Theses - Materials Science and Metallurgy

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Investigation of a heirarchical collagen scaffold/3D printed lattice structure designed for use in total knee replacements
Meek, Matthew; Meek, Matthew [0000-0001-9013-5168]
Aseptic loosening is a common cause of late-stage failure in total knee replacement (TKR) surgery. This multifactorial condition is exacerbated by stress shielding and micromotion at the bone/implant interface. Stress shielding can be reduced using implant materials with a similar modulus to that of the surrounding bone, such as Ultra High Molecular Weight Polyethylene (UHMWPE). Micromotion is reduced by improved implant osseointegration. A new design for the tibial component of a prosthesis based on macromolecular materials was therefore proposed. A fused deposition modelling (FDM) 3D printed poly ether ketone ketone (PEKK) lattice structure formed the load bearing component of the construct. This contained a lyophilised collagen scaffold filling the lattice voids, to encourage osseointegration. To understand the effects of printing parameters on the structure and properties of prepared samples, the bonds formed between FDM printed rasters were explored. Samples printed from poly lactic acid (PLA) and PEKK polymers were investigated. X-ray microtomography provided information on structural defects (including porosity) and tensile testing was used to investigate the relationships between printing parameters and the ultimate tensile strength (UTS) and modulus. The rasters of dumbbell specimens were aligned perpendicular to the applied load, such that the inter-raster bonds were strained during testing. Increased print speed and raster width were associated with reduced porosity and increased UTS and modulus in both PLA and PEKK. Based on these findings, optimised printing parameter sets were defined. Next, a lattice design was developed to match the compressive modulus of cancellous bone. Based on a combination of theoretical modelling and experimental testing, the effects of lattice volume fraction and geometry on compressive modulus were determined for a range of different lattice morphologies. A sheet diamond lattice morphology exhibited the highest modulus over a range of volume fractions and was taken forward for further experiments. A method of modulating local lattice volume fraction based on medical computed tomography (CT) data was developed. A modulus-matched tibial component derived from 3D printed PEKK sheet diamond lattices was designed using CT data and mechanical testing data. Having investigated the mechanical properties of the load-bearing component of the proposed construct, focus then moved to production and characterisation of the collagen component of the design. Lyophilisation was used to create highly porous collagen structures, which were characterised using X-ray microtomography. A method was developed to accurately evaluate pore size, avoiding artefacts that had previously provided erroneous measurements. Collagen scaffolds were produced in the absence and presence of the polymer lattice and it was found that the average pore size increased from 168 micro m to 229 micro m due, in part, to poor interfacial bonding between the collagen and the lattice struts. To address this issue, two different approaches to improve collagen scaffold/polymer lattice interfacial bonding were investigated. First, a range of different polymer surface topographies were produced using FDM printing. Polymer surfaces with increased topographical roughness were found to exhibit a collagen/polymer interfacial strength, in the dry state, of up to 28.1 kPa, compared with 5.5 kPa for less rough polymer surfaces. The architectural properties of these interfaces were investigated using micro-CT, which demonstrated the presence of interlocking collagen fibrils and polymer. However, it was found that when the interfacial strength of samples was tested in the hydrated state (to mimic the situation in-vivo) there was 18-fold reduction in interfacial strength. Next the effects of collagen crosslinking and the addition of hydroxyapatite (HA) into the collagen scaffold (to create a closer chemical analogue with bone) were explored. Collagen crosslinking (using 5:2:3.3 EDC:NHS:collagen) resulted in a 4 fold increase in the interfacial bond strength while the addition of 20 wt.% HA to collagen scaffolds by blending doubled the interfacial strength as compared with pure collagen scaffolds. Cellular response to the constructs was assessed using MG-63 osteosarcoma cell culture. Cell attachment was increased on collagen-PEKK constructs compared with pure collagen scaffolds. Cell proliferation, migration and metabolism were similar on the construct and pure collagen scaffolds. Increased osteoblast-specific metabolic activity was observed on the collagen component of collagen/PEKK samples compared with the PEKK component, indicating that the collagen component might be more biologically favourable than the PEKK component, in vitro. The work in this thesis demonstrates the mechanical and biological potential of a hierarchical collagen scaffold/3D printed PEKK lattice construct for application in TKR tibial components. The results offer important new insights on methods to improve the longevity of TKRs.
Embargo
Grain boundaries and creep deformation mechanisms in a high performance nickel-based superalloy
Monni, Francesco
This dissertation is focused on the description of creep deformation in the new Rolls-Royce Alloy 12-1 polycrystalline nickel-based superalloy for disc application in gas turbine engines. In particular, this study aims to progress the understanding of the effect of grain boundaries in relation to the content of minor additions such as C, B and Zr. This was achieved by creep testing at conditions representative of service in gas turbine engines and extensive use of electron microscopy. Alloy 12-1 microstructure was characterised before and after interrupted and failed creep tests at 700°C. Grains exhibited intragranular precipitation of large cubic MC carbides and tetragonal M₃B₂ borides. Grain boundaries showed a serrated character with large γˈ particles and fine tetragonal M₅B₃ precipitates before creep testing. Analysis after creep testing revealed a progressive evolution of grain boundary microstructure which is related to the localised accumulation of strain. Orthorhombic M₂B, tetragonal M₂B and orthorhombic MB borides were detected at the grain boundaries after creep deformation. It is suggested that the combination of strain accumulation and atomic diffusion during creep leads to the formation of these types of secondary precipitates. Analysis of the deformation mechanisms revealed that these varied depending on the testing temperature. Dislocation and stacking fault formation are the main mechanisms at 700°C. or below, whereas perfect dislocations glide and bypass of secondary γˈ particles is predominant at 800°C. However, deformation always initiated at grain boundaries independently from the testing conditions. Grain boundary serrations promoted the formation of dislocation networks which were linked to both the nucleation of stacking faults and the inhibition of their impingement on opposite grain boundaries. This promoted an homogeneous stress distribution preventing grain boundary sliding and cavitation, the latter being limited to triple points. Grain boundary migration and γˈdirectional coarsening were commonly observed after creep at 800°C. However, grain boundary borides showed a pinning effect that limited the migration. Finally four alloys based on Alloy 12-1 composition and varying contents of C, B and Zr were creep tested at 700°C and at 800°C. Despite showing a similar initial microstructure, mechanical testing results showed that grain boundary composition strongly affects creep resistance. A lower B content was found to be beneficial during creep at 700°C and 800 MPa, whereas increasing the amount of C was found to improve creep resistance for any testing condition. This was attributed to the increased carbide and boride intragranular precipitation which disrupted dislocation and stacking fault propagation. Presence of Zr was found to be critical to promote the precipitation of carbides and borides. A reduced content of Zr was associated with higher grain boundary mobility, particularly during creep at higher temperature.
Open Access
On the Plasticity of Layered Crystals
Pürstl, Julia Theresa
Progress in modern applications in the aerospace or energy generation industries is increasingly determined by access to higher temperatures. Increasing effort is thus put into research of ceramics, which generally provide superior high temperature chemical and structural stability compared to their metallic counterparts. The governing factor is here the control of their intrinsic brittleness, which is required for their use in a structural environment. One way to alleviate ceramic brittleness is to promote plastic processes. A model system in this regard are the MAX phases, ternary carbides and nitrides with a hexagonal layered crystal structure, which show a critical resolved shear stress for basal plane slip of as low as 77 MPa. A possible explanation for this unusual ductility was proposed on the basis of electronegativity differences, which promote electron density shifts and locally ease the motion of dislocations. In theory, this effect may be modified through compositional substitutions within the layers, and as such it was proposed as a design basis for other ceramic layered systems, for example with a cubic crystal structure. Yet, experimental evidence to support this approach is scarce. The present work was aimed at further experimental investigation of the concept of modifying plasticity in layered ceramic systems by means of compositional variation between layers. This was facilitated by single crystal micromechanical testing of two model systems, the MAX phases and the cubic Th₆Mn₂₃ structure. To evaluate the effects of compositional variation a total of three MAX phases, Ti₃SiC₂, Ti₃AlC₂ and Cr₂AlC, and four Th₆Mn₂₃ phases, Al₁₆Co₇Ti₆, Al₁₆Ni₇Ti₆, Al₁₆Co₇Zr₆ and Co₁₆Zr₆Si₇, were considered. The MAX phases were tested via micro-compression at room and cryogenic temperatures, where the main aim was to assess a value for the friction stress, and the possibility of its variation with temperature. In preparation of this work, a detailed analysis of micropillar deformation was performed in Cr₂AlC using digital image correlation assisted pillar strain mapping, along with considerations of pillar size and crystal orientation, to ensure accurate assessment of the yield stress. In the Th₆Mn₂₃ structure type, exploratory testing was carried out using nanoindentation, aiming to compare changes in hardness as an estimate of plasticity. These data were supported by transmission electron microscopy, to link experimental observations to the underlying material defects. The results of the MAX phase micropillar compression studies suggest that deformation is governed by the motion of dislocation sources, and a link was established for the extraction of afriction stress from compression experiments on this basis. This was furthermore shown to vary significantly with orientation, suggesting pronounced non-Schmid effects, which were further explained on the basis of dislocation mobility and taken into account for the design of tests on the basis of composition. Testing of different MAX phase compositions at room and cryogenic temperatures suggested an increase in strength for Cr₂AlC compared to Ti₃SiC₂ and Ti₃AlC₂, The tests at cryogenic temperatures further indicate an increase in strength with a decrease in temperature for all components, which was furthermore most pronounced for Ti₃AlC₂ and Cr₂AlC. Yet, pronounced scatter in the data was observed for both studies. The hardness measurements in the Th₆Mn₂₃ structure confirmed a variation with layer composition, with analysis by transmission electron microscopy supporting the claim that plasticity is mediated by dislocations moving parallel to the layers. The ranking in hardness data was slightly offset with regards to what was predicted from variations in electronegativity, from which an alternative scaling on the basis of layer electronegativity was proposed.
Open Access
Gradients in Collagen Films: Effects of Mechanical and Chemical Properties on Directional Cell Response
Vriend, Eleonora
Mammalian cells are highly sensitive to variations in cues provided by their surroundings. Directed cell migration is key to many physiological processes and there is a need for tissue engineering materials and structures to encourage optimised cell response. This thesis describes the creation of different types of gradients in collagen films and investigates their physicochemical properties and resultant cell behaviour. Crosslinking gradients were formed by gradual immersion of collagen films into different crosslinking solutions. The use of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride with N-hydroxysuccinimide (EDC/NHS) influenced both the mechanical and chemical properties, while crosslinking using genipin altered mechanical behaviour without changing surface chemistry. By adjusting the rate of immersion and solution concentration, gradient shape could be controlled. The effect of crosslinking chemistry was evaluated on uniformly treated films, using tensile testing, atomic force microscopy, and (2,4,6-Trinitrobenzene-1-sulfonic acid). An increase in stiffness from 3.5 to 26.6 MPa was obtained by varying the concentration of EDC/NHS solutions and from 3.5 to 23.9 MPa in the case of genipin. Cell response to mechanical and chemical variations based on crosslinking and peptide dendrimer (IKVAV) attachment was tested using human dermal fibroblasts (HDFs), fibrosarcoma cells (HT1080s) and rat Schwann cells (RSCs). HDFs showed an unfavourable response to high EDC/NHS crosslinking levels, but increased proliferation with increasing genipin crosslinking. The presence of IKVAV dendrimers enhanced RSC adhesion and proliferation. On gradient films, the variation in chemistry and stiffness created by EDC/NHS presented competing effects. The response of HT1080s was dependent on the type and profile of crosslinking gradient. On IKVAV gradients, migration of RSCs towards higher concentrations was promoted, whereas HDF movement was discouraged. This body of work demonstrates, for the first time, the use of this method in forming property gradients in collagen, which can provide cues to achieve directional cell response.
Open Access
Production of Dermal Inspired Collagen Architectures by Lyophilisation and Electrophoretic Deposition
Smith, Patricia
Collagen scaffolds have been used successfully as dermal regeneration templates in clinical applications, but most do not recapitulate the natural tissue structure and are often combined with synthetic membranes to afford additional barrier properties. This work aimed to produce dermal-inspired collagen structures that better replicate the native dermal and epidermal structure and function. Aqueous suspensions of 1% w/v insoluble collagen type I were freeze-dried to create porous collagen sponges c. 32-35mm in diameter with varying thicknesses. Freeze-drying comprised an ice nucleation and growth phase, followed by sublimation to leave an interconnected pore network. The effects of changing process parameters on the resulting pore structure were analysed using micro-computed X-ray tomography and scanning electron microscopy. The target pore structure consisted of a majority "reticular" region of <125µm pores with a thin upper region of >150µm pores to mimic the open papillary dermal ECM. An "annealing" step at -20°C for 24h following ice nucleation increased mean pore size by 20-30µm in samples over 6mm thick but had no effect on samples thinner than this. Extending time at equilibrium (TAE) during freezing increased pore size significantly, with TAE and pore size following a power law relationship, the exponent 1/n experimentally determined to be n=6.21. Annealing temperature was identified as a critical parameter influencing the rate of ice crystal coarsening and a heat source was introduced to increase the temperature gradient through the scaffold thickness after freezing. After process optimisation, differential coarsening through the thickness of c. 4-5mm thick scaffolds was observed in samples annealed above c. -3°C. Annealing for 100min near 0°C produced scaffolds with upper regions of mean pore size above 160µm, while lower regions remained comparable with the 95-105µm mean pore size of non-annealed controls. To address the requirement for barrier properties to mimic the native epidermis, direct current electrophoretic deposition (DC-EPD) was investigated as a method to produce collagen membranes. DC-EPD was applied to suspended collagen scaffolds to produce well-adhered, continuous, and defect-free scaffold-film bilayers. Carbodiimide crosslinking of scaffolds to 10% of the standard 5:1 carbodiimide:COO-(Col) ratio was found to be sufficient to maintain the scaffold structure during DC-EPD processing. By controlling the overall resistance of the deposition cell, deposition yields approaching 100% were achieved in under 20min. The barrier properties of collagen scaffold-film bilayers produced by freeze-drying and DC-EPD were assessed. The water vapour transmission rate (WVTR) across scaffolds and scaffold-film bilayers was compared. The film addition reduced the WVTR from 0.862-0.660g/Pa/m²/d, the latter within the recommended range for wound cover. Finally, human dermal fibroblast (HDF) infiltration into nude scaffolds and scaffold-film bilayers (seeded either on the film, or scaffold side) was investigated. The film-seeded bilayers significantly impeded HDF infiltration compared to nude scaffolds, whereas the scaffold-seeded bilayers showed a small reduction in infiltration compared to nude controls. Bilayers seeded on both surfaces showed similar HDF population counts on both surfaces after 14 days.
Open Access
Computational modelling of polymer network formation
Jenei, Mark
There exists a number of methods for the computational simulation of polymerisation processes. This thesis provides an overview of these methods, and establishes a connection between them on a theoretical level, based on the First Shell Substitution Effect (FSSE) model of polymerisation. The FSSE framework was used to evaluate a coarse-grained simulational method, benchmarked against a fine-grained, atomistic model that was constructed using Reactive Molecular Dynamics (RMD). A modified version of RMD was implemented using a non-reactive force field, and used it to estimate the physical properties of a 3D printing thermosetting-thermoplastic copolymer, in order to validate the method. When comparing to experimental results, we found a good matching for the glass transition temperature, but inaccuracies when estimating elastic properties. We believe that this is due to the limited system size we had to resort to, when using the computationally intensive fully atomistic models. Next, we assessed whether the FSSE model can capture the most relevant properties of the forming polymer network in step-growth polymerisation. As model systems, epoxy-amine copolymers were used, with different monomer functionalities of the hardener molecules. Using numerical simulations on a graph, which is essentially a simulated random process where edges are step-wise added to it, we showed that the FSSE model is indeed capable of reproducing the results of the benchmark RMD simulations, in terms of network evolution. A core property within this model is the monomer reactivity, which is a function of both the monomer degree and the overall functional group conversion. The conversion dependent monomer reactivities provide a basis for an intuitive comparison of different methods modelling the polymerisation process. Finally, we implemented a novel coarse-graining method, Trajectory Matching, which can be used to scale up simulations. Using the FSSE framework, we looked at whether the coarse-grained model can reproduce the dynamics of network evolution throughout the polymerisation process. We used identical epoxy-amine systems as for the FSSE study, and found good matching between the results of coarse-grained DPD, and benchmark RMD simulations. We also showed that steric effects only become significant for higher degree monomers and at high conversions, while the reactivity of low degree monomers are diffusivity controlled.
Open Access
Influence of surface hardening on single crystal nickel-based superalloys
Bogachev, Ivan
This thesis is concerned with the influence of two surface hardening procedures, mechanical shot peening (MSP) and deep cold rolling (DCR), on the microstructures of single crystal nickel-based superalloys. These alloys have excellent high temperature properties, and consist of a randomly substituted face centred cubic matrix phase known as γ, together with a large volume fraction of an ordered precipitate phase, most often the primitive cubic γ' phase. Superalloys cast as single crystals are the premier materials for the manufacture of turbine blades in aerospace jet engines, where the material experiences high temperatures and fatigue stresses for prolonged periods of time in service. Turbine blade superalloys have traditionally been optimised for creep resistance. However, a continual industry drive to improve turbine jet engine efficiency has led to greater low cycle fatigue damage being accumulated by the single crystal turbine blades, as metal stresses and temperatures rise. A case in point is the poor fatigue performance of intermediate pressure turbine blades in the Rolls Royce Trent 1000 engine, which was a major industrial issue at the time of writing. The application of MSP and DCR was identified as a prospective way to improve turbine blade fatigue resistance. This called for a systematic investigation of the effects of these procedures on single crystal superalloy materials, which had previously been largely unexplored. Surface hardening procedures such as MSP and DCR function by plastically deforming the surface of a workpiece, generating a surface compressive residual stress and a layer of cold worked material. Both the residual stress and cold work are thought to impede the fatigue propagation of surface cracks, but previous studies have established that at high cycling stresses and high temperatures, the cold work confers the main benefit to fatigue life. In MSP, the surface of the material is bombarded by a stream of small hard shot, while in DCR, the surface is deformed by a roller under hydrostatic pressure. Each procedure is defined by several key process parameters: intensity, coverage and shot size for MSP, and pressure and roller ball diameter for DCR, and setting the correct combinations of these parameters is important for optimising a particular procedure for a given application. The ultimate goal of the Work described in this thesis was to enable the informed application of surface hardening to single crystal turbine blades, so as to achieve the greatest improvement in fatigue behaviour. The specific brief of this Work was to examine the effects of MSP and DCR on the microstructures of single crystal superalloys, and to characterise how different procedure parameters affected the resultant work hardened layer of material, particularly the distribution of microscopic cold work. Changes in the as hardened microstructure following high temperature heat treatments were also to be assessed, since thermal stability is a key requirement for the turbine blade. Local misorientation (LM) analysis of electron backscattered diffraction (EBSD) data was the principal tool for characterising cold work structures produced by MSP and DCR. The development of the LM analysis methodology for single crystal superalloys and the creation of a Python script to implement this methodology were major outcomes of this Work. Cold work depths and magnitudes, the latter being the amount of cold work per depth, were defined on the basis of LM data and were used to quantify the average extents and densities of the cold worked layers. Test samples made from the superalloy CMSX-4, treated by MSP or DCR with varied combinations of parameters, were examined using LM analysis and backscattered electron imaging. It was shown that in MSP, greater intensity increased the depth of cold work, while peening coverage chiefly influenced the amount of cold work per depth. Greater shot size tended to produce deeper cold work but with less magnitude. Likewise, in DCR, greater roller diameter and greater rolling pressure both led to greater cold work depth, with the rolling pressure having a further significant influence over the amount of cold work per depth. Both the depth and the magnitude of cold work were also seen to depend somewhat on the secondary crystallographic orientation of the crystal matrix in a given sample. Overall, DCR resulted in much deeper cold worked layers than MSP, with much lower magnitudes. The lateral cold work distributions produced by MSP and DCR were also observed to differ substantially. In MSP, a thick continuous surface layer of cold work, ~35 µm in depth, was seen, while in DCR, cold work was concentrated into narrow surface streaks. Sets of slip bands, denser but more shallow in MSP than DCR, penetrated in depth from the hardened surface, often past the depth of the bulk cold work. To assess the response of the peened and rolled sample microstructures to highly elevated temperatures, samples were heat treated at 900 and 1100 °C under vacuum with no externally applied stress. Significant amounts of recrystallisation (RX) and topologically close packed phase (TCP) precipitation were observed and quantified in samples treated at 900 °C, both phenomena being promoted by the slip bands induced in the material. After 500 h at 900 °C, samples with sufficiently low cold work magnitudes displayed RX depths fairly in line with a control sample, though almost all MSP samples displayed RX grains of ~30 µm or more after 1000 h. At 1100 °C RX occurred more far more rapidly in all samples, whereas TCP formation was hindered by the recovery of slip bands and the lower thermodynamic stability of TCPs at this temperature. Samples treated by DCR generally displayed smaller RX depths and lower rates of TCP formation than those treated by MSP. Directional coarsening, or rafting, of the γ/γ' microstructure was noted at 1100 °C. The extent of rafting was independent of the hardening conditions, and increased with depth before reaching a transition to the bulk microstructure. It was surmised that rafting was driven by residual stresses generated within the material by the surface hardening, and that cold work impeded rafting. The latter conclusion was supported by the invariable occurrence of peak rafting after the measured cold work depth, highlighting the difference between the related, but distinct phenomena of cold work and residual stress. Finally, work was carried out on turbine blades manufactured from the superalloy RR3010 and treated by MSP, to validate the results obtained in this Work. The cold work structures of the blades were seen to correspond well with those in test samples peened under similar conditions. Preliminary investigation was also carried out on peened blades with Pt Cr diffusion coatings, which are proposed to be used in conjunction with MSP to increase the corrosion and oxidation resistance of intermediate pressure turbine blades. The coatings did not themselves change the underlying cold work structure significantly. However, they were found to occupy the area of greatest cold work magnitude in the cold work profile typical of MSP, so that the amount of cold work transmitted to the alloy beneath was generally low. The present Work has resulted in development of greater understanding of the effects of MSP and DCR on single crystal nickel based superalloys, including the variation of cold work depth and magnitude with procedure parameter, the differences between the cold work structures produced by the two procedures, and the microstructural consequences of exposing the work hardened material to high temperatures. This will aid industry in rationalising and refining hardening procedures when applied to turbine blades, and will lead to blades having greater fatigue resistance and longevity, ultimately improving jet engine efficiency. This thesis consists of seven chapters and an appendix. A brief summary of the contents is given at the beginning of each chapter. Chapter 1 is the introduction, where the industrial context, relevant background information and a review of the known effects of surface hardening procedures on superalloys are given. Chapter 2 contains the experimental details of the materials and methods employed throughout the Work. Chapters 3 and 4 deal with the as hardened states of the MSP and DCR samples, respectively. The effects of static heat treatments on these samples are described and discussed in Chapter 5. Chapter 6 is concerned with the studies performed on both coated and uncoated turbine blade specimens. The conclusions drawn from the Work are summarised in Chapter 7, together with suggestions for possible future work in this field. Appendix A contains the Python script used to conduct the LM analysis and a discussion of its functionality.
Open Access
Understanding nanomagnetism from all angles: Developments in magnetic electron tomography
Lewis, George; Lewis, George [0000-0001-9232-4253]
This thesis aims to develop transmission electron microscopy (TEM) techniques to study magnetic nanomaterials in three dimensions (3D). At the nanoscale, magnetic materials exhibit various intriguing behaviours such as topological protection in skyrmions (which may be used in next-generation computers), vortex states in nanorings (which are being developed to treat cancer), and long-term stability in paleomagnetic samples (which allow researchers to study the formation of planetary bodies in the solar system). Many of these behaviours are intrinsically 3D in nature, and so a 3D characterisation method is required to fully understand them. This thesis explores how magnetic imaging in the TEM can be combined with electron tomography to provide a tool for studying magnetic nanomaterials in 3D. The thesis begins by introducing the motivation, theory, and background literature relevant to magnetic electron tomography. This is then followed by a detailed study on magnetite nanorings which showcases the types of insights that can be gained through 3D magnetic imaging. Following this, it is demonstrated how prior knowledge about magnetic fields can be used to improve magnetic electron tomography experiments both through improvements to the data acquisition process as well as through changes to the data reconstruction process. At the end of the thesis, the individual strands of prior knowledge, electron tomography, and image analysis are applied independently to different projects in a section which showcases how magnetic electron tomography is intricately interconnected with the development of complementary research fields. Taken together, the content of this thesis contributes towards the maturing of magnetic electron tomography as a powerful technique for understanding magnetic nanomaterials in 3D.
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.
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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.
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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.
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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.
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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.
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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.