Iron-based superconductors (IBS) have great potential for high-power applications due to their prominent high-field properties. IBS long wires made by the powder-in-tube (PIT) method have already been made into coils. However, there is still considerable scope for the increase of critical current density, J_c. One of the central issues is to reveal the roles and limitations of grain boundaries in supercurrent transport in IBS. Here, we finely tuned the electronic properties of grain boundaries by doping Ba_1-xK_xFe₂As₂ superconductors in a wide range (0.25≤x≤0.598). It is found that the intra-grain J_c^intra peaks near x~0.287, while the inter-grain J_c^inter has a maximum at about x~0.458. Remarkably, the grain boundary transparency parameter defined as ε=J_c^inter/J_c^intra rises monotonically with doping. Through detailed microscopic analysis, we suggest that the FeAs segregation phase commonly existing at grain boundaries and the adjacent grains constitute superconductor-normal metal-superconductor (SNS) Josephson junctions which play a key role in transporting supercurrent. A sandwich model based on the proximity effect and the SNS junction is proposed to well interpret our data. It is found that overdoping in superconducting grains largely strengthens the proximity effect and consequently enhances the intergrain supercurrent. Our results will shed new insights and inspirations for improving the application parameters of iron-based superconductors by grain boundary engineering.

Keywords: Iron-based superconducting wires, Critical current density, Grain boundary engineering