MgB_2 inclusions in Bi-2223 matrix: The evaluation of microstructural, mechanical and superconducting properties of new system, Bi-2223 + MgB_2

摘要

This study reports the effect of MgB2 inclusions on the microstructural, electrical, mechanical and superconducting properties of Bi_(1.8)Pb_(o.4)Sr_2(MgB_2_)xCa_(2.2)Cu_(3.o)O_y ceramics with x = 0,0.01,0.03,0.05,0.1,0.3,0.5 and 1.0 by use of bulk density, dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transport critical current density (J_c) and Vickers microhardness (H_v) measurements. The samples studied in this work are prepared by conventional solid-state reaction method at the annealing temperature of 840 °C for 48 h. For the potential technological and industrial applications, the important characteristics such as the normal state resistivity, density, porosity, critical (onset and offset) transition temperature, variation of critical temperature, self-field critical current density, crystallinity, phase purity, lattice parameter, texturing, surface morphology, Vickers microhardness and elastic modulus values are obtained for the pure and MgB_2-doped samples and compared with each other. It is found that all the properties given above are sensitively dependent upon the MgB_2 concentration in Bi-2223 matrix. The critical transition temperature and critical current density values of the samples are observed to improve significantly for the optimum doping level of x = 0.05. The maximum T_c~(onset) of 121.3 K, T_c~(offset)of 114.1 K and J_c of 463 A/cm2 for the sample doped with x = 0.05 whereas the minimum values are found to be 118.3 K, 101.3 K and 235 A/cm2 for the material doped with x = 1.0 as against 118.6 K, 109.4 K and 306 A/cm2, respectively, for the undoped sample. This may be attributed to the fact that the Bi-2223 system changes from the optimally doped to an underdoped position with the excess of the MgB_2 nanoparticles. Furthermore, XRD measurements illustrate that both pure and MgB_2 doped superconductors (new system) contain only Bi-2223 and Bi-2212 phases (no different phase of MgB2 or any other cation) and exhibit the polycrystalline superconducting phases with the variable intensity of diffraction lines, confirming that the MgB2 inclusions are incorporated into the Bi-2223 crystal structure in the form of nanoparticles. Besides, the smallest (largest) lattice parameter a (c) is noticed to belong to the sample doped with x = 0.05 while the largest (smallest) cell parameter a (c) is observed for the maximum doping level of MgB2 (x = 1.0). As for SEM images, similar to the XRD investigations, the former sample has the best crystallinity, texturing, grain connectivity, lowest porosity and largest grain size while the worst surface morphology is observed for the latter sample. Additionally, the bulk porosity analyses for the samples demonstrate that the bulk porosity decreases monotonously with the MgB2 content in the Bi-2223 system up to x = 0.05 after which the porosity starts to increase regularly towards to maximum value, leading to the degradation of the grain connectivity. At the same time, the Vickers microhardness values indicate that the samples doped with x = 0.01, 0.03 and 0.05 (towards optimally doped state) exhibit reverse indentation size effect (RISE) behavior; on the other hand, the others (towards underdoped state) present indentation size effect (ISE) feature. To sum up, the MgB_2 addition in the Bi-2223 superconducting system promotes the velocity of the Bi-2223 phase formation up to the concentration level of x = 0.05 beyond which the microstructural, electrical, mechanical and superconducting properties of the Bi-2223 materials studied are found to degrade systematically and in fact reach to their local minimum points for the doping level of x = 1.0.