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Investigation of Microcrack Growth in [0/90]s Graphite Epoxy Composite Laminates Using X-Ray MicrotomographyTatiparthi, Arun 21 May 2004 (has links)
Graphite epoxy composites are being used in aerospace industry and spacecraft applications for their light weight and high strength. As a matter of fact these materials also have some disadvantages like damage which is hazardous when used in cryogenic application. Composite materials IM7/977-2, IM7/5555 and IM7/5276-1 are of interest for the aerospace industry and this research concentrates on study of microcracking, delamination and other defects in the [0/90]s composite laminates of the above materials. These materials were uni-axially tested to pre-determined stress levels and the damages in the material were recorded in the form of microcrack density at different stress levels. In this research work the use of X-Ray Microtomography has proven to be an excellent tool to characterize the crack connectivity and damage information three dimensionally. Dye penetrant technique was also used in this work to enhance the visibility of the cracks.
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Crystallographic investigation of n-Aminopyridinium perhalometallatesLawrence, Estee 06 February 2012 (has links)
MSc., Faculty of Science, University of the Witwatersrand, 2011 / This dissertation presents the results of a crystallographic investigation into nineteen
novel n-aminopyridinium perhalometallates of the general formula (C5H7N2
+)2MX4
2-
where n = 2, 3 or 4, M = Zn, Cd or Hg and X = Cl, Br or I. The aim was to identify
positional effects of the cation hydrogen bonding donor groups on the structures and to
possibly identify robust synthons. Overall structural trends were also studied. The
hybrid crystals were synthesized using temperature controlled crystallization methods
and were characterized using X- ray diffraction techniques. Eighteen of the nineteen
hybrid crystals displayed isolated tetrahedral perhalometallate anions. The results were
classified into six different classes of isostructural compounds. Eight of the nineteen
hybrid structures (42%) contained water. The 42% include two 3-ammoniopyridinium
structures and all cadmium and mercury containing 4-aminopyridinium structures. The
effect of a change in metal and/ or halide on the structure was investigated, and the
overall structural trends in each n-aminopyridinium family were identified. Four
different synthons were observed in the 2-aminopyridinium series while one synthon
was observed throughout the 4-aminopyridinium series. A common synthon was
observed in both the 3-aminopyridinium and 3-ammoniopyridinium series.
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XRF analysis of base metals prepared by fused bead methodEngelbrecht, Chantelle 27 February 2012 (has links)
M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / The objective of the study was to investigate the preparation of glass beads for base metal analysis of mining samples prior to x-ray fluorescence analysis.
The research method used included the investigation of different fluxes, oxidising, non-wetting agents, fusion temperature and time. The experiments were carried out using different fusion instruments: Electrofluxer and Katanax followed by both EDXRF and WDXRF analysis. The x-ray spectrometers were calibrated with standards prepared from pure oxides and the results compared to values determined by alternative techniques. Different statistical methods were used to validate the experiments including factorial designs.
Not all the elements and oxides were recovered successfully, however, perfect glass beads were prepared. The two areas of concern were addressed successfully: firstly the loss of copper was overcome by using an alternative heating mechanism of the Katanax and sodium iodide as the non-wetting agent. Secondly, the sulphur was successfully oxidised and retained in the glass beads.
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Systhesis and characterization of analogues of the antimalaria alkaloid febrifuginePienaar, Daniel Petzer 15 November 2006 (has links)
Student Number : 0011001D -
PhD thesis -
School of Chemistry -
Faculty of Science / The subject of this thesis is the development of a potentially simple, general and
economical synthetic protocol for the potent antimalarial alkaloid febrifugine (1) and
its analogues. In Chapter 1, the interesting history of 1, which includes a description
of several reported total syntheses of 1, is discussed. Natural products derived from 1,
as well as promising synthetic derivatives that display good antimalarial activity, are
also discussed. The structure-activity relationships determined to date for 1 and its
derivatives are presented in order to substantiate the need for the development of new
synthetic strategies towards derivatives of 1 and its 3″-unsubstituted analogue, (±)-
deoxyfebrifugine (14).
A brief overview of the general strategies used in the Organic Chemistry Group at the
University of the Witwatersrand for the synthesis of alkaloids is given in Chapter 2.
The Eschenmoser sulfide contraction reaction between a thiolactam and an α-
bromocarbonyl compound results in the formation of a vinylogous amide (or
“enaminone”) product, which can be further manipulated to produce commonly
encountered alkaloidal molecular skeletons. The chosen approach to 1 is based on
reaction between the pivotal bromide 3-(3-bromo-2-oxopropyl)quinazolin-4(3H)-one
(105) and suitable 3-substituted piperidine-2-thiones.
A series of model studies, described in Chapter 3, was performed in order to test the
feasibility of the synthetic strategy. These studies resulted in a new preparation of the
key bromide 105 and a new and efficient synthesis of (±)-deoxyfebrifugine (14).
Enaminone analogues derived from five-, seven-, eight-, nine- and thirteen-membered
thiolactams were also prepared for comparison. An interesting difference in the
sensitivity of the five- and six-membered model cyclic vinylogous amides towards
reducing agents was observed. Whereas three piperidine analogues of 14 could be
prepared in high yields by the chemoselective hydrogenation of six-membered cyclic
vinylogous amide precursors, the five-membered analogues either reacted nonselectively
under various standard hydrogenation conditions, or resisted reduction
entirely.
An extension of the approach towards the synthesis of a 3″-amino analogue of
febrifugine (1) from L-ornithine is discussed in Chapter 4. Several 3-aminated
piperidin-2-ones and thiones were prepared, but the subsequent enaminones were
stable and characterizable only when the piperidinylidene ring nitrogen was alkylated.
However, chemoselective reduction of the enaminone C=C bond in 3-{(E)-3-[(3S)-3-
(tert-butoxycarbonylamino)-1-(4-methoxybenzyl)piperidin-2-ylidene]-2-oxopropyl}-
quinazolin-4(3H)-one (221), an interesting 3-acylamino dehydro analogue of 1, did
not give the desired azafebrifugine, but instead yielded a 5,6,7,8-tetrahydro-1Hpyrido[
3,2-c][1,2]oxazine derivative.
Several approaches to febrifugine (1) itself based on the successful model studies are
discussed in Chapter 5. Initially, stereoselective α-bromination and subsequent
substitution by oxygen of piperidin-2-ones derived from the chiral auxiliary (S)-
phenylglycinol was explored. Unexpected racemization occurred at the α-position of
the lactam during this route. A second approach to 3-hydroxypiperidin-2-one from Larginine
was also problematic. Finally, the utility of α-hydroxylation methodology
(including Davis methodology) on N-substituted piperidin-2-ones was explored. This
route yielded a range of 3-oxygenated lactams and thiolactams, many of which
displayed optical activity. The crystal structures of several 3-substituted thiolactams
were determined and compared. However, attempts to apply the sulfide contraction
procedure to these precursors were unsuccessful.
Some investigations designed to explore the structural differences between
vinylogous amides derived from the quinazoline-containing bromide 105 and
thiolactams of different ring sizes are discussed in Chapter 6. Single crystal X-ray
diffraction and NMR spectral data are compared for this series of compounds, the
results revealing that the enaminone group in the five-membered ring derivative 3-
[(3Z)-2-oxo-3-(pyrrolidin-2-ylidene)propyl]quinazolin-4(3H)-one 155 possesses a
significantly different electronic distribution to the other analogues in the series.
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Electronic phase behaviors in spin-orbit coupled magnets at the localized and itinerant limitsChen, Xiang January 2018 (has links)
Thesis advisor: Stephen Wilson / The magnetic interaction in materials generally can be categorized into two extremes: localized and itinerant. This work will focus on the electronic and magnetic properties of two prototypical magnetic compounds, which fall into the opposite extremes, i:e:, the spin-orbit coupled Mott insulator Sr₂IrO₄ (Sr214) described by the localized Heisenberg model and the itinerant helical (nearly-ferromagnetic) metal MnSi pictured with band or Stoner magnetism. The single layered cuprate analogue Sr₂IrO₄ has attracted considerable attentions in recent years, due to its unusual electronic and magnetic properties and the potential to access superconducting states. The exotic jeff = 1/2 ground state for the Ir⁴⁺ (5d⁵) ions results from the delicate balance of competing/cooperating energy scales, such as the stronger spin-orbit coupling (SOC) in 5d materials as compared to 3d transition metal oxides (TMOs), crystal electric field (CEF) splitting and electron-electron correlations. Superconducting states are theoretically predicted to be achievable if sufficient carriers are introduced into this spin-orbit assisted compound, which later triggers tremendous experimental works toward the realization of superconductivity. Here in this work a combined study of various probes, such as transport, magnetization, X-ray and neutron scattering measurements, focusing on the electronic and magnetic properties, is presented in the perturbed spin-orbit coupled Mott (SOM) state. Specifically in electron doped (Sr₁₋ₓLaₓ)₂IrO₄, a detailed mapping of magnetism with respect to electron doping is presented, demonstrating the gradual transition from long range magnetic order in parent state, to intermediate short range order, and eventually into the incommensurate (IC) spin density wave (SDW) state with increasing electron doping. Our picture supports the conjecture that the quenched Mott phases in electron-doped Sr₂IrO₄ and hole doped La₂CuO₄ share common competing electronic phases. On the other hand, the prototypical itinerant metal MnSi is examined by inelastic neutron scattering (INS). Our experimental data directly demonstrate the collapse of linear spin wave theory for localized Heisenberg magnets in the large energy limit, although the low energy dispersion is still described by the ferromagnetic spin wave theory. Most importantly, our observations display the chimney-like dispersion spectrum up to the energy scale of at least 240 meV, which is more than one order of magnitude larger than the Heisenberg interaction energy scale. For the first time, solid characterizations of Stoner excitations in itinerant helimagnet (nearly ferromagnetic) have been demonstrated up to an exceedingly large energy scale. Our intriguing results will greatly promote further understanding and exploration of Stoner excitations in itinerant magnets. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.
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Study of Allosteric Regulation of Escherichia coli Aspartate TranscarbamoylaseZheng, Yunan January 2013 (has links)
Thesis advisor: Evan R. Kantrowitz / For nearly 60 years the ATP activation and the CTP inhibition of Escherichia coli aspartate transcarbamoylase (ATCase) has been the textbook example of allosteric regulation. We present kinetic data and 5 X-ray structures determined in the absence and presence of a Mg2+ concentration within the physiological range. In the presence of 2 mM divalent cations (Mg2+, Ca2+, Zn2+) CTP does not significantly inhibit the enzyme while the allosteric activation by ATP is enhanced. The data suggest that the actual allosteric inhibitor in vivo of ATCase is the combination of CTP, UTP and a M2+ cation and the actual allosteric activator is ATP and M2+ or ATP, GTP and M2+. The structural data reveals that two NTPs can bind to each allosteric site with a Mg2+ ion acting as a bridge between the triphosphates. Thus the regulation of ATCase is far more complex than previously believed and calls many previous studies into question. The X-ray structures reveal the catalytic chains undergo essentially no alternations, however, several regions of the regulatory chains undergo significant structural changes. Most significant is that the N-terminal regions of the regulatory chains exist in different conformations in the allosterically activated and inhibited forms of the enzyme. Here, a new model of allosteric regulation is proposed. / Thesis (MS) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.
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Derivative spectrophotomtric and x-ray fluorescence spectrometric methods for the simultaneous determination of metals in alloys and water samples.January 1991 (has links)
by Ho Sing Yiu. / Thesis (M. Phil.)--Chinese University of Hong Kong, 1991. / Includes bibliographical references. / ACKNOWLEDGEMENT --- p.i / ABSTRACT --- p.ii / Chapter CHAPTER 1 --- GENERAL INTRODUCTION --- p.1 / Chapter PART I --- DERIVATIVE SPECT3ROPHOTOMETRIC METHOD FOR THE SIMULTANEOUS DETERMINATION OF COPPER AND NICKEL IN ALLOYS / Chapter CHAPTER 2 --- INTRODUCTION --- p.5 / Chapter CHAPTER 3 --- EXPERIMENTAL --- p.14 / Chapter CHAPTER 4 --- RESULTS AND DISCUSSION --- p.18 / Chapter PART II --- X-RAY FLUORESCENCE SPECTROMETRIC METHOD FOR THE SIMULTANEOUS DETERMINATION OF SEVEN METALS IN WATER SAMPLES / Chapter CHAPTER 5 --- INTRODUCTION --- p.43 / Chapter CHAPTER 6 --- EXPERIMENTAL --- p.58 / Chapter CHAPTER 7 --- RESULTS AND DISCUSSION --- p.65 / Chapter CHAPTER 8 --- CONCLUSION --- p.99 / REFERENCES --- p.101
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Three-dimensional structure reconstruction from tomographic views.January 1996 (has links)
by Ho, Chi-Kin. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 62-64). / Chapter 1 --- Introduction / Chapter 2 --- Previous Work --- p.2-1 / Chapter 2.1 --- Thresholding --- p.2-1 / Chapter 2.2 --- Edge Detection --- p.2-2 / Chapter 2.3 --- Region Growing --- p.2-2 / Chapter 2.4 --- Radial Contour Model --- p.2-3 / Chapter 2.5 --- Regularized Region Contrast --- p.2-3 / Chapter 2.6 --- Deformable Model --- p.2-4 / Chapter 3 --- The ODD-Balloons Model --- p.3-1 / Chapter 3.1 --- Design Rationale --- p.3-1 / Chapter 3.2 --- Overview --- p.3-5 / Chapter 3.3 --- 2-D Deformations --- p.3-8 / Chapter 3.4 --- Orthogonal Cut and Volume Transfer --- p.3-11 / Chapter 3.5 --- Smoothing Operation --- p.3-17 / Chapter 3.6 --- Properties --- p.3-20 / Chapter 3.6.1 --- Conformation to 3-D Shape --- p.3-20 / Chapter 3.6.2 --- Noise Sensitivity --- p.3-20 / Chapter 3.6.3 --- Convergence and Efficiency --- p.3-22 / Chapter 3.6.4 --- Easy-to-Use --- p.3-23 / Chapter 3.7 --- Summary --- p.3-24 / Chapter 4 --- Experiment Results --- p.4-1 / Chapter 4.1 --- Synthetic Data Experiments --- p.4-1 / Chapter 4.2 --- Real Data Experiment --- p.4-3 / Chapter 4.3 --- Discussions --- p.4-6 / Chapter 5 --- Conclusion and Future Work --- p.5-1 / Chapter 5.1 --- Conclusion --- p.5-1 / Chapter 5.2 --- Recommended Future Work --- p.5-2 / Appendix A Discrete Implementation of 2-D Deformation --- p.A-1 / Appendix B Choosing Elasticity and Rigidity Coefficients of 2-D Deformation --- p.B-1 / Bibliography --- p.BIB-1
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Towards high throughput single crystal neutron diffraction of hydrogen bonded molecular complexesJones, Andrew January 2012 (has links)
This work presents findings from experiments carried out using the neutron Laue method in tandem with laboratory source X-ray diffraction to characterise a series of organic molecular complexes which exhibit interesting, and potentially “tunable”, temperature dependent charge transfer effects, such as proton migration and proton disorder within hydrogen bonded networks. These subtle processes are studied by variable temperature neutron diffraction studies, allowing the positional and anisotropic displacement parameters of the hydrogen atoms to be refined accurately and their evolution with temperature followed. The hydrogen atom behaviour is found to be influenced by the local environment, including weak intermolecular interactions in the vicinity of the hydrogen bond under study. Complexes of urea and methyl substituted ureas with small organic acids are presented, which show robust and reproducible structural motifs. In favourable circumstances, these contain short, strong hydrogen bonds (SSHBs) within which the proton may undergo temperature dependent migration. By synthesising a number of complexes containing SSHBs, potential routes to the design of proton migration complexes are found, which utilise crystal engineering principles and pKa matching. Variable temperature studies conducted on these complexes also show unusual thermal expansion properties and phase transitions in urea-acid complexes which do not display proton migration. Systems containing hydrogen bonded dimers of 3,5-dinitrobenzoic acid are also studied, and shown to contain temperature proton disorder within moderate strength hydrogen bonds linking the dimers. The presence and potential onset temperature of any disorder is found to be influenced by interactions around the acid dimers and potential routes to controlling proton disorder are discussed. Complexes of the proton sponge, 1,8-bis(dimethylamino)napthalene (DMAN), with organic acids are also presented, in which the structures have been determined using neutron diffraction. DMAN readily accepts a proton from the acid co-molecules used in forming the complexes, forming a strong intramolecular SSHB within the protonated DMAN. Strong intermolecular hydrogen bonds are also induced between the acid molecules in many cases. The neutron studies presented here investigate the effect of weak interactions on the behaviour of hydrogen atoms located within these SSHBs, and also indicate over what distance such interactions significantly affect the hydrogen atom behaviour.
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Correlating structural and opto-electrical properties of perovskite solar cellsAlsari Almheiri, Mejd January 2019 (has links)
Perovskite photovoltaics is one of the fastest growing opto-electronic technologies with device efficiencies currently exceeding 23%. The opportunity to deposit these abundant materials with large area solution processing techniques could make perovskites viable for low-cost production. However, since perovskite materials are prone to degradation, their lifetime needs to be improved to that of silicon solar cells before these devices can be commercialized. Moreover, unlike most semiconductors, trap densities in polycrystalline perovskite films in high-performing devices have been determined to be relatively large, suggesting a remarkable defect tolerance in perovskite films that needs to be understood in the context of the nature of the trap states and any residual non- radiative losses. These non-radiative losses are observed as photoluminescence heterogeneity within perovskite films, even for high-performing perovskite systems. In this work, we explore the degradation kinetics of perovskite devices under stress conditions and find that further stability improvements should focus on the mitigation of trap generation during ageing. Furthermore, we fabricate perovskite solar cells with a novel back-contact structure, in which electron- and hole-selective electrodes are co-positioned on the back side of the cell and spaced by 100 μm. By utilising grazing-incidence X-ray diffraction, we show that even in the earliest stages of conversion of precursors to perovskite we achieve remarkably high open-circuit voltages, suggesting that the defect tolerance of perovskites appears at an early stage in the conversion process. Moreover, we employ scanning X-ray diffraction with nanofocused beam and obtain detailed information, revealing overlapping grains located at different depths within perovskite films. We find that the critical grain size is the longer-range structural super-grains rather than the grains viewed with conventional microscopy techniques. These findings further highlight the presence of structural defects in perovskite materials and provide important insights towards improving the optoelectronic behaviour of these materials.
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