Tailorable porous ceramics via directional freeze casting
Graduate Student: Sarah Miller

Porosity in ceramics has long been considered detrimental because pores are known to be strength-limiting in brittle materials. Recently, however, numerous uses for porous ceramics have been identified, including liquid, molten metal or gas particulate filters, ceramic-metal composite preforms, solar radiation converters, catalytic reaction supports, high temperature thermal insulation, and as electrodes in energy devices, among others. Ceramic porosity has, instead of being the primary flaw, become the primary enabler for such applications. To tailor materials for a given application, it necessary to customize the pore network– pore fraction, roughness, and tortuosity – to meet requirements for flow, filtering, or insulation.

One method of producing porous materials is freeze casting, a subset of processes using sacrificial templates. Pores are formed by employing a fugitive phase in a ceramic slurry, and removing the phase after drying, resulting in a ceramic with pore identical to the sacrificial phase In freeze casting, the sacrificial phase is part of the ceramic slurry. The slurry is frozen, and the particles are rejected from the solidified dispersion medium and compacted between adjacent crystals. The frozen dispersion medium is then sublimed from the structure, leaving behind a porous structure. The process is finished by sintering the ceramic for particle fusion and enhanced mechanical integrity.

This research project is focused on understanding the relationships between processing conditions, like temperatures used in the freezing stage, on pore network characteristics, specifically, pore size and shape, network tortuosity, and specific surface area. Using Al2O3 as a model system for freeze casting, samples are characterized using scanning electron microscopy, mercury intrusion porosimetry, BET nitrogen adsorption, and by X-ray computed tomography, an invaluable tool allowing for visualization of the complex porous structure. By understanding the relationships between processing and structure, freeze-cast ceramics can be tailored by application, resulting in optimized ceramic devices.



Three-dimensional reconstruction of a water-based freeze-cast sample.


This research was funded by a NASA Science and Technology Research Training Fellowship (NSTRF), Grant# NNX11AM91H.