MULLITE FORMATION FROM NONSTOICHIOMETRIC SLOW HYDROLYZED SINGLE-PHASEGELS

A comparative dynamic x-ray diffraction (DXRD) and differential therma l analysis (DTA) study was performed in the investigation of mullite a nd spinel formation from slowly hydrolyzed single phase gels with Al/S i ratios ranging from 1/1 to 14/1. Both metastable tetragonal mullite and spinel were observed to form at temperatures <1000 degrees C in th e gels with Al/Si ratios <8/1 and mullite transformed to the orthorhom bic structure at similar to 1250 degrees C. However, at higher Al/Si r atios, spinel was the only crystalline phase detected at <1000 degrees C and orthorhombic mullite formed directly at temperatures >1250 degr ees C. As the Al/Si ratio increases, both the tetragonal mullite and s pinel formation temperatures decrease while the orthorhombic mullite f ormation temperature increases. Based on the Al/Si composition where t he formation extents of tetragonal mullite and spinel were maximum, th eir compositions are estimated to be 2Al(2)O(3) . SiO2 and 6Al(2)O(3) . SiO2, respectively.