Patent Description:
As will be understood by those skilled in the art, high temperature superconductor (HTS) materials can carry extremely large amounts of current with extremely low loss. HTS materials lose all resistance to the flow of direct electrical current and nearly all resistance to the flow of alternating current when cooled below a critical temperature. The development of HTS wires (the term "wires" as used herein is intended to include a variety of conductors, including tape-like conductors) using these materials promises a new generation of high efficiency, compact, and environmentally friendly electrical equipment, which has the potential to revolutionize electric power grids, transportation, materials processing, and other industries. However, a commercially-viable product has stringent engineering requirements, which has complicated the implementation of the technology in commercial applications.

In commercially-available second generation HTS wires, the HTS material is generally a polycrystalline rare-earth/alkaline-earth/copper oxide, e.g., yttrium barium-copper oxide (YBCO). The current carrying capability of the HTS material is strongly related to its crystalline alignment or texture. Typically, HTS materials are fabricated with a high degree of crystallographic alignment or texture over large areas by growing a thin layer of the material epitaxially on top of a flexible tape-shaped substrate. The HTS material is preferably fabricated so that it has a high degree of crystallographic texture at its surface. When the crystalline HTS material is grown epitaxially on this surface, the crystal alignment of the HTS material grows to match the texture of the substrate. In other words, the substrate texture provides a template for the epitaxial growth of the crystalline HTS material. Further, the substrate provides structural integrity to the HTS layer. In the current state of the technology, the substrate is an integral structural component of the commercially-available second generation HTS wires.

The substrate is typically made of highly resilient structural materials that may include refractory alloys based on nickel. Although the superconductor layer is only about <NUM> thick in these tapes, the substrate material and buffers can be as much as <NUM> to <NUM> thick. The typical width of the tape during manufacture is in the range of <NUM> wide. Depending on the intended application, this wide tape is then cut into smaller widths, typically from <NUM> down to about <NUM>, by mechanical slicing. Although even narrower strips are desirable in certain applications, the ability to cut narrower widths is limited due to damage caused to the integrity of the HTS tape by mechanical slicing tools.

<FIG> shows the construction of a commercially-available RABiTS-based <NUM> wire (product of AMSC Corp. marketed as Amperium wire), referred to herein as tape <NUM>. Tape <NUM> includes an approximately <NUM> thick metal substrate <NUM>. Substrate <NUM> is coated with an oxide buffer <NUM>, which is formed from a sequence of layers of various oxides, for example yttrium oxide, yttrium-zirconium oxide and cerium oxide. Oxide buffer <NUM> is typically deposited by a vacuum deposition method, such as reactive sputtering or electron beam evaporation. A layer of yttrium barium copper oxide superconductor ReBa<NUM>Cu<NUM>O<NUM>-x, referred to herein as superconducting layer <NUM>, is grown on oxide buffer <NUM>. In this commercially-available product, Re is a rare-earth metal, such as Y, Dy, Gd, Nd, and x is the oxygen index, with x<<NUM>. A protective silver layer <NUM> is deposited on top of superconducting layer <NUM> by magnetron sputtering. Finally, tape <NUM> is solder-plated with opposing top and bottom metal foils, forming stabilizing layers <NUM> and <NUM>, respectively. The stabilizing layers <NUM>, <NUM> are <NUM>-<NUM> wider than the remainder of the tape, so a pair of opposing solder fillets <NUM> are formed in order to join the two stabilizer foils.

Many practical applications of HTS, such as transformers, fault current limiters, energy storage magnets, magnets for fusion energy, and poles for rotating machinery, require high AC current carrying capacity and high mechanical flexibility to enable dense and uniform wrapping of the wire on a coilform or center core. Although the prior art discloses certain processes for manufacturing a cable from second-generation tapes, the disclosed cables still lack the necessary current carrying capacity and high mechanical flexibility for various applications. For example, <CIT> describes a process for manufacturing a twisted cable from a stack of <NUM> tapes, but because of the limited flexibility of the disclosed cable, the magnet is required to have a special hexagon shape to allow proper winding. For example, the limited flexibility of prior art wires made from stacked tape segments allows for a twisting pitch of no less than about <NUM> (<NPL>.

An important aspect of any superconducting device is its resistance to a quench event. A quench event is a spontaneous transition of some part of the superconductor to a normal (non-superconducting) state. During a quench event, magnetic energy stored in the device is dissipated within a very small length, typically less than <NUM>. The associated temperature rise can be over <NUM>, which can destroy the superconducting material. Thus, existing HTS wires made from a tape having a single superconducting layer (such as Amperium tape from AMSC Corp) are prone to failure during a quench event. Likewise, HTS wires made from a plurality of stacked tape segments (e.g., the wires disclosed in <CIT>) are also prone to failure during a quench event because the high level of contact resistance between the individual superconducting layers prevents/limits any current sharing between the separate layers. For example, the typical contact resistance (i.e., voltage drop per unit current density) for a prior art HTS wire including an oxide buffer layer is > <NUM>µΩcm<NUM>, a level which greatly exceeds the level which would allow current sharing between adjacent stacked tape segments.

There is therefore a need in the art for a HTS cable that provides improved AC current carrying capacity and flexibility for various applications, including dense and uniform wrapping. There is a further need in the art for a multi-filament HTS cable which allows current sharing between the separate superconducting layers such that the current passing through a superconducting layer experiencing a quench event can shift to and pass through adjacent superconducting layers thereby protecting the original superconducting layer from failure and/or destruction. There is a further need in the art for a method of joining two sections of a multi-filament HTS cable while maintaining the integrity of the cable.

<NPL>shows stacking exfoliated HTSC tape filaments to form a HTSC cable.

<CIT> and <CIT> show stacking and soldering of (non exfoliated) tape conductors to a cable.

The present invention, which addresses the needs of the prior art, provides a multi-filament HTS cable according to claim <NUM>.

The present invention further relates to a method for manufacturing multi-filament HTS cable according to claim <NUM>.

As a result, the present invention provides a multi-filament HTS cable that provides improved AC current carrying capacity and flexibility, as well as a method of manufacturing such cable. The present invention further provides a multi-filament HTS cable that allows current sharing between the individual superconducting layers such that the current passing through a superconducting layer experiencing a quench event can shift to and pass through adjacent superconducting layers thereby protecting the original superconducting layer from failure and/or destruction.

A partially exfoliated second generation (<NUM>) wire, i.e., tape <NUM> is shown in <FIG>. As described in detail in commonly-owned international publication <CIT>, tape <NUM> includes a metal substrate <NUM>, a buffer layer <NUM>, a superconducting layer <NUM>, and a stabilizing metal layer <NUM>. The stabilizer layer <NUM> can be made of copper, stainless steel, bronze or another conductive metal. In a preferred embodiment, the tape is subjected to an external action, which increases the stress level between the superconducting layer <NUM> and the buffer layer <NUM>. This external action can be accomplished by, for example, rapid heating by an external source, such as an inductive coil, infrared radiation or radio-frequency radiation or by rapid cooling, such as quenching in a cryogenic liquid, for example liquid nitrogen. The external action can also be accomplished by a mechanical deformation of the tape, such as bending. The stress level is preferably raised to a level where the substrate <NUM> and the buffer layer <NUM> can be mechanically separated from the superconducting layer <NUM> without damaging the latter in a process referred to as exfoliation.

Because the exfoliated HTS tape is ultra thin, it can be sliced by laser rather than mechanical tools. Stated differently, an HTS tape, which includes the substrate and buffer layers, is substantially thicker, and not suitable for cutting by laser. The laser-sliced HTS filaments preferably have a width of from about <NUM> to about <NUM>. Laser slicing reduces the waste that is caused by mechanical slicing, and greatly reduces the mechanical stress on the polycrystalline HTS material that accompanies mechanical slicing. Laser slicing is also capable of providing filaments with improved edge straightness to that accomplished by mechanical cutting (which tends to roll the edges of the tape as it is being cut). Additionally, mechanical cutting is known to introduce cracks in the superconducting layer due to bending of the tape edge. These cracks can produce a non-superconducting band (up to an approximately <NUM> wide) along the tape edge, thus reducing the effective cross section of the filament. These cracks can also propagate into the interior of the tape during usage of the tape. It is important to note that the HTS filaments that result from laser slicing of the tape do not include fillets or structural components along their edges. This unique filament architecture results in a very flexible wire with much tighter bending radius.

The narrow HTS filaments also facilitate the current carrying capability of the resultant cable. As will be appreciated by those skilled in the art, the transport current in a superconductor generates a magnetic field around the conductor, which is called the self-field. With an alternating transport current, the alternating self-field penetrates the superconductor during each current cycle. Even if there is no external magnetic field, the variation of the self-field inside the material causes a hysteresis loss, which is called self-field loss. The hysteresis or self-field loss can be reduced by decreasing the width dimension of the superconductor.

However, the current capacity of the superconductor is proportional to the width of the tape. Hence a <NUM> wide tape will carry about <NUM>/5th the current of the <NUM> wide tape. So, although hysteresis losses are reduced by filamentizing, the current capacity is also reduced. This loss in width can be compensated for and addressed by stacking the filaments to provide a multi-filament cable. In one preferred embodiment, this multi-filament cable is wrapped in an insulating or high-resistance sheath that can provide inter-winding insulation in a magnetic structure.

As mentioned, stacking filaments increases the current capacity of a given length of cable. However, in AC applications, the magnetic field of the stacked filaments will produce "shielding currents" in the stack that cause AC losses and will reduce the current carrying capacity. To reduce these losses, the stacked array of filaments is typically twisted along the axis of the cable. The twist reduces the shielding currents that would otherwise be generated and reduces the overall losses in the cable. A tighter pitch results in lower losses. The highly flexible filaments which are provided by exfoliation and laser slicing allow for significantly tighter pitches, and thus provide a structure with significantly lower overall loss due to shielding currents.

Referring now to <FIG>, the new HTS cable of the present invention is formed by stacking a plurality of exfoliated filaments <NUM>. Each of these exfoliated filaments includes a superconducting layer and a stabilizing metal layer, but does not include a substrate layer or a buffer layer. A metal foil <NUM>, e.g., copper, stainless steel or high carbon steel, is preferably positioned at both the bottom and top of the stack of filaments. The metal foils provide mechanical strength to the resultant cable structure. The material selected for the foil may vary depending on the intended purpose of the cable. The metal foil also provides added electrical stability to the cable. That is, in case of a quench event (i.e., transition to normal state), the current that was previously supported by the superconductor, will be diverted to the normal metal. The normal metal is also more effective at transferring heat away from the structure. Thus, the more normal metal that is in the cable, the greater the stability of the cable during a quench event. Of course, the addition of normal metal to the stack (for a given cross-sectional area) reduces the overall current carrying capacity of such cable. Nonetheless, this reduction in current carrying capacity may be justified in certain critical applications, such as MRI magnets, where a quench event is highly undesirable. The stack of exfoliated filaments and metal foils is then wrapped, for example with a wire <NUM>. Wire <NUM> may be a metal wire, such as copper, nichrome, stainless steel or regular steel, or a synthetic polymer thread, such as nylon. The wrapping ensures intimate contact of the filaments, and additionally provides insulation between the winding layers in a magnet. In one preferred embodiment, the wire wrapping tension is at least about <NUM> N, and more preferably from about <NUM> N - <NUM> N.

According to the invention, the exfoliated filaments <NUM> are coated with a low-temperature solder prior to stacking. The solder layer is thin, preferably < <NUM>, such that the filament flexibility is not impaired. The preferred solder formulation provides good wetting of the filament surface, and at the same time, the solder coat should dissolve the silver coating by the amalgamation process. For example, Sn62Pb36Ag2 solder has demonstrated the required performance. It is has been discovered herein that the electrical connectivity between the filaments can be improved by reflowing the solder layer previously applied to the filaments. The solder reflow is preferably performed after the twisting of the stack. In this way, the individual filaments are free to slide during the twisting process. The stack is then heated to a temperature above the melting point of the low-temperature solder, which results in the melting of the low-temperature solder, and the flow of the molted solder into the gaps between the filament due to capillary action, and to the subsequent bonding of the adjacent filaments. In one preferred embodiment, the stack is heated to a temperature of approximately <NUM> for approximately <NUM> minutes. This soldering (or fusing) of the adjacent filaments provides a low resistance electrical connection between the adjacent filaments, thus allowing for an uninterrupted flow of current across the stack, i.e. current sharing. In one preferred embodiment, the individual filaments are fused after the cable has been wrapped and/or incorporated into a superconducting device. For example, the solder reflow can be accomplished by heating the item, such as the magnet coil, to a temperature higher than the melting temperature of the solder after the winding of the magnet coil with the cable. Alternatively, the reflow can be accomplished by localized heating of the stack during the winding process, either by an inductive coil or by a laser.

The current sharing between the superconducting layers in the cable is generally dependent on the contact resistance between the adjacent filaments. According to the invention, the level of contact resistance between the adjacent filaments is below about 10µΩcm<NUM>, and more preferably below about <NUM>µΩcm<NUM>. In one particularly preferred embodiment, the level of contact resistance between the adjacent filaments is below about <NUM>µΩcm<NUM>. In a more particularly preferred embodiment, the level of contact resistance between the adjacent filaments is below about <NUM>µΩcm<NUM>. This inter-filament connectivity can be achieved through mechanical contact between adjacent stacked filaments, e.g., through the mechanical pressure exerted on the stack by the exterior winding described herein. In one example outside the scope of the present invention, the filaments within the cable are not fused to one another, but rather are secured in intimate contact with one another via the external winding surrounding the stack. Preferably, the winding is sufficiently taught to ensure that the level of contact resistance between the adjacent filaments is consistent and uniform, and is below about <NUM>µΩcm<NUM>, and more preferably below about <NUM>µΩcm<NUM>, and most preferably below about <NUM>µΩcm<NUM>.

<FIG> illustrates the assembled structure, i.e., cable <NUM>, after twisting. The twisting pitch is chosen so that the strain limit of the YBCO layer is not exceeded. For a <NUM> wide cable, the twisting pitch is preferably from about <NUM> to about <NUM> (see <FIG>). For narrower strips, the twisting pitch can be even tighter. For example, the twisting pitch for a cable formed from <NUM> wide filaments can be as tight as <NUM>. The stacked filaments are also subjected to a tensile force during the twisting step. This "drawing" of the filaments during twisting ensures that all the filaments in the stack are under the same tension, and prevents/limits any bulging of the stack. In one preferred embodiment, the tension force is on the order of about <NUM> N.

<FIG> shows current-voltage curves of a single <NUM> wide exfoliated filament, a cable comprised of two filaments and of three filaments. A proportional enhancement of the critical current density of the stack was observed.

A cable splice, as such not forming part of the invention, is illustrated in <FIG>. The cable splice is accomplished by trimming the exfoliated filaments 301a of HTS cable 300a in a step-like configuration, and by trimming the exfoliated filaments 301b of HTS cable 300b in a corresponding step-like configuration. As such, the exposed surface of each filament in the first HTS cable electrically contacts the exposed surface of the filament in the second HTS cable. Preferably, the stabilizing metal layers of each filament are exposed, such that the stabilizing metal layers of filaments 301a engage the stabilizing metal layers of filaments 301b when cable 300a is spliced to cable 300b (because the YBCO-silver interface on the stabilizer side of the superconducting layer has lower resistivity as compared to the other side of the superconducting layer). This is because the YBCO-silver interface on the stabilizer side is annealed before exfoliation at a temperature exceeding <NUM>, a process which is known to deliver very low contact resistance at the YBCO-silver interface. The silver layer deposited on the other side of the YBCO layer after exfoliation is annealed at a temperature not exceeding the melting temperature of the solder used to attach the stabilizer to the YBCO layer. The melting temperature of the solder used to attach the stabilizer is typically <NUM>° C. Thus, the silver deposited on the other side of the YBCO layer after exfoliation must be annealed at a temperature below <NUM>, which results in greater contact resistance of that surface. The spliced filaments can be secured together via low temperature solder and/or external winding of the splice. A cross-sectional view through one of the spliced filaments is shown in <FIG>. Both a high temperature solder, such as Pb64Sn34Ag2, used to bond the stabilizing metal layer to the superconducting layer and the low temperature solder used to fuse adjacent filaments are shown in <FIG>.

The use of exfoliated filaments to create a multi-filament superconducting cable provides an improved fill factor (useful cross-section occupied by the superconductor). This is because the substrate and buffer layers, which account for the majority of the original HTS tape, are removed via exfoliation. The resultant structure provides improved flexibility, thus allowing for a smaller pitch period when twisted, and for a smaller bending radius when winding magnets.

In another embodiment (as shown in <FIG>), filaments <NUM> are stacked such that the height H<NUM> of stack <NUM> is substantially equal to the width W<NUM> of the individual filaments. As such, the resultant stack <NUM> of filaments <NUM> defines a substantially square cross-section. The filaments in stack <NUM> may be soldered to one another and/or wrapped with a wire or thread as described hereinabove. In certain applications, stack <NUM> may be twisted as described hereinabove. In other applications, the twisting of stack <NUM> may be unnecessary or undesirable. In still other applications, material may be removed from the outer edges of stack <NUM> (via a cutting tool) to provide a stack with a substantially circular cross-section which may be desirable in certain applications.

In another embodiment (as shown is <FIG>), the width of the filaments is varied to provide a stack <NUM> defining a substantially circular cross-section C<NUM>. For example, the width of filament 301c > 301b > 301a. Filaments 301a, 301b and 301c can represent discreet filaments or they may represent a pre-assembled stack of individual filaments of common width. The filaments in stack <NUM> may be soldered to one another and/or wrapped with a wire or thread as described hereinabove. In one particularly preferred embodiment, the step-shaped gaps <NUM> surrounding the circumference of stack <NUM> are filled with solder, either by dipping/coating of the stack prior to winding or by flow of the solder from between the individual filaments during the subsequent heating step. In certain applications, stack <NUM> may be twisted as described hereinabove. In other applications, the twisting of stack <NUM> may be unnecessary or undesirable.

Despite the advances that have been made in current HTS manufacturing processes, today's HTS tapes still contain defects which can adversely affect the current carrying capacity of the tape. Traditionally, when a defect is identified by an in-line quality control method, such as TapeStar (product of Theva GmbH), the defect is cut out of the tape and a defect-free portion is spliced therein. This method is practical only for tapes wider than <NUM>. It has been discovered herein that the stacking of exfoliated HTS filaments as described herein addresses the issue of manufacturing defects in today's HTS tapes, particularly in tapes having a width of less than <NUM>, and more particularly, in tapes having a width of less than <NUM>. The present disclosure allows a HTS cable capable of carrying a designated level of current to be readily designed. First, the average defect percentage for a known width/length of a selected HTS tape is estimated and/or calculated based on existing manufacturing data. The filament stack can tolerate defects that are spaced further than the current transfer length, which is <NUM>-<NUM> in the examples disclosed herein. Next, the allowable cable dissipation is calculated using the known inter-filament resistivity and the expected defect density. The dissipation level per a defect, Q, can be calculated using the formula, <MAT>, where If is the filament current, Rs is the arial contact resistance, w is the filament width and λ is the current transfer length. Thereafter, the number of individual superconducting filaments is calculated to provide the necessary cross-sectional area for carrying the current. Finally, the number of individual superconducting filaments is adjusted (e.g., increased) per statistical analysis based on the known defect percentage in HTS tape of that width/length to ensure that the resultant HTS cable is capable of carrying the designated level of current without risk. Using this approach, an operating safety factor can easily be designed into the cable. The minimum number of individual superconducting filaments for a particular application can also be readily calculated.

The illustrative embodiments described herein are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the scope of the subject matter as defined by the claims. It will be readily understood that the aspects of the present invention, as generally described herein and illustrated in the figures can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure.

A <NUM> wide HTS tape (AMSC Corp Amperium tape) was exfoliated to provide a high-temperature superconducting layer of YBCO materials secured to a <NUM> thick stabilizing metal layer of copper. The exfoliated tape was sliced into <NUM> filaments using a <NUM> W CO<NUM> laser (Kern Lasers HSE model). The filaments (<NUM> in <FIG>) were stacked, and a <NUM> thick, <NUM> wide stainless steel foil was positioned at the top and bottom of the stack (<NUM> in <FIG>). The stack was wrapped with an AWG <NUM> tinned copper wire (<NUM> in <FIG>) using a wire wrapping machine (ACME Mechatronics). The wrap speed and the tape advance were matched to provide a <NUM> wire wrap pitch. The exfoliated filaments were coated with a low-temperature solder (Sn62Pb36Ag2) prior to stacking. The low temperature solder layer was less than <NUM>. A tension force of approximately <NUM> N was applied to the stack during twisting. The stack was then subjected to a temperature of <NUM> for approximately <NUM> minutes to melt the low temperature solder, thereby fusing the individual filaments to one another.

A <NUM> wide YBCO tape was exfoliated to provide a <NUM> thick YBCO layer secured to a <NUM> wide, <NUM> thick copper foil. The exposed YBCO face was coated with <NUM> of silver by magnetron sputtering. The combined YBCO layer/copper foil was then sliced into <NUM> wide filaments using a <NUM> W CO<NUM> laser. After slicing, the filaments were coated with 62Sn 36Pb 2Ag solder using a dip coating method. Briefly, the filaments were immersed in a bath or organic acid flux, Kester <NUM>-ZX, and transported into a bath of molten solder kept at a constant temperature <NUM>. <FIG> shows the effect of the filament linear advance speed on the solder coating thickness. In this example, we used <NUM>/s linear speed, which delivered a coating having a thickness of approximately <NUM> on each side of the filament, thus the total added thickness of the coating was approximately <NUM>. The <NUM> wide exfoliated filaments were assembled into a <NUM>-layer stack in a cabling machine. The stack was then clad with a <NUM> thick <NUM> stainless steel foil on both sides to provide mechanical protection. The whole stack (~ <NUM> thick) was then wrapped with a AWG <NUM> nichrome wire at a <NUM> pitch to ensure uniform mechanical contact between the filaments in the stack, thereby providing a cable. Several <NUM>" diameter coils were manufactured from <NUM> long coupons of the cable. After winding, the critical current of the coil was measured at <NUM> using the standard <NUM>-point transport method; the voltage tap was placed at <NUM> distance from the current lead. <FIG> shows the current-voltage curves of as-wound coils. As-wound coils exhibited unpredictable current-voltage characteristics, as witnessed by voltage jumps and reduced critical current density. The coils were subsequently heated in a low-temperature oven at <NUM> for <NUM> minutes in order to partially melt the solder and electrically connect the filaments. The current-voltage characteristics of the coils after the heat-treatment exhibited a marked improvement. The voltage spikes were practically absent and the critical current of three coils was within <NUM>% of each other (see <FIG>). This experiment demonstrates the improved stability due to current sharing in the electrically-coupled filaments.

Claim 1:
A multi-filament high temperature superconducting cable, comprising: a plurality of stacked exfoliated filaments (<NUM>), each of said filaments including a superconducting layer (<NUM>) and a stabilizing metal layer (<NUM>) in the absence of a substrate layer and a buffer layer, and wherein the adjacent stacked filaments are soldered to one another with a low temperature solder and maintained in uniform mechanical contact with each other to allow the uninterrupted flow of current from the superconducting layer of one filament to the superconducting layer of an adjacent layer, and wherein said uniform mechanical contact provides a level of contact resistance between adjacent stacked filaments of less than about <NUM>µΩcm<NUM>.