N. Kaneda

S. Yamada

Y. Hishinuma

Y. Yamada

National Institute for Fusion Science (NIFS-Repository)

1. Introduction Since the discovery of superconductivity in MgB2 with critical temperature Tc of 39 K, developments of?conductors as well as fundamental researches on the new superconductor have been performed. In particular, improvements of critical current density Jc in MgB2superconducting wires and tapes have gained world-wide interest for practical applications1)-3).In present work, superconducting properties and structure of MgB2 wires synthesized with external Mg diffusion process have been reported. 2. Experimental Fig. 1 shows preparation procedure of MgB2 wires by external Mg diffusion process. A pure Mg tube of 6/4 and 6/3.5 mm in outside/inside diameter was inserted into a pure iron tube of 12/6 mm, and then amorphous B powder mixed with 5 mol% SiC nano-sized powder addition was encased in the Mg tube to form Fe/Mg/B(powder) composite. The composite was fabricated into 1.8 mm square wire through grooved-rolling and then drawn into round wires of 1.0 and 0.8 mm in diameter without intermediate annealing. The heat-treatment was performed   at 630? for 5 h in Ar gas atmosphere.The critical current Ic at 4.2 K of specimens wasmeasured by a four-probe resistive method, the criterion of the Ic measurement being 1 µV/cm. The core Jc was calculated by dividing Ic by the cross-sectional area of the MgB2 core. 3. Results and Discussion Fig. 2 shows SEM image taken on the fractured cross- section of MgB2 wire of 0.8 mm. The MgB2superconducting core was synthesized in a center of the wire through diffusion reaction between Mg metal and B powder with SiC addition. The MgB2 core forms denser structure without voids in comparison with conventional in-situ PIT processed wires and tapes. The residual Mg which has not reacted with B powder remains around iron sheath. According to EPMA analysis, the composition of MgB2 core is slightly B richer compare to stoichiometric composition of 1:2. Magnetic field dependence of core Jc at 4.2 K for the MgB2 wires is shown in Fig. 3. The Ic at 4.2 K for the MgB2 wire of 0.8 mm (Mg 6/4) are 187 A at 5 T and 21 A at 10 T, which Ic values correspond to core Jc of 3740 A/mm2 and 420 A/mm2, respectively. The Jc values are one order of magnitude higher than that of PIT processed wires. The higher Jc results from denser structure without voids synthesized by diffusion process.  1) Y. Yamada, et al. : IEEE Trans. Appl. Supercond., 22(2012) 6200304.2) Y. Yamada, et al. : ICEC24-ICMC2012, 15P-P09-09, 80.3) M. Son, et al. : IUMRS-ICEM2012, B-4-O26-022.Fig. 1.  Preparation procedure of MgB2 wires by external Mg diffusion process.              Fig. 2. SEM image of the fractured cross-section of MgB2wire.  Fig. 3. Magnetic field dependence of Jc at 4.2 K for the MgB2 wires.  306§19. Superconducting Properties of MgB2 Wires Synthesized with External Mg Diffusion ProcessYamada, Y., Kaneda, N. (Tokai Univ.),  Hishinuma, Y., Yamada, S.

Superconducting Properties of MgB2 Wires Synthesized with External Mg Diffusion Process

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