Theses - Materials Science and Metallurgy

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Open Access
Characterisation of radiation effects in ultra-thin GaAs solar cells for space applications
Barthel, Armin
Ultra-thin solar cells show promise for application in space power systems for particularly harsh radiation environments, due to their high intrinsic radiation tolerance. The work presented in this thesis focuses on such a cell design, incorporating an 80 nm thick GaAs absorber layer, and furthers the understanding of the electrical behaviour of ultra-thin solar cells, as well as the effects of radiation on these devices and the materials comprising them. This was achieved through irradiation of the devices with a variety of radiation types and their characterisation by cathodoluminescence, both continuous wave and time-resolved, and current-voltage measurements. It is demonstrated that the preservation of short-circuit current up to high dose in ultra-thin solar cells can be attributed to the carrier lifetime remaining greater than the time taken for carriers to traverse the absorber layer, even after the lifetime is degraded severely by radiation damage. The eventual collapse in short-circuit current beyond 2 × 10¹³ MeV g⁻¹ is found to likely stem from the rapid increase in traverse time, due to carrier removal in the passivation layers. This study identifies the significant contribution of trap-assisted tunneling to the carrier recombination rate in these devices. This effect accounts for the difference in radiation-induced open-circuit voltage degradation rate between devices with absorber layer doping concentrations of 1 × 10¹⁷ and 1 × 10¹⁸ cm⁻³. Trap-assisted tunneling also partially explains the high ideality factors (> 2) of ultra-thin solar cells. The observed variation in ideality factor with radiation type is correlated with corresponding trends in open-circuit voltage degradation rate, carrier lifetime damage constants and appearance of radiation-induced defects in cathodoluminescence intensity maps. These trends are indicative of the variation in defect type with radiation type and suggest that the presence and size of defect clusters has a significant effect on the electrical behaviour of ultra-thin solar cells.
Embargo
Thermal Evolution of MAS, YSZ and GZO Thermal Barrier Coating Materials in Benign and CMAS Rich Environments
Vreeswijk, Maxim
Abradable thermal barrier coatings (TBCs) play a crucial role as thermal insulators and dynamic seals in turbine engines. However, the conventional choice, yttria-stabilised zirconia (YSZ), exhibits vulnerabilities, such as sintering induced stiffening, phase transformations, and susceptibility to molten siliceous debris. This study investigates the potential of magnesia-alumina spinel (MAS) as an alternative top coat material for TBCs due to its superior resistance to sintering and remarkable high temperature stability. The study comprises an extensive examination of the thermal evolution in MAS, encompassing stiffness, density, microstructure, and phase stability, in benign environments. Comparative analysis with YSZ showed that MAS demonstrates enhanced resistance against stiffening. This stiffening phenomenon primarily resulted from microcrack closure during initial thermal exposure, with subsequent dominance of density driven stiffening. The gradual formation of alumina phase was observed within MAS, but had minimal impact on mechanical properties due to its gradual development. *In situ* characterisation of MAS during initial thermal exposure revealed an early initiation of stiffening at low temperatures, leading to a twofold increase in Young’s modulus at 1200°C. An intermediate temperature effect (520-900°C) was observed, characterised by a sudden increase in thermal expansion and the release of energy, accompanied by shifting oxygen positions indicative of structural changes. Furthermore, the interaction between YSZ, MAS, and gadolinium zirconate (GZO) with molten debris, specifically calcia-magnesia-alumina-silica (CMAS) mixtures, was explored. YSZ exhibited significant deterioration in the presence of CMAS, with solution-reprecipitation leading to the occurrence of the tetragonal-to-monoclinic phase transformation and a rise in stiffness. In contrast, MAS and GZO displayed excellent chemical resistance to CMAS. In MAS this was attributed to the formation of a protective anorthite layer and the chemical similarities between CMAS and MAS. The CMAS-GZO interaction results in an apatite-fluorite reaction layer, limiting CMAS infiltration. Despite their merits, both MAS and GZO experience substantial stiffening during thermal exposures. In MAS, CMAS induced stiffening occurred due to the formation of a stiff CMAS layer, limiting compliance. Conversely, GZO exhibited limited CMAS induced stiffening but suffered from intrinsic stiffening. In conclusion, MAS demonstrates improved performance over YSZ in benign and CMAS rich environments. However, addressing intrinsic stiffening in the MAS system and further optimising CMAS resistance are essential to mitigate further against in service spallation.
Open Access
On the application of machine-learning to the study of glass-formation in metallic systems: Prediction, optimisation, and characterisation of novel glassy alloys
Forrest, Robert; Forrest, Robert [0000-0001-6256-888X]
Metallic glasses (MGs) are amorphous materials created by quenching a molten alloy mixture so quickly that crystalline phases do not have time to nucleate and grow, leaving a solid material with liquid-like structural disorder. This lack of crystalline ordering in MGs gives rise to a variety of desirable properties, including strong corrosion resistance and excellent soft magnetism, and leads to diverse potential applications, from golf clubs to transformer cores. The proclivity of any particular alloy composition to form a glass is known as the glass-forming ability (GFA). This is, however, poorly understood, greatly restricting the development of novel alloys. Many empirical rules and criteria aiming to describe the GFA of alloy compositions have emerged since MGs were first discovered in 1959, but they have limited predictive power and do little to demonstrate fundamental understanding. This thesis explores the application of the ‘fourth paradigm’ of scientific research, that being the use of machine-learning (ML) to model relationships within large datasets, to the study of glass formation in metallic systems, with the dual aims of improving understanding of the phenomenon of glass formation itself, and accelerating the identification of novel glass-forming alloys. In this thesis, a neural-network model is applied to predict simultaneously the liquidus temperature, glass-transition temperature, crystallisation-onset temperature, maximum glassy casting diameter, and probability of glass formation, for any given alloy. Feature permutation is used to identify the features of importance in the black-box neural network, recovering Inoue’s empirical rules, and highlighting the effect of discontinuous Wigner-Seitz boundary electron-densities on atomic radii. With certain combinations of elements, atomic radii of different species contract and expand to balance electron-density discontinuities such that the overall difference in atomic radii increases, improving GFA. Adjusted atomic radii are calculated via the Thomas-Fermi model and this insight is used to propose promising novel glass-forming alloy systems. Building on the predictive capabilities enabled by the ensemble neural-network model, further new glass-forming alloys are identified by means of a genetic algorithm (GA); the genetic operators of competition, recombination, and mutation to a population of trial alloy compositions, with the goal of evolving towards candidates with excellent glass-forming ability. The size of composition space means even coarse grid-based searches for interesting alloys are infeasible unless heavily constrained, which requires prior knowledge and reduces the probability of making novel discoveries. GAs provide a practical alternative to brute-force searching, by rapidly homing in on fruitful regions and discarding others. In this work, optimisation focuses on the maximum casting diameter of a fully glassy rod, the width of the supercooled region, and the price-per-kilogramme, to identify commercially viable novel glass-formers. The GA is also applied with specific constraints, to identify aluminium-based and copper–zirconium-based glass-forming alloys, and to optimise existing zirconium-based alloys. In particular, aluminium-lanthanide alloys are identified to be of relatively low cost-per-kilogramme and high GFA. Finally, this thesis studies the properties of crystalline and glassy alloys via atomistic simulations. Ab initio quantum-mechanical calculations are typically too computationally expensive for investigations of glassy materials, requiring the use of approximate inter-atomic potentials. Due to the novelty of the alloy composition selected from GA output, namely aluminium-ytterbium, the creation of new interatomic potentials is required. Classical embedded-atom-method (EAM) potentials and machine-learning-based Gaussian approximation potentials (GAPs) and spectral-neighbour-analysis potentials (SNAPs) are generated, and used to model the material properties of Al-Yb in crystalline and glassy forms, producing predictions including equilibrium densities and elastic moduli. Characterisation of the properties of these materials enables the suggestion of engineering applications for which they would be suitable, with the glasses predicted to be competitive with crystalline materials for a variety of uses.
Open Access
Predicting microstructural heterogeneities in the laser powder bed fusion of γ′ strengthened nickel-base superalloys
Velasco Castro, Mauro
This work addresses the formation of microstructural heterogeneities during the laser powder bed fusion (LPBF) of CM247LC, a nickel-base superalloy with high γ′ content and hence, highly prone to weld-cracking. Multiple cracking mechanisms have been identified as active in the LPBF of various precipitation-strengthened superalloys. However, there is no precise understanding of how LPBF thermal processing conditions induce the development of specific cracking characteristics. Through a thorough revision of the literature, discrepancies in the formation and distribution of various microstructural heterogeneities were first identified. These primarily correspond to the coarsening mechanism of small γ′ precipitates, and the apparent nucleation of the γ/γ′ eutectic phase. It is proposed here that a closer examination of the development of these microstructural features can lead to new insights into the physical mechanisms driving the formation of cracks upon rapid solidification. To study the previously mentioned, a multiscale modelling framework has been cooperatively utilised to capture the influence of LPBF thermal processing conditions on the solute partitioning behaviour, which ultimately dictates the nucleation of secondary phases and the evolution of the microstructure. The integrated modelling framework (IMF) comprises three different numerical methods: (1) CALPHAD approach, (2) finite element analysis (FEA), and (3) multi-phase field modelling. CALPHAD-based models were utilised to explore how alloying additions affect the material's printability properties due to changes in phase transitions, phase-fraction evolution, and solute partitioning tendency. From this analysis, a solid-state transformation from a supersaturated γ solid-solution is proposed to better explain the inconsistent documentation of γ/γ′ eutectic, among other microstructural heterogeneities. To link thermal processing conditions with the microstructure evolution, a heat transfer analysis was performed to elucidate the thermal history induced in LPBF processing. This study delivered a more consistent description of the thermal history when incorporating the thermal conductivity, κ_tc(T), and heat capacity, CP (T), as function of temperature. The nonlinear temperature profiles delivered by this heat transfer analysis were subsequently coupled to the multi-phase field numerical framework. The coupling of a nonlinear thermal history demonstrated that no proportional correlation between the scanning speed, ν, and the induced cooling rate, $\dot T$, can be anticipated, as it is conventionally assumed in the 3D printing of metals. This coupling scheme delivered compelling insights into the development of various microstructural heterogeneities. These simulations were contrasted against the experimental results reported here. In the present work, the as-built microstructure, cracking characteristics and chemical analysis were examined via high-spatial resolution imaging in electronic microscopy. Particular examination was directed to regions in the vicinity of a crack feature. The solute partitioning tendency was found to be dissimilar at regions distant and close to the notch of the crack. This phenomenon was extensively discussed and compared with multi-phase field predictions. Qualitative agreement between experiments and simulations is consistent when examining the solute partitioning tendency. These results delivered a more consistent description of the development of microstructural heterogeneities and their connection to the formation of cracks.
Embargo
The potential for superlattice precipitate reinforcement in titanium-iron alloys
Mellor, Rosie
In the pursuit of novel structural metallic alloys, the concept of alloys featuring ordered B2 precipitates within a BCC matrix offers a promising possibility due to the potential synergistic benefits of a matrix with superlattice precipitates. The titanium-iron system is selected for this purpose, particularly as both are abundant elements, which should address cost and sustainability concerns. Titanium is an ideal base element owing to its high specific strength and solubility for a range of transition metals, though the BCC phase is metastable at lower temperatures. Additions of Fe serve to stabilise this phase and allow for formation of the desired superlattice phase precipitates. However, the lattice misfit between these phases is large in the binary system, which limits precipitate-matrix coherency, and may prevent full exploitation of the superlattice benefits. In principle, the lattice misfit may be tuned through compositional modification. Consideration must also be given to alloying that ensures sufficient environmental resistance and phase stability. This work explores the possibility of viable titanium-iron-based superlattice precipitate reinforced alloys through the introduction of ternary additions. The phase equilibria in Ti-Fe-X systems were investigated, particularly in the vicinity of the BCC + B2 two-phase field. In Chapter 4, the inclusion of Cu is shown to lead to a BCC + B2 two-phase field that is relatively large in compositional extent, particularly at higher temperatures. Such a result should allow considerable additions of Cu to titanium-iron-based BCC+B2 alloys, which could be particularly beneficial for the mechanical properties. In Chapter 5, the BCC+B2 two-phase field in the Ti-Fe-Co system is found to be limited in compositional extent by a Ti-Co intermetallic phase that has particularly high Fe solubility, significantly limiting the potential addition of Co in BCC+B2 alloys. Investigation of the phase equilibria in the Ti-Fe-Al system in Chapter 6 shows that the intermetallic Ti3Al phase is present in all alloys with an Al content of greater than 5 at.% at 800 °C and below. However, at 1000 °C, the majority of the alloys were in a stable BCC+B2 two-phase field, as desired. Lattice parameter measurements demonstrate that Al significantly reduces the lattice misfit between BCC and B2 phases, whilst Cu increases the lattice parameters of both phases simultaneously, leading to negligible effect on the overall lattice misfit. The effect of Co on the lattice misfit is inconclusive owing to limited experimental investigation of alloys containing a BCC+B2 microstructure. The inherent metastability of the BCC phase in the Ti-Fe-Al alloys leads to a range of nanoscale modulated structures that form by apparent decomposition of the BCC phase, resulting in hierarchical microstructures, and very high hardness. These effects are explored in Chapter 7. The findings reveal opportunities for exploring alternative design concepts, and highlight the versatility of these systems as the basis for novel high-strength metallic alloys.
Open Access
Conformable and robust microfluidic force sensors for orthopaedic surgery
Ives, Liam; Ives, Liam [0000-0001-8705-7269]
Quantitative force feedback for orthopaedic surgeons is crucial for the accurate positioning of implants during total hip and knee arthroplasty (THA and TKA respectively). During the trial stage of THA, the surgeon manually determines the implant size, positioning, and range of joint motion of the femoral (ball and stem) and acetabular (socket) parts of the hip implant using a trial acetabular liner, before discarding the liner and replacing it with the final liner. Poor implant positioning during this stage can lead to implant failure, which harms patient well-being and increases pressure on healthcare systems. As a result, incorporating force sensors within this trial hip liner during the trial stage of THA could improve surgical outcomes, by giving surgeons real-time quantitative force feedback to aid implant positioning, and by acting as a guidance tool for trainee surgeons. However, existing force sensors cannot be easily incorporated into the trial hip implant, due to the small and complex geometry of the joint, so there is a crucial need for an alternative technology to be developed. Here, a novel thin, flexible, conformable, and robust microfluidic force sensor is developed which can be incorporated into the hip implant liner during the trial phase of total hip arthroplasty. The sensor comprises a thin, flexible polyimide (Kapton, PI) substrate, onto which silver electrodes are deposited using aerosol-jet printing (AJP). The substrateis bonded to a microfluidic chip, which comprises a flexible deformable elastomer with an embedded fluid reservoir and channel. When an external force is applied to the reservoir portion of the microfluidic chip, the reservoir is compressed and displaces fluid through the microfluidic channel. As the fluid is displaced through the channel, it overlaps with the electrodes, changing the dielectric properties of the channel, and therefore changing the electrical impedance measured by the electrodes. The AJP printing parameters were optimised for the silver electrodes, and the resistance of the printed silver changed by up to 25% after 500 cycles of flexion. For the microfluidic chip, a study was done to compare the suitability of several elastomers such as polydimethylsiloxane (PDMS), Flexdym, and stereolithography 3D printing (SLA) 3D printing resins using a range of characterisation techniques. By conducting a series of T-peel adhesion tests, Flexdym was found to have the largest and most tuneable bonding strength to Kapton, compared to PDMS and SLA resin. Hyperelastic models were used to describe the mechanical behaviour based on tensile and compressive testing, with the Mooney-Rivlin 5-parameter model being the most suitable for all elastomers. Profilometry and contact angle goniometry indicated a high dependence of elastomer fabrication parameters on the wetting interaction between the liquid and the channel. Rheometry results indicate that PDMS has the shortest stress relaxation time, but its low stiffness makes it unsuitable for large applied forces, while the 3D printing resin has the opposite issue. The sensor was calibrated using a linear motor, which consists of a stationary part with attached load cell and a translating part with an attached pressing arm. The sensor was mounted onto the stationary part, and a sinusoidal force with known frequency and amplitude was applied to the fluid reservoir using the pressing arm. The resulting impedance change was measured using an impedance analyser, and was determined to have a high dependence on the type of elastomer, the device dimensions, and the magnitude and frequency of the applied force. The required properties of the sensor, such as the maximum detectable force, were optimised by material choice and device design, both experimentally and using finite element modelling, and it was found that the sensors could be reliably calibrated for forces up to approximately 20N, which is a couple of orders of magnitude below the forces applied during THA and TKA. To mimic the hip joint geometry, a novel trial hip implant liner which incorporates six microfluidic force sensors was designed using computer-aided design software, and prototypes were fabricated using SLA 3D printing from Flexible Resin and Durable Resin (Formlabs, United States). To calibrate the sensors within this prototype trial liner, a bespoke mechanical testing rig was developed using a combination of machined and SLA-printed components. The rig consisted of a polycarbonate base and an SLA-printed (Clear Resin, Formlabs) insert which housed the prototype hip liner. An aluminium rod containing a ceramic (aluminium oxide) femoral head was attached to a mechanical testing machine and used to apply forces of up to 1 kN to the prototype liner. The insert was rotated inside the rig to orient the liner from −30◦ to 30◦ to the femoral component of the test rig, to represent a surgeon applying forces at a range of angles to the liner during the trial phase of THA. By taking advantage of the force shielding effect of incorporating sensors into the stiff trial liner, the sensors were reliably calibrated up to approximately 1 kN. In order to assess balance in the test rig, the changes in capacitance were compared to finite element simulations of a symmetrical, balanced, ball and socket joint. Such a high force range is novel for a microfluidics-based force sensor, and so this technology is a potentially powerful surgical tool to guide orthopaedic surgeons.
Restricted
Effects of environment and mechanical loading on metallic coatings on single crystal nickel superalloys in hot corrosion conditions
Poli, Eleonore
[Restricted]
Open Access
Improving Oxidation Resistance in Polycrystalline Ni-based Superalloys for High-Temperature Applications
Wo, Jackson; Wo, Jackson [0000-0002-7635-947X]
The design of new polycrystalline Ni-based superalloys with enhanced oxidation resistance can enable higher operating temperatures in aeroengines with increased fuel efficiencies and minimised CO₂ emissions. This thesis investigated the oxidation behaviour in polycrystalline Ni-based superalloys with a combination of experimental and computational methods. The transient oxidation behaviour of the commercial alloy, RR1000, was investigated at 800°C with several characterisation techniques. The results showed a systematic evolution of oxide spectra with increasing oxidation times that could be related to the pseudo-linear kinetics. The sub-parabolic oxidation kinetics of RR1000 also differed significantly from similar studies. Wagner’s criteria for the internal-to-external oxidation transition for the formation and maintenance of a continuous Al₂O₃ scale were used in computational models to assess commercial alloys and an experimental alloy (Alloy X). The models successfully predicted the oxidation behaviour of the commercial alloys but underpredicted the transition temperature of Alloy X by 50-100°C, which may have been due to complex oxide formation. The model was used to design a pre-oxidation treatment for instigating an internal-to-external oxidation transition in a new Cr₂O₃-forming Ni-based superalloy (C19). The pre-oxidation treatment at 1100°C for 1 hour resulted in a continuous Al₂O₃ scale in C19, which dramatically reduced oxidation damage during further exposure at 800°C for 100 hours. The γ′ size evolution and topologically close-packed phase formation in C19 were also investigated and compared to RR1000. C19 exhibited insignificant coarsening at 700°C but showed more coarsening at 800°C after 1000 hours. The σ phase was also detected after 800°C and 1000 hours. The results suggested that C19 was less prone to coarsening than RR1000 but more work is needed to demonstrate its commercial viability. The oxidation performance of C19 at 800°C was further examined with systematic substitutions of Nb, Ta, and Ti for Al concentrations. The addition of Nb improved the oxidation resistance, which differed from the literature and may have promoted the formation of a CrTaO₄ layer as a diffusion barrier to oxygen ingress. The addition of Ta significantly improved the oxidation resistance and may have been due to the formation of AlTaO₄. The addition of Ti did not significantly affect the oxidation resistance, which may have been due to the relatively low Ti concentrations in the investigated alloys. Several of the samples with modified Ta and Ti concentrations also exhibited regions of continuous Al₂O₃ scale formation, suggesting that Type III oxidation behaviour could be induced in C19 with further refinement.
Open Access
Nanoscale Characterisation of Heterointerfaces in 2D Materials
Ramsden, Hugh
For the last 80 years, bulk materials, such as silicon, have underpinned electronic devices[1]. Whilst remarkable improvements to performance have been achieved through miniaturisation, progress is slowing as we approach the physical limitations of bulk materials[1, 2]. As materials which naturally exist as 3D crystals, thinning them means detrimental surface effects become more and more dominant. Therefore, to continue pushing device performance, a new class of materials must be used, one that intrinsically lends itself to miniaturisation. Layered materials naturally consist of individual stacked sheets. When thinned down to the monolayer, the resulting ‘2D’ crystals are a few atoms thick, but maintain their crystal structure, allowing the physical limits of bulk materials to be side-stepped. For this reason, to continue making progress, leading chip manufacturers are projecting that by 2028, 2D materials will be utilised at the core of semiconductor device technology[3]. However, to realise widespread adoption of 2D materials for device applications, some key challenges need to be overcome. Whilst intrinsically these materials possess the properties needed to supersede bulk materials, to form devices, these materials must be integrated into larger systems. Increasingly, work is showing that the nature of the interfaces formed during integration are a key factor that is limiting device performance. At present however, characterising these interfaces is challenging. As atomically thin materials, naturally, effects that influence device performance on the macroscale may only be detectable on the nanoscale. Whilst some techniques exist that can reach these length-scales, they often suffer from issues such as requiring destructive, complex sample preparation or direct contact with a pristine surface. In this thesis we develop and demonstrate a range of approaches for performing nanoscale characterisation of heterointerfaces in 2D materials. We first examine interfaces in 2D lateral heterostructures of monolayer transition metal dichalcogenides (1L-TMDs). Studies on the synthesis of these structures have shown a range of interface widths can occur, which will influence their properties. We show that information about the width of the interface can be determined by examining the line-shape of micro-Raman and photoluminescence measurements, even if the interface varies at length-scales below the probe width. This offers a facile non-destructive route to assessing interfacial widths. We then move on to assessing van der Waals heterostructures using scanning electron microscope cathodoluminescence (SEM-CL). With this, we identify nanoscale inhomogeneities in dielectric environment and strain, which we believe are a result of sample fabrication. Furthermore, we show adopting more advanced fabrication processes improve these effects. We also show that these techniques can be employed on complex device structures comprising multiple integrated 2D materials. The next part of this thesis focused on employing a set of techniques collectively known as conductive mode SEM (CM-SEM). These techniques allow the mapping of the flow of carriers through a device. Prior to this work, CM-SEM had not been employed on 1L-TMDs in van der Waals heterostructures. To ensure proper interpretation of results, we performed Monte-Carlo simulations of electron sample interactions for such samples, finding depending on the acceleration voltage, up to 70 % of beam electrons can be deposited into the sample. We then used CM-SEM to observe this process, confirming electrons injected into the sample can accumulate in both hBN and 1L- WSe₂. We then find that this accumulation of charge appears to increase the electrical conductivity of hBN and decrease the luminescent efficiency of WSe₂. We then study a transistor device structure through CM-SEM and map out the electric fields caused by Schottky barriers at the contacts, finding nanoscale variations. Finally, we perform CL on a 2D lateral heterostructure sample, allowing characterisation of its interface with a nanoscale probe. We use these signals to demonstrate the universality of the spectral interface modelling put forward, allowing identification of interfaces below 250 nm wide. To summarise, the findings in this thesis detail approaches for the characterisation of both lateral and van der Waals heterostructures. With these, it is hoped new insights can be gained that allow for the rational optimization and design of devices comprising 2D materials. They also may allow for the uncovering of new fundamental effects. [1] M. Mitchell Waldrop. The chips are down for Moore’s law. Nature News, 530(7589):144, February 2016.
[2] Stuart Thomas. An industry view on two-dimensional materials in electronics. Nature Electronics, 4(12):856–857, December 2021.
[3] IRDS™ 2022: More Moore. Technical report, The International Roadmap for Devices and Systems.
Open Access
Ultraviolet-Assisted Atmospheric-Pressure Spatial Atomic Layer Depositions for Tuning ZnO Properties
Raninga, Ravi
This thesis investigates the development of UV-assisted Atmospheric Pressure Spatial Atomic Layer Deposition (UV-APSALD) as a technique for depositing ZnO films. A study of the current literature identifies the need for high-quality metal oxide thin films; metal oxides are near ubiquitous in modern day electronic applications, from optoelectronics and transistors, to superconductors and energy storage. As the electronic properties are strongly tied to the microstructure and chemistry of the metal oxides, it is vital to have a deposition method whereby these properties can be tuned. A review of the current deposition methods identifies a niche for an open-air, energy-assisted thin film deposition method. A significant proportion of this doctorate was devoted to the design, construct and assembly of the components to enable the UV-assisted depositions. These design and engineering considerations are discussed herein. A systematic study of the substrate-manifold distance *versus* film thickness indicates that changing the substrate-manifold distance is a key factor in determining whether the reactor operates in atomic layer deposition (ALD) mode or chemical layer deposition (CVD) mode. UV-assisted depositions are shown to alter the crystallographic and electrical properties of ZnO; *in-situ* UV light, is found to suppress ZnO growth in the (002) direction, and increase the resistivity by two orders of magnitude. When UV-assisted depositions are coupled with N doping, the films are approximately five orders of magnitude more resistive. It is proposed that the UV-light-assisted depositions increase the presence of charged reaction intermediates which sterically hinder the polar (002) facet, thereby promoting growth in the non-polar (100) direction. These resistive UV-ZnO:N films are incorporated as the channel layer into functioning thin-film transistors, further demonstrating the utility of the UV-APSALD deposition system. Finally, epitaxial ZnO is grown on c-sapphire at 250 oC; to the best of my knowledge, this is the first demonstration of open-air epitaxial growth of ZnO via CVD or ALD. Further, the degree of epitaxy is improved by reducing the growth rate to get a more controlled deposition.
Embargo
Understanding the formation and influence of the omega phase in metastable beta Ti-Nb based alloys
Talbot, Christian; Talbot, Christian [0000-0003-1619-5423]
Metastable β-Ti alloys have potential applications ranging from low modulus biomedical alloys to vibration damping in aerospace. This is, in part, due to the ability of certain compositions to undergo a stress induced transformation to the martensitic α″ phase, enabling superelasticity. However, the inability to control key properties of the transformation prevents industrial uptake, with the presence of the hexagonal ω phase often reported as a primary cause for such challenges. Despite over 70 years active research, the formation of ω is difficult to predict and there is much contention over its influence on β phase decomposition and subsequent properties. These issues are compounded by the existence of two crystallographically identical, but mechanistically distinct, forms; athermal ω_ath and isothermal ω_iso. This work studied the ω phase within the Ti-Nb alloy system through in situ synchrotron X-ray diffraction, with the aim of investigating its formation, stability and influence, particularly with respect to superelasticity. It was shown that ωath readily formed through a metastable mechanism. The diffusional form, ω_iso, was shown to be a transient of the more stable α phase, with α being the direct decomposition product in alloys with sufficiently high internal strains. The formation of ω_iso was found to be suppressed by the addition of Zr, which reduced both the intragranular strain and interphase misfit of the evolving ω_iso phase. Whilst the addition 4 at.% Sn to Ti-24Nb prevented ω_iso growth entirely, potentially due to the electronic structure of Sn. Crucially, ω_ath did not prevent superelasticity. Instead ω_ath was readily consumed by the growth of α″ during mechanical loading, this is important as it highlights that a number of the issues surrounding superelasticity in these alloys cannot be attributed to the presence of ω_ath. In contrast, ω_iso prevented superelasticity in larger volume fractions. This knowledge was extended into the commercial alloy system, Ti-2448 (Ti-24Nb-4Zr-8Sn, wt%), where the presence of Zr and Sn was shown to significantly suppress ω_iso evolution, especially at low temperatures. This suppressive effect was subsequently utilised to study the effect of smaller ω_iso volume fractions on superelasticity, which altered key characteristics of the transformation, potentially to the benefit of specific applications. These insights significantly expand our understanding of ω. They highlight that, with respect to superelasticity, ω_ath is not as problematic as reported in sections of the literature - a critical observation given the ubiquity of this form of ω. Additionally the addition of Sn and Zr, whose efficacy in suppressing ω has recently been questioned, identified potential mechanisms by which ω_iso formation can be controlled or prevented, opening new avenues for alloy design and improving the tolerance of this class of alloys to ω formation.
Embargo
Vertically Aligned Nanocomposite Thin Films for Micro-Battery and Nanoionic Applications
Lovett, Adam; Lovett, Adam [0000-0002-3076-2992]
High energy and power density rechargeable micro-batteries are a necessity for powering the next generation of flexible electronics, internet of things and MedTech devices. In theory, significant improvements in the capacity, current and power densities of micro-batteries would result if 3-dimensional architectures were used, as they have both enhanced interdigitated component interface areas and shortened ion diffusion path lengths. Vertically aligned nanocomposite (VAN) films, an example of a thin film 3D architecture, have shown promise in solid oxide fuel cells devices displaying enhanced ionic conductivity, reduced areal surface resistances and improved cell performance by enhancing the interfacial surface area. These VAN attributes may be transferable to solid-state micro-batteries, enabling improvements in the aforementioned battery properties, while also compensating for intrinsic low diffusivity due to nanoscale path lengths. VANs can be grown with high control of the crystallographic and interface orientation and act as a scaffold to stabilize challenging phases; hence, systems can be optimized to maximize capacity and performance, particularly when working with materials with anisotropic properties. Thus, VANs may allow a wider selection of materials to be utilised in miniaturised batteries. In the first part of this thesis, two battery based vertically aligned nanocomposite thin films are presented. The first system, a LL(Nb,Ti)O-(Ti,Nb)O₂ VAN, showcases the essential physical properties of a VAN film that could be incorporated into an all solid-state battery. Namely, high Li⁺ ionic conductivity and distinct regions of electron conducting nanocolumns embedded in an insulating matrix. The second system, a LiMn₂O₄-SrRuO₃ VAN, is comprised of LiMn₂O₄ cathode nanocolumns embedded in a SrRuO₃ current collecting matrix. This system exhibits high discharge capacities and excellent rate performance. It is demonstrated that the cycling performance is dependent on both the LiMn₂O₄ columnar orientation and dimensions. In the third part of this thesis, a new potential current collector is explored, NiCo₂O₄. Preliminary work on epitaxial LiMn₂O₄ films grown on NiCo₂O₄ current collector is presented, demonstrating that it is possible to grow epitaxial LiMn₂O₄ at 360 °C, ∼ 200 °C lower than previously reported. This cathode system also displays a high discharge capacity, > 100 mAh g⁻¹ for 6000 cycles. The final chapter deviates away from batteries and explores the stabilisation of oxide ion conducting fluorite δ-Bi₂O₃. A series of systems are discussed, cumulating in the development of a dysprosium stabilised δ-Bi₂O₃-DyMnO₃ VAN exhibiting an ionic conductivity of 10⁻³ S cm⁻¹ at 500 °C. This thesis communicates important advancements for the micro-battery community. First, two new battery oxide VAN systems are introduced, expanding the total systems reported to three (at the time of writing). With the first VAN system discussed, LL(Nb,Ti)O-(Ti,Nb)O₂, promising physical properties are reported alongside key criteria that should be met in order to develop a battery VAN that exhibits clear redox behaviour. These criteria are successfully implemented in the second VAN presented, LiMn₂O₄-SrRuO₃. This VAN system exhibits clear LiMn₂O₄ redox, the first VAN to achieve this feat, and demonstrates remarkable capacity retention under high-rate regimes. It is also shown that the electrochemical performance is dependent on both the LMO pillar crystallographic orientation and dimensions. The latter dependence has not been shown before and it has very important ramifications for the development and future of 3D architectured thin film VAN batteries. Beyond VAN batteries, the LiMn₂O₄/NiCo₂O₄ system reduces the epitaxial growth temperature of LiMn₂O₄ to within the complementary metal-oxide-semiconductor stability window (< 450 °C). This shows a route towards and may enable the implementation of epitaxial cathodes in next-generation micro-batteries.
Open Access
Ultrasonic Welding of Glassy Thermoplastic Polymers
Wise, Roger Jeremy
Ultrasonic welding of thermoplastic polymers has been practiced industrially for many years without being fully understood. This study includes an analysis of the energy dissipation mechanisms responsible for the formation of ultrasonic welds in PMMA including viscoelastic loss and by plastic deformation. The physical effects contributing to the formation of an ultrasonic weld in glassy thermoplastics are identified and used in the derivation of a classification for all welding techniques for these materials. Ultrasonic welds were studied using high speed photography, the measurement of polymer chain orientation and residual stress, the measurement of contact dynamics at the initiation of welding and the influence of polymer chain interdiffusion by reference to simple thermal welds. A description of defects present in an ultrasonic weld is given and this is extended to a proposed scheme for defects in welds made using all welding techniques.
Open Access
Computational modelling of interfacial failure behaviours in polymer–metal joints
Suganuma, Yoshitake; Suganuma, Yoshitake [0000-0003-2410-9214]
This thesis is broadly aimed at developing a better strategy to achieve enhanced interfacial properties in polymer–metal/metal oxide joints using computational techniques such as density functional theory (DFT), molecular dynamics (MD), and dissipative particle dynamics (DPD) methods. Our calculations provide support to a valuable design principle that increasing polymer stiffness can improve the strength of polymer–metal/metal oxide joints even when interfacial failures are observed. Additionally, we reveal that, as the polymer stiffness increases, the chemical functionality within polymers can work more significantly to improve the interfacial strength of polymer–metal/metal oxide joints. Bonding technologies between polymers and metals/metal oxides are required in many industries such as the transportation sector, including automotive, aviation, and maritime industries, where multi-material architectures should be achieved with the aim of weight reductions to decrease global greenhouse gas emissions. However, there is not yet a full understanding of the contributions of influential parameters such as stiffness and chemical functionality of polymers to the interfacial properties, which makes designing joining processes difficult. This thesis focuses on polymer–metal joints consisting of isotactic polypropylene (iPP), iPP grafted with maleic anhydride (iPPgMA), or iPP grafted with amine groups (iPPgNH₂) and hydroxylated γ-Al₂O₃, which is a model for an oxidised aluminium surface, and investigates the contributions of the stiffnesses of iPP, iPPgMA, and iPPgNH₂, and chemical functionalities (MA and NH₂ groups) in iPPgMA and iPPgNH₂ to the interfacial failure behaviours. In Part I containing between Chapters 2 and 4, using joints models between iPP, iPPgMA, or iPPgNH₂ and a flat surface of hydroxylated γ-Al₂O₃, we investigate the influences of the stiffness of a polymer component and the chemical functionalities within two grafted polymers, iPPgMA and iPPgNH₂, on the interfacial failure behaviours on a flat surface. Our calculations reveal that higher Young’s moduli of iPP, iPPgMA, and iPPgNH₂ lead to a higher tensile strength of the joint models even in interfacial failures. Moreover, as the Young’s moduli of iPPgMA and iPPgNH₂ increase, their functional groups of MA and NH₂ groups improve their interfacial strengths more significantly. Additionally, based on these findings, we suggest a protocol that requires much reduced computational resources to compare different functional groups within polymers with regard to their bonding abilities to metal/metal oxide substrates. This proposed procedure successfully provides consistent results that iPPgNH₂ shows the highest interfacial strength with an alumina surface, followed by iPPgMA, and then non-grafted iPP, relative to experimental observations. In Part II consisting of between Chapters 5 and 7, a coarse-grained joint model between iPP and an alumina surface is developed as a preparation for investigating the effect of the stiffness of iPP on the interfacial failure behaviours on a porous surface in the DPD method. DPD parameters between iPP beads and those between iPP and surface beads are derived by Bayesian optimisation and validated from the comparison between MD and DPD methods. Finally, in Chapter 8 in Part III, using a coarse-grained joint model between iPP and a porous alumina surface, the effect of the stiffness of iPP on the interfacial failure behaviours on a porous surface is examined. The results from DPD simulations demonstrate that a higher Young’s modulus of iPP results in an increased interfacial strength on a porous surface even in interfacial failures. Generally, when interfacial failures are observed in mechanical tests on polymer–metal/metal oxide joints, improving the interfacial interactions may seem to be the most effective way to enhance the strengths. Nevertheless, our findings offer another useful strategy of increasing the polymer stiffness. Some chapters of this thesis are based on the following manuscripts published in journals. 1. Y. Suganuma and J. A. Elliott. Effect of Varying Stiffness and Functionalization on the Interfacial Failure Behavior of Isotactic Polypropylene on Hydroxylated γ-Al₂O₃ by MD Simulation. ACS Applied Materials and Interfaces, 15(4): 6133–6141, 2023. https://doi.org/10.1021/acsami.2c19593 2. Y.Suganuma and J.A.Elliott. Isolating the Effect of Crosslink Densities on Mechanical Properties of Isotactic Polypropylene Using Dissipative Particle Dynamics. Macro- molecular Theory and Simulations, 2300014, 2023. https://doi.org/10.1002/MATS.202300014
Restricted
Development of zincblende GaN grown on 3C-SiC/Si substrates for LED applications
Gundimeda, Abhiram; Gundimeda, Abhiram [0000-0001-5208-1920]
[Restricted]
Open Access
Improvement of Cathode Material for Solid Oxide Fuel Cell through Surface Infiltration and Electrospinning
Gao, Chenlong
In this study, two methods have been used to enhance the catalytic property of the La_0.6Sr_0.4Co_0.2Fe_0.8O₃/Ce_0.9Gd_0.1O₂ (LSCF/CGO; LSCF:CGO = 1:1) cathode in solid oxide fuel cell applications. One method involved altering the surface of the LSCF/CGO by including metal oxide nanoparticles. By utilizing an infiltration approach, Co₃O₄, NiO and CuO nanoparticles with a particle size of 10–20 nm has been effectively deposited onto the surface of the LSCF/CGO composite cathode. The as infiltrated cathode showed significant reduction in the overall area-specific resistance (ASR) at 500^oC (1.72Ω•cm², 2.5Ω•cm², and 3.9Ω•cm² respectively), which was nearly 4-10 times smaller than the non-infiltrated LSCF/CGO (15.5Ω•cm²) at 500^oC. The enhanced electro-catalytic active sites that these nano decorations on the cathode's surface can provide were ascribed to this improvement. It was discovered that these metal oxides increased the density of reaction sites on the surface, encouraging improved surface oxygen ion exchange sites at the interface between the basic cathode materials and the catalytic oxides. The oxygen ions supplied by these metal oxides assisted in reducing the oxygen vacancy concentration on the surface of LSCF and suppressing the Sr surface segregation, which was verified by SEM and XPS, therefore slowing the pace of cell performance degradation. Impedance experiments utilizing symmetrical cells, where the performance degradation rate of LSCF/CGO at 500^oC dramatically decreased because of Co₃O₄, NiO, and CuO nanoparticles metal oxide infiltration, verified the cathode's performance. In this work, it has also been recognized how the silver particles on the LSCF/CGO surface affect the cell's overall catalytic properties. The silver current collector was also discovered to be partially responsible for the decline in cell function with age. By infiltrating Co₃O₄ nanoparticles into both the silver current collector and the LSCF/CGO, the cell degradation rate declined from 1.78 Ω•cm²/hour to 0.06 Ω•cm²/hour at 500^oC. In a different method, an electro-spun cathode was employed in place of a conventional powder-formed cathode. This cathode had a fibrous structure. By meticulously regulating the electrospinning parameters, such as voltage, solution concentration, and the distance between the needle and the collector, the method of creating nanofibres has been researched and perfected. By coating the electrolyte with uniform CGO fibres covered with LSCF nanoparticles, a cathode was created. The fibrous cathode demonstrated much reduced ASR (about 5 times smaller) than the powder-formed cathode in the temperature range of 500^oC to 650^oC. It has also been researched and adjusted how the loading of LSCF affects CGO fibre. The quantity of LSCF has been proposed by modeling and experimental evidence.
Open Access
Progress towards quantitative dopant profiling with the scanning electron microscope
Kazemian, Payam
One of the ten most important challenges facing the semiconductor industry is to obtain an accurate quantitative two-dimensional dopant profiling technique with high spatial resolution. Secondary electron (SE) imaging in the scanning electron microscope (SEM) has been shown to be a very promising technique for dopant profiling in the past. However, the limited accuracy obtained with SE imaging (> ±20% dopant sensitivity) has hindered this technique from being used for quantitative studies. In this dissertation, we develop the novel technique of SE energy-filtering in a field-emission gun SEM (FEG-SEM). We demonstrate that energy-filtered imaging can be used to increase the prediction accuracy of dopant concentrations (~8.5 % dopant sensitivity) because it allows one to measure the shift in SE energy distributions that are caused by the potential difference across the pn-junction. The surface potential difference across a pn-junction is a function of the dopant concentration change across the device, hence enabling us to quantify the dopant concentration. This method provides much more accuracy and reproducibility compared to conventional SE imaging where often all SEs are allowed to be detected. In addition, we explore the spatial resolution limits of dopant profiling with the FEG-SEM. The effect of some of the imaging conditions on the dopant contrast (that is SE yield differences) observed from SE images has been examined, and recommendations for optimum conditions for the quantification of dopant profiling are given. Also, the potential of site-specific sample preparation for dopant profiling in the SEM using a focused ion beam (FIB) workstation has been explored. Contrast was observed in SE images in the SEM after FIB milling. These experimental results have provided insight into some of the SE emission mechanisms responsible for the observed dopant contrast.
Controlled Access
Steady State and Time-Resolved Cathodoluminescence Analysis of III-Nitride Semiconductors
Loeto, Kagiso; Loeto, Kagiso [0000-0002-1694-2102]
Cathodoluminescence is a tool that is used to investigate optical emission from semiconductors. The technique operates by exciting the sample with a focused electron beam such that it becomes excited and emits photons which are then analysed. The technique can be operated with a continuous electron beam but can also be utilised in time-resolved mode by pulsing the electron beam. In this thesis, steady state and time-resolved cathodoluminescence are used to investigate the properties of III-nitride semiconductors. The instigation is split into three main research topics which will now be summarised. Firstly, cathodoluminescence will be applied to the investigation of InGaN/GaN core-shell nanorods for the purposes of light emission as light-emitting diodes. This work focuses on understanding the structural and optical emission inhomogeneities across the nanorods and their relationship to their fabrication processes. Importantly, the nanorods have exposed non-polar, semi-polar and polar surfaces. By using time-resolved cathodoluminescence, the difference in charge carrier dynamics at these exposed crystallographic facets will be investigated. This is carried out to assess the effect of polarity on the degree of carrier recombination. Secondly, cathodoluminescence will be used as a novel quantification technique in buffer structures of high-electron-mobility transistors. The idea here is to quantify the change in the alloy composition from the AlGaN layer of the structure and the carbon doping concentration from the GaN layer. The change in alloy composition is quantified by considering the change in emission wavelength while the carbon concentration is evaluated by utilising the change in emission intensity. Lastly, cathodoluminescence is applied to assess the radiation damage in GaN layers from Ga and Xe ion milling processes. Here, the emission intensity is used to compare the level of damage produced by the two milling species.
Open Access
Developing next generation, non-toxic, inorganic materials for photovoltaics and thin-film transistors
Huq, Tahmida
The focus of this thesis is on developing two next-generation inorganic materials for thin-film device applications, namely photovoltaics and thin-film transistors. Both of these device applications are crucial in today’s technology-based society with photovoltaics enabling sustainable generation of electricity whilst advancements in thin-film transistors allow for development of low-power, efficient electronic devices. BiOI, a non-toxic, perovskite-inspired material is investigated for photovoltaics (PVs) whilst Cu₂O, with a reasonably high predicted hole mobility is developed for *p*-type thin-film transistors (TFTs). These novel materials are fabricated with scalable processing techniques which enable lower manufacturing costs and improve energy efficiency. In the first results chapter, the suitability of non-toxic BiOI as a photovoltaic material is investigated. Dense BiOI films grown by thermal chemical vapour deposition (CVD) incorporated into an all-inorganic ITO/NiO_*x*/BiOI/ZnO/Al stack demonstrate high external quantum efficiencies (80% at 450 nm wavelength). However, the 1.9 eV band gap of BiOI is not matched to terrestrial solar spectra; the PVs achieve 1.8% power conversion efficiency. Owing to improved spectral matching with indoor light spectra, BiOI devices improve in efficiency to 4.37% under 1000 lux white light emitting diode indoor illumination, and millimetre-area BiOI devices are sufficient to power novel carbon nanotube inverters. The factor limiting further efficiency gains is downwards band-bending at the BiOI/NiO_*x* interface owing to NiO_*x* having a lower work function. In the second chapter, MoS₂ is investigated as an alternative to NiO_*x* where the work function of MoS₂ is tuned through oxygen plasma treatment to increase its work function. The experimental examination of defect tolerance of BiOI is conducted in chapter three. BiOI films are vacuum-annealed to induce surface composition changes. Large changes in surface atomic fractions (reduction in iodine and bismuth by 40% and 5% respectively, and increase in oxygen by >45%) are observed. These significant changes do not affect the electronic and optoelectronic properties, in contrast to traditional covalent semiconductors. The applicability of low-temperature (≤ 200 °C) atmospheric pressure spatial atomic layer deposited (AP-SALD) Cu₂O for use in *p*-type TFTs is explored in chapter four. The performance of AP-SALD Cu₂O is comparable to atomic layer deposition (ALD) grown Cu₂O with an I_*ON*/I_*OFF* ratio of 10³, and field-effect mobility between 10^-4 - 10^-3 cm²·V^-1·s^-1, illustrating the potential of AP-SALD grown films for integration with flexible substrates.
Open Access
A multiscale study on the origins of the Portevin-Le Chatelier effect in polycrystalline nickel-based superalloys
Rowlands, Bradley
Various engineering alloys display load drops and strain localisation during constant strain rate tensile testing, termed the Portevin-Le Chatelier (PLC) effect. This includes nickel-based superalloys, between approximately 200 °C and 600 °C. The PLC effect is widely attributed to Dynamic Strain Ageing (DSA), referring to dislocation pinning effects caused by the diffusion of solute to dislocations, driven by elastic interactions. DSA is widely adopted to explain the PLC effect in superalloys, but was initially applied to model the effect of interstitial solutes in carbon-containing steel. There is often an implicit, mostly unsubstantiated assumption that substitutional solutes can create a strong pinning effect. Doubts remain over whether substitutional solute atoms are sufficiently mobile in the temperature regime of the PLC effect, and details of the atomistic mechanisms in superalloys have not been experimentally elucidated. The origins of the effect warrant attention, given possible effects of the accompanying strain localisation on the ductility and fatigue life of polycrystalline superalloys at in-service temperatures in the turbine engine. This thesis presents a wide experimental account of the PLC effect in a nickel-based superalloy, RR1000. A binary alloy, Ni-20Cr (weight %), is also investigated to facilitate understanding. A characterisation of the macroscopic serration and strain localisation characteristics is first presented, through optical Digital Image Correlation (DIC). From this, load drops are unambiguously associated with the presence of bursts of localised strain within so-called Lüders bands. Wide similarities between the behaviour of the superalloy and the binary alloy suggest that the microstructural complexity of the superalloy is not necessary to explain the effect. The presence of history dependent effects upon a change in strain rate or isothermal hold are also characterised, demonstrating that the PLC effect likely originates from a time-dependent strengthening effect. Scanning electron microscope based DIC techniques are subsequently employed across large (0.2 mm × 6 mm) areas spanning a Lüders band, providing the most detailed image of strain localisation within a Lüders band to date. Results from fast Fourier transform based techniques for slip band detection and quantification demonstrate little variation in the distributions of slip band spacings between tests where the PLC effect is present or absent. The results challenge the view sometimes held, that the PLC effect is directly associated with the accumulation of enhanced strain localisation at the microscale. An investigation of the accompanying defect structures demonstrates that coupled anti phase boundary configurations remain dominant when the PLC effect is present, disputing the view that the PLC effect in superalloys is caused by the growth of stacking faults. Finally, preliminary experimental results are provided of the compositional profiles around strain localisation features, through a combination of atom probe tomography and electron microscopy techniques. Results do not demonstrate clear solute enrichment around dislocations. Rather, electron microscopy techniques provide preliminary evidence for the formation of short-range ordered domains in both alloys, an alternative theorised cause of the PLC effect. The results suggest a need for closer scrutiny of the wide attribution of the PLC effect in superalloys to DSA, with the ultimate goal of providing an increased physical understanding of how to control the extent of the PLC effect and its influence on the mechanical properties of hot components of the turbine engine, such as the fatigue life.

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