Effect of annealing conditions on the superconducting properties of nano-sized metallic Au-added Bi_1.8Sr₂Au_0.2Ca_1.1Cu_2.1O_y (Bi-2212) ceramics

In this study, the effects of annealing process on the phase formation and electrical and magnetic properties of nano-sized metallic Au-added Bi-2212 ceramics have been investigated. The Bi_1.8Sr₂Au_0.2Ca_1.1Cu_2.1O_y including x = 0.2 Au content as starting composition was chosen due to the improved transport properties in the Bi-2212 system, as known in the literature. The samples for the post-annealing technique were exposed at different dwell times (10 m, 30 m, 1 h, and 2.5 h) at a fixed temperature of 870 °C after the application of normal heat treatment of 850 °C for 120 h. The XRD diagrams clearly indicate that a significant part of the phases in all the samples belongs to Bi-2212 phase. SEM results show that all the samples have randomly oriented grain forms as the main characteristic of applied solid-state method. In all cases, the post-annealed samples showed better superconducting properties of Bi-2212 phase, indicating that the increases in the dwell time positively affect its granular polycrystalline character such as inter-connectivity of the grains, the nucleation, and growth of the Bi-2212 phase. Moreover, the sample with dwell time of 2.5 h at 870 °C reveals significant enhancement in the M–H loops compared to the sample with less dwell time, despite the fact that the Bi-2212 phase can easily decompose into secondary phases such as Cu-free and Bi-free due to thermodynamic instabilities of Bi-2212 at temperatures above 860 °C. Finally, the intragranular critical current density values (J_c) have been obtained by using the data from M-H measurements according to the critical Bean model. It has been found that the J_c value (16.5 × 10⁴ A/cm² in 0.15 T at 15 K) of the sample in the dwell time of 2.5 h at 870 °C is higher than that of the sample (13.4 × 10⁴ A/cm² in 0.15 T at 10 K) with the same starting composition reported in the literature.