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 $J_{\rm c}$ distribution and the two directional $J_{\rm c}$ 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 $J_{\rm c}$ but also $n$-value.