Dissertations / Theses on the topic 'Dioxide de niobium'

Le dioxyde de niobium (NbO2) a récemment suscité un vif intérêt dans les domaines de la physique des solides et nano-dispositifs technologiques. D'une part, le NbO2 isolant à température ambiante subit une distorsion structurelle accompagnée d'une transition de phase électronique par laquelle il devient métallique à des températures supérieures à 1080 K. D'autre part, NbO2 présente une phase de résistance différentielle négative sous l'application du courant électrique, un phénomène connu sous le nom de résistance différentielle négative contrôlée par le courant CC-NDR. Dans cette thèse, nous avons fabriqué des films minces de NbO2 par la technique de pulvérisation RF-mangnétron sur des substrats amorphes et cristallins (verre et silicium). Les films déposés sont toujours amorphe, et un traitement de recuit des films déposés est nécessaire pour atteindre la cristallinité. Lors des études électroniques sur NbO2, nous avons observé la CC-NDR avec une hystérèse dans les courbes V(I). Nous avons montré que cette hystérèse dynamique est due à l'inhomogénéité de la température. Les mesures de transport électronique en fonction de la température montrent qu'il n'y a pas de transition métal-isolant associée à la CC-NDR. Par ailleurs, nous avons montré qu’il y avait un changement similaire de la conductivité lié à la température dans les échantillons amorphes et cristallins, cependant, l'échantillon amorphe est un meilleur conducteur électronique et thermique. Enfin, nous avons prouvé que la CC-NDR peut être simplement expliqué par la création de porteurs excités thermiquement dans un semi-conducteur, sans la nécessité de faire appel à des mécanismes de transport plus complexes
Niobium dioxide (NbO2) has been recently gaining a lot of interest in the fields of solid state physics and technological nano-devices. On one hand, NbO2 undergoes a structural distortion accompanied by an electronic phase transition where the material changes from an insulating state at room temperature into a metallic state at temperatures above ∼ 1080 K. On the other hand, NbO2 exhibits a negative differential resistance phase under the application of electric current, a phenomenon known as current-controlled negative differential resistance CC-NDR. In this thesis, we have fabricated thin films of NbO2 by RF-mangentron sputtering technique on amorphous and crystalline substrates (glass and silicon). The deposited films were always amorphous, and annealing treatment of the as-deposited films was necessary to achieve crystallinity. . Upon performing electronic studies on NbO2, we witnessed CC-NDR with a hysteresis in the V(I) curves. We showed that hysteresis in CC-NDR is due to temperature inhomogeneity. Simultaneous electronic transport and Raman measurements show that CC-NDR is not associated to a phase transition. Moreover, we showed that there is a similar temperature driven change in conductivity in both the amorphous and the crystalline samples, however, the amorphous sample is a better electronic and thermal conductor. Finally, we proved that the CC-NDR may be simplyexplained by the creation of carriers by temperature in a semiconductor, without the need for invoking more complicated transport mechanisms

Niobium dioxide films have been prepared by the radio frequency sputtering of Niobium dioxide powder targets. Polycrystalline films have been deposited onto Silicon substrates maintained at temperatures exceeding 900 K during preparation. Characterisation by X-ray diffraction and E.S.C.A. indicates that the Argon environment during sputtering must be doped with approximately 500 to 1000 ppm of Hydrogen for the growth of Niobium dioxide films. Optical measurements were carried out on single crystals and thin films of Niobium dioxide as well as Vanadium dioxide films. The optical measurements were carried out over a wavelength range of 0.5 to 15 um for Niobium dioxide and 0.5 to 6 um for Vanadium dioxide. The temperature range over which measurements were made on Niobium dioxide (in a vacuum) was from 90 to 1200 K and for Vanadium dioxide from 300 to 370 K. The optical properties (refractive index and extinction coefficient) of Niobium dioxide and Vanadium dioxide were calculated across both of their respective thermally excited transitions. It was found that there was an abrupt change in the optical properties of both Niobium dioxide and Vanadium dioxide at their respective transition temperatures.

Cette thèse vise la réalisation et la caractérisation structurale, optique et électrique de couches minces de matériaux oxydes ayant des propriétés de transition isolant-métal (MIT), plus particulièrement des oxydes de vanadium (VO2 et VO2 dopé avec W) et du dioxyde de niobium (NbO2). Le dépôt de ces couches minces a été effectué par pulvérisation magnétron réactive dans une atmosphère Ar/ O2. Le processus d'optimisation des films minces a permis d'obtenir des informations précieuses sur les effets du processus de fabrication sur les caractéristiques structurelles, optiques et électriques. En outre, les films ont été intégrés dans divers dispositifs tels que des modulateurs THz activés thermiquement/électriquement, des métacanvases reconfigurables optiquement et des dispositifs oscillants. Les informations fournies dans ce manuscrit sont d'un grand intérêt pour l'intégration de films de VO2 et de VO2 dopé avec W dans des dispositifs pouvant fonctionner dans différents domaines. Ce travail permet également d'approfondir notre compréhension des films de NbO2, un domaine relativement moins étudié mais particulièrement intéressant pour les applications nécessitant une plus grande stabilité à la température
This thesis aims at the realization and structural, optical, and electrical characterization of thin films of oxide materials with insulator-to-metal transition (MIT) properties, more specifically vanadium oxides (VO2 and W-doped VO2) and niobium dioxide (NbO2). The deposition of these thin films was performed using reactive magnetron sputtering in an Ar/ O2 atmosphere. During the optimization process of the thin films, valuable insights were gained into the effects the fabrication process has over the structural, optical and electrical characteristics. Furthermore, the films were integrated into various devices such as thermally/ electrically activated THz modulators, optically reconfigurable metacanvases and oscillating devices. The information provided in this manuscript is of high interest for the integration of VO2 and W-doped VO2 films into devices that can operate in various fields. This work also deepens our understanding of NbO2 films, an area with comparatively less research but of particular interest for applications requiring higher temperature stability

The gradual depletion of and dependence on fossil fuels, air pollution and global warming have all accelerated the development of alternative energy systems which use hydrogen as an energy carrier. The hybrid sulphur cycle (HyS) is the foremost electrothermochemical process that can produce hydrogen as the energy carrier. The HyS cycle consists of two units, namely the sulphuric acid decomposition reactor and the sulphur dioxide electrolyser (SDE). The SDE is responsible for the SO2 electrooxidation to sulphuric acid and protons at the anode and the electro–reduction of protons to hydrogen at the cathode. This research study focuses on the kinetic data collected from the prepared catalysts for SO2 electro–oxidation at the anode. Platinum dispersed on carbon, niobium pentoxide, tantalum pentoxide and zirconium dioxide as electrocatalysts were prepared using sodium borohydride as a reducing agent. These electrocatalysts were characterized using transmission electron microscopy and x–ray diffraction. Cyclic voltammetry was used to study the electrochemical active surface area (EAS) and the results showed that Pt/ZrO2–C had a higher EAS area than Pt/Ta2O5–C, Pt/Nb2O5–C and Pt/C. The high EAS of Pt/ZrO2–C can be explained by the low crystal size however after a series of linear polarisation scans Pt/ZrO2–C experiences a much greater area loss than all the other catalysts. Linear polarisation scans for each of the catalysts revealed that the influence of increased temperature and sulphuric acid concentration were showed improved results. Levich and Koutecky–Levich plots revealed that the SO2 oxidation is a multistep reaction on all the prepared catalysts and that there are regions which are kinetic and diffusion controlled and diffusion–only controlled. Pt/Ta2O5–C catalysts exhibited superior catalytic activity and stability compared Pt/Nb2O5–C, Pt/ZrO2–C and Pt/C. The Pt/ZrO2–C exhibited the most inferior catalytic activity and stability.
Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2011.

Diese Arbeit beschäftigt sich mit den elektrischen Eigenschaften der resistiven Schaltmaterialien SrTiO3 und NbO2. Im ersten Teil werden NbO2 (001)-Dünnschichten untersucht. Bisher sind die für NbO2-Dünnschichten in der isolierenden Phase gemessenen spezifische Widerstände um einen Faktor von 200 niedriger als der in NbO2-Einkristallen gemessene 10 kΩ cm Widerstand. In dieser Arbeit wird der spezifische Widerstand von NbO2-Dünnschichten auf 945 Ω cm erhöht. Es wird gezeigt, dass leitfähige Perkolationspfade entlang der Korngrenzen für die Abnahme des spezifischen Widerstandes verantwortlich sind. Durch temperaturabhängige Leitfähigkeitsmessungen wurden Defektzustände identifiziert, die für die Verringerung des spezifischen Widerstandes gegenüber dem theoretischen Wert verantwortlich sind. Im zweiten Teil wird der Einfluss des Ti-Antisite Defekts auf das resistive Schalten in SrTiO3 Dünnschichten untersucht, welche mit metallorganischer Dampfphasenepitaxie gezüchtet wurden. Dabei werden sowohl stoichiometrische als auch Strontium defizitäre Schichten untersucht. Es wird über temperaturabhängige Permittivitätsmessungen gezeigt, dass durch Kristalldefekte die weiche Phononenmode gestört wird und bei stark strontiumverarmten Schichten polare Nanoregionen gebildet werden, was auf die Bildung des TiSr Defekts zurückgeführt wurde. Darüber hinaus wird gezeigt, dass stark strontiumdefiziente SrTiO3 -Schichten ein stabiles resistives Schalten mit einem Ein-Aus-Verhältnis von 2e7 bei 10 K aufweisen, während stöchiometrische Dünnschichten kein stabiles Schalten zeigen. Es wird ein diodenartiger Transportmechanismus, der im hochohmigen Zustand auf Schottkyemission beruht und ihm niederohmigen Zustand durch defektassistierten Tunnelstrom dominiert wird, identifiziert. Daraus wurde ein neues Modell für das resistive Schalten, basierend auf dem TiSr Defekt und der induzierten Ferroelektrizität, entwickelt.
In this work, the impact of crystal defects on the resistive switching materials SrTiO3 and NbO2 is investigated. The work is divided into two parts. In the first part, NbO2 (001) thin films are studied. So far, resistivities measured for NbO2 thin films in the insulating phase are by a factor of 200 lower than the 10 kΩ cm resistivity measured in NbO2 single crystals. To make this material applicable for resistive switching, the resistivity in the insulating phase has to be increased to effectively block the current in the high resistive state. Throughout the investigations presented in this work, the resistivity of NbO2 thin films is increased to 945 Ω cm. It is shown that conductive percolation paths along the grain boundaries are responsible for the decrease in resistivity. Temperature-dependent conductivity measurements identified defect states responsible for the reduction in resistivity from the theoretical value. In the second part of this work, the influence of the Ti anti-site defect on resistive switching in SrTiO3 thin films grown by metal-organic vapor phase epitaxy is studied. Both stoichiometric and strontium deficient thin films are studied. It is shown via temperature-dependent permittivity measurements that crystal defects harden the soft phonon mode and polar nano regions are formed in highly strontium deficient films, which was attributed to the formation of Ti antisite defects. In addition, highly strontium deficient SrTiO3 films are shown to exhibit stable resistive switching with an on-off ratio of 2e7 at 10 K, whereas stoichiometric thin-films do not show stable switching. A diode-like transport mechanism based on Schottky emission in the high-resistance state and dominated by defect-assisted tunneling current in the low-resistance state is identified. From this, a new model for resistive switching based on the Ti antisite defect and the induced ferroelectricity is developed.

L’amélioration des performances du combustible nucléaire nécessite, aussi bien en régime nominal que lors de rampes de puissance ou de transitoires accidentels, d’être en mesure de maîtriser les effets délétères induits par la réactivité chimique des produits corrosifs (générés par la fission) susceptibles de provoquer des ruptures de gaine. Dans cet objectif, ce travail vise à maîtriser le potentiel d'oxygène de fonctionnement du combustible dans un domaine stabilisant les formes non corrosives de l’iode, par l’usage in-situ de systèmes tampons d’oxydo-réduction appropriés. En fonctionnement nominal, le potentiel en oxygène est assez proche de celui imposé par le tampon (MoO2/Mo). Par conséquent, le premier objectif de ce travail a été de clarifier la thermochimie (solubilité et spéciation dans UO2) du couple (MoO2/Mo). Nous avons étudié la transition MoO2 ⇌ Mo dans UO2 en fonction de la pO2 et de la température, par spectroscopie d'absorption des rayons X (XAS) in situ au synchrotron KARA (Allemagne). Nous montrons que les réactions d’oxydation du molybdène sont toujours possibles thermodynamiquement, quelles que soient les conditions de température mais avec une cinétique lente à 800°C. La vitesse d’oxydation augmente avec la température. De plus, la solubilité de Mo dans UO2 a été mesurée par SIMS (spectrométrie de masse des ions secondaires) et EPMA (microsonde électronique). Nous proposons une limite de solubilité de l’ordre de10-4 mol Mo/mol U, soit 0,01 % m de Mo/UO2. Ensuite, le niobium, sous ses formes redox Nb2O5/NbO2, a été identifié comme un candidat prometteur de par ses propriétés physico-chimiques et neutroniques. La première étape a consisté à préparer des céramiques UO2 contenant le couple tampon redox. Un procédé de fabrication spécifique du combustible UO2 tamponné, dopé par le couple (Nb2O5/NbO2), a été développé afin de préserver les deux formes oxydo-réductrices du tampon oxygène dans les pastilles lors du frittage.Ce processus a été également validé pour UO2 dopé par le couple (MoO2/Mo). Nous avons aussi montré quel e tampon (Nb2O5/NbO2) introduit initialement se transformait lors du frittage en (UNb2O7/NbO2),présentant un équilibre très proche de celui du tampon initial et donc apte à remplir le rôle de tampon depO2 dans le domaine visé. Ensuite, l’objectif final a été la validation de l’effet tampon lié à l’ajout du couple(Nb2O5/NbO2) dans UO2. Nous avons donc d’abord étudié par diffraction des rayons X à haute résolution et haute température (HR HT XRD) et par diffraction des neutrons (DN) la variation du paramètre de maille de la matrice UO2 fluorite en présence du couple oxydo-réducteur qui va agir comme un tampon local pour stabiliser la pO2 au sein du système et la stœchiométrie de la matrice UO2+x. Nous montrons également que le système U-Nb-O présente de nombreuses phases ternaires incluant les états d'oxydation Nb4+ etNb5+, toutes stables sur un domaine étroit de pO2. Ainsi, ces différentes phases ternaires participent à des systèmes tampons, qui permettent de réguler les perturbations oxydo-réductrices pouvant être imposées au matériau sans modification notable de la stœchiométrie du combustible (stabilité du paramètre de maille). Ensuite, les pastilles UO2 élaborées avec le couple tampon Nb2O5/NbO2 ont été implantées avec des ions césium, iode, molybdène et tellure dans une couche superficielle (environ 1 μm). La spéciation de ces espèces a été caractérisée par spectroscopie d'absorption des rayons X (synchrotron KARA)
Improvement of nuclear fuel performances requires reducing the stability of corrosion fission gas, in particular the speciation of potentially corrosive fission products (Cs, I, Te). Doping uranium dioxide with an oxydo-redox buffer couple is the improvement axis considered in this work. It aims at improving redox buffer couple effect on pO2 (oxygen partial pressure) and their impact on properties of speciation of fission products in UO2. Actually, in typical nominal operation, the oxygen potential is quite close to that imposed by the (Mo/MoO2) buffer. The first objective of this work was to clarify the thermochemistry (solubility and speciation in UO2) of the (Mo/MoO2) couple. We studied the MoO2 ⇌ Mo transition in UO2 as a function of pO2 and temperature by X-ray Absorption Spectroscopy (XAS) in situ at the KARA synchrotron (Germany). We show that the oxidation reactions of molybdenum are always thermodynamically possible, whatever the temperature conditions but with a slow kinetics at 800°C. The oxidation rate increases with temperature. In addition, the solubility of Mo in UO2 was measured by SIMS (Secondary Ion Mass Spectrometry) and EPMA (Electron probe micro-analyzer). We found a solubility limit of the order of 10-4 mol Mo/mol U, i.e. 0.01 %weight of Mo/UO2.Then niobium, with its redox forms Nb2O5/NbO2, has been identified as a promising candidate due to different selection criteria (physico-chemical and neutronic properties). The first step was to prepare UO2 with this buffer. A specific manufacturing process of the buffered UO2 fuel, doped with the (Nb2O5/NbO2) couple has been developed in order to preserve both oxido-reducing forms of the oxygen buffer in the pellets while sintering. This process has been also validated for UO2 doped with the (MoO2/Mo) couple. We have shown that the initially introduced (Nb2O5/NbO2) buffer was transformed during sintering into (UNb2O7/NbO2). The equilibrium of the latter couple is very close to that of the initial buffer and therefore it can fulfill its role of pO2 control in the targeted domain.Then, the final objective was the validation of the buffer effect related to the addition of (Nb2O5/NbO2) in UO2. So we first studied by means of High Resolution X-Ray diffraction at high temperature (HR HT XRD) and Neutron Diffraction (DN) the variation of the lattice parameter of the UO2 fluorite matrix in presence of the niobium pO2 buffer. The latter can act as a local buffer enforcing a stabilization of the pO2 within the system and therefore of the stoichiometry of the UO2+x matrix. We also show that the U-Nb-O system has many ternary phases including Nb4+ and Nb5+cations, all stable on a narrow domain of pO2. Thus, these different ternary phases participate to buffer systems, which make it possible to regulate the oxido-reducing disturbances imposed on the material without any notable modification of the pO2 (stabilization of the lattice parameter).Then, the UO2 pellets elaborated with the (Nb2O5/NbO2) buffer couple were implanted with cesium, iodine, molybdenum and tellurium ions in a thin surface layer (1 µm approximately). Speciation were characterized by X-Ray Absorption Spectroscopy, using synchrotron radiation (KARA)

"Charge transport in nanostructured SnO2:SiO2 thin films": Silica based nanostructured thin films grown on silicon substrates are promising materials for novel light emitter devices. In particular, tin dioxide is a wide band gap n-type semiconductor (Eg=3.6 eV) with an expected band-to-band emission centered in the ultraviolet (344 nm) region of the electromagnetic spectrum. Our group succesfully demonstrated UV emission from such systems, but at the beginning of my work many issues in charge transport processes needed to be explained. Aim of this project was to clarify electric transport and charge trapping mechanisms. As a result of a materials science approach we can now interpret the experimental data through specific relationships between synthesis conditions, clustering morphology (nanoparticle (NP) size distribution and volumic concetration, interphase substoichiometry, film thickness), and electric response. The observed phenomena have been analyzed within the percolation theory. Main results concern: electric transport of both holes and electrons is sustained by NP-to-NP hopping events and dielectric enhancement results from oscillating charges (holes) on NPs. "Second Harmonic Generation in potassium niobium silicate glasses": Second harmonic generation (SHG) is a non linear optical process largely employed in current laser technology and photonics. However in almost every application the material employed for these purposes are single crystals. Therefore the possibility to achieve large SHG in amorphous systems may lead to devices with innovative configurations. SHG may occur only if the system is non-centrosymmetric, therefore for glasses it is forbidden due to intrinsic isotropy. The inversion symmetry can be broken up with poling treatments. They consist in applying strong electrostatic field while the sample is stressed by external perturbation (typically heat, electron beam or laser light). We have explored the effect of thermal poling treatment on potassium niobium silicate glasses on inducing non linear optical properties. The results have revealed a strong SHG associated with structural modifications. The proposed mechanism involves a rearrangement of niobium oxide groups mediated by non bridging oxygen and potassium ion transport across the glass. These new charge arrangements form a non-centrosymmetric region underneath the anodic contact responsible of the detected SH signal. "Tapered silica optical microfibres for gas sensors": In the last years, tapered silica fibres have attracted much interest in photonic research, because of peculiar properties emerging in waveguides with lateral dimensions of the same order of the guided modes. In particular, in these structures the large evanescent field enables some interesting properties, such as microfluidic sensors and high Q optical resonators (coiling the tapered fibre), non-linear effects and supercontinuum generation. In this project, carried out at the University of Southampton (UK) in the group of Dr. Gilberto Brambilla, we have explored the feasibility of an innovative optical absorption device, based on ring down spectroscopy. In this case we are interested in a sensor for in-line application: a fluidic channel wrapped with tapered fibre in which the analyte can flow. The large power fraction outside the fibre interacts with the flowing medium and any change in the surrounding optical properties (refractive index or absorption coefficient) leads to a modification of the recorded light intensity propagating in the fibre. The idea is to exploit ring down time of a silica tapered microcoil resonator as an indicator of the absorption coefficient of a gas (or a liquid) flowing in the channel.

Ce travail a pour objet l'étude de films d'oxydes Ti-x Nbx O₂ et Ti -x Vx O₂ amorphes (0 ≤x ≤ 1) obtenus par pulvérisation cathodique R. F. La première partie traite de l'élaboration et de la caractérisation chimique des dépôts amorphes. Une analyse des différents paramètres de la pulvérisation nous a permis d'optimiser le protocole expérimental de fabrication des couches. Nous avons ensuite déterminé les propriétés structurales de ces matériaux après recuit : identification des phases apparaissant au-dessus de la température de transition amorphe →cristal et caractérisation de la microstructure. L'absorption X des échantillons Ti-x Nbx O₂ amorphes a été mesurée par RéflEXAFS et détection d'électrons au seuil K du titane. L'analyse des seuils permet d'envisager un transfert électronique partiel entre atomes métalliques. Les mesures de résistivité et de pouvoir thermoélectrique sur les films Ti-x Nbx O₂ amorphes montrent que les porteurs sont localisés et que la conduction s'effectue par sauts selon le mécanisme décrit par Hurd. La similitude entre le comportement des phases amorphes et celui des phases cristallines haute température entraîne que, pour ces dernières, le transport électronique s'effectue également par sauts.

L'objectif de l'étude est d'examiner la genèse de l'acidité des solides WOx/ZrO2, NbOx/ZrO2, WOx/TiO2 et NbOx/TiO2 en relation avec la structure des espèces superficielles et l'activité catalytique. Les mesures d'activité catalytique des solides à base de W en déshydratation du propan-2-ol indiquent la présence d'une teneur seuil (~ 1,2 at. W/nmø) pour l'apparition de l'activité. Une évolution similaire de l'acidité de Brønsted "forte" et de l'abondance des espèces WOx fortement polymérisées est observée. De plus les résultats montrent une relation directe entre l'activité catalytique, l'abondance des sites acides de Brønsted "forts" et celle des espèces de WOx fortement polymérisées. L'origine de cette acidité est attribuée à la stabilisation de protons sur les espèces WOx fortement polymérisées. Le comportement des solides NbOx/ZrO2 est similaire à celui des catalyseurs WOx/ZrO2 : teneur seuil (1,2 at. Nb/nmø) pour le développement de l'activité catalytique, de l'acidité de Brønsted (dans ce cas faible) et des espèces NbOx fortement polymérisées. Par contre, le système NbOx/TiO2 montre un développement progressif de ces paramètres dès les faibles teneurs en Nb. Toutefois, comme pour les systèmes à base de W, les résultats obtenus pour les deux systèmes à base de Nb montrent une relation directe entre l'activité catalytique, l'abondance des sites acides de Brønsted et celle des espèces NbOx fortement polymérisées. L'étude de l'influence de la nature de la phase active montre que l'activité catalytique des systèmes à base de W est ~ 2 ordres de grandeur supérieure à celle des solides à base de Nb. Ceci est attribué à l'acidité de Brønsted "forte" des solides base de W

La température et la pression partielle d’oxygène (PO2) constituent les paramètres majeurs contrôlantles évolutions thermochimiques en réacteur des combustibles nucléaires de type oxyde, et notammentla spéciation des produits de fission potentiellement corrosifs (Cs, I, Te). Pour limiter les risques derupture de la gaine en Zr par corrosion, une solution innovante consiste à imposer au combustible defonctionner dans un domaine de PO2 où les espèces chimiques des gaz de fission sont inoffensives, pardopage in-situ avec un tampon oxydo-réducteur solide. Le niobium, avec ses couples redoxNbO2/NbO et Nb2O5/NbO2, a été identifié comme le candidat le plus prometteur. Un procédé defabrication d’un combustible dopé niobium répondant à cet objectif et conforme aux spécificationsd’usage (densité, microstructure) a été optimisé. L’étude expérimentale du système UO2-NbOx a révélél’existence à 810°C d’une phase liquide entre UO2 et NbO2, non identifiée à ce jour. La caractérisationdes phases solides et en solution du niobium nous a conduit à proposer un modèle thermodynamiquede solubilité du dopant dans UO2 à 1700°C. Une étude approfondie de la spéciation du niobiumprécipité a permis d’identifier la présence simultanée dans le matériau des phases majeures NbO2 etNbO, ainsi que Nb en moindre teneur. La coexistence du niobium sous deux degrés d’oxydationdifférents constitue un élément-clé de démonstration d’un possible effet tampon in-situ, dont l’impactest observé sur certaines propriétés du combustible dépendantes de la PO2, la densification notamment.Les résultats confirment le potentiel prometteur des combustibles tamponnés en PO2 au regard de sesperformances en réacteur
Temperature and oxygen partial pressure (PO2) of nuclear oxide fuels are the main parametersgoverning both their thermochemical evolution in reactor and the speciation of volatile fissionproducts such as Cs, I or Te. An innovative way to limit the risk of cladding rupture by corrosionunder irradiation consists in buffering the oxygen partial pressure of the fuel under operation in a PO2domain where the fission gas are harmless towards Zr clad, by using solid redox buffers as additives.Niobium, with its NbO2/NbO and Nb2O5/NbO2 redox couples has been found to be a promisingcandidate to this end. A manufacturing process of a buffered UO2 fuel, doped with niobium has beenoptimized, in order to fulfill usual specifications (density, microstructure). The experimental study ofthe UO2-NbOx system has shown the existence of a liquid phase between UO2 and NbOx at 810°C,which was not reported in the literature. The characterization of Nb containing phases present in UO2both in solid solution and as precipitates has lead us to propose a solubility thermodynamic model ofniobium in UO2 at 1700°C. An extensive study of the niobium precipitates shows the co-existence inthe fuel of NbO2 and NbO as major phases, together with small amounts of metallic Nb. The coexistenceof niobium under two oxidation states inside the fuel is a key element of demonstration of apossible in-situ buffering effect, which is likely to impact some properties of the material that aredependent upon PO2, such as densification. These results confirm the promising potential of oxygenbuffered fuels as regard to their performance in reactor

Le travail de recherche présenté dans ce manuscrit est dédié à la synthèse et la caractérisation structurale, électrique et spectroscopique de nouveaux composés à base d'ion uranyle, d'oxoanions de métaux de transition (Niobates, tantalates et antimonates) et des métaux alcalins, susceptibles de se former en fin de cycle du combustible nucléaire. Une large partie de cette étude a été consacrée a la famille de niobates A1-xUNbO6-x/2,(X = 0 et 0,5 et A = alcalin). Cette famille de composés adopte deux types d'architectures structurales: structures tridimensionnelles présentant des tunnels ou structures bidimensionnelles avec empilement de couches, où les contrecations monovalents sont localisés dans les tunnels ou entre les couches. Le rôle du cation alcalin dans la connectivité et la dimensionnalité des structures, ainsi que sur la stabilité de ces phases a été également mis en évidence, en réalisant différentes substitutions entre cations alcalins: Cs1.-xKxUNbO6, CS0.5-xRbxUNbO5.75 et Cs1-xRbxUNb06 ou entre oxoanions : CS0.5UNb1-xTixO5.75-x/2 et CS0.5UNb1-xWxO5.7S+x/2. L'étude s'est également étendue à d'autres oxoanions comme les tantalates et antimoinates. La résolution structurale de nouvelles phases AUMO6 (A = K et Cs ;M = Ta, Sb) et A0,5UTaO5,75 (A = Rb, Cs) a été effectuée à partir des données de diffraction des rayons X sur poudre
The work presented in this manuscript was dedicated to the synthesis and to the structural, electric and spectroscopic characterization of new compounds containing ion uranyl, transition metal oxoanions (Niobates, tantalates and antimonates) and alkaline metals. The first part of this study was devoted to the niobate family A1-xUNbO6-x/2, with x = 0 and 0,5 and A = alkaline; ln this family, the compounds can adopt two types of structural architectures: three-dimensional structures presenting large tunnels, or two-dimensional structures with a stacking of layers, where the monovalent cations are localized in the tunnels or between the layers. The role of the alkaline cation in the structural connectivity and dimensionality, as also in the stability of these phases, has been evidenced by carrying out various substitutions between alkaline cations: Cs1.- xKxUNbO6, CS0.5-xRbxUNbO5.75 and Cs1-xRbxUNbO6 or between oxoanions: CS0.5UNb1-xTixO5.75-x/2 and CS0.5UNb1-xWxOS.7S+x/2. The last part of that study was devoted to the alkaline uranyl tantalates and antimoinates. The structures of these new compounds AUMO6 (A = K et Cs; M = Ta, Sb) and A0,5UTaO5,75 (A = Rb, Cs) have been determined from powder X-Ray diffraction data

The present project has made a comprehensive assessment of the effect of Nb doping on various charge-transfer related properties of TiO2. Of particular focus, the electrical properties of Nb-doped TiO2 (0.65 at %) have been investigated using the simultaneous measurement of electrical conductivity and thermoelectric power. This investigation was undertaken at elevated temperatures (1073 K -- 1298 K) in equilibrium with a gas phase of controlled oxygen activity (10-10 Pa < p(O2) < 75 kPa). In addition, the effect of segregation on the surface versus bulk composition of Nb-doped TiO2 was also investigated at a function of temperature and oxygen activity. Specifically, the following determinations were undertaken: The effect of oxygen activity, p(O2) and temperature on both electrical conductivity and thermoelectric power The effect of Nb on the defect disorder and related electrical properties of TiO2 The determination of equilibration kinetics and the associated chemical diffusion data for Nb-doped TiO2 The determination of Nb bulk diffusion in TiO2 The effect of p(O2), temperature and dopant content on Nb segregation and the related surface composition of Nb-doped TiO2 The obtained electrical properties enable the determination of a defect disorder model for Nb-doped TiO2, which may be considered within the following p(O2) regimes: Strongly Reduced Regime. In this regime, the predominant ionic defect was anticipated to be oxygen vacancies compensated electronically by electrons. While the transition to this regime (from higher p(O2)) was clearly observed, the predominant defect disorder existing beyond this transition was not confirmed due to an inability to obtain sufficiently low oxygen activity. Metallic-type conductivity behaviour was observed within this transition region. Reduced Regime I. In this regime, the predominate defect disorder defined by the electronic compensation of incorporated Nb ions by electrons was clearly observed. Reduced Regime II. In this regime, the predominate defect disorder defined by the ionic compensation of incorporated Nb ions by quadruply-charged titanium vacancies, was clearly observed. The present project included the determination of diffusion data which included: Temperature dependence of 93Nb tracer diffusion in single crystal TiO2 over the temperature range 1073 K -- 1573 K Chemical diffusion coefficient over the temperature range 1073 K -- 1298 K and oxygen activity range, 10-10 Pa < p(O2) < 75 kPa These pioneering studies are significant as they enable the prediction of the processing conditions required to reliably 1) incorporate Nb into the TiO2 lattice, and 2) achieve equilibrium with the gas phase. Finally, the present project included investigations on the effect of Nb segregation on the surface composition of Nb-doped TiO2, with the following outcomes: Due to segregation, the surface can be significantly enriched in Nb compared to the bulk The extent of enrichment increases as the bulk Nb content or the oxygen activity is decreased Following enrichment, the surface Nb concentration could be sufficiently high to assume a unique surface phase The outcomes of the present project are significant as they can enable the processing of TiO2 with enhanced charge transport and controlled surface properties.

For this thesis niobium oxides (NbO [subscript x]) with the goal of obtaining phase pure epitaxial NbO₂ were grown under variable conditions by molecular beam epitaxy (MBE) on different substrates, mostly (111) oriented strontium titanate. The films were characterized by X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) and from that their epitaxial relationship to the substrate was determined. With X-ray photoelectron spectroscopy (XPS) the oxidation states and stoichiometry of the films were determined. Asymmetric NbO₂ Nb 3d core-level spectra are clearly seen for the first time in XPS. Additionally, the film thickness was measured by X-ray reflectivity (XRR). Judging from the XPS spectra and diffraction data the goal to grow epitaxial NbO₂ of high phase-purity was achieved.
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碩士
輔仁大學
物理學系碩士班
104
In this study, optical films of silicon dioxide and niobium pentoxide were coated by using an RF sputtering system aided with a plasma source. Dioxide need in multilayer optical applications usually is a low refractive index material, and; niobium pentoxide is a high refractive index material. In this study, we expected that the silicon dioxide and niobium pentoxide films have a good stable refractive index and low optical absorption to develop the useful optical filters. We use home-made plasma source system to replace end-Hall ion source used in our previous study. The advantages of this plasma source are no overheating, and less contamination from the filament of Wolfram assembled in the end-Hall source. There is no issue of burning of filament during the deposition. This plasma source is derived by using a high DC voltage to dissociate the reactive gas in the source, such as oxygen. The substrate temperature significantly reduces in the deposition process, when compared with that in ion assisted deposition by end-Hall source. The optical properties of the samples are illustrated by ellipsometry and spectroscopy measurements to, analyze their refractive index and extinction coefficients and to identify the suitable parameters for coating optical thin films of silicon dioxide and niobium pentoxide. The results show that the silicon dioxide film has the best refractive index and the lowest optical absorption when deposited at the oxygen partial pressure of 70% with the ion current 1.2 mA. Moreover, the niobium pentoxide film at the oxygen partial pressure of 60% with the ion current 1.5 mA. In this study, we confirmed the plasma in the sputtering process can effectively increase the film optical quality. Keywords: silicon dioxide, niobium pentoxide, optical thin film, ellipsometry, spectroscopy

Since the landmark paper in 1972 by Fujishima and Honda [1], TiO2 has become one of the most promising candidates of a new generation of solar energy materials capable of generating clean hydrogen fuel using only sunlight (photo-electrochemically) to dissociate water. TiO2 has both bulk properties and surface properties which contribute to its functional performance. Considering that all of the electrochemical reactions induced by light occur at the surface of TiO2, it becomes clear that understanding the surface properties of TiO2 is of crucial importance for its performance; specifically the conversion of solar energy into chemical energy. The surface phase of TiO2 can be substantially different from that of the bulk phase as a result of a phenomenon known as segregation. Segregation involves the transport of certain lattice species from the bulk phase to the surface, driven by excess surface energy. To date, developments in the understanding of TiO2 solid solutions and related properties have mainly been centred on bulk properties. In comparison, relatively little work has been reported on segregation in TiO2 solid solutions and its influence on functional properties, such as reactivity and photoreactivity. The present work has studied the effect of indium (acceptor-type ion) and niobium (donor-type ion) segregation on the surface chemistry of well-defined In-doped and Nb-doped TiO2 solid solutions. Specifically, examining the relationship between imposed sample processing conditions, such as the gas phase oxygen activity, on segregation-induced surface enrichment. This was achieved using a range of complimentary analysis techniques including X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Rutherford backscattering (RBS) and proton-induced X-ray emission (PIXE).

碩士
國立雲林科技大學
電子工程系
105
Transparent conductive oxide(TCO) with high transmittance and excellent conductivity, It has been widely used as photovoltaic components. Because of the cost of materials is lower than the commonly used indium tin oxide(ITO), niobium doped titanium dioxide(TNO) is a promising transparent conducting oxide material In this study, transparent conductive Nb-doped TiO2 (TNO) thin films were deposited onto glass substrate, using the electron beam gun evaporation method. There are two kinds of penetration way, one is slug type and the other is powder type. Nb-doped TiO2 thin films with different doping concentration and different annealing temperature were researched.The TNO thin films were measured thickness by α-step. Structural and composition were analyzed by energy dispersive spectrometer (EDS) and X-ray diffractometer (XRD). And then measured by UV-Visible spectrometer and Hall effect measurement system to observed transmittance and electrical properties. The results show the best Nb-doping concentration is 10wt% of Nb2O5, the ratio of atom of Ti and Nb, Nb/(Ti+Nb), is 9.13at%. After annealed in 400℃, the structural changed into anatase from amorphous. The average transmittance is 70% in visible region. The lowest resistivity is 2.33×10-3Ω-cm.