Morphologically Textured Mullite in Sintered Tape-Cast Kaolin.

A commercial Malaysian kaolin powder made into an aqueous slurry was cast into green tapes ∼200 lm 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 16001C/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 16001C into mullite when the texture is developed, and sintering is initially facilitated by an SiO2-Al2O3 liquid formed at > 1260°C, the eutectic temperature of metastable equilibrium. Further densification is promoted by additional particle rearrangement in an SiO2-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. [ABSTRACT FROM AUTHOR]