Achievements in developing high performance iron-based superconducting wires
Recently, by employing new mechanical pressing process, Prof. Yanwei Ma’s group in IEECAS achieved the highest transport critical current density (Jc) in Sr1-xKxFe2As2 (Sr-122) iron-based superconducting tapes. The current density reached over 10^5 A/cm^2 at 4.2 K and 10 T. This achievement sets a record in Jc value for iron-pnictide wires and this is considered as a technical breakthrough in reaching the Jc level desired for practical applications.
After preparing the world’s first iron-based superconducting wire in 2008, Ma’s group has continuously enhanced the transport Jc for iron-based superconducting wires and tapes, through systematical research on the sheath materials, deformation process, heat treatment, chemical doping and grain texture. In Sept. 2013, they processed the as-rolled tapes by hot pressing combined with high-quality precursors, which significantly increased the mess density of the superconducting core, and eliminated the residual micro-cracks induced during the defamation process, thus greatly improved the transport Jc of Sr-122 tapes up to 5.1×10^4 A/cm^2 (4.2 K, 10 T). This result was published in Scientific Reports 4, 4465 (2014). Soon after this, by further optimizing the pressing process, they obtained highly dense textured Sr-122 tapes, whose transport Jc achieved the practical level of 10^5 A/cm^2 (4.2 K, 10 T) for the first time. The grain boundary structure was investigated on atomic scale using high resolution TEM, it was found that the Sr-122 phase inside the tapes had good grain connectivity and lots of clean grain boundaries with low misorientation angle. The superior Jc in these Sr-122 tapes can be attributed to the combination of significantly improved grain connectivity, good grain texture and strong pinning characteristics. This work was recently reported by Appl. Phys. Lett. 104, 202601 (2014), and was highly commented by the referee as “The highest transport Jc reported in Sr122 tape is very encouraging for the superconductivity community”. In order to further promote the application of iron-based superconducting wires, Ma’s group also developed high-performance 7- and 19-core multifilamentary wires. The best transport Jc of 7-core Sr-122 tapes was 6.1×10^4 A/cm^2 (4.2 K, 10 T), which is the highest value ever reported for iron-based superconducting multifilamentary wires and tapes. On the other hand, compared with 122 type iron-based superconductors, SmFeAsO1-xFx (Sm-1111) has higher transition temperatures and upper critical fields. However, due to higher synthesis temperature, it is still very challenging to prepare high-Jc Sm-1111 tapes. Recently, by add Sn powder into Sm-1111 precursor during the sintering process, Ma’s group successfully reduced the FeAs impurity phase between Sm-1111 grains, and set a new record of 3.45×10^4 A/cm^2 (4.2 K, 10 T) for the transport Jc of Sm-1111 tapes, just as reported in Appl. Phys. Lett. 104, 172601 (2014) . These works were partially supported by the National ‘973’ Program, the National Natural Science Foundation of China and the Beijing Municipal Natural Science Foundation.
The field dependent transport Jc of mono-core, 7-core and 19-core Sr-122 iron-based superconducting tapes. The transport Jc of mono-core Sr-122 tapes achieved the practical level of 10^5 A/cm^2 at 4.2 K and 10 T.