Thesis (M.S.)--University of Texas at El Paso, 2009. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

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We have developed a new coarsening theory that more completely describes grain growth of a system of anisotropic particles such as completely faceted crystals by Ostwald ripening. Our model takes the anisotropy of surface energies on dissimilar facets into account, and the particle sizes are described by a distribution function. The theory is applied to study the coarsening of ,B-silicon nitride in a liquid medium due to the anisotropic growth of grains in different crystallographic directions. A model of the growth of silicon nitride grains is obtained based on the numerical solution of the equations of the new theory. Computer experiments are performed to determine how the distribution function evolves, to investigate the influence of various parameters such as diffusion and interfacial reaction constants on grain growth and to extract grain growth exponents from this model in order to determine the growth mechanisms that are responsible for the anisotropic growth behaviour. Only preliminary numerical results are available thus far due to 1/r instabilities that occur in the theory as r → 0. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2002.

Polyhedral boranes and carboranes have acquired their popularity for constructing meso and nano-size structures for an array of applications from pharmaceuticals to material science. These three-dimensional boranes range from 4 to 22 boron atom per molecules with delocalized bonding analogous to aromatic compounds. The unique vibrational spectroscopy of the BH function allows for possible application of these species to bioimaging. Silver nanoparticles functionalized with ortho-carboranes have been reported for bioimaging using Surface-enhanced Raman scattering (SERS). The silver nanoparticles were functionalized with antibodies specific to cancer cell receptors. Bonding thiol-substituted carboranes to these particles allowed for observation of enhanced Raman signals as the imaging mode. Here, attempts to synthesize second generation carborane molecules with additional Raman-active group such as nitrile were conducted. Hybrid diblock copolymers have the ability to self-assemble in different morphological patterns depending on the type and ratio of monomers and the compatibilities in various solvents. Linear hybrid diblock copolymers were synthesized by ring-opening metathesis polymerization (ROMP) reactions with norbornenyl-based decaborane and various amounts of norbornene and norbornenyl-ester derivative monomers. Their self-assembly behaviour in various solvents were characterized by NMR, TGA, DSC, and SEM. P(norbornene)60-b-p(norbornenyl-decaborane)40 polymers showed lamellar morphology patterns when slowly evaporated from chloroform. Based on results and the SEM images, a few of these diblock copolymers were used as ceramic precursors and pyrolyzed to elevated temperatures forming boron nitride and boron carbonitride nano-ordered ceramics

Two equalisation or reheating temperatures (1050°C and 1150°C) and three equalisation or reheating times (53 min, 318 min, and 1333 min) were applied to three V- and V-Ti- microalloyed steels, which contain a low carbon content (0.067 ~ 0.073 wt%) and high nitrogen content (0.017 ~ 0.021 wt%), by the simulated direct charging or conventional cold charging processes. The experimental results show that in directly charged V-microalloyed steels, MnS provides the main pinning effect but does not have a sufficient ability to prevent the austenite grains from growing during the equalisation. Because of the slow precipitation, AIN only precipitates at the longest equalisation time and is the main compound which has an obvious pinning effect on the austenite grain growth in V-microalloyed steels. Austenite grain growth appears more likely to be abnormal as a result of the direct charging than of the cold charging due to the precipitation of fine AIN particles during the phase transformations. The experimental results also show that the longer equalisation or reheating time in the furnace does not result in a significant change in the microstructures and in the austenite grain size, because of the precipitation of AIN during equalisation or reheating. However, in V-Ti-microalloyed steels, the existence of titanium can promote the precipitation of complex (Ti×V₁₋×)N particles during equalisation or reheating. In the cold charged V-Ti-N microalloyed steel, the fine austenite grains produced (<10μm) can be observed after the reheating and water-quenching due to the significant pinning effect of (Ti×V₁₋×)N. When the reheating time of V-Ti-N microalloyed steels at 1150°C is longer than 318 min, the coarsening of (Ti×V₁₋×)N precipitates occurs and leads to abnormal austenite grain growth. The TEM results also show that cruciform (Ti×V₁₋×)N particles can only be found indirectly charged steels but not in cold charged steels.

Group-III nitride materials are increasingly important, because of their semiconducting properties and bandgaps tuneable across a wide range from the infrared to ultraviolet. They are of particular interest for optoelectronic and power electronic applications. The studies on nitride materials are comprehensive, and one way to categorise them is based on the scale of the material, namely: (a) 3D bulk materials, for example the development of 3D bulk nitride substrate; (b) epitaxial layers, for example GaN/InGaN 2D quantum well based light emitting diodes (LEDs); (c) 1D nitride nanowires and (d) 0D quantum dots, for example InGaN quantum dot based single photon sources. This thesis uses a multimicroscopy concept to investigate various group-III nitride nanowires and hillocks. Multiple different microscopy techniques were applied to the same specific nanostructure or defect. This allows the properties of the materials of interest to be linked directly to the nanostructures or defects, providing a more complete picture of the samples that have been studied. The multiple microscopy techniques used to conduct the work in this thesis include (scanning) transmission electron microscopy ((S)TEM), cathodoluminescence (CL), focused ion beam (FIB) and atomic force microscopy (AFM). Specifically, AFM was used to characterise the morphology of the sample on a sub-nanometer scale. The crystalline structures were characterised using (S)TEM, and the in-situ energy dispersive X-ray spectroscopy (EDS) was used to conduct compositional analysis of the selected sites. CL was used to reveal the optoelectronic properties by analysing the emission wavelengths of the materials, excited by the electron beam. FIB was the technique used to prepare site-specific samples to be measured in (S)TEM. A detailed explanation of these characterisation techniques was also included. In the context of the studies on nitride materials, nitride nanowires and their heterostructures are a particular research focus. They combine the unique properties of III-nitride materials together with the advantages induced by the nanowire geometry. This thesis explores three different nanowire systems: a GaN nanowire structure incorporating a GaN/Sc$_x$Ga$_{1-x}$N axial heterostructure grown by molecular beam epitaxy (MBE); GaN/InGaN core-shell nanowires fabricated by a hybrid approach combining metalorganic vapour phase epitaxy (MOVPE) and dry etching techniques; and AlGaN nanowires on free standing AlGaN substrates fabricated by MBE and inductively coupled plasma (ICP) etching. The optoelectronic properties, compositions and structures of these nanowires were studied in detail. Moreover, a comprehensive review on the properties, growth methods and applications of group-III nitride nanowires is also included in this thesis. Apart from nanowires, a lot of effort has been focusing on the improvement of the quality of epitaxial layers of GaN and its alloys, and they currently have an even wider perspective than nitride nanowires. The understanding of defects within the epitaxial layers is crucial in order to mitigate the their adverse effects, leading to the increased emphasis on defect analysis. Hillocks are a type of defects found on GaN epilayers, which are less well studied than other defects such as dislocations and stacking faults. As a consequence, the formation mechanisms of hillocks remain controversial. In this context, after a review on the past studies on GaN hillocks, this thesis also investigates two types of hillocks, i.e. hillocks on GaN p-i-n diodes and hillocks on GaN grown on patterned sapphire substrates (PSS). Their nanoscale structures, properties and formation mechanisms are studied.