Preparation And Study Of Polyetherimide Matrix Field Grading Composites With Low Percolation Threshold

The high voltage involved in high-voltage direct current transmission technology tends to induce local electric field concentration and distortion.Field grading composites which possess field-dependent nonlinear conductive characteristics show good insulation under normal operating conditions at low electric fields.When the electric field exceed a critical value,conductivity of the composite will rapidly increase to alleviate local field strength to realize field grading.A prerequisite for the composite to exhibit stable and continuous nonlinear conductive characteristics is that the filler content exceeds the percolation threshold.A high filler content is unfavorable for flexibility of the composite and will increase weight of the material.In addition,it is prone to cause overheating at high electric fields,limiting the application scope of the composite.Therefore,studying field grading composites with low percolation threshold is of great significance and value.In the work of this dissertation,composite films were fabricated by using polyetherimide（PEI）as matrix and adopting SiC whisker,Fe₃O₄ microplate,MoS₂ and Ti₃C₂T_x as fillers.Morphologies and structures of the fillers and composite films were characterized.Field-dependent nonlinear conductive characteristics and dielectric properties of the composite films were studied and related mechanisms were investigated.（1）SiC whisker/PEI and SiC particle/PEI composite films were prepared by blade coating.Percolation threshold of SiC whisker/PEI composite film was 7 phr,which was much lower than that of SiC particle/PEI.At percolation threshold,nonlinear coefficient of the SiC whisker/PEI composite was 4.87 and its conductivity increased for 3magnitudes before conductivity saturation.This study indicates that fillers with high aspect ratio are good for preparing field grading composites with low percolation thresholds.（2）Two-dimensional Fe₃O₄ microplates were prepared by a polyvinylpyrrolidone（PVP）-assisted hydrothermal method.A small amount of PVP adsorbed on the surface of the Fe₃O₄ microplate provided good compatibility between it and the matrix.Percolation threshold of Fe₃O₄ microplate/PEI composite film was about 8 phr,and with this filler content,its nonlinear coefficient was 5.37.（3）High-power sonication（300 W,3h）treatment was adopted for exfoliating MoS₂ particles.The treated MoS₂ particle（HPS-MoS₂）was transformed into microplates and a small amount of nanosheets.The percolation thresholds of the MoS₂ particle/PEI and HPS-MoS₂/PEI were both 15 phr and at this filler content,their nonlinear coefficients were 6.92and 7.38,respectively,and their switching fields were 17.2 k V/mm and 8.8k V/mm,respectively.It indicates that the high-power sonication treatment on MoS₂ particles can tailor the nonlinear conductive characteristics of the MoS₂/PEI composites.The dielectric constant of HPS-MoS₂/PEI was higher than that of MoS₂ particle/PEI at the same filler content.（4）Ti₃C₂T_x nanosheets were prepared by etching Ti₃AlC₂ with HCl/LiF.Ti₃C₂T_x/PEI composite films were made by drop casting.Percolation threshold of Ti₃C₂T_x/PEI was as low as 2 phr and nonlinear coefficient of the composite film reached to 10.18 at filler content of 6 phr,while its conductivity increased for over 3 magnitudes before conductivity saturation and its dielectric constant reached to 13.8 at 1 k Hz.Its grading effect on electric field distribution in cable terminal was shown by COMSOL Multiphysics simulation.Nonlinear conductive characteristics of the composite film can be regulated by conducting hydrothermal treatment on Ti₃C₂T_x to vary its oxidation degree.Nonlinear conductive characteristics of the PEI matrix composite films filled separately by SiC whisker,Fe₃O₄ microplate,MoS₂ and Ti₃C₂T_x were driven by Fowler-Nordheim tunneling effect or Schottky emission.The PEI matrix composite films filled by SiC whisker,Fe₃O₄microplate,MoS₂ and Ti₃C₂T_x were competitive in both low percolation threshold and nonlinear conductive characteristics.The work of this dissertation revealed that these fillers have the potential of being adopted for field grading composites with low percolation threshold and low filler content and provided valuable references for the study of high-performance field grading composites.