U.S. patent application number 15/204915 was filed with the patent office on 2017-04-20 for synthesis of superconducting nb-sn.
This patent application is currently assigned to Fermi Research Alliance, LLC. The applicant listed for this patent is Massimiliano Bestetti, Fermi Research Alliance, LLC, Silvia Franz. Invention is credited to Emanuela Barzi, Massimiliano Bestetti, Silvia Franz.
Application Number | 20170107636 15/204915 |
Document ID | / |
Family ID | 58522853 |
Filed Date | 2017-04-20 |
United States Patent
Application |
20170107636 |
Kind Code |
A1 |
Barzi; Emanuela ; et
al. |
April 20, 2017 |
SYNTHESIS OF SUPERCONDUCTING NB-SN
Abstract
A method comprising: electrodepositing a film comprising a
Nb--Sn material onto a copper substrate surface from an electrolyte
bath comprising (a) SnCl.sub.2, (b) NbCl.sub.5, and (c) (i)
1-Ethyl-3-methylimidazolium chloride (EMIC), (ii)
1-Butyl-3-methylimidazolium chloride (BMIC), or (iii) a mixture
thereof.
Inventors: |
Barzi; Emanuela; (Batavia,
IL) ; Bestetti; Massimiliano; (Milan, IT) ;
Franz; Silvia; (Milan, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bestetti; Massimiliano
Franz; Silvia
Fermi Research Alliance, LLC |
Batavia |
IL |
US
US
US |
|
|
Assignee: |
Fermi Research Alliance,
LLC
Batavia
IL
|
Family ID: |
58522853 |
Appl. No.: |
15/204915 |
Filed: |
July 7, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62190199 |
Jul 8, 2015 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 5/18 20130101; C25D
5/505 20130101; C25D 5/38 20130101; C23C 10/28 20130101; C25D 3/665
20130101; C25D 3/38 20130101; C25D 3/30 20130101; C25D 5/10
20130101; C22C 27/02 20130101; C22C 1/00 20130101 |
International
Class: |
C25D 5/50 20060101
C25D005/50; C25D 3/32 20060101 C25D003/32; C23C 10/28 20060101
C23C010/28; C22C 27/02 20060101 C22C027/02; C22C 1/00 20060101
C22C001/00; C25D 3/38 20060101 C25D003/38; C25D 5/10 20060101
C25D005/10 |
Claims
1. A method comprising: electrodepositing a film comprising a
Nb--Sn material onto a copper substrate surface from an electrolyte
bath comprising (a) SnCl.sub.2, (b) NbCl.sub.5, and (c) (i)
1-Ethyl-3-methylimidazolium chloride (EMIC), (ii)
1-Butyl-3-methylimidazolium chloride (BMIC), or (iii) a mixture
thereof.
2. The method of claim 1, wherein the bath comprises (a)
SnCl.sub.2, (b) NbCl.sub.5, and (c) 1-Butyl-3-methylimidazolium
chloride (BMIC).
3. The method of claim 1, wherein prior to initiation of
electrodeposition (a) is present in an amount of 1 mol % to 50 mol
%, (b) is present in an amount of 1 mol % to 50 mol %, and (c) is
present in an amount of 1 to 99 mol %.
4. The method of claim 1, wherein the film comprises
Nb.sub.3Sn.
5. The method of claim 1, wherein the electrodeposition occurs at 1
to 1000 mA/cm.sup.2 and 0 to 150.degree. C. for 1 to 7200
seconds.
6. The method of claim 1, wherein the Nb--Sn film has a structure
of cubic Nb.sub.3Sn, orthorhombic NbSn.sub.2, .eta.
Cu.sub.6Sn.sub.5 and .epsilon. Cu.sub.3Sn.
7. A method comprising: electrodepositing a seed copper layer onto
a surface of a Nb substrate; electrodepositing a tin layer onto the
seed copper layer; electrodepositing a copper barrier layer onto
the tin layer to form an intermediate construct; and heating the
intermediate construct to form a Nb.sub.3Sn coating.
8. The method of claim 7, wherein the heating of the intermediate
construct is from 10 to 90.degree. C.
9. The method of claim 7, wherein the Nb.sub.3Sn coating has a
structure of cubic Nb.sub.3Sn, orthorhombic NbSn.sub.2, .eta.
Cu.sub.6Sn.sub.5 and .epsilon. Cu.sub.3Sn.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 62/190,199, filed Jul. 8, 2015, which is
incorporated herein by reference.
BACKGROUND
[0002] The Nb.sub.3Sn intermetallic compound is a high performing
superconductive material which finds wide application in Nuclear
Resonance Magnetic devices, high field lab magnets, but also fusion
and accelerator magnets.
[0003] Manufacturing a film-based superconducting radio frequency
(SRF) structure remains a "holy grail" for accelerator physicists.
The main reason for aiming at thin film surface coating is that one
can, in principle, improve performance and save on costs, since
with penetration depths of 40 to 100 nm only .about.1 .mu.m in
thickness would be needed for the superconducting film. Within the
new EUCARD Program, which was started in Europe last year, the thin
film activity is distributed between four labs, including
Helmholtz-Zentrum Berlin fur Materialien und Energie (HZB), CEA
Saclay, INP Grenoble and the European Organization for Nuclear
Research (CERN). The most successful film-based Cu cavities are
those made at CERN, which have reached accelerating gradients in
excess of 20 MV/m. More recently promising results for Nb/Cu
cavities were obtained with High Impulse Power Magnetron Sputtering
(HIPIMS) as compared to the standard dc Magnetron Sputtering.
[0004] On the other hand, the manufacturing of superconductive
Nb.sub.3Sn films is still in its infancy, and film-based SRF
cavities have not yet equaled the performance of conventional bulk
niobium cavities. There are two main problems with film-based
cavities. The first is the presence of defects within one or two
penetration depths from the surface. In fact H.sub.c1 (at defects)
is expected to be much lower than H.sub.c1 (bulk). Defects are
particularly numerous in films produced at low temperatures. Oxygen
and hydrogen will be trapped at defects, and could have significant
negative impact at high gradients. The second issue concerns the
grain size for the Nb/Cu films, which is about 100 nm, i e 10,000
times smaller than the grain size of conventional SRF cavities.
Grain boundary diffusion and trapping of oxygen and hydrogen are
much faster than impurity diffusion in bulk Nb. It is known that
grain boundary scattering is the main reason for a low residual
resistivity ratio (RRR) in films compared to bulk niobium of
similar purity. Yet the surface resistance of film-based SRF
cavities at low currents is at least as low as for the bulk
materials. This means that the quality factor Q.sub.0 of film based
cavities could be at least as good as for high quality bulk Nb
cavities. However, the present performance of film-based cavities
at high acceleration gradients is at lower Q values.
[0005] Kolosov et al. deposited Nb and Nb3Sn from high-temperature
molten salt solutions containing LiF--NaF--KF, NaCl--KCI and
LiCI--KCI (e.g., Kolosov, V. N. and Matychenko, E. S., Evaluation
of High Frequency Superconductivity of Niobium Coatings Prepared by
Electrodeposition Process in Molten Salts, in Refractory Metals in
Molten Salts, Dordrecht: Kluwer, 1998, pp. 231-238). However due to
the aggressive condition of deposition (corrosivity and temperature
from 400 to 1000 K) only few substrates can be utilized. The most
promising alternative to those high temperature electrolytes are
low temperature ionic liquids. Early attempts to deposit Nb--Sn
alloys from ionic liquids were performed by Koura et al. (Ito H.
Koura N., Ling G. Electrodeposition of Nb--Sn alloy from ambient
temperature molten salt electrolytes, Hyoumen Gijutsu, 46(12),
1162-1166(1995). They recorded cyclic voltammetries for
1-butylpyridinium chloride (BPC)-NbCl.sub.5 and
BPC-NbCl.sub.5--SnCl.sub.2 solutions. Moreover they investigated
the deposition of Nb--Sn alloys from 53.8% SnCl.sub.2--7.7%
NbCl.sub.5--38.5% BPC solution at 130.degree. C., at 5 mA/cm.sup.2,
using a copper cathode and a tin anode. The resulting Nb--Sn film
contained about 14.8 wt % Nb. The deposition from 7.7%
SnCl.sub.2--15.4% NbCl.sub.5--76.9% BPC bath at 40 mA/cm.sup.2 and
130.degree. C. resulted in a Nb composition of 27.9 wt %. In a
second paper the same group evaluated the effects of a pulse
electrolysis on a 28.6% SnCl.sub.2--14.3% NbCl.sub.5--57.1% BPC
bath. Ito H. Koura N., Ling G. Electrodeposition of Nb--Sn alloy
from ambient temperature molten salt electrolytes by pulse
electrolysis, Hyoumen Gijutsu, 48 (4), 454-459 (1997). They
concluded that the niobium content in the Nb--Sn electrodeposit was
affected by pulse period, current density and duty ratio:
decreasing the duty ratio and increasing the current density
increased the deposited Nb content. They deposited a Nb--Sn alloy
containing 44.3 wt % Nb, at current density of 60 mA/cm.sup.2, t=50
ms and a duty ratio of 0.2. They also deposited a 41.3 wt % Nb
alloy from a 7.7% SnCl.sub.2--15.4% NbCl.sub.5--76.9% BPC bath at
60 mA/cm.sup.2, t=10 ms and a duty ratio of 0.2. However, none of
the papers gave evidence of the presence of a superconductive
phase. The same group also reported that the electrodeposition of a
Nb--Sn alloy can be done from a SnCl.sub.2--NbCl.sub.5 solutions in
1-ethyl-3-methylimidazolium chloride (EMIC). N. Koura, T
Umebayashi, Y Idemoto and Gouping Ling, Electrodeposition of Nb--Sn
Alloy from SnCl.sub.2--NbCl.sub.5-EMIC Ambient Temperature Molten
Salts, Electrochemistry, 67(6), 689(1999). Electrodeposition
carried out in constant current resulted in a very low niobium
content in the alloy. On the contrary, pulse plating from an acidic
melt with a 2.8% SnCl.sub.2--68.6% NbCl.sub.5--28.6% EMIC with a
pulse period of 10 ms, duty ratio of 0.2 at 160.degree. C.,
increased the Nb concentration to 69.1 wt %. XRD analysis and
resistivity tests demonstrated that a superconductive Nb.sub.3Sn
phase could be obtained. However, the same authors declared that
the reproducibility of the process was not acceptable.
[0006] To overcome the limits of Lewis acidic ionic liquids, more
recently Koichi et al also tested a Lewis basic melt consisting in
4.4% SnCl.sub.2--95.6% EMIC. Koichi Ui, Sakai H, Takeuchi K., Ling
G., Koura N., Electrodeposition of Nb.sub.3Sn Alloy Film from Lewis
basic SnCl.sub.2--NbCl.sub.5-EMIC melt, Electrochemistry, 77 (9)
798-800 (2009). Cyclic voltammetries were carried out at
130.degree. C. and 10 mV/s and reduction and oxidation waves were
clearly observable in the potential range (vs Al(III)/Al) from
-1.07 V to -1.30 V and from 1.05 V to -0.75 V respectively. The
cyclic voltammetry of the 11.9% NbCl.sub.5--88.1% EMIC basic melt
showed reduction waves from 0.21 V and -0.77 V vs Al(III)/Al. A
similar profile was obtained by Sun et al. from a 49.0%
AlCl.sub.3--51.0% EMIC containing NbCl.sub.5. I-Wen Sun and Charles
L. Hussey, Electrochemistry of Niobium Chloride and Oxide Chloride
Complexes in the Basic Aluminum
Chloride-1-Methyl-3-ethylimidazoliumChloride Room-Temperature Ionic
Liquid, Inorg. Chem. 1989, 28, 2731-2737. Koichi et al. performed
cyclic voltammetry of the 19.2% NbCl.sub.5--10.0% SnCl.sub.2--70.8%
EMIC Lewis basic melt and observed two reduction waves at a
potential lower than -0.5 V vs Al(III)/Al. The reduction wave of
niobium ionic species was identified as the one at -0.4 V vs
Al(III)/Al, while the second peak at -0.8 V vs Al(III)/Al was
attributed to the reduction of Sn, indicating that the
co-deposition reaction of Nb and Sn might occur because both
reduction potentials were very close to each other. The
electrodeposition from this melt was attempted using a constant
current pulse method at 0.1 A/cm.sup.2 with a pulse period of 10
ms, duty ratio of 0.20, and electricity of 5 C/cm.sup.2 at
130.degree. C. The electrodeposition cell consisted in a copper
working electrode, a Sn counter electrode and an Al(III)/Al quasi
reference electrode. The XRD pattern of the electrodeposit revealed
the presence of Nb.sub.3Sn along with metallic Sn and
Cu.sub.10Sn.sub.3 alloy, which was attributed to the solid phase
interdiffusion between the copper substrate and Sn atoms due to the
high bath temperature. However, the presence of Sn and Cu--Sn
phases could also be attributed to the dissolution of the Sn anode
during the electrodeposition process. While anode dissolution might
be advantageous for some metal deposition, in this case the choice
of Sn appears to be problematic. In fact, Sn already shows good
solubility in the solvents and it is more prone to reduce at the
cathode than it is Nb. Furthermore, the aluminum wire was also
soluble in the same electrolyte, making it an unsuitable material
as quasi-reference electrode. Finally, no indication of the total
thickness of the film was given, which is a property of great
interest to make the approach eligible for applications and further
development to the industrial scale.
[0007] In addition, the intermetallic compound Nb.sub.3Sn is a type
II superconductor having a well-defined stoichiometry and the A15
crystal structure. It has a critical temperature T.sub.c0 of up to
18.3 K and an upper critical magnetic field B.sub.c20 of up to 30
T. As a comparison, the ductile alloy NbTi has a T.sub.c0 of 9.3K
and a B.sub.c20 of 15 T, which make NbTi adequate only up to
operational magnetic fields of 8 to 9 T, as in the case of the
Large Hadron Collider (LHC) at the European Organization for
Nuclear Research (CERN, Switzerland), whose NbTi magnets operate in
superfluid helium at 1.9K to bend and collide proton beams and
eventually reach an energy of 14 TeV in the center of mass.
Superconducting materials have found a wide range of applications
in science and society. Their unique properties and exquisite
sensitivity have been exploited in many science disciplines.
Superconductivity is used in detectors for dark matter, for the
cosmic microwave background radiation and for national security
purposes. Superconducting magnets and radio frequency (SRF)
structures are at the heart of most particle accelerators for
fundamental science, as well as accelerators for medical isotope
production and ion therapy treatment. Superconductivity is also
being explored for use in biosensors and quantum computing. Thanks
to Nb.sub.3Sn's stronger superconducting properties, it enables
magnets above 10 T, which for instance is a larger field than any
existing in present NbTi particle accelerators. Nb.sub.3Sn is also
the superconductor of choice for high field magnets to be used for
plasma confinement in fusion reactors. The International
Thermonuclear Fusion Research and Engineering project (ITER,
France) uses a Central Solenoid of 13.5 T. But perhaps the most
extensive use of Nb.sub.3Sn is for Nuclear Magnetic Resonance (NMR)
spectrometers, which have become a key analysis tool in modern
biomedicine, chemistry and materials science. These systems use
fields up to 23.5 T, which correspond to a Larmor frequency of 1000
MHz.
[0008] Some of the challenges are that Nb.sub.3Sn requires
high-temperature processing, which makes it brittle, and its
critical current is strain sensitive, i.e. high strain on the
sample may reduce or totally destroy its superconductivity. In the
last decades, several manufacturing processes have been developed
to produce superconductive Nb.sub.3Sn wires, including the bronze
route, the powder-in-tube method, and internal tin, which includes
as variants the modified jelly roll and the Restacked Rod processes
(RRP.RTM.). In the last 15 years, Fermi National Accelerator
Laboratory (Fermilab, US) has used these wires and developed
superconducting cables to perform Nb.sub.3Sn research for high
field accelerator magnets. The Fermilab High-Field Magnet Group
built the first reproducible series in the world of single-aperture
10 to 12 T accelerator-quality dipoles made of Nb.sub.3Sn,
establishing a strong foundation for the LHC luminosity upgrade at
CERN. More recently, the first successful twin-aperture accelerator
magnet made of Nb.sub.3Sn and developed and fabricated at Fermilab
reached its design field of 11.5 Tesla at 1.9K.
SUMMARY
[0009] Disclosed herein is a method comprising:
[0010] electrodepositing a film comprising a Nb--Sn material onto a
copper substrate surface from an electrolyte bath comprising (a)
SnCl.sub.2, (b) NbCl.sub.5, and (c) (i) 1-Ethyl-3-methylimidazolium
chloride (EMIC), (ii) 1-Butyl-3-methylimidazolium chloride (BMIC),
or (iii) a mixture thereof.
[0011] Also disclosed herein is a method comprising:
[0012] electrodepositing a seed copper layer onto a surface of a Nb
substrate;
[0013] electrodepositing a tin layer onto the seed copper
layer;
[0014] electrodepositing a copper barrier layer onto the tin layer
to form an intermediate construct; and
[0015] heating the intermediate construct to form a Nb.sub.3Sn
coating.
[0016] The foregoing will become more apparent from the following
detailed description, which proceeds with reference to the
accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIGS. 1A-1C. CV curves of pure Emim-Cl compared to: FIG.
1A--10% SnCl2 90% Emim-Cl; FIG. 1B--25% NbCl5 75% Emim-Cl; FIG.
1C--75% Emim-Cl--21% NbCl.sub.5--4% SnCl.sub.2.
[0018] FIGS. 2A-2C2. FIG. 2A: polarization curves at constant
current in the range 10-50 mA/cm.sup.2; FIG. 2B and FIG. 2C: SEM
micrograph and XRD pattern of a Nb--Sn coating deposited from Bath
1 at 40 mA=cm2 and 130.degree. C.
[0019] FIGS. 3A-3C. CV curves of pure Bmim-Cl compared to: FIG.
3A--10% SnCl.sub.2 90% Bmim-Cl; FIG. 3B--25% NbCl5 75% Bmim-Cl;
FIG. 3C--75% Bmim-Cl--21% NbCl.sub.5--4% SnCl.sub.2.
[0020] FIG. 4. XRD patterns of Nb--Sn films electrodeposited onto
Cu substrates from from Bath 2 at 40 mA/cm2: 28at. % Nb, film
thickness 750 nm.
[0021] FIGS. 5A-5D. SEM micrographs (5A and 5B) and XRD patterns
(FIGS. 5C and 5D) of Nb--Sn films electrodeposited onto Cu
substrates from Bath 2 at 400 mA/cm.sup.2: 17at. % Nb, film
thickness 200 nm.
[0022] FIGS. 6A-6D. Nb--Sn film on Cu substrate, from Bath 3 at
130.degree. C. and 40 mA/cm.sup.2 for 4 h 30 min: FIG. 6A--XRd
pattern; FIG. 6B--AFM micrograph; FIG. 6C--SEM micrograph; FIG.
6D--In depth concentration profile by EDX analysis.
[0023] FIGS. 7A-7C. Nb--Sn film on Cu substrate, from Bath 4 at
130.degree. C. and 400 mA/cm.sup.2 for 25 min: FIG. 7A--SEM
micrograph; FIG. 7B--GDOES in-depth profile; FIG. 7C--XRD
pattern.
[0024] FIG. 8. Thickness of Sn coatings on copper substrates (a)
and on Nb substrates (b) as a function of deposition time.
[0025] FIGS. 9A-9D. Copper seed layer onto Nb substrate (FIG. 9A),
Sn coating onto Nb/Cu substrates (FIG. 9B), XRD pattern of a
Nb/Cu/Sn sample (FIG. 9C) and final sample design for
superconductivity tests (FIG. 9D).
[0026] FIG. 10. Thickness of the Nb-Sn phase as a function of the
duration of the heat treatment, compared to data reported in
literature.
[0027] FIG. 11. Temperature vs. time profile of heat treatments
performed on Nb/Cu/Sn/Cu samples.
[0028] FIGS. 12A-12B. XRD pattern (FIG. 12A) and GDOES analysis
(FIG. 12B) of sample consisting of Nb/Sn(10 .mu.m)/Cu(10 .mu.m)
after thermal treatment (sample 53).
[0029] FIGS. 13A-13B. XRD pattern (FIG. 13A) and GDOES analysis
(FIG. 13B) of a Nb/Sn(15 .mu.m)/Cu(15 .mu.m) sample after thermal
treatment (sample 56).
[0030] FIGS. 14A-14B. GDOES analysis (FIG. 14A) and XRD pattern
(FIG. 14B) of a Nb/Sn(20 .mu.m)/Cu(15 .mu.m) sample after thermal
treatment (sample 59).
[0031] FIGS. 15A-15B. Critical current density of Nb--Sn thin films
as a function of magnetic field in the perpendicular (FIG. 15A) and
parallel (FIG. 15B) orientations.
DETAILED DESCRIPTION
[0032] As used herein, "Nb--Sn" refers to a material that comprises
Nb and Sn (in preferred embodiments the material contains only Nb
and Sn, and substantially no other element or contaminant). The
Nb--Sn material may be present in various stoichiometric or
nonstoichiometric ratios (e.g., Nb.sub.3Sn and/or NbSn.sub.2) and
phases.
Electrodepositing Nb--Sn Coatings on Copper Substrates
[0033] Disclosed herein are method for electrodepositing Nb--Sn
coatings on copper substrates from SnCl.sub.2--NbCl.sub.5-EMIC and
SnCl.sub.2-(NbCl.sub.5)-BMIC ionic liquids. In certain embodiments,
the copper substrate is substantially pure copper. Two different
approaches were followed: Nb cations were provided to the
electrolyte either by direct addition of NbCl.sub.5 salt or by
electrochemical dissolution from the Nb anode. Cyclic voltammetric
curves were recorded to investigate the electrochemical behavior of
the electrolyte. Reduction waves of Nb and Sn ionic species were
clearly identified. Electrodeposition was performed in constant
current mode at 40 and 400 mA/cm.sup.2 and at a temperature of
130.degree. C. Phase structure and texture, composition and
morphology were determined by X-ray diffraction (XRD), X-Ray
Fluorescence (XRF), glow discharge optical emission spectrometry
(GDOES), Laser profilometry, Scanning electron microscopy (SEM) and
Energy Dispersive X-Ray (EDX) analysis. The maximum niobium content
in electrodeposited Nb-Sn films was 63 at %. Film thickness was in
the range from 200 to 4000 nm and average surface roughness in the
range of 0.230/2.113 .mu.m depending on the operating parameters.
The electrodeposited coatings showed a cubic Nb.sub.3Sn phase with
(211) preferred orientation and an orthorhombic NbSn.sub.2 phase.
These results demonstrate depositing thin superconducting layers on
copper surfaces with a relatively simple and inexpensive method. In
certain embodiments, the minimal layer thickness for SRF
applications is 1 micron, more particularly 1 to 5 microns.
[0034] The electrochemical methods disclosed herein do not require
high temperature processing. Use of SnCl.sub.2 and NbCl.sub.5 and
the ionic liquids overcomes the technological difficulty of using
large area cathodes in controlled environments, and to control
thickness uniformity. The main feature of electrochemical
deposition techniques is that they allow molecule formation beyond
the standard metallurgical phase diagrams. For instance, in the
technique herein described, the Nb.sub.3Sn film is directly
produced in molecular form and at a temperature of 130.degree. C.
to 150.degree. C., which is much lower than in any known
state-of-the-art method, which is by Nb--Sn solid diffusion at
temperatures greater than 650.degree. C. Also, electrochemical
deposition techniques have the advantage that they can be
successfully performed on complex 3D shapes having high aspect
ratio, which in the case of Nb.sub.3Sn is impossible to obtain with
the classical metallurgical techniques due to the intrinsic
brittleness of Nb.sub.3Sn. Furthermore, in principle
electrodeposition techniques enable synthesizing of Nb.sub.3Sn
coatings in only one step.
[0035] Disclosed herein is the electrodeposition of Nb--Sn thin
films from a bath containing SnCl.sub.2, NbCl.sub.5 and
1-Ethyl-3-methylimidazolium chloride (EMIC) and from a novel Lewis
basic ionic liquid consisting in SnCl.sub.2, NbCl.sub.5 and
1-Butyl-3-methylimidazolium chloride (BMIC) (both belonging to the
so-called first generation ionic liquids).
1-butyl-3-methylimidazolium and 1-ethyl-3-methylimidazolium ions
have good viscosity and high conductivity (see Table 1). The
synthesis of BMIC is an easier process than the synthesis of EMIC,
which requires a pressurized reaction vessel, in fact BMIC is less
expensive than EMIC. Ionic liquids containing BMIM
tetrafluoroborates were successfully used to electrodeposit a
variety of metals, such as Zn, Fe and Mg, BMIM hexafluorophospates
were used to electrodeposit Ge, Lewis acid BMIC/chloroaluminate
ionic liquids were used for the electrodeposition of Al alloys,
while Lewis basic BMIC/chloroaluminate ionic liquids were used for
the electrodeposition of Pd.
TABLE-US-00001 TABLE 1 Melting point (Tm), thermal decomposition
point (Td), ionic conductivity (.sigma.), viscosity (.eta.) and
diffusion coefficient of EMIC, BMIC, EMIM+ and BMIM+ [E]. EMIC BMIC
EMIM+ BMIM+ Water Tm (.degree. C.) 89 65 Td (.degree. C.) 285 250
.rho. (g cm.sup.-3) 1.256 (30.degree. C.) .sigma. (mS cm.sup.-1)
6.5 11 4.6 (30.degree. C.) (30.degree. C.) (30.degree. C.) .eta.
(cP) 47 27 40 0.89 (30.degree. C.) (30.degree. C.) (30.degree. C.)
(25.degree. C.) D/10-11 (m.sup.2 s.sup.-1) 6.2 3.4 (30.degree. C.)
(30.degree. C.)
Experimental Setup
[0036] The experiments were carried out using two ionic liquids
based on either EMIC (>98%, Sigma-Aldrich) or BMIC (>98%,
Sigma-Aldrich). Anhydrous stannous chloride (SnCl.sub.2, 98%,
Sigma-Aldrich) and anhydrous niobium chloride (NbCl.sub.5, 99%,
Sigma-Aldrich) were added to the ionic liquids to obtain the Lewis
basic melts. The chemicals were mixed in a glove box (argon or
nitrogen atmosphere) by magnetic stirring at 100.degree. C. for 24
hours. Subsequent electrochemical measurements and deposition
experiments were done in an electrochemical cell sealed to the
ambient atmosphere. In the following, % stands for mol. %. The
SnCl.sub.2/EMIC mixtures were prepared mixing EMIC and SnCl.sub.2
and subsequently heating at 100.degree. C. with a light stirring.
Heating the mixture is required, since both the chemicals are solid
at room temperature. At 70.degree. C. EMIC melts, and SnCl.sub.2
easily dissolves into it by forming a transparent greenish
solution. Cyclic voltammetries (CVs) were performed at 100.degree.
C. and 10 mV/s using a three-electrode cell configuration. Cu
sheets (99.95%, thickness 700 .mu.m) were used as cathode. The
exposed area into electrolyte was 1-2 cm.sup.2. The counter
electrode was a Sn sheet. A Pt wire was employed as a reference
electrode Unlike EMIC-SnCl.sub.2, the solution formed with niobium
salt is liquid at room temperature at concentrations ranging from
4% to more than 30% NbCl.sub.5. Cyclic voltammetry was performed on
a 25% NbCl.sub.5--75% EMIC solution at 100.degree. C. and 10 mV/s
in a three-electrode cell similar to the previous one with niobium
instead of tin as counter electrode.
[0037] The SnCl.sub.2--NbCl.sub.5-EMIC solution was obtained by
adding NbCl.sub.5 to an EMIC-SnCl.sub.2 solution. The addition of
niobium salt greatly increased the melting point of the solution,
which was solid at ambient temperature and assumed a dark brown
color. Electrolytes with higher niobium salts concentrations were
not tested, since it was observed that they are solid even at
temperatures up to 150.degree. C. A CV was performed on a 75%
EMIC-21% NbCl.sub.5--4% SnCl.sub.2 at 130.degree. C. by using
niobium metal as counter electrode. Platinum was chosen as
reference electrode for our experiments, while aluminum or tin were
used by Koura and Koichi. Platinum resulted to be stable in ionic
liquids solutions, while aluminum and tin reacted when in contact
with the electrolyte.
[0038] Electrodeposition from EMIC based electrolytes was carried
out from a 85% EMIC-10% NbCl.sub.5--5% SnCl.sub.2 (in the following
called Bath 1) solution at 120.degree. C. using a two electrodes
cell configuration. Cyclic voltammetries, which are electrochemical
measurements, were done with a three electrode system.
Electrochemical deposition experiments were done using a two
electrode cell. The cathode was pure copper, the anode was pure tin
or pure niobium. Galvanostatic mode was tested, but potentiostatic
and pulsed modes could also be used. Cu sheets (99.95%, thickness
700 .mu.m) were used as substrates. The exposed area into
electrolyte was 1-2 cm.sup.2. The anode was a Nb sheet. Experiments
were carried out at a constant current density of 40 mA/cm.sup.2
applied for 240 s.
[0039] A similar procedure was followed for the investigation on
BMIC solutions. The CV curves were recorded on 10% SnCl.sub.2--90%
BMIC electrolyte, which was obtained by mixing the chemicals in a
glove box by magnetic stirring at 100.degree. C. for 24 hours. The
as prepared solution was transparent and uncolored, with the
advantage of a lower viscosity compared to the EMIC-based
solutions. The 75% BMIC-25% NbCl.sub.5 eutectic was prepared by
adding NbCl.sub.5 to BMIC at 100.degree. C. under magnetic stirring
in glove-box. The solution showed a light brown color, lower
viscosity if compared to EMIC and it was liquid at ambient
temperature. The SnCl.sub.2--NbCl.sub.5-BMIC eutectic was obtained
by adding NbCl.sub.5 to an EMIC-SnCl.sub.2 solution. The CV curves
were recorded using a solution of composition 75% BMIC-21%
NbCl.sub.5--4% SnCl.sub.2.
[0040] The electrodeposition from BMIC-based electrolytes was
carried out from a 85% BMIC-10% NbCl.sub.5--5% SnCl.sub.2 solution
(in the following called Bath 2) in galvanostatic mode at current
densities of 40 and 400 mA/cm.sup.2 for times from 600 s to 1800 s
at 130.degree. C. The deposition of the Bn--Sn alloy was done on
copper substrates, the exposed area being .about.1-2 cm.sup.2. The
counter electrode was a Nb sheet. It has to be considered that the
addition of the niobium chloride greatly increased the melting
point of the solution, which was solid at ambient temperature and
assumed a color ranging from orange (10% NbCl.sub.5) to dark brown
(25% NbCl.sub.5). In the following, this approach will also be
referred to as "approach 1". Alternatively, Nb was added to the
electrolyte by electrolytic dissolution of the Nb anode during the
electrochemical polarization. The electrolyte composition was 90%
BMIC-10% SnCl.sub.2 (Bath 3). In the following, this approach will
also be referred to as "approach 2". Despite the lower melting
temperature (the commercial BMIC in particular is liquid at room
temperature while EMIC is solid), electrodeposition tests were
carried out at T>100.degree. C. In fact, .sup.13C and .sup.35Cl
NMR spectra demonstrated that by increasing the operating
temperature the geometry of the coordination complex changes and a
more dissociated structure is favored, which in turn has beneficial
effects on metal deposition.
[0041] Karl-Fischer analysis was performed to determine the water
content in the as-prepared electrolyte using a Mettler Toledo
titrator (Model DL31). The water content was found to be about 0.44
wt. % for the as prepared electrolyte containing 85% BMIC-10%
NbCl.sub.5--5% SnCl.sub.2.
[0042] For electrochemical measurements, a potentiostat/galvanostat
(Solartron Analytical ModuLab ECS) was used. The surface morphology
was investigated by means of Scanning Electron Microscopy
(SEM-Zeiss.RTM. EVO 50) equipped with LaB6 source, operated at 20
kV accelerating voltage. Phase structure and texture of the Nb--Sn
coatings were assessed by acquiring X-ray diffraction (XRD)
patterns with Cu K.alpha. radiation (.lamda.=1.5405 .ANG.) and a
powder goniometer (Philips PW-1830) in the 2.theta. angular range
of 10-90.degree.. XRD patterns were explained by means of powder
diffraction references. Grain size was estimated by calculating the
crystal coherence extensions according to Scherrer equation.
Surface roughness was measured by generating 2D profiles using a
UBM Mikrofocus.RTM. laser profilometer (UBM Messtechnik GmbH). Film
thickness was measured by X-ray fluorescence using a
Fischerscope-XAN.RTM.-FD BC instrument. Glow discharge optical
emission spectrometry (GD-OES) depth profiling analysis was
performed with a Spectruma GDA750 analyser using argon ions for
sputtering with a beam spot size of 2.5 mm. Film composition was
also assessed by EDX analysis.
Results and Discussion
[0043] The cathodic behavior of the ionic liquid solution was
investigated by CV with the objective to define the potential range
for alloy deposition.
[0044] FIG. 1 shows CV curves over the potential range from -0.5 to
-3.7 V corresponding to the pure EMIC (a), EMIC with addition of
10% SnCl.sub.2 (a), EMIC with addition of 25% NbCl.sub.5 (b), and
EMIC containing 5% SnCl.sub.2 and 21% NbCl.sub.5 (c). In the case
of the base electrolyte, a low current density (c.d.) was observed
over the potential range from OCP to -2.0 V, significantly
increasing towards the lower limit of the scanning range, possibly
due to the reduction of ionic liquid itself or to the
electrochemical decomposition of the water contained in the
electrolyte. In fact, a main shortcoming of the first generation
ionic liquids (EMIC and BMIC) is that the organic halides can
easily contain more than 1000 ppm of water (up to 10 000 ppm of
water), this water possibly reacting even with metal chlorides.
However pure EMIC shows a relatively wide potential window. By
adding 10% SnCl.sub.2 to pure EMIC (FIG. 1-a), a broad cathodic
peak appears at about -0.75 V, with a peak of about 1.6
mA/cm.sup.2. Since the melt is basic, SnCl.sub.3.sup.- is the main
anionic species in the melt, and Sn deposits or dissolves according
to the following equation:
SnCl.sub.3.sup.-+2e.fwdarw.Sn+3Cl.sup.- Eq. 1
[0045] At higher cathodic potential, the c.d. approaches the value
observed for the base electrolyte, shifting water or EMIC
decomposition at -0.83 V. Galvanostatic deposition experiments
carried out at 80.degree. C. and 10 mA/cm.sup.2 for 600 s resulted
in a bright and uniform metallic Sn film (not shown).
[0046] By adding 25% NbCl.sub.5 to pure EMIC (FIG. 1-b), two
reduction peaks were observed at -0.86 V and c.d. 4.0 mA/cm.sup.2,
and -1.83 V vs Pt and c.d. of 5.6 mA/cm.sup.2. A similar profile
was obtained by Sun et al. from a 49.0% AlCl.sub.3--51.0% EMIC
containing NbCl.sub.5. Sun et al. attributed the two reduction
peaks to reactions involving the niobium anionic species according
to the subsequent equations:
NbCl.sub.6.sup.-+e.fwdarw.NbCl.sub.6.sup.2- Eq. 2
NbCl.sub.6.sup.2-+e.fwdarw.NbCl.sub.6.sup.3- Eq. 3
[0047] The attribution of the two reduction waves to partial
reduction of the niobium-containing anionic species was also
confirmed by Koichi, who reported that the main niobium anionic
species in the 33.3% NbCl.sub.5--66.7% EMIC melt was
NbCl.sub.6.sup.-, forming a complex with EMI.sup.+ cations.
Therefore, the two reduction waves at -0.86 V and -1.83 V vs Pt.
can be attributed to Nb(V)/Nb(IV) and to Nb(IV)/Nb(III) redox
couples, respectively. The reduction wave peak at -0.86 V vs. Pt is
very close to the Sn reduction peak recorded in SnCl-EMIC
solutions, suggesting the possibility of co-deposition of the two
elements. Compared to Sn, more intense current density peaks were
measured. Further increasing the cathodic potentials resulted in
c.d. values approaching those of pure EMIC, with a shifting of the
current increase from -2.0 V in pure EMIC to -2.7 V. Galvanostatic
deposition experiments were performed on this solution in constant
current mode at c.d. values of 10-30-50-100 mA/cm.sup.2, by varying
the quantity of NbCl.sub.5 in the melt from 10% to 30%. No metallic
coating were obtained even after 3600 s.
[0048] The CV of 75% EMIC-21% NbCl.sub.5--4% SnCl.sub.2, showed one
pronounced reduction peak at -1.83 V and c.d. of 13.4 mA/cm.sup.2
(FIG. 1-c), probably corresponding to the reduction of Nb(IV) to
Nb(III), indicating that the potential for alloy deposition is
slightly shifted towards more negative values compared to the
reduction potential of either cations from the respective
electrolyte. Notably, the peak is preceded by a slowly rising c.d.
in the potential range where--according to curve (a)--Sn2+ ions can
be reduced to Sn and Nb(V) ions to Nb(IV) ions. A number of
galvanostatic tests were performed on EMIC based electrolytes,
varying the quantity of NbCl.sub.5 and SnCl.sub.2 salts, and using
both niobium and tin anodes. Sn anodes visibly dissolved by simple
immersion in the electrolyte and were severely consumed at the end
of any electrodeposition process. For example, electrochemical
deposition carried out at constant current density of 10
mA/cm.sup.2 and 130.degree. C. for 10 minutes gave a bright film,
having thickness of about 1.2 .quadrature.m thick and a niobium
content of 3 wt %. In the same experimental conditions the niobium
anode was not chemically etched by the electrolyte nor did it
appear to be damaged after the deposition test. Correspondingly,
niobium content in the resulting film increased up to 8 at %, and
thickness was 550 .mu.m. Therefore, it was concluded that the Nb
sheet should be preferred to Sn as anodic material in the
considered process.
[0049] FIGS. 2A-2C2-a shows the effect of a galvanostatic
polarization on the cathodic potential: at 10 mA/cm.sup.2 and 30
mA/cm.sup.2 the potential stabilized immediately after the
beginning of the experiment, while a pronounced polarization at
negative potential values appeared at 50 mA/cm.sup.2, the current
reaching the plateau more slowly. Niobium content in the coating
increased with current density up to 35 at % at 50 mA/cm.sup.2.
Correspondingly the thickness of the coating decreased to 150 nm.
In FIGS. 2A-2C2-b it is reported the surface morphology of a Nb--Sn
coating obtained from obtained from a 85% EMIC-10% NbCl.sub.5--5%
SnCl.sub.2 solution at 120.degree. C. on a copper substrate and at
a constant current density of 40 mA/cm.sup.2 applied for 240 s. The
coating appeared to be porous. The maximum niobium content in the
sample was 18 at %, and the thickness was 1 .mu.m. FIGS. 2A-2C2-c
shows the XRD pattern of Nb--Sn films deposited at 40 mA/cm.sup.2
and 120.degree. C. for 450 s from the same electrolyte. The Nb
content in the deposits was about 35 at %, film thickness was 500
nm. As revealed by the XRD pattern and the powder diffraction
references, the Nb--Sn thin film showed a cubic Nb.sub.3Sn
structure (A15 phase) with a strong (211) preferred orientation, no
NbSn.sub.2 phase was detected.
[0050] FIGS. 3A-3C show CV curves over the potential range from
-0.4 to -3.5 V corresponding to the pure BMIC (a), BMIC with
addition of 10%. SnCl.sub.2 (a), BMIC with addition of 25%.
NbCl.sub.5 (b), and BMIC containing 5%.SnCl.sub.2 and
21%.NbCl.sub.5 (c). In the case of pure BMIC, a low current density
(c.d.) was observed over the potential range from OCP to -1.9 V,
which can be considered a suitable potential window for Nb--Sn
deposition. As for EMIC, the significant increase of C.d. towards
the lower limit of the scanning range might be attributed to the
electrochemical decomposition of the water contained in the
electrolyte or to the reduction of ionic liquid itself.
[0051] By adding 10%.SnCl.sub.2 to BMIC, a broad reduction wave
peaking at about -1.14 V and c.d. of 6.0 mA/cm.sup.2 was observed.
As in the case of EMIC, the melt is basic and SnCl.sub.3 .sup.- is
the main anionic specie present in the melt, therefore it reacts
according to Eq. 1. Compared to the EMIC-based electrolyte, the
reduction of Sn(II) ions shifted to more cathodic potentials and
higher c.d, suggesting the possibility of better co-deposition with
Nb. Electrodeposition experiments carried out from this solution on
copper substrates at 20 mA/cm.sup.2 and 80.degree. C. for 600 s
resulted in a uniform and adherent tin coating (not shown).
[0052] By addition of 25% NbCl.sub.5 to BMIC (FIG. 3-b) two
pronounced reduction waves were observed peaking at -0.6 V with
c.d. of 1.5 mA/cm.sup.2, and at -1.96 V vs. Pt with c.d. 4.1
mA/cm.sup.2, corresponding to partial reduction of Nb(V) and Nb(IV)
species respectively (see Eq. 2 and Eq. 3). Compared to EMIC
electrolytes, the reduction of the couple Nb(V)/Nb(IV) occurs at
less cathodic potentials and the reduction of the couple
Nb(IV)/Nb(III) to more cathodic potentials, with lower c.d. values.
Furthermore, the first reduction peak for Nb(V) appears at
potential values less cathodic than the reduction peak of Sn(II).
Compared to EMIC electrolytes, the better overlapping of the
reduction waves of Sn(II) and Nb(V) can be expected, possibly
leading to higher Nb content in the electrodeposited Nb--Sn
alloy.
[0053] Finally, the cathodic behavior of 75% BMIM-21%
NbCl.sub.5--4% SnCl.sub.2 electrolyte was investigated (FIG. 3-c).
Two reduction waves peaking at about -1.1 V with c.d. of 1.9 mA/cm2
and -2.0 V with c.d. of 4.5 mA/cm2 were observed. The first
reduction wave peaks at potential values very close to that of
Sn(II), while the peak owing to the reduction of Nb(V) to Nb(IV) is
not evident.
[0054] Several deposition tests were carried out from a 85%
Bmim-Cl--10% NbCl5--5% SnCl.sub.2 bath. Electrodeposition at 40
mA/cm.sup.2 for 600 s at 130.degree. C. on a copper cathode and
using a niobium anode resulted in a 750 nm film, having a nominal
Nb content of 28 wt. %. According to XRD pattern in Error!
Reference source not found., the Nb--Sn film includes a cubic
Nb.sub.3Sn structure (A15 phase) with strong (211) preferred
orientation (reflection at 41.78.degree.), along with a disordered
orthorhombic NbSn.sub.2 structure (reflections at
28.64.degree.-29.86 .degree.-57.87.degree.). By increasing the
deposition c.d. at 400 mA/cm.sup.2 for 30 minutes at 130.degree.
C., Nb--Sn film containing a maximum of 17 at % Nb and an average
Nb content of about 9 at. % was obtained. A significant amount of
chloride (up to 6 at %) was also measured. The film thickness was
about 200 nm. The sample surface was unevenly porous. The SEM
micrograph was not well-defined because of a film of electrolyte
covering the surface (Error! Reference source not found.-a and -b).
The XRD pattern and the powder diffraction references revealed the
presence of a cubic Nb.sub.3Sn phase having (211) preferred
orientation. An orthorhombic NbSn.sub.2 phase with a slight (422)
preferred orientation was also detected. GD-OES depth profiling
analysis was carried out in order to assess the thickness of the
Nb--Sn layer and the oxygen content. As shown in Error! Reference
source not found.-c, the Nb-Sn layer was about 50 nm thick,
followed by a Nb--Sn--Cu and Sn--Cu layers. The presence of a
Nb--Sn--Cu layer can be explained considering the thermal
interdiffusion occurring at the operating temperature. A very low
oxygen content was measured, confirming the good quality of the
film. All samples presented the .eta. Cu.sub.6Sn.sub.5 phase. Due
to the low signal to noise ratio, it is not possible to estimate
the relative amount of the Nb.sub.3Sn and NbSn.sub.2 phases.
However, there is evidence of higher NbSn.sub.2 volume percentage
in samples obtained at 400 mA/cm.sup.2.
[0055] An alternative approach (in the following also named
"approach 2") was also used, consisting in electrolytic dissolution
of the Nb anode during prolonged polarization in a BMIC-SnCl.sub.2
electrolytes. As showed by the SEM micrographs in FIGS. 6-a and -b,
samples obtained at 40 mA/cm.sup.2 for 4 h 30 min were
characterized by a rough and uneven surface, with bumps surrounded
by a fine grained material. The EDX analysis (not shown) revealed
that the bumps mainly consisted in pure tin plus 4 at % Cl and 0.4
at. % Nb. The fine grained phase surrounding the bumps consisted in
a Nb--Sn phase containing up to 63at % Nb. a significant Cl content
was also measured (up to 11 at %), which can be attributed to
precipitation of Nb and Sn chlorides. The average composition was 4
at % Cl, 23 at % Nb and 73 at % Sn. The XRD pattern in FIG. 6-d
showed the presence of Cu.sub.6Sn.sub.5, Cu.sub.3Sn and NbSn.sub.2
(2theta=37.63.degree.; 53.degree.; 83.degree.) phases. The
reflection at 41.8.degree. can be attributed both at Nb.sub.3Sn and
Cu.sub.3Sn phases. GDOES analysis evidenced a Nb and Sn overlapping
region about 4 .mu.m thick, and a Sn--Cu layer about 3 .mu.m thick
laying underneath, given by Cu--Sn interdiffusion.
[0056] Also at 400 mA/cm.sup.2 the sample surface appeared rough,
grainy and inhomogeneous (FIGS. 7-a and -b). EDX analysis (not
shown) evidenced that the globular shapes on the surface consisted
in pure tin. The average composition was 2 at % Nb, 3 at % Cl and
95 at % Sn. GDOES analysis (FIG. 7-c) revealed a surface Nb--Sn
layer about 0.5 .mu.m thick, followed by Sn-rich layer of about 6
.mu.m and by Cu and Sn overlapping region about 8 .mu.m. The XRD
pattern (FIG. 7-d) revealed the presence of cubic Nb.sub.3Sn with
(211) preferred orientation and of the Cu.sub.6Sn.sub.5 phase.
There was no evidence of the orthorhombic NbSn.sub.2 phase. It was
concluded that the Nb.sub.3Sn phase was unevenly distributed on the
sample surface, between the Sn bumps.
[0057] The crystallite size .tau. was estimated by the Scherrer's
equation:
.tau.=K.lamda./.beta.2 cos .theta., Eq. 4
where K is the shape factor (taken as 0.94 for cubic crystals),
.lamda. is the X-ray wavelength (1.54 for Cu K.alpha. radiation),
.beta. is the line broadening (full width at half maximum, FWHM),
and .theta. is the Bragg angle. The average crystallite size of
Nb--Sn coatings electrodeposited from the four baths are reported
in Table 2. The crystallite size of Nb--Sn films was affected by
the c.d. value rather than by bath composition. Films
electrodeposited from either EMIC or BMIC based ionic liquids
containing NbCl.sub.5 showed a crystallite size in the range 3/4 nm
at c.d. of 40 mA/cm.sup.2, and of about 15 nm at c.d. of 400
mA/cm.sup.2. Films deposited by anodic dissolution of Nb showed
higher crystallite size, of about 25 nm at c.d. of 40 mA/cm.sup.2
and about 62 nm at c.d. of 400 mA/cm.sup.2. The average surface
roughness was about 0.233 .mu.m in the former case, and in the
range 0.696/2.113 .mu.m using approach 2.
TABLE-US-00002 TABLE 2 Bath type, deposition current density
(c.d.), crystallite size (.tau.), thickness (t), average roughness
(Ra) and root mean square roughness (Rq) of Nb--Sn films. Bath c.d.
.tau. t Ra Rq type (mA/cm.sup.2) (nm) (.mu.m) (.mu.m) (.mu.m) 1 40
3 1.000 -- -- (XRF) 2 40 4 0.750 -- -- (XRF) 2 400 15 0.200 0.233
.+-. 0.026 0.306 .+-. 0.054 (XRF) 3 40 25 1.446 .+-. 0.308 1.928
.+-. 0.229 4 40 24 4.000 0.696 .+-. 0.051 0.886 .+-. 0.054 (GDOES)
4 400 62 0.500 2.113 .+-. 0.676 2.934 .+-. 1.229 (GDOES)
[0058] Electrodeposition of Nb--Sn thin films from ionic liquids
and without the need of high temperature heat treatment is
disclosed herein. Electrolytes consisted of either EMIC or BMIC
with addition of SnCl.sub.2 and NbCl.sub.5 salts. Cyclic
voltammetry (CV) demonstrated that the selected ionic liquids had a
sufficiently wide potential window to allow the electrodeposition
of Nb--Sn alloys, notwithstanding a relative high water content in
the electrolyte.
[0059] The electrodeposited Nb-Sn thin films with average Nb
content up to 63 at % showed a cubic Nb.sub.3Sn structure (A15
phase) with (211) preferred orientation. Other phases were also
observed, in particular the disordered orthorhombic NbSn.sub.3
phase, the pure Sn phase and Cu.sub.6Sn.sub.5 structure, depending
on the operating parameters. Realization of improved intra-crystal
structure and inter-grain boundary characteristics in the
Nb.sub.3Sn material layer in a controlled environment using pulsed
electrodeposition, stringent temperature control of the
electrodeposition bath, and optimization of the galvanic cell
design for better thickness uniformity and layer microstructure are
the chief challenges at this stage.
[0060] Overall, the electrodeposition of Nb--Sn from EMIC and BMIC
based ionic liquids, even in the presence of a relatively high
content of water, was shown to be a promising process for the
deposition of Nb.sub.3Sn thin films on copper substrates. As
electrochemical deposition is controllable on curved surfaces and
is also scalable in size, in principle this technique could allow
using superconductors as surface coatings as opposed to bulk, wires
and cables.
Synthesis of Superconducting Nb.sub.3Sn Coatings on Nb
Substrates
[0061] Superconducting Nb.sub.3Sn films are obtained by
electrodeposition of Sn layers and Cu intermediate layers onto Nb
substrates followed by high temperature diffusion in inert
atmosphere. Electrodeposition was performed from aqueous solutions
at current densities in the 20 to 50 mA/cm.sup.2 range and at
temperatures between 40 and 50.degree. C. Subsequent thermal
treatments were realized to obtain the Nb.sub.3Sn superconductive
phase. Glow discharge optical emission spectrometry (GDOES)
demonstrated that after thermal treatment interdiffusion of Nb and
Sn occurred across a thickness of about 13 .mu.m, where the
Nb.sub.3Sn phase was about 5 .mu.m thick. X-ray diffraction (XRD)
patterns confirmed the presence of a cubic Nb.sub.3Sn phase (A15
structure) having (200) preferred orientation. Electrical
superconductivity tests measured a maximum J.sub.c (4.2 K, 12 T) of
600 A/mm.sup.2 in perpendicular magnetic field. The J.sub.c (4.2 K,
12 T) in parallel magnetic field was 736 A/mm.sup.2. With the
procedure described herein, coating complex shapes cost effectively
becomes possible, which is typical of electrochemical techniques.
Furthermore, this approach can be implemented in classical wire
processes such as "Jelly Rod" or "Rod in Tube", or directly used
for producing superconducting surfaces.
[0062] Disclosed herein is a combination of thermal diffusion
processes and electrochemical techniques to obtain thick
superconductive Nb.sub.3Sn coatings onto Nb substrates. In certain
embodiments, Nb.sub.3Sn coatings of at least 5 microns can be
obtained. The approach was to electrodeposit a seed copper layer
onto the Nb substrate, followed by a tin layer and a copper barrier
layer. The electrodeposition processes were carried out using
aqueous solutions working at near-room temperatures and atmospheric
pressure. Samples were then heat treated and characterized. Details
of the fabrication process are given in the following.
Experimental Setup
[0063] Electrodeposition tests were carried out on niobium foils of
1 cm.times.3 cm having thickness of 25 .mu.m and of 250 .mu.m.
Prior to deposition, the niobium foils were degreased in acetone
and cleaned in diluted acid to reduce the presence of niobium
oxides on the surface. In fact, niobium oxides could reduce the
adhesion of electrodeposited metals and act as a diffusional
barrier layer during the heat treatment, hindering the formation of
the superconductive phase. The electrodeposition of tin was
performed using the commercial bath Solderon.TM. MHS-W at a current
density of 50 mA/cm.sup.2 and bath temperature of 50.degree. C.
Copper seed layers were electrodeposited at 30 mA/cm.sup.2 and
40.degree. C. using a sulphate-based electrolyte whose composition
is reported in Table 3. Copper barrier layers were deposited from a
pyrophosphate-based electrolyte whose composition is reported in
Table 4. The pH of the electrolyte was 8.5. Electrodeposition
experiments were carried out at 20 mA/cm.sup.2 and 50.degree. C.
Electrolytes were prepared from analytical grade chemicals and
deionized water. Electrodeposition experiments were made in a two
electrodes cell, where the anode was a copper sheet and the cathode
a Nb foil. Deposition times ranged from 1 to 25 min.
TABLE-US-00003 TABLE 3 Composition of electrolyte used for the
deposition of the copper seed layer. Chemicals Concentration (g/l)
CuSO.sub.4 60 H.sub.2SO.sub.4 200 g/l HCl 40 g/l
TABLE-US-00004 TABLE 4 Composition of the electrolyte used for the
deposition of the copper barrier layer. Chemicals Concentration
(g/l) Cu.sub.2P.sub.2O.sub.7 26 NaNO.sub.3 5 Na.sub.4P.sub.2O.sub.7
180
[0064] Heat treatments were performed in a computer controlled
tubular furnace. The oven was equipped with three separately
programmable induction resistances. Temperature was continuously
monitored and maintained constant by means of two thermocouples.
Heat treatments were performed in argon atmosphere. To determine
the diffusional parameters, samples were observed using an optical
microscope. The samples were prepared for optical microscope
observation by means of classical metallurgical techniques: they
were placed in an epoxy resin and accurately sliced by means of a
metallographic sectioning saw. The exposed surface was grinded by
means of a Buheler HandiMetr roll grinder, using sandy papers from
a 240 grit to 600 grit. The final polishing was performed by an
automatic grinding and polishing system (LECO GPX-300). Samples
were observed after heat treatment using an inverted metallurgical
microscope (Nikon ECLIPSE MA200,) connected to a computer with a
camera control unit. The Imaging Software used for the analysis was
"NIS-Elements", which gives the possibility to apply smart filters
to the image, such as different phase and grain boundary
recognition. The maximum optical magnification was 500.times..
[0065] The crystallographic structure of the Nb--Sn coatings was
assessed by X-ray diffraction (XRD) using a Philips PW1830
instrument, with Cu K.alpha.1 radiation and Bragg-Brentano
geometry. XRD was performed in the 2.theta. angular range of 10 to
90.degree.. An approximate measure of the grain size was evaluated
by calculating the crystal coherence extensions according to
Scherrer equation. Glow discharge optical emission spectrometry
(GDOES) depth profiling analyses were performed with a Spectruma
GDA750 analyser using argon ions for sputtering with a beam spot
size of 2.5 mm. For superconductivity tests, a commercial
magneto-cryostat equipped with a Variable Temperature Insert (VTI)
was used, whose operation temperature was in the range of 1.5 to
200 K. Since the Nb--Sn--Cu films after reaction could not be
soldered, the original soldered contacts used to transfer the
current from the Cu current leads to the samples were replaced by a
sample holder with pressure contacts. For the same reason, the
voltage tap wires were attached to stainless steel screws that were
put in contact with the sample. Using the modified setup, film
samples were tested for critical current I.sub.c in liquid He at
4.2 K and in magnetic fields from 0 T up to 14 T. The tests were
performed both in a field parallel and perpendicular to the tape.
An electrical field criterion of E.sub.c=1 .mu.V/m was used to
define the transition voltage as:
V c = E c .rho. c 1 S , ##EQU00001##
where S is the superconducting tape cross section and 1 is the
distance between the voltage taps used during the I.sub.c
measurement. Thickness and width values were measured with
micrometer and caliber respectively in five positions along the
samples. The average length of the samples was 37.41.+-.0.51
mm.
Results
Electrodeposition of Sn and Cu Layers on Nb Substrates
[0066] Preliminary electrodeposition experiments of Sn layers were
carried onto Nb substrates at 50 mA/cm.sup.2 and 50.degree. C. The
coating showed high roughness and scarce adhesion on the Nb. On the
other hand, the electrodeposition of Sn onto copper substrates
resulted in a bright Sn coating with good adhesion. In addition, as
shown in FIG. 8, the growth rate of the Sn film on copper
substrates was about 1.25 .mu.m/min), higher than that onto Nb
substrates (about 0.63 .mu.m/min). The most efficient thermal
techniques for obtaining superconductive Nb.sub.3Sn wires through
solid diffusion at high temperature require the presence of copper
and the formation of a Cu--Sn phase prior to the growing of the
Nb.sub.3Sn phase. Due to the presence of copper, a Nb--Sn--Cu
ternary system forms. The diffusion path from the Cu--Sn solid
solution to the Nb--Sn solid solution passes only through the A15
phase field, destabilizing the formation of the non-superconductive
phases NbSn.sub.2 and Nb.sub.6Sn.sub.5. Thus Nb.sub.3Sn is the only
phase formed at the interface between Nb and a Cu--Sn solid
solution. In short, the addition of Cu lowers the A15 formation
temperature from well above 930.degree. C. to any other that is
deemed practical thereby limiting grain growth and thus retaining a
higher grain boundary density required for flux pinning Although Cu
can be detected in the A15 layers, it is generally assumed to exist
only at the grain boundaries and not to appear in the A15 grains,
allowing to use the binary A15 phase diagram to qualitatively
interpret compositional analysis in wires. Also, to the first
order, the addition of Cu does not dramatically change the
superconducting behavior of wires as compared to binary systems.
Therefore, the conclusion was made that a copper seed layer could
be conveniently deposited onto the Nb substrates prior to the
deposition of the Sn film.
[0067] Electrodeposition of copper on Nb was carried out from a
sulphate-based electrolyte at 30 mA/cm.sup.2 and 40.degree. C. The
resulting coating was bright and adherent (FIG. 9-a). As expected,
the subsequent deposition of tin on the copper seed layer resulted
in a coating adherent to the substrate (FIG. 9-b). The XRD pattern
revealed that the Sn coating onto the copper substrate had a
crystalline tetragonal structure with a slight (211) preferred
orientation.
[0068] Thermal treatments on Nb/Cu/Sn samples (see the following
section) evidenced that Sn coalesces into small lumps during
heating, producing a severe inhomogeneous tin distribution. This
issue was addressed by changing sample design. A copper barrier
layer was deposited onto the Sn coating in order to restrain the
coalescence effect and maintain the coverage of the Nb substrate
homogeneous. The copper barrier layer was deposited from a
pyrophosphate-based electrolyte at 20 mA/cm.sup.2 and 50.degree. C.
Thermal treatments for superconductivity tests were carried out on
samples having the sandwiched structure represented in FIG. 9-d. As
reported in Table 7, three types of samples were fabricated, based
on the thickness of the tin and copper barrier layers. The
thickness of the Sn layer ranged between 10 and 20 .mu.m, and the
thickness of the copper barrier layer was either 10 .mu.m or 15
.mu.m.
Calculation of Diffusional Parameters
[0069] The diffusional parameters were determined by annealing
Nb/Cu/Sn samples. In the case of Nb--Sn systems, a parabolic growth
rate was suggested in literature for the newly forming
superconductive layer. This behavior is derived from first Fick's
law, assuming a constant concentration of the diffusing component
at both the boundaries of the interlayer, and a constant
concentration gradient across the interlayer:
J i = - D i .differential. C i .differential. x , Eq . 5
##EQU00002##
where J is the diffusional flux (mol/.mu.m.sup.2s), D is the
diffusion coefficient (.mu.m.sup.2/s), C is the concentration
(mol/.mu.m.sup.3) and x is the width of the concentration gradient
(.mu.m). More precisely the parabolic growth can be described by
the simple law:
L = 2 Dt n , Eq . 6 ##EQU00003##
where L is the thickness of the new phase created after the heat
treatment (.mu.m), n is usually considered equal to 2, D
(.mu.m.sup.2/s) is the interdiffusion coefficient and t is the
duration of the heat treatment in seconds. Large deviations from
the parabolic growth rate are primarily due to cracks in the layers
for n<2, and to depletion of Sn in the matrix for n>2. The
interdiffusion coefficient can be written in an Arrhenius form
as:
D = D 0 exp ( - Q 0 RT ) , Eq . 7 ##EQU00004##
where D.sub.0 (.mu.m.sup.2/s) is the diffusion frequency, Q.sub.0
(kJ/mole) is the activation energy for diffusion, R is the gas
constant (kJ/Kmol) and T (K) is the reaction temperature. The
thickness of the newly formed phase was sampled in ten different
locations and the average value was calculated (Table 5).
TABLE-US-00005 TABLE 5 Temperature and duration of thermal
treatments, thickness of the resulting Nb--Sn phase and
corresponding standard deviation. Temperature Duration Thickness of
Nb--Sn Sample (.degree. C.) (h) phase (.mu.m) 1 220 20 No diffusion
2 400 20 No diffusion 3 800 40 4.2 .+-. 0.36 4 800 20 3.9 .+-. 0.44
5 950 20 12.7 .+-. 0.48
[0070] At temperatures below the melting point of tin (232.degree.
C.), diffusion was negligible. At higher temperatures, diffusion
becomes significant, but the tin layer coalesced forming small
domains and leaving the niobium substrate partly uncovered. The
thickness of the newly formed Nb--Sn phase could be evaluated in
the areas where good coverage was maintained. However, in view of
further analysis and practical applications, the samples design was
later changed (see following section).
[0071] In FIG. 10, the experimental thickness of the Nb--Sn phase
is reported as a function of the duration of the heat treatment,
and it is compared to curves calculated based on data reported in
literature. In Table 6 the calculated values for D.sub.0 and
Q.sub.0 are reported and compared with others found in literature.
The experimental activation energy for diffusion Q.sub.0 was about
202 kJ/mol. Values reported in literature are between 221 kJ/mol
and 404 kJ/mol. High values of Q.sub.0 are not desired because it
means lower diffusion rates and higher duration of heat treatments,
with excessive grain growth and loss of the superconductive
properties. As shown in Table 6, the value of Q.sub.0 determined in
the present work is lower than that measured for Nb surrounded by a
bronze matrix.
TABLE-US-00006 TABLE 6 Experimental D.sub.0 and Q.sub.0, and values
of Q.sub.0 reported in literature. Sample design before TT D.sub.0
(.mu.m.sup.2/s) Q.sub.0 (kJ/mol) Sn/Nb -- 221 CuSn7/Nb -- 404
CuSn8/Nb -- 279 This work 2 .times. 10.sup.9 202
Heat Treatment of Nb/Cu/Sn/Cu Samples
[0072] As mentioned before, it was observed that during thermal
treatments of Nb/Cu/Sn samples tin melts at temperatures higher
than 232.degree. C. and coalesces on the Nb surface. To overcome
this problem a Cu barrier layer was electrodeposited onto the Sn
coating. Three different types of samples were produced, whose
design is shown in FIG. 9-d and the thickness of each layer is
reported in Table 7. The thickness of the Sn layer ranged between
10 and 20 .mu.m, the thickness of the copper barrier layer was
either 10 .mu.m or 15 .mu.m.
[0073] Nb/Cu/Sn/Cu samples were processed following the thermal
profile shown in FIG. 11. The initial step was carried out at a
temperature of 214.+-.2.degree. C. for 72 hours, slightly lower
than the Sn melting point, to allow for relaxation of the internal
stresses in the metal layers and to start the diffusion between Cu
and Sn. According to literature, during this step a 3 .mu.m .eta.
phase should form. The second intermediate step at 458.+-.2.degree.
C. for 10 hours was done to allow the formation of a liquid tin
phase and start the interdiffusion with niobium and copper. This
intermediate step was necessary in order to avoid the Kirkendall
effect and consequently a severe degradation of the materials
properties. Furthermore, higher temperatures would induce higher Sn
pressures onto the Cu barrier layer, with a possible damage of the
same and consequent Sn leakage. According to literature, after ten
hours at .about.450.degree. C., a bronze E phase forms on the
surface, and an .eta. phase develops underneath. Finally, the
temperature was increased to 700.+-.1.degree. C. for a duration of
24 hours to form the Nb.sub.3Sn superconducting phase with a mean
grain size of about 80 .mu.m. After the heat treatment, small tin
islands were observed only on the surface of type 1 samples,
probably because of the lower thickness of the copper barrier
layer.
TABLE-US-00007 TABLE 7 Thickness of Sn and Cu layers in Nb/Cu/Sn/Cu
samples. Sample Thickness of Sn layer (.mu.m) Thickness of Cu layer
(.mu.m) Type 1 10 10 Type 2 15 15 Type 3 20 15
Characterization of Nb/Cu/Sn/Cu Samples
[0074] Nb/Cu/Sn/Cu samples after thermal treatment were
characterized by means of GDOES, XRD and electrical tests. By means
of GDOES analysis, the region where Sn and Nb are superimposed was
defined and, in some cases, the possible position and thickness of
the Nb.sub.3Sn phase was inferred. It must be noted that the in the
present work the relative intensities of the GDOES signal do not
give indication on the relative amount of the elements.
[0075] According to the experimental diffusional parameters, the
expected thickness of the Nb--Sn alloy after thermal treatment was
about 3.5 .mu.m. In FIG. 12-a the qualitative composition profile
of a type 1 sample consisting of a Nb/Cu/Sn(10 .mu.m)/Cu(10 .mu.m)
multilayered structure is shown. The thickness of the Nb.sub.3Sn
phase was about 5 .mu.m, located at a depth of about 10 .mu.m from
the surface. The corresponding XRD pattern (FIG. 12-b) revealed the
presence of a crystalline cubic Nb.sub.3Sn phase (A15 structure).
Other phases were also detected: NbSn.sub.2, .beta. Sn, .epsilon.
Cu.sub.3Sn (2.theta.=43.65.degree.), NbO and NbO.sub.2
(36.98.degree. and 74.84.degree.).
[0076] In FIG. 13-a the GDOES analysis of a type 2 sample
consisting of a Nb/Cu/Sn(15 .mu.m)/Cu(15 .mu.m) multilayered
structure after thermal treatment is shown. The composition
gradient did not allow to infer the thickness nor the position of
the Nb.sub.3Sn phase. However, the XRD pattern (FIG. 13-b) shows
the reflection of a crystalline cubic Nb.sub.3Sn phase (A15
structure). Other reflections can be attributed to NbSn.sub.2,
.beta. Sn, Cu--Sn phases, NbO and NbO.sub.2.
[0077] In FIG. 14 the GDOES analysis and corresponding XRD pattern
of type 3 sample (Nb/Cu/Sn(20 .mu.m)/Cu(15 .mu.m)) after thermal
treatment are reported. Based on the GDOES analysis, the thickness
of the Nb.sub.3Sn phase may be about 5 .mu.m. According to the XRD
pattern, a Nb.sub.3Sn phase having cubic structure with a strong
(210) preferred orientation is present. Compared to the other XRD
patterns, the signal to noise ratio increased, probably due to
mechanical cleaning of the sample surface before XRD analysis.
NbSn.sub.2 (2.eta.=18.71.degree. and 58.19.degree.), NbO.sub.2
(26.14.degree.-35.28.degree.-52.13.degree., Cu.sub.6Sn.sub.5)
(68.27.degree.), .beta. Sn (30.24.degree.), Cu (30.24.degree.) were
also detected. In all cases, the GDOES analysis revealed that the
region where Sn and Nb elements are superimposed was about 13 .mu.m
thick and confirmed the presence of oxygen in the outer 3
.mu.m.
[0078] Since the dominant source of flux pinning in Nb.sub.3Sn
appears to be grain boundaries, in order to obtain high critical
current densities it is necessary to produce a fine grained
structure. The crystallite size .tau. of samples after thermal
treatment was estimated by the Scherrer's equation:
.tau.=K.lamda./.beta.2 cos .theta., Eq. 8
[0079] where K is the shape factor (taken as 0.94 for cubic
crystals), .lamda. is the X-ray wavelength (1.54 for Cu K.alpha.
radiation), .beta. is the line broadening (full width at half
maximum, FWHM), and .theta. is the Bragg angle. The average
crystallite size of the electrodeposited Nb--Sn alloys was about 27
nm for type 1 sample, 24 nm for type 2 and 32 nm for type 3 sample.
In similar conditions Verhoeven obtained grain size in the range
100-110 nm.
[0080] The electrical tests were performed both in a field parallel
and perpendicular to the tape. To calculate the critical current
density J.sub.c from the measured currents I.sub.c, a thickness of
5 .mu.m was assumed for the Nb.sub.3Sn phase in all samples. FIG.
15 shows the critical current density as a function of the magnetic
field in the perpendicular (a) and parallel (b) orientations. A
superconductive behavior was observed. The largest J.sub.c (4.2 K,
12 T) in perpendicular magnetic field of 600 A/mm.sup.2 was
obtained for a type 2 sample. The corresponding J.sub.c (4.2 K, 12
T) in parallel magnetic field was 736 A/mm.sup.2. This is
consistent with crystallite size values for unoriented samples
(Table 8). Despite higher crystallite size, type 3 sample showed
intermediate electrical properties, which might be explained
considering that these samples have a strong (210) preferred
orientation, which is an additional parameter affecting electrical
properties.
TABLE-US-00008 TABLE 8 Preferred orientation (P.O.), crystallite
size, critical current and critical current density of Nb.sub.3Sn
thin films. Cristallite size I.sub.c J.sub.c (4.2 K, 12 T) J.sub.c
(4.2 K, 12 T) Sample (nm) (A) (A/mm.sup.2) perp. (A/mm.sup.2) par.
Type 1 27 6.5 148 -- Type 2 24 24 600 736 Type 3 31 16* 420 --
[0081] The results of the synthesis of Nb.sub.3Sn thin films onto
Nb substrates were presented. Superconductive coatings were
obtained by combining thermal treatments and the electrochemical
technique for thin film deposition. Samples were fabricated by
electrodeposition of a Cu seed layer onto the Nb substrate,
followed by deposition of a Sn layer (10-20 .mu.m) and a Cu barrier
layer (10-15 .mu.m). Subsequent thermal treatments were carried out
to form the Nb.sub.3Sn phase. The copper seed layer improved
adhesion of tin onto the substrate, while the copper barrier layer
limited tin coalescence during thermal treatments. Both layers were
expected to favor the formation of the Nb.sub.3Sn phase.
[0082] Diffusional parameters were determined, indicating a
thickness of the Nb--Sn phase after thermal treatment of about 3.5
.mu.m. GDOES analysis revealed that the region where Sn and Nb are
superimposed was about 13 .mu.m thick. In some cases it was
possible to infer that the thickness of the Nb.sub.3Sn phase was
about 5 .mu.m, at about 10 .mu.m from the sample surface. The XRD
patterns revealed the presence of both Nb.sub.3Sn+NbSn.sub.2
crystalline phases and of Cu--Sn phases. Electrical tests showed
superconductive behavior. The largest J.sub.c (4.2 K, 12 T) in
perpendicular magnetic field was 600 A/mm.sup.2 and the J.sub.c
(4.2 K, 12 T) in parallel magnetic field was 736 A/mm.sup.2.
* * * * *