An aqueous, all nitrate, solution-based preparation of BaTiO 3 is reported here. Rapid freezing of a barium and titanyl nitrate solution, followed by low temperature sublimitation of the solvent, yielded a freeze-dried nitrate precursor which was thermally processed to produce BaTiO 3 . XRD revealed that 10 min at temperatures >600 ± C resulted in the formation of phase pure nanocrystalline BaTiO 3 . TEM revealed that the material was uniform and nanocrystalline (10 -15 nm). The high surface to volume ratio inherent in these small particles stabilized the cubic phase of BaTiO 3 at room temperature. It was also found that the average particle size of the BaTiO 3 produced was highly dependent upon calcination temperature and only slightly dependent upon annealing time. This result suggests a means of selection of particle size of the product through judicious choice of calcination temperature. The experimental details of the freeze-dried precursor preparation, thermal processing of the precursor, product formation, and product morphology are discussed.
YBCO was synthesized using atomically mixed nitrate precursors. Atomic mixing was achieved using a freeze-dried process which is fully discussed here. The thermal processing behavior of these precursors is fundamentally different from that of mechanical mixtures of the Y, Ba, and Cu oxides and it is examined in detail. Ba2Cu305+A: (normally considered a high oxygen pressure phase) and Y2O3 formed as nitrate decomposition products at ambient atmospheric conditions. Subsequent reaction of these materials (2 h at 925 °C) produced polycrystalline YBCO. Without any post-processing of the powders, product YBCO powders consisted of 30 fim agglomerates composed of crystals 1-3 jum on an edge. The powdered products exhibited a magnetic susceptibility greater than 90% -1/4 77.
The synthesis of Bi2−xPbxSr2Ca2Cu3O10 (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.
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