Matthias Meffert

Fast mapping of the cobalt-valence state in Ba0.5Sr0.5Co0.8Fe0.2O3-d by electron energy loss spectroscopy.

A fast method for determination of the Co-valence state by electron energy loss spectroscopy in a transmission electron microscope is presented. We suggest the distance between the Co-L3 and Co-L2 white-lines as a reliable property for the determination of Co-valence states between 2+ and 3+. The determination of the Co-L2,3 white-line distance can be automated and is therefore well suited for the evaluation of large data sets that are collected for line scans and mappings. Data with a low signal-to-noise due to short acquisition times can be processed by applying principal component analysis. The new technique was applied to study the Co-valence state of Ba0.5Sr0.5Co0.8Fe0.2O3-d (BSCF), which is hampered by the superposition of the Ba-M4,5 white-lines on the Co-L2,3 white-lines. The Co-valence state of the cubic BSCF phase was determined to be 2.2+ (±0.2) after annealing for 100 h at 650°C, compared to an increased valence state of 2.8+ (±0.2) for the hexagonal phase. These results support models that correlate the instability of the cubic BSCF phase with an increased Co-valence state at temperatures below 840°C.

Dopant-Site Determination in Y- and Sc-Doped (Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ by Atom Location by Channeling Enhanced Microanalysis and the Role of Dopant Site on Secondary Phase Formation.

(Ba0.5Sr0.5)(Co0.8Fe0.2)O3-δ (BSCF) is a promising material with mixed ionic and electronic conductivity which is considered for oxygen separation membranes. Selective improvement of material properties, e.g. oxygen diffusivity or suppression of secondary phase formation, can be achieved by B-site doping. This study is concerned with the formation of Co-oxide precipitates in undoped BSCF at typical homogenization temperatures of 1,000°C, which act as undesirable nucleation sites for other secondary phases in the application-relevant temperature range. Y-doping successfully suppresses Co-oxide formation, whereas only minor improvements are achieved by Sc-doping. To understand the reason for the different behavior of Y and Sc, the lattice sites of dopant cations in BSCF were experimentally determined in this work. Energy-dispersive X-ray spectroscopy in a transmission electron microscope was applied to locate dopant sites exploiting the atom location by channeling enhanced microanalysis technique. It is shown that Sc exclusively occupies B-cation sites, whereas Y is detected on A- and B-cation sites in Y-doped BSCF, although solely B-site doping was intended. A model is presented for the suppression of Co-oxide formation in Y-doped BSCF based on Y double-site occupancy.

Grain-size dependence of the deterioration of oxygen transport for pure and 3 mol% Zr-doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ induced by thermal annealing

In this study, the influence of long-term annealing at intermediate temperatures on oxygen transport of Ba0.5Sr0.5Co0.8Fe0.2O3 d (BSCF) and 3 mol% Zr-doped BSCF (BSCF-Z3) ceramics with different grain sizes was studied by means of in situ electrical conductivity relaxation (ECR) measurements. Ceramics with different grain sizes in the range of 3–80 mm were obtained by varying the temperature and dwell time during sintering. For both compositions, the apparent values of the chemical diffusion coefficient Dchem and surface exchange coefficient kchem extracted from the data of ECR measurements are found to decrease with the time of annealing. The strongly correlated decreases in Dchem and kchem, being greater in magnitude at a lower grain size and temperature, are observed significantly more pronounced for BSCF than for BSCF-Z3. The results from microstructural analysis provide a clear rationale for the observations from ECR. High-resolution transmission electron microscopy (TEM) images recorded before and after annealing under pure oxygen at 700 C for 13 d show excessive formation of hexagonal and plate-like lamellar precipitates at the grain boundaries and in the interior of grains of BSCF ceramics during the annealing process, whilst secondary phase formation is restricted solely to the grain boundary regions in BSCF-Z3. The possible importance of grain boundaries in determining the oxygen surface exchange kinetics is emphasized.

Anisotropic electronic transport of the two-dimensional electron system in Al₂O₃/SrTiO₃ heterostructures

Transport measurements on the two dimensional electron system in Al2O3 SrTiO3 heterostructures indicate significant noncrystalline anisotropic behavior below T = 30 K. Lattice dislocations in SrTiO3 and interfacial steps are suggested to be the main sources for electronic anisotropy. Anisotropic defect scattering likewise alters magnetoresistance at low temperature remarkably and influences spin-orbit coupling significantly by the Elliot Yafet mechanism of spin relaxation resulting in anisotropic weak localization. Applying a magnetic field parallel to the interface results in an additional field induced anisotropy of the conductance, which can be attributed to Rashba spin orbit interaction. Compared to LaAlO3 SrTiO3, Rashba coupling seems to be reduced indicating a weaker polarity in Al2O3 SrTiO3 heterostructures.

Galvanic Exchange in Colloidal Metal/Metal-Oxide Core/Shell Nanocrystals

While galvanic exchange is commonly applied to metallic nanoparticles, recently its applicability was expanded to metal-oxides. Here the galvanic exchange is studied in metal/metal-oxide core/shell nanocrystals. In particular Sn/SnO2 is treated by Ag+, Pt2+, Pt4+, and Pd2+. The conversion dynamics is monitored by in situ synchrotron X-ray diffraction. The Ag+ treatment converts the Sn cores to the intermetallic AgxSn (x ∼ 4) phase, by changing the core’s crystal structure. For the analogous treatment by Pt2+, Pt4+, and Pd2+, such a galvanic exchange is not observed. This different behavior is caused by the semipermeability of the naturally formed SnO2 shell, which allows diffusion of Ag+ but protects the nanocrystal cores from oxidation by Pt and Pd ions.

Effect of Yttrium (Y) and Zirconium (Zr) Doping on the Thermodynamical Stability of the Cubic Ba0.5Sr0.5Co0.8Fe0.2O3-δ Phase

Mixed ionic-electronic conducting (MIEC) ceramic materials are currently considered as membranes for oxygen production due to their oxygen-ion and electronic conductivity. Among other MIECs Ba0.5Sr0.5Co0.8Fe0.2O3-δ (denoted BSCF) possesses superior oxygen permeation properties due to its high oxygen non-stoichiometry. However, the inherent instability of the cubic phase during long-time operation in the temperature range of 700 800 °C is a major drawback. Below 825 °C, a gradual decline in oxygen permeation over time is associated with partial decomposition of the cubic phase into a hexagonal phase [1,2]. The formation of the hexagonal phase can be correlated with the increase of the oxygen concentration at lower temperatures, which requires an increase of the average valence state for the multivalent Co-cations [3]. TEM studies also revealed additional secondary phases in BSCF with low crystal symmetry and plate-like morphology. These plate-like phases consist of lamellar stackings of the cubic, hexagonal and Ban+1ConO3n+3(Co8O8) (n 2) phase [3]. Moreover, the Co-cation in BSCF has a high tendency to diffuse out of the cubic lattice forming CoO precipitates which might act as nuclei for the lamellar phases. To counteract the cation valence change and, hence, decomposition of the cubic phase B-site doping with monovalent transition metals was proposed. Recent studies have shown beneficial effects for doping with Y or Zr [4,5]. However, the impact regarding the microstructure is not well understood yet. Therefore, an extensive SEM and TEM study was performed on Yand Zr-doped BSCF which includes the variation of numerous parameters like sintering temperature, annealing temperature and dopant concentration. To verify that the dopant is incorporated on the desired B-type lattice site, the technique of atom location by channeling enhanced microanalysis was used [6].