Field and temperature dependences of critical current in direct-heated Nb-tube processed Nb/sub 3/Al wires

High-field performance of Nb-tube processed Nb/sub 3/Al conductors is improved and allows these conductors for use above 20 T, when continuous direct-heating is applied before final annealing in the fabrication process. In the present study we prepared 0.74 mm/spl phi/ Nb/sub 3/Al wires containing 121/spl times/121 7-core elementary Nb/Al-Mg composites by this modified method and measured their critical current, magnetization and susceptibility as a function of field and temperature. From these measurements, we have found that the volume pinning force, F/sub p/ of these wires simply follows the Kramer´s scaling law, i.e. F/sub p//spl prop/B/sub c2//sup 2.5/b/sup 0.5/(1-b)/sup 2/ w here b=B/B/sub c2/, for the entire range of temperature measured, 4.2/spl sim/16 K. Since A15 phase grains formed are very fine, the most probable pinning source seems to be grain boundaries which are dense compared to other possible sources such as interfaces between Nb-Al alloy phase and Nb/sub 3/Al filaments.