Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-08-17
2001-08-14
Arbes, Carl J. (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C505S431000, C505S433000
Reexamination Certificate
active
06272732
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a superconducting wire and manufacturing method thereof, as well as to an oxide superconducting coil and a cable conductor. More specifically, the present invention relates to an oxide superconducting wire, manufacturing method thereof, an oxide superconducting coil and a cable conductor having high critical current density and low a.c. loss.
2. Description of the Background Art
In recent years, superconducting materials of ceramics, i.e., oxide superconducting materials, are watched with interest due to higher critical temperatures thereof. Among these materials, yttrium, bismuth and thallium oxide superconducting materials which exhibit high critical temperatures of about 90K, 110K and 120K respectively, are expected for practical application.
A single-filamentary oxide superconducting wire having high critical current density formed of such oxide superconducting materials is obtained by heat treating and then covering with a metal sheath the material powder, drawing, rolling and by further heat treatment. Similarly, an oxide superconducting multi-filamentary wire having high critical current density is obtained by heat treating powder mainly consisting of oxide superconducting material, then covering the same with a metal sheath, drawing and putting together the as-obtained wires to provide a multi-filamentary wire, and further by drawing, rolling and heat treating the same.
It has been conventionally known that an oxide superconducting wire having higher critical current density can be obtained by repeating several times the steps of rolling and heat treatment in manufacturing such an oxide superconducting wire.
If such an oxide superconducting wire is to be applied to an a.c. cable or magnet, it must have low a.c. loss, high strength and superior property under bending-strain, in addition to the high critical current.
The single-filamentary and the multi-filamentary oxide superconducting wires manufactured through the conventional method described above have as high a critical density as 30000 A/cm
2
or higher.
However when an a.c. current is applied with the wire wound in a coil, there is generated an a.c. loss heat radiation. This is because an induced current flows between the metal sheath and the ceramics when an a.c. current is applied, resulting in heat radiation caused by a.c. loss of normal conduction resistance of the metal sheath, as compared to the case when a d.c. current is applied, in which case current flows only through the ceramic portions. Since the temperature of the coil as a whole increases, critical current density is decreased.
Accordingly, the operational frequency of the coil manufactured in accordance with the conventional method has been about 0.1 Hz at the highest.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above described problems and to provide a long oxide superconducting wire for a coil or a cable, manufacturing method thereof, an oxide superconducting coil and a cable conductor which have high operational frequency.
Another object of the present invention is to provide a long oxide superconducting wire for a coil or a cable, manufacturing method thereof, an oxide superconducting coil and a cable conductor having high operational frequency and high strength.
A further object of the present invention is to provide a long oxide superconducting wire for a coil or a cable, manufacturing method thereof, an oxide superconducting coil and a cable conductor having high operational frequency and superior property under bending-strain.
According to one aspect of the present invention, an oxide superconducting wire is provided, which wire is a tape-like oxide superconducting wire including a plurality of filaments of oxide superconductor embedded in a matrix, with the filament twisted spirally along the longitudinal direction of the tape wire.
As mentioned above, the filament of oxide superconductor embedded in the matrix is twisted spirally along the longitudinal direction of the wire. Therefore, the induced current flowing between the matrix and the filament is cut at every twist pitch and flows in a small loop, so that the magnitude of the current is limited. As a result, heat radiation caused by a.c. loss of the matrix can be avoided. This function will be discussed in detail with reference to the figures.
FIG. 8
is a perspective view showing a conventional oxide superconducting multi-filamentary wire.
Referring to
FIG. 8
, the multi-filamentary wire is constituted by filaments
11
a
,
11
b
,
11
c
and
11
d
of oxide superconductor embedded in a silver matrix
2
.
When the multi-filamentary wire structured in this manner experiences a change in the magnetic field dB/dt generated, for example, by a coil, there is generated a large induced current loop
13
other than the applied current between filaments
11
a
and
11
b
because of an induced electromotive force, and therefore a large loop current I flows. Accordingly, heat radiation derived from the superconducting resistance of silver matrix
2
increases in proportion to (dB/dt)
2
with the frequency.
By contrast,
FIG. 7
is a perspective view showing an oxide superconducting multi-filamentary wire of one example of the present invention.
Referring to
FIG. 7
, the multi-core wire is constituted by filaments
1
a
,
1
b
,
1
c
and
1
d
of oxide superconductor embedded in silver matrix
2
, with each of the filaments
1
a
,
1
b
,
1
c
and
1
d
twisted spirally along the longitudinal direction of the multi-filamentary wire.
When the multi-filamentary wire structured in this manner experiences the change in the magnetic field dB/dt, the induced current loop
3
is limited by the length L
P
of the twist pitch of the filaments
1
a
and
1
b
. Consequently, the magnitude of loop current I
P
also decreases, and the a.c. loss decreases as the length L
P
of the twist pitch decreases.
Preferably, the pitch of the twist should be at least the width of the wire. This prevents disconnection of the wire during twisting, rolling and drawing.
Preferably, the matrix may be silver or silver alloy, since the matrix of silver or silver alloy can serve as a stabilizer. The a.c. loss mentioned above is in reverse proportion to the resistance value of the matrix. In order to reduce the a.c. loss, it is preferable to provide a matrix having high resistance, by using silver alloy.
According to another aspect of the present invention, an oxide superconducting wire is provided which is similar to the wire of the aforementioned aspect and additionally characterized in that each of the plurality of filaments is covered by silver or silver alloy, and a barrier layer of metal or metal alloy having the resistance value at a room temperature higher than that of silver alloy and in the range of 10
−6
to 10
−10
&OHgr;m is provided to surround one or more filaments covered by silver or silver alloy in the longitudinal direction of the wire.
More preferably, the barrier layer should be formed of metal or metal alloy having the resistance value at a room temperature in the range of from 10
−7
to 10
−9
&OHgr;m.
As described above, a barrier layer of high resistance is provided to surround the filament in the longitudinal direction of the wire is provided. The barrier layer may be provided thin over the surface or at the interface between the oxide superconducting material and the metal sheath before the single-filamentary wires are put together, and as it experiences work hardening during the subsequent steps of drawing, twisting and rolling, disconnection is not caused. In addition, since the barrier material experiences appropriate work hardening, tensile strength and bending strength of the finished tape are improved, and therefore an oxide superconducting wire, which has high strength and high resistance against high electromagnetic stress when it is wound into a coil and current is applied, can be obtained.
According to a still another aspect of the present inventio
Ohkura Kengo
Sato Ken'ichi
Arbes Carl J.
Pennie & Edmonds LLP
Sumitomo Electric Industries Ltd.
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