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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
141

Electric arc-contact interaction in high current gasblast circuit breakers

Nielsen, Torbjörn January 2001 (has links)
No description available.
142

"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 DEVICES

Higashi, 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.
143

Preparation and characterization of granular magnetic cobalt silver thin film.

January 2000 (has links)
by Chiah Man Fat. / Thesis submitted in: September 1999. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references (leaves 94-97). / Abstracts in English and Chinese. / Acknowledgements --- p.2 / Abstract --- p.3 / Table of Contents --- p.5 / List of Figures --- p.7 / List of Tables --- p.13 / Chapter Chapter 1 --- Introduction --- p.14 / Chapter 1.1. --- Overview --- p.14 / Chapter 1.2. --- Giant Magnetoresistance (GMR) --- p.15 / Chapter 1.3. --- Application of GMR Materials --- p.20 / Chapter 1.4. --- Preparation Methods --- p.22 / Chapter 1.5. --- This Thesis --- p.23 / Chapter Chapter 2 --- Sample Preparation and Experimental Methods --- p.24 / Chapter 2.1. --- MEVVA Ion Source Implanter --- p.24 / Chapter 2.2. --- The Pulsed Filtered Cathodic Arc Co-deposition System --- p.26 / Chapter 2.3. --- Sample Preparation --- p.29 / Chapter 2.3.1 --- Implantation Condition --- p.29 / Chapter 2.3.2 --- Co-deposition Conditions --- p.31 / Chapter 2.4. --- Characterization methods --- p.32 / Chapter 2.4.1 --- Magnetoresistance Measurement --- p.32 / Chapter 2.4.2 --- Atomic Force Microscopy and Magnetic Force Microscopy --- p.34 / Chapter 2.4.3 --- Rutherford Backscattering Spectroscopy (RBS) --- p.37 / Chapter 2.4.4 --- SQUID Magnetometer --- p.38 / Chapter Chapter 3 --- Characterization of Implanted Samples --- p.39 / Chapter 3.1. --- Introduction --- p.39 / Chapter 3.2. --- Results and Discussion --- p.39 / Chapter 3.2.1 --- Ag Film Thickness Dependence --- p.39 / Chapter 3.2.2 --- Dose Dependence --- p.44 / Chapter 3.2.3 --- Extraction Voltage Dependence --- p.46 / Chapter 3.2.4 --- Annealing Temperature Dependence --- p.49 / Chapter 3.2.5 --- Thicker Layer Formation --- p.56 / Chapter 3.2.6 --- AFM and MFM Measurements --- p.58 / Chapter 3.3. --- Summary --- p.64 / Chapter Chapter 4 --- Characterization of Co-deposited Samples --- p.65 / Chapter 4.1. --- Introduction --- p.65 / Chapter 4.2. --- Results and discussion --- p.65 / Chapter 4.2.1 --- RBS Measurement --- p.65 / Chapter 4.2.2 --- Magnetoresistance Measurement --- p.66 / Chapter 4.2.3 --- AFM Measurement --- p.69 / Chapter 4.2.4 --- MFM Measurement --- p.76 / Chapter 4.3. --- Summary --- p.84 / Chapter Chapter 5 --- Conclusion --- p.85 / Chapter 5.1. --- Main Results of This Work --- p.85 / Chapter 5.2. --- Suggestions on Future Works --- p.87 / Appendix --- p.89 / Reference --- p.94 / Publications --- p.97
144

Characterization of the Near Plume Region of Hexaboride and Barium Oxide Hollow Cathodes operating on Xenon and Iodine

Taillefer, Zachary R 24 January 2018 (has links)
The use of electric propulsion for spacecraft primary propulsion, attitude control and station-keeping is ever-increasing as the technology matures and is qualified for flight. In addition, alternative propellants are under investigation, which have the potential to offer systems-level benefits that can enable particular classes of missions. Condensable propellants, particularly iodine, have the potential to significantly reduce the propellant storage system volume and mass. Some of the most widely used electric thrusters are electrostatic thrusters, which require a thermionic hollow cathode electron source to ionize the propellant for the main discharge and for beam neutralization. Failure of the hollow cathode, which often needs to operate for thousands of hours, is one of the main life-limiting factors of an electrostatic propulsion system. Common failure modes for hollow cathodes include poisoning or evaporation of the thermionic emitter material and erosion of electrodes due to sputtering. The mechanism responsible for the high energy ion production resulting in sputtering is not well understood, nor is the compatibility of traditional thermionic hollow cathodes with alternative propellants such as iodine. This work uses both an emissive probe and Langmuir probe to characterize the near-plume of several hollow cathodes operating on both xenon and iodine by measuring the plasma potential, plasma density, electron temperature and electron energy distribution function (EEDF). Using the EEDF the reaction rate coefficients for relevant collisional processes are calculated. A low current (< 5 A discharge current) hollow cathode with two different hexaboride emitters, lanthanum hexaboride (LaB6) and cerium hexaboride (CeB6), was operated on xenon propellant. The plasma potential, plasma density, electron temperature, EEDF and reaction rate coefficients were measured for both hexaboride emitter materials at a single cathode orifice diameter. The time-resolved plasma potential measurements showed low frequency oscillations (<100 kHz) of the plasma potential at low cathode flow rates (<4 SCCM) and spot mode operation between approximately 5 SCCM and 7 SCCM. The CeB6 and LaB6 emitters behave similarly in terms of discharge power (keeper and anode voltage) and plasma potential, based on results from a cathode with a 0.020�-diameter. Both emitters show almost identical operating conditions corresponding to the spot mode regime, reaction rates, as well as mean and RMS plasma potentials for the 0.020� orifice diameter at a flow rate of 6 SCCM and the same discharge current. The near-keeper region plasma was also characterized for several cathode orifice diameters using the CeB6 emitter over a range of propellant flow rates. The spot-plume mode transition appears to occur at lower flow rates as orifice size is increased, but has a minimum flow rate for stable operation. For two orifice diameters, the EEDF was measured in the near-plume region and reaction rate coefficients calculated for several electron- driven collisional processes. For the cathode with the larger orifice diameter (0.040�), the EEDFs show higher electron temperatures and drift velocities. The data for these cathodes also show lower reaction rate coefficients for specific electron transitions and ionization. To investigate the compatibility of a traditional thermionic emitter with iodine propellant, a low-power barium oxide (BaO) cathode was operated on xenon and iodine propellants. This required the construction and demonstration of a low flow rate iodine feed system. The cathode operating conditions are reported for both propellants. The emitter surface was inspected using a scanning electron microscope after various exposures to xenon and iodine propellants. The results of the inspection of the emitter surface are presented. Another low current (< 5 A), BaO hollow cathode was operated on xenon and iodine propellants. Its discharge current and voltage, and plume properties are reported for xenon and iodine with the cathode at similar operating conditions for each. The overall performance of the BaO cathode on iodine was comparable to xenon. The cathode operating on iodine required slightly higher power for ignition and discharge maintenance compared to xenon, as evident by the higher keeper and anode potentials. Plasma properties in the near- plume region were measured using an emissive probe and single Langmuir probe. For both propellants, the plasma density, electron energy distribution function (EEDF), electron temperature, select reaction rate coefficients and time-resolved plasma potentials are reported. For both propellants the cathode operated the same keeper (0.25 A) and discharge current (3.1 A), but the keeper and anode potentials were higher with iodine; 27 V and 51 V for xenon, and 30 V and 65 V for iodine, respectively. For xenon, the mean electron energy and electron temperature were 7.5 eV and 0.7 eV, with bulk drift energy of 6.6 eV. For iodine, the mean electron energy and electron temperature were 6.3 eV and 1.3 eV, with a bulk drift energy of 4.2 eV. A literature review of relevant collisional processes and associated cross sections for an iodine plasma is also presented.
145

Electron Energy Distribution Measurements in the Plume Region of a Low Current Hollow Cathode

Behlman, Nicholas James 12 January 2010 (has links)
A hollow cathode is an electron source used in a number of different electric thrusters for space propulsion. One important component of the device that helps initiate and sustain the discharge is called the keeper electrode. Cathode keeper erosion is one of the main limiting factors in the lifetime of electric thrusters. Sputtering due to high-energy ion bombardment is believed to be responsible for keeper erosion. Existing models of the cathode plume, including the OrCa2D code developed at Jet Propulsion Laboratory, do not predict these high-energy ions and experimental measurement of the electron energy distribution function (EEDF) could provide useful information for the development of a high fidelity model of the plume region. Understanding of the mechanism by which these high-energy ions are produced could lead to improvements in the design of hollow cathodes. The primary focus of this work is to determine the EEDF in the cathode plume. A single Langmuir probe is used to measure the current-voltage (I-V) characteristic of the plasma plume from a low current hollow cathode in the region downstream of the keeper orifice. The EEDF is obtained using the Druyvesteyn procedure (based on interpretation of the second derivative of the I-V curve), and parameters such as electron temperature, plasma density and plasma potential are also obtained. The dependence of the EEDF and other parameters on the radial position in the plume is examined. Results show that the EEDF deviates from the Maxwellian distribution, and is more accurately described by the Druyvesteyn distribution directly downstream of the cathode. Off-axis measurements of the EEDF indicate the presence of fast electrons, most likely due to the anode geometry. The cathode used in these tests is representative of the cathode used in a 200W class Hall thruster. Data is presented for a hollow cathode operating on argon gas for two cases with different discharge currents.
146

Crystal chemistry of vanadium phosphates as positive electrode materials for Li-ion and Na-ion batteries / Cristallochimie de phosphates de vanadium comme électrodes positives pour batteries Li-ion et Na-ion

Boivin, Édouard 24 November 2017 (has links)
Ce travail de thèse a pour but d'explorer de nouveaux matériaux de type structural Tavorite et de revisiter certains déjà bien connus. Dans un premier temps, les synthèses de compositions ciblées ont été réalisées selon des procédures variées (voies tout solide, hydrothermale, céramique assistée par sol-gel, broyage mécanique) afin de stabiliser d'éventuelles phases métastables et d'ajuster la microstructure impactant fortement les performances électrochimiques de tels matériaux polyanioniques. Ces matériaux ont ensuite été décrits en profondeur, dans leurs états originaux, depuis leurs structures moyennes, grâce aux techniques de diffraction (diffraction des rayons X sur poudres ou sur monocristaux et diffraction des neutrons) jusqu'aux environnements locaux, en utilisant des techniques de spectroscopie (résonance magnétique nucléaire à l'état solide, absorption des rayons X, infra-rouge et Raman). Par la suite, les diagrammes de phases et les processus d'oxydoréduction impliqués pendant l'activité électrochimique des matériaux ont été étudiés grâce à des techniques operando (diffraction et absorption des rayons X). La compréhension des mécanismes impliqués pendant le cyclage permet de mettre en évidence les raisons de leurs limitations électrochimiques : La synthèse de nouveaux matériaux (composition, structure, microstructure) peut maintenant être développée afin de contrepasser ces limitations et de tendre vers de meilleures performances / This PhD work aims at exploring new Tavorite-type materials and at revisiting some of the well-known ones. The syntheses of targeted compositions were firstly performed using various ways (all solid state, hydrothermal, sol-gel assisted ceramic, ball milling) in order to stabilize eventual metastable phases and tune the microstructure impacting strongly the electrochemical performances of such polyanionic compounds. The materials were then described in-depth, at the pristine state, from their average long range structures, thanks to diffraction techniques (powder X-rays, single crystal X-rays and neutrons diffraction), to their local environments, using spectroscopy techniques (solid state Nuclear Magnetic Resonance, X-rays Absorption Spectroscopy, Infra-Red and/or Raman). Thereafter, the phase diagrams and the redox processes involved during electrochemical operation of the materials were investigated thanks to operando techniques (SXRPD and XAS). The in-depth understanding of the mechanisms involved during cycling allows to highlight the reasons of their electrochemical limitations: the synthesis of new materials (composition, structure and microstructure) can now be developed to overcome these limitations and tend toward better performance.
147

Síntese e caracterização de LaFe1-xNixO3-δpara utilização como cátodo em células a combustível

Ramos, Kethlinn 30 January 2014 (has links)
Made available in DSpace on 2017-07-21T20:42:45Z (GMT). No. of bitstreams: 1 Kethlinn Ramos.pdf: 3306187 bytes, checksum: 9ad9682e26dbce661bb2882e6aee6988 (MD5) Previous issue date: 2014-01-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The fuel cell are an alternative of production clean and efficient energy, because converts chemical energy in electrical energy. A fuel cell is formed basically by an electrolyte, a cathode and an anode. The fuel fed the anode and the oxidant fed the cathode. The fuel cells are classified according to the electrolyte used. The solid oxide fuel cell (SOFC) has all the components in solid state and work in high temperatures. The main electrolyte used for SOFC manufacturing is the ZrO2-Y2O3 owing your high ionic conductivity. The materials for electrode manufacturing must possess thermal expansion characteristics close to electrolyte and have high electrical conductivity in operating temperature, besides chemical and mechanical stability during operate and manufacture. High accomplishment cathodes have been studied, highlighting those with mixed conductivity, most ceramics with perovskite structure and lanthanide ions in your composition. Among these oxides, the perovskite LaFe1-xNixO3-δ has attracted interest for application as cathode in SOFC’s, to operate at intermediate temperatures. Chemical methods of production of ceramic powders are very efficient for obtaining powders with high reactivity and chemical homogeneity. Among the methods of chemical synthesis used to obtain powders, the Pechini method has pointed as a promising alternative process for obtaining nanometric powders. This work aimed to obtain LaFe1-xNixO3-δ powders by Pechini method, studying their behavior when changed processing conditions. The samples were synthetized, calcined and sintered. The characterizations accomplished were thermogravimetric analysis, differential thermal analysis, infrared spectroscopy, x-ray diffraction, scanning energy microscopy, spectroscopy dispersive energy, helium picnometry, surface area measured by BET method, density, porosity and water absorption by Arquimedes method, impedance spectropy and mercury porosimetry.Based on the characterizations performed, the LaFe1-xNixO3-δ obtained by the method based on the Pechini method proved viable for a possible application as cathode fuel cell solid oxide. / As células a combustível são uma alternativa à produção de energia elétrica limpa e eficiente, pois convertem energia química em energia elétrica. Uma célula a combustível é composta basicamente por um eletrólito, um cátodo e um ânodo. O combustível alimenta o ânodo e o oxidante alimenta o cátodo. As células a combustível são classificadas de acordo com o eletrólito utilizado. A célula a combustível de óxido sólido (CaCOS) tem todos seus componentes no estado sólido e opera em altas temperaturas. O principal eletrólito utilizado para a fabricação de uma CaCOS é o ZrO2-Y2O3 devido a alta condutividade iônica. Os materiais para a fabricação dos eletrodos devem possuir características de dilatação térmica próximas a do eletrólito e possuírem alta condutividade elétrica na temperatura de operação, além de estabilidade química e mecânica durante a operação e fabricação. Cátodos de alto desempenho vêm sendo estudados, chamando a atenção aqueles com condutividade mista, em sua maioria cerâmicos com estrutura perovisquita e com íons lantanídeo em sua composição. Entre esses óxidos, a perovisquita LaFe1-xNixO3-d tem atraído o interesse para a aplicação como cátodo nas CaCOSs, para operar em temperaturas intermediárias. Os métodos químicos de obtenção de pós cerâmicos são muito eficientes para a obtenção de pós com alta reatividade e homogeneidade química. Dentre os métodos de síntese química utilizados para a obtenção de pós, o método Pechini tem se destacado como um processo alternativo e promissor para a obtenção de pós nanométricos. Este trabalho teve como objetivo obter pós de LaFe1-xNixO3-d por meio do método Pechini, estudando seu comportamento quando alteradas condições de processamento. As amostras foram sintetizadas, calcinadas e sinterizadas. As caracterizações realizadas foram análise termogravimétrica, análise térmica diferencial, espectrometria no infravermelho, difração de raios x, microscopia eletrônica de varredura, espectrometria de energia dispersiva, picnometria a hélio, medida de área superficial pelo método BET, medidas de densidade, porosidade e absorção de água pelo método de Arquimedes, espectroscopia de impedância e porosimetria de mercúrio. Com base nas caracterizações realizadas o LaFe1-xNixO3-d obtido pelo método baseado no método Pechini, mostrou-se viável para uma possível aplicação como cátodo de células a combustível de óxido sólido.
148

OBTENÇÃO DE COMPÓSITOS COM CONDUTIVIDADE MISTA ELETRÔNICA-PROTÔNICA PARA CÁTODOS DE CÉLULAS A COMBUSTÍVEL / OBTENÇÃO DE COMPÓSITOS COM CONDUTIVIDADE MISTA ELETRÔNICA-PROTÔNICA PARA CÁTODOS DE CÉLULAS A COMBUSTÍVEL

Kabbas Junior, Tufy 27 January 2017 (has links)
Made available in DSpace on 2017-07-21T20:43:51Z (GMT). No. of bitstreams: 1 Tufy Kabbas Junior.pdf: 2988725 bytes, checksum: bd676d57a21df0604e836ad177676653 (MD5) Previous issue date: 2017-01-27 / Fundação Araucária de Apoio ao Desenvolvimento Científico e Tecnológico do Paraná / Due to the need for clean energy, an alternative that has gained worldwide prominence are like fuel cells. As proton conductive ceramics have an operating temperature in the range of 600 to 800 ° C, making them especially interesting for a fuel cell manufacturing. In this way, the objective of this work was to study a peroxide production with mixed proton-electronic conductivity to update as cell cathodes a solid oxide fuel with proton conductivity. These composites are produced using the mechanical mixture from a quality database, which has electronic conductivity in the proportions 25/75, 50/50 and 75/25, sintered at 1400 ° C. The composite 25 / 75 and 75/25 showed to be only of an electronic and ionic conductor (oxygen ions), respectively, showing no mixed-protonic conductivity. The 50/50 composite, through the obtained results, leads to believe that the mixed proton-electronic conductivity occurred. / Devido à necessidade de se produzir energia limpa, uma alternativa que tem ganho destaque mundial são as células a combustível. As cerâmicas condutoras protônicas possuem uma temperatura de operação na faixa de 600 a 800ºC, tornando-as especialmente interessantes para a fabricação de células a combustível. Desta forma, este trabalho teve por objetivo estudar a obtenção de perovisquitas com condutividade mista protônica-eletrônica para atuar como cátodos de células a combustível de óxido sólido com condutividade protônica. Estes compósitos foram produzidos utilizando-se mistura mecânica, da perovisquita BaCe0,2Zr0,7Y0,1O3-d (BCZY), a qual possui condutividade protônica, com a perovisquita LaNi0,5Cr0,5O3 (LNC), que possui condutividade eletrônica, nas proporções 25/75, 50/50 e 75/25, sinterizadas à 1400°C. Os compósitos 25/75 e 75/25 demonstraram ser apenas de um condutor eletrônico e iônico (íons oxigênio) respectivamente, não mostrando condutividade mista eletrônica-protônica. Já o compósito 50/50, através dos resultados obtidos através de mapeamento químico e espectroscopia de impedância, demonstram um provável aparecimento de condutividade mista protônica-eletrônica.
149

Obtenção de fibras de La0,6Sr0,4Co1-yFeyO3 pela técnica de electrospinning e sua caracterização para aplicação como cátodo em células a combustível

Lubini, Marcieli January 2016 (has links)
Neste trabalho, investigou-se a obtenção de fibras de La0,6Sr0,4Co1-yFeyO3 pela técnica de electrospinning e sua caracterização visando a sua aplicação como cátodo em células a combustível de óxido sólido de temperatura intermediária (SOFC-IT). Foram obtidos 5 compostos perovskitas LaxSr1-xCo1-yFeyO3 (LSCF) variando-se a quantidade de cobalto na composição (La0,6Sr0,4Co1-yFeyO3, sendo y = 1,0; 0,8; 0,6; 0,4; 0,2). As fibras LSCF, após tratamento térmico de 1000 ºC, apresentaram diâmetro médio em torno de 1 μm e estrutura perovskita com simetria romboédrica, com exceção do composto La0,6Sr0,4FeO3, que apresentou estrutura ortorrômbica. Foram avaliadas as propriedades elétricas das fibras sem compactação, compactada e sinterizada no intervalo de temperatura de 25-900 ºC. A condutividade elétrica das fibras LSCF aumentou com a compactação e sinterização das fibras e com o aumento do conteúdo de cobalto. As fibras sem compactação apresentaram valores de condutividade elétrica entre 0,23 S.cm-1 para La0,6Sr0,4FeO3 (LSF) à 0,43 S.cm-1 para La0,6Sr0,4Co0,8Fe0,2O3 (LSCF82) a 600 °C. Nas fibras compactadas os valores de condutividade elétrica aumentaram de 0,90 S.cm-1 para LSF à 9,06 S.cm-1 para LSCF82 a 600 °C. As fibras sinterizadas apresentaram os maiores valores de condutividade elétrica, 71 S.cm-1 para LSF e 832 S.cm-1 para LSCF82 em 600 ºC. A avaliação do desempenho eletroquímico das fibras LSCF como cátodo foi estudada por espectroscopia de impedância em células simétricas, contendo o eletrólito de céria dopada com gadolínio (CGO) e cátodos LSCF infiltrados com CGO. As medidas de impedância mostraram que os diagramas de Nyquist são compostos de dois a três semicírculos, dependendo da temperatura da medida. Os cátodos LSCF com maior conteúdo de cobalto apresentaram menor resistência de polarização. O cátodo La0,6Sr0,4Co0,8Fe0,2O3 apresentou a menor resistência de polarização entre 500 e 900 °C, classificando este cátodo compósito como um promissor material para SOFC de temperatura intermediária baseado em eletrólito CGO. / In this work, the preparation of La0.6Sr0.4Co1-yFeyO3 fibers by electrospinning and its characterization was investigated aiming the production of cathodes for Intermediate Temperature Solid Oxide Fuel Cell (SOFC-IT). Five compounds of the family LaxSr1-xCo1-yFeyO3 (LSCF) were obtained varying the cobalt content (La0.6Sr0.4Co1-yFeyO3, where y = 1.0; 0.8; 0.6; 0.4; 0.2). The electrospun La0.6Sr0.4Co1-yFeyO3 (y=0.2-1.0) fibers resulted in an average diameter of about 1 μm and perovskite crystalline structure with rhombohedral symmetry after heat treatment at 1000 °C, except for La0.6Sr0.4FeO3 that crystallized in an orthorhombic structure. The electrical properties of the fibers in the non-compacted, compacted and sintered forms were investigated in the temperature range of 25-900 °C. The electrical conductivity of LSCF fibers increases with the compaction and sintering of the fibers and with the increase of cobalt content. The non-compacted fibers showed electrical conductivities ranging from 0.23 S.cm-1 for La0.6Sr0.4FeO3 (LSF) up to 0.43 S.cm-1 for La0.6Sr0.4Co0.8Fe0.2O3 (LSCF82) at 600 °C. The electrical conductivity increased in compacted fiber samples to 0.90 S.cm-1 for LSF and to 9.06 S.cm-1 for LSCF82 at 600 °C. The sintered fibers showed the highest electrical conductivity for all samples, 71 S.cm-1 for LSF and 832 S.cm-1 for LSCF82 at 600 ºC. The electrochemical performance of LSCF fibers as cathode was studied by impedance spectroscopy in symmetrical cells containing gadolinium doped ceria (CGO) electrolyte and LSCF cathode infiltrated with CGO. Impedance measurements showed that the Nyquist diagrams have two or three semicircles, depending on the measurement temperature. The LSCF cathodes with higher cobalt content exhibit lower polarization resistance and the La0.6Sr0.4Co0.8Fe0.2O3 cathode had the lowest polarization resistance between 500 and 900 °C, classifying this composite cathode as a promising material for intermediate temperature SOFC based on CGO electrolyte.
150

THE PHYSICAL BEHAVIOR AND CHARACTERIZATION OF NANOPOROUS SILICON AND DISPENSER CATHODE SURFACES

Maxwell, Tyler Lucius Corey 01 January 2018 (has links)
Nanostructured materials have received a surge of interest in recent years since it has become apparent that reducing the size of a material often leads to heightened mechanical behavior. From a fundamental standpoint, this stems from the confinement of dislocations. Applications in microelectromechanical devices, lithium ion batteries, gas sensing and catalysis are realized by combining the improvements in mechanical behavior from material size reduction with the heightened chemical activity offered by materials with a high surface-area-to-volume ratio. In this study, films of nanoporous Si-Mg were produced through magnetron sputtering, followed by dealloying using an environmentally benign process with distilled water. The film composition and structure was characterized both at the surface and throughout the film thickness, while the mechanical behavior was explored with nanoindentation. Dispenser cathodes operate via thermionic emission and are an important area of interest in vacuum electron devices. While scientists have known for many years what elemental constituents are used to manufacture dispenser cathodes of excellent emission behavior, a fundamental understanding has yet to be realized. In this study, components of a scandate cathode that exhibited excellent emission behavior were characterized and used to inform the study of model thin films. Isolating relevant components of the scandate cathode for careful study could help inform future breakthroughs in understanding the working mechanism(s) of the scandate cathode. The structure, composition and electronic behavior of model W-Al alloy films were characterized experimentally and compared to computation. Moreover, a unique vacuum chamber was designed to activate modern thermionic dispenser cathodes, observe residual gas species present, and measure the work function through various state-of-the-art techniques.

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