Effect of secondary phases in the precursor powders on the transformation to the (Bi,Pb)-2223 phase
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Effect of secondary phases in the precursor powders on the transformation to the (Bi,Pb)-2223 phase R. Gundakaram1, S.C. Chang1, R.S. Liu1, L. Woodall2 and M. Gerards2 1
Department of Chemistry, National Taiwan University, TAIPEI 106, TAIWAN, R.O.C. 2
Merck KGaA, DARMSTADT, GERMANY
ABSTRACT The (Bi,Pb)2Sr2Ca2Cu3Oy system [also know as (Bi,Pb)-2223] has shown promise for use in superconducting tapes and wires due to its high transition temperature and high critical current density. The tapes and wires are usually fabricated by the powder-in-tube method, after which they are subjected to thermal and mechanical treatments. Depending on the nature of the heat treatment, phase transformations occur in the powder and it is of paramount importance to understand the response of the powders to different processing conditions. In this study, we have synthesized a precursor powder of the nominal composition Bi1.8Pb0.33Sr1.87Ca2Cu3Oy by spray pyrolysis. The powder was then calcined under controlled conditions between 760 and 800ºC in an atmosphere between 0 and 21% O2 for 2 to 24 hours with controlled heating and cooling, such that different amounts of Pb are incorporated into the majority phase. The resultant powders were then converted to the (Bi,Pb)-2223 phase with suitable heat treatments. Using powder Xray diffraction and magnetization measurements, the volume fractions of the secondary phases in the precursors were estimated and the effect of the phases on the conversion to the (Bi,Pb)-2223 phase was studied. We show that the ratio of the intensity of the (020) and (115) reflections of the (Bi,Pb)-2212 phase can be used to tune the precursor powders for optimal conversion. While a lower fraction of the secondary phases such as CaO and CuO is desirable in the precursor powders, a higher volume fraction of Ca2PbO4 seems to help in the rapid conversion to the (Bi,Pb)-2223 phase. INTRODUCTION Of the three superconducting phases of the system Bi2Sr2Can-1CunO2(n+2), the n = 3 member, also known by the acronym 2223, shows the highest value of the transition temperature (110 K) [1]. Synthesis of this composition in single phase is difficult due to the formation of the n = 2 member (2212) as an impurity. It was discovered that the substitution of Pb at the Bi site facilitates the speedy formation of the 2223 phase [2]. In addition to a high transition temperature, it is also desirable for the materials to possess a high critical current density (Jc) for practical applications. Amongst the superconducting members of the series mentioned above, the (Bi,Pb)-2223 phase is again the material of choice due to its high Jc [3]. In applications such as power transmission, superconductors are used in the form of a tape or a wire. The powder-in-tube method is usually used for the fabrication of wires and tapes. In this technique, the superconducting precursor powder is filled into a metal tube (usually Ag) and E3.6.1
then drawn into a wire. The diameter of the wire is decreased in successive steps and rolled into a tape of desired thick
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