PHASE-EQUILIBRIA, STRUCTURE AND PROPERTIES OF Y-BA CERAMIC

X-ray diffraction (XRD), microscopy, chemical and activation analysis, together with measurement of Tc and some other properties, have been used to study the changes in phase composition, microstructure, crystal structure, texture and "composition-property" diagrams of the high temperature Y-Ba superconductors (single-crystal and polycrystalline specimens and cold-rolled strips) with variations in the conditions of preparation, heat treatment, deformation and temperature. When prepared by the conventional solid state reactions, specimens of the Y-Ba ceramic are generally polyphase (the orthorhombic superconducting 123 phase, the 2115 phase, sometimes BaCuO2, and residual amounts of the initial oxides). The non-equilibrium specimens also contain YxBayOz-type phases. Homogeneous single-phase (as shown by XRD) specimens of the 123-type phase with Tc = 88-92 K (containing under 2-5 vol.% second phase, predominantly 2115) are obtained by repeated wet grinding, mixing and solid state annealing at 920-930 °C, in air and under oxygen, of the initial mixture of oxides, including BaO2. The manner in which the lattice periods of the orthorhombic 123 phase (Tc = 92 K) vary with temperature displays an anomaly near Tc that correlates with the thermal behaviour of the Debye temperature. The lattice parameters of a YBa2Cu3O6.6 orthorhombic crystal have been studied at 91 and 293 K to ascertain the occupancy of the oxygen positions. Cooling the orthorhombic crystals involves the compression of the triple layers formed by two sheets of CuO5 pyramids, with their vertices facing the Cul-Ol-Cul chains. The compression is mainly due to a reduction in the Cul-O2 distance. Centrally located in the "a" edges, the oxygen atoms may "trigger" the interchain interaction. Rolling of the 123 phase powder and strip with the addition of a plasticizer causes brittle cleavage of the crystals and gives rise to the (001) [110] basal texture. Reducing the amount of binder to 2%-3% intensifies the process of basal texture development and increases the basal density from 4.5 to 13-15 arbitrary units. An axial basal texture is formed with a small scattering angle (within ± 7° normal to the plane of the specimens) and a weak preferred orientation in the "b", "a" or "a + b" direction along the rolling direction. The orthorhombic structure is distorted and transformed to tetragonal by cold work. This is accompanied by the loss of superconductivity which is reduced by the subsequent annealing. © 1990.