Theses - Materials Science and Metallurgy

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Open Access
The Design, Preparation and Characterisation of Light-Responsive Pickering Emulsions
Richards, Kieran
Light-responsive particle-stabilised (Pickering) emulsions can, in principle, be selectively emulsiﬁed or demulsiﬁed on-demand through the remote application of light. However, despite their wide-ranging potential in applications such as drug delivery and biphasic catalysis, their rational design is extremely challenging and there are very few examples to date. In this thesis, we investigate a model system based on silica particles functionalised with azobenzene-derived photoswitches to understand the key factors that determine the characteristics of light-responsive Pickering emulsions. We address their design on three distinct length scales: the photoswitch at the molecular level, the photoswitchable particle at the sub-micron scale, and light-responsive emulsion at the tens of micron scale. Key to the creation of light-responsive Pickering emulsions is the design of photo-switchable molecules that can impart light-responsive behaviour. These molecules are attached to the particle-stabilisers and upon switching change the hydrophobicity of the particle, resulting in a change in the emulsion’s stability. Two groups of photoswitches are investigated in this work, which belong to the azobenzene and arylazopyrazole families. When irradiated with either UV or blue light, these molecules isomerise from a more hydrophobic trans state to a less hydrophobic cis state. Both groups are derivatised with hydrophobic or hydrophilic terminal modiﬁcations and by the addition of a carbon chain spacer. The arylazopyrazoles are also inherently more hydrophilic. Investigation of the optical properties of both families show successful photoswitching and, for arylazopyrazoles, an exceptionally high photostationary state (>90 %) and half-lives as long as 24 days. The photoswitchable molecules are then appended to fumed silica particle at three diﬀerent grafting concentrations. The eﬀect of the grafting density and type of photoswitch on the hydrophobicity of the particle is ﬁrst analysed by surface energy analysis, before and after trans-cis photoisomerisation. It is found that the length of the carbon spacer is the most important factor in controlling particle hydrophobicity and the azobenzene-modiﬁed particles show a greater diﬀerence in hydrophobicity when isomerised compared to the arylazopyrazoles. Emulsions are then produced using oils of diﬀerent polarity and their stability and morphology assessed by optical microscopy. A computer-vision application is produced to help extract droplet size information from microscopy images using the circle Hough-transform. This method is benchmarked against the most commonly used alternative, a region growing technique, and it is found to have higher accuracy, recall and precision. The light-responsive behaviour of the emulsions is also assessed and, for the ﬁrst time, a reversible transition between emulsiﬁed water-in-oil droplets and demulsiﬁed water and oil phases is observed with the application of either UV or blue light, which can be repeatedly cycled. Using the observed trends and data from the surface energy analysis of the particles, a set of design rules are presented which will help facilitate the rational design and, therefore, more widespread application of light-responsive Pickering emulsions.
Open Access
Understanding the Stability of Retained Austenite in High-Carbon Steels: Modelling and Alloy Design
Wong, Adriel
In high-carbon steels, the austenite-to-martensite transformation under applied stress initiates at a critical value, contrary to low-carbon steels where the transformation occurs at the early stages of deformation. There are few models that account for the delayed transformation, because previous models were mostly developed from low-carbon steels that exhibit martensite transformation behaviour that is different from high-carbon steels. Also, few studies have been done on methods of tailoring chemical composition and processing to design steels that possess optimal austenite stability, but do not require too many expensive austenite stabilising elements such as nickel. This research aims to develop equations for modelling the critical stress and progress of deformation-induced martensite transformation in high-carbon steels. Another aim is to design a new high-manganese steel that is comparable to the high-nickel AISI 3310 carburising steel in terms of austenite stability and mechanical properties. The research investigated carburising-grade steels used in bearing applications. Various thermomechanical tests were conducted to investigate the influence of chemical composition and microstructure on retained austenite stability. Dilatometry experiments showed that the martensite-start temperature decreased with increasing grain size when the steels were austenitised below the A_cm temperature, but increased with grain size above the A_cm temperature. These observations were linked to the amount of carbon dissolved in austenite. Analytical models that describe the critical stress and progress of martensite transformation in high-carbon steels were developed. The critical stress model is a function of chemical composition, deformation temperature, and initial retained austenite fraction. Higher austenite stabiliser concentrations or deformation temperatures result in smaller magnitudes of the chemical driving force, leading to a higher critical stress predicted and represent higher austenite stability. A lower initial austenite fraction represents fewer martensitic nucleation sites, resulting in deformation-induced martensite transformation that occurs at a higher critical stress. The transformed amounts of austenite under applied stress depends on the critical stress for martensite transformation. The predicted results were in good agreement with experimental data. Model applications were demonstrated through examples that involve the determination of alloying combinations and service conditions that are suitable for desired range of critical stresses and austenite fractions predicted. The examples highlight the utility of these models as tools for alloy design. A new high-manganese carburised steel was developed based on thermodynamic modelling and literature-informed design criteria. Samples of the new steel and AISI 3310 steel were carburised and subjected to martensitic quench-and-temper heat treatments. The mechanical properties and austenite mechanical stability of the new steel were found to be not on par with the 3310 steel. The reasons include a lower bulk carbon content, and the presence of massive carbides and oxide inclusions in the new steel. Practical solutions to improve the low-pressure carburisation process of the new steel are suggested in Section 7.5.2. The possibility of designing new carburising steels with lower costs by tailoring composition and processing routes based on enhancing austenite stability was explored and demonstrated. Process improvement aspects identified from the carburisation and heat treatment of the new steel can be used to inform the processing of such steels in the future.
Open Access
Controlling the Synthesis of Plasmonic Magnesium Nanoparticles and Understanding Crystal Structures Beyond Face-Centred Cubic
Hopper, Elizabeth
At the nanoscale, materials’ properties differ vastly from the bulk. For example, nanoparticles (NPs) of plasmonic materials can interact strongly with light and sustain resonant oscillations of their free electron density, giving rise to absorption, scattering and local electric field enhancement. These phenomena enable applications in a variety of fields such as sensing, surface-enhanced spectroscopies, photothermal cancer therapy and photocatalysis. Many properties of NPs, including the optical properties of plasmonic NPs, are strongly dependent on NP size and shape. Understanding the shapes of NPs, in particular those arising from twins in the crystal structure, is vital to understanding their properties. Much NP technology is dominated by face centred cubic (FCC) metals, including Au, Ag and Pd. In this thesis, the synthesis and NP shapes of non-FCC nanocrystals are studied. First, the synthesis of hexagonal close packed (HCP) Mg NPs, an earth-abundant plasmonic material, is investigated. The NP shapes are explained and the effects of reaction parameters on the products of a colloidal synthesis are systematically probed. NP sizes are selected between 80 nm and over a micrometre by varying the reaction time, overall reaction concentration, temperature, electron carrier, and metal salt additives. Next, seed mediated growth syntheses of Mg NPs are developed. These methods are a common technique to manipulate NP size and shape for other metallic NPs. The reaction starts with a rapid nucleation step using Li biphenyl as a strong reducing agent. Then the NPs are slowly grown by converting the reducing agent to Li phenanthrene, which is less strongly reducing than Li biphenyl, and using an ice bath to suppress further nucleation. However, control over the amount of growth remains a challenge. The reaction is then further adapted by employing a new reducing agent. By investigating the synthesis route, small Mg NPs with new morphologies are formed. These small seeds are then again grown by altering the reducing agent during the synthesis. Finally, based on these batch syntheses, Mg NPs are synthesised for the first time in a continuous flow system. These systems present opportunities for enhanced control over the reaction products by facilitating more efficient initial mixing. A stable system for reproducible synthesis of Mg NPs is reported, with the capability to form the first coloured colloids of Mg NPs, indicating improved size and shape dispersity. Beyond HCP crystallinity, other crystal systems are also gaining interest in several fields. For instance, body centred cubic (BCC) NPs such as Fe and W have applications in catalysis, and body centred tetragonal (BCT) NPs such as In have plasmonic applications. Both the catalytic and plasmonic properties of NPs are strongly dependent on the NP morphology, including twinning. The shapes of BCC and BCT NPs are investigated computationally, showing that distinguishing twinned NPs from their single crystal counterparts is challenging for many BCC NPs, but often more straightforward for BCT NPs. Modelling the electron diffraction signal shows that distinguishing twinned BCC and BCT NPs from single crystal signals is impossible if NPs are measured along many of the most likely viewing directions, as the diffraction spots from each twin overlap. This insight into NP shapes with crystallographies beyond the more readily understood FCC system may reveal why twinned BCC NPs have not yet been reported. Based on these results, reliable methods are suggested to diagnose twinning in BCC and BCT NPs. This thesis paves the way for the use of alternative, non-FCC materials in nanotechnology, including for the large-scale use of Mg as a low-cost and sustainable plasmonic material and for the characterisation of BCC and BCT NPs, opening the door to improved control and understanding of novel NP shapes.
Embargo
Extreme Disorder in Metal–Organic Frameworks
Sapnik, Adam
Metal–organic frameworks are emerging as a highly functional class of hybrid materials. Once believed to be rigid and well-ordered structures, a growing number of studies are proving this is not always the case. This thesis focuses on the atomic structure, physical properties and formation mechanism of the amorphous Fe-BTC framework whose structure is very poorly understood, compared to its crystalline counterpart MIL-100, despite its catalytic and industrial significance. Chapters 1 and 2 introduce metal–organic frameworks, the concept of disorder, and the techniques necessary for characterising amorphous MOFs. Chapter 3 explores the structure of Fe-BTC. Using a combination of X-ray, electron and neutron-based techniques, coupled with computational modelling, the first model for Fe-BTC is produced. Electron microscopy reveals the presence of a nanocomposite structure that is dominated by an amorphous phase, while X-ray pair distribution function analysis reveals a mixed hierarchical local structure. The Fe-BTC model enables us to establish key structure–property relationships. Chapter 4 investigates how the disordered structure of Fe-BTC impacts its gas sorption ability compared to MIL-100. While the porous network within MIL-100 facilitates high uptakes of gas, the disordered structure of Fe-BTC leads to the emergence of highly sought-after propane/propene separation capabilities. A range of gases are measured and coupled with surface area, virial, ideal adsorption solution theory and non-local density functional theory analyses. The results suggest disorder can be used to tune the adsorption capacity and selectivity in framework materials. Chapter 5 studies how disorder can be progressively introduced into the MIL-100 framework as motivated by the previous chapter. Disorder is introduced using mechanical milling to break the metal–linker bonds and ultimately cause the collapse of the framework. Analysis of the pair distribution functions of the series of disordered MIL-100 materials reveals a stepwise collapse of the hierarchical structure with retention of the local structural building unit. Progressive milling of MIL-100 is found to be a facile route to tune the degree of long-range order and porosity. Chapter 6 addresses one of the key challenges faced in this thesis, namely the sheer complexity of MIL-100’s crystallographic unit cell. Using multivariate analysis, mechanisms of structural disordering are studied using pair distribution function data in a manner inaccessible to computational modelling techniques. Rich kinetic and mechanistic information is found to be deeply encoded within the pair distribution functions. The analysis methodology is then applied to a very different disordered system to explore the approach’s generality. Chapter 7 investigates the formation of Fe-BTC using in situ X-ray absorption spectroscopy and UV-vis spectroscopy to probe the redox behaviour that occurs during Fe-BTC formation. Kinetic and mechanistic analysis reveals a non-linear mechanism that occurs via the formation of a transient intermediate. These results are used to propose the first mechanism for Fe-BTC formation. Chapter 8 summarises the results and discusses the avenues for future research.
Embargo
Development of Low-Cost Polycrystalline Nickel-Base Superalloys for Gas Turbine Applications
Wise, George; Wise, George [0000-0001-8001-811X]
Polycrystalline nickel-base superalloys have found extensive application throughout the aerospace, petrochemical and power generation sectors, due to their exceptional balance of high temperature mechanical properties, thermal stability and oxidation resistance. Extensive alloy development has produced a diverse class of alloys suited to a wide range of operating conditions. This work details research performed to develop new low-cost polycrystalline Ni-base superalloys, intended for intermediate temperature and loading environments. There is currently significant industrial demand for alloys of this type, that offer improved thermal stability and mechanical properties compared to current commercial alloys such as Inconel 718 and ATI 718Plus, whilst retaining good amenability to deformation processing and joining operations. The important properties required of polycrystalline Ni-base superalloys are discussed alongside an initial comparison of current commercially available materials and a potential class of candidate alloys suitable for further development. Compositional modifications were made to these alloys in order to improve their thermal stability and reduce their cost. Characterisation was performed after a standard ageing heat treatment, and after long term thermal exposure at 700˚C, revealing significantly improved resistance to precipitation of the δ phase in these alloys compared to Inconel 718. Despite this, comparable measurements of the hardness values indicated similar mechanical strength. The oxidation resistance of the candidate alloys was also assessed through 1000~hour exposures in air at 700˚C and 800˚C. The alloys were found to be chromia forming, with internal oxidation of Al forming a discontinuous subscale. Significant microstructural degradation was observed at 800˚C due to the formation of the δ phase, with the differences in oxidation behaviour rationalised using Wagnerian diffusional analyses. So that further investigation of fundamental alloy behaviour could be performed, industrial quantities of three candidate alloys were produced. The initial characterisation of these materials is described, including a detailed study of the ageing response after a controlled solution heat treatment performed using a quenching dilatometer. The observed precipitate coarsening was well described using classic models, and was correlated to the hardening response by comparison to the contributions from strong- and weak-pair dislocation coupling. The deformation behaviour of small-scale thermomechanical compression specimens of the preferred candidate alloy was studied under both sub- and super-solvus forging operations. Subsequent microstructural examination was used to assess the extent of recrystallisation, and characterise the microstructural evolution that took place during deformation. In the supersolvus trials, fully recrystallised microstructures were obtained, with partially recrystallised microstructures evident in the sub-solvus trials. These results, combined with the flow curve data were used to specify large-scale forging of the industrially produced compacts. In an exploratory study of the weldability of these alloys, electron beam welding was used to produce autogenous bead-on-plate welds using three different welding conditions. All the welds produced were fully penetrating, with no deleterious defects characterised in any of the conditions studied. Using scanning electron microscopy and large area hardness contour mapping, the effects of a post-weld heat treatment on the as-welded microstructure were assessed. The restoration of mechanical properties across the weld cross-section was successfully correlated with the local distribution of precipitates within the fusion zone after heat treatment. The results indicated that the alloys were amenable to joining via this process. The research described details the development of new low-cost polycrystalline Ni-base superalloys from initial compositional design, through to industrial production, including detailed studies of the phase equilibria, thermal stability, microstructural evolution and oxidation resistance. In addition to demonstrating superior microstructural stability compared to current commercial alloys, the alloys developed appear to be readily formed via conventional deformation processing routes, and can be joined without producing defects using electron beam welding. With additional work to verify the detailed mechanical response of the alloys produced, it is hoped that they will find industrial application in the near future.
Embargo
A Mechanistic Understanding of the β to α′′ Transformation in Superelastic Ti-24Nb-4Zr-8Sn (wt%)
Church, Nicole; Church, Nicole [0000-0001-9551-8125]
Metastable β titanium alloys have potential for application in vibration damping systems, due to their wide mechanical hysteresis and large recoverable strains, arising from a re- versible martensitic transformation between a parent bcc β phase and an orthorhombic α′′ martensite. Of these alloys, those based on the Ti–Nb binary system such as Ti2448 (Ti-24Nb-4Zr-8Sn, wt%) are of particular interest, as with appropriate alloying and ther- momechanical processing their properties can be tuned for application over a wide range of temperatures. However, their use is currently limited by a significant variability in both thermal and mechanical transformation parameters. These are critical to defining the superelastic response and vibration damping potential of the material, which can additionally be shown to vary with both thermal and mechanical cycling. This study aims to rationalise how processing and loading parameters affects different aspects of the martensitic transformation, via a combination of in situ and ex situ techniques. Whilst the superelastic transformation was shown to largely be independent of grain size, it was found to be highly sensitive to other parameters. In particular the sensitivity of the microstructure to thermal history was identified, and it was shown that this sensitivity could not be rationalised by currently held theories. In light of this, the mechanical be- haviour of different microstructural conditions was identified to ascertain whether this sensitivity to thermal history was the origin of many discrepancies in the mechanical be- haviour. However, it was shown that the mechanical response was only dependent on the initial microstructure for low applied stresses, with microstructural changes occuring during testing of the material in response to the applied load. Consequently, an alterna- tive mechanism was proposed, consistent with a total stress approach, which was further shown to be consistent with variations in microstructure and mechanical properties for samples of varying thicknesses. In further characterising the mechanical response at low applied stresses, it was found that the behaviour of these materials actually changes via a two-step mechanism, driven by the accumulation of transformation related defects and their associated stress fields, again consistent with a total stress model. This understanding highlighted that both the thermal and mechanical behaviours of the alloy may be mechanistically linked, where both effects could be rationalised by a total stress approach, driven by a changing distribution of dislocation strain fields. This link was confirmed through two experiments, that probed both mechanical and thermal aspects of the transformation. This understanding is able to rationalise many discrepan- cies within the literature and paves the way for more successful alloy design, which will ultimately make this class of alloy more commercially viable.
Open Access
Wear of Polycrystalline Cubic Boron Nitride Cutting Tools when Machining Nickel-based Superalloys
Brug, Eleanor
Cubic boron nitride is the second hardest material known, after diamond. Polycrystalline cubic boron nitride, PcBN, is a composite consisting of cubic boron nitride grains and either a metallic or ceramic binder phase. It is currently used as a cutting tool material for several difficult to machine alloys where diamond’s reactivity towards ferrous materials precludes its use. High tool wear rates are observed when using PcBN to machine Ni-based superalloys - in particular Inconel 718. While there have been other studies on the machining of this alloy using PcBN tools, and the types of wear that occur have been described, the underlying mechanisms are still not well understood. With a better understanding of the underlying processes, PcBN tools could be further refined so that they become an economical replacement for tungsten carbide cobalt-based tools, which are limited to low cutting speeds. Tungsten and cobalt are also subject to limited reserves and environmental concerns, while PcBN requires little more than energy to produce. Nine grades of PcBN with cBN concentrations from 30 % to 90% were characterised using a range of techniques including X-ray diffraction and electron microscopy. Of these, seven grades were used to machine Inconel 718. High pressure coolant was used in longitudinal finish turning tests at cutting speeds from 150m/min to 1200m/min. The wear rates and relative prevalence of flank, notch, and crater wear for the PcBN grades were measured using optical and electron microscopy and the cutting forces were analysed. While ion beam milling into the layer of adhered workpiece material on the tool showed some chemical reaction, the phases formed could not be adequately characterised in situ due to their size. In order to better understand the reactions occurring at the interface, static diffusion tests were used to simulate the diffusive and chemical wear processes. Diffusion couples were made from all grades of PcBN, and Inconel 718, as well as three other Ni-based superalloys with different minor elements. The samples were treated at 1200 ◦C in a low oxygen environment for 24 hours to encourage coarsening of the phases and therefore aid characterisation. The extent of the reactions at the interfaces as well as the identity of the new phases were analysed using energy dispersive X-ray spectroscopy. While the boron from the tool diffused hundreds of microns through the alloy before forming Nb, Mo, and Cr-rich phases, primarily along grain boundaries, the nitrogen from the cBN led to the formation near the interface of bands of cubic TiN precipitates containing other minor elements from the alloys. The morphology and quantity of these phases was dependent on both the PcBN grade and the alloy composition.
Open Access
Phase stability and the Portevin-Le Chatelier effect in Cr-Mn-Fe-Co-Ni High-Entropy Alloys
Bloomfield, Maximilian; Bloomfield, Maximilian [0000-0002-6529-261X]
High-Entropy Alloys (HEAs) are a new class of metallic materials based on the combination of multiple principal elements, often with the intention to form a concentrated solid solution. It has been suggested that these solid solution phases could offer enhanced mechanical properties, due to fluctuations in their local atomic environments. However, at present, the thermodynamic stability of the solid solution phases which form in HEAs is poorly understood. This study aims to improve our understanding of phase stability in HEAs and investigate possible differences in the mechanical behaviour of dilute and concentrated solid solutions. Firstly, a systematic series of experiments is presented, which establish the effect of Co and Fe on the phase equilibria of the widely studied CrMnFeCoNi system. Both Co and Fe were found to stabilise the A1 matrix relative to the A2 and D8b phases at elevated temperatures but did not prevent the formation of ordered phases at 500°C. Alongside literature data, the results show that stable single-phase alloys are rare in the CrMnFeCoNi system. Secondly, a systematic assessment of the AlxCrFeCoNi alloy system showed that Al promotes the formation of B2, D8b and A2 phases, and improved our understanding of phase stability in this system, which has strong potential for developing alloys with desirable mechanical properties. In each study, experimental observations were used to test the fidelity of thermodynamic predictions from the latest CALPHAD databases. Whilst the predictions generally provided a close approximation of the observed phase equilibria, inaccuracies were common, especially at lower temperatures. Phase stability investigations also served to identify thermodynamically stable alloys, suitable for the investigation of the Portevin-Le Chatelier (PLC) effect - a discontinuous yielding phenomenon widely attributed to the repeated pinning of dislocations by mobile solute atoms over certain regimes of temperature and strain-rate. Under mechanical testing, the effect is generally associated with serrations in the materials flow curve and the localisation of strain into discrete bands. Serrated flow has been observed in several HEAs but the associated strain localisation behaviour has not yet been reported, so it is uncertain whether this behaviour is similar to the PLC effect in other alloy systems. Thus, a series of alloys based on the stable equiatomic quaternary alloy, CrFeCoNi, were selected to investigate the effect of compositional complexity on the PLC effect. Tensile testing, combined with digital image correlation, enabled simultaneous recording of the materials stress and local strain response across a range of temperatures. The results showed close similarities between the alloys at each temperature, indicating that compositional complexity did not have a dominant influence on the PLC effect. This suggested that the dislocation-solute interactions in the complex, concentrated solid solutions were not substantially different from those of more dilute solutions. Calorimetric investigations also revealed evidence of a varying degree of short-range order among the alloys, the influence of which warrants further investigation.
Open Access
Visualising Energy Landscapes Through Manifold Learning
Shires, Benjamin
Potential energy surfaces (PESs) or landscapes provide a conceptual framework for structure prediction, and a detailed understanding of their topological features is necessary to develop efficient methods for their exploration. The ability to visualise these surfaces is essential, but the high dimensionality of the corresponding configuration spaces makes this visualisation difficult. This thesis is concerned with the potential energy surfaces sampled during the structure prediction of materials and molecules, with a focus on random structure searching. The key contribution is a novel approach to energy landscape visualisation. The proposed method, SHEAP (stochastic hyperspace embedding and projection), is an algorithm for dimensionality reduction constructed by adapting existing, state-of-the-art manifold learning approaches to better deal with the structural datasets obtained from searching. Also discussed is a method for basin volume computation, previously applied to model periodic systems with fixed unit cells. Adapting this method to deal with realistic crystalline systems with variable unit cells was the initial intention for the research reported here, and we provide an overview of the early difficulties encountered. Ultimately, a different direction was pursued. In this thesis, the common framework of state-of-the-art manifold learning-based approaches to dimensionality reduction for visualisation, such as t-SNE and UMAP, is described. Following this outline, we provide motivation for adapting these existing approaches in order to deal with structural data, before presenting our new method designed for application to PESs: SHEAP. SHEAP is compared to the methods on which it is based using test datasets standard in the machine learning community, showing comparable performance. SHEAP is then applied to a variety of structural datasets. First considered are simple model systems: Lennard-Jones clusters, for which the energy surfaces are generally well understood. These systems are used to demonstrate how SHEAP can reproduce well-known features, such as funnels, and provide detail beyond, or complementary to, previous methods. For the 38-atom cluster, the impact of varying the structure descriptor on SHEAP’s representation of the dataset is explored. Also studied are periodic and finite systems of atoms described by first-principles density functional theory. Through the visualisations provided by SHEAP, key features of the energy landscapes considered are discussed and compared to one another. In addition, the number of dimensions required for SHEAP to faithfully depict the captured features of these energy landscapes is examined and revealed to often be no more than three, the maximum that we can visually comprehend. Two structure searching case studies, which use SHEAP to aid in the analysis and depiction of the search results, are also presented. The first is a search for metallic structures with random structure searching, using orbital-free density functional theory for the electronic structure calculations. This study demonstrates how orbital-free density functional theory can be used to drive accelerated searching for metallic systems. SHEAP maps for solid-state Li, Na, Mg, and Al provide clear and concise summaries of the search results for each system, and facilitate comparison between key features of their energy landscapes. Of particular note, this analysis highlights that each PES possesses a band of low-energy, close-packed structures, with face-centred cubic and hexagonal close-packed at either end. The second case study is a search for lithium-ion battery cathode materials, conducted using random structure searching with conventional Kohn-Sham density functional theory. Study of LiCoO2, a well-established cathode material, is used to demonstrate the impact of varying the parameters of a search, and the advantage that prior knowledge of a system can provide. SHEAP proves to be an effective tool through which to illustrate and interpret the difference between various searches on this system. Using the findings for this system as a basis, a systematic method for approaching entirely unexplored systems, for which one has no prior knowledge, is also discussed. In addition, an established polyanionic cathode material, LiFePO4, is studied, revealing a more complex PES than LiCoO2, and demonstrating the benefit of using pre-defined structural units with AIRSS, where appropriate. A relatively unexplored potential LIB cathode material is also considered: Li2Fe(C2O4)2. Using the exploratory searching framework laid out in the discussion of LiCoO2, a new layered phase of Li2Fe(C2O4)2 is discovered, with lower energy and higher theoretical rate capability than the previously discovered experimental phase.
Embargo
Exploring new materials for caloric cooling
Dilshad, Melony
Caloric materials produce nominally reversible thermal changes when subjected to an applied driving field. These materials are widely considered a potential replacement to the environmentally harmful refrigerant fluids that are currently used in vapour compression systems. Barocaloric materials that are driven by changes in hydrostatic pressure are comparatively the most promising type of caloric material, as they display the largest thermal changes, are practically more cost-effective, and can withstand large cyclic changes in hydrostatic pressure without breaking down or decreasing performance. This work reports on a selection of superionic conductors and metal-organic frameworks that were chosen based on their promising thermodynamic properties for barocaloric studies. Furthermore, novel ways of driving caloric effects are explored by exploiting latent heat from first-order phase transitions in a smart polymer driven by stimuli unexplored in the field of calorics thus far. Despite displaying large thermally driven entropy changes (|∆S₀|) near superionic transitions, the superionic conductors studied here showed a tendency to exist in many stoichiometric compositions and metastable states, which hindered their barocaloric performance. Complex calorimetric data from synthesised Ag-Cu-S ternary materials were understood to be an effect of variation in the pressure-dependence of the transition in different stoichiometries of the individual Ag-Cu-S ternary phases present. Furthermore, both conventional and inverse barocaloric effects were produced within each individual sample, which resulted from two different phases of the ternary system. Similar phase-related complexity was demonstrated for Ag₂Se and Cu₂Se. The metal-organic frameworks studied display improved barocaloric performance. Large reversible isothermal entropy changes |∆S(it)| ∼ 14 J K¯¹ kg¯¹ and adiabatic temperature changes |∆T(ad)| ∼ 5 K due to changes in applied pressure |∆p| ∼ 1 kbar were evaluated in a nitrogenated sample of the zeolitic imidazolate framework ZIF-4(Zn) near its first-order phase transition at T₀ ∼ 185 K. These barocaloric effects could be driven over an exceptionally wide range of starting temperatures (∼75 K when driven by |∆p| ∼ 1 kbar) due to the large tunability of T₀ with pressure in this compound. While initial calculations yielded |∆S₀| ∼ 16 J K¯¹ kg¯¹, extended analysis correlating time-dependent calorimetric measurements with x-ray diffraction data and the Clausius-Clapeyron relation indicated that a certain fraction of the sample does not undergo the transition, which therefore largely dwarfed |∆S₀| due to mass normalisation. By correcting for this inactive mass, a maximum barocaloric response of |∆S(it)| ∼ 155 J K¯¹ kg¯¹ and |∆T(ad)| ∼ 50 K driven by |∆p| ∼ 1 kbar was evaluated. Furthermore, a novel type of caloric effect, tentatively termed barosorptiocaloric in this work, was investigated in MIL-53(Fe) near a sorption driven first-order phase transition, in which adsorption and desorption of water molecules in the porous MIL-53(Fe) framework was regulated by changes in hydrostatic pressure. This new driving mechanism delivered |∆S(it)| ∼ 10 J K¯¹ kg¯¹ and |∆T(ad)| ∼ 7 K near room temperature, with nominally no hysteresis. Two more types of novel caloric effects were investigated in the smart polymer, poly(N-isopropylacrylamide) [p(NIPAM)], and its acidic functional derivative, by using changes in solvent concentrations and pH to drive a first-order conformational phase transition. The caloric responses from these previously unexplored caloric driving fields, herein termed solvocaloric effects and pH-caloric effects respectively, were evaluated. Changes in pH only shift the transition temperature marginally, but changes in ethanol:water solvent ratios led to a large shift in transition temperature of ∼20 °C, leading to large solvocaloric changes in entropy of ∼16 J K¯¹ kg¯¹ near room temperature in p(NIPAM), with nominally no hysteresis. Direct measurements of adiabatic temperature change were performed to demonstrate this novel effect.
Open Access
The Effect of Framework Structure and Chemical Functionality on Melting in Zeolitic Imidazolate Frameworks
Bumstead, Alice
Interest in the amorphous phases of metal–organic frameworks (MOFs) has increased in recent years. Special consideration has been given to melt-quenched MOF glasses: the first new category of glass discovered in 50 years. Zeolitic imidazolate frameworks (ZIFs) are the most common MOF family that have been found to undergo melt-quenching. They are composed of tetrahedrally coordinated metal ions connected to imidazolate linkers. The dynamic nature of the melting mechanism in ZIFs has been demonstrated, with melting occurring via de-coordination and re-coordination of the imidazolate linkers at high temperatures. A wide variety of ZIF crystal structures have been reported to date. However, at present, the number of ZIFs that can undergo melt-quenching remains limited. This thesis aims to provide a better understanding of melting in ZIFs as well as the different factors that control the melting process so that, ultimately, novel melt-quenched MOF glasses can be prepared. Initially, four closely related ZIFs were studied to systematically investigate how linker chemistry and framework structure influence the melting process. Importantly, dense framework structures — specifically those displaying the cag network topology — were found to be crucial for melting. Moreover, their presence could initiate melting in more open framework structures. As dense frameworks were found to be essential for melting, the thermal behaviour of ZIFs displaying ultra-high framework densities, specifically those exhibiting the zni network topology, were investigated. Melting in these ZIFs was found to occur at higher temperatures than in cag topology systems. Furthermore, melting was found to be highly sensitive to chemical composition, with a 0.25% change in linker composition capable of eliciting a 7 °C change in melting temperature. We then demonstrate, for the first time, the possibility of further altering the chemistry in a ZIF glass by post-synthetic modification (PSM). A novel amine-functionalised ZIF glass was prepared that would be an ideal candidate for PSM. As a proof of concept, this amine-functionalised ZIF glass was reacted with octyl isocyanate, resulting in a urea-functionalised glass surface and a change in its surface wetting behaviour from hydrophilic to hydrophobic. Finally, we further expand the possible chemistries that can be incorporated in ZIF glasses by the inclusion of purine in a novel ZIF structure. The resulting glass forming ZIF was found to have one of the lowest melting temperatures reported for any ZIF. This represents a reduction in the melting temperature of over 250 °C compared to some of the early reports of ZIF melting. Evidently, judicious control of both linker chemistry and framework structure can be utilised to alter the thermal behaviour of ZIFs and to prepare novel melt-quenched glasses.
Embargo
Structure and Ionic Conductivity Properties of Ionic Liquid@Metal—Organic Framework Composites
Tuffnell, Joshua Mark; Tuffnell, Joshua [0000-0003-3069-1466]
IL@MOF (IL: ionic liquid; MOF: metal-organic framework) materials have been proposed as a candidate for solid-state electrolytes, combining the inherent non-flammability, high ionic conductivity, and high thermal and chemical stability of the ionic liquid with the host-guest interactions of the MOF. Although there is a large degree of chemical tunability of the MOF and IL components, the ionic conductivity of the IL in the composite is greatly reduced compared to the bulk due to the nanoconfinement of the IL within the nanopores of the MOF. In the literature, studies have been limited to modifications to the organic linkers or metal ions of the MOF, however, the structuring of the pore architecture can be modified in several ways, explored in this thesis. Firstly, the addition of disordered mesopores from sol-gel synthesis conditions resulted in a hierarchically porous MOF superstructure containing both micropores (innate to the MOF) and mesopores (formed by controlled drying of the sol-gel system) and affords greater IL filling capacities as measured by nitrogen gas sorption experiments. Electrochemical impedance spectroscopy was used to compare the ionic conductivities of hierarchically porous IL@MOF composite with a standard microcrystalline IL@MOF composite. The theme of superstructures was further explored by utilising artificial opal polystyrene templates to generate ordered (inverse opal) and disordered macroporous networks within the MOF particles. The limited mechanical stability of the inverse opal MOF structure meant that gentle IL infiltration conditions were required for successful composite formation. Finally, a study on how the interaction between the IL and MOF components affects the structural transitions in ‘breathing’ MOF materials using variable temperature X-ray diffraction was carried out. In particular, the presence of the ionic liquid was demonstrated to lead to a distinct crystal structure which undergoes a similar phase transformation, but at a lower temperature.
Open Access
Towards sustainable plasmonics: a study on the shapes and plasmonic properties of magnesium nanoparticles
Boukouvala, Christina
The growing interest in nanoparticles and their increasingly diverse applications is fuelled by the ability to tune properties via material selection and shape control, promoting intense experimental and theoretical research. For plasmonics, where nanostructures are exploited to focus and channel light in the nanoscale, material and structure effects are studied in relation to optical properties. Understanding such light-matter interactions can inspire the design and synthesis of nanoparticles tailored for various applications, such as in sensing, photocatalysis and biomedicine, to name a few. Modelling shape effects can accelerate this progress and relies on coupling the nanoparticle’s, crystallographically dependent, geometrical representation with electromagnetic simulations. Material selection is also critical. Noble metals, which have been the pillars of plasmonics since the nascence of the field, have certain limitations and do not satisfy the sustainability requirements of modern societies, hence triggering intense research into alternative plasmonic materials. The objective of the current research is thus twofold: to provide a platform for user-friendly and crystallographically correct nanoparticle shape modelling, compatible with available electromagnetic simulation tools, and, helped by this tool, to investigate the shapes and structure-plasmonic property relations for nanoparticles made out of magnesium, a recently introduced plasmonic metal. In this work, the reader is first introduced to the concepts of localised surface plasmon resonances, Wulff-based approaches to nanocrystal shape modelling as well as the challenges and opportunities of magnesium nanoparticles in plasmonics and beyond. The next three chapters focus on nanoparticle shape modelling commencing by a summary of the already available tools. This is followed by the description of Crystal Creator, a GUI developed to facilitate the generation of shape input for electromagnetic simulations and to model the twinned nanoparticle shapes for various crystallographies. The applicability of Crystal Creator on a variety of nanocrystal structures is demonstrated via the calculation of the optical response of well-known face-centred cubic plasmonic metals as well as the investigation of the single crystal and twinned nanoparticle shapes of metals that adopt body-centered cubic and tetragonal crystallographic configurations. The hexagonal close packed adaptation is then used to understand and predict the nanoparticle shapes of magnesium. The two subsequent chapters focus on the plasmonic properties of the modelled magnesium nanoparticles. Here, electromagnetic simulations in the discrete dipole approximation and single-particle dark-field scattering measurements are employed to unravel size, shape and environment effects in the near-field and far-field optical response of the nanoparticles. Finally, a summary and a concluding discussion on the findings of this work, along with an outlook for future research, are provided in the last chapter.
Open Access
Electron microscopy study of degradation mechanisms in Ni-rich transition metal oxide cathodes for Li-ion batteries
Morzy, Jędrzej; Morzy, Jędrzej [0000-0003-0770-461X]
Li-ion batteries have a pivotal role in the transition towards electric transportation. This drastic societal change depends on high energy, sustainable, and long lifetime batteries. Among the most promising cathode active materials that enable such batteries is the class of Ni-rich layered transition metal oxides such as LiNi0.8Mn0.1Co0.1O2 (NMC811). However, they exhibit complex degradation mechanisms that impair their longevity and impede their commercial application. The degradation often has its roots in nanoscale processes that require careful characterisation with high spatial resolution techniques. The aim of this thesis is to advance the understanding of these degradation mechanisms using electron microscopy techniques. The relative importance of various aspects of NMC811 degradation is probed using tailored electrochemical protocols. Electron energy loss spectroscopy in a scanning transmission electron microscope, as well as focused-ion beam scanning electron microscopy cross sectional imaging allow for investigating the role of intergranular cracks and reduced surface layers in NMC811. The reduced surface layer evolution is found to be correlated with the impedance rise of NMC811, which suggests the importance of the surface layers for the performance of NMC811 based cells. Moreover, the underlying mechanisms of intragranular crack formation are investigated using electron energy loss spectroscopy, energy dispersive X-ray spectroscopy and high-resolution imaging in a scanning transmission electron microscope. Briefly, it is found that local stresses can lead to opening of intragranular cracks in the charged state already in the first few cycles. Initially, the cracks are mostly reversible. However, over longer cycling they can become detrimental to the battery performance due to plane gliding and fragmentation of particles. Lastly, a step towards more reliable operando electrochemical transmission electron microscopy techniques is developed. Using aerosol-jet printing, microbatteries consisting of commercially available battery active material powders are fabricated. This way, high resolution, placement precision and the ability to deposit arbitrary shapes as well as combine materials in a facile way is achieved. Overall, this thesis provides important insights into the degradation mechanisms and their relative importance for Ni-rich layered transition metal oxide cathodes and lays foundation towards operando electrochemical electron microscopy studies of industrially relevant batteries.
Restricted
Methods and Applications for Nanometrology using Scanning Precession Electron Diffraction
Crout, Phillip
Scanning electron diffraction offers researchers relatively easy access to the nanoscale and is often used to map sample properties in crystalline materials. Data quality can often be improved by introducing double conical rocking of the beam (commonly called precession). In this thesis, routes to improvement for two key areas within the field - orientation mapping and strain mapping - are demonstrated, allowing for significant increases in both throughput and accuracy. Comparisons are made between different electron-based techniques including the extremely popular electron backscatter diffraction method. We present results investigating the changes that occur under a nano-indent in the ultra-hard material cubic boron nitride, illustrating the well-suitedness of scanning electron diffraction to the investigation of such highly deformed materials. Our findings indicated that a lobe like orientation pattern is formed, similar to that seen by other researchers conducting experiments with copper, a substantially softer material. Following this we provide a number of technique agnostic machine learning approaches for working with orientations, a feature of crystalline samples that is often of interest to researchers as texture can have a profound effect on material properties. Several other samples, both studied and potential are then considered in the closing chapters, giving the reader a broad sense of the range of investigations that scanning electron diffraction can facilitate.
Open Access
Ultra-thin GaAs Photovoltaics for Space Applications
Sayre, Larkin
Ultra-thin photovoltaics (<100 nm) have shown an intrinsic tolerance to radiation-induced damage which makes them a potentially advantageous power source for spacecraft which need to withstand harsh environments outside Earth’s atmosphere. In the ultra-thin regime, high transmission losses can be mitigated by integrating light management structures with nanoscale features. A new type of ultra-thin single-junction GaAs solar cell was designed using drift-diffusion simulations with an 80 nm absorber layer thickness and optimised passivation layers. In particular, the use of InGaP as the front surface passivation layer, instead of the more widely used AlGaAs, produced optimal front surface passivation and performance despite being a direct band-gap semiconductor. The annealed n-type contact was optimised using a transmission line measurement study to minimise series resistance at the metal-semiconductor interface while avoiding excess diffusion of Au into the active layers of the device which degrades shunt resistance. Periodic metal-dielectric nanostructures were simulated and optimised for light management in 80 nm devices using rigorous coupled-wave analysis. Displacement Talbot lithography (DTL) was used for the first time in a photovoltaic application to produce these nanostructures. DTL is a non-contact, wafer-scale interference lithography technique that produces periodic features with excellent uniformity over significant topography in a single exposure. A hexagonal array of Ag pillars in a SiN layer was patterned on the back surface of the ultra-thin devices to increase the optical path length of photons through the active layers. A wafer lift-off process using an epoxy bond and substrate etch back technique was developed to remove the devices from their growth wafers. This lifted-off design produced an AM0 short circuit current of 15.35 mA/cm² and an AM0 efficiency of 9.08%, a 68% increase over the planar on-wafer equivalent. Optical simulations confirmed the contributions of Fabry-Perot and waveguide modes to this current increase. Simulated fabrication and design improvements showed a feasible pathway to 16% AM0 efficiency. Planar on and off-wafer 80 nm ultra-thin devices were then exposed to 68 MeV and 3 MeV proton radiation to test their resilience in the space environment. Irradiation results for on-wafer devices have shown boosted absorption of light compared to previous 80 nm onwafer ultra-thin designs in the literature. Maximum power values for off-wafer devices with integrated back surface planar mirror also exceeded cells that are two orders of magnitude thicker from 3×10¹¹ p⁺/cm², the lowest 3 MeV proton fluence that was tested. Devices with 3500 nm thickness produced just 53% of pre-exposure short circuit current at an equivalent fluence of 7.21×10¹² p⁺/cm². However, there was no degradation in short-circuit current for 80 nm devices up to 2×10¹⁴ p⁺/cm² . Time-resolved cathodoluminescence analysis was carried out on radiation damaged devices and was used to correlate the onset of short circuit current degradation with the point when extrapolated carrier lifetime drops below the calculated time for carriers to traverse the junction. This is the first evidence in the literature that suggests the intrinsic radiation tolerance of ultra-thin cells is due to carrier lifetimes remaining long in relation to junction traverse time even after radiation-induced defects are introduced.
Embargo
Electrocaloric and barocaloric effects in organic materials
Liu, Zipeng
Electrocaloric (EC) and barocaloric (BC) materials undergo reversible thermal changes in response to changes in applied electric field and pressure, respectively. These materials could potentially be exploited in novel solid-state cooling systems that may replace current vapour-compression systems, which are environmentally harmful, noisy, and relatively energy inefficient. In this work, I studied EC effect and BC effects in multicaloric organic materials, which promise large caloric effects near room temperature. The dissertation is structured as follows. Chapter 1 introduces the background for conventional refrigeration and traditional caloric materials. Chapter 2 then surveys the literature on EC materials and BC materials, as well as the literature on EC prototype devices. Chapter 3 reviews the experimental and modelling methods used for this work. These include sample preparation methods, dielectric spectroscopy and ferroelectric polarisation measurements, calorimetry and infrared imaging, dilatometry and Landau models. Chapter 4 describes the study of EC effects in two dabco-based organic salts, namely [Hdabco][BF4] and [AH][ReO4], where dabco is 1,4-diazabicyclo[2.2.2]octane and AH is a variant of dabco, 1-azabicyclo[2.2.1]heptanium. Experiments and modelling demonstrates that [Hdabco][BF4] shows giant EC effects (isothermal entropy change |∆S| = 15.5 J K-1 kg-1 for |∆E| = 12 kV cm-1) that are one order-of-magnitude larger than those observed in traditional EC oxides such as BaTiO3 (|∆S|= 2.1 J K-1 kg-1 for |∆E| = 4 kV cm-1). [AH][ReO4] shows smaller EC effect of |∆S|~ 7.5 J K-1 kg-1 for |∆E| = 11.2 kV cm-1, but displays better mechanical integrity and operates closer to room temperature. It is concluded that dabco-based organic salts exhibit promising performance due to their large EC effects, non-toxicity and great tunability via chemical alterations. However, electrical leakage and breakdown remains an important challenge to be overcome for their use as EC working bodies. Chapter 5 describes the study of BC effects in the aforementioned dabco-based organic salts. BC effects in these materials have the advantage to be driven using hydrostatic pressure instead of electric field, thus avoiding issues related to electrical leakage and breakdown. Three compositions were selected for BC studies, namely [Hdabco][BF4], [Hdabco][ClO4] and [Hdabco][ReO4]. Among these, [Hdabco][ClO4] shows the largest reversible BC effects, |∆S| = 73.2 J K-1 kg-1 for |∆p| = 1200 bar, which compare well with those reported in state-of-the-art BC materials. Notably, BC effects in [Hdabco][BF4] largely outperform EC effects in the same compound, thus demonstrating that pressure is a useful driving parameter for leaky organic ferroelectrics. Chapter 6 describes BC studies in ureasil polymeric materials. These compounds show large changes in entropy when transforming from liquid to solid. By exploiting a gelation method, the liquid to solid phase transition in these compounds is transformed to a gel-to-solid phase transition, which is desirable for some caloric applications. By driving these transitions using pressure, very large BC effects of |ΔS| = 263 J K−1 kg−1 for |∆p| = 1200 bar are found, which are similar to those observed in commercial vapour-compression refrigerants, e.g. R134a. Moreover, the studied polymers have other advantages, in terms of being stretchable, non-toxic, inexpensive and have great tunability of transitions temperatures. Finally, chapter 7 summarises the main results of this work and discusses interesting avenues for future work.
Open Access
Diffraction Between the Spots: Scanning Electron Diffraction of Beam-sensitive Disordered Materials
Laulainen, Joonatan; Laulainen, Joonatan [0000-0003-0596-1663]
Crystallography loves order, but many organic materials are disordered and only partially, if at all, crystalline. Nonetheless, these disordered materials are functionally complex and require characterisation. Their lack of crystallinity poses not only fundamental questions about how to best describe such structures, but also blunts the typically precise tools of crystallography in describing atomic order. Yet, disordered structures not only have structural characteristics, but also complex micro- and nanostructures, defects, and phase distributions. Much of this can be gleaned from diffraction spots, but even more of the information lies in-between the spots in a diffraction pattern. The diffracted intensity outside of the diffracted spots contains the necessary information to not only obtain structural information, but to also characterise the disorder present. Recent developments in transmission electron microscopy (TEM) have enabled the collection of numerous spatially separated diffraction patterns across a specimen, and when combined with computational tools opened a new space for the crystallographic analysis of disordered materials. In scanning electron diffraction (SED), a two-dimensional diffraction pattern is acquired at each probe position in a two-dimensional scan across a specimen. This four-dimensional (4D) data set can be extensively manipulated post-acquisition using computational tools, enabling the acquisition of multiple correlated conventional TEM experiments at once. Yet this is just the tip of the iceberg. Within such a 4D data set, any pixel can be correlated with another, even ones that may seem at first glance unphysical. In this work, such computational microscopy is applied to SED data to characterise the structure of disordered materials. In this work, the requisite computational methods are developed and applied to extract crystallographic information in metal-organic frameworks through pair distribution function analysis, in pharmaceutical cocrystals through nanoscale twist characterisation, and in polymers through semi-crystalline variance and correlation analyses. As all information is contained within a single scan, all of this analysis is done at doses low enough to avoid irradiation damage in the probed beam-sensitive samples.
Open Access
Development of Nanostructured Light Emitters in Gallium Nitride
Jarman, John; Jarman, John [0000-0001-8095-8603]
Nanostructured light emitters in gallium nitride (GaN), including quantum wells (QWs) and quantum dots (QDs), are of widespread importance as the key technology enabling high- brightness blue light-emitting diodes (LEDs) and lasers. GaN-based QDs also show promise as novel polarised single-photon sources that can operate at room temperature. This thesis explores methods of structural and optical characterisation of such nanostructures using electron microscopy and in-situ cathodoluminescence, resolving structural features that correlate with cathodoluminescence images, and providing insight into the differences between QD samples grown by quasi-two-temperature and modified droplet epitaxy methods. Refractive index engineering of GaN via the manufacture of porous structures is explored, and the integration of InGaN QWs with porous distributed Bragg reflectors (DBRs) is reported, with a corresponding increase in LED efficiency owing to improved light extraction. Porous DBRs are also integrated with InGaN QDs to create prototype optical cavity structures, combined with a vertical etching process to create nano- and micropillar cavities. Improved light extraction and background suppression from InGaN QDs embedded in such structures is seen, giving a single-photon emission purity of 96%, a record for these QDs. The integration of InGaN QDs into vertical and planar contact geometries also enables the investigation of the behaviour of these QDs under applied electric fields. Finally, the engineering of a full LED structure containing InGaN QDs is reported, leading to the first measurement of electroluminescence from InGaN QDs and the demonstration of electrically- excited single photon emission from this system.
Open Access
Superconducting phase transitions in hybrid superconducting systems with ferromagnets and spin-orbit coupling
Olde Olthof, Linde
This PhD thesis investigates theoretically the proximity-coupling of superconductivity with ferromagnetism and/or spin-orbit coupling (SOC) in hybrid superconductor systems. The results are summarised in three results chapters which assess the proximity effect in hybrid superconductor systems through calculations of the superconducting phase transition (i.e., the critical temperature T_c and critical fields h_c1 and h_c2). Chapter 3 investigates a ferromagnetic (F) strip on a thin film superconductor (S) with interfacial Rashba SOC in the Ginzburg-Landau formalism. In the presence of SOC, h_c1 has a positive vortex contribution and a negative contribution from the interaction between vortices and SOC. Since the latter is negative, SOC lowers h_c1. When the SOC is strong enough, h_c1 becomes zero and vortices are generated in the absence of a magnetic field. Chapter 4 considers the phase diagram of a thin film S with SOC in the Usadel formalism. Comparing infinite films with and without SOC, the SOC renormalises the magnetic field, effectively increasing h_c2. In finite sized samples, singlet-to-triplet conversion results in spin magnetisation at the sample edges. This edge effect suppresses the phase transition. Due to the sample size-dependence, the transition can be controlled in shape-anisotropic samples by rotating the applied magnetic field direction. Finally, chapter 6 explores an s-wave superconductor (S) / chiral p-wave superconductor (P) junction in the Bogoliubov-de Gennes lattice model. In a S/P junction, the singlet Cooper pairs in S cannot mix with the triplet Cooper pairs in P and T_c of both layers remains the same. However, in a S/F/P junction, F converts singlet pairs to triplet pairs, which boosts the P T_c. By rotating the F layer magnetisation, the singlet pairs convert into a different type of triplet pair state that cannot enter P and T_c is unaffected. Hence, in a S/F/P junction, P T_c is magnetisation-orientation-dependent.