Point boron carbides

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

When boron is heated to high temperatures with carbon, it forms boron carbide, B12C3, a solid with a high melting point that is almost as hard as diamond. The solid consists of B12 groups that are pinned together by C atoms. When boron is heated to white heat in ammonia, boron nitride, BN, is formed as a fluffy, slippery powder ... 

B4C boron carbide has a melting point of 2450 °C and a hardness somewhere between those of SiC and diamond. This makes the material a suitable abrasive. It is used in heads of sand blasting equipment, in mortars and in armour plating. For the latter application a B4C plate is provided on both sides with a plastic which has been reinforced with glass fibre. This is done to reduce the risk of splintering. Boron carbide is also used as the raw material for many other boron compounds ... 

Boron also forms important compounds with two other elements, carbon and nitrogen. Boron carbide (B4C) and boron nitride (BN) are important compounds because of their hardness. In fact, boron nitride may be the hardest substance known. Both compounds have very high melting points 4,262°F (2,350°C) for boron carbide and more than 5,432°F (3,000°C) for boron nitride. 

Because boron carbide can be used as the control rod material in a nuclear reactor, in order to interpret its performance it is necessary to establish nature of grown-in and neutron-radiation-induced lattice defects in boron carbide. It was found that the dose received by the irradiated specimen corresponds to transmutation of about eight B nuclei per unit cell in equal number of both Li and " He nuclei (Ashbee 1971). It is believed that the formation of the partial dislocation loops resulting from the agglomeration of point defects are introduced during neutron irradiation. 

Gaseous boric acid removes a boron oxide film. The rates of formation and removal of the B2O3 film are equal at 550-600°C in air with a dew point of 25-70°C and at 650°C with a dew point of 88°C. At higher temperatures, B2O3 is formed at a higher rate than it is removed by the interaction with water vapor. Therefore, at low temperatures boron carbide is oxidized with water vapor more rapidly than with dry air, at high temperatures the situation is quite the opposite [2]. 

This material usually contains only small amounts of residual carbon or boron carbide but no metals, and is thus the favored process for the technical synthesis of less contaminated borides. The process is carried out in tunnel furnaces under hydrogen or in a vacuum at 1600-2000°C, i.e., below the melting point of the boride. It is thus a reaction sintering procedure yielding a high-porosity product which can easily be crushed and milled. Additional refinement is obtained by multiple vacuum treatments with metallic or B4C additives to compensate nonstoichiometries. The final product is then called vacuum quality . 

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