The interactions between different components of solid oxide cells (SOCs) are critical issues for achieving the tens of thousands of hour's goal for long-term performance stability and lifetime. The interactions between the ceramic electrolyte, porous ceramic air electrode, and metallic interconnect materials — including solid state interfacial reactions and vaporization/deposition of some volatile elements — have been investigated in the simulated SOC operating environment. The interactions demonstrate the material degradation mechanisms of the cell components and the effects of different factors such as chemical composition and microstructure of the materials, as well as atmosphere and current load on the air electrode side. In the aspect of materials, this work contributes to the degradation mechanism on the air electrode side and provides practical material design criteria for long-term SOC operation. In this research, an yttria-stabilized zirconia electrolyte (YSZ)/strontium-doped lanthanum manganite electrode (LSM)/AISI 441 stainless steel interconnect tri-layer structure has been fabricated in order to simulate the air electrode working environment of a real cell. The tri-layer samples have been treated in dry/moist air atmospheres at 800°C for up to 500 h. The LSM air electrode shows slight grain growth, but the growth is less in moist atmospheres. The amount of Cr deposition on the LSM surface is slightly more for the samples thermally treated in the moist atmospheres. At the YSZ/LSM interface, La enrichment is significant while Mn depletion occurs. The Cr deposition at the YSZ/LSM interface is observed. The stoichiometry of the air electrode is an important factor for the interactions. The air electrode composition has been varied by changing the x value in (La0.8Sr0.2)xMnO₃ from 0.95 to 1.05 (LSM95, LSM100, and LSM105). The enrichment of La at the YSZ/LSM interface inhibits the Cr deposition. The mechanisms of Cr poisoning and LSM elemental surface segregation are discussed. A 200 mA·cm-2 current load have been applied on the simulated cells. Mn is a key element for Cr deposition under polarization. Excessive Mn in the LSM lessens the formation of La-containing phases at the YSZ/LSM interface and accelerates Cr deposition. Deficient Mn in LSM leads to extensive interfacial reaction with YSZ forming more La-containing phase and inhibiting Cr deposition. / Ph. D.

Complex oxides are an intriguing field of solid-state research, as they can exhibit a wide variety of functional properties, such as ferroelasticity, ferroelectricity, ferro- and antiferromagnetism or an even more complicated type of magnetic ordering, the combination or interaction of those ferroic properties (multiferroicity), high spin polarization, or high-temperature superconductivity. Thus they are prospective candidates for future materials in microelectronics. It is a matter of fact that the performance of such oxide-based devices depends mainly on transport properties, which in turn depend on the distribution and density of intrinsic or extrinsic electronic interface states across the device structure. The present thesis focuses on the identification and characterization of such electronic properties by two different photoassisted spectroscopy techniques: surface photovoltage spectroscopy and photoelectron spectroscopy. This work especially deals with perovskite oxides, namely with the model perovskite strontium titanate (SrTiO3) as a substrate and three differently doped lanthanum manganite thin films (10-15 nm thickness) grown by pulsed laser deposition (PLD) on the SrTiO3 substrate(La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). The first part aims at the identification of electronic surface and interface states at the free SrTiO3 surface as well as at the three different lanthanum manganite/SrTiO3 interfaces. For that purpose three different experimental realizations of the surface photovoltage spectroscopy technique were implemented and employed: photoelectron spectroscopy under additional optical excitation, the capacitive detection of the photoinduced displacement current in a parallel-plate capacitor geometry under modulated optical excitation, and the classical Kelvin probe technique. The methods are evaluated comparatively with respect to their suitability to analyze the given oxidic interfaces. The main result of this first part is a map of the energetic positions and relaxation time constants of the surface states at the SrTiO3 surface as well as of the interface states at the lanthanum manganite/SrTiO3 interfaces within the SrTiO3 bandgap. The interface states were classified into film- and substrate-induced states and it could be demonstrated that an appropriate annealing procedure can dramatically decrease their densities. The second part tackles the problem of the manganese valence and the doping type of di- and tetravalent-ion-doped LaMnO3. The question whether the insulating parent compound LaMnO3 becomes an electron-doped semiconductor after doping with tetravalent cations such as Ce4+ - which would be in analogy to the well-established hole doping after partial substitution of La3+ by divalent cations such as Sr2+ or Ca2+ - has been discussed controversially in the literature so far. Due to the physics of the manganite crystal lattice the question can also be formulated in a different way: Can part of the manganese ions be driven from the Mn3+ state towards the Mn2+ state without any crystal instabilities or phase separation phenomena? In order to contribute to the clarification of this question, an extensive X-ray- and UV-photoelectron spectroscopy (XPS/UPS) investigation was performed. The three differently doped lanthanum manganite thin films were comparatively studied considering the exchange splitting of the Mn 3s core level line, which is a linear function of the Mn valence, as measured by XPS and the work function as extracted from UPS. All measurements were performed at different states of deoxygenation after heating in ultrahigh vacuum and reoxidation after heating in a pure oxygen atmosphere. Strong evidence for electron doping of the La0.7Ce0.3MnO3 film after deoxygenation was found. Furthermore, the reversible tunability of the Mn valence by variation of the oxygen content could be demonstrated for both tetravalent- and divalent-ion-doped lanthanum manganite films. / Oxidische Komplexverbindungen können eine Vielzahl an funktionellen Eigenschaften, wie z.B. Ferroelastizität, Ferroelektrizität, Ferro- und Antiferromagnetismus sowie kompliziertere magnetische Ordnungen, die Kombination und Interaktion solcher ferroischer Eigenschaften (Multiferroizität), hohe Spinpolarisation oder Hochtemperatursupraleitung aufweisen und gelten daher als aussichtsreiche Materialien für die zukünftige Mikroelektronik. Entscheidend für die Funktionsfähigkeit oxidischer Bauelemente sind deren elektronische Transporteigenschaften, die in äußerst sensibler Weise von der Verteilung und Dichte von ex- oder intrinsischen elektronischen Defektzuständen an Grenz- und Oberflächen innerhalb der Bauelementstruktur abhängen. Die vorliegende Arbeit beschäftigt sich mit der Spektroskopie solcher elektronischer Eigenschaften mittels photonenbasierter Methoden. Im Fokus stehen dabei perowskitische Oxide , speziell das Modellperowskit Strontiumtitanat (SrTiO3) als Substrat und darauf mittels gepulster Laserdeposition (PLD) abgeschiedene dünne Filme (10-15 nm Dicke) dotierter Lanthanmanganate (La0.7Sr0.3MnO, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). Im Rahmen einer halbleiterphysikalischen Interpretation widmet sich der erste Teilder Identifikation elektronischer Ober- und Grenzflächenzustände an der SrTiO3-Oberfläche sowie an verschiedenen Lanthanmanganat/SrTiO3-Grenzflächen mittels dreier unterschiedlicher experimenteller Methoden zur Vermessung der Oberflächenphotospannung: der Photoelektronenspektroskopie unter zusätzlicher optischer Anregung, einer kapazitiven Detektionsmethode in Plattenkondensatorgeometrie unter modulierter optischer Anregung und der optischen Kelvin-Sonde. Neben einem auf die bei oxidischen Ober- und Grenzflächen auftretenden besonderen Herausforderungen zugeschnittenen Methodenvergleich werden Grenzflächenzustände bezüglich ihrer energetischen Position in der Bandlücke des SrTiO3 und ihres Relaxationsverhaltens analysiert, als substrat- oder filminduziert klassifiziert, und die Verringerung ihrer Dichte nach geeigneter Ausheilprozedur wird nachgewiesen. Der zweite Teil der Arbeit befasst sich mit der in der Literatur bisher kontrovers diskutierten Frage, ob sich die isolierende Stammverbindung LaMnO3 durch Dotierung mit tetravalenten Kationen, wie z.B. Ce4+, in einen elektronendotierten Halbleiter verwandeln lässt - analog zur Herstellung lochdotierter Lanthanmanganate durch Dotierung mit divalenten Kationen, wie z.B. Sr2+ oder Ca2+. Die Frage ist äquivalent zur Betrachtung, ob unter Beibehaltung der Stabilität des Kristallgitters ein Teil der Manganionen vom Mn3+-Zustand in den Mn2+-Zustand übergehen kann. Um einen Beitrag zur Klärung dieses Problems zu leisten, wurden als elektronisch sensitive Methoden die Röntgen- und UV-Photoelektronenspektroskopie (XPS/UPS) gewählt. Die oben genannten Lanthanmanganatfilme wurden dazu hinsichtlich der Austauschaufspaltung der Mangan-3s-Linie im XP-Spektrum, die in linearer Weise von der Manganvalenz abhängt, und der anhand der Breite des UP-Spektrums ermittelten Austrittsarbeit jeweils nach Reinigung der Oberfläche im Ultrahochvakuum (UHV) vergleichend untersucht. Die Messungen wurden nach unterschiedlich starker Desoxidation durch Heizen im UHV und Reoxidierung durch Heizen in Sauerstoffatmosphäre durchgeführt. Es konnte nachgewiesen werden, dass eine Elektronendotierung des La0.7Ce0.3MnO3-Films bei geeigneter Einstellung des Sauerstoffgehalts tatsächlich möglich ist. Außerdem wurde gezeigt, dass sich sowohl in di- als auch in tetravalent dotierten Lanthanmanganatfilmen die Manganvalenz und damit der Dotierungstyp reversibel durchstimmen lässt.

Complex oxides are an intriguing field of solid-state research, as they can exhibit a wide variety of functional properties, such as ferroelasticity, ferroelectricity, ferro- and antiferromagnetism or an even more complicated type of magnetic ordering, the combination or interaction of those ferroic properties (multiferroicity), high spin polarization, or high-temperature superconductivity. Thus they are prospective candidates for future materials in microelectronics. It is a matter of fact that the performance of such oxide-based devices depends mainly on transport properties, which in turn depend on the distribution and density of intrinsic or extrinsic electronic interface states across the device structure. The present thesis focuses on the identification and characterization of such electronic properties by two different photoassisted spectroscopy techniques: surface photovoltage spectroscopy and photoelectron spectroscopy. This work especially deals with perovskite oxides, namely with the model perovskite strontium titanate (SrTiO3) as a substrate and three differently doped lanthanum manganite thin films (10-15 nm thickness) grown by pulsed laser deposition (PLD) on the SrTiO3 substrate(La0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). The first part aims at the identification of electronic surface and interface states at the free SrTiO3 surface as well as at the three different lanthanum manganite/SrTiO3 interfaces. For that purpose three different experimental realizations of the surface photovoltage spectroscopy technique were implemented and employed: photoelectron spectroscopy under additional optical excitation, the capacitive detection of the photoinduced displacement current in a parallel-plate capacitor geometry under modulated optical excitation, and the classical Kelvin probe technique. The methods are evaluated comparatively with respect to their suitability to analyze the given oxidic interfaces. The main result of this first part is a map of the energetic positions and relaxation time constants of the surface states at the SrTiO3 surface as well as of the interface states at the lanthanum manganite/SrTiO3 interfaces within the SrTiO3 bandgap. The interface states were classified into film- and substrate-induced states and it could be demonstrated that an appropriate annealing procedure can dramatically decrease their densities. The second part tackles the problem of the manganese valence and the doping type of di- and tetravalent-ion-doped LaMnO3. The question whether the insulating parent compound LaMnO3 becomes an electron-doped semiconductor after doping with tetravalent cations such as Ce4+ - which would be in analogy to the well-established hole doping after partial substitution of La3+ by divalent cations such as Sr2+ or Ca2+ - has been discussed controversially in the literature so far. Due to the physics of the manganite crystal lattice the question can also be formulated in a different way: Can part of the manganese ions be driven from the Mn3+ state towards the Mn2+ state without any crystal instabilities or phase separation phenomena? In order to contribute to the clarification of this question, an extensive X-ray- and UV-photoelectron spectroscopy (XPS/UPS) investigation was performed. The three differently doped lanthanum manganite thin films were comparatively studied considering the exchange splitting of the Mn 3s core level line, which is a linear function of the Mn valence, as measured by XPS and the work function as extracted from UPS. All measurements were performed at different states of deoxygenation after heating in ultrahigh vacuum and reoxidation after heating in a pure oxygen atmosphere. Strong evidence for electron doping of the La0.7Ce0.3MnO3 film after deoxygenation was found. Furthermore, the reversible tunability of the Mn valence by variation of the oxygen content could be demonstrated for both tetravalent- and divalent-ion-doped lanthanum manganite films. / Oxidische Komplexverbindungen können eine Vielzahl an funktionellen Eigenschaften, wie z.B. Ferroelastizität, Ferroelektrizität, Ferro- und Antiferromagnetismus sowie kompliziertere magnetische Ordnungen, die Kombination und Interaktion solcher ferroischer Eigenschaften (Multiferroizität), hohe Spinpolarisation oder Hochtemperatursupraleitung aufweisen und gelten daher als aussichtsreiche Materialien für die zukünftige Mikroelektronik. Entscheidend für die Funktionsfähigkeit oxidischer Bauelemente sind deren elektronische Transporteigenschaften, die in äußerst sensibler Weise von der Verteilung und Dichte von ex- oder intrinsischen elektronischen Defektzuständen an Grenz- und Oberflächen innerhalb der Bauelementstruktur abhängen. Die vorliegende Arbeit beschäftigt sich mit der Spektroskopie solcher elektronischer Eigenschaften mittels photonenbasierter Methoden. Im Fokus stehen dabei perowskitische Oxide , speziell das Modellperowskit Strontiumtitanat (SrTiO3) als Substrat und darauf mittels gepulster Laserdeposition (PLD) abgeschiedene dünne Filme (10-15 nm Dicke) dotierter Lanthanmanganate (La0.7Sr0.3MnO, La0.7Ca0.3MnO3, La0.7Ce0.3MnO3). Im Rahmen einer halbleiterphysikalischen Interpretation widmet sich der erste Teilder Identifikation elektronischer Ober- und Grenzflächenzustände an der SrTiO3-Oberfläche sowie an verschiedenen Lanthanmanganat/SrTiO3-Grenzflächen mittels dreier unterschiedlicher experimenteller Methoden zur Vermessung der Oberflächenphotospannung: der Photoelektronenspektroskopie unter zusätzlicher optischer Anregung, einer kapazitiven Detektionsmethode in Plattenkondensatorgeometrie unter modulierter optischer Anregung und der optischen Kelvin-Sonde. Neben einem auf die bei oxidischen Ober- und Grenzflächen auftretenden besonderen Herausforderungen zugeschnittenen Methodenvergleich werden Grenzflächenzustände bezüglich ihrer energetischen Position in der Bandlücke des SrTiO3 und ihres Relaxationsverhaltens analysiert, als substrat- oder filminduziert klassifiziert, und die Verringerung ihrer Dichte nach geeigneter Ausheilprozedur wird nachgewiesen. Der zweite Teil der Arbeit befasst sich mit der in der Literatur bisher kontrovers diskutierten Frage, ob sich die isolierende Stammverbindung LaMnO3 durch Dotierung mit tetravalenten Kationen, wie z.B. Ce4+, in einen elektronendotierten Halbleiter verwandeln lässt - analog zur Herstellung lochdotierter Lanthanmanganate durch Dotierung mit divalenten Kationen, wie z.B. Sr2+ oder Ca2+. Die Frage ist äquivalent zur Betrachtung, ob unter Beibehaltung der Stabilität des Kristallgitters ein Teil der Manganionen vom Mn3+-Zustand in den Mn2+-Zustand übergehen kann. Um einen Beitrag zur Klärung dieses Problems zu leisten, wurden als elektronisch sensitive Methoden die Röntgen- und UV-Photoelektronenspektroskopie (XPS/UPS) gewählt. Die oben genannten Lanthanmanganatfilme wurden dazu hinsichtlich der Austauschaufspaltung der Mangan-3s-Linie im XP-Spektrum, die in linearer Weise von der Manganvalenz abhängt, und der anhand der Breite des UP-Spektrums ermittelten Austrittsarbeit jeweils nach Reinigung der Oberfläche im Ultrahochvakuum (UHV) vergleichend untersucht. Die Messungen wurden nach unterschiedlich starker Desoxidation durch Heizen im UHV und Reoxidierung durch Heizen in Sauerstoffatmosphäre durchgeführt. Es konnte nachgewiesen werden, dass eine Elektronendotierung des La0.7Ce0.3MnO3-Films bei geeigneter Einstellung des Sauerstoffgehalts tatsächlich möglich ist. Außerdem wurde gezeigt, dass sich sowohl in di- als auch in tetravalent dotierten Lanthanmanganatfilmen die Manganvalenz und damit der Dotierungstyp reversibel durchstimmen lässt.

Os filmes cerâmicos de manganito de lantânio dopado com estrôncio (LSM) e de manganito de lantânio dopado com estrôncio/zircônia estabilizada com ítria (LSM/YSZ) são utilizados como catodos das células a combustível de óxido sólido de temperatura alta (CaCOSTA). Estes filmes cerâmicos porosos foram depositados sobre o substrato cerâmico denso de YSZ, utilizado como eletrólito, componente estrutural do módulo, assim conferindo uma configuração de meia-célula denominada auto-suporte. O estudo da meia-célula é fundamental, pois na interface catodo/eletrólito ocorre a reação de redução do oxigênio, conseqüentemente influenciando no desempenho da CaCOSTA. Neste sentido, o presente trabalho contribui para a síntese de pós de LSM e LSM/YSZ e para o processamento de filmes finos, utilizando a técnica de pulverização de pó úmido, adotada para a conformação dos filmes cerâmicos por permitir a obtenção de camadas porosas com espessuras variadas na ordem de micrômetros. Os pós de LSM foram sintetizados pela técnica de citratos e os pós de LSM/YSZ pela técnica de mistura de sólidos. Na etapa de conformação foram preparadas suspensões orgânicas de LSM e LSM/YSZ alimentada por gravidade em um aerógrafo manual. Para a conformação do substrato de YSZ utilizou-se uma prensa uniaxial hidráulica. Foram possíveis a obtenção das meia-células de óxido sólido catodo/eletrólito de estruturas cristalinas hexagonal para a fase LSM e cúbica para a fase YSZ. E as micrografias das meia-células mostram que o substrato YSZ é denso, suficiente para ser utilizado como eletrólito sólido, e os filmes de LSM e LSM/YSZ apresentam-se porosos com espessura de aproximadamente 30 μm e com boa aderência entre os catodos e o eletrólito. A presença do catodo compósito entre o catodo LSM e o substrato YSZ, possibilitou um aumento no desempenho eletroquímico na reação de redução do oxigênio. / The ceramic films of strontium-doped lanthanum manganite (LSM) and strontiumdoped lanthanum manganite/yttria-stabilized zirconia (LSM/YSZ) are used as cathodes of the high temperature solid oxide fuel cells (HTSOFC). These porous ceramic films had been deposited on the YSZ dense ceramic substrate, used as electrolyte, structural component of the module, thus conferring a configuration of half-cell called auto-support. The study of the half-cell it is basic, therefore in the interface cathode/electrolyte occurs the oxygen reduction reaction, consequently influencing in the performance of the HTSOFC. In this direction, the present work contributes for the processing of thin films, using the wet powder spraying technique, adopted for the conformation of the ceramic films for allowing the attainment of porous layers with thicknesses varied in the order of micrometers. The LSM powders were synthesized by the citrate technique and the LSM/YSZ powders synthesized by the solid mixture technique. In the stage of formation were prepared organic suspensions of LSM and LSM/YSZ fed by gravity in a manual aerograph. For the formation of the YSZ substrate was used a hydraulical uniaxial press. The attainment of solid oxide half-cells cathode/electrolyte was possible of crystalline structures hexagonal for phase LSM and cubic for phase YSZ. The half-cells micrographs show that the YSZ substrate is dense, enough to be used as solid electrolyte, and the LSM and LSM/YSZ films are presented porous with approximately 30 μm of thickness and good adherence between the cathodes and the electrolyte. The presence of composite cathode between the LSM cathode and YSZ substrate, presented an increase in the electrochemical performance in the oxygen reduction reaction.

Os filmes cerâmicos de manganito de lantânio dopado com estrôncio (LSM) e de manganito de lantânio dopado com estrôncio/zircônia estabilizada com ítria (LSM/YSZ) são utilizados como catodos das células a combustível de óxido sólido de temperatura alta (CaCOSTA). Estes filmes cerâmicos porosos foram depositados sobre o substrato cerâmico denso de YSZ, utilizado como eletrólito, componente estrutural do módulo, assim conferindo uma configuração de meia-célula denominada auto-suporte. O estudo da meia-célula é fundamental, pois na interface catodo/eletrólito ocorre a reação de redução do oxigênio, conseqüentemente influenciando no desempenho da CaCOSTA. Neste sentido, o presente trabalho contribui para a síntese de pós de LSM e LSM/YSZ e para o processamento de filmes finos, utilizando a técnica de pulverização de pó úmido, adotada para a conformação dos filmes cerâmicos por permitir a obtenção de camadas porosas com espessuras variadas na ordem de micrômetros. Os pós de LSM foram sintetizados pela técnica de citratos e os pós de LSM/YSZ pela técnica de mistura de sólidos. Na etapa de conformação foram preparadas suspensões orgânicas de LSM e LSM/YSZ alimentada por gravidade em um aerógrafo manual. Para a conformação do substrato de YSZ utilizou-se uma prensa uniaxial hidráulica. Foram possíveis a obtenção das meia-células de óxido sólido catodo/eletrólito de estruturas cristalinas hexagonal para a fase LSM e cúbica para a fase YSZ. E as micrografias das meia-células mostram que o substrato YSZ é denso, suficiente para ser utilizado como eletrólito sólido, e os filmes de LSM e LSM/YSZ apresentam-se porosos com espessura de aproximadamente 30 μm e com boa aderência entre os catodos e o eletrólito. A presença do catodo compósito entre o catodo LSM e o substrato YSZ, possibilitou um aumento no desempenho eletroquímico na reação de redução do oxigênio. / The ceramic films of strontium-doped lanthanum manganite (LSM) and strontiumdoped lanthanum manganite/yttria-stabilized zirconia (LSM/YSZ) are used as cathodes of the high temperature solid oxide fuel cells (HTSOFC). These porous ceramic films had been deposited on the YSZ dense ceramic substrate, used as electrolyte, structural component of the module, thus conferring a configuration of half-cell called auto-support. The study of the half-cell it is basic, therefore in the interface cathode/electrolyte occurs the oxygen reduction reaction, consequently influencing in the performance of the HTSOFC. In this direction, the present work contributes for the processing of thin films, using the wet powder spraying technique, adopted for the conformation of the ceramic films for allowing the attainment of porous layers with thicknesses varied in the order of micrometers. The LSM powders were synthesized by the citrate technique and the LSM/YSZ powders synthesized by the solid mixture technique. In the stage of formation were prepared organic suspensions of LSM and LSM/YSZ fed by gravity in a manual aerograph. For the formation of the YSZ substrate was used a hydraulical uniaxial press. The attainment of solid oxide half-cells cathode/electrolyte was possible of crystalline structures hexagonal for phase LSM and cubic for phase YSZ. The half-cells micrographs show that the YSZ substrate is dense, enough to be used as solid electrolyte, and the LSM and LSM/YSZ films are presented porous with approximately 30 μm of thickness and good adherence between the cathodes and the electrolyte. The presence of composite cathode between the LSM cathode and YSZ substrate, presented an increase in the electrochemical performance in the oxygen reduction reaction.