Spherical shaped spray-dried admixtures of chemical pure and very fine alpha -Al//2O//3 and quartz powders with mullite composition (72 wt% Al//2O//3, 28 wt% SiO//2) were used as starting materials. The spray-dried powders (10-100 mu m) were melted in a nitrogen plasma flame and subsequently quenched in water thus producing spherical, hollow, and porous particles ( less than equivalent to 100 mu m). The as-quenched spherules consist of mullite glass, some residual alpha -Al//2O//3 and quartz, and a very low amount of newly formed mullite. Double quenching of the material increases the glass content to greater than 90 wt.%.**2**7Al and **2**9Si MAS NMR studies show that the rapidly quenched mullite glass is composed of a network of (SiO)-tetrahedra and (AlO)-octahedra, -pentahedra, and-tetrahedra. The frequency distribution of (AlO)-structural units is similar to those in metakaolinite, type I (polymer) mullite precursors, and in other melt-quenched aluminium-silicate glasses suggesting strong structural similarities of these phases. This has been supported by the exothermic mullite crystallization process taking place at approximately equals 980 degree C in all cases. (Author abstract) 12 Refs.
Schmuecker, M. Schneider, H. Poorteman, M. Cambier, F. Meinhold, R.