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Coming dissertations at TekNat

  • Bound to the past: Historical contingency in aquatic microbial metacommunities

    Author: Máté Vass
    Publication date: 2020-01-16 12:24

    The composition of ecological communities differs due to a combination of different processes, which includes selection by local environmental conditions, dispersal from the regional species pool and random events. Additionally, historical processes such as past dispersal events may leave their imprint on communities as well, resulting in historically contingent communities. However, in most ecological studies the existence and the effect of historical processes remained hidden, even though they could be important predictors of contemporary variations in ecological communities.

    This thesis focuses on how historical processes could influence aquatic microbial metacommunities by investigating when and where history matters, and which factors may regulate historical contingency.

    Using null model approaches, evidence for historical contingency was found in natural ecosystems, more specifically rock pool metacommunities, and appeared to be more likely to influence bacterial than microeukaryotic communities.

    The thesis further used an outdoor mesocosm experiment to test how ecosystem-sized induced differences in environmental fluctuations influenced community...

  • Towards atomically resolved magnetic measurements in the transmission electron microscope : A study of structure and magnetic moments in thin films

    Author: Hasan Ali
    Publication date: 2020-01-15 09:09

    The magnetic properties of thin metallic films are significantly different from the bulk properties due to the presence of interfaces. The properties shown by such thin films are influenced by the atomic level structure of the films and the interfaces. Transmission electron microscope (TEM) has the potential to analyse the structure and the magnetic properties of such systems with atomic resolution. In this work, the TEM is employed to characterize the structure of the Fe/V and Fe/Ni multilayers and the technique of electron magnetic circular dichroism (EMCD) is developed to obtain the quantitative magnetic measurements with high spatial resolution.

    From TEM analysis of short period Fe/V  multilayers, a coherent superlattice structure is found. In short period Fe/Ni multilayer samples with different repeat frequency, only the TEM technique could verify the existence of the multilayer structure in the thinnest layers. The methods of scanning TEM imaging and electron energy loss spectroscopy (EELS) results were used and refined to determine interdiffusion at the interfaces. The confirmation of the multilayer structure helped to explain the saturation magnetization of these...

  • Computational Modeling of the Structure, Function and Dynamics of Biomolecular Systems

    Author: Yashraj Kulkarni
    Publication date: 2020-01-14 10:38

    Proteins are a structurally diverse and functionally versatile class of biomolecules. They perform a variety of life-sustaining biological processes with utmost efficiency. A profound understanding of protein function requires knowledge of its structure. Experimentally determined protein structures can serve as a starting point for computer simulations in order to study their dynamic behavior at a molecular level. In this thesis, computational methods have been used to understand structure-function relationships in two classes of proteins - intrinsically disordered proteins (IDP) and enzymes.

    Misfolding and subsequent aggregation of the amyloid beta (Aβ) peptide, an IDP, is associated with the progression of Alzheimer’s disease. Besides enriching our understanding of structural dynamics, computational studies on a medically relevant IDP such as Aβ can potentially guide therapeutic development. In the present work, binding interactions of the monomeric form of this peptide with biologically relevant molecular species such as divalent metal ions (Zn2+, Cu2+, Mn2+) and amphiphilic surfactants were characterized using long timescale molecular dynamics (MD) simulations. Among...

  • Atomic Scale Modelling in Photoelectrocatalysis : Towards the Development of Efficient Materials for Solar Fuel Production

    Author: Giane B. Damas
    Publication date: 2020-01-10 12:10

    Using sunlight to produce valuable chemicals has been pointed out as an interesting alternative to deal with the well-known environmental problem related to the use of fossil fuels for energy generation. Thus, it is crucial for this field the development of novel photocatalysts that could drive the uphill reactions with high efficiency while presenting low price and toxicity. In this context, conjugated polymers with a donor-acceptor architecture have shown good photoactivity for the hydrogen evolution reaction (HER) due to their advantageous properties, including a broad UV-Vis absorption spectrum and thermodynamic driving force to carry out the charge transfer processes. In this thesis, a series of fluorene- and benzothiadiazole-based polymers are evaluated by means of ab initio methods as potential candidates for photocatalytic HER. A set of small-molecules with well-defined molecular weight have also been considered for this application. In general, tailoring a chemical unit has enabled an improvement of the absorption capacity in benzo(triazole-thiadiazole)-based polymers and cyclopentadithiophene-based polymers, with a higher impact exhibited upon acceptor tailoring. On...

  • High Power Radio Frequency Solid-State Amplifiers and Combiners for Particle Accelerators : From module to system design approach

    Author: Long Hoang
    Publication date: 2019-12-20 13:13

    The rise of Big Science projects brings issues related to the energy consumption and the associated environmental impacts of such large-scale facilities. Therefore, environmentally-sustainable developments are undertaken towards the adoption of energy savings and improved energy-efficient approaches. The advent of the superconducting (SC) radio frequency (RF) accelerating cavity is bringing answers to these issues. Such superconducting RF (SRF) cavity is made of niobium that allows much higher accelerating gradients with a minimization of the energy consumption. The SC RF technology is increasingly used in many modern particle accelerators, including: the European Spallation Source (ESS), the X-ray Free Electron Laser (XFEL), the Linac Coherent Light Source (LCLS)-II and the proposed International Linear Collider (ILC).

    The innovation of solid state PA technology pushes limits regarding packaging, efficiency, frequency capability, thermal stability, making them more attractive than other well-established alternative technologies, such as vacuum tube technology in mid-range power applications. Through the investigations of designs and techniques, this research goal of the...

  • Synthesis and Tuning of Multifunctional Materials at High Pressure

    Author: Lei Liu
    Publication date: 2019-12-19 13:38

    At the present stage, human society is developing at an unprecedented speed, facing an emergence of highly pressing challenges, e.g., information explosion, energy production problems, environmental pollution, climate problems. Functional materials with tailored properties are considered as holding a key to solving these problems. In this thesis, high-pressure techniques were employed to synthesize and tune the properties of multiferroic materials relevant to spintronic and light-harvesting applications, and multifunctional high-entropy alloys.

    Melanostibite (Mn2FeSbO6, MFSO) is a very rare mineral discovered in Sweden. Previous studies indicate it is a potential multiferroic material with foreseen applications in information storage and spintronic devices. However, its multiferroic phase has not been synthesized yet. Herein, the structural evolution of MFSO was studied up to ~50 GPa, and the LiNbO3-type MFSO was synthesized at high pressure and moderate temperature. As a polar structure material, the LiNbO3-type MFSO represents a promising candidate for multiferroic materials. The double perovskite, Pb2CoTeO6, was also compressed to ~60 GPa, while no polar phase was...

  • Interface Studies for Gold-based Electrochemical DNA Sensors

    Author: Xingxing Xu
    Publication date: 2019-12-18 12:57

    Gold based label-free electrochemical DNA sensors have been widely studied for biomarker diagnostics. The sensitivity and reproducibility of these sensors are determined by the sensing interface: the DNA modified gold surfaces. This thesis systematically studies the preparation processes of the DNA sensor interfaces as well as their effects on the sensor performance. First, three pretreatment methods to clean the gold electrode surface and their influence on the subsequent binding of thiolated molecules were carefully investigated. As we found that the surface pretreatment method involving cyclic voltammetry (CV) in H2SO4 may induce structural changes to the gold surface, thus greatly impacting the thiolated molecule binding, the factors influencing this pretreatment method were studied. Practical guidelines were summarized for preparing a clean and reproducible gold surface prior to functionalization. Afterwards, the effects of the surface coverage density of probe DNA and the salt concentration on the probe-target DNA hybridization on a gold sensing surface were systematically investigated using surface plasmon resonance (SPR) analysis. Based on the SPR results, the maximum...

  • Where our feet have taken us : Examples of human contact, migration, and adaptation as revealed by ancient DNA

    Author: Alexandra Coutinho
    Publication date: 2019-12-16 08:47

    In spite of our extensive knowledge of the human past, certain key questions remain to be answered about human prehistory. One involves the nature of cultural change in material culture through time from the perspective of how different ancient human groups interacted with one another. The other is how humans have adapted to the different environments as they migrated and populated the rest of the world from their origin in Africa. For my thesis I have investigated examples of human evolutionary history using genetic information from ancient human remains. Chapter 1 focused on the nature of possible interaction between the Pitted Ware Culture (PWC) and Battle Axe Culture (BAC) on the island of Gotland, in the Baltic Sea. Through the analysis of 4500 year old human remains from three PWC burial sites, I found that the existence of BAC influences in these burial sites was the result of cultural and not demic influence from the BAC. In chapter 2, I investigated the ancestry of a Late Stone Age individual from the southwestern Cape of South Africa. Population genetic analyses revealed that this individual was genetically affiliated with Khoe groups in southern Africa, a genetic...

  • Becoming a physics teacher : Disciplinary discourses and the development of professional identity

    Author: Johanna Larsson
    Publication date: 2019-12-09 10:15

    In this Licentiate thesis I examine the system of physics teacher education. Physics teacher education is important because it is one of the main ways to influence how physics is taught in schools. By extension, physics teacher education has the potential to affect both who chooses to pursue physics as a career and how physics is perceived by Swedish society as a whole. In order to approach this problem, I chose to investigate the professional discourses of Swedish physics teacher educators. I focus on how these discourses potentially afford and constrain trainees’ possibilities of performing a professional physics teacher identity. While the topic of teacher identity has been extensively explored in the literature, the influence of the educational environment on what it means to become a physics teacher has remained very sparsely researched. Theoretically, I view identity as socially constructed in discourse. I connect identity to trainee learning by arguing that what trainees learn will be dependent on their possibilities to perform professional physics teacher identities in their educational programme. Using discourse analysis of...

  • Development of an analytic Beam Model with non-linear Space-Charge Forces and Envelope Stability Studies

    Author: Michael Holz
    Publication date: 2019-11-28 08:03

    Space-charge—the repelling force of same-charged particles—is a major challenge in the development and application of high-current linear accelerators like spallation sources or accumulator rings. Understanding the dynamical behavior of the beam in its presence is imperative to prevent beam loss and subsequent detrimental effects. There are several established methods to examine the beam’s behavior in the presence of space-charge. On one hand, there are fast, analytic methods describing the beam as a collective, thereby describing coherent beam envelope modes. An example of such a method is the widely used, so-called envelope equation. They have been extensively employed in the frame of modeling space-charge in high-current linear and circular accelerators. On the other hand, numerical methods resolve the dynamics of single particles that comprise the beam. These methods include the incoherent dynamics of single particles, but are therefore computationally expensive. The two approaches also reveal adichotomy in particle beam modeling in which the collective beam behavior is contrasted with that of individual particles. In order to complement the established methods, an analytic...