Strain-induced structure and oxygen transport interactions in epitaxial La 0.6 Sr 0.4 CoO 3− δ thin films

Abstract: The possibility to control oxygen transport in one of the most promising solid oxide fuel cell cathode materials, La0.6Sr0.4CoO3−δ, by controlling lattice strain raises questions regarding the contribution of atomic scale effects. Here, high-resolution transmission electron microscopy revealed the different atomic structures in La0.6Sr0.4CoO3−δ thin films grown under tensile and compressive strain conditions. The atomic structure of the tensile-strained film indicated significant local concentration of the oxygen vacancies, with the average value of the oxygen non-stoichiometry being much larger than for the compressive-strained film. In addition to the vacancy concentration differences that are measured by isotope exchange depth profiling, significant vacancy ordering was found in tensile-strained films. This understanding might be useful for tuning the atomic structure of La0.6Sr0.4CoO3−δ thin films to optimize cathode performance

Author Correction: Strain-induced structure and oxygen transport interactions in epitaxial La 0.6 Sr 0.4 CoO 3−δ thin films

An amendment to this paper has been published and can be accessed via a link at the top of the paper

Elektrochemische Charakterisierung von (La,Sr,Ca)2Co2-xMnxO6-δ als Kathodenmaterial in Festoxidbrennstoffzellen

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersIntermediate temperature fuel cells, with a working temperature between 500 °C and 700 °C, suffer from the slow reduction kinetics of oxygen at the cathode side which is responsible for most of the resistive losses. The aim of this work was the investigation of the electrical properties, oxygen exchange kinetics and diffusion processes of 18O tracer ions in advanced cathode materials, such as perovskites with the generalized structure (La,Ca,Sr)2Co2-xMnxO6-¿. First, targets materials were synthesized by a Pechini-type method. Subsequently thin films of different compositions were grown by pulsed laser deposition (PLD) under various p(O2) on three different single crystal substrates, since the background pressure in the PLD chamber and the used substrate have a massive impact on the thin film structure and properties. For ionic conductivity determination, some of the samples were microstcructured by UV-lithography and impedance spectroscopy was performed. The electrical properties of bulk and thin film samples were investigated by van der Pauw method. Keeping the Co/Mn ratio 1:1, a higher conductivity for Ca and Sr doped samples is observed due to p-type doping. Moreover, by changing the relative amount of Co without A-site doping conductivity can also be enhanced. All produced and investigated bulk samples possess a lower electrical conductivity and higher activation energy than La0.8Sr0.2MnO3 (LSM). For applicable cathode materials, a good oxygen diffusion is crucial. Hence, La2CoMnO6 thin film were investigated by performing 18O exchange experiments and ToF-SIMS depth profiling. With these techniques, the diffusion coefficient D* and the surface exchange coefficient k* were determined. In La2CoMnO6 samples, grain boundary diffusion plays a vital role. Diffusion coefficients of the grain boundaries are one to two orders of magnitude higher than the bulk diffusion coefficients and stay constant when p(O2) in the PLD-chamber during thin film growth is increased. Additionally, a high p(O2) leads to an additional diffusion process and is most likely related to the dislocation concentration. For the determination of the ionic area specific resistance of La2CoMnO6 impedance spectroscopy was carried out. Activation energies below 550 °C indicate a triple phase boundary path of oxygen reduction. If the samples were heated for the first time above 500 °C irreversible changes took place and the activation energies and resistivity rose tremendously in further measurements.8

Oxygen stoichiometry changes in SrTiO3 under UV illumination

Arbeit an der Bibliothek noch nicht eingelangt - Daten nicht geprüftAbweichender Titel nach Übersetzung der Verfasserin/des VerfassersIn den kommenden Jahren wird der Bedarf an kleinen, vom Stromnetz entkoppelten Geräten als Teil des “Internet of Things“ stetig zunehmen. Die Bereitstellung von elektrischer Energie für diese Geräte ist eine große Herausforderung. Unter extremen Bedingungen könnten hier hochtemperatur-photovoltaische Zellen von Nutzen sein. Für solche Zellen ist SrTiO3 ein vielversprechendes Absorbermaterial. Allerdings ergeben sich damit auch viele Fragen z.B. bezüglich der Stabilität und der Beleuchtung und der Eigenschaft von möglicherweise einsetzbaren Dünnfilmen. Daher war das Ziel der vorliegenden Arbeit, die Auswirkungen des UV-Lichtes auf die Defektchemie von SrTiO3 Einkristallen und Dünnfilmen unter erhöhter Temperatur zu untersuchen. Darüber hinaus wurde das pseudo-intrinsische Verhalten von SrTiO3 Dünnfilmen untersucht.In Fe dotiertem SrTiO3 führte die Beleuchtung mit UV-Licht zu einem beschleunigten Sauerstoffeinbau. Dadurch wurden Sauerstoffleerstellen gefüllt und Fe3+ zu Fe4+ oxidiert. Dies führte zu einer Farbänderung von bräunlich-transparent zu schwarz. Die Schwärzung war bei Raumtemperatur irreversibel und die ursprüngliche Färbung konnte erst mit Hilfe einer Auslagerung bei 700 °C an Luft wieder erreicht werden. Die beschriebenen Veränderungen wurden mit UV/Vis-Spektroskopie zeitabhängig verfolgt. Dies ermöglichte die Bestimmung von Diffusionskoeffizienten. Zusätzlich zu diesem hochtemperatur-photochromen Effekt führte das Beleuchten der Proben mit UV-Licht zu einer signifikanten Zunahme der in-plane Leitfähigkeit, welche mittels Impedanzspektroskopie und van der Pauw Messungen bestimmt wurde. Eine Verbesserung der Leitfähigkeit an Luft um mehr als das Fünffache konnte auf eine entsprechende Erhöhung der Lochkonzentration zurückgeführt werden. Die entsprechende Verschiebung des chemischen Potentials des Sauerstoffes entsprach einem Sauerstoffpartial-druck von 104 bar im Material.In undotierten SrTiO3 Einkristallen kam es durch die UV-Beleuchtung an Luft zwischen 280 und 450 °C sogar zu einer Verbesserung der in-plane Leitfähigkeit um mehrere Größenordnungen. Zusätzlich führte solch eine Beleuchtung zu einer Verschiebung im Röntgenabsorptions-spektrum der Ti L2,3-Kante um +0.1 eV. Dies spricht für eine Oxidation unter UV-Licht. Im nächsten Schritt wurde der Sauerstoffpartialdruck drastisch abgesenkt, sodass SrTiO3 von einem p- zu einem n-Halbleiter wurde. In trockener Wasserstoffatmosphäre und bei 350 °C reduzierte sich die Leitfähigkeit von SrTiO3 Einkristallen nach einer Beleuchtung mit UV-Licht um über 50 %. Die entsprechende Verringerung der Elektronenkonzentration war verursacht durch Sauerstoffeinbau aufgrund von photokatalytischer Wasserspaltung. Während der UV-Beleuchtung konnten zwei Regionen mit unterschiedlichen Eigenschaften identifiziert werden. In einer sehr dünnen direkt an der Oberfläche gelegenen Zone wurde das UV-Licht absorbiert und Elektron-Loch-Paare gebildet; dadurch stieg die Photo-Leitfähigkeit stark an. In der viel dickeren darunter befindlichen Schicht nahm die Leitfähigkeit durch den Sauerstoffeinbau jedoch ab.In einem nächsten Schritt wurden SrTiO3 Dünnfilme untersucht, indem die Massezunahme durch den Einbau von Sauerstoff mit Hilfe von GaPO4 Mikrowaagen bestimmt bzw. nachgewiesen wurde. Ein neuer Messstand wurde zu diesem Zweck in der Arbeitsgruppe gebaut und mittels Gasaustauschexperimenten getestet. Dabei wurde zwischen O2 und N2 Atmosphären gewechselt und die Masseänderungen von La0.6Sr0.4CoO3, undotierten und Fe dotierten SrTiO3 Dünnfilmen gemessen. Die Veränderungen lagen allesamt im ng-Bereich und wurden mittels Literaturwerten bzw. durch defektchemische Rechnungen plausibilisiert.Schließlich wurde mit Impedanzspektroskopie die pseudo-intrinsische Leitfähigkeit von undotierten und mit unterschiedlichen Metallen (Al, Ni, Fe) dotierten SrTiO3 Dünnfilmen untersucht. Diese Messungen wurden an Luft bei Temperaturen zwischen ca. 300 °C und 700 °C durchgeführt. Alle Schichten wurden mittels Laserablation hergestellt und wuchsen epitaktisch auf SrTiO3 oder Nb dotierten SrTiO3 Einkristallen. Die Abscheidung selbst erfolgte von stöchiometrischen, polykristallinen Pellets. Die Proben wurden einer ausgedehnten Analyse unterzogen; so wurde „Positron annihilation lifetime“ Spektroskopie, „reciprocal space maps“, und induktiv gekoppelte Plasmaemissionsspektroskopie durchgeführt und Sr Leerstellen als vorherrschende Punktdefekte identifiziert. Im weiteren Verlauf diente Fe dotiertes SrTiO3 als Beispielverbindung. Fe konnte sowohl am A- als auch am B-Platz mit Hilfe von Röntgenabsorptionsspektroskopie und „X-ray standing wave“ Messungen nachgewiesen werden. Somit wirkt dieses Dotierelement sowohl als Akzeptor- als auch Donordotierung. Zusätzlich wurde ein Modell entwickelt, mit welchem das pseudo-intrinsische Verhalten der erwähnten Schichten durch eine Fixierung des Fermi-Niveaus in die Mitte der Bandlücke erklärt wird. Verantwortlich hierfür ist das Zusammenspiel von Sr Leerstellen und Ti Substitutionsatomen auf einem Sr Gitterplatz.SrTiO3 is a promising material for high temperature photovoltaic cells, which might be used for powering small off-grid devices such as sensors. However, questions arose regarding the stability of SrTiO3 under UV irradiation and properties of SrTiO3 thin films, possibly usable in such solar cells. The main aim of this thesis was to investigate the impact of UV irradiation on the defect chemistry of SrTiO3 single crystals and thin films at elevated temperatures. Moreover, the pseudo-intrinsic behavior of SrTiO3 thin films was examined.In Fe-doped SrTiO3, UV irradiation results in an enhanced oxygen incorporation rate which leads to a filling of oxygen vacancies and to an oxidation of Fe3+ to Fe4+. The corresponding change in the oxygen stoichiometry causes a color change of the samples (darkening), which was found to be irreversible at room temperature. The original transparent/brownish color can only be restored after annealing at 700 °C in air. The time dependence of these changes was monitored by UV/Vis spectroscopy which allowed determination of an oxygen chemical diffusion coefficient. In addition, to this high temperature photochromic effect, UV irradiation results in a significant increase of the in-plane bulk conductivity measured by impedance spectroscopy and van der Paw experiments. An enhancement by more than a factor of five was revealed, due to an increased electron hole concentration under UV illumination in air. The corresponding shift of the oxygen chemical potential was estimated, yielding an equivalent of 104 bar.The increase of in-plane conductivity of undoped SrTiO3 single crystals was even more pronounced, with increases by several orders of magnitude upon UV irradiation between 280 and 450 °C. After UV illumination the X-ray absorption spectrum of the Ti L2,3-edge was shifted by +0.1 eV, again suggesting an oxidation due to UV light. In a next step, the was drastically reduced, and SrTiO3 became an n-type semiconductor. In dry H2 at 350 °C the bulk in-plane conductivity dropped to 40 % of the initial (dark) value after UV irradiation. Oxygen was still incorporated under UV irradiation, thus increasing the oxygen chemical potential. This reduces the electron (now main electronic charge carrier) concentration and thus the conductivity. Here, however, H2O rather than O2 was photocatalytically split. Interestingly, after UV irradiation different characteristics could be identified in two regions of the SrTiO3 single crystals. One region was located near the surface, where the UV light was absorbed, and electron-hole pairs led to a strongly increased photoconductivity. The conductivity of the second much thicker layer beneath the absorption zone was lowered due to the chemical potential shift.SrTiO3 thin films were investigated as well and at first, the mass gain during UV exposure due to the oxygen incorporation was probed by GaPO4 microbalances. A measurement set-up was built and verified by gas exchange experiments. Thereby the atmosphere was changed back and forth from O2 to N2 and the mass changes of La0.6Sr0.4CoO3, undoped and Fe-doped SrTiO3 specimens were monitored. The measured changes were in the ng range and in good accordance with oxygen vacancy concentration changes obtained from defect chemical calculations and literature data.Finally, ultra-low (pseudo-intrinsic) conductivities of undoped and metal doped (Al, Ni, Fe) SrTiO3 thin films were investigated by impedance spectroscopy between approx. 300-750 °C in air. They were epitaxially grown, by pulsed laser deposition from stochiometric targets. Positron annihilation lifetime spectroscopy, reciprocal space maps, and inductively-coupled plasma optical emission spectroscopy revealed Sr vacancies as predominant point defects. Fe-doped SrTiO3 was used as model compound and further investigated by X-ray absorption spectroscopy and X-ray standing wave measurements. Fe was found to be located on both the A- and the B-site. Thus, the dopant seems to act as donor or acceptor depending on the site occupation. The pseudo-intrinsic behavior was explained by a model suggesting that the Fermi level is pinned to the band gap center by the interplay of Sr vacancies and Ti antisite defects both caused by a lack of Sr in the thin films.14

Strain-induced structure and oxygen transport interactions in epitaxial La0.6Sr0.4CoO3−δ thin films

AbstractThe possibility to control oxygen transport in one of the most promising solid oxide fuel cell cathode materials, La0.6Sr0.4CoO3−δ, by controlling lattice strain raises questions regarding the contribution of atomic scale effects. Here, high-resolution transmission electron microscopy revealed the different atomic structures in La0.6Sr0.4CoO3−δ thin films grown under tensile and compressive strain conditions. The atomic structure of the tensile-strained film indicated significant local concentration of the oxygen vacancies, with the average value of the oxygen non-stoichiometry being much larger than for the compressive-strained film. In addition to the vacancy concentration differences that are measured by isotope exchange depth profiling, significant vacancy ordering was found in tensile-strained films. This understanding might be useful for tuning the atomic structure of La0.6Sr0.4CoO3−δ thin films to optimize cathode performance.</jats:p

Quasi-Simultaneous In-Line Flue Gas Monitoring of NO and NO₂ Emissions at a Caloric Power Plant Employing Mid-IR Laser Spectroscopy

Two pulsed thermoelectrically cooled mid-infrared distributed feedback quantum cascade lasers (QCLs) were used for the quasi-simultaneous in-line determination of NO and NO₂ at the caloric power plant Dürnrohr (Austria). The QCL beams were combined using a bifurcated hollow fiber, sent through the flue tube (inside diameter: 5.5 m), reflected by a retro-reflector and recorded using a fast thermoelectrically cooled mercury–cadmium–telluride detector. The thermal chirp during 300 ns pulses was about 1.2 cm^–1 and allowed scanning of rotational vibrational doublets of the analytes. On the basis of the thermal chirp and the temporal resolution of data acquisition, a spectral resolution of approximately 0.02 cm^–1 was achieved. The recorded rotational vibrational absorption lines were centered at 1900 cm^–1 for NO and 1630 cm^–1 for NO₂. Despite water content in the range of 152–235 g/m³ and an average particle load of 15.8 mg/m³ in the flue gas, in-line measurements were possible achieving limits of detection of 73 ppb for NO and 91 ppb for NO₂ while optimizing for a single analyte. Quasi-simultaneous measurements resulted in limits of detection of 219 ppb for NO and 164 ppb for NO₂, respectively. Influences of temperature and pressure on the data evaluation are discussed, and results are compared to an established reference method based on the extractive measurements presented

Unravelling the Origin of Ultra-Low Conductivity in SrTiO3 Thin Films: Sr Vacancies and Ti on A-Sites Cause Fermi Level Pinning

Different SrTiO3 thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO3 films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO3 films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (Fe-Sr and Ti-Sr) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably TiSr center dot center dot$${\rm{Ti}}_{{\rm{Sr}}}{ \bullet \bullet }$$) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping.ISSN:1616-3028ISSN:1616-301

Unravelling the Origin of Ultra‐Low Conductivity in SrTiO3_3 Thin Films: Sr Vacancies and Ti on A‐Sites Cause Fermi Level Pinning

Different SrTiO$_3$ thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO$_3$ films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO$_3$ films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (Fe$_{Sr}$ and Ti$_{Sr}$) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably Ti$_{Sr}^{\bullet\bullet}$) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping