Study On The Influence Of Pore Defects Of Alumina Ceramics For Electric Vacuum Devices On Dielectric Breakdown

Electric vacuum devices have the advantages of strong load capacity,strong radiation resistance and strong stability,etc.They are the core devices in many equipment and systems and widely used in high-frequency and high-power fields.The leakage failure caused by the dielectric breakdown of the insulating shell of the electric vacuum device is an important factor restricting the performance and reliability of the electric vacuum device.Therefore,the insulating shell material is required to have a high dielectric breakdown strength.Features of high mechanical strength,high temperature resistance,high resistivity,moderate dielectric constant,low dielectric loss,high dielectric breakdown strength and the ability to be sealed to metal parts through metallization and brazing,make alumina ceramics an excellent material of electrical vacuum device insulation shell.In recent years,with the development of high technology,in order to meet growing demands of high performance in various fields,electrical vacuum devices are developing towards the direction of high power and high voltage,which puts forward higher requirements for dielectric breakdown strength of alumina ceramics used in electric vacuum devices.The dielectric breakdown strength of alumina ceramics for electrical vacuum devices is affected by many factors,among which pore defects are the main influencing factor,which limits the improvement of its insulating ability and the improvement of its performance stability.Therefore,it is of great scientific and engineering significance to explore the methods to improve the dielectric breakdown strength of alumina ceramics by studying the influence mechanism of pores on dielectric breakdown of alumina ceramics.In this paper,a series of alumina ceramics with different pore states are prepared by dry pressing,cold isostatic pressing and hot die casting.Micro-CT is mainly used to characterize the dielectric breakdown channels of alumina ceramics with different pore states.The influence of pore defects on the dielectric breakdown strength of alumina ceramics and the dielectric breakdown mechanism of alumina ceramics are studied.The main research results obtained are as follows:(1)The influence of porosity and pore size on the dielectric breakdown strength of alumina ceramics was studied,revealing that the dielectric strength of alumina ceramics not only decreases with the increase of porosity,but also decreases with the increase of pore size.Further through the electric field simulation and theoretical analysis based on Paschen’s law,combined with the experimental results,it is found that for alumina ceramics with dielectric breakdown strength below 14.5 kV/mm under power frequency voltage,when the porosity is below 6.54%,and the pore size is at the same time below 25 μm,the pores weakly influence the dielectric breakdown strength of the alumina ceramic.(2)Oxygen atmosphere sintering can effectively reduce the porosity of alumina ceramics.The research results show that the porosity of alumina ceramics prepared by dry pressing is 4.10%after sintering in air atmosphere,and the porosity is reduced to 2.82%after sintering in oxygen atmosphere.For hot die casting method,the porosity of the alumina ceramic is reduced from 6.41%to 3.59%when oxygen atmosphere was used instead of air atmosphere.(3)Through the design of alumina ceramics with "sandwich" structure,the influence of the different distributions of pore positions on the dielectric breakdown strength and breakdown point distribution of alumina ceramics was studied.The research results show that the influence of pores on the dielectric breakdown of ceramics is a result of the combined effect of surface effect and body effect,and there is a mutually reinforcing relationship between surface effect and body effect.The study found that pores distributing near the surface of alumina ceramics have a stronger weakening effect on the dielectric breakdown strength of alumina ceramics than the pores distributing inside alumina ceramics.This is due to the "prebreakdown"caused by the pores distributed on the surface of alumina ceramics.(4)Through electric field simulation and experimental results,it is found that the dielectric breakdown strength of alumina ceramics is not determined by the maximum electric field strength in the alumina ceramic body,but the electric field distribution caused by the electrode structure plays a major role.Therefore,the dielectric breakdown strength of alumina ceramics can be improved through reasonable electrode structure design.It was found that plating a metal conductive layer on the surface of alumina ceramics can increase the dielectric breakdown strength of alumina ceramics from 13.1 kV/mm to 14.8 kV/mm.(5)The process of dielectric breakdown of alumina ceramics was collected by a high-speed camera.The study found that before the dielectric breakdown of alumina ceramics,the insulating oil first began to discharge sparks on the two surfaces of the alumina ceramics.The process lasted approximately 2.35 s,accompanied by a rustling sound.Based on the results of electric fields simulation,the study found that the local dielectric breakdown of the insulating oil before the dielectric breakdown of the alumina ceramic will cause the conductivity and the dielectric constant of the insulating oil to change,thereby causing the maximum intensity of electric field in the alumina ceramic changes a lot,and the result of the electrostatic field simulation without considering the dielectric breakdown of the insulating oil is no longer suitable for calculating the electric field strength that the alumina ceramics can withstand.(6)The characterization technology of alumina ceramic breakdown channel was explored,and a Micro-CT three-dimensional reconstruction microscopic analysis method of alumina ceramic breakdown channel was established,which laid a foundation for the analysis of the dielectric breakdown process of alumina ceramics.The basis of the method is useful to analyze the relationship between the electric field distribution and the breakdown channel in alumina ceramics,and to analyze the dielectric breakdown mechanism of alumina ceramics.(7)Through the characterization of the micro-morphology of the breakdown channel by Micro-CT and SEM,the study found that when the dielectric breakdown of alumina ceramics occurs,a large amount of heat energy is generated instantaneously to melt the material around the breakdown channel,which erupts under high internal stress to the surface and cools to form a "crater",and finally leaves a breakdown channel inside.The "crater" is often produced at the cathode,which is related to the emission of electrons at the cathode.And the size of the "crater" has no obvious relationship with the porosity of alumina ceramics,but the position distribution of the pores will affect the formation of the breakdown channel and the shape of the breakdown channel.(8)Based on the characterization of breakdown channel characteristics of alumina by Micro-CT,it was found that the dielectric breakdown of alumina ceramics develops from one end to the other end of the surface,and always passes through the area of electric field distortion.The breakdown point often appears at the three-phase point where the electric field distortion is strong.The development is promoted by the weak dielectric breakdown strength area,while hindered by the high dielectric breakdown strength area.