Method for producing superconductive wires

The present invention relates to a method for producing multifilament superconductive wires of Nb.sub.3 Sn or V.sub.3 Ga filaments embedded in a Cu or Cu alloy matrix, with the wires containing metal additive elements from the group including Ti, Zr, Hf, V, Nb, Ta, Fe, Co, Ni in the filaments and/or in the matrix. The superconductive characteristics of the wires are predetermined for medium magnetic fields (below 12 T) as well as for high magnetic fields (12 T and above). The percentage of the additive (or additives) can be set to a predetermined value and thus the superconductive properties can be set as desired. This is accomplished by mixing metal additives in powder form with a powder of niobium or a niobium alloy, or of vanadium or a vanadium alloy, in a defined grain size and in a defined quantity. The resulting powder mixture is compacted in a container of copper or a copper alloy, the compacted container is shaped into a wire and, upon removal of the container layer, is processed further into wires. Depending on the desired superconductive properties, the final reaction heat treatment of the wires is performed at a selected temperature in a range from 500.degree. C. to 1000.degree. C. and at a selected heat treatment duration in a range from 48 to 300 hours.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for producing multifilament 
superconductive wires of Nb.sub.3 Sn or V.sub.3 Ga filaments having the 
A15 crystal structure embedded in a Cu or Cu alloy matrix, with the wires 
containing metal additive elements from the group including Ti, Zr, Hf, V, 
Nb, Ta, Fe, Co, Ni in the filaments and/or in the matrix, and the 
superconductive characteristics of the wires are predetermined for medium 
magnetic fields (below 12 Tesla (H=12T)) as well as for high magnetic 
fields (12 T and above). The starting materials, with or without cladding, 
are subjected to one or a plurality of repeated mechanical deformation 
steps and to a final reaction heat treatment in a temperature range 
between 500.degree. C. and 1000.degree. C. 
2. Technology Review 
Superconductive wires that are used to produce high magnetic fields (&gt;12 T) 
are usually composed of Nb.sub.3 Sn or V.sub.3 Ga filaments embedded in a 
Cu or Cu alloy matrix. To improve their superconductive properties for 
high magnetic fields, ternary or quaternary additives have been added to 
the superconductive intermetallic compounds. 
Multifilament superconductive wires based on bronze-Nb.sub.3 Sn with 
ternary or quaternary additives of, for example, uranium, titanium, 
zirconium, hafnium, vanadium, tantalum, iron, nickel, palladium, aluminum 
or others, are disclosed in published European Patent Application No. 
48,313. These additives serve to reduce the so-called prestress effect and 
are alloyed to the niobium and/or copper or to the bronze, respectively. 
This effect is a result of the different degree of contraction in the 
Cu-bronze and in the A15 filament during cooling from 1000.degree. K. to 
4.2.degree. K. and primarily affects the current carrying capability for 
high magnetic fields. Primarily, the critical current density J.sub.c is 
thus reduced considerably compared to the voltage-free state. If the 
extraneous magnetic field is the same, the reduction is noticeably 
weakened by the additives. 
Tantalum or also titanium and nickel are usually alloyed as additive 
elements into Nb or V starting material (0.3 to 30 weight % additive 
element). Since these elements are metals that have a high melting point, 
the alloys must be produced in an electron beam furnace. Melting processes 
performed in an electron beam furnace are very complicated and costly. To 
realize homogeneous distribution of the additive element, repeated 
remelting in the electron beam furnace is necessary. 
SUMMARY OF THE INVENTION 
The present invention to provides a method for producing multifilament 
superconductive wires of Nb.sub.3 Sn or V.sub.3 Ga filaments embedded in a 
Cu or Cu alloy matrix with which, on the one hand, the large amounts of 
time and energy required to introduce ternary or multinary additives into 
the superconductive compound are avoided and, on the other hand, the 
percentage of the additive (or additives) can be set to a predetermined 
value and thus the superconductive properties can be set as desired.

DETAILED DESCRIPTION OF THE INVENTION 
This is accomplished by the present invention in that 
(a) a powder of one or a plurality of metal additive elements or one or a 
plurality of alloys composed of at least two additive elements are mixed 
with a powder composed of 
(a.sub.1) niobium; or 
(a.sub.2) one or a plurality of Nb alloys from the group including Nb-Ta, 
Nb-Ti, Nb-Zr; or 
(a.sub.3) vanadium: or 
(a.sub.4) one or a plurality of V alloys from the group including V-Ta, 
V-Nb, V-Ti, V-Zr, 
with all powders having grain sizes in a range between 0.1 .mu.m and 500 
.mu.m diameter and the percentage y of the additive metal(s) in the powder 
mixture corresponds to a weight percentage within the limits of 0.1&lt;y&lt;50; 
(b) the powder mixture obtained from a) is filled into a compactable and 
evacuatable container of copper or a copper alloy, the enclosed quantity 
of air is removed and then the powder mixture and the closed container are 
compacted jointly by means of simple or isostatic pressing until a powder 
density of more than 90% theoretical density (T.D.) is obtained; 
(c) the compacted container is shaped in a known manner into a wire which 
has a diameter in a range from 0.5 mm to 15 mm and, after removing the 
outer Cu or Cu-alloy layer, is processed further, according to a known 
method, into Nb.sub.3 Sn wires or V.sub.3 Ga wires; and 
(d) depending on the desired superconductive properties, the final reaction 
heat treatment of the wires is performed at a selected temperature in a 
range from 500.degree. C. to 1000.degree. C. and for a selected duration 
in a range from 48 to 100 hours. 
In an advantageous embodiment of the invention, the powder(s) of the 
additive element(s) and the powder containing Nb or V as a component have 
the same or similar ductilities as defined by the respective Vickers 
microhardness values not differing by more than factor 2. 
The powder mixture is pressed in a Cu or Cu alloy tube and processed into a 
rod which is then processed further in the same manner as in the known 
methods. 
With the known methods for producing Nb.sub.3 Sn or V.sub.3 Ga 
multifilament superconductive wires products are obtained which have 
critical current density of 10.sup.4 A/cm.sup.2 at high magnetic fields 
(18 to 20 T). 
To achieve homogeneous distribution, the additive material inclusions 
should be smaller than 0.5 .mu.m. In the reaction heat treatment (about 
500.degree.-750.degree. C., 3 to 10 days), in which the respective 
superconductive Nb.sub.3 Sn or V.sub.3 Ga phase is produced, the alloy 
with the additive material is simultaneously formed as a result of 
solid-state diffusion. Depending on the selection of the reaction 
conditions, more or less additive material is dissolved in the 
superconductive phase. 
The process according to the invention can be used to produce ternary or 
quaternary superconductive wires, avoiding the complicated electron beam 
melting of the alloy (insofar as additive material is alloyed to the Nb 
core). Moreover, the percentage of additive material alloyed in is 
established only at the end of the manufacturing process, by the selection 
of the temperature and duration of the heat treatment. 
The invention will be described in greater detail below with reference to 
an example. The example is intended to illustrate the process of the 
present invention, and not to limit the scope of the invention of the 
claims. 
EXAMPLE 
The samples were produced from niobium powder having grain sizes between 
106 and 125 .mu.m. Its Viokers hardness H.sub.V was about 85 kg/mm.sup.2. 
The additive substance was tantalum powder (about 20 .mu.m in diameter; 
H.sub.V 100-120 kg/mm.sup.2). Weighed quantities of powder were mixed and 
then carefully filled--so that the mixture did not degrade--through a 
funnel into a Cu-Zr tube which was closed at the bottom and had a length 
of about 7 cm and a diameter of 9 mm. To realize extensive compaction, the 
powder was already prepressed with about 200 Megapascal (MPa) when the 
tube was partially filled. The tube was closed by a Cr-Zr plug which was 
pressed in from the top at about 300 MPa (1 MPa=10.sup.6 Newton/m.sup.2 
=10 bar). 
Thereafter, the wire was produced, primarily by hammering, in part also by 
rolling. To finely distribute the inclusions, it is necessary to reduce 
the cross sectional area to R.sub.a =10.sup.6 (R.sub.a =cross section 
areal reduction rate). Because of the small diameters of the samples, it 
was necessary to bundle several conductors together. To do this, the 
cladding material was etched away with diluted nitric acid when the wire 
diameter was about 1 mm and then 19 pieces of the conductor were bundled 
in one Cu-Zr tube. The second bundling was made in a bronze tube which was 
then hammered to a final diameter of 0.6 mm. 
The final reaction heat treatment in which the Nb.sub.3 Sn-A15 structure 
was produced, took place at temperatures between 675.degree. C. and 
750.degree. C. and for a period in a range from 64 to 100 hours. The 
curves in the attached drawing figure (1 to 3 from the process according 
to the present invention; 4 to 6 from the prior art processes) show that 
the addition of 15 weight % Ta, with reference to the Nb+Ta percentage in 
the (Nb,Ta).sub.3 Sn in conjunction with the reaction temperature and the 
reaction time permits the setting of predetermined properties for the 
superconductive wires for fields below 12 T as well as for fields of 12 T 
and above. 
1. (85 weight % Nb, 15 weight % Ta; 675.degree. C., 100 h) 
2. (85 weight % Nb, 15 weight % Ta; 700.degree. C., 64 h) 
3. (85 weight % Nb, 15 weight % Ta; 750.degree. C., 64 h) 
4. (100 weight % Nb, i.e. Nb.sub.3 Sn without additive elements; 
700.degree. C., 64 h) 
5. (92.5 weight % Nb, 7.5 weight % Ta; 700.degree. C., 64 h) 
6. (90.7 weight % Nb, 9.3 weight % Ta; 750.degree. C., 96 h). 
Curves 4 and 5 reflect the results from E. Drost, F. Flukiger, W. Specking, 
in Cryogenics 24, 622 (1984); curve 6 reflects the results from J. D. 
Livingston, IEEE Trans. Magn. MAG-14, 611 (1978). For reactions at 
temperatures below 700.degree. C., current carrying capability values are 
obtained similarly to those of good binary conductors, above 700.degree. 
C. as for ternary or multinary conductors. 
The manufacturing process is simplified if the starting diameter is 
selected to be large enough so that bundling several wires is no longer 
necessary. 
The processibility of the other known additive alloy materials (such as, 
for example, Ta, Ti, Zr, Hf, V, Fe, Co, Ni or Nb for V.sub.3 Ga) is 
similar to that of tantalum. Impurities from oxygen, carbon and nitrogen 
in the starting materials should be avoided. The Vickers hardnesses of the 
mixed powders are preferably approximately the same so that both types of 
powder will deform uniformly. 
The manufacturing process described was performed according to the bronze 
method; however, the desired superconductive wire could also be produced 
according to the similar method of internal tin diffusion. 
The invention described above is also described in patent application filed 
in the Federal Republic of Germany on Sept. 6th, 1985, application No. P 
35 31 769.8, in our entire specification of which is incorporated herein 
by reference. 
It is understood that various other modifications will be apparent to and 
can readily be made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.