Charge Accumulation Mechanism And Its Effect Of Polyimide Under Temperature Cycling And Electron Irradiation

The rapid development of the aerospace industry has brought huge social and economic benefits.However,the space radiation and temperature environment of spacecraft are severe,which lead to the charge accumulation and discharge effect of dielectrics,and seriously restrict the development of high-voltage and high-power spacecraft.At present,the research on the charge accumulation characteristics of spacecraft dielectrics under the radiation of particles is mostly based on the room temperature.However,considering the temperature cycle of spacecraft surface and the temperature sensitive charge accumulation characteristics of the dielectrics,it is necessary to study the charge accumulation mechanism of the dielectrics under the electron radiation in different temperature and temperature cycles,and the effect of the charge on the flashover along the surface.In this thesis,aiming at the environment of space radiation and temperature cycle in which the low orbit spacecraft works,a platform for measuring the surface potential of dielectrics under temperature cycle and electron irradiation is developed.At 1643?6623 eV electron energy,0?25 nA beam current,243?343 K temperature(the maximum temperature change rate is 2 K/min),the joint experiment of electron irradiation and surface potential distribution measurement under constant temperature or temperature cycle is realized.The space charge recovery algorithm is improved according to the continuous variable temperature test conditions.The space charge distribution test of insulating dielectrics is realized in the temperature range of 243?343 K with the maximum variable temperature rate of 2K/min.Polyimide,a common dielectric for spacecraft,is taken as the research object.Through the measurement of the surface potential distribution characteristics,electron incident deposition characteristics and space charge characteristics under electron irradiation,the charge accumulation mechanism of polyimide under electron irradiation in the temperature cycle is studied,and the influence of charge on the flashover characteristics along the surface is investigated by using the active control charge accumulation mode,The main achievements are as follows:The charge accumulation characteristics of polyimide under electron irradiation at constant temperature are obtained.There are three modes of charge accumulation in the temperature range of 243?343 K and 1643?6623 eV:"positive" mode,"negative-peak"mode and "negative-crater" mode.By calculating the distribution of electron incident deposition,the relationship between the critical electron energy of polyimide and the temperature is obtained.When the incident energy is less than the critical electron energy,the secondary electron escapes from the surface of the dielectric,causing the hole accumulation,showing a "positive" mode;when the incident energy is greater than the critical electron energy,the incident electron is deposited in the dielectric,showing a"negative" charge accumulation.When forming the self-built electric field exceeds the injection threshold,hole injection appears at the electrode/dielectric interface.When the dielectric temperature is high,the injection threshold decreased,which makes the hole injection area is large,and the charge accumulation presents a "negative-peak" mode;as the dielectric temperature decreases,the injection threshold increases,the hole injection area reduces,and the charge accumulation presents a "negative-crater" mode.At the same temperature,with the increase of electron irradiation energy,the self-built electric field formed by the incident electron increases,and the charge accumulation mode changes from"negative-peak" to "negative-crater".The change rule of the conductivity of polyimide in the range of 243?343 K and the rate of 2 K/min was obtained.When the temperature was higher than the critical temperature in the same cycle,the conductivity of polyimide showed a positive correlation with the temperature;with the increase of temperature cycles,the conductivity decreased gradually at the same temperature.By testing the space charge distribution and trap characteristics of polyimide in the temperature cycle,it is found that the thermal stimulation induced by the temperature change inhibits the charge trapping process,so the charge accumulation in the temperature cycle is dominated by hetero-polarity;with the increase of the number of temperature cycles,the space charge accumulation gradually increases to saturation at the same temperature,and dominated by the deep trapped charge above 0.55 eV,which restrains the effective transport of carriers.Based on the above analysis,the conductivity of polyimide under temperature cycling is caused by both trap filling and thermal stimulation.Based on the dynamic process of charge trapping/detrapping,an effective carrier transport model considering trap filling and thermally stimulated detrapping is established,and a method for calculating the conductivity of dielectric materials in the temperature cycle is proposed.The charge accumulation characteristics of polyimide under the temperature cycling and electron irradiation at a rate of 2 K/min are obtained.The thermal stimulation induced by the temperature change enhances the migration and dissipation process of the incident electrons,and the incident energy required for the transition of the charge accumulation mode from "peak shape" to "crater shape" is higher than that in the constant temperature.For the "negative peak" charge accumulation mode,due to the thermal stimulation caused by the temperature change,the surface potential is lower than that under the constant temperature,the amount of hole injection at the electrode/dielectric interface is reduced,and the accumulated charge is mainly the charge incident at the vacuum/dielectric interface.In the "negative-crater" charge accumulation mode,due to the trap filling effect,which hinders the dissipation of the incident electrons,the accumulated charge is significantly increased compared with the constant temperature,forming a larger self-built electric field,and more holes are injected into the electrode/dielectric interface.Under the active control charge accumulation mode,the flashover experiments in constant temperature and temperature cycles are carried out,and the influence of three charge accumulation modes on the flashover characteristics along the surface is obtained.In the constant temperature and "positive" mode,the secondary electron escape inhibited the secondary electron multiplication process along the surface flashover,and the flashover voltage was increased;in the constant temperature and "negative" mode,the hole injected into the electrode/dielectric interface enhance the field strength of the vacuum/electrode/dielectric triple junction point,thus significantly promoting the flashover process.In the "negative-peak" mode of the temperature cycle,the self-built electric field is low due to the thermal stimulation of the detrapping effect,and the accumulated charge is mainly the incident electron at the vacuum/dielectric interface,so the flashover voltage changes less than that of the non-irradiated;while in the "negative-crater" mode,the accumulated charge includes the irradiated incident electron and the injected hole from the electrode,which is restrained by the trap filling effeet.Compared with the constant temperature,the charge and dissipation process increase the amount of hole injection,promote the initiation and development of flashover,and significantly increase the decrease of flashover voltage along the surface.The results obtained in this paper provide a theoretical basis for the suppression methods of spacecraft electrostatic discharge in China.