Near-net shaping of aqueous alumina slurries using carrageenan

The production of alumina pieces by gelation of concentrated aqueous slurries using carrageenan is described. The potassium carrageenan forms a low viscosity solution when heated, that provides a strong gel on cooling. Aqueous suspensions of alumina are prepared at a solid loading of 50 vol.% at different polyelectrolyte concentrations. The optimum dispersing conditions are selected from zeta potential, particle size distribution and viscosity measurements. Afterwards, the effect of temperature on the rheological behaviour is studied. The rheological properties of carrageenan solutions are studied for different concentrations and the gelation hystheresis is determined. The evolution of viscosity of the alumina slurries containing 0.25 wt.% carrageenan is studied on cooling. Casting the hot blend in a refrigerated non-porous mould allows us to obtain pieces with high green strength in a few seconds. The green density of the casts is 58.2% of theoretical and after sintering at 1500°C/2 h a final density of 98% is reached.