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(2022)ThesisTwo-dimensional transition metal dichalcogenide (TMD) nanocrystals (NCs) exhibit unique optical and electrocatalytic properties. However, the growth of uniform and high-quality NCs of monolayer TMD remains a challenge. Until now, most of them are synthesized via solution-based hydrothermal process or ultrasonic exfoliation method, in which the capping ligands introduced from organic solution often quench the optical and electrocatalytic properties of TMD NCs. Moreover, it is difficult to homogeneously disperse the solution-based TMD NCs on a substrate for device fabrication since the dispersed NCs can easily aggregate. Here, we put forward a novel CVD method to grow closely-spaced TMD NCs and explored the growth mechanism and attempts on the size control. Their applications acting as electrocatalysts and adhesion layer for Au film deposition have been also well displayed. Through the whole chapters of this thesis, the following aspects are highlighted: 1. MoS2 and other TMD nanocrystals have been grown on the c-plane sapphire. The surface oxygen vacancies determine the density of TMD nanocrystals. The MoS2 nanocrystals demonstrate excellent hydrogen evolution reaction and surface-enhanced Raman scattering performance owing to the abundant edges. 2. Deep insights into the growth of MoS2 nanograins have been explored. The surface step edges and lattice structures of the underlying sapphire substrates have a significant influence on the growth behaviors. The step edges could modulate the aggregation of MoS2 nanograins to form unidirectional triangular islands. The Raman spectra of MoS2 demonstrate a linear relationship with the crystal size of MoS2. 3. The orientation of sapphire substrate has an of importance effect on the critical size of MoS2 nanocrystals. The MoS2 nanocrystals have the smallest size on the r-plane sapphire, besides, the MoS2 on r-plane sapphire demonstrates the sintering-resistance feature, which is attributed to the edge-pinning effect when MoS2 edges are anchored on the sapphire surface. 4. The MoS2 nanocrystalline layer was utilized as the adhesion layer for Au film depositing on a sapphire substrate. The Au films on MoS2 displayed superior transmittance and electrical conductivity as well as outstanding thermal stability, which lay in the strong binding of Au film with MoS2 nanocrystalline layer.
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(2023)ThesisBiodegradable 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.
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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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(2023)ThesisThe high cooling rates in metal additive manufacturing (AM) of high-entropy alloys (HEAs) can not only prohibit the formation of intermetallic compounds and detrimental elemental segregation but also facilitate microstructural refinement, thereby providing improved mechanical properties. However, there are still many challenges, including the presence of AM fabrication defects and residual stresses as well as anisotropic properties. Thus, further understanding is needed regarding applying post heat treatment to release residual stresses in as-built HEAs, and the techniques that can be employed to reduce or minimize the property anisotropy in as-built HEAs. In this thesis, a model Cantor alloy was fabricated and subject to a series of post heat treatment conditions. The results showed that annealing at a temperature lower than 900 °C gave rise to the formation of M23C6 carbides while recrystallization took place at temperatures over 900 °C. Interestingly, three distinct types of microstructures were exhibited at the annealing temperature of 900 °C. This is primarily attributable to the different thermal histories undergone through the LPBF process among these areas where heterogeneity in chemical elements and microstructure was revealed. The second group of samples were fabricated to develop a machine learning coupled microstructure mapping approach for tailoring mechanical property anisotropy. Three distinct types of microstructures were generated by varying the laser power and scanning speed, namely, herringbone, bimodal, and columnar microstructures. Epitaxial grain growth and side-branching in the melt pool center and overlapped regions are the main reasons responsible for the different microstructures. The underlying mechanisms were discussed in terms of effective grain size, heterogeneous microstructure, and the twinning induced plasticity (TWIP) effect. Subsequently, pre-deforamtion was applied before post heat treatment to study the influence of grain boundary engineering (GBE) on the recrystallization and resultant deformation behavior. Twinning-assisted recrystallization nucleation accelerated the formation of a nearly a fully recrystallized microstructure in the heavily deformed sample. As a result, the present strategy of GBE can provide further insight regarding the manipulation of the post heat treatment in as-built alloys particularly for those precipitation hardenable alloys where precipitation kinetics can be varied.
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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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(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.
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(2023)ThesisPower systems today are facing increasing challenges from the uptake of inverter based resources (IBRs) aiming at zero carbon emission targets and sustainability through electrification of multiple major emission intensive sectors such as the transportation sector. This transition has driven the power grid into an era of new operating norms with decreasing system strength with respect to voltage stability together with the needs for frequency stability enhancement given the decommissioning of conventional synchronous machines including synchronous generators and synchronous condensers. The recent primary energy supply crisis with soring volatile prices for gas and coal adds further uncertainties to resilient energy supply which had resulted into doubled or even trebled spot market prices in many electricity markets worldwide. In Australia the national electricity market (NEM) even stopped normal operation into suspension under administered price cap of $300/MWh over 16-24 June 2023 because the operator was unable to ensure secure system operation under a combined impact from outage, high commodity prices, low renewable energy generation and high demand. Behind all the factors, system strength is a critical measure for stable system operations and resilience against system faults. While the weak system strength in many power grids require less renewable energy source grid integration, however, on the other hand, the overall emission reduction initiative means invariably more renewable IBRs will be grid connected, although the current industrial practices are either costly or inadequate to ensure system strength to achieve the IBR connection needs. Clearly, there is an urgent need to address the system strength problem for the power grid. At the same time, emission reduction through the future power grid requires a systems approach in order to achieve efficiency and effectiveness in achieving the targets. As such, in addition to solving the technical challenges of achieving emission reduction by more grid uptake of IBRs, mechanisms through emission trading and the energy market are equally important for the sector, especially considering the ultimate clean energy options based on green hydrogen trading. Research presented in the thesis covers comprehensive research findings in better understanding of the renewable energy IBR grid connection issues, nature and effective measures of system strength for IBR grid connection studies, operational as well as planning options considering system strength. The thesis also present research contributions exploring the future opportunities brought by the ultimate clean energy sources – green hydrogen. Green hydrogen serves as clean energy storage for midterm and longer term energy storage. It also achieves financial effectiveness by utilizing the renewable energy combined with volatile electricity market prices. This actually opens up new emission reduction trading opportunities beyond existing carbon emission trading through Clean Development Mechanism (CDM) and Joint Implementation (JI). A green hydrogen trading framework has been developed to form a comprehensive framework in achieving a clean and stable future power grid.