Evolution of intergranular microstructure and critical current properties of polycrystalline Co-doped BaFe₂As₂through high-energy milling

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.