Publications

Members of TU Berlin can publish different kinds of (text) publications on DepositOnce:

  • publications with academic content such as monographs and collected editions, journal articles and book chapters, conference proceedings and conference papers, research reports,
  • publications that must be made public in connection with examination regulations (doctoral theses and post-doctoral theses),
  • theses of TU Berlin students if the thesis has received the grade "very good" and the publication is endorsed by the supervisor or referee.

The following publication types can be selected: Doctoral thesis, Habilitation, Master thesis, Bachelor thesis, Book, Conference Proceedings, Periodical Part, Preprint, Report, Research Paper, Article, Book Part, Conference Object

For further information on requirements and workflows see Checklists and Hints for Publishing.

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Recent Submissions

Now showing 1 - 20 of 16433
  • Global coal phaseout and its implications for just transitions at the local level

    This dissertation focuses on the global coal phaseout and its implications for just transitions at the local level, with a supply-side perspective for coal-dependent countries and regions in the Global South. It deals with four core research questions: 1) What is the current mid to long-term trajectory of global coal demand and how does it compare to Paris Agreement compatible trajectories? 2) how are changes in international coal demand and initiatives to phaseout coal impacting producing countries? 3) how vulnerable are coal-producing countries and regions to the global energy transition and what are their key vulnerabilities? and 4) what are the key challenges and policy options for Just Transition processes in coal-dependent economies? To answer these questions, this dissertation navigates back and forth between the global, national, and sub-national levels to reflect on the existing interdependence and potential future feedback loops between those levels as the global energy transition unfolds, coal transitions accelerate, and its impacts materialize at each of those levels. It uses an interdisciplinary approach that combines quantitative and qualitative research frameworks and applies a variety of methods. These include interviews and surveys, modelling of the international coal trade, ethnological observations, press and literature review and analysis, and statistical data analysis. This interdisciplinary approach allows for the study of the complex issue of coal phaseout from different perspectives, to gain a more comprehensive understanding of drivers and barriers for phasing out coal and provide policy recommendations to accelerate this process while actively pursuing a just transition. This dissertation describes and quantifies the scenario space of thermal coal demand and supply; evaluates key vulnerabilities, impacts and potential reactions of producer countries under that scenario space; and takes a forward-looking approach to suggest policy responses and strategies to tackle the challenges of just coal transitions. It contributes to the literature on supply-side climate change mitigation, coal phaseout, and just transitions, by improving the understanding of the effects of current developments and trends on thermal coal production and exports; providing up-to-date coal trade projections that consider demand and supply interactions, as well as geopolitical factors; introducing a novel methodology to measure holistically the vulnerability of individual coal countries to the transition; and improving the understanding of the specific just transition challenges in coal-dependent regions of the Global South.
  • Foregrounding learner voice: Chinese international students' interpretations of strategy use during oral discourse socialization in Germany

    Although multilingualism is becoming a more important topic of inquiry, a closer look at intercultural studies on Chinese overseas students reveals that they are inclined to contrast the learning behaviors of these students with Western learning conceptions and practices and interpret their learning ways based on Western assumptions. The study used qualitative and quantitative methods to investigate how this cohort of minoritized learners leveraged a material‐centered approach highlighting repetition, reading aloud, rote learning, and translanguaging to increase the salience and processing of the target language input. Additionally, this study performed a comparative analysis of the use frequency and interpretations of learning strategies between self‐reported successful and currently less successful students from China. The results indicate that the former cohort was more concerned with self‐directed activities in favor of input processing and the model of input‐output connection. Lastly, this highlights the importance of legitimization of their approach resource repertoire by foregrounding their interpretations of their own learning behaviors in the new academic environment.
  • Global dryland aridity changes indicated by atmospheric, hydrological, and vegetation observations at meteorological stations

    In the context of global warming, an increase in atmospheric aridity and global dryland expansion under the future climate has been expected in previous studies. However, this conflicts with observed greening over drylands and the insignificant increase in hydrological and ecological aridity from the ecohydrology perspective. Combining climatic, hydrological, and vegetation data, this study evaluated global dryland aridity changes at meteorological stations from 2003 to 2019. A decoupling between atmospheric, hydrological, and vegetation aridity was found. Atmospheric aridity represented by the vapor pressure deficit (VPD) increased, hydrological aridity indicated by machine-learning-based precipitation minus evapotranspiration (P − ET) data did not change significantly, and ecological aridity represented by the leaf area index (LAI) decreased. P − ET showed nonsignificant changes in most of the dominant combinations of the VPD, LAI, and P − ET. This study highlights the added value of using station-scale data to assess dryland change as a complement to results based on coarse-resolution reanalysis data and land surface models.
  • Behaviour of Acoustically Levitated Drops in Mid-Water

    A low-impact acoustic levitation system has been developed to study immobilised single drops in liquid–liquid systems. The ability to observe liquid drops several millimetres in diameter for days enables fundamental research into a wide range of mechanisms. Non-invasive optical measurements with excellent optical accessibility are possible. This experimental work provides the basis for mass transfer studies, emphasizing the precise volume determination, signal noise, reproducibility, and the impact of the acoustic field on the drop and its surrounding environment. The setup can be effectively controlled and proves beneficial for research objectives provided that all liquid phases are entirely degassed, and there are no compressible voids present within the liquids. In addition to the precise, uniform, and reliable measurement conditions, we observed no acoustic streaming in the proximity of the drop and there was no significant vibration of the drop. Qualitative observations using rainbow schlieren deflectometry indicate that the nodal or anti-nodal planes of the standing waves can act as barriers to the dispersion of inhomogeneous dissolved substances in the continuous phase.
  • Extrusion-Based Additive Manufacturing-Driven Design and Testing of the Snapping Interlocking Metasurface Mechanism ShroomLock

    This study presents the manufacturing process-driven development of an interlocking metasurface; (ILM) mechanism for fused filament fabrication; (FFF) with a focus on open-source accessibility. The presented ILM is designed to enable strong contact between two planar surfaces. The mechanism consists of spring elements and locking pins which snap together when forced into contact. The mechanism is designed to deliver optimized mechanical properties, functionality, and printability with common FFF printers. The mechanism is printed from a thermoplastic polyurethane; (TPU) filament which was selected for its flexibility, which is necessary for the proper functioning of the spring elements. To characterize the designed mechanism, a tensile test is carried out to assess the holding force of the ILM. The force-displacement profiles are analyzed and categorized into distinct phases, highlighting the interplay between spring deformation, sliding, and disengagement. Finally, from the measurements of multiple printed specimens, a representative holding force is determined through averaging and assigned to the mechanism. The resulting tolerance, which can be attributed to geometric and material-related factors, is discussed. The testing results are discussed and compared with a numerical simulation carried out with a frictionless approach with a nonlinear Neo-Hookean material law. The study underscores the importance of meticulous parameter control in three-dimensional (3D) printing for the consistent and reliable performance of interlocking metasurface mechanisms. The investigation leads to a scalable model of an ILM element pair with distinct three-phase snapping characteristics ensuring reliable holding capabilities.
  • Untangling the Effect of Carbonaceous Materials on the Photoelectrochemical Performance of BaTaO2N

    The water oxidation reaction is a rate-determining step in solar water splitting. The number of surviving photoexcited holes is one of the most influencing factors affecting the photoelectrochemical water oxidation efficiency of photocatalysts. The solar-to-hydrogen energy conversion efficiency of BaTaO2N is still far below the benchmark efficiency set for practical applications, notwithstanding its potential as a 600 nm-class photocatalyst in solar water splitting. To improve its efficiency in photoelectrochemical water splitting, this study offers a straightforward route to develop photocatalytic materials based on the combination of BaTaO2N and carbonaceous materials with different dimensions. The impact of diverse carbonaceous materials, such as fullerene, g-C3N4, graphene, carbon nanohorns, and carbon nanotubes, on the photoelectrochemical behavior of BaTaO2N has been examined. Notably, the use of graphene and g-C3N4 remarkably improves the photoelectrochemical performance of the composite photocatalysts through a higher photocurrent and acting as electron reservoirs. Consequently, a marked reduction in recombination rates, even at low overpotentials, leads to a higher accumulation of photoexcited holes, resulting in 2.6- and 1.7-fold increased BaTaO2N photocurrent densities using graphene and g-C3N4, respectively. The observed trends in the dark for the oxygen reduction reaction (ORR) potential align with the increase in the photocurrent density, revealing a good correlation between opposite phenomena. Importantly, the enhancement observed implies an underlying accumulation phenomenon. The verification of this concept lies in the evidence provided by oxygen reduction and is in line with photoredox flux matching during photocatalysis. This research underscores the intricate interplay between carbonaceous materials and oxynitride photocatalysts, offering a strategic approach to enhancing various photocatalytic capabilities.
  • Phoneme dependence of horizontal asymmetries in voice directivity

    Human voice directivity shows horizontal asymmetries caused by the shape of the lips or the position of the tooth and tongue during vocalization. This study presents and analyzes the asymmetries of voice directivity datasets of 23 different phonemes. The asymmetries were determined from datasets obtained in previous measurements with 13 subjects in a surrounding spherical microphone array. The results show that asymmetries are inherent to human voice production and that they differ between the phoneme groups with the strongest effect on the [s], the [l], and the nasals [m], [n], and [ŋ]. The least asymmetries were found for the plosives.
  • How Chromophore Labels Shape the Structure and Dynamics of a Peptide Hydrogel

    Biocompatible and functionalizable hydrogels have a wide range of (potential) medicinal applications. The hydrogelation process, particularly for systems with very low polymer weight percentages (<1 wt %), remains poorly understood, making it challenging to predict the self-assembly of a given molecular building block into a hydrogel. This severely hinders the rational design of self-assembled hydrogels. In this study, we demonstrate the impact of an N-terminal group on the self-assembly and rheology of the peptide hydrogel hFF03 (hydrogelating, fibril forming peptide 03) using molecular dynamics simulations, oscillatory shear rheology, and circular dichroism spectroscopy. We find that the chromophore and even its specific regioisomers have a significant influence on the microscopic structure and dynamics of the self-assembled fibril, and on the macroscopic mechanical properties. This is because the chromophore influences the possible salt bridges, which form and stabilize the fibril formation. Furthermore, we find that the solvation shell fibrils by itself cannot explain the viscoelasticity of hFF03 hydrogels. Our atomistic model of the hFF03 fibril formation enables a more rational design of these hydrogels. In particular, altering the N-terminal chromophore emerges as a design strategy to tune the mechanic properties of these self-assembled peptide hydrogels.
  • Force renormalization for probes immersed in an active bath

    Langevin equations or generalized Langevin equations (GLEs) are popular models for describing the motion of a particle in a fluid medium in an effective manner. Here we examine particles immersed in an inherently nonequilibrium fluid, i.e., an active bath, which are subject to an external force. Specifically, we consider two types of forces that are highly relevant for microrheological studies: A harmonic, trapping force and a constant, “drag” force. We study such systems by molecular simulations and use the simulation data to extract an effective GLE description. We find that within this description, in an active bath, the external force in the GLE is not equal to the physical external force, but rather a renormalized external force, which can be significantly smaller. The effect cannot be attributed to the mere temperature renormalization, which is also observed.
  • Talking about Familial Breast and Ovarian Cancer Risk—Evaluation of a Psychosocial Training Module for Gynecologists in Germany

    Primary care gynecologists are increasingly integrated into the care of patients with hereditary breast and ovarian cancer (HBOC) risks. These physicians should not only have basic genetic knowledge; they should also feel able to sensitively address an increased HBOC risk and deal with emotional, stressful situations in this context. Our project aimed at developing a training module, ‘iKNOWgynetics’, addressing psychosocial challenges in the context of HBOC care for primary care gynecologists. We developed the psychosocial training module in three phases: first, we conducted an online survey with n = 35 women with a family history of breast or ovarian cancer to assess patients’ experiences and needs. Second, based on the results of the needs assessment, we developed the training module. Third, we evaluated the training by assessing physicians’ (n = 109) self-efficacy with regard to communication skills in the context of HBOC before and after the training. In the needs assessment, seven psychosocial themes emerged. These themes, complementing a review of the literature, informed the training curriculum. The training was divided into two parts: (1) communicating with women before genetic testing and (2) care co-management for women with HBOC after genetic testing. After the training, participants reported a significant increase in self-efficacy in three domains: communicating empathetically, educating patients in a comprehensible way and dealing with emotionally challenging situations. Our results highlight the relevance of psychosocial issues for patients with HBOC. A genetic literacy training module that integrates aspects of psychosocial care increases physicians’ confidence in dealing with emotionally challenging situations before and after their patients’ genetic testing. Thus, such trainings may improve the care of women with hereditary cancer risks.
  • Suspension of a point-mass-loaded filament in non-uniform flows: Passive dynamics of a ballooning spider

    Spiders utilize their fine silk fibers for their aerial dispersal, known as ballooning. With this method, spiders can disperse hundreds of kilometers, reaching as high as 4.5 km. However, the passive dynamics of a ballooning model (a highly flexible filament and a spider body at the end of it) are not well understood. Here, we introduce a bead–spring model that takes into account the anisotropic drag of a fiber to investigate the passive dynamics by the various non-uniform flows: (i) a shear flow, (ii) a periodic vortex flow field, and (iii) a homogeneous turbulent flow. For the analysis of the wide range of parameters, we defined a dimensionless parameter, which is called “a ballooning number.” The ballooning number is defined as the ratio of Stokes’ fluid-dynamic force on a fiber by the non-uniform flow field to the gravitational force of a body. Our simulations show that the present model in a homogeneous turbulent flow exhibits the biased characteristic of slow settling with increasing turbulence. Upon investigating this phenomenon for a shear flows, it was found that the drag anisotropy of the filament structure is the main cause of the slow settling. Particularly, the cause of slow settling speed lies not only in the deformed geometrical shape but also in its generation of fluid-dynamic force in a non-uniform flow. Additionally, we found that the ballooning structure could become trapped in a vortex flow. These results help deepen our understanding of the passive dynamics of spiders ballooning in the atmospheric boundary layer.
  • Assessment of overheating risk in free-running residential buildings in Palestine under future climate

    This paper addresses the impact of climate change on residential buildings in Palestine, which recently faced an increased risk of overheating. The study investigates the effect of the thermal properties of the building envelope of a single detached house on increasing the building's resilience to climate change. The overheating risk is evaluated using ASHRAE 55 standard under typical historical and future years (2035, 2065, and 2090) based on RCP-4.5 and RCP-8.5 emission scenarios in three climate zones in Palestine (2A,3A and 2B based on ASHRAE 169-2020). The simulation results reveal that the Medium Energy Efficient Building (MEEB) is more effective in enhancing the thermal comfort of the building compared to the Low Energy Efficient Building (LEEB). However, the risk of overheating increases in future climates, particularly in vulnerable populations and specific locations in the hot, dry zones, such as 2B. This necessitates the implementation of combined mitigation strategies, including both active and passive cooling strategies, highlighting the importance of improving the building’s indoor environment and envelope. The findings emphasize the need to incorporate the impact of climate change into building design to ensure energy efficiency, thermal comfort and promote climate-resilient buildings.
  • Formation of amyloid fibrils from ovalbumin under Ohmic heating

    Ohmic heating (OH) is an alternative sustainable heating technology that has demonstrated its potential to modify protein structures and aggregates. Furthermore, certain protein aggregates, namely amyloid fibrils (AF), are associated with an enhanced protein functionality, such as gelation. This study evaluates how Ohmic heating (OH) influences the formation of AF structures from ovalbumin source under two electric field strength levels, 8.5 to 10.5 and 24.0–31.0 V/cm, respectively. Hence, AF aggregate formation was assessed over holding times ranging from 30 to 1200 sunder various environmental conditions (3.45 and 67.95 mM NaCl, 80, 85 and 90 °C, pH = 7). AF were formed under all conditions. SDS-PAGE revealed that OH had a higher tendency to preserve native ovalbumin molecules. Furthermore, Congo Red and Thioflavin T stainings indicated that OH reduces the amount of AF structures. This finding was supported by FTIR measurements, which showed OH samples to contain lower amounts of beta-sheets. Field flow fractioning revealed smaller-sized aggregates or aggregate clusters occurred after OH treatment. In contrast, prolonged holding time or higher treatment temperatures increased ThT fluorescence, beta-sheet structures and aggregate as well as cluster sizes. Ionic strength was found to dominate the effects of electric field strength under different environmental conditions.
  • A robust communication protocol over ad hoc networks for collaborative driver assistance systems

    Advanced driver assistance systems (ADAS) have begun to reach conditional automation in level-3 functions such as the highway pilot introduced in the 2022 Mercedes EQS [1], [2]. At the same time, vehicles are increasingly connected, and vehicle-to-vehicle communication with wireless ad hoc networks is gaining momentum with first mass-market introduction in the 2020 VW Golf [3]. Nevertheless, the combination of vehicle automation and communication remains a critical research topic because an inherently unreliable technology (i.e., wireless communication) appears incapable of ensuring the functional safety required by ADAS. In this thesis, we investigate the combination of wireless communication and vehicle automation to enable the novel class of Cooperative Driver Assistance Systems (CoDAS). Through communication, automated vehicles can extend their perception range beyond their line of sight, gain insights into other vehicles’ planning, and even negotiate and perform collaborative driving maneuvers. In this thesis we examine possible collaborative driving maneuvers, analyze the current state-of-the-art work, and formulate the need for a novel role-based cooperation approach. Because current wireless vehicular communication is unsuitable for CoDAS, we present a novel approach in the Collaborative Maneuver Protocol (CMP). On the functional level, CMP introduces the concept of role-based cooperation, in which vehicles negotiate roles and behavior in collaborative functions instead of cooperative driving trajectories, as proposed in comparable approaches. This ensures self-reliance in planning and better tolerance to external disturbances by other traffic. CMP introduces the concept of session-based cooperation as the basis for the distributed Functional State Machine (FSM). We present and formalize a protocol to enable the synchronization of this FSM and a negotiation mechanism to simultaneously advance the FSM state among cooperating participants. We prove the formal correctness of CMP even under Byzantine network conditions. We further show that failure in communication exceeding a defined threshold acts as a fail-safe on the functional level. We present an implementation of CMP as well as the message set used, and we show how CMP integrates with the existing ETSI ITS G5 standard [4]. A platooning function is used to investigate and evaluate the applicability of CMP. In particular, we analyze the protocol behavior and functional behavior in faulty networks and under malicious attacks. We show that CMP is able to handle up to 35% packet loss and will fail safely under further network degradation or when attacked with falsified packets. We present a detailed discussion of the trade-offs in protocol design between robustness and network usage. Furthermore, we show that CMP adds only negligible network overhead, even for larger numbers of participants. The role-based collaboration approach presented in this thesis is currently implemented in the IMAGinE project to enable real-world collaboration of automated vehicles [5].
  • Cycling towards sustainability: The transformative potential of urban design thinking in a sustainable living lab

    The recent proliferation of Sustainable Living Labs as experimental arenas within and through which contemporary cities try to tackle today’s sustainability challenges, has led to a systematic integration of user’s consideration in the design and testing of Sustainable Product-Service Systems (S.PSSs). Therefore, it is crucial to co-create with users throughout the whole innovation process of Sustainable Living Labs and to discover user needs and shape S.PSSs according to how they fit into users’ daily life. Yet, it remains understudied how the co-creation knowledge in a multi-stakeholder environment of a Sustainable Living Lab can be facilitated methodically to fully utilize the transformative potential and to accelerate sustainability transition in line with the SDGs. Hence, this paper focuses on the integration of the co-creation method of Urban Design Thinking in a Sustainable Living Lab, its associated knowledge generation and application, and how different stages of Urban Design Thinking contribute to the potential of Sustainable Living Labs to accelerate urban sustainability transitions via S.PSSs. Therefore, the author analyzed empirical data from a series of in-depth interviews and written surveys and was actively involved within a cycling Sustainable Living Lab in Berlin, Germany.
  • Influence of moderate electric fields on globular protein gelation and the use of Ohmic heating as an alternative heating technology

    Ohmic heating (OH) is an emerging alternative heating technology for foods offering advantages over conventional heating processes (COV), e.g., in areas of sustainability, energy conversion or time efficiency. During OH, the food is exposed to an external voltage, thus, to an electric field (EF), which causes charged particles (here: ions) to move, resulting in the conversion of electrical into thermal energy. Electrostatic forces present in the EF during OH also interact with larger molecules showing a charge, e.g., globular proteins. When exposed to heat, globular proteins usually denature and subsequently (depending on their concentration) form aggregates and/ or gel network structures. Only a low amount of data on the influence of OH or the interactions between the EF and globular proteins during thermal gelation is available. Although studies have been published in recent years that examine the influence of EF on the denaturation and aggregation of globular proteins, the effects of OH on the thermally-induced gelation of globular proteins and the resulting gel properties of self-standing gels have not yet been described in the literature. The aim of this dissertation is therefore to systematically evaluate the influence of electric fields on the various steps of thermal gelation and the resulting gel properties of self-standing gels. On this basis, a statement can be made about the use of OH technology as an alternative to COV for thermally-induced gel formation. To evaluate the influence of EF on globular proteins, two model proteins were selected (egg white and potato protein), from which self-standing gels were produced (via OH and COV) and analyzed. In this work, it was shown that the use of OH affects resulting protein structures in all three steps of gelation (i.e., denaturation, aggregation and formation of gel network structures). This was indicated e.g., in a lower amount of denatured proteins, the formation of smaller aggregates and less amyloid fibrillar structures within aggregates under OH. It was also shown that the formation of intermolecular beta sheets is less pronounced when using OH compared to COV. In addition, it was found that gels created under OH exhibit fewer hydrophobic interactions and (in the case of egg white gels) also fewer disulfide bonds in the gel network. The resulting gel structures showed a more porous network, having a reduced gel strength and lower water holding capacities. Differences in the resulting protein structures (i.e., aggregates as well as gels) were attributed to the interactions of the EF with the proteins, i.e., non-thermal effects, which led to a less denatured conformation as well as to an altered overall motion of protein molecules. Although it was shown that differences occur in protein aggregates/ gels after OH and COV treatment, these differences were classified as negligible, since overall comparable gel structures and gel properties were achieved. Thus, the influence of the EF is rated less strong than the influence e.g., of temperature. It could also be shown in this work that the electric field strength of the EF and differences in the physicochemical properties of the two proteins have no additional influence on gel formation (under conditions applied, i.e, at high electrical frequencies). In this dissertation, it was thus possible to validate the statement that OH is suitable as a (sustainable) heating alternative to COV to produce comparable self-standing gels. To further evaluate the exact interactions between the protein molecules and the EF, additional software-supported modelling studies will be needed in the future.
  • Photoluminescence of diamondoids: experiment and theory

    In this thesis, the photoluminescence of diamondoids has been investigated. Diamondoids are a class of purely sp3-hybridized hydrocarbons with a carbon framework congruent with the bulk diamond lattice. The valences at the diamondoid surface are passivated with hydrogen. Diamondoids are size- and shape-selectable, which makes them ideal model systems to investigate the influence of size and shape on optical and electronic properties in nanoparticles. In this work, the vibrationally resolved photoluminescence of twelve different structures with five different sizes has been investigated. For this purpose, an existing experimental setup was improved and upgraded by the construction and implementation of a new sample cell with a heating and cooling system. Furthermore, the setup was equipped with a new high-resolution spectrometer and modified for the use with a new laser system. For the investigations, samples of single structural isomers were brought into the gas phase and excited with laser or synchrotron radiation. The radiation emitted in the subsequent relaxation process was spectroscopically analyzed. The photoluminescence spectra of the diamondoids show vibrational fine structure unique to each isomer. To understand the photophysics behind the spectra, quantum chemical calculations with density functional theory and time-dependent density functional theory have been performed. It was possible to simulate and reproduce the experimentally observed vibrational structure to a very high degree. With the help of the calculations, it could be shown that the composition of the spectra exhibits a size dependence. The spectra of smaller diamondoids are dominated by a relaxation into several overtones of a few ground state normal modes and combinations thereof, whereas the spectra of larger diamondoids are almost exclusively combinations of fundamental transitions of many vibrational ground state modes. The calculations also show that the main contributors to the spectra are, in all cases, CH wagging and CH2 twisting modes. This can be traced back to the rigidity of the carbon framework and changes of the CCH- and the HCH-angles upon excitation. In some cases these excitation induced structural changes result in a symmetry reduction and seem to have a size-dependent component. Larger particles seem to undergo smaller structural changes upon excitation compared to the smaller particles.
  • Influencing factors on the separation efficiency of particles due to negatively as well as bipolarly charged air ions in slowly flowing air

    Cleaning indoor air from particles is essential to providing a healthy environment because particles can cause health problems and can additionally be carriers of pathogens. Currently, mechanical filtration is the most commonly used technique for air filtration, but because this results in an increase in energy consumption and there are rooms without mechanical ventilation systems, further techniques may be able to contribute. In the current work the influence of different parameters on the particle separation efficiency of air ions was investigated. Air speed, temperature, relative humidity, ion source (combining emission rate and polarity) and particle source (combining emission rate and material) were assessed to display which parameters influence the separation efficiency and the extent of this influence. In addition, the influence of the surroundings (grounding of surfaces, installations in the air flow) was reviewed. The interaction between the different parameters was also considered. The ion source was seen to have the highest influence on the separation efficiency. However the surroundings also demonstrated an impact, in particular the grounding of the surfaces significantly influenced the separation efficiency under the influence of bipolarly charged particles. In general, the simultaneous introduction of negatively and positively charged air ions (bipolar ionization) showed a much better performance in terms of particulate air cleaning than solely introducing negative air ions. The ion concentration did also show a huge impact in interaction with other parameters, like grounding, absolute humidity and temperature.
  • Structure-property relationships of mixed matrix membranes based on PIM-1 and covalent organic frameworks ─ molecular mobility, physical aging and gas transport

    Commercialized polymeric membranes for gas separation have demonstrated promising feasibility in e.g., natural gas processing, and biogas upgrading. However, the performance optimization of polymeric separation membranes is restricted by a tradeoff relationship: more permeable polymers are generally less selective and vice versa. Furthermore, most of polymeric membranes show a strong tendency to physical aging. This leads to a continuous change in their internal structure and a loss of the good performance of these polymers over time. Up to now, it is not fully understood how this subsequent drawback is connected to the molecular mobility of the polymer systems. In the first part of this thesis, the molecular mobility and the physical aging of a microporous high-performance polymer, a polymer of intrinsic microporosity (PIM), were studied. PIMs are promising candidates for the active layer of gas separation membranes because of their high permeability and reasonable permselectivity values. These appealing properties originate from a microporous structure as a result of an inefficient packing of the polymer segments when the polymer solidifies in the condensed state. The inefficient segment packing is due to a combination of a ladder-like rigid backbone and sites of contortion. Nevertheless, PIMs suffer from a decrease in performance with time due to physical aging. The initial microporous structures approach a denser state via local molecular rearrangements, leading to a reduction of the permeability. Hence a characterization of the molecular mobility in such materials can provide valuable information about physical aging. The dielectric behavior of PIM-1 films (the archetypal PIM) was studied by isothermal frequency scans during different heating/cooling cycles in a broad temperature range between 133 K and 523 K (– 140 °C-250 °C) by broadband dielectric spectroscopy (BDS). In addition, the obtained results were compared with data from samples that were measured after an annealing at ambient temperature for various annealing times. Multiple dielectric processes were observed such as different relaxations related to localized fluctuations and a Maxwell-Wagner-Sillars polarization effect due to the microporosity. The temperature dependence of the rates of all processes follows the Arrhenius law where the estimated activation energy depends on the process. The influence of the thermal history on the processes is discussed in detail. Moreover, X-ray scattering patterns were measured as a function of temperature during heating/cooling in a temperature range corresponding to that covered by BDS. The X-ray scattering intensity increases with increasing temperature. The observed increase in the scattering intensity of the SAXS (small-angel X-ray scattering) region is not completely reversible. This irreversible effect refers to changes in the microporous structure of the samples during annealing. The next aim of the project aims to develop PIM-1 based mixed matrix membranes (MMMs) by incorporating advanced covalent organic frameworks (COFs) filler to enhance both the gas separation performance and long-term stability. COFs are a class of materials that form two- or three-dimensional structures through reactions between organic precursors resulting in strong, covalent bonds to afford porous and stable materials. To prepare MMMs anticipating good separation performance, COF fillers containing λ5-phosphinine moieties denoted as CPSF-MeO and CPSF-EtO will be used as a filler. The morphologies for both COFs were first studied by scanning electron microcopy and X-ray scattering combining SAXS and WAXS data. The maximum of the pore size distribution was found for CPSF-MeO to be 3.5 nm and for CPSF-EtO to be 4 nm. Furthermore, CPSF-MeO and CPSF-EtO fillers were investigated by BDS and fast scanning calorimetry to establish the structure-property relationship of these new innovative materials. It was found out for the first time that both materials undergo a glass transition where the glass transition temperature of CPSF-EtO is found to be 100 K higher than for CPSF-MeO probably due to a more rigid structure of CPSF-EtO than CPSF-MeO. Mixed matrix membranes based on PIM-1 and various concentrations of CPSF-EtO fillers were prepared by solution casting method. The MMMs were characterized by X-ray scattering, transmission scanning electron microscopy (T-SEM), and thermogravimetric analysis (TGA) to address morphological changes of PIM-1 induced by COF filler. Aggregates up to 100 nm were observed in T-SEM images of MMMs. X-ray scattering investigations confirm these aggregates further assemble into larger secondary aggregates at higher filler concentrations. A quantitative analysis of the X-ray patterns in the SAXS region provided a size distribution of the spherical scattering objects ranging up to sizes of 62 nm indicating that the COF particles were incorporated in PIM-1 matrix on a nanoscopic length scale. Moreover, the molecular mobility of the PIM-1/CPSF-EtO mixed matrix membranes with different COF concentrations was investigated by BDS to establish a correlation between molecular mobility and gas diffusion in PIM-1. Like pure PIM-1, several dielectrically active processes were found for the MMMs composites. In contrast to pure PIM-1, for all MMMs a broadened Maxwell-Wagner-Sillars polarization process was found at elevated temperatures due to the presence of an additional mechanism for blocking the charge carriers in composites. It was found out as a main result of BDS investigations that COF aggregates create further free volume elements at the interface to the PIM-1 matrix. Gas transport data shows that the separation performance of MMMs has been enhanced in terms of permeability without a significant loss in the selectivity. As key results result for PIM-1/CPSF-EtO MMMs, the CO2 permeability increases by approximately 50% and selectivity of CO2/N2 increase by 27% for 7 wt% CPSF-EtO filler incorporation. Moreover, the influence of CPSF-EtO fillers on the physical aging of the PIM-1 matrix in MMM containing 5 wt% filler was investigated. These results are important for comprehending the fundamental mechanisms of gas transport in glassy microporous polymers. Moreover, these findings shed light on the potential insights into the influence of COF nanofillers when incorporated into microporous host polymers.
  • Quantitative MRI for assessment of myocardial fat infiltration at ultra-high field strength

    This work aims to provide respiratory motion compensated (MC) 3D fat-water images of the heart at 7T. Imaging at ultra-high fields (UHF >= 7T) promises both an increased signal-to-noise ratio and higher spectral resolution allowing for more accurate measurement of smaller fat signals compared to low field strengths. The combination of both advantages should allow faster image acquisition with simultaneous robust separation of fat and water. A challenge accompanied of an increased field strength to 7T is an increase in the Larmor frequency of the hydrogen protons to 298MHz and, thus, a decrease in the wavelength to 12 cm in the body. therefore, he transmitted radio frequency (RF) fields (B1+ ) necessary for excitation are subject to interference effects that can cause complete signal cancellation in parts of the target area. Furthermore, the magnetic field inhomogeneities ΔB0 also increase, which can lead to errors in the fat-water separation. For these challenges, solutions are presented to take advantage of an increase in field strength. To accomplish this, methods are developed to measure and compensate for B1+ and ΔB0 variations across the heart in 3D under free breathing. The B1+ method developed in this work is validated in phantoms and applied to eleven healthy volunteers. With these data sets, the breathing influences on the B+1 distribution are described. For shallow breathing it is shown that 3D B+1 phase shimming can be performed with non-respiration resolved B1+ maps. Therefore, the 3D non-respiration resolved B1+ maps were used to correct the B1+ field in the following study. In this second study, measurements are performed in eleven healthy volunteers with a 3D bipolar triple-echo GRE sequence with radial phase encoding (RPE) trajectory. In both the first and the second study, the volunteers had a wide range of body mass indexes (BMI; 19.9 - 34.0 kg/m2). The bipolar-corrected triple-echo GRE-RPE data is divided into different respiratory phases (self-navigation) and used to estimate nonrigid motion vector fields and respiration resolved ΔB0 maps. Respiration resolved ΔB0 maps, and motion compensated ΔB0 maps are compared against a reference respiratory phase to assess respiratory-related changes. It is found that, local respiration-related frequency shifts in motion compensated ΔB0 maps are small compared to chemical shifts of the fat peaks. Subsequently, cardiac binned fat-water images are generated using a model-based, respiratory motion-corrected image reconstruction. The motion compensated and cardiac binned fat-water reconstruction of the heart reduces respiration-induced blurring in the fat-water images, and flow artifacts are reduced in the end-diastolic fat-water separated images. This work demonstrates the feasibility of 3D fat-water imaging at UHF for the entire human heart despite spatial and temporal B1+ and ΔB0 variations due to respiration.