Use of amino rare-earth fluorides in magnetic refrigeration

Abstract

The present invention belongs to the field of magnetic functional materials. Disclosed are the use of amino rare-earth fluorides in magnetic refrigeration, a magnetic refrigeration method, and a magnetic refrigeration device. The magnetic card materials are amino rare-earth fluorides, and the chemical formulae thereof are NH4TbF4, NH4GdF4 and NH4SmF4. By applying a variable magnetic field of 0-4 kOe, 0-6 kOe, 0-10 kOe or 0-20 kOe to the magnetic card materials, the magnetic card materials can achieve a large magnetocaloric effect under a low driving magnetic field. In a variable magnetic field of 0-20 kOe, isothermal magnetic entropy changes with the sizes of 14.39 J·kg-1·K-1, 51.58 J·kg-1·K-1 and 0.47 J·kg-1·K-1 are respectively obtained at the positions of 2 K, 1.4 K and 2 K. In the case of no external magnetic fields, the specific heat peak value of NH4GdF4 at a low temperature of 0.8 K reaches 66.27 J·kg-1·K-1.

Description

Application of amino rare earth fluorides in magnetic refrigeration Technical Field

[0001] This invention belongs to the field of magnetic functional materials, specifically relating to a giant magnetic card effect material that can be driven by a small magnetic field in the low-temperature range. Background Technology

[0002] In recent years, with the rapid development of science and technology, cryogenic refrigeration technology has become increasingly indispensable in many basic science and engineering fields, such as spintronics and quantum computing, nuclear magnetic resonance imaging, space exploration, aerospace, and biomedicine. Especially in the field of space exploration, extremely low temperatures at the mK level are typically required, making research into cryogenic and ultra-low temperature technologies increasingly important. Currently, liquid nitrogen is commonly used to achieve cryogenic environments above 80K, and liquid helium (4He) is used to achieve cryogenic environments above 2K and below 20K. However, achieving ultra-low temperatures below 1K or even at the 20mK level requires the use of the scarce strategic resource helium-3 (3He). Magnetic refrigeration technology can not only achieve cryogenic, cryogenic, and ultra-low temperature refrigeration, but also does not rely on the expensive and scarce 3He resource, thus having broad application prospects in the field of cryogenic refrigeration. Therefore, a great deal of research has been conducted in the field of cryogenic magnetic refrigeration technology.

[0003] Currently, the most mature magnetic working materials for cryogenic magnetic refrigeration are the garnet series (Gd3Ga5O12 (GGG) and Dy3Al5O12 (DAG)). However, in practical applications, to obtain a large magnetic entropy change, a large varying magnetic field (0-40kOe, 0-50kOe, or 0-70kOe) is usually required for the magnetic working material. Such a magnetic field necessitates large electromagnets and superconducting magnets, magnetic shielding devices, etc., which severely hinders the widespread application of magnetic refrigeration technology. Therefore, finding a magnetic working material that can achieve a large magnetic entropy change under a small magnetic field within a low-temperature range is a key issue in cryogenic magnetic refrigeration.

[0004] To address the aforementioned problems, this invention provides a magnetic card material that can be driven by a small magnetic field to achieve large magnetic entropy change and large adiabatic temperature change. This magnetic card material is an amino rare earth fluoride polycrystalline powder. Summary of the Invention

[0005] The purpose of this invention is to disclose the application of a rare-earth fluoride polycrystalline powder in magnetic refrigeration. This rare-earth fluoride is an amino-rare-earth fluoride polycrystalline powder. By applying a varying magnetic field of 0-10 kOe or 0-20 kOe to this material within a low-temperature range, a large magnetic entropy change can be obtained. The discovery of this material opens up possibilities for the simplification and widespread application of low-temperature magnetic refrigeration devices.

[0006] The technical solution of this invention is as follows:

[0007] Application of an amino-rare-earth fluoride polycrystalline powder in magnetic refrigeration.

[0008] When an amino-rare-earth fluoride polycrystalline powder sample is subjected to an external magnetic field, the magnetic moments change from disordered to ordered. The interaction energy between atomic magnetic moments and between the atomic magnetic moments and the external magnetic field decreases, its magnetic entropy decreases, and it releases heat to the surroundings. Conversely, when the magnetic field is removed, the magnetic moments of the amino-rare-earth fluoride polycrystalline sample change from ordered to disordered, its magnetic entropy increases, and it absorbs energy from the surroundings. Under adiabatic conditions, this manifests as a temperature change within the amino-rare-earth fluoride polycrystalline sample itself, i.e., adiabatic temperature change.

[0009] As a preferred technical solution:

[0010] The chemical formula of the amino rare earth fluoride is NH4REF4, where RE represents a rare earth element.

[0011] The rare earth element RE is one or more of gadolinium (Gd), terbium (Tb), and samarium (Sm).

[0012] The amino rare earth fluoride is a polycrystalline powder with an orthorhombic crystal structure.

[0013] Among them, NH4TbF4 and NH4SmF4 exhibit paramagnetism, while NH4GdF4 exhibits ferromagnetic order at Curie temperatures below 0.8K.

[0014] The application temperature range of amino rare earth fluoride polycrystalline powder is the low temperature range below 20K.

[0015] The maximum isothermal magnetic entropy change ΔSm is an important parameter for evaluating the performance of a magnetic card material. Therefore, this invention uses a superconducting quantum interference device (SQI) to measure the magnetization curves of polycrystalline amino-rare-earth fluoride samples at different temperatures. Then, the obtained magnetization curve data at different temperatures are processed using Maxwell's relation to determine the relationship between magnetic entropy change and temperature. By applying a small varying magnetic field of 0-20 kOe to the polycrystalline amino-rare-earth fluoride samples, the polycrystalline amino-rare-earth fluoride samples exhibited large isothermal magnetic entropy changes. Specifically, NH4TbF4, NH4GdF4, and NH4SmF4 obtained isothermal magnetic entropy changes of 14.39 J·kg⁻¹·K⁻¹, 51.58 J·kg⁻¹·K⁻¹, and 0.47 J·kg⁻¹·K⁻¹ at 2 K, 1.4 K, and 2 K, respectively.

[0016] In the field of solid-state magnetic refrigeration, the change in specific heat of magnetic card materials under different magnetic fields with temperature indirectly reflects the magnetocaloric effect of the material. By calculating the total entropy of the measurement data, the change in the total entropy of the material under different magnetic fields can be determined. The specific heat of polycrystalline amino-rare-earth fluorides was tested using the specific heat option of a comprehensive physical property measurement system. Since the specific heat of polycrystalline amino-rare-earth fluoride samples is sensitive to the measurement environment, measurements were performed in a high-vacuum environment to reduce errors, and the samples were prepared in the form of polycrystalline powder compacts. NH4GdF4 reached a peak specific heat of 66.27 J·kg⁻¹·K⁻¹ at a low temperature of 0.8 K without an external magnetic field.

[0017] A magnetic refrigeration method based on amino-rare-earth fluoride polycrystalline material is characterized by using amino-rare-earth fluoride polycrystalline material as the magnetic working medium. By applying and removing magnetic fields to the magnetic working medium, the temperature of the magnetic working medium changes and heat is transferred with the heat exchanger, thereby achieving the purpose of refrigeration.

[0018] As a preferred technical solution:

[0019] The magnetic working material, amino-rare-earth fluoride polycrystalline material, can be one or more of NH4TbF4, NH4GdF4, and NH4SmF4; when it is multiple, the low-temperature refrigeration range can be extended by mixing and arranging different amino-rare-earth fluoride polycrystalline blocks.

[0020] The magnetic working material can also be an amino-rare-earth fluoride single crystal. When the magnetic working material is a single crystal, the magnetic field direction needs to be parallel to the easy magnetization direction of the amino-rare-earth fluoride single crystal to achieve higher low-temperature refrigeration performance.

[0021] Two or more rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) can also be incorporated into the preparation of amino rare earth fluorides to extend the cryogenic refrigeration range.

[0022] A magnetic refrigeration device driven by a small magnetic field, the device comprising amino rare earth fluoride polycrystalline material as magnetic working fluid.

[0023] As a preferred technical solution:

[0024] The magnetic working material, amino-rare-earth fluoride polycrystalline, can be one or more of NH4TbF4, NH4GdF4, and NH4SmF4; when it is multiple, the low-temperature refrigeration range can be extended by mixing and arranging different amino-rare-earth fluoride polycrystalline powders in bulk.

[0025] Two or more rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) can also be incorporated into the preparation of polycrystalline amino rare earth fluorides to extend the cryogenic cooling range.

[0026] The refrigeration device also includes a magnetic field application component, a thermal switch, a heat sink, and a load. The magnetic field application component can be a permanent magnet, an electromagnet, or a superconducting magnet, with a permanent magnet being preferred. This can effectively simplify the device size and optimize the design. The thermal switch can be a mechanical contact type, a superconducting type, a gas type, or a magnetoresistive type thermal switch, with a superconducting thermal switch being preferred.

[0027] The technical effects of this invention are as follows:

[0028] 1. Amino rare earth fluoride polycrystalline materials undergo a phase transition at low temperatures. Therefore, when used as a magnetic working medium in a magnetic refrigeration device, they can keep the refrigeration temperature range of the magnetic refrigeration device in a low-temperature range of less than 20K.

[0029] 2. Amino rare earth fluoride polycrystalline materials have extremely strong magnetic response, and can reach saturation with a small magnetic field. Components that provide small magnetic fields, such as common permanent magnets, are inexpensive and small in size. This can not only greatly reduce the cost of refrigeration devices, but also optimize the design of refrigeration devices. It can expand the application range of magnetic refrigeration devices to large scientific experimental devices and any devices and operations that require low temperature and extremely low temperature environments.

[0030] 3. This invention effectively expands the refrigeration range by mixing and arranging various amino-rare-earth fluoride polycrystalline powders in a compacted form, meeting the needs of different refrigeration ranges and further expanding the application scope. Alternatively, the refrigeration range can also be broadened by incorporating a certain amount of other rare-earth elements (Gd, Tb, Dy, Ho, Er, Tm, and Yb) during the preparation of amino-rare-earth fluoride polycrystalline powders.

[0031] 4. Compared to the 18.8 J·kg⁻¹·K⁻¹ isothermal magnetic entropy change of the currently mainstream gadolinium gallium garnet (Gd₃Ga₅O₁₂) single crystal material at 0-20 kOe, the polycrystalline NH₄TbF₄, NH₄GdF₄ and NH₄SmF₄ exhibit isothermal magnetic entropy changes of 14.39 J·kg⁻¹·K⁻¹, 51.58 J·kg⁻¹·K⁻¹ and 0.47 J·kg⁻¹·K⁻¹ at 2 K, 1.4 K and 2 K, respectively.

[0032] Among them, the superior performance of NH4GdF4 makes it a complete replacement for these mainstream materials, and it can be widely used in cryogenic magnetic refrigeration fields such as gas liquefaction, ultra-low temperature scientific devices, and aerospace, to achieve low-cost, high-efficiency, and pollution-free cryogenic magnetic refrigeration. At the same time, NH4GdF4 achieves a specific heat peak of 66.27 J·kg-1·K-1 at a low temperature of 0.8 K without an external magnetic field.

[0033] 5. The preparation process of polycrystalline amino rare earth fluorides is simple and mature, enabling 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 promoted and applied. Attached Figure Description

[0034] Figure 1 shows the crystal structure of amino rare earth fluorides.

[0035] Figure 2 shows the X-ray diffraction pattern of amino rare earth fluorides. The bottom curve is the calculated result of NH4GdF4 (Example 1).

[0036] Figure 3 is a schematic diagram of the magnetic refrigeration principle.

[0037] Figure 4 shows the magnetic entropy change curves of gadolinium gallium garnet (Gd3Ga5O12) (GGG) single crystal material and amino rare earth fluoride polycrystalline material under a magnetic field of 0-10kOe (Examples 2-3).

[0038] Figure 5 shows the magnetic entropy change curves of gadolinium gallium garnet (Gd3Ga5O12) (GGG) single crystal material and amino rare earth fluoride polycrystalline material under a magnetic field of 0-20kOe (Examples 2-3).

[0039] Figure 6 shows the magnetic entropy change curves of gadolinium gallium garnet (Gd3Ga5O12) (GGG) single crystal and NH4GdF4 polycrystalline material under varying magnetic fields of 0-10kOe and 0-20kOe (Example 4).

[0040] Figure 7 shows the specific heat of NH4GdF4 polycrystalline material as a function of temperature under different magnetic fields (Example 4).

[0041] Figure 8 is a schematic diagram of the magnetic refrigeration device. Detailed Implementation

[0042] An application device for amino rare earth fluoride polycrystalline materials, specifically a magnetic refrigeration device driven by a magnetic field, wherein the magnetic working medium is selected from amino rare earth fluoride polycrystalline materials, specifically one or more of NH4TbF4, NH4GdF4 and NH4SmF4.

[0043] The device also includes a magnetic field application component, a thermal switch, a heat sink (a miniature heat sink), and a load (the end to be cooled, such as a copper cooling box). The magnetic field application component can be one or more of permanent magnets, electromagnets, or superconducting magnets; the thermal switch can be one or more of mechanical contact, superconducting, gas, or magnetoresistive thermal switches; the load is used to achieve cooling; and the heat sink provides heat dissipation. Initially, the amino-rare-earth fluoride polycrystalline material has the same temperature as the heat sink in a zero magnetic field state. Then, with the thermal switch closed, the magnetic field is increased. During isothermal magnetization, heat flows from the magnetic working fluid to the heat sink, and entropy decreases. When the magnetic field reaches its maximum, the thermal switch is opened, and then the magnetic field is decreased. Even when the temperature of the magnetic working fluid drops to the temperature required by the load, some magnetic field remains. As the magnetic field continues to decrease, the magnetic working fluid absorbs heat from the load to cool it, thus achieving the cooling function of the entire device.

[0044] When multiple different amino-rare-earth fluoride polycrystalline powders are mixed and arranged, the cooling range can be effectively expanded. For example, in one embodiment, NH4TbF4 and NH4GdF4 powders are mixed and arranged, so that the cooling range is expanded from the temperature range below 6K when NH4GdF4 is used alone to the temperature range below 15K. In another embodiment, by incorporating a certain amount of other rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) during the preparation of amino-rare-earth fluorides, amino-rare-earth fluorides containing two or more rare earth elements can also be prepared, thereby expanding the low-temperature cooling range.

[0045] The following examples demonstrate the magnetic card performance of NH4TbF4, NH4GdF4, and NH4SmF4. The test results show that, within the low-temperature range below 20K, under a magnetic field variation of 0-20kOe, the polycrystalline NH4GdF4 exhibits a superior isothermal magnetic entropy change of 18.8 J·kg⁻¹·K⁻¹ compared to the single-crystal material of gadolinium gallium garnet (Gd₃Ga₅O₁₂ (GGG)). Furthermore, NH4GdF4 achieves a peak specific heat of 66.27 J·kg⁻¹·K⁻¹ at a low temperature of 0.8K without an external magnetic field.

[0046] Example 1

[0047] Preparation method of NH4REF4 (RE represents samarium, gadolinium, and terbium) sample (solution method): First, dissolve 1 mmol of Gd(NO3)3·6H2O, Tb(NO3)3·6H2O, or Sm(NO3)3·6H2O (purity 99.99%) and 1 mmol of NH4F (purity 99.9%) in 15 ml of anhydrous methanol, respectively. Then, mix the two solutions and place them on a magnetic stirrer, stirring for 10 minutes. Pour the resulting solution into a 50 ml reaction vessel liner, cover it, place the liner into the reaction vessel, tighten the reaction vessel lid to seal, and then place it in a vacuum drying oven. Heat to 150°C and react at 150°C for 4 days, then allow it to cool naturally. Subsequently, filter the resulting solution using a funnel and filter paper, and simultaneously add 80 ml of deionized water and 20 ml of anhydrous ethanol (added in 4 portions, no specific order) to wash the sample. The samples left on the filter paper were allowed to dry naturally. After complete drying, the samples were recovered to obtain NH4GdF4, NH4TbF4, or NH4SmF4, respectively. The crystal structures of this series of materials were characterized by X-ray diffraction. The results showed that NH4GdF4, NH4TbF4, and NH4SmF4 all have orthorhombic crystal structures (Figure 2).

[0048] Example 2

[0049] Magnetic card performance test of NH4SmF4 polycrystalline powder

[0050] In the magnetic testing, a portion of the NH4SmF4 powder sample was first taken out and weighed using an electronic balance. The sample mass was 9.52 mg. The sample was then placed into a transparent plastic capsule (a cylindrical capsule with an open top, 1.5 cm long and 0.56 cm in diameter; containing approximately 0.15 g of sample). 0.2 ml of curing adhesive (GE / IMI 7031VARNISH; CMR direct) was added over the sample to cover it. The capsule was left to dry naturally for 6-12 hours, fixing the sample to the bottom. The capsule containing the sample was then fixed into a plastic sample tube. Next, a superconducting quantum interference magnetic measurement (SQUID; Quantum Design) was used to perform magnetization curve (MH) tests (M is the magnetic moment, H is the magnetic field). Within a temperature range of 2-20 K, an MH curve was measured every 1 K (Figures 4-5 only show data from 0-10 K). Within each tested MH curve, a data point was measured every 2000 Oe under a magnetic field of 0-50 kOe.

[0051] Based on the MH curves obtained from the magnetic tests above at different temperatures, the relationship between the magnetic entropy change and temperature after applying varying magnetic fields of 0-10 kOe, 0-20 kOe, or 0-50 kOe to the NH4SmF4 sample was calculated using Maxwell's equations (Reference: Phahul Zhemas Zul Nehan et al., The magnetocaloric effect properties for potential applications of magnetic refrigerator technology: a review, Physical Chemistry Chemical Physics, 2024-04-29, DOI:10.1039 / d4cp01077a). The NH4SmF4 sample obtained magnetic entropy changes of 0.13 J·kg⁻¹·K⁻¹, 0.47 J·kg⁻¹·K⁻¹, and 2.58 J·kg⁻¹·K⁻¹ under varying magnetic fields of 0-10 kOe, 0-20 kOe, and 0-50 kOe, respectively.

[0052] Example 3

[0053] Magnetic card performance test of NH4TbF4 polycrystalline powder

[0054] In the magnetic testing, a portion of the NH4TbF4 powder sample was first taken and weighed using an electronic balance; the sample mass was 1.61 mg. The sample was then placed into a transparent plastic capsule (a cylindrical capsule with an open top, 1.5 cm long and 0.56 cm in diameter; containing approximately 0.15 g of sample). 0.2 ml of curing adhesive (GE / IMI 7031VARNISH; CMR direct) was added over the sample to cover it, and the capsule was allowed to dry naturally. The sample was then fixed to the bottom of the plastic capsule, and the capsule containing the sample was then fixed into a plastic sample tube. Next, a magnetization curve (MH) test (M is the magnetic moment, H is the magnetic field) was performed using superconducting quantum interference magnetic measurement (SQUID; Quantum Design). Within the temperature range of 2-20 K, an MH curve was measured every 1 K (Figures 4-5 only show data from 0-10 K). Within each tested MH curve, a data point was measured every 2000 Oe under a magnetic field of 0-50 kOe.

[0055] Based on the MH curves obtained from the magnetic tests above at different temperatures, the relationship between the magnetic entropy change and temperature after applying varying magnetic fields of 0-10 kOe, 0-20 kOe, or 0-50 kOe to the NH4TbF4 sample was calculated using Maxwell's equations (Reference: Phahul Zhemas Zul Nehan et al., The magnetocaloric effect properties for potential applications of magnetic refrigerator technology: a review, Physical Chemistry Chemical Physics, 2024-04-29, DOI:10.1039 / d4cp01077a). The NH4TbF4 sample obtained magnetic entropy changes of 6.31 J·kg⁻¹·K⁻¹, 14.39 J·kg⁻¹·K⁻¹, and 20.65 J·kg⁻¹·K⁻¹ under varying magnetic fields of 0-10 kOe, 0-20 kOe, and 0-50 kOe, respectively.

[0056] Example 4

[0057] Magnetic card performance test of NH4GdF4 polycrystalline powder

[0058] In the magnetic test, a portion of the NH4GdF4 powder sample was first taken out and pressed into a block (applied pressure of 2MPa). Then, a small piece of the sample (length*width*height of 2mm*1mm*0.5mm) was cut out and weighed by an electronic balance. The sample mass was 2.51mg. The sample was then fixed in a plastic sample tube using double-sided tape. Next, the magnetic measurement system (MPMS3; Quantum Design) was used to perform the MH test. (1) MH test in the 0.4-2.0K temperature range (using the extended function option: He3 refrigerator option; Quantum Design): MH curves were measured at 0.4K and 0.5K respectively, and then MH curves were measured every 0.2K between 0.6-2K. In each MH curve, a data point was measured every 1000Oe under a magnetic field of 0-2kOe and every 2000Oe under a magnetic field of 2-50kOe. (2) MH test in the 2-8K temperature range: MH curves are measured every 1K in the temperature range of 2-8K. In each MH curve, a data point is measured every 1000Oe under a magnetic field of 0-2kOe and every 2000Oe under a magnetic field of 2-50kOe.

[0059] Based on the MH curves obtained from the magnetic tests above at different temperatures, the relationship between the magnetic entropy change and temperature after applying varying magnetic fields of 0-10 kOe, 0-20 kOe, or 0-50 kOe to the NH4GdF4 sample was calculated using Maxwell's equations. The NH4GdF4 sample exhibited magnetic entropy changes of 38.19 J·kg⁻¹·K⁻¹, 51.58 J·kg⁻¹·K⁻¹, and 64.93 J·kg⁻¹·K⁻¹ under these varying magnetic fields.

[0060] Utilizing a comprehensive material property measurement system ( DynaCool TM Specific heat tests of NH4GdF4 polycrystalline material under different magnetic fields were conducted using Quantum Design. A portion of the NH4GdF4 powder sample was taken and pressed into a block (applied pressure 2 MPa), and then a small sample (length * width * height 1 mm * 1 mm * 0.5 mm) was cut out and weighed using an electronic balance; the sample mass was 1.12 mg. The sample was fixed in the center of the sample stage using 0.1 ml of low-temperature vacuum grease (Apiezon N grease; the specific heat variation data of the low-temperature vacuum grease with temperature has been determined and will be used as a baseline). Subsequently, the measurement environment was evacuated to a high vacuum state (vacuum degree 1 × 10⁻⁵ Torr) to measure the specific heat at different temperatures. Then, the specific heat variation with temperature was measured under an applied constant magnetic field of 5 kOe, 10 kOe, 20 kOe, and 50 kOe. For the ultra-low temperature measurement, the extended function option: He3 refrigerator option (Quantum Design), with a temperature range of 0.1-4 K, and one data point measured at a temperature interval of 0.1 K for each curve. In the relatively high-temperature regions of 2K-300K (without magnetic field) and 2K-50K (with external magnetic field), the He3 refrigeration option was not used. For each curve, the temperature interval was 1K for 2-14K, 3K for 14-50K, and 5K for 50-300K. For example, NH4GdF4 reached a specific heat peak of 66.27 J·kg-1·K-1 at a low temperature of 0.8K without an external magnetic field.

[0061] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

[0062] Furthermore, descriptions of well-known structures and techniques are omitted herein to avoid unnecessarily obscuring the concepts of the present invention.

Claims

1. An application of an amino-rare earth fluoride in magnetic refrigeration.

2. Use according to claim 1, characterized in that: The chemical formula of the amino rare earth fluoride is NH4REF4, where RE represents a rare earth element.

3. Use according to claim 1 or 2, characterized in that: The rare earth element RE is one or more of samarium, gadolinium, and terbium.

4. Use according to claim 3, characterized in that: The amino rare earth fluoride is a polycrystalline powder with an orthorhombic crystal structure. NH4TbF4 and NH4SmF4 exhibit paramagnetism in the temperature range above 2K, while NH4GdF4 exhibits ferromagnetic order at Curie temperatures below 0.8K.

5. Use according to any one of claims 1 to 4, characterized in that: The application temperature range of amino rare earth fluoride polycrystalline powder is the low temperature range below 20K.

6. Use according to any one of claims 1 to 4, characterized in that: When a varying magnetic field of 0-20 kOe is applied to polycrystalline powder samples of amino rare earth fluorides, the amino rare earth fluorides exhibit large isothermal magnetic entropy changes. Among them, NH4TbF4, NH4GdF4 and NH4SmF4 obtain isothermal magnetic entropy changes of 14.39 J·kg-1·K-1, 51.58 J·kg-1·K-1 and 0.47 J·kg-1·K-1 at 2 K, 1.4 K and 2 K, respectively. The specific heat peak of NH4GdF4 at a low temperature of 0.8K without an external magnetic field reaches 66.27 J·kg-1·K-1.

7. The application according to any one of claims 1-4, characterized in that: The magnetic refrigeration method based on amino rare earth fluoride polycrystalline powder is as follows: amino rare earth fluoride polycrystalline powder is pressed into a block (2-20MPa, preferably 6-12MPa, more preferably 6-8MPa) as the magnetic working medium. By applying and removing magnetic fields to the magnetic working medium, the temperature of the magnetic working medium changes and heat is transferred with the heat exchanger, thereby achieving the purpose of refrigeration.

8. Use according to claim 7, characterized in that: The amino rare earth fluoride polycrystalline material is one or more of NH4TbF4, NH4GdF4 and NH4SmF4; when it is multiple, the low temperature refrigeration range is expanded by pressing and mixing different amino rare earth fluoride polycrystalline powders separately. Alternatively, the chemical formula of the amino rare earth fluoride is NH4REF4, where RE is a rare earth element, and RE is derived from a mixture of at least two rare earth elements selected from Tb, Gd, and Sm to extend the cryogenic refrigeration range. Alternatively, by incorporating 1% to 10% of other rare earth elements (other rare earth elements refer to one or more of Tb, Gd, and Sm) during the preparation of polycrystalline amino rare earth fluorides (where rare earth refers to one or more of Gd, Tb, Dy, Ho, Er, Tm, and Yb), an amino rare earth fluoride containing two or more rare earth elements can be prepared, thus extending the cryogenic cooling range.

9. A magnetic refrigeration device driven by a small magnetic field, characterized by The device includes amino rare earth fluoride polycrystalline material as the magnetic working medium. The magnetic working material amino-rare earth fluoride polycrystalline is one or more of NH4TbF4, NH4GdF4 and NH4SmF4; when it is multiple, the low temperature refrigeration range is extended by mixing and arranging different amino-rare earth fluoride polycrystalline blocks. Alternatively, the chemical formula of the amino rare earth fluoride is NH4REF4, where RE is a rare earth element, and RE is derived from a mixture of at least two rare earth elements selected from Tb, Gd, and Sm. Alternatively, by incorporating 1% to 10% of other rare earth elements (other rare earth elements refer to one or more of Tb, Gd, and Sm) during the preparation of polycrystalline amino rare earth fluorides (where rare earth refers to one or more of Gd, Tb, Dy, Ho, Er, Tm, and Yb), an amino rare earth fluoride containing two or more rare earth elements can be prepared, thus extending the cryogenic cooling range.

10. The magnetic refrigeration device according to claim 9, characterized in that: The refrigeration device also includes a magnetic field application component, a thermal switch, a heat sink, and a load; The magnetic field application component can be one or more of permanent magnets, electromagnets, or superconducting magnets, with permanent magnets being preferred; the thermal switch can be one or more of mechanical contact type, superconducting type, gas type, or magnetoresistive type, with superconducting type being preferred; the load function is used to achieve cooling; the heat sink plays a role in heat dissipation.

Images