05_Laurent_chrgtransport - S. Le Roy et al.: Charge...

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Unformatted text preview: S. Le Roy et al.: Charge Transport and Dissipative Processes in Insulating Polymers: Experiments and Model 1070-9878/05/$20.00 © 2005 IEEE 644 Charge Transport and Dissipative Processes in Insulating Polymers: Experiments and Model S. Le Roy, G. Teyssedre and C. Laurent Laboratoire de Génie Electrique de Toulouse CNRS-Toulouse III University, 118, route de Narbonne 31062, Toulouse, France. ABSTRACT Electroluminescence in insulating polymers signs the existence of potentially harmful excited states because the excess energy can be released through degraded pathways, opening the way to chemical degradation. Excitation mechanisms involve impact excitation/ionization by hot carriers or bi-polar charge recombination. In order to discuss the relative contribution of these processes to light emission in polyethylene, we introduce a numerical model of charge transport postulating a recombination- controlled electroluminescence. Model outputs and experimental measurements are compared for three different protocols of dc voltage application considering especially the simulated recombination rate and the measured light emission. A fairly good agreement is found between model and experiment when considering electroluminescence under constant dc stress. The model fails to describe the measurement under time-varying dc stress suggesting that excitation by hot carriers should be taken into account. Index Terms - Charge carrier processes, electroluminescence, modeling, plastics. 1 INTRODUCTION Polymers submitted to thermo/electrical stress suffer from aging that can drastically affect their functional behavior [1]. Understanding the physical/chemical processes at play during ageing and defining transport regimes in which these mechanisms start to be critical is therefore a prime goal to prevent degradation and to develop new formulations or new materials with improved properties. It is thought that a way to define these critical regimes is to investigate under which conditions (in terms of stress parameters) light is generated in the material by electroluminescence (EL). This can happen through impact excitation/ionization involving hot carriers or upon bi-polar charge recombination [2]. This definition excludes light from partial discharges, which would sign an advanced stage in the degradation process [3]. In a first part of the paper, we shortly discuss the relationship between EL emission and chemical degradation (aging). It is shown that EL is a proof of potentially harmful excited states which can dissipate their excess energy by degraded pathways, opening the way to chemical degradation. In a second part, we briefly review the EL phenomenology under dc, taking polyethylene as a case study. In a third part, a numerical model of charge transport postulating a recombination-controlled electro- luminescence is presented and critically evaluated with special Manuscript received on 17 February 2005, in final form 13 May 2005....
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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05_Laurent_chrgtransport - S. Le Roy et al.: Charge...

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