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Charge packets in insulating polymers have been reported by many groups within the last two decades, especially in polyethylene-based materials. They consist in a pulse of net charge that remains in the form of a pulse as it crosses the insulation. In spite of a variety of characteristics depending on material properties and experimental conditions, one of the puzzling aspects of the packets is their repetitive character until they eventually die away. Several theories have been proposed to explain their formation and propagation. Two of them have the advantage of simplicity and of being physically based, being the existence of an hysteresis loop in the injection mechanism or a negative differential mobility of carriers with the electric field. Based on these descriptions, some progress has been done recently by discussing the shape of the packets during their propagation but none of the concepts has been incorporated into a transport model to describe the full evolution from the packet generation to their vanishing. Here, we used a simplified transport model featuring bipolar charge injection and transport coupled to specific conditions in charge injection or carrier mobility to reproduce experimental results. One of the salient features of the results is that both models are able to reproduce the repetitive character and the dying away of the packets that appear to be linked with the internal field distribution modulated by a bipolar space charge. [ABSTRACT FROM AUTHOR]

Modeling charge transport and linking charge dynamics with dissipative processes responsible for electrical ageing are a challenging objective for developing a mature approach in insulation design. Such an approach is exemplified here for polyethylene-based materials by introducing two models describing bipolar Space Charge Limited Current in transient and steady states. They rely on two classical descriptions of the distribution function of the energy levels of trap states –single trapping level or exponential distribution. Their predictions are discussed as regards the experimental behavior. They notably highlight the importance of recombination processes in explaining the sigmoidal shape of the steady-state current-voltage characteristic. Also, bipolar charge transport seems to be a necessary factor for the observed features of oscillatory charge packets. The energetic features of charge dynamics is reviewed with particular emphasis on recombination phenomena because the latter promote electronically excited states that are chemically reactive and could be involved in ageing reaction. The relationship between charge recombination and electroluminescence is highlighted through experiments and simulation. The spectral analysis of the emitted light advocates the existence of massive chemical/physical degradation in the electrical regime where recombination is a major factor of the Space Charge Limited Current (SCLC). [ABSTRACT FROM PUBLISHER]