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

TL;DR

This study demonstrates that high energy ball milling of C-doped MgB2 significantly refines grain size, enhances irreversibility field, and greatly improves critical current density at high magnetic fields, with optimal milling times around 1200 minutes.

Contribution

It reveals that prolonged milling induces grain refinement and MgO nano-dispersion, leading to superior high-field superconducting performance in C-doped MgB2.

Findings

Irreversibility field increases linearly with inverse grain size.

Critical current density at 8 T and 4.2 K peaks at over 80,000 A/cm2 after 1200 min milling.

Grain size reduces to less than 30 nm with increased milling time.

Abstract

Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 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 less than 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 Jc benefited greatly from lengthy powder milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling compared with only ~ 26,000 A/cm2 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.