Influence of temperature on the colloidal processing of electrostatically stabilised alumina suspensions

Electrostatically stabilised aqueous alumina suspensions were prepared at various solid loadings. The effects of temperature and solid volume fraction on the rheology and casting rate of the suspensions, and on the drying-shrinkage behaviour, green density and pore size distributions of the consolidated bodies were evaluated. Stress-sweep experiments showed an increase in viscosity, and in the shear- and time-dependence character of the alumina suspensions with increasing temperature, especially at high solid loading. These effects were attributed to a destabilising effect of temperature, derived from a decrease of the surface charge density of the particles and of the dielectric constant of the liquid medium. The flocculating effect of temperature was further confirmed by steady-shear experiments, with an increase of equilibrium viscosity with increasing temperature in the low shear rate range. Concerning the microstructure of slip cast bodies, increasing temperature lead to a decrease in green density (increase in porosity), particularly noticed at high solid loadings. The casting rate was enhanced with increasing temperature as a consequence of the concomitant decrease in water viscosity and the improved permeability of the cake. Overall, good correlation was found among the green microstructure, the rheology of suspensions and the particle interaction forces.