Reaction pathways for the formation of the YBa₂Cu₃O_7−x compound

Abstract

The kinetics of the reactions that lead to the formation of YBa₂Cu₃O_7−x was investigated starting from various powder mixtures containing different reactants as sources of Ba, Cu, and Y (BaCO₃, BaO, CuO, Y₂O₃. Y₂Cu₃O₅. and BaCuO₂). These powders were calcined at 940 °C for various time intervals. Quantitative x-ray analysis was used to determine the relative amounts of the compounds present in the calcined powders. When BaCO₃ was used as the source of barium, long calcination times were required to obtain the single phased YBa₂Cu₃O_7−x because of the slow decomposition of BaCO₃. This leads to large particles and broad particle size distributions and hence to a lower density of the sintered superconducting pellets. Procedures to attain high density superconducting material are identified on the basis of the kinetic studies. Each of them isolates the slow reaction in a first step and prepares the superconducting material via a rapid reaction in a second step. The kinetic studies reveal the reaction mechanisms that were followed in different cases.