Optical emission due to space charge effects in electrically stressed polymers

Electrical wear-out and dielectric breakdown of insulating polymers involve dissipation to the lattice of the energy of mobile or trapped space charges. Optical emissions are proof of such energy releases and can be used to define the onset of electrical aging and to investigate the dissipation mechanisms. They also could provide a basis for monitoring the degradation rate. The situation considered is that of a material subjected to a voltage stress in the absence of any gas discharge, i.e., the light does not originate from a gaseous plasma. Depending on the type of material and experimental configuration, light emission can be due to excited states of the solid itself and/or of dissolved or adsorbed gas molecules. The polymers of concern are practical materials used as electrical insulation in cables, capacitors, motors, transformers, etc. This review attempts to bring together the published data on electroluminescence of insulating polymers and to discuss the relationship between light emission, space charge, and polymer degradation.