Dependence of the contents and the states of residual carbon in YBa2Cu3O7−x powders synthesized by the sol-gel method on heating conditions, and effects of residual carbon on the superconducting properties were investigated. The carbon content of the powder heated at 950 °C for 20 h was reduced to 0.02%. It has been found that a large number of carbonaceous materials in the starting materials that are used in the sol-gel method does not affect the carbon content of synthesized superconducting powders. The volume of diamagnetism was increased and the Tc,onset was raised as the carbon contents were reduced to 0.04%. These properties were further improved by O2–HIP treatment at 500 °C.

Using Bi-based oxide superconductors of the Bi0.96Pb0.24-xSbxSr1.0Ca 1.1 Cu1.6Oy composition which were prepared by the sol-gel method, production of the high-Tc phase and also microstructures of the high-Tc and the low-Tc phases were investigated using the XRD method and the TEM technique. Two superconducting phases showed good linearities of shrinkages of the a, b, and c lattice parameters as the Sb content increased, and so it has been found that Sb can be included in superconducting crystal structures. However, all shrinkages were below 1%. The activation energy of Sb diffusion was estimated from time dependence of changes of the lattice parameters using the Arrhenius plots. Its energy was from 510 to 593 Kj/mol.

YBa2Cu3O7−x superconducting films have been prepared on a YSZ ceramic substrate through the sol-gel method using metal alkoxides of Y, Ba, and Cu. A homogeneous solution containing the three metal alkoxides was prepared by dissolving the copper alkoxide, which is otherwise insoluble, in diaminoalcohol. The gel film, formed by dipping the substrate into the sol of 80 °C and withdrawing it up, was converted to a ceramic film consisting of YBa2Cu3O7−x phase by sintering at 850 °C for 30 min. The resulting film of about 10 μm in thickness showed zero resistivity (Tc end) at 54 K. Cold isostatic pressing of the film before sintering remarkably improved the Tc end to 85 K.

We investigate the crystal growth of YBa2Cu4O8 and Y2Ba4Cu7C7O15−σ in Al2O3 crucibles at P(O2) = 20 MPa employing commercial O2-HIP equipment. Large crystals of YBa2Cu4O8 up to 1 × 0.5 × 0.05 mm3 are obtained from starting compositions of about 65–67 mol % CuO. When we use a composition of Y: Ba: Cu = 2:6:15, single crystals of YBa2Cu4O8, free from contamination, are obtained at a growth temperature of 1000°C, and those of the Y2Ba4Cu7O15 phase with Al, having the maximum size of 3 × 1.5 × 0.05 mm3, are obtained at a temperature of 1050°C. YBa2Cu4O8 crystals exhibit a superconducting transition temperature Tc of 75 K. Y2Ba4Cu7O15−σ crystals have a Tc of 20 K because of the contamination of aluminum from the crucible.