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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Novel synthesis methods of carbon-nanotube-based composite materials

McCafferty, Liam January 2014 (has links)
This thesis reports novel methods for carbon nanotube synthesis, purification and decoration, which have been designed to be facile processes, with high potential for scale-up manufacture. The synthesis routes outlined compares with methodologies highlighted in the recent literature, with respect to the quality of materials produced, and ease of sample preparation with reduced processing steps. Decoration of carbon nanotubes with magnetic iron nanoparticles has been achieved using a novel synthesis route with cyclopentadienyl iron dicarbonyl dimer as a precursor to the nanoparticle formation. The thermal decomposition protocol for the material is key to the beneficial properties of the final carbon nanotube hybrid material being realised. Decomposition forms iron nanoparticles that have shown catalytic activity and absorb carbon (also provided by the compound) which is subsequently exuded as a protective graphitic shell. A wide range of synthesis temperatures have been studied, 250°C to 1200°C, and led to varying degrees of graphitization, analysis of the protective abilities of the shell to the "core" nanoparticles has been studied. Resistance to acid dissolution has been shown and a potential application for removal of organic materials (rhodamine dye) from water has also been demonstrated. The synthesis of single and few-walled carbon nanotubes of very high quality has also been shown, using the organometallic compound used to decorate CNTs, the first time this has been demonstrated for this compound. Synthesis has been carried out on silicon and quartz substrates in a photo-thermal chemical vapour deposition (PT-CVD) chamber, with minimal sample preparation required. The quality of the CNTs produced has been assessed by Raman Spectroscopy (ID/IG ratio) showing extremely high CNT quality, some of the highest reported using a CVD based technique. The defect concentration has been shown to be lower than some commercially available products. Simplification of the sample preparation required for carbon nanotube synthesis has been achieved; the compound and method used in this study has the potential for scale-up manufacture and to be a competitive high quality carbon nanotube product.
22

Examining the effects of nanomaterials in epoxy systems

Tang, Winnie January 2014 (has links)
Fibre reinforced composites have been gaining wide acceptance in a variety of applications within the transport and aerospace sectors. The performances of these composites have a tendency to suffer when the material is exposed to adverse environments for long periods of time. This research focuses on the reinforcement of existing epoxy systems used in aerospace composite with nanofillers such as polyhedral oligomeric silsesquioxane (POSS) and functionalised carbon nanotubes (CNTs). The thermo-mechanical properties, cure kinetics, moisture properties and electrical conductivity have been investigated. Two epoxy systems with different curing agents were investigated: diglycidyl ether of bisphenol A cured with an amine and an anhydride curing agent. The incorporation of an amine functionalised POSS into the epoxy-amine system and an epoxy functionalised POSS into the epoxy-anhydride were investigated. The results showed that in both systems, the addition of POSS increased the Tg, cross-link density, final char yield at 800°C, whilst reducing the maximum moisture absorption when compared to the unmodified resin. In a composite system, the addition ofPOSS displayed an improvement in the reduction of moisture absorption. The addition of carboxyl-functionalised CNTs into the epoxy-anhydride system increased the electrical conductivity reaching an optimised percolation threshold at 2 wt-% addition. Improvements in the thermo-mechanical properties and cross-link density were also observed. The addition of CNTs in a composite system also displayed an improvement in both the electrical and thermal conductivity with little changes in the Tg and degradation temperature. A combination of POSS and CNT was also incorporated into the epoxy-anhydride system and was found that at small quantities (0.5 wt-% CNT with 0.5 wt-% POSS) the electrical conductivity reached values close to samples with 1 wt-% CNT. This suggests that the addition of both POSS and CNT are complementary and beneficially increases the dispersion within the matrix. Molecular modelling was also carried out in parallel to the experimental work and it can be applied to predict the properties of epoxies. The simulations were performed on cross-linked networks of diglycidyl ether of bisphenol A (DGEBA) and a dimethyl-methylene bis( cyclohexylamine) curing agent using condensed-phased optimised molecular potentials for atomistic simulation studies (COMPASS). The Tg and the effect of water on the Tg was calculated and the results were found to be in close agreement with experimental findings.
23

Monitoring the interaction of multiple mode deterioration mechanisms in concrete

Backus, Jonathon January 2014 (has links)
The current Eurocodes (British Standards 2005b) and American Standards (ACI Committee 318 2004) stipulate single deterioration mechanism design that caters to the worst case scenario, However, in certain environments such as coastal structures and road structures where de-icing salts are used, combinations of chlorides, sulfates and carbon dioxide are possible. Thus as current codes of practice provide no guidance on how to deal with multiple modes of deterioration this is the main subject of this thesis, In this research project the effects of combined deterioration mechanisms in concrete have been measured over time, Although the effects of chloride, carbon dioxide ingress have been widely researched previously, their interactions and effect on durability of concrete has not been comprehensively covered for common cementitious materials, In this study an accelerated exposure regime was established, This allowed the interactions between carbon dioxide and chloride, and chloride and sulfate, and their effects on the concrete to be assessed. The progression was measured using the following techniques: chloride, sulfate and pH profiles, phenolphthalein indicator, Autoclaim air permeability, compressive strength and X-ray diffraction analysis, It has been shown that combined mechanisms can occur and can have a detrimental effect on the durability of concrete, Thus structures designed to the current standards may deteriorate prematurely, It is recommended that further research is carried out to check that the experimental data correlates with results for existing structures, If these trends hold true then the standards need to be updated to account for these interactions,
24

Influence of structural cracks in concrete on transport properties and chloride-induced corrosion of steel reinforcement

Wang, Junjie January 2015 (has links)
Chloride-induced corrosion of steel in reinforced concrete structures is one of the main problems affecting their durability, but most previous research projects and case studies have focused on concretes without cracks or not subjected to any structural load. Although it has been recognised that structural cracks do influence the chloride transport and chloride induced corrosion in reinforced concrete structures, there is little published work on the influence of microcracks due to service loads on these properties. Therefore, the main objective of this research was to identify to what extent microcracks due to structural loads affect the chloride transport and chloride-induced corrosion in reinforced concrete structures and thereby make recommendations for any change of service life designs of such structures. Three stages of experiments were carried out. They were the influence of stresses and microcracks on chloride migration, diffusion, sorption and chloride-induced corrosion of steel in concrete. The main findings from this project are: chloride migration coefficients of concrete changed little when the stress level was below 50% of the ultimate stress (fu). A recovery of around 50% of the increased chloride migration coefficient was found in the case of concretes subjected to 75% of the fu when the load was removed. The influence of concrete mixes on chloride diffusion coefficient of concrete could be more significant than that of stress levels when the concrete was subjected to ponding of salt water. The role of cracks was found to be more important when the concrete was subjected to 'sorption + diffusion' compared with concrete subjected to diffusion alone. These finds suggest that the effect of microcracks is very significant for the service life of reinforced concrete structures in aggressive environments. This means that for the service life design of reinforced concrete structures in chloride exposure environments, consideration should be given to microcracks at service loads.
25

Bio-sensing with gold nano-structured arrays

Hill, Breandán James January 2015 (has links)
Gold nanostructures, with exotic optical behaviours, have been shown to be suitable as high performance refractive index sensors for biochemical reactions. Extensive research has focused both on the fabrication and characterisation of the nanostructures, as well as the surface chemistries of the sensor. Here we present a novel biosensor design, based on the excitation of localised surface plasmon resonances (LSPR) in gold nanowire arrays. Nanostructures arrays are produced by the electrode position of gold into self-assembled porous aluminium oxide templates. Nanostructure growth is precisely monitored by in-situ optical measurements to provide fine tuning of the optical behaviour. This fabrication technique is easily repeatable and scalable. Nanostructure arrays are characterised using high resolution imaging and theoretical models are developed to help understand the rich optical behaviour. Finite element modelling carried out using COMSOL is used to investigate fluid behaviour and localised electric fields in the array. A Retarded Dipole Interaction Model (RDIM) and a Maxwell-Garnett effective medium theory are then used to determine the underlying mechanisms behind the optical behaviour and also to predict future behaviours. These models show good agreement with experimental results. Nanowire arrays are prepared for biochemical studies by the growth self-assembled monolayers (SAMs) on the gold surface. Various monolayers have been investigated to develop a non-fouling surface including simple alkanethiols, synthesised peptides and PEG-thiol derivatives. Activated chips are then integrated into a cuvette based prototype biosensor. This prototype has been used to investigate various binding events e.g. antigen/antibody. Various proteins have been immobilised on the sensor surface using NHS/EDC chemistry and the binding kinetics determined. Binding coefficients calculated have been shown to be comparable with current commercial biosensors, with a simplified experimental setup and a significantly lower cost.
26

Electrical control of bacterial adherence to conducting polymers

Freebairn, David Alexander January 2016 (has links)
In this thesis, low direct currents (DC) have been shown to successfully reduce bacterial adherence to conducting polymers in unique electrically modified flow devices without the aid of an antimicrobial agent. However, alternating currents (AC) and radio-frequency currents (RF) were not found to be effective. The design and manufacture of these new DC and RF flow devices has been comprehensively documented within, accompanied by relevant standard operating procedures and experimental designs. Additionally, the thesis includes a review of seminal bioelectric literature and discusses the potential for exciting future developments in this multidisciplinary field of research. These findings are ultimately intended to facilitate the design of new indwelling medical devices (IMDs) as well as electrically sterilized polymer surfaces for a wide range of far-reaching applications in industries where bacterial biofilms proliferate
27

The effect of Maleic Anhydride-grafted-high density polyethylene combatibilisers on the structure and properties of high density polyethylene/montmorillonite nanocomposities

Lutton, R. January 2014 (has links)
Nanocomposites have received a great deal of attention in recent years; particularly polymer layered silicate nanocomposites such as montmorillonite, where the surface to volume ratio is exceptionally high. As a result of their ultrafine phase dimensions, these layered silicate nanocomposites possess unique properties typically not shared by more conventional microcomposites and, therefore, offer new technology and business opportunities. However, despite the attention, the influence of compatibilisers on the nanocomposites is not well understood; more critically, the literature exposes a large degree of disagreement as to the required compatibiliser type for optimal function. This study examines the effect of Maleic Anhydride grafted HDPE compatibilisers, with differing MFls and grafting contents, on the mechanical, thermal an.d rheological properties of HDPE. The materials were compounded in a twin screw extruder before being compression moulded into plaques for characterisation and analysis; this consisted of structural analysis utilising XRD and tensile testing; thermal and rheological characterisation using power compensated DSC, DMTA and a rotational rheometer. The results confirm organo-modification of MMT as sufficient in producing a miscible polymer system; however addition of compatibiliser is best. In an unfilled system, the incorporation of MAH groups cause axis distortion and rearrangement of chain conformation and lattice structure, crosslin king and increase RAF. With the addition of MMT, the MAH groups preferentially interact with the clay changing the crystal type to transcrystalline layers, lowering the Tg whilst simultaneously increasing the modulus. The most effective compatibilisers have a characteristic enabling a more dominant exfoliation mechanism. Intercalated nanocomposites presented the more improved systems, with 4:1 and 5:1, plus 5% clay, being the optimal due to crystallisation between layers. Changes in MFI and grafting are a complicated matrix but higher mobility and grafting is preferred: as opportunity for interaction increases, the likelihood of formation of nanocomposites, and improvement in material properties, increases.
28

Modelling, simulation and experimental investigation of the effects of material microstructure on the micro-endmiling process

Elkaseer, Ahmed Abd-Elrahman January 2011 (has links)
Recently it has been revealed that workpiece microstructure has dominant effects on the performance of the micro-machining process. However, so far, there has been no detailed study of these effects on micro-endmilling. In this research, the influence of the microstructure on the matters such as cutting regime, tool wear and surface quality has been investigated. Initially, an experimental investigation has been carried out to identify the machining response of materials metallurgically and mechanically modified at the micro-scale. Tests have been conducted that involved micro-milling slots in coarse-grained (CG) Cu99.9E with an average grain size of 30 μm and ultrafine-grained (UFG) Cu99.9E with an average grain size of 200 nm. Then, a method of assessing the homogeneity of the material microstructure has been proposed based on Atomic Force Microscope (AFM) measurements of the coefficient of friction at the atomic scale, enabling a comparative evaluation of the modified microstructures. The investigation has shown that, by refining the material microstructure, the minimum chip thickness can be reduced and a better surface finish can be achieved. Also, the homogeneity of the microstructure can be improved which in turn reduces surface defects. Furthermore, a new model to simulate the surface generation process during micro- endmilling of dual-phase materials has been developed. The proposed model considers the effects of the following factors: the geometry of the cutting tool, the feed rate, and the workpiece microstructure. In particular, variations of the minimum chip thickness at phase boundaries are considered by feeding maps of the microstructure into the model. Thus, the model takes into account these variations that alter the machining mechanism from a proper cutting to ploughing and vice versa, and are the main cause of micro-burr formation. By applying the proposed model it is possible to estimate more accurately the resulting roughness owing to the dominance of the micro-burrs formation during the surface generation process in micro-milling of multi-phase materials. The model has been experimentally validated by machining two different samples of dual-phase steel, AISI 1040 and AISI 8620, under a range of chip-loads. The results have shown that the proposed model accurately predicts the roughness of the machined surfaces with average errors of 14.5% and 17.4% for the AISI 1040 and AISI 8620 samples, respectively. The developed model successfully elucidates the mechanism of micro-burr formation at the phase boundaries, and quantitatively describes its contributions to the resulting surface roughness after micro-endmilling. (Abstract shortened by UMI.).
29

Large-eddy simulation of wall-bounded flows subjected to curvature and rotation

Guleren, Kursad Melih January 2007 (has links)
This PhD thesis considers the analysis and the interpretation of the complex turbulent flows subjected to curvature and rotation effects. To achieve this goal, largeeddy simulation (LES) is performed for various wall-bounded flow problems. For the validation and verification purposes, the adopted finite-volume code is tested by considering fully developed channel and duct flow problems. The behaviour of the subgrid-scale (SOS) models and the spatial schemes are investigated in detail for the channel and duct flows subjected to orthogonal rotation. Among the tested SOS models and the spatial schemes, the Wall-Adapting Local Eddy Viscosity (WALE) model and the bounded central differencing (BCD) scheme are found to perform the best. During the validation and verification processes, the turbulence mechanism in the channel and duct flows for various rotation rates are reviewed and the laminarization process due to Coriolis force is revealed by considering a wide range of data processing. Using the experience gained from the rotating channel and duct flow cases, more challenging flow cases are considered. The flow in the square-sectioned U-duct and the centrifugal compressor are simulated with LES at high Reynolds numbers. Predictions are extensively validated for both flow problems with the available experimental data. Grid convergence and appropriate near-wall resolutions are provided in order to avoid errors associated with the filter width and the wall functions. For both flow problems, Reynolds-averaged Navier-Stokes (RANS) results are included to determine the impact level of LES. Upon encouraging results obtained via LES, the effects of strong curvature and Coriolis forces are explored on mean, secondary flows and turbulence.
30

International competitiveness in the advanced materials sector : the case of carbon fibre

Russell, Celia January 1996 (has links)
Carbon fibre is the most commercially significant of the advanced materials, and its development has been driven by both civil and defence interests. With the end of the Cold War, demand from the defence sector virtually collapsed. The data presented in this thesis reveal the consequent global restructuring of the industry. Over the five year period 1990-1995, European market share fell over twenty percentage points, while that of Japan increased markedly. Meanwhile, US production levels faltered and then recovered following government intervention to stabilise this dual-use technology. This thesis examines the subsequent international shift in the location and ownership of carbon fibre production capacity and the variation in corporate response over this turbulent time. It is found that the national business systems in which this particular high technology sector operates have played a fundamental role in shaping the eventual competitive structure of the industry.

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