On the role of precursor powder composition in controlling microstructure, flux pinning, and the critical current density of Ag/Bi₂Sr₂CaCu₂O_x conductors

Precursor powder composition is known to strongly affect the critical current density (J _c) of Ag/Bi₂Sr₂CaCu₂O_x (Bi-2212) wires. However, reasons for such J _c dependence have not yet been fully understood, compromising our ability to achieve further optimization. We systematically examined superconducting properties, microstructural evolution and phase transformation, and grain boundaries of Bi-2212 conductors fabricated from precursor powders with a range of compositions using a combination of transport-current measurements, a quench technique to freeze microstructures at high temperatures during heat treatment, and scanning transmission electron microscopy (STEM). Samples include both dip-coated tapes and round wires, among which a commercial round wire carries a high J _c of 7600 A mm^-2 at 4.2 K, self-field and 2600 A mm^-2 at 4.2 K, 20 T, respectively. In the melt, this high-J _c conductor, made using a composition of Bi_2.17Sr_1.94Ca_0.89Cu₂O_x, contains a uniform dispersion of fine alkaline-earth cuprate (AEC) and copper-free solid phases, whereas several low-J _c conductors contain large AEC particles. Such significant differences in the phase morphologies in the melt are accompanied by a drastic difference in the formation kinetics of Bi-2212 during recrystallization cooling. STEM studies show that Bi-2212 grain colonies in the high-J _c conductors have a high density of Bi₂Sr₂CuO_y (Bi-2201) intergrowths, whereas a low-J _c conductor, made using Bi_2.14Sr_1.66Ca_1.24Cu_1.96O_x, is nearly free of them. STEM investigation shows grain boundaries in low-J _c conductors are often insulated with a Bi-rich amorphous phase. High-J _c conductors also show higher flux-pinning strength, which we ascribe to their higher Bi-2201 intergrowth density.