ボロンファイバーを用いて作製したMgB<SUB>2</SUB>テープの組織と超伝導特性

Abstract

MgB2 tapes were fabricated using B fibers and Mg powder. B fibers and Mg powder were wrapped with Fe or Ta foils to form tapes and heat-treated at 700-900°C for 2 hours in a vacuum. MgB2 layers were formed around the B fibers for all the tapes, although the cores of the B fibers were remained un-reacted. For the heat treatment temperature of 900°C needle-like Fe2B compound was formed around MgB2 layers when used Fe foils. For the 700 and 800°C, however, no Fe-B compound was formed and only the MgB2 layers were obtained. Broad superconducting transitions with onset temperatures of 27-35 K were observed for Fe foils. In the MgB2/Ta tapes, on the other hand, only MgB2 layer was formed at 900°C and onset Tc of 36 K with sharper transition was obtained. Thus, the lower Tc and broader transitions of MgB2/Fe tapes can be explained by the inclusion of Fe in the MgB2 layers. Jc values at 4.2 K of the MgB2/Fe tapes were ∼109 A/m2 in a zero magnetic field and ∼108 A/m2 in 2 T. These Jc values of the MgB2 tape were comparable to those of MgB2 wires fabricated by heat-treating B fibers under Mg vapor atmosphere, but lower than those of the powder-in-tube processed MgB2 tapes.