Superconductor technology: apparatus – material – process – High temperature – per se – Free metal containing
Reexamination Certificate
2000-08-25
2002-04-02
Kopec, Mark (Department: 1751)
Superconductor technology: apparatus, material, process
High temperature , per se
Free metal containing
C505S236000, C505S450000, C505S729000, C505S785000
Reexamination Certificate
active
06365553
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxide superconductor having an excellent crack-resistant property, capable of inhibiting occurrence of cracks due to internal or external force such as electromagnetic force, thermal stress, and so forth, and maintaining a high trapped magnetic field over a long period of time, and also to a process of producing the same.
The oxide superconductor according to the invention can be put to a wide variety of applications such as a magnetic levitation train, an energy storage system, and so forth, and is expected to play an important role in enhancement of performance of apparatuses such as those mentioned above.
2. Description of the Related Art
Since there have been found metal oxide superconducting materials having a relatively high critical temperature (Tc), such as LiTi
2
O
3
, Ba(Bi, Pb)O
3
and (Ba, K)BiO
3
in recent years, the research has been spurred more and there have successively been created copper oxide superconducting materials having a relatively high critical temperature (Tc) 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
O10, TiBa
2
Ca
2
Cu
3
O
10
and HgBa
2
Ca
2
Cu
3
O
8
.
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 a superconductor in question.
Particularly, accompanying the enhanced characteristics and large scale operation of relatively high temperature bulk superconductors (particularly, copper oxide superconductors) prepared by a melting process, 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. Since an 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 the improvement in mechanical properties rather than a further improvement in superconducting properties.
Further, although it is necessary that the oxide superconductor is cooled to temperatures (at least less than the temperature of liquid nitrogen) to allow the foregoing oxide superconductor to exhibit intended characteristics, the thermal shock generated at that time in the case of oxide bulk superconducting materials becomes a major cause of the occurrence of cracks.
It being so, the following two proposals have been made as a means for reinforcing an oxide bulk superconductor prepared by a melting process.
One proposal includes a method of preparing an oxide bulk superconductor from a row material to which Ag is added. It is said that the oxide bulk superconductor obtained by this method exhibits an excellent strength (e.g., “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 bulk superconducting material by fitting the bulk superconducting material with a metallic ring at the outer periphery (refer to “Extended Abstract of ISTEC International Workshop” 1998, pp. 115 to 118). It is said that by taking this method, an improvement is brought about on 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.
However, according to the method of reinforcing the superconductor by adding Ag, it is disadvantageous in cost, and it is impossible to dispel fears that the crack-prevention effect is not sufficient when the material is put to a use where a large force is applied to the material and also it was very difficult to restrain cracks from occurring when the material is used over a long period of time. to restrain cracks from occurring when the material is used over a long period of time.
Meanwhile, according to the above mentioned method of fitting the material with a metallic ring at the outer periphery, it takes unexpected labor for fitting the material with a metallic ring, and the surface of the material to be fitted with the metallic ring is restricted because it is difficult to cover the entire surface of the material with the metallic ring, and hence it was difficult to obtain a sufficient intended crack resistance or prevention effect.
The object of the invention, taking into consideration of the foregoing, is to establish a method for readily providing at a low cost, an oxide superconductor prepared by a melting process capable of sufficiently withstanding an external force or inner stress 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 over a long period of time.
SUMMARY OF THE INVENTION
The inventors have conducted intensive studies in order to achieve the object described above, and as a result, the following novel information has been obtained.
(a) An oxide bulk superconductor prepared by a melting process is a ceramic in the state of a pseudo-single crystal, however, it is practically difficult to prevent microcracks and pores from being formed therein in a process of preparing the same. In particular, there is a tendency of microcracks and pores being formed in a subsurface layer thereof. formed in a subsurface layer thereof.
(b) When such an oxide bulk superconductor is subjected to “a large mechanical impact force, a large thermal impact force due to sudden change in temperature, a large electromagnetic force”, and so forth, stress concentration occurs in the microcracks and the pores as described above, whereupon relatively large cracks which are propagated from the microcracks and the pores as starting points are developed.
(c) With an oxide bulk superconductor, once the relatively large cracks as described above are developed, this will inhibit flow of superconductive current, resulting in a large decrease in a trapped magnetic field value.
(d) However, when an oxide bulk superconductor is brought into contact with a low melting metal (low melting metal having a melting point at about 200° C. or lower, or at about 300° C. or lower) in molten state in an atmosphere of reduced pressure, this will cause the low melting metal to be smoothly permeated inside the oxide bulk superconductor through the microcracks and the pores formed in the surface thereof even if the oxide bulk superconductor is one produced by a melting method, having a high density of constituent material, thus resulting in occurrence of a phenomenon wherein the microcracks and the pores inside the oxide bulk superconductor are filled up with the low melting metal. With the oxide bulk superconductor having the microcracks and the pores formed therein, filled up with the low melting metal as described above, occurrence of stress concentration in regions of the microcracks and the pores is alleviated, and propagation of cracks starting from the microcracks and the pores can be inhibited, so that a situation where there occurs a large degradation in the trapped magnetic field can be avoided even if “a large mechanical impact force, a large thermal impact force due to sudden change in temperature, a large electromagnetic force”, or so forth is applied thereto.
(e) In this case, even if impregnation with the low melting metal does not reach the center of the oxide bulk superconductor, staying in a l
Murakami Masato
Tomita Masaru
Flynn ,Thiel, Boutell & Tanis, P.C.
International Superconductivity Technology Center
Kopec Mark
LandOfFree
Oxide superconductor having excellent crack-resistant... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Oxide superconductor having excellent crack-resistant..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Oxide superconductor having excellent crack-resistant... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2916550