JP2005089740A - Magnetic refrigeration material and cold storage material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new magnetic refrigeration material substitutive for an intermetallic compound, especially, the magnetic refrigeration material capable of being used in generating a cold temperature suitable for liquifying hydrogen. <P>SOLUTION: The magnetic refrigeration material contains at least one kind of rare earth element nitride expressed by the formula: MN (M expresses one or more rare earth metal elements). One or more kinds of elements selected from Nd, Gd, Dy, Ho, Tb and Er are preferably used as the rare earth elements. A nitride expressed by the formula: Gd<SB>x</SB>Dy<SB>1-x</SB>N ((x) is not less than 0 and not more than 1) is concretely exemplified as the rare earth element nitride. The nitride is used in a magnetic refrigeration system, especially in the refrigeration system for a liquid hydrogen production plant, as the magnetic refrigeration material producing a magneto-caloric effect. The rare earth element nitride is used as a cold storage material. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to nitrides of rare earth elements, magnetic refrigeration materials (or magnetic refrigeration working materials) containing such nitrides, cold storage materials using large specific heat near the magnetic transition temperature, and refrigeration using such materials. The present invention relates to a system and a hydrogen liquefaction plant having such a system.

  Hydrogen is useful as a new energy in the near future society, and social infrastructure facilities are necessary for its use. In order to efficiently store and transport hydrogen as a fuel, hydrogen liquefaction technology is essential. Therefore, it is desired to significantly improve the energy efficiency required for liquefaction over the current cooling with a gaseous refrigerant using ammonia or chlorofluorocarbon gas. It has been proposed to use a magnetic refrigeration system for such improvement, but in order to make the system effective, it is desired to develop an optimal magnetic refrigeration material that can be used in the system.

Various magnetic intermetallic compounds of rare earth elements and transition metals have been proposed as magnetic refrigeration materials that can be used in such a magnetic refrigeration system. Examples of such metal compounds include DyNi ₂ , HoAl ₂ , ErCo ₂ , DyAl ₂ , (Dy, Ho) Al _{2, and the} like (see Non-Patent Documents 1 to 4 below). As can be understood from the fact that such an intermetallic compound can also be used for a hydrogen storage alloy, the reactivity with hydrogen is high, and it becomes a hydride and changes magnetic properties such as a magnetic transition temperature. When liquefying hydrogen, the magnetic refrigeration material is desirably used directly with hydrogen in order to increase its thermal efficiency. However, in the case of direct contact, due to the above-described reactivity, the magnetic refrigeration material cannot be used as a magnetic refrigeration material that operates stably in a desired temperature range over a long period of time. In addition, some intermetallic compounds such as ErCo2 undergo a primary phase transition, and the crystal structure changes at the magnetic transition temperature. Therefore, the material becomes brittle due to repetitive exposure to a temperature change through the magnetic transition temperature accompanying excitation and demagnetization, and the function as a magnetic refrigeration material is reduced (see Non-Patent Documents 5 to 8 below).

  By the way, a cold storage material type refrigerator having a cold storage function has been proposed as an apparatus for efficiently realizing a cryogenic temperature such as a liquid helium temperature, and is already on the market. This refrigerator is used for cooling superconducting coils for generating a strong magnetic field, such as MRI of medical equipment and magnetic levitation vehicles. Although a large specific heat is required for the regenerator material, the specific heat of the substance is extremely small as compared with He used as a refrigerant at an extremely low temperature of 20K or less. For this reason, materials having a large specific heat even at 20K or less have been investigated, and intermetallic compounds containing rare earth elements have been developed and are now on the market. These intermetallic compounds undergo a magnetic transition at an extremely low temperature and exhibit a large magnetic specific heat near the transition temperature. This large magnetic specific heat is used for heat storage.

In general, the magnetic specific heat tends to decrease significantly in a magnetic field atmosphere as compared to a non-magnetic field atmosphere. Therefore, when a low temperature is emitted and maintained in an atmosphere in which a magnetic field exists, a cold storage material having a large specific heat in the magnetic field atmosphere is required. For example, as mentioned above, a cryogenic refrigerator is often used in the vicinity of a device that generates a strong magnetic field, so that it is exposed to a certain amount of leakage magnetic field. It is. Although specific heat at a very low temperature of a part of rare earth nitrides (for example, ErN, DyN, etc.) has been evaluated (see Non-Patent Document 9 below), there is no teaching or suggestion about the influence of a magnetic field.
T. Hashimoto, K. Matusmoto, T. Kurihara, T. Numazawa, A. Tomokiyo, H. Yayama, T. Goto, S. Toda, M. Sahashi, Adv. Cryog. Eng., 32 (1986) 279. A. Tomokiyo, H. Yayama, H. Wakabayashi, T. Kuzuhara, T. Hashimoto, M. Ssahashi, K. Inomata, Adv. Cryog. Eng., 32 (1986) 295. H. Wada, S. Tomekawa, M. Shiga, Cryogenics, 39 (1999) 915. NH Duc, DT Kim Anh, PE Brommer, Physica B, 319 (2002) 1. RL Cohen, KW West, F. Oliver, KHJ Buschow, Phys. Rev. B21 (1980) 941 F. Pourarian, WE Wallace, SK Malik, J. Magn. Magn. Mater. 25 (1982) 299 NV Mushnikov, T. Goto, VS Gaviko, NK Zajikov, J. Alloys Comp. 292 (1999) 51 F. Pourarian, Physica B 321 (2002) 18 W. Stutius, Phys. Kondens. Materie 10, 152-185 (1969)

  Therefore, the problem to be solved by the present invention is that a new magnetic refrigeration material and cold storage material replacing the intermetallic compound as described above, particularly a magnetic refrigeration material or magnetic field that can be used to generate a low temperature convenient for hydrogen liquefaction. It is to provide a cold storage material that is convenient for a refrigerator for cooling a generator. Another problem to be solved by the present invention is to provide a refrigeration system using such a magnetic refrigeration material or a cold storage material, and a hydrogen liquefaction plant using such a system.

  As a result of intensive studies on the above problems, the inventors manufactured nitrides of rare earth elements, measured their magnetic property data, and processed the data to make such rare earth element nitrides magnetically refrigerated. It has been found that it can be used as a material and as a cold storage material exhibiting high heat storage characteristics even in a magnetic field atmosphere, thereby finding that the above-mentioned problems can be solved.

  Accordingly, in a first aspect, the present invention provides a magnetic refrigeration material or a cold storage material comprising a nitride of at least one rare earth element. Such nitrides can be represented by the composition formula: MN, where M represents one or more rare earth elements. The magnetic refrigeration material is a material used as a working substance in a well-known magnetic refrigeration technique, and shows a magnetocaloric effect by changing the magnetic entropy under the influence of a magnetic field. Generally, when a magnetic field is applied to a magnetic refrigeration material, its magnetic entropy decreases, and as a result, heat is released to the surroundings. Conversely, when a magnetic refrigeration material is demagnetized from the applied state, its magnetic entropy increases. As a result, it absorbs heat from the surroundings.

In the magnetic refrigeration material or the cold storage material of the present invention, the rare earth element in the form of nitride, that is, the rare earth element nitride has a NaCl structure. As apparent from the periodic table, the rare earth element contains 17 kinds of elements, and the rare earth element constituting the nitride contained in the magnetic refrigeration material of the present invention may be any one of them, Moreover, any combination of at least two types may be used. The magnetic refrigeration material of the present invention has the property that the change in magnetization M with respect to temperature T when magnetic field H is constant (∂M / ∂T) _H has an extreme value at a certain temperature. Therefore, a rare earth element single nitride such as Sc (scandium), Y (yttrium), La (lanthanum), cerium (Ce), Yb (ytterbium) and Lu (lutetium) does not have this property. And excluded from the magnetic refrigeration material of the present invention. However, when these rare earth elements are combined with one or more other rare earth elements that can exhibit this property, the nitride as a whole has a temperature at which (∂M / ∂T) _H is an extreme value. To do. Therefore, such a combination of rare earth elements is included in the magnetic refrigeration material of the present invention.

  The rare earth element constituting the nitride of the magnetic refrigeration material or the cold storage material of the present invention may be at least one kind of the rare earth element as described above, and the number thereof is not particularly limited. Two or three types. Particularly preferred rare earth elements are Nd (neodymium), Gd (gadolinium), Dy (dysprosium), Ho (holmium), Tb (terbium) and Er (erbium), and nitrides of any one of these elements (Ie, a single-component nitride) or a nitride of a plurality of elements (ie, a solid solution), for example, a nitride of two elements (ie, a binary nitride). In the case of a nitride of a plurality of rare earth elements, any appropriate combination of the rare earth elements contained in the nitride and the ratio thereof may be used. These can be selected according to, for example, the magnetic properties of the nitride, the low temperature intended to be obtained using the magnetic refrigeration material, the availability of raw materials for producing the nitride, the production cost, and the like.

Therefore, GdN, DyN, HoN, TbN and ErN can be exemplified as the rare earth element nitride contained in the magnetic refrigeration material or the cold storage material of the present invention. As the solid _{solution, Gd X Dy 1-X N} (0 <X <1) can be exemplified. Such a magnetic refrigeration material containing a nitride may be in any suitable form depending on the use mode, for example, a powder form, a bulk form or a bulk form, a porous form, and the like. In one example, it may be a sintered body having a predetermined shape.

  The magnetic refrigeration material or the cold storage material of the present invention may contain, in addition to the rare earth element nitrides described above, components inevitably mixed, for example, components derived from impurities coexisting in the raw materials used in the production of nitrides. Moreover, as long as the target magnetic refrigeration effect or the cool storage effect can be acquired, another component may be included for another purpose. Such other components include, for example, carbon added for the reduction of rare earth oxides in the synthesis of nitrides by the carbothermal reduction method, and inevitable removal of oxygen and the phase of rare earth nitrides. Components added to (e.g., boron, silicon, molybdenum, sodium, calcium, tantalum, zirconium, these compounds, or any combination thereof), sintering aids (e.g., vanadium, bismuth, indium, these compounds, or Any combination of these may be exemplified and may be included in an amount necessary for such another purpose.

  Moreover, this invention provides the magnetic refrigeration system using the magnetic refrigeration material of the above-mentioned 1st summary in the 2nd summary. There are various types of magnetic refrigeration systems that use magnetic refrigeration materials. In such known magnetic refrigeration systems, the magnetic refrigeration materials of the first aspect described above can be used as magnetic refrigeration materials. Moreover, the refrigerating system using the cool storage material of said 1st summary is provided. Various refrigeration systems using a cold storage material are known. In such a known refrigeration system, the cold storage material of the first aspect described above can be used as the cold storage material to be used.

  Furthermore, in the third aspect, the present invention provides a liquid hydrogen production method and a liquid hydrogen production plant using the magnetic refrigeration system of the second aspect described above. Various methods for producing gaseous hydrogen by cooling gaseous hydrogen and plants for carrying out the method are known. In such a method and plant, the magnetic refrigeration system of the second aspect described above can be used to obtain the low temperature required for liquefaction. In addition, the present invention provides a magnetic field generator using the regenerative refrigeration system of the second aspect described above. Various magnetic field generators using a refrigeration system are known. In such a method and system, the refrigeration system of the second aspect described above can be used to obtain the low temperature required to generate the magnetic field. Specifically, the magnetic field generator is an MRI apparatus (nuclear magnetic resonance diagnostic apparatus), a high vacuum pump for a semiconductor manufacturing apparatus, a radio observatory, various X-rays of a semiconductor inspection apparatus, cooling of an infrared sensor, a protein analyzer, a superconductivity Used in a device such as a generator, a superconducting power storage device, or a superconducting filter for communication. Therefore, the present invention is an MRI apparatus using the rare earth nitride of the present invention as a cold storage material, a high vacuum pump for semiconductor manufacturing equipment, radio observatory, various X-rays of semiconductor inspection equipment, cooling of infrared sensors, protein analysis equipment Also provided are a superconducting generator, a superconducting power storage device, a superconducting filter for communication, and the like.

  As far as the inventors know, any single rare earth nitride is known, but at least two rare earth nitrides are not known in many cases. For example, known material systems in which crystal structure parameters have been reported for binary rare earth nitrides include the following: (La, Ce) N, (La, Nd) N, (La, Gd) N, (Ce, Pr) N, (Ce, Nd) N, (Ce, Gd) N, (Ce, Tb) N, (Pr, Gd) N, (Nd, Gd) N, (Eu, Gd) N , (Gd, Yb) N, (Y, Gd) N, (Y, Er) N. There are no reports on binary systems other than these. Also, in order to evaluate the magnetocaloric effect of single and multiple rare earth nitrides, the magnetic properties of any such nitrides have not been measured and the results examined. Therefore, the nitrides of at least two rare earth elements provided by the present invention, other than the above-mentioned material systems, are novel per se.

Therefore, the present invention is, for example, Gd _X Dy _1-X N, Gd _X Tb _1-X N, Gd _X Ho _1-X N, Gd _X Er _1-X N, Gd _X Nd _1-X N, Tb _X Dy. _1-X N, Tb _X Nd _1-X N, Tb _X Ho _1-X N, Tb _X Er _1-X N, Dy _X Ho _1-X N, Dy _X Er _1-x N, Dy _X Nd _{1- X N, Ho X Er 1-} x N, Ho X Nd 1-x N, and _Er X _{Nd 1-x} N (none 0 <X <1) providing a rare earth element nitride itself like. In the present specification, it can be said that X is a composition of rare earth elements.

  As described herein, the inventors have produced nitrides of Gd (gadolinium), Dy (dysprosium), Ho (holmium), Tb (terbium) and Er (erbium), and The magnetic properties of these nitrides were evaluated under various temperatures and magnetic fields. In addition, nitride solid solutions of various combinations of Gd (gadolinium) and Dy (dysprosium) were actually produced, and similarly the magnetic properties of these nitrides were evaluated. From these findings and as exemplified in the present specification, GdN and DyN are completely dissolved, and their lattice constant and magnetic transition temperature change almost linearly with respect to the composition. Therefore, it can be easily analogized that two or more kinds of rare earth nitrides are completely dissolved. It is clear that the magnetic transition temperature also changes depending on the composition of the rare earth nitride. For nitrides that do not undergo magnetic transition, it is reasonable to consider that the magnetic transition temperature is 0K. Based on the fact that the Gd-Dy nitride solid solution can be produced in this way and the crystal structure or magnetic evaluation results thereof, it is clear that the solid solution nitride can be produced similarly for other rare earth element combinations. It is also clear that these solid solution nitrides can be used as magnetic refrigeration materials.

The rare earth element contained in such a novel nitride of at least two rare earth elements can be any of the above-mentioned rare earth elements as long as there is a temperature at which (∂M / ∂T) _H is an extreme value as a whole material. It may be a combination. For example, a combination comprising at least one selected from Nd (neodymium), Gd (gadolinium), Dy (dysprosium), Ho (holmium), Tb (terbium), and Er (erbium) can be exemplified. A nitride containing only these rare earth elements is preferable.

  In addition, the composition of the rare earth element when a plurality of rare earth elements are contained is not particularly limited, and may be any composition depending on the purpose of using the nitride. As will be described later, in the production of nitrides, the raw material compounds containing these rare earth elements (for example, oxides of rare earth elements) are mixed in a predetermined amount according to the intended composition, so that any composition is obtained. Is also possible.

  The rare earth nitride contained in the magnetic refrigeration material of the present invention undergoes a secondary phase transition. As a result, compared to the intermetallic compound described above, it does not become brittle with respect to repeated temperature changes through the magnetic transition temperature. It is also stable under a hydrogen atmosphere. Furthermore, as will be described later, most of the rare earth nitrides in the magnetic refrigeration material of the present invention undergo a phase transition in the range of nitrogen boiling point (77K) to hydrogen boiling point (20K). Therefore, the low temperature required for liquefying hydrogen can be efficiently generated, and even when the magnetic refrigeration material is in direct contact with hydrogen, it can function stably as a magnetic refrigeration material.

BEST MODE FOR CARRYING OUT THE INVENTION

The production of the rare earth nitrides of the present invention may be carried out by any suitable method known for producing such nitrides. For example, it can be produced by a carbothermal reduction method of a rare earth element oxide (for example, Gd ₂ O ₃ , Dy ₂ O _3, etc.) in a nitrogen or ammonia stream. An inert gas such as Ar, He, Ne, and Kr may be mixed as long as the atmosphere during the carbothermal reduction can maintain a sufficient amount of nitrogen to react. Further, hydrogen gas may be mixed for the purpose of removing carbon remaining in the solid phase. In the case of a solid solution nitride, at least two kinds of rare earth element oxides are mixed and similarly subjected to carbothermal reduction. The ratio of the rare earth element contained in the mixed oxide substantially corresponds to the ratio of the rare earth element in the solid solution. That is, the rare earth oxide mixture is used so that the amount of the rare earth element in the rare earth oxide mixture stoichiometrically corresponds to the composition of the rare earth element in the target solid solution. In another method, a hot isostatic press (HIP) that heats a rare earth element metal piece at a high temperature (eg, 1600 ° C.) in a high-purity (eg, 1400 atm) high-purity (eg, purity 99.9999%) nitrogen gas atmosphere. Rare earth nitrides can also be produced by this method. Rare earth element nitrides can also be produced by a Hot Isostatic Press (HIP) method in which an alloy piece composed of two or more kinds of rare earth elements is heated at a high pressure (eg, 1400 atm) at a high temperature (eg, 1600 ° C.). The proportion of rare earth elements contained in the alloy substantially corresponds to the proportion of rare earth elements in the solid solution.

The carbothermal reduction method under a nitrogen atmosphere can be represented by the following formula:
M ₂ O ₃ + 3C + N ₂ → 2MN + 3CO
(In the formula, M is a rare earth element, which may be a single element or a plurality of elements.)

For example, in the case of gadolinium,
Gd ₂ O ₃ + 3C + N ₂ → 2GdN + 3CO
It becomes.

In the case of binary nitrides of gadolinium and dysprosium,
XGd ₂ O ₃ + (1-X) Dy ₂ O ₃ + 3C + N ₂ → 2Gd _X Dy _1-X N + 3CO
(Where 0 <X <1). The reaction proceeds similarly for other nitrides.

Of the rare earth elements, terbium (Tb) exists more stably as Tb ₄ O ₇ at normal temperature and pressure. Therefore, the carbothermal reduction method using Tb ₄ O ₇ as a raw material can be represented by the following formula:
Tb ₄ O ₇ + 7C + 2N ₂ → 4TbN + 7CO
Thus, for binary systems containing Tb, the carbothermal reduction method can be represented by the following formula:
_{2XLn 2 O 3 + (1-} X) Tb 4 O 7 + (7-X) C + 2N 2
→ 4Ln _X Tb _1-X N + (7-X) CO
(However, Ln represents a rare earth element other than Tb, and 0 <X <1.)

Thus, for example, for the binary system of gadolinium and terbium (Gd _X Tb _1-X N),
_{2XGd 2 O 3 + (1-} X) Tb 4 O 7 + (7-X) C + 2N 2
→ 4Gd _X Tb _1-X N + (7-X) CO
For example, for the binary system of holmium and terbium (Ho _X Tb _1-X N),
_{2XHo 2 O 3 + (1-} X) Tb 4 O 7 + (7-X) C + 2N 2
→ 4Ho _X Tb _1-X N + (7-X) CO
It becomes.

  The carbothermic reduction method under a nitrogen stream is a well-known technique, and carbon or a carbon supply material such as phenol resin or citric acid and rare earth element oxide are mixed in a powder state to form a pellet. Can be carried out by heating to a high temperature (for example, 1500 ° C.) under a nitrogen stream. The amount of carbon to be reacted is preferably used in excess of the stoichiometric amount in the above reaction formula, for example, twice as much.

  Regarding the evaluation of the magnetocaloric effect of the rare earth element nitride contained in the magnetic refrigeration material of the present invention, magnetization characteristic data can be obtained for the obtained nitride, and the obtained data can be processed as follows.

  The magnetocaloric effect is evaluated by calculating the magnetic entropy change ΔS caused by demagnetizing the magnetic field from H to zero in an isothermal state. This ΔS is expressed by the following equation (1) using the magnetic field H and the temperature T:

（尚、式中、Ｍは磁化を表す）
を代入すると、

となる。 In this formula (1), Maxwell's relational expression:

(In the formula, M represents magnetization)
Substituting

It becomes.

  Accordingly, for the magnetic refrigeration material of the present invention, a data set of magnetization M at the magnetization M, temperature T and magnetic field H (ie, M (T, H)) is experimentally obtained, and numerical calculation is performed using the obtained data. By doing so, the entropy change ΔS (T, H) can be obtained.

(Production of rare earth element nitrides)
Gd ₂ O ₃ and Dy ₂ O ₃ powders (purchased by: Rare Metallic, purity: 99.99% each) in a given quantitative ratio, and amorphous carbon were pulverized in a mortar and mixed thoroughly by a planetary ball mill did. Carbon was added in an amount twice the stoichiometric amount in the above reaction formula to make the total amount of oxide 3 g.

  The mixed powder is formed into cylindrical pellets (diameter 7 mm, mass about 80 mg), placed in an alumina reaction tube, and heated in an electric oven at 1500 ° C. for 15 hours in a nitrogen stream. And cooled to room temperature. In this way, nitrides sintered with Gd alone, Dy alone, and their solid solutions (X = 0.1, 0.3, 0.5, 0.7, and 0.9) were obtained.

  In order to analyze the crystal structure of the obtained nitride, the nitride was powdered in a mortar to obtain an X-ray diffraction (XRD) pattern. In this case, RINT Ultima + from Rigaku Corporation was used and Cu—Kα line was used.

The measurement conditions were divergent slit: 1.00 °, scan speed: 3.00 ° / min, sampling width: 0.02 °, and diffraction angle 2θ: 20-120 °.
In addition to Gd and Dy, Er, Ho, and Tb nitrides were similarly produced using these oxides as raw materials. The measurement result of the X-ray diffraction pattern for GdN, TbN, DyN, HoN and ErN is shown in FIG. 1 (where the vertical axis is the diffraction intensity (Intensity, arbitrary unit)).

  Further, nitrides of the following binary systems: Gd-Tb and Ho-Tb were produced in the same manner while changing the composition (X), and the X-ray diffraction patterns were measured in the same manner. The measurement results of Gd-DyN, Gd-TbN and Ho-TbN are similarly shown in FIGS. 9 to 11 (where X means the ratio of Gd or Ho, ie, the composition).

In addition, since no special treatment was performed to remove excess carbon, it is considered that some carbon remained in the nitride. However, as will be described later, the magnetic data and crystal structure parameters of GdN and DyN measured by the present inventors are already reported (for example, DX Li, Y. Haga, H. Shida, T. Suzuki). , TS Kwon, G. Kido, J. Phys. Condens. Mater. 9 (1997) 10777; G. Busch, J. Appl. Phys. 38 (1967) 1386; O. Vogt, K. Matternberger, J. Alloys Comp 223 (1995) 226; and RJ Gambino, TR McGuire, in: Processing of the 7th Rare Earth Research Conference, Coronado, CA, vol. 1, 1968, p.233). It is considered that nitride is not substantially formed. Further, it has been reported that the Curie temperature of GdN _0.88 C _0.12 is about 190 K (RJ Gambino, TR McGuire, in: Processing of the 7th Rare Earth Research Conference, Coronado, CA, vol. 1, 1968). This is much higher than the Curie temperature of GdN, which is a pure nitride. Therefore, it is considered that the carbon remaining in the solid phase does not exist magnetically. However, although the mass of the obtained nitride is slightly affected, the amount is about several percent by mass.

FIG. 1 shows that only a NaCl-type diffraction pattern is observed in any nitride. As the atomic number of the rare earth element increases, the diffraction peak position shifts to the higher angle side. This corresponds to the fact that the lattice constant is reduced, and is consistent with the reports in the above literature. . Further, FIGS. 9 to 11 clearly show that the obtained binary nitride (eg, Gd _0.5 Dy _0.5 N) is a mononitride having a single phase structure having a NaCl structure. Show. Therefore, it was found that the rare-earth element nitrides have the same structure because the same diffraction pattern is obtained regardless of whether the nitride is a single element or a binary element.

  2 and 12 show lattice parameters (Lattice Parameters) calculated from the X-ray diffraction pattern. The lattice constant was calculated using the Cohen method by extracting a diffraction peak called a main peak having a high intensity. For binary nitrides, the lattice constant increases linearly as the ratio X of one element (for example, Gd) increases for any rare earth element from FIG. 2, that is, according to Vegard's law. I understand that. This means that in the rare earth element nitride of the present invention, binary nitrides of rare earth elements such as GdN—DyN, Gd—TbN and Ho—TbN form a continuous region of a solid solution. Note that the lattice constants of GdN, TbN, HoN, ErN and DyN (O. Vogt, K. Matternberger, J. Alloys Comp. 223 (1995) 226; and A. Vendle, J. Nucl. Mater. 79 (1979) 246) is also shown in FIG. 12, but the values obtained by the inventors are almost identical to these.

  The linearity as shown in FIG. 2 indicates that the interaction between Gd-N and Dy-N and the interaction between Gd-Gd, Dy-Dy and Dy-Gd are very similar. This means that both rare earth elements are very similar, in particular very similar with respect to the ionic radius. Further, the difference in lattice constant between GdN and DyN is only 1.5%, which indicates that local internal strain caused by rare earth element substitution is a serious problem in the binary nitride of the present invention. Indicates not. In addition, the difference in lattice constants is similar for other binary systems (difference in lattice constant between GdN and TbN is 1.0%, difference in lattice constant between TbN and HoN is 1.3%), and the like. This is true.

When the theoretical density of the rare earth element nitride of the present invention was calculated from the lattice constant, GdN was 9.18 × 10 ³ kg / m ³ , DyN was 9.96 × 10 ³ kg / m ³ , TbN Was 9.58 × 10 ³ kg / m ³ , HoN was 10.3 × 10 ³ kg / m ³ , and ErN was 10.6 × 10 ³ kg / m ³ . The rare earth atom density of each nitride is as follows: GdN is 3.23 × 10 ²⁸ RE (rare earth element) m ⁻³ , DyN is 3.40 × 10 ²⁸ REm ⁻³ , and TbN is 3.34 × 10 ²⁸ REm ^−3. , HoN is 3.47 × 10 ²⁸ REm ⁻³ , and ErN is 3.52 × 10 ²⁸ REm ⁻³ . These rare earth atom densities are significantly greater than the rare earth atom densities of the respective rare earth metals. The rare earth atom density of Gd metal is 3.02 × 10 ²⁸ REm ⁻³ (theoretical density 7.90 × 10 ³ kg / m ³ ), and that of Dy metal is 3.17 × 10 ²⁸ REm ⁻³ (theoretical density 8.55). × 10 ³ kg / m ³ ), Tb metal 3.12 × 10 ²⁸ REm ⁻³ (theoretical density 8.23 × 10 ³ kg / m ³ ), Ho metal 3.21 × 10 ²⁸ REm ⁻³ (theoretical) Density is 8.80 × 10 ³ kg / m ³ ), and Er metal is 3.26 × 10 ²⁸ REm ⁻³ (theoretical density: 9.07 × 10 ³ kg / m ³ ).

(Measurement of magnetization of rare earth nitride)
Magnetization measurement of the rare earth element nitride obtained in Example 1 was performed. In order to prevent oxidation of the resulting nitride pellets, about 9 mg of small pieces obtained by crushing and weighing the pellets in a glove box (connected to the reaction system) in an Ar atmosphere were sealed with paraffin in a quartz tube. The magnetism was measured using a SQUID magnetometer (superconducting quantum interference device magnetometer, MPMS (Magnetic Property Measurement System), manufactured by Quantum Design) without being exposed to the atmosphere. In the hysteresis measurement, applied magnetic field of -5 to 5 T (tesla), measurement temperature of 5K, and evaluation of magnetocaloric effect were applied with magnetization measured at applied magnetic field of 0 to 5 T and measurement temperature of 5 to 200 K. The SQUID magnetometer is a dc type that includes two junctions in a superconducting ring and is driven by a DC bias current.

FIG. 13 shows the relationship between magnetization (M) and magnetic field (H) measured at 5K for a simple rare earth nitride. For the sake of clarity, the measurement results for each nitride are shown so as not to overlap, and in FIG. 13, the broken line crossing the center of each nitride plot shows zero magnetization, ordinate length between the dashed and the adjacent dashed line corresponds to a 5 [mu] _B. Although almost no hysteresis is observed in GdN, hysteresis is observed as the atomic number increases to Tb and Dy. It can be seen that as the atomic number increases further, the hysteresis decreases. In addition, although any nitride is not perfect at 5T, there is a tendency that the magnetization is almost saturated.

FIG. 3 shows, as an example, the relationship between the magnetization (M) and the magnetic field (H) measured at 5K for rare earth nitrides having various compositions X of the binary system Gd-DyN. In FIG. 3, for the sake of clarity, the measurement results for each composition are shown so as not to overlap, and in FIG. 3, the broken line crossing the center of each composition plot shows zero magnetization. It indicates the length of the longitudinal axis between the dashed and the adjacent dashed line corresponds to a 5 [mu] _B. As can be seen from FIG. 3, as the ratio of Gd increases, the width of the hysteresis decreases, and the hysteresis is almost extinguished in GdN. When the magnetic field is increased, the magnetization is almost saturated in any sample. As inset of FIG. 3 shows the value of magnetic moment per rare earth element, the value is about 7～8Myu _B, which is almost all of the moment is perfectly aligned along the external magnetic field Means that.

  Next, Curie Temperature Tc was determined from an Arlot plot for simple rare earth nitrides and binary rare earth element nitrides (Gd-DyN, Gd-TbN and Ho-TbN), and the results are shown in FIG. And shown in FIG. In FIG. 4, this Curie temperature is shown by ● TC from Arrott plot, and the horizontal axis is an atomic number. In FIG. 14, the Curie temperature from the allot plot is indicated by a black symbol, and the horizontal axis is the composition X. 4 and 14, the peak temperature of ΔS of magnetic entropy change is also plotted (■ in FIG. 4, white symbols in FIG. 14), and the literature values are also shown in FIG. 4 and 14 show that the Curie temperature and the peak temperature are in good agreement. It is also in good agreement with the literature value for Curie temperature. This means that the present inventor is suitable for the production of rare earth element nitrides by the carbothermal reduction method as described above, and that the magnetization measurement is also suitable.

The literature values are those described in the following literature:
O. Vogt, K. Matternberger, J. Alloys Comp. 223 (1995) 226
G. Busch, P. Junod, O. Vogt, Phys. Lett., 6 (1963) 79.
DX Li, Y. Haga, H. Shida, T. Suzuki, TS Kwon, G. Kido, J. Phys. Condens. Mater. 9 (1997) 10777
HR Child M. K Wilkinson, JW Cable, WC Koehler, EO Wollan, Phs. Rev. 131 (1963) 922.

  Further, as is apparent from FIG. 14, for the binary rare earth element nitride, the Curie temperature Tc is the composition X (total number of rare earth elements) between single element nitrides (for example, between GdN and DyN). The ratio of the number of moles of Gd to the total). This means that by selecting an appropriate composition, it is possible to design a rare earth element nitride that maximizes ΔS at an appropriate temperature, and design a magnetic refrigeration system that uses such a nitride as a magnetic refrigeration material. This means that the degree of freedom is high.

  The Curie temperature 61K of GdN and the Curie temperature 21K of DyN determined by the inventors are the Curie temperatures 58-75K and 18K already reported (DX Li, Y. Haga, H. Shida, T. Suzuki). , TS Kwon, G. Kido, J. Phys. Condens. Mater. 9 (1997) 10777; G. Busch, J. Appl. Phys. 38 (1967) 1386; RJ Gambino, TR McGuire, in: Processing of the 7th Rare Earth Research Conference, Coronado, CA, vol. 1, 1968, p.233; TR McGuire, RJ Gambino, SJ Pickart, J. Appl. Phys. 41 (1970) 933; and P. Junod, F. Vevy, Phys Lett. 23 (1966) 624) is in a reasonable range. Such coincidence also means that the production of rare earth element nitrides, measurement of data relating to nitrides, and data processing performed by the inventors are reliable.

  FIG. 5A shows the relationship between magnetization (M), Magnetization vs. temperature (T), and Temperature when a magnetic field of up to 5T is applied to a DyN sample (however, Dy atoms are normalized). Here, the sample is kept constant at a certain measurement temperature, the magnetic field is intermittently raised from 0T to 5T, and the magnetization at that time is measured. Also, once the applied magnetic field was set to 0 T, the temperature of the sample was raised to the next measurement temperature. The measurement temperature range is 5 to 100K. Incidentally, at a temperature of 19 K or higher, neither residual magnetism nor coercive force was observed.

  Numerical calculation was performed from the data of FIG. 5A using the above equation (3) to calculate a magnetic entropy change (ΔS) and a magnetic entropy change. The result is shown in FIG. Regardless of the magnitude of the magnetic field applied before degaussing, the magnetic entropy change (ΔS) vs. temperature (T) curve shows a peak around 21-23K. That is, the magnetic entropy change is maximized in such a temperature range. Such a temperature is called a peak temperature.

  Similar measurements were performed on nitrides of other simple rare earth metal elements, and the change in magnetic entropy was calculated. The results are shown in FIGS. 15 to 18 as in FIG. The peak temperatures are 53 to 59K for GdN, 33 to 36K for TbN, 13 to 16K for HoN, and 6 to 7K for ErN.

  Both results show that the peak temperature hardly changes with the magnetic field. Moreover, it is shown that the half-value width of the peak is wider as the magnetic field is stronger. This indicates that when a higher magnetic field is demagnetized as a magnetic refrigeration material, a high magnetocaloric effect can be obtained in a wider temperature range.

  From FIG. 5, DyN is 10 to 33K when demagnetizing a magnetic field less than 2T, preferably 15 to 30K, more preferably 18 to 27K, and 10 to 10 when demagnetizing a magnetic field of 2T or more and less than 3T. 39K, preferably 12-33K, more preferably 18-28K. When demagnetizing from a magnetic field of 3T or more and less than 4T, the temperature range is 10-56K, preferably 10-42K, more preferably 14-38K. When demagnetizing a magnetic field of 4T or more and less than 5T, it is 10 to 64K, preferably 10 to 52K, more preferably 13 to 42K. When demagnetizing a magnetic field of 5T or more, 10 to 80K, preferably 10 It can be seen that it operates efficiently as a magnetic refrigeration material in a temperature range of ~ 64K, more preferably 13-52K.

  From FIG. 15, GdN has a temperature range of 32 to 71K, preferably 38 to 68K, more preferably 42 to 64K when demagnetizing a magnetic field of less than 2T, and 29 to 29 when demagnetizing a magnetic field of 2T or more and less than 3T. 84K, preferably 32 to 70K, more preferably 42 to 64K. When demagnetizing from a magnetic field of 3T or more and less than 4T, the temperature range is 29 to 84K, preferably 32 to 70K, more preferably 42 to 64K. Even when a magnetic field of 4T or more and less than 5T is demagnetized, even when a magnetic field of 5T or more is demagnetized in a temperature range of 29 to 84K, preferably 32 to 70K, more preferably 42 to 64K, it is 29 to 84K, preferably 32. It can be seen that it operates efficiently as a magnetic refrigeration material in a temperature range of ~ 70K, more preferably 42-64K.

  From FIG. 16, TbN is 20 to 43K when demagnetizing a magnetic field of less than 2T, preferably 22 to 41K, more preferably 28 to 35K, and 18 to less than 2T to 3T. When degaussing from a magnetic field of 3T or more and less than 4T in a temperature range of 52K, preferably 20-49K, more preferably 28-40K, a temperature range of 18-56K, preferably 20-52K, more preferably 24-45K. When demagnetizing a magnetic field of 4T or more and less than 5T, it is 18 to 67K, preferably 20 to 62K, more preferably 24 to 51K. When demagnetizing a magnetic field of 5T or more, 18 to 75K, preferably 20 It can be seen that it operates efficiently as a magnetic refrigeration material in a temperature range of ~ 68K, more preferably 22-61K.

  From FIG. 17, HoN has a temperature range of 6 to 28K, preferably 6 to 22K, more preferably 10 to 18K when demagnetizing a magnetic field of less than 2T, and 6 to 6 when demagnetizing a magnetic field of 2T or more and less than 3T. When degaussing from a magnetic field of 3T or more and less than 4T in a temperature range of 37K, preferably 6-24K, more preferably 10-21K, a temperature range of 6-52K, preferably 6-30K, more preferably 8-27K. In the case of demagnetizing a magnetic field of 4T or more and less than 5T, it is 6 to 57K, preferably 6 to 42K, more preferably 6 to 35K. In the case of demagnetizing a magnetic field of 5T or more, 6 to 65K, preferably 6 It can be seen that it operates efficiently as a magnetic refrigeration material in a temperature range of ~ 51K, more preferably 6-42K.

  From FIG. 18, ErN is 2 to 15 K when demagnetizing a magnetic field of less than 2T, preferably 2 to 12 K, more preferably 2 to 10 K when demagnetizing a magnetic field of 2 T or more and less than 3 T in a temperature range of 2 to 10 K. When demagnetizing from a magnetic field of 3T or more and less than 4T in a temperature range of 28K, preferably 2 to 20K, more preferably 2 to 15K, a temperature range of 2 to 34K, preferably 2 to 25K, more preferably 2 to 18K. When demagnetizing a magnetic field of 4T or more and less than 5T, the temperature range is 2 to 42K, preferably 2 to 32K, more preferably 2 to 20K, and when demagnetizing a magnetic field of 5T or more, 2 to 48K, preferably 2 It can be seen that it operates efficiently as a magnetic refrigeration material in the temperature range of ~ 35K, more preferably 2-22K.

Similar processing was performed, and the same graph as FIG. 5 was obtained for various rare earth element nitrides Gd-DyN of X. Among them, the result in the case of X = 0.5 (Gd _0.5 Dy _0.5 N) is shown as an example in FIG. 6 (however, the rare earth element is normalized). In this case, although very similar to the case of FIG. 5, the magnetic entropy change (ΔS) vs. temperature (T) curve has a peak in the vicinity of about 40K.

From FIG. 6, Gd _0.5 Dy _0.5 N is a magnetic field of 2T or more and less than 3T in a temperature range of 22 to 58K, preferably 26 to 52K, more preferably 27 to 45K when demagnetizing a magnetic field of less than 2T. When degaussing from 20 to 60K, preferably 25 to 56K, more preferably 29 to 51K, and when demagnetizing from a magnetic field of 3T or more and less than 4T, 16 to 68K, preferably 20 to 41K. Preferably when demagnetizing a magnetic field of 4T or more and less than 5T in a temperature range of 27 to 55K, demagnetizing a magnetic field of 5T or more in a temperature range of 10 to 71K, preferably 18 to 64K, more preferably 22 to 58K Thus, it can be seen that the magnetic refrigeration material operates efficiently in a temperature range of 8 to 74K, preferably 17 to 64K, and more preferably 20 to 60K.

  Similar measurement and calculation results for nitrides of other binary systems (Gd-TbN and Ho-TbN, X = 0.5) are shown in FIGS. In FIG. 19, the magnetic entropy change (ΔS) vs. temperature (T) curve has a peak around 44 to 46K, and in FIG. 20, the magnetic entropy change (ΔS) vs. temperature (T) curve is around 23 to 26K. Have a peak.

From FIG. 19, Gd _0.5 Tb _0.5 N is a magnetic field of 2T or more and less than 3T in a temperature range of 22 to 62K, preferably 28 to 58K, more preferably 35 to 54K when demagnetizing a magnetic field of less than 2T. Is demagnetized, it is 18 to 72K, preferably 23 to 65K, more preferably 30 to 60K. Demagnetizing from a magnetic field of 3T or more and less than 4T is 16 to 77K, preferably 20 to 70K. When demagnetizing a magnetic field of 4T or more and less than 5T, preferably in a temperature range of 27 to 65K, when demagnetizing a magnetic field of 5T or more in a temperature range of 12 to 82K, preferably 19 to 73K, more preferably 24 to 68K. It can be seen that the magnetic refrigeration material operates efficiently in the temperature range of 10 to 84K, preferably 18 to 80K, more preferably 25 to 70K.

From FIG. 20, Ho _0.5 Tb _0.5 N is a magnetic field of 2T or more and less than 3T in a temperature range of 7 to 38K, preferably 7 to 33K, more preferably 12 to 28K when demagnetizing a magnetic field of less than 2T. When degaussing from 7 to 48K, preferably 7 to 42K, more preferably 10 to 33K, and when demagnetizing from a magnetic field of 3T or more and less than 4T, 7 to 52K, preferably 7 to 46K. When demagnetizing a magnetic field of 4T or more and less than 5T, preferably in a temperature range of 7-40K, and demagnetizing a magnetic field of 5T or more in a temperature range of 7-60K, preferably 7-52K, more preferably 7-43K It can be seen that the magnetic refrigeration material operates efficiently in a temperature range of 7 to 66K, preferably 7 to 56K, more preferably 7 to 48K.

For all samples with different composition X, 2-way system rare earth nitrides when demagnetized from _{5T (Gd X Dy 1-X} N, Gd X Tb 1-X N, Ho X Tb 1-X N) magnetic The entropy change (ΔS) is calculated and shown as a function of temperature in FIGS. 7, 21 and 22. It can be seen that the ΔS curve with respect to temperature changes smoothly as the composition X changes. This means that the peak temperature of ΔS can be controlled by changing the composition X for any nitride. Moreover, it can be seen that ΔS near the peak is large in any composition and is suitable as a magnetic refrigeration material.

From this result, Gd _X Dy _1-X N (0 ≦ X ≦ 1) has a phase transition temperature in the temperature range of about 20 K to about 80 K, and the entropy change ΔS near the phase transition temperature is about 100. It can be seen that it can be used with a magnetic refrigeration material to obtain a low temperature of about 20K to about 80K, which is quite large, up to about 150 kJK-1m ^{- 3} . Gd _X Tb _1-X N (0 ≦ X ≦ 1) has a phase transition temperature in the temperature range of about 25 K to about 80 K, and the entropy change ΔS near the phase transition temperature is about 100 to about It can be seen that it can be used with a magnetic refrigeration material to obtain a low temperature of about 25K to about 80K, which is considerably large as 200 kJK-1m ^{- 3} . Further, Ho _X Tb _1-X N (0 ≦ X ≦ 1) has a phase transition temperature in the temperature range of about 10K to about 50K, and the entropy change ΔS near the phase transition temperature is about 150 to about It can be seen that it can be used with a magnetic refrigeration material to obtain a low temperature of about 10K to about 50K, as large as 300 kJK-1m ^{- 3} .

FIG. 7 shows that in Gd _X Dy _1-X N, the composition of X ≦ 0.1 when the temperature T is 29K or less, and 0.1 ≦ X ≦ 0.3 as the preferable composition when 29K ≦ T ≦ 42K, In 37K ≦ T ≦ 49K, 0.3 ≦ X ≦ 0.5 is preferable as a composition, 0.5 ≦ X ≦ 0.7 is preferable in 43K ≦ T ≦ 48K, and 0.5 ≦ X is satisfied in 48K ≦ T. It shows that it is suitable as a magnetic refrigeration material.

Furthermore, from FIG. 21, in Gd _X Tb _1-X N, when the temperature T is 43K or less, the composition of X ≦ 0.25 is preferred, and when 43K ≦ X ≦ 50K, 0.25 ≦ X ≦ 0.5, 50K ≦ It can be seen that 0.5 ≦ X ≦ 0.75 is preferable as a composition at T ≦ 62K, and 0.75 ≦ X is excellent as a magnetic refrigeration material at 62K ≦ T.

In addition, in FIG. 22, in Ho _X Tb _1-X N, if the temperature T is 26K or less, the composition of X ≦ 0.25 is particularly 0.25 ≦ X ≦ 0. It can be seen that a composition of 0.75 ≦ X is suitable as a magnetic refrigeration material at a temperature of 75 or higher.

  The peak temperatures in FIGS. 7, 21, and 22 are also indicated by white symbols (記号, Δ, or □) in the graph of FIG. As is clear from FIG. 14, the peak temperature and the Curie temperature are in good agreement. This is because the peak temperature can be changed by changing the composition X. Therefore, in the case of nitrides of a plurality of rare earth elements, various temperature characteristics (especially the largest ΔS) can be changed by changing the composition of the rare earth elements. This means that a rare-earth element nitride having a temperature indicating (that is, a peak temperature) can be obtained.

  This means that only by changing the composition X, magnetic refrigeration materials having different peak temperatures can be obtained, that is, the peak temperature can be controlled by the composition X. In the vicinity of the peak temperature, the absolute value of the magnetic entropy change ΔS is large. Therefore, the amount of heat absorbed and released by the magnetic refrigeration material is large due to the magnetic entropy change. Is the most efficient. Therefore, a rare earth element nitride having a desired peak temperature can be obtained by appropriately selecting the rare earth element composition X in the nitride, and as a result, using the nitride containing the rare earth element in various different compositions. Various low temperatures can be obtained efficiently.

When the magnetic field is changed from 5T to 0T, the largest value of the magnetic entropy change ΔS of the rare earth element nitride of the present invention, that is, ΔS at the peak temperature is 3.0 JK ^{− in} the case of DyN, as can be seen from FIG. ¹ mol ⁻¹ . Table 1 below shows this value along with ΔS for known materials having a transition temperature in the range of 20-70K. The values of known substances are reported in the ΔS vs. T curve (T. Hashimoto, K. Matsumoto, T. Kurihara, T. Numazawa, A. Tomokiyo, H. Hayama, T. Goto, S. Todo, M. Sahashi, Adv. Cryog. Eng. 32 (1986) 279; A. Tomokiyo, H. Yayama, H. Wakabayashi, T. Kuzuhara, T. Hashimoto, M. Sahashi, I. Inomata, Adv. Cryog. Eng. 32 (1986) 295; and H. Wada, S. Tomekawa, M. Shiga, Cryogenics 39 (1999) 915). Although ΔS is shown in three types of units, this conversion is based on the lattice constant of each substance. Table 1 also shows GdN, TbN, HoN, ErN, Gd-DyN systems, Gd-TbN systems, and Ho-TbN systems.

As can be seen from Table 1, the rare earth nitride of the present invention is not larger than the other materials in the case of the unit JK ⁻¹ mol ⁻¹ , but in the case of the unit JK ⁻¹ m ⁻³ , some other A substance is equivalent or better. This is due to the fact that the rare earth element nitride of the present invention has a large density, as described above. Therefore, considering the fact that it is a stable nitride, the rare earth element nitride of the present invention is more useful than known magnetic refrigeration materials, particularly when hydrogen is liquefied, the problem of hydrogen embrittlement. Therefore, it can be used as a very useful magnetic refrigeration material.

  FIG. 8 summarizes the relationship between magnetic entropy change (ΔS) and temperature (T) of a simple rare earth element nitride. FIG. 8 shows the result when the magnetic field is changed from 5T to 0T.

  As is apparent from FIG. 8, the magnetic entropy change at the peak temperature of the rare earth element nitride is large, and in particular, the magnetic entropy change at the peak temperatures of HoN, TbN, and ErN is larger than the magnetic entropy change of GdN or DyN. Quite big. In addition, the magnetic entropy change of the binary rare earth nitride is also quite large, particularly for Gd-TbN and Ho-TbN.

  Also, considering the similarities between Gd and Dy described above and the entropy change ΔS of other rare earth nitrides being much larger than GdN and DyN, these other rare earth elements also have their solid solutions It can also be seen from this that it has an entropy change that can be used as a magnetic refrigeration material at temperatures between the peak temperatures of the single elemental nitrides that comprise it.

Therefore, the similarity of Gd and Dy described above, GdN, DyN and Gd _X Dy _1-X N magnetic measurement data, magnetic entropy change and peak temperature obtained from the data, magnetic entropy of other rare earth nitrides Considering the change and peak temperature, it can be seen that any one or more rare earth nitrides can be used as a useful magnetic refrigeration material, indicating that GdN, TbN and Gd _X Tb _1-X N This is supported by the magnetic measurement data, the magnetic measurement data of HoN, TbN, and Ho _X Tb _1-X N, and the magnetic entropy change and peak temperature evaluated using these data.

(Measurement of specific heat)
(1) Manufacture of TbN, DyN, HoN, and ErN samples Tb, Dy, Ho, and Er metal pieces (purity: 99.9% each; dimensions: about 1 mm × 1 mm × 0.1 mm) high purity nitrogen of 140 MPa Sample pieces of TbN, DyN, HoN, and ErN were manufactured by a Hot Isostatic press (HIP) method in which heating was performed at 1600 ° C. in a gas atmosphere (purity 99.9999%) (heating time: 2 hours). In the HIP method, a heating device O2-Dr. HIP was used.

  The measurement result of the X-ray diffraction pattern of each nitride sample obtained is shown in FIG. 23 as in FIG. As can be seen from comparison with FIG. 1, the diffraction pattern of each obtained nitride sample is equivalent to the diffraction pattern of the nitride obtained by the carbothermal reduction method. This means that a single-phase nitride could be synthesized by the HIP method, and that it was confirmed that the rare-earth nitride could be synthesized without being substantially affected by excess carbon even by the carbothermal reduction method. I can say that.

(2) Specific heat measurement of nitride sample The specific heat of each sample obtained by the HIP method was measured in a magnetic field atmosphere and a 5T magnetic field atmosphere using Heat Capacity System of Oxford Instruments. The measurement temperature ranges were 2 to 70K for TbN, 2 to 43K for DyN, 2 to 40K for HoN, and 2 to 25K for ErN. First, the specific heat was measured at each temperature up to 2K, starting from the high temperature side in a non-magnetic field atmosphere. After raising the temperature again, the specific heat was measured at each temperature up to 2K by applying a magnetic field of 5T. The result of each specific heat measurement is shown in FIGS. In these drawings, the vertical axis represents specific heat C (specific heat), and the horizontal axis represents temperature.

These measurement results are shown together in FIG. 28 together with specific heats of helium and Pb and Er ₃ Ni currently used as cold storage materials. As is clear from FIG. 28, it can be seen that these rare earth metal nitrides have a very large specific heat and have a cold storage capacity equal to or higher than that of the materials used as the cold storage materials currently used. Therefore, such a nitride can be used as a cold storage material having a very large cold storage capacity. The cold storage material is a material that has a role of temporarily storing heat by sufficiently exchanging heat with a working fluid (for example, helium gas) inside a low temperature generator (for example, a refrigerator). It is a heat sink for holding, and is required to have a large volume specific heat and high thermal conductivity. Particularly in the cryogenic region of 20K or less, the performance of the cold storage material has a significant effect on the refrigeration efficiency of the refrigerator. A material having this function is also called a cold storage material, and these matters are well known.

（式中、Ｓ_Ｈは磁場Ｈでの絶対エントロピーを、Ｃ_Ｈは温度Ｔを変数とした磁場Ｈでの比熱を表す。）
に基づいて絶対エントロピーを算出できる。そこで、図２４〜図２７に示す比熱（Ｃ_Ｈ）ｖｓ温度（Ｔ）データを用いて積分すると、H＝０TおよびH＝５Tの場合の絶対エントロピーを算出できる。そのように算出した絶対エントロピーを図２９〜図３２黒塗り記号で示す(図面中、縦軸は絶対エントロピー（Entropy S）であり、横軸は絶対温度（Temperature）である)。尚、測定した最低温度２Ｋ以下の比熱は、デバイの比熱則に従うとして、Ｔ^３に比例すると仮定して算出している。 (3) Calculation of absolute entropy When the value of specific heat with respect to temperature is known,

(Wherein, the absolute entropy in S _H are the magnetic field H, C _H represents the specific heat at a magnetic field H in which the temperature T as a variable.)
Based on the absolute entropy can be calculated. Therefore, by integrating using the specific heat (C _H ) vs. temperature (T) data shown in FIGS. 24 to 27, the absolute entropy when H = 0T and H = 5T can be calculated. The absolute entropy calculated in this manner is indicated by black symbols in FIGS. 29 to 32 (in the drawing, the vertical axis is absolute entropy (Entropy S) and the horizontal axis is absolute temperature (Temperature)). Incidentally, the lowest temperature 2K following specific heat measured as follow Debye specific heat law, is calculated by assuming that proportional to T ^3.

Incidentally, the magnetic entropy change ΔS from the difference between the absolute entropy S _H in absolute entropy S ₀ phrase field H in the field-free can be calculated by the following equation:

On the other hand, as described above with reference to the equations (1) to (3), ΔS can also be calculated from the magnetization measurement. Thus, knowing the S _{0 (T)} by specific heat measurements, using ΔS determined from the magnetization measurements, it is possible to determine the absolute entropy S _H in each field H

The results are shown by white symbols in FIGS. Then, if Motomare is S _H in each field, by differentiating equation (4) at a temperature T, it can be determined specific heat C _H at a given magnetic field H _(T) is as follows:

  Thus, the specific heat in another magnetic field atmosphere was calculated | required and the result is shown in FIGS. From this result, even when the rare earth element nitride is affected by the magnetic field, the decrease in specific heat is not so great. Therefore, it can be seen that even when such a nitride is used in a magnetic field atmosphere, it sufficiently functions as a cold storage material.

  FIG. 37 shows the magnetic entropy change of the rare earth nitride calculated based on the formula (5) from the specific heat measurement result described above. This is the value when the magnetic field is demagnetized from 5T. Although there are some errors in absolute values, the peak temperature is in good agreement with the temperature dependence of the magnetic entropy change that occurs when the rare earth nitride is demagnetized from 5T, as shown in FIG. ing.

  As explained above, based on the fact that the specific heat in an arbitrary magnetic field can be reproduced from the magnetic entropy change and specific heat data, the fact that the magnetic entropy change of the binary nitride continuously changes is based on the fact that It is considered that the specific heat of the binary rare earth nitride also changes continuously corresponding to the binary system and the composition. Therefore, not only a simple rare earth nitride but also a binary rare earth nitride can be used as an effective cold storage material.

Therefore, the present invention
Composition formula: MN (wherein M represents one or more rare earth elements)
A cold storage material comprising a nitride of at least one rare earth element represented by:
Particularly preferred rare earth nitrides are DyN, ErN, HoN and TbN and nitride solid solutions of two rare earth elements (eg Dy-HoN, Dy-TbN, Tb-ErN, Dy-ErN, Ho-ErN and Ho-TbN). It is. These cold storage materials are suitable for use as a cold storage material in a magnetic field atmosphere, preferably in a magnetic field atmosphere of 5T or less, more preferably in a magnetic field atmosphere of 1T or less. Moreover, when using as a cool storage material, it can point out that it uses as a cool storage material in the low temperature of 2-40K, Preferably 4-30K, More preferably, 4-20K.

Based on the above measurement results, DyN can function as a suitable heat storage material in a magnetic field atmosphere of 5T or less at a temperature of 12 to 42K. ErN can function as a suitable heat storage material in a magnetic field atmosphere of 5T or less at a temperature of 2 to 20K. HoN can function as a suitable heat storage material in a magnetic field atmosphere of 5T or less at a temperature of 8 to 26K. TbN can function as a suitable heat storage material in a magnetic field atmosphere of 5T or less at a temperature of 20-60K.
Furthermore, for binary nitrides, Ho _X Er _1-X N (0 <X <1) can function as a suitable heat storage material in a magnetic field atmosphere of 5 T or less at a temperature of 2 to 26K. Dy _X Er _1-X N (0 <X <1) can function as a suitable heat storage material in a magnetic field atmosphere of 5 T or less at a temperature of 2 to 42K. _{Tb X Er 1-X N (} 0 <X <1) can function as a suitable storage material in a following field atmosphere 5T at a temperature of 2～60K. Dy _X Ho _1-X N (0 <X <1) can function as a suitable heat storage material in a magnetic field atmosphere of 5 T or less at a temperature of 12 to 26K. _{Tb X Ho 1-X N (} 0 <X <1) can function as a suitable storage material in a following field atmosphere 5T at a temperature of 8～60K. _{Tb X Dy 1-X N (} 0 <X <1) can function as a suitable storage material in a following field atmosphere 5T at a temperature of 12～60K. In the present specification, the magnetic field atmosphere does not include a no magnetic field atmosphere.

  The rare earth element nitride of the present invention can be used as a stable magnetic refrigeration material. Moreover, considering the peak temperature with the largest magnetic entropy change, it is possible to obtain a low temperature in the region of 10 to 80 K, and the low temperature necessary for liquefaction of hydrogen can be provided by the rare earth element nitride of the present invention. It becomes.

  Therefore, the magnetic refrigeration material of the present invention can constitute a magnetic refrigeration system using the same, and such a system can be advantageously applied to a hydrogen liquefaction plant.

  Moreover, since the rare earth element nitride of the present invention has a large specific heat at low temperatures, it can function effectively as a cold storage material. Therefore, the magnetic refrigeration material of the present invention not only brings about a low temperature, but can effectively keep the produced low temperature. In addition, the rare earth element nitride of the present invention often does not decrease so much even when the specific heat is reduced in a magnetic field atmosphere, so cold storage is required in an atmosphere under the influence of a magnetic field. Is particularly effective. Therefore, the present invention provides a method for storing cold in a magnetic field atmosphere, and the method is characterized by using the rare earth nitride described above as a cold storage material. Therefore, the present invention provides a refrigeration system that uses the rare earth nitride of the present invention as a cold storage material, preferably the rare earth nitride of the present invention as a cold storage material in a magnetic field atmosphere in order to maintain the low temperature generated by the low temperature generator. A refrigeration system to be used is provided, and a low-temperature utilization apparatus (such as an MRI apparatus) having such a refrigeration system is also provided.

  Therefore, the present invention provides a method for generating a low temperature by using the rare earth nitride as described above as a magnetic refrigeration material, a device for generating a low temperature, a low-temperature generating apparatus including such a device, and a The present invention also provides a method for storing or holding a low temperature by using a rare earth nitride as a cold storage material, a device for storing or holding a low temperature, and a low-temperature generating apparatus including such a device.

FIG. 1 is a graph showing X-ray diffraction patterns of GdN, TbN, DyN, HoN and ErN, which are examples of the rare earth element nitride of the present invention synthesized by a carbothermal reduction method. Gd _X Dy _1-X N (0 ≦ X ≦ 1), Gd _X Tb _1-X N (0 ≦ X ≦ 1), and Ho _X Tb _1-X N (0 It is a graph which shows the lattice constant computed from the X-ray-diffraction pattern of <= X <= 1. For an example _{Gd X Dy 1-X N (} 0 ≦ X ≦ 1) of a rare earth element nitride of the present invention, is a graph showing the relationship between the magnetization measured at 5K and (M) and magnetic field (H). It is a graph which shows the Curie temperature Tc calculated | required from the Arrott plot about GdN, TbN, DyN, HoN, and ErN which is an example of the rare earth element nitride of this invention with a literature value. FIG. 5A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a DyN sample. FIG. 5B is a graph showing the relationship between FIG. 5 is a graph showing a magnetic entropy change ΔS when each magnetic field calculated by numerical calculation based on the data (3) is demagnetized. FIG. 6A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a sample with Gd _0.5 Dy _0.5 N. FIG. ) Is a graph showing the magnetic entropy change (ΔS) when the external magnetic field calculated by the numerical calculation based on the formula (3) from the data of FIG. 5A is demagnetized. Is a graph showing the relationship between the magnetic entropy change and temperature obtained when demagnetized an external magnetic field 5T for _{Gd X Dy 1-X N (} 0 ≦ X ≦ 1). It is a graph which shows the relationship between the magnetic entropy change and temperature which are obtained when it degausses from 5T about GdN, TbN, DyN, HoN, and ErN. It is a graph showing the X-ray diffraction pattern of an example _{Gd X Dy 1-X N (} 0 ≦ X ≦ 1) of a rare earth element nitride of the present invention. It is a graph showing the X-ray diffraction pattern of an example _{Gd X Tb 1-X N (} 0 ≦ X ≦ 1) of a rare earth element nitride of the present invention. Is a graph showing the X-ray diffraction pattern of an example _{Ho X Tb 1-X N (} 0 ≦ X ≦ 1) of a rare earth element nitride of the present invention. It is the figure which showed the lattice constant of GdN, TbN, DyN, HoN, and ErN computed from the X-ray diffraction pattern shown in FIG. 1 with the literature value. It is a graph which shows the relationship between the magnetization (M) measured at 5K, and the magnetic field (H) about GdN, TbN, DyN, HoN and ErN. Gd _X Dy _1-X N (0 ≦ X ≦ 1), Gd _X Tb _1-X N (0 ≦ X ≦ 1) and Ho _X Tb _1-X N (0 It is a graph which shows both the Curie temperature Tc calculated | required from the Arrott plot, and the peak temperature of the magnetic entropy change calculated from the magnetization measurement about (<= X <= 1). FIG. 15A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a GdN sample, and FIG. It is a graph which shows the magnetic entropy change ((DELTA) S) at the time of demagnetizing the external magnetic field computed by numerical calculation based on Formula (3) from data of (3). FIG. 16A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a TbN sample, and FIG. 16B is a graph of FIG. It is a graph which shows the magnetic entropy change ((DELTA) S) at the time of demagnetizing the external magnetic field computed by numerical calculation based on Formula (3) from data of (3). FIG. 17A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a HoN sample, and FIG. 17B is a graph showing FIG. It is a graph which shows the magnetic entropy change ((DELTA) S) at the time of demagnetizing the external magnetic field computed by numerical calculation based on Formula (3) from data of (3). FIG. 18A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to an ErN sample. FIG. 18B is a graph showing the relationship between FIG. It is a graph which shows the magnetic entropy change ((DELTA) S) at the time of demagnetizing the external magnetic field computed by numerical calculation based on Formula (3) from data of (3). FIG. 19A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a sample with Gd _0.5 Tb _0.5 N. FIG. ) Is a graph showing the magnetic entropy change (ΔS) when the external magnetic field calculated by the numerical calculation based on the equation (3) from the data of FIG. 19A is demagnetized. FIG. 20A is a graph showing the relationship of magnetization (M) vs. temperature (T) when a magnetic field of up to 5T is applied to a sample of Ho _0.5 Tb _0.5 N. FIG. ) Is a graph showing the magnetic entropy change (ΔS) when the external magnetic field calculated by the numerical calculation based on the formula (3) from the data of FIG. Is a graph showing the relationship between the magnetic entropy change and temperature obtained when demagnetized an external magnetic field 5T for _{Gd X Tb 1-X N (} 0 ≦ X ≦ 1). It is a graph showing the relationship between the magnetic entropy change and temperature obtained when demagnetized an external magnetic field 5T for _{Ho X Tb 1-X N (} 0 ≦ X ≦ 1). It is a graph which shows the X-ray-diffraction pattern of TbN, DyN, HoN, and ErN synthesize | combined by HIP method. It is a graph which shows the temperature dependence of the specific heat C of ErN measured in the non-magnetic field and the magnetic field of 5T. It is a graph which shows the temperature dependence of the specific heat C of HoN measured in the magnetic field of no magnetic field, and 5T. It is a graph which shows the temperature dependence of the specific heat C of DyN measured in the magnetic field of no magnetic field, and 5T. It is a graph which shows the temperature dependence of the specific heat C of TbN measured in the magnetic field of no magnetic field and 5T. FIG. 28 is a graph summarizing changes in specific heat of rare earth nitrides in a magnetic field of 5 T and in a non-magnetic field, and changes in specific heat of helium and Pb and Er ₃ Ni currently used as a cold storage material. It also shows. Absolute value calculated by numerical calculation based on equation (4) from measured data of magnetic entropy change ΔS when demagnetizing the external magnetic field calculated from numerical measurement based on equation (3) from magnetization measurement data and specific heat C in the absence of magnetic field It is the graph which showed the temperature change of the absolute entropy S in each magnetic field of ErN computed using Formula (6) from entropy. Absolute value calculated by numerical calculation based on equation (4) from measured data of magnetic entropy change ΔS when demagnetizing the external magnetic field calculated from numerical measurement based on equation (3) from magnetization measurement data and specific heat C in the absence of magnetic field It is the graph which showed the temperature change of the absolute entropy S in each magnetic field of HoN computed using Formula (6) from entropy. Absolute value calculated by numerical calculation based on equation (4) from measured data of magnetic entropy change ΔS when demagnetizing the external magnetic field calculated from numerical measurement based on equation (3) from magnetization measurement data and specific heat C in the absence of magnetic field It is the graph which showed the temperature change of the absolute entropy S in each magnetic field of DyN computed using Formula (6) from entropy. Absolute value calculated by numerical calculation based on equation (4) from measured data of magnetic entropy change ΔS when demagnetizing the external magnetic field calculated from numerical measurement based on equation (3) from magnetization measurement data and specific heat C in the absence of magnetic field It is the graph which showed the temperature change of absolute entropy S in each magnetic field of TbN computed using Formula (6) from entropy. FIG. 33 shows a plot of the temperature dependence of the specific heat in each magnetic field of ErN calculated from the data of FIG. 29 and the numerical calculation based on equation (7). FIG. 33 shows a plot of the temperature dependence of the specific heat in each magnetic field of HoN calculated from the data of FIG. 29 and the numerical calculation based on equation (7). FIG. 33 shows a plot of the temperature dependence of the specific heat in each magnetic field of DyN calculated from the data of FIG. 29 and the numerical calculation based on equation (7). FIG. 33 shows a plot of the temperature dependence of the specific heat in each magnetic field of TbN calculated from the data of FIG. 29 and the numerical calculation based on equation (7). FIG. 37 is a graph showing the temperature dependence of the magnetic entropy change ΔS of the rare earth nitride calculated from the specific heat measurement data by numerical calculation based on the equations (4) and (5). The value when the applied magnetic field is demagnetized from 5T is shown.

Claims (18)

Composition formula: MN (wherein M represents one or more rare earth elements)
A magnetic refrigeration material comprising a nitride of at least one rare earth element represented by:   The magnetic refrigeration material according to claim 1, which has a NaCl structure.   The rare earth element is one or more elements selected from Nd (neodymium), Gd (gadolinium), Dy (dysprosium), Ho (holmium), Tb (terbium) and Er (erbium). The magnetic refrigeration material according to 1 or 2.   The magnetic refrigeration material according to claim 1, comprising a nitride of at least two rare earth elements. Nitride has a composition formula: Gd _X Dy _1-X N, Gd _X Tb _1-X N and Ho _X Tb _1-X N (where X is a numerical value of 0 or more and 1 or less). The magnetic refrigeration material according to any one of claims 1 to 4, wherein the magnetic refrigeration material is at least one of nitrides represented by any of the above.   The magnetic refrigeration system which uses the magnetic refrigeration material in any one of Claims 1-5 as a magnetic refrigeration material which shows a magnetocaloric effect.   The liquid hydrogen production plant which obtains the low temperature required when liquefying hydrogen with the magnetic refrigeration system of Claim 6. Composition formula: MN (wherein M represents one or more rare earth elements)
A cold storage material comprising a nitride of at least one rare earth element represented by:   The cold storage material according to claim 8, which has a NaCl structure.   The rare earth element is one or more elements selected from Nd (neodymium), Gd (gadolinium), Dy (dysprosium), Ho (holmium), Tb (terbium) and Er (erbium). The cold storage material according to 8 or 9.   The cold storage material according to any one of claims 8 to 10, comprising a nitride of at least two rare earth elements. Nitride has a composition formula: Gd _X Dy _1-X N, Gd _X Tb _1-X N and Ho _X Tb _1-X N (where X is a numerical value of 0 or more and 1 or less). The cold storage material according to any one of claims 8 to 11, which is at least one of the nitrides represented by any of the above.   A refrigeration system comprising the cold storage material according to any one of claims 8 to 12.   The liquid hydrogen production plant which obtains the low temperature required when liquefying hydrogen with the refrigeration system of Claim 13. Composition formula: MN (wherein M represents one or more rare earth elements)
The method of using at least 1 sort of rare earth element nitride represented by these as a cool storage material in a magnetic field atmosphere. Composition formula: MN (wherein M represents one or more rare earth elements)
The apparatus which uses low temperature in magnetic field atmosphere which has the nitride of at least 1 sort (s) of rare earth elements represented by these as a cool storage material.   The apparatus according to claim 16, wherein the apparatus is an MRI apparatus. Composition formula: MN (wherein M represents one or more rare earth elements)
The apparatus which generate | occur | produces low temperature in a magnetic field atmosphere which has the nitride of at least 1 sort (s) of rare earth elements represented by these as a cool storage material.

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