DK2041_07 - FIELD ENHANCED CONDUCTION T he dielectric...

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FIELD ENHANCED CONDUCTION T he dielectric properties which we have discussed so far mainly consider the influence of temperature and frequency on and s" and relate the observed variation to the structure and morphology by invoking the concept of dielectric relaxation. The magnitude of the macroscopic electric field which we considered was necessarily low since the voltage applied for measuring the dielectric constant and loss factor are in the range of a few volts. We shift our orientation to high electric fields, which implies that the frequency under discussion is the power frequency which is 50 Hz or 60 Hz, as the case may be. Since the conduction processes are independent of frequency only direct fields are considered except where the discussion demands reference to higher frequencies. Conduction current experiments under high electric fields are usually carried out on thin films because the voltages required are low and structurally more uniform samples are easily obtained. In this chapter we describe the various conduction mechanisms and refer to experimental data where the theories are applied. To limit the scope of consideration photoelectric conduction is not included. 7.1 SOME GENERAL COMMENTS Application of a reasonably high voltage -500-1000V to a dielectric generates a current and let's define the macroscopic conductivity, for limited purposes, using Ohm's law. The dc conductivity is given by the simple expression C7=— AE

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330 Chapter? where a is the conductivity expressed in (Q m)" 1 , A the area in m 2 , and E the electric field in V m" 1 . The relationship of the conductivity to the dielectric constant has not been theoretically derived though this relationship has been noted for a long time. Fig. 7.1 shows a collection of data 1 for a range of materials from gases to metals with the dielectric constant varying over four orders of magnitude, and the temperature from 15K to 3000K. Note the change in resistivity which ranges from 10 26 to 10~ 14 Q m. Three linear relationships are relationship is given as noticed in barest conformity. For good conductors the log p + 3 log e' = 7.7 For poor conductors, semi-conductors and insulators the relationship is (7.2) 2 O I 2 X o >-" H > (/> </> UJ (E TITANATES FERRO-ELECTRICS © CARBON AT 0°C GRAPHITE AT 0°C COPPER AT 500°C SILVER AT 15° K GLYCERINE / AT 800° C Sn-Bi TUNGSTEN AT 3500°K / SILVER AT 0°C SUPERCONDUCTORS COPPER AT 15° K (7.3) I0 2 I0 3 I0 4 DIELECTRIC CONSTANT Fig. 7.1 Relationship between resistivity and dielectric constant (Saums and Pendleton, 1978, with permission of Haydon Book Co.) Ferro-electrics fall outside the range by a wide margin. The region separating the insulators and semi-conductors is said to show "shot-gun" effect. Ceramics have a higher dielectric constant than that given by equation (7.3) while organic insulators have lower dielectric constant. Gases are asymptotic to the y-axis with very large resistivity and s' is close to one. Ionized gases have resistivity in the semi-conductor region.
From the definition of complex dielectric constant (ch. 3), we

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This note was uploaded on 03/03/2010 for the course POWER 332 taught by Professor Dr during the Spring '10 term at Ain Shams University.

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DK2041_07 - FIELD ENHANCED CONDUCTION T he dielectric...

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