|Monthly Tech-Tip |
Alternate Names: Alumina Calcined, Calcnd Alum, Ground Alumina, Corundum
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. Amazingly, ceramic bodies containing 95% or more alumina are being employed to produce ceramic parts for a wide range industries (fired to 1400C or more). Fabrication methods and glazing vary according to application.
Alumina has a very high melting temperature (about 2000C) and alumina ceramics can maintain up to 90% of their strength above 1100C. They are thus employed in many refractory materials (i.e. Calcium Aluminate Cements have PCEs above cone 35) and used to make parts that must withstand high temperature.
Calcined (or alpha) alumina is made by roasting a source alumina powder at 1200-1300C to convert it to pure Al2O3 (when calcined near 2000C large hexagonal, elongated tablet shaped crystals form as "Tabular Alumina"). The alpha form is the densest and most stable crystalline form of alumina. It is insoluble in water but is soluble in hydrofluoric acid and potassium bisulfate. 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). In addition, for glazes that have alot of melt fluidity, an addition of pure calcined alumina powder can stabilize the melt while maintaining most of the visual effect.
Unlike hydrated alumina, the calcined material has no loss in weight on firing. Thus it produces no gases of decomposition.
Fired alumina ceramic parts can be harder than tungsten carbide or zircon, two to four times as strong as electrical porcelain, and very resistant to abrasion. Alumina is thus used in grinding media, cutting tools, high temperature bearings, and a wide variety of mechanical parts. Compared to zircon it has a high thermal conductivity and a higher thermal expansion.
Alumina (preferably in the calcined form) can be used in clay bodies as an aggregate and filler in place of quartz. This can increase the firing range, decrease quartz inversion firing problems, and increase hardness and whiteness in the fired body. However, alumina is much more expensive.
Pure Ravenscrag Slip is glaze-like by itself (thus tolerating the alumina addition while still melting as a glaze). It was applied on a buff stoneware which was then fired at cone 10R (by Kat Valenzuela). This same test was done using equal additions of calcined alumina. The results suggest that the hydrated version is decomposing to yield some of its Al2O3, as an oxide, to the glaze melt. By 15% it is matting and producing a silky surface. However crazing also starts at 10%. The more Al2O3 added the lower the glaze expansion should be, so why is this happening? It appears that the disassociation is not complete, raw material remains to impose its high expansion.
The Ravenscag:Alumina mix was applied to a buff stoneware fired at cone 10R (by Kat Valenzuela). Matting begins at only 5% producing a very dry surface by 15%. This "psuedo matte" surface is simply a product of the refractory nature of the alumina as a material, it does not disassociate in the melt to yield its Al2O3 as an oxide (as would a feldspar, frit or clay). The same test using alumina hydrate demonstrates that it disassociates somewhat better (although not completely).
It is made from 96.5% calcined alumina and 3.5% Veegum (to provide plasticity for forming). At cone 6, with no prior firing to a higher temperature, a 5mm thick slice can support a piece like this. The larger the span the higher it should be prefired to get needed hot strength. This is just typical sintered alumina, it does not have nearly the thermal shock resistance that fully crystallized tabular alumina has.
The home-made kiln shelf (left) was fired it at cone 10. It is half the weight (and thickness) of the cordierite one (but remember that it does not have the thermal shock resistance of cordierite). It is made from a body consisting of 96.25% calcined alumina and 3.75% Veegum. It rolls out nicely and dries perfectly flat over about three days. But the Veegum does not give up its water easily. I cut it 1/4" larger than the other and it has fired to the same size; this body has incredibly low shrinkage.
These are glazed test bars of two fritted white clay bodies fired at cone 03. The difference: The one on the right contains 13% 200 mesh quartz, the one on the left substitutes that for 13% 200 mesh calcined alumina. Quartz has the highest thermal expansion of any traditional ceramic material, alumina has the lowest. As a result the alumina body does not "squeeze" the glaze (put it under some compression). The result is crazing. There is one other big difference: The silica body has 3% porosity at cone 03, the alumina one has 10%!
I mixed a cone 6 porcelain body and a cone 6 clear glaze 50:50 and added 10% Mason 6666 black stain. The material was plastic enough to slurry, dewater and wedge like a clay, so I dried a slab and broke it up into small pieces. I then melted them at cone 6 in a zircopax crucible (I make these by mixing alumina or zircopax with veegum and throwing them on the wheel). Because this black material does not completely melt it is easy to break the crucible away from it. As you can see no zircon sticks to the black. I then break this up with a special flat metal crusher we made, size them on sieves and add them to glazes for artificial speckle. As it turned out, this mix produced specks that fused too much, so a lower percentage of glaze is needed. I can thus fine tune the recipe and particle size to theoretically duplicate the appearance of reduction speckle.
This is due to its inability to withstand thermal gradients across its width. Typical sintered alumina is refractory, but it is not thermal shock resistant like tabular alumina. The inner part of the shelf was being protected from the rising heat because of this heavy, slow-to-rise calcimine vessel on top of it. The moment of the crack was so dramatic that, in spite of the weight on top of it, the shelf blew apart leaving 4 pieces with an inch-gap separating them.
Calcining is simply firing a ceramic material to create a powder of new physical properties. Often it is done to kill the plasticity or burn away the hydrates, carbonates, sulfates of a clay or refractory material.
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.
In ceramics, potters make crucibles to melt frits, stains and other materials. Crucibles are made from refractory materials that are stable against the material being melted in them.
Coorstek Ad-94 94% Alumina body
An overview of the hazards of calcined and hydrated alumina materials in the ceramic glazes and clay bodies
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.
Low Expansion Material
Materials used to make bodies requiring low expansion (e.g. flameware, refractories). The individual particles of these materials have low expansion. Some of theme even expand at certain temperature ranges.
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
|Bulk Density g/cc (Packed)||1.0-1.3|
|Density, loose packed (lbs/cu fut)||0.7-1.0|
|Index of Refraction||1.765|
|Frit Softening Point||2040C|
|Density (Specific Gravity)||3.75-3.90|
|Surface Area (m2/gm)||0.5-25|
|Body Thermal Expansion||A very low expansion material.|