A comparative study of the electrical properties and microstructure of polycrystalline YBaCuO and thallium-based thin films

Thin films of YBCO and thallium-based superconductor compounds have been deposited with very similar polycrystalline structures. The critical current densities carried by the YBCO films are much lower than measured in the thallium films (>10³, as opposed to >10⁴ A/cm²). Grain boundaries in these films have been studied to correlate microstructure with the measured electrical properties. High defect densities and frequent microcracking have been observed at and around the boundaries in the YBCO films, but these defects are not seen in the thallium-based films. We suppose that this difference is because of the higher differential thermal expansion stresses set up in YBCO during cooling. Our observations imply that eventual application of polycrystalline superconducting films prepared by an ex situ process is more likely for the thallium-based materials than for YBCO.