This 3D-rendered tomogram shows a cross-section through the solidified sample, in which two phases have separated: the pure ice crystal phase in blue and the sugar phase in red. The lamellar structure formed by the snowflake-like ice crystals is clearly visible. Image: HZB/PSI.Freeze-casting processes can be used to produce highly porous and hierarchically structured materials with large surface areas. They are suitable for a wide variety of applications, including as electrodes for batteries, as catalyst materials or in biomedicine.
Now a team led by Ulrike Wegst at Northeastern University and Francisco Moreno at Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany have used a newly developed X-ray tomoscopy technique to observe in real time and at high resolution how the process of structure formation takes place during freeze casting. They used a sugar solution as their model system.
Freeze-casting involves several steps. First, substances are dissolved or suspended in a solvent and then frozen in a mold by applying cooling to the bottom (directional solidification). After freezing, the solid solvent phase is removed by sublimation. What remains are the previously dissolved solute molecules and suspended particles, which form the cell walls of the resulting complex, highly porous architecture.
Freeze-cast materials can be used for many applications. For example, due to their enormous internal surface areas, they can be used as battery electrodes or catalysts. Because of their aligned porosity, they can also be used as scaffolds for peripheral nerve repair. However, exactly how the ice templates the complex architecture during freezing, and how the desired honeycomb-like aligned porosity and the various surface features on the cell walls are formed, has remained little understood until now.
Moreno and his team at HZB developed a method for observing these highly dynamic processes in detail. "Using X-ray tomoscopy, we can image the formation of structures in situ with high spatial and temporal resolution and even observe transient phenomena and transitional structures," he explains.
Using an ultrafast turntable, intense X-rays, an extremely fast detector and software for rapid analysis of the X-ray data, the HZB team, together with colleagues at the Swiss Light Source of the Paul Scherrer Institute, studied freeze casting on a model system and demonstrated the high performance of the method.
"For this study, we developed a new measuring cell with sensors to precisely record the temperature gradient," says Paul Kamm from HZB, who is lead author of a paper on this work in Advanced Functional Materials. The team was able to generate a 3D tomogram with a spatial resolution of 6µm per second and document the entire freezing process over 270 seconds.
Wegst suggested using an aqueous sugar solution as a polymeric model system, since this system can be simulated computationally and aqueous solutions still dominate the freeze casting process. "We are now able to experimentally observe for the first time the dynamics of directional ice crystal grow from the liquid phase," she says. "In doing so, the images document how instabilities form during crystal growth, how these shape the sugar phase and how characteristic, organic-looking structures are formed on the cell walls that are reminiscent of jellyfish and tentacles."
This story is adapted from material from Helmholtz-Zentrum Berlin für Materialien und Energie, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.