Borides

compounds of boron with metals. Borides have physical properties characteristic of metallic substances (increase of the coefficient of electrical resistance with increase in temperature, high values for electrical and heat conductivity, and a metallic luster), as well as those of nonmetallic substances with semiconductor properties. Borides of the transitional metals are an intermediate class between inter-metallic compounds (of the beryllide type) and the so-called embedded phases. A characteristic crystallochemical trait of borides is the presence of isolated configurations of boron atoms in their structures. The chemical stability of borides is determined largely by the strength of the boron-to-boron bonds in the boride lattices; it increases with increased boron content in them. The greatest chemical stability (according to the speed of hydrolytic decomposition) is observed in hexaborides and dodecaborides. Most borides are resistant to acids—for example, even boiling aqua regia has no effect on TaB₂.

Figure 1. Dependence of heat capacity of diborides on temperature

The diborides (MeB₂) are used most widely in engineering. The most important indicator of these materials is the change in their basic properties with temperature (see Figures 1, 2, and 3). The most important physical properties of some borides of refractory metals are shown in Table 1. A large group is formed by borides of rare-earth metals-the lantha-nides and scandium and yttrium, whose properties are similar to those of the lanthanides. The most interesting in this group are the hexaborides (MeB₆). (See Table 2.) The structure of hexaborides is double in character. The crystal lattice of hexaborides may be regarded as a simple cubic lattice of metal atoms centered by an octahedron of boron atoms or as a cubic lattice of boron atom complexes with the metal atoms located freely in the center.

Borides have negligible plasticity and very great hardness

Figure 2. Dependence of coefficient of linear expansion of diborides on temperature

(microhardness 20–30 giganewtons per sq m [GN/m²]). The tensile strength of TiB₂ at a porosity of 2–3 percent is 380 meganewtons per sq m (MN/m²), and at a porosity of 7–9 percent, 140 MN/m² (1 GN/m² = 100 kilograms-force per sq mm [kgf/mm²]; 1 MN/m² = 0.1 kgf/mm²). The high heat resistence of this diboride is characterized by a relatively slow creep rate (at a tension of 90 MN/m² the creep rate at temperatures of 1920°, 2080°, and 2270° C is 1.5, 9.2, and 57 microns/min respectively). The modulus of elasticity obtained in nonporous specimens by measuring the velocity of longitudinal ultrasonic oscillations is 650 GN/m² for NbB₂, 700 for TaB₂, and 685 for Mo₂B₅; and 790 GN/m² for W₂B₅.

Borides are obtained by several methods, the most important of which include the reduction of metal oxides with a mixture of boron carbide and carbon black according to the reaction MeO + B₄C + C → MeB + CO; the reduction of mixtures of metal oxides with boric anhydride by carbon black according to the reaction MeO + B₂O₃ + C → MeB + CO; and reduction by the magnesium-thermal method according to the reaction MeOx + nBO_1.5 + (1.5n + x)Mg → MeB_n + (1.5n + x) · MgO.

Solid products are obtained from boride powder by pressing with subsequent sintering or by hot forming. Borides are

Figure 3. Dependence of heat conductivity of diborides on temperature

widely used in technology. Because of their emission properties, they are used in radio electronics—for example, lanthanum hexaboride is used to make cathodes of high-powered generator apparatus and instruments. Because of their high neutron capture cross section, borides are used in nuclear engineering as regulating materials and for protection against nuclear radiation. Their great hardness, wear resistance, and grinding capability make them useful in machine and instrument building. The ability of some borides to maintain their properties in a medium of molten metals has made it possible to use zirconium boride—for example, in metallurgy for making thermocouple tips, which provided the possibility of automatic control of the temperature of steel in open-hearth furnaces. Future uses of borides include in high-strength and high-modulus continuous fibers and threadlike crystals for reinforcing composition materials.