Oxide superconductor and process for producing same

Details Oxide superconductor and process for producing same Oxide superconductor and process for producing same

2000-03-08

2002-07-02

Dunn, Tom (Department: 1725)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C505S238000

Reexamination Certificate

active

06413624

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide superconductor which is capable of trapping a high magnetic field and maintaining its performance for a long period of time without being affected by internal or external forces such as an electromagnetic force or thermal stress or by corrosive environments, and also to a process for producing said oxide superconductor.
Regarding a superconducting material, which has a high critical current density as compared with an ordinary conducting material and is capable of passing a large electric current without any loss, research and development have vigorously been conventionally carried out on its application in the field of experimental equipment for nuclear fusion, superconductive MRI for medical diagnosis, magnetic levitation trains, electric generators, energy storage units, magnetometers and the like, and since there have been found metal oxide superconducting materials having a relatively high critical temperature (T) such as LiTi
2
O
3
, Ba(Bi,Pb)O
3
and (Ba,K)BiO
3
in recent years and there have successively been created copper oxide superconducting materials having such a relatively high (T) that had never been anticipated before, such as (La,Sr)
2
CuO
4
, REBa
2
Cu
3
O
7
(RE is a rare earth element), Bi
2
Sr
2
Ca
2
Cu
3
O
10
, Ti
2
Ba
2
Ca
2
Cu
3
O
10
and HgBa
2
Ca
2
Cu
3
O
8
, the research has been spurred more.
A superconducting material has a high critical current density as compared with an ordinary conducting material, and thus is capable of passing a large electric current without any loss. However, it is known that in the case of passing such a large electric current, a material is sometimes destroyed, depending upon its strength, since a large electromagnetic force acts on the superconductor in question.
Accompanying the enhanced characteristics and large scale operation of relatively high temperature bulk superconductors (particularly, copper oxide superconductor), the magnitude of a magnetic field capable of being trapped in a bulk superconductor has recently been drastically enhanced, for instance, to the extent that a magnetic flux density exceeding 5 tesla (T) has come to be trapped (refer to “Superconductor Science and Technology” 11, 1998, pp. 1345 to 1347). Since the electromagnetic force applied to a superconductor increases with an increase in a trapped magnetic field, there has recently been brought about such a situation in that a restriction is imposed on a trapped magnetic field depending upon a material's strength. Under such circumstances, importance is attached to an improvement in mechanical properties rather than a further improvement in superconducting properties (refer to “Physica C” vol. 7, No. 9, 1991, pp. 4989 to 4994).
It being so, the following two proposals have been made as a means for reinforcing an oxide bulk superconductor.
One proposal includes a method in which Ag is added to a material in question. It is said that by taking such a measure, a remarkable improvement is brought about in the mechanical strength of an oxide bulk superconductor (refer to “Japanese Journal of Applied Physics” vol. 70, No. 9, 1991, pp. 4989 to 4994 and “Superconductor Science and Technology” 11, 1998, pp. 1345 to 1347).
The other proposal includes a method in which a compression strain is applied in advance to a material in question by fitting a bulk superconducting material with a metallic ring (refer to “Extended Abstract of ISTEC International Workshop” 1998, pp. 115 to 118). It is said that by taking such a measure, an improvement is brought about in the trapped magnetic field, since the tensile stress caused at the time of trapping the magnetic field is alleviated by the compression strain which was applied in advance, thereby suppressing the destruction of the material.
Nevertheless, the above-mentioned methods including the reinforcement with Ag addition and reinforcement with a metallic ring are desired to make further improvements in the aspects of workability and manufacturing cost. Moreover, the problem has been recognized in that the reinforced performance is deteriorated by long-term use under a corrosive environment.
SUMMARY OF THE INVENTION
To solve the problem described above, the inventors have researched repeatedly a method for readily providing at a low cost, an oxide superconductor which is capable of sufficiently withstanding internal or external forces, such as a large electromagnetic force or a thermal stress accompanying a sudden rise or drop in temperature at the time of use, and further capable of exhibiting a high trapped magnetic field for a long period of time without being adversely influenced by a corrosive environment and the findings as described as a)-f) hereinafter have been obtained at the midpoint of the research.
Other objects of the present invention will be obvious from the text of this specification hereinafter disclosed.
In these circumstances, intensive research and investigation were accumulated by the present inventors in order to achieve the above-mentioned objects. As a result, novel information and findings as described hereunder have been obtained.
(a) An oxide bulk superconductor is a ceramic in the state of pseudo-single crystal. It is difficult in practice to prevent microcracks or micropores from being internally included during the manufacture thereof.
(b) When such an oxide bulk superconductor is subjected to “a strong mechanical impact”, “thermal impact due to sudden temperature variation”, “a large electromagnetic force (Lorentz force)” or the like, a stress concentration occurs in the aforesaid microcracks or whereby the microcracks or micropores as starting points progress and expand to relatively large cracks.
(c) In the case where the oxide bulk superconductor is exposed for a long time in a corrosive atmosphere containing a large amount of moisture or carbon dioxide gas, the materials for the oxide bulk superconductor deteriorate, or a reaction phase is formed resulting in the generation of new cracks, which progress and expand to relatively large cracks.
(d) The aforesaid relatively large cracks, when being formed, inhibit the flow of the superconductive current, thus greatly decreasing the trapped magnetic field.
(e) However, even if an oxide bulk superconductor is one which has been believed that there is by no means any possibility of internal permeation of a coating material or the like because of an extremely high density due to its generally being produced by a melting method, contact with a resin in liquid form in an atmosphere of reduced pressure enables said superconductor to maintain a high trapped magnetic field. This is due to the mechanism that the resin permeates not only into the microcracks having openings on the surface, but also into the whole surface layer and, further, the inside of the bulk body through the microcracks having openings, whereby the corrosion resistance of the surface is markedly improved and besides, the mechanical strength of the bulk superconductor itself is drastically enhanced, thereby suppressing, to the utmost, internal forces, external stresses and the propagation of cracking due to corrosion.
(f) In addition, since there is not recognized at all a deterioration, due to the resin impregnation, of the superconductivity characteristics of the bulk body matrix, the above-mentioned method is an extremely advantageous means for improving the mechanical properties and corrosion resistance, while maintaining the excellent superconductivity characteristics of the oxide superconductor.
Such being the case, on the basis of the above-mentioned information and findings, the present inventors previously proposed an oxide superconductor which comprises an oxide superconductive bulk body (for instance, a copper oxide superconductive bulk body containing at least one rare earth element) having a resin-impregnated layer, as an oxide superconductor which is minimized in the generation of cracks due to an external force or internal stress, is hardly adversely influ

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