Ion Implantation And Deposition On The Surface Of Alumina Ceramic And Surface Flashover Characteristics

Alumina ceramics are widely used in high voltage and electrical vacuum devices as solid insulator for its excellent thermal conductivity,dielectric protecties and high temperature resistance.However,when operated in a high electric field and complex environment,a large amount of charge would be easily accumulated on the surface of alumina ceramic due to its high surface resistivity and secondary electron emission coefficient,resulting in a lower surface flashover voltage and consequently discharge accidents.Therefore,it was of great scientific and engineering significance to explore ways to suppress the accumulation of charge on the dielectric surface and improve flashover performance by studying the development process of surface flashover of alumina ceramic in vacuum.In this paper,both simulation and experimental methods were used to investigate the formation and development of surface flashover,and the flashover characteristics of alumina ceramic were improved via the methods of ion implantation and deposition.Titanium ion implantation and chromic oxide film deposition were used to modify the surface of alumina ceramic.During the ion implantation,the phenomenon of "flashover" on the surface of the alumina ceramic was eliminated through a combination of assisted metal grid and radio frequency argon plasma method.The effect of ion implantation time on the chemical composition and microstructure of the surface layer of alumina ceramic was studied.The concentration of the implanted titanium element in the depth direction was Gaussian-like shape and the peak concentration of the titanium element was at a depth of 30 nm.An amorphous modified layer was formed on the alumina ceramic surface.The effects of substrate temperature,oxygen flow rate,and deposition time on the microstructure of chromic oxide films were studied.The film began to crystallize when the substrate temperature was higher than 100 ?.With the increase of oxygen flow rate,the deposition rate of the chromic oxide film,the crystallinity of the film decreased,the stoichiometric ratio of the film transformed from an under-oxygen state to an oxygen-rich state.When the substrate temperature reached 300 ? and the oxygen flow rate was 5 sccm,the composition of deposited chromic oxide film reached stoichiometric ratio 2:3.The film was composed of closely-arranged nanocolumn polycrystals.With the increase of deposition time,the film deposition rate remained stable and the grain size gradually increased.The surface electrical properties of alumina ceramics before and after ion implantation and deposition were tested.With the increase of titanium ion implantation time,the surface resistivity of alumina ceramic decreased,and the volume resistivity,relative permittivity,and secondary electron emission coefficient remained stable.As the thickness of the deposited chromic oxide film increased,the surface resistivity of the alumina ceramic was almost the same with that of the bulk chromic oxide,but the secondary electron emission coefficient decreased from 7.8 to 1.93.Through pin-plate DC corona discharge experiments,it was found that the main dissipated path for the accumulated charge on the surface of alumina ceramic was surface conduction after the titanium ion implantation and deposition treatment.Decreased surface resistivity and shallower trap energy levels suppressed the surface charge accumulation of alumina ceramic and promoted surface charge dissipation.Through vacuum surface flashover test,it was found that the flashover voltage of alumina ceramic increased first and then decreased with the increase of titanium ion implantation time.For the deposited chromic oxide film,the flashover voltage of alumina ceramic increased and then stabilized as the thickness of the deposited chromic oxide film increased.A two-dimensional PIC/MCC(Particle-in-cell/Monte Carlo collision)simulation method was used to establish a vacuum surface flashover simulation model of alumina ceramic,the surface flashover process was simulated under different conditions.When the surface outgassing was considered,due to collision ionization of electrons and desorbed gas,the number of space electrons and ions increased exponentially with time after the multipactor reaches saturation,leading to the gas avalanche breakdown.If the surface outgassing was ignored,the secondary electron multiplication had a saturation stage,and the number of space electrons had a quadratic relationship with the applied electric field strength;the surface accumulated positive charge density increased linearly with the increased of applied electric field strength.The accumulation of positive charges on the surface of alumina ceramic would promote the development of the multipactor and increasing the accumulation of surface positive charges in the saturation stage.The number of space electrons and surface positive charge density increased exponentially with the increase of the secondary electron emission coefficient.Lower the secondary electron emission coefficient on the surface of alumina ceramic to a value less than 3 could effectively suppress the multipactor effect and reduce the surface positive charge density.Under the effect of the vacuum pulse voltage,the surface saturation potential of the alumina ceramic decreased slightly after titanium ion implantation treatment,and the surface potential distribution range was similar to that of the untreated alumina ceramic.The surface saturation potential and distribution range of the alumina ceramic were significantly reduced after a 120 min deposition of chromic oxide film.