Mullite: Crystal Structure and Related Properties

• Published in: Journal of the American Ceramic Society Vol. 98; no. 10; pp. 2948 - 2967
• Main Authors: Schneider, Hartmut, Fischer, Reinhard X., Schreuer, Jürgen
• Format: Journal Article
• Language: English
• Published: Columbus Blackwell Publishing Ltd 01.10.2015; Wiley Subscription Services, Inc
• Subjects: Aluminosilicates; Aluminum oxide; Aluminum silicates; Atomic structure; Cations; Ceramics; Compressibility; Creep (materials); Crystal structure; Diffusion; Elasticity; Electrical resistivity; Heat conductivity; Mechanical properties; Mullite; Optical properties; Oxygen; Physical properties; Silicon dioxide; Sillimanite; Single crystals; Thermal conductivity; Thermal expansion; Thermodynamic properties
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.13817

Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al2(Al2+2xSi2‐2x)O10‐x. Main members are sillimanite (x = 0), stoichiometric 3/2‐mullite (x = 0.25), 2/1‐mullite (x = 0.40), and the SiO2‐free phase ι‐alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite‐type structural backbone of parallel chains consisting of edge‐sharing MO6 octahedra and their specific cross‐linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.