Structure and properties of mullite - A review

Abstract

Due to its high temperature but low pressure formation conditions, mullite occurs very rarely in nature. It has been found at the contact of superheated magma intrusions with Al2O3-rich sediments, as on the Island of Mull (Scotland), where the name mullite comes from. Mullite has also been described in high temperature metamorphosed rocks of the sanidinite facies1 and in hornfelses (porcellanite), e.g. at the contact of bauxites with olivine dolerite intrusions. Special and rare occurrences of mullite are in alumino silicate lechatelerite glasses produced by lightening impact in sandstones2, and in small druses of volcanic rocks (e.g. in the Eifel mountain, Western Germany), where it probably grew under moderate hydrothermal conditions (Figs. 1 a and b). In spite of its rare occurrence in natural rocks mullite is perhaps one of the most important phases in both traditional and advanced ceramics. The importance of mullite and mullite ceramics is documented by the big number of publications, which appeared in recent years (Fig. 2) [Add reference]. The outstanding scientific and technical importance of mullite can be explained by • Its high thermal stability and the favorable properties like low thermal expansion and conductivity, high creep resistance and corrosion stability together with suitable strength and fracture toughness (Table 1). • The fact that the starting materials (e.g. α-alumina plus silica, alumino silicates of the composition Al2SiO5, i.e. sillimanite, andalusite and kyanite, refractory-grade bauxite, Al2O3-rich sheet silicates and clays) are available in big quantities on earth. Thereby kaolinite and other clay-based materials achieved high importance, since they allow multiple shaping procedures of components and structures in the green state. • Its ability to form mixed crystals in a wide Al2O3/SiO2 range and to incorporate a large variety of foreign cations into the structure. • The fact that the structural principles of mullite senso stricto can be extended to a large number of phases belonging to the family of mullite-type structures. Mullite and mullite ceramics display a large variety of appearances, reaching from Czochralski-grown single crystals to polycrystalline and polyphase ceramics, and from very large refractory products to very tiny engineering components of high purity and homogeneity (Fig. 3). Basically three types of polycrystalline mullite ceramics may be distinguished: Monolithic mullite ceramics, mullite coatings and mullite matrix composites.