The plasma arc production of Si-based ceramic whiskers
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J. M. Toguri Department of Metallurgy and Materials Science, University of Toronto, Toronto, Ontario, M5S 1A4, Canada (Received 16 December 1992; accepted 23 March 1993)
Silicon-based ceramic whiskers, such as silicon carbide and silicon nitride, were produced in a plasma arc reactor. Silicon monoxide gas was generated by reacting briquettes of silica and carbon according to the following reaction: SiO2(s or i) + C(S) = SiO(g) + CO(g). The effects of operating variables on the production of silicon monoxide are discussed. The silicon monoxide gas was then back-reacted with carbon monoxide to produce silicon carbide whiskers according to the following reaction: SiO(g) + 3CO(g) = SiC(s) + 2CO2(g). In the presence of nitrogen, the product contained some silicon nitride and silicon oxynitride whiskers. The effects of gas composition and flow rate, crucible mass, and alkali additions on the amount and morphology of the whiskers were determined. It is concluded that the whiskers grew by a vapor-liquid-solid mechanism.
I. INTRODUCTION At the present time, a considerable amount of research effort is being devoted to reinforced composite materials. This is due to the high strength and low density of these materials as compared to the conventional engineering materials. There are a number of reinforcing agents such as fibers, platelets, whiskers, and various shapes of particulates. In comparison to most reinforcing agents, whiskers have unique properties. Whiskers exhibit high mechanical strength and have diameters of the order of a micron.1 Thus, whiskers have considerable potential as reinforcing agents for metals, ceramics, and polymers. Whiskers have large length-to-diameter (l/d) ratios of about 10 to 10000. In addition, they are single crystals. These two factors are responsible for the high strength of these materials. It has been shown that as the diameter of a whisker becomes smaller, its strength increases rapidly.2 In addition, reducing the size of the whisker also increases the interfacial area per unit volume between the reinforcing agent and the matrix. Thus, the strength of the composite will increase. Also, ceramic whiskers such as silicon carbide and silicon nitride are stable at high temperatures. Therefore, they could be employed as reinforcing agents in materials that are subjected to hostile environments at high temperatures. A number of techniques have been devised for the growth of silicon carbide whiskers. These include (1) sublimation of silicon carbide followed by nucleation and growth from catalytic sites,3 (2) hydrogen reduction of methyltrichlorosilane (CH3SiCl3) on a carbon substrate,4'5 (3) reaction of silica and carbon in 1996
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J. Mater. Res., Vol. 8, No. 8, Aug 1993
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rice hulls,6-7 (4) a vapor-liquid-solid (VLS) process that employs a silicon monoxide generator and a catalyst that provides nucleation sites for the whiskers,8"10 and (5) carbothermal reduction of silica using a molten halide bath.11"13 There is very little information av
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