Environmental effects on luminescence yield of superconducting YBa 2 Cu 3 O x
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Environmental effects on luminescence yield of superconducting YBa2Cu3Ox M.S. Jahan,a) D. W. Cooke, H. Sheinberg, and J.L. Smith Los Alamos National Laboratory, Los Alamos, New Mexico 87545
D. P. Lianos W. J. Schafer Associates, Inc., Huntsville, Alabama 35868 (Received 8 November 1988; accepted 24 March 1989) Gamma-ray-induced surface defects in YBa2Cu30^ (x = 7) have been investigated by thermally stimulated luminescence (TSL) following storage of the samples in dry (vacuum), oxygen, and 98% relative humidity environments. Irradiation of samples stored in either vacuum or oxygen environments shows that no insulating chemical species are formed on the surface of the superconductor. In contrast, exposure to the humid environment produces various chemical components on the surface, and, consequently, enhances the TSL yield. Comparison of the YBa2Cu30^ TSL glow curve and emission spectra with those of BaCO3 suggests that one of the components contributing to the purported YBa2Cu30;c luminescence is BaCO3. Evidence for formation of other chemical species and the suitability of the TSL technique for their determination are discussed.
I. INTRODUCTION Polycrystalline oxide superconductors possess grain boundaries, and, quite frequently, impurity phases, both of which are important in determining electrical, magnetic, and mechanical properties. Chemical and environmental reactions may also degrade the surface and consequently alter the crystal structure, 1 ' 2 thus circumventing superconductivity at high temperatures. Gao et al.3 investigated the stability of La-Sr-Cu-0 and YBa2Cu3O^ surfaces under exposure to vacuum, prolonged electron bombardment, oxygen exposure, and x-ray irradiation. Although the surface was found to be reasonably stable under low-energy electron bombardment at a pressure of 1 x 10"10 Torr, chemisorption of oxygen was observed, the reaction being limited to the outermost 10-20 A of the superconductor. Also, they showed that during high-energy electron irradiation oxygen dissociated from near the surface and was replenished by oxygen transport in YBa 2 Cu 3 0 J . but not in L a - S r - C u - O . Oxygen diffusion both in and out of YBa2Cu30^ has been reported by Robinson.4 Bansal and Sandkuhl5 and Garland6 have reported surface degradation of YBa2Cu3Oi exposed to a humid environment. Previously, we reported TSL of x-ray-induced surface defects in RBa2Cu30JC (R = Gd, Ho, or Eu) in the temperature intervals 80-300 K7 and 25-400 °C.8 It was suggested that F- and V-type surface centers were formed by the radiation and that their subsequent recombination produced the observed luminescence. Although TSL was found to be independent of the particular rare-earth constituent, the glow peak temperatures, as well as the emission maxima, exhibited large sample-to-sample variations. "'Current address: Department of Physics, Memphis State University, Memphis, Tennessee 38152.
Moreover, irreversible quenching of the TSL was observed in many samples, an effect attributed to the reaction of adsorbed oxygen with the r
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