Formation of Nb3Al in Nb-Al superconductors by powder process

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Abstract
The critical current density of Nb-Al superconductors made by powder process is limited predominantly by the volume fraction and composition of the superconducting A15 phase (Nb3Al). Reacted Nb-Al powder wires contain A15, sigma phase, and unreacted Nb. The Nb and Al particles in an unreacted wire draw down to thin sandwiched ribbons. The layers in the wire are not uniform. In some regions, Nb particles have agglomerated to form superlayers which limit the extent of A15 formation. In the reacted wire, continuous regions of A15 up to 1 μm wide are seen in TEM. Other regions form layered arrangements of Nb/A15/σ/A15/Nb. The presence and volume fraction of these phases can be understood in light of recent work on thin Nb/Al multilayers made by sputter deposition. To characterize the average Nb layer thickness in the unreacted wires, a Nb "drawing index" is defined as (Nb powder size)/(R)1/2. The critical current density is found to scale well with this parameter for a wide range of wires. To decrease the agglomeration of Nb particles, a set of wires were made in which the loose Nb powders were precoated with Al by evaporation.