SYNTHESIS, PROCESSING BEHAVIOR, AND CHARACTERIZATION OF BISMUTH SUPERCONDUCTORS USING FREEZE-DRIED NITRATE PRECURSORS

The synthesis of Bi2-xPbxSr2Ca2CU3O10 (Bi2223) powders from a freeze d ried nitrate precursor is reported here. We examine the composition an d morphology of the precursor material, describe the chemistry and kin etics of product formation, and evaluate the phase composition and sup erconducting properties of the products. A nitrate solution containing the appropriate ratio of cations was rapidly frozen and then freeze d ried at low temperatures to form an atomic mixture of the component sa lts. 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-substitut ed bismuthates and strontium-substituted calcium cuprate, not Bi2201, are the intermediates between the nitrates and the superconducting pro ducts. These highly disordered phases rapidly transform into Bi2212 or Bi2223 at higher temperatures (>790 degrees C). The kinetics of produ ct formation was studied using XRD analysis and magnetic susceptibilit y. The kinetics were shown to follow the nucleation and growth mechani sm. Bi2223 formed after only 30 min at a few degrees below the melting point, and after 37 h Bi2223 products exhibited excellent phase compo sition and magnetic susceptibility characteristics.