Nb3(Al,Ge) multifilamentary wires made by the rapidly-heating/quenching process

Nb₃(Al,Ge) wires, made through the rapid-heating/quenching process, are very promising for the applications in high fields. The precursor wires were fabricated by a rod-in-tube method, and several kinds of Al-Ge alloys (Al-2at%, 5at%, 10at% and 20at%Ge) were used for the core materials. In the case of Al-20at%Ge alloy core, the disordered A15 phases formed directly with the rapid-heating/quenching. It is necessary for the as-quenched wires to be additionally annealed in order to improve the superconducting properties through the recovery of the long-range order of A15 crystal structure. The best values of T_c and H_c2 (4.2 K) for the Nb ₃(Al,Ge) wires were 19.4 K and 39 T, respectively. Furthermore, J_c apparently increased with decreasing the Al-Ge alloy core size in the precursor wires. J_c (4.2 K, 25 T) of 0.3 μm-core Nb₃(Al,Ge) wire exceeds 150 A/mm² , which was about two times larger than that of 1.5 μm-core Nb ₃(Al,Ge) wire. The results of TEM observation revealed that the reduction of Al-Ge alloy diameter enhanced the volume fraction of the A15-Nb₃(Al,Ge) phases through increasing of diffusion pairs density in the composite. The diameter of A15 crystal grains is about 100 nm, and scarcely dependent on the Al-Ge alloy core size