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(2006)Conference PaperThe topics of ACUN-5 will cover all aspects of the science and technology of composite materials, from materials fabrication, processing, manufacture, structural and property characterisation, theoretical analysis, modelling and simulation, materials design to a variety of applications, such as aerospace, automotive, infrastructure, packaging, ship-building, and recreational products. ACUN-5 will bring together the latest research and developments of the complete range of composite materials, including biocomposites, medical-composites, functional and smart composites, gradient and layered composites, nanocomposites, structural composites and mimicking natural materials. The reinforcements will range from nano-, micro-, meso- to macro-scale in polymer, metal, ceramic and cementitious matrices.
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(2011)Journal ArticleThick anatase films were fabricated on graphite substrates using a method of anodic aqueous electrophoretic-deposition using oxalic acid as a dispersant. Thick films were subsequently fired in air and in nitrogen at a range of temperatures. The morphology and phase composition were assessed and the photocatalytic performance was examined by the inactivation of Escherichia coli in water. It was found that the transformation of anatase to rutile is enhanced by the presence of a graphite substrate through reduction effects. The use of a nitrogen atmosphere allows higher firing temperatures, results in less cracking of the films and yields superior bactericidal performance in comparison with firing in air. The beneficial effects of a nitrogen firing atmosphere on the photocatalytic performance of the material are likely to be a result of the diffusion of nitrogen and carbon into the TiO2 lattice and the consequent creation of new valence band states.
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(2008)Journal ArticleThe designers of a large scale video-on-demand system face an optimization problem of deciding how to assign movies to multiple disks (servers) such that the request blocking probability is minimized subject to capacity constraints. To solve this problem, it is essential to develop scalable and accurate analytical means to evaluate the blocking performance of the system for a given file assignment. The performance analysis is made more complicated by the fact that the request blocking probability depends also on how disks are selected to serve user requests for multicopy movies. In this paper, we analyze several efficient resource selection schemes. Numerical results demonstrate that our analysis is scalable and sufficiently accurate to support the task of file assignment optimization in such a system. © 2008 IEEE.
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(2022)ThesisA three-dimensional multi-scale finite element analysis is performed to ascertain the effects of geometrical variations at multiple structural scales on the mechanical properties, including the stiffness, strength and onset of damage, of cortical bone. Finite element models are developed, with reference to experimental and numerical data from existing literature, to account for cortical bone’s anisotropy and viscoelastic behaviour from the most fundamental level of cortical bone consisting of mineralised collagen fibrils, up to the macroscopic bone consisting of osteons and Haversian canals. A user-defined material subroutine is developed to account for the viscoelastic and anisotropic properties of cortical bone in a three-dimensional setting, at multiple length scales. Further, the Taguchi-ANOVA statistical approach is applied to perform sensitivity analyses on the effects of geometrical parameters on the effective material properties, including stiffness and strength, of cortical bone at each structural scale. A cohesive zone based finite element model is further incorporated to examine the effects of geometrical variations on the damage onset and strength properties of cortical bone at multiple structural scales. Numerical results indicate that there is a positive correlation between the mineral volume fraction and the effective stiffness constants, as well as tensile and shear strength, at each length scale. Variations in the effective geometrical parameters at each structural scale also contributed to changes in the damage initiation sites and damage mechanisms, particularly at the lower length scales. Further, numerical results indicate that cortical bone exhibits a two-phase stress relaxation process: a fast and a slow response relaxation process, which can be mathematically represented by the Generalised Maxwell Model. Numerical results also indicate that the anisotropic and hierarchical structure of cortical bone contribute to significant changes in the stress relaxation behaviour, damage onset, and strength properties of cortical bone at each structural scale.
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(2024)ThesisOsteochondral tissue comprises of complex chemical, cellular and physical gradients which pose an immense challenge for repair via traditional tissue engineering approaches. Granular hydrogels have emerged as an exciting new opportunity for control of micro- and macro- scale properties for fabrication of complex tissue environments. This thesis explores the utility of gelMA-microgel based granular scaffolds for tissue engineering, with a focus on the osteochondral interface. First, scaffolds were optimised for MSC chondrogenesis by manipulating interstitial filler volume. Higher volumes were found to promote cell sphericity, proliferation, and aggregation, which led to enhanced chondrogenic matrix deposition. Second, incorporation of bioactive Laponite® -RD and -XLG nanosilicates is explored for MSC osteogenesis. Nanosilicate inclusion significantly enhanced MSC osteogenesis but was not found to be osteo-inductive. Third, embedded extrusion printing was utilised for deposition of a bone-mimetic ink in optimised MSC-laden microgel suspensions. Photo-crosslinkable gelMA microgels allowed tuning of ink properties, thus improving robustness. Over 21 days, MSCs exhibited gradient-like chondrogenic ECM deposition away from the bone and mineralisation close to it. Lastly, vascular structures were fabricated using a casting approach for straight channels and direct writing with a sacrificial ink for complex geometries. Design of a novel PDMS-based reactor allowed channel fabrication and long-term perfusion of constructs within the same setup. Taken together, this thesis demonstrates the modularity and versatility of gelMA microgel based granular suspensions and lays the foundation for their use in the fabrication of complex tissue models replete with vasculature.
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(2018)ThesisGraphene oxide is a single layer of carbon atoms with decorated oxygen functional groups. Stacked monolayers in the laminate form create an interlayer space of sub-nanometer scale with oxygenated functional group to attract water molecules, and graphitic domains to allow frictionless flow of water molecules and achieve maximum efficiency of water transportation. The research reported herein is aimed to understand and explore characteristics of the diffusion-dependent mass transportation across an array of cascading nanochannels confined by graphene oxide laminates at sub-nanometer level. This dissertation has 6 Chapters. Chapter 1 is the introduction and Chapter 2 reports the recent progress in graphene oxide for mass transport application. Chapter 3 discusses efforts of engineering the channel confinement, which is represented by the interlayer spacing in between graphene oxide laminates. By adjusting the fundamental factors of graphene oxide suspension, the interlayer spacing can be controlled at 0.7 to 0.8 nm. Based on the engineered interlayer spacing, separation of vaporous mixture by graphene oxide membrane is studied in Chapter 4. Numerical description of nanochannels enclosed by graphene oxide monolayers is determined by time lag analysis. The feature of ethanol vapor transportation with the support of water vapor is revealed, showing accelerated transportation of non-permeable matter, which enriches the existing knowledge. A geometrical model of graphene oxide membrane for vapor separation was established and analyzed. In Chapter 5, adsorption and intercalated of molecules and solvated ions are studied and proved as a size-dependent enlargement of graphene oxide nanochannels. Carriers such as water and ethanol are used for transporting ions and molecules into graphene oxide slits. Taking the adsorption into consideration, permeation of vaporous substances through adsorbed graphene oxide membrane is investigated in Chapter 6. The research initiates researching crystallization of adsorbed matters in graphene oxide interlayer structure. A simplified model was directed to predict the water vapor permeation behavior of intercalated graphene oxide membrane. Such efforts not only lead to a better understating of graphene oxide membrane for gas separation but also give a hint of spatially efficient matter transport in achieving excellent electrochemical devices with graphene oxide components.
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(2023)ThesisTumbling ball milling is a critical comminution process in materials and mineral processing industries. It is an energy intensive process with low energy efficiency. It is important that ball mills and the milling process are properly designed and operated. To achieve this, models at different scales are needed to provide accurate prediction of mill performance under various conditions. This study aimed to develop a combined discrete element method (DEM) and machine learning (ML) modelling framework to link mill design, operation parameters with particle flow and mill efficiency. A scale-up model was developed based on DEM simulations to link mill size ratio, rotation rate, and filling level with power draw and grinding rate. Then, an ML model using the Support Vector Machine (SVM) algorithm was developed to predict the angle of repose (AoR) and collision energy based on various operation conditions. The ML model was trained by the data generated from the DEM simulations and able to predict the AoR and collision energy. In the process monitoring, an artificial neural network (ANN) was firstly proposed to predict internal particle flow properties of a rotating mill based on acoustic emission (AE) signal generated using the DEM. Main features of AE signals and power draw were fed into the ANN to predict flow properties such as particle size distributions, collision energy distribution and filling levels. Further, a convolution neural network (CNN) was used to replace the ANN to extract more efficient features of AE signals non-linearly based on different local frequency ranges in a ball milling process partially filled with steel balls and grinding particles. Last, a physics-informed ML model was developed based on continuous convolution neural network (CCNN) to learn particle contact mechanisms provided by DEM data at different rotation speeds. The ML model coupled with DEM simulation can accelerate DEM simulation to accurately predict particle flow in a long time series. In summary, this work has demonstrated that combining physics-based numerical models DEM to ML models not only improves the efficiency and accuracy of predictions of complicated processes but also provides more insight to the process and makes predictions more transparent.
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(2023)ThesisEmerging photovoltaic materials such as kesterite and perovskite for optoelectronic devices have inevitable defects that affect the device performance. Therefore, examining the optoelectronic properties of defects is crucial to improve the device performance. Scanning probe microscopy (SPM) is one of the essential techniques to investigate various properties of materials at nanoscale with high resolution. This thesis focuses on characterising kesterite and perovskite using SPM technique to elucidate nanoscale charge transport properties. This primary objective of the first part of this thesis is to understand the effect of compositional variation of kesterite on optoelectronic properties at nanoscale by controlling tin content during the precursor process. This work demonstrated the highest A-type [ZnCu + VCu] defect and altered distribution of sulfur with high near-surface accumulation at an optimised compositional ratio. This synergetic outcome facilitated carrier separation across grain boundaries (GBs). Moreover, the open-circuit voltage deficit was significantly reduced at an optimum compositional ratio owing to improving charge transport through GBs, thereby exhibiting an improved power conversion efficiency. Furthermore, this thesis examines the effect of passivation strategies in halide perovskite for indoor applications using phenethylammonium iodide (PEAI) and lead(II) chloride (PbCl2). A homogenous charge separation across a surface of perovskite was observed when an adequate amount of PEAI was deposited on the perovskite surface. In addition, incorporation of PbCl2 facilitated effective charge transport through GBs. Both strategies facilitated carrier transport towards the surface of perovskite with less ion migration and reduced non-radiative recombination, thereby improving the performance of indoor perovskite solar cells. Finally, another aspect of this thesis is to investigate the optoelectronic properties of two-dimensional perovskite single crystal (butylammonium lead bromide, BA2PbBr4) and elucidate their effect of these optoelectronic properties on the performance of photodetectors (PDs). Accumulation of charge carriers increased at edges with increasing the edge height. Furthermore, the existence of multiple sub-bandgap states in BA2PbBr4, and increasing electron transitions from sub-bandgap states with higher edge height were observed with increasing edge height. This work suggests that edge-height dependence of charge-carrier behaviour in BA2PbBr4 can be utilised in broadband PDs.
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(2022)ThesisOver the past two decades, Bismuth ferrite (BiFeO3, BFO) thin films have attracted significant attention on account of their attractive multifunctional properties, ranging from room-temperature multiferroicity to robust high Curie temperature (Tc), large switchable spontaneous polarization (Ps) and enhanced electromechanical response, and ability to generate photovoltaic current etc. In particular, the past decade has seen huge efforts devoted to engineering the phase structure of BFO films to achieve a morphotropic phase boundary -(MPB)-like materials system with excellent ferroelectric properties and high electromechanical response. In this thesis, strain engineering and site engineering are applied to tailor the phase composition of epitaxial BFO films through a chemical solution deposition (CSD) method. The compressive epitaxial strain provided by the lattice misfit between the film and the lanthanum aluminate (LaAlO3, LAO) substrate can stabilize a tetragonal-like (T’) phase. The chemical pressure produced by the A-site substitution of smaller Sm element can induce a ferroelectric rhombohedral (R) to paraelectric orthorhombic (O) phase transition. The coexistence of these diverse polymorphs of BFO is expected to generate an MPB effect, where a maximum electromechanical property usually reports. The thesis first employs the misfit strain engineering (BFO films grown on LAO substrates) to tune the phase fraction in mixed-phase (rhombohedral-like (R’)-T’ coexistence) BFO films by altering the synthesis parameters via the CSD method. It shows that the T’-phase fraction ranging from 10% to 35% is achieved by decreasing the spin-coated layers from four to one layer. In a two-layer configuration, the T’-phase fraction is further found that can be varied from 8% to 38% by changing the precursor concentration and heating treatment parameters. The mixed-phase BFO films show a typical polydomain structure with a polarization-orientation dependent conduction behavior whereby poled-up (polarization pointing away from the lower electrode) domains have higher conductivity. An optimized film with a T’-phase fraction of ~ 28% shows the lowest leakage current of <0.1 A/cm2 up to field strengths of 500 kV/cm. Upon increasing electric field, the mixed-phase film shows an interface-limited Schottky emission to bulk-limited space-charge-limited-conduction (SCLC) mechanism predominate leakage current transition. The thesis then applies site engineering (A-site substitution with Sm) to tailor the phase structure of BFO films deposited on strontium titanate (SrTiO3, STO, the lattice parameter is similar to that of BFO) substrates. The role of the Sm on the precursor gelation chemistry is first studied. It is found that the electronegativity of the cation species in metal nitrates affects the reaction rate of the hydrolysis reaction and esterification reaction. The structural investigation of the crystalline films shows that the phase transition occurs at x = 0.10 with paraelectric O phase and antipolar phase appearance. The domain contrast of as-grown BFO/BSFO (BSFO: Bi1-xSmxFeO3) films reduces gradually with the increase of the Sm composition. More importantly, Sm introduction greatly improves the ferroelectric properties of BFO films. At an Sm ratio of 0.14, a fully developed polarization hysteresis loop is achieved. When the Sm ratio is increased to 0.15, an electric-field-induced distorted double hysteresis loop is observed. Then strain engineering and site engineering are combined to construct a complex phase configuration in BSFO films. A structural evolution from T’-R’ to T’-R’-O, and then to T’-O phases is demonstrated with the increase of the Sm ratio. The synergetic effects of misfit strain and chemical pressure drive the phase transition composition to a higher value of 0.14 compared to that of strain-free BSFO/LSMO//STO (LSMO: La0.67Sr0.33MnO3) films at 0.10. Likewise, Sm doping in the A-site leads to the decrease of the piezoresponse force microscopy (PFM) amplitude. While an enhanced domain switching behavior is attained at MPB. The ferroelectric properties show a transition from a single ferroelectric square-shaped to a slightly distorted double hysteresis loop with the Sm3+ doping content increasing from 0.14 to 0.16. The investigation of the resistive switching behavior shows an interesting transition of the current flow under the external bias from “high resistance state (HRS)-> low resistance state (LRS)->HRS->LRS” to “HRS->LRS->LRS->HRS”. This thesis provides a comprehensive understanding of the effects of epitaxial strain or/and chemical pressure on the phase composition of BFO films and their multifunctional properties. The appealing physical properties induced by the structure evolution promote the seeking of novel phase structure of perovskite oxides in thin-film form.
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(2023)ThesisClean chemical fuels such as molecular hydrogen are promising eco-friendly alternatives to CO2-emitting fossil fuels ; hydrogen can be generated using solar-driven photoelectrochemical water splitting with an efficient semiconductor photoelectrode. However, to optimize the process and reduce costs, exploring new materials and photoelectrode structures, as well as understanding their fundamental characteristics and impact on energy conversion, is crucial. Sulfide-based semiconductors such as CuS and ZnS have gained interest due to their earth abundance, non-toxicity, and suitable band gaps, but have limitations such as rapid recombination of photoexcited charge carriers and susceptibility to (photo)corrosion for CuS, and a wide band gap for ZnS. This thesis explores various approaches, such as varying synthesis conditions for the deposition of CuS thin films, forming CuS-ZnS mixtures, and doping, to improve the performance of both semiconductors. Firstly, CuxS thin films were synthesized using pulsed laser deposition. The deposition conditions such as substrate temperature and background gas pressure were varied to analyze their impact on structural and optoelectronic properties. Substrate temperature had a more significant effect on film growth than background gas pressure, and the highest temperature used (500°C) yielded the best performance with a reduced band gap and higher photocurrent density. However, poor stability was observed. To enhance the stability and overall performance of CuS, mixed films of CuS-ZnS were synthesized; they showed better optoelectronic properties and enhanced photoelectrode performance compared to films of CuS and ZnS on their own. Computational studies using DFT were also used to investigate the CuS-ZnS mixtures, which confirmed the enhanced charge separation as well as reduced band gaps compared to bulk CuS and ZnS. Altering the thickness of the ZnS layer also affected both stability and band gap, attributed to changes in interfacial bond lengths and atomic charges. From DFT calculations, it was found that doping CuS with transition metal and alkaline earth metal dopants affects the bonding behavior and can switch CuS between plasmonic and typical fluorescent semiconductor behavior. Overall, the thesis highlights the importance of exploring new materials and photoelectrode structures and understanding their fundamental characteristics to optimize the process of solar-driven water splitting and reduce costs.