Abstract
It is known that the critical current density of Ba2YCu3O6+x superconductors made by the solid state reaction method is much lower than that of conventional metallic superconductors. Recently, there has been considerable interest drawn to the improvement of the critical current density for their practical applications. In this report, we investigated the factors which enhance the critical current density of the Ag sheathed tape prepared by powder in tube method.
The composite consisting of the calcined powder sheathed with the Ag tube was cold-rolled or pressed to form a tape and then sintered. In the case of the Ag-sheathed composites, crack generates during a heat-treatment because of thermal stress due to a distinction of thermal expansion coefficient between oxide and Ag sheath. In a part of experiments, the calcined powder was reduced to a partially metallic mixture by hydrogen gas in order to eliminate the generation of cracks. The critical current density and the microstructure have been investigated.
The usage of the hydrogen gas reduced powder was effective for eliminating a serious degradation of the critical current density for the thin tape specimens. The critical current density tended to increase with decreasing tape thickness. The improvement was attributed to the formation of textured structure and densification. From the analysis of the pole figure and the observation of the microstructure, the c axis of grains was deduced to incline by about 10 degrees to the tape surface normal. The pressing technique was found to be more effective for improving the critical current density than the cold-rolling. The present investigation confirmed that the higher critical current density can be obtained for the thinner tape prepared by the pressing technique.
