JPS6316335B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a magnetic working material for a cryogenic magnetic refrigerator. Prior Art Recently, with the progress of superconducting technology, the range of practical applications for cryogenic applications has expanded, and there has been a demand for products with good refrigeration efficiency in cryogenic environments. In the conventional refrigeration method, which repeatedly compresses and expands gas,
The efficiency decreases significantly below 20K, especially from liquid helium (4.2K) to superfluid helium (2.0K).
There was a problem in that a large decompression pump was required to obtain this. Therefore, magnetic refrigeration, which is based on a completely new principle, has attracted attention. Generally, heat generation occurs when a magnetic material is inserted into a strong magnetic field and its magnetic spins are aligned. After removing this heat to the outside, the magnetic material is pulled out from the strong magnetic field and the magnetic spin is disturbed, causing heat absorption, which takes heat from the external object to be frozen and freezes it. Magnetic refrigeration utilizes this principle; mechanically, the alignment and disorder of magnetic spins in magnetic refrigeration correspond to the compression and expansion of gas in gas refrigeration, resulting in high refrigeration efficiency. Compared to conventional gas refrigeration methods, this magnetic refrigeration method eliminates the need for a compressor, reducing noise and vibration.
It has many excellent features such as being smaller and lighter, can be controlled by a computer, and saves helium resources. In order to put such an excellent magnetic refrigeration method into practical use, it is essential to develop high-performance magnetic refrigeration materials. Magnetic refrigeration materials in the temperature range below 20K have the following properties: (1) large magnetic moment, (2) low magnetic transformation point, (3) large thermal conductivity, and (4) small specific heat. characteristics are required. Particularly important properties are a large magnetic moment and high thermal conductivity. The larger the magnetic moment, the larger the amount of heat absorbed from the object to be frozen in one refrigeration cycle. Thermal conductivity is also a major factor that determines the operating speed of a magnetic refrigerator, and the higher it is, the higher the number of cycles the refrigerator can perform. Further, this thermal conductivity depends on the inherent properties of the substance and is closely related to defects and grain boundaries in the crystal, and the fewer defects there are, the higher the thermal conductivity becomes. From these viewpoints, compounds containing the rare earth metals gadolinium (Gd) and disprosium (Dy), which have large magnetic moments, are considered promising. Currently, Gd ₃ Ga ₅ O ₁₂ garnet is considered to have excellent properties as a magnetic refrigeration material in the temperature range below 20K, and magnetic refrigeration tests are being conducted using it. As a result, high refrigeration efficiency can be obtained at the lower part of the refrigeration cycle, and the maximum refrigeration efficiency is 3 terraces (T) with a magnetic field strength of 0.3 cycles.
It is near Hz. If the magnetic field strength or the number of cycles of the refrigerator is increased in order to increase the refrigerating capacity, the efficiency of the refrigerator will actually decrease. This means that the shape of the working material is 20 mm in diameter
This is due to the fact that heat exchange of heat generation and heat absorption accompanying magnetization and demagnetization cannot be performed sufficiently because it requires a diameter of mmφ or more and a length of 40 mm or more. OBJECTS OF THE INVENTION The object of the present invention is to overcome the drawbacks of conventional Gd ₃ Ga ₅ O ₁₂ garnet and to provide a magnetic refrigeration material having higher thermal conductivity than garnet. Structure of _the Invention As _a result of research to achieve the above object, the present inventors found that _{the general formula Gd 3 (} Ga ₁ -xAlx) ₅ O ₁₂ (However, ,X
It has been found that gadolinium-gallium-aluminum garnet, represented by 0.01 to 0.6, is useful as a magnetic refrigeration material with high thermal conductivity. Also, the garnet
If 1 to 20% by weight of Ag or Cu is distributed in the grain boundaries, the thermal conductivity can be further increased. Furthermore, it has been found that thermal conductivity can be significantly increased by single crystallization without the presence of grain boundaries that impede heat conduction, and that refrigeration capacity can be increased. The present invention was completed based on these findings. The gist of the present invention is as follows: (1) General formula Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ (where X is
Magnetic refrigeration material for cryogenic temperatures consisting of gadolinium, gallium, aluminum, and garnet represented by 0.01 to 0.6. (2) General formula Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ (X is
(representing 0.01 to 0.6) in gadolinium gallium aluminum garnet.
A cryogenic magnetic refrigeration material containing 1 to 20% by weight of Ag or Cu. (3) General formula Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ (X is
A magnetic refrigeration material for cryogenic temperatures consisting of a single crystal of gadolinium, gallium, aluminum, and garnet, expressed as 0.01 to 0.6. It is in. The amount of Ga to be substituted with Al in the Gd ₃ Ga ₅ O ₁₂ garnet is such that X in the above general formula is in the range of 0.01 to 0.6. If X is less than 0.01, there will be little effect on improving thermal conductivity. On the other hand, X
If it exceeds 0.6, _three phases of GdAlO with a perovskite structure will crystallize in the garnet structure, resulting in a decrease in thermal conductivity. A preferred range of X is 0.3 to 0.5. Ag with high thermal conductivity is present in the grain boundaries of the gadolinium-gallium-aluminum garnet of the present invention.
Alternatively, if Cu is distributed in a range of 1 to 20% by weight, the thermal conductivity can be further increased. If the amount is less than 1% by weight, there is almost no effect of improving thermal conductivity, while if it exceeds 20% by weight, thermal conductivity will increase, but on the other hand, the magnetocaloric amount will decrease, so 1 to 20% by weight. It is necessary to be within the range of . Furthermore, when made into a single crystal, there are no grain boundaries that impede heat conduction, so thermal conductivity can be significantly increased. The gadolinium-gallium-aluminum garnet of the present invention is made by blending the component raw materials with the composition,
It can be produced by kneading and sintering with a hot press. Further, the single crystal is produced by, for example, melting the sintered body by high frequency heating, immersing a seed crystal in it, and pulling it in a predetermined crystal axis direction in a nitrogen gas atmosphere containing 0 to 3% oxygen gas. It can be produced by a so-called pulling method. Examples Example 1 Gadolinium oxide (Gd ₂ O ₃ ), gallium oxide (Ga ₂ O ₃ ), and aluminum oxide (Al ₂ O ₃ ) powders with a diameter of approximately 5 μm and a purity of 99.99% were mixed as shown in Table 1. 420g of Gd ₃ (Ga ₁ −
xAlx) ₅ O ₁₂ garnet was produced.

[Table] A rectangular rod-shaped sample with a cross section of 4 x 4 mm and a length of 35 mm and a cylindrical sample with a diameter of 25 mmφ and a height of 8 mm were made from these samples, and the thermal conductivity and magnetic caloric content of each were measured. The results were as shown in Table 1 above.
Thermal conductivity is determined by setting the temperature of the rod sample to 4.2K or 20K.
After adjusting the temperature, heat is applied to one end using a heater so that the heat flows steadily toward the other end, and the temperature is measured at two points on the sample. Steady heat flow is calculated from the temperature difference. Required by law. The magnetocaloric amount is determined by fixing a cylindrical sample in a superconducting magnet and magnetizing it by applying a magnetic field of 5 terraces (T).
The temperature of the sample was adjusted to 4.2K or 20K, and it was determined by the adiabatic demagnetization method, which calculates from the temperature change that occurs in the sample when it is demagnetized at a rate of 5 seconds to 0 terraces (T). As shown in Table 1, when X is smaller than 0.01, almost no improvement in thermal conductivity is observed. On the other hand, X
When it exceeds 0.6, the thermal conductivity decreases rapidly. 0.3～
A range of 0.5 indicates high thermal conductivity. Example 2 The same raw material powder as in Example 1 was used as Gd ₃ (Ga _0.6 Al _0.4 ).
Blend and knead to have a composition of ₅ O ₁₂ , press and sinter at 1250℃, and use a ball mill to mill it into diameters of approximately
It was ground to a powder of 5 μm. This powder was kneaded with Ag or Cu powder in the amount shown in Table 2, and 420 g was sintered at 1100°C using a hot press to make a sample with a diameter of 30 mmφ. A sample of the same size as in Example 1 was prepared from this sample, and its thermal conductivity and magnetocaloric content were measured in the same manner. The results are shown in Table 2.

[Table] As shown in this result, when the amount is less than 1% by weight, there is almost no improvement in heat conduction, but when it is more than 1% by weight, the heat conduction is improved. On the other hand, if it exceeds 20% by weight, the magnetocaloric amount decreases significantly, resulting in a decrease in refrigeration efficiency. Therefore,
It can be seen that the blending amount of Ag or Cu is preferably 1 to 20% by weight. Example 3 Using the same raw material powder as in Example 1, it was blended and kneaded to have the composition shown in Table 3 below, and after pressing
A garnet structure was created by firing at 1250℃. Pour this 420g into an iridium crucible (diameter 50mmφ,
A seed crystal is immersed in this by high-frequency heating in a 50 mm height), and pulled up in the <111> crystal axis direction in a nitrogen gas atmosphere containing 0 to 3% oxygen gas, with a diameter of approximately 28 mmφ and length. A single crystal of approximately 50 mm was produced. The single crystal obtained (when X=0.3) was as shown in FIG. Samples of the same size as in Example 1 were prepared from these single crystals, and their thermal conductivity and magnetocaloric content were measured in the same manner. The results were as shown in Table 3.

[Table] Comparing this result with Table 1 of the results for the sintered sample, it is clear that the single crystal has no grain boundaries and has extremely few defects, so its thermal conductivity is significantly higher than that of the sintered sample. Become. Further, the change in properties with respect to composition shows almost the same tendency as in Example 1. Note that oxygen gas contained in the nitrogen gas atmosphere during single crystal growth prevents evaporation of Ga ₂ O ₃ from the melt and eliminates compositional fluctuations in the crystal. Since the oxygen gas concentration can be decreased as X increases,
Iridium crucibles are less consumed by oxidation. Effects of the Invention As described above, the Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ of the present invention
(However, X is the same as above.) Garnet has magnetic properties with a large magnetocaloric effect and a low magnetic transformation temperature, and also has high thermal conductivity. Therefore, when used as the working material of a magnetic refrigerator that creates an extremely low temperature environment such as in the production of liquid helium or superfluid helium, the magnetic refrigerator can be operated at a high refrigeration cycle, improving refrigeration capacity and efficiency. be able to. Furthermore, the garnet of the present invention is different from the conventional garnet.
Gd ₃ Ga ₅ O ₁₂ Since expensive Ga in garnet is replaced with cheap Al, the price can be lowered.
Moreover, excellent effects can be achieved by allowing conventional manufacturing equipment and techniques to be used as they are. Furthermore, in growing a single crystal, the oxygen concentration contained in the growing nitrogen gas atmosphere can be reduced as X increases, so consumption of an expensive iridium crucible due to oxidation is reduced, and manufacturing costs can be reduced. (In addition, to grow a single crystal of an oxide such as garnet, which has a high melting point of 1700°C or higher, by the pulling method, an iridium crucible with a high melting point is required.)

[Brief explanation of the drawing]

Figure 1 shows a photograph of a single crystal when X=0.3.

Claims (1)

[Claims] 1 General formula Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ (where X is
Gadolinium (expressing 0.01 to 0.6)
A magnetic refrigeration material for cryogenic temperatures consisting of gallium, aluminum, and garnet. 2 General formula Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ (X is
Gadolinium (expressing 0.01 to 0.6)
A cryogenic magnetic refrigeration material made of gallium aluminum garnet mixed with 1 to 20% by weight of Ag or Cu. 3 General formula Gd ₃ (Ga ₁ −xAlx) ₅ O ₁₂ (where X is
Gadolinium (expressing 0.01 to 0.6)
A cryogenic magnetic refrigeration material made of single crystals of gallium, aluminum, and garnet.