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Thesis (Ph. D.)--University of Chicago, 1920. / "Private edition, distributed by the University of Chicago libraries, Chicago, Illinois." "Reprinted from the Physical review, n.s., vol. XVII, no. 3, March, 1921." Also available on the Internet.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1976. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Bibliography: leaves 87-88.

Electronic processes, such as electronic breakdown and electron emission, in gases and liquids have implications in microplasmas, laser applications, water purification, biomedical applications, geographical mapping, and radiation detection. Electron emission and breakdown mechanisms are heavily researched and characterized in gases. Much of the current research into these mechanisms is focused on unifying breakdown and emission mechanisms. For electron emission, these mechanisms include field emission (FE), space-charge-limited emission (SCLE), thermionic emission (TE), and photoemission (PE), while gas breakdown emission mechanisms include Paschen’s law (PL) and Townsend breakdown (TB)with ion-enhanced FE becoming important at microscale. This research first unified SCLE and FE in vacuum and has been extended to include SCLE with collisions (for a gas at non-vacuum) and TE. This thesis extends this approach in electron emission unification, referred to as “nexus” theory, in two directions. First, we will apply this theory to liquids to examine the transition from FE to SCLE and hypothesize about the implications should there be a phase change. Second, we will incorporate PE, which becomes important with increasing interest in ultrafast laser phenomena at nanoscale and development of solar cells, with SCLE, TE, and FE.<div><br></div><div>Initial nexus theory studies included gas at non-vacuum pressures by including electron mobility in the electron force law. In principle, this behavior should be the same whether the medium is air or liquid. Electron emission and breakdown, which can arise from field emission, are increasingly important in plasma water treatment, pulsed power systems, radiation detection, and even understanding the physics of high electric fields applied to liquid helium for the Spallation Neutron Source. To demonstrate the applicability of nexus theory to liquids, we fit experimental data for electron emission in hydrocarbons to the full theory unifying FE to SCLE with and without collisions. The measured current followed Fowler-Nordheim scaling for FE at lower voltages with space charge beginning to contribute at higher voltages; none of the hydrocarbons study fully transitioned to Mott-Gurney (SCLE with collisions) scaling within the experimentally studied parameter range. Considering a higher mobility representative of a vapor in the theory demonstrates the feasibility of achieving Child-Langmuir (SCLE in vacuum)scaling for the gaps of the size considered experimentally. Thus, this approach may ultimately be applied to model electron emission during both phases changes and transitions between the mechanisms.<br></div><div><br></div><div>We next extended the gas nexus theory to analyze the transitions between PE and the other emission mechanisms. We modified the previous theory that used the generalized thermal-field emission (GTF) theory for electron current to instead use the generalized thermal-field photoemission (GTFP) theory. Using this, we obtained exact solutions for current as a function of applied voltage and demonstrated the asymptotic behavior with regard to the modified Fowler DuBridge (MFD) equation, which models PE. We combined the MFD equation with the other asymptotic solutions to develop state diagrams unifying the various emission mechanisms to provide guidance to the mechanisms and transitions relevant under various conditions of mobility, gap distance, temperature, and laser energy/wavelength/frequency. These diagrams provide guidance on which asymptotic solution or more detailed theory would be necessary to accurately relate current and voltage under various operating conditions.<br></div>

Secondary electron (SE) emission from thin carbon foils induced by 1-20 keV positrons has been investigated over a range of nominal foil thicknesses from 1.0 to 5.0 μg/cm2. The measurement of SEs was carried out in forward geometry using a microchannel plate as a detector. The SE yield γ has been measured as a function of beam energy and compared with some Monte Carlo simulation results. We also present in this thesis the material parameter Λ=? / (dE / dx) and the emitted SE energy spectra. Forincident positron energy of 5 keV or higher, the distribution is found to be characterizedby the Sickafus form, AEm and m is close to 1. For low energy incident positrons however, another form, Bexp(E / t) , is proposed for describing the SE distribution. The maximum scattering angle for SEs emitted from 5.0 μg/cm2 is found to be around 60°. Measurements of energy loss and energy loss straggling for 1-10 keV positrons passing through thin carbon foils of different thicknesses ranging from 1.0 to 5.0 μg/cm2 are present in this thesis. The stopping power dE / dx and positron transmission coefficient have also been investigated as a function of incident positron energy and foil thickness. Particularly, our experimental results are compared with those from Monte Carlo simulation and theory with a view to providing a way to determine the real thickness of carbon foil. The ratio of the energy loss straggling to the foil thickness seems to have a linear relation with the beam energy. The transmitted positrons after passing through 5.0 μg/cm2 C-foil have a small scattering angle which is less than 10°. Titanium dioxide nanotube arrays fabricated by anodization of titanium foil and annealed at different temperatures were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and positron annihilation spectroscopy (PAS). The crystallization process and morphological change of the nanotubes have been discussed. It is found that anatase (101) only appeared on the walls of the nanotube. The atomic concentration of fluoride and the ratio of Ti/O decreased when the annealing temperature increased. Vacancy type defects were found to diffuse toward the surface when the samples were annealed at 200°C and 400°C and healing of vacancies occurred at 600°C. In addition, the fluoride may form some complexes with vacancies on the surface hence lowering the value of the S parameter. A new design of the lifetime positron beam based on the SE emission is proposed. The observed experimental results and simulation results make it possible to construct a new type of high resolution (<250ps) lifetime positron beam. In addition, how rastering of the positron beam can be used to accurately locate the position of the C-foil is also shown; a feature that will be of value in setting up a positron lifetime system. Such a lifetime system is expected to be much simpler in construction than existing beam based lifetime spectroscopy systems. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

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Carbon Nanotubes (CNTs) have great potentials for Field Electron Emission (FEE) and Flow Boiling Heat Transfer (FBHT) applications. However, their weak adhesion on metallic substrates limits the development of CNTs in both applications. Diamond has high thermal conductivity and develops strong bonding with CNTs. The development of a diamond interlayer between CNTs and substrates is a feasible approach to address the adhesion problems. The purpose of this research was to develop a new CNT-based materials with a diamond interlayer for FEE and FBHT applications by focusing on four objectives: (1) enhancement of diamond thin film adhesion on a Cu substrate, (2) improvement of the CNT FEE stability, (3) reduction of the CNT FEE turn-on field, and (4) investigation of the FBHT performance of CNT based structures. The CNTs and diamond thin films in this thesis were prepared by Microwave Plasma enhanced Chemical Vapor Deposition (MPCVD) and Hot Filament enhanced Chemical Vapor Deposition (HFCVD). The structure and chemical states of the diamond films and CNTs were characterized by Scanning Electron Microscopy (SEM), cross-sectional Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), Raman spectroscopy, synchrotron based X-ray Absorption Spectroscopy (XAS). To deposit diamond thin films on a Cu substrate with sufficient adhesion strength, a sandblasting pretreatment and alloying with a tiny amount of Al were investigated. The adhesion of diamond thin films to substrates was evaluated by Vickers micro-hardness indentation. The FEE stability and turn-on field were measured by a Keithley 237 high voltage measuring unit. The FBHT property of the structures was tested repeatedly at different flow velocities to explore the dependence of heat transfer performance on certain parameters, including the flow patterns, Critical Heat Flux (CHF), and stability. The results show that sandblasting pretreatment increases the surface roughness and surface defect density, thereby increasing diamond nucleation density and adhesion to the Cu substrate. Al alloying appears to inhibit the formation of graphite at the interface between diamond and the Cu substrate, which improves the chemical bonding between diamond and the Cu substrate and increases the adhesion strength between them. The FEE testing results show that ultra-high FEE stability (more than 5000 minutes) was achieved for the CNTs with a diamond interlayer. This is attributed to the good contact at the diamond-CNT and diamond-substrate interfaces. The main factors that affect the CNT FEE turn-on field were also studied. By optimizing the structure, an FEE turn-on field of 5.1 V/μm was achieved and an emission barrier model for CNTs with a diamond interlayer on Cu substrate was used to explain the results. FBHT testing was done on CNTs with different structures and the results show that high heat transfer efficiency can be achieved on CNTs with a diamond interlayer at low mass fluxes.

Biological damage of tissue due to intermediate energy neutrons is generally known to be very important in radiobiology and radiation protection. However, there is no suitable method to determine the quality of these neutrons in particular in the working environment of mixed (n,y) radiation fields. In this thesis, an attempt is made to develop a dosimeter based on microdosimetric principles which has the capability for such a purpose. With this object the basic concepts of microdosimetry are reviewed and discussed with emphasis on their application for radiation protection and in designing of the dosimeter. Microdosimetry based on low pressure tissue-equivalent proportional counters (TEPCs) is a powerful technique for determining microscopic distributions of energy deposition and quality of ionizing radiations. However the energy deposition spectra of intermediate energy neutrons in mixed fields of fast neutrons can only be measured using TEPC in co-axial double cylindrical form by an appropriate choice for the thickness of the common tissue-equivalent (TE) dividing wall separating the inner and outer counters and by appropriate use of coincidence/anti-coincidence pulse arrangements. An analytical calculation for the response of the inner counter operating in coincidence/anti-coincidence modes with the outer counter was developed. However there will be some events, due to fast neutrons, which will contribute to the signals from intermediate energy neutrons and which cannot be removed by anti-coincidence. For these analytical corrections must be made. Also, the events associated with the dividing wall inherent in the system can contribute to the response of the inner counter and must be corrected by calculation. The calculation was possible due to the fact that recoil particles from intermediate energy neutron interactions have effective stopping powers and projected ranges which differ significantly from the continuous slowing down approximation (CSDA) values. By incorporating these the basic CSDA formulae for energy deposition spectra of neutrons could be extended down to intermediate energy neutrons of about 1 keV. A prototype co-axial double cylindrical TEPC capable of separating the component of neutrons (≤ 850 keV) in mixed (n,y) radiation fields was manufactured and tested. The thin wall dividing the inner and outer counters was fabricated from the standard A-150 TE plastic with the thickness equivalent to the range of 850 keV protons. The operational characteristics of the dosimeter were studied to determine its applicability for use in microdosimetry. The gas gain of the inner and outer TEPCs was measured at various simulated mean chord lengths and applied voltages. The results can be expressed according to Campion's equation within a given range of the electric field strength. The resolution of the inner TEPC measured at the operating voltages is in agreement with the theoretical prediction. A series of microdosimetric experiments were performed with mixed fields of 60Co gamma-rays and neutrons from the UTR-300 nuclear reactor and from 252Cf and 241Am-Be radioactive sources. Discrimination against fast neutrons of energy > 850 keV was achieved using an anti-coincidence unit specially designed for better efficiency of data acquisition. Discrimination against fast electrons due to photon interactions was also achieved. Spectra with anti-coincidence are dominated by slow protons and electrons. Their mean lineal energies are higher than those of spectra without anti-coincidence. The quality factor and dose equivalent for spectra with anti-coincidence are higher than the spectra without anticoincidence indicating the importance of intermediate energy neutrons in mixed fields. The quality factor and the corresponding dose equivalent corrected for saturation of lineal energy corresponding to 2 nm of ionization spacing is consistently higher than those derived from the absorbed dose based formulae, the biophysical implications of which are discussed. Suggestion for future developments for microdosimetry of intermediate energy neutrons in mixed fields are made and discussed.

The question of whether the small bandgaps of semiconductors play a significant role in their secondary electron emission properties is investigated by studying evaporated graphitic amorphous carbon, which has a roughly 0.5 eV bandgap, in comparison with microcrystalline graphite, which has zero bandgap. The graphitic amorphous carbon is found to have a 30% increase in its maximum secondary electron yield over that of two microcrystalline graphite samples with comparable secondary electron yields: highly oriented pyrolytic graphite and colloidal graphite. The potentially confounding influence of the vacuum level has been isolated through the measurement of the photoelectron onset energy of the materials. Other less significant materials parameters are also isolated and discussed. Based on these measurements, it is concluded the magnitude of bandgap may have an appreciable effect on the magnitude of the secondary electron yield and further studies of this effect with annealed graphitic amorphous carbon are warranted. In support of this work, a hemispherical two-grid, retarding field electron energy analyzer has been designed, constructed, and characterized for the present work. The advantages and disadvantages of the analyzer are discussed in comparison to other methods of measuring secondary electron emission. The analyzer has a resolution of ±(1.5 eV + 4% of the incident electron energy). A novel effort to derive theoretical, absolute correction factors that compensate for electron losses within the analyzer, mainly due to the grid transmission, is presented. The corrected secondary electron yield of polycrystalline gold is found to be 30% above comparable experimental studies. The corrected backscattered electron yield of polycrystalline gold is found to be 14% above comparable experimental studies. Corrected secondary yields for the microcrystalline graphite samples are found to range from 35-70% above those found in five experimental studies in the literature. The theoretical correction factors are estimated to have a 4-6% uncertainty. Reasons for the large discrepancy in yield measurements with the analyzer are discussed and thought to be due mainly to the lack of similar corrective factors in the previous studies. The supporting instrumentation is fully characterized, including a detailed error analysis.