Critical assessment of the process of growth of a YBa2Cu3O7-δ layer on Y2BaCuO5

A layer of YBa2Cu3O7-δ (123) can be grown on a Y2 (211) substrate by reaction with a mixture of 3BaCuO2+2CuO. This reaction has been studied in the temperature range of 930 to 9800C. The reaction is never complete but levels off when the CuO present in the reactant mixture is exhausted. The liquid dissolves an excess of CuO and the 211 substrate becomes covered by co-precipitation of 123 grains and CuO inclusions. The formation of 124 and related polytypoids is favoured by the Cu-rich stoichiometry of the reactant liquid. The growth of the 123 grains follows a parabolic law. The presence of a thermal gradient during reaction does not affect the orientation of the grains but has an influence on the thickness of the 123 layer. (author)