Dissertations / Theses: '620.1/1'

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Delgado, Sánchez Clara. "Nouvelles méthodes d’optimisation et de caractérisation de mousses à base de tanins pour l’isolation thermique du bâtiment." Thesis, Université de Lorraine, 2017. http://www.theses.fr/2017LORR0246/document.

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Dans cette thèse, des mousses produites à plus de 90% à partir de produits naturels et à très faible conductivité thermique ont été étudiées en détail. L’objectif principal de ce travail était d’améliorer certaines faiblesses de ces matériaux et de résoudre les problèmes qui pourraient être rencontrés lors de leur utilisation, pour leur permettre de concurrencer d’autres mousses synthétiques actuellement sur le marché de l’isolation thermique. De nouvelles méthodologies ont été proposées pour optimiser les mousses à base de tanin de différents points de vue. Tout d’abord, des mousses liquides ont été analysées en termes de stabilité et de processus de polymérisation à l’aide d’un analyseur de lumière rétrodiffusée, afin de les transformer en mousses rigides de tanin plus performantes. Des plans d’expériences ont également été utilisés pour améliorer les propriétés mécaniques de mousses rigides, produites par moussage physique, sans porter préjudice à leur conductivité thermique. D’autre part, des traitements d’hydrophobisation ont été réalisés pour réduire la sensibilité de ces mousses à l’eau, qu’elle soit sous forme liquide ou vapeur, et l’effet des ingrédients des formulations sur leurs propriétés au feu a été élucidé. Enfin, deux techniques de caractérisation mécanique ont été étudiées et comparées, ce qui a permis de déterminer le coefficient de Poisson et le facteur de perte, et de mettre en évidence les précautions à prendre pour caractériser les mousses fragiles
In this thesis, foams based on more than 90% of natural products and with an exceptionally low thermal conductivity have been studied in depth. The main objective of this work was to improve some of the weaknesses of those materials and to solve problems that might be encountered during use, for allowing them to compete with other synthetic foams that are currently on the thermal insulation market. New methodologies have been proposed to optimise tannin-based foams from different points of view. First, liquid foams were analysed in terms of stability and polymerisation process using a backscattered light analyser, in order to convert them into improved rigid tannin-based foams. Experimental design was also used to improve the mechanical properties of physically blown rigid foams without prejudicing their thermal conductivity. On the other hand, hydrophobisation treatments were suggested for reducing the sensitivity of those foams to water in liquid or vapour form, and the effect of formulations’ ingredients on their fire properties were elucidated. Finally, two techniques of mechanical characterisation were investigated and compared, leading to Poisson’s coefficient and loss factor, and evidencing the precautions to be taken for characterising brittle foams

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Daly, Michael Andre John. "Advanced imaging and mechanistic modelling of ductile fracture." Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/advanced-imaging-and-mechanistic-modelling-of-ductile-fracture(6d00e179-cb90-4225-b334-b9a21e2b95f2).html.

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Nuclear Reactor Pressure Vessels (RPV) are manufactured from medium strength low alloy ferritic steel, specifically selected for its high toughness and good weldability. The ability of the RPV material to resist crack growth is crucial given that it is one of the fundamental containment safety systems of nuclear power plants. For most of their lifetime, the RPV operates at sufficiently elevated temperatures to ensure the material is ductile. However, the development of ductile damage, in the form of voids, and the ability to predict ductile tearing in RPV materials using a mechanistically-based model remains difficult. The Gurson-Tvergaard-Needleman (GTN) model of ductile tearing provides one such tool for predicting ductile damage development in RPV materials. The difficulty in using the GTN model lies in the ability to calibrate the model parameters in a robust manner. The parameters are typically calibrated data, derived from fracture tests and relying on an iterative “trial and error” procedure of numerical simulations and comparison with test data until the model reproduces the experimental behaviour with sufficient accuracy. This research has addressed the development of a mechanistically-based approach to the calibration of the GTN model by developing a new understanding of the ductile fracture mechanism in RPV material through conventional metallography and 3D X-ray computed tomography to image the initiation, growth and coalescence of ductile voids. The metallographic and tomographic data were analysed in a quantitative manner to establish a direct link between the microstructural features and void evolution and the key parameters of the GTN model. This approach has established a more robust mechanistically based method for the calibration of the GTN model that will enhance the conventional iterative calibration procedure. The calibrated model was applied to predict ductile tearing behaviour in compact-tension and notched-tensile specimens. The results showed good agreement with test data and also reproduced the morphology and branching of crack extension observed in practise. Whilst these observations were due, in part, to the numerical solving procedure, they enabled new insights to be gained regarding the development of non-uniform void volume fraction distributions in tested specimensThe results from this research will strengthen the guidance provided to structural integrity engineers in industry regarding the calibration and application of ductile damage mechanics models such as the GTN model for predicting ductile initiation and growth in RPV materials.

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Papantonis, Stergios. "Investigation of passive electromagnetic components with metamaterials." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/34313.

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The main goal of this work is the design and analysis of passive components employing metamaterial structures and in particular the wire medium metamaterial. Although there has been a lot of research interest in the physics of such metamaterial structures, there are not many resources available describing the behaviour of classical components, such as waveguides and cavity resonators, that are formed by metamaterials. Therefore, the aforementioned widely used devices, are realized with the deployment of the "Fakirs bed of nails" and their performance is analyzed. Our motivation is to expand existing analytical models and their applications to commonly used passive electromagnetic components, with a view to explore potentially new applications. As a means of study analytical techniques together with numerical simulations and measurements were used. This thesis is structured in the following chapters. The first chapter is an introduction to the basic principles of electromagnetics and their use on the framework of metamaterials; as illustrations some state of the art applications are presented. The next chapter is a literature review covering the work that has been done in the area of our main research interest (i.e., the Fakir's bed of nails as a metamaterial). An overview of the mathematics describing its behaviour is given as well as applications of the proposed structure. Attention has been paid on the latest studies because they provide complete physical insight. Some results from this chapter are used later as background knowledge for the analysis of passive components. This chapter is intended to lay the foundations for the reader to continue reading the rest of this work without the need to look in the literature. Chapter three investigates the dispersion effects in parallel-plate waveguides with both plates being realized by the Fakir's bed of nails. This chapter serves as an example as to how the Fakir's bed of nails can be used to form components. An analytical solution describing the behaviour of the waveguide is presented and compared against full wave numerical simulations. Chapter four presents a theoretical study of the resonant behaviour of metallic nanorods. A clear analogy between the coupled rods and the split rings/split squares is shown. The decline in the resonant frequency as the gap decreases, previously described in terms of self-capacitance, is interpreted by surface plasmons coupled across the gap. Chapter five presents a new enabling technology for implementing tunable rectangular waveguide components and circuits with the use of 2D and 3D metamaterials; a holey metal surface and wire media, respectively. As proof of concepts, results for tunable rectangular waveguide filters are presented with the use of pin block inductive irises and capacitive posts. Furthermore, by adapting the traditional metal-pipe rectangular waveguide for tunability, regions of the solid metal walls are replaced by holey metasurfaces. Prototype tunable structures were measured for verification and good agreement is achieved between full-wave numerical simulations and measurements. Chapter six analyzes a radically new design of waveguide verification device, suitable for measuring instruments such as Vector Network Analyzers. The device is designed to enable its roperties to be changed, by known amounts, after the device has been connected to the system that requires verification. The performance of the device is based on introducing relative changes in the transmitted and reflected signals and so is insensitive to errors introduced by waveguide flange imperfections. This makes the technique, in principle, ideally suited for waveguide VNAs operating at millimeter- and submillimeter-wave frequencies where these flange errors can dominate the measurements. A verification device is designed, simulated and tested in WR-15 waveguide (50-75 GHz). The last part of this thesis presents a rigorous analysis of lossy spherical cavity resonators starting from first principles. The electromagnetic field inside the spherical cavity is expanded in normal waveguide modes and the eigenfrequencies of the cavity resonator are obtained analytically by enforcing the appropriate boundary conditions at the cavity wall. Unlike perturbation techniques, used when low losses are present, there are no inherent limitations in the presented analysis and, therefore, its applicability range is much broader. Exact analytical results, acting as a benchmark reference standard, are compared to those generated independently by two commercial full-wave simulation software packages (HFSS and COMSOL). When the wall transforms from being a perfect electrical conductor to free space, as its intrinsic conductivity decreases from infinity to zero, it is found that the eigenmode solvers with both software packages increasingly fail. With both software packages, all possible modeling strategies have been investigated and their associated limitations identified. Moreover, a plane-wave approximation model is proposed that accurately predicts the numerical simulation results.

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Zhao, Fan. "Modelling of gas-solid flows with non-spherical particles." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/34398.

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Dispersed multiphase flows are common in nature and industry and are governed by complex physical phenomena. The complex features of the turbulence continuity carrier phase and the dispersed phase make the problem of a dispersed multiphase flow much more complex than a single phase flow. This research work focuses on modelling and analysing one type of dispersed multiphase flows: solid particles suspended in a turbulent channel flow. The aim of this thesis is to numerically investigate the effects of Stokes number, particle shape and particle volume fraction on the behaviour of gas-solid turbulent channel flows with non-spherical particles. This study not only considers spherical particles but also studies non-spherical fibre-like ellipsoids suspended in the channel flow. To fully describe the complex dynamics of non- spherical particles, the rotational motion and orientation is efficiently and accurately re- solved by applying unit Quaternions. To address inevitable numerical errors caused by the Quaternion integration algorithms in previous studies, a novel Quaternion integration method is derived, validated and applied for more accurately updating the unit Quaternions. This work also derives a new Quaternion equation to relate second order tensor variables between different frameworks. This research work applies four-way coupling to accurately model the complex gas-solid turbulent channel flows, and the fluid-particle, particle-particle and particle-wall interactions are all taken into account. Important conclusions from this work are summarized as follows. In four-way coupled simulations, the average viscosity of the fluid flow is not affected by the particles, whereas the turbulence intensity is reduced by adding small heavy particles. The average direct dissipation by the particles is negligible, and the primary mechanism by which the particles affect the flow is by altering the turbulence structure near and around the turbulence kinetic energy peak. For non-spherical particles, the distributions of the orientation angles clearly demonstrate that ellipsoids tend to align within the plane that lies perpendicular to the span-wise direction in the very near wall region, follow the stream-wise direction in the buffer layer, and almost randomly distribute in the central region of the channel.

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Nejati, Morteza. "Finite element modeling of frictional contact and stress intensity factors in three-dimensional fractured media using unstructured tetrahedral meshes." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/34348.

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This thesis introduces a three-dimensional (3D) finite element (FE) formulation to model the linear elastic deformation of fractured media under tensile and compressive loadings. The FE model is based on unstructured meshes using quadratic tetrahedral elements, and includes several novel components: (i) The singular stress field near the crack front is modeled using quarter-point tetrahedral finite elements. (ii) The frictional contact between the crack faces is modeled using isoparametric contact discretization and a gap-based augmented Lagrangian method. (iii) Accurate stress intensity factors (SIFs) of 3D cracks computed using the two novel approaches of displacement correlation and disk-shaped domain integral. The main contributions in the FE modeling of 3D cracks are: (i) It is mathematically proven that quarter-point tetrahedral finite elements (QPTs) reproduce the square root strain singularity of crack problems. (ii) A displacement correlation (DC) scheme is proposed in combination with QPTs to compute SIFs from unstructured meshes. (iii) A novel domain integral approach is introduced for the accurate computation of the pointwise $J$-integral and the SIFs using tetrahedral elements. The main contributions in the contact algorithm are: (i) A square root singular variation of the penalty parameter near the crack front is proposed to accurately model the contact tractions near the crack front. (ii) A gap-based augmented Lagrangian algorithm is introduced for updating the contact forces obtained from the penalty method to more accurate estimates. The results of contact and stress intensity factors are validated for several numerical examples of cubes containing single and multiple cracks. Finally, two applications of this numerical methodology are discussed: (i) Understanding the hysteretic behavior in rock deformation; and (ii) Simulating 3D brittle crack growth. The results in this thesis provide significant evidence that tetrahedral elements are efficient, reliable and robust instruments for accurate linear elastic fracture mechanics calculations.

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Pham, Kien Cuong. "Nano-structured carbon materials for energy generation and storage." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/33734.

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A nano-structured carbon material referred to as Graphene-Carbon Nanotube hybrid is developed for electrochemical energy conversion and storage devices. The hybrid is obtained by catalyst-free growth of free-standing graphene on CNT scaffolds. The hybrid combines the advantageous properties of constituent materials, including an ultra-high density of graphitic edges of graphene and a porous structure of CNTs. As a catalyst support for platinum in PEM fuel cells, the hybrid shows both enhanced catalytic activity and superior stability compared to a commercial carbon black-supported platinum catalyst. The hybrid is also used as a support material for amorphous molybdenum sulfide in supercapacitor and hydrogen evolution reaction catalyst applications. As a supercapacitor electrode material, the hybrid shows high specific capacitance and good stability. As a hydrogen evolution reaction catalyst, the hybrid is one of the most active non-precious catalysts ever reported. FIB-SEM tomography is used to reconstruct the porous 3D structure of carbon electrodes.

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Hernando, Quintanilla Francisco. "Pseudospectral collocation method for viscoelastic guided wave problems in generally anisotropic media." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/34915.

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In Non-Destructive Evaluation (NDE) applications guided waves are attractive to perform rapid inspections of long lengths and large areas. However, they are complicated, therefore it is important to have as much information and understanding about their physical properties as possible in order to design the most efficient and robust inspection process as well as to draw the correct conclusions from the measurement results. The main piece of information to gain insight into the guided wave's properties is dispersion curves which, for isotropic structures such as plates and cylinders, have been available for many years. There are many robust algorithms which are currently used to compute them: finite element simulations, partial wave based root finding routines (PWRF) and semi-analytical finite element simulations (SAFE). These methodologies have been generalized and also used to study and compute dispersion curves of more complicated anisotropic materials though the range of tractable cases was limited. Although robust, all these approaches present several challenges, mostly computational, such as missing modes (PWRF), the so called "large-fd" problem (PWRF), artificially increased stiffness (FE, SAFE) or improvement of dispersion curve tracing routines (FE, PWRF, SAFE). In addition, when studying complicated anisotropic materials with a low degree of symmetry or unusual axes configurations where propagation does not take place along any of the principal axes, PWRF routines are frequently unreliable and one must resort to specific SAFE simulations which also present their own challenges and, depending on the SAFE scheme used, can yield spurious modes which need to be carefully filtered. Recently, Pseudospectral Methods (Galerkin and Collocation schemes), were introduced in the field of elastic guided waves, providing a powerful, yet strikingly and conceptually simple alternative to the above algorithms by successfully finding the dispersion curves in isotropic structures and in some simple anisotropic problems. However, a systematic and general approach for accurately and robustly computing dispersion curves of guided waves in anisotropic media, up to the most general case of triclinic symmetry, has not yet been developed. The goal of the work presented in this thesis is to develop such a tool by means of the Pseudospectral Collocation Method (SCM) and to take advantage of its particular features to make it as robust as possible. Firstly, a PSCM scheme is developed for computing dispersion curves of guided waves in anisotropic elastic media by finding all the frequencies for a given value of the real wavenumber. The results are validated with the existing literature as well as with the results provided by the software DISPERSE developed in the NDT group at Imperial College London. Many of the most remarkable features of the PSCM (spectral accuracy, speed, and its failure to miss modes for instance) are already observed in this simple, yet important, class of problems in elastic media. Secondly, guided waves in viscoelastic anisotropic media are studied. In this case, modes present attenuation due to material damping which is reflected in the wavenumber being complex. In order to handle complex wavenumbers the PSCM schemes developed for elastic materials are appropriately extended by means of the Companion Matrix Method. It will be seen that, apart from lowly attenuated propagating modes, all the other highly attenuated modes are found, yielding the full three-dimensional spectrum of the problem under consideration. Moreover, when the PSCM schemes for viscoelastic media are used to compute the dispersion curves of guided waves in an elastic medium, all the remaining, imaginary as well as complex, roots of the elastic problem which were not computed by the simpler PSCM elastic schemes are found, providing the full three-dimensional picture of the dispersion curves. These PSCM schemes, as any other of the aforementioned approaches, only find pairs (\omega,k). If dispersion curves are to be plotted, those pairs must be linked correctly in order to plot the desired dispersion curves, which is non-trivial when crossings amongst modes occur. Motivated by this, an investigation of the parity and coupling properties of guided wave solutions is carried out in detail for all crystal classes. This investigation provides a robust alternative to conventional tracing routines and avoids the problem of mode crossings by exploiting the parity and coupling properties of the solutions. Finally, the most complicated problems involving embedded structures are investigated by including a Perfectly Matched Layer (PML) in the previously developed PSCM schemes for viscoelastic media. The dispersion curves for leaky and trapped modes in an isotropic elastic plate and in a similar cylinder immersed in an infinite ideal fluid are found, showing very good agreement with the results given by PWRF routines in a large range of frequencies. Last, but not least, an illustration of a two-dimensional PSCM scheme is presented to study a vibrating membrane. The results are compared with the available analytical solution showing again excellent agreement.

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Shah, Saurabh Mahesh Kumar. "Multi-scale imaging of porous media and flow simulation at the pore scale." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/34323.

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In the last decade, the fundamental understanding of pore-scale flow in porous media has been undergoing a revolution through the recent development of new pore-scale imaging techniques, reconstruction of three-dimensional pore space images, and advances in the computational methods for solving complex fluid flow equations directly or indirectly on the reconstructed three-dimensional pore space images. Important applications include hydrocarbon recovery from - and CO2 storage in - reservoir rock formations. Of particular importance is the consideration of carbonate reservoirs, as our understanding of carbonates with respect to geometry and fluid flow processes is still very limited in comparison with sandstone reservoirs. This thesis consists of work mainly performed within the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC) project, focusing on development of three dimensional imaging techniques for accurately characterizing and predicting flow/transport properties in both complex benchmark carbonate and sandstone rock samples. Firstly, the thesis presents advances in the application of Confocal Laser Scanning Microscopy (CLSM), including the improvement of existing sample preparation techniques and a step-by step guide for imaging heterogeneous rock samples exhibiting sub-micron resolution pores. A novel method has been developed combining CLSM with sequential grinding and polishing to obtain deep 3D pore-scale images. This overcomes a traditional limitation of CLSM, where the depth information in a single slice is limited by attenuation of the laser light. Other features of this new method include a wide field of view at high resolution to arbitrary depth; fewer grinding steps than conventional serial sectioning using 2D microscopy; the image quality does not degrade with sample size, as e.g. in micro-computed tomography (micro- CT) imaging. Secondly, it presents two fundamental issues - Representative Element of Volume (REV) and scale dependency which are addressed with qualitative and quantitative solutions for rocks increasing in heterogeneity from beadpacks to sandpacks to sandstone to carbonate rocks. The REV is predicted using the mathematical concept of the Convex Hull, CH, and the Lorenz coefficient, LC, to investigate the relation between two macroscopic properties simultaneously, in this case porosity and absolute permeability. The effect of voxel resolution is then studied on the segmented macro-pore phase (macro-porosity) and intermediate phase (micro-porosity) and the fluid flow properties of the connected macro-pore space using lattice-Boltzmann (LB) and pore network (PN) modelling methods. A numerical coarsening (up-scaling) algorithm have also been applied to reduce the computational power and time required to accurately predict the flow properties using the LB and PN methods. Finally, a quantitative methodology has been developed to predict petrophysical properties, including porosity and absolute permeability for X-ray medical computed tomography (CT) carbonate core images of length 120 meters using image based analysis. The porosity is calculated using a simple segmentation based on intensity grey values and the absolute permeability using the Kozeny-Carman equation. The calculated petrophysical properties were validated with the experimental plug data.

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Fert, Marcin Maciej. "An investigation of the mechanical performance of Z-pin reinforced composites." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/33729.

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Fibrous composites, having excellent mechanical properties in the direction of the fibres, have lower mechanical properties in the through thickness direction, controlled by resin. Z-pinning improves the delamination toughness (up to 500%) with a relatively modest reduction to the in-plane mechanical properties (typically 5-15%). This experimental study investigates the mechanical performance of Z-Pins bridging an existing delamination in fibre reinforced resin composites under pull-out (Mode I), shear-out (Mode II) and mixed mode loading conditions using a specially designed testing rig. In Mode II the opening displacement was restricted and measured by springs of three different stiffnesses. A new technique of needle assisted Z-Pin insertion was developed, in which prepreg panels were perforated with a steel needle in order to insert Z-Pins. This technique ensured the desired orientation of Z-Pins, improved pinning quality and removed the necessity of costly preforms used in the traditional UAZ method. Test specimens were blocks (15 mm x 15 mm x 6mm thick) of carbon-epoxy IM7/8552 composite in unidirectional (UD) and quasi-isotropic (QI) stacking sequences, with PTFE delamination film in the mid-plane recreating an existing crack, bridged with a single T300/9310 Z-Pin or a group of four pins of either 0.28 mm or 0.51 mm diameter. Three phases of pull-out were identified: Linear Phase (linear force-displacement curve), Crack Formation (unstable crack propagation phase) and Frictional Sliding (friction-controlled pull-out). Two phases of shear-out were identified: Linear Phase (with no energy loss) and Breaking Phase (where the fibrous structure of the Z-Pins is fractured, ending with Z-Pin breakage). In mixed mode specimens behaved similarly to pull-out for the pin angles up to 45°. For higher angles the behaviour was more similar to pure shear-out. The influence of the Z-Pin diameter, z-pinning depth, distance between adjacent Z-Pins, composite stacking sequence and pull-out speed on the Z-Pins behaviour were investigated. The results will be useful in the formulation of improved Z-Pin bridging laws for use in finite element models.

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Moghaddasi-Tafreshi, Azamolsadat. "Design optimization using the boundary integral equation method." Thesis, Imperial College London, 1990. http://hdl.handle.net/10044/1/46451.

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Bhutani, Gaurav. "Numerical modelling of polydispersed flows using an adaptive-mesh finite element method with application to froth flotation." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/39046.

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An efficient numerical framework for the macroscale simulation of three-phase polydispersed flows is presented in this thesis. The primary focus of this research is on modelling the polydispersity in multiphase flows ensuring the tractability of the solution framework. Fluidity, an open-source adaptive-mesh finite element code, has been used for solving the coupled equations efficiently. Froth flotation is one of the most widely used mineral processing operations. The multiphase, turbulent and polydispersed nature of flow in the pulp phase in froth flotation makes it all the more challenging to model this process. Considering that two of the three phases in froth flotation are polydispersed, modelling this polydispersity is particularly important for an accurate prediction of the overall process. The direct quadrature method of moments (DQMOM) is implemented in the Fluidity code to solve the population balance equation (PBE) for modelling the polydispersity of the gas bubbles. The PBE is coupled to the Eulerian--Eulerian flow equations for the liquid and gas phases. Polydispersed solids are modelled using separate transport equations for the free and attached mineral particles for each size class. The PBE has been solved using DQMOM in a finite element framework for the first time in this work. The behaviour of various finite element and control volume discretisation schemes in the solution of the PBE is analysed. Rigorous verification and benchmarking is presented along with model validation on turbulent gravity-driven flow in a bubble column. This research also establishes the importance of modelling the polydispersity of solids in flotation columns, which is undertaken for the first time, for an accurate prediction of the flotation rate. The application of fully-unstructured anisotropic mesh adaptivity to the polydispersed framework is also analysed for the first time. Significant improvement in the solution efficiency is reported through its use.

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Anthony, David Benbow. "Improved synthesis of carbon nanotube grafted carbon fibre : towards continuous production." Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/39371.

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Grafting carbon nanotubes (CNTs) onto reinforcing fibre surfaces has been shown to improve composite structural performance, through improved interfacial bonding of the matrix and reinforcement. Sourcing a suitable amount of CNT-grafted fibre has currently limited test coupons geometry and development in the area. The scale-up of current synthesis procedures for grafting CNTs onto carbon fibre (CF) surfaces, using low intensity processing techniques (minimal processing of fibre substrate) compatible with industrial practices has not yet been reported. CNT growth from CF surface (CNT-g-CF) without damaging the mechanical parent fibre properties is a challenge as chemical vapour deposition (CVD) CNT growth typically results in catalyst pitting and surface defects occurring. In this thesis I attempt to address concerns detailed above; through the development of a catalyst system which is easily deposited onto CF, uses a CVD CNT-synthesis method which does not damage the original fibre properties in a potentially continuous scalable manner. I present a simple incipient wetness technique for loading a bi-catalyst precursor mixture onto CF. CF pre-deposited with bi-catalyst precursor under the application of an electric field, using CF as an electrode, in-situ during conventional thermal-CVD demonstrated significant promotion of CNT-synthesis directly from the CF surface. Electric field applied during CVD CNT-synthesis produces CNT-g-CF without apparent mechanical degradation to the parent fibre retaining original mechanical properties. When CVD CNT-synthesis is undertaken without the application of an electric field, degradation of original mechanical properties are witnessed. Batch CVD process was adapted, in an attempt to demonstrate the feasibility of continuous production of CNT-g-CF in a bespoke continuous CVD set-up. Alternative routes for CNT-g-CF including a novel silicon oxide based CNT-synthesis are also discussed.

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Yang, Haoliang. "Creep age forming investigation on aluminum alloy 2219 and related studies." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/39352.

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By the middle of the 20th Century, traditional mechanical metal forming methods were showing to be inadequate for producing components comprising of large high strength aluminium alloy panels with complex curvatures, such as those used in modern aircraft and aerospace metal structures. To deal with this problem, a new forming method was conceived by Textron, which has proven to be very useful for forming components with these shape characteristics and good mechanical properties. The method is called Creep Age Forming (CAF). The research described in this thesis is a study of CAF of a 2219 aluminium alloy, which is used for fabricating the isogrid structure for fuel tanks of launch vehicles. The main aim of the research is to develop experimental and modelling tools for CAF of AA2219 sheet structures. A series of creep-ageing tests and stress-relaxation tests have been conducted on AA2219 at 175 °C. The age-hardening, creep deformation and stress relaxation behaviour of AA2219 have been investigated. Based on the experimental investigation, a novel set of physically based, unified creep constitutive equations has been established. A small scale CAF test rig was designed to validate the springback prediction from FE simulation. The experimental result and simulation are in good agreement. Development of FE procedures for simulating creep-ageing behaviour of the material and springback has been performed to predict and assess the springback behaviour of metal sheet in typical forming tools.

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Burridge, Henry Charles. "The behaviour of miscible Boussinesq fountains in uniform quiescent environments." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/39380.

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This research focuses on the fundamental fluid mechanics of fountains. Attention is restricted to fountains formed by the vertical injection of fluid into a uniform environment of marginally differing density such that the momentum of the ejection is opposed by buoyancy. The small density dif- ference between the two miscible fluids permits dilution of the ejected fluid and, for sufficiently energetic ejections, turbulent entrainment occurs. Our experimental study of dense saline solution forced vertically upwards into a quiescent freshwater environment examines fountains arising over a broad range of source conditions. We combine experimental results with dimensional reasoning and theoretical arguments to provide meaningful and significant additions to the existing knowledge of fountain dynamics. The results presented enhance our understanding through the study of rise heights, the fluctuations in rise heights and the scale and balance of bulk fluxes within fountains over the range of source conditions. A study of transient and steady-state behaviours and associated fountain rise ratio (peak relative to mean rise heights) identifies previously unreported dynam- ics which extend published results - these include the pinch-off of the initial vortex and rise height ratios (unexpectedly) below unity. Analysis of the rise height fluctuations during the quasi-steady state provides characteristic length, time and velocity scales distinct to the varied classes of fountain flow. Our results for the rise height ratio and indeed rise height fluctuations reveal a number of differing classes of fountain behaviour; behaviour which highlights the changing and complex nature of fountains. This results in a consistent classification which extends the previous three classes of fountain behaviour to five. Some of the implications of these new behaviours to practical engineering and geophysical applications of fountains are discussed.

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Zastawny, Marian. "Fundamental understanding of turbulent gas-solid flows." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/40287.

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Gas-solid flows are abundant both in nature and in industrial applications, therefore the ability to accurately predict their behaviour is of crucial importance. The main goal of the research project presented in this thesis was to develop a methodology for eff cient true direct numerical simulations (DNS) of turbulent flows with solid particles. True DNS (TDNS) in this case implies that not only all the spatial and temporal scales of the f ow f eld are directly computed, but also that the appropriate boundary conditions are imposed on surfaces of the particles allowing for the boundary layer development. The designed approach is strongly based on the ideas of the Immersed Boundary Method (IBM). In this technique, two separate computational grids are used: a fixed fluid grid and a moving triangulated one, representing the body surface. The flow equations are modif ed in the regions were the grids overlap. Various implementations of the IBM are discussed, along with the most common diff culties encountered while using this approach. These challenges include accurate imposition of the boundary conditions, evaluation of the fluid-particle momentum transfer and spurious pressure oscillations observed in the case of moving bodies. A number of improvements, designed for addressing the main IBM challenges, are proposed and evaluated on a set of test cases. Additionally, a parallel triangulation library, MFTL, designed in the course of the research project is presented. The IBM technique is subsequently adopted for the investigation of flows past non-spherical particles at a range of Reynolds numbers and orientations. The results of this study lead to the development of shape-specif c correlations evaluating the drag, lift and torques on non-spherical particles as functions of Reynolds number and the angle of incidence. Also, an approach for describing the motion of such particles is presented as well.

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Kahk, Juhan Matthias. "Spectroscopic studies of IrO2 and Bi2Ir2O7." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/40495.

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The oxides of iridium, a 5d transition metal, have recently attracted interest in a number of scientific disciplines, ranging from fundamental solid state physics, to more applied areas of research such as spintronics and catalysis. The metallic oxides IrO2 and Bi2Ir2O7, in particular, are known to be good catalysts of the commercially important oxygen evolution reaction; IrO2 has also been identified as a promising material for spin current detection, and Bi2Ir2O7 has received attention due to its unusual magnetic response at low temperatures. In the work reported in this thesis, X-ray photoelectron spectroscopy using an Al Kα photon source (XPS), synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES), X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and resonant inelastic X-ray scattering (RIXS) were used to characterize the electronic structures of IrO2 and Bi2Ir2O7. The results were compared to simulated spectra derived from the results of density functional theory calculations performed by collaborators, and analyzed in terms of qualitative models of the electronic structure. Excellent agreement between theory and experiment was observed, especially if the effects of final state lifetime broadening were accounted for. A new formalism was derived that allows final state lifetime effects to be included in band structure based RIXS simulations. The results of the theoretical calculations were also used to analyze the properties of the low energy electronic states in IrO2 and Bi2Ir2O7, and it was found that in both cases there are strong deviations from the predictions of the popular jeff = 1/2 model. The results of preliminary high pressure photoemission measurements of IrO2 are also presented in this thesis, alongside a more detailed discussion of fundamental aspects of this relatively new technique. In particular, the issue of the pressure profile that is formed around the sample and the first aperture in differentially pumped spectrometers is addressed using a combination of experimental measurements and computational fluid dynamics simulations. For the flow of N2 through a 0.3 mm aperture, the calculated pressures at the plane of the sample are tabulated for a range of sample-to-cone distances and pressures of 5.0 mbar, 9.4 mbar and 30 mbar.

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Kelly, Mark. "Comparing the blast tolerance of different composite structures." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/40428.

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Large surface ships have traditionally used steel for their construction, which provides good blast resistance and predictable behaviour in use. This research project, however, considered the use of polymeric foam core sandwich panels for the construction of ship hulls, with the intention of reducing the radar signature of the vessel; increasing the maximum speed; reducing fuel consumption; and providing control over desired mechanical properties for specific applications. This project specifically considered the resistance of the sandwich composites to non-contact explosives, specifically sea mines and areal blast. This research project firstly presents air blast testing performed on polymeric foam core sandwich panels with glass fibre face-sheets. The foam polymer type, the effect of the face-sheet material and the effect of using a graded density foam core for blast wave attenuation were all investigated. When subjected to air blast loading it was found that by grading the core density a smoother back face-sheet displacement was witnessed. This was a significant discovery as protecting the back face-sheet is key to maintaining structural integrity of the ship hulls. In a comparison of foam polymers in the cores of the sandwich panel it was concluded that styrene acrylonitrile offers optimum fracture and adhesion properties. Furthermore, it was found that by interleaving high modulus polypropylene fibres between the glass fibre front face-sheets, front face-sheet cracking and delamination can be prevented, restricting water ingress into the sandwich panel if it were subjected to blast loading. Advances in using 3D high speed imaging for digital image correlation were also achieved, whereby the results were used to estimate core shear strain during deformation, and the kinetic energy and strain energy in the sandwich panel were estimated by comparison with blast wave pressure simulations. The residual flexural and edgewise compression properties of the air blasted sandwich panels were then determined to calculate the flexural stiffness and strength and edgewise stiffness and strength with varying damage. It was found that the construction of the sandwich panel properties were unaffected by the construction, and that debonding of the face-sheets and the core was the key performance reducing damage mechanism. Residual properties were determined for the sandwich materials, with the intention that these could be used in simulations to predict performance of the ship hulls after successfully withstanding a blast. In underwater blast scenarios it was discovered that using a graded density foam core reduced the central deflection of the sandwich panel due to the increase in energy absorption due to crushing of the stepwise increase in foam core density. This effect was greater in carbon fibre sandwich panels than in glass fibre panels, due to the increased stiffness of the face-sheets. The central deflection was calculated by averaging the strains measured on the two face-sheets with electronic strain gauges, which was a novel technique and took advantage of the symmetry present in the square sandwich panels. As the focus of this research project was on foam core properties, the polymeric foam materials were characterised in quasi-static and dynamic tension, and quasi-static and dynamic compression. Dynamic compression tests were performed using a split-Hopkinson pressure bar which was designed specifically for testing low density foam materials. Stress equilibrium was achieved in these tests using a textured polypropylene pulse shaper. Stress equilibrium was checked for by performing high speed digital image correlation during deformation, to ensure that core crushing did not begin at the incident bar. The final stage of the research project was to construct finite element simulations of the air blast tests, using the polymeric foam material properties measured in the quasi-static and dynamic tests. A large number of simulations were performed with varying charge sizes and stand-off distances, to determine a combination of blast peak pressure and impulse values at which the sandwich panels failed. Developments were achieved in the use of a brittle cracking material model and the effect of the boundary conditions on the simulations was also studied. This research project furthered the understanding of sandwich composite materials to air and underwater blast loading. Advancements were made into the use of stepwise graded density foam cores and in characterising foams at dynamic tensile and compressive rates. A major conclusion of the project is that the use of graded density cores can be utilised to prevent damage of the sandwich panel on the rear side and can be used to absorb blast energy due to core crushing. The results of the project aid in simulating the response of composite ship hulls to blast loading and better predict the usability of the ships after being subjected to a blast event.

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D'Elia, Eleonora. "Self-healing organic/inorganic composites." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/42229.

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Nature provides us with amazingly complex and clever systems, structures and substances that make up the world we see around us. We can refer to nature, borrowing its ingenious solutions to solve engineering challenges or improve existing man-made materials. The process of assimilating real- world biological examples into technology is called 'bio-inspiration', and for many years scientists have been attempting to imitate the design of natural materials. This project seeks to mimic some of the complex architectures with outstanding properties found in nature: the shells of molluscs, with extraordinary toughness due to a highly hierarchical structure of platelets on the micro- and nano- scale, and human bone, with its ability to self-heal and regenerate its complex composite organic/inorganic microstructure after fracture. In this work it will therefore be investigated the effect of composite polymer/ceramic structures obtained via a manufacturing technique called freeze-casting, it is observed and optimised the role of the thin interface in self-healing organic/inorganic composites and the composition of the soft supramolecular phase and the inorganic phase is varied in order to obtain structures with properties closer to the behaviour of natural ones. The study couples interface and composite design with mechanical tests to determine interfacial adhesion in order to understand the factors that control the strength of the composite and the effectiveness and timescale of its self-healing. The same self-healing polymer is moreover used in the production of an innovative light composite exhibiting electrical conductivity and compression and flexion sensing capabilities in the attempt to mimic the outstanding properties of skin.

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Bell, Robert Valentine. "Emulsion-based supracolloidal materials stabilized by specifically designed branched copolymers." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/41836.

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The possible applications of branched copolymers are far reaching because of their various combinations of functionality and architectural diversity. More importantly, the domains and chain-end functionalities of the branched copolymers can be readily varied, via the simple and scalable Strathclyde route, to optimize/tailor the properties of the polymers for a specific application by careful choice of monofunctional monomers, branching monomers, and chain transfer agents. In the present thesis, branched copolymers were utilized as emulsifying agents for the production of oil-in-water emulsion droplets. These emulsion droplets were used as a platform to create novel emulsion-based supracolloidal materials. The chemical composition and architectural structure of the branched copolymers were specifically chosen to create stable emulsions and provide the correct functionalities required for the application. Calcium phosphate (CaP) microcapsules were fabricated by utilizing oil-in-water emulsion droplets, stabilized with branched copolymer, as templates. The branched copolymer was designed to provide a suitable architecture and functionality to produce stable emulsion droplets, and permit the mineralization of CaP at the surface of the oil droplet. These CaP capsules were made fluorescent by post-functionalization of the CaP shell with a fluorescent conjugate. Oil-in-water emulsion droplets stabilized with Laponite clay disc functionalized with pH-responsive branched copolymers were microfluidically spun into supracolloidal fibers. These supracolloidal fibers can be used as a tool to delivery volatile compounds in a time-controlled manner. The dried fibers created were low-weight porous materials. It was also discovered that these supracolloidal fibers can be utilized as a storage material for emulsion droplets, where emulsion droplets are 'locked' in the fiber structure under acidic condition, and are released from the fiber upon basification of the system. The release of emulsion droplets from the fiber can be time-controlled by programming the transient acidic pH states of the system by combining a fast acidic promoter with a feedback-driven biocatalytically controlled slow generation of base in a close system.

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Gurrutxaga, Lerma Beñat. "A dynamic discrete dislocation plasticity model for the study of plastic relaxation under shock loading." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/42360.

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This thesis concerns with Dynamic Discrete Dislocation Plasticity (D3P), a planar method of discrete dis- location dynamics aimed at the study of plastic relaxation processes in crystalline materials subjected to weak shock loading and high strain rates. Traditionally, the study of plasticity under these condi- tions was based on experimental measurement of the macroscopic response of the material. Using these data, well-known macroscopic constitutive laws and equations of state have been formulated. However, direct simulation of dislocations as the dynamic agents of plasticity in those circumstances remains a challenge. In discrete dislocation dynamics (DDD) methods, in particular planar discrete dislocation plasticity (DDP), dislocations are modelled as discrete discontinuities in an elastic contin- uum. Current DDP and DDD methods are unable to adequately simulate plastic relaxation because they treat dislocation motion quasistatically, neglecting the time-dependent nature of the elastic fields and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. This thesis proves that under shock loading, this assumption leads to models that invariably break causality. This thesis posits that these limitations can only be overcome with a fully time-dependent formulation of the elastic fields of dislocations. A truly dynamic formulation for the creation, annihi- lation, and nonuniform motion of straight edge dislocations is derived, extending the DDP framework to a fully elastodynamic formulation, D3P. This thesis describes the changes in paradigm that D3P poses, including retardation effects in dislocation interactions and the effect of the dislocation past history. The thesis then builds an account of all the methodological aspects of D3P that have to be modified from DDP, including mobility laws, generation rules, etc. Finally, the thesis explores the ap- plications D3P has to the study of plasticity under shock loading. It is found that, D3P elastodynamic formulation is able to explain the attenuation of the dynamic yield stress in a shock as a cumulative interference of elastic waves.

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21

Yu, Shaoxi. "Large eddy simulation of deflagration to detonation transition." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/41080.

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Deflagration to detonation transition (DDT) is a very important research project for both national defense and energy industry. It is the process where a subsonic deflagration transits into a supersonic detonation, which generates shock waves. In the past years, the simulations of DDT were limited in a small domain, usually sev- eral cubic centimeters. If we want to simulate it in a larger space without improving the numerical method, we need to use the more powerful computer. When the computing resources are limited, we must improve the numerical method to achieve the big-domain simulating. There are two technical paths, one is the adaptive mesh refinement and the other is the large eddy simulation. Both of them are difficult to realize. In this project, we focus on the usage of the LES method for simulating DDT. The main challenge in this work is to develop a reliable model. In this research, a new approach for LES modelling was developed. It is a fully compressible variant of the artificial thickened flame model, which adopts the opt- ing functions on the reference flame thickness. This method ensures that the flame is not over-thickened in deflagration or detonation. To control the options on the flame thickness, a detonation sensor is utilized during the computing.

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Wearing, David. "Investigation of the nucleation of aluminium by atomic simulation." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/50788.

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The aim of this project is to investigate the nucleation of aluminium off the substrate titanium diboride, by using surface and interfacial energies derived from Density Functional Theory (DFT) calculations, and bulk energies derived from thermodynamic data & theory, to calculate the total Gibbs energy change according to classical nucleation theory. It has been known for a long time that TiB2 particles are essential for the nucleation of aluminium in industry. However, the detailed mechanism of how TiB2 aids nucleation is still not known. In particular, it is not known why there needs to be excess dissolved Ti in the melt. Ground state DFT calculations are performed to calculate the interfacial energies of four different systems at 0K: TiB2(0001)//Al(111) and TiB2(0001)//Al3Ti(112), for both Ti and B terminations of TiB2. The DFT calculations for the TiB2//Al interfacial energies are improved over the existing literature results, due to the consideration of strain at the interface, while the TiB2//Al3Ti interfacial energies have not been calculated anywhere else. The DFT results are augmented with thermodynamic data and classical nucleation theory to compare the Gibbs energy of formation of solid on the TiB2 substrate occurring via each of these 4 systems, as a function of titanium concentration in the melt. Thus an evaluation of the comparative likelihoods of the four candidate mechanisms for the nucleation of aluminium is made. Finally, the surface and interfacial energies of the TiB2(0001)//Al(111) systems are investigated at finite temperature, using Density Functional Perturbation Theory under the harmonic approximation, to take account of the vibrational phonon energies occurring at finite temperature. The temperature dependent interfacial energies of TiB2(0001)//Al(111) are estimated, and used to refine the evaluation of the nucleation mechanism of aluminium.

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Scatigno, Giuseppe Giovanni. "Chloride-induced transgranular stress corrosion cracking of austenitic stainless steel 304L." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/51506.

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Stress corrosion cracking (SCC) of austenitic stainless steels has been a known failure mode for more than 80 years and it continues to be a major cause of concern in the nuclear industry. The so-called nuclear grades, such as 304L, contain low levels of C and are therefore hard to sensitise, which is a major problem with high C grades, and these low C grades mainly fail by transgranular SCC. The effect of cold work (CW) has long been known to have a detrimental effect on SCC performance of a stainless steel component. CW is readily introduced in engineering components, through manufacturing history, or implementation, i.e. welding and hammering during fitting. The aim of this thesis is to systematically assess the role of CW in Cl-induced atmospheric SCC in 304L grade austenitic stainless steel. 304L is widely used in the nuclear industry, for both the primary cooling system of nuclear power plants and dry casks for interim storage of spent nuclear fuel. CW was applied in uniaxial tension to levels of 0, 0.5, 1, 2, 5 10, 20, and 40%. The specimens were loaded in a jig to produce a uniform stress of 60 MPa on the top surface and corroded under atmospheric conditions at 75°C, 70% relative humidity, using MgCl2, for 20 days. The role of applied stress (from 60-180 MPa), on SCC susceptibility was investigated at a fixed level of CW (chosen as 10% CW after preliminary experiments) using indicators such as crack density. Secondary and transmission electron microscopy, electron back-scattered diffraction, focused ion beam and secondary ion spectroscopy were the main characterisation techniques used. The maximum susceptibility to SCC was observed between 0.5-5% CW, while 20 and 40% CW did not exhibit cracking. The characterisation of the samples tested provided evidence that Cl is found ahead of the crack tip, whereas oxygen is not, which was never previously observed in the literature. Secondary ion mass spectroscopy and transmission electron microscopy were both used to observe and study the presence of Cl. Simulations such as SRIM and Casino 3.2 were used to confirm that the findings were not a technique artefact. Evidence of dealloying was also observed during the characterisation. Dealloying has long been deemed unlikely in Cl-SCC of austenitic stainless steel, but recent work showed that this may also be an available mechanism for SCC as more and more of the characteristics features of dealloying are observed. The dealloying signs observed were: nanoporosity, found on fracture surfaces; severe striations, heavy dissolution of slip planes; element migration (areas of light and dark contrast in back scattered electron images, dictated by the migration of Cr); cleavage failure; Cr and Ni migration around the crack. The role of salt loading was investigated. Different levels of salt deposition were tested in order to obtain an engineering threshold for salt deposition, namely: low ( < 5.70 x 10-3 g cm-2), medium (5.70 x 10-3–1.42 x 10-2 g cm-2) and high ( > 1.42 x 10-2 g cm-2). A linear relationship was observed between level of salt deposited and both crack density and corrosion area. However, more work is necessary to obtain a threshold.

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Page, Jacob. "The dynamics of vortical perturbations in viscoelastic shear flows." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/51515.

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Polymer additives have a striking impact on vortical motions in transitional and fully turbulent flows. Coherent structures which are familiar from Newtonian dynamics are altered in both strength and size while new, elasto-inertial phenomenology also emerges. In this work three canonical problems are used to explain the formation and evolution of particular flow structures in terms of the coupling between vorticity and polymer torque. Viscoelastic streak amplification is investigated first by forcing laminar Couette flow with a row of streamwise rolls. The streak response is shown to be determined by the ratio of the polymer relaxation time to the disturbance diffusion timescale in the solvent. When the timescales are disparate, quasi-Newtonian and elastic dynamics can be distinguished. When the timescales are commensurate, the fluid supports the propagation of vorticity waves whose reflection and superposition result in spanwise-travelling streaks which re- energise cyclically. In the second problem the receptivity of viscoelastic Couette flow to surface roughness on the lower wall is examined. The vorticity response is classified using a phase diagram, which is parameterised by the ratios of the channel depth and of the vorticity waves’ critical layer depth to the wavelength of the surface waviness. In shallow flows the bulk streamline distortion matches the topography of the lower wall, while large vorticity fluctuations are contained in an upper-wall solvent boundary layer. In deep channels, vorticity fluctuations are generated by a kinematic amplification mechanism at the critical layer. Finally, the evolution of a weak Gaussian vortex in a viscoelastic shear flow is computed to examine the dynamics of spanwise vorticity. Vorticity wave propagation along the tensioned mean streamlines causes the vortex to split into two as it is reoriented by the background flow. In addition, the analysis identifies the existence of a ‘reverse-Orr’ mechanism, whereby the vorticity amplifies as structures are tipped forward with the shear. The vorticity-polymer torque framework provides a unifying framework for studying these coherent motions. It also provides a grounding for understanding more complex, multiscale flows, such as elastic and inertial turbulence at low- and high-Reynolds numbers.

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Khawaja, Mohammed. "Modelling the permeability of nitrile rubber." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/51510.

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Elastomer seals are widely used in the oil and gas industry and form small but crucial parts of critical mechanical and electronic components. Ideally, to prevent damage to such components, the seals should act as impermeable barriers. However, under the high pressures and temperatures found downhole, they are liable to suffer two main types of permeation-driven failure, either by gases permeating through the entire seal, or by dissolved gases causing swelling and rupture, in a process known as explosive decompression. Neither failure mode is particularly well understood, and experimental approaches have encountered difficulties in replicating service conditions and providing insight into the mechanisms responsible. In this work, I use a molecular-simulation-based approach to investigate the drivers of and trends in the permeation of gases as a function of the underlying polymer chemistry and the environmental conditions. I focus on two elastomers in particular that are widely used in the oil and gas industry due to their resistance to hydrocarbons and to thermal degradation: nitrile butadiene rubber, or NBR, which is a statistical copolymer of acrylonitrile and butadiene, and hydrogenated-NBR, or HNBR, which is a polymer derived from the cross-linking and saturation of NBR. For NBR, I first develop a fully atomistic model using the OPLS-AA force-field. I demonstrate that solubility increases with acrylonitrile content, the cyano group plays a crucial role in the enhanced solubility of polar gases, and that the effects of pressure and temperature are heavily gas-dependent. In contrast, the diffusivity is found to decrease with increasing acrylonitrile content. For HNBR, I show that, while the diffusivity decreases with increasing cross-link fraction, counter-intuitively, the solubility increases. The work in this thesis provides a set of molecular-level design principles for the future development of corrosion- and decompression-resistant elastomers.

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Bell, Benjamin. "The influence of alloying elements on the corrosion of Zr-based nuclear fuel cladding using density functional theory." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/51546.

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Zr-based alloys are used primarily as fuel cladding in water-cooled nuclear fission reactors. This is due to their good thermal and mechanical properties and low capture cross section for thermal neutrons. In this work, density functional theory (DFT) simulations were performed to investigate the behaviour of dopant elements in the cladding alloy oxide, with the aim of furthering the understanding of corrosion and hydrogen pick-up in Zr-based alloys. Simulations were performed in both monoclinic and tetragonal ZrO2 on single isolated defects and defect clusters, with the effect of compressive stress on some systems also simulated. Brouwer diagrams were constructed to model the interaction of multiple defect types within a system, in both equilibrium and non-equilibrium overall charge states. The concentration of the dopant elements in the oxide was fixed, and the chemical potential allowed to vary from the reference state, allowing the variation in oxidation state under varying oxygen partial pressure within the oxide to be investigated. Sn was shown to exist as Sn4+ under high oxygen partial pressures in tetragonal ZrO2, however at lower partial pressures a cluster with an oxygen vacancy (VO) and Sn2+ was the dominant defect type. As corrosion progresses, the oxygen partial pressure around a given immobile defect within the oxide layer will increase. Thus, as corrosion progresses the Sn2+:VO cluster will transition to Sn4+, with a consequent reduction in oxygen vacancy concentration. Oxygen vacancies help stabilise the tetragonal phase, and so this transition may cause a transformation from tetragonal to monoclinic to occur. The associated increase in volume (monoclinic phase has a volume ~4% larger) is proposed to be a contributing factor to the early transition observed in Sn-containing alloys. Nb was shown to exist in oxidation states ranging from Nb2+ to Nb5+ in the tetragonal phase, but only as Nb5+ in the monoclinic. In the lower oxidation states, Nb is able to mitigate the space charge which builds up in the oxide layer during corrosion, as a result of the lower oxygen vacancy diffusion rate compared to electrons. By mitigating the space charge, the corrosion kinetics are able to approach parabolic and the HPUF is lowered, both observations are in excellent agreement with experimental work which has shown Zr-Nb alloys to be unique in these properties. At transition, a large proportion of the tetragonal phase present in the oxide layer will transform to monoclinic as compressive stress is relieved. Since Nb was predicted to only exist in the 5+ state in the monoclinic phase, any lower states will oxidise upon phase transformation, releasing electrons into the oxide layer during transition. This oxidation process is proposed as a possible explanation for the sudden drop in HPUF that has been experimentally observed to occur around transition. Sc was shown to exist only as Sc3+ in both tetragonal and monoclinic phases, charge balanced by an increase in the concentration of oxygen vacancies. Since a stable cluster involving Sc3+ and VO would require the close proximity of two Sc3+ defects, it is assumed that at the low doping level in the model alloys produced alongside this work (0.2-0.4 %. wt.) stable clusters are unlikely to form in significant concentrations. Thus, the inclusion of Sc as a dopant increases the concentration of unbound (i.e. mobile) oxygen vacancies in the oxide layer. This is proposed as the reason for the extremely high corrosion rate observed in the Sc-containing model alloys produced and tested alongside this work. It is also assumed that the tetragonal phase fraction in the oxide layer of the Sc-containing alloys is likely to be high due to the increased oxygen vacancy concentration, however experimental testing to verify this prediction has not yet taken place. Sb was shown to exist as Sb5+ at high oxygen partial pressures, transitioning to Sb3+ at lower partial pressures in both the tetragonal and monoclinic phases. The application of a non-equilbrium charge state to the Brouwer diagrams showed that the oxygen partial pressure at which the transition between Sb5+ and Sb3+ occurs is able to smoothly change in order to counteract the applied space charge. This behaviour was observed in both phases, and implies that Sb may be able to act as a space charge compensation mechanism, in a similar fashion to Nb. The Zr-Nb-Sb model alloys produced alongside this work exhibited a lower corrosion rate and HPUF than the Zr-Nb alloys, suggesting that Sb may improve the beneficial effects already observed in Zr-Nb alloys.

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Gray, Farrel. "Simulating flow and reactive transport in porous media." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/51505.

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In this work, we developed and applied computational methods for simulating flow, transport and reactive transport in porous media. This comprised four main components: single-phase flow calculation; chemical transport calculation; the coupling to reaction kinetics at mineral surfaces and resulting structural changes; and the use of parallel and GPU computing to make the calculation practicable on realistic rock geometries. Single phase flow was calculated using the Lattice Boltzmann (LB) method. We used the multi-relaxation-time (MRT) operator for its superior stability and viscosity-independence. A sparse memory approach was employed which improves the efficiency of calculations performed on low-permeability rock pore-space images. We also extended an idea proposed by Skordos in which the lattice Boltzmann densities were transformed to increase the number of floating point bits retained in the calculation. We showed that this enhances the numerical precision of the calculation considerably, where the original paper found no appreciable benefit. We showed how this now permits the 4-byte datatype to be used reliably in slow flowing, heterogeneous domains. The LB algorithm was implemented for the use of parallel GPUs (Graphics Processors) using the MPI (Message Passing Interface) and shown to give strong scaling on a cluster of 24 Tesla K40 GPUs. A study of single phase permeability on micro-CT images of sandstone and carbonate rock pore structures of varying degrees of heterogeneity was carried out. Good agreement with experiment was found for the simpler pore spaces, while discrepancies in the micro-porous samples was attributed to two causes: 1) the exclusion of flow through unresolved micro-porosity and 2) unrepresentative sample sizes used in the simulation. The effect of image resolution and segmentation was studied by comparing single phase permeability computed in 1) scans of the same volume obtained at different voxel sizes, individually segmented and 2) numerically coarsened images from a high resolution segmented image. Numerical coarsening from a high resolution segmented image was found to be much more consistent than 1) and was shown to preserve porosity and permeability down to lower voxel size images unlike the images scanned and segmented at different voxel sizes. Finally, representative elementary volume (REV) was investigated for the rock samples. A statistical method was used in which porosity and permeability were obtained from sub-volumes sampled from the domain. The convergence of these parameters with sub-volume size was used to obtain characteristic length scales and measures of heterogeneity. The image sizes used were found to be unrepresentative for the complex microporous carbonates. Transport curves (propagators) were computed in three different porous media samples of increasing heterogeneity (a bead-pack; sandstone; and carbonate) and found to agree with experiment. Questions about the origins of stagnant transport zones in the microporous carbonate were pursued by investigating the effects of image segmentation. The effects of the image segmentation techniques, in which grey-scale micro-porosity in a scanned pore image is binarised into fluid or mineral, were quantified by computing the fraction of trapped solute (stagnant zones) for segmentations of varying porosity. Physical differences between experiment and calculation were clarified, and we suggest alternative approaches for the treatment of micro-porous rocks. A pore-scale reactive flow model was put together by coupling flow calculation and solute transport methods with changes in pore-structure through chemical kinetics. Convection and diffusion in this model was solved using a finite-volume approach: a second order transport model with a flux limiter function made the model suitable for high Peclet number transport calculations. We also proposed a method for counteracting errors associated with the staircase representation of diagonal surfaces in the Cartesian grid in which exposed grid surfaces are associated with a rescaling factor. First order reaction kinetics were included at mineral surfaces and the dissolution of a sphere was shown to give different dissolution profiles with different dimensionless transport and reaction parameters. The dissolution model was applied to the reaction between HCl acid and calcite mineral under the assumption that products of the reaction could be neglected. An experimental system in which HCl acid was injected through a flow cell containing a calcite block was simulated and the normalised volume of undissolved calcite was compared with the experimental data, as well as resulting morphologies obtained by micro-CT scanning. Good agreement with the experimental dissolution rate was obtained, however some differences in the resulting morphologies were found. This was attributed to neglecting the influence of product ions on the diffusion behaviour of the reactant and was discussed. By obtaining the concentration of H+ reactant on the surface of calcite block, the process could be concluded to be strongly transport-controlled. This enabled the definition of a new effective Damkohler number in terms of the reactant surface concentration which no longer required approximating length scales or separating convection or diffusion rates. Finally, the dissolution of a Ketton carbonate sample was computed. The injection process mirrored that of a strong acid flowing through the pore-space at a given flow rate, and having an intrinsic surface reaction rate with the rock mineral. It was found that the flow rate strongly affected the resulting dissolution pattern, in line with experimental observation. This lead to drastically altered flow properties, including single-phase permeability which was quantified.

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Del, Rosso Stefano. "Micro-scale hybrid fibres for low cost polymer armours." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/50702.

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High performance polymer fibres are extensively used to make personal protective textiles and as reinforcing phase in polymer reinforced composite materials. This because of their excellent mechanical properties such as high tenacity and toughness, low elongation to break, and their ability to dissipate shock waves over a large area in a short amount of time. Over the past decades, braiding has become a very popular manufacturing technique and widely exploited in the composite industry for the production of advanced engineering parts. Braided reinforced structures offer certain advantages over conventional reinforced composite materials such as damage and impact resistance, high delamination resistance due to a greater through-the-thickness reinforcement. Moreover, the investment and labour cost can be minimised due to the inexpensive machinery, high rate of production and level of automation. This work aimed at studying the mechanical response of polymer structures rein- forced by high performance microbraids for ballistic impact applications. The mechanical properties of different high performance fibres were experimentally investigated under different conditions, from quasi-static to dynamic loads. Quasi-static tensile tests were performed on yarns and microbraids at constant strain rate and the effect of different gauge lengths studied; high strain rate tests were performed by transversely impacting yarns and microbraids using flat-face lead projectiles. Tensile tests performed on dry microbraids showed a significant improvement up to 96.2% over the constituent materials in terms of toughness and energy absorption. Dry microbraids also proved to offer greater ability to capture impactors with respect to unidirectional fibres, although with greater deformation. A novel and unique manufacturing technique was also developed for the manufacture of microbraid reinforced polymer composites (mBRPC). The microbraids were wound, at controlled tension, onto a spinning plate via a robotised lament winding system. The dry fabric was then consolidated into a prepreg via the hot-press technique using a thermoplastic resin film. The final composites were manufactured by stacking the prepregs in a cross-ply fashion. Quasi-static tests performed on mBRPC showed a different mechanical behaviour with respect to cross-ply composites manufactured using the same technique but with unidirectionally aligned fibres. mBRPC showed enhanced tensile strength, strain to failure and toughness for certain materials and architectures with respect to the UD counterpart. The ballistic performance of mBRPC were assessed by firing mild steel balls using a gas gun. Two high speed cameras were employed to record the behaviour of the mBRPC during the impact event. Test results showed a higher ballistic limit, up to 19%, for certain mBRPC with respect to the UD counterpart.

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Bovo, Gianluca. "Room temperature spin crossover in molecular and polymeric materials." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/51493.

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Spin crossover materials represent a remarkable model of molecular spin switching, where a transition between low spin and high spin configurations can be reversibly triggered by suitable external stimuli and is often accompanied by a hysteresis of various physical properties, holding potential for application in memory devices and optical switches. This thesis work investigates the thermal spin crossover behaviour of a class of Fe(II)-triazole polymers where the chemical properties can be tailored to achieve solubility in common solvents and one mononuclear Fe(II) complex deemed suitable for thermal evaporation. A broad spectrum of experimental techniques including magnetic, optical, calorimetric and structural assessments are applied to characterise the spin crossover phenomena in powders, thick films and pellets, highlighting the impact of the molecular environment and the effects of the processing methods on the spin switching. Additionally, thermal expansion measurements with a capacitance dilatometer provide a macroscopic evaluation of the mechanical changes associated with the transitions, which mirror the expansion/contraction of the Fe coordination sphere at the molecular level. Solution processing and vacuum deposition of the materials further enables the preparation and study of stable thin films, better suited to device application, with optical characterisation confirming the retention of spin crossover thermochromic properties. As a first step to deploy the iron-polymers inside simple electronic devices, MIM structures (metal-insulator-metal) are fabricated and tested by impedance spectroscopy, showing the presence of a thermal hysteresis loop in the dielectric function around room temperature. We demonstrate that in addition to the previous studies on bulk powders, optical and dielectric bistability with sharp spin transitions can also be achieved for thin films devices, thus opening interesting new perspectives for the application of spin crossover polymeric and molecular materials.

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30

Argyropoulos, Christos. "A combined immersed boundary/phase-field method for simulating two-phase pipe flows." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/51089.

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The investigation of the flow in a pipe is a major issue for the pipeline capacity but also plays an important role for the control and prevention of phenomena that could damage the pipe, such as corrosion, erosion, and the potential formation of wax or their deposits. Therefore, the characterization of the flow patterns is also a major issue for the prediction of the distribution over the cross-section of the pipe, in order to understand any problems that may interrupt or shut down the operation of the production line. The main purpose of the present effort is to develop an appropriate numerical method for simulating two-phase pipe flows. Advanced Computational Fluid Dynamics (CFD) methods are employed as Navier-Stokes solver, while a Phase-Field method is used to simulate the interfacial region between the two fluids. A Ghost-Cell Immersed Boundary Method (GCIBM) was developed and implemented for the reconstruction of smooth rigid boundaries (pipe wall) based on the work of Tseng and Ferziger (2003). The method was also modified in order to incorporate appropriate boundary conditions for coupling the Phase-Field and Navier-Stokes solvers for two-phase pipe flows. Tseng and Ferziger (2003) used the GCIBM for turbulent single-phase flows; the present modified version comprises a continuation of the method for handling two-phase pipe flows. The computational model is capable of handling large density and viscosity ratios with good accuracy. The developed GCIBM algorithm was validated against analytical solutions for single and two-phase pipe flow, presenting very good agreement. The computational model was compared to available experimental data from the literature for single rising bubbles and bubble coalescence in vertical pipe also with good agreement. The numerical method was used to investigate the lateral wall effects of a 3-D single bubble in a viscous liquid for different pipe diameters and bubble flow regimes. The dynamics of 3-D Taylor bubbles was also examined in vertical pipes for different properties of fluids (e.g. air-water system) and dimensionless parameters relevant to the problem (e.g. ReB, Eo, Mo). The numerical results were compared with available experimental and numerical data from the literature, presenting good agreement.

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Kardoulaki, Erofili. "Damage modelling of leaded free cutting steel under hot forming conditions." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/52638.

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In this thesis the influence of stress-state on ductile damage in free cutting steel under hot forming conditions is examined. The industrial motivation for the project focuses on edge cracking in hot rolling. A brief outline of the hot rolling process conditions is necessary to define the important parameters affecting edge cracking. Triaxiality was, thus, identified as the key parameter relating to damage under hot rolling conditions. Based on a detailed literature review, the appropriate testing and modelling methodology were identified for this body of work. A high temperature, uniaxial tension test program was implemented to identify the effect of triaxiality on damage under hot forming conditions. Double notched bars with varying notch radii were utilised, thus inducing different stress triaxialities due to geometrical constraints. Based on the resultant stress-strain data the effect of triaxiality on ductility and the strain to failure was investigated. Subsequently, unbroken notches from tested double notched samples were sectioned and optically examined to reveal damage initiation sites. Interesting damage features were identified and correlated with sample geometry (i.e. triaxiality) and testing conditions. Finite element analysis of the double notched samples revealed the effect of triaxiality on the local stress-state. The accuracy of the mechanical analysis from such simulations was improved by incorporating the thermal gradients induced during high temperature Gleeble tests. Three stress parameters were examined in relation to their effect on the experimentally observed damage; maximum principal stress, effective stress and hydrostatic stress. The maximum principal stress and equivalent stress were most clearly correlated to damage development under multiaxial conditions for this particular free cutting steel. Based on the results of the stress-state investigation of the double notched samples, a multiaxial damage expression was developed that reproduced the experimentally observed damage characteristics. The new multiaxial damage model was calibrated using a combination of uniaxial and multiaxial stress-strain data and damage profiles. The model was shown to have good accuracy in predicting both the stress-strain data and the damage initiation sites as a function of geometry and damage conditions. Finally, an extensive range of temperature and strain rate conditions were simulated for all tested sample geometries, and an additional sample geometry, to fully understand how testing conditions affect damage characteristics and under what triaxialities this is prone to happen.

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Tear, Gareth Richard. "Shock properties of homogeneous anisotropic dielectrics." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/53828.

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Anisotropy, the directional dependence of a physical quantity, is present in numerous physical processes involved in the shock compression of solid materials. The effect that a particular property’s anisotropy has on the propagation of a shock wave is obscured by other effects such as those from strain rate and material heterogeneity. Recent studies have focussed on single- and bi-crystal metals to understand the effect of crystal anisotropy on the mechanics of shock wave propagation. This thesis extends this work to optically transparent non-metallic dielectric single crystals by developing an optical model for anisotropic dielectrics and performing experimental measurements to test the validity of that optical model. Current optical models for shock compressed materials use an isotropic Gladstone- Dale model or isotropic modifications of the Gladstone-Dale model. This thesis extends the isotropic Gladstone-Dale model to an anisotropic photoelastic model for the optical behaviour of linear anisotropic materials under shock compression in the elastic regime. The model uses static photoelastic tensor values available in the literature to predict material response under uniaxial strain in an arbitrary crystal orientation. The effect of varying photoelastic tensor values is studied using Monte Carlo techniques, and confidence intervals on dynamic predictions are presented. Polarimetry is applied to experimentally measure birefringence under shock compression delivered using plate impact on a single stage light gas gun. This method is used to validate the linear photoelastic model developed in this thesis. Experiments were performed on < 10-10 > (a-cut) sapphire up to 15 GPa longitudinal stress and < 10-10 > (a-cut) calcite up to 2 GPa longitudinal stress. It was found that the birefringence of a-cut sapphire under shock compression behaved in agreement with the model in the elastic regime for a 5% error on the photoelastic tensor. Furthermore it was found that birefringence predictions for a-cut calcite as given by the same model did not agree with experimentally measured results. The discrepancy was 0.3% at 1.2 GPa, in excess of 5 standard deviations. Possible reasons for the discrepancy are put forward. Current optical models for shock compressed materials use an isotropic Gladstone-Dale model or isotropic modifications of the Gladstone-Dale model. This thesis extends the isotropic Gladstone-Dale model to an anisotropic photoelastic model for the optical behaviour of linear anisotropic materials under shock compression. The model uses static photoelastic tensor values available in the literature to predict material response under uniaxial strain in an arbitrary crystal orientation. The effect of varying photoelastic tensor values is studied using Monte Carlo techniques, and confidence intervals on dynamic predictions are presented. Polarimetry is applied to experimentally measure birefringence under shock compression. This method is used to validate the linear photoelastic model developed in this thesis. Experiments were performed on ⟨1 0 1 0⟩ (a-cut) sapphire and ⟨1 0 1 0⟩ (a-cut) calcite. It was found that the birefringence of a-cut sapphire under shock compression behaved in agreement with the model. Furthermore it was found that birefringence predictions for a-cut calcite as given by the same model did not agree with experimentally measured results. Possible reasons for the discrepancy are put forward.

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Ibarra-Hernandez, Roberto. "Horizontal and low-inclination oil-water flow investigations using laser-based diagnostic techniques." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/53121.

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The extraction of detailed phase and velocity-fields structures, which have not been fully explored in the literature, represents key information that can assist with the fundamental understanding of multiphase flows and be used to improve, develop, or validate closure relationships. This work is focused on the investigation of co-current oil-water flows in horizontal and low-inclination pipes in terms of flow regimes, in situ phase fractions, pressure gradients, and interface and flow velocity information. This was achieved by the implementation of shadowgraphy and laser-based techniques, along with pressure drop measurements. The laser-based experimental methodologies consist of a novel two-line light sheet arrangement which allows the extraction of simultaneous two-dimensional spatio-temporally resolved fluid-phase and velocity information in stratified flows for fluids with different refractive indices whose physical properties correspond directly to real field-industrial applications. Analysis of the experimental data reveals interesting flow structures and interactions between the fluids. The distribution of the phases is dependent on the flow velocities, the inclination of the pipe, and the design of the inlet section. The latter was found to have a significant effect on the flow instabilities either by generating droplets or promoting separation which in turn affects the pressure gradient along the pipe. Instantaneous velocity fields, mean velocity profiles, turbulence characteristics, and interface profiles are extracted from the laser-based measurements. Mean axial velocity profiles show characteristics of laminar and turbulent flows while wall-normal velocity profiles suggests that secondary flows (structures or vortices in the azimuthal direction), which are attributed to have an influence on the unsteadiness in the flow, are present in the investigated flows as single- or counter-rotating vortices generating a region of high shear where the vortices interact. Moreover, Reynolds stresses and mixing lengths profiles are constructed in order to offer further information on the level of turbulence in the flow. Experiments in upward pipe inclinations reveal that instabilities at the interfacial region are enhanced as a result of complex flow structures (e.g. large vortices and regions with only wall-normal velocity component). Mean and fluctuating velocity profiles, along with Reynolds stresses, at a given flow condition are strongly dependent on the pipe inclination for low mixture velocities; however, weakly dependent as the mixture velocity increases. In general, space- and time-resolve phase and velocity information has been obtained for stratified oil-water flows revealing detailed and complex interfacial and flow structures on both liquid phases.

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Xu, Yilun. "On the development of a multi-scale modelling framework to study plasticity and damage through the coupling of finite element crystal plasticity and discrete dislocation plasticity." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/52630.

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The microstructure of polycrystalline materials crucially determines their mechanical performance in engineering applications. A multi-scale modelling approach is capable of representing the microstructure and thus capturing the material performance for various resolution requirement at different scales. Besides, the application of multi-scale modelling effectively reduces expense and improves efficiency of computations without loss of accuracy at sensitive zones. A method of concurrent coupling of planar discrete dislocation plasticity (DDP) and a crystal plasticity finite element (CPFE) method was devised for simulating plastic deformation in large polycrystals with discrete dislocation resolution in a single grain or cluster of grains for computational efficiency; computation time using the coupling method can be reduced by an order of magnitude compared to DDP. The method is based on an iterative scheme initiated by a sub-model calculation, which ensures displacement and traction compatibility at all nodes at the interface between the DDP and CPFE domains. The proposed coupling approach is demonstrated using two plane strain problems: (i) uniaxial tension of a bi-crystal film and (ii) indentation of a thin film on a substrate. The latter demonstrated that the rigid substrate assumption used in earlier discrete dislocation plasticity studies is inadequate for indentation depths that are large compared to the film thickness, i.e. the effect of the polycrystalline plastic substrate modelled using CPFE becomes important. The coupling method can be used to study a wider range of indentation depths than previously possible using DDP alone, without sacrificing the indentation size effect regime captured by DDP. A comprehensive indentation pressure formula has been developed by applying the proposed multi-scale modelling approach on a polycrystalline coating system. Planar nano-sliding and fretting calculations have been performed on thin films modelling by CPFE and DDP at different scales. Results of CPFE simulations provide an understanding of the role of microstructure on the plasticity and crack initiation during a contact problem. Beside, a new DDP computational framework has been proposed for a nano-fretting problem which is able to capture the contact size effect, simulate the dislocation evolution and predict the surface profile variation of thin films. Calculations of DDP simulations potentially provide CPFE simulations with fatigue parameters that is of more physical significance. The method is general and can be applied to any problem where finer resolution of dislocation mediated plasticity is required to study the mechanical response of polycrystalline materials, e.g. to capture size effects locally within a larger elastic/plastic boundary value problem. Also, the model described here will provide further opportunities for directly coupled, three-tiered multi-scale models compromising an overall macroscopic continua having embedded crystal plasticity and discrete dislocation plasticity models, respectively, as the length scale decreases in the area of interest. Finally, the methodology of the proposed coupling method will shed light on archiving a general compatibility of sub-regions and thus benefit other researchers who are working on coupling methods among other scales.

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35

Dimela, Nefeli. "Numerical simulations of primary break-up in two-phase flows." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/52425.

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Liquid-Gas interactions and break-up processes are found in many technological and environmental applications, from Internal Combustion and Gas Turbine engines to food processing and manufacturing. Their complete characterisation at realistic Weber and Reynolds numbers is not possible, due to the vast range of scales integrated and the requirement of a ’minimum’ computational mesh size to capture these scales. To this day, a number of questions remain unanswered, with relative research still ongoing. It is crucial to understand such phenomena so that any technological applications can be optimised and the environmental impact can be reduced. Currently, there is a high need to develop appropriate numerical modelling tools that provide both mass conservation and accurate interface topological properties. Two common interface modelling approaches are the Volume of Fluid and the Level Set, typically coupled into CLSVOF methods to ensure improved surface representation and good mass properties. In this work, a novel in-house Mass Conservative Level Set (CMLS) method is developed and validated extensively. The CMLS novelty is in the Level Set coupling with the Volume of Fluid, being processed only when necessary, providing a faster and more robust approach. Doing so, some numerically imposed limitations due to the ’physics’ and ’stability’, are overcome. The novel CMLS is employed for primary break-up investigations, in a single liquid droplet and jets. Single droplet break-up remains a benchmark test case, as it provides good foundations for liquid jet break-up and spray atomisation modelling. In such processes, the main effective parameters considered are the Weber and Reynolds numbers, along with the Ohneshorge (droplets) and Dynamic Pressure ratio (jets). Contrary to most studies, this work employs the surface density evolution using the Σ − Y model. The droplet break-up cases, show a strong correlation between the break-up initiation time and the Ohneshorge number, whilst as the Weber increases so does the droplet complete break-up time. This is of particular interest as at higher Weber numbers, surface density effects be- come negligible and thus by definition the complete break-up time should in fact decrease. However, similar behaviours were noted in previous studies. The droplets surface density evolution shows a ’quasi-independent’ relationship with the gas Weber. In the jets, a strong correlation between the surface density and ligament formation exists. However, the surface density is ’quasi-independent’ of the liquid Reynolds and the gas Weber. The gas boundary layer presence in jets, shows to both reduce and delay any liquid/gas inter- face perturbations and the potential break-up. To summarise, the present investigations are generally in good agreement with previous studies, with minimal contradictions in cases. The novel CMLS capabilities show promising results both in the two- and three- dimensional space. This work provides good foundations for a slightly alternative research approach in two-phase flows modelling.

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36

de, La Porte Jacoba. "Sensitivities to component characterizations of heavy oil viscosity in numerical reservoir simulation of steam-injection processes." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/11056.

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This work examines heavy oil viscosity modelling during simulation of steam injection processes, such as steam-line-drive and SAGD, and the sensitivity of oil recovery predictions to the uncertainty in the oil viscosity. Analytical models to predict the sensitivity have been developed, confirmed by numerical simulation. Heavy oil compositional component viscosities are modelled with the Free Volume model. The model is extended in this thesis to estimate the viscosities of long-chain n-alkanes from C6H14 to C45H92 within an accuracy of 10% in the temperature range 27 to 300 °C (80 to 575 °F) and pressure range 0.1 to 120 MPa (14.5 to 17,400 psi). It estimates viscosities of long-chain n-alkanes up to C64H130 to within 30%. Extrapolated Free Volume molecular characteristic parameters, optimised based on available viscosity measurements for n-alkanes up to C64H130, are provided, and are the recommended values for use in heavy oil simulation. A heavy pseudo-component, representing a combination of asphaltenes and resins, which are the compounds responsible for the high viscosities observed in heavy oil, is characterised in terms of molecular weight, shape and activation energy for viscous flow. A method to predict its viscosity as a function of its physical properties, pressure and temperature, using the Free Volume model, is demonstrated. A density model based on the Tait model is extended, to predict the long-chain heavy oil compositional component densities within an accuracy of 3%, in the same temperature and pressure ranges as above. A grouping procedure is demonstrated to achieve oil recovery results comparable to a 24-component simulation case, using two pseudo-components. Key is the mixing equation used to calculate the oil phase viscosity as components are grouped. The Arrhenius mixing equation is evaluated for accuracy in predicting hydrocarbon mixture viscosities. Guidelines for accurate use are provided, while mixtures with CO2 are shown to require a different method.

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Soin, Preetma Kaur. "First principles modelling of dislocations in BCC iron." Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/11752.

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This work centers around employing magnetic tight binding to study defects in body centred cubic iron, a material of potentially great importance as a structural component in hydrogen fusion power plants. An existing d-band tight binding model was extended to include charge. The first task was to develop a working scheme for calculating spins and charges self consistently. This was achieved using an extended form of the Harris-Foulkes functional and implemented with a generalisation of the Newton-Raphson minimisation procedure. Having established such a scheme it was tested on bulk structures, point defects and straight dislocations. The motion of dislocations and the opposition to this was also considered though calculation of Peierls barriers.

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38

Woolley, Russell. "Ruddlesden-Popper phases as solid oxide fuel cell cathodes : electrochemical performance and in situ characterisation." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/14370.

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The aim of this work was to develop oxide fuel cell (SOFC) cathodes made from (LaNiO3)nLaO Ruddlesden-Popper (R-P) phases, and to investigate novel in situ characterisation techniques for SOFC cathodes. Cathodes were developed from La2NiO4+δ (L2N1) and La4Ni3O10-δ (L4N3), R-P phases known to have attractive conductivities at SOFC temperatures. These phases were shown to be chemically stable, both with each other and with the common electrolyte material La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM). LSGM-supported symmetrical cells were fabricated with electrodes of single phase L2N1 and L4N3, and a range of L2N1+L4N3 composites. The performance of these was tested from 500 – 700 °C with the composites giving the lowest area-specific resistance (ASR); a 50:50 wt.% L2N1:L4N3 composition being optimal. Functionally graded electrodes were developed consisting of a thin compact L2N1 layer deposited onto the LSGM, topped by a thicker porous L2N1+L4N3 composite layer, completed by a thin porous L4N3 current collector. These gave a lower ASR than the ungraded electrodes. Using a 50:50 composite was optimal with ASRs of 15.59, 2.29, and 0.53 Ωcm2 at 500, 600, and 700 °C respectively; amongst the best-in-class for electrodes made from this type of material. X-ray absorption near-edge spectroscopy was chosen as a method to gain in situ information on the redox chemistry of elements within SOFC materials. Initial studies were carried out on powder samples of L2N1 and L4N3; the nickel oxidation state in these was found to reduce on heating to SOFC operating temperatures. Bespoke equipment was developed to enable such studies to be carried out on symmetrical cells under polarisation and with simultaneous AC impedance spectroscopy. The bulk nickel redox chemistry was correlated with the changing concentration of ionic charge carriers in the materials, and was found to be dominated by thermal effects. These techniques were then used to explore in situ chromium poisoning of state-of-the-art perovskite cathodes. The surface chemistry of SOFC materials is key to performance. Low-energy ion scattering was used to find the composition of the outer monolayer for the entire (LaNiO3)nLaO R-P series; lanthanum termination was found for each phase.

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Evans, Christabel. "Micromechanisms and micromechanics of Zircaloy-4." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/14335.

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The micromechanisms of Zircaloy-4 are investigated in relation to texture evolution, hydride formation and fatigue. The Zircaloy-4 plate used throughout this thesis was provided by Rolls- Royce plc, Derby, and was annealed post unidirectional rolling. The effect of strain rate on the texture evolution of Zircaloy-4 was investigated to understand how different processing methods would effect the final texture. Texture evolution during high temperature (550◦C) compression and tension tests were investigated using synchrotron X- ray diffraction in the transverse and rolling directions (TD and RD) at strain rates ranging from 10−4s−1 to 10−1s−1. The post deformation microstructures showed the presence of twins at the higher strain rates (10−1s−1 to 10−2s−1 ), with minimal twinning seen at the slower rates. The pole figures obtained throughout testing showed no texture evolution during tensile testing, regardless of strain rate and the basal poles remained orientated ±30◦between the normal direction (ND) and the transverse direction (TD), which is the original texture for the as received material. During the compression tests specimens tested in the RD showed an evolution in the pole figures as strain rate was increased. At a strain rate of 10−1s−1 a reorientation of the basal poles to lie almost solely in the RD was seen, indicative of twinning. As the strain rate was reduced, this effect diminished and at a strain rate of 10−4s−1only a slight rotation of the basal poles was observed. The Kearns’ factor evolution with strain confirmed this result. These results were then used in an elasto-plastic self-consistent model to simulate the slip and twin levels during deformation. The computational results were consistent with the notion that increasing the strain rate increased twin density, as shown in the post deformation microstructures. To understand the micromechanical effects hydride precipitates have on the alloy, a section of the alloy was charged with hydrogen in a vacuum furnace to 375 ppm ± 50 ppm. Microstructural characterisation of the material indicated that high levels of hydrides forming predominantly at grain boundaries. Nanoindentation tests were carried out at room temperature on individual hydride packets, the surrounding matrix and the as received material to characterise the me- chanical properties. The results obtained from these tests were used in computational modelling scenarios to determine more accurate mechanical properties. The nano-hardness of the matrix was found to be highest (4.64 GPa), followed by the matrix and the as received material (3.62 GPa and 2.74 GPa respectively). As part of the initial scope of this thesis it was the author’s original intention to understand how the presence of hydrides affects dislocation propagation and micro-deformation mechanisms. However, since carrying out the experimental procedures and results analysis, a number of papers have come to the author’s attention which outline the importance of the final processing steps prior to testing. It has been found that mechanical polishing as a method for material preparation induces work hardening into the surface of the material. Although this does not have an affect in macro and indeed micro scale hardness testing, where the tested layer is in the scale of a few microns, this work hardened layer does have a major effect in nano-hardness tests, where the testing layer is in the region of nanometers. As a result of this no dislocation analysis was carried out as it would be impossible to distinguish between dislocations present from mechanical polishing and those induced by the presence of hydrides. In spite of the work hardened layer rendering the absolute hardness values invalid, the relative values in relation to the matrix, hydride and as received material are still of interest. High cycle fatigue tests were carried out on samples taken from the rolling and the transverse direction of the material. Fractographic examination of the samples showed facets in the area immediately surrounding the initiation site. There were only found to be between 10-20 faceted grains, which were confined to this region. These features showed feather-like characteristics, indicative of plastic deformation. Site specific transmission electron microscopy (TEM) was carried out on the initiation facets, showing mostly dislocations, although and imperfect dislocations segments were also found to be present. The low dislocation density in these features compared to that of titanium suggests that these features may be quite brittle in nature. Crack propagation was found to occur via striated crack growth. The direction of the striations appear to be affected by grain orientation. TEM analysis of the underlying grain did not show the presence of any dislocations. It is thought that this may be a result of image stresses causing the dislocation to evaporate out the TEM specimen once it is removed from the fracture surface, although further work needs to be done to confirm this.

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Taub, Samuel. "Transition metal oxide doping of ceria-based solid solutions." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/18845.

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The effects of low concentration Co, Cr and Mn oxide, singly and in combination, on the sintering and electrical properties of Ce0.9Gd0.1O1.95 (CGO) have been investigated with possible mechanisms suggested to explain this modified behaviour. The influence of these dopants on the densification kinetics of CGO were primarily investigated using constant heating rate dilatometry. Whilst low concentration Co and Mn-oxide were found to improve the sinterability of CGO, the addition of Cr-oxide was found to inhibit the densification kinetics of the material. The location and concentration of these dopants were investigated as a function of relative density using scanning transmission electron microscopy combined with energy dispersive x-ray mapping. All materials showed a gradual reduction in the grain boundary dopant concentration with sintering time, leading eventually to the formation of a second phase that was subsequently analysed by either electron energy loss spectroscopy or synchrotron x-ray powder diffraction. The improved densification of both the Co-doped and Mn-doped materials was believed to be related to an increased rate of lattice and grain boundary cation diffusion, associated with the segregation of the transition metal dopant to the grain boundary. In both cases the onset of rapid densification was correlated with the reduction of the transition metal cation leading to an increase in cerium interstitials, which are suggested to be the defects responsible for cerium diffusion. The inhibiting effects of Cr-addition were similarly related to changes in the defect chemistry, with the Cr ions creating a blocking effect that hindered the dominant grain boundary pathway for cation diffusion. The effects of these dopants on the electrical conductivity of CGO were examined using a combination of AC impedance spectroscopy and Hebb-Wagner polarisation measurements. Whilst Co-doping was found to enhance the specific grain boundary conductivity of CGO, the addition of either Cr or Mn resulted in an approximate 2 orders of magnitude decrease, even at dopant concentrations as low as 100 ppm. Despite these differences in ionic conductivity, both Co and Cr-doping were found to significantly enhance the electronic contribution to the conductivity along the boundaries, particularly within the p-type regime. The modified electrical behaviour was related to the formation of a continuous, transition metal-enriched grain boundary pathway and a change in the driving force for grain boundary Gd segregation, leading to a depletion of oxygen vacancies within the space charge regions and the consequent reduction of oxygen transport across the boundaries. The effects of this segregation were finally examined with mono-layer sensitivity using low energy ion scattering incorporating a novel method of self-standardisation. These analyses provided strong support for the conductivity mechanisms previously outlined.

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Stechert, Thorsten Roland. "Glasses for energy applications : atomic scale network structure and properties." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/18940.

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Glass is used for the vitrification of high level waste that results from the reprocessing of spent nuclear fuel. A better understanding of the structure of vitrified wastes may lead to insights into the observed compositional flexibility. It is also the starting point for studies of the self-irradiation behaviour of glasses under long-term repository conditions. Appropriate models need to be employed for the study of glasses when using molecular dynamics. The nature of nuclear waste necessitates an accurate structure prediction for a range of compositions and parameters. To this end, the suitability of established potential sets have been compared. The established potential models were used to investigate the structure of zinc containing sodium silicate glass. Once validated, this structure was used to investigate structural changes observed during simulated self-irradiation, where significant changes were observed on the atomic scale. This will provide the basis for further studies of radiation damage, glass-crystal interfaces and damage across glass-crystal interfaces. In order to further enhance the understanding of potential models, a novel glass of composition LiAlF4 has been successfully described, and may become relevant in the future as a thin film coating in Li-ion batteries.

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Hadjicosti, Krystallo. "Surface waves in magnetic metamaterials : phenomena and applications." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/49781.

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Surface waves in metamaterials are responsible for a range of phenomena, and understanding their properties is essential for the development of metamaterial applications. This thesis first investigates the propagation of surface magneto-inductive waves along a defect in an array of split-ring resonators. A new approach is demonstrated in which the defect is formed by changing the resonant frequency of the defect elements. The dispersion relation was derived and its dependence on the sign and the strength of the inter-element coupling and the relationship between the resonant frequencies was studied. Waveguides, couplers and filters were simulated, and the theoretical results were verified experimentally. The thesis further investigates the effect of inter-element coupling on surface magneto-inductive polaritons. Using two models, one relying on the effective-medium approximation and the other on equivalent circuits, we studied theoretically surface polaritons propagating along an interface between air and a magnetic metamaterial. With a moderate longitudinal coupling, a single polariton is supported similar to the un-coupled case. However, the presence of a transverse coupling changed the polariton dispersion dramatically. The effective-medium model yielded two branches of polariton dispersion at low values of transverse coupling. As the coupling increased, both polaritons disappeared. The validity of the effective-medium model was further tested by employing the circuit model. In this model, surface polaritons could exist in the presence of a transverse coupling only if the boundary layer of the metamaterial included additional impedances, which could become non-Foster circuits. The results reveal that the inter-element coupling is a major mechanism affecting the properties of the polaritons. They also highlight the limitations of using bulk effective-medium parameters for interface problems in metamaterials.

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43

Zhang, Ruoyu. "Experimental and numerical investigation of structure-mechanical property relationships in alumina trihydrate reinforced poly (methyl methacrylate) composites." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/49255.

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This study mainly investigated how the mechanical behaviour of alumina trihydrate (ATH) filled Poly (methyl methacrylate) (PMMA) composites is affected by changes in certain parameters such as the volume fraction of and the size of the ATH particles. Although numerous studies have been conducted in this area, the effect of the microstructure is very difficult to explain because of the many factors which are involved and affect the mechanical behaviour of particle-filled composites. In this study, a systematic study, where volume fraction and particle diameter are varied, is presented. Modelling technique, both micromechanical analytical and finite element models, are used to achieve understanding of the experimentally observed mechanical behaviour of the composites. Five groups of composites were tested: Composite A: 15 μm, 34.7 vol. %, B: 8 μm, 39.4 vol. %, C: 15 μm, 39.4 vol. %, D: 25 μm, 39.4 vol. % and E: 15 μm, 44.4 vol. %. Flexural tests, uniaxial tensile tests, single edge notch bending (SENB) tests were performed at different temperatures. The elastic modulus of the composites increased as the filler content increased, whilst the change in particle diameter hardly affected the elastic modulus. Due to the competition between the reinforcing effect of the rigid particles and the weakening effect of the particle agglomeration on the composites, the increase in filler content showed little effect on the strength of the composites. On the other hand, as the particle diameter increased, the strength of the composite decreased. The fracture toughness, G_IC, of the composites is negatively correlated with the filler content, whilst increased with the increasing particle diameter. The FE simulation has great advantages comparing to micromechanical analytical models. For predicting the elastic modulus of the particle filled composites, the FE simulation can make as good prediction as the Lielens model, the prediction of which agreed well with the experimental data. The FE simulation can also predict the strength of particle-filled composites if an appropriate crack initiation stress of the matrix is used. The stress field obtained using the FE simulation led to a better understanding of how the microstructures of the composites affect their mechanical behaviour. In addition, the application of the SEM image-converted RVEs, i.e. real microstructure approach, takes into consideration the effect of particle agglomeration, which is another big improvement comparing to micromechanical analytical models.

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Thong, Aaron. "Electrical characteristics of single molecule fullerene based devices." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/49253.

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This thesis investigates the design of a unimolecular donor-acceptor system (4TPA-C60) for the purpose of developing biomimetic Turin IETS sensors. The gas phase 4TPA-C60 molecule is calculated to have a localised double-well electronic structure, which is similar to that of a nanowire resonant tunnelling device. 4TPA-C60 decorated gold surfaces are prepared from scratch, and characterised at each step. STM imaging confirms successful grafting of the molecular species to the gold surface. Scanning tunneling spectroscopy performed on these molecular double-barrier tunnel junctions show a slightly asymmetric I(V ) profile, similar to predictions from ab initio calculations. DFT calculations also reveal that the behaviour of the device is strongly dependent on the supramolecular couplings at both metal/molecule interfaces. A new phenomenon is identified, where pinning of the LUMO to the HOMO states maintains the resonant transmission channel and prevents crossing of the frontier molecular orbitals at higher biases. The HOMO-LUMO pinning effect is determined to arise from charge accumulation on the C60 cage, due to smaller coupling at the C60/metal interface. By preventing crossing of the states, HOMO-LUMO pinning delays the onset of the NDR feature, resulting in a wider current peak and lower resolution of the sensor. Based on the charge transport mechanism, several alternative systems are proposed in which HOMO-LUMO pinning can be minimized. An endohedral fullerene derivative, 4TPA-F@C60, is found to be the most promising candidate, displaying much narrower NDR peaks. These findings not only help to improve future designs of molecular Turin IETS sensors, but also contribute significantly to our understanding in the broader field of molecular devices in general.

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de, Luca Francois. "Fibre-reinforced composites with nacre-inspired interphase : a route towards high performance toughened hierarchical composites." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/49423.

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Conventional fibre-reinforced polymer composite materials are well known for their high strength, stiffness, low weight and chemical resistance but composites do fail catastrophically, in a brittle manner, with little prior warning. When a fibre breaks in tension, shear stresses transfer load previously carried by the broken fibre to neighbouring fibres through the matrix, leading to local stress concentrations. As tensile loading continues, fibre breaks accumulate in the composite, eventually leading to the formation of a critical cluster, which triggers the failure of the composite. The aim of this research was to develop a novel hierarchical composite architecture consisting of fibres decorated with a nanostructured coating embedded in a matrix. A high performance and tough nanostructured composite interphase, inspired by nacre, should provide additional toughness in tension. A Layer-by-Layer assembly method was used to assemble inorganic nanometre-wide platelets and a polyelectrolyte into a well-organised nanostructure, mimicking the “brick-and-mortar” architecture of nacre, which was developed and characterised. The nanostructure was successfully deposited around conventional reinforcing-fibres, such as carbon and glass fibres, and allowed for absorption of the energy arising from fibre breaks and substantial increase in debonding toughness in single fibre composite models. Impregnated fibre bundle composites were manufactured and tested in tension, which exhibited an increased tensile strength, strain to failure and work of fracture when the nanostructured composite interphase was incorporated. This work was part of the HiPerDuCT programme grant, collaboration between the departments of Aeronautics, Chemical Engineering, Chemistry and Mechanical Engineering of Imperial College London and the University of Bristol.

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Eriksen, Daniel. "Molecular-based approaches to modelling carbonate-reservoir fluids : electrolyte phase equilibria, and the description of the fluid-fluid interface." Thesis, Imperial College London, 2017. http://hdl.handle.net/10044/1/49242.

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In this thesis, a new approach to parameterization of the intermolecular potential models of ionic species in electrolyte solutions for the SAFT-VRE Mie theory is presented. Additionally, a predictive approach to the description of the fluid-fluid interface of non-electrolytic, non-associating mixtures is presented. These approaches are intended to support an integrated workflow for the study of the fluid systems relevant for carbon capture and sequestration. The parameterization methodology developed for the intermolecular potential models of ionic species in the SAFT-VRE Mie theory reduces the parameters to be estimated from solution data to a single interaction-energy per solvent-ion pair. This is achieved through the use of literature values for the ion-size parameter, and theoretical estimates for the ion-ion interaction energy. Additionally, the Born diameters of the ion models are taken to be those of Rashin and Honig, and not estimated from data. This approach is applied to the monovalent halides as well as select divalent ions. The resulting models reproduce the solvation energy in H2O to within 5 % error at standard conditions for the monovalent halides. Furthermore, the electrolyte models are demonstrated to provide a fair description of aqueous electrolytes when considering the limited parameterization. The predictive description of the fluid-fluid interface, is achieved by an approach in which the Square Gradient Theory (SGT) and the SAFT-VR Mie EOS are combined. The SGT influence parameter is mapped to the SAFT-VR Mie intermolecular model parameters through the relationship with the direct correlation function. The resulting model is parametrized by matching simulation data for the interfacial tension of λr-6 Mie monomeric fluids. A final evaluation of the model is carried out against non-associating systems of up to 4 species, for which predictive capabilities are demonstrated.

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Pang, Jing Sheng. "Engineered nanostructures for metal enhanced fluorescence applications in the near-infrared." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/43157.

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Recent advancements in fabrication techniques allow construction of nanostructures with well-defined features in nanometres scale. Tiny nanostructures that have features below the resolution of optical diffraction limit can now be made in the laboratory. The specific properties of those nanostructures with specific properties made from variety of materials allow us to study and explore many different properties that have never been observed while they are in bulk. One such phenomenon is localised surface plasmon resonance effect, which is exhibited by certain materials when in nanometric size. Their peculiar interaction with light is in such a way that the optical properties such as reflection and transmission deviate from typical characteristics and change according to the material involved and their shapes. Furthermore, this effect could also enhance the electric field in a specific area of the structure. This thesis is motivated by the attractiveness of the tunability of localised surface plasmon resonance and aims at exploring those properties by fabricating multiple types of nanostructures through a low-cost and versatile technique called nanosphere lithography. By improving the technique and combining with other fabrication techniques (such as oxygen plasma etching and argon ion milling), a large variety of nanostructures with hexagonal lattice like as nanocones, nanopencils, and nanofins arrays have been successfully created. Among them, three main types of nanostructure were selected for detailed study: nanotriangle, nanodisc, and nanohole-disc arrays. The distance between the adjacent nanoparticles were changed in those structures and strong interparticle coupling behaviours were observed as the distance between them becomes shorter. Current portable biosensing devices for in vitro studies are limited by the sensitivity limit of the detector, the poor quality of emitters and the size of the devices. In this thesis, the application of localised surface plasmon resonance for near infrared in vitro biosensing is explored. This is achieved through a mechanism called metal enhanced fluorescence. The techniques take advantage of the high electrical field strength and the resonance condition of the plasmon to enable a fluorophore to achieve brighter emission. The greater the resonance and electrical field are, the greater the emission amplification would be. Such effect makes it highly attractive for near infrared in vitro studies, which benefits from high optical penetration of common biology components such as water and lipids, but suffer from poor emission of existing fluorophores. Thus, enhancement of the emission signals through metal enhance fluorescence mechanism is an attractive route to obtain better signal to noise ratio in medical diagnostic, and improve detectability while at the same time reduce the need of a high sensitivity detector which can be costly and large in size. The three chosen nanostructures, i.e. nanotriangular arrays, nanodisc arrays and nanohole-disc arrays have shown marked enhancement in the emission of attached fluorophores up to 83x, 235x, and 411x respectively, making them highly attractive nanostructures for such application.

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48

Tsui, Hei Chit Leo. "Characterisation of scandium-based III-nitride thin films." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/43378.

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Wurtzite III-nitrides are widely used in optoelectronic applications. However, the external quantum efficiency of III-nitride-based light emitting diodes in the deep-UV region is extremely low compared to those emitting in the visible region. This problem has motivated the search for better materials that can help. Theoretical calculations predict that alloying ScN and GaN can produce wurtzite-structure semiconducting ScxGa1-xN films with direct band gaps in the UV region and a lattice parameter – band gap relationship that differs significantly from that of the AlxGa1-xN alloys used conventionally. Therefore, this thesis investigates the growth, composition, microstructure and optical properties of ScxGa1-xN thin films. Epitaxial ScxGa1-xN (0 ≤ x ≤ 0.5) thin films were grown using molecular beam epitaxy under metal-rich conditions. The alloy composition was determined by four different techniques. A linear relationship was established between the Sc flux measured in the growth chamber and the Sc content measured by Rutherford backscattering, whereas the compositions measured by X-ray photoelectron spectroscopy were 5–8% lower than that by Rutherford backscattering and information obtained from X-ray and electron diffraction (i.e. the a and c lattice parameters and the c/a ratio) cannot provide a reliable estimation of the Sc content. Structural analysis confirmed that ScxGa1-xN can be stabilised in the wurtzite structure up to x = 0.26 using metal-rich growth conditions, which is in line with theoretical predictions and is significantly greater than the value of x = 0.17 for the ScxGa1-xN films grown under N-rich conditions reported previously. UV-Vis absorption measurements showed that the direct optical band gap of ScxGa1-xN increases from 3.33 eV to 3.89 eV as Sc content increases from x = 0 to x = 0.26. This trend is consistent with theoretical predictions but contradicts the observations reported by other groups. Instead, nanoscale cubic inclusions, revealed by aberration-corrected scanning transmission electron microscopy, are responsible for the observed decrease in band gap as the Sc content increases. Finally, valence band offset measurements indicated that type I and type II heterojunctions can be formed by depositing ScxGa1-xN on AlN and on GaN respectively.

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49

Hill, Jonathan. "Theory and applications to elastic wave sensors for interpretation of material properties by remote sensing methods." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/43329.

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The mathematical treatment of a transient wave theory with complex structural interactions has been investigated in various elastic problems as a tool for detection and interpretation of material properties by remote sensing methods. These problems are of great importance when attempting to gain information of an enclosed material when there is no direct access, with particular significance in worldwide applications including down hole oil exploration and screening of containers. Here we concentrate on the development of a transient analysis of such elastic wave sensor problems, employing both rigorous analytical and numerical methods. The thesis begins with a model problem of a single elastic solid layer under antiplane deformation. With the availability of analytic solutions, the understanding of the transient response is aided considerably. A forcing piston theory is thus formulated by distributing the original line load formulation over part of the free surface of the layer material. From this we evaluate the transient response of the problem numerically using various receiver sensor outputs, with either the layer thickness or the density of the layer material identified. Along with providing an overview of some of the main techniques used in the later chapters, the model problem introduces an averaging method formulation that develops an approximate form of solution, which is utilised throughout the thesis. The second of these problems presented is a two-dimensional analysis of elastic solid layers enclosing a channel comprising of a stationary and compressible viscous fluid. A forcing piston of a given displacement is applied to both of the free elastic surfaces to create a transient disturbance that propagates throughout the layered structure. If the force is applied with some discrete time-signature, the receiver signal processing mechanism is the measurement of the surface displacement either away from the piston, or at the piston location itself, at a later time. A number of ancillary and mathematical tools have been developed here so that various checks on the calculations can be made. The transient response of the problem is then examined numerically in an attempt to detect variation in the material parameters of the viscous fluid channel enclosed within the elastic layers. The two problems considered later in this thesis concentrate on the separate antiplane displacements and plane strain motions of a partially filled cylinder. This annular structure may be arbitrarily filled anywhere between empty and fully filled in a simple two phase system. The sensor is modelled as a long line load which reduces the problem to a two-dimensional analysis at any cross section of the cylindrical pipe, with displacement measurements made at either the source or elsewhere along the external pipe boundary. The piston theory is again introduced to provide further insight into the response mechanism of the cylinder structure. The transient response of the problem is then evaluated numerically to allow detection and quantitative determination of some characteristic material properties, this being the depth of the partial filling material or the density of the content that is enclosed within the cylinder.

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Tarnowski, Keith. "Measuring crack initiation and growth in the presence of large strains using the potential drop technique." Thesis, Imperial College London, 2016. http://hdl.handle.net/10044/1/42986.

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Accurate laboratory measurements of crack initiation and growth are of vital importance for characterising material behaviour for use in the residual life assessment of structural components. The Potential Drop (PD) technique is one of the most common methods of performing these measurements, but such measurements are also sensitive to large inelastic strains which are often erroneously interpreted as crack growth. Despite the maturity of the PD technique, the extent of these errors is not fully understood and the most appropriate method of suppressing them is unknown. In this thesis typical errors in the measurement of crack extension due to large inelastic strains have been quantified experimentally. These errors depend on the PD configuration and in some cases the configurations recommended in the standards are susceptible to particularly large errors. Optimum configurations for common fracture specimens have been identified but despite these mitigating measures, when testing ductile materials, the errors due to strain remain large compared to other sources of error common to the PD technique. A sequentially coupled structural-electrical FE modelling approach has been developed which is capable of predicting the influence of strain on PD. This provides a powerful tool for decoupling the effects of strain from crack extension. It has been used in conjunction with experimental measurements, performed using a novel low frequency ACPD system (which behaves in a quasi-DC manner), to develop procedures for accurately measuring crack initiation and growth during fracture toughness and creep crack growth testing. It is demonstrated that some of the common methods of interpreting PD measurements during these tests are not fit for purpose. The proposed method of interpreting creep crack growth data has been used to re-validate creep crack initiation prediction models provided in the R5 assessment procedure.

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Dissertations / Theses on the topic '620.1/1'

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