Patent Description:
The advent of second generation (<NUM>) YBa<NUM>Cu<NUM>O<NUM> (YBCO) wire technology has spawned impressive technological progress since the first meter of <NUM> wire was manufactured in <NUM>. Further developments in the field have been driven by existing and emerging applications, such as fault current limiters, transformers, and wind turbines. The second generation superconducting (<NUM>) wires have record high upper critical field and critical temperature, which could be used in various industrial and commercial applications. The core <NUM> wire technology can be described as a thin (<<NUM>) YBCO layer deposited on a <NUM>-<NUM> thick metal substrate. <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 Y-ReBa<NUM>Cu<NUM>O<NUM>-x (YBCO), 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 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.

Commercially-available <NUM> wires, such as the wire shown in <FIG>, are often delivered as high-aspect ratio tapes with the wide side being on the order of <NUM>-<NUM> and the thickness being on the order of <NUM>-<NUM>, whereas the thickness of superconducting layer <NUM> is on the order of <NUM>-<NUM>. The architecture associated with known commercially-available wires presents a number of problems, particularly when this architecture is used in magnet applications:.

Therefore, the manufacture of a multi-strand cable requires an elaborate splicing procedure that ensures that only top stabilizer surfaces are in contact with each other.

There is therefore a need in the art for high-temperature superconducting filaments which can be used in various industrial and commercial applications. There is a further need in the art for a method of manufacturing high-temperature superconducting filaments and cables with reduced/eliminated risk of delamination of the superconducting layer, which exhibit more uniform electrical properties, which exhibit enhanced mechanical properties, and which may be readily spliced together.

The present invention is directed to a method according to Claim <NUM> and according to Claim <NUM>. The present invention relates to a method for manufacturing high-temperature superconducting filaments from a <NUM> wire using a reel-to-reel exfoliation process. The reel-to-reel exfoliation process includes the step of loading the <NUM> wire on a first reel. The <NUM> wire includes a superconducting layer positioned upon a metal substrate, wherein the superconducting layer is coated with a protective metal layer, such as silver. The wire is passed over an inductive coil - which rapidly heats the wire, resulting in the partial or complete exfoliation of the superconducting layer from the substrate. The separated superconducting layer is spooled on a second reel and the substrate is spooled on a third reel.

The present invention further relates to a method for continuous separation of the substrate from the superconducting layer. The method includes the step of tensioning the wire with a predetermined load applied at a predetermined angle to achieve clean separation of the substrate from the superconducting layer.

The present invention further relates to a method of reel-to-reel slicing of the exfoliated filament. The slicing is preferably performed by an industrial CO2 laser. The slicing is performed by synchronizing the motion of the laser beam and the tape. A single laser head will perform the tape slicing into filaments that are at least <NUM> wide.

The present invention further relates to a method of coating the exfoliated filaments with a silver layer, and thereafter processing the silver layer in order to achieve low resistivity. The silver layer processing step involves the application of high-frequency radiation that is confined to the silver layer.

The present invention further relates to a method of reel-to-reel coating of the filaments with a solder layer that will allow fusing of the filaments in an inter-connected filament stack. The filaments are coated with a solder paste and passed through a high-temperature zone, where the solder is melted, thus forming a continuous solder layer.

The present invention further relates to a method of reel-to-reel exfoliation utilizing an air blade to facilitate the separation of the YBCO layer(s) from the substrate. One or more air blades may be utilized depending on the application. The air forming the air blade can be heated or cooled. According to the invention, a separation angle β is formed between the YBCO layer(s) and the substrate, the separation angle β ranging from <NUM> to <NUM> degrees.

A partially exfoliated second generation (<NUM>) wire, i.e., tape <NUM> is shown in <FIG>. Tape <NUM> is preferably on the order of <NUM> - <NUM> meters long and <NUM> - <NUM> wide. 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. 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.

<FIG> illustrates the exfoliation process of a two-sided <NUM> tape <NUM>. As discussed above, an external action is applied to substrates <NUM> so that both upper and lower substrates are separated. The remaining structure is formed from stabilizer layer <NUM> and two superconducting layers <NUM> attached to the top and bottom sides of the stabilizer layer.

<FIG> is a schematic presentation of the reel-to-reel exfoliation process of a one-sided tape, such as the tape shown in <FIG>. The <NUM> tape is wound on a reel <NUM> which is connected to a shaft of a stepper motor <NUM>. In an alternative embodiment, the <NUM> tape is fed directly into the process from a continuous source of tape. An idler roller <NUM> maintains constant position of the tape with respect to a tape guide <NUM>. An inductive coil <NUM> is placed directly over the tape. Idler rollers <NUM> and <NUM> feed the separated tape layers towards reel <NUM> and reel <NUM>, respectively, The substrate and buffer layers are then spooled on reel <NUM>, which is attached to a torque motor. The stabilizer with the YBCO layer is then spooled on reel <NUM>, which is also attached to a torque motor. The position of reels <NUM> and <NUM> determines the separation angle α. Separation angle α is preferably in the range of from about <NUM> degrees to about <NUM> degrees. The torque applied by one or both of the torque motors generates a force F that has a component normal to the tape Fz and lateral component Fx. It is the action of the normal component Fz, together with the heating from the inductive coil, which causes separation of the substrate from the superconducting layer.

<FIG> is a schematic presentation of the reel-to-reel exfoliation process of a two-sided tape, such as the tape shown in <FIG>. In an alternative embodiment, the <NUM> tape is fed directly into the process from a continuous source of tape. Here, inductive coils <NUM>' apply heating to the top and bottom substrate/buffer layers. The position of reels <NUM> and <NUM> determine the angle of separation of the top and the bottom substrate/buffer layers from the tape. The exfoliated <NUM>-sided tape is spooled to a reel <NUM>. Torque applied by the torque motors connected to reels <NUM>, <NUM> and <NUM> determines the absolute tape tension and the separation force magnitude for both top and bottom substrate/buffer layers.

Referring now to <FIG>, the reel-to-reel exfoliation process can utilize an "air blade" to facilitate the separation of the YBCO layer(s) from the substrate. An air blade is a sheath of compressed air which is formed by directing air through a flat, slit-like nozzle. The air pressure facilitates the uniform and even separation of the layers of the starting material. Of course, more than one air blade may be utilized if, for example, a <NUM>-sided tape is being exfoliated. It is contemplated herein that the air blade method can be used in conjunction with the application of tension to the starting material. Additionally, the air coming from the air blade can be either heated or cooled, whereby thermal stress can be applied to the exfoliated area, which may be desirable in certain exfoliation processes.

In the air-assisted exfoliation process shown in <FIG>, the tape is supplied by spool <NUM>. The take-up spools <NUM> and <NUM> are positioned to provide a separation angle β between the YBCO layer(s) and the substrate. The separation angle β determines the normal, or pull pressure, exerted on a tape, and preferably ranges from <NUM> to <NUM> degrees. In one preferred embodiment, a tension of up to about <NUM> N is applied to the tape during the reel-to reel process. Depending on the strength of the substrate-YBCO interface, the preferred range of <NUM> to <NUM> degrees for separation angle β will typically provide the desired separation pressure to the tape.

An air blade <NUM> preferably delivers air into the gap formed between the exfoliated YBCO layer(s) and the substrate, thus creating an additional separating action. In order to deliver pressure on the order of <NUM>-<NUM> MPa, flow on the order of <NUM>/min of compressed air is required. It has been discovered herein that the air-assisted separation technique provides a substantially uniform and smooth pressure field, which facilitates the separation of the YBCO layer(s) from the substrate.

After exfoliation, a protective metal layer is preferably deposited over the superconductor layer of the superconducting tape for the purposes of protecting the superconductor layer and providing a path for the electrical current if the superconductor becomes locally resistive due to overheating or spontaneous loss of superconductivity. The protective silver layer deposited on the exfoliated YBCO surface needs to be treated in order to improve adhesion of silver to YBCO, and reduce the interfacial resistance. Conventionally, a short annealing step at approximately <NUM> is performed, resulting in a Ag-YBCO interface resistivity reduction from <NUM>-<NUM> Ω*cm<NUM> to < <NUM>-<NUM> Ω*cm<NUM>. However, this approach is not appropriate for the present exfoliated filament because the YBCO layer is typically attached to the stabilizer with a low-temperature (<NUM> - <NUM> melting point) solder. It has been discovered herein that a short application of GHz-range microwave power to the coated tape delivers structural improvement identical to thermal processing, but at a significantly reduced temperature. The effect is explained by the electro-migration of metal ions under large (<NUM><NUM> A/m<NUM>) inductive eddy current that can be easily generated in a conductive film by modern magnetron sources. This is because at <NUM>, the common industrial processing frequency, the skin depth of silver (specific resistivity <NUM>×<NUM>-<NUM> Q*cm) is <NUM> (as shown in <FIG>), which means that the inductive currents are confined in the silver layer. A <NUM> system has several advantages. First, the magnetrons and waveguide components are mass-manufactured for household (microwave ovens) and industrial markets. Second, the waveguides and applicators are large, for example a popular WR340 waveguide is <NUM> × <NUM>, allowing for uniform treatment of <NUM> wide strip.

<FIG> shows a schematic layout of the <NUM> processing system <NUM>. The magnetron head <NUM> is powered by a high-voltage power supply <NUM>. The magnetron is protected from a reverse power surge by a circulator <NUM>. Tuner <NUM> is adjusted to match the impedance of the generator with the load. The exfoliated tape <NUM> is transported through the multimode applicator <NUM> using a set of reels <NUM> and <NUM>. The power meter <NUM> provides the direct and reflected RF power reading. The terminal load <NUM> absorbs the unused radiofrequency radiation. The applicator <NUM> is preferably operated in a transverse electric mode (TE103) wherein the magnetic field has a well-defined maximum in the middle of the resonator. The sample is placed in the area of maximum magnetic field, where the largest inductive currents can be achieved. A short pulse (< <NUM>) of microwave power is applied by modulation of the anode voltage of the magnetron. The optimum processing conditions are determined by varying the magnetron power and the pulse application duration.

The table shown in <FIG> summarizes the thermal properties of the layers comprising the filament. It is to be noted that the thermal diffusion time for the present filament is <NUM> x <NUM>-<NUM>, which is approximately <NUM>. In other words, the sample thermalizes practically instantly. Thus, under adiabatic conditions the temperature would rise to <NUM> after absorption of <NUM> J of energy. This amount of energy can be delivered by the application of a <NUM>,<NUM> W pulse over <NUM>.

Claim 1:
A method for exfoliating a superconducting tape (<NUM>), said tape including a superconducting layer (<NUM>) and a substrate layer (<NUM>), said tape further including a buffer layer (<NUM>) positioned between said superconducting layer and said substrate layer, comprising:
a) providing a continuous length of said tape;
b) subjecting said tape to an external action which increases the stress level between said superconducting layer and said buffer layer whereby said superconducting layer separates from said buffer and substrate layers; and
c) spooling said separated superconducting layer and said separated buffer and substrate layers; and
further comprising the step of tensioning said tape with a predetermined load applied at a separation angle α to achieve clean separation of said superconducting layer from said buffer and substrate layers;
wherein said separation angle α is from <NUM> degrees to <NUM> degrees,
and wherein said predetermined load is up to <NUM> Newtons.