Oxygen permeability, electronic conductivity and stability of La 0.3Sr 0.7CoO 3-based perovskites

Abstract

The electrical properties of perovskite-type (La 0.3Sr 0.7) 1− y CoO 3− δ ( y = 0–0.03) and La 0.3Sr 0.7Co 0.8M 0.2O 3− 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 Al 3+ or Ga 3+ 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 Co 4+ 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 La 0.3Sr 0.7CoO 3− δ . 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 10 2 to 10 4 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.