Effect of ambient environment and pressure on the formation of YBa₂Cu₃O_6+x from Y₂O₃, BaCO₃ and CuO

The reaction pathway through which YBa₂Cu₃O_6+x (123) is formed from a precursor material containing finely mixed Y₂O₃, BaCO₃ and CuO prepared by a spray pyrolysis process has been studied. The precursor powder was loosely packed in cylindrical crucibles and heated for different lengths of time in N₂ and in O₂ at different temperatures and pressures. The samples were then layered and analyzed by XRD to assess the spatial variations in the composition. The tetragonal 123 phase was the primary reaction product that could be identified by XRD. At 700-750°C and atmospheric pressure, the formation of the 123 phase was found to occur at first in the outer layer of the packing and subsequently this front moved into the specimen, in both N₂ and O₂ environments. Experiments carried out at lower pressures revealed that a decrease in pressure resulted in an enhancement in the conversion. Similar trends were obtained with dry-pressed compacts of the precursor as well. This shows clearly that the rate of removal of CO₂ from the interior of the sample affects the progress of these reactions in a profound way. Two binary mixtures 2BaCO₃ + 3CuO and 4BaCO₃ + Y₂O₃ corresponding to the stoichimetry as in the 123 compound were prepared by a spray pyrolysis technique. Thermogravimetric analysis of these binary mixtures and the precursor for 123 revealed that the rate-limiting step in the formation of 123 is the reaction between BaCO₃ and CuO.