Acetyltriphenylgermane, (C₆H₅)₃GeCOCH₃, crystallizes in the monoclinic system with a = 15.30, b = 14.53, c = 7.68Å, and β = 94.8°. The space group is P2₁/c and there are four molecules per unit cell, thus each molecule forms an asymmetric unit in the cell. The intensities of 2537 reflections were measured by means of a scintillation counter using CuKα radiation. The structure was determined by heavy atom Patterson and Fourier synthesis and refinement was by least-squares methods. The final discrepancy, R, for 1834 observed reflections is 0.075. The compound was found to be tetrahedral about the germanium atom, with only small deviations caused by the spreading of the phenyl rings. The phenyl rings are planar with a mean C-C bond distance of 1.383 Å, mean C-C-C bond angle of 120.0°, and a mean C-H bond distance of 1.09 Å. Intermolecular interaction causes one ring to deviate from a symmetric propeller orientation. Two Ge-C bond distances were found: Ge-Cphenyl distance of 1.945 Å and Ge-Cacetyl distance of 2.011 Å. The longer Ge-C bond is attributed to contribution from a resonance structure in which there is no formal bond between germanium and the carbonyl carbon, resulting in a partial negative charge on the oxygen and a partial positive charge on the germanium. This is supported by the electronegativity difference between carbon and germanium. The C=O bond distance is 1.20 Å. / Science, Faculty of / Chemistry, Department of / Graduate

We envisage a fundamental study of the physical mechanisms (dislocation slip versus deformation twinning) involved in plastic deformation of hexagonal close-packed (HCP) metals like titanium and magnesium. A novel combination of X-ray imaging and diffraction techniques, termed X-ray diffraction contrast tomography (DCT), will be used to investigate details of the deformation process in the bulk of polycrystalline specimen. DCT provides access to the position, 3D shape, (average) orientation and elastic strain tensor of grains in polycrystalline sample volumes containing up to 1000 grains and more. Ultimately, an extension of the X-ray DCT technique is associated with a section topography methodology on the same instrument. This combination enables the measurement of local orientation and elastic strain tensors inside selected bulk grains. A very preliminary study of this approach is carried out on a magnesium alloy, underlying the current limitations and possible improvements of such approach. In this thesis, the data acquisition and analysis procedures required for this type of combined characterisation approach have been developed. The work is supported by the use of neutron diffraction, for an in-situ loading experiments, and two-dimensional electron backscatter diffraction (EBSD), for the initial microstructure of the materials and cross-validation of the results obtained with the X-ray DCT technique.

Unsteady shock waves are formed by the coalescence of pressure waves. The attenuationof pressure and shock waves in general is of great importance in a wide varietyof application such as vehicle performance, health and safety. Previous researcheshave been carried out on a variety of geometries to understand the physics. Theaim of this project is to advance the previous-state-of-the-art and to shed furtherlight into the fundamental physics associated with the shock wave interactions andphenomena. Shock wave attenuation was studied by using rough walls in a three-pipe system. The roughness at the walls is added by placing grooves on the upper and lower wallsof the junctions. The angles of the branch pipe were varied from 30 to 150 degrees. Shock wave interactions with a co-flow jet were also examined. All the experimentswere performed for driver gas (air) pressures of 4, 8, and 12 bar and atmosphericpressure within the driven section, giving theoretical Mach number of 1.34, 1.54, and1.66, respectively. Three different velocities, 114, 138, and 178 m/s, were used forthe co-flow jet. High-speed schlieren photography, particle image velocimetry (PIV),and pressure measurements techniques were employed to visualise and quantify theflow field. Expansion and compression waves produced by the grooves led to a highly unsteadyflow field, an increase to the pressure upstream, and the formation of asecondary shock wave. The pressure of the incident shock front was reduced by anestimated 20%. A maximum of 10% reduction of velocity of the shock front at theexit was achieved. The shock vortex/ structure led to multiple reflections, distortionof the vortical field, a lambda-shock configuration and pressure fluctuations. Theinfluence of the co-flow jet dissipated the shock/vortex structure, and attenuatedthe pressure peaks caused by multiple reflections. Complementing this investigation the testing of pressure sensitive paints (PSP)for the use of unsteady and high speed flows was carried out. The results showedthat the use of luminophores with high intensity output, and pressure sensitivityapplied on a porous material were the most suitable PSPs for these applications.

The history of diffraction theory, exact frequency domain solutions and selected past time domain solutions are briefly reviewed. Exact time domain solutions for scattering of plane, cylindrical and spherical acoustic waves by a half plane are derived by inverse Fourier transforming the frequency domain integral solutions. The solutions consist of two diffraction terms, a reflected term and a direct term. The diffracting edge induces step function discontinuities in the direct and reflected terms at two shadow boundaries. At each boundary, the associated diffraction term reaches a maximum amplitude of half the geometrical optics term and has a signum function discontinuity, so that the total field remains continuous. A physical interpretation is developed in terms of Huygen's principle near the diffracting edge. Solutions for practical point source configurations are evaluated by numerically convolving the impulse diffraction responses with a wavelet. The numerical problems presented by convolution with a singular, truncated operator are solved by analytically derived correction techniques, which are favourably compared to those used by earlier authors. The diffraction solution collapses into a compact discretized formulation. The half plane is shown to be a limiting form of wedge solutions, which can thus be computed using similar algorithms. Two zero offset sections are produced and compared to approximate Kirchhoff integral solutions. The exact diffraction hyperbola is noticeably non-symmetric, with higher amplitudes on the reflector side of the edge. Near the apex of the hyperbola the Kirchhoff solution is nearly equivalent to the exact diffraction term symmetric in amplitude about the reflection shadow boundary but fails to describe the other, low amplitude, term equivalent to half the response of a line scatterer. The differences are more noticeable on the flanks of the hyperbola, where the two terms are comparable in amplitude, and at shallow depths, due to an aperture effect. Increasing either the depth of the edge or the angle of the seismic line to the normal to the edge results in a flatter diffraction hyperbola showing little amplitude variation with moveout. As the seismic line becomes parallel to the edge the diffraction curve becomes flat and is indistinguishable from a reflection event. At great depth diffraction events may be mistaken for reflection events as well. Examples of CDP and CSP gathers, when compared to the Common Offset (CO) gathers, demonstrate that CO gathers are optimal for diffraction processing. Also, since the diffraction moveout and reflection moveout curves differ widely except for depth points near the edge, normal moveout stacking will distort the diffractions and diffraction stacking is essential to retain diffraction information. Strips of varying width are modelled by superposition of half planes to demonstrate resolution effects and show that the limit of a strip is a line scatterer. A dipping strip and an offset half plane model are produced and added for later comparison with wedge solutions. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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