Oxide solid electrolytes under non-equilibrium conditions - Interfaces and ageing

This paper gives an overview of the materials transport processes which occur in oxide solid electrolytes subjected to a generalized thermodynamical potential gradient. The general equations allowing to describe the transport processes on the anionic and cationic sublattices are given. Kinetic demixing processes and microstructural changes leading to surface instabilities are analysed. Experimental results obtained with yttria-doped zirconia and gadolinium doped ceria illustrate the importance of these phenomena on the transport properties of the materials but also on their long term stability in operating conditions

PHYSICO CHEMICAL PROPERTIES OF NICKEL OXIDE AT HIGH TEMPERATURE

Une étude de l'écart à la stoechiométrie dans l'oxyde de nickel pulvérulent a été effectuée par titrage coulométrique, dans une gamme de pressions d'oxygène s'étendant de 0,012 à 0,21 atm et aux deux températures de 881° et 896°C. Les résultats semblent montrer l'existence, dans le domaine étudié, de lacunes de Nickel ionisées une fois comme défaut prédominant. Le coefficient de diffusion chimique dans l'oxyde de nickel monocristallin a été déterminé par la méthode de la conductivité électrique entre 1 000°et 1 400°C, et pour des pressions d'oxygène s'étendant de po2 = 10-4 atm à po2= 1 atm. De plus, le comportement anormal de la conductivité électrique a été expliqué par la présence simultanée, dans le domaine étudié, de lacunes de Nickel une fois et deux fois ionisées. Un modèle thermodynamique est proposé sur cette base.A study of the deviation from stoichiometry by coulometric titration has been performed on nickel oxide powder, from po2 = 0.012 atm to po2 = 0.21 atm, at 881° and 896°C. The results are in agreement with the existence of singly ionized nickel vacancy as the predominant defect in the investigated range. Chemical diffusion coefficient in nickel oxide single crystals has been determined by the electrical conductivity technique, between 1 000°and 1 400°C, from po2 = 10-4 atm to po2 = 1 atm. Moreover, the anomalous behaviour of the electrical conductivity has been explained by the simultaneous presence of singly and doubly ionized nickel vacancies in the investigated range. A thermodynamic model is proposed on this basis

Electrical conductivity and thermoelectric power of uranium dioxide

The electrical conductivity and thermoelectric power of an uranium dioxide single crystal were measured between 908 and 1697 K and for P-O2, included between 10(-24) atm and the boundary UO2+x/U4O9. For T = 1273 K, the conductivity exhibits a minimum. The Seebeck coefficient (Q) over bar (UO2), also exhibits a behavior change when the temperature is increased. For T 1200 K, (Q) over bar (UO2), changes from negative to positive values when P-O2, increases. This set of results shows that the stoichiometric oxide presents a p extrinsic to intrinsic transition near 1273 K. At T 1273 K, the electrical conductivity minima are characteristic of a p -> n transition. A band gap energy of 2.0 eV has been calculated from the temperature dependence of these conductivity minima values. Furthermore, this set of results has allowed us to determine the relative oxygen partial molar free energy of stoichiometric UO

Influence of the kinetic demixing of cations on ceramic ageing and alloy corrosion

This paper concerns the kinetic demixing of cations in semiconducting oxides. From the general equations of matter transport, time-dependent cation redistributions are given for (Co, Mg)O and (Fe, Cr)O solid solutions for which thermodynamical and kinetic data are available. The cation kinetic demixing effects on the reaction rates are described. The conclusions are used to analyse the behavior of (Fe, Cr)O solid solutions under a chemical potential gradient and the effect of Cr on the internal oxidation of Ni-Cr alloys

Anionic and cationic diffusion in ionic conducting oxides

This paper concerns an analysis of the transport processes at high temperature in anionic conducting oxides subjected to a chemical potential gradient or an applied electrical field. The general equations are given. The principle of the cationic kinetic demixing under a "generalized" thermodynamical potential gradient is reviewed. Experimental results obtained with yttria-doped zirconia are reported. An experimental procedure for the determination of the oxygen diffusion coefficient in ionic and semiconducting oxides is also described. The results obtained with yttriastabilized zirconia are compared to both self diffusion and conductivity data. This has allowed us to obtain information concerning the defect structure

Nanostructure, nanochemistry and grain boundary conductivity of yttria-doped zirconia

This work was directed at a comprehensive study of the role of the nanostructure and nanochemistry on the transport properties of yttria-stabilized zirconia. Alumina additions lead to a decrease Of sigma(gb) when the samples have clean grain boundaries, while sigma(gb) goes through a maximum in samples having glassy grain boundaries. The differences were attributed to the strong interaction between Al2O3 and SiO2 impurities leading to a glassy phase depletion at the grain-boundaries, due to a change in wettability. Moreover, XPS analyses show that Si and Y segregate near these interfaces according to a kinetic demixing process, explaining why a faster cooling rate after sintering has a beneficial effect on sigma(gb)