Kinetics of alumina segregation in mullite ceramics

Dense polycrystalline mullite was equilibrated for 6 h in air at 1800 °C and then quenched to room temperature. During subsequent annealing at 1600 °C a gradual decrease of the Al2O3 concentration in the grains occurs which approaches an equilibrium concentration after about 100 h annealing time. A simplified model of spherical grains of uniform size is applied to describe the observed kinetics of the Al2O3 concentration decrease in the mullite grains. This model allows to determine a chemical diffusion coefficient of Al2O3 from the measured kinetics data. This chemical diffusion coefficient of Al2O3 is compared to the ambipolar diffusion coefficient of Al2O3 calculated from our tracer diffusivity data in single crystalline 2/1-mullite. The resulting thermodynamic factor is in reasonable agreement with the value calculated from literature data for mullite formation in a solid state reaction.