Boron powder, method for producing magnesium diboride, and magnesium diboride bulk

Abstract

The present disclosure is a boron powder having two or more peaks in the number distribution, wherein the ratio of the peak frequency of a peak 2 that is the peak having the second highest frequency to the peak frequency of a peak 1 that is the peak having the highest frequency exceeds 0.12. The peak diameter of the peak 1 is preferably smaller than the peak diameter of the peak 2.

Description

Boron powder, manufacturing method of magnesium diboride, and magnesium diboride bulk

[0001] The present disclosure relates to boron powder, a method for producing magnesium diboride, and bulk magnesium diboride.

[0002] Magnesium diboride (hereinafter sometimes referred to as "MgB2") has a high critical temperature of 39 K and is expected to be a superconducting material powered by liquid hydrogen refrigerant. MgB2 is produced as bulk or wire by reacting a mixed powder of boron and magnesium through sintering. However, compared to current superconducting materials such as niobium-titanium alloys, the MgB2 reported to date has poor superconducting properties and has not yet been used as a replacement. Therefore, various studies are being conducted with the aim of improving the superconducting properties of MgB2.

[0003] For example, Patent Document 1 reports that adding titanium to MgB2 can achieve a critical current density (JC) of 1.7 x 104 A / cm2 (20 K, 3 T) to 2.0 x 106 A / cm2 (5 K, 0 T) in a low magnetic field. Also, Patent Document 2 reports that adding silver to MgB2 can achieve a critical current density of 8.15 x 105 A / cm2 (37 K, 3 T) in a low magnetic field.

[0004] In contrast, it has been disclosed that densified MgB2 can be obtained using mechanically activated crystallized boron as a raw material, as MgB2 with improved critical current density without using any additive elements, and that the electric field current density of this MgB2 in a high magnetic field is 0.29 A / cm2 (4.2 K, 9 T).

[0005] The MgB2 of Patent Document 3 has a higher critical current density than conventional MgB2 without using any additional elements. However, even the MgB2 of Patent Document 3 still has a low critical current density in a low magnetic field.

[0006] JP 2003-095650 A JP 2008-120659 A JP 2005-508278 A

[0007] The present disclosure aims to provide at least one of a boron powder, a method for producing magnesium diboride, and a magnesium diboride bulk body that provides magnesium diboride with improved critical current density in a low magnetic field without requiring any additional elements.

[0008] In this disclosure, we have investigated a method for improving the critical current density of magnesium diboride without using any additional elements, focusing on its precursor, particularly boron powder. As a result, we have found that by using boron powder with controlled physical properties as a precursor, magnesium diboride that exhibits a practical critical current density in a low magnetic field can be obtained.

[0009] That is, the present invention is as set forth in the claims, and the gist of the present disclosure is as follows. [1] A boron powder having two or more peaks in a number distribution, wherein the peak frequency of Peak 2, the peak having the second highest frequency, relative to the peak frequency of Peak 1, the peak having the highest frequency, exceeds 0.12. [2] The boron powder according to [1] above, wherein the peak diameter of Peak 1 is smaller than the peak diameter of Peak 2. [3] The boron powder according to [1] or [2] above, wherein the peak diameter of Peak 1 is less than 1 μm. [4] The boron powder according to any one of [1] to [3] above, wherein the peak diameter of Peak 2 is 1 μm or more. [5] The boron powder according to any one of [1] to [4] above, wherein the ratio of the peak diameter of Peak 2 to the peak diameter of Peak 1 is 10 or more. [6] The boron powder according to any one of [1] to [5] above, wherein the oxygen content is 1.5 mass% or more. [7] The boron powder according to any one of [1] to [6] above, having an oxygen content of less than 3.5 mass%. [8] A method for producing magnesium diboride using the boron powder according to any one of [1] to [7] above. [9] A magnesium diboride bulk body, the magnesium diboride bulk body having a grain boundary and at least one particle in the grain boundary, the atomic weight ratio of boron to magnesium, B / Mg, being 0.7 or more.

[10] The magnesium diboride bulk body according to [9] above, wherein the particle has a maximum size of 150 nm or less in the direction perpendicular to the grain boundary.

[11] A magnesium diboride bulk body according to [9] or

[10] above, having at least one region in which, in the crystal grain boundary within a radius of 200 nm from the center position of the particle, the proportion of oxygen atoms in a 40 nm square region including the crystal grain boundary is 6 atom % or less, when the total atomic weight of magnesium, boron, and oxygen is 100 atom %.

[12] A magnesium diboride bulk body according to [9] or

[10] above, having at least one region in which, in the crystal grain boundary within a radius of 200 nm from the center position of the particle, the proportion of oxygen atoms in a 40 nm square region including the crystal grain boundary is greater than 6 atom %, when the total atomic weight of magnesium, boron, and oxygen is 100 atom %.

[0010] According to the present disclosure, it is possible to provide a boron powder that provides magnesium diboride with improved critical current density in a low magnetic field without requiring any additional elements, a method for producing magnesium diboride using the same, and a magnesium diboride bulk body.

[0011] 1 is a number distribution of the mixed boron powder described in Example 1. FIG. 2 is a number distribution of boron powder 2 described in Comparative Example 1. FIG. 3 is a number distribution of boron powder 2 described in Comparative Example 2. FIG. 4 is a number distribution of the mixed boron powder described in Example 2. FIG. 5 is a scanning electron microscope (SEM) image of the MgB2 bulk body described in Example 2. FIG. 6 is a scanning electron microscope (SEM) image of the MgB2 bulk body described in Comparative Example 3. FIG. 7 is a scanning transmission electron microscope (STEM) image of the MgB2 bulk body described in Example 2. FIG. 8 is a scanning transmission electron microscope (STEM) image of the MgB2 bulk body described in Comparative Example 3.

[0012] The present disclosure will be described below with reference to an example embodiment. Any combination of the configurations and parameters in this specification is included in the present disclosure, and any combination of the upper and lower limits of the values ​​disclosed in this specification is also included in the present disclosure. The main terms used in this embodiment are described below.

[0013] The "number distribution" refers to the frequency distribution of particles constituting a powder, and is measured by a laser diffraction / scattering method and is expressed by the number-based frequency and particle diameter. The number distribution can be determined using a common nanoparticle size distribution measurement device (e.g., SALD-7100, manufactured by Shimadzu Corporation) under the following conditions: Calculation mode: Manual measurement Number of measurements: 1 Measurement interval: 2 seconds The refractive index of the sample used in the measurement can be a value selected based on the LDR method (light intensity distribution reproduction method). That is, first, five points with the highest fitting rate are extracted from multiple refractive indices obtained by fitting, and then the refractive index at which the particle diameter corresponding to a number frequency of 10.000% (the so-called D10% diameter) is the largest among the extracted five refractive indices can be determined as the refractive index of the sample. In addition, if the D10% diameter is the same value, the refractive indexes of the extracted five points are examined in the same order, starting with the D20% diameter, followed by the D30% diameter, etc., and the refractive index at which the particle diameter corresponding to any of the number frequencies is the largest is taken as the refractive index of the sample. The refractive index can be determined using analysis software included with the nanoparticle size distribution measurement device (e.g., WingSALDII, manufactured by SHIMADZU). The measurement sample can be a slurry prepared by mixing 0.001±0.00098 g of powder with 30±25 mL of pure water and ultrasonically dispersing this for 1 minute. The obtained number distribution can be analyzed using the analysis software included with the nanoparticle size distribution measurement device to detect peaks and calculate the peak frequency and particle diameter.

[0014] A "peak" is a frequency peak of particle diameters detected by analyzing the number distribution, a "peak frequency" is the height [%] of the peak, and a "peak diameter" is the particle diameter [nm or μm] corresponding to the diameter of the peak top of the peak.

[0015] [Boron Powder] This embodiment is a boron powder having two or more peaks in a number distribution, and in which the peak frequency of Peak 2, which is the peak having the second highest frequency, relative to the peak frequency of Peak 1, which is the peak having the highest frequency (hereinafter also referred to as "frequency peak ratio") is greater than 0.12. This will be specifically described below.

[0016] The boron powder of this embodiment is preferably a powder consisting only of boron, but may contain inevitable impurities. Examples of impurities contained in the powder of this embodiment include one or more metal impurities selected from the group consisting of Al, Mn, Ti, and Ag. In this case, the boron powder of this embodiment may be 100 mass% boron and metal impurities. Examples of the mass ratio of the metal impurities to the boron powder include 5 mass% or less, 3 mass% or less, 2 mass% or less, 1 mass% or less, or 0.5 mass% or less. It is preferable that the amount of metal impurities is small, and examples of the mass ratio of the metal impurities to the boron powder include 0 mass% or more or more than 0 mass%. Examples of the mass ratio of the metal impurities to the boron powder include 0 mass% or more and 5 mass% or less, 0 mass% or more and 3 mass% or less, or more than 0 mass% and 2 mass% or less.

[0017] The boron powder of this embodiment has two or more peaks in the number distribution. Having two or more peaks means that the powder of this embodiment has a particle distribution including two or more particle groups with different particle diameters. By using such a powder as a precursor to MgB2, MgB2 exhibiting a practical critical current density (JC) in a low magnetic field can be obtained regardless of the subsequent manufacturing method. The boron powder of this embodiment may have two or more peaks, for example, two to five peaks, or may have two peaks. In this embodiment, the peaks may be numbered as Peak 1, Peak 2, ... Peak n (n is an integer) in order of decreasing peak frequency. For convenience, peaks with the same peak frequency may be numbered in order of decreasing particle diameter. For convenience, the peak frequency and peak diameter of each peak are referred to as the "Pn frequency" and "Pn diameter," respectively.

[0018] In the boron powder of this embodiment, the peak frequency of Peak 2, which is the peak with the second highest frequency, relative to the peak frequency of Peak 1, which is the peak with the highest frequency, is greater than 0.12. A frequency peak ratio of greater than 0.12 improves the packing of boron particles, resulting in a high density of the resulting MgB2. The frequency peak ratio is preferably 0.5 or greater or 0.7 or greater. Furthermore, the frequency peak ratio is preferably less than 1.0 or 0.99 or less, more preferably greater than 0.12 and less than 1.0, even more preferably 0.7 or greater and less than 1.0, and particularly preferably 0.7 or greater and 0.99 or less.

[0019] In the boron powder of this embodiment, the peak diameter of Peak 1 is preferably smaller than the peak diameter of Peak 2. When the peak diameter of Peak 1 (P1 diameter) is smaller than the peak diameter of Peak 2 (P2 diameter), the boron particles having smaller particle diameters are more likely to fill the gaps formed between the boron particles having larger particle diameters, which makes it easier to improve the particle filling rate during compaction.

[0020] The P1 diameter is, for example, less than 1 μm, 0.5 μm or less, or 0.3 μm or less, or 0.03 μm or more, 0.05 μm or more, or 0.07 μm or more. The P1 diameter is preferably 0.03 μm or more and less than 1 μm, more preferably 0.05 μm or more and 0.5 μm or less, and even more preferably 0.07 μm or more and 0.3 μm or less.

[0021] The P2 diameter is, for example, 1 μm or more, 2 μm or more, or 3 μm or more, and 50 μm or less, 25 μm or less, or 10 μm or less. The P2 diameter is preferably 1 μm or more and 50 μm or less, more preferably 2 μm or more and 25 μm or less, and even more preferably 3 μm or more and 10 μm or less.

[0022] The ratio of the P2 diameter to the P1 diameter (hereinafter also referred to as the "peak diameter ratio") is preferably 10 or more. If the peak diameter ratio is 10 or more, the small particles will be more likely to fill the voids of the large particles. The peak diameter ratio may be 20 or more or 30 or more, and may be 50 or less or 40 or less. The peak diameter ratio is preferably 10 or more and 50 or less, and more preferably 20 or more and 40 or less.

[0023] The peak frequency of peak 1 (P1 frequency) and the peak frequency of peak 2 (P2 frequency) may satisfy the above-mentioned frequency peaks. For example, the P1 frequency may be 5% or more, 9% or more, or 9.5% or more, and less than 25.5% or less than 25%. The P1 frequency is preferably 5% or more and less than 25.5%, and more preferably 9.5% or more and less than 25%. Similarly, the P2 frequency is lower than the P1 frequency and may be 3.1% or more, 5% or more, or 7% or more, and less than 20% or less than 15%. The P2 frequency is preferably 3.5% or more and less than 20%, and more preferably 7% or more and less than 15%. The boron powder of this embodiment preferably has an oxygen content of 1.5% by mass or more, and preferably 1.8% by mass or more or 2.0% by mass or more. When the oxygen content is 1.5% by mass or more, the decrease in magnetic flux pinning centers when the MgB2 bulk body is formed is suppressed, and the critical current density is less likely to decrease. The upper limit of the oxygen content can be, for example, less than 3.5% by mass or 3.0% by mass or less. The boron powder of this embodiment preferably has an oxygen content of 1.5% by mass or more and less than 3.5% by mass, more preferably 1.8% by mass or more and less than 3.5% by mass, even more preferably 2.0% by mass or more and less than 3.5% by mass, and particularly preferably 2.0% by mass or more and less than 3.0% by mass.

[0024] [Method for producing boron powder] The boron powder of this embodiment may be produced by any method as long as it satisfies the above-mentioned requirements. A preferred method includes, for example, a mixing step of mixing boron powder having a peak diameter of less than 1 μm in its number distribution (hereinafter also referred to as "fine powder") with boron powder having a peak diameter of 1 μm or more in its number distribution (hereinafter also referred to as "coarse powder").

[0025] The fine powder is boron powder having a peak diameter of less than 1 μm in the number distribution. By using boron powder with such a peak diameter in the mixing step, the peak diameter of Peak 1 becomes less than 1 μm. The peak diameter of the fine powder may be less than 1 μm, 0.5 μm or less, or 0.3 μm or less, or may be 0.03 μm or more, 0.05 μm or more, or 0.07 μm or more. The peak diameter of the fine powder is preferably 0.03 μm or more and less than 1 μm, more preferably 0.05 μm or more and 0.5 μm or less, and even more preferably 0.07 μm or more and 0.3 μm or less.

[0026] The fine powder may be a commercially available boron powder, but is preferably a powder produced by a production method including a step of treating boron powder having a peak diameter of 1 μm or more (hereinafter also referred to as a “treatment step”).

[0027] The treatment method in the treatment step can be one or more selected from the group consisting of pulverization with a bead mill, vapor deposition, liquid phase reduction, hydrothermal synthesis, neutralization decomposition, hydrolysis, sol-gel method, laser treatment, and plasma treatment, and plasma treatment is preferred.

[0028] The plasma treatment is a treatment in which powder is introduced into high-temperature plasma, vaporized, and rapidly cooled to reduce the size of the powder, and is preferably a plasma treatment in which the powder is reduced to nanometer size using alternating current (RF) plasma.

[0029] The RF output in the plasma treatment can be, for example, 3 kW or more, preferably 3 kW or more and 200 kW or less, and more preferably 6 kW or more and 150 kW or less.

[0030] The boron supply rate in the plasma treatment can be, for example, 0.1 g / min or more, preferably 0.1 g / min to 10 g / min, and more preferably 0.2 g / min to 2.0 g / min. The larger the boron supply rate, the larger the particle size of the boron powder after plasma treatment. The working gas in the plasma treatment can be argon, hydrogen, or a mixed gas of argon and hydrogen, with a mixed gas of argon and hydrogen being preferred. The ratio of hydrogen gas in the mixed gas can be, for example, more than 0.5 vol% or 0.7 vol% or more, and can be, for example, 4.0 vol% or less or 3.5 vol% or less. The ratio of hydrogen gas in the mixed gas can be, for example, more than 0.5 vol% to 4.0 vol%, and more preferably 1.0 vol% to 3.5 vol%.

[0031] The coarse powder is boron powder having a peak diameter of 1 μm or more in the number distribution. By using boron powder having such a peak diameter in the mixing step, the peak diameter of Peak 2 becomes 1 μm or more. The peak diameter of the coarse powder may be 50 μm or less, 30 μm or less, or 25 μm or less, or may be 1 μm or more and 50 μm or less, or 2 μm or more and 25 μm or less.

[0032] The boron purity of the fine powder and the coarse powder can be, for example, 95% or more or 99% or more, and 100% or less or less than 100%. The oxygen content of the fine powder can be, for example, 1.5% by mass or more, 2.0% by mass or more, or 2.5% by mass or more. The upper limit of the oxygen content can be, for example, 5.0% by mass or less, 4.5% by mass or less, or 4.0% by mass or less. The oxygen content of the fine powder is preferably 1.5% by mass or more and 5.0% by mass or less, more preferably 2.0% by mass or more and 4.5% by mass or less, and even more preferably 2.5% by mass or more and 4.0% by mass or less. The oxygen content of the coarse powder can be, for example, 0.5% by mass or more, 1.0% by mass or more, or 1.5% by mass or more. The upper limit of the oxygen content can be, for example, 5.0% by mass or less, 4.5% by mass or less, or 4.0% by mass or less. The oxygen content of the coarse powder is preferably 0.5% by mass or more and 5.0% by mass or less, more preferably 1.0% by mass or more and 4.5% by mass or less, and even more preferably 1.5% by mass or more and 4.0% by mass or less.

[0033] The boron powder of this embodiment is obtained by mixing fine boron powder and coarse boron powder.

[0034] The mixing may be any method that mixes fine powders and coarse powders without pulverizing them, and is preferably dry mixing, more preferably dry mixing using at least one of a rotary cylindrical mixer and a V-type mixer, and even more preferably dry mixing using a V-type mixer.

[0035] The atmosphere for mixing may be one or more selected from the group consisting of an oxidizing atmosphere, an air atmosphere, a reducing atmosphere and an inert atmosphere, and an inert atmosphere or an air atmosphere is preferred.

[0036] The fine powder and the coarse powder may be mixed in a ratio such that the frequency peak ratio in the resulting powder falls within the above-mentioned range. To achieve the above-mentioned frequency peak ratio, the fine powder is preferably mixed in an amount of 5% by mass to 95% by mass, or even 20% by mass to 50% by mass, and the coarse powder is preferably mixed in an amount of 5% by mass to 95% by mass, or even 50% by mass to 80% by mass, so that the total of the fine powder and the coarse powder is 100% by mass. The ratio of the oxygen content of the fine powder to the oxygen content of the coarse powder may be 1.1 or more, 1.5 or more, or 1.9 or more, and may be 2.7 or less, 2.6 or less, or 2.5 or less. The ratio of the oxygen content of the fine powder to the oxygen content of the coarse powder is preferably 1.1 or more, 1.5 or more, or 1.9 or more, and more preferably 1.1 or more, 2.7 or less, more preferably 1.5 or more, 2.6 or less, and even more preferably 1.9 or more, 2.5 or less. If the oxygen content ratio is within the above range, when an MgB2 bulk body is formed, the bonding of the crystal grain boundaries of magnesium diboride becomes dense, making it easier to improve the critical current density.

[0037] [Method for producing magnesium diboride] The boron powder of this embodiment can be used for known applications of boron powder, but is preferably used as a precursor of magnesium diboride. Hereinafter, a method for producing magnesium diboride using the boron powder of this embodiment will be described.

[0038] As a method for producing magnesium diboride of this embodiment, known methods for producing magnesium diboride (MgB2) can be applied except for using the boron powder of this embodiment. Examples include a method of press-molding and heating a mixed powder of boron powder and magnesium of this embodiment (hereinafter also referred to as the "in-situ method"), and a method of molding and heating a mixed powder of the powder of this embodiment, magnesium powder, and magnesium diboride (hereinafter also referred to as the "Premix method"). The method for producing magnesium diboride of this embodiment is preferably the Premix method, because the resulting magnesium diboride is more likely to exhibit a higher critical current density in a low magnetic field. The method for producing magnesium diboride of this embodiment is preferably the in-situ method, because it allows magnesium diboride to be produced by a simpler method.

[0039] In the in-situ method, magnesium diboride is produced by heat-treating a mixed powder of the boron powder of this embodiment and magnesium powder. The mixed powder may be any powder obtained by mixing magnesium powder and boron powder of this embodiment so that the substance ratio (nMg / nB) is 0.5 or more or 0.505 or more and 0.55 or less or 0.53 or less.

[0040] Examples of pressure molding conditions in the in-situ method include the following: Pressure conditions: 0.5 GPa or more or 1.0 GPa or more, 3.0 GPa or less or 2.5 GPa or less The pressure conditions in the in-situ method are preferably 0.5 GPa or more and 3.0 GPa or less, and more preferably 1.0 GPa or more and 2.5 GPa or less.

[0041] Examples of heating conditions in the in-situ method include the following: Heating temperature: 550°C or higher or 600°C or higher, and 950°C or lower or 900°C or lower Heating time: 1 hour or higher or 2 hours or higher, 24 hours or lower or 12 hours or lower The heating temperature in the in-situ method is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The heating time in the in-situ method is preferably 1 hour or higher and 24 hours or lower, and more preferably 2 hours or higher and 12 hours or lower.

[0042] (Premix Method) In the Premix method, magnesium diboride is produced by heat-treating a mixed powder of the boron powder of this embodiment, magnesium powder, and magnesium diboride. The mixed powder may be a powder obtained by mixing the magnesium powder and the boron powder of this embodiment such that the substance ratio (nMg / nB) of the magnesium powder to the boron powder of this embodiment is 0.5 or more or 0.505 or more and 0.55 or less or 0.53 or less, and the substance ratio (nMg / nMgB2) of the magnesium powder to the MgB2 powder is 0.8 or more or 0.9 or more and 1.5 or less or 1.4 or less.

[0043] Examples of pressure molding conditions in the Premix method include the following: Pressure conditions: 0.5 GPa or more or 1.0 GPa or more, 3.0 GPa or less or 2.5 GPa or less The pressure conditions in the Premix method are preferably 0.5 GPa or more and 3.0 GPa or less, and more preferably 1.0 GPa or more and 2.5 GPa or less.

[0044] Examples of heating conditions in the Premix method include the following: Heating temperature: 550°C or higher or 650°C or higher, and 900°C or lower or 850°C or lower Heating time: 1 hour or higher or 6 hours or higher, 36 hours or lower or 24 hours or lower The heating temperature in the Premix method is preferably 550°C or higher and 900°C or lower, and more preferably 650°C or higher and 850°C or lower. The heating time in the Premix method is preferably 1 hour or higher and 36 hours or lower, and more preferably 6 hours or higher and 24 hours or lower.

[0045] -Preparation of Magnesium Diboride Seed Crystals- In the Premix method, it is preferable to pretreat magnesium diboride before mixing with a carboxylic acid to prepare magnesium diboride seed crystals, and then use these magnesium diboride seed crystals to produce magnesium diboride. Pretreatment with a carboxylic acid allows for the removal of magnesium oxide and magnesium contained in the magnesium diboride. The carboxylic acid may be one or more selected from the group consisting of monocarboxylic acids, dicarboxylic acids, and tricarboxylic acids. Specifically, one or more selected from the group consisting of acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, benzoic acid, phthalic acid, oxalic acid, maleic acid, and anhydrides thereof are preferred, one or more selected from the group consisting of acetic acid, propionic acid, valeric acid, and caproic acid are more preferred, and propionic acid is even more preferred. When the carboxylic acid is a liquid at room temperature (25°C), it may be used directly in the pretreatment, or it may be mixed with an organic solvent and then used in the pretreatment. When the carboxylic acid is a solid at room temperature (25°C), it is dissolved in an organic solvent before the pretreatment. As a method of the pretreatment, it is preferable to immerse the magnesium diboride before mixing in the carboxylic acid, more preferably in acetic acid, propionic acid, valeric acid, or caproic acid, and even more preferably in propionic acid. In the immersion treatment, the treatment temperature and treatment time are not particularly limited as long as the carboxylic acid does not volatilize or decompose. However, if the carboxylic acid is liquid at room temperature (25°C), it is preferable to set the temperature below its boiling point, and if the carboxylic acid is solid at room temperature (25°C), it is preferable to set the temperature below its thermal decomposition temperature. When immersing in propionic acid, the treatment temperature and treatment time can be exemplified as follows: Treatment temperature: 80°C or higher or 100°C or higher, and 140°C or lower or 130°C or lower Treatment time: 0.5 hours or higher or 1 hour or higher, 5 hours or lower or 3 hours or lower By setting the treatment temperature to 80°C or higher, magnesium oxide and magnesium contained in the magnesium diboride can be efficiently removed. When the film is immersed in propionic acid, the heating temperature is preferably 80°C or higher and 140°C or lower, and more preferably 100°C or higher and 130°C or lower.When the substrate is immersed in propionic acid, the treatment time is preferably from 0.5 hours to 5 hours, more preferably from 1 hour to 3 hours.

[0046] [Magnesium diboride] The magnesium diboride obtained from the boron powder of this embodiment has a pore ratio of 12% or less and an average crystal grain size of 60 nm to 1,300 nm (hereinafter also referred to as "magnesium diboride of this embodiment"). Such magnesium diboride exhibits a practical critical current density in a low magnetic field.

[0047] The magnesium diboride (MgB2) of this embodiment is preferably a bulk body, and is also preferably contained in a wire. From the viewpoint of installation in a small superconducting device such as a benchtop NMR, a bulk body is preferable, while from the viewpoint of installation in a large superconducting device such as a superconducting coil or superconducting electric wire, a wire is preferable. Here, the bulk body is a magnesium diboride sintered body, and the wire is a material in which magnesium diboride is filled inside a metal tube such as an SUS tube.

[0048] (Porosity) The magnesium diboride of this embodiment has a pore ratio of 12% or less, preferably 10% or less. A pore ratio of 10% or less allows a practical critical current density to be exhibited in a low magnetic field. A low pore ratio is preferable, and examples of the pore ratio include 3% or more or 5% or more, and examples of the pore ratio include 3% or more and 12% or less, or 5% or more and 10% or less.

[0049] (Average Crystal Grain Size) The average crystal grain size of the magnesium diboride of this embodiment is preferably 60 nm or more and 1300 nm or less. Having an average crystal grain size of 60 nm or more can prevent the number of grain boundaries of magnesium diboride from increasing, thereby reducing the influence of impurities such as oxides unevenly distributed at the grain boundaries and improving the critical current density. Furthermore, having an average crystal grain size of 1300 nm or less can prevent the density of grain boundaries that become magnetic flux pinning centers from decreasing, thereby improving the critical current density. The average crystal grain size may be 100 nm or more or 150 nm or more, and may be 500 nm or less or 300 nm or less. Furthermore, the average crystal grain size is preferably 100 nm or more and 500 nm or less, or 150 nm or more and 300 nm or less.

[0050] The maximum value of the crystal grain size of the magnesium diboride of this embodiment is 7 μm or less, 6.5 μm or less, or 6 μm or less, and the minimum value is 30 nm or more, 35 nm or more, or 40 nm or more. The crystal grain size of the magnesium diboride of this embodiment is 30 nm or more and 7 μm or less, 35 nm or more and 6.5 μm or less, or 40 nm or more and 6 μm or less.

[0051] -Method of measuring pore ratio, average crystal grain size, and crystal grain size distribution- The pore ratio, average crystal grain size, and crystal grain size distribution are values ​​determined from SEM / EBSD observation images of magnesium diboride obtained under the following conditions using a Schottky field emission scanning electron microscope equipped with electron backscatter diffraction (EBSD) (for example, JSM-IT800SHL, manufactured by JEOL Ltd.). For observation, it is preferable to perform pretreatment by argon ion milling. <SEM / EBSD observation conditions> Acceleration voltage: 15 kV Observation magnification: 1000x or 5000x Sample tilt: 70° <Argon ion milling conditions> Acceleration voltage: 6 kV

[0052] The average crystal grain size is determined by identifying crystal grains by regarding the portion at the crystal interface where the difference in crystal orientation is 5° or more as the crystal grain boundary, measuring the longest diameter of each crystal grain, and using this as the crystal grain size. The crystal grain sizes of 100±50 crystal grains are measured, and the average is taken as the average crystal grain size. The maximum and minimum values ​​of the crystal grain size are the maximum and minimum values, respectively, of the 100±50 crystal grain sizes measured using the above method.

[0053] The pore ratio is the proportion of pores determined from an observation image of a cross section of magnesium diboride, and is the proportion of pores measured by image analysis of an SEM observation image of the surface of magnesium diboride processed by argon ion milling. Observation magnification: 5000x Observation field: 5 fields The pore ratio (%) of each field can be determined by determining the proportion (%) of black areas to the total area of ​​black and white areas in each observation image. The average value of the pore ratios of each observed field can be used as the pore ratio. The area of ​​the black areas can be calculated by image analysis. Image analysis can be performed using general-purpose image analysis software (e.g., Image-Pro, manufactured by Media Cybernetics) by setting a threshold so that areas with a pixel value of 100 or less in the SEM image are black, performing binarization, and calculating the area. The following conditions can be used for image analysis. Threshold (pixel value): 100

[0054] (Critical Current Density) The magnesium diboride of this embodiment can exhibit a practical critical current density in a low magnetic field. A low magnetic field can be 0 T or more and 5 T or less. For example, the critical current density in a magnetic field of 0 T can be 1.05 x 10 A cm or more, 1.1 x 10 A cm or more, or 1.2 x 10 A cm or more. When the magnesium diboride of this embodiment is a bulk body, the oxygen content can be 1 mass% or more, 2 mass% or more, 3 mass% or more, 3.5 mass% or more, or 3.8 mass% or more, with the upper limit being 20 mass% or less, 15 mass% or less, or 10 mass% or less. When the magnesium diboride of this embodiment is a bulk body, the oxygen content can be 3 mass% or more and 20 mass% or less, 3.5 mass% or more and 15 mass% or less, or 3.8 mass% or more and 10 mass% or less.

[0055] -Method of measuring critical current density- The critical current density in this embodiment is a value calculated by an extended Bean model from the sample size and hysteresis width from a magnetization hysteresis curve obtained using a general superconducting quantum interference magnetometer (for example, MPMS XL-5s, manufactured by Quantum Design) under the following conditions: Cooling gas: Helium Sample size: 1.5 mm x 1.5 mm x 0.5 mm Measurement temperature: 15 K Magnetic field sweep: 0 T → 5 T → -1 T

[0056] In the magnesium diboride bulk of this embodiment, the maximum size of the grains in the direction perpendicular to the grain boundary may be 150 nm or less, 100 nm or less, or 70 nm or less. The "grain boundary" in the magnesium diboride bulk may be defined as the line connecting two points where a particle and a grain boundary meet, with the longest length of the line connecting the two points.

[0057] (Oxygen Atomic Ratio) The magnesium diboride bulk of this embodiment may have at least one region where the ratio of oxygen atoms at the grain boundaries is 6 atom% or less, 4 atom% or less, or 3 atom% or less, when the total atomic weight of magnesium, boron, and oxygen is 100 atom%. Furthermore, the magnesium diboride bulk of this embodiment may have at least one region where the ratio of oxygen atoms at the grain boundaries is more than 6 atom%, more than 7 atom%, more than 8 atom%, or more than 10 atom%, when the total atomic weight of magnesium, boron, and oxygen is 100 atom%.

[0058] -Method of determining the oxygen atomic ratio- In this embodiment, the ratio of oxygen atoms at the grain boundaries can be determined by using a field emission transmission electron microscope (model number: JEM-F200, manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray analyzer (for example, JED-2300 Silicon Drift Detector 100 mm2, manufactured by JEOL Ltd.) and analyzing the size of particles present at the grain boundaries of the MgB2 bulk body from an STEM / EDS observation diagram of magnesium diboride obtained under the following conditions using analysis software (for example, Analysis Station, manufactured by JEOL Ltd.) by a thin film approximation method to determine the atomic percentages (atom %) of B (boron atoms), Mg (magnesium atoms), and O (oxygen atoms) in the particles present at the MgB2 grain boundaries and in the MgB2 grain boundary region. <STEM observation conditions> Sample thickness: 100 nm or less Acceleration voltage: 200 kV Observation magnification: 500,000 times <EDS observation conditions> Acceleration voltage: 200 kV Observation magnification: 500,000 times Area analysis range: square with a side length of 40±1 nm

[0059] (Ratio of magnesium oxide contained in magnesium diboride) The ratio of magnesium oxide contained in magnesium diboride is determined by performing XRD measurement of a magnesium diboride bulk body using a powder X-ray diffractometer (for example, device name: RINT Ultima III, manufactured by RIGAKU Corporation) under the following measurement conditions, and determining the ratio of the peak intensity derived from the oxide (magnesium oxide) observed at 43 to 44.5 degrees to the peak intensity derived from MgB2 observed at 42.5 degrees. The peak intensity ratio may be 0.2 or less or 0.1 or less, and may be 0 or more or 0.001 or more. The ratio of the peak intensity derived from the oxide (magnesium oxide) observed at 43 to 44.5 degrees to the peak intensity derived from MgB2 observed at 42.5 degrees is preferably 0 or more and 0.2 or less, more preferably 0 or more and 0.1 or less, and even more preferably 0.001 or more and 0.1 or less. <XRD measurement conditions> Accelerating current and voltage: 40 mA, 40 kV Radiation source: CuKα radiation (λ = 0.15405 nm) Measurement mode: Continuous scan Scan conditions: 5° / min Measurement range: 2θ = 20° to 50° Divergence vertical limiting slit: 10 mm Divergence / entrance slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used

[0060] The present disclosure will be described below with reference to examples, but the present disclosure is not limited thereto.

[0061] (Particle size distribution) A particle size distribution based on volume was obtained using a particle size distribution measuring device (device name: SALD-7100, manufactured by Shimadzu Corporation). Prior to particle size distribution measurement, pure water was added to the powder sample to form a slurry, which was then subjected to ultrasonic dispersion for 1 minute as a pretreatment. Measurements were performed under the following conditions while dispersing the sample using the sample circulator attached to the device. Calculation mode: Manual measurement Number of measurements: 1 Measurement interval: 2 seconds The refractive index of the measured sample was determined using the analysis software attached to the device (WingSALDII, manufactured by Shimadzu Corporation) using the automatic refractive index calculation function based on the LDR method (light intensity distribution reproduction method). Among the five values ​​with the highest fitting rates obtained by the automatic refractive index calculation function of the software, the refractive index at which the particle diameter corresponding to any of the number frequencies was largest in the order of D10% diameter, D20% diameter, D30% diameter, etc. was determined as the refractive index of the sample. The obtained number distribution was analyzed using the analysis software described above.

[0062] (Average Crystal Grain Size) A Schottky field emission scanning electron microscope (instrument name: JSM-IT800SHL, manufactured by JEOL Ltd.) equipped with electron backscatter diffraction (EBSD) was used to obtain SEM images of the interior of the sample under the following conditions: Acceleration voltage: 15 kV Observation magnification: 1000x or 5000x Sample tilt: 70° In the obtained SEM images, portions where the difference in crystal orientation at the crystal interface was 5° or more were regarded as crystal grain boundaries, and crystal grains were identified. The longest diameter of each crystal grain was measured, and this was taken as the crystal grain size. The crystal grain sizes of 100±50 grains were measured, and the average was used to determine the average crystal grain size. The maximum diameter was used as the maximum crystal grain size, and the minimum diameter was used as the minimum crystal grain size to determine the grain size distribution.

[0063] (Pore Ratio) The pore ratio was determined from the proportion of pores measured by image analysis of an SEM observation image of the surface of magnesium diboride processed by argon ion milling. Observation magnification: 5000x Observation field: 5 fields The pore ratio (%) of each field was determined from the proportion (%) of black areas to the total area of ​​black and white areas in each observation image. The average value of the pore ratios of each observed field was taken as the pore ratio. The area of ​​the black areas was calculated by image analysis. Image analysis was performed using general-purpose image analysis software (e.g., Image-Pro, manufactured by Media Cybernetics), with a threshold set so that areas with a pixel value of 100 or less in the SEM image were black, and binary processing was performed, and the area was calculated under the following image analysis conditions. Threshold (pixel value): 100

[0064] (Oxygen Content) SEM / EDS observation images of magnesium diboride were obtained under the following conditions using a field emission scanning electron microscope (model number: JSM-IT800SHL, manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray microanalyzer (model number: Ultim Max65, manufactured by Oxford Instruments). The oxygen content (mass%) in magnesium diboride was calculated by the XPP correction calculation method using analysis software (AZtec 5.0, manufactured by Oxford Instruments) for the obtained SEM / EDS observation images. For observation, pretreatment was performed by argon ion milling. <EDS observation conditions> Acceleration voltage: 15 kV Observation magnification: 2000x <Argon ion milling processing conditions> Acceleration voltage: 6 kV

[0065] (Critical current density) A superconducting quantum interference magnetometer (model: MPMS XL-5s, manufactured by Quantum Design) was used to measure the critical current density (JC) of the MgB2 bulk body under the following conditions. The critical current density was measured by sweeping the magnetic field, stopping the magnetic field sweep every 1,000 Oe, and measuring the magnetization. A magnetization hysteresis curve was obtained by sweeping the magnetic field from 0 T to 5 T to -1 T, and JC was calculated using the extended Bean model from the sample size and hysteresis width. Cooling gas: Helium Sample size: 1.5 mm x 1.5 mm x 0.5 mm Measurement temperature: 15 K

[0066] (STEM / EDS Measurement) A field emission transmission electron microscope (model number: JEM-F200, manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray analyzer (JED-2300 silicon drift detector 100 mm2, manufactured by JEOL Ltd.) was used to obtain STEM / EDS observation images of magnesium diboride under the following conditions. <STEM observation conditions> Sample thickness: 100 nm or less Acceleration voltage: 200 kV Observation magnification: 500,000x <EDS observation conditions> Acceleration voltage: 200 kV Observation magnification: 500,000x Area analysis range: square with side length of 40±1 nm

[0067] (B, Mg, and O content) The obtained STEM / EDS observation image was analyzed using analysis software (Analysis Station, manufactured by JEOL Ltd.) to determine the size of particles present at the grain boundaries of the MgB2 bulk body, as well as the atomic percentages (atom %) of B (boron atoms), Mg (magnesium atoms), and O (oxygen atoms) in the particles present at the MgB2 grain boundaries and in the MgB2 grain boundary region. To determine the atomic percentages, EDS area analysis was performed on a square with a side length of 40 nm ± 1 nm. When measuring particles present at the MgB2 grain boundaries, EDS area analysis was performed by placing the particle so that its center position was included within the square. Note that the "particle center position" is the midpoint of the line connecting the two contact points between the particle and the grain boundary. In the case of a particle spanning multiple grain boundaries, the midpoint of the longest line connecting the two contact points between the particle and the grain boundary is used. In this case, the particle center is determined to be a point other than the center of gravity of the particle. When measuring the MgB2 grain boundary, the EDS area analysis was performed by setting the square so that the MgB2 grain boundary within a radius of 200 nm from the center position of the particle present at the MgB2 grain boundary was included within the square. For observation, pretreatment was performed using focused ion beam processing and argon ion milling processing, and the sample thickness was set to 100 nm or less. <Focused ion beam processing conditions> Acceleration voltage: 30 kV Ion source: Ga <Argon ion milling processing conditions> Acceleration voltage: 1 kV

[0068] (XRD Measurement) XRD measurement of MgB2 bulk was performed using a general powder X-ray diffractometer (device name: RINT Ultima III, manufactured by RIGAKU Corporation). The measurement conditions are as follows: Acceleration current / voltage: 40 mA / 40 kV Radiation source: CuKα ray (λ = 0.15405 nm) Measurement mode: Continuous scan Scan condition: 5° / min Measurement range: 2θ = 20° to 50° Divergence vertical limiting slit: 10 mm Divergence / entrance slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used

[0069] Example 1 Commercially available boron powder (particle size: 45 μm or less, product name: BBE01PB, manufactured by Kojundo Chemical Co., Ltd.) was used as a raw material and vaporized by introducing it into a plasma (RF output: 6 kW, boron supply rate: 1 g / min) using a mixed gas of argon and hydrogen (argon:hydrogen = 98.5 vol%:1.5 vol%) as the working gas. The vaporized boron was cooled to obtain boron powder with a D50 diameter of 156 nm, designated as Boron Powder 1. The oxygen content of Boron Powder 1 was 3.6 mass%. Meanwhile, commercially available boron (300 mesh, manufactured by Furuuchi Chemical Co., Ltd.) was designated as Boron Powder 2. The oxygen content of Boron Powder 2 was 1.8 mass%. Boron Powder 1 and Boron Powder 2 were weighed out so that each was 50 mass%, and then dry-mixed in an agate mortar in an air atmosphere to produce a mixed boron powder. It was confirmed that the mixed boron powder had a bimodal volume particle size distribution with peaks at particle sizes of 119 nm and 1.73 μm. The volume particle size distribution of the mixed boron powder is shown in Figure 1. The peak frequency of the 119 nm particle size peak (P1) was 12.4%, and the peak frequency of the 1.73 μm particle size peak (P2) was 9.8%, resulting in a frequency peak ratio of 0.79. The oxygen content of the mixed boron powder was 2.7% by mass.

[0070] Using the resulting mixed boron powder, an MgB2 bulk body was fabricated by the in-situ powder-in-tube method. Specifically, magnesium powder (100 mesh, manufactured by Furuuchi Chemical Co., Ltd.) and mixed boron powder were weighed and mixed in an air atmosphere to obtain an Mg:B mixed powder in a mass ratio of 1.03:2.00. The Mg-B mixed powder was then introduced into an SUS tube, one end of which was closed by uniaxial pressing. The open end was then uniaxially pressed to seal the Mg-B mixed powder into the SUS tube, and the center of the SUS tube was pressed at a pressure of 1.5 GPa. The SUS tube was then sealed in a quartz glass tube, which was then heated at 850°C for 3 hours to obtain a columnar MgB2 bulk body measuring 1.5 mm x 1.5 mm x 0.5 mm thick.

[0071] Comparative Example 1 An MgB2 bulk body of this comparative example was obtained in the same manner as in Example 1, except that boron powder 2 was used instead of the mixed boron powder. The particle size distribution of boron powder 2 is shown in Figure 2. It was confirmed that boron powder 2 had a peak at a particle size of 2.61 μm. It was confirmed that boron powder 2 had no peak at particle sizes below 250 nm, and had a monomodal particle size distribution with a peak at a particle size of 2.62 μm. The peak frequency of the peak (P1) at a particle size of 2.62 μm was 12.0%, and the frequency peak ratio was 0.

[0072] Comparative Example 2 An MgB2 bulk body of this comparative example was obtained in the same manner as in Example 1, except that boron powder 1 was used instead of the mixed boron powder. The particle size distribution of boron powder 1 is shown in Figure 3. It was confirmed that boron powder 1 had a bimodal particle size distribution with peaks at particle sizes of 179 nm and 933 nm. The peak frequency of the peak at 179 nm (P1) was 25.5%, and the peak frequency of the peak at 933 nm (P2) was 3.0%, resulting in a frequency-peak ratio of 0.12.

[0073] The results of measuring the critical current density of Example 1 and Comparative Examples 1 and 2 are shown in Table 1.

[0074]

[0075] Example 2 A mixed boron powder having a trimodal particle size distribution with peaks at particle sizes of 119 nm, 933 nm, and 3.95 μm was prepared in the same manner as in Example 1, except that boron powder 1 and boron powder 2 were weighed out in a ratio of 30% by mass to 70% by mass. The particle size distribution of the mixed boron powder is shown in FIG. 4 . The peak frequency of the 119 nm peak (P1) was 9.8%, the peak frequency of the 933 nm peak (P2) was 2.5%, and the peak frequency of the 3.95 μm peak (P3) was 9.6%, resulting in a frequency-peak ratio of 0.98. The oxygen content of the mixed boron powder was 2.3% by mass.

[0076] Using the obtained mixed boron powder, an MgB2 bulk body was produced by the premix-powder-in-closed-tube method. Specifically, magnesium powder (100 mesh, manufactured by Furuuchi Chemical Co., Ltd.), mixed boron powder, and MgB2 powder (100 mesh, manufactured by Furuuchi Chemical Co., Ltd.) were weighed in an air atmosphere to achieve a mass ratio of Mg:B:MgB2 = 1.2:2.0:1.0.

[0077] One end of the tube was closed by uniaxial pressing, and the mixture was introduced into the tube in the order Mg / MgB2 + B / Mg, with a mass ratio of Mg:MgB2 + B:Mg = 0.6:3.0:0.6. The open end was uniaxially pressed to seal the Mg-B-MgB2 mixed powder into the tube, and the center of the tube was pressed with a pressure of 1.5 GPa. The pressed body was then sealed in a quartz glass tube and heated at 800 °C for 24 hours to obtain an MgB2 bulk body.

[0078] Comparative Example 3 An MgB2 bulk body of this comparative example was obtained in the same manner as in Example 2, except that boron powder 2 was used instead of the mixed boron powder.

[0079] For Example 2 and Comparative Example 3, the measurement results of the critical current density are shown in Table 2, and the measurement results of the pore ratio, average crystal grain size, maximum crystal grain size, minimum crystal grain size, and oxygen content are shown in Table 3. Scanning electron microscope images of the MgB2 bulk bodies of Example 2 and Comparative Example 3 are shown in Figures 5 and 6.

[0080]

[0081]

[0082] STEM observation images of the MgB2 bulk bodies of Example 2 and Comparative Example 3 are shown in Figures 7 and 8, respectively. Table 4 shows the results of STEM / EDS measurement of the particles present at the grain boundaries of the MgB2 bulk body in Example 2, and the contents of B, Mg, and O at the grain boundaries in Comparative Example 3. The analysis region for Example 2 in Table 4 is region 001 shown in Figure 7, and the analysis region for Comparative Example 3 is region 001 shown in Figure 8. Note that the above regions are 40 nm square.

[0083]

[0084] For Example 2, the size of particles present at the grain boundaries of the MgB2 bulk body and the contents of B, Mg, and O were measured by STEM / EDS and are shown in Table 5. Note that regions 001, 002, and 003 in Table 5 are the 40 nm square regions shown in FIG.

[0085]

[0086] As shown in the table above, it was confirmed that the critical current density of magnesium diboride was improved in the MgB2 bulk body of Example 2 compared to the case where the particle size distribution had one peak in each magnetic field. Furthermore, as shown in Figures 5 and 6, it was found that the MgB2 bulk body of Example 2 formed a sintered body with few voids.

[0087] Example 3: Commercially available boron powder (particle size: 45 μm or less, product name: BBE01PB, manufactured by Kojundo Chemical Laboratory Co., Ltd.) was used as a raw material and vaporized by introducing it into a plasma (RF output: 6 kW, boron supply rate: 1 g / min) using a mixed gas of argon and hydrogen (argon:hydrogen = 98.8 vol%:1.2 vol%) as the working gas. The vaporized boron was cooled to obtain boron powder with a D50 diameter of 134 nm, designated as Boron Powder 3. The oxygen content of Boron Powder 3 was 3.0 mass%. Boron Powder 3 and Boron Powder 2 were weighed to be 50 mass% each, and then dry-mixed in an agate mortar in an air atmosphere to produce a mixed boron powder. The oxygen content of the mixed boron powder was 2.4 mass%. A magnesium diboride bulk was synthesized using the same method as in Example 2, and the critical current density was evaluated. The results are shown in Table 6.

[0088] Example 4: Commercially available boron powder (particle size: 45 μm or less, product name: BBE01PB, manufactured by Kojundo Chemical Laboratory Co., Ltd.) was used as a raw material and vaporized by introducing it into a plasma (RF output: 6 kW, boron supply rate: 1 g / min) using a mixed gas of argon and hydrogen (argon:hydrogen = 99.5 vol%:0.5 vol%) as the working gas. The vaporized boron was cooled to obtain boron powder with a D50 diameter of 134 nm, designated as Boron Powder 4. The oxygen content of Boron Powder 4 was 5.1 mass%. Boron Powder 4 and Boron Powder 2 were weighed to be 50 mass% each, and then dry-mixed in an agate mortar in an air atmosphere to produce a mixed boron powder. The oxygen content of the mixed boron powder was 3.5 mass%. A magnesium diboride bulk was synthesized using the same method as in Example 2, and the critical current density was evaluated. The results are shown in Table 6.

[0089]

[0090] (Example 5) 10 g of the magnesium diboride bulk obtained in Example 1 was pulverized in a mortar until the average particle size was 2 μm or less. The pulverized magnesium diboride was placed in a glass container, and 100 mL of propionic acid (purity: 98%, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added. The mixture was heated to 110 ° C and held for 1 hour. The glass container was then allowed to cool, and the propionic acid was removed by decantation. 100 mL of ethanol was then added and mixed, followed by decantation again. This ethanol decantation was performed three times, and the resulting magnesium diboride powder was dried at room temperature (25 ° C) and used as magnesium diboride seed crystals. Using the magnesium diboride seed crystals, an MgB2 bulk was produced by the premix-powder-in-closed-tube method. Specifically, magnesium powder (100 mesh, manufactured by Furuuchi Chemical Co., Ltd.), boron powder 1, and MgB2 powder (100 mesh, manufactured by Furuuchi Chemical Co., Ltd.) were weighed in an air atmosphere to achieve a mass ratio of Mg:B:MgB2 = 1.2:2.0:1.0. The powder was introduced into a SUS tube, one end of which was closed by uniaxial pressing, in the order Mg / MgB2 + B / Mg, to achieve a mass ratio of Mg:MgB2 + B:Mg = 0.6:3.0:0.6. The open end was uniaxially pressed to seal the Mg-B-MgB2 mixed powder into the SUS tube, and the center of the SUS tube was pressed at a pressure of 1.5 GPa. The pressed body was then sealed in a quartz glass tube and heated at 850 °C for 12 hours to obtain an MgB2 bulk body. In the X-ray diffraction of the obtained MgB2 bulk body, the ratio of the peak intensity due to oxide (magnesium oxide) around 43° to the peak intensity due to MgB2 at 42.5° was 0. The measurement results of the critical current density of this bulk body are shown in Table 7.

[0091] Example 6 An MgB2 bulk was produced in the same manner as in Example 5, except that the magnesium diboride seed crystal was not etched with propionic acid. In the X-ray diffraction of the obtained MgB2 bulk, the ratio of the peak intensity at 43 to 44.5° derived from the oxide (magnesium oxide) to the peak intensity at 42.5° derived from MgB2 was 0.18. The measurement results of the critical current density of this bulk are shown in Table 7.

[0092] This application is based on a Japanese patent application filed on November 17, 2023 (Patent Application No. 2023-195915) and a Japanese patent application filed on March 29, 2024 (Patent Application No. 2024-057471), the entire contents of which are incorporated by reference. In addition, all references cited herein are incorporated in their entirety.

Claims

1. A boron powder having two or more peaks in a number distribution, and in which the peak frequency of peak 2, which is the peak having the second highest frequency, relative to the peak frequency of peak 1, which is the peak having the highest frequency, is greater than 0.

12.

2. The boron powder according to claim 1, wherein the peak diameter of said peak 1 is smaller than the peak diameter of said peak 2.

3. The boron powder according to claim 1 or 2, wherein the peak diameter of said peak 1 is less than 1 μm.

4. The boron powder according to any one of claims 1 to 3, wherein the peak diameter of said peak 2 is 1 µm or more.

5. The boron powder according to any one of claims 1 to 4, wherein the ratio of the peak diameter of said peak 2 to the peak diameter of said peak 1 is 10 or more.

6. The boron powder according to any one of claims 1 to 5, having an oxygen content of 1.5 mass% or more.

7. The boron powder according to any one of claims 1 to 6, having an oxygen content of less than 3.5 mass%.

8. A method for producing magnesium diboride using the boron powder according to any one of claims 1 to 7.

9. A magnesium diboride bulk body, the magnesium diboride bulk body having a crystal grain boundary, and at least one grain in the crystal grain boundary, in which the atomic ratio of boron to magnesium, B / Mg, is 0.7 or more.

10. A magnesium diboride bulk body according to claim 9, wherein the grains have a maximum dimension perpendicular to the grain boundaries of 150 nm or less.

11. A magnesium diboride bulk body as described in claim 9 or 10, having at least one region in which the proportion of oxygen atoms in a 40 nm square region including the crystal grain boundary within a radius of 200 nm from the center position of the particle is 6 atom % or less, when the total atomic weight of magnesium, boron, and oxygen is 100 atom %.

12. A magnesium diboride bulk body as described in claim 9 or 10, having at least one region in which the proportion of oxygen atoms in a 40 nm square region including the crystal grain boundary within a radius of 200 nm from the center position of the particle exceeds 6 atom %, when the total atomic weight of magnesium, boron, and oxygen is 100 atom %.

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