Microstructural Effect on Critical Current Density in (Bi,Pb)2Sr2Ca2Cu3Ox Ag Sheathed Tape

Microstructure controlling the weak links is the primary determining factors for the critical current density, Jc, in Ag sheathed (Bi,Pb)2Sr2Ca2Cu3Ox superconductors. Especially, we focused on cold working process and the bulk density effect on the Jc. High bulk density resulted in less weak-link microstructure with a small amount of voids, small secondary phases, high degree of orientation and smooth interface between oxide core and Ag sheath. Hg amalgam method of removing Ag sheath revealed as-processed microstructures without any damage. The tape with the highest density showed high critical current density of 6.6÷104 A/cm2 at 77K and 0T. Moreover, the bulk density varied over the cross section of the oxide core implying the variation of Jc. This was confirmed by measuring critical current for polished samples in the width direction: the center region with high bulk density showed twice higher Jc than the edge region with lower bulk density. This means that a higher Jc than the present record would be possible in the powder-in- tube method by improving the microstructure. Higher bulk density was obtained by Ag-Mg alloy sheath material. Although a higher Jc is not yet obtained due to differences in the conditions from the case of pure Ag sheath, the mechanical strength of 0.2% yield strength which is another important practical property was much improved by fine MgO dispersion from 20 MPa to 200 MPa which is comparative to the value of practical Nb3Sn conductors.