Static electrification and conductivity

Strella (1970, 1971) based his triboelectric series on contact potential differences. Table 11.5 shows a reproduction of it. 

Electric conductivity, a (in S/cm or in Q-1 cm-1), of materials in general extends over a wide range, from 10-20 to 106 S/cm (see Fig. 11.8). This range is subdivided into conductors (cr = 104-106), semi-conductors ( x = 10-4-102) and insulators (a = 10-2°-10-5). The tremendous 

9 Schematic representation of the band theory of electronic conduction (A) insulators (B) semi-conductors and (C) conducting materials. The hatched bands are allowed bands, the cross hatched bands are occupied. 

Metals are the classical conductors. Most organic solids, and notably polymers, are insulators by nature. In the usual polymers the electrical resistance is very high their conductivity probably results partially from the presence of ionic impurities, whose mobility is limited by the very high viscosity of the medium. 

At sufficiently low temperatures (i.e. below a typical transition temperature Tc most metals lose their electrical resistance completely and become superconductors. Also a number of inorganic semi-conductors, viz. complex oxides of two- and three-valent metals, become superconductors below their Tc, which may be as high as 100 K or more. 

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