Unidirectional Partial Melting and Solidification of SmBCO Superconductor
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Unidirectional partial melting and solidification of SmBCO superconductor M. Sumida Department of Metallurgy, Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113, Japan
Y. Nakamuraa) and Y. Shiohara Superconductivity Research Laboratory – International Superconductivity Technology Center, 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 20 June 1996; accepted 22 January 1997)
Microstructure control of the SmBCO superconductor was carried out using the floating zone partial melting and solidification method. It is generally recognized that finely and uniformly dispersed nonsuperconductive high temperature stable phase (Sm211) particles included in the superconductive Sm123 matrix act as effective pinning centers. Microstructure formation of the partial molten mixture (Sm211 particles and BaO–CuO liquid) by decomposition of the precursor Sm123 on melting and solidification of Sm123 from the mixture have to be controlled concurrently to fabricate the 123y211 composite fiber with the optimum microstructure. During unidirectional solidification, planar crystal growth which provides the single crystal growth of Sm123 becomes unstable with increased growth rate. During unidirectional melting, the mean diameter of aligned Sm211 particles behind the melting interface decreases with increased growth rate and with decreased temperature gradient at the melting interface. Initial composition of the precursor significantly affects the formation behavior of Sm211 particles. The contribution of process parameters to the microstructure formation is also briefly discussed.
I. INTRODUCTION
Many rare earth 123 type superconductors (RE1 Ba2 Cu3 O72d : RE123, RE: rare earth metals, Sm, Nd, Yb . . . ) have been discovered since the first YBa2 Cu3 O72d (Y123, hereafter) was identified. To widen the applications of bulk high T c superconductors at liquid nitrogen temperature, much effort has been devoted to materials processing. As contamination by impurity elements greatly reduces superconducting properties,1,2 the Floating Zone melting method (FZ method) is an advantageous technique since it needs no crucible and it is possible to fabricate a large bulk material. To fabricate bulk high T c superconductive materials with high critical current density (J c ), it is necessary to introduce pinning centers into the interior of the superconductive phase crystal grain. The following defects and/or inclusions are known to act as pinning centers at the mixed state, oxygen defects, dislocations, stacking faults, second a)
Present address: Railway Technical Research Institute, 2-8-38, Hikari-cho, Kokubunji, Tokyo, 185, Japan. J. Mater. Res., Vol. 12, No. 8, Aug 1997
phase particles, and grain boundaries. Furthermore, due to the weak-link electric characteristics of high angle grain boundaries which is due to the shor
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