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The Chemistry of HO{Sub 2}NO{Sub 2} and the Photochemistry of the HO{Sub X}-NO{Sub X}-CO{Sub X} System

Description: The objectives of this work were: (1) to investigate formation and destruction reactions of peroxynitric acid, HO{sub 2}NO{sub 2} and (2) to study the photochemistry and kinetics of the HO{sub x} - NO{sub x} - CO{sub x} system were also studied. Investigations were done using infrared spectroscopy to observe the behavior of species involved with the reactions of interest. Several techniques were used, including expansion of reactants into an evacuated cell, static cell decays under constant illumination, and periodic photolysis of flow systems, Infrared absorption cross sections were obtained for CO, CO{sub 2}, NO, NO{sub 2}, N{sub 2}O{sub 5}, HNO{sub 3}, H{sub 2}O{sub 2}, and H{sub 2}O. The spectroscopic measurements were used to derive kinetic information on the systems under study,
Date: August 1, 1980
Creator: Littlejohn, David
Partner: UNT Libraries Government Documents Department

Cluster Chemistry

Description: Metal cluster chemistry is one of the most rapidly developing areas of inorganic and organometallic chemistry. Prior to 1960 only a few metal clusters were well characterized. However, shortly after the early development of boron cluster chemistry, the field of metal cluster chemistry began to grow at a very rapid rate and a structural and a qualitative theoretical understanding of clusters came quickly. Analyzed here is the chemistry and the general significance of clusters with particular emphasis on the cluster research within my group. The importance of coordinately unsaturated, very reactive metal clusters is the major subject of discussion.
Date: May 1, 1980
Creator: Muetterties, Earl L.
Partner: UNT Libraries Government Documents Department

Building Structural Complexity in Semiconductor Nanocrystals through Chemical Transformations

Description: Methods are presented for synthesizing nanocrystal heterostructures comprised of two semiconductor materials epitaxially attached within individual nanostructures. The chemical transformation of cation exchange, where the cations within the lattice of an ionic nanocrystal are replaced with a different metal ion species, is used to alter the chemical composition at specific regions ofa nanocrystal. Partial cation exchange was performed in cadmium sulfide (CdS) nanorods of well-defined size and shape to examine the spatial organization of materials within the resulting nanocrystal heterostructures. The selectivity for cation exchange to take place at different facets of the nanocrystal plays an important role in determining the resulting morphology of the binary heterostructure. The exchange of copper (I) (Cu+) cations in CdS nanorods occurs preferentially at the ends of the nanorods. Theoretical modeling of epitaxial attachments between different facets of CdS and Cu2S indicate that the selectivity for cation exchange at the ends of the nanorods is a result of the low formation energy of the interfaces produced. During silver (I) (Ag+) cation exchange in CdS nanorods, non-selective nucleation of silver sulfide (Ag2S), followed by partial phase segregation leads to significant changes in the spatial arrangement of CdS and Ag2S regions at the exchange reaction proceeds through the nanocrystal. A well-ordered striped pattern of alternating CdS and Ag2S segments is found at intermediate fractions of exchange. The forces mediating this spontaneous process are a combination of Ostwald ripening to reduce the interfacial area along with a strain-induced repulsive interaction between Ag2S segments. To elucidate why Cu+ and Ag+ cation exchange with CdS nanorods produce different morphologies, models for epitaxial attachments between various facets of CdS with Cu2S or Ag2S lattices were used to calculate interface formation energies. The formation energies indicate the favorability for interface nucleation at different facets of the nanorod and the stability of ...
Date: May 20, 2009
Creator: Sadtler, Bryce F.
Partner: UNT Libraries Government Documents Department

Charge Imbalance

Description: The purpose of this article is to review the theory of charge imbalance, and to discuss its relevance to a number of experimental situations. We introduce the concepts of quasiparticle charge and charge imbalance, and discuss the generation and detection of charge imbalance by tunneling. We describe the relaxation of the injected charge imbalance by inelastic scattering processes, and show how the Boltzmann equation can be solved to obtain the steady state quasiparticle distribution and the charge relaxation rate. Details are given of experiments to measure charge imbalance and the charge relaxation rate when inelastic scattering is the predominant relaxation mechanism. Experiments on and theories of other charge relaxation mechanisms are discussed, namely relaxation via elastic scattering in the presence of energy gap anisotropy, or in the presence of a pair breaking mechanism such as magnetic impurities or an applied supercurrent or magnetic field. We describe three other situations in which charge imbalance occurs, namely the resistance of the NS interface, phase slip centers, and the flow of a supercurrent in the presence of a temperature gradient.
Date: September 1, 1980
Creator: Clarke, John
Partner: UNT Libraries Government Documents Department

Development of extreme ultraviolet and soft x-ray multilayer optics for scientific studies with femtosecond/attosecond sources

Description: The development of multilayer optics for extreme ultraviolet (EUV) radiation has led to advancements in many areas of science and technology, including materials studies, EUV lithography, water window microscopy, plasma imaging, and orbiting solar physics imaging. Recent developments in femtosecond and attosecond EUV pulse generation from sources such as high harmonic generation lasers, combined with the elemental and chemical specificity provided by EUV radiation, are opening new opportunities to study fundamental dynamic processes in materials. Critical to these efforts is the design and fabrication of multilayer optics to transport, focus, shape and image these ultra-fast pulses This thesis describes the design, fabrication, characterization, and application of multilayer optics for EUV femtosecond and attosecond scientific studies. Multilayer mirrors for bandwidth control, pulse shaping and compression, tri-material multilayers, and multilayers for polarization control are described. Characterization of multilayer optics, including measurement of material optical constants, reflectivity of multilayer mirrors, and metrology of reflected phases of the multilayer, which is critical to maintaining pulse size and shape, were performed. Two applications of these multilayer mirrors are detailed in the thesis. In the first application, broad bandwidth multilayers were used to characterize and measure sub-100 attosecond pulses from a high harmonic generation source and was performed in collaboration with the Max-Planck institute for Quantum Optics and Ludwig- Maximilians University in Garching, Germany, with Professors Krausz and Kleineberg. In the second application, multilayer mirrors with polarization control are useful to study femtosecond spin dynamics in an ongoing collaboration with the T-REX group of Professor Parmigiani at Elettra in Trieste, Italy. As new ultrafast x-ray sources become available, for example free electron lasers, the multilayer designs described in this thesis can be extended to higher photon energies, and such designs can be used with those sources to enable new scientific studies, such as molecular bonding, phonon, ...
Date: May 21, 2009
Creator: Aquila, Andrew Lee
Partner: UNT Libraries Government Documents Department

The Coordination Chemistry of Metal Surfaces

Description: In coordinately unsaturated molecular metal complexes, carbon-hydrogen bonds of the peripheral ligands may, if the stereochemistry allows, closely approach a metal center so as to develop a three-center two-electron bond between the carbon, the hydrogen, and the metal atoms, C-H-M. In some instances, the interaction .is followed by a scission of the C-H bond whereby the metal is effectively oxidized and discrete M-H and M-C {sigma} bonds are forrned. This class of metal-hydrogen-carbon interactions and reactions is shown to be a common phenomenon in metal surface chemistry. Ultra high vacuum studies of nickel and platinum with simple organic molecules like olefins, and arenes are described. These surface chemistry studies were done as a function of surface crystallography and surface composition. The discussion is largely limited to the chemistry of methyl isocyanide, acetonitrile, benzene and toluene. Molecular orbital calculations are presented that support the experimental identification of the importance of C-H-M metal bonding for metal surfaces.
Date: October 1, 1980
Creator: Muetterties, Earl L.
Partner: UNT Libraries Government Documents Department

Correlation method for the measure of mask-induced line-edge roughness in extreme ultraviolet lithography

Description: As critical dimensions for leading-edge semiconductor devices shrink, line-edge roughness (LER) requirements are pushing well into the single digit nanometer regime. At these scales many new sources of LER must be considered. In the case of extreme ultraviolet (EUV) lithography, modeling has shown the lithographic mask to be a source of significant concern. Here we present a correlation-based methodology for experimentally measuring the magnitude of mask contributors to printed LER. The method is applied to recent printing results from a 0.3 numerical aperture EUV microfield exposure tool. The measurements demonstrate that such effects are indeed present and of significant magnitude. The method is also used to explore the effects of illumination coherence and defocus and has been used to verify model-based predictions of mask-induced LER.
Date: May 25, 2009
Creator: Naulleau, Patrick
Partner: UNT Libraries Government Documents Department

CRADA Final Report: Process development for hybrid solar cells

Description: TCF funding of a CRADA between LBNL and RSLE leveraged RSLE's original $1M investment in LBNL research and led to development of a solar cell fabrication process that will bring the high efficiency, high voltage hybrid tandem solar cell closer to commercialization. RSLE has already built a pilot line at its Phoenix, Arizona site.
Date: February 14, 2011
Creator: Ager, Joel W.
Partner: UNT Libraries Government Documents Department

Extreme ultraviolet lithography: A few more pieces of the puzzle

Description: The work described in this dissertation has improved three essential components of extreme ultraviolet (EUV) lithography: exposure tools, photoresist, and metrology. Exposure tools. A field-averaging illumination stage is presented that enables nonuniform, high-coherence sources to be used in applications where highly uniform illumination is required. In an EUV implementation, it is shown that the illuminator achieves a 6.5% peak-to-valley intensity variation across the entire design field of view. In addition, a design for a stand-alone EUV printing tool capable of delivering 15 nm half-pitch sinusoidal fringes with available sources, gratings and nano-positioning stages is presented. It is shown that the proposed design delivers a near zero line-edge-rougness (LER) aerial image, something extremely attractive for the application of resist testing. Photoresist. Two new methods of quantifying the deprotection blur of EUV photoresists are described and experimentally demonstrated. The deprotection blur, LER, and sensitivity parameters of several EUV photoresists are quantified simultaneously as base weight percent, photoacid generator (PAG) weight percent, and post-exposure bake (PEB) temperature are varied. Two surprising results are found: (1) changing base weight percent does not significantly affect the deprotection blur of EUV photoresist, and (2) increasing PAG weight percent can simultaneously reduce LER and E-size in EUV photoresist. The latter result motivates the development of an EUV exposure statistics model that includes the effects of photon shot noise, the PAG spatial distribution, and the changing of the PAG distribution during the exposure. In addition, a shot noise + deprotection blur model is used to show that as deprotection blur becomes large relative to the size of the printed feature, LER reduction from improved counting statistics becomes dominated by an increase in LER due to reduced deprotection contrast. Metrology. Finally, this dissertation describes MOSAIC, a new wavefront metrology that enables complete wavefront recovery from print or aerial image ...
Date: May 20, 2009
Creator: Anderson, Christopher N.
Partner: UNT Libraries Government Documents Department

A Brief History of Industrial Catalysis

Description: This history covers: catalytic cracking and other acid catalysed reactions; zeolite catalysis; dual functional catalysis; hydrogenation catalysis and hydrogen production; catalytic hydrocarbon dehydrogenation; catalytic alkylation and dealkylation; catalytic coal liquefaction and gasification; heterogeneous oxidation, arnmoxidation, chlorination, and oxychlorination catalysis; olefin disproportionation catalysis; industrial homogeneous catalysis; catalytic polymerization; catalysis for motor vehicle emission control; fuel cell catalysis; and the profession of the catalytic chemist or engineer. The discussion is mostly limited to the rapid growth of industrial catalysis between the second World War and 1978.
Date: June 1, 1979
Creator: Heinemann, Heinz
Partner: UNT Libraries Government Documents Department

Broadband Excitation in Nuclear Magnetic Resonance

Description: Theoretical methods for designing sequences of radio frequency (rf) radiation pulses for broadband excitation of spin systems in nuclear magnetic resonance (NMR) are described. The sequences excite spins uniformly over large ranges of resonant frequencies arising from static magnetic field inhomogeneity, chemical shift differences, or spin couplings, or over large ranges of rf field amplitudes. Specific sequences for creating a population inversion or transverse magnetization are derived and demonstrated experimentally in liquid and solid state NMR. One approach to broadband excitation is based on principles of coherent averaging theory. A general formalism for deriving pulse sequences is given, along with computational methods for specific cases. This approach leads to sequences that produce strictly constant transformations of a spin system. The importance of this feature in NMR applications is discussed. A second approach to broadband excitation makes use of iterative schemes, i.e. sets of operations that are applied repetitively to a given initial pulse sequences, generating a series of increasingly complex sequences with increasingly desirable properties. A general mathematical framework for analyzing iterative schemes is developed. An iterative scheme is treated as a function that acts on a space of operators corresponding to the transformations produced by all possible pulse sequences. The fixed points of the function and the stability of the fixed points are shown to determine the essential behavior of the scheme. Iterative schemes for broadband population inversion are treated in detail. Algebraic and numerical methods for performing the mathematical analysis are presented. Two additional topics are treated. The first is the construction of sequences for uniform excitation of double-quantum coherence and for uniform polarization transfer over a range of spin couplings. Double-quantum excitation sequences are demonstrated in a liquid crystal system. The second additional topic is the construction of iterative schemes for narrowband population inversion. The use of ...
Date: October 1, 1984
Creator: Tycko, R.
Partner: UNT Libraries Government Documents Department

Bubble Dynamics at Gas-Evolving Electrodes

Description: Nucleation of bubbles, their growth by diffusion of dissolved gas to the bubble surface and by coalescence, and their detachment from the electrode are all very fast phenomena; furthermore, electrolytically generated bubbles range in size from ten to a few hundred microns; therefore, magnification and high speed cinematography are required to observe bubbles and the phenomena of their growth on the electrode surface. Viewing the action from the front side (the surface on which the bubbles form) is complicated because the most important events occur close to the surface and are obscured by other bubbles passing between the camera and the electrode; therefore, oxygen was evolved on a transparent tin oxide "window" electrode and the events were viewed from the backside. The movies showed that coalescence of bubbles is very important for determining the size of bubbles and in the chain of transport processes; growth by diffusion and by coalescence proceeds in series and parallel; coalescing bubbles cause significant fluid motion close to the electrode; bubbles can leave and reattach; and bubbles evolve in a cycle of growth by diffusion and different modes of coalescence. An analytical solution for the primary potential and current distribution around a spherical bubble in contact with a plane electrode is presented. Zero at the contact point, the current density reaches only one percent of its undisturbed value at 30 percent of the radius from that point and goes through a shallow maximum two radii away. The solution obtained for spherical bubbles is shown to apply for the small bubbles of electrolytic processes. The incremental resistance in ohms caused by sparse arrays of bubbles is given by {Delta}R = 1.352 af/kS where f is the void fraction of gas in the bubble layer, a is the bubble layer thickness, k is the conductivity of gas ...
Date: December 1, 1980
Creator: Sides, Paul J.
Partner: UNT Libraries Government Documents Department

The Erosion Behavior of Steel as a Function of Microstructure on Solid Particle Erosion

Description: The effects of the microstructure of two ductile steels on their solid particle erosion were determined. The steels chosen allowed microstructural changes to be made without drastically changing their hardness, which is reported to be a direct function of erosion resistance. The steels used were plain carbon 1075 and 1020 in the coarse pearlite, fine pearlite, and spheroidized forms for the 1075 and in three spherodized conditions for the 1020 steel. Single particle and multiple particle erosion tests were conducted using 240 {micro}m diameter SiC particles, angles of impingement of 15°, 30°, and 90° and velocities of 30.5 mps (100fps) and 61 mps (200fps). Both surface and subsurface analyses were conducted using scanning electron microscopy. In the room temperature erosion tests, the spheroidized microstructure of the 1075 steel eroded less than either of the two pearlitic microstructures. It was found that the pearlitic steels exhibited cracking at the eroded surface as well as beneath it, causing greater material removal. The spheroidized structure showed no surface cracking; however, cracking did occur at a depth of approximately 20 {micro}m below the surface. The carbide particle spacing in the 1020 spheroidized steel also had a measureable effect on the erosion rate, The hardness of the various microstructures had an inverse relation to the erosion rate,
Date: April 1, 1980
Creator: Levy, Alan V.
Partner: UNT Libraries Government Documents Department

A Study of Fundamental Reaction Pathways for Transition Metal Alkyl Complexes: [Parts 1 and 2]

Description: I. This study reports the rapid reaction under mild conditions of internal or terminal alkynes with methyl (acetyl~ acetonato) (triphenylphosphine) nickel (1) in either aromatic or ether solvents. In all cases vinylnickel products 2 are formed by insertion of the alkyne into the nickel=methyl bond. These complexes may be converted into a variety of organic products (e.g. alkenes, esters, vinyl halides) by treatment with appropriate reagents. Unsymmetrical alkynes give selectively the one regioisomer with the sterically largest substituent next to the nickel atom. In order to investigate the stereochemistry of the initial insertion, a x-ray diffraction study of the reaction of 1 with diphenylacetylene was carried out. This showed that the vinylnickel complex formed by overall trans insertion was the product of the reaction. Furthermore, subsequent slow isomerization of this complex, to a mixture of it and the corresponding cis isomer, demonstrated that this trans addition product is the kinetic product of the reaction. In studies with other alkynes, the product of trans addition was not always exclusively (or even predominantly) formed, but the ratio of the stereoisomers formed kinetically was substantially different from the thermodynamic ratio. Isotope labeling, added phosphine, and other experiments have allowed us to conclude that the mechanism of this reaction does involve initial cis addition. However, a coordinatively unsaturated vinylnickel complex is initially formed which can undergo rapid, phosphine-catalyzed cis-trans isomerization in competition with its conversion to the isolable phosphine-substituted kinetic reaction products. II. The reaction of CpMo(CO){sub 3}H (1a) with CpMo(CO){sub 3}R (2, R= CH{sub 3}, C{sub 2}H{sub 5}) at 50{degrees} C in THF gives the aldehyde RCHO and the dimers [CpMo(CO){sub 3}]{sub 2} (3a) and [CpMo(CO){sub 2}]{sub 2} (4a). Labeling one of the reactants with a methylcyclopentadienyl ligand it was possible to show that the mixed dimers MeCpMo(CO){sub 3}-(CO){sub 3}MoCp (3b) and MeCpMo(CO){sub ...
Date: June 1, 1980
Creator: Huggins, John Mitchell
Partner: UNT Libraries Government Documents Department

Symmetry Simplifications of Space Types in Configuration Interaction Induced by Orbital Degeneracy

Description: Symmetry simplifications are introduced in configuration interaction (CI) by reducing the number of symmetry-allowed space types if there is degeneracy in some of the molecular orbitals by constructing the unique space types. A new symmetry group which we call the configuration symmetry group is defined and is shown to be expressible as a generalized wreath product group. Generating functions are derived for enumerating the equivalence classes of space types wherein all the space types in the same equivalence class with identical spin coupling make identical contributions to correlation. A double coset method is expounded which constructs the representatives of all equivalence classes of space types using the cycle index of generalized wreath product and the double cosets of label subgroup with generalized wreath product in the symmetric group S{sub n}, if n is the number of electrons. Method is illustrated with CI using the localized Lowdin orbitals of polyenes and CI in benzene for several reference states.
Date: November 1, 1980
Creator: Balasubramanian, K.
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

Ultrafast Magnetization Dynamics of SrRuO3 Thin Films

Description: Itinerant ferromagnet SrRuO3 has drawn interest from physicists due to its unusual transport and magnetic properties as well as from engineers due to its low resistivity and good lattice-matching to other oxide materials. The exact electronic structure remains a mystery, as well as details of the interactions between magnetic and electron transport properties. This thesis describes the use of time-resolved magneto-optical Kerr spectroscopy to study the ferromagnetic resonance of SrRuO3 thin films, where the ferromagnetic resonance is initiated by a sudden change in the easy axis direction in response to a pump pulse. The rotation of the easy axis is induced by laser heating, taking advantage of a temperature-dependent easy axis direction in SrRuO3 thin films. By measuring the change in temperature of the magnetic system in response to the laser pulse, we find that the specific heat is dominated by magnons up to unusually high temperature, ~;;100 K, and thermal diffusion is limited by a boundary resistance between the film and the substrate that is not consistent with standard phonon reflection and scattering models. We observe a high FMR frequency, 250 GHz, and large Gilbert damping parameter, alpha ~;; 1, consistent with strong spin-orbit coupling. We observe a time-dependent change in the easy axis direction on a ps time-scale, and we find that parameters associated with the change in easy axis, as well as the damping parameter, have a non-monotonic temperature dependence similar to that observed in anomalous Hall measurements.
Date: May 19, 2009
Creator: Langner, Matthew C.
Partner: UNT Libraries Government Documents Department

The Use of Isotope Crossover Experiments in Investigating Carbon-Carbon Bond Forming Reactions of Binuclear Dialkyl Cobalt Complexes

Description: Our present understanding of the mechanisms of organometallic reactions stems almost completely from investigation of complexes containing only one metal. Recently interest has been increasing in the synthesis, structure elucidation and reaction mechanisms of polynuclear clusters, complexes containing more than one metal. This attention derives partially from the possibility that polynuclear catalysts and reagents might be designed in such a way that the metals could interact, generating cooperative systems which might be much more selective than their mononuclear analogs. Another stimulant to this work has been the relationship of cluster complexes to larger multi-metal systems, such as heterogeneous catalysts. Many polynuclear clusters have been prepared and characterized, and some of these have been found to function as unique catalysts or catalyst precursors. However, very little is yet known about how chemical transformations take place at multinuclear reaction centers. Given this paucity of information, they decided a few years ago to initiate mechanistic study of simple cluster systems containing two metal centers, in which each of the metals has a {sigma}-bound organic ligand attached to it. They also choose to focus on reactions of these complexes in which new carbon-carbon or carbon-hydrogen bonds are formed. This Account describes the work on such a system: a binuclear alkyl cobalt complex capable of transferring both alkyl groups to a molecule of carbon monoxide. In this work they have adopted as one of our highest priorities the determination of whether the cluster 'holds together' during its reactions, a question that is in our opinion too often ignored in such studies. They have found that isotope crossover experiments provide a powerful tool for investigating this structural integrity questions, and in this Account they outline a number of examples in which such crossover experiments have provided important, and occasionally surprising, informationa bout the mechanisms involved in ...
Date: January 1, 1980
Creator: Bergman, Robert G.
Partner: UNT Libraries Government Documents Department

Viewing spin structures with soft x-ray microscopy

Description: The spin of the electron and its associated magnetic moment marks the basic unit for magnetic properties of matter. Magnetism, in particular ferromagnetism and antiferromagnetism is described by a collective order of these spins, where the interaction between individual spins reflects a competition between exchange, anisotropy and dipolar energy terms. As a result the energetically favored ground state of a ferromagnetic system is a rather complex spin configuration, the magnetic domain structure. Magnetism is one of the eldest scientific phenomena, yet it is one of the most powerful and versatile utilized physical effects in modern technologies, such as in magnetic storage and sensor devices. To achieve highest storage density, the relevant length scales, such as the bit size in disk drives is now approaching the nanoscale and as such further developments have to deal with nanoscience phenomena. Advanced characterization tools are required to fully understand the underlying physical principles. Magnetic microscopes using polarized soft X-rays offer a close-up view into magnetism with unique features, these include elemental sensitivity due to X-ray magnetic dichroism effects as contrast mechanism, high spatial resolution provided by state-of-the-art X-ray optics and fast time resolution limited by the inherent time structure of current X-ray sources, which will be overcome with the introduction of ultrafast and high brilliant X-ray sources.
Date: June 1, 2010
Creator: Fischer, Peter
Partner: UNT Libraries Government Documents Department

The Theory of Uranium Enrichment by the Gas Centrifuge

Description: Onsager's analysis of the hydrodynamics of fluid circulation in the boundary layer on the rotor wall of a gas centrifuge is reviewed. The description of the flow in the boundary layers on the top and bottom end caps due to Carrier and Maslen is summarized. The method developed by Wood and Morton of coupling the flow models in the rotor wall and end cap boundary layers to complete the hydrodynamic analysis of the centrifuge is presented. Mechanical and thermal methods of driving the internal gas circulation are described. The isotope enrichment which results from the superposition of the elementary separation effect due to the centrifugal field in the gas and its internal circulation is analyzed by the Onsager-Cohen theory. The performance function representing the optimized separative power of a centrifuge as a function of throughput and cut is calculated for several simplified internal flow models. The use of asymmetric ideal cascades to exploit the distinctive features of centrifuge performance functions is illustrated.
Date: March 1, 1981
Creator: Olander, Donald R.
Partner: UNT Libraries Government Documents Department

Size-dependent structure of silver nanoparticles under high pressure

Description: Silver noble metal nanoparticles that are<10 nm often possess multiply twinned grains allowing them to adopt shapes and atomic structures not observed in bulk materials. The properties exhibited by particles with multiply twinned polycrystalline structures are often far different from those of single-crystalline particles and from the bulk. I will present experimental evidence that silver nanoparticles<10 nm undergo a reversible structural transformation under hydrostatic pressures up to 10 GPa. Results for nanoparticles in the intermediate size range of 5 to 10 nm suggest a reversible linear pressure-dependent rhombohedral distortion which has not been previously observed in bulk silver. I propose a mechanism for this transitiion that considers the bond-length distribution in idealized multiply twinned icosahedral particles. Results for nanoparticles of 3.9 nm suggest a reversible linear pressure-dependent orthorhombic distortion. This distortion is interpreted in the context of idealized decahedral particles. In addition, given these size-dependent measurements of silver nanoparticle compression with pressure, we have constructed a pressure calibration curve. Encapsulating these silver nanoparticles in hollow metal oxide nanospheres then allows us to measure the pressure inside a nanoshell using x-ray diffraction. We demonstrate the measurement of pressure gradients across nanoshells and show that these nanoshells have maximum resolved shear strengths on the order of 500 MPa to IGPa.
Date: December 31, 2008
Creator: Koski, Kristie Jo
Partner: UNT Libraries Government Documents Department

Soft x-ray microscopy - a powerful analytical tool to image magnetism down to fundamental length and times scales

Description: The magnetic properties of low dimensional solid state matter is of the utmost interest both scientifically as well as technologically. In addition to the charge of the electron which is the base for current electronics, by taking into account the spin degree of freedom in future spintronics applications open a new avenue. Progress towards a better physical understanding of the mechanism and principles involved as well as potential applications of nanomagnetic devices can only be achieved with advanced analytical tools. Soft X-ray microscopy providing a spatial resolution towards 10nm, a time resolution currently in the sub-ns regime and inherent elemental sensitivity is a very promising technique for that. This article reviews the recent achievements of magnetic soft X-ray microscopy by selected examples of spin torque phenomena, stochastical behavior on the nanoscale and spin dynamics in magnetic nanopatterns. The future potential with regard to addressing fundamental magnetic length and time scales, e.g. imaging fsec spin dynamics at upcoming X-ray sources is pointed out.
Date: August 1, 2008
Creator: Fischer, Peter
Partner: UNT Libraries Government Documents Department

Spec-Doc: A User's Guide to Spectrometer Software

Description: SPEC is the name of the operating system designed to control the NMR spectrometers in lab. SPEC is actually one large program which handles many functions necessary to control each spectrometer. The program handles all I/O with peripheral devices such as the console ('terminal' or 'CRT'). The program carries out its operations by accepting commands which each invoke specific subroutines to perform their function. There are a total of 60 commands in SPEC, each carrying out a different function. Because so many commands make SPEC a very large program, not all of the program is core resident. Rather, each command calls in an overlay handler which loads into memory the appropriate overlay from the disk and begins execution of the command. Thus SPEC is an independent disk based operating system. The commands in SPEC are capable of operating the microprocessor based pulse programmer, starting and acquiring data from the spectrometer data acquisition system, storing data on disk and manipulating it mathematically, displaying and plotting data. All arithmetic operations within SPEC are performed on integers. Since the DATA GENERAL computers are 16 bit machines operating in two's complement mode, the integer range is +32767. Many of the mathematical operations of SPEC are done in double precision integer mode with the final result always scaled to the above range. For many of the commands, integer overflow is detected and reported as an error message. Overflowed points are set to +32767. SPEC accepts command input from the console or reads a string of commands previously entered on the disk. The later command structure is called a macro. Macros may be nested and may have constants passed to them at execution time, thus allowing for a powerful supercommand structure. Both forms of commands are discussed in the next section. SPEC is designed to run ...
Date: May 1, 1983
Creator: Sinton, S.
Partner: UNT Libraries Government Documents Department