abstract
Incorporation of gallium into the perovskite lattice of La0.3Sr0.7CoO3-delta leads to increasing unit cell volume and to decreasing thermal expansion, total conductivity and oxygen permeability. At 973 - 1223 K, the oxygen permeation fluxes through La0.3Sr0.7Co0.8Ga0.2O3-delta ceramics with 96.5% density are determined by the bulk ionic conduction and surface exchange rates. The total conductivity of La0.3Sr0.7Co0.8Ga0.2O3-delta, predominantly p-type electronic, exhibits an apparent pseudometallic behavior due to oxygen losses on heating, whereas the p(O-2) dependencies of the conductivity and Seebeck coefficient suggest a small-polaron mechanism of hole transport. The average thermal expansion coefficients in air are 15.9 x 10(-6) K-1 at 360 - 710 K and 27.9 x 10(-6) K-1 at 710 - 1030 K. On decreasing oxygen pressure down to 4 - 30 Pa at 973 - 1223 K, perovskite-type La0.3Sr0.7Co0.8Ga0.2O3-delta transforms into a brownmillerite-like modi. cation, whose electrical properties are essentially p( O2) independent. Further reduction results in the decomposition of the brownmillerite into a multiphase oxide mixture at p(O-2)= 8 x 10(-10) - 3 x 10(-4) Pa, and then in the segregation of metallic cobalt. Due to surface-limited oxygen transport, La0.3Sr0.7Co0.8Ga0.2O3-delta membranes are, however, kinetically stable under an air/CH4 gradient up to 1223 K. The conversion of dry methane in model membrane reactors increases with oxygen permeation flux and temperature, but yields high CO2 concentrations (> 90%), indicating a dominant role of complete CH4 oxidation on the membrane surface.
keywords
PEROVSKITE-TYPE OXIDES; OXYGEN NONSTOICHIOMETRY; ION-TRANSPORT; CONDUCTIVITY; METHANE; PERMEABILITY; TRANSITIONS; EXPANSION
subject category
Electrochemistry
authors
Kharton, VV; Tsipis, EV; Marozau, IP; Yaremchenko, AA; Valente, AA; Viskup, AP; Frade, JR; Naumovich, EN; Rocha, J
our authors
Aleksey Yaremchenko
Principal Researcher
Anabela Tavares Aguiar Valente
Principal Researcher
João Rocha
Full Professor
Jorge Ribeiro Frade
Retired Professor