Resistivity and Magneto-Optical Evidence for Variable and Incomplete   Connectivity in Dense, High Critical Current Density C-alloyed Magnesium   Diboride
  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. 
B. J. Senkowicz, A. A. Polyanskii, R. J. Mungall, E. E. Hellstrom, and   D. C. Larbalestier