Role of preparation conditions of Bi-2223 ceramic materials and optimization of Bi-2223 phase in bulk materials with experimental and statistical approaches

摘要

This study deals with the determination of optimum preparation conditions (press load, annealing temperature and time) for the Bi-2223 superconducting compound with the aid of both the experimental methods regarding dc resistivity, transport critical current density and powder X-ray diffraction measurements and statistical approaches including response surface explorer based on Box-Behnken designs for the first time. It is found that the Bi-2223 polycrystalline compound prepared under 300 MPa pressure load at 840 degrees C for 48 h presents the highest formation of Bi-2223 phase due to the considerable elimination of the impurity scattering and lattice strain in the crystal structure. On the other hand, the sample prepared at 860 degrees C under 350 MPa for 48 h exhibits the worst characteristic features of high T-c-phase as a consequence of the enhancement in local structural distortions, dislocations, defects and disorders in the Cu-O-2 consecutively stacked layers. Thus, the latter condition causes to both the degradation in amplitude of pair wave function (decrease in the overlapping of Cu 3d and O 2p functions) and metastability due to the reduction of hole trap energy. In this respect, the best material displays the largest T-c(onset) of 110.63 K and T-c(offset) of 108.46 K values while the worst material obtains the smallest values (T-c(onset) of 95.45 K and T-c(offset) of 45.32 K). Similarly, the maximum J(c) value of 818 A/cm(2) is experimentally observed for the best compound whereas the worst sample obtains the smallest value of 112 A/cm(2). The decrement in the Jc parameter is attributed to the regression of intergrain coupling and flux pinning vortices in the Bi-2223 crystal structure due to the increment of misorientations and especially grain boundary weak-interactions in the crystal system. The XRD results also reveal that the combination of 300 MPa press load, 840 degrees C annealing temperature and 48 h annealing time for the preparation condition promotes seriously the high T-c-phase as a consequence of the enhancement in the average crystallite size and lattice parameter c or decrement in the a-axis length. Accordingly, the best material obtains the largest c-axis length of about 37.22 angstrom and average grain size of 71.4 nm but the smallest lattice parameter a of about 5.29 angstrom. At the same time, the surface response designs of statistical analyses show that the optimum preparation conditions are defined to be 306.5657 MPa press load, 840 degrees C annealing temperature and 48 h annealing time to maximize the high T-c-phase (with the maximum T-c(onset) ; T-c(offset) and J(c) parameters. (C) 2016 Elsevier B.V. All rights reserved.