A series of ex situ Cu-sheathed powder-in-tube MgB2 wires was fabricated, with 20% by volume of Ag, Pb, In and Ga added to the MgB2 powder. It was found that the transport critical current of the wires increased significantly with the addition of metals to the core filament. In particular, the critical current density, JC, of the MgB2/Ga(20%) wire exceeded 5 x 104A/cm2 at 10K in self-field. This was nearly 50 times higher than that of MgB2/Ag(20%) wire. The temperature dependent JC of all wires was well described as an ensemble of clean S/N/S junctions in which the relevant parameters were the average thickness of the N layer, the critical temperature of the S layer, and a scaling term related to JC at zero temperature. Eliminating the differences in the filament microstructure as the primary cause of the enhanced JC, it was suggest that JC was determined by the magnitude of the proximity effect induced superconductivity in the normal metal layer, which was known to be proportional to the electron–electron interaction in N. One-dimensional material specific calculations were presented that supported this, and zero-field cooled direct-current magnetic susceptibility data confirmed an increased number of well-connected superconducting grains existed in the composite wires that contained metal additions with large electron–electron interactions and long electron mean free paths.

Supercurrents in Magnesium Diboride/Metal Composite Wire. M.J.Holcomb: Physica C, 2005, 423[3-4], 103-18