Effect of strain on ac power loss of Bi-2223/Ag superconducting tapes

The ac power losses of monofilament and multifilament Bi-2223/Ag composite tapes were investigated to determine the effect of mechanical strain on the loss and to identify the loss mechanisms. Measurement of the self-field losses was performed at 77 K and 60 Hz as a function of ac current amplitude (0–100 A) for tapes in their as-prepared or virgin state and after being subjected to applied strain by temperature cycling or bending to small radii of curvature ( R=1–50 mm). For good quality virgin tapes the experimental data are well described by the Norris equation P∼ I m n for the dependence of the power loss P on the amplitude I m of the ac transport current, with n=3 for monofilament and n=4 for multifilament tapes. Applied strain causes the loss to increase by several orders of magnitude and the exponent n to decrease below the Norris values. At strains very much greater than the irreversible strain limit, the loss becomes purely ohmic (i.e., I 2 R loss) and n=2. Examination by SEM of the transverse cross-section of tapes reveals a variety of strain-induced structural defects including transverse cracks that sever the filaments and extended regions where the superconductor core has delaminated from the silver sheath.