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Coagulation of concentrated aqueous alumina slurries prepared using an ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC). A small amount of MgO (0.2 wt% of alumina) increased the viscosity of the concentrated alumina slurry with time and finally transformed it into a stiff gel. The mechanism of coagulation is proposed such that the time‐delayed in situ generation of Mg2+ ions from the sparingly soluble MgO forms Mg–poly(acrylate) with the unadsorbed ammonium poly(acrylate) molecules in solution that shift the poly(acrylate) adsorption equilibrium toward the left by depleting the poly(acrylate) molecules adsorbed on the alumina particle surface. This leads to insufficient dispersant coverage on the particle surface and coagulation of the slurry. DCC using MgO is possible only if the slurry is prepared at a dispersant concentration higher than that required for optimum dispersion as the slurries prepared at the optimum dispersant concentration underwent premature coagulation. The gelation time could be tailored within 20 min to a few hours by maintaining the temperature in the range of 70°–30°C. The wet coagulated bodies prepared from 50 vol% alumina slurry showed a compressive strength of nearly 0.05 MPa.

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High strength and Young's modulus of wet‐coagulated bodies is key to the success of a direct coagulation casting (DCC) process. The yield strength and Young's modulus of wet‐coagulated alumina bodies prepared at various concentrations of ammonium poly(acrylate) dispersant and MgO coagulating agent has been evaluated. The yield strength and Young's modulus of the wet‐coagulated bodies, prepared at a MgO concentration equivalent to react with the dispersant, increased with an increase in the dispersant concentration due to the binding of alumina particles by the Mg‐poly(acrylate) formed by the reaction between the dispersant and MgO. Addition of MgO higher than the equivalent amount to react with the dispersant increased the yield strength, Young's modulus, and brittleness of the wet‐coagulated bodies. This is due to a combination of an increase in the cohesive force between particles and a decrease in homogeneity of the particle network in the wet‐coagulated body induced by heterocoagulation of alumina and MgO particles having opposite surface charges. The high yield strength (up to 472 kPa) and Young's modulus (up to 110 MPa) achieved would facilitate easy and successful removal of the wet‐coagulated bodies even from intricate shape molds.

A porous NiO-YSZ substrate for anode-supported solid oxide fuel cells has been prepared by gelcasting of NiO-YSZ powders using urea-formaldehyde monomers, followed by humidity-controlled drying, binder removal, and sintering of the gelled bodies. The gelled bodies had sufficient strength to remove even 2-mmthick samples from the mold immediately after gelation. A gelcast NiO-YSZ sample sintered at 14501C for 2 h showed an open porosity of B53 vol%, and the porosity increased to B58% upon reduction with hydrogen. Pore sizes measured on the scanning electron microscopy photomicrograph of NiO-YSZ and Ni-YSZ cermet substrates are in the range of 2-5 lm. Urea-formaldehyde polymer, present in a high amount (B13 wt%) in the gelcast body, acts as a template for pores. P. Holtappels-contributing editor