Oxygen permeability, thermal expansion and mixed conductivity of GdxCe0.8−xPr0.2O2−δ, x=0, 0.15, 0.2

The non-linear thermal expansion behaviour observed in Ce_1−yPr_yO_2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system Gd_xCe_0.8−xPr_0.2O_2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr⁴⁺. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values t_o=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce_0.8Pr_0.2O_2−δ, with specific oxygen permeability values approximately 2×10⁻⁹ mol s⁻¹ cm⁻¹ at 950 °C, but offering much better thermal expansion properties.