[Show abstract][Hide abstract] ABSTRACT: The thermal stability of kaolinite and the microstructure of its thermal products strongly depend on random defects (R2) rather than crystalline defects (HI). Kaolinite with lower random defect density is more stable than that with higher defect density during dehydroxylation and the derived metakaolinite can be directly transformed into orthorhombic mullite (3/2-mullite). However, for kaolinite with higher random defect density, there is a cubic phase occurring in the transformation from metakaolinite to primary mullite. Primary mullite will be transformed into orthorhombic mullite as temperature increases. AlV is universally present in the metakaolinite and the relative amounts of AlVI, AlV and AlIV vary with the random defect density of the parent kaolinite.
Full-text · Article · Apr 2005 · Journal of the American Ceramic Society
[Show abstract][Hide abstract] ABSTRACT: Mullites synthesized from four kaolinites with different random defect densities have been studied by 27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) and X-ray diffraction (XRD). All these mullites show the same XRD pattern. However, 29Si and 27Al MAS NMR spectra reveal that the mullites derived from kaolinites with high defect densities, have a sillimanite-type Al/Si ordering scheme and are low in silica, whereas those mullites derived from kaolinites with low defect densities, consist of both sillimanite- and mullite-type Al/Si ordering schemes and are rich in silica.
Full-text · Article · May 2004 · Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy