On the mechanisms of J c increment and degradation in high-J c Ba122 tapes made by different processing methods

We compared the grain and grain boundary (GB) nanostructures in two Ba122 tapes with similarly high J c. The Ag-sheathed tape made by hot pressing has larger, more plate-like grains with better c-axis alignment but has more GBs blocked by FeAs and Ba–O. In contrast, the tape made by cold pressing with an Ag-Sn/stainless steel sheath possesses fewer plate-like grains and weaker grain alignment but has more continuous current paths with clean physically well-connected GBs. Our nanostructural comparison emphasizes the strong need to achieve both good grain alignment and clean GBs for further J c improvement of Ba122 tapes.

T he possibility of whether Fe-based superconductors (FBS) can be transformed into practical superconductors has been investigated since the first FBS compound LaFeAsO 0.89 F 0.11 exhibited superconductivity at a critical temperature T c ∼26 K. 1) Several different families of FBS compounds were later discovered with T c up to 55 K and an estimated critical field H c2 exceeding 100 T. [2][3][4][5][6][7][8] In particular, BaFe 2 As 2 (Ba122), especially K-doped Ba122 (K-Ba122) has the potential to become a cost-effective high-field superconductor because of its low raw material costs in addition to its T c (∼38 K), H c2 (∼90 T) and low H c2 anisotropy (γ = 1-2).7) A high critical current density J c has been seen in many single-crystal and thin-film studies, demonstrating that high vortex pinning is possible in the Ba122 system.[9][10][11][12][13][14][15][16] Foreseeing practical applications, the most versatile form is a long-length wire in which the materials are inevitably polycrystalline; thus J c across the grain boundary (GB) network, the so-called intergrain J c , becomes the very important factor.17,18) There are both intrinsic and extrinsic GB current-blocking mechanisms in FBS.[19][20][21][22] But to properly address the intrinsic mechanism we need to rule out the extrinsic effects caused by impurity phases introduced by processing.Indeed, recent studies have suggested that the intergrain connectivity of Ba122 can be easily degraded by extrinsic factors such as local or global impurity concentrations and GB cracks [23][24][25][26][27][28][29] that can lead to poor reproducibility of the intergrain J c .Oxygen contamination, even at low levels, degrade GB connectivity in K-Ba122.24,25,30) The highest J c of polycrystalline Ba122 reported to date is found in flat tape conductors.It is considered that such a high J c in the tapes is correlated to uniaxially textured Ba122 grains, [31][32][33][34][35][36][37] although the effects of uniaxial texture on J c across the GBs are not as simple as in [001]-tilt bicrystals due to more complex inter/intra-grain current paths rather than along the ab-planes.19,22) Also, the high packing density of Ba122 in tape after high-pressure pressing may also significantly enhance the physical connectivity.32,38) The important question is whether the high J c in the tapes is induced by the same mechanism, as a result of GBs free of extrinsic current blockages.To address such a question, we utilized analytical scanning transmission electron microscopy (S/TEM) to investigate the grain and GB nanostructures in similarly high J c K-Ba122 tapes made by the different fabrication processes.
We examined two K-Ba122 tapes, both of which were made by the powder-in-tube method.The first tape is Ag-sheathed and was prepared by the hot pressing method described in Ref. 37 (sample Ag-HP).The second tape is double-sheathed with Ag-Sn alloy and stainless steel, made by cold pressing followed by heat treatment at ambient pressure 31) (sample Ag-SS).The J c values of samples Ag-HP and Ag-SS are 1500 A mm −2 and 1400 A mm −2 at 4.2 K and 10 T parallel to the tape planes, respectively.T c of Ag-HP and Ag-SS is 37.6 K and 37.0 K, respectively.The crosssectional specimens for scanning transmission electron microscopy (STEM) were prepared by a focused ion beam in a FEI Helios G4 UC.The STEM imaging and elemental mapping by energy dispersive X-ray spectroscopy (EDS) were performed in a JEOL ARM200cF.
Figure 1 shows the grain structure in the Ag-HP sample.As seen in Fig. 1(a), with bright field (BF) STEM the grains in Ag-HP have a plate-like shape with some equiaxial small grains that are also found in electron backscattered diffraction imaging. 39)There is no porosity in the observed region, indicating the high density of the Ba122 flat core, consistent with the earlier study. 39) Despite the minor difference in J c (only ∼7%), the Ag-SS sample showed a rather different grain and GB nanostructure compared with Ag-HP (Fig. 3).The diffraction contrasts of Ba122 grains indicate that most of the Ba122 grains in Ag-SS are equiaxial rather than plate-like as seen in the Ag-HP sample.The grains are approximately 0.5-1.0μm in size.Judged from the shape of the grains and their configuration, some large grains appear very slightly uniaxially textured, although most of the small grains appear rather randomly oriented, which appears to be consistent with the trend found in the EBSD study. 39)Also, there are many porosities of ∼0.2-0.5 μm in size, indicating the lower density of the Ba122 core in Ag-SS than in Ag-HP; however, such porosities do not extend along the GBs.As seen in Fig. 3(b), there are continuous clean GB networks in Ag-SS in contrast to Ag-HP.The HAADF-STEM imaging in Fig. 3(b) showed no dark Z-contrast traces at many of the GBs, strongly implying that most of the grains are physically well-connected without losing local density, such as with nanocracks found in some Ba122 bulks, 25) and there is no compositional variation caused by secondary phases or oxide byproducts at such physically connected GBs.Interestingly, despite the presence of porosity, Ag-SS showed a higher Vickers hardness (Hv)  013004-2 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd value than Ag-HP, 31,37) maybe implying that clean, wellconnected GBs contribute to a high Hv value in addition to the effects of physical core density.
Although the clean GB networks are much more dominant, the Ag-SS sample is not free from disconnected GBs.As evidenced in Fig. 4, some of GBs parallel to the direction of the tape are physically disconnected by the impurity phase segregation.The higher-magnification STEM image in Fig. 4(b) reveals that, in the Ag-SS sample, such an impurity phase clusters at the GBs as fine particles rather than the continuous layer as seen in the Ag-HP sample.The corresponding EDS elemental maps in Fig. 4(b) show a sharp increase in Fe and As, as well as a sharp reduction in Ba and K at the disconnected GBs, strongly suggesting that these impurity clusters are made of FeAs.Interestingly the EDS mapping detected almost no oxygen, excluding the presence of oxide byproducts and indicating that the dark contrasts around the FeAs clusters at the GBs have a very small porosity.
The overall J c performance of K-Ba122 polycrystals, including flat tapes, is essentially determined by the network of GBs with good superconducting connectivity.Extrinsic and intrinsic current-limiting mechanisms can both affect the superconducting connectivity at GBs.The number of current paths would be physically reduced by an extrinsic current blocker at the GBs, including FeAs, Ba oxide and/or nanocracks which are dependent on the synthesis and processing. 25,26,29,30,40)Concerning intrinsic current limiters, it appears highly plausible that J c across the GBs in Fe-based superconductors including Ba122 decays as a function of GB misorientation angle, 19,20) although it is still largely unknown how much reduction in J c actually occurs in the GBs of K-Ba122.Nevertheless, previous studies suggested that grain alignment such as uniaxial or concentric texturing is  013004-3 © 2024 The Author(s).Published on behalf of The Japan Society of Applied Physics by IOP Publishing Ltd beneficial for raising J c , 36,39) presumably due to improvement of intrinsic GB connectivity by reducing GB misorientation.
Comparing the two similarly high J c K-Ba122 tapes [J c (4.2 K, 10 T) = 1500-1400 A mm −2 ], two different factors play the key role for J c : the connectivity at each GB and the number of GB connections.The Ag-HP sample possesses many highly uniaxially textured plate-like Ba122 grains that can result in intrinsically strong GB connectivity when the GBs are cleanly connected without contaminants or porosities.However, the physical connection of many GBs is still highly compromised by extrinsic factors of impurity segregation, such as the continuous layers of FeAs and the Ba oxide byproduct, indicating that the number of effective GB connections is suppressed in Ag-HP.Indeed, the Fe particles found in Fig. 1 also imply As loss during fabrication, suggesting a local compositional shift from the optimum stoichiometry that has been improved more recently. 41)On the other hand, in the Ag-SS sample, the larger number of GBs are physically well-connected without impurity segregation but with some density reduction, except for the minor GBs disconnected by the FeAs clusters.However, the Ba122 grains are more equiaxial and their grain texture appears weaker, implying that the connectivity of each GB in Ag-SS might not be as intrinsically strong as the textured GBs found in Ag-HP.Our nanostructural analysis suggested that the high J c in Ag-HP and Ag-SS is essentially derived from very different mechanisms, the former from the connectivity at each GB and the latter from the number of physically connected GBs.In summary, we compared the grain and GB nanostructures in two K-Ba122 tapes, the J c of which is among the highest ever reported. 31,37)Essentially there are the two key factors for a high J c -the quality of GB connectivity and the number of GB connections.The former can be achieved by grain alignment and the latter can be increased by clean synthesis and processing.It was revealed that even state-ofart high J c tapes utilize only one of these mechanisms, implying that the true potential J c of K-Ba122 has not been fully explored yet.A clear direction for further increment in J c would be the development of clean processing with controlling the grain alignment.

Figure 1 (
b) represents high-angle annular dark field (HAADF) STEM on the same region of Fig. 1(a).In contrast to the BF STEM image showing each Ba122 grain, the HAADF-STEM image of Fig. 1(b) shows the Z-contrasts which represent local compositional and/or density variations.Strikingly, many GBs appear either dark or bright in Z-contrast compared with the inside of the Ba122 grains, indicating a very different local chemical composition (i.e.not Ba122) and/or lower density at the GBs.The EDS elemental mapping in Fig. 2 revealed that such different Z-contrasts in Fig. 1(b) of Ag-HP are caused by different compositional segregations: a continuous bright contrast in the center of GBs and a discontinuous dark contrasts between the Ba122 grains and the bright bands at the center of the GBs.The EDS maps identify the former as made by continuous Fe and As segregation along the GBs, indicating that the bright Z-contrast at the GBs is FeAs.The EDS of K and O shows significant O segregation and K depletion at the GBs too.Judged by the O location and Ba distribution, the discontinuous dark contrast at the GBs is consistent with Ba-O segregation.

Fig. 1 .
Fig. 1.(a) Bright field scanning transmission electron microscopy (STEM) image and (b) high-angle annular dark field STEM image showing the grain and grain boundary structures in the same region in Ag-sheathed K-doped Ba122 tape made by hot pressing (sample Ag-HP).

Fig. 2 .
Fig. 2. Energy dispersive X-ray spectroscopy elemental maps showing the compositional segregations in the Ag-HP sample.At the grain boundaries, the continuous bright contrast is FeAs, whereas the discontinuous dark contrast is Ba-O.

Fig. 3 .
Fig. 3. (a) Bright field scanning transmission electron microscopy (STEM) image and (b) high-angle annular dark field STEM image showing the grain and grain boundary structures in Ag-stainless steel double-sheathed K-doped Ba122 tape made by cold pressing (sample Ag-SS).

Fig. 4 .
Fig. 4. (a) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image showing some disconnected grain boundaries (GBs) in the Ag-SS sample.(b) High-magnification HAADF-STEM image and corresponding energy dispersive X-ray spectroscopy elemental maps of a junction of representative disconnected GBs.