Fabrication of MgB₂ Wires by the In-situ PIT Method using Cu and Cu-alloy Sheaths

Abstract

We investigated the applicability of Cu and Cu-alloy sheaths to MgB₂ wires prepared by the in-situ synthesis powder-in-tube (PIT) method. Raw materials pellets of Mg and B mixed with ZrH₂ and/or C powder were inserted into Cu, Cu-Zr, Cu-Cr, Cu-Sn and Cu-Be sheath tubes. We fabricated round MgB₂ wires 1.0mm in diameter utilizing rotary swaging, drawing and two-axial rolling at room temperature. The wires were annealed at 500 - 800°C for 2 h in an Ar gas atmosphere. Compared with Cu sheath wires, the critical current characteristics of some of the Cu-alloy sheath wires were improved. For example, the four-probe transport measurements at 4.2 K in a self-field showed that a MgB₂/Cu short sample with a 1.0mm diameter had an I_C of 726 A (J_C = 545 kA/cm²), while that of the MgB₂/Cu-Cr sample was 846 A (J_C = 635 kA/cm²). This improvement may be due to the higher core density of the MgB₂ wires fabricated using Cu-Cr sheaths with higher hardness. Futhermore, decreasing the Cu:Core ratio in the cross-section of the MgB₂/Cu wire from 4.9 to 2.0, is was possible to increase the I_C from 726 A to 1219 A.