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Coming dissertations at TekNat

  • The formation of successful physics students : Discourse and identity perspectives on university physics

    Author: Anders Johansson
    Publication date: 2018-09-13 11:40

    In university physics education, unequal student participation has always been an issue. An example is the fact that men constitute 70–80% of the student body in most countries. In recent years, physics education research has started to explore issues of participation, diversity, and identity, but more research and theoretical and methodological development is needed. The work presented in this thesis adopts a discursive perspective on students’ physics identity, building on developments in gender studies and related fields. Focusing on several important steps in physics education, the study explores what it means to become a physicist by asking how norms about being a successful physics student are constructed in the discourses of the education. The methodology is qualitative and interpretative, using participant observation and interviews to explore classroom discourse and student narratives. These theoretical and methodological tools combined with a detailed focus on physics education practice, provide a framework for a deeper understanding of identity in physics. A general conclusion of this study is that physics courses, when taught from a narrow physics perspective, may...

  • Causal MMSE Filters for Personal Audio : A Polynomial Matrix Approach

    Author: Simon Widmark
    Publication date: 2018-09-12 11:31

    This thesis is devoted to the study of synthesis of causal filters for generating personal sound zones. Personal sound zones, personal audio or personal sound is the theory and practice of steering sound in such a way that it is amplified in one region and suppressed in another. In lower frequencies, this is accomplished by designing filters that control the complex pattern of constructive and destructive interference that a set of sound sources gives rise to. There are many ways of designing such filters and the field of personal audio has been rather active in the last few decades. As the algorithms approach actual production line implementation, the aspects of realisability gain in importance. It has been noted in the literature that filters that are causal by design solve many of the problems associated with implementability and may increase the subjective sound quality in the bright zone. However, the problem of synthesis of causal filters for personal audio has received less attention than that of non-causal filters. In this thesis, synthesis of causal filters for personal audio with emphasis on implementability is explored. Several different approaches, with varying...

  • Biotic and abiotic drivers of life-history and demographic variation in boreal orchids

    Author: Matthew R. Tye
    Publication date: 2018-09-12 10:36

    Environmental interactions, including both abiotic and biotic factors, are key drivers of life-history and population dynamics. Despite this, most population studies do not explicitly link variation in environmental factors and vital rates. Here, I combined long-term demographic data and short-term experiments in four species of long-lived boreal orchids (Dactylorhiza incarnata, Dactylorhiza lapponica, Dactylorhiza maculata, and Gymnadenia conopsea) at two sites (Nordmarka, a coastal site, and Sølendet, an inland site) in Norway to understand the effects of biotic and abiotic drivers of variation in vital rates, population dynamics and life history. More specifically, I examined if (I) demographic responses to climate are sensitive to small changes in species life history or location, (II) floral traits are under selection due to limitations of resources or pollinators, (III) pollen limited orchid species compensate for short-term reductions in pollination success, and (IV) there is a demographic advantage to multi-year gaps in reproduction.I found that relationships between demography and climate were highly variable, both among species and sites. This highlights the...

  • Contributions to the understanding of diversity and evolution in the genus Coreomyces

    Author: Henrik Sundberg
    Publication date: 2018-09-11 13:37

    The Laboulbeniales is an order of enigmatic ascomycete fungi that are obligate microscopic parasites of arthropods, which are usually non-detrimental. They typically display a high degree of host specificity, and the genus Coreomyces, which is the recurring theme of this thesis, is no exception. Coreomyces is a small aquatic genus parasitising water boatmen (Corixidae). Members in this genus present something that is unique for Laboulbeniales, position specificity. This means that a parasite is restricted to a specific position on the host. This thesis aims at elucidating the patterns of host and position specificity present in the genus Coreomyces by the use of molecular markers. A second aim is to describe new taxa encountered during the course of the project. Finally we wanted to reveal what mating system is present in a member of the Laboulbeniales, something that has never been done before. Understanding the mode of sexual reproduction and to get insights into mating type organisation can give important clues to how the species patterns we observe may have evolved. To achieve these goals we first had to develop a robust and simple molecular...

  • Overcoming the Mass Transport Limitations of Dye-Sensitised Solar Cells : Towards highly efficient and robust alternative redox mediator electrolyte DSSCs

    Author: Roger Jiang
    Publication date: 2018-09-11 10:56

    Dye-sensitised solar cells (DSSCs) have been subject to decadelong research interest, and may have some potential niche market applications. Recently, major advances in power conversion efficiencies have been achieved following the transition to cobalt polypyridine based redox mediators. Unfortunately, these champion devices remained unfeasible for commercialisation due to heavy reliance on flammable volatile acetonitrile electrolytes. Overcoming this issue has been a challenge due to increased mass transport limitations, which are especially exacerbated in more stable albeit viscous electrolytes. The overall aim of this thesis is to better understand the contributing factors to mass transport, and then to develop practical solutions to overcome these limitations. In particular, specific mass transport limitations pertaining to the bulk electrolyte medium and electrolyte-infiltrated TiO2 medium, are investigated. The effect of non-uniform photogeneration profile across the photoanode is explored with respect to mass transport using photocurrent transient techniques. Practical devices engineering for optimal mass transport is explored in terms of minimising the inter-electrode...

  • The Multiple Faces of Interfaces : Electron microscopy analysis of CuInSe2 thin-film solar cells

    Author: Olivier Donzel-Gargand
    Publication date: 2018-09-07 14:57

    The CIS solar cell family features both a high stability and world-class performances. They can be deposited on a wide variety of substrates and absorb the entire solar spectrum only using a thickness of a few micrometers. These particularities allow them to feature the most positive Energy returned on energy invested (EROI) values and the shortest Energy payback times (EPBT) of all the main photovoltaic solar cells. Using mainly electron microscopy characterization techniques, this thesis has explored the questions related to the interface control in thin-film photovoltaic solar cells based on CuInSe2 (CIS) absorber materials. Indeed, a better understanding of the interfaces is essential to further improve the solar cell conversion efficiency (currently around 23%), but also to introduce alternative substrates, to implement various alloying (Ga-CIS (CIGS), Ag-CIGS (ACIGS)…) or even to assess alternative buffer layers.

    The thread of this work is the understanding and the improvement of the interface control. To do so, the passivation potential of Al2O3 interlayers has been studied in one part of the thesis. While positive changes were generally measured, a subsequent...

  • Radial Basis Function generated Finite Difference Methods for Pricing of Financial Derivatives

    Author: Slobodan Milovanović
    Publication date: 2018-09-06 13:55

    The purpose of this thesis is to present state of the art in radial basis function generated finite difference (RBF-FD) methods for pricing of financial derivatives. This work provides a detailed overview of RBF-FD properties and challenges that arise when the RBF-FD methods are used in financial applications.

    Across the financial markets of the world, financial derivatives such as futures, options, and others, are traded in substantial volumes. Knowing the prices of those financial instruments at any given time is of utmost importance. Many of the theoretical pricing models for financial derivatives can be represented using multidimensional PDEs, which are in most cases analytically unsolvable.

    We present RBF-FD as a recent numerical method with the potential to efficiently approximate solutions of PDEs in finance. As its name suggests, the RBF-FD method is of a finite difference (FD) type, from the radial basis function (RBF) group of methods. When used to approximate differential operators, the method is featured with a sparse differentiation matrix, and it is relatively simple to implement — like the standard FD methods. Moreover, the method is mesh-free,...

  • Mechanisms of speciation in Silene

    Author: Xiaodong Liu
    Publication date: 2018-09-06 13:44

    A fundamental question in the field of evolutionary biology is how new species originate. Investigating speciation benefits from an integrated approach, which requires a solid understanding of ecology, reproductive biology, geographical distribution, underlying genetic architecture of reproductive isolation (RI), demographic history and genomic divergence. In this thesis, I studied the evolution of reproductive isolation in the sister species Silene dioica (L.) Clairv. and S. latifolia Poiret. The aims of the thesis are to investigate (1) the individual and cumulative contributions of extrinsic and intrinsic reproductive barriers between S. dioica and S. latifolia, (2) the genetic architecture of traits associated with reproductive barriers (3) the demographic history of lineage-split between the two species, (4) genomic patterns of divergence between the species.

    I found that multiple extrinsic pre- and postzygotic barriers resulting from ecological differentiation contributed most to total RI, while intrinsic barriers had substantial individual strength but contributed weakly to total RI. QTL mapping revealed evidence for genetic...

  • Calcium Phosphate Based Biomaterials for Bone Augmentation

    Author: Jun Luo
    Publication date: 2018-09-05 11:58

    Basic (apatite-based) calcium phosphate cements (CPCs), and acidic (brushite and monetite-based) CPCs are used as bone replacement materials because of their bioactivity, mouldability and ability to harden in place. However, their application is limited by their inherent brittleness and difficulties related to their handling. The current thesis aimed to provide solutions to these limitations.

    To assess the baseline, the mechanical properties of two promising experimental and two commercially available apatite and brushite cements were investigated. The two experimental CPCs exhibited significantly higher mechanical strengths than the two commercially available ones, warranting further advancement of the former towards clinical use.

    The setting reaction of brushite cements was, for the first time, quantitatively studied in the first seconds and minutes, using synchrotron X-ray diffraction. The reaction was found to include a fast nucleation induction period (<9 s), nucleation (<18 s), brushite content increase and setting completion. The effect of the commonly used retardant citric acid – which usually also gives stronger brushite cements - was also evaluated,...

  • Vacua in String Theory : de Sitter Space and Stability in Flux Compactifications

    Author: Sergio C. Vargas
    Publication date: 2018-09-05 09:01

    Our understanding of cosmology has evolved radically in the last decades. Current models demand the presence of dark energy in our universe and the most favored candidate behind this component is a small positive cosmological constant that characterizes a de Sitter (dS) spacetime. Simultaneously, theoretical physicists have stood up to the challenge of building a consistent theory of quantum gravity and string theory has raised as a strong contender.

    In this thesis we present some explorations within supergravity, a low energy limit of string theory, studying non-supersymmetric vacua, its stability, and the possibility of finding dS.

    We study the landscape of flux compactifications to produce dS with non-geometric fluxes. We find precise analytic procedures to find perturbatively stable dS near supersymmetric and no-scale Minkowski in a potential derived from type IIB compactifications. We also provide analytical evidence of naked singularities being produced in supergravity backgrounds after the introduction of anti-Dp-branes, at both vanishing and finite temperature.

    In order to study the problem of semi-classical stability, we explore compactifications with...