Kinetic study on the anisotropic grain growth of elongated iron-containing mullite

Elongated iron-containing mullite exhibiting anisotropic grain growth was prepared using fused silica, α-alumina, and Fe2O3 powders as raw materials at elevated temperatures. The effects of different Fe2O3 contents on the anisotropic growth of mullite columnar crystals were investigated, and phase compositional analysis and structural characterisation were carried out by X-ray diffraction and field emission scanning electron microscopy. The kinetic index and activation energy for the anisotropic grain growth of elongated mullite crystals were calculated, and the surface energy of each crystal plane of the elongated crystal and the solid solution behavior of the iron ions in the mullite were calculated using the density functional theory (DFT) approach. It was found that the addition of Fe2O3 promotes the anisotropic grain growth of mullite, and the aspect ratio of the elongated crystals increased upon increasing the Fe2O3 content. The solid solubility upper limit of the iron-containing mullite in this system is 9 wt% of iron addition. After sintering at 1973 K, for Fe2O3 contents of 3, 6, and 9 wt%, the growth kinetics indices obtained by calculations in the longitudinal direction were 2.22, 2.50, and 3.85, while the reaction activation energies were 457.3, 442.1, and 411.9 kJ mol−1. The calculation results on the surface energy of the elongated mullite crystal showed that the smallest surface energy is on the (001) crystal plane, which is the preferred growth direction. Upon comparison of the surface energies for different Fe2O3 contents on the (001) surface, a decrease was observed upon increasing the quantity of Fe2O3, and both the growth tendency of the mullite crystals and the aspect ratio of the elongated crystals also increased. The refractoriness under load of the prepared elongated iron-containing mullite was 1664 °C, demonstrating its excellent high-temperature mechanical properties.