Metal oxide bulk body, metal oxide superconducting bulk body, and method for manufacturing metal oxide bulk body

Abstract

The object of the present invention is to provide a homogeneous metal oxide bulk and a homogeneous metal oxide superconducting bulk, as well as a method for producing the metal oxide bulk. The metal oxide bulk of the present invention is REBa 2 Cu 3 O 7-x (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) 2 BaCuO 5 The crystal has a composition represented by the formula: 2 BaCuO 5 The volume fraction of the first surface decreases with increasing distance from the first surface.

Description

[Technical field]

[0001] The present invention relates to a metal oxide bulk, a metal oxide superconducting bulk, and a method for producing a metal oxide bulk. [Background technology]

[0002] REBa2Cu3O 7-x (REBCO, where RE stands for rare earth elements) exhibits a critical temperature (~90K) higher than the temperature of liquid nitrogen (77K) and a high critical current density (J c ) and therefore research and development of it as a superconductor material is being actively pursued.

[0003] Compared to low-temperature superconductors that are cooled to liquid helium temperatures (approximately 4K), REBCO is a high-temperature superconductor that can be used at relatively high temperatures, such as liquid nitrogen temperatures, and is therefore expected to reduce cooling costs by reducing cooling energy and simplifying cooling systems. In addition, a molten, solidified bulk in which non-superconducting RE2BaCuO5 is dispersed in REBCO can capture strong magnetic fields even in a small size. Taking advantage of these characteristics, the development of small-sized devices that do not require liquid helium is being considered, for example, by applying REBCO to NMR (Nuclear Magnetic Resonance) and MRI (Magnetic Resonance Imaging), which require strong magnetic fields. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5297148 [Non-patent literature]

[0005] [Non-Patent Document 1] Superconductor Science and Technology 35 (2022)094003(7pp) [Non-Patent Document 2] Ceramics International 49(2023) 39280-39288 [Non-Patent Document 3] Ceramics International 49(2023) 22177-22186 Summary of the Invention [Problem to be solved by the invention]

[0006] For example, when applying the above-mentioned REBCO-based superconducting molten solidified bulk to NMR or MRI devices, the uniformity (spatial uniformity) of the trapped magnetic field distribution is important to ensure high resolution. In order to obtain a high uniformity of the trapped magnetic field distribution, the REBCO-based superconducting molten solidified bulk itself needs to be highly homogeneous. However, at present, it is difficult to obtain a highly homogeneous bulk, and it has been an issue that the uniformity of the trapped magnetic field distribution cannot be sufficiently obtained.

[0007] The present invention has been made in consideration of the above-mentioned points, and has an object to provide a homogeneous metal oxide bulk, a metal oxide superconducting bulk, and a method for manufacturing a homogeneous metal oxide bulk. [Means for solving the problem]

[0008] The metal oxide bulk of the present invention is REBa2Cu3O 7-x (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and a crystal having a composition represented by RE2BaCuO5, characterized in that it has one face with the (001) plane exposed, the crystallinity gradually improves with increasing distance from the one face, and the volume fraction of the RE2BaCuO5 decreases with increasing distance from the one face.

[0009] The metal oxide bulk of the present invention is REBa2Cu3O 7-x(RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and a crystal having a composition represented by RE2BaCuO5, and is characterized in that it has a hollow cylindrical shape or a hollow truncated cone shape, and the proportion of the area of ​​voids in the cross-sectional area decreases from the inside of the hollow cylindrical shape or the hollow truncated cone shape toward the inner surface and the outer surface.

[0010] The metal oxide bulk of the present invention is REBa2Cu3O 7-x (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE2BaCuO5, and is characterized in that the area of ​​voids in the cross-sectional area in a cross section passing through the center and taken along the c-axis is 10% or less.

[0011] The method for producing a metal oxide bulk material of the present invention is to 7-x and a crystal having a composition represented by RE2BaCuO5 (RE represents one or more rare earth elements, and x satisfies 0≦x≦1), the method comprising the steps of: preparing a hollow cylindrical molded body made of a mixture of REBa2Cu3O7-x and RE2BaCuO5; arranging the molded body on a seed substrate made of a material having a peritectic temperature higher than that of REBa2Cu3O7-x; heating the molded body for a predetermined period of time to maintain a temperature that is lower than the peritectic temperature of the seed substrate and higher than the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify it, thereby obtaining the metal oxide bulk body; and separating the metal oxide bulk body from the seed substrate.

[0012] The method for producing a metal oxide bulk material of the present invention is also described. 7-xand a crystal having a composition represented by RE2BaCuO5, wherein RE is one or more rare earth elements and x satisfies 0≦x≦1. The method includes the steps of: preparing a molded body made of a mixture of REBa2Cu3O7-x and RE2BaCuO5; arranging the molded body on a seed substrate made of a material having a peritectic temperature higher than that of REBa2Cu3O7-x; heating the molded body for a predetermined period of time, maintaining the temperature at or below the peritectic temperature of the seed substrate and at or above the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify it, thereby obtaining the metal oxide bulk body; and separating the metal oxide bulk body from the seed substrate. In the step of preparing the molded body, a mixed powder of REBa2Cu3O7-x powder and RE2BaCuO5 powder is pressurized and molded at a pressure of less than 50 MPa.

[0013] The method for producing a metal oxide bulk material of the present invention is also described. 7-x and a crystal having a composition represented by REBa2Cu3O7-x (where RE represents one or more rare earth elements and x satisfies 0≦x≦1), and a metal oxide bulk body having a composition represented by RE2BaCuO5, the method comprising the steps of: preparing a molded body made of a mixture of REBa2Cu3O7-x and RE2BaCuO5; arranging the molded body on a seed substrate made of a material having a peritectic temperature higher than that of REBa2Cu3O7-x; heating the molded body for a predetermined period of time, maintaining the temperature at or below the peritectic temperature of the seed substrate and at or above the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify it, thereby obtaining the metal oxide bulk body; and separating the metal oxide bulk body from the seed substrate, wherein in the melting and solidifying step, the temperature of the molded body is controlled by applying a temperature gradient in a direction perpendicular to the seed substrate so that the temperature is lower in a portion of the molded body closer to the surface of the molded body in contact with the seed substrate.

[0014] The method for producing a metal oxide bulk material of the present invention is also described. 7-xa step of placing the molded body on a seed substrate made of a material having a peritectic temperature higher than that of the REBa2Cu3O7-x; a step of heating the molded body for a predetermined period of time to maintain a temperature that is lower than the peritectic temperature of the seed substrate and higher than the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify it, thereby obtaining the metal oxide bulk body; and a step of separating the metal oxide bulk body from the seed substrate, the step of using the metal oxide bulk produced by the method as the seed substrate to produce a new metal oxide bulk body by the method.

[0015] The method for producing a metal oxide bulk material of the present invention is also described. 7-x (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and a metal oxide bulk body composed of a crystal having a composition represented by REBaCuO5, the method comprising the steps of: preparing a molded body composed of a mixture of REBaCuO7-x and REBaCuO5; arranging the molded body on a seed substrate made of a material having a peritectic temperature higher than that of REBaCuO7-x; and heating the molded body for a predetermined time to a temperature equal to or lower than the peritectic temperature of the seed substrate and to obtain a crystal having a composition represented by REBaCuO5. The method includes a melting and solidifying process in which the metal oxide bulk is obtained by maintaining the temperature at or above the peritectic temperature and then cooling to melt and solidify, a process in which the metal oxide bulk is separated from the seed substrate, and a process in which the metal oxide bulk is prepared by heating a mixed powder of the REBa2Cu3O7-x powder and the RE2BaCuO5 powder for a predetermined period of time to densify the molded body, and the densification process is characterized in that the molded body is heated for a predetermined period of time in a vacuum or an oxygen atmosphere.

[0016] The bulk metal oxide superconductor of the present invention is REBa2Cu3O7-x It consists of crystals of a composition represented by 7-x (where RE represents one or more elements among rare earth elements and x satisfies 0 ≦ x ≦ 1) and crystals of a composition represented by RE2BaCuO5. When the perimeter of the equipotential line of the trapped magnetic field is L, the area of the region surrounded by the equipotential line is S, and the circularity C of the trapped magnetic field distribution is 4πS / L2 (0 < C ≦ 1), the average value of the circularity C for a plurality of the equipotential lines is 0.98 or more.

Brief Description of the Drawings

[0017] [Figure 1] It is a perspective view showing a metal oxide bulk body and a seed substrate according to Example 1 of the present invention. [Diagram 2] It is a flowchart showing an example of a method for manufacturing an oxide superconducting bulk body of the present invention. [Diagram 3] It is an explanatory diagram schematically showing the crystal growth direction by the SDMG method. [Figure 4] It is a diagram showing the measurement points and measurement results of the rocking curve measurement by the X-ray diffraction method for the oxide superconducting bulk body according to Example 1. [Diagram 5] It is a diagram showing the measurement points and measurement results of the rocking curve measurement by the X-ray diffraction method for the oxide superconducting bulk body according to the comparative example. [Figure 6] It is an explanatory diagram schematically showing the crystal growth direction of the oxide superconducting bulk body by the TSMG method. [Figure 7] It is a perspective view showing a metal oxide bulk body and a seed substrate according to a modified example of Example 1. [Figure 8] It is a perspective view showing a metal oxide bulk body and a seed substrate according to Example 2 of the present invention. [Figure 9] It is a diagram showing the measurement points and measurement results of the rocking curve measurement by the X-ray diffraction method for the metal oxide bulk body according to Example 1. [Figure 10] It is a diagram showing a cross-sectional observation image of the metal oxide bulk body according to Example 2. [Figure 11] It is a diagram showing a cross-sectional observation image of the metal oxide bulk body according to Example 3. [Figure 12]FIG. 13 is a diagram showing a cross-sectional observation image of a metal oxide bulk material according to Example 3. [Figure 13] FIG. 11 is a flow chart showing another example of the method for producing a metal oxide bulk material according to Example 4. [Figure 14] 13 is a diagram showing measurement points and measurement results of rocking curve measurement by X-ray diffraction method of the oxide superconducting bulk body of Example 4. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In the following, preferred embodiments of the present invention will be described, which may be modified and combined as appropriate. In the following description and accompanying drawings, substantially the same or equivalent parts are designated by the same reference numerals. EXAMPLES

[0019] The metal oxide bulk of the present invention is REBa2Cu3O 7-x (RE indicates one or more rare earth elements) and a bulk body (hereinafter also referred to as REBCO bulk body) consisting of a crystal having a composition represented by REBaCuO5. 7-x In the composition, x satisfies 0≦x≦1. In the following description, REBa2Cu3O 7-x The substance having the composition represented by the formula is also called REBCO or RE123. The substance having the composition represented by the formula RE2BaCuO5 is also called RE211. The rare earth elements include Sc, Y, and lanthanoid elements (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).

[0020] FIG. 1 is a perspective view showing a state in which a metal oxide bulk body 13 according to Example 1 of the present invention is formed on a seed substrate (hereinafter also referred to as a seed plate) 11.

[0021] As shown in Fig. 1, the seed substrate 11 has a disk shape. The seed substrate 11 is made of REBa2Cu3O 7-xand RE2BaCuO5. In this embodiment, the seed substrate 11 is a EuBCO bulk body with Eu (europium) as the RE.

[0022] The metal oxide bulk body 13 is a REBa2Cu3O 7-x and RE2BaCuO5. In this embodiment, the metal oxide bulk body 13 has a cylindrical shape. In this embodiment, the metal oxide bulk body 13 is a GdBCO bulk body in which the RE is Gd (gadolinium).

[0023] The metal oxide bulk body 13 is formed by a Single Direction Melt Growth (SDMG) method (hereinafter referred to as the SDMG method) in which a molded body obtained by pressing raw material powder is placed on a seed substrate 11 as a precursor, brought into a partially molten state on the seed substrate 11, and then cooled to melt and solidify, thereby causing crystal growth from the lower surface of the precursor in contact with the seed substrate 11 toward the upper surface. In this embodiment, the molded body used to form the metal oxide bulk body 13 has the same shape as the metal oxide bulk body 13, i.e., a cylindrical shape in this embodiment.

[0024] In the SDMG method, a seed substrate made of a REBCO bulk body having a higher peritectic temperature than the green body, which is the precursor of the REBCO bulk body to be created, is used. When the green body, which is the precursor of the metal oxide bulk body 13, is placed on the upper surface of the seed substrate 11 and heated together with the seed substrate 11 to a temperature lower than the peritectic temperature of the seed substrate and higher than the peritectic temperature of the green body, the seed substrate does not undergo a peritectic reaction, i.e., does not melt, and RE123 contained in the green body becomes a partially molten body by a peritectic reaction. In this respect, the SDMG method utilizes the difference in peritectic temperature between the green body, which is the precursor, and the seed substrate. After that, in the process of cooling the seed substrate 11 and the green body, crystal growth proceeds starting from the seed substrate 11, and the metal oxide bulk body 13, which is a molten solidified body in which RE211 is dispersed in RE123, is generated.

[0025] The portion of the upper surface of seed substrate 11 that contacts the molded body becomes the starting point of crystal growth. In this embodiment, the entire lower surface of the molded body contacts seed substrate 11. In other words, the upper surface of seed substrate 11 has a size and shape such that the outer edge surrounds the outer edge of the lower surface of the molded body when viewed from above.

[0026] After melting and solidifying, the resulting metal oxide bulk body 13 is separated from the seed substrate. After being separated from the seed substrate, the metal oxide bulk body 13 is subjected to oxygen enrichment treatment by oxygen annealing, thereby becoming doped and exhibiting superconductivity. Note that oxygen annealing may be performed before the metal oxide bulk body 13 is separated from the seed substrate.

[0027] [Manufacturing method of bulk body] The method for producing the metal oxide bulk body 13 will be described in detail with reference to Fig. 2. Fig. 2 is a flow diagram showing an example of the method for producing the metal oxide bulk body 13. First, a compact made of raw material powder of the metal oxide bulk body 13 is prepared (step S11). The raw material powder is REBa2Cu3O 7-x A mixture of (RE123) powder and RE2BaCuO5 (RE211) powder in a predetermined ratio is used. In other words, the compact is made of a mixture of RE123 and RE211.

[0028] For example, RE123 and RE211 are mixed at a molar ratio of 6:4 to 8:2 to obtain a mixed powder. For example, when RE is Y (yttrium) or Dy (dysprosium), the molar ratio of RE123 and RE211 is preferably 7:3. For example, when RE is Gd, the molar ratio of RE123 and RE211 is preferably 7.5:2.5. In this embodiment, GdBa2Cu3O 7-x A mixed powder of (Gd123) and RE2BaCuO5(Gd211) with a molar ratio of 7.5:2.5 was used.

[0029] Also, a mixed powder containing a predetermined ratio of RE123 and RE211 can be obtained by mixing and firing a rare earth oxide, BaCO3, and CuO in a predetermined molar ratio. For example, when RE is yttrium, a mixed powder with a molar ratio of Y123 (YBCO):Y211 of 7:3 can be obtained by mixing Y2O3, BaCO3, and CuO in a molar ratio of 1.3:1.7:2.4 and calcining the mixture.

[0030] Ag2O may also be added to the raw material powder. The addition of Ag2O can lower the peritectic temperature of the compact, which is the precursor of the metal oxide bulk body 13. The addition of Ag2O can also suppress voids in the metal oxide bulk body 13 that are formed, and can increase the mechanical strength of the bulk body. For example, by making the raw material powder contain about 10 wt% Ag2O, the peritectic temperature of the precursor can be lowered by about 20°C. In this embodiment, the proportion of Ag2O in the raw material powder was set to 10 wt%.

[0031] Also, Pt or CeO2 may be added to the raw material powder. By adding Pt or CeO2, the grain growth of RE211 can be suppressed. For example, when Pt is added, it is preferable that the ratio of Pt in the raw material powder is about 0.1 wt%. Also, for example, when CeO2 is added, it is preferable that the ratio of CeO2 in the raw material powder is about 0.5 wt%. Also, both Pt and CeO2 may be added to the raw material powder. In this embodiment, CeO2 is set to 0.5 wt%.

[0032] [Pressure molding] Thereafter, the raw material powder is pressure molded. The raw material powder is filled in a mold and pressed in a uniaxial direction. There is no particular restriction on the shape of the mold, and a mold of any shape according to the shape of the target molded body can be used. For example, the shape of the molded body may be a cylindrical shape (also called a disk shape) or a hollow cylindrical shape (also called a ring shape). The shape of the molded body may be a rectangular parallelepiped or a shape in which multiple solids are combined. In this embodiment, a cylindrical molded body is formed, so the mold is also cylindrical. The molded body is formed by uniaxial molding in which the raw material powder is filled in a cylindrical mold, one opening of the cylinder is closed, and the raw material content inside is pressed from the other opening. In other words, the molded body is molded by pressing one bottom surface of the molded body to be molded into a cylindrical or cylindrical shape.

[0033] The pressure during the press molding (hereinafter also referred to as the pressing pressure) is not particularly limited as long as the molded body can retain its shape during handling until it is placed on the seed substrate 11, but can be, for example, about 100 MPa or less. The higher the pressure during the press molding, the denser the molded body can be, but if the pressure is too high, cracks may easily occur in the bulk body after melting and solidifying. The inventors of the present invention have also found that the larger the diameter of the molded body, that is, the maximum width in plan view from the pressing direction (c-axis direction) of the uniaxial molding, the more likely it is that cracks will occur. It has also been found that this phenomenon is more noticeable when a molded body with a diameter exceeding 50 mm is used. This is thought to be due to the influence of in-plane variations in pressure.

[0034] From the viewpoint of suppressing cracks, the pressure during pressure molding is preferably low. For example, the pressure during pressure molding is preferably 80 MPa or less, more preferably 50 MPa or less, and even more preferably 25 MPa or less. For example, it has been confirmed that molding and production of a metal oxide bulk body are possible even at a pressure of 20 MPa or less, for example, about 12.5 MPa.

[0035] For example, if a small molded body has a diameter of about 25 to 35 mm, the above-mentioned crack problem does not occur even if the molded body is pressurized at a pressure of about 100 MPa. For example, in the case of a large molded body having a diameter of 60 to 80 mm or more, the inventors of the present application have found that the lower the pressure of the molded body, the lower the probability of crack occurrence, and a high-quality metal oxide bulk body can be produced with good reproducibility. In this example, a cylindrical (disc-shaped) molded body was produced by pressurizing at 25 MPa using a mold with a diameter of 80 mm.

[0036] [Densification by heating] After the pressure molding, the green body is heated to densify (densification step). In the densification step, the green body is heated at a temperature higher than the peritectic temperature of the green body, and the RE123 melts to densify the green body (hereinafter, such a densification process involving melting of the green body is also referred to as pre-melting or pre-melting process). In the densification step, the green body is not yet placed on the seed substrate 11, so it can be heated to a temperature higher than the peritectic temperature of the seed substrate 11.

[0037] In the densification step, the temperature is raised to the target temperature over 6 to 12 hours (about 12 to 72 hours in the case of a large compact so as not to cause cracks), and after holding for about 2 to 24 hours (the holding time is extended depending on the size), the temperature is lowered. For example, the densification is performed in an air atmosphere during heating. In this embodiment, a box furnace (muffle furnace) was used to heat the substrate in an air atmosphere at 1055°C, which is higher than the peritectic temperature of the seed substrate 11 (Ag-doped EuBCO) of 1020°C, for 2 hours, to perform densification by melting.

[0038] In the densification process, the molded body may be densified by heating at a temperature lower than the peritectic temperature of the molded body, which is known as sintering (hereinafter, such a process for densification involving sintering of the molded body is also referred to as pre-sintering or pre-sintering treatment).

[0039] In the densification step, if the green body is heated at a temperature higher than the peritectic temperature and the RE123 is melted, the dimensions of the green body are reduced by about 20%. If the green body is heated at a temperature lower than the peritectic temperature and only sintering is performed without melting in the densification step, the dimensions are reduced by about 5 to 15%. In this case, the green body undergoes a melting and solidification step described below, which causes further dimensional changes and results in the dimensions of the final metal oxide bulk body 13.

[0040] [Polish] After the densification step, the entire surface of the molded body that contacts the seed substrate was mirror-polished. By polishing the surface of the molded body, the contact area with the seed substrate can be increased.

[0041] As described above, a green body was prepared through the pressure molding step and the densification step in step S11.

[0042] [Placement on seed substrate] Thereafter, the compact is placed on a seed substrate 11 (step S12). In step S12, a seed substrate 11 made of a REBCO bulk body having a peritectic temperature higher than that of the compact is prepared. In this embodiment, a seed substrate grown by a top seeded melt growth (TSMG) method is used.

[0043] More specifically, a plate-shaped bulk body cut out from a EuBCO bulk body (EuBCO bulk material manufactured by Nippon Steel Corporation) manufactured by the TSMG method so that the surface was the (001) plane was used as the seed substrate 11. The surface of the seed substrate 11 where the (001) plane was exposed was mirror-polished to serve as the surface on which the molded body was placed. The thickness of the seed substrate may be 0.5 mm or more from the viewpoint of preventing damage during handling.

[0044] Thereafter, a slurry or paste was applied to the mirror-polished surface of the molded body, and the molded body was attached to the mirror-polished surface of the seed substrate 11, thereby disposing the molded body on the seed substrate 11. For example, a slurry in which raw material powder of the molded body is mixed and dispersed in an organic solvent such as ethanol or 1-butanol is used. In this embodiment, a paste in which raw material powder of the molded body is mixed and dispersed in polyvinyl resin polyacrylate was used.

[0045] [Melting and solidifying] After placing the green body on the seed substrate 11, the seed substrate 11 and the green body were heated in an air atmosphere in a heating furnace under a predetermined temperature control until the temperature reached a temperature equal to or higher than the peritectic temperature of the green body and equal to or lower than the peritectic temperature of the seed substrate 11, and then cooled to melt and solidify the green body (step S13). The melt and solidify step yielded a metal oxide bulk body 13 with crystal growth on the seed substrate 11 (FIG. 1). In the melt and solidify step, the temperature is controlled, for example, as follows.

[0046] First, while the compact is being heated, desolvation is performed to decompose and remove the organic components contained in the slurry or paste. For example, the temperature is raised to 500°C in 6 hours, and the compact is held at 500°C for 6 hours in a dry air stream. To prevent CO2 generated during the desolvation process from accumulating, the compact is held in the dry air stream.

[0047] Then, the temperature is raised to a temperature (T1) that is equal to or higher than the peritectic temperature of the compact and equal to or lower than the peritectic temperature of the seed substrate 11 in about 6 hours, and the temperature is maintained for 0-6 hours to partially melt the RE123 in the compact. Then, the temperature is lowered to a temperature (T2) close to the peritectic temperature of the compact in 1-3 hours. For example, the temperature T2 is set to a temperature slightly lower than the peritectic temperature of the compact. Then, the temperature is lowered (slowly cooled) to a temperature (T3) that is about 30°C lower than T2 in 80 hours or more (for example, 100 hours), and crystal growth is performed starting from the seed substrate 11. Then, the temperature is lowered to room temperature in about 8 hours. The time required for the melting and solidifying process is about 100-200 hours in total.

[0048] For example, the temperatures T1 and T2 are set taking into consideration that the peritectic temperature of RE123 decreases by about 20° C. due to the addition of Ag.

[0049] For example, when RE is Y, taking into consideration that the peritectic temperature of Y123 is 1000°C, and the addition of Ag lowers the peritectic temperature by about 20°C, the actual measured temperature T1 is set to 990°C and the actual measured temperature T2 is set to 975°C. The actual measured temperature is the temperature measured at a position near the side of the sample (molded body) in the furnace, approximately half the total height of the sample. Hereinafter, the actual measured temperature is the actual measured temperature at the same measurement location.

[0050] In this embodiment, since RE is Gd, the peritectic temperature of Gd211 is 1030°C, and the peritectic temperature is reduced by about 20°C by adding Ag. Therefore, the measured temperature T1 was set to 1015°C and the measured temperature T2 was set to 1000°C.

[0051] -Temperature gradient In addition, in the melting and solidifying step, when the molded body is heated in the heating furnace, it is preferable to control the temperature of the molded body by applying a temperature gradient in a direction perpendicular to the seed substrate 11 so that the temperature is lower in the portion of the molded body closer to the surface in contact with the seed substrate 11. This allows the temperature to shift from higher to lower than the peritectic temperature in order from the seed substrate 11 side, and allows crystal growth to proceed.

[0052] In this embodiment, melting and solidifying were performed with a temperature gradient. Specifically, a box furnace equipped with heaters on the upper, side, and lower surfaces was used, and the temperature was set so that the temperature of the lower heater was the lowest and the temperature of the upper heater was the highest. After the temperature rise process to temperature T1, heating and cooling in the melting and solidifying process were performed so that this relationship was always maintained. The dimensions of the box furnace used were 200 mm wide, 200 mm high, and 300 mm deep.

[0053] More specifically, the temperature of the bottom heater was set 25°C lower than that of the side heater, and the temperature of the top heater was set 25°C higher than that of the side heater. In other words, a temperature gradient of 50°C was applied to the set temperature so that the temperature of the bottom heater was the lowest. At this set temperature, the sample was heated so that the measured temperature near the side was a temperature (T1) higher than the peritectic temperature of the compact. Thereafter, while maintaining the difference in set temperature between adjacent heaters at 25°C, cooling (slow cooling) was performed to a temperature (T3) lower than the peritectic temperature of the compact, and then to room temperature.

[0054] In particular, the larger the bulk, the more uneven the crystal growth may be. By providing the above-mentioned temperature gradient during melting and solidification, uneven crystal growth can be prevented. In this embodiment, since a large molded body with a diameter of 80 mm is used, uneven crystal growth is prevented by providing a temperature gradient and melting and solidifying.

[0055] [Detach] After melting and solidifying, the metal oxide bulk body 13 was cut off from the seed substrate 11 at the interface with the seed substrate 11 using a diamond saw (step S14). Through the above steps, the metal oxide bulk body 13 was obtained. In addition, the cut surface of the metal oxide bulk body 13 cut off from the seed substrate 11 was polished. Note that the seed substrate 11 from which the metal oxide bulk body 13 has been cut off can be reused as a seed substrate for manufacturing the metal oxide bulk body 13 by the above-mentioned SDMG method.

[0056] In addition, after removing (also referred to as cutting out) a portion of the bulk body from the surface (referred to as the growth end) opposite the seed substrate to about 1 mm in the thickness direction of the melt-solidified bulk body, the surface was polished to expose the (001) plane. During melt-solidification by the SDMG method, RE211 is pushed out and accumulates on the uncrystallized side. In the region from the growth end to a depth of 1 mm, there is a tendency for RE211 to be abundant and RE123 to be non-oriented. In the region where RE123 is non-oriented, it is possible that superconducting properties are unlikely to be expressed, so it is preferable to remove about 1 to 2 mm from the growth end side. In the bulk body after melt-solidification, the proportion of RE211 increases with increasing distance from the seed substrate. This is thought to be due to the phenomenon that RE211 is pushed out and accumulates on the non-crystallized growth end side during melt-solidification by the SDMG method as described above.

[0057] As described above, when the green body is melted in the densification step, the dimensions of the green body are reduced by about 20%. In that case, the dimensions change very little through melting and solidification, resulting in a bulk body. On the other hand, when only sintering is performed in the densification step or when densification is not performed, the dimensions are reduced by about 20% during subsequent melting and solidification compared to when the green body was formed. In either case, a bulk body that is about 20% smaller than the green body is ultimately produced. In this embodiment, a metal oxide bulk body 13 with a diameter of 67 mm was obtained from a green body with a diameter of 80 mm.

[0058] As described above, the larger the diameter of the molded body, the more likely it is that cracks will occur or that uneven crystal growth will occur. For example, the tendency for cracks to occur immediately after molding, or after subsequent heating for densification or further subsequent melting and solidification, begins to appear when the diameter of the molded body is about 50 mm, and the frequency of such occurrences tends to increase as the diameter increases. Therefore, when forming a bulk using a molded body with a diameter of about 50 mm or more, a more stable and high-quality bulk can be produced by reducing the pressure during pressurization as described above and controlling the temperature with a temperature gradient during melting and solidification. In other words, the inventors of the present invention have found that in order to produce high-quality bulk with a high yield, manufacturing measures such as crack countermeasures are required at the boundary of the diameter of the molded body of about 50 mm.

[0059] From this viewpoint, in this specification, a shaped body having a diameter of 50 mm or more is considered to be large. Furthermore, a metal oxide bulk body formed using the shaped body as a precursor is considered to be large when it has a diameter of 40 mm or more. When a metal oxide bulk body is produced using a 50 mm precursor, the size is reduced during the formation process, resulting in a size of about 40 mm, and this definition is adopted. Furthermore, the shape of the metal oxide bulk body of the present invention is not limited to a cylindrical shape, and can be any shape. When the shape of the metal oxide bulk body is not cylindrical, a bulk body having a maximum width of 40 mm or more in a plan view from the c-axis direction is considered to be large.

[0060] The bulk body thus obtained has a slightly smaller dimension from the surface on the cut surface side (called the seed side) of the seed substrate to the surface opposite the cut surface (called the growth end side), and has a tapered shape with the side surface inclined from the outside to the inside. In the case of a cylindrical shape, the diameter on the growth end side is about 1-3% smaller than the diameter on the seed side, and up to about 5% smaller.

[0061] The tapered shape or platform shape (tapering) that narrows toward the top as described above occurs during melting and melting and solidifying in the densification process. For example, tapering can be reduced by turning the molded body upside down when placing it on the seed substrate after melting it in the densification process. Also, if only sintering is performed at a temperature at which the molded body does not melt in the densification process, the molded body does not taper, but does taper in the subsequent melting and solidifying process.

[0062] The degree of tapering, i.e., the inclination of the side surface and the dimensional difference between the top and bottom surfaces, tends to increase as the grain size of the raw material powder decreases. In addition, since the inclination of the side surface within a bulk body is considered to be constant, the dimensional difference between the top and bottom surfaces tends to increase as the thickness of the bulk body increases. It is also considered that the temperature and holding time of the pre-melting also have an effect. Taking such tapering into consideration, the shape of a metal oxide bulk body produced using a cylindrical molded body can be said to be cylindrical or truncated cone.

[0063] In step S11, crystal growth is possible by the SDMG method even without performing densification by melting or sintering in the densification step by heating after the above-mentioned pressure molding, and melting occurs and densification progresses when the temperature of the molded body is raised to or above the peritectic temperature in the melting and solidification step.

[0064] However, if densification by heating is not performed, the surface of the molded body cannot be polished before being placed on the seed substrate 11. By performing densification by heating, the surface of the molded body that contacts the seed substrate can be polished before being placed on the seed substrate 11, which is thought to increase the contact area with the seed substrate, making it easier to align the crystals of the seed substrate and resulting in higher crystallinity. This can improve the uniformity and reproducibility of the obtained bulk body.

[0065] In addition, in the densification process before placement on the seed substrate, the temperature can be raised above the peritectic temperature of the seed substrate, whereas in the melting and solidifying process, the temperature cannot be raised above the peritectic temperature of the seed substrate because the crystallinity of 13 deteriorates if a peritectic reaction occurs in the seed substrate. Therefore, it is advantageous to perform the densification process in that the density of the bulk body can be increased. For example, in the densification before melting and solidifying (pre-melting), the temperature can be raised up to, for example, 1100°C, whereas the temperature during melting and solidifying is about 10°C lower than the peritectic temperature of the seed substrate, so the maximum temperature of melting and solidifying is about 1010°C.

[0066] In addition, when densification is performed by melting, the size of the compact decreases as described above, so that the change in size during melting and solidification is small. As a result, for example, the size of the seed substrate can be effectively utilized.

[0067] [Reduction annealing] Thereafter, the metal oxide bulk body 13 was heated in a reducing gas flow for a predetermined time to perform reduction annealing (step S15, reduction annealing step). In step S15, for example, the metal oxide bulk body 13 was heated in an Ar gas flow with an oxygen concentration of 1% at 800-900°C for 12-48 hours. In this embodiment, the metal oxide bulk body 13 was heated in an Ar gas flow with an oxygen concentration of 1% for 24 hours at 850°C.

[0068] Note that reduction annealing is not essential, and even if reduction annealing is not performed, superconductivity can be exhibited in the metal oxide bulk body 13 by performing oxygen annealing. By performing reduction annealing, the superconducting properties after the subsequent oxygen annealing are improved. By performing reduction annealing, substitution of RE with Ba sites (RE / Ba substitution) can be suppressed. RE / Ba substitution in RE123 leads to deterioration of the superconducting properties.

[0069] [Oxygen annealing] After the reduction annealing, oxygen annealing was performed by heating the metal oxide bulk 13 in an oxygen stream (step S16, oxygen annealing step). Since the REBCO bulk has oxygen non-stoichiometry, the oxygen content is usually adjusted to an optimum level by heat treatment (annealing) to develop superconducting properties. The metal oxide bulk of the present invention also develops superconductivity by annealing in an oxygen stream (oxygen annealing). Specifically, the electrical resistance becomes zero below the critical temperature, and the property of being magnetized by an external magnetic field is developed. The metal oxide bulk 13 after oxygen annealing is a superconductor, a metal oxide superconducting bulk.

[0070] In the oxygen annealing step, for example, the metal oxide bulk body 13 is heated in an oxygen flow at 425° C. for 200 hours or more. In this embodiment, the metal oxide bulk body 13 is heated in an oxygen flow at temperatures between 450° C. and 350° C. for 200 hours. By the above manufacturing method, a metal oxide superconducting bulk body was obtained.

[0071] [Crystal growth direction in SDMG method] The crystal growth direction of the metal oxide bulk body 13 will be described with reference to Fig. 3. Fig. 3 is a schematic diagram showing the crystal growth direction when the metal oxide bulk body 13 is formed by the SDMG method. As described above, the metal oxide bulk body 13 is formed by partially melting the precursor compact and then growing crystals starting from the seed substrate 11.

[0072] As described above, the entire lower surface of the compact is in contact with the seed substrate 11, so that as shown in Fig. 3, the compact grows uniformly from any part of the lower surface thereof in the c-axis direction starting from the seed substrate 11. The metal oxide bulk body 13 is a single crystal growth region as a whole, and the whole is a c-axis growth region. In REBCO bulk superconductors, the c-axis growth region is known to have higher crystallinity and superior critical current density characteristics in a magnetic field than the a-axis growth region.

[0073] [Crystallization] The crystallinity evaluation results for the metal oxide bulk 13 and the comparative example will be described with reference to Figures 4 and 5. For the metal oxide bulk 13 and the comparative example, rocking curve measurements (ω measurements) were performed for the (005) plane (i.e., for the diffraction plane that produces a (005) peak) using the X-ray diffraction method of RE123, and the full width at half maximum was compared. The full width at half maximum of the ω measurement reflects the variation in the orientation of the crystal plane with respect to the c-axis direction, and a smaller full width at half maximum indicates higher crystallinity.

[0074] Fig. 4 is a diagram showing the measurement points and the measurement results of the rocking curve measurement for sample 1 of metal oxide bulk body 13. For sample 1, the seed-side surface 13S, which is the surface on the side that was in contact with seed substrate 11, was measured. The left side of Fig. 4 shows a schematic diagram of the positions of the measurement points on seed-side surface 13S of sample 1. As shown in Fig. 4, the measurement points were the center P1, P3 close to the outer periphery, and P2 in the middle.

[0075] The graph on the right side of Figure 4 shows the results of rocking curve measurement. In the graph on the right side of Figure 4, the horizontal axis indicates the angle ω between the incident X-ray and the sample surface, and the vertical axis indicates normalized intensity. As shown in the graph in Figure 4, sharp peaks were obtained for all measurement points, and the full width at half maximum (FWHM) was 0.64°, 0.46°, and 0.52° for measurement points P1, P2, and P3, respectively.

[0076] FIG. 5 shows the measurement points and results of rocking curve measurements performed on a EuBCO bulk body 100 (EuBCO bulk material manufactured by Nippon Steel Corporation) manufactured by the TSMG method as a comparative example, in the same manner as for the metal oxide bulk body 13, for the (005) plane.

[0077] In the TSMG method, a seed crystal 101 is placed at the center of a precursor and melted and solidified. For the EuBCO bulk body 100, the surface on which the seed crystal 101 is placed is defined as a seed-side surface 100S, and measurements were taken at measurement points P1 to P3 similar to the measurement points on the seed-side surface 13S shown in FIG.

[0078] Furthermore, for the EuBCO bulk body 100, rocking curve measurements similar to those for P1 to P3 were also performed for measurement points P4 and P5 in the bulk, which are 13 mm away in the depth direction (height direction) from measurement points P1 and P3 on the seed-side surface 100S. Note that for measurement points P4 and P5, a cross section of the EuBCO bulk body 100 cut along a plane passing through the center of the seed-side surface 100S and aligned along the c-axis was measured.

[0079] The graph in the center of Fig. 5 shows the results of rocking curve measurement for measurement points P1 to P3 of the EuBCO bulk body 100. As shown in the graph in the center of Fig. 5, a sharp peak was obtained for measurement point P1, with a full width at half maximum of 0.36°. For measurement points P2 and P3, broad peaks were obtained, with full width at half maximum of 2.80° and 2.24°, respectively.

[0080] The graph on the right side of Fig. 5 shows the results of rocking curve measurements at measurement points P4 and P5 of the EuBCO bulk body 100. Although measurement point P4 has two peaks, the full width at half maximum was 0.51° for the peak at ω ≒ 19° and 0.68 for the peak near ω ≒ 17°. Measurement point P5 has a sharper peak than measurement point P3, and the full width at half maximum was 0.52° for the peak at ω ≒ 19° and 0.71 for the peak near ω ≒ 18°.

[0081] The measurement results shown in Fig. 4 show that sample 1 has high crystallinity at all of measurement points P1 to P3 on the seed-side surface 13S. In contrast, the graph in the center of Fig. 5 shows that in the EuBCO bulk body 100 of the comparative example, only measurement point P1 at the center of the seed-side surface 100S has high crystallinity, and measurement points P2 and P3 have significantly lower crystallinity than sample 1. It was confirmed that the crystallinity of the seed-side surface is higher in the bulk of this example.

[0082] 5, the crystallinity is higher at positions farther from the seed-side surface 100S in the region near the outer periphery of the EuBCO bulk body 100 of the comparative example. These results are considered to be due to the crystal growth direction. The crystal growth direction during melting and solidification by the TSMG method will be described below.

[0083] [Crystal growth direction in TSMG method] 6 is a diagram showing the crystal growth direction when a REBCO bulk is formed by the TSMG method as a comparative example, taking an example of an EuBCO bulk 100. As described above, the EuBCO bulk 100 is formed by placing a seed crystal 101 at the center of the upper surface of a molded body, which is a precursor, when the molded body is melted and then solidified, and by growing the crystal with the seed crystal 101 as the starting point.

[0084] 6, when a bulk body is formed by the TSMG method of the comparative example, growth in the a-axis direction as well as growth in the c-axis direction occurs starting from a seed crystal 101. Growth in the a-axis direction proceeds in four directions, and boundaries are formed between adjacent a-axis growth regions (a-growth in the figure).

[0085] Due to this growth direction, EuBCO bulk 100 formed by the TSMG method has a relatively high crystallinity in the center (measurement point P1), which is the c-axis growth region (c-growth in the figure), but in the region close to seed crystal 101, an a-axis growth region exists on the periphery, and the a-axis growth region on the periphery has lower crystallinity than the c-axis growth region. In contrast, metal oxide bulk 13, as shown in Figure 3, is a c-axis growth region throughout, and can be said to have high crystallinity and homogeneity throughout.

[0086] [Trapping magnetic field distribution] Sample 1 of the metal oxide bulk material 13 was magnetized by applying an external magnetic field of 1.5 T (tesla) at liquid nitrogen temperature (77K) and the trapped magnetic field distribution was evaluated, and a uniform trapped magnetic field distribution was obtained.

[0087] Let the perimeter of the equipotential magnetic field lines of the trapped magnetic field be L, the area of the region surrounded by the equipotential magnetic field lines be S, and the circularity C of the trapped magnetic field distribution be C = 4πS / L 2 (0 < C ≤ 1) (C = 1 in the case of a perfect circle) was calculated for a plurality of equipotential magnetic field lines at intervals of 0.02 T from 0 T to the maximum trapped magnetic field. When the average value of the circularity C of the plurality of equipotential magnetic field lines was obtained, it was 0.95 or more.

[0088] As described above, the metal oxide bulk body 13 according to Example 1 is a metal oxide bulk body that was grown on a plate-shaped seed substrate and then separated from the seed substrate at the interface with the seed substrate, and is REBa2Cu3O 7-x It consists of crystals of the composition represented by (RE represents one or more elements among rare earth elements, and x satisfies 0 ≤ x ≤ 1) and crystals of the composition represented by RE2BaCuO5. Further, the metal oxide bulk body 13 has a c-axis growth region extending in a direction substantially perpendicular to the cut surface from the cut surface when separated from the seed substrate. In other words, the metal oxide bulk body 13 is a metal oxide bulk body composed of a single growth region without a boundary between the a-axis growth region and the c-axis growth region.

[0089] Regarding sample 1 of the metal oxide bulk body 13, when the crystallinity of the surface on the seed substrate side (seed side surface) was evaluated, results were obtained indicating that the crystallinity was higher compared to the surface on the seed crystal side of the bulk body produced by the TSMG method of the comparative example. Also, the crystallinity of the central part and the outer peripheral part of the seed side surface of sample 1 was equivalent. That is, sample 1 had high crystallinity and uniform crystallinity in the plane of the seed side surface. Also, the trapped magnetic field distribution was uniform. From these facts, it can be said that the metal oxide bulk body 13 is a homogeneous REBCO bulk body.

[0090] Further, the metal oxide bulk body 13 has a diameter of 67 mm and is a large-sized bulk body compared to, for example, small-sized bulk bodies of about 20 mm to 30 mm. Such a large size is a size at which it is difficult to obtain sufficient superconducting characteristics in REBCO bulks manufactured by the TSMG method. According to this example, even for a large-sized bulk body, a trapped magnetic field distribution with high uniformity was obtained, and sufficient superconducting characteristics were obtained.

[0091] Furthermore, even when manufacturing REBCO bulk bodies using the SDMG method, as the diameter increases, cracks may occur and uneven crystal growth may occur, posing issues with reproducibility and yield.

[0092] According to the manufacturing method of this embodiment, when the metal oxide bulk body 13 is manufactured by the SDMG method, the pressure molding is performed at a lower pressure than in the past, and the occurrence of cracks can be suppressed. In addition, by providing a temperature gradient so that the temperature is lower in the portion closer to the seed substrate during melting and solidification, non-uniform crystal growth can be prevented.

[0093] Therefore, according to this embodiment, it is possible to provide a homogeneous and high-yield metal oxide bulk, a metal oxide superconducting bulk, and a method for producing the metal oxide bulk.

[0094] In the pressure molding process for producing the metal oxide bulk body 13 of this embodiment, a binder may be mixed with the raw material powder before molding. When a binder is mixed, even if the compact is large, cracks in the bulk body are unlikely to occur even if the pressure during pressure molding is about 100 MPa. Since RE123 may react with water and decompose, an organic solvent is used as the binder solvent instead of water. For example, an acrylic acid ester soluble in an organic solvent can be used as the binder.

[0095] However, when a binder is used, it is necessary to carry out desolvation to decompose and remove the binder component before the densification step. For example, the desolvation is carried out by heating in the air at 100 to 500°C for about 100 to 250 hours. For example, the desolvation may be carried out during the temperature increase for densification. The desolvation takes time for production.

[0096] The effect of pressure during compaction on the likelihood of cracking of the bulk body varies depending on the particle size of the raw material powder. In this example, RE123 powder with an average particle size of 1 to 2 μm and RE211 powder with an average particle size of 1 to 2 μm were used.

[0097] [Variations] Fig. 7 is a perspective view showing a seed substrate 11 and a molded body 14 which is a precursor of a metal oxide bulk body according to a modified example of Example 1. In the above-mentioned manufacturing method, the use of a molded body having a size such that the entire lower surface is covered by the upper surface of the seed substrate 11 is described, but the present invention is not limited to this. As shown in Fig. 7, a metal oxide bulk body may be produced using a molded body 14 which is larger than the seed substrate 11, in other words, which has an outer edge which surrounds the outer edge of the seed substrate 11 in a plan view.

[0098] For example, in a plan view, the shape and size of molded body 14 may be such that the outer edge of molded body 14 approximately matches the outer edge of seed substrate 11. For example, in a plan view, the outer edge of molded body 14 does not have to completely surround the outer edge of seed substrate 11, and a part of the outer edge of molded body 14 may protrude beyond the outer edge of seed substrate 11.

[0099] According to this modification, for example, it is possible to prepare a metal oxide bulk body whose outer edge is large enough to surround the outer edge of seed substrate 11 in a plan view. Note that an a-axis growth region may occur in a portion of compact 14 that protrudes from seed substrate 11. In order to prevent the occurrence of an a-axis growth region, for example, a compact is used whose diameter after melting and solidifying is equivalent to the diameter of the seed substrate. For example, compact 14 that has been densified by melting in the densification step and has a diameter equivalent to the diameter of seed substrate 11 can be used. EXAMPLES

[0100] 8 is a perspective view showing a state in which a metal oxide bulk 15 according to Example 2 is grown on a seed substrate 11 similar to that of Example 1. The metal oxide bulk 15 differs from the metal oxide bulk 13 of Example 1 only in shape and RE element, and is otherwise similar, so some of the explanations of overlapping parts will be omitted. As in Example 1, the seed substrate 11 is a EuBCO bulk produced by the TSMG method.

[0101] 8, the metal oxide bulk body 15 has a hollow cylindrical shape (ring shape). The metal oxide bulk body 15 is produced by melting and solidifying a ring-shaped body as a precursor by the above-mentioned SDMG method.

[0102] Even in the case of a ring-shaped molded body, as shown in Figure 3, crystal growth progresses in the c-axis direction extending approximately perpendicular to the seed substrate from the side in contact with the seed substrate 11 (seed side), and the entire body becomes a metal oxide bulk body 15 which is a c-axis growth region.

[0103] Furthermore, even when melting and solidifying is performed using a hollow cylindrical shaped body as a precursor, as described above, the bulk body after melting and solidifying using the SDMG method may have a tapered shape, so it can be said that the metal oxide bulk body 15 has a hollow cylindrical shape or a hollow truncated cone shape.

[0104] Two samples of the metal oxide bulk body 15 of this embodiment were prepared, and designated as samples 2 and 3. Sample 2 is a YBCO bulk body formed by the SDMG method on a seed substrate 11 using a ring-shaped compact having an outer diameter of 80 mm and an inner diameter of 35 mm, with Y (yttrium) as the RE. The pressing pressure for the pressure molding was 25 MPa, and melting and solidifying were performed under a temperature gradient. The finished size (i.e., the size after melting and solidifying, cutting and polishing) was an outer diameter of 65 mm, an inner diameter of 30 mm, and a height of 17 mm.

[0105] Sample 3 is a YBCO bulk body formed by the SDMG method on a seed substrate 11 using a ring-shaped compact with an outer diameter of 50 mm and an inner diameter of 25 mm, with Y (yttrium) as the RE. The pressing pressure for the press molding was 100 MPa, and melting and solidification were performed under a temperature gradient. The finished size was an outer diameter of 42 mm, an inner diameter of 21 mm, and a height of 9 mm.

[0106] After cutting each of Sample 2 and Sample 3 from the seed substrate 11, the area up to about 1 mm from the surface on the growth end side was removed, and the cut surface and the growth end side were polished. As a result, the (001) plane was exposed on the growth end side of the sample. Sample 2 was evaluated for crystallinity, and Sample 3 was evaluated for cross-sectional structure. The evaluation results are explained below.

[0107] [Crystallization] Fig. 9 is a diagram showing the measurement points and the measurement results of the rocking curve measurement for sample 2 of the metal oxide bulk body 15 in the same manner as in Example 1. As shown on the left side of Fig. 9, the measurement points were P6 on the seed-side surface 15S, P7 located on the outer periphery side of P6, P6, and P9 in the bulk, which were 13 mm away in the depth direction (height direction) from P7, respectively.

[0108] The graph in the center of Figure 9 shows the results of rocking curve measurements for measurement points P6 and P7. The full width at half maximum of the two peaks at measurement point P6 was 0.83° and 0.76° from the low angle side, respectively. The full width at half maximum of the peak at measurement point P7 was 0.72°. For measurement points P6 and P7, the full width at half maximum was 1° or less, indicating high crystallinity.

[0109] 9 shows the results of rocking curve measurements for measurement points P8 and P9. The full width at half maximum of the peak at measurement point P8 was 1.38°, and the full width at half maximum of the peak at measurement point P9 was 1.91°.

[0110] From the above measurement results, it can be said that for measurement point P8 located on the inner circumference and measurement point P9 located on the outer circumference when viewed from above, the full width at half maximum of the peak in the rocking curve measurement is 2° or less, and there is either one peak or the center position of the peak is within a range of 2 degrees.

[0111] 9, the peaks are broader and the full width at half maximum is larger inside the bulk (P8, P9) than on the seed-side surface (P6, P7). It can be said that the crystallinity of sample 2 of the metal oxide bulk body 15 tends to decrease with increasing distance from the seed-side surface, i.e., the cut surface cut from the seed substrate.

[0112] In addition, it can be said that the (001) plane is exposed on the growth end side of the metal oxide bulk 15 of sample 2. Therefore, it can be said that sample 2 has a surface with the (001) plane exposed, and that the crystallinity gradually improves with distance from the (001) plane, i.e., with distance from the seed-side surface.

[0113] The results for measurement points P8 and P9 show a full width at half maximum of 2° or less, which indicates that the crystallinity is high compared to the measurement results for the seed crystal side of the comparative example shown in the center graph of Figure 5. Therefore, although the crystallinity tends to decrease with increasing distance from the seed side surface (cut surface), the product can be said to be a homogeneous bulk body with high crystallinity overall.

[0114] In addition, a similar tendency was confirmed for a cylindrical YBCO bulk (referred to as sample 4) produced by the SDMG method, which is smaller in size than sample 2 of metal oxide bulk 15, with a diameter of 23 mm and a height of 12 mm, namely, the crystallinity tends to decrease with increasing distance from the seed side surface (cut surface).

[0115] Specifically, rocking curve measurements were performed at the measurement point (0 mm) at the center of the seed-side surface of Sample 4, which is a cylindrical YBCO bulk body, as well as at a point 5 mm away from there in the depth direction and at a point 8 mm deep. The peaks became broader and the full width at half maximum also gradually increased with increasing distance from the seed-side surface, and were 0.67°, 1.11°, and 1.51°, respectively, from the seed side. Note that Sample 4 is small, with a diameter of 23 mm, so the pressure (press pressure) during press molding was set to 100 MPa, and it was melted and solidified using a general-purpose box furnace without applying a temperature gradient.

[0116] Therefore, it can be said that bulk bodies grown by the SDMG method, regardless of size or shape, tend to have higher crystallinity near the seed side surface and lower crystallinity as they move away from the seed side surface. This trend is different from that of bulk bodies produced by the TSMG method. As shown in Figure 5, in the case of bulk bodies produced by the TSMG method, the crystallinity increases as they move away from the seed crystal, which is the opposite trend to that of the SDMG method.

[0117] One of the reasons why the crystallinity decreases with increasing distance from the seed-side surface is that the later the crystal growth, the more likely it is that random crystal growth occurs. More specifically, the peritectic reaction in which crystals precipitate from a partially molten state initially causes crystal growth to match the crystal orientation of the seed substrate. In the latter half of the crystal growth, the degree of supercooling of the partially molten RE123 increases, so that random crystal growth occurs more easily, in which crystal growth proceeds even if the crystal orientation does not necessarily match the seed substrate. In addition, since crystal growth proceeds from the seed substrate side, the closer the crystal growth approaches the end, the more likely it is that random crystal growth occurs in the part farther from the seed substrate. From the above, it is considered that the more likely it is that random crystal growth occurs in the part farther from the seed substrate.

[0118] In addition, as the crystal growth by the SDMG method progresses and the melting and solidifying process progresses to the latter half, the proportion of the solid phase component RE211 in the non-crystallized portion increases. As a result, the orientation misalignment caused by the incorporation of RE211 in the RE123 crystal becomes more likely to occur in the latter half of the melting and solidifying process, which is also considered to be one of the reasons why the crystallinity decreases with increasing distance from the seed side surface. It is noted that Non-Patent Document 2 describes that the volume fraction of Dy211 particles in the DyBCO bulk grown by the SDMG method increases with increasing distance from the seed substrate.

[0119] Incidentally, Non-Patent Document 3 also describes that in YBCO bulks produced by the TSMG method, the volume fraction of Y211 particles in the bulk increases as the distance from the seed crystal increases.

[0120] As described above, in the REBCO bulk crystal grown by the SDMG method, the proportion of RE211 increases and the crystallinity decreases as the distance from the seed substrate increases. In other words, if the surface on the growth end side opposite to the seed substrate is surface 1 with the (001) surface exposed, the crystallinity gradually improves as it moves away from surface 1, and the volume fraction of RE211 decreases as it moves away from surface 1. It can be said that the decrease in crystallinity as it moves away from the seed side surface is a tendency specific to the bulk grown by the SDMG method, which grows in one direction starting from the seed substrate.

[0121] [Cross-sectional structure evaluation] 10 is a diagram showing a cross-sectional observation image of sample 3, a YBCO bulk body different from the above-mentioned sample 2 of the metal oxide bulk body 15. The YBCO bulk body of sample 3 has a hollow cylindrical shape like the metal oxide bulk body 15, but its size is different, with an outer diameter of 42 mm, an inner diameter of 21 mm, and a height of 9 mm.

[0122] The image shown in Fig. 10 shows one side of a cross section cut along a cross section passing through the center of the ring shape and along the c-axis of Sample 3. As shown in Fig. 10, the side surface of YBCO Sample 3 is inclined from the outside to the inside, forming a tapered shape.

[0123] The scattered black areas in the image of Fig. 10 are voids remaining in the bulk of Sample 3. As can be seen from the image of Fig. 10, many voids are seen in the inner part of the cross section of Sample 3, whereas there are fewer voids near the top surface, outer surface, and inner surface on the growth end side.

[0124] It is believed that the voids in the bulk that grow by melting and solidifying are caused by inert gases such as nitrogen and Ar that are left behind in the bulk. Also, during crystal growth by melting and solidifying, bubbles of inert gases are expelled from the surface by diffusion, so there are fewer voids near the surface.

[0125] In sample 3 in Fig. 10, not only are there few voids near the surface (upper surface) on the growth end side and near the outer surface, but there are also few voids near the surface on the inner surface side. This is because a ring-shaped bulk body was formed from a ring-shaped compact by the SDMG method, and air bubbles were expelled from the inner surface of the ring shape during melting and solidification.

[0126] On the other hand, when making a ring-shaped bulk body by the TSMG method, a seed crystal is first placed in the center of the top surface of a disk-shaped compact, melted and solidified, and then a hole is drilled in the center by machining to make it into a ring shape. Therefore, the area near the inner surface of the ring shape after machining is the part that was inside the compact when melted and solidified, and becomes an area with many voids. In addition, the mechanical strength of the REBCO bulk body may be reduced due to damage during machining after crystal growth by such melting and solidification.

[0127] In contrast, the link-shaped metal oxide bulk body 15 of this embodiment has a c-axis growth region that extends in a direction approximately perpendicular to the cut surface from the cut surface of the seed substrate 11, as described above, and the proportion of the void area in the cross-sectional area decreases from the inside toward the inner and outer surfaces. Furthermore, since the metal oxide bulk body 15 has few voids near the inner peripheral surface and does not require machining, high mechanical strength can be ensured.

[0128] Such characteristics are specific to REBCO bulk bodies grown by the SDMG method, and if such characteristics are present, it can be said that the entire REBCO bulk body is a c-axis growth region and is a homogeneous REBCO bulk body.

[0129] [Trapping magnetic field distribution] A ring-shaped bulk body manufactured by the same manufacturing method as metal oxide bulk body 15 was magnetized by applying an external magnetic field of 1.5 T (tesla) at liquid nitrogen temperature (77 K) and the trapped magnetic field distribution was evaluated, and a uniform trapped magnetic field distribution was obtained. The average circularity C of the isomagnetic field lines calculated at intervals of 0.02 T from 0 T to the maximum trapped magnetic field was calculated as the circularity of the trapped magnetic field distribution, and a result of 0.98 to 0.99 or more was obtained.

[0130] For example, the circularity of the trapped magnetic field distribution of a YBCO bulk body (referred to as sample 5) with an outer diameter of 41 mm, inner diameter of 17 mm, and height of 10 mm was 0.987. Furthermore, the circularity of the trapped magnetic field distribution of a DyBCO bulk body (referred to as sample 6) with an outer diameter of 41 mm, inner diameter of 13 mm, and height of 10 mm was 0.996. Many other bulk bodies with a circularity of the trapped magnetic field distribution of 0.98 or more have also been produced. In this way, ring-shaped bulk bodies with extremely uniform trapped magnetic field distribution have been obtained with good reproducibility.

[0131] Therefore, according to this embodiment, it is possible to provide a metal oxide bulk and a metal oxide superconducting bulk which are hollow cylindrical or hollow truncated cone shaped, are homogeneous, have high mechanical strength, and have a high yield, as well as a method for manufacturing a metal oxide bulk.

[0132] Furthermore, Sample 2 of this Example 2 has a finished diameter of 65 mm, which is a large YBCO bulk body compared to a typical bulk body having a diameter of about 20 to 30 mm.

[0133] In addition, Y is inexpensive among the rare earth elements, and compared to REBCO bulk bodies that use other rare earth elements as RE, YBCO is non-magnetic and suitable for generating a homogeneous magnetic field, making it promising for applications that require a homogeneous magnetic field. However, since YBCO has a low peritectic temperature and a slow growth rate, it is difficult to grow large-sized YBCO using the TSMG method, which requires longer growth times as the size increases. In addition, it is difficult to obtain homogeneous bulks using the TSMG method as the size increases.

[0134] As mentioned above, the uniformity of the trapped magnetic field distribution is important in NMR and MRI. The shape of the superconductor used in NMR and MRI that most easily achieves the uniformity of the trapped magnetic field distribution is a ring shape.

[0135] According to the SDMG method of this embodiment, the growth time does not change even when the in-plane size increases, and since the growth direction is single, it is possible to efficiently produce large, homogeneous YBCO bulk bodies even in complex shapes such as ring shapes.

[0136] In this embodiment, the size of the ring-shaped bulk body is not limited to the above example, and any size of link-shaped bulk body can be produced by SDMG method. For example, as an example of the size of the REBCO bulk body suitable for NMR use, the inner diameter of the hollow cylinder shape is 5 mm or more, and the outer diameter is 20 to 200 mm. Also, the difference between the inner and outer diameters is 5 mm or more. Also, the height is 50 mm or less. For example, if a REBCO bulk body with a larger height is required, multiple bulk bodies may be stacked and used. EXAMPLES

[0137] [Densification in a vacuum] In Example 1, the densification is performed by heating in the atmosphere, but the inside of the heating furnace during densification may be evacuated. The vacuum referred to here refers to the vacuum defined in the JIS standard, which refers to a state in a space filled with a gas whose pressure is lower than atmospheric pressure. The inventors of the present application have found that by evacuating the inside of the heating furnace during densification, the amount of voids in the generated metal oxide bulk body 13 can be reduced. The fewer the voids in the bulk body, the higher the mechanical strength of the bulk body.

[0138] It is believed that the number of voids in the metal oxide bulk body 13 is reduced for the following reason. It is believed that the voids are generated when gas in the furnace gets into the bulk and is left behind during the densification process or the subsequent melting and solidification process. For example, it is believed that after gas gets into the voids in the compact during the densification process, the voids become closed spaces through sintering or melting.

[0139] By creating a vacuum inside the heating furnace during densification, it is possible to reduce voids, either by making it difficult for voids to form or, even if voids do exist, by making the voids themselves a vacuum.

[0140] However, when the inside of the heating furnace is kept in a vacuum state during densification, temperature control is important. When the densification process is performed in a vacuum state, melting is considered to occur at a lower temperature from the viewpoint that the lower the oxygen partial pressure, the lower the peritectic temperature. On the other hand, depending on the degree of vacuum, the decomposition temperature of the raw materials (RE123 and RE211) becomes lower than the peritectic temperature, and the decomposition of the raw materials causes precipitation of Cu2O, etc. Decomposition of the raw materials affects the subsequent melting and solidification, so it is preferable that it does not occur. Therefore, when the decomposition temperature of the raw materials is lower than the peritectic temperature of the molded body, it is preferable to perform densification at a temperature that suppresses decomposition and allows sintering without melting the molded body. From the viewpoint of suppressing the decomposition of the raw materials, a high degree of vacuum is not preferable.

[0141] For example, in order to suppress decomposition and reduce voids, it is preferable to perform densification in a low vacuum. Here, low vacuum refers to a low vacuum as defined by the JIS standard, and is 10 5Pa~10 2 This refers to a vacuum of 100 Pa. When densification is performed in a low vacuum, sintering at 800°C or higher is considered possible.

[0142] For example, the densification process is carried out at a pressure of about 0.2 atm (≒2.0×10 4 When the densification process is carried out at a pressure of 10 Pa, it is believed that decomposition can be suppressed and sintering can be achieved by setting the heating temperature at 800°C or higher and 1050°C or lower. -3 Atmospheric pressure (≒10 2 When the heating is performed at 1000 MPa (Pa), it is believed that decomposition can be suppressed and sintering is possible by setting the heating temperature at 800°C or higher and 900°C or lower.

[0143] [Void evaluation] 11 and 12, samples 7 and 8 of the metal oxide bulk according to Example 3 will be described. Samples 7 and 8 differ from each other only in the processing conditions of the densification step, and were otherwise manufactured under similar conditions, and are disk-shaped metal oxide bulks manufactured in the same manner as metal oxide bulk 13 of Example 1. As precursors of samples 7 and 8, two GdBCO compacts molded into disk shapes with a diameter of 30 mm were prepared.

[0144] One of the two compacts was used as a precursor of sample 7, and densification was performed by heating in the air. Specifically, the temperature was raised to 1050°C in an air atmosphere at normal pressure in a box furnace, and then the temperature was maintained for 2 hours, thereby performing densification in the air.

[0145] The other green body was used as the precursor of sample 8, and densification was performed by heating in a vacuum. Specifically, the temperature was raised to 950°C and held for 2 hours in a tubular furnace while constantly evacuating, and densification was performed in a vacuum. The degree of vacuum during the temperature hold at 950°C was approximately 0.1 atm (≒10 4 Pa).

[0146] It is also possible to evacuate the furnace for about 10 minutes before heating, and then heat the furnace without evacuating it, but in that case, oxygen will be generated from the compact due to the oxygen non-stoichiometry of RE123, and the degree of vacuum will change depending on the amount of sample. Therefore, it is preferable to always evacuate the furnace during heating, including during the temperature rise.

[0147] Thereafter, the two densified compacts were placed on the same seed substrate 11 (EuBCO) and melted and solidified by the SDMG method to obtain two GdBCO bulk bodies, Samples 7 and 8. The finished size of the bulk bodies of Samples 7 and 8 was both 25 mm in diameter. For each of the two bulk bodies, a cross section cut along a plane passing through the center of the disk shape and parallel to the c-axis was observed with an optical microscope to evaluate the void ratio.

[0148] The void ratio was evaluated by binarizing the image observed under an optical microscope and calculating the area of ​​the black parts. Note that since the black parts correspond to voids and cracks, the cracks were also included in the evaluation.

[0149] Figure 11 shows a cross-sectional observation image (Figure 11(a)) and a binarized image (Figure 11(b)) of sample 7. For metal oxide bulk body 17 pretreated in air, the area ratio of black parts corresponding to voids and cracks in the binarized image was 17.12%.

[0150] FIG. 12 shows a cross-sectional observation image ((a) in FIG. 12) and a binarized image ((b) in FIG. 12) of Sample 8. For Sample 8, which was densified in a vacuum, the area of ​​the black parts corresponding to voids and cracks in the binarized image was 7.73%. By densifying in a vacuum, the area occupied by voids and cracks in Sample 8 was reduced by about 55% compared to Sample 7.

[0151] In addition, in sample 8, the area including voids and cracks was 7.73%, so the area of ​​voids excluding cracks is even smaller. Therefore, in a cross section cut along a plane passing through the center of the disk shape and parallel to the c-axis, the area of ​​voids is 10% or less.

[0152] These two bulk bodies were magnetized by applying an external magnetic field of 1.5 T (tesla) at liquid nitrogen temperature (77 K), and the trapped magnetic field distribution was evaluated. The distributions of the trapped magnetic field were found to be equivalent, and the trapped magnetic field was uniform and sufficiently high.

[0153] Therefore, it was found that densification by heating in a vacuum (pre-vacuum heat treatment) can significantly reduce voids and cracks without affecting the trapped magnetic field distribution. Reduction of voids and cracks improves mechanical strength. Naturally, the same can be said for large bulks with a diameter of 40 mm or more, which are larger than Sample 8 in size, by making the heating furnace in a vacuum state during densification, voids themselves are less likely to occur, or even if voids exist, the voids are made vacuum, so that voids can be reduced.

[0154] [Densification in an oxygen atmosphere] Also, voids in the bulk body can be suppressed by densifying the bulk body by heating in an oxygen atmosphere. Since oxygen is absorbed by the REBCO bulk, it is considered that if the gas in the voids left behind (isolated from the outside) by sintering or melting is oxygen, it is unlikely to become a void after melting and solidifying. In addition, since carbon dioxide and water vapor also cause a decrease in the superconducting properties, it is preferable that they are not present in the densifying atmosphere. Considering the above, it is preferable to perform the densification in a pure oxygen atmosphere.

[0155] When densification is performed in an oxygen atmosphere, at the start of heating, the voids in the compact are connected to the outside, and the gas in the voids is replaced with oxygen. When the voids are subsequently isolated from the outside by sintering or melting, oxygen is left behind in the voids. When the oxygen is subsequently absorbed into the surrounding bulk during the melting and solidification process, the voids disappear or become smaller, and the amount of voids is thought to be reduced.

[0156] On the other hand, when the oxygen partial pressure is high, the peritectic temperature becomes high, and the temperature must be raised to melt the compact, which makes the raw material more likely to decompose.In addition, when the oxygen partial pressure is high, the RE / Ba solid solution (RE / Ba substitution) of RE123 tends to progress, which is undesirable because it deteriorates the superconducting properties.

[0157] Considering these factors, when densification by heating in an oxygen atmosphere is performed, it is preferable to perform the densification at a temperature that can suppress decomposition and at a pressure lower than atmospheric pressure. For example, in the case of a pure oxygen atmosphere, 5 Pa~10 2 It is preferable to carry out the process at a low vacuum of about 100 Pa, for example about 0.2 atm or less.

[0158] For example, when the densification step is carried out in a pure oxygen atmosphere of 0.2 atm or less, it is believed that decomposition can be suppressed and sintering can be achieved by setting the heating temperature to 800° C. or higher and 1050° C. or lower.

[0159] In addition, the mechanical strength of the REBCO bulk is also important to prevent damage caused by the effects of strong magnetic fields. For example, voids and cracks that occur in the REBCO bulk cause a decrease in mechanical strength.

[0160] In the densification step of the manufacturing method of this embodiment, voids and cracks in the metal oxide bulk can be reduced by heating the compact in a vacuum or a low-pressure oxygen atmosphere. In fact, by performing densification in a vacuum, the area of ​​voids in the cross-sectional area of ​​the cross section passing through the center of the metal oxide bulk and along the c-axis can be reduced to 10% or less. This can improve the mechanical strength of the metal oxide bulk. Furthermore, even when densification is performed in a vacuum, voids and cracks can be reduced while maintaining the homogeneity of the trapped magnetic field distribution. EXAMPLES

[0161] 13 and 14, the manufacturing method of the metal oxide bulk body 17 according to Example 4 and the evaluation results of the metal oxide bulk body will be described. As described above, in Examples 1 and 2, the REBCO bulk body manufactured by the TSMG method was used as the seed substrate 11. In Example 4, a bulk body was manufactured by the SDMG method (also referred to as the SDMG bulk body), and the SDMG bulk body was used as a seed substrate (also referred to as the SDMG seed substrate) to further manufacture a metal oxide bulk body 17 as a new REBCO bulk body by the SDMG method.

[0162] Fig. 13 is a flow chart showing an example of a method for manufacturing a metal oxide bulk body according to Example 4. Steps S21 to S24 proceed in the same manner as S11 to S14 in Fig. 2, and therefore some of the explanations will be omitted. In this example, steps S21 to S24 produced a metal oxide bulk body 13, which is a GdBCO bulk body having a cylindrical shape similar to that of Example 1. In step S24, the metal oxide bulk body 13 was separated from the seed substrate 11, which is a TSMG seed substrate. In addition, a region up to 1 mm in the height direction from the surface of the growth end side of the metal oxide bulk body 13 opposite to the cut surface from the seed substrate was removed, and a bulk body (sample 1) with a diameter of 67 mm and a height of 4 mm was obtained.

[0163] Next, a new compact was prepared (step S25). As the raw material powder of RE123 for the compact prepared in step S25, a material having a peritectic temperature lower than that of the bulk body to be used as the seed substrate, i.e., the metal oxide bulk body 13 obtained in step S24, was selected.

[0164] In this embodiment, a YBCO compact having a lower peritectic temperature than GdBCO was prepared. 7-x The raw material powder used was a mixture of powder containing (Y123) and Y2BaCuO5 (Y211) in a molar ratio of 7:3, 10 wt% Ag2O and 0.5 wt% CeO2 (or 0.1 wt% Pt).

[0165] This raw material powder was pressure molded using a hollow cylindrical mold to obtain a molded body with an outer diameter of 80 mm and an inner diameter of 35 mm. The molded body was densified (pre-melted) by melting it by heating, and the surface that would later come into contact with the seed substrate was mirror-polished. In the pressure molding process of this example, an acrylic acid ester that is soluble in an organic solvent was used as the binder. The pressure for pressure molding was 50 MPa. By densification, the outer diameter was reduced from 80 mm to approximately 65 mm, which was equivalent to the diameter of the metal oxide bulk body 13.

[0166] Thereafter, the molded body prepared in step S25 was placed on the metal oxide bulk body 13, which was the SDMG bulk body obtained in step S24 (step S26, placing step). In step S26, the surface of the metal oxide bulk body 13 on which the molded body was placed was set to the (001) plane, and the surface on which the (001) plane was exposed was mirror-polished.

[0167] The seed substrate may be a portion cut out of the metal oxide bulk body 13. For example, a cylindrical metal oxide bulk body 13 is cut along the ab plane perpendicular to the c axis so that the surface is the (001) plane, thereby obtaining a plurality of plate-shaped seed substrates from the metal oxide bulk body 13. The thickness of the seed substrate may be 0.5 mm or more in order to prevent damage during handling.

[0168] Thereafter, the same paste as in Example 1 was applied to the mirror-polished surface of the molded body and adhered to the mirror-polished surface of a metal oxide bulk body 13, which is an SDMG seed substrate, thereby placing the molded body on the SDMG seed substrate.

[0169] After step S26, the new molded body was melted and solidified in the same manner as in Example 1, and a YBCO bulk body was crystal-grown starting from the SDMG seed substrate (GdBCO). The temperature control during melting and solidification was performed with a temperature gradient, as in Example 1. Specifically, the set temperature of the lower heater was set to the lowest and the set temperature of the upper heater was set to the highest, and a temperature gradient of 50°C was applied from the upper surface to the lower surface.

[0170] Regarding the temperature (T1) above the peritectic temperature of the green body and below the peritectic temperature of the SDMG seed substrate, and the temperature (T2) close to the peritectic temperature of the green body, the actual measured temperatures in the furnace near the sample were set to T1 and T2. Specifically, taking into consideration the peritectic temperature of Ag-added Y123 of 980°C and the peritectic temperature of Ag-added SDMG seed substrate (GdBCO) of 1010°C, the actual measured temperature T1 was set to 990°C and the actual measured temperature T2 was set to 975°C.

[0171] After step 27, a diamond saw was used to separate a new bulk body (YBCO bulk body) from the SDMG seed substrate (GdBCO) at the interface with the SDMG seed substrate (step S28). In this way, a YBCO bulk body (metal oxide bulk body 17, designated as sample 9) grown on the SDMG seed substrate was obtained. A 1 mm region was removed from the growth end of the YBCO bulk body of sample 9, and the surface was polished to obtain a bulk body with an outer diameter of 65 mm, an inner diameter of 29 mm, and a height of 13 mm. The SDMG seed substrate can also be reused as a seed substrate for the SDMG method, just like the TSMG seed substrate.

[0172] After step S28, reduction annealing (step S29) was performed, and then oxygen annealing (step S30) was performed to cause sample 9 to exhibit superconducting properties, thereby forming a superconductor.

[0173] [Crystallization] Fig. 14 is a diagram showing the measurement points and measurement results of rocking curve measurement for the (005) plane by X-ray diffraction method for a YBCO bulk (sample 9) grown on an SDMG seed substrate, which is the metal oxide bulk 17 of Example 4. The measurement method is the same as in Example 1. As shown in Fig. 14, the measurement points were measurement points P11 to P14, which are midpoints between the inner circumference and the outer circumference on the seed-side surface 17S of the metal oxide bulk 17.

[0174] The graph on the right side of FIG. 14 shows the results of rocking curve measurements for measurement points P11 to P14. The full width at half maximum of the two peaks at measurement point P11 were 0.30° and 0.47° from the low angle side, respectively. The full width at half maximum of the peak at measurement point P12 was 0.51°. The full width at half maximum of the two peaks at P13 were 0.35° and 0.33° from the low angle side, respectively. The full width at half maximum of the peak at measurement point P14 was 0.72°.

[0175] As described above, the full width at half maximum of the peak in the rocking curve measurement on the seed side surface of the metal oxide bulk body 15, which is a ring-shaped YBCO bulk body melted and solidified on a TSMG seed substrate, was 0.83° and 0.76° at measurement point P6, and 0.72° at measurement point P7 (see FIG. 9 ). In this example, the full width at half maximum of the similar measurement results was at most 0.72°, and it can be said that the metal oxide bulk body 21 has high crystallinity equivalent to or higher than that of the metal oxide bulk body 15.

[0176] This is believed to be because the SDMG seed substrate has high crystallinity, and the SDMG method allows crystal growth to proceed in line with the crystals of the seed substrate.

[0177] [Superconducting properties evaluation] Metal oxide bulk 17 was magnetized by applying an external magnetic field of 1.5 T (tesla) at liquid nitrogen temperature (77 K) and the trapped magnetic field distribution was evaluated, and a uniform trapped magnetic field distribution was obtained. Specifically, the average circularity C of the magnetic field contours calculated at intervals of 0.02 T from 0 T to the maximum trapped magnetic field was calculated and found to be 0.98 or more.

[0178] Therefore, according to the manufacturing method of this embodiment, a metal oxide bulk body produced by the SDMG method can be used as a seed substrate to produce a new metal oxide bulk body by the SDMG method, thereby producing a bulk with higher homogeneity.

[0179] In the manufacturing method of the bulk body by the SDMG method in the above-mentioned Examples 1 to 3, a seed substrate may be placed on the upper surface of the precursor. In that case, when a temperature gradient is applied during melting and solidification, the temperature on the upper surface side is always lower. For example, heating may be performed only by a lower surface heater. In that case, it is preferable to use a seed substrate of a shape and size that covers the entire upper surface of the molded body to be manufactured.

[0180] It is believed that the crystallinity of the TSMG seed substrate also has an effect on the SDMG method. For example, when comparing a portion of the TSMG seed substrate with a larger c-axis growth region and a portion of the TSMG seed substrate with a larger a-axis growth region, the SDMG bulk grown on the TSMG seed substrate with a larger c-axis growth region tends to have higher crystallinity.

[0181] Table 1 shows a list of each sample of the metal oxide bulk material evaluated in the above examples.

[0182] [Table 1]

[0183] The configurations and manufacturing methods in the above-described embodiments are merely examples and can be modified as appropriate depending on the application, etc.

[0184] In the above examples, the description has focused on metal oxide bulk bodies having a cylindrical shape and a hollow cylindrical shape (ring shape), but the shape of the metal oxide bulk body is not limited to this. As described above, by using molded bodies having various shapes, such as a rectangular parallelepiped, as a precursor, metal oxide bulk bodies having various shapes can be grown by the SDMG method.

[0185] [Increase in size] In addition, in the modified example of Example 1, a metal oxide bulk body is produced using a molded body larger than the seed substrate. Another method for increasing the size of the metal oxide bulk body of the present invention is to combine multiple seed substrates. Multiple seed substrates whose upper surfaces serve as the growth surfaces of the bulk are cut into square or hexagonal prism shapes, and are arranged without gaps so that the growth surfaces are flush with each other, and a large molded body is placed on the multiple growth surfaces to grow a metal oxide bulk body by the SDMG method. According to this method, for example, a metal oxide bulk body whose maximum width at the bottom surface is larger than the maximum width of the growth surface of each seed substrate can be grown.

[0186] For example, seed substrates with a diameter of approximately 65 mm or more are generally difficult to obtain, but by combining multiple seed substrates as described above, it is possible to grow a rectangular metal oxide bulk (hereinafter also referred to as a rectangular bulk) having a bottom surface that faces the seed substrate during growth, with the long side (length) of the bottom surface being 50 to 100 mm or more, a short side (width) being approximately 10 mm to 100 mm, and a height (thickness) being 50 mm or less.

[0187] When multiple seed substrates are arranged in this manner, in order to grow a bulk body in a single growth region, it is necessary that the same crystal plane is exposed on the multiple growth surfaces, for example, all of the growth surfaces are (001) planes. From the viewpoint of the in-plane uniformity of the crystal in the bulk to be grown, it is preferable that when multiple seed substrates are arranged, the deviation in the directions of the a-axes between adjacent seed substrates is 5° or less.

[0188] [Crystal growth direction] Furthermore, the above-mentioned rectangular metal oxide bulk can be applied to a bulk current lead, which is a current-carrying conductor for supplying current from a room temperature power source to a superconducting coil at a very low temperature, by taking advantage of its lower thermal conductivity than copper, etc. When using the rectangular bulk as a bulk current lead, the rectangular bulk is cut along the long side direction to obtain the desired dimensions, taking advantage of the length of the long side. For example, it is cut into a rod or plate shape with a long side of 100 mm, a short side of about 5 mm, and a height of about 5 mm, and used by passing a current along the long side direction.

[0189] As described above, the REBCO bulk metal oxide bulk has the property that the electric current flows along the ab plane and does not flow in the c-axis direction. Therefore, in order to make the electric current flow in the long side direction of the rectangular bulk, it is preferable to make the ab plane parallel to the long side. For example, when the growth surface of the seed substrate is the (001) plane and the entire surface is the c-axis growth region to grow a rectangular bulk, if the compact is placed so that the long side of the rectangular bulk is parallel to the growth surface of the seed substrate, the ab plane perpendicular to the c-axis and the long side of the rectangular bulk will be parallel. This allows the electric current to flow along the long side of the rectangular bulk.

[0190] The rectangular bulk as described above may be grown with the seed substrate's growth surface being an ac plane, i.e., (100), (010) or (110). In that case, the entire bulk will be an a-axis growth region (a-growth region). Even in this case, the long sides of the obtained rectangular bulk can be parallel to the ab plane, and a current can be passed in the long side direction.

[0191] For example, when placing the compact on a seed substrate, the long side of the bottom surface of the rectangular parallelepiped is parallel to the a-axis parallel to the growth surface of the seed substrate, that is, the long side extends in the direction along the a-axis parallel to the growth surface, so that either the short side of the bottom surface or the height of the rectangular parallelepiped becomes parallel to the c-axis. This allows the long side of the bottom surface of the rectangular parallelepiped bulk to be parallel to the ab-plane.

[0192] Metal oxide superconducting bulk materials are often used as magnetic sources for MRI and NMR. The reasons why the cylindrical or cylindrical metal bulk materials of the above-mentioned embodiments of the present application are suitable for application to MRI or NMR are described below. When using a metal oxide superconducting bulk material as a superconductor after oxygen annealing, in order to obtain sufficient superconducting properties, the direction of the current (superconducting current) flowing in the bulk when magnetized needs to be parallel to the bottom or top surface of the cylindrical or ring shape.

[0193] Furthermore, for a metal oxide superconducting bulk, having a cylindrical or columnar shape is preferable in order to make the trapped magnetic field distribution uniform.

[0194] Considering these points, it is preferable to grow the crystal so that the bottom or top surface of the cylindrical or ring-shaped body has an ab-plane parallel to it, and it is preferable that the c-axis perpendicular to the ab-plane is aligned along the height direction. Therefore, when manufacturing a cylindrical or ring-shaped superconducting bulk body, it is preferable to grow the crystal along the height direction by the SDMG method to make the entire body a c-axis growth region.

[0195] Therefore, the cylindrical or ring-shaped metal oxide bulk bodies described in the examples of this application, whose entire bodies are c-axis growth regions, are suitable for applications to NMR and MRI because, due to their shape, the direction parallel to the bottom surface is the a-axis, and therefore the current flows parallel to the bottom surface.

[0196] As described above, by taking into consideration the shape of the bulk and the direction of the current depending on the application, and by selecting the crystal plane of the growth surface of the seed substrate, a metal oxide bulk with a single growth region that suits the purpose can be obtained. [Explanation of symbols]

[0197] 11 types of substrate 13, 15, 17 Bulk metal oxides 14 Molded body

Claims

1. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) ２ BaCuO ５ The crystals are made of a composition represented by the formula: The end face 1 exposed at the growth end is a (001) plane, and the crystallinity is gradually improved with increasing distance from the end face 1, and the RE ２ BaCuO ５ The volume fraction of decreases with increasing distance from the end face of the first portion, The maximum width of the planar shape when viewed from the c-axis direction is 40 mm or more. A metal oxide bulk body comprising:

2. A hollow cylindrical or hollow truncated cone shape, 2. The metal oxide bulk body according to claim 1, wherein the proportion of void area in the cross-sectional area decreases from the inside of the hollow cylindrical or hollow truncated cone toward the inner and outer surfaces.

3. REBa measured at measurement points located on the outer periphery when viewed from above by X-ray diffraction method ２ Cu ３ O ７－ｘ 3. The metal oxide bulk material according to claim 1, characterized in that the full width at half maximum of the peak in a rocking curve measurement for the (005) plane is 2 degrees or less.

4. REBa measured at measurement points located on the outer periphery when viewed from above by X-ray diffraction method ２ Cu ３ O ７－ｘ A metal oxide bulk body as described in claim 1 or 2, characterized in that the rocking curve measurement for the (005) plane has one peak or the central positions of multiple peaks are within a range of 2 degrees.

5. 3. The metal oxide bulk according to claim 1, which comprises a single growth region with no boundary between an a-axis growth region and a c-axis growth region.

6. A crystal having a composition represented by REBa 2 Cu 3 O 7-x (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and a crystal having a composition represented by RE 2 BaCuO 5 , The end face 1 exposed at the growth end is a (001) plane, the crystallinity gradually improves with increasing distance from the end face 1, and the volume fraction of the RE 2 BaCuO 5 decreases with increasing distance from the end face 1; A metal oxide bulk body, characterized in that the area of ​​voids in a cross section passing through the center and along the c-axis is 10% or less.

7. A metal oxide bulk material according to any one of claims 1, 2 and 6, characterized in that it is a superconductor.

8. A superconductor comprising: When the perimeter of each isomagnetic field line in the trapped magnetic field distribution is L and the area of ​​the region surrounded by each isomagnetic field line is S, C = 4πS / L ２ 3. The metal oxide bulk body according to claim 1, wherein the average circularity C of each of the isomagnetic field lines expressed as:

9. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) ２ BaCuO ５ A method for producing a metal oxide bulk body having a crystal composition represented by the formula: The REBa ２ Cu ３ O ７－ｘ and the above RE ２ BaCuO ５ preparing a molded body consisting of a mixture of The molded body is ２ Cu ３ O ７－ｘ a step of placing the semiconductor substrate on a seed substrate made of a material having a peritectic temperature higher than that of the semiconductor substrate; a melting and solidifying step of heating the molded body for a predetermined time, maintaining the temperature at or below the peritectic temperature of the seed substrate and at or above the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify the molded body, thereby obtaining the metal oxide bulk body; Separating the metal oxide bulk body from the seed substrate; In the step of preparing the molded body, ２ Cu ３ O ７－ｘ The powder and the RE ２ BaCuO ５ A method for producing a metal oxide bulk body, comprising compressing and molding a mixed powder of the above powder at a pressure of less than 50 MPa.

10. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) ２ BaCuO ５ A method for producing a metal oxide bulk body having a crystal composition represented by the formula: The REBa ２ Cu ３ O ７－ｘ and the RE ２ BaCuO ５ preparing a molded body consisting of a mixture of The molded body is ２ Cu ３ O ７－ｘ a step of placing the semiconductor substrate on a seed substrate made of a material having a peritectic temperature higher than that of the semiconductor substrate; a melting and solidifying step of heating the molded body for a predetermined time, maintaining the temperature at or below the peritectic temperature of the seed substrate and at or above the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify the molded body, thereby obtaining the metal oxide bulk body; Separating the metal oxide bulk body from the seed substrate; A method for producing a metal oxide bulk body, characterized in that in the melting and solidifying process, the temperature of the molded body is controlled by creating a temperature gradient in a direction perpendicular to the seed substrate so that the temperature is lower in the portion of the molded body closer to the surface in contact with the seed substrate.

11. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) ２ BaCuO ５ A method for producing a metal oxide bulk body having a crystal composition represented by the formula: The REBa ２ Cu ３ O ７－ｘ and the above RE ２ BaCuO ５ preparing a molded body consisting of a mixture of The molded body is ２ Cu ３ O ７－ｘ a step of placing the semiconductor substrate on a seed substrate made of a material having a peritectic temperature higher than that of the semiconductor substrate; a melting and solidifying step of heating the molded body for a predetermined time, maintaining the temperature at or below the peritectic temperature of the seed substrate and at or above the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify the molded body, thereby obtaining the metal oxide bulk body; Separating the metal oxide bulk body from the seed substrate; A method for producing a metal oxide bulk body, characterized in that a metal oxide bulk body produced by a manufacturing method comprising the steps of: using a metal oxide bulk body produced by the manufacturing method comprising the steps of: as the seed substrate, and producing a new metal oxide bulk body by the same manufacturing method.

12. A method for producing a metal oxide bulk body described in any one of claims 9 to 11, characterized in that in the process of preparing the molded body, the molded body is made to have a hollow cylindrical shape.

13. A method for manufacturing a metal oxide bulk body described in any one of claims 9 to 11, characterized in that in the process of preparing the molded body, the molded body is molded to a size such that the maximum width of its planar shape when viewed in a direction perpendicular to the surface on which it is placed on the seed substrate is 50 mm or more.

14. The step of preparing the molded body includes the step of ２ Cu ３ O ７－ｘ The powder and the RE ２ BaCuO ５ A method for producing a metal oxide bulk body described in any one of claims 9 to 11, characterized in that it further comprises a densification process of heating a compact formed from a mixed powder of the powder and the powder of the above for a predetermined period of time to densify the compact.

15. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) ２ BaCuO ５ A method for producing a metal oxide bulk body having a crystal composition represented by the formula: The REBa ２ Cu ３ O ７－ｘ and the above RE ２ BaCuO ５ preparing a molded body consisting of a mixture of The molded body is ２ Cu ３ O ７－ｘ a step of placing the semiconductor substrate on a seed substrate made of a material having a peritectic temperature higher than that of the semiconductor substrate; a melting and solidifying step of heating the molded body for a predetermined time, maintaining the temperature at or below the peritectic temperature of the seed substrate and at or above the peritectic temperature of the molded body, and then cooling the molded body to melt and solidify the molded body, thereby obtaining the metal oxide bulk body; Separating the metal oxide bulk from the seed substrate; The step of preparing the molded body includes the step of ２ Cu ３ O ７－ｘ The powder and the RE ２ BaCuO ５ a densification step of heating the mixture of the powder and the powder for a predetermined period of time to densify the compact; A method for producing a metal oxide bulk body, characterized in that in the densification step, the molded body is heated in a vacuum or in an oxygen atmosphere for a predetermined period of time.

16. The method for producing a metal oxide bulk body according to claim 15, characterized in that in the densification step, the compact is heated at a temperature of 800°C or higher in a low vacuum.

17. A method for producing a metal oxide bulk body described in any one of claims 9 to 11, 15 and 16, further comprising an oxygen annealing step of heating the metal oxide bulk body separated from the seed substrate by the separation step in an oxygen flow for a predetermined period of time.

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