Current Transport Mechanism and Control of the $n$-Value for Ag/Bi2212 Wires and Tapes
In order to study the mechanism of the small n-value for Bi2212, E- J characteristics were measured carefully for various kinds of Bi2212 tapes and round wires in high magnetic fields. Some of wires were heat-treated in a magnetic field of 5 T. We found that the n -values of all the samples increase by the in-field heat-treatment at 5 T. In addition, we analyzed the E- J properties by the combination of the percolation model based on the local Jc distribution and the two directional Jc distribution model. The improvement of the n-value by the in-field heat treatment can be understood by the change of the aspect-ratio of the grains. Therefore, the aspect-ratio of the grains plays an important role in the current transport influencing not only Jc but also n-value.