Preparation,characterization And Properties Of Polyimide/Nano SiO₂-Al₂O₃ Composite Corona Resistant Film

Polyimide?PI?is widely used in aerospace and electrical and electronic fields due to its excellent thermal stability,mechanical properties and electrical insulating properties.In recent years,with the wide application of AC frequency conversion speed regulating motor,the requirements for corona performance of insulating materials has been further improved.Design and development a kind of new materials have become a hot issue in the study of material science.In this paper,nano SiO₂-Al₂O₃ materials were doped into PI matrix and nanometer dielectric materials with high corona resistance were prepared.Nano SiO₂-Al₂O₃ dispersions were prepared by the combination of sol-gel method and hydrothermal method.The nano SiO₂-Al₂O₃ and PI were hybride by in-situ polymerization,and PI/nano SiO₂-Al₂O₃ monolayer and multilayer composite films were prepared by means of casting and dipping method.The microstructure of nanoparticles and composite films was studied by means of X ray diffraction,infrared spectroscopy and projection electron microscopy.The results show that the nano SiO₂-Al₂O₃ is an amorphous structure,and the structure of Si—O—Al is formed,the average particle size is about 15²0nm.The introduction of nano SiO₂-Al₂O₃ particles reduces the degree of ordering of the PI molecular chain,but has little influence on the degree of imidization of PI matrix.The dispersibility of nano SiO₂-Al₂O₃ particles in PI matrix,and the average particle size is less than 30 nm.There is no crack and porosity in multilayer composite film.The study of thermal stability shows that the thermal stability of PI/nano SiO₂-Al₂O₃ composite films can be improved by the increase of nano SiO₂-Al₂O₃doping,and the thermal properties of the three layer composite films are better than that of single layer films.The analysis of the mechanical properties of the film shows that the introduction of nano SiO₂-Al₂O₃ particles will reduce the tensile strength and elongation at break of the PI/nano SiO₂-Al₂O₃ composite film.Compared with the monolayer film,the three layer composite films prepared by dipping method have higher tensile strength and elongation at break,and five layer composite structure can further improve the mechanical properties,the tensile strength and elongation at break of the five layer composite film A₁₈A₂₂PA₂₂A₁₈are 147 MPa and 45%respectively.The thermal stability study shows that the thermal stability of the PI/nano SiO₂-Al₂O₃ monolayer and three layer composite film is higher than that of the pure PI film.Broadband dielectric spectroscopy shows that the dielectric constant and dielectric loss of the three layer PI/nano SiO₂-Al₂O₃ composite films increase with the increase of SiO₂-Al₂O₃ doping,and when the doping amount of nano particles is the same,the dielectric constant and dielectric loss of the three layer composite films are lower then the monolayer film.The dielectric strength analysis shows that the dielectric strength of the three layer PI/nano SiO₂-Al₂O₃ composite thin film is better than that of the monolayer film.The five layer composite structure can further improve the dielectric strength characteristics of the material while ensuring the doping amount of nanoparticles.The conductivity test shows that the three layer PI/nano SiO₂-Al₂O₃ composite film has good insulation properties,and the limited carrier density of the composite film increases with the increase of the amount of nano SiO₂-Al₂O₃ doping.The research of corona aging characteristics shows that the PI molecule end group,the dispersion state of nanoparticles and the laminated structure of composite will affect the corona resistance of composite films.The effect of the PI molecular end group on the corona resistance of the film is:end phenyl>end anhydride group>end amido.The corona resistance life of the film will be improved after secondary particles orientation.When the tensile ratio is 10%,the corona resistance life of the three layer composite film with 15wt%doping is14.7h,under the conditon of 155?,80kV/mm,which is 41%higher than that of original film.When the SiO₂-Al₂O₃ doping amount is the same,the corona resistance life of the composite can be improved by the design of the laminated structure of the film.The corona test results under different electric field intensity show that the corona resistance life of 20wt%doped three layer PI/nano SiO₂-Al₂O₃ composite films prepared by dipping method is 3230.05 h and 3.65 times of the Kapton100 CR at 155?.The corona lifetime is related to the activation energy E_a,and the E_a value increases with the increase of nano SiO₂-Al₂O₃ doping amount.Based on the analysis of the results of a large number of corona tests and the study of the changes in the morphology and structure of the films before and after the corona aging,the model of the nanoparticles,the polymer matrix and the laminated structure was established.It is considered that the corona resistance mechanism of PI/nano SiO₂-Al₂O₃ composite film can be summarized as the effect of space charge against electric field,the shielding effect of inorganic particles,the dredging effect of nanoparticles on the charge and the partial pressure of the laminated structure.