Reproducible, Large-Scale Production of Thallium-Based High-Temperature Superconductors
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REPRODUCIBLE, LARGE-SCALE PRODUCTION OF THALLIUM-BASED HIGH-TEMPERATURE SUPERCONDUCTORS RICHARD L. GAY*, D. STELMAN*, J. C. NEWCOMB*, L. F. GRANTHAM*, and G. D. SCHNITTGRUND* *Rockwell International Corp, Rocketdyne Division, 6633 Canoga Avenue, Canoga Park, CA 91303 ABSTRACT The Rocketdyne Division of Rockwell International has developed a large scale spraycalcination technique generic to the preparation of ceramic high-temperature superconductor (HTSC) powders. Among the advantages of the technique is that of producing uniformly mixed metal oxides on a fine scale. Production of both yttrium and thallium-based HTSCs has been demonstrated using this technique. Inthe spray calciner, solutions of the desired composition are atomized as a fine mist into a hot gas. Evaporation and calcination are instantaneous, yielding an extremely fine, uniform oxide powder. The calciner is 76 cm indiameter and can produce metal oxide powder at relatively large rates (approximately 100 g/h) without contamination. Bulk samples of thallium-based powder were prepared by spray calcination, collected, cold-pressed, and sintered. An optimized temperature schedule was developed to produce TI2Ba2Ca2Cu3Ox. Samples were examined by X-ray diffraction and characterized by measurements of critical transition temperature, Tc. A complete transition to zero resistance at 112 K was measured. INTRODUCTION A significant feature that characterizes the new generation of HTSC materials is that they are ceramic. Since the discovery of these HTSC materials two years ago, scientists and engineers at Rockwell have addressed issues related to large-scale production, processing, characterization, and applications. One successful achievement inthis effort is the development and demonstration of a unique spray calcination technique for production at relatively large rates (approximately 100 g/h) of metal oxide powders that are uniform on a fine scale. The spray calcination technique is generic and may be applied to yttrium, bismuth, or thallium-based superconductors. Inthis paper we describe the results of initial development and testing with thallium-based materials. THE SPRAY CALCINATION POWDER PRODUCTION PROCESS Conventional techniques for preparing the new ceramic superconductor precursor powders from solid mixtures of constituent oxides, carbonates, nitrates, etc., may often produce material of inhomogeneous chemical compositions, particularly on a very fine scale. After sintering, such methods yield mixtures of the desired superconductors, together with undesirable nonsuperconducting phases. Ina high-temperature spray calciner, metal nitrate solutions (as opposed to solid mixtures) of the desired composition are atomized as a fine mist into a hot gas stream. Evaporation and calcination are instantaneous, yielding an extremely fine, uniform metal oxide powder of the desired HTSC composition. The use of nitrate solutions for preparation of yttrium-based HTSC powder has been demonstrated by radiant-tube heated aerosol reactors111 and heated-substrate pyrolysis
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