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Now showing 1 - 9 of 9
  • (2021) Myekhlai, Munkhshur
    The electrochemical water splitting reaction, which consists of hydrogen reduction at the cathode and oxygen evolution (OER) at the anode, is one of the core processes for the utilization of sustainable and green energy sources. However, the sluggish kinetics of the oxygen evolution reaction requires a higher overpotential than the theoretical potential (1.23 V). Engineering a high-performance electrocatalyst is an avenue to improve the reaction kinetics for OER. Bimetallic branched nanoparticles offer substantial benefits for OER electrocatalysts; which include a greatly increased exposed surface area, highly crystalline hcp branches and stable surfaces. This thesis aims to design branched nanoparticles as electrocatalysts for enhanced OER in the following ways: (i) extending the cubic-core hexagonal-branch growth approach for a new bimetallic (Co, Au) system, (ii) leveraging the advantages of the Ru-Pd branched nanoparticles by tuning the surface facets and branch number, and (iii) making branched nanoparticles consisting of a cubic core (Pd) and alloyed branches (RuCo). Chapter one discusses the literature on the oxygen evolution reaction and Co- and Ru-based electrocatalysts for OER as well as the organic solution-phase synthesis method. The limiting factors of Co- and Ru- based catalysts and the strategies for improving their catalytic performance are also summarized. Also outlined is the fundamental understanding for synthesizing metallic nanoparticles using a seed-mediated growth approach in an organic solution phase and controlling the shape and size of the final products. Chapter two describes the synthetic methodology, sample purification, ink preparation for electrochemical measurements and characterization techniques in more detail. Chapter three provides the synthetic approaches and challenges in making Co and Ru branched nanoparticles. Chapter four compares the OER catalytic activity and stability of the Co-Au branched nanoparticles with the Co-Au core-shell and Co3O4 nanoparticles in alkaline media. The improved catalytic performance of the branched nanoparticles can be attributed to the formation of an active and stable oxide layer on the branch surface. Chapter five investigates the effect of branch number, and surface facets on the catalytic properties of the Ru-Pd branched nanoparticles with tunable branch number and surface facets. It is found that tuning surface facets and branch length is essential for enhancing catalytic performance by increasing the exposure of more active sites and improving the accessibility of the catalytic surface to the catalytic reaction. Chapter six explores alloyed branched nanoparticles consisting of a Pd core and RuCo branches and assesses their catalytic activity for OER electrocatalysts. It is demonstrated that Co leaching during catalytic activation in acid solution increases the exposure of highly catalytically active sites on the branch surface resulting in enhanced catalytic activity. Chapter seven concludes the overall results and achievements of this thesis and also discusses future opportunities.

  • (2021) Djuandhi, Lisa
    With a theoretical capacity of 1672 mA h g-1, more than five times higher than any commercially available lithium-ion (Li-ion) cell systems, the lithium-sulfur (Li-S) cell is an attractive candidate for next generation energy storage. Despite this high theoretical capacity, Li-S cells generally suffer from poor capacity retention and working lifetimes that prevent them from mass commercialisation. This is mainly due to current limitations in managing the inherent Li-S redox reactions which involve diffusion and migration of electrochemically active polysulfides. One approach to prevent polysulfide migration is by rational design of the sulfur electrode framework. The aim of this research is to investigate the electrochemical implications of using different frameworks for entrapment of redox active species, mainly designed for the Li-S cell system. The two types of frameworks investigated are: (1) mixed-morphology carbon feeds derived from waste sources wherein the intention is for the carbon to purely act as a structural framework to trap lithium polysulfides, and (2) sulfur-rich copolymers wherein redox active sulfur is covalently bound within the framework. More specifically, the goals involve determining: (1) whether carbon acts purely as a structural framework to trap redox active species during electrochemistry, and (2) whether sulfur-rich copolymers act purely as a sulfur feed. Achieving these goals requires a thorough understanding of what properties in each framework are ideal for the Li-S cell. The main conclusion drawn from this work is that neither of the materials studied behaved as pure structural or covalent frameworks partaking in various side processes. Using specialised techniques such as X-ray powder diffraction, solid-state NMR, and X-ray absorption near-edge structure spectroscopy, the beneficial and parasitic side processes involved in each framework are able to be determined. Overall, a significantly enhanced understanding of the Li S cell chemistry when using these materials is presented in this work.

  • (2021) Rathbone, Harry
    Photosynthesis has played a key role in the evolutionary trajectory of life on Earth. The ways in which organisms harvest sunlight have diversified over the billions of years since photosynthesis emerged in the quest for more efficient use of this energy source. The evolutionary origins of some organisms’ light harvesting apparatus, however, have remained elusive as have the causes for stark architectural changes between evolutionarily related organisms. In this thesis, I firstly provide a detailed exploration of published data describing photosynthetic efficiency through the lens of structural biology and quantum mechanics, examining observations from a range of antenna systems. After having built a framework for how an efficient photosynthetic antenna may be constructed, the rest of this thesis explores the evolutionary trajectory of the light harvesting antenna of the cryptophyte algae. Cryptophytes are a clade of secondary endosymbiotic algae which gained their photosynthetic chloroplasts from an engulfed red alga, but produced in an architecturally distinct antenna. Red algae have an antenna comprised of stacked protein rings that form an energetic funnel to the photosynthetic reaction centre which generates chemical energy from photon excitations. Cryptophytes took this energy funnel and dismantled it; complexing one of its component proteins with a peptide of unknown origin (‘cryptophyte alpha’) and packing them at high density within the chloroplast. By examining recently published cryo-electron microscopy maps of red algal antennas, I have discovered the evolutionary ancestor of the unique cryptophyte alpha subunit. Through this discovery, I reveal possible evolutionary events following secondary endosymbiosis leading to the origin of the cryptophyte light harvesting system. Finally, I examine the light harvesting antenna of a particular cryptophyte species, Hemiselmis andersenii, isolating multiple protein components and determining their crystal structures at high resolution. Through this, I discover a more complex antenna than previously thought with multiple protein components and a rich energetic structure. Some of these antenna proteins show previously unrecognised spectral properties and chromophore architecture. This structural data aids in understanding the architectural change between the red algal and cryptophyte light harvesting antennas and further diversification within the cryptophyte clade.

  • (2021) Kong, Scarlet
    Piezoelectric single crystals provide a large electro-mechanical response that is desired for sensor and actuator applications. However, the complexity and cost of single crystal growth inhibits their widespread use. Alternatively, polycrystalline piezoelectrics can be easily fabricated at low cost. Due to the anisotropic piezoelectric response, the electro-mechanical response of polycrystals is largely limited by the elastic coupling of randomly aligned grains. Microstructural engineering via crystallographic texturing of polycrystalline ceramics offers an alternative method that both enhances the electro-mechanical response while maintaining an economical fabrication process. There is currently limited understanding on how the changes in the microstructure from the texturing process, such as the addition of heterogenous templates and changes in grain orientation distribution, impacts the electro-mechanical response mechanisms. Understanding the structural and electro-mechanical mechanism that enhances the piezoelectric response will benefit sensor and actuator applications such as sonar systems and medical imaging instruments. In this thesis, a range of techniques were used to study the microstructural changes and the strain mechanisms from crystallographic texturing, and its impact on the electro-mechanical response. The local response of electro-mechanical phase-change ceramics was investigated by theoretical calculations to see how texturing changed the strain heterogeneity at the grain-scale. Textured ceramics of Pb(Mg1/3Nb2/3)O3− x%PbTiO3 (PMN-PT) were fabricated by tape casting and templated grain growth, using BaTiO3 platelet templates. The structure of these textured ceramics was characterised using synchrotron x-ray diffraction to gain a more comprehensive understanding on the texture that develops. In situ electric-field dependent diffraction measurements were then used to study how texture affects the electro-mechanical response mechanism. And lastly, the piezoelectric non-linearity and response stability over time was analysed in the direct piezoelectric response mode. Changes in electric-field-induced phase transformation local strain response in polycrystalline piezoelectrics of varying grain orientation distribution was studied through theoretical calculations. The March-Dollase function was used to generate texture distributions in a model polycrystal, and strains associated with phase transitions from pseudo-cubic to tetragonal, rhombohedral and orthorhombic symmetries were calculated. In the tetragonal system, the overall strain response was improved by 60% with crystallographic texture, at very strong texture (of March Dollase distribution r = 0.05); and the local strain heterogeneity was similar to the random system. However, moderate increases in texture had a negative impact on the strain heterogeneity. In crystallographic symmetries with higher numbers of possible polarization (and thus spontaneous strain) directions, the magnitude of the strain response increased, and the heterogeneity of the system decreased. Using high energy x-ray diffraction and in situ electric-field dependent measurements, the microstructural modifications due to crystallographic texturing and the resulting electro-mechanical response mechanisms were investigated. Structural analysis of textured ceramics, by calculating the orientation distribution function (ODF), showed that sheet-like texture developed in the piezoelectric. Texture characterisation, by calculating the ODF, was able to provide a more comprehensive quantification of texture than the Lotgering factor, where multiple samples with a 95% Lotgering factor showed different ODFs and maximum Multiples of Random Distribution (MRD) values of 12.8 and 16.54. In addition to the crystallographic texture observed in the diffraction pattern, a ferroelastic texture was also seen. Termed the ‘self-poling effect’, this unique structural distortion reduced the remanent strain of the textured piezoelectric to 0.13% after poling to 2 kV/mm, while the random (untextured) ceramic produced a remanent strain of 0.18% after poling to 2 kV/mm. On a subsequent unipolar strain cycle, however, the textured ceramic achieved a larger response of 0.16% compared to 0.096% of random ceramic. A new strain mechanism model was proposed to explain the relationship between the self-poling effect and the observed strain behaviour. Finally, the direct piezoelectric response in textured ceramics was measured to understand the piezoelectric non-linearity and response stability under application-like conditions. By applying a static offset and dynamic sinusoidal compressive loading, the d33 and d32 response were measured. The piezoelectric non-linear behaviour increased with crystallographic texturing. In particular, the piezoelectric property in the d33 mode, deteriorated faster and was more unstable, losing around 60% of its original d33 value once the offset load was released. On the other hand, the d32 response of texture ceramics was able to recover 90% of its original response after loading. Furthermore, the stability of the piezoelectric response in textured piezoelectric was observed to be compositionally dependent, where the d32 response decayed over time faster in PMN-31PT than in PMN-32PT. The results from this thesis shows that crystallographic texturing alters the local strain environment, affecting the electro-mechanical response mechanisms, both enhancing the response but also potentially degrading the expected lifetime and performance of these ceramics. Further work in understanding the microstructural interaction with the electro-mechanical response will help to optimise texture fabrication of piezoelectric materials for industrial applicability.

  • (2021) Gnanasambandapillai, Vikkitharan
    “The enjoyment of the highest attainable standard of health is one of the fundamental rights of every human being without distinction of race, religion, political belief, economic or social condition” [56]. Genomics (the study of the entire DNA) provides such a standard of health for people with rare diseases and helps control the spread of pandemics. Still, millions of human beings are unable to access genomics due to its cost, and portability. In genomics, DNA sequencers digitise DNA information, and computers analyse the digitised information. We have desktop and thumb-sized DNA sequencers, that digitise the DNA data rapidly. But computations necessary for the analysis of this data are inevitably performed on high-performance computers (HPCs) and cloud computers. These computations not only require powerful computers but also necessitate high-speed networks since the data generated are in the hundreds of gigabytes. Relying on HPCs and high-speed networks, deny the benefits that can be reaped by genomics for the masses who live in remote areas and in poorer nations. Developing a low-cost and portable genomics computation platform would provide personalised treatment based on an individual’s DNA and identify the source of the fast-spreading epidemics in remote areas and areas without HPC or network infrastructure. But developing a low-cost and portable genome analysing computing platform is a challenging task. This thesis develops novel computer architecture solutions to assemble the whole human DNA and COVID-19 virus RNA on a low-cost and portable platform. The first phase of the solution describes a ring-pipelined processor architecture for a key genome assembly algorithm. The human genome is partitioned to fit into the small memory footprint of embedded processors. These techniques allow an entire human genome to be assembled using highly portable and low-cost embedded processor cores. These processor cores can be housed within a single chip. Each processor was only 0.08 mm 2 and consumed just 37.5 mW. It has only 2 GB memory, 32-bit instruction width, and a clock with a 1 GHz frequency. The second phase of the solution describes how application-specific instruction-set processors can be sped up to execute a key genome assembly algorithm. A fully automated design system is presented, which improves the performance of large applications (such as genome assembly algorithm) and generates application-specific instructions for a commercial processor design tool (Xtensa). The tool enhances the base processor, which was used in the ring pipeline processor architecture. Thus, the alignment algorithms execute 2.1 times faster with only 11% additional hardware. The energy-delay product was reduced by 7.3× compared to the base processor. This tool is the only one of its type which can handle applications which are large. The third phase of the solution designs a portable low-cost genome assembly computer (PGA). PGA enhances the ring pipeline architecture with the customised processor found in phase two and with improved inter-processor communication. The results show that the COVID-19 virus RNA can be assembled in under 10 minutes and the whole human genome can be assembled in 11 days on a portable platform (HPC take around two days) for 30× coverage. PGA has an area footprint of just 5.68 mm 2 in a 28 nm technology node and is far smaller than a high-performance computer processor chip. The PGA consumes only 4W of power, which is lower than the power requirement of a high-performance processor chip. The manufacturing cost of the PGA also would be much cheaper than the high-performance system cost, when produced in volume. The developed solution can be powered by a USB port of a laptop. This thesis is the first of its type to show the design of a single-chip solution to be able to process a complex genomic problem. This thesis contributes to attaining one of the fundamental rights of every human being wherever they may live.

  • (2021) Badami, Maisie
    This research explores and investigates strategies towards automation of the systematic literature review (SLR) process. SLR is a valuable research method that follows a comprehensive, transparent, and reproducible research methodology. SLRs are at the heart of evidence-based research in various research domains, from healthcare to software engineering. They allow researchers to systematically collect and integrate empirical evidence in response to a focused research question, setting the foundation for future research. SLRs are also beneficial to researchers in learning about the state of the art of research and enriching their knowledge of a topic of research. Given their demonstrated value, SLRs are becoming an increasingly popular type of publication in different disciplines. Despite the valuable contributions of SLRs to science, performing timely, reliable, comprehensive, and unbiased SLRs is a challenging endeavour. With the rapid growth in primary research published every year, SLRs might fail to provide complete coverage of existing evidence and even end up being outdated by the time of publication. These challenges have sparked motivation and discussion in research communities to explore automation techniques to support the SLR process. In investigating automatic methods for supporting the systematic review process, this thesis develops three main areas. First, by conducting a systematic literature review, we found the state of the art of automation techniques that are employed to facilitate the systematic review process. Then, in the second study, we identified the real challenges researchers face when conducting SLRs, through an empirical study. Moreover, we distinguished solutions that help researchers to overcome these challenges. We also identified the researchers' concerns regarding adopting automation techniques in SLR practice. Finally, in the third study, we leveraged the findings of our previous studies to investigate a solution to facilitate the SLR search process. We evaluated our proposed method by running some experiments.

  • (2021) Jia, Hong
    To enable mobile devices to perform in-the-wild sports analytics, particularly swing tracking, remains an open question. A crucial challenge is to develop robust methods that can operate across various sports (e.g., golf and tennis), different sensors (cameras and IMU), and diverse human users. Traditional approaches typically rely on vision-based or IMU-based methods to extract key points from subjects in order to estimate trajectory predictions. However, these methods struggle to generate accurate swing tracking, as vision-based techniques are susceptible to occlusion, and IMU sensors are notorious for accumulated errors. In this thesis, we propose several innovative solutions by leveraging AIoT, including the IoT with ubiquitous wearable devices such as smartphones and smart wristbands, and harnessing the power of AI such as deep neural networks, to achieve ubiquitous sports analytics. We make three main technical contributions: a tailored deep neural network design, network model automatic search, and model domain adaptation to address the problem of heterogeneity among devices, human subjects, and sports for ubiquitous sports analytics. In Chapter 2, we begin with the design of a prototype that combines IMU and depth sensor fusion, along with a tailored deep neural network, to address the occlusion problems faced by depth sensors during swings. To recover swing trajectories with fine-grained details, we propose a CNN-LSTM architecture that learns multi-modalities within depth and IMU sensor fusion. In Chapter 3, we develop a framework to reduce the overhead of model design for new devices, sports, and human users. By designing a regression-based stochastic NAS method, we improve swing-tracking algorithms through automatic model generation. We also extend our studies to include unseen human users, sensor devices, and sports. Leveraging a domain adaptation method, we propose a framework that eliminates the need for tedious training data collection and labeling for new users, devices, and sports via adversarial learning. In Chapter 4, we present a framework to alleviate the model parameter selection process in NAS, as introduced in Chapter 3. By employing zero-cost proxies, we search for the optimal swing tracking architecture without training, in a significantly larger candidate model pool. We demonstrate that the proposed method outperforms state-of-the-art approaches in swing tracking, as well as in adapting to different subjects, sports, and devices. Overall, this thesis develops a series of innovative machine learning algorithms to enable ubiquitous IoT wearable devices to perform accurate swing analytics (e.g., tracking, analysis, and assessment) in real-world conditions.

  • (2021) Cai, Lin
    This thesis consists of three chapters that investigate the linkage between uncertainty and corporate investment decisions on an international basis. In first chapter, I investigate the extent of U.S. policy-related spillovers into 22 other real economies. I find that, after accounting for factors previously used to explain corporate investment, US Economic Policy Uncertainty (US EPU, hereafter) fluctuations affect foreign corporate investments through two channels. First, the single effect of US EPU on international corporate investment shows an unequivocal negative relation (the direct channel). Second, an increase in US EPU also attenuates the negative sensitivity of corporate investment towards the cost of capital (the indirect channel). Further, I find that while the direct channel of US EPU on corporate investment persists across several subsamples, its indirect channel relates to a high degree of dependence on the U.S. economy and opacity exhibited by local economies. The second chapter reconciles the contrary views on the foreign investors using local disaster shocks from 46 countries over the period 1998-2018. I find that local disaster shocks cause significant disruptions to corporate investments, but foreign institutional investors attenuate the costs of disaster risks. The benefits associated with foreign institutional investors are not uniformly held across all economies, where the role of foreign institutional investors is particularly measurable in countries with well-developed institutional environment. The third chapter focuses on the uncertainty at domestic level using national elections across 23 different countries. I find that the corporate investment cycle corresponds with the timing of national elections, but there is a cross-sectional difference in the firm-level investment sensitivity to elections. During election periods, while firms temporarily reduce investment expenditures relative to nonelection years, the decline is mainly sourced from firms with greater political exposures. Further, I find that the investment cycles are more volatile when the election outcomes are uncertain, and the institutional environments are weaker.

  • (2021) Webb, David
    Planetary waves play a role in a large variety of oceanic and climate dynamics. In particular, Kelvin waves can provide rapid teleconnections from large-scale climate and weather events to remote regions of the globe. Kelvin waves may be partially responsible for linking climatic changes in Southern Ocean winds to increases in subsurface warming around Antarctica that can lead to glacial ice-melt and increases in global sea level rise. Kelvin waves may also link changes in Southern Ocean winds to increases in North Atlantic Deep Water (NADW) formation and an enhancement of the Atlantic Meridional Overturning Circulation (AMOC), which is responsible for circulating a vast amount of the ocean’s heat and nutrient content. However, the exact role of Kelvin waves in these processes is unclear. This thesis aims to further clarify the role that Kelvin waves play in these high-latitude climate processes. First, we use a suite of idealized models in order to better understand the dynamics of barotropic Kelvin waves around Antarctica. We find that super-inertial (high frequency) barotropic Kelvin waves are nearly completely scattered away from the Antarctic coastline due to a combination of coastal geometry and bathymetry. Sub-inertial (low frequency) barotropic Kelvin waves are mostly scattered away from the Antarctic coastline due to bathymetry, however a significant amount of barotropic Kelvin wave energy remains at the Antarctic coastline after one circumnavigation of the continent, enabling a gradual build-up of energy along the coast and the ability to sustain a barotropic Kelvin wave signal around Antarctica over time. Secondly, we perform a diagnostic study using theory and a range of varying resolution model simulations to quantify the amount of subsurface warming along the West Antarctic Peninsula caused by barotropic Kelvin waves via an induced bottom Ekman flow that advects warm Circumpolar Deep Water onto the Antarctic continental shelf. We find that barotropic Kelvin waves can account for a substantial amount of warming within one year, depending on the background temperature gradients and thickness of the bottom Ekman layer. Lastly, we explore the role of Kelvin waves in linking Southern Ocean wind-stress to NADW formation and the AMOC by analysing ensemble simulations from a fully-coupled ocean-sea-ice model at 1/4 degree horizontal resolution (50 vertical levels). We find first mode baroclinic Kelvin waves to propagate along a global coastal and equatorial waveguide from the Southern Ocean forcing region to the North Atlantic, where downwelling waves initiate an enhancement of the AMOC by making surface waters denser.