23,009 Matching Results

Search Results

Nucleation and dynamics of vortices in type-II superconductors

Description: The one- and two-dimensional Ginzburg-Landau equations are numerically integrated in a slab geometry, which is appropriate for comparison to experimental work done on films. When two-dimensional variations become energetically favorable, a vortex is found to nucleate and move to the center of the film with the Gibbs free energy decreasing during the process. An important process by which the energy is lowered during this nucleation procedure is found to be the savings in condensation energy arising from the shrinking size of the vortex core as it moves to the center of the film. The solutions of the Ginzburg-Landau equations are used to explain anomalies observed experimentally in the tunneling characteristics of thin films of PbIn. Excellent agreement between theory and experiment is found with the Ginzburg-Landau equations correctly predicting the field at which flux would first enter the films. We then use the Clem model of an isolated vortex to model vortex nucleation and dynamics under the influence of a transport current. The entry fields predicted by the model are found to be off by almost a factor of two but have the advantage of requiring simple computer programs for their solution, while the Ginzburg-Landau solutions require substantially more numerical work.
Date: March 1, 1977
Creator: Balley, R. E.
Partner: UNT Libraries Government Documents Department

Studies of design parameters in the fabrication of Nb--Al--Ge superconductors by the powder metallurgy infiltration method

Description: Experimental studies have been carried out in which the A15 phase of the Nb-Al-Ge system has been synthesized in the form of thin filaments contained in form rolled wires. A powder metallurgy approach has been used to achieve controlled porosity in compacts of sintered niobium powder. Infiltration with an aluminum-germanium eutectic alloy followed by mechanical deformation has produced small interconnected filaments embedded in the Nb matrix. Diffusion heat treatment for a short time transforms them into the A15 superconducting compound with a size range of 1-5 microns at 1300/sup 0/C and 1-9 microns at 1750/sup 0/C. The superconducting properties T/sub c/ and J/sub c/ were evaluated for samples subjected to different condition of time and temperature. The influence of certain parameters involved in the process has been investigated. The microstructure and microhardness of the Al-Ge eutectic alloy cooled at high and low cooling rates from temperatures between 900-400/sup 0/C have been evaluated. Optical and electron beam metallographic results are presented for the analysis of the different phases and relative compositions. The critical temperature measured inductively is between 16.4/sup 0/K and 18.1/sup 0/K depending on the heat treatment. The critical current density as a function of the applied magnetic field is reported. These data were obtained using a pulsed magnetic field technique that measured J/sub c/ at 4.2/sup 0/K in fields up to 100 kG. J/sub c/ was 8.8 x 10/sup 4/ amp/cm/sup 2/ at 20 kG and 3.6 x 10/sup 4/ amp/cm/sup 2/ at 60 kG for specimens containing approximately 20% A15 phase.
Date: December 1, 1976
Creator: Granda, J. J.
Partner: UNT Libraries Government Documents Department

Characterization of sialon-type materials

Description: Four sialon-type materials using volcanic ash as a raw material were characterized and some of their properties were determined. The M3 and M4 materials were identified as ..beta../sup 1/--Si/sub 3/N/sub 4/ sialons; their principal constituent is silicon. The M2 material was identified as a 15R-A1N polytype sialon whose principal constituent is aluminum. The M1 material is a mixture of the two types. An overview of results showing the general structural formulae and the relative order of the materials with respect to various properties as determined by the investigation is presented. It is concluded that of the materials tested, the M2 material shows the most promise as a candidate for meeting some of the current needs for high-temperature materials. It is also concluded that more research is needed in order to explain the low resistance of these materials to thermal shock since their coefficients of thermal expansion are relatively low.
Date: June 1, 1977
Creator: Spencer, P. N.
Partner: UNT Libraries Government Documents Department

Interrelationships between thermal history and mechanical properties of a secondary hardening steel. [Vasco MA steel]

Description: Commercial secondary hardening steel, VASCO MA, has been subjected to modified heat treatments to introduce mixed microstructures of martensite and lower bainite. Dilatometry, tensile testing, hardness measurements and slow-bend testing have been carried out and the mechanical properties obtained have been correlated to microstructure using scanning electron microscopy and metallography. Duplex microstructures containing small amounts of lower bainite are found to show an increased toughness (at similar strength levels) on tempering in the secondary hardening range, as compared to initially fully martensitic microstructures. As the percentage of lower bainite in the duplex microstructures is increased, the secondary hardening peak is found to attenuate. The effect of different austenitization treatments on the secondary hardening behavior, has also been studied. Thermal cycling (for grain refinement) at lower austenitization temperatures is found to reprecipitate out carbon, which has been taken into solution in earlier high temperature austenitization, thereby leading to a significant depletion in the strength of this secondary hardening steel. Alloy carbides, in the secondary hardening stage, have also been analyzed using energy dispersive analysis of x-rays.
Date: December 1, 1976
Creator: Kar, N. J.
Partner: UNT Libraries Government Documents Department

Relationship of structure to effectiveness of some organophosphorus insecticides and the crystal and molecular structures of tris(bicarbonato) tetraaquoholmium (III) dihydrate and tris(ethylenediamine-cobalt (III) tetrakis (isothiocyanato cobaltate(II) nitrate

Description: Results are reported from an investigation of correlations between molecular structural parameters of selected organophosphorus insecticides and their corresponding toxic effectiveness. The crystal and molecular structures of azinphos-methyl, emidithion, and tetrachlorvinphos were determined via three-dimensional x-ray analysis. Acetylcholinesterase (AChE) in nerve cells was identified as the target for organophosphorus insecticides.
Date: June 1, 1977
Creator: Rohrbaugh, W. J.
Partner: UNT Libraries Government Documents Department

Effects of hydrogen on the single crystalline elastic constants of niobium

Description: A special hydriding system was designed and constructed to satisfy conditions for hydriding niobium. This system controlled the temperature and hydrogen atmosphere surrounding the niobium while ultrasonic measurements were recorded. Ultrasonic wave velocities were determined by measurement of the times for ultrasonic pulses to transit and then echo through known dimensions of test specimens. The method which was employed is commonly known as the pulse-echo-overlap method. This study confirmed the general trends of earlier investigations. In this study C' continued to decrease and C/sub 44/ continued to increase up to 4.69 atomic percent hydrogen which is the maximum concentration which has yet been examined. In the case of the niobium-hydrogen system the Snoek effect may well be a contributory factor to the decrease of C' with increasing hydrogen concentration. However, crystallographic considerations preclude this effect from contributing a concentration dependence to C/sub 44/ or B. The observation of the present work implies that other factors must also be contributing to the overall behavior.
Date: June 1, 1977
Creator: Schlader, D. M.
Partner: UNT Libraries Government Documents Department

Polarized electronic spectra for the crystals of three compounds, potassium tetrabromoplatinate(II) dihydrate, tetraethylammonium hexabromodiplatinate(II), and tetra-. mu. -glycine-dimolybdenum (II) sulfate tetrahydrate

Description: The polarized absorption spectra for K/sub 2/PtBr/sub 4/ . 2H/sub 2/O, (N(C/sub 2/H/sub 5/)/sub 4/)/sub 2/PtBr/sub 6/, and Mo/sub 2/(O/sub 2/CCH/sub 2/NH/sub 3/)/sub 4/(SO/sub 4/)/sub 2/ . 4H/sub 2/O have been recorded at 300 and 15/sup 0/K. In K/sub 2/PtBr/sub 4/ . 2H/sub 2/O the bands at 24,000 and 27,000 cm/sup -1/ in both a- and b-polarizations appear to be vibronically induced. The energy spacing of the vibrational structure was noted to be somewhat higher at 180 cm/sup -1/ than for the analogous structure of K/sub 2/PtBr/sub 4/. The presence of a Pt/sub 2/Br/sub 6//sup 2 -/ impurity gave rise to red sections, which evidently were due to the electron transfer, Pt(IV)/reverse arrow/Pt(II), occurring in c-polarization. Very weak spin-forbidden bands were observable in all three polarizations below 23,500 cm/sup -1/. In the crystal spectra of (N(C/sub 2/H/sub 5/)/sub 4/)/sub 2/Pt/sub 2/Br/sub 6/ the transitions were defined with respect to the three molecular axes of the Pt/sub 2/Br/sub 6//sup 2 -/ ion. Excited states were assigned under the D/sub 2//sub h/ point group symmetry of the ion. The delocalization of the d electrons gave rise to strongly enhanced intensities for both spin-forbidden and spin-allowed d/reverse arrow/d transitions. The M/reverse arrow/L charge-transfer transitions occur at lower energies in the case of Pt/sub 2/Br/sub 6//sup 2 -/ than in PtBr/sub 4//sup 2 -/. These charge transfer transitions were found from the polarizations to originate from the terminal bromides. The crystal spectra of Mo/sub 2/(O/sub 2/CCH/sub 2/NH/sub 3/)/sub 4/(SO/sub 4/)/sub 2/ . 4H/sub 2/O were recorded through the region 20,000 - 25,000 cm/sup -1/. The spectra at 15K revealed the vibrational structure in the two recorded polarizations. The electronic transition observed in these spectra was forbidden under the local symmetry of D/sub 4//sub h/.
Date: June 1, 1977
Creator: Peters, T. J.
Partner: UNT Libraries Government Documents Department

Separated-pair independent particle model and the generalized Brillouin theorem: ab initio calculations on the dissociation of polyatomic molecules

Description: A method is developed to optimize the separated-pair independent particle (SPIP) wave function; it is a special case of the separated-pair theory obtained by using two-term natural expansions of the geminals. The orbitals are optimized by a theory based on the generalized Brillouin theorem and iterative configuration interaction (CI) calculations in the space of the SPIP function and its single excitations. The geminal expansion coefficients are optimized by serial 2 x 2 CI calculations. Formulas are derived for the matrix elements. An algorithm to implement the method is presented, and the work needed to evaluate the molecular integrals is discussed. (auth)
Date: January 1, 1976
Creator: Sundberg, K. R.
Partner: UNT Libraries Government Documents Department

Studies of fundamental properties of rutherfordium (Element 104) using organic complexing agents

Description: Chemical properties of rutherfordium (Rf) have been investigated with the organic ligands triisooctylamine (TIOA), tributylphosphate (TBP), and thenoyltrifluoroacetone (TTA). The TIOA studies showed that Rf behaves differently than Th and Eu and most similarly to Zr, only Zr and Rf extract from 12 M HCI. This result is further evidence that Rf is a Group 4 element. Studies with TBP showed that Rf chemical behavior differed from the other Group 4 elements. The extraction by TBP at different chloride concentrations showed that Rf at times behaves more like Pu{sup 4+} than Zr or Hf. At high chloride concentrations, Rf and Pu extraction decreased. Under the same conditions, Zr, Hf and Th extraction increased. In addition, Rf extraction by TBP was affected by hydrogen ion concentration, while Zr and Hf extraction was not. TTA extractions were used to determine the K{sub eq}, K{sub hyd}, and the ionic radius of Rf. The of K{sub eq} for Rf with TTA was calculated to be 3.18 {plus minus} 0.90. The first four log K{sub hyd's} for Rf are calculated to be {minus}2.6 {plus minus} 0.7, {minus}5.9 {plus minus} 1.7, {minus}10.2 {plus minus} 2.9, and {minus}14.5 {plus minus} 4.1. These hydrolysis constants indicate that Rf will not hydrolyze at conditions under which Zr, Hf, and Pu will. Calculations of the ionic radius were made with the log K{sub eq} for Rf derived with TTA. The ionic radius of Rf was calculated to be 91 {plus minus} 4 pm for the 6-coordinate species and 102 {plus minus} 4 pm for the 8-coordinate species. A search for the isotope {sup 263}Rf was conducted using TTA chemistry. In 300 experiments, seven SF events and no alpha events were observed in the Rf chemical fraction. The observed SF events attributed to {sup 263}Rf and had a half-life of 500 ...
Date: April 1, 1992
Creator: Czerwinski, K. R.
Partner: UNT Libraries Government Documents Department

Two-state and two-state plus continuum problems associated with the interaction of intense laser pulses with atoms

Description: Two mathematical methods are utilized (one a form of adiabatic approximation, and the other closely related to the Zener method from collision theory) in order to calculate the probability of three-photon ionization when strong counter propagating pulses are tuned very near a two-photon resonant state. In this case the inverted populations predicted by Grischkowsky and Loy for smooth laser pulses lead to larger ionization probabilities than would be obtained for a square pulse of equal peak power and energy per pulse. The line shape of the ionization probability is also quite unusual in this problem. A sharp onset in the ionization probability occurs as the lasers are tuned through the exact unperturbed two-photon resonance. Under proper conditions, the change can be from a very small value to one near unity. It occurs in a very small frequency range determined by the larger of the residual Doppler effect and the reciprocal duration of the pulse. Thus, the line shape retains a Doppler-free aspect even at power levels such that power broadening would dwarf even the full Doppler effect in the case of a square pulse of equal energy and peak power. The same mathematical methods have been used to calculate line shapes for the two-photon excitation of fluorescence when the atoms see a pulsed field due to their time of passage across a tightly focused cw laser beam. Thus,the mathematical methods used above permitted accurate analytical calculations under a set of very interesting conditions.
Date: February 1, 1977
Creator: Choi, C. W. & Payne, M. G.
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of metal--metal bonded dimers of tantalum and tungsten

Description: A search for innovative synthetic routes to new metal-metal bonded metal-halide clusters resulted in isolation of two completely independent dimeric species. The syntheses and characterizations of Ta/sub 2/X/sub 6/(SC/sub 4/H/sub 8/)/sub 3/ (X = Cl, Br) and ((n-C/sub 3/H/sub 7/)/sub 4/N)/sub 2/(W/sub 2/Br/sub 9/) followed entirely different experimental routes, yet occasionally similar properties linked the two projects. (auth)
Date: December 1, 1975
Creator: Templeton, J. L.
Partner: UNT Libraries Government Documents Department

Crystal and molecular structure of organophosphorus insecticides

Description: The crystal and molecular structures of the following organophosphorus (OP) insecticides have been determined by three-dimensional X-ray analysis: ronnel (0,0-dimethyl 0-2,4,5 trichlorophenyl phosphorothioate), ronnel oxon (0,0-dimethyl 0-2,4,5 trichlorophenyl phosphate), bomophos (0-(4-bromo-2,5-dichlorophenyl) 0,0-dimethyl phosphorothioate), Ruelene (0-(4-tert-Butyl-2-chlorophenyl)-0-methyl-N-methyl phosphoroamidate), fospirate (0,0-dimethyl 0-3,5,6-trichloro-2-pyridyl phosphate) and chlorpyrifos (0,0-diethyl 0-3,5,6-trichloro-2-pyridyl phosphorothioate). Phosphorus to meta hydrogen distances for the respective compounds are: 5.51, 5.49, 5.42, (5.68, 5.13), 5.79 and 5.78 A. All of these distances fall well within the range of literature values cited for the intramolecular active site-separation distance for insect acetylcholinesterase (AChE), yet are well outside that for mammalian AChE. In addition the crystal structure of ronnel displays a hydrogen-sulfur intermolecular interaction in the b direction and a phosphorus which is readily accessible for phosphorylation of AChE. The structure of ronnel oxon is quite similar to that of ronnel. CNDO 11 molecular orbital calculations are presented to show the charge distribution in the compound. A likely intramolecular hydrogen bond in bromophos restricts rotation about the phenolic C-O bond. The configuration of Ruelene is substantiated by CNDO 11 molecular orbital calculations and van der Waals arguments and features a possible weak intramolecular hydrogen bond which somewhat restricts rotation about the phenolic C-O bond. Charge density information is also presented. The heterocyclic portion of fospirate and chlorpyrifos allow torsional angles about the pyridoxic C-O bond to be substantially different from the pnenoxy OP's. CNDO 11 molecular orbital calculations are presented.
Date: October 1, 1977
Creator: Baughman, R. G.
Partner: UNT Libraries Government Documents Department

Chemical shift anisotropies of /sup 1/H in H/sub 2/O(s), H/sub 2/S(s), and C/sub 6/H/sub 6/(s)

Description: The proton NMR in powdered samples of H/sub 2/O(s), H/sub 2/S(s), and C/sub 6/H/sub 6/(s) have been studied by multiple pulse line narrowing techniques. The resultant spectra provide nuclear magnetic shielding tensors that are (at least approximately) axially symmetric. The anisotropy is 34.2 +- 1.0 ppM for ice, 11.1 +- 1.0 ppM for the highest-temperature phase of solid hydrogen sulfide, and -5.3 +- 0.3 for benzene. Comparisons are made with previous experimental and theoretical work.
Date: October 1, 1977
Creator: Ryan, L. M.
Partner: UNT Libraries Government Documents Department

Investigations in silicate glasses. I. Radiation damage. II. Optical nonlinearity. [Gamma rays and electrons]

Description: The investigation of two poorly understood but technologically important physical properties of silicate glasses and related materials is described. The use of Electron Paramagnetic Resonance to investigate the nature of radiation-induced damage in glasses exposed to a variety of high-energy radiation sources is discussed first. Second, the measurement of the nonlinear index of refraction coefficient in a variety of optical materials related to the design of high-power laser systems is described. The radiation damage investigations rely heavily on the comparison of experimental results for different experimental situations. The comparison of EPR lineshapes, absolute spin densities and power saturation behavior is used to probe a variety of microscopic and macroscopic aspects of radiation damage in glasses. Comparison of radiation damage associated with exposure to gamma rays and fast neutrons (and combinations thereof) are interpreted in terms of the microscopic damage mechanisms which are expected to be associated with the specific radiations. Comparison of radiation damage behavior in different types of glasses is also interpreted in terms of the behavior expected for the specific materials. The body of data which is generated is found to be internally self-consistent and is also generally consistent with the radiation damage behavior expected for specific situations. A new and versatile technique for measuring the nonlinear index of refraction coefficient, n/sub 2/, in optical materials is described. The technique utilizes a 1 ns pulsed neodymium-glass laser system and time-resolved interferometry to determine the ratio of the coefficient n/sub 2/ of sample materials to the n/sub 2/ of CS/sub 2/. This method avoids some of the complications associated with performing absolute measurements of n/sub 2/ and allows the use of a relatively simple experimental technique. The measurements determine the nonlinear index ratios of the samples with an accuracy of about +-15 percent.
Date: November 15, 1976
Creator: Moran, Michael James
Partner: UNT Libraries Government Documents Department

Selectivity in multiple quantum nuclear magnetic resonance

Description: The observation of multiple-quantum nuclear magnetic resonance transitions in isotropic or anisotropic liquids is shown to give readily interpretable information on molecular configurations, rates of motional processes, and intramolecular interactions. However, the observed intensity of high multiple-quantum transitions falls off dramatically as the number of coupled spins increases. The theory of multiple-quantum NMR is developed through the density matrix formalism, and exact intensities are derived for several cases (isotropic first-order systems and anisotropic systems with high symmetry) to shown that this intensity decrease is expected if standard multiple-quantum pulse sequences are used. New pulse sequences are developed which excite coherences and produce population inversions only between selected states, even though other transitions are simultaneously resonant. One type of selective excitation presented only allows molecules to absorb and emit photons in groups of n. Coherent averaging theory is extended to describe these selective sequences, and to design sequences which are selective to arbitrarily high order in the Magnus expansion. This theory and computer calculations both show that extremely good selectivity and large signal enhancements are possible.
Date: November 1, 1980
Creator: Warren, W. S.
Partner: UNT Libraries Government Documents Department

Comparison between phase shift derived and exactly calculated nucleon--nucleon interaction matrix elements. [Tables]

Description: A comparison is made between matrix elements calculated using the uncoupled channel Sussex approach to second order in DWBA and matrix elements calculated using a square well potential. The square well potential illustrated the problem of the determining parameter independence balanced with the concept of phase shift difference. The supersoft core potential was used to discuss the systematics of the Sussex approach as a function of angular momentum as well as the relation between Sussex generated and effective interaction matrix elements. In the uncoupled channels the original Sussex method of extracting effective interaction matrix elements was found to be satisfactory. In the coupled channels emphasis was placed upon the /sup 3/S/sub 1/-/sup 3/D/sub 1/ coupled channel matrix elements. Comparison is made between exactly calculated matrix elements, and matrix elements derived using an extended formulation of the coupled channel Sussex method. For simplicity the potential used is a nonseparable cut-off oscillator. The eigenphases of this potential can be made to approximate the realistic nucleon-nucleon phase shifts at low energies. By using the cut-off oscillator test potential, the original coupled channel Sussex method of determining parameter independence was shown to be incapable of accurately reproducing the exact cut-off oscillator matrix elements. The extended Sussex method was found to be accurate to within 10%. The extended method is based upon more general coupled channel DWBA and a noninfinite oscillator wave function solution to the cut-off oscillator auxiliary potential. A comparison is made in the coupled channels between matrix elements generated using the original Sussex method and the extended method. Tables of matrix elements generated using the original uncoupled channel Sussex method and the extended coupled channel Sussex method are presented for all necessary angular momentum channels.
Date: January 1, 1975
Creator: Gregersen, A. W.
Partner: UNT Libraries Government Documents Department

Magnetic properties of MnPt

Description: The magnetic properties of quenched and annealed powder specimens of MnPt were investigated using x-ray diffraction, magnetization measurements and neutron diffraction techniques. Thin films of MnPt were prepared by rf sputtering techniques and the films were investigated by x-ray diffraction and magnetization measurements. The powder specimens of MnPt were found to be typically antiferromagnetic in an L1/sub 0/ structure with Mn atoms occupying the (001) planes. The Mn moments were antiparallel to their nearest neighbors in the (001) planes. The atomic moment of Mn is (4.1 +- .2) ..mu../sub B/ for the annealed specimens and (3.9 +- .2) ..mu../sub B/ for the quenched specimens. Neutron data failed to determine whether or not Pt carries a moment. Assuming a moment associated with the Pt atoms, the maximum value is 0.2 ..mu../sub B/ per atom. The quenched specimens contained small ferromagnetic regions probably with a composition of 18-32 at. percent Mn. The sputtered films of MnPt were highly disordered in all but one specimen, which was amorphous. The films were ferromagnetic with the deduced Mn moment of (3.0 +- .3) ..mu../sub B/ per atom, which may not represent the true value because the films were not entirely ferromagnetic.
Date: June 1, 1977
Creator: Severin, C. S.
Partner: UNT Libraries Government Documents Department

Interactions of hydrogen with ethylene and ethane on iridium

Description: In an effort to determine the details of reaction mechanisms, kinetic parameters are obtained for the following two catalytic reactions, C/sub 2/H/sub 4/ + H/sub 2/ ..-->.. C/sub 2/H/sub 6/ and C/sub 2/H/sub 6/ + H/sub 2/ ..-->.. 2 CH/sub 4/. The first reaction is carried out, for the most part, under reaction conditions (e.g. 110-200 K) which prevent complications caused by a side reaction, the surface dehydrogenation of adsorbed ethylene. The second reaction is carried out at somewhat higher temperatures (80 to 200/sup 0/C). Both reactions are studied in the pressure range 0.5 to 1000 ..mu... Extensive isotope labeling experiments are also carried out, which together with the kinetic measurements support in a self-consistent way the following mechanisms of hydrogen addition. The adsorbed species C/sub 2/H/sub 5/(a) and H(a) are found to be intermediates in both the hydrogenation and hydrogenolysis reactions. In the case of the hydrogenation reaction, the rate limiting step is found to be the irreversible addition of an adsorbed hydrogen atom to an adsorbed ethylene molecule to produce C/sub 2/H/sub 5/(a) which is further hydrogenated to produce ethane. The hydrogenolysis occurs by dissociative adsorption of ethane to produce C/sub 2/H/sub 5/(a) and H(a). In this case the final kinetically significant elementary step is the reaction of an adsorbed hydrogen atom with one of the methyl hydrogen atoms of C/sub 2/H/sub 5/(a), which produces a hydrogen molecule and is accompanied by the breaking of the carbon-carbon bond. Other processes which are kinetically significant for the hydrogenolysis reaction include slow (the sticking coefficient approximately 10/sup -5/) ethane adsorption, slow ethane desorption (by reaction of C/sub 2/H/sub 5/(a) with H(a)), the reversible dehydrogenation of C/sub 2/H/sub 5/(a) to produce C/sub 2/H/sub 4/(a) and competition of hydrogen for the surface sites on which the hydrocarbon species are adsorbed.
Date: June 1, 1977
Creator: Mahaffy, P. R.
Partner: UNT Libraries Government Documents Department

Aerosol Property Comparison Within and Above the ABL at the ARM Program SGP Site

Description: This thesis determines what, if any, measurements of aerosol properties made at the Earth surface are representative of those within the entire air column. Data from the Atmospheric Radiation Measurement site at the Southern Great Plains, the only location in the world where ground-based and in situ airborne measurements are routinely made. Flight legs during the one-year period from March 2000 were categorized as either within or above the atmospheric boundary layer (ABL) by use of an objective mixing height determination technique. Correlations between aerosol properties measured at the surface and those within and above the ABL were computed. Aerosol extensive and intensive properties measured at the surface were found representative of values within the ABL, but not of within the free atmosphere.
Date: May 1, 2002
Creator: Delle Monache, L
Partner: UNT Libraries Government Documents Department

The Effect of Initial Conditions on the Nonlinear Evolution of Perturbed Interfaces Driven by Strong Blast Waves

Description: In core-collapse supernovae, strong blast waves drive interfaces susceptible to Rayleigh-Taylor (RT), Richtmyer-Meshkov (RM), and Kelvin-Helmholtz (KH) instabilities. In addition, perturbation growth can result from material expansion in large-scale velocity gradients behind the shock front. Laser-driven experiments are designed to produce a strongly shocked interface whose evolution is a scaled version of the unstable hydrogen-helium interface in core-collapse supernovae such as SN 1987A. The ultimate goal of this research is to develop an understanding of the effect of hydrodynamic instabilities and the resulting transition to turbulence on supernovae observables that remain as yet unexplained. In this dissertation, we present a computational study of unstable systems driven by high Mach number shock and blast waves. Using multi-physics radiation hydrodynamics codes and theoretical models, we consider the late nonlinear instability evolution of single mode, few mode, and multimode interfaces. We rely primarily on 2D calculations but present recent 3D results as well. For planar multimode systems, we show that compressibility effects preclude the emergence of a regime of self-similar instability growth independent of the initial conditions (IC's) by allowing for memory of the initial conditions to be retained in the mix-width at all times. The loss of transverse spectral information is demonstrated, however, along with the existence of a quasi-self-similar regime over short time intervals. Aspects of the IC's are shown to have a strong effect on the time to transition to the quasi-self-similar regime. With higher-dimensional blast waves, divergence restores the properties necessary for establishment of the self-similar state, but achieving it requires very high initial characteristic mode number and high Mach number for the incident blast wave. We point to recent stellar calculations that predict IC's we find incompatible with self-similarity, and emphasize the consequent importance of developing a sound understanding of the initial modal structure in the supernova progenitor. ...
Date: April 27, 2004
Creator: Miles, A
Partner: UNT Libraries Government Documents Department

Cubic Spline Collocation Method for the Simulation of Turbulent Thermal Convection in Compressible Fluids

Description: A collocation method using cubic splines is developed and applied to simulate steady and time-dependent, including turbulent, thermally convecting flows for two-dimensional compressible fluids. The state variables and the fluxes of the conserved quantities are approximated by cubic splines in both space direction. This method is shown to be numerically conservative and to have a local truncation error proportional to the fourth power of the grid spacing. A ''dual-staggered'' Cartesian grid, where energy and momentum are updated on one grid and mass density on the other, is used to discretize the flux form of the compressible Navier-Stokes equations. Each grid-line is staggered so that the fluxes, in each direction, are calculated at the grid midpoints. This numerical method is validated by simulating thermally convecting flows, from steady to turbulent, reproducing known results. Once validated, the method is used to investigate many aspects of thermal convection with high numerical accuracy. Simulations demonstrate that multiple steady solutions can coexist at the same Rayleigh number for compressible convection. As a system is driven further from equilibrium, a drop in the time-averaged dimensionless heat flux (and the dimensionless internal entropy production rate) occurs at the transition from laminar-periodic to chaotic flow. This observation is consistent with experiments of real convecting fluids. Near this transition, both harmonic and chaotic solutions may exist for the same Rayleigh number. The chaotic flow loses phase-space information at a greater rate, while the periodic flow transports heat (produces entropy) more effectively. A linear sum of the dimensionless forms of these rates connects the two flow morphologies over the entire range for which they coexist. For simulations of systems with higher Rayleigh numbers, a scaling relation exists relating the dimensionless heat flux to the two-seventh's power of the Rayleigh number, suggesting the existence of ''hard'' turbulence in two-dimensional compressible convection.
Date: January 12, 2005
Creator: Castillo, V M
Partner: UNT Libraries Government Documents Department

Characterization of Functionalized Self-Assembled Monolayers and Surface-Attached Interlocking Molecules Using Near-Edge X-ray Absorption Fine Structure Spectroscopy

Description: Quantitative knowledge of the fundamental structure and substrate binding, as well as the direct measurement of conformational changes, are essential to the development of self-assembled monolayers (SAMs) and surface-attached interlocking molecules, catenanes and rotaxanes. These monolayers are vital to development of nano-mechanical, molecular electronic, and biological/chemical sensor applications. This dissertation investigates properties of functionalized SAMs in sulfur-gold based adsorbed molecular monolayers using quantitative spectroscopic techniques including near-edge x-ray absorption fine structure spectroscopy (NEXAFS) and x-ray photoelectron spectroscopy (XPS). The stability of the gold-thiolate interface is addressed. A simple model SAM consisting of dodecanethiol adsorbed on Au(111) degrades significantly in less than 24 hours under ambient laboratory air. S 2p and O 1s XPS show the gold-bound thiolates oxidize to sulfinates and sulfonates. A reduction of organic material on the surface and a decrease in order are observed as the layer degrades. The effect of the carboxyl vs. carboxylate functionalization on SAM structure is investigated. Carboxyl-terminated layers consisting of long alkyl-chain thiols vs. thioctic acid with short, sterically separated, alkyl groups are compared and contrasted. NEXAFS shows a conformational change, or chemical switchability, with carboxyl groups tilted over and carboxylate endgroups more upright. Surface-attached loops and simple surface-attached rotaxanes are quantitatively characterized, and preparation conditions that lead to desired films are outlined. A dithiol is often insufficient to form a molecular species bound at each end to the substrate, while a structurally related disulfide-containing polymer yields surface-attached loops. Similarly, spectroscopic techniques show the successful production of a simple, surface-attached rotaxane that requires a ''molecular riveting'' step to hold the mechanically attached crown ether rings in place until disulfides in the molecules adsorb onto the gold. Finally, by introducing an aldehyde into the crown ether portion of this well-characterized surface-attached rotaxane, conformational changes are directly measured when the aldehyde is reacted with ...
Date: March 24, 2004
Creator: Willey, T & Willey, T
Partner: UNT Libraries Government Documents Department

Analysis of Hypothetical Promoter Domains of DKFZp564A1164, NPHS1 and HSPOX1 Genes

Description: For this study, a high throughput method for identifying and testing regulatory elements was examined. In addition, the validity of promoters predicted by FirstEF was tested. It was found that by combining computer based promoter and first exon predictions from FirstEF (Davuluri et al., 2001) with PCR-based cloning to generate luciferase reporter constructs, and by testing reporter activity in cultured mammalian cells plated in a 96 well format one could identify promoter activity in a relatively high throughput manner. The data generated in this study suggest that FirstEF predictions are sometimes incorrect. Therefore, having a strategy for defining which FirstEF predicted promoters to test first may accelerate the process. Initially testing promoters that are at a confirmed transcription start site for a gene, at a possible alternate transcription start site or in a region of conserved sequence would be the best candidates, while promoters predicted in gene desert regions may not be as easy to confirm. The luciferase assay lent itself very well to the high throughput search, however the subcloning did not always go smoothly. The numerous steps that this traditional subcloning method requires were time consuming and increased the opportunities for errors. A faster method that skips many of the traditional subcloning steps, such as the Creator{trademark} system by Clontech is currently being investigated by our lab. The development and testing of substantially larger enhancer/silencer regulatory elements may not be possible at this time using these high throughput methods. These regulatory elements are generally GC rich making them more difficult to PCR and subclone. Additionally, confirming upstream untranslated first exons was not possible within this time scale using the SMART RACE protocol. It will be necessary to further explore the limitations within these procedures in order to confirm these and future regulatory elements. Alterations and modifications to these ...
Date: November 29, 2003
Creator: Hammond, S S
Partner: UNT Libraries Government Documents Department

Single crystal electronic spectra for the compounds potassium octachlorodimolybdate(II)-dihydrate, potassium tetra-. mu. -sulfatodimolybdate(II)-dihydrate and potassium tetra-. mu. -sulfatodimolybdate(II,III)-3. 5 hydrate

Description: The polarized electronic absorption spectra for single crystals of potassium octachlorodimolybdate (II-dihydrate, potassium tetra-..mu..-sulfatodimolybdate (II)-dihydrate and potassium tetra-..mu..-sulfatodimolybdate (II,III)-3.5 hydrate have been obtained at room and liquid helium temperatures.
Date: June 1, 1977
Creator: Fanwick, P. E.
Partner: UNT Libraries Government Documents Department