Enhanced critical current density in the nano-sized metallic Au-added Bi1.8Sr2Au0.3Ca1.1Cu2.1Oy (Bi-2212) ceramics by means of optimization of the dwell time at 870 °C

Abstract

This study investigates how the postannealing process applied to Bi1.8Sr2Au0.3Ca1.1Cu2.1Oy (Bi-2212) ceramics at 870 °C can improve their Jc values. Bi1.8Sr2Au0.3Ca1.1Cu2.1Oy 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 (Tcmag) 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 (Jcmag) has been obtained through the data from M–H loops according to the critical Bean model. The highest Jc value, 39.92 × 104 A/cm2 under 0.2 T at 15 K, is higher than the Jc 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.