SYNTHESIS AND MAGNETIC CHARACTERIZATION OF (TL0.5PB0.5)SR2CA2CU3O9 AND TL2BA2CACU2O8 BULK SUPERCONDUCTORS

Nearly single-phase bulk superconducting (Tl0.5Pb0.5)Sr2Ca2Cu3O9-delta (hereafter referred to as Tl,Pb-1223) samples were prepared readily from calcium- and copper-rich starting compositions of (Tl0.6Pb0.5)Sr2Ca2+ZCu3+ZOy (Z=0.4-0.5). The starting compounds were heated in a crimped gold tube at 850 degrees C for 2 h in air with one intermediate grinding, and quenched in liquid nitrogen. The T-c was found to be 108 K (measured as the diamagnetic onset temperature) for the ''as-synthesized'' samples. The crystal symmetry was found to be tetragonal with lattice parameters alpha=3.807(2) Angstrom A, and c=15.24(1) Angstrom A, similar to the literature values. Magnetic characterizations were conducted by using a DC-SQUID magnetometer. From the magnetization measurements, the intragrain critical-current density, J(cs) was calculated to be 6 X 10(5) A/cm(2) at 77 K and a field of 0.05 T. Flux pinning characteristics were investigated for Tl,Pb-1223 samples and the results are compared with that of Tl2Ba2CaCu2O8 (Tl-2212). The irreversibility fields and bulk flux-creep activation energies for Tl,Pb-1223 samples are found to be higher than for Tl-2212 at all investigated temperatures. The flux-line densities and the activation energy per flux are reported for the first time and reveal an increased defect density (serving as pinning centers) for Tl,Pb-1223 samples as compared to that for Tl-2212 samples at 35 K. These features are attributed to the three-dimensional behavior of Tl,Pb-1223 samples when compared to the two-dimensional (highly anisotropic) behavior of Tl-2212 samples.