Method of preparing an oxide superconducting conductor

Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors

Reexamination Certificate

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C029S599000, C505S431000, C505S887000

Reexamination Certificate

active

06215072

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide superconducting conductor, and more particularly, it relates to an oxide superconducting conductor having a high critical current density.
2. Description of the Background Art
In recent years, ceramic superconducting materials, i.e., oxide superconducting materials, exhibiting higher critical temperatures are watched with interest. In particular, yttrium, bismuth and thallium oxide superconducting materials exhibit high critical temperatures of about 90 K, 110 K and 120 K respectively, to remain in superconducting states under temperatures higher than the liquid nitrogen temperature. Thus, such oxide superconducting materials are expected for practical application to high temperature superconducting materials with cooling media of liquid nitrogen.
In relation to practical use of such an oxide superconducting material, study is now being made on application to a superconducting cable which is cooled with liquid nitrogen, for example. When a superconducting cable consisting of an oxide superconducting material is put into practice, it is possible to simplify a thermal protection system and to reduce the cooling cost since this cable requires no cooling with high-priced liquid helium, dissimilarly to a conventional superconducting cable utilizing a metal superconducting conductor.
The inventors have satisfied in development of an oxide superconducting wire which is excellent in bendability by bringing a superconductor into a multifilamentary state with silver, for example, as an exemplary oxide superconducting conductor which is applied to such a superconducting cable. They have discovered that it is possible to obtain a flexible oxide superconducting conductor having a high critical current density by assembling a plurality of such silver-covered superconducting multifilamentary wires on a flexible pipe which serves as a core called a former.
In an oxide superconducting conductor obtained in the aforementioned manner, however, there still remains a room to be improved in view of the critical current density. An oxide superconducting conductor which is applied to a cable or the like must have a higher critical current density.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an oxide superconducting conductor having a higher critical current density and a superconducting cable utilizing the oxide superconducting conductor, thereby solving the aforementioned problem.
According to an aspect of the present invention, provided is an oxide superconducting conductor consisting of a plurality of metal-covered multifilamentary superconducting strands which are assembled with each other. The present invention is characterized in that bending is applied to this superconducting conductor for improving its critical current density.
In general, an oxide superconductor consisting of a ceramic has been recognized as being extremely weak against bending also in a conductor state. Thus, it has been considered indispensable for practicalization of a superconducting cable to implement a superconducting conductor which is resistant against bending and provided with a high critical current density.
Through their particular experiments, the inventors have discovered that it is possible to obtain a flexible oxide superconducting conductor having a high critical current density as described above by assembling a plurality of metal-covered multifilamentary superconducting strands on a former.
In each of these experiments, the inventors temporarily bent the conductor, thereafter returned the same to a linear state and dipped the same in liquid nitrogen, in order to measure the critical current density of the superconducting conductor in a bent state.
On the other hand, the inventors made a new experiment by dipping a conductor which was maintained in a bent state in liquid nitrogen, for measuring its critical current density. In this case, improvement of about 10% was recognized in the critical current density as compared with the value which was measured in a linear state. Namely, the inventors have discovered through this experiment that the critical current density of a superconducting conductor consisting of an oxide superconductor is improved in a bent state as compared with that in a linear state, so long as the superconducting conductor is bendable. The present invention utilizes such an effect.
The superconducting conductor according to the present invention is characterized in that bending is applied to the same. Thus, its critical current density is improved as hereinabove described.
According to the present invention, the bending which is applied to the superconducting conductor is preferably at least 0.5 m in radius of curvature, more preferably at least 1.0 m and not more than 3.0 m in radius of curvature, in order to attain effective improvement of the critical current value.
The metal for covering the multifilamentary superconducting strands which are employed in the present invention is preferably prepared from silver or a silver alloy. The oxide superconductor, which may conceivably be prepared from a bismuth, thallium or yttrium superconductor, is preferably prepared from a bismuth oxide superconductor, in consideration of easiness of elongation, the high critical current density and the like. Further, the filament number of each multifilamentary strand is at least 7 and not more than 10,000.
When wires are spirally wound on the former in the present invention to be assembled with each other, it is preferable to wind the wires in a plurality of layers while reversing winding directions of the layers, in order to improve adhesion between tape-shaped superconducting multifilamentary wires by winding an insulating material. The most conspicuous effect is brought about when DC current is supplied to the super conducting conductor and this will be the most effective means in applying the same to the DC superconducting cable.
Preferably, an insulating material having a coefficient of heat contraction of at least twice that of the superconducting wires is wound on the surface of the superconducting conductor. Thus, the conductor is radially compressed in cooling, whereby adhesion between the superconducting wires is improved for improving the critical current density of the conductor.
According to the present invention, the insulating material which can radially compress the conductor in cooling must have a coefficient of heat contraction of at least twice, preferably at least five times that of the superconducting strands as employed. For example, a silver-covered Bi superconducting multifilamentary wire exhibits a coefficient of heat contraction of about 0.2% when the same is cooled from the room temperature to the liquid nitrogen temperature. Therefore, the insulating material is preferably prepared from a PPLP paper (polypropylene laminate paper), a PE film (polyethylene film), EP rubber (ethylene propylene rubber), a PE solid insulator (polyethylene solid insulator) or the like, which has a coefficient of heat contraction of at least 1%.
According to another aspect of the present invention, provided is an oxide superconducting cable which is formed by stranding a plurality of superconducting conductors. Each of the superconducting conductors consists of a plurality of metal-covered multifilamentary strands which are assembled with each other, while bending is applied thereto for improving its critical current density.
Namely, the superconducting cable according to the present invention is formed by a plurality of superconducting conductors which are stranded with each other. Due to such a stranded structure employed in preparation of the superconducting cable, bending is applied to the conductors for improving the critical current density of the superconducting cable.


REFERENCES:
patent: 3838503 (1974-10-01), Suenaga et al.
patent: 4377905 (1983-03-01), Agatsuma et al.
patent: 5098178 (1992-03-01), Ortabasi
patent: 1 813 397 (1970-06-01), None
patent: 23 45 779 (1974-

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