Coupled heat transfer and thermal stress in high-T_c thin-film superconductor devices

A coupled heat transfer and thermal stress analysis is developed for a thin-film high-T_c superconductor device. The thermal boundary resistance between the film and substrate, which is modelled as a function of interfacial peeling stress, is used to couple the structural and thermal sides of the model. The thermal stress generated from the temperature reduction from room to cryogenic operating temperatures, and the temperature field due to uniform Joule heating in the normal-state film, are calculated using finite element analysis. The resulting peeling stress, that is the normal stress at the film/substrate interface, and peeling stress intensity factor, are calculated for different substrate materials, different YBCO film lattice direction combinations, and different current densities. The finite element analysis demonstrates the importance of considering the thermal boundary resistance when calculating the peeling stress, although the peeling stress/thermal boundary resistance coupling is relatively insignificant for determining the resulting stress and temperature fields. The report suggests that a polycrystalline YBCO thin film with small grain size deposited on an MgO substrate is a good choice, since it has smaller stress in the film and better heat dissipation ability.