Freeze gelcasting of naphthalene-in-aqueous alumina slurry emulsions for the preparation of macroporous alumina ceramics

An emulsion freeze gelcasting process has been studied for the preparation of macroporous alumina ceramics. The naphthalene-in-aqueous alumina slurry emulsions prepared at 90 °C from a 30 vol% alumina slurry undergo gelation on cooling in a mould due to the solidification of naphthalene and physical cross-linking of the carrageenan present in the aqueous slurry. The medium internal phase emulsions with naphthalene to alumina slurry volume ratios in the range of 1 to 1.86 show shear thinning flow behavior with low viscosity and yield stress, suitable for casting. The room temperature drying of the gelled bodies followed by naphthalene removal at 70 °C and sintering at 1550 °C produces macroporous alumina ceramics with porosity in the range of 65 to 78%. The macropore morphology depends on the emulsifying agent (sodium dodecyl sulphate) concentration and naphthalene to alumina slurry volume ratio. The average macropore size (35.1 to 4.9 μm) decreases with an increase in the emulsifying agent concentration and naphthalene to alumina slurry volume ratio. The macroporous ceramics obtained have high compressive (7 to 28.75 MPa) strength and Young’s modulus (553 to 1230 MPa).