Thermal instability near planar defects in superconductors

It is shown that the local Joule heating due to planar defects, such as gra in boundaries, microcracks, etc., can cause thermal instabilities, which li mit the current-carrying capability of YBa2Cu3O7-coated conductors. Explici t instability criteria are obtained for a planar defect in a film and for a grain boundary. Thermal instabilities can be triggered by low-angle grain boundaries or planar defects, which block only a small fraction of the samp le cross section. Hot spots near small defects and overheating of grain bou ndaries are essential for interpretation of experimental data on ac losses and E-J curves of polycrystals. (C) 2001 American Institute of Physics.