The development of MgB2 superconducting wires fabricated with an internal Mg diffusion (IMD) process

In this review, we introduce the development of internal Mg diffusion (IMD)-processed MgB2 superconducting wires. First, a brief history of the IMD method (which has been given different names by different groups) is introduced and the mechanism of IMD MgB2 wire is discussed. Second, factors related to J(c) and J(e) including: boron (B) powder and carbon (C) addition, the 'effective MgB2 density' (the MgB2 layer excluding pores, unreacted B particles, MgO, and other impurities), grain size, flux pinning, and connectivity, maximum Mg infiltration distance, B layer thickness and wire diameter, along with other factors such as heat treatment conditions, sheath materials, pressure, and chemical addition are discussed. Third, fabrication processes for IMD long wires, IMD multi-filament wires, and a hybrid method combining IMD and PIT processes are introduced. Fourth, other research related to practical applications, such as the mechanical properties, AC loss of IMD MgB2 wires are introduced. Finally, future work, including the enhancement of J(c) (especially J(c) at 20-30 K) and the superconducting joint of IMD MgB2 wire are discussed.