急熱急冷変態法 Nb3Al 超伝導体の急熱急冷条件の最適化

Abstract

The RHQT (rapid-heating, quenching and transformation) processed Nb3Al superconducting wire has a larger critical current density Jc in high magnetic fields and better strain tolerance than Nb3Sn does. Thus, the Nb3Al wire is one of the most promising candidate superconductors for the large-scale and high-field applications. Although the RHQ optimization is a key to a high Jc through controlling the microchemstry and grain structure of bcc and hence A15 phases, investigated have been only the RHQ parameters of heating current IRHQ and wire speed vwire for a given electrode spacing (100 mm). From a view point of apparatus design a smaller IRHQ is favorable, but a heating time which it takes for a wire to move between electrodes is instead needed to become longer for heating the wire up to 2273 K. Thus vwire must become slower. However, such a slow vwire also causes a slower cooling rate of wire that would partly form undesirable A15 phase at the RHQ treatment. In the present study, an attempt has been made to extend the electrode spacing from 100 mm to 300 mm. This enables a relatively large vwire (high cooling rate) but a small IRHQ, which ensures a heating time enough to react the precursor. The Nb/Al precursor used has been prepared by the double-stacked rod-in-tube process, where the starting material is a 7-core Nb/Al composite of which Al was alloyed with 5 at% Mg. The final size of Al alloy core calculated is 0.56 μm, several times larger than the thickness of Al layer of jelly-roll Nb/Al precursor. The extension of electrode spacing from 100 mm to 300 mm did not degrade Tc of the Nb3Al transformed from bcc supersaturated-solid solution, which was 17.6 K, and allowed the IRHQ to be reduced to ∼1/√3 in comparison to the conventional IRHQ condition with the same vwire. The newly optimized RHQ condition of 0.33 m/s, 48 A for the electrode spacing of 300 mm eventually doubled the Jc (15 T, 4.2 K), at least, for the rod-in-tube processed Nb3Al that has not been mechanically deformed between RHQ and transformation annealing.