Engineering

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Now showing 1 - 8 of 8
  • (2022) Wang, Shuangyue
    Thesis
    Two-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.

  • (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) Kong, Hui
    Thesis
    The 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.

  • (2023) Lee, Minwoo
    Thesis
    Due to the unique photovoltaic properties and ease of fabrication, organic-inorganic halide perovskites have generated considerable research interest. The perovskite solar cell can be applied to many applications, by tuning the bandgap. Inter of Things (IoT) devices and tandem solar cell applications, in particular, have been required for the wide bandgap perovskite solar cells. However, wide bandgap perovskite solar cells have band alignment mismatch problems, leading to charge recombination at the interface of perovskite, resulting in encouraging low device performance and decrease device stability. The first part of this thesis includes the study of the structure and working mechanism of perovskite solar cells. In addition, the defect of the perovskite was explained about how the majority of defects formed. This is caused by shallow defect energies within the bandgap, low density of deep traps, and low trap-charge interaction cross-sections which are occurred during the interaction between traps and charges. After that, the explanation of the reason how wide bandgap is applied for the indoor application. There is previous work on the tuning of the band alignment between perovskite and hole transfer layer which improved the efficiency of hole transfer, resulting in high device performance under the low light intensity condition. Lastly, the experiment of the thesis is focused on the address of the band alignment mismatch by adding two dimensional (2D) BA2PbBr4 perovskite layer for the tunnelling effect between the electron transport layer (ETL) and perovskite layer. The tunnelling layer of 2D perovskite improved the 3D perovskite crystal quality and charge transport from the 3D perovskite to ETL. As a result, the power conversion efficiency under the 200 lux white light emitting diodes (LED) light for the IoT devices was 43.70% with around 1 V of open circuit voltage and improved the device stability under the 1000 lux of white LED up to 1200 hrs.

  • (2023) Abbasi, Roozbeh
    Thesis
    Low melting point post-transition metals are a class of materials that melt below 330 ℃. Their low melting points offer distinctive physical and chemical properties that are yet to be fully explored. The study of properties of low melting point post-transition metals in various organic or inorganic systems enable insights into the field of biomaterials. The first stage of this thesis reports the synthesis of a liquid metal-polymer system which has the potential for patterning of liquid metals on different substrates. A sonication method is utilised for dispersion of eutectic mix of gallium and indium (EGaIn) particles into a photo-polymer. After characterisation of this inorganic-organic system, patterning of the dispersion is demonstrated through conductive tracks on flexible and rigid substrates. This method provides straightforward patterning of conductive EGaIn liquid metal traces. In the second stage, the physical, mechanical and biocompatibility properties of another liquid metal system known as Field’s metal (a eutectic mix of indium, tin and bismuth) was explored. Field’s metal (FM) has a melting point of 61 ℃. Two eutectic mixes of FM and FM-with zinc were synthesised and compared. A proof-of-concept application was demonstrated for the two biocompatible materials shaped into body implants for clinical applications. The removal of these implants from within a tissue mimic was demonstrated by utilising a mild non-contact heat source. This approach was shown to negate the need for invasive surgery for removal of implants from the body to potentially improving the health of patients. In the third stage, another liquid metal system was investigated based on gallium as the reaction media. Magnesium/bismuth intermetallic was formed on the surface of gallium through selective solidification. The intermetallic system, with a very high intrinsic melting point, formed at low temperature and hexagonal shaped domains of intermetallics on the surface were established. This inorganic system was then studied to show favourable antibacterial properties in comparison to pure gallium control. The work was a successful experimental demonstration on the possibility of the usage of liquid metal media for the formation of various mixes and intermetallic species in mild thermal conditions. Altogether, the outcomes of this successful PhD thesis will provide fundamental insights into surface chemistry of liquid metals with potential benefits in biomedical applications.

  • (2022) Al-Farsi, Mo
    Thesis
    Multijunction solar cells based on silicon are predicted to achieve an efficiency of 40-45% for a top cell with a band gap of 1.6-1.9 eV. However, there are currently no known materials with suitable band gaps able to deliver high efficiencies. Two classes of materials that have been proposed for top cells are alloys of CuGaSe2 and alloyed oxide perovskites. CuGaSe2 has a suitable band gap (1.68 eV) for a top cell on silicon, but the maximum efficiency achieved is only 11%, while that of the closely-related CuInGaSe2 (band gap 1.14 eV) is 23.35%. The low efficiency of CuGaSe2 has been attributed to anti-site defects. Therefore, suppressing this defect formation is critical to achieving higher efficiencies. On the other hand, most oxide perovskites have band gaps that are too high (>2 eV) to be used as top cells on silicon, hence strategies such as alloying are required to lower their band gaps. In this work, the effects of alloying CuGaSe2 with Ag, Na, K, Al, In, La and S were investigated using Density Functional Theory (DFT) calculations. The band gaps of the alloyed compounds and formation energies of anti-site defects were calculated to find alloying elements that can increase the defect formation energy but maintain the band gap. CuGaSe2 alloyed with Al at 50at% showed the highest increase (compared to unalloyed CuGaSe2) in the defect formation energy (by ~0.20 eV) followed by Na (~0.15 eV) and S (~0.10 eV), both at 50at%. However, the band gap of the Al alloy (~2.15 eV) is too high for a top cell, while those of Na (~1.95 eV) and S (~1.91 eV) are slightly above the upper limit. Thus, alloying with these elements is not an ideal route towards significantly increasing the formation energy of anti-site defects while maintaining the band gap of CuGaSe2. However, some of the factors that influence the defect formation energy are identified, potentially leading to design rules for future work. Defect formation energies were found to be higher in structures with more positively charged Ga and negatively charged Se atoms. Analysis of bond lengths revealed a positive correlation between shorter Ga and Se bonds and higher defect formation energies. Band gaps of various alloyed oxide perovskites were calculated using DFT. BiFeO3 was alloyed with Y and Sb; LaFeO3 with Cr and Sb and YFeO3 with Bi and Sb. YFeO3 alloyed with Sb at 50at%, was found to have a band gap of 1.4-2.1 eV (depending on the basis set used) which is in the range for a top cell.

  • (2023) Islam, Md Shariful
    Thesis
    Tissue engineering aims to create functional tissues by cultivating cells in a laboratory setting. A primary area of focus to achieve that objective is the development of scaffolds capable of providing a suitable environment for cellular adhesion, growth, and the execution of fundamental cellular functions to establish tissue-scale properties. However, scaffold systems in the laboratory do not benefit from the dynamic forces that are exerted on tissues in an organism. In pursuit of this aim, the overall objective of this thesis was to develop a tissue engineering scaffold system mimicking the natural tissue-like environment, with in-built capabilities for external control of dynamic mechanical properties to modulate cell differentiation. We first developed a magnetic nanoparticle-loaded hydrogel system, where the modulus of the hydrogels can be reversibly altered by applying a magnetic field. To demonstrate versatility, we have used two popular hydrogel systems broadly used in tissue engineering: poly (ethylene glycol dimethacrylate) and gelatine methacryloyl. We analysed the effects of the field-induced change in stiffness on cell behaviour upon the attenuation of a magnetic field. Our studies demonstrate that adipose-derived stem cells (ADSC) and embryonic muscle cells (C2C12 cell line) can perceive these stiffness changes and differentiate towards myofibroblast and myoblast, respectively. We then developed a composite hydrogel system to segregate the magnetic particles within gelatin fibres, which simultaneously provides nanotopography to the adherent cells. We used electrospinning to synthesize magnetic gelatin nanofibers containing 5 wt/v% iron oxide nanoparticles. This concentration was selected to ensure maintenance of fiber morphology while simultaneously ensuring magnetic response. To stabilize the nanofiber structure, we used a crosslinking method involving citric acid and high temperature to stabilize the gelatin via amide bonds between strands. Introducing a magnetic nanofiber mat at the interface of the hydrogel system provides remote actuation of the nanotopography through an external magnetic field. We found that the nanotopography alone directed adipogenesis, while mechanical actuation of the interface drove osteogenesis in adherent ADSCs. The adhesion characteristics suggest that the field influences the nanofiber structure, greatly enhancing focal adhesion. The field induced actuation was also found to stimulate the formation of aligned multinucleated myotubes and markers associated with maturation in adherent C2C12. Finally, we integrated the magnetic nanofiber into hydrogels as a modular system that closely resembles the fibrous network in the natural extracellular matrix. These hydrogels can be reversibly stiffened in response to external magnetic fields within cell-laden 3D constructs. Including a small fraction of short nanofibers (<3%) can significantly influence ADSC and C2C12 differentiation. As before, nanotopography was beneficial to adipogenesis while stiffening promoted enhanced osteogenesis and myogenesis. Together, this body of work provides a modular platform with broad versatility in format to study the effects of nano-topography and dynamic mechanics in cell systems. Moreover, these hydrogels and the magnetic components are cytocompatible with scope for inclusion in tissue bioreactors as a means for dynamic stimulation of cell differentiation for tissue engineering.

  • (2024) Zhou, Lu
    Thesis
    The environment and energy are two areas of great concern today. Dye pollution control and research on lithium-ion batteries (LIBs) performance are branches of these two fields, respectively. The use of piezoelectric materials to achieve piezoelectric catalysis is a relatively novel method for degrading dyes. Barium titanate (BTO), as an excellent piezoelectric material, has been widely used in the field of piezoelectric catalysis. In addition, as a bimetallic oxide with excellent stability, it also has the potential to be used to protect the cathode material of LIBs. In this thesis, BTO nanoparticles were synthesized utilizing three different methodologies: sol-gel, hydrothermal and molten salt to control the phase and microstructure. BTO nanocubes, characterized by a diameter ranging between 20-30 nm, were procured through sol-gel and hydrothermal methods, and their tetragonal phase and spontaneous polarization were achieved through annealing and ball milling processes, respectively. A subsequent modification of BTO was carried out using a Ti3C2Tx (MXene) coating, resulting in an over 90% degradation ratio of Rhodamine B (RhB) within a span of 15 min. Compared to BTO, which degrades by 40% in 15 minutes, the efficiency has approximately doubled. Additionally, this prepared composite exhibited significant outcomes in the photocatalytic and synergistic (piezo-photo) catalytic degradation of RhB. The photocatalytic degradation efficiency of unmodified BTO stood at approximately 20% after 90 minutes. In contrast, the MXene-modified BTO achieved an efficiency of around 70%, representing a significant enhancement. When subjected to simultaneous light and ultrasound exposure, the degradation efficiency witnesses a marked increase. The MXene-modified BTO degraded over 90% of RhB in just 15 minutes, while the unmodified BTO achieved 70%. The MXene-modified BTO exhibited relatively good stability. After three cycles of usage, it sustained a degradation efficiency of approximately 80%. In the context of LIBs applications, a BTO coating was applied to the surface of the cathode electrode material, which is Li(Ni0.8Co0.15Al0.05)O2 (NCA), to enhance its electrochemical properties. With the BTO coating, a relatively uniform film formed on the surface of NCA. At a concentration fraction of 1%, this coating positively impacted NCA. Performance evaluations of the battery revealed that the incorporation of the BTO coating contributed to an enhanced cycle and rate performance. Remarkably, post 200 cycles, a discharge capacity of 144 mAh/g and a retention of 86.9% of the discharge capacity was observed. Compared with 64.6% of pristine NCA, it was a relatively good improvement. In conclusion, this work provides a facile approach to controlling the phase and morphology of barium titanate nanomaterials with enhanced piezoelectric catalysis. In addition, the as-prepared barium titanate nanomaterials have been utilized as cathode coatings to prevent the side reactions between the cathode materials and electrolytes. This work demonstrates that barium titanate is a promising, multifunctional material with wide applications in energy and environment related areas.