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Fabrication of advanced thermionic emitters using laser chemical vapor deposition-rapid prototypingFuhrman, Brian Thomas 08 1900 (has links)
No description available.
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Field enhanced thermionic emission from oxide coated carbon nanotubesDay, Christopher M. January 2006 (has links)
A cathode structure was demonstrated that utilizes aligned carbon nanotubes (CNTs) to improve the thermionic electron emission by increasing the field enhancement of the cathode surface. Aligned CNTs were grown on the surface of a tungsten substrate by plasma enhanced chemical vapor deposition. The tungsten-CNT structure was further coated with a thin film of low work function emissive materials by magnetron sputtering. Numerous cathodes with varying CNT morphology and oxide layer thickness were created. The field and thermionic emission of the cathodes were tested in order to study the effects of the surface properties on the emission characteristics. It was observed that the introduction of CNTs into an oxide cathode structure improves both the thermionic and field emission, even in cathodes with relatively low field enhancement factors. Because of the high field enhancement factors that are available for CNTs, there remains a potential for dramatically improved electron emission. / Department of Physics and Astronomy
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Carrier escape from semiconductor quantum wells.Takasaki, Bruce Warren. Preston, John S. Unknown Date (has links)
Thesis (Ph.D.)--McMaster University (Canada), 1996. / Source: Dissertation Abstracts International, Volume: 57-10, Section: B, page: 6328. Adviser: J. S. Preston.
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A GENERALIZED THERMIONIC-TUNNELING MODEL OF CARRIER INJECTION ACROSS A METAL/POLYMER INTERFACEMUTHIAH-NAKARAJAN, VENKATARAMAN 30 June 2003 (has links)
No description available.
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Surface photovoltage transients for p-type AlGaNPhumisithikul, Karen L 01 January 2015 (has links)
There is an understanding of surface photovoltage (SPV) behavior for GaN, yet little is known about the SPV behavior for AlGaN. In this work, a Kelvin probe was used to measure the SPV for p-type AlGaN. Very slow SPV transients were found in AlGaN, which could not be explained with a simple thermionic model. A possible explanation of this behavior is the segregation of impurities to the surface, which causes significant reduction of the depletion region width (down to 2 nm), with carrier tunneling and hopping becoming the dominant mechanisms responsible for the SPV transients. To verify this assumption, the near-surface defective region (about 40 nm) has been removed through the ICP-RIE process. After the etching, the SPV transients became fast and increased in magnitude by about 0.6 eV. By using the thermionic model, band bending was estimated to be -1 eV.
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"Investigação do processo de obtenção de aluminatos de bário e cálcio para construção e caracterização de catodos termiônicos impregnados para aplicação em dispositivos de microondas de potência" / INVESTIGATION OF BARIUM-CALCIUM ALUMINATE PROCESS TO MANUFACTURE AND CHARACTERIZE IMPREGNATED THERMIONIC CATHODE FOR POWER MICROWAVE DEVICESHigashi, Cristiane 20 October 2006 (has links)
O presente trabalho descreve os processos de preparação do aluminato de bário e cálcio, material emissor de elétrons, empregados nos catodos do tipo impregnado para utilização em uma válvula de microondas do tipo TWT. Os catodos investigados constituem-se de uma pastilha de tungstênio porosa impregnada com aluminato de bário e cálcio com proporção molar 5:3:2. Para a síntese do aluminato, utilizaram-se três diferentes métodos: reação em estado sólido, precipitação e cristalização. A termogravimetria auxiliou na consolidação dos procedimentos de preparação dos aluminatos de modo a definir os parâmetros de pirólise/calcinação. Verificou-se que a técnica que apresentou melhores características de síntese foi o método da cristalização, pois esta apresentou uma menor temperatura de formação do aluminato (800ºC) em atmosfera oxidante (O2), quando comparada às técnicas de reação em estado sólido e de precipitação (temperatura de 1000ºC em atmosfera redutora H2). Utilizou-se o conceito da distribuição da função trabalho prática (PWFD) de Miram para a caracterização termiônica dos catodos impregnados. Empregando-se este método, foi possível traçar o perfil termiônico do catodo com aluminato de bário e cálcio. As curvas PWFD apresentaram a função trabalho média do catodo aluminato de, aproximadamente, 2,00 eV. / In the present work it is described the barium calcium aluminate manufacture processes employed to produce impregnated cathodes to be used in a traveling-wave tube (TWT). The cathodes were developed using a tungsten body impregnated with barium and calcium aluminate with a 5:3:2 proportion (molar). Three different processes were investigated to obtain this material: solid-state reaction, precipitation and crystallization. Thermal analysis, thermogravimetry specifically, supported to determine an adequate preparation procedure (taking into account temperature, time and pirolisys atmosphere). It was verified that the crystallization showed a better result when compared to those investigated (solid-state reaction and precipitation techniques formation temperature is about 1000ºC in hydrogen atmosphere), whereas it presented the lower formation temperature (800ºC) in oxidizing atmosphere (O2). It was used the practical work function distribution theory (PWFD) of Miram to characterize thermionic impregnated cathode. The PWFD curves were used to characterize the barium-calcium aluminate cathode. PWFD curves shown that the aluminate cathode work function is about 2,00 eV.
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Development of systems analysis program for space reactor studiesLewis, Bryan R. 14 June 1993 (has links)
An overall systems design code was developed to model
an advanced in-core thermionic energy conversion based
nuclear reactor system for space applications at power
levels of 10 to 50 kWe. The purpose of this work was to
provide the overall shell for the systems code and to also
provide the detailed neutronic analysis section of the code.
The design code that was developed is to be used to evaluate
a reactor system based upon a single cell thermionic fuel
element which uses advanced technology to enhance the
performance of single cell thermionic fuel elements.
A literature survey provided information concerning how
other organizations performed system studies on similar
space reactor designs. / Graduation date: 1994
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System modeling and reactor design study of an advanced incore thermionic space reactorLee, Hsing Hui 12 October 1992 (has links)
Incore thermionic space reactor design concepts which operate at a
nominal power output range of 20 to 50 kWe are described. Details of the
neutronic, thermionic, thermal hydraulics and shielding performance are
presented. Due to the strong absorption of thermal neutrons by natural
tungsten, and the large amount of that material within the reactor core,
two designs are considered.
An overall system design code has been developed at Oregon State
University to model advanced incore thermionic energy conversion based
nuclear reactor systems for space applications. The code modules include
neutronics and core criticality, a thermionic fuel element performance
module with integral thermal hydraulics calculation capability, a
radiation shielding module, and a module for the waste heat rejection.
The results show that the driverless single cell ATI configuration,
which does not have driver rods, proved to be more efficient than the
driven core, which has driver rods. It also shows that the inclusion of
the true axial and radial power distribution decrease the overall
conversion efficiency. The flattening of the radial power distribution by
three different methods would lead to a higher efficiency. The results
show that only one thermionic fuel element (TFE) works at the optimum
emitter temperature; all other TFEs are off the optimum performance and
result in 40 % decrease of the efficiency of the overall system. / Graduation date: 1993
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Nuclear design analysis of low-power (1-30 KWe) space nuclear reactor systemsGedeon, Stephen R. 23 November 1993 (has links)
Preliminary nuclear design studies have been completed on ten
configurations of nuclear reactors for low power (1-30 kWe) space
applications utilizing thermionic energy conversion. Additional design
studies have been conducted on the TRICE multimegawatt in-core
thermionic reactor configuration. In each of the cases, a reactor
configuration has been determined which has the potential for operating
7 years with sufficient reactivity margin. Additional safety
evaluations have been conducted on these configurations including the
determination of sufficient shutdown reactivity, and consideration of
water immersion, water flooding, sand burial, and reactor compaction
accident scenarios. It has been found, within the analysis conducted
using the MCNP Monte Carlo neutron transport code, that there are
configurations which are feasible and deserve further analysis. It has
also been found that solid core reactors which rely solely on conduction
for heat removal as well as pin type cores immersed in a liquid metal
bath have merit. The solid cores look attractive when flooding and
compaction accident scenarios are considered as there is little chance
for water to enter the core and cause significant neutron moderation. A
fuel volume fraction effect has also been found in the consideration of
the sand burial cases for the SP-100 derived configurations. / Graduation date: 1994
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Experimental and numerical studies of a new thermionic emitter structure based on oxide coated carbon nanotubes operating at large emission currentsLittle, Scott A. January 2007 (has links)
We have developed a thermionic cathode capable of high emission currents. The structure of this cathode is oxide coated carbon nanotubes (CNTs) on a tungsten (W) substrate. This cathode was superior in emission due to the combination of the field enhancement effect from the CNTs and the lowered work function from the semiconducting oxide surface. Such oxide coated CNTs were excellent electron emitters. Conventional electron emission theories, such as Richardson's and Fowler-Nordheim's, did not accurately describe the field enhanced thermionic emission from such emitters. A unified electron emission theory was adopted and numerical simulations were performed to explain the deviation of electron emission from conventional field and thermionic emission theories. Also, the thermionic measurement system and measurement methods were improved in order to measure and characterize the strong electron emission from this new cathode. Large electron emission current from such structures also made a new thermionic cooling device a possibility. Cooling due to the electron emission was measured in terms of temperature drop, and a large temperature drop was observed from this cathode structure. Finally, applications of this cathode in plasma discharge devices were explored. This new cathode was tested in a plasma environment and initial results were obtained. / Department of Physics and Astronomy
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