Freeze casting processes can be used to produce highly porous, hierarchically structured materials that have a large surface area. They are suitable for a wide variety of applications, such as battery electrodes, catalyst materials or in biomedicine. Now a team led by Professor Ulrike GK Wegst, Northeastern University, Boston, MA, USA and Dr. Francisco García Moreno of the Helmholtz-Zentrum Berlin now they have used the new development X-ray tomoscopy technique at the Swiss Light Source of the Paul Scherrer Institute to observe in real time and in high resolution how the structure formation process occurs during freezing. A sugar solution served as a model system.
Freeze-drying requires several steps: first, substances are dissolved or suspended in a solvent and then frozen in a mold with a cooling rate applied 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 and highly porous architecture. Freeze-dried materials can be used for many applications: for example, due to their huge internal surface area as battery electrodes or catalysts or due to their aligned porosity in biomedical applications, for example as scaffolds for peripheral nerve repair. However, until now it has not been understood exactly how ice shapes the complex architecture during freezing, and how the desired honeycomb-aligned porosity and cell walls with their various surface features are formed.
Moreno and his team at the Helmholtz-Zentrum Berlin have developed a method to observe these highly dynamic processes in detail. “Using X-ray tomoscopy we can visualize the formation of structures in the place with high spatial and temporal resolution and even observe transient phenomena and transition structures,” explains the physicist. Using an ultra-fast rotating plate, intense X-rays, an extremely fast detector and software for rapid analysis of the X-ray data, the team from HZB, together with colleagues from the Swiss light source of the Paul Scherrer Institute, studied freeze casting in a model system and demonstrated the high performance of the method: “For this study, we developed a new measurement cell with sensors to record accurately the temperature gradient,” says Dr. Paul Kamm (HZB), lead author of the study. A 3D tomography was generated with a spatial resolution of 6 µm per second. The entire freezing process was documented for 270 seconds.
Wegst of Northeastern University, USA, had suggested an aqueous sugar solution as a polymer model system, since this system can be simulated computationally and because aqueous solutions still dominate the freeze-casting process. “We can now observe experimentally for the first time the dynamics of the directional growth of ice crystals from the liquid phase,” says Wegst. “The images document how instabilities form during crystal growth, how these shape the sugar phase, and how characteristic, organic-looking structures form in the cell walls, reminiscent of jellyfish and tentacles.” It is also interesting to note that some of these structures may disappear again.
– This press release was provided by Berlin Helmholtz Center