Es wurden die Zusammenhänge zwischen den Herstellungsbedingungen und dem nanostrukturellen Aufbau von Multischichten, mit Rücksicht auf das mechanische Verhalten aufgeklärt. Dazu wurden durch plasmaunterstützte Gasphasenabscheidung (PACVD) Hartmetallsubstrate mit Viellagen beschichtet und vorrangig mittels analytischer Transmissionselektronenmikroskopie (TEM) charakterisiert. Als Schichkomponenten wurden hauptsächlich TiN und Al2O3 untersucht, daneben aber auch Schichtsysteme der Komponenten AlON, (TiAl)N, und (Ti,Al)ON. Darüber hinaus wurden noch TiC-aC (TiC mit amorphem Kohlenstoffanteil)-Schichten einbezogen. Ziel waren gleichmäßige Multischichten mit Korngrößen von einigen Nanometern, geringer Testur und geringer Mikrorissdichte, die hart sind und gut haften. Die TEM-Untersuchungen dienten insbesondere der Aufklärung der Nanostruktur in den Interface-Bereichen der Schichtsysteme, wobei an Hand der Elektronenenergie-Verlustspektroskopie (EELS) sowohl element- als auch phasenspezifische Signale ausgewertet wurden. Zur verbesserten Bewertung der anfallenden Datenmengen wurden z. T. faktoranalytische Methoden eingesetzt. Je nach Prozessführung der Schichtherstellung kommt es in den Interface-Bereichen zur Durchmischung der Komponenten. Insbesondere führt diffundierender Sauerstoff zur Bildung von TiO2, was sich nachteilig auf die Qualität der Schichten auswirkt. Die Tiefe der "gestörten" Zonen begrenzt die wünschenswerte Verringerung der Einzelschichtdicken. Als wirkungsvolle Gegenmaßnahme hat sich der Einbau von Kohlenstoff erwiesen, wodurch sich dünnere Einzelschichten verwirklichen lassen.

The submerged entry nozzle (SEN) has been used to transport the molten steel from tundish to the mould. The main purpose of the SEN usage is both to prevent oxygen and nitrogen pick-up by molten steel and to achieve the desired flow condition in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. Furthermore, the steelmaking processes occur at high temperatures around 1873 K so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during pre-heating and casting is necessary for the design of the steelmaking processes. The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during pre-heating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied in supplement 1. The results of the study indicated the penetration of the formed alkaline-rich glaze into the Alumina/graphite base refractory during pre-heating. More specifically, the alkaline-rich glaze reacts with graphite to form carbon monoxide gas. Thereafter, dissociation of CO at the SEN/molten metal interface takes place. This leads to reoxidation of dissolved REM (Rare Earth Metal), which form the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to a formation of a high-viscous alumina-rich glaze during the SEN pre-heating process. This in turn, creates a very uneven surface at the SEN internal surface. The “In Situ” formation of the REM oxides together with the uneven internal surface of the SEN may facilitate the accumulation of the primary inclusions. Supplement 1 revealed the disadvantages of the glass/silicon powder layer. On the other hand the carbon oxidation is a main industrial problem for un-coated Alumina/Graphite Submerged Entry Nozzles (SEN) during pre-heating. This led to the proposal of a new refractory material for the SEN. In supplement 2, the effect of ZrSi2 antioxidant and the coexistence of antioxidant additive and (4B2O3 ·BaO) glass powder on carbon oxidation were investigated at simulated non-isothermal heating conditions in a  controlled atmosphere. Also, the effect of ZrSi2 antioxidant on carbon oxidation was investigated at isothermal temperatures at 1473 K and 1773 K. The specimens’ weight losses and temperatures were plotted versus time and compared to each others. The thickness of the oxide areas were measured and also examined using XRD, FEG-SEM and EDS. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina/Graphite base refractory materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by (4B2O3 ·BaO) glaze during green body sintering led to an excellent carbon oxidation resistance. In supplement 3, decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials constituting a commercial Submerged Entry Nozzle (SEN), have been investigated in different gas atmosphere consisting of CO2, O2 and Ar. The (CO2/O2) ratio values were kept the same as it is in propane combustion flue gas at Air Fuel Ratio (AFR) values equal to 1.5 and 1 for both Air-fuel and Oxygen-fuel combustions. Laboratory experiments were carried out non-isothermally in the temperature range 873 K to 1473 K at 15 K/min followed by isothermal heating at 1473 K for 60 min. The decarburization ratio (α) values of the three refractory types were determined by measuring the weight losses of the samples. The results showed that the decarburization ratio (α) values of the MgO-C refractory became 3.1 times higher for oxygen-fuel combustion compared to air-fuel combustion both at AFR equal to 1.5 in the temperature range 873 K to 1473 K. The decarburization ratio (α) values for Al2O3-C samples were the same as for the isothermal heating at 1473 K and non-isothermal heating in the temperature range 473  to 1773 K with a 15 K/min heating rate. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times instead of heating at lower temperatures for longer holding times. Jander’s diffusion model was proposed for estimating the decarburization rate of Al2O3-C refractory in the SEN. The activation energy for Al2O3-C samples heated at AFR equal to 1.5, for air-fuel and oxygen-fuel combustions were found to be 84.5 KJ/mol and 95.5 KJ/mol respectively during non-isothermal heating in the temperature range 873 K to 1473 K. / QC 20101008

No description available.

Κατά τις τελευταίες δεκαετίες παρατηρείται συνεχής αύξηση της έντασης του φαινομένου του θερμοκηπίου γεγονός που προκαλεί σημαντικές συνέπειες στο περιβάλλον και στη ζωή μας γενικότερα. Συνεπώς, είναι απαραίτητη η μείωση της ανθρωπογενούς εκπομπής των αερίων που συμβάλλουν στην αύξηση του φαινομένου αυτού. Η εκμετάλλευση και χρήση των δύο πιο σημαντικών θερμοκηπικών αερίων, του μεθανίου και του διοξειδίου του άνθρακα, μπορεί να επιτευχθεί με την αναμόρφωση του CH4 με CO2 ή αλλιώς ξηρή αναμόρφωση του μεθανίου (Dry Reforming of Methane-DRM). Με τη διεργασία DRM τα δύο συγκεκριμένα αέρια μετατρέπονται σε αέριο σύνθεσης (synthesis gas), το οποίο χρησιμοποιείται είτε για τη σύνθεση πληθώρας οργανικών ενώσεων, είτε για την παραγωγή Η2 για ενεργειακούς σκοπούς. Η DRM παρουσιάζει σημαντικά πλεονεκτήματα: δεν απαιτείται η χρήση ύδατος, φθηνό σχετικά κόστος εγκαταστάσεων, χρησιμοποιείται σε χημικά συστήματα μεταφοράς ενέργειας, ενώ και το αέριο σύνθεσης που παράγεται έχει ακόμα κατάλληλη αναλογία για συνθέσεις Fischer–Tropsch. Παρόλα αυτά η DRM δεν έχει εκτεταμένη βιομηχανική εφαρμογή επειδή αντιμετωπίζει ένα σημαντικό μειονέκτημα: ο καταλύτης μετά από κάποιο χρόνο λειτουργίας απενεργοποιείται λόγω του άνθρακα που αποτίθεται πάνω του. Στην παρούσα εργασία μελετήθηκε ο state of the art καταλύτης Ni/Al2O3, τον οποίο τροποποιήσαμε με βόριο σε διάφορους λόγους [Β/(B+Νi)] με κύριο στόχο τη μείωση των ανθρακούχων αποθέσεων. Οι τροποποιημένοι καταλύτες συντέθηκαν με τη μέθοδο του υγρού συνεμποτισμού και χαρακτηρίστηκαν φυσικοχημικά με διάφορες τεχνικές, ώστε να μελετήσουμε την επίδραση του βορίου στην υφή τους (ΒΕΤ, porosimetry, SEM, TEM), στη δομή τους (XRD, UV-Vis DRS) και στην αναγωγιμότητά τους (H2-TPR). Η καταλυτική συμπεριφορά τους για την αντίδραση της ξηρής αναμόρφωσης του μεθανίου αξιολογήθηκε σε αντιδραστήρα σταθερής κλίνης, για 24h, σε συνθήκες: 973Κ, 1 atm, τροφοδοσία 50%CH4-50%CO2. Ο άνθρακας που αποτέθηκε στους χρησιμοποιημένους καταλύτες μετρήθηκε με τη μέθοδο της θερμοπρογραμματισμένης υδρογόνωσης (TPH). Τα ανηγμένα και χρησιμοποιημένα στην DRM καταλυτικά δείγματα μελετήθηκαν επίσης με ηλεκτρονικό μικροσκόπιο σάρωσης (SEM με αναλυτή EDS) και ηλεκτρονικό μικροσκόπιο διαπερατότητας (ΤΕΜ). Βρέθηκε ότι η παρουσία του Β μειώνει σημαντικά την ποσότητα του αποτιθέμενου άνθρακα στους καταλύτες Ni/Al2O3, σε ποσοστό έως και 86%, χωρίς να επηρεάζει ιδιαίτερα τη δραστικότητα και την εκλεκτικότητα των καταλυτών. Σημαντικό ρόλο παίζει το ποσοστό του Β στον καταλύτη, με τον καταλύτη με λόγο Β/(B+Νi) = 0,5 να εμφανίζει τη βέλτιστη συμπεριφορά. Τα αποτελέσματα μας έδειξαν ότι η ιδιαίτερη θετική επίδραση του βορίου οφείλεται κυρίως στο γεγονός ότι ευνοεί τη διασπορά του μεταλλικού νικελίου. Τροποποίηση με βόριο, σε κατάλληλη περιοχή φορτίσεων, του καταλύτη Ni/Al2O3 μεγιστοποιεί το πλήθος των νανοσωματιδίων νικελίου με μέση διάσταση < 6.0 nm, τα οποία, ως γνωστόν, ελαχιστοποιούν την απόθεση άνθρακα. / The intensity of the greenhouse effect is constantly increasing in the last few decades with an adverse effect both on the environment and the humanity. In order to decrease the effect, human-caused emissions should be minimized. The two most important greenhouse gases, methane and carbon dioxide, can be used in the DRM (Dry Reforming of Methane) process. With this process the above mentioned gases are converted to synthesis gas, which is then used for the synthesis of a great number of organic compounds and synthetic fuels (through the Fisher-Tropsch syntheses) or for the production hydrogen to be used as a fuel (energy purposes). The DRM process presents a number of advantages, namely: no water is required, relatively low cost of process plants,it can be used as a Chemical Energy Transfer System and, finally, the produced synthesis gas has adequate CO/H2 ratio for Fisher-Tropsch syntheses. Although DRM is a promising process, its industrial application is hindered by a major drawback: the catalysts are rapidly deactivating due to coking. In the current study, the state of the art catalyst Ni/Al2O3 was studied and modified with boron, using different ratios of Β/(B+Νi). Our primary objective was to reduce coking. The modified catalysts were synthesized by wet co-impregnation and physicochemically characterized in their oxidic, reduced and used forms, using various techniques, in order to investigate the influence of boron on the texture (BET, Porosimetry, SEM, TEM), structure (XRD, UV-Vis DRS) and reducibility (H2-TPR) of the catalysts. The catalytic performance for the DRM process was studied under stable conditions (973Κ, 1 atm and 50%CH4-50%CO2 undiluted feed), for 24h, using a fixed bed reactor. Carbonaceous deposits on the used catalysts were determined by Temperature Programmed Hydrogenation (TPH). Scanning Electron Microscopy (SEM) with EDS analyser and Transmission Electron Microscopy (TEM) were also used in the study of reduced and used catalytic samples. Modifying Ni/Al2O3 catalysts with boron results in a great decrease of the deposited coke (up to 86%), without any significantly influence on the activity and selectivity of the catalysts. A major factor influencing the catalyst is the B loading, with the ratio Β/(B+Νi)=0,5 giving the best results. Boron’s positive effect was mainly attributed to its ability to increase Ni dispersion. Modification of Ni/Al2O3 catalysts, by using the appropriate boron loading, resulted to an increase of the amount of nickel nanoparticles with an average dimension under 6.0 nm, which are known to minimize coke deposition.

Les techniques de frittage ultrarapide « naturel » (sans charge) comme le frittage par micro-ondes ou par induction présentent de nombreux avantages. Toutefois, le développement de ces techniques passe par une compréhension et une maitrise des mécanismes mis en jeu. A la fois similaires et complémentaires, ces procédés de chauffage-frittage ont été étudiées par une approche expérimentale afin d’approfondir les connaissances dans le domaine du Frittage Ultrarapide Naturel (FUN). Au cours de cette thèse, l’étude du frittage par micro-ondes de composites céramiques Al2O3-(Y)ZrO2 (3 à 40%vol.) a été menée parallèlement à celle du frittage par induction d’une poudre métallique micronique de nickel. La démarche expérimentale adoptée a consisté à réaliser des expériences de frittage à vitesses de chauffage imposées (de 25 à 1000°C/min) sur ces matériaux modèles et des pièces de petites dimensions, en se référant aux comportements en frittage conventionnel tant au niveau macroscopique qu’au niveau microscopique. / The techniques of ultrafast pressureless sintering as microwave or induction sintering offer manyadvantages. However, the development of these techniques requires an understanding and a control ofthe mechanisms involved. Both similar and complementary, these processes of heating-sintering havebeen studied by an experimental approach to increase knowledge in the field of Ultrafast PressurelessSintering.In this thesis, the study of microwave sintering of Al2O3-(Y)ZrO2 composites has been conductedin parallel with induction sintering of a submicronic nickel powder. The experimental approach usedconsisted in carrying out sintering experiments at imposed heating rates (from 25 to 1000°C/min) onchosen materials and small parts, referring to conventional sintering behavior at the macroscopic andmicroscopic scale.

[pt] No presente trabalho foram realizadas medidas de capacidade térmica à pressão constante do espinélio Al2MnO4 na faixa entre 2 e 873 K. No intervalo entre 2 e 300 K empregou-se um calorímetro de relaxação térmica. Os dados evidenciaram a presença de uma anomalia em torno de 33 K, cuja componente magnética pôde ser constatada mediante medidas de capacidade calorífica com campo magnético constante, bem como também medidas de magnetização específica como função da temperatura. A contribuição entrópica associada à mencionada anomalia foi considerada no cálculo da entropia molar a 298.15 K do óxido em questão (116.05 mais ou menos 5.2 J/mol.K), valor este consistente com valores da literatura para outros espinélios. Na faixa entre 323 e 873 K empregou-se um calorímetro diferencial de varredura. Os dados foram ajustados quantitativamente com o modelo de Berman e Brown, incluindo-se no ajuste o valor de capacidade térmica a 298.15 K, obtido via calorimetria de relaxação térmica. Empregando-se o valor de entropia molar determinado no presente trabalho, os parâmetros do modelo de Berman e Brown estimados com os dados em temperaturas elevadas, e uma estimativa disponível na literatura para a entalpia de formação do óxido Al2MnO4, construiu-se um modelo para a dependência térmica da energia de Gibbs do referido composto válido na faixa entre 298.15 e 2114 K. O modelo foi testado com sucesso no acesso termodinâmico das propriedades do sistema Al2O3-MnO. / [en] In the present work the constant pressure molar heat capacity of the spinel Al2MnO4 was measured between 2 K and 873 K. In the interval between 2 K and 300 K a relaxation calorimeter was employed. The data indicated the presence of a thermal anomaly around 33 K, whose magnetic component could be evidenced through measurements of the heat capacity with a constant applied magnetic field, and also through specific magnetization data as a function of temperature. The entropic contribution of the thermal anomaly was considered in the calculation of the molar entropy of the oxide at 298.15 K (116.05 more or less 5.2 J/mol.K), and the calculated value has proven to be consistent with values published earlier for other spinel compounds. In the interval between 323 and 873 K a differential scanning calorimeter was employed. The data were quantitatively modeled with the function proposed by Berman and Brown, including the heat capacity value obtained at 298.15 K accessed through the relaxation calorimeter route. By using the molar entropy at 298.15 K, the values of the parameter estimated for the Berman and Brown model with the heat capacity data at elevated temperatures, and an estimative for the heat of formation of the spinel Al2MnO4 extracted from the literature, it was possible to construct a model for the thermal dependence of the Gibbs energy of this compound valid between 298.15 K and 2114 K. The model was successfully tested in the thermodynamic assessment of the properties of the system Al2O3-MnO.

Les potentialités du nitrure de gallium (GaN) et notamment de l'hétérostructure AlGaN/GaN, semiconducteur à large bande interdite, en font un matériau particulièrement intéressant en électronique de puissance, notamment pour des applications haute tension, haute température et haute fréquence. L'objectif de ce travail de thèse était de développer et d'optimiser l'étape d'isolation de la grille lors la réalisation de transistors MIS-HEMT de puissance sur hétérostructure AlGaN/GaN, le but étant de réduire les courants de fuite de grille sans perturber les propriétés du transistor. Après avoir évaluation, le choix s'est porté sur l'alumine Al2O3 déposé par ALD comme diélectrique de grille. L'étude s'est d'abord concentrée sur l'analyse de l'influence de traitements, chimiques ou plasma, sur la contamination de la surface d'AlGaN au travers d'analyses XPS et AFM. Puis, l'influence du diélectrique de grille a été évalué à travers la réalisation et la mesure électrique de dispositifs, diodes et transistors, en variant les méthodes de dépôt par ALD. Enfin, l'impact d'un recess par gravure ICP-RIE partielle ou complètes de la barrière d'AlGaN sous la grille a été étudiée. La réalisation d'un HEMT passe par l'étape critique du dépôt du diélectrique de grille sur le semiconducteur, et le contrôle de la qualité de l'interface " diélectrique/AlGaN " est donc une étape fondamentale car elle influe sur les propriétés électriques du composant. Ce contrôle comprend le traitement de surface du semiconducteur, mais aussi la nature et la technique de dépôt du diélectrique. Ainsi il apparaît à travers l'étude qu'un traitement de surface à l'ammoniaque à haute température est le plus efficaces pour retirer les contamination en oxydes natifs. Les mesures électriques, C(V) et Id(Vg), ont quant à elle montrés la supériorité de la PEALD par rapport à un dépôt thermique conventionnel. Ceci peut s'expliquer par le fait que le plasma oxygène qui entre jeu lors du dépôt de l'alumine par PEALD semble nettoyer la surface lors des premiers cycles, retirant notamment la contamination carbone. Cela permet d'avoir une meilleure interface entre l'alumine et le semi-conducteur, limitant les pièges à l'interface et dans l'oxyde. Cela a réduit de manière considérable les courants de fuite de grille, sans détériorer la qualité et la rapidité de la transition entre l'état on et off. De plus, les HEMTs réalisé étant de type normally-off, le recess de grille par gravure ICP-RIE a été implémenté afin de rendre moins négative la tension de pincement. Cela a été réalisé avec succès, notamment avec la réalisation d'un composant de type noramlly-off grâce à un recess total de la barrière d'AlGaN sous la grille. Des résultats à l'état de l'art ont été obtenus à travers une approche simple, et un processus de création de transistors robuste et hautement reproductible, avec une réduction importante des courants de fuite de grille et une pente sous le seuil record. Afin de compléter l'étude il conviendra par la suite de réaliser des études de fiabilité, notamment à travers des mesures dynamiques pour évaluer notamment les phénomènes de dégradation du Ron. / With its large band gap, Gallium Nitride (GaN) semiconductor is one of the most promising materials for new power devices generation thanks to its outstanding material properties for high voltage, temperature and frequency applications. The main objective of this thesis was the development and optimization of the insulating step taking place in the elaboration of MIS-HEMT transistors on an AlGaN/GaN heterstroctructure. In order to reduce gate leakage currents without degrading the device properties, alumina Al2O3 deposited by ALD was chosen as a gate dielectric. The study was first centered on the influence of surface treatments, chemical or plasma, regarding surface contamination. Their impact was analyzed through XPS and AFM. Secondly, electrical measures were performed on complete MIS-HEMT diodes and transistors to evaluate the influence of the alumina insulating layer depending on the ALD deposition method. Lastly, partial and full recess of the AlGaN barrier was studied via ICP-RIE etching. The gate dielectric deposition is one of the crucial steps intervening in the HEMT creation process. The quality and control at the Al2O2/AlGaN interface being paramount, it will directly influence the device's electric properties. This involves control ing the semiconductor surface, but also the nature and deposition technique of the dielectric. As such, an ammonia-based treatment at high temperature appears to be the most efficient in reducing native oxygen contamination. Regarding electric performances, C(V) and Id(Vg) measures showed the superiority of PEALD compared to traditional thermal ALD deposition. This can be explained by the fact that the oxygen plasma used as oxydant during the alumina deposition by PEALD seems to clean the surface during the first cycles, mostly by reducing carbon contamination. This allowed to achieve a better interface between the semiconductor and the insulting layer, thus limiting traps at the interface or in the oxyde. This allows to considerably reduce gate leakage currents, without degrading the quality and transition sharpness between the on and off state. Moreover, the realized HEMTs being normally-off, gate recess etching via ICP-RIE was implemented in order to make the threshold voltage less negative. This was successfully achieved, especially through the realization of a normally-off transistors thanks to a full recess of the AlGaN barrier under the gate. State of the art results were achieved through a simple approach, and a robust and highly reproducible transistor elaboration process, with great reduction of gate leakage currents and a record sub-threshold slope. In order to complete the study, it will be necessary in the future to proceed to viability studies, especially through dynamic electric evaluation, in order to evaluate for instance Ron degradation phenomenons.

The submerged entry nozzle (SEN) is used to transport the molten steel from a tundish to a mould. The main purpose of its usage is to prevent oxygen and nitrogen pick-up by molten steel from the gas. Furthermore, to achieve the desired flow conditions in the mould. Therefore, the SEN can be considered as a vital factor for a stable casting process and the steel quality. In addition, the steelmaking processes occur at high temperatures around 1873 K, so the interaction between the refractory materials of the SEN and molten steel is unavoidable. Therefore, the knowledge of the SEN behaviors during preheating and casting processes is necessary for the design of the steelmaking processes  The internal surfaces of modern SENs are coated with a glass/silicon powder layer to prevent the SEN graphite oxidation during preheating. The effects of the interaction between the coating layer and the SEN base refractory materials on clogging were studied. A large number of accretion samples formed inside alumina-graphite clogged SENs were examined using FEG-SEM-EDS and Feature analysis. The internal coated SENs were used for continuous casting of stainless steel grades alloyed with Rare Earth Metals (REM). The post-mortem study results clearly revealed the formation of a multi-layer accretion. A harmful effect of the SENs decarburization on the accretion thickness was also indicated. In addition, the results indicated a penetration of the formed alkaline-rich glaze into the alumina-graphite base refractory. More specifically, the alkaline-rich glaze reacts with graphite to form a carbon monoxide gas. Thereafter, dissociation of CO at the interface between SEN and molten metal takes place. This leads to reoxidation of dissolved alloying elements such as REM (Rare Earth Metal). This reoxidation forms the “In Situ” REM oxides at the interface between the SEN and the REM alloyed molten steel. Also, the interaction of the penetrated glaze with alumina in the SEN base refractory materials leads to the formation of a high-viscous alumina-rich glaze during the SEN preheating process. This, in turn, creates a very uneven surface at the SEN internal surface. Furthermore, these uneven areas react with dissolved REM in molten steel to form REM aluminates, REM silicates and REM alumina-silicates. The formation of the large “in-situ” REM oxides and the reaction of the REM alloying elements with the previously mentioned SEN´s uneven areas may provide a large REM-rich surface in contact with the primary inclusions in molten steel. This may facilitate the attraction and agglomeration of the primary REM oxide inclusions on the SEN internal surface and thereafter the clogging. The study revealed the disadvantages of the glass/silicon powder coating applications and the SEN decarburization. The decarburization behaviors of Al2O3-C, ZrO2-C and MgO-C refractory materials from a commercial Submerged Entry Nozzle (SEN), were also investigated for different gas atmospheres consisting of CO2, O2 and Ar. The gas ratio values were kept the same as it is in a propane combustion flue gas at different Air-Fuel-Ratio (AFR) values for both Air-Fuel and Oxygen-Fuel combustion systems. Laboratory experiments were carried out under nonisothermal conditions followed by isothermal heating. The decarburization ratio (α) values of all three refractory types were determined by measuring the real time weight losses of the samples. The results showed the higher decarburization ratio (α) values increasing for MgO-C refractory when changing the Air-Fuel combustion to Oxygen-Fuel combustion at the same AFR value. It substantiates the SEN preheating advantage at higher temperatures for shorter holding times compared to heating at lower temperatures during longer holding times for Al2O3-C samples. Diffusion models were proposed for estimation of the decarburization rate of an Al2O3-C refractory in the SEN. Two different methods were studied to prevent the SEN decarburization during preheating: The effect of an ZrSi2 antioxidant and the coexistence of an antioxidant additive and a (4B2O3 ·BaO) glass powder on carbon oxidation for non-isothermal and isothermal heating conditions in a controlled atmosphere. The coexistence of 8 wt% ZrSi2 and 15 wt% (4B2O3 ·BaO) glass powder of the total alumina-graphite refractory base materials, presented the most effective resistance to carbon oxidation. The 121% volume expansion due to the Zircon formation during heating and filling up the open pores by a (4B2O3 ·BaO) glaze during the green body sintering led to an excellent carbon oxidation resistance. The effects of the plasma spray-PVD coating of the Yttria Stabilized Zirconia (YSZ) powder on the carbon oxidation of the Al2O3-C coated samples were investigated. Trials were performed at non-isothermal heating conditions in a controlled atmosphere. Also, the applied temperature profile for the laboratory trials were defined based on the industrial preheating trials. The controlled atmospheres consisted of CO2, O2 and Ar. The thicknesses of the decarburized layers were measured and examined using light optic microscopy, FEG-SEM and EDS. A 250-290 μm YSZ coating is suggested to be an appropriate coating, as it provides both an even surface as well as prevention of the decarburization even during heating in air. In addition, the interactions between the YSZ coated alumina-graphite refractory base materials in contact with a cerium alloyed molten stainless steel were surveyed. The YSZ coating provided a total prevention of the alumina reduction by cerium. Therefore, the prevention of the first clogging product formed on the surface of the SEN refractory base materials. Therefore, the YSZ plasma-PVD coating can be recommended for coating of the hot surface of the commercial SENs. / <p>QC 20111014</p>

Alumino-silicate systems are of great interest for materials scientists and geochemists. Thermodynamic knowledge of these systems is useful in steel and ceramic industries, and for understanding geochemical processes. A popular and efficient approach used to obtain a self-consistent thermodynamic dataset is called CALPHAD. It couples phase diagram information and thermochemical data with the assistance of computer models. The CALPHAD approach is applied in this thesis to the thermodynamic modelling and assessments of the CaO-Al2O3-SiO2, MgO-Al2O3-SiO2 and Y2O3-Al2O3-SiO2 systems and their subsystems. The compound energy formalism is used for all the solution phases including mullite, YAM, spinel and halite. In particular, the ionic two sub-lattice model is applied to the liquid solution phase. Based both on recent experimental investigations and theoretical studies, a new species, AlO2-1, is introduced to model liquid Al2O3. Thus, the liquid model corresponding for a ternary Al2O3-SiO2-M2Om system has the formula (Al+3,M+m)P (AlO2-1,O-2, SiO4-4,SiO20)Q, where M+m stands for Ca+2, Mg+2 or Y+3. This model overcomes the long-existing difficulty of suppressing the liquid miscibility gap in the ternary systems originating from the Al2O3-free side during the assessments. All the available and updated experimental information in these systems are critically evaluated and finally a self-consistent thermodynamic dataset is achieved. The database can be used along with software for Gibbs energy minimization to calculate any type of phase diagram and all thermodynamic properties. Various phase diagrams, isothermal and isoplethal sections, and thermochemical properties are presented and compared with the experimental data. Model calculated site fractions of species are also discussed. All optimization processes and calculations are performed using the Thermo-Calc software package. / QC 20100607

Les industries aéronautiques tentent de mettre au point de nouvelles structures plus résistantes et surtout réutilisables en vue d'accélérer la conquête de l'espace. A cet effet, le matériau composite à matrice intermétallique NiAl renforcée par des fibres longues d'alumine semble prometteur et adapté à ces applications. Les travaux présentés concernent a la fois l' élaboration du composite par infiltration liquide sous pression de gaz et l'étude du couplage fibre-matrice (F/M), phénomène prépondérant qui gouverne les caractéristiques macroscopiques du matériau. Le couplage a été étudié par le biais de la microscopie électronique en transmission et de la spectroscopie de photo-électrons X ; une approche thermodynamique du couplage a ete obtenue au travers de l'étude de mouillabilite du renfort par la matrice. Ces différentes caractérisations des interfaces et interphases sont correlées les unes avec les autres pour permettre une compréhension globale des phénomenes de couplage a l'interface F/M.