Transformation of Yttrium-Doped Hydrated Zirconium into Tetragonal and Cubic Nanocrystalline Zirconia

Nanostructured yttrium-stabilized zirconia powders, with yttria concentrations between 0.0 and 10.0 mol%, were prepared via the hydrolysis of an aqueous solution of zirconyl and yttrium chloride, and ammonium hydroxide. Powder phases were characterized by using X-ray powder diffraction; their crystalline structures were refined with the Rietveld technique. When samples were annealed below 200°C, their diffraction patterns corresponded to an amorphous atom distribution and were independent of yttria concentration. The doped amorphous phases crystallized, at 400°C, into tetragonal or cubic nanocrystalline zirconia, which were stabilized by yttrium. These results suggest that yttrium atoms served as a substitute for zirconium atoms not only in the crystalline phases but also in the amorphous phases, which are determined by the fast condensation of zirconyl clusters. Nondoped samples contained a mixture of monoclinic and tetragonal nanocrystalline zirconia; those with 2.5 to 5.0 mol% yttria contained only the tetragonal zirconia nanophase, and those with 7.5 to 10.0 mol% had only the nanocrystalline cubic phase. The average crystallite size of the nanophases diminished when Y₂O₃concentration was increased.