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Coming dissertations at TekNat

  • Grid frequency stability from a hydropower perspective

    Author: Elin Dahlborg
    Publication date: 2021-07-02 06:41

    Many AC grids suffer from decreased frequency stability due to less system inertia. This has increased the risk of large-scale blackouts. This thesis and its papers address the frequency stability problem from a hydropower perspective.

    Grid frequency stability assessments often require accurate system inertia estimates. One approach is to estimate the inertia of all individual power plants and sum up the results. We implemented three inertia estimation methods on a Kaplan unit and compared their results. The generator contributed with 92-96% of the unit inertia, which verified the results from previous studies. However, the three methods estimated slightly different values for the unit inertia, which raises the question of when to use which method.

    Hydropower often deliver frequency control, yet we found no studies which validate Kaplan turbine models for large grid frequency disturbances on strong grids. Therefore, we performed frequency control tests on a Kaplan unit, implemented three hydropower models, and compared the simulation results to the measurement data. The models overestimated the change in output power and energy delivered within the first few seconds...

  • Multiscale Modeling in Systems Biology : Methods and Perspectives

    Author: Adrien Coulier
    Publication date: 2021-06-11 10:09

    In the last decades, mathematical and computational models have become ubiquitous to the field of systems biology. Specifically, the multiscale nature of biological processes makes the design and simulation of such models challenging. In this thesis we offer a perspective on available methods to study and simulate such models and how they can be combined to handle biological processes evolving at different scales.

    The contribution of this thesis is threefold. First, we introduce Orchestral, a multiscale modular framework to simulate multicellular models. By decoupling intracellular chemical kinetics, cell-cell signaling, and cellular mechanics by means of operator-splitting, it is able to combine existing software into one massively parallel simulation.  Its modular structure makes it easy to replace its components, e.g. to adjust the level of modeling details. We demonstrate the scalability of our framework on both high performance clusters and in a cloud environment.

    We then explore how center-based models can be used to study cellular mechanics in biological tissues. We show how modeling and numerical choices can affect the results of the simulation and mislead...

  • The effects of exposure to microplastics and pollutants on the arthropod microbiome

    Author: Javier Edo Varg
    Publication date: 2021-06-08 14:39

    Anthropogenic pollution is widespread across various ecosystems. This disturbance can alter the interaction between a host and its associated microbiome, with repercussions for hosts traits such as health, behavior, and host evolution. The thesis aim is to understand the effects of inert microplastics and other pollutants, as pesticides, detergents, and toxic metals, on the host-microbiota of different freshwater invertebrates. Specifically, this thesis explores the secondary effects of stress factors on the host, trophic interactions, and free-living microbes. Chapter I tested the effects of microplastics and the pesticide esfenvalerate on Chironomus riparius survival, emergence, and its microbiome. Chapter II tested the effects of microplastics and the pesticide deltamethrin on a trophic chain of three organisms:  Daphnia magna, damselfly larva Ischnura elegans, and wild dragonfly larva Aeshna cyanea. Chapter III tested the effects of microplastics and sodium dodecyl sulfate on the microbiome of wild water boatman from the family Corixidae. Chapter IV tested the...

  • Synthesis–Structure–Property Relationships in Li- and Mn-rich Layered Oxides

    Author: Ashok S. Menon
    Publication date: 2021-05-24 09:20

    The commercialisation of Li-ion batteries over the last decade has provided additional impetus for the improvement of existing energy storage technologies. Towards this, a major portion of the global efforts includes exploratory research aimed at the development of new material chemistries. Aligning with this theme, this Thesis explores the synthesis–structure–property relationships in Li- and Mn-rich layered oxides, a cost-effective high-capacity material system that shows promise as a positive electrode material for future Li-ion batteries. The compositional and crystallographic diversity of Li- and Mn-rich layered oxides make them particularly susceptible to synthesis-dependent variations and exacerbates structural characterisation. Therefore, understanding how synthetic variations influence their structural and electrochemical properties is a crucial step in realising their potential as positive electrode materials.

    Even for simple compositions like Li2MnO3, dissimilar crystallographic ordering and particle morphologies are produced depending on whether a solid-state or sol-gel synthesis approach was implemented. Subsequently, due to the higher degree of...

  • Studies of Topological Superconductors via Defects

    Author: Oladunjoye Awoga
    Publication date: 2021-05-21 14:05

    Defects from impurities, edges, junctions or domain walls have local detrimental effects on the superconducting state. Defects are also important in topological superconductors, and useful in studying the properties of the superconducting order parameter. In the former case, gapless edge states appear as a consequence of the bulk-edge correspondence, which relates the edge states to the change in bulk topological invariants across the edge. In the latter case impurity scattering of electrons modifies the density of states locally and may (or not) lead to the appear-ance of subgap states depending on the nature of the impurity and the superconductor. Due to these telltale effects of defects in topological superconductors, this Thesis uses defects to probe topological superconductors. The studies are divided into two parts. In the first part Majo-rana bound states at the ends of one-dimensional topological superconductors are studied. The Majorana bound states at the ends of magnetic chains are found very stable in a disordered su-perconducting medium, as long as the superconducting order parameter does not vanish locally. Also junction formed in a nanowire in proximity to two...

  • Magnetic Properties of Epitaxial Metal/Oxide Heterostructures

    Author: Tobias Warnatz
    Publication date: 2021-05-21 08:59

    The work in this dissertation is devoted to tailoring and studying magnetic properties of epitaxial metal/oxide heterostructures. The aim is to understand the fundamental principles governing these properties and how they affect each other. The acquired knowledge can prove useful for the development of future spintronic devices. A variety of experimental techniques is used to fabricate and characterize the epitaxial structures. For fabrication, a combination of direct-current and radio-frequency sputtering is used, whereas x-ray reflectivity and diffraction measurements are the main tools for the structural characterization of the heterostructures. The magnetic characterization of these structures is done by a combination of longitudinal magneto-optical Kerr-effect measurements, Kerr-microscopy and polarized neutron reflectometry. 

    First, it is shown how strain affects the magnetic properties of metal/oxide heterostructures by comparing Fe/MgO and Fe/MgAl2O4 superlattices. Subsequently, an antiferromagnetic interlayer exchange coupling in  Fe/MgO superlattices is revealed and attributed to a spin-polarized-tunneling mechanism. The coupling strength can be tuned by...

  • Deciphering the Mechanistic Diversity of Proton-Coupled Electron Transfer Reactions

    Author: Robin Tyburski
    Publication date: 2021-05-20 14:54

    Proton coupled electron transfer is ubiquitous in biological and artificial reaction systems. Much has been done in order to describe the occurrence of such reactions. However, PCET reactivity is often very complex. For instance, there are multiple (stepwise and concerted) mechanistic pathways through which PCET may occur. The aim of this thesis is to further describe factors and underlying principles governing PCET reactivity. The contents of this thesis can be summarized in three parts:

    In the first part (chapter 4), the competition between different PCET mechanisms is discussed. Considering all mathematical expressions for the dependence of the rate constants on The Gibbs free energy changes (driving forces) associated with electron and proton transfer, mechanistic Zone-Diagrams are constructed. These show which of the mechanistic pathways is dominant, given a certain electron and proton transfer driving force. It is shown, how these diagrams simplify discussion of PCET reactivity. Strategies for modifying the mechanistic landscape, suppressing or favoring a CEPT mechanism, are demonstrated in the PCET oxidation of 4-methoxyphenol by photogenerated Ru(III) oxidants in...

  • Nanocellulose and Metal-Organic Framework-Based Composites : Synthesis, Characterization, and Applications

    Author: Shengyang Zhou
    Publication date: 2021-05-19 14:17

    Nanocellulose is one of the most promising of the green materials available for use in a sustainable economy because of its natural abundance and renewability. Compared to petroleum‒derived synthetic polymers, nanocellulose has many unparalleled advantages such as its unique nanofibrous structure, high thermal stability, mechanical flexibility, rich surface chemistry, biocompatibility, and biodegradability. The tremendous potential of nanocellulose has recently been realised in its use as a building block substrate for multifunctional applications such as energy storage devices, flexible electronic devices, and advanced filtration units. In future, more insight will be gained into the fundamental structure−function relationships of nanocellulose‒based functional materials, with subsequent advantages for the materials industry.

    Metal–organic frameworks (MOFs) are an emerging family of coordination polymers with unique crystalline porous features. Because of their diverse design principles and facile chemical synthesis processes, thousands of MOFs are currently under development. MOFs have found huge application value in many fields, including gas separation and storage,...

  • Galaxies in the epoch of reionization : Investigating the high-redshift galaxy population through simulations and observations

    Author: Christian Binggeli
    Publication date: 2021-05-19 13:15

    The cosmic reionization is the last major gas phase transition in cosmic history, yet it remains poorly understood. Current constraints indicate that early star-forming galaxies drove the reionization process through producing and releasing large numbers of ionizing photons into the intergalactic medium. However, our understanding of the ionizing escape fraction (fesc) and the general properties of high-redshift galaxies is still limited.

    In this thesis, simulated galaxies and observations are used to investigate epoch-of-reionization galaxies and to explore methods that can aid future investigations of such objects. Using simulations, we have shown that it may be possible to constrain fesc in epoch-of-reionization galaxies using quite simple diagnostics that should be observable with the upcoming James Webb Space Telescope (JWST). We also show that variations in star formation activity larger than those predicted in our simulations may lead to a possible degeneracy with high fesc. However, auxiliary information obtained with the JWST may allow us to disentangle variations in the star formation activity from high f...

  • Exploring the Frontiers of Polymer Electrolytes for Battery Applications : From Surface to Bulk

    Author: Christofer Sångeland
    Publication date: 2021-05-19 13:07

    Lithium-ion batteries have dominated the market since their inception in 1991 due to their unparalleled energy and power densities, but are now faced with new challenges. Growing demand for battery materials for energy intense applications and large-scale interim energy storage have emphasized the need for safe and sustainable battery electrolytes. In this context, non-flammable solid polymer electrolytes (SPEs) are a promising alternative to address the shortcomings of conventional liquid electrolytes. Despite its significance, little research has thus far been devoted to understanding the electrochemical stability of SPEs under the harsh conditions exerted by next-generation electrode materials.

    In this thesis, the stability and ramifications of interfaces in polycarbonate- and polyester-based SPEs have been investigated. The polycarbonate exhibited severe degradation upon contact with lithium compared to its ester counterpart. Volatile species stemming from polycarbonate and salt decomposition were observed independent of irreversible current response, thus also highlighting the limitations of voltammetry techniques to determine the electrochemical stability. Two novel...