CHARACTERIZATION OF MONOPHASIC AND DIPHASIC MULLITE PRECURSOR POWDERSPREPARED BY AQUEOUS ROUTES - AL-27 AND SI-29 MAS-NMR SPECTROSCOPY INVESTIGATIONS

The structural evolution from amorphous to crystalline mullite, for di fferent 3Al(2)O(3) . 2SiO(2) mono- and diphasic precursors, has been i nvestigated by Si-29 and Al-27 magic angle spinning nuclear magnetic r esonance (MAS NMR) spectroscopy. The crystallization has also been stu died by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the Si-29 NMR spectra. A close agreement is found with the compo sition deduced from the lattice parameters measured by XRD. For monoph asic precursors the amount of hexa-coordinated aluminium atoms decreas es when the temperature increases while AI(IV) and AI(V) increase. AI( VI) practically completely disappears just before the crystallization at 980 degrees C. An alumina-rich mullite 2Al(2)O(3) . SiO2 (2:1 mulli te) is then formed through a strong exotherm. An enthalpy of 75 kJ per mot is determined for the crystallization of the 2:1 mullite. At high er temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the Si-29 NMR spectr oscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO6 octahedra bef ore the crystallization at 980 degrees C is higher. Spinel crystallize s and continues to become richer in alumina until it reacts with silic a to form the stoichiometric 3:2 mullite at 1260-1275 degrees C. The n ature of the crystallization is related to the local composition of th e amorphous alumino-silicate network and to the amount of AlO6 octahed ra on approaching 980 degrees C.