In 2012 the HZDR, and in consequence also the Institute of Ion Beam Physics and Materials Research (IIM) including its Ion Beam Center (IBC), has undergone a scientific evaluation. The evaluation committee composed of the Scientific Advisory Board and numerous external experts in our field of research concluded that “the overall quality of the scientific work is excellent”, that “there are an impressive number of young scientists working enthusiastically on a variety of high-level projects” and that “the choice of these projects represents a clear underlying strategy and vision”. We feel honored and are proud that the external view on our scientific achievements is that extraordinary. In view of this outstanding result we would like to express our gratitude to all our staff members for their commitment and efforts! In the past year, we continued our integration into the Helmholtz Association of German Research Centers (HGF) with our Institute mostly active in the research area “Matter”, but also involved in a number of activities in the research area “Energy”. In this respect, many consultations were held with the Helmholtz centers contributing to common research areas to precisely define the role we will play in the newly established HGF program “From Matter to Materials and Life” (see schematic below). Our IBC has been recognized as a large-scale user facility for ion beam analysis and modification of materials, i.e., specializing on materials science. In particular, the IBC plays a prominent role in the recently approved Helmholtz Energy Materials Characterization Platform (HEMCP), which mainly concentrates on the development of dedicated analytical tools for the characterization of materials required for future energy technologies. The successes achieved by the IBC allows us to invest 7200 k€ to further improve and strengthen the ion beam capabilities at the Institute. In addition to this infrastructure-related grant, we were also successful in our funding application for the establishment of the International Helmholtz Research School for Nanoelectronic Networks (IHRS NANONET), aiming at promoting the next generation of leading scientists in the field of nanoelectronics. The IHRS NANONET is coordinated by our Institute and offers a well-structured PhD program to outstanding students of all nationalities with emphasis on interdisciplinary research and comprehensive training in technical and professional skills.

The year 2013 was the third year of HZDR as a member of the Helmholtz Association (HGF), and we have made progress of integrating ourselves into this research environment of national Research centers. In particular, we were preparing for the evaluation in the framework of the so-called program oriented funding (POF), which will hopefully provide us with a stable funding for the next five years (2015 – 2019). In particular, last fall we have submitted a large proposal in collaboration with several other research centers. The actual evaluation will take place this spring. Most of our activities are assigned to the program “From Matter to Materials and Life” (within the research area “Matter”). A large fraction of this program is related to the operation of large-scale research infrastructures (or user facilities), one of which is our Ion Beam Center (IBC). The second large part of our research is labelled “in-house research”, reflecting the work driven through our researchers without external users, but still mostly utilizing our large-scale facilities such as the IBC, and, to a lesser extent, the free-electron laser. Our in-house research is performed in three so-called research themes, as depicted in the schematic below. What is missing there for simplicity is a small part of our activities in the program “Nuclear Waste Management and Safety” (within the research area “Energy”).

This past year 2014 was the year when we finally completely arrived as a “full member” in the Helmholtz Association. This is related to the successfully passed research evaluation in the framework of the Program Oriented Funding (POF), which will give us a stable and predictable funding for the next five years (2015 – 2019). This is particularly true for our large-scale user facilities, like the Ion Beam Center (IBC) and the electron accelerator ELBE with the free-electron laser. Most of our activities are assigned to the program “From Matter to Materials and Life” within the research area “Matter”, in cooperation with several other German Helmholtz Centers. Our in-house research is performed in three so-called research themes, as depicted in the schematic below. What is missing there for simplicity is a small part of our activities in the program “Nuclear Waste Management and Safety” within the research area “Energy”. Our research and facilities were well appreciated by the evaluation committee, who made the following judgement about the Ion Beam Center: “The Ion Beam Centre (IBC) of HZDR is an internationally leading ion-beam facility (with ion energies ranging from several eV to several tens of MeV). At both the national and international level it is one of the key players and is unique in its kind. The synergy between forefront research and user service has been leading to a very good publication output for both in-house research and user research. … The very broad range of beam energies, the versatility of techniques and applications – both for ion beam modification of materials and for ion-beam analysis – makes the IBC unique in its kind. … The strength of IBC is that its activities are based on a combination of forefront research and user service, which mutually interact in synergy and strengthen one another. In turn, this synergy has been leading to a very good publication output for both in-house research and user research.” In order to make our Annual Report a bit more compact, we have decided to include only four full journal papers this year. This was also triggered by the fact that our publication activities have turned out be become more diverse, in more diverse journals than in the past, and often through longer papers, which would be too long to reprint them here. However, apart from the constantly quantitatively high publication output, we succeeded to publish in excellent journals such as Nature Physics, Nano Letters and Physical Review Letters, in fields as diverse as ion beam physics, magnetism and terahertz spectroscopy. Two of our scientists, Dr. Artur Erbe and Dr. Alexej Pashkin obtained their Habilitation in 2014, both at University of Konstanz. For the first time, we are hosting an Emmy Noether Young Investigator Group funded by the Deutsche Forschungsgemeinschaft (DFG); the group works on the hot topic of magnonics and is headed by Dr. Helmut Schultheiß. Finally we would like to cordially thank all partners, friends, and organizations who supported our progress in 2014. Special thanks are due to the Executive Board of the Helmholtz-Zentrum Dresden-Rossendorf, the Minister of Science and Arts of the Free State of Saxony, and the Minister of Education and Research of the Federal Government of Germany. Numerous partners from universities, industry and research institutes all around the world contributed essentially, and play a crucial role for the further development of the institute. Last but not least, the directors would like to thank again all IIM staff for their efforts and excellent contributions in 2014.

The pursuit of high quality, large area graphene has been a major research focus of contemporary materials science research, in the wake of the discovery of the multitude of exceptional properties exhibited by the material. The DPhil project was undertaken with the objective of developing an understanding of the growth of large graphene sheets by chemical vapour deposition (CVD), and also in the subsequent characterisation of their material properties. By conducting atmospheric pressure CVD growth at high methane flow rates, it was found that few-layered graphene (FLG) could be deposited on a copper catalyst. It is demonstrated that the self-limiting property of a copper catalyst is not universal to all deposition conditions, and shown that FLG grows in a terrace-like configuration. In depth transmission electron microscopy (TEM) studies were carried out on FLG. By selective image reconstruction from the inverse power spectrum of the TEM images, it was possible to elucidate the inter-grain connectivity of few-layer graphenes. It was determined that there were two possible inter-grain configurations possible; specifically an overlap of graphene layers or a discrete atomic bonding edge. The perturbation of the few-layer structure when subject to an out of plane distortion was found to incur a shift in the conventional AB-Bernal stacking of FLG. By utilising the aberration corrected TEM (AC-TEM) at Oxford it was possible to resolve atomic detail in CVD synthesised monolayer films, including atomic bond rotations and vacancies. The use of a high current density at low accelerating voltage (80 kV) was demonstrated to allow for the controlled defect creation in graphene sheets. This permitted the creation of monovacancies and iron doped vacancy complexes suitable for further study. The behaviour of these two defect types under electron beam irradiation was subsequently studied.

Ce mémoire s'intéresse au comportement hydro-mécanique de l'argilite. On montre que la théorie classique de Biot n'est pas du tout adaptée au cas de l'argilite. Une loi de comportement originale est alors construite grâce à des outils d'homogénéisation, en intégrant la microstructure de l'argilite et des phénomènes physiques existant aux échelles microscopiques, tels la surpression de gonflement au niveau des particules d'argile et les effets capillaires au niveau du réseau poreux. Cette loi de comportement permet d'expliquer tout un jeu d'expériences qui ne l'était pas par la théorie classique de Biot. Elle est ensuite enrichie pour intégrer les données expérimentales que sont la dépendance du tenseur d'élasticite au degré de saturation et la présence d'une porosité autour des inclusions. Cette loi de comportement pertinente, exploitée à partir du suivi dimensionnel d'échantillons sous chargement hydrique, permet de proposer une estimation pour le tenseur de Biot pour l'argilite. Afin d'alimenter cette équation d'état, on s'intéresse par ailleurs aux variations du degré de saturation d'un échantillon lors d'un séchage. Deux modèles de séchage sont étudiés et comparés, puis une modélisation du réseau poreux est proposée afin d'expliquer les mesures atypiques de perméabilité.

Une démarche globale pour l'estimation et l'élaboration de la fiabilité de systèmes mécaniques aux différentes étapes de leur processus de conception est proposée. Elle repose sur une approche multidisciplinaire exploitant une représentation système, physique et probabiliste des scénarios de défaillance. Combinant leurs interprétations système sans physique et physique sans système, elle assure la construction de modèles de fiabilité à la fois physiques et systèmes. Plusieurs stratégies d'évaluation sont alors déployées. Utilisant les outils du couplage mécano-fiabiliste (e.g. chaos polynomial, FORM/SORM), elles fournissent des mesures de fiabilité traditionnelles et permettent également la définition de chaînes de mesures d'importance originales. Celles-ci détaillent le poids et l'influence de chaque scénario et de chacun de leurs paramètres sur la réalisation des défaillances. L'optimisation de la conception est alors en partie assurée par l'analyse de fiabilité elle-même

Microscopic beams of penetrating synchrotron radiation provide a unique tool for the analysis of material structure and deformation. This thesis describes my contributions to the development of new synchrotron X-ray micro-beam diffraction experimental techniques and data interpretation, and the use of experimental results for the validation of material deformation models. To study deeply buried material volumes in thick samples, the micro-beam Laue technique was extended to higher photon energies. Through-thickness resolution was achieved either by a wire scanning approach similar to Differential Aperture X-ray Microscopy (DAXM), or by applying tomographic reconstruction principles to grain-specific Laue pattern intensity. Both techniques gave promising first results. For reliable micro-beam Laue diffraction measurements of elastic strains in individual grains of a polycrystal, understanding of the error sources is vital. A novel simulation-based error analysis framework allowed the assessment of individual contributions to the total measurement error. This provides a rational basis for the further improvement of experimental setups. For direct comparison of experimental measurements and dislocation dynamics simulations, diffraction post-processing of dislocation models in two and three dimensions was developed. Simulated diffraction patterns of two-dimensional dislocation cell/wall type structures captured correctly some of the features observed experimentally in reciprocal space maps of a large-grained, lightly deformed aluminium alloy sample. Crystal lattice rotations computed from three-dimensional dislocation dynamics simulations of a Frank-Read source showed anisotropic orientation spread similar to that observed in micro-beam Laue experiments. For the experimental study of crystal lattice distortion, a novel technique was proposed that combines micro-beam Laue diffraction with scanning white-beam topography. Diffraction topography allows the study of lattice rotation at scales smaller than the scanning beam size. The new technique makes it possible to apply classical topography methods to deformed samples.

L'utilisation de polymères thermoplastiques renforcés par des fibres de verre courtes (SGFRT) est en constante progression dans le domaine automobile car ils offrent de nombreux avantages : gains de masse, intégration de fonctions combinée à des géométries complexes et gain de productivité du fait de cadences élevées. Le Polyamide renforcé à 50% en masse, par des fibres de verre courtes, propose un très bon compromis technico-économique pour des applications sous capot moteur, y compris pour des composants structuraux. Ces applications nécessitent un dimensionnement à la fatigue fiable et optimisé. Les campagnes de caractérisation classiques se heurtent à la fois à la sensibilité thermomécanique à la fréquence de sollicitation et à la multiplicité des facteurs influents (process, environnement, multi-axialité, cumul, ...). Nous avons donc mis au point un protocole de caractérisation rapide à base d'essais d'auto-échauffement : cette technique, accompagnée d'un critère énergétique, permet un gain de temps considérable pour l'évaluation des différentes courbes d'endurance. Elle permet l'obtention d'une courbe d'endurance déterministe en moins de 2 jours et offre ainsi la possibilité d'étudier différents paramètres influents (géométrie, reprise en humidité, orientation des fibres, longueur des fibres, ligne de soudure ...). Son application à la caractérisation d'éprouvettes de structures et d'une structure industrielle a également illustré la pertinence de cette approche.

Le comportement thermomécanique des matériaux à mémoire de forme est étudié dans la plage de leur pseudoélasticité. En effet, le modèle Zaki-Moumni initialement validé dans les conditions de chargements quasi statiques, est étendu à l'effet de la vitesse de chargement par la prise en compte du couplage thermomécanique. Les résultats de la simulation de cette extension sont comparés à l'expérience et montre une bonne reproduction qualitative des observations à différentes vitesses de chargements et à température initiale fixée. Le modèle tridimensionnel est ensuite réduit pour analyser les oscillations unidimensionnelles d'un dispositif en matériaux à mémoire de forme. La réponse en fréquences de l'oscillateur forcé est alors investie autour de la résonance. Les oscillations isothermes et non isothermes ainsi que la dissymétrie en traction-compression de la force de rappel, sont prises en compte. Des phénomènes dynamiques tels que le chaos, le doublement de périodes et les sauts autour de bifurcations dans la solution, apparaissent pour des amplitudes de chargement élevées et pour des paramètres matériaux donnés.

De nos jours, l'utilisation de tunneliers à front pressurisé est souvent pleinement justifiée, en particulier en contexte urbain, où les terrains sont meubles, partiellement ou totalement saturés, et où la préservation du bâti impose une limitation drastique des pertes de volume au cours de l'excavation. Ce travail de thèse vise à améliorer la compréhension de la phénoménologie de ce mode de creusement et à développer de nouveaux outils d'aide à la justification des ouvrages, au dimensionnement des machines et au pilotage des chantiers. Dans un premier temps, des analyses sont menées à partir d'essais sur un modèle réduit de tunnelier à pression de terre original à l'échelle internationale. Celles-ci concernent le comportement en contraintes et déplacements du massif au cours du creusement en régime idéal, l'évaluation de la stabilité du front de taille, et l'analyse des grandeurs-machine du tunnelier. Les analyses réalisées sont menées en massifs de sol homogènes ainsi qu'en massifs stratifiés (bicouches/tricouches) configurations fréquemment rencontrées sur chantier. Dans un second temps, les analyses effectuées à l'échelle du modèle réduit sont confrontées à des données de chantier pour validation de la modélisation physique effectuée. Enfin, la modélisation théorique du creusement pressurisé des tunnels est étudiée. D'une part, les capacités et limites des outils analytiques (calcul à la rupture) et numériques d'évaluation de la stabilité du front de taille sont évaluées par confrontation avec nos résultats expérimentaux. D'autre part, une procédure de modélisation numérique du processus de creusement en régime idéal, calée sur la modélisation physique, est proposée.