Sr-122 Tapes Research Articles

Enhanced transport critical current of iron-based superconducting joints

Superconducting joints are one of the crucial components to make Sr1−xKxFe2As2 (Sr-122) superconducting wires or tapes successful for future high-field, high-homogeneity magnetic application. In this paper, the hot-pressing process for an iron-based superconducting joint of Sr-122 tapes was optimized. The microstructures, superconducting properties and element distribution in the connection areas were researched. The transport properties of the iron-based superconducting joints were enhanced by prolonging the holding time at high pressure, which prevents potassium loss in the iron-based superconducting joint. The best transport critical current that was achieved was 57 A at 4.2 K and 10 T; meanwhile, the critical current ratio (CCR = Icjoint/Icnon-joint) of the joint was realized to be 63.3%, which is the highest value of an iron-based superconducting joint reported so far. These results clearly demonstrate that iron-based superconductors are very promising for high-field magnet applications.

Transport current density at temperatures up to 25 K of Cu/Ag composite sheathed 122-type tapes and wires

The fabrication of iron-based superconductors with high transport critical current density (Jc) and low cost is a crucial determinant of whether they can be used for practical applications. In this paper, Cu/Ag composite sheathed Sr0.6K0.4Fe2As2 (Sr122) tapes and Ba0.6K0.4Fe2As2 (Ba122) round wires were fabricated via an ex situ powder-in-tube method and heat-treated by the hot pressing and hot isostatic pressing process respectively. In order to thoroughly reveal the application potential of Cu/Ag composite sheathed ‘122’ iron pnictide superconductors, transport Jc of tapes and wires in high fields at temperatures up to 25 K was measured. High transport Jc of 4.4 × 104 A cm−2 at 4.2 K and 3.6 × 103 A cm−2 at 20 K in 10 T was achieved in Cu/Ag composite sheathed Sr122 tapes. Transport Jc of Ba122 wires is 9.4 × 103 A cm−2 at 4.2 K and 1.9 × 103 A cm−2 at 20 K in 10 T. These results demonstrate the great potential of Cu/Ag composite sheathed ‘122’ iron pnictide superconducting tapes and wires for high-field applications at intermediate temperatures around 20 K, which can be easily obtained by using cryocoolers.

Superconducting Properties of 100-m Class Sr0.6K0.4Fe2As2 Tape and Pancake Coils

Iron-pnictides are hotly studied since 2008 in the superconducting materials research area, due to their special properties and unclear mechanism. Big achievement has been made in the pnictide research during the past years. For practical uses, pnictide superconductor should be fabricated in a long wire form which can be used for different devices. In this work, 100 m class 7-core Sr0.6K0.4Fe2As2 (Sr122) tapes have been made using the powder-in-tube technique, which is reported for the first time. Clearly, an average Jc of 1.3x10^4 A/cm^2 at 10 T was reached over the 115 meter length, showing a high property and good uniformity of 100 meter level Sr122 tapes. Using the 10 m long Sr122 tapes, two double-pancake coils were fabricated by a wind and reaction technique. No transport current could be measured for the coil made from the mono-filamentary tape. However, transport Ic was obtained in the coil made from the 7-filamentary tape. The factors which affect the superconducting property of the coil were discussed in this work.

Large transport J(c) in Cu-sheathed Sr(0.6)K(0.4)Fe2As2 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.

Investigation of <inline-formula> <tex-math notation="TeX">$J_{c}$</tex-math></inline-formula>-Suppressing Factors in Flat-Rolled <inline-formula> <tex-math notation="TeX">$\hbox{Sr}_{0.6}\hbox{K}_{0.4}\hbox{Fe}_{2}\hbox{As}_{2}/\hbox{Fe}$</tex-math></inline-formula> Tapes Via Microstructure Analysis

Pnictide superconductors will be very promising for applications if wires with high critical current density $J_{c}$ can allow reel-to-reel large-scale fabrication at low costs. To understand the mechanism(s) that limited $J_{c}$ in flat-rolled $\hbox{Sr}_{0.6}\hbox{K}_{0.4}\hbox{Fe}_{2}\hbox{As}_{2}$ (Sr122) tapes, microstructure analysis has been considered the most direct and efficient way. Here, we report on high-resolution microstructure imaging and analysis on Fe-sheathed flat-rolled Sr122 tapes, which have a $J_{c}$ as high as $2.3\times 10^{4} \hbox{A/cm}^{2}$ at 10 T and 4.2 K. The overlapping nature of the Sr122 plates was clearly observed. Transmission electron microscopy/scanning transmission electron microscopy analysis showed that, besides the cracks formed during the fabrication process, the $\hbox{SrO}_{2}$ phase and cavities caused by the inhomogeneously dispersed Sr and K are the other important factors suppressing $J_{c}$ . The wetting phase FeAs at the grain boundaries can be partially substituted by Sn in Sn-added samples. Our findings provide insights that pave the way to further enhance the critical current of the rolled 122 tapes up to the practical level.

Electromechanical properties of iron and silver sheathed Sr0.6K0.4Fe2As2 tapes

This work describes the electromechanical behaviour of iron- and silver-sheathed ex situ Sr0.6Ka0.4Fe2As2 (Sr-122) tape superconductors. Free-standing Sr-122 tapes were stressed by axial tension at 4.2 K under external fields at 5–6 T, and critical currents have been simultaneously measured. An attempt to reinforce Sr-122/Ag mechanically was done by simple soldering of two stainless steel strips of 40 μm to both sides of the tape. The obtained results are compared with the electromechanical property of MgB2/Fe. The mechanical limits of Sr-122/Fe and Sr-122/Ag regarding possible high field applications are also discussed.

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.

Effects of heating condition and Sn addition on the microstructure and superconducting properties of Sr0.6K0.4Fe2As2 tapes

We report a systematic investigation on the effects of sintering temperature and Sn addition on the phase formation, resistivity transition, critical current density and microstructure of Sr-122 tapes. With increasing sintering temperature from 800 to 950°C, the a-lattice parameter of Sr122+Sn samples slightly elongates while c-lattice parameter shrinks. The composition of superconducting phase was affected by different sintering conditions with Sn addition. It is found that the grain connectivity of Sr122+Sn samples is greatly enhanced while higher content of FeSn impurity and structure defects are introduced at higher sintering temperature. The samples sintered at 900°C show a maximal transport Jc value in self-field whereas the field independence of Jc for the samples sintered at 850°C is optimal. On the other hand, comparing with the pure samples, the Sn-added samples present well connected cake-like grains, growth steps and a large number of dislocations, resulting in high transport Jc properties. It is proposed that molten Sn enters the rim of Sr-122 phase and greatly accelerates the dissolution of misfit at the edges of grains within a short sintering time, and subsequently reduces the interfacial energy. Our results indicate that moderate Sn addition in combination with optimized heating conditions is very promising for high Jc-H performance.

Large transport critical currents and magneto-optical imaging of textured Sr1−xKxFe2As2 superconducting tapes

We report the temperature dependence of the transport critical current density (Jc) in textured Sr1−xKxFe2As2/Fe (Sr122) tapes fabricated by an ex situ powder-in-tube process. Critical currents were measured in magnetic fields in the range 0–15 T and/or the temperature range 4.2–30 K by using a dc four-probe method. It was found that textured Sr122 tapes heat-treated at low temperatures showed higher transport Jc performance due to much improved intergrain connections. At temperatures of 20 K, easily obtained using a cryocooler, Jc reached ∼104 A cm−2 in self-field, which is the highest transport value of ferropnictide wires and tapes reported so far. Magneto-optical imaging observations further revealed significant and well distributed global Jc at 20 K in our tapes. These results demonstrate that 122 type superconducting tapes are promising for high-field applications at around 20 K.