No abstract

Measurements of levitation forces on melt-quenched and free sintered YBa2Cu3O x superconductors show that melt-quenched samples produce forces from 96-170% higher than sintered specimens. The force increases are correlated with higher magnetization values for the melt-quenched material. Similar increases in the magnetic stiffness were also measured. Vertical levitation force relaxation showed less than 5% decrease in one hour. These results have important favorable implications for application to superconducting bearings.

The discovery of high Tc superconductivity in layered cuprate ceramics has so far led to the identification of about 35 distinct superconducting cuprate systems, the latest of which is the 133 K superconducting system Hg-Ba-Ca-Cu-O. For most of their proposed applications, high Tc ceramics have to be resistant to environmental degradation both with respect to atmospheric water vapor, e.g., in storage, and to liquid water (produced by condensation on warm-up from cryogenic conditions). The presence of CO2 is an important factor in both environments. Increasing environmental stability involves improving the processing methods to eliminate pores, cracks, and other macroscopic defects (e.g., highly leachable impurity phases) which are prevalent in materials prepared by solid-state sintering. Furthermore, protective coatings and hermetic seals are necessary in many applications involving films because of small film thickness. (Wires are usually drawn inside metal tubes, which provide protection.)

In the first part of this study, fly ash has been used as a main component in developing glass compositions which are cellulated through carbonization and oxidation processes. The resulting foam glasses have highly uniform closed pore structure and negligible water permeability with densities ranging from 140 kg/m3 to >800 kg/m3. Foam glasses with open pore structure have also been produced and characterized. In the second part, mechanical and thermal properties are discussed. These include the stressing rate effect on the fracture strength in compressive and flexural loading for different densities. Variations of thermal conductivity and overall strength with density and pore size are also described.