Thermal conversion of gels to YBa₂Cu₃O_x, Bi₂Sr₂CaCu₂O_x, and (Bi, Pb)₂Sr₂Ca₂Cu₃O_x and their decarbonization by low-temperature treatment with nitric acid

Thermal conversion of acetate-derived gels to YBa₂Cu₃O_x (Y–123), Bi₂Sr₂CaCu₂O_x (Bi–2212), and (Bi, Pb)₂Sr₂Ca₂Cu₃O_x (Bi-2223) has been studied by thermal analysis, x-ray diffraction, and infrared spectroscopy. Carbonates formed above 200 °C during thermal treatment of all gels. Decomposition of the carbonates proved to be more difficult for Y-123 than for Bi-2212 or Bi-2223. However, all of the gels that were heated contained significant amounts of carbon after calcination. Complete decarbonization of materials was attained by treating the intermediate phases (e.g., those formed after calcination at 600 °C) with nitric acid and then subjecting them to a final thermal treatment. Removal of carbonates from the intermediate phases strongly accelerated formation of the superconducting compounds.