Transport currents of two-axially rolled and post-annealed MgB2/Fe wires at 4.2 K

Single- and four-core MgB2 wires have been made by the powder-in-tube (PIT) method using a commercial MgB2 powder (Alfa Aesar) and two-axial rolling deformation in an Fe sheath. The deformed wires were subjected to annealing at temperatures from 950 degreesC up to 1100 degreesC for 30 minutes in argon. The interface layer thickness of 10-25.4 mum caused by inter-diffusion and reaction between the MgB2 core and the Fe sheath has been observed in all annealed wires. Transport currents of as-deformed and post-annealed wires were measured at the temperature 4.2 k and external magnetic field B = 0-5.5 T. The presence of a ferromagnetic sheath in MgB2 composite wire influences the I-c(B) characteristics leading to Ic hysteresis and I-c anisotropy, which is more pronounced for those that have not been annealed (weakly linked grains) in comparison to well-connected MgB2 grains after recrystallization. A comparison of the overall current densities versus the external magnetic field of NbTi, Nb3Sn and Bi-2223 superconductors with four-core MgB2 wire is also presented.