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Formula: Al2O3
Chemistry %
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| DENS - Density (Specific Gravity) | 3.75-3.90 |
| XREF - Index of Refraction | 1.765 |
| HMOH - Hardness (Moh) | 9.0 |
| DNLP - Density, loose packed (lbs/cu fut) | 0.7-1.0 |
| MLPT - Melting Point (MP) | 2040C |
| SAMG - Surface Area (m2/gm) | 0.5-25 |
| BDGC - Bulk Density g/cc (Packed) | 1.0-1.3 |
Calcined alumina is generally used in the manufacture of high-grade ceramic shapes, refractories and fused alumina abrasives. It can be compressed to produce a fired density of 3.8 or more.
Calcined (or alpha) alumina is made by calcining a source alumina powder at 1200-1300C to convert it to pure Al2O3. This is the densest and most stable crystalline form of alumina. It is insoluble in water but is soluble in hydrofluoric acid and potassium bisulfate. When nearly 100% of the material converts to the large hexagonal, elongated tablet shaped crystals associated with the alpha phase, the product is referred to as "Tabular Alumina". Unground calcined aluminas are typically 100-300 mesh, but much finer grades (often called "Ground Alumina") are produced by milling. Calcined aluminas are available in numerous grades based on the heat treatment applied, crystal size, soda content, and degree of thermal conversion to alpha phase. Soda content is a major factor in determining the final use (low soda materials are used for electronic applications, medium soda for electrical insulation and porcelains, high soda for glass, glaze, fiberglass and electrical porcelain).
Some exceptionally fine 'super ground' grades are available which can be made into casting slurries of very high specific gravity and which cast well with very low shrinkage (even though alumina powder is not a plastic material). Deflocculation can be achieved using a low pH (3.5-4.5) positive anion mechanism employing hydrochloric or nitric acid, a high pH (11-12) cation mechanism with alkali hydroxide salt additions, or with the addition of standard alkali polyelectrolyte dispersants. With the addition of organic binders, alumina bodies can be cast and pressed into a wide variety of shapes requiring heat and abrasion resistance. Alumina parts are then sintered to permit discrete crystals to react with each other to form larger ones. Coorstek AD-94 is an example of a very high alumina content body, they publish alot of physical data about the material.
Calcined alumina can be substituted for silica filler in porcelain bodies (325 mesh). It reduces shrinkage, increases thixotropy, provides strength in the kiln minimizing warping, benefits glaze fit, and adds fired strength. The book "Clay Bodies" by Robert Tichane has more information on this.
Although it might seem logical to calculate a chemically equivalent substitute of alumina and silica for part of the kaolin in a recipe (i.e. to reduce glaze shrinkage in high kaolin recipes) this will likely not work unless the alumina is ground to micron sizes (very expensive). This is because the high melting temperature of the raw alumina, it will simply act as a matting agent. Notwithstanding this, alumina is added to glazes in the tile industry to impart matteness and texture (depending on particle size).
Unlike hydrated alumina, the calcined material has no loss in weight on firing. Thus it produces no gases of decomposition.
Mechanisms
Body Thermal Expansion - Low
A very low expansion material.
Out Bound Links
Zirconium Silicate
Alum Hydrate, Alumina Trihydrate, Hydrated Alumina, Alpha Aluminum Trihydroxide, Gibbsite
Aluminum Oxide
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In Bound Links
Mulite
<?xml version="1.0" encoding="UTF-8"?> <material name="Calcined Alumina" descrip="Aluminum oxide" searchkey="Alumina Calcined, Calcnd Alum, Ground Alumina, Corundum" loi="0.00" casnumber="1302-74-5"> <oxides> <oxide symbol="Al2O3" name="Aluminum Oxide, Alumina" status="" percent="100.000" tolerance=""/> </oxides> </material>
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