Dominant effect of residual secondary phase of powders on Jc and microstructure of Bi-2212 superconducting wires

Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) superconducting round wires exhibited great potential for use in high-field applications. The purity of the precursor powders is critical for the transport current of the wires. However, the role of the residual secondary phase in the precursor powders is not fully understood. Here, the origin of the secondary phase was investigated in precursor powders that were prepared using ultrasonic spray pyrolysis (USP) and calcination processing. The microstructure and phase evolution of the precursor powders during the crystallization process were analyzed. Moreover, the effects that the residual secondary phase has on melting behavior, morphology properties, and the supercurrent flow of Bi-2212 multi-filamentary wires are systematically discussed. The residual secondary phase in the filament caused further crystallization, and this led to the formation of more and larger Bi-2201 grains at the onset of the melting process. The poor microstructure and low critical current of the final Bi-2212 wires can be attributed to the presence of the residual copper-rich phase. Bi-2212 wires that were prepared with fully crystallized powders had a high critical current density (J_c) of 6773 A/mm² at 4.2 K, self-field. It was revealed that control of the secondary phases in precursor powders is greatly significant for achieving superior values of J_c.