Fe2As2 Tapes Research Articles

Low field anomaly in the critical current of Ba1− x K x Fe2As2 tapes

Ba1−x K x Fe2As2 superconductors have strong potential for magnet applications through their very high upper critical field, relatively high superconducting transition temperature and manufacturability through the powder-in-tube (PIT) route. However, the critical current density in PIT tapes is still low compared to the incumbent technologies, so a greater understanding of the limiting factors is required. We have measured the in-field critical currents (I c) of stainless steel and silver double-sheathed monofilament Ba0.6K0.4Fe2As2 superconductor tapes at elevated temperatures from 15 K to 35 K. At 20 K, the critical current density is up to 140 kA cm−2 in low (optimal) field and 22 kA cm−2 in 8 T. In the low-field region we observe an anomalous and sharp suppression of I c centred at the zero field. This feature is non-hysteretic for lower temperatures and perpendicular fields, but becomes hysteretic for higher temperatures in perpendicular fields and for all temperatures in parallel fields. The low-field suppression is also reflected in the n-values which can otherwise be very high, in excess of 100 in the optimal field. Magnetic-field hysteresis of I c is generally attributed to flux exclusion/flux trapping in granular superconductors and this is likely to be the case also in the present conductors. The low-field I c anomaly also likely has its origin in planar granularity, while magnetic phases in grains or grain boundaries may also play a role.

High transport Jc in 7-filament Cu/Ag sheathed Ba0.6K0.4Fe2As2 tapes manufactured by ambient pressure sintering

In regard to iron-based superconductors (IBS), the multi-filament Cu/Ag composite sheathed Ba-122 tape is particularly attractive for industrial high-field applications owing to its prominent properties. By a cost-effective and scalable way, ambient pressure sintering, a high transport critical current density (Jc) of 7-filament Cu/Ag sheathed Ba-122 tapes reached 4.5 × 104 A cm−2 at 4.2 K and 10 T, whose corresponding engineering critical current density (Je) has achieved to 104 A cm−2. An optimized powder-in-tube (PIT) method was adopted aiming at overcoming the difficulty of ambient pressure sintering to obtain high-density superconducting core and grain alignment. Improvements in the manufacturing process, such as drawing without intermediate annealing and rolling from larger initial diameter to thinner tapes, have resulted in high grain connectivity and good c-axis texture, which are the major factors contributing to high Jc. Our results demonstrate that the ambient pressure sintering process is a viable method to fabricate long-length multifilamentary Cu/Ag composite sheathed IBS tapes with high Jc for practical high-field applications.

Critical current degradation behavior of 7-filamentary Ba1−x K x Fe2As2 tapes under uniaxial strain

122-type iron-based superconductors (IBS) have attracted extensive attention due to their excellent performance. The 100 m class Ba1−x K x Fe2As2 (Ba-122) 7-filamentary IBS tapes were successfully fabricated by the Institute of Electrical Engineering, Chinese Academy of Sciences. In this study, we have performed tests of the 7-filamentary Ba-122 short tape samples (Bas samples) and the 100 m class 7-filamentary Ba-122 tape samples (Ba100 samples) under various kinds of strain using U-shaped bending spring devices. Both types of samples were still reversible even when the applied compressive strain reached −0.65%. In addition, the reversible tensile strain limit of the Bas and Ba100 samples were 0.25% and 0.28%, which increased to 0.38% and 0.41% after considering the effect of the cooling process.

A bi-layer barrier design for 122-type iron-based superconducting wires and tapes

Iron-based superconducting wires and tapes hold great promise for high-field magnet applications. A promising design for 122-type wires and tapes based on the powder-in-tube method is using silver and copper double-layer sheaths. For this design a heat treatment temperature below ∼ 779 °C is required to prevent Ag-Cu liquid formation. However, this may be below the optimal heat treatment temperature for the critical current density, and still cannot prevent Ag-Cu interdiffusion occurring in the solid state. In this work we propose adding a niobium or tantalum or vanadium (or their alloys) barrier layer between the Ag and Cu to solve the Ag-Cu interdiffusion issue, given that the group-VB metals (vanadium, niobium, tantalum) are relatively inert to both Ag and Cu. To investigate the effectiveness of this design, BaFe1.84Co0.16As2 wires and tapes with Ag/Cu and Ag/Ta/Cu sheaths, as well as Ba0.6K0.4Fe2As2 wires and tapes with Ag/Cu and Ag/Nb/Cu sheaths, were fabricated. It was found that both the Ta and Nb layers kept integral after wire drawing, but after a large flat-rolling reduction the Ta layer broke while the Nb layer kept integral. In the tapes with Ag/Cu sheaths (without the Ta or Nb layer) Cu diffused through the Ag layer and into the powder cores during 740 °C heat treatment, while in the tapes with Ag/Nb/Cu sheaths the Nb layer effectively blocked Ag-Cu interdiffusion even at 900 °C. This work demonstrates that Ta is a suitable barrier material for 122-type wires, while Nb is suitable for both wires and tapes. In this design using Ag/Nb (or Ta)/Cu sheaths, we can regard the outer Cu as the conductor matrix while the Ag and Nb (or Ta) serve as two layers of barriers that suppress reactions between the components. Thus, we call this design a “bi-layer barrier” design for 122-type wires and tapes.

Significant enhancement of transport Jc in Cu/Ag-sheathed (Ba,K)Fe2As2 superconducting tapes by pre-composite technique

Iron-based superconductors (IBSs) have received extensive attention in superconductivity mechanisms and high-field applications based on their ultra-high upper critical fields and low anisotropy. A major goal in the application research of IBSs is the fabrication of superconducting wires and tapes with high critical current density (Jc) and low cost. The use of Cu/Ag composite sheath can reduce the use of expensive silver, thereby reducing the cost of the tapes. In this paper, (Ba,K)Fe2As2 (Ba-122) tapes with Cu/Ag composite sheath were fabricated by a pre-composite process, which provides a feasible method for fabricating high-performance iron-based superconducting wires and tapes. Pre-composite tapes sintered at 750°C for 3 h achieved a high Jc of 6.2 × 104 A cm−2 at 10 T and 4.2 K. It is found that the high-density superconducting cores, large-size grains and uniform element distribution are the major causes of the high Jc. The results suggest that the low-cost Ba-122 tapes with pre-composite Cu/Ag sheath have great practical application prospect in the future.

Transverse distribution of critical current density in Ag-sheathed Ba1− x K x Fe2As2 tapes

The Ag-sheathed Ba1−x K x Fe2As2 (Ba-122) monofilamentary tapes were prepared by the ex-situ powder-in-tube method. The variation of the microstructure and superconducting properties with the thickness of the superconducting core in the cross-section of sintered tapes is studied. At the same time, the reason is studied in comparison with the unsintered tapes. The research results show that the magnetic J c of the iron-based superconducting tapes increases continuously with distance from the core-sheath interface, which is the complete opposite of the Bi-based superconductor. The magnetic J c of central layers for final Ba-122 tapes is about 33% higher than the Jc of the whole tape at 4.2 K and 7 T. We have found that the center of the superconducting core shows higher hardness and better texture. In addition, it is also found that there is a reaction layer at the Ag-superconductor interface. These reasons may result in the reduction of the critical current density near the interface in the tapes. Moreover, we also found the presence of a reaction layer in the hot-pressed (HP) high-performance samples. However, no unevenness was found in the unsintered samples. Therefore, the superconductivity of Ba-122 tapes will be better by reducing the reaction layer and eliminating inhomogeneity at the core-sheath interface of the sintered tapes.

Fabrication and Characterization of (Ba,Na)Fe2As2 Wires and Tapes

In order to enhance J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> in (Ba,Na)Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires and tapes further by improving texturing, we examined the following approaches; (1) Ag-sheathed HIP tapes, (2) AgSn-sheathed cold-pressed tapes, and (3) multifilamentary HIP wires with each filament having tape-like cross section. In addition, we attempted to increase I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> by fabricating (Ba,Na)Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires with larger core areas, and constructed a small demo coil using one of these wires with length up to 9 m. J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> characteristics of these new wires and the performance of the demo coil with and without an electrical insulation are reported.

Properties of seven-filament Cu/Ag-sheathed (Ba,K)Fe2As2 tapes fabricated from round and square wires

Properties of seven-filament Cu/Ag-sheathed (Ba,K)Fe2As2 tapes fabricated from round and square wires

Enhancing Transport Performance in 7-filamentary Ba0.6K0.4Fe2As2 Wires and Tapes via Hot Isostatic Pressing

Iron-based superconductors (IBS) are considered as potential materials for manufacturing high-field magnets, for which developing multi-filamentary conductors with high performance and high strength is essential. Herein, 7-filamentary Cu/Ag composite sheathed Ba0.6K0.4Fe2As2 (Ba122) round wires and tapes were successfully prepared through the ex situ powder-in-tube (PIT) method and treated by hot isostatic pressing (HIP) process. Pure Ba122 phase with roughly homogeneous element distribution was obtained in the superconducting filaments. The wires and tapes show very small low-temperature normal-state resistivity of 0.12 μΩ cm and 0.17 μΩ cm respectively owing to the high electrical conductivity of copper and silver. The HIP process greatly enhances the mass density of the superconducting filaments and promotes the formation of well-grown plate-like Ba122 grains. Vickers hardness measurements on the cross sections of these filaments reveal a rather good uniformity of the mass density. The transport critical current density (Jc) in the 7-filamentary Ba122/Ag/Cu round wires and tapes reached 1.3 × 104 A cm−2 and 4.8 × 104 A cm−2 at 4.2 K in 10 T respectively. These results indicate that hot isostatic pressing is advantageous in developing high-performance multi-filamentary iron-based superconductors.

Visualization of the grain structure in high-performance Ba1−x K x Fe2As2 superconducting tapes

122 type iron-based superconductors (IBSs) are potential for high-field applications, and the fabrication of high-performance IBS wires and tapes is essential. By using the powder-in-tube method, transport critical current density (J c) of hot-pressed (HP) silver-sheathed Ba1−x K x Fe2As2 (Ba-122) tapes have reached 1.5 × 105 A cm−2 at 4.2 K and 10 T. However, the J c of flat-rolled (FR) tapes is 6.2 × 104 A cm−2 (4.2 K, 10 T), less than half of the former. The grain orientation, grain size, and grain shape are important parameters for the understanding of the superconducting properties of IBS tapes. Such grain structure data can be provided by the electron backscatter diffraction (EBSD) technique. In this work, we extensively adopted EBSD to visualize the microstructure of state-of-the-art HP and FR Ba-122 tapes. The grain orientation, grain connectivity, grain size, and grain shape aspect ratio of these two types of tapes are quantitatively analyzed. The c-axis texture is commonly found in both HP and FR tapes, but no in-plane texture is discovered. The texture and grain connectivity in HP tapes are better than that in FR tapes. The grain size of FR tapes is smaller than that of HP tapes, and the hot-pressing processes can promote the growth of grains along the ab plane. We recommend that strong texture will promote the growth of grains, while the small-sized grains will return to limit the formation of texture. Therefore, finding a balance point that plays a synergistic promote effect of grain orientation and grain size is a strategy to improve the transport properties of IBS tapes further. The present results demonstrate that there is still margined to enhance the properties of IBS tapes.

Strong flux pinning and anomalous anisotropy of Sr0.6K0.4Fe2As2 superconducting tapes

A comprehensive study on the resistivity, magnetization and microstructure is carried out to understand the superconductivity and flux pinning behavior of hot-pressed Sr0.6K0.4Fe2As2 tapes. The Sr0.6K0.4Fe2As2 tapes possess excellent properties of high T c = 36.2 K, large transport J c of ∼105A cm−2 at 10 T and 4.2 K, and high pinning potential U 0/k B with small field-dependence, demonstrating a great potential in high-field applications. The magnetic J c at various temperatures and flux pinning mechanism are exhibited for both B//ab and B//c directions. Notably, a point pinning in combination with surface pinning appears in our samples, resulting in a strong flux pinning force. And the characteristic ratio B irr //c(0 K)/B irr //ab(0 K) is as low as 1.2, indicating that the vortex pinning abilities are nearly isotropic. Further transmission electron microscope characterization reveals that the anomalous pinning behavior is originated from the largely increased small-sized defects and the morphological differences along two directions. These positive results indicate that tailoring the pinning characteristic by creating micro-defects is crucial to improving the J c value and suppressing the anisotropy in iron pnictides.

High-critical-current-ratio superconducting joint between Ba0.6K0.4Fe2As2 tapes fabricated by angle-polishing method

Superconducting joints of Ba0.6K0.4Fe2As2 (BaK-122) tapes were fabricated by a simple cold-pressing method. The interfaces of the superconducting cores were prepared by two different methods; (i) peeling off a part of the Ag sheath (PO joint) and (ii) angle polishing of one edge of each tape (AP joint). For the PO joint, a critical current ratio (CCR =Icjoint/Ictape, where Ictape and Icjoint are critical currents in tapes and joint parts, respectively) of 30% at 4.2 K and 3.5 T was obtained. Scanning electron microscopy revealed that the jointing area was not fully connected when the applied pressure was insufficient, and that cracks were induced in the superconducting core around the ends of the joint when the pressure was too high. These are considered to be factors that caused lowering of a CCR value. On the other hand, in the case of the AP joint, the formation of such gaps in the joint interface as well as inhomogeneous deformations and cracking around the joint-ends were successfully avoided. As a result, high CCR values of 80 ∼ 90% at 4.2 K under 0.5 ∼ 3.5 T were achieved, demonstrating that superconducting wires/tapes with sufficient length can be fabricated based on iron-based superconductors, which possibly opens up the potential for high-field magnet application.

Enhancement of the critical current density in Cu/Ag composite sheathed (Ba, K)Fe2As2 tapes by pre-annealing process

Iron-based superconductors, a promising candidate for high-field applications, have aroused rising attentions. In the present study, a pre-annealing method was used to promote the transport critical current density (Jc) in Cu/Ag composite sheathed (Ba,K)Fe2As2 tapes. It was found that tapes sintered at 740 °C for 3 h under ambient pressure showed a Jc up to 4.9 × 104 A cm−2 (4.2 K, 10 T). Also, a transport Jc of 6.5 × 104 A cm−2 (4.2 K, 10 T) was achieved using a hot-press technique. The factors affecting the Jc of tapes were systematically analyzed. The well-constructed crystallization and grain connectivity obtained in pre-annealed tapes were considered as the major causes of the high Jc. These results highlight the great potential to increase the Jc in multiple metal composite sheathed Ba-122 tapes.

High critical current density in Cu/Ag composited sheathed Ba0.6K0.4Fe2As2 tapes prepared via hot isostatic pressing

Iron-based superconductors (IBSs) with ultrahigh upper critical fields and low anisotropies have attracted significant attention in terms of the novel mechanism of superconductivity and high-field applications. A major concern for practical research is the fabrication of long wires with enhanced critical current density and low cost. In this paper, Cu/Ag composited sheathed Ba0.6K0.4Fe2As2 (Ba122) tapes were fabricated through a hot isostatic pressing method, which is feasible for long-wire manufacturing. The Cu/Ag composite sheath can lower the tape cost by reducing the use of expensive Ag. A high-transport critical current density (Jc) up to 5.8 × 104 A cm−2 under 10 T at 4.2 K was achieved in our tapes. Evidence has shown that these tapes have pure Ba122 phase, homogeneous element distribution, orientated grains and good grain connectivity. Our work shows that low-cost Cu/Ag-sheathed IBS tapes have great promise for practical applications in the future.

Enhancement of critical current density in AgSn-sheathed (Sr,Na)Fe2As2 superconducting tapes

We have fabricated (Sr,Na)Fe2As2 tapes by using AgSn alloy as a sheath material. In the tape sintered at 750°C, transport Jc reached 47 kA/cm2 at 4.2 K and 50 kOe, which is larger than that of the Ag-sheathed (Sr,Na)Fe2As2 tape. This result indicates that we have succeeded in reducing the sintering temperature by more than 100°C keeping the high Jc value, which is very promising for practical applications. In the SEM images of polished longitudinal cross section of the tapes, the improvement of interface flatness by using AgSn alloy as a sheath material is confirmed.

Improvements of fabrication processes and enhancement of critical current densities in (Ba,K)Fe2As2 HIP wires and tapes

We fabricated (Ba,K)Fe2As2 superconducting wires and tapes using the powder-in-tube method and hot isostatic pressing (HIP). HIP wires and tapes showed a high value of transport critical current density (Jc) exceeding 100 kAcm−2 at T = 4.2 K and the self-field. Transport Jc in the HIP wire reached 38 kAcm−2 in a high magnetic field of 100 kOe. This value is almost twice larger than the previous highest value of Jc among round wires using iron-based superconductors. Enhancement of Jc in the wires and tapes was caused by improvement of the drawing process, which caused degradation of the core, formation of microcracks, weak links between grains, and random orientation of grains. Details of the effect of the improved fabrication processes on the Jc are discussed.

High transport current superconductivity in powder-in-tube Ba0.6K0.4Fe2As2 tapes at 27 T

The high upper critical field and low anisotropy of iron-based superconductors (IBS) make them particularly attractive for high-field applications, especially for the construction of next-generation nuclear magnetic resonance spectrometers, particle accelerators and high-field magnets. However, for practical use it is essential to make IBS materials into wire and tape conductors with sufficient current carrying capability, which is limited by misaligned grains inside the conductors. Here, based on a simple and low-cost powder-in-tube (PIT) method, we demonstrate a high transport critical current density (Jc) reaching 1.5 × 105 A cm−2 (Ic = 437 A) at 4.2 K and 10 T in Ba0.6K0.4Fe2As2 (Ba-122) tapes by texturing the grain orientation with optimized hot-press technique. The transport Jc measured at 4.2 K under high magnetic fields of 27 T is still on the level of 5.5 × 104 A cm−2. Moreover, at 20 K and 5 T the transport Jc is also as high as 5.4 × 104 A cm−2, showing a promising application potential in moderate temperature range which can be reached by liquid hydrogen or cryogenic cooling. All these Jc values are the highest ever reported for IBS wires and tapes. The high-performance PIT Ba-122 tapes in this work suggest IBS to be a strong potential competitor of cuprate superconductors for the race of high-field applications in the future.

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.

Large and field-insensitive critical current densities in (Sr,Na)Fe2As2 superconducting tapes

We have fabricated (Sr,Na)Fe2As2 tapes by optimizing the sintering temperature and time, and characterized their transport Jc at 4.2 K up to 14 T. The transport Jc for the (Sr,Na)Fe2As2 tape sintered at 875°C reaches 4.9 × 104 A/cm2 under self-field, and it is suppressed only down to 1.9 × 104 A/cm2 even at 14 T. We succeeded in fabricating very field-insensitive tapes, which are very promising for practical applications. We also report compositional distributions and magneto-optical images of current distribution in tapes. Magneto-optical imaging of the core of the tape clarified inhomogeneous intergranular current distributions.

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.