Qianjun Zhang

Realization of practical level current densities in Sr0.6K0.4Fe2As2 tape conductors for high-field applications

122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields Hc2 (> 100 T) and low anisotropy γ (<2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density Jc, which is limited by poor grain coupling and weak-link behavior at grain boundaries. Here, we report our recent achievement in the developing Sr0.6K0.4Fe2As2 tapes with transport Jc up to 0.1 MA/cm2 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 Jc performance exhibited in our samples. This advanced Jc result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magne...

Hot pressing to enhance the transport Jc of Sr0.6K0.4Fe2As2 superconducting tapes

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.

Strongly enhanced current densities in Sr0.6K0.4Fe2As2 + Sn superconducting tapes

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.

Microstructure and transport critical current in Sr0.6K0.4Fe2As2 superconducting tapes prepared by cold pressing

Fe-clad Sr0.6K0.4Fe2As2 (Sr-122) superconducting tapes were fabricated by cold uniaxial pressing. It is found that the pressing can improve the mass density and induce c-axis texture in the Sr-122 core. With an increase of pressure from 0.6 to 1.8 GPa, the degree of texture increases monotonically. The transport current density (Jc) is correspondingly improved until the applied pressure comes up to 1.4 GPa, and finally becomes saturated at 1.8 GPa. A transport Jc as high as 16.8 kA cm−2 (at 4.2 K in self-field) is achieved at 1.4 GPa. The saturation effect of transport Jc can be ascribed to the microcracks in the grains induced by the strong deformation force when the applied pressure is high.

Fabrication and transport properties of Sr0.6K0.4Fe2As2 multifilamentary superconducting wires

Seven-core Ag/Fe sheathed Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires were produced by the ex situ powder-in-tube method. The relationship between the cold-work deformation process and the superconducting properties of the wires was systematically studied. It was found that flat rolling can efficiently increase the mass density of the superconducting core, thus, significantly improving the transport critical current density (Jc) of the as-drawn wires. The transport Jc of the best sample achieved 21.1 kA/cm2 at 4.2 K in self field, and showed very weak magnetic field dependence at high fields. Our result suggested a promising future of multifilamentary iron-based superconductors in practical applications.

Enhancement of transport critical current density of SmFeAsO1−xFx tapes fabricated by an ex-situ powder-in-tube method with a Sn-presintering process

SmFeAsO1−xFx (Sm1111) tapes were prepared by an ex-situ powder-in-tube method with a Sn-presintering process. Scanning electron microscopy revealed apparent difference in microstructure between Sn-presintered tapes and the previously reported polycrystalline Sm1111 bulk, since Sn has reduced FeAs wetting phase and filled the voids between Sm1111 grains. The Sn-presintered tapes showed significant enhanced field dependences of transport Jc compared with Sn-added tapes. A highest transport critical current density (Jc) of 3.45 × 104 A cm−2 at 4.2 K and self-field is achieved. Magneto-optical imaging further confirmed large and well-distributed global and intergranular Jc in Sn-presintered Sm1111 tapes.

Combined effect of Sn addition and post-rolling sintering on the superconducting properties of SmFeAsO1−xFx tapes fabricated by an ex-situ powder-in-tube process

Polycrystalline Sn-added SmFeAsO1−xFx tapes were prepared by an ex-situ powder-in-tube method. It is found that the transport critical current density (Jc) of the SmFeAsO1−xFx tapes can be significantly improved with 10-30 wt. % Sn addition. A transport Jc of 2.11 × 104 A cm−2 at 4.2 K in self-field was obtained in SmFeAsO1−xFx + 30 wt. % Sn sample. Most interestingly, Sn-added SmFeAsO1−xFx tapes without any sintering also showed substantial transport Jc of 9.81 × 103 A cm−2, which is reported for the first time. X-ray diffraction and scanning electron microscopy characterization revealed that the elemental Sn dispersed in the sample transformed into FeSn2 during the post-rolling sintering (PRS). The reaction between Sn and FeAs helps to eliminate the detrimental FeAs wetting phase, which blocks the supercurrent path between SmFeAsO1−xFx grains. The combined impact of fluorine loss and Sn reaction was discussed in this work by varying the Sn addition quantity and PRS condition.

Large transport Jc in Cu-sheathed Sr0.6K0.4Fe2As2 superconducting tape conductors

Copper sheath is the first choice for manufacturing high-Tc superconducting wires and tapes because of its high electrical and thermal conductivities, low-cost and good mechanical properties. However, Cu can easily react with superconducting cores, such as BSCCO, MgB2 and pnictides, and therefore drastically decrease the transport Jc. Here, we report the fabrication of Cu-sheathed Sr1−xKxFe2As2 tapes with superior Jc performance using a simple hot pressing method that is capable of eliminating the lengthy high-temperature sintering. We obtained high-quality Sr1−xKxFe2As2 tapes with processing at 800 oC for 30 minutes and measured high Tc and sharp transition. By this rapid fabrication, Cu sheath does not give rise to apparent reaction layer, and only slightly diffuses into Sr-122 core. As a consequence, we achieved high transport Jc of 3.1 × 104 A/cm2 in 10 T and 2.7 × 104 A/cm2 in 14 T at 4.2 K. The in-field Jc performance is by far the highest reported for Cu-sheathed high-Tc conductors. More importantly, Cu-sheathed Sr-122 tapes also showed a high Je value of 1.0 × 104 A/cm2 in 10 T at 4.2 K, which has reached the widely accepted practical level for applications. These results demonstrate that Cu is a very promising sheath for the practical application of pnictide conductors.

Vortex pinning and dynamics in high performance Sr0.6K0.4Fe2As2 superconductor

We have studied vortex pinning and dynamics in a Sr0.6K0.4Fe2As2 superconducting tape with critical current density Jc ∼ 0.1 MA/cm2 at 4.2 K and 10 T. It is found that grain boundary pinning is dominant in the vortex pinning mechanism. Furthermore, we observe large density of dislocations which can also serve as effective pinning centers. We find that the temperature dependence of critical current density is in agreement with the model of vortices pinned via spatial fluctuation of charge carrier mean free path. Magnetic relaxation measurement indicates that the magnetization depends on time in a logarithmic way. The relaxation rate in the low and intermediate temperature region is small, and it exhibits a weak temperature and field dependence. A crossover from elastic creep to plastic creep regime is observed. Finally, we conclude a vortex phase diagram for the high performance Sr0.6K0.4Fe2As2 superconducting tape.

Critical Current Analysis of an YBCO Insert for Ultrahigh-Field All-Superconducting Magnet

With the development of superconducting magnet technology, high magnetic field experimental conditions can be more easily achieved than ever. Recently, a superconducting magnet with the center field of 25 T at 4.2 K and a worm bore size of 32 mm in diameter has been designed. The superconducting magnet consists of NbTi, Nb3Sn coils, and YBCO insert. The YBCO insert has been designed, fabricated, and tested in the liquid helium at the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS), to prove the technical feasibility to achieve the target of 25 T. The YBCO insert has the inner diameter, outer diameter, and height of 41, 124, and 115 mm, respectively. The critical current distribution of the YBCO insert at 77 and 4.2 K, self-field, and in a 15-T background field is given in detail, based on the electrical properties of an YBCO-coated conductor. An analysis on the critical current shows that the YBCO insert has a minimum critical current of 19.4 A at 77 K, 119 A at 4.2 K, at the self-field, respectively, and 177 A at 4.2 K in a 15-T background field. Preliminary tests in the liquid nitrogen and liquid helium show that the analysis on the critical current is reasonable and agrees well with the test. The analysis on critical current at 4.2 K shows that it is much reasonable for the YBCO insert to obtain a central magnetic field of up to 24 T in a 15-T background field. In this paper, the design, critical current analysis, and preliminary experiment are reported.