Browsing by Subject "Lexan"
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Item Charge injection threshold in Lexan and Acrylite(Texas Tech University, 1996-12) McCuistian, Brian TrentBulk 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.Item Reduction of the surface electric potential on Lexan and Teflon(Texas Tech University, 1997-05) Demirturk, TayfunA method of discharging the surface of insulating materials has been investigated in this research. The method described here wall minimize charging of the material during sample preparation and installation, and produce zero surface potential so that one could measure the first crossover energy point in the Secondary Electron Emission (SEE) experiment with a small error for the polymers. Lexan and Teflon virgin samples were treated with heat, humidity and vacuum. The best results were obtained using room temperature and 50-60% humidity and resulted in zero surface potential.Item Secondary electron emission coefficient from lexan: the low energy crossover(Texas Tech University, 1993-05) Adamson, Elise Rene BoerwinkleThe energy location of the first crossover point of the secondary electron emission curve of the polymer insulator Lexan (polycarbonate) has been experimentally investigated. The secondary electron emission coefficient (SEEC) is the ratio of the number of secondary electrons leaving the surface to the number of incident primaries. At high and low energies the SEEC is less than one, between the energy extremes the SEEC is greater than one. The energy at which the (SEEC) is first equal to one is called the first crossover point. The Lexan sample is treated as a parallel plate capacitor. The sign of the replacement current to the metalized back of the sample shows whether the SEEC is above or below one, and the incident electron energy is above or below the first crossover point. Successive choices of incident electron energy bracketed the first crossover point for Lexan, at about 44 eV.Item The effect of magnetic field on coated insualtors(Texas Tech University, 1995-08) Hao, Jing-FangThe insulator surface flashover potential increases when a magnetic field with the proper orientation is present at the surface of the dielectric (magnetic insulation effect). In order to observe an obvious magnetic insulation effect, a critical magnetic field has to be reached. Earlier studies have shown that the insulator surface flashover potential increases after a vacuum spark discharge surface treatment. This research is to study the magnetic effect on a treated sample. A cylindrical Lexan sample and brass plate-plate electrodes are used in this study. Small, cubic, lightweight permanent magnets which can produce up to 0.38 T magnetic fleld at their surface are arranged in proper configuration to create a magnetic field along the sample surface. Three experimental configurations are used in this experiment. This thesis first gives some background information about the surface flashover, then introduces the experimental setup and procedure. Finally results are presented and explained.