MECHANICAL AMORPHIZATION OF MULLITE AND THERMAL RECRYSTALLIZATION

Commercial fused-mullite was ball-milled up to 240 h and its structura l changes were monitored by XRD, TEM and solid-state Al-27 MAS NMR. Mi lling for 240 h reduces the mullite XRD reflections to about 10% of th eir original intensity. Broadening of the XRD lines occurs, suggesting drastic crystal size reduction, but TEM indicates that the loss of XR D intensity is due to amorphization rather than to the effect of parti cle size or lattice strain. MAS NMR spectra show that the amorphous ma terial retains the tetrahedral and octahedral Al environments of the o riginal mullite. However, the Al NMR signal near 40 ppm in mullite whi ch is associated with the tetrahedral tricluster Al sites is graduall y replaced by a resonance at about 30 ppm. This NMR signal also appear s in alumina-rich gels and glasses acid has been explained either by t he presence of fivefold coordinated aluminum or by distorted tricluste red AlO4 tetrahedra. Thermal treatment at > 700 degrees C causes gradu al recrystallization of the non-crystalline material to mullite with t he simultaneous replacement of the 30 ppm Al-27 NMR resonance by the A l resonance at about 40 ppm. Complete recrystallization of the ground material leads to extremely small mullite grains indicating higher nu cleation densities than in crystallized mullite gels and glasses. The low activation energy of recrystallization in mechanically amorphized mullite and the low crystallization temperature suggest that ball-mill -produced mullite 'glass' has a structural short-range-order similar t o that of crystalline mullite. (C) 1998 Published by Elsevier Science B.V. All rights reserved.