Open cellular alumina ceramics with additional strut porosity were prepared by a two-step sponge-replication based manufacturing process. In the first step open cellular ceramic foams were prepared following the Schwarzwalder replication process. Therefor, 20 ppi polymer foam templates were coated with an aqueous alumina slurry with a solid load of 20, 30 or 40 vol%, respectively. In a second step a pore network was generated inside the foam struts by a subsequent freeze-casting process, whereby the ice crystals generated during controlled freezing act as sacrificial pore formers. The influence of the parameters chosen for the freeze-casting step and the slurry solid load on the microstructure, freeze-casting pore size and compressive strength was studied.
Results show, that the hierarchical structure of the cell pores originating from the initial PU template and the strut pores generated by the freeze-casting step in the ceramic foams remain intact after sublimation drying, template removal and sintering. The foams were analyzed by µ-computed tomography concerning the microstructure and the pore size distribution of the strut pores. The size and morphology of the strut pores depend on the solid load of the slurry and on the freezing temperature: with decreasing freezing temperature and increasing solid loading the lamellar strut pores became smaller and more isolated. Finally, the processing parameters were correlated to the overall porosity and the compressive strength of the hierarchical-porous foams. Cellular structures with a strut porosity between 50 % and 60 % and a total porosity exceeding 90 % were prepared. Depending on the relative density and strut pore morphology, the foams have a compressive strength of 0.4 - 0.6 MPa.
Their specific surface area was increased by factor 2 to 3, or from 70 cm²g-1 to 200 cm²g-1, respectively. Due to this additional strut porosity these foams are interesting candidates for applications in catalysis, and in energy storage due to their ability to load the strut pores with active components.