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This paper focuses on the effects of the interface between epoxy resin and nano-or micro-scale silica filler. Experiments were carried out on epoxy resin containing nano-or micro-scale silica fillers (nano-composite or micro-composite) to investigate the effects of the epoxy/filler interface on the properties of water absorption, electrical insulation and fracture toughness. The results indicated that epoxy/silica filler interface has an impact on the properties of the nano-and micro-composites. Nano-composites were shown to have higher water absorption than micro-composites because water accumulated over a larger interface area. Moreover, the nano-composites with interfaces treated with a silane coupling agent showed insulation breakdown properties and fracture toughness superior to those of the base epoxy resin and the micro-composites. The chemical bonding by the silane coupling agent at the epoxy/nano-scale silica filler interface played a more important role in water absorption and insulation breakdown properties than for the micro-scale silica filler. The filler size reduction to nano-scale needs appropriate interface treatment because of its tremendously large interface area. In addition, belt-shaped area models demonstrated the improvement of insulation breakdown properties and fracture toughness from the viewpoint of encountering frequency between electrical treeing or initial cracking and fillers. It is assumed that an increase in encountering frequency prevents treeing or cracking from propagating efficiently and thus improves insulation breakdown properties and fracture toughness in the nano-composite.

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A primary concern in recent nanocomposite research is practical application. In this study, various kinds of epoxybased nanocomposites were made and their properties evaluated to determine their applicability as insulating materials for heavy electric apparatuses. Experimental results demonstrated that nano-fillers enhance insulation breakdown properties in nanocomposites. Moreover, nano-and micro-filler combinations were adopted as an approach toward practical application of nanocomposite insulating materials. These nano-and microfiller mixed composites had the same low thermal expansion as aluminum, and insulation breakdown properties superior to those of conventional insulating materials. Consequently, an aluminum conductor and a vacuum interrupter were molded by the nano-and micro-filler mixed composites for the first time in nanocomposite research.