Thesis (Ph. D.)--Drexel University, 2006. / Includes abstract and vita. Includes bibliographical references (leaves 174-191).

Thesis (Ph. D.)--Drexel University, 2006. / Includes abstract and vita. Includes bibliographical references (leaves 52-56).

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / Recently, the rare earth nitrides have received a considerable attention from theorists and experimentalists due to their potential applications in spintronic, piezoelectric, and thermoelectric devices. In this work, erbium nitride (ErN) and scandium nitride (ScN) crystals were grown and characterized experimentally, and the growth process was modeled and simulated. Erbium nitride (ErN) is a rare earth nitride notable for its magnetic and optical properties. Here we report on its growth on a non-native substrate, tungsten foil, via physical vapor transport, and its characterization. The source material was erbrium metal that was converted to ErN by heating in nitrogen. Subsequently, it was sublimed to form the ErN crystals. The operating conditions were 1620-1770 ⁰C and 150-330 Torr in pure nitrogen. The growth rate increased exponentially with temperature with an activation energy of 508 kJ/mol, and inversely with pressure. X-ray diffraction revealed the ErN preferentially adopted a (100) orientation, the same as the dominant orientation of the tungsten sheet. The lattice constant was 4.853 Å. The crystal shapes and sizes were dependent on the temperature, as revealed by SEM and optical microscopy. The ErN crystals were highly faceted, bound by (100) and (111) crystal planes. The ErN compound deviated from stoichiometry: the Er:N atomic ratio ranged from 1:1.15 to 1:1.2 according to EDX and XPS elemental analysis. Raman spectra was in good agreement with theoretical predictions. Scandium nitride single crystals (14–90 µm thick) were grown on tungsten (100) single crystal substrate by physical vapor transport in the temperature range of 1850-2000 ⁰C and pressure of 15-35 Torr. Epitaxial growth was confirmed using in-plane ɸ scan and out-of-plane x-ray diffraction techniques which revealed that ScN exhibited cube-on-cube growth with a plane relationship ScN (001) || W (001) and normal direction ScN [100] || W [110]. Atomic force microscopy revealed the surface roughness decreased from 83 nm to 18 nm as the growth temperature was increased. X-ray diffraction (XRD) rocking curves widths decreased indicating the crystal quality improved with increasing growth temperature. The lowest XRC FWHM was 821 arcsec, which is so far the lowest value reported for ScN. Scanning electron microscopy (SEM) exhibited the formation of macrosteps and cracks on the crystal surface with latter due to the mismatch of ScN’s and tungsten’s coefficients of thermal expansion . In general for crystal growth, material should deposit on the seed crystal and not on any adjacent supporting structures. This efficiently uses the source material and avoids the possibility of spurious polycrystals encroaching on, and interfering with the single crystal growth. To achieve this goal, a new crucible design with a cooling fin in contact with the seed was simulated and experimentally demonstrated on the physical vapor transport (PVT) crystal growth of scandium nitride. The heat transfer of the growth cavity for a conventional crucible and a modified crucible with the cooling fin were modeled theoretically via computational fluid dynamics (CFD) with FLUENT. The CFD results showed that the seed in the modified crucible was approximately 10 °C cooler than the crucible lid, while in the conventional crucible the temperature of the seed and lid were uniform. The experimental results showed that increasing the temperature gradient between the source and the seed by employing the cooling fin led to a dramatic increase in the growth rate of ScN on the seed and reduced growth on the lid. The relative growth rates were 80 % and 20 % on the seed and lid respectively, in the modified crucible, compared to 25% and 75% with the conventional crucible. Thus, the modified crucible improved the process by increasing the species transporting to the seed by sublimation.

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Hyperuricemia and local temperature changes in the joints of the extremities are known to be responsible, in part, for the development of gouty arthritis. No satisfactory explanation is yet available for (1) the selective deposition of monosodium urate monohydrate (MSUM) crystals in connective tissues (2) the increased incidence of gout in the later years of life and (3) the increased incidence of MSUM crystal deposition in connective tissues after trauma and in joints with preexisting disease. It is possible that the alterations in composition of the non-fibrillar matrix of cartilage and synovial fluid which are thought to occur with ageing, trauma or preexisting disease, may predispose these tissues to crystal deposition. The objectives of this study were to determine the effect of the cartilage and synovial fluid components, chondroitin sulfate, hyaluronic acid, proteoglycan monomer, proteoglycan aggregate, phospholipids and albumin on the growth of MSUM. The degradation of MSUM solutions was studied under sterile and non-sterile conditions to determine the possible causes of degradation and to define the time span of crystal growth experiments. The rate of degradation of MSUM solutions increased with an increase in temperature. The concentration of MSUM in solution fell sharply after autoclaving and solutions stored in containers with rubber closures showed greater degradation of MSUM than autoclaved solutions stored in all-glass containers. Rubber stoppers apparently absorbed MSUM from solution. The degradation of MSUM solutions was thought to be due to both bacterial consumption and chemical decomposition in non-sterile solutions but was due only to chemical decomposition in sterile solutions. The aqueous solubility of MSUM was determined at different temperatures and in the presence of varying concentrations of sodium chloride. Sodium chloride suppressed MSUM solubility. The aqueous solubility of MSUM was also determined in the presence of several connective tissue components at 37°. Chondroitin sulfate (CS) decreased the saturation solubility of MSUM probably due to the sodium present in the CS samples. Proteoglycan aggregate, proteoglycan monomer, hyaluronic acid an albumin resulted in very slight increases in the solubility of MSUM. The growth kinetics of MSUM was studied using the seeded growth technique. An equation of the general form: [formula omitted] was used to determine the overall growth rate constant, [formula omitted]. Linear plots of the integrated form of the second order growth equation gave the best fit between the points and gave reasonably constant values for [formula omitted] determined at a given initial supersaturation concentration and varying seed amounts. An induction period or a period of slow growth was observed at both the initial supersaturation concentrations studied. The length of the induction period was inversely proportional to the added seed amount. Differing concentrations of additives were included in the growth medium and K' determined. Chondroitin sulfate (CS) significantly increased the growth rate constant for MSUM growth. However, the proportion of CS decreases in aged and osteoarthritic cartilage and thus a decreasing proportion of a growth accelerator is unlikely to be a factor in the deposition of MSUM in cartilage. CS has been found in the synovial fluid of arthritic joints and may act as an MSUM growth accelerator in this medium. Hyaluronic acid (HA) and albumin caused significant inhibition of the growth of MSUM crystals. This effect may be due to the adsorption of these molecules onto the MSUM seed crystals resulting in the poisoning of the active growth sites on the crystal surface. Cartilage HA and synovial fluid albumin levels are increased in aged and/or diseased joints. Increased proportions of growth inhibitors do not offer likely explanations of crystal deposition in joint tissues. At concentrations of 0.1-1.0 mg mL⁻¹ proteoglycan monomer (PGM) and proteoglycan aggregate (PGA) slightly increased the MSUM growth rate constant but this increase was statistically insignificant. The two phospholipids, phosphatidylcholine and phospha-tidylserine increased the growth rate constant of MSUM. Phosphatidylserine, however, did not significantly increase the growth rate constant at the concentrations studied. It is possible that the raised levels of phospholipids in aged or diseased cartilage and synovial fluid could accelerate the growth of MSUM crystals resulting in MSUM deposition in these tissues. / Pharmaceutical Sciences, Faculty of / Graduate

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