Reseach On Microstructure And Electrical Properties Of Polyimide Composites Modified By Zero And Two Dimensional Nanomaterials

Polyimide are widely used in frequency conversion motors,integrated circuits,fuel cells,aerospace and other fields due to its excellent insulation,mechanical properties and thermal stability.In this paper,polyimide was synergistically modified by zero-dimensional SiO₂,Al₂O₃ nanoparticles and two-dimensional BN nanosheets with unique structure and properties.PI/SiO₂,PI/Al₂O₃,PI/BN and PI/BN+Al₂O₃composite films with 5%,10%,15%and 20%components were designed and prepared by in-situ polymerization,and their microstructures,electrical properties,thermal properties and corona resistance mechanism were compared and studied.The particle sizes of zero-dimensional nano SiO₂ and Al₂O₃ are about 7.5 nm and 13.0 nm,respectively.SiO₂ surface is negatively charged and Al₂O₃ surface is positively charged.They are uniformly dispersed in the polyimide matrix.Large number of polyimide molecular chains are adsorbed on the surface of the particles and form organic-inorganic interfacial layer.SAXS and TEM tests confirm the existence of the interfacial layer.The thickness of the interfacial layer tested is about2 nm.The experimental results show that the dielectric constants of the composite films increase,the conductivity and dielectric loss increase slightly,with the increase of SiO₂ and Al₂O₃ components.Under high electric field,SiO₂ and Al₂O₃ play a protective role on the polyimide molecular chain and prevent its decomposition into amino and hydroxyl groups.The corona aging life and AC breakdown electric field of the composite films increase significantly.With the same composition,the corona aging life of PI/Al₂O₃ is higher than that of PI/SiO₂,the corona aging life of PI/Al₂O₃20%is about 25 times of that of pure PI.With the increase of SiO₂ and Al₂O₃components,the thermal stability and thermal conductivity of the composite films were improved.With the same composition,the thermal stability and thermal conductivity of PI/Al₂O₃ were higher than those of PI/SiO₂.Two-dimensional nano BN has an irregular lamellar structure,with planar dimension about 100-250 nm and thickness less than 7 nm.BN was induced to align in Polyimide by multilayer coating,mechanical stress,electric field induction and heat curing.BN dispersed uniformly and achieved horizontal alignment under low component conditions.With the increase of BN composition,the dielectric constant of PI/BN increases and then decreases,the conductivity and dielectric loss increases slightly,the thermal conductivity and the thermal stability were significantly improved,is better than that of PI/SiO₂ and PI/Al₂O₃.Under high electric field,BN has layer-blocking hole carrier effect,the corona aging life and AC breakdown electric field of PI/BN are significantly improved,but decrease due to the poor dispersion of BN.In PI/BN+Al₂O₃ composite films,Two-dimensional nano is oriented horizontally,and Al₂O₃ surrounds BN independently and effectively fills in polyimide between BN to form a multi-disperse system of Al₂O₃ independently distributed around BN and BN+Al₂O₃ form uniform distribution.With the increase of BN+Al₂O₃component,the dielectric constant of PI/BN+Al₂O₃ increases rapidly.With the increase of BN+Al₂O₃ composition,the relative dielectric constant of PI/BN+Al₂O₃increases rapidly,the electrical conductivity and dielectric loss increase little,and the thermal stability and thermal conductivity are obviously improved.When the composition of BN+Al₂O₃ is 15%,the corona aging life and the AC breakdown field strength are increased to 28 times and 1.5 times of pure PI,respectively;when the composition of BN+Al₂O₃ is 20%,the corona aging life and the AC breakdown field strength are both decline.In this paper,the surface potential,space charge,thermally stimulated current and conductive current of the composite films are analyzed and tested from the micro point of view.The corona aging resistance mechanism of the composite films is revealed by combining the results of the microstructures,dielectric properties and thermal performances of the composite films.The results show that the dielectric constant of Al₂O₃?BN?BN+Al₂O₃ is higher,and the surface potential is lower than PI,the electric field lines near Al₂O₃?BN?BN+Al₂O₃ are denser,and the carriers mainly reach Al₂O₃?BN?BN+Al₂O₃ and their interface areas.BN blocks hole carrier transport to the interior,and the interface layer formed by Al₂O₃ and polyimide produces more deep trap levels,which become carrier transport and space charge composite channels.BN and Al₂O₃ limit polyimide orientation polarization,maintain insulation performance and lower dielectric loss.At the same time,the electrical conductivity and the aging threshold electric field are improved,and the space charge is prevented from accumulating.At the same time,the composite film has better thermal conductivity and heat aging performance,and the corona aging resistance is remarkably improved.In PI/BN+Al₂O₃,BN and Al₂O₃ play a synergistic role,in which BN blocks hole carriers and resists high-energy carrier bombardment of polyimide;Al₂O₃ effectively prevents BN agglomeration and forms a polydisperse system with BN,which facilitates transport of carriers,complex space charge,and has excellent thermal stability,further improving the polyimide corona aging life.