Low AC loss high temperature superconductor tape

A superconductor tape includes a plurality of conductive strips having respective long directions parallel to a long tape direction of the superconductor tape, where each of the plurality of conductive strips separated from one another by a inter-strip region. The superconductor tape further includes a superconductor layer disposed adjacent the plurality of conductive strips, having a length along the long tape direction, where the superconductor layer comprises a plurality of superconductor strips disposed under the respective plurality of conductive strips, and a non-superconductor strip disposed adjacent the inter-strip region.

FIELD

The present embodiments relate to superconducting materials and, more particularly, to superconducting tape and fabrication techniques therefore.

BACKGROUND

Superconducting wires or tapes have been developed based upon high temperature superconducting (HTc or HTS) materials which may have critical temperatures TCabove 77 K, facilitating their use in cryogenic systems cooled by liquid nitrogen. In particular, superconducting tapes have been developed in which a layer of superconducting material is integrated into a stack of conductive and/or non-conductive layers that form the tape.

When used to conduct alternating current (AC) a superconducting tape generates a magnetic field along the edges of the superconducting tape. When the polarity of current in the superconducting tape switches with the AC signal, magnetic flux switches polarity and exhibits a hysteretic behavior (Hp), which contributes to an energy loss that is often termed “AC loss.” The AC loss depends upon the aspect ratio of the superconducting tape in which the aspect ratio is defined as the thickness of the superconducting layer divided by the width of the superconducting tape. In particular, as the aspect ratio decreases, the magnetic hysteresis and AC loss increases. A typical superconducting tape may have a width of about 1 cm and superconducting layer thickness of about 1 μm, thereby exhibiting an extremely low aspect ratio. Although attempts have been made to improve the aspect ratio by etching superconductor tape to form narrower superconducting lines, such processes may not be ideally suited to high volume manufacturing. It is with respect to these and other considerations that the present improvements are needed.

SUMMARY

In one embodiment, a superconductor tape includes a plurality of conductive strips having respective long directions parallel to a long tape direction of the superconductor tape, where each of the plurality of conductive strips separated from one another by a inter-strip region. The superconductor tape further includes a superconductor layer disposed adjacent the plurality of conductive strips, having a length along the long tape direction, where the superconductor layer comprises a plurality of superconductor strips disposed under the respective plurality of conductive strips, and a non-superconductor strip disposed adjacent the inter-strip region.

In an additional embodiment, a method form a superconductor tape includes forming a superconductor layer comprising a superconductor material on a tape substrate layer, the tape substrate layer having a long tape direction; forming a plurality of conductive strips on the superconductor layer, the conductive strips having respective long directions parallel to the long tape direction, the plurality of conductive strips separated from one another by at least one inter-strip region that defines respective one of an exposed superconductor region; and treating the exposed superconductor region to render the exposed superconductor region into non-superconductor material.

DETAILED DESCRIPTION

The present embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which some embodiments are shown. This subject matter, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the subject matter to those skilled in the art. In the drawings, like numbers refer to like elements throughout.

To address some of the deficiencies in the aforementioned superconductor tapes, embodiments are described herein that provide improved structure for superconductor tapes as well as improved techniques for forming superconductor tapes. These embodiments may be especially suited to applications of superconductor tapes in which the tapes are subject to an AC voltage including in fault current limiters and other applications.

To address this situation, the present embodiments in particular provide techniques to generate a superconductor tape configuration that imparts an effectively lower aspect ratio to the superconductor tape. The resultant superconductor tapes maintain the superconducting layer intact unlike in earlier approaches. The terms “superconductor” “superconductor element” or “superconductor material” as used herein, refer to a substance or object that has the capability of conducting electrical current without resistance. Thus a material such as YBa2Cu3O7−x(also referred to herein as “YBCO”) may be referred to a superconductor or superconductor material even when subject to a room temperature environment in which the material is not superconducting, since YBCO does become superconducting at temperatures below about 91 K.

The term “superconducting” or “superconducting layer” on the other hand, are used herein to refer to properties of a tape or material. Thus, YBCO is superconducting under certain conditions, such as temperatures below 91 K or when current conducted by the YBCO material is below a critical current. Moreover, the term “non-superconducting” and “non-superconducting state” as used herein both refer to the state of a superconductor material in which the superconductor material does not have superconducting properties, such as when the superconductor material is subject to room temperature ambient.

In addition, the term “non-superconductor” as used herein, may refer to a material that is not capable of being superconducting. For example, a non-superconductor may include a material derived from a superconductor material such as YBCO, in which the material is altered, either chemically or structurally, from the parent superconductor material in a manner to render it incapable of becoming superconducting. Thus, a superconductor material may exist in superconducting state or non-superconducting state depending on conditions including temperature, electrical current density for current being conducted by the superconductor material, and the magnetic field applied to the material, etc. A non-superconductor material, on the other hand, may exist in a non-superconducting state regardless of temperature or other factors.

Finally, the terms “superconductor tape” and “superconductor layer” as used herein refer to a tape or layer in which at least a portion of the respective tape or layer contains a superconductor material. Thus, a “superconductor tape” may include one or more regions of superconductor material and optionally one or more regions of non-superconductor material.

FIG. 1depicts an architecture of a superconductiong fault current limiter (SCFCL) system100consistent with embodiments of the disclosure. The SCFCL system100includes an SCFCL102, which may be a conventional SCFCL except as otherwise noted herein with respect to the description to follow. The SCFCL system further includes a protection element106that contains superconducting tapes arranged according to various embodiments. The SCFCL102is arranged in series with a transmission line108that conducts AC current generated by an AC source110. In operation, the SCFCL system100provides fault current protection by limiting fault current (not separately shown) that may develop along the transmission line108. Under normal operation mode an AC load current may periodically, occasionally, or continuously pass through the SCFCL system100. The AC load current in normal operation mode exhibits a current level such that the superconductor elements104remain in a superconducting state and therefore transmit the load current through the superconductor elements104with zero resistance when the load current passes through the SCFCL102. Accordingly, the load current is transmitted with relatively lower resistance through the SCFCL, which includes resistive points including normal state metals and connection points. During a fault condition in which an excessive fault current may be rapidly generated, the superconductor elements104react to the excessive fault current by transforming from a superconducting state to a non-superconducting state, which places a large overall impedance to the excessive fault current, thereby limiting the fault current during the fault condition. Subsequently, the superconductor elements104may return to a superconducting state to regulate current by limiting current in future fault events.

Consistent with the present embodiments the superconductor elements104include superconductor tape having a novel structure that facilitates a reduction in AC loss by decreasing the aforementioned magnetic hysteresis that develops in response to conduction of alternating current through the superdonductor tape. In this manner the SCFCL operates more efficiently in normal operation.

In particular embodiments as detailed below the magnetic hysteresis may be reduced up to about 70% or so as compared to conventional superconductor tapes. This is accomplished by subdividing a superconductor layer of a superconductor tape into a plurality of superconductor strips that are isolated from one another so as to increase the effective aspect ratio of superconductor portions of the superconductor tape. Advantageously, as detailed below the present embodiments provide robust techniques to fabricate this subdivided structure consistent with high volume manufacturing.

FIG. 2illustrates the improved properties of superconductor tapes afforded by the present embodiments. In particular, there is illustrated the general relationship between magnetic hysteresis (HP) induced by an AC current in the superconductor tape and the aspect ratio of the superconductor tape. The value of HPis proportional to the AC loss and it is therefore desirable to minimize HP. As illustrated, the aspect ratio is defined by the height or thickness (t) divided by a width d of a cross section of the superconductor element (layer). With reference to the Cartesian coordinate system shown h is the superconductor layer dimension along the Y direction, d is the superconductor layer dimension along the X direction, and AC current flows into or out of the page along the length of the superconductor tape parallel to the Z-direction. Conventional superconductor tapes have typical widths (d) on the order of one centimeter, such as about 1 or 2 cm, and have thickness t on the order of one micrometer, such as about 0.5-2 μm. This imparts an aspect ratio t/d of about 0.000025 to about 0.0002 for a conventional superconductor tape. Even at the “high” aspect ratio of 0.0002 shown by the vertical dotted line inFIG. 2, the relative level202of HPis about 6. In contrast the present embodiments provide a superconductor tape divided into a plurality of superconductor regions having widths that are much narrower than the overall tape width. In various embodiments, a superconductor tape is divided into one or more superconductor strips in which the strip width (dS) is much narrower than the overall tape width, thereby reducing the aspect ratio t/d for any given superconductor path (strip), where d=dS. For example, in some implementations the value of dSmay range between about 20 μm and 500 μm. This leads to aspect ratios over the range204for typical superconductor layer thickness in the range of 0.5 to 2.0 μm. In turn the range204corresponds to relative HPvalues in a range206of between about 4.5 and 2 for the scale used inFIG. 2, which corresponds to a reduction in magnetic hysteresis of about 25% to 67%. The values shown inFIG. 2are merely exemplary and the embodiments are not limited in this context.

FIG. 3Adepicts an end cross-sectional view of one embodiment of a superconductor tape300having a plurality of isolated superconductor strips. The cross-sectional view is perpendicular to the long direction of the tape and therefore perpendicular to the direction of current flow.FIG. 3Bdepicts a top plan view of the superconductor tape ofFIG. 3A.FIG. 4depicts a top plan view of the superconductor tape ofFIG. 3Bwith a conductive strip layer removed for clarity. As illustrated in the FIGS., the superconductor strips308are formed on a substrate302.

FIG. 5Adepicts one embodiment of substrate layers andFIG. 5Bdepicts another embodiment of substrate layers of a superconducting tape, each of which represent variants of the substrate302. The embodiments are not limited in this context. For example, referring toFIG. 5A, the substrate302A includes a base metal alloy layer508, Y203layer506disposed on the base metal alloy layer508, yttrium stabilized zirconia (YSZ) layer504disposed upon the Y203layer506, and CeO2layer502diposed upon the YSZ layer504. The CeO2layer502represents the layer upon which the superconductor layer304is disposed. In the example ofFIG. 5B, the substrate302B includes a base metal alloy layer508, Al2O3/Y2O3Buffer Layer517, MgO layer formed by ion beam-assisted deposition (IBAD MgO516), homoepitaxial MgO layer514disposed the IBAD MgO516, and epitaxial LaMnO3(LMO)) layer512. The epitaxial LMO layer512represents the layer upon which the superconductor layer304is disposed. Notably, the embodiments are not limited to the specific layer stacks depicted inFIGS. 5A and 5B.

Referring again toFIGS. 3A and 3B, the superconductor layer304includes a plurality of superconductor strips308that run parallel to a long direction of the superconductor tape (Y-direction) and are separated by non-superconducting strips310. Disposed on the plurality of superconductor strips308are a respective plurality of conductive strips306. Multiple conductive strips306, which may have a strip width dSthat is about 20 μm to about 500 μm in various embodiments, may be formed within a superconductor tape300whose tape width dTAPEis about 0.5 to 5 μm. In various embodiments the superconductor tape including the conductive strips306and superconductor strips308may extend for many hundreds of centimeters or many meters along the Y-direction. It is to be noted that in operation the superconductor tape300may be bent or curved such that the absolute direction of what is shown as the Y-direction with respect to a fixed coordinate system may vary. However, in all portions of a superconductor tape current may be conducted parallel to the instantaneous Y-direction.

The conductive strips306may include multiple layers such as in a conventional superconductor tape structure. In one example the conductive strip306includes an underlayer312made of silver that contacts the superconductor strip308, and an overlayer314made of copper. In operation, under normal conditions, the superconductor strips308may conduct current such as AC current when cooled below the critical temperature for the superconductor material that makes up the superconductor strips308, provided that the AC current conducted by the superconductor tape300is below the critical current JCfor transforming the superconductor material of the superconductor tape300into a non-superconducting state.

As discussed below, the superconductor strips308are characterized by a strip width dSthat is defined by the width of the conductive strip306formed upon the superconductor layer304. Because the strip width dSof the superconductor strips308is less than the tape width dTAPEof the superconductor tape300, the aspect ratio t/d where d is either dTAPEor dS, may be substantially less than in a conventional tape in which the superconductor layer spans the width dTAPEof the tape. Accordingly, in normal operation conditions where the AC current is less than JCthe AC current is conducted with lower AC loss due to the reduced magnetic hysteresis resulting from the lower aspect ratio t/d for superconductor tape300as compared to a tape having a single superconductor layer that spans the tape width dTAPE.

Under fault conditions, the AC current exceeds JCand causes the superconductor strips308to transition to a non-superconducting state in which resistance is sufficient in the superconductor strips308(temporarily in a non-superconducting state) that most current is temporarily conducted through the conductive strips306.

As further illustrated inFIGS. 3A and 3Bthe non-superconducting strips310are disposed adjacent the inter-strip regions316defined between the conductive strips306. As detailed below, the non-superconducting strips310are formed to define the superconductor strips308by a simplified process that is compatible with high volume manufacturing.

FIGS. 6A-6Ddepict four different stages of fabrication of a superconductor tape according to an embodiment of the disclosure. At one stage of fabrication shown inFIG. 6Aa substrate302is provided. As noted previously, the substrate302may comprise multiple layers. In some embodiments, a base layer is a metal alloy such as a nickel based alloy material. Additional layers may include oxide layers as discussed, for example, with respect toFIGS. 5A and 5B.

Each subsequent stage of processing may be carried out in a manner consistent with high volume manufacturing may be performed to process the superconductor tape structure. At a subsequent stage of fabrication depicted atFIG. 6B, a superconductor layer602is formed upon the substrate302. The superconductor layer602may be formed by known techniques including metal organic deposition (MOD), metal organic chemical vapor deposition (MOCVD), or other convenient technique. In various embodiments, the thickness of the superconductor layer may range between about 0.5 μm and about 2 μm. Suitable material for the superconductor layer include YBCO based material as well as bismuth strontium calcium copper oxide materials which can be represented by the general formula Bi2Sr2Can−CunO4+2n+x. The embodiments are not limited in this context.

Subsequent to formation of the superconductor layer602,FIG. 6Cdepicts the formation of strips604upon the superconductor layer602. In some embodiments, the strips604are formed by a screen printing process. In other embodiments, the strips604are formed by extrusion printing. In various embodiments, the total thickness of the strips604may range up to about 30 μm, while the width dCSof the strips604ranges between about 20 μm and 50 μm. As discussed previously, the strips604may contain multiple different layers, such as an underlayer606and layer608. In one example the strip604includes an underlayer606made of silver that is deposited as a paste or other silver-containing medium to directly contact the superconductor layer602. The strip604may also include a layer608that is a copper containing layer that is separated from the superconductor layer602by the underlayer606. The printed or extruded strips604may be subsequently subject to thermal treatment to form the conductive strips306ofFIG. 3A. For example the strips604may be in a metal paste form that includes various solvents and binders that are removed when subject to thermal treatment, leaving a residual metallic conductive strip306. As illustrated, after formation, the strips604define the aforementioned inter-strip regions316.

FIG. 6Ddepicts another stage of fabrication of a superconducting tape that takes place subsequently to the stage illustrated inFIG. 6C. As illustrated, the substrate302with conductive strips306in place is subject to energetic treatment605, which is represented schematically by the arrows shown. The energetic treatment605may include, for example, any combination of heat treatment, ion treatment, and gas phase treatment that is effective to selectively alter exposed regions610of the superconductor layer602that are adjacent the inter-strip regions316and are therefore not covered by the conductive strips306. The alteration of the exposed regions610renders them into non-superconductor material that defines the superconductor strips308(612).

As a result of the processes illustrated inFIGS. 6A-6Da superconductor tape, such as the superconductor tape300ofFIG. 3Amay be fabricated with a series of superconductor strips that each present a smaller aspect ratio t/d than a conventional superconductor tape can be fabricated. An advantage of this technique is that it does not require removal of superconductor tape materials such as the conductive material deposited on the superconductor layer as well as the superconductor layer. In a previous scheme that has been proposed to pattern narrower structures in superconductor tapes, patterning takes place after depositing a blanket metal overlayer on a superconductor layer over the entire tape surface. In the previous scheme an etch pattern is defined on top of the superconductor tape, followed by etching of the metal overlayer, which may be several tens of micrometers thick. Subsequently, to define patterned superconductor features, the superconductor layer is etched through its entire thickness. In contrast, in the present embodiments, no etching need be performed after the conductive strips306are formed.

Moreover, the use of deposition processes such as screen printing or extrusion printing facilitates formation of an array of superconductor strips that are closely spaced even for narrow strip widths in order to maximize JCunder normal operating conditions. For example, in one specific embodiment, conductive strips having a width of 50 μm may be printed with a spacing between conductive strips of about 10 μm, yielding strip width and spacing of similar dimensions in the underlying superconductor layer.

In addition, the energetic treatment to form non-superconductor strips (and thereby define the adjacent superconductor strips) may take place in various apparatus suitable for high volume manufacturing including furnance annealing apparatus, plasma chamber or reactive gas chamber, or ion implantation apparatus. The embodiments are not limited in this context.

FIG. 7depicts one variant of the energetic treatment605in which ions702are directed toward a superconductor tape after a superconductor layer704is formed on the substrate302, and conductive strips306formed on the superconductor layer704. The ions702are provided as an ion species and in an ion dose and ion energy that is effective to render the superconductor layer704into a non-superconducting material in exposed regions706that are not covered by the conductive strips306. For example nitrogen, boron or other low atomic weight ions may implant to depths of about 0.5 μm to 1 μm for ion energies in the range of 300 kV to 1 meV. Concomitant damage to superconductor material implanted with such ions may extend to greater depths such as about 1-2 μm. Accordingly, for superconductor tapes having a superconductor layer thickness in the range of 0.5-2 μm the process depicted inFIG. 7may be conveniently performed in a medium energy or high energy beamline ion implantation apparatus in some embodiments. It is to be noted that for a high temperature superconductor material such as YBa2Cu3O7−xthe material in exposed regions706need not be amorphized in order for those regions to be rendered as non-superconductor material. This is because the superconductivity properties are particularly sensitive to changes in the crystalline structure and stochiometry of the YBCO material.

Advantageously, the regions708disposed underneath conductive strips306are screened from any damage from the ions702inasmuch as the thickness of the conductive strips306may be on the order of 20 μm. Accordingly, a relatively small upper portion of the conductive strips306may be altered by implantation of ions702. Moreover, even an ion dose sufficient to transform the exposed regions706into a non-superconducting material may increase electrical resistivity just in the implanted portions of the conductive strips306resulting in a marginal increase in overall electrical resistance of the conductive strips306.

FIG. 8depicts another variant of the energetic treatment605in which heat flux802is directed toward a superconductor tape800after a superconductor layer804is formed on the substrate302, and conductive strips306formed on the superconductor layer804. The heat flux may be in the form of conductive heating, convective heating or radiative heating or any combination thereof. In some embodiments the superconductor layer804is RBa2Cu3O7−xwhere R is a rare earth element. Such materials exhibit a strong dependence of superconductivity on oxygen stochiometry such that lower the oxygen content causes the material to exhibit poorer superconductivity or no superconductivity. In addition, oxygen mobility in such structures is relatively high such that heating under certain conditions may generate the release of oxygen, thereby reducing the oxygen content within the crystalline structure. Accordingly, the heat flux802may be provided to deplete oxygen from RBa2Cu3O7−xmaterial that is disposed in exposed regions806that are not covered with the conductive strips306.

The heating of superconductive tape800may be tailored such that exposed regions806become sufficiently depleted of oxygen to be rendered non-superconducting while regions808, which are protected by the conductive strips306, remain superconductive. For example, in the prototype YBa2Cu3O7−xthe material exhibits an orthorhombic structure at room temperature and becomes superconducting at low temperature for values of x between 0 and about 0.20. At higher values of x the YBa2Cu3O7−xmaterial no longer is superconductive while at values of x above about 0.5 the YBa2Cu3O7−xis tetragonal at room temperature. Accordingly, heating may be arranged to outdiffuse oxygen species810to deplete YBa2Cu3O7material disposed in exposed regions806of about 0.2 mole fraction of oxygen or more, while leaving YBa2Cu3O7material disposed in regions808substantially fully oxygenated, that is, where the value of x remains at zero or less than about 0.05.

In one embodiment, the heating of superconductor tape800may be performed in vacuum at elevated temperatures such as at 300° C. or greater. However in other embodiments an ambient may be provided that includes reducing gases to effect removal of oxygen as shown inFIG. 9. As shown therein, while heat flux802is provided, an ambient containing gas species902is supplied to the superconductor tape900to react with superconductor material of the superconductor layer904disposed in exposed regions906, causing the release of oxygen species810. This causes the exposed regions906to become non-superconductive while leaving the regions908as superconductive material. In various embodiments, the gas species902may be provided as forming gas (N2/H2mixture) or alternative in an NH3plasma. The embodiments are not limited in this context.

Although the disclosed embodiments detail formation of a superconductor tape containing multiple superconductor strips and non-superconductor strips, in other embodiments a superconductor tape may be defined by a single superconductor strip and single non-superconductor strip. In order to form such a structure a single metal layer may be used to mask a portion of an underlying superconductor tape such that the exposed portion of the superconductor tape is rendered into a non-superconductor strip. In further embodiments, a superconductor tape may include a single superconductor strip and multiple non-superconductor strips or multiple superconductor strips and a single non-superconductor strip. The embodiments are not limited in this context.

In summary, the present embodiments provide techniques and structure for superconductor tapes that provide multiple advantages over conventional superconductor tapes. By using deposited conductive strips to define in an underlying superconductor layer superconductor strips having smaller width than the superconductor tape width, the aspect ratio of superconducting structures is reduced, thereby decreasing magnetic hysteresis and AC loss when the superconductor tape transmits AC current. Moreover, the use of printing techniques to form conductive strips provides a manufacturable approach to defining larger aspect ratio superconductor structures in a superconductor tape. In particular, etch processes are avoided, which save process time and materials cost, which may result in considerable cost reduction for superconductor tape technology that employ high cost materials such as silver.