Interfaces and Space Charge in Polymeric Insulating Materials
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Interfaces and Space Charge in Polymeric Insulating Materials G. Chen School of Electronics and Computer Science University of Southampton, UK
ABSTRACT In this paper, the influence of interface between electrode and polymer or polymer and polymer on space charge dynamics has been studied. Planar samples of low density polyethylene (LDPE) were subjected to high dc electric stresses for extended periods of time and space charge measurements were taken using the pulsed electroacoustic (PEA) technique. Common electrode materials used in either laboratory or power cable industry were selected (i.e. aluminium (Al), gold (Au) and carbon loaded XLPE (Sc)). Experimental results demonstrated that charge injection processes take place in all cases once the applied electric stress has exceeded a threshold value. However, the amount of charge and polarity of the dominant injected charges showed a significant dependence on the electrode materials (under the same applied electric stress). Having establishing the influence of electrode materials on charge accumulation, our attention was then focused on the effect of polymer/polymer interface on charge dynamics. Unlike our previous approach where two different polymeric materials were used, this time the polymer/polymer interface was formed by using two layer of LDPE films cut from the same sheet. Sc and Al were used as electrodes to form different combinations. The results clearly indicated that the interface between two layers of LDPE acts as traps for electrons but not for positive charge carriers. The charge distribution in the bulk of the sample strongly depends on the electrode materials. INTRODUCTION Interfaces are often encountered in practical insulation systems and have become a growing area of interest due to their influence on the electrical performance of whole system. Unlike the case of semiconductors where transistors and diodes utilize characteristics of interfaces (p-n junctions), the interfaces in high electrically stressed materials, whether polymer/metal or polymer/polymer, can cause accumulation of space charge which can lead to unwanted electric field modification. For example, the interfaces in cable joints and terminations of extruded high voltage cables have been identified as crucial parts as their failure probability is much higher than cable itself. It has been proposed [1] that the accumulated space charge at the interface is a more significant factor with regard to electric field enhancement than in the bulk. Despite of increasing interests in the interfaces, the underlying mechanisms related to charge formation and electrical ageing are not well understood and research into the best interfacial materials and its binding structure is becoming increasingly important. Initially, thermally stimulated discharge current (TSDC) technique has been used to study polymer/polymer, polymer/mica and polymer/oil interfaces [2-5]. Suzuoki et al [4], through their research on the low density polyethylene
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(LDPE)/ethylene vinyl-acetate (
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