Optimization of Operating Conditions for a RHQT (Rapid-Heating, Quenching and Transformation) Processed Nb₃Al Superconductors

Abstract

  The RHQT (rapid-heating, quenching and transformation) processed Nb₃Al superconducting wire has a larger critical current density J_c 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 J_c through controlling the microchemstry and grain structure of bcc and hence A15 phases, investigated have been only the RHQ parameters of heating current I_RHQ and wire speed v_wire for a given electrode spacing (100 mm). From a view point of apparatus design a smaller I_RHQ 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 v_wire must become slower. However, such a slow v_wire 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 v_wire (high cooling rate) but a small I_RHQ, 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 T_c of the Nb₃Al transformed from bcc supersaturated-solid solution, which was 17.6 K, and allowed the I_RHQ to be reduced to ∼1/√3 in comparison to the conventional I_RHQ condition with the same v_wire. The newly optimized RHQ condition of 0.33 m/s, 48 A for the electrode spacing of 300 mm eventually doubled the J_c (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.