Ice Crystal Growth and the Dynamics of Ice Templating by X-Ray Tomoscopy

The physics of ice-crystal growth from the liquid phase and the complex dynamics of microstructure formation by ice-templating during the directional solidification of aqueous solutions and slurries, also termed freeze casting, has to date received comparatively little research attention. Recent advances in synchrotron-based X-ray tomoscopy, which is time-resolved X-ray tomography, enable us now to investigate in situ under well-defined experimental conditions the formation of the hierarchical architecture of freeze-cast materials. We can quantify in 3D ice crystal growth at high spatial and temporal resolution and observe the ice-templating mechanisms, which determine the morphology of the freeze-cast material. The performance-defining morphology is defined by a honeycomb-like material structure with lamellar cell walls that frequently are decorated with a range of unilateral surface features. Analyzing the tomoscopy data, we can determine, which features of the freeze-cast materials are templated by the slow faceted growth along the c-axis of the hexagonal ice-crystal, and which are templated by the slightly anisotropic fast crystal growth along the preferred a- and t-axes, both of which are characterized by atomically rough ice-water interfaces. Additionally, we can observe, which role secondary instabilities play in the formation of cell wall surface features. Important advantages, which in situ X-ray tomoscopy offers over post-mortem imaging and state-of-the-art simulation techniques, are that the dynamics of the structural evolution can be monitored over several minutes in a sample volume of several cubic millimeters so that also transient phenomena are captured.