L’étude des impacts hypervéloces (IHV) est essentielle dans de nombreux domaines tels que l’aérospatial, la cosmologie ou l’armement. Pour les reproduire en laboratoire, il est usuel d’utiliser des lanceurs à gaz ou à poudre. Toutefois, ce type de moyen se limite à des vitesses d’impact de l’ordre de 10 km/s pour des projectiles millimétriques. Afin d’étudier des vitesses plus élevées, il faut se tourner vers des moyens alternatifs. Dans cette étude, nous démontrons qu’une analogie est possible entre irradiations laser et IHV. Pour parvenir à ce résultat, des données expérimentales ont été obtenues sur le lanceur HERMES et sur l’installation laser GCLT. Deux matériaux cibles ont été considérés : l’aluminium 6061-T6 et l’EDM3, un graphite poreux. Par simulation numérique, nous avons caractérisé spatialement et temporellement les champs de pression générés en surface des cibles par un projectile et par un laser. Cela nous a permis de proposer et de valider une procédure permettant de lier IHV et essais laser. Pour finir, une campagne expérimentale été réalisée sur l’installation laser du LULI2000 afin d’étudier des vitesses d’impact pouvant atteindre 32 km/s. / The study of hypervelocity impacts (HVI) is essential in many fields such as aerospace, cosmology or defense. To reproduce them in laboratory, it is usual to use gas or powder launchers. However, this type of facility is limited to impact velocities under 10 km/s for projectiles of millimeter size. In order to study higher velocities, it is necessary to consider alternative means. In this study, we demonstrate that an analogy is possible between laser irradiations and HVI. To do this, experimental data were obtained on the HERMES launcher and the GCLT laser facility. Two target materials were considered: 6061-T6 aluminum and EDM3, a porous graphite. By numerical simulation, we spatially and temporally characterized the pressure fields generated on the surface of the targets by a projectile and a laser. It allowed us to propose and validate a procedure to link HVI and laser shots. Finally, an experimental campaign was carried out on the LULI2000 laser facility to study impact velocities up to 32 km/s.

Graduation date: 2005

Graduation date: 1999

The metal/graphite interface is interesting due to the typically large disparity in the characteristics of the electronic structure (e.g. Fermi wavelength and Fermi energy) and dimensionality (3D in the metal versus quasi-2D in graphite). The goal of this work is to determine how the contact conductance to graphite depends on the metal contact area for nanometer-scale contacts. From this we deduce the effect of electronic screening in the graphite. Three different metals were chosen for this work: Solid Cu and Al, and liquid Ga. Liquid Ga provided a unique opportunity to reduce the effect of mechanical interactions to near zero, while Cu and Al were chosen for their different electronic structures. At the interface between the metal and graphite, the large Fermi wavevector of Al should allow phase matching of Al states to those in graphite, while the Cu Fermi surface lies inside of all available graphite wavevector states.

This study investigates the development of a strategy to simulate nuclear graphite oxidation with Computational Fluid Dynamics (CFD) to determine an estimate of graphite lost. The task was achieved by comparing the results of the CFD approach with a number of different experiments. For molecular diffusion, simulated results were compared to analytical solutions. Mass flow rates under conditions of natural convection were sourced from the 2002 NACOK experiment. Experimental data from the KAIST facility were sourced for the basic oxidation of graphite in a controlled environment. Tests included the reactions of carbon with oxygen and with carbon dioxide. Finally, the tests at NACOK from 2004 and 2005 were chosen for comparison for the simulation of oxidation. The 2005 test considered two reacting pebble bed regions at different temperatures. The 2004 test included multiple detailed structural graphite. Comparison of results indicated that the phenomenon of diffusion can be correctly simulated. The general trends of the mass flow rates under conditions of natural convection were obtained. Surface reaction rates were defined with user functions in Fluent. Good comparisons of the simulated and the KAIST experimental results were obtained. For the 2005 NACOK comparison, the pebble bed regions were simulated with a porous medium approach. Results showed that correct trends and areas of oxidation were estimated. The 2004 tests were with a combination of a porous medium and surface reaction approaches. More detailed oxidation experimental data would possibly improve the accuracy of the results. This research has shown that the CFD approach developed in the present study can identify areas of maximum oxidation although the accuracy needs to be improved. Both the porous and detailed surface reaction approaches produced consistent results. The limitations of the approach were discussed. These included transient phenomena which were estimated with steady state simulations, and the effects of change in geometry were not considered. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010.

This study investigates the development of a strategy to simulate nuclear graphite oxidation with Computational Fluid Dynamics (CFD) to determine an estimate of graphite lost. The task was achieved by comparing the results of the CFD approach with a number of different experiments. For molecular diffusion, simulated results were compared to analytical solutions. Mass flow rates under conditions of natural convection were sourced from the 2002 NACOK experiment. Experimental data from the KAIST facility were sourced for the basic oxidation of graphite in a controlled environment. Tests included the reactions of carbon with oxygen and with carbon dioxide. Finally, the tests at NACOK from 2004 and 2005 were chosen for comparison for the simulation of oxidation. The 2005 test considered two reacting pebble bed regions at different temperatures. The 2004 test included multiple detailed structural graphite. Comparison of results indicated that the phenomenon of diffusion can be correctly simulated. The general trends of the mass flow rates under conditions of natural convection were obtained. Surface reaction rates were defined with user functions in Fluent. Good comparisons of the simulated and the KAIST experimental results were obtained. For the 2005 NACOK comparison, the pebble bed regions were simulated with a porous medium approach. Results showed that correct trends and areas of oxidation were estimated. The 2004 tests were with a combination of a porous medium and surface reaction approaches. More detailed oxidation experimental data would possibly improve the accuracy of the results. This research has shown that the CFD approach developed in the present study can identify areas of maximum oxidation although the accuracy needs to be improved. Both the porous and detailed surface reaction approaches produced consistent results. The limitations of the approach were discussed. These included transient phenomena which were estimated with steady state simulations, and the effects of change in geometry were not considered. / Thesis (M.Ing. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2010.

After three decades of commercialization, graphite remains the preferred active material for intercalation-type Li-ion battery anodes. Still, the characterization of staging continues to be elusive at the sub-micro- and nano-scales, the typical dimensions of graphite crystallites. Here, the intercalation of Al-based anions in graphitic materials was studied using X-ray powder diffraction (XRD) and Raman spectroscopy. While, in the first case, the analysis was done ex-situ and in mm3-samples, a more localized view was provided by the laser probe which could, furthermore, interrogate the electrochemical process in real-time (in-situ). To do this, an electrochemical cell for Raman studies was custom-made for Al batteries working with non-aqueous electrolytes. Two C materials were used: natural graphite (NG) and processed expandable graphite (EG). Owing to the smaller flake size, higher graphitization degree and larger crystallites of the NG, the Al/NG cells exhibited better performance than the Al/EG ones. Interestingly, discrepancies were observed in the stage numbers estimated from XRD and Raman. These were thought to arise from the, respectively, long- and short-range atomic order scales that are analyzed by those two techniques. To confirm this, in-situ Raman multi-point studies were performed. The results show the presence of domains with mixed stage graphite intercalation when the cells were fully charged, explaining the staging discrepancies.

Vita. / Digitized by Kansas Correctional Industries

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