Carbides of transition metals

Titanium Carbide. Carbides of transition metals are known for their hardness, wear resistance and also for their high electrical conductivity, which makes them attractive as a refractory coating material for cutting tools or bearings. Only little work has been done on the electrochemical stability of transition metal carbides with the exception of TiC, where a corrosion and passivation mechanism was suggested by Hintermann et al. [119,120]. This mechanism was confirmed on amorphous TiC produced by metal-... 

The following carbides of transition metals were used TiC, ZrC, HfC, VC, NbC, TaC, WC, Mo2C, Cr7C3, Cr3C2 (supplied by the Donetsk Chemical Agent Plant, Ukraine), plus Fe3C prepared under laboratory conditions. 

Carbides of transition metals as catalysts for oxidation reactions... 

We have developed solvothermal synthesis as an important method in research of metastable structures. In the benzene-thermal synthesis of nanocrystalline GaN at 280°C through the metathesis reaction of GaClj and U3N, the ultrahigh pressure rocksalt type GaN metastable phase, which was previously prepared at 37 GPa, was obtained at ambient condition [5]. Diamond crystallites were prepared from catalytic reduction of CCI4 by metallic sodium in an autoclave at 700°C (Fig.l) [6]. In our recent studies, diamond was also prepared via the solvothermal process. In the solvothermal catalytic metathesis reaction of carbides of transition metals and CX4 (X = F, Cl, Br) at 600-700°C, Raman spectrum of the prepared sample shows a sharp peak at 1330 cm" (Fig. 1), indicating existence of diamond. In another process, multiwalled carbon nanotubes were synthesized at 350°C by the solvothermal catalytic reaction of CgCle with metallic potassium (Fig. 2) [7]. 

Recently, there has also been an increase in the importance of melts in their use as a reaction medium for chemical and electrochemical synthesis of compounds for functional and construction ceramics, e.g. double oxides with spinellitic and perowskite structure and binary compounds with prevailing covalent bond character, mainly borides and carbides of transition metals. 

Oxidation of TiC is similar to that of TiN. Many carbides of transition metals form carbon at the carbide-oxide interface. In the case of TiC, no carbon was formed [182]. Oxidation of titanium and carbon at 1000°C occurs with the same rate [183]. 

Oxidation. Carbides of transition metals catalyze a number of oxidation reactions such as, those of H2, CO, NH3, and CH4 (80). Catalytic activity is generally lower than those of pure metals, but higher than oxides containing the same metal. TMCs chemisorb oxygen and dissolve it at 573-673 K, and they are burned violently when exposed to air. Hence, the control of reaction conditions is important. 

Metallic nitrides can be alloyed with other nitrides and carbides of transition metals to give solid solutions, as shown in Table 3 (4). Complete solid solubility has been demonstrated for a great number of combinations. Continuous alloys seem to be formed when the crystal structure and bonding in the materials are similar and when the parent metals by themselves form solid solutions with each other. For example, all cubic monocarbides and mononitrides of group 4 and 5 metals show complete miscibility with the exception of the pairs ZrN-VN, HfN-VN, ZrN-VC, HfN-VC, and HfC-VN (11). Recent developments in ternary and quaternary nitride chemistry have also been reviewed (16). Similarly, materials of mixed C, N, and O compositions are common, and oxycarbides, oxynitrides, and oxycarbonitrides have been reported (17). Oxygen is particularly ubiquitous, and even materials that are normally pure may contain substantial amounts of dissolved oxygen. 

I Higashi, Y Takahashi, T Atoda. Crystal growth of borides and carbides of transition metals from molten aluminum solutions. J Cryst Growth 33 207, 1976. 

Hue] Huetter, L.J., Stadelmaier, H.H., Ternary Carbides of Transition Metals with Aluminium and Magnesium , Acta Metall., 6, 367-370 (1958) (Crys. Stracture, Review, 6)... 

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