Optimizing internal Mg diffusion processed MgB2 superconductors: The minimal activation strategy

Abstract

In this study, the effect of nanoscale copper (Cu) coating thickness on the phase formation kinetics, microstructural homogeneity, and superconducting properties of Internal Mg Diffusion (IMD) processed MgB₂ wires was systematically investigated. Central magnesium (Mg) rods were coated with Cu layers ranging from 200 nm to 800 nm to act as in situ activators. Differential thermal analysis (DTA) indicated that the Cu coating promotes low-temperature MgB₂ formation by facilitating the formation of a local Mg-Cu eutectic interactions that accelerate Mg diffusion, enabling phase formation even at 575 °C. Resistivity measurements showed that the 200 nm Cu-coated wires exhibited broader superconducting transitions (ΔT_c ≈ 1.8 K) at low annealing temperatures, whereas the 800 nm Cu-coated wires showed sharper transitions (ΔT_c ≈ 1.0 K) and denser microstructures, indicating improved phase homogeneity due to enhanced Mg diffusion. However, transport critical current density (J_c) measurements revealed that excessively thick Cu coatings adversely affect inter-granular current transport, likely due to the segregation of Cu-rich impurity phases such as MgCu₂ at grain boundaries. Among all samples, the 200 nm Cu-coated wire annealed at 625 °C exhibited the highest J_c value of 3.9 × 10⁴ A/cm². These results demonstrate that careful optimization of Cu coating thickness provides an effective interface-engineering strategy for improving the performance of IMD-MgB₂ wires and supports the use of Cu activation as a practical route for low-cost, high-performance superconducting applications.