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A Search for new physics in photon-lepton events in proton-antiproton collisions at {radical} s = 1.8 TeV

Description: We present the results of a search in p{bar p} collisions at {radical}s = 1.8 TeV for anomalous production of events containing a photon with large transverse energy and a lepton (e or {mu}) with large transverse energy, using 86 pb{sup -1} of data collected at the Collider Detector at Fermilab during the 1994-95 collider run at the Fermilab Tevatron. The presence of large missing transverse energy (E{sub T}), additional photons, or additional leptons in these events is also analyzed. The results are consistent with standard model expectations, with the possible exception of photon-lepton events with large E{sub T}, for which the probability of a statistical fluctuation of the standard model expectation up to and above the observed level is 0.7%.
Date: April 30, 2001
Creator: Berryhill, J. W.
Partner: UNT Libraries Government Documents Department

Studies in ion source development for application in heavy ion fusion

Description: The overall purpose of these experiments is to contribute to the development of ion injector technology in order to produce a driver for use in a heavy-ion-fusion (HIF) power generating facility. The overall beam requirements for HIF are quite demanding; a short list of the constraints is the following: (1) Low cost (a large portion of overall cost will come from the beam system); (2) Bright, low emittance beam; (3) Total beam energy 5MJ; (4) Spot size 3mm (radius); (5) Pulse Duration 10ns; (6) Current on target 40kA; (7) Repetition Rate 5Hz; (8) Standoff from target 5m; and (9) Transverse Temp < 1 keV. The reasons for employing ion beams in inertial fusion systems become obvious when the repetition rate required is considered. While laser drivers are useful in producing a proof-of-concept, they will be incapable of application in power generation. Consequently attempts in the U.S. to achieve a power generating system make use of linear ion accelerators. It is apparent that the accelerator system requires the highest quality input as obtainable. Therefore injector design is an essential portion of the entire inertial fusion system. At Lawrence Berkeley and Lawrence Livermore National Laboratories experiments are being conducted using two injector formats. For this project I have conducted a series of studies using both. The next two sections provide a brief description of the sources used for my experiments.
Date: May 30, 2004
Creator: Kapica, Jonathan G.
Partner: UNT Libraries Government Documents Department

Photon-counting single-molecule spectroscopy for studying conformational dynamics and macromolecular interactions

Description: Single-molecule methods have the potential to provide information about conformational dynamics and molecular interactions that cannot be obtained by other methods. Removal of ensemble averaging provides several benefits, including the ability to detect heterogeneous populations and the ability to observe asynchronous reactions. Single-molecule diffusion methodologies using fluorescence resonance energy transfer (FRET) are developed to monitor conformational dynamics while minimizing perturbations introduced by interactions between molecules and surfaces. These methods are used to perform studies of the folding of Chymotrypsin Inhibitor 2, a small, single-domain protein, and of single-stranded DNA (ssDNA) homopolymers. Confocal microscopy is used in combination with sensitive detectors to detect bursts of photons from fluorescently labeled biomolecules as they diffuse through the focal volume. These bursts are analyzed to extract fluorescence resonance energy transfer (FRET) efficiency. Advances in data acquisition and analysis techniques that are providing a more complete picture of the accessible molecular information are discussed. Photon Arrival-time Interval Distribution (PAID) analysis is a new method for monitoring macromolecular interactions by fluorescence detection with simultaneous determination of coincidence, brightness, diffusion time, and occupancy (proportional to concentration) of fluorescently-labeled molecules undergoing diffusion in a confocal detection volume. This method is based on recording the time of arrival of all detected photons, and then plotting the two-dimensional histogram of photon pairs, where one axis is the time interval between each pair of photons 1 and 2, and the second axis is the number of other photons detected in the time interval between photons 1 and 2. PAID is related to Fluorescence Correlation Spectroscopy (FCS) by a collapse of this histogram onto the time interval axis. PAID extends auto- and cross-correlation FCS by measuring the brightness of fluorescent species. A data-fitting model is developed, which is used to simultaneously determine coincidence, brightness, diffusion time, and occupancy from experiments performed on ...
Date: July 30, 2002
Creator: Laurence, Ted Alfred
Partner: UNT Libraries Government Documents Department

High-Resolution NMR of Quadrupolar Nuclei in the Solid State

Description: This dissertation describes recent developments in solid state nuclear magnetic resonance (NMR), for the most part involving the use of dynamic-angle spinning (DAS) NMR to study quadrupolar nuclei. Chapter 1 introduces some of the basic concepts and theory that will be referred to in later chapters, such as the density operator, product operators, rotations, coherence transfer pathways, phase cycling, and the various nuclear spin interactions, including the quadrupolar interaction. Chapter 2 describes the theory behind motional averaging experiments, including DAS, which is a technique where a sample is spun sequentially about two axis oriented at different angles with respect to the external magnetic field such that the chemical shift and quadrupolar anisotropy are averaged to zero. Work done on various rubidium-87 salts is presented as a demonstration of DAS. Chapter 3 explains how to remove sidebands from DAS and magic-angle spinning (MAS) experiments, which result from the time-dependence of the Hamiltonian under sample spinning conditions, using rotor-synchronized {pi}-pulses. Data from these experiments, known as DAH-180 and MAH-180, respectively, are presented for both rubidium and lead salts. In addition, the applicability of this technique to double rotation (DOR) experiments is discussed. Chapter 4 concerns the addition of cross-polarization to DAS (CPDAS). The theory behind spin locking and cross polarizing quadrupolar nuclei is explained and a method of avoiding the resulting problems by performing cross polarization at 0{sup o} (parallel) with respect to the magnetic field is presented. Experimental results are shown for a sodium-23 compound, sodium pyruvate, and for oxygen-17 labeled L-akmine. In Chapter 5, a method for broadening the Hartmann-Hahn matching condition under MAS, called variable effective field cross-polarization (VEFCI?), is presented, along with experimental work on adamantane and polycarbonate.
Date: November 30, 1995
Creator: Gann, Sheryl Lee
Partner: UNT Libraries Government Documents Department

Organic Light-Emitting Diodes (OLEDs) and Optically-Detected Magnetic Resonance (ODMR) studies on organic materials

Description: Organic semiconductors have evolved rapidly over the last decades and currently are considered as the next-generation technology for many applications, such as organic light-emitting diodes (OLEDs) in flat-panel displays (FPDs) and solid state lighting (SSL), and organic solar cells (OSCs) in clean renewable energy. This dissertation focuses mainly on OLEDs. Although the commercialization of the OLED technology in FPDs is growing and appears to be just around the corner for SSL, there are still several key issues that need to be addressed: (1) the cost of OLEDs is very high, largely due to the costly current manufacturing process; (2) the efficiency of OLEDs needs to be improved. This is vital to the success of OLEDs in the FPD and SSL industries; (3) the lifetime of OLEDs, especially blue OLEDs, is the biggest technical challenge. All these issues raise the demand for new organic materials, new device structures, and continued lower-cost fabrication methods. In an attempt to address these issues, we used solution-processing methods to fabricate highly efficient small molecule OLEDs (SMOLEDs); this approach is costeffective in comparison to the more common thermal vacuum evaporation. We also successfully made efficient indium tin oxide (ITO)-free SMOLEDs to further improve the efficiency of the OLEDs. We employed the spin-dependent optically-detected magnetic resonance (ODMR) technique to study the luminescence quenching processes in OLEDs and organic materials in order to understand the intrinsic degradation mechanisms. We also fabricated polymer LEDs (PLEDs) based on a new electron-accepting blue-emitting polymer and studied the effect of molecular weight on the efficiency of PLEDs. All these studies helped us to better understand the underlying relationship between the organic semiconductor materials and the OLEDs’ performance, and will subsequently assist in further enhancing the efficiency of OLEDs. With strongly improved device performance (in addition to other OLEDs' attributes such as mechanical ...
Date: November 30, 2011
Creator: Cai, Min
Partner: UNT Libraries Government Documents Department

Laser ablation-inductively coupled plasma-mass spectrometry: Examinations of the origins of polyatomic ions and advances in the sampling of particulates

Description: This dissertation provides a general introduction to Inductively coupled plasma-mass spectrometry (ICP-MS) and laser ablation (LA) sampling, with an examination of analytical challenges in the employment of this technique. It discusses the origin of metal oxide ions (MO+) in LA-ICP-MS, as well as the effect of introducing helium and nitrogen to the aerosol gas flow on the formation of these polyatomic interferences. It extends the study of polyatomic ions in LA-ICP-MS to metal argide (MAr+) species, an additional source of possible significant interferences in the spectrum. It describes the application of fs-LA-ICP-MS to the determination of uranium isotope ratios in particulate samples.
Date: November 30, 2011
Creator: Witte, Travis
Partner: UNT Libraries Government Documents Department

Search for Resonances in the Photoproduction of Proton-Antiproton Pairs

Description: Results are reported on the reaction {gamma}p {yields} p{bar p}p with beam energy in the range 4.8-5.5 GeV. The data were collected at the Thomas Jefferson National Accelerator Facility in CLAS experiment E01-017(G6C). The focus of this study is an understanding of the mechanisms of photoproduction of proton-antiproton pairs, and to search for intermediate resonances, both narrow and broad, which decay to p{bar p}. The total measured cross section in the photon energy range 4.8-5.5 GeV is {sigma} = 33 {+-} 2 nb. Measurement of the cross section as a function of energy is provided. An upper limit on the production of a narrow resonance state previously observed with a mass of 2.02 GeV/c{sup 2} is placed at 0.35 nb. No intermediate resonance states were observed. Meson exchange production appears to dominate the production of the proton-antiproton pairs.
Date: June 30, 2006
Creator: Stokes, Burnham
Partner: UNT Libraries Government Documents Department

Development of high-spatial and high-mass resolution mass spectrometric imaging (MSI) and its application to the study of small metabolites and endogenous molecules of plants

Description: High-spatial and high-mass resolution laser desorption ionization (LDI) mass spectrometric (MS) imaging technology was developed for the attainment of MS images of higher quality containing more information on the relevant cellular and molecular biology in unprecedented depth. The distribution of plant metabolites is asymmetric throughout the cells and tissues, and therefore the increase in the spatial resolution was pursued to reveal the localization of plant metabolites at the cellular level by MS imaging. For achieving high-spatial resolution, the laser beam size was reduced by utilizing an optical fiber with small core diameter (25 μm) in a vacuum matrix-assisted laser desorption ionization-linear ion trap (vMALDI-LTQ) mass spectrometer. Matrix application was greatly improved using oscillating capillary nebulizer. As a result, single cell level spatial resolution of ~ 12 μm was achieved. MS imaging at this high spatial resolution was directly applied to a whole Arabidopsis flower and the substructures of an anther and single pollen grains at the stigma and anther were successfully visualized. MS imaging of high spatial resolution was also demonstrated to the secondary roots of Arabidopsis thaliana and a high degree of localization of detected metabolites was successfully unveiled. This was the first MS imaging on the root for molecular species. MS imaging with high mass resolution was also achieved by utilizing the LTQ-Orbitrap mass spectrometer for the direct identification of the surface metabolites on the Arabidopsis stem and root and differentiation of isobaric ions having the same nominal mass with no need of tandem mass spectrometry (MS/MS). MS imaging at high-spatial and high-mass resolution was also applied to cer1 mutant of the model system Arabidopsis thaliana to demonstrate its usefulness in biological studies and reveal associated metabolite changes in terms of spatial distribution and/or abundances compared to those of wild-type. The spatial distribution of targeted metabolites, mainly waxes ...
Date: November 30, 2011
Creator: Jun, Ji Hyun
Partner: UNT Libraries Government Documents Department

Circular magnetic x-ray dichroism in rare earth compounds

Description: This report discusses the following topics: Circular magnetic x-ray dichroism at the ER L{sub 3} Edge; angular dependence of circular magnetic x-ray dichroism in rare earth compounds: and circular magnetic x-ray dichroism in crystalline and amorphous GDFE{sub 2}.
Date: September 30, 1993
Creator: Jonathan, L.
Partner: UNT Libraries Government Documents Department

Transverse Space-Charge Effects in Circular Accelerators

Description: The particles in an accelerator interact with one another by electromagnetic forces and are held together by external focusing forces. Such a many-body system has a large number of transverse modes of oscillation (plasma oscillations) that can be excited at characteristic frequencies by errors in the external guide field. In Part I we examine one mode of oscillation in detail, namely the quadrupole mode that is excited in uniformly charged beams by gradient errors. We derive self-consistent equations of motion for the beam envelope and solve these equations for the case in which the space-charge force is much less than the external focusing force, i.e., for strong-focusing synchrotrons. We find that the resonance intensity is shifted from the value predicted by the usual transverse incoherent space-charge limit; moreover, because the space-charge force depends on the shape and size of the beam, the beam growth in always limited. For gradient errors of the magnitude normally present in strong-focusing synchrotrons, the increase in beam size is small provided the beam parameters are properly chosen; otherwise the growth may be large. Thus gradient errors need not impose a limit on the number of particles that can be accelerated. In Part II we examine the other modes of collective oscillation that are excited by machine imperfections. For simplicity we consider only one-dimensional beams that are confined by harmonic potentials, and only small-amplitude oscillations. The linearized Vlasov and Poisson equations are used to find the twofold infinity of normal modes and eigenfrequencies for the stationary distribution that has uniform charge density in real space. In practice, only the low-order modes (the dipole, quadrupole, sextupole, and one or two additional modes) will be serious, and the resonant conditions for these modes are located on a tune diagram. These results, which are valid for all beam intensities, ...
Date: October 30, 1968
Creator: Sacherer, Frank James
Partner: UNT Libraries Government Documents Department

A Measurement of the Top Quark Mass in 1.96 TeV Proton-Antiproton Collisions Using a Novel Matrix Element Method

Description: A measurement of the top quark mass in t{bar t} {yields} l + jets candidate events, obtained from p{bar p} collisions at {radical}s = 1.96 TeV at the Fermilab Tevatron using the CDF II detector, is presented. The measurement approach is that of a matrix element method. For each candidate event, a two dimensional likelihood is calculated in the top pole mass and a constant scale factor, 'JES', where JES multiplies the input particle jet momenta and is designed to account for the systematic uncertainty of the jet momentum reconstruction. As with all matrix element techniques, the method involves an integration using the Standard Model matrix element for t{bar t} production and decay. However, the technique presented is unique in that the matrix element is modified to compensate for kinematic assumptions which are made to reduce computation time. Background events are dealt with through use of an event observable which distinguishes signal from background, as well as through a cut on the value of an event's maximum likelihood. Results are based on a 955 pb{sup -1} data sample, using events with a high-p{sub T} lepton and exactly four high-energy jets, at least one of which is tagged as coming from a b quark; 149 events pass all the selection requirements. They find M{sub meas} = 169.8 {+-} 2.3(stat.) {+-} 1.4(syst.) GeV/c{sup 2}.
Date: September 30, 2007
Creator: CDF Collaboration
Partner: UNT Libraries Government Documents Department

Low Temperature Constrained Sintering of Cerium Gadolinium OxideFilms for Solid Oxide Fuel Cell Applications

Description: Cerium gadolinium oxide (CGO) has been identified as an acceptable solid oxide fuel cell (SOFC) electrolyte at temperatures (500-700 C) where cheap, rigid, stainless steel interconnect substrates can be used. Unfortunately, both the high sintering temperature of pure CGO, >1200 C, and the fact that constraint during sintering often results in cracked, low density ceramic films, have complicated development of metal supported CGO SOFCs. The aim of this work was to find new sintering aids for Ce{sub 0.9}Gd{sub 0.1}O{sub 1.95}, and to evaluate whether they could be used to produce dense, constrained Ce{sub 0.9}Gd{sub 0.1}O{sub 1.95} films at temperatures below 1000 C. To find the optimal sintering aid, Ce{sub 0.9}Gd{sub 0.1}O{sub 1.95} was doped with a variety of elements, of which lithium was found to be the most effective. Dilatometric studies indicated that by doping CGO with 3mol% lithium nitrate, it was possible to sinter pellets to a relative density of 98.5% at 800 C--a full one hundred degrees below the previous low temperature sintering record for CGO. Further, it was also found that a sintering aid's effectiveness could be explained in terms of its size, charge and high temperature mobility. A closer examination of lithium doped Ce0.9Gd0.1O1.95 indicated that lithium affects sintering by producing a Li{sub 2}O-Gd{sub 2}O{sub 3}-CeO{sub 2} liquid at the CGO grain boundaries. Due to this liquid phase sintering, it was possible to produce dense, crack-free constrained films of CGO at the record low temperature of 950 C using cheap, colloidal spray deposition processes. This is the first time dense constrained CGO films have been produced below 1000 C and could help commercialize metal supported ceria based solid oxide fuel cells.
Date: June 30, 2007
Creator: Nicholas, Jason.D.
Partner: UNT Libraries Government Documents Department

Spectra of $gamma$-rays from capture of 2 eV to 9 x 10$sup 4$ eV neutrons by $sup 181$Ta

Description: Using new experimental techniques, the spectra of $gamma$-rays from the capture of neutrons by $sup 181$Ta were measured at the Livermore 100-MeV linac for neutrons from 2 eV to 9 x 10$sup 4$ eV with a (Ge(Li)-NaI) three-crystal spectrometer. Individual primary $gamma$-ray lines were resolved to 1778-keV excitation in $sup 182$Ta. Neutron resonances were resolved to 200-eV neutron energy. Data analysis techniques and codes were developed to extract positions and intensities of resolved transitions from the large data matrices accumulated in this experiment. Techniques were developed to unfold the unresolved $gamma$- ray spectra using the simple response of the three-crystal spectrometer. The resolved transition data were used to place 110 states with spin and parity assignments in the $sup 182$Ta level diagram below 1780-keV excitation. A set of 1240 E1 transition strengths were analyzed to extract 1.38 +- 0.11 degrees of freedom for the most likely chisquared fit to the distribution of widths. The E1 strength function was extracted for E/sub gamma/ = 4 to 6 MeV and compared with previous results. The $gamma$-ray spectra for E/sub gamma/ = 1.5 to 6.1 MeV were unfolded for neutron energy groups between 20 and 9 x 10$sup 4$ eV. Below 5-MeV $gamma$-ray energy no dependence of the spectral shape on neutron energy was observed. (30 figures, 4 tables) (auth)
Date: April 30, 1976
Creator: Stelts, M.L.
Partner: UNT Libraries Government Documents Department

Chemical and nuclear properties of Rutherfordium (Element 104)

Description: The chemical-properties of rutherfordium (Rf) and its group 4 homologs were studied by sorption on glass support surfaces coated with cobalt(II)ferrocyanide and by solvent extraction with tributylphosphate (TBP) and triisooctylamine (TIOA). The surface studies showed that the hydrolysis trend in the group 4 elements and the pseudogroup 4 element, lb, decreases in the order Rf>Zr{approx}Hf>Th. This trend was attributed to relativistic effects which predicted that Rf would be more prone to having a coordination number of 6 than 8 in most aqueous solutions due to a destabilization of the 6d{sub 5/2} shell and a stabilization of the 7p{sub l/2} shell. This hydrolysis trend was confirmed in the TBP/HBr solvent extraction studies which showed that the extraction trend decreased in the order Zr>Hf>Rf?Ti for HBr, showing that Rf and Ti did not extract as well because they hydrolyzed more easily than Zr and Hf. The TIOA/HF solvent extraction studies showed that the extraction trend for the group 4 elements decreased in the order Ti>Zr{approx}Hf>Rf, in inverse order from the trend of ionic radii Rf>Zr{approx}Hf>Ti. An attempt was made to produce {sup 263}Rf (a) via the {sup 248}Cm({sup 22}Ne, {alpha}3n) reaction employing thenoyltrifluoroacetone (TTA) solvent extraction chemistry and (b) via the {sup 249}Bk({sup 18}O,4n) reaction employing the Automated Rapid Chemistry Apparatus (ARCA). In the TTA studies, 16 fissions were observed but were all attributed to {sup 256}Fm. No alpha events were observed in the Rf chemical fraction. A 0.2 nb upper limit production cross section for the {sup 248}Cm({sup 22}Ne, {alpha}3n){sup 263}Rf reaction was calculated assuming the 500-sec half-life reported previously by Czerwinski et al. [CZE92A].
Date: October 30, 1995
Creator: Kacher, C.D.
Partner: UNT Libraries Government Documents Department

Analysis of the application of decontamination technologies to radioactive metal waste minimization using expert systems

Description: Radioactive metal waste makes up a significant portion of the waste currently being sent for disposal. Recovery of this metal as a valuable resource is possible through the use of decontamination technologies. Through the development and use of expert systems a comparison can be made of laser decontamination, a technology currently under development at Ames Laboratory, with currently available decontamination technologies for applicability to the types of metal waste being generated and the effectiveness of these versus simply disposing of the waste. These technologies can be technically and economically evaluated by the use of expert systems techniques to provide a waste management decision making tool that generates, given an identified metal waste, waste management recommendations. The user enters waste characteristic information as input and the system then recommends decontamination technologies, determines residual contamination levels and possible waste management strategies, carries out a cost analysis and then ranks, according to cost, the possibilities for management of the waste. The expert system was developed using information from literature and personnel experienced in the use of decontamination technologies and requires validation by human experts and assignment of confidence factors to the knowledge represented within.
Date: September 30, 1993
Creator: Bayrakal, S.
Partner: UNT Libraries Government Documents Department

Fourier transform photoelectron diffraction and its application to molecular orbitals and surface structure

Description: Photoemission intensities from the molecular orbitals of c(2x2)CO/Pt(111) over a wide photon energy range were measured and analyzed by the same methods developed for structural studies using core levels. The 4{sigma} orbital center of gravity is found to be concentrated between the C and O atoms, while that of the 5{sigma} orbital lies between the C atom and the Pt surface. The C 1s photoelectron diffraction was used to determine the adsorption geometry. The earlier ambiguity that multiple scattering is needed to correctly model a {chi} curve while single scattering is sufficient for understanding major peaks in the ARPEFS-FTS is clarified by studying the clean Ni(111) surface. In the normal emission case, several different combinations of scattering events have similar path length differences (PLDs), and can either cancel each other or enhance the corresponding FT peak. In the off-normal case the degeneracy is greatly reduced due to the lower degree of symmetry. In normal emission AR PEFS, up to third order multiple scattering is needed to describe fully both the {chi} curve and its FT spectrum. To improve the spectral resolution in the ARPEFS-FT analysis, several new spectral analysis methods are introduced. With both autocorrelation autoregression (ACAR) and autocorrelation eigenvector (ACE), we can produce a reliable power spectrum by following the order-closing procedure. The best spectra are usually obtained when the autocorrelation sequence is computed with lags up to half the data range. A simple way of determining surface adsorption sites is proposed as follows: First use a single scattering cluster for possible adsorption sites to construct the geometrical PLDs from the strong backscattering events; then compare these PLDs with those obtained from the ARPEFS-FT analysis of the experimental data. After the preferred adsorption site is determined, fine tune the interlayer distances according to the positional R-factor.
Date: November 30, 1998
Creator: Zhou, Xin
Partner: UNT Libraries Government Documents Department

Physical Properties of Intermetallic FE2VA1

Description: Fe{sub 2}VAl has recently been discovered to have a negative temperature coefficient of resistivity, moderately enhanced specific heat coefficient, and a large DOS at the Fermi level by photoemission. This triggered a round of heated research to understand the ground state of this material, both theoretically and experimentally. here they report a comprehensive characterization of Fe{sub 2}VAl. X-ray diffraction exhibited appreciable antisite disorder in all of our samples. FTIR spectroscopy measurements showed that the carrier density and scattering time had little sample-to-sample variation or temperature dependence for near-stoichiometric samples. FTIR and DC resistivity suggest that the transport properties of Fe{sub 2}VAl are influenced by both localized and delocalized carriers, with the former primarily responsible for the negative temperature coefficient of resistivity. Magnetization measurements reveal that near-stoichiometric samples have superparamagnetic clusters with at least two sizes of moments. X-ray photoemission from Fe core level showed localized magnetic moments on site-exchanged Fe. They conclude that in Fe{sub 2}VAl, antisite disorder causes significant modification to the semi-metallic band structure proposed by LDA calculations. With antisite disorder considered, they are now able to explain most of the physical properties of Fe{sub 2}VAl.
Date: May 30, 2002
Creator: Feng, Ye
Partner: UNT Libraries Government Documents Department

Hydrogen Storage Properties of Lithium Aluminohydride modified by dopants and mechanochemistry

Description: Alkali metal aluminohydrides have high potential as solid hydrogen storage materials. They have been known for their irreversible dehydrogenation process below 100 atm until Bogdanovic et al [1, 2] succeeded in the re-hydrogenation of NaAlH{sub 4} below 70 atm. They achieved 4 wt.% H{sub 2} reversible capacity by doping NaAlH{sub 4} with Ti and/or Fe organo-metalic compounds as catalysts. This suggests that other alkali and, possibly alkaline earth metal aluminohydrides can be used for reversible hydrogen storage when modified by proper dopants. In this research, Zr{sub 27}Ti{sub 9}Ni{sub 38}V{sub 5}Mn{sub 16}Cr{sub 5}, LaNi{sub 4.85}Sn{sub 0.15}, Al{sub 3}Ti, and PdCl{sub 2} were combined with LiAlH{sub 4} by ball-milling to study whether or not LiAlH{sub 4} is capable to both absorb and desorb hydrogen near ambient conditions. X-ray powder diffraction, differential thermal analysis, and scanning electron microscopy were employed for sample characterizations. All four compounds worked as catalysts in the dehydrogenation reactions of both LiAlH{sub 4} and Li{sub 3}AlH{sub 6} by inducing the decomposition at lower temperature. However, none of them was applicable as catalyst in the reverse hydrogenation reaction at low to moderate hydrogen pressure.
Date: May 30, 2002
Creator: Hosokawa, Keita
Partner: UNT Libraries Government Documents Department

Measurement of the Neutron electric form factor at Q2=0.8 2(GeV\\c)

Description: Nucleon form factors allow a sensitive test for models of the nucleon. Recent experiments utilising polarisation observables have resulted, for the first time, in a model-independent determination of the neutron electric form factor GnE. This method employed an 80% longitudinally polarised, high intensity (10 uA) electon beam (883 MeV) that was quasi-elastically scattered off a liquid deuterium target in the reaction D (e, en)p. A neutron polarimeter was designed and installed to measure the ratio of transverse-to-longitudinal polarisation using neutron scattering asymmetries. This ratio allowed a determination of the neutron elastic form factor, GnE, free of the previous large systematic uncertainties associated with the deuterium wave function. The experiment took place in the A1 experimental hall at MAMI taking advantage of a high resolution magnetic spectrometer. A detailed investigation was carried out into the performance of the neutron polarimeter.
Date: September 30, 2007
Creator: Glazier, Derek
Partner: UNT Libraries Government Documents Department

A RANS/DES Numerical Procedure for Axisymmetric Flows with and without Strong Rotation

Description: A RANS/DES numerical procedure with an extended Lax-Wendroff control-volume scheme and turbulence model is described for the accurate simulation of internal/external axisymmetric flow with and without strong rotation. This new procedure is an extension, from Cartesian to cylindrical coordinates, of (1) a second order accurate multi-grid, control-volume integration scheme, and (2) a k-{omega} turbulence model. This paper outlines both the axisymmetric corrections to the mentioned numerical schemes and the developments of techniques pertaining to numerical dissipation, multi-block connectivity, parallelization, etc. Furthermore, analytical and experimental case studies are presented to demonstrate accuracy and computational efficiency. Notes are also made toward numerical stability of highly rotational flows.
Date: October 30, 2007
Creator: Andrade, A J
Partner: UNT Libraries Government Documents Department

Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides

Description: A series of in situ toughened, Al, B and C containing, silicon carbide ceramics (ABC-SiC) has been examined with Al contents varying from 3 to 7 wt%. With increasing Al additions, the grain morphology in the as-processed microstructures varied from elongated to bimodal to equiaxed, with a change in the nature of the grain-boundary film from amorphous to partially crystalline to fully crystalline. Fracture toughness and cyclic fatigue tests on these microstructures revealed that although the 7 wt.% Al containing material (7ABC) was extremely brittle, the 3 and particularly 5 wt.% Al materials (3ABC and 5ABC, respectively) displayed excellent crack-growth resistance at both ambient (25 C) and elevated (1300 C) temperatures. Indeed, no evidence of creep damage, in the form of grain-boundary cavitation, was seen at temperatures at 1300 C or below. The enhanced toughness of the higher Al-containing materials was associated with extensive crack bridging from both interlocking grains (in 3ABC) and uncracked ligaments (in 5ABC); in contrast, the 7ABC SiC showed no such bridging, concomitant with a marked reduction in the volume fraction of elongated grains. Mechanistically, cyclic fatigue-crack growth in 3ABC and 5ABC SiC involved the progressive degradation of such bridging ligaments in the crack wake, with the difference in the degree of elastic vs. frictional bridging affecting the slope, i.e., Paris law exponent, of the crack-growth curve. In addition an investigation of fracture resistance in non-transforming ceramics toughened by grain bridging mechanism is presented using linear elastic fracture mechanics (LEFM). Linear superposition theorems are used for the superposition of crack opening displacements, as well as stress intensity factors, resulting from the external tractions and the internal compressive bridging stresses. Specifically weight functions are used to relate the CODs, stress intensity factors, and tractions and the bridging stress. Expressions are derived for apparent material resistance, the bridging ...
Date: August 30, 2004
Creator: Yuan, Rong
Partner: UNT Libraries Government Documents Department