Heterocoagulation moulding of alumina powder suspensions prepared using citrate dispersant

Coagulation characteristics of aqueous alumina suspensions prepared using di-ammonium hydrogen citrate (DHC) dispersant has been studied using MgO coagulating agent for direct coagulation casting (DCC). Mg–citrate complexes formed by the reaction between the DHC and MgO act as dispersant for alumina at pH near its iso-electric point. Setting of the alumina suspensions takes place, at MgO concentrations higher than the equivalent amount required to react with the DHC, by heterocoagulation of the alumina and MgO particles due to their opposite surface charges. The yield strength and Young's modulus of the wet-coagulated alumina bodies increased with aging time due to the hydration of the excess MgO. The minimum time required for mould removal decreases and yield strength and Young's modulus of the wet-coagulated bodies increases with MgO concentration. Alumina green bodies prepared at MgO concentrations in the range of 0.2–0.5 wt% sintered to ∼97% theoretical density at 1550 °C and the sintered ceramics showed more or less similar microstructures irrespective of MgO concentrations.