HOMOGENEOUS MULLITE-FORMING POWDERS FROM SPRAY-DRYING AQUEOUS-SOLUTIONS

A mullite precursor solution was made by hydrolyzing TEOS in a diluted aqueous solution of aluminum nitrate. This solution was spray-dried at a relatively low temperature (200-degrees-C), producing a chemically homogeneous powder crystallizing completely into mullite at 975-degrees-C. By aging this solution at 100-degrees-C, a diphasic precursor sol made of colloidal silica and aluminum nitrate was made. The resulting spray-dried powder was less homogeneous, crystallizing into mullite and spinel or only spinel. The aging by thermal treatment may thus be a way to control the heterogeneity of the powder.