Devido à relevância de filmes finos, as técnicas que são utilizadas para produzi-los e também para caracteriza-los tem se tornado importante. Neste contexto, foram analisados filmes finos de até 100 nm, de duas ligas metálicas (cromel e alumel), obtidos a partir da deposição por plasma de Arco Catódico Filtrado em Vácuo (Filtered Cathodic Vacuum Arc - FCVA). O objetivo deste projeto foi avaliar a similaridade em composição elementar entre os materiais utilizados para deposição, que operam como cátodos, e os filmes finos depositados, a partir de medições obtidas pela técnica de microanálise quantitativa Energy Dispersive Spectroscopy (EDS). Para comparação entre resultados e apreciação de compatibilidade, foi realizada avaliação estatística considerando o Teste t, no qual a estatística do teste é dada pela Distribuição t de Student, adotando nível de significância de 5%. Os valores obtidos por EDS Quantitativo para os cátodos foram de (em wt%) (90,3 ± 0,5)% de Ni e (9,72 ± 0,19)% de Cr para o cromel e (95,1 ± 0,8)% de Ni, (2,02 ± 0,14)% de Mn, (1,65 ± 0,04)% de Si e (1,15 ± 0,05)% de Al para o alumel. Já para os filmes finos, foram de (90,2 ± 0,5)% de Ni e (9,8 ± 0,5)% de Cr para o cromel e (95,2 ± 0,4)% de Ni, (2,8 ± 0,4)% de Mn, (0,77 ± 0,17)% de Si e (1,08 ± 0,09)% de Al para o Alumel, ambos apresentando compatibilidade com as medidas por Rutherford Backscattering Spectrometry (RBS) - técnica comumente utilizada para este tipo de espécime. Verificou-se que a composição elementar do filme fino de cromel não apresentou diferença significativa com o cátodo da mesma liga. Entretanto, para o filme fino de alumel, houve evidências de diferença significativa com relação ao cátodo, apontada pelo elemento silício. / Due the relevance of thin films, the techniques used to produce and also to characterize them has become important. In this context, it was analyzed thin films up to 100 nm of two alloys (Chromel and Alumel) obtained by plasma deposition using Filtered Cathodic Vacuum Arc (FCVA). The objective of this project was to evaluate the similarity in elemental concentration of the materials used for deposition, which act as cathodes, and the deposited thin films, through measurements obtained by quantitative microanalysis technique Energy Dispersive Spectroscopy (EDS). In order to compare results and compatibility assessment was performed statistical analysis considering the t-test in which the test statistic is given by the Student\'s t - distribution, adopting a significance level of 5%. The values obtained by Quantitative EDS for the cathodes were (in wt%) (90.3 ± 0.5)% of Ni and (9.72 ± 0.19)% of Cr for the Chromel and (95.1 ± 0.8)% of Ni, (2.02 ± 0.14)% of Mn, (1.65 ± 0.04)% of Si and (1.15 ± 0.05)% of Al for the Alumel. As for the thin films, they were (90.2 ± 0.5)% of Ni and (9.8 ± 0.5)% of Cr for the Chromel and (95.2 ± 0.4)% of Ni, (2.8 ± 0.4)% of Mn, (0.77 ± 0.17)% of Si and (1.08 ± 0.09)% for Al Alumel, both featuring compatibility with the measures by Rutherford Backscattering Spectrometry (RBS) - technique commonly used for this type of specimen. It was verified that the elemental concentration of the thin film of Chromel presented no significant difference with the cathode of the same alloy. However, for the Alumel thin film, there was evidence of a significant difference with respect to the cathode, appointed by element Silicon.

The optimisation of the specular reflectance of solar collectors is a key parameter to increase the global yield of concentrated solar power (CSP) plants. In this work, the influence of filtered cathodic vacuum arc deposition parameters, particularly working pressure and deposition time, on the specular and diffuse reflectance of aluminium thin films, was studied. Changes in specular reflectance, measured by ultraviolet–visible and near-infrared spectroscopy (UV-vis-NIR) spectrophotometry, were directly correlated with thin film elemental concentration depth profiles, obtained by Rutherford backscattering spectrometry (RBS), and surface and cross-sectional morphologies as measured by scanning electron microscopy (SEM) and profilometry. Finally, atomic force microscopy (AFM) provided information on the roughness and growth mechanism of the films. The two contributions to the total reflectance of the films, namely diffuse and specular reflectance, were found to be deeply influenced by deposition conditions. It was proven that working pressure and deposition time directly determine the predominant factor. Specular reflectance varied from 12 to 99.8% of the total reflectance for films grown at the same working pressure of 0.1 Pa and with different deposition times. This transformation could not be attributed to an oxidation of the films as stated by RBS, but was correlated with a progressive modification of the roughness, surface, and bulk morphology of the samples over the deposition time. Hence, the evolution in the final optical properties of the films is driven by different growth mechanisms and the resulting microstructures. In addition to the originally addressed CSP applications the potential of the developed aluminium films for other application rather than CSP, such as, for example, reference material for spectroscopic diffuse reflectance measurements, is also discussed.

We have designed a new type of plasma gun ion source for a Malmberg-Penning trap based on Metal Vapor Vacuum Arc (MeVVA) ion source designs. Our primary intent with this MeVVA-type source is to create a confinable beryllium-7 (7-Be) plasma. 7-Be is a peculiar isotope due to its varying radioactive decay half-life in different electro-chemical configurations. It is also found in an unexpected abundance at high altitudes of the Earth's atmosphere. It is possible ioniziation affects the radioactivity of the isotope, partly explaining this discrepancy with atmospheric models. The short half-life of 7-Be requires us to replace the sample inside the ion source on a regular basis. Our design makes it possible to easily remove the cathode of the ion source from an ultra-high vacuum trap and exchange 7-Be samples while only needing to repressurize a small chamber rather than the entire trap. This design has an added benefit of being capable of generating plasmas from a wide variety of metals by simply exchanging the source target in the removable cathode. Because of this wide compatibility, we will be able to use our trap for studying any number of different plasmas, including other radioactive types. Testing of the ion source design shows we are able to extract more than a sufficient number of ions at reasonable energies for confinement.

Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / This thesis describes the development of a vacuum arc thruster (VAT) to be used as a potential low mass (< 500 g), low power (< 5–10W) propulsion system for nanosatellites. The thruster uses a high voltage capacitive circuit to initiate and power the arc process with a 400 ns high current (150–800A) pulse. A one-dimensional steady state analyticalmodel describing the cathode region of the vacuum arc was developed. The model made use of mass and energy balances at the sheath region and cathode surface respectively to predict key quantities such as thrust, ion velocity, ion-to-arc current ratio and erosion rate. Predicted results were shown to be within the limits of reported literature (∼63 μN/A, 26.12 km/s, 0.077 and 110 μg/C respectively). A sensitivity analysis of the analytical model found that a high electric field in the cathode region impedes and decelerates ion flow, which is used for thrust. This was confirmed experimentally for thrust values at arc voltages greater than 2000 V. Both direct and indirect means of measuring thrust were achieved by using a deflecting cantilever beam and an ion collector system, respectively. The transient response of the cantilever beam to impulsive thrust was analytically modeled, whilst the ion current was found by measuring the current induced on a plate subject to ion bombardment. Knowledge of the ion current density distribution was successfully used to approximate the effective normal thrust vector. Direct and indirect thrust levels were roughly 140 and 82 μN/A of average arc current, respectively. Measured thrust was found to be higher than predicted thrust due to thrust contributions fromthe ablation of Teflon insulation. The discrepancy is also due to the uncertainty in quantifying free parameters in the analytical model such as the fraction of generated ions flowing away from the cathode region. The thrust-topower ratio, specific impulse and efficiency of the vacuum arc thruster at an average arc current of 200 A was measured to be 0.6 μN/W, 160 s and 0.05 %, respectively. A thruster performance analysis and specification showed that the VAT is capable of achieving specific orbital and slew manoeuvres within a constant 5–10 W average power. It was concluded that thruster performance could be improved by using a two-stage arc circuit consisting of a high voltage, low current, short pulse trigger and a low voltage, high current, long pulse driver.

A vacuum interrupter is a switching device used in vacuum circuit breakers, which are widely employed in medium voltage circuits for interrupting the short-circuit fault currents. The vacuum interrupter is the chamber in which the arc extinction and hence the current interruption takes place. On the occurrence of a fault, the breaker mechanism separates the contacts of the vacuum interrupter. As the contacts separate, an arc is established between the contacts. The arc evolves in the contact space and extinguishes at or near the current zero, thus interrupting the current. The processes of arc ignition, evolution and extinction are very complex. These processes are fundamental to the design and the performance of the vacuum interrupter and hence the circuit breaker. The evolution of the arc predominantly depends on the short-circuit current, the design and metallurgy of the contacts. The evolution of the vacuum arc has been the focal point of considerable research activity. Significant effort has been concentrated to understand the various modes of the arc, the transition between the modes, the arc movement and the dependency on the contact design and finally the effect of the arc evolution on the current interruption performance of the vacuum interrupter. The voltage across the contacts during the arcing, termed as the arc voltage, has been a focal point of several research projects. Research has shown that the arc voltage depends strongly on the mode and the evolution process of the arc. The dependency is observed with respect to the magnitude and the nature of the arc voltage. This dependency has been established through the comparison of the arc voltage trace and the actual arc photographs. The arc voltage is thus an important parameter in understanding the arcing process in the interrupter. Arc voltage could also be utilised to compare the arcing behaviour in vacuum interrupters with different contact geometries and metallurgies. Having understood how the arc voltage depends on the arc modes and how it can be used to analyse the arcing performance of the interrupter, this work aims to establish experimentally the dependency of the arc voltage on fundamental parameters of the short-circuit current and the contact design. The variation of the arc voltage is studied with respect to the magnitude of the short-circuit current. It is seen that the magnitude of the arc voltage is higher, for a higher short-circuit current. This dependency is also reflected in the nature of the arc voltage waveform. The effect of cumulative short- circuit operations has been understood through the study of arc voltage variation with respect to the accumulated arcing time. It has been found that the arc voltage consistently decreases as the accumulated arcing time increases. The effect of the contact diameter on the arc evolution has been studied by comparing the arc voltage variations for contacts of different diameters for the same short-circuit current. It is observed that the variation of arc voltage with respect to the contact diameter depends on the type of contact. In the case of radial magnetic field contacts, it has been observed that the arc voltage is lower for a contact with lower diameter. Whereas in the case of axial magnetic field contacts there is an inverse relation between the contact diameter and the arc voltage. Finally, the effect of the type and distribution of the magnetic field on the arc voltage variation as well as the contact erosion has been studied. In general, the observations show that the arc voltage magnitude for the radial magnetic field geometry is higher than the axial magnetic field geometry. Also, there is a significant difference in the appearance of the arc voltage waveforms for the arcs under the two types of magnetic fields. Finite element simulations and short-circuit evaluations have shown that the axial magnetic field contact system with 90 deg coil orientations yield a more uniform distribution of the flux density and hence lower erosion of the contacts. These results show a clear dependence of the arc voltage on the various above mentioned parameters. Thus the arc voltage could be utilised as a diagnostic parameter during the evaluation of the vacuum interrupter. In the present scenario, significant research is being done to increase the breaking capacity of the interrupters. This calls for optimization of design of the existing contacts and the design of novel contact geometries. The arc voltage would be used as an important diagnostic tool in this process. Also, the utilization of vacuum interrupter in high voltage and extra high voltage circuits is being explored. This application requires increase in the contact gap or series connection of gaps. The arc behaviour in longer gaps and gaps connected in series would be an important research area. Again the arc voltage could be used to study the arc evolution in these specialised conditions. The experiments in this research work have been performed on commercial vacuum interrupters. For a dedicated research on vacuum arcs and vacuum interrupter contacts, development of a vacuum arc research facility has also been attempted as a part of this research work.

Solar selective coatings based on carbon transition metal carbide nanocomposite absorber layers were designed. Pulsed filtered cathodic arc was used for depositing amorphous carbon: metal carbide (a-C:MeC, Me = V, Mo) thin films. Composition and structure of the samples were characterized by ion beam analysis, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The optical properties were determined by ellipsometry and spectrophotometry. Three effective medium approximations (EMA), namely Maxwell-Garnett, Bruggeman, and Bergman, were applied to simulate the optical behaviour of the nanocomposite thin films. Excellent agreement was achieved between simulated and measured reflectance spectra in the entire wavelength range by using the Bergman approach, where in-depth knowledge of the nanocomposite thin film microstructure is included. The reflectance is shown to be a function of the metal carbide volume fraction and its degree of percolation, but not dependent on whether the nanocomposite microstructure is homogeneous or a self-organized multilayer. Solar selective coatings based on an optimized a-C:MeC absorber layer were designed exhibiting a maximum solar absorptance of 96% and a low thermal emittance of ~5 and 15% at 25 and 600ºC, respectively. The results of this study can be considered as predictive design tool for nanomaterial-based optical coatings in general.

This thesis explores the potential of utilizing metal-based plasma sources as a sustainable solution for high-powered electric propulsion and its implications for future interplanetary travel. Focusing on the Vacuum Arc Thruster and the Variable Specific Impulse Magnetoplasma Rocket engine, the study encompasses numerical simulations, analytical comparisons, and performance analyses to assess the feasibility of metal plasma fuels in space missions.The numerical analysis employs COMSOL Multiphysics to delve into the magnetohydrodynamics behavior within the VAT. Such simulation setup could provide valuable insights. Although the numerical results are disappointing for this paper, there exist possibilities within future work. The main hurdle is the simulation of vacuum. There are workarounds in COMSOL's Vacuum System Modeling tool which was not available for this thesis. Also, the used material properties were not suited for this high temperature plasma environment. The lack of material properties is a consequence of the insufficient research in the metal plasma field.Performance analysis is conducted on both the VAT and VASIMR engine, exploring efficiency, thrust capabilities, and feasibility for interplanetary missions. The results demonstrate the potential of metal-based plasma sources to reduce dependence on Earth for refueling and decrease mission costs. It is found that aluminum and magnesium have similar performance as the argon gas used in the VASIMR.Although challenges exist, such as integration problems and availability of material properties for metals in plasma states, the study underscores the promise of metal plasma fuels for sustainable space exploration. By advancing high-powered electric propulsion technologies, we move closer to realizing humanity's ambitious journey to distant celestial bodies. This research paves the way for future innovations, enabling a more self-sustaining space economy and unlocking new horizons of interplanetary travel.

Gegenstand dieser Arbeit ist die retrograde Bewegung des Vakuumbogens, einer Bewegung des Bogenfußes entgegen der Lorentz-Kraft. Die retrograde Bewegung wird auf die Bewegung der Brennflecke des Kathodenansatzes zurückgeführt. Es wurden die Unterstrukturen der Brennflecke und ihre Bewegung im Magnetfeld untersucht. Dabei wurden bekannte makroskopische Erscheinungen (Plasmakante entlang der Magnetfeldlinien) erstmalig beobachteten mikroskopischen Erscheinungen (Jets) zugeordnet. In der vorliegenden Arbeit wurden die Eigenschaften der Jets bestimmt. Desweiteren wurde aus Untersuchungen zur Schrittweite und zeitlichen Abfolge der Brennfleckbewegung unter dem Einfluß des externen Magnetfeldes abgeleitet, daß die zufällige Bewegung des Brennflecks und seiner Bestandteile im magnetfeldfreien Fall auf kleineren Orts- und Zeitskalen als die retrograde Bewegung des Brennflecks (200...300µm) und (1...4µs) erfolgt und damit auch verschiedene Mechanismen ihre Ursache sind. Die beobachteten Jets, die aus dem Brennfleckkern emittiert werden, liefern eine Erklärung für die retrograde Bewegung. In den Jets werden elektrische Felder generiert, welche zur Neuzündung von Brennflecken beitragen. Sie sind der Hauptgrund für die Asymmetrie auf der retrograden Seite (Jets) und der Vorwärtsrichtung (keine Jets). Außerdem konnte gezeigt werden, daß bei heißen Kathoden keine Bewegungsumkehr der retrograden Bewegung erfolgt. Dadurch konnte Klarheit in Bezug auf ältere Veröffentlichungen geschaffen werden, die der retrograden Bewegung eine solche Erscheinung zugeordnet haben. / This thesis treats the phenomenon of retrograde motion of vacuum arcs, the motion of the arc root in the opposite direction of the Lorentz-force. The retrograde motion is attributed to the motion of the cathode spots. The sub-structures of cathodes spots and their motion in a magnetic field were examined. Known macroscopic phenomena (plasma edge along the magnet field lines) were related to observed microscopic phenomena (jets). These jets have been observed for the first time and their properties have been determined. From investigations of typical step widths and time constants of the cathode spots' motion in an external magnetic field it was derived, that random walk and retrograde motion of cathode spots and its fragments in the magnetic field-free case occur on smaller spatial (200...300µm) and temporal (1...4µs) scales than the retrograde motion of the spot. This leads to the conclusion that the mechanisms of both types of motion are different. The observed jets, which are emitted from the spot core, supply an explanation for the retrograde motion. Within the jets electrical fields are generated, which contribute to the ignition of new cathode spots. The jets are the reason for asymmetry on the retrograde side (jets) and the forward direction (no jets). Additionally it could be shown that with hot cathodes no reversal of the retrograde motion occurs. This cleared the assertion of older publications that a reversal of the retrograde motion is linked to the cathode temperature.

Dans le procédé de refusion à l’arc sous vide, la structure et la dynamique de l’arc électrique conditionnent les distributions spatiales d’énergie et de courant au sommet du lingot refondu. Ces distributions impactent fortement les champs de température et de vitesse du métal liquide, qui gouvernent les conditions de solidification du lingot et donc la qualité finale du produit. Une étude par mesures optiques, de la dynamique de l’arc aux longues échelles de temps lors de refusions industrielles d’alliages de titane a été entreprise. Cette analyse a été effectuée par une méthode qualitative, à l’aide d’enregistrements vidéo, et par une nouvelle méthode de diagnostic quantitative développée spécifiquement dans ce travail, à base de photodiodes. L’analyse de la dynamique de l’arc confirme la présence d’un mouvement d’ensemble de l’arc électrique lors de refusions d’alliages de titane et montre une forte corrélation entre la dynamique observée et l’intensité du brassage électromagnétique imposé. Nos résultats ont de plus permis de prédire la dynamique probable de l’arc dans des conditions non étudiées et de proposer de possibles origines à la dynamique observée. Enfin, une première approche de modélisation de cette dynamique dans le logiciel de simulation RAVEL est proposée afin d’étudier son impact sur la solidification du lingot. Ce travail présente également une modélisation détaillée des rayonnements thermiques émis au sommet du lingot, basée sur la méthode des radiosités / In the vacuum arc remelting process, the structure and dynamics of the electric arc are responsible for the spatial distributions of energy and current at the top of the remelted ingot. Those distributions strongly impact the ingot temperature field and the liquid metal velocity field, which govern the ingot solidification conditions, hence the final product quality. A study based on the optical measurement of the electric arc dynamics at a long time-scale during vacuum arc remelting of titanium alloys was undertaken. This analysis was performed either qualitatively using melt video recordings, or quantitatively by a new specifically developed diagnostic technique based on the use of photodiodes. The analysis of the electric arc dynamics confirmed the presence of an ensemble arc motion during vacuum arc remelting of titanium alloys and showed a strong correlation between the observed dynamics and the magnitude of the imposed electromagnetic stirring. Furthermore, our results allow to predict the arc dynamics in non-studied conditions and to propose possible origins for the observed behaviors. Finally, a first simulation of the influence of the arc behavior is proposed in order to study its impact on the ingot solidification. This work comprises also a detailed modelling of the thermal radiation at the ingot top, based on the radiosity method

Le procédé VAR (Vacuum Arc Remelting ou refusion à l'arc sous vide en français) est employé dans la production d'alliages à haute performance pour les industries aéronautique (aciers spéciaux, superalliages et alliages de titane) et nucléaire (alliage de zirconium). Comme pour tous les procédés de fonderie, la maîtrise de l'homogénéité chimique et de la structure métallurgique des lingots coulés par le procédé VAR constitue un enjeu industriel important. Les travaux présentés dans ce mémoire visent à identifier, pour les alliages de zirconium en particulier, les effets de la convection naturelle et de la convection forcée due au brassage électromagnétique sur la macroségrégation. Dans ce but, un modèle numérique a été développé. Il est basé sur la résolution couplée des équations de conservation d'énergie, de quantité de mouvement et de solutés, dans des conditions d'écoulement laminaire ou turbulent. La modélisation de la solidification tient compte du couplage fort entre le transport d'énergie et de solutés dans la zone pâteuse. Afin de décrire la microségrégation, la diffusion restreinte des solutés dans les phases liquides et solides peut être prise en compte. Parallèlement, deux électrodes chimiquement homogènes d'alliages Zircaloy-4 et M5® ont été spécialement refondues dans un four VAR industriel sur le site de CEZUS à Ugine (Savoie, France). La macroségrégation des lingots obtenus a été caractérisée.La comparaison entre les mesures expérimentales et les résultats de simulation a montré que pour un alliage dont l'intervalle de solidification est important (comme l'alliage Zircaloy-4), la convection solutale dans la zone pâteuse peut avoir une influence essentielle sur la macroségrégation de la région centrale du lingot. Par ailleurs, le mouvement de grains équiaxes lors de l'application d'un brassage électromagnétique de forte intensité semble accentuer significativement la macroségrégation dans la région externe du lingot. Pour un alliage dont l'intervalle de solidification est faible (comme l'alliage M5®), nous avons montré que la macroségrégation dépend plus spécifiquement de la convection forcée due au mode de brassage électromagnétique appliqué au cours de la refusion / Vacuum Arc Remelting (VAR) is used to produce high performance alloys for the aeronautic (special steels, superalloys, titanium alloys) and nuclear (zirconium alloys) industries. As for all casting processes, the control of the chemical homogeneity and the metallurgical structure in VAR ingots is an important industrial issue. The goal of this thesis is to identify, for zirconium alloys in particular, the effects of the natural convection and the forced convection due to the electromagnetic stirring on macrosegregation. To this purpose, a numerical model has been developed. It is based on the solution of the coupled transient energy, momentum and solute transport equations, under laminar or turbulent flow conditions. The solidification modeling accounts for a full coupling between energy and solute transport in the mushy zone. The finite diffusion of solutes in both solid and liquid phases can be taken into account to describe microsegregation. In addition, chemically homogeneous Zircaloy-4 and M5® electrodes have been specially remelted in an industrial VAR furnace at the CEZUS plant in Ugine (Savoie, France). The macrosegregation of the ingots has been measured. The comparison between the experimental measurements and the simulation results showed that for an alloy with a large solidification interval (like Zircaloy-4), the solutal convection in the mushy zone could have an essential influence on the macrosegregation in the inner part of the ingot. Furthermore, the motion of equiaxed grains caused by a strong stirring seems to seriously intensify macrosegregation in the outer part of the ingot. For an alloy with a small solidification interval (like M5®), we have shown that the macrosegregation depends more specifically on the forced convection due to the type of stirring applied during the remelting