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Coming theses from other universities

Please note that the date and time given on these pages is the time of electronic publication, and not the date of the public defense. To find the time and venue of the public defense, please follow the link to DiVA of the thesis in question.
  • Isotope-based constraints on sources and processing of black carbon, carbon monoxide, and brown carbon in South Asia

    Author: Sanjeev Dasari
    Publication date: 2021-03-08 08:00

    The highly populated South Asian region is facing rapid economic growth and urbanization. Here, both climate- and health-affecting atmospheric agents such as light-absorbing aerosols black carbon (BC) and brown carbon (BrC), trace gas carbon monoxide (CO), are often found in relatively high levels compared to in other regions. However, atmospheric chemistry-transport/climate models are unable to fully capture the extent of the abundance of BC, CO, and BrC in the regional atmosphere during winter. The Thesis aims to address potentially important uncertainties that may be contributing to the model-observation offset — uncertainties in the ambient optical properties of BrC, uncertainties in the relative source contributions of BC (biomass burning vs. fossil fuel combustion) and CO (direct emission-derived vs. atmospheric chemical oxidation-derived), uncertainties in the regional lifetime and absolute emission fluxes of BC. For the Thesis work, field sampling was conducted at three sites, megacity Delhi (key source region), the Bangladesh Climate Observatory–Bhola Island (BCOB; receptor site for the highly-polluted Indo-Gangetic Plain) and the Maldives Climate Observatory–Hanimaadhoo Island (MCOH; receptor site for wider South Asia).

    The light-absorptivity of water-soluble BrC is found to decrease by ~84% during transport of haze from source-to-receptor regions i.e., Delhi-to-BCOB-to-MCOH — much greater than estimated in chamber studies and accounted in models. Atmospheric photochemical oxidation is found to be a likely driver for the loss of water-soluble BrC light-absorption in the S Asian outflow (with an estimated bleaching rate of 0.20±0.05 day−1) (Paper I). Radiocarbon (Δ14C)-based source apportionment of BC aerosols shows a stark similarity in the relative contributions of fossil (~50%) and biomass sources (~50%) at BCOB as well as at MCOH, suggesting a regional homogeneity in BC source contributions. However, a distinct stable isotopic fingerprint (δ13C) of BC in the N Indian Ocean is found to be arising from a small yet significant contribution (upto 10%) from C4-biomass burning in peninsular India (region south of 23.4°N) (Paper II). Comparison of source-segregated observed and emission inventory-driven modeled BC concentrations indicates regional offsets in the anthropogenic emission fluxes of BC in emission inventories—overestimated fossil-BC in the Indo-Gangetic Plain and underestimated biomass-BC in peninsular India (Paper II). Dual-isotope (δ13C, δ18O)-based source apportionment of CO shows a significantly large contribution (~80%) from direct emissions of primary sources (biomass burning and fossil fuel combustion) in South Asia, in contrast to modeled CO budget (Paper III). The BC-to-CO ratio in South Asia is found to be higher, by a factor of 2-3, than in other polluted regions such as in East Asia during winter. The regional lifetime and emission flux of BC are estimated to be 8±0.5 days (higher than values used in models) and ~2.4±1 Tg/yr (significantly higher than estimated in current emission inventories), respectively (Paper IV).

    Taken together, for convergence between models and observations in wintertime South Asia, i) the ‘dynamic’ nature of BrC light-absorption should be considered in models, ii) improvements in emission information of BC and CO are needed for better-simulating concentrations. Controls on activities such as open burning (such as agricultural crop residue burning, domestic burning of wood and dung as fuel) in South Asia could enable a reduction in BC, CO, and BrC, thereby leading to improved air quality and paving the way for achieving some of the key sustainable development goals outlined by the United Nations.

  • Ensembles and Open Quantum Systems in Polaritonic Chemistry

    Author: Eric Davidsson
    Publication date: 2021-03-03 08:25

    Optical cavities—where spatial confinement can focus electromagnetic excitations—are able to couple matter to light with enough strength that the behaviour of the combined system is best understood in terms of polaritonic states; mixtures of excitations in both light and matter. This polaritonic regime provides a novel approach for modification and control of chemical reactions, and lately, experimental advancements are realising this potential.

    There are however many challenges with creating useful theoretical models of the prominent quantum behaviour in these systems, which (with exception of some simple cases) require numerical simulations. In this thesis, we engage with two such challenges from polaritonic chemistry: Modelling cavities containing an ensemble of matter systems, and modelling the open quantum systems that arise from losses in the cavity. We learn what influence an ensemble of atoms has on molecular dissociation, and we characterise multiple physical phenomena in bad cavities.

  • Thermal Conductivity of Hygroscopic Foams Based on Cellulose Nanomaterials

    Author: Varvara Apostolopoulou-Kalkavoura
    Publication date: 2021-03-03 08:00

    Biobased super-insulating materials could mitigate climate change by minimizing the use of petroleum-based materials, creating artificial carbon sinks and minimizing the energy needed to maintain pleasant interior conditions. Cellulose nanomaterials (CNM) produced from abundantly available cellulose sources constitute versatile, highly anisotropic raw materials with tunable surface chemistry and high strength. This thesis includes the evaluation of the thermal conductivity of isotropic and anisotropic CNM-based foams and aerogels and analysis of the dominant heat transfer mechanisms. 

    We have developed a customized measurement cell for hygroscopic materials in which the humidity and temperature are carefully controlled while the thermal conductivity is measured. Anisotropic cellulose nanofibrils (CNF) foams with varying diameters showed a super-insulating behavior perpendicular (radial) to the nanofibril direction, that depended non-linearly on the relative humidity (RH) and foam density. Molecular simulations revealed that the very low thermal conductivity is related to phonon scattering due to the increase of the inter-fibrillar gap with increasing RH that resulted in a 6-fold decrease of the thermal boundary conductance. The moisture-induced swelling exceeds the thermal conductivity increase due to water uptake at low and intermediate RH and resulted in a minimum thermal conductivity of 14 mW m-1 K-1 at 35% RH and 295 K for the foams based on the thinnest CNF.

    The density-dependency of the thermal conductivity of cellulose nanocrystal (CNC) foams with densities of 25 to 129 kg m-3 was investigated and a volume-weighted modelling of the solid and gas thermal conductivity contributions suggested that phonon scattering was essential to explain the low radial thermal conductivity, whereas the replacement of air with water and the Knudsen effect related to the nanoporosity in the foam walls had a small effect. Intermediate-density CNC foams (34 kg m-3) exhibited a radial thermal conductivity of 24 mW m-1 K-1 at 295 K and 20% RH, which is below the value for air.

    The moisture uptake of foams based on CNMs with different degree of crystallinity and surface modifications decreased significantly with increasing crystallinity and temperature. Molecular simulations showed that the narrow pore size distribution of the amorphous cellulose film, and the relatively low water adsorption in the hydration cell around the oxygen of the carboxyl group play an important role for the moisture uptake of amorphous and crystalline CNM-based materials.

    Isotropic CNF- and polyoxamer based foams as well as CNF-AL-MIL-53 (an aluminum‑based metal-organic framework) foams were both moderately insulating (>40 mW m-1 K-1) and comparable with commercial expanded polystyrene. The thermal conductivity of CNF and polyoxamer foams displayed a very strong RH dependency that was modelled with a modified Künzel’s model. The presence of hydrophobic AL-MIL-53 decreased the moisture uptake of CNF-AL-MIL-53 aerogels by 42% compared to CNF-polyoxamer foams.

    Solid and gas conduction are the main heat transfer mechanisms in hygroscopic nanofibrillar foams and aerogels that depend on the interfacial phonon scattering, Knudsen effect and water uptake. It is essential that the thermal conductivity measurements of hygroscopic CNM-based foams and aerogels are determined at controlled RH and that parameters such as the temperature, density, nanoporosity, fibril dimensions and alignment are characterized and controlled for systematic development and upscaling of biobased foams for applications in building insulation and packaging.

  • Desires for mathematics teachers and their knowledge : Practicum, practices, and policy in mathematics teacher education

    Author: Lisa Österling
    Publication date: 2021-03-02 08:00

    This dissertation is driven by questions about images of desired teachers, privileged teacher knowledge, and access to knowledge in teacher education. My position is that images of particular teachers restrict access to teacher education, while visible knowledge increases epistemic access. 

    A particular focus is practicum, where images of desired teachers and privileged knowledge are negotiated between the three arenas of school, university, and policy. Four papers are included, and each paper is a separate study.

    Two studies engage images of desired teachers. The first study engages lesson observation protocols from the practicum part of teacher education in six countries. The result is four different images of desired teachers: the knowledgeable, the knowledge-transforming, the efficient, and the constantly-improving teacher. The second study is an analysis of Swedish policy reports prepared for political decisions on teacher education, at a national level. The analysis targets mathematics knowledge and mathematics teachers as constructed in the reform. The images of desired teachers constructed in policy were the born, the interested, the knowledgeable, and the skilful teacher. The privileged mathematical knowledge was skills and facts. 

    The next two studies engage privileged knowledge. The third study uses practicum tasks from two programmes in the same institution, and engages an analysis of a third space, where the practice-based context and conceptual objects can integrate. The result is that the visibility of conceptual knowledge, and particularly mathematical knowledge, decreased from the former to the more recent programme, and the third space for theory and practice to integrate, diminished. The fourth study is an analysis of mentor conversations in the school arena, focusing on de-ritualising prompts in teaching. Mentors were found to privilege learners’ agentive participation in learning mathematics and hence the production of narratives and flexible routines.

    In the studies, the images of desired teachers and privileged knowledge are compared across arenas. The image of the knowledgeable teacher and the image of the efficient teacher who successfully obtains goals, permeated all arenas. There were four differences: one, the images of born, interested, and skilful teachers were visible only in the policy arena; two, the privileged mathematical knowledge in policy was skills and facts to be memorised, while for mentors in schools, learner participation in mathematics discourse was privileged; three, the third space was not generated in practicum tasks, whereas the complex joint labour in teaching and learning mathematics was foregrounded by mentors; four, the image of the constantly improving teacher was found only in the practicum instruments of teacher education.

    Although the image of a knowledgeable teacher was visible across the arenas, a disagreement on privileged knowledge was found. Student teachers are asked to self-improve, but are at the same time made responsible for recognising invisible knowledge. I claim that more can be done in mathematics teacher education to promote visible knowledge in practicum, and thereby increase epistemic access. I also claim that the image of the born teacher is based on normalisations which are often irrelevant for appraising teachers.

  • Modelling of stably-stratified, convective and transitional atmospheric boundary layers using the explicit algebraic Reynolds-stress model

    Author: Velibor Zeli
    Publication date: 2021-02-28 07:51

    The atmospheric boundary layer (ABL) is in continuous turbulent motion. The heating and cooling of the Earth’s surface drives mechanic and thermodynamic processes in the ABL through enhancing and damping of atmospheric turbulence. The surface forcing has a profound effect on the diurnal cycle of temperature,wind and related variables in the ABL. Efforts have been made to model atmospheric turbulence with linear algebraic relations such as the eddy-viscosity hypothesis. Modelling of atmospheric turbulence, however, still remains a great challenge and forms an important problem in the context of numerical weather prediction and climate models. In this thesis a recently developed non-linear turbulence model, the so-called explicit algebraic Reynolds-stress (EARS) model, implemented in the context of a single-column model is used to simulate dry, stratified ABLs.

    We propose a new boundary-condition treatment in the EARS model. The boundary conditions correspond to the relations for vanishing buoyancy effects that are valid close to the ground. In the simulation of an idealized diurnal cycle the solutions for the stratified surface layer is in agreement with the surface scaling physics and the Monin–Obukhov functions.

    We have carried out simulations of the ABL with varying levels of stratification using the EARS model implemented in the context of a single-column model. We use the same model formulation and coefficients in these simulations with different thermal stratifications of the ABL. Even in the SCM formulation the EARS model solution produces a full Reynolds-stress tensor and heat flux vector. The set-up of the numerical experiments are taken from previously published large-eddy simulation (LES) studies of ABL.

    Simulations of stably-stratified ABL show that the EARS model is able to accurately predict the development of a low-level jet and wind turning for different levels of stratification. In addition to first-order statistics, the model also predicts more intricate features of the turbulent ABL such as the relation between vertical and horizontal fluctuations for different stratifications and horizontal heat fluxes caused by wind shear. In the simulations of convective ABL the EARS model correctly predicts the horizontal wind speed and potential temperature profiles. The study also shows that the non-gradient term in the vertical heat flux equation, that naturally appears in the model formulations, gives a large contribution to the heat flux and has a significant influence on the predicted potential temperature profile of the convective ABL. Finally, we study the effects of transitional turbulence in the simulation of diurnal cycle extended to several days. The comparison with the LES shows that the EARS model correctly predicts the mean profiles and surface fluxes at different times of the day, including the low-level jet close to the surface. The model also predicts residual turbulence near the top of the ABL at night. The study demonstrates that the EARS model is able to capture key features of stably-stratified and convective ABLs as well as transitional processes that drive the ABL from one stratification to another.

  • EMF Exposure and Radiation Performance of Millimeter-Wave Antennas in 5G Mobile Terminals

    Author: Bo Xu
    Publication date: 2021-02-25 07:33

    Since 2019, 5G has been rolled out in many countries. To support the demand of increasing traffic capacity, for the first time, the millimeter-wave (mmWave) frequency spectrum is exploited for the mobile wireless telecommunication technologies. For the telecommunication industry,  many questions are raised with the advent of 5G mmWave, including what would impact the performance of mmWave antennas in a mobile terminal and how to evaluate the impacts/effects. This thesis focuses on two topics about 5G mmWave mobile antenna performance. One is the radiation performance for mmWave antennas integrated in the mobile terminal. The other is electromagnetic field (EMF) exposure from mmWave mobile antennas. 

    When integrated into a mobile terminal, the radiation performance of mmWave antennas can be affected by the housing conditions, for example, phone casing and display, etc. By detailed step-by-step simulation analyses, different types of housing effects, as well as the effects of the user's hand, are investigated. The effects of realistic housing conditions are also examined with far-field measurements and near-field antenna diagnose based on the solution to the inverse problem. The analyses provide useful insights into mmWave mobile antenna design and measurements in realistic housing environments. 

    The mobile terminal needs to comply with regulations on EMF exposure before putting them on the market. By carrying out multi-physics simulation hybridizing the electromagnetic problem and the thermal problem, the correlation between tissue temperature rise and incident power density generated by the mmWave antennas is studied. Various field combination methods for EMF exposure from array antenna elements are investigated with simulations and measurements, and methods for calculating the upper bound of EMF exposure from the mmWave antenna array are developed. 

    Recently, the international EMF exposure guidelines have been revised, including the changes of the EMF exposure limits in the mmWave frequencies. The implications of the revised limits are investigated by assessing the maximum power and maximum equivalent isotropically radiated power (EIRP) that are allowed to be transmitted from mmWave mobile antennas in the ideal as well as realistic scenarios. The obtained results provide valuable input to the device manufacturer, network operators, and standardization bodies.

  • Numerical and experimental investigations of fluid-surface interaction

    Author: Aidan Rinehart
    Publication date: 2021-02-23 15:20

     Fluid-structure interactions play a central role in an overwhelming number of physical phenomena. All fluid dynamic students are familiar with the common assumption of a "smooth boundary". While this assumption often is enough to provide a high level understanding of the dynamics and physics at hand in practice this is not true.  Much of the detail and the unique phenomena can be traced back to surface properties that deviate from this elementary assumption.  In this work we investigate three problems all motivated by the existence of non-smooth surfaces. 

     

     The first paper considers how inhomogeneous surfaces can generate a lift force for lubricated contacts. This work showcases how subtle changes to surface texture or chemistry modeled by a slip length can invoke non-trivial forces. These forces result in striking particle trajectories not possible in the presence of a smooth no-slip wall.  

     

     The next work focuses on porous surfaces. Often the geometry of surfaces in nature and industry are complex covering a wide range of length scales. Resolving all the scales of motion arising from fluid interaction with such surfaces are computationally expensive. Effective equations are often applied to reduce the cost of such simulations. The Brinkman equation is one common model choice for free-fluid and porous surface interface. Despite the common application of the Brinkman equation, fundamental questions about what the effective viscosity should be remain open. We compare pore-scale Stokes flow solutions to the Brinkman model for several porous surfaces. This study provides a scaling for the effective viscosity as well as error quantification of the Brinkman model.  

     

     Lastly, we investigate how porous surfaces modify a turbulent boundary layer. Streamwise preferential porous surfaces have recently been suggested as a surface modification that has the potential to reduce drag. We compare particle image velocimetry measurements with direct numerical simulations focusing on the near wall features that are modified from the canonical smooth wall case. We present some preliminary turbulent statistics and flow visualizations in the current report.

  • Hemicellulose-based wood adhesives

    Author: Tijana Todorovic
    Publication date: 2021-02-23 10:06

    The increasing sustainability concern encourages the society to use sustainable resources. In the wood industry the main component is from a renewable resource, but others are not. One of the fossil-based components is the adhesive. The wood adhesive industry is, besides the sustainability issue, facing environmental and health issues since most adhesives contain formaldehyde. Many biobased polymers show promising results as wood adhesives, and could therefore be used in the wood industry.

    Wood hemicelluloses that originate from the pulp industry as a sidestream could be used in the adhesive production, but their bonding performances need to be improved. When used alone, wood hemicelluloses do not have sufficient bond strength. On the other hand, locust bean gum which has similar structure to hemicelluloses but substantially higher molecular weight gives a significantly higher bond strength. The investigation of the molecular-weight effect on bonding properties shows that solely low molecular weight hemicelluloses do not show sufficiently high strength to be used as wood adhesives. Hemicellulose-like compounds with very high molecular weight, over 1,000 kDa, show higher strength, but their dispersions are very viscous, which limits the applicability and the penetration of the adhesive into the wood. A proper balance between adhesive properties, such as molecular weight, dry content and viscosity, and the application method, leads to an improved bond strength.

    When wood hemicelluloses are used together with high molecular weight amino-functional polymers, the bond strength is further increased. Additionally, the water resistance is significantly improved. The hypothesis is that these improvements are the results of synergetic effects of high molecular weight, electrostatic interactions and hydrogen bonding between the amino-functional polymer and the hemicelluloses in the adhesives, and between the adhesive and wood.

  • Online Dimensionality Reduction

    Author: Kaito Ariu
    Publication date: 2021-02-23 06:13
  • Creep modeling and first-principles investigation of high-temperature alloys

    Author: Jing Zhang
    Publication date: 2021-02-22 09:18

    Stainless steels and nickel-based superalloys are materials that have been widely used to manufacture components servicing at high temperatures. Creep strength is one of the most important properties in such conditions. High creep strength generally comes from a combination of solid solution hardening, precipitation hardening and dislocation hardening. However, some details of the mechanism of solid solution hardening are still not fully understood. The present thesis can be separated into two parts.

    In the first part, fundamental creep models are used in an investigation of creep rate of nickel-based alloys. The fundamental models are based on dislocation theories without using any adjustable parameters to describe the creep data. All parameters in the models have been derived from experimental data or using computational approaches such as ab initio methods. In the models, the effects of stacking faults, strain-induced vacancies and pipe diffusion have been taken into account. 

    W is a vital element to create solid solution hardening and improve creep strength of nickel-based alloys. W readily dissolves in nickel to form a solid solution and to provide a significant effect on the creep strength. Moreover, many ternary and more complex systems of nickel-based alloys are developed starting from Ni-W solid solutions. The developed models can describe the dramatic reduction of the creep rate due to W, which has not been possible in the past. The reduction has a close correlation with the stacking fault energy and drag stress.

    In the second part, the exact muffin-tin orbital method combined with the coherent potential approximation has been interfaced with a quasi-harmonic Debye model to predict the elastic and other thermodynamic properties of selected metallic alloys at high temperature. The knowledge of such properties is very useful in modeling the behavior of materials servicing at high temperatures. However, few experimental studies have been focused on measurements of thermo-mechanical properties at such temperatures. Ab initio methods based on density functional theory is an alternative way to obtain information about thermo-mechanical properties. 

    Therefore, in the present work, ab initio based studies of the elastic and thermodynamic properties of pure nickel, nickel-based solid solutions and Fe25Cr20NiMnNb austenitic stainless steel have been performed. Although the modeling technique cannot fully reproduce the temperature dependencies in some of the considered cases where experimental data are available, the computed values of such properties as shear moduli, thermal expansion coefficients and entropy are close to the experimentally derived values.

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