No description available.

In this study, a deposition system called EAVD was made to deposite porous LSCF films used as cathode material in the solid oxide fuel cell (SOFC). The relation of deposition parameters to morphology was discussed. Porous La0.8Sr0.2Co0.2Fe0.8O3 films were successfully deposited on Corning glass and ceria substrates, and a pseudo-cubic perovskite phase was obtained after a post-calcination at 750¢J for 2 hrs. Deposition parameters, such as deposition time, deposition temperature, flow rate, voltage applied, different kinds of set-ups (downward spraying or upward spraying), were discussed. The obtained calcined films were characterized by X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). On the other hand, surface and cross-section morphology were examined using SEM. In the series using downward spraying system, deposition temperature and deposition time showed profound effect on morphology. With increasing the extent of these two factors, porous films were obtained. With decreasing the extent of these two factors, however, dense films were obtained. The effects of other parameters to morphology were less obvious. Under proper conditions, cauliflower-like films with high porosity were obtained. In the series using upward spraying system (vertical set-up), reticular films were successfully obtained using deposition temperature ranging from 275~320¢J, flow rate 1.0~1.5 ml/hr, and deposition time within 2 hrs. In the series of flow rate, the pores of reticular structure seemed to grow up with increasing flow rate. Under the condition of prolonged deposition (4 hrs), a stalactitc structure with micropores on it was obtained. The highly porous structures obtained in this study are very suitable for applications in gas sensor and electrodes in SOFC.

No description available.

Une des solutions envisagée pour éviter la réactivité entre la cathode de LSCF et l'électrolyte de YSZ est l'intercalation d'une couche barrière de CGO. Une étude de la réactivité interfaciale par DRX et ToF-SIMS entre CGO, déposée par atomisation électrostatique, et YSZ a montré qu'un traitement thermique au-dessus de 1100 C sous air induit une détérioration de la couche de CGO par la formation d'une solution solide. Le vieillissement du système LSCF/CGO/YSZ a été étudié en fonction de l'épaisseur de la couche de CGO de 0,11 à 2 µm, par spectroscopie d'impédance complexe, à 700 °C sous air à l'abandon. Les mesures ont montré que l'épaisseur de cette couche est un facteur influençant les propriétés électriques des différents systèmes. L'introduction d'une couche mince de CGO, déposée par pulvérisation cathodique, a conduit à une diminution de la résistance série du système ainsi qu'une diminution de la dégradation de l'électrode LSCF. La dégradation de l'électrode de LSCF est liée à la ségrégation du Sr au niveau de l'interface LSCF/YSZ. / One of the considered solutions to avoid the reactivity between LSCF cathode and YSZ electrolyte is the intercalation of a CGO buffer layer. A study of the interfacial reactivity by XRD and by ToF-SIMS between CGO, by ESD, and YSZ has shown that an heat treatment above 1100 °C in air leads to the chemical degradation of the CGO layer leads to the formation of a solid solution. An ageing investigation of LSCF/CGO/YSZ half cell was performed for different CGO layer thicknesses coated by DC magnetron sputtering from 0.11 to 2 µm, by electrochemical impedance spectroscopy at 700 °C in air at OCV. EIS measurements have shown that the thickness of this coating has a strong effect on the electrical properties of these systems. The introduction of a thin CGO buffer layer has lead to the decrease of the initial value of the series resistance and to the reduction of the LSCF electrode degradation. This degradation electrode is due to the diffusion of Sr at the LSCF/YSZ interface as shown by microanalyses X.

Les méthodes hybrides alliant la mécanique quantique et la mécanique moléculaire (QM/MM) sont des outils adéquats pour traiter des systèmes biologiques. Les phénomènes électroniques souvent étudiés sur des petites molécules ont, dès lors pu être envisagés dans des environnements macromoléculaires. Ce travail explore trois phénomènes électroniques en présence d'un environnement protéique: l'absorption, la capture électronique et les ionisations de coeur. Nous avons employé les derniers développements de la méthode QM/MM {Local Self-Consistent Field} (LSCF) pour traiter la jonction covalente entre la partie QM et la partie MM ainsi que le couplage QM/MM avec le PCM. En premier lieu, nous nous sommes focalisés sur les spectres d'absorption de chromophores présents dans des macromolécules. Nous avons mis au point un couplage entre les méthodes LSCF/MM et PCM pour prendre en compte la polarisation électronique de l'environnement suite à l'absorption d'un photon par le chromophore. Ce modèle, le LSCF/MM\string:ERS, a été testé et validé sur le spectre d'absorption du complexe de squaraine-tétralactame. Une étude plus poussée a ensuite été réalisée sur une protéine fluorescente en décomposant la longueur d'onde d'absorption maximale en trois contributions physiques. Les effets de la substitution du chromophore ont aussi été évalués. Dans une seconde partie, nous avons étudié la capture électronique par un cyclotide contenant trois ponts disulfures, qui, après irradiation, forment des demi-liaisons caractéristiques (2c-3e). La dernière partie est consacrée à la validation de l'approche de la projection asymptotique. Elle a été réalisée dans le cadre d'une étude sur les ionisations de coeur d'un ensemble de molécules pour, par la suite, étudier les ionisations de coeur de la glycine présentes dans des systèmes de plus en plus complexes jusqu'à la Sérum-Albumine Humaine. / Hybrid methods that combine quantum mechanics and molecular mechanics (QM/MM) provide a near-ideal treatment of biological system reactivity and spectroscopy. Many electronic phenomena often studied on small systems can be now forseen in macromolecular surroundings. This work considers the treatment with QM/MM tools of three electronic phenomena in biosystem: absoprtion, electronic attachment and core ionization. Latest developments of the Local Self-Consistent Field formalism (LSCF) have been used to treat delicate covalent junctions between the QM part and the MM part and the coupling of QM/MM and PCM methods. Firstly we have focussed on absorption spectra of chromophores embedded in macromolecules. The combined LSCF/MM and PCM approach have been employed to account for the electronic polarization when the chromophore absorbs one photon. This new method, called LSCF/MM\string:ERS has been tested and validated with the study of the absorption spectra on the squarain-tetralactam complex. Then, we have considered a fluorescent protein and decomposed the maximum absorption wavelength into several physical contributions. We have also analyzed the substitution effect of the chromophore. Secondly, the electronic capture have been studied on a cyclotide containing three disulfide bonds, which forms caracteristic hemi-bond (2c-3e) after irradiation. The last electronic phenomena studied is the core ionization tackled within the framework of the asymptotic projection approach. The latter has been implemented, then tested and validated on a set of molecules. This method have been employed to analyze the specific core ionization of glycine-containing systemes of increasing complexity, up to the Human Serum Albumin

L'énergie produite par des matières fossiles, pétrole et charbon, va se raréfier de manière inéluctable et couter de plus en plus cher à moyen terme. Pour pallier à la fin des matières fossiles, le développement d'énergies alternatives est indispensable. Parmi celles-ci, la production d'électricité et de chaleur à partir d'hydrogène commence à se développer grâce aux piles à combustible (PAC) depuis les très faibles puissances (des microwatts pour alimenter les capteurs) jusqu'aux fortes puissances (des Mégawatts pour l'industrie) en passant par des puissances moyennes (des kilowatts pour le résidentiel). Une PAC est constituée de 3 éléments : 2 électrodes (anode et cathode) séparées par un électrolyte. Dans cette thèse, ces 3 éléments sont constitués d'oxydes solides et la pile est appelée SOFC (Solid Oxide Fuel Cell). Les piles SOFC actuellement commercialisées fonctionnent à de très hautes températures, typiquement supérieures à 800°C. L'objectif du travail a été d'élaborer des oxydes pour diminuer cette température vers 600°C ce qui permet d'utiliser de l'acier pour contenir ces piles. Pour que la pile SOFC fonctionne à cette température, il est impératif de diminuer la résistance électrique des 2 électrodes et de l'électrolyte de manière à récupérer une tension électrique continue maximale aux bornes de la pile et aussi à faire passer un courant électrique élevé dans celle-ci. La cathode, en contact avec l'oxygène de l'air, est l'élément le plus critique à optimiser. Nous avons choisi comme matériau de cathode un matériau déjà étudié, La₀.₆Sr₀.₄Co₀.₈Fe₀.₂O₃ (LSCF) et comme électrolyte Ce₀.₉Gd₀.₁O₂ (CGO) connu comme performant en dessous de 650 °C. Nous avons élaboré ces matériaux par une méthode de chimie douce, la méthode sol-gel Péchini, et caractérisé ceuxi-ci par diffraction de rayons X et microscopie électronique à balayage. Une part importante du travail a été la caractérisation électrique à l'aide de mesures d'impédance complexe dans une large gamme de fréquence (0,05 Hz à 2 MHz) et de température (300°C à 700 °C). Le meilleur résultat a été obtenu avec une cathode composite poreuse d'épaisseur 40 µm constituée à masses égales de LSCF et de CGO déposée par sérigraphie sur une céramique dense de CGO d'épaisseur 1,5 mm. De plus, un film mince dense de LSCF d'épaisseur 0,1 µm environ a été déposé par centrifugation pour améliorer l'interface entre la cathode et l'électrolyte. À 600 °C la résistance de cette cathode a été mesurée à 0,13 Ω pour 1 cm² de cathode : cette valeur est à l'état de l'art. Une étude du vieillissement de cette cathode et de l'électrolyte a été effectuée à 600 °C pendant 1000 h en continu sous air : cela s'est traduit par une augmentation de la résistance de la cathode de 32%. Ceci peut être lié à la différence de valeurs des coefficients d'expansion thermique des matériaux de cathode et d'électrolyte. / Energy made from fossil fuels, oil or coal, is becoming increasingly rare and its price will increase in the near future. Developing alternative energy sources could compensate the use of fossil fuel. Particularly, an alternative form of energy is being developed through fuel cells, through the production of electricity and heat from hydrogen. Fuel cells can provide low wattage (microwatts for sensor applications), medium wattage (kilowatts for residential applications) and high wattage (megawatts for the industry). A fuel cell consists of 3 components : 2 electrodes (anode and cathode) separated by an electrolyte. In my work, I use solid pxide materials for these three elements in order to expand on the literature of Solid Oxide Fuel Cell (SOFC). Commercialized SOFCs currently operate at very high temperatures, typically above 800°C. The objective of this study was to develop oxides that could decrease the working temperature of the cell to 600°C, which would allow the use of steel to contain these fuel cells. In order to enable the SOFC to operate at this temperature, it is imperative to decrease the electrical resistances of the two electrodes and electrolyte in order to collect a continuous voltage which is maximal at the terminals of the fuel cell, and also to have a high electric current going through the fuel cell. The cathode, in contact with the oxygen present in the atmosphere, is the most critical element to be optimized. I close as a cathode material La₀.₆Sr₀.₄Co₀.₈Fe₀.₂O₃ (LSCF), which has already been studied. As electrolyte, I used Ce₀.₉Gd₀.₁O₂ (CGO) which is known to work below 650°C. I synthesized these materials through the Pechini method, a soft chemistry sol-gel method. The materials were characterized by X-ray diffraction and scanning electron microscopy. An important aspect of this work was the electrical characterization using complex impedance measurements in a wide frequency range (0,05 Hz to 2 MHz) and temperature (300°C to 700°C). The best result was obtained with a 40 µm thick, porous, composite cathode (LSCF/CGO 50/50 wt%) was deposited by screen printing on a 1,5 mm thick and dense CGO ceramic. In addition, a dense thin film of LSCF with a thickness of about 0,1 µm was spin-coated between the cathode and the electrolyte to improve the interface. At 600°C the measured resistance of the cathode was 0,13 Ω for 1 cm² : this value is similar to the results found in the state of the art. An aging study of the cathode and the electrolyte was carried out at 600 °C for 1000 h in air : the resistance of the cathode increased of 32%. This may be related to the different values of the thermal expansion coefficients of the cathode and electrolyte materials.

Comprendre, contrôler et optimiser le mécanisme de la réaction de réduction de l’oxygène à la cathode, cette démarche devient une nécessité pour améliorer les dispositifs de conversion d'énergie de haute performance tels que les piles à combustible à oxyde électrolyte solide (PAC). Des films poreux à conduction mixte, ionique et électronique (MIEC) et leurs composites comprenant un conducteur ionique offrent des propriétés uniques. Cependant, la corrélation des propriétés intrinsèques des composants d'électrodes aux caractéristiques microstructurales reste une tâche difficile. Dans le cadre de cette thèse, la couche fonctionnelle de cathode de La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) pure et du composite LSCF/Ce0.9Gd0.1O2-δ (CGO) a été élaborée par la technique d’atomisation électrostatique. Une microstructure à porosité hiérarchique a été obtenue dans un domaine nanométrique à micrométrique. Les films ont été recouverts d’un collecteur de courant (CCL), LSCF, par sérigraphie. Une étude paramétrique a été réalisée expérimentalement pour optimiser la double couche en termes de taille de particules, de composition et d'épaisseur des couches de CFL et CCL. En se basant sur ces résultats, un modèle éléments finis 3D a été développé en utilisant les paramètres microstructuraux déterminés par tomographie de FIB-SEM dans une géométrie simple, similaire à des caractéristiques colonnaires. Dans ce travail, un guide de conception du matériau d’électrode a été proposé reliant des performances électrochimiques optimisées à la microstructure et aux propriétés du massif en combinant une étude expérimentale et une étude théorique de modélisation. Une cellule complète de PAC intégrant la cathode optimisée double couche de LSCF a été testée dans des conditions réelles d'exploitation. / Understanding, controlling and optimizing the mechanism of oxygen reduction reaction at the cathode need to be addressed for high performance energy conversion devices such as solid oxide fuel cells (SOFCs). Structured porous films of mixed ionic electronic conductors (MIECs) and their composites with addition of a pure ionic conductor offer unique properties. However, correlating the intrinsic properties of electrode components to microstructural features remains a challenging task. In this PhD thesis, cathode functional layers (CFL) of La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and LSCF/Ce0.9Gd0.1O2-δ (CGO) composite cathodes with hierarchical porosity from nano- to micro-range are fabricated by electrostatic spray deposition technique. The films were topped with LSCF as a current collecting layer (CCL) by screen printing technique. A parametric optimization study was conducted experimentally in terms of particle size, composition, and thickness of CFL and CCL layers. The experimental results were supported by a numerical 3D Finite Element Model (FEM). Microstructural parameters determined by FIB-SEM tomography were used in a simple geometry similar to experimentally observed columnar features. In this work, experimental results and modelling were combined to provide design guidelines relating optimized electrochemical performances to the microstructure and bulk material properties. A complete fuel cell with optimized cathode film was tested in real SOFC operational conditions.

Made available in DSpace on 2014-12-17T14:58:15Z (GMT). No. of bitstreams: 1 LaureniaMPG_DISSERT.pdf: 2747629 bytes, checksum: a8b9f0209bf9acabd46832a08265ed7f (MD5) Previous issue date: 2012-05-08 / The lanthanum strontium cobalt iron oxide (La1-xSrxCo1-yFeyO3 LSCF) is the most commonly used material for application as cathode in Solid Oxide Fuel Cells (SOFCs), mainly due to their high mixed ionic electronic conductivity between 600 and 800?C. In this study, LSCF powders with different compositions were synthesized via a combination between citrate and hydrothermal methods. As-prepared powders were calcined from 700 to 900?C and then characterized by X-ray fluorescence, X-ray diffraction, thermal analyses, particle size analyses, nitrogen adsorption (BET) and scanning electronic microscopy. Films of composition La0,6Sr0,4Co0,2Fe0,8O3 (LSCF6428), powders calcined at 900?C, were screen-printed on gadolinium doped ceria (CGO) substrates and sintered between 1150 and 1200?C. The effects of level of sintering on the microstructure and electrochemical performance of electrodes were evaluated by scanning electronic microscopy and impedance spectroscopy. Area specific resistance (ASR) exhibited strong relation with the microstructure of the electrodes. The best electrochemical performance (0.18 ohm.cm2 at 800?C) was obtained for the cathode sintered at 1200?C for 2 h. The electrochemical activity can be further improved through surface activation by impregnation with PrOx, in this case the electrode area specific resistance decreases to values as low as 0.12 ohm.cm2 (800?C), 0.17 ohm.cm2 (750?C) and 0.31 ohm.cm2 (700?C). The results indicate that the citrate-hydrothermal method is suitable for the attainment of LSCF particulates with potential application as cathode component in intermediate temperature solid oxide fuel cells (IT-SOFCs) / A cobaltita de lant?nio dopada com estr?ncio e ferro (La1-xSrxCo1-yFeyO3 LSCF) ? comumente o material mais utilizado para aplica??o como catodo em c?lula a combust?vel de ?xido s?lido (SOFC), principalmente devido a sua elevada condutividade mista i?nica e eletr?nica entre 600 e 800?C. Neste trabalho, p?s de LSCF de diferentes composi??es foram sintetizados via uma combina??o entre os m?todos citrato e hidrotermal. Os p?s como obtidos foram calcinados entre 700 e 900 ?C e caracterizados por fluoresc?ncia de raios X, difratometria de raios X, an?lises t?rmicas, distribui??o de tamanho de part?cula, adsor??o gasosa (BET) e microscopia eletr?nica de varredura. Filmes de composi??o La0,6Sr0,4Co0,2Fe0,8O3 (LSCF6428) foram obtidos por serigrafia de p?s calcinados a 900?C. Os filmes foram depositados sobre substratos de c?ria dopada com gadol?nia (CGO) e ent?o sinterizados entre 1150 e 1200?C. Os efeitos do patamar de sinteriza??o na microestrutura e no desempenho eletroqu?mico dos eletrodos foram avaliados por microscopia eletr?nica de varredura e espectroscopia de imped?ncia. A resist?ncia espec?fica por ?rea apresentou forte rela??o com a microestrutura dos eletrodos. O melhor desempenho eletroqu?mico (0,18 ohm.cm2 a 800?C) foi obtido para o catodo sinterizado a 1200?C por 2 horas. A atividade eletroqu?mica pode ainda ser melhorada mediante ativa??o superficial por impregna??o com PrOx, neste caso a resist?ncia espec?fica por ?rea do eletrodo diminui para valores t?o baixos como 0,12 ohm.cm2 (800?C), 0,17 ohm.cm2 (750?C) e 0,31 ohm.cm2 (700?C). Os resultados obtidos indicam que o m?todo citrato-hidrotermal ? adequado para a prepara??o de particulados de LSCF com potencial aplica??o como catodo em c?lulas a combust?vel de ?xido s?lido de temperatura intermedi?ria (600-800?C)

Este trabalho investigou a deposição dos filmes finos de ferritas de lantânio dopado com estrôncio e cobalto La1-xSrxCo1-yFeyO3-δ (LSCF) através da técnica de spray pirólise empregando precursores inorgânicos e silício como substrato. Foram utilizados água e etanol na proporção de (3:1) como solvente na preparação da solução a ser aspergida. As deposições foram realizadas em diferentes temperaturas: 130ºC, 150ºC, 170ºC e 200ºC. Os filmes de LSCF antes e após o tratamento térmico foram analisados por difração de raios X (DRX) e microscopia eletrônica de varredura (MEV). O spray de deposição dos filmes em função das soluções com diferentes proporções de etanol (0%, 50%, 75% e 100%) foi investigado utilizado uma câmera rápida de alta resolução e um laser com comprimento de onda 532 nm. Os resultados evidenciaram que a temperatura de ebulição do solvente e de deposição são parâmetros importantes na determinação da morfologia dos filmes. Maiores teores de etanol promovem um aumento nos valores do ângulo de cone e que a propriedade da tensão superficial exerce significativamente maior influência nos aspectos que envolvem a estrutura do spray, a atomização do líquido, formação de gotas e no ponto de ruptura das soluções Os resultados de DRX apresentaram a fase cristalina majoritária de LSCF após o tratamento térmico a 750°C por 2 horas e a fase secundária de La2O3. As imagens de MEV evidenciaram a formação de filmes para as distâncias de deposição de 120 mm, embora apenas na temperatura de 200°C apresentasse uma estrutura porosa antes e após o tratamento térmico. A presença de interdifusão e microporosidade foi observada para esse filme. Para as diferentes composições de LSCF nas condições de 120 mm e a 200°C, apenas as composições de La1-xSrxCo0,2Fe0,8 com x = 0,6, 0,9 e 0,7 apresentaram estrutura porosa após o tratamento térmico. O filme de La0,8Sr0,2Co0,2Fe0,8 apresentou maior condutividade iônica/elétrica e energia de ativação. A presença de fases secundárias (La2O3, SrO2 e La2CoO3) contribuiu para a redução da condutividade elétrica e iônica e a energia de ativação. / This work investigated the deposition conditions of thin lanthanum ferrite films doped with strontium and cobalt – La1-xSrxCo1-yFeyO3-δ (LSCF) through the spray pyrolysis technique using inorganic precursors and silicon as a substrate. Water and ethanol were used in the proportion (3:1) as solvent in the preparation of the solution to be sprinkled. The depositions were carried out at different temperatures: 130°C, 150°C, 170°C and 200°C. The films of LSCF were analyzed by X-ray diffraction and scanning electron microscopy before and after thermal treatment. The XRD results showed the obtainment of crystalline phase of LSCF after thermal treatment at 750°C/2hours and secondary phase of La2O3. The sprays of solutions with different proportions of ethanol (0%, 50%, 75% and 100%) were analyzed using a fast high-resolution camera and a laser with a wavelength of 532nm.The results showed those deposition temperature and solvente boiling points are important parameters to determine the thin morphology Higher ethanol levels promote an increase in the cone angle values and that the property of the surface tension exerts significantly greater influence on issues involving the structure of the spray, the atomization of the liquid, the droplets formation and the breaking point of the solutions. The XRD results showed the obtainment of majority crystalline phase of LSCF after thermal treatment at 750°C/2 hours with the presence of secondary phase of La2O3. The SEM images showed formation of films for the distance of 120 mm, although only at the temperature of 200°C it presents porous morphology before and after thermal treatment. The presence of interdiffusion and microporosity was observed for this film. For different LSCF compositions under the conditions of 120 mm and 200°C, the films of La1-xSrx Co0,2Fe0,8 with x = 0,6; 0,9 and 0,7 presented porous morphology after thermal treatment. Otherwise, the film of La0,8Sr0,2Co0,2Fe0,8 showed the highest ionic/electrical conductivity. The presence of secondary phase (La2O3, SrO2 and La2CoO3) evidenced by XRD promoted a decrease in ionic/electrical conductivity and in the activation energy.

Atmospheric plasma spraying is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (600 to 750°C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. Processes were developed to manufacture metal-supported SOFC cathodes by axial-injection plasma spraying. Cathodes consisted of LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ) or SSC (Sm0.5Sr0.5CoO3) as the primary material. Initially, the plasma spray process parameters were varied, and x-ray diffraction analyses were performed on the cathode coatings to detect material decomposition and the formation of undesired phases. These results determined the envelope of plasma spray parameters in which coatings of LSCF and SSC can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured. Subsequently, composite cathodes were fabricated by mixing up to 40 wt. % of the ionic conducting SDC (Ce0.8Sm0.2O1.9) material into the feedstock. The deposition efficiencies of these cathodes were calculated based on the mass of the sprayed cathode. Particle surface temperatures were measured in-flight to enhance understanding of the relationship between spray parameters, microstructure, and deposition efficiency. Electrochemical impedance spectroscopy was performed in symmetrical cells: at 750°C, LSCF-SDC cathodes had polarization resistances as low as 0.101 Ωcm², and SSC cathodes had polarization resistances as low as 0.0056 Ωcm². Finer mixing of the ceramic phases was achieved by using a nano-structured feedstock that contained both LSCF and SDC phases agglomerated together in larger particles. Fuel cells containing a yttria-stabilized zirconia (YSZ) electrolyte and a nickel-YSZ anode were fabricated, and the effect of the cathode microstructure on cell impedance was studied using the analysis of differential impedance spectra. The degradation of composite LSCF-SDC cathodes on porous 430 stainless steel supports was also investigated. To reduce degradation, La2O3 and Y2O3 reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. As a result, polarization resistance degradation rates as low as 0.00256 Ω·cm2 /1000 h were observed over 100 hours on coated substrates, compared to 0.1 Ω·cm2 /1000 h on uncoated substrates.