L'objectif de cette thèse était de synthétiser des particules de carbonate de calcium (CaCO3) poreuses pour applications industrielles comme charge dans du papier à cigarette, ainsi que pour l'encapsulation d’'arômes. Nous avons cherché à maîtriser les paramètres de synthèse pour obtenir de la vatérite de taille contrôlée. Nous avons étudié sa transformation à haute température et dans l’'eau, car ce polymorphe est métastable. La transition de phase vatérite/calcite a été étudié par DRX et imagerie par diffraction des rayons X cohérents qui permet d’accéder à l’'image en 3D des particules. Nous avons montré que la vatérite de taille 1 à 2µm présentant 20% de porosité peut être synthétisée de façon reproductible. Les particules préparées ont été introduites comme charge dans du papier à cigarette pour évaluer l’'impact de nouvelles formes de CaCO3 sur les propriétés physiques du papier ainsi que sur la réduction des certains composées nocifs contenus dans la fumée. Nous avons développé l’analyse conjointe de l’'absorption et de la diffraction des rayons X pour estimer la charge réelle introduite ainsi que la porosité des papiers. Nous avons démontré que l’'utilisation de CaCO3 sous forme des sphères poreuses permet d’'augmenter la diffusivité du papier et de réduire l’'émission de CO dans la fumée principale.L’encapsulation d'arômes par la co-cristallisation et l'inclusion moléculaire dans le carbonate de calcium a été aussi étudiée. Nous avons montré que CaCO3 peut être utilisé comme matrice d’'imprégnation d'arômes avec une efficacité d’'encapsulation de plus que 55%. Les particules aromatiques ont été après ajoutées dans le papier pour évaluation sensorielle. / The aim of this thesis was to synthesize porous calcium carbonate (CaCO3) particles for industrial applications as fillers for cigarette paper as well as a matrix for flavour encapsulation. We show that we can control the fabrication of porous particles of vaterite with a given size by tuning the parameters of synthesis. After the synthesis, the stability of vaterite in aqueous solution and at high temperature was studied. The phase transition was analyzed by XRD and coherent X-ray diffraction imaging that allows us to have a 3D-image of the particles. Finally, particles of 1-2 μm size with 20% porosity were reproducibly synthesized. Prepared vaterite particles were introduced as a filler in cigarette paper, with the goal to evaluate their impact on the physical properties of papers as well as on the reduction of some harmful compounds during the smoking. It was demonstrated that the use of vaterite can increase the diffusivity of paper and reduce the CO emission in the mainstream smoke. We also show that the use of X-ray absorption and diffraction can provide an estimation of the filler fraction and porosity of the papers in a non-destructive way. The encapsulation of flavours in CaCO3 particles was performed by co-crystallization and molecular inclusion. It was demonstrated that CaCO3 can be used as a matrix for flavour impregnation with more than 55% of encapsulation efficiency. Flavoured particles was added in paper for sensory evaluation. We shown that it is possible, to flavour the final product with flavoured calcium carbonate particles.

Nous avons étudié par STM, SQUID, Réflectivité X, Diffraction des rayons X, absorption optique et XAS des échantillons de Fe(phen)2(NCS)2 et Fe{[3,5-dimethylpyrazolyl]3BH}2 déposé par évaporation thermique sur des substrats de Cu(100), Co(100) et SiO2, et comparé avec des échantillons en poudre. Nous avons confirmé l'existence de l'effet de piégeage d'état de spin induit par les rayons X (SOXIESST), et étudié ses propriétés, en particulier dynamiques. Celui-ci dépend de l'intensité et de la structure du faisceau X appliqué, et est non-résonant. Nous suggérons que son efficacité est influencée également par les états de transfert de charge métal-ligand (MLCT). L'étude des molécules isolées a montré que l'on pouvait les faire transiter par une impulsion électrique, et construire ainsi des dispositifs memrésistifs, mais seulement si l'influence du substrat est suffisamment réduite. À l'aide d'un modèle thermodynamique simple, nous avons alors étudié les couches minces et montré que la coopérativité est réduite et que la température de transition est modifiée (plus grande pour la Fe-phen, plus faible pour la Fe-pyrz). Enfin, nous utilisons ces résultats pour construire des dispositifs multicouche verticaux Au/Fe-phen/Au dont les propriétés électriques, d'après nos résultats préliminaires, sont dépendantes des stimuli extérieurs (température, champ magnétique). Notamment, ils présentent un effet « diode » à la transition de spin. / We have studied by STM, SQUID, X-ray reflectivity, X-ray diffraction, optical absorption and XAS Fe(phen)2(NCS)2 and Fe{[3,5-dimethylpyrazolyl]3BH}2 samples deposited by thermal evaporation on Cu(100), Co(100) and SiO2 substrates, and compared with results on powder samples. We have confirmed the existence of the soft X-ray induced excited spin state trapping (SOXIESST), and investigated its properties, in particular dynamic aspects. The effect is sensitive to the intensity and the structure of the applied X-ray beam, and is non-resonant. We suggest that its efficiency is also governed by metal-ligand charge transfer states (MLCT). The study of single molecules has revealed that they could be switched by voltage pulses, and by this way building memristive devices, but only if the influence of the substrate is sufficiently reduced. We have then investigated thin films with the help from a simple thermodynamic model, and evidenced that the cooperativity was reduced and the transition temperature is modified (higher for Fe-phen, and lower for Fe-pyrz). Finally, we use these results to build multilayer vertical devices Au/Fe-phen/Au, and its electrical properties depends, according to our preliminary results, on the external stimuli (temperature, magnetic field). Notably, they present a “diode” effect at the spin transition.

O titanato e zirconato de chumbo \'PB\'(\'ZR\'1-y\'TI\'y)\'O IND.3\' é um material ferroelétrico de estrutura perovskita que tem sido aplicado como transdutores, amplificadores, sensores piezoelétricos, piroelétricos e memórias ferroelétricas. É bem estabelecido que a incorporação de íons de \'LA POT.3+\' ou \'BA POT.2+\' nos sítios ocupados pelo \'PB\' no sistema \'PB\'(\'ZR\'1-y\'TI\'y)\'O\' (PZT), formando os sistemas \'PB\'1-x\'LA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PLZT) e \'PB\'1-x\'BA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PBZT), provoca mudanças significativas nas suas propriedades. No entanto, poucos trabalhos tem sido dedicados a esses sistemas contendo altas concentrações de \'TI\', principalmente no que se refere à estrutura desses materiais. Assim, este trabalho teve por objetivo analisar as propriedades estruturais e suas correlações com as propriedades dielétricas dos sistemas \'PB\'1-x\'LA\'x\'ZR\'0,40\'TI\'0,60\'O IND.3\' (PLZT100x) e \'PB\'1-x\'BA\'x\'ZR\'0,40\'TI\'0,60\'O IND.3\' (PBZT100x) em função da composição e da temperatura. Foram preparadas amostras cerâmicas por meio de sinterização convencional com x variando entre 0,05 e 0,21 para o sistema PLZT e entre 0,10 e 0,50 para o sistema PBZT. Em relação à estrutura a longa distância, medidas de difração de raios X mostraram uma diminuição no grau de tetragonalidade com o aumento da concentração dos cátions substituintes, que foi atribuída à formação de vacâncias no sítio A (caso do \'LA\') e diferença entre o raio iônico (caso do \'BA\'). Estas alterações estruturais em função da composição foram também responsáveis pelo aumento do grau de difusidade das curvas de permissividade dielétrica e pela observação de um estado ferroelétrico relaxor nas amostras contendo altas concentrações de \'LA\' e \'BA\'. Em relação à estrutural local, os resultados obtidos através da técnica de espectroscopia de absorção de raios X (XAS) nas bordas \'K\' do \'TI\' e LIII do \'PB\' mostraram que a incorporação de átomos de \'LA\' ou \'BA\' à estrutura do PZT leva a uma redução no deslocamento do átomo de \'TI\' em relação ao centro do octaedro \'TI\'O IND.6\' e mudanças na ordem local do átomo de \'PB\'. No que tange as composições contendo 21% at. de \'LA\' e 50% at. de \'BA\', diferentemente dos resultados de DRX que mostraram uma simetria cúbica, a técnica de XAS mostrou uma simetria local tetragonal. Em bom acordo com os resultados obtidos pela técnica de espectroscopia Raman, espectros EXAFS medidos em altas temperaturas mostraram também que a estrutura local não é compatível com uma estrutura de simetria cúbica. Espectros XANES medidos na borda \'K\' do oxigênio revelaram uma redução no grau de hibridização entre os estados 2p do \'O\' com 6sp do \'PB\' à medida que a concentração de \'LA\' ou \'BA\' aumenta, que estaria relacionada com o surgimento de comportamento relaxor. Amostras cerâmicas densas nanoestruturadas de composição PZT, PLZT11 e PBZT10 foram preparadas pelo método de spark plasma sintering (SPS) a fim de analisar a influência do tamanho de grão. Foi verificado que as amostras sinterizadas por SPS apresentam tamanho de grão em torno de 60 nm. A caracterização dielétrica destas amostras mostra que a redução do tamanho de grão causa uma redução no valor de máximo da permissividade dielétrica e características difusas da permissividade em função da temperatura devido ao aumento das regiões de contorno de grão. / Lead titanate zirconate (\'PB\'(\'ZR\'1-x\'TI\'x)\'O IND.3\') are ferroelectric materials with perovskite structure which has been used as transducers, capacitors, piezoelectric and pyroelectric sensors and ferroelectric memories. The substitution of \'PB POT.+2\' ions by \'LA POT.+3\' or \'BA POT.+2\' ions in the \'PB\'(\'ZR\'1-x\'TI\'x)\'O IND.3\' (PZT) system, which leads to the formation of the \'PB\'1-x\'LA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PLZT) and the \'PB\'1-x\'BA\'x\'ZR\'1-y\'TI\'y\'O IND.3\' (PBZT) systems, induces several changes in the electric and structural properties of these materials. However, PLZT or PBZT systems based on \'TI\'-rich compositions have not been thoroughly investigated and the literature contains few reports concerning their structure. Thus, the main objectives of this doctoral thesis were the synthesis and structural characterization of \'PB\'1-x\'R\'x\'ZR\'0.40\'TI\'0.60\'O IND.3\' ferroelectric ceramic samples, with R = \'BA\' and \'LA\' and x between 0.00 to 0.50 (PLZT100x and PBZT100x). The characterization with X-ray diffraction technique of these samples showed a decrease of the tetragonality degree with increase of the doping cation concentration, which was related to the appearance of defects caused by the incorporation of \'LA\' or \'BA\' cations. These structural modifications were also responsible by the increase of the diffuseness at the dielectric permittivity and a relaxor behavior as a function of the \'LA\' or \'BA\' concentration. Concerning the local structure, XANES spectra in the absorption edge of various elements in PLZT and PBZT samples were performed. In the cases of \'TI\' \'K\'-edge absorption, the doping of \'LA\' and \'BA\' atoms in the PZT structure leads to a reduction of the displacement of \'TI\' atom in the center of the \'TI\'O IND.6\' octahedron. However, even when the crystal structure is cubic, a local octahedron distortion remains. EXAFS measurements in \'PB\' LIII-edge and \'ZR\' \'K\'-edge were performed and also indicate that local structure around lead or zirconium atoms is also affected by the introduction of \'LA\' and \'BA\' atoms in the PZT structure. In addition, XANES spectra measured at \'O\' \'K\'-edge revealed a reduction in the hybridization degree between \'O\' 2p and \'PB\' 6sp states with the addition of \'LA\' or \'BA\' atoms to the structure of PZT. It has been shown that hybridization between these states is essential to ferroelectricity and this reduction would be related to the relaxor behavior. PLZT and PBZT systems were also studied depending on the size of particle size in a nanometer scale. Thus samples PZT, PLZT11 and PBZT10 compositions were prepared using the synthesis method of precursor polymers and the process of sintering by spark plasma. A pronounced decrease in the values of maximum permittivity was observed and the dielectric curve as a function of the temperature exhibits a diffuse behavior. This size-induced diffuse phase transition and the reduction of the permittivity magnitude could be related to the differences between the core grain and the grain boundaries.

Orientador: Carlos Kenichi Suzuki / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-11T16:06:32Z (GMT). No. of bitstreams: 1 Nunes_ClaudiaCarvalho_M.pdf: 7074659 bytes, checksum: 4dec2d1e75a640341af023ba7541f5cb (MD5) Previous issue date: 2008 / Resumo: O sistema binário sílica titânia, SiO2-TiO2,apresenta grande interesse tecnológico devido as suas propriedades: ultra-baixo coeficiente de expansão térmico e alto índice de refração. As propriedades são dependentes da quantidade de dopante presente no material. A SiO2-TiO2 é usada na indústria de componentes fotônicos, tais como, lentes, micro-lentes, espelhos e fibras ópticas especiais. Um importante material óptico para litografia no extremo ultravioleta (EUVL) é caracterizado por concentrações próximas a 8 % em peso de TiO2, com estrutura amorfa e transparente num amplo espectro da região UV, visível e IV. A indústria óptico-eletrônica necessita de fibras ópticas sensoras com alto índice de refração. Este tipo de fibra pode ser obtido através da fabricação de uma fibra de SiO2-TiO2. Amostras com concentrações que variam de 1 a 14 % em peso TiO2 fabricadas pelo método do aerosol em chama foram caracterizadas quanto a suas propriedades estruturais e ópticas. As amostras com concentrações superiores a 7,5 % em peso apresentaram-se translúcidas ou opacas, e com estrutura octaédrica segundo dados de XANES (Absorção de Raios-X próximo à estrutura da borda). A difração de raios-X identificou a fase anatásio da titânia como estrutura octaédrica. Através de tratamentos térmicos a altas temperaturas em chama (> 1500 ºC) obtivemos amostras transparentes no visível, amorfa, contendo 8,2 % em peso. Foi obtido um material com índice de refração de 1,48 para concentração de 10 % em peso de TiO2. Portanto a SiO2-TiO2 é viável para a fabricação de uma fibra com alto índice de refração / Abstract: The binary SiO2-TiO2 system presents a great technological importance due to its special properties: ultra low thermal expansion and high refractive index. The properties depend on the amount of dopant present in material. The SiO2-TiO2 is used for the components of photonic industries, such as lenses, mirrors and special fiber optics. As an important optical material for extreme ultraviolet lithography (EUVL), it is characterized by concentrations of ~ 8 wt. % of TiO2, with amorphous structure and transparent for a large UV, visible and IR spectra. The optoelectronic industries needs optical fiber sensor that presents high refractive index. This kind of fiber can be produce by SiO2-TiO2 fiber production. Samples with concentrations varying in the range 1 to 14 wt. % TiO2 made by flame aerosol technique were characterized in terms of structural and optical properties. As-consolidated samples with concentrations up to 7.5 wt. % TiO2 presented itself translucent or opaque with octahedral structure, according to XANES data (X-ray absorption near-edge structure). The anatase phase of titania was identified by X-ray diffraction patterns. However, by high temperature flame heat treatments (> 1500 ºC), it was possible to obtain transparent SiO2-TiO2 materials in the visible range with concentration up to 8.2 wt. % TiO2. The material presents refractive index 1.48 with concentration about 10 wt. % TiO2. Therefore SiO2-TiO2 is feasible to fibers fabrication with high refractive index / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Ce travail se place dans le cadre des réacteurs nucléaires de 4ème génération tels que les réacteurs à neutrons rapides et caloporteur gaz (RNR-G), pour lesquels des matériaux réfractaires comme le nitrure de titane (TiN) enroberont le combustible afin de permettre la rétention des produits de fission. Cette étude a porté sur le comportement thermique intragranulaire du xénon dans des échantillons de TiN obtenus par frittage à chaud sous charge. Le rôle de la microstructure sur le comportement thermique du xénon a été étudié. Plusieurs lots ont ainsi été synthétisés sous différentes conditions de température et de composition de la poudre initiale. Le xénon a été introduit par implantation ionique. Les échantillons ont ensuite subi des traitements thermiques entre 1300°C et 1600°C, soient les températures accidentelles envisagées. Un transport majoritaire du xénon vers la surface a été mis en évidence. Ce dernier est ralenti lorsque la température de frittage augmente. Des différences de comportement ont été observées selon les poudres mises en oeuvre dans la synthèse et selon l'orientation cristalline du grain considéré. Le relâchement du xénon a également été corrélé à l’oxydation de TiN. Des bulles de Xe dès 0,38 % atomique ont été observées. Leur taille est proportionnelle à la concentration en Xe et augmente avec la température de recuit, d’où une certaine mobilité du Xe au sein de TiN. Plusieurs mécanismes pouvant expliquer cette mobilité sont proposés. En complément, des calculs ab initio ont confirmé le caractère fortement insoluble du Xe dans TiN et révélé que les bilacunes sont les plus favorables à l'incorporation du xénon au sein de ce matériau. / This work concerns the generation IV future nuclear reactors such as gas-cooled fast reactor (GFR) for which refractory materials as titanium nitride (TiN) are needed to surround fuel and act as a fission product diffusion barrier. This study is about Xe thermal behavior in sintered titanium nitride. Microstructure effects on Xe behavior have been studied. In this purpose, several syntheses have been performed using differents sintering temperatures and initial powder compositions. Xenon species have been introduced into samples by ionic implantation. Then, samples were annealed in temperature range from 1300°C to 1600°C, these temperatures being the accidental awaited temperature. A transport of xenon towards sample surface has been observed. Transport rate seems to be slow down when increasing sintering temperature. The composition of initial powder and the crystallographic orientation of each considered grain also influence xenon thermal behavior. Xenon release has been correlated with material oxidation during annealing. Xenon bubbles were observed. Their size is proportional with xenon concentration and increases with annealing temperature. Several mechanisms which could explain Xe intragranular mobility in TiN are proposed. In addition with experiments, very low Xe solubility in TiN has been confirmed by ab initio calculations. So, bivacancies were found to be the most favoured Xe incorporation sites in this material.

Exploiting coordination bonding of aromatic carboxylic acids at metal surfaces, this thesis explores new directions in the design and application of self-assembled monolayers (SAMs). The SAMs are investigated using a multi-technique approach comprising of a complementary combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In addition, the X-ray standing wave technique (XSW) was used to characterise the substrates. The process of layer formation and the final structures of the SAMs are found to be strikingly dependent on the combination of molecule and substrate, which is discussed in terms of the intermolecular and molecule-substrate interactions, bonding geometries and symmetry of the organic molecules. This is illustrated by the dramatic difference between molecular adsorption on Ag and Cu for molecules such as biphenyl-3,4',5-tricarboxylic acid and biphenyl-4-acetic acid. In the case of self-assembly on Cu, the molecule-substrate interactions play a decisive role in the resulting SAM structure, whereas on Ag, the intermolecular interactions dominate over the weaker molecule-substrate binding. This exploration of the balance of interactions that lead to the formation of these SAM structures lays the foundation for a systematic design of the structures and properties of aromatic carboxylic acid based monolayers. Finally, different applications and properties of some SAMs were investigated, namely coordination of a Pd(II) complex to a pyridine/pyrazole terminated molecule adsorbed on Ag. Evidence of coordination of Pd(II) to single molecules was provided by STM, XPS and NEXAFS spectroscopy. Additionally, controlled STM tip induced modification of local areas of a 1,3,5-tris(4-carboxyphenyl)benzene SAM on Ag was performed, opening an exciting prospect for nanoscale molecular manipulation.

Um neue Halbleiter-Bauelemente zu entwickeln und die Effizienz bereits existierender zu verbessern, müssen neue Materialien erkundet und untersucht werden. Für Anwendungen in Hochleistungselektronik und UV-Optoelektronik ist Ga2O3 mit seiner ultra-weiten Bandlücke von 4.8 eV ein vielversprechender Kandidat. Diese Anwendung haben zu wachsendem Interesse an seinen fundamentalen elektronischen und optischen Eigenschaften geführt. Diese Dissertation präsentiert eine umfassende ab initio-Untersuchung der elektronischen Anregungen in Ga2O3, um zu dem Verständnis dieser fundamentalen Eigenschaften beizutragen. Die Arbeit besteht aus zwei Teilen: Im ersten Teil präsentieren wir eine Vielteilchen-Störungstheorie Methode zur konsistenten Berechnung der neutralen Anregungen von Valenz- und Kernelektronen in kristallinen Halbleitern. Diese ermöglicht die präzise Berechnung von Absorptions- und Streuungsspektren vom optischen bis zum Röntgenbereich. Zusätzlich präsentieren wir einen neuartigen Ausdruck für resonante inelastische Röntgenstreuung (RIXS) innerhalb unseres Vielteilchen-Formalismus, der eine detaillierte Analyse dieser Streuung erlaubt. Mit ausgewählten Beispielen demonstrieren wir das Potential unserer Implementation, die Spektren dieser verschiedenen spektroskopischen Methoden zu berechnen, zu analysieren und zu interpretieren. Im zweiten Teil der Dissertation verwenden wir unsere Methode, um die Anregungen der Valenzelektronen, sowie der Ga 1s-, Ga 2p- und O 1s-Elektronen in Ga2O3 zu berechnen. Wir finden ausgeprägte Unterschiede in den diversen Röntgenabsorptionsspektren von Ga2O3 -Polymorphen, die von der unterschiedlichen lokalen elektronischen Struktur stammen. Wir bestimmen die Zusammensetzung der Valenz- und Kernanregungen und analysieren ihre Signatur in den verschiedenen Absorptions- und Streuungsspektren. Abschließend demonstrieren wir wie RIXS einen zusätzlichen Blickwinkel auf die Valenz- und Kernanregungen und deren Wechselwirkungen ermöglicht. / To develop new semiconductor devices and improve the performance of existing ones, the exploration and understanding of novel materials is required. With an ultra-wide band gap of around 4.8 eV, Ga2O3 is a promising candidate for applications in UV-optoelectronics and power electronics. These applications have led to an increasing interest in its fundamental electronic and optical properties. In this thesis, we present a comprehensive first-principles study of the electronic excitations of Ga2O3 to contribute to the understanding of these fundamental properties. The thesis consists of two parts: In the first part, we present an all-electron many-body perturbation theory (MBPT) approach for consistent calculations of neutral core and valence excitations. It enables accurate calculation of absorption and inelastic scattering spectra in the optical, UV, and x-ray region. While these spectroscopic techniques probe either the valence or core excitations, resonant inelastic x-ray scattering (RIXS) reveals the interplay between the two. We present a novel expression for the RIXS cross section within our all-electron many-body formalism that allows for a detailed analysis of this interplay. We demonstrate the capability of our implementation to compute, analyze, and interpret the different spectroscopic techniques with selected examples of prototypical insulators. In the second part, we apply our approach to study valence excitations, as well as excitations of various core states, i.e. the gallium 1s, gallium 2p, and oxygen 1s states in Ga2O3 . Comparing the core spectra of Ga2O3 polymorphs, we find distinct differences that originate from their local environments. We determine the composition of valence and core excitons, and analyze their signatures in the various absorption and scattering spectra. Finally, we demonstrate how RIXS can be employed to provide a different viewpoint on the core and valence excitations and unravel the interplay between them.

L’identification des mécanismes de relâchement des Produits de Fission (PF) hors de combustible nucléaire irradié lors d’un accident grave est primordiale pour le développement de codes capables d’estimer précisément le terme source (nature et quantité des radionucléides émis dans l’environnement). Parmi les différents PF, les Ba, Cs, Mo et le Ru sont particulièrement intéressants, car ils peuvent interagir entre eux ou avec d’autres éléments et donc affecter leur relâchement. Dans le cadre de cette thèse, deux axes de travail ont été mis en place avec l’objectif d’identifier les phases chimiques présentes avant l’accident et leur évolution au cours de l’accident lui-même. L’approche expérimentale a consisté à reproduire les conditions d’un accident nucléaire à l’échelle du laboratoire, en utilisant des échantillons de combustibles irradiés et des matériaux modèles (UO₂ vierge dopés en 12 PF). Le principal avantage de ces derniers est l’utilisation de méthodes de spéciation chimique comme la Spectroscopie d’Absorption des rayons X, qui n’est pas aujourd’hui encore disponible pour les combustibles irradiés. Trois échantillons de combustible irradié ont été étudies, représentatifs de l’état initial (i.e. avant l’accident), d’une étape intermédiaire en température (1773K) et d’un état avancé d’accident nucléaire (2873K). Pour les matériaux modèles, plusieurs séquences accidentelles (de 573K à 1973K) ont été réalisés. Les résultats expérimentaux ont permis d’établir un nouveau mécanisme de relâchement des PF en en fonction des conditions oxydantes et réductrices du scénario accidentel. Ces résultats ont démontré aussi l’importance des matériaux modèles pour l’étude des accidents nucléaires graves, en complémentarité aux combustibles irradiés. / The identification of Fission Products (FP) release mechanism from irradiated nuclear fuels during a severe accident is of main importance for the development of codes for the estimation of the source-term (nature and quantity of radionuclides released into the environment). Among the many FP Ba, Cs, Mo and Ru present a particular interest, since they may interact with each other or other elements and thus affect their release. In the framework of this thesis, two work axes have been set up in order to identify, firstly, the chemical phases initially present before the accident and, secondly, their evolution during the accident itself. The experimental approach consisted in reproducing nuclear severe accidents conditions at laboratory scale using both irradiated fuels and model materials (natural UO₂ doped with 12 FP). The advantage of these latter is the possibility of using characterization methods such as X-ray Absorption Spectroscopy which are not available for irradiated fuels. Three irradiated fuel samples have been studied, representative to an initial state (before the accident), to an intermediate stage (1773K) and to an advanced stage (2873K) of a nuclear severe accident. Regarding to model materials, many accident sequences have been carried out, from 573 to 1973K. Experimental results have allowed to establish a new release mechanism, considering both reducing and oxidizing conditions during an accident. These results have also demonstrated the importance of model materials as a complement to irradiated nuclear fuels in the study of nuclear severe accidents.

Les travaux de recherche concernent l'étude des propriétés magnétiques des nanoparticules de coordination et de complexes à transition de spin assemblés en monocouches sur différents substrats métalliques par spectroscopie d’absorption des rayons X (XAS) aux seuils L₂,₃ des atomes métalliques. Nous utilisons le dichroïsme magnétique circulaire des rayons X (XMCD) et le dichroïsme naturel linéaire des rayons X (XNLD). La simulation des données expérimentales est réalisée par la méthode des multiplets en champ de ligands (LFM). Cette thèse est divisée en deux parties.La première partie vise à étudier l'anisotropie magnétique des nanoparticules magnétiques de coordination assemblées en une monocouche sur du graphite pyrolytique hautement orienté. Les mesures XAS, XNLD et XMCD révèlent la présence d'une anisotropie électronique et magnétique par l'ouverture d'une boucle d'hystérèse magnétique à basse température. En combinant les données expérimentales avec les calculs LFM, nous proposons un modèle structural de la surface des particules qui permet de rendre compte de l'anisotropie électronique et magnétique due à l'interface nanoparticules/vide.Dans la deuxième partie, on étudie la transition de spin thermique et photo-induite de complexes de Fe(II) assemblés en sous-monocouches sur des substrats métalliques. Les calculs LFM et les données expérimentales permettent de démêler les différents effets que les paramètres externes (température, rayons X, lumière bleue et rouge) induisent sur la nature et l'ampleur de la transition de spin. Une transition "anormale" de l’état haut spin vers bas spin, induite par la lumière bleue est découverte et analysée comme étant due à une interaction à l'interface entre les molécules et les substrats métalliques, cette interaction apportant l’énergie nécessaire à cette transition. / This thesis is focused on the investigation of the magnetic properties of magnetic nanoparticles and spin crossover complexes assembled as monolayers on different metallic substrates by X-ray Absorption Spectroscopy (XAS) at the L₂,₃ edges of the metal atoms. We use X-ray Magnetic Circular Dichroism (XMCD) and X-ray Natural Linear Dichroism (XNLD) to do so. The simulation of the experimental data is carried out by Ligand Field Multiplet (LFM) calculations. This thesis is divided into two parts.The first part aims at studying the magnetic anisotropy of Prussian blue analog magnetic coordination nanoparticles that were assembled as a single layer on Highly Oriented Pyrolytic Graphite. XAS, XNLD and XMCD measurements disclose the presence of electronic and magnetic anisotropy by the opening of a magnetic hysteresis loop at low temperature. Combining experimental data with LFM calculations, we propose a structural model of the surface of the particles that allows accounting for electronic and magnetic anisotropy and that is proposed to be due to the single layer/vacuum interface.In the second part, thermal and light-induced spin crossover behavior of Fe(II) containing molecules have been assembled as a sub-monolayer on metallic substrates. Complementary LFM calculations and experimental XAS data allow unravelling the nature and the magnitude of the spin crossover as a function of the external stimuli (temperature, X-rays, blue and red light). An anomalous blue light effect allowing a crossover at low temperature from the high to low spin states is discovered and analyzed as being due to the interface between the molecules and the metallic substrates.

Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of nanochain-containing hydrogels, as a preliminary stage for the preparation of aerogels, take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd–Pt aerogels at the nano-scale as well as at the macro-scale is still unclear. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd–Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd–Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. The features of mono-metallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd–Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd–Pt aerogels for application in electrocatalysis, we used the electrochemical CO stripping method. Due to their high porosity and extended network structure, the resulting values of the ECSA for the Pd–Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd–Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.