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

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 °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 °C under 350 MPa for 48 h exhibits the worst characteristic features of high Tc-phase as a consequence of the enhancement in local structural distortions, dislocations, defects and disorders in the Cu–O2 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 Tconset of 110.63 K and Tcoffset of 108.46 K values while the worst material obtains the smallest values (Tconset of 95.45 K and Tcoffset of 45.32 K). Similarly, the maximum Jc value of 818 A/cm2 is experimentally observed for the best compound whereas the worst sample obtains the smallest value of 112 A/cm2. 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 °C annealing temperature and 48 h annealing time for the preparation condition promotes seriously the high Tc-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 Å and average grain size of 71.4 nm but the smallest lattice parameter a of about 5.29 Å. 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 °C annealing temperature and 48 h annealing