Polymer insulation materials such as epoxy resins generate micro-scale damage under long-term electrical-thermal aging and mechanical stress, which induces insulation failure and seriously threatens equipment operation safety. Microcapsule technology realizes the independent repair of micro-scale damage of insulating materials, however, most of the existing microcapsule self-repairing technology adopts liquid repairing agent, which has the problems of irreversible curing and only single repairing, and also needs a strong external force to be triggered passively. In this paper, microcapsules with magnetic field targeting effect composed of the phase change material octacosane ware prepared by using Fe₃O₄@SiO₂ nanoparticles as Pickering emulsion stabilizers, which were composited with normal temperature curing epoxy resin to form an insulating material with self-repair function. The solid-liquid transformation of the phase change material octacosane repair agent is reversible, which gives the composite material the property of being able to be repaired repeatedly.
Ultrafine Fe₃O₄@SiO₂ nanoparticles were used as the oil-in-water emulsifier. During the formation of phase change microcapsules, Fe₃O₄@SiO₂ nanoparticles were at the junction of water and oil (melted eicosanoids), and when the eicosanoids were cooled down to room temperature, the Fe₃O₄@SiO₂ nanoparticles were wrapped around and embedded in the outer surface of solid eicosanoids, thus forming phase change microcapsules. The Fe₃O₄@SiO₂ nanoparticles embedded on the surface of the solid eicosanoids reparative material have the dual functions of targeted migration in response to a directional magnetic field and focused infrared targeted heating, which can attract the microcapsules to the damage-prone parts of the composite material under the action of a directional magnetic field. At the same time, an appropriate amount of silane coupling agent-modified Al₂O₃ nanoparticles were introduced into the epoxy resin matrix to enhance the thermal conductivity and insulation strength of the composite material. When microdamage was generated, focused infrared light was applied artificially and conveniently under charged conditions to target heating of the microcapsules, which induced the phase change microcapsules in the damaged area to melt and flowed out rapidly to fill the damaged channels. Upon cooling and solidification, the material realized autonomous repair. In this paper, the microstructure and thermal stability of the microcapsules and the insulating and thermal conductivity of microcapsules/nano-Al₂O₃/epoxy composites were experimentally investigated, and finally the self-repairing performance of the composite insulating material was tested on the surface of the mechanical scratch damage to verify the self-repairing characteristics of the composite material.
The following conclusions can be reached from test analysis: (1) The particle size of the microcapsules is uniformly distributed, and the cumulative 80% frequency range is concentrated in 50.02~138.56 μm. Additionally,they maintain stability and do not decompose thermally below 200℃. (2) The magnetic targeting induction technology can improve the self-repairing efficiency of the material and reduce the doping amount of the microcapsules, so as to maintain the good intrinsic performance of the substrate; The doping of nano-Al₂O₃ particles endows the composite material with excellent thermal response characteristics for targeted infrared radiation heating. (3) The relative dielectric constant of the 2% microcapsules/1% Al₂O₃/EP composites is approximately equal to that of the pure epoxy resin, and the dielectric strength has been improved by 2.32%. The composites are capable of repairing mechanical scratch damage autonomously, and can fully fill the scratch damage channels on the material surface. The insulation strength can be restored to 90.78% of the undamaged one.