APPLICATION OF ENERGY-DISPERSIVE X-RAY ELEMENTAL MAPPING TO PROBE THE HOMOGENEITY OF SOL-GEL DERIVED YBA2CU3O7-DELTA AND RELATED PHASES

Energy-dispersive X-ray (EDX) elemental mapping has been used to assess the homogeneity of solid state products of a sol-gel synthetic route developed for the synthesis of YBa2Cu3O7-delta. Transmission electron microscopy (TEM), X-ray diffraction (XRD), FTIR, iodometric titration, elemental analysis, and magnetic susceptibility have been used to probe the homogeneity and phase composition at various stages of the sol-gel process. The starting solutions contain stoichiometric amounts of the metal 2-(2-methoxyethoxy)ethoxide components in 2-(2-methoxyethoxy)ethanol and appear to be homogeneous by TEM with particle sizes less than 2 nm. While XRD suggests that products of sol-gel synthesis and conventional solid-state synthesis fired to 950 degrees C are single-phase orthorhombic YBa2Cu3O7-delta, magnetic susceptibility measurements indicate that both samples have two superconducting phases, and the EDX elemental maps of each show considerable micrometer-scale inhomogeneities. Sol-gel derived products processed at 700 degrees C for 48 h appear to be tetragonal by XRD; additional data suggest that the dominant phase is the tetragonal oxycarbonate phase (YBa2Cu3(CO3)(0.2)O-6.7). Elemental maps show a more homogeneous distribution of the component metals in this material, but some zoning is seen in the irregularly shaped particles, suggesting a nonequilibrium product. When the gel is fired at 700 degrees C for 2 weeks, an orthorhombic YBa2Cu3O7-delta XRD pattern is observed, and magnetic susceptibility shows a single superconducting phase, with an 88 K transition temperature. Elemental maps show the product to be highly homogeneous in elemental distribution within particles. These results indicate that this alkoxide sol-gel route can yield high-quality YBa2Cu3O7-delta at 700 degrees C but that kinetic effects are important in this low-temperature synthetic route.