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This thesis analyzes the influence of process gases on the formation and the characteristics of process byproducts that emerge during additive manufacturing of an aluminum alloy belonging to the 2000 series.  In order to address the influence, four pure gases, argon, nitrogen, helium, and carbon dioxide, were used as a shielding gas on the same parameter sets.  The interaction of the laser beam with the powder bed under each shielding condition was recorded by a camera.  The humidity, particle size distribution, and chemistry of the spatters produced after each job was analyzed.  The chemistry of small cylinders printed, was determined.  The density of the produced cubic samples was determined following the Archimedes principle, as well as through the analysis of the  cross-section.   Lastly,  the  embedded  and  polished  samples  were  etched,  and  the penetration depth of the laser was determined.  Under argon and nitrogen shielding, the process looked the same and the produced spatters show similar results.  Under helium shielding, less incandescent spatters were seen, and the particle size distribution is smaller than under argon or nitrogen. Carbon dioxide resulted in the highest number of incandescent particles and a change of the color of the rays from red to yellow.  The chemical analysis shows that a slight increase of nitrogen in the spatters and the bulk material can be seen under nitrogen shielding.  Oxygen and hydrogen content was sim- ilar under argon, nitrogen, and helium shielding.  Carbon dioxide shielding resulted in the highest oxygen content in the spatter and the bulk material. The density is highest under helium shielding, and lowest under carbon dioxide shielding.  Under argon and nitrogen shielding, the density was similar.  The study concluded that the choice of a process gas is not an arbitrary one but should be selected with care. / Denna avhandling analyserar processgasernas påverkan på bildandet och egenskaperna hos process biprodukter som uppstår vid additiv tillverkning av en aluminiumlegering som tillhör 2000-serien. För att hantera inflytandet användes fyra rena gaser, argon, kväve,  helium  och  koldioxid  som  skyddsgas  på  samma  parameteruppsättningar. Interaktionen mellan  laserstrålen och  pulverbädden under  varje skärmningsförhållande registrerades  av  en  kamera.  Fuktigheten, partikelstorleksfördelningen  och  kemin  hos stänkarna som producerades efter varje jobb analyserades. Kemien hos de små cylindrarna  som  trycktes  bestämdes. Densiteten  hos  de  producerade  kubikproven  bestämdes enligt Archimedes princip, liksom genom analys av tvärsnittet. Slutligen etsades de inbäddade och polerade proverna och laserns penetrationsdjup bestämdes. Under argon- och kväveavskärmning såg processen likadan ut och de producerade stänkarna visar liknande resultat. Under heliumskärmning sågs mindre glödande stänk och partikelstorleksfördelningen är mindre än under argon eller kväve. Koldioxid resulterade i det högsta antalet glödande partiklar och en förändring av strålarnas färg från rött till gult. Den kemiska analysen visar att en liten ökning av kväve i stänkarna och bulkmaterialet kan ses under kväveavskärmning. Syre- och väteinnehållet var liknande under argon-,  kväve-  och  heliumskärmning. Koldioxidavskärmning  resulterade  i  det  högsta syreinnehållet i stänk och bulkmaterial. Densiteten är högst under heliumskärmning och lägst under koldioxidskärmning. Under argon- och kväveavskärmning var densiteten densamma. Studien drog slutsatsen att valet av en processgas inte är godtyckligt utan bör väljas med omsorg.

<p>We propose a physically based model that describes the density and size of voids in silicon introduced via high dose helium ion implantation and subsequent annealing. The model takes into account interactions between vacancies, interstitials, small vacancy clusters, and voids. Void evolution in silicon occurs mainly by a migration and coalescence process. Various factors such as implantation energy and dose, anneal temperature, atmospheric pressure, and impurity level in silicon can influence the migration and coalescence mechanism and thus play a role in the void evolution process. Values for model parameters are consistent with known values for point defect parameters and assumed diffusion limited reaction rates. A single “fitting parameter” represents the rate of bubble migration and coalescence and is therefore related to surface diffusion of adatoms. Results obtained from simulations based upon the model were compared to our experimental results and to previously reported experimental results obtained over a wide range of conditions.</p> <p>Our own experiments involved the implantation of silicon samples and samples with a thin Si_1-xGe_x (x = 0.05, 0.09) epilayer on silicon with 30 keV, 5×10¹⁶ cm^-2 helium. Anneals were done in the range 960-1110°C for 15-30 minutes in nitrogen and dry oxygen. Void size distributions were measured from transmission electron microscopy images. Average void diameter and void density values and void size distribution did not show any significant differences between the samples annealed in nitrogen and dry oxygen. However, the presence of Si_1-xGe_x epilayer on silicon resulted in increased average void diameter and reduced average void density when compared with Si samples as well as more selective void size distribution.</p> <p>Data from the literature included experiments with helium ion implantation energies in the range 30 - 300 keV, doses of 1×10¹⁶ - 1×10¹⁷ cm^-2, subsequent annealing temperatures in the range 700 - 1200°C, and annealing duration in the range 15 minutes - 2 hours. Excellent agreement is found between the simulated results and those from reported experiments. The extracted migration and coalescence rate parameter shows an activation energy consistent with surface diffusivity of silicon. It shows a linear dependence on helium dose, and increases with decreased implantation energy, decreased ambient pressure, decreased substrate impurities, increased temperature ramp rate, or increased Ge fraction in cavity layer, all consistent with the proposed physical mechanism. Our mathematical model specifically ignores the long time saturation in void size, although we propose a simple explanation consistent with the physical picture. Similarly, we give physical reasons for a threshold implant dose resulting in the formation of small vacancy clusters during implant. But in modeling void growth we simply show that when such clusters exist voids will evolve according to our model.</p> <p>In our experiments, the presence of a Si_0.95Ge_0.05 epilayer on silicon resulted in retarded B diffusion when compared with Si samples. This phenomenon is correlated to the role of the Si_0.95Ge_0.05 epilayer on silicon in the void evolution mechanism and both are attributed to Ge interdiffusing from the epilayer into the Si bulk. The B diffusion data also allows us to predict conditions for the SiGe epilayer to modify the injection of interstitials from surface during dry oxidizing anneal.</p> / Doctor of Philosophy (PhD)

In this dissertation, I explore elements of the changing Arctic Ocean through the application of Stable Isotope, Noble Gas Isotopes, and sulfur hexafluoride (SF6) to better understand ice dynamics for freshwater balance, air/sea gas exchange and ocean circulation. For the tracer studies of stable isotope fractionation, our approach is to use sea ice core data to determine the stable oxygen isotope effective fractionation coefficient. The result is an average value close to 2.2 ‰, which is compared to literature values. For the tracer studies of air/sea gas exchange, we use Neon (Ne) and Helium (He) isotope data sets collected in the ‘Switchyard’ region of the Arctic Ocean between 2005 and 2013 and in the Greenland and Norwegian seas between 1994 and 1999. The Switchyard data show a distinct excess in Ne concentrations in the upper waters. We hypothesize that rejection of Ne during sea ice formation accounts for the Ne excess in the Switchyard area of the Arctic Ocean. Based on this hypothesis we estimate sea-ice formation rates by integrating the Ne excess from the surface to the Atlantic Water layer. The resulting amount of excess Ne corresponds to formation of a nearly 4 m thick sea ice layer. We compare the sea ice formation obtained from the Ne excess method with an independent estimate based on oxygen isotope ratio anomalies ?18O, which is nearly 6.07 m. The difference in the sea ice formation estimated by these two methods indicates loss of Ne through leads. We estimate that the gas exchange rate through the sea-ice cover is ca. 11.3 percent per year. The gas exchange rate through sea-ice covered water would be 0.015 meters per day. For the tracer studies of mean residence times, we analyzed tritium (3H), helium isotope (3He and 4He) and sulfur hexafluoride (SF6) samples collected in the ‘Switchyard’ region of the Arctic Ocean between 2008 and 2013. We calculated apparent tracer ages using the 3H/3He ratios and the partial pressure of SF6 and compare their values for the depth interval between the surface and the core of the Atlantic Water layer. The apparent tracer ages range from zero to about 30 years. Generally, the linear correlation between the 3H/3He and SF6 apparent ages was strong, with the coefficient of determination R2 of 0.94. We explore deviations from this linear trend and discuss them in the context of mixing, air-sea gas exchange, and the impact of sea ice formation on the helium and SF6 gas balances in the surface mixed layer.

The paper presents new results concerning the influence of the gas density and porous plug diameter on the nature of liquid steel stirring with an inert gas in the ladle. The tests were carried out on a cold model of a 30t ladle using particle image velocimetry (PIV) with a high-speed camera to analyse the plume zone formed during the supply of argon and helium as a stirring gas. The similarity criteria for the investigation of stirring processes in cold model in the past were discussed and compared. The modified Morton number was used in this paper to relate the gas flow rate in the model with real objects. The presented results constitute complete documentation of the influence of the plug diameter and gas density on the size of formed gas bubbles and the velocity of gas bubbles rising in different zones of the plume, plume, and spout geometry, including the expansion angle, spout height, open eye area, and gas hold-up.

HOLCIM has various cement production plants across India. These plants struggle to produce the projected amount of cement due to electricity shortages. Although coal is abundant in India, the production thereof is in short supply. It is proposed that a thorium HTR (100 MWt) combined with a PCU (Rankine cycle) be constructed to supply a cement production plant with the required energy. The Portland cement production process is investigated and it is found that process heat integration is not feasible. The problem is that for the feasibility of this IPP to be assessed, a Rankine cycle needs to be adapted and optimised to suit the limitations and requirements of a 100 MWt thorium HTR. Advantages of the small thorium HTR (100 MWt) include: on-site construction; a naturally safe design and low energy production costs. The reactor delivers high temperature helium (750°C) at a mass flow of 38.55 kg/s. Helium re-en ters the reactor core at 250°C. Since the location of the cement production plant is unknown, both wet and dry cooling tower options are investigated. An overall average ambient temperature of India is used as input for the cooling tower calculations. EES software is used to construct a simulation model with the capability of optimising the Rankine cycle for maximum efficiency while accommodating various out of the norm input parameters. Various limitations are enforced by the simulation model. Various cycle configurations are optimised (EES) and weighed against each other. The accuracy of the EES simulation model is verified using FlowNex while the optimised cycle results are verified using Excel’s X-Steam macro. It is recommended that a wet cooling tower is implemented if possible. The 85% effective heat exchanger delivers the techno-economically optimum Rankine cycle configuration. For this combination of cooling tower and heat exchanger, it is recommended that the cycle configuration consists of one de-aerator and two closed feed heaters (one specified). After the Rankine cycle (PCU) has been designed and optimised, it is evident that the small thorium HTR (100 MWt) can supply the HOLCIM plant with the required energy. The optimum cycle configuration, as recommended, operates with a cycle efficiency of 42.4% while producing 39.867 MWe. A minimum of 10 MWe can be sold to the Indian distribution network at all times, thus generating revenue. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

HOLCIM has various cement production plants across India. These plants struggle to produce the projected amount of cement due to electricity shortages. Although coal is abundant in India, the production thereof is in short supply. It is proposed that a thorium HTR (100 MWt) combined with a PCU (Rankine cycle) be constructed to supply a cement production plant with the required energy. The Portland cement production process is investigated and it is found that process heat integration is not feasible. The problem is that for the feasibility of this IPP to be assessed, a Rankine cycle needs to be adapted and optimised to suit the limitations and requirements of a 100 MWt thorium HTR. Advantages of the small thorium HTR (100 MWt) include: on-site construction; a naturally safe design and low energy production costs. The reactor delivers high temperature helium (750°C) at a mass flow of 38.55 kg/s. Helium re-en ters the reactor core at 250°C. Since the location of the cement production plant is unknown, both wet and dry cooling tower options are investigated. An overall average ambient temperature of India is used as input for the cooling tower calculations. EES software is used to construct a simulation model with the capability of optimising the Rankine cycle for maximum efficiency while accommodating various out of the norm input parameters. Various limitations are enforced by the simulation model. Various cycle configurations are optimised (EES) and weighed against each other. The accuracy of the EES simulation model is verified using FlowNex while the optimised cycle results are verified using Excel’s X-Steam macro. It is recommended that a wet cooling tower is implemented if possible. The 85% effective heat exchanger delivers the techno-economically optimum Rankine cycle configuration. For this combination of cooling tower and heat exchanger, it is recommended that the cycle configuration consists of one de-aerator and two closed feed heaters (one specified). After the Rankine cycle (PCU) has been designed and optimised, it is evident that the small thorium HTR (100 MWt) can supply the HOLCIM plant with the required energy. The optimum cycle configuration, as recommended, operates with a cycle efficiency of 42.4% while producing 39.867 MWe. A minimum of 10 MWe can be sold to the Indian distribution network at all times, thus generating revenue. / MIng (Mechanical Engineering), North-West University, Potchefstroom Campus, 2014

Polarized helium-3 is used as a contrast agent for lungs magnetic resonance imaging that has recently reached the pre-clinical applications. One method to hyperpolarize 3He is the metastability exchange optical pumping (MEOP). Optical pumping is performed in standard conditions at low pressure (~ 1 mbar) and low magnetic field (~ 1 Gauss). In this work, the complete update of a low field polarizer dedicated to small animal lungs imaging is presented. The implementation of a new 10 W laser, new peristaltic compressor and others components resulted in a production of 3-4 scc/min for a polarization between 30 to 40%. Images of rat lungs made with better resolution and a new dynamic radial sequence are presented as a validation of the system. Since few years, MEOP has also been studied at higher pressures and higher magnetic fields in small sealed cells. It showed that, thanks to hyperfine decoupling effect induced by high magnetic field, it was also possible to efficiently polarize at higher pressure (67 mbar). Experiments done at 4.7 and 1.5 T are reported in this work. The first ones show a benefic (higher polarization values) and a negative effect (lower production rates) of the magnetic field. The seconds highlight the advantage of using an annular beam shape of the laser that matches the distribution of 23S state atoms at higher pressure. Nuclear polarization values of 66.4% at 32 mbar and 31% at 267 mbar were obtained in 20 mL sealed cells and a 10 times increase in the production rate compare to best standard conditions. These promising results were the first motivation for building a high-field polarizer working inside MRI scanner in hospital. The design and the construction of such a polarizer are described in detail in the last part of the dissertation. The polarizer produces hyperpolarize 3He at 30-40% with a 4 times higher flow than the low field polarizer (10-15 scc/min). The first good quality human lungs images made in Poland with healthy volunteers are the main result of this work.

When bulk helium-4 is cooled below T = 2.18 K, it undergoes a phase transition to a superfluid, characterized by a complex wave function with a macroscopic phase and exhibits inviscid, quantized flow. The macroscopic phase coherence can be probed in a container filled with helium-4, by reducing one or more of its dimensions until they are smaller than the coherence length, the spatial distance over which order propagates. As this dimensional reduction occurs, enhanced thermal and quantum fluctuations push the transition to the superfluid state to lower temperatures. However, this trend can be countered via the proximity effect, where a bulk 3-dimensional (3d) superfluid is coupled to a low (2d) dimensional superfluid via a weak link producing superfluid correlations in the film at temperatures above the Kosterlitz-Thouless temperature. Recent experiments probing the coupling between 3d and 2d superfluid helium-4 have uncovered an anomalously large proximity effect, leading to an enhanced superfluid density that cannot be explained using the correlation length alone. In this work, we have determined the origin of this enhanced proximity effect via large scale quantum Monte Carlo simulations of helium-4 in a topologically non-trivial geometry that incorporates the important aspects of the experiments. We find that due to the bosonic symmetry of helium-4, identical particle permutations lead to correlations between contiguous spatial regions at a length scale greater than the coherence length. We show that quantum exchange plays a large role in explaining the anomalous experimental results while simultaneously showing how classical arguments fall short of this task.

L'étude expérimentale des propriétés radiatives de plasmas d'hélium hors équilibres peut être réalisée par spectroscopie d'émission, un diagnostic non intrusif qui ne perturbe pas le système. Dans cette thèse, ce diagnostic a été utilisé sur des plasmas créés dans des tubes à décharges et dans une machine linéaire magnétisée MISTRAL. Les données expérimentales ont été confrontées aux résultats théoriques issus du code de physique atomique SOPHIA (modèle collisionnel-radiatif). Les tubes à décharge se caractérisent par des températures électroniques de l'ordre de l'électron-Volt et des densités électroniques de l'ordre de 10 exposant 12 cm-3. La première partie des résultats de cette thèse a permis de montrer que, dans ces conditions, le modèle C.C.C. (Convergent Close Coupling) reproduit au mieux la dynamique atomique d'un tel système. Dans la machine MISTRAL, la deuxième partie des résultats de cette thèse met en évidence la signature spectroscopique (rapports d'intensités de raies et étutés de raies et études radiales), de la présence d'électrons énergétiques ionisants et de la diffusion des particules chargées à travers un champ magnétique. / The experimental study of the radiative properties of plasmas of helium out of equilibrium can be achieved by emission spectroscopy, a non-invasive diagnostic which does not affect the system. In this thesis, this diagnostic was used on plasmas created in discharge tubes and in the linear magnetized machine MISTRAL. The experimental data were compared with theoretical results coming from the atomic physics code SOPHIA (collisional-radiative model). The discharge tubes are characterized by electron temperatures of the order of the electron volt and electron densities in the range of 10 12 cm-3. The first part of the results of this thesis has shown that under these conditions, the CCC model (Convergent Close Coupling) reproduces in the best way the atomic dynamics of such a system. The second part of the results reveals the spectroscopic signatures (line intensities ratios and radial studies), in the machine MISTRAL, the presence of ionizing energetic electrons and the diffusion of charged particles through a magnetic field.