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

We have fabricated ex situ processed MgB₂ round wires and tapes sheathed with various metals using chemically treated powders. The critical current density (J_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_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 MgB₂ and slight enhancement in magnetization hysteresis when using Ta sheath, compared with Fe sheath. In contrast, the transport J_c properties of the wires sheathed with Ta as barrier are drastically deteriorated by heat treatment, whereas Fe-sheathed tapes show drastic J_c enhancement. Microstructural observation reveals that many cracks are generated in the MgB₂ core by heat treatment only for the heat treated wires sheathed with Ta as barrier. Such cracks act as obstacles for supercurrent flow.