Abstract
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 Al 2O 3 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 Al 2O 3 concentration decrease in the mullite grains. This model allows to determine a chemical diffusion coefficient of Al 2O 3 from the measured kinetics data. This chemical diffusion coefficient of Al 2O 3 is compared to the ambipolar diffusion coefficient of Al 2O 3 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.
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