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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.
11

The preparation and characterisation of mixed-anion and non-oxide materials

Headspith, David Andrew January 2009 (has links)
Traditionally, research in solid-state chemistry has focused largely on the chemistry of oxides and on chemical tailoring of the structure and physical properties via cationic substitutions. Consequentially, the chemistry of non-oxide ompounds and other means of chemical tailoring, such as anionic substitutions, have been comparatively overlooked by the scientific community. Non-oxides offer a wide diversity of chemistry, most noticeable in unusual oxidation states and coordination geometries found for metals in these compounds. Furthermore, the development of anionic substitutions could open up an alternative avenue for the modification of the structure and properties in solids and the preparation of novel compounds. The work reported here covers: (1) the preparation of non-oxide compounds (Ba2CoS3, Ba2MnS3, Ce2MnN3, Ce2-xLaxMnN3) and mixed-anion compounds (Ce2MnN3F2-δ, apatite oxide-nitrides) via direct synthesis and/or cationic and anionic substitutions; (2) the characterisation of their structure and (3) the characterisation of selected physical properties. The three-dimensional magnetic cell of Ba2CoS3, which undergoes a transition to long-range order at 46 K, was found to be double the size of the crystallographic unit cell, along the c-axis. A conclusive representation of the magnetic cell of Ba2MnS3, with similar structure to Ba2CoS3, could not be achieved and two possible models are proposed. A one-step synthetic route for Ce2MnN3, more convenient than the route reported in the literature, was developed in this work. Cationic substitutions led to the preparation of the solid solution Ce2-xLaxMnN3 and anionic manipulation of the lattice, via fluorination, led to the preparation of the first example of a quaternary nitride-fluoride Ce2MnN3F2-δ. The structure of Ce2MnN3F2-δshowed layers of distorted MnN5F octahedra and staged fluorine occupancy of the interstitial sites. A range of novel oxide-nitrides were prepared via reaction of the apatite-type oxides La9.33Si6O26, La8+xSr2−xM6O26+x/2 (M = Si, Ge; 0 less than or equal to x less than or equal to 2), La9.67Si5CoO26 and La10M5CoO26.5 (M = Si, Ge) with gaseous ammonia at temperatures above 700°C, most retaining the apatite structure with nitrogen located in interstitial sites.
12

Integrated DNA extraction and amplification on a microfluidic device

Shaw, Kirsty Jane January 2009 (has links)
An evaluation of DNA extraction and amplification performed in microfluidic systems was carried out, with the aim of integrating the two processes in a single microfluidic device. This integrated device will then be incorporated upstream of capillary gel electrophoresis and fluorescence-based detection for development of a completely integrated genetic analysis system. DNA extraction was performed using a silica substrate with both hydrodynamic and electro-osmotic pumping (EOP), resulting in maximum DNA extraction efficiencies of 82% and 52% respectively under optimised conditions. While the DNA extraction efficiency was lower using EOP, this method eliminates the need for external pumps and ensures easier mechanical connection to the microfluidic device. The use of thermally activated silica monoliths as the solid-phase resulted in superior DNA extraction efficiencies compared to when photo-initiated monoliths and silica beads were used. DNA amplification of up to nine forensically relevant loci was successfully achieved on the microfluidic device in volumes as low as 1.1 microlitres using Peltier heating. A combination of silanisation and dynamic passivation was required to prevent PCR inhibition resulting from DNA polymerase adsorption. A custom-built microwave heating system was also evaluated, which was capable of heating and cooling rates of 65degC/second and 58degC/second, respectively. EOP was used in the generation of an integrated microfluidic device, for DNA extraction and amplification. The silica monolith used as the solid-phase for DNA extraction also acted as a pump for electrokinetic movement. All necessary reagents for carrying out both DNA extraction and amplification were encapsulated in agarose gel and pre-loaded onto the microfluidic device creating a self-contained, ready-to-use system. Following addition of the biological sample to the microfluidic device, all electrokinetic movement and thermal cycling was controlled using a custom-built operating system.
13

Nematic liquid crystals for nano-structured organic photovoltaic

Alkhalifah, Manea S. January 2010 (has links)
A range of novel liquid crystals and amorphous organic conjugated semiconductors were synthesised by the chemistry branch of the Organophotonics group at Hull University. These compounds are studied electrochemically and optically to investigate the suitability of these materials as good donor/acceptor with suitable pairs energy levels for use in organic photovoltaic devices. Liquid crystal compounds with a fluorene-thiophene structure were identified as potential electron donors in combination with perylene based compounds as electron acceptors. Time-of-flight was used to study the charge transport of organic semiconductors in this thesis. The nature of the functional groups of the molecules was found to have a significant influence on the charge carrier mobility. The incorporation of a reactive end group with spacer affected the charge carrier mobility of electron donors negatively, suggesting that the mobility depends on the intermolecular separation. Based on the need to correlate the charge transport of the donor/acceptor blends with photovoltaic devices, the electron and hole mobility were studied for blends. For all blends the hole mobility is lower than that of the pure electron donor. The electron mobility of the blends is much higher than that of the pure electron acceptor. The thermal activation of charge transport in the liquid crystals is investigated by applying the Gill model. The result shows that charge transport in the liquid crystals is thermally activated and the activation energy is field dependent. The Gaussian disorder model and correlated disorder model were used to analyze the mobility data of four liquid crystals compounds with the same conjugated core and different end groups. We show that the thin film nanoscale morphology and the phase separation of the donors/acceptors blends depend on the chemical structures of donors and acceptors, the casting solvents and the annealing temperature of the film. The functional groups of the perylene bisimide are found to influence the roughness. The surface roughness of the blended thin film is minimum and its phase separation finest when the electron donors component has short terminal aliphatic groups rather than long polymerisable chains. Chlorobenzene shows the best performance as coasting solvent. The annealing temperature is significant in controlling the nanoscale morphology and the phase separation of an intermixed network of the blends. We successfully demonstrate photovoltaic performance using blends of our novel donors and acceptors. The annealing temperature is very important to optimise the solar cell performance by optimisation of the phase separation. The perylene based liquid crystals have disappointing performance as electron acceptors. The donor with the shortest terminal end group gives the best result. The device performance fully correlates with the blend nanoscale morphology of the blends; the blend with the smallest domains gives best power conversion efficiency; the best device has a value of 1.1%.
14

Electro-catalytic reactions

Lledo-Fernandez, Carlos January 2009 (has links)
This thesis discusses and demonstrates the utility and advantages of redox catalytic reactions and small scale synthetic applications. In particular electro-catalytic reactions are investigated along with electrogenerated chemiluminescence (ECL). ECL is used both as a probe for investigating the redox reactions and as an analytical method. The first chapter reviews the current techniques and achievements found in the area of dynamic electrochemistry and electrogenerated chemiluminescence and discusses occurrence, mechanism and potentially useful reactions worthy of further investigation for analytical applications. In Chapter 3 the electrocatalytic ECL reaction of Ru(bpy)32+ with tertiary amines in particular with codeine as a model compound was investigated. The aim of the work was to develop a portable drug testing device, that gave good sensitivity and reproducibility. The Ru(bpy)32+ was immobilized within the sol-gel matrix to develop a sensor which could be used directly for measuring tertiary amine containing drugs in buffer solution without the need to add reagents. A system of three electrodes was used in which Ru(bpy)32+ was immobilised on to a glassy carbon working electrode. Initial work involved physical entrapment of the Ru(bpy)32+ within the sol-gel matrix however, problems occurred with leaching of the reagent from this matrix. This problem was overcome by the covalent attachment of a novel Ru(bpy)32+ derivative to the sol-gel matrix. Using this approach a calibration was obtained using ECL for the determination of codeine over the range 1E-3 to 1E-7 M in aqueous buffer with a limit of detection of 2.65E-6 M for codeine. Covalent attachment, as compared to the physical entrapment of CL reagent, was advantageous as it ensured homogeneous distribution of the reagent within the matrix and prevented leaching. This reduced analysis costs, extended sensor lifetime and gave a reproducible analyte responses. This method was, however, only suitable for drugs that were soluble in aqueous solution and an alternative approach was needed for insoluble compounds. Chapter 4 describes an alternative approach for ECL reaction of tris(2,2’-bipyridyl)ruthenium(II) with tertiary amine compounds. In this approach the electrode was modified using microdroplets of a highly hydrophobic tertiary amine (trioctyamine). As well as allowing for the analysis of compounds that were insoluble in aqueous solution, this setup allowed the investigation of the electron transfer reaction occurring on liquid|liquid interfaces. This mechanism was studied in fully protonated and deprotonated conditions. The extent of the electrochemiluminescence production was shown to be dependent on the degree of the interfacial protonation. Moreover, the data obtained enabled the estimation of the biphasic pKa, which was found to be approximately 10.8. Furthermore, the mechanism was studied in fully deuterated and dedeuterated conditions, in order to investigate the effect of the deuterium on the biphasic pKa. Results suggest that the pKa increases to 13.18 when deuterium is used instead of protons. In Chapter 5 a further electrocatalytic reaction was investigated. The reaction was that of vitamin B12a with trans 1,2-dibromocyclohexane (DBCH) in a homogeneous dimethylformamide media. The reaction was studied by cyclic voltammetry. Four peaks were seen due to the two chemically reversible redox (two electron process) couples for vitamin B12a/B12r (Co(III)/CO(II)L) and B12r/B12s (Co(II)L/Co(I)). When the bulk electrocatalytic reaction was tried in a “one pot” system, the reduction could not, however, be achieved; the cathodically synthesised Co(I)L was thought to be reoxidised at the anode. A “two pot” system did not have sufficient potential control and, therefore, chemical reduction was investigated instead. Four reducing agents (Na/amalgam, NaBH4/NaOH, DL-cysteine/alkali solution and Zn dust/NH4CL) were studied to reduce B12 to B12s for a simple biphasic batch reaction of vitamin B12s with DBCH. The mild reducing agent Na/amalgam was not successful but the other three methods were shown to give 100% yield if the reaction vials were rigorously shaken. This simple type of green, surfactant free reaction has not been previously reported. The reaction was then investigated in a microfluidic system. For this work the vitamin B12 was reduced before being introduced into the microfluidic device. NaBH4/NaOH and DL-cysteine/alkali solution were selected as the most compatible reducing agents for the microfluidic device. Two types of microfluidic device were employed, one with a T-shape channel, and one with a serpentine channel. The conversions obtained with the microfluidic device were much lower (approximately 10%) than for the simple batch reactions (100%). The yield increased as the flow rate decreased, and the residence time increased. Using the serpentine channel made no noticeable difference to the conversion rate. Problems were also seen due to the use of excess reducing agent prior to introduction of the reduced vitamin B12 into the microfluidic device, as this could cause blockages and bubbles. The main problem was, however, the lack of mixing in the device. One way to overcome this would be to use an ultrasonic transducer with the microfluidic device but although preliminary experiments were carried out, there was not time to fully investigate this approach.
15

Liquid crystals for light emitting diodes

Liedtke, Alicia January 2009 (has links)
In this work a series of new semiconducting liquid crystals (LCs), which are applicable for organic light emitting diodes (OLEDs), were investigated. Semiempirical calculations were carried out on monomers and anti-cofacial dimers built from our molecules, representing molecules in solution and thin film respectively. Compared to the monomer a doubling of the oscillator strength in the dimer was found for longitudinal offsets larger than 20 A. Smaller shifts showed a forbidden absorption transition from ground to the lowest excited state. Assuming that the absorption transition is equivalent to the emissive transition, this might explain the reduced optical quantum efficiency observed for all of our materials in the solid state. OLEDs made from blends of three different blue/green emitters with a red component showed white light emission with voltage independent CIE coordinates close to the ideal white. With polarised microscopy nematic phases frozen in a glassy state at room temperature were observed for all blends. Thus the blends were homogeneous and no phase separation occurred. This is important for homogeneous white emission and the alignment of the LCs due to a rubbed alignment layer below. Polarised white electroluminescence with an average polarisation ratio of 8:1 was shown from an OLED made with a blend deposited onto an alignment layer. Polarised background light for LC displays is desirable as this minimises the losses at the polarisers in the display and thus increases its brightness or lowers the power consumption. The low efficiency of the red emitter however limited the OLED performance. Surface relief gratings (SRGs) with periods of a few hundred nm and a maximum depth of 66 nm and periods in the nm-range with a depth of 140 nm were spontaneously induced on our films. They were formed through molecular mass transport from the dark to bright regions during crosslinking by irradiation with a sinusoidal light pattern created by a phase mask. The anisotropic properties of LCs are shown to enhance transport. SRGs were formed at room temperature and an elevated sample temperature of 65deg. They are suitable feedback structures for optically pumped organic lasers and can also be employed to enhance the outcoupling of OLEDs.
16

Parallel kinetic resolutions using active esters

Coulbeck, Eliot January 2009 (has links)
Obtaining enantiomerically pure compounds is of major importance in modern organic chemistry; the resolution of racemic compounds is a very useful and practical method of achieving this. Parallel kinetic resolutions are an interesting variation on the more classical resolution methods; this method has been recently introduced to the scientific community by Vedejs and there are currently very few examples of successful parallel kinetic resolutions in the literature.The aim of the project, outlined in this report, was to investigate the use of parallel kinetic resolution methodology to resolve racemic carboxylic acids and secondary alcohols. This aim was achieved and in total four distinct parallel kinetic resolution methods were developed; one for the resolution of carboxylic acids, one for the resolution of secondary alcohols, and two that can be used to resolve either carboxylic acids or secondary alcohols. The development process is described for each of these distinct resolutions and their relative scope and limitations are discussed. This report also details the possible reasons for the levels of selectivity found in these reactions, and discusses what effect the reaction conditions have on the level ofstereocontrol. Similarities between all four of the resolutions are described, and the possibility of a generic stereoselective pathway is discussed.This report however is not limited only to findings directly related to parallel kinetic resolutions; it also encompasses all findings from the above mentioned studies. As such, it also describes an unusual observation; the fact that the sign of optical rotation for the common resolving agent 4-isopropyl oxazolidinone is solvent dependant. The discovery and further exploration of a method for synthesising a range of optical pure secondary alcohols from the commercial available (S)-enantiomer of 1- (2-bromo-phenyl)-ethanol is also described. An interesting method for the determination of enantiomeric excess of carboxylic acids is also discussed.
17

Optimization of Equipment for Tomographic Measurements of Void Distributions using Fast Neutrons

Andersson, Peter January 2011 (has links)
This licentiate thesis describes a novel nondestructive measuring technique for determiningspatial distributions of two-phase water flows. In Boiling Water Reactors, which compose themajority of the world's commercial nuclear reactors, this so called void distribution is of importance for safe operation. The presented measurement technique relies on fast neutron transmission tomography using portable neutron generators. Varying hardware options for such an instrument based on this technique and a prototype instrument, which is under construction, are described. The main design parameters are detailed and motivated from a performance point of view. A Paretomultiple objective optimization of the count rate and image unsharpness is presented. The resulting instrument design comprises an array of plastic scintillators for neutron detection. Such detector elements allow for spectroscopic data acquisition and subsequent reduction of background events at low energy by means of introducing an energy threshold in the analysis. The thesis includes two papers: In paper I, the recoil proton energy deposition distribution resulting from the interaction of the incoming neutrons is investigated for thin plastic scintillator elements. It is shown that the recoil proton losses have a large effect on the pulse height distribution and the intrinsic neutron detection efficiency is calculated for varying energy thresholds. In paper II the performance of the planned FANTOM device is investigated using the particle transport code MCNP5. An axially symmetric phantom void distribution is modeled and there construction is compared with the correct solution. According to the solutions, the phantom model can be reconstructed with 10 equal size ring-shaped picture elements, with a precision of better than 5 void percent units using a deuterium-tritium neutron generator with a yield of 3 · 107 neutrons per second and a measurement time of 13 h. However, it should be noted that commercial neutron generators with a factor of 103 higher yields exist and that the measurement time could decrease to less than a minute if such a neutron generator would beutilized. / STUNT
18

Adsorption studies by pulsed streaming potentials in microfluidic channels

Nguyen, ThuTrang 16 July 2010 (has links)
In this work, an instrument for measuring pulsed streaming potentials was constructed and optimized for analytical and teaching applications. This thesis is divided in three chapters, the first one deals with the construction of the instrument, the second describes a microfluidic experiment designed for undergraduate and high school students using this instrument, and the third one shows an application of pulsed streaming potential measurements in the detection of heparin. Streaming potential is the electric field generated when a liquid is forced to flow by a pressure gradient through a channel or other stationary charged surfaces.1,2 These measurements were done in microfluidic channels built with commodity plastics such polycarbonate (PC) and cyclic olefin copolymer (COC). Microfluidics studies the changing behaviors of fluids within small volumes, (nL, pL, fL), or small sizes, (channel size is about 100 nanometers to several hundred micrometers).3 With low level of complexity in instrumentation, low cost, and easy way to implement, the system is ideally suited as a teaching instrument in high 2 school and undergraduate labs. By creating simple experiments with suitable processing time, our goal is to introduce to students several fundamental concepts related to ionic solutions, electrochemical potentials, and charged surfaces. By doing the experiments, students can improve their analytical skills, and problem solving skills. They can learn many useful techniques, such as measuring pH, measuring conductivity, and calculating zeta potential. For these experiments, Polycarbonate (PC) is chosen as microfluidic platform because it is commercially available and the cost is low enough for a school budget. PC microfluidic channels are modified by different charged species, which are the anionic poly (sodium 4-styrenesulfonate) (PSS), the cationic poly(allylamine) hydrochloride (PAH), and bovine serum albumin (BSA). Since the relative polarity of streaming potential is determined by the surface charge, the signal detected is the reverse of streaming potential with different charged modified surfaces. With the same strategy, heparin is detected by real time monitoring adsorption on COC and PC microfluidic channels modified by protamine. The results on the two kinds of channels are compared. For COC channels, linear correlation of initial adsorption rates is found in the range between 0.00074 units/ml and 0.050 units/ml. For PC channels it is between 0.00074 units/mL and 0.074 units/mL. Streaming potential measurements have been useful for determining the charge of such surfaces as capillaries, 1 membranes, 4 and other porous materials.5 There has been no work done using pulsed streaming potential measurements for sensing purposes in microfluidic channels. With our sensing device, no referent electrode is needed since the signal acquisition is made using pulsed flow, so drifting of the measure voltages can be avoided. In addition, no such fluorescent, electrochemical, or radioactive labeling is required for detection.
19

Electronic Structure Based Classification of Neurotransmitters and Related Drugs

Jha, Amrita 06 October 2010 (has links)
A fundamental understanding of the relationship between structure and activity of neurotransmitters in the human brain is of vital importance for the design targeted drugs. Using density functional theory and hybrid exchange correlation energy functionals we have studied the structure-activity-relationships of some important neurotransmitters and selected drugs by calculating their absolute hardness (η) and absolute electronegativity (χ). A plot of the η- χ diagram allowed us to assign them into three distinct groups, namely, (i) Acetylcholine analogs (positively charged structure), (ii) GABA analogs (zwitterionic structures) and (iii) monoamines. The results suggest that brain stem is chemically soft because of distribution of monoaminergic nerve pathways. Prefrontal cortex is also chemically soft due to secretion of dopamine from mesocortical dopaminergic nerve A10, whereas neocortex is chemically hard due to presence of zwitterionic neurotransmitters. Target drugs (agonists/antagonists) can also be predicted by comparing the η- χ diagram of neurotransmitters with those of the drugs.
20

Allocating Homeland Security Screening Resources Using Knapsack Problem Models

Dreiding, Rebecca 29 October 2010 (has links)
Since the events of September 11, 2001, the federal government is focused on homeland security and the fight against terrorism. This thesis addresses the idea of terrorist groups smuggling nuclear weapons through the borders of the United States. Security screening decisions are analyzed within maritime and aviation domains using discrete optimization models, specifically knapsack problems. The focus of the maritime chapters involves a risk-based approach for prescreening intelligence classifications for primary and secondary screening decisions given limited budget and resources. Results reveal that screening decisions are dependent on prescreening classification and the efficacy of the screening technologies. The screening decisions in the aviation security chapter highlight different performance measures to quantify the effectiveness of covering flights with the intent of covering targets. Results reveal that given scarce resources, such as screening devices capacities and budget, flights and targets can be covered with minimal expense to the system.

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