Detrimental Effects Depending on the Sheath Materials in Ex Situ Processed Heat Treated $hbox{MgB}_{2}$ Wires and Tapes

摘要

We have fabricated ex situ processed $hbox{MgB}_{2}$ round wires and tapes sheathed with various metals using chemically treated powders. The critical current density $(J_{rm c})$ properties of the as-fabricated wires and tapes are not so different, that is, independent of the sheath materials. On the other hand, the transport $J_{rm c}$ properties of the heat-treated wires and tapes strongly depend on the sheath materials. Heat treatment of those wires and tapes brings about the sintering of $hbox{MgB}_{2}$ and slight enhancement in magnetization hysteresis when using Ta sheath, compared with Fe sheath. In contrast, the transport $J_{rm c}$ properties of the wires sheathed with Ta as barrier are drastically deteriorated by heat treatment, whereas Fe-sheathed tapes show drastic $J_{rm c}$ enhancement. Microstructural observation reveals that many cracks are generated in the $hbox{MgB}_{2}$ core by heat treatment only for the heat treated wires sheathed with Ta as barrier. Such cracks act as obstacles for supercurrent flow.