Agglomeration of alumina submicronparticles by silica nanoparticles: application to processing spheres by colloidal route

In aqueous media, heterocoagulation between submicronic alumina (400 nm) and nanometric silica (25 nm) leads to the adsorption of silica on the alumina surface. By controlling the coverage rate of alumina particles, this adsorption destabilizes the suspension that leads to a very porous network of agglomerated particles. This work shows that the structure is all the more open as the density of charge carried by the two oxides is high and the ionic strength in the suspension low. From such a flocculated suspension, a new colloidal process to fabricate ceramic spheres is proposed which is based on a size increase of agglomerates. Under a controlled rotation of the vessel, electrostatic attraction between the surface charges of opposite polarity induces a size increase of agglomerates until the formation of spheres occurs. It has been shown that the mechanism of growth is poisoned by species adsorbed such as ions. Nevertheless, this new process proves very promising because it leads to a narrow size distribution of spheres by colloidal way, which can be subsequently consolidated by sintering, with a smooth surface.