The Influence Of Nanostructured Particles On The Electrical Properties Of Silicone Rubber

Silicone rubber（SR）,with excellent dielectric properties,thermal stability and hydrophobicity,has been widely used in high-voltage insulation fields such as composite insulator,anti-pollution flashover coating and cable stress cone.With the continuous improvement of the voltage level of the power transmission in our country,high-capacity and long-distance transmission lines put forward higher requirements on the mechanical properties and electrical insulating properties of SR.A few percent nanoparticles can significantly improve the electrical properties of the composites,such as reducing the dielectric constant of the composites,increasing the volume resistivity,and improving the breakdown strength,due to their large specific surface area.However,in the study of nanodielectrics,nanofillers are easy to agglomerate and disperse unevenly in the matrix.How to get better dispersion of nanoparticles has always been the difficulty and emphasis in the research of nanodielectrics.In our study,two methods were utilized to enhance the dispersion of particles:1)For nanoparticles,organic surface modification was carried out to improve the dispersion of nanoparticles;2)Using submicron or micron nanostructured particles,which retain the advantages of nanoparticles,and easy to disperse.The main research contents and results are as follows:（1）Effects of nano MgO on the electrical properties of SRBy the sol-gel method and sintering process under high temperature（800 °C or 1000℃）,nano MgO（marked as MgO800 and MgO 1000）with different surface activity were prepared.Modified surface of nano MgO（marked as MgOC12）was prepared using dodecyltriethoxysilane（C12）.The results demonstrated that by boosting the calcination temperature,the particle size and crystallinity of MgO increased,and the surface activity of particles decreased as well,accordingly the adsorption of oxygen and water became low.After surface modification,the surface of MgOC12 turned hydrophobic,hence the moisture resistant of the particles improved.Due to the poor compatibility between particles and the matrix,MgO800 and MgO 1000 almost had no reinforcing effects on the mechanical properties of SR.Even if MgOC12 particles were evenly dispersed into the matrix after surface modification,the tensile properties of SR/MgOC12 composites remainded weak.When the filler loading kept less than 1 phr（parts per hundred rubber）,nano MgO increased the volume resistivity of SR by the order of MgO800>MgO1000>MgOC12.This phenomenon occured because the particles with high surface activity could effectively capture carriers and improve the volume resistivity of composites.For untreated nano MgO,as the filler content maintained more than 1 phr,the volume resistivity of composites commenced to decrease owing to the aggregation of the particles.Since MgOC 12 was uniformly dispersed in the matrix and held the filler loading at 3 phr as well,the volume resistivity of composites reached the maximum(7.7×10¹⁶ Ω·cm).The interface interaction between nano MgO and SR matrix was weak,so the breakdown strength of SR/MgO composites had no obvious change even after surface modification of the particles.When the loading of MgO800 and MgO 1000 kept high,the breakdown strength of the composites decreased slightly.（2）Effect of hollow silica spheres（HSS）on the electrical properties of SRUsing polystyrene microspheres as templates,HSS with wall thickness of 15 nm and 35 nm（marked as HSS 15 and HSS35）were prepared by sol-gel method and sintering process.Also,monodisperse solid silica（marked as SS）was prepared by stober method.The results revealed that the HSS and SS particles with sub-micron size were uniformly dispersed in the matrix without agglomeration.Fumed silica was easy to form agglomerates in the matrix despite the strong shear of the three-roll extruder.Due to its high specific surface area,the rough surface and the holes on the shell,HSS 15 can introduce a strong interface interaction into the composite.Therefore,the tensile properties and thermal stability of SR/HSS15 composites were remarkably improved.Since the molecular chains and dipoles were restrained by the interface zone,and the dielectric constant of air inside the microspheres was low,dielectric constant of the composites was reduced.Because of the binding effect of the interface zone on the carriers,the concentration and migration rate of carriers were diminished,so the volume resistivity of SR/HSS15 composites greatly increased.Compared to pure SR,the breakdown strength of SR/HSS15 composites increased by 37.7%at most,which was attributed to the strong interface interaction between HSS 15 and the matrix,the blocking effect of particles on the development of electron destruction channel and the protection of HSS 15 on the matrix.Owing to the introduction of Maxwell-Wagner interfacial polarization in SR/SS composites,the dielectric constant and dielectric loss of the composite increased;the volume resistivity and breakdown strength also decreased with increasing the filler loading and defects.（3）Effect of porous glass microspheres on the electrical properties of SRPorous glass microspheres,namely porous surface HGM（marked as HGM-1）and the through-hole structure HGM（marked as HGM-2）were prepared by alkali etching under hydrothermal conditions using hollow glass microspheres（HGM）as a raw material.Due to the abundant pore structure and high specific surface area of HGM-2,the interface bonding strength among HGM-2 and the matrix was strengthened,so the tensile strength and elongation at break of SR were significantly improved.Moreover,the thermal stability of the composites was improved because of the strong interfacial bonding and the protection of the microspheres on the matrix.Volume resistivity of SR with a few HGM-2 loading（<5 phr）was increased because of the limitation of carrier migration in the interface zone.Furthermore,on account of the protective effect of HGM-2 on the matrix,the blocking of the electron destruction channel by the microspheres and the strong interface interaction,the breakdown strength of SR/HGM-2 composites was notably improved,which is rare for polymer composites doping with micron particles.However,the dipole and interfacial polarization were accelerated owing to the large particle size,pore structure complexity and increasing of hydroxyl ions and hydroxyl groups on the surface of HGM-2,which increased the dielectric constant and dielectric loss of the composite.The HGM-1 porous structure existed on the surface,so the tensile properties of SR/HGM-1 was not as good as that of SR/HGM-2.The volume resistivity and breakdown strength of SR/HGM-1 composites decreased slightly because of the presence of impurity ions.For HGM,the impurity ions on the surface of the particles and the weak interface interaction led to the rapid decrease of the volume resistivity and breakdown strength of the composites.