An experimental study of superconductor stabilization has resulted in the successful application of the concepts of filamentary structure and conductor twist to Nb3Sn ribbon. The Nb3Sn is formed in parallel, helical paths, which are continuous around the ribbon. Short lengths (12-18cm) of 1.27 cm wide superconductive ribbon were produced. The filamentary and twist characteristics are incorporated in the ribbon by means of an inert mask formed on the ribbon surface early in the fabrication process. Diffusion reaction of the niobium and tin is prevented at the filament boundaries. Described are the conductor methods of fabrication, and test results obtained. The technology required to adapt the processes for the production of long lengths of ribbon is available

Coles, W. D.

English

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TWISTED, MULTIFILAMEIMT Nb3Sn SUPERCONDUCTIVE RIBBON
by Willard D. Coles
Lewis Research Center
Cleveland. Ohio
TECHNICAL PAPER proposed for presentation at
Fourth International Conference on Magnet Technology
Upton, New York, September 19-22,. 1972.
https://ntrs.nasa.gov/search.jsp?R=19720025065 2020-03-23T10:16:24+00:00Z
TWISTED, MULTIFILAMENT Nb.Sn SUPERCONDUCTIVE RIBBON
by Willard D. Coles
Lewis Research Center
Cleveland, Ohio
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Abstract
An experimental study of superconductor sta-
bilization has resulted in the successful applica-
tion of the concepts of filamentary structure and
conductor twist to Nb3Sn ribbon. The Nb3Sn is
formed in parallel, helical paths, which are con-
tinuous around the ribbon. Short lengths (12-18
cm) of 1.27 cm wide superconductive ribbon have
been produced. The filamentary and twist charac-
teristics are incorporated in the ribbon by means
of an inert mask formed on the ribbon surface
early in the fabrication process. Diffusion reac-
tion of the niobium and tin is prevented at the
filament boundaries. Described are the conductor
methods of fabrication, and test results obtained.
The technology required to adapt the processes for
the production of long lengths of ribbon is avail-
able.
Introduction
Because of the excellent work of many theo-
rists, experimenters and materials fabricators,
e.g., references 1 to 5, high-field superconduc-
tive materials have been improved at a very rapid
pace in.the scant 10 years of their commercial ex-
istence. Many of the "coil degradation" problems
of the early years were solved for the ductile al-
loys with adoption of the concepts of copper ma-
trix composites, multifilaments, and twisted con-
ductors. The brittle intermetallic superconduc-
tors have been made flexible and have been
strengthened with stainless steel and stabilized
with copper and aluminum. The attainment of a
twisted, multifilament configuration for the in-
termetallic cpmpounds has, however, been elusive.
The use of very fine, insulated filaments in
a composite conductor has .been shown to provide an
intrinsic stabilization resulting from a more fav-
orable dissipation of heat. By imparting a twist
to the filaments, the path length of induced mag-
netization currents is reduced, which limits the
energy available locally to quantities which can
be absorbed at lower temperatures (ref. 4) . Mag-
netization currents in a superconductor are field-
induced circulating persistent current loops.
These currents are in addition to the transport
current and may be many times higher in current
magnitude.-- - - _
The very thin ribbon geometry developed to
provide flexibility for the brittle intermetallic
compound superconductors, unfortunately, is par-
ticularly subject to induced magnetization cur--
rents. With no means available to limit the cur-
rent loop path length in ribbon conductors, the
loops can become very long and contain consider-
able energy. The effects of these persistent cur-
rent loops are (1) increased likelihood of
transition to the normal state, (2) high residual
field values (field remaining in a magnet after
the power supply current is. reduced to zero,
(3) variable current-field relationship (history
dependent), and (4) variable field homogeneity.
The obvious improvements realized with the
ductile alloy composites led to an experimental
study with the purpose of applying the multifila-
ment and twist concepts to Nb3Sn ribbon. These
characteristics have been successfully incorpor-
ated into short lengths (12-18 cm) of 1.27 cm
wide Nb3Sn ribbon. The Nb3$n is formed in par-
allel, helical paths which are continuous around
the ribbon. The filamentary and twist character-
istics are incorporated in the ribbon by means of
an inert mask formed on the ribbon surface early
in the fabrication process. Diffusion reaction of.
the niobium and tin is prevented at the inert mask
forming the filament boundaries. Described are
the conductor, methods of fabrication, and test
results obtained.
In the diffusion reaction process for forming
the compound Nb3Sn, tin is diffused into niobium
at temperatures between 900 and 1200 C (ref. 6).
By means of an inert shielding material, formed in
a pattern to outline desired current paths on the
niobium surface, the tin can be prevented from ui-
rectly contacting the niobium in the masked re-
gions. Thus by outlining continuous filamentary
paths spiraling around the ribbon, diagonally
across the faces and around the edges, Fig. 1, a
base is established for reacting the tin and nio- •
bium in the twisted, filamentary structure as in-
dicated in Fig. 2. Any nonsuperconductive mate-
rial which could be bonded to the niobium, was in-
soluble.in tin at the high temperatures, and could
be selectively applied or selectively removed,
might qualify as a mask material. .
Procedure
Not many materials meet' the previously stated
shield material requirements. Ceramic materials
might, but the application process and adherence
present problems. Only a few metals (mostly the
refractories) are sufficiently insoluble in molten
tin to be considered, but techniques for their ap-
plication are not available. Fortunately (for
this application) ,' niobium oxide meets the re-
quirements and is readily formed. To oxidize the
niobium ribbon selectively in the desired configu-i
ration, the portion of the ribbon which is later ;
required to react with the tin must be protected
from oxidation or the oxide removed from it. Pro-
tection of the surfaces was considered to be more
readily accomplished.
The following steps and techniques were uti-
lized to process the the prototype conductor.
1. Niobium ribbon 1.27 cm wide by 0.40025 cm
thick (1/2 in. x 1 mil) was cleaned, flashed with
an evaporatively deposited copper layer and copper
plated to a thickness of 0.0006 cm (1/4 mil).
Post plating heat treatment was used to improve
the copper-niobium bond, but this step and the
evaporative layer may not be necessary.
2. The plated ribbon was spray coated on all
surfaces with a commercial photoresist material
and force dried.
3. Computer generated grids reproduced on
photographic film as shown in Fig. 3 were placed
on each side of the ribbon and indexed to match at
the ribbon edges. This grid geometry essentially
eliminates the edge match problem that would exist
for simple diagonal lines, especially with varia-
tions in ribbon thickness or with appreciable rib-
bon curvature. Note that the diagonal lines span
slightly less than the ribbon width. The coated
ribbon was then exposed to ultraviolet light
through the film mask and the photoresist devel-
oped to remove the etch resistant material cover-
• ing the copper in the non-exposed regions (the
lines).
The unprotected copper was then etched off
using a conventional etchant. Modification of the
process by printing the desired pattern on the ::
plated ribbon using an etch-resistant material
would eliminate step 2 and most of step 3. Alter-
natively, an ink mask printed over the photoresist
coating would eliminate the film mask.
4. Niobium oxide was formed on the noncopper
covered regions by passing the ribbon through a
furnace at 370 C in an air atmosphere. The re-
maining etch-resist and copper-oxide layer were
removed in a dilute acid bath leaving only niobium
oxide and bare copper on the ribbon surface.
5. The diffusion reaction apparatus is shown
in Fig. 4. A prepared ribbon sample is attached
to a sliding rod mechanism which allows the ribbon
to be lowered into and withdrawn from the furnace
containing the molten tin. No attempt was made to
optimize the reaction times or temperatures be-
cause this has undoubtedly been thoroughly ex-
plored in the commercial operations. Plain nio-
bium ribbons and copper-plated niobium ribbons
were also reacted for comparison purposes. Con-
tamination of the tin bath by the copper is not
considered -to be-a .problem, because 30 percent tin
in copper is apparently used in the' commercial
processing (ref. 6).
Results and Discussion
The gaometry selected for the filamentary
paths consisted of 10 parallel filaments (5 on
each side of the ribbon), having a twist pitch of
2.54 on and with a ratio of superconductor to fil-
ament separator width of 3:1. A photograph of the
conductor is shown in Fig. 5. The superconductive
cross section for this geometry is equivalent to
about one face of a plain ribbon of equal width.
X-ray and electron microprobe analyses were
made on some of the ribbon samples. The X-ray
analysis' showed that Nb3Sn was formed in the de-
sired regions and indicated that no residual cop-
per remained. The electron microprobe also showed
no trace of copper indicating that the copper was
totally displaced by the tin.
Examination using an optical microscope indi-
cated generally good filament separation and fila-
ment formation on both faces and edges of the rib-
bon. Some defects do occur in the mask resulting
in interfilament shorting or interruption of a
filament. Random defects of this nature involving
only a small part of the ribbon at any one loca-
tion would probably not be of serious consequence
especially if the ribbon were copper clad.
The superconductive transition temperature
was measured and was found to be greater than
17.95° K. Critical current - critical field tests
made over a range of magnetic field strengths from
5 to 8 teslas gave the results indicated in
Fig. 6. Magnetic field direction was perpendicu-
lar to the ribbon face. Results are shown for
both the twisted multifilament and the plain Nb3Sn
ribbons. Critical current values are considerably
below those for commercially available ribbons.
This result is not considered to be of particular
significance because no attempt was made to maxi-
mize performance.
The apparent superior performance of the
twisted multifilament conductor in comparison with
the plain conductor cannot be definitively ex-
plained at this time. Short sample 1^ vs HC tests
of this nature are not expected to demonstrate
coil capabilities. Processing of the plain sam-
ples was not identical to that used for the multi-
filament material in that in one case no copper
plating was used and in the other a thinner copper
plate had been applied Without the evaporative
layer or the post-plating heat-treatment. Further
tests will be necessary to determine the effect of
the omissions of these parts of the process. If
inclusion of these steps show Ip - HC results as
high or higher than the multifilament results one
might conclude that the copper plating improves
the diffusion reaction process.
All of the techniques used in the fabrication
of the twisted multifilament ribbon were adapted
from prior art. Extension and modification of
these techniques to produce long lengths of such
ribbon appear to be-straightforward. Solder con-
nections to the ribbon were easily made and clad-
ding with copper should present no difficulty.
Finer filaments than were produced and fine fila-
ment separator strips should easily be possible.
Longer twist pitch lengths and finer filament sep-
arators would result in increased superconductor
area, but may not be desirable for reasons of
stability.
I • .' '• . Conclusions
 :
' The experimental pieces of twisted, multifil-
;ament Nb3Sn ribbon conductor which have teen pro- ;
:duced indicated that the methods and techniques '
-were sound. Various tests made on the finished
ribbon showed that (1) Nb3Sn had been formed, ;
(2) the critical temperature was greater than •
17.95° K and (3) rc-Hc"characteristics were below I
!commercially attainable values for plain ribbon, ]
•but were encouraging for nonoptimized processing
parameters. I
: Fabrication of longer lengths of ribbon of .
this type should be possible by adapting available
technology to the processes required.
References •
1. A.R. Kantrowitz, and Z.J.J. Stekly, Appl.
; Phys. Letters 6^  56 (1965).'
2. P.F. Chester, Rep. Prog. Phys. 30. pt. 2,
1967.
3. Anon., Proceedings of the 1968 Summer Study on
Superconducting Devices and Accelerators,"
Brookhaven National Laboratory Report BNL-
50155, pts. 1-3 (1968)._
4. M.N. Wilson, C.R. Walters, J.D. Lewin, P.F.
Smith, and A.H. Spurway., J. Phys. D.-Appl.
Phys. 3., 1517 (1970). '"
5. R. Hancox, Culham Laboratory Report No. CLM-P
\ 121. Also Proc. 10th Int. Cpnf. on Low Temp.
! Phys., Moscow (1966).
6. J.P. Denny, and R.W. Hardt, "Superconductors
| and Method for the Preparation Thereof," U.S. '
Patent 3,181,936 (may 4, 1964).
i
3
MASK
CS-62311
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Figure 1. - Mask forming twisted path on Nb ribbon.
SUPERCONDUCTIVE CURRENT PATH
CS-62312
Nt^Sn
Figure2. - Twisted filamentary Nb^Sn ribbon.
(a) SINGLE GRID.
(b) TOP AND BOTTOM GRIDS SUPER-IMPOSED.
(c) TOP AND BOTTOM GRIDS WITH RIBBON BETWEEN.
Figure 3. - Photographic film masks used with photoresist process.
V-CAP AND SEAL
n
^HIED^VAC GAGE
GATE VALVE =£---3
VAC GAGE^MT~>
LT MAGNETIC POSISTIONER
TIN B)
•TO VAC PUMP
TO VAC PUMP
MASKED Nb RIBBON
Figure 4. - NbjSn diffusion reaction apparatus.
-H-I TEST SOLDER JOINTS
f\J
o
o
'-NIOBIUM
OXIDE
1000
900
800
700
a. 600
< 500
~° 400
z
UJ
300
I 200
100
Figure5. -Twisted, multifilament ribbon.
O TWISTED MULTIFILAMENT
D PLAIN-WITHTHINCU
O PLAIN -NO CU
, GSCC7/%
DIFFUSION,
REACTION TIME,
MIN
%4;-
%//
^NIOBIUM
OXIDE
C-72-1350
3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0
H, TESLAS
Figure 6. - Ic vs H for 1.27 cm wide Nl^ Sn ribbon. ~
NASA-Lewis


Twisted, multifilament Nb3Sn superconductive ribbon

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