SiO(g) formation from SiC in mixed oxidizing-reducing gases

The formation of SiO(g) from SiC by either active oxidation or an oxidation-reduction process is discussed. The Wagner criterion for the transition from active to passive oxidation is generalized for any oxidant. Kinetic modeling of both active oxidation and oxidation-reduction is described.Symbols - - MW_SiC/MW_Si - _CO CO(g) boundary layer thickness - _ox gaseous-oxidant, boundary layer thickness - stoichiometric factor from Eqs. (2)–(4), # of CO produced/# of oxidant (on oxygen atom basis) - gas viscosity - concentration of diffusing gas species in boundary layer - concentration of major gas species in boundary layer - _oxide density of SiO₂ - D diffusion coefficient of diffusing species in gas-boundary layer - D_CO diffusion coefficient of CO(g) - D_ox gas diffusion coefficient of oxidant - J flux, rate of weight loss limited by diffusion in gas-boundary layer - J_CO flux of CO(g) - J_ox flux of gaseous oxidant - K_g linear oxide growth constant, weight/(length² time) - k_g linear oxide growth constant, length/time - k_l linear volatilization constant for SiO₂,k_lo+k_ls, weight/(length² time) - k_lo linear volatilization constant for oxygen from SiO₂, weight/(length² time) - k_ls linear volatilization constant for silicon from SiO₂, weight/(length² time) - k_l linear volatilization constant for SiO₂, length/time - k_p parabolic oxide growth constant, weight²/(length⁴ time) - k_p parabolic oxide growth constant, length²/time - L sample length parallel to gas flow direction - (M/A)₁ specific weight change due to oxygen gain and associated carbon loss in paralinear oxidation - (M/A)_1L limiting value of weight change due to oxygen gain and associated carbon loss in paralinear oxidation - (M/A)₂ specific weight change due to silicon loss and associated carbon loss in paralinear oxidation - MW_C molecular weight of carbon - molecular weight of O₂ - MW_Si molecular weight of silicon - MW_SiC molecular weight of silicon carbide - molecular weight of silica - n number of oxygen atoms per oxidant molecule - P_CO^eq eqiilibrium CO(g) pressure - P_CO^g CO(g) pressure outside of boundary layer - P_COⁱ CO(g) pressure at SiC-gas interface - P_ox^g oxidant gas pressure outside of boundary layer - P_oxⁱ oxidant gas pressure at SiC-gas interface - R gas constant - t time - T absolute temperature - v linear gas velocity - x oxide thickness - x_L limiting oxide thickness achieved in paralinear oxidation - x_t oxide thickness at which transition from linear to parabolic growth occurs