Behaviour of space charge correlated with electroluminescence in cross-linked polyethylene

A fundamental study to correlate the transient space charge and electroluminescence (EL) emission in cross-linked polyethylene under high dc stress (ca 136 kV mm(-1)) is proposed. Space charge profiles were analysed by the pulsed electro-acoustic method under transient conditions in order to infer from where carriers are generated, how they are transported and how the space charge decays after being formed. The information is used to interpret transient EL signals obtained under similar stress conditions. It is shown that both positive and negative charges are injected from the electrodes. Injected positive and negative charges propagate with similar speed and meet in the bulk about 600 s after the application of the step voltage. A strong luminescence is observed soon after the step voltage application and decays quickly within several tens of seconds. Subsequently, the EL intensity shows a gradual increase. The first intense emission was interpreted as an interfacial phenomenon, probably a hot electron effect, associated with heterogeneities in the electrode/insulation interface. The gradual increase of the EL was correlated with the recombination of two kinds of carriers injected from both electrodes. Periodical application of the stress leads to decreasing EL intensity as a function of time, which was the result of cumulative relaxation of the field around the surface heterogeneities.