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 resist ance between the film and substrate, which is modelled as a function o f interfacial peeling stress, is used to couple the structural and the rmal sides of the model. The thermal stress generated from the tempera ture reduction from room to cryogenic operating temperatures, and the temperature field due to uniform Joule heating in the normal-state fil m, are calculated using finite element analysis. The resulting peeling stress, that is the normal stress at the film/substrate interface, an d peeling stress intensity factor, are calculated for different substr ate 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 ca lculating the peeling stress, although the peeling stress/thermal boun dary resistance coupling is relatively insignificant for determining t he 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 fil m and better heat dissipation ability. (C) 1998 Elsevier Science Ltd. All rights reserved.