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An introduction to the fundamental concepts of X-ray physics and X-ray tube design is given. This discussion also includes a brief description of various X-ray tube types available commercially for a number of different industrial applications. The design of a high-energy MgO cold cathode X-ray tube, which is to be used in an X-ray fluorescence (XRF) spectrometer, is described in detail with emphasis placed on the electron beam focusing mechanism and the theory of operation as well as the construction of the X-ray tube MgO cold cathode, which functioned as the electron emitter of the device. A detailed account is also given of the output characteristics of the X-ray tube power supply, which has a direct effect on the design requirements and consequently the performance of the X-ray tube. An investigation into the manufacture of the vacuum envelope with particular attention focused on the production of reliable metal-to-ceramic seals was performed. A number of tests were conducted especially with regard to the maximum temperature that such seals may withstand without becoming permanently damaged. These tests were essential, since high temperature gradients tend to develop in an X-ray tube during operation, which the metal-to-cerarnic seals of the tube must be capable of withstanding if damage to the device is to be avoided. The set-up of the XRF spectrometer in which the completed X-ray tube was tested is discussed, in which the X-ray current and voltage measuring techniques are described. Furthermore a detailed account of the operation of the X-ray detector system and the multichannel analyser is given, which was used to detect and record spectra of the sample elements excited by the primary radiation of the X-ray tube. Finally the measured X-ray tube performance characteristics are discussed and compared to the predicted results. / Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.

Dispenser cathodes emit electrons through thermionic emission and are a critical component of space-based and telecommunication devices. The emission of electrons is enhanced when coated with a refractory metal such as osmium (Os), osmium-ruthenium (Os-Ru), or iridium (Ir). In this work the microstructure, thermionic emission, and work function of thin film Os-Ru coatings were studied in order to relate microstructural properties and thermionic emission. Os-Ru thin film coatings were prepared through magnetron sputtering and substrate biasing to produce films with an array of preferred orientations, or texture. The effect of texture on thermionic emission was studied in detail through closely-spaced diode testing, SEM imaging, and x-ray diffraction. Results indicated that there was a strong correlation with emission behavior and specific preferred orientations. An ultra-high vacuum compatible Kelvin Probe was used to measure the work function of W-Os-Ru ternary alloy films to determine the effect W interdiffusion has on work function. The results indicated that a high work function alloy coating corresponded to low work function cathodes, as expected. It was inferred that a high work function alloy coating results in a low work function cathode because it aligns more closely with ionization energy of Ba. The results also proved that this method of evaluating dispenser cathode coatings can distinguish small variations in microstructure and composition and may be a beneficial tool in the development of improved dispenser cathode coatings. A novel experimental apparatus was constructed to measure the work function of dispenser cathode coatings in-vacuo using the ultra-high vacuum Kelvin Probe. The apparatus is capable of activating cathodes at high temperature and measuring the work function at elevated temperature. The design of this apparatus allows for more rapid evaluation of dispenser cathode coatings.

Au Québec, les alumineries sont de grandes consommatrices d’énergie électrique, soit près de 14% de la puissance installée d’Hydro-Québec. Dans ce contexte, des petits gains en efficacité énergétique des cuves d’électrolyse pourraient avoir un impact important sur la réduction globale de la consommation d’électricité. Le projet de maîtrise décrit dans cette étude répond à la problématique suivante : comment l’optimisation de la géométrie d’un bloc cathodique en vue d’uniformiser la densité de courant peut augmenter l’efficacité énergétique et la durée de vie de la cuve d’aluminium? Le but premier du projet est de modifier la géométrie en vue d’améliorer le comportement thermoélectrique des blocs cathodiques et d’accroître par le fait même l’efficacité énergétique du procédé de production d’aluminium. La mauvaise distribution de la densité de courant dans la cuve est responsable de certains problèmes énergétiques ayant des impacts négatifs sur l’économie et l’environnement. Cette non-uniformité de la distribution du courant induit une usure prématurée de la surface de la cathode et contribue à réduire la stabilité magnétohydrodynamique de la nappe de métal liquide. Afin de quantifier les impacts que peut avoir l’uniformisation de la densité de courant à travers le bloc cathodique, un modèle d’un bloc cathodique d’une cuve de la technologie AP-30 a été conçu et analysé par éléments finis. À partir de son comportement thermoélectrique et de données expérimentales d’une cuve AP-30 tirées de la littérature, une corrélation entre le profil de densité de courant à la surface du bloc et le taux d’érosion local au même endroit a été créée. Cette relation correspond au modèle prédictif de la durée de vie de tout bloc du même matériau à partir de son profil de densité de courant. Ensuite, une programmation a été faite incorporant dans une même fonction coût les impacts économiques de la durée de vie, de la chute de voltage cathodique et de l’utilisation de nouveaux matériaux. Ceci a permis d’évaluer les bénéfices faits à partir d’un bloc modifié par rapport au bloc de référence. Plusieurs paramètres géométriques du bloc sont variables sur un domaine réaliste et l’intégration d’un composant en matériau plus conducteur y a également été étudiée. Utilisant des outils mathématiques d’optimisation, un design de bloc optimal a pu être trouvé. Les résultats démontrent qu’il est possible de générer des économies à partir de la modification du bloc. Il est également prouvé que l’uniformisation de la densité de courant à travers le bloc peut apporter de grands avantages économiques et environnementaux dans le procédé d’électrolyse de l’aluminium. Les résultats de cette étude serviront d’arguments pour les chercheurs dans l’industrie à savoir s’il vaut la peine d’investir ou non dans la fabrication d’un prototype expérimental souvent très coûteux.

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Atmospheric plasma spraying is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (600 to 750°C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. Processes were developed to manufacture metal-supported SOFC cathodes by axial-injection plasma spraying. Cathodes consisted of LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ) or SSC (Sm0.5Sr0.5CoO3) as the primary material. Initially, the plasma spray process parameters were varied, and x-ray diffraction analyses were performed on the cathode coatings to detect material decomposition and the formation of undesired phases. These results determined the envelope of plasma spray parameters in which coatings of LSCF and SSC can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured. Subsequently, composite cathodes were fabricated by mixing up to 40 wt. % of the ionic conducting SDC (Ce0.8Sm0.2O1.9) material into the feedstock. The deposition efficiencies of these cathodes were calculated based on the mass of the sprayed cathode. Particle surface temperatures were measured in-flight to enhance understanding of the relationship between spray parameters, microstructure, and deposition efficiency. Electrochemical impedance spectroscopy was performed in symmetrical cells: at 750°C, LSCF-SDC cathodes had polarization resistances as low as 0.101 Ωcm², and SSC cathodes had polarization resistances as low as 0.0056 Ωcm². Finer mixing of the ceramic phases was achieved by using a nano-structured feedstock that contained both LSCF and SDC phases agglomerated together in larger particles. Fuel cells containing a yttria-stabilized zirconia (YSZ) electrolyte and a nickel-YSZ anode were fabricated, and the effect of the cathode microstructure on cell impedance was studied using the analysis of differential impedance spectra. The degradation of composite LSCF-SDC cathodes on porous 430 stainless steel supports was also investigated. To reduce degradation, La2O3 and Y2O3 reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. As a result, polarization resistance degradation rates as low as 0.00256 Ω·cm2 /1000 h were observed over 100 hours on coated substrates, compared to 0.1 Ω·cm2 /1000 h on uncoated substrates.

Atmospheric plasma spraying is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (600 to 750°C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. Processes were developed to manufacture metal-supported SOFC cathodes by axial-injection plasma spraying. Cathodes consisted of LSCF (La0.6Sr0.4Co0.2Fe0.8O3-δ) or SSC (Sm0.5Sr0.5CoO3) as the primary material. Initially, the plasma spray process parameters were varied, and x-ray diffraction analyses were performed on the cathode coatings to detect material decomposition and the formation of undesired phases. These results determined the envelope of plasma spray parameters in which coatings of LSCF and SSC can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured. Subsequently, composite cathodes were fabricated by mixing up to 40 wt. % of the ionic conducting SDC (Ce0.8Sm0.2O1.9) material into the feedstock. The deposition efficiencies of these cathodes were calculated based on the mass of the sprayed cathode. Particle surface temperatures were measured in-flight to enhance understanding of the relationship between spray parameters, microstructure, and deposition efficiency. Electrochemical impedance spectroscopy was performed in symmetrical cells: at 750°C, LSCF-SDC cathodes had polarization resistances as low as 0.101 Ωcm², and SSC cathodes had polarization resistances as low as 0.0056 Ωcm². Finer mixing of the ceramic phases was achieved by using a nano-structured feedstock that contained both LSCF and SDC phases agglomerated together in larger particles. Fuel cells containing a yttria-stabilized zirconia (YSZ) electrolyte and a nickel-YSZ anode were fabricated, and the effect of the cathode microstructure on cell impedance was studied using the analysis of differential impedance spectra. The degradation of composite LSCF-SDC cathodes on porous 430 stainless steel supports was also investigated. To reduce degradation, La2O3 and Y2O3 reactive element oxide coatings were deposited on the internal pore surfaces of the metal supports. As a result, polarization resistance degradation rates as low as 0.00256 Ω·cm2 /1000 h were observed over 100 hours on coated substrates, compared to 0.1 Ω·cm2 /1000 h on uncoated substrates.

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.