What are the properties of mullite? How is it used in aluminum silicon silicon carbide refractory materials?
Mullite is a mineral formed from aluminosilicates at high temperatures. It is formed when aluminosilicates are artificially heated and is often used to produce high-temperature refractory materials.The chemical formula of it is 3Al2O3·2SiO2. Theoretically, the Al2O3 content in pure mullite is about 71.8%. Under normal pressure, It is the only compound that can exist stably in the Al2O3-SiO2 binary phase diagram. The crystal structure of mullite is an orthorhombic crystal system. The average structure is composed of octahedral shared edges connected into a chain parallel to the C axis. It is located at the four vertices and center of the projection plane of the unit unit cell. At Z= of each unit cell The octahedral chain at 1/2 is connected to the [SiO4] and [AlO4] tetrahedrons, and the tetrahedrons form a double chain parallel to the c-axis.
Mullite is a chain-like arrangement of needle-like or columnar crystals extending along the c-axis. These needle-like or columnar crystals intersect with each other to form an interlaced and solid network structure. This feature gives mullites products Relatively excellent performance. It has the following properties: its hardness is high, about 6.7 (Mohs hardness), its melting point can reach 1870°C, its thermal conductivity is 13.8KJ/m·h·K, and its linear expansion is small, for example, at 20~1000°C The linear expansion rate of mullites is 5.3×10-6/K, and the elastic modulus is low, between 160-200Gpa, which is about half that of Al2O3 and SiC.
Mullite has excellent physical and chemical properties, such as high refractoriness, good thermal shock stability and creep resistance, high load softening temperature, good volume stability and corrosion resistance. Another characteristic of it is that its high-temperature strength and fracture properties will increase with the increase of temperature and will not decrease. For example, its strength at a high temperature of 1300°C is 1.7 times that at room temperature. When the temperature rises above 1800°C, trace amounts of liquid phase begin to form inside the mullite. When the temperature reaches 1850°C, the mullite completely melts. Therefore, mullite is widely used in industries such as cement, glass, ceramics, metallurgy, chemistry, electric power, national defense and gas, becoming an advanced refractory material that meets the conditions of use.
In aluminum silicon silicon carbide refractory materials, columnar and needle-shaped mullite crystals are interspersed with each other to form a staggered network structure. This network structure can form a tight combination with the surrounding particles, thus making the refractory material relatively high in strength. The presence of mullite can improve the thermal shock stability and load softening temperature of the material, allowing the aluminum silicon carbide refractory material to be used in locations with higher temperatures and frequent changes.
