Highly c-axis orientated superconducting core and large critical current density in Ba0.6Na0.4Fe2As2 powder-in-tube tape

Improvement of the critical current density (Jc) of superconducting wires/tapes is one of the key issues in the field of superconductivity applications. Here we report the fabrication of a silver-sheathed Ba1−xNaxFe2As2 (BaNa-122) superconducting tape by using a powder-in-tube technique and its superconducting properties, in particular transport Jc, as well as the tape-core texture. The optimally-doped BaNa-122 tape with Na concentration x = 0.4 exhibits the superconducting critical temperature (Tc) of 33.7 K and high transport Jc of 4 × 104 A/cm2 at 4.2 K in a magnetic field of 4 T. Patterns of x-ray diffraction for the superconducting core show that the degree of c-axis orientation is significantly enhanced through the tape fabrication process. The tendency of c-axis orientation is advantageous for achieving higher Jc, suggesting the high potential of BaNa-122 for superconducting wire/tape applications.


X-ray diffraction analysis.
shows XRD patterns of the BaNa-122 polycrystalline powder (black) and tape (red) samples. All the visible peaks can be indexed based on the 122 structure (ThCr 2 Si 2 type, I4/mmm). No impurity peaks were observed in the range of resolution. The lattice constants of the powder sample estimated from the d values of peaks are a = 3.902 Å and c = 13.13 Å. These values correspond to x = 0. 4 21 , indicating that the Na composition of the present powder was successfully controlled. The peak positions of XRD patterns of the superconducting core surface are almost identical to those for polycrystalline powders within the experimental certainty, indicating that the Na concentration was not significantly changed through the tape fabrication process. On the other hand, the intensities of the 00 l peaks of tape core are largely enhanced in comparison with the polycrystalline powder. This indicates that the c-axis of BaNa-122 grains is well-oriented perpendicularly to the tape surface, that is, along the direction of the uniaxial pressure.
Magnetic susceptibility. Figure 2 shows the temperature (T) dependence of magnetic susceptibility for polycrystalline powders (black) and a 3 mm-long tape (red) measured under the field of 10 Oe. The filled and open symbols indicate the data taken by the zero-field-cool and field-cool processes, respectively. The T c values were determined as a temperature where the susceptibility dropped by 10% of the superconducting transition in the zero-field-cool branch. The determined T c of the polycrystalline powder is 34.2 K, which is in good agreement with the reported T c of BaNa-122 with x = 0.4 21 , supporting that the Na concentration was successfully controlled. After the fabrication process, T c ( = 33.7 K) of the tape is found to decrease by 0.5 K from that of powder, while the width of superconducting transition is preserved.
The degradation in T c after wire/tape fabrication process has been also reported in previous works 23 . An explanation for the reduction in T c is the loss of Na during the fabrication process, although the Na concentration is seemingly unchanged within the resolution of the XRD and the EDX measurements. On the other hand, the reduction in T c is possibly caused by the mechanical stress during the wire/tape fabrication process. It was pointed that T c possibly decreases if the local structures such as pnictogen height and As-Fe-As bond angle are changed by the mechanical stress because T c of IBSs is sensitive to these local structures. SEM-EDX analysis. Figure 4(a,b) show SEM images of the cross section of the BaNa-122 tape core obtained at different magnifications. EDX analyses show that the average atomic composition in the core, Ba: Na: Fe: As, is approximately 12: 8.8: 37: 40. This atomic ratio corresponds to (Ba 0.62 Na 0.43 )Fe 1.8 As 2.0 , indicating that the Na concentration is ~0.4, which is close to the nominal composition of the starting powder. The SEM images show the presence of Fe 2 As impurities (dark gray spots), which are not detected by the XRD measurements. The amount of Fe 2 As was estimated to be ~3.7% from the cross-section area of the SEM images. The black regions correspond to voids or cracks with the sizes of a few micrometers. Such voids/cracks have been considered as one of the main  www.nature.com/scientificreports www.nature.com/scientificreports/ limiting factors of J c . The blight rod-like spots are found to be Ag-As alloys. Note that Ag was not added to the starting powders and a similar phenomenon was reported for the SrNa-122 tape 19 .

Discussion
A high-performance IBS tape was successfully fabricated by using BaNa-122, demonstrating that BaNa-122 is also a good candidate material for superconducting wire/tape applications. In particular, an advantageous characteristic is found, i.e. the superconducting core of the BaNa-122 tape is strongly c-axis oriented perpendicularly to the tape surface. Note that the high c-axis orientation has been considered as an important factor for improving J c 14,15 . To quantify the degree of c-axis orientation, the orientation factor (F) was estimated based on the XRD patterns by using the Lotgering method 24 , i.e. F = (ρ -ρ 0 )/(1 -ρ 0 ) with ρ = ΣI(00 l)/ΣI(hkl) and ρ 0 = ΣI 0 (00l)/ ΣI 0 (hkl), where I(hkl) (I 0 (hkl)) is the intensity of the hkl peak for tapes (polycrystalline powders). The estimated F of the present BaNa-122 tape is 0.61. This value is larger than that of our BaK-122 tape (F = 0.37) 25 , suggestive of a better c-axis orientation of the BaNa-122 core. For further comparison, in Fig. 5, we plotted the F values of various BaK-122 and SrK-122 tapes taken from references. The F value of the BaNa-122 tape is found to be considerably high among those of 122 tapes. It should be noted that the high F values (>0.5) of previous BaK-122 and SrK-122 tapes were achieved using advanced techniques efficient for obtaining the c-axis orientation; (i) application of large uniaxial pressure (~4 GPa, which is 4 times larger than that in this work), (ii) application of hot-press technique, or (iii) use of Ag-Sn alloy sheath or Ag/stainless steel (SS) double sheath (which are harder than the pure Ag sheath used in this work). Thus, the comparison of F values between the BaNa-122 tape and various BaK-/SrK-122 tapes demonstrates that the use of BaNa-122 material is promising for obtaining a better c-axis orientation of the tape core.
The high c-axis orientation found for the BaNa-122 core is possibly associated with the strain in the crystal structure. It is reported that the c axis of BaNa-122 shows a non-monotonic x dependence violating the Vegard's law (the c-axis parameter increases up to x ~ 0.4 while decreases at higher x) owing to the large mismatch of ionic radii between Ba 2+ and Na + 26 . This suggests that BaNa-122 has a significant strain in the crystal structure compared with other 122 materials, which possibly causes an easy cleavage of the grains during the flat-rolling and  www.nature.com/scientificreports www.nature.com/scientificreports/ uniaxial-pressing processes. Such cleaved grains likely align easily and thus result in the high c-axis orientation of the tape core.
In addition, the precipitation of Ag-As alloys was observed in the BaNa-122 tape core similarly to the case of SrNa-122 tape 19 , which has not been reported for K-doped 122 tapes. The Ag-As alloys occupy about 0.6% of the cross-section area for the present BaNa-122 tape, which is smaller than that estimated for the previous SrNa-122 tape (~3%). Meanwhile, the amount of Fe 2 As impurities is also smaller for BaNa-122 (~3.7%) than that for SrNa-122 tape (~10%). For BaK-122 and SrK-122, the Ag-As alloys have not been observed and the amount of Fe 2 As impurities is much smaller. There seems a correlation between the amounts of Ag-As alloys and Fe 2 As impurities, suggesting that the precipitation of Ag-As alloys is the resultant of the chemical reaction between 122 phase and the Ag sheath. The degree of chemical reaction between 122 and Ag sheath is possibly associated with the chemical stability of 122 phase. Note that AENa-122 phase has a solubility limit at some Na concentration because Na-122 (x = 1) is a metastable phase 21,27,28 , while AEK-122 phase can be obtained for any K concentration. Therefore, AENa-122 is likely more unstable and hence reactive compared with AEK-122. In addition, SrNa-122 used for the tape contains a higher Na concentration than that of BaNa-122 because T c is optimized at x = 0.55 and 0.4 for SrNa-122 and BaNa-122, respectively. Then, SrNa-122 is closer to the solubility limit hence more unstable compared with BaNa-122. Thus, the precipitation of Ag-As alloys is most prominent for the most unstable SrNa-122 core.
Regarding the Ag-As precipitation, two opposing effects on the grain connectivity, which is one of the key factors determining J c , are considered; (i) the Ag-As alloys fill voids and cracks in the core and thus improve the grain connectivity, and (ii) the accompanying Fe 2 As impurities disturb the grain connectivity. Then, J c is possibly increased by balancing the positive and negative effects. For the optimization of the precipitation of Ag-As alloys and Fe 2 As impurities, the fine adjustments of the heat-treatment temperature and time as well as the chemical composition of the BaNa-122 powder should be considered.
Meanwhile, the transport J c of the present BaNa-122 tape does not reach the practical-level J c (=10 5 A/cm 2 ) in spite of the considerably high F value. To understand the limiting factor of J c , we checked the Vickers hardness (H V ) of the BaNa-122 core, which is often used as a measure of the core density. The obtained H V ~ 90 is relatively small compared with the other 122 tapes with higher J c . Therefore, we concluded that the low core density owing to the presence of crack and voids mainly limits J c of the present BaNa-122 tape. Such cracks and voids are expected to be reduced by using the techniques mentioned above, i.e. application of a large uniaxial pressure, hot press, and hard sheath materials. Note that these techniques are also efficient for the enhancement of c-axis orientation. Thus, the use of BaNa-122, which provides a better c-axis orientation, possibly leads to the further enhancement of J c of 122-based PIT tapes. On the other hand, it should be also noted that the cold-and hot-press techniques limit the length of the tape hence they are not suitable for the mass production (in the present study, the length of the tape (~50 mm) is limited by the uniaxial-press process). Then, the application of hot rolling and over-pressure sintering, which have been used for the production of high-T c BSCCO tapes, should be interesting in terms of the scale up of the tape length.
Finally, regarding the H dependence of transport J c of the present BaNa-122 tape, a weak H dependence and a small anisotropy against the H orientation were observed similarly to BaK-122 and SrK-122 tapes, which are important properties from the application viewpoint. Note that the J c anisotropy is smaller than (or inversed from) that expected from the anisotropy of H c2 , namely, the effective mass model. The inverse anisotropy has been also reported for BaK-122 11 and SrK-122 tapes 29 . To account for this behavior, two scenarios have been proposed; (i) correlated pinning centers, i.e. elongated perpendicular to the tape surface, or (ii) pinning centers larger than the coherence length dominate the pinning properties. To elucidate the origin of the inverse anisotropy of J c , it is desired to perform the transmission-electron-microscopy measurement, which is a powerful tool to investigate the defect structures determining vortex pinning properties.

Summary
An iron-based superconducting PIT tape was fabricated using BaNa-122. High J c values of 5.0 × 10 4 A/cm 2 at 4.2 K under a self-field and 1.0 × 10 4 A/cm 2 at 20 K under a magnetic field of 4 T were achieved, which are comparable to those in BaK-122 and SrK-122 tapes. Also, a weak H dependence of J c and a small anisotropy of J c against the H orientation were observed similarly to the cases of BaK-122 and SrK-122 tapes, which are important properties for high-field applications. Moreover, we found an advantageous characteristic of BaNa-122 tape; the superconducting core tends to show a high c-axis orientation. Meanwhile, a substantial amount of voids and cracks are observed in the core, which are likely to be the main limiting factor of the transport J c of present BaNa-122 tape. Then, there still remains room for the further enhancement of J c in BaNa-122 tapes by taking advantage of the better c-axis orientation.

Methods
Polycrystalline powders of BaNa-122 were synthesized by a solid-state reaction. Prior to the synthesis, BaAs, NaAs, and Fe 2 As powders were prepared as precursors by mixing elemental Ba, Na, Fe, and As chunks at stoichiometric molar ratio, sealing them into quartz tubes (BaAs and Fe 2 As) or stainless-steel container (NaAs), and heating at 650 °C (BaAs and NaAs) and 750 °C (Fe 2 As). The precursors were mixed in the ratio of BaAs: NaAs: Fe 2 As = 0.6: 0.45: 1. Here, the target Na concentration was x = 0.4 so as to yield highest T c of 34.2 K. Extra NaAs was added considering the possible loss of Na and As during the synthesis process. The mixed powders were put in an alumina crucible and then sealed in a stainless-steel container, then sintered at 770 °C for 15 hours. The polycrystalline powders were pressed into a pellet by applying a pressure of about 100 MPa. The pellet was packed into a silver tube having an outer diameter of 10 mm and an inner diameter of 4.3 mm. The silver tube was groove-rolled into a square wire with the 1.2 × 1.2 mm 2 cross-sectional and subsequently flat-rolled into a tape with a thickness of 0.48 mm. The flat rolled tape was uniaxial cold-pressed between stainless steel disks under a