Freeze casting is a sublime, cost-effective manufacturing technique to provide highly porous materials with custom-designed hierarchical architectures, well-defined pore orientation, and multifunctional surface structures. Freeze-cast materials are suitable for a lot of applications, from biomedicine to environmental engineering and energy technologies. An article in Nature Reviews Methods Primer now provides a guide to freeze-casting methods that features an summary on current and future applications and highlights characterization techniques with a deal with X-ray tomoscopy.
“We were delighted when the world-renowned journal Nature offered us the chance to organize a Nature Reviews Methods Primer with instructions and an summary of the method,” says materials scientist Prof. Ulrike Wegst (Northeastern University, Boston, MA, USA and TU Berlin). “Along with tomoscopy experts Dr. Francisco GarcÃa-Moreno und Dr. Paul Kamm (each HZB and TU Berlin), Dr Kaiyang Yin (now Humboldt Research Fellow on the University of Freiburg) and I had just performed first in situ experiments and discovered latest ice crystal growth and templating phenomena. It due to this fact appeared timely to mix in our Freeze Casting guide for Nature Reviews Methods Primers (impact factor 39.8), experimental methods of freeze casting with techniques for process and materials evaluation.”
Observing the method with X-Ray tomoscopy
Following an introduction to the varied batch and continuous freeze casting processes, and a temporary outline of lyophilization (freeze drying), the Primer provides an summary on the various characterization techniques for the evaluation of the complex, hierarchical material architectures and material properties. Highlighted are the unique capabilities and strengths of X-ray tomoscopy, which allows to analyse crystal growth and the dynamics of structure formation in all classes of materials (polymers, ceramics, metals, and their composites) during solidification in real time and 3D. “This is especially attractive once we want to quantify anisotropic crystal growth, equivalent to that in aqueous solutions and slurries, by which crystals extend in different crystal directions at different velocities,” says GarcÃa-Moreno.
From tissue scaffolds to porous electrodes
The freeze-casting process was developed greater than 40 years ago for the production of tissue scaffolds. It soon became apparent that freeze-cast materials, attributable to their highly porous structure, could integrate well with host tissues and support healing processes. Today, freeze-cast materials are widely used not only in biomedicine but in addition in engineering, from progressive catalysts to highly porous electrodes for batteries or fuel cells. A wide selection of solvents, solutes and particles could be used to create the specified structures, shapes and functionalities.
How does freeze casting work?
First, a substance is dissolved or suspended in a solvent, here water, and placed in a mold. Then a well-defined cooling rate is applied to the copper mold bottom to directionally solidify the sample. Upon solidification, a phase separation right into a pure solvent, here ice, and a solute and particles occurs, with the ice templating the solute/particle phase. Once the sample has been fully solidified, the solid solvent is removed by sublimation during lyophilization. Lyophilisation reveals the highly porous, ice-templated scaffold, a cellular solid, whose cell partitions are composed of the solute/particle that self-assembled during solidification. The dimensions and variety of pores, their geometry and orientation, the packaging of particles and the surface characteristics of the cell partitions and with it the mechanical, thermal, magnetic and other properties of the fabric could be tailored for a desired application.
Outlook: Latest insights into the method under microgravity
To realize further information on the basic science of freeze casting, experiments to be performed on the International Space Station are planned. It is because ISS microgravity, i.e. an enormously reduced gravitational force, minimizes effects of sedimentation and convection on structure formation. The experts expect this to guide to further advances within the understanding of freeze casting processes and the manufacture of custom-designed, defect-free materials.
