On the role of precursor powder composition in controlling microstructure, flux pinning, and the critical current density of Ag/Bi₂Sr₂CaCu₂O_xconductors

Abstract

Precursor powder composition is known to strongly affect the critical current density (J _c) of Ag/Bi$_2$Sr$_2$CaCu$_2$O$_x$ (Bi-2212) wires. However, reasons for such J _c dependence have not yet been fully understood, compromising our ability to achieve further optimization. In this paper, 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. Finally, high-J _c conductors also show higher flux-pinning strength, which we ascribe to their higher Bi-2201 intergrowth density.