Method of forming high-temperature superconducting single crystal and high-temperature superconducting single crystal produced thereby

Abstract

To provide a method of forming a high-temperature superconducting single crystal that facilitates the growth of a high-temperature superconducting single crystal containing rare-earth metals using a multilayer seed, as well as a high-temperature superconducting single crystal produced by the method.SOLUTION: A method for producing a high-temperature superconducting single crystal of the present invention is a method for producing a ReBCO-based high-temperature superconducting single crystal containing rare-earth metals, the method including a precursor preparation step of preparing a ReBCO-based precursor containing rare-earth metals, a seed preparation step of preparing a plurality of seeds having mutually different lattice constants, a seed placement step of placing the prepared seeds in a stacked manner on an upper surface of the precursor, a melting step of heating the precursor to a temperature equal to or higher than a peritectic temperature of the precursor to melt a part of the precursor, and a growth step of cooling the precursor to a crystal growth temperature of the precursor to grow a single crystal in accordance with a crystal orientation of the seeds.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a high-temperature superconductor single crystal and a high-temperature superconductor single crystal manufactured thereby. More specifically, the present invention relates to a method for manufacturing a high-temperature superconductor single crystal containing two or more rare earth metals, which can easily grow a high-temperature superconductor single crystal using a multi-stage seed, and a high-temperature superconductor single crystal manufactured thereby.

Background Art

[0002] In 1911, Heike Kamerlingh-Onnes of Leiden University in the Netherlands conducted an experiment to measure the electrical resistance of mercury and discovered the phenomenon that the electrical resistance suddenly disappeared at an absolute temperature of 4.2 K (-268.8 °C). Starting from this, low-temperature superconductors such as NbTi and SnTi began to be used in applications that require high magnetic fields. In 1987, high-temperature superconductors were discovered by Johannes Bednorz and Karl Muller in Switzerland. Since then, research has been actively carried out to improve the critical temperature and critical current density of high-temperature superconductors. Typical compositions of high-temperature superconductors are BSCCO and REBCO. Here, Re means Rare Earth, which represents rare earth elements. Since the oxygen content in copper oxide (Cu Oxide) is in a chemically unstable state (Non-Stoichiometric compound) in both compositions, it is important to control the oxygen atmosphere during manufacturing and use. The matters described as the above background art are only for enhancing the understanding of the background of the present invention, and it should not be accepted as admitting that they correspond to the prior art already known to those having ordinary knowledge in the technical field.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present invention provides a method for manufacturing a high-temperature superconductor single crystal capable of easily growing a high-temperature superconductor single crystal containing two or more rare earth metals using multi-stage seeds, and a high-temperature superconductor single crystal manufactured thereby. The technical problems to be solved by the present invention are not limited to the above-described technical problems, and other technical problems not described above will be clearly understood by those having ordinary knowledge in the technical field to which the present invention pertains from the following description.

Means for Solving the Problems

[0005] The method for manufacturing a high-temperature superconductor single crystal of the present invention includes the steps of disposing a plurality of seeds having different lattice constants in a ReBCO-based precursor containing a rare earth metal, heating the precursor to a temperature equal to or higher than the peritectic temperature of the precursor to melt a part of the precursor, and cooling the precursor to the crystal growth temperature of the precursor to grow a single crystal in accordance with the crystal orientation of the seeds.

Effects of the Invention

[0006] According to the present invention, the following effects can be expected. First, when using the superconducting material as a wire, the critical current density of the wire applied to the driving motor can be increased, so that the magnetic field of the superconducting coil (race track) can be increased compared to the current level of the wire and the output density can be maximized. Second, when used as a superconducting bulk magnet, an increase in the critical current density leads to a maximization of the magnetic field at which magnetization occurs, and the output density of the motor can be maximized. Third, directly, it can have advantages in the manufacture of large-area single crystal superconductors and the manufacture of new substances without the development of new seeds.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0008] Hereinafter, with reference to the accompanying drawings, the embodiments disclosed in this specification will be described in detail. However, regardless of the drawing reference numerals, the same or similar components are given the same reference numerals, and duplicate explanations thereof are omitted. In the following description, the suffixes "module" and "section" for components are given or used interchangeably only for the ease of preparing the specification, and do not have meanings or roles that distinguish them from each other by themselves. In describing the embodiments disclosed in this specification, if it is determined that a specific description of related known technologies may obscure the gist of the embodiments disclosed in this specification, the detailed description thereof will be omitted. Also, the accompanying drawings are only for facilitating the understanding of the embodiments disclosed in this specification, and the technical idea disclosed in this specification is not limited by the accompanying drawings, and it should be understood to include all modifications, equivalents, or alternatives included in the idea and technical scope of the present invention.

[0009] Terms including ordinal numbers such as "first", "second", etc. can be used to describe various components, but these components are not limited by these terms. These terms are only used for the purpose of distinguishing one component from another. When a component is said to be "connected to" or "attached to" another component, it should be understood that it may be directly connected or attached to the other component, but also includes the case where another component is interposed therebetween. In contrast, when a component is said to be "directly connected to" or "directly attached to" another component, it should be understood that no other component is interposed therebetween. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "including" or "having" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should not be construed as precluding the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0010] The present invention relates to a method for manufacturing a high-temperature superconductor single crystal capable of easily growing a high-temperature superconductor single crystal containing a rare earth metal using a multi-stage seed that relaxes a large difference between the lattice constants of a precursor and a seed in an upper-seed melt growth method, and to a high-temperature superconductor single crystal manufactured thereby.

[0011] Figure 1 is a schematic diagram of stepwise multi-stage seed growth using the method for manufacturing a high-temperature superconductor single crystal of the present invention. Figure 2 is a flowchart showing an example of the method for manufacturing a high-temperature superconductor single crystal of the present invention. Figure 3 is a graph showing the lattice constants of the a-axis and b-axis of the (Gd 1-x Ho x )Ba2Cu3O 7-δ (0.4 < x < 0.6, δ < 7) precursor and the NdBCO seed crystal, measured using X-ray diffraction. Figure 4 is a diagram showing the temperature change during the heat treatment process of the method for manufacturing a high-temperature superconductor single crystal of the present invention. Figure 5 is a photograph showing stepwise multi-stage seed growth using the method for manufacturing a high-temperature superconductor single crystal of the present invention. Figure 6 is a graph showing the magnetic susceptibility according to the Ho content of a single-crystal high-temperature superconductor at a temperature of 10K. Figure 7 is a graph showing the critical current density according to the Ho content of a single-crystal high-temperature superconductor at a temperature of 10K, shown by the Extended Bean Critical State Model. Figure 8 is a graph showing the critical current density according to the Ho content of a polycrystalline and a single-crystal high-temperature superconductor at a temperature of 10K.

[0012] As shown in Figure 1, a plurality of seeds having different lattice constants are arranged in a ReBCO-based precursor containing a rare earth metal. At this time, the precursor may be a ReBCO-based precursor containing a rare earth metal. For example, it may be a (Gd, Ho)BCO-based precursor containing Gd and Ho as rare earth metals, but is not limited thereto. Also, the (Gd, Ho)BCO-based precursor may be (Gd 1-x Ho x )Ba2Cu3O 7-δ (0.4 < x < 0.6, δ < 7) in which the superconducting properties are improved. On the other hand, the precursor may be a ReBCO-based polycrystal.

[0013] The plurality of seeds (crystals) can be composed of a buffer seed (crystal) and a seed crystal (crystal). A buffer seed can be placed between a precursor and a seed crystal in an upper-seed melt growth method to mitigate the differences in lattice constants and peritectic temperatures between the seed crystal and the precursor. The difference in lattice constants between the precursor and the buffer seed may be smaller than the difference in lattice constants between the precursor and the seed crystal. The difference in lattice constants between the precursor and the buffer seed can be 0.8% or less based on the lattice constant of the precursor. The difference in peritectic temperature between the precursor and the buffer seed may be smaller than the difference in lattice constants between the precursor and the seed crystal. The peritectic temperature of the seed crystal may be higher than the peritectic temperature of the precursor. The buffer seed can be GdBa2Cu3O 7-δ (0.4 < x < 0.6, δ < 7). Also, the seed crystal can be NdBa2Cu3O 7-δ (0.4 < x < 0.6, δ < 7).

[0014] For ReBCO used in high-temperature superconductors, the critical current varies significantly depending on the microstructure. A single crystal has a higher critical current value than a polycrystal, which implies excellent high-temperature superconductor performance. Single-crystal superconductors are manufactured by the upper-seed melt growth method based on polycrystalline superconductors. The upper-seed melt growth method involves placing a single-crystal seed with the same crystal structure and a higher melting temperature on top of a precursor that is a polycrystalline superconductor. After raising the test piece above the peritectic temperature and then immediately lowering it below the peritectic temperature again, it is gradually cooled to produce a single-crystal ReBCO high-temperature superconductor from the seed.

[0015] At this time, the single-crystal ReBCO high-temperature superconductor grows epitaxially with directionality. Epitaxial growth refers to the phenomenon where a crystalline film with directionality grows on a crystal substrate.

[0016] In the case of GdBCO or YBCO, generally, high-quality single-crystal superconductors are manufactured using NdBCO or SmBCO seeds. However, at this time, when the difference in lattice constant and peritectic temperature between the polycrystalline ReBCO precursor and the seed is above a certain level, there is a problem that a single crystal superconductor cannot be manufactured.

[0017] FIG. 3 is a graph showing the a-axis and b-axis lattice constants of the (Gd 1-x Ho x )Ba2Cu3O 7-δ (0.4 < x < 0.6, δ < 7) precursor and the NdBCO seed. Tables 1 and 2 are tables analyzing this.

[0018]

Table 1

[0019]

Table 2

[0020] Next, with reference to this, a method for manufacturing a high-temperature superconductor single crystal of the present invention will be described. (Gd 1-x Ho x )Ba2Cu3O 7-δ (0.4 < x < 0.6, δ < 7) The lattice constant of the precursor shows a tendency to decrease in proportion to the Ho addition amount as shown in FIG. 3, and it is confirmed that the difference in lattice constant from the NdBCO seed increases accordingly. In the top-seed melt growth method, when there is a large difference in lattice constants between the top seed and the precursor, epitaxial growth in which a crystalline film with directionality grows on the crystal substrate does not occur smoothly, so a high-quality single-crystal superconductor cannot be manufactured. If the difference in the magnitude of such lattice constants is 0.8% or more at maximum, epitaxial growth does not occur smoothly. Therefore, in the case of a general top-seed melt growth method, (Gd 0.5 , Ho 0.5 )BCO precursor (x = 0.5) and the NdBCO seed have an a-axis lattice constant error rate of 1.03% and a b-axis lattice constant error rate of 0.84%, and smooth epitaxial growth is impossible.

[0021] However, in the case of the method for manufacturing a high-temperature superconductor single crystal of the present invention, a GdBCO buffer seed is disposed between the (Gd 0.5 , Ho 0.5 )BCO precursor (x = 0.5) and the NdBCO seed to relax the difference in lattice constants. Specifically, the (Gd 0.5 , Ho 0.5 )BCO precursor and the GdBCO buffer seed have an a-axis lattice constant error rate of 0.26% and a b-axis lattice constant error rate of 0.17%. Also, the NdBCO seed and the GdBCO buffer seed have an a-axis lattice constant error rate of 0.77% and a b-axis lattice constant error rate of 0.66%. In this way, the GdBCO buffer seed shows a low difference in lattice constants of 0.8% or less with both the precursor and the seed. This is because the GdBCO buffer seed effectively relaxes the difference in lattice constants between the (Gd 0.5 , Ho 0.5 )BCO precursor and the NdBCO seed. When using the method for manufacturing a high-temperature superconductor single crystal of the present invention, despite the large difference in lattice constants between the (Gd 0.5 , Ho 0.5 )BCO precursor and the NdBO seed, smooth epitaxial growth is possible.

[0022] Figure 2 is a flowchart showing an example of a method for manufacturing a high-temperature superconducting single crystal of the present invention. Next, with reference to this, the method for manufacturing a high-temperature superconducting single crystal of the present invention will be described.

[0023] It includes a step of arranging a plurality of seeds having different lattice constants in a ReBCO-based precursor containing a rare-earth metal (S110). The ReBCO-based precursor may be a (Gd, Ho)BCO-based precursor containing Gd and Ho as rare-earth metals. At this time, the (Ge, Ho)BCO-based precursor is (Gd 1-x Ho x )Ba2Cu3O 7-δ (0.4 < x < 0.6, δ < 7) may be used. Also, the step of arranging on the precursor can include a step of preparing a polycrystalline ReBCO precursor, and the step of preparing the precursor can be the same as the following method. However, this is not limited to being exemplary.

[0024] [Method for manufacturing polycrystalline precursor] Mix 99.9% or more of Gd2O3, BaCO3, CuO, and Ho powder according to the chemical composition amounts. Mix well in a mortar for 10 minutes so that the powder mixing is performed well. To make a green body from the mixed powder, a molded body is manufactured by applying a pressure of 5 to 20 MPa into a jig.

[0025] Heat treatment is performed to make a superconductor from the manufactured molded body. For the primary heat treatment, the temperature is raised at 120°C per hour, heat-treated at 880°C for 20 hours, and then the temperature is lowered at 120°C per hour again. This is for controlling the content of C contained as an impurity in the oxide material. For the secondary heat treatment, the temperature is raised to 120°C per hour, heat-treated at 900°C for 20 hours, and then the temperature is lowered at 120°C per hour again. This is for ensuring (Gd, Ho)BCO. The three - step heat treatment involves heating at a rate of 120°C per hour to 925°C, performing a heat treatment at 925°C for 15 hours, and then cooling at a rate of 120°C per hour again. This is for increasing the crystal grain size of (Gd, Ho)BCO and ensuring stable crystallinity.

[0026] On the ReBCO precursor, a plurality of seeds having different lattice constants from each other are laminated on the surface of the precursor. The plurality of seeds is composed of buffer seeds with a difference in lattice constant from the precursor below a certain level and seed seeds with a difference in lattice constant from the precursor above a certain level. At this time, the difference in lattice constant between the precursor and the buffer seeds can be 0.8% or less. Also, buffer seeds with a difference in lattice constant from the precursor below a certain level are arranged close to the precursor, and seed seeds with a difference in lattice constant from the precursor above a certain level are arranged far from the precursor. On the other hand, the seed resin can be NdBa2Cu3O 7-δ (0.4 < x < 0.6, δ < 7). The buffer seeds can be GdBa2Cu3O 7-δ (0.4 < x < 0.6, δ < 7).

[0027] The method includes a step of heating the precursor to a temperature above the peritectic temperature of the precursor to melt a part of the precursor (S120). Figure 4 is an example of the method for manufacturing a high - temperature superconductor single crystal of the present invention (Ho 0.4 Gd 0.6 )Ba2Cu3O 7-δ showing the temperature profile. Referring to this, an example of the method for manufacturing a high - temperature superconductor single crystal of the present invention will be described.

[0028] The upper seed melting growth method includes a cold seeding method and a hot seeding method. In the case of cold seeding, it is a method of placing seeds on the precursor before heating the precursor to the peritectic temperature. In the case of hot seeding, it is a method of placing seeds after heating the precursor to a temperature above the peritectic temperature. The method for manufacturing a high-temperature superconductor single crystal of the present invention can use both methods. Preferably, the cold seeding process is used for the convenience of the process.

[0029] The step of melting a part of the precursor includes a first heating process of heating the precursor to a first heating temperature and a second heating process of heating the precursor to a second heating temperature higher than the first heating temperature. At this time, the first heating temperature can be a temperature lower than the peritectic temperature of the precursor. After the first heating temperature, the temperature is raised to the second heating temperature. The second heating temperature may be higher than the peritectic temperature of the precursor and lower than the peritectic temperature of the seed crystal. It may also be near the peritectic temperature of the buffer seed. This may be for semi-melting the precursor. For example, the first heating temperature may be 1000°C, which is lower than the peritectic temperature of 1022°C of the precursor (Ho 0.4 Gd 0.6 )Ba2Cu3O 7-δ And the second heating temperature may be 1045°C, which is higher than the peritectic temperature of 1022°C of the precursor (Ho 0.4 Gd 0.6 )Ba2Cu3O 7-δ The step (S130) of cooling the precursor to the crystal growth temperature of the precursor and growing a single crystal in accordance with the crystal orientation of the seed is included.

[0030] FIG. 4 is a diagram showing the temperature profile of the method for manufacturing a high-temperature superconductor single crystal of the present invention. With reference to this, an example of the method for manufacturing a high-temperature superconductor single crystal of the present invention will be described. The growth steps include a first growth process of cooling the seed to a first cooling temperature, a second growth process of cooling the seed to a second growth temperature lower than the first growth process to grow a high-temperature superconductor single crystal, and a third growth process of cooling to room temperature. Through a heating process, the precursor is heated to a second heating temperature higher than the peritectic temperature of the precursor to be semi-molten, and then undergoes a first growth process of cooling the semi-molten precursor to a first growth temperature which is the peritectic temperature of the precursor. Thereafter, it undergoes a second growth process of cooling from the first growth temperature to the second growth temperature. At this time, the first growth temperature can be the peritectic temperature of the precursor. Also, the second growth temperature can be a crystal growth temperature lower than the peritectic temperature of the precursor. Also, the cooling rate of the second growth process may be slower than the cooling rate of the first growth process. This can achieve the efficiency of the process by rapidly cooling from the second heating temperature to the first growth temperature, and achieve the stable growth of the ReBCO high-temperature superconductor single crystal by gently cooling from the first growth temperature to the second growth temperature with a gentle temperature change from the peritectic temperature of the precursor to the crystal growth temperature. Thereafter, it undergoes a third growth process of cooling to room temperature. After semi-melting the precursor in this way, the epitaxial growth of the single crystal is effectively enabled through a slow cooling process from the peritectic temperature, effectively providing a ReBCO high-temperature superconductor single crystal.

[0031] For example, the first growth temperature is around 1022°C, which is the peritectic temperature of the precursor (Ho 0.4 Gd 0.6 )Ba2Cu3O 7-δ , and the second growth temperature can be around 986°C, which is a crystal growth temperature lower than 1022°C, the peritectic temperature of the precursor (Ho 0.4 Gd 0.6 )Ba2Cu3O 7-δ . Also, the cooling rate of the first growth process of cooling from the first heating temperature to the first growth temperature is 50°C / h, and the cooling rate of the second growth process of cooling from the first growth temperature to the second growth temperature is 0.50°C / h, which may be lower than the cooling rate of the first growth process. Thereafter, it includes a step of heat-treating the ReBCO high-temperature superconductor single crystal (S140). The grown single crystal superconductor requires sufficient heat treatment for the effective manifestation of superconducting properties. At this time, the heat treatment step is performed in an oxygen atmosphere. The temperature of the heat treatment step can be 400 - 600 °C, preferably 500 °C. The time of the heat treatment step can be 150 - 170 hours, preferably 160 hours. By such a method for manufacturing a high-temperature superconductor single crystal, a ReBCO high-temperature superconductor single crystal having excellent performance is provided.

[0032] Hereinafter, in order to explain the method for manufacturing a high-temperature superconductor single crystal of the present invention and the effects of the high-temperature superconductor single crystal manufactured thereby, experimental data using examples will be described.

[0033] Example As shown in FIG. 1, the buffer seed 200 is disposed on the precursor 100. The seed crystal 300 is disposed on the buffer seed. Thereafter, by the method for manufacturing a high-temperature superconductor single crystal of the present invention, Ho 0.4 Gd 0.6 Ba2Cu3O 7-δ A high-temperature superconductor single crystal was manufactured (peritectic temperature: 1022 °C). At this time, the precursor 100 is composed of 70 wt% of polycrystalline Ho 0.823 Gd 0.177 Ba2Cu3O 7-δ and 30 wt% of polycrystalline Gd2BaCuO5. The buffer seed 200 is composed of 70 wt% of GdBa2Cu3O7 and 30 wt% of GdBaCuO5. The seed crystal 300 consists of a single crystal NdBa2Cu3O7 seed crystal. As a result, a high-quality (Gd 0.6 , Ho 0.4 )BCO high-temperature superconductor single crystal sample as shown in FIG. 5 was manufactured.

[0034] The physical properties of the grown single-crystalline (Gd, Ho)BCO superconductors were evaluated by MPMS (Magnetic Properties Measurement System), and the critical current density was evaluated by the Extended Bean Critical State Model.

[0035] Figure 6 is a graph showing the magnetic susceptibility according to the Ho content of the single-crystalline high-temperature superconductor at a temperature of 10K, and Figure 7 is a graph showing the critical current density according to the Ho content of the single-crystalline high-temperature superconductor at a temperature of 10K by the Extended Bean Critical State Model. The y-axis in Figure 7 indicates the critical current density. Thus, Ho 0.823 Gd 0.177 Ba2Cu3O 7-δ single-crystalline high-temperature superconductors produced by the method for producing a high-temperature superconductor single crystal of the present invention 7-δ were confirmed to have a critical current density superior to that of conventional GdBa2Cu3O

[0036] Figure 8 is a graph showing the critical current density according to the Ho content of the polycrystalline and single-crystalline high-temperature superconductors at a temperature of 10K. The y-axis in Figure 8 indicates the critical current density. Thus, Ho x Gd 1-x Ba2Cu3O 7-δ single-crystalline high-temperature superconductors produced by the method for producing a high-temperature superconductor single crystal of the present invention x Gd 1-x Ba2Cu3O 7-δ were confirmed to have a critical current density superior to that of conventional Ho

[0037] Therefore, by the method for producing a high-temperature superconductor single crystal of the present invention, a high-temperature superconductor single crystal with excellent performance can be provided, which is made possible by the buffer seed relaxing the difference in lattice constants between the seed and the precursor.

[0038] Although the present invention has been described with reference to the accompanying drawings and the above-described preferred embodiments, the present invention is not limited thereto and is limited by the claims described later. Therefore, those having ordinary knowledge in the technical field can variously modify and correct the present invention without departing from the technical idea of the claims described later.

Explanation of Signs

[0039] 100 Precursor 200 Buffer Seed 300 Seed

Claims

1. disposing a plurality of seeds having different lattice constants in a ReBCO-based precursor containing a rare earth metal; heating the ReBCO-based precursor to a temperature equal to or higher than the peritectic temperature of the ReBCO-based precursor to melt a portion of the ReBCO-based precursor; and a step of cooling the ReBCO-based precursor melted in the step to a crystal growth temperature of the ReBCO-based precursor to grow a single crystal in accordance with the crystal orientation of a seed.

2. 2. The method for producing a high-temperature superconductor single crystal according to claim 1, wherein the ReBCO-based precursor is a (Gd, Ho)BCO-based precursor containing Gd and Ho as rare earth metals.

3. The (Gd,Ho)BCO precursor is (Gd １－ｘ Ho ｘ ) Ba ２ Cu ３ O ７－δ 3. The method for producing a high-temperature superconductor single crystal according to claim 2, characterized in that (0.4<x<0.6, δ<7).

4. The ReBCO-based precursor is G.D. ２ O ３ , BaCO ３ and a mixing step of mixing the CuO powder and the Ho-containing powder in accordance with the chemical composition to prepare a mixed powder; 2. The method for producing a high temperature superconductor single crystal according to claim 1, wherein the ReBCO precursor is prepared by a molding process in which the mixed powder is compressed and molded to prepare the ReBCO precursor.

5. The plurality of seeds comprises a seed seed and a buffer seed; 2. The method for producing a high-temperature superconductor single crystal according to claim 1, wherein a difference in lattice constant between the ReBCO precursor and the buffer seed is smaller than a difference in lattice constant between the ReBCO precursor and the seed.

6. 6. The method for producing a high-temperature superconductor single crystal according to claim 5, wherein a difference in peritectic temperature between the ReBCO-based precursor and the buffer seed is smaller than a difference in peritectic temperature between the ReBCO-based precursor and the seed.

7. 6. The method for producing a high-temperature superconductor single crystal according to claim 5, wherein the difference in lattice constant between the buffer seed and the ReBCO-based precursor is 0.8% or less.

8. The seed resin is NdBa ２ Cu ３ O ７－δ 6. The method for producing a high-temperature superconductor single crystal according to claim 5, wherein δ is less than 7.

9. The buffer seed is GdBa ２ Cu ３ O ７－δ 6. The method for producing a high-temperature superconductor single crystal according to claim 5, wherein δ is less than 7.

10. In the step of placing, The method for producing a high temperature superconductor single crystal according to claim 5, characterized in that the buffer seed is disposed close to the ReBCO-based precursor and the seed seed is disposed far from the ReBCO-based precursor.

11. The step of melting a portion of the precursor comprises: a first heating step of heating the precursor to a first heating temperature; and a second heating step of heating the precursor to a second heating temperature higher than the first heating temperature.

12. the first heating temperature is lower than the peritectic temperature of the precursor; 12. The method for producing a high temperature superconductor single crystal according to claim 11, wherein the second heating temperature is higher than the peritectic temperature of the precursor and lower than the peritectic temperature of the seed.

13. The growing step comprises: a first growing stage, cooling the seeds to a first growing temperature; a second growth step of growing a high temperature superconductor single crystal by cooling the seed to a second growth temperature lower than the first growth temperature; 2. The method for producing a high-temperature superconductor single crystal according to claim 1, further comprising a third growth step of cooling the crystal to room temperature.

14. a first growth temperature in the first growth step is the peritectic temperature of the ReBCO-based precursor; 14. The method for producing a high temperature superconductor single crystal according to claim 13, wherein the second growth temperature in the second growth step is a crystal growth temperature lower than the peritectic temperature of the ReBCO-based precursor.

15. 14. The method for producing a high temperature superconductor single crystal according to claim 13, wherein a cooling rate in the first growth step is higher than a cooling rate in the second growth step.

16. (Gd １－ｘ Ho ｘ ) Ba ２ Cu ３ O ７－δ (0.4<x<0.6, δ<7).

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