The focus of this research is the development of high strength, high conductivity copper films. Pure copper is soft and traditional strengthening mechanisms cause substantial decrease in conductivity. To address the challenge, epitaxial nanotwinned copper films are synthesized on HF etched Si (110) substrates. These films show high hardness (~ 2.8 GPa) due to high density of coherent twin boundaries (CTBs) which effectively block the motion of dislocations similar to grain boundaries (GBs). Resistivity of CTBs is calculated to be an order of magnitude lower than that of GBs. Hence, conductivity of nanotwinned copper is still comparable to that of pure copper. In addition, it is shown that average twin spacing can be controlled by adjusting deposition rate. Analytical studies together with experimental evidence show that nanotwins can improve the strength-to-resistivity ratio significantly in copper. In general, nanocrystalline metals suffer from low ductility. To study plastic deformation via rolling, thick polycrystalline nanotwinned copper foils are sputtered on SiO2 and then peeled off the substrate. Despite the high strength, room temperature rolling experiments show that nanotwinned copper films exhibit stable plastic flow with no shear localization or fracture even at thickness reduction of over 50%. Postdeformation studies of microstructure reveals that the plastic deformation is facilitated by the migration of CTBs normal to the twin boundary plane due to the glide of twinning dislocations in the twin plane. X-ray pole figure measurements show insignificant out of plane rotation as a result of 50% rolling thickness reduction. Thermal stability of nanocrystalline metals is also a concern. Free standing nanotwinned polycrystalline copper films show remarkable thermal stability after annealing at 800 degrees C. The driving force for twin growth is much lower than that for grain coarsening because the energy stored in CTBs is an order of magnitude lower than that of GBs. As a result, the average twin spacing stays below 20 nm after annealing. Such high thermal stability of nanotwins leads to the retention of hardness of 2.2 GPa. Low energy twin boundary may provide a unique way to achieve both high strength and high temperature thermal stability in certain metallic materials.

Polymer nanofibers have attracted a lot of industrial interest in the past decade. In general, these fibers need to be thermally stable for many applications, such as in the aerospace industry. However, most of these polymer nanofibers suffer from low temperature degradation, limiting their use in many potential applications. Graphene, which is one sheet of graphite, has unique properties such as high conductivity, and high thermal stability. This exceptional material can be incorporated into the polymer nanofibers as nanofillers in order to enhance their thermal properties. The aim of this dissertation is to investigate the effect of adding graphene nanofillers into the polymer fiber on the resulting fibers’ thermal properties. For that purpose, polyvinyl alcohol (PVA), a non-conductive polymer and a different source of graphene, namely graphene foam, expendable graphite and graphite powder were used. The growth technique was the electrospinning technique which offers a variety of parameters that need to be optimized. For this includes, the amount of PVA in the water solvent, the flow rate, the applied voltage, the growth time, and the tip/collector distance. In summary, it has been optimized that the best conditions for growth of fibers will be as follows: PVA concentration will be fixed at 10 wt%, flow rate will be 3 ml/h, applied voltage will be 30 kV, growth time of 60 s and tip/collector distance will be fixed at 12 cm. The resulted PVA fibers from these conditions were smooth continuous and hollow with diameter ranging between 190-340 nm, while PVA/graphene nano-fibers are much thinner with diameter ranging between 132 - 235 nm when the same parameters were used with only graphene concentration varied. The fiber obtained with PVA showed a hollow structure which is desirable for incorporation of graphene nanofillers. The dispersion of the different source of graphene sheets in the starting PVA solution showed enhanced thermal stability compared to the PVA fibers alone. Furthermore, an increase in the thermal stability is observed with increasing concentration of graphene nanofillers. This work shows the promising use of graphene as nanofillers for PVA fibers. This can be expended to other non-conductive and conductive polymers in order to broaden the application of these fibers in the industries, where thermal stability is a prerequisite. / Dissertation (MSc)--University of Pretoria, 2013. / gm2014 / Physics / unrestricted

The "slot-drill" completion method, which utilizes a mechanically cut high-conductivity "slot" in the target formation created using a tensioned abrasive cable, has been proposed as an alternative stimulation technique for shale-gas and other low/ultra-low permeability formations. This thesis provides a comprehensive numerical simulation study on the "slot drill" completion technique. Using a Voronoi gridding scheme, I created representative grid systems for the slot-drill completion, as well as for the case of a vertical well with a single fracture, the case of a horizontal well with multiple hydraulic fractures, and various combinations of these completions. I also created a rectangular slot configuration, which is a simplified approximation of the actual "slot-drill" geometry, and investigated the ability of this rectangular approximation to model flow from the more complicated (actual) slot-drill configuration(s). To obtain the maximum possible diagnostic and analytical value, I simulated up to 3,000 years of production, allowing the assessment of production up to the point of depletion (or boundary-dominated flow). These scenarios provided insights into all the various flow regimes, as well as provided a quantitative evaluation of all completion schemes considered in the study. The results of my study illustrated that the "slot-drill" completion technique was not, in general, competitive in terms of reservoir performance and recovery compared to the more traditional completion techniques presently in use. Based on my modeling, it appears that the larger surface area to flow that multistage hydraulic fracturing provides is much more significant than the higher conductivity achieved using the slot-drill technique. This work provides quantitative results and diagnostic interpretations of productivity and flow behavior for low and ultra-low permeability formations completed using the slot-drill method. The results of this study can be used to (a) help evaluate the possible application of the "slot-drill" technique from the perspective of performance and recovery, and (b) to establish aggregated economic factors for comparing the slot-drill technique to more conventional completion and stimulation techniques applied to low and ultra-low permeability reservoirs.

O presente trabalho utiliza o método Constructal Design para desenvolver o estudo numérico da configuração de materiais de alta condutividade térmica em forma de “Y” que minimiza a resistência ao fluxo de calor, quando áreas ocupadas pelos materiais de alta e baixa condutividades são mantidas constantes. Para a solução numérica da equação diferencial da difusão do calor e suas respectivas condições de contorno, foi utilizado o software MATLAB ®, mais especificamente a ferramenta PDETOOL, Partial Differential Equations Tool. O objetivo deste trabalho é a minimização da resistência térmica do sistema gerador de calor com baixa condutividade térmica com a utilização de vias em formato de Y com material de alta condutividade térmica e volume constante, sendo variáveis os comprimentos e espessuras do material dos ramos simples e bifurcados. Todas as possibilidades geométricas foram avaliadas e a geometria ótima foi aquela que conduziu a menor resistência térmica. Duas condições são apresentadas, a primeira tem os ramos e a base da geometria “Y” com igual condutividade térmica. Os resultados para esta configuração mostram que existem valores específicos para os graus de liberdade que minimizam a resistência térmica. Nesse caso, os ramos se degeneraram e a configuração ótima tem a forma de um “V”. A segunda configuração apresenta combinações de condutividade térmica diferentes, para os ramos e a bases. Para estes casos obteve-se um valor otimizado próximo de 1 para a razão entre os comprimentos dos ramos simples e bifurcados, indicando que a configuração otimizada tem realmente a forma de um “Y” o que demonstra a dependência entre a geometria e as condições impostas pelo meio. Embora o design inicial do Y possa assumir diversas configurações, quando comparado o primeiro design com o design final, no caso do Y com iguais condutividades térmicas se conseguiu uma melhora superior a 28% e no caso do Y com condutividades diferentes mais de 30 %. Finalmente, este trabalho mostra que a geometria otimizada é aquela que melhor distribui as imperfeições, isto é, os pontos quentes (pontos de temperatura máxima), o que está de acordo com o princípio da ótima distribuição das imperfeições. / The present work used the method Constructal Design to develop numerical analyses of pathways of high thermal conductivity in "Y" shape which minimizes the thermal resistance when areas occupied by the materials of high and low conductivities are kept constant. For the numerical solution of the differential equations of heat diffusion and their boundary conditions, we used the MATLAB ® software, specifically the PDETOOL tool. The aim was to minimize the thermal resistance of the heat generator system with low thermal conductivity with the use of Y-shaped pathways with high thermal conductivity and constant volume, with variable lengths and thicknesses of material from stem and forked branches. All geometric possibilities were evaluated and the optimal geometry was that which resulted in lower thermal resistance. Two conditions were studied. In the first one the stem and branches of the "Y" have equal thermal conductivity. The results for this configuration show that there are specific values for the degrees of freedom to minimize the thermal resistance. In this case, the branches have degenerated and the optimum configuration has the shape of a "V". The second configuration offers different combinations of thermal conductivity, for branches and bases. For these cases we obtained a optimized value close to 1 for the ratio between the lengths of stem and bifurcated branches, indicating that the optimized configuration actually has the shape of a "Y" which shows the dependency of geometry and condition imposed by the environment. Although the initial design of Y can take various configurations, when compared the first design to the final design, in the case of Y with equal thermal conductivity it this improvement was achieved an improvement greater than 28% and in the case of Y with different conductivities over 30%. Finally, this study showed that the optimized geometry is the one that better distributes imperfections, this is, hot spots (points of maximum temperature), which is in accordance with the principle of the optimal distribution of imperfections.

O presente trabalho utiliza o método Constructal Design para desenvolver o estudo numérico da configuração de materiais de alta condutividade térmica em forma de “Y” que minimiza a resistência ao fluxo de calor, quando áreas ocupadas pelos materiais de alta e baixa condutividades são mantidas constantes. Para a solução numérica da equação diferencial da difusão do calor e suas respectivas condições de contorno, foi utilizado o software MATLAB ®, mais especificamente a ferramenta PDETOOL, Partial Differential Equations Tool. O objetivo deste trabalho é a minimização da resistência térmica do sistema gerador de calor com baixa condutividade térmica com a utilização de vias em formato de Y com material de alta condutividade térmica e volume constante, sendo variáveis os comprimentos e espessuras do material dos ramos simples e bifurcados. Todas as possibilidades geométricas foram avaliadas e a geometria ótima foi aquela que conduziu a menor resistência térmica. Duas condições são apresentadas, a primeira tem os ramos e a base da geometria “Y” com igual condutividade térmica. Os resultados para esta configuração mostram que existem valores específicos para os graus de liberdade que minimizam a resistência térmica. Nesse caso, os ramos se degeneraram e a configuração ótima tem a forma de um “V”. A segunda configuração apresenta combinações de condutividade térmica diferentes, para os ramos e a bases. Para estes casos obteve-se um valor otimizado próximo de 1 para a razão entre os comprimentos dos ramos simples e bifurcados, indicando que a configuração otimizada tem realmente a forma de um “Y” o que demonstra a dependência entre a geometria e as condições impostas pelo meio. Embora o design inicial do Y possa assumir diversas configurações, quando comparado o primeiro design com o design final, no caso do Y com iguais condutividades térmicas se conseguiu uma melhora superior a 28% e no caso do Y com condutividades diferentes mais de 30 %. Finalmente, este trabalho mostra que a geometria otimizada é aquela que melhor distribui as imperfeições, isto é, os pontos quentes (pontos de temperatura máxima), o que está de acordo com o princípio da ótima distribuição das imperfeições. / The present work used the method Constructal Design to develop numerical analyses of pathways of high thermal conductivity in "Y" shape which minimizes the thermal resistance when areas occupied by the materials of high and low conductivities are kept constant. For the numerical solution of the differential equations of heat diffusion and their boundary conditions, we used the MATLAB ® software, specifically the PDETOOL tool. The aim was to minimize the thermal resistance of the heat generator system with low thermal conductivity with the use of Y-shaped pathways with high thermal conductivity and constant volume, with variable lengths and thicknesses of material from stem and forked branches. All geometric possibilities were evaluated and the optimal geometry was that which resulted in lower thermal resistance. Two conditions were studied. In the first one the stem and branches of the "Y" have equal thermal conductivity. The results for this configuration show that there are specific values for the degrees of freedom to minimize the thermal resistance. In this case, the branches have degenerated and the optimum configuration has the shape of a "V". The second configuration offers different combinations of thermal conductivity, for branches and bases. For these cases we obtained a optimized value close to 1 for the ratio between the lengths of stem and bifurcated branches, indicating that the optimized configuration actually has the shape of a "Y" which shows the dependency of geometry and condition imposed by the environment. Although the initial design of Y can take various configurations, when compared the first design to the final design, in the case of Y with equal thermal conductivity it this improvement was achieved an improvement greater than 28% and in the case of Y with different conductivities over 30%. Finally, this study showed that the optimized geometry is the one that better distributes imperfections, this is, hot spots (points of maximum temperature), which is in accordance with the principle of the optimal distribution of imperfections.

O presente trabalho utiliza o método Constructal Design para desenvolver o estudo numérico da configuração de materiais de alta condutividade térmica em forma de “Y” que minimiza a resistência ao fluxo de calor, quando áreas ocupadas pelos materiais de alta e baixa condutividades são mantidas constantes. Para a solução numérica da equação diferencial da difusão do calor e suas respectivas condições de contorno, foi utilizado o software MATLAB ®, mais especificamente a ferramenta PDETOOL, Partial Differential Equations Tool. O objetivo deste trabalho é a minimização da resistência térmica do sistema gerador de calor com baixa condutividade térmica com a utilização de vias em formato de Y com material de alta condutividade térmica e volume constante, sendo variáveis os comprimentos e espessuras do material dos ramos simples e bifurcados. Todas as possibilidades geométricas foram avaliadas e a geometria ótima foi aquela que conduziu a menor resistência térmica. Duas condições são apresentadas, a primeira tem os ramos e a base da geometria “Y” com igual condutividade térmica. Os resultados para esta configuração mostram que existem valores específicos para os graus de liberdade que minimizam a resistência térmica. Nesse caso, os ramos se degeneraram e a configuração ótima tem a forma de um “V”. A segunda configuração apresenta combinações de condutividade térmica diferentes, para os ramos e a bases. Para estes casos obteve-se um valor otimizado próximo de 1 para a razão entre os comprimentos dos ramos simples e bifurcados, indicando que a configuração otimizada tem realmente a forma de um “Y” o que demonstra a dependência entre a geometria e as condições impostas pelo meio. Embora o design inicial do Y possa assumir diversas configurações, quando comparado o primeiro design com o design final, no caso do Y com iguais condutividades térmicas se conseguiu uma melhora superior a 28% e no caso do Y com condutividades diferentes mais de 30 %. Finalmente, este trabalho mostra que a geometria otimizada é aquela que melhor distribui as imperfeições, isto é, os pontos quentes (pontos de temperatura máxima), o que está de acordo com o princípio da ótima distribuição das imperfeições. / The present work used the method Constructal Design to develop numerical analyses of pathways of high thermal conductivity in "Y" shape which minimizes the thermal resistance when areas occupied by the materials of high and low conductivities are kept constant. For the numerical solution of the differential equations of heat diffusion and their boundary conditions, we used the MATLAB ® software, specifically the PDETOOL tool. The aim was to minimize the thermal resistance of the heat generator system with low thermal conductivity with the use of Y-shaped pathways with high thermal conductivity and constant volume, with variable lengths and thicknesses of material from stem and forked branches. All geometric possibilities were evaluated and the optimal geometry was that which resulted in lower thermal resistance. Two conditions were studied. In the first one the stem and branches of the "Y" have equal thermal conductivity. The results for this configuration show that there are specific values for the degrees of freedom to minimize the thermal resistance. In this case, the branches have degenerated and the optimum configuration has the shape of a "V". The second configuration offers different combinations of thermal conductivity, for branches and bases. For these cases we obtained a optimized value close to 1 for the ratio between the lengths of stem and bifurcated branches, indicating that the optimized configuration actually has the shape of a "Y" which shows the dependency of geometry and condition imposed by the environment. Although the initial design of Y can take various configurations, when compared the first design to the final design, in the case of Y with equal thermal conductivity it this improvement was achieved an improvement greater than 28% and in the case of Y with different conductivities over 30%. Finally, this study showed that the optimized geometry is the one that better distributes imperfections, this is, hot spots (points of maximum temperature), which is in accordance with the principle of the optimal distribution of imperfections.

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