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Mullite

Alternate Names: Mulite

Description: Calcined Mullite

Oxide Analysis Formula
K2O 0.20% -
Fe2O3 1.00% 0.01
Na2O 0.20% 0.01
TiO2 0.60% 0.01
Al2O3 57.00% 1.00
SiO2 41.00% 1.22
Oxide Weight 178.95
Formula Weight 178.95

Notes

Mullite is a mineral of long interlocking needle-like crystal structure that is very resistant to thermal shock failure (has a low thermal expansion). It also has a low thermal conductivity and is very refractory. The theoretical formula of 71.8% alumina and 28.2% silica bears little resemblance to a real-world material (we have provided a typical non-theoretical analysis).

Mullite is found rarely in nature, it is named after a deposit on the Isle of Mull in Scotland. On a scale of lowest to highest thermal expansions at 2000F (where fused silica is almost zero and quartz is 1.5%), mullite is about one third of the way. It has a lower expansion than fused alumina (0.9%) and stabilized zircon (0.8%).

Mullite crystals can also be formed within special-purpose low-feldspar porcelains by incorporating similar minerals into the recipe and firing to the necessary degree to decompose them into mullite. These include andalusite (cone 13), kyanite (cone 12), and sillimanite (cone 20). The resulting bodies display low thermal expansion and are useful in spark plugs, laboratory ware, etc. and in thermal shock-resistant refractories.

The firing of ordinary kaolin-containing stoneware bodies can provide the necessary temperature to produce mullite crystals from the decomposition of kaolinite. The resulting lattice is potentially much stronger than the simple glass-weld bonds of low-fire ceramics.

The chemistry of mullite depends on the parent material. Impurities tend to be TiO2, Fe2O3, Na2O, K2O.

Related Information

Links

URLs http://www.azom.com/details.asp?ArticleID=925
Overview of Mullite at Azom.com
URLs https://en.wikipedia.org/wiki/Mullite
Mullite at Wikipedia
Materials M70 Mullite
Materials Kyanite
Kyanite is a granular material used in the manufacture of ceramics and abrasives. It is notable for low thermal expansion and one-way expansion on heating.
Materials Pyrophyllite
Materials Virginia Kyanite
Materials Calcined Alumina
Materials Mulcoa 70 Mullite
Materials Silica Sand
Materials Sillimanite
Typecodes Generic Material
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
Typecodes Aluminum Silicate
Materials not classifiable as commonly known aluminum silicates. For example, kaolin is a common aluminum silicate.
Typecodes Refractory
Materials that melt at high temperatures. These are normally used for kiln bricks, furniture, etc. or for ceramics that must withstand high temperatures during service.
Glossary Mineralogy
Raw ceramic materials are minerals or mixtures of minerals. By taking the characteristics of these into account technicians can rationalize the application of glaze chemistry.
Minerals Mullite
A fibrous alumina silicate mineral of very low thermal expansion. Mullite is formed within porcelain

Data

Hardness (Moh) 6-7
Frit Softening Point 1810C

Mechanisms

Body Thermal ExpansionThis material has a very low thermal expansion and high melting temperature.
By Tony Hansen
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