Structural inhomogeneity of superconducting ex situ MgB2/Cu wires made by the powder-in-tube technique

Abstract

Single core MgB2 wires have been made by the powder-in-tube (PIT) method using commercial MgB2 powder (Alfa Aesar). Using the two-axial rolling process, composites have been made in a Cu sheath. Energy dispersive x-ray (EDX) mapping and scanning electron microscopy backscattered electron observations prove that there is high element segregation (phase inhomogeneity) for as-rolled and as-sintered wires, although the wire after sintering is more inhomogeneous than the as-rolled (non-sintered) wire. X-ray diffraction diffractograms confirm the presence of isotropy and the absence of texturization in the ‘macroscopic’ phase within the MgB2 cores, despite their thermomechanical history. The observed decrease in the value of Jc in the wire after sintering was attributed to macroscopic cracking and the worsening of the intergrain connectivity. EDX line scans have also shown high anisotropy and high inhomogeneity in the distribution of elements within the MgB2 cores. Inhomogeneity in the final product (wires) has resulted from the high macroscopic inhomogeneity observed in the starting MgB2 powder. The Cu sheath has been shown to be neutral (non-poisoning) for MgB2 cores made ex situ. However, due to its softness, it cannot properly constrain the MgB2 core or give enough support required for adequate grain connectivity.