Shape, size and composition of mullite nanocrystals from a rapidly sintered kaolin

The characteristics of mullite crystallites from a kaolin raw material were determined at the early stage of crystallisation by X-ray diffractometry. The equipment was a Debye–Scherrer system equipped with a curved position sensitive detector. We measured the size and shape of mullite crystallites in relation to the temperature (1100–1150°C) and the temperature rate (3–20°C/min). In particular, a temperature rate influence was found on crystallites anisotropy. This result was satisfactorily correlated with TEM observations of sintered materials. The mullite cells constants were also measured, they vary significantly with sintering conditions. Cell constants were correlated with mullite stœchiometry using the Cameron relationship. Results indicated a narrow composition domain (62–65% Al2O3) for all thermal cycles. In each case, the orthorhombic phase was identified, in spite of the low temperature range studied and the relatively high alumina content.