Study of AC magnetic loss in grain boundaries in melt-processed and granular high temperature superconductors using levitation and AC susceptibility techniques.

摘要

The first part of this thesis studies the intrinsic ac-magnetic loss mechanisms in the new high-temperature-superconducting levitators. Measurements of both the AC magnetic susceptibility in Y-Ba-Cu-O and frictional losses of spinning permanent magnets levitated above Y-Ba-Cu-O are presented. A direct correlation is observed only when large susceptibility samples are studied indicative of the importance of grain boundary loss mechanisms. The increasing loss per revolution with increasing frequency for a spinning permanent magnet, and the dependence of the loss on the height of the magnet over the superconductor are explored.; We show that Josephson vortices between large grains have much different characteristics than Josephson vortices between small grains. Long grain boundaries behave as long (R{dollar}rm sb{lcub}grain{rcub} > lambdasb{lcub}Josephson{rcub}{dollar}) Josephson Junctions (JJ), where R{dollar}rm sb{lcub}g{rcub}{dollar} is the grain radius and {dollar}lambda sb{lcub}rm Josephson{rcub}{dollar} is the Josephson vortex size. Long JJ have little pinning, and behave like a metal for large dc fields. Short JJ have many pinning sites, and so act like a weak critical state with a frequency dependence derived from the relaxation of the critical state.; The second part studies the relationship between mechanical effects and the magnetic response of the levitators. Specifically, both the experimental and theoretical aspects of the precession of a freely levitated spinning magnet over both a melt-processed and sintered high temperature superconductor are explored. The levitated magnet slowly precesses about a position at least close to the center of mass of the magnet, and the tilt angle decays back to the original orientation. A simple torque equation is used to obtain the applied torque from the rate of change of the tilt angle and the precession angle. We discuss how to relate the torques to the moments in the superconductor.