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Fe-based superconductors (FBS) show great promise due to their high critical temperatures (Tc) and critical current densities (Jc). Research has revealed that controlled grain boundaries and defects in FBSs can significantly enhance their properties by pinning magnetic flux vortices. However, high-angle grain boundaries (HAGB) often limit the Jc in polycrystalline FBS materials. Therefore, understanding detailed microstructures of FBSs, including crystal orientations, the structure and density of defects and grain boundaries, and their impact on properties, will be of great interest. This report focused on two types of FBSs: epitaxial growth of FBS thin films and sintered FBS polycrystalline bulks.
The first part studied NdFeAs(O,F) films grown on IBAD-MgO/Y2O3/Hastelloy substrates [1] and K-doped BaFe2As2 films on single-crystal CaF2 substrates [2], by molecular beam epitaxy (MBE). The films had textured columnar grain structures and high-density low-angle grain boundary (LAGB) networks, contributing to high critical current properties and strong pinning efficiency. In addition, the triple junctions of LAGBs are found to be effective pinning centers even at temperatures close to the Tc. The study suggests potential for grain boundary engineering in improving thin film performance. The second part investigated the microstructure of polycrystalline Co-doped BaFe2As2 superconducting bulks synthesized by ball milling and spark plasma sintering (SPS) [3]. A strong correlation was observed between microstructure and synthesis conditions. Specific ball milling energies resulted in nanograins with high densities of planar defects in bulks, exhibiting high Jc under both magnetic and self-fields. These planar defects are rich in barium and exhibit obvious local strain fields at the edges, which are believed to act as volumetric pinning centers in polycrystalline bulks. Additionally, two potential microstructural evolution process during the fabrication of Ba122 bulks were proposed. Intentional control of fabrication conditions could introduce a high density of planar defects in polycrystalline Ba122 bulks, enhancing superconducting properties for applications like superconducting magnets.
[1] Z. Guo, H. Gao, K. Kondo, et. al., ACS Appl. Electron. Mater. 3, 3158–3166 (2021).
[2] K. Iida, D. Qin, C. Tarantini, T. et.al., NPG Asia Materials 13, 1–9 (2021).
[3] Z. Guo, K. Muraoka, H. Gao, et. al., Acta Materialia 266, 119648 (2024).
Dr. Zimeng Guo was a postdoctoral researcher in Department of Advanced Materials Science and Engineering at Kyushu University in Japan, until January 2024. He obtained his bachelor's degree from Shenyang University of Technology in 2015, and master's degree from University of Science and Technology Beijing in 2018, respectively. Then he moved to Kyushu University in Japan for doctoral program on Fe-based superconductors, and obtained PhD degree in September 2022, under the mentorship of Prof. Satoshi Hata. His key research area is the nanoscale crystal structural characterization and property analysis of Fe-based superconductors, as a major researcher in a JST-CREST project. His research interests encompass advanced electron microscope techniques, superconducting thin films and bulks, materials microstructure and property analysis, and so on.
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