Influence of porosity on the bulk and grain-boundary electrical properties of Gd-doped ceria

The effects of porosity on the grain and grain-boundary transport properties of the solid-state electrolyte Ce0.9Gd0.1O1.95 (CGO) have been analysed with a modified brick-layer model. Ceramics of CGO with varying degrees of densification were obtained on sintering compacted green bodies of commercially produced powder in the range 1200 ≤ Ts ≤ 1500 °C. Impedance spectroscopy was employed to determine the transport properties of the microstructural components in the range 150–1000 °C for samples pertaining to the sintering regimes of densification (Ts = 1200–1300 °C) and grain growth (1300–1500 °C). The bulk resistance is dependent only on porosity throughout the sintering process and decreases monotonously with increasing relative density (r.d.). The grain-boundary transport is dependent, however, on both porosity and grain size, which is reflected by a sharp increase in conductance on crossing from densification to grain-growth regimes. Since the effect of porosity on capacitance is opposite to that of resistance, neither bulk nor grain-boundary “specific” conductivity is affected by densification for levels above 75% r.d. Accordingly, activation energies for the conductance of both components are independent of the degree of porosity. The total conductivity could be well correlated with the bulk and grain-boundary components by means of a porosity factor introduced in the brick-layer model.