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.

Coagulation of aqueous alumina slurries prepared using various concentrations (0.43–1.04 wt% based on alumina) of ammonium poly(acrylate) dispersant by MgO has been studied for direct coagulation casting (DCC) of alumina. The slurries prepared at dispersant concentration below 0.84 wt% are not suitable for DCC at room temperature (∼30°C) as they undergo premature coagulation. Mixing the slurry with MgO at a low temperature of nearly 5°C slows down the reactions leading to coagulation and keeps the slurry viscosity low for a sufficient period of time. Coagulation of slurries prepared at a dispersant concentration of 0.92 wt% and above at room temperature requires MgO concentrations much higher than the equivalent amount required for reaction with the dispersant. This anomalous behavior at higher dispersant concentration is explained such that the Mg‐poly(acrylate) formed by the reaction between ammonium poly(acrylate) and MgO formed a sheath over the remaining MgO particles and prevented them from further dissolution at room temperature. Faster coagulation could be achieved by heating the slurries after casting in closed molds. The Mg‐poly(acrylate) acts as a binder and stabilizes the coagulated bodies as their strength and stability against oscillatory stresses increase with an increase in dispersant concentration.

The effect of lead zirconate titanate (PZT) powder loading in the slurry on wall thickness of PZT microtubes formed by dip coating on vermicelli of diameters about 500 and 1000 mm has been reported. The vermicelli of 1000 mm diameter withstand longer dipping time (8 min) compared to vermicelli of diameter 500 mm (2 min). The wall thickness of the PZT microtubes formed by dip coating increased with both slurry loading and dipping time. The PZT microtubes by dip coating on vermicelli of 1000 mm diameter have thicker walls than that obtained with vermicelli of diameter y500 mm due to higher water absorption capacity of the former. Microtubes of PZT with wall thickness in the ranges of about 125 to 280 mm and about 220 to 480 mm could be prepared by dip coating of slurries of PZT powder loading in the range of 40?2 to 46?5 vol.-% on vermicelli of diameters 500 and 1000 mm respectively.

PZT microtubes have been fabricated by dip coating of aqueous PZT powder slurry on vermicelli, followed by burnout of the vermicelli and sintering. The water-absorbing character of the vermicelli made a thin layer of PZT slurry to gel on its surface during dip coating. Thickness of the gel on the surface of the vermicelli depends on the dipping time. The green PZT layer thickness in the range of 140-190 lm could be achieved for a dipping time of 30-120 s. Microtubes having an inner diameter of nearly 500 lm with wall thickness in the range of 120-160 lm could be prepared by this process.M. Kuwabara-contributing editor

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