Nb3(Al.Ge) superconductors prepared by hot-substrate liquid-quenching and subsequent annealing

The A15 Nb
3
(Al.Ge) and Nb
3
Al superconductors with high critical current density J
c
have been successfully prepared by the liquid-quenching on a hot-substrate and subsequent annealing. The hot-substrate liquid quenching has a higher cooling rate than that of the conventional liquid-quenching techniques and facilitates the formation of completely supersaturated bcc solid solution of Nb
3
(Al
1-x
Ge
x
) up to x=0.3. The quenched materials are then transformed to the A15 structure by annealing above 750°C. Following the transformation, the critical temperature T
c
rises above 18 K, exact values depending on the composition and the heat treatment condition. The A15 phase converted from the bcc phase has an extremely small grain size of hundreds of angstroms, resulting in a high J
c
in high magnetic fields. The J
c
of the Nb
3
(Al.Ge) is higher than that of the Nb
3
Al probably due to the smaller grain size. J
c
of 3.5 × 10
⁵
A/cm
²
at 4.2 K and 16 T has been achieved in the Nb
3
(Al
0.8
Ge
0.2
sample annealed at 850°C for 7 hr, which is much higher than the highest value measured in the V
3
Ga superconductors. It is demonstrated that this technique of hot-substmate liquid-quenching process is potentially useful for tape conductor fabrication, because the scaling-up is straightforward and the thin copper substrate which strongly adheres to the A15 layer can serve directly as a stabilizing material.