A novel synthetic method for the preparation of oxide superconductors: Anionic oxidation-reduction

Abstract

A new chemical method for the bulk synthesis of Ba 2YCu 3O 7 powder is described and is applicable to the synthesis of other superconducting oxides. Ionic salt precursors are used which are intimately mixed by spray-drying an aqueous solution of these salts. The anions in the spray-dried mixtures (which in this study are mixtures of nitrate and acetate salts) participate in a low-temperature oxidation-reduction reaction. The exothermic redox reaction and the heat evolved from the reaction completely convert the precursors into their corresponding mixed oxides at temperatures below 300°C in one reaction step. Phase segregation (normally apparent in the decomposition of organic precursors) is not observed in the decomposition of the mixed anion (nitrate/acetate) precursors. The enthalpy of this reaction is strongly dependent upon acetate content in the mixture: −ΔH (9:4, acetate:nitrate) = 4016 (70) kJ/mole, whereas −ΔH (5:8, acetate:nitrate) = 890 (70) kJ/mole. These precursors can be reacted at 910°C for 10 min to produce single-phase Ba 2YCu 3O x . Sintered disks of Ba 2YCu 3O 7, prepared from Ba 2YCu 3O x powder, show onset T c's of 94–95 K and are fully superconducting by 90.5–91 K.