  The influence of the nature of the boron precursor on the superconducting
properties of polycrystalline MgB2 was studied. Critical current densities for
the 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 Jc compared to those made from amorphous precursors. X-ray, Tc
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.
