Constitution of Mullite Glasses Produced by Ultra-rapid Quenching of Plasma-sprayed Melts

摘要

Spherical shaped spray-dried admixtures of chemical pure and very fine α-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 μm) were melted in a nitrogen plasma flame and subsequently quenched in water thus producing spherical, hollow, and porous particles (≤ 100 μm). The as-quenched spherules consist of mullite glass, some residual α-Al_2O_3 and quartz, and a very low amount of newly formed mullite. Double quenching of the material increases the glass content to > 90 wt.%. ~(27)Al and ~(29)Si 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 ≈ 980℃ in all cases.