Crystallization processes are extensively utilized for the purification and separation of a variety of final and intermediate products. Despite its widespread use, in the fine chemicals, pharmaceutical and food industries, crystallization still has many unknowns which can affect the purity, quality and quantity of the crystals produced. As a result, monitoring crystallization processes in order to better understand the underlying science is of utmost importance. To this end, various tools have been developed. They are, however, not without their flaws. The work in this PhD project aims to develop and demonstrate the viability of process video imaging as a reliable methodology for the monitoring, recording and analysis of crystallization processes which either minimizes or removes these flaws altogether. Experimental investigations to test the robustness of the process video imaging focused on three commonly found crystallization processes: melt crystallization in the form of clathrate hydrates, anti-solvent crystallization and cooling crystallization. The results from these investigations were compared to previously reported findings of similar systems in order to examine the degree of accuracy of the process video imaging tool. The significant findings from this work are that the process video imaging can be successfully implemented into melt, anti-solvent and cooling crystallization systems with relative ease. This allowed key crystallization parameters to be determined; including but not limited to: solubility, metastable zone width, crystallization rate and growth kinetics.

No description available.

No description available.

Absorption spectra of tetragonal single crystals of Cs₃MnCl₅ have been measured at 4.2K over the wavelength range 5200-2600Å. Spin forbidden bands of low oscillator strengths were observed for the ⁴Γ ← ⁶A₁ transitions of the MnCl₄^2- chromophore. All the intensity arose from the electric dipole mechanism, but for small magnetic dipole contributions to the intensities of ⁴E(G) and ⁴T₂(F). Magnetic circular dichroism spectra measured at 4.2K in a field of 4.75 Teslas were dominated by C-term signals. The band intensities were assumed to arise via the spin orbit interaction, allowing "intensity borrowing" from higher energy charge transfer transitions. A model based on tetrahedral symmetry for the chromophore ion allowed the calculation of MCD:absorption intensity ratios for the quartet states. Good agreement was found with the experimental values for most quartets. Crystal Field calculations were performed in support of the MCD-based band assignments. Covalency was introduced into Stout's electrostatic matrices via the Koide and Pryce parameter andepsilon;. The following parameter values were found: B = 600cm^-1 , C = 2940cm^-1, Dq = 350cm^-1, and ε = 0.03, giving the correct ordering of the states and an rms. deviation from the observed energies of 520cm^-1. ⁴A₂(F) was placed above ⁴T₁(F) in energy, a departure from the "conventional" ⁴F term assignment. A simplified model for the perturbation of the cubic states by the tetragonal component of the crystal field was developed to account for the observed linear polarisations. The spectra were examined in detail to identify the electronic origins and assign them using a tetrahedral or tetragonal scheme. The latter appeared to be the more realistic model for most states. Assisted by data from the Raman spectrum, the vibrational sidebands were also assigned, When very sharp origin lines were present, some of the sidebands showed small splittings attributable to the presence of the two chlorine isotopes.

This thesis describes the application of the well established technique of X-ray diffraction topography to a variety of problems, and includes considerations of the optimum conditions for taking rapid topographs. Chapter I contains a brief review of the subject together with an indication of the range of applicability. Several modifications of X-ray topography exist and several are briefly described to illustrate the principles and mechanisms of image formation. Contrast is formed in one or both of two ways. Regions of crystal may be so badly misoriented from the bulk, that no beams from the source can satisfy the reflection condition formulated by Bragg. This type of contrast is known as orientation contrast. The second type of contrast arises from the point to point lattice displacements giving a different reflecting power around the defect. This second type of contrast, termed extinction contrast, is dependent on the perfect crystal diffraction. A description of the different types of extinction contrast is given in terms of the dynamical theory of diffraction in a perfect crystal and the significance of the direct, intermediate, and dynamical images stressed. The chapter concludes by sketching some of the important fields of application. The second chapter is concerned with the relative merits of scanning and wide beam X-ray topography. A description of the technique of wide beam topography using the Kβ line is given and it is demonstrated that the resolution is as good as that using Lang's scanning technique. A new wide beam method using the tungsten Landalpha;₁ line is proposed and demonstrated. From expressions derived for the exposure times, it is seen that the important parameter to maximise is the power per unit horizontal length of the X-ray source. The speeds of the two techniques are compared for a variety of generators and the advantage of using an Elliott GX6 is made clear. A direct imaging system is clearly preferable in principle to film recording and Chapter III is devoted to a discussion of such systems. Following a review of previously developed imaging systems, a new method of directly displaying X-ray images using a channel plate is demonstrated. The remainder of the chapter gives detailed calculations of the efficiency of a channel plate to X-rays in the wavelength region 0.5-2 andAring;. Using published data, the efficiency is calculated for several cases and compared with previous experimental work. To better than an order of magnitude, these efficiencies may be used to predict the intensity of the image produced on the image converter screen. Chapter IV falls into two sections. The first describes an experiment to determine the minimum thickness at which dislocations are visible in crystals using normal topographic techniques. It was found that the value was dependent on the fraction of the material thickness taken up by misoriented material. This explanation was found to be in qualitative agreement with experiment, the minimum thickness fell from O.4 of an extinction distance for the low order reflections to 0.2 of an extinction distance for high order reflections. The second part consists of a comparison between experimental image profiles and those simulated on a computer. Quite reasonable agreement was found and the sources of error in the method of computing and ways to avoid them are discussed in terms of practical and basic limitations. The remainder of this thesis describes application of X-ray topography to four different types of material. Chapter V is devoted to a study of defects in a silicon slice following device fabrication. Interest in the effect of crystallographic defects on the performance of integrated circuit devices has led to a large number of investigations over the last decade and these are reviewed and their conclusions summarised. An attempt is made to measure the stress at the junction edge, produced by the mismatch in the ionic radii between dopant and matrix. As a result of approximations in the theory and measurements, the force per unit length of junction, measured to be 2 andtimes; 10⁴ dynes/cm, must be taken as order of magnitude only. Similarly, only an order of magnitude estimate of the stress is permissible. There follows a description of a combined Scanning Electron Microscope and X-ray topographic experiment to determine the cause of breakdown fingers, sometimes seen at the junction edges in the beam induced conductivity mode in the S.E.M. It is concluded that they are due to scratches in the oxide mask prior to diffusion. The rest of the chapter is given to a detailed description of dislocations and their contrast observed in a silicon slice heavily doped with boron. The slip behaviour is not analysed in detail, but seems to support the conclusion that this deformation, due to thermal shock, is independent of the doping. Interactions between 'emitter edge' dislocations are described and the anomalously narrow image widths of such dislocations explained by considering the effects of overlapping strain fields. Burgers vectors of dislocations inside the diffused regions are determined, and the contrast of these dislocations at high density is interpreted as an effect due to overlapping images. Interactions between the 'inside' and 'emitter edge' dislocations are described. In regions where the dislocation density was low, the dislocations exhibited a reversal of contrast on reversal of the diffraction vector. With the diffraction vector parallel to the dislocation line, the contrast was black-white, reversing with the diffraction vector. With the diffraction vector at 60° to the line, the contrast was either black or white, reversing with the diffraction vector. This effect is interpreted in terms of surface relaxation and computations performed using the Penning-Polder theory are in qualitative agreement with the experimental results. Chapter VI describes the application of X-ray topography to the study of defects in natural fluorite. Dislocations are identified with Burgers vectors parallel to andlang;101andrang; directions. These are nearly pure edge in character and the Burgers vector is presumed to be ¹andfrasl;₂ andlang;101andrang;. A direct correspondence is observed between bundles of dislocations and regions of birefringence contrast. A uniform birefringence contrast in the matrix, which can not be explained in terms of dislocation stress, is also observed. This is explained in terms of impurity atoms, sited between {111} planes during growth, distorting the crystal normal to the {111} faces. The boundaries between regions of birefringence lie along inclined andlang;110andrang; directions and on this model, no strain is associated with these boundaries, in agreement with the X-ray topographic evidence. Extensive planar faults, lying on {111} planes and with fault vector a non integral value of the lattice spacing, are postulated to be thin lamellae of material containing markedly different impurity concentration. In Chapter VII, some vapour grown layer compounds are studied. Individual dislocations are resolved in SnSe₂, TiSe₂, SnS₂, TiS₂, ZrS₂ and HfS₂. The Burgers vectors of the dislocations in SnS₂ were determined and their origins discussed. Large area stacking faults bounded by ¹andfrasl;₃ andlang;1 1</span>00andrang; partial dislocations are also observed in these specimens. Dislocation and defect configurations in the other compounds are described and a modified divergent beam method for taking topographs of bent crystals is demonstrated. Some preliminary experiments on pure iron are described in Chapter VIII. Contrast is observed suggestive of dislocation helices and rows of loops in these strain-anneal grown crystals. Arrays of dislocations of pure edge type and Burgers vector parallel to [100] are interpreted as being contained in a low angle boundary wall, following Futagami. The tilt angle across such boundaries is measured to be a few seconds of arc. Finally, Chapter IX suggests a few directions for further investigation.

No description available.

No description available.

This thesis is concerned with the structure determination of 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP⁺ oxidoreductase (decarboxylating), E.C.1.1.1.44) from sheep liver. This protein is the second enzyme in the pentose phosphate pathway and catalyses the conversion of 6-phosphogluconate to ribulose-5-phosphate and CO₂. Chapter 1 gives an introduction to the chemical properties of 6-phosphogluconate dehydrogenase (6PGDH) and compares these properties for the enzyme from a variety of sources. The common structural features of the NAD⁺-dependent dehydrogenases are discussed in terms of divergent and convergent evolution. Details of an NADP⁺-dependent enzyme, E.coli dihydrofolate reductase are also given. This is the only published high resolution structure of an NADP⁺-dependent enzyme to date (Matthews et al. (1977)). The crystallographic techniques used in the course of this work are discussed in Chapter 2. Particular attention is paid to the application of synchrotron radiation in protein crystallography in optimising heavy atom anomalous scattering. Appendix I also gives some experimental details relevant to this. Chapters 3, 4, and 5 give details of the structural results. In Chapter 3 the low resolution (6 andAring;) structure is presented with a full analysis of the derivatives (KAu(CN)₂ and K₂Pt(CN)₄) used in the double derivative phase determination. The 6 andAring; electron density map could be interpreted. It showed the overall shape of the monomer and enabled the dimer axis to be identified. It also yielded the structurally interesting features of several long columns of density of the correct cross section for andalpha;-helix and also a cleft region. Chapter 4 is concerned with the determination of the coenzyme (NADP⁺) binding position at 6 andAring; resolution. Difference electron density maps calculated from X-ray measurements on crystals soaked in the oxidised coenzyme indicated the binding of NADP⁺ in the cleft region. Also, by using an iodinated derivative of the coenzyme, iodopyridine adenine dinucleotide phosphate (IADP⁺), it was possible to identify the nicotinamide moiety of the coenzyme and thus orient the coenzyme with respect to the enzyme. Chapter 5 describes the screenless rotation method and its application to the 2.6 andAring; crystallographic study of 6PGDH. In particular, the collection of the data for the native enzyme crystals and the K₂Pt(CN)₄ derivative. The phases calculated at 6 andAring; with two derivatives (as described in andsect;3) were extended to 2.7 andAring; by calculating phases, using the photographic data, by the method of single isomorphous replacement with anomalous scattering. This enabled an electron density map to be calculated to 2.7 andAring;. Preliminary details of the quality of this electron density map are given.

This thesis presents the results and discussions of an investigation into nucleation mechanisms in a cooling crystallization of sodium chlorate in both a stirred tank crystallizer (STC) and an oscillatory baffled crystallizer (OBC) under various crystallizer configurations and operational conditions. The key question to be addressed was why nucleation took place in an OBC without seeds, while seeds were essential in an STC for the same chemistry and at the same process conditions. Various hypotheses have been initiated, tested and verified in both primary and secondary nucleation experiments, and new scientific insights and better understanding have been achieved on the parameters that have influenced the nucleation mechanisms and some explanations as to why seeds were not necessary in the OBC are put forward. For the seeded nucleation, the fluid dynamic environment and mixing mechanics were responsible for dictating the nature of the nucleation mechanism. The unique scraping action of the baffles against the crystallizer wall in the OBC enabled a different enantiomorphism of the product crystals compared to the seed crystal. Removing such a motion in the OBC provided product crystals similar to that seen in the STC. The degree and the means of mixing near the single seed crystal also affected the crystal handedness. For the primary nucleation experiments, it was found that the handedness of the product crystals in the STC was strongly orientated towards a single enantiomorph, while both enantiomorphs were formed in the OBC. The results were suggestive of a lower free energy barrier in the OBC, in turn resulting in primary nucleation being favoured in the OBC as opposed to rapid secondary nucleation being dominant in the STC.

With the increasing capacity requirements of telecommunication systems, the ability to increase capacity density is of great importance for optical transmission technologies. This thesis presents several integrated photonic devices (semiconductor laser diodes and silicon devices) aimed at boosting the capacity density of the optical transmission systems. The first part of the thesis is about four wave mixing (FWM) effects in semiconductor ring lasers. Mode beating via third order nonlinearity in semiconductor ring lasers has been analysed using a frequency-domain multi-mode rate equation model. Compared with Fabry-Perot lasers, semiconductor ring lasers are found to be 1.33, 2, and 4 times more efficient in self-gain compression, cross-gain compression and four-wave mixing processes, respectively, due to its travelling-wave nature. It is shown that, using dual (pump and signal) external optical injections into the ring laser cavity, multiple modes can be locked in phase via the strong four wave mixing phenomenon. This results in modulation of the light wave at the mode beating frequencies which could be used for RF optical catrier generation. Secondly, following Bristol's research on compact optical vortex beam emitter based on silicon photonic micro-ring resonators, a different approach is demonstrated to simultaneously generate a pair of orbital angular momentum (OAM) modes with opposite topological charge by integrating a micro-ring OAM resonator with simple waveguide devices. The relative phase between two vortices can be actively modulated on the chip by thermo-optical controls. Furthermore, based on the ring cavity structure, OAM ring lasers on AlGaInAs/InP wafer are also developed. Detailed designs, fabrication processes and characterization of the device are discussed. In the last part of the thesis, a new approach is to propose and demonstrate directly generated optical OAM beams, by integrating a micro-scale spiral phase plate (SPP) on top of a vertical-cavity surface-emitting laser (VCSEL). The presence of the multi-level SPP transforms the linearly polarized Gaussian beam to a beam carrying specific OAM modes and their superposition states. The emitted OAM beams are characterized by usmg a spatial light modulator (SLM), and show good agreement with semi-analytical numerical simulation. The innovative OAM emitter opens a new horizon in the field of OAM-based optical and quantum communications, especially for low-cost short reach interconnects.