Lowering the sintering temperature and enhancing densification by epitaxy in structurally diphasic Al₂O₃ and Al₂O₃MgO xerogels

The densification and sintering behavior of diphasic xerogels derived from boehmite and Al₂O₃MgO compositions seeded with α-Al₂O₃, spinel (MgAl₂O₄), α-Fe₂O₃, Cr₂O₃, kaolinite and Ludox was studied by measuring densities and by microstructural characterization of sintered materials. Isostructural seeding with the final equilibrium phase, i.e. α-Al₂O₃ in boehmite and α-Al₂O₃ and spinel in Al₂O₃MgO gels, resulted in enhanced densification as well as sintering at lower temperatures. Solid state epitaxy appears to be the mechanism by which isostructural seeding affects the densification and sintering behavior. Microstructural characterization revealed uniform grain growth with little or no porosity in the gels with isostructural seeds, while non-uniform grain growth and porosity were seen in unseeded gels as well as in gel with nonisostructural seeds. Isostructural seeding may become a generalizable process for enhancing densification and sintering in sol-gel ceramic processing.