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

The A15 Nb3(Al.Ge) and Nb3Al superconductors with high critical current density Jchave 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 Nb3(Al1-xGex) 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 Tcrises 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 Jcin high magnetic fields. The Jcof the Nb3(Al.Ge) is higher than that of the Nb3Al probably due to the smaller grain size. Jcof 3.5 × 10⁵A/cm²at 4.2 K and 16 T has been achieved in the Nb3(Al0.8Ge0.2sample annealed at 850°C for 7 hr, which is much higher than the highest value measured in the V3Ga 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.