Science

Publication Search Results

Now showing 1 - 10 of 22
  • (2022) Zhao, Runqing
    Thesis
    Emerging modes of air transport such as autonomous airport shuttle and air taxi are potentially efficient alternatives to current transport practices such as bus and train. This thesis examines bus shuttle service within an airport and air metro as two examples of network design. Within an airport, the bus shuttle serves passengers between the terminals, train stations, parking lots, hotels, and shopping areas. Air metro is a type of pre-planned service in urban air mobility that accommodates passengers for intra- or inter-city trips. The problems are to optimise the service, and the outputs including the optimal fleet size, dispatch pattern and schedule. Based on the proposed time-space networks, the service network design problems are formulated as mixed integer linear programs. The heterogeneous multi-type bus fleet case and stochastic demand case are extended for the airport shuttle case, while a rolling horizon optimisation is adopted for the air metro case. In the autonomous airport inter-terminal bus shuttle case, a Monte Carlo simulation-based approach is proposed to solve the case with demand stochasticity, which is then further embedded into an "effective" passenger demand framework. The "effective" demand is the summation of mean demand value and a safety margin. By comparing the proposed airport shuttle service to the current one, it is found that the proposed service can save approximately 27% of the total system cost. The results for stochastic problem suggest estimating the safety margin to be 0.3675 times of the standard deviation brings the best performance. For the second case, the service network design is extended with a pilot scheduling layer and simulation is undertaken to compare the autonomous (pilot-less) and piloted service design. The results suggest that an autonomous air metro service would be preferable if the price of an autonomous aircraft is less than 1.6 times the price of a human-driven one. The results for rolling horizon optimisation suggest to confirm the actual demand at least 45 minutes prior to departure. Based on data from the Sydney (Australia) region, the thesis provides information directly relevant for the service network design of emerging modes of air transport in the city.

  • (2022) Boldbaatar, Jigmidmaa
    Thesis
    Sexual conflict could play an important role in shaping spatial variation in sex ratio in facultatively parthenogenetic species. This is because if females of such species avoid mating, they can establish all-female populations via parthenogenetic reproduction. By contrast, if females cannot avoid mating (i.e., males), they reproduce offspring of both sexes and establish mixed-sex populations in the wild. In Australia, natural populations of Megacrania batesii exhibit extreme spatial variation in sex ratio, with all-female and mixed-sex populations occurring over a small scale. However, it is unclear how facultatively parthenogenetic females avoid mating and establish all-female populations. Sex-specific patterns of dispersal could play a role because, depending on whether females are mated or unmated, they could establish mixed-sex and all-female populations if they successfully disperse to new areas. However, if males invade all-female populations, they could convert these populations into mixed-sex populations via mating. In chapter one, I therefore investigated dispersal rate of females and males from a mixed-sex population and single females from an all-female population. I found that females from the all-female population were less dispersive than females and males from the mixed-sex population. I also found that dispersal was limited to movement within habitat patches. Additionally, in chapter two, I examined whether costs associated with mate guarding (which is widespread and long-lasting in M. batesii) imposes energetic costs on females (i.e., reduction in foraging rate) because guarded females carry males. If females’ foraging rates were impacted by guarding males, mate guarding could be a manifestation of sexual conflict in M. batesii. However, I found little evidence that laboratory-reared females from mixed-sex populations fed more while unpaired than while paired with males. Overall, these studies contribute insights into how sex-specific dispersal and costs of mating and guarding might contribute to sexual conflict and variation in sex ratio in natural populations of the facultative parthenogenetic species, M. batesii.

  • (2022) Zhang, Jun Ze
    Thesis
    This thesis investigates some properties of complex structures on Lie algebras. In particular, we focus on nilpotent complex structures that are characterized by a suitable J-invariant ascending or descending central series dj and dj respectively. In this thesis, we introduce a new descending series pj and use it to give proof of a new characterization of nilpotent complex structures. We examine also whether nilpotent complex structures on stratified Lie algebras preserve the strata. We find that there exists a J-invariant stratification on a step 2 nilpotent Lie algebra with a complex structure.

  • (2022) Chou, Yu-Hsiang
    Thesis
    The traditional ITO-based transparent conductive films and electrodes have been widely applied in many fields and used in various electronics in the past decades. However, its brittle property and costly manufacturing process limit the development of ITO for next-generation electronic devices, which need the features of light weight, flexibility, and low cost. Silver nanowires have attracted considerable attention from researchers among many candidates due to their outstanding electrical, mechanical, and optical properties. Many synthesis methods of AgNWs have been demonstrated recently, but there are still some parameters that are unclear and need further investigations. In this thesis, one-step solvothermal synthesis of AgNWs has been studied and explored. AgNWs with a high aspect ratio (~2000) were successfully obtained by using this method. The morphology of AgNWs was significantly affected when tuning the different factors, including the heating temperature of PVP dissolution, the molar ratio of PVP/AgNO3, the molecular rate of PVP, and the concentration of ionic assistants. Moreover, another latest modified polyol method, the Maillard reaction process, has also been introduced and researched in this thesis. The ultra-long AgNWs (~100 μm) were successfully generated through the Millard reaction method using ammonium chloride and glucose as reacting agents of MRPs. Effects of experimental parameters such as reaction time and pressure environment on the morphology of AgNWs have also been investigated. Besides, the extra salt additive (NaBr) was employed to see if the NW diameter would become thinner to achieve the high aspect ratio AgNWs. The NW diameter was greatly reduced when the molar ratio of NH4Cl/ NaBr was at 40:1; however, a large amount of Ag nanoparticles were generated, which could significantly affect the overall performance of AgNWs. The AgNWs-based TCFs were fabricated in this project. With the help of annealing post-treatment, TCFs with low sheet resistance (~22 Ω/sq) and high transmittance (86%) were prepared using the high aspect ratio AgNWs. In addition, the AgNWs were applied to the electrochemical reduction of CO2 to syngas and showed the potential of syngas production. Nevertheless, more factors and studies need to be carried out to improve the Faradic efficiency of syngas production.

  • (2022) Sherow, Brie
    Thesis
    Anthropogenic debris has been documented in all habitats of the ocean that have been explored by humankind. In order to reduce the flow of debris to the ocean we must understand its sources, pathways, and distribution. The distribution of debris is determined by how it enters the marine environment and where it accumulates. Urban debris often enters the marine ecosystem via stormwater networks or is deposited during recreational activities such as fishing and boating. Here we show that debris abundance is higher under piers than adjacent soft sediment benthic habitats and appears to be related to the type of activity occurring at piers, specifically recreational fishing. Subtidal benthic habitats recorded high amounts of fishing related debris items, particularly under piers in the outer harbour. Land use and intensity of activity drive debris type and abundance entering stormwater drains. The onset of the COVID-19 pandemic allowed us to investigate changes in debris type and abundance entering drains during government-mandated lockdown periods, when human activity was vastly reduced in urban areas. We found that single-use plastics and cigarette butts were prevalent debris items in areas of high human activity, such as city centres, transport hubs, and shopping centres, and that lockdown measures impacted debris abundance in these zones. Debris abundance in industrial zones was less impacted by lockdown measures, and microplastics were abundant in these zones both before and during COVID-19 lockdowns. Our results demonstrate that marine debris reduction and remediation efforts must be targeted to the type and intensity of activity at the source of debris. For example, source reduction strategies are needed in industrial zones to manage microplastics escape into stormwater drains. Problematic debris types, such as cigarettes and single-use plastics, necessitate targeted management strategies in areas with high foot traffic, and recreational fishing debris can be targeted at piers in urbanised estuaries. Successful efforts to manage marine debris must incorporate broadscale policy changes and individual action. We anticipate these studies will inform public and private sector management interventions, and educational or awareness campaigns at a societal level.

  • (2022) Han, Bruce
    Thesis
    The Hawkes process was first proposed by Alan G. Hawkes in which the arrival of events exhibits a self-exciting behaviour. One extension of the classical Hawkes process is the renewal Hawkes process, which allows the underlying process for background events to be a renewal process, rather than the homogeneous Poisson process in the classical Hawkes process. The renewal Hawkes process is stationary in nature, so it is not suitable in situations where there are systematic trends in event occurrence rate. Therefore, in this thesis, we propose a renewal Hawkes process in which a trend function is employed to account for the systematic patterns in the event occurrence rate. We term the process the modulated renewal Hawkes process. Due to the lack of an explicit expression for the intensity process, likelihood evaluation for the modulated renewal Hawkes process model is not trivial. However, by modifying the likelihood evaluation algorithm for renewal Hawkes process in Chen & Stindl (2018), we are able to propose an algorithm to evaluate the exact likelihood of the modulated renewal Hawkes process model. The evaluated likelihood can then be maximised to obtain the maximum likelihood estimator (MLE) of the model parameters. We also propose a method to obtain fast and accurate approximations to the likelihood. In the case where a suitable parametric form of the trend function is not available, we approximate the trend function using B-spline functions. We also derive the Rosenblatt residuals of the modulated renewal Hawkes process, which can serve as a basis for assessing the goodness-of-fit of the model. Simulation experiments were conducted to assess the performance of the MLE of the modulated renewal Hawkes process with either exact or approximate likelihood evaluation, both in the parametric model and in the semiparametric model with an unspecified trend function. We also present an application of the modulated renewal Hawkes process model to the analysis of cryptocurrency data. The modulated renewal Hawkes process model with a B-spline trend function is applied to model extreme intraday negative returns on several cryptocurrencies. The estimated trend function suggests an inverse U-shaped trend in the intraday occurrence times of extreme negative returns on cryptocurrencies. We also compared the model fitting results with several simpler models, such as the nonstationary Hawkes process and the renewal Hawkes process. On most of the cryptocurrency data sets considered in this work, the modulated renewal Hawkes process was found to provide the best fit both by the Rosenblatt residuals based goodness-of-fit check and by the Akaike Information Criterion.

  • (2022) Dou, Zehao
    Thesis
    Resistive random-access memory (RRAM) is a kind of highly promising non-volatile memory technology. Recently, halide perovskites have aroused attention worldwide because of their outstanding resistive switching performance and ease of fabrication. The advantages of the halide perovskite devices include high ON/OFF ratio and low operation voltage, enabling excellent device performance with low power consumption. Currently, the most widely studied halide perovskites contain lead, which is a toxic element that may incur serious environmental problems and significant harm to human health. In order to address these issues, there is a pressing need to develop lead-free halide perovskites and their derivatives possessing comparable functional properties to their lead-based counterparts. Bismuth-based halide perovskites have emerged as a promising lead-free alternative for applications in RRAM. A great advantage of bismuth-based halide perovskites lies in their high solubility for various elements, thus offering the possibility of the formation of modified compositions to tailor the resistive switching behaviours including ON/OFF ratio, endurance and retention. Cs3Bi2I9 and MA3Bi2I9 (MA = methylammonium) are two common lead-free perovskite halides that have been widely studied for RRAM. However, doping in Cs3Bi2I9 and MA3Bi2I9 is normally conducted on a single chemical site (either A-site or X site) and the impact of co-doping on their resistive switching properties remains less explored. In this project, thin films of several co-doped compositions namely MA2CsBi2BrxI9-x (x=2, 3, 4, 5, 6, 7, 8) were prepared to investigate the double doping (Cs on A-site, Br on X-site) effects on their structural, morphological and electrical properties. In addition, the effect of different top electrodes (Ag and Au) on the electrical performance of the MA2CsBi2BrxI9-x thin films was also studied. It was found that more uniform and denser thin films could be obtained with an increase in Br content. Among the several compositions under investigation, MA2CsBi2Br8I-based thin film with Au top electrodes exhibited typical resistive switching behaviour and an interface-type conduction mechanism. When the perovskites layer was covered by Ag top electrodes, the distinct resistive switching behaviour could be observed with the increase of I content, which could be attributed to the redox reaction of Ag electrodes and iodide ions at the interface between electrodes and the active layer. Compared to other compositions, MA2CsBi2Br2I7-based thin film with Ag electrodes exhibited an outstanding ON/OFF ratio of around 105. Since the MA2CsBi2Br8I perovskite had good endurance and full-coverage surface, the MA2CsBi2Br8I perovskite was employed for further study. Au/MA2CsBi2Br8I/ITO devices with different thicknesses (290 nm, 307 nm, 341 nm and 435 nm) showed stable bipolar resistive switching behaviours. With the increasing thickness, the SET electric field remains around 6.5 V/μm, which is nearly independent of film thickness. When the thickness of the MA2CsBi2Br8I perovskite layer increased from 136 nm to 307 nm, the device demonstrated better stability over 100 cycles and a higher ON/OFF ratio (~10) at a low reading voltage of 0.27 V.

  • (2022) Ross, James
    Thesis
    Graphs are combinatorial objects commonly used to model relationships between pairs of entities. Hypergraphs are a generalization of graphs in which edges connect an arbitrary number of vertices. We consider hypergraphs in which each edge has size k, each vertex has a degree specified by a degree sequence d, and all edges are unique. These are known as simple k-uniform hypergraphs with degree sequence d. We focus on algorithms for sampling these hypergraphs, particularly when the degree sequence is approximately regular and sufficiently sparse. The goal is an algorithm which produces approximately uniform output with expected running time that is polynomial in the number of vertices. We first discuss an algorithm for this problem which used a rejection sampling approach and a black-box bipartite graph sampler. This algorithm was presented in a paper by myself and co-authors: my specific contributions to the publication are described. As a new contribution (not contained in the paper), the rejection sampling approach is extended to give an algorithm for sampling linear hypergraphs, which are hypergraphs in which no two distinct edges share more than one common vertex. We also define and analyse an algorithm for sampling simple k-uniform hypergraphs with degree sequence d. Our algorithm uses a black-box sampler A for producing (possibly non-simple) hypergraphs and a ‘switchings’ process to remove any repeated edges from the hypergraph. This analysis additionally produces explicit tail bounds for the number and multiplicity of repeated edges in uniformly distributed random hypergraphs, under certain conditions for d and k. We show that our algorithm is asymptotically approximately uniform and has an expected running time that is polynomial in the number of vertices for a large range of degree sequences d, provided d is near-regular. This extends the range of degree sequences for which efficient sampling schemes are known.

  • (2022) Xiao, Yang
    Thesis
    The urgency of developing sustainable and renewable energy sources has been heightened by limited fossil fuel reserves as well as environmental concerns. Hydrogen is an efficient alternative to traditional fossil fuels because of its clean combustion emissions and high energy density. Water splitting is a clean method for producing green hydrogen but is limited by the slow reaction kinetics of oxygen evolution reaction (OER). In recent years, multi-metallic electrocatalysts with high activity and minimum energy consumption have been developed for efficient OER. The objective of this thesis is to take advantage of the spontaneous corrosion electrochemistry to make nonprecious multi-metallic hydroxides for efficient oxygen evolution reaction and evaluate their performance in industrial alkaline water electrolysis. First, oxygen- and sulfate-mediated corrosion engineering were developed by using macroporous iron foam to produce Cr-doped FeNi and FeCo ternary hydroxides. This strategy was successfully achieved to modulate and accelerate the in-situ phase transition of the Ni-O species to the oxyhydroxide active phase which results in high surface-intermediate interactions and intrinsic electrocatalytic activity. The promising long-term OER stability, reproducible performance at high current densities indicates the great potential of designing efficient polymetallic electrocatalysts for water splitting technology through corrosion engineering. In addition, the long-term stability of FeNiCr under industrially relevant conditions was further investigated. It was found that the doping of Cr reduced the OER overpotential of the FeNi binary system, however, at the same time increased the sensitivity of the FeNi binary system to high pH and temperature. After characterization and testing before and after the OER reaction, it was found that the causes of catalyst deactivation include dissolution of the active material at high potentials and high pH values, surface morphology changes, phase transitions, and physical detachment. The results obtained in this thesis demonstrate that corrosion engineering and Cr doping are efficient strategies to design nonprecious multi-metallic electrocatalysts for OER. The deactivation mechanism of FeNiCr ternary catalysts under industrial conditions suggests that high pH and temperature are important factors for the evaluation of OER catalysts for industrial applications.

  • (2022) Loo, Daniel
    Thesis
    Prior to the emergence of cellular life, there must have been an abiotic analogue to the cell -- also known as the protocell. To flourish, they should have performed key functions enabling growth, and ensuring their survival and propagation. While researchers study a variety of model protocell systems, the study of model protocells assembled from fatty acid vesicles is lacking in comparison to the abundant literature on phospholipid vesicles. There is evidence that the existence of fatty acids predated life on Earth. Moreover, the highly dynamic nature of fatty acid vesicles is not well documented in the origins of life context. There is a need to explore the behaviour of fatty acid vesicles in systems which mimic real world environments. I explored the flocculation behaviour of vesicles in environments of mechanical stress, monovalent salt and particles including minerals. I also explored the effect of RNA-relevant divalent cations on fatty acid vesicles, as well as changes due to pH fluctuation. The findings of this project suggest that fatty acid vesicles are able to flocculate under quite low concentrations of monovalent salt, and that such flocs can incorporate particles including mineral particles which are relevant for catalysing RNA-related reactions. Furthermore, I found that after vesicles have been formed, they are surprisingly robust in conditions that inhibit vesicle formation. These results provide evidence that protocells assembled from simple building blocks such as fatty acids are quite robust and may persist in challenging environments, and that their flocculation behaviour may have co-localised other processes pertaining to the origins of life.