Transition nitrides formation

The only reactions of molecular nitrogen at ambient temperature are the formation of lithium nitride Li3N, reactions with certain transition metal complexes, and nitrogen fixation with nitrogenase in the bacteria of the root nodules of legumes and in blue algae (Sections 14.1.1 and 14.1.2). Above 500 °C nitrogen reacts with some elements, especially with metals (nitride formation). 

Li3(BN2) have already demonstrated the decomposition of (BN2) ions into boron nitride. The remaining nitride can lead to the formation of a binary metal nitride or reduce the transition metal ion under the formation of N2. Both mechanisms have been obtained experimentally, depending on the stability of the metal nitride. For instance niobium pentachloride forms NbN, titanium trichloride forms TiN, and nickel dichloride forms Ni, plus BN and nitrogen, respectively, in reactions with Li3(BN)2 (at 300-600°C) [24]. 

Heating borazine in vacuo at 70°C yields poly(borazylene) polymers, which are soluble in solvents such as tetrahydrofuran or glyme and could be thermolyzed to boron nitride in good yields (120). Other soluble preceramic polymers were produced by transition-metal catalyzed formation of B-alkenylborazines (eq. 34) which were thermally polymerized under mild conditions to poly(alkenylborazines). The latter yielded boron nitride having low carbon contents when thermolyzed in an ammonia atmosphere (121). 

A less explored area of transition metal catalysis involves bond formation between Group 14 elements and nitrogen. In direct analogy to previously discussed areas of research, silicon-nitrogen bonds can be formed by dehydrocoupling, hydrosilylation, and dehydrogenative silylation. The compounds produced are valuable for use in organic synthesis or as polymer precursors to silicon nitride ceramics. 

Munir and Holt examined theoretically the effect of nitrogen pressure and sample porosity in the simultaneous formation of nitrides and solid solutions. Using thermodynamic arguments, they derived minimum values of nitrogen pressures required for the nitridation of various transition metals however, several experimental studies demonstrated that nitridation could take place at much lower pressures than those predicted by these authors. 

In the following sections some examples are given of the ways in which these principles have been utilized. The first example is the use of these techniques for the low temperature preparation of oxide ceramics such as silica. This process can also be used to produce alumina, titanium oxide, or other metal oxides. The second example describes the conversion of organic polymers to carbon fiber, a process that was probably the inspiration for the later development of routes to a range of non-oxide ceramics. Following this are brief reviews of processes that lead to the formation of silicon carbide, silicon nitride, boron nitride, and aluminum nitride, plus an introduction to the synthesis of other ceramics such as phosphorus nitride, nitrogen-phosphorus-boron materials, and an example of a transition metal-containing ceramic material. 

It seems relevant to remind once again that in the case of formation of a single-phase compound layer, the reverse (parabolic-to-linear) transition is impossible. From a physicochemical viewpoint, it is only possible during the simultaneous occurrence of two or more compound layers, as is indeed observed experimentally. Parabolic-to-linear growth kinetics are thus indicative of the formation of multiple layers of oxides, nitrides, sulphides, etc., even though some of them may be unindentifiable due to their extremely small thickness. 

As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

Table 2. Heat and free energy of formation from the elements of transition metal nitrides (kJ mol-1) calculated from data in Ref. 22.

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

Figure 4 Richardson diagram of various transition metal mononitrides (together with other nitrides) giving the free energy of formation as a function of temperature. (Ref 4. Reproduced by permission of Wiley VCH)...

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