Numerical modeling of space charge dynamics and electrical breakdown in solid dielectrics.

摘要

A general numerical model has been developed to predict the dielectric breakdown strength of insulating materials. Low-alkali boroaluminosilicate glasses are of interest for electrostatic energy storage since they have exceptionally high dielectric breakdown strength. Polymer dielectric such as polyethylene is the state-of-the art material for power transmission cables. Therefore, understanding their breakdown mechanisms and predicting their strengths are important theoretically as well as practically. This research focuses on understanding electrical conduction and space charge dynamics and their effects on electrical breakdown strengths. Conduction mechanisms for AF45 glass (one kind of low-alkali BAS) below 473 K were studied using Schottky, Poole-Frenkel, space-charge-limited current, and ionic hopping conduction mechanism. This study showed that the electrical conduction in low-alkali BAS glass is governed by a combination of two or more conduction mechanisms. Cation depletion phenomena during thermal poling of low-alkali BAS is an important precursor to dielectric breakdown. Numerical models including multiple charge carriers such as Na+, nonbridging oxygen ion, H3O+/H+, Ba2+ were developed to predict depletion widths under anode and electric field distribution within the glass. These numerical models accurately predicted widths of 2.1 mum, which were very close to the experimentally determined values. Moreover, the calculated electric field from a numerical model assuming Na+, H3O+/H +, Ba2+ migration could reproduce the experimentally determined electric field distribution. Numerical breakdown models were developed assuming electronic conduction or ionic redistribution and electronic breakdown for low-alkali BAS glass. The numerical model assuming electronic conduction predicted weakly thickness dependent breakdown strengths below 20 mum although it cannot predict strongly thickness dependent breakdown strengths above 20 mum. Another combined breakdown model assuming ionic redistribution and electronic breakdown predicted two distinct regions in AF45 glass for thickness dependence of breakdown strengths. Temperature dependence of breakdown strengths for AF45 glass predicted by the model assuming ionic redistribution and electronic breakdown agreed well with experimental results. This model showed that the change in breakdown strengths with temperature depended on the initial mobile sodium ion concentration. Thermal and electronic breakdown combined model was also applied to low-density polyethylene where electrical conduction is dominated by electrons and holes. Upon high electric fields these carriers move and produce space charges which enhance local electric field near the anode. This breakdown model predicts weakly thickness dependent breakdown strengths at room temperature which is also proved by other researchers. Furthermore, the relationship between the breakdown strength and voltage ramp rate can be reproduced in this model.