Environmental Stability of High T c Superconducting Ceramics
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tailed understanding of surface alteration mechanisms and their effects on the microstructure and on electric and magnetic properties and, on the other hand, to assuring enough water resistance for high Tc cuprate ceramics in various applications. Most recent studies have concentrated on the two systems that currently form the basis for applied work, viz. the Y-Ba-Cu-O and Bi(Pb)-Sr-Ca-Cu-O systems. Further studies are necessary to complete the quantitative understanding of the corrosion kinetics, to relate the corrosion kinetics to those of other ceramic materials, and to characterize the behavior of high-quality high Tc superconducting ceramics under service conditions.
Reactions of YBa2Cu3O7.x with Water and Humid Atmospheres Based on early studies of the products of degradation of YBa2Cu3O7.x (YBCO) in humid atmospheres, as well as in liquid water, the following mechanism was proposed:4 3H2O + 2YBa2Cu3O7^Y2BaCuO5 + 3Ba(OH)2 + 5CuO + 0.5O2 (1) Ba(OH)2 + CO2 -> BaCO3 + H2O
(2)
Several other corrosion products have also been suggested. Y2BaCuO5 may subsequently break down into BaCuO2 (or Ba 2 Cu(OH) 6 ) 15 and Y2O3,16 with the latter oxide undergoing hydration to form Y(OH)3. Y(OH)3 has been observed in several studies.61719 As in the cases of other ceramics and glasses, thermodynamic predictions of the nature of the identity of the corrosion products of ceramic products do
not always agree with observations. For instance, while it was expected that CuO would undergo hydration to form Cu(OH)2 and eventually become converted into CuCO3 • Cu(OH)2,2" CuO is generally observed.4'6*""18-21"24 Some of the corrosion products are poorly crystalline or amorphous,412'23'24 and secondary nucleation and crystal growth follow the initial attack. Listings of the observed corrosion products12 in studies of the leaching and weathering of YBa2Cu3O7_v show that the products most frequently observed between room temperature and 90°C are BaCO3, CuO, and Y2BaCuO5 or the decomposition products of the latter, viz. BaCuO2 and Y(OH)3. Initial observations supporting this conclusion8 have been confirmed by recent work.15'25 Accordingly, the mechanism represented by Reactions 1 and 2, with slight variations, has been widely accepted in the interpretation of the corrosion of YBCO. The chemical identity of the corrosion products does not provide a complete picture of the consequences of the contact between YBCO and water. As with other ceramics and glasses, the corrosion process leads to the formation of surface layers that are quite different in composition from that of the original specimen. Because of the selective solubilization of barium, a barium-rich deposit develops on the outer surface of the material.23'24'26 Careful analysis of elemental profiles in the surface regions of both bulk specimens and thin films of YBCO16'27 has shown that, on being exposed to humid environments, the surface composition undergoes alteration, with gradual enrichment in both Ba and Y and a corresponding depletion of copper on approaching the outer surf
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