Abstract
A detailed study of the non-isothermal and the isothermal sinterability of aluminium hydroxide gel powders has been made. It has been confirmed that the gel powders consist of boehmite phase and on dehydroxylation transform to gamma alumina. The formation of other intermediate transition aluminas like delta and theta is morphology dependent. A fibrillar gel (Baymal) studied transformed from gamma to theta to alpha, whereas other two gel powders studied, which consisted of spherical crystallites, transformed to both the delta and the theta phase before converting to the alpha phase. All the gel powders yield alpha alumina in the temperature range 1150--1300°C. The gel powders exhibit poor non-isothermal and isothermal sinterability in spite of extremely fine crystallite size of the as received gel powders and the gamma alumina powders produced from them. It has been shown that poor sinterability of the gel compacts is associated with the simultaneous polymorphic phase transformation to the stable alpha, phase during sintering. The polymorphic phase transformation occurs by a nucleation and growth process which is diffusional in character. The electron diffraction studies have revealed that the metastable delta + theta or theta phases directly transform to the alpha phase, without the formation of any intermediate amorphous phase. The transmission electron microscopy has revealed that considerable redistribution of the fine porosity within the metastable phase matrix occurs into the form of elongated interconnected pores which are left behind within the growing alpha grains, and are effectively trapped. It is concluded that the poor sinterability of the gel compacts is due to considerable grain growth and effective trapping of the pores which occurs during the phase transformation. A geometrical model has been proposed to account for the observed redistribution of the porosity. It is concluded on the basis of model and interface migration kinetics considerations , that the interface remains pinned by the pores allowing pores to redistribute by vacancy diffusion along the interface from smaller to larger pores. The effect of rate of heating, calcination, impurity addition and atmosphere on the non-isothermal sinterability of the gel compacts has also been studied. It has been demonstrated that considerable improvement in the densification of the gel compacts can be achieved by the addition of phase stabilizing impurities like thoria.