Preparation of 94 K-superconducting NdBa₂Cu₃O_7-δ without post-anneal in oxygen

The preparation of the polycrystalline NdBa₂Cu₃O_7-δ superconductor using Ba₂Cu₃O_5+x as a precursor has been studied. Ba₂Cu₃O_5+x was prepared by the sol-gel process using metal acetates as starting materials, tartaric acid as an chelating reagent and water as solvent. The gel was heated at 780°C for 60 h in O₂ to form the Ba₂Cu₃O_5+x. precursor. The value of x in Ba₂Cu₃O_5+x was determined to be 0.5; that is, copper was in a highly oxidized state over +2. NdBa₂Cu₃O_7-δ was obtained by the solid-state reaction between Nd₂O₃ and Ba₂Cu₃O_5+x heated at 900°C for 20 h in N₂. Although the sample did not undergo post-anneal in oxygen, it showed good superconducting properties; T_c of 94 K and the intragranular J_c of 3.7 × 10⁵ A/cm² under a field of 50 kOe at 20 K. The advantage of using Ba₂Cu₃O_5+x is considered to be its sufficient amount of oxygen as well as prevention of the Nd³⁺/Ba²⁺ substitution.