Silicon carbide active oxidation

Kharatyan, S. L., and Nersisyan, H. H., Combustion synthesis of silicon carbide under oxidative activation conditions. Int. J. SHS, 3,17 (1994). 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

Zinc oxide (ZnO) is widely used as an active filler in rubber and as a weatherability improver in polyolefins and polyesters. Titanium dioxide (TiOj) is widely used as a white pigment and as a weatherability improver in many polymers. Ground barites (BaS04) yield x-ray-opaque plastics with controlled densities. The addition of finely divided calcined alumina or silicon carbide produces abrasive composites. Zirconia, zirconium silicate, and iron oxide, which have specific gravities greater than 4.5, are used to produce plastics with controlled high densities. 

At high temperature, silicon carbide exhibits either active or passive oxidation behavior depending on the ambient oxygen potential (65,66). When the partial pressure of oxygen is high, passive oxidation occurs and a protective layer of Si02 is formed on the surface. 

A fresh surface of silicon carbide is thus constandy being exposed to the oxidizing atmosphere. Active oxidation takes place at and below approximately 30 Pa (0.23 mm Hg) oxygen pressure at 1400°C (66). Passive oxidation is determined primarily by the nature and concentration of impurities (67). 

Shimoo, T., Morisada, Y., Okamura, K., (2002), Active-to-passive oxidation transition for polycarbosilane-derived silicon carbide fibers heated in Ar-02 gas mixtures , J. Mater. Sci., 37, 1793-1800. 

Schneider, B., Guette, A., Naslain, R., Cataldi, M., Costecalde, A., 1998), Atheoretical and experimental approach to the active-to-passive transition in the oxidation of silicon carbide , J. Mater. Sci., 33, 535-547. 

Narushima, T., Goto, T., Iguchi, Y., Hirai, T., (1991), High-temperature active oxidation of chemically vapor-deposited silicon carbide in an Ar-02 atmosphere , J. Am. Ceram. Soc., 74, 2583-2586. 

The main functions of a carrier or support are usually to lend mechanical strength, increase stability to sintering and provide a larger active surface area than would otherwise be available. There is evidence that, in many instances, compound or complex formation takes place between the catalyst and the support, with a consequent effect on the catalytic properties. The most commonly used support materials are silica, alumina, silica-alumina, titania, silicon carbide, diatomaceous earths, magnesia, zinc oxide, iron oxide and activated carbon. 

The synthesis of nanophase ceramics is one of these concepts, it allows micro-porous ceramic materials with ceramic grains in the nanometer range to be obtained. Research in the field of nanophase materials is very active. A number of results on the control of microstructure and temperature stability of metal oxide ceramics can be applied to membrane preparation. Works carried out on non-oxide ceramics such as silicon carbide, silicon oxinitride or aluminum nitride should be regarded in order to extend the domain of available membrane materials. 

The catalyst systems employed are based on molybdenum and phosphorus. They also contain Various additives (oxides of bismuth, antimony, thorium, chromium, copper, zirconium, etc.) and occur in the form of complex phosphomolybdates, or preferably heteropolyacids deposited on an inert support (silicon carbide, a-alumina, diatomaceous earths, titanium dioxide, etc.). This makes them quite different from the catalysts used to produce acrylic acid, which do not offer sufficient activity in this case. With residence times of 2 to 5 s, once-through conversion is better than 90 to 95 per cent, and the molar yield of methacrylic acid is up to 85 to 90 per cent The main by-products formed are acetic add, acetone, acrylic add, CO, C02, etc. The major developments in this area were conducted by Asahi Glass, Daicel, Japan Catalytic Chemical, Japanese Gem, Mitsubishi Rayon, Nippon Kayaku, Standard Oil, Sumitomo Chemical, Toyo Soda, Ube, etc. A number of liquid phase processes, operating at about 30°C, in die presence of a catalyst based on silver or cobalt in alkaline medium, have been developed by ARCO (Atlantic Richfield Co,), Asahi, Sumitomo, Union Carbide, etc. 

A theoretical and experimental approach to the active-to-passive transition in the oxidation of silicon carbide./. Mater, Sci, 33, 535-547. 

Other unusual additives include oxetanes, vinylic macromono-mers, silicon carbide, superconductive carbon blacks, silver-coated fly ash, metal oxides, Tb for green emission, antibacterial agents, and organic-inorganic hybrid copolymer fibers. In an interesting reversal, transition element acetylacetonate salts were decomposed in a PDMS matrix to give membranes with catalytic activity. ... 

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