Presently, there are several techniques that can be used to prepare ceramics with microstructural control, however, each technique possesses certain strengths and weaknesses. Such methods include; gel casting, chemical or physical deposition and tape/slip casting. A great deal of literature already exists for most of these techniques. Ice-templating is becoming a popular preparation route for macroporous materials and has been widely applied in various classes of materials, particularly ceramics. This method entails redistribution of suspended ceramic particles by ice crystals as they grow within a temperature gradient in an aqueous slurry, thereby effectively templating the ceramic. The ice is later removed by freeze-drying, and thus is a fugitive template. The ice crystal growth is an intricate affair that depends on many factors. For example, the components in the suspension to be freeze cast must be well dispersed to prevent premature settling. The dispersants and other additives employed provide control of the porous morphology. Systematic studies regarding the additives and the role they play in this process are still at early stages.
A team of researchers led by Professor Mary Anne White at Dalhousie University in Canada investigated the role of various additives and procedures in tailoring the morphology of ice-templated, freeze-cast alumina ceramics. Specifically, they set out to unveil the role of important additives: zirconium acetate, and zirconium acetate with polyvinyl alcohol. In addition, they investigated the role of epitaxial growth of the freeze-cast ice on an oriented ice surface. Their work is recently published in Journal of American Ceramic Society.
The research team prepared the ice-templated ceramics by dissolving the additives in water then dispersing the ceramic powder in the solution. Next, the resultant slurry was poured into a mold and placed on a cold plate operating at a -20±0.5 °C, until all the solvent was solidified. The growth rate along the ice crystal front was 16±3 μm/s. The frozen samples were then freeze dried to ensure complete sublimation of the ice, then sintered in a furnace. The morphologies of the samples were studied using scanning electron microscopy.
The authors observed that the presence zirconium acetate in the slurry beyond a threshold concentration had the potential to alter the structure of the morphology of the ice-templated, freeze-cast alumina ceramics from the usual lamellar structure to columnar pores, whereas polyvinyl alcohol had the ability to act as both a binder and as a modifier of the ice surface. Moreover, the latter was seen to compete with zirconium acetate to provide cooperative effects and a wide range of porosities.
Additionally, the resultant porous alumina ceramic structure can be influenced by epitaxial growth on an oriented layer of ice. Overall, their results show that freeze-cast preparation techniques using a diversity of additives can allowing control of morphology of the resulting porous alumina ceramics.
Mary Anne White, Jacy Conrad, Sarah N. Ellis, Ran Chen. Investigations of ice-structuring agents in ice-templated ceramics. Journal of American Ceramic Society. 2017; volume 100: pages 5066–5074.
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