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

The effects of MgB 4 addition on the superconducting properties and the microstructure of in situ processed MgB bulk superconductors were studied. MgB 4 powder of 1-20 wt.% was mixed with (Mg + 2B) powder and then pressed into pellets. The pellets of (Mg + 2B + xMgB 4 ) 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 MgB 2 and the minor phases were MgB 4 and MgO. The full width at half maximum (FWHM) values showed that the grain size of MgB 2 decreased as the amount of MgB 4 addition increased. MgB 4 particles included in a MgB 2 matrix is considered to suppress the grain growth of MgB 2 . The onset temperatures ( T c,onset ) of MgB 2 with MgB 4 addition (0-10 wt.%) was between 37-38 K. The 20 wt.% MgB 4 addition slightly reduced the T c,onset of MgB 2 to 36.5 K. This result indicates that MgB 4 addition did not influence the superconducting transition temperature ( T c ) of MgB 2 significantly. On the other hand, the small additions of 1-5 wt.% MgB 4 increased the critical current density ( J c ) of MgB 2 . The J c enhancement by MgB 4 addition is attributed not only to the grain size refinement but also to the possible flux pinning of MgB 4 particles dispersed in a MgB 2 matrix.