Influence of ageing on space charge and electroluminescence of epoxy resin

The occurrence of light emission under high ac fields, known as electroluminescence (EL), is attributed to the interaction between mobile and trapped charges within the dielectric. Images of EL for 200 micron thick virgin and aged epoxy samples at different voltages were recorded using a CCD camera. The images were obtained by applying ac electric fields across the samples and exposing the CCD for 600s. The images were constructed from the read-out data using the same grey level for comparison. Images of the virgin and aged samples showed a difference in the emission with the virgin sample producing a more homogeneous emission compared with a rather inhomogeneous emission from the aged sample. Comparison of EL with field strength showed virgin samples having an intensity level around 3 times greater than aged samples at 6kV. The EL emission follows an exponential increase due to the greater injection of charge carriers into the material. The reduction in EL intensity may be related to molecular degradation of the resin causing a greater trapping of charge carriers at the interface and therefore a reduction in the electric field, resulting in less charge injection. This reduction in EL could be due to some chemical changes in samples that cause deep traps, which require a higher electric field for injection. Accordingly there is not much increase in the EL emission compared to the one seen in LDPE, for example. Also, the reduction in EL might be due to some physical changes in the surface which limit charge injection, due to an increase in shallow trapping sites once they become saturated.