Freeze casting allows relatively dense ceramics to be made without the need for high temperature sintering and the associated shrinkage and dimensional change.
Freeze casting involves the production of a dispersion of nano-sized solid particles usually in water. When the water is frozen, these solid sol particles are rejected by the growing ice front because of their zero solubility in ice, and they gradually increase in local concentration in the inter-dendritic regions between the ice crystals. Eventually, as the ice fraction increases further, the solid particles begin to interact with one another at a very fine scale, forced together by the growing and expanding ice and overcoming their previous mutual repulsion. At this point, the solid particles become irreversibly bonded to one another to form a solid.
If further other solid particles are present, they become "locked in" to this solid framework. When the ice is then removed by melting and heating, the solid remains intact with almost zero shrinkage. At low loadings of sol, the solid network is quite fragile. Mechanical integrity and strength can be increased by heat treatment that leads to sintering, although this results in some shrinkage as porosity is eliminated, and an inevitable loss of dimensional control.
The materials studied in Oxford all used a sillica sol, together with alumina, copper and hydroxide powders of micron sizes. Casting is performed under vibration when the freeze-cast slurry behaves thixotropically and viscosity is reduced sufficiently to allow the slurry to replicate fine surface features and complex shaped master patterns or moulds.
Where sacrificial moulds are used, the possibility arises for the manufacture of high dimensional accuracy ceramics with relatively complex internal or surface features. In order to expand the possible range of applications for these 3D ceramics, various approaches to manipulating the ceramic thermophysical properties are being investigated, for instance by the mixing of novel constituents into the ceramic matrix. In each case, the new approaches to composition and the effect of processing on the microstructure and properties of the freeze castingsare under investigation.
The freeze cast microstructure is always porous and current research is studying the effect of directional solidification conditions on the orientation of porosity in the freeze cast microstructure. By analogy to the key casting variables in the casting of metals, the effects of undercooling, cooling rate and local temperature gradient are being studied, together with the ability of the moving solid/liquid ice front to "push" nano-sized silica sol particles, as well as larger micron-scale alumina powder. In contrast to previous work elsewhere that has tended to focus on relatively low fractions of solid to produce relatively open but rather weak porous structures, this project builds on our experience of very high solid loadings to try to manufacture ceramic forms of good mechanical integrity containing highly directional pores of controlled size. The picture at the top of this page shows micron scale pores in alumina freeze castings obtained by manipulating freezing conditions from some of our preliminary results.