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 MgB2 was studied. Critical current densities (J(c)) for MgB2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB2 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 B2O3. The different reactivity rates of the precursor powders accounts for this anomaly.