MULLITE EVOLUTION IN CERAMICS DERIVED FROM KAOLINITE, KAOLINITE WITH ADDED ALPHA-ALUMINA, AND SOL-GEL PRECURSORS

The microstructural evolution of mullite prepared from sol-gel (boehmi te and colloidal silica), kaolinite, and kaolinite-alpha-alumina has b een investigated. For the sol-gel route, calcination at 600 degrees C for 1 h produced a powder consisting of gamma-alumina and amorphous si lica. The density of sintered compacts increased with temperature to a maximum of 3 g cm(-3) after 2 h at 1700 degrees C. Calcination of sol -gel powder at 1400 degrees C for 2 h resulted in mullite powder which exhibited inferior sinterability, achieving a maximum density of 2.6 g cm(-3) after 2 h at 1700 degrees C. Both sintered samples had an equ iaxed grain structure with some glassy phase located mainly at triple junctions. The maximum density achieved in kaolinite derived ceramics was 2.68 g cm(-3) on sintering at 1450 degrees C for 2 h, and the resu ltant microstructure consisted of high aspect ratio mullite grains in a glassy matrix. Addition of alpha-alumina to the kaolinite to give a total alumina content of similar to 72 (wt-)%, led to a density of 2.9 2 g cm(-3) on sintering at 1700 degrees C for 2 h. The microstructure consisted of primary mullite derived from the kaolinite, and secondary mullite produced by precipitation from a liquid phase. (C) 1997 The I nstitute of Materials.