| MadSci Network: Engineering |
Dear Duncan,
There's no magic here. Just consider alumina as you would any other ceramic. The key is the process of sintering. That is the particle:particle bonding which occurs at high temperatures due to increased ionic mobility within the crystal structure. Thermodynamics controls this loss of undesirable, relatively large surface area of individual particles ( example).
It is not necessary to melt a material to convert a "green" piece into a hard, brittle ceramic. In fact, the resulting properties are somewhat different, and melting is restricted to forming glassy materials (e.g. common glass, glazes, enamel, etc). Neverthless, it is possible to take advantage of the formation of glasses (by melting of contaminants or dopants) to "stick" the alumina grains together: (example)
Reaction sintering is an interesting twist... take alumina (aluminum oxide) and zircon (zirconium silicate), mix them together and fire them at high tempaerture. There is an exchange reaction which results in zirconia being distributed throughout aluminum silicate. This "zirconia toughened mullite" is a durable "high-tech" ceramic (another example).
So, any normal ceramic forming method (moulding, pressing, pressure- casting, extrusion) could be used in theory. Binders are normally added to the alumina (plus some mobile phase such as water) to make a plastic (thats plastic, not polymeric) mass to hold everything together during the drying step. These binders are burnt out during the high temperature phase. Slip-casting is can also be used, as can gel-casting.
The correct shape can be therefore be pre-formed, but not necessarily the correct size!. It is usual that the green ceramic will shrink when sintered. Therefore the change in size has to be calculated and accommodated for in the manufacturing process. Small pieces, such as thermal analyzer pans, might be machined down to their final size after firing.
Alumina crucibles are interesting as you say. Because alumina has a very high melting point, and is largely inert, it is used to contain materials in solid-state reaction studies. That is where two solids are intimately mixed and heated so as to cause a reaction to produce a third substance, without going through any liquid or solution state.
I hope that has clarified some of your thoughts on alumina crucibles.
Happy sintering!
Additional Reading
Alumina as a Ceramic Material, Walter H. Gitzen, editor
Try the links in the MadSci Library for more information on Engineering.