Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – Superconductor next to superconductor
Patent
1994-09-30
1998-07-28
Yamnitzky, Marie
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
Superconductor next to superconductor
505450, 505451, 505490, 505491, 505500, 505701, 505704, 505927, 428701, 428702, 428930, H01L 3912, H01L 3924
Patent
active
057863040
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a REBa.sub.2 Cu.sub.3 O.sub.x -based superconducting joined product having a high critical current density and a process for producing the same.
BACKGROUND ART
REBa.sub.2 Cu.sub.3 O.sub.x -based superconductors have a high critical temperature exceeding the liquid nitrogen temperature and hence can offer a great economical benefit when they could be put to practical use. In these superconductors, however, a crystal grain boundary serves as a weak link, so that a high superconducting current cannot be flowed across the crystal grain boundary and, in particular, the critical current density is remarkably lowered in a magnetic field. For this reason, the practical use of the REBa.sub.2 Cu.sub.3 O.sub.x -based superconductors could have not been realized in the field of heavy current.
The production of a REBa.sub.2 Cu.sub.3 O.sub.x -based superconducting bulk material comprising large crystal grains having a volume of not less than 50 cm.sup.3 has become possible (see M. Morita. et al: Advances in superconductivity III, Springer-Verlag, Tokyo, 1990, pp. 733-736) by the melt process, which is a process for producing an oxide superconducting material, including the QMG process (quench and melt growth process) (see U.S. patent application Ser. No. 07/735,105, now U.S. Pat. No. 5,278,137, or Japanese Unexamined Patent Publication (Kokai) No. 2-153,803). The melt process basically comprises heating a starting material for a REBa.sub.2 Cu.sub.3 O.sub.x -based superconductor to a semi-melted state wherein a RE.sub.2 BaCuO.sub.5 phase is present together with a liquid phase composed mainly of Ba, Cu and O; and gradually cooling the system from just above the peritectic temperature to conduct crystal growth of the REBa.sub.2 Cu.sub.3 O.sub.x phase. Large grains prepared by this process contains a small angle grain boundary having a crystal misorientation of several degrees but no grain boundary having a large angle which serves as a weak link. For this reason, in this material, the critical current density within the grains is as high as 10000 A/cm.sup.2 or more at 77 K and 1 T, so that the material is considered to be applicable as materials for superconducting coils, bulk magnets, magnetic shields and current leads.
In order that the REBa.sub.2 Cu.sub.3 O.sub.x -based superconducting bulk material prepared by the melt process can be used as these materials, it is necessary to increase the size, area or length. Since, however, the melt process is basically a technique for producing an oxide superconductor having no grain boundary by gradual cooling from the melted state, there is a limitation on an increase in size. Further, the material produced by the melt process cannot be subjected to working which breaks crystal grains, such as rolling and bending. Therefore, it is considered that if it becomes possible to prepare a junction having a high critical current density, an increase in size of the material and the shaping become easy, which enables the material to be used in the above applications.
Joining methods for an oxide superconductor, which have been reported in the art, include a method which utilizes solid phase diffusion and a method wherein a liquid phase is formed to conduct joining. Most of these joining methods are intended for use in joining between sintered bodies, and no consideration is taken for the prevention of the formation of a grain boundary in the junction. Therefore, the formation of a grain boundary in the junction is unavoidable. In this case, the crystal grain boundary serves as a weak link, and no high critical current density can be obtained. Further, also when the material is used as a magnetic shielding material, no good shielding property can be obtained because the grain boundary permits a magnetic field to penetrate. No joining process has hitherto been reported which is intended for use in a material free from a grain boundary in the junction and having a high critical current density. Specific examples of the convention process w
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Hashimoto Misao
Kimura Keiichi
Miyamoto Katuyoshi
Nippon Steel Corporation
Yamnitzky Marie
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