Abstract. Magnesium diboride bulk pellets were fabricated from pre-reacted MgB 2 powder ball milled with different amounts of exposure to air. Evidence of increased electron scattering including increased resistivity, depressed T c , and enhanced H c2 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 variability of "undoped" MgB 2 properties extant in the literature.
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