Células a combustível de óxido sólido são sistemas capazes de gerar energia elétrica por meio da oxidação de moléculas hidrogenadas. Normalmente os sistemas planares e tubulares, são compostos por quatro constituintes bem definidos: cátodo, ânodo, eletrólito e selante. Este último componente é o foco do presente estudo, sendo que suas principais características são estabilidade química na temperatura de operação da célula, isolamento elétrico e coeficiente de expansão térmica compatível com os outros constituintes, além da viscosidade elevada e resistência química em atmosferas oxidantes e redutoras. Devido à geometria planar e de multicamadas da célula se optou por usar como selante vidros niobofosfatos. A selagem foi realizada a partir de dispersão de pó de vidro em álcool etílico, gerando uma solução viscosa que foi aplicada sobre o substrato. Posteriormente realizou se um tratamento térmico para a consolidação do selamento. Os vidros estudados foram denominados de Nb30, Nb37, Nb40 e Nb44, de acordo com o teor nominal de óxido de nióbio utilizado na composição. O objetivo desse trabalho foi caracterizar, a partir de precursores os selantes a base de vidros niobofosfatos para aplicar em células a combustível de óxido sólido do tipo planar. Foram feitos caracterizações dos pós dos vidros e de pastilhas cristalizadas para determinar os coeficientes de expansão térmica (CET), resistividade elétrica, difração de raio X e microscopia eletrônica de varredura (MEV), além de, caracterizar visualmente sua adesividade, molhabilidade, resistência mecânica em substratos de alumina e em conjunto com os componentes das SOFC, sendo também testados os selantes em operação nas unidades previamente formadas de SOFC (ciclos térmicos). / Solid oxide fuel cells (SOFC) are devices which generate d.c. power by the oxidation of hydrogen molecules. These devices can have a multilayer plane design containing a cathode, an anode, a solid electrolyte, and a sealing material. The sealing, which is the subject of this study, has to be chemically stable at relatively SOFC operational condition in oxi-redox atmospheres, electrical insulator, with a thermal expansion coefficient matching other components, and, in of glass, the viscosity must be relatively high. The aim of the present work is to characterize niobophosphate glasses which will be used as sealant precursors of Solid Oxide Fuel Cell with a plane design. Niobophosphate glasses, named Nb30, Nb37, Nb40, and Nb44 according to the niobium content, were investigated for this purpose. The sealing was performed by mixing glass powder with ethanol which was applied over the substrate. Later, a heat treatment was performed to consolidate the sealing. Glass powder and devitrified glass pellets were characterized by different techniques. The thermal expansion coefficient, electrical resistivity, and the X-rays diffraction pattern were determined for these materials. Scanning electron microscopy was also used to visualize the sealing/ substrate interface, and to evaluate the adhesiveness, wetability, apparent mechanical resistance in alumina substrates and in other SOFC components. The sealants were tested in SOFC, and also submitted to simulating thermal cycles.

Células a combustível de óxido sólido são sistemas capazes de gerar energia elétrica por meio da oxidação de moléculas hidrogenadas. Normalmente os sistemas planares e tubulares, são compostos por quatro constituintes bem definidos: cátodo, ânodo, eletrólito e selante. Este último componente é o foco do presente estudo, sendo que suas principais características são estabilidade química na temperatura de operação da célula, isolamento elétrico e coeficiente de expansão térmica compatível com os outros constituintes, além da viscosidade elevada e resistência química em atmosferas oxidantes e redutoras. Devido à geometria planar e de multicamadas da célula se optou por usar como selante vidros niobofosfatos. A selagem foi realizada a partir de dispersão de pó de vidro em álcool etílico, gerando uma solução viscosa que foi aplicada sobre o substrato. Posteriormente realizou se um tratamento térmico para a consolidação do selamento. Os vidros estudados foram denominados de Nb30, Nb37, Nb40 e Nb44, de acordo com o teor nominal de óxido de nióbio utilizado na composição. O objetivo desse trabalho foi caracterizar, a partir de precursores os selantes a base de vidros niobofosfatos para aplicar em células a combustível de óxido sólido do tipo planar. Foram feitos caracterizações dos pós dos vidros e de pastilhas cristalizadas para determinar os coeficientes de expansão térmica (CET), resistividade elétrica, difração de raio X e microscopia eletrônica de varredura (MEV), além de, caracterizar visualmente sua adesividade, molhabilidade, resistência mecânica em substratos de alumina e em conjunto com os componentes das SOFC, sendo também testados os selantes em operação nas unidades previamente formadas de SOFC (ciclos térmicos). / Solid oxide fuel cells (SOFC) are devices which generate d.c. power by the oxidation of hydrogen molecules. These devices can have a multilayer plane design containing a cathode, an anode, a solid electrolyte, and a sealing material. The sealing, which is the subject of this study, has to be chemically stable at relatively SOFC operational condition in oxi-redox atmospheres, electrical insulator, with a thermal expansion coefficient matching other components, and, in of glass, the viscosity must be relatively high. The aim of the present work is to characterize niobophosphate glasses which will be used as sealant precursors of Solid Oxide Fuel Cell with a plane design. Niobophosphate glasses, named Nb30, Nb37, Nb40, and Nb44 according to the niobium content, were investigated for this purpose. The sealing was performed by mixing glass powder with ethanol which was applied over the substrate. Later, a heat treatment was performed to consolidate the sealing. Glass powder and devitrified glass pellets were characterized by different techniques. The thermal expansion coefficient, electrical resistivity, and the X-rays diffraction pattern were determined for these materials. Scanning electron microscopy was also used to visualize the sealing/ substrate interface, and to evaluate the adhesiveness, wetability, apparent mechanical resistance in alumina substrates and in other SOFC components. The sealants were tested in SOFC, and also submitted to simulating thermal cycles.

Detailed thermodynamic, kinetic, geometric, and cost models are developed, implemented, and validated for the synthesis/design and operational analysis of hybrid solid oxide fuel cell (SOFC) – gas turbine (GT) – steam turbine (ST) systems ranging in size from 1.5 MWe to 10 MWe. The fuel cell model used in this thesis is based on a tubular Siemens-Westinghouse-type SOFC, which is integrated with a gas turbine and a heat recovery steam generator (HRSG) integrated in turn with a steam turbine cycle. The SOFC/GT subsystem is based on previous work done by Francesco Calise during his doctoral research (Calise, 2005). In that work, a HRSG is not used. Instead, the gas turbine exhaust is used by a number of heat exchangers to preheat the air and fuel entering the fuel cell and to provide energy for district heating. The current work considers instead the possible benefits of using the exhaust gases in an HRSG in order to produce steam which drives a steam turbine for additional power output. Four different steam turbine cycles are considered in this M.S. thesis work: a single-pressure, a dual-pressure, a triple-pressure, and a triple-pressure with reheat. The models have been developed to function both at design (full load) and off-design (partial load) conditions. In addition, different solid oxide fuel cell sizes are examined to assure a proper selection of SOFC size based on efficiency or cost. The thermoeconomic analysis includes cost functions developed specifically for the different system and component sizes (capacities) analyzed. A parametric study is used to determine the most viable system/component syntheses/designs based on maximizing total system efficiency or minimizing total system life cycle cost. / Master of Science

O desenho planar para as células a Combustível de Óxido Sólido (SOFC) é melhor do que o tubular devido a sua maior densidade de corrente e menor custo de fabricação. No entanto, o projeto de SOFC planar requer selantes para evitar o vazamento de combustível e a mistura de gases em altas temperaturas. Os vidros e os vitrocerâmicos têm demonstrado serem os mais adequados por apresentarem boa compatibilidade com outros componentes da célula nas temperaturas de trabalho das SOFCs (700-1000°C). No presente estudo, uma série de composições pertencentes ao sistema BaO-Al2O3-SiO2 (BAS) com a adição de B2O3 foram sintetizados tomando as proporções apropriadas de cada óxido constituinte. Propôs-se melhorar este sistema utilizando-se formadores e teores relevantes de modificadores estruturais, de forma a compatibilizar tanto o desempenho térmico por meio do coeficiente de expansão térmica (CET) como a compatibilidade química com os demais componentes da célula. A originalidade deste estudo está na busca destas características em regiões de composições ainda não exploradas, localizadas dentro do triangulo de compatibilidade BS-B2S-BAS2 na região rica em bário do sistema ternário. Entre estes vidros sintetizados quatro composições (BAS-4, BAS-5, BAS-6 e BAS-7) foram escolhidas porque são as mais adequadas às solicitações termomecânicas exigidas para um material vítreo atuar como selante em SOFC. / The design for planar cells Fuel Solid Oxide (SOFC) is better than the tubular due to its higher current density and lower manufacturing cost. However, the design of planar SOFC requires sealant to prevent leakage of fuel and the mixture of gases at high temperatures. Glasses and glass-ceramics have proven to be the most suitable because they have good compatibility with the other components of the cell at working temperature (700-1000°C). In the present study, a series of compositions belonging to the BaO-Al2O3-SiO2 (BAS) system with the addition of B2O3 were synthesized having the appropriate proportions of each component oxide. It was proposed to improve this system using relevant levels of formers and structural modifiers oxides, in order to match both the thermal performance of thermal expansion coefficient (TEC) and chemical compatibility with other components of the cell. The originality of this study is to search for these characteristics in regions of compositions not yet explored, located inside the compatibility triangle BS-B2S-BAS2 at the barium rich part of the ternary diagram. Among the synthesized glasses four batch compositions (BAS-4, BAS-5, 6-BAS, BAS-7) were chosen because best matched the thermo-mechanical required for a glassy material to act as SOFCs sealant.

O desenho planar para as células a Combustível de Óxido Sólido (SOFC) é melhor do que o tubular devido a sua maior densidade de corrente e menor custo de fabricação. No entanto, o projeto de SOFC planar requer selantes para evitar o vazamento de combustível e a mistura de gases em altas temperaturas. Os vidros e os vitrocerâmicos têm demonstrado serem os mais adequados por apresentarem boa compatibilidade com outros componentes da célula nas temperaturas de trabalho das SOFCs (700-1000°C). No presente estudo, uma série de composições pertencentes ao sistema BaO-Al2O3-SiO2 (BAS) com a adição de B2O3 foram sintetizados tomando as proporções apropriadas de cada óxido constituinte. Propôs-se melhorar este sistema utilizando-se formadores e teores relevantes de modificadores estruturais, de forma a compatibilizar tanto o desempenho térmico por meio do coeficiente de expansão térmica (CET) como a compatibilidade química com os demais componentes da célula. A originalidade deste estudo está na busca destas características em regiões de composições ainda não exploradas, localizadas dentro do triangulo de compatibilidade BS-B2S-BAS2 na região rica em bário do sistema ternário. Entre estes vidros sintetizados quatro composições (BAS-4, BAS-5, BAS-6 e BAS-7) foram escolhidas porque são as mais adequadas às solicitações termomecânicas exigidas para um material vítreo atuar como selante em SOFC. / The design for planar cells Fuel Solid Oxide (SOFC) is better than the tubular due to its higher current density and lower manufacturing cost. However, the design of planar SOFC requires sealant to prevent leakage of fuel and the mixture of gases at high temperatures. Glasses and glass-ceramics have proven to be the most suitable because they have good compatibility with the other components of the cell at working temperature (700-1000°C). In the present study, a series of compositions belonging to the BaO-Al2O3-SiO2 (BAS) system with the addition of B2O3 were synthesized having the appropriate proportions of each component oxide. It was proposed to improve this system using relevant levels of formers and structural modifiers oxides, in order to match both the thermal performance of thermal expansion coefficient (TEC) and chemical compatibility with other components of the cell. The originality of this study is to search for these characteristics in regions of compositions not yet explored, located inside the compatibility triangle BS-B2S-BAS2 at the barium rich part of the ternary diagram. Among the synthesized glasses four batch compositions (BAS-4, BAS-5, 6-BAS, BAS-7) were chosen because best matched the thermo-mechanical required for a glassy material to act as SOFCs sealant.

Τα προηγμένα (δομικά ή λειτουργικά) κεραμικά θεωρούνται ως τα πλέον κατάλληλα υλικά για εφαρμογές όπου απαιτούνται υψηλές θερμοκρασίες. Διαθέτουν μία σειρά από πλεονεκτήματα όπως π.χ. αντοχή σε θερμικούς αιφνιδιασμούς, υψηλή σκληρότητα, αντοχή σε φθορά και διάβρωση και μεγάλο εύρος στις τιμές των ηλεκτρικών τους ιδιοτήτων. Από τεχνολογική άποψη ενδιαφέρον παρουσιάζει ο συνδυασμός τους με μεταλλικές φάσεις με στόχο την συνένωση υλικών ή την παρασκευή σύνθετων κεραμομεταλλικών υλικών. Κεραμικές ενώσεις οξειδίων μπορεί να χρησιμοποιηθούν στην τεχνολογία των κελιών καυσίμου στερεού ηλεκτρολύτη (SOFC) ως μονωτικά ή στεγανωτικά υλικά. Ιδιαίτερο ενδιαφέρον παρουσιάζει η αλληλεπίδραση τους στην διεπιφάνεια σε επαφή με άργυρο και κράματα με βάση τον άργυρο για χρήση ως εναλλακτικών, σε αντικατάσταση των υαλοκεραμικών, συγκολλητικών μεταξύ των στρώσεων των μεμονωμένων στοιβάδων των SOFC. Σημαντικό ρόλο στη μικροδομή και τις ιδιότητες των υλικών αυτών παίζουν τα φαινόμενα διαβροχής και η ισχύς του δεσμού που αναπτύσσεται στη διεπιφάνεια κεραμικού / μετάλλου, καθώς και οι επιφανειακές και διεπιφανειακές ενέργειες των υλικών ή των συστημάτων των υλικών που βρίσκονται σε επαφή. Για το λόγο αυτό η γνώση των επιφανειακών και διεπιφανειακών μεγεθών είναι απαραίτητη για την πρόβλεψη των ιδιοτήτων των συστημάτων σε επαφή. Σκοπός της παρούσας εργασίας είναι η μελέτη της συνάφειας και των διεπιφανειακών ιδιοτήτων σε συστήματα κεραμικών οξειδίων σε επαφή με ρευστές μεταλλικές φάσεις και ιδιαίτερα σε συστήματα του κεραμικών οξειδίων σε επαφή με ρευστές μεταλλικές φάσεις αργύρου, με τελικό σκοπό την εφαρμογή των συστημάτων αυτών στην τεχνολογία των SOFC. Στο πρώτο μέρος της εργασίας, εξετάσθηκε η επίδραση του διαλυτοποιημένου οξυγόνου στην επιφανειακή ενέργεια του ρευστού άργυρου και του ρευστού χαλκού. Από τις εξισώσεις που εξήχθησαν είναι δυνατός ο προσδιορισμός της επιφανειακής ενέργειας τους για δεδομένη θερμοκρασία και μερική πίεση οξυγόνου. Υπολογίσθηκε η ελεύθερη ενέργεια προσρόφησης του οξυγόνου στην επιφάνεια του ρευστού χαλκού, μέχρι τον κορεσμό. Διατυπώθηκε επίσης μία σχέση για τον υπολογισμό της διαλυτότητας ενός οξειδίου στα ρευστά μέταλλα σε εξάρτηση με την θερμοκρασία και την μερική πίεση του οξυγόνου στην ατμόσφαιρα του πειράματος. Στη συνέχεια, με χρήση ενός συνδυασμού βιβλιογραφικών και πειραματικών δεδομένων σχετικά με τις τιμές της επιφανειακής ενέργειας και τις γωνίας επαφής σε συστήματα κεραμικών οξειδίων σε επαφή με διάφορα ρευστά μέταλλα βελτιστοποιήθηκε μια εμπειρική σχέση η οποία, σε δεδομένη θερμοκρασία, συνδέει άμεσα την επιφανειακή ενέργεια των στερεών οξειδίων με την επιφανειακή ενέργεια των ρευστών μετάλλων και τη γωνία επαφής. Μέσω αυτής της σχέσης είναι δυνατή η εκτίμηση της επιφανειακής ενέργειας ενός στερεού οξειδίου ή της γωνίας επαφής σε μη διαβρέχοντα και μη αντιδρώντα συστήματα κεραμικών οξειδίων / ρευστών μετάλλων, με την προϋπόθεση ότι η μερική διαλυτοποίηση οξυγόνου του κεραμικού μέσα στο ρευστό μέταλλο δεν επηρεάζει τις διεπιφανειακές ιδιότητες του συστήματος. Η σχέση αυτή επαληθεύθηκε για διάφορα συστήματα κεραμικών οξειδίων / ρευστών μετάλλων και επιπλέον εφαρμόσθηκε για τον προσδιορισμό της επιφανειακής ενέργειας του πολυκρυσταλλικού οξειδίου Y2O3 μετά από πειράματα διαβροχής από ρευστό άργυρο, του πολυκρυσταλλικού οξειδίου 3YTZ (3mol% Yttria partial stabilized zirconia) και του μικτού πολυκρυσταλλικού οξειδίου 85wt% MgO + 15 wt% MgAl2O4, μετά από πειράματα διαβροχής με ρευστό άργυρο. Στο δεύτερο μέρος της εργασίας πραγματοποιήθηκαν πειράματα διαβροχής κεραμικών οξειδίων από τήγμα αργύρου σε οξειδωτικές συνθήκες (αέρας) για να εξετασθεί η επίδραση του οξυγόνου στις διεπιφανειακές ιδιότητες του συστήματος, καθώς η τεχνολογία των SOFC απαιτεί οι διεργασίες αυτές να πραγματοποιούνται σε συνθήκες περιβάλλοντος. Διαπιστώθηκε ότι η παρουσία οξυγόνου βελτιώνει τη διαβρεξιμότητα στα συστήματα κεραμικών / μετάλλου αυξάνοντας την ισχύ του δεσμού στην διεπιφάνεια, όμως η γωνία θ παραμένει θ > 90◦ (κακή διαβροχή). Σημαντική ελάττωση της γωνίας επαφής επιτυγχάνεται με προσθήκη διεπιφανειακά ενεργών συστατικών στο τήγμα του συγκολλητικού μετάλλου αυξάνοντας σημαντικά το έργο συνάφειας και ως εκ τούτου την ισχύ του δεσμού στην διεπιφάνεια κεραμικού/μετάλλου. Για τον ίδιο λόγο πραγματοποιήθηκαν πειράματα διαβροχής κεραμικών οξειδίων από οξείδια με βάση το βόριο και το λίθιο, στον αέρα, με σκοπό να εξετασθεί η συνοχή μεταξύ των φάσεων σε επαφή. Τέλος εξετάσθηκε η διαβροχή του χάλυβα Crofer 22 APU, ο οποίος χρησιμοποιείται στην τεχνολογία των SOFC, από τις ίδιες ρευστές φάσεις, με στόχο να εξετασθεί η δυνατότητα χρήσης τους ως συγκολλητικές φάσεις σε κελιά καυσίμου στερεού ηλεκτρολύτη. / Advanced ceramics (structural or functional) are considered to be the most suitable for use in high temperature applications. They have a number of advantages, such as resistance to thermal shocks, high hardness, wear and corrosion resistance and a wide range in the values of their electrical properties. Special interest is being manifested in the compounds of ceramics with metals and metal alloys, in the field of materials joining and the production of composite materials. Ceramic compounds are used in the field of solid oxide fuel cells (SOFC) as insulators and sealing materials. Particular interest has been stimulated in the interaction in the interface of ceramics in contact with liquid silver and silver based alloys, as alternatives to the glass-ceramics sealing materials in SOFC stacks. In all of these cases the surface and interfacial energies of the materials or the materials systems used, as well as the wetting and bonding phenomena at the interface, play a key role in obtaining materials with the desired properties and microstructure. The aim of the present work is the study of adhesion and interfacial properties in ceramic oxide / liquid metal systems, particularly in systems of ceramic oxides in contact with liquid silver and silver-based alloys, with the ultimate aim of implementing such systems in the SOFC technology. In the first part of this work, the effect of the dissoluted oxygen on the surface energy of liquid copper and liquid silver was examined. The equations that were deriverd can be used to calculate their surface energy as a function of the temperature and the partial pressure of the oxygen. The free energy of the oxygen adsorption in the surface of the liquid copper was calculated, until saturation. Also, an equation that allows to calculate the solubility of an oxide in a liquid metal was deriverd, as a function of the temperature and the oxygen partial pressure. Moreover, from the combination of literature and experimental data of interfacial energies and contact angles in non-wetting and non-reactive ceramic oxide/liquid metal systems where the limited solubility of oxygen of the ceramic oxides into the liquid metalls has no effect on the interfacial properties, has led to an empirical relationship which correlates at a given temperature the surface energy of the oxides with the contact angle and the surface energy of the liquid metal. This relationship allows either the calculation of the surface energy of an oxide from known values of the surface energy of a liquid metal and the contact angle, or conversely, the estimation of the contact angle value, as well as the work of adhesion, for known surface energy of the oxide. The formulated empirical relationship has been applied to additional non-wetting and non-reactive systems of oxides in contact with liquid metals and the results showed good agreement with literature data. In addition, the empirical formula was used to calculate the surface energies of the polycrystalline oxides Y2O3 and 3YTZ (3mol% Yttria partial stabilized zirconia) as well as the 85wt% MgO + 15 wt% MgAl2O4 mixed oxide, after wetting experiments with liquid copper and/or liquid silver in an Ar- 4%H2 atmosphere. In the second part of this work, the effect of the oxygen on the the interfacial properties of the ceramics / liquid silver systems was examined by wetting experiments, in order to achieve conditions similar to the SOFC operating conditions. The results showed that the presence of the oxygen improves the wetability in the ceramic / liquid metal systems, increasing the bond in the interface but the angle remains θ > 90◦ (non wetting systems). The addition of interfacial active compounds in the liquid metal led to a significant decrease in the contact angle value, with the simultaneous increase in the work of adhesion, and so to the increase in the strength of the bond. For this purpose and in order to examine the adhesion between the two phases, wetting experiments with lithium and borium based oxides took place. Finally, the above liquid phases were used in wetting experiments on steel substrate (Crofer 22 APU) in order to investigate the potential usage of them as sealing and insulators in SOFC technology.

Le développement actuel des piles à combustible SOFC fonctionnant à température intermédiaire suppose l'optimisation des méthodes de synthèse et de mise en forme pour les matériaux nouveaux développés au cours des dernières années. En effet, les propriétés électrochimiques de ces dispositifs sont étroitement liées aux caractéristiques des poudres de départ ainsi qu'à la microstructure des électrodes (ou de l'électrolyte) après leur mise en forme. Une amélioration significative des dites propriétés peut être obtenue par la nanostructuration des matériaux. Dans ce contexte, ce travail de thèse est consacré à l’élaboration du matériau de cathode Nd2NiO4+d ainsi que du matériau d'électrolyte Ce1-xAxO2-d. Les méthodes mises en œuvre sont la synthèse de nanopoudres en milieux éthanol/eau supercritiques et par voie pyrosol ainsi que le dépôt de couches minces en milieu CO2 supercritique. Les objets obtenus ont enfin été caractérisés par spectroscopie d'impédance électrochimique afin de quantifier leur performance pour l’application SOFC. / The ongoing development of Intermediate Temperature Solid Oxide Fuel Cells implies the optimization of the synthesis and deposition methods for the new materials developed these past years. Indeed, electrochemical properties of these materials are closely linked to the initial powder characteristics as well as the electrode (or electrolyte) microstructure after deposition. Significant improvement of the aforementioned properties can be obtained via nanostructuration of the materials. Thus, this thesis is dedicated to the synthesis of the cathode material Nd2NiO4+d and the electrolyte material Ce1-xAxO2-d. Methods employed are namely nanopowder synthesis in water/ethanol supercritical mixtures and spray pyrolysis as well as thin film deposition in supercritical fluids. The obtained objects have finally been characterized by electrochemical impedance spectroscopy in order to assess their performance for the SOFC application.