A process has been developed to make the dielectric layer for organic thin-film transistors (OTFTs) in a roll-to-roll vacuum web coater environment. This dielectric layer combined with an organic semiconductor layer and metal layer deposited in vacuum allows a solvent-free process to make organic/inorganic multilayer structures for thin-film electronic devices on a flexible substrate at, potentially, high speed. The polymeric gate dielectric layers were fabricated by flash evaporation of acrylic monomers onto a polymer film with pre-patterned metal gates followed by radiation curing by electron beam, ultra-violent light (UV) or plasma. With a non-polar dielectric surface, charge carrier mobility (μ) of 1 cm²-V^-1s^-1; on/off curren ratio of 10⁸, sub-threshold swing (SS) of 0.3 V/decade and saturated output curve were routinely achieved in dinaphtho-[2,3-b:2'3'-f]thieno[3,2-b]thiophene (DNTT) transistors with dielectric layer of tripropylene glycol diacrylate (TPGDA) of ~400 nm. Apart from the TPGDA, monomer formulas including 1,6-Hexanediol diacrylate (HDDA) as well as several commercial acrylic resins have been used to make the dielectric layer. The highest areal capacitance of 41nF-cm^-2 was achieved with a pin-hole free film of less than 100 nm made of an acrylate mixture resin. A non-polar dielectric surface treatment layer has been developed based on flash evaporation of lauryl acrylate and HDDA mixture. The transistors with the buffer layer showed constant performance and a mobility fivefold greater than those of untreated samples. The effect of humidity, oxygen, and light during switching cycles of both pentacene and DNTT transistors were studied. Water and oxygen/illumination had a distinct effect on both pentacene and DNTT transistors. Oxygen leads to acceptor-like charge traps under illumination, which shifted the turn-on voltage (V_to) to more positive values. In contrast, water in transistors gave rise to donor-like charge traps, which shifted the V_to and the threshold voltage (V_T) more negatively. The DNTT devices showed good stability in dry air without encapsulation, while pentacene transistors degraded with either repeating measurement or long term storage. A DNTT transistor with a PS-coated TPGDA dielectric layer showed stable drain current (I_d) of ~105A under bias stress of the gate voltage (em>V_g) of -20V and the drain voltage (em>V_d) of -20V for at least 144 hours. The V_to shift after the stress was less than 5 V and was recoverable when the device was kept in dry air for a few days. Possible reasons for the V_to shift have been discussed.

This thesis investigates the mechanisms behind natural silk spinning by integrating rheology, spectroscopy and small angle scattering to better understand this process and to guide our efforts towards mimicking Nature’s ways of producing high performance fibres. As a result of natural selection, arthropods such as spiders and moths have evolved the ability to excrete silk proteins in a highly controlled manner. Spun from liquid feedstocks, silk fibres are used ex vivo to build structures with mechanical properties currently unmatched by industrial filaments. As yet, relatively little attention has been directed to the investigation of spinning under biologically relevant conditions. To better understand how and why silk is spun, this thesis bridges the gap between liquid silk flow properties and structure development. To directly connect the two, I have developed and deployed novel experimental platforms that combine infrared spectroscopy and small angle scattering with rheology. This approach has clarified long-standing ambiguities on the structural root of silk’s apparently complex flow properties. Small angle scattering revealed the length scales involved in the flow induced solidification under a range of spinning conditions. Mo reover, infrared spectroscopy offered a unique perspective into silk’s formation process immediately after excretion. In a similar manner to the post-extrusion tuning of the properties of partly solidified spider silk filaments, this thesis has revealed that silkworm silk fibres are far from completely formed once excreted. One might describe the filaments of mulberry silkworm as seeded molten polymers that form its hydrogen bonding network and crystallises slowly on site. Consequently, it enlightens that post-spinning conditions are equally paramount for silkworm silk, giving an explanation for the relatively poorer mechanical properties. The comparison of silks from a range of species, allowed this hypothesis to be extended to wild silkworm silk. My insights into spinning had the fortuitous repercussion of facilitating silk fibre solubilisation leading to the development of better artificial silk feedstocks flowing like native silks. With these findings, I believe we are now in an improved position to conceive artificial fibres with properties rivalling those of Nature.

Organic photovoltaics have attracted considerable research and commercial interest due to their lightness, mechanical flexibility and low production costs. There are two main approaches for the fabrication of organic solar cells – solution and vacuum processing. The former relies on morphology control in polymer-fullerene blends resulting from natural phase separation in these systems. The latter takes advantage of solvent-free processing allowing highly complex multi-junction architectures similar to inorganic solar cells. This work aims to combine the benefits of both by depositing conjugated polymers using vacuum thermal evaporation. By employing this unconventional approach it aims to enhance the efficiency of organic photovoltaics through increased complexity of the thin-film architecture while improving the nanoscale morphology control of the individual active layers. The thesis explores the vacuum thermal deposition of polythiophenes, mainly poly(3-hexylthiophene) (P3HT) and side-group free poly(thiophene) (PTh). A variety of chemical techniques, such as NMR, FT-IR, GPC, DSC and TGA, are used to examine the effect of heating on chemical structure of the polymers. Optimal processing parameters are identified and related to the resulting thin-film morphology and charge transport properties. Efficient photovoltaic devices based on polythiophene donors and fullerene acceptors are fabricated. Materials science techniques AFM, XRD, SEM, TEM and MicroXAM are used to characterize topography and morphology of the thin films, and UV-Vis, EQE, I-V and C-V measurements relate these to the optical and electronic properties. The results of the study show that polymer side groups have a strong influence on molecular packing and charge extraction in vacuum-deposited polymer thin films. Unlike P3HT, evaporated PTh forms highly crystalline films. This leads to enhanced charge transport properties with hole mobility two orders of magnitude higher than that in P3HT. The effect of molecular order is demonstrated on polymer/fullerene planar heterojunction solar cells. PTh-based devices have significantly better current and recombination characteristics, resulting in improved overall power conversion efficiency (PCE) by 70% as compared to P3HT. This confirms that the chemical structure of the molecule is a crucial parameter in deposition of large organic semiconductors. It is also the first-ever example of vacuum-deposited polymer photovoltaic cell. Next, vacuum co-deposited PTh:C60 bulk heterojunctions with different donor-acceptor compositions are fabricated, and the effect of post-production thermal annealing on their photovoltaic performance and morphology is studied. Co-deposition of blended mixtures leads to 60% higher photocurrents than in thickness-optimized PTh/C60 planar heterojunction counterparts. Furthermore, by annealing the devices post-situ the PCE is improved by as much as 80%, achieving performance comparable to previously reported polythiophene and oligothiophene equivalents processed in solution and vacuum, respectively. The enhanced photo-response is a result of favourable morphological development of PTh upon annealing. In contrast to standard vacuum-processed molecular blends, annealing-induced phase separation in PTh:C60 does not lead to the formation of coarse morphology but rather to an incremental improvement of the already established interpenetrated nanoscale network. The morphological response of the evaporated PTh within the blend is further verified to positively differ from that of its small-molecule counterpart sexithiophene. This illustrates the morphological advantage of polymer-fullerene combination over all other vacuum-processable material systems. In conclusion, this processing approach outlines the conceptual path towards the most beneficial combination of solution/polymer- and vacuum-based photovoltaics. It opens up a fabrication method with considerable potential to enhance the efficiency of large-scale organic solar cells production.

Reduced-activation ferritic steels are considered as the primary candidate materials for structural applications within nuclear fusion power plants. It is known that by mechanically alloying ferritic steel powder with Y (usually in the form of Y₂O₃) then consolidating the material by hot isostatic pressing, a nanoscale dispersion of oxygen rich nanoclusters as small as ~2nm is introduced into the microstructure. This vastly improves high temperature strength and creep resistance, and the nanoclusters also act as trapping sites for helium and point defects produced under irradiation. In this thesis, the evolution of the oxide nanoclusters in a Fe-14Cr-2W-0.3Ti & 0.3Y₂O₃ ODS alloy was investigated primarily using atom probe tomography. The microstructure was characterised at various points during processing to give an insight into the factors influencing the formation of the nanoclusters. It was found that the nanoclusters nucleated during the mechanical alloying stage, then followed near classical nucleation and growth mechanisms keeping the same composition of ~8%Y, ~12%Ti,~25%O and ~45%Cr throughout. The formation and evolution of 5-15nm grain boundary oxides was also observed, and these were shown to form first as Cr₂O₃ particles that subsequently transform into a Y-Ti-O based oxide on further processing. The influence of mechanical alloying with 0.5wt.%Fe₂Y rather than 0.3wt.%Y₂O₃ was also investigated, and this showed that there was no difference in the final microstructure produced provided the level of Ti in the starting powder was tightly controlled. Without sufficient Ti, the nanoclusters were Y-O based and ~6nm diameter. Both the Y-O and Y-Ti-O nanoclusters were moderately stable on annealing at 1200°C for up to 100 hours, with only minimal coarsening observed. Ti was found not to influence the coarsening rate of the nanoclusters significantly. The stability of the oxide nanoclusters under irradiation was investigated by using Fe²⁺ ion irradiation to simulate displacement cascade damage in the ODS-Eurofer material (the official European candidate material for testing in the ITER fusion test reactor). Doses up to ~6 dpa at 400°C were used, and there was no significant change to the nanocluster distribution. However segregation of Mn to dislocations was observed after irradiation. These results indicate that ODS steels are good candidate structural materials, as the microstructure is stable at high temperature and under irradiation. The starting powders, and processing parameters need to be tightly controlled in order to produce the optimal material for use in service.

The factors controlling the reliability of Pb-free solders when subject to thermomechanical regimes relevant to the harsh aerospace environment have been studied. Ball grid array (BGAs) typical of microelectronic devices have been manufactured in-house and subjected to isothermal ageing and thermal cycling. The BGAs comprised both Cu and Ni-Au metallizations, Pb-free Sn-Ag-Cu 400 and 600μm solder balls, FR4 and Al₂O₃ boards, and included circuits to measure resistance changes due to damage in the joints during thermal cycling. Microstructural evolution within the solders balls and complex interfacial reactions were studied in all configurations using various types of electron microscopy. The mechanical properties of the different phases formed within solder joints were studied using nanoindentation at room and elevated temperatures up to 175°C for the first time. Intermetallic compounds (IMCs) were stiff, hard and brittle with very low creep rates, while the softer primary Sn, eutectic regions and Cu metallization readily underwent creep. Two-dimensional finite element analysis (FEA) of nanoindentation was used to understand better the physical meaning of nanoindentation creep data. Reliability experiments comprised both thermal cycling and FEA of BGAs. The difference in coefficient of thermal expansion (CTE) in the BGA materials caused interfacial fatigue damage in the solder joints, which was detected primarily at the solder/metallization interface of the outermost, most strained solder joint. Accumulated creep strain per cycle at this interface was evaluated using 3D FEA of the stress-strain state of the BGA and results calibrated against experimental BGA mean lifetimes using the Coffin-Mason relationship. Nanoindentation combined with FEA has been shown to be a viable route for the rapid assessment of creep performance and lifetime in lead-free solders under aerospace thermal cycles.

This thesis describes the fabrication and characterization of ex situ magnesium diboride (MgB<sub>2<) bulk material to study its sintering behaviour. Since the discovery of superconductivity in magnesium diboride in 2001, many research studies have identified the attractive properties of this easy-to-fabricate, low cost superconductor which can attain high critical current density even without heat-treatment. However there is little consensus in the literature on the processing requirements to produce high quality MgB<sub>2< material with low impurity content and high density. In this work, the key parameters in the production of dense ex situ MgB<sub>2< produced from Alfa Aesar MgB<sub>2< powder are established by examining the effect of modifying the characteristics of the starting material and the processing parameters during pressureless and pressure assisted heat-treatment. The particle size distribution, impurity content and particle morphology of Alfa Aesar MgB<sub>2< powder were determined using laser dffraction, X-ray diffraction, X-ray photoelectron spectroscopy, electron dispersive spectroscopy, scanning electron microscopy and transmission electron microscopy. This powder was also modified by separation (sieving and sedimentation) and milling (ball milling and attrition milling), with changes made to the powder determined by the same techniques. A pressureless heat-treatment method using a magnesium diboride powder bed was developed. This minimised MgO formation in samples produced from as-purchased MgB<sub>2< powder to less than 8 wt.% for heat-treatment at 1100°C. MgO content was determined by X-ray diffraction using calibrated standards. MgB<sub>2< bulk material was produced from as-purchased and modified powders by pressureless heat-treatment under Ar gas, and characterized using Archimedes' density method, X-ray diffraction, Vickers hardness testing, scanning electron microscopy, and magnetization measurements. Very limited densification was observed for all samples prepared by pressureless heat-treatment, with only limited increases in connectivity observed for some samples heat-treated at 1100°C. Pressure-assisted bulk samples were prepared from as-purchased MgB<sub>2< and selected modified powders using resistive sintering, spark plasma sintering, and hot pressing. These were characterized using the same techniques, which indicated much more extensive densification with similar levels of impurity formation as for pressureless heat-treatment at 1100°C. The results indicate that densification and applied pressure are strongly correlated, while the effect of temperature is less significant. The optimum processing environment (inert gas or vacuum) was dependent on the technique used. These results indicate that pressure-assisted heat-treatment is required in order to produce dense bulk MgB<sub>2<.

Pre-cellularised scaffolds are limited in volume due to the constraints of the time delay required for angiogenic cells ingrowth forming a vascular network and allowing for delivery of nutrients and waste exchange. Channels have the potential to improve the time taken for cellular penetration. The effectiveness of channels in improving angiogenic cells penetration was assessed in vitro and in vivo in porous 3-D collagen scaffolds. Initial studies conducted in vitro demonstrated that the scaffolds supported angiogenic cells ingrowth in culture and the channels improved the depth of penetration of cells into the scaffold. The cells reside mainly around the channels and migrate along the channels. In vivo, channels increased cell migration into the scaffolds and in particular angiogenic cells resulting in a clear branched vascular network of micro vessels in the channelled samples which was not apparent in the non-channelled samples. This correlated well with macrophage invasion into scaffolds since angiogenesis in vivo is usually accompanied by infiltration of macrophages which participate in organization of angiogenesis, and in regulation of tissue regeneration. Thus, macrophage-mediated biodegradation of collagen scaffolds in vitro was also assessed. Furthermore, pre-seeding channelled collagen scaffolds with endothelial cells implantation has potential of speeding up vascularisation of scaffolds compared to human bone marrow stromal cells.

Compressive residual stresses (CRS) are beneficial for enhancing the fatigue life of metal components. Shot Peening (SP) is an industrial cold working process that is applied to induce a field of CRS and modify the mechanical properties of the metal component. The SP process involves impacting a surface with tiny shots with forces sufficient to create plastic deformation. The process is governed by a number of important parameters such as the shot size, angle of attack, initial velocity, mass flow rate and the distance from the shot nozzle to the surface being peened. The relationship between the optimal peening outcome, particularly the residual stress distribution of the treated surface, and the peening parameters is still unknown and needs to be investigated further. Manufacturers are interested in producing a uniform peening process for complex geometries which optimises the SP parameters. Modelling the process is complex as it involves the interaction of a metallic surface with a large number of shots of very small diameter. Conventionally, such problems are solved using finite element software to predict stresses and strains of a single shot impact then applying superposition. At the moment there are no Finite Element Method (FEM) modelling solutions involving more than tens of shots. The number of shots and elements required for such a modelling process made the approach unfeasible prior to the work described herein. The objective of this work is to develop an appropriate numerical modelling approach that can better simulate the real SP process. The model will be provided by combining Discrete Element Method (DEM) with FEM. The DEM is employed to get a distribution of impact velocities over space and time which are then implemented into a FEM analysis. A discrete element model with randomly distributed steel shots bombarding a steel component at various velocities has been developed as benchmark example. With this model the SP shot - shot interaction, the shot - target interaction, the surface coverage, angle of impingement, shot size, impact velocity and the overall shot flow can be parametrically studied in details and with little computational effort. The novel approach also proposes a new method to dynamically change the coefficient of restitution for repeated impacts during the simulation and predicts the CRS more effectively. The effects of SP on different materials of relevance to gas turbine engine components will be investigated in order to improve the understanding of the interaction between the shots and the targeted material. Initially, an uncoupled analysis was peforned, in order to assess the capabilities of the two modelling systems, DEM and FEM, to delivery an improved solutuion when combining two commercially available codes. This parametric analysis is performed using the state-of-the-art Discrete Element (DE) application EDEM. In the subsequent part of this work, a dynamic Finite Element (FE) application Abaqus will be used to investigate single shot impacts and to obtain the residual stress distribution. This gives us a prescribed residual stress distribution and peening coverage. A Combined DEM/FEM tool (DEST) is proposed that eliminates any manual pre-processing required for linking/coupling, eliminating the use of two different applications and provide an integrated solution for the simulation of the Shot Peening process. In the subsequent chapter, the implementation of essential tools for the enchanced modelling of Shot Peening process functionalities, such as the nozzle, bounding box, coverage and intensity is described. A number of computational improvements are also implemented to reduce the computation time. The existing binary search is enhanced to self-balancing search tree and further improved to allow insertion and deletion of elements. A bounding box feature which removes shots that move out of the domain during the course of the simulation is also implemented. Experiments featuring single shot impacts are performed to gain better understanding the deformation process in the target material subjected to impact conditions to those occurring in the production peening. The single shot impacts are experimentally examined using SEM and EBSD. During final chapter, case studies are performed to compare the results of the simulations with large-scale experimental work. The coverage of peening of single and multiple nozzles with different angle of impingements are assessed. Finally, possible directions for further research concerning the accurate quantification of material responses to SP are identified in the report.

O objetivo deste trabalho foi o estudo das propriedades mecânicas e térmicas do PHB e de compósitos de PHB/pó de madeira processados, reprocessados e irradiados. Foram preparados compósitos com PHB e pó de madeira com concentrações de PHB/pó de madeira de 90/10, 80/20 e 70/30 (m/m). Foram realizados ensaios de propriedades mecânicas, HDT, ponto de amolecimento Vicat, TGA e DSC. A incorporação do pó de madeira aumentou o grau de cristalinidade e a temperatura de cristalização do polímero, e nos compósitos PHB/pó de madeira 80/20 e 70/30 a rigidez do material aumentou em relação ao polímero puro. O compósito PHB/pó de madeira 70/30 apresentou os melhores resultados em termos econômicos, de processamento e de estabilidade em relação ao reprocessamento. A irradiação após o processamento, em dose de 30 kGy, provocou aumento da rigidez do PHB puro e dos compósitos PHB/pó de madeira 90/10 e 80/20, embora as outras propriedades tenham decrescido. O compósito PHB/pó de madeira 70/30 apresentou queda de todas as propriedades com a irradiação. / The purpose of this work was the study of the mechanical and thermal properties of processed, reprocessed and irradiated PHB, as well as PHB/wood flour composites. PHB/wood flour composites were prepared with PHB/wood relation of 90/10, 80/20 e 70/30 (w/w). Mechanical properties, HDT, Vicat melting point and thermo analyses (TGA and DSC) tests were carried out. The introduction of the wood flour increased both polymer crystallinity and crystallization temperature. The material stiffness increased in PHB/wood flour composites (80/20 and 70/30) resulting in higher values in some properties when compared with pure PHB. The PHB/wood flour composite 70/30 showed the best results in terms of economy, processing and stability during reprocessing. The irradiation after processing in 30kGy doses led to a stiffness increasing of pure PHB and PHB/wood flour composites (90/10 and 80/20) while other properties have decreased. The PHB/wood flour 70/30 showed decreasing in all properties after irradiation.

O objetivo deste trabalho foi o estudo das propriedades mecânicas e térmicas do PHB e de compósitos de PHB/pó de madeira processados, reprocessados e irradiados. Foram preparados compósitos com PHB e pó de madeira com concentrações de PHB/pó de madeira de 90/10, 80/20 e 70/30 (m/m). Foram realizados ensaios de propriedades mecânicas, HDT, ponto de amolecimento Vicat, TGA e DSC. A incorporação do pó de madeira aumentou o grau de cristalinidade e a temperatura de cristalização do polímero, e nos compósitos PHB/pó de madeira 80/20 e 70/30 a rigidez do material aumentou em relação ao polímero puro. O compósito PHB/pó de madeira 70/30 apresentou os melhores resultados em termos econômicos, de processamento e de estabilidade em relação ao reprocessamento. A irradiação após o processamento, em dose de 30 kGy, provocou aumento da rigidez do PHB puro e dos compósitos PHB/pó de madeira 90/10 e 80/20, embora as outras propriedades tenham decrescido. O compósito PHB/pó de madeira 70/30 apresentou queda de todas as propriedades com a irradiação. / The purpose of this work was the study of the mechanical and thermal properties of processed, reprocessed and irradiated PHB, as well as PHB/wood flour composites. PHB/wood flour composites were prepared with PHB/wood relation of 90/10, 80/20 e 70/30 (w/w). Mechanical properties, HDT, Vicat melting point and thermo analyses (TGA and DSC) tests were carried out. The introduction of the wood flour increased both polymer crystallinity and crystallization temperature. The material stiffness increased in PHB/wood flour composites (80/20 and 70/30) resulting in higher values in some properties when compared with pure PHB. The PHB/wood flour composite 70/30 showed the best results in terms of economy, processing and stability during reprocessing. The irradiation after processing in 30kGy doses led to a stiffness increasing of pure PHB and PHB/wood flour composites (90/10 and 80/20) while other properties have decreased. The PHB/wood flour 70/30 showed decreasing in all properties after irradiation.