Oxygen Permeation Through Cobalt-Containing Perovskites: Surface Oxygen Exchange vs. Lattice Oxygen Diffusion

The oxygen permeation fluxes from p′O2 to pnO2 (p′O2\u3epnO2) across cobalt-containing perovskite ceramic membranes La1−xSrxCoO3−δ and SrCo0.8Fe0.2O3−δ were measured by gas chromatography as functions of oxygen chemical potential gradient, temperature, thickness, and catalytic activity on the surface. Power indexes 0.5\u3en\u3e0 for uncatalyzed La1−xSrxCoO3−δ and 1\u3en\u3e0.5 for SrCo0.8Fe0.2O3−δ were obtained when JO2 vs. p′nO2−p\u27′nO2 was plotted as a straight line. The results clearly indicate an overall permeation process controlled by both surface oxygen exchange and bulk oxygen diffusion for uncatalyzed La1−xSrxCoO3−δ and SrCo0.8Fe0.2O3−δ. Application of a thin layer of catalytically active SrCo0.8Fe0.2O3−δ on the feeding-gas surface of La0.5Sr0.5CoO3−δ under the condition of a fixed p′O2=0.21 atm and a varied p′\u27O2 not only increases remarkably the overall oxygen flux, but also changes a mixed control to a bulk diffusion control. This enables evaluation of the bulk transport properties of the mixed conductors. A coat of SrCo0.8Fe0.2O3−δ on the permeate side has little catalytic effect, especially at low p′\u27O2 range, due to the formation of a poorly conducting brownmillerite phase. The results explicitly show a higher activation energy for the surface exchange kinetics than for the ambipolar transport in the mixed conductors. The mechanism of the surface exchange is discussed, and an analytic expression that agrees well with the experimental results is obtained

Solid oxide fuel cell and doped perovskite lanthanum gallate electrolyte therefor

A perovskite lanthanum gallate electrolyte doped with strontium and magnesium and a solid oxide fuel cell incorporating a doped lanthanum gallate electrolyte with a cathode on one side, an anode on the other side and a buffer layer comprising a mixed electronic and oxide-ion conductor between the anodes and/or the cathode and the electrolyte to block unwanted chemical reactions while permitting electronic and oxide-ion transport.Board of Regents, University of Texas Syste

Anisotropic magnetoresistance in antiferromagnetic Sr2IrO4

We report point-contact measurements of anisotropic magnetoresistance (AMR) in a single crystal of antiferromagnetic (AFM) Mott insulator Sr2IrO4. The point-contact technique is used here as a local probe of magnetotransport properties on the nanoscale. The measurements at liquid nitrogen temperature revealed negative magnetoresistances (MRs) (up to 28%) for modest magnetic fields (250 mT) applied within the IrO2 a-b plane and electric currents flowing perpendicular to the plane. The angular dependence of MR shows a crossover from four-fold to two-fold symmetry in response to an increasing magnetic field with angular variations in resistance from 1-14%. We tentatively attribute the four-fold symmetry to the crystalline component of AMR and the field-induced transition to the effects of applied field on the canting of AFM-coupled moments in Sr2IrO4. The observed AMR is very large compared to the crystalline AMRs in 3d transition metal alloys/oxides (0.1-0.5%) and can be associated with the large spin-orbit interactions in this 5d oxide while the transition provides evidence of correlations between electronic transport, magnetic order and orbital states. The finding of this work opens an entirely new avenue to not only gain a new insight into physics associated with spin-orbit coupling but also better harness the power of spintronics in a more technically favorable fashion.Comment: 13 pages, 3 figure

Oxygen Permeation Through Composite Oxide-Ion and Electronic Conductors

Oxygen permeation through composites consisting of four well-known oxide-ion conductors and a noble metal, Pd or Ag, is reported. The oxides were Zr0.9Y0.1O1.95 (YSZ), (Bi1.75Y0.25O3)0.95(CeO2)0.05 (BYC5), Ce0.8Sm0.2O1.9 (SSC), and La0.8Sr0.2Ga0.83Mg0.17O2.815 (LSGM). The results show that (BYC5 + Ag) yields the highest oxygen permeation flux, but the composite deteriorates with time. The composites (SSC + Pd), (LSGM + Pd), and (YSZ + Pd) give stable, but relatively lower oxygen permeation flux in the order of (SSC + Pd) \u3e (LSGM + Pd) \u3e (YSZ + Pd). The composite microstructures indicate that (BYC5 + Ag) has the best percolating network for both oxide-ion and electronic pathways while (SSC + Pd) has the longest triple-phase boundary lengths with the smallest grains, which is beneficial to the surface oxygen exchange. It is shown that the microstructure of the composites, which strongly influences the competition between surface reaction and bulk diffusion, is technically as important as the oxide-ion conductivity. The activation energy appears to be related more to the morphology of the metallic phase than to that of the oxide phase. These results suggest that (SSC + Pd) is a promising composite mixed conductor for applications requiring oxygen separation

Thoughts on Incremental Permeability

After a short discussion of the factors which influence the incremental permeability, three different methods are discussed which should give a large change in incremental permeability with a change of the biasing field. The first requires a grain-oriented magnetic tape and take advantage of demagnetizing fields. The second requires a material with a square B-H loop, a low coercive force, and a large maximum permeability. The third requires a material with low crystalline anisotropy. Finally some currently available materials are suggested

A Theory of Perovskite-Type Manganites (La,M(II))MnO₃

Semicovalence and its effects on indirect magnetic-exchange interaction are reviewed and applied to the manganites. These considerations lead to qualitative predictions which are in complete accord with the following experimentally observed facts, where x is taken as the percentage of manganese ions which are Mn[superscript 4+]: (1) At x = 0, LaMnO₃ is an orthorhombic perovskite-type lattice with a₁ = a₃ > a₂. The ratio a₂/a₃ increases to 1 at x = 1/4, and for 0.40.75 the lattice is cubic. (2) At x = 0 the lattice is composed of ferromagnetic layers parallel to a (110) face perpendicular to the a₂ axis; these layers are stacked antiferromagnetically. As x increases to 1/4, the lattice becomes ferromagnetic. For x>0.4 various antiferromagnetic phases form, the magnetic configurations varying with x. (3) In the range 0.25≤x≤0.35 the saturation moment corresponds to ferromagnetically coupled spin-only values of the manganese ions; it drops off sharply toward zero around x = 0.1 and x = 0.5. (4) The electrical resistivity is a minimum and the Curie temperature a maximum at x≈0.3. (5) The maximum Curie temperature increases markedly from (La,Ca)MnO₃ to (La,Sr)MnO₃ and (La,Ba)MnO₃

Graphical Summary of Core Data in the MgO-Fe₂O₃-MnO System

A graphical summary of some of the magnetic data which were collected during 1953 on the MgO-Fe₂O₃-MnO System is presented

Sr‐ and Ni‐Doped LaCoO\u3csub\u3e3\u3c/sub\u3e and LaFeO\u3csub\u3e3\u3c/sub\u3e Perovskites: New Cathode Materials for Solid‐Oxide Fuel Cells

An improved cathode material for a solid‐oxide fuel cell would be a mixed electronic and oxide‐ion conductor with a good catalytic activity for oxygen reduction at an operating temperature T op ≥ 700°C and a thermal expansion matched to that of the electrolyte and interconnect. We report on the properties of Sr‐ and Ni‐doped LaCoO3 and LaFeO3 perovskites that meet these criteria. Single‐phase regions were determined by X‐ray diffraction, and thermogravimetric analysis measurements were used to obtain the temperatures above which oxygen loss, and hence oxide‐ion conductivity, occurs. The conductivity and Seebeck measurements indicate the coexistence of both p‐type and n‐type polaronic charge carriers resulting from an overlap of the NiIII/Ni2+ redox couple with the low‐spin/intermediate‐spin CoIV/Coiii and high‐spin Fe4+/Fe3+ redox couples. Motional enthalpies ΔHm = 0.03, 0.02, and 0.08 eV, respectively, were estimated for Ni2+, CoIV, and Fe4+ polarons. Optimal compositions have percolation pathways between dopants. Comparisons with transport data for the conventional cathode materials La1-xSrxCoO3-δ and La1-xSrxMnO3 indicate superior cathode performance can be expected

Increasing Power Density of LSGM-Based Solid Oxide Fuel Cells Using New Anode Materials

Chemical reactions between the superior perovskite oxide-ion conductor Sr- and Mg-doped LaGaO3 (LSGM), CeO2, and NiO have been studied by powder X-ray diffraction. The results showed that an extensive reactivity occurs as a result of La migration driven by a gradient of La chemical activity. La migration across the LSGM/electrode interfaces in a fuel cell leads to the formation of resistive phases at the interface, either LaSrGa3O7 or LaSrGaO4. Use of 40 mol % La2O3 -doped CeO2 as an interlayer between anode and electrolyte as well as in the NiO-containing anode prevents all reactions found. Consequently, the air-H2 cell maximum power density was increased to nearly 900 mW/cm2 at 800°C with a 600 μm thick LSGM electrolyte. No sign of degradation was observed at 800°C over 2 weeks for an interlayered cell under a loading current density of 250 mA/cm2