Chemical stability and superconductivity in Ag-sheathed CaKFe4As4 superconducting tapes

Ag-sheathed CaKFe4As4 superconducting tapes have been fabricated via the ex situ powder-in-tube method. X-ray diffraction analyses suggest that the CaKFe4As4 phase reacts strongly with the silver sheath at temperature above 500 degrees C. We therefore anneal the tape at 500 degrees C and obtain a transport critical current density J(c)(4.2 K, 0 T) similar to 2.7 x 10(4) A cm(-2). The pinning potential derived from magnetoresistance measurements is one order of magnitude lower than that of the (Ba/Sr)(1-x)KxFe2As2 tapes. Combining with the scanning electron microscopy and magneto-optical imaging results, we suggest that bad connectivity between superconducting grains caused by the low sintering temperature is the main factor responsible for the low J(c). However, this system is still a promising candidate for superconducting wires and tapes if we find a compatible sheath material and further optimize the post-annealing process to achieve better grain connectivity.