LEVITATION FORCES, STIFFNESS AND FORCE-CREEP IN YBCO HIGH-T-C SUPERCONDUCTING THIN-FILMS

The applicability of YBCO high-T-c superconducting thin films to magnetic levitation has been investigated. Here we present fundamental studies employing cylindrical permanent magnets Roaring above laser ablated YBCO thin films typically about 0.3 mu m thick acid of 10 mm length and width. With permanent magnetization values of 0.4 T, interactions between magnet and superconductors are observable for distances up to about 10 mm, showing maximum force values of several mN (up to 4 mN) for minimum magnet heights. During vertical magnet motion highly hysteretic force-distance relations exhibit remarkable symmetry of the approaching (repulsive) and withdrawing (attractive) branch with maximum values of attractive Forces about 80% of maximum repulsive forces. Quasistatic stiffness data for both vertical as well as lateral motion are shown to be up to an order of magnitude higher than those known from melt-quenched material. The experimental data are supported by phenomenological models. By means of the latter. basic force and stiffness properties of the arrangement can be described. In strong contrast to bulk materials distinct limitations with respect to usable force levels are found to be caused by a pronounced force-creep effect, which is in the range of 30% up to 60% of the initial value after delay times of typically 12 min. Preliminary tests indicated additional creep components to be expected arising From mechanical activation, such as vibration or shock treatment. Apart from low-mass applications in e.g. microturbine rotors, micromotors and microgyros, thin films seem to be promisingly applicable to devices, where-without need for levitation (e.g. aerospace environment)-mechanical stiffness and damping is required.