High performance MgB2 superconducting wires fabricated by improved internal Mg diffusion process at a low temperature

Abstract

Internal Mg diffusion (IMD) process can produce MgB₂ superconducting wires with engineering critical current density several times higher than that of traditional powder in tube processed wires, which makes it an attractive and promising method for the mass production of practical MgB₂ wires. However, the MgB₂ layer growth stops shortly after the onset of the heat treatment and unreacted B always remains in the MgB₂ layer within MgB₂ wires due to slow Mg diffusion, negatively affecting the J_e performance. In the present study, to solve the problem of slow Mg diffusion and fabricate high performance MgB₂ superconducting wires, a Cu coating technique was innovatively introduced into internal Mg diffusion (IMD) process. It was found that Cu coating first reacts with the Mg rod forming Mg–Cu liquid at a low temperature during heating process. This Mg–Cu liquid locally concentrates between Mg rod and B powder and can provide higher transport for the diffusion of Mg atoms into B. Consequently, Mg diffusion rate and distance can be increased significantly and the formation of MgB₂ layer is accelerated dramatically. Complete dense MgB₂ layer without B-rich or unreacted B regions was successfully synthesized within Cu coated IMD wires with a larger diameter (1.03 mm) at a temperature as low as 600 °C (below Mg melting point). The engineering critical current density of these Cu coated IMD MgB₂ wires is even comparable to the best J_e obtained in IMD MgB₂ wires with a small diameter prepared by heating at a high temperature (above Mg melting point) in other groups. The Cu coating technique proposed in the present study opens a promising way to fabricate practical high performance MgB₂ superconducting wires with a large diameter at a low temperature.