Effects Of Plasma Modified BNNSs On Their Dispersion And AC Breakdown Strength Of EP Composites

Filling epoxy resin（EP）with boron nitride nanosheets（BNNSs）can effectively improve the thermal conductivity of epoxy resin composites.However,increasing the amount of BNNSs to a certain extent will cause agglomeration problems and reduce the AC breakdown strength of epoxy resin composites,which limits the further improvement of thermal conductivity of epoxy resin composites and its practical application.Modification with a surface modifier such as silane coupling agent can effectively improve the dispersion of the nanoparticles,but there are few hydroxyl groups on the surface of BNNSs,and the effect of directly modifying with coupling agent is not good,while plasma modification can increase the hydroxyl content on the surface of material.Therefore,in this paper,BNNSs were modified by Ar+H₂O bipolar nanosecond pulse discharge plasma hydroxylation,and then modified by silane coupling agent KH560 to improve their dispersion in epoxy resin.The influence mechanism of plasma modification on the AC breakdown strength of BNNSs/EP nanocomposites was studied,and more BNNSs could be filled on the premise of maintaining certain insulation strength,thereby improving the thermal conductivity of the composites.Firstly,a plasma modification platform was set up,and X-ray photoelectron spectroscopy（XPS）was used to study the modification effect under different humidity,and the appropriate humidity point was obtained for subsequent experiments.Then,the surface chemical properties of BNNSs modified by coupling agent and plasma+coupling agent were compared by fourier transform infrared spectroscopy（FTIR）and thermogravimetric analysis（TGA）.The results show that plasma modification can effectively improve the hydroxyl content on the surface of BNNSs,and the hydroxyl content increases first and then decreases with the increase of relative humidity.Plasma+coupling agent modified BNNSs have more chemical bonding with coupling agent,and the coating rate of coupling agent is also higher.Secondly,the minimum neighborhood distance and area distribution probability method were used to analyze the cross-section scanning electron microscope（SEM）pictures of BNNSs/EP nanocomposites,and directly evaluate the dispersion of BNNSs.The dispersion was indirectly characterized by the transmittance and impact strength of the composites.The influence mechanism was analyzed based on the the research results of surface chemical properties.The results show that the dispersion of BNNSs modified by plasma+coupling agent is better than that modified by coupling agent only.With the increase of BNNSs content,the degree of agglomeration increases,and the improvement of dispersion by plasma modification weakens,so the difference in dispersion between the two kinds of composites gradually decreases.Finally,the AC breakdown strength and thermal conductivity of BNNSs/EP nanocomposites were tested.The trap characteristics of the composites were tested by thermally stimulated depolarization current method,the space charge distribution of the composites was tested by pulse electroacoustic method,and the influence mechanism was analyzed based on the dispersion and nano-dielectric interface theory.The results show that with the increase of BNNSs filling amount,the deep trap density decreases,the trapping and binding effect on carriers weakens,and the space charge near the negative electrode changes from negative（inhibition charge injection）to positive（promotion charge injection）,so the AC breakdown strength of the two composites decreases.At the same filling amount,the interface between BNNSs modified by plasma+coupling agent and epoxy resin is stronger,the dispersion is better,and the volume fraction of deep trap area is larger,so the deep trap density is larger,and the AC breakdown strength is also larger.The interface thermal resistance between BNNSs modified by plasma+coupling agent and epoxy resin is smaller,and the better dispersion is conducive to the formation of thermal conduction paths,so the thermal conductivity of BNNSs/EP nanocomposites modified by plasma+coupling agent is larger.