The effect of colonies of aligned grains on critical current in high-temperature superconductors

X-ray microdiffraction has been used to map the orientational distribution of grains in a 4 × 5 mm² polycrystalline film of Tl_1-yBa₂Cu₂Cu₃O_x (Tl-1223) grown on yttira-stabilized zirconia (YSZ). The film consists of "colonies", each containing ∼ 10⁴ grains with their a-axes aligned; the c-axes of all grains are aligned normal to the film, but there is no overall a-axis alignment. While the grain boundaries within a colony are small angle, colonies are separated by large-angle tilt boundaries. The typical colony size is 0.4 mm. Measured critical currents are compared with simulations based on observed distributions of grain alignment; we find that variations in grain alignment account for the observed variations in the critical current density. Simulations over longer distances indicate that a colony structure can substantially increase the critical current, especially in magnetic fields.