Influence of Sr/Ca on the phase evolution mechanism, phase purity and critical current density of Bi-2212 wires

Abstract

Two batches of Bi-2212 precursor powder with Sr/Ca = 2.24, 2.33 were prepared by the spray pyrolysis technology. Then two Bi-2212 superconducting wires marked as Line4 and Line8-2 were prepared by the above powders. Line4 with Sr/Ca = 2.24 showed much higher phase purity, higher texture and its critical current density(J_c) was 1.5 times that of Line8-2 with Sr/Ca = 2.33. Their micro-structure evolution was scrutinized by quenching each wire at 7 instantaneous moments during the partial melting process(PMP). Bi-2212 was found to decompose into (Sr,Ca)₁₄Cu₂₄O_x(14:24AEC), Bi₉Sr₁₁Ca₅O_x(9:16CF), Bi-rich liquid Bi-2212 and Bi-rich solid Bi-2212 at the initial stage of PMP. When Bi-2212 began to solidify, the above four phases reacted to generate Bi-2212. By analyzing the particle size, the content and the composition variation for 14:24AEC, 9:16CF as well as the composition variation for Bi-2212 matrix in above 7 moments of PMP, the phase evolution’s difference between two wires was finally confirmed. The formation energy of 14:24AEC was smaller compared with 9:16CF, while 9:16CF was faster on dynamics. 14:24AEC determined the whole synthetic reaction’s rate of Bi-2212, and Sr/Ca as well as its value fluctuation in Bi-2212 precursor powder can decide both the timeline and the driving force of PMP. A larger Sr/Ca in Line8-2 made it melt earlier compared with Line4, which led to its earlier timeline during the melting stage of PMP. While the more consistent phase evolution’s pace between 14:24AEC and 9:16CF in Line4 finally contributed a larger Sr/Ca after solidification. Both the larger Sr/Ca and its larger fluctuation in Line4 finally contributed to its faster phase evolution’s pace, higher phase purity, better texture and higher J_c. The deep logic driving the phase evolution mechanism in Bi-2212 wires was disclosed for the first time, which will be very helpful to the future improvement of J_c for Bi-2212 wires.