Catalytic combustion is a promising technology for heat and power applications, especially gas turbines. By using catalytic combustion ultra low emissions of nitrogen oxides (NOX), carbon monoxide (CO) and unburned hydrocarbons (UHC) can be reached simultaneously, which is very difficult with conventional combustion technologies. Besides achieving low emission levels, catalytic combustion can stabilize the combustion and thereby be used to obtain stable combustion with low heating-value gases. This thesis is focused on the high-temperature part of the catalytic combustor. The level of performance demanded on this part has proven hard to achieve. In order to make the catalytic combustor an alternative to the conventional flame combustor, more stable catalysts with higher activity have to be developed. The objective of this work was to develop catalysts with higher activity and stability, suitable for the high-temperature part of a catalytic combustor fueled by natural gas. Two template-based preparation methods were developed for this purpose. One method was based on soft templates (microemulsion) and the other on hard templates (carbon). Supports known for their stability, magnesia and hexaaluminate, were prepared using the developed methods. Catalytically active materials, perovskite (LaMnO3) and ceria (CeO2), were added to the supports in order to obtain catalysts with high activities and stabilities. The supports were impregnated with active materials by using a conventional technique as well as by using the microemulsion technique. It was shown that the microemulsion method can be used to prepare catalysts with higher activity compared to the conventional methods. Furthermore, by using a microemulsion to apply active materials onto the support a significantly higher activity was obtained than when using the conventional impregnation technique. Since the catalysts will operate in the catalytic combustor for extended periods of time under harsh conditions, an aging study was performed on selected catalysts prepared by the microemulsion technique. The stability of the catalysts was assessed by measuring the activity before and after aging at 1000 C in humid air for 100 h. One of the most stable catalysts reported in the literature, LMHA (manganese-substituted lanthanum hexaaluminate), was included in the study for comparative purposes. The results showed that LMHA deactivated much more strongly compared to several of the catalysts consisting of ceria supported on lanthanum hexaaluminate prepared by the developed microemulsion method. Carbon templating was shown be a very good technique for the preparation of high-surface-area hexaaluminates with excellent sintering resistance. It was found that the pore size distribution of the carbon used as template was a crucial parameter in the preparation of hexaaluminates. When a carbon with small pores was used as template, the formation of the hexaaluminate crystals was strongly inhibited. This resulted in a material with poor sintering resistance. On the other hand, if a carbon with larger pores was used as template, it was possible to prepare materials with hexaaluminate as the major phase. These materials were, after accelerated aging at 1400 C in humid air, shown to retain surface areas twice as high as reported for conventionally prepared materials. / QC 20100719

Une étude systématique du phénomène de relaxation diélectrique dans les gammes de fréquence 1 MHZ-1GHZ et de température de 250-500 K, a été effectuée sur des céramiques de titanate de baryum substituées. Il existe, quelle que soit la composition, un minimum de la fréquence de relaxation et un maximum de la dispersion diélectrique à chaque température de transition de phases. La relaxation est de type dipolaire ; elle est liée au déplacement des ions, en site octaédrique disposés suivant des chaînes (ou volume) de corrélation.

Ce travail se place dans le cadre de la recherche de céramiques susceptibles de fonctionner à plus hautes température que la zircone yttriée, le système le plus employé actuellement pour constituer la couche isolante des barrières thermiques de turbines, aéronautiques ou terrestres. Une analyse des critères auxquels doit répondre une telle céramique a conduit à choisir la famille des perovskites, et, en conclusion de calculs de conductivité thermique par dynamique moléculaire, à sélectionner BaMg1/3Ta2/3O3 (BMT), un composé dont le point de fusion est supérieur à 2900°C. Un mode d'élaboration par voie solide a été développé pour obtenir cet oxyde sous forme dense. En guise de référence, des matériaux denses de zircones yttriées complètement stabilisées ont également été élaborés. Ces deux oxydes ont ensuite été caractérisées expérimentalement : structure par diffraction de rayons X, conductivité thermique jusqu'à 1200°C à partir de la mesure de la diffusivité thermiques par méthode flash laser, coefficient de dilatation entre 200 et 1200°C. Une étu de du gravage thermique de joints de grains, avec suivi par microscopie à force atomique du profil de joints après traitements entre 1100°C et 1400°C, a permis dans certains cas de déterminer le coefficient de diffusion en surface. Il ressort de cette étude que, si la perovskite BMT possède un ensemble de propriétés intéressantes pour constituer une barrière thermique : stabilité structurale, conductivité thermique à 1200°C du même ordre que celle de la zircone yttriée (autour de 2 W.m-1.K-1), coefficient de dilatation élevé (11.10-5 /K entre 200 et 1200°C), des phénomènes de sublima tion sont susceptibles de perturber la stabilité de couches à très hautes températures.

L'ensemble des résultats décrits dans la thèse constitue une contribution a l' etude des relaxations diélectriques dans des céramiques de structure perovskite qu'elles soient férroelectriques ou relaxeurs. Ils ont montré les relations étroites entre fréquence de relaxation, température de transition, nanostructure et liaison chimique. Ils permettent d'entrevoir des lois qui régissent la variation des caractéristiques des relaxations avec la nature des substitutions cationiques, le rapport entre le nombre d'atomes B' et B'' en site octaédrique, l'ordre ou le désordre

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L' intérêt des phases oxydes à base de terres rares est certes multiple et leurs propriétésont été explorées depuis longtemps: mais nous nous intéressons ici aux comportements de cesphases en tant que phases thermiques, catalytiques ou conductrices ioniques à hautetempérature. Il s'agit en particulier de développer des systèmes innovants de matériauxpouvant intervenir dans la conception de dispositifs pour la microélectronique, pour capteursde gaz ou membranes sélectives ou pour systèmes dépolluants.Les phases de structure pyrochlore ou fluorine de type TR2Ce207 où TR désigne unélément de Terre Rare présentent divers potentiels d' applications: elles ont été considéréescomme des phases pouvant résister à de hautes températures. Les oxydes de structurespérovskites, de formule générale ABO), présentent de multiples applications potentielles,notamment en tant que phases diélectriques pour condensateurs, ou phases conductricesioniques (en ions oxygène ou en protons) pour électrolytes solides, du fait même de leur hautestabilité chimique à haute température.Ce travail a été divisé en deux parties. La première a consisté à élaborer la phase" thermique)} de type pyrochlore TR2Ce20 7 (TR = La, Ce, ... ) en utilisant un minéralcomplexe à base d'allanite-monazite et de silico-aluminates issus de déchets industriels, doncà bas coût.La deuxième partie a consisté à élaborer BaCe03 et à étudier ses propriétéscatalytiques et conductimétriques en fonction de la température. Une nouvelle méthode desynthèse reposant sur l' utilisation du mélange EDTA-citrate a été utilisée afin d'élaborer unprécurseur, qui, traité thermiquement à 950°C, a permis d'élaborer des poudressubmicroniques de la phase BaCe03. L'activité catalytique du composé BaCe03 démarre à450°C pour atteindre la conversion totale à 675°C : dans cette gamme de température,l'efficacité catalytique de la phase BaCe03 est maximale. L'évolution de la conductivité enfonction de la température de pastilles compactées de BaCe03 a révélé l'existence d'une sériede modifications électriques fortement corrélées aux transitions structurales connues pourBaCe03 dans la littérature. À basse température (300 à 450°C), la faible conductivité de laphase orthorhombique, associée à la faible énergie d'activation, peut être liée à la migrationdes défauts extrinsèques (gaz adsorbés). Cependant, au-dessus de 500°C, la conductivité de laseconde phase orthorhombique augmente: ceci pourrait être attribué à une mobilité croissantedes atomes d'oxygène.

The literature review indicates that studies on the structures and properties of perovskites materials have shown attractive applications in the fields of energy, catalysts and sensing materials for fuel cell and sensing industries. Kinetic phenomena in thin film systems, such as solid state amorphization and interface reactions due to diffusion during thin film deposition, are introduced indicating that thermal stability is a concern in thin film sensor device application. The principle of traditional methods of materials characterization is briefly discussed. Emphasis is placed on the functions of analytical transmission electron microscopy. / The SrFeO3/SiO2/Si and SrFeO3/Al 2O3 thin film systems have been studied using transmission electron microscopy. The thin films of SrFeO3 were grown by pulsed laser deposition. For the SrFeO3/SiO2/Si system, TEM characterization showed that the microstructure of the film deposited at room temperature contained crystalline and amorphous layers. Silicon diffusion into SrFeO3 films occurred at the SiO2 interface. The silicon-induced interfacial reactions resulted in phase transformations and the growth of complex crystalline and amorphous phases. The principal compositions of these phases were Sr(Fe,Si)12O19, SrOx and amorphous [Sr-Fe-Si-O]. / The films in the SrFeO3/Al2O3 system were deposited onto single crystal and sintered polycrystalline Al2O 3 substrates at room temperature and 700°C and subjected to annealing for various periods of time at 700-1000°C. TEM characterization showed that the morphology of the film varied with changes in deposition temperature: a columnar structure was produced at room temperature and layers containing crystalline grains were produced at 700°C. The interfacial structures of the films remained unchanged below 700°C. Interfacial reactions were observed following annealing at 850°C for 5 hours. The phase transformation at the interface was characterized for the film annealed at 1000°C for 5 hours, for which the principal phases were identified as SrAl2-xFe xO4 and SrFe12-yAlyO19. As a result, an isothermal section at 1000°C of a ternary phase diagram for SrO-Al2O3-Fe2O3 is proposed. Evaluation for thin film conductometric sensing applications indicated that the untreated films deposited at 700°C onto both single crystal and sintered Al2O3 substrates exhibited a similar temperature dependency of conductivity in air and a p-type gas sensor response to oxygen and propane at 500°C.

Šiame darbe vienfazis perovskito struktūros gadolinio aliuminatas GdAlO3 zolių-gelių metodu buvo gautas 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, kompleksus sudarančiu reagentu – 1,2 etandiolį, natūralią vyno rūgštį, citrinų rūgštį arba citrinų rūgšties ir etanolamino mišinį. Sintetinant GdAlO3, gautų tyrimų rezultatai parodė, kaip yra svarbu zolių gelių procese pasirinkti tinkamas pradines medžiagas bei kompleksus su metalais sudarančius reagentus: gadolinio jonų šaltiniu naudojant Gd2O3 buvo gauti geresni rezultatai nei naudojant Gd(NO3)3•6H2O, vienfazis GdAlO3 gali būti sėkmingai susintetintas kompleksus sudarančiu reagentu naudojant tiek natūralią vyno rūgštį, tiek 1,2 etandiolį, tiek citrinų rūgštį, tiek citrinų rūgšties ir etanolamino mišinį. Nustatyta, kad sintetinant stronciu pakeistus gadolinio aliuminatus Gd1–xSrxAlO3– (x = 0,10, 0,25, 0,50, 0,75), tinkamesnis gadolinio jonų šaltinis yra gadolinio oksidas nei gadolinio nitratas heksahidratas, kompleksus sudarantis reagentas – EDTA nei natūrali vyno rūgštis. Vienfazis perovskito struktūros stronciu pakeistas gadolinio aliuminatas Gd0,90Sr0,10AlO3– buvo susintetintas zolių-gelių metodu 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, stroncio jonų šaltiniu – Sr(NO3)2, kompleksus sudarančiu reagentu – EDTA. Be to, 800 ºC iškaitinimo temperatūra yra per maža šiems junginiams sintetinti. Pirmą kartą pasiūlytas vandeninis zolių gelių metodas lantano ir gadolinio kobaltatams... [toliau žr. visą tekstą] / In the present thesis the reported results let us to conclude that the developed aqueous sol–gel procedure could be successfully used for the low-temperature synthesis of monophasic perovskite gadolinium aluminate ceramics. It was also demonstrated that the selection of raw materials for the sol–gel processing should be done with care. Monophasic perovskite structure gadolinium aluminate GdAlO3 has been synthesized by sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material and 1.2 ethanediol, natural tartaric acid, citric acid or mixture of citric acid and ethanolamine as complexing agent. According to the XRD results, the Gd2O3 instead of Gd(NO3)36H2O preferably should be used as starting material for the preparation of GdAlO3. Monophasic perovskite structure strontium substituted gadolinium aluminate Gd0.90Sr0.10AlO3– was synthesized by the same sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material, strontium nitrate as strontium ion source and EDTA as complexing agent. It was proved that better gadolinium ion source is gadolinium oxide than gadolinium nitate hexahydrate, better complexing agent – EDTA than natural tartaric acid in the synthesis of GdAlO3 and strontium substituted Gd1–xSrxAlO3– (x = 0.10, 0.25, 0.50, 0.75) samples. According to the XRD results, increasing amount of strontium reduces amount of peaks of perovskite structure gadolinium aluminate. Besides, 800 ºC temperature is too low for the... [to full text]

In the present thesis the reported results let us to conclude that the developed aqueous sol–gel procedure could be successfully used for the low-temperature synthesis of monophasic perovskite gadolinium aluminate ceramics. It was also demonstrated that the selection of raw materials for the sol–gel processing should be done with care. Monophasic perovskite structure gadolinium aluminate GdAlO3 has been synthesized by sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material and 1.2 ethanediol, natural tartaric acid, citric acid or mixture of citric acid and ethanolamine as complexing agent. According to the XRD results, the Gd2O3 instead of Gd(NO3)36H2O preferably should be used as starting material for the preparation of GdAlO3. Monophasic perovskite structure strontium substituted gadolinium aluminate Gd0.90Sr0.10AlO3– was synthesized by the same sol gel synthesis method at 1000 oC, using gadolinium oxide as gadolinium raw material, strontium nitrate as strontium ion source and EDTA as complexing agent. It was proved that better gadolinium ion source is gadolinium oxide than gadolinium nitate hexahydrate, better complexing agent – EDTA than natural tartaric acid in the synthesis of GdAlO3 and strontium substituted Gd1–xSrxAlO3– (x = 0.10, 0.25, 0.50, 0.75) samples. According to the XRD results, increasing amount of strontium reduces amount of peaks of perovskite structure gadolinium aluminate. Besides, 800 ºC temperature is too low for the... [to full text] / Šiame darbe vienfazis perovskito struktūros gadolinio aliuminatas GdAlO3 zolių-gelių metodu buvo gautas 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, kompleksus sudarančiu reagentu – 1,2 etandiolį, natūralią vyno rūgštį, citrinų rūgštį arba citrinų rūgšties ir etanolamino mišinį. Sintetinant GdAlO3, gautų tyrimų rezultatai parodė, kaip yra svarbu zolių gelių procese pasirinkti tinkamas pradines medžiagas bei kompleksus su metalais sudarančius reagentus: gadolinio jonų šaltiniu naudojant Gd2O3 buvo gauti geresni rezultatai nei naudojant Gd(NO3)3•6H2O, vienfazis GdAlO3 gali būti sėkmingai susintetintas kompleksus sudarančiu reagentu naudojant tiek natūralią vyno rūgštį, tiek 1,2 etandiolį, tiek citrinų rūgštį, tiek citrinų rūgšties ir etanolamino mišinį. Nustatyta, kad sintetinant stronciu pakeistus gadolinio aliuminatus Gd1–xSrxAlO3– (x = 0,10, 0,25, 0,50, 0,75), tinkamesnis gadolinio jonų šaltinis yra gadolinio oksidas nei gadolinio nitratas heksahidratas, kompleksus sudarantis reagentas – EDTA nei natūrali vyno rūgštis. Vienfazis perovskito struktūros stronciu pakeistas gadolinio aliuminatas Gd0,90Sr0,10AlO3– buvo susintetintas zolių-gelių metodu 1000 oC temperatūroje, gadolinio jonų šaltiniu naudojant Gd2O3, stroncio jonų šaltiniu – Sr(NO3)2, kompleksus sudarančiu reagentu – EDTA. Be to, 800 ºC iškaitinimo temperatūra yra per maža šiems junginiams sintetinti. Pirmą kartą pasiūlytas vandeninis zolių gelių metodas lantano ir gadolinio kobaltatams... [toliau žr. visą tekstą]

Carbonatites are igneous rocks comprising more than 50 modal percent of carbonate minerals and characterized by highly variable modal compositions. The majority of carbonatites are confined to intra-continental rifts, whereas occurrences associated with plate margins and orogenic settings are less common. Petrogenesis of carbonatites has been a matter of intense debate for several decades. The possible genetic models include crystallisation from a primary carbonatite magma, liquid immiscibility and crystal fractionation from carbonate-rich silicate magma. In contrast to the voluminous bulk-rock trace-element data and major-element analyses of minerals from carbonatites available in the literature, there has been no systematic study concerned with the trace-element signatures of the most common constituents of these rocks. This work is the first comprehensive study of the interrelations between the trace-element chemistry of the most common constituents of carbonatites, the geochemistry of these rocks, and their tectonic setting. The rock samples examined represent 21 different localities worldwide. The extent of major- and trace-element substitutions in amphibole, clinopyroxene, trioctahedral micas, dolomite, magnetite and perovskite is investigated in detail. The silicate minerals from carbonatites exhibit much larger compositional diversity than previously recognized. They can incorporate significant amounts of such petrogenetically important elements as Sr, REE, Zr, Nb and Ta. The majority of studied clino-amphibole- and clinopyroxene-group minerals exhibit previously unrecognized a bimodal distribution patterns of REE, which can be explained in terms of crystal chemistry of these phases. The trace-element signature of phlogopite from carbonatites, particularly Nb, Mn, Ni and Cr, is distinctly different from that of phlogopite from kimberlites, and can be used as a reliable petrogenetic indicator. Compositional variations in dolomite reflect magmatic and subsolidus processes in carbonatites. Magnetite from carbonatites follows a well-defined magmatic and previously unrecognized reaction trend. Contrary to prior studies, this mineral is only a minor host of HFSE in carbonatitic rocks. The U-Pb age data, trace-element and Sr-isotopic composition of perovskite from the Afrikanda carbonatite and clinopyroxenite suggest that the two rocks are not related by crystal fractionation. This study underlines the importance of a systematic approach in petrogenetic studies based on trace-element distribution.