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DK2041_10 - 10 THERMALLY STIMULATED PROCESSES hermally...

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10 THERMALLY STIMULATED PROCESSES T hermally stimulated processes (TSP) are comprised of catalyzing the processes of charge generation and its storage in the condensed phase at a relatively higher temperature and freezing the created charges, mainly in the bulk of the dielectric material, at a lower temperature. The agency for creation of charges may be derived by using a number of different techniques; Luminescence, x-rays, high electric fields corona discharge, etc. The external agency is removed after the charges are frozen in and the material is heated in a controlled manner during which drift and redistribution of charges occur within the volume. During heating one or more of the parameters are measured to understand the processes of charge generation. The measured parameter, in most cases the current, is a function of time or temperature and the resulting curve is variously called as the glow curve, thermogram or the heating curve. In the study of thermoluminescence the charge carriers are generated in the insulator or semiconductor at room temperature using the photoelectric effect. The experimental aspects of TSP are relatively simple though the number of parameters available for controlling is quite large. The temperature at which the generation processes are catalyzed, usually called the poling temperature, the poling field, the time duration of poling, the freezing temperature (also called the annealing temperature), and the rate of heating are examples of variables that can be controlled. Failure to take into account the influences of these parameters in the measured thermograms has led to conflicting interpretations and in extreme cases, even the validity of the concept of TSP itself has been questioned. In this chapter we provide an introduction to the techniques that have been adopted in obtaining the thermograms and the methods applied for their analysis. Results obtained in specific materials have been used to exemplify the approaches adopted and indicate
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the limitations of the TSP techniques 1 ' 2 ' 3 . To limit the scope of the chapter we limit ourselves to the presentation of the Thermally Stimulated Depolarization (TSD) Current. In what follows we adopt the following terminology: The electric field, which is applied to the material at the higher temperature, is called the poling field. The temperature at which the generation of charges is accelerated is called the poling temperature and, in polymers, mostly the approximate glass transition temperature is chosen as the poling temperature. The temperature at which the electric field is removed after poling is complete is called the initial temperature because heating is initiated at this temperature. The temperature at which the material is kept short circuited to remove stray charges, after attaining the initial temperature and the poling field is removed, is called the annealing temperature. The annealing temperature may or may not be the initial temperature.
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