Interdiffusion-Induced Phase Changes of Co1-xO/Zirconia Composites

Co1-xO powders mixed with pure zirconia (ZrO2) or yttira (Y2O3) partially stabilized zirconia (Y-PSZ) were sintered and annealed at 1300°C for 300 h, and the Co1-xO/Y-PSZ composite also at 1600°C for 1–100 h to investigate interdiffusioninduced phase changes. Analytical electron microscopic observations indicated the formation of (Zr,Y)-codoped Co3O4 spinel from intra- and intergranular Co1-xO particles for all the fired composites. The spinel formation above the equilibrium temperature 900°C for pure Co1-xO/Co3O4 pair can be rationalized by the substitution of Zr4+ (<2mol%) and Y3+ (<1mol%) for Co2+ to generate a considerable number of charge- and volume-compensating defects and paracrystalline array of defect clusters in the Co1-xO lattice. The (111) faulting of (Zr,Y)-codoped Co3O4 implies the possible presence of Zinc blende-type defect clusters with cation vacancies assembled along oxygen close packed (111) plane of Co1-xO. About 6mol% Co2+ dissolution of Y-PSZ was found to stabilize the cubic-phase of zirconia upon cooling to room temperature. A relatively larger flux of Co2+ from Co1-xO into Y-PSZ than the opposite flux of Zr4+ and Y3+ causes a net vacancy flux, hence the formation of Kirkendall pores around the Co1-xO particles.