Phase transformation and growth of mullite in kaolin ceramics

The phase transformation and growth of mullite (3Al2O3·2SiO2) in kaolin ceramics have been investigated using X-ray diffractometer, transmission electron microscope, select area electron diffractometer, energy dispersion spectrometer and differential thermal analysis. The mullite which was transformed from kaolin appears at 1050 °C by XRD and tallies with DTA. The initial mullite crystal showed a plate-like morphology. The Al2O3 content in mullite crystal increased from 49.57 to 71.37 wt.% but the lattice parameters of a, b and c axes decreased from 8.085, 8.106 and 3.215 Å to 7.882, 7.974 and 2.946 Å, respectively, with the grain width increasing from 20 to 70 nm when the kaolin was sintered at 1300 °C for 30 min. The nonisothermal activation energy of mullite crystallization in kaolin ceramics was 1182.3 kJ mol−1. The growth morphology parameters n and m were both about 2.0, indicating that the bulk nucleation was dominant in mullitization and the crystal growth was controlled by diffusion. Seemingly, this study has been attempted to provide an integrative presentation of the thermal–structural characterization together with detailed kinetic and mechanistic interpretations.