Effect of Plasmas Hydroxylation Modified Nano-SiO2 Particles on Insulation Characteristics of Insulating Papers

In the oil-immersed transformer, the cellulose insulating paper is prone to partial discharge, which makes the insulating paper easy to break down and causes security risks. Therefore, it is important to improve the insulation performance of insulating paper. At present, the primary method is to add inorganic nanoparticles to insulating paper. By doping nanoparticles with a high specific surface area and surface energy in insulating paper, charge carriers can be adsorbed to reduce the energy and thus improve the insulation characteristics of insulating paper. However, the nanoparticle is easy to agglomerate, accumulating charge carriers in the nanoparticle aggregation area, resulting in partial discharge and breakdown. The silane coupling agent is often used to improve the agglomeration of nanoparticles. One end of the silane coupling agent can be combined with the hydroxyl group (−OH) on the surface of nanoparticles through a condensation reaction. The other end of the amino group can be combined with the hydroxyl group on the surface of insulating paper through hydrogen bonding force. Thus, the dispersion of nanoparticles on the surface of insulating paper is improved. However, the hydroxyl content on the surface of inorganic nanoparticles is low. The effect of direct modification with a silane coupling agent could be better, and the activity of the silane coupling agent will decrease with time. If the number of hydroxyls on the surface of nanoparticles can be increased, the effect of the silane coupling agent can be improved. Plasma grafting can graft corresponding functional groups on the surface of materials according to the application requirements, widely used in material surface modification. This paper uses high-frequency AC power to drive the dielectric barrier discharge (DBD) reactor to generate plasma in the humid nitrogen and oxygen mixture. The influence of the surface modification method of nano-SiO2 particles on the dispersion of nano-SiO2 particles and the insulation characteristics of insulating paper is studied. By grafting hydroxyl free radicals (·OH) from water molecule ionization onto the surface of nano-SiO2 particles, nano-SiO2 particles could be modified by hydroxylation, and more silane coupling agents could be grafted to improve the agglomeration. Accordingly, the cellulose insulating paper doped with nano-SiO2 particles was prepared by the in-situ polymerization method. The chemical composition, morphology, functional groups, and the number of grafted hydroxyl groups on the surface of nano-SiO2 particles before and after plasma modification were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Fourier transform infrared spectrometer (FTIR), and specific surface and porosity analyzer (BET) combined with titration. The effects of plasma treatment conditions and modification steps on the breakdown field and volume resistivity were investigated. The results show that the content of oxygen (O) element and the hydroxyl absorption peak on the surface of nano-SiO2 particles are significantly enhanced after plasma treatment, and the number of hydroxyl on the surface of nano-SiO2 particles reaches the highest when the plasma modification time is 5 min and the air relative humidity is 75%. SEM observation shows that the dispersion of nano-SiO2 particles in insulating paper is improved after plasma hydroxylation modification. When the relative humidity of air is 75% and the mass fraction of nano-SiO2 particles is 3%, the DC breakdown field and volume resistivity of insulating paper reach the maximum. The best modification procedure is to treat nano-SiO2 particles with plasma modification and silane coupling agent to obtain the optimal breakdown field strength and volume resistivity of insulating paper. It is confirmed that plasma hydroxylation-modified nano-SiO2 particles can improve their dispersion on the surface of insulating paper and improve the limiting effect on charge carriers. © 2023 Chinese Machine Press. All rights reserved.