Charge injection threshold in Lexan and Acrylite
McCuistian, Brian Trent
Bulk breakdown in dielectrics occurs when the applied electric field exceeds a critical field and the insulating dielectric becomes conducting. This breakdown event damages the dielectric by producing a permanent channel of fractal dimension inside the dielectric. The resulting conducting structure is referred to as a tree because of its treelike structure. The production of electric field induced microvoids, or precursors, has been theorized to precede the breakdown event. Electric breakdown through solid dielectrics is heavily influenced by charge injected into the dielectric from electrodes. An experimental apparatus to study dielectrics stressed by impulse electric fields in a vacuum has been constructed. This apparatus is used to study several physical processes related to the initiation of electrical breakdown in polycarbonate (PC) and polymethylmethacrylate (PMMA) dielectrics. A critical field is found to exist below which no charge is injected into the dielectrics. This critical field for charge injection is slightly below the critical field for bulk breakdown. When an applied voltage produces fields greater than the critical field for charge injection, a space charge cloud is injected into the dielectric. Values of the critical field necessary for charge injection, total amount of injected charge, radius of the space charge cloud region and a limit on the time required to produce the space charge cloud region are obtained in this experiment. From these parameters, limits on the high field mobility of charge carriers, and trap densities in the materials are determined. After the electrical fields are applied to the dielectrics, the dielectrics are studied with an optical microscope to detect damage caused by charge injection. This damage may be experimental detection of precursors hypothesized by one theory of bulk breakdown. In addition, a computer simulation of dielectric breakdown, that introduces variables representing physical phenomena such as electron impact ionization, field ionization, and breakdown threshold, as well as the random structural nature of polymers and defects has been developed. These computer generated fractal breakdown structures are similar to experimentally produced breakdown trees.