WO1999007003A1

WO1999007003A1 - A high temperature superconductor and method of making and using same

Info

Publication number: WO1999007003A1
Application number: PCT/US1998/016069
Authority: WO
Inventors: Michael J. Tomsic; Martin S. Lubell; Jinwun W. Lue; Jonathan A. Demko
Original assignee: Eurus Technologies, Inc.
Priority date: 1997-08-01
Filing date: 1998-07-30
Publication date: 1999-02-11
Also published as: AU8764698A

Abstract

A stack of HTS tapes (14) are bundled together, encapsulated with a silver or non-silver sheath (12) by a continuous tube filling and forming process without welding, and sintering to form the final product. A large current carrying conductor is achieved with the stacking of multiple HTS tapes. The bundling process is implemented using a continuous tube forming and filling process, which restacks round wires or flat tapes inside another tube. After restacking, the encapsulated tape is subjected to a conventional rolling operation to convert the tape to a desired thickness. For AC applications, a resistive barrier is introduced between HTS tapes to prevent coupling loss therebetween. To reduce sheath costs, a non-silver alloy may be used such as Cu-Al-Mn-Ni, stainless steel/nickel based compounds, like inconels, Cu-Ni, or Cu-Al. In one embodiment, an HTS tape is reinforced with an alloy strip by diffusion bonding and sintering in a single step. The backing strip is placed on the outside of the winding such that most of the tape is placed in compression when made to follow a tortious path.

Description

A HIGH TEMPERATURE SUPERCONDUCTOR AND METHOD OF MAKING AND USING SAME

BACKGROUND

1. Field of the Invention

The present invention relates generally to superconductors, and more particularly, to improved high temperature superconductors and methods of manufacture which maximize the critical current carrying capacity I_c thereof.

2. Description of the Prior Art

Superconductors exhibit diamagnetism and have a zero voltage drop along their length, irrespective of the passage of current therethrough. Applications for superconductors include magnetic hydrodynamic ("MHD") generation of electricity, transmission and storage of electric power, magnetic levitation of trains and electromagnetic ship propulsion, and various uses in instrumentation, including NMR, pi-meson sources for medical treatment, and supersensitive sensors for magnetic fields, microwaves, radioactive beams and the like. Superconductors are also being employed as high speed switching elements such as Josephson junction devices.

In the past, superconductivity had only been observed at very low temperatures, on the order of less than 23.2°K with Nb₃Ge. Thus, such superconductors required liquid helium to cool the superconductor material to a temperature low enough to exhibit superconductivity. Liquid helium has a boiling point of only 4.2 °K. As a result, the large technical and cost burdens associated with such an arrangement had for a long time prevented the practical implementation of this technology. Developments in superconductors have brought about the use of sintered bodies consisting of oxides of elements in groups Ha or Ilia of the periodic table. These materials can produce a superconductor having a very high critical temperature T_c. Such a superconductor(s) is referred to as a high temperature superconductor (hereinafter "HTS"). Composite oxides having a pseudo-perovskite type crystalline structure such as [La, Ba]₂CuO₄ or [La, Sr]₂CuO₄ have an orthorhombic structure and the like, which is similar to the crystalline structure of perovskite type oxides. These have exhibited a T_c of 30 °K to 50°K. Furthermore, a T_c in excess of 70°K has been associated with superconducting materials formed of oxides such as Ba, Y and Cu.

The bodies are produced by a sintering-rolling process in which heat and pressure are applied to the material. An example of a prior art superconductor wire comprising a normal conductor and a HTS is disclosed in U.S. Patent No.4,906,609 ("the '609 Patent"). The '609 Patent teaches a method of manufacture comprising the steps of continuously forming a strip of material into a U-shaped strip; filling the strip with raw material powders; closing up the strip, forming a tube; butt welding the resulting seam; and sintering the resulting tube containing the raw material powders to produce the final product. The raw material powders include a mixture of oxides, nitrides, fluorides, carbonates, nitrates, oxalates or sulfates of a first element selected from group Ila in the periodic table, a second element selected from group Ilia in the periodic table, and a third element selected from groups lb, lib, Illb, IVa and Villa in the periodic table, or powders of composite oxides obtained by sintering and pulverizing these materials. The '609 Patent also discloses continuously forming a strip of metal into a flume-like shape and then filling the resulting concave member with the oxide superconducting material. Thereafter, the conductor is almost closed, leaving a gap between opposed marginal edges to surround the superconducting material, and then sintered and formed to the desired final shape and size. Prior art HTS suffer several drawbacks. Because of fundamental processing requirements, ceramic HTS made in the from of thin tape exhibit extreme fragility and degradation in I_c resulting from tensile bending strain. It has been found that 90% of the straight length I_c can be retained if the bending strain is limited to 0.2%.

Another problem with present HTS is the high cost associated with the use of silver material. Silver also has a lower tensile and yield strength than can be achieved with other alloys as described in accordance with the present invention hereinbelow.

SUMMARY OF THE INVENTION In view of the disadvantages in the prior art, it is an object of the present invention to provide an improved HTS having superior strength and durability.

It is another object of the present invention to provide a means by which the HTS can be mechanically strengthened to facilitate manufacture and prevent a reduction in I_c when the superconductor is wound on the outer diameter of a winding.

It is yet another object of the present invention to increase the I_c of the superconductor such that fewer layers of tape are required while still satisfying the basic requirements for kilo-amp class cables.

It is another object of the present invention to provide a HTS which is strengthened by diffusion bonding a backing material to the superconductor tape.

It is still another object of the present invention to provide a HTS in which the reinforcing material is not soldered to the superconductor tape.

It is yet another object of the present invention to provide a HTS consisting of a bundle of superconductor tapes.

It is still another object of the present invention to provide a HTS in which flat tapes are stacked in a rectangular tube to provide increased surface area at the interface between the powder material and the tube.

It is still another object of the present invention to provide a HTS bundle encapsulated with a sheath by a continuous tube filling and forming process, and sintered to form the final product without the need to weld the tube. It is yet another object of the present invention to provide a HTS in which the manufacturing process includes an extra pressing step between sintering operations to facilitate densification.

It is still another object of the present invention to provide a HTS having a resistive barrier disposed between layers of superconducting tapes to minimize AC losses.

It is yet another object of the present invention to provide a HTS having a non-silver alloy sheath.

In accordance with the above objects and additional objects that will become apparent hereinafter, the present invention provides the following embodiments of improved HTS.

Bundling An Encapsulated HTS

A stack of HTS tapes are bundled together, encapsulated with a silver or non-silver sheath by a continuous tube filling and forming process, and sintered to form the final product. A large current carrying conductor is achieved with the stacking of multiple HTS tapes. This bundling and encapsulating technique can be applied to commercially finished HTS tapes. In accordance with this process, the present invention provides an encapsulated high temperature superconductor bundle, comprising: a plurality of high temperature superconducting tapes; and an outer sheath encapsulating the plurality of high temperature superconducting tapes to form a bundled superconductor. The high temperature superconducting tapes are stacked flat, and a resistive barrier can be interposed between contiguous high temperature superconducting tapes. In one embodiment, the high temperature superconducting tapes are stacked side-by- side, and bundled within a single sheath.

Continuous Tube Filling & Forming Process f" CTFF")

The bundling process described above is implemented using the CTFF process, which restacks round wires or flat tapes inside another tube. After restacking, the encapsulated tape is subjected to a conventional rolling operation to convert the tape to a desired thickness. The process involves either restacking round wire in a round tube, or flat tapes in a rectangular tube. Using flat tapes is the preferred method and is referred to as the uniform transverse and longitudinal filament ("UTLF") process. Such tapes typically comprise 6 to 18 transverse superconducting filaments that yield a final thickness of 10 - 15um. The UTLF process facilitates superconductivity by maximizing current density for a given precursor powder at a given filament thickness, since research indicates that most of the superconducting current is carried at the interface between the silver and the powder. For AC applications, a resistive barrier is introduced between HTS tapes to prevent coupling loss therebetween. In accordance with the CTTF process, the present invention provides a method of fabricating high temperature superconducting tapes, comprising the steps of: dispensing a strip of conductive material and passing the strip through forming rolls; filling the strip with precursor powder; and passing the strip through closing rolls to roll the strip into a tube without welding and drawing the strip to increase the density of the powder within the tube. The method further comprises the steps of: encapsulating a plurality of tapes formed as above in an outer sheath to form a multi-filament bundle; and sintering and densifying the multi-filament bundle. In one embodiment, the sintering and densifying step comprises pressing the bundle between sintering steps. Using this process, round wires may be encapsulated inside a round sheath, and flat tapes may be encapsulated inside a rectangular sheath.

Overlap Pressing

After the multifilament tape is constructed, it is densified between sintering operations and then subjected to a final sintering operation. Prior art tape manufacturing methods have utilized sintering-rolling-sintering-rolling- sintering and then sintering-pressing-sintering-pressing-sintering procedures to avoid pressure-induced tape damage. The present invention provides improved current density by pressing between sintering operations. Thus, the process may involve sintering-rolling-sintering-pressing-sintering. The critical sintering or heat-treating step is not a continuous process, but rather a batch operation. Non-Silver Sheath Material To reduce sheath costs, the amount of silver may be reduced. Exemplary tapes exhibiting critical transport currents I_c of 20 - 22 A at 77 °K and engineering current densities J_e of 2000 - 2200 A/cm² have been fabricated from 34% superconductor, 36% silver, and 30% non-silver material as compared to a more typical 33% superconductor/67% silver. This is a reduction in silver of

44%, while realizing a 500% increase in yield strength as compared to pure annealed silver. The higher resistivity of the non-silver sheath (several times that of silver) is beneficial in AC applications. The alloy may comprise Cu-Al-Mn- Ni, stainless steel/nickel based compounds, like inconels, Cu-Ni, or Cu-Al. Diffusion Bonding of a Reinforcing Strip

An HTS tape is reinforced with an alloyed strip by diffusion bonding and sintering in a single step. The backing strip is placed on the outside of the winding such that most of the tape is placed in compression when made to follow a tortious path. Normally, tensile strain causes a reduction in critical current capacity I_c. By reinforcing the tape with the backing strip, bending strain induced critical current degradation is minimized by taking up tensile loads on the OD of the bend.

BRIEF DESCRIPTION OF THE DRAWINGS In accordance with the above, the present invention will now be described in detail with particular reference to the accompanying drawings.

FIG. la is a cross sectional view of a 3-tape encapsulated bundle superconductor;

FIG. lb is a cross sectional view of a 3 -tape encapsulated bundle superconductor with a resistive barrier interposed between adjacent HTS tapes; FIG. 1 c is a cross sectional view of a 6-tape encapsulated bundle superconductor comprised of two 3-tape stacks disposed side-by-side;

FIG. 2 is a flow chart of a representative process for manufacturing superconducting Bi:2223 tapes;

FIG. 3 is a schematic of the CTFF process; FIG. 4 is a schematic of a prior art powder-in-tube process; FIGS. 5a-5d are illustrative views of photographs of transverse and longitudinal cross sections of tapes fabricated in accordance with the UTLF process;

FIG. 6 is a schematic of an exemplary continuous pressing machine in accordance with the present invention;

FIGS.7a and 7b are illustrative views of photographs which depict the transverse cross section of both silver and reduced silver tapes that were processed with identical powder and processing steps;

FIG. 8 graphically depicts I-V curves of a 3-tape bundle of CTFF tapes both in straight length and coiled onto successively smaller diameter holders;

FIG. 9 graphically depicts I-V curves of a 3-tape bundle of third party tapes both in straight length and coiled onto successively smaller diameter holders with the conductor wound with the double-sheath disposed on the ID side of the winding;

FIG. 10 graphically depicts I-V curves of a 3-tape bundle of third party tapes both in straight length and coiled onto successively smaller diameter holders with the conductor wound with the double-sheath disposed on the OD side of the winding; FIG. 11 graphically depicts critical currents as a function of bending strain for the conductor tested and shown in FIGS. 9 and 10. The strains were calculated with the thickness of the superconductor in tension only;

FIG. 12 graphically depicts reduced critical currents as a function of bending strain for a single control tape and the bundle conductor tested and shown in FIGS. 9 and 10;

FIG. 13a is a cross sectional view of an HTS tape having a diffusion bonded backing strip;

FIG. 13b is a cross sectional view of an HTS tape having a diffusion bonded alloy plated backing strip; and FIG. 13c is a cross sectional view of a multi-tape HTS having a diffusion bonded backing strip. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides an improved HTS and method for making and using same.

Bundling An Encapsulated HTS In the process of developing HTS transmission cables, it was found that the mechanical strength of the superconducting tape was the most crucial property and needed improvement. It is also desirable to increase the current carrying capacity of the conductor so that fewer layers are required to make the kilo-amp class cables used in electric utility applications. The present invention provides a process of encapsulating a stack of Bi-2223/Ag tapes with a silver or non-silver sheath to form a strengthened bundle superconductor. This process was applied to HTS tapes made by a Continuous Tube Forming and Filling (hereinafter "CTFF") technique described below, and off the shelf tapes obtained from other manufacturers. Conductor bundles of 2 to 6 tapes have been fabricated in accordance with the inventive method. The finished bundle conductor maintained or even surpassed the sum of the original critical currents I_c of the individual tapes.

A stack of HTS tapes are bundled together, encapsulated with a silver or non-silver sheath by the CTFF process, and sintered to finish the assembly. A large current carrying conductor is achieved with the stacking of multiple HTS tapes. A variety of strengthening materials can be used for the sheath to form a robust conductor. Bundled superconductors of up to 6 HTS tapes have been fabricated with silver and non-silver sheathing. The finished HTS tapes are protected from the external degradation and possible damage caused by liquid nitrogen penetration. FIG. la depicts a cross section of an illustrative 3- tape encapsulated HTS bundle 10 comprising an outer sheath 12 and stacked flat- filament HTS tapes 14. FIG. lb shows a cross section of a 3-tape encapsulated HTS bundle 10 of the type depicted in FIG. la, further having a resistive barrier 16 interposed between contiguous HTS tapes 14 to minimize AC losses. FIG. 1 c illustrates a 6-tape encapsulated HTS bundle 10 comprising two stacks of 3 HTS tapes 14 disposed side-by-side. The specifics of the encapsulating technique are described below.

Continuous Tube Filling and Forming Process (CTFF)

A large number of practical applications using high-temperature, oxide superconducting materials have been identified and demonstrated by manufacturing prototype conductors (tapes) and installing them into devices. This has been accomplished most successfully with the Bi:2212 and Bi:2223 (BSCCO) tapes. While systems using these tapes have been demonstrated, there exists a need in the superconductivity field for tapes with increased performance and reduced costs.

Several processing steps are used in the manufacture of superconducting Bi:2223 tapes. FIG. 2 shows the main processing steps in the manufacture of the BSCCO tape. The first innovation for BSCCO tape was the development of the CTFF process shown in FIG. 3. With this process, a silver strip 18 is used in lieu of a tube. The strip 18 is dispensed from a strip pay-out reel 19 and passes through a plurality of "U" shaped forming rolls 22. The strip 18 is formed and filled continuously with the precursor powder 20 as it is dispensed from powder feed 24. Shorter strips can be welded prior to powder filling to produce strips of any desired length, which permits fabrication of unlimited length wires having the powder contained therein. The filled tube initially starts out at a very small diameter, typically on the order 4.0 mm, thereby reducing the number of drawing passes needed to obtain the desired characteristics. A similar process has been used for a long time in the welding industry to make tubular welding wire. After filling the tube by the CTFF process, conventional continuous drawing and rolling steps are employed to increase the density of the powder inside the tube. The strip 18 is passed through a plurality of "U" shaped closing rolls 26, a draw die 28 and thereafter received on a wire take-up reel 30. As a result, wires and tapes may be fabricated with cross sections that are indistinguishable from those made by a conventional powder-in-tube process, as depicted schematically in FIG. 4 without the need to butt weld a seam as in the prior art.

In addition to monofilament tapes, the present invention provides for the manufacture of multi-filament tapes using the CTFF process. Restacking round wires or flat tapes inside another tube has been found to be even more productive than filling the tube with powder. In this regard, the restacking step is less dependent on speed than the powder filling step. Here again, length is not an issue as the process is continuous and uniform. After restacking, the encapsulated tape is subjected to a conventional rolling operation to convert the tape to desired thickness.

There are two embodiments of this process: restacking round wire in a round tube, and restacking flat tapes in a rectangular tube. After restacking, the rectangular tube can be shaped into a round wire if desired. The flat-filament embodiment is preferred for several reasons, and is referred to as the UTLF process. Exemplary tapes fabricated by this process have 6 to 18 transverse superconducting filaments that produce a final filament thickness of ~ 10- 15 μm. Tapes with uniform flat filaments ~7-10 m thick have been processed. Copies of photographs of transverse and longitudinal cross sections of such tapes are depicted in FIGS. 5a-5d. The UTLF process produces a superconductor in which most of the superconducting current is carried at the interface between the silver and the powder. By restacking flat filaments, the powder-silver interface area is maximized. It has also been found that the current density for a given precursor powder can be maximized for a given filament thickness. In contrast, maintaining a uniform filament thickness with restacked round wires is very difficult since the filament is always thick in the middle and tapers off to zero at the corners. Thus, for a given filament thickness, only a portion of that filament is at the optimal thickness. With properly processed UTLF tapes, more of the powder can be located at the ideal filament thickness and the superconductor to sheath ratio can be maximized. Overlap Pressing After the multi-filament tape is constructed, it is subjected to densification steps between sintering operations, and then to a final sintering operation. Most researchers in the field have been performing repeated sintering operations with short static pressing steps interposed between each sintering step. In an attempt to produce longer-length tapes, most tape manufacturers have been performing sintering-rolling-sintering procedures to avoid pressure-induced tape damage. The present invention provides increased current densities by pressing between sinters on a high-speed, continuous pressing machine 32, an example of which is depicted schematically in FIG. 6. This machine comprises a pay-off spool 34, press 36 and take-up spool 38. The tape assembly is dispensed and pressed in steps between components of press 36 as the tape assembly passes from pay-off spool 34 to take-up spool 38 between sintering steps. This arrangement permits tape pressing in long continuous lengths at speeds comparable to the rolling operation. Among all tape manufacturing steps, one process that is not continuous is the critical sintering or heat-treating step. This is still a batch process, which means that the only limitation on length is the size that be wound up and sintered at one time in the furnace. Current ovens can accommodate tape lengths in the 200 meter range, however larger ovens will allow processing of 1000-2000 meter long tapes. The challenge of economically implementing the sintering process is not the power required, gas atmosphere, or capital equipment cost for the oven, but rather the equipment and labor costs incurred in connection with winding and unwinding tapes on fixtures that are placed inside the ovens. With proper winding and fixturing equipment, this operation can be automated to reduce labor costs.

Non-Silver Sheath Materials

The large silver content in BSCCO/Ag tape has kept costs high. It is appreciated by those skilled in the art that even if the powder and labor were free, a tape with 70-75% silver cannot compete with more conventional superconductors on a dollar per kilo amp-meter basis. The use of strips instead of tubes somewhat mitigates the silver costs. Silver strips typically cost $6 - $7 per troy oz., with silver tubes in the $12- $15 range. As discussed above, there is a requirement for conductors with a stronger sheath to facilitate ease of winding, and higher mechanical strength in a magnetic field. Neither of these objectives are economically feasible using pure silver as the sheath material.

A significant reduction in sheath costs may be realized by using a non-silver sheath. In this connection, FIGS 7a and 7b show the transverse cross section of two tapes that were processed with identical powder and processing steps. Both tapes had similar transport currents (I_c) of ~ 20-22A at 77 °K in self field, and similar engineering current densities (J_e) of ~ 2000-200 A/cm². The tape 10a on the left consists of 33% superconductor and 67% silver. The tape 10b on the right contains 34% superconductor, 36% silver, and 30% non-silver sheath material. This represents a 44% reduction in the amount of silver. Moreover, the non-silver sheath has a tensile strength of 480 Mpa and a yield strength of 270

Mpa - a 500% increase in yield strength as compared to pure annealed silver. The resistivity of the non-silver sheath is several times that of silver, which benefits the tape in AC applications. The alloy may comprise Cu-Al-Mn-Ni, stainless steel/nickel based compounds, like inconels, Cu-Ni, or Cu-Al. It is believed that further improvements may see production of tapes with as little as 15% silver content.

There exists a need for a much more robust conductor having a higher Ic for large coils (8 inch bore coils and larger), particularly for applications such as fault current limiters, transformers, and magnetic separators. To produce such large coils, the CTFF process has been applied to encapsulate various tapes to fabricate a robust conductor. Typically, individual tapes have been produced with I_c's of ~ 20-25 A at 77°K in self field. By encapsulating 2-6 tapes into a robust conductor, I_c levels of ~ 40-100 A at 77° K in self field can be obtained. An 80 A conductor at 77 °K in self field will typically produce an I_c of ~ 240 A at 26 °K in a 2T field. The CTFF encapsulation process provides the capability to customize the conductor to the desired amperage at the magnetic field level for a particular application.

The unique procedure of sintering after the encapsulation makes the present technique also suitable to make bundle conductors out of finished HTS tapes. No undesirable soldering process that may damage the HTS tapes is used.

Bending Tests Of The Bundled Conductor

Bundle Conductor With Plastronic Tapes

DC I-V curves were measured on a bundle conductor made of three 8-filament Plastronic CTFF tapes encapsulated in an Ag-sheath. The measurement was first made in liquid nitrogen with the sample in a straight length. The conductor was then coiled onto a 2.54-cm diameter holder, and the I-V curve was measured again. FIG. 8 depicts the measured I-V curves for both conditions. There is a significant reduction of the current carrying capacity I_c of the conductor after bending. The voltage taps were separated by 20 cm. This conductor exhibits an I_c of 34.5 A in a straight length and 18.5 A in a 2.54-cm diameter holder. Accordingly, there is a 46% reduction in I_c after the superconductor is subjected to bending strain.

Bundle Conductor With Third Party Tapes As discussed in the foregoing, the present bundling and encapsulating technique can be applied to commercially finished HTS tapes. A sample was made with third party tapes by encapsulating a 3-HTS-tape bundle with a non-silver sheath. The conductor's bending tolerance was tested by coiling it onto successively smaller diameter holders. Since the conductor sheath 12 has a first thickness on one side of the bundle and a second double thickness on the other side (see FIG. 1 a), the sample was cut in half. One half of the conductor 10 was tested with the double sheath thickness on the ID of the winding, and the other half with the sheath disposed on the OD of the winding. A lay angle of 45 degrees was used in this series of tests. FIG. 9 shows the I-V curves of the half tested with the double sheath side on the ID of the winding. A gradual diminution in the current carrying capacity I_c of the conductor was observed. FIG. 10 shows the I-V curves of the other half tested with the double sheath side on the OD of the winding. A similar but less severe reduction in current carrying capacity I_c was observed in that case.

Nearly identical critical currents of 51.4 A and 52.8 A were respectively measured for the two halves of the sample in a straight length. The basic tape has a nominal I_c of 15-16 amps. Thus, the encapsulated bundle conductor has a greater I_c than the original I_c of the individual tapes.

The degradation of I_c due to bending with the double sheath thickness on the ID of the winding was greater than with the double sheath thickness disposed on the OD of the winding. For example, at the 7.62cm diameter bend, the I_c dropped to 31.1 A in the former case, but to only 41.5 A in the latter case. Thus, even though the conductors were bent on the same diameters, the degree of bending on the superconductor itself were different under these two bending conditions. The average bending strain, e on a conductor subj ected to bending on a radius R may be expressed by the following equation e = tV2 Sine (1)

R where the lay angle θ is measured from the axis of the forming medium and t is the thickness of the conductor. The exemplary 3-HTS-tape bundle thickness is about 0.66 mm, the sheath thickness about 0.178 mm and the total conductor thickness about 1.194 mm. Thus, the neutral plane in this example was located

0.597 mm from the ID of the conductor when bent. If the double sheath thickness was located on the ID of the winding, the measured thickness of the HTS tapes under tension was 0.419 mm. When the double sheath thickness was disposed on the OD, the measured thickness of the HTS tape under tension was O.141 mm, i.e. less superconductor was under tension in the latter case. By using these thickness' of the superconductor in tension as the t/2 in Eq. (1), the respective bending strains of the superconductor were calculated under the two bending conditions. The strain levels were 74% higher when the double sheath thickness was bent on the ID of the winding. FIG. 11 shows the measured two halves of I_c as a function of the calculated bending strains. The two sets of data show a single trend of degradation. Thus, the portion of the superconductor that is subjected to bending tension contributes to most of the degradation in I_c. Note that because no data was taken at strains less than 0.3 %, the linear drop indicated by the line connecting to the zero strain point may be fortuitous. Comparison with Single Tape Bending Results

As a control, a third party tape was subjected to the same re- sintering process as the bundle conductor. This tape was also tested for its bending tolerance by coiling it onto successively smaller diameter holders, and I-V curves similar to those shown in FIGS. 9 and 10 were obtained. The bending strain at each holder diameter was again calculated with Eq. (1) using the tape thickness for "t." FIG. 12 shows the reduced I_c, I_c/I_co as a function of the bending strain for this reprocessed tape and the bundle conductor described above. The bundle conductor data is located along the curve drawn through the single tape data. Thus, the I_c degradation is due to bending strain, and in particular, the portion of the superconductor that is subjected to bending tension, irrespective of whether it is in a single tape or bundled assembly. More than 90% of the straight length I_c can be retained if the bending strain is limited to 0.2%

If the conductor is bent to a specified diameter, it is subjected to a higher bending strain and consequent higher I_c degradation because of its larger thickness. However, due to its greater strength and stiffness, it may not suffer additional degradation from strain attributable to handling and winding. Possible variations of the superconductor bundles are: 1 . Different sheathing materials, Ag or non-Ag containing alloys can be used as desired for different strengths and different thermal contraction considerations.

2. A resistive barrier can be placed between the HTS tapes to reduce AC losses as shown in FIG. lb.

3. Different stacking arrays of HTS tapes can be used to make very large current carrying conductors or to suit smaller winding diameters.

Diffusion Bonding Of A Backing Reinforcing Strip Another embodiment of the present invention covers an overlapped or rolled HTS tape backed with an alloyed reinforcing strip which is diffusion bonded during sintering to finish or restore the superconductor to its full currentlevel. FIGS. 13a- 13c are cross sectional schematics ofHTS tapes having a diffusion bonded backing strip 40. The backing strip 40 may have alloy plating

42, e.g., Ag and the like. In FIG 13a, an alloyed backing strip 40 is diffusion bonded 41 to one side of an HTS tape 14. In FIG. 13b, an alloy plating 42 is applied to the backing strip 40. In FIG. 13c, the alloy plated backing stip 40 is diffusion bonded 41 to one of a pair of HTS tapes 14. The backing strip 40 is located on the side of the assembly corresponding to the OD of the winding, so that most of the superconductor can be placed in compression, as discussed above with respect to the encapsulated bundle embodiment. This acts to minimize bending strain induced critical current degradation. The unique procedure of pressing the HTS tape and sintering the same during the diffusion process, makes the present invention suitable to make strengthened conductors out of finished

HTS tapes.

It will be appreciated by those skilled in the art that different backing materials can be diffusion bonded to the HTS tape. Depending on the desired strength, different alloys or Ag-plated high strength materials can be employed. The only requirement is material compatibility to create a diffusion bond - which is governed by a combination of time, temperature and pressure. For large current carrying conductors, multiple HTS tapes can also be diffusion bonded together with the backing material.

This arrangement overcomes the following problems in making a workable strengthened HTS conductor:

1. A strengthened conductor is achieved through diffusion bonding to a backing material.

2. No soldering of the HTS tapes that can damage the superconducting property of the HTS is required. 3. The unique procedure of pressing the HTS tape and sintering simultaneously not only preserves but can also enhance the critical current I_c of the original HTS tape.

4. Different backing materials can be used to meet strength and winding requirements for different applications.

The present invention has been shown and described in what are considered to be the most practical and preferred embodiments. It is anticipated, however, that departures can be made therefrom and that obvious modifications will be implemented by persons skilled in the art.

Claims

1. An encapsulated high temperature superconductor bundle, comprising: a plurality of high temperature superconducting tapes; and an outer sheath encapsulating said plurality of high temperature superconducting tapes to form a bundled superconductor.

2. The high temperature superconductor bundle recited in Claim 1 , wherein said high temperature superconducting tapes are stacked flat.

3. The high temperature superconductor bundle recited in Claim 1, further comprising a resistive barrier interposed between contiguous high temperature superconducting tapes.

4. The high temperature superconductor bundle recited in Claim 2, wherein said high temperature superconducting tapes are stacked side-by-side.

5. The high temperature superconductor bundle recited in Claim 1 , wherein said sheath is made of a non-silver material.

6. The high temperature superconductor bundle recited in Claim 1 , wherein said outer sheath is bundled around said high temperature superconducting tapes such that at least one side of said bundle has a plurality of sheath layers disposed over said tapes.

7. The high temperature superconductor bundle recited in Claim 1 , wherein said bundle has a backing reinforcing strip disposed on at least one side of the bundle.

8. The high temperature superconductor bundle recited in Claim 6, wherein said backing strip includes alloy plating.

9. The high temperature superconductor bundle recited in Claim 6, wherein said backing reinforcing strip is diffusion bonded to said bundle.

10. A method of fabricating high temperature superconducting tapes in a continuous tube filling and forming process, comprising the steps of: a. dispensing a strip of conductive material and passing said strip through forming rolls; b. filling said strip with precursor powder; and c. passing said strip through closing rolls to roll said strip into a tube and drawing said strip to increase the density of the powder within the tube.

11. The method recited in Claim 10, further comprising the steps of: d. encapsulating a plurality of tapes formed in step (c) in an outer sheath to form a multi-filament bundle; and e. sintering and densifying said multi-filament bundle.

12. The method recited in Claim 1 1, wherein said sintering and densifying step comprises pressing said bundle between sintering steps.

13. The method recited in Claim 11, wherein step (d) comprises encapsulating round wires inside a round sheath.

14. The method recited in Claim 11, wherein step (d) comprises encapsulating flat tapes inside a rectangular sheath.

15. The method recited in Claim 14, further comprising the step of shaping said rectangular sheath into a round wire.

16. The method recited in Claim.11 , further comprising the step of placing a resistive barrier between contiguous tapes prior to said encapsulating step.

17. A method of fabricating high temperature superconducting tapes in a continuous tube filling and forming process, comprising the steps of: a. dispensing a strip of conductive material and passing said strip through forming rolls; b. filling said strip with precursor powder; and c. passing said strip through closing rolls to roll said strip into a tube and drawing said strip to increase the density of the powder within the tube; d. encapsulating a plurality of tapes formed in step (c) in an outer sheath to form a multi-filament bundle; and e. sintering and densifying said multi-filament bundle.