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Coming dissertations at TekNat

  • Past environment and sediment dynamics in the Black Sea-Caspian Sea region from Southern Russian loess sequences

    Author: Chiara Amalia Költringer
    Publication date: 2021-05-18 14:08

    Loess deposits are excellent past climate and environment archives and contain records of aeolian mineral dust accumulation. Loess deposits stretch across Eurasia and sequences along the Lower Volga in the Northern Caspian lowland in the South East European Plain represent a key link between European and Asian loess. They were deposited during phases of Caspian Sea sea-level low stands in the Late Quaternary and are confined by deposits of marine sediments from transgressive phases. These sea-level fluctuations of the Caspian Sea and their driving factors are not well resolved, and one obstacle is the lack of palaeoclimate information during phases of sea-level low stands. In contrast to the marine sediments, the continental deposits are understudied and the origin and formation of the material is debated. As a consequence, loess along the Lower Volga is often left out from palaeoclimate reconstructions in the region. This thesis aims to resolve the origin, formation and post-depositional modification of this loess and determines its aeolian origin. Provenance analyses show that Lower Volga loess particles were formed due to glacial grinding and were transported over long...

  • On the gap statistics of directions in planar quasicrystals

    Author: Gustav Hammarhjelm
    Publication date: 2021-05-18 09:56
  • Electrochemical characterisations of TiO2 nanotube and lithium-metal electrodes for lithium-based batteries

    Author: Charlotte Ihrfors
    Publication date: 2021-05-17 13:59

    Lithium-ion batteries have since their introduction on the commercial market in the early 1990s played an important role within the field of energy storage. In this thesis both titanium dioxide, TiO2, and lithium metal are investigated based on their properties as anodes for lithium-based battery technologies. TiO2 nanotubes with lengths between 4.5 and 40 µm were synthesized through a two-step anodization of titanium metal foil in a fluoride containing electrolyte. The formed nanotubes were investigated using scanning electron microscopy and electrochemically evaluated in pouch-cell batteries. 

    For the long nanotubes, especially the 40 µm long ones, the results indicate that the lithiation and delithiation processes are limited by the solid state diffusion of lithium ions within the electrode material for the employed cycling conditions. Studies of the TiO2 nanotubes at elevated temperatures, 80 °C, showed a good temperature stability both in a conventional organic electrolyte and in an ionic liquid based electrolyte. In full cells the ionic liquid electrolyte was needed to get a good performance mainly due to the LiFePO4 cathode. Despite a good general performance, the...

  • Expanding the Chlamydiae tree : Insights into genome diversity and evolution

    Author: Jennah E. Dharamshi
    Publication date: 2021-05-12 13:55

    Chlamydiae is a phylum of obligate intracellular bacteria. They have a conserved lifecycle and infect eukaryotic hosts, ranging from animals to amoeba. Chlamydiae includes pathogens, and is well-studied from a medical perspective. However, the vast majority of chlamydiae diversity exists in environmental samples as part of the uncultivated microbial majority. 

    Exploration of microbial diversity in anoxic deep marine sediments revealed diverse chlamydiae with high relative abundances. Using genome-resolved metagenomics various marine sediment chlamydiae genomes were obtained, which significantly expanded genomic sampling of Chlamydiae diversity. These genomes formed several new clades in phylogenomic analyses, and included Chlamydiaceae relatives. Despite endosymbiosis-associated genomic features, hosts were not identified, suggesting chlamydiae with alternate lifestyles.

    Genomic investigation of Anoxychlamydiales, newly described here, uncovered genes for hydrogen metabolism and anaerobiosis, suggesting they engage in syntrophic interactions. Anaerobic metabolism is found across modern eukaryotes, and syntrophic hydrogen exchange is central in many hypotheses for...

  • Full circle : Rise and fate of genetic variation in Marasmius oreades fairy rings

    Author: Markus Hiltunen
    Publication date: 2021-05-12 10:41

    Genetic variation is a prerequisite for evolution. The degree of variability within a species is governed by forces including mutation, recombination and selection. In the kingdom of fungi, where periodic sexual reproduction may be interleaved with extended vegetative phases, generators of variability are not restricted to act only during sexual cycles. Such generators may be in the form of mutations to the genome, affecting single base pairs up to large-scale rearrangements, movement of transposable elements, or non-meiotic shuffling of genetic variants by mitotic recombination or parasexuality. Particularly in mushroom-forming fungi, where mycelia may become large and old, the evolutionary potential of variation acquired over vegetative growth is expected to be large. In this thesis, I have studied the rise and fate of variation gained during vegetative growth in the mushroom-forming fungus Marasmius oreades: a non-model species known for growing in ‘fairy rings’. By taking advantage of state-of-the-art genome sequencing technology and developing new bioinformatics methods, the genome sequence of M. oreades was successfully reconstructed. This resource was...

  • Hunting Hydrogen : Structure-property relations in High Entropy Alloy-based metal hydrides

    Author: Gustav Ek
    Publication date: 2021-05-12 08:58

    Metal hydrides have many uses when switching the energy system from fossil fuels to renewable sources, such as rechargeable batteries, hydrogen storage, hydrogen compression and thermal storage. State of the art materials for these applications such as LaNi5 and TiFe, however, suffer certain limitations such as degradation during repeated hydrogen cycling and harsh activation conditions for initial hydrogen uptake, promoting the need for novel materials.  One class of materials that are interesting options are High Entropy Alloys (HEA), which are solid solutions where typically four or more different elements occupy a single crystallographic site in a simple structure such as body centered cubic (bcc) or cubic close packed (ccp). Due to the random distribution of the elements, there is a large variety of local environments for hydrogen, potentially unlocking sites that are unavailable in conventional transition metal hydrides. There is also the possibility of vast chemical tunability when using this many principal elements. It is therefore imperative to establish design rules to enable tuning of the hydrogen sorption properties of these materials by changing the composition....

  • Experimental methods of neutron diagnostics and fast ion physics for fusion devices

    Author: Andrea Sperduti
    Publication date: 2021-05-12 08:48

    Measurements of the neutron emission resulting from nuclear fusion reactions provide an abundance of information on the underlying spatial, temporal and energetic distributions of reacting ions and how they are affected by a wide range of MagnetoHydroDynamic (MHD) instabilities.

    This thesis focuses on studies of the neutron emission and fast ion physics at the Mega Ampere Spherical Tokamak (MAST), its upgrade MAST-U, the Joint European Torus (JET) and the Divertor Tokamak Test (DTT). In particular, measurements and simulations of neutron emissivity and neutron rates by collimated neutron flux monitor are here discussed and applied to study the properties of the plasma and of the fast ion distribution.

    The first part of the thesis describes plasma measurement methods based on neutron diagnostics. In particular, the design of the neutron camera upgrade on MAST-U is here presented and possible outcomes from its future measurements are discussed. MAST and MAST-U, due to their low plasma temperature, are suitable for fast ion studies and in order to relate neutron measurements with the fast ion distributions the weight functions of the neutron camera on MAST are...

  • Large-scale simulation-based experiments with stochastic models using machine learning-assisted approaches : Applications in systems biology using Markov jump processes

    Author: Fredrik Wrede
    Publication date: 2021-05-11 13:26

    Discrete and stochastic models in systems biology, such as biochemical reaction networks, can be modeled as Markov jump processes. The chemical master equation describes how the probability distribution of a biochemical system's states evolves. Unfortunately, solutions to the chemical master equation only exist for trivial problems. However, the stochastic simulation algorithm (SSA) can generate exact sample paths. Large-scale simulation-based experiments involving variations to the model's parameters are computationally intensive and hinder modelers from exploring and inferring their models due to high-dimensional models.

    This thesis proposes methodologies and tools for model exploration and approximate parameter inference of high-dimensional stochastic models simulated via the SSA.  We propose a smart computational workflow using machine learning-assisted approaches to enable model exploration of gene regulatory networks where the objective is to assess different qualitative behaviors present in the model. 

    An artificial neural network is proposed for learning summary statistics used in approximate parameter inference.  The neural network can find distinct local...

  • Extreme wave conditions and the impact on wave energy converters

    Author: Eirini Katsidoniotaki
    Publication date: 2021-05-11 09:31

    The amount of energy enclosed in ocean waves has been classified as one of the most promising renewable energy sources. Nowadays, different wave energy conversion (WEC) systems are being investigated, but only a few concepts have been operated in a sea environment. One of the largest challenges is to guarantee the offshore survivability of the devices in extreme wave conditions. However, there are large uncertainties related to the prediction of extreme wave loads on WECs.  Highfidelity computational fluid dynamics (CFD) simulations can resolve nonlinear hydrodynamic effects associated with wave-structure interaction (WSI).

    This thesis explores the point-absorbing WEC developed by Uppsala University in extreme wave conditions. The dynamic response and the forces on key components (mooring line, buoy, generator's end-stop spring) of the device are studied and compared. The high nonlinear phenomena accompany the steep and high waves, i.e., breaking behavior, slamming loads can be well-captured by the highfidelity CFD simulations. A commonly used methodology for extreme waves selection, recommended by technical specifications and guidelines, is the environmental contour...

  • Trainee teacher identities in the discourses of physics teacher education : Going against the flow of university physics

    Author: Johanna Larsson
    Publication date: 2021-05-11 09:18

    This thesis investigates what is involved in being recognized as a legitimate physics teacher-to-be in a Swedish physics teacher programme. Drawing on in-depth, qualitative interviews with 17 physics teacher educators and 17 trainee physics teachers, this thesis sees learning to become a physics teacher as a process of performing professional identities. It demonstrates aligning discourses of educators and trainees, and outlines a number of challenges that trainees have to negotiate when learning to become physics teachers.

    The first part of the project analyzes the discourses of teacher educators. Four discourse models are identified which demonstrate how the talk of physics lecturers portrays the default goal of learning physics as becoming a researcher. Choosing to become a teacher in this system, means diverting from the expected path of a physics student, and moving backwards towards school physics. In such a system, trainee physics teachers are described as less competent and ambitious than other physics students, and can be understood to be incomprehensibly “going against the flow” of university physics by aiming towards school physics.

    The second part of the...