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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SUMMARY. When diamond is synthesized at conditions of comparatively high temperature and pressure, the nucleation rate is high, as is the growth rate of the nuclei. Consequently the product is usually an aggregate of crystals with dendritic or skeletal structure. In this study the presence of gold or silver as an additive mixed with a catalyst was found to have the effect of suppressing nucleation. When a homogeneous mixture of graphite, catalyst, and additive was treated at conditions where skeletons and dendrites were produced in the absence of additive, euhedral crystals of octahedra were formed. When a special cell assemblage for high pressure experiments, in which the graphite was placed inside a cylinder of catalyst coated with additive, was used, prismatic and tabular crystals were synthesized. S I N G L E crystals of diamond commonly occurring in nature are octahedral, dodecahedral, or cubic in shape, but those with prismatic or tabular morphology have been rarely found (Orlov, I977). In the laboratory, octahedral, dodecahedral, and cubic crystals are also formed when synthesis is attempted in the diamond-stable region close to the thermodynamic equilibrium between graphite and diamond (Berman and Simon, I955). If the conditions are far from the equilibrium, the initial rate of nucleation is promoted and the growth rate of these nuclei is accelerated. Consequently the crystals produced are usually dendrites or skeletal structures (Bovenkerk, I96I ). ExperimentalA cubic anvil-type device was used for the synthesis experiments (Osugi et al., i964). Pressures applied to samples were calibrated by the abrupt change of electric conductivities of Bi (2.5 and Present address: