published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / abstract / Physics / Master / Master of Philosophy

Dynamic abnormal grain growth (DAGG) is a phenomenon that produces abnormal grain growth at elevated temperatures during plastic deformation. It is distinct from classically studied static abnormal grain growth phenomena in that it only occurs during plastic deformation. Previous investigations of DAGG in a Mo sheet material produced using powder metallurgy techniques observed DAGG grains to grow more rapidly near the sheet surface than near the sheet center. This phenomenon is explored in the present study. A Mo sheet material produced using arc melting techniques is also studied to determine the morphology of DAGG grains. A preference for growth near the sheet center is observed in this material. The through-thickness variations in texture and grain size for both the arc-melted and powder-metallurgy Mo sheet materials are investigated. The preference for growth near the surface in the powder-metallurgy material is due to a through-thickness variation in grain size, with smaller grains near the surface and larger grains near the center. The preference for DAGG grain growth at the center of the arc-melted sheet material is because of very large grains that grow near the sheet surface. These large grains may be the product of multiple abnormal grains occurring near the sheet surface because of texture variation through the sheet thickness. Regardless, the DAGG grain cannot consume these large grains and leaves them as island grains decorating the region near the sheet surface. These results suggest that DAGG is driven primarily by grain boundary curvature. Microstructures that include DAGG grains are investigated with electron backscatter diffraction (EBSD). A new method to evaluate geometrically necessary dislocation densities using EBSD data is derived. DAGG grains are relatively undeformed compared to the polycrystalline microstructure. DAGG grains are not oriented either favorably or unfavorably for slip. Results of the analysis of the grain boundaries between DAGG grains and normal grains do not indicate any special character preference for these grain boundaries. / text

This thesis contains two main parts: the first part focusses on an electron diffraction study on unsupported antimony (Sb) clusters, while in the second part the design and development of a time-of-flight mass spectrometer (TOFMS) is discussed. Electron diffraction is an ideal tool to study the structure of clusters entrained in a beam. The main advantage of this technique is the ability to study the clusters in situ and in an interaction-free environment. It is therefore not necessary to remove the particles from the vacuum system which would lead to oxidation. Since the particles do not have to be deposited on a sample for further investigation, there is also no substrate which could influence the cluster structure. An additional advantage is the short exposure to the electron beam, thereby minimising the likelihood of damaging the particles. Sb clusters were produced using an inert-gas aggregation source. To control the cluster properties the source temperature, pressure and type of cooling gas can be adjusted. In the range of source parameters tested, Sb clusters with three different structures were observed: a crystalline structure corresponding to the rhombohedral structure of bulk Sb, an amorphous structure equivalent to the structure of amorphous Sb thin films, and a structure with the same diffraction signature as Sb4 (Sb evaporates mainly as Sb4). This last structure was found to belong to large particles consisting of randomly oriented Sb4 units. In order to study the size distributions and morphologies of the Sb clusters, the clusters were deposited onto substrates and studied under an electron microscope. The crystalline particles showed a wide variety of strongly faceted shapes. Depending on source conditions, the average cluster diameters ranged from 15 to 130 nm. There was a considerable disagreement between these values and the size estimates from the diffraction results with the latter being smaller by an order of magnitude. This might be due to the existence of domains inside the clusters. The amorphous particles were all found to be spherical with mean sizes between 27 and 45 nm. The Sb4 particles showed a liquid-like morphology and tended to coalesce easily. Their sizes ranged from 18 to 35 nm. To obtain an independent method for determining the cluster size, a TOFMS was designed and developed in collaboration with Dr Bernhard Kaiser. However, the TOFMS failed to detect a cluster signal in the original set-up which is most likely due to a defective ioniser and underestimated cluster energies. Further tests were performed in a new vacuum system and mass spectra for palladium clusters were successfully recorded.

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This dissertation includes three separate studies of related waveguide grating phenomena. These studies deal with a numerical improvement of the integral method of diffraction grating theory, the theoretical analysis of waveguide gratings, and fabrication techniques for photoresist grating masks. The first topic addresses the acceleration of the convergence of the integral kernels. To improve the performance of the integral method for calculating diffraction grating efficiencies, the convergence of the integral kernels is studied. A nonlinear sequence transformation, Levin's u-transformation, is successfully applied to accelerate the convergence of the integral kernels. The computer execution time saving is significant. The application details and many numerical examples are given. The second subject is the ray optics theory of waveguide grating analysis. To establish a linkage between the analysis of diffraction gratings and the analysis of waveguide gratings, a new rigorous ray optics theory is developed. It takes into account phase changes on diffraction, multiple diffraction processes, depletion of the incident guided wave, and lateral shifts. A general characteristic equation that determines the waveguide grating attenuation (coupling) coefficient is derived. The symmetry properties of grating diffraction are applied to waveguide grating analysis for the first time. Lateral shifts of optical rays at a periodically corrugated interface similar to the Goos-Haenchen shift at a planar interface are suggested. The third subject is the in situ control of the development of photoresist grating masks. The existing method for monitoring and modeling photoresist grating development are modified and extended to monitoring and modeling photoresist grating mask development. Experimental examples, detailed theoretical considerations, and computer simulations are presented.