| Discharge in vacuum solid insulation system usually occurs on the solid surface first.The surface flashover of alumina ceramic dielectric in vacuum environment is the main reason that restricts the application of vacuum circuit breakers,pulse power devices and other devices to higher voltage levels.It is of great significance to study the surface flashover phenomenon and its development mechanism of solid dielectric for improving the insulating properties of alumina ceramics.At present,it is generally believed that the surface flashover is mainly caused by local electric field concentration.The use of functional gradient materials(FGM)to mitigate the electric field distortion is an effective method to improve the surface flashover threshold voltage.The surface modification of insulating materials by plasma surface modification technology has become a research hotspot.In this paper,the surface of alumina ceramics was modified by low temperature plasma coating deposition technology to improve its surface insulation properties.Firstly,the principle of functional gradient materials(FGM)to improve electric field distribution and alleviate electric field concentration is analyzed,and the FGM structure is designed based on this principle;Using COMSOL simulation software,taking the ceramic with dielectric constant of 9 as the substrate,the surface is evenly divided into three sections and different dielectric constant coatings are set respectively.Comparing the electric field mitigation effect,the three FGM structures with dielectric constant of 100-50-9 are finally selected.Secondly,based on the above simulation results,the functional coatings were selected,and the coatings were determined to be composed of titanium dioxide anatase phase and titanium dioxide rutile phase.The effects of substrate temperature and power output power on the crystal growth of titanium dioxide anatase were studied in the atmospheric pressure plasma jet environment.It was determined that the coating growth was the best when the substrate temperature was 400℃ and the power output power was 700 W;On this basis,anatase phase was calcined to 950℃ to obtain titanium dioxide rutile phase;The crystal structure,surface morphology,chemical composition and coating thickness of the two crystalline coatings were characterized.The two kinds of coatings were deposited on the ceramic surface in sections,and formed a three-stage FGM Structure with the ceramic itself.The results show that the initial flashover voltage of the treated samples is 48.6%higher than that of the untreated ceramic samples,and the average surface flashover voltage is 36%.Moreover,the flashover voltage of the treated samples did not decrease significantly after 120 days and 180 days,indicating that the atmospheric pressure low-temperature plasma deposition coating has good stability.Finally,the simulation model of electrostatic field is established by COMSOL.The simulation results show that in the untreated ceramic samples,the local electric field is concentrated at the three junction points near the high voltage electrode and the grounding electrode,while for the FGM Structure samples,the maximum electric field near the high voltage electrode is reduced by 66%;At the same time,the electric field and potential distribution at different positions of the samples were measured in the vacuum and atmospheric environment.The results showed that the electric field concentration of FGM samples was improved to a certain extent compared with untreated ceramic samples in the vacuum environment,and the electric field changes of the two samples in the atmospheric environment were small. |
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