Tuning of grain boundaries in MgB2 by boron ultra-sonication in 2-propanol – A way to low-cost high-Jc bulk superconducting magnets

Abstract

Viscosity of solvent has been found to influence particle refinement during high-energy ultra-sonic treatment. 2-propanol, a solvent with viscosity ∼ 2.07 cP, was used to ultra-sonically refine boron powder. Unlike ball-milling, boron powder refined this way seem to be oxide free in nature as observed via X-ray diffraction (XRD). Field-emission scanning electron microscope (FE-SEM) showed a boron powder refinement up to nanoscale as well as morphological changes to the particles. FE-SEM of the bulk MgB2 fabricated from the ultra-sonically refined boron exhibited a principal MgB2 phase with scarce impurities of MgO. Lattice parameters ‘a’ and ‘c’ calculated from XRD patterns remained constant around 3.086 and 3.523 Å respectively, ruling out any carbon intrusion from solvent into bulks. Simultaneously all the bulks had Tc,onset around 38.5 K, with a sharp transition (ΔTc ∼ 0.5 K). Ultra-sonic treatment of boron for 45 min led to high self-field critical current density (Jc) of 500 and 380 kAcm−2 at 10 and 20 K, respectively. Microstructural analysis revealed nanometer sized MgB2 grains in the matrix, which improved the grain boundary pinning also supported by Dew-Hughes approximations leading to almost 80% increase in Jc compared to MgB2 bulk prepared from pristine boron. This Jc values are on par with MgB2 prepared of the expensive amorphous nano-boron and the trapped field (TF) simulations hinted at potential TF bulk magnets operatable at 2–3 T.