Nitrides boron

Boron carbide is either prepared from boron ores or from pure boron. The process involves the reduction of a boron compound. Usually, boron carbide is obtained by reacting boric acid or boron oxide and carbon at ca. 2500°C in an electric-arc furnace. 

Boron nitride [10043-11-5], chemical formula BN, exists as three different poly-morphs alpha-boron nitride (a-BN), a soft and ductile polymorph (p = 2280 kg.m and m.p. = 2700°C) 

Cubic BN, or borazon, is produced by subjecting hexagonal BN to extreme pressure and heat in a process similar to that used to produce synthetic diamonds. Melting of either phase is possible only with a high nitrogen overpressure. The alpha-phase decomposes above 2700 C at atmospheric pressure and at ca. 1980 C in a vacuum. 

The major industrial applications of hexagonal boron nitride rely on its high thermal conductivity, excellent dielectric properties, self-lubrication, chemical inertness, nontoxicity, and ease of machining. These are, for instance, mold wash for releasing molds, high-temperature lubricants, insulating filler material in composite materials, as an additive in silicone oils and synthetic resins, as filler for tubular heaters, and in neutron absorbers. On the other hand, the industrial applications of cubic boron nitride rely on its high hardness and are mainly as abrasives. 

Boron nitride (BN) is a chemical compound consisting of equal numbers of boron and nitrogen atoms. It is not found in Nature, and is therefore produced synthetically the first synthesis of hexagonal BN was described in 1842 by Balmain [123]. 

Antimony Oxide Graphite II Graphite I Boron Nitride 

Graphite is one of the oldest and the most widely used (due to its lower price compared to M0S2 or BN) solid lubricants. It is obtained both as a soft mineral and as a man-made (synthetic) product. Graphite powders are used as solid lubricants in three ways (1) in dry films, (2) as an additive in liquids (oils) or semisolids (greases), and (3) as a component of self-lubricating (internally lubricated) composites. A NIOSH web site [8] has listed a total of 55 synonyms for graphite. 

Thin hlms of a-BN (which possesses the layer structure shown in Fignre 13.20) can be deposited by CVD using reactions of NH3 with volatile boron compounds such as BCI3 (equation 28.13) or BF3 at temperatures of sslOOOK. 

At high temperatures (1800°C) and pressures (8.6 GPa), g-BN goes over to cubic c-BN, also known as borazon, which has the normal diamond 

Finally, a report that pure BN nanotubes are formed in an electric discharge between a BN-packed tungsten electrode and a cooled copper electrode extends the list of formal similarities between BN and carbon, and implies that much interesting BN chemistry remains to be discovered. 

The reaction of KCN with B2O3 at 1100°C leads to a rhombohedral BN modification which is an ordered /3-graphite version [57a]. 

In BN all excess valence electrons not used for the sp bonds are, in the ionic limit, well localized on a doubly occupied nitrogen p-orbital. Hence the one 

Symmetrical (A) and non-symmetrical (B) carbon analogs and corresponding (C, D) cross compounds with second- and third-row elements (SiC analogs, n = 2.5). 

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Riter J R Jr 1973 Shock-induced graphite.far.wurtzite phase transformation in boron nitride and implications for stacking graphitic boron nitride J. Chem. Phys. 59 1538... 

Yoo C S, Akella J, Nicol M and Cynn H 1997 Direct elementary synthesis of hexagonal and cubic boron nitrides at high pressures and temperatures Phys. Rev. B 56 140... 

Boron nitride can be prepared by allowing ammonia to react with boron trichloride. The first product is boron amide which decomposes on heating to give the nitride ... 

Boron nitride is chemically unreactive, and can be melted at 3000 K by heating under pressure. It is a covalent compound, but the lack of volatility is due to the formation of giant molecules as in graphite or diamond (p. 163). The bond B—N is isoelectronic with C—C. 

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 ... 

The isotope boron-10 is used as a control for nuclear reactors, as a shield for nuclear radiation, and in instruments used for detecting neutrons. Boron nitride has remarkable properties and can be used to make a material as hard as diamond. The nitride also behaves like an electrical insulator but conducts heat like a metal. 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

Diamond. Diamond [7782 0-3] is the hardest substance known (see Carbon, diamond, natural). It has a Knoop hardness of 78—80 kN/m (8000—8200 kgf/m ). The next hardest substance is cubic boron nitride with a Knoop value of 46 kN/m, and its inventor, Wentorf, beheves that no manufactured material will ever exceed diamond s hardness (17). In 1987 the world production of natural industrial diamonds (4) was about 110 t (1 g = 5 carats). It should be noted that whereas the United States was the leading consumer of industrial diamonds in 1987 (140 t) only 260 kg of natural industrial diamonds were consumed this is the lowest figure in 48 years (4), illustrating the impact that synthetic diamonds have made on the natural diamond abrasive market. 

Lithium Nitride. Lithium nitride [26134-62-3], Li N, is prepared from the strongly exothermic direct reaction of lithium and nitrogen. The reaction proceeds to completion even when the temperature is kept below the melting point of lithium metal. The lithium ion is extremely mobile in the hexagonal lattice resulting in one of the highest known soHd ionic conductivities. Lithium nitride in combination with other compounds is used as a catalyst for the conversion of hexagonal boron nitride to the cubic form. The properties of lithium nitride have been extensively reviewed (66). 

The interelectrode insulators, an integral part of the electrode wall stmcture, are required to stand off interelectrode voltages and resist attack by slag. Well cooled, by contact with neighboring copper electrodes, thin insulators have proven to be very effective, particularly those made of alumina or boron nitride. Alumina is cheaper and also provides good anchoring points for the slag layer. Boron nitride has superior thermal conductivity and thermal shock resistance. 

Fig. 16. Insulator wall designs (a) peg wall (b) conducting bar wall and (c) segmented bar wall. The gas-side materials are tungsten and tungsten—copper composite, the base material, copper, and the insulators, boron nitride. Slagging grooves are shown.

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