Superconductor, method for producing superconductor, liquid level sensor for liquid hydrogen, and liquid level gauge for liquid hydrogen

Abstract

[Problem] The present invention addresses the problem in realizing a superconducting material which can be used for a liquid level gauge for liquid hydrogen, enabling a simple production method, and achieving reproducibility. [Solution] The present invention provides a superconductor which is composed of a metal compound that has a molar ratio of magnesium (Mg) to boron (B) of 1:2, and does not contain SiC. This superconductor is characterized in that the critical temperature (Tc) thereof, at which the electric resistance is substantially zero, is about 32K to about 34K.　As a result, it is possible to realize a liquid level gauge for liquid hydrogen, which enables a simple production method and achieving reproducibility.

Description

Superconductor, method for manufacturing superconductor, liquid hydrogen level sensor, and liquid hydrogen level gauge

[0001] The present invention relates to a superconductor, a method for manufacturing a superconductor, a liquid hydrogen level sensor, and a liquid hydrogen level gauge.

[0002] There is a growing global awareness that global warming is a common challenge we must address now and in the future. A major challenge is the movement toward decarbonization, which reduces fossil fuel consumption. One option is the use and utilization of hydrogen. In Japan, the "Basic Policy for Realizing GX (Green Transformation)" was approved by the Cabinet in February 2023, promoting the introduction of hydrogen. However, accurate measurement of the amount of liquid hydrogen during its transportation, storage, and consumption is required.

[0003] Liquid hydrogen is an extremely low-temperature substance with a boiling point of approximately 20 K under normal pressure, and there has been a demand for a material that exhibits superconductivity at temperatures higher than the temperature of liquid hydrogen as a sensor material for liquid hydrogen level gauges that utilize changes in electrical resistance.At the beginning of the 21st century, the relatively high-temperature superconductivity of MgB2 (magnesium diboride) was reported as an intermetallic compound, and efforts are underway to develop liquid hydrogen level gauges using MgB2.

[0004] Patent Document 1 describes an intermetallic compound made of magnesium (Mg) and boron (B), which has a critical temperature (Tc) of 39 K at which the electrical resistance becomes substantially zero. Patent Document 2 describes a liquid hydrogen level gauge constructed using a long superconductor in which some of the B atoms in MgB2 are replaced with C atoms. Patent Document 3 describes a superconducting wire produced by filling one end of a metal sheath with a first powder made by mixing Mg and B with an average particle size of less than 2 μm, filling the other end of the metal sheath with a second powder made by mixing Mg and B with an average particle size of 2 μm or more but less than 15 μm, and then performing wire drawing and heat processing.

[0005] Patent Document 1: JP 2002-211916 A Patent Document 2: JP 2000-175034 A Patent Document 3: JP 2014 / 162379 A

[0006] However, the intermetallic compound described in Patent Document 1 has a high critical temperature (Tc) of 39 K, which is too high for a superconductor to be used in a level gauge for liquid hydrogen, whose boiling point under atmospheric pressure is approximately 20 K.

[0007] Furthermore, the superconductor described in Patent Document 2 in which some of the B atoms of MgB2 are substituted with C atoms has the problem that some of the B atoms must be substituted with C atoms, making the manufacturing method complicated and making it difficult to achieve reproducibility.

[0008] Furthermore, the superconducting wire material described in Patent Document 3 also has a problem in that the manufacturing method is complicated because two types of B having different average particle sizes are used.

[0009] An object of the present invention is to solve the above problems and to realize a superconducting material that can be used in a liquid hydrogen level gauge and that can be manufactured by a simple and reproducible method.

[0010] In order to solve the above problems, the present invention provides a superconductor that is made of a metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, does not contain SiC, and has a critical temperature (Tc) of about 32K to about 34K at which the electrical resistance becomes substantially zero.

[0011] This configuration makes it possible to realize a superconductor that can be used in a liquid hydrogen level gauge and that can be manufactured simply and reproducibly. Here, the temperature of liquid hydrogen is 20 K at normal pressure, but rises to 30 K at 1 MPaG. Therefore, a superconductor for a liquid hydrogen level gauge must have a critical temperature (Tc) of at least 31 K or higher to accommodate a liquid hydrogen temperature of 20 to 30 K. The applicant then discovered that, as described below, a superconductor with a Tc of approximately 32 K to approximately 34 K can be realized by mixing magnesium (Mg) with boron (B) having an average particle size of 17 μm to 7 μm in a molar ratio of 1 part magnesium (Mg) to 2 parts boron (B).

[0012] In order to solve the above problems, the present invention provides a method for producing a superconductor having a critical temperature (Tc) of approximately 32K to approximately 34K at which the electrical resistance becomes substantially zero, the method comprising: a mixing step of mixing magnesium (Mg) with boron (B) having an average particle size of 17μm to 7μm in a molar ratio of 1 part Mg to 2 parts B; and a firing step of firing the mixture of Mg and B mixed in the mixing step.

[0013] This configuration makes it possible to realize a superconducting material that can be used in a liquid hydrogen level gauge and that can be manufactured by a simple and reproducible method.

[0014] In order to solve the above problems, the present invention also provides a liquid hydrogen level sensor comprising: a long superconductor which is a metal compound made of MgB2 and does not contain SiC, and which has a critical temperature (Tc) of approximately 32 K to approximately 34 K at which the electrical resistance becomes substantially zero; and a coating metal which covers the surface of the superconductor.

[0015] This configuration makes it possible to realize a liquid hydrogen level sensor that can be used in a liquid hydrogen level gauge and includes a superconducting material body that can be manufactured simply and with high reproducibility.

[0016] Furthermore, in order to solve the above-mentioned problems, the present invention provides a liquid hydrogen level sensor comprising: a long superconductor which is a metal compound made of MgB2 containing no SiC and has a critical temperature (Tc) of about 32K to about 34K at which the electrical resistance becomes substantially zero; and a coating metal which covers the surface of the superconductor; a heater which heats the liquid hydrogen level sensor; a power source which passes a current through the liquid hydrogen level sensor; and a voltmeter which measures the voltage at the liquid hydrogen level sensor.

[0017] This configuration makes it possible to realize a liquid hydrogen level gauge that can be manufactured simply and with high reproducibility.

[0018] The superconductor and the method for manufacturing a superconductor of the present invention make it possible to realize a superconductor that can be manufactured simply and reproducibly, and therefore liquid hydrogen level sensors and liquid hydrogen level gauges can be easily realized.

[0019] 1 shows measurement data of the critical temperature (Tc) of the superconductor in Example 1 of the present invention (including comparative data). FIG. 2 is a diagram illustrating a liquid hydrogen level sensor in Example 1 of the present invention. FIG. 3 is a diagram illustrating a liquid hydrogen level gauge in Example 1 of the present invention. FIG. 4 is a graph showing the relationship between the average particle size of boron and the critical temperature in Example 1 of the present invention.

[0020] Example 1 of the present invention will be described with reference to Figs. 1 to 4. Fig. 1 shows measurement data of the critical temperature (Tc) of the superconductor in Example 1 of the present invention (including comparative examples). Fig. 2 is a diagram illustrating a liquid hydrogen level sensor in Example 1 of the present invention. Fig. 3 is a diagram illustrating a liquid hydrogen level gauge in Example 1 of the present invention. Fig. 4 is a graph showing the relationship between the average particle size of boron and the critical temperature in Example 1 of the present invention.

[0021] (Superconductor) The superconductor in Example 1 can be used in a liquid hydrogen level gauge. The critical temperature (Tc) at which the electrical resistance of liquid hydrogen becomes substantially zero is 20 K at normal pressure, but rises to 30 K at 1 MPaG. Therefore, a superconductor with Tc = 32.4 K to 34.3 K was realized to correspond to the critical temperature Tc = 20 to 30 K for a liquid hydrogen level gauge.

[0022] The superconductor in Example 1 does not use SiC, but is made of magnesium (Mg) and boron (B). A mixture of magnesium (Mg) and boron (B) in a molar ratio of 1:2 is filled into a hollow metal tube, drawn, and then heat-treated in an argon atmosphere at approximately 600°C to produce an intermetallic compound. The critical temperature (Tc) of the superconductor is 32.4 K for (d) and 34.3 K for (c), as shown in the experimental data in Figure 1. The horizontal axis of Figure 1 represents temperature (K), and the vertical axis represents output voltage (mV). When the output voltage becomes substantially zero, the resistance becomes zero, and the temperature at that time is the critical temperature (Tc). 1, graph (d) shows the results of measuring the critical temperature (Tc) of 32.4 K when boron with an average particle size of 14 μm is mixed with magnesium, and graph (c) shows the results of measuring the critical temperature (Tc) of 34.3 K when boron with an average particle size of 2 μm is mixed with magnesium. The magnesium used here was 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm).

[0023] Referring to Figure 1, we will explain the results of an experiment in which the critical temperature (Tc) was measured comparatively while varying the average particle size of boron. Graphs (a), (b), (e), (f), and (g) in Figure 1 are all experimental data (comparison data) in which the critical temperature (Tc) was outside the range of 32 K to 34 K. That is, graph (a) shows the result of mixing 1 part magnesium with 2 parts boron having an average particle size of 0.7 μm, and the critical temperature (Tc) was 35.6 K. Graph (b) shows the result of mixing 1 part magnesium with 2 parts boron having an average particle size of 1 μm, and the critical temperature (Tc) was 35.5 K. In other words, the critical temperatures (Tc) of the data in (a) and (b) are higher than 35 K, and therefore cannot be used as superconductors for liquid hydrogen level gauges.

[0024] Graph (e) in Figure 1 shows the results of mixing 1 part magnesium with 2 parts boron having an average particle size of 36 μm, with a critical temperature (Tc) of 27.3 K. Graph (f) shows the results of mixing 1 part magnesium with 2 parts boron having an average particle size of 38 μm, with a critical temperature (Tc) of 29.2 K. Graph (g) shows the results of mixing 1 part magnesium with 2 parts boron having an average particle size of 39 μm, with a critical temperature (Tc) of 26.9 K. In other words, the data in (e), (f), and (g) have critical temperatures (Tc) lower than 32 K and cannot be used as superconductors for liquid hydrogen level gauges.

[0025] The graph in Figure 1 (d) shows the results of mixing magnesium with boron having an average particle size of 14 μm in a ratio of 1 part magnesium to 2 parts boron, with a critical temperature (Tc) of 32.4 K. The graph in (c) shows the results of mixing magnesium with boron having an average particle size of 2 μm in a ratio of 1 part magnesium to 2 parts boron, with a critical temperature (Tc) of 34.3 K. In other words, the data in (c) and (d) show values ​​close to the target for a superconductor for a liquid hydrogen level gauge.

[0026] Here, we will explain how to determine the average particle size and critical temperature (Tc) shown in the experimental data. The average particle size was determined by measurement using a Shimadzu SALD-2300 (laser diffraction particle size distribution analyzer). The critical temperature (Tc) was not determined according to JIS H7309, but rather by approximating the measurement data (25K to 45K) of a short sample using a cryostat device with a sigmoid curve (transition curve) using the least squares method, and the value at 50% of the height was taken as Tc.

[0027] From the above experimental data, it was discovered that the smaller the average particle size of boron, the higher the critical temperature (Tc), and the larger the average particle size, the lower the critical temperature (Tc). Next, we investigated what the average particle size of boron should be to achieve a critical temperature (Tc) of approximately 32K to approximately 34K. The graph shown in Figure 4 has the average particle size (μm) of boron on the horizontal axis and the critical temperature (Tc) on the vertical axis, and seven data points from the above experimental results are plotted on this graph. As can be seen from Figure 4, these data have a very strong linear approximation relationship as shown in Equation 1. Equation 1: Tc temperature = -0.2 x average particle size (μm) + 35.35

[0028] The average particle size of boron needed to obtain a critical temperature (Tc) of approximately 32K to approximately 34K, obtained from this approximation line, is 7 to 17 μm. In other words, if one part of magnesium is mixed with two parts of boron with an average particle size of 7 to 17 μm, a superconductor with a critical temperature (Tc) of 32K to 34K can be obtained.

[0029] In Example 1, the particle size of boron was expressed as an average particle size. The average particle size is a representative diameter obtained by averaging the measurements of the SALD-2300 described above, and is calculated by multiplying the particle size on the horizontal axis of the histogram by the frequency on the vertical axis, and then adding them together. However, if the particle size distribution, which represents the particle size distribution of the particulate matter, differs, the critical temperature (Tc) may differ. Therefore, in this application, the critical temperature (Tc) obtained from the approximation line is expressed as approximately 32 K to approximately 34 K, with the word "approximately" added.

[0030] (Method of manufacturing a superconductor) First, a mixing step is carried out in which 1 part magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) is mixed with 2 parts boron (B) of an average particle size of 14 μm. Next, a firing step is carried out in which the mixture of Mg and B mixed in the mixing step is fired in an argon atmosphere at 600°C for 1 hour to complete an MgB2 superconductor. Note that, as described below, when making a long liquid hydrogen level sensor, the fired MgB2 can be powdered and placed in a long tube and subjected to wiredrawing, or the mixed Mg and B powder can be wiredrawn and then fired.

[0031] The boron (B) used here was purchased from Nilaco with a particle size of 40 μm, and the average particle size measured using the laser diffraction particle size distribution analyzer was 14 μm. The magnesium (Mg) used here was purchased from Alfa Aesar with a 325 mesh (particle size of approximately 44 μm).

[0032] Magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) was mixed with boron (B) of an average particle size of 14 μm in a molar ratio of 1 part, and the mixture was subjected to a wiredrawing process. The critical temperature (Tc) of the sintered superconductor was 32.4 K (see Figure 1).

[0033] A mixing step is also carried out in which boron (B) having an average particle size of 2 μm is mixed with 1 part magnesium (Mg) having a 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) in a molar ratio of 2. Next, a firing step is carried out in which the mixture of Mg and B mixed in the mixing step is fired at 600° C. in an argon atmosphere for 1 hour.

[0034] Here, boron (B) was purchased and used from nanogaraphi technology with a particle size of 10 μm, but the average particle size measured with the laser diffraction particle size distribution analyzer was 2 μm. Magnesium (Mg) was purchased and used from alfa Aesar with a 325 mesh (particle size of approximately 44 μm).

[0035] Magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) was mixed with boron (B) of an average particle size of 2 μm in a molar ratio of 1:2, and the sintered superconductor had a critical temperature (Tc) of 34.3 K (see Figure 1).

[0036] In Example 1, the magnet was manufactured without containing any substance other than MgB2, but this is not necessarily limited to this and can be changed as appropriate. For example, the magnet may contain SiC or other substances that do not affect the critical temperature (Tc).

[0037] After mixing boron (B) and magnesium (Mg), and before the firing process, the Mg and B powder particles are in contact at their contact points. When the firing process is carried out in this state, MgB2 crystal nuclei are generated from the contact points and these nuclei grow. The number of MgB2 crystal nuclei generated is affected by the number of contact points, which varies depending on the particle size ratio of the powders being mixed. When there are many crystal nuclei (a large number of contact points), the overall reaction is fast and adjacent crystals collide at the interface, fuse, and grow, resulting in a complete crystal with no unreacted powder as impurities.

[0038] Therefore, in the particle size range of several μm to several tens of μm, the smaller the particle size of B is compared to the particle size of Mg, the more contact points there are, and the generated MgB2 becomes a complete crystal, so the critical temperature (Tc) becomes higher. This supports the relationship between the average particle size of B and the critical temperature (Tc) explained in Figure 4 above.

[0039] As a result of the above, it has become possible to produce superconductors with reproducibility. However, to ensure even greater reproducibility, it is necessary to strictly control the heat treatment (rate and time of temperature increase, constant temperature, and temperature decrease), the particle size and particle size ratio of Mg and B, particle size distribution, etc., in the production to maintain the same conditions.

[0040] (Liquid Hydrogen Level Sensor) The liquid hydrogen level sensor 10 in Example 1 is composed of a superconductor 11 and a metal coating 12, and forms a long core wire with a length of approximately 1 m and a diameter of approximately 0.3 mm, as shown in Figure 2. The superconductor 11 is made by mixing magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) with boron (B) of an average particle size of 14 μm in a molar ratio of 1:2 (mixing process), which is then drawn in a wiredrawing process and then sintered to form an MgB2 superconductor. In the wiredrawing process, the mixture of Mg and B obtained in the mixing process is powdered and placed in a tube approximately 1 m long, which is then drawn to form a long wire.

[0041] In Example 1, the mixing step, wire drawing step, and firing step are performed in this order, but this is not necessarily limited to this and can be modified as appropriate. For example, the firing step may be performed after the mixing step, followed by a powdering step for powdering, and finally the wire drawing step. Also, the heat treatment step may be performed multiple times.

[0042] In Example 1, a long core wire having a length of about 1 m and a diameter of about 0.3 mm is formed, but this is not necessarily limited to this and can be changed as appropriate. For example, the length may be about 5 m or 0.5 m, as long as it is prepared according to the depth of the liquid hydrogen. Also, in Example 1, the diameter is about 0.3 mm, but this is not necessarily limited to this and can be changed as appropriate. For example, it may be 0.5 mm or 0.2 mm, and the diameter may be selected taking into account the critical temperature (Tc).

[0043] Furthermore, the metal coating 12 may be any metal having high conductivity. Although cupronickel was used in Example 1, other metals such as stainless steel may also be used.

[0044] In Example 1, boron (B) having an average particle size of 14 μm was mixed with magnesium (Mg) having a mesh size of 325 (manufactured by Alfa Aesar, particle size approximately 44 μm) in a molar ratio of 2, but this is not necessarily limited to this and can be changed as appropriate. For example, boron (B) having an average particle size of 2 μm may be mixed with magnesium (Mg) having a mesh size of 325 (manufactured by Alfa Aesar, particle size approximately 44 μm) in a molar ratio of 2.

[0045] The resistance value of the liquid hydrogen level sensor 10 in the first embodiment at room temperature is 1 Ω / m to 6 Ω / m, but is not limited to this and can be changed as appropriate.

[0046] In addition, in the first embodiment, the length of the liquid hydrogen level sensor 10 is set to approximately 1 m, but this is not necessarily limited to this and can be changed as appropriate. For example, the length may be set to approximately 5 m or approximately 0.5 m. It is sufficient if the length is at least long enough to measure the liquid hydrogen level.

[0047] (Liquid hydrogen level gauge) The configuration of the liquid hydrogen level gauge 100 in Example 1 is shown in Figure 3. A tank T is filled with liquid hydrogen M, and a liquid hydrogen level sensor 10 is installed vertically in the tank T. The liquid hydrogen level sensor 10 is placed deep in the liquid hydrogen M.

[0048] The liquid hydrogen level sensor 10 in the room temperature area above the liquid surface can generate resistance heat by passing a current from the heater power supply 21 through the heater 20, thereby heating the liquid hydrogen level sensor 10. This eliminates the influence of the temperature of the liquid hydrogen M and makes it possible to maintain a normal conducting state above the liquid surface.

[0049] A power supply 30 and a voltmeter 40 are connected to both ends of the liquid hydrogen level sensor 10. Then, a current is passed from the power supply 10 to the liquid hydrogen level sensor 10, and the voltage across the liquid hydrogen level sensor 10 is measured by the voltmeter 40 and stored in the processing unit 50. Then, the liquid level position is calculated from the stored voltage based on a predetermined known relational expression.

[0050] Thus, in Example 1, a superconductor is made of a metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, does not contain SiC, and has a critical temperature (Tc) at which the electrical resistance becomes substantially zero of about 32 K to about 34 K. This superconductor can be used in a liquid hydrogen level gauge, and can be manufactured by a simple method that allows for reproducibility.

[0051] Moreover, in Example 1, a method for manufacturing a superconductor having a critical temperature (Tc) at which the electrical resistance becomes substantially zero of about 32 K to about 34 K is provided, the method comprising: a mixing step of mixing magnesium (Mg) with boron (B) having an average particle size of 17 μm to 7 μm in a molar ratio of 2 to 1; and a firing step of firing the mixture of Mg and B mixed in the mixing step. This method makes it possible to realize a superconductor for a liquid hydrogen level gauge that is simply manufactured and is highly reproducible.

[0052] Furthermore, in Example 1, a liquid hydrogen level sensor is provided which is characterized by comprising a long superconductor which is a metal compound made of MgB2 containing no SiC and has a critical temperature (Tc) of about 32 K to about 34 K at which the electrical resistance becomes substantially zero, and a coating metal which covers the surface of the superconductor, thereby making it possible to realize a liquid hydrogen level gauge which can be manufactured simply and with high reproducibility.

[0053] Furthermore, in Example 1, a liquid hydrogen level sensor comprising: a long superconductor which is a metal compound made of MgB2 containing no SiC and has a critical temperature (Tc) of about 32 K to about 34 K at which the electrical resistance becomes substantially zero; and a coating metal which covers the surface of the superconductor; a heater which heats the liquid hydrogen level sensor; a power source which passes a current through the liquid hydrogen level sensor; and a voltmeter which measures the voltage at the liquid hydrogen level sensor, can be realized by a liquid hydrogen level sensor which is simple to manufacture and provides reproducibility.

[0054] The superconductor, the method for manufacturing the superconductor, the liquid hydrogen level sensor, and the liquid hydrogen level gauge of the present invention can be widely applied in the field of liquid hydrogen level gauges.

[0055] 10: Liquid hydrogen level sensor 11: Superconductor 12: Coated metal 20: Heater section 21: Heater power supply 30: Power supply 40: Voltmeter 50: Processing section 100: Liquid hydrogen level gauge

Claims

1. A superconductor consisting of a metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, not containing SiC, characterized in that the critical temperature (Tc) at which the electrical resistance becomes substantially zero is approximately 32K to approximately 34K.

2. A method for producing a superconductor having a critical temperature (Tc) of approximately 32K to approximately 34K, at which the electrical resistance becomes substantially zero, comprising: a mixing step of mixing magnesium (Mg) with boron (B) having an average particle size of 17μm to 7μm in a molar ratio of 2 parts, and a firing step of firing the mixture of Mg and B mixed in the mixing step.

3. A liquid hydrogen level sensor comprising: a long superconductor which is a metal compound made of MgB2 containing no SiC and has a critical temperature (Tc) of about 32K to about 34K at which the electrical resistance becomes substantially zero; and a coating metal which covers the surface of the superconductor.

4. A liquid hydrogen level sensor comprising: a long superconductor which is a metal compound made of MgB2 containing no SiC and has a critical temperature (Tc) of about 32K to about 34K at which the electrical resistance becomes substantially zero; and a coating metal which covers the surface of the superconductor; a heater which heats the liquid hydrogen level sensor; a power source which passes a current through the liquid hydrogen level sensor; and a voltmeter which measures the voltage at the liquid hydrogen level sensor.

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