Freeze Casting - Applications

Applications

To consider the applications of freeze-casting, we should consider the properties of the freeze-cast component. First, and critically, it is not fully dense. It contains only about 60-70% solid matter, the remainder being air in the form of porosity. This is turn leads to an interesting property of freeze-castings - they are often porous, not merely at the surface, but throughout their thickness. A fluid will penetrate through the pores in the casting and eventually soak through, like a sponge. This is because at porosity percentages above the 'pore percolation threshold', pores link up into continuous channels. The pore percolation threshold depends on the characteristics of the material, but it is normally very roughly around 20%. A 60% dense component has 40% porosity.

As we might expect, this amount of air in the component reduces its strength a lot. Pure, fully dense alumina, for example, is as strong as steel - far stronger if processed carefully - but freeze-cast alumina components are of similar strength to concrete. The freeze-cast component also tends to be brittle, fracturing easily.

It is unlikely then that freeze-cast components could be used structurally (without further processing - more later), but they have other properties that make them useful. They are rather light, with freeze-cast alumina components having a density somewhere in the region of 2.5 g/cm³, similar to aluminium. They are easy and cheap to make, from inexpensive and safe ingredients and using no dangerous equipment. They can take complex shapes, as they are cast, rather than machined. They can also be very large, probably larger than monolithic ceramic components made by any other process. Finally, and crucially, their porosity means that they can be infiltrated by materials with useful properties, or processed with other materials in. For example, the component could be dipped in molten copper, such that the copper is drawn up by capillary action into the porosity, increasing the conductivity of the component vastly. Alternatively, copper powder could be used as a filler powder in place of some alumina to the same end.

Freeze-cast components, in their basic form, are ideal for use as heat-resisting objects. In this way, they can be useful in metalwork, as molds or as substrates for metal spray-forming. However, with suitable post-processing, they could fulfil many other applications, such as silicon chip mounts, or even engine blocks.