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 Degree Discipline: Physics
The Concept of Collision Strength and Its Applications

The Concept of Collision Strength and Its Applications

Date: May 2004
Creator: Chang, Yongbin
Description: Collision strength, the measure of strength for a binary collision, hasn't been defined clearly. In practice, many physical arguments have been employed for the purpose and taken for granted. A scattering angle has been widely and intensively used as a measure of collision strength in plasma physics for years. The result of this is complication and unnecessary approximation in deriving some of the basic kinetic equations and in calculating some of the basic physical terms. The Boltzmann equation has a five-fold integral collision term that is complicated. Chandrasekhar and Spitzer's approaches to the linear Fokker-Planck coefficients have several approximations. An effective variable-change technique has been developed in this dissertation as an alternative to scattering angle as the measure of collision strength. By introducing the square of the reduced impulse or its equivalencies as a collision strength variable, many plasma calculations have been simplified. The five-fold linear Boltzmann collision integral and linearized Boltzmann collision integral are simplified to three-fold integrals. The arbitrary order linear Fokker-Planck coefficients are calculated and expressed in a uniform expression. The new theory provides a simple and exact method for describing the equilibrium plasma collision rate, and a precise calculation of the equilibrium relaxation time. It generalizes ...
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Microstructure and Electronic Structures of Er-Doped Si Nano-particles Synthesized by Vapor Phase Pyrolysis

Microstructure and Electronic Structures of Er-Doped Si Nano-particles Synthesized by Vapor Phase Pyrolysis

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Date: May 2000
Creator: Chen, Yandong
Description: Si nanoparticles are new prospective optoelectronic materials. Unlike bulk Si cry-stals, Si nanoparticles display intriguing room-temperature photoluminescence. A major challenge in the fabrication of Si nanoparticles is the control of their size distribution. The rare-earth element Er has unique photo emission properties, including low pumping power, and a temperature independent, sharp spectrum. The emission wavelength matches the transmission window of optical fibers used in the telecommunications industry. Therefore, the study of Er-doped Si nanoparticles may have practical significance. The goals of the research described in this dissertation are to investigate vapor phase pyrolysis methods and to characterize the microstructure and associated defects, particles size distributions and photoluminescence efficiencies of doped and undoped Si nanoparticles using analytical transmission electron microscopy, high resolution electron microscopy, and optical spectroscopy. Er-doped and undoped Si nanoparticles were synthesized via vapor-phase pyrolysis of disilane at Texas Christian University. To achieve monodisperse size distributions, a process with fast nucleation and slow growth was employed. Disilane was diluted to 0.48% with helium. A horizontal pyrolysis oven was maintained at a temperature of 1000 °C. The oven length was varied from 1.5 cm to 6.0 cm to investigate the influence of oven length on the properties of the nanoparticles. ...
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Synthesis and Study of Engineered Heterogenous Polymer Gels

Synthesis and Study of Engineered Heterogenous Polymer Gels

Date: August 1998
Creator: Chen, Yuanye
Description: This dissertation studies physical properties and technological applications of engineered heterogenous polymer gels. Such gels are synthesized based on modulation of gel chemical nature in space. The shape memory gels have been developed in this study by using the modulated gel technology. At room temperature, they form a straight line. As the temperature is increased, they spontaneously bend or curl into a predetermined shape such as a letter of the alphabet, a numerical number, a spiral, a square, or a fish. The shape changes are reversible. The heterogenous structures have been also obtained on the gel surface. The central idea is to cover a dehydrated gel surface with a patterned mask, then to sputter-deposit a gold film onto it. After removing the mask, a gold pattern is left on the gel surface. Periodical surface array can serve as gratings to diffract light. The grating constant can be continuously changed by the external environmental stimuli such as temperature and electric field. Several applications of gels with periodic surface arrays as sensors for measuring gel swelling ratio, internal strain under an uniaxial stress, and shear modulus have been demonstrated. The porous NIPA gels have been synthesized by suspension technique. Microstructures of newly ...
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Theoretical and Experimental Investigations of Peg Based Thermo Sensitive Hydro Microgel

Theoretical and Experimental Investigations of Peg Based Thermo Sensitive Hydro Microgel

Date: December 2012
Creator: Chi, Chenglin
Description: Poly ethylene glycol (PEG) based microgels were synthesized and investigated. The PEG microgel has the same phase transition as the traditional poly N-isopropylacrylamide (PNIPAM). As a good substitute of PNIPAM, PEG microgel exhibits many advantages: it is easier to control the lower critical solution temperature (LCST) of the microgel by changing the component of copolymers; it has a more solid spherical core-shell structure to have a double thermo sensitivity; it is straightforward to add other sensitivities such as pH, magnetic field or organic functional groups; it readily forms a photonic crystal structure exhibiting Bragg diffraction; and, most importantly, the PEG microgel is biocompatible with human body and has been approved by FDA while PNIPAM has not. PEG microgels with core-shell structure are synthesized with a two-step free radical polymerization and characterized with DLS, SLS and UV–Vis. The dynamic mechanics of melting and recrystallizing of the PEG core-shell microgel are presented and discussed. Photonic crystals of PEG microgels were synthesized and characterized. The crystal can be isolated in a thin film or a bulk column. The phase transition of PEG microgel was simulated with the mean field theory. The enthalpy and entropy of phase transition can be estimated from the best ...
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Magnetically Driven Instabilities in Gas Discharges

Magnetically Driven Instabilities in Gas Discharges

Date: August 1963
Creator: Choate, Jimmie W.
Description: In the present experiment a gas discharge plasma generator was designed and constructed and a search was made for evidence of a plasma instability due to the influence of an externally applied magnetic field. The evidence for such an unstable mode of operation is too indirect to make a possible conclusion, but an approach to more certain identification will be indicated.
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Expulsion of Carriers from the Double-Barrier Quantum Well and Investigation of Its Spectral and Transport Consequences

Expulsion of Carriers from the Double-Barrier Quantum Well and Investigation of Its Spectral and Transport Consequences

Date: March 1992
Creator: Chyla, Wojciech Tadeusz
Description: In this work I investigate the expulsion of carriers from nanostructures using the double-barrier quantum well (DBQW) as an example and discuss manifestations of this effect in the spectrum of the DBQW in absence of bias, and in the tunneling current in presence of bias. Assuming equality of the Fermi energy in all regions of the considered system, I compute the relative density of carriers localized in the DBQW and conclude that a fraction of carriers is expelled from this nanostructure.
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Transport Processes in Synchrotrons

Transport Processes in Synchrotrons

Date: May 1994
Creator: Cole, Benjamin H. (Benjamin Holland)
Description: This thesis examines the evolution of beams in synchrotrons. Following an introduction to accelerator physics in Chapter 1, in Chapter 2 I describe the Fermilab E778 'diffusion' experiment. Families of sextupoles were powered to drive the 2/5 resonance, and a beam was then kicked to populate a nonlinear region of the transverse phase space. The beam was then observed over periods of approximately 30 minutes for a variety of kick amplitudes and physical apertures. In Chapter 3 comments about the analytic treatment of such systems are discussed, including the assumptions inherent in the conventional treatment. I motivate my use of a simplified model in Chapter 4 after examining common computational methods. Deriving the model from the formalism of traditional accelerator physics, I discuss its implementation on a massively parallel computer, the Intel iPSC/860 hypercube, and examine the performance of this algorithm in detail. Using the simple model to perform the numerical experiment equivalent to E778 is the subject of Chapter 5. I derive the parameters needed for the simple model based upon the physical experiment. Both three dimensional cases and cases with reduced dimensionality are run. From power supply ripple data and an electrical model of the magnet string, I ...
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Temperature Dependence of Line Widths of the Inversion Spectra of Ammonia

Temperature Dependence of Line Widths of the Inversion Spectra of Ammonia

Date: August 1969
Creator: Cook, Charles E.
Description: One of the purposes of this work is to investigate modifications that have to be made to a standard source-modulation microwave spectrograph so that it can be used to study gases at various temperatures. Another objective in this work is to determine experimentally the function of temperature that describes how the line widths of microwave spectral lines vary with changing temperature. The most important segment of the study is the temperature dependence of the line width since from an accurate knowledge of this temperature dependence one is able to determine what molecular force fields are present and the relative importance of parts of the molecular force field.
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Quantized Hydrodynamics

Quantized Hydrodynamics

Date: August 1972
Creator: Coomer, Grant C.
Description: The object of this paper is to derive Landau's theory of quantized hydrodynamics from the many-particle Schroedinger equation. Landau's results are obtained, together with an additional term in the Hamiltonian.
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Growth and Characterization of β-Iron Disilicide, β-Iron Silicon Germanide, and Osmium Silicides

Growth and Characterization of β-Iron Disilicide, β-Iron Silicon Germanide, and Osmium Silicides

Date: August 2009
Creator: Cottier, Ryan James
Description: The semiconducting silicides offer significant potential for use in optoelectronic devices. Full implementation of the materials, however, requires the ability to tailor the energy gap and band structure to permit the synthesis of heterojunctions. One promising approach is to alloy the silicides with Ge. As part of an investigation into the synthesis of semiconducting silicide heterostructures, a series of β-Fe(Si1−xGex)2 epilayer samples, with nominal alloy content in the range 0 < x < 0.15, have been prepared by molecular beam epitaxy on Si(100). I present results of the epitaxial and crystalline quality of the films, as determined by reflection high-energy electron diffraction, Rutherford backscattering spectroscopy, and double crystal x-ray diffraction, and of the band gap dependence on the alloy composition, as determined by Fourier transform infrared spectroscopy. A reduction in band gap was observed with increasing Ge content, in agreement with previous theoretical predictions. However Ge segregation was also observed in β-Fe(Si1−xGex)2 epilayers when x > 0.04. Osmium silicide films have been grown by molecular beam epitaxy on Si(100). The silicides have been grown using e-beam evaporation sources for both Os and Si onto Si(100) substrates at varying growth rates and temperatures ranging from 600-700ºC. The resulting films have been ...
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