Large critical current densities in a silver-sheathed (Sr,Na)Fe2As2tape

We report the first superconducting tape using an iron-based superconductor (Sr,Na)Fe2As2 fabricated by the powder-in-tube (PIT) technique. Polycrystalline (Sr1−x Na x )Fe2As2 (x = 0.55), which we recently succeeded in synthesizing, was used as the starting powder. First, a silver (Ag) tube was groove-rolled into a square wire. The wire was then flat-rolled into tape form, followed by uniaxial cold pressing. The heat-treated tape exhibits a sharp superconducting transition at 34.5 K. Large transport critical current densities (J c) of over 1.0 × 104 A cm−2 are achieved at 20 K under a magnetic field of 2.5 T. J c at 4.2 K, as estimated from magnetic hysteresis curves, is 1.0 × 105 A cm−2 in 2.0 T, which is comparable to the transport J c reported for (AE,K)Fe2As2 (AE = Ba or Sr) tapes. SEM-EDX observations indicate that some Ag and As-rich material is present between (Sr,Na)Fe2As2 grains, even though no Ag was added to the starting powder. This Ag and As-rich material may strengthen grain connectivity and help to improve the transport J c.