Sedimentation, rheology and particle-packing structure of aqueous Al2O3 suspensions

Gravity settling, rheology and particle-packing structure of aqueous alumina suspensions have been investigated using submicrometer α-Al2O3 powders with an average particle size ∼0.2 μm dispersed in pure water. The suspension pH varied from 2 to 11. The interface that separated the supernatant from the sediment appeared to settle linearly with time during the sedimentation experiment, followed then by a gradual reach toward a minimal sediment height; to which, the height was pH and solids loading dependent. The suspensions tended to form a continuous particulate network that defined the gravity settling as φ⩾0.1 at pH 11. This resulted in a flocculated suspension structure which barely settled even after 24 h without disturbance. The flocculated suspensions exhibited correspondingly a shear-thinning flow character over given shear-rate range (γ=1–1000 s−1) examined, in contrast to an apparent transition of flow toward shear thickening from the shear thinning for the better-dispersed suspensions (pH=2) at relatively high shear rates (γ⩾100 s−1). This finding suggested a breakdown of the particulate network into smaller flow units for the flocculated suspensions as shear rate was increased. Films made from the flocculated suspensions showed apparent cracking after drying. Microstructural examinations revealed that a pronounced grain growth occurred in the films made from the pH 2 suspensions after isothermal sintering.