Synthesis of High-Purity YBa₂Cu₃O_7-d Superconductors by Polymerized Complex Method

High-purity ceramic samples of the superconducting YBa₂Cu₃O_7-d (123) phase have been prepared by the polymerized complex method starting either from metal carbonates dissolved into ethyleneglycol (EG) with large excess of citric acid (CA) or from metal nitrates dissolved into a mixed solvent of water and EG with an approximately equivalent amount of CA. The method is based on the formation of a polymeric gel which is obtained through polyesterification between metal citrate complexes and EG. It has turned out that the large excess of citric acid facilitates direct gelation without passing through formation of colloid or sol, whereas coagulation of a colloid (sol) has occurred when the amount of citric acid is insufficient. X-ray diffraction, Raman scattering and complex magnetic susceptibility measurements have shown better aspects with regard to purity and superconducting properties in samples prepared via the direct gelation than those prepared via the colloid (sol) coagulation. The onset temperature of superconducting transition for the best 123 sample was 91.5 K and the transition width (10-90%) of full diamagnetism was 0.5 K for a measuring field of 10 mOe.