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  • (2023) Dela Cruz, Michael Leo
    Thesis
    Biodegradable implant materials are more appropriate for temporary support applications compared with their inert counterparts since the former requires no removal surgery because they naturally degrade and eventually dissolve completely during healing. Iron and its alloys are a possible substitute for the commercial magnesium biodegradable implants because of their superior mechanical properties and slower corrosion rates. The addition of manganese and silicon in iron imparts another interesting property to the material–the shape memory effect. There is copious research on the structure and properties of the biodegradable face centred cubic (FCC) Fe-30Mn-6Si shape memory alloy (SMA) that exhibits the reversible FCC austenite to hexagonal close packed (HCP) ε-martensite transformation. However, recent advances in additive manufacturing of metals, brought by the development of the laser powder bed fusion (LPBF) technique, warrant the need for an investigation on the adaptability of the technique in fabricating this alloy composition. The LPBF technique is limited by the need for specialty raw material powder, and this thesis extends the application of the technique in fabricating the Fe-30Mn-6Si shape memory alloy (SMA) from homogenised powder precursors. More so, LPBF processing of Fe-30Mn-6Si alloy from either pre-alloyed powder or blended powder has not been reported. To successfully fabricate a Fe-30Mn-6Si LPBF product, the influence of key LPBF processing parameters on product quality was identified as a major challenge. This was addressed by investigating the influence of laser power, laser scan speed, laser re-scanning, and their equivalent input energy on the relative density and defect formation. A relative density of over 99% with few processing defects was achieved using the optimised parameters of 175 W laser power, 400 mm/s scan speed, and no re-scanning. The influence of these parameters on the solidification microstructure was also investigated using key techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) in conjunction with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). Further, the simulated thermal profile of the melt pool region as a function of process parameters via single scan track experiments was calculated using the finite element method (FEM). These data were used to explain the key microstructural features observed in the as-solidified microstructure of the LPBF alloy as a function of the processing parameters. The mechanical properties of the LPBF alloy were then assessed by hardness and tensile testing and then compared with a reference alloy produced by arc melting. The hardness of the LPBF as-built alloy was ∼20% higher than the reference alloy. To identify the factors affecting the increased hardness of the former, the influence of grain size and morphology, crystallographic texture, phase constituents (mainly austenite and martensite), and residual strain were investigated. The hardness of the reference alloy was affected mainly by the grain size and residual strain, but for the LPBF-built alloy, the relative volume fractions of austenite and martensite strongly influenced the hardness. Meanwhile, the tensile properties of the LPBF alloy, such as the yield stress, ultimate tensile stress, and ductility, were adversely affected by the internal defects present, such that high temperature homogenisation and hot isostatic pressing (HIP) post-process treatments were investigated to improve these properties. The homogenisation and HIP treatments increased both the tensile strength and ductility of the LPBF-built alloy. Homogenisation altered the grain morphology by promoting recrystallisation and grain growth, and this increased the tensile strength by ∼80%. The hardness, however, decreased due to a reduction in the volume fraction of HCP martensite in the FCC austenitic microstructure. HIP retained some of the columnar microstructure generated by the LPBF process, marginally increased the density, and increased the tensile strength by ∼65%. The improvement in tensile properties through these post-process treatments allowed for the measurement of LPBF alloy’s shape memory behaviour, whereby a tensile recovery strain of 2% was achieved for the HIP-treated alloy. Finally, the biocorrosion behaviour of the LPBF-processed and HIP-treated alloy was investigated, whereby the in vitro corrosion potential and current density of the alloy were determined to be -769 mV and 5.6 μA/cm2, respectively, indicating a reasonable corrosion rate for this material. Overall, this thesis enabled the first demonstration of the shape memory effect in an LPBF-built Fe-based alloy fabricated from homogenised powder, an alloy which also exhibits biodegradable properties.

  • (2023) Zhou, Yingze
    Thesis
    In recent years, the urgent need for efficient energy technologies such as electrocatalysis and energy storage has attracted great attention around the world. Among them, the electrocatalytic is one of the most promising green hydrogen production approaches. For consideration of limited earth reserves, exploring the earth-abundant non-noble metal catalysts for highly efficient is necessary. Manganese-based electrocatalysts including manganese oxides and manganese sulphides are promising candidates owing to their high natural abundance and cheap price. However, some key factors such as exploring facile and effective methods to synthesize Mn-based catalysts, optimize the phase and morphology, enrich the active sites and vaccines, and control the electronic structure still need deep study in the current stage. Compared to the hydrogen evolution reaction (HER) process, the oxygen evolution reaction (OER) process requires the transfer of four electrons with a higher reaction energy barrier. It is very important to investigate the highly efficient OER catalysts to break through the technology of hydrogen production from overall electrocatalytic. Hence, in this work, the controllable phases of MnO2 catalysts for bifunctional water splitting, the modified MnO2 catalyst with rich oxygen vacancies for OER, Co doped MnO catalyst for OER and MnS/Ni3S2 catalyst for OER have been designed and deeply studied. The as prepared samples show superior catalytic performance with low overpotential, small Tafel slopes, enhanced conductivity, and outstanding durability. The mechanisms of enhanced electrocatalytic performance have been investigated as well. It is found that both crystalline structures and electronic structures have a strong effect on the catalytic performance of Mn-based catalysts. Firstly, the phases and morphologies directly affect the exposure of the active sites. Secondly, the high content of Mn3+ sites and oxygen vacancies could be highly improved the water splitting performance. Thirdly, superior band alignment tunability allows for efficient improvement of the electrocatalytic performance. Finally, the specific element doping results in the lower work function could accelerate the charge transfer process of the OER activity. This work has demonstrated feasible ways to fabricate efficient electrocatalysts, which may provide key strategies to design advanced metal oxides for practical water splitting devices in the future.

  • (2023) Xirocostas, Zoe
    Thesis
    Plant introductions to novel environments, whether intentional or accidental, have occurred for centuries and are the precursor to the thousands of invasions that are currently threatening ecosystems across the globe. One of the common, well-studied mechanisms that is thought to aid in successful introduction is known as the enemy release hypothesis, which explains that organisms may thrive in their new environments as they have escaped their co-evolved natural enemies. While enemy release may facilitate introduction in some species or situations, half of the time it does not, and we did not understand the circumstances that lead to its occurrence. Using a robust, biogeographical approach, I quantified herbivore damage across 16 plant species at varying sites across their native and introduced ranges and found that neither time, space, climate, or leaf palatability explained patterns of enemy release. Most research on invasion ecology tends to focus on the negative interactions that are missed in the introduced range and fails to consider how positive interactions are affected. Here, I provide the first broad test of the missed mutualist hypothesis across ten plant species in their native and introduced ranges, that accounts for variation between locations. Following over 120 hours of in-situ observations I found plants to be visited 2.6 times less frequently and with 1.8 times lower richness of pollinators in their introduced range in comparison to their native range. I also introduce the ZAX Herbivory Trainer, a globally accessible software that can reduce researchers’ inaccuracy of herbivory estimates by 7% in less than 10 minutes, which can be retained for up to 3 months. My thesis deepens our understanding of the mechanisms that facilitate and hinder successful introduction and provides an effective tool scientists can use to further this area of research at even larger scales.

  • (2023) Hu, Guangyu
    Thesis
    With the evolution of ever-changing intelligent electronics and the increasingly severe electricity shortages, substantial efforts have been made to explore new technologies for powering electronics. Moist-electric generation (MEG) devices, which can extract chemical energy in moisture to generate electricity, have attracted intensive interest. However, the electric outputs of the most reported MEG devices are still low. Herein, we present a novel strategy of coupling graphene oxide (GO) based MEG device with the electrochemical cell (i.e., GO/galvanic MEG device) to boost power outputs. HCl and HNO3 acids are employed to enhance the power outputs of the hybrid MEG device through unique acidification treatments. The GO/galvanic MEG device is fabricated through a simple solvent evaporation method. Polyethylene terephthalate (PET) plastic film, multi walled carbon nanotube (MWCNT), GO, and metal sheets are all the components of the device, which reflects the low-cost advantage. The power outputs of the GO/galvanic MEG device are collected using a Keysight SourceMeter. Scanning electron microscope (SEM), x-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) are utilized to characterize the device. After optimizing the fabrication parameters and using the unique acidification treatments, the hybrid MEG device generated exceptional power outputs based on the synergetic mechanisms of proton diffusion and galvanic oxidation. A single hybrid GO/galvanic MEG device stably generates a maximum voltage output of 1.69 V and a highest current density of 182 μA∙cm-2 under 80% RH at room temperature. Notably, the voltage output in this study is the apex among the reported GO-based MEG devices, while the current density output is top-ranked. Impressively, in room humidity, the single GO/galvanic MEG device directly powers a CASIO calculator, or a pressure sensor, or a LED light. Additionally, the simple integration of several hybrid MEG units with a capacitor easily and efficiently drives the water electrolysis and a commercial GPS tracker. This study demonstrates the vast potential of the GO/galvanic MEG device for driving practical electronics by harvesting energy from ambient moisture.

  • (2023) Chen, Junhong
    Thesis
    Transparent conductive films (TCFs) and electrodes based on indium tin oxide (ITO) dominate the majority of the world electronics market in the past few decades. Although the manufacture techniques of ITO are mature and relatively low cost when comparing to other TCF materials, the inherent brittleness and the utilization of scarce element (indium) imply that ITO is not suitable for next-generation device applications, which require low cost, mechanical flexibility, reliability and stability. Metal nanowires such as silver nanowires have drawn significant interest among researchers as they could achieve excellent electrical, optical and mechanical properties when coated or printed onto a variety of substrates. Many silver nanowire synthesis methods and related applications have been explored in the recent years, but some parameters and underlying mechanisms are still unclear, which requires further study and improvement. In this thesis, polymer-free synthesis of silver nanowires has been studied and explored. Nanowires with aspect ratio around 900 were synthesized by a polymer-free method. The morphology of nanowires was investigated by tuning the concentration of exotic additives, temperature and reaction time. In addition, silver nanowire-based TCFs with high transparency (84.1%) and good electrical conductivity (44.2 ohms per square) were fabricated in this project. Zinc oxide was also uniformly coated onto the nanowires through a low temperature process (150 °C) to enhance the performance. After introduction of zinc oxide layer, the TCFs could maintain its electrical and optical properties after scratching. Moreover, zinc oxide coating enhanced the thermal stability of the device and no distinct resistance change was observed in 50 °C environment for 40 days.

  • (2023) Jia, Haowei
    Thesis
    Oxygen evolution reaction (OER) is thought to play an essential part in electrochemical water splitting (EWS), metal-air batteries, energy storage, etc. Therefore, exploring an advanced anode material is necessary for the efficient OER process. Although manganese dioxide (MnO2) as the anode catalyst has been widely researched in recent centuries, most of the research concentrated on the specific phases of MnO2 (α-MnO2, δ-MnO2, and γ-MnO2). Very few reports were related to the ε-MnO2. In the meantime, the lower states of manganese oxides (MnOx 1 ≤ x ≤ 2) are still worth exploring. The multiple chemical states and large oxygen vacancies of MnOx will apply outstanding electrochemical performance. Moreover, the components of MnOx with other advanced materials, such as MXene, also show great catalytic activities. In addition, heat treatment was always adopted to modify the phase transformation of MnO2 samples. This dissertation mainly involved three experimental chapters: (1) ε-MnO2 was successfully covered on the carbon cloth (CC) substrate surface by electrodeposition strategy at different depositing times and temperatures. The morphological changes and OER performance of these as-prepared samples have been investigated. The results demonstrated that when the preparation duration was 30 min and the temperature was 50 °C, the as-prepared MnO2/carbon cloth (MnO2/CC) exhibited the best OER performance. From the microscopic characterization, the MnO2 was uniformly and firmly grown on the carbon cloth without binder usage. All the as-prepared samples displayed a core-shell structure, and the morphology of MnO2/CC did not have significant changes after the long-time stability test. (2) Ni-doped MnO2/CC (NMO/CC) was successfully synthesized by adding different concentrations of Ni cations into the precursor solution. Doping Ni cations increased the OER performance, which attributed to the appearance of more oxygen vacancies and the increased active surface area. In addition, MXene (MX) was used to deposit on the surface of NMO/CC to form the compounds of Ni-doped MnO2 and MXene electrode (MX-NMO/CC), which further decreased the resistance of the samples and increased their OER performance. (3) Thermal treatment was adopted to enhance the OER performance of as-prepared MnO2/CC samples. Three different heating temperatures were applied (250, 350, and 450 °C), and the duration was 1 h. The sample treated at 350 °C (MnO2/CC 350) acquired the best OER performance. The phase transition can be detected when the temperature reached 350 °C, and the effects of phase transition on OER performance were researched. This work explored a simple binder-free electrodeposition strategy to prepare different core-shell structured MnOx-based catalysts for an efficient OER process.

  • (2023) Wu, Winton
    Thesis
    Staphylococcus aureus is a major opportunistic human pathogen and a leading cause of bacteraemia, infective endocarditis, and medical device-related infections. The emergence of multidrug-resistant S. aureus (MRSA) is a global public health concern as these S. aureus isolates are resistant to several antibiotics. Current treatment is dependent on the efficacy of last line antibiotics like vancomycin. However, isolates with intermediate resistance (MIC 4-8 µg/mL) to vancomycin are increasingly detected worldwide and are associated with treatment failure. These vancomycin-intermediate S. aureus (VISA) isolates appear to arise from the acquisition of a disparate series of point mutations that commonly lead to physiological changes including cell wall thickening and reduced autolysis. Transcriptional profiling has revealed that antibiotic treatment drives conserved changes in small RNA (sRNA) expression in S. aureus and may contribute to the VISA phenotype. However, the function of hundreds of sRNAs in S. aureus are still poorly understood. To address this, the endoribonuclease RNase III, which processes sRNA-RNA duplexes, was used as a scaffold to capture sRNA-RNA interactions in VISA using a proximity-dependant ligation and sequencing technique termed CLASH. RNase III-CLASH recovered 215 unique sRNA-RNA interactions in vivo and 135 (63%) of these interactions are novel when compared to the vancomycin-susceptible S. aureus (VSSA) sRNA interactome. Gene ontological analyses revealed that these sRNA-RNA interactions are enriched for functions associated with reproduction, citrate transport, and cellular responses to oxidative stress. To determine whether some of these sRNAs possess coding potential, ribosome profiling was performed to delineate novel ORFs in VISA. Fifty-two potential ORF-containing sRNAs were discovered, seven of which have been confirmed in past literature, and a potential dual-function RNA Sau6072 was revealed. In addition, we identified two novel ribosome-associated noncoding RNAs PemZ1 and PemZ2 which are derived from the 3’ UTR of two different type II toxin-antitoxin operons. By mapping the translational landscape of the VISA genome with ribosome profiling, it can provide insight into the additional uncharacterised functions of these regulatory RNAs. Small RNA interactome techniques have enriched our understanding of sRNA-mRNA interactions and their roles in several physiological functions to conferring antibiotic tolerance, nutrient adaptation, and oligopeptide transport. However, one major challenge is determining how these sRNAs affect the expression of their target mRNAs. To address this, we examined the correlation between gene transcript abundance (RNA-seq), ribosome occupancy (Ribo-seq), and protein levels (proteomics) to identify sRNA-mRNA interactions that post-transcriptionally regulate mRNA translation. We used the machine learning technique Self-Organising Maps to cluster genes with similar transcription and translation patterns and identified a cluster of mRNAs that appeared to be post-transcriptionally repressed. By overlaying our sRNA-mRNA interaction network on these clusters we identified sRNAs that may be mediating this post-transcriptional repression. Two of these sRNA-mRNA interactions are mediated by RsaOI, a sRNA that is highly upregulated in the presence of vancomycin. CRISPRi knockdown of RsaOI resulted in increased vancomycin sensitivity in two different VISA strains and we discovered that RsaOI post-transcriptionally represses the expression of the major autolysin Atl. This autolysin has been previously implicated in vancomycin tolerance whereby its expression is reduced in several VISA isolates compared to their VSSA parent strains. We postulate that this interaction contributes to vancomycin tolerance in VISA as this could result in reduced autolytic activity, which is a common phenotypic hallmark of VISA. Further, we have confirmed that RsaOI post-transcriptionally represses the sugar phospotransferase component PtsH and Arginase. These results collectively suggest that RsaOI is a global regulator of several biological functions including arginase catabolism, sugar transport and cell wall turnover. Taken together, the work in this thesis has contributed to a greater understanding of the functions of regulatory RNAs in S. aureus. Our multi-omics analyses have provided insight into how sRNA-responsive networks induce changes in response to vancomycin treatment and adapt to antibiotic stress. This research supports the development of novel and robust antimicrobial therapies by developing an enhanced understanding of post-transcriptional regulatory mechanisms in S. aureus and their roles in antibiotic tolerance.

  • (2023) Ma, Mingyou
    Thesis
    With the rapid growth of e-commerce, the surging freight traffic is imposing unprecedented pressure on urban transport systems. To mitigate negative impacts of urban freight traffic, the integrated public transport system, i.e., urban co-modality, has been proposed to utilize the existing urban passenger transport system to also carry freight during off-peak hours. Despite the benefits, the co-modal system might reduce public transport reliability and demand due to freight loading/unloading and transshipment operations. This thesis focuses on understanding and modelling the emerging integrated co-modal system for passengers and freight, and investigating and managing its system-wide impacts. This thesis first uses the smart transit card data to understand the travel behaviour of public transport users, and quantify the impact of public transport reliability on users’ day-to-day travel choices. We find that public transport users tend to reserve safety margin for the unforeseen service unreliability. Besides, we also find that there was under-utilized capacity in transit services operating during off-peak hours, which indicates the potential for transporting freight in the public transport system. With the understanding of service-reliability-based travel choices, this thesis then models the mixed freight-passenger cross-type flow and strategic interactions among operators and users in a standalone co-modal system. We first construct a fundamental game-theoretical model based on the essential characteristics of the co-modal system, such as negative impacts of freight on passenger demand. In the fundamental model, we examine the strategic interaction between a transit operator and a freight operator. We show that introducing the co-modality has the potential to generate Pareto-improving outcomes for the operators. This model is extended by considering the endogenous interactions among freight customers, passengers, freight and transit operators. We find that the co-modal system may enhance levels of services for both passengers and freight customers. Building upon these, this thesis further explores the impact of the co-modal system on the freight transport market with outsourcing arrangements. The non-cooperative and cooperative games among a freight carrier, a freight integrator, and a transit operator are modelled, and the co-modal system performance is quantified.

  • (2023) Pollo, Pietro
    Thesis
    Evolutionary biology literature often suggests that the sexes express reproductive behaviours completely differently from one another, with stereotypical representations such as choosy females and competitive males. This thesis explores this concept at multiple levels from examining whether this is the actual perception of the research community to investigating overlooked behaviours like male mate choice and female-female competition. In chapter 2, I conducted a survey in which I asked participants about their perceptions on sex differences in reproductive behaviours in non-human animals. I found that although people agree with the stereotypical roles proposed in the literature, they understand variation exists around these stereotypes. More importantly, I found that personal and research experiences from participants were associated with their perception about sex differences, revealing potential sources of biases about this topic. In chapter 3, I assessed whether male mate choice occurs in the praying mantis Miomantis caffra, in which females often cannibalize males before copula. I found evidence that males express mate preferences in that species and that males’ personality (activity) was associated with differences in approaching behaviour to females, showing that the common stereotype of males always being eager to mate is not true. In chapter 4, I conducted a meta-analysis across all animals to evaluate whether male mate choice varies among individuals. I found that, on average, higher quality males (e.g. larger and in better body condition) are choosier than their counterparts. Finally, in chapter 5, I explored whether mate competition in humans impacts their same-gender friendships. More specifically, I hypothesized that physical similarity in same-gender friendships would be more prominent for women than for men. This is because physical traits are often mentioned by men to determine mates’ attractiveness and thus could be a potential source of rivalry in friendships between women. I found no support that men and women choose friends differently. Altogether, I show that the simplifications of sex differences that stereotypes convey can conceal complexities found in nature.

  • (2023) Wilson, David
    Thesis
    This thesis investigates issues important to the aviation industry, quantifying the likelihood of flights arriving in unsafe or inefficient circumstances from inaccurate airport weather forecasts (TAF, TTF and TAF3). Key issues include investigating: the likelihood of recurrence for an accident which occurred at Norfolk Island in 2009; the likelihood of adverse situations during aeromedical flights arriving at Australian remote islands and capital city aerodromes; and the potential impact of TAF3 use for major airport arrivals. Unsafe situations, or ‘misses’, where flights arrive during unplanned unsuitable weather, may result in landings below safe limits, or aircraft safety incidents. Inefficient situations, or ‘false alarms’, where pilots have planned for adverse weather that does not eventuate, have economic and environmental implications due to extra unnecessary fuel being carried. No suitable existing methodology was identified to achieve the research objectives, requiring development of a new utility-based weather forecast verification model. This model compares simulated decisions of historical aerodrome weather forecast use with actual meteorological conditions. The key element of the model is the measurement of likelihood of unsafe or inefficient situations based on the time prior to arrival (‘time-offset’) that forecasts were available to pilots. Results are calculated using a computer-based implementation of this model. The odds of a ‘miss’ at Norfolk Island were found to be 1 in 859 when simulating the flight plan that was used in the 2009 accident. These odds were predicted to be 1.8 times lower if the ‘time-offset’ between forecast promulgation and planned arrival time was reduced from 5 to 2 hours, the latter time being at the point of last safe diversion. For aeromedical flights to Australian remote islands and to the five busiest capital city aerodromes, strong linear relationships between ‘miss’ likelihood and ‘time-offsets’ between 0 and 12 hours were identified at all locations except for Melbourne and Sydney, with strong second-order relationships identified for ‘false alarms’. ‘False alarms’ are predicted to be considerably more likely at remote islands compared to capital cities, although there was no discernible trend between these groups for ‘misses’. When using all TAF3 information for planning, the replacement of TTF with TAF3 forecasts at Australia’s five busiest major aerodromes is predicted to reduce ‘miss’ arrivals by 34% (enhancing safety), and increase ‘false alarm’ arrivals (when flight have excessive fuel) by 91%. TAF3 PROB segments are predicted to contribute to at least 80% of this change due to the proportion of time these had an operational effect.