Large critical current densities in a silver-sheathed (Sr,Na)Fe sub(2)As sub(2) tape

We report the first superconducting tape using an iron-based superconductor (Sr,Na)Fe sub(2)As sub(2) fabricated by the powder-in-tube (PIT) technique. Polycrystalline (Sr sub(1-x)Na sub(x))Fe sub(2)As sub(2) (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 sub(c)) of over 1.0 x 10 super(4) A cm super(-2) are achieved at 20 K under a magnetic field of 2.5 T. J sub(c) at 4.2 K, as estimated from magnetic hysteresis curves, is 1.0 x 10 super(5) A cm super(-2) in 2.0 T, which is comparable to the transport J sub(c) reported for (AE,K)Fe sub(2)As sub(2) (AE = Ba or Sr) tapes. SEM-EDX observations indicate that some Ag and As-rich material is present between (Sr,Na)Fe sub(2)As sub(2) 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 sub(c).