U.S. patent application number 10/455415 was filed with the patent office on 2003-11-06 for oxide superconducting wire, solenoid coil, magnetic field generating apparatus, and process for production of oxide superconducting wire.
Invention is credited to Fukushima, Keiji, Okada, Michiya, Tanaka, Kazuhide, Wakuda, Tsuyoshi.
Application Number | 20030205403 10/455415 |
Document ID | / |
Family ID | 18329324 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030205403 |
Kind Code |
A1 |
Tanaka, Kazuhide ; et
al. |
November 6, 2003 |
Oxide superconducting wire, solenoid coil, magnetic field
generating apparatus, and process for production of oxide
superconducting wire
Abstract
The cross section of a wire is round and is composed of several
units, each consisting of tape-like superconductors laminated in an
approximately rhombic shape, which are arranged such that they form
a hexagon as a whole. Oxide superconducting tape wires each
consisting of a plurality of oxide superconducting filaments are
arranged in rotational symmetry to a core. The oxide
superconducting filaments have the cross section such that the
average thickness is 3 to 20 .mu.m and the average aspect ratio is
larger than 2 and smaller than 10. A step of arranging the oxide
superconducting tape-like wires in rotational symmetry is
accomplished when the multi-core tape-like wires are packed in a
third metal pipe which becomes a metal sheath later. Since the
multi-core tape wires having oxide superconducting filaments are
arranged in rotational symmetry, the oxide superconductor in the
oxide superconducting filaments permits its c axis to orient in
various directions. This makes it possible to prevent the critical
current from decreasing irrespective of the direction in which the
magnetic field is applied and to increase the critical current
density (Jc) because the oxide superconducting filament has an
optimal size. The oxide superconductor should be a bismuth-based
oxide superconductor, preferably be the one which has a composition
of Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x.
Inventors: |
Tanaka, Kazuhide; (Hitachi,
JP) ; Okada, Michiya; (Mito, JP) ; Fukushima,
Keiji; (Hitachi, JP) ; Wakuda, Tsuyoshi;
(Hitachi, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-9889
US
|
Family ID: |
18329324 |
Appl. No.: |
10/455415 |
Filed: |
June 6, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10455415 |
Jun 6, 2003 |
|
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09555222 |
May 26, 2000 |
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6591120 |
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Current U.S.
Class: |
174/125.1 ;
257/E39.018 |
Current CPC
Class: |
H01B 12/06 20130101;
Y02E 40/60 20130101; Y10S 505/705 20130101; H01L 39/143 20130101;
Y10T 29/49014 20150115 |
Class at
Publication: |
174/125.1 |
International
Class: |
H01B 012/00 |
Claims
1. An oxide superconducting wire which is characterized in that the
wire has an approximately round cross section perpendicular to its
lengthwise direction, the cross section is composed of several
units, each unit being composed of a plurality of elements in the
form of tape-like oxide superconductors which are laminated
stepwise on top of each other in the direction perpendicular to the
lengthwise direction at an angle of about 60 degrees with respect
to the element surface within said cross section, said unit having
an approximately rhombic shape within said cross section, said
cross section having at least three different units which are
arranged such that adjacent units have a rotational symmetry at
about 120 degrees with respect to the direction of tape lamination
and at least one side of a unit of rhombic shape is in contact with
an adjacent unit.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
09/555,222, filed May 26, 2000.
TECHNICAL FIELD
[0002] The present invention relates to an apparatus for generating
a strong magnetic field by utilizing an oxide superconductor. More
particularly, the present invention relates to a new oxide
superconducting wire which is suitable for those apparatuses which
need a strong, uniform magnetic field, such as scientific
instruments, NMR analyzers, and medical MRI apparatus. It relates
also to a solenoid coil and a magnetic field generating apparatus
with said oxide superconducting wire and a process for producing
said oxide superconducting wire.
BACKGROUND OF THE INVENTION
[0003] Superconducting magnetic apparatuses generally find use in
two fields: one in which no consideration need be given to the
uniformity and stability of the magnetic field so long as a
magnetic field is generated, and the other in which the quality of
the magnetic field (or the uniformity and stability of the magnetic
field) is a matter of important concern. A highly uniform, highly
stable magnetic field is usually required for research work, such
as measurements of physical properties and magnetic field
generating apparatuses for medical use. It is essential for the
technology of high-quality superconducting magnets. Such
superconducting magnets have conventionally been produced with
accurately worked wires made of a metal superconductor, such as a
niobium-titanium superconductor and a niobium-tin superconductor.
Superconducting wires are wound into a solenoid under stringent
quality control. The resulting superconducting magnet is run with
an extremely stable power supply or in a permanent current mode.
The thus generated magnetic field is highly stable time-wise and
space-wise. Recent years have seen a remarkable advance in magnets
generating a highly uniform magnetic field. Magnets capable of
generating as high an intensity as 20T have appeared to meet the
need for a NMR apparatus. Unfortunately, the intensity of the
magnetic field that can be generated by the conventional metal
superconductor is limited to about 20T on account of the critical
magnetic field inherent in the material used. In order to exceed
this limit, it is essential to resort to an oxide
superconductor.
[0004] A magnetic field generating apparatus has been developed
with an oxide superconductor in which the coil is formed by winding
a tape-like wire into a double pancake shape, as reported in
Journal of Applied Physics, Vol. 35, 1996, Part 2, L623 to 626.
This coil is suitable for generating a magnetic field stronger than
22T, which has never been attained with conventional metal
superconductors. However, this coil suffers the disadvantage of
being unable to generate a uniform magnetic field. The reason for
this it that the conductor is in the form of a tape, and the tape
is wound into a coil of pancake shape in such a way that more than
one coil is placed on top of the other. One way to address this
problem is to use a conductor with a round cross section and wind
it into a solenoid coil for the magnet, as reported in Journal of
Material Science, Vo. 30, 1995, pp. 3200 to 3206. Such a conductor
generates a comparatively strong, uniform magnetic field; however,
it cannot generate a desired magnetic field because its critical
current density (Jc) is only one-fifth that of the tape-like wire.
It is possible to secure a strong magnetic field easily by winding
a tape-like wire into a solenoid, but it is difficult to secure a
sufficiently uniform magnetic field in this way on account of the
outer dimensional accuracy of tape-like wire.
[0005] A wire with a round cross section is usually produced by
drawing a metal through a die, and it has a higher accuracy (of the
order of microns) compared with a tape-like wire. Therefore, such a
wire is suitable for the generation of uniform magnetic fields. By
contrast, a tape-like wire is produced by rolling, and its working
accuracy (thickness and width) is usually limited to the order of
10 microns. Therefore, when it is wound in thousands of turns, the
number of turns varies from one place to another because of the
uneven working accuracy. This results in an uneven magnetic field.
For this reason, there has been a demand for an oxide
superconducting wire which meets requirements for both high current
density and mechanical working accuracy so that it generates a
strong, uniform magnetic field.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide an oxide
superconducting wire which has high dimensional accuracy and
generates a strong, uniform magnetic field. It is another object of
the present invention to provide a solenoid coil formed from the
oxide superconducting wire. The term "uniform magnetic field" means
that the intensity of the magnetic field varies less than 0.1%,
preferably less than 0.01%, and the term "strong magnetic field"
means that the intensity of the magnetic field is higher than 22T.
In the past, it was difficult to generate such a strong, uniform
magnetic field; it was only possible to achieve it by means of a
magnet formed from an oxide superconducting wire.
[0007] The basic reason for this is that an oxide superconductor is
a greatly anisotropic substance. When it is made into a tape-like
wire by rolling, the resulting wire permits a coil current density
of about 50 to 100 A/mm.sup.2 in a magnetic field stronger than
10T. However, when it is made into a round wire by drawing, the
resulting wire permits a current density of only 10 to 20
A/mm.sup.2.
[0008] An object of the present invention is to provide an oxide
superconducting wire which has high dimensional accuracy and
generates a strong, uniform magnetic field with a high critical
current density. Another object of the present invention is to
provide a solenoid coil and a magnetic field generating apparatus
formed from the oxide superconducting wire.
[0009] According to the present invention, it is possible to
produce an oxide superconducting wire which has high dimensional
accuracy without a decrease in current density. The first aspect of
the present invention is directed to an oxide superconducting wire
which is characterized in that the wire has an approximately round
cross section perpendicular to its lengthwise direction, the cross
section is composed of several units, each unit being composed of a
plurality of tape-like oxide superconductors, the tapes in each
unit being laminated stepwise on top of the other in the direction
perpendicular to the lengthwise direction at an angle of about 60
degrees with respect to the tape surface within the cross section,
each unit having an approximately rhombic shape within the cross
section, the cross section having at least three different units
which are arranged such that adjacent units have a rotational
symmetry through about 120 degrees with respect to the direction of
tape lamination and at least one side of the rhombic shape of a
unit is in contact with an adjacent unit.
[0010] In other words, the oxide superconducting wire of the
present invention is characterized in that it has a round cross
section perpendicular to its lengthwise direction and the cross
section is composed of oxide cores which are geometrically arranged
at rotationally symmetric positions. This rotational symmetry may
be established by 3 rotations, 4 rotations, or 6 rotations within
the round cross section. For the oxide to be packed most closely,
it is desirable to arrange the cores in triangular symmetry. It
follows, therefore, that the most efficient packing ratio (or the
ratio of the sectional area of the oxide to the total sectional
area) is achieved when three rhombi are arranged in rotational
symmetry, each rhombus being composed of two regular triangles.
[0011] According to the present invention, the oxide superconductor
has a tape-like shape, and these tapes are laminated on top of the
other to form a conductor. However, it is very difficult to obtain
a conductor of ideal configuration because of the limitation
accuracy in tape rolling and tape assembling. In view of this, the
angle of lamination direction should be about 60 degrees or 120
degrees. The closer to ideal the shape is, the better the
performance will be. As the shape departs from the ideal, the
performance of the oxide decreases. The allowance of the angle is
about 5 degrees. If the allowance exceeds this limit, the
performance will decrease to 1/2 to 1/3.
[0012] The second aspect of the present invention is directed to an
oxide superconducting wire with a round cross section which, in its
cross section perpendicular to its Lengthwise direction, is
composed of three units, each unit consisting of laminated
tape-like oxide superconductors, the assembly of the units being
concentrated to form a closest-packed shape at the center of the
cross section, with all the tapes having at least one end thereof
in contact with the sheathing material constituting the periphery
of the wire.
[0013] The above-mentioned rotational symmetry will be satisfactory
so long as it is formed within three adjacent units; thus, it is
not necessary that all the units form the rotational symmetry in
the cross section. Symmetry is essential to prevent the shape from
being disturbed during drawing for isotropic reduction in cross
section. Symmetry with three units is not a must; but it is easiest
to form. The contact with the sheathing material is not a direct
concern of the present invention; it is mentioned here merely from
a geometrical point of view.
[0014] According to the present invention, the tape-like oxide
superconductors may be formed from multi-core wires.
[0015] According to the present invention, the tape-like oxide
superconductors may be multi-core wires which are twisted.
[0016] According to the present invention, the tape-like oxide
superconductors may be multi-core wires which are twisted and the
tapes are arranged with a high-resistance layer interposed between
them.
[0017] According to the present invention, the tape-like oxide
superconductors may be multi-core wires which are twisted and the
tapes are arranged with a high-resistance layer interposed between
them, and the superconducting wire is twisted.
[0018] According to the present invention, the oxide
superconducting wire mentioned above is characterized in that the
oxide superconductor should preferably be
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x. This oxide superconductor
may be replaced by others such as (Bi,Pb).sub.2Sr.sub.2Ca.-
sub.2Cu.sub.3O.sub.x and a thallium-based superconductor.
[0019] Examples of the oxide superconductors that can be used in
the present invention include:
[0020] Bi--Sr--Ca--Cu-0 type:
[0021] Bi.sub.1.5-2.2--Sr.sub.5-2.2--Cu.sub.0.5-1.3-0.sub.5-7,
[0022]
Bi.sub.1.5-1.2--Sr.sub.1.5-2.2--Ca.sub.0.5-1.3--Cu.sub.1.5-2.3-0.su-
b.7-9,
[0023]
Bi.sub.1.5-2.2--Sr.sub.1.5-2.3--Ca.sub.1.5-2.3--Cu.sub.2.5-3.3--O.s-
ub.9-11,
[0024] Bi--Pb--Sr--Ca--Cu-0 type:
[0025]
(Bi.sub.y--Pb.sub.1-y).sub.1.5-2.2--Sr.sub.1.5-2.2--Cu0.sub.0.5-1.3-
--O.sub.5-7,
[0026]
(Bi.sub.y--Pb.sub.1-y).sub.1.5-1.2--Sr.sub.1.5-2.2--Ca.sub.0.5-1.3--
-Cu.sub.1.5-2.3--O.sub.7-9,
[0027]
(Bi.sub.y--Pb.sub.1-y).sub.1.5-2.2--Sr.sub.1.5-2.3--Ca.sub.1.5-2.3--
-CU.sub.2.5-3.3--O.sub.9-11.
[0028] where y=0-1 to 0.9.
[0029] The third aspect of the present invention is directed to an
oxide superconducting wire characterized in that the cross section
of the wire is round and is composed of several units, each
consisting of tape-like superconductors laminated in an
approximately rhombic shape, which are arranged such that they form
a hexagon as a whole. In other words, the oxide superconducting
wire has a round cross section perpendicular to its lengthwise
direction, within the plane of the cross section, the oxide
superconductor is composed of several units, each consisting of a
plurality of tape-like superconductors laminated one over another.
The laminated tapes form an approximately rhombic shape. Within the
plane of the cross section, there are at least three units, which
are rotationally symmetric with adjacent units in the direction of
tape lamination. At least one side of the rhombus is opposite to
the adjacent unit.
[0030] The fourth aspect of the present invention is directed to an
oxide superconducting wire composed of a metal sheath and a core as
an assembly of oxide superconducting filaments, characterized in
that the core is made up of multi-core tape wires, each consisting
of oxide superconducting filaments, which are arranged in
rotational symmetry, the oxide superconducting filaments having a
cross section such that the average thickness is 3 to 20 .mu.m and
the average aspect ratio is larger than 2 and smaller than 10. The
oxide superconducting tape-like wires are arranged in rotational
symmetry. This step is accomplished when the multi-core tape-like
wires are packed in the third metal pipe which becomes the metal
sheath later.
[0031] The fact that the multi-core tape-like wires are arranged in
rotational symmetry offers the advantage that the oxide
superconductor in the oxide superconducting filaments permits its c
axis to orient in various directions. This makes it possible to
prevent the critical current from decreasing irrespective of the
direction in which the magnetic field is applied and to increase
the critical current density (Jc) because the oxide superconducting
filament has an optimal size. The oxide superconductor should be a
bismuth-based oxide superconductor, and preferably be one which has
a composition of Bi.sub.2Sr.sub.2Ca.sub.1Cu.s- ub.2O.sub.x.
[0032] The metal sheath may be formed from silver or a silver
alloy, such that the ratio of the metal sheath to the oxide
superconducting filaments is greater than 3 and smaller than 7.
This makes it possible to increase further the critical current
density (Jc).
[0033] In the case where the oxide superconducting wire has a
rectangular shape, its cross section should have an aspect ratio
greater than 1 and smaller than 6.
[0034] The oxide superconducting wire should be formed from an
oxide superconductor (or a raw material thereof) in the form of
powder having an average particle diameter smaller than 3 .mu.m, so
that it is comparable to the conventional tape-like oxide
superconducting wire in current flow characteristics and can be
formed continuously.
[0035] According to the present invention, the oxide
superconducting wire has better working accuracy compared with the
conventional tape-like oxide superconducting wire. When the oxide
superconducting wire is made into a solenoid, the resulting
solenoid has a smaller deviation (in the axial and circumferential
direction) than the pancake coil formed from the conventional
tape-like oxide superconducting wire. Therefore, such a solenoid
coil can produce a strong, uniform magnetic field.
BRIEF DESCRIPTION OF DRAWINGS
[0036] FIG. 1 is a diagram showing the cross section of an oxide
superconducting wire representing a first example of the present
invention;
[0037] FIG. 2 is a diagram showing the cross section of the oxide
superconducting wire representing a second example of the present
invention;
[0038] FIG. 3 is a diagram showing the cross section of the oxide
superconducting wire representing a third example of the present
invention;
[0039] FIG. 4 is a diagram showing the cross section of the oxide
superconducting wire representing a fourth example of the present
invention;
[0040] FIG. 5 is a diagram showing the cross section of the oxide
superconducting wire representing a fifth example of the present
invention;
[0041] FIG. 6 is a diagram showing the cross section of the oxide
superconducting wire representing a sixth example of the present
invention;
[0042] FIG. 7 is a flow diagram showing the process of producing
the oxide superconducting wire in the sixth example;
[0043] FIG. 8 is a diagram showing the oxide superconducting
multi-core wire 6 representing a seventh example of the present
invention;
[0044] FIG. 9 is a flow diagram showing the process of producing
the oxide superconducting multi-core wire 6;
[0045] FIG. 10 is a diagram showing the oxide superconducting
multi-core wire 20 in rectangular form;
[0046] FIG. 11 is a graph showing the relation between the
thickness of the oxide filament 9 and the critical current density
(Jc);
[0047] FIG. 12 is a graph showing the dependence on magnetic field
of the critical current density (Jc) of the oxide superconducting
multi-core wire 6;
[0048] FIG. 13 is a graph in which the critical current density
(Jc) is plotted against the silver (Ag) ratio in the oxide
superconducting multi-core wire 6;
[0049] FIG. 14 is a graph in which the critical current density
(Jc) is plotted against the sectional area of the oxide
superconducting multi-core wire 6 and the oxide superconducting
multi-core wire 20;
[0050] FIG. 15 is a graph showing the distribution of the critical
current density (Jc) which varies depending on the average particle
diameter (1, 3, 4.5, and 6 .mu.m) of the superconducting
powder;
[0051] FIGS. 16 to 22 are diagrams showing the cross section of
other oxide superconducting multi-core wires;
[0052] FIG. 23 is a diagram showing the solenoid coil 13
representing an eighth example of the present invention; and
[0053] FIG. 24 is a diagram showing the solenoid coil 14.
BEST MODE FOR CARRYING OUT THE INVENTION
[0054] The invention will be described in more detail with
reference to the following examples, which are not intended to
restrict the scope thereof.
EXAMPLE 1
[0055] The first example of the invention is shown in FIG. 1.
According to the present invention, the oxide superconducting wire
is produced by drawing. It has a uniform, round cross section. It
is composed of a silver sheath 1 and a superconductor 2 of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.- 2O.sub.x in the form of tape-like
filaments. The tape-like filaments are laminated on top of another
to constitute one unit and three units are united into one.
Lamination in each unit is accomplished stepwise so that the
laminated seven layers form a rhombus in the cross section
perpendicular to the lengthwise direction. The laminate having a
rhombic cross section constitutes one unit. Three units are
combined into one, so that the core is formed from 21 layers. All
the layers as a whole constitute in cross section a regular
hexagon. The three units are arranged in a rotational symmetry
through 120 degrees. In FIG. 1, the hatched parts denote the
tape-like filaments of a superconductor, and the white parts other
than the black tape denote the silver sheath.
[0056] In this example, seven tapes (layers) are laminated;
however, the number of layers is not specifically restricted, but
should be determined according to the workability of the material,
the volume ratio of the superconductor, and the application of the
wire. The wire should preferably have a diameter of 1 to 2 mm;
however, the diameter may vary depending on the application of the
wire. If the wire is to be used for AC, it may be twisted at a
pitch of 10 to 100 mm. The reason why the wire should preferably
have an outside diameter of 1 to 2 mm is that this diameter is
common for wires used for superconducting coils and this diameter
permits a critical current of about 1000A. With an excessively
large diameter, the wire presents difficulties in coil winding and
has an excessively large current value. The reason why the twisting
pitch should preferably be 10 to 100 mm for AC use is that it is
necessary to keep AC loss below 0.1 W/m for a commercial frequency
of 50 to 60 Hz.
EXAMPLE 2
[0057] The second example of the invention is shown in FIG. 2.
According to the present invention, the oxide superconducting wire
is produced by drawing. It has a uniform, round cross section. It
is composed of a silver sheath 1 and a superconductor 2 of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.- 2O.sub.x in the form of tape-like
filaments. The tape-like filaments are laminated on top of another
to constitute one unit and three units are united into one.
Lamination in each unit is accomplished stepwise, as in Example 1,
so that the laminated seven layers form a rhombus in the cross
section perpendicular to the lengthwise direction. The laminate
having a rhombic cross section constitutes one unit. Three units
are combined into one, so that the core is formed from 21 layers.
All the layers as a whole constitute in cross section a regular
hexagon. The three units are arranged in a rotational symmetry
through 120 degrees. Three units constitute one segment with a
hexagonal shape. Seven segments in total are united closely. The
wire should preferably have a diameter of 1 to 2 mm; however, the
diameter may vary depending on the application of the wire. If the
wire is to be used for AC, it may be twisted at a pitch of 10 to
100 mm. The term "united closely" means that the united segments
constitute a honeycomb structure.
EXAMPLE 3
[0058] The third example of the invention is shown in FIG. 3.
According to the present invention the oxide superconducting wire
is produced by drawing. It has a uniform, round cross section. It
is composed of a silver sheath 1 and a superconductor 2 of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.- 2O.sub.x in the form of tape-like
filaments. The tape-like filaments are laminated on top of another
to constitute one unit and three units are united into one.
Lamination in each unit is accomplished stepwise, as in Example 1,
so that the laminated seven layers form a rhombus in the cross
section perpendicular to the lengthwise direction. The laminate
having a rhombic cross section constitutes one unit. Three units
are combined into one, so that the core is formed from 21 layers.
All the layers as a whole constitute in cross section a regular
hexagon. The three units are arranged in a rotational symmetry
through 120 degrees. Three units constitute one segment with a
hexagonal shape. Fifty-five segments in total are united closely.
The wire should preferably have a diameter of 1 to 2 mm; however,
the diameter may vary depending on the application of the wire. If
the wire is to be used for AC, it may be twisted at a pitch of 10
to 100 mm.
EXAMPLE 4
[0059] The fourth example of the invention is shown in FIG. 4.
According to the present invention, the oxide superconducting wire
is produced by drawing. It has a uniform, round cross section. It
is composed of a silver sheath 1 and a superconductor 2 of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.- 2O.sub.x in the form of tape-like
filaments. The tape-like filaments are laminated on top of another
to constitute one unit and three units are united into one.
Lamination in each unit is accomplished stepwise, as in Example 1,
so that the laminated seven layers form a rhombus in the cross
section perpendicular to the lengthwise direction. The laminate
having a rhombic cross section constitutes one unit. Three units
are combined into one, so that the core is formed from 21 layers.
All the layers as a whole constitute in cross section a regular
hexagon. The three units are arranged in a rotational symmetry
through 120 degrees. Three units constitute one segment with a
hexagonal shape. Forty-eight segments in total are united closely.
At the center of the cross section is a metal layer 3 to enhance
the thermal stability and the mechanical strength of the wire. This
metal layer may be formed from a silver alloy containing 0.5 wt %
of magnesium, which is superior to silver in mechanical strength
and is comparable to silver in thermal conductivity. The wire
should preferably have a diameter of 1 to 2 mm; however, the
diameter may vary depending on the application of the wire. If the
wire is to be used for AC, it may be twisted at a pitch of 10 to
100 mm.
EXAMPLE 5
[0060] The fifth example of the invention is shown in FIG. 5.
According to the present invention, the oxide superconducting wire
is produced by drawing. It has a uniform, round cross section. It
is composed of a silver sheath 1 and a superconductor 2 of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.- 2O.sub.x in the form of tape-like
filaments. The tape-like filaments are laminated on top of an other
to constitute one unit and three units are united into one.
Lamination in each unit is accomplished stepwise, as in Example 1,
so that the laminated seven layers form a rhombus in the cross
section perpendicular to the lengthwise direction. The laminate
having a rhombic cross section constitutes one unit. Three units
are combined into one, so that the core is formed from 21 layers.
All the layers as a whole constitute in cross section a regular
hexagon. The three units are arranged in a rotational symmetry
through 120 degrees. Three units constitute one segment with a
hexagonal shape. Fifty-five segments in total are united closely.
The periphery 4 of the wire is made of reinforced silver alloy
containing 0.5 wt % of magnesium, so that the wire has an allowable
stress increased to 200 MPa. The wire should preferably have a
diameter of 1 to 2 mm; however, the diameter may vary depending on
the application of the wire. If the wire is to be used for AC, it
may be twisted at a pitch of 10 to 100 mm.
EXAMPLE 6
[0061] The sixth example of the invention is shown in FIG. 6.
According to the present invention, the oxide superconducting wire
is produced by drawing. It has a uniform, round cross section. It
is composed of a silver sheath 1 and a superconductor 2 of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.- 2O.sub.x in the form of tape-like
multi-core wires 5. The tape-like multi-core wires are laminated on
top of another to constitute one unit and three units are united
into one. Lamination in each unit is accomplished stepwise so that
the laminated seven layers form a rhombus in the cross section
perpendicular to the lengthwise direction. The laminate having a
rhombic cross section constitutes one unit. Three units are
combined into one, so that the core is formed from 21 layers. All
the layers as a whole constitute in cross section a regular
hexagon. The three units are arranged in a rotational symmetry
through 120 degrees.
[0062] In this example, seven tape wires (layers) are laminated;
however, the number of layers is not specifically restricted, but
should be determined according to the workability of the material,
the volume ratio of the superconductor, and the application of the
wire. The wire should preferably have a diameter of 1 to 2 mm;
however, the diameter may vary depending on the application of the
wire. If the wire is to be used for AC, it may be twisted at a
pitch of 10 to 100 mm.
[0063] FIG. 7 demonstrates the process for producing the oxide
superconducting wire according to the present invention. A
previously prepared powder of oxide superconductor is packed into a
sheath (or pipe) of silver or silver alloy. The packed sheath is
drawn to produce a single core wire. If necessary, drawing may be
performed again on more than one single core wire bundled together.
The resulting wire is formed into a tape, 0.1 to 0.3 mm thick and 2
to 5 mm wide, by rolling. Seven tapes were laminated in the
direction of thickness. The laminated tapes were placed in another
silver pipe such that their geometrical arrangement is shown in
FIG. 1. The silver pipe underwent hydrostatic extrusion, which was
followed by drawing. Thus, there was obtained a long wire, 0.5 to 2
mm in outside diameter and 100 to 1000 m in length.
[0064] For dimensional accuracy, the final drawing was carried out
by using a specially calibrated diamond die. In this way it was
possible to obtain a wire which has a specific outer shape (for
example, the shape for a solenoid) necessary for the generation of
uniform magnetic field.
[0065] In this example, the rolled tape is 0.1 to 0.3 mm thick and
2 to 5 mm wide. This tape is easy to handle and unite into a
rhombic shape. The tape should preferably have an aspect ratio
greater than 10. With an aspect ratio smaller than 10, the critical
current density of the wire will be decreased by more than
half.
[0066] The wire shown in Example 2 was made into a solenoid coil,
60 mm in inside diameter, 130 mm in outside diameter, and 600 mm in
height, with the insulating material being an alumina sleeve. The
solenoid coil underwent heat treatment (for partial melting) at 870
to 885.degree. C. for 10 to 30 minutes. This heat treatment was
followed by carrier concentration adjustment in a mixture gas of 5%
oxygen and 95% argon at 800.degree. C. Finally, the solenoid coil
was impregnated with epoxy resin for reinforcement.
[0067] In a first comparative example, a double-pancake coil, 60 mm
in inside diameter, 130 mm in outside diameter, and 15 mm in
height, was formed by winding a tape-like wire. Forty
double-pancake coils were placed on top of the other. In a second
comparative example, a single-core wire (with a silver sheath and a
round cross section) was prepared by drawing from an oxide
superconductor (Bi-2212 type). This wire was made into a solenoid
coil, 60 mm in inside diameter, 130 mm in outside diameter, and 600
mm in height, with the insulating material being alumina sleeve. In
the first comparative example, one magnet is formed from 40 coils
placed on top of one another. It generates a strong magnetic field,
but the magnetic field lacks uniformity.
[0068] In a second comparative example, a single-core wire (with a
silver sheath and a round cross section) was prepared by drawing
from an oxide superconductor (Bi-2212 type). This wire was made
into a solenoid coil, 60 mm in inside diameter, 130 mm in outside
diameter, and 600 mm in height, with the insulating material being
an alumina sleeve. This coil was tested for its performance at 4.2K
in a backup magnetic field of 21T. The results of the test are
shown in Table 1.
1 TABLE 1 Coil Intensity of current Uniformity of magnetic field
Operating density magnetic field generated (T) in current
(A/mm.sup.2) (%) 21 T (A) Working 100 0.01 3.3 250 Example
Comparative 125 0.3 3.8 280 Example Comparative 25 0.01 0.8 30
Example 2
[0069] The coil current density is the current density per unit
area of the cross section of the coil. The uniformity of magnetic
field is the fluctuation of the magnetic field which occurs within
30 mm (in diameter) at the center of the coil. The magnetic field
generated is the value measured at the center of the coil. The
operating current is defined as the maximum value of flowing
current. The sample in Comparative Example 1 is superior to the
sample in the Working Example in the intensity of magnetic field
generated. However, it cannot generate a uniform magnetic field
because the number of windings varies from one double-pancake to
another. The reason for this is that the tape wire varies in
thickness due to poor working accuracy. The tape thickness is in
the range of 0.1 to 0.3 mm, whereas the thickness variation is
about 10%. By contrast, the wire with a round cross section in
Comparative Example 2 is superior in dimensional accuracy and gives
a coil which generates a uniform magnetic field. However, the
magnetic field is weak because the oxide superconductor lacks
crystal orientation.
[0070] The wire in this working example has a round cross section
and hence generates a uniform, strong magnetic field. In addition,
the oxide superconductor in the wire has a tape-like shape, and the
c axis of the crystal orients along the tape surface. Therefore,
the wire produces good superconducting characteristics.
[0071] This working example demonstrates a magnet according to the
W&R method which involves heat-treatment after winding.
However, this working example may also be applied to a magnet
according to the R&W method in which the wire is wound after
heat treatment. In this case, enamel coating or formal coating may
be used as the insulating material. The resulting solenoid coil
generates a uniform magnetic field. In the case of a magnet
according to the R&W method, bending is carried out after heat
treatment. Therefore, the wire is damaged by winding and the
resulting coil decreases in performance. On the other hand,
however, it offers the advantage that the insulating material can
be made thin and the coil deformation due to heat treatment can be
prevented. Therefore, it is an advantageous method for producing a
uniform magnetic field, except for damage to the wire. There has
been no high-performance wire with a round cross section. The
present invention provides a wire with a round cross section which
permits a high critical current density Jc in a strong magnetic
field. A round wire is produced with high precision and generates a
uniform magnetic field when wound into a solenoid. According to the
present invention, it is possible to generate a uniform magnetic
field stronger than 2T in a magnetic field stronger than 21T. Thus,
the present invention makes feasible various apparatus (such as
scientific instruments, NMR analyzers, and medical MRI apparatus)
that employ an oxide superconducting magnet.
EXAMPLE 7
[0072] The seventh example of the invention relates to an oxide
superconducting multi-core wire 6 as shown in FIG. 8. The oxide
superconducting multi-core wire 6 is composed of 37-core tape wires
5, which are arranged in rotational symmetry in the cross section
perpendicular to the lengthwise direction. The tape wires are
covered with a silver sheath 1. In this example, six 37-core tape
wires 5 are laminated and combined into single sets. Six sets are
combined into one segment 7. Three segments are arranged in
rotational symmetry through 120 degrees, so that a hexagonal core 8
is formed. The 37-core tape wire 5 is formed by covering 37 oxide
superconducting filaments 9 with a silver sheath 10. Therefore, one
oxide superconducting multi-core wire 6 contains 666 oxide
superconducting filaments 9 (37'6'3=666). The number of oxide
superconducting filaments 9 may be properly selected according to
the shape of the wire.
[0073] (Production Method)
[0074] The oxide superconducting multi-core wire 6 in this example
is produced in the following manner.
[0075] The process starts with producing the oxide superconducting
filament 9.
[0076] (1) The starting materials for the oxides are strontium
oxide (SrO) calcium oxide (CaO), and copper oxide (CuO), all having
a purity higher than 99%. They are weighed such that the atomic
molar ratio of strontium (Sr), calcium (Ca), and copper (Cu) is
2.0:1.0:2.0. The resulting mixture is placed in a centrifugal ball
mill and mixed for 20 minutes.
[0077] (2) The mixture undergoes heat treatment in the atmosphere
at 900.degree. C. for 20 hours.
[0078] (3) The heat-treated mixture is cooled to room temperature
and then crushed and mixed again in a centrifugal ball mill for 20
minutes.
[0079] (4) The resulting powder is given a prescribed amount of
bismuth oxide (Bi.sub.2O.sub.3) so that the mixture contains
bismuth (Bi), strontium (Sr), calcium (Ca), and copper (Cu) in an
atomic molar ratio of 2.0:2.0:1.0:2.0. The mixture is mixed in a
centrifugal ball mill for 20 minutes.
[0080] (5) The thus obtained powder is heated in the atmosphere at
800 to 850.degree. C. for 10 hours. In this way there is obtained a
superconducting powder.
[0081] This superconducting powder was examined by powder X-ray
diffractometry and observed under a scanning electron microscope
(SEM). It was found to contain some strontium oxide (SrO) and
copper oxide (CuO) and a small amount of unreacted
non-superconducting phase which was not identified.
[0082] (6) The superconducting powder is crushed and mixed in a
centrifugal ball mill so as to give a superconducting fine powder
having an average particle diameter smaller than 3 .mu.m.
[0083] (7) The thus obtained superconducting fine powder is packed
in a first pure silver pipe which has a round cross section, 21.0
mm in outside diameter and 17.5 mm in inside diameter.
[0084] (8) This first pure silver (Ag) pipe is drawn by means of a
draw bench such that the reduction in area is 11 to 13% until the
outside diameter is reduced to 2.5 mm. The resulting product is
then made into the oxide superconducting filament 9.
[0085] (9) The first pure silver (Ag) pipe, which has been reduced
in diameter, is then cut into 37 equal lengths. The 37 cut pieces
are packed into a second pure silver (Ag) pipe, 21.0 mm in outside
diameter and 18.2 mm in inside diameter. The number of the cut
pieces should be properly adjusted according to the intended
use.
[0086] (10) This second pure silver (Ag) pipe is drawn by means of
a draw bench such that the reduction in area is 11 to 13% until the
outside diameter is reduced to 1.5 to 2.0 mm.
[0087] (11) The second pure silver (Ag) pipe, which has been
reduced in diameter, is then rolled into a 37-core tape-like wire 5
which has a flat cross section, 0.10 to 0.50 mm thick and 1.0 to
5.0 mm wide.
[0088] Subsequently, the 37-core tape wire 5 is made into the oxide
superconducting multi-core wire 6.
[0089] (12) The 37-core tape wire 5 is cut into 18 pieces, which
are subsequently divided into three groups, each consisting of 6
pieces. Six pieces are laminated on top of each other so as to form
the rhombic segment 7. Three sets of the segment 7 are packed into
a third pure silver (Ag) pipe, 21.0 mm in outside diameter and 18.2
mm in inside diameter. (They are arranged in rotational symmetry in
the pipe.) The 37-core tape-like wire 5 of the same size permits
the reduction of manufacturing steps for the oxide superconducting
multi-core wire 6 and also permits cost reduction.
[0090] The bismuth-based oxide superconductor is characterized in
that its crystal grains grow, upon heating, in the direction
perpendicular to the c axis. Consequently, it decreases in critical
current in a magnetic field parallel to the c axis. If the 37-core
tape-like wire 5 is arranged in rotational symmetry, it is possible
to prevent the critical current from decreasing irrespective of the
direction in which a magnetic field is applied.
[0091] (13) The third pure silver (Ag) pipe, in which the 37-core
tape-like wire 5 has been packed, is then drawn until the outside
diameter of the pipe is reduced to 0.75 to 2.5 mm. Thus, there is
obtained the oxide superconducting multi-core wire 6. The third
pure silver (Ag) pipe becomes the silver sheath 1; the first silver
pipe and the second silver pipe become the silver sheath 10; and
the oxide superconductor packed in the first silver pipe becomes
the oxide filament 9. The oxide filament 9 has a desired thickness
and aspect ratio if the tape-like wire is selected adequately.
Drawing and rolling give the wire the desired cross section and
densify the oxide filament 9. Incidentally, if rolling is carried
out so as to promote extension in the widthwise direction while
minimizing elongation in the lengthwise direction, it is possible
to densify the oxide filament 9 even more.
[0092] The oxide superconducting multi-core wire 6 optionally
undergoes rolling or drawing through a cassette roller die so as to
provide the oxide superconducting multi-core wire 11 which has a
rectangular cross section as shown in FIG. 10.
[0093] Then, the oxide superconducting multi-core wire 6 undergoes
heat treatment so as to impart superconductivity. (14) The oxide
superconducting multi-core wire 6 is placed in pure oxygen (with an
oxygen partial pressure of 1 atm) and heated at 875 to 900.degree.
C. for 5 to 60 minutes. This temperature is slightly higher than
the decomposition temperature of the oxide superconductor
(Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x in this example) of the
oxide filament 9. This heat treatment brings about partial melting
of the oxide superconductor. After heat treatment, the wire is
cooled to room temperature. Thus, the oxide superconducting
multi-core wire 6 is given superconductivity.
[0094] The wire undergoes optional annealing at 800 to 840.degree.
C. for 5 to 50 hours in an atmosphere containing 1 to 20% oxygen
(with an oxygen partial pressure of 0.01 to 0.2 atm). This
temperature is slightly lower than the decomposition temperature of
the oxide superconductor.
[0095] In this example, the oxide superconductor having the
composition of Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x is made from
a raw material powder composed of bismuth (Bi) compound, strontium
(Sr) compound, calcium (Ca) compound, and copper (Cu) compound.
However, the raw material powder may additionally contain a lead
(Pb) compound and/or barium (Ba) compound, if necessary. These
compounds may be in the form of oxide, hydroxide, carbonate,
nitrate, borate, and acetate. The technology of this example may be
applied to oxide superconductors of any other type than the bismuth
(Bi) type, such as thallium (Tl)-type superconductor and mercury
(Hg) type superconductor.
[0096] The metal sheath material should preferably be silver or
silver alloy which resists corrosion during heat treatment. Silver
may be alloyed with gold (Au), antimony (Sb), platinum (Pt),
magnesium (Mg), titanium (Ti), manganese (Mn), nickel (Ni), copper
(Cu), aluminum (Al), etc.
[0097] The preparation of the oxide superconducting powder and the
heat treatment for intermediate firing should be carried cut at 700
to 950.degree. C. The composition of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x may be incorporated with or
replaced with a third element according to need. The resulting
oxide superconductor is heated at a temperature at which it partly
melts. In the cooling step, non-superconducting phases are
dispersed into the crystal grains of the superconducting phase.
This enhances the pinning effect.
[0098] In this example, the oxide superconducting multi-core wire 6
is formed by a "powder-in-tube" method; however, it is possible to
employ a "rod-in-tube" method, a doctor blade method, a dip coating
method, a spray pyrolysis method, a screen printing method, or a
jelly roll method.
[0099] The oxide superconducting multi-core wire 6 produced as
mentioned above was tested for superconductivity.
[0100] (Results of Measurement 1)
[0101] The oxide superconducting multi-core wire 6, with the oxide
filament 9 varying in average thickness from 1.5 .mu.m to 40 .mu.m,
was tested for critical current density (Jc) at 4.2K in the absence
of an applied magnetic field. The results are shown in Table 2.
2TABLE 2 Average thickness of 1.5 2.5 3 5 10 15 20 25 40 thickness
of oxide filament (.mu.m) Jc (at 4.2 K, 0 T) 150 185 230 245 235
260 248 190 180 (A/mm.sup.2)
[0102] It is noted that the critical current density (Jc) is as
high as 230 to 260 A/mm.sup.2 if the oxide filament 9 has an
average thickness of 3 to 20 .mu.m. However, Jc decreases if the
average thickness is smaller than 3 .mu.m or larger than 25
.mu.m.
[0103] This result suggests that the critical current density (Jc)
can be made high if the oxide filament 9 has an average thickness
in the range of 3 to 20 .mu.m.
[0104] (Results of Measurement 2)
[0105] The oxide superconducting multi-core wire 6, with the oxide
filament 9 varying in thickness accounting for more than 50%, was
tested for critical current density (Jc). The results are shown in
FIG. 11.
[0106] It is noted from FIG. 11 that the oxide superconducting
multi-core wire 6 invariably gives a critical current density (Jc)
of 230 A/mm.sup.2 if more than 50% of oxide filament 9 is in the
range of 3 to 15 .mu.m.
[0107] (Results of Measurement 3)
[0108] The oxide superconducting multi-core wire 6, with the oxide
filament 9 varying in aspect ratio from 1 to 20, was tested for
critical current density (Jc) at 4.2K in the absence of an applied
magnetic field. The results are shown in Table 3. The aspect ratio
is the ratio of the vertical side to the horizontal side of the
oxide filament 9.
3TABLE 3 Average aspect ratio 1 1.5 2 2.5 5 7.5 10 12.5 20 of oxide
filament Jc (at 4.2 K, 150 185 225 245 225 230 240 190 190 0 T)
(A/mm.sup.2)
[0109] It is noted that the critical current density (Jc) is as
high as 225 to 245 A/mm.sup.2 if the oxide filament 9 has, an
average aspect ratio in the range of 2 to 10. However, Jc decreases
if the average aspect ratio is outside this range.
[0110] This result suggests that the critical current density (Jc)
can be made high if the oxide filament 9 has an average aspect
ratio in the range of 2 to 10 .mu.m.
[0111] In addition, the oxide superconducting multi-core wire 6
invariably gives a critical current density (Jc) higher than 200
A/mm.sup.2 if more than 90% of the oxide filaments 9 have an aspect
ratio in the range of 2 to 10.
[0112] (Results of Measurement 4)
[0113] The rectangular oxide superconducting multi-core wire 20,
with its aspect ratio varying from 1 to 30, was tested for critical
current density (Jc) at 4.2K in the absence of magnetic field
applied. The results are shown in Table 4.
4TABLE 4 Average aspect ratio 1 2 4 5 6 8 12 16 20 Jc (at 4.2 K,
220 225 235 245 250 240 250 245 245 0 T) (A/mm.sup.2)
[0114] It is noted that the critical current density (Jc) is as
high as 220 to 250 A/mm.sup.2 if the aspect ratio is in the range
of 1 to 15. However, Jc decreases if the aspect ratio exceeds 6.
Moreover, the oxide superconducting multi-core wire 6 is liable to
breakage during fabrication into a rectangular shape if its aspect
ratio exceeds 6.
[0115] Therefore, the oxide superconducting multi-core wire 20
should have an aspect ratio in the range of 1 to 10.
[0116] (Results of Measurement 5)
[0117] The oxide superconducting multi-core wire 6 was tested for
the dependence of critical current density (Jc) on the intensity of
an applied magnetic field. The results are shown in FIG. 12. The
solid line represents the oxide superconducting multi-core wire 6
of the working example, and the dotted line represents the oxide
superconducting multi-core wire for comparison in which the oxide
filament 9 is made round instead of tape-like.
[0118] It is noted from FIG. 12 that the oxide superconducting
multi-core wire 6 gives a critical current density (Jc) of 250
A/mm.sup.2 at 4.2K in the absence of an applied magnetic field,
whereas the comparative wire gives a Jc of 180 A/mm.sup.2. Also,
the oxide superconducting multi-core wire 6 gives a critical
current density (Jc) of 125 A/mm.sup.2 at 4.2K in the presence of a
magnetic field of 20T, whereas the comparative wire gives a Jc of
40 A/mm.sup.2. These results suggest that the oxide superconducting
multi-core wire 6 gives a higher critical current density (Jc) than
the comparative wire in the presence of an applied magnetic
field.
[0119] (Results of Measurement 6)
[0120] The oxide superconducting multi-core wire 6, with the ratio
of silver (Ag) therein varied, was tested for critical current
density (Jc). The results are shown in FIG. 13. The ratio of silver
(Ag) is expressed in terms of the ratio of silver (As) as the metal
matrix to the oxide filament 9 in the cross section of the oxide
superconducting wire 4. In FIG. 13, the vertical axis represents
the critical current density (Jc) in A/mm.sup.2 at 4.2K in the
absence of an applied magnetic field, and the horizontal axis
represents the ratio of silver.
[0121] It is noted from FIG. 13 that the critical current density
(Jc) at 4.2K in the absence of an applied magnetic field is 220 to
245 A/mm.sup.2 if the ratio of silver is in the range of 3 to 7,
whereas it decreases if the ratio of silver is outside this
range.
[0122] The results mentioned above suggest that it is possible to
increase the critical current density (Jc) when the ratio of silver
(Ag) in the oxide superconducting multi-core wire 6 is in the range
of 3 to 7.
[0123] (Results of Measurement 7)
[0124] The oxide superconducting multi-core wire 6 having a round
cross section and the oxide superconducting multi-core wire 20
having a rectangular cross section, with their sectional area
varied, were tested for critical current density (Jc). The results
are shown in FIG. 14. It is noted from FIG. 14 that the critical
current density (Jc) at 4.2K in the absence of an applied magnetic
field is lower than 200 A/mm.sup.2 if the sectional area is smaller
than 1 mm.sup.2, whereas the critical current density (Jc) is
higher than 230 A/mm.sup.2 if the sectional area is larger than 1
mm.sup.2.
[0125] The results mentioned above suggest that it is possible to
increase the critical current density (Jc) when the oxide
superconducting multi-core wire has a sectional area larger than 1
mm.sup.2.
[0126] (Results of Measurement 8)
[0127] The oxide superconducting multi-core wire 6 was tested for
the distribution of the critical current density (Jc) in the
lengthwise direction at 4.2K in the absence of an applied magnetic
field, with the average particle diameter of the superconducting
powder packed into the first pure silver (Ag) pipe varied (3, 1, 3,
4.5, and 6 .mu.m). The voltage terminals are 30 mm apart. The
results are shown in FIG. 15.
[0128] It is noted from FIG. 15 that the distribution of the
critical current density (Jc) becomes wider and uneven as the
particle diameter of the superconducting powder increases. To be
more specific, in the case where the average particle diameter of
the superconducting powder is 6 .mu.m, the critical current density
(Jc) fluctuates between 270 A/mm.sup.2 and 190 A/mm.sup.2. In the
case where the average particle diameter of the superconducting
powder is 4.5 .mu.m, the critical current density (Jc) fluctuates
between 290 A/mm.sup.2 and 200 A/mm.sup.2. In the case where the
average particle diameter of the superconducting powder is 1 .mu.m
or 3 .mu.m, the critical current density (Jc) fluctuates between
240 A/mm.sup.2 and 290 mm.sup.2. In the last case, the critical
current density (Jc) is high and uniform.
[0129] The cross section of the oxide superconducting multi-core
wire 6 was observed under a scanning electron microscope. It was
found that non-superconducting phase becomes coarser as the average
particle diameter of the superconducting powder becomes larger. It
is considered that the coarse non-superconducting phase interrupts
the current flow, thereby decreasing the critical current density
(Jc).
[0130] For the reasons mentioned above, it is desirable that the
superconducting powder to be packed into the first metal pipe
should be crushed to such an extent that its average particle
diameter is smaller than 3 .mu.m. A superconducting powder having
an average particle diameter smaller than 3 .mu.m gives the long
oxide superconducting multi-core wire 6 which is comparable in
current flow characteristics to the conventional tape-like oxide
superconducting wire.
[0131] In this example, the hexagonal core 8 is formed from three
segments 7 arranged in rotational symmetry, each segment consisting
of 37-core tape-like wire 5 laminated on top of each other. The
arrangement of the 37-core tape-like wire 5 may be modified as
shown in FIGS. 16 to 22. The illustrated laminate structure permits
the oxide filaments 9 to orient in various directions, so that the
resulting wire is less anisotropic and keeps the critical current
high irrespective of the direction of the magnetic field. The
tape-like wires may be arranged closely as shown in FIGS. 2 to
5.
[0132] Another arrangement is shown in FIG. 18. A fourth metal pipe
or metal wire 12 (of silver or silver alloy) is placed at the
center of the oxide superconducting multi-core wire 6. This metal
pipe or wire 12 in place of the oxide filament 9 does not greatly
change the critical current (Jc), because the oxide filament 9
placed at the center of the oxide superconducting multi-core wire 6
is less dense than the oxide filament 9 placed at the surface even
though after rolling. In fact, no significant difference in
critical current density (Jc) was observed irrespective of the
oxide filament 9 and the metal pipe or wire 12. In addition, the
fourth metal pipe contributes to mechanical strength and uniform
cross section. When the oxide superconducting multi-core wire 6 is
twisted for the production of twisted conductor, the center is not
virtually twisted; therefore, it is not concerned with the
reduction of AC loss and it is not necessarily an oxide filament
9.
[0133] The inside of the third metal pipe is not necessarily
required to be round; it may be polygonal so that the 37-core
tape-like wire 5 is packed more densely. A hexagonal cross section
is most suitable for densest packing and regular sectional
shape.
[0134] The above-mentioned oxide superconducting multi-core wire
will be widely used for superconducting apparatuses exemplified
below to increase their efficiency. Examples are a power
transmitting cable, bus bar, long conductor, permanent current
switching element, large magnet, NMR analyzer, medical NMR
diagnosing apparatus, superconducting electric power storing
apparatus, magnetic separating apparatus, apparatus for pulling up
a single crystal in a magnetic field, cryogenic superconducting
magnet, superconductive energy storage, superconducting generator,
magnet for nuclear fusion reactor, accelerator, current lead, and
current limiting relay.
[0135] The oxide superconducting multi-core wire of the present
invention may be cooled with liquid helium as well as liquid
nitrogen or a freezer. Therefore, it helps reduce the running cost
of the apparatus and merely needs simple provisions to prevent
quench (rapid transition from superconducting state to normal
conducting state, leading to destruction). In addition, it improves
the reliability of superconductivity.
EXAMPLE 8
[0136] This example demonstrates a solenoid coil shown in FIG. 23.
The solenoid coil 11 (35 mm in inside diameter, 70 mm in outside
diameter, and 150 mm in height) consists of a bobbin 6 of heat
resistant metal and an oxide superconducting multi-core wire 6
having a round cross section (1.5 mm in diameter) which is wound
around the bobbin. This solenoid coil was heated at 890.degree. C.
for 10 minutes so that the oxide superconductor partly melts. (This
heating temperature is slightly higher than the decomposition
temperature of the oxide superconductor having a composition of
Bi.sub.2Sr.sub.2Ca.sub.1Cu.sub.2O.sub.x.) Upon cooling to room
temperature, the solenoid coil exhibited superconductivity. For
enhancement of superconductivity, the solenoid coil underwent
annealing at 800.degree. C. for 50 hours in an atmosphere
containing 10% oxygen.
[0137] The solenoid coil 11 was tested for excitation in an
external magnetic field of 20T. It generated a magnetic field of
1.5T. The uniformity of magnetic field measured for the coil alone
was 0.005 ppm at 20 mm.phi.). In other words, it achieved a very
high uniformity of magnetic field.
[0138] FIG. 24 shows a solenoid coil 14 (35 mm in inside diameter,
70 mm in outside diameter, 150 mm in height), which consists of a
bobbin 21 of heat resistant metal and an oxide superconducting
multi-core wire 20 having a rectangular cross section, 1.1 mm thick
and 2.2 mm wide, wound around the bobbin. The solenoid coil 14
underwent heat treatment in the same way as the solenoid coil 13 so
that it exhibited superconductivity.
[0139] The solenoid coil 12 was tested for excitation in an
external magnetic field of 20T. It generated a magnetic field of
1.7T. The uniformity of magnetic field measured for the coil alone
was 0.007 ppm at 20 mm.phi.. In other words, it achieved a very
high uniformity of magnetic field which was not achieved by the
conventional laminated pancake coil.
[0140] The oxide superconducting multi-core wire 6 having a round
cross section or the oxide superconducting multi-core wire 20
having a rectangular cross section can be produced easily with
higher accuracy than conventional tape-like oxide superconducting
wires. Therefore, the solenoid coils 13 and 14 made from them can
be made more accurately in the axial and circumferential directions
than the pancake coil made from conventional tape-like oxide
superconducting wires. Therefore, the solenoid coils 11 and 14 can
be applied to the high-resolution NMR magnet which needs a strong,
uniform magnetic field.
[0141] According to the present invention, it is possible to
provide an oxide superconducting wire which has good accuracy of
outer shape and is capable of generating a strong, uniform magnetic
field at a high critical current density (Jc), and it is also
possible to provide a solenoid coil and a magnetic field generating
apparatus made therefrom.
[0142] According to the present invention, the oxide
superconducting wire is composed of multi-core tape-like wires
which are arranged in rotational symmetry. Therefore, the critical
current is prevented from decreasing irrespective of the direction
in which the magnetic field is applied. In addition, it has a high
critical current density (Jc) because the oxide superconducting
filament has an optimal size.
[0143] If a metal sheath of silver or silver alloy is used and the
ratio of the metal sheath to the oxide superconducting filament is
larger than 3 and smaller than 7, it is possible to increase the
critical current density Jc) more.
[0144] In the case of oxide superconducting wire having a
rectangular cross section, the aspect ratio of the cross section
should preferably be larger than 1 and smaller than 6.
[0145] If the oxide superconducting wire is made from an oxide
superconductor or a raw material of oxide superconductor in the
form of powder having an average particle diameter smaller than 3
.mu.m, it is possible to obtain a long oxide superconducting wire
which is comparable in current flowing characteristics to
conventional tape-like oxide superconducting wires.
[0146] Solenoid coils made from the oxide superconducting wire of
the present invention generate a strong, uniform magnetic
field.
[0147] The oxide superconducting wire of the present invention has
good accuracy of outer shape and generates a high critical current
density; therefore, it can be used for apparatuses requiring a
strong, uniform magnetic field, such as scientific instruments, NMR
analyzers, and medical MRI apparatuses.
* * * * *