Electroplating process for connectorizing superconducting cables

Abstract

An example method for connectorizing a superconducting cable is described herein. The method can include depositing an oxide layer on a surface of a superconducting cable, electroplating a metal layer on the surface of the superconducting cable, and soldering a connector to the metal layer coated on the surface of the superconducting cable. The oxide layer allows the metal layer to adhere to the surface of the superconducting cable.

Claims

1. A method for connectorizing a superconducting coaxial cable, comprising: providing the superconducting coaxial cable, the superconducting coaxial cable comprising one or more electrically-conductive components made of niobium-titanium (NbTi) superconducting material; depositing an oxide layer on a connectorizing surface of the superconducting coaxial cable; electroplating a metal layer on the connectorizing surface of the superconducting coaxial cable, wherein the oxide layer allows the metal layer to adhere to the connectorizing surface of the superconducting coaxial cable; and soldering a radio frequency connector to the metal layer coated on the connectorizing surface of the superconducting coaxial cable.

2. The method of claim 1, wherein the oxide layer has a thickness greater than or equal to about 10 nanometers (nm).

3. The method of claim 2, wherein the oxide layer has a thickness of less than about 35 nm.

4. The method of claim 1, wherein the metal layer is made of a non-magnetic metal.

5. The method of claim 1, wherein the metal layer is made of at least one of copper, silver, gold, or alloys thereof.

6. The method of claim 1, wherein the oxide layer is deposited on the connectorizing surface of the superconducting coaxial cable using a first electrolytic cell, and wherein the metal layer on the connectorizing surface of the superconducting coaxial cable is deposited using a second electrolytic cell.

7. The method of claim 6, wherein the first electrolytic cell includes an oxidizing solution.

8. The method of claim 7, wherein the oxidizing solution comprises sodium hydroxide (NaOH) in deionized water.

9. The method of claim 6, wherein depositing the oxide layer on the connectorizing surface of the superconducting coaxial cable comprises: placing the connectorizing surface of the superconducting coaxial cable into the first electrolytic cell; and applying a first electrical signal to a circuit comprising a cathode and an anode, wherein the anode is the connectorizing surface of the superconducting coaxial cable.

10. The method of claim 9, wherein the first electrical signal has a voltage between about 5V and about 15V.

11. The method of claim 6, wherein the metal layer is made of copper, and wherein the second electrolytic cell includes an acidic copper electroplating solution.

12. The method of claim 11, wherein the acidic copper plating solution comprises copper sulfate pentahydrate (CuSO.sub.4.5H.sub.2O) with sulfuric acid (H.sub.2SO.sub.4) in deionized water.

13. The method of claim 6, wherein depositing the metal layer on the connectorizing surface of the superconducting coaxial cable using a second electrolytic cell comprises: placing the connectorizing surface of the superconducting coaxial cable into the second electrolytic cell; and applying a second electrical signal to a circuit comprising a cathode and an anode, wherein the cathode is the connectorizing surface of the superconducting coaxial cable.

14. The method of claim 13, wherein the second electrical signal delivers current per area between about 5 $\frac{mA}{{\mathrm{cm}}^2}$ and about 60 $\frac{mA}{{\mathrm{cm}}^2}.$

15. The method of claim 1, wherein the superconducting coaxial cable is made of niobium, titanium, or alloys thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The components in the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding parts throughout the several views.

(2) FIG. 1A illustrates an oxidized NbTi cable 100. The oxide layer is shown with reference number 102. FIG. 1B illustrates the NbTi cable 100 electroplated with copper 104 that is ready to be connectorized. FIG. 1C illustrates the electroplated NbTi cable 100 with a soldered SMA connector 106.

(3) FIG. 2 is a graph illustrating reflection of an electrical pulse sent through a cryostat containing two sections of electroplated NbTi cable. The transmitted pulse height is 0.38 V.

(4) FIG. 3 is a flow chart illustrating the electroplating process for connectorizing superconducting cables according to an implementation described herein.

DETAILED DESCRIPTION

(5) Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure. As used in the specification, and in the appended claims, the singular forms “a,” “an,” “the” include plural referents unless the context clearly dictates otherwise. The term “comprising” and variations thereof as used herein is used synonymously with the term “including” and variations thereof and are open, non-limiting terms. The terms “optional” or “optionally” used herein mean that the subsequently described feature, event or circumstance may or may not occur, and that the description includes instances where said feature, event or circumstance occurs and instances where it does not. Ranges may be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, an aspect includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another aspect. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. While implementations will be described for connectorizing niobium-titanium (NbTi) superconducting cables, it will become evident to those skilled in the art that the implementations are not limited thereto.

(6) As used herein, the terms “about” or “approximately”, when used in reference to a linear dimension (e.g., thickness of an oxide layer) or voltage/current (e.g., voltage or current per area of an electrical signal), mean within plus or minus 10 percent of the referenced parameter.

(7) Experiments done at cryogenic temperatures below the 4.2 K temperature of liquid helium frequently require superconducting coaxial cables to efficiently transmit high-frequency signals while minimizing heat transfer to the experiment's cold stage. These cables are often made of niobium-titanium alloy (NbTi) which is difficult to solder directly. Described herein is an electroplating procedure for plating NbTi coaxial cables directly with copper, which enables connector attachment matching the structural and electrical properties of standard coaxial cables. Here, a cable is first electrochemically coated with a thin oxide layer and then electroplated with copper in an acidic copper sulfate solution. The procedure has modest safety requirements and may be carried out in a standard vented laboratory fume hood.

(8) As described above, attaching radio frequency (RF) connectors to a superconducting cable presents challenges. Conventionally, there are two ways that cables are connectorized: mechanically joining (crimping) the connector onto the cable conductors or, alternatively, making solder connections. An alternative for connectorization is electroplating onto the cable a surface (e.g., a metal layer) that can be soldered. One approach that is used commercially is electroplating nickel onto NbTi cables, using procedures similar to that described in a 1961 patent by Saubestre and Bulan.sup.9 for plating niobium and its alloys. However, this technique has two significant drawbacks. First, nickel is ferromagnetic, which is a potential problem for experiments sensitive to the magnetic environment including some cryogenic qubit proposals. Second, to promote strong adhesion of the nickel plating to the niobium, the Saubestre and Bulan procedure uses a high temperature bake that is well above the 327° C. melting point of polytetrafluoroethylene (PTFE)..sup.10 As PTFE is the dielectric usually used for NbTi cables, this is a fatal drawback for using this as a connectorizing process.

(9) Plating the cables directly with copper, which is only weakly diamagnetic and is readily solderable, is described herein. Existing recipes for electroplating titanium and its alloys with copper use a solution of sodium dichromate (Na.sub.2Cr.sub.2O.sub.7) and highly concentrated hydrofluoric acid (HF)..sup.11 Sodium dichromate is an OSHA-regulated carcinogen, and is not allowed in our local materials science laboratory/cleanroom facility..sup.12

(10) To overcome these obstacles, an electroplating procedure for directly electroplating copper onto NbTi cables has been developed and is described below. It should be understood that copper is only one example material that can be used with the electroplating procedure described herein. This disclosure contemplates using non-magnetic materials including, but not limited to copper, silver, gold, or alloys thereof with the electroplating procedure described below. In embodiments, the electroplating procedure described herein can be used on other oxide producing metals, such as aluminum or magnesium, for example. The electroplating procedure described below uses comparatively safe chemicals and can be carried out in a standard vented laboratory fume hood. Additionally, the electroplating procedure does not require a high temperature bake. It should be understood that NbTi is only one example superconductive material. This disclosure contemplates that the superconducting cables described herein can be other materials including, but not limited to, niobium, titanium, or alloys thereof.

(11) Referring now to FIG. 3, an example method for connectorizing a cable (e.g., the NbTi coaxial cable shown in FIGS. 1A-1C) is disclosed herein. At step 302, the method can include depositing an oxide layer on the surface of a superconducting cable. It should be understood that some metals, including superconductors, oxidize when exposed to air. In conventional process, the oxide layer is removed from the superconducting cable before it is electroplated with copper or nickel. In contrast to such conventional processes, the oxide layer is deposited (e.g., the thickness is increased) at step 302. The oxide layer allows the metal layer, which is deposited at step 304, to adhere to the surface of the superconducting cable. The oxide layer can be deposited to increase its thickness to greater than about 1 nanometer (nm). As described herein, the thickness of the oxide layer can be determined based on the color of the oxide layer. The oxide layer can have a thickness between about 10 nm and about 35 nm. Optionally, the oxide layer has a thickness from about 10 nm to about 15 nm (e.g., gray colored). Optionally, the oxide layer has a thickness from about 15 nm to about 20 nm (e.g., dark gray colored). Optionally, the oxide layer has a thickness from about 20 nm to about 30 nm (e.g., straw colored). Optionally, the oxide layer has a thickness from about 25 nm to about 30 nm (e.g., yellow colored). Optionally, the oxide layer has a thickness from about 30 nm to about 35 nm (e.g., brass colored). In some implementations, the oxide layer can have a thickness less than about 35 nm. An oxide layer with a thickness of less than about 35 nm has been shown to have strong adhesion. The thickness of the oxide layer should be somewhere between yellow (˜20 nm) and brown (˜35 nm) but not purple (˜45 nm). Additionally, the oxide layer can be deposited on the surface of the superconducting cable using a first electrolytic cell as described below.

(12) At step 304, a metal layer is deposited on the surface of the superconducting cable, for example, using a second electrolytic cell as described below. As described herein, the metal layer be a non-magnetic metal such as copper, silver, gold, or alloys thereof. Copper, which is solderable, is the material described in the example below. This disclosure contemplates that the electroplating procedure described herein can be used to deposit metal other than copper. The procedure described herein can therefore use two electrolytic cells. One holds a basic oxidizing solution (e.g., first electrolytic solution used at step 302) and the other holds an acidic copper plating solution (e.g., second electrolytic solution used at step 304) that does the actual plating. Initially, the surface of the NbTi cable is thoroughly cleaned and degreased, for example in an ultrasonic cleaner. Then, at step 302, the first electrolytic cell is used to deposit a special oxide layer on the NbTi cable that will allow the copper to adhere to the titanium. The first electrolytic cell is filled with any alkaline bath—a bath having a pH of at least 7. In embodiments the bath can be made of about 110 g/L of sodium hydroxide (NaOH) in deionized water. The NbTi substrate is connected as the anode in the oxidizing solution with a copper sheet as the cathode with a typical spacing of 5 cm. A first electrical signal is then applied to the circuit. Optionally, the first electrical signal can be a stepped voltage signal, for example, 5 V is applied to this circuit for about 15 seconds and then 10 V for about 30 seconds. Optionally, the first electrical signal can have a voltage from about 5V to about 15V. This stepped up voltage can be applied until the substrate darkens noticeably from the gray of the NbTi to a yellow, tan, or brown color, as shown in FIG. 1A. The oxide layer produces a structural color..sup.13 A very thin oxide layer of less than about 35 nm leads to the strongest adhesion. It should be understood that chemicals, concentrations, and/or voltages described above are provided only as examples. This disclosure contemplates that other chemicals, concentrations, and/or voltages can be used to deposit the oxide layer using the first electrolytic solution.

(13) After rinsing in deionized water, the NbTi cable can be transferred to the second electrolytic cell to perform step 304. This bath can be an acidic copper plating solution made of 220 g/L of copper sulfate pentahydrate (CuSO4.5H2O) with about 40 mL/L of sulfuric acid (H2SO4) in deionized water. The NbTi substrate is connected as the cathode with a metal sheet as the anode. In embodiments, the metal sheet can be made of copper. A second electrical signal is applied to the circuit to plate the NbTi cable to achieve the desired thickness of copper (e.g., about 120 seconds) at 0.032 A/cm2, producing the uniform coating of copper shown in FIG. 1B. For the copper electrode, oxygen-free copper can be used to limit impurities and increase the lifetime of the CuSO4.5H2O solution. In embodiments, the second electrical signal can deliver current per area between about 5 mA/cm.sup.2 and about 60 mA/cm.sup.2. Optionally, the signal delivers current per area of about 5 mA/cm.sup.2. Optionally, the signal delivers current per area of about 5 mA/cm.sup.2 to about 10 mA/cm.sup.2. Optionally, the signal delivers current per area of about 10 mA/cm.sup.2 to about 20 mA/cm.sup.2. Optionally, the signal delivers current per area of about 20 mA/cm.sup.2 to about 30 mA/cm.sup.2. Optionally, the signal delivers current per area of about 30 mA/cm.sup.2 to about 40 mA/cm.sup.2. Optionally, the signal delivers current per area of about 40 mA/cm.sup.2 to about 50 mA/cm.sup.2. Optionally, the signal delivers current per area of about 50 mA/cm.sup.2 to about 60 mA/cm.sup.2. The current per area can be varied depending on the materials and/or different shapes of the object being plated. It should be understood that chemicals, concentrations, and/or currents per are described above are provided only as examples. This disclosure contemplates that other chemicals, concentrations, and/or voltages can be used to deposit the oxide layer using the first electrolytic solution.

(14) As noted above, this disclosure contemplate plating the NbTi cable with other non-magnetic metal such as silver or gold. Alternatively, in embodiments, nickel can be used as a plating metal. Where nickel is used as a plating metal, this disclosure contemplates that step 302 is performed as described above. At step 304, however, the second electrolytic cell can be filled with a standard nickel-plating solution (as opposed to a copper plating solution). An example nickel plating solution is a mixture of NiCl2 and HCl and water, for example, as described in U.S. Pat. No. 2,999,799. According to the process described in U.S. Pat. No. 2,999,799, a baking step is incorporated in an electroplating process, after an object is electroplated. But, such a baking step is accomplished at a temperature which would melt an insulator material found inside many superconducting cables, making such a process unideal for electroplating cables. The electroplating procedure described herein can be accomplished with no baking step, and as such, preserves the insulator inside the NbTi cable. For large or continuous operations, the plating solution can be continuously filtered and regularly replaced.

(15) It should be understood that steps 302 and 304 of the procedure described herein require only standard chemical safety procedures and protective equipment including gloves, a lab coat, and chemical safety goggles. All steps can be performed in a standard laboratory fume hood. Neoprene gloves should be worn when handling concentrated H2SO4 and NaOH.

(16) At step 306, a connector (e.g., SubMiniature version A (SMA) type connectors) can then be attached to the NbTi cable. Optionally, the connector can be an RF connector. The connector can be soldered to the copper (e.g., metal layer) deposited at step 304. This step is the same as if the NbTi cable was a standard coaxial cable. A cable with an SMA connector attached is shown in FIG. 10. It should be understood that SMA type connectors are provided only as an example and that other types of RF connectors can be used.

(17) After producing connectorized superconducting cables using the process described above with regard to FIG. 3, the cables were tested for thermal cycling by thermally shocking them in liquid nitrogen and warming them to room temperature. This cycle was repeated twenty times over the course of about ten minutes. After cycling, the cables were tested for structural integrity and electrical conductivity. None of the connectorized cables tested to date have failed.

(18) “Pull tests” were also performed to characterize the mechanical integrity of the connector-cable junction. Retention force tests were also carried out on four cables by pulling on the SMA connector in a controlled, measured fashion until the connector broke free from the cable.14 Following the military's specifications for the regular room temperature version of these particular RF cables (MIL-PRF-39012E15), the cable-connector junction was found to withstand 60 pounds of axial force. In the four tests carried out, the junctions failed at 61, 61, 67, and 69 pounds of force. Thus, these cables are mechanically robust and may be handled just like similar cables using conventional conductors.

(19) Finally, a test of the electrical properties of the connectorized cables was carried out by creating an electrical loop circuit of superconducting cable. The circuit went into and out of the cryostat through standard coaxial cables, but, in the coldest stage, the two superconducting cables under test were joined together to send the signal back up another leg of standard cable out of the cryostat. All cables and connectors had a nominal characteristic impedance of 50Ω. A ˜700 ps full width at half maximum (FWHM) pulse—representative of signals in our SNSPD application—was sent into the loop and recorded on an oscilloscope the transmitted and reflected signals.

(20) The latter was extracted by using an RF circulator on the incident leg of the loop before it entered the cryostat, which would redirect any reflections coming back out of the chamber into the oscilloscope. By looking at the time dependence of the reflected signal, different reflections were assigned to particular segments of the loop. The results are shown in FIG. 2. In effect, the loop, including the superconducting cables at the link, faithfully transmitted the incident signal. The reflection coefficient from the superconducting segment was <1%, corresponding to a reflection loss of better than −40 dB. Indeed, the connectorized superconducting cable was better impedance matched than other parts of the loop. Specifically, the first peak in the reflection curve of FIG. 2 is from a standard 50Ω SMA vacuum feedthrough and is three times as large as the reflections from the connectorized cables.

(21) In conclusion, a procedure for connectorizing superconducting cables is described herein. The mechanical and electrical integrity of the joints has also been demonstrated. While direct experience is with SMA connectors, this method can be readily adaptable to connecting any type of RF connector to a superconducting cable. Finally, the copper on NbTi electroplating recipe detailed herein may have uses beyond attaching RF connectors to superconducting cables. For example, this disclosure contemplates that the process described in FIG. 3 can be used to electroplate a layer of metal on the surface of another metal that oxidizes when exposed to air (e.g., magnesium or aluminum). For example, the method can include depositing an oxide layer on a surface of a first metal using a first electrolytic cell (e.g., using step 302 of FIG. 3); and depositing a layer of a second metal on the surface of the first metal using a second electrolytic cell (e.g., using step 302 of FIG. 3). The oxide layer allows the layer of the second metal to adhere to the surface of the first metal

REFERENCES

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(23) Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.