|Monthly Tech-Tip |
Alternate Names: Mulite
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.
Overview of Mullite at Azom.com
Mullite at Wikipedia
|Materials||Mulcoa 70 Mullite|
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.
Materials not classifiable as commonly known aluminum silicates. For example, kaolin is a common aluminum silicate.
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.
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.
|Frit Softening Point||1810C|
|Body Thermal Expansion||This material has a very low thermal expansion and high melting temperature.|