Surface degradation of high Tc superconductors

Surface degradation studies of the YBa₂Cu₃O _7-x (123) and Bi-Sr-Ca-Cu-O (2223) superconducting ceramics were performed in N₂, Ar₂, O₂, wet and dry air. By using a novel noncontact room-temperature technique the time dependence of the surface impedance of both 123 and Bi-Sr-Ca-Cu-O bulk ceramics was measured from 100 Hz to 20 kHz. In humid environments, the rate of degradation is dependent on the humidity and appears to be unaffected by the carrier gas. The surface impedance change due to humidity follows a power law and does not appear to saturate at a final value. In environments without water present, the surface impedance follows an exponential curve of the form 1+e^-at, where a is approximately 150 s. This implies that the surface of the ceramic is relatively stationary and that it is losing oxygen. Oxygen loss converts the surface from a highly conductive state to an insulating state. This implication is supported by electron spectroscopy for chemical analysis (ESCA) spectra of the exposed surface and freshly abraded or broken samples´ interiors. In the ESCA spectra of the exposed surface, there is a lack of Cu-O bond peaks; in the center of the same sample the Cu-O peaks are present