Transmission electron microscopy of air-processed Gd123 superconductors with the addition of Gd₂Ba₄MCuO_x (M ≤ Mo, Zr)

We have studied the microstructure of melt-growth GdBa₂Cu ₃O_7-δ (Gd123) bulk superconductors with the addition of Gd₂Ba₄CuMO_x (abbreviated as M2411 with M ≤ Mo or Zr) particles by transmission electron microscopy. Gd123 bulks are prepared with nominal compositions Gd123+(0.4-x)Gd211+xM2411+0.1BaO₂ in a molar ratio with x ≤ 0-0.2. The addition of M2411 particles induced nanosized particles inside bulks. They are apparently distributed well under seeding positions, where fewer Gd211 particles are located. The observed nanosized particles assigned as M2411 may play the role of flux pinning centres, leading to an enhanced J_c. However, the addition of M2411 with x>0.12 leads to the appearance of excess liquid and macrodefects in the bulks. Enhanced interfacial stress induced by the excess M2411 addition was found, together with modified local chemical compositions. These results account for both the suppression of the superconducting properties and the formation of the band structure in the present system.