Chemical stability and superconductivity in Ag-sheathed CaKFe4As4 superconducting tapes

TL;DR

This study fabricates Ag-sheathed CaKFe4As4 superconducting tapes, analyzes their stability and superconducting properties, and identifies grain connectivity as a key factor affecting their critical current density, suggesting further optimization for practical use.

Contribution

It introduces a fabrication method for CaKFe4As4 tapes, investigates their thermal stability and superconducting performance, and highlights the importance of grain connectivity for improving critical current density.

Findings

CaKFe4As4 phase is unstable at high temperatures.

Critical current density Jc is approximately 2.7x10^4 A/cm2 at 4.2 K.

Low sintering temperature leads to poor grain connectivity.

Abstract

Ag-sheathed CaKFe4As4 superconducting tapes have been fabricated via the ex-situ powder-in-tube method. Thermal and X-ray diffraction analyses suggest that the CaKFe4As4 phase is unstable at high temperatures. It decomposes into the CaAgAs phase which reacts strongly with the silver sheath. We therefore sintered the tape at 500C and obtain a transport critical current density Jc(4.2 K, 0 T)~ 2.7x10^4 A/cm2. The pinning potential derived from magnetoresistance measurements is one order of magnitude lower than that of the (Ba/Sr)1-xKxFe2As2 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 Jc. However, this system is still a promising candidate for superconducting wires and tapes if we further optimize the post-annealing process to achieve better grain connectivity.