Transport numbers and oxygen permeability of SrCe(Y)O3-based ceramics under oxidising conditions

Partial conductivities in the SrCe(Y)O3-delta system have been studied in oxidising conditions in the temperature range 923-1273K. Compositions with variable Y content (5 and 10at.%), Sr deficiency (3at.%), and with the addition of Fe2O3 as sintering aid (2mol%) were analysed. A modified Faradaic efficiency method and oxygen permeation measurements were employed to appraise the oxide-ionic transport. Oxide-ion transference numbers in air lie in the range 0.19-0.80 and decrease with increasing temperature in the range 973-1223K. Modelling of total conductivity as a function of oxygen partial pressure (p(O2)) confirmed that protonic transport is minor under the studied conditions. SrCe0.95Y0.05O3-delta exhibits greater oxide-ion conductivity than SrCe0.9Y0.1O3-delta, indicative of dopant-vacancy association at high dopant contents. Conversely, oxygen permeability is slightly higher for SrCe0.9Y0.1O3-delta as a result of faster surface-exchange kinetics. The oxygen flux through Fe-free membranes is dominated by the bulk in low p(O2) gradients, when the permeate-side p(O2) is higher than 0.03atm, but surface exchange plays an increasing role with increasing p(O2) gradient. Addition of Fe2O3 to SrCe(Y)O3-delta lowers the sintering temperature by 100K but results in the formation of intergranular second phases which block oxide-ionic and electronic transport, and thus oxygen permeation. The average thermal expansion coefficients (TECs) are (10.8-11.6)X10-6K-1 in the temperature range 373-1373K for all studied compositions.