>Magister Scientiae - MSc / Due to the negative impact on the environment and the associated biological risks on human and animal life, the need for eco-friendly synthetic protocols is critical. With the rapid advancement in nanotechnology, this extends to the synthesis of nanomaterials. Eco-friendly nanoparticle synthesis protocols have led to the use of fungi, plants and other biological substances, due to their remarkable ability in reducing metal ions. This led to the formation of very efficient hybrid catalysts, which are partially organic/inorganic composites. Palladium nanoparticles have drawn much interest due to its potential in catalytic applications and in photovoltaic cell development. In this study, the brown marine algae, Ecklonia radiata, was employed as a putative palladium nanoparticle bioreactor. Aqueous extracts of the algae were used as a supporting matrix for the synthesis of palladium nanoparticle (AE-PdNPs) catalysts according to the principles of green chemistry. The catalysts were then assessed for their capability in various carbon-carbon coupling reactions such as Suzuki-Miyaura, Sonogashira, and Heck coupling reactions. Selectivity studies were also performed. The PdNPs were compared to “model” polyvinylpyrrolidone palladium nanoparticles (PVP-PdNPs), synthesized according to literature methods. A variety of spectroscopic techniques were used to characterize the nanoparticles and the organic reaction products, including HRTEM, EDX, NMR, FTIR, DLS, TGA, UV-Vis, ICP-AES, GC-MS and XRD spectroscopy. qNMR was used to determine the product % yields. The aqueous extracts were characterised using NMR and a variety of assays, including total antioxidant potential, total reducing power and radical scavenging ability) to assess its ability to reduce the Pd metal salt. 2D NMR revealed polysaccharides and polyphenols to be the major and minor components, respectively, present in the extract. HRTEM images revealed the average size of the AE-PdNPs and PVP-PdNPs to be 12 nm and 8 nm, respectively. The images also showed the shapes of the NPs to be cubic for the AE-PdNPs and cubic or triangular for the PVP-PdNPs. SAED and XRD spectroscopy revealed the face-centred cubic phase and polycrystalline nature of the AE-PdNPs. No reliable data, other than the HRTEM images was obtained for the PVP-PdNPs. Zeta potential and DLS measurements confirmed the negative charge present on the surface of the nanoparticles, while the hydrodynamic radii were found to be 65 nm and 99 nm for the AE- and PVP-PdNPs, respectively, substantiating the presence of the capping agents. ICP-AES analysis revealed the Pd content of the NPs to be 48.8 and 28.9 ppm for the AE- and PVP-PdNPs. Following characterization, the PdNPs were assessed as potential catalysts in the Suzuki-Miyaura, Heck and Sonogashira carbon-carbon coupling reactions. Bromo and iodo substrates were employed, together with sterically hindered substrates, with a nitro moiety in the ortho or para positions. For the Suzuki-Miyaura reactions, both sets of PdNPs revealed slightly higher yields for the products synthesized using the bromo substrate (>90%), while low yields (40 – 55% yields) were obtained for the ortho substituted substrate in comparison to the para substrate (>90% yields). The Heck coupling reactions with butyl acrylate and 4-iodoacetphenone were successful (~70% yields), while reactions with 4-bromoacetophenone failed. However, the Sonogashira couplings did not proceed at all. With the series of reactions NPs showed some selectivity, with the AE-PdNPs consistently producing higher yields for the products obtained. This may be due to overall nature of the NPs, or due to the higher platinum loading content for the AE-PdNPs.

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Heat treatment for the purpose of material strengthening is accompanied by residual stresses and distortion. During these processing steps, steel alloys experience a phase change that in turn modify their overall mechanical response. To properly account for the cumulative composite behavior, the mechanical response, transformation kinetics and subsequent interaction of each phase have to be properly accounted for. Of interest to material designers and fabricators is modeling and simulating the evolutionary process a part undergoes for the sake of capturing the observable residual stress states and geometric distortion accumulated after processing. In an attempt to capture the aforementioned physical phenomena, this investigation is premised upon a consistent thermodynamic framework. Following this, the single phase Evolving Microstructural Model of Inelasticity state variable model is extended to accommodate the occurrence of multiphases, affirming that the interaction between coexisting phases is through an interfacial stress. Since the efficacy of a multiphase model is dependent on its ability to capture the behavior of constituents phases and their subsequent interaction, we introduce a physically based self-consistent strain partitioning algorithm. With synthesis of the aforementioned ideas, the additional transformation induced plasticity is numerically accounted for by modifying each phase’s flowrule to accommodate an interfacial stress. In addition, for simulating the cohabitation of two phases, the mechanical multiphase model equations is coupled with a previously developed non-diffusional phase transformation kinetics model. A qualitative assessment of the material response based on a Taylor, Sachs and self-consistent polycrystalline approximation is carried out. Further analysis of the multiphase model and its interaction with transformation kinetics is evaluated.

A material model capable of reproducing the anisotropic behavior of polycrystalline materials will prove to be useful in simulations in which directional properties are of key importance. The primary contributor to anisotropic behavior in polycrystalline materials is the development of texture through the rotation and alignment of slip systems due to plastic deformation. A large concentration of aligned slip systems will influence the glide of dislocations in the respective global deformation direction resulting in a directionally dependent flow stress. The Evolving Microstructural Model of Inelasticity (EMMI) is modified to account for evolving anisotropy due to the development of texture. Texture is characterized via a second order orientation tensor and is incorporated into EMMI through various modifications to the EMMI equations based on physical assumptions. Evolving anisotropy is captured via a static yield surface through a modification to the flow rule based on the assumption loading is entirely elastic within the yield surface. A separate modification to EMMI captures evolving anisotropy through an apparent yield surface via a modification to the EMMI internal state variable evolution equations. The apparent yield surface is the result of a smaller yield surface translating through stress space and assumes the state of the material is disturbed at stresses much lower than indicated by experimental yield surfaces.

Polycrystalline diamond (PCD) is used as a cutting tool in many industries because of its superior wear resistance compared to single crystal diamond. Engineers who design new PCD materials must have an understanding of the tradeoffs between material properties in order to tailor a product for different applications. Two competing material properties that are often encountered in PCD are transverse rupture strength and thermal-resistance. Thermal-resistance is directly related to the cobalt content of PCD, and is the ability of the material to withstand thermally induced degradation. In this thesis, we characterize the tradeoff boundary between transverse rupture strength and cobalt content of PCD. We also characterize the tradeoff boundary between cost and cobalt content, and show how both of these tradeoff boundaries can be used to manage product development, which adds value for managers in both engineering and business. In order to characterize these tradeoffs, empirical models are developed for each material property in terms of the design variables of sintering pressure and diamond grain size, where the pressure ranges from 55 kbar to 77 kbar and the grain size ranges from 12 μm to 70 μm in diameter. Then the models are used as optimization objectives in the normal constraint method to generate the tradeoff boundary. Finally, the tradeoff boundary is validated through additional experiments. The tradeoff boundary shows that the relationship between transverse rupture strength and cobalt content is not linear. It also shows that the optimal PCD designs can occur over a wide range of pressures and grain sizes, but pressures above 66 kbar and grain sizes between 20 and 30 μm appear to offer the best compromise between these material properties. These results are compared to the wear rates of PCD compacts in rock cutting tests. The rock cutting test results confirm that the designs with the best compromise between transverse rupture strength and cobalt content also have the highest wear resistance. In general, the designs that offer the best compromise between the properties are also the most expensive to manufacture.

<p> A unified approach, employing effective phonon frequency distributions, is used to investigate effects of phonon anisotropy in the hcp metals.</p> <p> Phonon information is included by means of empirical force constant models, and pseudopotentials are used to describe the electron-ion interaction.</p> <p> Zinc and thallium superconducting gaps are determined as a function of position on the Fermi surface. The gap anisotropy is used to calculate thermodynamic properties.</p> <p> The normal state electron-phonon mass enhancement and the imaginary part of the electron self-energy are calculated as a function of temperature and Fermi surface position. Anisotropic transport scattering times are defined, calculated and used to obtain the polycrystalline and single crystal resistivities. Comparison is made with resistivities obtained by the variational approach.</p> / Thesis / Doctor of Philosophy (PhD)

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