No description available.

The effect of a magnetic field on copper electrodeposition was investigated. Copper was electrodeposited onto square copper cathodes 1 sq cm in area from an aqueous solution (0.5 M CuSO4, 0.5 M H2SO. A glass cell was placed between the pole pieces of an electromagnet, and the magnetic fields applied were in the range from 0 to 12.5 kG. The current density was in the range from 80 mA/sq cm to 880 mA/sq cm. In each of the experiments, cell current, cell voltage, and cell temperature were monitored with a microcomputer. The weight change, deposit surface and cross section morphology, and the hardness were also found. Anodes used in the experiments were studied to see the effect of various conditions on the surface finish. Copper was also electrodeposited onto copper grids in order to study how the uniformity of the deposit is affected by an applied magnetic field.

It is accepted worldwide that the best final solution for spent nuclear fuel is to bury it in deep geological repositories. Despite the physical and chemical barriers that are supposed to isolate the nuclear waste for at least 100.000 years, some uncertainty factors may cause underground water to get in contact with the nuclear waste. Due to radioactivity and oxidation under air, dissolution experiments using UO2 pellets are difficult and frequently lead to incoherent results. Therefore, to enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel over time, it is necessary to produce analogues that closely resemble nuclear fuel in terms of crystallography and microstructure. At the same time, in-depth understanding of dissolution phenomena is crucial to geological processes such as dissolution precipitation creep and solvent mediated phase transformations. My thesis is based in two manuscripts. Paper I reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240°C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900°C and 1000°C. Paper II reports how differences of surface chemistry and crystal symmetry, characteristics of each surface orientation, affect the topography of CaF2 pellets described in paper I during dissolution. I propose that every orientation of the fluorite structure can be decomposed in the three reference surfaces {100}, {110} and {111}. The {111} is the most stable surface with a dissolution rate of the top surface of 1,13x10-9 mol.m-2.s-1, and {112} the less stable surface with a dissolution rate 34 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plan dissolve faster, possibly because the calcium is more susceptible to be solvated. The faster dissolving surfaces are replaced by the more stable {111} and {100} surfaces which causes the development of roughness on the top surface and stabilizes the surface on high energy sites; i.e. pores or grain boundaries. The main consequences of these observations are i) the increase of the total surface area; ii) the decrease of the overall surface energy. I present a dissolution model for surfaces of crystal with different surface energies. The main conclusions are: a) dissolution rates calculated from surface area are over estimated to the real dissolution rate; b) dissolution rates are faster at the beginning of dissolution and tend to diminish with time until a minimum value is reached.

No description available.

The behaviors of short fatigue crack (SFC) propagation through grain boundaries (GBs) were monitored during high cycle fatigue in an Al-Li alloy AA8090. The growth behaviors of SFCs were found to be mainly controlled by the twist components (α) of crack plane deflection across each of up to first 20 GBs along the crack path. The crack plane twist at the GB can result in a resistance against SFC growth; therefore SFC propagation preferred to follow a path with minimum α at each GB. In addition to the grain orientation, the tilting of GB could also affect α. An experiment focusing on quantifying GB-resistance was conducted on an Al-Cu alloy AA2024-T351. With a focused ion beam (FIB) and electron backscatter diffraction (EBSD), the micro-notches were made in front of the selected GBs which had a wide range of α, followed by monitoring the interaction of crack propagation from the notches with the GBs during fatigue. The crack growth rate was observed to decrease at each GB it had passed; and such growth-rate decrease was proportional to α. The resistance of the GB was determined to vary as a Weibull-type function of α. Based on these discoveries, a microstructure-based 3-D model was developed to quantify the SFC growth in high-strength Al alloys, allowing the prediction of crack front advancement in 3-D and the quantification of growth rate along the crack front. The simulation results yielded a good agreement with the experimental results about the SFC growth rate on the surface of the AA8090 Al alloy. The model was also used to predict the life of SFC growth statistically in different textures, showing potential application to texture design of alloys. Fatigue crack initiation at constituent particles (β-phase) was preliminarily studied in the AA2024-T351 Al alloy. Cross-sectioning with the FIB revealed that the 3-D geometry, especially the thickness, of fractured constituent particles (β-phase) was the key factor controlling the driving force for micro-crack growth. The resistance to micro-crack growth, mainly associated with crack plane twist at the particle/matrix interface, also influenced the growth behaviors of the micro-cracks at the particles on the surface.

The objective of this dissertation centers on gaining a better understanding of the structure - property - performance relations of nickel and copper through the advanced multiscale theoretical framework and integrated computational methods. The goal of this dissertation also includes to combine material science and computational mechanics to acquire a transformative understanding of how the different crystal orientations, size scales, and penetration velocities affect plastic deformation and damage behavior of metallic materials during high strain rate (> 103s-1) processes. A multiscale computational framework for understanding plasticity and shearing mechanisms of metallic materials during the high rate process was developed, which for the first time reveals micromechanical insights on how different crystal orientations, size scales, and penetration velocities affect the atomistic simulations which render structure property information for plasticity, shearing and damage mechanisms. The contributions of this dissertation include: (1) Comprehensive understanding of the plasticity and shearing mechanisms between the nickel penetrator and copper target under high strain rates (2) Development of a multiscale study of a nickel penetrator striking a copper plate by employing macroscale simulations and atomistic simulations to better understand the micromechanisms. (3) An essential description of how different crystal orientations, size scales, and strain rates affect the plasticity and shearing mechanisms.

No description available.

Donor-acceptor (D-A) copolymers have revolutionized the field of organic electronics over the last decade. Comprised of a electron rich and an electron deficient molecular unit, these copolymers facilitate the systematic modification of the material's optoelectronic properties. The ability to tune the optical band gap and to optimize the molecular frontier orbitals as well as the manifold of structural sites that enable chemical modifications has created a tremendous variety of copolymer structures. Today, these materials reach or even exceed the performance of amorphous inorganic semiconductors. Most impressively, the charge carrier mobility of D-A copolymers has been pushed to the technologically important value of 10 cm^{2}V^{-1}s^{-1}. Furthermore, owed to their enormous variability they are the material of choice for the donor component in organic solar cells, which have recently surpassed the efficiency threshold of 10%. Because of the great number of available D-A copolymers and due to their fast chemical evolution, there is a significant lack of understanding of the fundamental physical properties of these materials. Furthermore, the complex chemical and electronic structure of D-A copolymers in combination with their semi-crystalline morphology impede a straightforward identification of the microscopic origin of their superior performance. In this thesis, two aspects of prototype D-A copolymers were analysed. These are the investigation of electron transport in several copolymers and the application of low band gap copolymers as acceptor component in organic solar cells. In the first part, the investigation of a series of chemically modified fluorene-based copolymers is presented. The charge carrier mobility varies strongly between the different derivatives, although only moderate structural changes on the copolymers structure were made. Furthermore, rather unusual photocurrent transients were observed for one of the copolymers. Numerical simulations of the experimental results reveal that this behavior arises from a severe trapping of electrons in an exponential distribution of trap states. Based on the comparison of simulation and experiment, the general impact of charge carrier trapping on the shape of photo-CELIV and time-of-flight transients is discussed. In addition, the high performance naphthalenediimide (NDI)-based copolymer P(NDI2OD-T2) was characterized. It is shown that the copolymer posses one of the highest electron mobilities reported so far, which makes it attractive to be used as the electron accepting component in organic photovoltaic cells.par Solar cells were prepared from two NDI-containing copolymers, blended with the hole transporting polymer P3HT. I demonstrate that the use of appropriate, high boiling point solvents can significantly increase the power conversion efficiency of these devices. Spectroscopic studies reveal that the pre-aggregation of the copolymers is suppressed in these solvents, which has a strong impact on the blend morphology. Finally, a systematic study of P3HT:P(NDI2OD-T2) blends is presented, which quantifies the processes that limit the efficiency of devices. The major loss channel for excited states was determined by transient and steady state spectroscopic investigations: the majority of initially generated electron-hole pairs is annihilated by an ultrafast geminate recombination process. Furthermore, exciton self-trapping in P(NDI2OD-T2) domains account for an additional reduction of the efficiency. The correlation of the photocurrent to microscopic morphology parameters was used to disclose the factors that limit the charge generation efficiency. Our results suggest that the orientation of the donor and acceptor crystallites relative to each other represents the main factor that determines the free charge carrier yield in this material system. This provides an explanation for the overall low efficiencies that are generally observed in all-polymer solar cells. / Donator-Akzeptor (D-A) Copolymere haben das Feld der organischen Elektronik revolutioniert. Bestehend aus einer elektronen-reichen und einer elektronen-armen molekularen Einheit,ermöglichen diese Polymere die systematische Anpassung ihrer optischen und elektronischen Eigenschaften. Zu diesen zählen insbesondere die optische Bandlücke und die Lage der Energiezustände. Dabei lassen sie sich sehr vielseitig chemisch modifizieren, was zu einer imensen Anzahl an unterschiedlichen Polymerstrukturen geführt hat. Dies hat entscheidend dazu beigetragen, dass D-A-Copolymere heute in Bezug auf ihren Ladungstransport die Effizienz von anorganischen Halbleitern erreichen oder bereits übetreffen. Des Weiteren lassen sich diese Materialien auch hervorragend in Organischen Solarzellen verwenden, welche jüngst eine Effizienz von über 10% überschritten haben. Als Folge der beträchtlichen Anzahl an unterschiedlichen D-A-Copolymeren konnte das physikalische Verständnis ihrer Eigenschaften bisher nicht mit dieser rasanten Entwicklung Schritt halten. Dies liegt nicht zuletzt an der komplexen chemischen und mikroskopischen Struktur im Film, in welchem die Polymere in einem teil-kristallinen Zustand vorliegen. Um ein besseres Verständnis der grundlegenden Funktionsweise zu erlangen, habe ich in meiner Arbeit sowohl den Ladungstransport als auch die photovoltaischen Eigenschaften einer Reihe von prototypischen, elektronen-transportierenden D-A Copolymeren beleuchtet. Im ersten Teil wurden Copolymere mit geringfügigen chemischen Variationen untersucht. Diese Variationen führen zu einer starken Änderung des Ladungstransportverhaltens. Besonders auffällig waren hier die Ergebnisse eines Polymers, welches sehr ungewöhnliche transiente Strom-Charakteristiken zeigte. Die nähere Untersuchung ergab, dass in diesem Material elektrisch aktive Fallenzustände existieren. Dieser Effekt wurde dann benutzt um den Einfluss solcher Fallen auf transiente Messung im Allgemeinen zu beschreiben. Zusätzlich wurde der Elektronentransport in einem neuartigen Copolymer untersucht, welche die bis dato größte gemesse Elektronenmobilität für konjugierte Polymere zeigte. Darauf basierend wurde versucht, die neuartigen Copolymere als Akzeptoren in Organischen Solarzellen zu implementieren. Die Optimierung dieser Zellen erwies sich jedoch als schwierig, konnte aber erreicht werden, indem die Lösungseigenschaften der Copolymere untersucht und systematisch gesteuert wurden. Im Weiteren werden umfangreiche Untersuchungen zu den relevanten Verlustprozessen gezeigt. Besonders hervorzuheben ist hier die Beobachtung, dass hohe Effizienzen nur bei einer coplanaren Packung der Donator/Akzeptor-Kristalle erreicht werden können. Diese Struktureigenschaft wird hier zum ersten Mal beschrieben und stellt einen wichtigen Erkenntnisgewinn zum Verständnis von Polymersolarzellen dar.

As former studies reveal, the nanoporous confinement could have influence on polymorphic drug crystallization. However, little attention has been paid to the question how crystallization of the commonly polymorphic drugs in nanoporous matrices influences the drug release. As a consequence, sufficient information about the crystallization conditions and their influence on phase behavior, crystal texture, and stability of polymorphs should be retrieved prior to drug delivery experiments. Drug release should be polymorph-selective and even crystal face-specific. Therefore, the topic of this PhD thesis is the systematic investigation of crystallization parameters (e.g., pore morphology, thermal history, presence or absence of a bulk surface reservoir) and their influence on the nucleation and crystal growth of the two selected model compounds in nanoporous matrices: acetaminophen (ACE) and n-tetracosane. Both are confined to two host-systems: AAO containing aligned cylindrical, isolated pores and CPG containing curved, interconnected pores. The guest materials inside the two model matrices have been investigated with X-ray diffraction (WAXS) and differential scanning calorimetry. In the first part it is shown that the nanopore morphology of the host systems determines into which polymorphic form ACE crystallizes. Moreover, the pore morphology influences the kinetics of solid/solid transitions. In AAO uniformly oriented form III crystals are converted into also uniformly oriented form II crystals by a solid/solid transition. Such a phase transition is kinetically suppressed in CPG membranes due to the curved pore morphology. In the second step, polymorph-specific release experiments with ACE from AAO membranes reveal that the drug dissolution is not exclusively diffusion-limited and can be described by the Korsmeyer-Peppas model. Dissolution of crystalline ACE having rough crystal faces exposed to the environment is nearly as fast as release of amorphous ACE. Encapsulating of ACE in AAO nanopores with a PLLA polymer retard the drug dissolution but does not modify the release kinetics. In the third part of this thesis crystallization of n-tetracosane, a saturated hydrocarbon, in nanoporous matrices was studied. n-Tetracosane shows inside AAO membranes the rotator phase sequence: triclinic−RV−RI−RII−liquid. Further, the long axes of the n-tetracosane molecules are oriented normal to the AAO pore axes. In general, n-tetracosane under confinement shows a more complex phase behavior than the polymeric analogue polyethylene. The presented work expands the available strategies for mesoscopic crystal engineering. The methods might be transferred into other areas of interest such as polymorphism screening or preparation of different types of nanowires with customized optoelectronic or ferroelectric properties.

L’objectif principal de ce travail est d’optimise la tomographie par coupe sériée dans un microscope ‘FIB’, en utilisant soit l’imagerie électronique du microscope à balayage (tomographie FIB-MEB), soit la diffraction des électrons rétrodiffusés (tomographie dite EBSD 3D). Dans les 2 cas, des couches successives de l’objet d’étude sont abrasées à l’aide du faisceau ionique, et les images MEB ou EBSD ainsi acquises séquentiellement sont utilisées pour reconstruire le volume du matériau. A cause de différentes sources de perturbation incontrôlées, des dérives sont généralement présentes durant l'acquisition en tomographie FIB-MEB. Nous avons ainsi développé une procédure in situ de correction des dérives afin de garder automatiquement la zone d'intérêt (ROI) dans le champ de vue. Afin de reconstruction le volume exploré, un alignement post-mortem aussi précis que possible est requis. Les méthodes actuelles utilisant la corrélation-croisée, pour robuste que soit cette technique numérique, présente de sévères limitations car il est difficile, sinon parfois impossible de se fier à une référence absolue. Ceci a été démontré par des expériences spécifiques ; nous proposons ainsi 2 méthodes alternatives qui permettent un bon alignement. Concernant la tomographie EBSD 3D, les difficultés techniques liées au pilotage de la sonde ionique pour l'abrasion précise et au repositionnement géométrique correct de l’échantillon entre les positions d'abrasion et d’EBSD conduisent à une limitation importante de la résolution spatiale avec les systèmes commerciaux (environ 50 nm)3. L’EBSD 3D souffre par ailleurs de limites théoriques (grand volume d'interaction électrons-solide et effets d'abrasion. Une nouvelle approche, qui couple l'imagerie MEB de bonne résolution en basse tension, et la cartographie d'orientation cristalline en EBSD avec des tensions élevées de MEB est proposée. Elle a nécessité le développement de scripts informatiques permettant de piloter à la fois les opérations d’abrasion par FIB et l’acquisition des images MEB et des cartes EBSD. L’intérêt et la faisabilité de notre approche est démontrée sur un cas concret (superalliage de nickel). En dernier lieu, s’agissant de cartographie d’orientation cristalline, une méthode alternative à l’EBSD a été testée, qui repose sur l’influence des effets de canalisation (ions ou électrons) sur les contrastes en imagerie d’électrons secondaires. Cette méthode corrèle à des simulations la variation d’intensité de chaque grain dans une série d’images expérimentales obtenues en inclinant et/ou tournant l’échantillon sous le faisceau primaire. Là encore, la méthode est testée sur un cas réel (polycritsal de TiN) et montre, par comparaison avec une cartographie EBSD, une désorientation maximale d'environ 4° pour les angles d’Euler. Les perspectives d’application de cette approche, potentiellement beaucoup plus rapide que l’EBSD, sont évoquées. / The aim of current work is to optimize the serial-sectioning based tomography in a dual-beam focused ion beam (FIB) microscope, either by imaging in scanning electron microscopy (so-called FIB-SEM tomography), or by electron backscatter diffraction (so-called 3D-EBSD tomography). In both two cases, successive layers of studying object are eroded with the help of ion beam, and sequentially acquired SEM or EBSD images are utilized to reconstruct material volume. Because of different uncontrolled disruptions, drifts are generally presented during the acquisition of FIB-SEM tomography. We have developed thus a live drift correction procedure to keep automatically the region of interest (ROI) in the field of view. For the reconstruction of investigated volume, a highly precise post-mortem alignment is desired. Current methods using the cross-correlation, expected to be robust as this digital technique, show severe limitations as it is difficult, even impossible sometimes to trust an absolute reference. This has been demonstrated by specially-prepared experiments; we suggest therefore two alternative methods, which allow good-quality alignment and lie respectively on obtaining the surface topography by a stereoscopic approach, independent of the acquisition of FIB-SEM tomography, and realisation of a crossed ‘hole’ thanks to the ion beam. As for 3D-EBSD tomography, technical problems, linked to the driving the ion beam for accurate machining and correct geometrical repositioning of the sample between milling and EBSD position, lead to an important limitation of spatial resolution in commercial softwares (~ 50 nm)3. Moreover, 3D EBSD suffers from theoretical limits (large electron-solid interaction volume for EBSD and FIB milling effects), and seems so fastidious because of very long time to implement. A new approach, coupling SEM imaging of good resolution (a few nanometres for X and Y directions) at low SEM voltage and crystal orientation mapping with EBSD at high SEM voltage, is proposed. This method requested the development of computer scripts, which allow to drive the milling of FIB, the acquisition of SEM images and EBSD maps. The interest and feasibility of our approaches are demonstrated by a concrete case (nickel super-alloy). Finally, as regards crystal orientation mapping, an alternative way to EBSD has been tested; which works on the influence of channelling effects (ions or electrons) on the imaging contrast of secondary electrons. This new method correlates the simulations with the intensity variation of each grain within an experimental image series obtained by tilting and/or rotating the sample under the primary beam. This routine is applied again on a real case (polycrystal TiN), and shows a max misorientation of about 4° for Euler angles, compared to an EBSD map. The application perspectives of this approach, potentially faster than EBSD, are also evoked.