Metal oxide bulk , metal oxide superconducting bulk , and method for producing metal oxide bulk

Abstract

The purpose is to provide a homogeneous metal oxide bulk and metal oxide superconducting bulk, and a method for producing a metal oxide bulk.　A metal oxide bulk body according to the present invention comprises crystals of a composition represented by REBa2Cu3O7-x (RE represents one or more of the rare earth elements and x satisfies 0≤x≤1) and crystals of a composition represented by RE2BaCuO5 and is characterized in that there is one surface having an exposed (001) plane, the crystallinity improves with distance from the one surface, and the volume fraction of the RE2BaCuO5 decreases with distance from the one surface.

Description

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

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

[0002] REBa ２ Cu ３ O ７－ｘ (REBCO, where RE represents a rare earth element) exhibits a critical temperature (up to 90 K) higher than the liquid nitrogen temperature (77 K) and a high critical current density (J ｃ ) and is therefore being actively researched and developed as a superconductor material.

[0003] Compared to low-temperature superconductors that are cooled to around the temperature of liquid helium (approximately 4K), REBCO is a high-temperature superconductor that can be used at relatively high temperatures, around the temperature of liquid nitrogen, so it is expected to reduce cooling costs by reducing cooling energy and simplifying the cooling system. ２ BaCuO ５ A molten and solidified bulk with REBCO dispersed therein can capture a strong magnetic field even in a small size. By utilizing these characteristics, the development of small-sized devices that do not require liquid helium and can be applied to, for example, nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), which require a strong magnetic field, is being considered.

[0004] Patent No. 5297148

[0005] Superconductor Science and Technology 35 (2022)094003(7pp)Ceramics International 49(2023) 39280-39288Ceramics International 49(2023) 22177-22186

[0006] For example, when applying the above-mentioned REBCO-based superconducting molten solidified bulk to an NMR or MRI device, uniformity (spatial uniformity) of the trapped magnetic field distribution is important to ensure high resolution. To achieve high uniformity of the trapped magnetic field distribution, the REBCO-based superconducting molten solidified bulk itself must be highly homogeneous. However, at present, it is difficult to obtain a highly homogeneous bulk, and the inability to obtain sufficient uniformity of the trapped magnetic field distribution has been an issue.

[0007] The present invention has been made in view of the above points, and has as its object to provide a homogeneous metal oxide bulk, a metal oxide superconducting bulk, and a method for manufacturing the metal oxide bulk.

[0008] The metal oxide bulk of the present invention is ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ The RE has one surface in which the (001) plane is exposed, and the crystallinity gradually improves with increasing distance from the one surface. ２ BaCuO ５ The volume fraction of the first surface decreases with increasing distance from the first surface.

[0009] The metal oxide bulk of the present invention is also ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ The crystals are composed of a composition represented by the formula (I) and have a hollow cylindrical or hollow truncated conical shape, and the proportion of the area of ​​voids in the cross-sectional area decreases from the inside of the hollow cylindrical or hollow truncated conical shape toward the inner and outer surfaces.

[0010] The metal oxide bulk of the present invention is also ２ Cu ３ O ７－ｘ(RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ and the area of ​​voids 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 ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body consisting of crystals of a composition represented by the formula: ３ preparing a hollow cylindrical compact made of a mixture of REBaCuO7-x and the REBaCuO5; ３ The method comprises a step of placing the formed body on a seed substrate made of a material having a peritectic temperature higher than that of O7-x; a step of heating the formed 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 formed body, and then cooling the formed 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.

[0012] The method for producing a metal oxide bulk material of the present invention further comprises the steps of: ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body consisting of crystals of a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３a melting and solidifying step of heating the compact for a predetermined time to maintain a temperature that is equal to or lower than the peritectic temperature of the seed substrate and equal to or higher than the peritectic temperature of the compact, and then cooling the compact to melt and solidify the compact, and a step of separating the bulk metal oxide from the seed substrate, wherein in the step of preparing the compact, ３ The mixed powder of the O7-x powder and the RE2BaCuO5 powder is press-molded at a pressure of less than 50 MPa.

[0013] The method for producing a metal oxide bulk material of the present invention further comprises the steps of: ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body consisting of crystals of a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ The method includes a placement step of placing the formed body on a seed substrate made of a material having a peritectic temperature higher than that of O7-x; a melting and solidifying step of heating the formed 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 formed body, and then cooling it to melt and solidify, thereby obtaining the metal oxide bulk body; and a step of separating the metal oxide bulk body from the seed substrate, wherein in the melting and solidifying step, the temperature of the formed body is controlled by applying a temperature gradient in a direction perpendicular to the seed substrate so that the temperature is lower in the portion of the formed body closer to the surface that contacts the seed substrate.

[0014] The method for producing a metal oxide bulk material of the present invention further comprises the steps of: ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５A method for producing a metal oxide bulk body consisting of crystals of a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ The present invention is characterized in that a metal oxide bulk produced by a manufacturing method including: a placement step of placing the formed body on a seed substrate made of a material having a peritectic temperature higher than that of O7-x; a melting and solidifying step of heating the formed 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 formed body, and then cooling the formed body to melt and solidify it, thereby obtaining the metal oxide bulk; and a step of separating the metal oxide bulk from the seed substrate, and using the metal oxide bulk produced by the manufacturing method as the seed substrate to produce a new metal oxide bulk by the manufacturing method.

[0015] The method for producing a metal oxide bulk material of the present invention further comprises the steps of: ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body consisting of crystals of a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ a melting and solidifying step of heating the compact for a predetermined time to maintain a temperature below the peritectic temperature of the seed substrate and above the peritectic temperature of the compact, and then cooling the compact to melt and solidify it, thereby obtaining the metal oxide bulk; a step of separating the metal oxide bulk from the seed substrate; and a step of preparing the metal oxide bulk. ３ The method includes a densification step in which a mixed powder of O7-x powder and the RE2BaCuO5 powder is press-molded into a compact, which is heated for a predetermined period of time to densify the compact, and the compact is heated in a vacuum or in an oxygen atmosphere for the predetermined period of time in the densification step.

[0016] The bulk metal oxide superconductor of the present invention is ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ wherein the average value of the circularity C of a plurality of the isomagnetic field lines is 0.98 or more, where L is the perimeter of the isomagnetic field lines, S is the area of ​​the region surrounded by the isomagnetic field lines, and C is 4πS / L2 (0<C≦1).

[0017] 1 is a perspective view showing a metal oxide bulk and a seed substrate according to Example 1 of the present invention; FIG. 2 is a flowchart showing an example of a method for manufacturing an oxide superconductor bulk according to the present invention; FIG. 3 is an explanatory diagram schematically showing the crystal growth direction by the SDMG method; FIG. 4 is a diagram showing measurement points and measurement results of rocking curve measurement by X-ray diffraction of an oxide superconductor bulk according to Example 1; FIG. 5 is a diagram showing measurement points and measurement results of rocking curve measurement by X-ray diffraction of an oxide superconductor bulk according to a comparative example; FIG. 6 is an explanatory diagram schematically showing the crystal growth direction of an oxide superconductor bulk according to the TSMG method; FIG. 7 is a perspective view showing a metal oxide bulk and a seed substrate according to a modified example of Example 1; FIG. 8 is a perspective view showing a metal oxide bulk and a seed substrate according to Example 2 of the present invention; FIG. 9 is a diagram showing measurement points and measurement results of rocking curve measurement by X-ray diffraction of a metal oxide bulk according to Example 1; FIG. 10 is a diagram showing a cross-sectional observation image of a metal oxide bulk according to Example 2; FIG. 11 is a diagram showing a cross-sectional observation image of a metal oxide bulk according to Example 3; FIG. 12 is a diagram showing a cross-sectional observation image of a metal oxide bulk according to Example 3; 10 is a diagram showing measurement points and measurement results of rocking curve measurement by X-ray diffraction method of an oxide superconductor bulk body according to Example 4. FIG.

[0018] In the following, preferred embodiments of the present invention will be described, but these 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.

[0019] The metal oxide bulk of the present invention is ２ Cu ３ O ７－ｘ (RE represents one or more elements of rare earth elements) and ２ BaCuO ５ REBCO is a bulk material (hereinafter also referred to as REBCO bulk material) consisting of crystals having a composition represented by the formula: ２ Cu ３ O ７－ｘ The x in the composition satisfies 0≦x≦1. ２ Cu ３ O ７－ｘ The substance having the composition represented by the formula is also called REBCO or RE123. ２ BaCuO ５ The substance having the composition represented by the formula (I) is also referred to as 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, and Lu).

[0020] FIG. 1 is a perspective view showing a state in which a metal oxide bulk body 13 according to a first embodiment 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. ２ Cu ３ O ７－ｘ and RE2BaCuO ５ In this example, the seed substrate 11 is a EuBCO bulk body containing Eu (europium) as RE.

[0022] The metal oxide bulk body 13 is formed on the seed substrate 11 by REBa. ２ Cu ３ O ７－ｘ and R.E. ２ BaCuO ５ In this example, the metal oxide bulk body 13 has a cylindrical shape. In this example, the metal oxide bulk body 13 is a GdBCO bulk body in which RE is Gd (gadolinium).

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

[0024] The SDMG method uses a seed substrate made of a REBCO bulk having a higher peritectic temperature than the green body, which is the precursor of the REBCO bulk to be produced. When the green body, which is the precursor of the metal oxide bulk 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 but higher than the peritectic temperature of the green body, the seed substrate does not undergo a peritectic reaction, i.e., does not melt, and the RE123 contained in the green body becomes a partially molten material through 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. Thereafter, as the seed substrate 11 and the green body are cooled, crystal growth proceeds starting from the seed substrate 11, and the metal oxide bulk 13, which is a molten solidified material in which RE211 is dispersed in RE123, is produced.

[0025] The portion of the upper surface of seed substrate 11 that contacts the molded body serves as the starting point for 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 on the metal oxide bulk body 13 before being separated from the seed substrate.

[0027] [Method for Producing Bulk Body] A method for producing the metal oxide bulk body 13 will be described in detail with reference to Fig. 2. Fig. 2 is a flow chart showing an example of a 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 REBa. ２ Cu ３ O ７－ｘ (RE123) powder and RE ２ BaCuO ５ In other words, the molded body 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 to RE211 is preferably 7:3. For example, when RE is Gd, the molar ratio of RE123 to RE211 is preferably 7.5:2.5. In this example, GdBa ２ Cu ３ O ７－ｘ (Gd123) and RE ２ BaCuO ５ A mixed powder with a molar ratio of 7.5:2.5 to (Gd211) was used.

[0029] Also, oxides of rare earth elements, such as BaCO ３ , and CuO in a predetermined molar ratio and then fired to obtain a mixed powder containing RE123 and RE211 in a predetermined ratio. For example, when RE is yttrium, Y ２ O ３ , BaCO ３ and CuO in a molar ratio of 1.3:1.7:2.4, and calcination is performed to obtain a mixed powder with a molar ratio of Y123 (YBCO):Y211 of 7:3.

[0030] The raw material powder also contains Ag ２ O may be added. Ag ２By adding O, the peritectic temperature of the molded body, which is the precursor of the metal oxide bulk body 13, can be lowered. ２ By adding O, it is possible to suppress voids in the formed metal oxide bulk body 13, and to increase the mechanical strength of the bulk body. ２ By adjusting the O content to about 10 wt %, the peritectic temperature of the precursor can be lowered by about 20° C. In this example, Ag in the raw material powder ２ The proportion of O was set to 10 wt %.

[0031] The raw material powder contains Pt or CeO ２ Pt or CeO may be added. ２ By adding Pt, for example, it is preferable to set the ratio of Pt in the raw material powder to about 0.1 wt %. ２ When adding CeO ２ It is preferable that the ratio of Pt and CeO is about 0.5 wt %. ２ Both of these may be added to the raw material powder. ２ was set to 0.5 wt%.

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

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

[0034] From the viewpoint of suppressing cracks, a low pressure during pressure molding is preferable. 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 compact having a diameter of about 25 to 35 mm is subjected to pressure molding at a pressure of about 100 MPa, the above-mentioned cracking problem does not occur. For example, in the case of a large compact having a diameter of 60 to 80 mm or more, the inventors of the present application have discovered that the lower the pressure molding, the lower the probability of cracking, making it possible to produce a high-quality metal oxide bulk body with good reproducibility. In this example, a cylindrical (disc-shaped) compact was produced by pressure molding at 25 MPa using a mold having a diameter of 80 mm.

[0036] [Densification by Heating] After pressure molding, the compact is heated to densify it (densification step). In the densification step, the compact is heated at a temperature higher than the peritectic temperature of the compact, thereby melting the RE123 and densifying it (hereinafter, such a densification process involving melting of the compact is also referred to as pre-melting or pre-melting process). Note that in the densification step, the compact 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 molded body is heated to the target temperature over a period of 6 to 12 hours (approximately 12 to 72 hours in the case of a large molded body to prevent cracks), held for approximately 2 to 24 hours (the holding time is extended depending on the size), and then cooled. For example, densification is performed in an air atmosphere during heating. In this example, a box furnace (muffle furnace) was used to heat the molded body in an air atmosphere at 1055°C, which is higher than the peritectic temperature of the seed substrate 11 (Ag-doped EuBCO), 1020°C, for 2 hours, thereby performing 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 of densifying the molded body that involves sintering will also be referred to as pre-sintering or pre-sintering treatment).

[0039] In the densification step, if the molded body is heated at a temperature higher than the peritectic temperature and RE123 is melted, the dimensions of the molded body will be reduced by approximately 20%. If the molded body is heated at a temperature lower than the peritectic temperature and only sintered without melting in the densification step, the dimensions will be reduced by approximately 5 to 15%. In this case, the molded body will undergo further dimensional changes as it undergoes the melting and solidification step described below, and will reach the dimensions of the final metal oxide bulk body 13.

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

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

[0042] [Placement on Seed Substrate] The compact is then 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 example, a seed substrate grown by the top seeded melt growth (TSMG) method was used.

[0043] More specifically, a plate-shaped bulk body was cut out from a EuBCO bulk body (EuBCO bulk material manufactured by Nippon Steel Corporation) produced by the TSMG method so that the surface was the (001) plane, and 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 compact was placed. Note that the thickness of the seed substrate should be 0.5 mm or more in order to prevent breakage during handling.

[0044] Thereafter, a slurry or paste was applied to the mirror-polished surface of the molded body, and the molded body was then attached to the mirror-polished surface of the seed substrate 11, thereby placing 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 example, 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 molded body on the seed substrate 11, the seed substrate 11 and the molded body were heated in a heating furnace in an air atmosphere under predetermined temperature control until the temperature reached a temperature above the peritectic temperature of the molded body and below the peritectic temperature of the seed substrate 11, and then cooled to melt and solidify the molded body (step S13). The melting and solidifying step yielded a metal oxide bulk body 13 with crystal growth on the seed substrate 11 ( FIG. 1 ). In the melting and solidifying step, the temperature was controlled as follows, for example.

[0046] First, while the temperature of the compact is being increased, solvent removal is performed to decompose and remove organic components contained in the slurry or paste. For example, the temperature is increased to 500°C in 6 hours, and then the compact is held at 500°C for 6 hours in a dry air stream. CO generated during the solvent removal process is removed. ２To prevent the molded body from stagnation, the molded body is held in a stream of dry air.

[0047] The temperature is then raised to a temperature (T1) above the peritectic temperature of the compact and below the peritectic temperature of the seed substrate 11 over approximately 6 hours, and maintained at that temperature for 0-6 hours, thereby partially melting the RE123 in the compact. The temperature is then lowered to a temperature (T2) close to the peritectic temperature of the compact over 1-3 hours. For example, the temperature T2 is set to a temperature slightly below the peritectic temperature of the compact. The temperature is then lowered (slowly cooled) to a temperature (T3) about 30°C lower than T2 over 80 hours or more (e.g., 100 hours), allowing crystal growth to occur starting from the seed substrate 11. The temperature is then lowered to room temperature over approximately 8 hours. The total time required for the melting and solidification process is approximately 100-200 hours.

[0048] For example, the temperatures T1 and T2 are set taking into consideration that the peritectic temperature of RE123 is lowered by about 20° C. by adding 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. Note that the actual measured temperature value is the actual measured value when the temperature is measured near the side of the sample (molded body) in the furnace, at a position about half the total height of the sample. Hereinafter, the actual measured temperature will be the actual measured value at the same measurement location.

[0050] In this example, RE is Gd, and therefore, taking into consideration that the peritectic temperature of Gd211 is 1030°C, the addition of Ag lowers the peritectic temperature by about 20°C, the measured temperature T1 was set to 1015°C and the measured temperature T2 was set to 1000°C.

[0051] - Temperature Gradient 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 transition from higher to lower than the peritectic temperature in order from the seed substrate 11 side, thereby allowing crystal growth to proceed.

[0052] In this example, melting and solidifying were performed with a temperature gradient. Specifically, a box furnace equipped with heaters on the top, side, and bottom surfaces was used, and the temperatures were set so that the temperature of the bottom heater was the lowest and the temperature of the top heater was the highest. After the temperature rise process to temperature T1, heating and cooling were performed in the melting and solidifying process 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 heaters, and the temperature of the top heater was set 25°C higher than that of the side heaters. In other words, a temperature gradient of 50°C was applied to the set temperature so that the temperature of the bottom heater would be the lowest. At this set temperature, the sample was heated so that the measured temperature near the side of the sample reached a temperature (T1) higher than the peritectic temperature of the compact. Thereafter, while maintaining the difference in set temperatures between adjacent heaters at 25°C, the sample was cooled (slowly cooled) to a temperature (T3) lower than the peritectic temperature of the compact, and then cooled to room temperature.

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

[0055] [Separation] After melting and solidifying, the metal oxide bulk body 13 was separated 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 separated from the seed substrate 11 was polished. Note that the seed substrate 11 after the metal oxide bulk body 13 has been separated can be reused as a seed substrate for manufacturing the metal oxide bulk body 13 by the above-mentioned SDMG method.

[0056] After melting and solidifying, the bulk body was removed (also referred to as "cutting") from the surface opposite the seed substrate to approximately 1 mm in the thickness direction, and the surface was then polished to expose the (001) plane. During melting and solidifying using the SDMG method, RE211 is extruded and accumulates on the uncrystallized side. The region from the growth end to a depth of 1 mm tends to be rich in RE211, with RE123 becoming unoriented. Since the region where RE123 is unoriented may be less likely to exhibit superconducting properties, it is preferable to remove approximately 1 to 2 mm from the growth end. In the bulk body after melting and solidifying, the proportion of RE211 increases with increasing distance from the seed substrate. This is thought to be due to the phenomenon of RE211 being extruded and accumulating on the uncrystallized growth end side during melting and solidifying using the SDMG method, as described above.

[0057] As mentioned above, when the compact is melted in the densification process, the dimensions of the compact are reduced by about 20%. In this case, the dimensions change little by melting and solidifying to form a bulk body. On the other hand, when only sintering is performed in the densification process or when densification is not performed, the dimensions are reduced by about 20% compared to when the compact is subsequently melted and solidified. In either case, a bulk body that is about 20% smaller than the compact is ultimately produced. In this example, a metal oxide bulk body 13 with a diameter of 67 mm was obtained from a compact with a diameter of 80 mm.

[0058] As mentioned above, the larger the diameter of the compact, the more likely it is that cracks will occur and uneven crystal growth will occur. For example, cracks will occur immediately after compaction, or after subsequent heating for densification or subsequent melting and solidification. These tendencies begin to appear when the diameter of the compact is around 50 mm, and tend to occur more frequently as the diameter increases. Therefore, when forming a bulk using a compact with a diameter of about 50 mm or more, more stable, high-quality bulk can be produced by reducing the pressure during pressurization and controlling the temperature with a temperature gradient during melting and solidification, as described above. In other words, the inventors of the present invention have discovered that in order to produce high-quality bulk with a high yield, manufacturing measures such as crack prevention are required once the diameter of the compact reaches about 50 mm.

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

[0060] The obtained bulk body 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 surfaces sloping from the outside to the inside. In the case of a cylindrical shape, the diameter on the growth end side is about 1-3%, and up to about 5%, smaller than the diameter on the seed side.

[0061] The tapered or trapezoidal shape (tapering) that narrows toward the top as described above occurs during melting and melt-solidification in the densification process. For example, tapering can be reduced by inverting the compact upside down when placing it on a seed substrate after melting it in the densification process. Furthermore, if only sintering is performed at a temperature that does not melt the compact in the densification process, no tapering occurs, but tapering occurs in the subsequent melt-solidification process.

[0062] The degree of tapering, i.e., the slope of the side surface and the dimensional difference between the top and bottom surfaces, tends to increase as the particle size of the raw material powder decreases. Furthermore, since the slope 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. The temperature and holding time of pre-melting are also considered to have an effect. Considering this tapering, the shape of a metal oxide bulk body produced using a cylindrical molded body can be said to be cylindrical or truncated conical.

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

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

[0065] Furthermore, in the densification step 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 step, the temperature cannot be raised above the peritectic temperature of the seed substrate because the peritectic reaction would deteriorate the crystallinity of 13. Therefore, performing the densification step is advantageous in that the density of the bulk body can be increased. For example, in the densification step before melting and solidifying (pre-melting), the temperature can be raised up to, for example, 1100°C, whereas during melting and solidifying, the temperature is set to be about 10°C lower than the peritectic temperature of the seed substrate, so the maximum temperature for melting and solidifying is about 1010°C.

[0066] Furthermore, 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] [Reducing Annealing] The metal oxide bulk body 13 was then heated in a reducing gas flow for a predetermined time to perform reducing annealing (step S15, reducing annealing step). In step S15, for example, the metal oxide bulk body 13 is heated in an Ar gas flow with an oxygen concentration of 1% at 800-900°C for 12-48 hours. In this example, the metal oxide bulk body 13 was heated in an Ar gas flow with an oxygen concentration of 1% at 850°C for 24 hours.

[0068] Note that reduction annealing is not essential, and even without reduction annealing, superconductivity can be exhibited in the metal oxide bulk 13 by performing oxygen annealing. By performing reduction annealing, the superconducting properties after subsequent oxygen annealing are improved. By performing reduction annealing, substitution of RE into Ba sites (RE / Ba substitution) can be suppressed. RE / Ba substitution in RE123 leads to deterioration of superconducting properties.

[0069] [Oxygen Annealing] After the reduction annealing, the metal oxide bulk 13 was heated in an oxygen stream to undergo oxygen annealing (step S16, oxygen annealing step). Because 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 bulk exhibits the property of being magnetized by an external magnetic field. 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 example, the metal oxide bulk body 13 was heated in an oxygen flow at temperatures ranging from 450°C to 350°C for 200 hours. A metal oxide superconductor bulk body was obtained by the above manufacturing method.

[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 diagram schematically 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 a 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, and therefore, as shown in Fig. 3, growth occurs uniformly from any location on the lower surface of the compact in the c-axis direction, starting from the seed substrate 11. The entire metal oxide bulk 13 is a single crystal growth region, and the entire region 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] 4 and 5, the crystallinity evaluation results for the metal oxide bulk 13 and the comparative example will be described. 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 producing the (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 in 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] Figure 4 shows the measurement points and measurement results of the rocking curve measurement for sample 1 of the metal oxide bulk body 13. For sample 1, the seed-side surface 13S, which is the surface that was in contact with the seed substrate 11, was measured. The left side of Figure 4 shows a schematic diagram of the positions of the measurement points on the seed-side surface 13S of sample 1. As shown in Figure 4, the measurement points were the center P1, P3 near the periphery, and P2 in the middle.

[0075] The graph on the right side of Fig. 4 shows the results of rocking curve measurements. In the graph on the right side of Fig. 4, the horizontal axis represents the angle ω between the incident X-ray and the sample surface, and the vertical axis represents normalized intensity. As shown in the graph in Fig. 4, sharp peaks were obtained at all measurement points, and the full widths at half maximum (FWHM) were 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 for the (005) plane of a EuBCO bulk material 100 (EuBCO bulk material manufactured by Nippon Steel Corporation) produced by the TSMG method as a comparative example, in the same manner as for the metal oxide bulk material 13.

[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 designated as the 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 at measurement points P4 and P5 within the bulk, which were 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, measurements were taken on a cross section of the EuBCO bulk body 100 cut along a plane passing through the center of the seed-side surface 100S and along the c-axis.

[0079] The graph in the center of Fig. 5 shows the results of rocking curve measurements at 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 at measurement point P1, with a full width at half maximum of 0.36°. Broad peaks were obtained at measurement points P2 and P3, with full widths at half maximum of 2.80° and 2.24°, respectively.

[0080] The graph on the right side of Figure 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, with the full width at half maximum being 0.52° for the peak at ω ≒ 19° and 0.71° for the peak near ω ≒ 18°.

[0081] The measurement results shown in Figure 4 indicate 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 Figure 5 indicates that the EuBCO bulk body 100 of the comparative example has high crystallinity only at measurement point P1, which is located in the center of the seed-side surface 100S, and that the crystallinity at measurement points P2 and P3 is significantly lower than that of Sample 1. It was confirmed that the crystallinity of the seed-side surface is higher in the bulk of this example.

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

[0083] 6 is a diagram schematically illustrating the crystal growth direction when a REBCO bulk body is formed by the TSMG method as a comparative example, using an EuBCO bulk body 100 as an example. As described above, the EuBCO bulk body 100 is formed by placing a seed crystal 101 in the center of the upper surface of a precursor compact when the compact is melted and then solidified, and crystal growth is initiated from the seed crystal 101.

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

[0086] [Trapped magnetic field distribution] Sample 1 of the metal oxide bulk body 13 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.

[0087] Let L be the perimeter of the isomagnetic field line of the trapped magnetic field, and S be the area of ​​the region surrounded by the isomagnetic field line. The circularity of the trapped magnetic field distribution is C = 4πS / L. ２ (0<C≦1) (C=1 for a perfect circle) was calculated for multiple isomagnetic field lines at intervals of 0.02 T from 0 T to the maximum trapped magnetic field. The average value of the circularity C of the multiple isomagnetic field lines was calculated and was found to be 0.95 or more.

[0088] As described above, the metal oxide bulk 13 according to Example 1 is a metal oxide bulk grown on a plate-shaped seed substrate and then separated from the seed substrate at the interface with the seed substrate. ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ The metal oxide bulk 13 is made of crystals having a composition represented by the formula: In addition, the metal oxide bulk 13 has a c-axis growth region extending from the cut surface formed when the bulk is separated from the seed substrate in a direction substantially perpendicular to the cut surface. In other words, the metal oxide bulk 13 is a metal oxide bulk consisting of a single growth region with no boundary between the a-axis growth region and the c-axis growth region.

[0089] When the crystallinity of the surface (seed-side surface) of sample 1 of metal oxide bulk 13 on the seed substrate side was evaluated, results were obtained showing that the crystallinity was higher than that of the surface on the seed crystal side of the bulk produced by the TSMG method of the comparative example. Furthermore, the crystallinity of the central and peripheral portions of the seed-side surface of sample 1 was equivalent. In other words, sample 1 had high crystallinity and uniform crystallinity within the plane of the seed-side surface. Furthermore, the trapped magnetic field distribution was also uniform. From these facts, it can be said that metal oxide bulk 13 is a homogeneous REBCO bulk.

[0090] Furthermore, the metal oxide bulk 13 has a diameter of 67 mm, which is large compared to small bulks of, for example, 20 mm to 30 mm. Such a large size makes it difficult to obtain sufficient superconducting properties with REBCO bulk produced by the TSMG method. According to this example, even with a large bulk, a highly uniform trapped magnetic field distribution was obtained, and sufficient superconducting properties were obtained.

[0091] Furthermore, even when producing REBCO bulk bodies using the SDMG method, the larger the diameter, the more likely it is that cracks will occur and uneven crystal growth will occur, posing problems in terms of reproducibility and yield.

[0092] According to the manufacturing method of this embodiment, when manufacturing the metal oxide bulk body 13 by the SDMG method, pressure molding is performed at a lower pressure than conventional methods, thereby making it possible to suppress the occurrence of cracks. Furthermore, by creating a temperature gradient so that the temperature is lower in the area closer to the seed substrate during melting and solidification, it is possible to prevent non-uniform crystal growth.

[0093] Therefore, according to this embodiment, it is possible to provide a homogeneous, high-yield metal oxide bulk, a metal oxide superconducting bulk, and a method for manufacturing a 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 in the case of a large molded body, cracks are less likely to occur in the bulk body even when the pressure during pressure molding is about 100 MPa. In addition, since there is a risk that RE123 will react with water and decompose, an organic solvent is used instead of water as the binder solvent. 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 perform desolvation to decompose and remove the binder component before the densification step. For example, desolvation is performed by heating in the atmosphere at 100 to 500°C for about 100 to 250 hours. For example, desolvation may be performed during the temperature increase for densification. The desolvation takes time for production.

[0096] The effect of pressure during compaction on the likelihood of cracking in 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] [Modification] Figure 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 modification of Example 1. In the above-described 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 this is not limited to this. As shown in Figure 7, a metal oxide bulk body may be produced using a molded body 14 that is larger than the seed substrate 11, in other words, whose outer edge surrounds the outer edge of the seed substrate 11 in a planar view.

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

[0099] According to this modification, for example, it is possible to produce a metal oxide bulk body whose outer edge surrounds the outer edge of seed substrate 11 in a plan view. Note that an a-axis growth region may occur in the portion of compact 14 that protrudes from seed substrate 11. 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, a 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.

[0100] 8 is a perspective view showing a state in which a metal oxide bulk 15 according to Example 2 has been 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 its shape and RE element, and is otherwise similar, so some of the overlapping parts will not be described. 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 using the SDMG method described above.

[0102] Even in the case of a ring-shaped molded body, as shown in Figure 3, crystal growth progresses from the side in contact with the seed substrate 11 (seed side) in the c-axis direction extending approximately perpendicular to the seed substrate, 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 molded 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 example were prepared, designated as Samples 2 and 3. Sample 2 was a YBCO bulk body formed by the SDMG method on a seed substrate 11 using a ring-shaped compact with an outer diameter of 80 mm and an inner diameter of 35 mm, with Y (yttrium) as the RE. The pressing pressure for pressure molding was 25 MPa, and melting and solidification were performed under a temperature gradient. The finished size (i.e., the size after melting and solidification, 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 pressure 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, a region extending 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 described below.

[0107] [Crystallinity Evaluation] Figure 9 shows the measurement points and measurement results of rocking curve measurement for sample 2 of the metal oxide bulk body 15, similar to Example 1. As shown on the left side of Figure 9, the measurement points were P6 on the seed-side surface 15S, P7 located on the outer periphery of P6, and measurement points P8 and P9 within the bulk, each 13 mm away in the depth direction (height direction) from P6 and P7.

[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, which indicates high crystallinity.

[0109] The graph on the right side of Figure 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 the metal oxide bulk body 15, the full width at half maximum of the peak in the rocking curve measurement is 2° or less for measurement point P8 located on the inner periphery when viewed from above and measurement point P9 located on the outer periphery, and that 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 are 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] It can also 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 increasing distance from the (001) plane, i.e., with increasing proximity to 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), it can be said that the bulk material is homogeneous and highly crystalline overall.

[0114] In addition, a similar trend 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, a tendency for crystallinity 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 was a cylindrical YBCO bulk body, as well as at a point 5 mm away from there in the depth direction and a point 8 mm deep. The peaks broadened and the full width at half maximum gradually increased with increasing distance from the seed-side surface, with the angles being 0.67°, 1.11°, and 1.51°, respectively, from the seed side. Because Sample 4 was small, with a diameter of 23 mm, 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 toward the seed-side surface and lower crystallinity with increasing distance from the seed-side surface. This trend differs from that of bulk bodies grown by the TSMG method. As shown in Figure 5, in the case of bulk bodies grown by the TSMG method, the crystallinity increases with increasing distance 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 thought to be that random crystal growth becomes more likely to occur in the latter half of crystal growth. More specifically, due to the peritectic reaction in which crystals precipitate from a partially molten state, crystal growth initially proceeds in alignment with the crystal orientation of the seed substrate. In the latter half of crystal growth, the degree of supercooling of the partially molten RE123 increases, so that crystal growth proceeds even if the crystal orientation with the seed substrate does not necessarily align, making random crystal growth more likely to occur. Furthermore, because crystal growth proceeds from the seed substrate side, crystal growth occurs in parts farther from the seed substrate as crystal growth approaches its end. From the above, it is thought that random crystal growth becomes more likely to occur in parts farther from the seed substrate.

[0118] Furthermore, as crystal growth by the SDMG method progresses and the melting and solidification process progresses to the latter half, the proportion of RE211, which is a solid phase component, increases in the non-crystallized portion. As a result, the later the melting and solidification process progresses, the more likely it is that misorientation due to the incorporation of RE211 into the RE123 crystals will occur, which is thought to be one of the reasons why crystallinity decreases with increasing distance from the seed-side surface. Note that Non-Patent Document 2 describes that for DyBCO bulk grown by the SDMG method, the volume fraction of Dy211 particles in the bulk increases with increasing distance from the seed substrate.

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

[0120] As explained above, in a 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 the seed substrate is defined as plane 1 where the (001) plane is exposed, the crystallinity gradually improves with increasing distance from plane 1, and the volume fraction of RE211 decreases with increasing distance from plane 1. The decrease in crystallinity with increasing distance from the seed-side surface can be said to be a tendency specific to bulk crystals grown by the SDMG method, which grows in one direction starting from the seed substrate.

[0121] [Cross-sectional structure evaluation] Figure 10 shows a cross-sectional observation image of Sample 3, a YBCO bulk body different from Sample 2 of the metal oxide bulk body 15. The YBCO bulk body of Sample 3 has a hollow cylindrical shape similar to 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 taken along a plane 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, while there are fewer voids near the top surface, outer surface, and inner surface on the growth end side.

[0124] It is believed that voids in the bulk grown by melting and solidifying are caused by inert gases such as nitrogen and Ar remaining in the bulk. Furthermore, during crystal growth by melting and solidifying, bubbles of inert gases are expelled from the surface by diffusion, resulting in fewer voids near the surface.

[0125] 10, sample 3 not only has few voids near the surface (upper surface) on the growth end side and near the outer surface, but also has 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 bubbles were expelled from the inner surface of the ring shape during melting and solidification.

[0126] On the other hand, when producing a ring-shaped bulk body by the TSMG method, a seed crystal is first placed at the center of the upper surface of a disk-shaped compact, which is then melted and solidified, and then a hole is drilled in the center by machining to form 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 a region with many voids. In addition, damage during machining after crystal growth of the REBCO bulk body by such melting and solidification may reduce its mechanical strength.

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

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

[0129] [Trapped 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 evaluated for the trapped magnetic field distribution. A uniform trapped magnetic field distribution was obtained. The average circularity C of the isomagnetic field lines calculated at 0.02 T intervals 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, an inner diameter of 17 mm, and a 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, an inner diameter of 13 mm, and a height of 10 mm was 0.996. Many other bulk bodies have also been produced with a circularity of trapped magnetic field distribution of 0.98 or more. 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 that 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 with a diameter of about 20 to 30 mm.

[0133] Furthermore, Y is inexpensive among 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 requiring a homogeneous magnetic field. However, since YBCO has a low peritectic temperature and a slow growth rate, it has been difficult to grow it large using the TSMG method, which requires a long time for growth as the size increases. Furthermore, it has been difficult to obtain a homogeneous bulk using the TSMG method as the size increases.

[0134] As mentioned above, 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 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 with 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 a link-shaped bulk body of any size can be produced by the SDMG method. For example, an example of the size of a REBCO bulk body suitable for NMR applications is a hollow cylindrical shape with an inner diameter of 5 mm or more and an outer diameter of 20 to 200 mm. The difference between the inner and outer diameters is 5 mm or more. 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.

[0137] [Densification in a vacuum] In Example 1, densification by heating in the atmosphere was described, but the heating furnace may be evacuated during densification. The term "vacuum" used here refers to the vacuum defined in the JIS standard, which refers to a space filled with a gas at a pressure lower than atmospheric pressure. The inventors of the present application have discovered that evacuating the heating furnace during densification can reduce the number of voids in the metal oxide bulk body 13 produced. The fewer 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 decreases for the following reason. It is believed that the voids are generated when gas in the furnace enters 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 enters the voids in the compact during the densification process, these voids become closed spaces through sintering or melting.

[0139] By creating a vacuum inside the heating furnace during densification, voids themselves are less likely to occur, or even if voids do exist, they are made into a vacuum, thereby reducing the number of voids.

[0140] However, when the inside of the heating furnace is kept in a vacuum state during densification, temperature control becomes important. When the densification process is carried out in a vacuum state, it is thought that melting occurs 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 Cu is produced by the decomposition of the raw materials. ２O precipitation occurs. 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 compact, it is preferable to perform densification at a temperature that suppresses decomposition and allows sintering without melting the compact. From the viewpoint of suppressing 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, the low vacuum refers to the low vacuum defined in the JIS standard, and is 10 ５ Pa to 10 ２ This refers to a vacuum of 800 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 atmospheres (≒ 2.0 × 10 ４ When the densification process is carried out at a temperature of 800°C or higher and 1050°C or lower, it is believed that decomposition can be suppressed and sintering can be achieved. －３ Atmospheric pressure (≒10 ２ When the heating is carried out at 800° C. or higher and 900° C. or lower, it is believed that decomposition can be suppressed and sintering can be achieved.

[0143] [Void Evaluation] Samples 7 and 8 of the metal oxide bulk body according to Example 3 will be described with reference to Figures 11 and 12. Samples 7 and 8 differ from each other only in the processing conditions of the densification step, but were otherwise manufactured under similar conditions, and are disk-shaped metal oxide bulk bodies manufactured in the same manner as metal oxide bulk body 13 of Example 1. As precursors for 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 for Sample 7, and densification was performed by heating in the atmosphere. Specifically, the compact was heated to 1,050°C in an atmospheric air atmosphere at normal pressure in a box furnace, and then maintained at that temperature for 2 hours, thereby performing densification in the atmosphere.

[0145] The other compact was used as a precursor for sample 8 and was densified 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. The degree of vacuum during the temperature hold at 950°C was approximately 0.1 atmospheres (≒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 evacuate the furnace at all times during heating, including during temperature rise.

[0147] The two densified compacts were then 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 bulk bodies of Samples 7 and 8 both had a diameter of 25 mm. 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 fraction.

[0148] The void ratio was evaluated by binarizing the image observed with 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 ((a) in Figure 11) and a binarized image ((b) in Figure 11) of sample 7. For the metal oxide bulk body 17 pretreated in air, the area ratio of the black areas 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 approximately 55% compared to Sample 7.

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

[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 trapped magnetic field distribution was found to be equivalent, and showed a uniform and sufficiently high trapped magnetic field.

[0153] Therefore, it was found that densification by heating in a vacuum (vacuum pre-heat treatment) can significantly reduce voids and cracks without affecting the trapped magnetic field distribution. Reducing voids and cracks improves mechanical strength. Naturally, the same applies to large bulk samples with a diameter of 40 mm or more, which are larger than Sample 8, because creating a vacuum inside the heating furnace during densification makes it difficult for voids to occur, or even if voids exist, the voids are evacuated, thereby reducing the number of voids.

[0154] [Densification in an Oxygen Atmosphere] Densification by heating in an oxygen atmosphere can also suppress voids in the bulk body. Since oxygen is absorbed by REBCO bulk, if the gas in the voids (isolated from the outside) left behind by sintering or melting is oxygen, it is thought that voids are less likely to form after melting and solidifying. Furthermore, since carbon dioxide and water vapor also cause a decrease in superconducting properties, it is preferable that they are not present in the densification atmosphere. Considering the above, it is preferable to perform densification in a pure oxygen atmosphere.

[0155] When densification is performed in an oxygen atmosphere, the voids in the compact are connected to the outside at the start of heating, 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. Also, when the oxygen partial pressure is high, RE / Ba solid solution (RE / Ba substitution) of RE123 tends to progress, which is undesirable because it deteriorates the superconducting properties.

[0157] Considering these points, when densification is performed by heating in an oxygen atmosphere, 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, 10 ５ Pa to 10 ２ It is preferable to carry out the treatment in a low vacuum of about 0.2 Pa, for example.

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

[0159] Furthermore, the mechanical strength of the REBCO bulk is also important to prevent damage due to the influence of a strong magnetic field. For example, voids and cracks occurring in the REBCO bulk cause a decrease in mechanical strength.

[0160] In the densification process of the manufacturing method of this example, heating the compact in a vacuum or a low-pressure oxygen atmosphere can reduce voids and cracks in the metal oxide bulk. In fact, by performing densification in a vacuum, the void area of ​​the cross-sectional area of ​​the metal oxide bulk in a cross section passing through the center 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.

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

[0162] FIG. 13 is a flowchart showing an example of a method for manufacturing a metal oxide bulk according to Example 4. Steps S21 to S24 proceed in the same manner as steps S11 to S14 in FIG. 2, and therefore some of the explanations will be omitted. In this example, steps S21 to S24 were used to produce a metal oxide bulk 13, which was a GdBCO bulk having a cylindrical shape similar to that of Example 1. In step S24, the metal oxide bulk 13 was separated from the seed substrate 11, which was the TSMG seed substrate. In addition, a region up to 1 mm in height from the surface of the growth end side of the metal oxide bulk 13 opposite the cut surface from the seed substrate was removed, and a bulk (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 lower peritectic temperature than 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 example, a YBCO compact having a lower peritectic temperature than GdBCO was prepared. ２ Cu ３ O ７－ｘ (Y123) and Y ２ BaCuO ５ (Y211) in a molar ratio of 7:3, and 10 wt% Ag ２ O and 0.5 wt% CeO ２ (or 0.1 wt % Pt) was mixed into the raw material powder.

[0165] This raw material powder was pressure-molded using a hollow cylindrical mold to produce 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 heating and melting, 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 soluble in an organic solvent was used as the binder. The pressure for pressure-molding was 50 MPa. As a result of 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 compact 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, placement step). In step S26, the surface on which the compact of the metal oxide bulk body 13 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 flat seed substrates from the metal oxide bulk body 13. The thickness of the seed substrate should be 0.5 mm or more to prevent breakage during handling.

[0168] Then, the same paste as in Example 1 was applied to the mirror-polished surface of the molded body, and the molded body was placed on the SDMG seed substrate by adhering it to the mirror-polished surface of the metal oxide bulk body 13, which is 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). Temperature control during melting and solidification was performed with a temperature gradient, as in Example 1. Specifically, the set temperature of the bottom heater was set to the lowest and the set temperature of the top heater to the highest, creating a temperature gradient of 50°C from the top to bottom surfaces.

[0170] The temperature (T1) above the peritectic temperature of the compact and below the peritectic temperature of the SDMG seed substrate and the temperature (T2) close to the peritectic temperature of the compact were set so that the actual measured temperatures in the furnace near the sample were T1 and T2. Specifically, taking into account the peritectic temperature of Ag-doped Y123 (980°C) and the peritectic temperature of Ag-doped SDMG seed substrate (GdBCO) (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 S27, a diamond saw was used to separate a new bulk (YBCO bulk) from the SDMG seed substrate (GdBCO) at the interface with the SDMG seed substrate (step S28). In this way, a YBCO bulk (metal oxide bulk 17, designated sample 9) grown on the SDMG seed substrate was obtained. A 1 mm region was removed from the growth end of the YBCO bulk of sample 9, and the surface was polished to obtain a bulk with an outer diameter of 65 mm, an inner diameter of 29 mm, and a height of 13 mm. The SDMG seed substrate, like the TSMG seed substrate, can be reused as a seed substrate for the SDMG method.

[0172] After step S28, reduction annealing (step S29) was performed, and then oxygen annealing (step S30) was performed to develop superconducting properties in sample 9, thereby making it a superconductor.

[0173] [Crystalline Quality Evaluation] Figure 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 Figure 14, measurement points P11 to P14, which are midpoints between the inner and outer circumferences on the seed-side surface 17S of the metal oxide bulk 17, were used as the measurement points.

[0174] The graph on the right side of Figure 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 was 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 was 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 15, which is a ring-shaped YBCO bulk melted and solidified on the TSMG seed substrate, was 0.83° and 0.76° at measurement point P6, and 0.72° at measurement point P7 (see Figure 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 21 has high crystallinity equivalent to or higher than that of the metal oxide bulk 15.

[0176] This is thought 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] [Evaluation of Superconducting Properties] The metal oxide bulk body 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. A uniform trapped magnetic field distribution was obtained. Specifically, 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 found to be 0.98 or more.

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

[0179] In the manufacturing method of a bulk body by the SDMG method in Examples 1 to 3 above, a seed substrate may be placed on the upper surface of the precursor. In this case, if a temperature gradient is created during melting and solidification, the temperature on the upper surface side should always be lower. For example, heating may be performed using only a lower surface heater. In this 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 also considered that the SDMG method is influenced by the crystallinity of the TSMG seed substrate. For example, when a portion of the TSMG seed substrate with a predominant c-axis growth region is used, the SDMG bulk grown on the TSMG seed substrate with a predominant a-axis growth region tends to have higher crystallinity than when a portion of the TSMG seed substrate with a predominant c-axis growth region is used.

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

[0182]

[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 (ring-shaped) shape, but the shape of the metal oxide bulk body is not limited to this. As described above, by using a molded body 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] [Increased Size] In addition, in the modified example of Example 1 above, it was explained that a metal oxide bulk was produced using a molded body larger than the seed substrate. Another method for increasing the size of the metal oxide bulk of the present invention is to combine multiple seed substrates. Multiple seed substrates, whose upper surfaces serve as the growth surfaces of the bulk, can be cut into square or hexagonal prism shapes, arranged without gaps so that the growth surfaces are flush, and a large molded body can be placed on the multiple growth surfaces to grow a metal oxide bulk using the SDMG method. With this method, it is possible to grow, for example, a metal oxide bulk whose maximum width at the bottom surface is larger than the maximum width of the growth surface of each seed substrate.

[0186] For example, seed substrates with a diameter of approximately 65 mm or more are generally difficult to obtain. However, 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 size in which, for example, the surface facing the seed substrate during growth serves as the bottom surface, the long side (length) of the bottom surface is 50 to 100 mm or more, the short side (width) is approximately 10 mm to 100 mm, and the height (thickness) is 50 mm or less.

[0187] In this case, when multiple seed substrates are used side by side, 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. Furthermore, from the viewpoint of in-plane uniformity of the crystal in the bulk to be grown, when multiple seed substrates are arranged, it is preferable that the deviation in the directions of the a-axes of adjacent seed substrates be 5° or less.

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

[0189] As described above, the REBCO bulk metal oxide bulk has the property that current flows along the ab plane and not along the c-axis. Therefore, in order to allow current to flow along the long side of the rectangular bulk, it is preferable to make the ab plane parallel to the long side. For example, when growing a rectangular bulk with the growth surface of the seed substrate as the (001) plane and the entire surface as the c-axis growth region, 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 current to flow along the long side of the rectangular bulk.

[0190] Furthermore, the rectangular bulk as described above may be grown with the seed substrate's growth surface set to the ac plane (100), (010), or (110). In this case, the entire metal oxide bulk will be an a-axis growth region. Even in this case, the long sides of the resulting rectangular bulk can be aligned parallel to the ab plane, allowing current to flow along the long sides.

[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, i.e., by extending in the direction along the a-axis parallel to the growth surface, 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 a-b 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 examples 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 within 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, it is preferable that it has a cylindrical or columnar shape in order to make the trapped magnetic field distribution uniform.

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

[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 in NMR and MRI because, due to their shape and the direction parallel to the bottom surface being the a-axis, 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 body with a single growth region that suits the purpose can be obtained.

[0197] 11 seed substrate 13, 15, 17 metal oxide bulk body 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) and RE ２ BaCuO ５ The RE has one surface in which the (001) plane is exposed, and the crystallinity gradually improves with increasing distance from the one surface. ２ BaCuO ５ A metal oxide bulk body characterized in that the volume fraction of the metal oxide particles decreases with increasing distance from the first surface.

2. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A metal oxide bulk body, characterized in that it is made of crystals having a composition represented by the formula: and has a hollow cylindrical or hollow truncated conical shape, and the proportion of void area in the cross-sectional area decreases from the inside of the hollow cylindrical or hollow truncated conical shape toward the inner and outer surfaces.

3. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A metal oxide bulk body comprising a crystal having a composition represented by the formula: wherein the area of ​​voids in a cross section passing through the center and along the c-axis is 10% or less.

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

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

6. A metal oxide bulk body according to any one of claims 1 to 5, characterized in that the maximum width in plan view from the c-axis direction is 40 mm or more.

7. A metal oxide bulk according to any one of claims 1 to 5, characterized in that it consists of a single growth region with no boundary between an a-axis growth region and a c-axis growth region.

8. A metal oxide bulk according to any one of claims 1 to 5, which is a superconductor.

9. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body made of crystals having a composition represented by the formula: ３ preparing a hollow cylindrical compact made of a mixture of REBaCuO7-x and the REBaCuO5; ３ A method for producing a metal oxide bulk, comprising: a placement step of placing the formed body on a seed substrate made of a material having a peritectic temperature higher than that of O7-x; a melting and solidifying step of heating the formed body for a predetermined period of time, maintaining the temperature below the peritectic temperature of the seed substrate and above the peritectic temperature of the formed body, and then cooling it to melt and solidify it, thereby obtaining the metal oxide bulk; and a step of separating the metal oxide bulk from the seed substrate.

10. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body made of crystals having a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ a melting and solidifying step of heating the compact for a predetermined time to maintain a temperature that is equal to or lower than the peritectic temperature of the seed substrate and equal to or higher than the peritectic temperature of the compact, and then cooling the compact to melt and solidify it, thereby obtaining the metal oxide bulk; and a step of separating the metal oxide bulk from the seed substrate, wherein in the step of preparing the compact, ３ A method for producing a metal oxide bulk body, comprising press-molding a mixed powder of powder of O7-x and powder of RE2BaCuO5 at a pressure of less than 50 MPa.

11. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body made of crystals having a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ a melting and solidifying step of heating the formed body for a predetermined period of time, maintaining the temperature below the peritectic temperature of the seed substrate and above the peritectic temperature of the formed body, and then cooling the formed 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, wherein in the melting and solidifying step, the temperature of the formed body is controlled by applying a temperature gradient in a direction perpendicular to the seed substrate so that the temperature is lower in the portion of the formed body closer to the surface that contacts the seed substrate.

12. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body made of crystals having a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ A method for producing a metal oxide bulk body, characterized in that a metal oxide bulk body produced by a production method including: a placement step of placing the molded body on a seed substrate made of a material having a peritectic temperature higher than that of O7-x; a melting and solidifying step of heating the molded body for a predetermined period of time, maintaining the temperature below the peritectic temperature of the seed substrate and above the peritectic temperature of the molded body, and then cooling it to melt and solidify, thereby obtaining the metal oxide bulk body; and a step of separating the metal oxide bulk body from the seed substrate, is used as the seed substrate to produce a new metal oxide bulk body by the production method.

13. The step of preparing the compact comprises: ３ A method for producing a metal oxide bulk body described in any one of claims 9 to 12, characterized in that it further comprises a densification process in which a compact formed from a mixed powder of O7-x powder and the RE2BaCuO5 powder is heated for a predetermined period of time to densify the compact.

14. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ A method for producing a metal oxide bulk body made of crystals having a composition represented by the formula: ３ preparing a compact comprising a mixture of REBaCuO7-x and the REBaCuO5; ３ a melting and solidifying step of heating the compact 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 compact, and then cooling the compact to melt and solidify it, thereby obtaining the metal oxide bulk; a step of separating the metal oxide bulk from the seed substrate; and a metal oxide bulk, wherein the step of preparing the compact is performed by using the REBa2Cu ３ A method for producing a metal oxide bulk body, comprising a densification step of heating a compact obtained by pressurizing a mixed powder of O7-x powder and the RE2BaCuO5 powder for a predetermined period of time to densify the compact, wherein in the densification step, the compact is heated in a vacuum or in an oxygen atmosphere for the predetermined period of time.

15. A method for producing a metal oxide bulk body according to claim 14, characterized in that in the densification step, the compact is heated in a low vacuum at a temperature of 800°C or higher and 1050°C or lower.

16. A method for producing a metal oxide bulk body described in any one of claims 9 to 12, 14 and 15, 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.

17. REBa ２ Cu ３ O ７－ｘ (RE represents one or more rare earth elements, and x satisfies 0≦x≦1) and RE ２ BaCuO ５ The circumferential length of the isomagnetic field lines of the trapped magnetic field is L, the area of ​​the region surrounded by the isomagnetic field lines is S, and the circularity C of the trapped magnetic field distribution is 4πS / L. ２ A bulk metal oxide superconductor characterized in that, when (0<C≦1), the average value of the circularity C for a plurality of the isomagnetic field lines is 0.98 or more.

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