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

The influence of the nature of the boron precursor on the superconducting properties of polycrystalline MgB 2 was studied. Critical current densities ( J c ) for MgB 2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB 2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower J c compared to those made from amorphous precursors. X-ray, T c and resistivity studies indicate that this is as a result of reduced current cross-section due to the formation of (Mg)B–O phases. The samples made from amorphous B contain far fewer Mg(B)–O phases than crystalline B despite the fact that the amorphous B contains more B 2 O 3 . The different reactivity rates of the precursor powders accounts for this anomaly.