Materials

3.4.1.4 Mullite Ceramics

By varying the chemical and mineralogical composition of the Al2O3–SiO2 system, specific modifications to the properties of the mullite ceramic can be achieved. Pure mullite (3Al2O3 . 2SiO2) consists of 82.7 % Al2O3 by mass and 17.3 % SiO2. Dense sintering of pure mullite cannot be achieved with conventional sintering techniques.
Mullite ceramics have a microstructure whose mineral phases consist of mullite, corundum (Al2O3) and glass (SiO2).

Sintered mullite usually contains up to 10 % glass phase material. The overall porosity, however, is still around 10 % by volume. M72 and M85 are typical of these materials. They are compared in the following table.

Table 2: Chemical composition and mineral phases

Porous mullite ceramics containing little glass phase material have a relatively high strength, comparatively low thermal expansion, and are therefore highly resistant to thermal shock. Resistance to creep at high temperatures is superior to that of pure aluminium oxide ceramic.
Applications include kiln furniture for temperatures up to 1,700°C – even in oxidising atmospheres – and carrier rollers in high-temperature furnaces. Because of its low thermal conductivity and high resistance to corrosion, porous mullite is also used as an industrial refractory material.

The porosity can be significantly reduced by increasing the proportion of glass phase material (> 10 %). Densely sintered mullite ceramic combines high-strength, good thermal shock resistance and a useful resistance to creep. One important application, for instance, is protective tubes for thermocouples.