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Coming dissertations at TekNat

  • In the confines of Cu(In,Ga)Se2 thin film solar cells with rear surface passivating oxide layers

    Author: Dorothea Ledinek
    Publication date: 2019-09-09 11:11

    The material supply to build renewable energy conversion systems needs to be considered from both a cost and an energy security perspective. For Cu(In,Ga)Se2 (CIGS) thin film solar cells the use of indium in the absorber layer is most problematic. The material input per service unit can be reduced, if the absorber layers are thinned down without a loss in power conversion efficiency.

    Thinning down absorber layers can increase the conversion efficiency. However, for real CIGS solar cells absorption losses and recombination rates at the rear surface between the CIGS absorber and the Mo rear contact as well as shunt-like behavior increase. Thus, both rear surface passivation and optical management are essential for maintaining high power conversion efficiencies.

    In this work, thin oxide layers, so-called passivation layers, are introduced between the CIGS absorber layer and the Mo contact. They can passivate the CIGS surface, if the CIGS-oxide interface has a lower defect density than the CIGS-Mo interface and/or if they contain a negative fixed oxide charge, which increases the hole concentration and reduces the electron concentration in the CIGS in the vicinity of...

  • Anion redox processes in novel battery cathode materials investigated by soft X-ray spectroscopy

    Author: Felix Massel
    Publication date: 2019-09-06 08:14

    This thesis presents experimental investigations of the electronic structure of emerging and novel cathode materials used in lithium- and sodium-ion batteries. The investigated materials include a range of oxide materials containing the elements nickel and manganese. Central goals are to find fundamental explanations for favorable, respectively, unfavorable electrochemical cycling behavior and to arrive at a better understanding of the roles that the different elemental constituents of the compounds play. The experiments are based on the application of X-ray Absorption Spectroscopy (XAS) and Resonant Inelastic X-ray Scattering (RIXS) in the soft X-ray region and have been performed at synchrotron radiation facilities such as The Advanced Light Source (USA), The Swiss Light Source (Switzerland) and SPring-8 (Japan).

     XAS and RIXS of spinel LiNi0.44Mn1.56O4 at the O K-edge as well as the Ni and Mn L-edges were measured for two different crystal structures, namely, transition-metal-ordered and -disordered, respectively. The results show that both Ni and O contribute strongly as redox centers for the charge compensation during electrochemical cycling. The Ni L-RIXS spectra...

  • Design flood estimation under uncertainty

    Author: Kenechukwu Okoli
    Publication date: 2019-09-04 10:29

    Att bestämma dimensionerande flöden, d.v.s. sannolikheten för att vat-tenföringen i ett vattendrag överskrider ett givet värde, är ett allmänt hydrologiskt problem som exempelvis används för att utvärdera över-svämningsrisker i vattendrag samt för att dimensionera hydrauliska kon-struktioner. Vanliga tillvägagångsätt för att bestämma dimensionerande flöden är baserade på: (i) hydrologiska metoder (t.ex. avrinningsmodel-lering), eller (ii) statistiska metoder (t.ex. anpassning av en sannolikhets-fördelning till en tidsserie med årliga vattenföringstoppar). I denna av-handling så jämförs dessa två tillvägagångssätt då olika osäkerhetskällor för dimensionerande flöden tillgodoses; valet av tillvägagångsätt, osä-kerhet i modellens uppbyggnad, osäkerheter i vattenföringsmätningar, samt mätfrekvens av dimensionerande flöden, begrundades i denna avhandling. Sannolikhetsfördelningen av vattenföringen i ett hypotetisk vattendrag antogs vara känt sedan tidigare; en uppsättning av virtuella experiment (’numeriska experiment där modellen antas vara sann och beskriva den modellerade processen korrekt’) utvecklades och tillämpa-des för båda tillvägagångssätten som utvärderades utifrån hur...

  • A search for leptoquarks with the ATLAS detector and hardware tracking at the High-Luminosity LHC

    Author: Mikael Mårtensson
    Publication date: 2019-09-04 09:36

    This thesis presents a search for pair-production of scalar leptoquarks, decaying into third-generation particles, using proton-proton collisions delivered by the Large Hadron Collider (LHC) and recorded by the ATLAS detector. It also presents the development of a hardware track trigger for the Phase-II upgrade of the ATLAS experiment.

    The search for pair-production of leptoquarks is performed using data collected by the ATLAS detector in 2015 and 2016 at a center-of-mass energy of 13 TeV. The total integrated luminosity of this data set amounts to 36.1 fb−1 . The search sensitivity is optimized for up-type leptoquarks where both leptoquarks decay to a b-quark and a τ -lepton. However, it also proves sensitive to down-type leptoquarks where both leptoquarks decay to a top-quark and a τ -lepton. The data is found to be compatible with the Standard Model, so exclusion limits are set on the leptoquark mass.

    After the High Luminosity upgrade of the LHC, the ATLAS detector faces a 5–7 times increase in the number of simultaneous proton-proton collisions. To benefit from this increase in luminosity, the ATLAS detector has to maintain low trigger thresholds while keeping...

  • Modelling the Molecular World of Electrolytes and Interfaces : Delving into Li-Metal Batteries

    Author: Mahsa Ebadi
    Publication date: 2019-08-30 11:29

    Lithium metal batteries (LMBs) are potential candidates for powering portable electronic devices and for electromobility. However, utilizing the reactive Li metal electrode means tackling serious challenges in terms of safety risks. A better understanding of electrolytes and solid electrolyte interphase (SEI) formation are highly important in order to improve these issues.

    In this thesis, density functional theory (DFT) and molecular dynamics (MD) are used to explore novel electrolyte systems and the interfacial chemistry of electrolyte/Li metal surfaces. In the first part, the electronic structure and possible decompositions pathways of organic carbonates at the Li metal surface are investigated, which provide information about initial SEI formation. Computed X-ray photoelectron spectroscopy (XPS) for these interfacial compounds is used as a tool to find likely electrolyte decomposition pathways and are supported by direct comparison with the experimental results. The electronic structure and computed XPS spectra of electrolyte solvents and the LiNO3 additive on Li metal by DFT provide atomistic insights into the interphase layer.

    Solid polymer electrolytes (SPEs)...

  • Population divergence at different spatial scales in a wide-spread amphibian

    Author: Patrik Rödin Mörch
    Publication date: 2019-08-29 14:16

    To study the distribution of genetic and phenotypic variation in different environments and at different spatial scales is important in order to understand the process of local adaptation and how populations will respond to future climate change. In my thesis I study populations of moor frogs (Rana arvalis) at different spatial scales, first along a 1700 km latitudinal gradient (Paper I, II, IV) and, second, in a system of inter-connected wetlands (III, IV). In Paper I, I present evidence for a major latitudinal break-point in larval life-history traits which is linked to a post glacial contact zone between two lineages that colonized Scandinavia after the last ice age. Using QST-FST comparisons I found divergent selection acting on life-history traits, where a major source of differentiation comes from the two colonization routes. In Paper II I focus on genomic variation, demographic history and selection along the gradient. Using demographic modeling I confirm the proposed demographic history and show historical signatures of gene flow between regions and over the contact zone. In terms of genetic...

  • 7D supersymmetric Yang-Mills theory on toric and hypertoric manifolds

    Author: Andreas Rocén
    Publication date: 2019-08-28 14:14

    This thesis consists of an introduction and three research papers in the general area of geometry and physics. In particular we study 7D supersymmetric Yang-Mills theory and related topics in toric and hypertoric geometry. Yang-Mills theory is used to describe particle interactions in physics but it also plays an important role in mathematics. For example, Yang-Mills theory can be used to formulate topological invariants in differential geometry. In Paper I we formulate 7D maximally supersymmetric Yang-Mills theory on curved manifolds that admit positive Killing spinors. For the case of Sasaki-Einstein manifolds we perform a localisation calculation and find the perturbative partition function of the theory. For toric Sasaki-Einstein manifolds we can write the answer in terms of a special function that count integer lattice points inside a cone determined by the toric action. In Papers II and III we consider 7D maximally supersymmetric Yang-Mills theory on hypertoric 3-Sasakian manifolds. We show that the perturbative partition function can again be formulated in terms of a special function counting integer lattice points in a cone, similar to the toric case. We also present a...

  • Exploration of extended Higgs sectors, development of a displaced track trigger, and improvements in GRID middleware

    Author: Max Isacson
    Publication date: 2019-08-28 14:08

    Two possible solutions to the Standard Model (SM) fine-tuning problem are presented in this thesis, MSSM and Compositeness. The Higgs sector of the MSSM can be tested by searching for charged Higgs bosons. Two direct searches for charged Higgs bosons decaying into a top- and bottom-quark are performed using 13.2 and 36.1 fb–¹ respectively of pp collision data collected at √s = 13 TeV with the ATLAS detector. No significant excess over the SM background is observed and upper limits on σ(pp→tbH+)×B(H+→tb) between 0.07 and 2.9 pb are set at a 95% confidence level in the charged Higgs boson mass range 200–2000 GeV using the CLs method. The phenomenology of a vector-like top-partner decaying into a BSM (pseudo-) scalar is explored and interpreted in 2HDM and Compositeness models. The experimental reach is evaluated using γγ and Zγ final states, and the LHC is determined to be sensitive to top-partner and (pseudo-) scalar masses in the 1–2 TeV range both in the Run 2 and expected Run 3 datasets. Two hardware based hit collection methods targeting long-lived particles are studied for a regional track trigger in anticipation of the HL-LHC upgrade. The efficiency is...

  • Mechanical analyses of trabecular bone and its interaction with implants

    Author: Dan Wu
    Publication date: 2019-08-28 13:24

    Bone substitute materials or implants are commonly used in the surgical treatment of bone fractures. However, severe complications are sometimes reported. In order to improve fracture treatment where the interior, porous trabecular bone is involved, it is important to better understand the mechanical properties of this bone and how it interacts with the substitutes/implants, and this was the aim of this thesis.

    Since one of the key mechanical properties of trabecular bone, i.e. the elastic moduli at the tissue level, was not consistently reported in the literature, the results from four widely applied methods were first summarized and presented in a review paper.

    Furthermore, to improve the analysis of the mechanical behavior of bone and its interaction with implants, a new digital volume correlation (DVC) technique was proposed based on higher-order finite elements.

    We further proposed a method to estimate the elastic modulus at the tissue level by compression of single trabeculae within a synchrotron radiation micro-computed tomograph (SRµCT). Full-field displacements were estimated by DVC, which also provided boundary conditions for a finite element model....

  • The Electrochemistry of LiNi0.5-xMn1.5+xO4-δ in Li-ion Batteries : Structure, Side-reactions and Cross-talk

    Author: Burak Aktekin
    Publication date: 2019-08-23 10:20

    The use of Li-ion batteries in portable electronic products is today widespread and on-going research is extensively dedicated to improve their performance and energy density for use in electric vehicles. The largest contribution to the overall cell weight comes from the positive electrode material, and improvements regarding this component thereby render a high potential for the development of these types of batteries. A promising candidate is LiNi0.5Mn1.5O4 (LMNO), which offers both high power capability and energy density. However, the instability of conventional electrolytes at the high operating potential (~4.7 V vs. Li+/Li) associated with this electrode material currently prevents its use in commercial applications.

    This thesis work aims to investigate practical approaches which have the potential of overcoming issues related to fast degradation of LNMO-based batteries. This, in turn, necessitates a comprehensive understanding of degradation mechanisms. First, the effect of a well-known electrolyte additive, fluoroethylene carbonate is investigated in LNMO-Li4Ti5O12 (LTO) cells with a focus on the positive electrode. Relatively poor cycling performance is found...

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