Preparation And Electrical Properties Of Silicon-based Nano-porous Insulating Paper Reinforced By Ceramic Fibers

The continuous development of high performance advanced electric equipment requests strictly on the performance of insulating materials.The research and development of high insulating materials are related to the miniaturization,low cost and safe operation of electrical equipment.Nano-composite technology is considered as an important method to improve the dielectric insulating properties.At present,most researches focus on filled composite,which is adding inorganic oxides into polymer matrices.However,this kind of material,developed more than 20 years ago,still has many problems.For example,breakdown strength improvement of dielectric is limited(generally lower than 30%),the dielectric loss is high,and the long-term stability and dispersion uniformity of nanoparticles are poor.These problems seriously restrict their industrial application.Based on the above research background,this paper bypassed the conventional nano-composite technology and proposed a novel silicon-based nano-porous dielectric material by hydrolytic polycondensation method.The experimental study and theoretical analysis were carried out on the mechanism of multi-performance parameters and internal mechanism.The specific research work and conclusions are as follows,(1)Ceramic fiber-porous silicon composite insulating paper was prepared by hydrolytic polycondensation and atmospheric pressure drying.The hydrolytic condensation reaction of the reactant of ethyl orthosilicate precursor was induced in situ polymerization to produce ceramic fiber-porous silicon composite insulating paper.Changing the concentration of the precursor reactants to regulate the pore structure of the dielectric.The basic physical properties,microscopic properties and thermal properties of the dielectric were determined.It was found that the composite dielectric prepared by the mol ratio of TEOS:H₂O=1:3 had a uniform structure with an average pore size of 14nm.The thermal stability of the dielectric can be effectively improved by constructing the nanostructure with a hydrophobic angle of 141°.(2)The electrical properties of ceramic fiber-porous silicon composite insulating paper were experimentally studied.Ceramic fiber-porous silicon composite insulating paper with different pore characteristics was prepared by changing the concentration of the precursor reactants.The influence of pore characteristics on the AC/DC breakdown characteristics and dielectric characteristics was studied.The results showed that the breakdown field strength can be greatly improved by constructing the nanostructure,and the breakdown field strength increases with the decrease of the pore size of dielectric.Compared with the dry samples,the breakdown field strength of ceramic fiber-porous silicon composite insulating paper after impregnated with insulating oil can achieve greater promotion which increased by 120.5%(under DC voltage)and 90.4%(under power frequency voltage).The relative dielectric constant,dielectric loss and volume conductivity of the dielectric can be reduced to some extent by constructing the nanostructure.At 50 Hz,the relative dielectric constant,dielectric loss factor and volume conductivity of ceramic fiber-porous silicon composite insulating paper decreased by12.4%,39.2%and 52.5%respectively,compared with ceramic fiber paper.(3)Based on the scale effect and space charges transport characteristics of the nano-porous structure,the microscopic mechanism of improving the electrical performance of the ceramic fiber-porous silicon composite insulating paper was revealed.The effects of the nanostructure on the mean free path of gas electron,the number of molecules and the size of the head of electron avalanche were calculated and analyzed.It was considered that the scale effect of nanostructure will hinder the development of electron collapse in the process of dielectric breakdown in nanopore.The intrinsic breakdown strength of the dielectric was increased by inhibiting initial ionization.The space charges accumulation and dissipation characteristics in ceramic fiber-porous silicon composite insulating paper were obtained by the electroacoustic pulse method.The energy level and distribution characteristics of traps in ceramic fiber-porous silicon composite insulating paper were obtained by TSC test.It was found that the nanostructure can reduce the energy level and density of traps in the dielectric.It is concluded that nanostructure can enhance the electron injection barrier which contributes to restrain the injection of space charges and accelerate the dissipation of space charges.Reducing the local electric field distortion and thus resulted in improving the electrical properties of the dielectric.