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Mullite

Calcined Mullite

Formula: 3Al2O3.2SiO2
Alternate Names: Mulite

OxideAnalysisFormula
K2O0.20%0.004
Na2O0.20%0.006
TiO20.60%0.013
Al2O357.00%1.000
SiO241.00%1.221
Fe2O31.00%0.011
Oxide Weight178.95
Formula Weight178.95
If this formula is not unified correctly please contact us.
HMOH - Hardness (Moh) 6-7
GSPT - Frit Softening Point 1810C

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 is also has a low thermal conductivity and is very refractory thus the theoretical formula (of 71.8% alumina and 28.2% silica) bears little resemblance to the 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. However, it can be synthesized by calcining kyanite, bauxite or alumina/kaolin mixtures of proper Al2O3:SiO2 ratio.

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 porcelains by incorporating similar minerals into the recipe and firing to the necessary temperature and heating curve to decompose them into mullite. These include andalusite (cone 13), kyanite (cone 12), sillimanite (cone 20). The resulting bodies display low thermal expansion and a useful in spark plugs, laboratory ware, etc. and in thermal shock resistant refractories.

Firing of ordinary stoneware bodies provides the necessary temperature and adequate kaolin to produce mullite crystals from the decomposition of kaolinite (kaolinite looses some silica and the remaining higher alumina reorients itself to a higher melting compound). The resulting lattice of crystals 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.


Mechanisms

Out Bound Links

In Bound Links


By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="Mullite" descrip="Calcined Mullite" searchkey="Mulite" loi="0.00" casnumber="55964-99-3"> <oxides> <oxide symbol="K2O" name="Potassium Oxide" status="" percent="0.200" tolerance=""/> <oxide symbol="Na2O" name="Sodium Oxide, Soda" status="" percent="0.200" tolerance=""/> <oxide symbol="TiO2" name="Titanium Dioxide, Titania" status="" percent="0.600" tolerance=""/> <oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="57.000" tolerance=""/> <oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="41.000" tolerance=""/> <oxide symbol="Fe2O3" name="Iron Oxide, Ferric Oxide" status="" percent="1.000" tolerance=""/> </oxides> </material>


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