Crystal growth of MgB 2 from Mg–Cu–B melt flux and superconducting properties

Abstract

A new method for the preparation of single crystals of the superconducting intermetallic MgB 2 compound from a Mg–Cu–B melt flux is presented. The high vapour pressure of Mg at elevated temperature is a serious challenge of the preparation process. The approximate thermodynamic calculations of the ternary Mg–Cu–B phase diagram show a beneficial effect of Cu, which extends the range of formation of MgB 2 to lower temperatures. Within the as-solidified Mg–Cu–B melt flux the MgB 2 compound forms plate-like single crystals up to a size of 0.2×0.2×0.05 mm 3 or alternatively rims peritectically grown around Mg B 4 particles. AC-susceptibility measurements were conducted with specimens selected from different parts of the as-solidified flux containing MgB 2 particles. Peritectically formed MgB 2-particles display the highest transition temperature of T c=39.2 K and a relatively narrow transition width of Δ T c=1.3 K. Other sections of the sample exhibit various superconducting transitions from T c=39 K to 7.2 K. This variation of T c is attributed to a finite homogeneity range of the MgB 2 compound whereas significant Cu solid solubility in MgB 2 can be excluded.