Thermal behaviour of cobalt oxides doped with ZrO2 and ThO2 and the possible cubic phase stabilization of zirconia

The effects of doping cobalt oxides with different amounts of ZrO₂ and ThO₂ (1.5–9 mol%) on the thermal stability of Co₃O₄ and the re-oxidation of CoO by O₂ to Co₃O₄ were investigated. The techniques employed were DTA, with a controlled rate of heating and cooling, X-ray diffraction, and IR spectrometry.The results obtained by DTA revealed that the addition of both Th⁴⁺ and Zr⁴⁺ (up to 6 mol%) exerted no appreciable effect on the thermal stability of Co₃O₄. Increasing the amount of the dopant ions to 9% resulted in no further change in the thermal stability of Co₃O₄ in the case of Th⁴⁺, and an increase of 16% in case of Zr⁴⁺-doping. However, ThO₂-doping of cobalt oxide was accompanied by an enhancement in the reactivity of CoO towards re-oxidation by O₂ to Co₃O₄ to an extent proportional to the amount of dopant oxide.The X-ray investigation of ZrO₂-doped cobalt oxides calcined in air at 1000°C revealed the presence of highly crystalline and stable zirconia in the cubic form. Such a stable phase could not be obtained at temperatures below 2370°C in the absence of stabilizing agents.X-ray and IR investigations of different solids showed the presence of free thoria and zirconia together with new thorium—cobalt and zirconium—cobalt compounds. However, the slow cooling of Zr-treated cobalt oxides from 1000°C to room temperature led to the decomposition of the newly formed compound. The d-spacings and absorption bands of the newly formed compounds were determined.