Two Common Forms of Mullite in Mullite Materials

Home » Two Common Forms of Mullite in Mullite Materials

When producing mullite materials, they can generally be synthesized directly using kaolin, silimanite minerals, alumina hydrate or alumina oxide, and silicon dioxide. The reaction between clay substances and alumina oxide or silimanite minerals and industrial alumina under heating conditions results in the formation of primary and secondary mullite. Primary mullite forms within the temperature range of 1000 to 1200°C, and further increasing the temperature only enlarges the crystals. The formation of secondary mullite typically completes at 1650°C. The two-step sintering method is commonly used to produce dense mullite products.

Mullite exists in two crystalline forms: acicular (needle-like) and prismatic (columnar). Acicular mullite reinforces the glass phase, and materials with acicular mullite have higher refractoriness compared to those with prismatic mullite when the chemical composition is the same. Heating kaolin rapidly above 1400°C results in the formation of acicular mullite. Otherwise, slow heating to lower temperatures leads to the formation of prismatic mullite. There are also reports of tubular and spherical mullite. The former is speculated to be caused by tension resulting from the size mismatch between silicon-oxygen and aluminum-oxygen tetrahedra, while the latter is known as nitrogen-containing mullite. The anisotropic thermal expansion of mullite contributes to its excellent thermal stability. When used as components in advanced mullite materials for feeding machines, they can be directly replaced on running machines without preheating.

Mullite

What Roles and Effects do Mullites Have?

Regarding the aluminum-silicon ratio and impurity composition in the production of fused mullite, what roles and effects do they have?

According to the Al₂O₃-SiO₂ binary phase diagram, the composition of mullite is favorable at approximately 79% Al₂O₃ and 21% SiO₂. However, considering the presence of certain impurity components in the raw materials, they react with Al₂O₃ and SiO₂ at high temperatures, altering the Al₂O₃/SiO₂ ratio. Therefore, during actual batching, the Al₂O₃/SiO₂ ratio is slightly lower.

As for impurities like TiO₂, CaO, MgO, and R₂O, except for a small amount of titanium that can enter the mullite solid solution and exist in the glass phase, the rest cannot be reduced or removed. Thus, an increase in impurity content leads to an increase in the glassy and corundum content, resulting in coarse crystalline mullite structure, higher porosity, and reduced thermal shock stability and erosion resistance, making it prone to cracking.

Zirconia Mullite

To further improve the chemical erosion resistance and thermal shock resistance while reducing the expansion coefficient of mullite, zirconium dioxide (ZrO₂) can be introduced into the Al₂O₃-SiO₂ system to improve the mullite structure. Mullite containing zirconia is called zirconia mullite. Zirconia mullite is typically produced by the electric melting method. The introduction of ZrO₂ into mullite has two effects: 1. It forms a solid solution, activates the lattice, and creates vacancies, promoting sintering. By utilizing the toughening mechanism of ZrO₂ phase transformation, it enhances high-temperature mechanical properties. When the mass fraction of ZrO₂ is between 15% and 30%, stress-induced phase transformation toughening is the primary mechanism. When ZrO₂ exceeds 30%, microcracking toughening becomes dominant.

Zirconia mullite is mainly used in key areas of new steel casting ladle slide gates, continuous casting nozzles, tundish dams, and glass furnace linings.

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