Transport Critical Current Density of $\hbox{Sr}_{0.6}\hbox{K}_{0.4}\hbox{Fe}_{2}\hbox{As}_{2}\hbox{/}\hbox{Ag}$ Superconducting Tapes Processed by Flat Rolling and Uniaxial Pressing

Ag-sheathed Sr_0.6K_0.4Fe₂As₂ superconducting tapes with various thicknesses were produced by flat rolling and cold uniaxial pressing based on an ex-situ powder-in-tube method. The transport critical current density (J_c) for both rolled and pressed tapes was found to increase with the decrease of tape thickness. Compared with flat rolling, cold pressing can further enhance the transport Jc of Sr_0.6K_0.4Fe₂As₂ tapes. A transport J_c as high as 4.8 × 10⁴ A/cm² at 4.2 K and 10 T was achieved by pressing Sr_0.6K_0.4Fe₂As₂ tapes at about 1.0 GPa. This superior transport J_c in the pressed tapes can be ascribed to improved mass density and grain texture of Sr_0.6K_0.4Fe₂As₂ superconducting cores.

I. Introduction

AMONG a variety of iron-based superconductors [1], the K doped 122 pnictides ( , or alkali earth elements) are very promising for high-field applications, because of their relatively high superconducting transition temperatures ( ) up to 38 K [2], very high upper critical fields ( ) above 100 T [3], [4], small anisotropy with being about 1.5–2 [5], [6], and large critical current density ( ) over in thin films [7]– [9]. As a convenient and low-cost way to prepare Bi-based cuprates and superconducting wires and tapes, the powder-in-tube (PIT) method has been widely considered to be suitable for large-scale applications. Therefore, the PIT method was used to develop iron-based superconducting wires and tapes since 2008. With lots of endeavors being made to overcome the non-superconducting layer between sheath materials and superconducting cores, defects such as cracks, pores and impurities inside superconducting cores, and weak links at high-angle grain boundaries, the transport has been significantly improved, especially for the 122 iron-pnictide tapes [10].