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PROCESSING AND FABRICATION OF YBa 2 Cu 4 0 8 AND YBa 2 Cu 4 Os/YBa 2 Cu 3 Ox COMPOSITES U. Balachandran, M. E. Biznek, K. C. Goretta, B. W. Veal, and R. B. Poeppel Argonne National Laboratory, Argonne, IL 60439 and T. 0. Mason Northwestern University, Evanston, IL 60201

ABSTRACT Powders of YBa 2 Cu 4 0 8 ("124") were prepared via solid state reaction of Y 2 0 3 , BaCO 3 , and CuO. The mixed precursors were heated in flowing oxygen of reduced total pressure, followed by cooling and annealing at 750°C under ambient pressure oxygen. The procedure produced orthorhombic 124 as the main phase, with YBa2Cu3Ox ("123") as a minor impurity phase. Phase purity improved, and nearly phase-pure 124 was obtained, upon annealing the as-calcined powder in flowing oxygen at 800°C. INTRODUCTION The superconducting ceramic YBa2Cu3Ox ("123") has been studied for nearly three years, and many advances have been made in this oxide. Recently, YBa 2 Cu 4 0 8 ("124") -- with a transition temperature -80 K -- was observed, first as a lattice defect in partly decomposed 123 powders [1], and then as an ordered defect structure in 123 thin films [2-4]. The crystal structure of 124 differs from 123 in that the single Cu-O chain has been replaced by a double Cu-O ribbon [5]. The 124 superconductor has a more thermally stable oxygen content up to -850°C [6-8]. The 124 has been synthesized in bulk by two methods: (1) high oxygen pressures (3-40 MPa) in the temperature range of 900-1000°C [6, 7, 9], and (2) a two-step process in ambient-pressure oxygen using an equal volume of alkali carbonates as catalyst. The materials made by these methods are in general not single-phase. We report here a novel synthesis route to obtain nearly phase-pure orthorhombic 124 powders without the use of alkali carbonates. EXPERIMENTAL The 124 compound was prepared via solid-state reaction of a stoichiometric mixture of Y 2 0 3 , BaCO3 , and CuO. The mixed powder was calcined in flowing oxygen with a total pressure of 2.7 x 102 Pa according to the following heating schedule: 120°C/h to 625°C, 20°C/h to 750°C, 4 h hold at 750°C, 20°C/h to 800°C, and 1 h hold at 800°C. Diffusional kinetics are enhanced in reduced oxygen pressures. A partial vacuum was used, instead of a mixture of oxygen and a noble gas, because CO 2 was removed with increased efficiency [10]. During cooling, the vacuum was discontinued and ambient-pressure oxygen was introduced. A 20-h hold at 750°C was incorporated into the cooling schedule. Earlier experimental results [11] indicated that the 750°C hold is essential to

Mat. Res. Soc. Symp. Proc. Vol. 169. @1990 Materials Research Society

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synthesize the 124 phase. The as-calcined 124 powder was annealed in flowing oxygen at 800°C for 24 h under ambient pressure to improve its phase purity. X-ray studies were performed on the as-calcined and annealed powders, and on powders subsequently heat treated at the decomposition temperature. Thermal analyses were performed at a heating rate of 3°C/min in flowing oxygen at ambient pressure. Magnetization m