Mullite: Crystal Structure and Related Properties

Abstract

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 Al 2 (Al 2+2x Si 2-2x )O 10-x . Main members are sillimanite (x = 0), stoichiometric 3/2-mullite (x = 0.25), 2/1-mullite (x = 0.40), and the SiO 2 -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 MO 6 octahedra and their specific cross-linkage by TO 4 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.

Preparation and Characterization of Mullite-Alumina Structures Formed "In Situ" from Calcined Alumina and Different Grades of Synthetic Amorphous Silica

“…RFS is the ratio between the experimental results of FS and the theoretical value obtained for a 100% dense sample (for mullite [2][3][4]16,30,34,51 , FS Theoretical = 180 MPa). Such an approach is useful for investigating the mechanical behavior of porous (RD ≤ 50 %) and semi-porous structures (95% ≤ RD > 50 %).…”

“…Mullite (3Al 2 O 3 .2SiO 2 or Al 6 Si 2 O 13 ), one of the main raw materials for the ceramic industry [1][2][3][4] , is an input for the production of refractory materials 1,[5][6][7][8][9] , catalyst support 10,11 , electronic and optic devices [12][13][14] , thermal insulators 8,9,15 , hot gas filters (above 600°C) [16][17][18] and biological and biomedical scaffolds [19][20][21] . Its formation is a complex set of high-temperature (800-1400°C) solid state reactions [22][23][24] between silica (SiO 2 ) and alumina (Al 2 O 3 ) [25][26][27][28][29] based on the mutual diffusion of Al 3+ , Si 4+ and O -2 ions amongst particles [30][31][32][33] .…”

Mullite‐glass and mullite‐mullite interfaces: Analysis by molecular dynamics (MD) simulation and high‐resolution TEM

“…Thus, the mullite structure is similar to that of sillimanite, which consists of AlO 6 octahedra chains that are parallel to the c axis and linked by SiO 4 and AlO 4 tetrahedra. During this reaction, the substitution of Si by Al is charge compensated by the removal of O and consequent O vacancy formation, which results in the disordering of the tetrahedrally‐coordinated cations …”

Application of Oxide Coatings for Improved Steel Filtration with the Aid of a Metal Casting Simulator

“…Due to its high thermal stability and other positive properties like thermal expansion and conductivity, high creep resistance, and corrosion stability. [10] Nevertheless in some case mullite offers poor stability against CO-containing reducing atmospheres, carbon and iron based melts. Naghizadeh et al investigate the stability of mullite under reducing conditions.…”