Selection of Fe as a barrier for manufacturing low-cost MgB₂ multifilament wires – Advanced microscopy study between Fe and B reaction

The high cost of using the niobium (Nb) barrier for manufacturing magnesium diboride (MgB₂) mono-and multi-filamentary wires for large-scale applications has become one of the barriers to replacing current commercial niobium-titanium superconductors. The potential of replacing the Nb barrier with a low-cost iron (Fe) barrier for multifilament MgB₂ superconducting wires is investigated in this manuscript. Therefore, MgB₂ wires with Fe barrier sintered with different temperatures are studied (from 650 ℃ to 900 ℃ for 1 h) to investigate the non-superconducting reaction phase of Fe-B. Their superconducting performance including engineering critical current density (J_e) and n-value are tested at 4.2 K in various external magnetic fields. The best sample sintered at 650 ℃ for 1 h has achieved a J_e value of 3.64 × 10⁴ A cm⁻² and an n-value of 61 in 2 T magnetic field due to the reduced formation of Fe₂B, better grain connectivity and homogenous microstructure. For microstructural analysis, the focused ion beam (FIB) is utilised for the first time to acquire three-dimensional microstructures and elemental mappings of the interface between the Fe barrier and MgB₂ core of different wires. The results have shown that if the sintering temperature can be controlled properly, the J_e and n-value of the wire are still acceptable for magnet applications. The formation of Fe₂B is identified along the edge of MgB₂, as the temperature increases, the content of Fe₂B also increases which causes the degradation in the performance of wires.