Oxygen Content Variation in Cation-Deficient and Chemically-Doped High-T C Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O Superconductors
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OXYGEN CONTENT VARIATION IN CATION-DEFICIENT AND CHEMICALLY-DOPED HIGH-Tc Y-Ba-Cu-O AND Bi-Sr-Ca-Cu-O SUPERCONDUCTORS Y. H. KAO*, L. W. SONG*, Y. D. YAO**, Z. TAO***, AND K. W. JONES*** * State University of New York at Buffalo, Department of Physics and New York State Institute on Superconductivity, Buffalo, New York 14260 ** Institute of Physics, Academia Sinica, Taipei, Taiwan ***Brookhaven National Laboratory, Upton, New York 11973 ABSTRACT Oxygen content in high-Tc superconductors has been determined by using a charged particle activation method. Changes of oxygen content in Cu-deficient Y-Ba-Cu-O and Cu/Bi-deficient Bi-Sr-Ca-Cu-O compounds were obtained and used as a basis for discussing the variations in other cation-deficient materials. It is well established that oxygen atoms play an extremely important role in all the high-Tc superconductors known to date. Oxygen is the only common element in all these materials, and its position and density are closely related to various superconducting properties. In this note, we report results on the variation of oxygen content in Y-Ba-Cu-O (YBCO) and Bi-Sr-Ca-Cu-O (BSCCO) systems obtained by using a nuclear charged particle activation (CPA) method. Details of this experimental technique were reported elsewhere [1]. This technique can be employed to investigate the variation of oxygen content in various cation-deficient superconducting materials. The oxide superconductors used in this experiment were all prepared by solid state reaction. Starting compounds were chosen from technical-grade high purity powders of Y2 0 3 , CuO, Bi2 0 3 , CaCO 3 , SrCO 3, and BaCO 3 , with required proportion for each nominal composition under study. These powders were thoroughly mixed and placed in alumina crucibles for heat treatment. Heating is usually between 10 and 24 hours at 820 to 990 OC in flowing oxygen. The furnace temperature and annealing time were varied for different samples. Pulverizing, mixing, and calcining were repeated several times until high homogeneity of the resulting material was ensured. The mixture after heat treatment was then pressed into cylindrical pellets for final sintering in flowing oxygen for more than 10 hours at temperatures around 980 °C. Repeated measurements of the oxygen content for samples prepared under the same conditions indicated that the amount of oxygen in each sample under investigation was saturated and remained constant. Hence, the value of oxygen content in the specimens with various cation deficiency represents the maximum amount of oxygen that was accomodated in the bulk material during the reaction process. X-ray studies indicated that for the compounds with small cation deficiency, the original phase of YBa 2Cu 3Oy (123) or Bi2 Sr 2CaCu 2 Oy (2212) is the major phase in the material. In YBCO (123), it has been known for sometime that the superconductive transition temperature Tc depends sensitively on the oxygen content y. By using the CPA method, it has been shown that y decreases monotonically when Cu deficiency was increased in t
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