Morphologically Textured Mullite in Sintered Tape-Cast Kaolin
A commercial Malaysian kaolin powder made into an aqueous slurry was cast into green tapes ∼200 m 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., 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 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.
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