Internal oxidation in solid solutions Bi 2.1-x Pb x Sr 2-y Ca 1-z R y+z Cu 2 O 8 +d (R = Y, Nd or La).
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Internal oxidation in solid solutions Bi2.1-xPbxSr2-yCa1-zRy+zCu2O8+d (R = Y, Nd or La). A.V. Garshev1, A.V. Knotko2, M.N. Pulkin1, D.I. Kirdyankin1, A.V. Geyer2, V.I. Putlayev2. 1 - Department of Materials Sciences, Moscow State University, Moscow 119992, Pussia 2 - Department of Chemistry, Moscow State University, Moscow 119992, Pussia ABSTRACT Bi2-xPbxSr2CaCu2O8+d solid solutions with substitution of Sr or Ca by Y, Nd or La were fabricated at 760 - 790°C in N2-flow. The as-synthesized solutions were oxidized in air under isothermal condition and tested with XRD, TGA chemical analysis, TEM, XANES. Effect of rare-earth element content on oxidation kinetics and microstructure of the product is discussed. INTRODUCTION The internal oxidation of Bi2Sr2CaCu2O8 (Bi-2212) solid solutions coupled with formation of fine precipitates in the bulk is a prospective way to create nanocomposite materials ‘superconductive matrix – nonconductive inclusion’ with improved critical current density. To control the oxidation process kinetics and the obtained materials micromorpholodgy one can carry out certain substitution in cation sites of Bi-2212 structure, e.g., replacement of Bi by Pb, and Ca (or Sr) by rare-earth elements (RE). EXPERIMENTAL DETAILS Samples of Bi2.1-xPbxSr2-qCa1-y-z-pYyNdz+qLapCu2O8+d solid solutions were prepared via conventional nitrate route, i.e., nitrate mixture after evaporation of acidic solutions with adequate stoichiometry was calcined at 700°С for 15-20 min in air followed by annealing at 760°С in nitrogen flow for 24-48 hrs or at 850°С in air depending on RE content. Oxidation kinetics was studied by thermogravimetric analysis (TGA) (MOM Q-1500D, Hungary) under isothermal annealing in air at 650 - 750°С up to 24 hrs long. Microstructure of the oxidized solutions was evidenced with transmission electron microscopy (TEM) (JEM2000FXII, JEOL, Japan, operated at 200 kV). Phase composition was examined with X-ray diffraction (DRON-3M, Russia) using Cu Kαm radiation. Fine structure of near-edge area of Xray absorption band (XANES) was revealed using facilities of synchrotron source at Taiwan National Center of Synchrotron Research. Amount of excessive oxidant (assuming O-2, Cu+2, Bi+3 and Pb+2 formal charges) was accessed through conventional technique of iodometric titration [1]. DISCUSSION According to the results of iodometric titration, the amount of excessive oxidant in the as-synthesized solid solutions (i.e., the overall amount of Cu+3, Bi+5, Pb+4, O-1) increased with increasing of Pb-content. With the same time, the content of oxygen drops with increasing of Pb-fraction in the solution. It is worth noting that substitution of Ca by Y caused an opposite effect: the oxygen content increased with dropping of excessive oxidant amount (figure1). It is
Mater. Res. Soc. Symp. Proc. Vol. 848 © 2005 Materials Research Society
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widely accepted to treat the amount of over-stoichiometric oxygen in such materials as a concentration of holes. Sometimes the term ‘hole’ is loaded by the same meani
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