Note: Descriptions are shown in the official language in which they were submitted.
1036338
The invention concerns generally a method for the manufacture
of a superconductor and more particularly, the manufacture of one having
a superconductive intermetallic compound.
Superconductive intermetallic compounds consisting of two elements,
of the type A3B, e.g., Nb3Sn or V3Ga, which have an A-15 crystal structure,
exhibit very good superconduction properties and are distinguished partic-
ularly by a high critical magnetic field, a high transition temperature
and a high critical current density. They are therefore highly suitable
for use as superconductor coils for generating strong magnetic fields such
as those needed for research purposes. Other applications are in super-
conducting magnets for the suspension guidance of magnetic suspension
railroads or in windings of electric machines.
Several methods for manufacturing these superconductors in
the form of long wires or ribbons are known. They are particularly
employed in the manufacture of so-called multi-core conductors having
wires, particularly of Nb3Sn and V3Ga, arranged in a normal-conducting
matrix. Generally in these methods a ductile element of the compound to
be manufactured in wire forn4 e.g., a niobium or a vanadium wire, is
surrounded by a jacket of an alloy comprising a ductile carrier metal
and the other elements of the compound, for instance, a copper-tin alloy
or a copper-gallium alloy. In particular, a multiplicity of such wires
are embedded in a matrix of the alloy. The structure so obtained is then
subjected to a cross section-reducing process. This results in a long
wire such as is required for coils wherein the diameter of the niobium
or vanadium wire is reduced to a value on the order of about 30 to 50 pm
or even less. The latter is desirable in view of the superconduction
properties of the conductor. One further seeks to obtain through the
cross section-reducing processing the best possible metallurgical bond
between the wire and the surrounding matrix material of the alloy, without
1~)36338
the occurrence of reactions that lead to an embrittlement of the conductor.
After the cross section-reducing processing, the structure is subjected
to a heat treatment such that the desired compound is formed through
reaction of the wire material, with the other element contained in the
surrounding matrix--in one example the tin or gallium. The element con-
tained in the matrix diffuses into the wire material and reacts with the
latter, forming a layer of the desired compound (German Offenlegungsschriften
2,044,660; 2,052,323 and 2,105,828).
These known methods are not satisfactory for a number of reasons.
First, the diffusion process cannot be directed in such a manner that all
the gallium or tin present in the matrix for forming the intermetallic
compound is consumed. It is therefore not possible to build up V3Ga or
Nb3Sn layers of any desired thickness. The diffusion of gallium or tin
in the direction toward the vanadium or niobium cores comes to a standstill
when the activity of the elements gallium and tin in the copper matrix
is equal to their activity in the produced intermetallic compounds V3Ga
or Nb3Sn. In other words no further V3Ga or Nb3Sn is formed if the con-
centration of the gallium or tin in the copper matrix has dropped to a
certain value due to the diffusion of gallium or tin into the cores. For
instance, if one diffuses gallium into vanadium cores from a copper-gallium
matrix with 18 atom-percent of gallium at a temperature of 700C, the
equilibrium condition mentioned is reached when the gallium content of
the matrix has dropped to about 12 atom-percent. This means that only
about 38% of the gallium available in the matrix has been converted to V3Ga.
The thickness of the Nb3Sn or V3Ga layers formed in a multi-
core conductor is therefore dependent not only on the annealing time,
the annealing temperature and the composition of the copper-gallium or
copper-tin alloy, but also by the total amount of tin or gallium available
for each core, i.e., on the volume of the part of the matrix available for
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each individual core.
In order to achieve a high effective critical current density,
i.e., a high critical current density referred to the entire conductor
cross section, however, layers thick as possible of the intermetallic
compound to be prepared are required. Employing the prior art methods
this can only be achieved by making the ratio of the matrix component to
the core component of the total cross section area of the conductor such
that the growth of the layer is not limited by the available gallium or
tin content in the alloy. In other words, a core spacing as large as
possible is necessary. In multi-core conductors of given cross section,
however, this requirement can only be met either by drawing the cores,
if the number of cores is fixed, particularly thin in the cross section-
reducing processing, or by reducing the number of the cores, if the core
cross section is fixed. Either solution is not very satisfactory. On
the one hand the drawing of the cores ~o ~rm very thin filaments presents
considerable difficulties and is quite expensive. On the other hand, if
the number of cores is reduced, the effective current density is decreased
and may only be offset generally by the thicker diffusion layers that
may possibly be obtained. Finally, an arbitrary increase of the core
spacing is not possible for technical reasons inherent in the deformation
process. This is due to the fact that if one wishes to draw a larger
number of vanadium or niobium cores uniformly thin in such a manner that
their cross sections remain equal, the core spacing must not be too great.
A further difficulty with the known methods is that the matrix
. .,
material containing the embedded cores and consisting of the carrier
metal and the other elements of the compound to be manufactured is
re~atively hard to deform, particularly with higher concentrations of
these elements. These matrix materials have the property that they harden
very quickly in a cross section-reducing, cold-working process and become
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very difficult to deform further. With these methods it is therefore neces-
sary to subject the conductor structure consisting of the cores and the
matrix material, after relatively small deformation steps to an intermediate
anneal for recuperating and recrystallizing the matrix structure. Although
these heat treatments can be performed at temperatures and with annealing
times at which the superconductive compound to be manufactured will not form,
they are very time-consuming because of the required frequency. This increas-
ing degradation of the deformability of the matrix material with increasing
content of the remaining elements of the compound to be manufactured limits
their concentration and thus the thickness of the intermetallic compound layer.
Further, with increasing concentration of these elements the melting point of
the matrix material decreases. At very high concentrations this leads to
problems in the heat treatment process necessary for forming the intermetallic
compounds. Furthermore, these elements can form undesired intermetallic phases
with the carrier metal, if their concentration is too high.
There are known methods for avoiding the aforementioned, repeated
intermediate anneals. In these methods, one or several cores of the ductile
element are embedded in a ductile matrix material, e.g., copper, silver or
nickel, which itself contains no element of the compound to be produced, or
only very small amounts of such an element. The structure consisting of the
cores and this matrix material is then processed without any intermediate
anneal into a thin wire by a cross section-reducing process, as for example
the cold-working process. After the last cross-section-reducing step, the
remaining elements of the compound to be produced, i.e., tin in the case of
Nb3Sn, are then applied to the matrix material. This is accomplished by
immersing the wire briefly into a tin melt, so that a thin tin layer is form-
ed on the matrix material, or by evaporating a tin layer onto the matrix mate-
rial. Subsequently a heat treatment is then performed, in which the elements
applied to the matrix material, are first diffused into and through the mat-
rix material and then form the desired superconductive compound through
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reaction with the cores (I'Applied Physics Letters" 20 (t972), pages 443 to
445; German Offenlegungsschrift 2,~05,308).
However, in this method only relatively small amounts of the element
can be applied to the matrix. If larger amounts of tin are applied, unde-
sirable brittle intermediate phases of copper and the element can readily form
at the temperature re~uired for the diffusion of the element into the copper
matrix. Further, if too much of the elemen~ is applied, the element itself
or a surface area of the matrix can melt at the tempe~atures necessary for the
diffusion of the element and can then easily drip off or run off from the
matrix surface. The result is that only a limi~ed amount of the lower-melting
element is therefore available for the formation of the desired intermetallic
compound.
German Offenlegungsschrift 2,205,308 teachers an expensive, multi-
step process for converting all the niobium contained within a copper matrix
into Nb3Sn. m e process calls for repeating the individual process steps of
coating the matrix with tin and reacting the tin contained in the matrix with
the niobium cores.
The continuous method for the manufacture of multi-core conductors
of Nb3Sn described in the German Offenlegungsschrift 2,205,308 is a method
wherein the conductor structure in wire form, consisting of a copper matrix
and embedded niobium cores, is continuously conducted through an oven, in
which several vessels with melted tin are arranged side by side. The con-
ductor structure runs successively through the part of the oven interior
situated above these vessels and then exits from the oven. The tin melt,
through whose corresponding vapor space the conductor structure first runs
is at a temperature of 1500 C. The other tin melts, are at a temperature-of
1000C. The conductor itself is held by the oven at a temperature of 850C~
Apparently, the tin vapor pressure in the vapor space over the first tin melt
is so high that the transfer or deposition rate of the tin exceeds the solid
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diffusion rate of the tin into the copper matrix. Thus, a tin cancen-
tration gradient builds up quickly across the wire radius. The conductor
structure is held above the tin melt of the higher temperature until suf-
ficient tin, determined by the desired mean matrix composition, is applied.
When the conductor structure next passes over the lower temperature melts, the
tin vapor pressure is just high enough that the tin supply rate is reduced
to a value which is equal to that at which the tin diffuses through the copper
matrix and arrives at the surfaces or the niobium cores. The solid diffusion
itself takes place at the temperature of 850C. This temperature is chosen
in order to prevent the tin from evaporating from the matrix and to prevent
the matrix from melting. This method, however, is also extremely expensive
because of the three different temperatures required for the tin melts and the
conductor structure and because the conductor structure itself must be held
accurately positioned during the relatively laborious process.
The following therefore are some of the objects of this invention:
to provide a process for manufacturing a superconductor wherein
the layer thickness of the superconductive, intermetallic compound is not
limited by the process:
to employ a ductile matrix material that can be deformed cold with-
out intermediate anneals:to form the finished product employing but one heat treatment temper-
ature and thereby simplify the procedure substantially:
to provide a process for producing a superconductor having a super-
conductive intermetallic compound layer comprising at least two elements which
;~ conductor can have various forms;
to provide a process whereby the thickness of the layers of superconductive
intermetallic compounds can be controlled by varying the individual process
parameters;and
to provide an apparatus for implementing the process of the invention
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which does not inhibit access of the metal vapor to the conductor surface.
~bwards the accomplishment of these objects and others which will
become apparent from a reading of the specification, applicant provides a
method for manufacturing a superconductor comprising a superconductive,
intermetallic compound including at least two elements. A ductile component
of at least one element of the compound is brought into contact with a
ductile carrier metal bearing the other elements of the compound. The
structure so obtained is heat-treated in a vapor formed over a separate melt
of the elements found in the carrier metal in such a manner that the inter-
metallic compound is formed through reaction of the vapor, diffusing through
the carrier metal, with the ductile component. The heat treatment applied
until the ductile components and the other elements reach the same temper-
ature is performed in a vacuum with a residual maximum gas pressure of lO 2
Torr or in an inert gas at a maximum gas pressure of 500 Torr.
Towards the accomplishment of the above process, since the heat
treatment takes a relatively long time~ and the constancy of the pressure is
important, a closed chamber is provided. The chamber is adapted so that it
can be evacuated and, if desired, filled with an inert gas. A spoke wheel
arrangement is positioned in the chamber to take up the conductor which is
to be exposed to the heat treatment. Further, a vessel is arranged in the
chamber to receive the elements which form a part of the intermetallic com-
pound and which are to be melted. A heating device surrounds the chamber
and provides the heat necessary to perform the process.
In accordance with the broadest aspect of the invention there is
provided an improved method for manufacturing a superconductor in the form
of a superconductive intermetallic compound of the type A3B with an A-15
crystal structure including at least two elements the first of which is a
ductile element having a higher melting point and the second an element
having a lower melting point comprising the steps of:
a) providing a single melt consisting of the second element;
b) providing a starting structure consisting of the first element
surrounded by and embedded in a ductile carrier metal;
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c) placing said melt and said starting structure together in a
chamber with said starting structure disposed above said melt;
d) providing an inert atmosphere in said chamber by one of evac-
uating said chamber to a maximum residual gas pressure of 10 2 torr and
pressurizing said chamber with an inert gas to a maximum pressure of 500
torr; and
e) heating both said single melt and starting structure to the
same temperature, said temperature being between 600 and 950C, and main-
taining said temperature such that said melt forms a vapor of said second
element said vapor reacting with said starting structure such that said
superconductive intermetallic compound is formed through the reaction of
said vapor of said second element, diffusing through said ductile carrier
metal, with said first element.
Reference is now made to the accompanying drawings for a better
understanding of the nature and objects of the invention. In the drawings:
: Figure 1 shows schematically, in cross section, a conductor
structure for a multi-core conductor prior to the heat treatment, to be
manufactured by the method according to the invention.
Figure 2 shows the conductor structure of Figure 1 after the final
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heat treatment according to the invention.
Figure 3 shows schematically, in cross section, a further example
of an embodiment of a multi-core conductor manufactured in accordance with
the invention:
Figure 4 shows schematically, in cross section, a preferred embodi-
ment of an apparatus for carrying out the method according to the invention.
The method according to the invention is particularly well suited
for the manufacture of a superconductor which comprises a superconductive
intermetallic compound, consisting of two elements, of the type A3B with an
\0 A-15 crystal structure. In the preparation of such compounds the first com-
ponent consists of the higher-melting element of the compound, while the heat
treatment is performed in the vapor of the lower-melting element.
Particularly good cold-formability of a conductor structure con-
sisting of the first and the second component is achieved if the second com-
ponent consists only of the carrier metal. However, in this case the heat
treatment required for forming thicker layers of the intermetallic conpound
takes a relatively long time. The time of the heat treatment required for
producing a given layer thickness can be shortened if the second component
contains, in addition to the carrier metal, a share of the remaining elements
of the compound to be produced.
Copper and silver or a ductile alloy of these metals are particularly
well suited as the carrier metal for the second component. In some cases,
other ductile metals are also suitable. These must allow a sufficient diffu-
sion of the other elements towards the first component of the compound and
must have no interfering reaction with the elements of the compound to be
produced.
The thickness of the layers of superconductive intermetallic com-
pounds produced by the method according to the invention can be controlled by
varying the individual process parameters. For a given vacuum or inert gas
103~338
pressure and a given composition of the second component containing either
the carrier metal or if applicable, the carrier metal and the other elements
of the compound, the layer thickness increases with increasing diffusion time
or increasing temperature of the heat treatment. If the temperature and the
diffusion time are given, the layer thickness can be controlled by a corre-
sponding choice of the vacuum or the inert gas and the inert gas pressure.
Inert gases are understood here to be gases which do not react in the heat
treatment with the metals participating in the reaction. Particularly well
suited inert gases for this method are the rare gases and preferably helium
or argon. The largest layer thicknesses of the intermetallic compound are
obtained if the heat treatment is performed in a vacuum. If particularly
large layer thicknesses are desired a vacuum with a residual gas pressure of
about 10 5 Torr or less and application of the heat treatment for at least 45 ~-
hours has been found advantageous.
If the heat treatment is performed in a rare gas, the compound layers
formed become thicker with decreasing pressure in the reaction space. With
the pressure in the reaction space constant, the layers formed become thicker
if a gas having a smaller atomic weight is used. Thus, if V3Ga or Nb3Sn layers
with a thickness of S ~m or more are desired, the gas pressure should be at
most 500 Torr for a heat treatment in helium. The heat treatment would have -
to last at least 100 hours. For a comparable heat treatment in argon, one
need only work with a gas pressure of at most 100 Torr to achieve the same
layer thickness.
If the second component contains, in addition to the carrier metal,
elements of the compound to be produced, i.e., consists, for instance, of a
copper-gallium alloy or a copper-tin alloy instead of pure copper, thicker
compound layers are obtained, for otherwise equal conditions. However, the
.. . . . .
gallium or tin content of the second component should not be too high for the
aforementioned reasons. Further, if the gallium or tin content of the carrier
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metal is too high, there's a likelihood it will be difficult to bend the
finished conductors.
With regard to the heat treatment required, it has been found that
; a temperature no higher than that required for the treatment of the conductor
itself is necessary. It has been found that the temperature required for the
treatment of the conductor structure is below the melting temperature of the
alloy formed, during the treatment, from the carrier metal and the other
elements of the compound. However, it is high enough to cause a quantity of
vapor of the other elements to form above the melt which, in an appropriate
vacuum or inert gas, is sufficient for forming the desired layer thickness of
the intermetallic compound once it diffuses into the carrier metal.
A preferable adaptation of the method according to the invention
is its application to the manufacture of multi-core conductors. In this ad-
apta~ion, several cores of the first component are first embedded in a matrix
material of the second component which, together, is then processed for cross
section reduction. After the last cross section-reducing process step, the
heat treatment is then performed ln the vapor of the other elements of the
compound to be produced.
The cores of the multi-core conductors to be fabricated by the method
according to the invention, incidentally, need not be solid cores of, for in-
stance, vanadium or niobium. The core alternately may contain a center metal
which has a high, thermal and electric conductivity and which has normal
electrical conductive properties at the operating temperature of the super-
conductor, and a surrounding jacket which comprises at least one element of
the compound. The center metal used, further, must not react during the heat
treatment process to form interfering layers. Lastly, problems with the
process are simplified if the center material can be the same material as the
matrix material forning the carrier metal for the other elements. Particularly
well suited for the core center materlal is copper and silver.
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Since the lleat treatment takes a relatively long time, it is pre-
ferable that the invention be performed in a closed chamber. Further, in a
closed chamber it is easier to keep the pressure conditions which, as mentioned,
influence the course of the reaction considerably constant over an extended
period of time.
A suitable apparatus, e~oecially where very long conductors are to
be made according to the invention, is shown diagramatically in Figure 4. It
consists essentially of a chamber which is composed of a lower part 31 and a
removable upper part 32. The chamber can be evacuated and, if required, filled
with an inert gas via the pipe connection 33. A spoke wheel 34 is arranged
and is adapted to take up the conductor structure which is to be submitted to
the process. This arrangement is advantageous in that only a small section
of the conductor contacts the rungs thereby providing virtually uninhibited
access of the metal vapor to the conductor surface. Towards this end adjacent
turns on the wheel are wound so that they don't touch each other. The rungs
35 of the wheel should be made from a temperature-resistant material which
is inert with regard to the materials of the conductor. Ceramic rungs have
been found suitable.
In the chamber there is further a vessel 37 to receive the gallium
or tin supply 38 which is to be melted. On the outside, the chamber is su-
rounded by an oven 39, for instance, an electric resistance furnace by means
of which it can be heated to the temperature required for the heat treatment.
Particular examples illustrative of how the method according to
the invention can be used to fabricate superconductors will now be shown.
Generally the examples described concern formation of the intermetallic com-
pound V3Ga or Nb3Sn. It is to be realized of course that the method has ap-
plication to the formation of any superconductive intermetallic compound of
the type A3B, comprising two elements and having an A-15 crystal structure.
For the manufacture of a superconductor having an intermetallic
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compound, V3Ga, the first component consists, of course, of vanadium and the
second component comprising a carrier metal of copper, silver or a copper-
silvèr alloy, and containing 0 to 23 atom-percent of gallium. If good cold-
formability is desired, it should contain no more than 15 atom-percent of
gallium. Particularly good cold-formability up to a cross section reduction
of about 99% is obtained if the gallium content of the second component is at
most 12 atom-percent. The heat treatment for producing a superconductor with
V3Ga is best performed at a temperature between 600 to 950C. The temperature
treatment and the gallium content of the second component must be matched so
that the second component does not melt during the heat treatment due to dif-
~; fusion of additional gallium from the vapor phase. Temperatures between 600
and 750C have been found particularly advantageous for the heat treatment.
Some examples of the formation of a V3Ga layer will now be illustrated.
Example 1
For fabricating a V3Ga single-core conductor, a vanadium rod with
a diameter of about 7 mm was placed in a copper tube with an outside diametert
of about 20 mm. In a number of cross section-reducing cold-drawing passes this
structure was processed into a wire with an outside diameter of 0.4 mm and a
diameter of the vanadium core of 0.15 mm. A piece of the wire made in this
manner was placed together with a supply of gallium in a quartz ampoule.
After purging with helium, this quartz ampoule was evacuated to a residual
gas pressure of 10-5 Torr and then sealed off. The gallium supply was arranged
so that the piece of wire could not come into contact with the liquid gallium.
The sealed ampoule was then heated for 48 hours to a temperature of 700C.
After this heat treatment, in which the wire as well as the gallium melt were
at the same temperature of 700C, the ampoule was opened and the wire examined.
The examination showed that a V3Ga layer 8 ~m thick had formed at the surface
of the vanadium core. m e vaporous gallium therefore diffused into the copper
jacket of the wire and through the latter and reacted with the vanadium core,
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forming a relatively thick V3Ga layer. The gallium vapor pressure over the
melt is about 10-7 to 10-6 Torr at the temperature of 700C.
Example 2
Another piece of the wire made in accordance with Example 1 with
a vanadium core and a copper jacket was heat-treated together with a supply
of gallium in a vacuum of about 10 5 Torr for 46 hours at a temperature of
660C. The V3Ga layer formed at the surface of the vanadium core had a thick-
ness of about 2 ~m.
Example 3
For fabricating another V3Ga single-core conductor, a vanadium rod
with a diameter of about 10 mm was placed in a tube of a copper-gallium alloy
with 10 atom-percent of gallium and the remainder copper. The latter had
an outside diameter of about 20 mm. In a number of cold-working passes,this
structure was then processed into a wire with an outside diameter of 0.4 mm -
and a diameter of the vanadium core of 0.2 mm. Because of the relatively low
gallium content of the copper-gallium tube, no intermediate annealing between
the individual cross section-reducing cold-working passes was necessary. A
piece of the wire fabricated in this manner was then kept, together with a
supply of gallium, for 48 hours in a vacuum of about 10 5 Torr at a temperature
or 700C. In this heat treatment, a V3Ga layer with a thickness of 10 pm was
formed at the surface of the vanadium core. As a control experiment, another
; piece of the wire consisting of the vanadium core and the copper-gallium tube
- was also heated at a temperature of 700C for 48 hours in the same vacuum, but
without the presence of the gallium supply. Examination of this wire showed
that no V3Ga layer at all had formed at the surface of the vanadium core.
This was so because the gallium concentration in the copper-gallium jacket was
too low for the formation of such a layer.
Example 4
A wire prepared in accordance with Example 3 was heated together with
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a supply of gallium for 47 hours in a vacuum of about to~5 Torr at a temper-
ature of 660C. The V3Ga layer was 4 ym thick.
Example 5
Another piece of the wire fabricated in accordance with Example 3
was heated for 63 hours together with a supply of gallium in a quartz ampoule
filled with helium of a vapor pressure of about 500 Torr to 700C. The V3Ga
layer formed at the surface of the vanadium core was about 3Jum thick. With
a heat treatment of 100 hours, a V3Ga layer thickness of more than 5 pm was
obtained under otherwise equal conditions.
Example 6
For manufacturing a V3Ga multi-core conductor a vanadium rod with a
diameter of about 10 mm was first placed in a tube of a copper-gallium alloy
with 18 atom-percent of gallium and the remainder copper, and an outside di-
ameter of about 20 mm. m is structure was then processed by cross section-
reducing cold-working passes into a wire with an outside diameter of about
1 mm. Because of the relatively high gallium content of the gallium-copper
alloy, an intermediate anneal was performed (30 minutes at about 550C) after
each deformation by 30%. 60 pieces of wire obtained in this manner were then
placed in a copper tube and, with intermediate annealing after each 30% of
deformation, were processed for cross section reduction so far until a conductor
structure with an outside diameter of 0.4 mm was obtained. This conductor
structure contained 60 vanadium cores with an average core diameter of 37 pm.
me average core spacing was 7 ym. A piece of the conductor structure made
in this manner was then heated for 112 hours in a vacuum of about 10-5 Torr,
together with a supply of gallium, to a temperature of 700C. Subsequent
examination showed that in this heat treatment all the vanadium cores of the
conductor structure were converted into V3Ga over their entire cross section.
Example 7
A piece of the conductor structure made in accordance with Example 6
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with 60 vanadium cores was heated for 112 hours in argon with a pressure of
about 500 Torr, together with a gallium supply, at a temperature of 700C.
In this heat treatment, a V3Ga layer with a thickness of about 3 ym was formed
at the surface of each individual vanadiwn core.
Example 8
A piece of the conductor structure made in accordance with Example
7 was heated for 112 hours in argon with a pressure of about 100 Torr in the
presence of a gallium supply at a temperature of 700C. In this heat treatment,
V3Ga layers with a thickness of about 5 ~m were formed at the surface of each
individual vanadium core.
Example 9 ~- -
Another piece of the conductor structure made in accordance with
Example 6 was heated, together with a gallium supply, for 46 hours in a vacuum
of about 10-5 Torr at a temperature of 660C. In this heat treatment a V3Ga ~ -
layer with a thickness of about 7 ym was formed at the surface of each in-
dividual vanadium core.
m e effective critical current density of the resulting supercon-
ductor was checked at a temperature of 4.2 R in an external magnetic field
having a magnetic flux density of 5 Tesla. It was found to be 1.2x105 A/cm2.
In a control experiment under the same conditions, but without the
gallium supply, V3Ga layer thickness was about 1.3 ym. ThiS multi-core con-
ductor with the reduced layer thickness was found to have an effective critical
current density of only 3 x 104 A/cm2 in an external magnetic field of 5
Teslas at 4.2 Ko
The manufacture of a conductor according to Example 9 is illustrated
- still further in Figures 1 and 2. For reasons of clarity fewer vanadium cores
are shown in the figures than the specific example described above. Figure 1
shows the conductor structure after the last cross section-reducing processing
step, but prior to the heat treatment. A number of vanadium cores 2 are embed-
ded in a copper-gallium matrix 1. The condition of the finished conductor
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after the heat treatment is shown in Figure 2. nuring the heat tr0atment,
the V3Ga layers 3 were formed at the surface of the individual vanadium cores
2.
Example 10
Another piece of the conductor structure with 60 vanadium cores
fabricated in accordance with Example 6 was heated together with a gallium sup-
ply for 46 hours in argon with a pressure of about 100 Torr, at a temperature
of 660~. A V3Ga layer about 2 ~m thick was formed at the surface of each
vanadium core.
Example 1t
In this example, the manufacture of a V3Ga multi-core conductor will
be illustrated, in which the individual vanadium cores contain centers of cop-
per. To fabricate such a conductor, a copper core in wire form with a vanadium
jacket and a copper-gallium sheath ~ith 18 atom-percent of gallium and the
remainder copper was made first. For this purpose a copper rod was first
placed in a tubular vanadium jacket and the latter in turn in a tube of copper-
gallium alloy and the structure obtained in this manner was processed for cross
section reduction. Sixty of these wires were combined in a bundle and placed
in a copper tube to facilitate the subsequent further cross section-reducing
processing steps; the copper tube was removed again after the cross section-
reducing processing of the conductor. me conductor structure obtained in
this manner was processed by further cross section-reducing processing steps
into a wire with an outside diameter of about 0.4 mm. After this treatment,
the individual vanadium jackets had an outside diameter of about 35 pm and a
wall thickness of about 7.5 ~m. me outside diameter of the copper centers
was about 20 ~m. The thickness of the copper-gallium layer present between
the individual vanadium jackets was about 13 ym. The wire fabricated in this
manner was heated for 49 hours, together with a gallium supply, in a vacuum
of about 10-5 Torr at a temperature of 660C. V3Ga layers with a thickness of
. : .
, ~ ' ,' ' , :
: ~ . .
1~3~;338
about 3 ~m were formed at the surface of the vanadium jackets of the individual
cores. Critical currents of 76 to 90 A were measured when the flnished wire
was placed in an external magnetic field of 5 Teslas at 4.2K. For comparison
a piece of the same wire was subjected to the same heat treatment under the
same conditions, but without the supply of gallium. The V3Ga layers formed
in this control experiment had a thickness of only about 1 ~m or less. In a
magnetic field of 5 Teslas, this wire had a critical current of only 20 A at
4.2K.
A multi-core conductor fabricated in accordance with Example 11 is
shown diagramatically in Figure 3. A number of cores with a copper center 21
and a vanadium jacket 22 are embedded in a copper-gallium matrix 23. The
V3Ga layers formed at the surface of the vanadium jackets 22 are designated with
24. In the manufacture of such a conductor one can also s~art, of course, with
a pure copper matrix instead of with the copper-gallium matrix.
The method according to the invention is also well suited for the
manufacture of superconductors with the intermetallic compound Nb3Sn. For
the manufacture of a superconductor with this compound, one starts out with
a structure whose first component consists of niobium and the second component
of copper, silver or a copper-silver alloy, having a tin content of 0 to 8.5
atom-percent. If good cold-formability is desired, the tin content should not
exceed 4 atom-percent. The heat treatment~ of course~ is performed at a tem-
perature at which the second component will not melt. This lies in a range
between 600 and 850C, and preferably between 600C and 800C.
Example 12
For manufacturing an Nb3Sn multi-core conductor, a niobium rod was
first placed in a copper tube and this structure was then drawn into a long
wire without intermediate annealing. Twenty sections of this wire were then
combined in a bundle, again placed in a copper tube and drawn by cross section-
reducing cold-formation to a wire with an outside diameter of about 0.65 mm.
17.
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:~ .
103~i338
The twenty niobium cores contained in the wire had a diameter of about 50 ~m
each. The average core spacing was 35 ~m. A piece of the conductor structure
made in this manner was then heated for 63 hours together with a supply of tin
in a vacuum of about 10-5 Torr at a temperature of 700C. In this heat treat-
ment, Nb3Sn layers with a layer thickness of about S ~m were formed at the
surface of the niobiurn cores. The ~in vapor pressure at 700C is about 10 6
to 10 7 Torr.
Example 13
Another piece of the conductor structure made in accordance with
Example 12 was kept at a temperature of 750C for 46 hours in a vacuum of
about 10-5 Torr, together with a supply of tin. In this heat treatment Nb3Sn
layers with a thickness of about 14 ~m were formed at the surfaces of the ni-
obium cores.
Example 14
For manufacturing an Nb3Sn single-core conductor, a niobium rod
was placed in a copper tube and processed together with the latter by cross
section-reducing cold-formation into a wire with an outside diameter of 0.55
mm. The diameter of the niobium core in this wire was 0.15 mm. A piece of the
wire made in this manner was then heated for 46 hours, together with a tin
supply, in a vacuum to a temperature of 750 C. An Nb3Sn layer with a thickness
of about 7 ~m was formed at the surface of the niobium core.
As already mentioned, the method according to the invention is
suited not only for the manufacture of superconductors components of wire form,
; but also for the manufacture of superconducting components of different form.
A superconducting shielding baffle or a superconducting shielding cylinder
with a V3Ga layer can, for instance, be manufactured by providing a vanadium
plate or a vanadium cylinder with a copper layer on one side and heating the
arrangement obtained in this manner for S0 hours to a temperature of about
700C in a vacuum of 10 5 Torr in the presence of a supply of gallium. Gallium
18~
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then diffuses into the copper layer and reacts with the adjoining vanadium,
forming a V3Ga layer On the copper-free side of the vanadium component,
however, practically no reaction between vanadium and gallium takes place
under the conditions indicated, so that no V3Ga layer is formed there.
The first component with a higher-melting element of the compound -~
to be produced need not consist of a single metal in the method according to
the invention, but may also contain additives. For instance, titanium, zir-
conium or tantalum in quantities of up to about 30% by weight may be admixed
to the niobium or the vanadium. Additions of hafnium are also possible. A
vanadium-niobium alloy can also be used. Instead of only one other element
of the compound to be produced, such as gallium or tin being present in the
carrier metal several such elements can also be contained therein, i.e. both
tin and gallium could be present. This is also true of the metal vapor to be
used in the heat treatment, so that both tin and gallium could be present
side by side.
Other variations in the above embodiments, not illustrated, will be
obvious to those skilled in the art without deviating from the scope of the
invention as defined in the appended claims.
