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The role of surface reactivity in geochemical processes: numerical investigations of adsorption and dissolution mechanisms
No full textSurface reactivity is a fundamental driving force of geochemical processes such as adsorption or dissolution reactions at the mineral-fluid interface. This dissertation combines numerical approaches to simulate and predict mineral surface processes at scales ranging from single nanometers to hundreds of micrometers. By focusing on the interaction between intrinsic surface reactivity heterogeneity and extrinsic factors such as fluid transport, this thesis provides new and fundamental insights and tools for applications in both natural and engineered settings.
The influence of heterogeneous surface reactivity on adsorption processes is investigated on nanotopographic surfaces. Focusing on sheet silicates as important mineral component in host rocks for nuclear waste repositories, the study uses density functional theory (DFT) calculations to quantify the energy barrier for adsorption at surface sites such as steps, kinks, and terraces. Resulting energy values are used to parameterize a kinetic Monte Carlo (KMC) simulation to study the adsorption on nanotopographic mineral surfaces. Low-barrier sites such as steps and kinks are shown to enhance radionuclide retention by up to three times compared to flat surfaces, while desorption occurs primarily at terrace sites. This mechanistic understanding of adsorption variability offers new insight into sorption efficiency, which is important in applications such as nuclear waste containment.
Pulsating dissolution is a recently discovered, dynamic process driven by the self-organization of reactive surface features. KMC simulations in combination with high-resolution rate maps demonstrate that rhythmic variations in local dissolution rates are caused by intrinsic surface-controlled mechanisms. This study rules out extrinsic factors such as transport control, emphasizing the importance of nanotopographic surface architecture as an intrinsic factor controlling surface reactions. This phenomenon challenges conventional steady-state dissolution models.
The influence of surface topography on reactivity and its integration into predictive models is a key problem. Dissolution processes influenced by varying surface reactivity can be parameterized by a nano- or microtopographic surface slope parameter in reactive transport models (RTM) where the simulation of individual surface atoms is no longer feasible. By correlating the surface slope with the density of reactive sites, this parameterization captures the spatial variability of dissolution rates and accurately predicts the evolution of topography, surface reactivity, and geometry. This description of reactivity is critical for an accurate simulation of dissolution at the pore scale. These results demonstrate the importance of including intrinsic parameters for surface-controlled
processes in RTMs where transport processes are not sufficient to describe mineral dissolution. This significantly improves the predictive capabilities of RTMs for complex geochemical systems.
By unifying these insights, this doctoral thesis establishes a methodology for modeling surface reactivity in adsorption and dissolution systems across multiple scales. The integration of nanoscale heterogeneity into RTMs bridges atomistic mechanisms with macroscopic predictions, addressing key challenges in applied fields such as nuclear waste management or carbon capture. These advances provide a foundation for the further development of predictive tools that enhance the understanding and management of complex geochemical processes across time and length scales
Resource orientation as a principle in educational work with unaccompanied young migrants with refugee experiences – Evaluation and conclusions of a model project in teacher training at the University of Bremen
No full textUnabhängig von ihrem Literalitätsgrad verfügen jugendliche Migrant*innen mit Fluchterfahrungen über Wissensbestände und Kompetenzen, die sie durch formale und non formale Bildung vor dem Erreichen des Aufnahmelandes erworben haben. Im Fokus der Studie stehen Gelingensbedingungen für das didaktische Aufgreifen und das mehrsprachigkeitsorientierte Erweitern dieser Potenziale. Um zu markieren, dass im Fachunterricht der Herkunftsländer bereits Fachkenntnisse angeeignet und kommuniziert wurden, wurde für die Studie der Begriff „Herkunftsunterrichtssprache“ konstruiert.
Die Studie ist als Aktionsforschungsprojekt im Praxisangebot für Lehramtsstudierende der Universität Bremen „Lehrlernwerkstatt Fach, Sprache, Migration“ (LLW) entstanden.
Die Autorin hat als pädagogische Leiterin der LLW in einem Ko-Konstruktionsprozess mit studentischen Förderlehrkräften den zweitsprachendidaktischen Ansatz „Ressourcenorientierter Anschlussunterricht für unbegleitete jugendliche Migrant*innen“ (RAuM) entwickelt. Mit diesem Ansatz wird das Ziele verfolgt, sowohl Diskursfähigkeit im Regelunterricht durch sprachsensiblen und mehrsprachigkeitsorientierten Fachunterricht zu entwickeln als auch dazu beizutragen, ein fluchtbedingt potenziell erschüttertes Selbstkonzept durch Selbstwirksamkeitserfahrungen im Unterricht sowie die Vermittlung von Kompetenzbewusstheit zu stärken.
Der RAuM-Ansatz wurde in einem außerschulischen Modellprojekt von studentischen Förderlehrkräften in der Universität erprobt.
Vor Projektstart, während und nach Ende des Projekts wurden empirische Daten erhoben. Das Datenkorpus enthält Interviews mit teilnehmenden Jugendlichen und studentischen Förderlehrkräften, Expert*inneninterviews, vorstrukturierte Fragebögen, Briefe und Protokolle. Die Daten wurden inhaltsanalytisch im Hinblick auf Gelingensbedingungen für den RAuM-Ansatz ausgewertet
Essays on characteristic-based portfolio optimization
No full textIn my dissertation, I develop new portfolio optimization methodologies designed to enhance empirical performance in practical applications. Instead of relying on asset characteristics as intermediary variables to estimate the distribution of asset returns—an approach that introduces estimation errors—this work explores methods that directly incorporate asset characteristics as input variables in the optimization program. By bypassing the estimation step, these approaches aim to improve robustness and efficiency in portfolio construction. This dissertation contributes to both active and passive portfolio management
Optimising the recovery of water isotope signals in deep ice cores
No full textUnderstanding the Earth's climate is a complex and challenging task, especially considering that direct observations of climate parameters have only been documented relatively recently. Information on longer timescales has to be gleaned from proxies, preserved physical characteristics which serve as natural recorders of the climate. One such proxy is the isotopic composition of water molecules stored as snow and ice in ice sheets, as they relate to past atmospheric temperatures. The retrieval of ice cores from Earth's largest ice sheets, Greenland and Antarctica, provides continuous isotopic records dating back up to hundreds of thousands of years. However, this information is not perfectly preserved within the ice sheet, due to the constant random motion of the water molecules. This movement, called diffusion, homogenises the initial climate signal, generating a smoothing effect that dampens high-frequency variability. Additional isotopic perturbations are introduced during the sampling and measurement processes, further obscuring the desired climate information.
Consequently, for accurate interpretation of the proxy, these effects must first be considered. Diffusion can be corrected for, but the efficacy of such a correction depends on two key factors. Firstly, the magnitude of diffusion (or "diffusion length") should be accurately known, to reliably amplify the attenuated frequencies without overcompensating or underestimating the effect. The diffusion length can be determined through models constrained using knowledge of the diffusive process and the physical parameters, or it can be statistically estimated from the isotopic data itself through inspection of the variability on different timescales. Secondly, it is important to obtain precise water isotope measurements, as the correction process cannot separate the climate signal from the measurement noise. High frequency information which has undergone significant diffusion can be dominated by this subsequently added measurement noise, rendering the signal irretrievable and limiting the maximum obtainable resolution. This dissertation investigates both of these factors, and explores the additional effects of discrete sampling, with a focus on deep ice cores.
In this thesis, an adapted method for statistically estimating the diffusion length in deep ice is described, which assumes more realistic climate variability structure than conventional approaches. The method is applied to the deepest, oldest isotopic data from the EPICA Dome C ice core and finds a notable reduction in diffusion length compared to previous estimates, in closer agreement with the physical models. Analysis of the resolution limitation imposed by measurement noise is performed, resulting in a method to calculate the potentially recoverable timescales of an ice core for different measurement precisions. Application of the method to the Beyond EPICA - Oldest Ice Core suggests 10,000 year cycles are realistically retrievable from 1.5 million year old ice if care is taken to optimise precision. Expressions outlining the quantitative effect of discrete sampling are also derived, and simulations reveal how sample size impacts the representativeness of the isotopic record and the accuracy of diffusion length estimations. The results suggest a sample size equal to half the diffusion length is sufficient for both purposes. The thesis concludes with a discussion of all the results and their collective implications, detailing the necessary considerations for optimal water isotope signal recovery from ice cores
Advancing data infrastructures on democratic representation: an open-source version of ParlGov
No full textThis research note presents an open-source version of the ParlGov website and discusses the state of cross-national data on parties, elections, and cabinets. The ParlGov project is a prominent political science data infrastructure with information for EU and OECD democracies since 1900. The data has been publicly available since 2010. ParlGov is an example of a new type of data infrastructure in political science that uses modern software tools, collaborative data collection, and open data principles. In this research note, I introduce a new open-source software of ParlGov that demonstrates how modern software practices enhance the accessibility, transparency, and reliability of political science data. In addition, I assess the contribution of the ParlGov project to advances in comparative political data collection by comparing it with four key datasets on elections and cabinets (EJPR-PDY, PPEG, V-Party, and PAGED). I emphasize the importance and benefits of open science principles, arguing that their full potential has yet to be realized in political science. By adhering to open science principles, ParlGov provides a reference point for future data projects aiming to increase the availability and interoperability of research data in political science.3
Tailoring of filtration matrix design in dielectrophoretic filtration: towards selective separation for industrial tasks
No full textSelective separation and recovery of micro and sub-micro particles is a challenging task for many industries. Among the others techniques to tackle this task, Dielectrophoretic (DEP) filtration established itself for the particle manipulation. While being present for mostly biomedical fields, DEP separators can also be applied towards non-biological particles, such as the ones which come from the electronics scrap.
Dielectrophoresis operates in the inhomogeneous electric field and moves polarizable particles, depending on their specific properties. Examples of these properties can be the size of these particles, their shape or material. Dielectrophoretic filtration is based on the generation of this inhomogeneous field with a porous bulk filter and features pressure-driven flow. In previous works on this topic, it was already shown that the separation efficiency of the system would mainly depend on three different clusters of factors. One of the factors is related to the filter itself and its parameters, such as structural or material ones. They were studied in detail in the microfluidic context, however, a clear link to the bulk filters in the context of dielectrophoretic filtration was not established yet.
This thesis performed experimental research on modification of the filter matrix. It aimed to connect the separation efficiencies of different types of the packed bed filters to the grain morphology of these packed bed filters. Additionally, the goal was to expand an application scope of the DEP towards non-biological filtration tasks, and separate the type of particles which resemble the ones that come from battery recycling slurry
Overt and covert attention to emotional faces in realistic social situations
No full textSociality is an essential aspect of humanity. Coexisting with others requires shifting and sustaining attention to individuals’ critical social cues, including emotional expressions. Gaze serves as a tool to gather information from other people and the environment, and to communicate intent. Therefore, understanding gaze behavior and neural mechanisms underlying attention in social contexts is relevant for comprehensively understanding human interactions. However, a large proportion of attention research has been conducted in controlled laboratory settings, reducing the ecological validity of findings. To address this real-world gap in social attention research, we conducted three studies aiming to investigate both gaze behavior and neural mechanisms involved in the intersection of social and emotion-driven attention in both realistic situations and laboratory settings. The three projects varied in their level of naturalness of the context, to investigate whether findings from laboratory experiments could be translated to real-life situations, and vice-versa. Study 1 was conducted in a naturalistic setting (waiting room), half of the participants (n = 24) saw a live confederate in the room and the other half (24) viewed a prerecorded video of the same confederate. The confederate displayed positive, neutral and negative facial expressions and participants’ gaze behavior was tracked via a mobile eye-tracker. Results showed that participants looked more at the video of the confederate, than to the live confederate and that emotional expressions did not modulate gaze behavior in both contexts. Study 2 was conducted in a fully controlled laboratory setting, where EEG and eye-tracking were co-registered to assess participants’ gaze behavior and neural activity. Participants (n = 48) viewed static images of faces displaying neutral, happy and angry expressions. Across three blocks, participants were to either direct their gaze toward peripheral faces (overt attention), keep their gaze fixed at the center of the screen (covert attention), or look freely around the screen (uninstructed natural attention). We found that emotional expressions were processed in the brain earlier and longer during natural attention shifts, whereas gaze was not modulated by emotional content, but only by instruction type. Finally, Study 3 provided an intermediate level of naturalness, blending a laboratory setting with a social manipulation. We investigated gaze behavior and neural activity by co-registering EEG and eye-tracking. Participants (n = 74) performed a difficult discrimination task and received feedback in the form of 1-second videoclips of a confederate displaying positive, neutral, or negative expressions. Participants believed that the feedback was either automatically generated by a computer (non-social context) or selected by an experimenter in the adjacent room (social context). The results showed that social context did not influence gaze behavior or brain activity, though positive expressions elicited distinct neural responses in late brain components. Altogether, these findings demonstrate that stimulus relevance, cognitive resource availability and direct overt attention modulate attention to emotions in social and non-social contexts. Additionally, they showed how laboratory and naturalistic studies may complement each other to draw a comprehensive picture of attention mechanisms in everyday life. The current thesis creates a bridge between real-life an laboratory studies on social and emotion-driven attention
Instrumente für die Förderung bürgerschaftlicher Nahwärme im urbanen Bestand
No full textDer Beitrag ist Teil der Arbeit des von der DBU geförderten Projektes „Urbane Anergienetze als Instrument der Wärmewende in Bremen“. Das Projekt zielt darauf, Erkenntnisse über den möglichen Beitrag „kalter Nahwärme“ auf der Basis flacher Geothermie (Erdwärme) in dicht bebauten Wohnquartieren für die klimaneutrale Wärmeversorgung zu identifizieren.
Für dicht bebaute Stadtgebiete gelten gebäudebezogene Wärmepumpen aufgrund von Platzproblemen und Lärm in der Regel als ungeeignet; der Ausbau perspektivisch dekarbonisierbarer Fernwärmenetze wird jedoch auf baulich sehr verdichtete Stadtgebiete und große Einzelabnehmer (z.B. öffentliche Einrichtungen und größere Mehrfamilienhäuser) beschränkt bleiben. Die verbleibenden dicht bebauten urbanen Gebiete werden die Kommunen bei der Umsetzung der Wärmewende vor erhebliche Herausforderungen stellen. Hier müssen innovative klimaneutrale Lösungen gefunden werden, die die Literatur bei auf erneuerbaren Energien basierende Nahwärmenetzen sieht. Ausgangspunkt des Arbeitspapiers ist die Situation in Bremen. Hier verfolgen bürgerschaftliche Initiativen das Vorhaben, eine klimaneutrale Wärmeversorgung durch ein „kaltes Nahwärmenetz“ mit Erdwärme aus Bohrungen im öffentlichen Raum als Energieträger in genossenschaftlicher oder Vereins-Trägerschaft zu etablieren.
Eine Aufgabe des Projektes ist die Analyse beispielhafter Handlungsansätze, Politikinstrumente und Policies insbesondere der Kommunen, aber auch höherer staatlicher Ebenen, die die Transformationen im Wärmesektor aktiv fördern können und die auch in der kommunalen Wärmeplanung (KWP) geforderte Umsetzung befördern können. Der Beitrag identifiziert Handlungsansätze und Politikinstrumente im Handlungsfeld, die verschiedene Formen der staatlichen Steuerung nutzen, und diskutiert, auf welcher Ebene diese zu verorten sind. Der Fokus liegt dabei auf der kommunalen Ebene und ihren Potenzialen.
Methodisch stützt sich der Beitrag auf eine Literatur- und Dokumentenanalyse sowie hieran anschließende qualitative Erhebung zur Handlungsorientierung der Akteure des Wärmesektors in Bremen und der Entwicklungsbedingungen für „kalte Nahwärme“ in Bremen.
Die Analyse kommt zu dem Ergebnis, dass Handlungsansätze erforderlich sind in den Bereichen
• Absicherung der hohen Anfangsinvestitionen in Wärmenetze durch sehr langfristig orientierte Instrumente,
• die proaktive Koordination der Entwicklung von Nahwärmeprojekten einschließlich einer Bündelung von Genehmigungsprozessen,
• die Entwicklung von rechtssicheren Ausschreibungsformen mit hohen Anforderungen an Gemeinwohl und den Einsatz erneuerbarer Energieträger,
• kooperative Betreibermodelle mit kommunaler Beteiligung,
• sowie eine aktive Beteiligung kommunaler Gesellschaften an der Entwicklung von anspruchsvollen gemeinwohlorientierten Nahwärmenetzen.
Während Kommunen insbesondere die Potenziale ihrer eigenen Verwaltungen und der Einrichtungen der Daseinsvorsorge nutzen können, sind sie bei der Weiterentwicklung rechtlicher Anforderungen für die Versorgungswirtschaft und der Förderinstrumente im Bereich der Wärmewende jedoch auf die Unterstützung z.B. des Bundes angewiesen.
Die Erkenntnisse dieser Analyse lassen sich auf andere Städte mit vergleichbaren Siedlungsstrukturen übertragen.22, 202
Enabling ultra-high vacuum for quantum optical instruments in space
No full textClouds of ultra-cold atoms or a Bose-Einstein Condensate (BEC), perhaps borne in microgravity, can be used as test objects for high-precision matter wave interferometers. Quantum optical instruments based on this technology have the potential to validate scientific theories, such as the unification of the standard model and general relativity, or to measure inertial forces for advanced navigation of vehicles. The point-like nature of a BEC and the unlimited time of free-fall in space increase the achievable accuracy of matter wave interferometers. Any disturbance of the ultracold atoms can trigger the interferometer to lose precision. Following, Ultra-High Vacuum (UHV) is fundamental to achieve a BEC and perform high-precision matter wave interferometry. The hits with the background gas in the vacuum system result in impulse changes and thereby a loss rate, which in turn causes loss of interferometer contrast. In the scope of this thesis, various key
technologies for ultra-cold atom UHV systems in space are investigated, simulated, and tested. The advantages and disadvantages of pump systems and their usage in cold atom instruments are shown. A focus is set on the lifetime verification for filaments of a Titanium Sublimation Pump (TiSub) and the optimization of gas species dependent pumping. Numerical analysis of the filament degradation is used to configure a TiSub for long-term missions. Usually, the limiting factor for pump rates is, in the regime of molecular flow, the transmission probability and conduction of appliances. Simulation work with a Test Particle Monte-Carlo (TPMC) method is performed to gather detailed information on transmission probabilities for in-vacuum components. Several geometries and their dimensions were modified and analyzed. The optimal parameters for meshes, sintered filters, atom chip holders, and a Differential Pumping Stage (DPS) were found to maximize the effective pump rate. Experimental setups were developed to verify and validate important parameters of UHV systems. An outgassing rate measurement testbed is built, characterized, and used for the measurement
of assemblies and materials. Here, challenges of the measurement setup are tackled and elaborated. Material specific outgassing rates were measured and are in good agreement to literature values. First assembly measurements showed a significant influence of heat and driving currents on the outgassing rate. Three laboratory setups were developed, to investigate on thermal, dynamic mechanical, and corrosive properties for all-metal seals of UHV windows. Results show the leakage rate of Indium (In), Lead (Pb), Gold (Au), and glue sealed windows. For leak tight connections, the leakage rate was investigated for various material combinations and surface qualities under representative dynamic mechanical and thermal loads — with no significant increase for In, Pb, and glue as seal. With another test setup ovens for alkali metal evaporation were characterized by their partial pressure and temperature behaviour. Results showed clearly that a valve is perfect to enable an immediate alkali metal partial pressure shut down. On the other hand, thermal ovens with cracked ampule and no valve were not suitable to completely shut down the alkali metal pressure, and in addition, needed longer activation times. In experiments using cracked ampules, the long times to establish a high partial pressure were probably caused by the disadvantageous heat transport and transmission of the alkali metal through the cracked glass. The measurements for filled, valve-regulated, and cracked ovens point out that there is no systematic root cause for the delay. The laboratory setups and simulation results were used to develop the International Space Station (ISS) payload Bose-Einstein-Condensate and Cold Atom Laboratory (BECCAL) and especially the UHV system for this space instrument. An overview of BECCAL and the UHV system in detail is given. Here, the ISS as platform, the goals of BECCAL, and the payload setup are shown. Finally, the pressure distribution of the UHV system were analyzed and simulated. With new methods and numerical simulations, investigations of pressure differences in the vacuum system are performed and thus help to verify scientific data and validate requirements. The analysis proves that the requirement of < 1 × 10−8 Pa (< 1 × 10−10 hPa) at the location of ultra-cold atoms can be achieved. In a prototype setup, the validity of the analysis is shown.
The results of this thesis lay the groundwork for the development and optimization of future UHV systems in space dedicated for quantum optical instruments
Wohnungspolitik und Wohnsituation im Land Bremen 2025
No full textIm Sommer 2024 haben die Arbeitnehmerkammer und das
Statistische Landesamt Haushalte im Land Bremen zu ihrer
Wohnsituation und zur Belastung durch Wohnkosten befragt.
Die Ergebnisse zeigen, dass in der Stadt Bremen etwa drei von
zehn Haushalten (in Bremerhaven: zwei von zehn) Wohnkos-
ten tragen müssen, die im Verhältnis zum Einkommen zu hoch
sind. Bei Haushalten mit besonders niedrigem Einkommen sind
sogar acht von zehn betroffen.
Die Ergebnisse sind eingebettet in aktuelle wohnungspolitische
Diskurse und Entwicklungen auf dem Wohnungsmarkt auf der
Grundlage des empirica Immobilienpreisindex. Abschließend
werden wohnungspolitische Handlungsbedarfe sowie Forde-
rungen abgeleit