Mullit

Mullit, commonly spelled mullite in English, is a high-temperature aluminosilicate ceramic phase with the approximate formula Al6Si2O13 (3Al2O3·2SiO2). It occurs as a crystalline solid in the system formed by aluminum oxide (Al2O3) and silicon dioxide (SiO2). Mullite is valued for its very high melting point, low thermal expansion, excellent thermal shock resistance, and good chemical stability, making it a key component in high-temperature ceramics and refractories.

Natural mullite is rare as a mineral. Most mullite used commercially is synthetic, produced by high-temperature reactions between alumina and silica or as a secondary phase formed during firing of aluminosilicate ceramics. It may appear as elongated crystals or as a fine-grained solid solution in fired materials.

Applications include refractory bricks and castables for kilns and furnaces, kiln furniture, and other furnace linings that operate above 1000°C. Mullite is also used in porcelain and whiteware bodies to improve thermal stability and reduce shrinkage and thermal expansion. In addition, mullite fibers and composites are employed for insulation and reinforced ceramic components, while mullite-based ceramics are explored for applications in engines, heat exchangers, and other high-temperature environments.

Manufacturing methods typically involve sintering or reacting alumina with silica under controlled conditions to form the mullite phase, sometimes as part of a ceramic matrix or as a dispersed phase in composites.