W. Sitte

Montanuniversität Leoben, Leoben, Styria, Austria

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Publications (119)267.69 Total impact

  • W. Preis · J. Hofer · W. Sitte ·
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    ABSTRACT: The electrical properties of bulk and grain boundaries of donor-doped barium titanate ceramics have been characterized as a function of temperature (50-350 °C) and voltage load (up to 140 V) by application of impedance spectroscopy. Both the grain boundary resistivities and the steepness of the R-T characteristics are diminished significantly with increasing voltage load. While the grain boundary resistances are strongly affected by the applied electric field, the grain boundary capacitance is almost independent of the dc-bias. The non-linearity of the resistivity of n-conducting BaTiO3 has been investigated in detail by impedance spectroscopy as a function of dc-bias and a small ac-voltage signal as well as impedance measurements with high ac-voltage amplitudes (zero bias). The non-linear current response to high ac-voltage amplitudes at low frequencies (0.01 Hz) has been determined experimentally and analyzed by means of fast Fourier transform (FFT) as well as Lissajous analyses. Moreover, a finite element model (FEM) has been developed for the simulation of the ac-current response. The FEM calculations are in close agreement with the experimentally determined data for the variation of the grain boundary resistance with ac-voltage amplitude.
    Journal of Solid State Electrochemistry 08/2015; 19(8). DOI:10.1007/s10008-015-2896-6 · 2.45 Impact Factor
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    ABSTRACT: Long-term degradation effects of combined Cr- and Si-poisoning for the promising intermediate temperature solid oxide fuel cell (IT-SOFC) cathode materials La0.6Sr0.4CoO3 − δ (LSC) and La2NiO4 + δ (LNO) were investigated at 700 °C in dry and humid atmospheres for subsequent periods of 1000 h by using dc-conductivity relaxation measurements. Changes in the chemical composition and morphology of the contaminated sample surfaces were studied by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy and wavelength dispersive X-ray analysis and by atomic force microscopy (AFM). Upon exposure to dry and humid gas flows containing Cr- and Si-species both materials exhibit a strong decrease of the chemical surface exchange coefficient of oxygen by a factor of 110 and 40 for La0.6Sr0.4CoO3 − δ and La2NiO4 + δ, respectively, which can be attributed to the formation of Cr-containing crystallites on the degraded sample surfaces. AFM-measurements reveal faceted crystallites of lateral extensions of up to 2 μm and heights of up to 500 nm on the surface of La0.6Sr0.4CoO3 − δ. XPS-analyses confirm the presence of high levels of Cr and Sr in these crystals, indicating the decomposition of the perovskite phase through SrCrO4-formation. The significantly smaller crystallites that are found on the surface of La2NiO4 + δ also contain Cr, as well as elevated levels of La. For both compounds, silicon is found to spread in small patches across the entire sample surface, as determined by EDXS elemental mapping analysis.
    Solid State Ionics 08/2015; 276:62-71. DOI:10.1016/j.ssi.2015.03.035 · 2.56 Impact Factor
  • A. Egger · N. Schrodl · W. Sitte ·
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    ABSTRACT: La2NiO4 is characterised as potential anode material in solid oxide electrolyser cells (SOECs). Investigations are performed at 800°C and 0.1-0.2 bar oxygen partial pressure over a period of several thousand hours. Trends in chemical surface exchange coefficient and electrical conductivity of a densely sintered sample are identified. In addition, changes in polarisation resistance of a porous anode layer of La2NiO4 on a button cell under constant anodic current bias of - 410 mAcm-2 are recorded. Special emphasis is laid on the impact of Cr-impurities in the surrounding gas phase on the oxygen transport properties and SOEC anode performance. Cr-poisoning is found to depend strongly on the level of humidity in the gas phase as well as on the polarisation conditions.
    ECS Transactions 07/2015; 68(1):3345-3358. DOI:10.1149/06801.3345ecst
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    Martin Perz · Edith Bucher · Werner Sitte · Thomas Griesser ·
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    ABSTRACT: Two synthesis routes for approximately 100 nm thick layers of LaNi0.6Fe0.4O3-δ (LNF64) are presented. Aqueous solutions of acetate or nitrate-precursors were deposited on Ce0.9Gd0.1O2-δ substrates by spin coating. After thermal annealing at 700 °C for 3 h, the thin films were characterized by X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The DC electrical conductivity of the LNF64 thin films was determined by four-point van der Pauw measurements as a function of temperature (20 ≤ T/°C ≤ 400) at 1 × 10− 3 ≤ pO2/bar ≤ 1. At the surface and cross section a nanocrystalline microstructure with grain sizes of approximately 50-100 nm was observed. The DC electrical conductivity of the thin films increases with increasing temperature and increasing pO2. A maximum electrical conductivity of 320 S/cm was found at 400 °C and pO2 = 1 bar which agrees with the literature data for LNF64 bulk samples.
    Solid State Ionics 05/2015; 273:30-34. DOI:10.1016/j.ssi.2014.10.027 · 2.56 Impact Factor
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    Edith Bucher · Christian Gspan · Werner Sitte ·
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    ABSTRACT: The impact of long-term exposure of La0.6Sr0.4CoO3 − δ to SO2-containing atmospheres was investigated. In-situ dc-conductivity relaxation measurements showed a decrease in the chemical surface exchange coefficient of oxygen (kchem) during the course of 1000 h in an atmosphere with either a few ppb or 2 ppm SO2 at 700 °C. Post-test analyses by scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and analytical transmission electron microscopy indicated that SrSO4 crystals with diameters of 100 nm-1 μm are formed during the degradation, in addition to a nanocrystalline 100-300 nm thick multi-phase layer and LaCoO3 − δ at grain boundaries. In order to regenerate the degraded sample, a thermal treatment was applied. It could be shown that a partial re-activation of the degraded specimen takes place at 750-850 °C even in an atmosphere with 2 ppm SO2. This regeneration is ascribed to the in-situ formation of catalytically active LaCoO3 − δ nanoparticles at the surface. However, a subsequent degradation follows independently of the SO2 content of the atmosphere when the sample is kept for 1000 h at 850-900 °C. Post-test analyses indicate that this effect is due to a strong grain growth of the LaCoO3 − δ nanocrystals which leads to a loss in the catalytic activity.
    Solid State Ionics 04/2015; 272:112-120. DOI:10.1016/j.ssi.2015.01.009 · 2.56 Impact Factor
  • Andreas Egger · Nina Schrödl · Werner Sitte ·
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    ABSTRACT: La2NiO4+δ is characterised as an example for a potential anode material for high-temperature solid oxide electrolyser cells (SOECs). Short-term characterisation is performed from 700°C to 850°C between 0.01 and 1 bar oxygen partial pressure (pO2) on asymmetrical cells using Ce0.9Gd0.1O2-δ as electrolyte. Long-term degradation studies over more than 3000 hours are conducted at 800°C and 0.2 bar pO2 in dry and humid atmospheres with and without a Cr-source placed in close vicinity to the cell. The SOEC anode performance is investigated by current-voltage curves combined with impedance spectroscopy. Current densities of up to -410 mAcm-2 are applied in current-voltage measurements and during long-term degradation studies. A total increase in anode resistance by 350% is observed over the course of the degradation measurements in increasingly harsh environment. Post-test analyses by SEM/EDX on a polished cross section of the cell show the presence of several contaminants in the electrode structure. However, chromium has not been identified by EDX even after prolonged exposure to Cr-sources in humid atmospheres, which is attributed to the anodic polarisation of the electrode. Electrode delamination appears to be the main factor for the strong loss in performance.
    Faraday Discussions 03/2015; DOI:10.1039/C5FD00021A · 4.61 Impact Factor
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    ABSTRACT: The oxygen tracer diffusion coefficient, Db⁎, and the oxygen tracer surface exchange coefficient, k, were measured in Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF5582) over the temperature range of 310–800 °C and the oxygen partial pressure range of 1.3 × 10− 3–0.21 bar. Several measurement techniques were used: isotope exchange followed by depth profiling (IEDP) within individual single grains or line scanning (IELS) along the sample cross-section and gas-phase analysis (GPA). Surface exchange kinetics was initially found to be slow and presumably inhibited by the formation of a passivating layer on the sample surface. High temperature pre-anneals (900–950 °C) changed the nature of this layer and enhanced surface exchange. Fast bulk oxygen diffusion and surface exchange kinetics were observed after high temperature pre-anneals within the temperature range studied. The activation energies for 18O tracer diffusion and surface exchange at 0.21 bar were 0.72 ± 0.05 and 1.10 ± 0.15 eV, respectively. The tracer diffusion coefficient showed weak dependence upon oxygen partial pressure, whereas the surface exchange coefficient exhibited strong oxygen partial pressure dependence. The microstructure of the samples (the porosity and grain size) had a profound effect on the measured tracer diffusion coefficient.
    Solid State Ionics 12/2014; 268:102–109. DOI:10.1016/j.ssi.2014.09.031 · 2.56 Impact Factor
  • Wolfgang Preis · Werner Sitte ·
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    ABSTRACT: A double Schottky barrier model suitable for the description of the grain boundary resistivity of n-conducting BaTiO3 ceramics has been modified by taking account of frozen-in diffusion profiles of cation vacancies at the grain boundaries which are formed during the cooling process after sintering. The space charge model has been extended in order to predict the electrical properties of PTC ceramics (positive temperature coefficient of resistivity) under voltage load for maximum dc-bias values around 03 - 0.5 V/grain boundary. The effect of the voltage drop across the Schottky barrier on the concentration profiles of electrons in the depletion zone as well as the space charge potential has been elaborated in detail.
    Solid State Ionics 09/2014; 262:486-489. DOI:10.1016/j.ssi.2013.10.062 · 2.56 Impact Factor
  • 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. DOI:10.1016/j.ssi.2014.01.038 · 2.56 Impact Factor
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    Edith Bucher · Christian Gspan · Ferdinand Hofer · Werner Sitte ·
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    ABSTRACT: The oxygen exchange kinetics of La0.6Sr0.4CoO3-δ was investigated at 700 °C and p(O2) = 0.10 bar by in-situ conductivity relaxation measurements. The experiments were performed for 1000 h in an O2–Ar atmosphere with and without 50 ppm SO2. For post-test analyses, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDXS) and analytic transmission electron microscopy (TEM) were applied. Elemental depth profiles were obtained by X-ray photoelectron spectroscopy (XPS). The decrease in the chemical oxygen exchange coefficient kchem by a factor of 10 during the course of 1000 h indicated that an accumulation of traces of sulphur (estimated to the range of ppb) in the form of SrSO4 can lead to a significant degradation even in nominally pure O2–Ar. The exposure to 50 ppm of SO2 for 1000 h caused a severe degradation in kchem by a factor of 3000. Post-test analyses showed that the perovskite phase La0.6Sr0.4CoO3-δ is decomposed into SrSO4 and La2O2SO4 within the first 500 nm of the surface, and a nano-crystalline Co-rich phase, which is located in the depth range of 500–1400 nm.
    Solid State Ionics 05/2013; 238:15–23. DOI:10.1016/j.ssi.2013.03.007 · 2.56 Impact Factor
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    Edith Bucher · Christian Gspan · Ferdinand Hofer · Werner Sitte ·
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    ABSTRACT: A degraded sample of the solid oxide fuel cell cathode material La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) is investigated by analytical transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS). In the present study TEM is applied in order to analyse the relevant surface-near regions of an LSCF sample which was pre-treated for 1000 h in a dry O2–Ar atmosphere and an additional 1000 h in a humidified atmosphere in the vicinity of a silicon source. The results show that Si contamination occurs in an approximately 20 nm thick layer (with local variations from 4 to 35 nm) at the surface of the degraded LSCF sample. In addition, TEM gives evidence of isolated nanocrystals of SrSO4 with diameters in the range of 200–500 nm. A local decomposition of the perovskite phase is found within depths of 500–600 nm from the surface. The decomposition products are ternary oxides which contain either Sr–La–O or Co–Fe–O.
    Solid State Ionics 01/2013; 230:7-11. DOI:10.1016/j.ssi.2012.08.013 · 2.56 Impact Factor
  • Jörg Waldhäusl · Wolfgang Preis · Werner Sitte ·
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    ABSTRACT: The aim of this work was the determination of the total, the electronic and the ionic conductivity of gadolinia-doped ceria (GDC), Gd0.1Ce0.9O1.95-δ, as a function of oxygen partial pressure at 700 °C. Two complementary techniques (impedance spectroscopy and a galvanostatic dc polarization method) were applied to samples with different thicknesses. From the dc measurements only the electronic conductivity could be extracted, while impedance spectroscopy delivered data for both the electronic and ionic conductivity of the mixed ionic-electronic conductor under reducing conditions, p(O2) < 10− 15 bar. Furthermore, the two methods enabledthe determination of the chemical diffusion coefficient in reducing atmospheres with typical values around 10− 4 cm2 s− 1 at 700 °C.
    Solid State Ionics 10/2012; 225:453–456. DOI:10.1016/j.ssi.2012.01.016 · 2.56 Impact Factor
  • Andreas Egger · Edith Bucher · Min Yang · Werner Sitte ·
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    ABSTRACT: La0.5Sr0.5CoO3 − δ and La0.6Sr0.4CoO3 − δ are characterized with respect to application as cathode materials in intermediate temperature solid oxide fuel cells (IT-SOFCs). Surface exchange and transport parameters of oxygen are determined by the conductivity relaxation technique between 525 °C and 725 °C at oxygen partial pressures of 0.1, 0.01 and 0.001 bar. Electrical conductivities of both compounds range between 1000 and 2400 S cm- 1 and are slightly higher for La0.6Sr0.4CoO3 − δ. However, La0.5Sr0.5CoO3 − δ shows superior performance with regard to oxygen diffusion, ionic conductivity and oxygen surface exchange within the investigated range of temperatures and oxygen partial pressures. At 725 °C the chemical surface exchange coefficient of oxygen is 2 × 10- 3 cm s- 1 for La0.5Sr0.5CoO3 − δ and 4 × 10- 4 cm s- 1 for La0.6Sr0.4CoO3 − δ. At lower oxygen partial pressures a strong decrease in the surface exchange coefficient is observed and diffusion data cannot be obtained from relaxation measurements. Oxygen vacancy diffusion coefficients are similar for both compounds and range between 10- 6 and 10- 8 cm2 s- 1 with activation energies around 100 kJmol- 1. At 725 °C and 0.1 bar oxygen partial pressure, the ionic conductivity of La0.5Sr0.5CoO3 − δ is 1 × 10- 2 S cm- 1 with an activation energy of 118 kJmol- 1. The results show that both compositions meet the requirements for the application as IT-SOFC cathodes in the short-term range.
    Solid State Ionics 10/2012; 225:55–60. DOI:10.1016/j.ssi.2012.02.050 · 2.56 Impact Factor
  • Peter Waldner · Werner Sitte ·
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    ABSTRACT: Gibbs energy modeling of the intermediate copper–sulfur phase digenite is performed at a total pressure of 1 bar using experimental sulfur activity data from the literature. A two-sublattice approach used in the framework of the compound energy formalism takes into account structural considerations. A limited set of optimized model quantities is obtained, which allows reproduction of all sulfur activity data available in the literature over a wide homogeneity and temperature range of the phase. Phase equilibria calculations applying the modeled Gibbs energy for digenite give very satisfactory phase relations in accordance with experimental data. A partial phase diagram within the composition range 0.0 ≤ x S ≤ 0.7 up to elevated temperatures is calculated. Graphical Abstract
    Monatshefte fuer Chemie/Chemical Monthly 09/2012; 143(9). DOI:10.1007/s00706-012-0751-3 · 1.22 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). DOI:10.1002/fuce.201200021 · 2.08 Impact Factor
  • Edith Bucher · Werner Sitte · Frederik Klauser · Erminald Bertel ·
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    ABSTRACT: In-situ investigations of the long-term degradation of the oxygen exchange kinetics of the promising SOFC cathode material La0.6Sr0.4CoO3 − δ are performed at 600 °C in dry and in humid O2–Ar atmospheres during subsequent time intervals of 1000 h. Extensive pre- and post-test analyses using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) are performed in order to assess the changes in the relevant surface-near zones with respect to elemental composition and morphology. The decrease of the chemical surface exchange coefficient of oxygen kchem during the exposure of the sample to a dry atmosphere at 600 °C for 1000 h is correlated to the formation of a 2 nm thick region at the surface, which is enriched with Sr and O, as well as to modifications of the surface morphology. The underlying mechanism cannot be unambiguously explained from this work. A possible interpretation would be the decomposition of La0.6Sr0.4CoO3 − δ at the surface into a Sr-enriched perovskite La1 − xSrxCoO3 − δ (0.4 < × < 1) and La2O3. During the treatment in humid atmospheres at 600 °C, a significant decrease in the kinetic parameters for oxygen exchange occurs. This is ascribed to the presence of a humid atmosphere together with a Si-source. Poisoning of the surface with silicon and a strong enrichment with Sr and O at depths of 10–90 nm, depending on the exposure time are evident in the XPS elementary depth profiles. It is assumed that the reaction of Si with Sr is a strong driving force for the accumulation of Sr at the surface. When, instead of distilled water, a saturated aqueous NaCl solution is used for the humidification of the carrier gas no significant increase in the degradation rate is observed. The degradation is also accompanied by a re-structuring of the surface morphology. An increase in surface roughness by about one order of magnitude due to the progressive growth of crystallites up to 200 nm in diameter is detected during the course of 5000 h. The targeted technical application of La0.6Sr0.4CoO3 − δ in IT-SOFCs operating at temperatures of around 600 °C involves ambient (humid) air as an oxidant and the presence of Si-sources (glass seals, thermal insulation materials). Under these circumstances, the mechanisms shown in this study can induce a significant decrease of the activity for oxygen exchange over time. The use of pre-dried air is suggested in order to increase the life-time of the SOFC cathode, since a significant transport of Si via the gas phase occurs almost exclusively in the presence of H2O(g). However, a time-dependent decrease of the performance of these cathodes can occur during several 1000 h periods of operation, even under dry conditions.
    Solid State Ionics 02/2012; 208:43–51. DOI:10.1016/j.ssi.2011.12.005 · 2.56 Impact Factor
  • E. Bucher · M. Yang · W. Sitte ·
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    ABSTRACT: The long-term stability of the oxygen exchange properties of the IT-SOFC cathode materials La0.6Sr0.4CoO3-delta and La0.58Sr0.4Co0.2Fe0.8O3-delta vs. chromium poisoning is investigated at 600 degrees C by in-situ measurements of the chemical surface exchange coefficient of oxygen k(chem) using the dc-conductivity relaxation method. The as-prepared samples show high values of k(chem), which decrease moderately during the treatment in a dry atmosphere during 1000 h without and with the presence of a Cr-source. Humidification of the carrier gas results in a strong decrease in k(chem) of both materials. Post-test surface analysis of the degraded specimens by X-ray photoelectron spectroscopy identifies Cr- and Si-poisoning as well as Sr- and O-enrichment in depth ranges of about 100 nm as the origin of the degradation in a humid atmosphere. These results can be correlated with the strong enhancement of the gas phase transport of volatile Cr- and Si-species in the presence of H2O(g). It can be concluded that La0.6Sr0.4CoO3-delta (and to a lesser extent La0.58Sr0.4Co0.2Fe0.8O3-delta) cathodes retain a relatively high stability of the chemical surface exchange coefficients of oxygen at 600 degrees C in dry atmospheres, even in the vicinity of Cr- and Si-sources. However, significant performance degradation of (La, Sr)(Co, Fe)O3-delta-based cathodes has to be expected in humid atmospheres.
    Journal of The Electrochemical Society 01/2012; 159(5):B592. DOI:10.1149/2.027206jes · 3.27 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 10/2011; 23(2). DOI:10.1007/s10832-009-9577-8 · 1.74 Impact Factor
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    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. DOI:10.1016/j.jpowsour.2010.10.064 · 6.22 Impact Factor
  • T. Frömling · J. Hou · W. Preis · W. Sitte · H. Hutter · J. Fleig ·
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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. DOI:10.1063/1.3626054 · 2.18 Impact Factor

Publication Stats

1k Citations
267.69 Total Impact Points


  • 2001-2015
    • Montanuniversität Leoben
      • • Chair of Physical Chemistry
      • • Department General, Analytical and Physical Chemistry
      Leoben, Styria, Austria
  • 2006
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 1988-2001
    • Graz University of Technology
      • Institute of Physical and Theoretical Chemistry
      Gratz, Styria, Austria
  • 1985
    • Max Planck Institute for Solid State Research
      Stuttgart, Baden-Württemberg, Germany