Browsing by Author "Yamauchi, Y"

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    Evaluation of isotopic boron (11B) for the fabrication of low activation Mg11B2 superconductor for next generation fusion magnets
    (John Wiley & Sons, Inc., 2020-04-13) Jie, H; Luzin, V; Zaman, M; Abdulsalam, AV; Chae, KH; Choi, H; Levchenko, VA; Nijhius, A; Kim, JH; Mustapić, M; Dou, SX; Yamauchi, Y; Khan, A; Hossain, A
    In this study, we analyze the properties of boron isotope (11B)‐rich powders from three different sources, that is, American, Cambridge, and Pavezyum, to fabricate the bulk Mg11B2 superconductors and evaluate their superconducting properties. While 11B‐rich powder is an essential precursor to fabricate Mg11B2 superconductors for fusion magnet applications, the properties of the 11B powder turned out to be critical to determine the quality of the final superconducting product. Therefore, appropriate control of processing conditions is needed to comply with the requirements of the nuclear fusion application. Analysis of the B isotope ratio by accelerator mass spectroscopy and neutron transmission revealed that all three types of powder are enriched with 11B to better than 99 at % quality. In addition, Pavezyum's 11B shows the lowest crystallinity and smallest crystalline domain size as evidenced by the high‐resolution X‐ray diffractometer and scanning electron microscopy. The chemical states of the boron isotope investigated with near edge X‐ray absorption fine structure spectroscopy and X‐ray photoemission spectroscopy also reveals that Pavezyum boron has amorphous structure. Mg11B2 bulks and multi‐filamentary (12‐filament) wires have been manufactured, sintered at different temperatures and characterized via the transport critical current density. The wire with Pavezyum 11B shows three times higher current carrying capacity at a particular magnetic field compared to the wire using Cambridge 11B and hence, Pavezyum 11B boron has the potential for manufacturing fusion grade Mg11B2 based magnets. The results of this study demonstrated that Boron powders with higher purity, smaller grain size and lower crystallinity are critical for improving the superconducting and electronic properties of Mg11B2 samples fabricated from the powder. Thus, the low‐neutron‐activation Mg11B2 is possibly an affordable and technically viable candidate to replace NbTi superconductors in the low field poloidal field and correction coils for the next‐generation fusion reactors. © 1999-2020 John Wiley & Sons, Inc.
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    Significant reduction in thermal conductivity and improved thermopower of electron‐doped Ba1–xLaxTiO3 with nanostructured rectangular pores
    (Wiley, 2021-04) Ahmed, AlJ; Cortie, DL; Yun, FF; Rahman, Y; Islam, KN; Bake, A; Konstantinov, K; Hossain, SA; Alowasheeir, A; Yamauchi, Y; Wang, X
    Electron‐doped BaTiO3 is a less studied n‐type metal oxide thermoelectric material. In this work, the electrical conductivity of BaTiO3 samples has been improved by introducing La to yield an n‐type Ba1–xLaxTiO3 semiconducting material. Density functional theory calculations show that the optimal electron‐doping occurs at x = 0.2, and this is also confirmed experimentally. To improve the thermoelectric properties further, nanostructured cuboidal pores are introduced into the bulk Ba1–xLaxTiO3 using F127 surfactant micelles for a chemical templating process, followed by spark plasma sintering. Interestingly, transmission electron microscopy images and X‐ray powder diffraction analysis confirms that our fabricated samples are cubic BaTiO3 perovskite phase with the nanostructured rectangular‐prism pores of >4 nm. Scanning electron microscopy images show that all the samples have similar grain boundaries and uniform La doping, which suggests that the large reduction in the lattice thermal conductivity in the F127‐treated samples arises primarily from the pore distribution, which introduces anisotropic phonon scattering within the unique nanoarchitecture. The sample with 20 at% La doping and nanopores also shows a thermopower that is doubled compared to the related sample without porosity. Together with the lattice thermal conductivity, enables a significant improvement in figure of merit, zT compared to the other samples. © 2021 Wiley-VCH GmbH.