Secondary Mullite Formation in Kaolin–Al₂O₃ Ceramics

Secondary mullite has been formed in 60 wt% kaolin–40 wt% Al₂O₃ ceramics by solution-precipitation in the glassy phase in contact with Al₂O₃ particle, showing a plate-like morphology, at temperatures above 1673 K. The lattice constant modification of secondary mullite has been determined: the lattice parameters of orthorhombic structure of a, b, and c axes decrease from 8.653, 8.770, and 3.167 Å to 7.958, 8.064, and 2.926 Å, respectively. The Al₂O₃ content in the secondary mullite crystals increases from 59.69 to 70.41 wt% when the grain width increases from 15 to 40 nm. Both the formation of secondary mullite and its grain growth are intimately related to the presence of the glassy phase originated from impurity (K₂O) in the kaolin. Their locations as well as their chemical composition of mullite grains, Al₂O₃ grains, and glassy phase are provided by the meticulous experiments using high-resolution transmission electron microscopy, selected-area electron diffraction, and energy dispersive spectroscopy techniques.