THE EFFECT OF COLONIES OF ALIGNED GRAINS ON CRITICAL-CURRENT IN HIGH-TEMPERATURE SUPERCONDUCTORS

X-ray microdiffraction has been used to map the orientational distribu tion of grains in a 4X5 mm(2) polycrystalline film of Tl1-yBa2Cu2Cu3Ox (Tl-1223) grown on yttria-stabilized zirconia (YSZ). The film consist s of ''colonies'', each containing similar to 10(4) grains with their a-axes aligned; the c-axes of all grains are aligned normal to the fil m, but there is no overall a-axis alignment. While the grain boundarie s within a colony are small angle, colonies are separated by large-ang le tilt boundaries. The typical colony size is 0.4 mm. Measured critic al currents are compared with simulations based on observed distributi ons of grain alignment; we find that variations in grain alignment acc ount for the observed variations in the critical current density. Simu lations over longer distances indicate that a colony structure can sub stantially increase the critical current, especially in magnetic field s.