Electrical Characteristics Of Multilayer Polyimide Nanocomposite Films For High Voltage Motors

Polyimide(PI) is widely used in internal insulation of inverter-fed motors of high-speed units due to its high-performing electrical,thermal,and mechanical properties.As the speed of these high-speed units increases,the power of inverter-fed motors increases.Consequently,PI film needs to bear strong electrical stresses in high-temperature environments.In such severe environments,partial discharge and space charge would cause irreversible chemical and physical degradation of PI film,resulting in premature failure.Nanoparticles can significantly improve the property of materials due to large specific surface areas;thus,they have received considerable attention to date.Therefore,introducing nanoparticles into polymers is now a prevailing area of research.However,factors like the distribution of nanoparticles,type of nanoparticles,interfacial zone between nanoparticles and polymer matrix,physical structure of nanocomposites,and so on,have a great influence on the physical,chemical,electrical,and thermal properties of nanocomposites.The research regarding these issues will have significant theoretical as well as engineering influence on the production of a well-formed PI nanocomposite that can be applied on an industrial level.In this thesis,we first prepared pure PI film and PI nanocomposite film and analyzed their chemical and physical structures using a scanning electron microscope(SEM)and Fourier-transform infrared spectroscopy(FTIR),respectively.We also conducted surface discharge experiments under different rise times and frequencies of pulse voltage.The result shows that nano Al₂O₃ can connect with a PI matrix through a covalent bond,thereby leading to the formation of a stable interfacial region.After surface discharge aging,nano Al₂O₃ would deposit on the film's surface to hinder further electrical erosion.In addition,a PI nanocomposite with a higher charge-dissipating ability can reduce the number of residual charges after each discharge event,resulting in mitigation of overlapped electric fields as the pulse voltage reverses,which can reduce the released discharge energy;thus,the surface discharge resistance of the PI nanocomposite is better than that of PI.Second,we prepared multilayer PI films doped with different nanoparticles(Si O₂ and Al₂O₃)based on a modified in-situ method and measured their dielectric frequency spectroscopy through a frequency spectroscopy tester.We then analyzed the distribution of nanoparticles using an atomic force microscope(AFM)and an electrostatic force microscope(EFM)and discussed the general law of mechanical as well as electrical breakdown strength.The results show that both the real part and the imaginary part of complex permittivity of triple-layer Al₂O₃-PI-Al₂O₃ film are the biggest,while those of single-layer PI/Si O₂ are the smallest.It is found that,in comparison to Si O2,the compatibility of Al₂O₃ in PI is better.In addition,the mechanical and electrical breakdown strength of films are influenced by the interparticle distance in nanocomposite films.Third,we proposed a model consisting of triple-layers of PI with air gaps and used it to analyze the influence of interface on an electric field.We also characterized the space charge behavior of triple-layer PI nanocomposites and calculated the trap energy levels using charge decay data and thermally stimulated current data,and the calculated results were compared with each other.The results showed that the polarity of space charge in interface depends on the electronic state of interface material.Thus,the electric field in interface may increase or reduce and,consequently,influence partial discharge inception voltage.Additionally,triple-layer Al₂O₃-PI-Al₂O₃film can suppress the accumulation of space charge,and most charges are in traps with a low energy level(1.126 e V),while in double-layer Al₂O₃-Si O₂ film,traps in a high energy level(1.26 e V)and a high trap density are observed.On the other hand,the charges in triple-layer films decay faster than those in double-and single-layer film.It can be concluded that the type of nanoparticles and structure of interface will greatly influence the effect of interface on suppressing space charge.