Detailed introduction to Mullite
Published Time:
2023-04-20
Mullite is a fire-resistant material mainly composed of crystalline phases. Mullite is divided into two categories: natural mullite and synthetic mullite.
1. What is Mullite?
Mullite is a refractory material mainly composed of crystalline mullite. Mullite is divided into two categories: natural mullite and synthetic mullite. Natural mullite is very rare, and synthetic mullite is generally used.
Mullite is composed of 71.8% Al2O3 and 28.2% SiO2. The mineral structure is orthorhombic, and the crystals are in the form of long columns, scales, and chains. The scaly mullite forms a strong framework in the product through cross-linking.
Mullite is divided into three types: α-Mullite, equivalent to pure 3Al2O3·2SiO2, commonly known as 3:2 type; β-Mullite, with excess Al2O3 produced by heat treatment and tempering, slightly expanded properties, commonly known as 2:1 type; γ-Mullite, with a small amount of TiO2 and Fe2O3 after heat treatment and tempering. Mullite has stable physical properties and is insoluble in HF. Its relative density is 3.03 g/cm3, hardness 6-7, melting point 1870 ℃, thermal conductivity (1000 ℃) is 13.8 W/(m·K), thermal expansion coefficient (20-1000 ℃) is 5.3 × 10-6 ℃, and elastic modulus is 1.47 × 1010 Pa.
Mullite has good high-temperature structural mechanics and thermodynamics properties. Therefore, synthetic mullite and its products have advantages such as high relative density and content, high high-temperature compressive strength, low high-temperature stress relaxation rate, low thermal deformation rate, strong chemical corrosion resistance, and good thermal shock resistance.
2. Appearance of Mullite
Raw materials for producing mullite can generally use kaolin, sillimanite minerals, magnesium hydroxide, or directly synthesize aluminum oxide and silicon dioxide. Clay materials and aluminum oxide or sillimanite minerals react with industrial aluminum oxide under high-temperature conditions to produce primary and secondary mullite. Primary mullite is produced in the range of 1000-1200 ℃. Further increasing the temperature will cause crystal growth. The formation of secondary mullite is usually completed at 1650 ℃. In order to obtain high-density mullite products, a two-step sintering method is commonly used.
Mullite has two crystal forms: scaly and prismatic. Scaly mullite strengthens the glass phase and raw material composition phase. At the same time, scaly mullite raw materials have higher refractoriness than prismatic mullite raw materials. Rapid heating of kaolin to above 1400 ℃ produces scaly mullite.
Otherwise, slow heating to a lower temperature produces prismatic mullite. There are also reports of tubular and spherical mullite. The former is speculated to be due to the mismatch of silicon-oxygen and aluminum-oxygen tetrahedra, resulting in support forming a tubular shape, while the latter is the so-called nitrogen mullite. The various advantages of mullite's thermal deformation give it good heat resistance, and high-end mullite raw materials are ideal raw materials for refractory materials.
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