Abstract
We succeeded to synthesize pure polycrystalline diamond by direct conversion from graphite at very high pressure and high temperature using Kawai-type multianvil apparatus (KMA), which was found to be made of nano-sized crystals with peculiar fine textures and have extraordinary high hardness. Thus synthesized nano-polycrystalline diamond (NPD or "Hime-diamond") has following features in addition to its outstanding hardness: higher fracture toughness, higher thermal resistance, and relatively low thermal conductivity, etc., relative to single crystal diamond. NPD is highly transparent, but exhibits rather yellowish in color due to absorption of optical lights in shorter wave lengths, probably because of grain boundary scattering/absorption of sintered diamond crystals. Application of NPD to various high-pressure apparatus has been attempted, in addition to its industrial applications, yielding some promising results particularly in higher pressure generation using diamond anvil cell with relatively large culet size, X-ray absorption studies at high pressure combined with synchrotron radiation, and high pressure generation in multianvil apparatus. The techniques to synthesize NPD are also applied to synthesize high-pressure phases, yielding some novel polycrystalline materials, such as nano-polycrystalline stishovite, nano-polycrystalline cubic BN, and nano- to micro-polycrystalline garnets, some of which are highly transparent and named as "poly-crystalline gems (poly gems) ". Thus, the KMA has opened a new field of making novel functional ceramic materials utilizing its relatively large sample volumes and capability of generation of pressures far higher than 10 GPa and temperatures well exceeding 2500K.
