Transition Metal Oxide Doping of Ceria-based Solid Solutions

Abstract

The effects of low concentration Co, Cr and Mn oxide, singly and in combination, on the sintering and electrical properties of Ce0.9Gd0.1O1.95 (CGO) have been investigated with possible mechanisms suggested to explain this modified behaviour. The influence of these dopants on the densification kinetics of CGO were primarily investigated using constant heating rate dilatometry. Whilst low concentration Co and Mn-oxide were found to improve the sinterability of CGO, the addition of Cr-oxide was found to inhibit the densification kinetics of the material. The location and concentration of these dopants were investigated as a function of relative density using scanning transmission electron microscopy combined with energy dispersive x-ray mapping. All materials showed a gradual reduction in the grain boundary dopant concentration with sintering time, leading eventually to the formation of a second phase that was subsequently analysed by either electron energy loss spectroscopy or synchrotron x-ray powder diffraction. The improved densification of both the Co-doped and Mn-doped materials was believed to be related to an increased rate of lattice and grain boundary cation diffusion, associated with the segregation of the transition metal dopant to the grain boundary. In both cases the onset of rapid densification was correlated with the reduction of the transition metal cation leading to an increase in cerium interstitials, which are suggested to be the defects responsible for cerium diffusion. The inhibiting effects of Cr-addition were similarly related to changes in the defect chemistry, with the Cr ions creating a blocking effect that hindered the dominant grain boundary pathway for cation diffusion. The effects of these dopants on the electrical conductivity of CGO were examined using a combination of AC impedance spectroscopy and Hebb-Wagner polarisation measurements. Whilst Co-doping was found to enhance the specific grain boundary conductivity of CGO, the addition of either Cr or Mn resulted in an approximate 2 orders of magnitude decrease, even at dopant concentrations as low as 100 ppm. Despite these differences in ionic conductivity, both Co and Cr-doping were found to significantly enhance the electronic contribution to the conductivity along the boundaries, particularly within the p-type regime. The modified electrical behaviour was related to the formation of a continuous, transition metal-enriched grain boundary pathway and a change in the driving force for grain boundary Gd segregation, leading to a depletion of oxygen vacancies within the space charge regions and the consequent reduction of oxygen transport across the boundaries. The effects of this segregation were finally examined with mono-layer sensitivity using low energy ion scattering incorporating a novel method of self-standardisation. These analyses provided strong support for the conductivity mechanisms previously outlined.