Freeze-cast alumina pore networks: Effects of freezing conditions and dispersion medium

Abstract

Alumina ceramics were freeze-cast from water-and camphene-based slurries under varying freezing conditions and examined using X-ray computed tomography (XCT). Pore network characteristics, i.e., porosity, pore size, geometric surface area, and tortuosity, were measured from XCT reconstructions and the data were used to develop a model to predict feature size from processing conditions. Classical solidification theory was used to examine relationships between pore size, temperature gradients, and freezing front velocity. Freezing front velocity was subsequently predicted from casting conditions via the two-phase Stefan problem. Resulting models for water-based samples agreed with solidification-based theories predicting lamellar spacing of binary eutectic alloys, and models for camphene-based samples concurred with those for dendritic growth. Relationships between freezing conditions and geometric surface area were also modeled by considering the inverse relationship between pore size and surface area. Tortuosity was determined to be dependent primarily on the type of dispersion medium.

Anisotropic properties of highly textured porous alumina formed from platelets

“…Anisotropic particles such as platelets or fibers are suitable starting materials for the formation of such materials. So far, porous ceramics with pore networks developed in one direction have been fabricated by water-freezing methods [19,20]. However, these conventional porous materials have shown no dependence of thermal conductivity on direction, since large pores hasten convective heat transfer at higher temperatures.…”

Freeze‐cast yttria‐stabilized zirconia pore networks: Effects of alcohol additives

“…GSA measurements of the control sample show a significant decrease along sample height, inversely related to the trends in pore size, as has been well established. 4,26 In contrast, alcohol additive samples show no significant changes in GSA through the thickness.…”

“…[1][2][3] Pores within the freeze-cast structures are negative copies of the solidified liquid within the slurry, meaning characteristics of the pore network such as porosity, pore size and shape, surface area, and tortuosity are highly dependent on the type of liquid, or dispersion medium, and freezing conditions used. 4 For example, changing the dispersion medium from water to camphene to tert-butyl alcohol changes the pore morphology from lamellar to dendritic to prismatic, respectively. 5 Pore size can be altered as well, achieving smaller pores as freezing velocity increases.…”

“…5 Pore size can be altered as well, achieving smaller pores as freezing velocity increases. 3,4 Freeze-cast pore structures can further be altered through the use of additives in the dispersion medium. Cryoprotector additives and antifreeze proteins decrease the freezing point of water and are used to alter ice growth kinetics and change the structure of the ice crystals themselves, resulting in different pore shapes when used in freeze casting.…”

The lure of ice-templating: Recent trends and opportunities for porous materials

“…Another possibly unique asset is the availability of many processing parameters to adjust the architecture and microstructure of ice-templated materials and thus decouple, to some extent, structure and properties. However, because many processing parameters are interdependent, such adjustments require careful, exhaustive studies and a good understanding and control of the process [36,37]. Ice-templating is also mostly material-agnostic.…”