The formation of mullite from kaolinite under various reaction atmospheres

Abstract

In addition to its importance as a high-technology ceramic, mullite is also an intermediate phase in the formation of non-oxide ceramics (sialons) by carbothermal reduction of clay minerals. This reaction involved the formation of mullite under non-oxidizing atmospheres and in the presence of carbon. The effect of eight different reaction atmospheres on mullite formation from kaolinite in the presence and absence of carbon was studied by X-ray diffraction, 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Generally, a greater amount of mullite is formed at 1200 °C under vacuum and reducing atmospheres, which also produce mullites that are slightly more silica-rich (as deduced from X-ray lattice parameter measurements) but contain higher proportions of tetrahedral 27Al NMR resonance at ≈ 46 ppm, probably associated with Al ∗ sites. Reaction systems containing either nitrogen or ammonia in the presence of carbon show NMR evidence of previously unreported early stage formation of SiON bonds. Thermodynamic calculations are presented which suggest that the formation of SiO is an important factor in the complex interactions between the various solid and gaseous reactants.