Effect of initial composition on distribution of RE211 (422) particles in RE123 superconductors
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Effect of initial composition on distribution of RE211 (422) particles in RE123 superconductors M. Kambara and Y. Watanabe Department of Metallurgy, Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan
K. Miyake Department of Materials, Graduate School of Engineering, Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108, Japan
A. Endo Superconductivity Research Laboratory, ISTEC, 1-10-13, Shinonome, Koto-ku, Tokyo 135, Japan
K. Murata Department of Materials, Graduate School of Engineering, Shibaura Institute of Technology, 3-9-14, Shibaura, Minato-ku, Tokyo 108, Japan
Y. Shiohara Superconductivity Research Laboratory, ISTEC, 1-10-13, Shinonome, Koto-ku, Tokyo 135, Japan
T. Umeda Department of Metallurgy, Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113, Japan (Received 21 April 1997; accepted 2 July 1997)
Microstructure in melt-textured bulk RE1 Ba2 Cu3 O6+d crystals (RE123; RE Sm, Nd) was investigated, changing the initial composition from the tie-line composition of RE123–Sm2 Ba1 Cu1 O5 (Sm211)yNd4 Ba2 Cu2 O10 (Nd422) to the Ba-enriched side. It was found that the Sm211yNd422 particle size decreased in the liquid with increasing the BayCu ratio of the initial composition, and this tendency was also found in the grown Sm123 crystals. Composition of the Sm123 grown crystal could be controlled by selecting the Ba-enriched initial composition to obtain an almost stoichiometric compound, which resulted in higher Tc values. Furthermore, the Jc values also increased under low magnetic fields due to the significant decrease of Sm211 particle size. Therefore, changing the initial composition toward the Ba-enriched side was found to be a new process to enhance both Jc and Tc values simultaneously.
I. INTRODUCTION
REBa2 Cu3 O6+d oxide superconductors (RE123, RE Sm, Nd, . . ) produced in air have been known to show relatively low critical superconducting temperature sTc d values because the RE123 phase is a solid solution due to substitution of RE ions at Ba sites.1–3 Furthermore, it has been reported that Tc values decrease with increasing substitution and that RE123 crystals are no longer superconducting materials when the substitution ratio is more than about 0.5.4–6 Recently, Nd123 and Sm123 crystals fabricated by melt processing under a low oxygen partial pressure atmosphere (OCMG) were reported to exhibit higher Tc values.7–9 It was clarified that the substitution of RE ions was suppressed in a reduced oxygen partial pressure atmosphere, which resulted in higher Tc values.10,11 Therefore, the oxygen partial pressure is found to be an effective parameter to obtain high Tc values as a result of depression of the substitution ratio. However, only a few ternary phase J. Mater. Res., Vol. 12, No. 11, Nov 1997
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diagrams were reported even in air at elevated temperatures where the RE123 phase and the liquid
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