1. Field of Invention
The invention relates to the field of thin films of iron-based superconductors and, in particular, to thin films of these superconductors on textured substrates. The invention also relates to methods of fabricating thin films of iron-based superconductors on textured substrates.
2. Background of the Related Art
High field applications of superconductors have been dominated by Nb3Sn, a material which allows magnetic fields up to 20 T to be achieved at 4.2 K. However, Nb3Sn wires typically require a post-winding heat-treatment, which is a technically-challenging manufacturing step. Although high temperature superconducting oxides (HTS) offer excellent superconducting properties, their characteristically high anisotropies and brittle textures, in addition to the high manufacturing costs, have limited their applications. In 2008, a new family of iron-based superconductors was found, which are semi-metallic low anisotropy materials with transition temperatures, Tc's, up to 55 K (Ren, et al. Europhys. Lett. 83, 17002 (2008); incorporated herein by reference in its entirety). The combination of extremely high upper critical fields Hc2(0) (˜100 T), moderate anisotropies of Hc2ab/Hc2a, and high irreversibility fields, Hirr, makes this class of superconductors appealing for high field applications.
These iron-based superconductors can further be divided into those that belong to iron pnictides ((LaFeAsO, SrFe2As2, BaFe2As2, etc.) and those that belong to iron chalcogenides (FeTe, FeSe, etc.). Both have very attractive properties. A more detailed discussion of iron-based superconductors is provided in Balatsky et al. (Physics 2, 59 2009) and Xia et al. (Phys. Rev. Lett. 103, 037002, 2009). Each of the aforementioned publications is incorporated by reference in its entirety as if fully set forth in this specification.
However, chalcogenides hold several practical advantages over the pnictides. Although the Tc's of chalcogenides are typically below 20 K, they exhibit lower anisotropies ˜2 with Hc2(0)' s approaching 50 T. The exceptionally high upper critical magnetic fields of chalcogenides are important for high-field applications such as MRI magnets and accelerator magnets. They also have the simplest structure among the iron-based superconductors and contain only two or three elements, which greatly simplifies their handling, unlike pnictides that contain toxic arsenic.
A lot of effort has gone into making high quality thin films of such materials. However, these films were made on crystalline substrates, which cannot be used to make superconducting tapes or wires for large scale applications. For practical applications, superconductors of this class must be made on substrates, which provide support and can be made in long tapes or wires. However, due to the lattice mismatch between these substrates and those materials, it is very difficult to grow such films.
There is therefore a continuing need to develop manufacturing methods that would allow the formation of iron-based superconductors such as iron chalcogenides and iron pnictides into films, wires or tapes that can be used for industrial and research use, e.g., to wind superconducting magnets.
Recognizing the challenges of obtaining high-quality thin films of iron-based superconductors on substrates, the technology described herein offers a way of fabricating thin films of iron chalcogenide- and iron pnictides-based superconductors on textured substrates and discloses structures that result from employing the technology.
Thus, in some embodiments, growth of iron-based superconductors on textured substrates is described. In some embodiments, the iron-based superconductors are iron chalcogenide-based superconductors, while in other embodiments, the iron-based superconductors are iron pnictides-based superconductors. The textured substrates preferably have similar in-plane lattice constants as the superconductors, although it is especially preferred if the textured substrates are nearly lattice-matched to the in-plane lattice constants of the superconductors.
In some embodiments, the iron-based superconductors are iron chalcogenides that comprise FezSexTe1-x, where 0≦x≦1 and 0.7≦z≦1.3. In some embodiments, the superconducting material comprises FeSySexTe1-x-y, where 0≦x+y≦1. In some cases, the iron chalcogenide superconductor is doped with various dopants, including oxygen.
In some embodiments, the iron-based superconductor is an iron pnictide, either an oxypnictide or a non-oxypnictide. The iron-oxypnictide can be expressed as M—FeyAsO1-xFx, where 0≦x≦1, 0.4≦y≦1.6 and M is one or more of rare-earth metals selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, one or more of alkali metals selected from Li, Na, K, Rb, or Cs, or one or more alkali-earth metals selected from Be, Mg, Ca, Sr, or Ba although La is preferred. The stoichiometric composition of M is preferably 1, e.g., La 0.5Y0.5. The iron-nonoxypnictide can be expressed as M—FeyAsxFz, where 1≦x≦2, 0.6≦y≦2.0 and 0≦z≦1. As with iron-oxypnictides, M for iron-nonoxypnictides is selected from one or more rare-earth metals, one or more alkali metals, or one or more alkali-earth metals. In some cases, the iron pnictide superconductor may be doped with various dopants, preferably fluorine.
In some embodiments, substrates comprise layers of buffer materials that improve the texture of the base to render it more suitable for formation of iron-based superconductor films thereupon. In some cases, a single layer of buffer material is used; in other cases, multiple layers of buffer materials are used.
In some embodiments, use of magnesium oxide (MgO) as a buffer layer is described. In other embodiments, cerium oxide (CeO2) is used to texture the surface of a substrate. In yet other embodiments, textured substrates comprise layers of yttrium oxide (Y2O3) and/or yttria-stabilized zirconia (YSZ).
In some embodiments, methods for fabricating thin films of iron-based superconductors on buffered metal substrates are presented. In some embodiments substrates comprise oxides, polymers, including metallized and conducting polymers, and/or semiconductors.
In some embodiments, the superconducting thin films retain their inherent superconducting properties, including critical electrical currents, critical magnetic fields, and critical superconducting transition temperatures, and these properties are on par with those of films of similar composition and thickness to films grown on single-crystal substrates. In some cases, the superconducting properties of the thin films are better than those of bulk materials having the same composition.
In some embodiments, thin films of iron-based superconductors such as iron chalcogenide-based superconductors, on textured metal substrates are described.
In some embodiments, the superconducting structures described may be used in magnetic, electronic, and superconducting devices.
It should be understood that the foregoing, being a summary, is necessarily a brief description of some aspects of the invention, which may be better understood with reference to the drawings and the following detailed description.
As is common practice in the art, the following figures may not be drawn to scale. Schematic depictions are used to emphasize the particular features of the invention and as a reference for their description.
a is a plot that shows Jc's of FeSe0.5Te0.5 films on LAO substrate at various temperatures with magnetic field parallel (solid symbols) and perpendicular (open symbols) to the ab plane (tape surface).
b is a plot that shows Jc's of FeSe0.5Te0.5 films on IBAD coated conductor at various temperatures with magnetic field parallel (solid symbols) and perpendicular (open symbols) to the ab plane (tape surface).
a is a plot that shows Jc at about 4.2 K of FeSe0.5Te0.5 films compared with the data of 2G YBCO wire, TCP Nb47Ti and Nb3Sn. For YBCO and FeSe0.5Te0.5 the field direction is parallel to the c-axis.
b is a plot that shows volume pinning force Fp at about 4.2 K of FeSe0.5Te0.5 films compared with the data of 2G YBCO wire, TCP Nb47Ti and Nb3Sn. For YBCO and FeSe0.5Te0.5 the field direction is parallel to the c-axis. Solid lines are Kramer's scaling approximations.
The method described herein offers a way of fabricating thin films of iron-based superconductors, such as iron chalcogenides and pnictides, on textured substrates, although iron chalcogenides are preferred because they do not contain a toxic arsenic component. Preferably, the intrinsic electronic and magnetic properties of the superconducting structure generated by the disclosed method(s) are at least on par with those of a thin film of iron-based superconductor with the same composition and thickness formed on a bulk single crystal substrate.
Generally, the method encompasses preparing a textured substrate having an in-plane lattice constant, i.e., the distance between unit cells in a crystal lattice, similar to, or preferably closely lattice-matched with, the in-plane lattice constant of the superconductor, and forming a film of iron-based superconductor on the textured substrate, preferably by pulsed laser deposition. As provided in the specification, the term “similar” may be interpreted as having a mismatch of no more than ±10%, while a mismatch of less than ±5% is considered to be closely matched and is more preferred. Alternatively, it is preferred to have closely matched lattice constant value defined as being within ±0.2Å, although it is more preferable to have the lattice constant values within ±0.1Å.
In a preferred embodiment, the textured substrate is prepared by depositing a buffer layer on a base of the substrate in order to provide a template for growth of high-quality thin films of iron-based superconductors on the surface of the base layer.
Throughout this specification, the superconducting structures and processes for their manufacture are described with reference to one or more most preferred embodiments. However, it is to be understood that those skilled in the art may develop other combinatorial, structural, and functional modifications to the disclosed techniques of fabricating thin films of iron-based superconductors, e.g., iron chalcogenides, on textured substrates without significantly departing from the spirit and scope of the disclosed invention.
To maintain the superconducting properties of the iron-based superconducting material, the substrates should be chosen to have an in-plane lattice constant similar, or alternatively closely lattice-matched, to the in-plane lattice constant of the superconductor and preferably shaped into a ribbon, a tape or a wire. In a preferred embodiment, the substrate includes a base and a buffer, although the substrates only having a base textured to be similar to or to more closely match the in-plane lattice constant of the superconductor material are also envisioned. If the substrate has the base and the buffer, any compound can be used as the base material since the surface texture is created by the buffer. Examples of appropriate substrates include oxides, semiconductors, metallized and conducting polymers, and metals whose surfaces have been textured using buffer materials to have a similar or closely matched in-plane lattice constant of the superconductor material. The substrates may also be flexible and polycrystalline in nature. In a preferred embodiment, nickel and Ni alloys, such as Hastelloy® superalloys (Haynes Inter. Inc., Indiana), may be selected for their formability.
For use in electronic devices that use a planar configuration, silicon, silicon dioxide, silicon nitride, and glass may be useful when their surface is textured by deposition of an appropriate buffer material.
The buffer layer is selected to provide a template for growth of high-quality thin films of iron-based superconductors. These materials should have a lattice constant close to that of iron-based superconductors. Examples of suitable compounds that may function as a buffer layer to provide a template for growth of iron-based superconductors include, but are not limited to, oxides, such as magnesium oxide (MgO), yttria-stabilized zirconia (YSZ), ceria (CeO2), yttria (Y2O3), and a combination thereof. Preferably, the buffer layer has a thickness between 1 nm and 10 μm.
The buffer layer may be deposited on the substrate by any suitable method known in the art to produce layers having the desired properties. Preferably, the buffer layer may be deposited on the substrate by either a rolling-assisted biaxially textured substrate (RABiTS) technique or an ion beam-assisted deposition (IBAD) technique. The buffer material may be deposited in a single layer on which the iron-based superconductor is grown. In alternative, it may be deposited in a multilayer of the same or different buffer material to maintain high quality growth of the final layer, on which the iron-based superconductor is grown. In certain embodiments, several different layers of buffer materials may be necessary in order to maintain the best lattice match on substrates such as a metal or metal alloy. For example, in rolling-assisted biaxially textured substrate (RABiTS) or ion beam-assisted deposition (IBAD), yttria stabilized zirconia (YSZ) and ceria (CeO2) may be used in series to form a much better buffer layer between the underlying metal of the substrates and the superconducting thin films, because CeO2 is more closely lattice-matched with the superconductor and it is easier to form a textured structure of YSZ on metal or alloy substrates.
The buffer layer must also be grown in texture (biaxially aligned) on the selected substrates. For example, CeO2 is fairly closely lattice-matched to FeSe0.5Te0.5, one of the iron-based superconductors having a relatively high superconducting transition temperature (Tc) and very large upper critical magnetic fields (Hc2). In a preferred embodiment, it can be grown in texture on Ni or Ni alloy using RABiTS or IBAD.
In an exemplary embodiment with reference to
The iron-based superconductors generated on the textured substrate by the disclosed method can be selected from iron chalcogenides or iron pnictides.
The iron chalcogenide based superconductors generated on the textured substrate by the disclosed method have a general formula FezSexTe1-x, where 0≦x≦1 and 0.7≦z≦1.3. In other embodiments, the iron chalcogenide based superconductors generated on the textured substrate by the disclosed method have a general formula FeSySexTe1-x-y, where 0≦x+y≦1. Examples of such superconductors include, but are not limited to, FeTe, FeSe, FeSe0.5Te0.5, although, FeSe0.5Te0.5.is being preferred. The iron chalcogenide superconductor may also be doped with various dopants, although oxygen (e.g., FeTe:Ox) is preferred. For example, oxygen doping may be accomplished under oxygen pressure, during growth, of between 10−2 to 10−7 Torr, more preferably between 10−3 to 10−6 Torr, and most preferably under pressure of about 10−4 Torr.
The iron pnictides based superconductors generated on the textured substrate by the disclosed method may be selected from oxypnictide or non-oxypnictide. The iron-oxypnictide can be expressed as M—FeyAsO1-xFx, where 0≦x≦1, 0.4≦y≦1.6 and M is one or more of rare-earth metals selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, one or more of alkali metals selected from Li, Na, K, Rb, or Cs, or one or more alkali-earth metals selected from Be, Mg, Ca, Sr, or Ba, although, La is being preferred. The stoichiometric composition of M is preferably 1, e.g., La0.5Y0.5. The iron-nonoxypnictide can be expressed as M—FeyAsxFz, where 1≦x≦2, 0.6≦y≦2.0 and 0≦z≦1. As with iron-oxypnictides, M for iron-nonoxypnictides is selected from one or more rare-earth metals, one or more alkali metals, or one or more alkali-earth metals. Examples of iron pnictides include LaOFeAs, LiFeAs, and BaFe2As2. Similar to iron chalcogenide, the iron pnictide superconductor may also be doped with various dopants, although fluorine is preferred.
In the exemplary embodiment, the iron chalcogenide based superconductor may be fabricated on the surface of the textured substrate by any suitable method known in the art to produce layers having the desired properties. In a preferred embodiment, the iron chalcogenide based superconductor is deposited by pulsed laser deposition (PLD). In an exemplary embodiment, the iron chalcogenide based superconductor, e.g., FeSe0.5Te0.5, may be fabricated by placing the substrate into a deposition chamber; evacuating the deposition chamber to a pressure of about 10−6 Torr; heating the substrates to between 350° C. and 450° C.; hitting a target of a desired iron chalcogenide composition with a laser beam for a selected time period, where the laser beam has an energy density of about 3 J/cm2 and a repetition rate of about 5 Hz; and turning off the substrate heater. The target of the desired iron chalcogenide may be prepared by inductive melting of Fe, Se, Te of desired stoichiometry at 650-750° C. Alternatively, without departing from the scope and spirit of the disclosed invention, the iron chalcogenide can be substituted with iron pnictide in the above described method.
The films depicted in
The templates were manufactured in two steps. First, an Y2O3 layer was made on unpolished Hastelloy® by sequential solution deposition to reduce the roughness of the tape surface, then a bi-axially textured MgO layer was deposited on top by the IBAD technique. (Matias, et al. J. Mater. Res. 24, 125 (2009); incorporated herein by reference in its entirety.) The very high tensile strength of Hastelloy® C-276 (0.8 GPa) allows the composite conductor to withstand the very high Lorentz force stresses produced by the 20-30 T magnetic fields.
Resistivity was measured by the standard four-probe method in a physical property measurement system (Quantum Design, PPMS) and magnetization was measured in a superconducting quantum interference device (Quantum Design, MPMS).
The conformation of the crystal lattice of the FeSe0.5Te0.5 superconductors grown by PLD on the STO substrate was studied using X-ray diffraction spectroscopy.
The iron chalcogenide FeTe superconductor was prepared with and without oxygen doping (Fe1.08Te:Ox).
In
It was reported that the grain boundary in a Ba(Fe1 xCox)2As2 system could reduce the Jc significantly. Without being bound by theory, the results seem to suggest that the grain boundaries in iron chalcogenides may behave differently, since they do not have a charge reservoir layer as in cuprates or Ba(Fe1 xCox)2As2, where carrier depletion occurs. Measurements of FeSe0.5Te0.5 films grown on bi-crystalline substrates are most desirable to provide direct information on the misorientation angle dependence of Jc.
It is also possible that the relatively lower Jc's in IBAD films is simply due to the lower Tc's compared to those of the films on LAO, a result of the larger lattice mismatch between Mg0 and FeSe0.5Te0.5. An additional buffer layer of CeO2, which has a better lattice match with FeSe0.5Te0.5, may improve the Tc, and hence raise the Jc. Alternatively, the elaborate oxide buffer structure, partially designed to protect the metal template from oxidation for 2G HTS wires, may not be needed since FeSe0.5Te0.5 is made in vacuum. Growing a FeSe0.5Te0.5 coating directly on textured metal tapes may be possible, potentially simplifying the synthesis procedure with a reduction of production costs. Wire applications require much thicker (over several lm) films, which may be grown by using a more scalable deposition technique, such as a low-cost web-coating process for 2G HTS wire.
In
While the foregoing description has been made with reference to individual embodiments of the invention, it should be understood that those skilled in the art, making use of the teaching herein, may propose various changes and modifications without departing from the invention in its broader aspects.
The foregoing description being illustrative, the invention is limited only by the claims appended hereto.
This invention was made with Government support under contract number DE-AC 02-98CH10886, awarded by the U.S. Department of Energy. The Government has certain rights in the invention.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US11/46312 | 8/2/2011 | WO | 00 | 4/12/2013 |
Number | Date | Country | |
---|---|---|---|
61370165 | Aug 2010 | US |