Method for enhancing critical current of triniobium tin

A method is described which increases the critical current of triniobium tin by bonding thermal contraction control layers to the triniobium tin superconducting articles at a process temperature to form a composite, and subsequently cooling the composite to a test temperature.

FIELD OF THE INVENTION 
This invention is directed to the enhancement of the critical current of 
triniobium tin superconductoring articles. In particular, the invention 
relates to the use of thermal contraction control layers to enhance the 
critical current in superconducting devices of triniobium tin. 
BACKGROUND OF THE INVENTION 
The effect of strain on the superconducting properties of triniobium tin 
(Nb.sub.3 Sn) has been studied for nearly thirty years. It is well known 
in the art that compressive strains in triniobium tin protect the brittle 
material from damage that may be encountered during handling, coiling, or 
from electromagnetic field forces in an energized magnet. 
Compressive strains are usually imparted to the triniobium tin by the 
materials that encase the superconductor. Such materials include bronze in 
bronze-processed multifilamentary wire or copper foil in laminated 
liquid-phase diffusion-processed trinibium tin foil. The compressive 
strain is caused by the difference in thermal expansion coefficients of 
the copper or bronze cladding and the triniobium tin superconductor. This 
difference leads to compression of the triniobium tin when the composite 
is cooled from the processing temperature. 
Most triniobium tin superconductor strain-effect research has been 
performed at high fields on mono- and multi- filamentary bronze-processed 
wire, which is formed by solid-phase diffusion process. 
For instance, a strain scaling relation has been developed by J. W. Ekin, 
"Strain Scaling Law and the Prediction of Uniaxial and Bending Strain 
Effects in Multifilamentary A15 Superconductors," Multifilamentary A15 
Superconductors, M. Suenaga and A. F. Clark, ed., Plenum Press, New York, 
1980, 187-203. Ekin's relation predicts the effect of strain on the 
critical current, I.sub.c, based on the axial strain dependence of the 
magnetic critical field, B.sub.c2, for multifilamentary superconductors. 
Ekin's paper explains that when uniaxial strain is applied to a 
multifilamentary triniobium tin conductor, the critical current density, 
J.sub.c, typically increases to a maximum value J.sub.cm at some strain 
and then decreases under further uniaxial tension. This maximum in 
critical current density arises from compressive prestress which the 
bronze matrix exerts on the triniobium tin reaction layer because of the 
difference in thermal contraction between the two materials on cooldown 
after the reaction heat treatment. It is thought that the maximum occurs 
where the triniobium tin experiences the smallest magnitude of intrinsic 
strain. 
There is a need to develop a method to enhance the critical current of 
triniobium tin superconducting articles formed by liquid-phase and 
solid-phase diffusion processes. 
BRIEF DESCRIPTION OF THE INVENTION 
This invention fulfills this need by providing a method to increase the 
critical current of a triniobium tin superconducting article comprising: 
heat treating the triniobium tin superconducting article with at least one 
thermal contraction control layer, said thermal contraction control layer 
being at least about as thick as said triniobium tin superconducting 
article, at a process temperature to form a composite, said composite 
comprising the triniobium tin superconducting article and the thermal 
contraction control layer; and cooling the composite to a test temperature 
lower than the process temperature, to reduce thermal strains and increase 
the critical current in the triniobium tin superconducting article. 
The term "triniobium tin superconducting article" may include the 
following: a triniobium tin superconductor, a superconducting foil 
comprising triniobium tin and niobium; a superconducting tape comprising 
the preceding foil laminated with a protective coating; a superconducting 
wire comprising triniobium tin and niobium clad with a protective coating; 
and other such articles comprising triniobium tin that are found to be 
superconducting. The triniobium tin superconducting article may be made by 
methods known to those skilled in the art, such as liquid-phase and 
solid-phase diffusion processes. 
Foil, where the triniobium tin superconductor is formed by a liquid-phase 
diffusion process, is a suitable article for this invention. Generally, 
the foil is about one mil (0.001 inch) thick. Herein, the term "triniobium 
tin foil" means foil comprising triniobium tin and niobium. 
In a preferred embodiment of this invention, the above-mentioned foil has a 
protective coating of copper laminated to the top and bottom surfaces of 
the foil, thus forming "tape" as the triniobium tin superconducting 
article. Such tape usually has a thickness of about seven mils (0.007 
inches). The triniobium tin foil is about one mil (0.001 inch) thick and 
the copper laminate on each side is about three mils (0.003 inches) thick. 
The term "thermal contraction control layer" means a layer or layers of 
material bonded to the triniobium tin superconducting article to reduce 
thermal stresses and increase the critical current of the superconductor. 
One method of bonding is soldering the thermal contraction control layer 
to the triniobium tin superconducting article. Appropriate soldering 
temperatures may be between about 423-1023K. 
Generally, two layers of the thermal contraction control material are 
applied to the triniobium tin superconducting article, so that the article 
is sandwiched between the two layers, having a symmetrical configuration. 
For example, in the case of tape, the thermal contraction control layer is 
bonded to the top and bottom sides of the tape. 
The practice of this invention is not limited to using only two thermal 
contraction control layers. Multiple layers bonded to each other may be 
used, as well as a single layer bonded to the triniobium tin 
superconducting article. When a single layer of the thermal contraction 
control layer is used, its thickness must be sufficient to reduce thermal 
strains in the triniobium tin superconducting article to increase the 
critical current. A single thermal contraction control layer provides an 
unsymmetric configuration. However, the thermal residual strain for 
laminated triniobium tin on a single substrate is not significantly 
different from that on double substrates. 
The term "process temperature" herein means the temperature used to heat 
treat the thermal contraction control layer and the triniobium tin 
superconducting article so that a composite is formed. For instance, when 
soldering is used to bond the thermal contraction control layers and the 
triniobium tin superconducting article to form a composite, the process 
temperature depends on the composition of the solder process and may be 
about 456K (183.degree. C.), as an example. Likewise, the term "test 
temperature" refers to a temperature less than the process temperature 
where the difference between the process temperature and test temperature 
is sufficient to impart thermal strains to the triniobium tin 
superconductor upon cooling from the heat treat temperature. Temperatures 
between about 4.2-18K and where the triniobium tin is superconducting may 
be used for the test temperature. 
By practicing this invention by using niobium as the thermal contraction 
control layer bonded to triniobium tin tape, the critical current of 
triniobium tin superconductor can be increased as much as sixty percent in 
a transverse field of 5T and at a temperature of 4.2K.

DETAILED DESCRIPTION OF THE INVENTION 
In this invention the difference in thermal expansion properties between 
the thermal contraction control layers and the triniobium tin 
superconducting articles induces uniform compressive or tensile strains in 
the superconducting layer. This occurs during cooling from the processing 
temperature used to heat treat the thermal contraction, control layer and 
the triniobium tin superconducting article to form a composite. The 
critical current reaches a maximum in the triniobium tin superconductor 
article when there is zero strain. 
When the triniobium tin superconducting article includes copper as a 
protective coating, the thermal contraction control layers utilized in 
this invention are materials having thermal coefficients of expansion 
about equal to or less than copper. Such thermal contraction control 
layers include, but are not limited to, niobium, tantalum, tungsten, 
molybdenum, stainless steel, Hastelloy alloys, Inconel alloys, nickel, 
titanium, steel, chromium, antimony, palladium, platinum, and mixtures 
thereof. Other materials, including alloys of the above-mentioned base 
metals, can be used if the thermal coefficient of expansion is about equal 
to or less than that of copper. 
In cases where the triniobium tin superconducting article includes a 
protective coating other than copper, such as a bronze matrix, then the 
thermal contraction control layer has a thermal coefficient of expansion 
less than or equal to the protective coating. For instance, if the 
protective coating around the triniobium tin is bronze, then the thermal 
contraction control layers have a thermal coefficient of expansion less 
than or equal to that of bronze. Such thermal contraction control layers 
include, but are not limited to, niobium, tantalum, tungsten, molybdenum, 
stainless steel, Hastelloy alloys, Inconel alloys, nickel, titanium, 
steel, chromium, antimony, palladium, platinum, and mixtures thereof. 
Other materials, including alloys of the above-mentioned base metals, can 
be used if the thermal coefficient of expansion is about equal to or less 
than that of bronze. 
Additionally, in cases where the triniobium tin superconducting article 
comprises triniobium tin foil without a protective coating, the thermal 
contraction control layer has a coefficient of expansion matched as 
closely to that of triniobium tin as possible so as to impart minimal 
thermal stress to the superconductor. Ideally, the maximum critical 
current is obtained when there is zero strain imparted to the triniobium 
tin superconductor. 
In one embodiment of this invention, a triniobium tin superconducting 
article, such as tape (the tape comprising triniobium tin foil with a 
copper protective coating), is bonded by soldering between two flat, 
significantly thicker, thermal contraction control layers. The minimum 
thickness of each thermal contraction control layer should be between 
about one to five times the thickness of the triniobium tin 
superconducting article, depending on the material chosen as the thermal 
contraction control layer. The maximum thickness may be up to twenty times 
or more of that of the superconducting article. For example, if the 
triniobium tin tape is about seven mils (0.007 inches) thick, then each 
thermal contraction control layer is between about seven to thirty-five 
mils (0.007-0.035 inches) thick. It is noted that the contribution of the 
solder to the overall thickness of the above-mentioned composite is 
negligible. 
As a manner of demonstrating, the stacking of the layers for the 
above-mentioned composite, comprising the triniobium tin tape bonded 
between two thermal contraction control layers, would be "c-b-a-b-c", 
where "c" equals the thermal contraction control layer, "b+a" equals the 
triniobium tin tape where "b" equals the copper layer and "a" equals the 
inner triniobium tin superconductor. 
Thermal contraction control layers are chosen so that the difference in 
thermal expansion properties of the thermal contraction control layer and 
the triniobium tin superconducting article induces uniform compressive or 
tensile strains in the superconducting layer during cooling from the 
processing temperature to the test temperature. Generally, a thermal 
contraction control layer of the same material is placed on each side of 
the triniobium tin superconducting article. This is to prevent the bending 
that would occur during the cooling of a bi-metal strip. 
The following discussion further demonstrates the invention using 
triniobium tin made by liquid-phase diffusion techniques known to those 
skilled in the art. For example, 25.4 micrometer (0.001 inch) thick 
niobium alloy foil (Nb-1 atomic % Zr-2 atomic % O) is dipped in a tin 
alloy melt (Sn-17 atomic % Cu). The tin-coated niobium alloy foil is then 
reacted at 1050.degree.C. for about 200 seconds. At this temperature, the 
tin alloy coating is liquid and tin alloy diffuses through the forming 
triniobium tin layer to react with the solid niobium alloy core. The 
process forms a layer of fine-grained, superconducting triniobium tin 
about 7 micrometers thick on both sides of the remaining niobium foil. 
Strips of copper foil are then soldered to the reacted foil to form a 
triniobium tin-copper laminate, i.e. tape. This triniobium tin tape is the 
triniobium tin article. 
Differential thermal contraction is achieved by soldering samples of 
triniobium tin tape between two plates of eight different materials, and 
cooling the composite structures from the soldering temperature to the 
test temperature. In the example, brass, copper, stainless steel, 
Hastelloy X, Inconel 600, nickel, niobium, and tungsten plates are used. 
Residual strains in the triniobium tin result from cooling the composite 
from the solder solidification temperature 456K (183.degree. C.) to liquid 
helium temperature, 4.2K. An additional residual strain is induced in the 
triniobium tin during cooling the triniobium tin foil from the reaction 
temperature, 1323K (1050.degree. C.), to the soldering temperature. At the 
reaction temperature, there is a liquid intergranular phase. This second 
phase provides no resistance to shear deformation before the phase 
solidifies, and thus no straining occurs in the triniobium tin. The liquid 
phase, which is approximately 65 atomic percent tin-30 atomic percent 
niobium-5 atomic percent copper, is assumed to solidify at 1223K 
(950.degree. C.). The resulting residual thermal strain in the triniobium 
tin due to cooling from 950.degree. C. to 183.degree. C., is approximately 
+0.03%. This is based on assuming constant moduli for the triniobium tin 
and niobium in the temperature range. 
In a transverse field of 5 Telsa and at a test temperature of 4.2K, a 
reduction in the thermally applied axial strain of 0.4% increased the 
critical current by 60%. An axial tensile bending strain of the same 
magnitude resulted in a critical current increase of only about 9%. 
The following examples further demonstrate the invention by tabulating test 
results for eight different thermal contraction control layers. 
EXAMPLES 1-8 
Table 1 gives the data for the calculated strains in triniobium tin-copper 
foil soldered to various substrates. The metals used for substrates 
include brass, copper, 304L stainless steel, Hastelloy X, Inconel 600, 
nickel, niobium, and tungsten. The thermally induced axial strain is 
represented by .epsilon..sub.x. The data show a sixty percent increase in 
critical current, I.sub.c, for a change in thermal strain of 
.epsilon..sub.x =-0.39 to .epsilon..sub.x =0.0. This represents a 
difference in I.sub.c of 235 amperes for triniobium tin foil soldered to 
brass and triniobium tin foil soldered to niobium. 
In Table 1, the composite triniobium tin laminates (the triniobium 
tin-copper foil soldered to various substrates) were tested for critical 
current in liquid helium, 4.2K, in a field of 5 Telsa. The temperature and 
magnetic field strength were not varied. For the thermal strain 
measurements, the magnetic field was oriented along the z-axis. In all of 
the experiments, the current flows in the axial direction. Voltage probes 
were placed on the middle one centimeter of the samples and the critical 
current was defined by a voltage difference of 0.2 microvolts. 
TABLE 1 
______________________________________ 
Properties of Substrate Materials, 
Calculated Residual Thermal Strains in 
Nb.sub.3 Sn, and Critical Current Measurements. 
Modulus I.sub.c 4.2 K., 
.DELTA.L/L.sub.o for 
Ex- Substrate 
elastcity 
.epsilon..sub.x 
5 T T = 4.2 K.- 
ample Material (GPa).sup.a 
(%) Amperes 
456 K (%) 
______________________________________ 
1 Brass 105 -0.393 393 0.676 
2 Copper 112 -0.312 424 0.596 
3 Stainless 
190 -0.306 451 0.590 
Steel 
4 Hastelloy 
200 -0.185 500 0.471 
5 Inconel 200 -0.177 511 0.463 
6 Nickel 207 -0.169 528 0.456 
7 Niobium 103 0.002 628 0.263 
8 Tungsten 407 0.116 565 0.161 
Nb.sub.3 Sn 
165 0.282 
______________________________________ 
.DELTA.L/L.sub.o stands for the change in length (.DELTA.L) over the 
original length (L.sub.o).