Portland State University. Environmental Sciences and Resources Ph. D. Program.
Date of Award
Doctor of Philosophy (Ph.D.) in Environmental Sciences and Resources: Physics
Environmental Science and Management
3, xi, 111 leaves: ill. 28 cm.
Bismuth compounds, Superconductivity
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.
Duan, Qing, "The Effects of Rapid Heating and Cooling on the Composition, Structure, and Superconducting Properties of Bi-Sr-Ca-Cu-O Compounds" (1993). Dissertations and Theses. Paper 1322.