The chemistry and superconducting properties of species in the system Bi-Ca-Sr-Cu-O
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Experimental results are presented on efforts to maximize the content of the phase responsible for the 110 K superconducting transition, Bi2Ca2Sr2Cu3Oz or (2223), in the system Bi-Ca-Sr-Cu-O. It was found that the content of this phase depends not only on the initial composition of the reacting mixture, but also on the presence of PbO, the temperature of reaction, and the duration of the reaction.
I. INTRODUCTION
The report of superconducting transition temperatures above 100 K in multi-phased samples of the BiCa-Sr-Cu-O system by Maeda et al.1 and in the Tl-Ca-Ba-Cu-O system by Sheng and Hermann2 has led to considerable effort toward the preparation of phase-pure specimens of the desired species. An additional motivation has been the recognition that these systems contain the potential for still higher transition temperature compositions.3'4 The present study was undertaken to determine the chemical parameters in the Bi-Ca-Sr-Cu-O system controlling phase purity. The primary synthetic route has generally been the reaction of the alkaline earth carbonates with bismuth and copper oxides at high temperatures in an air atmosphere. Initial experiments in this laboratory showed there was the potential for significant liquid phase formation prior to the complete decomposition of the carbonate components, leading to erratic product characteristics. This was due in part to the macroscopic segregation of transient carbonate-oxide fluids with compositions different from the bulk composition of the samples. This melting occurred at temperatures less than 840 °C, although the exact liquidus of this fluid was not specifically determined. Once the carbonate decomposition was complete, it was generally impossible to rehomogenize the sample by solid state diffusion within reasonable times. The obvious solution to this difficulty was to heat the reacting materials slowly so as to allow the decomposition to go to completion below the liquidus temperature of the fluids. This approach raised concerns regarding possible oxidation of the Bi(III) to Bi(V), with evaporative loss of this species. Instead, an alternate synthetic approach was adopted, modeled on the work of Hermann5 and Hermann and Sheng2'6'7 for the preparation of materials in the Ba-Y-Cu-O and Tl-Ca-BaCu-O systems. Intimate mixtures, compounds, and possibly solid solutions, of the divalent cation oxides, CaO, SrO, and CuO, were prepared separately, then comJ. Mater. Res., Vol. 5, No. 1, Jan 1990
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bined with the Bi2O3 and reacted to the final product. An additional benefit of this approach was that no gas was liberated with the final reaction, providing increased latitude in the fabrication of useful conductor configurations, such as sheathed wires. II. MATERIAL PREPARATION
The first step in the sample preparation was that of the base divalent cation oxides, the compositions of which are represented by the convention [Ca:Sr:Cu], with integers being the cation numbers of each species in the overall formula of the mat
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