Improvement of strain dependent critical current in Ag- and Ag/Cu-sheathed Bi(2223) superconducting tapes

Applications of Bi(2223) tapes in superconducting devices require a reduction of critical current degradation induced by mechanical or thermal stress. Critical currents have been measured dependent on thermocycles and axial strain. The monofilament Ag-sheathed tapes show a degradation of 10% at a critical strain epsilon=0.3%, while multifilament tapes are nearly stable to epsilon=0.6%. A considerable improvement could be reached using Ag90Cu10 sheathes. In opposite to the abrupt irreversible degradation in Ag-sheathed tapes the process in Cu hardened tapes is more gradual and the critical E-values are doubled approximately. Micrographs show that microcracks are the reason for the critical current degradation. Their density is dependent on core properties.