The influence of microstructure on the transport critical current density in MgB2 wires obtained by PIT methods

Four decades after the discovery of high-temperature superconductivity by Bednorz and Müller, magnesium diboride, which has the highest critical temperature among conventional phonon-mediated superconductors, remains one of the most promising materials for practical, low-cost superconducting wires operating above liquid-helium temperature. This article reviews how various factors, such as the purity and grain size of Mg and B powders, diffusion barriers, dopants, and high-pressure processing, influence the formation of the superconducting MgB2 phase and its microstructure, and consequently the transport critical current density (Jc) measured at 4.2 and 20 K in in situ powder-in-tube (PIT) MgB2 wires. Moreover, a method of preparing samples for scanning electron microscopy based on fracturing rather than polishing is presented, which enables a substantially improved characterization of the microstructure of the in situ MgB2 material in superconducting wires.