GRANULARITY, MICROSTRUCTURE, AND FLUX-CREEP IN TLBA2CA2CU3O9-SHEATHEDTAPES(DELTA SUPERCONDUCTING POWDER AND SILVER)

Silver-sheathed TlBa2Ca2Cu3O9+delta (TI-1223) tapes, with a transport critical current density, J(ct), of 6200 A/cm2 at 75 K under zero magn etic field, were fabricated by the oxide-powder-in-tube (OPIT) method and characterized using an X-ray diffractometer (XRD), scanning electr on microscope (SEM), transmission electron microscope (TEM), and super conducting quantum interference device (SQUID) magnetometer. The resul ts of the magnetization and SEM study indicate that, in these tapes, i ndividual grains are distributed randomly in orientation and weak link s 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 therm omechanical processing, magnetic relaxation at 1 and 2 T over 5-50 K w as measured. The tape shows slightly lower normalized relaxation rates (S = - (1/M0) dM/dln t) than the cauliflower-like precursor powder. A nalyses of the relaxation data obtained from the tape, after incorpora ting temperature dependence and field scaling, yield an expression for the effective pinning energy, U(eff)(J, T, H) = (U1/H-0.54) [1 - (T/7 2.5)2]4 (J/J(i))mu. This result was compared with the prediction of th e collective flux-creep model, which suggests that Tl-1223 has a three -dimension-like (3D-like) vortex lattice. Presumably, a more plate-lik e powder morphology may result in improved texturing by the OPIT proce ss. Tl0.5Bi0.5Sr2CaCu2O7+delta ((Tl, Bi)-1212) powder with this morpho logy was therefore synthesized for comparison.