Effect of Nb addition on magnetic, structural and superconducting properties of (Bi, Pb)-2223 superconductors

摘要

In this work, the effects of Nb_2O_5 addition with different ratios on the structural and magnetic properties of Bi_(1.7-x)Pb_(0.3)Nb_xSr_2Ca_2Cu_3O_y (x = 0.00-0.20) superconducting samples were investigated. (Bi, Pb)-2223 superconducting samples were prepared by conventional solid-state reaction method. The phase formation, phase fraction and lattice parameters were determined from X-ray powder diffraction (XRD) measurements, the microstructure, surface morphology analyses of the samples were carried out using scanning electron microscope (SEM). Additionally, ac susceptibility measurements were done in order to determine the critical current density (J_c) and hole concentration (p) of the samples. AC susceptibility measurements were done at various ac fields (ranging from 20 to 160 A/m) to understand the effect of Nb addition on magnetic properties of Bi_(1.7-x)Pb_(0.3)Nb_xSr_2Ca_2Cu_3O_y superconductor. Critical onset (T_c~(on)) and loss peak temperatures (T_p) were estimated from the ac susceptibility curves. It was observed from ac susceptibility measurements that the critical onset temperatures decreased from about 108-98 K with increasing Nb addition (x = 0.00-0.20). The imaginary part of susceptibility was used to calculate the intergranular critical current density (J_c) by means of the Bean's model. X-ray diffraction analysis revealed that the samples consisted of a mixture of Bi-2223 and Bi-2212 phases as the major constituents and non-superconducting phase Ca_2PbO_4 as the minor. It was also shown from XRD measurements that volume fraction of high-T_c phase decreases with increasing Nb addition up to x = 0.20. The sample with Nb addition of x = 0.20 showed the highest volume fraction of Bi-2223 phase (86 %). When Nb addition was increased, the surface morphology and grain connectivity are found to degrade, the grain sizes decrease and porosity of the samples were observed to increase from SEM images except the sample with x = 0.20 Nb addition.