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

Abstract

Magnesium diboride (MgB 2 ) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B 0.95 C 0.05 ) 2 and then sintered at 1000 o C in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes. Full C incorporation required only 30-60 min of milling. Grain size of sintered samples decreased with increased milling time to <30 nm for 20-50 hrs of milling. Milling had a weak detrimental effect on connectivity. Strong irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to increased milling time was observed and correlated linearly with inverse grain size (1/d). As a result, high field J c benefited greatly from lengthy powder milling. J c (8 T, 4.2 K) peaked at > 80,000 A/cm 2 with 1200 min of milling compared with only ~ 26,000 A/cm 2 for 60 min of milling. This non-compositional performance increase is attributed to grain refinement of the unsintered powder by milling, and to the probable suppression of grain growth by milling-induced MgO nano-dispersions.

Improved Transport J c in MgB2 Tapes by Graphene Doping

“…This is consistent with the conclusion that residual C phases exist at the grain boundaries will have an obstruction effect to the current flowing [27].…”

Superconducting properties of MgB2 prepared from attrition ball-milled boron powder

“…FWHM of the peaks is a quite interesting factor used to investigate grain size and lattice strain [15,16,26]. However, the (1 0 1) peak of MgB 2 (2Â ≈ 42.21 • ) is not really suitable to reveal grain size, even with strong intensity, because this almost overlaps the (2 0 0) MgO peak (2Â ≈ 42.92 • ).…”

“…Specifically, most efforts have been made to introduce dopants into the MgB 2 structure to elucidate how to change the crystal structure, as well as how to improve the superconducting properties. Recently, enhanced J c results were obtained by means of ball milling [14][15][16][17]. High-energy ball milling can introduce small grain sizes and better reactivity [18].…”

Hollow carbon spheres as an efficient dopant for enhancing the critical current density of MgB2-based tapes

“…The reduction in grain size is probably explained by the inhibition of grain growth by finely dispersed second phases introduced by the doping 5 and milling process [25]. In order to further clarify effects of grain refinement, the SEM images for pure and doped samples sintered at 800 °C were also shown in Fig.5 (e) and (f).…”