Synthesis, processing behavior, and characterization of bismuth superconductors using freeze dried nitrate precursors
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J. Brynestad Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6100 (Received 20 December 1993; accepted 17 June 1994)
The synthesis of Bi2-xPbA;Sr2Ca2Cu301o (Bi2223) powders from a freeze dried nitrate precursor is reported here. We examine the composition and morphology of the precursor material, describe the chemistry and kinetics of product formation, and evaluate the phase composition and superconducting properties of the products. A nitrate solution containing the appropriate ratio of cations was rapidly frozen and then freeze dried at low temperatures to form an atomic mixture of the component salts. The thermal processing of the freeze dried material consisted of three steps: (i) dehydration, (ii) denitration, and (iii) solid state reaction to form the Bi2223 superconducting product. Calcium-substituted bismuthates and strontium-substituted calcium cuprate, not Bi2201, are the intermediates between the nitrates and the superconducting products. These highly disordered phases rapidly transform into Bi2212 or Bi2223 at higher temperatures (>790 °C). The kinetics of product formation was studied using XRD analysis and magnetic susceptibility. The kinetics were shown to follow the nucleation and growth mechanism. Bi2223 formed after only 30 min at a few degrees below the melting point, and after 37 h Bi2223 products exhibited excellent phase composition and magnetic susceptibility characteristics.
I. INTRODUCTION Presented here is the synthesis of Bi2-xPbxSr2Ca2 Cu3Oio (Bi2223) powder from freeze dried nitrate precursors. Freeze-drying technology has been applied to the fabrication of ceramic materials,1"3 and has been applied to the synthesis of YBa 2 Cu 3 07_ x , YBa 2 Cu 4 0 8 , and related materials.4'5 Recently this approach has been applied6 to the synthesis of Bi2Sr2Ca2Cu3O10, but questions regarding the processing chemistry and optimization of superconducting properties remain. In this report, we examine the composition and morphology of the precursor material, describe the product formation process, and evaluate the phase composition and superconducting properties. II. EXPERIMENTAL Starting materials were analyzed using thermogravimetric analysis (TGA) to determine precisely the metal/ weight ratio. Bismuth, lead, and copper oxides (Johnson Matthey, 99.999%) and calcium and strontium carbonates (Johnson Matthey, 99.999%) were dissolved in nitric acid (Baker Ultrex, HN0 3 ). The final solution concentration was 0.03 M in Bi, the pH was 0.5, and the stoichiometry was 1.74 Bi, 0.34 Pb, 1.94 Sr, 2.01 Ca, and 3.07 Cu. Enough solution was prepared to make between 100 g and 500 g of oxide product. Solutions 2510
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J. Mater. Res., Vol. 9, No. 10, Oct 1994
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were filtered/degassed. The nitrate solution was atomized using an ultrasonic spray nozzle and rapidly frozen by allowing the aerosol to fall into stirred liquid nitrogen. The apparatus for preparing frozen solutions has been described in detail elsewhere.7 The frozen nitrate solutions as prepa
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