Enhanced critical current density in the nano-sized metallic Au-added Bi_1.8Sr₂Au_0.3Ca_1.1Cu_2.1O_y (Bi-2212) ceramics by means of optimization of the dwell time at 870 °C

This study investigates how the postannealing process applied to Bi_1.8Sr₂Au_0.3Ca_1.1Cu_2.1O_y (Bi-2212) ceramics at 870 °C can improve their J_c values. Bi_1.8Sr₂Au_0.3Ca_1.1Cu_2.1O_y has been selected in the present work as initial composition because nano-sized metallic Au additions at large scale positively affect the superconducting properties of Bi-2212 ceramics due to the enhancement of intergranular coupling between the grains, as reported in the literature. These Bi-2212 ceramics were prepared by the solid-state reaction method, including a heat treatment at 860 °C for 96 h for sintering. Then, the samples in the postannealing stage are exposed at different dwell times (8 h, 16 h, and 32 h) at 870 °C. XRD analysis showed that all the samples have similar chemical composition, with Bi-2212 phase as the major one, and the presence of minor impurity phases such as Au, Bi₂CaO₄ and Bi₄Sr₄CaCu₃O₄. The crystalline size of the samples calculated according to the Debye–Scherrer equation increases with increasing dwell time. Magnetization–temperature (M–T) curves were used to determine the superconducting transition temperatures (T_c^mag) of all the samples, showing that they are around 70 K. This means that the dwell-time process applied at 870 °C in the present study does not degrade their transition temperatures. To observe how both diamagnetic character and grain growth of the Bi-2212 phases affect magnetic properties, as a function of the different dwell times at 870 °C, magnetic–hysteresis measurements (M–H) at 15 K were performed. It has been found that the thermal treatment at 870 °C for 32 h favors the formation of the widest hysteresis loop due to better intergrain coupling between grains. Finally, the magnetic critical current density (J_c^mag) has been obtained through the data from M–H loops according to the critical Bean model. The highest J_c value, 39.92 × 10⁴ A/cm² under 0.2 T at 15 K, is higher than the J_c of for the samples with the same composition reported in the literature (13.42 × 10⁴ A/cm² under 0.15 T at 10 K). These results clearly show that appropriate postannealing processes can significantly improve superconducting properties of Bi-2212 ceramics.