The Influence Of Trap Distribution Of Alumina Ceramics On The Surface Flashover Performance In Vacuum

Surface flashover across the insulators in vacuum is the major limiting factor for electrical insulation system, since it typically takes place at applied electric field strengths much lower than both that of materials and the vacuum gap in the same dimension. It is of considerable importance to have a full understanding of surface flashover mechanism and seek the way to improve the flashover strength. Alumina ceramics are adopted in the dissertation as insulation material, which are used widely in vacuum insulation system. Different ceramics samples were prepared under different sintering temperature and varied additives. Their flashover and charging performance have been systematically investigated in vacuum under pulse voltage (0.7/4(s), together with their thermally stimulated current (TSC) characteristics. Intrinsic correlation is studied among the surface charging, applied voltage sequence, flashover performance and trap distribution based on energy band theory. Total twelve kinds of Alumina ceramics are investigated systematically by using the TSC test system developed by us. According to the experimental results, trap charge density and trap energy distribution are acquired. The further analysis indicated that sintering temperature and additives would influence the crystalline structure formation during the sintering process. Multiple steady chemical valence of the metal element of additive would possibly contribute to the shallow trap energy level appearing.The surface charging phenomena of Alumina insulators in vacuum has been studied under different voltage level. The whole process for charge polarity changing from negative to positive has been observed for ten kinds of ceramics samples. Analysis shows that it could be attributed to adsorbed atmosphere on the surface. Meanwhile the particularly negative charge distribution on the other two kinds of samples is also observed for the first time. It presents negative polarity for the whole applied voltage process no matter how the sample was treated by flashovers. The phenomenon is thought to result from its particular trap energy structure. According to the experimental results, a new mechanism of the surface charging of ceramic insulators in vacuum has been put forward in the dissertation. Under low electric field, surface charge distribution was mainly attributed to the metal electrode injection, and the free carriers could move in the surface solid state, which was caused by surface absorbed gas. By the enhancing of the electric field, free carriers would be emitted at the triple junction of electrode-insulator-vacuum. Along with the destroy of surface state by electron collision, the surface charging process was finally determined by the secondary electron emission process and the trap characteristics of dielectrics. The negative charge distribution of two kinds of ceramics could mainly be due to the particular trap structure, in which the shallow trap was dominant.The influence of applied voltage sequence on surface flashover performance has been studied at the first test circle. Experimental results show that the sequence plays the important role by changing the surface charge distribution. The flashover performance of samples treated by flashovers was found that it is not influenced by the sequence. It could be attributed to the disappearance of surface adsorbed gas. The influence of trap distribution on surface charge distribution and flashover performance has been analyzed in detail according to the experimental results. It was found that the more the trap density is in the material, the larger the surface charge density is, and the lower the flashover voltage is. The dominant shallow trap is thought to improve the surface flashover performance. The trapping & detrapping mechanism of trap carriers would play the important role during the developing process of flashover, together with the secondary electron emission mechanism.