02_dissado_elf_mechanism - Griseri e t ai.:...

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150 Griseri et ai.: Electroluminescence Excitation Mechanisms in an Epoxy Resin Under Divergent Electroluminescence Excitation Mechanisms in an Epoxy Resin Under Divergent and Uniform Field V. Griseri, L.A. Dissado, J.C. Fothergill University of Leicester, Department of Engineering, Leicester, LE1 7RH England G. Teyssedre and C. Laurent Universitt Paul Sabatier, Laboratoire de Gtnie Electrique, 118 route de Narbonne 31062 Toulouse, Cedex, France ABSTRACT Electroluminescence (EL) 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. EL under divergent field was stimulated by a pulse volt- age. Light was emitted on the positive and negative fronts of the pulses when the field exceeded 20 kV/mm at the wire surface, with equal intensity and without po- larity 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 experi- ments. A continuous level of EL 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. Homocharge 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 continu- ous EL by bipolar charge recombination. 1 INTRODUCTION sustain partial discharges. This needs careful control dur- LECTROLUMINESCENCE (EL) will be defined in E 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 delocal- ized when dealing with EL of semi-conducting 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 investi-
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02_dissado_elf_mechanism - Griseri e t ai.:...

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