Universität Stuttgart

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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Institut für Theoretische und Angewandte Physik (aufgelöst)Start date: 2010End date: 2015

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Now showing 1 - 10 of 21
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    Spin-echo resolved neutron scattering from self-organised polymer interfaces
    (2010) Nülle, Max; Dosch, Helmut (Prof. Dr.)
    This thesis focused on two main objectives: First, the clarification of the prospects of the spin-echo resolved grazing incidence neutron scattering method (SERGIS) for the investigation of buried interfaces. And second, the investigation of the self-organisation (i.e. microphase separation and dewetting) of ultrathin poly(styrene-block-isoprene) diblock copolymer films on silicon substrates by means of SERGIS and complementary techniques. SERGIS is a novel neutron scattering technique which was implemented and further developed at the new neutron / x-ray reflectometer N-REX+ at the FRM II (Garching, Germany). In contrast to conventional small-angle scattering methods, SERGIS characterises the lateral structure and morphology of interfaces and thin-film systems in real space. The technique uses a polarised primary beam, and the measured quantity is the integral polarisation of the scattered beam. By decoupling the measurement resolution and the beam divergence (in a first approximation), SERGIS aims at a good resolution and a good measurement statistics simultaneously. As a first systematic application of SERGIS to a real physical problem, the dewetting and internal structure of ultrathin poly(styrene-block-isoprene) diblock copolymer films were studied by means of SERGIS and complementary surface sensitive techniques, namely neutron and x ray reflectivity and atomic force microscopy (AFM).
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    In-situ X-ray studies of model electrode surfaces for solid oxide fuel cells
    (2010) Khorshidi, Navid; Dosch, Helmut (Prof. Dr.)
    Fuel cells are considered as a promising way to produce clean energy. These cells convert the chemical energy created by the reaction of hydrogen and oxygen to water into electric energy. Regarding the difficulties connected with the production and more importantly storage of hydrogen, solid oxide fuel cells (SOFCs) play an outstanding role among the diverse fuel cell types. SOFCs are able to use not only pure hydrogen as a fuel, but also hydrocarbons. This ability leads to impressive efficiencies and allows to integrate SOFCs into existing structures. SOFCs are usually operated at temperatures above 800°C which leads to extreme conditions for the used materials and limits the lifetime of the cells. One of the major goals in the future is thus to develop low temperature SOFCs. In order to achieve such a goal, an atomic understanding of the chemicals reactions on the electrodes is essential. A typical SOFC is made of a cathode consisting of lanthanum strontium manganate (LSM) and yttria-stabilized zirconia (YSZ). Due to its electronic and thermal isolation and the conduction of oxygen ions, YSZ also serves as an electrolyte and at the same time as a part of the anode, which in addition is covered with nickel particles. An atomic understanding of the respective chemical reactions thus requires to have atomic models of the YSZ surface being present at the cathode and the anode surface. Another fundamental component to know are the nickel particles covering the anode. The superb importance of the anode or fuel cathode is to be found by the diverse fuels to be processed here. The aim of this work is to experimentally determine the atomic surface structure of the two important orientations (111) and (100) of YSZ under relevant conditions. Additionally the growth and shape of Ni nano particles grown on these surfaces as well as their shape changes under related conditions are studied. The gathered knowledge can be assembled to a model anode and a part of a model cathode. The found results are also of importance for the growth of thin films, where YSZ is a frequent substrate. The main experimental tool is surface X-ray diffraction (SXRD), which allows to derive atomic structures of surfaces regardless of their conductivity properties. The experiments were carried out in a mobile ultra-high vacuum chamber using synchrotron radiation.
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    Lattice dynamics of complex metallic alloys
    (2011) Euchner, Holger; Trebin, Hans-Rainer (Prof. Dr.)
    Throughout this thesis the lattice dynamics in CMA phases with different structural and dynamical peculiarities have been studied in experiment and simulation. While inelastic neutron and X-ray scattering enabled an experimental approach to dynamical quantities as dispersion curves, vibrational density of states or dynamical structure factors, the theoretical approach was based on ab-initio and molecular dynamics simulations. Experimental results could be analyzed and interpreted by means of computer simulations, thus yielding insight into dynamical processes on an atomistic level. Indeed, this combination of experiment and simulation proved to be a powerful tool for the investigation of different dynamical phenomena. In the Mg-Zn system the impact of structural complexity on vibrational properties was studied. Pure hcp Zn and the MgZn2 Laves phase were used as rather simple reference structures and compared to the structurally more complex Mg2Zn11 Pauling triacontahedral phase. While MgZn2 showed the behavior of an almost perfectly harmonic solid, Mg2Zn11 turned out to exhibit quite unusual dynamical features. In the case of MgZn2 experimental results from INS could be reproduced with high accuracy. For Mg2Zn11 experimental results and DFT calculations first evidenced non-negligible discrepancies. After reinvestigating the structure of Mg2Zn11 with both, experimental and computational methods, a partially occupied Zn site could be spotted as possible source of the occurring discrepancies. Surprisingly, the partially vacant Zn1 position, at the center of the mini-Bergman cluster proved to exert a strong influence on stability and dynamics of this system. After taking vacancy disorder into account, the experimental results could be decently reproduced and differences could be understood. With this knowledge the experimental GVDOS was finally interpreted in terms of distinct atomic motions, thus connecting macroscopic properties with processes on atomistic scale. The second Zn-based CMA phase that was explored, is the ScZn6 1/1-approximant. The structure of this phase is closely related to the Cd-based binary icosahedral quasicrystals in the Cd-Yb and Cd-Ca system, thus making it an interesting phase with respect to structure and dynamics of quasicrystals like Mg-Zn-Sc. Secondly, the ScZn6 1/1-approximant evidences an order-disorder phase transition at about 150 K. The dynamical aspects of this phase transition were investigated throughout this work, using quasielastic neutron scattering and molecular dynamics methods. Interestingly, the phase transition could be shown to be closely related to a freezing in of the tetrahedral shell in the center of the Tsai-type cluster building blocks. In fact, experiment and calculation clearly evidenced a dynamic disorder of the tetrahedral shell above the transition temperature. The tetrahedral shell is constantly reorienting between different, energetically equivalent configurations. From neutron scattering experiments the residence time between two tetrahedron jumps could be estimated to be of the order of a few ps, while it was overestimated by the conducted MD simulations. These results thus answer the controversially debated question about the nature of the disorder in ScZn6 in favor of a dynamic process. Finally the dynamic reorientations of the tetrahedron are highly interesting with respect to entropical stabilization, a possible candidate for quasicrystal stabilization. In the last part of the thesis the clathrate system Ba-Ge-Ni, was studied with respect to its cage-like structure and the resulting effects on its dynamical properties. Inelastic neutron scattering experiments nicely evidenced a flat dispersionless optic-like phonon branch, which by means of DFT could be shown to stem from localized motions of the encaged Ba atoms - so-called rattling modes. The cage structure of the Ba-Ge-Ni clathrates furthermore made a decomposition into different subsystems possible, such that their contributions to the vibrational spectrum could be analyzed. A comparison to a hypothetical Ge46 structure could be used to elaborate the influence of the encaged Ba-atoms and the host-lattice, respectively. Interestingly, the introduction of Ba-atoms creates a localized, dispersionless phonon branch at rather low energy, which interacts with the acoustic modes of the host structure, resulting in a reduction of the velocity of sound. Thus the low lattice thermal conductivity in this phase seems to be related to both, rattling modes of Ba guest atoms and reduced velocity of sound of the host framework.
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    Phasonendynamik in dekagonalen Quasikristallen
    (2014) Lipp, Hansjörg; Trebin, Hans-Rainer (Prof. Dr.)
    Quasikristalle weisen mehr an Freiheitsgraden auf als die üblichen phononischen: Es gibt sogenannte phasonische Flips, atomare Sprünge zwischen nahe beieinanderliegenden Minima in der komplexen Energielandschaft dieser Festkörper. Sowohl diese anharmonischen Potentiale als auch die Existenz des phasonischen Freiheitsgrads haben großen Einfluss auf die thermodynamischen und mechanischen Eigenschaften der Quasikristalle. Hierzu gibt es experimentelle Befunde von K. Edagawa et al., die in der vorliegenden Arbeit theoretisch untersucht werden. Einerseits wurde eine über Dulong-Petit hinausgehende Wärmekapazität festgestellt, als deren Ursache der zusätzliche Freiheitsgrad vermutet wurde. Diese Arbeit untersucht eindimensionale Modellsysteme, die Teilchenflips zulassen und in denen Teilchen gemäß einem anharmonischen Doppelmuldenpotential wechselwirken. Dazu werden Molekulardynamiksimulationen durchgeführt und analytische Rechnungen angestellt. Dabei stellt sich heraus, dass die thermodynamischen Eigenschaften dieses Systems hauptsächlich vom Wechselwirkungspotential bestimmt werden. Die Wärmekapazität lässt sich daher analytisch berechnen und weist allein aufgrund der Anharmonizität einen erhöhten Wert auf. Der größere zweite Teil der Arbeit behandelt von Edagawas Gruppe durchgeführte elektronenmikroskopische Aufnahmen von dekagonalem Al-Cu-Co. Hier beobachtete Edagawa helle Flecken, die an den Vertices eines Tilings lagen und sich im Laufe der Zeit ähnlich einem Phasonenflip änderten: Sie erschienen und verschwanden auf erratische Weise. Bemerkenswert war hierbei die Zeitskala: Die Intensitätsschwankungen der Flecken erfolgen im Sekunden- und Minutenregime, während man aus Neutronenbeugungsexperimenten weiß, dass atomare phasonische Flips in diesen Systemen im Pikosekundenbereich vorkommen. Dekagonales Al-Cu-Co besteht aus periodisch angeordneten Doppelschichten. Jede Doppelschicht kann entsprechend einem Strukturmodell von Zeger et al. als Tiling von Rauten betrachtet werden, die mit Atomen dekoriert sind. In übereinander liegenden Rauten können die Atome zwischen verschiedenen Flippositionen springen. Dementsprechend werden statistische Modelle übereinander liegender Rauten erstellt, die verschiedene diskrete Zustände annehmen können. Diese Rauten können ihren Zustand entsprechend umgebungsabhängiger Sprungraten in einem Zufallsprozess ändern, wodurch die Schichten gekoppelt sind. Definiert man nun die mesoskopische Sichtbarkeit eines hellen Flecks über gemeinsame Zustände mehrerer übereinander liegender Rauten, so kann man die Sprungraten und Sichtbarkeitswahrscheinlichkeiten der Flecken untersuchen. Dies erfolgt in der vorliegenden Arbeit analytisch und mit Hilfe von Monte-Carlo-Simulationen. Dabei zeigt sich, dass sich im System übereinander liegende Rauten gleichen Zustands, sogenannte Cluster, bilden, welche die Sichtbarkeit der Flecken bestimmen. Diese Cluster werden in der Zeitentwicklung gebildet, zerstört oder verschoben. Die Verteilung der Clustergrößen weist einen stabilen Gleichgewichtszustand auf, der von der Kopplungsstärke der Rauten abhängt. Die Anzahl der Cluster fluktuiert um den Gleichgewichtswert. Die Abweichungen können über einen neuartigen Zufallsprozess, den "harmonischen Random Walk" beschrieben werden, bei dem die Sprungwahrscheinlichkeit proportional zur Entfernung vom Gleichgewicht ist. Dieser Zufallsprozess stabilisiert den Gleichgewichtszustand und wird zum besseren Verständnis des Systems analytisch und numerisch behandelt. Er erlaubt es, Sprungraten und Sichtbarkeitswahrscheinlichkeiten der Flecken in Simulation und analytischer Rechnung zu bestimmen. Daraufhin werden von K. Edagawa zur Verfügung gestellte Beobachtungsdaten eines HRTEM-Experiments analysiert. Sprungraten und Sichtbarkeitswahrscheinlichkeiten werden extrahiert und mit den theoretischen Ergebnissen verschiedener Modelle verglichen. Dabei zeigt sich eine gute Übereinstimmung des Experiments mit einem aus Doppelschichten aufgebauten System, in dem sich Ringe aus zehn Atomen durch kollektive Flips verschieben können. Die langsame Flipdynamik der Flecken lässt sich also statistisch dadurch erklären, dass sich Atome in vielen Schichten kollektiv bewegen müssen.
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    Magnetoelektrischer Effekt in metallischen Nanostrukturen : Ab-initio Elektronentheorie und atomistische Modellierung
    (2011) Subkow, Sergej; Fähnle, Manfred (Prof. Dr. rer. nat.)
    Der magnetoelektrische Effekt in metallischen Nanostrukturen wird untersucht. Dabei wird die Veränderung der magnetokristallinen Oberflächenanisotropieenergie dünner Metallfilme als Funktion externer elektrischer Felder im Rahmen der elektronischen Spindichtefunktionaltheorie berechnet. Dadurch wird eine Parametrisierung und Modellierung des Effekt in den Systemen möglich, die wegen ihrer größe einen direkten ab-initio Zugang ausschließen.
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    Zur Theorie der Approximanten in Quasikristallen
    (2012) Reschke, Margarita
    Die Arbeit zur Theorie der Approximanten in Quasikristallen dient dazu, einen Grundbaustein in der Diskussion von Approximanten zu legen. Sie setzt sich mit den geometrischen Aspekten der Konstruktion von Approximantenstrukturen auseinander und gibt eine Einführung zu Phasonen in Approximanten.
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    In-situ study of real time structural evolution during polymer/fullerene bulk heterojunction thin film formation
    (2011) Sanyal, Monamie; Dosch, Helmut (Prof. Dr. Dr.h.c.)
    This thesis was devoted to investigate the real-time structural evolution of a polymer/fullerene blend from solution using in-situ x-ray diffraction. In this thesis, the structural evolution of the blend of electron-donor semiconducting polymer poly-(3-hexylthiophene) (P3HT) with the electron-acceptor semiconducting fullerene-derivative [6,6]-phenyl- C61-butyric-acid-methyl-ester (PCBM) in a solution in solvent 1,2-dichlorobenzene (DCB) during solvent drying has been studied in-situ using x-ray scattering. The effect that processing parameters play on the nanostructural evolution of the P3HT:PCBM blend is largely unknown. Real-time x-ray scattering study of P3HT:PCBM blends during drying gives novel insight into the crystallisation and the associated evolution of the elastic properties of the blend film during the film formation. The blend microstructure has been shown to emerge from smectic liquid crystal phase to solid phase as the solvent evaporates. The bulk modulus and the bending rigidity modulus of the blend film was calculated at different drying times using diffuse x-ray scattering analysis. This enabled for the first time direct evaluation of the material properties of the blend film as it was drying. Drying temperature has been seen to play a very important effect on the structural evolution of doctor-bladed P3HT:PCBM blends in the transition from wet to solid blend film. Drying the P3HT:PCBM active layer at a lower temperature of 10°C was seen to lead to a good P3HT interchain pi-pi stacking, broader orientational distribution of P3HT enabled by the slow crystallization kinetics and a restricted phase separation of P3HT and PCBM due to lower molecular mobility giving rise to better nanomorphology with finer interpenetrated network. It had been previously observed that lower drying temperature during processing of the photoactive blend created better performing solar cells; however the nanomorphological and nanostructural reason behind this empirical finding was not known. Based on the study carried out in this thesis, it can be proposed that lowering the substrate temperature during coating and drying is a simple route for optimization of device efficiency in doctor-bladed solar cells. The composition of P3HT:PCBM blend has been observed to be vital for good solar cell performance. The structural evolution of P3HT: PCBM blend film has been observed to depend on the blend composition ratio. The work in this thesis shows that the mosaicity of P3HT decreases with increasing loading of PCBM in the P3HT: PCBM blend suggesting that the interlayer ordering of P3HT improves with higher PCBM content in the blend film. However, the interchain pi-pi stacking of P3HT deteriorates with increased loading of PCBM. In the P3HT:PCBM blend film with high PCBM content, the formation of a complex P3HT-PCBM structure was observed for the first time. All the above investigations conducted during this thesis work exhibited delicate ordering of pi-pi interchain stacking in P3HT along the substrate that depends on drying temperature, composition ratio of P3HT:PCBM blend and the nature of solvent including additives in the solvent used to process the blend. On the other hand, structural ordering of P3HT along [100] direction, which is normal to the substrate surface, is found to be quite resilient. It forms in the early part of drying and improves with higher temperature processing and when P3HT phase segregates from PCBM in the P3HT:PCBM blend. However, this phase segregation of P3HT from PCBM deteriorates the nanophase mixing of their blend and hence photovoltaic efficiency. The resilience of P3HT out-of-plane ordering along [100] was studied here following concepts used in the diffuse scattering study of liquid crystalline phases. The results obtained in this thesis work clearly show that out-of-plane ordering of P3HT along [100] forms first in these films and in-plane interchain π-π P3HT ordering do not form in the initial phase of drying and P3HT:PCBM blend films behave like a liquid crystalline film having long range order in the out-of-plane direction without any ordering in the in-plane direction. In-plane P3HT interchain pi-pi ordering forms in the later part of drying exhibiting emergence of P3HT (020) peak only if blend ratio and processing temperature are proper. It has also been found here that nanophase mixing of P3HT and PCBM exhibiting higher mosaicity in P3HT (100) peaks and sharper in--plane P3HT (020) peaks provide the red-shift of the solid-state absorption band having vibronic side bands related to P3HT pi-pi interchain interaction that results in more efficient solar cell performance.
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    Computer simulations of laser ablation from simple metals to complex metallic alloys
    (2011) Sonntag, Steffen; Trebin, Hans-Rainer (Prof. Dr.)
    In this work, a method for computer simulations of laser ablation in metals is presented. The ambitious task to model the physical processes, that occur on different time and length scales, is overcome to some extent by the combination of two techniques: Molecular dynamics and finite differences. The former is needed to achieve atomistic resolution of the processes involved. Material failure like melting, vaporization or spallation occur on the atomic scale. Light absorption and electronic heat conduction, which plays the major role in metals, is described by a generalized heat conduction equation solved by the finite differences method. From the so-called Two-Temperature Model temperature, density and pressure evolution - both in time and space - can be derived. With this, various studies on laser heated metals were done. For reasons discussed in more detail later, aluminum was chosen as a model system for most simulations on isotropic materials. As a more complex structure, the metallic alloy Al13Co4 was used because of its special material properties. As an approximant to the decagonal phase of Al-Ni-Co, the alloy shows an anisotropy in its transport properties, e.g. an anisotropic heat conduction. It will be shown, that the model is able to describe the physics in laser heated solids on time scales from 100 fs up to the ns-scale properly. Great insight was gained about the processes occuring during and shortly after the laser pulse. Many of the quantities interesting for experimentalists can be predicted by the theory. From the simulations relevant parameters like the electron-cooling time or the important ablation threshold were calculated. All values match their experimental counterpart very well.
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    In situ oxidation study of flat and stepped binary alloy surfaces
    (2010) Ellinger, Claus Steffen; Dosch, Helmut (Prof. Dr.)
    Binary intermetallic alloys like Fe3Al and NiAl are the basis for so called super-alloys. Super alloys are technologically used in hot and highly reactive environments, like jet engines. Here, the limit of mechanical stability for normal Fe-based steel is reached. To protect the alloys from further corrosion a thin, homogeneous, pure alumina layer is formed on their surfaces. From a scientific point of view low pressure (1E-6 mbar) oxidation of binary alloys is also interesting. On single crystal surfaces like NiAl (110) or CoGa (100) long-range ordered ultrathin surface oxides are formed, which are used as nanotemplates for catalytic reactions or in future microelectronic devices. Therefore fundamental knowledge of the initial oxidation process of binary alloy surfaces is necessary from a technological as well as a scientific point of view. Regarding this process this thesis answers two main questions: 1) How is the intermetallic ordering, responsible for the high mechanical strength, influenced by the formation of a protecting oxide layer? 2) How do surface steps influence the oxidation process? Is it possible to tailor oxide structures via regularly arranged steps? Four model systems were oxidized to answer these questions: the Fe3Al (110) surface, the vicinal NiAl (671) and (430) surfaces, and the stepped CoGa (100) surface with a 0.8 ° miscut. For a structural analysis of the systems in situ surface x-ray diffraction experiments are carried out at the European synchrotron source ESRF in Grenoble, the Angströmquelle Karlsruhe and the Swiss light source in Villingen. For a chemical analysis of the surfaces high resolution core level spectroscopy (HRCLS) measurements were conducted at MAX-Lab in Lund The x-ray data of the clean Fe3Al (110) surface showed, that, due to segregation, the topmost layer of the system has a different intermetallic ordering than the D03 ordered bulk underneath. Oxidations at 1E-6 mbar O2 and temperatures between 400 K and 720 K lead to a vanishing of the intermetallic order within the interfacial region. The oxide-alloy-interface gets slightly rougher but remains crystalline while an 8.4 Angstroms thick oxide film is formed. The analysis of the HRCLS spectra measured during oxidation shows that the oxide composition strongly depends on the oxidation rate and the Al segregation from the bulk. High oxidation rates at oxygen pressures of 1E-6 mbar lead to the formation of Al- and Fe-oxides. For low oxidation rates (1E-8 mbar) pure alumina can be formed. The experiments on the NiAl surfaces show that both clean, regularly stepped, vicinal surfaces are thermodynamically stable. After oxidation at 550 K and 1E-6 mbar O2 an amorphous, 5.5 Angstrom thick oxide film is formed on both surfaces. Annealing the samples above 1100 K leads to a massive (110) faceting and the formation of a long-range ordered oxide on both surfaces. On the (110) terraces of the (671) surface single domain surface oxide, which is known to grow in twin domains on flat NiAl (110), is observed. On the (110) terraces of the (430) surface theta-Al2O3-like oxide structures are observed. Due to preferential strain release at the oxide alloy interface in the direction of the steps it is therefore possible to trigger the growth of certain oxide structures as well as certain domains. Measurements of the oxide growth on the stepped CoGa (100) surface show the formation of the two oxide domains, also known from the flat surface. Thermally induced disorder on the surface might inhibit a single domain growth. However, in comparison with the flat surface the time-dependence of the oxide growth is different. A two-step exponentially reduced growth mode is observed.
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    Dynamics of complex fluids at liquid-solid interfaces
    (2010) Almenar Egea, Laura; Dietrich, Siegfried (Prof. Dr.)
    In this thesis the dynamics of complex fluids in thin channels in the presence of flow is studied. To do so, two kinds of complex fluids are considered: suspensions of hard as well as soft particles. The transport process is described taking into account direct as well as hydrodynamic interactions and the surface confinement. The steady state transport in a simplified model system of two particles diffusing through a two-dimensional narrow channel was analyzed in detail numerically using finite element methods.