Application of rare earth fluoride single crystals to magnetic refrigeration

Abstract

The present invention discloses the application of rare earth fluoride single crystals to magnetic refrigeration, a magnetic refrigeration method and a magnetic refrigeration device, which belong to the field of magnetic functional materials. The series of magnetocaloric materials are lithium rare earth fluorides, with the chemical formula LiREF4, where RE is one or more of the rare earth elements gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium. The single crystals of the series of magnetocaloric materials have extremely strong magnetic anisotropy, and by applying a changing magnetic field of 0 to 20 kOe in the direction of the easy axis of magnetocaloric materials, LiGdF4, LiTbF4, LiHoF4, LiDyF4, LiErF4, LiYbF4 and LiTmF4 have a magnetic anisotropy of 50.35 J kg at 1.8 K, 4 K, 4 K, 1.8 K, 1.8 K, 1.8 K and 18 K. -1 ·K -1 , 21.12 J kg -1 ·K -1 , 24.98 J kg -1 ·K -1 , 24.45 J kg -1 ·K -1 , 21.95 J kg -1 ·K -1 , 14.7 J kg -1 ·K -1 and 0.692 J kg -1 ·K -1 The thermal conductivity of LiTbF4 and LiErF4 at 25 K is 50 W K. -1 m -1 reaches.

Description

[Technical field]

[0001] The present invention relates to the field of magnetic functional materials, and more particularly to giant magnetocaloric effect materials that can be driven by a small magnetic field in a low temperature range. [Background technology]

[0002] At present, refrigeration technology is well known to people and is widely applied in daily life, such as air conditioners, refrigerators, agriculture and medicine. In refrigeration technology, the temperature range lower than that of liquid hydrogen is called the low-temperature range, which may be applied to space communications, quantum computers or gas liquefaction in the chemical industry, and ultra-sensitive detectors in space. All of these refrigeration technologies require the use of helium gas. For example, the superconducting magnet that provides the magnetic field for the adiabatic demagnetization refrigeration device needs to be cooled with liquid helium, and the 3He / 4He dilution refrigerator and the He adsorption refrigerator cannot function at all without helium gas. Compared with traditional refrigeration methods, the new magnetic refrigeration technology based on the magnetocaloric effect has the advantages of being environmentally friendly, having high refrigeration efficiency, low energy consumption, low noise, no pollution, stable and reliable, etc. Therefore, it has attracted the attention of researchers.

[0003] At present, the most mature magnetic materials for low-temperature magnetic refrigeration are the garnet series (Gd 3 Ga 5 O 12 (abbreviated as GGG), Dy 3 Al 5 O 12 However, in order to obtain a large magnetic entropy change, it is usually necessary to apply a large magnetic field (0-50 kOe) to the magnetic working material, and such a magnetic field must rely on large electromagnets and superconducting magnets. However, the high cost of the equipment, the complicated design, and the large size of the equipment significantly hinder the widespread use of magnetic refrigeration technology. Therefore, the key to low-temperature magnetic refrigeration is to find a magnetic working material that can be driven by a small magnetic field in the low-temperature range to obtain a large magnetic entropy change.

[0004] To solve the above problems, the present invention provides a magnetocaloric material that can be driven by a small magnetic field and can obtain a large magnetic entropy change, and the magnetocaloric material is a lithium rare earth fluoride single crystal. Summary of the Invention [Problem to be solved by the invention]

[0005] The purpose of the present invention is to disclose the application of rare earth fluoride single crystal to magnetic refrigeration, the rare earth fluoride single crystal is lithium rare earth fluoride single crystal, and by applying a changing magnetic field of 0-20 kOe in the easy magnetization direction of the material in the low temperature range, a large magnetic entropy change can be obtained, and the series of materials have good thermal conductivity ability. The discovery of this material brings the possibility of simplifying low-temperature magnetic refrigeration equipment and wide application. [Means for solving the problem]

[0006] The technical means of the present invention are as follows.

[0007] Application of lithium rare earth fluoride single crystals to magnetic refrigeration.

[0008] Under the action of an external magnetic field, the magnetic moment of the lithium rare earth fluoride single crystal sample changes from a disordered state to an ordered state, the interaction energy between the atomic magnetic moments and the external magnetic field is reduced, and its magnetic entropy is reduced and dissipates heat to the outside. Conversely, in the process of removing the magnetic field, the magnetic moment of the lithium rare earth fluoride single crystal changes from an ordered state to a disordered state, the magnetic entropy is increased, and energy is absorbed from the outside. In the case of sample insulation, it appears as a temperature change of the lithium rare earth fluoride single crystal sample itself.

[0009] As a preferred technical measure: The chemical formula of the lithium rare earth fluoride is LiREF 4 and RE is a rare earth element.

[0010] The rare earth element RE is one or more of gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm) and ytterbium (Yb).

[0011] The lithium rare earth fluoride has a tetragonal scheelite structure.

[0012] The lithium rare earth fluorides are single crystals having extremely strong magnetic anisotropy.

[0013] LiGdF 4 , LiTbF 4 and LiHoF 4 The easy magnetization direction of LiDyF is parallel to the C axis. 4 , LiErF 4 , LiYbF 4 and LiTmF 4 The easy magnetization direction of is perpendicular to the C axis.

[0014] LiGdF 4 is paramagnetic, LiTmF 4 exhibits Van Vleck paramagnetism and LiTbF 4 and LiHoF 4 exhibit ferromagnetic order below the Curie temperatures of 2.87 K and 1.53 K, respectively, and LiDyF 4 , LiErF 4 and LiYbF 4 show antiferromagnetic order below the Néel temperatures of 0.62 K, 0.38 K, and 0.128 K, respectively.

[0015] The applicable temperature range for lithium rare earth fluoride single crystals is the low temperature range below 20K.

[0016] The isothermal magnetic entropy change ΔS is an important parameter for evaluating the performance of magnetocaloric materials. Therefore, the present invention uses a superconducting quantum interference meter to measure the magnetization curves of lithium rare earth fluoride single crystal samples at different temperatures in the easy magnetic direction. Then, the obtained magnetization curve data at different temperatures is processed using the Maxwell equation to obtain the relationship of magnetic entropy change with temperature change. By applying a small changing magnetic field of 0-20 kOe to the easy magnetic direction of the lithium rare earth fluoride single crystal sample, the lithium rare earth fluoride single crystal exhibits a large isothermal magnetic entropy change, and LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 , LiErF 4 , LiYbF 4 , LiTmF 4 is 50.35 J kg at 1.8K, 4K, 4K, 1.8K, 1.8K, 1.8K, and 18K. -1 ·K -1 , 21.12 J kg -1 ·K -1 , 24.98 J kg -1 ·K -1 , 24.45 J kg -1 ·K -1 , 21.95 J kg -1 ·K -1 , 14.7 J kg -1 ·K -1 and 0.692 J kg -1 ·K -1 We obtain the isothermal magnetic entropy change of

[0017] In the field of solid-state magnetic refrigeration, the thermal conductivity of magnetocaloric materials is closely related to the refrigeration efficiency of the entire device. The thermal conductivity of lithium rare earth fluoride single crystals is measured using the heat transport option of the comprehensive physical property measurement system. The thermal conductivity of lithium rare earth fluoride single crystal samples is high at low temperatures, so they are measured using the four-wire method to reduce errors. The shape of the sample is a rectangular parallelepiped similar to a needle, and a temperature-reducing measurement method is used. LiTbF 4 and LiErF 4 The thermal conductivity of at 25 K is 50 W K. -1m -1 reaches.

[0018] The magnetic refrigeration method based on lithium rare earth fluoride single crystal is characterized in that the lithium rare earth fluoride single crystal is used as a magnetic working material, and the temperature of the magnetic working material is changed by applying and removing a magnetic field to the magnetic working material, thereby realizing the transfer of heat with a heat exchanger, thereby achieving the purpose of refrigeration.

[0019] As a preferred technical measure: The lithium rare earth fluoride single crystal which is the magnetic working material is LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 , LiErF 4 , LiYbF 4 and LiTmF 4 In the case of a plurality of types, different lithium rare earth fluoride single crystals are mixed and arranged to realize the expansion of the low-temperature refrigeration section.

[0020] In the process of producing lithium rare earth fluoride single crystals, two or more kinds of rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) can be added to realize the extension of the low temperature freezing section.

[0021] The small magnetic field driven magnetic refrigeration device is characterized by including a lithium rare earth fluoride single crystal as a magnetic working material.

[0022] As a preferred technical measure: The lithium rare earth fluoride single crystal which is the magnetic working material is LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 , LiErF 4 , LiYbF 4 and LiTmF 4 In the case of a plurality of types, different lithium rare earth fluoride single crystals are mixed and arranged to realize the expansion of the low-temperature refrigeration section.

[0023] In the process of producing lithium rare earth fluoride single crystals, two or more kinds of rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) can be added to realize the extension of the low temperature freezing section.

[0024] The refrigeration apparatus further includes a magnetic field applying member, a thermal switch, a heat sink and a load, wherein the magnetic field applying member can be a permanent magnet, an electromagnet or a superconducting magnetic material, preferably a permanent magnet, which can effectively simplify the volume of the apparatus and optimize the design, and the thermal switch can be a mechanical contact type, superconducting type, gas type or magnetic resistance type thermal switch, preferably a superconducting thermal switch. Effect of the Invention

[0025] The technical effects of the present invention are as follows:

[0026] 1. The lithium rare earth fluoride single crystal undergoes phase transition at low temperatures. When used as a magnetic working material in a magnetic refrigeration device, the refrigeration temperature range of the magnetic refrigeration device can be kept within a low temperature range of less than 20K.

[0027] 2. Lithium rare earth fluoride single crystals have extremely strong magnetic anisotropy and can reach saturation when a small magnetic field is applied in the direction of the easy axis of magnetization. The components providing the small magnetic field, such as ordinary permanent magnets, are inexpensive and small in volume, which not only greatly reduces the cost of the refrigeration equipment, but also optimizes the design of the refrigeration equipment, and expands the application range of magnetic refrigeration equipment to large scientific experimental equipment and any equipment and operations requiring low and ultra-low temperature environments.

[0028] 3. The present invention can effectively extend the freezing range by mixing and arranging multiple lithium rare earth fluoride single crystals, meet the needs of different freezing ranges, and further expand the scope of application. Alternatively, the freezing range can be extended by adding a certain amount of other rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) in the process of producing lithium rare earth fluoride single crystals.

[0029] 4. Currently used Gd 3 Ga 5 O 12 17.5 J kg -1 ·K -1 Compared with the isothermal magnetic entropy change of LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 , LiErF 4 , LiYbF 4 , and LiTmF 4 is 50.35 J kg at 1.8K, 4K, 4K, 1.8K, 1.8K, 1.8K, and 18K. -1 ·K -1 , 21.12 J kg -1 ·K -1 , 24.98 J kg -1 ·K -1 , 24.45 J kg -1 ·K -1 , 21.95 J kg -1 ·K -1 , 14.7 J kg -1 ·K -1 and 0.692 J kg -1 ·K -1 With these excellent properties, lithium rare earth fluoride single crystals can completely replace these mainstream materials, and are widely used in low-temperature magnetic refrigeration fields such as gas liquefaction, cryogenic scientific equipment, and aerospace, realizing low-cost, high-efficiency, and non-polluting low-temperature magnetic refrigeration. Lithium rare earth fluoride single crystals have high thermal conductivity in the low-temperature refrigeration range, and LiTbF 4 and LiErF 4 The thermal conductivity of at 25 K is 50 W K. -1 m-1 This accelerates the heat exchange rate of the magnetocaloric material, thereby improving the efficiency of the entire magnetic refrigeration device.

[0030] 5. The manufacturing process of lithium rare earth fluoride single crystal is mature and capable of large-scale industrial production, which greatly reduces the cost of using the material and the manufacturing cost of the refrigeration system, and can be rapidly and widely popularized and applied. [Brief description of the drawings]

[0031] [Figure 1] This is the crystal structure of lithium rare earth fluoride. [Diagram 2] FIG. 1 is a diagram illustrating the principle of magnetic refrigeration. [Diagram 3] 1 shows magnetic entropy change curves when a changing magnetic field of 0 to 10 kOe is applied along the easy axis direction of magnetization of gadolinium gallium garnet and lithium rare earth fluoride single crystals. [Figure 4] 1 shows magnetic entropy change curves when a changing magnetic field of 0 to 20 kOe is applied along the easy axis direction of magnetization of gadolinium gallium garnet and lithium rare earth fluoride single crystals. [Diagram 5] FIG. 1 is a diagram showing the relationship between the thermal conductivity of a LiTbF4 single crystal and temperature. [Figure 6] FIG. 1 is a diagram showing the relationship between the thermal conductivity of a LiErF4 single crystal and temperature. [Figure 7] FIG. 2 is a schematic diagram showing the structure of a magnetic refrigeration device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] This is a device that applies lithium rare earth fluoride single crystals, specifically a magnetic refrigeration device driven by a magnetic field. The lithium rare earth fluoride single crystal is selected as the magnetic working material, specifically LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 , LiErF 4 , LiYbF4 and LiTmF 4 One or more of the following may be used:

[0033] The device further includes a magnetic field applying member, a thermal switch, a heat sink, and a load. The magnetic field applying member can be a permanent magnet, an electromagnet, or a superconducting magnetic material, the thermal switch can be a mechanical contact type, a superconducting type, a gas type, or a magnetic resistance type thermal switch, and the function of the load is to realize refrigeration, and the heat sink has a heat dissipation effect. In the initial state, the lithium rare earth fluoride single crystal is in the same temperature state as the heat sink in the zero magnetic field state, and then the magnetic field is increased while the thermal switch remains in the off state, and in the isothermal magnetization process, heat flows from the magnetic working material to the heat sink, and the entropy is reduced. When the magnetic field rises to the maximum, the thermal switch is turned on, and then the magnetic field is reduced, and when the temperature of the magnetic working material drops to the temperature required by the load, there is still some magnetic field remaining, and with the continuous reduction of the magnetic field, the magnetic working material absorbs heat from the load to cool the load, thereby achieving the purpose of cooling, and realizing the refrigeration function of the entire device.

[0034] Mixing and arranging multiple different lithium rare earth fluoride single crystals can effectively extend the refrigeration range. For example, in one embodiment, LiErF 4 and LiTbF 4 By mixing and arranging the LiErF, the freezing section can be made of a single LiErF 4 In another embodiment, the temperature range can be extended from 6 K or less at the time of LiDyF to 15 K or less. 4 and LiTbF 4 By mixing and arranging the LiDyF, the freezing section can be made up of a single LiDyF 4 The low-temperature freezing range can be extended from a temperature range of 4K or less at the time of refrigeration to a temperature range of 15K or less, or by adding a certain amount of other rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) in the process of producing lithium rare earth fluoride single crystals, the low-temperature freezing range can be extended when producing lithium rare earth fluoride single crystals containing two or more rare earth elements.

[0035] The following examples are each LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 , LiErF 4 , LiYbF 4 and LiTmF 4 The magnetocaloric performance test was carried out on LiGdF 4 , LiTbF 4 , LiHoF 4 , LiDyF 4 and LiErF 4 are both 17.5 J kg -1 ·K -1 It shows a better isothermal magnetic entropy change. LiTbF 4 and LiErF 4 In the low temperature section, both are 50W·K -1 m -1 It exhibits a thermal conductivity of up to Example 1

[0036] LiGdF 4 Magnetocaloric performance testing of single crystals In the magnetic test, first, a large LiGdF 4 A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 7.35 mg. The sample is fixed in a plastic sample tube with double-sided tape. Then, MH measurements are performed along the easy magnetization direction of the sample, and the MH curve is measured at a temperature of 1.8 K, and the MH curve is measured every 1 K within the temperature range of 2 K to 10 K. The MH curve is measured every 2 K within the temperature range of 10 K to 24 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0037] Based on the MH curves at different temperatures obtained by the above magnetic tests, the Maxwell equations were used to determine the 4The magnetic entropy change with temperature after applying a varying magnetic field of 0-10 kOe and 0-20 kOe along the easy magnetization direction to the sample is obtained. 4 The sample was subjected to a magnetic field of 0 to 20 kOe with a resistance of 50.35 J kg -1 ·K -1 We obtain the magnetic entropy change of Example 2

[0038] LiTbF 4 Magnetocaloric performance testing of single crystals In magnetic testing, the measurement range of the instrument is limited, so it is necessary to use a scalpel to measure the large LiTbF 4 A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 5.4 mg. The sample is fixed to a plastic sample tube with double-sided tape. Then, MH measurements (i.e., magnetization curve measurements) are performed along the magnetization easy direction of the sample, and one MH curve is measured every 0.2 K within the temperature range of 1.85 K to 2.85 K. One MH curve is measured every 2 K within the temperature range of 4 K to 22 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0039] Based on the MH curves at different temperatures obtained by the above magnetic tests, the Maxwell equations were used to determine the 4 The magnetic entropy change with temperature after applying a varying magnetic field of 0-10 kOe and 0-20 kOe along the easy magnetization direction to the sample is examined. 4 The single crystal sample exhibited a magnetic field of 21.12 J kg under a changing magnetic field of 0 to 20 kOe. -1 ·K -1 We obtain the magnetic entropy change of

[0040] LiTbF 4 In measuring the thermal conductivity of a single crystal sample, the thermal transport option of the integrated physical property measurement system was used to measure the thermal conductivity of LiTbF 4The thermal conductivity along the C-axis direction of the sample was measured. The shape of the sample was rectangular. The cross-sectional area was 9.63 mm. 2 and has a surface area of ​​109.8 mm 3 The electrode distance is 1.5 mm, and the heat dissipation coefficient is 1. The temperature range is 25K to 50K, and the temperature drop rate is 0.1K / min. Then, the LiTbF 4 Obtain the relationship of the thermal conductivity of the sample with respect to temperature, and verify that the thermal conductivity at 25 K is 50 W K. -1 m -1 reaches. Example 3

[0041] LiDyF 4 Magnetocaloric performance testing of single crystals In the magnetic test, first, a scalpel was used to cut the large LiDyF 4 A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 1.48 mg. The sample is fixed to a plastic sample tube with double-sided tape. Then, MH measurements are performed along the easy magnetization direction of the sample, and the MH curve is measured at a temperature of 1.8 K, and the MH curve is measured every 1 K within the temperature range of 2 K to 10 K. The MH curve is measured every 2 K within the temperature range of 10 K to 24 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0042] Based on the MH curves at different temperatures obtained by the above magnetic tests, the LiDyF 4 The relationship between the magnetic entropy change and the temperature change after applying a changing magnetic field of 0 to 10 kOe and 0 to 20 kOe along the easy magnetization direction to the sample is obtained. 4 The sample was subjected to a magnetic field of 0 to 20 kOe with a resistance of 24.45 J kg -1 ·K -1 We obtain the magnetic entropy change of Example 4

[0043] LiHoF4 Magnetocaloric performance testing of single crystals In the magnetic test, first, a large LiHoF 4 A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 6.35 mg. The sample is fixed to a plastic sample tube with double-sided tape. Then, MH measurements are performed along the easy magnetization direction of the sample, and the MH curve is measured at a temperature of 1.8 K, and the MH curve is measured every 1 K within the temperature range of 2 K to 10 K. The MH curve is measured every 2 K within the temperature range of 10 K to 24 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0044] Based on the MH curves at different temperatures obtained by the above magnetic tests, the Maxwell equations were used to determine the 4 The relationship between the magnetic entropy change and the temperature change after applying a changing magnetic field of 0 to 10 kOe and 0 to 20 kOe along the easy magnetization direction to the sample is obtained. 4 The single crystal sample exhibited a magnetic field of 24.98 J kg under a changing magnetic field of 0 to 20 kOe. -1 ·K -1 We obtain the magnetic entropy change of Example 5

[0045] LiErF 4 Magnetocaloric performance testing of single crystals In the magnetic test, first, a large LiErF 4A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 2.55 mg. The sample is fixed to a plastic sample tube with double-sided tape. Then, MH measurements are performed along the easy magnetization direction of the sample, and the MH curve is measured at a temperature of 1.8 K, and the MH curve is measured every 1 K within the temperature range of 2 K to 10 K. The MH curve is measured every 2 K within the temperature range of 10 K to 24 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0046] Based on the MH curves at different temperatures obtained by the above magnetic tests, the LiErF 4 The relationship between the magnetic entropy change and the temperature change after applying a changing magnetic field of 0 to 10 kOe and 0 to 20 kOe along the easy magnetic direction to the sample is obtained. 4 The sample was subjected to a stress of 21.95 J kg at a temperature of 1.8 K and a varying magnetic field of 20 kOe. -1 ·K -1 We obtain the magnetic entropy change of

[0047] LiErF 4 The thermal conductivity of single crystals was measured, and the thermal transport option of the integrated physical property measurement system was used to measure the thermal conductivity of LiErF 4 The thermal conductivity along the C-axis direction of the sample was measured. The shape of the sample was rectangular. The cross-sectional area was 9.3 mm. 2 and has a surface area of ​​104.37 mm 3 The electrode distance is 1.87 mm, and the heat dissipation coefficient is 1. The four-wire method is used. The temperature range is 25K to 300K, and the temperature drop rate is 0.1K / min. After that, the LiErF 4 Obtain the relationship of the thermal conductivity of the sample with respect to temperature, and verify that the thermal conductivity at 25 K is 50 W K. -1 m -1 reaches. Example 6

[0048] LiTmF 4 Magnetocaloric performance testing of single crystals In the magnetic test, first, a scalpel was used to remove the large LiTmF 4 A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 82.63 mg. The sample is fixed in a plastic sample tube with double-sided tape. Then, MH measurements are performed along the easy magnetization direction of the sample, and the MH curve is measured at a temperature of 1.8 K, and the MH curve is measured every 1 K within the temperature range of 2 K to 10 K. The MH curve is measured every 2 K within the temperature range of 10 K to 24 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0049] Based on the MH curves at different temperatures obtained by the above magnetic tests, the Maxwell equations were used to determine the 4 The relationship between the magnetic entropy change and the temperature change after applying a changing magnetic field of 0 to 10 kOe and 0 to 20 kOe along the easy magnetization direction to the sample is obtained. 4 The sample had a magnetic field of 0.692 J kg under a varying magnetic field of 0 to 20 kOe. -1 ·K -1 We obtain the magnetic entropy change of Example 7

[0050] LiYbF 4 Magnetocaloric performance testing of single crystals In the magnetic test, first, a large LiYbF 4 A small sample is cut out from the single crystal sample and weighed on an electronic balance, and the mass of the sample is 4.81 mg. The sample is fixed to a plastic sample tube with double-sided tape. Then, MH measurements are performed along the easy magnetization direction of the sample, and the MH curve is measured at a temperature of 1.8 K, and the MH curve is measured every 1 K within the temperature range of 2 K to 10 K. The MH curve is measured every 2 K within the temperature range of 10 K to 24 K. In each measured MH curve, one data point is measured every 500 Oe in a magnetic field of 0 to 10 kOe, and one data point is measured every 1000 Oe in a magnetic field of 10 kOe to 20 kOe.

[0051] Based on the MH curves at different temperatures obtained by the above magnetic tests, the Maxwell equations were used to determine the 4 The magnetic entropy change with temperature after applying a varying magnetic field of 0-10 kOe and 0-20 kOe along the easy magnetic axis to the sample is obtained. LiYbF 4 The sample was subjected to a magnetic field of 0 to 20 kOe with a resistance of 14.7 J kg -1 ·K -1 We obtain the magnetic entropy change of

[0052] The above examples are merely intended to illustrate the technical ideas and features of the present invention so that those skilled in the art can understand and practice the contents of the present invention, and are not intended to limit the scope of protection of the present invention. Any equivalent modifications or improvements made based on the spirit of the present invention should be included within the scope of protection of the present invention.

[0053] In addition, in this specification, descriptions of known structures and techniques are omitted in order to avoid unnecessarily confusing the concepts of the present invention.

Claims

1. Application of lithium rare earth fluoride single crystals to magnetic refrigeration.

2. The chemical formula of the lithium rare earth fluoride is LiREF ４ 2. The application of claim 1, wherein RE is a rare earth element.

3. 3. The application according to claim 2, characterized in that the rare earth element RE is one or more of gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium.

4. 4. The application of claim 3, wherein the lithium rare earth fluoride is a single crystal having a tetragonal scheelite structure and having magnetic anisotropy.

5. LiGdF ４ , LiTbF ４ and LiHoF ４ The easy magnetization direction of LiDyF is parallel to the C axis. ４ , LiErF ４ , LiYbF ４ and LiTmF ４ 5. The application of claim 4, characterized in that the easy axis of magnetization of is perpendicular to the C-axis.

6. LiGdF ４ is paramagnetic, and LiTmF ４ exhibits Van Vleck paramagnetism, and LiTbF ４ and LiHoF ４ exhibit ferromagnetic order below the Curie temperatures of 2.87 K and 1.53 K, respectively, and LiDyF ４ , LiErF ４ and LiYbF ４ and exhibit antiferromagnetic order below the Neel temperatures of 0.62 K, 0.38 K and 0.128 K, respectively.

7. The application according to any one of claims 1 to 4, characterized in that the application temperature range of the lithium rare earth fluoride single crystal is a low temperature range below 20K.

8. When a varying magnetic field of 0 to 20 kOe is applied along the easy magnetic axis of a lithium rare earth fluoride single crystal sample, the lithium rare earth fluoride single crystal exhibits a large isothermal magnetic entropy change, LiGdF ４ , LiTbF ４ , LiHoF ４ , LiDyF ４ , LiErF ４ , LiYbF ４ , LiTmF ４ is 50.35 J kg at 1.8K, 4K, 4K, 1.8K, 1.8K, 1.8K, and 18K. －１ K －１ , 21.12J・kg －１ K －１ , 24.98J・kg －１ K －１ , 24.45J・kg －１ K －１ , 21.95J・kg －１ K －１ , 14.7 J kg －１ K －１ and 0.692 J kg －１ K －１ The application according to any one of claims 1 to 4, characterized in that an isothermal magnetic entropy change of

9. LiTbF ４ and LiErF ４ The thermal conductivity of at 25K is 50W・K. －１ ・m －１ 6. The application according to claim 5, characterized in that

10. A magnetic refrigeration method based on lithium rare earth fluoride single crystal, characterized in that the magnetic working material is a lithium rare earth fluoride single crystal, and the temperature of the magnetic working material is changed by applying and removing a magnetic field to the magnetic working material, thereby realizing the transfer of heat with a heat exchanger, thereby achieving the purpose of refrigeration.

11. The lithium rare earth fluoride single crystal is LiGdF ４ , LiTbF ４ , LiHoF ４ , LiDyF ４ , LiErF ４ , LiYbF ４ and LiTmF ４ One or more of the following: The magnetic refrigeration method according to claim 10, wherein in the case of a plurality of types, different lithium rare earth fluoride single crystals are mixed and arranged to realize an expansion of the low-temperature refrigeration section.

12. The chemical formula of the lithium rare earth fluoride is LiREF ４ 11. The magnetic refrigeration method according to claim 10, wherein RE is a rare earth element and is at least two rare earth elements selected from the group consisting of Gd, Tb, Dy, Ho, Er, Tm, and Yb, and the low-temperature refrigeration section is expanded.

13. A magnetic refrigeration device driven by a small magnetic field, comprising a lithium rare earth fluoride single crystal as a magnetic working material.

14. The lithium rare earth fluoride single crystal, which is a magnetic working material, is LiGdF ４ , LiTbF ４ , LiHoF ４ , LiDyF ４ , LiErF ４ , LiYbF ４ and LiTmF ４ One or more of the following:

14. The magnetic refrigeration apparatus according to claim 13, wherein in the case of a plurality of types, different lithium rare earth fluoride single crystals are mixed and arranged to realize an expansion of the low temperature refrigeration section.

15. The chemical formula of the lithium rare earth fluoride is LiREF ４ 14. The magnetic refrigeration apparatus according to claim 13, wherein RE is a rare earth element and is at least two rare earth elements selected from the group consisting of Gd, Tb, Dy, Ho, Er, Tm, and Yb.

16. 14. The magnetic refrigeration apparatus according to claim 13, further comprising a magnetic field applying member, a thermal switch, a heat sink, and a load.

17. 14. The magnetic refrigeration apparatus according to claim 13, wherein the magnetic field application member in the magnetic refrigeration apparatus uses a permanent magnet, and the thermal switch uses a superconducting thermal switch.

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