The effect of boron powder on the microstructure of MgB 2 filaments prepared by the modified internal magnesium diffusion technique

We have analyzed the microstructure of MgB 2 prepared by the modified internal magnesium diffusion technique resulting in filaments without a central hole. Three different amorphous boron powders with a nominal purity of 98.5%–99.0% and a different particle size were used. It was shown that the real purity of the powders was different from the nominal purity, and the microstructure as well as the critical current densities of the MgB 2 were affected by using boron. Larger B particles with a size of 400–500 nm led to the presence of large B-rich grains inside the MgB 2 matrix. For one wire series, boron powder containing residual chlorine from the boron preparation process was used. It was found that just a small quantity of chlorine in the B powder caused significant MgB 2 filament inhomogeneity, an apparent decrease in the critical current density and high reactivity with air. We estimated the ideal packing density for achieving a large volume of dense MgB 2 .