No description available.

No description available.

The crystallization kinetics of hydronium jarosite have been studied in the area of the Fe₂O₃-SO₃-H₂O phase diagram where hematite is the stable phase. Hydronium jarosite has been shown to be a kinetically favored intermediate to hematite over a wide range of chemical and thermal conditions. A model useful for predicting the crystal size distribution as a function of temperature, free acid and iron concentrations, and residence time has been developed. Hydronium, sodium, and potassium jarosite have been shown to convert, via a solid phase reaction, to hematite. A model useful for predicting the conversion rate as a function of temperature, free acid concentration, and particle size has been developed. A predictive model, the growing core model, has been developed. It is useful for predicting the crystal size distribution and the product split between hydronium jarosite and hematite when both crystallization and conversion are occurring simultaneously. The cardinal assumption in this model is that crystal growth and conversion occur at separate cites on the crystal surface simultaneously. The model, with only one adjustable constant, has been verified with experimental results. The effect of double draw off (DDO) operation in this system has been demonstrated. It has been shown, via the growing core model and experimental results, that DDO operation can produce a high iron, low sulfur product. Without DDO operation, this high product quality can only be achieved through higher operating temperature, high neutralization rates, or very large vessel size.

This thesis describes the growth, X-ray diffraction assessment and tensile deformation properties of dislocation-free copper single crystals. As such it has been possible to conveniently section the work carried out into these three main areas within this thesis. Consequently, each chapter may be read almost independently of the others with references and further work suggestions being incorporated at the end of each chapter. This format, it is felt, does not disjoint the work: rather, it enables the central theme (i.e. the title of this thesis) to be developed in a much more continuously clear way than is normally apparent in a thesis where conclusions, further work suggestions and references are not drawn together until the end of the volume. Chapter I opens with a brief outline of the crystal growth methods which could have been used to produce low dislocation density (< 10⁶ cm/cm³) crystals by utilising the three fundamental phase transitions, i.e. solid to solid, vapour to solid and liquid to solid. The Czochralski method is then discussed in detail since it was used by the author to produce dislocation-free copper single crystals. The technological problems in obtaining such crystals are extensively enumerated and solutions presented. For instance, melt surface vibrations were eliminated by using a continuous flow of cooling water, by standing the complete crystal puller on a bed of foam, and by rigidly clamping the R.F. coil. It is emphasised that these technological problems must be solved before the scientific aspects of the growth of dislocation-free crystals can be studied. It is further shown that the author's modified crystal growing technique can be reliably used to grow dislocation-free crystals of copper each time, providing adequate care is taken over growth rates (1.2 cm/hr); specimen shape (a long thin "double-neck" must precede the required crystal to eliminate dislocations propagating from the seed and to act as a heat flow resistance); and crystal cooling rates (a long "tail" allowed the crystal to reach the ambient temperature slowly thereby minimising dislocation by thermal stresses and/or vacancy condensation). In the future, it is suggested, an automated crystal pulling system would be advantageous and a study of crystal growth in a synchrotron X-ray beam could be potentially definitive experiment on crystal growth. Chapter II looks at the theoretical aspects of Czochralski crystal growth with the aid of a new model which has been analysed on the Oxford University Computer. The model assumes a crystal-neck-seed configuration to be "sitting" on a liquid and examines the influence of geometric changes of the seed-neck-crystal and of radiation changes on the temperature gradients, primarily at the growth front. Four elements were chosen for study in this way: Si, Ge, Ag, Cu, i.e. four elements which have been grown dislocation-free. It was found that the seed size and shape played a small part in determining the interfacial temperature gradients, DT_c(0), of all the elements. The neck, however, could have a marked influence on DT_c(0) values in metals but not so much in semiconductor crystals. By geometric control alone it was found that the best way to reduce DT_c(0) values was to grow a large diameter crystal. The influence of radiation losses was found to be marked for semiconductor crystals but not for metal crystals. Finally, it is concluded that to minimise DT_c(0) values then the seed and neck must be long and thin and the crystal fat. These results fit in well with experimental knowledge. Further work to be carried out could consider the influence of a varying ambient temperature and convective heat losses on the interfacial temperature gradients. Chapter III is concerned with the interaction of X-rays with perfect crystals. The Lang-Borrmann X-ray topography technique is examined, and the experimental methods used to obtain X-ray topographs taken throughout this work are discussed. The major part of the chapter takes up the discussion of the theoretical interaction of X-rays with a perfect crystal set to diffract such X-rays. It is demonstrated that for a plane-wave incident on a cylindrical crystal, for the boundary condition to be satisfied the dispersion surface tie-points are displaced as the crystal traverses the incident X-ray beam. Thus the crystal wave-vectors no longer exactly satisfy the Bragg condition. This effect, was never unambiguously monitored experimentally because of the incident beam divergence. It is suggested that a future study could consider this problem in more detail. The Takagi-Taupin-Uragami generalised X-ray diffraction theory is reviewed and then used to calculate the intensity in the diffracted beam of a traverse topograph from a perfect cylindrical copper crystal. This computer simulation is also compared to an experimental condition. In both cases it is found that the Bragg surface of the crystal produces a very intense reflection whilst the remainder of the crystal gives a much reduced diffracted intensity in comparison. This asymmetric profile is interpreted in terms of absorption mechanisms which are strong at the centre of the crystal but not at the surface. The general agreement between theory and experiment is considered to be good although a few discrepancies arose, e.g. the lateral extent of the Bragg surface peak was found to be larger for the experiment than for the theory. Continued research in this area must explore further these small discrepancies; and it is suggested that a possible line of future study would be to examine theoretically and experimentally the X-ray diffraction from dislocation-free cylindrical crystals which possess low absorption coefficients (e.g. aluminium, silicon) for harder radiations (e.g. M_oK₁, Ag_K₁). It should then be possible to produce interference fringes and then it should be possible to examine the effects of a strain gradient on fringe spacing and visibility. Chapter IV sets out to discuss the tensile deformation behaviour of [1-2-3] growth axis, dislocation-free, chromium plated copper single crystals. This is done using the results from two sets of coupled experiments: an Instron deformation study and a synchrotron deformation investigation. It is first shown that chromium plating can destroy the perfection of the crystal unless care is exercised over plating temperatures and times. It was finally found that after plating for 10 seconds at 55C (1 μm of Cr deposited) that there was no indication of lattice dislocation. The stress-strain curve of a chromium plated dislocation-free copper crystal (Cr thickness 1 μm) is shown to exhibit a yield point commensurate with that for a non-plated, dislocation-free copper crystal, i.e. 90 g/mm². The work hardening curve is shown to be comprised of three distinct regions: an initial rapid work hardening rate; a transitionary work hardening rate; and a stabilised work hardening rate up to a shear strain of 6%. In straining the crystal through these regions the work hardening rate progressively decreased. For a plated crystal which is dislocation-free initially it is shown that serrations in the flow curve occur, whilst for all other crystals these are shown to be absent. Such serrations are argued to occur by a source suppression and new source operation mechanism during the early deformation stages (1% shear strain), and then by a crack formation mechanism at the copper-chromium interface during the later stages of deformation. The initial work hardening rates (18 kg/mm²) were found to be independent of coating thickness (0.5 μm and 1 μm) but in the latter two regions the thicker coating imparted a higher work hardening rate to the crystal than the thinner layer, e.g. for a 0.5 μm chromium layer the work hardening rate was 1.65 kg/mm² in region iii and for a 1 μm chromium layer it was 2.97 kg/mm² in region iii. It is tentatively suggested that the chromium layer affected mobile dislocation motion rather than dislocation generation. The synchrotron work produced no evidence to support the argument that dislocation motion up the elastic line took place. Yielding was found to occur at a stress level similar to that measured in the Instron work. The complex stress system imposed by the deformation jig and the lack of resolution rendered it impossible to decide where the dislocation sources were located. Further increases in the load on the crystal produced double slip and this was argued to prematurely occur because of the combination of torsion, bending and tension which the crystal experienced. Certain interfacial dislocation activity was registered but this was not readily analysed in terms of surface sources. The yielding behaviour was inhomogeneous and appeared to be remote from the few slip bands induced by specimen transport. After yielding evidence was found for dislocation pile-up at the centre of the crystal. It is pointed out that future studies should use a better design of deformation jig so as to apply only a tensile stress to the crystal. Plating thickness and specimen size could be further explored.

No description available.

No description available.

The effect of silica on the crystallization and melting behavior of a highly isotactic, well characterized isotactic polystyrene (i-PS) have been investigated. The origins of the various endotherms obtained upon heating have been defined by partial scanning experiments and by a study of the effect of heating rate using differential scanning calorimetry (DSC). The presence of 1 part silica in 100 parts polymer (1 pph) decreases the maximum degree of crystallinity considerably but has a minimal effect on the rate of crystallization. Analysis by the Avrami method shows that the silica does not affect the overall rate of crystallization significantly. The decrease in the crystallinity indicates that silica affects the level of secondary crystallization, thus the crystal perfection. / The surface morphologies and growth rates of i-PS spherulites, as studied by photomicroscopy, were not affected by 1 pph of silica. The experimental data were fitted to a modified form of the Hoffman-Lauritzen equation. / The effect of silica on spherulite growth rates and surface morphologies of isotactic poly(propylene oxide) (i-PPO) have also been investigated by optical microscopy. Two distinct i-PPO samples of different molecular weights were used, each of which was highly isotactic. The addition of silica has a pronounced effect on the morphology of the spherulites, producing dendritic type morphology. Upon step-crystallization, the spherulites exhibited mixed morphologies, i.e., fibrillar and ringed. Silica depresses the spherulite growth rates throughout the entire temperature range. The effects were more profound as the quantity of filler increased. The growth rate-temperature behavior was analysed in terms of the classical Hoffman-Lauritzen equation and a modified version to take into account the polymer-filler interaction.

Thesis (Ph. D.)--University of Hawaii at Manoa, 1995. / Includes bibliographical references. / Microfiche. / xxi, 170 leaves, bound ill. 29 cm

by Chan Kim Wai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references. / by Chan Kim Wai. / Chapter Chapter I --- Introduction / Chapter 1.1 --- Rapid solidification / Chapter 1.1.1 --- Rapid quenching --- p.1-1 / Chapter 1.1.2 --- Undercooling --- p.1-2 / Chapter 1.2 --- Grain refinement / Chapter 1.2.1 --- What is grain refinement? --- p.1-5 / Chapter 1.2.2 --- Previous results in grain refinement / Chapter 1.2.2.1 --- Pure metals (or dilute alloys) --- p.1-5 / Chapter 1.2.2.2 --- Alloys --- p.1-9 / Chapter 1.2.2.3 --- Semiconductor --- p.1-10 / Chapter 1.2.3 --- Critical crystal growth velocity V* --- p.1-11 / Chapter 1.2.4 --- Proposed models to grain refinement / Chapter 1.2.4.1 --- Dynamic nucleation and cavitation --- p.1-12 / Chapter 1.2.4.2 --- Remelting (melt-back) --- p.1-14 / Chapter 1.2.4.3 --- Interdendritic fluid flow --- p.1-15 / Chapter 1.2.5 --- Volumetric manifestation of grain refinement --- p.1-15 / Chapter 1.3 --- Aim of this project --- p.1-16 / References / Figures / Chapter Chapter II --- Experimental / Chapter 2.1 --- Pure palladium / Chapter 2.1.1 --- Sample preparation and procedure --- p.2-1 / Chapter 2.1.2 --- Limitation and choice of flux --- p.2-2 / Chapter 2.1.3 --- High temperature furnace --- p.2-3 / Chapter 2.1.4 --- Measurement of specific volume / Chapter 2.1.4.1 --- Theory --- p.2-4 / Chapter 2.1.4.2 --- Setup --- p.2-5 / Chapter 2.1.5 --- Observing internal morphology --- p.2-5 / Chapter 2.2 --- Palladium with insoluble impurity / Chapter 2.2.1 --- Choice of insoluble impurities --- p.2-6 / Chapter 2.2.2 --- Sample preparation --- p.2-7 / References / Figures / Chapter Chapter III --- Results and Discussion / Results / Chapter 3.1 --- Pure palladium / Chapter 3.1.1 --- Specific volume --- p.3-1 / Chapter 3.1.2 --- Grain structure and internal voids --- p.3-2 / Chapter 3.2 --- Palladium with insoluble impurity / Chapter 3.2.1 --- Pinning effect of insoluble impurities --- p.3-3 / Chapter 3.2.2 --- Pd-Ni-S system / Chapter 3.2.2.1 --- Grain refinement in Pd99.9Ni-S)0.1 --- p.3-4 / Chapter 3.2.2.2 --- Change of ΔT* with addition of sulfur --- p.3-5 / Chapter 3.2.2.3 --- Internal voids --- p.3-5 / Discussion / Chapter 3.3 --- Dynamic nucleation of Pd-Ni-S system --- p.3-6 / Chapter 3.4 --- Void formation of pure palladium and Pd-Ni-S --- p.3-6 / Chapter 3.5 --- Grain refinement and specific volume --- p.3-7 / Reference / Figures

Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Materials Science and Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Vita. / Includes bibliographical references. / Ph.D.