Effect of temperature on the formation of thallium-based superconductors

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Abstract
An investigation of reaction pathways for the Tl-Ca-Ba-Cu-O system, starting with different stoichiometries (2:0:2:2, 2:1:2:2, 2:2:2:3, 1:1:2:2, 1:2:2:2:3, 1:3:2:4) and employing various temperatures (775, 825, 865 and 890 degrees C), was conducted. BaCuO2, CuO, CaO and Tl2O3 were used as reactants. The relative amounts of the various compounds present in the samples heated for various time intervals were determined by X-ray diffraction. Melting as well as rapid loss of thallium were observed in the samples heated at 890 degrees C. The reaction rates were very slow at 775 and 825 degrees C and the thallium loss was negligible at 775 degrees C. For the stoichiometries 2:1:2:2, 2:2:2:3, 1:2:2:3 and 1:3:2:4, the 2021 and the 2122 were the only superconducting phases detected in the samples heated at 775 and 825 degrees C for 5 min to several hours. The samples contained, however, numerous other compounds. For the 1:1:2:2 stoichiometry, the 2021, 2122 and 1122 superconducting phases were detected in the samples heated at 775 and 825 degrees C. For the 2:0:2:2 stoichiometry, the 2021 phase was detected at 775 and 825 degrees C. For samples with the 2:1:2:2 and the 1:1:2:2 stoichiometries calcined at 865 degrees C, the sequential transformations 2021 to 2122 to 1122 were observed. For the 2:2:2:3, 1:2:2:3 and 1:3:2:4 stoichiometries, the sequential transformations 2021 to 2122 to 2223 to 1324 occurred. The 1223 phase was not detected in any of the calcined samples, including the samples with the 1:2:2:3 stoichiometry. The 2122 and 2223 compounds were obtained in large amounts (2122 as a single phase) starting from the 2:1:2:2 and 2:2:2:3 stoichiometries, respectively. They are therefore most suitable to achieve high critical current densities.