Nitridation, of boron

Direct nitridation of boron in a furnace according to reaction ... 

As shown in subsequent chapters, most elements form carbides and nitrides and these can be divided into several types with different physico-chemical structures and characteristics. Of these, however, only the interstitial and covalent materials meet the refractory qualification. This includes the carbides and nitrides of the nine transition elements of Groups rv, V, and VI and the 4th, 5th, and 6th Periods, the carbides and nitrides of boron and silicon, and aluminum nitride. 

In the five categories listed above, only some of the interstitial and covalent nitrides qualify as refractory, i.e., the nitrides of the elements of Groups IV and V and the covalent nitrides of boron, aluminum, and silicon. These elements are shown in bold type in Table 9.1. Unlike the carbides of Group VI elements, the Group VI nitrides are not refractory and consequently are not considered in any depth in this book. 

It is still a matter of doubt in what form the boric add, thus obtained, occurs in the earth. ITie occurrence of ammoniacal salts and sulphide of ammonium, together with the boric acid, is veiy remarkable. The most probable hypothesis appears to be that of W6hler and Deville, according to which the acid is derived from the decomposition of a nitride of boron, BN. Boron is one of tlie elements which... 

Wentorf R FI 1957 Cubio form of boron nitride J. Chem. Phys. 26 956... 

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 ... 

Boric oxide is used as a catalyst ia many organic reactions. It also serves as an iatermediate ia the production of boron haUdes, esters, carbide, nitride, and metallic borides. 

Preparation. Hexagonal boron nitride can be prepared by heating boric oxide with ammonia, or by heating boric oxide, boric acid, or its salts with ammonium chloride, alkaU cyanides, or calcium cyanamide at atmospheric pressure. Elemental nitrogen does not react with boric oxide even in the presence of carbon, though it does react with elemental boron at high temperatures. Boron nitride obtained from the reaction of boron trichloride or boron trifluoride with ammonia is easily purified. 

The cubic 2inc blende form of boron nitride is usually prepared from the hexagonal or rhombohedral form at high (4—6 GPa (40—60 kbar)) pressures and temperatures (1400—1700°C). The reaction is accelerated by lithium or alkaline-earth nitrides or amides, which are the best catalysts, and form intermediate Hquid compounds with BN, which are molten under synthesis conditions (11,16). Many other substances can aid the transformation. At higher pressures (6—13 GPa) the cubic or wurt2itic forms are obtained without catalysts (17). 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

Next to Cr C2, TiC is the principal component for heat and oxidation-resistant cemented carbides. TiC-based boats, containing aluminum nitride, AIN, boron nitride, BN, and titanium boride, TiB2, have been found satisfactory for the evaporation of metals (see Boron compounds, refractory boron compounds Nitrides). 

Nakamura, K., Preparation and Properties of Boron Nitride Films by Metal Organic Chemical Vapor Deposition, /. Electochem. Soc., 133-6 120-1123 (1986)... 

The atomic and crystalline structure of the three covalent nitrides, aluminum, boron, and silicon nitrides, is less complex than that of the interstitial nitrides. Their bonding is essentially covalent. 

Poly crystalline boron nitride films, with a structure similar to rhombohedral boron carbide and a ratio of boron to nitrogen of 3 1, were produced by hot-filament CVD. This work indicates the possible existence of other boron-nitride structures. 

The first two examples of boron nitrides with N atoms branehing off a typical B—B zigzag chain are UBN (UBC type) and NG2BN. 

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. 

A pellet is pressed of an intimate mixture of finely divided reactants and reaction induced either by arc melting and high-T annealing or by solid-state sintering in an electrical or high-frequency furnace. Isolating the borides from reactive container components can be a problem. The use of boron nitride liners has proved effective. In some cases the protective liner is made of sintered boride containing the same elements as the boride in preparation. 

Miyake, S., Tribology of Carbon Nitride and Boron Nitride Nanoperiod Multilayer Films and Its Application to Nanoscale Processing," Thin Solid Films,Wo. 493,2005, pp. 160-169. 

C21-0047. Boron nitride (BN) is a planar covalent solid analogous to graphite. Write a portion of the Lewis structure and describe the bonding of boron nitride, which has alternating B and N atoms. 

Mixtures of boron or carbon with this peroxide are explosive. Boron nitride becomes incandescent. 

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