Improved critical current densities in Nb3Al based conductors

By reducing substantially the thickness of the Nb and Al reacting layers, very high current densities (Jc= 1.8×10⁹A/m²at 11 T and 4.2k) have been I T and 4.2 K) have been achieved in Nb3Al single-core conductors produced by the jelly-roll approach. This impressive improvement of Jcis larger than expected from previous results which correlate critical current densities with the thickness of the layered structure. The present characteristics of commercial Nb thin foils do not allow very high Jcmultifilamentary wires to be produced by the jelly-roll process, since a recrystallized 10-20 μm Nb foil containing less than 50 ppm of oxygen would be required. However, a new process, the intrinsic multifilamentary approach, IMF¹⁰, has been set up to obtain a multifilamentary wire by using commercial Nb thin sheets. According to this method, some IMF samples have been produced with critical current densities somewhat lower than the density of the jelly-roll conductors but comparable with Nb3Sn conductors. Improvements are still possible in order to approach the jelly-roll results. The effective filament diameters of IMF conductors, determined by magnetization measurements, are around 20 μm, suggesting that among the Nb3Al thin sheets there are some contacts, while some unreacted Nb could provide barriers in the radial direction. Similar measurements on monofilamentary jelly-roll conductors are in progress.