Improvement of Critical Current Density in NbTi/Nb/Cu Superconducting Multilayer Sheet by Rolling Elongation of Ti Precipitates

The critical current density (Jc) of NbTi/Nb/Cu superconducting sheet is determined by two types of Ti precipitate in NbTi layers. One type is approximately 160nm in thickness and 240nm in length, and is precipitated in the grain boundaries. The other is a few nanometers in thickness and 20 to 30nm in length, and is precipitated in crystal grains. But contribution to the Jc value due to the larger precipitate is relatively small, and consequently the Jc value is as small as one-third of that of commercial NbTi multifilamentary wires. In this study, the sheet was rolled by a reduction of 50% or more after the final aging treatment, and the large precipitates were elongated to a favorable size for pinning sites. Consequently the Jc value in the transverse direction at 5T is remarkably improved to twice that of the conventionally aged specimen. On the other hand, the increased value of the Jc in the longitudinal direction is small in comparison to that in the transverse specimen. This is because there are many deformed sections in the NbTi layers of the longitudinal specimen. The NbTi layers of the transverse specimen have fewer deformed sections than those of the longitudinal specimen. The mechanical elongation of the specimen rolled after aging treatment is decreased to less than 2%, but it is recovered by short heat treatment. The Jc value of the transverse specimen decreases by 5 to 15%, while that of the longitudinal specimen increases by 2 to 12% by applying recovery heat treatment. Since the tensile stress remaining between the Nb layers and Cu layers in the longitudinal direction due to the rolling is relieved by recovery heat treatment, the Jc value of the longitudinal specimen is increased.