Suspension- and solution-based freeze casting for porous ceramics

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 α-factor of the solvent; solvents with low α-factors yielded nonfaceted pore structures, while high α-factors produced more faceted structures. Intermediate α-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.