  Carbon-doped magnesium diboride was fabricated from pre-reacted pure MgB2 by
mechanical alloying. The sample set had excellent critical current densities
Jc(8T,4.2K) ranging from 15-60 kA/cm2, depending on composition.
Magneto-optical imaging detected regions up to 0.5 mm in size which were nearly
100% dense with Jc 2-6 times that of the matrix. Evaluation of resistivity
curves using the Rowell method predicts that only 10-50% of the cross sectional
area carries the normal state measurement current, suggesting that considerable
increases in Jc in these ~80% dense MgB2 samples would be possible with
complete grain and particle connectivity.
