US3243871A

US3243871A - Method of making ductile superconductors

Info

Publication number: US3243871A
Application number: US301494A
Authority: US
Inventors: Eugen J Saur
Original assignee: Nat Res Corp
Current assignee: National Research Corp
Priority date: 1963-08-12
Filing date: 1963-08-12
Publication date: 1966-04-05
Anticipated expiration: 1983-04-05

Description

April 5, 1966 E. J. SAUR METHOD OF MAKING DUCTILE SUPERCONDUOTORS Filed Aug. 12, 1963 WIND INTO Fig.8

INVENTOR. EU GEN J. SAUR United States Patent Ofiice 3,243,871 Patented Apr. 5, 1966 3,243,871 NLETHOD F AKING DUCTILE SUPERCONDUCTORS Eugen J. Saur, Giessen, Germanyg assignor to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed, Aug. 12, 1963, Ser. No. 301,494

Claims. (Cl. 29-1555) The invention is applicable to hard superconductor alloys comprising, as a higher melting point component,

indium.

For a fuller understanding of the nature and objects of the Invention, reference should be had to the following detailed description in conjunction with the accompanying FIG. 1 is a schematic view of the first step of the process of the invention;

FIG. 2 is a cross-section of the product of the invention at an early stage of treatment; 1

FIG. 3 is a cross-section of the product at a later stage 5' 7 of treatment; 1

FIG. 4 is a schematic view of a still further treatment; FIG. 5 is a cross section of'the product after the treatment indicated in FIG. 4, the section being typical of any cross-section along the length of the product except for the ends thereof;

FIGS. 6 and 7 are typical cross-sections of the product after further treatments;

I FIG. 8 is a block diagram of final treatments; and

FIG. 9 shows the resultant product in cross-section.

(2) Pressing into a disk.

(3) Spinning the disk into a tube (several passes with vacuum annealing after each pass).

(4) Vacuum casting tin into the tube.

(5) Furnace cooling.

(6) Rolling and drawing the tube down to wire.

Conventional metallurgical processes are utilized in all the steps. Steps 1 and 4 and the anneals of Step 3 are carried out m a vacuum furnace. The temperature of the niobium tube and molten tin in Step 4 should be maintained at about 300 C., after initially annealing the niobium at higher temperature, with the vacuum furnace method of Allen, Das andStaufler, disclosed in the copending United States applications, S.N. 193,281 filed May 8,1962 and S.N. 278,723, filed May 7, 1963. This The ingot 20 is pressed into the form of a disk as shown in FIG. 2. The disk is spun into the form of a tube 24, shown in FIG. 3, having a inch outer diameter and .040 inch wall thickness. The tube 24 is thus of high purity and free of seams or other join The niobium tube 24 is returned to the vacuum chamber 10. where it is annealed under a vacuum of 10- mm. V C. and 1400 C. for one hour. The chamber is provided-with a crucible 6 and crucible pivots 28. Tin is melted in the crucible at 300 C. and under a vacuum of 10- mm. Hg. Then :he crucible is tilted to vacuum cast the tin into tube 24. Tube 24 is cooled and then removed from the furnace. The resultant product is a cylinder 32 comprising a niobium sheath 24 and a solid tin core 30, as shown in FIG. 5. Cylinder 32 is capped by crimping the open end. Then cylinder 32 is rolled and drawn down to wire size to form the wire 34 of FIG. 6. The rolling and drawing is carried out in accord with the teachings of Allen, Das and Stautfer in their US. patent application, S.N. 193,281, filed May 8, 1962. The diameter of the resultant wire 34 is on the order of .010 to .020 inch after a reduction of cross-section area in excess of 30:1. The extensive reduction creates new surface at the niobium-tin interface, thus dispersing the residual contaminants therein to improve the cleanliness of the interface.

Wire 34 is flattened between pressure rolls to form the ribbon 36 of FIG. 7. Ribbon 36 is -then drawn to form a reduced thickness ribbon which is less than about .002 inch thick. The ribbon comprises an unbroken, sea-mless sheath of niobium surrounding a flattened core The ribbon is then fabricated into a solenoid according to the steps outlined in FIG. 8. The heating is carried out between 900 C. and 1200 C. for /z to two hours. The clean-niobium surface is readily wettable by molten tin and the reaction proceeds rapidly and uniformly. At the end of the heating cycle, the ribbon is removed from the furnace and cooled in air. The ribbon may then be insulated by a conventional plastic whereas the insulation of prior art core wires would be a special high temperature ceramic insulation because the prior art cores must be insulated before heat treatment.

FIG. 9 shows the ribbon at the end of the heat treatment step. The sheath 24 then comprises a diffusion layer 40 of Nb Sn at the inner surface of the sheath. The core 30 comprises residual tin and small void spaces. Essentially, no niobium or Nb Sn particlesare found in the core, apart from the niobium and Nb Sn trapped in the sheaths inner diffusion layer.

The present invention is particularly applicable to vanadium gallium which can only be formed with great difiiculty in prior art core wires and is a very brittle compound. In applying the technique of the present invention, the tube 24 of FIG. 4 (which would be vanadium) should be capped by inserting a stopper in the open end of the tube after pouring the gallium and indenting the tube into depressions in the stopper. This will hold the low melting gallium during the drawing process. The process of the present invention guards the highly reactive gallium from contamination.

A principal advantage of the present invention is that the ribbon is heat treated before insulating and forming into a coil. It is not necessary to apply an expensive ceramic insulation as in prior art core wires. A cheap plastic insulation can be used. A related advantage is thatthe reactive tin or gallium or indium, etc. is trapped inside the ribbon. The ribbon can be wound into a compact spiral, inserted into a furnace muflie, and then heat treated and readily unwound after heat treatment. A ribbon with the low melting component on an outer surface would pose problems of adjacent turns sticking if wound into a tight spiral and heated.

The above-described preferred embodiment has several advantages over the prior art. The flattened form of the final product permits a higher ratio of length of Nb Sn diffusion layer to total cross-section area than a round wire because the geometry of a ribbon is inherently more favorable and because ribbons can be cold worked to smaller thicknesses more readily than wires. The ribbon form inhibits the formation of voids during the heat treatment and is a more favorable shape than round wire for winding coils. The wire 34, comprising a soft core 30 of tin and no hard materials, is readily flattened.

The process permits the use of high purity sheath material and vacuum degassed core material. The clean core material is degassed while held in crucible 26 and while being transferred from vacuum degassing crucible 26 to tube 24. The tin is then trapped in the tube without exposure to oxidizing agents and other contaminants which would limit the effectiveness of the subsequent heat treatment.

Other advantages are afforded by maintaining a continuous sheath of pure niobium about the core throughout the cold work and heating steps. The leakage of tin is prevented and the niobium-tin interface is protected from contaminants.

In comparison to the known core wires of the prior art,

the present invention offers the further advantage of confining the superconductive alloy to a thin diffusion layer which permits bending of the finished product and entails shorter times of heat treatment compared to the '20 hour heat treatments needed for prior art core wires.

Since certain changes can be made in the above product and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of making ductile superconductors providing high current densities through an internal layer of an intermetallic alloy hard superconductor comprising the steps of placing a tube made of a higher melting component of the alloy in a non-oxidizing atmosphere and annealing it, maintaining the lower melting point component of the alloy in a non-oxidizing atmosphere in molten state to degas it, pouring the molten metal into the tube while maintaining their temperatures below the reaction temperature at which their brittle intermetallic alloys are formed, cooling the tube, cold working the tube down to wire size to extend the interface between the core and sheath thereby making the sheath readily wettable by the core materials, cold rolling the wire to flatten it to ribbon form and to further reduce its cross section thickness after flattening the wire to ribbon form, and then heating the ribbon at the reaction temperature to form the desired intermetallic, hard superconductor alloy as a diffusion layer inside the ribbon.

2. A method of making ductile superconductors providing high current densities through an internal layer of an intermetallic alloy hard superconductor comprising the steps of placing a tube made of a higher melting component of the alloy in a non-oxidizing atmosphere, maintaining the lower melting component of the alloy in said atmosphere, in molten state, pouring the molten metal into the tube while maintaining the temperatures of the tube and molten metal below the reaction temperature at which their brittle intermetallic alloys are formed, cooling the tube, then drawing the filled tube down to wire size, to extend the interface between the core and sheath thereby making the sheath readily wettable by the core materials, then flattening the tube to ribbon, and then heating the ribbon at the reaction temperature to form the desired intermetallic alloy as diffusion layer at the inner surface of the sheath.

3. The method of claim 2 wherein the higher melting component is niobium and the lower melting component is selected from the class consisting of tin, gallium, thallium, silicon, aluminum, germanium, indium, and mixtures thereof.

4. The method of claim 3 wherein the higher melting component is niobium and the lower melting component is tin and the reaction temperature is maintained between 900 and 1200 C. a

5. The method of claim 4 wherein the ribbon is cold rolled to reduce its cross section thickness to less than .002 inch before heat treating at the reaction temperature.

6 6. The method of claim 2 wherein the higher melting 2,887,763 5/1959 Snavely 29-1555 component is selected from the class consisting of niobium, 3,060,557 10/1962 Rostoker et a]. 29-194 tantalum and vanadium, and mixtures thereof. 3,057,048 10/1963 Hirakis 29-194 3,181,936 5/1965 Denny et a1. 7 References Cited by the Examiner 5 UNITED STATES PATENTS WHITMORE A. WILTZ, Primary Examiner. 2,877,539 3/1959 Kinnan 55 5 P. M. COHEN, Assistant Examiner.

Claims

1. A METHOD OF MAKING DUCTILE SUPERCONDUCTORS PROVIDING HIGH CURRENT DENSITIES THROUGH AN INTERNAL LAYER OF AN INTERMETALLIC ALLOY HARD SUPERCONDUCTOR COMPRISING THE STEPS OF PLACING A TUBE MADE OF A HIGHER MELTING COMPONENT OF THE ALLOY IN A NON-OXIDIZING ATMOSPHERE AND ANNEALING IT, MAINTAINING THE LOWER MELTING POINT COMPONENT OF THE ALLOY IN A NON-OXIDIZING ATMOSPHERE IN MOLTEN STATE TO DEGAS IT, POURING THE MOLTEN METAL INTO THE TUBE WHILE MAINTAINING THEIR TEMPERATURES BELOW THE REACTION TEMPERATURE AT WHICH THEIR BRITTLE INTERMETALLIC ALLOYS ARE FORMED, COOLING THE TUBE, COLD WORKING THE TUBE DOWN TO WIRE SIZE TO EXTEND THE INTERFACE BETWEEN THE CORE AND SHEATH THEREBY MAKING THE SHEATH READILY WETTABLE BY THE CORE MATERIALS, COLD ROLLING THEWIRE TO FLATTEN IT TO RIBBON FORM AND TO FURTHER REDUCE ITS CROSS SECTION THICKNESS AFTER FLATTENING THE WIRE TO RIBBON FORM, AND THEN HEATING THE RIBBON AT THE REACTION TEMPERATURE TO FROM THE DESIRED INTERMETALLIC, HARD SUPERCONDUCTOR ALLOY AS A DIFFUSION LAYER INSIDE THE RIBBON.