Microstructure and superconducting properties of nanocarbon-doped internal Mg diffusion-processed MgB₂wires fabricated using different boron powders

MgB 2 /Nb/Monel monofilament wires were fabricated using four different boron powders by an internal Mg diffusion (IMD) process. The microstructure, morphology and the critical current density ( J c ) of the used boron powders and the formative MgB 2 layers were analyzed and compared. It was found that the purity and particle size of the boron powder influence the superconducting properties of MgB 2 wires; further that the optimized heat-treatment condition also depends on the quality of the boron powder. The highest J c was obtained in the MgB 2 layer made using amorphous boron (AB) powder, although a certain amount of voids existed in the superconducting layer. The IMD-processed MgB 2 layer fabricated using high-purity boron (HB) powder had also a high J c compared with the powder-in-tube (PIT) process and a few unreacted boron particles remained in it. MgB 2 wire fabricated using low-purity boron (LB) powder had a high cost-performance ratio compared with the others, which is expected to allow the fabrication of large-scale and low-cost superconducting wires for practical application. However, the enhancement of the J c was not found in the MgB 2 layer manufactured using the ball-milled LB (MLB) powder as expected due to the increased percentage of impurity.