Atmospheric conditions and their effect on ball-milled magnesium diboride

TL;DR

This study investigates how atmospheric exposure during ball milling affects magnesium diboride's superconducting properties, revealing that air contact can induce doping-like effects and alter material performance.

Contribution

It demonstrates that air exposure during ball milling acts as a doping process, influencing MgB2's electrical and superconducting properties, which explains variability in literature reports.

Findings

Increased electron scattering and resistivity with air exposure

Depressed critical temperature (Tc) in air-exposed samples

Enhanced upper critical field (Hc2) due to air-induced doping

Abstract

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.