Degradation of high- T c superconductors by annealing in dry and moist atmospheres
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the realization of the practical applications of high Tc superconducting oxides, knowledge regarding their stability and property changes following treatments in ambient atmosphere is essential. Earlier studies of YBa2Cu3O7 showed that it reacts rapidly with water and degrades in atmospheric moisture, CO and CO2, to form carbonates and hydroxides.1"6 Specifically, its reaction with H2O results in the formation of CuO, Ba(OH)2, O2, and the nonsuperconducting phase Y2BaCuO5. The Ba(OH)2 then reacts with CO2 to form BaCO3. However, these studies were all carried out below 100 °C. Gallagher et aV did study the effect of annealing of YBa2Cu3O7 in various atmospheres up to 1000 °C. They concluded from weight change measurements that carbonates and hydroxides are formed below 400 °C and then decompose at higher temperatures. However, the effect of such anneals upon the normal state resistivity and superconducting behavior in bulk samples was not reported. Preliminary studies of degradation of normal state resistivity and superconducting transition in YBa2Cu3O78'9 and GdBa2Cu3O710 annealed in O2 between 200 °C and 500 °C were reported. The effect was attributed to the existence of a highly resistive boundary layer between the superconducting grains. The nature of the layer, however, is unclear. Similarly, surface degradation of La2_xSr^CuO4 near room temperature in an atmosphere containing H2O, CO2, and CO, though not as well documented as in YBa 2 Cu 3 O 7 , was also reported.11 Furthermore, the presence of absorbed water during sample preparation was shown to suppress appreciably the volume fraction of superconducting material.12 J. Mater. Res., Vol. 5, No. 9, Sep 1990
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In this work, we study the effect of annealing both La2_xSrJCuO4 and YBa2Cu3O7 in wet and dry O2 atmospheres at various temperatures between 200° and 950 °C. The effects of these treatments on both the superconducting and normal state properties of these materials are reported. While the major purpose was to study the effects of moisture, an additional unexpected degradation was observed for the case of YBa2Cu3O7. II. EXPERIMENTAL
YBa2Cu3O7 was prepared from Y2O3, BaCO3, and CuO. The materials were mixed, pulverized, and calcined in a Pt crucible at 900 °C in air for 24 h. The mixture was reground and recalcined under the same condition. Finally, the material was pulverized and cold pressed into pellets. The pellets were then sintered under flowing O2 at 950 °C for 24 h followed by furnace cooling to room temperature over several hours. La2_;cSrcCuO4 (x = 0,0.10,0.15) was prepared by mixing appropriate ratios of La2O3, CuO, and SrCO3. The mixture was pulverized and calcined over Pt foil at 1000 °C in air for 24 h. The resulting material was reground and recalcined as before. Finally the powder was pelletized and sintered at 1100 °C in air for 48 h. In the case of both materials, x-ray diffraction confirmed that single phase materials had been obtained. Sample treatments were done at different atmosphe
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