Making a superconductive tube

The method of U.S. Pat. No. 3,866,315 is improved by brushing copper particles also on the other side of the niobium ribbon prior to, e.g., rolling and by increasing the layer's thickness by an electrolytic process. That layer will be on the outside of the tube. Tin is subsequently deposited thereon; and by means of annealing, the tin is caused to diffuse through this outside copper layer and into the niobium to form a uniform Nb.sub.3 Sn layer.

BACKGROUND OF THE INVENTION 
The present invention relates to the making of a corrugated, 
copper-stabilized Nb.sub.3 Sn superconductor. 
Nb.sub.3 Sn is a superconductive compound which is highly suitable for 
cryogenic current conduction because its transition temperature of about 
18.degree. Kelvin is, for this material, unusually high and can, 
therefore, be readily operated with. 
U.S. Pat. No. 3,866,315 and German printed patent application No. 2,443,226 
relate to methods of making such a superconductor; a ribbon of niobium is 
worked in that copper particles are brushed onto the ribbon and, 
thereafter, a thick copper layer is electroplated onto the ribbon. Next, 
the ribbon is longitudinally folded in order to obtain a split tube, 
welded along its edges, and the resulting tube is corrugated. Thereafter, 
a tin layer is deposited onto the niobium and annealed in a vacuum for 
obtaining the compound Nb.sub.3 Sn in the interface of tin and niobium. A 
problem has arisen here concerning the depositing of the tin layer which 
should be thin, but must be continuous, without any holes or weak spots 
because the Nb.sub.3 Sn layer should be as uniform as possible for the 
purpose of obtaining a uniform current density in cryogenic use. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
method of making superconductors as basically outlined above, but in a 
manner that improves their yield. 
It is a feature of the present invention to continue the use of a niobium 
ribbon with brushed-on copper particles and enhancement of the thickness 
of this copper layer. Also, the ribbon is to be longitudinally folded for 
obtaining a split tube, welding the tube along adjoining ribbon edges, and 
corrugating the tube. 
In accordance with the preferred embodiment of the invention, it is now 
suggested to additionally brush copper particles also onto the other side 
of the niobium ribbon prior to tube-forming and to, possibly, enhance the 
thickness of that second brushed-on layer which will be the outer surface 
of the tube when formed. Subsequently, tin is applied to that outer layer 
which is followed, possibly immediately, by a heat treatment process which 
causes the tin to diffuse throughout the copper until reaching the 
niobium. Further heat treatment causes the tin to diffuse into the niobium 
so that Nb.sub.3 Sn is formed. The heat treatment is to be carried out at 
a temperature in excess of 600.degree. C., preferably even in excess of 
800.degree. C. 
It was surprisingly found that this thinly brushed-on copper layer 
facilitates greatly the diffusion process and results in a uniform 
diffusion of tin into the niobium. Also, the time it takes for the tin to 
complete this requisite diffusion is quite short so that one can 
manufacture continuously. 
Enhancement of the thickness of this second copper layer is preferably 
carried out after the copper-plated niobium layer with brushed-on copper 
particles on the second side is plastically deformed, e.g., by rolling, 
stretching, or otherwise. The resulting tubular conductor is really a 
niobium copper compound sheet tube having a relatively large copper cross 
section and a small, relative niobium content. The rolling actually 
improves the metallic copper-to-niobium bond.

Proceeding now to the detailed description of the drawings, the process 
begins with a thin niobium ribbon having copper particles of 0.1 mm to 1.0 
mm thickness. Fine copper particles are continuously brushed onto one side 
of that ribbon, as described, e.g., in U.S. Pat. No. 3,866,315 mentioned 
above. As a consequence, a very thin copper layer of about 10 mg/dm.sup.2 
(=0.1 mg/m.sup.2) on the average is provided on that one niobium ribbon 
side. Also as described in said patent, the ribbon is thereafter 
electroplated on that side in one or several, serially arranged 
electrolytic baths. Accordingly, a relatively thick copper layer of, say, 
0.5 mm thickness is grown on that one niobium ribbon side. This 
additional, stabilizing copper layer should be at least 0.3 mm thick, but 
not more than 0.8 mm. A range of 0.4 mm to 0.6 mm is preferred. 
Next, the copper-plated ribbon is rolled, with or without application of 
heat, at a degree of deformation in excess of 50% in order to obtain a 
uniform thickness. In lieu of rolling, one may stretch the ribbon. 
Generally speaking, the niobium-plus-copper ribbon should undergo some 
plastic deformation (of at least 50%) in such a manner that the metallic 
bond between the copper and the niobium is enhanced. Soft-annealing may 
follow this rolling (or other working) process. 
In accordance with specific features of the invention, the ribbon is now 
treated by a brushing-on of copper also on the other (niobium) side for 
obtaining a similarly thin copper particle layer. Conceivably, this copper 
layer may also be enhanced as to its thickness by an electrolytic plating 
process. This increase or growth in and of the second copper layer should 
amount to at least several .mu.. This second layer should be approximately 
10.mu. to 120.mu. thick, preferably 40.mu. to 80.mu.. By way of example, 
it may be approximately 60.mu. thick (combined layers 6 and 11 in FIG. 2, 
infra). This then is still considerably thinner than the first-mentioned 
copper layer (being combined layers 4 and 11 in FIG. 2). The resulting 
ribbon 1, e.g., is stored in a drum 2 until used in the production line 
shown in FIG. 1. 
The ribbon is longitudinally folded into a smooth wall-split tube, whereby 
the thicker copper layer faces the interior of the tubes, the second, thin 
copper layer faces outwardly. Adjoining edges of the folded ribbon are 
welded in 7 to close the tube. It should be noted that in a separate 
cutting process edge portions of the niobium ribbon may be trimmed off so 
that only the thick copper portions adjoin directly for welding, the 
trimmed niobium edges will be spaced a little. This will avoid any mutual 
interference of copper and niobium during the welding process. 
The closed tube is corrugated in a station 8 which may be a corrugating 
device for obtaining a helical corrugation or one for obtaining an annular 
corrugation pattern. 
The corrugated tube is next passed through a tank 9 which is a tin bath, 
for depositing tin onto the outer surface of the tube. Another downstream 
station (not shown) heats the tube sufficiently, e.g., at least up to 
600.degree. C., preferably up to 800.degree. C. The tin will now diffuse 
through the thin, outer copper layer and into the niobium whereupon 
Nb.sub.3 Sn is obtained. As stated earlier, the thin copper layer ensures 
an easy and uniform diffusion of the tin, first into the copper and then 
through that thin copper layer into the niobium. The resulting application 
of the tin to the niobium in that manner is surprisingly more uniform than 
in the case of direct application. 
Tin plating can be obtained also by an electrolytic process or by flame 
spraying. In the case of a tin bath as shown, there may immediately be 
provided a diffusion annealing for obtaining the migration of tin 
molecules into and through the thin copper layer and into the niobium. In 
other words, 8 can be construed to be a combined tin-depositing and 
diffusion-annealing station. 
FIG. 2 is a cross section through the completed conductor. The innermost 
layer 4 of this tube is the thick copper layer that was plated on first, 
onto a niobium ribbon 5. The interface of niobium and copper was 
originally the first brushed-on copper layer 10 which, however, will be 
more or less obliterated as an independent layer by the copper 
electroplating. 
The outer layer of the tubularly folded niobium ribbon 5 carries a thin, 
brushed-on copper layer 6. The illustrated thickness of the brushed-on 
layers is grossly exaggerated for ease of illustration. A thin copper 
layer 11 was galvanically, i.e. electrolytically, plated onto the outer 
brushed-on layer 6. Tin 12 constitutes the outer layer of the assembly. 
The desired composition Nb.sub.3 Sn is formed between the copper particle 
layer 6 and the niobium ribbon 5. The ribbon now is actually just one of 
several intimately joined strata of a multilayered tube.