Engineered porous ceramics using a directional freeze-drying process

It is shown, that directional freeze-drying is capable of forming dense and porous ceramic structures in one processing step. This promising technique makes for high porosities (up to 90%), the development of aligned pores and graded porosity while sintering shrinkage is reduced. Research on yttrium stabilized zirconia (YSZ) and subsequently fine copper particle systems, indicates that this method appears applicable to various material systems without major processing modifications. Graded functional ceramic and cermet layers, which are particularly important in porous anode-electrolyte assemblies in solid oxide fuel cells (SOFCs), could also be processed in one step using the freeze-drying approach. Scanning electron microscopy was used to study pore formation mechanisms and pore geometry. It could be shown, that the mechanism, which drives pore formation in directional freeze-drying, may result from convective motion of the water-based ceramic slurry at the onset of freezing