For borides

In contrast to the carbides and nitrides, there are a large variety of formulas and structure types for borides (from M4B to MB 5). Although it is possible to establish a comparison between metal-rich borides and carbides, B-rich borides have no counterpart in the carbides. 

Borides of Al, Ti, Zr and Hf are prepared according to reaction (a), although even with careful measuring of reactants it is not always possible possible to form the metal boride in stoichiometric ratio, and free metal B may be deposited . This method is not suitable when the free metal deposits at T below those required for boride formation, as in the cases of Nb, Ta, Mo and W. 

The reduction of a metal oxide or other metal compound by C, B or boron carbide requires higher T than in the other methods available for boride preparation. 

Crystal growth is relatively difficult for borides because they have high melting points and sometimes low thermal stability, as indicated in 6.7.2. 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

From the discussion in Section II,C it should be clear that there is a relationship between the structures of borides and boranes as far as the boron networks are concerned. A comparison of the literature shows that the known structural diversity of the boranes is much greater than that of the borides. This may well be due to the rigorous conditions required for boride preparation that is, only the most stable boron networks are formed. TTie low-temperature routes illustrated in the foregoing work suggest an approach to presently unknown compounds with novel boride networks. Hence, the multitude of known metalla-boranes constitute a stockpile of potential precursors to metal borides with new and perhaps useful properties. 

D.l Chemical Vapour Deposition Phase Digrams for Borides... 

Very important also are systems containing BF3, which are used as electrolytes for boriding of steels and as heat-bearing medium in secondary circuits of nuclear power plants. 

Some intermetallic or metalloid compounds may also be obtained by high-temperature electrolysis of liquid melts of the corresponding metal compounds. Secondary reactions sometimes play an Important role in this case. This method, developed mainly by Andrieux and Dodero [7], has so far been used for borides (see p. 1798), silicides (see p. 1796 f.), phosphides, arsenides and carbides. 

The mixture of starting materials is pressed into pellets and heated in a tubular carbon furnace imder high vacuum. Maximum temperatures of 1400-1900°C are required to produce the metal boride within a reasonable time. The method has been tested for borides of Ti, Zr, V, Nb, Ta and W. 

Besides the high-tech applications of novel materials, it should not be forgotten that there was and still is a tremendous market for borides in the metallurgy of steel and iron, e.g., for antioxidizing additives in refractory linings or as alloying ingredients for the metals. 

Spear calculated three chemical bonding parameters for borides of the AlB2-type, plotted in Figure 2. The parameter in the top graph is the ratio of the two elliptic axes of the metal atom and provides a measure of the amount of metal atom distortion. The middle... 

Some of the thermodynamic properties for a few diborides, including HfB2 and ZrB2 are listed in Tables 3 and 4. The data in these Tables are from Pankratz et al, who reviewed available data in the early 80 s and included only the data they deemed to be reliable. Enthalpies of formation are strongly correlated with Gibbs free energies of formation for borides, because the entropy terms are small. This also means that the free energy is relatively insensitive to temperature. ... 

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