Synthesis of Cuprate Ceramic Superconductors by Oxidation of Rapidly Quenched Alloy Precursors
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SYNTHESIS OF CUPRATE CERAMIC SUPERCONDUCTORS BY OXIDATION OF RAPIDLY QUENCHED ALLOY PRECURSORS J.S. LUO, D. MICHEL and J.-P. CHEVALIER Centre d'Etudes de Chimie M~tallurgique C.N.R.S., 15, rue G. Urbain, 94407 Vitry C~dex (France) ABSTRACT We have produced various high Tc superconductors by oxidation of rapidly quenched alloy precursors produced by planar flow casting. This method generally leads to reactive oxidized powders and improves homogeneity if there is a solubility domain in the liquid state for the alloy. The grain size distribution is often more uniform than that for specimens prepared by conventional methods from mixtures of constituent oxides and carbonates. INTRODUCTION Usual ceramic preparation of high Tc superconductors presents limitations in obtaining good electrical properties, notably high critical current densities, mainly due to chemical inhomogeneity, impurities at grain boundaries, as well as microcracking [1,2]. The melting and rapid quenching of alloy precursors has been reported as a means of obtaining a homogeneous cation distribution and of producing superconductors after oxidation [37]. This paper reports on the preparation of various superconducting phases, using this method, and on the resulting electrical properties as a function of microstructure and homogeneity. EXPERIMENTAL PROCEDURE Starting alloys of about 5 g are prepared by RF melting on a cooled copper hearth under helium. These ingots are remelted in quartz nozzles, and then quenched by planar flow casting on a rotating wheel under argon. After quenching, alloy fragments of 30-50 mm thick are obtained [6]. Samples are oxidized at temperatures ranging from 800*C to 900 0 C under a controlled oxygen partial pressure. These are then ground, compacted and sintered at similar temperatures. X-ray diffraction (XRD) spectra are recorded using Co Ka radiation. Samples are examined with a scanning electron microscope (SEM) The electrical equipped with Si-Li X-ray microanalysis. resistivity is measured on sintered bars using a standard fourprobe dc method with silver paint contacts. RESULTS AND DISCUSSION We have obtained the LBa 2 Cu 3 O7 (L=Y, Eu, Yb), Bi-Sr-Ca-Cu-0, Bi-Pb-Sr-Ca-Cu-O, as well as Pb 2 Sr 2 Ca 0 . 5 L0 . 5 Cu 3O 8 (L=Y, Eu, Yb) superconducting phases by oxidation of the corresponding alloy precursors produced through planar flow casting. The L-Ba-Cu system (L=Y.
Eu.
Yb)
For Y-Ba-Cu, a majority YBa 2 Cu 3 O7 phase is obtained after alloy oxidation despite the presence of a miscibility gap for the liquid ternary alloy. This result can be explained on the basis Mat. Res. Soc. Symp. Proc. Vol. 169. 01990 Materials Research Society
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of a fine scale dispersion of phases in the quenched alloy. Tc(onset) is 89K and zero resistivity is reached at 65K. For Eu and Yb based alloys, amorphous metallic fragments are mainly obtained after quenching the alloy from the melt. For YbBa 2 Cu 3 , after oxidation, only about one third of the sample consists of the orthorhombic superconducting phase, whilst, on the other hand, a pu
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