The gelation, phase transformation, and densification of a colloidal monolithic gel made from γ‐Al 2 O 3 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 γ‐to‐α phase transformation. TiO 2 is an effective sintering aid to overcome this bottleneck of densification because (1) it enhances the phase transformation rate so that the sintering of α‐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°C.
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Tsai et al. (1991) studied this question.
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