The kinetics mechanism of MgB2 layer formation within MgB2 superconducting wire fabricated using improved internal Mg diffusion process
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 mass producing practical MgB2 wires. However, the MgB2 layer growth stops shortly after the onset of the heat treatment and unreacted B always remained in the MgB2 layer within MgB2 wires, negatively affecting the Je performance. 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 MgB2 layer within IMD wires during heat treatment, and identify the critical factor that controls the formation rate of MgB2 layer. In present work, the kinetics mechanism of reaction between Mg and B forming MgB2 layer in the internal Mg diffusion (IMD) processed MgB2 wires were systemically studied in present work. It was found that the reaction between Mg and B forming MgB2 layer during the heating treatment is controlled by varied mechanisms. At initial stage, the formation of MgB2 layer is mainly determined by the rate of chemical reaction between liquid Mg and B powder. As the reaction processes and the thickness of synthesized MgB2 layer 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 MgB2 layer within wires. As a result, Cu coating can change the kinetics mechanism of MgB2 layer formation and enable the formation of MgB2 layer to get rid of the restriction of slow Mg diffusion. Complete dense MgB2 layer 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 MgB2 layer formation determined in our work can provide a valuable guide for optimizing processing parameters of IMD MgB2 wires. Moreover, the Cu coating technique proposed here opens a promising way to fabricate practical high performance IMD wires at low heating temperature.