Atmospheric conditions and their effect on ball-milled magnesium diboride

Abstract

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.

Synthesis of MgB2 superconductor under carbon induced virtual inert atmosphere

“…The T c observed in sample M3 is smaller than the standard value of T c = 39 K for pristine MgB 2 [1,42,43] samples and is in good agreement with the T c for the sample of the said carbon content as reported earlier [8,13,38,[44][45][46]. This lower value of T c is attributed mainly to increase in electron scattering due to carbon doping in the sample [7,8,[47][48][49]. The residual resistivity ratio (RRR), i.e., R (300 K)/R (40 K) was calculated and found to be 1.7 which is again smaller than that of pristine MgB 2 [47,48,50].…”

“…This lower value of T c is attributed mainly to increase in electron scattering due to carbon doping in the sample [7,8,[47][48][49]. The residual resistivity ratio (RRR), i.e., R (300 K)/R (40 K) was calculated and found to be 1.7 which is again smaller than that of pristine MgB 2 [47,48,50]. In general the observed lower values of RRR are due to presence of defects.…”

Nanoscale grains, high irreversibility field and large critical current density as a function of high-energy ball milling time in C-doped magnesium diboride

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] The vast majority of MgB 2 doping studies are carried out with the dopant present during the initial Mg + B MgB 2 reaction. That method is best suited for in situ conductor fabrication.…”

Superconducting Properties and Microstructure of Nanocrystalline MgB2 Bulk Prepared by Sintering of Ball-Milled Powders

“…However, the oxide impurities, such as MgO and B 2 O 3 , which are formed prior to the reaction between Mg and B, lead to the reduction of MgB 2 phase fraction or formation of non-stoichiometric compounds [21,22]. Nanocrystalline MgB 2 bulk samples with grain size distributed from 40 to 100 nm have been fabricated using ball-milled Mg and B powders and subsequent sintering, but a detailed microstructural characterization was not performed [23][24][25]. This wide range of grain size, as well as the difficulty to obtain the ultrafine grain size of less than 40 nm, can be attributed to the high recovery rate of Mg powders during ball milling [26].…”