High Temperature Neutron Diffraction Study of the Formation Reaction of Bismuth Superconductors
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HIGH TEMPERATURE NEUTRON DIFFRACTION STUDY OF THE FORMATION REACTION OF BISMUTH SUPERCONDUCTORS Mary F. Garbauskas and Ronald H. Arendt General Electric Corporate Research and Development, Schenectady, NY 12301 James D. Jorgensen and Richard L. Hitterman Materials Science Division, Argonne National Laboratory, Argonne, IL60439 ABSTRACT High temperature neutron powder diffraction has been used to study the formation of the 110K transition temperature material, Bi2Ca 2Sr 2Cu3Ox (2223), both from synthetic reaction mixtures and after partial melting. The results indicate that the 80K transition temperature material, Bi2CaSr 2Cu 2Ox (2122), is the precursor to the (2223) material. The reaction to form the (2122) occurs rapidly at 860 0C, while the conversion of this material to the (2223) is much slower. INTRODUCTION Since the discovery by Maeda, et. al. 1 of high-temperature superconductivity inthe Bi-Ca-Sr-Cu-O system, the richness of both the chemistry and crystallography of this system has been explored by a multitude of workers. There are at least three superconducting compounds inthis system: the single Cu-O layer Bi2Sr 2CuOx (2021), which has a transition temperature of approximately 10K, the double Cu-O layer Bi2CaSr2Cu 2Ox (2122), with a transition temperature of 80K, and the three layer Bi2Ca2Sr 2Cu3Ox (2223), possessing a transition temperature of 110K. A large amount of the work has centered on the (2223) material because of its higher transition temperature. Several common features of the (2223) formation chemistry which have been discovered both in this laboratory and elsewhere are (1) the need for Pb to promote the formation of (2223), (2) the need for excess Ca and Cu for the formation of the (2223) phase, (3) the need for long reaction times (many days) for complete reaction, and (4) the existence of intergrowths, as visible with transmission electron microscopy, with up to 7 Cu-O layers. In an effort to understand the chemistry behind these observations, there was an interest in studying the reaction in-situ. Quenched samples were considered, but since ceramic materials are relatively poor thermal conductors, there was no guarantee that the quenched material would be representative of the sample at temperature. Efforts to monitor this reaction using high-temperature x-ray diffraction were thwarted by the shallow penetration depth of Cu K-a x-rays in this system, and the presence of thin surface layers of Bi-depleted species. However, neutrons easily penetrate these samples. Thus, neutron powder diffraction was chosen as the technique for the in-situ study of this reaction. In addition to understanding the chemistry of this complex system, we were also interested in investigating the reactions which occur when the (2223) material melts and re-forms. Based upon experiences with the Ba2YCu 307-8 system, the formation of bulk material with high critical current densities will probably involve some sort of partial melting during processing. Neutron powder diffraction is well-suited for this investigation a
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