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Dispersion and Setting of Powder Suspensions in Concentrated Aqueous Urea Solutions for the Preparation of Porous Alumina Ceramics with Aligned Pores



Highly concentrated alumina powder suspensions have been prepared in aqueous urea solutions of concentrations in the range 200–360 g/100 mL using an ammonium poly(acrylate) dispersant at 80°C. The dispersant concentration for the suspension viscosity minimum in the urea solutions is higher than that in water due to the higher processing temperature. The urea solutions having higher dielectric constant than that of water offer higher interparticle potential that resulted in better dispersion of the powder as evidenced from the lower viscosity and yield stress of the suspensions. The decrease in temperature increased the suspension viscosity and the suspension formed a strong gel when cooled to room temperature due to the crystallization of urea. The minimum urea solution concentration for a 55 vol% alumina suspension to form a dimensionally stable gel is 240 g/100 mL. The compressive strength and Young's modulus of the gels increased with the increase in urea solution concentration. The alumina ceramics prepared by the urea removal followed by sintering at 1500°C had porosity in the range 28–36 vol% with the rectangular rod-shaped aligned pores.