Crystal borides

By using a large metal excess, which can also act as a flux, well-crystallized borides can be obtained. After reaction, the excess is eliminated either by distillation at low T (Na, 350°C K, 300°C) and in high vacuum (10 to 10 N m ), or by washing with NH3. 

Other interesting directions are hardening with intermetallics, quasi-crystals, borides, silicides, discrete fibres, creation of natural composites. Conscious regulation of structure and properties of such materials requires studying phase equlibria in multi-component systems, in particular on the basis of light metals like aluminum, magnesium, titanium. The important direction is also a creation of specially organized porous structures. To some extent these directions are presented in a number of papers of the present book. 

Even though TiC is much harder than WC at room temperature (3200 kg/mm for TiC, vs 1800 kg/mm for WC), at higher temperatures, TiC oxidi2es and loses its hardness rapidly. Figure 17 is a plot of the variation of hardness of single crystals of various monocarbides with temperature (44). No similar data is available for multicarbides or other refractory hard materials, such as nitrides, borides, oxides, or any combination of them. 

The crystal stmeture and stoichiometry of these materials is determined from two contributions, geometric and electronic. The geometric factor is an empirical one (8) simple interstitial carbides, nitrides, borides, and hydrides are formed for small ratios of nonmetal to metal radii, eg, < 0.59. 

Figure 6.1 The icosahedron and some of its symmetry elements, (a) An icosahedron has 12 vertices and 20 triangular faces defined by 30 edges, (b) The preferred pentagonal pyramidal coordination polyhedron for 6-coordinate boron in icosahedral structures as it is not possible to generate an infinite three-dimensional lattice on the basis of fivefold symmetry, various distortions, translations and voids occur in the actual crystal structures, (c) The distortion angle 0, which varies from 0° to 25°, for various boron atoms in crystalline boron and metal borides.

A systematic approach to the crystal chemistry of borides is possible on the simple basis of atom size considerations, as well as the tendency of B to form covalent skeletons. 

The crystal chemistry of the borides is discussed in 6.7.2 according to this scheme. General methods of preparation, single-crystal growth and sintering of borides is considered, respectively, in 6.7.3, 6.7.4 and 6.7.5. 

The crystal structures of the borides of the rare earth metals (M g) are describedand phase equilibria in ternary and higher order systems containing rare earths and B, including information on structures, magnetic and electrical properties as well as low-T phase equilibria, are available. Phase equilibria and crystal structure in binary and ternary systems containing an actinide metal and B are... 

Uncharacterized ternary and binary metal borides, lacking precise composition or crystal chemistry data (see Fig. 2, Table 2), are not covered in this context. Further-... 

Silico- and phosphorus borides crystallizing with ordered W5Si3-type structures, such as Ni4ftSi2B and Cr. 45P2B, are closely related structurally. ... 

Existence and Crystal Chemistry of Borides 6.7.2.1. Borides with Isolated Boron Atoms... 

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