An electron microscopy study of superconducting YbBa2Cu3Ox produced by oxidizing metallic precursors

Abstract

Abstract The microstructure of a superconducting YbBa2Cu3Ox/Ag microcomposite grown from a silver containing metallic ribbon precursor has been investigated mainly by transmission electron microscopy. The metallic precursor was oxidized at 900°C for 3 hours in air resulting in 90% of the oxide phase being the 1-2-3 oxide. The grain size was 2–5 μm and nearly equiaxed. A new phase Yb2Ba4Cu8O16−x (orthorhombic, a=3.81 A , b=3.87 A , c=27.1 A ), has been observed and has been analyzed as occurring through the periodic insertion of CuO layers into the normal 1-2-3 structure. This structure is common at (001)-faced grain boundaries. Except for these “2-4-8” areas, very few planar defects exist in samples oxidized at 900°C. However, this is not the case for metallic precursors oxidized at 800°C where many CuO layers are inserted randomly within 1-2-3 grains. These additional CuO layers are not regarded as the product of 1-2-3 phase decomposition or artifacts, but are believed to be grown during the oxidation process in order to absorb excess copper. Possible mechanisms by which conventional oxide processing situations could lead to the new 2-4-8 phase are described.