Metal halide carbidization

Carbides may also be prepared, either by dhect carburizing, as in the case of steel, in which a surface carbide film dissolves into the subsuate steel, or by refractoty metal carbide formation as in die cases when one of the refractory metal halides is mixed with methane in the plasma gas. 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

Cesium acetylide, 11 195 Cesium aluminum hydride, preparation and properties of, 8 323 Cesium carbide, 11 195 Cesium chloride, 2 5-6 gaseous metal halide complexes, 26 204-206... 

There is increasing experimental evidence for the superlattice ordering of vacant sites or interstitial atoms as a result of interactions between them. Superlattice ordering of point defects has been found in metal halides, oxides, sulphides, carbides and other systems, and the relation between such ordering and nonstoichiometry has been reviewed extensively (Anderson, 1974, 1984 Anderson Tilley, 1974). Superlattice ordering of point defects is also found in alloys and in some intermetallic compounds (Gleiter, 1983). We shall examine the features of some typical systems to illustrate this phenomenon, which has minimized the relevance of isolated point defects in many of the chemically interesting solids. 

A large variety of catalysts, both homogeneous and heterogeneous, has been found active for dehydrohalogenation. The catalysts include a number of Br nsted and Lewis acids (liquid or soluble, as well as solid), metal oxides, active carbon, carbides, nitrides and some metals. However, in the latter case, the actual catalysts are most probably surface metal halides... 

Because of the deficiency in carbon, it was expected that free carbon would not form on carbides made from transition metal halides and any of the above carbon-nitrogen compounds, that are rich in nitrogen. In the reaction ... 

Hydrogen reacts with metal borides, carbides, silicides, nitrides, phosphides, oxides, sulfides, and halides to form a solid solution of hydrogen in the compound with... 

The use of sacrificial metal anodes inevitably leads to the formation of metal halides as by-products. This problem can be solved by the use of a silicon carbide electrode. The homo coupling takes place smoothly in an undivided cell without formation of metal halides [185]. A hydrogen electrode is also effective for this purpose [185]. 

If single-source precursors are unavailable, carbides can be produced by reacting a metal halide or metal hydride with a hydrocarbon gas. Often the hydrocarbon is methane, and the carbide formation reaction may be written as follows ... 

CVD processes have a greater flexibility of using a wide range of chemical precursors such as halides, hydrides, organo-metallic compounds and so forth which enable the deposition of a large spectrum of materials, including metals, non-metallic elements, carbides, nitrides, oxides, sulphides, as well as polymers. Up to now, around 70% of elements in the periodic table have been deposited by the CVD technique, some of which are in the form of the pure element, however, more often the compound materials. 

The production of silicon nitride [16,17] and aluminum nitride [18-21] has been extensively studied plasma reactions [21-23] were also reported. Instead of ammonia nitrogen may be used [24,25]. Hydrocarbons and metal halides for the formation of carbides are worth mentioning [26,27]. 

The most metal-rich carbide halides have the formula R2XC and are hitherto only known for R = Gd. Their crystal structures can either be described as condensed layers of edge-sharing octahedra of Gd atoms separated by X atom layers, or they can be derived from the Gd2C structure by interleaving neighboring Gd atom bilayers... 

The CVD of other refractory metal carbides is essentially similar to that of TiC. The metal halide is reacted with a hydrocarbon, usually methane, although propane, propene and toluene have been used also. Pressure varies from 1 kPa to 1 atm. (composition closest to stoichiometry are usually obtained at the lower pressures). 

The volume edited by Quill contains ten papers by various authors, dealing with liquid-solid equilibria, temperature-composition diagrams of metal-metal halide systems, and properties of the elements, carbides, sulphides, silicides, phosphides, and halides. 

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