Transport properties of Ce-doped nickel oxide and relevance on the thermal growth of oxide scales on CeO2-coated nickel
Résumé
The growth of oxide scales on Ce02-coated Ni polycrystals, in the temperature range 800–1200 °C, is analyzed taking into account both the transport properties of Ce-doped Ni1-xO single crystals, under non-equilibrium conditions, and a formal analysis of transport properties of cation deficient p-type semi conducting oxides in presence of an external driving force. It was shown that a key feature of the beneficial influence of Ce02-coatings on the layer growth is the kinetic demixing of cations, leading to an enrichment of cerium in the inner oxidation scale. It follows a higher cationic vacancy concentration near the metal/oxide interface, leading to a lower vacancy concentration gradient across the layer and a decrease of the driving force of diffusion controlling the oxidation kinetics. Further, the lower cation mobility in the outer layer due to the depletion of Ce4+, leads to a blocking effect of the outward diffusion of cations. It follows a local excess of Ni2+ and the formation of new oxide units which also contribute to a decrease of the oxidation rate.