Investigation Into Charge Trapping Transport Properties And Mechanisms In Polymeric Insulating Materials

The charge transport properties of polymeric insulating materials determine the space charge accumulation properties.The accumulated space charges can distort electric fields.When the distorted electric fields exceed a threshold value,the electromechanical forces and energy transfer processes associated with charge detrapping can initiate aging,deterioration or electrical breakdown in the insulating materials,influencing the operational safety and reducing the lifespan of spacecrafts.The charge transport properties of dielectrics determine the space charge accumulation levels.However,the charge transport properties and mechanisms of some highly insulating materials,such as conductivity,and the formation and migration of space charges,are not very clear until now.Accordingly,it is significantly important to investigate the charge transport properties and mechanisms of polymeric insulating materials.The present work investigates the charge trapping/detrapping dynamic responses,steady state charge carrier mobility,and the relationship between the carrier mobility and trap distribution properties,charge density,temperature and electric field,and space charge limited current(SCLC)of polymeric insulating materials,when the materials exist single discrete trap,multiple discrete traps,and exponential distribution traps.We measure the two dimensional surface potential decay(SPD)properties of polyimide(PI)irradiated by electron beams.Then,a SPD model is established to calculate the surface and bulk charge transport properties.The bipolar charge transport model(BCT)is adopted to simulate the SPD experimental results,and calculate the surface charge decay characteristics,bulk charge density and electric field evolution properties.We also study the SCLC behaviors of polyimide at high temperatures and high electric fields,and analyze the charge transport properties at various SCLC regimes.Firstly,we study the influence of trap capture effects on the charge trapping/detrapping dynamic responses,charge transport properties and SCLC current.We propose a Coulombic trap potential barrier model,and derive the analytical equation of capture cross-section and trapping probability.Then,we obtain the analytical equation of effective carrier mobility and SCLC current,which are dependent on trap energy,carrier mobility edge,temperature,charge density and electric field.Secondly,we explore the SPD characteristics and charge transport properties of PI in the temperature range of 298-338 K and the electric field range of 106-5.4×107Vm-1.Considering the surface conduction and bulk conduction processes,we establish a two dimensional SPD model combined with genetic algorithm.Then,we calculate the effective surface resistivity,bulk Ohmic resistivity,and effective charge carrier mobility at various temperatures,and the activation energy of effective surface resistivity,bulk Ohmic resistivity,and deep trap energy of PI.Thirdly,the BCT model is used to analyze the SPD and space charge evolution properties of PI at room temperature and various initial surface potentials,and calculate the charge transport parameters.The BCT model with genetic algorithm simulates the SPD curves at various initial surface potentials,and optimizes the relaxation polarization and charge transport parameters.Substituting the optimized parameters into the BCT model,we obtain the time dependent surface deposited charge density,bulk free charge distribution,trapped charge distribution and electric field distribution properties during the SPD processes of PI.Fourthly,we investigate the charge dynamic response properties,and the trap distibution,charge density and temperature dependent charge carrier mobility in the case of multiple discrete trap levels and exponential distribution of trap levels.We analyze the charge dynamic responses theoretically and numerically,and obtain the equivalent circuits.Then,we obtain the relationship between trapped charge density and free charge density at steady state,and derive the analytical equations of effective carrier mobility in the case of exponential distribution of trap levels.Finally,we analyze the charge transport and space charge distribution characteristics of polyimide with the thickness of 1.4?m in the temperature range of 593-673 K and the electric field range of 107-3×108Vm-1.Investigating the SCLC current properties of the samples,we obtain the trap distribution parameters.Then,we calculate the charge density and temperature dependent quasi Fermi level and effective carrier mobility,and the charge density and electric field distribution properties at various SCLC regimes.