Physical Sciences and Mathematics Commons^™

Chemical And Electronic Structure Of Surfaces And Interfaces In Compound Semiconductors, Sujitra Pookpanratana

UNLV Theses, Dissertations, Professional Papers, and Capstones

The interface formation between two different materials is important in applications for optoelectronic devices. Often, the success or performance of these devices is dependent on the formation of these heterojunctions. In this work, the surface and interfaces in such materials for optoelectronic devices are investigated by a suite of X-ray analytical techniques including X-ray photoelectron (XPS), X-ray excited Auger electron (XAES), and X-ray emission (XES) spectroscopies to provide novel insight.

For the group III-nitrides (e.g., AlxGa1-xN) used in many light emitting devices, a significant challenge exists to form an Ohmic contact. The electron affinities and band gaps of GaN and …

Measurement System For High Pressure Characterizations Of Materials, Matthew K. Jacobsen

UNLV Theses, Dissertations, Professional Papers, and Capstones

Thermoelectric materials have long been investigated for possible use as power sources. This application was recently put to use in the Voyager space program, powering the deep space probes. Despite the usefulness of these materials, the use of pressure to investigate the material properties has only recently become interesting. As such, the work in this document was to developing a system for concurrently measuring the necessary properties. This system is capable of measuring the electrical resistivity, thermal conductivity, and Seebeck coefficient in the pressure range from 0 - 10 GPa. The results for zinc, almandine garnet, and nickel are presented …

Speciation And Spectroscopy Of The Uranyl And Tetravalent Plutonium Nitrate Systems: Fundamental Studies And Applications To Used Fuel Reprocessing, Nicholas A. Smith

UNLV Theses, Dissertations, Professional Papers, and Capstones

This dissertation explores the use of UV-Visible spectroscopy and Time Resolved Laser Induced Fluorescence spectroscopy as near real time process monitors of uranium and plutonium concentrations in aqueous reprocessing trains. The molar absorptivities and linear ranges of these metals were investigated under total nitrate and acid concentrations similar to those found in current reprocessing systems. Concurrent to this, a new multiple wavelength monitor was derived that is capable of determining the total nitrate concentration spectroscopically. This method uses the uranium absorbance spectrum to calculate the nitrate concentration in solution. When used as part of an Advanced Safeguard suite, this technique …

Vibrational Spectroscopic Study Of The 1,3,5,7-Cyclooctatetraene At High Static Pressures, Edward Romano

UNLV Theses, Dissertations, Professional Papers, and Capstones

1,3,5,7-cyclooctatetraene (COT) is a single-ringed hydrocarbon that has not been studied extensively under high static pressures. It was the intent of this study to reveal its high-pressure behavior through its vibrational spectra. To this end, its infrared spectra as a function of pressure were collected at the National Synchrotron Light Source (NSLS) U2A beamline, as well as its Raman spectra over pressure using a micro-Raman spectrometer.

The spectra show the typical stiffening of force constants with increasing pressure for most peaks, particularly those which would theoretically exhibit a mode-softening behavior under hypotheses of molecular planarization and aromatization. Additionally, the data …

Neutron Flux And Energy Characterization Of A Plutonium-Beryllium Isotopic Neutron Source By Monte Carlo Simulation With Verification By Neutron Activation Analysis, Zachary R. Harvey

UNLV Theses, Dissertations, Professional Papers, and Capstones

The purpose of this research was to characterize the neutron energy distribution and flux emitted from the UNLV plutonium-beryllium source, serial number MRC-N-W PuBe 453. This was accomplished through the use of the MCNPX/5 Monte-Carlo particle transport code to simulate radiation interactions within the physical environment of the source and its surroundings. The moderating drum currently containing the source as well as all of the sampling ports were accurately modeled in MCNPX/5. This geometry was then used to simulate the neutron interactions taking place in these geometries. The results of the simulations were then verified by the use of specifically …

Characterization Of Plutonium Particles Originating From The Bomarc Accident -1960, Richard Charles Gostic

UNLV Theses, Dissertations, Professional Papers, and Capstones

Within the U.S. arsenal, 32 accidents with nuclear weapons were reported between 1950 and 1980. One of these accidents occurred at McGuire AFB in 1960. A BOMARC missile armed with a nuclear warhead caught on fire and as a result the warhead was destroyed. Sub-millimeter particles consisting of weapons grade plutonium (WGPu) produced by this accident were distributed around the site and remained in the environment for 47 years. Soil cores known to contain WGPu particles produced by this accident were obtained. The particles were localized and removed from the soil with the aid of high resolution computed tomography. The …

Partial Phonon Density Of States Of 57-Iron And 161-Dysprosium In Dyfe3 By Nuclear Resonant Inelastic X-Ray Scattering Under High Pressure, Elizabeth Anne Tanis

UNLV Theses, Dissertations, Professional Papers, and Capstones

The dual partial phonon density of states (DOS) from two different Mossbauer isotopes (161Dy and 57Fe) in the same material (DyFe3) was successfully measured using the nuclear resonant inelastic x-ray scattering (NRIXS) technique at high pressure. Nuclear inelastic scattering measurements yield an in-depth understanding of the element-specific dynamic properties. The Debye temperatures , the Lamb-Mossbauer factor, and the vibrational contributions to the Helmholtz free energy, specific heat , entropy and internal energy are calculated directly from the phonon density of states.

Theoretical And Computational Study Of Time Dependent Scattering On A 2d Surface, Michael Sohn

UNLV Theses, Dissertations, Professional Papers, and Capstones

The quantum mechanical treatment of the elastic scattring of atoms from a crystal surface provides valuable information, such as surface properties and gas-surface interaction potentials. However, since it is based on the stationary state solution, it does not provide the details of the scattering process in the neighborhood of the surface, especially when atoms are physically adsorbed. In this thesis, the time evolution of the scattering process is treated in 2D with a model potential, V(x, z) = -|g|δ(z) + λδ(z)cos(2πx/a), using the Gaussian wave packet approach. The focus is on the case where the Gaussian wave packet makes a …

Photon Density Of States Of 47-Iron And 161-Dysprosium In Dyfe3 By Nuclear Resonant Inelastic X-Ray Scattering Under High Pressure, Elizabeth Anne Tanis

UNLV Theses, Dissertations, Professional Papers, and Capstones

The dual partial phonon density of states (DOS) from two different Mossbauer isotopes (161Dy and 57Fe) in the same material (DyFe3) was successfully measured using the nuclear resonant inelastic x-ray scattering (NRIXS) technique at high pressure. Nuclear inelastic scattering measurements yield an in-depth understanding of the element-specific dynamic properties. The Debye temperatures, the Lamb-Mossbauer factor (fLM), and the vibrational contributions to the Helmholtz free energy (Fvib), specific heat (cV ), entropy (Svib) and internal energy (Uvib) are calculated directly from the phonon density of states.

Finite Strain Studies Of Single Crystal Fe3p Under High Pressures, John William Howard

UNLV Theses, Dissertations, Professional Papers, and Capstones

Fe3P (synthetic schreibersite) is a phosphide occurring in iron alloys. Phosphorousis often considered an undesired impurity causing brittleness. Conversely, in some cases the addition of iron phosphides to certain materials is beneficial (e.g.properties of certain frictional materials are enhanced). In terrestrial rock, we do not find Fe3P, although (Fe;Ni)3P (natural schreibersite) is found in nearly all iron-containing meteorites. In this project, we examine the unit cell parameters of Fe3P as function of pressure and derive the respective axial and bulk compressibilities. Both Vinet and Birch-Murnaghan formulations were used to relate pressure and unit cell volume, and a comparison of each …