08 Fakultät Mathematik und Physik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/9
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Item Open Access Microscopic calculation of line tensions(2008) Merath, Rolf-Jürgen Christian; Dietrich, Siegfried (Prof. Dr.)In this work the line tension has been determinded with molecular resolution, which in this context marks the forefront of research. A semi-microscopic line tension theory based on the sharp-kink approximation has been further developed. The sharp-kink results concerning wetting and line tension behavior deviate considerably from the fully microscopic results. A hybrid line tension theory has been introduced, which employs an improved effective interface potential for the SK line tension calculation. For most of the studied cases the results from this hybrid method describe the fully microscopic line tension values semi-quantitatively. However, for a tailored system with relatively strong spatial variations of the substrate potential and of the solid-liquid interfacial density the hybrid method fails and does not predict the correct order of magnitude of the line tension values. Hence in general the fully microscopic approach is required, if one is interested in quantitatively reliable line tension values or/and if the validity of the hybrid method for the considered system has not been checked. The calculation of the line tension of a liquid wedge is an important contribution for understanding the shape of very small droplets (below the micrometer range). Furthermore a proposal is given, how axisymmetric sessile droplets can be addressed efficiently within DFT.Item Open Access Ambient pressure oxidation of Ag(111) surfaces : an in-situ X-ray study(2008) Reicho, Alexander; Dosch, Helmut (Prof. Dr.)The oxidation of metals plays an outstanding role in everyday life. Typical phenomena are the formation of rust on steel or oxide scales on copper, showing up as a green patina. The formation of metal oxides is not always an unwanted process. The functionality of many materials is directly related to their controlled oxidation. The most prominent examples are passivating oxide layers on stainless steel. Relevant for this thesis are industrially applied heterogeneous catalytic reactions for the synthesis of many chemical products, where gaseous reactants are in contact with the solid surface of the catalyst. Oxidation reactions are very important in this context, leading to a big need of understanding of these processes in research and development. Thereby, the active oxygen species on the surface and selectivity and poisoning of the catalyst have to be studied on an atomic scale. The high temperature and high pressure oxidation of the 4d transition metals Ru, Rh, Pd and Ag is a matter of particular interest, because these metals are widely used as oxidation catalysts. On Ruthenium one observes the formation of RuO2(110) bulk oxide islands at elevated temperatures and oxygen pressure. In the case of the Pd(100) and Rh(111) surface oxidation can lead to the formation of so-called surface oxides. These oxides are structurally related to the bulk oxide of the respective element. Furthermore, surface oxides are ultra thin oxides containing one metallic layer surrounded by two oxygen layers, giving rise to an oxygen-metal-oxygen sequence perpendicular to the surface plane. A future vision is to get a direct microscopic control of the emerging surface structures and ultimately of the real-time oxidation/reduction dynamics allowing one to tailor such catalytic reactions to better performance. A necessary prerequisite to the microscopic control is the full atomistic understanding of the surface structures which form at high temperature and at high oxygen pressures. Silver plays a unique role in heterogeneous catalysis. Supported Ag catalysts are used for the selective oxidation ('epoxidation') of ethylene and for the partial oxidation of methanol to formaldehyde. Ethylene oxide and its derivates are basic chemicals for industry, used in a many technologies with a world-wide production of more than 10 million tons as in medicine for disinfection, sterilization, or fumigation, or in transport and energy technologies for engine antifreeze and heat transfer. Because of its ability to kill most bacteria, formaldehyde is extensively used as disinfectant and as preservative in vaccinations. Therefore, the optimisation of these two Ag-supported catalytic reactions is of paramount importance. Current strategies employed in the industrial process to enhance selectivity include the empirical use of inhibitors (Cl) and promoters (Cs), however, on the way to a knowledge-based control of these reactions one has first to understand the surface structure of oxidized silver under relevant conditions in full detail. The formation of extended Ag(111) facets is observed on polycrystalline silver during the above industrial catalytic oxidation reactions, in turn fundamental research (experiment and theory) has been devoted to the detailed understanding of oxidation of this surface. The formation of an oxygen induced p(4x4) reconstruction on the Ag(111) surface is known since the early 70s. A surface oxide trilayer model, based on a three-layer slab of Ag2O(111), was proposed. Accordingly, the Ag(111) surface seemed to show a similar behaviour like Pd and Rh, being neighbours in the periodic table. Further theoretical calculations predicted the stability of this reconstruction under industrially relevant conditions. Nevertheless, several questions remained unsolved: the stability of the p(4x4) reconstruction under industrially relevant conditions was not checked experimentally, the structural model of the p(4x4) structure was not proven by a crystallographic method and previously unknown structures might play an important role for the catalytic activity of Ag(111) facets. Our experimental approach is based on the nowadays routinely available highly brilliant x-ray radiation produced by third generation synchrotron light sources. This radiation is used by us in three surface sensitive x-ray techniques. In-situ surface x-ray diffraction (SXRD) allows the identification and determination of structural models of surface reconstructions under industrially relevant conditions. This technique is combined with high resolution core level spectroscopy (HRCLS) and normal incidence x-ray standing wave absorption (NIXSW), giving insight into the local binding geometry of the oxygen and silver atoms.Item Open Access The role of dimensionality and geometry in quench-induced nonequilibrium forces(2021) Nejad, Mehrana Raeisian; Khalilian, Hamidreza; Rohwer, Christian M.; Moghaddam, Ali GhorbanzadehWe present an analytical formalism, supported by numerical simulations, for studying forces that act on curved walls following temperature quenches of the surrounding ideal Brownian fluid. We show that, for curved surfaces, the post-quench forces initially evolve rapidly to an extremal value, whereafter they approach their steady state value algebraically in time. In contrast to the previously-studied case of flat boundaries (lines or planes), the algebraic decay for curved geometries depends on the dimension of the system. Specifically, steady-state values of the force are approached in time as t-d/2 in d-dimensional spherical (curved) geometries. For systems consisting of concentric circles or spheres, the exponent does not change for the force on the outer circle or sphere. However, the force exerted on the inner circles or sphere experiences an overshoot and, as a result, does not evolve to the steady state in a simple algebraic manner. The extremal value of the force also depends on the dimension of the system, and originates from curved boundaries and the fact that particles inside a sphere or circle are locally more confined, and diffuse less freely than particles outside the circle or sphere.Item Open Access 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.Item Open Access 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.Item Open Access Tailored magnetic properties of exchange-spring and ultra hard nanomagnets(2017) Son, Kwanghyo; Schütz, Gisela (Prof. Dr.)Item Open Access Interplay between geometry and fluid properties(2005) König, Peter-Michael; Dietrich, Siegfried (Prof. Dr.)Many real systems feature a complex geometric shape. In order to develop a quantitative model for such systems one normally tries to simplify the geometry as much as possible to be able to apply analytic or numeric methods. However, in some cases, geometry plays an important role and is essential for the functioning of the system. As an example we study in this work key-lock systems which describe the role of enzymes in biological cells. Quantitative experiments show that the fluid particles surrounding both the key and the lock molecule are essential for the high efficiency of all enzymatic reaction. Apparently the surrounding macromolecules in the cytoplasm lead to a net attraction between key and lock, which is termed as effective interaction. Although there exist developed theories for their calculation, a direct application of these methods to complex geometries as in the present case is impossible from a practical viewpoint. In this work, however, we introduce and verify an indirect method that allows such kind of calculations. For this purpose we systematically study the influence of geometry on various properties of fluids. We start by analysing the dependence of a thermodynamic potentials on the geometry of a wall which bounds the fluid. For this we postulate an additive dependence on the shape for all thermodynamic potentials and find a so-called morphometric form for the grand potential. From this form we deduce the curvature dependence of all thermodynamic quantities such as the interfacial tension and the excess adsorption. These predictions are verified a posteriori by means of a large numerical study based on density-functional theory and Monte-Carlo simulations. Structural properties, such as the correlation function or the density profile near a wall, can be expanded analytically in powers of the local curvatures of the wall. Such an ansatz is motivated by the morphometric forms of the thermodynamic quantities and allows to determine the distribution of fluid particles even around complexly shaped objects. We verify this approximate approach numerically and find excellent agreement with quasi-exact results obtained via Monte-Carlo simulations. Based on the structural properties of a fluid near a wall enables we can, in a final stage, calculate also effective interaction potentials between complexly shaped objects. This is done using the so-called insertion method. We verify the accuracy of this approach by comparing our data to independently derived results of simple setups and eventually apply this method also to a simple model of a key-lock systems. A systematic study of the resulting potentials shows that spherical key-molecules are always repelled from the lock due to a very high energetic barrier. If, however, the shape of the key-molecule is sufficiently asymmetric, the barrier can be overcome by an appropriate free orientation of the key-particle.Item Open Access Linear response theory for equilibrium and nonequilibrium systems perturbed by nonconservative forces: the role of symmetries(2020) Asheichyk, Kiryl; Dietrich, Siegfried (Prof. Dr.)Item Open Access General properties of ionic complex fluids(2016) Bier, Markus; Dietrich, Siegfried (Prof. Dr.)Item Open Access XMCD investigation at M4,5 edges of the rare earth elements in high-performance permanent magnet(2018) Tripathi, Sapana; Goering, Eberhard (PD Dr.)