Evolution of mullite texture on firing tape-cast kaolin bodies

Kaolin undergoes a series of phase changes on heating to elevated temperature, proceeding through kaolin, metakaolin gamma-Al2O3 spinel, and mullite. The morphologic evolution of the kaolin-mullite reaction series was investigated in the present study. A highly textured kaolin green body was prepared by a tape-casting technique, and the morphology evolution from kaolin flakes to mullite aciculars on firing from 450degrees to 1600degreesC was then monitored using X-ray diffractometry and scanning electron microscopy. Equiaxed mullite nuclei first appeared at 1000degreesC; the aspect ratio of the mullite grains then increased with increased firing temperature. The mullite aciculars rearranged their orientation in a glassy phase until they impinged on each other. A highly textured mulfite specimen was prepared by firing the kaolin tape at a temperature >1500degreesC.