Enhanced critical current density of in situ processed MgB2 bulk superconductors with MgB4 additions

Abstract

The effects of MgB4 addition on the superconducting properties and the microstructure of in situ processed MgB2 bulk superconductors were studied. MgB4 powder of 1-20 wt.% was mixed with (Mg + 2B) powder and then pressed into pellets. The pellets of (Mg + 2B + xMgB4) were heat-treated at 650 ℃ for 1 h in flowing argon. The powder X-ray diffraction (XRD) analysis for the heat-treated samples showed that the major formed phase in all samples was MgB2 and the minor phases were MgB4 and MgO. The full width at half maximum (FWHM) values showed that the grain size of MgB2 decreased as the amount of MgB4 addition increased. MgB4 particles included in a MgB2 matrix is considered to suppress the grain growth of MgB2. The onset temperatures (Tc,onset) of MgB2 with MgB4 addition (0-10 wt.%) was between 37-38 K. The 20 wt.% MgB4 addition slightly reduced the Tc,onset of MgB2 to 36.5 K. This result indicates that MgB4 addition did not influence the superconducting transition temperature (Tc) of MgB2 significantly. On the other hand, the small additions of 1-5 wt.% MgB4 increased the critical current density (Jc) of MgB2. The Jc enhancement by MgB4 addition is attributed not only to the grain size refinement but also to the possible flux pinning of MgB4 particles dispersed in a MgB2 matrix.