U.S. patent application number 14/198618 was filed with the patent office on 2014-09-25 for monofilament for the production of an nb3sn superconductor wire.
The applicant listed for this patent is Bruker EAS GmbH. Invention is credited to Manfred Thoener.
Application Number | 20140287929 14/198618 |
Document ID | / |
Family ID | 47900807 |
Filed Date | 2014-09-25 |
United States Patent
Application |
20140287929 |
Kind Code |
A1 |
Thoener; Manfred |
September 25, 2014 |
Monofilament for the production of an Nb3Sn superconductor wire
Abstract
A monofilament (1) for the production of a superconducting wire
(20) has a powder core (3) that contains at least Sn and Cu, an
inner tube (2), made of Nb or an alloy containing Nb, that encloses
the powder core (3), and an outer tube (4) in which the inner tube
(2) is arranged. The outer side of the inner tube (2) is in contact
with the inner side of the outer tube (4) and the outer tube (4) is
produced from Nb or from an alloy containing Nb. The outer tube is
disposed in a cladding tube. The superconducting current carrying
capacity of the superconducting wire is thereby improved.
Inventors: |
Thoener; Manfred;
(Biebergemuend, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bruker EAS GmbH |
Hanau |
|
DE |
|
|
Family ID: |
47900807 |
Appl. No.: |
14/198618 |
Filed: |
March 6, 2014 |
Current U.S.
Class: |
505/510 ; 29/599;
419/66; 428/555 |
Current CPC
Class: |
B22F 5/12 20130101; C22C
30/02 20130101; H01L 39/2409 20130101; Y10T 428/12076 20150115;
B22F 2301/30 20130101; H01F 41/048 20130101; B22F 3/20 20130101;
C22C 13/00 20130101; H01F 6/06 20130101; Y10T 29/49014 20150115;
B22F 2303/01 20130101; B22F 7/04 20130101; B22F 2301/20 20130101;
B22F 2998/10 20130101; H01L 39/2403 20130101; H01L 39/12 20130101;
B22F 1/0003 20130101 |
Class at
Publication: |
505/510 ; 29/599;
419/66; 428/555 |
International
Class: |
H01L 39/24 20060101
H01L039/24 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2013 |
EP |
13 159 230.5 |
Claims
1. A monofilament for the production of a superconducting wire, the
monofilament comprising: a powder core that contains at least Sn
and Cu; an inner tube that encloses said powder core, said inner
tube being made of Nb or of an alloy containing Nb; an outer tube
in which said inner tube is disposed, wherein an outer side of said
inner tube is in contact with an inner side of said outer tube,
said outer tube being made of Nb or of an alloy containing Nb; and
a cladding tube in which said outer tube is disposed.
2. The monofilament of claim 1, wherein said cladding tube has a
hexagonal outer cross-section and a round inner cross-section.
3. The monofilament of claim 1, wherein said inner tube has a round
inner cross-section and a round outer cross-section and said outer
tube has a round inner cross-section and a round outer
cross-section.
4. The monofilament of claim 3, wherein
1.2.ltoreq.D.sup.outside.sub.outertube/D.sup.outside.sub.innertube.ltoreq-
.2.0 with D.sup.outside.sub.outertube: outer diameter of the outer
tube and D.sup.outside.sub.innertube: outer diameter of the inner
tube.
5. The monofilament of claim 3, wherein
4.ltoreq.W.sub.outertube/W.sub.innertube.ltoreq.50 with
W.sub.outertube: wall thickness outer tube and W.sub.innertube:
wall thickness inner tube.
6. The monofilament of claim 1, wherein said powder core has a
content of 2 weight % to 12 weight % of Cu, 3 to 9 weight % of Cu
or said powder core contains elementary Cu powder.
7. The monofilament of claim 1, wherein said powder core is
compacted in said inner tube or has a density of at least 40% or of
at least 50% of a theoretical density.
8. The monofilament of claim 1, wherein said powder core contains
NbSn.sub.2 and/or Nb.sub.6Sn.sub.5 and/or elementary Sn.
9. The monofilament of claim 1, wherein said inner tube and/or said
outer tube are produced from an alloy containing Nb and containing
Ta and/or Ti, have a summed content of at least 0.5 weight % of Ta
and/or Ti or have with a summed content of maximally 10 weight % of
Ta and/or Ti.
10. The monofilament of claim 1, wherein said inner tube and said
outer tube are produced from different materials.
11. The monofilament of claim 1, wherein said cladding tube is made
of Cu.
12. A method for producing the monofilament of claim 1, the method
comprising the steps of: a) filling the powder core into the inner
tube; b) drawing, following step a), the inner tube, thereby
compacting the powder core; c) inserting the outer tube into the
cladding tube; and d) inserting, following step b), the drawn,
filled inner tube into the outer tube, wherein step c) is performed
prior to or after step d).
13. The method of claim 12, wherein the cladding tube already has a
hexagonal outer cross-section prior to step c).
14. A method for producing a precursor of a superconducting wire
using the monofilament of claim 1, the method comprising the steps
of: a') drawing one or more of the monofilaments to produce
monofilaments having a hexagonal outer cross-section; b') bundling
a plurality of drawn monofilaments in a wire cladding tube; c')
extruding and/or drawing the wire cladding tube containing the
bundled and drawn monofilaments, thereby obtaining the precursor of
the superconducting wire.
15. The method of claim 14, wherein, in step a'), the monofilaments
already have a hexagonal outer cross-section of the cladding tube
and a round outer cross-section of the outer tube prior to drawing,
wherein the drawn monofilaments also have a hexagonal outer
cross-section of the cladding tube and a round outer cross-section
of the outer tube after drawing.
16. The precursor of a superconducting wire produced by the method
of claim 14.
17. A method for producing a superconducting wire using the
precursor of claim 16, the method comprising the steps of: d')
mechanically deforming the precursor or winding the precursor to
form a coil; and e') temperature-treating the deformed precursor or
temperature-treating the deformed precursor at a maximum
temperature of 700.degree. C. or less, wherein Nb from the inner
tube and the outer tube reacts with Sn from the powder core to form
Nb.sub.3Sn.
18. The method of claim 17, wherein temperature treatment is
terminated in step e') before a reaction front that advances to an
outside has reached a boundary surface between the outer tube and
the cladding tube.
19. The superconducting wire produced by the method of claim 17.
Description
[0001] This application claims Paris convention priority from EP 13
159 230.5 filed on Mar. 14, 2013, the entire disclosure of which is
hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] The invention concerns a monofilament for the production of
a superconducting wire, comprising [0003] a powder core that
contains at least Sn and Cu, [0004] an inner tube that encloses the
powder core, [0005] an outer tube in which the inner tube is
arranged, wherein the outer side of the inner tube is in contact
with the inner side of the outer tube, and wherein the outer tube
is made from Nb or an alloy containing Nb, and [0006] a cladding
tube in which the outer tube is arranged.
[0007] A monofilament of this type is disclosed by EP 0 169 596
A1.
[0008] Nb.sub.3Sn is a widely used superconducting material, in
particular, for the production of superconducting magnet coils. In
comparison with other metallic low-temperature superconducting
materials (such as NbTi), Nb.sub.3Sn superconducting wires achieve
technically relevant current densities in higher magnetic fields.
However, the production and processing of Nb.sub.3Sn
superconducting wires is difficult, since Nb.sub.3Sn is a
relatively brittle material and can therefore not be plastically
deformed (or only to a minimum extent).
[0009] Nb.sub.3Sn wires are usually produced in accordance with the
bronze route, the internal in diffusion technology, or the
powder-in-tube technology (PIT). In all three cases, the production
is divided into semi-finished product manufacture on the one hand
and reaction annealing on the other hand. Brittle Nb.sub.3Sn is
generated only during reaction annealing. Deformation of the
superconducting wire is normally not carried out after reaction
annealing.
[0010] With respect to powder-in-tube technology, as disclosed e.g.
by EP 0 169 596 A1, a powder mixture containing Sn is disposed in
an inner tube, which is normally made of copper, and the inner tube
is, in turn, introduced into an outer tube, which is normally made
of Nb. The outer tube is, in turn, arranged in a round conductive
matrix tube (cladding tube), which is normally made of Cu. A
plurality of these monofilaments can be bundled. Due to reaction
annealing, during which Sn from the powder mixture reacts with Nb
from the outer tube to form Nb.sub.3Sn with the catalytic
assistance of Cu from the inner tube or as an additive to the
powder mixture, one obtains a superconducting wire with high
superconducting current carrying capacity using that powder-in-tube
technology.
[0011] H. Veringa et al., Adv. Cryo. Eng. (Materials), 1984; 30,
pages 813-821 disclose a superconducting wire, wherein NbSn.sub.2
powder with addition of Cu is introduced into an Nb tube, the Nb
tube is introduced into a hexagonal copper tube, and several copper
tubes filled in this fashion are arranged in a bundling tube of
copper and are temperature-controlled after extrusion and drawing.
A similar procedure is also disclosed by A.C.A. van Wees et al.,
IEEE Trans. Magn., MAG 19, 556 (1983), pages 5-8.
[0012] It is the object of the present invention to further
increase the superconducting current carrying capacity of the
superconducting wire.
SUMMARY OF THE INVENTION
[0013] This object is achieved by a monofilament of the
above-mentioned type, which is characterized in that the inner tube
is made of Nb or an alloy containing Nb.
[0014] Due to the fact that the inner tube is produced from Nb or
an alloy containing Nb, one obtains a finer and more homogeneous
structure of Nb.sub.3Sn grains after reaction annealing, in
particular, in the area of the previous boundary surface between
the inner tube and the powder core compared with the use of an
inner tube of copper. Cavities (e.g. gaps) in this area are also
prevented or at least reduced after reaction annealing.
[0015] Both result in a significant increase in the superconducting
current carrying capacity of the finished superconducting wire. The
inventors have observed an increase in the critical current density
by a factor of 1.5 to 2.
[0016] The reaction front of Nb.sub.3Sn that advances to the
outside is moreover easier to control during reaction annealing. In
particular, breakthroughs of the reaction front into the cladding
tube (matrix) and consequently contamination of the cladding tube
(which is made of Cu in most cases) can be easily prevented. In
correspondence therewith, the residual resistance of the cladding
tube can be kept low (or the RRR value can be kept high).
[0017] The use of an inner tube of Nb or an alloy containing Nb
surprisingly did not entail noticeable problems with the
deformation behavior, e.g. during drawing of the filled inner tube
only, during drawing of the monofilament or during drawing of a
precursor of a superconducting wire with several drawn
monofilaments. In particular, there was no increase in filament
breakage in the reacted superconducting wire.
[0018] The powder core contained in the inner tube can be well
compacted by the inner tube in a drawing process prior to insertion
into the outer tube. The main part of the niobium required to form
Nb.sub.3Sn can be provided by the outer tube such that the inner
tube can be designed to have relatively thin walls.
[0019] The inner tube typically has an Nb content of at least 80
weight WO %, preferably at least 90 weight %. The inner tube is
moreover typically free of copper. The same applies to the outer
tube. The powder core typically contains a mixture of different
powders of different compositions, inter alia Cu which is required
to accelerate formation of Nb.sub.3Sn. Cu is preferably present in
elementary form whereas Sn is generally present at least partially
in the form of an alloy (preferably alloyed with Nb).
[0020] In one particularly advantageous embodiment of the inventive
monofilament, the cladding tube has a hexagonal outer cross-section
and a round inner cross-section. In this case, an outer tube having
a round outer cross-section (and round inner cross-section) can be
inserted, the outer cross-section of which does not have to be
changed within the scope of bundling processes. It is, in
particular, no longer necessary to impress a hexagonal outer
cross-section onto the cladding tube in a drawing step of the
monofilament, which would also result in an approximately hexagonal
outer cross-section of the previously round outer tube (the inner
cross-section would remain substantially round). For this reason,
the outer tube can maintain a uniform wall thickness, in
particular, also at the beginning of reaction annealing. Nb.sub.3Sn
can then be uniformly formed from the inside into the outer tube
without fear of premature breakthrough of Sn into the cladding tube
at a thin point, and no unreacted corner areas remain. A
particularly large portion of the cross-sectional area of the
monofilament can correspondingly react to Nb.sub.3Sn which is then
available for the superconducting current transport. Since the
reacted volume of Nb.sub.3Sn can moreover transport a relatively
high current density due to its fine structure, the advantages of
the invention exponentiate in this embodiment to yield a
particularly high current carrying capacity for each monofilament
or for the overall superconducting wire. As an alternative to this
embodiment, the cladding tube may also have a round outer
cross-section. For subsequent bundling, it is then necessary to
impress a hexagonal outer cross-section in a drawing step.
[0021] In one preferred embodiment, the inner tube has a round
inner cross-section and a round outer cross-section and the outer
tube has a round inner cross-section and a round outer
cross-section. In this case, the reaction of Sn with Nb to
Nb.sub.3Sn may be performed particularly evenly, in particular with
a (circular) round reaction front in the monofilament.
[0022] In a preferred further development of this embodiment, the
following applies:
1.2.ltoreq.D.sup.outside.sub.outertube/D.sup.outside.sub.innertube.ltore-
q.2.0 with D.sup.outside.sub.outertube: outer diameter of the outer
tube and D.sup.outside.sub.innertube: outer diameter of the inner
tube.
[0023] In another preferred further development
4.ltoreq.W.sub.outertube/W.sub.innertube.ltoreq.50 with
W.sub.outertube: Wall thickness outer tube and W.sub.innertube:Wall
thickness inner tube. These ranges of diameter ratios and wall
thickness ratios have turned out to be advantageous in practice. In
particular, there was no increase in filament breakage in the
reacted superconducting wire.
[0024] In another advantageous embodiment, the powder core has a
content of 2 weight % to 12 weight % of Cu, preferably 3 to 9
weight % of Cu, wherein the powder core preferably contains
elementary Cu powder. The
[0025] Cu addition in the powder core has a catalytic effect for
the formation of Nb.sub.3Sn and reduces the reaction temperature.
In practice, a copper content in the powder core of up to 5 weight
% is often already sufficient to achieve an efficient reaction. Due
to the omission of an inner tube of copper, the overall copper
content in the reaction area of the monofilament can be selected
relatively freely, in particular, to be lower than if an inner tube
of copper were used. Distribution of copper in the reaction area,
in particular uniformly in the powder core, is moreover also
improved.
[0026] In another preferred embodiment, the powder core is
compacted in the inner tube, in particular with a density of at
least 40%, preferably at least 50% of the theoretical density. The
compaction can, in particular, be realized by a drawing process of
the filled inner tube prior to insertion into the outer tube. The
(pre)compaction improves the overall plastic deformability of the
finished conductor (precursor of the superconducting wire) prior to
reaction annealing.
[0027] In one preferred embodiment, the powder core contains
NbSn.sub.2 and/or Nb.sub.6Sn.sub.5 and/or elementary Sn. These
materials can be well used as in sources within the scope of the
present invention. In most cases, a combination of two or also of
all thee materials is used.
[0028] In another advantageous embodiment, the inner tube and/or
the outer tube are produced from an alloy containing Nb and
containing Ta and/or Ti, in particular, with a summed content of at
least 0.5 weight % of Ta and/or Ti, and in particular, with a
summed content of maximally 10 weight % of Ta and/or Ti. Small
additions of tantalum and/or titanium have a positive influence on
the forming kinetics and on the structure of Nb.sub.3Sn and
increase the achievable critical current density. In accordance
with this embodiment, the overall additions of tantalum and
titanium should preferably maximally amount to 10 weight % in order
to avoid undesired phases. Hf or Zr could also be used as alloy
additions.
[0029] In another preferred embodiment, the inner tube and the
outer tube are produced from different materials. The inner and
outer cladding tubes can consequently be designed to have different
properties. The inner tube may, in particular, be selected through
suitable composition and/or structural adjustment such that the
outer side of the inner tube is minimally deformed during filling
in the powder core and during a drawing process in order to
facilitate subsequent insertion into the outer tube. The outer tube
can e.g. consist of NbTa7.5 and the inner tube of Nb. The materials
of the inner tube and of the outer tube could alternatively also be
selected to be the same.
[0030] In another advantageous embodiment, the cladding tube is
made of Cu. Copper has high conductivity and can therefore protect
the Nb.sub.3Sn filament as a parallel current path close to it in
case of a quench (loss of superconductivity).
[0031] The present invention also concerns a method for producing
an inventive monofilament as described above, characterized by the
following steps:
[0032] a) the powder core is initially filled into the undrawn
inner tube and the filled undrawn inner tube is subsequently drawn,
thereby compacting the powder core;
[0033] b) the outer tube is inserted into the cladding tube,
and
[0034] c) the drawn, filled inner tube is inserted into the outer
tube, wherein step
[0035] b) is performed prior to or after step c).
[0036] Step b) is preferably performed prior to step c). The
undrawn inner tube preferably has thin walls, e.g. with a ratio
between wall thickness and outer diameter of 1/25 to 1/60, thereby
achieving particularly high compaction of the powder core in step
a).
[0037] In one particularly advantageous variant of the inventive
method for producing a monofilament, the cladding tube already has
a hexagonal outer cross-section prior to step b). For this reason,
bundling of the monofilaments does not require drawing to obtain a
hexagonal outer cross-section and the outer tube can remain round
inside and outside. A large portion of the cross-section of each
monofilament can be used for forming Nb.sub.3Sn having a very
homogeneous and fine structure. This leads to particularly high
superconducting current carrying capacities (see above).
[0038] The present invention also concerns a method for producing a
precursor of a superconducting wire, characterized by the following
steps:
[0039] a') drawn monofilaments having a hexagonal outer
cross-section are produced by drawing one or more inventive
monofilaments;
[0040] b') a plurality of drawn monofilaments are bundled in a wire
cladding tube;
[0041] c') the wire cladding tube that contains the bundled and
drawn monofilaments, is extruded and/or drawn, thereby obtaining
the precursor of the superconducting wire. The contact between the
inner tube and the outer tube can be improved by the drawing step
according to a') (in particular, any gaps can be closed) and
further compaction can be achieved such that more monofilaments can
be bundled in step b'). The size of the monofilaments and, if
required, the shape of the monofilaments can be adjusted for step
b'). The precursor of the superconducting wire is given the size
and cross-sectional shape required for the application (e.g. a
magnet coil to be wound) in step c'). The wire cladding tube is
typically produced of Cu in order to ensure a low residual
resistance.
[0042] In one variant of the inventive method of producing a
precursor of a superconducting wire, in step a') the
monofilament(s) already has/have a hexagonal outer cross-section of
the cladding tube and a round outer cross-section of the outer tube
prior to drawing, and the drawn monofilaments also have a hexagonal
outer cross-section of the cladding tube and a round outer
cross-section of the outer tube after drawing. The monofilaments
are thus merely "radially compressed" during drawing in step a').
This is particularly simple and prevents an outer shape change of
the cladding tube from also being impressed inside, e.g. on the
outer tube.
[0043] The invention also concerns a precursor of a superconducting
wire, produced by an inventive method as described above. A
superconducting wire having particularly high current carrying
capacity can be produced from the precursor.
[0044] The invention also concerns a method for producing a
superconducting wire from an inventive precursor of a
superconducting wire as described above, characterized by the
following steps:
[0045] d') the precursor of the superconducting wire is
mechanically deformed, in particular, wound to form a coil;
[0046] e') the deformed precursor of the superconducting wire is
temperature-treated, in particular at a maximum temperature of
700.degree. C. or less, wherein Nb from the inner tube and the
outer tube reacts with Sn from the powder core to form Nb.sub.3Sn.
The superconducting wire produced in this fashion can achieve a
particularly high superconducting current carrying capacity. In
step d'), the precursor is brought into a form desired for the
required application (which cannot be changed again after reaction
annealing in step e')). Temperature treatment is subsequently
performed in this form in accordance with step e'). Typical
applications are magnet coils, in particular for spectroscopic NMR
apparatus and imaging MRI apparatus.
[0047] In one preferred variant of the inventive method of
producing a superconducting wire, temperature treatment is
terminated in step e') before a reaction front that advances to the
outside has reached the boundary surface between the outer tube and
the cladding tube. This prevents introduction of impurities into
the cladding tubes of the contained monofilaments such that the
residual resistance of the cladding tubes can be kept low. In case
of a quench, the cladding tubes then provide powerful current paths
parallel to the Nb.sub.3Sn filaments to protect the superconducting
wire from burning through. Since advance of the reaction front
during temperature treatment is relatively uniform, it is easy to
find a suitable point in time for cooling in order to terminate the
reaction advance for different wire geometries by means of a few
tests with different annealing periods. The reaction fronts can be
easily recognized e.g. in cross-section via a scanning electron
microscope.
[0048] The invention also concerns a superconducting wire produced
by an inventive method as described above. The superconducting wire
has a particularly high current carrying capacity as already
described above.
[0049] Further advantages of the invention can be extracted from
the description and the drawing. The features mentioned above and
below may be used in accordance with the invention either
individually or collectively in arbitrary combination. The
embodiments shown and described are not to be understood as
exhaustive enumeration, rather have exemplary character for
describing the invention.
[0050] The invention is illustrated in the drawing and is explained
in more detail with reference to embodiments. In the drawing:
BRIEF DESCRIPTION OF THE DRAWING
[0051] FIG. 1 shows a schematic cross-sectional view of a first
embodiment of an inventive monofilament with hexagonal cladding
tube;
[0052] FIG. 2 shows a schematic cross-sectional view of a second
embodiment of an inventive monofilament with round cladding
tube;
[0053] FIG. 3 shows a schematic cross-sectional view of an
embodiment of an inventive precursor of a superconducting wire;
[0054] FIG. 4 shows a schematic diagram of the production of an
inventive monofilament;
[0055] FIG. 5 shows a schematic diagram of the production of an
inventive superconducting wire;
[0056] FIG. 6 shows a schematic cross-sectional view of a section
of an inventive superconducting wire in the area of a
monofilament.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0057] FIG. 1 schematically shows a first embodiment of an
inventive monofilament 1 in cross-section perpendicular to its
longitudinal direction.
[0058] The monofilament 1 has an inner tube 2 which consists of
niobium in the illustrated embodiment. A powder core 3 of a mixture
of, in the present case, NbSn.sub.2 powder, Sn powder and Cu powder
is disposed in the inner tube 2. The content of Cu in the mixture
is approximately 5 weight % and the content of Sn in the mixture is
typically at least 50 weight %.
[0059] The inner tube 2 is arranged in an outer tube 4 which
consists of NbTa7.5 in the present case. The outer side of the
inner tube 2 thereby directly abuts the inner wall of the outer
tube 4. The inner tube 2 and the outer tube 4 each have (circular)
round inner and outer cross-sections.
[0060] The outer tube 4 is, in turn, arranged in a cladding tube 5
which consists of elementary copper in the present case. The
cladding tube 5 has a round inner cross-section in the illustrated
embodiment such that the outer tube 4 abuts the cladding tube 5
over the whole surface. The outer cross-section of the cladding
tube 5 is hexagonal ("hexagonal tube"), thereby increasing the area
portion of the finished superconducting wire that can be utilized
for Nb.sub.3Sn.
[0061] The main source of Nb for the reaction heat treatment is the
outer tube 4. The inner tube 2 enables good compaction of the
powder core 3 in a previous drawing step (see also FIG. 4 in this
context) due to its comparatively thin wall. By way of example, the
ratio VD between the outer diameter D.sup.outside.sub.outertube of
the outer tube 4 and the outer diameter D.sup.outside.sub.innertube
of the inner tube 2 is approximately 1.65 in the embodiment shown.
The ratio VW between the wall thickness W.sub.outertube of the
outer tube 4 and the wall thickness W.sub.innertube of the inner
tube 2 is moreover approximately 4.2. The inner tube 2, the outer
tube 4 and the cladding tube 5 are arranged concentrically. No
solder is required for the monofilament 1.
[0062] FIG. 2 schematically shows a second embodiment of an
inventive monofilament 1 in cross-section perpendicular to its
longitudinal extension.
[0063] The second embodiment of the monofilament 1 largely
resembles the embodiment of FIG. 1, in particular, with respect to
the inner tube 2, the powder core 3 and the outer tube 4.
[0064] However, the cladding tube 5 here is provided with a
(circular) round outer cross-section. For this reason, the
monofilament 1 is easy to manufacture. The cladding tube 5 is also
produced of elementary copper and has a (circular) round inner
cross-section.
[0065] FIG. 3 shows a cross-section perpendicular to its
longitudinal extension of an embodiment of an inventive precursor
10 of a superconducting wire.
[0066] A plurality of drawn monofilaments 11 (in the present case
seven as an example), which are each produced from one monofilament
by means of a filament drawing process, are bundled in a wire
cladding tube 12 ("casing tube") in the precursor 10, and are
subsequently subjected to an extrusion and/or wire drawing process
in order to reduce the cross-section.
[0067] The wire cladding tube 12 is preferably produced of
elementary copper.
[0068] Cavities at the inner edge of the wire cladding tube 12 are
prevented or filled by means of filling profiles 13 which are
preferably produced of elementary copper.
[0069] In the present case, the monofilaments had a hexagonal outer
cross-section (see FIG. 1) already prior to the filament drawing
process such that the monofilaments were only radially compressed
during the filament drawing process. The outer tubes 4 in the drawn
monofilaments 11 correspondingly still have a round outer
cross-section. The round outer cross-section of each outer tube 4
is maintained even after the extrusion and/or wire drawing process
of the precursor 10. For this reason, starting from the powder core
3, the reaction front of Nb.sub.3Sn can uniformly and
concentrically approach the outer edge of the outer tube 4 during
reaction annealing. There are no particularly thin points where Sn
could prematurely break through into the cladding tube 5. Nor are
there particularly thick points where residual Nb not utilized for
the reaction to Nb.sub.3Sn protrudes.
[0070] FIG. 4 shows a schematic diagram of the respective
cross-sections of the specified components for producing an
inventive monofilament 1.
[0071] Within the scope of the production variant illustrated here,
the inner tube 2 is filled with the powder core 3 and subjected to
a drawing step a). The cross-section of the comparatively
thin-walled inner tube 2 is thereby reduced and the powder core 3
is compacted. The non-filled outer tube 4 is furthermore inserted
into the cladding tube 5 (in the present case of hexagonal outer
cross-section) in one step b). The steps a) and b) can thereby be
performed in arbitrary order or also simultaneously. In step c),
the drawn and filled inner tube 2 is subsequently introduced into
the outer tube 4 which is already arranged in the cladding tube
5.
[0072] Alternatively, the drawn and filled inner tube may also be
initially inserted into the outer tube and the outer tube can
subsequently be inserted into the cladding tube (not separately
shown).
[0073] FIG. 5 schematically illustrates the production process of a
superconducting wire 20 from monofilaments 1 as produced e.g. in
accordance with FIG. 4.
[0074] A monofilament 1 is transformed into a drawn monofilament 21
by drawing in step a') ("filament drawing process"). The
cross-sectional surface area is thereby reduced. If the
monofilament 1 already has a hexagonal outer cross-section (as
illustrated in FIG. 5), drawing merely effects radial compression.
This is preferred since in this case, a round outer cross-section
of the outer tube 4 can be easily obtained after drawing. If the
monofilament 1 has a non-hexagonal outer cross-section (e.g. a
round outer cross-section) a hexagonal outer cross-section is also
impressed during drawing according to a').
[0075] A plurality of drawn monofilaments 21 are then bundled in a
wire cladding tube 12 in step b'). The number of drawn
monofilaments is thereby basically arbitrary. Seven drawn
monofilaments 21 are bundled in the illustrated variant. In the
bundled configuration, the monofilaments in the core area can be
replaced by hexagonal Cu elements.
[0076] Extrusion and/or drawing is subsequently performed in step
c') ("wire drawing process") which is again accompanied by a
reduction in cross-section, thereby obtaining a precursor 10 of a
superconductor. This precursor 10 already has the cross-sectional
shape and cross-sectional size of the subsequent superconducting
wire but can still be plastically deformed.
[0077] For finishing the superconducting wire, the precursor 10
must be shaped in step d') so as to have the shape required for the
superconducting wire as determined by the desired application. In
the illustrated variant, the application concerns a magnet coil 23.
The precursor 10 is correspondingly wound onto a carrier 22.
[0078] Temperature treatment ("reaction annealing") of the formed
precursor is subsequently carried out in step e'). Towards this
end, the magnet coil 23 is put into a furnace 24 that is heated to
a temperature of maximally 700.degree. C. Sn from the powder cores
reacts with Nb of the inner and outer tubes in the monofilaments of
the precursor to Nb.sub.3Sn at these temperatures. Temperature
treatment is terminated before the reaction front reaches the outer
edge of the outer tubes. The formed precursor has been transformed
into a superconducting wire 20 by means of the temperature
treatment, the Nb.sub.3Sn filaments of which can carry an
electrical current (with corresponding cooling e.g. with liquid
helium) practically without ohmic losses. The superconducting wire
20 should not be deformed again after temperature treatment in
order to prevent breaking of the enclosed brittle Nb.sub.3Sn
filaments.
[0079] FIG. 6 is a schematic cross-section illustrating a section
of the superconducting wire 20 in the area of a temperature-treated
monofilament 61.
[0080] A reaction front 62 has radially advanced from the inside to
the outside in a temperature-treated monofilament 61 and has
generated a relatively homogeneous fine-grained area 63 of
Nb.sub.3Sn. The reaction front 62, however, has not completely
crossed the outer tube 4 but has left a circumferential border 64
of non-reacted material of the outer cladding tube 4 (in the
present case of NbTa7.5, i.e. Nb with 7.5 weight % Ta). The border
64 has an approximately uniform thickness S over its entire
circumference. The thickness S is adjusted by the temperature
treatment program to be just sufficiently large in order to
reliably prevent breakthrough of Sn into the matrix 65 (formed from
previous cladding tubes) of copper, thereby maintaining the
electrical conductivity of the matrix 65 at a high level. Due to
the round outer cross-section of the outer tube 4, a large portion
of the cross-sectional area of the superconducting wire 20 can
react to Nb.sub.3Sn. In particular, there are no remaining useless
bulges of material of the outer tube 4 (as would be generated at
the edges of an outer tube having an outer hexagonal
cross-section).
[0081] The round outer cross-section of the outer tube 4 can
already be obtained prior to drawing of the monofilaments (see step
a') in FIG. 5) through a hexagonal outer cross-section of the
cladding tubes.
[0082] A residual core 66 resulting from the powder core with a
reduced amount of Sn generally remains in the temperature-treated
monofilament 61.
[0083] The temperature-treated monofilament 61 is substantially
free of gaps and cavities
LITERATURE
[0084] A. C. A. van Wees et al., IEEE Trans. Magn.; MAG 19, 556
(1983), pages 5-8;
[0085] H. Veringa et al., Adv. Cryo. Eng. (Materials), 1984, 30;
813-821;
[0086] W. L. Neijmeijer, B. H. Kolster, Journal of Less-Common
Metals, 160 (1990), 161-170;
[0087] EP 1 701 390 A2
[0088] U.S. Pat. No. 3,926,683
[0089] T. Wong, C. V. Renaud, IEEE Trans. Appl. Supercond., Vol.
11, No. 1, March 2001, 3584-3587;
[0090] JP 2006 252949 A
[0091] US 2009/0011941 A1
[0092] EP 0 169 596 A1
[0093] S. Murase et al., IEEE Trans. Magn., Vol. Mag-21, No. 2,
March 1985, pages 316-319;
[0094] DE 26 20 271 A1
[0095] D. Rodrigues Jr. et al., Materials Research Vol. 3, No. 4,
2000, pages 99-103;
[0096] JP 05 298 947 A
[0097] JP 2004 193019 A
[0098] JP 05 290 655 A
[0099] J. F. Kunzler et al., Phys. Rev. Lett. 6 (1961), pages
89-97;
[0100] J. D. Elen et al., IEEE Trans. Magn., MAG-13 (1977), No. 1,
pages 470-473;
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