Studies are described of vacancy clusters, twin boundary structures and vacancy migration at twin boundaries. Much of the work involves atomistic computer simulation techniques based on a short ranged, central, 'non-equilibrium' pair potential representing copper. This potential is matched to the experimental lattice parameter, the elastic constants and the vacancy formation and intrinsic stacking fault energies. A geometrical-mathematical method is adopted to study the structures of vacancy clusters and their transformations (growth, contraction and migration). The implications of collapse of these clusters are also studied. The relative values of cluster migration energies are estimated from the number of super nearest neighbours (atoms which lie at a distance less than the first nearest neighbour, from a migrating self interstitial atom). The structures of some low index twin boundaries e. g. (111), (113), (112) and (120), are investigated and their low energy configurations are established. The (112) and (120) boundaries contain some coalesced planes (planes formed from the combination of two neighbouring planes parallel to the interface). The (111) and (113) boundaries do not reveal this unusual behaviour. The effect is interpreted in terms of atomic density of the interface plane. The simple and important (111) twin boundary is selected to study vacancy migration. Three distinct paths are chosen, (a) in the boundary, (b) parallel and adjacent to the boundary, and (c) between the boundary and its adjacent plane. For each of these cases the migration process involves the simulation of a model in which a self interstitial atom is moved in steps between the two sites of a divacancy. At each step the model is allowed to relax with the motion of interstitial restricted to a plane normal to the axis of the divacancy. In this way it is possible to deduce the actual migration path. Reductions of 2-6% are found in the vacancy migration energy.

This thesis investigates the early stages of crystallisation in the bulk, at surfaces and in pores. The main motivation is to achieve a greater understanding of the crystallisation of proteins in the presence of disordered porous media, although this research is applicable to a wide range of research fields. Metropolis Monte Carlo simulation is used in conjunction with techniques such as forward flux sampling to measure the rate of nucleation of crystals. Initially, the 2d Ising model is used to simulate nucleation in rectangular pores with one open end. By calculating the rate of nucleation from pores with different pore widths we show that the overall rate of nucleation peaks at a certain pore width. To investigate effects from crystallisation we used a simple system of spherical particles interacting via the Lennard-Jones potential. Our typical conditions of study are below the triple point temperature where the equilibrium phase is crystal. As a pre-requisite to investigating crystallisation in pores we studied bulk transitions and surface transitions at planar surfaces. We find that, starting from a bulk supersaturated vapour, crystallisation occurs via two distinct steps. In the first step, a liquid droplet nucleates from the vapour. Then, the crystal phase nucleates in the liquid droplet, provided that this liquid droplet exceeds a minimum size. Since it has been shown that similar sized crystallites have lower free energies in highly defected structures such as icosahedral and decahedral ordering we investigated the role of defects in our- nucleated crystallites. Crystallites with defects patterns similar to those seen in decahedral ordering are found to have lower relative free energies than those with fcc ordering. We also observe non-equilibrium effect which we suggest are due to the slow dynamics associated with defect formation. At a planar surface crystallisation is studied from liquid phase. We find that the surface behaviour, such as freezing and pre-freezing transitions, directly effects the nature of crystallisation. Far below freezing nucleation is well described by Classical Nucleation Theory but as the surface-particle interaction becomes more attractive the nuclei become more 2 dimensional. At surfaces with a strongly attractive surface-particle interaction, the formation of a surface crystal phase removes the barriers to bulk crystallisation. Whilst crystallisation behaviour in the bulk and at a planar surface appears qualitatively similar to that of liquid nucleation, this thesis shows that crystal nucleation in a wedge shaped pore is drastically different. Due to intrinsic angles in the crystal lattice, crystallisation is enhanced at angles that allow a defect-free unstrained piece of crystal to fit into the wedge. At other angles the formation of defects and strain causes a decrease in the rate of crystallisation. Our simulation using both the Ising and Lennard-Jones model show that crystallisation is enhanced from pores with a particular size and shape. Therefore, disordered porous media with a wide distribution of pore sizes and shapes may be needed as there is a greater chance that they will contain pores with the required geometry.

X-ray diffraction methods have been employed to determine order parameters in polymeric materials (mainly polyethylene). Two types of order have been defined; structural order, (i.e. crystallinity, unit cell dimensions, crystallite size and perfection) and alignment order (preferred orientation). Existing methods of analysis have been examined, modified and compared for a range of polymer types. The effect of molecular weight, molecular weight distribution, branching and conditions of crystallisation have been shown to alter the order parameters. The parameters of structural order have been examined over a range of temperatures up to the melting point of the polymers. These reveal changes in order due to recrystallisation above about 110°C, and a further change a few degrees below the melting point. Computer data processing techniques have been developed to produce pole figure plots and to determine orientation functions. These enable both qualitative and quantitative characterisation of preferred orientation in deformed and processed material. (310) twinning has been identified in both cold drawn and rolled polyethylene. (110) twinning is observed in polyethylene sheet after rolling, annealing and then further drawing. The operative twinning mode has been explained in terms of the fold geometry of the crystalline regions. Cold rolled polyoxymethylene has been shown to undergo a phase transformation from the hexagonal to orthorhombic phase. Films of polyethylene produced by the commercial process of extrusion blow moulding have been examined. The orientation results from a stress crystallisation procedure and the actual orientation may be related to the machine variable. The molecular chain axis distribution is defined by means of the orientation functions. These enable a quantitative correlation of the orientation with film properties to be made.

No description available.

The key aim of this Thesis is the development and implementation of a set of simulation techniques for LCs capable of tackling mesoscopic phenomena. In this, we concentrate only on mesh-free particle numerical techniques. Two broad approaches are used, namely bottom-up and top-down. While adopting the bottom-up approach, we employ the DPD method as a foundation for devising a novel LC simulation technique. In this, we associate a traceless symmetric order tensor, Q, with each DPD particle. We then further extend the DPD forces to directly incorporate the Q tensor description so as to recover a more complete representation of LC behaviour. The devised model is verified against a number of qualitative examples and applied to the simulation of colloidal particles immersed in a nematic LC. We also discuss advantages of the developed model for simulation of dynamic mesoscopic LC phenomena. In the top-down approach, we utilise recently emergent numerical mesh-free methods. Specifically, we use the SPH method and its variants. The developed method includes hydrodynamics, variable order parameter and external electric and magnetic fields. The developed technique is validated against a number of analytical and numerical solutions. Subsequently, we apply our top-down methods to the simulation of the complex 3D post-aligned bistable nematic (PABN) device. This includes a smooth geometry representation in order to fully exploit the developed mesh-free numerical techniques. We study both the static and dynamic behaviours of the PABN device for a number of distinct post shapes.

Crystal polymorphism, where a molecule forms several different crystal lattices, is common, and often needs to be controlled. For example, crystalline drugs must be manufactured as one specified polymorph, so polymorph purity is essential to the pharmaceutical industry. This thesis is a quantitative study of the crystallization of glycine from aqueous solution, which focuses particularly on polymorphism. Crystallization is observed within a 96-well microplate, where each well is filled with 0.1 mL of supersaturated solution. We address the difficulty of obtaining reproducible nucleation data. This problem is difficult because induction times are extremely sensitive to factors such as how the crystallizing system is prepared, and small variations in the supersaturation. The appropriate statistical tests needed to show reproducibility are discussed. Glycine has two common polymorphs, alpha and gamma, the competition between these polymorphs is studied. We obtain data at multiple NaCl concentrations. Addition of NaCl is known to favour nucleation of the gamma polymorph. The polymorph of crystals are individually identified in-situ using Raman spectroscopy. At high salt concentrations, nucleation kinetics of the alpha and gamma polymorphs are qualitatively different. The gamma polymorph behaves like the hare in Aesop's story of the tortoise and the hare: Nucleation start off rapidly, but slows, while for the alpha polymorph, nucleation starts off slow but at later times almost overtakes that of the gamma polymorph. The opposite time dependencies of the nucleation of the competing polymorphs, allows optimisation of polymorph purity using time-dependent supersaturation. Growth of the two polymorphs is analysed. The alpha polymorph is observed to grow faster than the gamma polymorph. Growth rates were variable, so they were also analysed in relation to induction times and crystal habits. We show that crystals with long induction times tend to be needle-like, and needle-like morphologies tend to grow faster than non needle-like morphologies.

No description available.

The performance of DASH has been evaluated against powder X-ray diffraction data collected from 101 molecular crystal structures, representing the most comprehensive testing of a "structure determination from powder diffraction data" (SDPD) program carried out to date. These 101 structures cover a broad range of molecular complexities, from very simple (6 degrees of freedom) to very challenging (49 degrees of freedom). 95 of the crystal structures could be solved with the current version of DASH, going some way to explaining why the parameterisation of its simulated annealing (SA) algorithm has not been altered since the launch of the program in 1999. This thesis explores optimisation of key DASH SA parameters using the program irace. The irace runs, comprising 255,000 individual DASH runs and requiring approximately 1300 CPU days of compute time, produced six sets of SA parameters which differed greatly from the DASH default parameters and which markedly improved the performance of DASH. Further evaluation of these six sets against all 101 compounds (a further 2874 of days of CPU time), allowed selection of one best-performing set, which delivered an order of magnitude improvement in the success rate with which crystal structures were solved. The adoption of these parameter values as the defaults in future releases of DASH is strongly recommended and is expected to broaden the range of molecular complexities to which the program can be applied. Three distinct approaches to further improving DASH performance, based on introducing prior conformational knowledge derived from the Cambridge Structural Database (CSD), have also been assesed. The findings show that inclusion of conformational knowledge brings significant additional gains in SDPD performance, and that existing implementations of these approaches in the DASH / CSD System are close to being ready for routine use.

The unique combination of physical properties displayed by the arrangement of carbon atoms recognised as diamond has fascinated scientists for hundreds of years. The electronic properties in particular are significantly superior to conventional semiconducting materials in most instances. Sadly, the currently limited supply of device quality single crystal material makes commercial production of diamond based electronic components unviable at present. Nevertheless, recent advances in the production of synthetic diamond via chemical vapour deposition (CVD) have stimulated great interest in this as a source of electronic grade material. However, in optimising the material for device applications detailed information is often required concerning the properties of the localised defect states located within the energy gap. Although there is currently significant data concerning the mid-gap states, little has been published with respect to the shallower centres that are often responsible for limiting carrier drift mobility and associated electronic properties. This study presents the first detailed exploration of the presence and influence of such shallow states from the analysis of transient photoconductivity (TPC) measurements. It is demonstrated that such procedures provide a powerful method for the study of the localised states in CVD diamond and related materials. In particular, measurements show that even in very good quality CVD diamond films, an energetically broad but structured collection of localised states exists. The energy distributions of these localised states have been calculated by employing techniques developed, refined and proven in prior studies of disordered semiconductors. During the course of the TPC studies it was observed that specimens exposed to an applied field can accumulate a significant space-charge related residual field. Clearly, this could have a considerable impact to the operation of many potential devices. Consequently, a new variant of the TPC system was developed to allow the decay of this field to be monitored as a function of time following the removal of the applied voltage. Such data provide further information concerning the localised state distribution in which the space-charge is trapped, and are demonstrated to be at least qualitatively consistent with the distributions calculated from the TPC measurements.

A number of chelates of Al(III), Cr(III), Mn(II), Mn(III), Fe(III), Co(II), Co(III), Ni(II) and Cu(II) with the ligands tropolone, 3-bromotropolone and 4-methyltropolone, have been prepared. Some of these chelates have not previously been reported. Using visible, ultraviolet, infrared and mass spectroscopic techniques, the general spectroscopic properties of these chelates have been studied. The visible spectra of the Cr(III) chelates shows that the crystal field splitting caused by the ligands used here, is of the same order as that of acetylacetone. The IE spectra indicate that associated tropolonate chelates exhibit characteristic absorption bands in the 1590 cm[-1] region. Absorption bands also exist in the region 800 cm[-1] 900 cm[-1], which are characteristic of the coordinated ligands. The mass spectra show that fragmentation is predominantly due to loss of ligand molecules, but that significant proportions of fragments derived from the ligands, occur in association with the metal to give, in particular, metal phenolate and metal cyclopentadienyl entities. Two of the chelates were chosen for a more detailed structural investigation by single crystal, X-ray crystallographic analysis. The hydrated complex of nickel(II) with tropolone is dimeric and the hydrated complex of cobalt(II) with tropolone is a tetramer. These chelates are associated through tropolone oxygen atom bridges, and represent the first reported oxygen bridged structures with tropolone. It is apparent that the compact structure of the tropolone ligand is an important factor contributing toward their stability. The formation of a hydrogen bonded lattice with both of these chelates is facilitated by the presence of terminal water molecules, and by the compactness of tropolone. The spectroscopic data, together with the results obtained from the X-ray analyses, has been of further value toward proposing the structures of some other of the chelates studied.