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.
