The production of high performance YBa 2 Cu 3 O 7 using nitrogen dioxide
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Thomas R. Askew and Richard B. Flippen Central Research and Development Department, E. I. duPont de Nemours & Co., Inc., Wilmington, Delaware 19880-0304
Steven W. Keller and Angelica M. Stacy Department of Chemistry, University of California, Berkeley, and Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, California 94720 (Received 7 August 1989; accepted 18 September 1989) Energy Dispersive X-ray Spectroscopy (EDS) was used to show that the elemental homogeneity of YBa 2 Cu 3 O 7 powders can be improved substantially by heating the powder in a nitrogen dioxide-containing atmosphere (e.g., 950 °C), followed by annealing in oxygen at 950 °C, and slow-cooling to room temperature. The improved homogeneity results in a substantially larger flux exclusion signal for the NO2-treated powder, as measured by both ac and dc techniques. The experimental results suggest a mechanism which involves the formation of a small amount of molten Ba(NO3)2, which acts as a flux that dissolves the constituents and reprecipitates them as high purity YBa 2 Cu 3 O 7 .
I. INTRODUCTION Since the discovery of superconductivity above 90 K in the Y-Ba-Cu-O system,1 a variety of routes have been reported for the synthesis of YBa 2 Cu 3 O 7 powders.2"30 These routes have ranged from solid-state reaction of the mixed oxides and/or oxide precursors2"12 to various solution and precipitation methods.2"5'13"27 In each case, the material is usually heated above 900 °C to form the tetragonal YBa2Cu3O.t (x = 6.0-6.3) phase, which is a semiconductor. During slow-cooling in oxygen, the material absorbs oxygen and converts to the orthorhombic YBa 2 Cu 3 O 7 phase, which is a superconductor. In many cases, a prolonged oxygen anneal at approximately 500 °C is used to ensure full oxygenation. Marinenko et al.31 used wavelength-dispersive x-ray compositional mapping to study YBa 2 Cu 3 O 7 powders prepared by conventional solid-state reaction techniques. The elemental maps revealed a significant number of large areas within the particles where the stoichiometry did not correspond to an atomic ratio of 1:2:3 for Y : Ba: Cu. Moreover, in many cases, these regions were larger than the original particle size, indicating that these impurity phases may not be the result of incomplete reaction, but rather, form during processing. EDS (Energy Dispersive X-ray Spectroscopy) studies performed in this laboratory have yielded similar results on a variety of YBa 2 Cu 3 O 7 powders prepared a)Address
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J. Mater. Res., Vol. 5, No. 1, Jan 1990
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by solid-state reaction and by solution techniques. In each case gross segregation of the elements is observed within the individual particles on a micron or submicron scale. This elemental inhomogeneity results in a reduced flux exclusion signal and may degrade the performance of devices made from these powders. The elemental inhomogeneity of powders prepared by solution routes stems from the fact that the precipi
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