Synthesis of a superconducting oxide by oxidation of a metallic precursor
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I.
INTRODUCTION
T H E R E has been a burst of activity in the synthesis and characterization of superconducting oxides with high critical temperatures since reports of their existence during the latter half of 1986.1'2 Tremendous efforts have been expended in trying to understand the mechanisms of superconductivity in these oxides, in the search for new materials with higher critical temperatures, and in the processing of these materials into useful forms. One method for the synthesis of superconducting ceramics is high temperature oxidation of a metallic alloy (a metallic precursor) that contains the metallic constituents of the ceramic; e . g . , an alloy of La, Ba, and Cu can be the metallic precursor for the oxide La~ 85Ba0 ~5CuO4-~. It is quite simple to form oxides with the K2NiF4 crystal structure by oxidation of metallic precursors; 3'4 however, it is somewhat more difficult to ensure that the oxidation product has the composition and microstructure necessary to yield good superconducting properties. 4 Matsuzaki et al. 5 produced an amorphous alloy ribbon of La, St, and Cu, and oxidized the ribbon at 900 ~ for 5 to 60 minutes in air. The oxidation product formed after 20 minutes had the K2NiFa-type crystal structure. It exhibited an onset of superconductivity at about 36 K, and a broad transition to zero resistance at 9 K. Conventional ceramic processing techniques readily yield La~ ssSr0 lsCuO4-~ with a sharp transition to zero resistance at 36 K, and thus, it is unlikely that Matsuzaki et al. produced a uniform oxide that had the correct composition. Gruen e t al. 6 oxidized a 62 at. pct La-5 at. pct Sr33 at. pct Cu alloy, and an alloy that contained an excess of Cu (5 at. pct La-0.5 at. pct Sr-94.5 at. pct Cu) in air at 800 ~ for various times. The oxides formed in both cases were multiphase and contained La203 (in the La-rich alloy), and CuO (in the Cu-rich alloy) in addition to an orthorhombic perovskite phase. No attempts were made to measure the superconducting properties of the oxidation prodG. J. YUREK, Associate Professor, J. B. VANDER SANDE, Professor, D. A. RUDMAN, Assistant Professor, and M. M. MATTHIESEN, Graduate Student, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. W.-X. WANG, on leave from Shanghai Jlao Tong Umversity, Shanghai, People's Republic of China, is with M.I.T. as Visiting Scientist. Y. ZHANG, on leave from the Institute of Aeronautical Materials, Beijlng, People's Republic of China, is with M.I.T. as Visiting Scholar. Manuscript submitted July 16, 1987. METALLURGICAL TRANSACTIONS A
ucts; however, because the oxidation products were inhomogeneous, and because the perovskite phases that formed had orthorhombic crystal structures, at room temperature, it is unlikely that either oxidation product would have exhibited superconductivity; i . e . , lanthanum-based cuprates that exhibit superconductivity typically have a tetragonal crystal structure at room temperature. We report here the synthesis of a high-
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