Superconducting composite conductor and method of manufacturing same

In a superconducting composite conductor consisting of several strands with a superconductive intermetallic compound of at least two elements, and at least one strand of a thermally and electrically highly conductive stabilizing metal which is normally conducting at the operating temperature of the superconducting composite conductor, wherein the strands with the superconductive compound each contain a core of at least one higher melting point element of the compound, having, at least on its surface, a layer of the compound, embedded in an alloy of at least one lower melting point element of the compound and a carrier metal in the form of a cable, stranded wire or flat cable, the strand of the stabilizing metal is joined to the adjacent strands containing the superconductive compound by diffusion of at least one lower melting point element of the compound and contains at least one zone, which zone extends along the strand and is enclosed by a diffusion inhibiting layer.

BACKGROUND OF THE INVENTION 
This invention relates to superconductors in general and more particularly 
to a superconductor with improved stabilization. 
Superconducting composite conductors consisting of several strands with a 
superconductive intermetallic compound of at least two elements, and at 
least one strand of a thermally and electrically highly conductive 
stabilizing metal which is normally conducting at the operating 
temperature of the superconducting composite conductor, wherein the 
strands with the superconductive compound each contain a core of at least 
one higher melting point element of the compound, having, at least on its 
surface, a layer of the compound, embedded in an alloy of at least one 
lower melting point element of the compound and a carrier metal, in the 
form of a cable, stranded wire or flat cable, are known from German 
Auslegeschrift No. 23 45 779, especially column 6, line 16, to column 8, 
line 51, and German Offenlegungsschrift No. 26 54 924, especially page 30, 
para. 1, and page 64, last para., to page 67, para. 2. 
Starting out from an intermediate product which consists of an alloy of a 
carrier metal and at least one lower melting point element of the 
superconductive compound and one or more cores embedded in the alloy of at 
least one higher melting point element of the compound, the 
superconductive intermetallic compound is formed in such conductors by a 
heat treatment, in which the lower melting point element of the compound 
diffuses into the core of the higher melting point element and reacts with 
the core material, forming the compound. Depending on the composition of 
the alloy, the dimensions of the intermediate product and the duration of 
the heat treatment, a surface layer of the core or also the entire core, 
can be converted into the superconductive compound. 
In practice, the superconductive intermetallic compounds Nb.sub.3 Sn and 
V.sub.3 Ga, in particular, are used at present, both of which have A-15 
crystal structure. Both compounds have very good superconducting 
properties and are distinguished particularly by a high transition 
temperature, a high critical magnetic field and high critical current 
density. In order to manufacture superconductors with these compounds, one 
starts out, as a rule, with an intermediate product consisting of a matrix 
of a copper-tin alloy or of a copper-gallium alloy in which a multiplicity 
of niobium or vanadium cores is embedded. This intermediate product is 
first processed to reduce the cross section, drawing the cores into thin 
filaments. Subsequently, the heat treatment for forming the compound takes 
place. In addition to the two compounds mentioned, however, other 
compounds of two or more components with the same crystal structure, such 
as Nb.sub.3 Ga, Nb.sub.3 Al, V.sub.3 Ga, V.sub.3 Si or Nb.sub.3 
(Al.sub.0.8 Ge.sub.0.2) as well as intermetallic superconductive compounds 
with other cyrstal structures are also of interest. 
Certain difficulties arise in superconductors with superconductive 
intermetallic compounds due to the fact that the superconductive compounds 
are relatively brittle. The flexibility of the finished conductors is 
therefore lower, as a rule, than that of comparable conductors which 
contain cores of superconductive alloys such as niobium-titanium. One 
therefore attempts to make the layers of the superconductive compounds on 
the surface of the core and, also, the cores themselves as thin as 
possible. Since heavy cross section reductions of the intermediate 
products are required for this purpose, the conductor strands containing 
the cores themselves also have a relatively small cross section as a rule. 
This has the advantage that the cores within a conductor strand are 
located relatively close to the neutral axis when the conductor is bent, 
for instance, in winding a coil, so that the mechanical tensile and 
compression stresses occurring in the compound layers can be kept within 
limits even for relatively small bending radii. However, if large currents 
are to be obtained, a number of thin individual strands must be combined 
in a conductor of larger cross section in which the superconducting layers 
of the individual conductor strands are again farther removed from the 
neutral axis. 
In superconductors with intermetallic compounds, the electrical 
stabilization of the superconductors is a further problem. Stabilization 
requires a metal of high thermal and electric conductivity which is 
electrically normally conducting at the operating temperature of the 
superconductor. In contrast to superconductors, in which, for instance, 
thin filamentary cores of niobium-titanium are embedded in a copper 
matrix, the alloy material surrounding the cores with an intermetallic 
compound can be utilized for stabilization only with difficulty. For, 
since, even after the superconductive compound is formed, the alloy 
material still contains residues of the lower melting point element or 
several such elements of the compound, it has a substantially higher 
electrical resistance than, for instance, pure copper, at the operating 
temperature of the superconductor, which is below the critical temperature 
of the respective superconductor material, i.e., as a rule between about 1 
and 20 K. To achieve better stabilization, strands of stabilizing metal, 
for instance, copper, are provided in the superconducting composite 
conductors known from Auslegeschrift No. 23 45 779 and Offenlegungsschrift 
No. 26 54 924, in addition to the strands with the superconductive 
compound. So as to also achieve maximum flexibility of the conductor after 
the heat treatment for forming the superconductive compound, however, 
separator means which prevent adjacent strands from sticking together, 
especially due to diffusion during the heat treatment, are arranged in 
these known conductors between the individual strands. As a consequence, 
no intimate electrical and thermal contact is formed between the strands 
with the superconductive compound and the strands of stabilizing metal. 
This, in turn, can have a very unfavorable effect on the stabilizing 
action of the strands of stabilizing metal. In the known composite 
conductors, therefore, in order to provide a further improvement of the 
stabilization, the possibility of arranging additional zones of 
stabilizing metal, within the individual strands with the superconductive 
compound, which zones extend along the strand and are enclosed by a 
diffusion-inhibiting layer, for instance, of niobium, vanadium or 
tantalum, is disclosed. This diffusion inhibiting layer acts to prevent 
diffusion of the lower melting point element of the compound into the 
stabilizing metal during the heat treatment, and, thereby, prevents an 
increase of the electric resistance of the stabilizing metal (see, for 
instance, German Auslegeschrift No. 23 45 779, col. 8, line 55, to col. 
10, line 40, and German Offenlegungsschrift No. 26 54 924, page 28, para. 
3, to page 29, last line). 
Such stabilizing zones within the conductor strands, however, have the 
disadvantage, for one, that they increase the cross section of the strand, 
whereby the distance of the cores from the neutral axis of the conductor 
strand is increased, at least if the stabilizing zone is arranged in the 
center. This in turn also increases the distance of the layers of the 
superconducting intermetallic compound from the neutral axis. Secondly, 
the fabrication of such conductor strands is accompanied by great problems 
due to the different material properties of the cores, alloy jacket, 
diffusion inhibitor and stabilizing metal. In particular, cracks can 
readily occur in the diffusion inhibiting layer, through which cracks the 
lower melting point element of the compound can penetrate into the 
stabilizing metal. In such a case, the entire conductor strand, including 
the superconductor material, then becomes unusable. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve the electrical 
stabilization of the composite conductors mentioned at the outset, while 
zones of stabilizing metal within the individual strands with the 
superconductive compound are to be avoided to the extent possible. 
According to the present invention, this problem is solved by the provision 
that the strand of the stabilizing metal is joined to the adjacent strands 
containing the superconductive compound by diffusion of at least one lower 
melting point element of the compound and contains at least one zone, 
which zone extends along the strand and is enclosed by a diffusion 
inhibiting layer. 
Contrary to the known composite conductors, in which a bond between the 
individual strands due to diffusion is expressly to be avoided, such a 
bond is intentionally brought about in the conductor according to the 
present invention, which leads to an intimate thermal and electrical 
contact between the strands with the superconductive compound and the 
strands with the stabilizing metal. For, it has been found, contrary to 
all expectations, that the flexibility of the composite conductor is fully 
adequate for the usual applications of the composite conductor, for 
instance, for winding superconductor magnet coils, in spite of the 
diffusion bond between the strands. Otherwise, it is also possible to 
establish the diffusion bond between the conductor strands only after the 
conductor has been given its final form, for instance, after the conductor 
has been wound into a coil. So as not the disturb the formation of a 
superconductive compound, for instance, by the addition of a foreign 
element, a lower melting point element of the compound or several such 
elements are used for establishing the diffusion bond between the strands. 
The lower melting point element can then diffuse only into a surface layer 
of the strand of stabilizing metal, while the inner region thereof is 
protected against diffusion by the diffusion inhibiting layer and 
consequently retains its high thermal and electric conductivity. Normally, 
only one such region will be provided in a strand of stabilizing metal; in 
special cases, however, the strands can also contain several such regions. 
Since the strands with the superconductive compound and the strands of 
stabilizing metal can be produced in separate operations, up until their 
combination in one conductor, the technical difficulties in the 
manufacture of the conductor are considerably reduced. 
It is particularly advantageous for a good stabilizing effect if every 
strand with the superconductive compound is bonded to at least one strand 
of stabilizing metal. 
Especially for larger conductor cross sections or for building up the 
composite conductor from numerous conductor strands, it is further 
advantageous to construct the composite conductor as a flat cable. For, in 
such a flat cable, the individual cores are substantially closer to the 
neutral axis of the conductor in one bending direction than with a 
circular cable of corresponding cross section. 
The composite conductor can furthermore advantageously be designed in such 
a manner that the strands with the superconductive compound and the 
strands of stabilizing metal have the same cross section. Then the strands 
of both kinds can be mixed within the conductor without difficulty. 
Preferably, the strands with the superconductive compound will be the type 
which have a multiplicity of filamentary cores embedded in a matrix of the 
alloy, with a layer of the superconductive compound at least on the 
surfaces thereof. 
In the composite conductor according to the present invention, the 
compounds Nb.sub.3 Sn and V.sub.3 Ga are particularly advantageous as the 
superconducting compound. The higher melting point element of the 
superconductive compound is one of the metals niobium and vanadium and the 
lower melting point element is one of the elements tin and gallium. 
Suitable carrier metals for the alloy which contains the cores include, in 
principle, all metals which form a ductile alloy with the lower melting 
point elements of the compound and do not adversely affect the formation 
of the compound during the heat treatment, for instance, copper, silver, 
nickel or alloys of these metals. Also, for the stabilizing metal, all 
metals with high thermal and electric conductivity which are normally 
conducting at the operating temperature of the superconductor and do not 
disturb the formation of the superconductive compound, are suited in 
principle. Copper is particularly advantageous as the carrier metal of 
alloy as well as for the stabilizing metal. 
For the diffusion inhibiting layers inside the strands of stabilizing metal 
the metals tantalum, niobium or vanadium are suited especially. These 
metals, in particular, prevent the diffusion of tin and gallium. If there 
is danger that, during a heat treatment, sufficient tin or gallium will 
diffuse into the stabilizing conductors so that, at the surface of the 
diffusion inhibiting layers of niobium or vanadium itself, superconductive 
intermetallic compounds are formed again, additional diffusion inhibiting 
layers can be provided, as is known from German Offenlegungsschrift No. 25 
43 613, especially page 7, para. 2, to page 10, para. 1, in order to 
prevent the formation of a closed layer of the superconductive compound, 
enclosing the remaining region of stabilizing metal on all sides. For, 
such a superconducting layer, which encloses the stabilizing metal from 
all sides, can affect the stabilizing action adversely because of its 
shielding effect. For mechanically reinforcing the composite conductor, 
especially for taking up tensile forces, one or more strands of 
reinforcement material, for instance, alloy steel, can further be provided 
within the conductor. As a rule, these strands should have higher tensile 
strength than the other strands of the composite conductor. 
The composite conductor according to the present invention can 
advantageously be manufactured in such a manner that several strands which 
contain at least one core of at least one higher melting point element of 
the superconductive compound embedded in an alloy of a carrier metal and 
at least one lower melting point element of the compound, and at least one 
strand of stabilizing metal, which contains at least one region which 
extends along the strand and is enclosed by a diffusion inhibiting layer, 
are joined together in a conductor, wherein at least one kind of strand is 
coated with a layer of at least one lower melting point element of the 
compound, and that the conductor is heat treated for joining the strands 
together by diffusion and for producing the superconductive intermetallic 
compound. The layer of at least one lower melting point element of the 
compound which is to be applied on the strands with the elements of the 
superconductive compound or on the strands of stabilizing metal or on 
both, ensures a good diffusion bond between the conductors and prevents 
the lower melting point elements of the compound of the strands with the 
cores of the higher melting point elements of the compound, where they are 
needed primarily for forming the superconductive compound, from diffusing 
into the strands of the stabilizing metal. 
One can advantgeously further perform a first heat treatment for joining 
the strands by diffusion at a temperature below the formation temperature 
of the superconductive compound and a second heat treatment above this 
temperature for producing the superconductive compound. The first heat 
treatment can be combined particularly with a hot deformation of the 
strands for sizing and shaping the conductor. Two heat treatments are 
advantageous particularly in cases where the second heat treatment for 
producing the superconductive compound is performed only after the 
conductor is given its final form, for instance, when the conductor is 
already wound into a coil. Through the first heat treatment, the conductor 
strands then already adhere to each other firmly and cannot bend away from 
each other when the coil is wound.

DETAILED DESCRIPTION OF THE INVENTION 
In the conductor shown in FIG. 1, eight conductor strands 1 which contain, 
in an alloy matrix 2 consisting of a carrier metal and the lower melting 
point element of a superconductive intermetallic compound, a multiplicity 
of filamentary cores 3 of the higher melting point element of the 
compound, are stranded with four strands 4 of stabilizing metal to form a 
flat cable in such a manner that each strand 1 is in contact with a strand 
4. Each strand 4 contains an inner region 5 which extends along the strand 
and is enclosed by a diffusion inhibiting layer 6. The latter is again 
surrounded by an outer layer 7 of stabilizing metal. 
As is well known, the conductor strands 1 can be made, for instance, by 
bundling niobium rods surrounded by a copper-tin jacket and first 
hot-deforming the arrangement so obtained for producing an intimate 
metallurgical bond between the individual parts and subsequently drawing 
it into a thin wire in a series of cold working steps which may be 
interrupted by intermediate anneals for recovery of the structure of the 
alloy matrix. Toward the end of this deformation, this wire can further be 
twisted about its axis so that filamentary cores 3 describe helical paths 
about the axis of the wire. Such twisted strands have the advantage that 
the filamentary cores 3 are completely transposed within the stranded 
composite conductor, i.e., occupy all possible positions sequentially 
along the composite conductor. 
The strands 4 can be similarly made, starting out, for instance, with an 
intermediate product which consists of a copper rod which is surrounded by 
a tantalum jacket and is inserted into a copper tube. 
For the further fabrication of the conductor according to FIG. 1, the 
strands 4 can be tinned, for instance, and subsequently stranded with the 
strands 1, which are advantageously provided with flux, to form a flat 
cable. For sizing and shaping, the flat cable can be hot-rolled, for 
instance, at a temperature below 650.degree. C., at which Nb.sub.3 Sn is 
not yet formed. It is advisable to start at about 200.degree. C., so that 
the tin does not run off, and then to continue heating. In this heat 
treatment, tin diffuses from the surface of the strands 4 into the outer 
zone 7 of the strands 4 located outside the diffusion inhibiting layer 6 
as well as into the alloy matrix 2 of the strands 1 and establishes an 
intimate diffusion bond between the strands 1 and 4. In a second heat 
treatment at about 700.degree. C., the Nb.sub.3 Sn layers are then formed 
at the surface of the niobium cores 3. In addition, the diffusion bond 
between the strands 1 and 4 is further strengthened thereby. 
In a conductor according to FIG. 1, in which the diameter of the individual 
strands was 0.4 mm, the strands 1 contained 1615 niobium filaments 3 with 
a diameter of 4.7 .mu.m each in a copper-tin matrix. The strands 4 had a 
copper core 5 with a diameter of about 240 .mu.m, a diffusion inhibiting 
tantalum layer 6 with a thickness of about 30 .mu.m and an outer copper 
jacket 7 with a thickness of about 50 .mu.m. With such a composite 
conductor, the overall cross section of which was about 0.74.times.2.7 
mm.sup.2, a critical current of about 1200 A was obtained at a temperature 
of 4.2 K and without an external field, and a critical current of about 
590 A with an external magnetic induction of about 10 Tesla. These values 
were obtained with short wire samples as well as as with a conductor wound 
into a coil, in which the second heat treatment for forming the Nb.sub.2 
Sn layers was performed after the coil was wound. 
In the conductor shown in FIG. 2, twelve strands 21 containing a 
multiplicity of cores 23 in an alloy matrix 22, are stranded about a 
ribbon-shaped strand 24 of stabilizing metal, which contains a zone 26 
enclosed by a diffusion inhibiting layer 25. The strand 24 of stabilizing 
metal is touched by each of the strands 21. The conductor can be 
fabricated by using the methods described in detail in connection with 
FIG. 1. With a conductor according to FIG. 2, the strands 21 of which 
corresponded to the strands 1 of the conductor of FIG. 1, and the strand 
24 of which was made of copper and had a cross section of about 
0.3.times.1.5 mm.sup.2 and again contained a tantalum layer as a diffusion 
inhibitor, a critical current of 930 A was obtained at 4.2 K with an 
external induction of 10 Tesla. For small bending radii, however, the 
conductor design according to FIG. 2 is less advantageous than that of 
FIG. 1, since the filaments 23 are further removed from the neutral axis 
of the conductor than are the filaments 3. 
FIG. 3 shows diagrammatically a further embodiment of a conductor according 
to the present invention, in which strands 31 with a superconductive 
intermetallic compound and strands 32 of stabilizing metal are 
alternatingly stranded about a ribbon shaped strand 33 of reinforcement 
material, for instance, alloy steel. The strands 31 and 32 can optionally 
be connected to this strand 33 of reinforcement material by a diffusion 
joint or by soldering. Strands of reinforcement material which have the 
same diameter as the other strands of the conductor can also be used, of 
course, and can be stranded, for instance, with the other strands in the 
manner shown in FIG. 1. 
The conductor according to the present invention and its manufacture can be 
modified further in many ways from the examples. For instance, one can 
also provide the finished but not yet heat treated conductor with a layer 
of one or more of the lower melting point elements of the compound, 
instead of the individual strands. Instead of providing the strands of 
stabilizing metal with a layer of one or more of the lower melting point 
elements, one can also make the outer jacket surrounding the diffusion 
inhibiting layer of an alloy of this element or these elements with the 
stabilizing metal. In such a strand, a copper core would then be 
surrounded, for instance, by a tantalum layer and the latter, in turn, by 
a copper-tin or copper-gallium jacket. 
Instead of stranding the strands, they can also be braided into a litz 
wire. The cores embedded in the alloy matrix of the superconductor 
strands, furthermore, need not consist of a single metal but can 
optionally also contain additions; for instance, titanium or zirconium can 
also be admixed to the niobium or the vanadium in amounts of up to about 
30% by weight. In the alloy matrix, the elements tin and gallium may also 
be present side by side, for instance. 
The strands with the superconductive compound can also contain additional 
regions of stabilizing material surrounded by diffusion inhibiting layers. 
In general, however, this will not be necessary because of the good 
stabilizing effect of the strands of stabilizing metal joined to these 
strands. Furthermore, it is also not absolutely necessary to use an alloy 
of a carrier metal and at least one lower melting point element of the 
compound, in which at least one core of at least one higher melting point 
element of the compound is embedded, as starting bodies for the strands 
with the superconductive compound. One can rather start also with a strand 
of the carrier metal alone, which contains one or several cores, and 
diffuse the lower melting point elements into the carrier metal only 
during the heat treatment of the composite conductor built up from the 
strands. 
This can be achieved, for instance, by tinning a strand with niobium cores 
in a copper matrix before or after the stranding and then annealing it. It 
is furthermore possible to supply the tin during the heat treatment from 
the vapor or gas phase.