The effect of copper additions in the synthesis of in situ MgB2 Cu-sheathed wires

The powder-in-tube (PIT) technique has been used to fabricate copper-sheathed magnesium diboride (MgB2) wires using an insitu reaction method. The effect of copper powder additions, magnesium-boron molar ratio and heat treatment is studied by SEM, XRD, transport critical current Ic(B) and resistivity ρ(T, B) measurements. The results show that addition of copper powder to the core of the wire accelerates the formation of MgB2 and hence increases its amount and greatly decreases the amount of Mg-Cu intermetallic phases present in the core of the wire after heat treatment. Excess magnesium proved to be effective in compensating for Mg loss due to interdiffusion with the Cu of the wire sheath and resulted in less unreacted boron in the core for wires without added Cu, but seems to oppose the accelerated formation of MgB2 in Cu added wires. The highest critical current density, 2.8 × 104 A cm-2 at 3 T and 4.2 K, was achieved for a wire with a stoichiometric Mg:B ratio and 3 at.% added copper powder heat treated at 700 °C for 5 min.