Constitution of mullite glasses produced by ultra-rapid quenching of plasma-sprayed melts

Spherical shaped spray-dried admixtures of chemical pure and very fine alpha-Al2O3 and quartz powders with mullite composition (72 wt% Al2O3, 28 wt% SiO2) 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 or equal to 100 mu m). The as-quenched spherules consist of mullite glass, some residual alpha-Al2O3 and quartz, and a very low amount of newly formed mullite. Double quenching of the material increases the glass content to >90 wt.%. Al-27 and Si-29 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 approximate to 980 degrees C in all cases.