JPH11238615A - Magnetic material for magnetic cooling and magnetic cooling device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic cooling device, in which CFC gas need not be used at 100-400 K and which utilizes adiabatic demagnetization, and a magnetic material for magnetic cooling useful for the magnetic cooling device. SOLUTION: In this magnetic material for magnetic cooling, a composition is represented by R1-x AxMnO3 (R is represented by at least one kind or more from among La, Pr, Nd and Sm and A by Sr and/or Ca) at an atomic ratio and 0.1<=X<=0.6, and has a perovskite structure, and the magnetic material is composed of a ferromagnetic material having a Curic temperature of 200-400 K.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic material for cooling used in a cooling device which does not require Freon gas, and a magnetic cooling device using the same.

[0002]

2. Description of the Related Art In recent years, it has been increasingly recognized that ordinary refrigerators and air conditioners are important obstacles to human survival. This is because most of such equipment is freon (CFC).
s), also known as chlorofluorocarbons), which escape into the atmosphere during leaks, repairs, or disposal, and when they reach the stratosphere, attack and destroy the ozone layer, which blocks the ultraviolet rays of sunlight. As a result, the amount of irradiation of the ground surface with ultraviolet rays is rapidly increased, and there is a concern that adverse effects on human health may occur. Thus, the preservation of the stratospheric ozone layer and anticipation of global warming are driving the ban on freon use. For example, cooling devices such as refrigerators and air conditioners in which Freon is sealed are widely used, and have a great influence on the global environment. For this reason, it is essential to completely ban freons, and there is a demand for a cooling medium or a cooling device that can replace a freon-based cooling device.

[0003] An adiabatic demagnetization system using a magnetic material as one of the alternative devices can be considered. For example, a method of adiabatic demagnetization using a paramagnetic salt (for example, iron alum) which has been known for a long time as one of the cryogenic technologies can be considered. The reason why adiabatic demagnetization is used in the low-temperature technique for measuring physical properties near absolute zero is that, first, the entropy of the magnetic material in this region has little contribution from lattice vibration and mainly depends on spin. Because. Second, the spin entropy of a paramagnetic material has the largest magnetic field dependence at absolute zero. The latter suggests that if a ferromagnetic material having a finite Curie temperature is used, adiabatic demagnetization can be expected in a higher temperature region (for example, a room temperature region) than an extremely low temperature region. The former works in the opposite direction, but the realization of a strong magnetic field with high-performance magnets and superconducting magnets in recent years has further promoted the possibility of adiabatic demagnetization in a high-temperature region.

[0004] The idea of magnetically cooling or raising the temperature in a higher temperature region using a ferromagnetic material is Edison (1887) or Te.
sla (1890), and in 1976 a study on the magneto-thermal effect of Gd metal was reported as a candidate for adiabatic demagnetization (magnetization) in the room temperature range. However, the literature (GV Brown: J. Appl. Phys. 47 (1976), p3673)
According to the above, even in the case of Gd metal, even when a magnetic field of 1 T (tesla) is used, the temperature change is only 1.5 ° C.
Therefore, a magnetic field of at least several T is required for use as a cooling device such as an air conditioner or a refrigerator in a room temperature range, and practical use is difficult.

[0005]

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, an object of the present invention is to provide a magnetic cooling device utilizing adiabatic demagnetization which does not require the use of freon gas at 100 to 400K, and a magnetic cooling magnet useful therefor. Is to provide the material.

[0006]

According to theoretical considerations, the present inventor has found that in order to obtain a larger temperature change in adiabatic demagnetization in the room temperature range, the Debye temperature is high and the Curie temperature (Tc) is close to room temperature. It is important to use a substance having a large magnetic susceptibility above Tc, and furthermore, if the above conditions are satisfied and a plurality of substances having slightly different Tc are operated in order from the one with the larger Tc, a large temperature drop will occur. The inventors have found that they can be obtained, and have reached the present invention. Hereinafter, the present invention will be described in detail.

Adiabatic demagnetization using a magnetic material is understood by a process as shown in FIG. First, a strong magnetic field is applied to the magnetic material at the temperature T1 to reduce thermal oscillation of spin.
In this case, since the magnetic order of the system increases, if this process is performed isothermally, the entropy of the system decreases (FIG. 1).
(A)). Here, when the magnetic field is removed, the spins try to be free again. However, when this process is performed adiabatically (dQ = 0 when the heat quantity is Q), the entropy S is kept constant (dQ = 0 by dS = dQ / T).
In the case of dS = 0), the thermal oscillation of the spin is kept suppressed. Because spin keeps order state at zero magnetic field,
The system becomes cold (FIG. 1 (b)). Conversely, when the process of applying the magnetic field is performed adiabatically, the temperature rises from the condition of constant entropy corresponding to FIG. From the above, if the correlation between the entropy S and the temperature T in each magnetic field is theoretically known, the temperature change in the adiabatic demagnetization process can be predicted. Generally, the entropy S is obtained from the specific heat C by using the equation (1).

[0008]

(Equation 1) Here, the specific heat C is generally spin, lattice vibration (phonon)
And three parts of conduction electrons.

In the case where a spatially localized electron (in this case, an f-electron) is responsible for the magnetic moment in a magnetic material such as Gd metal, the molecular field approximation is not so bad for treating magnetic properties at a finite temperature. Under the molecular field approximation, the average magnetization per site is given by m = gμ _B σ (2) (g: Lande factor, μB: Bohr magneton).
σ is a Brillouin function: B _S (x) = (2S + 1) / 2S · coth (x (2
Σ = S · B _s (x) (3) using S + 1) / 2S) −1 / 2S · coth (x / 2S). S is the spin angular momentum at each site. x is
x = (λσ + gμ _B H) S / K _B T (4) In equation (4), k _B is the Woltzmann constant, T
Is the temperature, H is the applied magnetic field, λ is the molecular field parameter,
If the Curie temperature Tc is known, is determined from _{λ = (3k B Tc) /} S (S + 1) (5). Once the magnetization m (ie, σ) is determined, the internal energy US = −λ
Using [sigma] < ² > / 2-g [mu] _B H [sigma], the spin part Cs of specific heat can be obtained from Cs = dUs / dT (6). Formula Ss (6) as a spin portion S _S entropy by substituting the equation _{(1) = k B (ln} (sinh (x (2S + 1) / 2S) / sinh (x / 2S)) - xBs ( x)) (7) is obtained.

FIG. 2 shows the Gd metal (S =
(7/2, Tc = 293K) is the temperature dependence of the spin entropy per ion. At H = 0, T =
It bends at Tc, and when T ≧ Tc, S _S / k _B = ln (2S +
1) = 2.079 constant value. This is because when T ≧ Tc, the system becomes non-magnetic (σ = 0) and the spin state becomes 2S + 1.
= 8-fold degeneration. As H increases, the fluctuation of the spin is suppressed, and S _S decreases. Here, assuming that the system is composed only of spins, T
When ≧ Tc, the entropy at H = 0 takes a constant value. Therefore, when the process of FIG. 1 is applied, the temperature change due to adiabatic demagnetization in this temperature region becomes T−0 regardless of any magnetic field of H ≠ 0.
It is expected to be Tc or more. That is, for any magnetic field (if finite), the final temperature will be below Tc. However, considering phonons (lattice vibrations), the temperature change is much smaller. Because if the temperature is reduced by turning off the magnetic field, the spin system increases the spin entropy by removing heat from the phonons, and instead reduces the phonon entropy and tries to keep the total entropy constant. Therefore, when there is a phonon, the arrow moves to the upper left in the adiabatic process of FIG. 1B, so that the temperature change can be kept small.

FIG. 3 shows the temperature dependence of the total entropy obtained by using the Debye model for the phonon specific heat Cp and using the Debye temperature θ _D = 172 K of the Gd metal, which is in good agreement with the experimental results. . Compared to FIG. 2, H =
It can be seen that even at 0, the entropy has a finite slope when T ≧ Tc, and the slope further increases when H ≠ 0. Therefore, it can be seen that the temperature change due to adiabatic demagnetization is smaller than in the case of only the spin system, and the temperature change is about 10 ° C. even in a high magnetic field of H = 70 kOe.
This is mainly because the phonon number decreases (the phonon entropy decreases) and the spin fluctuation increases (the spin entropy increases due to the increase in the magnon number) as the temperature decreases.
It is due to. Here, phonon specific heat is low (T "rises in proportion to T ³ in theta _D), saturated with T～shita _D vicinity, T" theta classical value of Dulong-Petit in _D (3
k _B ). Therefore, the contribution of phonons to the specific heat and entropy higher Debye temperature theta _D is large material becomes smaller, the temperature change due to adiabatic demagnetization increases. Also,
This temperature change increases as the change in spin entropy due to the application of a magnetic field increases. Therefore, it is desired that the magnetic susceptibility in the paramagnetic state of T> Tc be high. From the above considerations, a material that causes a large temperature change by adiabatic demagnetization in the room temperature region is a ferromagnetic material having a Tc near room temperature, a substance having a large paramagnetic susceptibility, and a high Debye temperature θ _D. You can see that. Here, assuming that θ _D is the melting point T _M , molecular weight M, and molar volume V of the substance, θ _D = C / V ^1/3 × (T _M / M) ^1/2 (8) from Lindemann's relational expression. Express. c is a constant, about 200 for non-metals and about 137 for metals. From equation (8), a large θ _D is expected to be realized in a nonmetal having a high melting point. Generally, oxides have a high melting point. Therefore, θ _D is often large. From this, a large adiabatic demagnetizing effect is expected for an oxide ferromagnetic material having Tc near room temperature. The present inventors have paid attention to the fact that a certain kind of perovskite-type oxide satisfies the above conditions, and reached the present invention.

Generally, an oxide magnetic material containing a transition metal often exhibits antiferromagnetism or ferrimagnetism.
In the case of a group of perovskite oxides of RMnO ₃ (R is at least one kind of rare earth element) represented by nO ₃ , a part of R is replaced with Ca or Sr to become a ferromagnetic material. The present inventors have noticed that Tc of this substance can be accurately controlled in the range of about 200 to 400 K across room temperature by substitution with Ca or Sr, and that the magnetic susceptibility is extremely large above Tc. Figures 4 (a) and 4 (b) show Tokura et al.
kura, A. Urushibara, Y. Moritomo, T. Arima, A. Asami
tsu, G.Kido, andN.Furukawa: J.Phys.Soc.Jpn.vol.63 (199
4), La1 _-x Sr _x MnO ₃ (X
= 0.175) and T = 304K
3 shows a magnetization curve at. Tc is about 293K which is the same as Gd
Asked. The solid line in the figure is a calculation result by molecular field approximation obtained by substituting S = 4/2 and Tc = 293K in equations (1) to (5). Neither of them coincides with the measurement result, and in particular, the measured value of the magnetization curve of (b) greatly exceeds the result of the molecular field approximation. FIG. 5 shows the temperature change (decrease) due to the applied magnetic field of H = 1 to 7 T obtained from the same molecular field approximation equation (7) as a function of the initial temperature. Here, the contribution of phonons is included assuming that θ _D is 800K. In either case, the temperature change is only a few degrees Celsius,
The result is almost the same as in the case of Gd metal. But,
In the case of Gd metal, considering that the magnetization curve and the temperature change due to adiabatic demagnetization were well reproduced by molecular field approximation, FIG.
The deviation from the calculation of the actual measurement value is reflected in FIG. 5 as it is,
In practice, it is expected that a larger temperature change than obtained from the molecular field approximation will be obtained. Furthermore, the Tc of this system is S
If several materials having different Tc are continuously prepared and adiabatic demagnetization is performed in the order of larger Tc, utilizing the control by r-substitution and Ca-substitution, it is possible to efficiently and gradually cool to a desired temperature. It is thought possible. Based on the above calculation results and considerations, the present inventor has determined that La _1-X Sr _X M
When a single crystal of nO ₃ was prepared and adiabatic demagnetization was evaluated, a temperature change of 2 to 7 ° C. was observed in a sample having a composition of 0.1 ≦ X ≦ 0.6 at an applied magnetic field of 1 to 2T. The magnetic material for magnetic cooling of the present invention does not exhibit ferromagnetism when X <0.1 and is an antiferromagnetic insulator. When the substitution amount is X> 0.6, the perovskite structure is broken and a desired adiabatic demagnetizing effect is obtained. I can't.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to examples. (Example 1) La ₂ O ₃ , SrCO ₃ and MnCO ₃ powders were mixed at an appropriate ratio so as to constitute the above-mentioned material of the present invention, and then pressed into a rod-like shape. After sintering under the heating condition of 1300 ° C. × 20 hours, it was cooled to room temperature to obtain a sintered body. Next, the obtained sintered body was machined into a rod shape having a diameter of 10 mm and a length of 100 mm, and then La _{1 -X} was obtained by a floating zone method.
A single crystal rod-shaped sample of Sr _x MnO ₃ (X = 0.2) was prepared. This rod-shaped sample was cut out to a length of 15 mm for measurement, and used as a columnar sample. The Tc of this sample was 270K. Next, a hole having a diameter of 1 mm was formed in the axis of the cylindrical sample to provide a mounting hole for a thermocouple. An electromagnet for ESR (electron spin resonance) was used as a magnet for the applied magnetic field, and styrene foam was used as a heat insulating material for holding the sample. The measurement of adiabatic demagnetization was performed by setting the sample in the center of the gap of the pole piece of the electromagnet, applying a magnetic field for 20 seconds to reach room temperature. Was measured with a thermocouple. The initial temperature (room temperature) at this time was 31 ° C. FIG. 6 and Table 1 show the measurement results. Applied magnetic field 1T
, A temperature drop of about 7 ° C at 2T was observed. The solid line in FIG. 6 is a calculation result obtained from the molecular field approximation, but a result is obtained in which the measured value of the temperature change becomes larger as in the case of the magnetization curve of FIG.

(Examples 2 to 5) Samples having the compositions shown in Table 1 were prepared in the same manner as in Example 1 except that the composition was changed, and the temperature change ΔT (° C.) due to adiabatic demagnetization was measured.
As in Example 1, the initial temperature of the measurement is 31 ° C. The results are summarized in Table 1.

[0015]

[Table 1]

By using the material of the present invention, a magnetic cooling device utilizing adiabatic demagnetization over a temperature range of 100 to 400K can be constructed. When the temperature of the material of the present invention is lower than 100 K, the arrangement of magnetic spins in the cooling section remains aligned in a specific direction, and the cooling effect cannot be obtained. If it exceeds 400K, it does not show ferromagnetism and is not suitable. From Table 1 and other related results, it is expected that a magnetic cooling device that does not use freons can be configured at 223 to 323K, particularly preferably at 273 to 300K.

FIG. 7 is a cross-sectional view of a main part of a magnetic cooling device 100 showing one embodiment of the present invention. In FIG. 7, 5 is the single crystal rod-shaped sample of Example 1 (Tc = 260 K), and 6 is Example 5.
Is a single crystal rod-shaped sample (Tc = 220K), and 7 is a single crystal rod-shaped sample of Example 3 (Tc = 205K). The ferromagnetic materials 5 to 7 are arranged at a predetermined interval 1 on the moving member 50 held substantially in an adiabatic environment. The moving member 50 is moved up and down, and 5 to 7 ferromagnetic substances are alternately put in the magnetic field created by the electromagnets 1 and 2 alternately, and then sequentially put into the cooling chamber 20 held at 100 to 400K for cooling. This is a method of cooling the chamber 20. That is, first, the magnetic spins of the ferromagnetic material 5 are aligned in the direction of application of the magnetic field of the electromagnet 1 in the heat radiating unit 10. Thereafter, the moving member 50 is moved downward in this state, and the ferromagnetic material 5 is put into the cooling chamber 20.
Is disturbed to cool the cooling chamber 20. Ferromagnetic material 5
Is in the cooling chamber 20, the magnetic spin of the ferromagnetic material 6 is oriented by the applied magnetic field of the electromagnet 2 in the heat radiating section 30. Next, when the ferromagnetic material 5 moves from the cooling chamber 20 to the heat radiating section 10, the ferromagnetic substance 6 enters the cooling chamber 20 and the cooling chamber 2.
Cool 0. Next, when the ferromagnetic material 6 is in the cooling chamber 20, the magnetic spin of the ferromagnetic material 7 is oriented in the heat radiation part 30. When the ferromagnetic material 6 moves from the cooling chamber 20 to the heat radiating section 10, the ferromagnetic material 7 enters the cooling chamber 20. Thereafter, the state is returned to the state shown in FIG. 7, and one cycle of the cooling process is completed. If this processing is continued for a predetermined cycle, 10
A useful magnetic cooling device can be constructed at 0 to 400K. As described above, in order to realize efficient cooling, it is preferable that the moving member be moved so that the magnetic field is applied to the magnetic material having a large Tc sequentially from the magnetic material having a small Tc.

Although the electromagnets 1 and 2 are used in FIG. 7, permanent magnets may be used instead. Alternatively, a magnetic field applying device may be configured by using an electromagnet and a permanent magnet together. Alternatively, in FIG. 7, the cooling chamber 20,
The heat radiating units 10 and 30 may be cooled. Further, instead of the electromagnets 1 and 2, the magnetic field applying device may be configured using a superconductor that exhibits a superconducting phenomenon at a temperature of liquid nitrogen or higher.

[0019]

By using the magnetic material for magnetic cooling of the present invention, a magnetic cooling device utilizing adiabatic demagnetization can be constructed. Therefore, adverse effects on the global environment due to freon gas are eliminated, and it is expected that a refrigeration apparatus and a cooling apparatus that are particularly useful in a room temperature range can be configured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an adiabatic demagnetization and adiabatic magnetization process on an entropy-temperature curve.

FIG. 2 is a diagram showing calculation results of temperature dependence of spin entropy per Gd metal ion by molecular field approximation.

FIG. 3 shows the calculation results of the temperature dependence of the total entropy per Gd metal ion by the molecular field approximation and the Debye approximation, and the wavy line indicates the measurement result by Brown (J. Appl. Phys. 47 (1976), p3673). Show.

FIG. 4 (a) shows La _0.815 Sr _0.175 by molecular field approximation.
Calculation results showing the temperature dependence of the magnetic moment of MnO ₃ , and the wavy line is shown by Tokura et al. (J. Phys. Soc. Jpn. 63 (1994), p1
71), and (b) is a calculation result of the magnetic field dependence of the magnetic moment of La _0.815 Sr _0.175 MnO ₃ by molecular field approximation, and the wavy line is shown by Tokura et al. (J. Phys. So
c. Jpn. 63 (1994), p171).

FIG. 5: La _0.815 Sr _0.175 MnO ₃ by molecular field approximation
9 is a calculation result of the initial temperature dependence of a temperature change due to adiabatic demagnetization of a sample.

FIG. 6 is a diagram showing the applied magnetic field dependence of a temperature change due to adiabatic demagnetization of La _0.8 Sr _0.2 MnO ₃ , □ and ○ are measurement results at initial temperatures of 21 ° C. and 31 ° C., respectively, and solid lines are 21 ° C. It is a calculation result by molecular field approximation at 31 ° C and 41 ° C.

FIG. 7 is a cross-sectional view of a main part showing one embodiment of a magnetic cooling device using adiabatic demagnetization of the present invention.

[Explanation of symbols]

1, 2, electromagnets, 5, 6, 7 members made of magnetic material for magnetic cooling, 10, 30 heat radiating section (magnetic field orientation area), 20 cooling chamber (cooling section), 50 moving member, 100 magnetic cooling device.

Claims (4)

[Claims] The composition is represented by an atomic ratio of R _{1 -X} Ax MnO ₃ (R is at least one of La, Pr, Nd and Sm;
A is Sr and / or Ca), represented by 0.1 ≦ X ≦ 0.6, and has a perovskite structure. 2. The Curie temperature (Tc) according to claim 1,
Is a magnetic material for magnetic cooling composed of a ferromagnetic material at 200 to 400K. 3. A cooling unit which is kept substantially insulated;
A heat dissipating part provided with a magnetic field applying device, and a moving member passing through the cooling part and the heat dissipating part, wherein the moving member has a composition in atomic ratio of R _{1 -X} AX MnO.
₃ (R is at least one of La, Pr, Nd and Sm, A is Sr and / or Ca), 0.1 ≦ X ≦
An apparatus for magnetic cooling, wherein a ferromagnetic material represented by 0.6 and having a perovskite structure is arranged. 4. The Curie temperature (Tc) according to claim 3, wherein the ferromagnetic material is manufactured by changing the value of X.
A step of sequentially moving these ferromagnetic materials to the cooling unit after applying a magnetic field in the heat radiating unit to the cooling unit, and performing a predetermined cycle to set the cooling unit in a range of 100 to 400K. Magnetic cooling device to cool inside.