Carbothermic silicon carbides

Carbothermic Reduction. Silicon carbide is commercially produced by the electrochemical reaction of high grade silica sand (quartz) and carbon in an electric resistance furnace. The carbon is in the form of petroleum coke or anthracite coal. The overall reaction is... 

Benzene, ethylene and acetylene were the predominate observed volatiles at 550 °C whilst methane was evolved from 650 °C to 875 °C. An amorphous SiCO material was obtained at 1200 °C and bond redistribution and carbothermic reduction occurred up to 1800 °C to give a ceramic material composed of substantial amounts of crystalline fi-silicon carbide. The preparation of bulk ceramic components from materials in the system... 

Hollow silicon carbide (SiC) spheres have been synthesized by a microwave heating and carbothermal reduction method with carbon spheres as template and fly ash (a solid waste from coal-fired power plant) as silica source. X-ray diffraction and scanning electron microscope were employed to characterize the morphology, structure of the products. The results show that hollow spheres prepared at 1300 "C under argon atmosphere have a hollow core and SiC shell structure. The shell of a hollow SiC sphere is composed of a lot of irregular SiC nanowires with 5-20 pm in length and 50-500 nm in diameter which belongs to the p-SiC. Moreover, the formation mechanism of the hollow SiC spheres is also discussed. 

HP Martin, R Ecke, and E Muller, Synthesis of nanocrystalline silicon carbide powder by carbothermal reduction, J. Ear Ceram. Soc.,18,1737-1742(1998). 

It has already been well known for more than two decades that it is feasible to produce SiC powders and/or whiskers by pyrolyzing rice husk [178] under an inert atmosphere, since a considerable content of colloidal amorphous silica is incorporated in husks and straws of this vegetable. It means that nature by itself is supplying a similar intimate mixture of carbon-containing molecules (cellulose) and colloidal silica like those artificially made mixtures we discussed in the last section. Thus, also in this case, silicon carbide may be formed by a carbothermal reduction process. This process starts in the temperature range 1100-1400 °C ... 

The preparation of a porous silicon carbide has been described by Fox and co-workers (10). The synthesis is based on heating the organosilicon pol3nmer (0 1158iOj 5) at 1600 C under argon (see Fig. 1). The pyrolysis reaction results in an intramolecular carbothermic reduction, i.e. the carbon bonded to silicon is used to remove oxygen and to form the carbide (the commercial manufacture of silicon carbide uses an external source of carbon for example, by mixing quartz sand and petroleum cokes). The product is purified by oxidation to remove excess carbon, followed by treatment with HF to remove silica. 

Moshtaghioun BM, Monshi A, Abbasi MH, Karimzadeh F (2011) A study on the effects of silica particle size and milling time on synthesis of silicon carbide nanoparticles by carbothermic reduction. Int J Refract Metal Hard Mater 29 645-650... 

Rice hull hydrocarbons supply the carbon source and rice hull ash the silicon source. This is an early carbothermal process (Equ. 10a). At <900°C, coking (Equ. 10b and 10c) removes water and organics and yields a mixture of silica and carbon, and at <1700°C, pyrolysis yields silicon carbide whiskers (Equation lOd). The overall yield is 1 percent. Three variants of the rice hull process are known. Compacted rice hulls can be pyrolized at 1600°C without coking [14], thus directly yielding a mixture of graphite and SiC whiskers, 10-200 pm in length and... 

Cuboid and cycloid niobium monocarbide (NbC) whiskers, 0.1-2.0 pm in diameter and 5-100 pm in length, and having a square-shaped tip, were recently synthesized by heating mixtures of niobium oxide (Nb203) and carbon black at temperatures over 1100°C [38]. Silicon carbide nano-whiskers, 20-50 nm in diameter and 2-5 pm in length, were carbothermally synthesized by reducing ultrafine precipitated silica powders with ultrafine carbon black by microwave heating [29]. These processes proceed without addition of metal particle catalysts, and therefore by a VS phase transformation [14] [29] [38]. 

Silicon carbide whiskers can also be synthesized by carbothermal reduction of silicon nitride [10]. Silicon nitride decomposes >1300°C, silicon melts at 1410°C, and reacts with graphite. Whisker formation in this process is initiated >1400°C and can be completed between 1550... 

If this carbothermal process is brought to only partial completion (Equation 11a and 11b), a homogeneous mixture of silicon carbide whiskers and silicon nitride powder [10] is obtained which can be fired directly to yield whisker reinforced ceramics. Silicon carbide reinforced alumina composites and silicon carbide whisker reinforced zirconia composites [31] are also products of the "chemical mixing process". The whisker growth rate in the zirconia process can be accelerated by adding metal particle catalysts such as cobalt chloride, thus potentially facilitating a VLS phase transformation. 

Siiicon carbide whiskers have been made by metai cataiyzed chemical vapor deposition, a VLS process (Chapter 2.1.4). Silicon carbide whiskers have aiso been made by variants of the carbothermal reduction of silicon (or a siiicon precursor) with carbon, either by the chemicai mixing or by the rice huii process (Chapter 2.1.7). 

H. Wang, Y. Berta and G. S. Fishman, Microstructure of silicon carbide whiskers synthesized by carbothermal reduction of silicon nitride, J. Am. Ceram. Soc., 75,0,1080-1084 (1992). 

P. K. Panda, L. Mariappan, V. A. Jaleel, T. S. Kannan, A. Amroune, J. Dubois and G. Fantozzi, Preparation of zirconia and silicon carbide whisker biphasic powder mixtures by carbothermal reduction of zircon powders, Jour, of Mat. Science, 31 [16], 4277-4288 (1996). 

Chen, L.L., Ye, H.H., Gogotsi, Y, and McNallan, M.J. Carbothermal synthesis of boron nitride coatings on silicon carbide. Journal of the American Ceramic Society 86, 1830-1837, 2003. 

Preiss H., Berger L-M., Schultze D. Studies on the carbothermal preparation of titanium carbide fi-om different gel precursors. J. Eur. Ceram. Soc. 1999 19 195-206 Raman V., Paraschar V.K., Dhakate S.R. Synthesis ofsiUcon carbide whiskers from substituted silicon alkoxides and rayon fibers. J. Sol-Gel Sci. Technol. 2002 25 175-179... 

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