Transformation kinetics for mullite in kaolin–Al₂O₃ ceramics

Transformation kinetics of mullite formation in kaolin–Al₂O₃ ceramics was studied by x-ray diffraction, transmission electron microscopy, and energy dispersion spectrometry. The mullitization process of kaolin–Al₂O₃ ceramics is described by two stages; one is the primary mullite transformation at 1273 to 1573 K, and the other is the secondary mullite formation at 1573 to 1873 K. The activation energy of 1164.6 kJ mol^-1 obtained for the secondary mullite formation is lower than 1356.9 kJ mol^-1 for the primary mullite transformation by the general form of the Johnson–Mehl–Avrami equation. The lower value of growth morphology parameter strongly supports that in the secondary mullite formation the added alumina is dissolved into glassy phase and the mullite is then precipitated.