Bulk GdBa2Cu3O7-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 (Jc) was enhanced by the proper amount addition of ZrO2 /SnO2 additions. The highest Jc 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.
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