The influence of steam on mullite formation from sol-gel precursors

Abstract

The effect of steam on the reaction kinetics of both mullite and metastable spinel formation from sol-gel precursors, having an Al/Si atomic ratio of 3/1, was studied using dynamic x-ray diffraction (DXRD) coupled with differential thermal analysis (DTA). Steam was observed to accelerate the nucleation of tetragonal mullite in single phase gels at ~980 °C. When single phase gels are formed with a faster hydrolysis step, the presence of steam also increases the nucleation of metastable spinel at this temperature, possibly due to the in situ formation of boehmite in locally enriched alumina regions at <300 °C. In addition, steam also enhances the decomposition of metastable spinel in single phase gels and transient alumina in the diphasic gel at ~1200 °C, probably as a result of surface interactions of water with spinel or transient alumina grains. There was no observable effect of steam on second stage mullite formation (~1250 °C) in either the single or diphasic gels. This is attributed to the hypothesis that the rate-limiting step at this stage might be the dissolution of alumina into the silica-rich amorphous matrix, a step that is unaffected by the presence of steam.