Three-dimensional microstructure and critical current properties of ultrafine grain Ba(Fe,Co)2As2 bulk superconductors

• Published in: Journal of alloys and compounds 2022-11, Vol.923, p.166358, Article 166358
• Main Authors: Shimada, Yusuke, Tokuta, Shinnosuke, Yamanaka, Akinori, Yamamoto, Akiyasu, Konno, Toyohiko J.
• Format: Article
• Language: English
• Subjects: Agglomeration; Bulk density; Critical current density; Electron microscopy; Flux pinning; Grain boundaries; Grain size; Iron; Iron-based superconductor; Mechanical alloying; Microstructure; Polycrystalline bulk; Polycrystals; Sintering (powder metallurgy); Structure-property relationship; Superconductors; Three-dimensional observation; Transport properties; Ultrafines
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2022.166358

In iron-based superconductors, randomly oriented grain boundaries have a strong influence on the transport properties via intrinsic weak-link and flux pinning mechanisms. Herein we report the critical current density (Jc) and the three-dimensional microstructure of polycrystalline bulk Co-doped Ba122 (BaFe1.84Co0.16As2) superconductors, with highly dense grain boundaries (grain size smaller than 50 nm), produced by high-energy milling. Three-dimensional electron microscopy revealed that the anomalous growth of secondary particles (aggregation) and the inter-aggregation structures were significantly different in the samples with finer grains, which may have extrinsically limited Jc. These important microstructural features were quantified as two parameters—local thickness and total pore length—by reconstructing the three-dimensional structure of the superconducting phase using the adaptive thresholding method. The results obtained in this study suggest that understanding and controlling the microstructural formation process by sintering are instrumental for improving the Jc properties of 122 polycrystalline materials consisting of ultrafine grains. •The grain size of Ba-122 can be controlled by ball milling energy in several 10 nm size.•Self-sintered Ba-122 bulks are consisted of dense aggregate and narrow neck areas, and the necks suppress current property.•3D structural quantification by FIB-SEM method is effective for evaluating structural inhomogeneity on bulk superconductor.•The Jc of bulk superconductor depends more strongly on inhomogeneous macroscale structure than nano-grain structure.