Effect of microstructure and composition on ionic conductivity of rare-earth oxide-doped ceria

Polycrystalline ceria doped with several rare-earth oxides (Dy(2)O(3), Er(2)O(3), Gd(2)O(3), Ho(2)O(3), La(2)O(3), Nd(2)O(3), Sm(2)O(3), and Yb(2)O(3)) at various concentrations were fabricated. Conductivity was measured by ac admittance and de four-probe methods. The conductivity was separated into grain and grain boundary contributions using complex admittance technique as well as the grain size dependence of conductivity. The effect of dopant type and concentration on grain boundary conductivity was examined. Grain size dependence of polycrystalline conductivity, which can be adequately described by the so-called brick layer model, appears to give a more reliable measure of the grain conductivity compared to the complex admittance method. Polycrystalline resistivity (1/conductivity) increases linearly with the reciprocal of grain size. The intercept of resistivity vs. inverse grain size plot gives a measure of the grain resistivity, and the slope gives a measure of the grain boundary resistivity. Activation energies of polycrystalline conductivity, grain conductivity, and grain boundary conductivity were determined. A compositional analysis of fractured samples was carried out by Auger spectroscopy. The dopant concentration near grain boundaries was estimated to be lower than that in the bulk.