Gelcasting of alumina using urea-formaldehyde I. Preparation of concentrated aqueous slurries by particle treatment with hydrolysed aluminium

A treatment of α-alumina with hydrolysed aluminium (HA), generated in situ by homogenous hydrolysis of aqueous aluminium nitrate solution using hexamethylene tetramine (HMTA), has been evolved to enhance the surface activity of the powder towards complexation with H+ and thus to promote dispersion of its aqueous concentrated (>50 vol%) slurries using a mineral acid (nitric acid) with a view to prepare slurries having high solids loading and low viscosity for gelcasting. Viscosity minimum of the slurries was obtained at HA minimum (Al3+ added in the system)∼6×10−6 mol/m2 (BET surface area) of the powder. Slurries of solids loading >60 vol% having viscosity <1 Pa s at 93 s−1 could be prepared by HA treatment of α-aluminas of median particle size 0.3–0.8 μm to obtain green bodies with relative density of 65–71% by evaporational drying. ©