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

Abstract

This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent repository link: ABSTRACTElectroluminescence excitation mechanisms have been investigated in epoxy resin under divergent and uniform field situations. Metallic wires embedded in the resin were used to produce field divergence whereas film samples were metallised to obtain a uniform field. Electroluminescence under divergent field was stimulated by an impulse voltage. Light was emitted on the positive and negative fronts of the square pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without polarity dependence. There was evidence of space charge accumulation around the wires in multiple-pulse experiments. Charge injection and extraction occurring at both fronts of the pulse provide the condition for EL excitation. Further excitation of the EL during the plateau of the voltage pulse is prevented by the opposite field of the trapped charge. Field computation with and without space charge supports the proposed interpretation. A DC voltage was used for the uniform field experiments. A continuous level of electroluminescence is found at 175 kV/mm. Charging/discharging current measurements and space charge profile analyses using the pulsed electro-acoustic (PEA) technique were performed at different fields up to the EL level. Dipolar orientation generates a long lasting transient current that prevents the conduction level being reached within the experimental protocol (one hour poling time). The continuous EL emission is nevertheless associated with a regime where the conduction becomes dominant over the orientational polarization. Polarization and space charge contribute to the PEA charge profiles. Homo-charge injection at anode and cathode is seen at 20 kV/mm and a penetration of positive space charge in the bulk is detected above 100 kV/mm, suggesting an excitation of the continuous EL by bipolar charge recombination.2 INTRODUCTIONElectroluminescence (EL) will be defined in our context as the light emitted from a solid material when subjected to an electric stress. It results from the radiative relaxation of excited states that are created by the application of the field. These states can be delocalized when dealing with EL of semiconducting materials [1,2], or highly localized when dealing with large band gap materials such as polymers used as insulation in electrical engineering [3]. In the latter case, it has been known for a long time that the existence of internal gas-filled cavities can also be at the origin of a light emission due to the excitation of gaseous molecules accompanying discharge occurrence [4]. In this paper, we will focus on the investigation of the light emitted by the material by taking care not to introduce any internal cavities of a size suitable to sustain partial discharges. This needs careful control during sample preparation.EL detection in a given material is evidence of the generation of excited states of energy bet...

Decay of space charge in a glassy epoxy resin following voltage removal

“…These signals show that a positive homocharge peak has developed in the epoxy resin over the time under electrical stress see figure 1. These results are qualitatively the same as those found previously at much higher applied fields (18kV/mm to 180kV/mm) [13]. It seems clear from the position of the peaks and the measurements made during voltage application that the injected charge did not move very far into the epoxy film before being trapped.…”

Transient and steady-state currents in epoxy resin

“…Above this threshold, the I -E variations could be fitted as well by an E 4 law as by an exponential dependence in sinh(eEλ/2kT ) corresponding to charge hopping between neutral traps at distance λ. In [2], 1 h charging and discharging currents at ambient temperatures have been reported and the transient were attributed to dipole orientation. Lovell [3] reported transient and steady-state currents measured at various temperatures below T g .…”

Space charge behaviour in an epoxy resin: the influence of fillers, temperature and electrode material

“…Several literature reports support homocharge injection in epoxy resins. Bipolar injection combined to slow polarization has been reported by Griseri et al (2002) in a somewhat different epoxy resin. Iizuka et al (1999) have reported similar behaviour in an epoxy resin, with some effect of relative humidity.…”

“…Published reports focussed on the effect of partial discharges and electrical treeing Dodd 1995, Champion et al 1996), but there is comparatively scarce data on the possible influence of internal space charges generated by the thermo-electric stress itself (Guillermin et al 2004, Griseri et al 2002, Iisuka et al 1999. Conclusions of these investigations converge towards consistent results, picturing essentially injection of homocharge close to the electrodes.…”