JP5686314B2 - Rare earth magnetic refrigerant for magnetic refrigeration system - Google Patents

Description

  The present invention relates to a rare earth magnetic refrigerant used in a magnetic refrigeration system, and relates to a novel rare earth magnetic refrigerant for a magnetic refrigeration system containing boron.

  Magnetic refrigeration is a cooling technology that uses a magnetic material exhibiting a magnetocaloric effect as a refrigerant, causing a ferromagnetism / paramagnetic phase transition by a cycle of magnetic field increase / decrease, and freezing using an endothermic reaction / exothermic reaction generated there. Technology.

In general heat pump cooling, greenhouse gases such as CO ₂ and chlorofluorocarbon are used as refrigerants. By replacing them with magnetic refrigeration systems, environmentally friendly refrigeration such as refrigerators and air conditioners that do not use greenhouse gases is used. It is expected that the system can be realized. In addition, the heat pump system requires a compressor for expanding and compressing the refrigerant gas, but such a compressor is not necessary in the magnetic refrigeration system. For example, low noise and low vibration can be achieved in a refrigerator or an air conditioner. In addition, it can be applied to small refrigerators such as small air conditioners that do not require outdoor units and cooling of automobile engines.

  By the way, in a magnetic refrigeration system, development of a high-performance refrigerant (magnetic material) has become a major issue. For example, in a magnetic refrigeration system, the magnitude of the entropy change ΔS at the time of the magnetic phase transition determines the refrigerating capacity. The key to performance. Also, in the magnetic refrigeration system, the refrigerating capacity becomes the highest near the ferromagnetic phase transition temperature Tc. Therefore, if a refrigerant having the ferromagnetic phase transition temperature Tc near room temperature can be developed, for example, a home refrigerator is put to practical use. It is expected that

  From this point of view, research on a refrigerant (magnetic material) for a magnetic refrigeration system has been promoted in various directions, and various magnetic refrigerants have been developed (see, for example, Patent Document 1 and Patent Document 2). .

For example, Patent Document 1 discloses that a ferromagnetic phase has a NiAs-type hexagonal crystal structure, Mn as a first element, As as a second element, and a third element that can be substituted for the second element. In addition, a magnetic refrigeration working substance that causes a magnetic phase transition in a temperature range of 230 K or more and less than 318 K is disclosed, and Mn (As _1-x Sb _x ) is described as a specific composition formula. According to the invention described in Patent Document 1, a magnetic refrigeration working material that exhibits a large magnetocaloric effect near room temperature is provided.

On the other hand, Patent Document 2 discloses that in NaZn ₁₃ type La (Fe _1-x Si _x ) ₁₃ H _y , Ce is partially substituted, H is absorbed, and the composition thereof is NaZn ₁₃ type La _1-z Ce _z (Fe _{1− x} Si _{x) 13} H _y and the magnetic refrigerant material is disclosed. In the invention described in Patent Document 2, La (Fe _1-x Si _x ) ₁₃ and its hydrogen absorbing compound exhibiting a large magnetocaloric effect at an arbitrary temperature from around 170 K to around 340 K by controlling the hydrogen absorption amount It is said that the magnetocaloric effect can be improved by partially replacing La in Ce.

JP 2003-28532 A JP 2006-89839 A

However, for example, Mn (As _1-x Sb _x ) described in Patent Document 1 has a high risk of arsenic (As) being vaporized during production. Arsenic is highly toxic and difficult to handle. On the other hand, the hydrogenated product of La (Fe _1-x Si _x ) ₁₃ described in Patent Document 2 needs to be synthesized by heating in a hydrogen atmosphere, and there is a problem that it involves a risk in production.

The present invention has been proposed in view of such a conventional situation, and a magnetic material that can be stably produced by a simple and safe method with a large magnetic entropy change ΔS and a high ferromagnetic phase transition temperature Tc. An object is to provide a rare earth magnetic refrigerant for a refrigeration system.

The inventor paid attention to the rare earth ferromagnet RX ₂ having a large magnetic entropy change ΔS, and investigated how the magnetic entropy change ΔS and the ferromagnetic transition temperature Tc change due to addition of a light metal such as boron. As a result, a high-performance magnetic refrigerant having both a large magnetic entropy change ΔS and a high ferromagnetic transition temperature Tc has been synthesized.

The rare earth magnetic refrigerant for a magnetic refrigeration system of the present invention is represented by the general formula ErCo ₂ Bx, where Bx is 0.02 ≦ x ≦ 0.1 in terms of molar ratio, and the added B (boron) is It is characterized in that the volume is increased without impairing the crystal structure by entering the void of ErCo ₂ which is the base material . The present invention is characterized in that Bx in the above formula is 0.04 ≦ x ≦ 0.07 in molar ratio.

  In a magnetic alloy composed of the rare earth element R and the transition metal element X, the large magnetic moment of the rare earth element R is reflected, and the magnetic entropy change ΔS at a low temperature is large. Further, the ferromagnetic phase transition temperature Tc can be controlled by the composition of the rare earth element R and the transition metal element X. However, a composition having a ferromagnetic phase transition temperature Tc near room temperature has a drawback that the magnetic entropy change ΔS is suppressed and the thermal efficiency is poor.

  In the rare earth magnetic refrigerant for a magnetic refrigeration system of the present invention, the increase in crystal volume and sample hardening were simultaneously achieved by adding boron, and the above-mentioned drawbacks were successfully solved. In the magnetic alloy composed of the rare earth element R and the transition metal element X, when boron is added, the crystal volume increases while maintaining a stable crystal structure, and the ferromagnetic phase transition temperature Tc increases. Further, the magnetic entropy change ΔS increases due to sample hardening. That is, the ferromagnetic phase transition temperature Tc can be shifted to around room temperature while maintaining the large magnetic entropy change ΔS, which is an advantage of the magnetic alloy composed of the rare earth element R and the transition metal element X.

According to the present invention, it is possible to provide a high-performance rare earth magnetic refrigerant for a magnetic refrigeration system having both a large magnetic entropy change ΔS and a high ferromagnetic phase transition temperature Tc. Further, the rare earth magnetic refrigerant of the invention comprises the steps of dangerous as synthesis by heating in a hydrogen atmosphere is not required, without compromising safety, without increasing the manufacturing cost, in a simple way It is possible to manufacture stably.

It is a figure explaining the heat cycle in a magnetic refrigeration system. It is a diagram showing the relationship between ferromagnetic transition temperature Tc and the magnetic entropy change ΔS in RX ₂ compounds. Is a diagram showing the X-ray diffraction pattern upon addition of B to ErCo _2, (a) X-ray diffraction pattern of ErCo _2, (b) X-ray diffraction pattern of ErCo ₂ B _0.02, (c) Is an X-ray diffraction pattern of ErCo ₂ B _0.04 , (d) is an X-ray diffraction pattern of ErCo ₂ B _0.07 , and (e) is an X-ray diffraction pattern of ErCo ₂ B _0.1 . It is a diagram showing the relationship between B addition amount x and the lattice constant in ERCO _2. It is a figure which shows the relationship between a lattice constant and the ferromagnetic transition temperature Tc. It is a diagram showing the relationship between B addition amount x and the ferromagnetic transition temperature Tc in the ERCO _2.

  Hereinafter, a rare earth magnetic refrigerant for a magnetic refrigeration system to which the present invention is applied will be described in detail with reference to the drawings.

  The magnetic refrigeration system, unlike the gas refrigeration technology, is a refrigeration technology that uses the internal degree of freedom (entropy) of the magnetic material, causing a ferromagnetic / paramagnetic transition in the magnetic refrigerant depending on whether a magnetic field is applied, and the endothermic reaction that occurs there.・ Freeze using heat energy from exothermic reaction. For example, when a magnetic field is applied to a magnetic refrigerant, the directions of electron spin are aligned, magnetic entropy is reduced, and an exothermic reaction occurs. On the other hand, when the magnetic field is lowered and the electron spin state of the magnetic refrigerant is made random, the magnetic entropy increases and an endothermic reaction occurs. In the magnetic refrigeration system, cooling is performed using such a change in magnetic entropy (magnetocaloric effect).

FIG. 1 is a diagram showing a thermal cycle in a magnetic refrigeration system. In the magnetic refrigeration system, a magnetic material serving as a refrigerant is thermally cycled as shown in FIG. That is,
(A) Isothermal excitation at the ferromagnetic phase transition temperature Tc (ΔS)
(B) Cooling by adiabatic demagnetization (ΔT)
(C) Return to the original state with zero magnetic field.
In this circulation cycle, the heat quantity of Q ₁ -Q ₂ = W per cycle is exhausted. Therefore, in order to improve the thermal efficiency in the thermal cycle, the W may be increased.

Given the above, considering the conditions for putting the magnetic refrigeration system into practical use, the following is necessary.
(1) operating temperature of the thermal cycle, i.e. to increase the ferromagnetic phase transition temperature Tc.
(2) In order to increase the W, a material having a large ΔS and ΔT is selected.
(3) Consider material cost and safety.

In the present invention, RX ₂ compound (R is a rare earth element, X is a transition metal element) having a large magnetocaloric effect as based, both boron (B) large magnetic entropy change ΔS high ferromagnetic phase transition temperature Tc by addition Realize rare earth magnetic refrigerant.

The RX ₂ compound reflects the large magnetic moment of the rare earth element R, and has a large ΔS and ΔT at low temperature. By selecting an appropriate rare earth element R and transition metal element X as constituent elements, the magnitude of ΔS and ΔT can be obtained. Can be controlled. However, as shown in FIG. 2, generally, the magnetic entropy change ΔS tends to decrease as the ferromagnetic phase transition temperature Tc increases.

On the other hand, when boron is added to the RX ₂ compound to form RX ₂ B _x , the following effects can be obtained.
(1) Boron enters the voids of the parent material (RX ₂ compound), and the volume increases without impairing the crystal structure. In fact, in ErCo ₂ , it has been experimentally confirmed that the lattice volume increases by about 0.8% when 7% of B is added.
(2) Since the ferromagnetic phase transition temperature Tc is greatly influenced by the distance between the magnetic moments of the rare earth elements, the lattice constant and the volume, addition of boron is effective as a method for controlling the ferromagnetic phase transition temperature Tc.
(3) The entropy change ΔS of the magnetic material is expressed by the following equation.
ΔS = ΔS _mag + ΔS _ph
Generally, since the rare earth material is soft, the ratio of ΔS _ph to ΔS is large, and the magnetic entropy change ΔS _mag due to excitation / demagnetization flows to ΔS _ph and the thermal efficiency W tends to be lost. Here, in order to reduce the contribution of ΔS _ph , it is effective to increase the material hardness. However, by adding boron, the RX ₂ compound is expected to increase in hardness.
(4) By the above (2) and (3), an increase in the magnetic entropy change ΔS and an increase in the ferromagnetic phase transition temperature Tc are realized at the same time by adding boron.

Due to the boron addition, as shown in FIG. 2, the curve indicating the relationship between the magnetic entropy change ΔS and the ferromagnetic phase transition temperature Tc in RX ₂ B _x shifts to the right as compared with the case of the RX ₂ compound, resulting in a large magnetic entropy. The contradictory factors of the change ΔS and the increase of the ferromagnetic phase transition temperature Tc can be made compatible.

The rare earth magnetic refrigerant for a magnetic refrigeration system according to the present invention is based on the above-mentioned viewpoint, a magnetic alloy composed of a rare earth element R and a transition metal element X is used as a base compound, and boron (B) is added thereto. . Here the base compound, as long as the magnetic alloy comprising a rare earth element R and a transition metal element X may be any one can be exemplified a magnetic alloy represented e.g. by the general formula RX ₂ . Of course, not limited to this, for example, the ratio of the rare earth element R and the transition metal element X can be arbitrarily set. The rare earth element R may be composed of one kind of rare earth element or may be composed of two or more kinds of rare earth elements. Similarly, the transition metal element X may be composed of one kind of transition metal element or may be composed of two or more kinds of transition metal elements.

In the magnetic alloy that is the base material, any rare earth element can be used as the rare earth element R, and any transition metal element can be used as the transition metal element X. As a specific compound, for example, ErCo ₂ is suitable.

In addition, R ₃ Co, R ₂₄ Co ₁₁ , R ₂ Co, R ₃ Co ₂ , R ₄ Co ₃ , RCo, RCo ₂ , RCo ₃ , R ₂ Co ₇ , R ₅ Co ₁₉ , RCo ₅ , R ₂ Co ₁₇ , RCo ₁₃ , RMn ₂ , R ₆ Mn ₂₃ , RMn ₅ , RMn ₁₂ , RTc ₂ , RRe ₂ , R ₅ Re ₂₄ , RFe ₂ , RFe ₃ , R ₆ Fe ₂₃ , R ₂ Fe ₁₇ , RRu, RRu ₂ , _{ROs 2, R 3 Rh, R} 7 Rh 3, R 5 Rh 3, R 4 Rh 3, RRh, RRh 2, RRh 3, R 2 Rh 7, RRh 5, R 7 Ir 3, rIr, rIr 2, rIr 3 _{, R 2 Ir 7, rIr 5} , R 3 Ni, R 7 Ni 3, R 2 Ni, R 3 Ni 2, RNi, RNi 2, RNi 3, R 2 Ni 7, RNi 5, R 2 Ni 17 , R ₅ Pd ₂ , R ₇ Pd ₃ , R ₂ Pd, R ₃ Pd ₂ , RPd, R ₃ Pd ₄ , RPd ₃ , R ₇ Pt ₃ , RPt, R ₃ Pt ₄ , RPt ₂ , RPt ₃ , RPt ₅ , RCu, RCu ₂ , RCu ₅ , RCu ₆ , RCu ₇ , RAg, RAg ₂ , RAg _3.6 , R ₂ Au, RAu, RAu ₂ , RAu ₃ , RAu _3.6 , RAu ₄ , RAu ₆ , RZn, RZn ₂ , RZn ₃ , R ₃ Zn ₁₁ , RZn ₄ , R ₁₃ Zn ₅₈ , RZn ₅ , R ₃ Zn ₂₂ , R ₂ Zn ₁₇ , RZn ₁₁ , RZn ₁₂ , RZn ₁₃ , R ₃ Al, R ₂ Al, R ₃ Al ₂ , RAl, RAl ₂ , RAl ₃ , R ₃ Al ₁₁ , R ₃ Tl, R ₂ Tl, R ₅ Tl ₃ , RTl, RTl ₃ , R ₅ Si ₃ , R ₃ Si ₂ , R _{5 Si 4, RSi, RSi 2} -x, RSi 2, R 5 Ge 3, R 4 Ge 3, R 5 Ge 4, R 11 Ge 10, RGe, RGe 2-x, RGe 2, R 3 Sn, R 5 _{Sn 3, R 5 Sn 4,} R 11 Sn 10, RSn 2, RSn 3, R 3 Pb, R 5 Pb 3, R 4 Pb 3, R 5 Pb 4, R 11 Pb 10, RPb 2, RPb also ₃ etc. It can be used. Here, the rare earth element R is one or more selected from Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu. For example, in the case of RMn ₂ , Pr _0.5 Nd _0.5 Mn ₂ may be sufficient.

In the rare earth magnetic refrigerant of the present invention, the amount of boron added can be arbitrarily set, but is preferably 0.1 mol% or less. That is, when the rare earth magnetic refrigerant is expressed as RX ₂ B _x , it is preferable that 0 <x ≦ 0.1. If the value of x is zero, the effect of boron addition cannot be obtained. On the other hand, if the value of x exceeds 0.1, the ferromagnetic phase transition temperature Tc may be lower than when it is not added.

Table 1 shows the ferromagnetic phase transition temperature Tc and magnetic entropy changes ΔS and ΔT of various magnetic refrigerants .

  In addition, the rare earth magnetic refrigerant of the present invention does not contain highly toxic elements, does not require heating in a hydrogen atmosphere, and can be manufactured safely and stably with simple operations. For example, each element can be mixed at a predetermined ratio and synthesized by arc melting in argon atmosphere (a method of melting and mixing non-volatile metals at a high temperature). Therefore, the rare earth magnetic refrigerant of the present invention is advantageous also in terms of production cost.

  Hereinafter, specific examples to which the present invention is applied will be described based on experimental results.

Synthetic rare earth magnetic refrigerant E r is used as the rare earth element R, Co is used as the transition metal element X, Er and Co are weighed so that the molar ratio is E r : Co = 1: 2, and at a predetermined ratio Boron (B) was added and arc-dissolved in an argon atmosphere to prepare a rare earth magnetic refrigerant. Rare earth magnetic refrigerant produced _{is, ErCo 2, ErCo 2 B 0.02} , ErCo 2 B 0.04, ErCo 2 B 0.07, a five ErCo ₂ B _0.1.

Confirmation by X-ray diffraction Five kinds of rare earth magnetic refrigerants prepared were analyzed by X-ray diffraction. The X-ray diffraction patterns of each rare earth magnetic refrigerant are shown in FIGS. 3 (a) to 3 (e).

As is apparent from these drawings, it can be seen that the X-ray diffraction pattern is hardly changed by the addition of boron, and the crystal structure of the base magnetic material (ErCo ₂ ) is maintained.

Lattice constant was then calculated lattice constants of the sample. The results are shown in FIG.

As is apparent from FIG. 4 and Table 1, an increase in lattice constant is observed with boron addition. From these facts, it is considered that the added boron enters the ErCo ₂ void, which is the base material, and increases the volume without deteriorating the crystal structure. In fact, ErCo ₂ increased the lattice volume by 0.8% by adding 7% boron.

Ferromagnetic phase transition temperature Tc
Further, the ferromagnetic phase transition temperature Tc was measured for each of the prepared samples. 5 and Table 3 show the relationship between the lattice constant and the ferromagnetic phase transition temperature Tc, and FIG. 6 shows the relationship between the boron addition amount x and the ferromagnetic phase transition temperature Tc.

First, as is apparent from FIG. 5 and Table 3, the ferromagnetic phase transition temperature Tc increases as the lattice constant increases. Similarly, as apparent from FIG. 6, the ferromagnetic phase transition temperature Tc increases with the addition of boron. However, in each case, there is a maximum value, and the ferromagnetic phase transition temperature Tc is maximum in the vicinity of a lattice constant of 7.14% and a boron addition amount of 7% (x = 0.07).

Claims (2)

It is represented by the general formula ErCo ₂ Bx, and Bx in the formula is 0.02 ≦ x ≦ 0.1 in terms of molar ratio, and the added B (boron) enters the void of ErCo _{2 as} the base material, A rare earth magnetic refrigerant for a magnetic refrigeration system, wherein the volume is increased without impairing the crystal structure . The rare earth magnetic refrigerant for a magnetic refrigeration system according to claim 1, wherein Bx in the formula is 0.04 ≦ x ≦ 0.07 in terms of molar ratio.

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