Bednorz and Muller, Z. Phys. B64, 189 (1986), disclose a superconducting phase in the La-Ba-Cu-O system with a superconducting transition temperature of about 35 K. The presence of this phase was subsequently confirmed by a number of investigators [see, for example, Rao and Ganguly, Current Science, 56, 47 (1987), Chu et al., Science, 235, 567 (1987)]. Chu et al., Phys. Rev. Lett., 58, 405 (1987), Cava et al., Phys. Rev. Lett., 58, 408 (1987), Bednorz et al., Europhys. Lett., 3, 379 (1987)]. The superconducting phase has been identified as the composition La.sub.l-x (Ba,Sr,Ca).sub.x O.sub.4-y with the tetragonal K.sub.2 NiF.sub.4 -type structure and with x typically about 0.15 and y indicating oxygen vacancies.
Wu et al., Phys. Rev. Lett., 58, 908 (1987), disclose a superconducting phase in the Y-Ba-Cu-O system with a superconducting transition temperature of about 90 K. The compounds investigated were prepared with nominal compositions (Y.sub.1-x Ba.sub.x).sub.2 CuO.sub.4-y and x=0.4 by a solid-state reaction of appropriate amounts of Y.sub.2 O.sub.3, BaCO.sub.3 and CuO in a manner similar to that described in Chu et al., Phys. Rev. Lett., 58, 405 (1987). This reaction method comprised heating the oxides in a reduced oxygen atmosphere of 2.times.10.sup.-5 bars (2 Pa) at 900.degree. C. for 6 hours. The reacted mixture was pulverized and the heating step was repeated. The thoroughly reacted mixture was then pressed into 3/16 inch (0.5 cm) diameter cylinders for final sintering at 925.degree. C. for 24 hours in the same reduced oxygen atmosphere.
Hundreds of other papers have since disclosed similar solid state reaction processes. Other papers have disclosed various solution and precipitation methods for preparing the reactants to be heated at temperatures of 800.degree.-850.degree. C. and above.
Hirano et al., Chemistry Letters, 665, (1988), disclose a process for producing Y-Ba-Cu-O superconductors by the partial hydrolysis of a solution of Ba metal, Y(O-iPr).sub.3 and Cu-acetylacetonate or Cu-alkoxides in 2-methoxy or 2-ethoxy ethanol. The solution was stirred in dry nitrogen and heated at 60.degree. C. for 12 hours. The solution was then hydrolyzed by the slow addition of water diluted with solvent. Stirring and heating continued for several hours. Stirring continued while the solution was evaporated under vacuum at about 60.degree. C. and an amorphous precursor powder was obtained. The powder was calcined in flowing oxygen at temperatures between 800.degree. and 950.degree. C. for up to 24 hours. The calcined powder was pressed and sintered in flowing oxygen at temperatures up to 920.degree. C. and then annealed at temperatures between 450.degree. and 550.degree. C.
It is highly desirable to form precursors that can be used to produce powders that have small size particles, i.e., generally sub-micron in size, and that can be pressed into desired shapes, sintered and converted to superconducting Y-Ba-Cu-O.