Effect Of Plasma Treatment On Trap Characteristics Of Polyimide Films

As one of the core equipment of high-speed train,the inverter-fed traction motor adopts pulse width modulation technology（PWM）for speed regulation.Polyimide film is widely used in motor turn-to-turn insulation due to its excellent electrical performance.Under the action of high frequency steep pulses,the over-voltage at the end of the motor winding and the partial discharge that may occur at the inter-turn insulation air gap will aggravate the aging rate of the film,leading to premature failure of the insulation performance,which will eventually affect the safe and stable operation of the train.Doping inorganic nanoparticles in the film can improve its insulation performance,but with the increase of high-speed train speed,the power of variable-frequency traction motor increases,it’s very important to find a more economical and convenient method to improve the inter-turn insulation performance of inverter-fed traction motor for high-speed trainsThe surface modification of PI film and PI/Al₂O₃ nanocomposite film by plasma treatment is carried out in this thesis.The structural characteristics of the film surface were analyzed and the electrical properties of the film after plasma modification were tested.The influence mechanism of plasma treatment on the insulation properties of the film was studied from the perspective of film trap distribution characteristics.The main research work is as follows:（1）PI film and PI/Al₂O₃ nanocomposite film were prepared by in-situ polymerization.The film surfaces was modified by plasma treatment.The surface morphology and chemical structure of the film were analyzed by scanning electron microscopy（SEM）and Fourier transform infrared spectroscopy（FTIR）.In the early stage of plasma treatment,polar groups are introduced on the surface of the film to form a new cross-linked layer.As plasma processing time increased,the surface structure of the film will be destroyed.（2）The corona-resistant lifetime and breakdown field strength of PI film and PI/Al₂O₃nanocomposite film were tested.The experimental results show that the insulation performance of the film was improved in the early stage of plasma modification,and then was reduced when the treatment time was too long;The dielectric spectrum of the thin film after plasma treatment was tested.It is found that the relative dielectric constant increased by the change in the chemical structure of the film surface.An isothermal surface potential decay test platform was built to study the influence of plasma treatment on the surface potential decay characteristics of the film.The experimental results show that the film surface potential decay rate increases first and then decreases with the increase of plasma processing time.（3）Based on the isothermal surface potential decay method（ISPD）,the trap distribution characteristics of PI film and PI/Al₂O₃ nanocomposite film after plasma modification were calculated and analyzed,and then used peak separation method to obtain the distribution characteristics of deep and shallow traps.It is found that the total trap density and the trap center energy level in the film decrease first and then increase with the increase of plasma processing time.Then combined with the changes of surface structure and dielectric properties of the films,the effect of plasma treatment on corona resistance lifetime and breakdown field strength of thin films were investigated from the perspective of trap characteristics.