Nickel oxide-zirconium oxide: Ni2+ incorporation and its influence on the phase transition and sintering of zirconia

Nickel-containing zirconia samples (Ni[less-than-or-eq]16.85%), prepared by contacting hydrous zirconium oxide with an Ni2+ solution to a given pH were studied by various techniques (XRD, DTA, TG, XPS) to investigate the incorporation of Ni2+ in the zirconia structure in relation to the dispersion degree of the transition-metal ions. Both dried (383 K)'as prepared' and thermally treated (773 K, air) samples were investigated. The as-prepared samples were a mixture of amorphous particles of hydrous zirconium oxide and nickel basic nitrate crystallites, some very small in size. Heating at 773 K caused crystallization of the hydrous zirconium oxide and transformation of the nickel compound into nickel oxide. Concomitantly the nickel in the dispersed state entered the solid solution in the zirconia structure. The incorporation took place substitutionally in sites with trigonally distorted eightfold cubic symmetry. The presence of nickel favoured the formation of metastable tetragonal zirconia and affected the sintering process. The stability of the incorporated Ni2+ against reduction by hydrogen was also investigated.