Microstructure and superconducting properties of nanocarbon-doped internal Mg diffusion-processed MgB₂ wires fabricated using different boron powders

MgB₂/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 MgB₂ layers were analyzed and compared. It was found that the purity and particle size of the boron powder influence the superconducting properties of MgB₂ 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 MgB₂ layer made using amorphous boron (AB) powder, although a certain amount of voids existed in the superconducting layer. The IMD-processed MgB₂ 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. MgB₂ 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 MgB₂ layer manufactured using the ball-milled LB (MLB) powder as expected due to the increased percentage of impurity. (paper)