Ag/YBa 2 Cu 3 O 7-x Powder Generation by Aerosol Decomposition
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Ag/YBa2Cu3O7-x POWDER GENERATION BY AEROSOL DECOMPOSITION TOIVO T. KODAS*, ALTAF H. CARIM*, AND KEVIN C. OTT** *Center for Micro-Engineered Ceramics and Department of Chemical and Nuclear Engineering, University of New Mexico, Albuquerque, NM 87131 ** Exploratory Research and Development Center, Los Alamos National Laboratory, Los Alamos, NM 87545
ABSTRACT YBa2Cu3O7ox (123) powders containing silver have been prepared by aerosol decomposition. Metal nitrate solution droplets were decomposed at temperatures above and below the melting point of the Ag-O eutectic. In both cases, the Ag was present as a separate grain attached to YBa2Cu3O7Tx. Individual aerosol particles had dimensions of 50 - 1000 nm. Grain sizes of Ag and 123 crystallites within these particles were 10 to 100 nm. Larger 123 grain sizes could be obtained by varying the reactor operating conditions. The powders provide a source of material for generation of YBa2Cu307.x/Ag ceramics with smaller Ag and 123 grain sizes and more uniform composition than can be obtained by other methods.
INTRODUCTION Composite YBa2Cu307.x/Ag ceramics are of great interest for a number of reasons. YBa2Cu3O7-x/Ag composites may provide higher ductility [1,2] and lower normal-state resistance [3,4] than can be obtained in pure 123. In order to fabricate YBa2Cu3O7.x/Ag composites it is desirable to begin with powders that have a distribution of Ag through the 123 that is as uniform as possible. In most studies, the product ceramic has been produced by mixing together 123 powders and Ag20 powders with particle sizes larger than one micron, followed by annealing at temperatures on the order of 900'C. This method cannot provide the fine grained material that is needed in many cases. This paper discusses the production of Ag/123 powders with nanometer-size grains by aerosol decomposition. The properties of the powders and the influence of operating conditions on the properties is considered below. The small grain size and the uniformity of the 123/Ag mixture may allow generation of bulk ceramics with considerable improvements in mechanical and electrical properties.
EXPERIMENTAL PROCEDURE The experimental apparatus has been described previously [5-8]. Y, Ba, Cu, and Ag nitrates in water were passed through an aerosol generator to form micron-sized or smaller droplets. The droplets were flowed in oxygen through a hot-wall reactor where they reacted to produce the Ag/123 composite particles. Reactor residence times in the heated region were varied from less than 10 to 60 sec while temperatures were varied from 900 to 10000 C. The particles were then collected on silver membrane filters. Ag/123 powders with 0, 16, and 48 weight % Ag were generated. The phase purity and microstructure of the particles were examined using x-ray diffractometry (XRD) and high-resolution and analytical transmission electron microscopy (TEM). XRD was carried out on a Scintag diffractometer. TEM analysis was performed by dispersing the powder on a carbon film supported by an aluminum grid. The use of alum
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