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Nonstationary Impedance Measurements in Forced and Natural Dynamic Conditions using Dynamic Multi-Frequency Analysis
No full textThis thesis is concerned with the characterization of electrochemical systems by means of dynamic impedance spectroscopy. Electron transfer reactions across electrified interfaces involve a variety of processes, occurring at different time scales. A detailed understanding of the reaction kinetics requires the investigation of the system over a wide range of frequencies in order to capture all time constants related to the different underlying processes. In electrochemical impedance spectroscopy (EIS) a small-amplitude sinusoidal potential perturbation is imposed on the system by successively changing the frequency of the sinusoidal wave. The system's response to the perturbation signal provides an insight into the mechanism governing the reaction. EIS is a powerful tool that allows for the investigation of a variety of electrode processes in a single measurement. This method requires the stationarity of the system throughout the acquisition time of one full impedance spectrum. However, in many electrochemical systems a stable steady-state cannot be established, since the system undergoes irreversible changes during the measurement. Dynamic multi-frequency analysis (DMFA) uses a multi-sine waveform to apply all frequencies to the system simultaneously, thus allowing for the acquisition of impedance spectra in dynamic conditions. Here, DMFA is employed for the investigation of the nonstationary impedance behavior of different electrochemical systems, exhibiting either forced or natural reaction dynamics. Fundamental principles of data recording and analysis are examined by application of DMFA to a simple redox process. It is demonstrated that the temporal evolution of physical variables can be extracted from data analysis using suitable electrical equivalent circuits (EECs). Based on these results, different stages of the hydrogen evolution reaction (HER) are examined by means of DMFA. To do so, the system is subjected to forced dynamic conditions by combining quasi- voltammetry and multi-sine impedance spectroscopy, thus providing a concatenated picture of the overall process. The investigation of the temporal evolution of the kinetic variables gives new insight into the mechanism of the HER. Finally, DMFA is employed for the examination of self-sustained current oscillations during the anodic electrodissolution of p-type silicon in fluoride containing electrolytes. The analysis of the system in dynamic equilibrium conditions provides the foundation for the investigation of the electrodissolution process in the oscillatory regime. DMFA allows for the characterization of different types of current oscillations in terms of their oscillatory reaction dynamics
Nonlinear Model Predictive Control for Industrial Manufacturing Processes with Reconfigurable Machine Tools
Get PDFManufacturing companies are faced with challenges to respond to volatile market demands quickly and flexibly while maintaining a cost-effective level of production. Instead of flexible working times, we adopt Reconfigurable Machine Tools (RMTs) to compensate for unpredictable events in case of bottleneck. To include these tools effectively on the operational layer, we propose a complementing feedback approach using model predictive control (MPC) together with genetic algorithm and branch and bound to achieve a better compliance with logistics objectives and a sustainable demand oriented capacity allocation. Further, the system stability is guaranteed by a trajectory-based unconstrained MPC scheme associated with the principle of flexible Lyapunov functions. The effectiveness and plug-and-play availability of the proposed method is demonstrated via a four-workstation job-shop system, which shows that the work in process can be practically asymptotically stabilized by usage of RMTs
Versuch der vertraglichen Regelungen des Emissionsausstoßes von Kohlendioxid in einem ökologischen Supply Chain. Service Level Agreement & mit darauf aufbauenden KPIs und dessen Umsetzung
Get PDFDie vorliegende Masterthesis stellt ein Konzept dar, wie der CO2-Ausstoß des Transportunternehmens in einer gedachten Logistikkette zwischen Auftraggeber und Auftragnehmer zur Basis des Transportvertrages werden kann. Das dafür verwendete Service Level Agreement kann individuell und flexibel vereinbart werden und basiert auf messbaren Kennzahlen. Die Arbeit fokussiert die Entstehung und die Bestimmung dieser ökonomischen Kennzahlen sowie ihren Einsatz zur Bestimmung der erlaubten Emissionsgrenzwerte. Mit Hilfe des Bonus-Malus-Systems sind die Abweichungen zu den ermittelten Grenzwerten entweder bestraft oder belohnt. Abschließend bietet das Service Level Agreement einen Rahmen für das Gesamtkonzept, das in der Praxis anwendbar ist. Der Autor dieser Masterarbeit erhofft sich dadurch einen interdisziplinären Erkenntnisgewinn und einen Beitrag zu der Reduktion der Klimaschäden durch Transport. This master thesis presents a concept of how the CO2-emissions of a transport company in an imaginary logistics chain - between client and contractor - can become the basis of a transport contract. The service level agreement be applied in this concept, can be agreed individually and flexibly and, is based on measurable key performance indicators. The work focuses on the formation and determination of these indicators as well as their use for the permitted emission limit values. With the aid of the bonus-malus system, the deviations from the calculated thresholds are either penalized or rewarded. As a last point, a service level agreement provides a framework for the overall concept, which is applicable in practice. The author of this thesis hopes thereby an interdisciplinary knowledge gain and a contribution to a significant reduction of the climate damage by transport
Mechanobiological Control of Circular Dorsal Ruffle Dynamics
Get PDFDynamic structures of polymerized actin play a crucial role in different cellular processes. These include different kinds of actin waves in a multitude of cell types, like Dictyostelium, neutrophiles, macrophages and fibroblasts. These actin waves are connected to a remodeling of the cytoskeleton, cell protrusion and migration as well as the uptake of extracellular fluids, but their specific functions are still debated. One type of them are circular dorsal ruffles (CDRs), actin-based ring-like membrane undulations on the dorsal cell side of fibroblasts, which emerge after growth factor stimulation. A large number of macromolecules were shown to be localized in CDRs and to be crucial for CDR formation. However, to date, the detailed signaling pathway and the underlying mechanism of CDR formation including their molecular main players remain unknown. Different studies on CDRs described them as actin waves in an excitable system or as wavefronts in a bistable regime between two stable states of actin. However, other studies focused on the interaction between actin polymerization and the cell membrane via the interplay of curved membrane protein complexes. This thesis further investigates the mechanism underlying CDR formation. For this study, the morphology of cells is an essential effector for the dynamics of actin waves. Their complexity and dynamical remodeling pose a challenge to the comparability of data. Therefore, in this work, fibroblasts are shaped into well-defined morphologies by seeding them on disk-like adhesion patterns made of fibronectin. This enables to identify long-range interactions between different CDRs combined with the influence of stochastic perturbations and thus uncovers the important role of the membrane tension in CDR dynamics. In combination with microfluidics, the response of the actin wave machinery to biochemical interference with drugs that target different parts of the actin machinery is investigated. The system allows systematical measurements of CDR velocities, periodicities and lifetimes that are performed to carry out a before/after comparison of the treated cells for examining the influence of actin, PIP3 and N-WASP. It is observed a dependence of CDR velocities, periodicities and lifetimes on the total amount of actin leading to the conclusion of a direct regulating role of actin in CDR formation and propagation. Furthermore, it is found that the actin nucleator N-WASP plays a fundamental role in CDR formation but not in CDR propagation. Numerical solutions of wavefronts in a bistable regime of a model system on an annulus domain resemble experimentally gained data and further uncover a dependence of the stimulation threshold for propagating wavefronts on the total actin concentration. The results underline the hypothesis that CDRs can be considered as wavefronts in a bistable regime between two stable states of actin
Die Unsichtbaren : Menschen mit Trisomie 21 in Archäologie und Anthropologie
Get PDFMenschen mit Trisomie 21 sind nur selten in der archäologischen Gräberfeldliteratur zu finden und die Zuschreibung der Chromosomenanomalie erfolgte bei der anthropologischen Untersuchung bislang ausschließlich über morphologische Merkmale an Schädeln. Im Zusammenhang mit der Ausstellung Touchdown: Eine Ausstellung mit und über Menschen mit Down-Syndrom der Bundeskunsthalle Bonn rfolgte 2016 weltweit erstmals eine Überprüfung von Verdachtsfällen an archäologischem Skelettmaterial auf Trisomie 21 mit einer aDNA-Analyse. Hierzu wurden zwei Individuen beprobt, bei denen aufgrund ihrer morphologischen Besonderheiten das Vorliegen dieser Chromosomenanomalie vermutet wurde. Die Untersuchung der aDNA erbrachte überraschende Ergebnisse, beide untersuchten Skelette wiesen keine Trisomie 21 auf
Defects Studies in I -Ga2O3 Using an Optimized Hybrid Functional
Get PDFI -Ga2O3 is a very important semiconductor. However, there is still little known about the defects in I -Ga2O3. Experiment alone is not enough to understand, Deak et al. have found tthe hybrid functional with the parameters I /-= 0.26 and I = 0.00 is the optimal HSE for I -Ga2O3 defects studies.This optimized hybrid functional reproduces not only the band gap, but also satisfies the generalized Koopmansa theorem. Herein, the optimized hybrid functional and a modification of the charge correction process were utilized for I -Ga2O3 with the results as follows: a A consistent description of observed carrier trapping by intrinsic defects in I -Ga2O3 was given. a The UV bands can be explained by the self-trapped holes states. The blue band mainly originates from singly negative Ga-O vacancies, and the green band is caused dominantly by interstitial O atoms. In N-doped samples, a nitrogen substitutional on the tetrahedral oxygen site reproduces the observed red luminescence. a a The calculated hyperfine tensor agrees well with the EPR measurement. The calculated results confirm that Mg cannot act as a shallow acceptor in I -Ga2O3.
Turbulent Suspension of Sediments in Shallow Shelf Seas
Get PDFThe interaction between near-bed flow and sediment transport is one of the most attractive research areas for both hydraulic engineers and geomorphologists. Still, current knowledge on the suspension of sediment and near-bed sediment transport, are far from being resolved. This is due to the lack of understanding of near-bed hydrodynamics and the critical conditions at which the sediment starts to move. This thesis explores the near-bed flow-sediment interaction in highly dynamic coastal environments where the near-bed flow is governed by rotating tidal currents. During field campaigns in the German Bight in winter 2015, spring 2015 and summer 2016, data on the near-bed hydro- and sediment dynamics were collected using Acoustic Doppler Velocimeters and an Acoustic Doppler Current Profiler in different water depths, with different sediment characteristics and tidal phases. State of the art methods were used to analyze the data on large (tens of minutes) and process (few seconds) time scales. To portray the hydrodynamic conditions in large time scales, bed shear stress, i.e. the force exerted from the water flow against the seabed, and turbulent kinetic energy (TKE) and Reynolds stress, i.e. the two most common turbulent statistics characterizing turbulent conditions, are studied. In addition, in process time scale, turbulent events, i.e. short energetic velocity fluctuations describing the turbulent flow, are investigated. On large time scales, thresholding of suspended sediment concentration variations was used to estimate critical suspension and deposition stresses. Comparison of the computed suspension stresses with empirical equations predicting the critical condition, illustrates that the movability-based predictors yielded higher accuracy in tidal environments. As the bed shear stresses exceed the critical suspension stress, both bed shear stress and suspended sediment concentration closely tracked each other until bed shear stress fell below the deposition threshold. Results explicitly showed that the bed shear stresses calculated from the turbulent statistics, in comparison to those calculated from the average velocity profile, correlate better with the suspended sediment concentrations. Next, the near-bed turbulence was scrutinized on process time scales to investigate its role in the vertical mixing of suspended sediment. Turbulent events are defined as a sequence of strong velocity fluctuations classified in a quadrant plane (u', w') contributing to 90% of the Reynolds stress. It is shown that, turbulent events only occur in 25% of the time, with the rest of the time being occupied by small background fluctuations. These events are able to induce higher stresses than the average bed shear stress, resulting in suspension of sediments below the critical conditions obtain from mean velocity. Apart from the significance of ejection and sweep events in near-bed sediment dynamics, this thesis further shows the importance of outward interactions in moving sediments away from the bed. This is achieved using turbulent events characterization by their type, strength, duration, length and concentration of associated suspended sediments. Finally, this thesis allows inference from process time scales to large time scale considerations of flow-sediment interaction. Being detected in only 25% of the time, the turbulent events are responsible for more than 60% of the total suspended sediment movements with the rest likely governed by settling velocity. Even though small background fluctuations cannot move sediment in the water column, they impose a large effect on turbulent statistics and reduce bulk TKE and Reynolds stress estimates up to 1.6 and 3 times, respectively. Therefore, this thesis suggests that for the study of large scale sediment dynamics, TKE is a more appropriate descriptor of turbulence. However, for accurate prediction of sediment transport, one needs to consider turbulent events on process time scales
Entwicklung eines neuartigen Release-Caging Mechanismus für den GraviTower Bremen Prototypen
No full textDevelopment of a new kind of mechanical decoupling systems for microgravity platforms. This new kind of mechanisms allows a very fast decoupling and recoupling in any relative position. This work shows in detail all design criteria and the whole development process of the RCM Release-Caging mechanism which is used in the GraviTower Bremen prototype
Strukturierte und DUPLEX behandelte Werkzeuge für die Trockenbearbeitung beim Rundkneten
Get PDFRundkneten ist ein inkrementelles Massivumformverfahren zur Herstellung von Leichtbaukomponenten aus zylindrischen Halbzeugen. Das konventionelle Rundkneten setzt derzeit große Mengen von Schmiermitteln voraus, um sowohl den Werkzeugverschleiß zu minimieren als auch die gewünschte Bauteilqualität einzustellen. Es besteht Interesse den Rundknetprozess in einer trockenen Prozessführung zu realisieren und somit die Prozesskette bei der Weiterverarbeitung der Werkstücke um kostspielige Reinigungsprozesse zu verkürzen. Für eine trockene Prozessführung werden Makrostrukturen durch Fräsen mit Kugelkopffräsern in die Reduzierzonen der Werkzeuge eingebracht, um tribologische Bedingungen einzustellen, die auch bei trockener Prozessführung zu einem stabilen Prozess und qualitativ hochwertigen Werkstücken führen. Um diese Strukturen zu schützen sowie den Werkzeugverschleiß und die Entstehung von Abriebpartikeln zu minimieren, werden Wolfram dotierte, wasserstoffhaltige amorphe Kohlenstoffschichten (a-C:H(:W)) auf die gesamte Umformzone durch reaktives Magnetronsputtern appliziert. Werkstücke, die mit beschichteten Werkzeugen beim Trockenrundkneten hergestellt wurden, weisen zudem vergleichbare oder bessere Werkstückoberflächen auf als unter der konventionellen, geschmierten Prozessführung. Allerdings muss die Schichthaftung auf den Werkzeugoberflächen weiter gesteigert werden, da die Beschichtungen kritisch auf lokale Überbeanspruchungen reagiert. Um einer frühzeitigen Schichtermüdung beim Trockenrundkneten von Stahl effektiv zu begegnen, werden die Werkzeuge nach dem Vergüten zusätzlich plasmanitriert. Im Rahmen dieser Arbeit wird zunächst geprüft, ob eine Übertragbarkeit von plasmanitrierten und anschließend a-C:H(:W) beschichteten Laborproben auf die komplexe Rundknetwerkzeuggeometrie gegeben ist. Zudem werden zwei sogenannte DUPLEX behandelte Werkzeuge gegen Stahl in der trockenen Anwendung getestet und mit den Werkzeugverschleißresultaten der vergüteten Werkzeuge verglichen
Gefügeaufbau und mechanische Eigenschaften nach Einsatzhärten außerhalb des üblichen Parameterfeldes
Get PDFA large part of the components used today, for example in gearbox construction, such as gears or bearing components, is adjusted to the stress during operation in a mostly thermochemical heat treatment process (usually gas carburizing). The resulting, mainly martensitic structure has been optimized in recent decades only within relatively narrow limits, especially with regard to increased component reliability and overall cost reduction. Due to increasing scarcity of resources and increasing limitations in terms of size or component weight, the trend in the recent past goes in the direction of increased load capacity or higher power densities. In addition to the use of costly high-alloy and / or high-purity material concepts, the research of novel surface-layer compositions has been brought into focus by mostly thermochemical heat treatment processes (i. e. carbonitriding). Before a possible adoption of these new boundary layer concepts into the industrial practice, however, a series of studies for better assessment and for a better understanding of the material-technical basics and relationships is necessary to assess potentials and risks. Important subgoals are: Detailed characterization and description of today's uncommon surface layer structure Testing and comparing novel heat-treated model samples with the standard to demonstrate the potential of the new heat treatment variants Basic studies on the conversion behavior and the retained austenite stability of the new process variant