Oxygen non-stoichiometry, electrical conductivity, and oxygen transport property in mixed conducting perovskite oxides.

摘要

The experimental results from solid-state coulometric titrations and electrical conductivity relaxation measurements on SrFeO_3−δ are described. In coulometric titration experiments, two-phase behavior at T ≤ 850°C was observed as hysteresis in the oxygen non-stoichiometry (δ) isotherms. The p to n transition in the electrical conductivity (σ_tot) isotherms is found to occur at the composition, SrFeO_2.505 at 950°C and SrFeO_2.508 at 900°C, respectively. Values of vacancy diffusion coefficient ( D_ν) for SrFeO_3−δ is ≈1.5∼5 times smaller than for the La_1−xSr_xFeO_3−δ series probably due to stronger defect interactions. Values of oxygen ion diffusion coefficient ($DO2-$) and surface exchange coefficient (k_ex) for SrFeO_3−δ are much greater than for the La _1−xSr_xFeO_3−δ series, because of its higher δ-values.; Electrical conductivity data for La_0.2Sr_0.8Cr _0.2Fe_0.8O_3−δ were obtained at 752 ≤ T ≤ 1055°C and at 10−18 ≤ pO₂ ≤ 0.5 atm. The slope of a plot of log σ vs. log pO₂ is ∼1/5 in the p-type region. At T ≤ 940°C, a plateau in the conductivity isotherms indicates the presence of a two-phase region most probably corresponding to a mixture of perovskite and an oxygen vacancy ordered phases. Electrical conductivity relaxation measurements were performed to investigate the kinetics of oxygen transport at 800 ≤ T ≤ 1000°C and at 0.01 ≤ pO₂ ≤ 1 atm, having values of D_chem = 1.62 × 10 −4 ∼ 3.10 × 10−4 cm2/s at 950°C with an activation energy (E_a) of 84 ± 14 kJ/mol. The oxygen ion diffusion coefficient ($DO2-$) and surface exchange coefficient (k_ex) of La_0.2Sr_0.8Cr_0.2Fe_0.8O _3−δ are relatively high because of high oxygen vacancy concentration.; The experimental results of electrical conductivity measurements and solid-state coulometric titrations on La_0.5Sr_0.5Ga _0.2Fe_0.8O_3−δ are reported. The effect of oxygen partial pressure (pO₂) on electrical conductivities of La_0.5Sr_0.5Ga_0.2Fe_0.8O_3−δ was investigated at 750 ≤ T ≤ 1050°C and at 10−17 ≤ pO₂ ≤ 0.5 atm. At 10−5 ≤ pO₂ ≤ 0.1 atm the log σ_tot is almost proportional to log (pO₂) with a slope of ≈¼ slope. At T ≤ 850°C, a plateau and a rapid increase in the conductivity isotherms indicate a presence of a two-phase region. In a plot of log pO ₂ vs. oxygen non-stoichiometry discontinuity appears at T ≤ 900°C and at the pO₂ region where the two-phase region was observed from the conductivity results. The p to n transition in the conductivity isotherms occurs at the composition, La_0.5Sr_0.5Ga_0.2Fe _0.8O_{2.749∼2.750} at 850°C and La_0.5Sr _0.5Ga_0.2Fe_0.8O_{2.748∼2.749} at 900°C, respectively.; The oxygen transport kinetics in La_0.5Sr_0.5FeO _3−δ were investigated using the electrical conductivity relaxation technique at 788 ≤ T ≤ 9