The processes of grain boundary grooving and cathode voiding which are important in determining the life times of thin films connecting the transistors in an integrated circuit are investigated by introducing a new mathematical model, which flows from the fundamental postulates of irreversible thermodynamics, accounting for the effects of applied electric field and thermal stresses. The extensive computer studies on the triple junction displacement dynamics shows that it obeys the first order reaction kinetics at the transient stage, which is followed by the familiar time law as , in the normalized time and space domain, at the steady state regime in the absence of the electric field (EF). The application of EF doesn&rsquo / t modify this time law very / but puts only an abrupt upper limit for the groove depth and fixes the total elapse time for that event, which is found to be inversely proportional with the electron wind intensity parameter. The drift in the cathode edge due to the surface diffusion along the side walls is simulated under the constant current regime. An analytical formula is obtained in terms of system parameters, which shows well defined threshold level for the onset of electromigration induced cathode drift, showing an excellent agreement with the reported experimental values in the literature.

Solid oxide fuel cell (SOFC) research is currently underway to improve performance, cost and durability by lowering the operating temperature to ~600°C. One approach is to design fabrication processes capable of tailoring desirable cathode microstructures to enhance mass and charge transfer properties through the porous medium. The aim of this study is to develop a cost effective fabrication technique for deposition of novel microstructures, specifically, functionally graded thin films of LSM oxide with porosity graded structure for use as IT- SOFCs cathode. Spray pyrolysis method was chosen as a low-temperature processing technique for deposition of porous LSM films onto dense YSZ substrates. The effort was directed toward the optimization of the processing conditions for deposition of high quality LSM films with variety of morphologies in the range of dense to porous microstructures. Results of optimization studies on spray parameters revealed that the substrate surface temperature is the most critical parameter influencing the roughness and morphology, porosity, cracking and crystallinity of the film. Physical and chemical properties of deposited thin films such as porosity, morphology, phase crystallinity and compositional homogeneity have shown to be extensively dependent on the deposition temperature as well as solution flow rate and the type of precursor solution among other parameters. The LSM film prepared from organo-metallic precursor and organic solvent showed a homogeneous crack-free microstructure before and after heat treatment as opposed to aqueous solution. Also, increasing the deposition temperature and the solution flow rate, in the specific range of 520-580 ℃ and 0.73-1.58 ml/min, respectively, leads to change the microstructure from a dense to a highly porous film. Taking the advantage of simplicity of spray pyrolysis technique combined with using metal-organic compounds, the conventional ultrasonic spray system was modified to a novel system whereby highly crystalline multi-layered porosity graded LSM cathode with columnar morphology and good electrical conductivity in the range of 500-700 °C was fabricated through a multi-step spray and via applying optimum combination of spray parameters. This achievement for the current graded LSM cathode would allow its use in IT-SOFCs.

Electrochemical behavior of electrolytic manganese dioxide (EMD), chemically prepared battery grade manganese dioxide (BGM), titanium dioxide (TiO2), lithium iron phosphate (LiFePO4) and lithium manganese phosphate (LiMnPO4) in aqueous lithium hydroxide electrolyte has been investigated. These materials are commonly used as cathodes in non-aqueous electrolyte lithium batteries. The main aim of the work was to determine how the electroreduction/oxidation behavior of these materials in aqueous LiOH compares with that reported in the literature in non-aqueous electrolytes in connection with lithium batteries. An objective was to establish whether these materials could also be used to develop other battery systems using aqueous LiOH as electrolyte. The electrochemical characteristics of the above materials were investigated by subjecting them to slow scan cyclic voltammetry and determining the charge/discharge characteristics of Zn/cathode material-aqueous LiOH batteries. The products of electroreduction/oxidation were characterized by physical techniques using X-ray diffraction (XRD), scanning electron micrography (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), Thermogravimetric analysis (TG) and infra-red spectrometry (IR). The reduction of ã-MnO2 (EMD) in aqueous lithium hydroxide electrolyte is found to result in intercalation of Li+ into the host structure of ã-MnO2. The process was found to be reversible for many cycles. This is similar to what is known to occur for ã-MnO2 in non-aqueous electrolytes. The mechanism, however, differs from that for reduction/oxidation of ã-MnO2 in aqueous potassium hydroxide electrolyte. KOH electrolyte is used in the state-of-art aqueous alkaline Zn/MnO2 batteries. Alkaline batteries based on aqueous KOH as the electrolyte rely upon a mechanism other than K+ intercalation into MnO2. This mechanism is not reversible. This is explained in terms of the relative ionic sizes of Li+ and K+. The lithium-intercalated MnO2 lattice is stable because Li+ and Mn4+ are of approximately the same size and hence Li+ is accommodated nicely into the host lattice of MnO2. The K+ ion which has almost double the size of Li+ cannot be appropriately accommodated into the host structure and hence the K+ -intercalated MnO2 phase is not stable. Chemically prepared battery grade MnO2 (BGM) is found to undergo electroreduction/oxidation in aqueous LiOH via the same Li+ intercalation mechanism as for the EMD. While the Zn/BGM- aqueous LiOH cell discharges at a voltage higher than that for the Zn/EMD- aqueous LiOH cell under similar conditions, the rechargeability and the material utilization of the BGM cell is poorer. The cathodic behavior of TiO2 (anatase phase) in the presence of aqueous LiOH is not reversible. In addition to LiTiO2, Ti2O3 is also formed. The discharge voltage of the Zn/TiO2- aqueous LiOH cell and material utilization of the TiO2 as cathode are very low. Hence TiO2 is not suitable for use in any aqueous LiOH electrolyte battery. LiFePO4 (olivine-type structure) as a cathode undergoes electrooxidation in aqueous LiOH forming FePO4. However the subsequent reduction forms not only the original LiFePO4 but also Fe3O4. Thus the process is not completely reversible and hence LiFePO4 is not a suitable material for use as a cathode in aqueous battery systems. LiMnPO4 (olivine-type structure) undergoes reversible electrooxidation in aqueous LiOH forming MnPO4. The charge/discharge voltage profile of the Zn/MnPO4-aqueous LiOH cell, its coulombic efficiency and rechargeability are comparable to that of the cell using ã-MnO2. EMD and LiMnPO4 both have the potential for use in rechargeable batteries using aqueous LiOH as the electrolyte. Recommendations for further developmental work for such batteries are made.

Methods for examination of the crystal structure of crystalline materials include the X-Ray, neutron and synchrotron-radiation diffraction, electron backscattered diffraction in the scanning electron microscope, scanning transmission electron microscopy, transmission electron microscopy and focused ion beam microscopy. The scanning low energy electron microscopy (SLEEM) is less known as yet but already has proven itself very powerful tool for studies of the crystal lattice. By means of very slow electrons reflected from the sample and effectively detected in their full angular and energy distribution the crystalline structure is imaged at high spatial resolution and high contrast is obtained between differently oriented grains in polycrystals. Because of high sensitivity of the image signal to the inner potential distribution in the sample even details like subgrains or twins as well as strain at the microstructural level can be visualized. The aim of this thesis is to demonstrate the scanning low energy electron microscopy as an effective tool for investigation of wide range of materials like steels, non-ferrous alloys and ultra-fine grained materials.

In linear accelerator driven 4th generation Free Electron Lasers (FELs), the final beam quality is set by the linac and ultimately by its photoinjector and photocathode. Therefore, to deliver cutting-edge beam characteristics, there are stringent requirements for the photocathode used in the photoinjector. Understanding how surface properties of materials influence photocathode properties such as quantum efficiency (QE) and intrinsic emittance is critical for such sources. Metal photocathode research at Daresbury Laboratory (DL) is driven by our on-site accelerators VELA (Versatile Electron Linear Accelerator) and CLARA (Compact Linear Accelerator for Research and Applications), a free electron laser test facility. Metals offer the advantage of a fast response time which enable the generation of short electron pulses. Additionally, they are robust to conditions within the gun cavity. The main challenge with metal photocathodes is to maximise their (relatively) low electron yield. In this PhD thesis, the goal has been to carry out an experimental investigation on alternative metals to copper, correlating surface properties with photoemissive properties. A range of surface analysis techniques have been employed: surface composition was investigated using X-ray Photoelectron Spectroscopy and Medium Energy Ion Scattering, Kelvin Probe apparatus and Ultra-violet Photoelectron Spectroscopy were used to measure work function, and Atomic Force Microscopy and Interferometric microscope provided images characterising surface morphology. The photocathode properties studied include: QE measured using a 265 nm UV LED source that was later upgraded to a 266 nm UV LASER, and Mean Transverse Energy measured using the Transverse Energy Spread Spectrometer. As a result of this work, Mg, Nb, Pb, Ti and Zr have all been identified as photocathode candidate materials, each exhibiting a QE greater than Cu. Additionally, surface preparation procedures for optimising QE from a selection of metals has been explored; the findings of these experiments would suggest that ex-situ Ar plasma treatment followed by in-situ heat treatment is well suited to remove surface contaminants without altering the surface morphology of the cathode. As part of this work, metallic thin films produced by magnetron sputtering have been produced; ultimately the chosen cathode metal will be deposited onto a cathode plug which will be inserted into the electron gun that will drive CLARA. Thus the preparation of metal thin films has been investigated and the effect of different substrate materials on the film properties has been explored. Preliminary experiments studying the effects of surface roughness on photoelectron energy distribution have been conducted; the findings have not been conclusive, thus further systematic studies are required.

Addressing the low power generation of anodic biocatalysts is pertinent to the advancement of microbial fuel cell technology. While Propionibacterium freudenreichii ssp. shermanii has shown potential as a biocatalyst, its incomplete consumption of the anodic substrate is a persistent issue. This research aims to optimize substrate consumption to increase power generation using Propionibacterium freudenreichii ssp. shermanii as a biocatalyst. The effect of coculturing Geobacter sulfurreducens with Propionibacterium freudenreichii ssp. shermanii was investigated. The cocatalyst and pure culture performance was tested in an air-cathode microbial fuel cell. Geobacter sulfurreducens produced the highest maximum power density among the experimental cases. Power density produced by Propionibacterium freudenreichii ssp. shermanii was improved in the air-cathode design compared to previous experiments performed in an H-type design. The novel cocatalyst was shown to produce electricity, however a full characterization to elucidate the contribution to power generation by each microbe would be desirable to investigate.

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.

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.

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2014-11-10T12:39:03Z No. of bitstreams: 0 / Made available in DSpace on 2014-11-10T12:39:03Z (GMT). No. of bitstreams: 0 / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

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.