Evaluation of thermal expansion coefficient of twinned YBa2Cu3O7-delta film for prediction of crack formation on various substrates

The effective thermal expansion coefficient of the c-axis-oriented, twinned YBa2Cu3O7-delta (YBCO) film formed on SrTiO3 (STO) substrates by liquid ph ase epitaxy was evaluated by observing crack formation of the films. The cr itical thickness of the YBCO film grown on STO at 920 degrees C is about 1. 35 mu m. A simple fracture model with mechanical properties of materials yi elds the effective thermal expansion coefficient of YBCO film of about 14.0 x 10(-6)/K, as an engineering value for prediction of crack formation on v arious substrates. Temperature dependence in the thermal expansion of the s ubstrate and the film suggests that control of oxygenation conditions enabl es us to suppress maximum stress induced in the films. According to the est imation, we predict the critical film thickness on various substrates used widely in oxide superconductor research.