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Coming dissertations at TekNat

  • On numerical analyses of woven composite laminates : Homogenization, damage and fracture

    Author: Juan José Espadas Escalante
    Publication date: 2019-08-22 14:31

    This dissertation analyzes various mechanical properties of textile reinforced composite laminates.

    The dissertation is based on a total of six published works, which are essentially numerical, although experimental elements are available. The numerical methods used are based on high-resolution finite element models in combination with sophisticated phase-field theories for brittle fracture. A key result is that important mechanical properties in engineering applications, such as fracture or damage resistance, can be substantially affected by the arrangement of the constituent materials at the meso level.

  • Sphagnum limits : Physiology, morphology and climate

    Author: Charles Campbell
    Publication date: 2019-08-19 14:05

    Sphagnum is the most important plant genus in terms of terrestrial carbon cycling. It and the habitats it creates store an equivalent of ~68% of the CO2 in the atmosphere. The genus has little dispersal limitation and the mire habitats are functionally similar at global scales. Sphagnum species are limited by water deficit at local and biogeographic scales, but this alone is not sufficient to explain local and global scale species patterns. As Sphagnum shoots are long-lived they may be limited by stochastic periods of cold temperature. Within Europe, species are associated with climate gradients along north-south (cold-warm) and oceanic-continental (wet-dry) clines. Within mires, species are sorted along a moisture (hummock-hollow) gradient.

    In this thesis I examined species responses to and recovery from freezing (I). I compared species with different water level niches in traits related to water management of individual shoots and colonies (II). Using distribution modelling of GBIF data, I estimated how different aspects of climate contributed to Sphagnum species distributions in Europe (III). Combining the approaches in papers II and...

  • Genomics of population decline

    Author: Tom van der Valk
    Publication date: 2019-08-16 10:28

    With human populations forecasted to grow in the next decades, many mammals face increasing anthropogenic threats. The consequential population declines are a precursor to extinctions, as small populations are not only more sensitive to stochastic events, but reduction in population size is generally also followed by a decrease in genetic diversity, which in turn reduces adaptive potential and fitness of the population. By using molecular methods I aimed to estimate the magnitude of the genomic consequences as a result of rapid population declines with a focus on the endangered eastern gorillas. First, I genotyped Grauer’s gorilla (Gorilla beringei graueri) faecal samples, which revealed lower genetic diversity and high differentiation in the peripheral compared to the central populations, indicating a strong effect of genetic drift and limited gene flow among the small, isolated forest fragments (Chapter 1). Next, by using a target capture approach I obtained complete mitochondrial genomes from degraded Grauer’s and mountain (Gorilla beringei beringei) gorilla faecal and museum samples (Chapter 2) which showed a loss of...

  • Gene regulatory evolution in flycatchers: statistical approaches for the analysis of allele-specific expression

    Author: Mi Wang
    Publication date: 2019-08-13 14:43

    Understanding the molecular mechanisms underlying evolutionary changes in gene expression is a major research topic in biology. While a powerful approach to study this is the analysis of allele-specific expression (ASE), most of previously published methods can only be applied to lab organisms. In this thesis, to enable the analysis of ASE in natural organisms, I developed two methods for ASE detection. The first one was Bayesian negative binomial approach, and the second one was Read-backed Phasing-based ASE approach. Both methods performed well in simulations and comparisons. By applying those methods, I found that ASE was prevalent in natural flycatcher species. Combining the analyses of differential gene expression and ASE, I found a widespread cis-trans compensation and a critical role of tissue-specific regulatory mechanism during gene expression evolution. Moreover, for cis-regulatory sequences, there was a larger proportion of slightly deleterious mutations and weaker signatures of positive selection for genes with ASE than genes without ASE. For coding sequence, no such difference was observed. These results indicated that the evolution of gene expression and coding...

  • Solid-State Nanopores for Sensing : From Theory to Applications

    Author: Chenyu Wen
    Publication date: 2019-08-12 11:13

    Nanopore based sensing technology has been widely studied for a broad range of applications including DNA sequencing, protein profiling, metabolite molecules, and ions detection. The nanopore technology offers an unprecedented technological solution to meeting the demands of precision medicine on rapid, in-field, and low-cost biomolecule analysis. In general, nanopores are categorized in two families: solid-state nanopore (SSNP) and biological nanopore. The former is formed in a solid-state membrane made of SiNx, SiO2, silicon, graphene, MoS2, etc., while the latter represents natural protein ion-channels in cell membranes. Compared to biological pores, SSNPs are mechanically robust and their fabrication is compatible with traditional semiconductor processes, which may pave the way to their large-scale fabrication and high-density integration with standard control electronics. However, challenges remain for SSNPs, including poor stability, low repeatability, and relatively high background noise level. This thesis explores SSNPs from basic physical mechanisms to versatile applications, by entailing a balance between theory and experiment.

    The thesis...

  • Contacts and Interconnects for Germanium-based Monolithic 3D Integrated Circuits

    Author: Lukas Jablonka
    Publication date: 2019-07-15 11:30

    Three-dimensional integrated circuits have great potential for further increasing the number of transistors per area by stacking several device tiers on top of each other and without the need to continue the evermore complicated and expensive down-scaling of transistor dimensions. Among the different approaches towards the realization of such circuits, the monolithic approach, i.e. the tier-by-tier fabrication on a single substrate, is the most promising one in terms of integration density. Germanium is chosen as a substrate material instead of silicon in order to take advantage of its low fabrication temperatures as well as its high carrier mobilities. In this thesis, the work on two key components for the realization of such germanium-based three-dimensional integrated circuits is presented:the source/drain contacts to germanium the interconnects.

    As a potential source/drain contact material, nickel germanide is investigated.In particular, the process temperature windows for the fabrication of morphologically stable nickel germanide layers formed from initial nickel layers below 10 nm are identified and the reaction between nickel and germanium is further studied by...

  • The Extremes of Neutrino Astronomy : From Fermi Bubbles with IceCube to Ice Studies with ARIANNA

    Author: Elisabeth Unger
    Publication date: 2019-07-02 14:16

    The Fermi bubbles are extended regions of hard gamma-ray emission which were discovered with Fermi-LAT data to exist above and below the Galactic Center. In order to explain the origin of the gamma-rays, different theories are proposed. In particular, within hadronic models, highly-accelerated cosmic rays interact with interstellar matter and create the observed gamma-rays and in addition neutrinos. Data from the neutrino detector IceCube was analyzed using a maximum likelihood method. An upper limit on the possible neutrino flux from the Fermi bubbles at energies between 10 GeV and 200 GeV was determined.

    While this analysis is performed with the lowest energies IceCube can reach, the ARIANNA (Antarctic Ross Ice-shelf ANtenna Neutrino Array) experiment has the goal to detect the highest energy neutrinos by measuring radio wave radiation produced by their interaction products in the ice. With ARIANNA the propagation of radio waves in the firn (packed snow) of the Ross Ice Shelf was investigated. According to the classical approach the radio waves, produced in the firn, are supposed to bend down because of the changing density, and therefore changing refractive index, an...

  • Leveraging Existing Microarchitectural Structures to Improve First-Level Caching Efficiency

    Author: Ricardo Alves
    Publication date: 2019-06-11 13:49

    Low-latency data access is essential for performance. To achieve this, processors use fast first-level caches combined with out-of-order execution, to decrease and hide memory access latency respectively. While these approaches are effective for performance, they cost significant energy, leading to the development of many techniques that require designers to trade-off performance and efficiency.

    Way-prediction and filter caches are two of the most common strategies for improving first-level cache energy efficiency while still minimizing latency. They both have compromises as way-prediction trades off some latency for better energy efficiency, while filter caches trade off some energy efficiency for lower latency. However, these strategies are not mutually exclusive. By borrowing elements from both, and taking into account SRAM memory layout limitations, we proposed a novel MRU-L0 cache that mitigates many of their shortcomings while preserving their benefits. Moreover, while first-level caches are tightly integrated into the cpu pipeline, existing work on these techniques largely ignores the impact they have on instruction scheduling. We show that the variable hit latency...

  • Trans-crustal magma storage in contrasting tectonic settings

    Author: Harri Geiger
    Publication date: 2019-06-10 11:06

    Magmatic plumbing systems comprise magma chambers, sheet intrusions, and conduits which link the Earth’s deep interior with the Earth’s surface. As such, they are the structural framework of magma transport and storage that is governed by complex physical and chemical processes in magma reservoirs and through the interaction of magma bodies with surrounding crustal rocks over timescales from hours to millions of years. These geological processes, in turn, play a vital role in controlling eruptive behaviour and the magnitude of associated volcanic eruptions that impact the environment as well as human society. Our understanding of the nature and location of magmatic processes and plumbing system architecture remains, however, fragmentary. This lack of knowledge can partly be attributed to limits regarding the spatial resolution of geophysical methods and partly to geochemical uncertainties and errors in associated models. Ongoing advances in analytical techniques increase spatial, temporal, and chemical resolution, hence enabling us to gather more detailed knowledge on the structure and dynamics of magmatic systems, especially for individual volcanoes, but also in respect to the...

  • Electrochemical Characterizations of Conducting Redox Polymers : Electron Transport in PEDOT/Quinone Systems

    Author: Mia Sterby
    Publication date: 2019-06-10 10:28

    Organic electrode materials for rechargeable batteries have caught increasing attention since they can be used in new innovative applications such as flexible electronics and smart fabrics. They can provide safer and more environmentally friendly devices than traditional batteries made from metals. Conducting polymers constitute an interesting class of organic electrode materials that have been thoroughly studied for battery applications. They have high conductivity but are heavy relative to their energy storage ability and will hence form batteries with low weight capacity. Quinones, on the other hand, are low weight molecules that participate in electron transport in both animals and plants. They could provide batteries with high capacity but are easily dissolved in the electrolyte and have low conductivity. These two constituents can be combined into a conducting redox polymer that has both high conductivity and high capacity. In the present work, the conducting polymer PEDOT and the simplest quinone, benzoquinone, are covalently attached and form the conducting redox polymer used for most studies in this thesis. The charge transport mechanism is investigated by in situ...