  We investigate the ice-templating behaviour of alumina suspensions by in-situ
X-rays radiography and tomography. We focus here on the formation and structure
of the transitional zone, which takes place during the initials instants of
freezing. For many applications, this part is undesirable since 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 chlorhydric acid. We show that the
height and the morphology of the transitional zone are determined by the growth
of large dendritic ice-crystals growing in a supercooled state, and growing
much faster than the cellular freezing front. When the freezing temperature
decreases, the degree of supercooling increases. This results in a faster
freezing front velocity and increases 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 influence on the freezing temperature of suspensions, yielding a
predominance of cellular ice crystals instead of the usual lamellar crystals.
