Thermodynamic predictions for material processing in a plasma reactor using solid oxide feed materials
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INTRODUCTION
M A T E R I A L processing in plasma reactors has received considerable attention in recent years. A large number of ceramic powders, such as oxides, carbides, nitrides, and borides, have been synthesized successfully in laboratory-scale plasma reactors. [~,2,3j Plasma processing is characterized by high enthalpy, reactive species, and rapid quenching. Materials injected into the plasma flame may vaporize or dissociate rapidly, and the high temperatures may enhance the reaction kinetics by several orders of magnitude. Rapid quenching in plasma reactors provides for the possibility of producing very fine powders. These powders may have very low levels of contamination, which would increase their market value. These fine and pure ceramic powders may be easier to sinter than the coarser powders produced by other processing techniques. Though there has been success in the synthesis of ceramic powders in plasma reactors on a laboratory scale, the plasma processing of these ceramic powders on an industrial scale is still in the developmental stage. An important aspect of material processing in plasma reactors is the type of precursors used. Chlorides or hydrides are typically used as precursors for this synthesis. If oxides can be used as precursors, then the process may be made simpler, due to the absence of corrosive compounds (such as in the case of chlorides), and may contain less contamination. In any proposed chemical process, thermodynamics is the first consideration. Thermodynamic analysis gives general guidelines for the expected reactions and equilibrium composition of species at different temperatures. These predictions should be used along with kinetic considerations for any initial screening of a proposed reaction system. Thermodynamic calculations, however, sometimes can give a misleading picture, especially when considering the condensation of the species. Due to the rapid temperature drop possible in plasma reactors, equilibrium conditions may not exist in the quench section of reactors. So, the equilibrium prediction, based on the free energy minimization (FEM) calculations in P.R. TAYLOR, Professor, and S.A. P1RZADA, Postdoctoral Research Associate, are with the Department of Metallurgical Engineering, University of Idaho, Moscow, ID. M. MANRIQUE, Professor, is with the Department of Material Science, Simon Bolivar University, Caracas, Venezuela. Manuscript submitted February 1, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS B
the lower temperature range (3000 K), most of the species are in gas phase. Next to the vaporization zone is the reaction zone (1750 to 2750 K), where silicon carbide is the stable phase. The last zone is the quench zone. If the quenching is fast enough, the reoxidation of silicon carbide may be avoided and the final product will be fine silicon carbide. Based on FEM, some of the possible reactions for the formation of silicon carbide are as follows: SiO (g) + C2H2 (g) = SiC (s) + H2 (g) + CO (g) SiO (g) + C2H (g) = SiC (s) + 0.5H2 (g) + CO (g) Si (g) + C2H (g)
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