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Mullite
Alternate Names: Mulite
Description: Calcined Mullite
| Oxide | Analysis | Formula | Tolerance |
|---|---|---|---|
| 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 rare natural mineral, named after deposits on Isle of Mull, but nearly all mullite used industrially is synthetic. Commercial mullite sands, grogs, and powders are manufactured by calcining alumino-silicate materials at high temperature. Their grain shapes usually bear little resemblance to the fine needle-like mullite crystals that develop inside vitrifying kaolin-containing ceramic bodies.
Mullite is highly refractory, has low thermal conductivity, and exhibits relatively low thermal expansion compared with many other ceramic materials. The ideal mullite composition is about 71.8% Al2O3 and 28.2% SiO2, corresponding to 3Al2O3·2SiO2, but commercial mullites vary because they are produced from natural raw materials and often contain residual glass and minor impurities. When kaolin is fired to form mullite, free silica is also generated, so the fired product is never pure mullite alone.
Its thermal expansion is significantly lower than quartz and somewhat lower than fused alumina, helping explain its excellent thermal shock resistance. Mullite-bearing bodies therefore combine refractoriness with dimensional stability at high temperature.
Mullite can also be produced in special-purpose porcelains and refractories by firing minerals such as kyanite, andalusite, or sillimanite. These alumino-silicate minerals decompose at high temperature to yield mullite plus silica, producing bodies with low thermal expansion and high thermal shock resistance suitable for spark plugs, kiln furniture, laboratory ware, and refractories.
In ordinary stoneware and porcelain, mullite forms during high-temperature firing as kaolinite decomposes. The resulting interlocking crystal network strengthens the body by reinforcing the surrounding glassy phase, contributing much of the strength and durability associated with well-fired porcelains and stonewares.
The exact chemistry of industrial mullite depends on the parent raw material and manufacturing process. Minor impurities commonly include TiO2, Fe2O3, alkalis, and excess silica, much of which may reside in the accompanying glass phase rather than in the mullite crystal itself.
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
A refractory aluminum silicate mineral notable for low thermal expansion and one-way volume expansion as it converts to mullite at high temperatures. |
| Materials |
Pyrophyllite
A refractory aluminum silicate mineral often used in clay body recipes to lower thermal expansion, control fired maturity, mullite development catalyst, etc. |
| Materials |
Virginia Kyanite
|
| Materials |
Calcined Alumina
A high-purity Al2O3 used in technical ceramics, refractories, glazes, and bodies for its extreme hardness, high-temperature strength, and chemical stability. |
| Materials |
Mulcoa 70 Mullite
|
| Materials |
Silica Sand
|
| Materials |
Sillimanite
A low expansion aluminum silicate mineral that converts to mullite on repeated firings. |
| 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
Mullite (3Al₂O₃·2SiO₂) is a naturally occurring aluminum silicate crystalline mineral. It forms in h |
| Temperatures | Metakaolin converts to spinel phase (950-1050) |
| Temperatures | Spinel phase meta kaolin converts to mullite (1050-1200+) |
Data
| Hardness (Moh) | 6-7 |
|---|---|
| Frit Softening Point | 1810C |
Mechanisms
| Body Thermal Expansion | This material has a very low thermal expansion and high melting temperature. |
|---|
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