Corona Resistance Mechanism Of Nano-modified Polyimide

Polyimide（PI）is an widely used engineering plastic with good performance,which is embodied in mechanical properties,electrical properties,radiation resistance,electrical insulation properties and chemical stability.These excellent properties benefit from its own aromatic heterocyclic structure.In recent years,with the advancement of science and technology and the continuous convenience of transportation methods such as high-speed trains and steamships,the frequency conversion speed regulation motor is an significant components of high-speed rail,and its application has become more extensive.At the same time,the requirements for improving the corona resistance of the frequency conversion speed regulation motor have also been further improved.Therefore,it is of great significance to improve the corona resistance of polyimide film materials by nano-modification to delay the motor failure caused by short-term high-voltage breakdown.In this paper,nano-SiO₂dispersion was prepared by combination of microemulsification and supercritical,and nano-SiO₂/polyimide（PI）three-layer composite films were prepared by laboratory-made molds.Scanning electron microscope（SEM）and transmission electron microscope（TEM）were used to characterize the morphology of the composite film before corona,infrared spectroscopy（FT-IR）was used to characterize the structure of the composite film before corona,and scanning electron microscope（SEM）was used to characterize the morphology and structure near the breakdown point after corona.The results showed that there was no obvious agglomeration phenomenon,indicating that the nano-SiO₂particles had good dispersion in PI matrix.After the PI composite films were broken down by corona,the breakdown area was of corona damage.The perforation was mostly round or irregular ellipse.The matrix was eroded,and nanoparticles were precipitated on the surface of the film,which collected on the surface of the film and hinders the application of strong electric field.The corona resistance life of the film can be improved.Conductivity current test showed that the increase of nano-SiO₂content increases the electrical aging threshold of PI three-layer composite film.The electrical aging threshold reflects the minimum field strength when the material begins to age,and the electrical aging threshold increases,indicating that the electrical aging resistance of the material is improved.The dielectric spectrum tests showed that the dielectric constant of the three-layer composite film increased with the increase of the content of nano-SiO₂in the doping range of this experiment.When the content of nano-SiO₂components is the same,the dielectric constant decreased with the increase of test frequency.The dielectric loss of nano-SiO₂/PI three-layer composite film decreased with the increase of frequency at low frequency,but the dielectric loss and conductivity increased with the increase of frequency at high frequency.In this paper,the effect of electric field strength and temperature on the corona-resistant life of the film and the method of predicting the corona-resistant life of the three-layer composite film under the working electric field strength was were studied and discussed in combination with the electrical aging threshold.The results showed that the introduction of nano-SiO₂particles can greatly improve the corona resistance life of the material.At the temperature of155^oC,when the applied external electric field strength was extrapolated to 20k V/mm,the corona resistance life of the PI/nano-SiO₂three-layer composite film with 20wt%of nanoparticles was 8885.00 h.At room temperature（20^oC）,the corona resistance time of 20 wt%PI/SiO₂three-layer composite film was 113.73h,which was 28.50 times that of pure PI film.In this paper,combined with the structural characterization and dielectric properties of polyimide composite films,the obtained test results were analyzed separately,so as to explore the corona resistance performance of the composite film,and the relative method of corona resistance life is inferred according to the electrical aging threshold.