Atmospheric conditions and their effect on ball-milled magnesium   diboride
  Magnesium diboride bulk pellets were fabricated from pre-reacted MgB2 powder ball milled with different amounts of exposure to air. Evidence of increased electron scattering including increased resistivity, depressed Tc, and enhanced Hc2 of the milled and heat treated samples were observed as a result of increased contact with air. These and other data were consistent with alloying with carbon as a result of exposure to air. A less clear trend of decreased connectivity associated with air exposure was also observed. In making the case that exposure to air should be considered a doping process, these results may explain the wide varibability of "undoped" MgB2 properties extant in the literature. 
B. J. Senkowicz, R. Perez Moyet, R. J. Mungall, J. Hedstrom, O. N. C.   Uwakweh, E. E. Hellstrom, and D. C. Larbalestier