Oxygen permeability, electronic conductivity and stability of La0.3Sr0.7CoO3-based perovskites

The electrical properties of perovskite-type (La0.3Sr0.7)1−yCoO3−δ (y = 0–0.03) and La0.3Sr0.7Co0.8M0.2O3−δ (M = Al, Ga), promising parent materials of dense mixed-conducting membranes for oxygen separation, were studied in the oxygen partial pressure range from 10−14 to 0.5 atm. The steady-state oxygen permeation fluxes through cobaltite ceramics at 973–1223 K are limited by both bulk ionic conductivity and surface exchange kinetics. The substitution of cobalt with Al3+ or Ga3+ increases cubic perovskite unit cell volume, oxygen deficiency and Seebeck coefficient, whereas the thermal expansion, p-type electronic conductivity and oxygen permeability decrease. The creation of A-site cation vacancies, compensated by Co4+ formation, leads to higher p-type electronic conductivity and thermal expansion at temperatures above 700 K, whilst the ionic transport in A-site deficient cobaltite is lower than that in La0.3Sr0.7CoO3−δ. Reducing oxygen pressure down to approximately 10−5 atm results in transition into brownmillerite-type phases having essentially p(O2)-independent electrical properties until decomposition, which occurs at p(O2) values 102 to 104 times higher compared to CoO/Co boundary. The average thermal expansion coefficients of cobaltite ceramics in air are (15.9–19.6) × 10−6 K−1 at 300–750 K and (27.9–29.7) × 10−6 K−1 at 750–1240 K.