Electric boron carbides

The covalent carbides These include boron carbide B4C and silicon carbide SiC the latter is made by heating a mixture of silica and coke in an electric furnace to about 2000 K ... 

Research-grade material may be prepared by reaction of pelleted mixtures of titanium dioxide and boron at 1700°C in a vacuum furnace. Under these conditions, the oxygen is eliminated as a volatile boron oxide (17). Technical grade (purity > 98%) material may be made by the carbothermal reduction of titanium dioxide in the presence of boron or boron carbide. The endothermic reaction is carried out by heating briquettes made from a mixture of the reactants in electric furnaces at 2000°C (11,18,19). 

Preparation. Boron carbide is most commonly produced by the reduction of boric oxide with carbon in an electric furnace between 1400 and 2300°C. In the presence of carbon, magnesium reduces boric oxide to boron carbide at 1400—1800°C. The reaction is best carried out in a hydrogen atmosphere in a carbon tube furnace. By-product magnesium compounds are removed by acid treatment. 

Diamondlike Carbides. SiUcon and boron carbides form diamondlike carbides beryllium carbide, having a high degree of hardness, can also be iacluded. These materials have electrical resistivity ia the range of semiconductors (qv), and the bonding is largely covalent. Diamond itself may be considered a carbide of carbon because of its chemical stmeture, although its conductivity is low. 

Boron carbide (p. 149) is a most useful and economic source of B and will react with most metals or their oxides. It is produced in tonnage quantities by direct reduction of B2O3 with C at 1600° a C resistor is embedded in a mixture of B2O3 and C, and a heavy electric current passed. 

Surfaces of synthetic diamond, doped with boron, are electrically conducting and show promise as very inert elccfrode materials [24]. Boron carbide (B C) has been used as an anode material but tliis cannot be conveniently prepared with a large surface area [25]. 

Boron carbide is prepared by reduction of boric oxide either with carbon or with magnesium in presence of carbon in an electric furnace at a temperature above 1,400°C. When magnesium is used, the reaction may be carried out in a graphite furnace and the magnesium byproducts are removed by treatment with acid. 

Boron carbide (B4C) is also an extremely hard, infusible, and inert substance, made by reduction of B203 with carbon in an electric furnace at 2500°C, and has a very unusual structure. The C atoms occur in linear chains of 3, and the boron atoms in icosahedral groups of 12 (as in crystalline boron itself). These two units are then packed together in a sodium chloride-like array. There are, of course, covalent bonds between C and B atoms as well as between B atoms in different icosahedra. A graphite-like boron carbide (BQ) has been made by interaction of benzene and BC13 at 800°C. 

Boron Carbide. Boron carbide [12069-32-87, B4C, is produced by the reaction of boron oxide and coke in an electric arc furnace (70% B4C) or by that of carbon and boric anhydride in a carbon resistance furnace (80% B4C) (see Boron compounds, refractory BORON compounds). It is primarily used as a loose abrasive for grinding and lapping hard metals, gems, and optics (18).. Although B4C is oxidation-prone, the slow speed of lapping does not generate enough heat to oxidize the abrasive. 

Included in the term nonoxide ceramics are all non-electrically conducting materials in the boron-carbon-silicon-aluminum system. The industrially most important representatives, apart from carbon (see Section 5.7.4), are silicon carbide (SiC), silicon nitride (Si3N4), boron carbide (B4C), boron nitride (BN) and aluminum nitride (AIN). 

Boron carbide is similar in hardness to diamond, and boron nitride is similar in structure and mechanical properties to graphite, but, unlike graphite, boron nitride does not conduct electricity. -> A1 has widespread use in construction and aerospace industries. Because it is a soft metal, its strength is improved by alloy formation with Cu and Si. 

Starting from fine boron and carbon powders, the direct synthesis of stoichiometric boron carbide is possible -, either under vacuum at 2073K in an electric furnace, or at 2273 K by hot pressing under Ar. This method is inefficient economically and finds no practical application. The compositions in the phase homogeneity range B10.4C-B4C can be obtained by hot pressing mixtures of B C with boron . The diffusion diagram between B and C is established. ... 

Single crystals a few mm long are obtained by chemical vapor deposition (cf. 5.3.2.2.3), or by reduction of B2O3 by graphite in an electrical arc . A 6-mm diameter sintered boron carbide rod can be zone melted under Ar... 

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