70 Matching Results

Search Results

Results: 1 - 24 next
open access

Theory of ordering transformations in metals and minerals

Description: This dissertation presents an investigation of ordering in FCC based systems using the pair potential approximation in the ground state and mean field limits. The theoretical approach is used to explain the occurrence of observed equilibrium phases and characteristics of thermodynamic instabilities, in particular, spinodal ordering and decomposition. It is shown that the stability of non-integer domain sizes in long period superstructures such as Al{sub 3}Ti and Ag{sub 3}Mg may result from the … more
Date: July 1, 1991
Creator: Lindsey, Timothy Francis
Partner: UNT Libraries Government Documents Department
open access

Interpretation of time domain electromagnetic soundings near geological contacts

Description: Lateral changes in geology pose a serious problem in data interpretation for any surface geophysical method. Although many geophysical techniques are designed to probe vertically, the source signal invariably spreads laterally, so any lateral variations in geology will affect the measurements and interpretation. This problem is particularly acute for controlled source electromagnetic soundings because only a few techniques are available to interpret the data if lateral effects are present. In t… more
Date: December 1, 1991
Creator: Wilt, Michael Joseph
Partner: UNT Libraries Government Documents Department
open access

Cracking behavior of cored structures

Description: The effects of compositional gradients, are considered based on a thermodynamic analysis, referred to as the Cahn-Hillard analysis, which describes the degree to which a local surface energy is modified by the presence of a composition gradient. The analysis predicts that both ductile and brittle fracture mechanisms are enhanced by the presence of a composition gradient. Data on stress corrosion cracking and fatigue crack growth in selected FCC alloys are used to illustrate the significance of … more
Date: September 1991
Creator: Wahid, A.; Olson, D.L.; Matlock, D.K. (Colorado School of Mines, Golden, CO (United States). Center for Welding and Joining Research) & Kelly, T.J. (General Electric Aircraft Engines, Evendale, OH (United States))
Partner: UNT Libraries Government Documents Department
open access

Capacitance of edge plane of pyrolytic graphite in acetonitrile solutions

Description: The capacitance of the edge plane of pyrolytic graphite electrodes, in acetonitrile solutions, is measured by recording the current response to an applied triangular voltage sweep; TVS, and then fitting the current response with an appropriate function, (via a set of adjustable parameters). The pretreatment of the electrodes, the supporting electrolyte concentration used, and the frequency of the input TVS, were all found to affect the measured capacitance. In these experiments, a background cu… more
Date: May 1, 1991
Creator: Minick, S. K. & Ishida, Takanobu
Partner: UNT Libraries Government Documents Department
open access

1. Mono([8]annulene)Uranium(4) half-sandwich complexes, 2. Novel syntheses of symmetrically substituted cyclooctatetetraenes

Description: A reproducible, high-yield synthesis of mono([8]annulene)uranium(4)dichloride (1) is reported, along with the X-ray crystal structural of the bis(pyridine) adduct. Metathesis reactions of the half-sandwich complex 1 with a variety of simple alkyl and alkoxy reagents failed to generate any isolable mono-ring complexes. Reactions of 1 with polydentate, delocalized anions did produce stable derivatives, including mono([8]annulene)uranium(4)bis(acetylacetonate) (4). An X-ray crystal structure of 4 … more
Date: October 1, 1991
Creator: Boussie, T. R.
Partner: UNT Libraries Government Documents Department
open access

The application of psoralens to the study of DNA structure, function and dynamics

Description: A series of six nitroxide spin-labeled psoralens were designed, synthesized and tested as probes for DNA dynamics. The synthesis of these spin-labeled psoralen derivatives and their photoreactivity with double-stranded DNA fragments is described. The spin labels (nitroxides) were demonstrated to survive the uv irradiation required to bind the probe to the target DNA. EPR spectra of the photobound spin-labels indicate that they do not wobble with respect to the DNA on the time-scales investigate… more
Date: April 1, 1991
Creator: Spielmann, P. H.
Partner: UNT Libraries Government Documents Department
open access

The generation and acceleration of low emittance flat beams for future linear colliders

Description: Many future linear collider designs call for electron and positron beams with normalized rms horizontal and vertical emittances of {gamma}{epsilon}{sub x} = 3{times}10{sup {minus}6} m-rad and {gamma}{epsilon}{sub y} = 3{times}10{sup {minus}8} m-rad; these are a factor of 10 to 100 below those observed in the Stanford Linear Collider. In this dissertation, we examine the feasibility of achieving beams with these very small vertical emittances. We examine the limitations encountered during both t… more
Date: November 1, 1991
Creator: Raubenheimer, T. O.
Partner: UNT Libraries Government Documents Department
open access

Potential energy surfaces and reaction dynamics of polyatomic molecules

Description: A simple empirical valence bond (EVB) model approach is suggested for constructing global potential energy surfaces for reactions of polyatomic molecular systems. This approach produces smooth and continuous potential surfaces which can be directly utilized in a dynamical study. Two types of reactions are of special interest, the unimolecular dissociation and the unimolecular isomerization. For the first type, the molecular dissociation dynamics of formaldehyde on the ground electronic surface … more
Date: November 1, 1991
Creator: Chang, Yan-Tyng
Partner: UNT Libraries Government Documents Department
open access

An amplitude analyses of the K K-bar system (M < 2 GeV/C{sup 2}) produced in J/{Psi} radiative decay

Description: Using the 5.8 {times} 10{sup 6} J/{psi} events collected by the MARK 3 experiment on the SPEAR e{sup +}e{sup {minus}} storage ring at SLAC, a mass independent amplitude analysis of the J/{psi} {yields} {gamma} K{sub s}K{sub s} and J/{psi} {yields} K{sup +}K{sup {minus}} decays is presented. For K{bar K} systems having mass less than 2 GeV/c{sup 2}, the efficiency-corrected spherical harmonic moments of the J/{psi} joint decay angular distribution are measured. Fits are then performed in each in… more
Date: November 1, 1991
Creator: Chen, Liang-Ping
Partner: UNT Libraries Government Documents Department
open access

Characterization of electrochemically modified polycrystalline platinum surfaces

Description: The characterization of electrochemically modified polycrystalline platinum surfaces has been accomplished through the use of four major electrochemical techniques. These were chronoamperometry, chronopotentiommetry, cyclic voltammetry, and linear sweep voltammetry. A systematic study on the under-potential deposition of several transition metals has been performed. The most interesting of these were: Ag, Cu, Cd, and Pb. It was determined, by subjecting the platinum electrode surface to a singl… more
Date: December 1, 1991
Creator: Krebs, L. C. & Ishida, Takanobu
Partner: UNT Libraries Government Documents Department
open access

Bis(pentamethylcyclopentadienyl) ytterbium: Electron-transfer reactions with organotransition metal complexes

Description: The divalent lanthanide complex, (Me{sub 5}C{sub 5}){sub 2}Yb, reacts with methylcopper to produce the base-free, ytterbium-methyl complex, (Me{sub 5}C{sub 5}){sub 2}YbMe. This product forms a asymmetric, methyl-bridged dimer in the solid state. The bulky alkyl complex, (Me{sub 5}C{sub 5}){sub 2}YbCH(SiMe{sub 3}){sub 2}, displays similar chemistry to (Me{sub 5}C{sub 5}){sub 2}YbMe, but at a reduced reaction rate due to the limited accessibility of the metal in (Me{sub 5}C{sub 5}){sub 2}YbCH(SiM… more
Date: November 1, 1991
Creator: Matsunaga, P. T.
Partner: UNT Libraries Government Documents Department
open access

Quantum-limited detection of millimeter waves using superconducting tunnel junctions

Description: The quasiparticle tunneling current in a superconductor-insulator- superconductor (SIS) tunnel junction is highly nonlinear. Such a nonlinearity can be used to mix two millimeter wave signals to produce a signal at a much lower intermediate frequency. We have constructed several millimeter and sub-millimeter wave SIS mixers in order to study high frequency response of the quasiparticle tunneling current and the physics of high frequency mixing. We have made the first measurement of the out-of-p… more
Date: September 1, 1991
Creator: Mears, C. A.
Partner: UNT Libraries Government Documents Department
open access

Electromagnetic dissociation of {sup 238}U in heavy-ion collisions at 120 MeV/A

Description: This thesis describes a measurement of the heavy-ion induced electromagnetic dissociation of a 120 MeV/A {sup 238}U beam incident on five targets: {sup 9}Be, {sup 27}Al, {sup nat}Cu, {sup nat}Ag, and {sup nat}U. Electromagnetic dissociation at this beam energy is essentially a two step process involving the excitation of a giant resonance followed by particle decay. At 120 MeV/A there is predicted to be a significant contribution of the giant quadrupole resonance to the EMD cross sections. The … more
Date: April 1, 1991
Creator: Justice, M. L.
Partner: UNT Libraries Government Documents Department
open access

Model heterogeneous acid catalysts and metal-support interactions: A combined surface science and catalysis study

Description: This (<100 {Angstrom}) silica-alumina layers were tested as potential model heterogeneous acid catalysts for combined surface science and catalysis studies. Three preparation methods were used: oxidation of r3 {times} r3 R30 Al/Si(111) structure in UHV; deposition on Si(lll) from aqueous solution; and argon ion beam sputter deposition in UHV. The homogeneous thin layers are amorphous, and the chemical environment of surface atoms is similar to that of Si, Al and oxygen atoms on high surface are… more
Date: May 1, 1991
Creator: Boszormenyi, I.
Partner: UNT Libraries Government Documents Department
open access

Spatially variant tomographic imaging: Estimation, identification, and optimization

Description: This thesis is an investigation of methods for processing multidimensional signals acquired using modern tomography systems that have an anisotropic or spatially variant response function. The main result of this research is the discovery of a new method to obtain better estimators of an unknown spatial intensity distribution by incorporating detailed knowledge about the tomograph system response function and statistical properties of the acquired signal into a mathematical model.
Date: November 1, 1991
Creator: Baker, J. R.
Partner: UNT Libraries Government Documents Department
open access

Imaging earth`s interior: Tomographic inversions for mantle P-wave velocity structure

Description: A formalism is developed for the tomographic inversion of seismic travel time residuals. The travel time equations are solved both simultaneously, for velocity model terms and corrections to the source locations, and progressively, for each set of terms in succession. The methods differ primarily in their treatment of source mislocation terms. Additionally, the system of equations is solved directly, neglecting source terms. The efficacy of the algorithms is explored with synthetic data as we p… more
Date: July 1, 1991
Creator: Pulliam, R. J.
Partner: UNT Libraries Government Documents Department
open access

A Surface Science Investigation of Silicon Carbide: Oxidation, Crystal Growth and Surface Structural Analysis

Description: For the semiconductor SiC to fulfill its potential as an electronic material, methods must be developed to produce insulating surface oxide layers in a reproducible fashion. Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) were used to investigate the oxidation of single crystal {alpha}-SiC over a wide temperature and O{sub 2} pressure range. The {alpha}-SiC surface becomes graphitic at high temperatures and low O{sub 2} pressu… more
Date: November 1, 1991
Creator: Powers, J. M.
Partner: UNT Libraries Government Documents Department
open access

AC microcalorimetry of adsorbates on evaporated metal films: Orientational ordering of H₂ multilayers

Description: We have improved and extended a novel ac calorimetric technique for measuring the heat capacity of adsorbates on evaporated metal films. Metallic substrates are of particular interest in current studies of the thermodynamics of adsorbed molecules. The method described in the present work is only calorimetric technique which allows measurements of molecules on simple metallic surfaces. Among other improvements, we have achieved significant progress in the preparation and characterization of the … more
Date: November 1, 1991
Creator: Phelps, R. B.
Partner: UNT Libraries Government Documents Department
open access

Optical pumping and xenon NMR

Description: Nuclear Magnetic Resonance (NMR) spectroscopy of xenon has become an important tool for investigating a wide variety of materials, especially those with high surface area. The sensitivity of its chemical shift to environment, and its chemical inertness and adsorption properties make xenon a particularly useful NMR probe. This work discusses the application of optical pumping to enhance the sensitivity of xenon NMR experiments, thereby allowing them to be used in the study of systems with lower … more
Date: November 1, 1991
Creator: Raftery, M. D.
Partner: UNT Libraries Government Documents Department
open access

Oligodendroglial response to ionizing radiation: Dose and dose-rate response

Description: An in vitro system using neuroglia from neonatal rat brain was developed to examine the morphologic, immunocytochemical and biochemical response of oligodendroglia to ionizing radiation. Following acute {gamma}-irradiation at day-in-culture (DIC) 8, oligodendrocyte counts at DIC 14 were 55% to 65% of control values after 2 Gy, and 29% to 36% after 5 Gy. Counts increased to near-normal levels at DIC 21 in the 2 Gy group and to 75% of normal in the 5 Gy group. Myelin basic protein levels (MBP) at… more
Date: December 1, 1991
Creator: Levy, R. P.
Partner: UNT Libraries Government Documents Department
open access

Magnetic flux noise in copper oxide superconductors

Description: Magnetic flux noise and flux creep in thin films and single crystals of YBa{sub 2}Cu{sub 3}O{sub 7-x}, Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+x}, Tl{sub 2}Ca{sub 2}Ba{sub 2}Cu{sub 3}O{sub x}, and TlCa{sub 2}Ba{sub 2}Cu{sub 3}O{sub x} are measured with a superconducting quantum interference device (SQUID). The noise power spectrum generally scales as 1/f (f is frequency) from 1 Hz to 1 kHz, increases with temperature, and decreases in higher-quality films. It is proportional to the magnetic field … more
Date: November 1, 1991
Creator: Ferrari, M. J.
Partner: UNT Libraries Government Documents Department
open access

Mass and charge distributions in chlorine-induced nuclear reactions

Description: Projectile-like fragments were detected and characterized in terms of A, Z, and energy for the reactions {sup 37}Cl on {sup 40}Ca and {sup 209}Bi at E/A = 7.3 MeV, and {sup 35}Cl, on {sup 209}Bi at E/A = 15 MeV, at angles close to the grazing angle. Mass and charge distributions were generated in the N-Z plane as a function of energy loss, and have been parameterized in terms of their centroids, variances, and coefficients of correlation. Due to experimental problems, the mass resolution corres… more
Date: December 31, 1991
Creator: Marchetti, A. A.
Partner: UNT Libraries Government Documents Department
open access

Calculations of Bottom Quark Production at Hadron Colliders

Description: This thesis studies Monte Carlo simulations of QCD heavy flavor production processes (p{bar p} {yields} Q({anti Q})X) at hadron colliders. ISAJET bottom quark cross-sections are compared to the O({alpha} {sub s}{sup 3}) perturbative calculation of Nason, Dawson, and Ellis. These Monte Carlo cross-sections are computed from data samples which use different parton distribution functions and physics parameters. Distributions are presented in the heavy quark`s transverse momentum and rapidity. Corr… more
Date: June 29, 1991
Creator: Kuebel, D.
Partner: UNT Libraries Government Documents Department
open access

Topics in particle physics and cosmology

Description: The Standard Model of particle physics, together with the Big Bang model of the early universe, constitute a framework which encompasses our current understanding of fundamental laws and beginning of our universe. Despite recent speculative trends, quantum field theory remains the theoretical tool of choice for investigating new physics either at high energy colliders, or in the early universe. In this dissertation, several field theoretic phenomena relevant to cosmology or particle physics are… more
Date: August 2, 1991
Creator: Hsu, S. D. H.
Partner: UNT Libraries Government Documents Department
Back to Top of Screen