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

The electrical properties of perovskite-type (La0.3Sr0.7)(1-y)CoO3-delta (y=0-0.03) and La0.3Sr0.7Co0.8M0.2O3-delta (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-delta. Reducing oxygen pressure down to approximately 10(-5) atm results in transition into brownmillerite-type phases having essentially p(O-2)-independent electrical properties until decomposition, which occurs at p(O-2) 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) x 10(-6) K-1 at 300-750 K and (27.9-29.7) x 10(-6) K-1 at 750-1240 K. (c) 2006 Elsevier B.V. All rights reserved.