Refractory Boron Compounds

Abstract

AbstractBorides have metallic characteristics such as high electrical conductivity and positive coefficients of electrical resistivity. Many of them, particularly the borides of metals of Groups 4 (IVB), 5 (VB), and 6 (VIB), the MB6compounds of Groups 2(II) and 13(III), and the borides of aluminum and silicon, have high melting points, great hardness, low coefficients of thermal expansion, and good chemical stability. Borides are inert toward nonoxidizing acids; however, a few, such as Be2B and MgB2, react with aqueous acids to form boron hydrides. Most borides dissolve in oxidizing acids such as nitric or hot sulfuric acid and they are also readily attacked by hot alkaline salt melts or fused alkali peroxides. In dry air, borides are relatively resistant to oxidation. On a commercial scale, borides are prepared by the reduction of mixtures of metallic and boron oxides using aluminum, magnesium, carbon, boron, or boron carbide, followed by purification. Borides can also be synthesized by vapor‐phase reaction or electrolysis. They are used for resistance‐heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. Boron and carbon form one compound, boron carbide, B4C, although excess boron may dissolve in boron carbide, and a small amount of boron may dissolve in graphite. Boron carbide has a rhombohedral structure consisting of an array of nearly regular icosahedra, each having twelve boron atoms at the vertices and three carbon atoms in a linear chain outside the icosahedra. A descriptive chemical formula would be B12C3. Boron carbide is resistant to most acids but is rapidly attacked by molten alkalies. Hot‐pressed boron carbide finds use as wear parts, sandblast nozzles, seals, and ceramic armor plates. Boron carbide is used in the shielding and control of nuclear reactors. Boron and nitrogen form one compound, boron nitride, BN, which may exist in a hexagonal, graphite‐like form, hBN, or in two denser forms having tetrahedrally bonded B and N atoms on a cubic (zinc‐blend) or a hexagonal (wurtzite) lattice. Hot‐pressed hBN is useful for high temperature electric or thermal insulation, vessels, etc, and for special materials such as III‐V semiconductors. The greatest use of cubic boron nitride is as an abrasive under the name Borazon.