Polycrystalline rhombohedral boron nitride and method of produci

Chemistry of inorganic compounds – Boron or compound thereof – Binary compound

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148DIG113, 156DIG86, 20415741, 20415743, 20415745, 4272552, 501 96, C01B 21064

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049005260

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BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to polycrystalline boron nitride of high purity and high density comprising rhombohedral crystals and a method of producing the same. The present invention is to provide materials suitable not only as a starting material of high pressure phase cubic boron nitride but also as materials for cruciblas used in melting semiconductive compounds, various jigs for high temperature service, high frequency insulators and microwave transmission windows and as a diffusion source of boron in various semiconductors.


BACKGROUND ART

Boron nitride is a compound consisting of boron and nitrogen in an atomic ratio of 1:1. Boron nitride is known to exist in several crystal structures when formed at atmospheric pressure. The structures are an amorphous structure (including a turbostratic structure), a hexagonal crystal and a rhombohedral (or trigonal) crystal.
Among these structures, rhombohedral boron nitride is different from hexagonal boron nitride and amorphous boron nitride in the stacking mode of layers of hexagonal network formed by alternatively linked boron and nitrogen. More specifically, the hexagonal crystal structure is built up by a two-layered stacking sequence, such as ABABAB . . . ; rhombohedral crystal structure is composed of a stack of the period of three layers, such as ABCABCABC . . . ; and amorphous boron nitride does not have such regularity or periodicity in its stacked structure.
On the other hand, there have been known, as high pressure phase boron nitride, wurtzite type boron nitride and cubic boron nitride (or also referred to as zinc blend type boron nitride) and, of these two, particularly cubic boron nitride is highly valuable as an industrial material because of its super-high hardness and high thermal conductivity. It can be explained that the structures of wurtzite type boron nitride and cubic boron nitride are formed when the atoms constituting the hexagonal network structure of atmospheric pressure phase boron nitride alternatively deviate from the plane of the hexagonal network to form a zig-zag network in which wurtzite type boron nitride has a two-layered stacking sequence (ABABAB . . . . ) and cubic boron nitride has a three-layered stacking repetition (ABCABCABC . . . ). Therefore, it can be assumed from the comparison between the stacking sequences of the atmospheric phase boron nitride and the high pressure phase boron nitride that atmospheric phase rhombohedral boron nitride is liable to transform into cubic boron nitride. Factually, the validity of this assumption was experimentally justified by explosive shock compression described in Journal of American Ceramic Society, Vol. 65, No. 10, page c-162 (1982). As set forth above, rhombohedral boron nitride is very suitable as a starting material for the preparation of cubic boron nitride and thus has become increasingly important in industrial applications.
In Compt. Rend., Vol. 246, page 1866 (b 1958), it has been reported that the rhombohedral boron nitride set forth above can be prepared in powder form from the reaction of borax or boron oxide with potassium cyanide, together with hexagonal boron nitride powder.
Also, in Japanese Patent Application Laid-open No. 58-74 511 and J. Crystal Growth, Vol. 52, page 285 (1981), rhombohedral boron nitride is prepared as a white wool-like material by the reaction between an oxygen-containing boron compound and cyanide gas. However, J. Crystal Growth. Vol. 52, page 285 (1981) states that the wool-like boron nitride such prepared is an agglomerate composed of whiskers (i.e., single crystal) of rhombohedral boron nitride, whiskers of hexagonal boron nitride and filaments of boron nitride of poor crystallinity.
As described above, rhombohedral boron nitride obtained by any production method heretofore known is a mixture in a powder or wool-like form containing boron nitride having crystal structure different from rhombohedral structure and is entirely different from the bulk or thin film products of pure and high density polycrystalline boron nit

REFERENCES:
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Toshitsugu Matsuda et al., "Synthesis and Structure of Chemically Deposited Boron Nitride" J. Mater. Sci. 21(2) 649-658 (1986).
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