Nanocarbon-doped Fe-sheathed MgB 2 tapes with different doping levels were prepared by the in situ powder-in-tube method. Compared to the undoped tapes, J c for all the C-doped samples was enhanced by more than an order of magnitude in magnetic fields above 9 T. At 4.2 K, the transport J c for the 5 at% doped tapes reached 1.85×10 4 A/cm 2 at 10 T and 2.8×10 3 A/cm 2 at 14 T, respectively. Moreover, the critical temperature for the doped tapes decreased slightly. Transmission electron microscopy showed a number of intra-granular dislocations and the dispersed nanoparticles embedded within MgB 2 grains induced by the C doping. The mechanism for the enhancement of flux pinning is also discussed. These results indicate that powder-in-tube-processed MgB 2 tape is very promising for high-field applications.

122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields H c2 (> 100 T) and low anisotropy γ (<2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density J c , which is limited by poor grain coupling and weak-link behavior at grain boundaries.Here we report our recent achievement in the developing Sr 0.6 K 0.4 Fe 2 As 2 tapes with transport J c up to 0.1 MA/cm 2 at 10 T and 4.2 K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior J c performance exhibited in our samples. This advanced J c result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magnet applications.

The high upper critical field and low anisotropy of iron-based superconductors make them being particularly attractive for high-field applications. However, the current carrying capability needs to be enhanced by overcoming the weak-link effect between misaligned grains inside wire and tape conductors. Here we demonstrate a high transport critical current density (Jc) reaching 1.5  10 5 A/cm 2 (Ic = 437 A) at 4.2 K and 10 T in Ba0.6K0.4Fe2As2 (Ba-122) tapes prepared by a combination of conventional powder-in-tube method and optimized hot-press technique. The transport Jc measured 2 at 4.2 K under high magnetic fields of 27 T is still on the level of 5.5  10 4 A/cm 2 , which is much higher than those of low-temperature superconductors. This is the first report of hot-pressed Ba-122 superconducting tapes and these Jc values are by far the highest ever reported for iron-based superconducting wires and tapes. These highperformance tapes exhibit high degree of c-axis texture of Ba-122 grains and low anisotropy of transport Jc, showing great potential for construction of high-field superconducting magnets.

We demonstrate that Ta sheathed SmO 1-x F x FeAs wires were successfully fabricated by the powder-in-tube (PIT) method for the first time. Structural analysis by mean of x-ray diffraction shows that the main phase of SmO 1-x F x FeAs was obtained by this synthesis method. The transition temperature of the SmO 0.65 F 0.35 FeAs wires was confirmed to be as high as 52 K. Based on magnetization measurements, it is found that a globe current can flow on macroscopic sample dimensions with J c of ~3.9×10 3 A/cm 2 at 5 K and self field, while a high J c about 2×10 5 A/cm 2 is observed within the grains, suggesting that a significant improvement in the globle J c is possible. It should be noted that the J c exhibits a very weak field dependence behavior. Furthermore, the upper critical fields (H c2 ) determined according to the Werthamer-Helfand-Hohenberg formula are (T= 0 K) ≈ 120 T, indicating a very encouraging application of the new superconductors. 1 NdFeAsO 0.9 F 0.1 superconductor from local and global measurements. Preprint at<

High-performance Sr0.6K0.4Fe2As2 (Sr-122) tapes have been successfully fabricated using hot pressing (HP) process. The effect of HP temperatures (850–925°C) on the c-axis texture, resistivity, Vickers micro-hardness, microstructure and critical current properties has been systematically studied. Taking advantage of high degree of c-axis texture, well grain connectivity and large concentration of strong-pinning defects, we are able to obtain an excellent Jc of 1.2 × 105 A/cm2 at 4.2 K and 10 T for Sr-122 tapes. More importantly, the field dependence of Jc turns out to be very weak, such that in 14 T the Jc still remains ~ 1.0 × 105 A/cm2. These Jc values are the highest ever reported so far for iron-pnictide wires and tapes, achieving the level desired for practical applications. Our results clearly strengthen the position of iron-pnictide conductors as a competitor to the conventional and MgB2 superconductors for high field applications.

The discovery of iron-based superconductors, the first non-cuprate family of superconductors with T c above 40 K, has stimulated enormous interest in the field of superconductivity since last year. This remarkable discovery not only offers the opportunity to study the origin of superconductivity, but also opens up new possibilities of application. One of the most fascinating and useful properties of superconductors is the ability to carry electrical current with zero resistance. Here, we report the successful fabrication of dense Sr 0.6 K 0.4 Fe 2 As 2 superconducting wires using the ex situ powder-in-tube (PIT) method and demonstrate a transport J c of 3750 A cm −2 at 4.2 K. The connectivity of grains was improved upon doping (Ag or Pb) and the transport property of Sr 0.6 K 0.4 Fe 2 As 2 wires was enhanced for a lead-doped sample, especially in low fields, to a best I c of 37.5 A. Our results suggest that grain boundary properties require much greater attention when looking for applications of the high-T c iron-based superconductors.

EuFe 2 As 2 is a member of the ternary iron arsenide family. Similar to BaFe 2 As 2 and SrFe 2 As 2 , EuFe 2 As 2 exhibits a clear anomaly in resistivity near 200 K. Here we report the discovery of superconductivity in Eu 0.7 Na 0.3 Fe 2 As 2 by partial substitution of the europium site with sodium. ThCr 2 Si 2 tetragonal structure, as expected for EuFe 2 As 2 , is formed as the main phase for the composition Eu 0.7 Na 0.3 Fe 2 As 2 .Resistivity measurement reveals that the transition temperature T c as high as 34.7 K is observed in this compound. The rate of T c suppression with the applied magnetic field is 3.87 T / K, giving an extrapolated zero-temperature upper critical field of 90 T. It demonstrates a very encouraging application of the new superconductors.

Improving transport current has been the primary topic for practical application of superconducting wires and tapes. However, the porous nature of powder-in-tube (PIT) processed iron-based tapes is one of the important reasons for low critical current density (Jc) values. In this work, the superconducting core density of ex-situ Sr0.6K0.4Fe2As2 + Sn tapes, prepared from optimized precursors, was significantly improved by employing a simple hot pressing as an alternative route for final sintering. The resulting samples exhibited optimal critical temperature (Tc), sharp resistive transition, small resistivity and high Vickers hardness (Hv) value. Consequently, the transport Jc reached excellent values of 5.1 × 104 A/cm2 in 10 T and 4.3 × 104 A/cm2 in 14 T at 4.2 K, respectively. Our tapes also exhibited high upper critical field Hc2 and almost field-independent Jc. These results clearly demonstrate that PIT pnictide wire conductors are very promising for high-field magnet applications.