Suspension- and solution-based freeze casting for porous ceramics
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Katherine T. Fabera) Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125 (Received 30 January 2017; accepted 27 March 2017)
Freeze casting of traditional ceramic suspensions and freeze casting of preceramic polymer solutions were directly compared as methods for processing porous ceramics. Alumina and polymethylsiloxane were freeze cast with four different organic solvents (cyclooctane, cyclohexane, dioxane, and dimethyl carbonate) to obtain ceramics with ;70% porosity. Median pore sizes were smaller for solution freeze casting than for suspension freeze casting under identical processing conditions. The pore structures, which range from foam-like to lamellar, were correlated to the Jackson a-factor of the solvent; solvents with low a-factors yielded nonfaceted pore structures, while high a-factors produced more faceted structures. Intermediate a-factors resulted in dendritic pore structures and were most sensitive to the processing method. Small suspended particles ahead of a solid–liquid interface are hypothesized to destabilize the dendrite tip in suspension freeze casting resulting in more foam-like structures. Differences in processing details were highlighted, particularly regarding the improved freezing front observation possible with solution-based freeze casting.
I. INTRODUCTION
Ceramics with engineered porosity are required in a wide range of industries including energy, pharmaceutical, and manufacturing. While pore-free ceramics are sought after for improved optical, mechanical, and electrical performance,1 their porous counterparts are desired for their transport capabilities while providing high specific surface area, low thermal conductivity, and low relative density.2 These characteristics are required for use in fuel cells,3 bone scaffolds,4 and various filters and thermal management systems.5 For these reasons, research in processing science and technology for porous ceramics has been steadily growing.6,7 A key requirement for porous ceramics processing is the ability to tailor the pore network in accordance with the requirements of each application, specifically the pore size and shape, tortuosity, and connectivity.8 To achieve control over these parameters, various processing techniques such as partial sintering, replication, direct foaming, and sacrificial templating have been exploited.9 In recent years, freeze casting (also called ice templating), a type of sacrificial templating, has attracted considerable interest.10–15 In freeze casting, pore formation is based on phase segregation of a two-component system achieved via
Contributing Editor: Nahum Travitzky a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.133
solidification. The process starts with a uniformly dispersed mixture consisting of a solid phase (e.g., alumina particles) and a liquid phase, typically water, which acts as the sacrificial template. As the temperature of the mixture is decreased, the liquid phase solidifies, rejecting the insol
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