JP6839394B2 - Superconducting wire, liquid level sensor element for liquid hydrogen, and liquid level gauge for liquid hydrogen - Google Patents

Description

The present invention relates to a superconducting wire suitable for a liquid hydrogen level gauge that measures the liquid level of liquid hydrogen with high accuracy, a liquid hydrogen level sensor element provided with the superconducting wire, and a liquid hydrogen level gauge.

In recent years, hydrogen fuel has been attracting attention from the viewpoint of introducing clean energy. As a hydrogen storage method, liquid hydrogen is one of the effective usage forms because of its high storage density. When storing and transporting liquid hydrogen, a liquid level sensor element and a liquid level gauge capable of measuring the liquid level in the container are required from the viewpoint of grasping the amount of liquid and safety management.

_{Since MgB 2,} which is a superconductor, has a critical temperature of 39K, it can exhibit a superconducting state in liquid hydrogen having a boiling point of about 20K under atmospheric pressure. Therefore, so far, a liquid hydrogen level gauge that can measure the amount of liquid hydrogen in a storage container from the outside by using _{an MgB 2 superconductor as a liquid hydrogen level sensor element has been proposed (Patent Documents).} See 1 and 2). In Patent Document 1, a liquid level sensor for detecting the liquid level of liquid hydrogen, which includes a superconducting wire that is in a superconducting state at the boiling point of liquid hydrogen and a sheath that covers the superconducting wire. Is shown, and MgB ₂ is mentioned as a superconducting wire. In addition, LSCO (La _2-x Sr _x CuO ₄ ) is mentioned. In Patent Document 1, the sheath is coated with a superconducting wire and is formed of a material having a lower thermal conductivity than the superconducting wire. For example, the sheath is made of ceramics and / or stainless steel.

According to the prior art literature search related to the present application, the following documents are known regarding iron-based superconductors (superconducting materials and superconducting wires) (Patent Documents 3 and 4, Non-Patent Documents 1 and 2). reference). Both documents are characterized by having an excellent critical current characteristic for the application of a high magnetic field generating magnet.

In Patent Document 3, a manufacturing method that realizes excellent critical current characteristics is described for a so-called 122-series superconductor whose composition is represented by (Ba, K) Fe ₂ As ₂ or (Sr, K) Fe ₂ As _2. It is disclosed. In Patent Document 3, (Ba, K) Fe ₂ As ₂ or (Sr, K) Fe ₂ As ₂ is a 122-based compound having _{a ThCr 2} Si _{type 2} crystal structure showing a maximum Tc of about 38K. It is explained.

Patent Document 4 provides an iron-based superconducting material having a small dependence of the critical current density on the magnetic field angle, a superconducting layer using the superconducting material, and a wire rod provided with the superconducting layer that can be used at a low temperature and a high magnetic field. Is the purpose. Patent Document 4 describes an iron-based superconductor having _{a ThCr 2} Si _{2 crystal structure and a particle size represented by BaXO 3} (X represents one or more of Zr, Sn, Hf, and Ti). An iron-based superconducting material having nanoparticles of 30 nm or less and in which the nanoparticles are ^{dispersed at a volume density of 1 × 10 21} m ^{-3 or more is disclosed.}

Non-Patent Document 1 _{reports on the discovery that BaFe 1.8} Co _0.2 As ₂ transitions to a superconducting state at 22K. Non-Patent Document 2 _{reports that the superconducting critical temperature of Ba (Fe 1-x} Co _x ) ₂ As ₂ changes in the range of 0 to 25 K depending on the value of the contained Co concentration x. ..

Japanese Unexamined Patent Publication No. 2009-175034 Japanese Unexamined Patent Publication No. 2014-98656 Japanese Unexamined Patent Publication No. 2014-240521 Japanese Unexamined Patent Publication No. 2014-227329

“Superconductivity at 22 K in Co-Doped BaFe2As2 Crystals”, Phys. Rev. Lett. 101, 117004, 2008. “Effects of Co Substitution on thermodynamic and transport properties and anisotropic Hc2 in Ba (Fe1-xCox) 2Asx single crystals” Phys. Rev. B 78, 214515, 2008.

In recent years, it has been desired to measure the liquid level of liquid hydrogen with high accuracy. In order to provide a high-precision liquid hydrogen level gauge, it is desirable that the superconducting wire that serves as the liquid level sensor element has the following three conditions.
(Condition 1) The critical temperature for transitioning to the superconducting state (electrical resistance becomes zero) is close to the temperature of liquid hydrogen of 20K.
(Condition 2) The difference between the temperature at which the electrical resistance begins to decrease and the critical temperature (hereinafter referred to as the transition width) is small.
(Condition 3) The temperature dependence of electrical resistance in the normal conduction state is small.

However, the conventional MgB ₂ hydrogen level gauge has the following problems (1) to (4).
(1) The superconducting critical temperature is too high for the boiling point of liquid hydrogen of 20K (for example, exceeding 25K), and the liquid surface is actually superconducted to the part not immersed in liquid hydrogen (non-immersed part). Will overestimate. In order to improve the measurement accuracy, it is necessary to make the non-immersed part completely normal by heating the non-immersed part with a heater or the like.
(2) It is possible to adjust the critical temperature to 20 to 25K by adding C, Al, or the like to _{MgB 2, but the transition width becomes large and the measurement accuracy decreases.}
(3) Even when the added concentrations of C and Al are the same, the critical temperature and transition width of the superconducting wire depend on the rolling process (for example, the diameter of the wire after processing), and the reproducibility is lowered.
(4) The temperature dependence of the electrical resistance in the normal conduction state is relatively large, and the electrical resistance of the non-immersed portion changes depending on the temperature distribution in the gas, so that the measurement accuracy is lowered.

The present invention is intended to solve these problems, and the present invention provides a superconducting wire material suitable for a liquid level sensor element capable of measuring the liquid level height of liquid hydrogen with high accuracy. With the goal. Another object of the present invention is to provide a liquid hydrogen level sensor element including the superconducting wire and a liquid hydrogen level gauge including the liquid level sensor element.

The present invention has the following features in order to achieve the above object.

(1) The present invention is a superconducting wire having a superconducting portion, and the composition of the superconducting portion is _{represented by the chemical formula Ba (Fe 1-x} Co _x ) ₂ As ₂ , and x is 0.06 or more. The transition width is the difference between the temperature at which the critical temperature at which the electric resistance becomes zero is 20 K or more and 25 K or less, and the electric resistance starts to decrease toward the critical temperature. Is 2K or less.
(2) In the superconducting wire rod according to (1) above, when the electrical resistivity at a temperature of 300K is ρ (300K) and the electrical resistivity at a temperature of 30K is ρ (30K), 1- (ρ (30K) ) / Ρ (300K)), the value of the temperature dependence of the electrical resistivity in the normal conduction state is 0.5 or less.
(3) The superconducting wire according to (1) or (2), wherein the superconducting wire has a diameter of 2 mm or less.
(4) The superconducting wire according to any one of (1) to (3) above is for a liquid level sensor element for liquid hydrogen.
(5) The present invention is a liquid level sensor element for liquid hydrogen, and includes the superconducting wire according to any one of (1) to (3) above.
(6) The present invention is a liquid hydrogen voltmeter for measuring the liquid level of liquid hydrogen, which comprises the superconducting wire according to any one of (1) to (3) above. It is composed of a surface sensor element, a current source, and a voltmeter, and is characterized in that the height of the liquid level is obtained based on the measured voltage.

According to the superconducting wire of the present invention, a liquid level sensor element capable of measuring the liquid level of liquid hydrogen with high accuracy is realized, and a heating device is not required, so that the structure is smaller and simpler. Liquid level gauge can be provided.

According to the present invention, the superconductor Ba (Fe _1-x Co _x ) ₂ As ₂ constituting the superconducting wire material controls the Co content because the superconducting critical temperature changes depending on the Co content x. By doing so, the critical temperature can be adjusted to any temperature of 20 to 25K. Further, since the maximum value of the critical temperature of the superconductor constituting the superconducting wire of the present invention is about 25K, the transition width is very small, 2K or less, in the range of 20 to 25K.

Further, the superconducting wire rod of the present invention has a smaller temperature dependence of the electrical resistance in the normal conduction state than that of _{MgB 2.} Therefore, since the superconducting wire of the present invention simultaneously satisfies the three conditions required for the superconducting wire for a liquid hydrogen level sensor element, a liquid hydrogen level gauge with high accuracy and a simple design can be obtained.

Further, according to the superconducting wire of the present invention, it is not necessary to heat the heater and the measurement can be performed with a small energizing current, so that the evaporation of hydrogen at the time of measurement can be suppressed.

Further, the superconducting wire of the present invention can be subjected to heat treatment or the like to further reduce the temperature dependence of the electrical resistance in the normal conduction state. For example, a value of 1- (ρ (30K) / ρ (300K)) could be achieved to be 0.5 or less.

Indeed, was subjected to a performance verification using liquid hydrogen, to reflect the characteristics of the superconductor _{Ba (Fe 1-x Co x} ) 2 As 2, even without overheating the non-immersed portion by the heater, A linear proportional relationship with good reproducibility was obtained between the voltage and the liquid level, and a 1: 1 correspondence was obtained particularly with the composition of x = 0.09.

It is a figure which shows the crystal structure of the iron-based superconductor in the superconducting part of the superconducting wire of this invention. It is sectional drawing of the superconducting wire rod in embodiment of this invention. It is a conceptual diagram of the liquid level gauge of embodiment of this invention. It is a figure which shows the temperature dependence (near superconducting transition) of the electric resistance of the superconducting wire in the embodiment of this invention. It is a figure which shows the Co content dependence of the critical temperature of the superconducting wire in the embodiment of this invention. It is a figure which shows the temperature dependence (from the vicinity of a superconducting transition to 300 (K)) of the electric resistance of the superconducting wire in the embodiment of this invention. It is a figure which shows the relationship between the reading liquid level position by visual observation, and the standardized electric resistance measurement value in embodiment of this invention.

Embodiments of the present invention will be described below.

The present inventor has conducted research and development focusing on iron-based superconductors as superconductors suitable for liquid level gauges using superconducting wire rods, and has come to obtain superconducting wire rods having excellent characteristics for liquid level gauges. It was.

Ba known as an example of a 122-based compound _{(Fe 1-x Co x)} 2 As 2 is superconducting critical temperature is 20K vicinity. The critical temperature of Ba (Fe _1-x Co _x ) ₂ As ₂ changes depending on the Co content x, but the maximum value is about 25K, so the transition width is 0.7K or more in the range of 20 to 25K. Like 0.8K, it is 0.5 to 2K, which is very small. In addition, this group of substances has a smaller temperature dependence of electrical resistance in the normal conduction state than _{MgB 2.} Therefore, by employing the iron-based superconductors _{Ba (Fe 1-x Co x} ) 2 As 2 superconducting material of the superconducting portion, the three requirements for a superconducting wire for liquid hydrogen level sensor element , Meet at the same time.

_{FIG. 1 shows the ThCr 2} Si _{type 2} crystal structure of the superconducting wire rod which is the premise of the present invention. Although the arrangement of Ba, Fe, and As is shown in FIG. 1, in Ba (Fe _1-x Co _x ) ₂ As ₂ , a part of Fe is replaced by Co.

FIG. 2 shows the structure of the superconducting wire constituting the liquid level sensor element. As shown in FIGS. 2A, 2B, and 2C, the superconducting wire has at least a long rod-shaped core 1 made of a superconductor and a metal (Ag, which covers the core 1). It is composed of a sheath tube 2 of Ag—Sn alloy or the like). FIG. 2A is an example in which the superconducting wire is composed of the core 1 of the superconductor and the sheath tube 2. FIG. 2B is an example in which the reinforcing coating layer 3 (Fe, SUS (stainless steel), etc.) is arranged on the outside of the metal sheath tube. FIG. 2C shows an example in which a plurality of superconductive wires are bundled to form a stranded wire. Further, the insulation may be integrally molded with a resin or the like.

The structure of the superconducting wire of the present embodiment may be a structure in which an iron-based superconducting material is filled as a superconducting core in a metal sheath as shown in FIG. 2, or a metal tape group (not shown). An iron-based superconducting tape wire rod having an iron-based superconducting layer formed on the material may also be used. As the metal tape base material, what is known as a base material for ordinary superconducting wires can be used, and an iron-based superconducting layer is deposited on the metal tape base material via an intermediate layer by a pulse laser deposition method (PLD). Formed by law) etc. For example, on a metal tape substrate such as Ni-based alloy, prevent interfacial reaction layer (bed layer, Y ₂ O _3, etc.) or a layer for improving the crystal orientation of the film (orientation layer, MgO, etc.) as an intermediate layer After the provision, a superconducting layer may be formed, and a stabilizing layer for overcurrent protection and suppression of chemical reaction may be appropriately provided on the superconducting layer.

The superconducting wire in the present embodiment preferably has a diameter of 2 mm or less. The smaller the diameter, the more the required energizing current can be suppressed, and the smaller the heat capacity, the better the response. In the embodiment described later, the size is 0.5 mm-2 mm, but the size may be 0.5 mm or less. Further, from the viewpoint of the strength of the wire rod, 0.1 mm or more is preferable.

A liquid level sensor element and a liquid level gauge using a superconducting wire will be described with reference to FIG. FIG. 3 is a schematic view of the liquid level gauge. The liquid level meter of the present embodiment has a liquid level sensor element 10 for liquid hydrogen, a power source for passing a current through the liquid level sensor element, and substantially both ends of the liquid level sensor element (the liquid level height to be measured). A voltmeter is provided to measure the voltage at (determined by measurable length). The liquid level sensor element 10 is composed of the superconducting wire rod of the present invention. The liquid level sensor element is installed in a container in which liquid hydrogen is stored. In principle, the resistance value differs between the part of the liquid level sensor element 10 in the liquid hydrogen 12 (superconducting state) and the part outside the liquid hydrogen (ideally in the normal conducting state). It was done. The liquid level 11 level is detected by measuring the voltage when a current is passed through the liquid level sensor element 10 in a state of being partially immersed in the liquid hydrogen 12 with a voltmeter. Since the length of the region where the superconducting wire is in the superconducting state and the region where it is in the normal conduction state differs depending on the liquid level, the liquid level is detected by detecting the voltage of the wire and the electrical resistance. The level can be measured.

(First Embodiment)
In this embodiment, the case of using _{Ba (Fe 1-x Co x} ) 2 As 2 as superconducting wire will be described with reference to FIG. 4, 5, 6, 7.

_{[Ba (Fe 1-x Co} x) Preparation of ₂ As ₂ polycrystals]
_{A Ba (Fe 1-x} Co _x ) ₂ As ₂ polycrystal having a Co content x in the range of 0.05 to 0.11 was prepared. The raw material reagents (Ba, Fe, Co, As) are mixed at a ratio of Ba: Fe: Co: As = 1: (2-2x): 2x: 2 and calcined at 900 ° C. for 48 to 72 hours to obtain Ba (Ba (2-2x): 2x: 2). producing _{Fe 1-x Co x) 2} As 2 polycrystal. Alternatively, the precursors BaAs, Fe ₂ As, and Co ₂ As are prepared in advance _{, mixed at a ratio of BaAs: Fe 2} As: Co ₂ As = 1: (1-x): x, and 48 to 72 at 900 ° C. It may be fired in time. Ba As, Fe ₂ As, and Co ₂ As are mixed at a ratio of Ba: As = 1: 1, Fe: As = 2: 1, and Co: As = 2: 1, respectively, and are mixed at 650 ° C, 800 ° C, and 850 ° C, respectively. Obtained by firing in. In either case, at the time of firing, the quartz tube is vacuum-sealed or the metal tube is sealed in an inert gas (nitrogen, argon, etc.).

[Manufacturing of superconducting wire]
The superconducting wire can be manufactured by various methods and is not limited to a specific manufacturing method. The production method by the powder in-tube method is illustrated below.
_{Ba (Fe 1-x Co x} ) 2 As 2 polycrystal powder metal pipe such as silver (eg, silver-sheathed) packed in, wire drawing by rolling or swaging or the like, a heat treatment is performed. When using silver sheath, since sheath and _{Ba (Fe 1-x Co x} ) has no reaction with ₂ As ₂ during the heat treatment, more preferable. Further, if necessary, a reinforcing coating layer may be further provided on the outside of the silver sheath. Further, a plurality of wire rods may be bundled to form a stranded wire, or insulation may be integrally molded with a resin or the like.
The dimensions of the silver sheath before processing are, for example, an outer diameter of 6 mm and an inner diameter of 4 mm. By wire drawing this step by step, a wire rod having a cross-sectional diameter of 0.5 mm-2 mm and a length of 1000 mm or more can be obtained. Then, the desired superconducting wire is obtained by subjecting it to a heat treatment at 800 ° C. to 900 ° C. for 10 hours to 20 hours.

[Evaluation of superconducting properties of superconducting wires]
_{A superconducting wire consisting of Ba (Fe 1-x} Co _x ) ₂ As ₂ having a Co content x different in the range of 0.05 to 0.11 is cut and shortened to about 4 cm for evaluation, and the four-terminal method is used. The electrical resistance was measured using. FIG. 4 is a diagram showing the measurement results of the electrical resistance in the vicinity of the superconducting transition of the superconducting wire. In the figure, in each case of 0.07, 0.08, 0.09, and 0.10 as typical values of Co content x, corresponding to the temperature (K) in the range of about 19 to 26.5. , It was shown that the resistance takes a value of about zero or 0.28-0.40 mΩ. By adjusting the Co content to 0.06 or more and 0.10 or less, it can be seen that the critical temperature Tc can be adjusted to an arbitrary temperature of 20K-25K.
FIG. 5 is a diagram showing the Co content dependence of the critical temperature in the superconducting wire rod of the present embodiment. The dotted line in the figure is a line indicating the liquid hydrogen temperature (approximately 20K). As can be seen from the figure, x is 0.06 or more and 0.10 or less, and the critical temperature exceeds the liquid hydrogen temperature. Further, when x = 0.06, it is about 21K. However, when x = 0.07 or less, Tc decreases significantly toward the low concentration side, so that more precise composition control is required than when adjusting Tc with a composition of x = 0.07 or more. On the low concentration side, it is difficult to adjust Tc, but there is an advantage that 1- (ρ (30K) / ρ (300K)) in the normal conduction state, which will be described later, becomes smaller.
In cases other than the range of 0.06 or more and 0.10 or less (x <0.06, 0.10 <x), Tc falls below 20K and does not transition to the superconducting state even in liquid hydrogen, so the hydrogen liquid level It cannot be used as a total. Transition width is in the range of 0.5 K-2K, it showed a very sharp superconducting transition than the conventional MgB ₂ wire material.
Moreover, when measurements were made at a plurality of different points of the long wire rod, no significant difference was observed in the critical temperature and the transition width, and a highly homogeneous wire rod could be obtained.

[Evaluation of normal conduction characteristics of superconducting wires]
For example, when a silver sheath is used, it can be considered as follows. In a superconducting wire consisting of a silver sheath and a Ba (Fe _1-x Co _x ) ₂ As ₂ superconductor core, the electrical resistivity _{of Ba (Fe 1-x} Co _x ) ₂ As _{2 in the normal conduction state is silver.} Since it is about 100 times larger than that, it is considered that the characteristics of silver are reflected if the electrical resistance of the superconducting wire is measured in the normal conduction state. The electrical resistance of silver becomes very small at low temperatures. As described above, the superconducting wire using the silver sheath is expected to have a small change in electrical resistance between the normal conduction state and the superconducting state, and the accuracy as a liquid level gauge is not obtained.
In the embodiment of the present invention, the superconducting wire is drawn and then heat-treated as appropriate. By setting the heat treatment temperature to 850 ° C. or higher, the temperature dependence of the resistance in the normal conduction state can be further reduced.
FIG. 6 shows the resistance value in the range of about 10 (K) to 300 (K) when the Co content x = 0.09. When the degree of temperature dependence of the normal conduction resistance is defined as "1- (ρ (30K) / ρ (300K))", the value is 0.5 or less.

[Manufacturing of liquid level gauge for liquid hydrogen using long superconducting wire]
A liquid hydrogen level gauge is manufactured by soldering a current lead wire and a measurement wire to a liquid level sensor element made of a long superconducting wire of about 200 mm to 500 mm and connecting it to a current power source and a voltmeter. did. No heaters are installed.

[Performance verification of liquid level gauge for liquid hydrogen]
The prepared liquid level gauge was placed in liquid hydrogen, and the electric resistance (output voltage) of the liquid level sensor element was compared with the actual liquid level using a scale. FIG. 7 shows the relationship between the visually read liquid level (mm) and the normalized electrical resistance measurement value of the comparison result. In the figure, the case of the composition of x = 0.09 is indicated by a black circle, the case of the composition of x = 0.08 is indicated by a white circle, and the ideal characteristic line is indicated by a dotted line. In the wire rod with x = 0.09 and Tc of 21.6K, the actual liquid level position and the value obtained by converting the electrical resistance into the liquid level position were in good agreement. Further, it was found that when the Tc of x = 0.08 is 23.5 K, the evaluation is overestimated by about 40% (for example, when the actual liquid level position is 100 mm, the liquid level gauge indicates 140 mm). Even when x = 0.08, the electrical resistance changes linearly with respect to the liquid level position, so that it can be used by simple calibration. From this, it can be seen that in the case of the superconducting wire of the present embodiment, the liquid level can be detected with high accuracy without using the heater conventionally required.

Regarding the degree of temperature dependence of the critical temperature, transition width, and normal conduction resistance when the Co content x of the superconducting wire is 0.07 to 0.10 "1- (ρ (30K) / ρ (300K))" The results of the investigation are summarized in Table 1.

It should be noted that the examples shown in the above-described embodiments and the like are described for the purpose of making the invention easier to understand, and are not limited to this embodiment.

The superconducting wire rod of the present invention is suitable for a superconducting liquid level sensor that measures the liquid level of liquid hydrogen, has high-precision measurement performance, and can realize a liquid level gauge having a simple structure. Therefore, for example, it can be widely applied to containers (stationary liquid hydrogen tanks, lorry vehicles for liquid hydrogen transportation, ships, etc.) used during the production, storage, and transportation of liquid hydrogen, and is industrially useful.

1 Superconducting core 2 Sheath tube 3 Reinforcing coating layer 10 Liquid hydrogen liquid level sensor element 11 Liquid level 12 Liquid hydrogen

Claims (5)

A superconducting wire with a superconducting part
The composition of the superconducting portion is _{represented by the chemical formula Ba (Fe 1-x} Co _x ) ₂ As ₂ , the x is in the range of 0.06 or more and 0.10 or less, and the critical temperature at which the electric resistance becomes zero is high. and at 25K or less 20K or more, transition width electrical resistance is the difference between the temperature and the critical temperature at which begins to fall toward the critical temperature of Ri der less 2K,
A superconducting wire rod characterized by being used for a liquid level sensor element for liquid hydrogen. The superconducting wire is represented by 1- (ρ (30K) / ρ (300K)) when the electrical resistivity at a temperature of 300K is ρ (300K) and the electrical resistivity at a temperature of 30K is ρ (30K). The superconducting wire according to claim 1, wherein the value of the temperature dependence of the electrical resistivity in the normal conduction state is 0.5 or less. The superconducting wire according to claim 1 or 2, wherein the superconducting wire has a diameter of 2 mm or less. A liquid level sensor element for liquid hydrogen comprising the superconducting wire according to any one of claims 1 to 3. A liquid hydrogen level gauge for measuring the liquid level of liquid hydrogen, the liquid hydrogen level sensor element for liquid hydrogen provided with the superconducting wire according to any one of claims 1 to 3, a current source, and the like. A liquid hydrogen level gauge composed of a voltmeter and characterized in that the height of the liquid level is obtained based on the measured voltage.

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