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. (topical review)