Classical Interpretation of Conduction Currents in Crosslinked Polyethylene (XLPE) and a Comparison with Ideal Polyethylene

Abstract

Recent experimental conduction current data are numerically analyzed in terms of the established Richardson/Dushman-Schottky (1914) expression for electron emission rates, from a metal cathode, multiplied by the traditional Townsend (1915) derived electron avalanche sizes due to electron-molecule collision ionization in the bulk insulating dielectric for electric fields up to in excess of 1000 MV/m. Agreement between applied theory and experiment limited to 70 MV/m is excellent. Deduced Townsend primary ionization coefficients in ideal, low and high density polyethylene, in solid, liquid and gaseous ethylenes, air, solid Al2O3 and Nb2O5 are shown diagrammatically. From these it is postulated by calculation that 1.5 - 6% of the interelectrode gap distance in practical XLPE is occupied by air filled space rather than by total ideal polyethylene and that this free space is responsible for considerable ionization in the region of 30 MV/m which prevents 1000 MV/m to be attained. Such high dielectric strength should be achievable if in addition a low electron field emitter cathode such as ultra-pure aluminum is used. It is concluded from this and previous work that bonds between atoms/molecules in polymers are as important as the individual discrete particles by themselves in determining dielectric properties.