Abnormal oxidation behavior of YBa2Cu3O7-δ ceramics

The influence of sample thickness and cooling rate on the oxidation kinetics of dense small grain YBa2Cu3O7-delta ceramics is presented. The oxidation behavior is strongly dependent on the phase type exhibited by the material. At high temperature, in the tetragonal phase, the oxygen stoichiometry of the ceramic core is dependent on the diffusion length and isothermal treatments improve the overall. Value increase in the sample thickness decreases the oxygen content. At low temperature, in the orthorhombic phase, the oxidation during cooling becomes very fast and the sample thickness is not observed to be a limiting parameter. An ultrasonic determination of Young's modulus as a function of temperature shows that this behavior cannot be related to the formation of microcracks. The improvement in oxidation observed in this temperature range is considered as characteristic of a very fast diffusion of oxygen along the grain boundaries, which is enhanced below the tetragonal to orthorhombic transition by the stresses resulting from anisotropic thermal contraction.