes in the [3 relaxation of PU have been found in polyurethane/polystyrene IPNs by Pandit and Nadkarni [16]. 

---