Effect of ambient and evolved CO 2 on an equilibrium reaction in YBa 2CU 3O 6 + X synthesis

Abstract

The formation of YBa 2Cu 3O 6 + x in air involves two main reactions. The first is between BaCO 3 and CuO to form BaCuO 2 and CO 2. Increasing the partial pressure of carbon dioxide in air shifts the endothermic BaCO 3 reaction to higher temperatures. At a fixed carbon-dioxide concentration in air, the reaction onset-temperature of a thick sample (2 mm) determined by TG/DTA exceeds by approximately 200 °C that of a thin film (10 mm) determined by in situ HT-XRD. Evolved CO 2 in thick samples disguises the dependence of the intrinsic reaction onset-temperature and reaction rate on the ambient CO 2 concentrations. The intrinsic equilibrium constant and associated thermodynamic parameters for this reaction should be determined by experiments in which the impact of transport limitations is eliminated, i.e., use of a thin film and high gas-velocity next to the sample surface.