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Epoxy resin has been widely used in the power industry owning to its superior insulating properties.The addition of nanoparticles can improve the dielectric property of epoxy,i.e.decrease the permittivity and dielectric loss and enhance the partial discharge resistance.[1] However,according to the literature,the volume resistivity of epoxy is usually deteriorated by the introduction of nanoparticles which can be ascribed to decrease of the glass transition temperature(Tg).[2] In this work,we found that the introduction of untreated silica(SiO2)nanoparticles can increase the Tg but decrease the volume resistivity of epoxy,which may be due to the presence of the hydroxyl group on the surface of SiO2.In order to improve the volume resistivity of epoxy,alkyl chain with the different length(C8 and C16)was grafted on the surface of SiO2 to sterically shield the hydroxyl groups on the particles.FTIR,TGA and XPS analysis were carried out to characterize the chemical structure of the modified SiO2 nanoparticle.We found that the volume resistivity of epoxy was increased when the content of C8@SiO2 or C16@SiO2 was low(≤ 1wt%),but decreased at high filler content.C16@SiO2/epoxy nanocomposite exhibited superior volume resistivity compared to C8@SiO2/epoxy nanocomposite.The volume resistivity of all the samples at different temperatures ranging from 285 to 348 K was systematically measured.It was found that the volume resistivity decreased linearly with the increase of the temperature.The decreasing slope of the volume resistivity was found to be OH@SiO2/epoxy> epoxy>C8@SiO2/epoxy>C16@SiO2/epoxy.It can be concluded that the decrease of the volume resistivity in OH@SiO2/epoxy sample may be due to the existence of the hydroxyl groups which will enhance carrier conductivity.The long alkyl chain can effectively shield the hydroxyl groups and increase trap density as the polymer-nanoparticle interface thickness was increased.