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

TL;DR

This study examines how alumina suspension composition and supercooling influence ice crystal growth and the resulting heterogeneity in porosity during ice-templating, using in-situ X-ray imaging.

Contribution

It reveals how suspension composition affects the formation and morphology of the transitional zone during freezing, highlighting the role of supercooling and counter-ions.

Findings

Higher supercooling increases the size of the transitional zone.

Na+ ions promote cellular ice crystal formation over lamellar crystals.

Adjusting suspension composition controls porosity heterogeneity.

Abstract

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.