Boron Nitride Crystallization

The value of cBN as an abrasive lies in its much higher oxidation stability compared to that of diamond (1200°C compared to 600°C) and in its reduced chemical 

The recent developments in the boron nitride phase diagram, particularly with respect to the room temperature and pressure thermodynamic stability of cBN, have stimulated work aimed at producing cBN over a wider range of conditions [136]. In conjunction with this, it is now known that a wide range of nitrogenous 

3 Poly crystalline Diamond and Cubic Boron Nitride 

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The DTA measurements exhibit the stability of c-BN at standard conditions. Influences of grain size and purity (oxide content) of the cubic boron nitride crystals on the conversion temperature become obvious. Fine grained samples containing boron oxide show a significantly lower conversion temperature than coarse material (conversion at 900 °C for 1.5-pm c-BN containing boron oxide and 1500 °C for 600-pm pure c-BN). 

W. P. Chai, Y. S. Gu, M. Li, Z. H. Mai, Q. Z. Li, L. Yuan, and S. J. Pang, Orientation influence of cubic boron nitride crystal facets on the epitaxial growth of diamond film by microwave plasma chemical vapor deposition, J. Cryst. Growth, 135(3-... 

By subjecting boron nitride (a white powder) to high pressure and temperature small crystals of a substance harder than diamond, known as borazon, are obtained. This pressure-temperature treatment changes the structure from the original graphite-like layer structure (p. 163) to a diamond-like structure this hard form can withstand temperatures up to 2000 K. 

It is stable up to 2000 K and melts under pressure at 2500 K. The crystal structure of aluminium nitride resembles that of boron nitride and diamond, but unlike both of these it is rapidly and exothermically hydrolysed by cold water ... 

A wide range of cutting-tool materials is available. Properties, performance capabilities, and cost vary widely (2,7). Various steels (see Steel) cast cobalt alloys (see Cobalt and cobalt alloys) cemented, cast, and coated carbides (qv) ceramics (qv), sintered polycrystalline cubic boron nitride (cBN) (see Boron compounds) and sintered polycrystalline diamond tbin diamond coatings on cemented carbides and ceramics and single-crystal natural diamond (see Carbon) are all used as tool materials. Most tool materials used in the 1990s were developed during the twentieth century. The tool materials of the 1990s... 

The greatest use of cubic boron nitride is as an abrasive under the name Bora2on, in the form of small crystals, 1—500 p.m in si2e. Usually these crystals are incorporated in abrasive wheels and used to grind hard ferrous and nickel-based alloys, ranging from high speed steel tools and chilled cast-iron to gas turbine parts. The extreme hardness of the crystals and their resistance to attack by air and hot metal make the wheels very durable, and close tolerances can be maintained on the workpieces. 

Cp.m = bT2 2-dimensional layered crystals such as graphite and boron nitride (4.5)... 

Boron nitride has two crystalline forms, hexagonal (h-BN) and cubic (c-BN), with much different properties. Hexagonal BN is the more important and has many industrial applications. Its structure is similar to that of graphite which it resembles in many ways. It has a very large anisotropy in the crystal with resulting anisotropic properties. 

Saitoh, H., Yoshida, K., and Yarborough, W., Crystal Structure of New Composition Boron Rich Boron Nitride Using Raman Spectroscopy, J. Mater. Res., 8(1) 8-11 (Jan. 1993)... 

Elements dissolved in boron influence its crystal structure. Dissolved impurities also influenee the physical and chemical properties of boron, especially the electrical properties, because boron is a semiconductor. Preparation of solid solutions in jS-rh boron requires a careful choice of crucible material. To avoid contamination, boron nitride or a cold, coinage-metal crucible should be used or the levitation or floating-zone melting techniques applied. 

The macrostructure of the boron nitride obtained here is porous with pores 2 pm in diameter. There is no evidence for microporosity and the BET surface area 1s 35 m2 g-1. Transmission electron micrographs (Figure 4) show regions of well developed crystallinity. The crystalling grains are 5—10 nm on a side and 30-40 nm long. The BN (002) lattice fringes are clearly visible. 

Cubed compound, in PVC siding manufacture, 25 685 Cube lattice, 8 114t Cubic boron nitride, 1 8 4 654 grinding wheels, 1 21 hardness in various scales, l 3t physical properties of, 4 653t Cubic close-packed (CCP) structure, of spinel ferrites, 11 60 Cubic ferrites, 11 55-57 Cubic geometry, for metal coordination numbers, 7 574, 575t. See also Cubic structure Cubic symmetry Cubic silsesquioxanes (CSS), 13 539 Cubic structure, of ferroelectric crystals, 11 94-95, 96 Cubic symmetry, 8 114t Cubitron sol-gel abrasives, 1 7 Cucurbituril inclusion compounds,... 

Cubic Phase of Boron Nitride c-BN. The cubic phase of boron nitride (c-BN) is one of the hardest materials, second only to diamond and with similar crystal structure. It is the first example of a new material theoretically predicted and then synthesized in laboratory. From automated synthesis a microcrystalline phase of cubic boron nitride is recovered at ambient conditions in a metastable state, providing the basic material for a wide range of cutting and grinding applications. Synthetic polycrystalline diamonds and nitrides are principally used as abrasives but in spite of the greater hardness of diamond, its employment as a superabrasive is limited by a relatively low chemical and thermal stability. Cubic boron nitride, on the contrary, has only half the hardness of diamond but an extremely high thermal stability and inertness. 

Single crystals of cubic boron nitride, with excess boron, have been studied20 by X-band and W-band EPR. Two types of defect were detected called D1 and D2. The D1 centre exhibits local axial < 111 > symmetry and ground spin state S = 1/2, with g = 2.0033 and gL = 2.0094 at T = 10 K. A broad line from D2 was observed only at X-band at high temperature with g = 2.0084. It was attributed to transitions inside the excited levels of another boron related defect. 

The determination of trace impurities in boron nitride of the highest purity for the production of GaAs single crystals is hampered by the formation of disturbing polyatomic and cluster ions. 

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