Influence of synthesis conditions on phase formation and magnetic properties of Hg-based high temperature superconductorsстатья
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Аннотация:Experiments on ampoule synthesis of high temperature superconductors Hg1-xPbxBe2-ySryCan-1CunO8+delta (x similar to 0.2-0.3, 0.5 less than or equal to y less than or equal to 1.5) were carried out under different oxygen partial pressures (PO2) controlled by redox couples Cu2O/CuO, CoO/Co3O4 and Pd/PdO at temperature range 690 less than or equal to t(s) less than or equal to 850 degreesC. It was found that at temperatures t(s) > 700 degreesC and low PO2 the higher Hg-HTS members are more stable than the lower ones. Optimization of PO2 allowed to prepare the sample containing about 80% of 1234 phase (n = 4) and to decrease the synthesis temperature of 1223 phase (n = 3) down to 690 degreesC. Investigation of magnetic properties of synthesized samples has shown that appreciable decrease of critical temperature with Sr content y is not noticeable up to y = 1.0, but as y increasing as t(s) decreasing rises the irreversibility line H-irr(T) and enhances the intragrain critical current density under applied magnetic field. On the contrary, when y > 1 its increasing leads to an abrupt decrease of T-c and, as a consequence, to lowering of H-irr(T) line at temperatures T > 100 K. Wave-like intragrain microinhomogeneities, which could serve as pinning centers, were observed by TEM. (C) 2004 Elsevier B.V. All rights reserved.