Evolution of porosity by freeze casting and sintering of sol-gel derived ceramics

Freeze cast of aqueous ceramic powder slurries is described as a versatile process to fabricate complex-shaped ceramic parts. Since freezing of aqueous sols or powder suspensions include the nucleation and growth of ice crystals the evolving microstructure in particular the pore characteristics which are left behind after elimination of the solvent can be controlled by the freezing process. The freezing kinetics have then to be used to manifest the conditions for the formation of the intended porosity. The temperature profile in the freezing slurry was measured and calculated, in particular the movement of the freezing front through the slurry was determined. The results show that a homogeneous microstructure is reached in the surface region of the consolidated part. Individual ice crystals are detected within a distance of some hundred micrometers from the surface. The final pores are dendritic in shape with an elliptical cross section. The pores can grow up to several millimeters in length under the process conditions used in this study. The limits of freeze-sensitive slurry compositions should be investigated in further studies and the approach should be followed to increase the porosity by additional foaming steps.