Spray pyrolysis of YBCO precursors
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Spray pyrolysis of YBCO precursors Gideon S. Grader, Dario R. Machado, and Raphael Semiat Chemical Engineering Department and the Crown Center for Superconductivity, Technion, Haifa 32000, Israel (Received 31 January 1994; accepted 27 May 1994)
Acetate, nitrate, and oxalate precursors for YBCO have been spray pyrolyzed under different conditions. Shelled and nonhollow microparticles were obtained from acetate and nitrate precursors, while nonhollow agglomerates were obtained from the oxalate suspension. At low furnace temperatures, the temperature and residence time of the particles were insufficient for complete decomposition of the precursors leading to Cu2O and Cu metal in the product. At 900 °C and above, reduced forms of CuO were not detected by x-ray measurements, and up to ~60 wt. % YBCO was obtained. An approximate model predicting the particle and gas temperatures along the reactor under different operating conditions was developed. The model demonstrates that under the experimental conditions used here, the absorbed radiation heat by the particles from the furnace walls is significant in heating the gas. The gas and the particle temperatures are fairly close due to the effective heat transfer to the particles. At furnace temperatures of 700 °C, the maximum predicted particle temperature is about 500 CC (for ~ 1 s). This explains the incomplete reactions obtained under these conditions. Above 900 °C the reactions are predicted to be complete within the first half of the furnace, leaving sufficient residence time for partial conversion into YBCO. Finally, an approximate expression predicting the relative contribution to the gas heating by the walls and the aerosol has been developed.
I. INTRODUCTION Conventionally, high-temperature superconducting powders are prepared by solid-state reaction1"4 where precursors such as oxides, carbonates, or nitrates are mechanically milled, calcined, and finally remilled. Solution processes can yield fine, intimately mixed precursor powders, where the repeated milling steps, common to conventional process, are often eliminated. The powder contamination is thereby reduced. For example, the oxalate coprecipitation from alcoholic solutions5 yields homogeneous particles in the 0.1-0.2 ixm diameter range. Some liquid phase processes require pH adjusting agents (which are potentially contaminating) to maintain the stoichiometry. In contrast, aerosol processes are capable of producing high-purity powders, with desired stoichiometry and particle size,6 while minimizing the contamination and synthesis time of the powders. In the spray pyrolysis method,7"9 the precursors are first dissolved or dispersed, and then sprayed into a furnace where the solvent evaporates and the precursors decompose and react. This route has been used to produce micron-sized YBCO powders,9"19 films,10-17 and other HTSC powders. 9 - 152021
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