Visualization of the grain structure in high-performance Ba_1−x K _x Fe₂As₂ superconducting tapes

Abstract

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.

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

“…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.…”

“…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).…”

“…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 beneficial for raising J c , 36,39) presumably due to improvement of intrinsic GB connectivity by reducing GB misorientation.…”

“…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.…”

High-performance Ba1−x K x Fe2As2 superconducting joints for persistent current operation

“…The study of the transport properties of superconducting twin films shows that IBSs have the problem of weak grain boundary connection similar to copper oxide superconductors [19]. Although the former has less stringent requirements on grain boundary angle than the latter, the texture of superconducting core is still the key to realize the high transmission performance of IBSs.…”