The Effect of Gd Concentration on the Physical and Magnetic Properties of Bi_(1.7)Pb_(0.3-x)Gd_(x)Sr_(2)Ca_(3)Cu_(4)O_(12+y) Superconductors

摘要

Bi-Pb-Gd-Sr-Ca-Cu-O bulk samples with nominal composition Bi_(1.7)Pb_(0.3-x)Gd_(x)Sr_(2)Ca_(3)Cu_(4)O_(12+y) (x velence 0.01, 0.05, 0.075, 0.10) were prepared by the melt-quenching method. The effects of different Gd doping on the structure have been investigated by electrical resistance, scanning electron micrographs, XRD, magnetization and magnetic hysteresis loop measurements. The magnetization measurements have been carried out as a function of magnetic field for fields up to 5 kOe at temperatures well below the zero resistance temperatures of the annealed samples. It has been found that the high-T_(c) superconducting phase, (2223), is formed in the sample A with concentration x velence 0.01, annealed at 840 deg C for 120 h. However, with increasing Gd~(3+) doping for Pb~(2+) the (2223) phase gradually transforms into the (2212) phase. The magnitudes of magnetization and initial susceptibility, |M| and |dM/dH|, and the hysteresis loop areas decrease with increasing Gd concentration x and/or temperature T. The fast decreases in |M|, |dM/dH|, and the hysteresis loop areas related to the superconducting volume, with increasing x and/or T seem to imply an existence of flux pinning centres in our samples. In order to support this implication the critical current densities J_(c), of the samples, have been estimated at two fixed temperatures, 9 and 30 K. Our data have indicated that J_(c) decreases with increasing temperature and/or Gd concentration, as expected.