Electrical conductivity and thermal expansion of Ln-substituted SrTiO3 for solid oxide cell electrodes and interconnects: the effect of rare-earth cation size

Abstract

The present work aims to evaluate relevant effects of the size of substituting rare-earth cation on electrical conductivity and thermal expansion of donor-doped SrTiO3 as ceramic components of electrochemical solid electrolyte cells. A series of strontium titanate-based ceramics with a moderate doping level and different cation stoichiometry, Sr0.90Ln0.10TiO3±δ and Sr0.85Ln0.10TiO3±δ (Ln = La-Yb), are prepared under identical conditions with the final reductive treatment in 10%H2-N2 atmosphere at 1500°C for 10 h. The solubility of rare-earth cations in the strontium sublattice of perovskite-type SrTiO3 is found to decrease with reducing ionic radius leading to a segregation of secondary pyrochlore-type Ln2Ti2O7 phase in the case of smaller rare-earth cations (Ln = Dy-Yb). The results of electrical, dilatometric and thermogravimetric studies suggest that the size of donor Ln3+ cation and cation stoichiometry have no evident impact on the electrical conductivity or thermal expansion behavior as long as the content of Ln3+ is within the solubility limits. All reduced donor-doped titanate ceramics (except Yb-doped) show a similar level of electrical conductivity, 100-155 S cm-1 at 800°C, determined by the content of Ln3+ in the strontium sublattice, and moderate thermal expansion coefficients, 11.9-12.1 ppm K-1 at 25-1100°C, compatible with that of conventional solid electrolytes.