No description available.

No description available.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references (leaves 155-159). Also available via the Internet.

No description available.

Supercritical carbon dioxide was explored as a medium for nucleation and growth processes for ceramic oxide superconductors. The high temperature superconductor examined, YBa₂Cu₃O₇₋ₓ (YBC), was found to be stable upon limited exposure to CO₂. The solubility of the compound was studied in the region near the critical point using spectrophotometry. The species solubilized was indirectly identified by elemental analysis. Preliminary studies on the potential of the medium for metastable synthesis, comminution, and crystallization were conducted and showed promise for further development.

No description available.

Thesis (Ph. D.)--Lehigh University, 1998. / Includes vita. Bibliography: leaves 201-223.

Graduation date: 1994

PrBa₂Cu₃O₆₊_x is anomalous among the compounds which can be made by substituting different rare-earth ions for yttrium (Y) in YBa₂Cu₃O₆₊_x:it is the only compound which has the same structure as YBa₂Cu₃O₆₊_x, and yet does not superconduct. This unusual property makes it an important system to study, since the differences between the two compounds could produce theoretical insights into the mechanisms of cuprate, or high-temperature, superconductivity. This thesis describes neutron scattering investigations of the magnetic properties of PrBa₂Cu₃O₆₊_x. I have investigated the magnetic ordering in well-characterised, single-crystal samples, both with and without Al-contamination. In the data analysis, proper account is taken of the sample mosaicity and the different intrinsic peak shapes encountered. Some of the most striking new characteristics reported here are that (i), there is a clear interaction between the Pr ions and the Cu-O₂ planes, (ii), the ordered Pr moments tilt away from the c-axis (as suggested previously by Mossbauer spectroscopy), and (iii), there is two-dimensional ordering in the Al-containing crystals. I show how observed trends in the magnetic ordering, as O- and Al-contents vary, may be understood in terms of charge redistribution involving the hybridisation of the Pr ions. I have also examined the crystal field transitions of the praseodymium ion. Because of the difficulties previously encountered in measuring these in PrBa₂Cu₃O₆₊_x, I have used instead the compounds PrO₂ and Y_0.3Pr_0.7Ba₂Cu₄O₈, which contain praseodymium in a chemically similar environment. Y_0.3Pr_0.7Ba₂Cu₄O₈ shows the same broadening effects as PrBa₂Cu₃O₆₊_x; PrO₂ shows signs of mixed-valency, with temperature-dependent lifetime effects. The application of symmetry considerations and the point-charge model to the crystal field measurements is considered in some detail.

This thesis describes investigations performed into the growth and characterisation of Bi-based (Bi₂Sr₂Ca_n-1Cu_nO_{2n + 4 + x}, n=2, 3) ceramic superconducting material in the form of thin films, about 0.5μm thick, grown on single crystal MgO, LaAIO₃ and SrTiO₃ substrates by r.f. magnetron sputtering. The effect of oxygen content on the Pb doped Bi-2223 (n=3) phase was also studied by changing the cooling process and by annealing in different partial pressures of oxygen at ambient pressure. The films produced have been assessed by considering their initial composition where it is found that Bi/Sr ratios can be between 0.9<Bi/Sr<1 for the Bi-2212 (n=2) phase, while for the Bi-2223 phase the Bi-content should be below 1.9 or lower than the Sr-content, for the films not to peel off the substrate during high temperature annealing. T_{c- zero} of around 80K is achievable for (Ca + Sr)/Bi ratios between 1.4 and 1.65 while T_{c- onset} remains above 90K for Bi-2212 films. However, the best superconducting properties can be obtained for a (Ca + Sr)/Bi ratio which is quite close the nominal composition. The use of a heavily Pb doped target is an effective way of Pb doping Bi-2223 thin films. A Bi-content of 1.4<Bi<1.8 in as deposited films may provide almost single phase Bi-2223 thin films with Tc values running from 105.5K to 109.5K and Jc>10,sup>4</sup>A/cm² at 77K. The effect of the initial Pb content and annealing conditions on the formation of the Bi-2223 phase was investigated. It was found that high Pb content (0.9<Pb/Bi<1.5) lowers the formation temperature appreciably and increases the range of sintering temperature (to at least 10K). The Bi-2223 phase starts to form at 835°C from the initial phases (Bi-2212, CuO and Ca₂PbO₄) formed below 835°C and its fraction increases with increasing sintering temperature up to 862°C, while the fraction of initial phases decreases. An annealing duration of 30 min. has provided highly oriented films with c-axis perpendicular to the substrate surface and sharp superconducting transition (<5K). Although Pb/Bi ratio is not critical in the range studied, when it is above 1.3 slow heating and cooling is necessary to prevent retention of excess Ca₂PbO₄ in the film after sintering. On LaAIO₃ and SrTiO₃ perovskite substrates, T_c is at least 5K lower than in the case of MgO. Nevertheless, LaAIO₃ can provide good microstructure with a critical current density, of 5x10⁴A/cm² at 77K. The direction and the range of variation of T_c in Bi-2223 films with oxidising process can be related to both the film composition (especially Bi and Pb content) and initial oxygen content. The variation range of T_c with oxidising is controlled by the Pb content. However, the maximum variation is around 4K at ambient pressure. Radiation response measurements were carried out on films patterned into a 150μm wide, and 1 cm long meander-type structure using standard photolithography and wet chemical etching in EDTA. The results showed that the optical response using a continuous wave (cw) He-Ne laser is bolometric, while the microwave response using a 34.5 GHz Gunn diode microwave generator contains a non bolometric component. Such polycrystalline Bi-based high T_c thin films may have interesting applications as sensitive microwave detectors, but they are not particularly good for microwave applications because of their high surface resistance, Rs, at microwave frequencies.