Carbide oxides

Metal-Matrix Composites. A metal-matrix composite (MMC) is comprised of a metal ahoy, less than 50% by volume that is reinforced by one or more constituents with a significantly higher elastic modulus. Reinforcement materials include carbides, oxides, graphite, borides, intermetahics or even polymeric products. These materials can be used in the form of whiskers, continuous or discontinuous fibers, or particles. Matrices can be made from metal ahoys of Mg, Al, Ti, Cu, Ni or Fe. In addition, intermetahic compounds such as titanium and nickel aluminides, Ti Al and Ni Al, respectively, are also used as a matrix material (58,59). P/M MMC can be formed by a variety of full-density hot consolidation processes, including hot pressing, hot isostatic pressing, extmsion, or forging. 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

Increased hardness and wear resistance may also be achieved by incorporating approximately 25-50% by volume of small non-metallic particles. These may be carbides, oxides, borides or nitrides, and hardness values up to 560 Hy have been reported. ... 

Chemical Resistance. The chemical resistance of chromium carbide is superior to that of other interstitial carbides. Oxidation in air starts at 1000°C and a dense and strong oxide layer is formed. It is insoluble in cold HCl but dissolves in hot oxidizing acids. 

Chemical Resistance. Hafnium carbide oxidizes in air at 500°C. It is not as chemically resistant as TiC and is similar to ZrC in that respect. 

Carbides oxidize readily although less rapidly than the nitrides but more so than the borides. Oxidation becomes more rapid going from the Group IV carbides (TiC, ZrC, HfC) to those of Group VI (Cr3C7, MoC, WC). In some cases, a protective film of the metal oxide is formed. Such is the case with SiC, as reviewed in Sec. 5.7 below. 

It is now worthwhile to focus attention on the carbide-oxide reactions which may be identified as the specific variant of the carbothermy under pyrovacuum conditions for some metals. Such reactions can be represented as ... 

See Tungsten carbide Nitrogen oxides Ditungsten carbide Oxidants... 

Leclercq, L., Almazouari, A., Dufour, M., and Leclercq, G. 1996. Carbide-oxide interactions in bulk and supported tungsten carbide catalysts for alcohol synthesis. In Chemistry of transition metal carbides and nitrides, ed. S. T. Oyama, 345-61. Glasgow Blackie. 

Motojima, S., Asakura, S., Kasemura, T., Takeuchi, S. and Iwanaga, H., Catalytic effects of metal carbides oxides and Ni single crystal on the vapor growth of micro-coiled carbon fibers, Carbon, 1996,34, (3), 289 296. 

Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify... 

Pentadecacarbonylhexarhodium carbide oxidation reactions, 30 159 reductive condensation, 30 156 Pentadecacarbonylpentaosmium carbide... 

Pentadecacarbonylpentaruthenium carbide oxidative addition reactions, 30 206 reaction with phosphines and diphosphines, 30 191... 

The use of nitrides, along with sulfides and carbides, as catalysts for hydroprocessing has recently been extensively reviewed by Furimsky and will not be discussed in detail here. Subsequently, Al-Megren et have published a comparison of the activities of bulk CoMo carbide, oxide, nitride and sulfide catalysts for pyridine hydrodenitrogenation. Of these, the sulfide catalysts were reported to possess more stable activity, with the carbide being next, followed... 

With catalysts converted to fairly pure Hagg carbide or cementite, the rates of oxidation and elemental carbon deposition were very slow at 7.8 atm. (21). However, at 21.4 atm. these carbides oxidized at least as rapidly as metallic iron, the carbidic carbon being transformed to elemental carbon (21). 

Phillips and Timms [599] described a less general method. They converted germanium and silicon in alloys into hydrides and further into chlorides by contact with gold trichloride. They performed GC on a column packed with 13% of silicone 702 on Celite with the use of a gas-density balance for detection. Juvet and Fischer [600] developed a special reactor coupled directly to the chromatographic column, in which they fluorinated metals in alloys, carbides, oxides, sulphides and salts. In these samples, they determined quantitatively uranium, sulphur, selenium, technetium, tungsten, molybdenum, rhenium, silicon, boron, osmium, vanadium, iridium and platinum as fluorides. They performed the analysis on a PTFE column packed with 15% of Kel-F oil No. 10 on Chromosorb T. Prior to analysis the column was conditioned with fluorine and chlorine trifluoride in order to remove moisture and reactive organic compounds. The thermal conductivity detector was equipped with nickel-coated filaments resistant to corrosion with metal fluorides. Fig. 5.34 illustrates the analysis of tungsten, rhenium and osmium fluorides by this method. 

Reaction Products O, CO, CxHy, HD, SixHy, carbides, oxides... 

In the first experiments in which chemistry of metal clusters was demonstrated, the reactant was present in the carrier gas. The problem with this approach is that the reactant is also decomposed during the high-temperature vaporization process, and thus reactive radical fragments are present which may participate in the cluster growth process. Although not exploited, this may be a good method to synthesize metal cluster carbides, oxides, nitrides, sulfides, and hydrides, to name a few. It is not a viable mode of operation for the measurement of reactivity toward molecular species, but has been used to examine the stoichiometry of metal cluster compounds. ... 

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