Influences of material processing on the microstructure and inter-granular current properties of polycrystalline bulk Ba(Fe,Co)2As2

• Ba(Fe,Co)2As2 polycrystalline bulks were prepared by systematically varied condition.
• The use of refined starting powder yield microstructure with improved uniformity.
• 122 phase can be synthesized at lower temperature owing to an increased reactivity.
• The samples synthesized at low temperature showed improved intergranular Jc.

The phase formation, microstructure, magnetic, and transport properties of Ba(Fe,Co)2As2 polycrystalline bulks prepared by systematically varied synthesis conditions were studied to clarify the key issues for intergranular critical current properties. After optimization of heat treatment process, Ba(Fe0.92Co0.08)2As2 samples with high phase purity and Tc > 25 K were reproducibly obtained. Electron microscopy analyses showed that the use of refined starting powder by high-energy ball-milling yield microstructure with improved uniformity and Ba(Fe,Co)2As2 phase can be synthesized at lower temperature down to 500 °C owing to an increased reactivity. The samples synthesized at low temperature showed well-connected microstructure with fine grain size and intergranular critical current density progressively improved with lowering the heating temperature. Our results suggest that fine grain size and grain boundary structure formed at low temperature are favorable for increasing the area of effective transport current path, while they are electromagnetically weakly coupled and suppressed under external magnetic field.