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

Abstract

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.

DC conduction current, dielectric strength and electroluminescence of standard, nanofilled and microfilled polyetherimide varnishes

“…This is in good agreement with a previous work performed with 50 mthick PEI commercial films [7]. Two different mechanisms may induce EL under a DC high field: recombinations of charges facing each other in the sample or impact excitation/ionization of luminescent centres by hot charge carriers.…”

“…Transition voltages from ohmic to SCLC regime of filled varnishes have been logically found to be lower than for pure varnish (Table 1). The slopes of the high field conduction current curves for both standard (already found to be higher than 2 in PEI [7]) and filled varnishes are higher than two. This may be explained by a charge carriers electric field dependent mobility in the high field region.…”

“…Between two consecutive applied voltages, the samples have been short-circuited (short-circuit duration = polarization duration). The electroluminescence measurements have been carried out by using the experimental setup described in [7]. They have been performed in the same electric field range as conduction measurements in order to find any correlation.…”

Electroluminescence excitation mechanisms in an epoxy resin under divergent and uniform field

“…Analysis of conduction processes and investigation of the field dependence of space charge profiles provide a way to tackle the problem. Combined EL and current measurements performed on different polymeric materials have shown a good correlation between the onset for permanent EL excitation and a change in the conduction mechanism as denoted by a steep increase of the slope of the current-field characteristic in a log-log representation [27]. Similarly, investigation of the spatial distribution of the space charge at different fields revealed that the continuous EL is seen after bipolar injection from cathode and anode, thereby pointing towards a mechanism of excitation due to charge recombination [28,29].…”

Investigating electroluminescence excitation by space charge measurements under transient stress in cross linked polyethylene

“…In some way, it could explain why results are not fairly reproducible regarding the quantitative aspects of the signal being measured. Under AC stress, part of the signal being detected has been explained by surface plasmon resulting from collective excitations of carriers on the metallic electrodes [4]. Such kind of emission is localized on 'hot spots' in the sample plane.…”