Study on formation mechanism and morphology evolution of Iron-exsolution mullite

Abstract

In this study, an iron-exsolution mullite crystal is defined as a mullite crystal formed form an iron-rich mullite solid solution, wherein the iron ion is ex-dissolved. The samples are prepared using α-alumina, fused-silica, and Fe2O3 powders. The research results demonstrate that after sintering at 1700 °C, the Fe3+ ions of the iron-rich mullite solid solution in the closed pores of the sample are reduced to Fe2+ ions under the influence of the reducing atmosphere, resulting in Fe2+ exsolution and the formation of the iron-exsolution mullite. This “solution-exsolution” behavior changes the nucleation rate of the mullite and increases the internal lattice driving force in the iron-rich mullite solid solution with critical solution saturation, significantly promoting anisotropic grain growth along the (0 0 1) direction. As a result, an elongated column mullite crystal is formed, whose morphology is different from that of a spheroidal-shaped iron-rich mullite. The obtained results provide a theoretical foundation for designing the crystal structure of mullite according to the application condition, and indicate the solid-solubility limit of iron in mullite solid solution.