Electrical conduction and breakdown in insulating polymers

This paper describes our theoretical and experimental approaches to the mechanisms responsible for electrical conduction at low and high fields, electrical aging, partial discharge and breakdown phenomena in insulating polymers. Electrical conduction always involves carrier injection from electrical contacts and subsequently dissociative trapping and recombination. When injected carriers make a transition from one energy state to a lower one, an energy equal to the energy difference between the two states will be evolved, and this energy could be of the order of 4 eV and it will be dissipated in the breaking of the polymer bonds and the creation of free radicals or low weight molecules and hence new traps. It is this gradual degradation process that leads to electrical aging. Depending upon the band structure and the potential barrier profile of the carrier injecting contracts, the normally electron-dominant conduction may change to a hole-dominant conduction at high fields, such as in polyethylene films. Electrical conduction is filamentary at high fields. Internal discharges such as electrical treeing and breakdown are initiated by impact ionization within the low-density regions created by electron (or hole) trapping or recombination, and then followed by the development of thermal instability in the high current density region of the main conduction filaments (or channels). This process then leads to a rapid increase in carrier multiplication and final destruction of the material inside the filaments (or breakdown channels). Methods for suppressing carrier injection and for reducing energy release during carrier trapping or recombination are also briefly discussed.