Performance tuning and improvement of Jc–B characteristics for Bi-2212/Ag tapes by controlling the excess oxygen content

Abstract

The dependence of the Jc value on applied magnetic fields (Jc–B characteristics) is investigated for Bi-2212/Ag tapes prepared by the melt-solidification method with respect to the effect of annealing after heat treatment (post-annealing). The results of the present work show that annealing under a high-oxygen partial-pressure and low-temperature condition enhances the Jc–B characteristics of Bi-2212/Ag tapes, and the sample annealed under a condition of p(O2) = 1 atm and 300 °C exhibited the highest Jc–B characteristics. In this case, no change in microstructure was observed; however, the Tc value and c-axis length of the Bi-2212 layer apparently decreased by the oxygenation resulting from post-annealing. Therefore, the improvement of the Jc–B characteristics is considered to be due to the increase in the excess oxygen content of the parent oxide layer in the tape sample, i.e. the decrease in the electromagnetic anisotropy of the Bi-2212 phase by the increase in the hole doping level. Based on this result, we also succeeded in the reversible control of the Jc–B characteristics for the Bi-2212/Ag tape by post-annealing. Furthermore, it is also revealed that the oxygenation by post-annealing restores the Jc–B characteristics of the Bi-2212/Ag sample with a good microstructure and less oxygen content. This means that the Bi-2212/Ag tape with low Jc–B characteristics should not be completely dismissed as hopeless. It is concluded that post-annealing after heat treatment is an indispensable process for the systematic investigation of Bi-2212/Ag tapes.