The environmental degradation mechanism and protective organic thin film coatings on a high-temperature bismuth–cuprate superconductor

Abstract

The degradation mechanism for Bi 2Sr 2CaCu 2O x (Bi-2212) in a highly humid air has been investigated. Using diffuse reflectance infrared Fourier transform spectroscopy, different reaction steps for carbonate formation with aging time were found to occur on the surface of the superconductors. During the initial stage, the calcium carbonate species mainly formed due to the preferred hydration of calcium ions because of their lower ionic–dipole interaction energy. At a later stage, the strontium carbonate species became the more dominant species because of the kinetics of this step along with the higher molar ratio of Sr 2+ to Ca 2+. In addition to the carbonates, larger amounts of Sr(OH) 2 · nH 2O ( n=1 or 8) and CuBi 2O 4, along with relatively small amounts of Ca(OH) 2 were also found on the surface along with Sr(OH) 2. The formation of larger amounts of strontium-related degradation products also led to larger amounts of Bi 2CuO 4 being formed. The effect of stearic acid coating on the degradation of the superconductor was also investigated with respect to long term durability in a very humid atmosphere. These coatings strongly prevented the formation of carbonates on the surface of (Bi,Pb) 2Sr 2Ca 2Cu 3O y (Bi,Pb-2223). Strong complex bonds between the carboxylate groups of the organic acids and the metal ions on the surface of the superconductor were formed, which helped to retard the degradation process for the superconducting phase.