W. Sitte

Montanuniversität Leoben, Leoben, Styria, Austria

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Publications (95)158.49 Total impact

  • Andreas Egger, Werner Sitte
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    ABSTRACT: The oxygen surface exchange kinetics of the mixed ionic–electronic conductor La2NiO4+δ has been enhanced by coating the material with a 200 nm thin layer of silver. Due to the catalytic activity of silver the surface exchange coefficient (kchem) was increased by one order of magnitude at 600 °C. This method thus reduced the limiting role of the surface redox process permitting the determination of the chemical diffusion coefficient of oxygen (Dchem) by means of the conductivity relaxation technique between 600 and 850 °C at oxygen partial pressures of 0.1 and 0.01 bar. Dchem-values range between 2 × 10− 5 and 2 × 10− 4 cm2s− 1 with activation energies of ~ 50 kJmol− 1. Over the course of a complete temperature cycle the silver layer was removed via gas phase transport at high temperatures which allowed the comparison of oxygen surface exchange coefficients with and without surface activation. Depth profiles of the surface of a tested sample by X-ray photoelectron spectroscopy showed no evidence of silver within the topmost 500 nm, indicating that Ag-deposition did not affect the diffusivity of the material. Additionally, the morphology of deposited silver films was studied after annealing between 500 and 800 °C by SEM.
    Solid State Ionics 05/2014; 258:30–37. · 2.05 Impact Factor
  • W. Preis, E. Bucher, W. Sitte
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    ABSTRACT: A square grain model is proposed for the calculation of the area-specific resistance (ASR) of porous cathodes for solid oxide fuel cells (SOFCs) by means of the finite element approach. The grains and pores are represented by squares of equal side length. The grain boundaries are assumed to be thin slabs of uniform thickness. Both blocking conditions for the ionic current and fast transport of oxide ions along the grain boundaries have been taken into account. The results of the simulation suggest that highly active cathode materials could be developed by increasing the grain boundary ionic conductivity. In the case of an average grain size of 0.1 μm, a remarkable decrease of the ASR is predicted, if the ionic conductivity of the grain boundaries exceeds that of the bulk by a factor of 100. The model has been applied to simulate the increase of the ASR due to degradation of La0.6Sr0.4CoO3–δ in dry and humid atmospheres at 600 °C. A rapid increase of the ASR is predicted in H2O-containing atmospheres. The effect of Cr-poisoning on the ASR has been modeled for dry and humid atmospheres at 600 °C. The degradation owing to Cr-poisoning is most pronounced in atmospheres containing water vapor.
    Fuel Cells 08/2012; 12(4). · 1.55 Impact Factor
  • Min Yang, Edith Bucher, Werner Sitte
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    ABSTRACT: The influence of chromium poisoning on the long-term stability of the oxygen exchange kinetics of the promising IT-SOFC cathode materials La0.6Sr0.4CoO3−δ (LSC) and Nd2NiO4+δ (NDN) is investigated in-situ by dc-conductivity relaxation experiments. The as-prepared LSC and NDN samples show high chemical oxygen surface exchange coefficients kchem. After the deposition of a 10 nm thick Cr-layer onto the surface, kchem of LSC decreases to 50% of the initial value. Additional chromium deposition of 20 nm on LSC leads to a further decrease of kchem to 27% of the initial value. In contrast, the effect of a 10 nm thick Cr-layer on kchem of NDN is negligible. Even with additional 20 nm of chromium and a total testing time of 1750 h, the nickelate retains a kchem of 60% of the initial value. X-ray photoelectron spectroscopy (XPS) of the degraded. LSC shows a significantly altered surface cation composition with Sr-enrichment down to 30 nm depth while XPS analysis of the degraded NDN reveals a thin surface zone of approximately 30 nm containing nickel and chromium. In contrast to LSC, the changes in the surface composition of NDN due to Cr-poisoning ultimately had only a minor influence on the surface exchange properties.
    Journal of Power Sources 09/2011; 196(17):7313–7317. · 5.26 Impact Factor
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    ABSTRACT: Oxygen exchange with the ambient atmosphere and oxygen diffusion are assumed to play a decisive role in the re-oxidation process of positive temperature coefficient (PTC) resistors based on donor doped barium titanate. 18O tracer experiments with subsequent time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were thus carried out to investigate the oxygen diffusion properties of donor doped barium titanate. Fast grain boundary diffusion was found at temperatures between 750 °C and 900 °C. Moreover, evidence is given for a position dependent diffusion coefficient close to the surface. The secondary phase developing during the production process is shown to be Ti-rich and hardly any oxygen tracer exchange with this secondary phase could be observed. This suggests that grain boundary diffusion does not take place via such secondary phases. Rather, evidence of diffusion along an oxygen vacancy enriched space charge region is found.
    Journal of Applied Physics 08/2011; 110(4):043531-043531-7. · 2.21 Impact Factor
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    ABSTRACT: The time-dependent degradation of the oxygen exchange kinetics of the solid oxide fuel cell cathode material La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) is investigated at 600 °C. Special emphasis is placed on systematic long-term dc-conductivity relaxation measurements (t > 1000 h) in dry as well as in humidified atmospheres in order to obtain representative trends for the application of LSCF in intermediate-temperature SOFCs. The determination of the chemical surface exchange coefficient kchem of oxygen is combined with investigations of the elemental surface compositions and depth profiles of fresh and degraded samples by X-ray photoelectron spectroscopy (XPS), providing further insight into the mechanisms of degradation. The slow decrease of kchem by a factor of 2 during exposure of the sample to a dry O2–Ar reference atmosphere for 1000 h at 600 °C can be ascribed to an enrichment of La and Sr in correlation with an elevated oxygen concentration within about 30–35 nm depth. The interpretation of the XPS core level spectra indicates the formation of SrO and La2O3 secondary phases in this zone. The subsequent treatment in a humidified atmosphere for 1000 h results in a pronounced initial decrease of kchem by an additional factor of 10, followed by a time dependent decay of about 15% kh− 1. A Sr-rich silicate layer of about 10 nm thickness is identified by XPS as the major cause of the degradation in humidified atmosphere. The evidence of Si-poisoning over the whole sample surface could also be confirmed by post-test SEM analysis. In addition, indications of a re-structuring of the sample surface during the degradation are shown. These results indicate, that with LSCF as a cathode in ambient (humid) air in SOFC stacks containing various Si-sources, such as glass or glass-ceramic seals, and thermal insulation materials a significant decrease of the surface oxygen exchange coefficient can occur, even at temperatures as low as 600 °C. In order to prevent a severe Si-induced degradation, dry air should be used as an oxidant. However, even in dry atmosphere a minor decrease of kchem can occur during long-term operation due to changes in the relative cation and oxygen content at the surface.
    Solid State Ionics 06/2011; 191(1):61–67. · 2.05 Impact Factor
  • A. Egger, E. Bucher, W. Sitte
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    ABSTRACT: For the promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode material Nd2NiO4+δ, chemical surface exchange coefficients k chem and chemical diffusion coefficients D chem of oxygen have been determined by conductivity relaxation measurements and compared with results for La0.6Sr0.4CoO3-δ between 575 and 725°C. At 725°C and an oxygen partial pressure of 0.1 bar, k chem and D chem of Nd2NiO4+δ amount to 1 × 10−3 cm s−1 and 2 × 10−6 cm2 s−1, respectively, which are higher than those for La0.6Sr0.4CoO3-δ. However, due to high activation energies, a strong decrease of both kinetic parameters is observed for Nd2NiO4+δ upon temperature reduction. Activation energies of k chem and D chem are lower for La0.6Sr0.4CoO3-δ, leading to faster oxygen exchange compared to Nd2NiO4+δ at 600°C. Electronic conductivities of Nd2NiO4+δ amount to 100–125 S cm−1 while those of La0.6Sr0.4CoO3-δ are between 1600 and 2200 S cm−1 in the investigated temperature and oxygen partial pressure range. Ionic conductivities and surface exchange resistances, which were calculated from the kinetic parameters, further show superior oxygen transport properties of La0.6Sr0.4CoO3-δ compared to Nd2NiO4+δ in pure O2–Ar atmospheres.
    Journal of The Electrochemical Society. 04/2011; 158(5):B573-B579.
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    ABSTRACT: The electrical properties of positive temperature coefficient (PTC) ceramics are expected to strongly correlate with the potential barrier height at grain boundaries, which in turn may be influenced by the grain boundary structure and chemistry. In this study, n-conducting BaTiO3 ceramics co-doped by La and Mn were prepared, and the electrical properties were determined by impedance spectroscopy and dc four-point van der Pauw measurements. Detailed analysis of the grain boundary structure was performed by electron microscopy techniques across different length scales. The study revealed that the randomly oriented polycrystalline microstructure was dominated by large angle grain boundaries, which in the present case were dry although a secondary crystalline and glass phase formed at triple junctions. The relationship between the observed grain boundary atomic structures and electrical properties is briefly discussed.
    Journal of the European Ceramic Society 01/2011; 31(5):763-771. · 2.36 Impact Factor
  • W. Preis, W. Sitte
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    ABSTRACT: Grain boundary regions in n-conducting barium titanate (BaTiO3) are re-oxidized during the cooling process after sintering the ceramics in air. The kinetics of this re-oxidation process is determined by rapid transport of oxygen along the grain boundaries and slow (rate-determining) diffusion of cation vacancies from the grain boundaries into the grains until the diffusion process is frozen-in. Based on numerical calculations of frozen-in diffusion profiles of cation vacancies at grain boundary regions for various cooling rates, a modified Schottky-barrier model is introduced in order to calculate the grain boundary resistivity as a function of temperature from the Curie-point up to 900°C. A change of the activation energy at approximately 500°C is predicted owing to an enrichment of holes in the space charge layers at elevated temperatures. The modeling results are compared with experimental data for BaTiO3-based positive temperature coefficient resistors (PTCRs).
    Journal of Electroceramics 01/2011; 23. · 1.42 Impact Factor
  • Edith Bucher, Werner Sitte
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    ABSTRACT: The long-term degradation of the surface oxygen exchange kinetics of the IT-SOFC cathode material La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) is investigated in dry and wet O2–Ar atmospheres at 600–700°C. Conductivity relaxation measurements indicate high chemical surface exchange coefficients (kchem) in dry O2–Ar atmosphere at 600–800°C with Ea(kchem)∼200kJmol−1. However, during 1kh an exponential decrease of kchem is observed at 600°C in dry atmosphere. After switching to a H2O-containing atmosphere a further step-wise decrease of kchem by a factor of 2 occurs, followed by an approximately linear long-term degradation with a rate of ∼10%kh−1. At 700°C a pronounced decrease of the surface exchange coefficient by 2 orders of magnitude is found after switching from dry to wet atmospheres. Post-test XPS analysis indicates that the deterioration of the catalytic activity of LSCF originates in significant changes of the surface cation composition (especially Sr enrichment) and silicon poisoning.
    Solid State Ionics 01/2011; 192(1):480-482. · 2.05 Impact Factor
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    ABSTRACT: The electrical properties of bulk and grain boundaries of scandia-stabilized zirconia co-doped with yttria and ceria have been determined as a function of temperature (300
    Solid State Ionics 01/2011; 192(1):148-152. · 2.05 Impact Factor
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    ABSTRACT: The degradation of the solid oxide fuel cell cathode material with respect to the oxygen exchange kinetics in - and -containing atmospheres has been investigated at . The material exhibits excellent stability in dry atmosphere over 1000 h. No appreciable degradation has been observed after exposure to a -rich atmosphere, but a decrease in the chemical surface exchange coefficient of oxygen by 1 order of magnitude has been found after 1000 h under moist conditions (30% relative humidity). Post-test X-ray photoelectron spectroscopy depth analysis showed significant changes in the Nd:Ni cation ratio within the topmost 100 nm of the -degraded sample. It is concluded that the degradation mechanism involves hydroxides as intermediate species even if the predominant species in the degraded surface is .
    Journal of The Electrochemical Society. 10/2010; 157(11):B1537-B1541.
  • Source
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    ABSTRACT: The oxygen tracer diffusion coefficient, D*, and the oxygen tracer surface exchange coefficient, k*, were measured in La0.6Sr0.4CoO3−δ over the temperature range of 406–680°C and the oxygen partial pressure range 0.044–1.019bar. Distortions of the near surface regions of the diffusion profiles were observed in several specimens. 18O exchange experiments were modeled by the Finite Element method and the origin of the observed distortions was assigned to exchange of oxygen during sample cooling. The estimated error in the determination of D* and k* was found to be less than 20%. The activation energies of D* and k* were 1.84±0.02eV and 0.76±0.13eV, respectively. D* was found to decrease with oxygen partial pressure following the power law with a power of −0.55±0.10. The effect of Sr doping on the values of D* in Ln1−xSrxCoO3−δ (Ln = La, Sm) perovskites is discussed. The data obtained from isotopic exchange measurements at 500–600°C are in good agreement with the kinetic parameters derived from conductivity relaxation experiments on samples from the same batch.
    Solid State Ionics 01/2010; 181(17):819-826. · 2.05 Impact Factor
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    ABSTRACT: The system Y2O3–Sc2O3–ZrO2 has been extensively studied in the last decade, with the purpose of using it as an electrolyte for solid oxide fuel cells (SOFCs). Scandia stabilized zirconia (SSZ) materials have good ionic conductivity, and yttria, when introduced, stabilizes the SSZ against phase transformation. As cerium is a large ion, it is expected to stabilize the cubic fluorite phase and enhance the conductivity of the scandia zirconia system. The electrical properties of system (CexY0.2−xSc0.6)Zr3.2O8−δ were studied, observing how conductivity changes when cerium is introduced and the amount of yttrium is decreased. Compositions were produced by an innovative sol–gel combustion method in an attempt to obtain a compositionally homogeneous, dense material. This material has the potential to be used as an electrolyte, at intermediate temperatures, for SOFCs. Conductivity in the system (CexY0.2−xSc0.6)Zr3.2O8−δ, has been investigated as a function of partial pressure of oxygen down to 10−24bars, at 700°C. Samples with Ceria content have better conductivity at higher pO2 values. When they are subjected to lower partial pressures of oxygen, there is a clear drop in the conductivity. This drop in the conductivity could be a consequence of the clustering of vacancies, resulting in a decrease of ionic conductivity, as the number of vacancies increases with ceria content. For the samples tested in this system, there is no evidence of significant electronic conductivity. The temperature dependence of conductivity was determined at ambient pressure from 300 to 800°C. From the Arrhenius plot a phase transition is observed to occur between 550 and 600°C. Composition does not have a very significant effect on conductivity values for each temperature. At low temperatures, the dominant contribution to conductivity is due to the poor grain boundary conductivity.
    Solid State Ionics 01/2010; 181(29):1344-1348. · 2.05 Impact Factor
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    ABSTRACT: Chemical surface exchange coefficients k_chem and chemical diffusion coefficients D_chem of Nd2NiO4+δ have been determined by the conductivity relaxation technique at 575{less than or equal to}T/{degree sign}C{less than or equal to}725 between oxygen partial pressures of 0.01 and 0.02 bar. At 700 {degree sign}C values of 1E-4 cm/s and 6E-7 cm2/s were measured for k_chem and D_chem, respectively. A strong decrease upon cooling was observed for both kinetic parameters due to high activation energies. Electronic conductivities range between 100 and 120 S/cm and are found to decrease for increasing temperature in the investigated temperature region of 575{less than or equal to}T/{degree sign}C{less than or equal to}725. Thermogravimetry/mass spectroscopy was performed at 20
    ECS Transactions 09/2009; 25(2).
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    ABSTRACT: The electrical properties of bulk and grain boundaries of CexY0.2-xSc0.6Zr3.2O8-d (0 < x < 0.2) have been investigated as a function of temperature and oxygen partial pressure by application of impedance spectroscopy. All samples have been prepared by means of a sol-gel combustion process. The ionic conductivities of the bulk are in the range from 0.03 to 0.04 S cm-1 at 700{degree sign}C in air. In the case of ceria-doped samples a significant decrease of the ionic conductivities of both bulk and grain boundary regions is observed under reducing conditions [p(O2) < 1E-15 bar, 700{degree sign}C] which may be interpreted in terms of association or clustering of oxygen vacancies. In addition, ageing studies have been performed on scandia-stabilized zirconia, doped with 10 mol% Sc2O3 and 1.0 mol% CeO2, in reducing atmosphere (1%-H2/Ar) at 700{degree sign}C by application of a dc four-point technique according to van der Pauw and ac measurements.
    ECS Transactions 09/2009; 25(2).
  • Source
    Jürgen Fleig, Werner Sitte
    Monatshefte fuer Chemie/Chemical Monthly 01/2009; 140(9):973-974. · 1.63 Impact Factor
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    ABSTRACT: The oxygen nonstoichiometry of Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF5582) was investigated by thermogravimetry resulting in values of 2.40 < (3 − δ ) <2.57 for the equilibrium oxygen content in the range 600 ≤ T/°C ≤ 900 and 1E− 4 < pO2/bar < 0.4. The oxygen exchange kinetics was studied by electrical conductivity relaxation as a function of temperature in the range 550 ≤ T/°C ≤ 725 with chemical diffusion coefficients 1E− 6 < Dchem/cm2 s− 1 < 3E− 5 and surface exchange coefficients 2E− 4 < kchem/cm s− 1 < 3E− 3. The activation energies of the kinetic parameters amount to Ea(Dchem) = 86 ± 8 kJ mol− 1 and Ea(kchem) = 64 ± 12 kJ mol− 1. Self-diffusion and surface exchange coefficients, as well as ionic conductivities are estimated.
    Solid State Ionics 09/2008; 179:1032-1035. · 2.05 Impact Factor
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    ABSTRACT: The oxygen exchange and transport properties of the perovskite-type oxides Ba0.5Sr0.5Co0.8Fe0.2O3 − x (BSCF5582) and Ca0.5Sr0.5Mn0.8Fe0.2O3 − x (CSMF5582) are investigated by oxygen permeation as well as electrical conductivity relaxation experiments. The temperature dependence of the oxygen permeation flux JO2of BSCF5582 in the range 600 ≤ T/°C ≤ 950 shows a transition from a low temperature region with higher activation energy to a high temperature region with lower activation energy. Complementary, the chemical surface exchange coefficient kchem, the chemical diffusion coefficient Dchem and their activation energies are determined from conductivity relaxation experiments in the ranges 600 ≤ T/°C ≤ 750 and 70 < pO2/Pa < 3000.
    Solid State Ionics 05/2008; 179(s 11–12):385–392. · 2.05 Impact Factor
  • Peter Waldner, Werner Sitte
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    ABSTRACT: Gibbs energy modeling of iron–nickel pentlandite has been performed using experimental data of ternary phase equilibria. A three-sublattice approach in the framework of the Compound Energy Formalism is developed to refine a two-sublattice model of pentlandite recently applied within a complete assessment of the Fe–Ni–S system. Experimental data about the iron site fraction on the octahedral sublattice at 523.15K for the composition Fe5Ni4S8 as well as the enthalpy of formation at 298.15K for the composition Fe4.5Ni4.5S8 are predicted satisfactorily by the novel model. New possibilities to interpret experimental phase equilibrium data on complex phase relations with pentlandite are discussed together on the basis of the recent extension of a second high-temperature heazlewoodite phase to a ternary solution phase.
    Journal of Physics and Chemistry of Solids 01/2008; 69(4):923-927. · 1.53 Impact Factor
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    ABSTRACT: The structure of the oxygen-deficient perovskite La{sub 0.4}Sr{sub 0.6}CoO{sub 3-} (=0.29) was investigated by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). Domains between 50 and 250 nm in size were observed in the electron microscope. Weak superstructure reflections were found with both X-ray and electron diffraction. Investigations of these superstructure reflections by selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED) showed that the domains in a crystal are orientated in a 90 deg. relationship. High-resolution transmission electron microscopy (HRTEM) images from the domain boundary also revealed a 90 deg. orientation dependency. Using the symmetry of CBED patterns, the point group 4/mmm was determined. By comparing reflections from the SAED pattern with possible reflections, the space group I4/mmm (No. 139) could be isolated and finally the crystal structure was refined by Rietveld refinement. - Graphical abstract: Two adjacent domains show different orientation of the c-axis of the tetragonal unit cell in the according convergent beam electron diffraction patterns.
    Journal of Solid State Chemistry 01/2008; 181(11):2976-2982. · 2.04 Impact Factor

Publication Stats

188 Citations
158.49 Total Impact Points


  • 2001–2014
    • Montanuniversität Leoben
      • Chair of Physical Chemistry
      Leoben, Styria, Austria
  • 2006
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany
  • 1994–2001
    • Graz University of Technology
      • Institut für Physikalische und Theoretische Chemie
      Graz, Styria, Austria