Study On Surface Charge Distribution Characteristics Of Polyimide At 300K～120K

During normal operation of large-scale cryogenic equipment and aerospace equipment,the insulation system is in a low-temperature and high-vacuum environment.Long-term operation will cause the insulating medium to easily accumulate charge,which will cause flashover along the surface of medium.At present,there is no device for measuring surface charge at low temperature.Research on the characteristics of charge accumulation on the surface of medium at low temperature and its effect on flashover along is rarely carried out.So the research work in this paper is of great significance.In view of the problems that electronic components are easily damaged at low temperatures and the ordinary motion mechanisms cannot work normally,this paper has developed a three-dimensional scanning surface charge measuring device in the temperature range of 300 K to 120 K by optimizing the structure of passive electrostatic probes,transforming the amplifier circuit and taking heat preservation measures,and the measurement system has been calibrated at low temperature.The spatial resolution of the measurement system is 0.6777 mm2,the charge sensitivity is 0.044μC/(m2·mV),and the measurement interval between two adjacent points can be up to 0.5 seconds.The surface potential distribution characteristics of polyimide at 300 K,210 K and 120 K were obtained through testing.Under the action of DC voltage,the polarity of the surface potential of the sample is the same as the polarity of potential of high-voltage electrode,and the potential decreases from the high-voltage electrode to the ground electrode;the low temperature suppresses the increase of the surface potential;increasing the DC voltage will increase the amount of charge injection and migration distance.After analysis,it was found that the charge was injected into the sample from the electrode,and migrated along the surface under the action of the interface and the tangential electric field,and generated a surface potential of the same polarity.The surface potential attenuation characteristic shows that the charge decay time at low temperature is longer than at normal temperature.According to the characteristics of charge accumulation under low temperature and high vacuum,a charge accumulation calculation model has been established based on the carrier transport model.The charge density distribution,surface potential and electric field on the vacuum side at different temperatures and different times have been calculated under the action of DC voltage.It was found that the migration distance of the injected charge of the same polarity along the surface is short,which weakens the electric field near the cathode and enhances the electric field between the charge and the anode;the migration distance of the charge inside the medium at low temperature is extremely small,which can be considered to be mainly concentrated in the sample surface.Studying the effect of transport parameters on the results,it was found that the increase in surface mobility,the decrease in trap depth and density will increase the distance of charge migration on the surface of the medium and the amount of accumulated charge.After analyzing the effect of charge accumulation on the flashover along the surface under high vacuum,four influencing factors related to electron emission,collision energy,normal field strength and tangential field strength have been selected.Using the charge accumulation model in this paper,the values of the influence factors at 300 K,210 K,and 120 K were calculated,and it was found that the charge accumulation at low temperature is more conducive to promoting flashover.Finally,measures to suppress the flashover along the surface were proposed from the aspects of material properties,electric field distribution and electrode smoothness.