Fully interconnected porous Al2O3 scaffolds prepared by a fast cooling freeze casting method

A fast cooling freeze casting (FCFC) method was developed for preparation of porous Al2O3 scaffolds with improved interconnectivity, unidirectional pore channels and dendritic walls. As estimated using the Archimedes method and mercury porosimetry, total porosity 64.5–68.5 vol% and the closed porosity <0.6 vol% reveal that most pores are open. The bimodal pore size distribution shifted to larger diameters with increasing sodium-hexametaphosphate (SHMP) concentration from 0.1% to 0.5%. Scanning electronic microscope (SEM) images of the resultant porous Al2O3 showed clear transformation from lamellae to dendrite structures with increasing polyvinyl alcohol (PVA) concentration from 1% to 3%. To discover the mechanism, the freezing curve of the Al2O3 slip was achieved, showing its supercooling point at about −10 °C where most isolated water solidifies at an average ice front velocity of 0.42 μm/s. Together with the PVA effects on the microstructure transformation, the underlying mechanism could be ascribed to both the supercooling and PVA effects. The lamellae ice front was initially formed under this velocity, but with increasing PVA, the ceramic slurry got more viscous and the rearrangement of ceramic particles was hindered, hence the ice front growth in the preferred a direction was limited whereas the growth in the c direction was promoted. Meanwhile under the effects of the applied thermal gradient, the lamellae were transformed into dendrite structures.