Bulk and thin film samples of Bi₂₋[subscript x]Pb[subscript x]Sr₂Ca₂Cu₃O[subscript y] compounds were prepared by suitable methods. The thin film and bulk samples were exposed to argon gas at temperatures of about 2000 °C and pressures of about 300 atm. in a ballistic compressor (BC) and then cooled at a rate of about 10⁵ °C/sec. The samples before and after this treatment were examined and compared using a transmission electron microscope (TEM), a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS) , and an x-ray diffractometer (XRD). Resistance and magnetic susceptibility measurements were made to determine changes in superconducting temperature Tc. Resistance and AC susceptibility measurements of bulk and thin film samples demonstrated that the Tc increased 3- 6 K after the BC treatment, and step-shaped curves were often observed in the resistance versus temperature graphs. The x-ray diffraction study revealed that the relative intensities of lines of the two high Tc phases clearly increased, and the relative intensities of lines of the low Tc phase decreased after the BC treatment. Scanning electron micrographs of the exposed samples showed that surface melting had occurred, and the BC treatment affected the surface of samples to a depth of about 10 μm. Energy dispersive spectral analysis showed that oxygen loss occurred in the exposure process. Transmission electron diffraction patterns showed that the crystal structure of the Bi-based compounds have an incommensurate modulation along the b-axis with different periods of 25.4 Å, 38.7 Å and 72.6 Å. After the BC treatment, the lattice parameters of superconductors did not change, but the relative intensities of spots changed, the modulation of 72.6 Å disappeared, and the density of twist boundaries increased. These observations suggest that the treatment changed the density of structural defects and the atomic arrangement. These studies tentatively indicate that the increase in structural defects was caused by oxygen loss. A very small oxygen loss increases the fraction of copper in CU‴ oxidation state, and thus increases the density of hole carriers in the Cu-O planes. This may be the reason why the Tc of Bi-based cuprate superconductors increased after the BC treatment.

Graduation date: 1999

Thesis (Ph. D.)--Lehigh University, 1999. / Includes vita. Bibliography: leaves 110-115.

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment for the requirements for a degree in Masters of Science. November 2017, Johannesburg. / Metal complexes have a wide range of applications in fields such as medicinal, industrial, environmental and so on. Determining the formations constants for metal complexes is thus essential to gather information on complexes. Conventional methods used to study complexation include spectroscopic methods and the use of voltammetric techniques such as direct current polarography using a mercury drop electrode. In this work alternative voltammetric methods to study complex formation are investigated. Bismuth complexes have important medicinal applications but not much is known about them because of the difficulties in studying them due to extensive bismuth hydrolysis and precipitation of hydrolysis products from very low pHs. The aim of this work was to investigate whether using low concentrations of bismuth (10-6 M) would prevent precipitation in nitrate solutions as predicted in literature. Differential pulse anodic stripping voltammetry (DPASV) was the technique of choice because of its low detection limit, even though it has not been widely used in complex formation studies before. The study revealed that electrochemical response for Bi3+ was not fully reversible using this technique. Additionally, even at the low concentration precipitation was observed in the acidic region (investigated via pH titration) and was suspected to be the formation of BiONO3 species. The second alternative method investigated was the use of mercury film electrodes to replace the use of the toxic mercury drop electrodes which are being phased out worldwide. This work looked at using in situ and ex situ plated films, where the in situ measurements proved more reproducible. A number of challenges were encountered, such as film degradation, peak splitting and ligand adsorption to the film electrode. Nonetheless, formation constants of lead-glycine complexes were determined under different experimental conditions and these were found to be reasonably compared to the literature values. / LG2018

Graduation date: 2003

Thesis (Ph. D.)--Oregon State University, 2002. / Typescript (photocopy). Includes bibliographical references. Also available via the World Wide Web.

published_or_final_version / Physics / Master / Master of Philosophy

Thesis (Ph. D.)--University of Texas at Austin, 2005. / Vita. Includes bibliographical references.

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