Superconducting wire

Abstract

A superconducting wire 1 is provided with a superconducting layer 12 including a first superconducting layer 121 and a second superconducting layer 122 that are layered in arbitrary order without any other layer, wherein the first superconducting layer 121 has columnar pinning centers and the second superconducting layer 122 has point pinning centers.

Description

SUPERCONDUCTING WIRE

[0001] The present invention relates to a superconducting wire.

[0002] When a superconducting wire that can conduct superconducting current is used in an environment where a magnetic field exists, the magnitude of the critical current varies depending on the angle of the magnetic field penetrating the superconducting wire. In order for superconducting wires to be widely used in various technological applications, the dependency of the critical current on the angle of the magnetic field, i.e., the anisotropy of the critical current, must be reduced.

[0003] Recently, attempts have been made to control the anisotropy of the critical current of REBCO superconducting thin films by including Y2O3, Y2BaCuO5(Y211), BaHfO3, and BaSnO3in the same YBCO matrix (see Non-Patent Literatures 1 and 2). Here, REBCO is a copper oxide superconductor expressed by a composition formula REBa2Cu3Oy(RE is a rare earth element).

[0004] In recent years, attempts have been made to control the anisotropy of the critical current of the REBCO superconducting thin films by adjusting the irradiation energy, direction of incidence, and other conditions to implement heavy ion irradiation (see Non-Patent Literatures 3 and 4).

[0005] A. K. Jha et al., “Controlling the Critical Current Anisotropy of YBCO Superconducting Films by Incorporating Hybrid Artificial Pinning Centers”, IEEE Trans. Appl. Supercond., 26, 8000404 (2016)A. K. Jha et al., “Isotropic enhancement in the critical current density of YBCO thin films incorporating nanoscale Y2BaCuO5 inclusions”, J. Appl. Phys., 122, 093905 (2017)B. Gautam et al., “Microscopic adaptation of BaHfO3 and Y2O3 artificial pinning centers for strong and isotropic pinning landscape in YBa2Cu3O7-x thin films”, Supercond. Sci. Technol. 31, 025008 (2018).N. M. Strickland et al., “Near-isotropic enhancement of the 20 K critical current of REBa2Cu3O7 coated conductors from columnar defects”, Supercond. Sci. Technol. 36, 055001 (2023).

[0006] The object of the present invention is to provide a superconducting wire in which the anisotropy of the critical current of the superconducting layer is reduced by an unconventional method.

[0007] To achieve the above object, on aspect of the present invention provides the superconducting wires as described below.

[0008] [1] A superconducting wire having a superconducting layer including a first superconducting layer and a second superconducting layer layered in arbitrary order without any other layer, wherein the first superconducting layer has columnar pinning centers and the second superconducting layer has point pinning centers. [2] The superconducting wire according to [1], wherein the first superconducting layer includes REBa2Cu3O6+y(0≦y≦1) (RE is a rare earth element) as a base material and contains BaMO3(M is Hf, Zr, or Sn) as an impurity, and the second superconducting layer includes REBa2Cu3O6+y(0≦y≦1) (RE is a rare earth element) as a base material and contains RE2O3(RE is a rare earth element) as an impurity. [3] The superconducting wire according to [1], wherein the first superconducting layer includes EuBa2Cu3O6+y(0≦y≦1) as a base material and contains BaHfO3as an impurity, and the second superconducting layer includes YBa2Cu3O6+y(0≦y≦1) as a base material and contains Y2O3as an impurity, or includes NdBa2Cu3O6+y(0≦y≦1) as the base material and contains Nd2O3as the impurity. [4] The superconducting wire according to any one of [1] to [3], wherein the superconducting layer includes an alternating laminate structure in which the first superconducting layer and the second superconducting layer are alternately layered. [5] The superconducting wire according to any one of [1] to [3], wherein when a magnetic field with a flux density of 8T or less is applied to the first superconducting layer and the second superconducting layer under a temperature of 65K, the critical current of the first superconducting layer when the magnetic field is perpendicular to the surface of the first superconducting layer is greater than the critical current of the first superconducting layer when the magnetic field is parallel to the surface of the first superconducting layer, and the critical current of the second superconducting layer when the magnetic field is parallel to the surface of the second superconducting layer is greater than the critical current of the second superconducting layer when the magnetic field is perpendicular to the surface of the second superconducting layer.

[0009] According to the present invention, it is possible to provide a superconducting wire with reduced anisotropy of critical current in the superconducting layer by an unconventional method.

[0010] Fig. 1 is a vertical cross-sectional view schematically showing a configuration of a superconducting wire according to an embodiment of the present invention.Fig. 2 is a graph showing the dependency of a critical current Icon a magnetic field angle Θ of an EBHO / YBCO sample and two comparative samples under a magnetic field of 1.6 T and a temperature of 65 K.Fig. 3 is a graph showing the dependency of the critical current Icon the magnetic field angle Θ of an EBHO / NdBCO sample and two comparative samples under a magnetic field of 1T and a temperature of 77.3K.Fig. 4 is a graph showing the critical current Icmeasured under a temperature of 65 K at different magnetic field strengths and angles Θ applied to an EBHO sample.Fig. 5 is a graph showing the dependency of an anisotropy A of the critical current of the EBHO sample on the strength of the magnetic field at a temperature of 65 K.

[0011] (Configuration of a superconducting wire) Fig. 1 is a vertical cross-sectional view schematically showing a configuration of a superconducting wire 1 according to an embodiment of the present invention. The superconducting wire 1 is a tape-like wire capable of conducting a superconducting current and includes a substrate 10, a buffer layer 11 formed on the substrate 10, and a superconducting layer 12 formed on the buffer layer 11.

[0012] The substrate 10 is a tape-like substrate composed of an alloy or metal such as Hastelloy (registered trademark). For example, the substrate 10 has a thickness of 40 to 60 μm.

[0013] The buffer layer 11 serves as a buffer between the substrate 10 and the superconducting layer 12 and includes at least one layer selected from the group consisting of an Al2O3layer, a Y2O3layer, and a MgO layer, and preferably, two or three layers selected from the group. The buffer layer 11 has a thickness of, e.g., 300 to 500 nm. The buffer layer 11 is formed, e.g., by pulsed laser deposition (PLD).

[0014] In the example shown in Fig. 1, the buffer layer 11 comprises a laminate structure of an Al2O3layer 111, a Y2O3layer 112, an IBAD-MgO layer 113, and a LaMnO3layer 114. The IBAD-MgO layer 113 is consists of a MgO film formed by IBAD (Ion Beam Assisted Deposition).

[0015] The superconducting layer 12 includes a first superconducting layer 121 and a second superconducting layer 122 layered in arbitrary order without any other layers. The first superconducting layer 121 is composed of a superconductor as a base material and has columnar pinning centers. The second superconducting layer 122 is composed of a superconductor as a base material and has point pinning centers. The anisotropy of the critical current Icof the superconducting layer 12 can be reduced since the superconducting layer 12 includes a laminate structure of the first superconducting layer 121 and the second superconducting layer 122.

[0016] The superconducting layer 12 may include an alternating laminate structure in which the first superconducting layer 121 and the second superconducting layer 122 are alternately layered one by one, with the total number of the first superconducting layer(s) 121 and the second superconducting layer(s) 122 being three or more. In this case, the anisotropy of the critical current Icof the superconducting layer 12 can be changed (controlled) by adjusting the number of layers of the first superconducting layer 121 and the second superconducting layer 122, etc. Either the first superconducting layer 121 or the second superconducting layer 122 may be the lowermost layer of the alternating laminate structure. The number of layers of the first superconducting layer 121 and the second superconducting layer 122 in the alternating laminate structure may be the same, or one of the first superconducting layer 121 and the second superconducting layer 122 may have one more layer than the other.

[0017] Here, the pinning center is to prevent the superconducting state from being broken by fixing (pinning) the magnetic flux inside the superconductor when a magnetic field is applied to the superconductor, and the presence of the pinning centers inside the superconductor ensures a sufficient critical current even in a magnetic field. In the first superconducting layer 121, the critical current can be increased by a large pinning effect in the presence of a magnetic field along nanorods composed of BaMO3as columnar pinning centers, that is, a magnetic field perpendicular to the surface of the superconducting wire 1.

[0018] The anisotropy of the critical current is a property expressed as (Icmax(Θ)-Icmin(Θ)) / Icmin(Θ), when Icmax(Θ) and Icmin(Θ) respectively represent the maximum value and the minimum value of the critical current Icwhich is a function of the angle Θ of the magnetic field applied to the superconducting wire 1. The property is strongly related to the dependency of the critical current Icon the magnetic field angle Θ.

[0019] The first superconducting layer 121 is composed of, e.g., REBa2Cu3O6+y(0 ≦ y ≦ 1) (RE is a rare earth element), i.e. REBCO as a base material and contains BaMO3(M is Hf, Zr, or Sn) as an impurity forming columnar pinning centers. BaMO3, which is an impurity inside the REBCO superconductor, forms nanorods that are self-organized columnar defects. The nanorods composed of BaMO3have an orientation nearly perpendicular to the first superconducting layer 121 and serve as columnar pinning centers.

[0020] The first superconducting layer 121 has a thickness of, e.g., 1000 to 1700 nm (the sum of the thicknesses if multiple first superconducting layers 121 are included in the superconducting layer 12). The first superconducting layer 121 is formed, e.g., by a pulsed laser deposition (PLD).

[0021] The second superconducting layer 122 is, e.g., composed of REBCO as a base material and contains RE2O3(RE is a rare earth element) as an impurity that forms point pinning centers. In the REBCO superconductor, the impurity RE2O3forms point defects that function as point pinning centers. In the second superconducting layer 122, the point defects formed by RE2O3as point pinning centers, can produce a large pinning effect in the presence of a magnetic field parallel to the second superconducting layer 122, i.e. parallel to the surface of the superconducting wire 1, and can increase the critical current.

[0022] The second superconducting layer 122 has a thickness of, e.g., 1000 to 1700 nm (the sum of the thicknesses if multiple second superconducting layer 122 are included in the superconducting layer 12). The second superconducting layer 122 is formed, e.g., by PLD.

[0023] The rare earth elements in REBCO constituting the first superconducting layer 121 and in REBCO constituting the second superconducting layer 122 can be any rare earth elements, and they can be different from each other or the same rare earth elements. For example, the base material of the first superconducting layer 121 and the base material of the second superconducting layer 122 may be different from each other, e.g., YBCO or EuBCO, respectively, or be the same, e.g., a combination of YBCO and YBCO.

[0024] The thickness of the first superconducting layer 121 and the thickness of the second superconducting layer 122 may be the same or different. Also, when the superconducting layer 12 includes an alternating laminate structure of the first superconducting layer 121 and the second superconducting layer 122, the thickness per layer of the first superconducting layer 121 and the thickness per layer of the second superconducting layer 122 may be the same or different. It is expected that the anisotropy of the critical current Icof the superconducting layer 12 can be varied by adjusting the ratio of the thickness of the first superconducting layer 121 to the thickness of the second superconducting layer 122.

[0025] A protective layer may be provided over the superconducting layer 12 to protect them from the surrounding atmosphere and to provide good electrical contact. This protective layer comprises, e.g., a film including Ag.

[0026] The first superconducting layer 121 and the second superconducting layer 122 are formed by crystal growth in a c-axis direction. That is, the thickness directions of the first superconducting layer 121 and the second superconducting layer 122 are parallel to the c-axis, and their in-plane directions are parallel to an a-axis and a b-axis. In a case where the critical current is Ic(B / / c) when a magnetic field perpendicular to the surface of the first superconducting layer 121 and the second superconducting layer 122, i.e., parallel to the c-axis is applied, and the critical current is Ic(B / / ab) when a magnetic field parallel to the surface of the first superconducting layer 121 and the second superconducting layer 122, i.e., parallel to the a-axis and the b-axis is applied, it is desirable that Ic(B / / c) of the superconducting layer 121 be greater than its Ic(B / / ab) and that Ic(B / / ab) of the second superconducting layer 122 be greater than its Ic(B / / c), when a magnetic field of a given magnitude is applied to the first superconducting layer 121 and the second superconducting layer 122 under a given temperature condition (for example, when a magnetic field with a flux density of 8T or less is applied to the first superconducting layer 121 and the second superconducting layer 122 under a temperature of 65K). In this case, the anisotropy of the critical current of the superconducting wire 1 having the first superconducting layer 121 and the second superconducting layer 122 can be reduced more effectively.

[0027] (Evaluation of superconducting wire) An evaluation of the anisotropy of the critical current in the superconducting wire 1 was performed. The method and results are described below.

[0028] First, the superconducting wire 1 with a two-layered superconducting layer 12, in which the first superconducting layer 121 comprising EuBa2Cu3O6+y(0≦y≦1) as a base material and containing BaHfO3as an impurity forming columnar pinning centers, and the second superconducting layer 122 comprising YBa2Cu3O6+y(0≦y≦1) as a base material and containing Y2O3as an impurity forming point pinning centers were layered (referred to as an EBHO / YBCO sample), was manufactured.

[0029] In addition, the superconducting wire 1 with an eight-layered superconducting layer 12, in which the first superconducting layer 121 comprising EuBa2Cu3O6+y(0≦y≦1) as a base material and containing BaHfO3as an impurity forming columnar pinning centers and the second superconducting layer 122 comprising NdBa2Cu3O6+y(0≦y≦1) as a base material and containing Nd2O3as an impurity forming point pinning centers were alternately layered one by one (referred to as an EBHO / NdBCO sample), was manufactured. The thickness of one layer of the first superconducting layer 121 and that of the second superconducting layer 122 in the EBHO / NdBCO sample are both 0.37 μm.

[0030] The superconducting layer 12 of each of the EBHO / YBCO and EBHO / NdBCO samples was formed by PLD, using a tape-like substrate 10 composed of Hastelloy C276 with a thickness of 40 to 50 μm and a width of 12 mm, and a buffer layer 11 composed of an Al2O3layer 111, a Y2O3layer 112, an IBAD-MgO layer 113, and a LaMnO3layer 114 laminated together.

[0031] Then, a bridge of 30 μm width was formed on the superconducting layer 12 of each of the EBHO / YBCO sample and the EBHO / NdBCO sample, and the critical current was measured. The bridge was formed by micromachining the superconducting layer 12 with a 10-nanosecond pulsed laser at a wavelength of 539 nm and 200 kHz (300 mW).

[0032] Fig. 2 is a graph showing the dependency of the critical current Icon the magnetic field angle Θ of each of the EBHO / YBCO sample and two comparative examples (referred to as the EBHO sample and the YBCO sample) under a magnetic field of 1.6 T and a temperature of 65 K. Here, the EBHO sample is a superconducting wire having only the first superconducting layer 121 composed of EuBa2Cu3O6+y(0≦y≦1) as a base material and containing BaHfO3as an impurity that forms columnar pinning centers as a superconducting layer. The EBHO sample corresponds to the EBHO / YBCO sample in which the superconducting layer 12 is replaced by a single first superconducting layer 121. Also, the YBCO sample is a superconducting wire having only the second superconducting layer 122 composed of YBa2Cu3O6+y(0≦y≦1) as a base material and containing Y2O3as an impurity that forms point pinning centers as a superconducting layer. The YBCO sample corresponds to the EBHO / YBCO sample in which the superconducting layer 12 is replaced by a single second superconducting layer 122. The values of the critical current Icshown in Fig. 2 are the values per 4 mm width, converted to match the values of the critical current Icof a typical superconducting wire with a width of 4 mm.

[0033] The magnetic field angle Θ shows the angle of the magnetic field with respect to the direction perpendicular to the superconducting layer 12 through which the current flows. That is, the magnetic field angle (B / / c) is 0° when the direction of the magnetic field is parallel to the direction perpendicular to the superconducting layer 12 (the c-axis direction), and the magnetic field angle (B / / ab) is 90° when the direction of the magnetic field is parallel to the in-plane direction of the superconducting layer 12 (parallel to the a-axis and the b-axis).

[0034] Fig. 2 shows that when the magnetic field angle Θ is 90°, the critical current Icincreases in the YBCO sample having only the second superconducting layer 122 with point pinning centers as the superconducting layer, and the critical current Icdecreases in the EBHO sample having only the first superconducting layer 121 with columnar pinning centers as the superconducting layer. This creates a complementary effect in the dependency of the critical current Icon the magnetic field angle Θ between the first superconducting layer 121 and the second superconducting layer 122 in the EBHO / YBCO sample having the superconducting layer 12 in which the first superconducting layer 121 and the second superconducting layer 122 are layered, so that the strength of one layer compensates the weakness of the other layer, and vice versa.

[0035] For the EBHO / YBCO sample, the anisotropy A of the critical current Ic, which is expressed as (Icmax(Θ)-Icmin(Θ)) / Icmin(Θ) (Icmax(Θ) and Icmin(Θ) are respectively the maximum value and the minimum value of Icwhich is a function of Θ), is 0.35. This value is significantly reduced compared to the anisotropy A of the critical current Icof the EBHO and YBCO samples, 1.17 and 0.62 respectively. This result proved that the anisotropy A of the critical current Iccan be significantly reduced if the superconducting layer 12 has two layers, i.e., the first superconducting layer 121 and the second superconducting layer 122.

[0036] Additionally, according to Fig. 2, Ic(B / / c) is larger than Ic(B / / ab) in the EBHO sample, and Ic(B / / ab) is larger than Ic(B / / c) in the YBCO sample. Therefore, in the EBHO / YBCO sample where the first superconducting layer 121 constituting the EBHO sample and the second superconducting layer 122 constituting the YBCO sample are layered, it is assumed that the anisotropy A was effectively reduced because the first superconducting layer 121 and the second superconducting layer 122 complemented each other the respective values of Ic(B / / c) and Ic(B / / ab).

[0037] Fig. 3 is a graph showing the dependency of the critical current Icon the magnetic field angle Θ of the EBHO / NdBCO sample and two comparative samples (referred to as the EBHO sample and an NdBCO sample) under a magnetic field of 1T and a temperature of 77.3K. Here, the EBHO sample is, as described above, a superconducting wire having only the first superconducting layer 121 composed of EuBa2Cu3O6+y(0≦y≦1) as a base material and containing BaHfO3as an impurity forming columnar pinning centers as a superconducting layer. The EBHO sample corresponds to the EBHO / NdBCO sample in which the superconducting layer 12 is replaced by a single first superconducting layer 121. Also, the NdBCO sample is a superconducting wire having only the second superconducting layer 122 composed of NdBa2Cu3O6+y(0≦y≦1) as a base material and containing Nd2O3as an impurity forming point pinning centers as a superconducting layer. The NdBCO sample corresponds to the EBHO / NdBCO sample in which the superconducting layer 12 is replaced by a single second superconducting layer 122. The values of the critical current Icshown in Fig. 3 are the values per 4 mm width,

[0038] In the EBHO / NdBCO sample, the anisotropy A of the critical current Icis 0.39, which is significantly less than the anisotropy A of the critical current Icin the EBHO and NdBCO samples that are 0.93 and 1.12 respectively.

[0039] Furthermore, according to Fig. 3, Ic(B / / c) is larger than Ic(B / / ab) in the EBHO sample, and Ic(B / / ab) is larger than Ic(B / / c) in the NdBCO sample. Therefore, in the EBHO / NdBCO sample in which the first superconducting layer 121 constituting the EBHO sample and the second superconducting layer 122 constituting the NdBCO sample are layered, it is assumed that the anisotropy A was effectively reduced because the first superconducting layer 121 and the second superconducting layer 122 complemented each other the respective values of Ic(B / / c) and Ic(B / / ab).

[0040] Fig. 4 is a graph showing the values of the critical current Icmeasured under a temperature of 65 K at different magnetic field strengths and angles Θ applied to the EBHO sample, and it shows the dependency of the critical current Icon the field angle Θ for each field strength in the EBHO sample. The values of the critical current Icshown in Fig. 4 are the values per 4 mm width. Additionally, “2T” to “15T” shown in Fig. 4 indicate the magnitude of the magnetic flux density of the magnetic field applied to the EBHO sample.

[0041] Fig. 5 is a graph showing the dependency of the anisotropy A of the critical current Icof the EBHO sample on the strength of the magnetic field at a temperature of 65 K. The anisotropy A in Fig. 5 was measured based on the data in Fig. 4.

[0042] The range of low anisotropy A of the critical current Icin the EBHO sample is determined by the competition between columnar pinning and point pinning. At low magnetic field strength, a magnetic flux tends to align parallel to the columnar defects (nanorods), so that the columnar defects determine the critical current Icand its dependency on the magnetic field angle Θ. However, as the magnetic flux density increases, the interaction between the magnetic fluxes becomes larger than the interaction with the columnar defects.

[0043] At a certain magnetic field strength (B*), the predominance of columnar pinning decreases, and thus, at this magnetic field strength B*, the dependency of the critical current Icon the magnetic field angle Θ qualitatively changes from the characteristic behavior of columnar pinning to that of isotropic random pinning with point centers. That is, columnar pinning becomes dominant when the magnetic field strength is smaller than B*and point pinning becomes dominant when the magnetic field strength is larger than B.*

[0044] In Fig. 5, a chain line indicates the location of B*, where the transition from columnar pinning to point pinning occurs. According to Figs. 4 and 5, at a temperature of 65 K, B* is about 8T, and the anisotropy A of the critical current Icis minimum at B*.

[0045] (Advantageous Effects of the Embodiment) According to the above embodiment of the present invention, it is possible to provide the superconducting wire 1 with reduced critical current anisotropy by making the superconducting layer 12 include a laminate structure of the first superconducting layer 121 with columnar pinning centers and the second superconducting layer 122 with point pinning centers.

[0046] That is all for the description of the embodiment of the present invention. This invention is not limited to the above embodiment, but various modifications can be made without departing from the scope and spirit of the invention. Also, the components of the above embodiment can be arbitrarily combined without departing from the scope and spirit of the invention.

[0047] Furthermore, the above embodiment does not limit the invention according to the scope of claims. Also, it should be noted that not all combinations of features described in the embodiment are essential to the means for solving problems of the invention.

[0048] According to the present invention, it is possible to provide a superconducting wire with reduced anisotropy of critical current in the superconducting layer. Such a superconducting wire can be used widely for various technical applications.

[0049] 1 Superconducting wire 10 Substrate 11 Buffer layer 12 Superconducting layer 121 First superconducting layer 122 Second superconducting layer

Claims

1. A superconducting wire, comprising: a superconducting layer in which a first superconducting layer and a second superconducting layer are layered in arbitrary order without any other layer, wherein the first superconducting layer has columnar pinning centers, and wherein the second superconducting layer has point pinning centers.

2. The superconducting wire according to claim 1, wherein the first superconducting layer comprises REBa2Cu3O6+y(0≦y≦1) (RE is a rare earth element) as a base material and contains BaMO3(M is Hf, Zr, or Sn) as an impurity, and wherein the second superconducting layer comprises REBa2Cu3O6+y(0≦y≦1) (RE is a rare earth element) as a base material and contains RE2O3(RE is a rare earth element) as an impurity.

3. The superconducting wire according to claim 1, wherein the first superconducting layer comprises EuBa2Cu3O6+y(0≦y≦1) as a base material and contains BaHfO3as an impurity, and wherein the second superconducting layer comprises YBa2Cu3O6+y(0≦y≦1) as a base material and contains Y2O3as an impurity, or comprises NdBa2Cu3O6+y(0≦y≦1) as the base material and contains Nd2O3as the impurity.

4. The superconducting wire according to claim 1, wherein the superconducting layer comprises an alternating laminate structure in which the first superconducting layer and the second superconducting layer are alternately layered.

5. The superconducting wire according to claim 1, wherein when a magnetic field with a flux density of 8T or less is applied to the first superconducting layer and the second superconducting layer under a temperature of 65 K, a critical current of the first superconducting layer when the magnetic field is perpendicular to a surface of the first superconducting layer is greater than a critical current of the first superconducting layer when the magnetic field is parallel to the surface of the first superconducting layer, and a critical current of the second superconducting layer when the magnetic field is parallel to a surface of the second superconducting layer is greater than a critical current of the second superconducting layer when the magnetic field is perpendicular to the surface of the second superconducting layer.

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