Phase evolution and gas-phase particle size distributions during spray pyrolysis of (Bi, Pb)-Sr-Ca-Cu-O and Ag-(Bi, Pb)-Sr-Ca-Cu-O powders

Phase evolution, gas-phase particle size distributions and lead loss were studied during formation of (Bi,Pb)-Sr-Ca-Cu-O powders and their composites with silver by spray pyrolysis starting from nitrate solutions. The 10 wt% Ag/90 wt% Bi_1.8Pb_0.44Sr₂Ca_2.2Cu₃O_x composite powders made at 700°C consisted of 20–60 nm grains of silver and mixed-oxide phases with a fine dispersion of Ag grains within the particles. At 700°C, the primary phases present in (Bi,Pb)-Sr-Ca-Cu-O powders were (Bi,Pb)₂Sr₂CuO_x (2201), Ca₂PbO₄ (plumbate), (Bi,Pb)₂Sr₂CaCu₂O_x(2212), and (Bi,Pb)₃Sr₂Ca₂Cu₁O_x(3221). For T≥800°C, the powders were considerably depleted in lead, and the plumbate and 3221 phases were absent. For T = 900°C, a large number of ultrafine particles (<30 nm) were formed, probably from the PbO vapor released from the reactor walls. Using spray pyrolysis, it is easy to control stoichiometry and limit the phase segregation at the nanometer-scale so that homogeneous and phase-pure materials can be obtained rapidly during subsequent processing.