  Current transport in polycrystalline magnesium diboride is highly non-uniform
(percolative) due to the presence of secondary phases and also due to the
intrinsic anisotropy of the material. The influence of secondary phases on the
transport properties of MgB2 was investigated. Bulk samples were prepared from
a mixture of MgB2 and MgO powders by the ex-situ technique in order to vary the
MgO content systematically. The samples were characterized by resistive and
magnetization measurements. The reduced MgB2 fraction is modeled by a reduced
effective cross section (connectivity), which was assessed directly by the
experiments. The presence of MgO also increases the percolation threshold,
which reduces the zero resistivity (or irreversibility) field.
