GRANULARITY, MICROSTRUCTURE, AND FLUX-CREEP IN TLBA2CA2CU3O9+DELTA SUPERCONDUCTING POWDER AND SILVER-SHEATHED TAPES

Silver-sheathed TlBa2Ca2Cu3O9+delta (TI-1223) tapes, with a transport critical current density, J(ct), of 6200 A/cm2 at 75 K under zero magnetic field, were fabricated by the oxide-powder-in-tube (OPIT) method and characterized using an X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and superconducting quantum interference device (SQUID) magnetometer. The results of the magnetization and SEM study indicate that, in these tapes, individual grains are distributed randomly in orientation and weak links exist. However, strongly linked percolative current paths within the tape persisting in increasing fields, accompanied by strong intrinsic interplanar coupling, sustain a significant J(ct) at high fields and lead to the plateau in the J(ct)-H curve. Dislocation networks, which may act as effective pinning centers, are the common features observed in the tapes. To investigate the effects on flux pinning due to thermomechanical processing, magnetic relaxation at 1 and 2 T over 5-50 K was measured. The tape shows slightly lower normalized relaxation rates (S = - (1/M0) dM/dln t) than the cauliflower-like precursor powder. Analyses of the relaxation data obtained from the tape, after incorporating temperature dependence and field scaling, yield an expression for the effective pinning energy, U(eff)(J, T, H) = (U1/H-0.54) [1 - (T/72.5)2]4 (J/J(i))mu. This result was compared with the prediction of the collective flux-creep model, which suggests that Tl-1223 has a three-dimension-like (3D-like) vortex lattice. Presumably, a more plate-like powder morphology may result in improved texturing by the OPIT process. Tl0.5Bi0.5Sr2CaCu2O7+delta ((Tl, Bi)-1212) powder with this morphology was therefore synthesized for comparison.