Evolution of intergranular microstructure and critical current properties of polycrystalline Co-doped BaFe₂As₂ through high-energy milling

Abstract

The effect of microstructure control on intergranular critical current density (J c ) was evaluated using Co-doped Ba122 as a model material. Polycrystalline bulk samples were prepared by sintering mechanically alloyed powder prepared by planetary ball milling. Between low and intermediate milling energy, refinement of grain size, improvement in the uniformity of structure and composition, and a threefold improvement in J c were observed with increasing milling energy. These indicate that the high-energy milling is an effective method for the synthesis of single-phase Ba122 samples and improvement of J c by controlling the microstructure. At very high milling energy, the grain size of Ba122 reached as small as 30 nm, abnormal aggregates were formed, and the magnetic T c and J c decreased. Our results suggest that the suppression of the formation of large aggregates and control of grain boundary properties are necessary to achieve superior critical current properties.

Phase Formation of Iron-Based Superconductors during Mechanical Alloying

“…Further grinding did not show any significant positive effect on the formation of the Ba-122 phase. Similar results were observed for BaFe 2−x Ni x As 2 superconducting compound [ 48 , 49 ], where the authors claimed that in long term MA the large Ba-122 aggregate appears before the heat treatment, which leads to formation of gaps between the grains, poor grain connectivity of the aggregates and grains [ 48 ]. These factors cause deterioration of the overall powder quality and its SC properties.…”

Enhanced critical current density in K-doped Ba122 polycrystalline bulk superconductors via fast densification

“…Previously, we improved the density and purity of Co-doped Ba122 polycrystalline bulks fabricated under ambient pressure using a mechanically alloyed precursor powder prepared via high-energy ball milling. At 5 K under self-field conditions, the J c value of these bulks improved to a value approximately three times that reported in a previous study ( Tokuta et al., 2020 ). Furthermore, the H c2 value of Co-doped Ba122 polycrystalline bulks can be artificially enhanced and adjusted by introducing lattice defects into the grains via high-energy ball milling ( Tokuta and Yamamoto, 2019 ).…”

“…Elemental metals, namely, Ba (chunk, 99.9%), K (chunk, 98.0%), Fe (100 mesh, 99.9+%), and As (granule, 99.9999%), were weighed at a molar ratio of Ba:K:Fe:As = 0.6:0.4:2:2 in an Ar glove box. These metals were ground and mixed in a planetary ball-mill apparatus (Premium line P-7, Fritsch) to produce a mechanically alloyed K-doped Ba122 powder with a ball-mill energy of 200 MJ/kg ( Häßler et al., 2013 ; Tokuta et al., 2020 ; Tokuta and Yamamoto, 2019 ). The powder was filled into a graphite SPS mold with an inner diameter of 10 mm.…”

“…In this study, K-doped Ba122 polycrystalline bulks were synthesized via spark plasma sintering (SPS), a pressure-assisted sintering method expected to achieve fast densification, using a mechanically alloyed precursor powder prepared via high-energy ball milling ( Maltsev et al., 2019 ; Pervakov and Vlasenko, 2020 ; Tokuta et al., 2020 ; Tokuta and Yamamoto, 2019 ; Weiss et al., 2013 ). The density, microstructure, phase composition, and superconducting properties of the fabricated bulk samples were evaluated as a function of the sintering temperature.…”

3D microstructure and critical current properties of ultra-fine-grain Ba(Fe,Co)2As2 bulk superconductors

“…The ball-milling energy, E BM , which is a function of the milling duration and the number of rotations [24], was systematically varied as 50, 80, 230, and 590 MJ/kg. Compacted, alloyed powder samples were sintered at 600 °C for 48 h under vacuum [34]. The lattice parameter of the obtained bulk was measured by X-ray diffraction (D2 PHASER, Bruker, MA, USA) and the critical current density J c was obtained from magnetization measurements using a superconducting quantum interference device (SQUID) and vibrating sample magnetometer (VSM) (MPMS3, Quantum Design Inc., CA, USA).…”