Role of ZrO₂/SnO₂ nano-particles on superconducting properties and microstructure of melt-processed Gd123 superconductors

Bulk GdBa₂Cu₃O_7-δ (Gd123) superconductors were fabricated in air by adding ZrO₂/SnO₂ nanoparticles with an average size of 50 nm. The effect of addition on microstructure and superconducting properties was investigated by scanning electron microscopy (SEM)/transmission electron microscopy (TEM) and SQUID. SEM results show that the distribution of Gd₂BaCuO₅ (Gd211) particles is inhomogeneous around the seeding site of single domain, induced by the addition of nano-particles. Particles with size less than 50 nm were observed by TEM inside Gd123 single domain, while the typical size of fined Gd211 particles is about 100 nm. Critical current density (J_c) was enhanced by the proper amount addition of ZrO₂ /SnO₂ additions. The highest J_c of 100,000 A/cm² was presently achieved in the bulk with 0.4 mol % ZrO₂, under self-field at 77 K. Excess amount addition rather suppresses superconducting properties. The optimum addition amount is 0.2-0.8 mol % for ZrO₂ and 0.4-0.8 mol % for SnO₂, ratio to Gd123, respectively.