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Nanocrystalline films are promising in various fields such as microelectronics. Low temperature deposition techniques are desirable since they would enable the use of new substrates that are temperature sensitive, leading to a wide range of new applications. This thesis explores nanocrystalline silver film deposition by impacting nanoparticles (NP) onto a substrate, a technique that enables low process temperatures. This work aims at better understanding the physical parameters governing the sticking probability of NP upon impaction. To achieve this, various substrate materials have been used (metallic and non metallic) and the influence of the impacted substrate temperature has been studied, among other experiments. These parameters showed a significant influence on the collection efficiency of NPs. These experimental results are analyzed in light of published computer simulations studies predicting the behavior of impacting NP to deposit nanostructured films. Secondly, a study of the mechanical properties of the deposited films has been attempted. Compression tests have been carried out varying the applied load, loading time and process temperature. The produced films are nanocrystalline and porous (~70% relative density). Densification has been observed even at room temperature, and the goal of this study is to understand which mechanisms cause the densification to occur. The experimental densification data are compared with a model describing the densification of microparticles compacts by hot pressing that has been adapted to nanocrystalline silver, and the mechanisms leading to densification are discussed. / text

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PhD - Science / The relationships between densification and microstructure, and between microstructure and mechanical and electrical properties of liquid phase sintered silicon carbide were studied in detail using hot pressing, gas pressure sintering and ultra–high pressure sintering techniques. Silicon carbide was sintered with 10 mass-% addition of the Y2O3-Al2O3 system, with various molar ratios. Hot pressing was carried out at 1925oC under 30 MPa, in argon, for half an hour. Materials were gas pressure sintered at 1925oC, under a final gas pressure of 80 bars (8MPa), in argon, for an hour. Ultra-high pressure sintering was done at ca. 1550oC, under 5.5 GPa pressure, for 15 minutes. The hot pressed and gas pressure sintered materials were subsequently heat treated at 1925oC and 1975oC. Most of the silicon carbide materials were sintered to a density around 99% of theoretical density. The heat treatment of the hot pressed materials resulted in an increase in density not changing the porosity. The densities of the heat treated hot pressed materials corresponded to the density of the gas pressure sintered materials. This resulted from the difference in composition of grain boundary phases – yttrium silicates in the hot pressed materials and yttrium aluminates in the gas pressure sintered and heat treated materials. The average silicon carbide grain size in the materials strongly depended on the densification method. In gas pressure sintered and heat treated materials the mean grain size was up to three times higher than that in the hot pressed materials. Grain growth appeared to be higher in the highest alumina-content materials. The heat treatment at 1975 °C resulted in more pronounced anisotropic grain growth. The ratio of the silicon carbide polytypes of sintered materials and materials heat treated materials at 1925oC, did not change significantly. In the materials heat treated at 1975oC Rietveld analysis revealed a decrease in SiC-6H polytype and an increase in amount of 4H and 15R polytypes, compared to the materials heat treated at 1925oC. This can be attributed to the increase in diffusion rates of aluminium into the SiC lattice at 1975oC. Segregation patterns were observed in the high yttria content materials, with Y2O3:Al2O3 molar ratios greater than or equal to two, after gas pressure sintering and heat treatments. This was suggested to be due to he poor wetting of the silicon carbide grains by the yttria-rich grain boundary phase. On heat treatment, the Vickers hardness of hot pressed materials was found to be increased from 20 to 26 GPa and elastic modulus from 318 to 338 GPa. In addition, the log of the electrical conductivity of liquid phase sintered silicon carbide (measured at 330oC) ranged from 10-8 to 10-3 with the changes in grain boundary phases observed after the heat treatments. The grain boundary phase composition also influenced the strength of the materials, The highest strength, 657 + 50 MPa, was measured for the hot pressed material containing the YAG phase. Indentation fracture toughness was mostly influenced by the SiC grain growth during heat treatments. The most significant increase in fracture toughness, the largest being from 3.7 MPa.m1/2 up to 5.6 MPa.m1/2, was observed in the higher alumina content materials after heat treatment at 1975oC. The increase in fracture toughness was attributed to the presence of a higher amount of platelet-like SiC grains within a broader grain size distribution. These elongated grains increased fracture toughness by increasing crack path deflection and crack bridging. The electrical properties were evaluated by Impedance Spectroscopy measurements between room temperature and 330oC. The LPS SiC materials can be classified into three groups with different electrical properties. This classification could be related to the grain boundary phases present in the materials. The materials with the lowest conductivity were all hot pressed materials, containing crystalline silicates and amorphous grain boundary phases. The materials with intermediate conductivity include gas pressure sintered materials and a hot pressed material, which contained crystalline aluminates (Y3Al5O12, YAlO3 and Y4Al2O9) in their grain boundaries. The materials with the highest conductivity only contained the aluminates, YAlO3 and Y4Al2O9. A pseudopercolation model of conduction was proposed, in which electrons move along a path which goes through the thinner intergranular layers, with largest nearest neighbour contact. The temperature dependence of the log of the conductivity of hot pressed and gas pressure sintered materials showed that the conduction mechanism in these liquid-phase sintered silicon carbide materials was variable range hopping conduction of electrons between defect sites. The non-Arrhenius behaviour, together with the observed wide range of peak frequencies, led to the conclusion that the effect of silicon carbide itself was not observed in the impedance spectra. The 1/T0.25 log conductivity dependence showed that the Cole-Cole arcs are due to insulating grain boundary phases rather than semiconducting SiC. In the Cole-Cole plots of the hot pressed and heat treated hot pressed materials only the effect of one phase could be observed. In the gas pressure sintered materials and the hot pressed material containing mainly YAG phase, the effects of two phases were seen in the frequency range measured. Ultra-high pressure liquid-phase sintered silicon carbide materials showed ultra-fine SiC grains, which were highly inter-grown. Segregated grain boundary “core-rim” structures, consisting of an inner core of nonequilibrium yttria and outer rim of equilibrium yttrium silicate were observed in materials containing 4 mass-% to 15 mass-% sintering additives. The hardness of ultra-high pressure sintered 10 mass-% materials increased with alumina-content, from 20 GPa – 22 GPa, and increased with decrease in sintering additive, up to 23 GPa (for the 4 mass-% material).

The area of investigation for this study falls within a small urban island called Westbury. Situated 7km to the West of Johannesburg’s CBD, it is isolated from the adjacent urban fabric as a result of its historical and also recent development. Westbury itself also consists of a series of fragmented islands with undefined boundaries, weak urban blocks and a disorientated grid. The area has recently been identified as a high priority region for densification1 by the city of Johannesburg, supported by transport-oriented infra-structural investment. The questions raised by this study are contextualized against this backdrop. How could densification in Westbury be achieved towards the creation of a more inter-connected, cohesive, accessible and therefore sustainable urban environment? Following from this: How could Westbury be better integrated with the immediate surrounding urban fabric whilst combating its own fragmentation? What is the role of urban blocks and boundary conditions to help shape a future more integrated Westbury, and also towards meaningful place-making? In what ways can architecture contribute in order to improve the urban fabric that operates on various scales: from the very scale of the house to that of an urban boundary to that of the urban block and ultimately the greater urban network? The hypothesis outlined in this study is that architecture is too weak to stand in isolation, that a network of buildings is necessary to achieve a more sustainable, accessible, cohesive, and inter-connected urban environment. This is tested through a rigorous analysis of boundary conditions at different scales as reflected in the urban blocks of Westbury and the resultant architectural strategies. Finally, a block and its attendant boundaries is singled out to test the architectural contribution towards densification of the suburb, the making of place, and better inter-connectivity. The process is envisaged as driven from both the scale at which urban issues inform the architecture, and the reverse scale the architecture in Westbury informs the urban master plan. The architecture in style and scale sets the conditions for the proposed urban blocks. The boundary wall being the medium where urban meets architecture. “For these dreams to flourish in reality, we must recognise that there can be no ready-made solutions in housing, no recipes or / Mini Dissertation (MArch (Prof))--University of Pretoria, 2018. / Architecture / MArch (Prof) / Unrestricted

Remedial ground densification is commonly used to reduce the liquefaction susceptibility of loose, saturated sand deposits, wherein controlled liquefaction is typically induced as the first step in the densification process. Assuming that the extent of induced liquefaction is approximately equal to the extent of ground densification, the purpose of this research is to assess the feasibility of using earthquake liquefaction data in remedial ground densification design via energy-based concepts. The energy dissipated by frictional mechanisms during the relative movement of sand grains is hypothesized to be directly related to the ability of a soil to resist liquefaction (i.e., Capacity). This hypothesis is supported by energy-based pore pressure generation models, which functionally relate dissipated energy to residual excess pore pressures. Assuming a linearized hysteretic model, a "simplified" expression is derived for computing the energy dissipated in the soil during an earthquake (i.e., Demand). Using this expression, the cumulative energy dissipated per unit volume of soil and normalized by the initial mean effective confining stress (i.e., normalized energy demand: NED) is calculated for 126 earthquake case histories for which the occurrence or non-occurrence of liquefaction is known. By plotting the computed NED values as a function of their corresponding SPT penetration resistance, a correlation between the normalized energy capacity of the soil (NEC) and SPT penetration resistance is established by the boundary giving a reasonable separation of the liquefaction / no liquefaction data points. NEC is the cumulative energy dissipated per unit volume of soil up to initial liquefaction, normalized by the initial mean effective confining stress, and the NEC correlation with SPT penetration resistance is referred to as the Capacity curve. Because the motions induced during earthquake shaking and remedial ground densification significantly differ in amplitude, duration, and frequency content, the dependency of the derived Capacity curve on the nature of the loading needs to be established. Towards this end, the calibration parameters for energy-based pore pressure generation models are examined for their dependence on the amplitude of the applied loading. The premise being that if the relationship between dissipated energy and pore pressure generation is independent of the amplitude of loading, then the energy required to generate excess pore pressures equal to the initial effective confining stress should also be independent of the load amplitude. However, no conclusive statement could be made from results of this review. Next, first order numerical models are developed for computing the spatial distribution of the energy dissipated in the soil during treatment using the vibratory probe method, deep dynamic compaction, and explosive compaction. In conjunction with the earthquake-derived Capacity curves, the models are used to predict the spatial extent of induced liquefaction during soil treatment and compared with the predicted spatial extent of improvement using empirical expressions and guidelines. Although the proposed numerical models require further validation, the predicted extent of liquefaction and improvement are in very good agreement, thus giving credence to the feasibility of using the Capacity curve for remedial ground densification design. Although further work is required to develop energy-based remedial densification design procedures, the potential benefits of such procedures are as follows. By using the Capacity curve, the minimum dissipated energy required for successful treatment of the soil can be determined. Because there are physical limits on the magnitude of the energy that can be imparted by a given technique, such an approach may lead to improved feasibility assessments and initial designs of the densification programs. / Ph. D.

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Ce travail porte sur l’élaboration de céramiques transparentes composites à base de grenats d’yttrium et d’aluminium (Nd:YAG) avec différents ratios stœchiométriques (Nd+Y)/Al compris entre 0,40 et 0,70, autour du ratio correspondant à la composition stœchiométrique du Nd:YAG ((Nd+Y)/Al = 0,6000) par frittage-réaction sous vide. La mise en forme a été effectué par coulage sous pression de suspensions stables et homogènes de mélanges Al2O3-Y2O3-Nd2O3. L’étude des propriétés microstructurales, thermiques et optiques de ces céramiques a permis de mettre en évidence l’influence du ratio (Nd+Y)/Al sur les cinétiques de formation de la phase grenat et de densification du matériau ainsi que sur leurs propriétés optiques. Ainsi, il apparaît que pour des céramiques avec un ratio supérieur à 0,6000, correspondant à un matériau enrichi en yttrium, les cinétiques précédemment évoquées sont diminuées mais les propriétés de transparence et lasers apparaissent moins dégradées, contrairement au cas de céramiques enrichies en aluminium ((Nd+Y)/Al < 0,6000). De plus, l’étude de mélanges pulvérulents Nd:YAP-Al2O3 a permis de mettre en évidence un mécanisme de formation de la phase Nd:YAG suggérant l’existence d’une réaction solide-gaz avec évaporation-condensation de la phase réactive aluminée. Enfin, la faisabilité de céramiques de Nd:YAG à haut taux de dopage (4 % at.) a permis de montrer l’intérêt de ces matériaux dans la génération de propriétés optiques non linéaires du troisième ordre, particulièrement le phénomène d’autofocalisation de faisceaux lumineux par effet Kerr. / This work is devoted to the elaboration of yttrium and aluminum garnets-based transparent composite ceramics with different (Nd+Y)/Al stoichiometric ratios ranged from 0.40 to 0.70, around the ratio corresponding to the Nd:YAG stoichiometric composition ((Nd+Y)/Al = 0.6000) by under-vacuum reactive-sintering. Pressure-casting of stable and homogeneous suspensions of mixing Al2O3-Y2O3-Nd2O3 was achieved. Study of microstructural, thermal and optical properties of these ceramics has made it possible to highlight the influence of (Nd+Y)/Al ratio on garnet phase formation and densification kinetics, and on optical properties. Thus, it appears that for ceramic with a ratio higher than 0.6000, corresponding to an yttrium-rich sample, the previously mentioned kinetics are decreased but transparence and lasers properties are less degraded, contrary to aluminum-rich ceramics ((Nd+Y)/Al < 0.6000). Moreover, study of Nd:YAP-Al2O3 powders mixing brings out Nd:YAG phase formation mechanism, suggesting the existence of solid-gas reaction with evaporation-condensation of aluminum-rich reactive phase. Finally, feasibility of Nd:YAG ceramics with high dopant content (4 at. %) has shown the interest of these materials in order to generate third-order nonlinear optical properties, especially self-focusing phenomenon of light beams by Kerr effect.