• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 2227
  • 1008
  • 1006
  • 484
  • 3
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 9884
  • 3397
  • 1738
  • 1413
  • 1409
  • 1409
  • 1065
  • 663
  • 516
  • 284
  • 277
  • 273
  • 255
  • 246
  • 232
  • 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.
141

Neutron reflectometry studies of conducting polymer films

Beebee, Charlotte Anne Stillwell January 2015 (has links)
Within the field of conducting polymers, it is well understood that the variables in film formation, ranging from the polymerisation and deposition methodologies to the supporting electrolyte in the deposition solution, can produce a wide range of different electrochemical and structural properties for a given polymeric species. For the most part, studies into these effects rely on spectroscopic, electrochemical and surface characterisation techniques. In this work, neutron reflectometry is used to identify the extent of solvent content and spatial distribution within polymer films, providing insight into the internal structure. In conjunction with electrochemical methods, two types of variables are explored: the addition of carbon composites and the electrochemical control function used for polymerisation/deposition. The effect of the incorporation of carbon nanoparticles is explored in the case of composite poly(3,4-ethylenedioxythiophene)-nanodiamond films. These are revealed to be similarly electroactive to pristine poly(3,4-ethylenedioxythiophene) films but present a more ordered internal structure. The effect of the electrodeposition method is investigated by comparing films produced by potentiodynamic, potentiostatic and galvanostatic deposition in the case of two different polymers, polypyrrole and poly(3,4-ethylenedioxythiophene). The results show that the polymerisation efficiency for a given growth protocol differs depending on the polymer species, as do the structural variations, such as porosity or structural stability following charge/discharge processes.
142

Type III Deep Eutectic Solvents (DESS) as base lubricants

Ahmed, Essa Ismaeil January 2015 (has links)
Mineral oils are the most commonly used fraction of formulated lubricating oils except for some specialised applications where synthetic oils are employed. An alternative approach has been suggested using ionic liquids (ILs) due to their high viscosity index and high thermal stability. The aim of this study is to examine the use a different type of ionic liquids in the form of Deep Eutectic Solvents (DESs) as base lubricants as these have significantly improved environmental credentials. The first stage of the study involved the investigation of thermo-physical properties such as heat capacity, thermal stability, surface tension, viscosity index, melting point, conductivity and density to determine if the liquids are suitable fluids as lubricants. The data for a variety of imidazolium based ILs and standard mineral base oil were also determined and used for comparison sake. In addition the properties of DES mixtures with water were determined and self-diffusivity were measured using NMR spectroscopy. It was shown for the first time that aqueous DES mixtures are not homogeneous but instead they form bicontinuous micro-emulsions. This study has also been the first to quantify the corrosion rates of metals in DESs and ionic liquids. The corrosion of iron, aluminium and nickel was studied in four DESs and four ILs using both Tafel slope analysis and electrochemical impedance spectroscopy. The corrosion rate was found to change over time for some liquids and so the corrosion product films were characterised using Raman spectroscopy. The interfacial properties of DES and ILs are shown to be totally different from mineral base oil and the wettability in terms of contact angle and interfacial energies for various metals have been studied. The friction coefficient and wear volume were measured for DESs for dissimilar sliding couples. Finally, the change of both thermo-physical and mechanical properties due to the inclusion of two common surfactants sodium dodecylsulfate and cetyltrmethylammonium bromide are characterised in three DESs and shown to decrease the wear volume.
143

Improving PTR-ToF-MS : implementation of a radio frequency ion funnel and an investigation into buffer-gas doping

Barber, Shane Brian January 2015 (has links)
Volatile organic compounds (VOCs) are ubiquitous in the Earth’s atmosphere. VOCs are produced from biogenic sources such as forests, or anthropogenic sources such as fossil fuel combustion. Many areas of research involve measuring VOCs, from atmospheric science, to medical science and homeland security with implications for health, the environment, and safety. It is crucial that VOCs are detected quickly and with a high sensitivity: proton transfer reaction - mass spectrometry (PTR-MS) offers a solution and potential enhancements of the PTR-MS technique are discussed here. A drift tube capable of simultaneously functioning as an ion funnel is demonstrated in PTR-MS for the first time enabling a much higher proportion of ions to exit the drift tube and enter the mass spectrometer than would otherwise be the case. An increase in the detection sensitivity for VOCs of up to two orders of magnitude and an increase in Limit of Detection of one order of magnitude is delivered, allowing lower concentrations of VOCs to be detected. An alternate way to change how the drift tube behaves is to alter the buffer gas. The collision energy within the drift tube is investigated in order to ascertain what advantages changing the buffer gas from nitrogen to argon yields with respect to sensitivity and fragmentation of analytes. For several compounds, the sensitivity is increased and fragmentation reduced. If sensitivity can be increased and/or fragmentation reduced within a complex mixture of analyte ions, then analysis of these mixtures will be simplified. Finally, a standard PTR-MS instrument is compared with the ion funnel equipped PTR-MS instrument in an urban, megacity (ClearfLo campaign, London, UK) in order to test the instrument and enhanced sensitivity for field measurements.
144

Unsymmetrical, monoanionic pincer ligands for titanium and palladium

Wright, Luka January 2015 (has links)
In this thesis, the synthesis, characterisation and coordination chemistry of a series of related [N,NPyN] and [N,NPy O] monoanionic pincer ligands, together with some reaction chemistry of the metal complexes is described. In Chapters 2 and 3, the synthesis and characterisation of novel [N,NPyO] and [N,NPyN] pro-ligands and reaction chemistry with palladium(II) starting materials to form square planar complexes of the type [(N,NPyX)PdL] (X = O, NH, L = OAc, Cl, I, MeCN, Py, Ph) is described. The pro-ligands and the metal complexes have been characterised by a combination of multinuclear NMR spectroscopic techniques, IR spectroscopy, mass spectrometry and, for selected examples, by single crystal X-ray crystallography; salient spectroscopic and structural data are discussed. The stoichiometric reactivity of the [(N,NPyO)PdCl] and [(N,NPyN)PdPh] species towards [p -Tol2I][OTf] and the electrophilic fluorinating agents Selectfluor and XeF2 are disclosed. Thereby, an assessment of the ability of these pincer ligands to support transient high-valent palladium(III) and palladium(IV) complexes has been made. It is established that the N-(4-i-PrC6H4) substituted [(N,NPyO)PdCl] complexes are able to facilitate C-Cl bond forming reactions with greater conversions than their N-(2,6-i-PrC6H3) substituted analogues, and that the reaction of [(N,NPyN)PdPh] [Ar = (4-i-PrC6H4)] with Selectfluor and XeF2 yields biphenyl via complex reaction pathways. In Chapter 4, the use of the same pro-ligands in the synthesis of relatively rare examples of titanium(IV) complexes containing tridentate ligands, [(N,NPyO)TiX3] (X = F, Cl), is described. These complexes have been characterised by multinuclear NMR techniques, mass spectrometry and, for selected examples, by single crystal X-ray crystallography. Investigations reveal that both classes of titanium(IV) complexes are suitable for use as ethylene polymerisation pre-catalysts in the presence of the co-catalyst, MAO. The activity of chloride-containing catalysts is higher than their fluoride-containing analogues. However, the catalysts are highly ligand-dependent; an activity of 340 g(PE) mmol-1 h-1 bar-1 is obtained for an [(N,NPyO)TiF3] complex supported by a sterically bulky tridentate ligand system and these systems are the most active ethylene polymerisation catalysts known which comprise a Ti-F bond.
145

Electrochemical concentration gradients in deep eutectic solvents

Zaleski, Christopher John January 2015 (has links)
Concentration gradients present in the solution during the redox chemistry of selected metals and conducting polymer (poly(3,4ethylenedioxythiophene) (PEDOT)) films redox cycled in Deep Eutectic Solvents (DES) were observed for the first time through the application of the Probe Beam Deflection (PBD) technique combined with the Electrochemical Quartz Crystal Microbalance (EQCM). Suitability of choline chloride (ChCl) based DES for applications as electrolytes in PEDOT based charge storage devices has also been investigated using EQCM. The combination of carefully optimized experimental parameters (temporally extended chronoamperometry and slow scan rate voltammetry) with modified design of the instrument (reduced probe’s distance of approach) allowed for in-situ observations of electrochemically induced concentration gradients in DES based systems. During the studies of electroactive polymer films, complete determination of mobile species transfers in PEDOT/Ethaline and PEDOT/Propaline systems has been achieved. The application of PBD-EQCM technique in studies of metal electrodeposition from DES allowed for monitoring metal speciation in dynamic and quantitative fashion. EQCM study of ChCl based DES indicated Ethaline as the most promising potential electrolyte for PEDOT based charge storage devices. Additionally, an unusual mass exchange process has been detected in PEDOT/Propaline and PEDOT/Acetaline processes. This work has shown a novel, affordable and non-invasive route for observation of electrode/electrolyte interface processes in DES. The experimental protocol developed can potentially be implemented in further studies of DES as well as Ionic Liquids.
146

Electrochemistry and galvanic deposition of group 10 & 11 transition metals using deep eutectic solvents

Forrest, Gregory Charles Hereward January 2015 (has links)
The galvanic deposition of metals has wide ranging applications in many industries. One example is the printed circuit board industry, where galvanic coatings are used as both a catalyst for the activation of the surface of a substrate and protective coating to prevent the oxidation of the surface. Deep Eutectic Solvents (DESs) have received growing interest in the area of metal processing, due to the unique solvation properties and ability to dissolve metal salts, without the need for addition of acids or bases. The DESs have been widely applied in electrochemical processes, where electrochemistry can be performed without the need for addition of an electrolyte. In this work, the speciation of the group 10 and 11 metals has been characterised using UV – Vis spectroscopy. The electrochemistry of the group 10 and 11 elements has been characterised using cyclic and linear sweep voltammetry and related to the speciation of the metal centres. Further characterisation of the electrochemistry was performed using an electrochemical quartz crystal microbalance, allowing for the rate and current efficiency of the electrochemical reaction to be measured. Galvanically deposited coating of group 10 and 11 metals, were produced on a standardised copper substrate. The morphology of the coatings was characterised by atomic force (AFM) and scanning electron microscopy (SEM) and the composition by energy dispersive X – ray (EDX) and X – ray photoelectron spectroscopy (XPS). The mechanism by which the galvanic coatings are formed was probed. The reaction mechanism was predicted from the formal electrode potential of the metal redox couples. The predicted mechanisms were experimentally investigated using quartz crystal microbalance experiments. The quartz crystal microbalance also allowed for the determination of the rate of the galvanic reactions.
147

Physical and chemical investigations of starch based bio-plastics

Abolibda, Tariq Ziyad Y. January 2015 (has links)
Starch is one of the most common and easily obtained natural polymers, making it attractive as a potential bio-based alternative to synthetic polymers. The plasticisation of starch is complex due to the extensive hydrogen bonding between chains. This study shows that a simple quaternary ammonium salt combined with hydrogen bond donor (HBD) forms effective modifiers that produce flexible plastics with good mechanical properties that are comparable to some polyolefin plastics. Starch-based plastics can be formed by the same processes as current commercial plastics, giving similar mechanical strength to some polyolefin plastics. The processing conditions are shown to significantly affect the structure of the polymer which has a concomitant effect upon the mechanical and physical properties of the resulting plastic. Using a glycerol based modifier results in a totally sustainable and biodegradable material which can be formed by extrusion, pressing, vacuum forming and injection moulding. Most significantly, it is shown that these plastics are environmentally compatible, recyclable, bio-degradable and compostable. This study has demonstrated the optimisation of the parameters for a range of techniques that are currently used in processing this type of plastic. These include: temperature profiling for both compression moulding and extrusion, residence time in the extruder, drying time and pressing time. Glycerol/choline chloride is the plasticiser that was found to give optimal properties and has been used for most of this study. The optimum ratio is shown to be 1:3 by weight plasticiser: starch. In addition, five different systems of fillers have been added to the starch based plastics to study their effect on tensile strength and strain, glass transition temperature, viscoelasticity, crystallinity, morphology and rheology. The chemical changes have also been investigated after the addition of the five fillers which are eggshell, wood-flour, silk, zein and lignin. Furthermore, the fillers have an effect on stabilising water content in thermoplastic starch. Water uptake and water loss have been examined using thermogravimetric analysis, water absorption isotherms, water absorption capacity and contact angle. Finally, some applications have been presented for protecting these plastics from surrounding moisture.
148

Characterisation of FAGE apparatus for HOx detection and application in an environmental chamber

Winiberg, Frank Alexander Frederick January 2014 (has links)
The hydroxyl radical, OH, and the hydroperoxy radical, HO2 (known collectively as HOx), play a key role in tropospheric chemistry and are intricately related to chemical cycles that control the concentration of greenhouse gases and have important implications for air quality. Through accurate measurements of these two important radicals, and thorough investigation of the chemical mechanisms that control their formation and removal, we can develop a better understanding of atmosphere. Simulation chambers offer the unique ability to study these processes under atmospherically relevant conditions, using a wide variety of instrumentation to probe many different species. The Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC) is a stainless steel chamber based at the University of Leeds and was previously designed to operate over a range of temperatures and pressures. HIRAC was implemented to validate important oxidation mechanisms of volatile organic compounds, furthering mechanism databases, such as the Master Chemical Mechanism (MCM). This thesis concentrates on the continued development of a dedicated HOx radical detection instrument, based on laser induced fluorescence spectroscopy at low pressure (fluorescence assay by gas expansion (FAGE)), for use in an atmospheric simulation chamber. In the field, FAGE instruments are designed to operate on board aircraft, which subject the instrument to a range of external operating pressures. Thorough characterisation and calibration of the FAGE instrument was performed using traditional methods, accounting for several factors known to affect instrument sensitivity. This calibration procedure was successfully validated using two newly developed calibration methods for OH and HO2, which take advantage of the HIRAC chamber and its ability to operate over a range of temperatures and pressures. After thorough calibration, the instrument was implemented in the investigation of direct OH radical production from the reaction of HO2 with acetylperoxy radicals in the HIRAC chamber. Reactions of RO2 radicals with HO2 have previously been thought to be a radical sink in atmospherically pristine environments (i.e., low NOx). However, more recently, higher than anticipated concentrations of OH have been observed in areas where biogenic loadings are high. Recycling of OH from reactions of RO2 with HO2 could provide part of the current mechanism shortfall. Acetyl peroxy radicals are of particular importance as they are formed directly from the oxidation of MVK, a major product of isoprene oxidation. Reported here is the first study sensitive to products from all three branching pathways of the reaction.
149

Synthesis of novel functionalised β-ketoiminate and β-diketonate metal complexes for their use in anti-cancer treatment

Lord, Rianne Michaela January 2014 (has links)
This thesis is concerned with the synthesis, characterisation and evaluation of novel metal complexes for their application as anti-cancer agents. It contains the in vitro cell results, along with a range of other techniques to determine their biological relevance and their potential as anti-cancer agents. Chapter 1 contains an introduction to the project including a literature search, previously synthesised complexes and project aims Chapter 2 presents the synthesis and characterisation of novel β-diketonate and β-ketoiminate ligands. X-ray crystallographic data are analysed for some of the ligands. Chapter 3 discusses the synthesis and characterisation of novel β-ketoiminate ruthenium chloride complexes. X-ray crystallographic data are analysed for all of the complexes. Chapter 4 introduces the MTT technique for assessing cytotoxicity, and presents in vitro activities for the library of complexes synthesised in Chapter 3. Chapter 5 looks at modifications of the previous ruthenium (II) complexes, introducing new ligands and iridum metal centres. X-ray crystallographic data for all of these complexes has been discussed, along with in vitro activity against a range of cell lines. Chapter 6 introduces hypoxia and states the cytotoxicities of a range of complexes under 1.0% and 0.1% oxygen concentrations. Chapter 7 discusses mechanistic studies on the complexes, including hydrolysis, hydrophobicity, Comet assay, apoptosis and thioredoxin reductase inhibition. Chapter 8 introduces the previous group IV work within the group and an extension of the library by synthesis of β-ketoiminate titanium complexes. X-ray crystallographic analysis is discussed where applicable. Chapter 9 contains experimental details and characterisation data for all compounds described in Chapters 2, 3, 5 and 8. Also protocols for all the biological studies. Appendix presents a summary of X-ray crystallographic structure analysis for any crystals obtained within this work.
150

Oxidative gold catalysis

Ball, Liam Thomas January 2014 (has links)
The utility of aryltrimethylsilanes 1 as coupling partners in oxidative gold catalysis has been established through the development of first- and second-generation methodologies for alkene oxyarylation, and through the realisation of a protocol for the direct arylation of simple arenes. Oxidative oxyarylation of mono-substituted olefins 2 is achieved in the presence of a commercially-available precatalyst, Ph3P AuCl and a fluoronium oxidant, Selectfluor. The net addition of an O-nuc leophile and a silane-derived aryl moiety across the C=C double bond affords the conesponding homo benzylic alcohol derivative 3 with complete regioselectivity. Although diverse O-nucleophiles and arylating agents are tolerated, the substrate-scope is limited to alkenes bearing a single alkyl substituent due to the high reactivity of the oxidant. Relative to literature procedures employing arylboronic acids, the use of arylsilanes 1 significantly reduces the generation of symmetrical biaryls 4 via oxidative homocoupling of the arylating agent. Limitations imposed upon the first-generation protocol by Selectfluor prompted an extensive screen of reaction conditions that led to identification of l -hydroxy-I,2-benziodoxol-3-(lH)-one (IBA) as an efficient terminal oxidant for gold-catalysed oxyarylation. Use of IBA expands the substrate scope to include styrenes 5, thereby providing access to bibenzyls 6, and gem-disubstituted olefins 7, which afford tertiary homobenzylic alcohol derivatives 8. Whilst the range of suitable O-nucleophiles is largely resh'icted to methanol, sterically- and electronically-diverse arylsilanes 1 can be employed. Use of benzotrifluoride as the reaction solvent substantially reduces the generation of alylsilane homocoupling side-products 4.

Page generated in 0.0238 seconds