Investigation On Interlayer Interface Structures And Properties Of PI/Al₂O₃ Tri-layer Nanocomposite Films

Polyimide has been widely used in microelectronics,electrical insulation,aerospace and other fields because of its excellent mechanical,thermal and electrical properties.With the rapid development of power electronic technology and high speed railway,pulse width modulation?PWM?technology is widely used in inverter-fed motor of electric-multiple-units?EMUS?,where polyimide were widely used as corona resistance insulation materials.During its usage of PI in EMUS,a great discharge and space charge caused by high frequency pulse voltage often lead to the premature aging of insulation materials,and traditional mono-layer polyimide films have been unable to meet the insulation requirements of variable-frequency motor.Therefore,the modification and functionalization of polyimide materials by addition of inorganic nano-particles into polymide has become a new hot topic in recent years.In this paper,both of the mono-layer PI/Al₂O₃ composite films and tri-layer PI/Al₂O₃ composite films with sandwich structure are prepared by casting film method.It is observed by TEM that tri-layer PI/Al₂O₃ composite films have a complete tri-layer structure,and the interlayer interface can be seen clearly,without cracks,pores and other defects.FT-IR shows that PI/Al₂O₃ composite films still have the typical chemical structure of polyimide,the addition of nanoAl₂O₃ particles and the design of tri-layer structure do not affect the imidization reaction;XRD patterns show that the nano-Al₂O₃ particles have reduced the order degree of PI molecular chain.Mechanical properties tests show that the addition of nano-Al₂O₃ particles will reduce the tensile strength and elongation at break of PI/Al₂O₃ composite film.However,under the same doping content,the tri-layer composite films have higher tensile strength and elongation at break,when compared with the monolayer PI/Al₂O₃ composite films.Especially under high doping content,the advantage of tri-layer composite films is more obvious,the pure PI layer could enhance the mechanical properties of composite films,the interlayer interface can also block the development of the micro crack,and improve the tensile strength and elongation at break.Results of some thermal properties of monolayer and tri-layer composite films,includes TG,DSC and DMA,indicated that the addition of Al₂O₃ can prevent the thermal decomposition of PI molecule,resulted the increase in thermal stability of the composite films.Broadband dielectric spectroscopy shows that when the content of nanoAl₂O₃ particles is 12wt%,PI/Al₂O₃ composite films have the higher dielectric permittivity and dielectric loss.Under the same doping content,compared with the mono-layer PI/Al₂O₃ composite films,tri-layer PI/Al₂O₃ composite films have the lower dielectric constant and dielectric loss,the pure PI layer can reduce the quantity of nano-Al₂O₃ particles and nano-matrix interface structure in the unit volume,which decreases the dielectric permittivity of tri-layer PI/Al₂O₃ composite films.With the increase of nano-Al₂O₃ particles,breakdown strength of PI/Al₂O₃ composite films increases firstly and then decreases.Under the same doping content,breakdown strength of tri-layer PI/Al₂O₃ composite film is significantly higher than that of mono-layer composite film,which should be attributed to the distributional effects on electric field intensity by tri-layer structure of composite.The space charge characteristics and trap characteristics of PI/Al₂O₃ composite films are studied.Results show that the addition of nano-Al₂O₃ particles can effectively restrict the space charge injection in composite films,space charge in the composite films is much lower than that of pure PI film.Because of the existence of a large number of deep traps,space charges can be accumulated in interlayer interface,and form space charge packet in tri-layer PI/Al₂O₃ composite films.TSDC results show that,compared with the pure PI film,absorption peak becomes higher and moves to higher temperature in composite films,which shows that there are a large amount of deeper traps in composite films.Photo stimulated discharge?PSD?test results show that the addition of nanoparticles can improve trap density of tri-layer composite films.The trap density of tri-layer composite films with 16wt% contents is greater than that of single-layer films,with the same Al₂O₃ contents.Thus,the tri-layer composite film interface structure can introduce a greater trap density in PI films more easily,so it has a certain role in the regulation of the space charge injected by the electric field.Based on the results tested aboved,the corona resistances of mono-layer and tri-layer composite films are tested in this paper.Results show that the addition of nano-Al₂O₃ particles will greatly extend the corona resistant time of PI/Al₂O₃ composite films,when the content of nano-Al₂O₃ particles is 28wt%,compared with pure PI films,the corona resistant time of tri-layer and mono-layer PI/Al₂O₃ composite films increases 113 times and 64 times.Through theoretical analysis,the interlayer interface in tri-layer PI/Al₂O₃ composite film has a large number of deep traps,thereby prolonging the corona resistance time.The concentration difference of loose layer on both sides of interlayer interface in tri-layer PI/Al₂O₃ composite film can hinder the growth of corona discharge circuit.