The influence of the nature of the B precursor on the superconducting properties of polycrystalline MgB2 was studied. Critical current densities, JC, for MgB2 made from high purity amorphous B were at least a factor of 3 higher than typical values measured for standard MgB2 samples made from amorphous precursors. Two possible mechanisms were proposed to account for this difference. Samples made from crystalline B powders have around an order of magnitude lower JC compared to those made from amorphous precursors. X-ray, TC and resistivity studies indicated that this was 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.

Strong Influence of Boron Precursor Powder on the Critical Current Density of MgB2. S.K.Chen, K.A.Yates, M.G.Blamire, J.L.MacManus-Driscoll: Superconductor Science and Technology, 2005, 18, 1473-7