SYNTHESIS OF NANOPHASE OXALATE PRECURSORS OF YBACUO SUPERCONDUCTOR BY COPRECIPITATION IN MICROEMULSIONS

A microemulsion has been studied as a reaction medium to control the coprecipitation of oxalate precursors of the superconducting YBa2Cu3O7-delta ceramics. The oxalate coprecipitation of Y, Ba and Cu from nitrate solution was studied on basis of computer modelling of equilibrium systems. The optimum operating conditions, especially the pH range and concentrations of Y/Ba/Cu/C2O42-, have been estimated. Oxalate coprecipitation in stoichiometry of Y:Ba:Cu=1:2:3 is obtained in a pH range 3-4. Phase diagrams of several multicomponent systems: hydrocarbon-aerosol OT(surfactant)-water, in the absence and presence of nitric/oxalic acids and nitrates, have been systematically investigated. Among three hydrocarbons tested, heptane has been found most suitable for the coprecipitation of Y, Ba and Cu oxalate. Different stability regions of microemulsions have been determined. The chemical and morphological properties of the oxalate powders obtained from microemulsions have been examined by ICP, TG, XRD, BET surface area and SEM. These precursors are nanophase powders with high homogeneity, low crystallinity and exact stoichiometry. The average size of the dispersed particles is 50-70 nm, while the mean diameter of their agglomerates is around 300 nm. The best sintered bulk sample has T-c(R=0) at 92 K.