CONTROLLED GELATION AND SINTERING OF MONOLITHIC GELS PREPARED FROM GAMMA-ALUMINA FUME POWDER

The gelation, phase transformation, and densification of a colloidal monolithic gel made from gamma-Al2O3 fume powder are investigated. Among the six gelation agents that we use, formamide and urea are quick in causing gelation and easy to burn off. The densification rate of this gel decreases rapidly after the gamma-to-alpha phase transformation. TiO2 is an effective sintering aid to overcome this bottleneck of densification because (1) it enhances the phase transformation rate so that the sintering of alpha-alumina occurs at a lower temperature, and (2) it promotes sintering rates at the initial and intermediate stages after phase transformation. On the other hand, MgO has an inappreciable effect on gel sintering. The effect of MgO at the final sintering stage is obstructed by this densification barrier after transformation. The titania-doped gel monoliths can be sintered to high density and fine microstructure at 1400-degrees-C.