Oxygen semi-permeation and electrical conducting behavior of electronic and oxide ionic conducting ceramics

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Electrical conductivity of mixed conducting perovskite-type oxide, La_<1-x>Sr_xCo_<0.2>Fe_<0.8>O_<3-δ> (LSCF, 0.4【less than or equal】x【less than or equal】0.7) was determined as functions of Sr-content (x), temperature (T) and oxygen partial pressure [P(O_2)]. Crystal structure and oxygen nonstoichiometry (δ) of LSCF was evaluated and correlated with its electrical property. The electrical conductivity (σ) in LSCF was greatly affected by oxygen nonstoichiometry and B-O-B angle (B ; B-site elements) in the crystal. From σ measurement and crystal structure refinement of La_<1-x>Sr_xCo_<0.2>Fe_<0.8>O_<3-δ> in air, σ is found to be significantly dependent upon the expansion of B-O-B angle (θ), inducing broadening of the electronic bandwidths of the B-3d and O-2p electronic bands. Oxygen nonstoichiometry directly reduced σ because of a decrease in the concentration of p-type charge carriers especially at elevated temperatures or low P(O_2). It was found that σ was sharply affected by the frozen oxygen vacancies below a critical P(O_<>) of ca. 6.8×10^<-4> atm at low temperatures.In addition, Sr-deficient LSCF (La_<0.6>Sr_<0.4-y>Co_<0.2>Fe_<0.8>O_<3-δ>, 0<y【less than or equal】0.025) was developed to increase oxygen vacancy concentration (δ) so as to enhance its oxide ionic conductivity. It was found that δ was increased with increase in y, determined by iodometry, and the sample of y=0.025 is suitable for cathode materials in SOFCs and oxygen semi-permeable membranes.