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