Kinetics analysis of mullite formation reaction at high temperatures

A detailed kinetics analysis was performed on the mullite (3Al₂O₃·2SiO₂) formation reaction occurring at high temperatures ranging from ∼1600 to 1800°C. The counterdiffusion model of Al³⁺ and Si⁴⁺ fluxes was used to derive the kinetics equation of the mullite formation reaction. From the parabolic kinetics between the thickness of the mullite layer and time, the reaction rate constant (k) for the mullite formation was determined to be a function of the average diffusion coefficient of Si⁴⁺ ions. This kinetics equation can be used to estimate the mullite formation at any temperature. By substituting previous experimental data into the present kinetics equation, average diffusion coefficient values of Si⁴⁺ ions in the mullite layer were calculated and these values are in good agreement with the diffusion coefficient values calculated using Aksay's interdiffusion equation for mullite formation. The activation energy values for the diffusion of the Si⁴⁺ ions were estimated to range from 730 to 780 kJ/mol, which are close to those obtained from previous diffusion and creep experiments.