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Coming dissertations at TekNat

  • Predictor Antennas : Enabling channel prediction for fast-moving vehicles in wireless broadband systems

    Author: Joachim Björsell
    Publication date: 2022-04-22 08:47

    Many advanced transmission techniques utilize channel state information (CSI) at the transmitter (CSIT) to improve throughput, spectral efficiency, power efficiency, and other performance metrics. Estimating CSI accurately is important to fully benefit from many of these techniques. In situations where users travel at high speed, the channel can change rapidly, especially in small-scale fading environments. In many systems, there is also a delay between measuring CSI and using it for transmission. If the channel changes significantly during this delay, CSI becomes outdated and the benefits of advanced transmission techniques are typically negatively affected. Long-range channel prediction can be used to counteract this delay and enable advanced transmission to vehicles that travel at high velocity. Conventional prediction methods use channel extrapolation and have a limited prediction horizon that does not support high vehicular velocities for the current size of these delays. The predictor antenna concept has been shown to increase the prediction horizon by at least an order-of-magnitude. It does so by placing an antenna array on the exterior of a vehicle, in the direction of...

  • Hybrid sterility and genetic incompatibilities in Ficedula flycatchers

    Author: Carolina Segami
    Publication date: 2022-04-21 14:25

    Although the theory behind the mechanisms generating intrinsic post-zygotic isolation is well established, very few concrete examples of genetic incompatibilities have been described, especially in vertebrates. Consequently, our understanding of the evolutionary forces shaping the appearance of genetic incompatibilities between natural populations and the overall role of genetic incompatibilities in the speciation process is limited. In this doctoral thesis I will contribute to filling this gap in knowledge by using different approaches to investigate the causes and genetic basis of male hybrid sterility in a natural Ficedula flycatcher hybrid zone. I started by analyzing hybrid inviability patterns using 17 years of long-term monitoring data and found evidence for hybrid inviability at different life stages (Paper I). Early developmental failure of hybrids as revealed by the lower hatching success of mixed-species pairs suggesting emerging severe but non-fixed incompatibilities between the two species. Subtler differences in terms of lower growth potential and shorter lifespan indicate mito-nuclear incompatibilities as elevated metabolic rate can cause accumulation of toxic by...

  • Fiber Optic Sensors for Monitoring of Lithium- and Sodium-ion Batteries

    Author: Jonas Hedman
    Publication date: 2022-04-12 09:49

    Rechargeable batteries, particularly lithium-ion batteries, have rapidly evolved since their introduction and now dominate the market, owing primarily to their high energy and power densities. With growing demand for high-performance batteries in portable electronics and electric vehicles, the need for safe, efficient, and reliable batteries is crucial. Conventional battery management systems, which generally rely on parameters such as current, voltage, and temperature, provide limited information on the chemical and physical processes taking place in the battery during operation. The understanding of degradation processes and how they evolve with time is also limited due to the complex nature of batteries. In order to enhance the battery lifetime, safety, and reliability of current batteries as well as for emerging battery technologies, more detailed information from the cells is required. Developing sensors that can be used to probe the batteries could allow for optimized performance and a more accurate determination of cell state. In this regard, fiber optic sensors are promising candidates.

    This work explores the use of fiber optical evanescent wave (FOEW) sensors for...

  • Preprojective Algebras of d-Representation Finite Species with Relations

    Author: Christoffer Söderberg
    Publication date: 2022-04-06 17:03

    In this article we study the properties of preprojective algebras of representation finite species. To understand the structure of a preprojective algebra, one often studies its Nakayama automorphism. A complete description of the Nakayama automorphism is given by Brenner, Butler and King when the algebra is given by a path algebra. We partially generalize this result to the species case, i.e. we manage to describe the Nakayama automorphism up to an unknown constant.

    We show that the preprojective algebra of a representation finite species is an almost Koszul algebra. With this we know that almost Koszul complexes exist. It turns out that the almost Koszul complex for a representation finite species is given by a mapping cone of a certain chain map. We also study a higher dimensional analogue of representation finite hereditary algebras called d-representation finite algebras. One source of $d$-representation finite algebras comes from taking tensor products. By introducing a functor called the Segre product, we manage to give a complete description of the almost Koszul complex of the preprojective algebra of a tensor product of two species with relations with certain...

  • Local Effects On Icing Forecasts for Wind Power In Cold Climate

    Author: Erik Janzon
    Publication date: 2022-03-30 15:24

    This thesis will examine the local effects of land cover on icing forecasts. In Paper I, a single column model was used to test the sensitivity of icing forecasts to land cover fraction. Here, the ice accretion forecast was found to be highly sensitive to the wind magnitude response to the surface roughness. Diabatic effects related to the surface albedo played a secondary role, significant in cases with strong solar irradiance. Paper II examined the impact of 2-dimensional patterns of land cover heterogeneity on the effective surface roughness and blending height using large eddy simulation over a diurnal cycle of solar irradiance. The blending height--or the elevation at which the atmospheric response to the underlying land cover becomes horizontally homogeneous--has been proposed as a guide for coupling numerical weather models to surface parameterizations. In stable conditions, when the atmospheric boundary layer height was shallow, the blending height over surfaces with large heterogeneity length scale was found to be much lower than that of analytical models from previous studies. A new formula for a dynamic blending height was proposed taking this effect into account....

  • A molecular guide to efficient charge transport : Coordination materials for photovoltaic cells

    Author: Hannes Michaels
    Publication date: 2022-03-30 14:43

    Emerging solar energy conversion and energy storage technologies play a vital role in solving the present energy crisis and achieving carbon net zero. Currently, they are limited by the use of inefficient, unstable and expensive charge transport materials. The development of new charge transport materials is still far behind the efforts that have been made to develop the light-absorbing or other components. Metalorganic coordination compounds offer unique sets of properties as hybrids between conductive metals and tunable organic molecules. The coordination of the metal centers is crucial to control in order to maximise the solar cell efficiency - or undesired electronic recombination limits the power output. Tetradentate ligands allow copper complexes to dynamically switch between dimers or monomers, pending the oxidation state of the metal ions. The high energy barrier for the reduction of CuII monomers prevents electron transfer across the TiO2|dye|electrolyte interface: Interfacial recombination is reduced and the dye-sensitised solar cells achieve greater photovoltages. Coordination complexes linked into low-dimensional coordination polymers afford charge transport with an...

  • Efficient Large-­eddy Simulation for Wind Energy Applications

    Author: Henrik Asmuth
    Publication date: 2022-03-30 11:26

    Modelling the interaction of wind turbines with the ambient flow is essential for almost all technical aspects of wind energy exploitation. Large-eddy simulation (LES) is the most detailed approach feasible to model this complex interaction of wind turbines with the atmospheric boundary layer and the wakes of upstream turbines. Despite more than twenty years of fundamental research on wind turbine modelling with LES, applications of the method remain limited to academic use cases to date. The main bottleneck hindering a broader adoption of LES in the industrial practice is the large computational demand of the method. Nevertheless, it holds enormous potential for addressing various modelling challenges arising from current trends in wind energy.

    A promising alternative to classical numerical approaches for LES is the lattice Boltzmann method (LBM). In particular, GPU-based (graphics processing unit) implementations of the method provide significant performance gains and have enabled unprecedented computational efficiencies for LES in different fields of fluid dynamics. Still, the LBM´s potential for wind energy applications remains untapped due to open questions, some of...

  • Designing metallic glasses : Alloying, properties, and degrees of freedom

    Author: Maciej Kaplan
    Publication date: 2022-03-17 07:39

    The present design rules for metallic glasses with respect to glass formation, thermal stability (resistance to crystallisation), and material properties can still be improved. A main design rule is to use thermodynamic calculations to determine the composition of eutectic points in the phase space. The work in this thesis shows that thermodynamic calculations can be utilised to a larger extent than calculating liquidus temperatures. To gain additional insight about metallic glass forming alloys, they are herein classified into systems with a small and large negative enthalpy of mixing of the liquid. The mixing enthalpy influences the number of compound phases (intermetallic or ceramic) forming at equilibrium. A small negative enthalpy of mixing results in few compound phases at equilibrium, whereby the formation range of metallic glasses is dominated by the atomic size mismatch, for instance in ZrNbCrMo and VZr. A low thermal stability is observed for glasses with a chemical composition close to the amorphous-to-crystalline boundary if the alloys have a small negative enthalpy of mixing. Moreover, thermodynamic calculations are herein used to relate the crystallisation...

  • Single-cycle undulator light

    Author: Georgii Shamuilov
    Publication date: 2022-03-14 12:41

    The past decade has witnessed a sharp rise of interest in coherent terahertz (THz) light sources for applications in condensed-matter physics. These sources are a powerful tool for studying collective excitations in solid-state systems: THz light can directly couple to low-energy excitations on the meV-scale such as the collective excitations of spins and phonons. Furthermore, coherent excitation of the material spin or phonon subsystem by a THz light pulse allows for tailoring the material’s macroscopic properties. This enables the creation of materials with new dynamic functionalities. To fully exploit the potential of the control of materials’ properties, a new generation of versatile sources of intense short-pulse THz light is needed.

    This thesis addresses the principles of generation of intense single-cycle THz pulses in an accelerator-based light source. The overarching principle is the phase-locked coherent emission of frequency-chirped waveforms from a specially prepared train of electron bunches inside a tapered undulator. The first part of the thesis (Ch. 1-2) motivates the THz light source development. It surveys the available light sources and scientific...

  • Palaeoenvironmental reconstructions from modern and ancient plant DNA

    Author: Kevin Nota
    Publication date: 2022-03-14 10:28

    Palaeoecological studies on lake sediments and peat archives have provided fundamental knowledge about past environments, nevertheless, a lot remains to be learned. In this thesis, I focussed on plant ancient DNA extracted from sedimentary archives (sedaDNA), in combination with DNA from living trees with the aims of: (1) investigating different extraction methods and levels of inhibition in sediments, (2) investigating postglacial recolonisation history of Norway spruce in Fennoscandia, (3) comparing different sequencing and bioinformatic approaches to investigate past flora changes using sedaDNA and investigating past flora change in contrasting environments (southern Italy versus southern Sweden), and (4) communicating my work more broadly using art.

    Based on seven original methodological case studies investigating enzyme inhibition, the effect of freeze-thaw and oxygen exposure on microbial composition, and the effects of DNA extraction methods on biological diversity, we formulated guidelines and recommendations for future sedaDNA analyses. We analysed DNA traces of spruce in multiple sediments from northern Europe and confirmed that the...

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