The grain boundaries in superconductors affect strongly the electronic transport in the normal and most notably superconducting state. E.g., high-angle grain boundaries in high-Tc cuprates act as weak links and suppress the current-carrying capability even in small magnetic fields . To the contrary, in A15 compounds a high density of grain boundaries acting as dominant pinning sites enhances the critical current density, Jc, even in high magnetic fields . For the iron-based superconductors both effects exist : an exponential decay of Jc with increasing misalignment of adjacent grains (most studies performed on films of Co-doped BaFe2As2 or Te-doped FeSe on bicrystal substrates), as well as the surface pinning by planar defects, such as grain boundaries, dominating at low temperature in magnetic fields.
The effect of the grain boundary density on the electronic transport in an iron-based material is discussed here using the example of sputtered c-axis FeSe films on (110) SrTiO3 substrates . Unexpected anisotropy of the in-plane electronic transport occurs for currents applied along the perpendicular  and [1-10] substrate directions. ... mehr