EP3739075A1

EP3739075A1 - Magnetic material for magnetic refrigeration

Info

Publication number: EP3739075A1
Application number: EP18900539.0A
Authority: EP
Inventors: Asaya Fujita; Kimihiro Ozaki
Original assignee: National Institute of Advanced Industrial Science and Technology AIST
Current assignee: National Institute of Advanced Industrial Science and Technology AIST
Priority date: 2018-01-11
Filing date: 2018-10-25
Publication date: 2020-11-18
Anticipated expiration: 2038-10-25
Also published as: JPWO2019138648A1; EP3739075B1; WO2019138648A1; JP6995391B2; EP3739075A4

Abstract

A magnetic material for magnetic refrigeration containing a NaZn₁₃ compound represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x)₁₃H_w is characterized in that a total amount of a coexistence phase other than the coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%, in crystal particles of the NaZn₁₃ type compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to a logarithmic normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm is less than 50%.
(In the formula described above, an amount of Si is 0.100 ≤ x ≤ 0.130, amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, and an amount of H is 1.4 ≤ w ≤ 1.7.)

Accordingly, a NaZn₁₃ type single phase La(Fe,Si)₁₃-based magnetic material for magnetic refrigeration is provided in which a hydrogen redistribution is suppressed and a large magnetic entropy change is obtained.

Description

Technical Field

The present invention relates to a magnetic material for magnetic refrigeration exhibiting a magnetocaloric effect.

Background Art

Recently, a magnetic refrigeration system that is clean and has a high energy efficiency has been proposed as a refrigeration technology for eliminating Freon gas that causes environmental problems. In the magnetic refrigeration system, a magnetic refrigeration material is used as a solid refrigerant. In order to operate a magnetic refrigeration apparatus at a normal temperature, a magnetic material in which a magnetic entropy change that is a source of a thermal change has a large value in the vicinity of a room temperature is indispensable.
As a magnetic material exhibiting such properties suitable for magnetic refrigeration, a La(Fe,Si)₁₃-based compound having a NaZn₁₃ crystal structure is known. The La(Fe,Si)₁₃-based compound is capable of obtaining a large magnetic entropy change of greater than or equal to 20 J/kg·K per 2 T of a magnetic field, in the vicinity of a transition temperature at which a magnetic phase is changed, and contains inexpensive Fe as a main constituent element, and thus, is practically advantageous (for example, refer to Patent Literature 1 or Non Patent Literature 1).
In order to adjust a temperature at which an entropy change of the La(Fe,Si)₁₃-based compound can be generated, in accordance with intended use, it is preferable to control the transition temperature, and for this reason, for example, there is a method of substituting Fe with Co (refer to Patent Literature 2).
However, in a case where the content of a Fe element is not greater than or equal to 82 atom%, the La(Fe,Si)₁₃-based compound is not capable of exhibiting magnetic phase transition in which the magnetic entropy change increases, and the value of the entropy change increases as the concentration of Fe increases, but in order to increase the transition temperature to a room temperature by substituting the Fe element with a Co element, it is necessary to substitute greater than or equal to 6% of a total amount of Fe with the Co element, and thus, the entropy change also decreases to less than or equal to half.
In order to avoid reciprocity between an increase in the transition temperature and a decrease in the entropy change due to the Fe substitution, in Patent Literature 3, an H element is infiltrated into crystals by hydrogenation, and thus, the transition temperature is controlled such that the transition temperature is increased to higher than or equal to a room temperature. In the use of the H element, in a case where hydrogen is absorbed to a stable highest concentration in a condition of an ordinary temperature and the atmospheric pressure, the transition temperature increases to the vicinity of 60°C. In addition, in a case where the concentration of hydrogen is adjusted, it is possible to adjust the transition temperature to an arbitrary value in a range of about -75°C that is a transition temperature of an unhydrogenated material to 60°C that is the highest temperature.
However, in Non Patent Literature 2, it is reported that in a case where a hydrogenated material is left to stand in an environment where the temperature is identical to the transition temperature, initially, hydrogen that is homogeneously adjusted in the entire region in the material is redistributed over time, and is separated into two regions with a higher concentration and a lower concentration than the original concentration. In a case where the hydrogenated material is used in a refrigeration machine in a state where such a phenomenon occurs, there is a danger that in a case where the temperature in the machine is coincident with the transition temperature and the machine is stopped, the condition is coincident with a condition in which a hydrogen redistribution phenomenon occurs. In this case, material properties are deviated from the initial setting after the refrigeration machine is restarted, and hinders the operation of the machine.
The progress of the hydrogen redistribution depends on the concentration of hydrogen, and in particular, in a case where hydrogen is absorbed to the highest concentration, the hydrogen redistribution is stopped. Therefore, in Patent Literature 4, there is proposed means for finely adjusting a transition temperature by using a treatment of partially substituting a constituent element while using a treatment of increasing the transition temperature by retaining the entropy change with hydrogenation. As an example thereof, there is a treatment in which hydrogenation is performed such that H is approximately 1.6 moles with respect to 1 mole of the stoichiometric notation of La(Fe,Si)₁₃, and Fe is substituted with Mn at a maximum of approximately 20 atom%. In such a method, as with the case of the Co substitution, Fe decreases due to the substitution, and thus, the amount of entropy change decreases. Therefore, in Patent Literatures 4 and 5, a method is proposed in which a composite partial substitution between a rare-earth metal and a Fe element and hydrogen absorption are combined by focusing on the effect of partial substitution of a La element with other rare-earth metals in which the entropy change is increased while the transition temperature is decreased.

Citation List

Patent Literature

Patent Literature 1: JP 3715582 B2
Patent Literature 2: JP 2009-221494 A
Patent Literature 3: JP 3967572 B2
Patent Literature 4: JP 5739270 B2
Patent Literature 5: JP 2015-517023 A

Non Patent Literature

Non Patent Literature 1: "Magnetic Refrigeration near Room Temperature" Magnetics Japan, Vol.1, No.7 (2006), p308-315
Non Patent Literature 2: Journal of Applied Physics vol. 113, pp. 17A908-1 - 17A908-3, 2013

Summary of Invention

Technical Problem

According to the alloy design of a magnetic material disclosed in Patent Literatures 4 and 5, an effect of suppressing the hydrogen redistribution obviously appears in a hydrogenated La(Fe,Si)₁₃ composite partial substitution material, but in the amount of magnetic entropy change, the value decreases to less than or equal to half of the value before the hydrogen redistribution measures are carried out. This is contrary to a result that the rare-earth metal partial substitution or the hydrogenation does not significantly affect the magnetic entropy change in a homogeneous sample. Regarding such degradation of the magnetic entropy change, a material synthesis method disclosed in Patent Literature 4 or 5 is a method different from industrially-used melting and solidifying, and as described in Patent Literature 4, it is considered that in a rare-earth metal partial substitution treatment, a part of a NaZn₁₃ crystal structural phase having a large magnetic entropy change is decomposed and is changed to a heterophase.
Therefore, the present inventors have applied a method of using the composite partial substitution and the hydrogenation together to a material that is single-phased to a NaZn₁₃ phase by a melting and solidifying method and a homogenization heat treatment, and have found that the magnetic entropy change is approximately the same as that of a material before a hydrogen redistribution suppression treatment, but the degree of hydrogen redistribution suppression is different for each material, and some hydrogen redistributions proceed at a high speed. It has been found that in such a material, the decomposition of the NaZn₁₃ phase due to the partial substitution is suppressed in advance to less than or equal to a certain amount by the homogenization heat treatment, and thus, there is a hydrogen redistribution occurrence factor due to reasons other than the single-phasing in the sample, and none of the disclosed technologies of the related art clarify and control such a factor.
The present invention has been made in consideration of such problems of the technologies of the related art, and an object thereof is to provide a magnetic material for magnetic refrigeration in which in a NaZn₁₃ single phase La(Fe,Si)₁₃-based magnetic material for magnetic refrigeration that is subjected to a homogenization heat treatment after melting and solidifying, a hydrogen redistribution can be suppressed and a large magnetic entropy change can be obtained.

Solution to Problem

In order to attain the object described above, according to the present invention, the following inventions are provided.

(1) A magnetic material for magnetic refrigeration containing a NaZn₁₃ compound represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x)₁₃H_w, in which a total amount of a coexistence phase other than the coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%, and in crystal particles of the NaZn₁₃ compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to a logarithmic normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm is less than 50%.
(In the formula described above, an amount of Si is 0.100 ≤ x ≤ 0.130, amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, and an amount of H is 1.4 ≤ w ≤ 1.7.)
(2) The magnetic material for magnetic refrigeration in the first invention described above, in which in the case of a magnetic field change equivalent to 0.5 tesla, an absolute value of an entropy change is greater than or equal to 15 J/kg·K.
(3) A magnetic material for magnetic refrigeration represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x-vAl_v)₁₃H_w, in which a total amount of a coexistence phase other than a coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%, and in crystal particles of the NaZn₁₃ compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to a logarithmic normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm is less than 50%.
(In the formula described above, an amount of Si is 0.100 ≤ x ≤ 0.130, amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, an amount of H is 1.4 ≤ w ≤ 1.7, and an amount of Al is 0 < v ≤ 0.030.)
(4) The magnetic material for magnetic refrigeration in the third invention described above, in which in the case of a magnetic field change equivalent to 0.5 tesla, an absolute value of an entropy change is greater than or equal to 15 J/kg·K.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a magnetic material for refrigeration in which in a NaZn₁₃ single phase La(Fe,Si)₁₃-based magnetic material for refrigeration that is subjected to a homogenization heat treatment after melting and solidifying, a hydrogen redistribution can be suppressed and a large magnetic entropy change can be obtained.

Brief Description of Drawings

Fig. 1 is a diagram illustrating magnetic entropy changes of magnetic materials that are obtained in Examples 1, 2 and 3 according to the present invention.
Fig. 2 is a diagram illustrating a reflected electron image of the magnetic material that is obtained in Example 2 according to the present invention.
Fig. 3 is a diagram illustrating a reflected electron image of a magnetic material that is obtained in Comparative Example 1 according to the present invention.
Fig. 4 is a diagram illustrating a thermomagnetic curve of a magnetic material that is obtained in Comparative Example 2.
Fig. 5 is a diagram illustrating a thermomagnetic curve of a magnetic material that is obtained in Comparative Example 3.
Fig. 6 is a diagram illustrating a thermomagnetic curve of a magnetic material that is obtained in Comparative Example 4.
Fig. 7 is a diagram illustrating a thermomagnetic curve of a magnetic material that is obtained in Comparative Example 5.
Fig. 8 is a diagram illustrating a thermomagnetic curve of the magnetic material that is obtained in Example 2.
Fig. 9 is a diagram illustrating a thermomagnetic curve of a magnetic material that is obtained in Comparative Example 6.
Fig. 10 is a diagram illustrating a thermomagnetic curve of a magnetic material that is obtained in Comparative Example 7.
Fig. 11 is a diagram illustrating crystal particle diameter distributions of the magnetic materials that are obtained in Example 2 and Comparative Examples 1 to 6.
Fig. 12 is a diagram illustrating a magnetic entropy change of the magnetic material that is obtained in Example 4.

Description of Embodiments

Hereinafter, embodiments for carrying out the present invention will be described.
A magnetic material for magnetic refrigeration of the present invention is configured of a NaZn₁₃ compound represented by General Formula La_1-yPr_y(Fe_1-x-zMn_zSi_x)₁₃H_w.
(In the formula described above, the amount of Si is 0.100 ≤ x ≤ 0.130, the amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, and the amount of H is 1.4 ≤ w ≤ 1.7.)
x, y, and z in general formula described above represents an excellent magnetic entropy change in the range described above.
In addition, a hydrogen redistribution is suppressed as the value of w is closer to a maximum value w_max in a condition of a room temperature and the atmospheric pressure, and thus, 1.4 ≤ w ≤ 1.7 is desirable.
As represented by the general formula described above, in the magnetic material for magnetic refrigeration of the present invention, La is partially substituted with Pr, and Fe is partially substituted with Mn, while a NaZn₁₃ structure is retained.
A phase transition temperature at which a magnetic entropy change increases is changed in accordance with the combination of w, x, y, and z, and thus, the combination can be adjusted in accordance with an object. In order to obtain a preferred entropy change without a concern for a hydrogen redistribution, in a temperature range from 0°C to a temperature directly higher than a room temperature, an amount y of Pr is preferably 0.1 ≤ y ≤ 0.3, and is more preferably 0.2 ≤ y ≤ 0.3. In addition, in consideration of increasing the magnetic entropy change that is also obtained by applying a smaller magnetic field, an amount x of Si is preferably 0.100 ≤ x ≤ 0.120, and is more preferably 0.105 ≤ x ≤ 0.110. Further, in consideration of an operation in a temperature range of higher than or equal to 0°C, an amount z of Mn is preferably 0.005 ≤ z ≤ 0.025, and is more preferably 0.010 ≤ z ≤ 0.020.
In addition, in the magnetic material for magnetic refrigeration of the present invention, a total amount of a coexistence phase other than the coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%. Such a magnetic material for magnetic refrigeration is extremely homogenized, and thus, is capable of improving a hydrogen redistribution suppression effect.
In addition, in the magnetic material for magnetic refrigeration of the present invention, in crystal particles of the NaZn₁₃ compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to a logarithmic normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of greater than or equal to 40 µm is less than 50%. In this case, even in magnetic materials for magnetic refrigeration having the same composition, it is possible to further improve the hydrogen distribution suppression effect. In a case where the average particle diameter is greater than 200 µm, the perimeter of a grain boundary separating the crystal particles increases, and a mechanical strength significantly increases, and thus, it is difficult to retain a bulk state after hydrogen is absorbed.
In the present invention, a crystal particle diameter is converted into a volume equivalent sphere radius by observing the crystal particle diameter with a metallographic microscope to obtain an image, and by performing binarization processing such that crystal particles and a crystal grain boundary are separated from each other while visually determining an observation image, for example, on a PC display with image software, and then, by creating a histogram of a circle radius equivalent particle diameter with automatic calculation processing of software. This will be described below in detail.
Further, in the magnetic material for magnetic refrigeration of the present invention, in the case of a magnetic field change equivalent to 0.5 tesla (T), it is preferable that an absolute value of an entropy change is greater than or equal to 15 J/kg·K.
The magnetic material for magnetic refrigeration of the present invention, for example, can be synthesized by the following procedure.

(Melting and Solidifying Procedure)

In addition to La, Fe, and Si, simple elements of each of Pr and Mn necessary for a partial substitution are weighed to be a predetermined composition, and then, are left to stand in a melting furnace, and are simultaneously melted and mixed. At this time, the aspect of each of the simple elements is not particularly limited, and the mode of the melting furnace is not also limited, but a rare-earth metal element such as La or Pr is likely to be selectively oxidized during the melting, and thus, it is preferable to provide a mechanism in which the air in the chamber can be evacuated to a vacuum higher than 10^-3 Pa, before the melting is started. In addition, Mn is easily evaporated during the melting, and thus, it is desirable that inert gas such as argon can be introduced to approximately 10^-1 MPa during the melting. A molten metal of an alloy after being melted and mixed is cooled to a room temperature, and is taken out as an alloy lump. At this time, in order to reduce a time for the subsequent homogenization heat treatment, it is sufficient to make a metal texture that is obtained in a melting and solidifying procedure fine, and in order for such an object, for example, it is preferable to perform the melting on a water-cooled hearth, and it is more preferable to have a structure in which the molten metal after the melting is poured into a water-cooled copper mold.

(Homogenization Heat Treatment)

The alloy lump that is obtained in the melting and solidifying procedure is heated in a vacuum, and is retained at a predetermined temperature for a constant time, and thus, an ingot of the NaZn₁₃ compound having extremely high single phase properties is obtained. At this time, a vacuum environment for a heat treatment is not particularly limited, and for example, the ingot put in a quartz tube, the air in the quartz tube is evacuated to 10^-3 Pa, and then, the quartz tube is sealed, and thus, an ampoule is prepared, and the ampoule can be arranged in a soaking area of an electric furnace. A total amount of a phase other than the NaZn₁₃ compound, remaining in a treated material that is finally obtained, depends on a retention temperature and a retention time, and the total amount affects the size of a magnetic entropy change of the treated material. For this reason, it is preferable that the predetermined temperature of the heat treatment is higher than or equal to 1100°C, and in the case of using the quartz tube as described above, it is preferable that the predetermined temperature is lower than or equal to 1250°C such that a vacuum sealing state can be retained. In addition, the NaZn₁₃ type compound exhibits a decomposition reaction referred to as a peritectic reaction between a single phase existing temperature range and a melting point, and thus, the single phase is not capable of being obtained at a temperature higher than 1200°C. Therefore, it is more preferable that the temperature of the heat treatment is higher than or equal to 1120°C and lower than or equal to 1180°C. Further, the present inventors have found that in the case of using Pr as a rare-earth metal for partially substituting La, as with the present invention, a peritectic reaction start temperature is at a higher temperature side, compared to the case of using Ce as with the example of the related art. For this reason, in the case of the magnetic material for magnetic refrigeration represented by the chemical formula described above, the temperature of the heat treatment can also be higher than or equal to 1130°C. In this case, it is possible to reduce a time for the heat treatment for obtaining the NaZn₁₃ compound having extremely high single phase properties to be within 24 hours.
In addition, even in a case where the temperature of the heat treatment is set to be higher than that of the peritectic reaction, it is possible to rapidly eliminate the segregation in each position of an artificial constituent element distribution generated in the melting and solidifying procedure that is the previous step. Then, the present inventors also have found that in the case of texture unevenness based on the decomposition reaction according to a state diagram of the peritectic reaction, similarly, a reverse reaction according to the state diagram can be attained by the heat treatment at a temperature lower than or equal to the temperature of the peritectic reaction, and thus, it is possible to eliminate non-equilibrated texture unevenness due to artificial segregation at a temperature higher than or equal to the temperature of the peritectic reaction, and it is possible to eliminate the texture unevenness due to the peritectic reaction that occurs simultaneously by subsequently connecting a heat treatment step at a temperature lower than or equal to the temperature of the peritectic reaction. For this reason, for example, a heat treatment for performing retention at 1160°C for 6 hours is performed in the first stage, and then, a heat treatment for performing retention at 1120°C for 12 hours is performed in the consecutive second stage, and thus, it is possible to obtain the NaZn₁₃ compound having extremely high single phase properties.
Further, it is preferable that the crystal particle diameter can be sufficiently increased in a condition where single-phasing sufficiently proceeds. The present inventors have found that in the case of the NaZn₁₃ magnetic material for magnetic refrigeration, the hydrogen redistribution is suppressed only in a case where crystal particle diameter has a distribution having a size greater than or equal to a certain value that is determined in a manner that depends on a constituent element, and thus, have conducted intensive studies about a condition for attaining a material to which such a distribution is applied. As a result thereof, it has found that as an aspect of the homogenization heat treatment, a material having desired properties can be obtained by a heat treatment at 1140°C for 24 hours.
Note that, a heat treatment condition for adjusting the single phase properties and the crystal particle size is not limited to the condition described above, and various known methods can be used.

(Hydrogenation)

As an example of hydrogenation, for example, the ingot that is obtained in the previous procedure is coarsely pulverized, a particle aggregate that is obtained is left to stand in a sealed chamber, and the air of the chamber is evacuated, and then, the temperature increases to 280°C. Pure hydrogen gas of 0.1 MPa is introduced into the chamber after the temperature reaches a predetermined temperature. In such a state, retention is performed for 12 hours, and thus, hydrogen is absorbed. In a hydrogenation method, the temperature or the pressure is not limited to the condition described above, and various known methods such as performing a heat treatment in a hydrogen airflow by a method other than the sealed chamber can be used.
As described above, a test piece of the magnetic material for magnetic refrigeration is synthesized.
In addition, according to the present invention, in order to adjust a temperature change profile of a magnetic entropy change ΔS_m by selecting the constituent element, the magnetic material for magnetic refrigeration is also capable of having the following composition.
A magnetic material for magnetic refrigeration containing a NaZn₁₃ compound represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x-vAl_v)₁₃H_w is characterized in that a total amount of a coexistence phase other than the coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%.
(In the formula described above, the amount of Si is 0.100 ≤ x ≤ 0.130, the amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, the amount of H is 1.4 ≤ w ≤ 1.7, and the amount of Al is 0 < v ≤ 0.030.)
x, y, and z in the general formula described above represent an excellent magnetic entropy change in the range described above.
In addition, the hydrogen redistribution is suppressed as the value of w is closer to the maximum value w_max in a condition of a room temperature and the atmospheric pressure, and thus, 1.4 ≤ w ≤ 1.7 is desirable.
Further, in the case of containing Al, as with Example 4 described below, a maximum value directly higher than a ferromagnetic transition temperature Tc is slightly lower than that of the magnetic material for magnetic refrigeration not containing Al, but a temperature range appears in which the maximum value gradually decreases with respect to an increase in the temperature, and the profile of a temperature change is close to a trapezoidal shape. Such a change is preferable since there is a case where in a configuration referred to as a cascade method in which materials having different Tc are arranged in multi-stage, in order to expand a refrigeration temperature width at the time of configuring a magnetic refrigeration machine, heat transfer properties are more easily adjusted than those of a peak-shaped change as with the magnetic material for magnetic refrigeration not containing Al.
As described above, the phase transition temperature at which the magnetic entropy change increases is changed in accordance with the combination of w, x, y, and z, and thus, the combination can be adjusted in accordance with an object. In order to obtain a preferred entropy change without a concern for a hydrogen redistribution, in a temperature range from 0°C to a temperature directly higher than a room temperature, the amount y of Pr is preferably 0.1 ≤ y ≤ 0.3, and is more preferably 0.2 ≤ y ≤ 0.3. In addition, in consideration of increasing the magnetic entropy change that is also obtained by applying a smaller magnetic field, the amount x of Si is preferably 0.100 ≤ x ≤ 0.120, and is more preferably 0.105 ≤ x ≤ 0.110. Further, in consideration of an operation in a temperature range of higher than or equal to 0°C, the amount z of Mn is preferably 0.005 ≤ z ≤ 0.025, and is more preferably 0.010 ≤ z ≤ 0.020.
In the magnetic material for magnetic refrigeration containing Al, in order to obtain a profile in which the profile of the temperature change is in a trapezoidal shape, it is preferable that the concentration of Al is a higher concentration, but in a case where v is a value greater than 0.030, in a substance amount ratio represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x-vAl_v)₁₃H_w, an antiferromagnetic component is mixed in a magnetic phase, and a magnetocaloric effect decreases, and thus, 0.005 ≤ v ≤ 0.030 is preferable, and in order to have both of the trapezoidal profile and the maximum value having sufficient ΔS_m, 0.010 ≤ v ≤ 0.020 is more preferable.
In addition, in the magnetic material for magnetic refrigeration, the total amount of the coexistence phase other than the coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%. Such a magnetic material for magnetic refrigeration is extremely homogenized, and thus, is capable of improving the hydrogen redistribution suppression effect.
In addition, in the magnetic material for magnetic refrigeration, in crystal particles of the NaZn₁₃ compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to the corresponding normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm is less than 50%. In this case, even in magnetic materials for magnetic refrigeration having the same composition, it is possible to further improve the hydrogen distribution suppression effect. In a case where the average particle diameter is greater than 200 µm, the perimeter of a grain boundary separating the crystal particles increases, and a mechanical strength significantly increases, and thus, it is difficult to retain a bulk state after hydrogen is absorbed.
In addition, in the magnetic material for magnetic refrigeration, in the case of a magnetic field change equivalent to 0.5 tesla (T), it is also preferable that an absolute value of an entropy change is greater than or equal to 15 J/kg·K.
A preparation procedure or the like of the magnetic material for magnetic refrigeration containing Al is the same as that described above.

Examples

Hereinafter, specific examples will be described, but the present invention is not limited to the examples.

Magnetic materials of Examples 1 to 3 in which the composition was adjusted were manufactured by the following procedures, and the magnetocaloric properties thereof were evaluated.

Example 1: La_0.7Pr_0.3(Fe_0.885Mn_0.005Si_0.110)₁₃H_1.5

Example 2: La_0.7Pr_0.3(Fe_0.875Mn_0.015Si_0.110)₁₃H_1.5

Example 3: La_0.7Pr_0.3(Fe_0.865Mn_0.025Si_0.110)₁₃H_1.5

(Melting and Solidifying Procedure)

A commercially available iron chip (manufactured by Kojundo Chemical Lab. Co., Ltd., a purity of 3N), a Mn chip (manufactured by Wako Pure Chemical Industries, Ltd., a purity of 3N), La and Pr metal chips (both are manufactured by NIPPON YTTRIUM CO., LTD., a purity of 2N), and a Si powder (manufactured by Kojundo Chemical Lab. Co., Ltd., a purity of 4N) were weighed with an accuracy of up to 1 mg order such that a substance amount ratio was the ratio of the chemical formula described above and total weight fell within a range of 9 g to 11 g, and were left to stand in a melting dent on a water-cooled hearth of an arc melting furnace (ACM-S01F, manufactured by DIAVAC LIMITED).
In such a state, a chamber of the arc melting furnace was sealed, and the air of the chamber was evacuated to 5 × 10^-3 Pa by an oil-diffusion pump, and then, Ar gas was introduced until the internal pressure of the chamber was approximately 0.08 MPa. Arc discharge was generated from an electrode rod facing the hearth in which a raw material was set, and an arc silver point was applied to a mother element group arranged in the dent on the hearth to be melted. It was visually checked that the raw material in the dent was completely melted from a light shielding window, and then, the arc discharge was stopped. The chamber was opened at a time point when the solidifying and cooling of a molten metal was completed, and an ingot was reversed and was set again in the melting dent on the hearth. The procedure from the air evacuation and the introduction of the Ar gas to the melting and solidifying, described above, was further repeated two times in the completely same condition such that the bias of the elements due to a difference between a cooling hearth surface and an arc heat input surface from the top did not occur. The surface of the ingot obtained as described above was accompanied by surface oxidation due to the remaining gas in an arc furnace during the melting, and thus, a portion in the vicinity of the surface was divided and removed with a wire cutter. The core ingot remaining approximately 80% at a weight ratio was further divided into chunks of 3 grams to 4 grams per one in order for the next homogenization heat treatment, from the ingot before being removed.

(Homogenization Heat Treatment)

According to a state diagram in terms of the composition described above, a mixed phase derived from a decomposition reaction referred to as a peritectic reaction exists in a temperature zone between a liquid phase and a NaZn₁₃ phase, and thus, a ratio of the NaZn₁₃ phase to the ingot immediately after the solidifying is approximately zero, and a non-equilibrated mixed state of a Fe-rich phase and a Larich phase is obtained. It is necessary to perform a heat treatment in order to homogenize the mixed phase to the NaZn₁₃ phase in an equilibrated state. As a result of considering that the treatment is ended in a short time as the temperature of the heat treatment is set to a higher temperature since the homogenization to an equilibrated phase is a diffusion phenomenon, the homogenization heat treatment of Examples 1 to 3 was set to 1160°C. As described below, such a temperature is a preferred temperature in a case where an element that partially substitutes Pr is La, and for example, in a case where the element that partially substitutes Pr is Ce, another consideration is required.
In a high-temperature heat treatment, a chunk-shaped sample that is obtained in the previous step was wrapped with a Mo foil having a thickness of 0.05 mm such that a rare-earth metal in a sample was not selectively oxidized, and was put in a quartz tube closed at one end (Momentive 214), and then, the air inside was evacuated to 5 × 10^-3 Pa, and then, an air evacuation port side was sealed, and thus, a vacuum ampoule was formed.
The vacuum ampoule that was prepared was left to stand in a muffle furnace, the temperature was increased to 1160°C that is the temperature of the heat treatment determined as described above for 2 hours from the start of a temperature increase, and then, retention was performed at the temperature of the heat treatment for 24 hours. After the retention for 24 hours, the temperature was decreased by turning off heating and a power distribution with respect to the furnace and by following the natural cooling of the furnace body. In the ampoule taken out from the furnace, the outer quartz tube was pulverized, and thus, a homogenized magnetic material was obtained.
In order to identify a compound phase contained in the obtained magnetic material, an extracted portion that was obtained by chipping approximately 1 g from each ingot was pulverized with a mortar to be a fine powder, and an X-ray diffraction profile was measured, but in any of Examples 1 to 3, no scattered peak was observed other than a scattering peak representing a NaZn₁₃ structure. Then, the surface was further polished, and thus, a reflected electron image of a scanning electron microscope SEM (manufactured by Hitachi Technologies, Ltd., Model Code: TM3000) was observed. The observation was performed at a magnification of 500 times for each example, and on average, an island-shaped contrast having a diameter of approximately several micrometers was observed in one of four independent visual fields, and this was identified as a coexistence phase containing more rare-earth elements than the NaZn₁₃ phase, by an SEM-affiliated energy dispersive X-ray analyzer (manufactured by Bruker, Model Code: Quantax 700). Therefore, in the case of calculating a ratio of the coexistence phase from an area ratio, the average of three types of samples was about 0.1 volume%.

(Hydrogenation)

The ingot that was obtained in the homogenization heat treatment was coarsely pulverized in an agate mortar, and passed through a sieve having a standard opening size of 500 micrometers defined in JISZ8801 (1982), and powder-particles remaining in the sieve having an opening of 250 micrometers were collected and were subjected to hydrogenation. About 1.2 g of the sample of each of the examples was wrapped with an aluminum foil into the shape of a strip such that the particles were spread in the plane of the strip, and the particles were arranged not to overlap with each other in a direction perpendicular to the plane. This is because when the heat treatment in a hydrogen atmosphere is performed, hydrogen molecules are easily penetrated through the aluminum foil, but in a case where the particles overlap with each other, the exposed surface of hydrogen decreases. A packet that was prepared as described above was placed on a stainless steel plate boat provided in the center of a stainless steel furnace core tube (a length of 80 cm and a diameter of 5 cm), the furnace core tube was sealed, and the air was evacuated with a molecular turbopump. At this time, the attained degree of vacuum is about 5 × 10^-3 Pa. While the air evacuation was continued, the output of a tubular furnace that was provided outside the tube in the vicinity of the center of the stainless steel furnace core tube and was concentric with the furnace core tube was started, and a temperature increase was raised. After checking that a thermometer provided in the vicinity of the sample reached 180°C in approximately 2 hours from the start of the temperature increase, retention was performed at the temperature for about 1 hour such that a heat distribution in the furnace was homogeneous. After that, the air evacuation was stopped, and hydrogen gas was introduced into the furnace core tube from a gas introduction port until the pressure in the furnace was the atmospheric pressure (a gauge pressure of 0) in the display of an affiliated Bourdon gauge. At this time, the temperature in the furnace that slightly increases due to heat conduction in the furnace may be stabilized again by PID control for a heater, and an initial reaction between the sample and hydrogen may occur, and thus, a hydrogen pressure slightly decreases for approximately 5 minutes immediately after hydrogen is introduced. In a case where such a decrease subsided, the furnace core tube was replenished with hydrogen and was set again to the atmospheric pressure. In such a state, the heat treatment was continued for 12 hours, a valve connecting a suction port of a rotary pump that was operated in advance and the inside of the furnace core tube was opened for 5 seconds to discharge hydrogen gas, after a set time elapsed, and then, atmospheric air leakage was performed, and then, a flange was taken out by closing the furnace core tube. A sample vessel connected to the flange was taken out from the furnace for each of the samples, and a sample packet was put in liquid hydrogen stored in a resin vessel and was instantaneously cooled from the temperature of the heat treatment to a room temperature. This is because in the procedure after the elapse of a heat treatment time, in a case where the environment temperature around the sample is changed for a long time, a change in the total amount of the concentration of hydrogen or a concentration deviation for each location may occur.
The amount of absorbed hydrogen was determined from the weight of the sample before and after being absorbed. From the literature (J. Appl. Phys. vol. 102, pp. 023907:1-5, 2007), in (La,Pr)(Fe,Si)₁₃H_w, a maximum value w_max of w stable at an ordinary temperature and the atmospheric pressure is 1.6, but in this example, the estimated value was 87% thereof.

(Evaluation of Magnetocaloric Effect)

In the samples of Examples 1 to 3, obtained as described above, magnetization measurement was performed, and a magnetic entropy change ΔS_m was obtained by a Maxwell relation described below. $Δ S_{m} = μ_{0} \int_{0}^{H} \frac{\partial M}{\partial T} dH$
In the formula, H is a magnetic field, and T is a temperature. In addition, MPMS manufactured by Quantum Design, Inc. was used in the measurement of magnetization M. The efficiency of magnetic refrigeration is excellent as a large value of ΔS_m is obtained with a small change in H. In the graph of Fig. 1, a temperature change of ΔS_m that is estimated from measurement results of the samples of Examples 1 to 3 is illustrated. Note that, a change in a magnetic field H that is necessary for generating ΔS_m is equivalent to a change in an external magnetic flux density (µ₀H) of 0 T to 0.5 T. From the graph, a portion in which ΔS_m rises steeply can be defined as a magnetic phase transition temperature Tc, and Tc = 33°C in Example 1, Tc = 14°C in Example 2, and Tc = -6°C in Example 3 can be estimated. Refrigeration capacity at the time of being mounted on a refrigeration machine increases as ΔS_m increases, but in a case where a magnetic field change that generates ΔS_m is large, the value also increases. However, it is not easy to repeatedly generate a magnetic field change of approximately a maximum magnetic flux density µ₀H = 1 T at an operation frequency of the refrigeration machine, and thus, it is considered that a reciprocating motion of a permanent magnet is preferable, and in this case, a special magnetic circuit is necessary for generating a magnetic flux density change of greater than or equal to 1 T, and both the size and the cost increase. On the other hand, in a case where a large value is obtained at a change smaller than µ₀H = 1 T, a magnet volume is reduced from 1/5 to 1/10. Therefore, unless ΔS_m in a magnetic flux density region of a small magnet increases, it is meaningless even in a case where a value at a larger magnetic flux density increases. In this regard, the results of Examples 1 to 3 represent more preferred properties in a region of 0.5 T than other materials that are expected to be used in the same temperature range. For example, in the magnetic refrigeration material described in JP 5739270 B2 , the magnetic entropy change ΔS_m obtained at a magnetic flux density change of 0 T to 1.98 T is not greater than 13 J/kg·K at any transition temperature, but even in the results of Example 3 having the lowest value among the values of the examples, the magnetic entropy change ΔS_m reaches 17 J/kg·K at a magnetic flux density change of 0 T to 0.5 T. In addition, in the example described in JP 2015-517023 A , in all examples in which the rare-earth metal and Mn are subjected to composite substitution, even in the case of applying a magnetic flux density change at a maximum of 2 T, the magnetic entropy change ΔS_m is not greater than 18 J/kg·K, and this value is inferior to 19 J/kg·K that is the value in the case of the magnetic flux density change of 0.5 T in Example 1. In addition, in particular, in the vicinity of 0°C, in the example of the literature described above, even in the case of the magnetic flux density change of 2 T, ΔS_m = 7 J/kg·K is obtained, and thus, it is checked that the value at the magnetic flux density of 0.5 T in Example 3 having a similar transition temperature is greater than or equal to 2 times the value in the example of the literature described above.

Comparative Example 1 having the following composition was prepared by the same procedure as that of the melting and solidifying procedure and the homogenization heat treatment of Example 2.

Comparative Example 1: La_0.8Ce_0.2(Fe_0.875Mn_0.015Si_0.110)₁₃

In Comparative Example 1, the same observation as that of the texture identification using an SEM that was performed immediately after the homogenization heat treatment of Example 2 was performed for comparison. Fig. 2 is a metal texture immediately after the homogenization heat treatment of Example 2, and Fig. 3 is a metal texture immediately after the homogenization heat treatment of Comparative Example 1. Fig. 2 illustrates a gray texture that was uniformly formed except for the contrast of voids due to a defect of a surface, and it was checked that the gray texture was the NaZn₁₃ phase from the results of X-ray diffraction and SEM-affiliated EDX measurement that were separately performed with respect to the same sample. On the other hand, in the texture of Comparative Example 1 of Fig. 3, a black island-shaped texture surrounded by a white band-shaped texture was observed in addition to the gray texture. From the results of EDX, it was found that the black texture contained Fe at a high concentration, and the white texture had a composition in which a substance amount ratio of rare-earth elements, Fe, and Si is approximately the same. In the literature (Intermetallics vol. 20, 2012, pp. 160-169), it is reported that in a case where the temperature is higher than or equal to a peritectic temperature, La(Fe_0.89Si_0.11)₁₃ before partial substitution is performed in La and Fe sites is decomposed to bccFe and a LaFeSi compound by a peritectic reaction. That is, it is found that in Example 2 in which the homogenization heat treatment is performed at 1120°C, the peritectic reaction does not occur, but in Comparative Example 1 in which the heat treatment is performed at the same temperature, the peritectic reaction starts to occur, and thus, it is difficult to perform homogenization. In the case of La(Fe_0.89Si_0.11)₁₃ before the partial substitution is performed in the La and Fe sites, the peritectic temperature is about 1170°C, and thus, in the case of a composition containing Ce as with Comparative Example 1, the peritectic temperature decreases. On the other hand, from the texture observation of Example 2, in a case where La was partially substituted only with Pr, the influence on the peritectic temperature was small, and the heat treatment was capable of being performed at a high temperature, compared to a system containing Ce.
The homogenization is a procedure in which a texture that is peritectic-separated during solidifying from a liquid phase is adjusted to a NaZn₁₃ single phase that is an equilibrated phase, and a diffusion rate is limited. That is, in a case where the heat treatment is performed at the highest temperature, the homogenization can be attained in a short time. A high-speed heat treatment is advantageous not only in the suppression of the energy cost required for the heat treatment, but also in the acceleration of the homogenization in consideration of an increase in technically associated inhibition of single-phasing when the heat treatment time is extended, such as selective oxidization of a rare-earth metal due to oxygen remaining in the atmosphere. It was checked that Examples 1 to 3 illustrated in Fig. 1 exhibited a magnetocaloric effect of higher properties than other examples described in the patent literatures ( JP 5739270 B2 and JP 2015-517023 A ) since high homogenization was attained and a phase change proceeded mercurially.

Comparative Examples 2, 3, 4, and 5 in which the element and the composition were adjusted as follows were prepared by the same procedure as that of Examples 1 to 3, except that the temperature of the homogenization heat treatment and the heat treatment time were different, and the degree of suppression of a hydrogen redistribution phenomenon was compared with those of Example 2.

Comparative Example 2: La_0.8Ce_0.2(Fe_0.875Mn_0.015Si_0.110)₁₃H_1.5

Comparative Example 3: La_0.8Ce_0.2(Fe_0.879Mn_0.011Si_0.110)₁₃H_1.5

Comparative Example 4: La_0.8Ce_0.2(Fe_0.879Mn_0.011Si_0.110)₁₃H_1.5

Comparative Example 5: La_0.8Ce_0.2(Fe_0.879Mn_0.011S_0.110)₁₃H_1.5

Comparative Example 2 was obtained by the same preparation method as that of Example 2, except that in the homogenization heat treatment, the temperature of the heat treatment was 1090°C. Comparative Example 3 was obtained by the same preparation procedure as that of Comparative Example 2, except that the composition was different. Comparative Examples 4 and 5 were obtained by the same procedure as that of Comparative Example 3, except that the heat treatment time was 48 hours and 60 hours, respectively.
Further, Comparative Examples 6 and 7 having the following compositions were prepared by the same procedure as that of Example 2, except that the temperature of the homogenization heat treatment was different.

Comparative Example 6: La_0.7Pr_0.3(Fe_0.875Mn_0.015Si_0.110)₁₃H_1.5

Comparative Example 7: La_0.7Pr_0.3(Fe_0.875Mn_0.015Si_0.110)₁₃H_1.5

In the preparation of Comparative Examples 6 and 7, the temperature of the heat treatment in the homogenization heat treatment was set to 1125°C and 1130°C, respectively, and the homogenization heat treatment was performed for 24 hours.
In order to find out the hydrogen redistribution behavior of the samples, magnetization measurement was performed by the following procedures.

(Measurement of Initial Curve)

The sample was set in a magnetization measurement apparatus (MPMS, manufactured by Quantum Design, Inc.), was cooled to a temperature slightly lower than the phase transition temperature, and then, the magnetization measurement was performed while the temperature was increased by applying a magnetic field equivalent to a magnetic flux density of 0.1 T. A curve that was obtained was set to an initial curve, and the measured value of the phase transition temperature was determined. After the measurement, the applied magnetic field was rapidly returned to 0.

(Phase Transition Temperature Annealing)

In order to cause the hydrogen redistribution, a sample space temperature of MPMS was set to be coincident with the phase transition temperature.

(Thermomagnetic Measurement for Progress Determination)

After a certain period of time, the sample was cooled again to a temperature lower than or equal to the transition temperature, and then magnetization measurement was performed while the temperature was increased by applying the magnetic field equivalent to the magnetic flux density of 0.1 T. In the case of further performing annealing continuously after the measurement, the magnetic field was returned to 0, and then, the sample space temperature of MPMS was set again to be identical to the phase transition temperature.
The transition temperature annealing and the thermomagnetic measurement for progress determination were alternately repeated while the progress of the hydrogen redistribution was continuously observed. A total transition temperature retention time t_h is the accumulation of time spent for the transition temperature annealing, and a time in which temperature sweep is performed by the thermomagnetic measurement is not included as a factor not affecting the hydrogen redistribution. A start time of the transition temperature annealing is a time when an MPMS sample space is coincident with the transition temperature, and thus, is stable for the first time, by regarding the initial curve as a curve of t_h = 0.
Fig. 4 is a thermomagnetic curve of t_h = 0 hours and 12 hours that was measured with respect to Comparative Example 2 by the procedure described above. In the examples described in JP 2012-41631 A or JP 2015-517023 A , as with Comparative Example 2, in La(Fe_xSi_1-x)₁₃H_1.5, the composite substitution of Ce and Mn was not performed, and the hydrogen redistribution was remarkably suppressed, but from the results of Fig. 4, the hydrogen redistribution was not stopped only by the combination of elements or compositions. In order to check the influence of a difference in a substance amount ratio of Ce and Mn, a thermomagnetic curve was also measured by the same procedure in Comparative Example 3 that was prepared by the same method as that of Comparative Example 2, except that the composition was different, but as illustrated in Fig. 5, even in the hydrogen redistribution treatment of t_h = 12 hours, a change in the thermomagnetic curve was started to be seen. Therefore, Fig. 6 illustrates a thermal magnetization curve when the hydrogen redistribution heat treatment was performed with respect to Comparative Example 4 that was prepared by using the same composition as that of Comparative Example 3, and by extending the heat treatment time to 48 hours. A curve of t_h = 18 hours is largely coincident with the initial curve of t_h = 0, but a change in the magnetization is obviously seen at a temperature directly lower than the transition temperature, and thus, the hydrogen redistribution is gradually started to occur. In Comparative Example 5 in which only the heat treatment time was further extended to 60 hours, as illustrated in Fig. 7, it was determined that a curve after t_h = 30 hours was coincident with the initial curve in an accuracy error range of the magnetization measurement, and in order to attain the suppression of the hydrogen redistribution, it was necessary to perform the homogenization heat treatment with respect to a product under test that was prepared by adding Ce, for an extremely long time.
In Example 2, the degree of suppression of the hydrogen redistribution phenomenon was evaluated by the same procedure, and thus, as illustrated in Fig. 8, it was checked that there was no difference in the curve exceeding the error in the magnetization measurement even after 42 hours, and the hydrogen redistribution was considerably suppressed. However, in Comparative Examples 6 and 7, as illustrated in each of Fig. 9 and Fig. 10, the steepness of the transition itself was low, and even in the treatment of t_h = 12 hours, the magnetization curve was changed, and thus, the hydrogen redistribution occurred. In Comparative Examples 6 and 7 in which the temperature of the heat treatment was set to be a temperature higher than that of Example 2, and the homogenization heat treatment was performed for 24 hours, the hydrogen redistribution was suppressed compared to Comparative Examples 2 and 5 in which the heat treatment time was the same, but a change in the thermomagnetic curve was seen even at t_h = 12 hours, and the degree of suppression was slightly insufficient, compared to Example 2.
In Comparative Examples 6 and 7, an increase in a heterophase is not also seen from the X-ray diffraction and the SEM observation. In addition, in any of Comparative Examples 2 to 5 in which the product under test contains Ce, there is no difference in single phase properties at the level of the X-ray diffraction and the SEM observation, and it is not possible to describe the reason that the heat treatment for a long time is required to suppress the hydrogen redistribution phenomenon in the product under test only by using the homogeneity as an index. Therefore, the evaluation was performed by focusing on the crystal particle diameter as a metallurgical factor related to the temperature of the heat treatment or the heat treatment time.

In addition to Example 2, Comparative Examples 2, 3, 4, 5, 6, and 7 were set as a target material. A method used in the evaluation of the crystal particle diameter is as follows.

(Surface Polishing)

Distilled water suspended in an alumina powder for polishing was dropped on a rotating buff disk, and the surface of the sample was subjected to mirror polishing.

(Sample Surface Etching)

The surface of the sample that was subjected to the mirror polishing was dipped in a solution in which a nitric acid that is a commercially available reagent and ethyl alcohol were mixed at a volume ratio of 1 : 8, for 5 seconds, and was lifted, and then, immediately, was washed with water and rinsed with ethyl alcohol.

(Microscope Observation)

A simple polarizing filter was selected such that the crystal particle diameter was capable of being observed in a viewing field of an eyepiece, by using a metallographic microscope (Eclipse LV150, manufactured by Nikon Corporation), and an image that was observed was digitally converted and stored by a CCD camera that was provided at a position where the optical axis of a lens barrel was cut.

(Image Analysis)

The observation image that was digitized was subjected to binarization processing such that crystal particles and a crystal grain boundary were separated from each other while being visually determined on a PC display, by using image analysis software (WinROOF2015, manufactured by Mitani Corporation), and after that, a histogram of a circle radius equivalent particle diameter was created by automatic counting processing of the software.

(Numerical Value Analysis)

The histogram that was obtained as described above has a two-dimensional shape obtained by cutting a three-dimensional cube at an arbitrary plane, and thus, it is necessary to convert the histogram into a volume equivalent sphere radius. For this reason, statistics of spreadsheet software was converted into a histogram of the volume equivalent sphere radius by using a polyhedral composite model method (Kiyotaka MATSUURA, Doctoral Dissertation of Hokkaido University (1993)). Further, least square fitting was performed by considering that the crystal particles distribution followed the logarithmic normal distribution, and thus, a logarithmic normal type crystal particle diameter distribution function was determine.
A histogram distribution of the volume equivalent sphere radius and the particle diameter distribution function of the crystal particles that are obtained by the procedure described above are illustrated in Fig. 11. In Comparative Examples 3 to 5 in which the partial substitution of Ce was performed, it was checked that in a case where the heat treatment time increased, the average particle diameter and the minimum particle diameter increased even at the same composition and the same homogenization heat treatment temperature, but in a Ce partial substitution system, it was checked that in a case where the average particle diameter was greater than or equal to 50 µm and the minimum particle diameter was greater than or equal to 30 µm, the hydrogen redistribution was not suppressed, compared to the degree of hydrogen redistribution that is determined from Fig. 8. On the other hand, in Example 2 in which the hydrogen redistribution was significantly suppressed, it was checked that the value of the average particle diameter that was obtained by applying the logarithmic normal distribution was 41 µm, and the hydrogen redistribution was suppressed even at the average particle diameter smaller than that of the comparative example containing Ce. From the comparison between Example 2 and Comparative Examples 6 and 7, it is determined that the hydrogen redistribution can be suppressed in a case where the average particle diameter is greater than or equal to 40 µm and a cumulative distribution in a particle diameter greater than the average particle diameter is greater than or equal to 50%.

In the magnetic material for magnetic refrigeration, the temperature change profile of the magnetic entropy change ΔS_m can also be adjusted by selecting the constituent element. In Example 4, a raw material was adjusted as with the following chemical formula, and thus, a test piece was prepared. A preparation procedure is the same as that of Example 2, except that a substance amount ratio of the raw material is different.

Example 4: La_0.7Pr_0.3(Fe_0.875Mn_0.015Si_0.095Al_0.015)₁₃H_1.5

Fig. 12 illustrates temperature dependency of ΔS_m that was obtained in the case of a magnetic field change equivalent to a magnetic flux density 1.5 T, by the same procedure as the evaluation of the magnetocaloric effect described above, in Example 4. The results of Example 2 evaluated in the same condition are also illustrated for comparison. In Example 2, a steep maximum value peak appears at a temperature directly higher than Tc, and then, rapidly decreases along with an increase in the temperature. On the other hand, in Example 4, the maximum value at a temperature directly higher than Tc slightly decreases, compared to Example 2, but a temperature range appears in which the maximum value gradually decreases with respect to an increase in the temperature, and the profile of a temperature change is close to a trapezoidal shape. Such a change is preferable since there is a case where in a configuration referred to as a cascade method in which materials having different Tc are arranged in multi-state, in order to expand a refrigeration temperature width at the time of configuring a magnetic refrigeration machine, heat transfer properties are more easily adjusted than those of a peak-shaped change as with Example 2. In order to obtain a trapezoidal profile, it is preferable that the concentration of Al is a higher concentration, but in a case where v is a value greater than 0.030, in a substance amount ratio represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x-vAl_v)₁₃H_w, an antiferromagnetic component is mixed in a magnetic phase, and the magnetocaloric effect decreases, and thus, 0.005 ≤ v ≤ 0.030 is preferable, and in order to have both of the trapezoidal profile and the maximum value having sufficient ΔS_m, 0.010 ≤ v ≤ 0.020 is more preferable.
In the magnetic material for magnetic refrigeration, a total amount of a coexistence phase other than a coexisting NaZn₁₃ compound was less than or equal to 0.1 volume%, and in crystal particles of the NaZn₁₃ compound, the average particle diameter at the time of applying the spherical volume equivalent particle diameter distribution to the logarithmic normal distribution was greater than or equal to 40 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm was less than 50%. In addition, in the case of a magnetic field change equivalent to 0.5 tesla, an absolute value of an entropy change was greater than or equal to 15 J/kg·K.

Claims

A magnetic material for magnetic refrigeration containing a NaZn₁₃ compound represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x)₁₃H_w, wherein a total amount of a coexistence phase other than the coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%, and in crystal particles of the NaZn₁₃ compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to a logarithmic normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm is less than 50%, and
in the formula described above, an amount of Si is 0.100 ≤ x ≤ 0.130, amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, and an amount of H is 1.4 ≤ w ≤ 1.7.
The magnetic material for magnetic refrigeration according to claim 1, wherein in the case of a magnetic field change equivalent to 0.5 tesla, an absolute value of an entropy change is greater than or equal to 15 J/kg·K.
A magnetic material for magnetic refrigeration represented by La_1-yPr_y(Fe_1-x-zMn_zSi_x-vAl_v)₁₃H_w, wherein a total amount of a coexistence phase other than a coexisting NaZn₁₃ compound is less than or equal to 0.1 volume%, and in crystal particles of the NaZn₁₃ compound, an average particle diameter at the time of applying a spherical volume equivalent particle diameter distribution to a logarithmic normal distribution is greater than or equal to 40 µm and less than or equal to 200 µm, and a cumulative probability of a particle diameter distribution having a spherical volume equivalent particle diameter of less than or equal to 40 µm is less than 50%, and
in the formula described above, an amount of Si is 0.100 ≤ x ≤ 0.130, amounts of Pr and Mn are 0 < y ≤ 0.4 and 0 < z ≤ 0.030, an amount of H is 1.4 ≤ w ≤ 1.7, and an amount of Al is 0 < v ≤ 0.030.
The magnetic material for magnetic refrigeration according to claim 3, wherein in the case of a magnetic field change equivalent to 0.5 tesla, an absolute value of an entropy change is greater than or equal to 15 J/kg·K.