Strong influence of boron precursor powder on the critical current density of MgB₂

Abstract

The influence of the nature of the boron precursor on the superconducting\nproperties of polycrystalline MgB2 was studied. Critical current densities for\nthe MgB2 made from high purity amorphous boron are at least a factor of three\nhigher than typical values measured for standard MgB2 samples made from\namorphous precursors. Two possible mechanisms are proposed to account for this\ndifference. Samples made from crystalline boron powders have around an order of\nmagnitude lower Jc compared to those made from amorphous precursors. X-ray, Tc\nand resistivity studies indicate that this is as a result of reduced current\ncross section due to the formation of Mg-B-O phases. The samples made from\namorphous B contain far fewer Mg-B-O phases than crystalline B despite the fact\nthat the amorphous B contains more B2O3. The different reactivity rates of the\nprecursor powders accounts for this anomaly.\n