Patent Application
20170107636
The Nb3Sn 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.
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 ˜1 μm 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.
On the other hand, the manufacturing of superconductive Nb3Sn 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 Hc1 (at defects) is expected to be much lower than Hc1 (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 Q0 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.
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)-NbCl5 and BPC-NbCl5—SnCl2 solutions. Moreover they investigated the deposition of Nb—Sn alloys from 53.8% SnCl2—7.7% NbCl5—38.5% BPC solution at 130° C., at 5 mA/cm2, using a copper cathode and a tin anode. The resulting Nb—Sn film contained about 14.8 wt % Nb. The deposition from 7.7% SnCl2—15.4% NbCl5—76.9% BPC bath at 40 mA/cm2 and 130° 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% SnCl2—14.3% NbCl5—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/cm2, t=50 ms and a duty ratio of 0.2. They also deposited a 41.3 wt % Nb alloy from a 7.7% SnCl2—15.4% NbCl5—76.9% BPC bath at 60 mA/cm2, 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 SnCl2—NbCl5 solutions in 1-ethyl-3-methylimidazolium chloride (EMIC). N. Koura, T Umebayashi, Y Idemoto and Gouping Ling, Electrodeposition of Nb—Sn Alloy from SnCl2—NbCl5-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% SnCl2—68.6% NbCl5—28.6% EMIC with a pulse period of 10 ms, duty ratio of 0.2 at 160° C., increased the Nb concentration to 69.1 wt %. XRD analysis and resistivity tests demonstrated that a superconductive Nb3Sn phase could be obtained. However, the same authors declared that the reproducibility of the process was not acceptable.
To overcome the limits of Lewis acidic ionic liquids, more recently Koichi et al also tested a Lewis basic melt consisting in 4.4% SnCl2—95.6% EMIC. Koichi Ui, Sakai H, Takeuchi K., Ling G., Koura N., Electrodeposition of Nb3Sn Alloy Film from Lewis basic SnCl2—NbCl5-EMIC melt, Electrochemistry, 77 (9) 798-800 (2009). Cyclic voltammetries were carried out at 130° 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% NbCl5—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% AlCl3—51.0% EMIC containing NbCl5. 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% NbCl5—10.0% SnCl2—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/cm2 with a pulse period of 10 ms, duty ratio of 0.20, and electricity of 5 C/cm2 at 130° 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 Nb3Sn along with metallic Sn and Cu10Sn3 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.
In addition, the intermetallic compound Nb3Sn is a type II superconductor having a well-defined stoichiometry and the A15 crystal structure. It has a critical temperature Tc0 of up to 18.3 K and an upper critical magnetic field Bc20 of up to 30 T. As a comparison, the ductile alloy NbTi has a Tc0 of 9.3K and a Bc20 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 Nb3Sn'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. Nb3Sn 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 Nb3Sn 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.
Some of the challenges are that Nb3Sn 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 Nb3Sn 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®). In the last 15 years, Fermi National Accelerator Laboratory (Fermilab, US) has used these wires and developed superconducting cables to perform Nb3Sn 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 Nb3Sn, establishing a strong foundation for the LHC luminosity upgrade at CERN. More recently, the first successful twin-aperture accelerator magnet made of Nb3Sn and developed and fabricated at Fermilab reached its design field of 11.5 Tesla at 1.9K.
Disclosed herein is a method comprising:
electrodepositing a film comprising a Nb—Sn material onto a copper substrate surface from an electrolyte bath comprising (a) SnCl2, (b) NbCl5, and (c) (i) 1-Ethyl-3-methylimidazolium chloride (EMIC), (ii) 1-Butyl-3-methylimidazolium chloride (BMIC), or (iii) a mixture thereof.
Also disclosed herein is 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 Nb3Sn coating.
The foregoing will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.
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., Nb3Sn and/or NbSn2) and phases.
Disclosed herein are method for electrodepositing Nb—Sn coatings on copper substrates from SnCl2—NbCl5-EMIC and SnCl2-(NbCl5)-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 NbCl5 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/cm2 and at a temperature of 130° 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 μm depending on the operating parameters. The electrodeposited coatings showed a cubic Nb3Sn phase with (211) preferred orientation and an orthorhombic NbSn2 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.
The electrochemical methods disclosed herein do not require high temperature processing. Use of SnCl2 and NbCl5 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 Nb3Sn film is directly produced in molecular form and at a temperature of 130° C. to 150° 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° 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 Nb3Sn is impossible to obtain with the classical metallurgical techniques due to the intrinsic brittleness of Nb3Sn. Furthermore, in principle electrodeposition techniques enable synthesizing of Nb3Sn coatings in only one step.
Disclosed herein is the electrodeposition of Nb—Sn thin films from a bath containing SnCl2, NbCl5 and 1-Ethyl-3-methylimidazolium chloride (EMIC) and from a novel Lewis basic ionic liquid consisting in SnCl2, NbCl5 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.
The experiments were carried out using two ionic liquids based on either EMIC (>98%, Sigma-Aldrich) or BMIC (>98%, Sigma-Aldrich). Anhydrous stannous chloride (SnCl2, 98%, Sigma-Aldrich) and anhydrous niobium chloride (NbCl5, 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° 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 SnCl2/EMIC mixtures were prepared mixing EMIC and SnCl2 and subsequently heating at 100° C. with a light stirring. Heating the mixture is required, since both the chemicals are solid at room temperature. At 70° C. EMIC melts, and SnCl2 easily dissolves into it by forming a transparent greenish solution. Cyclic voltammetries (CVs) were performed at 100° C. and 10 mV/s using a three-electrode cell configuration. Cu sheets (99.95%, thickness 700 μm) were used as cathode. The exposed area into electrolyte was 1-2 cm2. The counter electrode was a Sn sheet. A Pt wire was employed as a reference electrode Unlike EMIC-SnCl2, the solution formed with niobium salt is liquid at room temperature at concentrations ranging from 4% to more than 30% NbCl5. Cyclic voltammetry was performed on a 25% NbCl5—75% EMIC solution at 100° C. and 10 mV/s in a three-electrode cell similar to the previous one with niobium instead of tin as counter electrode.
The SnCl2—NbCl5-EMIC solution was obtained by adding NbCl5 to an EMIC-SnCl2 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° C. A CV was performed on a 75% EMIC-21% NbCl5—4% SnCl2 at 130° 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.
Electrodeposition from EMIC based electrolytes was carried out from a 85% EMIC-10% NbCl5—5% SnCl2 (in the following called Bath 1) solution at 120° 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 μm) were used as substrates. The exposed area into electrolyte was 1-2 cm2. The anode was a Nb sheet. Experiments were carried out at a constant current density of 40 mA/cm2 applied for 240 s.
A similar procedure was followed for the investigation on BMIC solutions. The CV curves were recorded on 10% SnCl2—90% BMIC electrolyte, which was obtained by mixing the chemicals in a glove box by magnetic stirring at 100° 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% NbCl5 eutectic was prepared by adding NbCl5 to BMIC at 100° 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 SnCl2—NbCl5-BMIC eutectic was obtained by adding NbCl5 to an EMIC-SnCl2 solution. The CV curves were recorded using a solution of composition 75% BMIC-21% NbCl5—4% SnCl2.
The electrodeposition from BMIC-based electrolytes was carried out from a 85% BMIC-10% NbCl5—5% SnCl2 solution (in the following called Bath 2) in galvanostatic mode at current densities of 40 and 400 mA/cm2 for times from 600 s to 1800 s at 130° C. The deposition of the Bn—Sn alloy was done on copper substrates, the exposed area being ˜1-2 cm2. 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% NbCl5) to dark brown (25% NbCl5). 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% SnCl2 (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° C. In fact, 13C and 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.
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% NbCl5—5% SnCl2.
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® 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 Kα radiation (λ=1.5405 Å) and a powder goniometer (Philips PW-1830) in the 2θ angular range of 10-90°. 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® laser profilometer (UBM Messtechnik GmbH). Film thickness was measured by X-ray fluorescence using a Fischerscope-XAN®-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.
The cathodic behavior of the ionic liquid solution was investigated by CV with the objective to define the potential range for alloy deposition.
SnCl3−+2e→Sn+3Cl− Eq. 1
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° C. and 10 mA/cm2 for 600 s resulted in a bright and uniform metallic Sn film (not shown).
By adding 25% NbCl5 to pure EMIC (
NbCl6−+e→NbCl62− Eq. 2
NbCl62−+e→NbCl63− Eq. 3
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% NbCl5—66.7% EMIC melt was NbCl6−, forming a complex with EMI+ 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/cm2, by varying the quantity of NbCl5 in the melt from 10% to 30%. No metallic coating were obtained even after 3600 s.
The CV of 75% EMIC-21% NbCl5—4% SnCl2, showed one pronounced reduction peak at −1.83 V and c.d. of 13.4 mA/cm2 (
By adding 10%.SnCl2 to BMIC, a broad reduction wave peaking at about −1.14 V and c.d. of 6.0 mA/cm2 was observed. As in the case of EMIC, the melt is basic and SnCl3 − 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/cm2 and 80° C. for 600 s resulted in a uniform and adherent tin coating (not shown).
By addition of 25% NbCl5 to BMIC (
Finally, the cathodic behavior of 75% BMIM-21% NbCl5—4% SnCl2 electrolyte was investigated (
Several deposition tests were carried out from a 85% Bmim-Cl—10% NbCl5—5% SnCl2 bath. Electrodeposition at 40 mA/cm2 for 600 s at 130° 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 Nb3Sn structure (A15 phase) with strong (211) preferred orientation (reflection at 41.78°), along with a disordered orthorhombic NbSn2 structure (reflections at 28.64°-29.86 °-57.87°). By increasing the deposition c.d. at 400 mA/cm2 for 30 minutes at 130° 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 Nb3Sn phase having (211) preferred orientation. An orthorhombic NbSn2 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 η Cu6Sn5 phase. Due to the low signal to noise ratio, it is not possible to estimate the relative amount of the Nb3Sn and NbSn2 phases. However, there is evidence of higher NbSn2 volume percentage in samples obtained at 400 mA/cm2.
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-SnCl2 electrolytes. As showed by the SEM micrographs in
Also at 400 mA/cm2 the sample surface appeared rough, grainy and inhomogeneous (
The crystallite size τ was estimated by the Scherrer's equation:
τ=Kλ/β2 cos θ, Eq. 4
where K is the shape factor (taken as 0.94 for cubic crystals), λ is the X-ray wavelength (1.54 for Cu Kα radiation), β is the line broadening (full width at half maximum, FWHM), and θ 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 NbCl5 showed a crystallite size in the range 3÷4 nm at c.d. of 40 mA/cm2, and of about 15 nm at c.d. of 400 mA/cm2. Films deposited by anodic dissolution of Nb showed higher crystallite size, of about 25 nm at c.d. of 40 mA/cm2 and about 62 nm at c.d. of 400 mA/cm2. The average surface roughness was about 0.233 μm in the former case, and in the range 0.696÷2.113 μm using approach 2.
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 SnCl2 and NbCl5 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.
The electrodeposited Nb-Sn thin films with average Nb content up to 63 at % showed a cubic Nb3Sn structure (A15 phase) with (211) preferred orientation. Other phases were also observed, in particular the disordered orthorhombic NbSn3 phase, the pure Sn phase and Cu6Sn5 structure, depending on the operating parameters. Realization of improved intra-crystal structure and inter-grain boundary characteristics in the Nb3Sn 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.
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 Nb3Sn 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.
Superconducting Nb3Sn 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/cm2 range and at temperatures between 40 and 50° C. Subsequent thermal treatments were realized to obtain the Nb3Sn 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 μm, where the Nb3Sn phase was about 5 μm thick. X-ray diffraction (XRD) patterns confirmed the presence of a cubic Nb3Sn phase (A15 structure) having (200) preferred orientation. Electrical superconductivity tests measured a maximum Jc (4.2 K, 12 T) of 600 A/mm2 in perpendicular magnetic field. The Jc (4.2 K, 12 T) in parallel magnetic field was 736 A/mm2. 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.
Disclosed herein is a combination of thermal diffusion processes and electrochemical techniques to obtain thick superconductive Nb3Sn coatings onto Nb substrates. In certain embodiments, Nb3Sn 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.
Electrodeposition tests were carried out on niobium foils of 1 cm×3 cm having thickness of 25 μm and of 250 μm. 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™ MHS-W at a current density of 50 mA/cm2 and bath temperature of 50° C. Copper seed layers were electrodeposited at 30 mA/cm2 and 40° 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/cm2 and 50° 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.
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×.
The crystallographic structure of the Nb—Sn coatings was assessed by X-ray diffraction (XRD) using a Philips PW1830 instrument, with Cu Kα1 radiation and Bragg-Brentano geometry. XRD was performed in the 2θ angular range of 10 to 90°. 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 Ic 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 Ec=1 μV/m was used to define the transition voltage as:
where S is the superconducting tape cross section and 1 is the distance between the voltage taps used during the Ic 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.
Preliminary electrodeposition experiments of Sn layers were carried onto Nb substrates at 50 mA/cm2 and 50° 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
Electrodeposition of copper on Nb was carried out from a sulphate-based electrolyte at 30 mA/cm2 and 40° C. The resulting coating was bright and adherent (
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/cm2 and 50° C. Thermal treatments for superconductivity tests were carried out on samples having the sandwiched structure represented in
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:
where J is the diffusional flux (mol/μm2·s), D is the diffusion coefficient (μm2/s), C is the concentration (mol/μm3) and x is the width of the concentration gradient (μm). More precisely the parabolic growth can be described by the simple law:
where L is the thickness of the new phase created after the heat treatment (μm), n is usually considered equal to 2, D (μm2/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:
where D0 (μm2/s) is the diffusion frequency, Q0 (kJ/mole) is the activation energy for diffusion, R is the gas constant (kJ/K·mol) 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).
At temperatures below the melting point of tin (232° 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).
In
As mentioned before, it was observed that during thermal treatments of Nb/Cu/Sn samples tin melts at temperatures higher than 232° 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
Nb/Cu/Sn/Cu samples were processed following the thermal profile shown in
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 Nb3Sn 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.
According to the experimental diffusional parameters, the expected thickness of the Nb—Sn alloy after thermal treatment was about 3.5 μm. In
In
In
Since the dominant source of flux pinning in Nb3Sn 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 τ of samples after thermal treatment was estimated by the Scherrer's equation:
τ=Kλ/β2 cos θ, Eq. 8
where K is the shape factor (taken as 0.94 for cubic crystals), λ is the X-ray wavelength (1.54 for Cu Kα radiation), β is the line broadening (full width at half maximum, FWHM), and θ 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.
The electrical tests were performed both in a field parallel and perpendicular to the tape. To calculate the critical current density Jc from the measured currents Ic, a thickness of 5 μm was assumed for the Nb3Sn phase in all samples.
The results of the synthesis of Nb3Sn 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 μm) and a Cu barrier layer (10-15 μm). Subsequent thermal treatments were carried out to form the Nb3Sn 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 Nb3Sn phase.
Diffusional parameters were determined, indicating a thickness of the Nb—Sn phase after thermal treatment of about 3.5 μm. GDOES analysis revealed that the region where Sn and Nb are superimposed was about 13 μm thick. In some cases it was possible to infer that the thickness of the Nb3Sn phase was about 5 μm, at about 10 μm from the sample surface. The XRD patterns revealed the presence of both Nb3Sn+NbSn2 crystalline phases and of Cu—Sn phases. Electrical tests showed superconductive behavior. The largest Jc (4.2 K, 12 T) in perpendicular magnetic field was 600 A/mm2 and the Jc (4.2 K, 12 T) in parallel magnetic field was 736 A/mm2.
This application claims the benefit of U.S. Provisional Application No. 62/190,199, filed Jul. 8, 2015, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62190199 | Jul 2015 | US |
