Ice-Templating of Alumina Suspensions: Effect of Supercooling and Crystal Growth During the Initial Freezing Regime

We investigate the ice-templating behavior of alumina suspensions by in situ X-ray radiography and tomography. We focus herein on the formation and structure of the transitional zone, occurring during the initial instants of freezing. For many applications, this zone is undesirable as the resulting porosity is heterogeneous in size, morphology, and orientation. We investigate the influence of the composition of alumina suspensions on the formation of the transitional zone. Alumina particles are dispersed by three different dispersants, in various quantities, or by hydrochloric acid. We show that the dimensions and the morphology of the transitional zone are determined by the growth of large dendritic ice-crystals growing in a supercooled state much faster than the cellular freezing front. When the freezing temperature decreases, the degree of supercooling increases. This results in an initial faster freezing front velocity and increase in the dimensions of the transitional zone. It is therefore possible to adjust the dimensions of the transitional zone by changing the composition of alumina suspensions. The counter-ion Na+ has the most dramatic effect on the freezing temperature of suspensions, yielding a predominance of cellular ice crystals instead of the usual lamellar crystals.