Synthesis of nanocrystalline magnesium diboride (MgB 2) metallic superconductor by mechano-chemical reaction and post-annealing

Abstract

MgB 2 is recently discovered superconducting compound with a record-breaking transition temperature ( T c = 39 K) for a conventional metallic superconductor. Nanocrystallinity can improve its electrical properties by strong pinning. In the present work we report the results of the synthesis of nanocrystalline MgB 2 superconducting compound by mechano-chemical reaction followed by post-annealing. The first stage of synthesis was carried out from elemental crystalline Mg and amorphous B powders by controlled mechanical alloying (CMA) in the magneto-mill Uni-Ball-Mill 5. X-ray diffraction studies reveal that the nucleation of small amount of MgB 2 is initiated after two-step milling for combined 100 h under protective helium gas. Further reaction to form MgB 2 is accelerated by subsequent annealing of the milled powder at various temperatures. X-ray diffraction shows formation of a well-developed nanocrystalline MgB 2 after annealing of the pre-nucleated powder at the 630–650 °C range for a few hours.