Study On Space Charge And Breakdown Characteristics Of Polyimide Films Modified By Nano Particles Under Strong Electric Field

Polyimide（PI）film has become an ideal insulating material for electronic components and power equipment in extreme environments due to its excellent electrical,mechanical properties,as well as its excellent physical and chemical characteristics.It is widely used in multifarious electrical and electronic engineering fields.However,in practical applications,PI film is susceptible to accumulate charges and trigger electric field distortion under strong electric field.This can result in the breakdown of the insulating materials,which seriously restricting its application in high-voltage power equipment and the other fields.The development and application of nano-modification technology provide a new method to solve the abovementioned problems.However,the internal influence mechanism of nano-modification is still unclear,and the further research is required.This thesis aim is to improve the various performance indicators of PI films and prolong the service life of the power equipment.To achieve this goal,nano-modification technology was used to prepare the PI films modified by nano-SiO₂ and Al₂O₃ with different mass fractions.A dielectric characteristic test platform was then built to explore the dielectric properties and conductivity characteristics of different types of PI films modified by nano-particles.Additionally,a space charge test platform based on pulse electroacoustic method and an AC/DC breakdown test platform were used to investigate the impact of nano-modification on PI films,and the space charge mechanism of these films was revealed through analysis of their space charge and electric field intensity distribution characteristics under strong electric field.The values of AC/DC breakdown strengths for different nano-modified PI films were tested,and the effect of nano-modification on their breakdown characteristics was analyzed.The following main conclusions were drawn:（1）After incorporating nano-SiO₂ and Al₂O₃ particles into the PI film,the relative permittivity and electrical conductivity of the two modified PI films increase.The smallest relative permittivity and the largest electrical conductivity are observed for the two modified PI films when the nano-particles content is 5wt%.The result can be attributed to various factors,including the relation between relative dielectric constant,conductivity of the nanoparticles and the PI matrix,changes in the internal defects of the PI matrix,alterations in the number of carriers,and migration and transport of electric charges.（2）The appropriate introduction by adding nano-SiO₂ or nano-Al₂O₃ particles can inhibit the injection and accumulation of the space charges in PI films with a certain extent,optimize the internal electric field distribution and reduce the corresponding electric field distortion rate.The best modification effect of nano-SiO₂ and Al₂O₃ for the modified PI films is observed with the adding mass fractions of 5wt%and 7wt%,respectively.When the compatibility between the nanoparticles and the PI film substrate is optimal,the nanoparticles are uniformly dispersed in the film,and the most uniform trap structure is formed.A small part of the carriers in the PI matrix are trapped by deep traps,while the most carriers migrate along the trap structure,to increase the mobility of carriers in the PI matrix.It reduces the accumulation of internal space charge in the PI matrix,decreases the electric field distortion caused by space charge accumulation,and makes the internal electric field intensity distribution more uniform.（3）The AC and DC breakdown strengths of different PI films were tested,and it is observed that the values of AC and DC breakdown strength for the modified PI films first increase and then decrease as the nanoparticle content increasing,reaching a maximum of the samples with 5wt%adding mass fractions.Compared with the unmodified ones,the improvement rates of AC and DC breakdown strength of PI films modified by nano-SiO₂ with 5wt%adding mass fractions are 20.28%and 18.42%,respectively,while those of PI films modified by nano-Al₂O₃ with 5wt%adding mass fractions are 27.24%and 20.05%,respectively.Because of the adding of nanoparticles,the trap structure inside the PI matrix is improved,the electric field concentration and heat generation caused by charge accumulation are reduced,the breakdown strength of the samples is enhanced.