The formation and role of a phase with glassy appearance in the Tl-Ca-Ba-Cu system
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The formation and role of a phase with glassy appearance in the Tl-Ca-Ba-Cu system C.T. Cheung and E. Ruckensteina) Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, New York 14260 (Received 26 June 1989; accepted 15 September 1989) A phase with a glassy appearance has been detected in samples of the Tl-Ca-Ba-Cu system. The presence of such a phase is expected to have a negative effect on the critical current density in a bulk superconductor. Samples of 2-2-2-3 starting composition were prepared from T12O3, BaO2, and relatively large particles of CaO and CuO. These samples were then heat-treated at 867 °C for various periods of time. The use of large particles of CaO and CuO facilitates the examination of the morphological changes associated with the formation of the superconducting phases. Using scanning electron microscopy, a layer of the 2122 phase was found to form on the surface of the large CaO particles whereas near the surface of the large CuO particles, large voids were observed. By varying some of the experimental conditions, a liquid phase with a composition close to 2-0-2-3 was inferred to form. The voids observed in the surroundings of the CuO particles may be due to the formation of such a melt. This liquid phase, after cooling, apparently leads to the "glassy" phase detected in the reacted samples. This glassy phase may play a role as an intermediate in the formation of the 2122 phase, which could result by a direct reaction between the liquid phase and CaO. A part may remain, however, among the superconducting grains, thus having a negative effect on the critical current density.
I. INTRODUCTION
In our previous study1 of the reaction pathways in the Tl2CaB_1Ba2CunO2n+4 and Tl Ca^iBazCu^O^+3 series, stoichiometric amounts of the constituent oxides were used to prepare samples with the 2-2-2-3, 1-2-2-3, 1-3-2-4, and 1-1-2-2 compositions. In those experiments, fine powders of the various oxides have been employed. It has been observed, by using powder x-ray diffraction analysis, that for the formation of the 2223 phase, the reaction pathway at 867 °C follows the sequence: 2021 -* 2122 -* 2223. For the 1223 and 1324 phases, their formation is preceded by the sequential transformations 1021 -> 2021 -> 2122 -» 2223, while for the 1122 phase, by the 1021 -» 2021 -» 2122 transformations. The scanning electron micrographs indicated that the reacted samples contained a phase with a glassy appearance which may not be detectable by x-ray diffraction. Since the presence of this "glassy" phase may affect adversely the critical current density, it is important to learn more about its formation. In the previous study, fine powders of CaO and CuO were mixed with T12O3 and BaO2 in stoichiometric ratios to prepare the samples. The use of the fine powder minimizes the reaction time, but, nevertheless, makes it difficult to follow the morphological changes associated with the formation reaction as well as the a)Address
correspondence to this author. J. Mater. Res., Vol. 5, No. 2, F
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