The kinetics mechanism of MgB₂layer formation within MgB₂superconducting wire fabricated using improved internal Mg diffusion process

Internal Mg diffusion (IMD) process can produce MgB₂superconducting 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 mass producing practical MgB₂wires. However, the MgB₂layer growth stops shortly after the onset of the heat treatment and unreacted B always remained in the MgB₂layer within MgB₂wires, negatively affecting the J_eperformance. Thus it is of great importance to have an in-depth understanding of the mechanism of reaction between Mg rod and B powder forming the MgB₂layer within IMD wires during heat treatment, and identify the critical factor that controls the formation rate of MgB₂layer. In present work, the kinetics mechanism of reaction between Mg and B forming MgB₂layer in the internal Mg diffusion (IMD) processed MgB₂wires were systemically studied in present work. It was found that the reaction between Mg and B forming MgB₂layer during the heating treatment is controlled by varied mechanisms. At initial stage, the formation of MgB₂layer is mainly determined by the rate of chemical reaction between liquid Mg and B powder. As the reaction processes and the thickness of synthesized MgB₂layer increases, the slow Mg diffusion-limited mechanism gradually becomes dominant at final stage. On the basis, herein Mg rod with a thin Cu coating is proposed to replace normal Mg rod in IMD procedure to accelerate the formation of MgB₂layer within wires. As a result, Cu coating can change the kinetics mechanism of MgB₂layer formation and enable the formation of MgB₂layer to get rid of the restriction of slow Mg diffusion. Complete dense MgB₂layer without B-rich or unreacted B regions was successfully synthesized within Cu coated IMD wires with larger diameter (1.03 mm) at temperature as low as 600 °C (below Mg melting point). The kinetics mechanism of MgB₂layer formation determined in our work can provide a valuable guide for optimizing processing parameters of IMD MgB₂wires. Moreover, the Cu coating technique proposed here opens a promising way to fabricate practical high performance IMD wires at low heating temperature.