Fast sintering of nano-sized sol-gel oxide powders

Abstract

Sol–gel zirconia was prepared with zirconiumn-butoxide and HCl, H2SO4, C2H4O2, and NH4OH as hydrolysis catalysts. Samples were characterized with DTA and TG analysis, X-ray powder diffraction, and FTIR spectroscopy. The structure of the crystalline phases was refined by the Rietveld method. When samples were annealed below 300°C, they lost weight and had an amorphous structure that, by annealing at higher temperatures, crystallized into nanostructures. For H2SO4as hydrolysis catalyst, the amorphous structure was stable even at higher temperatures, which was probably caused by the presence of SOxions in the structure. The local order in the amorphous phase was similar to the local order in the tetragonal zirconia. Crystallization of the amorphous phase produced tetragonal and monoclinic nanophases, with the tetragonal as the main phase. Both phases had a similar average crystallite size. By annealing, the tetragonal nanophase, which was more stable when C2H4O2was the hydrolysis catalyst, was transformed into the monolinic nano-phase. Since not only OH−ions in the structure were detected with FTIR spectroscopy but also Zr vacancies were measured with X-ray powder diffraction in the zirconia crystalline structure, we propose that these defects stabilized the tetragonal phase. Both defects disappeared when samples were annealed at high temperatures, which brought about the irreversible transformation of the tetragonal into the monoclinic structure.