Influence of melting temperature on morphology of primary mullite in SiO₂-Al₂O₃ system

A series of melting and solidification experiments were carried out using high-purity SiO₂ and Al₂O₃ powders as the starting materials. The effect of melting temperature on the morphology of the primary mullite was investigated. The following conclusions were made: 1) The morphology of the primary mullite went through five stages as the melting temperature increased; massive, fibrous, faceted dendritic, feathery and fine feathery structures. These phenomena can be explained based on the number of effective nuclei in the melt. 2) The fibrous mullite grew from mullite precursors formed in the melt. Under highly supercooled conditions, the fibrous morphology becomes unstable and begins to branch and form a faceted dendritic crystal. 3) When the number of nuclei is extremely small, the mullite nucleated on the rhombic dodecahedron Mo originating from the molybdenum crucible and crystallizes out into a feathered structure.