Morphologically Textured Mullite in Sintered Tape‐Cast Kaolin

Abstract

A commercial Malaysian kaolin powder made into an aqueous slurry was cast into green tapes ∼200 μm in thickness using the doctor‐blade technique. A kaolinite (001) K(aolinite) texture was found on the green tapes using X‐ray diffractometry (XRD). Subsequent sintering at 1600°C/1 h induces the preferred crystallographic orientations of (100) M(ullite) , (110) M , (120) M , and (210) M in the mullite crystals thus formed by vitrification, as determined using electron backscatter diffraction. A nonunique orientation was observed on these crystals, suggesting that the texture is morphological with the c ‐axis, i.e., [001] M lying parallel to the sample surface, rather than crystallographical. An additional preferred orientation on (100) M , a forbidden reflection of the systematic absence from XRD is detected using pole figure. Kaolin is vitrified at 1600°C into mullite when the texture is developed, and sintering is initially facilitated by an SiO 2 –Al 2 O 3 liquid formed at >1260°C, the eutectic temperature of metastable equilibrium. Further densification is promoted by additional particle rearrangement in an SiO 2 –mullite liquid formed at >1587°C, the stable equilibrium eutectic temperature. Alumina used as a substrate for the kaolin sintering does not play a significant role in developing the mullite morphological texture.