Ac and dc electroluminescence in insulating polymers and implication for electrical ageing

Electroluminescence detected in many different polymers under uniform 50 Hz ac and dc fields exhibits a threshold-like character, i.e. the light emission increases supralinearly with the applied voltage above a critical value called the emission threshold. The significance of this threshold is not straightforward. It could be a true physical threshold corresponding to the onset of the luminescence excitation as opposed to a sensitivity-limited threshold without specific significance. This paper discusses this issue on the basis of electroluminescence analysis in several polymers used in electrical engineering, such as polyethylenes, polyesters and polyimides. The analysis relies on the consideration of the field dependence of the electroluminescence and its imaging in the sample plane. It is shown that the threshold-like character under a dc field reveals the onset of the electroluminescence excitation whereas it has no specific meaning under ac stress where the detection is sensitivity-limited. The significance of the luminescence-voltage characteristic, as, regards the electrical ageing and its possible use for diagnostic purposes, is further considered.