Mullite 3Al2O3.2SiO2

3. Composition Mullite is commonly denoted as 3Al2O3 .2SiO2 (i.e. 60 mol% Al2O3). However it is actually a solid solution with the equilibrium composition limits of 60 – 63 mol % Al2O3 below 1600?C.

4. Mullite Mullite is a good, low cost refractory material. The raw materials are easily obtainable and are reasonably priced. It has excellent high temperature properties with improved thermal shock and thermal stress owing to the low thermal expansion, good strength and interlocking grain structure.

5. Key properties Mullite has long been used as a refractory material. Its properties include:          Good high temperature strength          Good thermal shock resistance          Excellent thermal stability          Resistance to most chemical attack; it has excellent stability in acid metal slags and is insoluble in most acids          Resistance to oxidation and attack by furnace atmospheres          Resistance to abrasion          Good electrical resistivity The approximate limiting temperatures of use are 1800?C in air and 1600?C in vacuum.

6. Table-Typical physical and mechanical properties of mullite Density (g/cm3) 3.03 Young’s Modulus (GPa) 130 Fracture Toughness (MPa.m-1/2) 2-4 Modulus of Rupture (MPa) 160 Thermal Expansion Coefficient (x10-6 /°C) 4.5-5.6 Thermal Conductivity (W/m.K) 4.0-6.0 (100-1400°C) Maximum Operating Temperature (°C) 1725°C in air

9. Synthetic Mullite Various starting materials and preparation methods are used to make synthetic mullite ceramics. For example; a mixture of solids, a mixture of sols, or a mixture of sol and salt can each be used as the starting materials. Similarly, a variety of preparation methods exist, for example reaction sintering of mechanically mixed powders, hydrothermal treatment of mixtures of sols and chemical vapour deposition. The starting materials and preparation method influence the properties of the mullite. Reaction sintered mullite made from mechanically mixed powders is usually characterised by low strength (<200 MPa) and low fracture toughness (1 – 2 MPa m-1/2) due to amorphous grain boundary phases. In contrast, gelation routes produce intimately mixed sub-micrometer particles that can be sintered or hot pressed to produce single phase materials with superior mechanical properties. Mechanical properties can be improved further by producing composites. Additions of ZrO2 and SiC have produced fracture toughness at room temperature close to 7 MPam-1/2.

16. Mullite formation in a floor tile body

17. Mullite formation in a porcelain tableware body

35. Ceramics based on mullite-corundum Metal moulds for metal casting Fixing plates for melting crucibles Dies for pressing articles from titanium alloys Stoppers for metallurgical furnaces Bottom plates for the installations intended for isostatic pressing titanium alloys 

36. Zirconia Mullite Grain/ Powder  Designations: Chemical Name: Zirconia-mullite Chemical Formula: Al2O3 + ZrO2 + SiO2 General Description: A fused, high purity product produced in an electric arc furnace from Bayer alumina and zircon sand. It is composed of large needle-like mullite crystals containing co-precipitated monoclinic ZrO2. Typical Chemical Purities Available: Al2O3= 45.80%, ZrO2= 36.0% and SiO2= 17.50% Typical Applications: Used in specialty product applications like ceramic pressure casting tubes, refractory shapes that require resistance to molten slag and molten glass.