Electroluminescence and thermo-stimulated luminescence in an epoxy resin under uniform ac field

The critical field beyond which irreversible evolution of a material occurs and how this limit varies with temperature is of prime importance from the insulation designer´s standpoint. The present work concerns electroluminescence (EL) measurements for an epoxy resin under uniform 50 Hz ac field. We have studied the EL dependency on both temperature (20 to 72°C) and electric field (0 to 16 kV/mm) under ac stress. The material investigated does not contain mineral fillers, and has a glass transition temperature of 65°C. We have observed two regions in the EL-field characteristics: a low field regime in which the EL threshold appears limited by the sensitivity of the detection set-up, and a higher field regime, above a threshold in the range 8 to 14 kV/mm, with a higher slope in the characteristics. We have also observed a strong spontaneous light emission in the absence of applied voltage. The origin and consequences of this on EL are discussed.