DK2041_11 - 11 SPACE CHARGE IN SOLID DIELECTRICS T his...

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Unformatted text preview: 11 SPACE CHARGE IN SOLID DIELECTRICS T his chapter is devoted to the study of space charge build up and measurement of charge density within the dielectric in the condensed phase. When an electric field is applied to the dielectric polarization occurs, and so far we have treated the polarization mechanisms as uniform within the volume. However, in the presence of space charge the local internal field is both a function of time and space introducing non- linearities that influence the behavior of the dielectrics. This chapter is devoted to the recent advances in experimental techniques of measuring space charge, methods of calculation and the role of space charge in enhancing breakdown probability. A precise knowledge of the mechanism of space charge formation is invaluable in the analysis of the polarization processes and transport phenomena. 11.1 THE MEANING OF SPACE CHARGE Space charge occurs whenever the rate of charge accumulation is different from the rate of removal. The charge accumulation may be due to generation, trapping of charges, drift or diffusion into the volume. The space charge may be due to electrons or ions depending upon the mechanism of charge transfer. Space charge arises both due to moving charges and trapped charges. Fig. 11.1 shows the formation of space charge due to three processes in a dielectric that is subjected to an electric field 1 . (a) The electric field orients the dipoles in the case of a homogenous material and the associated space charge is a sharp step function with two peaks at the electrodes. (b) Ion migration occurs under the influence of the electric field, with negative charges migrating to the positive electrode and vice-versa. The mobility of the various carriers 515 are not equal and therefore the accumulation of negative charges in the top half is random. Similarly the accumulation space charge due to positive charges in the bottom portion is also random and the voltage due to this space charge is also arbitrary. The space charge is called "heterocharges". (c) Charges injected at the electrodes generate a space charge when the mobility is low. The charges have the same polarity as the electrode and are called "homocharges." V o Fig. 11.1 Development of charge distribution p (z) in a dielectric material subjected to an electric field, (a) dipole orientation, (b) ion migration, (c) charge transfer at the interfaces (Lewiner, 1986, © IEEE). A modern treatment of space charge phenomenon has been presented by Blaise and Sarjeant 2 who compare the space charge densities in metal oxide conductors (MOS) and high voltage capacitors (Table 11.1). The effect of moving charges is far less in charging of the dielectric and only the trapped charges influence the internal field. 11.2 POLARONS AND TRAPS The classical picture of a solid having trapping sites for both polarities of charge carriers is shown earlier in Fig. 1.11. The concept of a polaron is useful in understanding the change in polarization that occurs due to a moving charge.change in polarization that occurs due to a moving charge....
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DK2041_11 - 11 SPACE CHARGE IN SOLID DIELECTRICS T his...

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