Preparation of the 110 K single phase superconductor Bi 1.6 Pb 0.4 Ca 2 Sr 2 Cu 3 O x using a precursor matrix reaction
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M.J.G. Lee Department of Physics and Scarborough College, University of Toronto, Toronto, Ontario, Canada
S. Bagheri and J. Rutter Department of Metallurgy and Materials Science, University of Toronto, Toronto, Ontario, Canada (Received 27 August 1990; accepted 19 February 1991)
Nearly single phase sintered samples of the lead-doped high-Tc oxide superconductor of nominal composition Bii 6Pbo.4Ca2Sr2Cu30;c have been prepared using a precursor matrix reaction method. The experimental results indicate that in samples annealed under optimum conditions (200 h at 855 °C) the volume fraction of impurity phases is only about 3%. If the optimum annealing time is exceeded, the high-Tc phase begins to decompose into the low-Tc phase and nonsuperconducting phases. It is found that the precursor reaction matrix method minimizes loss of lead during processing.
I. INTRODUCTION Since the discovery of superconductivity in the B i - C a - S r - C u - 0 system,1 many articles have appeared describing the preparation and properties of the superconducting phases. Three different superconducting phases have been identified: the 2021 phase (Tc - 10 K), the 2122 phase (the low-Tc phase with Tc = 75-85 K), and the 2223 phase (the high-Tc phase with Tc = 110 K). 2 " 6 Depending on the preparation procedure and starting composition, mixtures of the superconducting phases with various insulating phases may be obtained. To achieve the optimum superconducting properties, it is important to maximize the volume fraction of the high-Tc phase. In early work, excess calcium and copper were added to the starting material, and the effects of prolonged annealing were studied.7'8 The most effective known way to increase the volume fraction of the high-Tc phase is to partially replace bismuth by lead. 9 " 11 A precursor matrix reaction method has been successfully applied to the preparation of single phase samples of the low-Tc phase,12 but when a similar method was applied to prepare the high-Tc phase, the 2223 phase constituted only about 70% of the resulting samples.13 In this paper we report the successful fabrication, by a precursor matrix reaction method, of nearly single phase samples of the high-Tc phase in which 20% of the bismuth is replaced by lead. For the optimum conditions of annealing (200 h at 855 °C) the impurity fraction was found to be about 3%.
precursor matrix, Ca2Sr2Cu3O7, was prepared by thoroughly mixing stoichiometric amounts of high purity (99.999% or better) CaO, SrCO 3 , and CuO, and reacting at 960 °C for 100 h with five intermediate grindings. X-ray powder diffraction measurements were carried out with a Philips 1380 diffractometer using Cu Ka radiation. Scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDXA) were done using a Hitachi S-570 equipped with a Link AN-10000 Energy Dispersive X-ray unit. X-ray diffraction and EDX analysis showed that Ca 2 Sr 2 Cu 3 07, the precursor of the high-Tc superconducting phase, makes up 80% of the volume of our matrix material, the remaining constituents being
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