SUPERCONDUCTOR FLUX PINNING WITHOUT COLUMNAR DEFECTS

Abstract

There is a superconducting article that includes a superconducting film comprising a substrate, one or more buffer layers, and a high temperature superconducting (HTS) layer. The superconducting layer may be comprised of the chemical composition REBa.sub.2Cu.sub.3O.sub.7−x, where RE is one or more rare earth elements, for example: Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The superconductor layer is produced using Photo-Assisted Metal Organic Chemical Vapor Deposition (PAMOCVD) and contains non-superconducting nanoparticles. The nanoparticles are substantially provided in the a-b plane and naturally oriented. The non-superconducting nanoparticles provide flux pinning centers that improve the critical current properties of the superconducting film.

Claims

1. A thin film composite high-temperature superconducting article comprising: a substrate; a buffer layer; and a high-temperature superconducting layer, wherein the high-temperature superconducting layer further comprises non-superconducting material distributed preferentially along the a-b plane coplanar with the superconducting layer.

2. The superconducting article of claim 1, wherein the non-superconducting material is randomly distributed in the a-b plane of the superconducting layer.

3. The superconducting article of claim 1, wherein the non-superconducting material distributed preferentially along the a-b plane coplanar with the superconducting layer lacks a substantial vertically oriented component.

4. The superconducting article of claim 1, wherein the non-superconducting material is comprised of nano-particulates.

5. The superconducting article of claim 1, wherein the non-superconducting material is non-crystalline.

6. The superconducting article of claim 1, wherein the non-superconducting material is comprised of RE.sub.2O.sub.3 where RE includes one or more of the following elements: Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

7. The superconducting article of claim 1, wherein the non-superconducting material is comprised of BaMO.sub.3 where M includes one or more of the following elements: Ti, Zr, Al, Hf, Ir, Sn, Nb, Mo, Ta, Ce, and V.

8. The superconducting article of claim 1, wherein the buffer layer and high-temperature superconducting layers are selected to ensure a lattice mismatch between the two layers.

9. A method of forming a high-temperature superconductor, the method comprising providing a substrate; depositing a buffer layer upon the substrate; depositing a high-temperature superconducting layer upon the buffer layer, and co-depositing a non-superconducting material distributed preferentially along the a-b plane coplanar with the superconducting layer, wherein the non-superconducting material is randomly distributed and lacks a substantial vertically oriented component.

10. The method of claim 9, wherein the non-superconducting material is comprised of RE.sub.2O.sub.3 where RE includes or more of the following elements: Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

11. The method of claim 9, wherein the non-superconducting material is comprised of BaMO.sub.3 where M includes one or more of the following elements: Ti, Zr, Al, Hf, Ir, Sn, Nb, Mo, Ta, Ce, and V.

12. The method of claim 9, wherein the non-superconducting material is deposited by introducing an atomic excess of RE during co-deposition with the superconducting layer.

13. The method of claim 9, wherein the non-superconducting material is deposited by introducing an atomic excess of Ba and new element M where M includes one or more of the following elements: Ti, Zr, Al, Hf, Ir, Sn, Nb, Mo, Ta, Ce, and V during co-deposition with the superconducting layer.

14. The method of claim 9, wherein the buffer layer, high-temperature superconducting layer, and non-superconducting material are deposited by photo-assisted MOCVD (PAMOCVD).

15. The method of claim 14, further wherein the high-temperature superconducting layer growth rate is 1.0 μm/min or greater.

16. A thin film composite high-temperature superconducting article comprising: a substrate; a buffer layer; a high-temperature superconducting layer; non-superconducting material distributed preferentially along the a-b plane coplanar with the superconducting layer; and a lift factor at 4K, 20T (Ic (4K, 20T)/Ic (77K, self-field)) of 2 or greater.

17. The superconducting article of claim 16, wherein the non-superconducting material is randomly dispersed in the a-b plane of the superconducting layer.

18. The superconducting article of claim 16, wherein the non-superconducting material distributed preferentially along the a-b plane coplanar with the superconducting layer lacks a substantial vertically oriented component.

19. The superconducting article of claim 16, wherein the lift factor is 3 or greater.

20. The superconducting article of claim 16, wherein the superconductor further comprises a critical current (Ic) of 450 A/cm-width or higher at 4K and 20T.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

[0022] FIG. 1 shows the axes and planes of reference relative to a high temperature superconducting film.

[0023] FIG. 2 shows a prior art superconductor with vertical columns of defects.

[0024] FIG. 3 shows an exemplary architecture of a high temperature superconductor.

[0025] FIG. 4 shows a Transmission Electron Microscope image of an exemplary YBCO superconductor material with nanoparticles produced by methods of the present invention.

[0026] FIG. 5 shows the X-ray diffraction (XRD) pattern for an exemplary HTS YBCO material of the present invention.

[0027] FIG. 6A-C shows different aspects of an exemplary PAMOCVD deposition system.

[0028] FIG. 7 shows the Lift Factor performance for example HTS wires produced by the presently disclosed methods.

DETAILED DESCRIPTION OF EXAMPLES OF THE INVENTION

[0029] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to a high-temperature superconductor with Y.sub.2O.sub.3 non-superconducting centers deposited using the fabrication technique of photo-assisted MOCVD. However, the embodiments discussed herein are not limited to such elements.

[0030] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0031] Embodiments of REBCO HTS superconductor tapes and wires of the present invention may include nano-sized particles distributed within the a-b plane of the superconducting layer of the wire to provide high Ic at high magnetic fields. In this context, said particles within the a-b plane shall mean within the plane that is coplanar to the superconducting layer (see FIG. 1). Within the a-b plane may include particles fully encompassed by the superconductor layer or particles extending above, below or upon an upper or lower boundary defined by the superconductor alayer's upper and lower thin film boundaries, or at multiple positions with the a-b plane.

[0032] Preferred embodiments of methods for manufacturing high Ic wire according to the present disclosure do not require a c-axis oriented HTS layer, or preferentially vertically aligned columns of nanodots or nanorods, nor does it require preferentially vertically aligned second phases, dopants or stacking faults as described by the prior art.

[0033] Embodiments of the methods disclosed herein yield non-superconducting nanoparticles being substantially preferentially distributed along the a-b plane within the HTS layer with no specific vertical or near vertical alignment. The presently disclosed nanoparticles captured in the a-b plane within the HTS layer and without specific vertical (c-axis) orientation can be used to obtain high Ic, at high magnetic fields and low temperatures. The reduction in criticality of nanoparticle c-axis alignment in-turn reduces cost as the production process is not restricted due to the need to orient the crystals nor the requirement to introduce rod shaped non-superconducting dopants into the process.

[0034] The epitaxial REBCO high temperature superconductor (HTS) wire is processed in certain preferred embodiments by using Metal Organic Chemical Vapor Deposition (MOCVD) or other suitable deposition process known in the art of superconductor fabrication. The wire typically has a thin film composite architecture an example of which is shown in FIG. 3. In this example, the architecture includes a substrate 300, at least one buffer layer (two are shown in this example as 310 and 320), at least one superconducting layer (one is shown in this example as 330), and at least one capping or stabilizing layer 340. Other layers are readily contemplated by those skilled in the art and may provide additional purpose to the basic architecture described herein.

[0035] The architecture may have a crystal orientation where the a and b axis are oriented along the surface of the film while the c axis is oriented normal to the film surface. This crystallographic orientation of the REBCO layer is typically obtained by using an atomically textured substrate 300 comprised of a metal foil with one or more atomically textured buffer layer(s) deposited on the metal.

[0036] The substrate metal 300 is typically in the form of a flexible foil or tape and is typically comprised of a metal-based alloy including but not limited to stainless steel alloys and nickel-based metal alloys. The metal-based substrates may have a tape structure with a high relative width and length as compared to its thickness. A typical width may be 12 mm, but can be more than 100 mm, while the length may be 100's of meters long and above. This metal substrate 300 may be processed to form a biaxial texture via use of rolling assisted biaxially textured substrate (“RABiTS”) process or other process known in the art to be suitable for texturing a metal substrate.

[0037] In certain preferred embodiments, the metal layer is non-textured, for example Hastelloy, Inconel or other alloy and instead of directly texturing the metal surface, the substrate metal layer 300 may have at least one or more deposited buffer layer(s) 310 and 320 atop the metal layer 300 that is/are biaxially textured. Such layers provide that the buffer's crystalline axes are aligned in plane and normal to the metal layers surface.

[0038] Deposition based biaxial texturing of the buffer layer or layers may be achieved via Ion Beam Assisted Deposition (IBAD), Pulsed Laser Deposition (PLD), or Inclined Substrate Deposition (ISD) or other methods. The biaxially textured film may have a rock salt (halite) like crystal structure. The biaxial texturing is necessary for proper crystallographic alignment of the REBCO superconductor layer when deposited upon the substrate 300 for optimum superconducting performance. The buffer material may be specified to ensure a desired lattice mismatch between the buffer (310, 320) and the REBCO HTS layer 330 to foster development of nanoparticles to be discussed later.

[0039] The high-temperature superconducting (HTS) layer 330 is typically comprised of HTS materials known in the art capable of generating superconducting behavior at 77K or above which corresponds to the temperature of liquid nitrogen. Suitable materials may include YBa.sub.2Cu.sub.3O.sub.7−x (YBCO) or BiSr.sub.2CaCu.sub.2 among others. Other stoichiometries of YBCO are known, including but not limited to Y.sub.2Ba.sub.4Cu.sub.7O.sub.14+x, YBa.sub.2Cu.sub.4O.sub.8 and others, which are also contemplated by the present disclosure and which are generally and henceforth will be referred to as YBCO material. In other embodiments, other rare earth elements may be substituted in place of Y, generally referred to as the family of materials REBa.sub.2CusO.sub.7−x (REBCO) where RE may include Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu.

[0040] For second generation (2G) high temperature superconductors (HTS), the flux pinning force is related to the density, size and dimensionality of the defects introduced. In preferred embodiments, the non-superconducting flux pinning particles are randomly dispersed within the superconducting layer. The material composition of the non-superconducting flux pinning sites can include but are not limited to RE.sub.2O.sub.3 and BaMO.sub.3. For RE.sub.2O.sub.3, RE may include Y, La, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu. In the case of BaMO.sub.3, BaMO.sub.3 nanoparticle formation in REBCO requires the additional element of M where M includes one or more of the following elements: Ti, Zr, Al, Hf, Ir, Sn, Nb, Mo, Ta, Ce, and V.

[0041] FIG. 4 shows a Transmission Electron Microscope image of an exemplary YBCO superconductor 400 material with non-superconducting nanoparticles 410 produced by methods of the present invention. The nanoparticles 410 are distributed along the crystallographic a-b plane 420 instead of the c-axis 430. In this example, note that the sample was prepared by cross-sectioning an HTS tape, thus exposing the HTS layer's depth (c-axis). Given that the layer may be in certain embodiments 10s to 1000s or more nanometers in thickness; more than one sub-plane within the a-b plane may be apparent. In this example, non-superconducting nanoparticles appear as dark striations randomly distributed within the a-b plane of the HTS superconductor. Also, in this example, with the c-axis out of the page, there is no observably substantial orientation nor distribution of non-superconducting particles along the c-axis 430 normal to the a-b plane 420.

[0042] FIG. 5 shows the X-ray diffraction (XRD) pattern 500 for an exemplary HTS YBCO material of the present invention that show high intensity (Y-axis 510) characteristic diffraction peaks 530 and 540 corresponding to the signature 2-theta (2Θ) positions (X-axis 520) of Y.sub.2O.sub.3 in the YBCO superconductor material 330. The XRD data demonstrates that the nanoparticles found distributed along the a-b plane 420 are composed of Y.sub.2O.sub.3 non-superconducting material.

[0043] The size of the non-superconducting flux pinning particles can range up to 100 nm or larger in diameter. RE.sub.2O.sub.3 nanoparticles form within the a-b planes of the REBCO layer without the need of additional elements beyond those typically contained in the precursor vapor source for growing REBCO superconducting material. Thus, in preferred embodiments, the non-superconducting flux pinning particles are co-deposited with the superconducting material without introduction of foreign material. It is a further feature of the presently disclosed superconducting wire and fabrication methods that the particles lack a substantial c-axis orientation.

[0044] The formation of these a-b plane distributed nanoparticles can be achieved in certain preferred embodiments using a Photo-Assisted Metal Organic Chemical Vapor Deposition (PAMOCVD) process without reducing the growth rate as commonly occurs with other growth methods that yield preferentially vertically orientated nanoparticles as shown in FIG. 6.

[0045] FIG. 6A shows an exemplary PAMOCVD system whereby the application of UV and visible light provides the energetic source to the reaction process to increase the mobility of the incoming atoms to form the non-superconducting nanoparticles during the deposition and distribution of both non-superconducting and superconducting material. The UV/visible radiation source 610 is typically enclosed within a low-pressure reaction chamber or vessel 620 maintained at a target pressure by one or more external vacuum pumps 630. The source 610 may be comprised of one or more lamps emitting a desired wavelength or range of wavelengths. The lamps may be arranged adjacent to or in proximity to the inlet showerhead 640 which provides injection of precursor from a feed line 650 for the precursor starting material. The source 610 is typically focused onto the growth surface of the moving metallic foil substrate 300. Such substrate is commonly provided in a reel to reel continuous feed system with the substrate passing through slits 660 in the walls of the reaction vessel 620.

[0046] The REBCO deposition surface in certain preferred embodiments is continually irradiated by the UV/visible radiation flux from the radiation source 610 while the REBCO film is growing with the radiation striking the tape substrate onto which a REBCO film is being grown at a substantially normal incident angle as shown in FIG. 6B. Normal orientation of radiation yields the highest radiation density at the surface as any off-normal radiation configuration yields a lower radiation density. When a radiation source or sources 610 are arranged in a hemispherical pattern around the inlet showerhead 640, the exposure may have both perpendicular and non-zero angular radiation striking the surface as shown, for example, in FIG. 6C.

[0047] The UV/visible radiation at the surface of the growing film energetically excites the surface atoms to enhance their surface mobility thus allowing for more rapid attainment of their lowest energy configuration consequently yielding highly crystalline structure for the growing film. It is this highly crystalline structure in the a-b plane (i.e predominantly within the plane of the substrate) for REBCO that promotes high current capacity and high performance. Further, the localization of the energy which is promoting growth of the REBCO film at the growth surface by supplying the energy from above the growing film eliminates any thermal lag associated with supply of energy from below the tape substrate as in the use of typical heated substrate susceptors.

[0048] The UV/visible radiation present at the growth surface of the growing REBCO layer greatly enhances the growth rate of highly textured REBCO. Rates of 1.2 microns/min (μm/min) or higher are possible while maintaining high performance quality of the REBCO tape. The high growth rates are proposed to be due to physio-chemical effects including surface diffusion enhancement of the alighting elements forming the REBCO unit cell on the buffer layer surface. It is important to note that for REBCO films, high performance (high current carrying capacity) is mainly defined by the atomic order of the atoms in the growing REBCO film. Enhancing diffusion of the atoms by UV/Visible radiation as they alight onto the growth surface allows for more rapid movement of atoms to their lowest energy positions on the surface, i.e., enhancing growth of a highly crystalline surface which is required for high performance REBCO films.

[0049] As stated above, the direct radiation exposure of the growth surface results in REBCO (for example, YBCO) films that can be grown at rates of 1.2 μm/min or higher and as low as 0.01 μm/min, if desired. The REBCO films are grown with a high degree of texturing as defined by x-ray diffraction parameters of Δϕ between 2° and 7°, and Δω between 1° and 4° in certain preferred exemplary embodiments. The performance of the resulting exemplary YBCO wires or tapes as measured by their current carrying capacity may exceed 500 A/cm-width or higher at 77K. Such high growth rates allow for industrial production of high performance REBCO wire with commercially attractive economics.

[0050] In other preferred embodiments, the flow rates and stoichiometry of the starting precursor material is controlled in order to co-produce RE.sub.2O.sub.3 or BaMO.sub.3 nanoparticles in the REBCO film for flux pinning. The growth rate is adjusted by control of precursor flow rates, and source energy inputs to ensure proper quantity, size and distribution of nanoparticles. Additionally, the stoichiometry of MOCVD precursor vapor contributes to the determination the composition of the secondary phase non-superconducting particles which act as the pinning centers. The non-superconducting particles of the present invention may in certain embodiments be generated by adding an excess of RE precursor or excess of Ba and introducing new M precursor into the vapor flow.

[0051] In one exemplary embodiment, Y.sub.2O.sub.3 non-superconducting particles are co-deposited in the YBCO as flux pining centers via PAMOCVD processing resulting in 20 atomic % excess Yttrium in the end coating. The deposition growth rate of HTS material in this example was approximately 0.2 μm/min upon a CeO.sub.2 capped IBAD buffered substrate. In another embodiment YBCO is deposited with 40 atomic % excess Yttrium in the coating. The deposition growth rate of HTS material in this example was approximately 0.25 μm/min upon a LaMnO.sub.3 capped IBAD buffered substrate.

[0052] As mentioned above, the density of pinning centers is an important factor in determining performance, and critical current may be limited by a low density of pinning centers. The presently disclosed methods permit tailoring performance by targeting specific densities of pinning centers via control of the above described process parameters. Presently disclosed non-superconducting particles deposited in the a-b plane without specific c-axis orientation at high growth rates permits significant gains in production rates and cost efficiencies.

[0053] As discussed above, the YBCO HTS material and with the non-superconducting flux pining centers can be produced by MOCVD from the precursor feed according to the following reactions:

Y.sub.2O.sub.3+4BaCO.sub.3+6CuO.fwdarw.2YBa.sub.2Cu.sub.3O.sub.6.5+4CO.sub.2

Y.sub.2O.sub.3(XS)+4BaCO.sub.3+6CuO.fwdarw.2YBa.sub.2Cu.sub.3O.sub.6.5+4CO.sub.2+Y.sub.2O.sub.3

[0054] Known systems for precursor delivery include gas, liquid, solid, and slurry-based approaches. In preferred embodiments using MOCVD, and in particular PAMOCVD based depositions, the precursors may be delivered as metal-organic compounds either as flash evaporated solids or as solvated vapor phase molecules using tetrahydrofuran (THF) or other suitable organic solvent.

[0055] The crystal structure of REBCO whereby the CuO.sub.2 component in the unit cell may be comprised of 2-dimensional planes flanking the RE atom also further enhances the forming of non-superconducting nanoparticles (principally oxide particles) between the two CuO.sub.2 planes as they can act as a capture mechanism for the particles in certain embodiments.

[0056] An important performance metric for this wire is to attain high critical current with the wire containing nanoparticles in the HTS layer for flux pinning which are distributed along a-b planes in the HTS layer with no specific vertical or near vertical alignment. Critical currents greater than 450 A/cm-width and 0.11 mm HTS tape thickness can be obtained at 4K and 19T when the magnetic field is perpendicular to the tape surface (H//c).

[0057] The Table below shows the Ic (A) measurement at 4K for magnetic fields perpendicular to the tape surface according to an embodiment of the present invention comprising the superconducting material having the composition YBa.sub.2Cu.sub.3O.sub.7−x. The tape width is 4 mm and the corresponding Ic/cm-width at 19T is, 463.8 A/cm-width in this embodiment.

TABLE-US-00001 TABLE 1 H(T) Ic(A) 2.0 498.4 4.0 404.9 6.0 353.8 8.0 312.2 10.0 280.7 12.0 252.9 13.0 239.6 14.0 229.0 15.0 218.3 16.0 209.3 17.0 200.7 18.0 192.1 19.0 185.5

[0058] The performance of the HTS wire in magnetic field is also often characterized by a measure commonly referred to as Lift Factor. The Lift Factor is typically defined as the ratio between the critical current at 77K, self-field and a separate temperature and field such as 4K and 20T. Unlike the critical current, which is an absolute value, the Lift Factor provides the relative relation of the two values. The wires of certain exemplary embodiments of the present disclosure have demonstrated lift at 4K, 20T (Ic (4K, 20T)/Ic (77K, self-field)), which corresponds to a Lift Factor of 2 or greater.

[0059] The ability to maintain high critical current performance at high growth rate is crucial towards commercial viability of HTS products. The thickness of the REBCO superconductor layer can have a growth rate of 0.2 μm/min, 1.0 μm/min, 1.2 μm/min, 1.5 μm/min and higher while retaining high flux pinning resulting in critical currents (Ic) above 450 A/cm-width at 4K and 20T and a corresponding engineering critical current density J.sub.E of 40,000 A/cm.sup.2 or greater, where the engineering critical current density J.sub.E is defined as the critical current Ic divided by the total cross-sectional area of the HTS layer.

[0060] FIG. 7 shows the Lift Factor performance for example HTS wires produced by the presently disclosed methods. The Lift Factor (Y-axis 710) Ic measured at 4.2K compared to Ic at 77K, self-field for a range of B field strengths measured in units Tesla (X-axis 720).