06_Dodd_phase_resolved_cct - 168 Dodd et al.: Phase Resoled

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Dodd et al.: Phase Resol © ed Electroluminescence Measurements in Thin Films of Low Density Polyethylene 168 Phase Resolved Electroluminescence Measurements in Thin Films of Low Density Polyethylene Using a Charge Coupled Device Camera S. J. Dodd, P. L. Lewin and K. I. Wong School of Electronics and Computer Science University of Southampton, Highfield, Southampton, SO17 1BJ, U.K. ABSTRACT () Electroluminescence EL produced by a commercially available additive free low density polyethylene film has been investigated under a 50Hz ac electrical stress. The spatial distribution, spectral characteristics and the phase angle relationship of EL with respect to the 50Hz applied ac voltage were studied using a sensitive Peltier cooled charge-couple device CCD camera. The experimental results from several test samples show that the EL can be divided into two different groups ac - cording to the EL spatial distribution: 1 homogeneous EL and 2 inhomoge - neous EL. At the same applied voltage, the spectra of the homogenous and inho - mogeneous EL were found to be significantly different. Homogeneous EL had max - imum emission intensity in the red and near infra-red ± 700nm , whereas the in - ( homogeneous emission had maximum intensity at shorter wavelengths 550 - 600 ) nm . Phase resolved measurements showed that homogeneous EL always produced ( equal emission intensity in the positive and negative half-cycles symmetric distri - ) bution , whereas, inhomogeneous EL produced an asymmetric intensity distribu - tion. To explain these results, it is suggested that homogeneous emission is associ - ated with charge injection and bipolar recombination in the surface states on both sides of the sample, whereas inhomogeneous emission is associated with field en - hancement at defects on one of the two sides of the film which allow charge to be injected into states more representative of the bulk material. Index Terms — Electroluminescence, thin Films, polyethylene, ac electrical stress. 1 INTRODUCTION LECTRICAL breakdown of polymeric insulation is a E prime concern of the electrical generation, transmis- sion and distribution industries. Polymer based insulation is becoming more popular, particularly for high voltage cables, owing to cost and environmental reasons. Today Ž. polymers such as polyethylene PE , and its cross-linked variant, XLPE are now widely used in cable insulation owing to its relative cheapness and ease of processing. These are gradually replacing oil r paper insulation sys- tems which are susceptible to oil leaks. Electrical breakdown in polymeric materials is often the result of slow material degradation in regions of high elec- trical stress that eventually gives rise to the formation of wx voids 1 . Subsequent electrical discharge activity within Manuscript recei ® ed on 23 No ® ember 2004, in final form 3 June 2005.
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06_Dodd_phase_resolved_cct - 168 Dodd et al.: Phase Resoled

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