SURFACE DEGRADATION AND AGING IN YBA2CU3O7-X CERAMICS

The time dependence of the surface impedance of YBa2Cu3O7-x (123) bulk ceramic superconductors was measured from 100 Hz to 20 kHz using a novel noncontact technique. The measurements were carried out at room temperature and in a variety of environments. All measurements were performed on freshly abraded 123 samples using a copper counter-electrode with an air-gap structure forming a 123-air-Cu capacitor. The results are divided into two general types by environment. In a dry environment the change in the surface dielectric constant appears to take the from of an exponential (1 + e-at). Ninety percent of the change takes place within 300 s, and the time constant is approximately 180 s. Experiments performed in oxygen and helium duplicated the dry environment results, which leads us to conclude that this degradation is not due to environmental interaction. In a high humidity environment the surface impedance change was reminiscent of the square law growth of SiO2 in a wet oxidation process and does not appear to saturate at a final value. Auger and XPS studies of samples after degradation indicates that the copper valence states at the surface and in the bulk of the 123 sample are different. The surface shows an XPS spectra similar to that of unoxidized copper (only d10 orbitals are present). An XPS spectra from the interior of the sample shows a definite correspondence with oxidized copper (both d9 and d10 orbitals are present). We determine two physical or chemical causes for the observed changes in the surface impedance of the 123 sample. (1) The high humidity results are strong evidence for the growth of a surface layer with a lower density than the 123 sample. (2) The results of the inert and dry atmospheres may be attributed to either slow oxygen loss from the 123 sample or dielectric aging.