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Feet on the potential energy surface, head in the pie clouds

Description: This work presents explorations of the potential energy surface of clusters of atoms and of the interactions between molecules. First, structures of small aluminum clusters are examined and classified as ground states, transition states, or higher-order saddle points. Subsequently, the focus shifts to dispersion-dominated π-π interactions when the potential energy surfaces of benzene, substituted benzene, and pyridine dimers are explored. Because DNA nucleotide bases can be thought of as substituted heterocycles, a natural extension of the substituted benzene and pyridine investigations is to model paired nucleotide bases. Finally, the success of the dispersion studies inspires the development of an extension to the computational method used, which will enable the dispersion energy to be modeled – and the potential energy surface explored – in additional chemical systems. The effective fragment potential (EFP) method is described, as well as various quantum mechanical methods. An ab inito quantum mechanical study of 13-atom aluminum clusters is described. EFP studies of aromatic dimers are reported in which dispersion energy makes a significant contribution to the attraction between monomers. Theory and code development toward a means of computing dispersion energy in mixed ab inito-EFP systems are described.
Date: July 12, 2012
Creator: Smith, Quentin
Partner: UNT Libraries Government Documents Department

Coupling of the 4f Electrons in Lanthanide Molecules

Description: (C5Me5)2LnOTf where Ln = La, Ce, Sm, Gd, and Yb have been synthesized and these derivatives are good starting materials for the synthesis of (C5Me5)2LnX derivatives. (C5Me5)2Ln(2,2'-bipyridine), where Ln = La, Ce, Sm, and Gd, along with several methylated bipyridine analogues have been synthesized and their magnetic moments have been measured as a function of temperature. In lanthanum, cerium, and gadolinium complexes the bipyridine ligand ligand is unequivocally the radical anion, and the observed magnetic moment is the result of intramolecular coupling of the unpaired electron on the lanthanide fragment with the unpaired electron on the bipyridine along with the intermolecular coupling between radicals. Comparison with the magnetic moments of the known compounds (C5Me5)2Sm(2,2'-bipyridine) and (C5Me5)2Yb(2,2'-bipyridine) leads to an understanding of the role of the Sm(II)/Sm(III) and Yb(II)/Yb(III) couple in the magnetic properties of (C5Me5)2Sm(2,2'-bipyridine) and (C5Me5)2Yb(2,2'-bipyridine). In addition, crystal structures of (C5Me5)2Ln(2,2'-bipyridine) and [(C5Me5)2Ln(2,2'-bipyridine)][BPh4](Ln= Ce and Gd), where the lanthanide is unequivocally in the +3 oxidation state, give the crystallographic characteristics of bipyridine as an anion and as a neutral ligand in the same coordination environment, respectively. Substituted bipyridine ligands coordinated to (C5Me5)2Yb are studied to further understand how the magnetic coupling in (C5Me5)2Yb(2,2'-bipyridine) changes with substitutions. In the cases of (C5Me5)2Yb(5,5'-dimethyl-2,2'-bipyridine) and (C5Me5)2Yb(6-methyl-2,2'-bipyridine), the valence, as measured by XANES, changes as a function of temperature. In general, the magnetism in complexes of the type (C5Me5)2Yb(bipy.-), where bipyo represents 2,2'-bipyridine and substituted 2,2'-bipyridine ligands, is described by a multiconfiguration model, in which the ground state is an open-shell singlet composed of two configurations: Yb(III, f13)(bipy.-) and Yb(II, f14)(bipyo). The relative contributions of the two configurations depends on the substituents on the bipyridine ligand.[(C5H4Me)3Ln]2(L) (Ln = Ce, Tb; L = 4,4'-bipyridine, 1,4-benzoquinone) are synthesized in order to study the effect of these ligands on the oxidation states of the metal ...
Date: September 12, 2008
Creator: Kazhdan, Daniel
Partner: UNT Libraries Government Documents Department

Far infrared spectroscopy of solids. I. Impurity states in Al$sub 2$O$sub 3$. II. Electron-hole droplets in Ge

Description: Far infrared Fourier transform spectroscopy was used to study the low lying vibronic states of Mn$sup 3+$ in Al$sub 2$O$sub 3$ and the plasma absorption of electron-hole droplets in Ge. The transmission of Mn-doped samples of Al$sub 2$O$sub 3$ was measured in the frequency range from 3 to 30 cm$sup -1$ in applied magnetic fields up to 50 kG. Absorption lines were observed due to both ground and excited state transitions. Polarization measurements established that these absorption lines were due to electric dipole transitions. Temperature dependence measurements were used to derive a level diagram for the low lying states of Mn$sup 3+$. A phenomenological model based on an electronic Hamiltonian was developed which successfully describes the data. The empirically determined trigonal field and spin-orbit quenching parameters of this model are 0.7 and 0.1 respectively. This quenching is attributed to the dynamic Jahn-- Teller interaction. The plasma absorption of small ($alpha$) electron-hole drops in Ge was measured in the frequency range from 30 to 300 cm$sup -1$. The observed absorption is in good agreement with measurements by Vavilov and other workers. A theoretical model which includes both intraband and interband contributions to the dielectric constant in the Rayleigh limit of Mie theory is used to describe the observed lineshape. Measurements of plasma absorption of large ($gamma$) drops in inhomogeneously stressed Ge were made in magnetic fields up to 50 kG. The lineshape at zero applied field was calculated in the large sphere limit of Mie theory including intraband terms and a zero-strain interband term. Qualitative agreement with experiment was obtained. The peak absorption shifted quadratically with applied magnetic field and the total plasma absorption increased. No oscillatory structure was observed in the field-dependence of the total absorption. (auth)
Date: September 12, 1975
Creator: Aurbauch, R.L.
Partner: UNT Libraries Government Documents Department

Precision Measurement of Neutrino Oscillation Parameters with KamLAND

Description: This dissertation describes a measurement of the neutrino oscillation parameters #1;{Delta}m{sup 2}{sub 21}, θ{sub 12} and constraints on θ{sub 13} based on a study of reactor antineutrinos at a baseline of ∼ 180 km with the KamLAND detector. The data presented here was collected between April 2002 and November 2009, and amounts to a total exposure of 2.64 � 0.07 � 10{sup 32} proton-years. For this exposure we expect 2140 � 74(syst) antineutrino candidates from reactors, assuming standard model neutrino behavior, and 350�88(syst) candidates from background. The number observed is 1614. The ratio of background-subtracted candidates observed to expected is (N{sub Obs} − N{sub Bkg})/N{sub Exp} = 0.59 � 0.02(stat) � 0.045(syst) which confirms reactor neutrino disappearance at greater than 5σ significance. Interpreting this deficit as being due to neutrino oscillation, the best-fit oscillation parameters from a three-flavor analysis are #1;{Delta}m{sup 2}{sub 21} = 7.60{sup +0.20}{sub −0.19}�10{sup −5}eV{sup 2}, θ{sub 12} = 32.5 � 2.9 degrees and sin{sup 2} θ{sub 13} = 0.025{sup +0.035}{sub −0.035}, the 95% confidence-level upper limit on sin{sup 2} θ{sub 13} is sin{sup 2} θ{sub 13} < 0.083. Assuming CPT invariance, a combined analysis of KamLAND and solar neutrino data yields best-fit values: #1;{Delta}m{sup 2}{sub 21} = 7.60{sup +0.20}{sub −0.20} � 10{sup −5}eV{sup 2}, θ{sub 12} = 33.5{sup +1.0}{sub −1.1} degrees, and sin{sup 2} θ{sub 13} = 0.013 � 0.028 or sin{sup 2} θ{sub 13} < 0.06 at the 95% confidence level.
Date: December 12, 2011
Creator: KamLAND, & O'Donnell, Thomas
Partner: UNT Libraries Government Documents Department

Ion Deflection for Final Optics in Laser Inertial Fusion Power Plants

Description: Left unprotected, both transmissive and reflective final optics in a laser-driven inertial fusion power plant would quickly fail from melting, pulsed thermal stress, or degradation of optical properties as a result of ion implantation. One potential option for mitigating this threat is to magnetically deflect the ions such that they are directed to a robust energy dump. In this paper we detail integrated studies that have been carried out to assess the viability of this approach for protecting final optics.
Date: December 12, 2005
Creator: Abbott, R P
Partner: UNT Libraries Government Documents Department

A Search for Direct CP Violation in Two-Body Cabibbo-Suppressed Decays of Neutral Charmed Mesons

Description: Presented are the results of a search for direct CP violation in Cabibbo-suppressed decays of D{sup 0} to two charged daughters. The analysis described was performed on {approx}230 fb{sup -1} of the BABAR data sample, recorded at the Stanford Linear Accelerator Center and using the PEP-II electron storage rings. We measure CP asymmetries for decay to the KK and {pi}{pi} final states, as well as for the branching ratio, and develop a new technique for tagging-efficiency correction using the Cabibbo-favored K{pi} final state. We find some evidence for CP violation in decays to the KK final state and results that suggest CP violation in the {pi}{pi} final state as well.
Date: November 12, 2007
Creator: Flacco, Christian Julienne & /UC, Santa Cruz
Partner: UNT Libraries Government Documents Department

Structure, ionic Conductivity and mobile Carrier Density in Fast Ionic Conducting Chalcogenide Glasses

Description: This thesis consists of six sections. The first section gives the basic research background on the ionic conduction mechanism in glass, polarization in the glass, and the method of determining the mobile carrier density in glass. The proposed work is also included in this section. The second section is a paper that characterizes the structure of MI + M{sub 2}S + (0.1 Ga{sub 2}S{sub 3} + 0.9 GeS{sub 2}) (M = Li, Na, K and Cs) glasses using Raman and IR spectroscopy. Since the ionic radius plays an important role in determining the ionic conductivity in glasses, the glass forming range for the addition of different alkalis into the basic glass forming system 0.1 Ga{sub 2}S{sub 3} + 0.9 GeS{sub 2} was studied. The study found that the change of the alkali radius for the same nominal composition causes significant structure change to the glasses. The third section is a paper that investigates the ionic conductivity of MI + M{sub 2}S + (0.1Ga{sub 2}S{sub 3} + 0.9 GeS{sub 2}) (M = Li, Na, K and Cs) glasses system. Corresponding to the compositional changes in these fast ionic conducting glasses, the ionic conductivity shows changes due to the induced structural changes. The ionic radius effect on the ionic conductivity in these glasses was investigated. The fourth section is a paper that examines the mobile carrier density based upon the measurements of space charge polarization. For the first time, the charge carrier number density in fast ionic conducting chalcogenide glasses was determined. The experimental impedance data were fitted using equivalent circuits and the obtained parameters were used to determine the mobile carrier density. The influence of mobile carrier density and mobility on the ionic conductivity was separated. The fifth section is a paper that studies the structures of low-alkali-content Na{sub 2}S + ...
Date: December 12, 2006
Creator: Yao, Wenlong
Partner: UNT Libraries Government Documents Department

One- and two-dimensional infrared spectroscopic studies of solution-phase homogeneous catalysis and spin-forbidden reactions

Description: Understanding chemical reactions requires the knowledge of the elementary steps of breaking and making bonds, and often a variety of experimental techniques are needed to achieve this goal. The initial steps occur on the femto- through picosecond time-scales, requiring the use of ultrafast spectroscopic methods, while the rate-limiting steps often occur more slowly, requiring alternative techniques. Ultrafast one and two-dimensional infrared and step-scan FTIR spectroscopies are used to investigate the photochemical reactions of four organometallic complexes. The analysis leads to a detailed understanding of mechanisms that are general in nature and may be applicable to a variety of reactions.
Date: December 12, 2008
Creator: Sawyer, Karma Rae
Partner: UNT Libraries Government Documents Department

Rational Ligand Design for U(VI) and Pu(IV)

Description: Nuclear power is an attractive alternative to hydrocarbon-based energy production at a time when moving away from carbon-producing processes is widely accepted as a significant developmental need. Hence, the radioactive actinide power sources for this industry are necessarily becoming more widespread, which is accompanied by the increased risk of exposure to both biological and environmental systems. This, in turn, requires the development of technology designed to remove such radioactive threats efficiently and selectively from contaminated material, whether that be contained nuclear waste streams or the human body. Raymond and coworkers (University of California, Berkeley) have for decades investigated the interaction of biologically-inspired, hard Lewis-base ligands with high-valent, early-actinide cations. It has been established that such ligands bind strongly to the hard Lewis-acidic early actinides, and many poly-bidentate ligands have been developed and shown to be effective chelators of actinide contaminants in vivo. Work reported herein explores the effect of ligand geometry on the linear U(IV) dioxo dication (uranyl, UO{sub 2}{sup 2+}). The goal is to utilize rational ligand design to develop ligands that exhibit shape selectivity towards linear dioxo cations and provides thermodynamically favorable binding interactions. The uranyl complexes with a series of tetradentate 3-hydroxy-pyridin-2-one (3,2-HOPO) ligands were studied in both the crystalline state as well as in solution. Despite significant geometric differences, the uranyl affinities of these ligands vary only slightly but are better than DTPA, the only FDA-approved chelation therapy for actinide contamination. The terepthalamide (TAM) moiety was combined into tris-beidentate ligands with 1,2- and 3,2-HOPO moieties were combined into hexadentate ligands whose structural preferences and solution thermodynamics were measured with the uranyl cation. In addition to achieving coordinative saturation, these ligands exhibited increased uranyl affinity compared to bis-Me-3,2-HOPO ligands. This result is due in part to their increased denticity, but is primarily the result of the presence ...
Date: August 12, 2009
Creator: Szigethy, Geza
Partner: UNT Libraries Government Documents Department

Measurement and Interpretation of Moments of the Combined Hadronic Mass and Energy Spectrum in Inclusive Semileptonic B-Meson Decays

Description: This thesis presents first measurements of moments of the hadronic n{sub X}{sup 2} distribution measured in inclusive semileptonic decays of B mesons to final states containing a charm quark, B {yields} X{sub c}{ell}{nu}. The variable n{sub X}{sup 2} is a combination of the invariant mass of the charmed meson m{sub X}, its energy in the B-meson rest-frame E{sub X;BRF}, and a constant {tilde {Lambda}} = 0.65 GeV, n{sub X}{sup 2} = m{sub X}{sup 2}c{sup 4}-2{tilde {Lambda}}E{sub X,BRF} + {tilde {Lambda}}{sup 2}. The moments <n{sub X}{sup k}> with k = 2,4,6 are measured as proposed by theory to constrain assumptions made in the theoretical description of inclusive observables in semileptonic B-meson decays. This description uses Heavy Quark Expansion (HQE), an effective QCD combined with an Operator Product Expansion. The measurement is based on a sample of 231.6 million e{sup +}e{sup -} {yields} {Upsilon}(4S) {yields} B{bar B} events recorded with the BABAR experiment at the PEP-II e{sup +}e{sup -}-storage rings at SLAC. We reconstruct the semileptonic decay by identifying a charged lepton in events tagged by a fully reconstructed hadronic decay of the second B meson. Correction procedures are derived from Monte Carlo simulations to ensure an unbiased measurement of the moments of the n{sub X}{sup 2} distribution. All moments are measured requiring minimum lepton momenta between 0.8 GeV/c and 1.9 GeV/c in the rest frame of the B meson. Performing a simultaneous fit to the measured moments <n{sub X}{sup k}> up to order k = 6 combined with other measurements of moments of the lepton-energy spectrum in decays B {yields} X{sub c}{ell}{nu} and moments of the photon-energy spectrum in decays B {yields} X{sub s}{gamma}, we determine the quark-mixing parameter |V{sub cb}|, the bottom and charm quark masses, the semileptonic branching fraction {Beta}(B {yields} X{sub c}{ell}{nu}), and four non-perturbative heavy quark parameters. ...
Date: August 12, 2011
Creator: Klose, Verena
Partner: UNT Libraries Government Documents Department

New Synthetic Methods for Hypericum Natural Products

Description: Organic chemistry has served as a solid foundation for interdisciplinary research areas, such as molecular biology and medicinal chemistry. An understanding of the biological activities and structural elucidations of natural products can lead to the development of clinically valuable therapeutic options. The advancements of modern synthetic methodologies allow for more elaborate and concise natural product syntheses. The theme of this study centers on the synthesis of natural products with particularly challenging structures and interesting biological activities. The synthetic expertise developed here will be applicable to analog syntheses and to other research problems.
Date: December 12, 2006
Creator: Jeon, Insik
Partner: UNT Libraries Government Documents Department

Novel Aryne Chemistry in Organic Synthesis

Description: Arynes are among the most intensively studied systems in chemistry. However, many aspects of the chemistry of these reactive intermediates are not well understood yet and their use as reagents in synthetic organic chemistry has been somewhat limited, due to the harsh conditions needed to generate arynes and the often uncontrolled reactivity exhibited by these species. Recently, o-silylaryl triflates, which can generate the corresponding arynes under very mild reaction conditions, have been found very useful in organic synthesis. This thesis describes several novel and useful methodologies by employing arynes, which generate from o-silylaryl triflates, in organic synthesis. An efficient, reliable method for the N-arylation of amines, sulfonamides and carbamates, and the O-arylation of phenols and carboxylic acids is described in Chapter 1. Amines, sulfonamides, phenols, and carboxylic acids are good nucleophiles, which can react with arynes generated from a-silylaryl triflates to afford the corresponding N- and O-arylated products in very high yields. The regioselectivity of unsymmetrical arynes has also been studied. A lot of useful, functional groups can tolerate our reaction conditions. Carbazoles and dibenzofurans are important heteroaromatic compounds, which have a variety of biological activities. A variety of substituted carbazoles and dibenzofwans are readily prepared in good to excellent yields starting with the corresponding o-iodoanilines or o-iodophenols and o-silylaryl triflates by a treatment with CsF, followed by a Pd-catalyzed cyclization, which overall provides a one-pot, two-step process. By using this methodology, the carbazole alkaloid mukonine has been concisely synthesized in a very good yield. Insertion of an aryne into a {sigma}-bond between a nucleophile and an electrophile (Nu-E) should potentially be a very beneficial process from the standpoint of organic synthesis. A variety of substituted ketones and sulfoxides have been synthesized in good yields via the intermolecular C-N {sigma}-bond addition of amides and S-N {sigma}-bond addition of sulfinamides to ...
Date: December 12, 2006
Creator: Liu, Zhijian
Partner: UNT Libraries Government Documents Department

Oxidations of Organic and Inorganic Substrates by Superoxo-, hydroperoxo-, and oxo-compounds of the transition metals.

Description: Chapters 1 and 2 dealt with the chemistry of superoxo-, hydroperoxo-, and oxo- complexes of chromium, rhodium and cobalt. Chapter 3 dealt with the mechanism of oxygen-atom transfer catalyzed by an oxo-complex of rhenium. In Chapter 1, it was shown that hydroperoxometal complexes of cobalt and rhodium react with superoxochromium and chromyl ions, generating reduced chromium species while oxidizing the hydroperoxometal ions to their corresponding superoxometal ions. It was shown that the chromyl and superoxochromium ions are the more powerful oxidants. Evidence supports hydrogen atom transfer from the hydroperoxometal ion to the oxidizing superoxochromium or chromyl ion as the reaction mechanism. There is a significant H/D kinetic isotope effect. Comparisons to the rate constants of other known hydrogen atom transfer reactions show the expected correlation with bond dissociation energies. In Chapter 2, it was found that the superoxometal complexes Cr{sub aq}OO{sup 2+} and Rh(NH{sub 3}){sub 4}(H{sub 2}O)OO{sup 2+} oxidize stable nitroxyl radicals of the TEMPO series with rate constants that correlate with the redox potentials of both the oxidant and reductant. These reactions fit the Marcus equation for electron transfer near the theoretical value. Acid catalysis is important to the reaction, especially the thermodynamically limited cases involving Rh(NH{sub 3}){sub 4}(H{sub 2}O)OO{sup 2+} as the oxidant. The rate constants are notably less than those measured in the reaction between the same nitroxyl radicals and other strong free-radical oxidants, an illustration of the delocalized and stabilized nature of the superoxometal ions. Chapter 3 showed that oxo-rhenium catalysts needed a nucleophile to complete the catalytic oxygen-atom transfer from substituted pyridine-N-oxides to triphenylphosphine. The reaction was studied by introducing various pyridine-derived nucleophiles and monitoring their effect on the rate, then fitting the observed rate constants to the Hammett correlation. It was found that the values of the Hammett reaction constant PN were -1.0(1) for ...
Date: December 12, 2006
Creator: Vasbinder, Michael John
Partner: UNT Libraries Government Documents Department

Phase Transformation in Cast Superaustenitic Stainless Steels

Description: Superaustenitic stainless steels constitute a group of Fe-based alloys that are compositionally balanced to have a purely austenitic matrix and exhibit favorable pitting and crevice corrosion resistant properties and mechanical strength. However, intermetallic precipitates such as sigma and Laves can form during casting or exposure to high-temperature processing, which degrade the corrosion and mechanical properties of the material. The goal of this study was to accurately characterize the solid-solid phase transformations seen in cast superaustenitic stainless steels. Heat treatments were performed to understand the time and temperature ranges for intermetallic phase formations in alloys CN3MN and CK3MCuN. Microstructures were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy and wavelength dispersive spectroscopy (EDS, WDS). The equilibrium microstructures, composed primarily of sigma and Laves within purely austenitic matrices, showed slow transformation kinetics. Factors that determine the extent of transformation, including diffusion, nucleation, and growth, are discussed.
Date: December 12, 2006
Creator: Phillips, Nathaniel Steven Lee
Partner: UNT Libraries Government Documents Department

Ultrafast Control of Magnetism in Ferromagnetic Semiconductors via Photoexcited Transient Carriers

Description: The field of spintronics offers perspectives for seamless integration of coupled and inter-tunable electrical and magnetic properties in a single device. For integration of the spin degree of freedom with current electronic technology, new semiconductors are needed that show electrically-tunable magnetic properties at room temperature and above. Dilute magnetic semiconductors derived from III-V compounds, like GaMnAs and InMnAs, show coupled and tunable magnetic, transport, and optical properties, due to the fact that their ferromagnetism is hole-mediated. These unconventional materials are ideal systems for manipulating the magnetic order by changing the carrier polarization, population density, and energy band distribution of the complementary subsystem of holes. This is the main theme we cover in this thesis. In particular, we develop a unique setup by use of ultraviolet pump, near-infrared probe femtosecond laser pulses, that allows for magneto-optical Kerr effect (MOKE) spectroscopy experiments. We photo-excite transient carriers in our samples, and measure the induced transient magnetization dynamics. One set of experiments performed allowed us to observe for the first time enhancement of the ferromagnetic order in GaMnAs, on an ultrafast time scale of hundreds of picoseconds. The corresponding transient increase of Curie temperature (Tc, the temperature above which a ferromagnetic material loses its permanent magnetism) of about 1 K for our experimental conditions is a very promising result for potential spintronics applications, especially since it is seconded by observation of an ultrafast ferromagnetic to paramagnetic phase transition above Tc. In a different set of experiments, we"write" the magnetization in a particular orientation in the sample plane. Using an ultrafast scheme, we alter the distribution of holes in the system and detect signatures of the particular memory state in the subsequent magnetization dynamics, with unprecedented hundreds of femtosecond detection speed. The femtosecond cooperative magnetic phenomena presented here further our understanding of Mn-hole correlations in ...
Date: December 12, 2008
Creator: Cotoros, Ingrid A.
Partner: UNT Libraries Government Documents Department

Single Molecule Screening of Disease DNA Without Amplification

Description: The potential of single molecule detection as an analysis tool in biological and medical fields is well recognized today. This fast evolving technique will provide fundamental sensitivity to pick up individual pathogen molecules, and therefore contribute to a more accurate diagnosis and a better chance for a complete cure. Many studies are being carried out to successfully apply this technique in real screening fields. In this dissertation, several attempts are shown that have been made to test and refine the application of the single molecule technique as a clinical screening method. A basic applicability was tested with a 100% target content sample, using electrophoretic mobility and multiple colors as identification tools. Both electrophoretic and spectral information of individual molecule were collected within a second, while the molecule travels along the flow in a capillary. Insertion of a transmission grating made the recording of the whole spectrum of a dye-stained molecule possible without adding complicated instrumental components. Collecting two kinds of information simultaneously and combining them allowed more thorough identification, up to 98.8% accuracy. Probing mRNA molecules with fluorescently labeled cDNA via hybridization was also carried out. The spectral differences among target, probe, and hybrid were interpreted in terms of dispersion distances after transmission grating, and used for the identification of each molecule. The probes were designed to have the least background when they are free, but have strong fluorescence after hybridization via fluorescence resonance energy transfer. The mRNA-cDNA hybrids were further imaged in whole blood, plasma, and saliva, to test how far a crude preparation can be tolerated. Imaging was possible with up to 50% of clear bio-matrix contents, suggesting a simple lysis and dilution would be sufficient for imaging for some cells. Real pathogen DNA of human papillomavirus (HPV) type-I6 in human genomic DNA was probed with fluorescently-labeled probe ...
Date: December 12, 2006
Creator: Lee, Ji-Young
Partner: UNT Libraries Government Documents Department

Mechanistic Selection and Growth of Twinned Bicrystalline Primary Si in Near Eutectic Al-Si Alloys

Description: Morphological evolution and selection of angular primary silicon is investigated in near-eutectic Al-Si alloys. Angular silicon arrays are grown directionally in a Bridgman furnace at velocities in the regime of 10{sup -3} m/sec and with a temperature gradient of 7.5 x 10{sup 3} K/m. Under these conditions, the primary Si phase grows as an array of twinned bicrystalline dendrites, where the twinning gives rise to a characteristic 8-pointed star-shaped primary morphology. While this primary Si remains largely faceted at the growth front, a complex structure of coherent symmetric twin boundaries enables various adjustment mechanisms which operate to optimize the characteristic spacings within the primary array. In the work presented here, this primary silicon growth morphology is examined in detail. In particular, this thesis describes the investigation of: (1) morphological selection of the twinned bicrystalline primary starshape morphology; (2) primary array behavior, including the lateral propagation of the starshape grains and the associated evolution of a strong <100> texture; (3) the detailed structure of the 8-pointed star-shaped primary morphology, including the twin boundary configuration within the central core; (4) the mechanisms of lateral propagation and spacing adjustment during array evolution; and (5) the thermosolutal conditions (i.e. operating state) at the primary growth front, including composition and phase fraction in the vicinity of the primary tip.
Date: December 12, 2006
Creator: Jung, Choonho
Partner: UNT Libraries Government Documents Department

High Pressure Scanning Tunneling Microscopy and High PressureX-ray Photoemission Spectroscopy Studies of Adsorbate Structure,Composition and Mobility during Catalytic Reactions on A Model SingleCrystal

Description: Our research focuses on taking advantage of the ability of scanning tunneling microscopy (STM) to operate at high-temperatures and high-pressures while still providing real-time atomic resolution images. We also utilize high-pressure x-ray photoelectron spectroscopy (HPXPS) to monitor systems under identical conditions thus giving us chemical information to compare and contrast with the structural and dynamic data provided by STM.
Date: May 12, 2006
Creator: Montano, M. O.
Partner: UNT Libraries Government Documents Department

Surface structures of Al-Pd-Mn and Al-Cu-Fe icosahedral quasicrystals

Description: In this dissertation, the author reports on the surface structure of i-Al-Pd-Mn twofold, threefold, fivefold and i-Al-Cu-Fe fivefold surfaces. The LEED studies indicate the existence of two distinct stages in the regrowth of all four surfaces after Ar{sup +} sputtering. In the first stage, upon annealing at relatively low temperature: 500K--800K (depending on different surfaces), a cubic phase appears. The cubic LEED patterns transform irreversibly to unreconstructed quasicrystalline patterns upon annealing to higher temperatures, indicating that the cubic overlayers are metastable. Based upon the data for three chemically-identical, but symmetrically-inequivalent surfaces, a model is developed for the relation between the cubic overlayers and the quasicrystalline substrate. The model is based upon the related symmetries of cubic close-packed and icosahedral-packed materials. These results may be general among Al-rich, icosahedral materials. STM study of Al-Pd-Mn fivefold surface shows that terrace-step-kink structures start to form on the surface after annealing above 700K. Large, atomic ally-flat terraces were formed after annealing at 900K. Fine structures with fivefold icosahedral symmetry were found on those terraces. Data analysis and comparison of the STM images and structure model of icosahedral Al-Pd-Mn suggest that the fine structures in the STM images may be the pseudo Mackay (PMI) clusters which are the structure units of the structure model. Based upon his results, he can conclude that quasicrystalline structures are the stable structures of quasicrystal surfaces. In other words, quasicrystalline structures extend from the bulk to the surface. As a result of the effort reported in this dissertation, he believes that he has increased his understanding of the surface structure of icosahedral quasicrystals to a new level.
Date: February 12, 1999
Creator: Shen, Z.
Partner: UNT Libraries Government Documents Department

Gas atomization processing of tin and silicon modified LaNi{sub 5} for nickel-metal hydride battery applications

Description: Numerous researchers have studied the relevant material properties of so-called AB{sub 5} alloys for battery applications. These studies involved LaNi{sub 5} substituted alloys which were prepared using conventional cast and crush alloying techniques. While valuable to the understanding of metal hydride effects, the previous work nearly ignored the potential for alternative direct powder production methods, like high pressure gas atomization (HPGA). Thus, there is a need to understand the relationship between gas atomization processes, powder particle solidification phases, and hydrogen absorption properties of ultra fine (< 25 {micro}m) atomized powders with high surface area for enhanced battery performance. Concurrently, development of a gas atomization nozzle that is more efficient than all current designs is needed to increase the yield of ultrafine AB{sub 5} alloy powder for further processing advantage. Gas atomization processing of the AB{sub 5} alloys was demonstrated to be effective in producing ultrafine spherical powders that were resilient to hydrogen cycling for the benefit of improving corrosion resistance in battery application. These ultrafine powders benefited from the rapid solidification process by having refined solute segregation in the microstructure of the gas atomized powders which enabled a rapid anneal treatment of the powders. The author has demonstrated the ability to produce high yields of ultrafine powder efficiently and cost effectively, using the new HPGA-III technology. Thus, the potential benefits of processing AB{sub 5} alloys using the new HPGA technology could reduce manufacturing cost of nickel-metal hydride powder. In the near future, the manufacture of AB{sub 5} alloy powders could become a continuous and rapid production process. The economic benefit of an improved AB{sub 5} production process may thereby encourage the use of nickel-metal hydride rechargeable batteries in electrical vehicle applications in the foreseeable future.
Date: February 12, 1999
Creator: Ting, J.
Partner: UNT Libraries Government Documents Department

The analysis of aqueous mixtures using liquid chromatography-electrospray mass spectrometry

Description: The focus of this dissertation is the use of chromatographic methods coupled with electrospray mass spectrometry (ES-MS) for the determination of both organic and inorganic compounds in aqueous solutions. The combination of liquid chromatography (LC) methods and ES-MS offers one of the foremost methods for determining compounds in complex aqueous solutions. In this work, LC-ES-MS methods are devised using ion exclusion chromatography, reversed phase chromatography, and ion exchange chromatography, as well as capillary electrophoresis (CE). For an aqueous sample, these LC-ES-MS and CE-ES-MS techniques require no sample preparation or analyte derivatization, which makes it possible to observe a wide variety of analytes as they exist in solution. The majority of this work focuses on the use of LC-ES-MS for the determination of unknown products and intermediates formed during electrochemical incineration (ECI), an experimental waste remediation process. This report contains a general introduction to the project and the general conclusions. Four chapters have been removed for separate processing. Titles are: Chapter 2: Determination of small carboxylic acids by ion exclusion chromatography with electrospray mass spectrometry; Chapter 3: Electrochemical incineration of benzoquinone in aqueous media using a quaternary metal oxide electrode in the absence of a soluble supporting electrolyte; Chapter 4: The determination of electrochemical incineration products of 4-chlorophenol by liquid chromatography-electrospray mass spectrometry; and Chapter 5: Determination of small carboxylic acids by capillary electrophoresis with electrospray mass spectrometry.
Date: February 12, 1999
Creator: Johnson, S.
Partner: UNT Libraries Government Documents Department