JPH0765137B2 - Magnetic sintered body - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a magnetic working material for magnetic refrigeration that utilizes the magnetocaloric effect for cooling.

[Technical background of the invention and its problems]

With the remarkable development of superconducting technology in recent years, its application is considered in a wide range of fields such as industrial electronics, information industry, and medical equipment. In order to use superconductivity technology, it is essential to develop a refrigerator that creates a cryogenic environment. A well-known refrigeration method is a substrate refrigeration method, but the efficiency is extremely low and the device becomes large. Therefore, as a new refrigeration method to replace this, a magnetic refrigeration method that uses the thermomagnetic amount effect of a magnetic material Research is actively carried out (Proceedi
ngs of ICEC 9 (1982, May); 26-29, Advances in Cryog
enic Engineering, 1984, Vol, 29, 581-587). In short, this utilizes the endothermic and heat radiating reactions due to the change in entropy between the spin alignment state when a magnetic field is applied to a magnetic substance and the disordered state of spins when the magnetic field is released. This magnetic substance is called a magnetic working substance. The magnetic refrigeration system is a promising system because it has a large size and a high efficiency. The efficiency of magnetic refrigeration is highly dependent on the magnetic working material. That is, high entropy and good thermal conductivity are required.

For example, Gd ₃ Ga ₅ O ₁₂ (GGG), Dy ₃ Al ₅ O _{12 is} used as a magnetic working substance for freezing in a temperature range of 20 K or lower.
(DAG) garnet-based oxide single crystal containing rare earth element, RAl ₂ Laves type intermetallic compound (R is rare earth element) for the temperature range of about 77 to 15K
Have been studied (Proceedings of ICEC (1982, Ma
y); 30-33 etc.).

This magnetic working substance is required to have a constant entropy change (ΔS) in the freezing temperature region. For example 77K
When considering a magnetic working material for magnetic refrigeration from a liquid nitrogen temperature that covers a wide temperature range up to ~ 15K, a large entropy change in a wide temperature range and a large entropy change within this temperature range in a material system having the same crystal structure It is necessary to have continuously different magnetic transition temperatures of. Examples of such a magnetic material include the above-mentioned RAl ₂ Laves type intermetallic compound.

Here, in consideration of the practicality of the magnetic working material, in addition to the above characteristics, the degree of freedom in workability and high accuracy are required. Therefore, it will be very effective if a sintered body satisfying the above characteristics is obtained.

Although there is no report on the sintering of the above RAl ₂ Laves type intermetallic compound, it is expected that the sinterability is poor because the melting point of RAl ₂ is as high as 1500 ° C. or higher. Further, considering the sintering at a high temperature of 1500 ° C. or higher, there are problems in terms of cost, cost problems due to containing a large amount of R component, and low thermal conductivity. Therefore, at present, a magnetic sintered body which is effective as a magnetic working material for magnetic refrigeration has not been obtained.

Further, in magnetic refrigeration intended for a temperature range of about 77K to 15K, a regenerator type cycle such as the Ericsson cycle is desirable because of large contribution of lattice entropy. In such a heat storage type refrigerator, heat transfer between the magnetic working substance and the regenerator material is indispensable, which greatly affects the cooling efficiency. At extremely low temperatures of 77 K or less, there is only a solid regenerator material such as lead, and it is necessary to make solid contact between the magnetic working substance and the regenerator material or form a narrow gap such as a He gas film to perform heat exchange. is there. Therefore, high precision processing such as mirror finishing and complex shape processing is required for both magnetic working materials and cold storage materials (Cryogenic Engineering Society, November 1984). As described above, it has been desired to develop a magnetic working material having particularly good workability for a cold storage refrigerator.

[Object of the Invention]

The present invention has been made in view of the above points, and has a large entropy change, a high thermal conductivity, and an excellent workability, particularly a magnetic sintered body useful as a magnetic working substance. The purpose is to provide.

[Outline of Invention]

The present inventors focused their attention on the use of a magnetic sinter having good workability as a magnetic work substance, and conducted research on the sinterability of RAl ₂ to find that a compound of R: Al = 1: 2 (molar ratio) was used. It was found that a good sintered body could not be obtained in the above, and a good sintered body could be obtained in the case of Al excess than the stoichiometric ratio, that is, RAl _{2 + x} (x> 0). Further, it was found that the obtained sintered body also had good thermomagnetic properties and thermal conductivity, and the present invention was created.

That is, in the present invention, at least one of rare earth elements from La to Yb and Y (hereinafter referred to as R) is 70 to 75 in the periodic table.
It is a magnetic sintered body having a composition between RAl _{2 and} RAl _{3 in} which the content is wt% and the balance is substantially Al, and it is effective as a magnetic working substance.

The amount of the rare earth element from La to Yb and Y is small, but if the amount is small, the thermomagnetism effect, that is, the magnetic entropy change is small, and the thermomagnetic characteristics deteriorate. Further, if the amount is too large or too small, the sinterability is remarkably deteriorated and the heat transfer property is deteriorated. Therefore, the amount is set to 70% by weight or more and 75% by weight or less. Further, among the rare earth elements, heavy rare earth elements of Gd to Yb have a large magnetic moment per rare earth element ion, so they are elements effective for improving the thermomagnetic effect and improving the magnetic refrigeration efficiency. preferable. The density of the sintered body changes depending on the sintering conditions and has a great influence on the heat transfer characteristics.
It is preferably 5 g / cm ³ or more. If it is too small, the heat transfer characteristics will be significantly deteriorated.

Such magnetic working material is manufactured as follows.

First, an R-Al alloy having a desired composition ratio is manufactured in an arc melting furnace or the like. Next, this alloy is pulverized to obtain fine particles of R-Al alloy. The particle size of these fine particles affects the sintering density, and
It is preferably in the range of up to 10 μm. If it is too large, the sintering density will decrease, and if it is too small, it will easily oxidize and the thermomagnetic effect will decrease, and the sintering density will also decrease.

The R-Al alloy fine particles are pressed into a desired shape and sintered. Sintering is performed in a non-oxidizing atmosphere such as an inert gas such as Ar gas. The sintering temperature is a main factor that determines the sintering density, but 900 to 1200 ° C is preferable. If it is too low, a high sintered density cannot be obtained, and if it becomes a high temperature, it becomes difficult to obtain a good sintered body due to oxidation, evaporation and the like. The inventors of the present invention, by utilizing the solid-liquid phase reaction line in the vicinity of 1000 ° C. existing between RAl _{2 and} RAl ₃ on the phase diagram, the packing rate exceeds 80%, and the density is 5 g / cm ³ or more, A dense sintered body with a maximum filling rate of more than 98% was obtained. This sintered body shows good thermomagnetic properties and thermal conductivity without being affected by the antiferromagnetic material RAl ₃ , and is a meltable material. They have found that it is possible to obtain the same characteristics as a certain RAl ₂ Laves type intermetallic compound.

In the present invention, although usually used industrial Al raw material is used, a small amount of Ca, Cu, Si, Fe, Mn, Mg, etc. which are usually contained.
Impurities such as Zn, Ti, C, N and O, or impurities that do not impair the effects of the present invention may be contained.

Further, in order to improve the heat conduction characteristics, it is possible to heat-treat at a temperature equal to or lower than the sintering temperature to adjust the texture.

〔The invention's effect〕

As described above, according to the present invention, it is possible to obtain a magnetic sintered body which is excellent in thermomagnetic properties and thermal conductivity and has a good workability and which is effective as a magnetic working material for magnetic refrigeration.

Further, since the magnetic sintered body of the present invention has a large degree of freedom in processing and enables complicated and high-precision processing, the contribution of the lattice entropy is large, and it is necessary to use the cold storage method from liquid nitrogen temperature such as Ericsson cycle. When used as a magnetic working substance for magnetic refrigeration, it is particularly effective because good heat transfer can be obtained.

Example of Invention

 Examples of the present invention will be described below.

A rare earth / aluminum alloy having a predetermined composition was produced in an arc melting furnace, pulverized by a ball mill method into fine powder having a particle size of about 3 μm, and then press-molded to obtain a green compact. This green compact was sintered in an Ar gas atmosphere.

The density (ρ) of the obtained sintered body, the number of effective bore magnetons (μ _eff ) obtained by measuring the paramagnetic coefficient, the Curie point (Tc), and the thermal conductivity (value at Tc) were measured. The measured values are shown in Table 1 together with the alloy composition and the sintering conditions.

Although Examples 1 to 8 are related to the present invention, it can be seen that all of them are excellent in both the effective bore magnet number and the thermal conductivity. Although Examples 1 to 6 contain 50 wt% or more of heavy rare earth elements, it is clear from comparison with Examples 7 and 8 that the effective bore magnet number is large and the thermomagnetic effect is excellent. Recognize.

Further, Comparative Examples 1, 2, and 4 are examples in which the composition is RAl ₂ and many rare earth elements are included, and Comparative Examples 3 and 5 are examples in which the composition is RAl _{3 and} few rare earth elements are included. It turns out that it is inferior in connection. Therefore, it has very poor thermal conductivity. This means that heat transfer does not take place effectively,
It is difficult to use as a magnetic working substance.

Further, the heat treatment of Example 1 was performed at 900 ° C. × 140 H.
Then, it was confirmed that the thermal conductivity was improved to 300 mw / cm · K. As described above, the present invention makes it possible to obtain a magnetic sintered body that can be used as a magnetic working material. Therefore, the performance of the magnetic refrigerator is improved and the regenerator type magnetic refrigerator is used. The contribution to the practical application of is enormous.

Claims (3)

[Claims] 1. A rare earth element from La to Yb in the periodic table of elements and at least one of Y (hereinafter referred to as R) is 70-
A magnetic sintered body characterized by using a sintered body having a composition between RAl _{2 and} RAl ₃ containing 75% by weight and the balance substantially consisting of Al. 2. The magnetic sintered body according to claim 1, wherein the sintered density of the sintered body is 5 g / cm ³ or more. 3. The magnetic sintered body according to claim 1 or 2, wherein the sintered body is for a magnetic refrigeration work substance.