Technical borides

With allowances for the actual metal content of zirconium (a small correction is also necessary for the hafnium content of about 2 /2%) and postulation of barium zirconate formation (see Table 27), reasonable accord between calculated and measured caloric output is established. However, the situation is more complex with boron mixtures where one encounters increase of heat output with increase of the percentage of boron in the mixtures much beyond the amounts of Equation (la). Thus, even with the reasonable assumption of secondary barium borate formation, the stoichiometry and heat output of the mixtures with more than about 10% of technical boron theoretical 8%) is obscure. Chromium boride formation may be a fector. 

Thus, this contribution is aimed at the state of the art in boride ceramics with their problems in densification, microstructural peculiarities and exceptional mechanical properties. Starting with the unique interaction of metallic, covalent and ionic types of bonding and the crystal structures of technically important compounds, phase diagrams will be presented as far as they are of technical interest. The major part consists of the description of the synthesis and properties of ceramics and cermets, reflecting the development of suitable sintering procedures and the consequent improvement of the thermal and mechanical properties. 

Figure 9. (a) The binary B-Si system according to experimental results with powders of technical purity. Homogeneity fields of the silicon borides according to Ettmayer et al. [76], Lugscheider et al. [77], Armas et al. 1981 [82], (b) The binary B-Si system according to recent calculations by Lim and Lukas [52],... 

Figures 16 and 17 show isothermal sections of the Ti-B-C system at 1400°C and 2100°C, respectively [104, 105]. The diagrams based upon the work of Rudy et al. [99] are wrong below 2100°C as they neglect the Ti3B4 phase. Technically interesting isopleths are shown in Fig. 18 (TiC-B), Fig. 19 (TiB2-B4Q, Fig. 20 (B4C-Ti), and Fig. 21 (B4C-TiC). The equilibria of the various titanium borides with TiC] ( have been studied in more detail by Brodkin and Barsoum [106].

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 . 

As discussed before, hard metal-like composites can be prepared by pressureless sintering of ternary borides with Fe, Ni, or Co melts. Materials with x-phase (M21M2 Bg, where = Fe, Ni, or Co, and M = Zr, Hf, Nb, Ta, or W with M as the matrix phase) have not been developed for technical use but Ni-based alloys with x are in applications as wear- and corrosion-resistant coatings on steels [355]. The x phase is also used for the improvement of the creep resistance of Ni-based superalloys. 

Coble, R. L. and Hobbs, H. A. in Investigation of Boride Compounds for Very High Temperature Applications, NTIS Report AD 428006, Kaufman, L. and Clougherty, E. V. (Eds), Clearinghouse for Federal Scientific and Technical Information, Springfield, VA, 1973,... 

The use of a particular catalyst for HOR in alkaline media depends on the operating conditions and cost. For example, non-noble metals such as nickel [37] have been used in H2 technical electrodes for AFCs. It is known that the catalytic activity of Raney Ni (porous Ni doped with Co or alloyed with Fe, Ti or Mo to improve its performance) approaches that of noble metals in these systems. Another catalyst that has been used is nickel boride (Ni2B). Sodium tungsten bronzes [38] and carbon supported Pt and Pt-Pd [39] have also been employed to eatalyze HOR in alkaline solutions. 

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