GB2257437A

GB2257437A - Method for forming triniobium tin superconductor

Info

Publication number: GB2257437A
Application number: GB9213163A
Authority: GB
Inventors: Mark Gilbert Benz; Lee Evan Rumaner
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1991-06-28
Filing date: 1992-06-22
Publication date: 1993-01-13
Anticipated expiration: 2012-06-22
Also published as: GB9213163D0; JPH0799652B2; GB2257437B; JPH05198227A

Abstract

A method of forming superconducting triniobium tin tapes by coating a niobium tape in a tin bath and reaction annealing the coated tape to form triniobium tin is improved by coating the niobium tape in a tin bath comprised of about 5 to 25 weight percent copper, about 5 to 25 weight percent lead, and the balance substantially tin. The niobium tape may comprise up to about 5 atomic percent of a solute selected from Zr, Hf, Mo, U, Re or mixtures thereof and up to about 10 atomic percent oxygen. The load, copper, and tin alloy coating promotes rapid formation of a fine grained triniobium tin during reaction annealing of the coated tape. The fine grain size in the triniobium tin provides improved current-carrying capacity in the superconducting tape. The lead, copper, and tin alloy coating that remains on the triniobium tin after reaction annealing also improves the solder bonding of a non-superconducting of copper outer laminae to the triniobium tin. As illustrated in Fig. 3 excess coating of tin alloy 4' covers the triniobium tin on the remaining tape 2'. <IMAGE>

Description

METHOD FOR FORMING: TRINTOBTUM TIN SEMICONDUCTOR Background of the Invention This invention relates to a method of forming a triniobium.tin superconductor having improved currentcarrying capacity, and improved triniobium tin tape.

Superconductivity is that characteristic of certain materials which permits them to conduct electric currents without resistance. A superconducting material exhibits this characteristic only when its temperature is below the superconducting critical temperature of the material, and then only if it is not subject either to a magnetic field greater than the superconducting critical magnetic field of the material or to an electric current greater than the superconducting critical current of the material.

Accordingly, superconductivity can be quenched, i.e., returned to a resistive state, by increasing the temperature, magnetic field, or current to which the superconducting element is subjected above the critical temperature, magnetic field, or current. Quenching of the superconductivity may occur abruptly or more gradually depending upon the particular material, i.e., the relative breadth of its superconducting transition state in terms of temperature, magnetic field, or current.

It is known that selected parent-metals, either pure or preferably containing minor alloying additions, are capable of being reacted with other metals and forming superconducting compounds or alloys that have a high currentcarrying capacity. Parent-metals niobium, tantalum, technetium, and vanadium can be reacted or alloyed with reactive-metals tin, aluminum, silicon, and gallium to form superconducting alloys, such as triniobium tin. As used herein, the term "triniobium tin" is a superconducting alloy in the form of an intermetallic compound comprised of about three niobium atoms per tin atom. Triniobium tin is the superconductor of interest in this invention.

Additionally, it is known that triniobium tin can be improved by first alloying the parent-metal, i.e., niobium with a minor amount of a solute metal having an atom diameter of at least 0.29 angstrom larger than the diameter of the parent-metal atom. U.S. Patent 3,429,032 incorporated by reference herein, discloses improved critical currents in triniobium tin formed by heating niobium comprised of up to about 25 percent zirconium in the presence of excess tin, and a non-metal selected from the group consisting of oxygen, nitrogen, and carbon. It is also known that the reactivemetal tin can be alloyed with copper to improve triniobium tin. In, "Enhancement of the Critical Current Density in Niobium-Tin" J.S.Caslaw, Cryogenics, February 1971, pp. 5759, incorporated by reference herein, the critical current density of triniobium tin was improved by reacting niobium with a reactive-metat comprised of up to about 45 weight percent copper and the balance tin.

The triniobium tin alloy has been fabricated in various forms, particularly wires and tapes, in efforts to produce devices such as high field superconducting electromagnets. One method for obtaining superconducting tape in a continuous fashion is that wherein a niobium or niobium alloy tape is continuously led through a bath of molten reactive-metal such as tin or tin alloy. The tape picks up a thin coating of the reactive-metal from the molten bath and the tape is subsequently heated in a reaction furnace to cause formation of a superconductive alloy on the surface of the parent-metal tape.

The superconducting alloy formed on the tape is fragile, and outer laminae of non-superconductive metal are applied to the tape to make a laminated superconductor that is strong and capable of being wound onto coils without damage to the superconductive material. For example, a relatively thin tape of niobium foil is treated with tin to form an adherent layer of triniobium tin on the surfaces of the tape, and copper tapes of substantially the same width are soft soldered to each of the major surfaces of the superconductive tape to form a symmetrically laminated structure. Because of the difference in the coefficient of thermal expansion of copper and the niobium-triniobium tin tape, the brittle intermetallic compound is placed in compression even at room temperature, minimizing the danger of mechanical fracture when coiling.

The copper outer laminae serve several other important functions on the tape. When the current load on a superconducting tape exceeds the critical current density, Jc, the tape is driven into the normal resistive state and a large amount of heat is generated which must be rapidly dissipated or the tape will be damaged. The copper outer laminae provide an alternate normal current path if the triniobium tin becomes non-superconducting, and reduces the heat generated. The copper outer laminae also provide a thermal cooling path for lowering the triniobium tin below the critical temperature of the superconductor. In order for the copper outer laminae to provide such benefits to the superconducting tape, a strong and uniform bond between the triniobium tin and the copper outer laminae is required.

An object of this invention is to provide a method for forming triniobium tin superconductors having improved current-carrying capacity.

Another object of the method of this invention is to provide an improved solder bond between the nonsuperconducting outer laminae and triniobium tin inner layer of a laminated triniobium tin tape.

Brief Description of the Invention Triniobium tin superconductors having improved current-carrying capacity are formed by the method of this invention. Superconducting triniobium tin tapes are formed by coating a niobium tape in a tin bath, and reaction annealing the coated tape to form triniobium tin. In the method of this invention the niobium tape is coated in a tin bath comprised of about 5 to 25 weight percent copper, about 5 to 25 weight percent lead, and the balance substantially tin. The lead, copper, and tin alloy coating promotes rapid formation of a fine grained triniobium tin during reaction annealing of the coated tape. The fine grain size in the triniobium tin provides improved current-carrying capacity in the superconducting tape.The lead, copper, and tin alloy coating that remains on the triniobium tin after reaction annealing also improves the solder bonding of a nonsuperconducting outer laminae to the triniobium tin.

Brief nescription of the Drawr ngs Fig. 1 is a side view of a niobium tape.

Fig. 2 is a side view of a niobium tape coated with a tin alloy.

Fig. 3 is a side view of a reaction annealed triniobium tin tape.

Fig. A is a side view. of a reaction annealed triniobium tin tape with non-superconducting outer laminae.

Fig. 5 is a cross section view of an apparatus for forming a laminated triniobium tin tape.

FIG. 6 is a graph comparing the current density versus reaction time for triniobium tin superconductors formed from niobium tapes coated with a copper-tin alloy, and niobium tapes coated with a lead-copper-tin alloy.

FIG. 7 is a graph comparing the current density versus grain size for triniobium tin superconductors formed from niobium tapes coated with a copper-tin alloy, and niobium tapes coated with a lead-copper-tin alloy.

Detailed nescrintion of the Invention Triniobium tin tapes are well known in the art being described, for example, in "Superconducting Properties of Diffusion Processed Niobium-Tin Tape," M. Benz, I.E.E.E.

Transactions of Magnetics, Vol. ItAG-2, No. 4, Dec. 1966, pp 7607764, incorporated by reference herein.

A method of forming continuous lengths of triniobium tin tapes is known, for example, being described in British patents 1,342,726 and 1,254,542, incorporated herein by reference. The method is briefly described by making reference to Figs. 1-4. A niobium tape 2 comprised of up to about 5 atomic percent of a metal selected from the group consisting of zirconium, aluminum, hafnium, titanium, and vanadium, and up to about 5000 parts per million oxygen is contacted with a molten tin bath comprised of up to about 45 weight percent copper to form a coating 4. Alternatively, the tin and copper may be deposited separately onto the niobium wire or tape at least partly by plating. Still further, the tin and copper may be applied to the niobium wire or tape by electrolytic or chemical processes.

The coated niobium tape 10 is reaction annealed at about 850 to 1100-C to react the niobium substrate with the coating and form laminae of triniobium tin 6 on niobium tape 2. Excess coating 4', as shown in Fig 3, covers the triniobium tin 6. The remaining niobium tape 2' is reduced in thickness from reaction with the coating to form the trioniobium tin 6. Non-superconducting outer laminae 8 can be bonded to the reacted tape, for example by soldering, to form a laminated triniobium tin tape 14 having improved strength and formability. For example, solder is applied to the outer laminae 8 and triniobium tin tape 12 and the outer laminae 8 and triniobium tin tape 12 are brought into contact while heated to a suitable soldering temperature to form the laminated triniobium tin tape 14.

The method of this invention is shown by again making reference to FIGS. 1-4. We have discovered that when the niobium tape 2 is coated in a tin bath comprised of about 8 to 10 weight percent lead, about 8 to 10 weight percent copper, and the balance substantially tin to form coating 4, the coated tape can be reactively annealed to form laminae of triniobium tin 6 having improved current-carrying capacity.

Preferably, the lead-copper-tin alloy coating 4 on the niobium tape 2 is uniform and continuous, and about 0.01 to 0.03 grams per square centimeter. In addition, the residual or excess coating 4' of the lead-copper-tin alloy remaining on the triniobium tin after reaction annealing provides an improved solder bond when triniobium tin tape 12 is soldered to outer laminae 8 to form laminated triniobium tin tape 14.

The niobium tape 2 may be comprised of up to about 5 atomic percent of a solute from the group consisting of hafnium, zirconium, molybdenum, uranium, rhenium, or mixtures thereof, up to about 10 atomic percent oxygen, and the balance substantially niobium. Preferably, the niobium tape is comprised of about 1 to 2 atomic percent solute, and about 2 to 4 atomic percent oxygen. Oxidation of the niobium tape surface and internal oxidation of the niobium tape can be preformed by conventional means known in the art, for example, as described in British patent 1,342,726.

Anodization followed by annealing in an inert atmosphere is a preferred means for introducing the oxygen into the niobium tape. A suitable niobium tape 2 is about 10 to 50 microns thick and about 0.076 to 5 centimeters in width.

In a suitable anodization process, a conductor, i.e. the niobium tape is connected as an anode in a suitable electrolytic bath. The electrolytic oxidation results in the formation of a niobium oxide, Nb2O5, film on the surface of the tape. The tape carrying the oxide film is heated in an inert atmosphere, such as argon, to cause the oxygen to diffuse into the tape. The desired concentration of oxygen, for example up to about 10 atomic percent, is diffused into the body of the tape leaving an oxide free tape surface. It is believed the oxygen combines with the solute-elements in the tape.

The niobium tape 2 can be coated by passing the tape through a molten tin bath comprised of the lead-coppertin alloy. Preferably, the coating 4 is chick enough to react with up to about 90 percent of the niobium tape, and leave a residual or excess coating 4' on the triniobium tin 6. The coated tape 10 is heat treated in an inert atmosphere for a time sufficient to form the triniobium tin superconductor 6, for example about 20 to 300 seconds.

Preferably, the reaction anneal is stopped before the niobium tape is fully reacted so that a niobium core 2' remains in the tape to support the brittle triniobium tin 6, and the residual coating 4' covers the triniobium tin 6.

The reacted tape 12 is soldered between nonsuperconducting outer laminae 8 of metal which have a greater coefficient of thermal expansion than the triniobium tin, preferably copper. The outer laminae 8 are about the same width as tape 12, and about 75 to 150 microns thick. Solder bonding can be performed as shown for example in FIG. 5.

Triniobium tin tape 12 and outer laminae 8 are led into a solder bath 20, heated to about 230 to 250 C. Alignment rollers symmetrically align and contact tape 12 with outer laminae 8 in the bath 20. Roller-wipers 22 remove excess solder while providing intimate contact under pressure to form the solder bond between tape 12 and outer laminae 8 to form laminated triniobium tin tape 14.

Referring back to FIGS. 3-4, preferably, a conventional lead-tin solder comprised of about 37 weight percent lead and the balance tin is used to bond the outer laminae 8 to the triniobium tin tape 12. The residual layer 4' of the lead-copper-tin coating provides an improved solder bonding between the triniobium tin tape 12 and outer laminae 8. The ease of forming the solder bond, and the uniformity and strength of the solder bond are improved by the residual coating 4' comprised of the lead-copper-tin alloy. For example, the solder bond can be formed at a lower temperature or in a shorter period of time with the residual coating 4' comprised of the lead-copper-tin alloy as compared to a residual coating 4' of tin or copper-tin alloy.As a result, the outer-laminae 8 are maintained at a higher strength so that triniobium tin 6 is maintained at a higher level of compression, and laminated triniobium tin tape 14 has greater formability.

Additional features and advantages of the method of this invention are shown in the following examples. Currentdensity of the triniobium tin tapes formed in the following Examples was measured by the conventional four probe resistance measurement technique well known in the art.

Briefly described, two voltage probes were soldered onto the superconducting tape a short distance apart. Current leads were soldered onto the superconducting tape at a further distance from each of the voltage probes. The triniobium tin tapes were cooled to 4.2 K by cooling in liquid nitrogen, followed by cooling in liquid helium. A magnet having a magnetic field of about 5 Tesla was aligned over the tapes so that the magnetic field was perpendicular to the current path in the superconducting tape. A current was passed through the tapes in increasing steps, and the voltage was recorded from the probes on the tapes. In this test, the critical current was defined as the current which caused a voltage differential of 0.2 microvolts between the probes.

Example 1 A niobium tape about 25 millimeters in width, 25 microns thick, and comprised of about 1.0 weight percent zirconium was obtained Teledyne Wah Chang, Albany, Oregon.

The niobium tape was anodized by passing the tape through an aqueous bath heated to about 90 C, and comprised of about 7 grams per liter of sodium sulfate. A potential of 145 volts was applied between the tape as the anode, and a stainless steel cathode. The tape was anodized in the bath at a rate of about 10 feet per minute. The anodized tape was passed at a rate of about 2 feet per minute through a tube furnace having a heating zone about 3 feet long, and heated to 1000-C in an argon atmosphere. The niobium oxide on the tape surface was decomposed to form substantially oxide-free tape surfaces of metallic niobium, and the oxygen diffused into the body of the tape.

Samples of the internally oxidized tape were contacted with a tin bath comprised of about 16.6 weight percent copper and the balance substantially tin. The tin bath was heated to about 1050-C, and the tape was passed through the bath at a rate of about 10 feet per minute to produce an adherent continuous film of about 0.03 grams per square centimeter of the tin-copper alloy on the tape surface. The tin coated tape was heat treated for various reaction times from 25 to 600 seconds in an argon atmosphere tube furnace at temperatures ranging from 1000 to 1150-C.

The reacted tape had a triniobium tin reaction layer of about 2.6 to 10.8 microns on each side of the tape, and about 1 to 22 microns of unreacted niobium at the core of the tape. The reaction temperature, reaction time, thickness of triniobium tin, and measured properties of critical current, and current density for each tape sample are shown below in Table I.

Table I.

Triniobium Tin Tape From Niobium Tape Coated in Tin Bath Comprised of 16.6 Weight Percent Copper Sample Reaction Reaction Nb3Sn Jc at 5 No. Temp Time Thickness Tesla ( C) (Sec.) (Microns) (A/cm2) 1 1000 150 2.6 7.01 2 1000 300 4.3 10.3 3 1000 500 9.7 8.76 4 1000 600 9.7 8.76 5 1050 180 6.1 8.04 6 1150 25 2.6 6.25 7 1150 50 4.1 9.15 8 1150 75 8.1 6.74 9 1150 100 10.8 7.41 Example2 Samples of triniobium tin superconducting tape were formed by the method in Example 1, except the tin bath was comprised of about 8.8 weight percent copper, 8.8 weight percent lead, and the balance substantially tin. The reaction temperature, time, thickness of triniobium tin, and resulting measured properties are shown below in Table II.

Tabletl Triniobium Tin Tape From Niobium Tape Coated in Tin Bath Comprised of 8.8 Weight Percent Copper, 8.8 Weight Percent Lead, Balance Substantially Tin.

Sample Reaction Reaction Nb3Sn Jc at 5 No. Temp Time Thickness Tesla ( C) (Sec.) (Microns) (A/cm2) 10 1050 150 3.3 11.47 11 1050 200 5.1 10.77 12 1050 250 6.8 11.84 By comparing the data in Tables I and II it can be seen that the triniobium tin formed from the lead-copper-tin alloy coating has an improved current density as compared to the triniobium tin formed from the copper-tin coating. The improved current-carrying capacity in triniobium tin tape formed by the method of this invention is further shown by making reference to FIGS. 6 and 7. FIGS. 6 and 7 are graphs comparing the current density versus reaction time for triniobium tin tapes formed by reacting niobium tapes coated with copper-tin alloy, and niobium tapes coated with leadcopper-tin alloy.

The current density of about 100 samples of triniobium tin tape formed by the method in Example 1 were averaged to produce the plot shown by the open circles in FIGS. 6 and 7. It can be seen that as reaction time increases for niobium reacted with copper-tin alloy coating, the current density of the triniobium tin decreases due to the growth of the grains in the triniobium tin layer.

However, the triniobium tin superconductor formed by reacting niobium with the lead-copper-tin alloy coating maintains a finer grain size as reaction time increases, and current density is improved over the prior art triniobium tin superconductor.

Claims

1. A method for improving the current-carrying capacity of triniobium tin superconducting tape formed by coating a niobium tape in a tin bath, and reacting the coated tape to form triniobium tin, comprising coating the niobium tape in a tin bath comprised of about 5 to 25 weight percent lead, about 5 to 25 weight percent copper, and the balance substantially tin.

2. The method of claim 1 wherein the niobium tape is comprised of about 1 to 2 atomic percent zirconium, and about 2 to 4 atomic percent oxygen.

3. A method of forming a laminated triniobium tin tape according to claim 1 wherein the triniobium tin tape has an excess layer of the tin bath coating and is soldered ta a copper outer laminae.

4. A method for improving the current-carrying capacity of triniobium tin superconducting tape formed from a niobium tape comprised of up to about 5 atomic percent of a solute from the group consisting of zirconium, hafnium, molybdenum, uranium, rhenium, or mixtures thereof and up to about 10 atomic percent oxygen, coating the tape in a tin bath, and reacting the coated tape to form triniobium tin, the improvement comprising: coating the niobium tape in a tin bath comprised of about 5 to 25 weight percent lead, about 5 to 25 weight percent copper, and the balance substantially tin.

5. A method of forming a laminated triniobium tin tape according to claim 4 wherein the triniobium tin tape has an excess layer of the tin bath coating and is soldered to a copper outer laminae.

6. A method according to Claim 1 substantially as described herein.