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Chemical Formula

3Al2O3 .2SiO2

Topics Covered



Synthetic Mullite

Mullite in Porcelains

Key Properties



Other Engineering Applications


Mullite is the mineralogical name given to the only chemically stable intermediate phase in the SiO2 - Al2O3 system. The natural mineral is rare, occurring on the Isle of Mull off the west coast of Scotland.


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 – 63mol % Al2O3 below 1600o C.

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 Zr2O and SiC have produced fracture toughness at room temperature close to 7 MPam-1/2.

Mullite in Porcelains

Mullite is also one of the important constituents of porcelain.  Clays with < 60% Al2O3 convert to mullite.  The amount of mullite produced is directly related to the amount of Al2O3 and the calcining temperature.

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 1800oC in air and 1600oC in vacuum.

Typical properties of mullite are given in table 1.

Table 1. Typical physical and mechanical properties of mullite.



Density (g/cm3)


Young’s Modulus (GPa)


Fracture Toughness (MPa.m-1/2)


Modulus of Rupture (MPa)


Thermal Expansion Co-Efficient (x10-6 /°C)


Thermal Conductivity (W/m.K)

4.0-6.0 (100-1400°C)

Maximum Operating Temperature (°C)

1725°C in air



By far the largest use of mullite based products is in refractories.  The glass and steel industries are two main markets.

The steel industry is the largest user, where refractoriness, high creep resistance and thermal shock resistance are important.  The main use of high-mullite based products is in hot blast stove checker bricks.  Many refractories in use in the steel industry have varying amounts of mullite based aggregates in them.  Steel ladles, lances, reheat furnaces and slide gates are examples of mullite aggregate based products with various alumina contents (figure 1).  The use of monolithic and precast shapes is increasing with the use of bricks declining.

Figure 1. A selection of mullite-based refractory shapes for the steel industry (photo courtesy of Dyson Precision Ceramics, UK)

The glass industry uses mullite based refractories in burner blocks, ports and in checker bricks as well as in the upper structure of the tanks where the glass is melted and in the drawing chambers.  Thermal shock resistance, chemical attack resistance, high hot strength and creep resistance are the primary properties valued by the industry.

Mullite based products are also resistant to particulate carryover into the glass melt.  This is particularly important in flat glass production, where contamination by low levels of Al2O3 is undesirable.

The next largest user of mullite is the ceramic industry mostly in kiln furniture items such as kiln setter slabs and posts for supporting ceramic ware during firing.

The aluminium and petrochemical industries also use mullite-based aggregates for applications requiring chemical attack resistance, thermal shock resistance and hot-load strength.

Other Engineering Applications

New mullite materials that have more controlled mechanical and physical properties and are providing opportunities for a wider use of the material. 

The good mechanical properties at high temperatures of high purity mullites have made them potential high temperature engineering ceramics, for example in turbine engine components.

Mullite is also a leading candidate material for high-strength infrared transmitting windows.

Other applications include electronic substrates and protective coatings.


Primary author: Ceram Research


For more information on this source please visit Ceram Research Ltd.


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