Effect of Ag on Microstructure and Superconducting Properties of Bi₂Sr₂CaCu₂O_x

Abstract

Transport critical current density J_c of polycrystalline high-T_c oxide superconductors are usually small due to the weak couling of grains. For the Bi-based oxide superconductors, grain coupling is significantly improved by the c-axis grain alignment and high transport J_c was obtained. Especially, for the Bi₂Sr₂CaCu₂O_X(Bi-2212), high degree of grain alignment is obtained by the slow cooling and solidification from the partially molten state on Ag substrate.¹ By this grain alignment, excellent transport J_c values over 10⁵A/cm² are attained at 4.2K even in high magnetic fields above 25T. In this solidification process, Ag substrate plays very important roles in the alignment of plate-like Bi-2212 grains.² During the heat treatment, Bi-2212 decomposes into partially molten state consisting of Bi-free phase, Cu-free phase and liquid phase.³ Ag enhances this decomposition of Bi-2212, and also lowers solidification temperature of Bi-2212 during slow cooling process.⁴ However, the effect of Ag on the microstructural changes during heating and cooling process is not well investigated yet. Such an investigation is important for the understanding of role of Ag on grain alignment.