Mechanical transition metal borides

In this case, B4C also undergoes a grain size refinement which is very beneficial for the mechanical properties. In Fig. 55, a micrograph of a reacted TiC-B powder mixture is presented, which still exhibits TiB2 B2C agglomerates of the size of the initial TiC particles. Note that the average particle size of both reaction products is approximately 1 pm. Generally, this reaction can be employed for most of the transition metal boride-boron carbide composites since the borides are usually more stable than the particular carbides [92],... 

Transition metal borides are mainly explored for their mechanical properties. Since they exhibit metallic transport properties such as high electric and thermal conductivity with a negative temperature coefficient they are also of interest as electrode materials, for heating elements and sensors. 

A pronounced bloating of the samples is observed due to the release of gaseous compoimds. Eq. (38) resembles the so-called boron carbide route for the production of the particular transition metal borides. Another limiting factor is the chemical, geometrical and mechanical destabilization of tetragonal zirconia if combined... 

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. 

Among electrode materials, the widest windows are known for transition metal oxides, borides, nitrides, and some specially fabricated carbon-based materials. It should be mentioned that, if the nature of electrode material can affect the formal potential value by changing the mechanism and kinetic parameters, the solvent frequently has a pronounced effect on the equilibrium potential, because of the solvation contribution to free Gibbs energy. 

A further study of Mo2NiB2-based cermets was dedicated to the influence of other transition-metal additives on the structure and habit of the ternary boride phase [365], Taking the previously developed 35.4 mass-% Ni + 6 mass-% B + 58.6 mass-% Mo cermet as a starting point, 10 mass-% of Cr, V, Fe, Co, Ti, Mn, Zr, Nb, and W were added at the expenses of Ni. In another series of experiments, V was added in steps of 2.5 mass-% up to 12.5 mass-%. The mechanical properties... 

Very interesting are the results of recent investigations on the mechanisms of Co(II) mediated reductions of nitriles, alkenes and alkyl halides by LiAlH4 and NaBH4. Those studies have unambiguously identified borides and aluminides of cobalt as catalysts in all three reductions, a finding clearly at odds with commonly held notions about the mechanisms of such processes and which could also be relevant to other transition-metal—hydride systems [12]. 

Subsequently, the most relevant data emerged from the investigations on Zr-, Hf- and Ta-carbides or borides are discnssed in order to understand the sintering mechanisms and the crucial role of MoSi or TaSi during densifieation. The interae-tion between carbides and borides with the two sdicides will be treated separately. For simplicity, Zr, Hf and Ta will be referred to as transition metal, TM. 

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