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 powder s (10-100 mu m) were melted in a nitrogen plasma flame and subsequentl y quenched in water thus producing spherical, hollow, and porous parti cles (less than or equal to 100 mu m). The as-quenched spherules consi st of mullite glass, some residual alpha-Al2O3 and quartz, and a very low amount of newly formed mullite. Double quenching of the material i ncreases the glass content to >90 wt.%. Al-27 and Si-29 MAS NMR studie s show that the rapidly quenched mullite glass is composed of a networ k of (SiO)-tetrahedra and (AlO)-octahedra, -pentahedra, and -tetrahedr a The frequency distribution of (AlO)-structural units is similar to t hose in metakaolinite, type I (polymer) mullite precursors, and in oth er melt-quenched aluminium-silicate glasses suggesting strong structur al similarities of these phases. This has been supported by the exothe rmic mullite crystallization process taking place at approximate to 98 0 degrees C in all cases.