Effect of freezing velocity and platelet size on structural parameters and morphology of freeze-cast porous alumina scaffolds

Abstract

In this article, we examine the effects of two different platelet sizes (4 and 8 μm respectively) on the architecture of freeze-cast sintered alumina scaffolds as a function of a wide range of freezing velocities, 5–57 μm s−1. The microstructural evolution along the freezing direction has been studied a-priori, explained on the basis of ice physics and the interaction of ceramic platelets with the advancing freezing front. An array of microstructures was produced to delve into the role of platelet sizes and freezing velocities on various structural parameters, viz. wavelength (λ), lamella thickness (δ), and bridge density (ρb). Regarding the pore morphology, transitions from lamellar to dendritic or isotropic structures were identified for the scaffolds containing smaller and bigger platelets, triggered by an increase in freezing velocity as well as platelet size. The different microstructures are quantified with a specific dimensionless parameter m. We identify the microstructure to be lamellar with low bridge density and m > 4. The wavelengths and bridge spacing were comparable for 2 < m < 4 and led to dendritic structure. For the morphologies characterized by m < 2, the spacing among the numerous interlamellar bridges was smaller than the structural wavelength and hence, the scaffolds revealed usually isotropic structure. Finally, the specific processing conditions that yield different morphologies and the parameter m are presented together in the form of a ‘morphology map’ to establish the different microstructural regimes.