New Experiments Elucidating the Current Limiting Mechanisms of Ag-Sheathed (Bi,Pb)_2Sr_2Ca_2Cu_3O_x Tapes

摘要

Multiple current limiting mechanisms exist from the nanometer to millimeter scale in Ag-sheathed (Bi,Pb)-2223 tapes. Recent studies of the zero-field critical current density (J_c (0T, 77K)), the irreversibility field (H~*) and the crack microstructure elucidate these properties. We show that H~* (77K) can vary significantly over the range approx 120-260 mT, independently of J_c (0T, 77K). Cracks, actual or incipient, exist on the sub to several hundred micron scale. Surface magneto optical imaging of whole tapes, correlated to subsequent ultrasonic fracture analysis of the bare 2223 filaments extracted by dissolving away the Ag shows that even composites having J_c (0T, 77K) vlaues of 60 kA/cm~2 exhibit strong signs of unhealed rolling damage. These combined studies show that today's very best 2223 tapes are still far from full optimization.