Tailorable Porous Ceramics via Freeze Casting

The goal of this work was to systematically explore and develop an understanding of the processing-pore network relationships in directionally freeze-cast aluminum oxide ceramics in order to tailor pore networks for desired applications. Freeze-cast alumina and yttria-stabilized zirconia (YSZ) samples were made from slurries varying in dispersion medium, solids loading, ceramic particle size, and additive content. Samples were cast under varying freezing conditions by altering the temperature of the slurry before casting and the temperature of the casting surface. Each parameter investigated has a different influence on characteristics of the resulting pore network, specifically porosity, pore size and shape, specific surface area, and pore tortuosity. Over 100 samples were cast under these varying conditions to determine how parameters influence the pore network. Scanning electron microscopy, X-ray computed tomography, Archimedes’ buoyancy method, and mercury intrusion porosimetry were employed to analyze the samples. Characteristics were measured using both physical techniques (e.g. mercury intrusion porosimetry) and image-based techniques (e.g. from X-ray computed tomography data). The data were modeled using a combination of mathematical modeling of solidification and classic solidification theory. By relating the two, a model was developed that described pore size from freezing conditions. Geometric considerations allowed extension of the model to geometric surface area and tortuosity. Additional samples were made to validate the model to create a method of determining the processing parameters required to produce a porous ceramic with a desired set of pore network characteristics. Results showed that pore size was inversely related to freezing front velocity, the magnitude of which could be estimated using the two-phase Stefan problem, slurry and freezing temperatures, and thermophysical properties of the materials frozen. The influence of other processing variables, such as solids loading and particle size, will be incorporated into the final model upon completion of the research project. The degree to be granted, Doctor of Philosophy in Materials Science and Engineering, will be received on 6/17/2016 (graduation). The thesis resulting from research performed over the course of the four year fellowship will be defended on 12/16/2015 and submitted for degree conferral by 12/31/2015. A copy will be provided upon completion. After degree conferral the grant fellow plans on pursuing a career in industry performing research and development in the field of ceramics, composites, and/or energy devices.