In der vorliegenden Arbeit wird dargestellt, wie sich mit Methoden der nuklearen Festkörperphysik Defekte und Deformationen selbst in kristallographisch komplexen Materialien wie den MAX-Phasen, einer Klasse von Komplexcarbiden bzw. -nitriden, in-situ nachweisen lassen. Die sensitive Messgröße bildet dabei der elektrische Feldgradient (EFG), der ein Maß für die Asymmetrie der den jeweiligen Sondenkern umgebenden Ladungsverteilung darstellt. Es werden zwei Wechselwirkungsmechanismen zwischen Defekt und EFG diskutiert: Einerseits die langreichweitigen Auswirkungen elastischer Verzerrungen, andererseits der direkte Einfluss eines Defektes auf seine lokale elektronische Umgebung. Die Bestimmung der elastischen Antwort des Feldgradienten erfolgt mittels Ab-initio-Methoden im Rahmen der Dichtefunktionaltheorie. Der dabei vorgestellte Ansatz erlaubt es, die Ursachen der Dehnungsabhängigkeit zu klären und andere, speziellere Dehnungsabhängigkeiten wie die Volumenabhängigkeit oder die Strukturabhängigkeit des Feldgradienten zu bestimmen. Die in der Umgebung bestimmter Defekte oder Deformationen auftretenden EFG-Verteilungen werden anhand der allgemeinen Dehnungsabhängigkeit mittels Monte-Carlo-Simulationen bestimmt. Die so vorhergesagten Verteilungen werden durch ein Experiment im Rahmen der gestörten $\gamma$-$\gamma$-Winkelkorrelation (PAC) sichtbar gemacht, indem polykristalline MAX-Phasen unter uniaxialer Last verformt werden. Eine quantitative Auswertung erlaubt es schließlich, Defektdichten in-situ abzuschätzen. Die lokalen Auswirkungen auf den EFG werden anhand verschiedener MAX-Phasen-Mischkristalle systematisch untersucht. Im Zuge dessen wird die Synthese eines bisher unbekannten MAX-Phasen-Mischkristalls, Ti$_2$(Al$_{0,5}$,In$_{0,5}$)C, beschrieben. Die Zugehörigen Gitterkonstanten werden mittels Röntgendiffraktometrie im Rahmen der Rietveld-Methode bestimmt.

Modeling and analysing materials with theoretical tools is of great use when finding new systems for applications, for example, semiconductors with point defects can be used for quantum applications, like single photon emitters. One important aspect to consider symmetry, which can yield useful information about the properties of a system. To perform symmetry analysis, a code was developed that takes the orbitals of atomic structures, as calculated with Density Functional Theory simulations, as input. Specifically, the orbitals of molecules, and defects in solids are in focus. The symmetry analysis code calculates overlap of orbitals and their symmetry transformed counterpart, maps these overlaps to characters, finds the irreducible representations, and also finds which optical transitions are allowed. The code was tested on CH4 and SF6 molecules, and the divacancy defect in 4H-SiC. The symmetry analysis is performed easily and produces results that coincide well with other theoretical results. Furthermore, symmetry matrices can be approximated to be integer matrices, and the wave functions can be approximated with less accurate plane wave expansions by reducing the cutoff energy, and thus reducing the number of plane waves. These approximations shorten the calculation time and do not compromise the accuracy of the overlap. The code automates the symmetry analysis and is intended to be used in a high-throughput manner. / <p>2021-10-12          </p><p>The student thesis was first published online. </p><p>2022-02-25          </p><p>The student thesis was updated with an errata list which is downloadable from the permanent link.</p>

<p> </p> <ol> <li>This dissertation investigates the thermodynamics and transport of vacancies and interstitials of oxygen (O) and thorium (Th) in thorium dioxide (ThO₂) with varying charge states from neutral to maximum, with respect to temperature and oxygen pressure. The study also explores the impact of varying fractions of uranium (U) as a cation (<em>y</em>) on the defect disorder in mixed oxide fuels (Th_1-<em>y</em>U_<em>y</em>O₂). Understanding the properties of point defects in these oxides lays a strong foundation, as defects influence the properties of bulk materials, such as thermal transport. To accomplish the stated objectives of this dissertation, the research is structured into three sections that employ first principles density functional theory (DFT) and phonon calculations. The first section focuses on the structure, internal energy of formation, and vibrational entropy of point defects in ThO₂. The results demonstrate that defect energetics increase with an increase in defect charge for O interstitials and Th vacancies, while the opposite is true for O vacancies and Th interstitials. The lowest internal energy of formation shifts from O vacancies of charge 2+ to O interstitials and Th vacancies at various temperature ranges of 0 to 600 K, 600 to 1300 K, and 1300 to 2000 K. The second section develops a model to calculate the defect disorder and off-stoichiometry in ThO_{2±<em>x</em>} and Th_1-<em>y</em>U_<em>y</em>O_{2±<em>x</em>}. The model shows that ThO₂ exists mainly as a hypo-stoichiometric oxide between 1200 K to 2900 K for oxygen pressures ranging from 10^-30 to 10 atm, with O defects dominating this off-stoichiometric regime. The addition of U increases the thermodynamic window over which Th_1-<em>y</em>U_<em>y</em>O2 is hyper-stoichiometric, with O vacancies dominating in the hypo-stoichiometric regime, and cation vacancies and O interstitials dominating at low and high temperatures, respectively. Specifically, at low U content and low temperatures, U vacancies dominate hyper-stoichiometry, while at high U content and low temperatures, Th vacancies are dominant. This research facilitates the comprehension of the intricate changes in structural and defect equilibria that take place during nuclear fuel irradiation, where the fuel is not in a stoichiometric condition. The third section of the dissertation investigates migration barriers and diffusivities of defects and of O and Th in ThO₂. Results indicate that the migration energy of a point defect is dependent on its charge state. The average diffusivity of O vacancies exceeds that of O interstitials, while the similar is true for Th vacancies and Th interstitials above 1650 K. The self-diffusion coefficient of O and Th increases with temperature and is influenced by oxygen pressure, showing a close agreement with experimental and molecular-dynamics-based computational data. At 1500 K, the self-diffusivity of O and Th in ThO2 is 7.47 x 10^-16 m²s^-1 and 4.48 x 10^-23 m²s^-1 , respectively, while at 2500 K, the values increase to 1.06 x 10^-12 m²s^-1  and 2.28 x 10^-17 m²s^-1 , respectively. The chemical diffusion coefficients of defects decrease initially and then plateau as the hypo-stoichiometry in the oxide increases. These findings serve as a fundamental framework for understanding the diffusion-controlled processes of defects, which affect the radiation tolerance and microstructural evolution of ThO₂ as a nuclear fuel.  </li> </ol>

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Ce travail de thèse traite de la détection des défauts mécaniques des roulements à billes par analyse de grandeurs mécaniques et électriques dans des machines synchrones à aimants permanents haute vitesse. Le domaine applicatif de ce travail concerne l'aéronautique. Généralement, pour surveiller l'état des roulements à billes dans un actionneur électrique, des mesures vibratoires sont réalisées. Elles permettent, en exploitant le spectre du signal vibratoire, de mettre facilement en évidence la détérioration du roulement. Cette méthode de surveillance est cependant relativement couteuse en termes d'instrumentation et le placement d'un capteur vibratoire dans des équipements à fort degré d'intégration est parfois difficile. Nous proposons dans ce mémoire d'utiliser d'autres grandeurs physiques prélevées sur le système pour réaliser la surveillance de ces défauts. Il peut s'agir de grandeurs mécaniques (vitesse, position par exemple) et de grandeurs électriques (courant statorique, courant onduleur par exemple). L'utilisation de données déjà disponibles dans l'équipement pour les besoins de la commande permet ainsi de supprimer le système d'acquisition vibratoire. A partir d'enregistrements temporels de données réalisées au cours de campagnes d'essais, nous proposons des méthodologies de traitement du signal permettant d'extraire automatiquement des informations sensibles au défaut à surveiller. L'idée finale est de construire des indicateurs de l'état de santé des roulements permettant de prendre « juste à temps » des décisions fiables relatives à la maintenance ou à la sécurisation de l'équipement. Pour construire ces indicateurs, les signatures spécifiques aux défauts de roulements sont étudiées de manière théorique et expérimentale, pour l'ensemble des grandeurs prélevées. Leurs propriétés sont mises en évidence, permettant ainsi de définir les bandes fréquentielles les plus contributives au diagnostic. L'extraction de ces signatures est réalisée dans le domaine fréquentiel selon plusieurs méthodes. Deux types d'indicateurs automatiques différents sont proposés. Le premier est construit directement à partir du spectre d'amplitude des grandeurs par extraction de l'amplitude des harmoniques dans des bandes fréquentielles particulières. Le second intègre une dimension statistique dans l'analyse en exploitant le caractère aléatoire de certains harmoniques pour détecter la présence du défaut. Des critères de comparaison sont définis et utilisés pour étudier les performances des indicateurs proposés pour deux campagnes d'essais avec des roulements artificiellement dégradés, pour différentes vitesses de fonctionnement et pour différents paramètres de réglage des indicateurs. / This Ph.D. thesis deals with detection of mechanical bearings faults by analysis of mechanical and electrical signals in high speed permanent magnet synchronous machine. The application domain of this work concerns aeronautics. Generally, to monitor the ball bearings status in electrical actuator, the vibration measurements are used. They allow, by extracting the vibration spectrum, to easily detect the deterioration of the bearing. This monitoring method is relatively expensive in terms of instrumentation and placing a vibration sensor in equipment with a high integration degree can be difficult. We propose in this paper to use other physical quantities taken from the system to perform the monitoring of these defects. It may be mechanical quantities (for example speed, position) and electrical quantities (for example stator current, power inverter). From time recording of data carried out during test campaigns, we propose signal processing methodologies to automatically extract information sensitive to the monitored fault. The final idea is to construct indicators of bearings health and make decisions relating to maintenance or equipment security. To construct these indicators, specific bearing defects signatures are studied theoretically and experimentally, for all collected variables. The extraction of these signatures is carried out in frequency domain. Two different types of automatic indicators are proposed. The first is constructed directly from the amplitude spectrum by extraction of the harmonic amplitude of the spectrum in particular frequency bands. The second includes a statistical dimension analysis by exploiting the random nature of some harmonics to detect fault presence. Criteria of comparison are defined and used to study the proposed indicators performances for two trial campaigns with artificially degraded bearings, for different speed functioning and for different regulation of indicators parameters.

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Fundamental understanding of the behaviour of nuclear fuel has been of great importance. Enhancing this knowledge not only by means of experimental observations, but also via multi-scale modelling is of current interest. The overall goal of this thesis is to understand the impact of atomic interactions on the nuclear fuel material properties. Two major topics are tackled in this thesis. The first topic deals with non-stoichiometry in uranium dioxide (UO2) to be addressed by empirical potential (EP) studies. The second fundamental question to be answered is the effect of the atomic fraction of americium (Am), neptunium (Np) containing uranium (U) and plutonium (Pu) mixed oxide (MOX) on the material properties.UO2 has been the reference fuel for the current fleet of nuclear reactors (Gen-II and Gen-III); it is also considered today by the Gen-IV International Forum for the first cores of the future generation of nuclear reactors on the roadmap towards minor actinide (MA) based fuel technology. The physical properties of UO2 highly depend on material stoichiometry. In particular, oxidation towards hyper stoichiometric UO2 – UO2+x – might be encountered at various stages of the nuclear fuel cycle if oxidative conditions are met; the impact of physical property changes upon stoichiometry should therefore be properly assessed to ensure safe and reliable operations. These physical properties are intimately linked to the arrangement of atomic defects in the crystalline structure. The first paper evaluates the evolution of defect concentration with environment parameters – oxygen partial pressure and temperature by means of a point defect model, with reaction energies being derived from EP based atomic scale simulations. Ultimately, results from the point defect model are discussed, and compared to experimental measurements of stoichiometry dependence on oxygen partial pressure and temperature. Such investigations will allow for future discussions about the solubility of different fission products and dopants in the UO2 matrix at EP level.While the first paper answers the central question regarding the dominating defects in non-stoichiometry in UO2, the focus of the second paper was on the EP prediction of the material properties, notably the lattice parameter of Am, Np containing U and Pu MOX as a function of atomic fractions.The configurational space of a complex U1-y-y’-y’’PuyAmy’Npy’’O2 system, was assessed via Metropolis-Monte Carlo techniques. From the predicted configuration, the relaxed lattice parameter of Am, Np bearing MOX fuel was investigated and compared with available literature data. As a result, a linear behaviour of the lattice parameter as a function of Am, Np content was observed, as expected for an ideal solid solution. These results will allow to support and increase current knowledge on Gen-IV fuel properties, such as melting temperature, for which preliminary results are presented in this thesis, and possibly thermal conductivity in the future. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished