Comparison of solidification behavior of synthetic mullite and mullite-forming raw materials by aero-acoustic levitation

Abstract

Fused mullite is used in refractories and as an abrasive due to its suitable properties. Mullite is formed over a wide range in the Al2O3–SiO2 phase system, preferably as 3:2 or 2:1 mullite. There are several options to tailor the features of fused mullite, and consequently its properties. In that sense, solidification at different Al2O3:SiO2 ratios, undercooling, and impurity content have a major influence on the microstructure.This report focuses on the difference between synthetic high-purity fused mullite and mullite-forming raw materials kyanite and andalusite, which are commonly used in refractories. The synthesis of high-purity samples is possible by applying containerless levitation. The used aero-acoustic levitator allows the melting and solidification of samples in a gas stream without any contamination from the crucibles. A high-speed camera enables the observation of molten samples and their solidification, while a pyrometer monitors the apparent temperature of the sample. This allows the continuous monitoring of the solidification process in terms of temperature, crystallization, and reaction during the whole process, including the release of latent heat. Further analysis is carried out by Scanning Electron Microscope (SEM) to examine the microstructure of the mullite phases and X-ray diffraction (XRD) to confirm the presence of mullite. Thermodynamic considerations are used to discuss these results.