Metal fusion bonding – Process – Critical work component – temperature – or pressure
Reexamination Certificate
2000-11-30
2003-05-13
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Critical work component, temperature, or pressure
C228S262200, C228S246000, C228S248100, C029S599000, C505S917000, C505S918000, C505S919000, C505S920000, C505S925000, C505S926000, C505S927000
Reexamination Certificate
active
06561412
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention relates to a method of joining together oxide superconductors for producing an oxide superconductor joined member having excellent electric current transmission performance, enabling electric current transmission to be effected at high efficiency, and to the oxide superconductor joined member having excellent electric current transmission performance, so that electric current lead wires and superconducting wire rods, excellent in electric current transmission performance, can be produced.
2. Description of the Related Art
There have recently been discovered oxide superconductors having high critical temperatures such as, for example, LiTi
2
O
3
, Ba(Bi, Pb)O
3
, (Ba, K) BiO
3
, (La, Sr)
2
CuO
4
, REBa
2
Cu
3
O
7
(RE refers to rare-earth elements), Bi
2
Sr
2
Ca
2
Cu
3
O
10
, Ti
2
Ba
2
Ca
2
Cu
3
O
10
or HgBa
2
Ca
2
Cu
3
O
8
and so forth, one after another. As a result, commercial application of these materials to powerful superconducting magnets, fly wheels, superconducting magnetic bearings, and so forth, in various sectors of the industry, has been under study.
Among these oxide superconductors, a RE 123-type oxide crystal, that is, a “RE Ba
2
Cu
3
O
y
oxide superconductor (RE refers to one kind of rare-earth element, or not less than two kinds of rare-earth elements, selected from the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb)” not only has a high critical temperature but also has come to attain a high critical current density in a magnetic field due to development of, and improvement on manufacturing techniques, so that the same is one of superconductors attracting most attention lately.
As one of the most common methods of preparing such an oxide crystal (bulk crystal) as described above, there has been well known a “melt-solidifying process” whereby a molten oxide material (crystal precursor material) is very slowly cooled from the neighborhood of a solidification starting temperature, thereby promoting solidification, and causing crystal growth to take place.
That is, the melt-solidifying process is a process utilizing solidification from a half- molten state, whereby a raw material for the bulk crystal from which the constitution of an RE 123-type oxide superconductor can be obtained normally in the atmosphere or in an atmosphere containing oxygen at low partial pressure is heated to not lower than a temperature (a peritectic point) at which a “123 phase (referred to hereinafter as RE 123 phase) of an RE Ba
2
Cu
3
O
x
oxide to be prepared” undergoes decomposition and melting, thereby causing the raw material to be decomposed and melted, and thereafter, is subjected to slow and gradual cooling at a temperature gradient or in an isothermal state, or is maintained in a supercooled temperature zone where growth of the RE 123 phase can take place, thereby promoting growth of the bulk crystal. In this case, a method of dipping a seed in a melt is commonly adopted in order to determine an orientation of the crystal to be grown.
And with the use of the melt-solidifying process, a RE 123 phase crystal (bulk crystal) of a large size can be stably obtained with relative ease.
Further, a “super cooled melt-solidifying process” aiming at shortening a crystal growth time has been well known as a method of preparing an oxide superconductor crystal (refer to Japanese Patent Laid-open No. H 6-211588).
The super cooled melt-solidifying process is a process whereby a molten precursor material is supercooled in as-molten state or as-half-molten state to a temperature zone below a solidification temperature, and is subjected to gradual cooling from a temperature reached as above or is kept at the temperature reached, thereby implementing crystal growth, and is a process intended to enhance a crystal growth speed by supercooling.
However, the oxide superconductor crystal (bulk crystal), obtained by those processes described above, and so forth, has been found to be material which is brittle and lacking in plasticity, and consequently, it has been difficult to form the same into a current lead wire, wire rod, and the like, of long length, by taking advantage of plasticity.
As a result, for production of the current lead wire, wire rod, and the like, of long length, with the use of the oxide superconductor crystal, it is necessary to produce “oxide superconductor rods” and “a conductive material coated with an oxide superconductor” by joining together a plurality of the oxide superconductor crystals.
As conventional means of joining together oxide superconducting crystals, there has been known a process of joining together oxide superconductor base materials by use of a solder (soldering material) of an oxide superconductor constitution, having a melting point (peritectic point) lower than that of the oxide superconductor base materials.
For example, in the text “Advances in Superconductivity VII”, pp. 681 to 684, published by Springer-Verlag Tokyo Co., 1995, there is shown a joining process whereby a powdered solder of Yb 123 superconductor material (Yb
1.2
Ba
2.1
Cu
3.1
O
y
) constitution, having a melting point (peritectic point) lower than that of base materials of Y 123-type superconducting bulk crystal (Y
1.8
Ba
2.4
Cu
3.4
O
y
) as produced by the melt-solidifying process, is sandwiched between the base materials, is heated up to an intermediate temperature between the melting point of the base materials and that of the solder, thereby rendering the solder in as half-molten state, and thereafter, is subjected to gradual cooling with the result that a crystal (Yb 123 crystal) of a soldering material is caused to make epitaxial growth on the surface of the respective base materials, and through the intermediary of the solder as crystallized, the base materials are joined together.
FIG. 1
is a schematic view illustrating a manner whereby the powdered solder of Yb 123 superconductor material constitution(that is, Yb211+Ba
3
Cu
5
O
x
) is held in a sandwich-like fashion between the base materials of the Y 123-type superconducting bulk crystal, and these materials are then placed in a heating furnace for joining together the base materials through heating and gradual cooling. In this case, heating and gradual cooling of the materials are carried out in the heating furnace according to a heating and cooling curve shown in
FIG. 2
by way of example.
Now, in the case of superconducting current lead wires, superconducting wire rods, and the like, current density for the whole length thereof is determined depending on that of a portion thereof in the longitudinal direction, where superconductive properties are at the lowest level. Accordingly, adjustment of the material thereof is required so as to be able to obtain a uniform superconductive properties throughout the whole length thereof. However, since it is extremely difficult to homogenize the superconductive properties at a junction of the material, the superconductive properties at the junction becomes extremely important in the case of the superconducting current lead wires, and superconducting wire rods.
Not with standing the above, it has been found that the following problem is inherent in the above-described process of joining together the superconducting bulk crystals, whereby base materials of a Y 123 superconducting bulk crystal are joined together by sandwiching a solder of Yb 123 superconductor constitution therebetween.
More specifically, when the base materials with the solder sandwiched therebetween are subjected to heating and gradual cooling, crystallization of a Yb 211 phase of the solder, which is turned into a half-molten state, gradually proceeds from the surface of the respective base materials of a Y 123 phase towards the center of the solder, and consequently, a finally solidified portion of the solder, in the form of a layer, naturally comes to reside at the central position of the thickness of the solder interposed between the base materials.
Meanwhile, since there exist a non-superconducting BaO—CuO melt, and the Yb 211 p
Izumi Teruo
Maeda Jyunya
Seiki Susumu
Shiohara Yuh
Dunn Tom
Edmondson L.
Flynn ,Thiel, Boutell & Tanis, P.C.
Superconductivity Research Laboratory
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