Freeze casting

Abstract

When solutions and slurries are directionally solidified, complex dynamics of solvent crystal growth and solvent templating determine the final hierarchical architecture of the freeze-cast material. With continuous X-ray tomoscopy, it is now possible to study in situ intricate and otherwise elusive ice crystal growth and solvent-templating phenomena. Quantifying these phenomena both time-resolved and in three dimensions provides novel insights into the formation of performance-defining features of freeze-cast cellular solids at several length scales: the material’s pore morphology (first hierarchical level), the molecular, fibrillar and particle self-assembly of components in the cell walls (second level) and the cell wall surface structures (third level). The freeze casting process is attractive because the features of the final hierarchical material architecture — which determine the material’s structural, mechanical and physical properties — can be custom designed for a given application. Overall porosity, pore size, geometry, orientation, particle packing in cell walls and cell wall surface features can be tailored for applications in, for example, biomedicine, environmental engineering, catalysis, power conversion, and energy generation and storage. Freeze casting involves directional solidification of solutions or slurries followed by removal of the solid solvent phase. This Primer introduces the freeze casting technique, including experimental and analysis methods, with a particular focus on using X-ray tomoscopy for real-time, 3D observations of freeze casting dynamics.