Effects of the size of Mg powder on the formation of MgB2and the superconducting properties

Abstract

The effect of the size and shape of magnesium(Mg) powder on the formation of MgB 2 and the critical current density( J c, ) of MgB 2 bulk was studied. As a precursor for the formation of MgB 2 , Mg and MgB 4 powder, which was synthesized through the reaction of boron (B) with Mg powders, was used. MgB 4 was mixed with Mg powders of various sizes, pressed into pellets and heat-treated at 650 ℃ -750 ℃ in flowing argon gas. The XRD analysis of the heat-treated MgB 2 samples showed that the volume fraction of MgB 2 was the highest as 92.74 % when spherical Mg powder with an average size of 25.7 µm was used, whereas the volume fraction was the lowest as 79.64 % when plate-like Mg powder with a size of 34.1 µm was used. The superconducting transition temperature ( T c ) of MgB 2 was not sensitive to the characteristics of the Mg powders used. All of the prepared MgB 2 samples showed a high T c of 38.3 K and a small superconducting transition width of 0.2 K-0.5 K. J c (5 K and 1 T) of MgB 2 was the highest as 3.93×10 4 A/cm 2 when spherical Mg powder with a size of 25.7 µm was used, whereas J c was the lowest as 2.18×10 4 A/cm 2 when plate-like Mg powder with a size of 34.1 µm was used. The relationship between the J c of MgB 2 and the characteristics of the Mg powders used was explained in terms of the volume fraction of MgB 2 and the apparent density of the MgB 2 pellets.