Thesis (M. Phil.)--University of Hong Kong, 1974.

Thesis (MTech (Mechanical Engineering))--Peninsula Technikon, 2002. / Word processed copy. Summary in English. Includes bibliographical references (leaves 142-143). Also available online.

A great deal of attention has been paid to semiconductor nanowires due to their compatibility of conventional silicon-based technology. Metal-catalytic vapor-liquidsolid (VLS) and various solution-based techniques have widely been used to synthesize silicon/germanium (Si/Ge) nanowires. It is well characterized that the crystallographic orientations, diameter sizes, and surface morphologies of semiconductor nanowires can be controlled by varying process conditions and metal catalysts. Earlier experimental and theoretical studies have identified mechanism underlying metal catalyzed Si/Ge nanowire growth, involving Si/Ge diffusion into a metal catalyst, eutectic Si/Ge-catalyst alloy formation, and Si/Ge precipitation at the catalyst-nanowire interface. However, little is known about the atomic-level details of the structure, energetics and dynamics of amorphous metal alloys such as gold-silicon (Au-Si) and gold-germanium (Au-Ge) despite their importance for well controlled synthesis of Si/Ge nanowires, which is essential for the success of Si/Ge nanowires-based applications. Experiments provide many clues to the fundamental aspects of the behavior and properties of metal alloys, but their interpretations often remain controversial due largely to difficulties in direct characterization. While current experimental techniques are still limited to providing complementary atomic-level, real space information, first principles based atomistic modeling has emerged as a powerful means to address the structure, function and properties of amorphous metallic alloys. This thesis work has focused on developing a detailed understanding of the atomic structure, energetics, and oxidation of Au-Si alloys, as well as molecular mechanisms underlying Au-catalyzed Si nanowire growth. In addition, the surface reconstruction and chemistry of Si nanowires has been examined, with comparisons to planar Si surfaces. In this dissertation, based on first principles atomistic simulations, we present: 1) the atomic structure, energetics, and chemical ordering of amorphous Au-Si alloys with varying Au:Si composition ratios; 2) the behavior of boron (B) in the Au-Si alloy, such as diffusion and agglomeration, and the effect of B addition on the atomic distribution of Si and Au, with implications for in-situ doping of Si nanowires; 3) the origin and structural ordering of Si surface segregation in the Au-Si alloy, providing important insights into the nucleation and early-stage growth of Si nanowires; 4) the interfacial interaction between the Au-Si alloy and various facets of crystalline Si, such as (111), (211), (110), (110), which explains well the underlying reasons for the growth direction of Si nanowires; 5) the oxidation of the Au-Si alloy; and 6) the surface reconstruction and chemistry of Si nanowires with comparisons to planar Si surfaces. Outcomes from the thesis work contribute to: clarifying the atomic structure, energetics and chemical ordering of amorphous bulk Au-Si alloys, as well as their surfaces and interfaces; better understanding molecular mechanisms underlying the Aucatalyzed synthesis of Si nanowires; and identifying the surface reconstruction and chemistry of Si nanowires. The improved understanding can provide invaluable guidance on the rational design and fabrication of Si nanowire-based future electronic, chemical, and biological devices. This thesis work also offers a theoretical platform for studying metal alloy systems with various applications. / text

In fracture management with open reduction and internal fixation with metallic implant, secondary procedure of removal of implant is often required. Such procedure causes additional surgical risks to patients, including anaesthetic risks, wound infection, bone infection, soft tissue adhesion and joint stiffness. The procedure is also costly to the patient and society. If the fixation implant is self-resorbable, the need for secondary surgery will be completely eliminated and the social resources can be saved. Making use of the corrosion process, metals can be developed into new generation of resorbable (or biocorrodible, biodegradable) implants. An ideal bioresorbable orthopaedic implant should provide adequate mechanical support that matches the bone healing process. The implant should resorb progressively as the bone heals. Many current resorbable materials are biomechanically inferior to conventional metallic implants. Magnesium based alloys are popularly studied because of their mechanical properties and biocompatibility. Implants made of magnesium based alloy are expected to resorb in the human body with no harmful effect. The major research challenge is to identify an alloy that performs satisfactorily in the following aspects: biocompatibility, degradation rate, hydrogen gas formation (gas product from the reaction between Mg and water), and mechanical strength. In addition, there is no standard evaluation method for the biodegradable alloys. It is because the interaction between the degradable implants and the physiological environment is too complicated to mimic. The in vitro and the in vivo results often mismatch. This research involved the design and the tests of three Mg based alloys. Zinc (Zn) and manganese (Mn) were chosen as the alloying elements for corrosion resistance and mechanical enhancement. Mg-1Zn-1Mn, Mg-3Zn-1Mn, Mg-5Zn-1Mn (in wt.%) were developed and compared. The study was divided into three parts: material characterization, in vitro studies, and in vivo (animal) studies. The SEM/EDX confirmed that the surface properties of the alloys were consistent after the surface treatment. From the mechanical test, the yield strengths and the densities of the alloys were found to be close to that of the natural bones. The theoretical calculation showed that the amount of Mn determined the threshold implant mass of the test alloys. The hydrogen evolution test showed that the Mg-1Zn-1Mn was the least corrodible. The elution test showed that the Mg-1Zn-1Mn was the least cytotoxic and the cytotoxicity was affected by the pH changes brought by the alloys. The live cell imaging captured the interaction between the alloys and the cells. The subcutaneous implantation showed that the Mg-3Zn-1Mn formed the smallest gas pocket. In the six-month femoral implantation study (Mg-3Zn-1Mn excluded), the Mg-1Zn-1Mn showed the least volume loss and the steadiest degradation behaviour. It was also found to associate with better bone responses. Concluding from all the results, the Mg-1Zn-1Mn demonstrated better potential to become biodegradable orthopaedic products. This work evaluated the potentials of the new alloys and proposed some suggestions for the mismatch results. Moreover, quantitative investigation of biomechanical properties, long term degradation behaviour, and toxicity are recommended to be carried out in the future. / published_or_final_version / Orthopaedics and Traumatology / Master / Master of Philosophy

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