Phase equilibria, structure and properties of YBa ceramics

X-ray diffraction, microscopy, chemical and activation analysis, as well as measurement of T c 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. The samples which were tested were single-crystal and polycrystalline specimens and coldrolled strips produced with variations in the conditions of preparation, heat treatment, deformation and temperature. Prepared by the conventional solid state reactions, specimens of YBa ceramics are generally polyphase (orthorhombic superconducting phase 123, phase 2115, sometimes BaCuO 2 and residual amounts of the initial oxides). The non-equilibrium specimens also contain phases of the Y x Ba y O z type. Homogeneous single-phase (as shown by X-ray diffraction) specimens of the 123 type with T c = 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 BaO 2. The manner in which the lattice periods of orthorhombic phase 123 with T c = 92 K vary with temperature displays an anomaly near the superconducting transition temperature that correlates with the thermal behaviour of the Debye temperature. The lattice parameters of a YBa 2Cu 3O 6.6 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 the CuO 5 pyramids, with their vertices facing the Cu1-O1-Cu1 chains. The compression is mainly due to the Cu1-O2 distance becoming shorter. Centrally located in the “ a” edges, the oxygen atoms may “trigger” the interchain interaction. Rolling of the phase 123 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° to ND—normal to the plane of the specimens) and a weak preferred orientation in the “ b”, “ a” or “ a + b” directions along the rolling direction. The orthorhombic structure is distorted and transformed to a tetragonal structure by cold