Application of potassium-based rare earth fluoride as magnetic material in magnetic refrigeration

Abstract

The invention discloses application of potassium-based rare earth fluoride in magnetic refrigeration and a magnetic refrigeration device, and belongs to the field of magnetic functional materials. The series of magnetic card materials are single-phase potassium-based gadolinium fluoride, and the chemical formula is KGdF4. The series of magnetic card materials can obtain a large magnetocaloric effect under a low driving magnetic field, and a 0-5kOe, 0-10kOe or 0-20kOe variable magnetic field is applied to the series of magnetic card materials. Under a magnetic field of 0-20 kOe, isothermal magnetic entropy changes of 29.18 J * kg <-1 > * K <-1 >, 27.92 J * kg <-1 > * K <-1 > and 31.99 J * kg <-1 > * K <-1 > are obtained at the temperature of 2K. When the cubic phase KGdF4 does not have an external magnetic field, the specific heat peak value at a low-temperature 2K position reaches 19.20 J * kg <-1 > * K <-1 >; and when the external magnetic field is increased to 2T, the low-temperature specific heat peak value reaches 72.15 J * kg <-1 > * K <-1 >.

Description

Technical Field

[0001] The invention belongs to the field of magnetic functional materials, and in particular relates to a giant magnetocaloric effect material which can be driven by a small magnetic field in a low temperature range. Background Art

[0002] With the development of society and technology, cryogenic refrigeration technology plays a vital role today, providing key support for scientific research, biomedical engineering, space exploration, aerospace, military industry, etc. At present, the most direct way to achieve low temperature still relies on 3 He dilution refrigeration, but 3 He has the disadvantages of being very expensive and unstable, especially in a gravity-free space environment. 3 He / 4 He dilution refrigerators are difficult to use. However, magnetic refrigeration technology can not only achieve low temperature, deep low temperature and ultra-low temperature refrigeration, but also does not rely on expensive and scarce 3 He resources, and thus have broad application prospects in the field of low-temperature refrigeration. In an adiabatic demagnetization refrigerator (ADR), the driving magnet does not need to contact the magnetic refrigerant, which can effectively reduce heat leakage to ensure the high efficiency of the refrigerator and have excellent performance at extremely low temperatures. At present, ADR is the only 3 He / 4 One of the mainstream low-temperature refrigeration technologies besides He dilution refrigerator. Therefore, researchers have carried out a lot of research work in the field of low-temperature magnetic refrigeration technology.

[0003] So far, the garnet series (Gd3Ga5O 12 (abbreviated as GGG) and Dy3Al5O 12 As a low-temperature refrigeration magnetic fluid, Dielectric Magnetic Refrigerant (DAG) has been favored by researchers and successfully applied in practical applications. However, in order to obtain a larger magnetic entropy change, it is usually necessary to apply a larger variable magnetic field (0-40kOe, 0-50kOe or 0-70kOe) to the magnetic fluid, and this magnetic field requires larger electromagnets and superconducting magnets, magnetic shielding devices, etc. This results in higher production costs and a more stringent working environment, which makes the development of magnetic refrigeration technology more restricted. Therefore, finding a magnetic fluid that can obtain a large magnetic entropy change in a low temperature range driven by a small magnetic field is a major goal of low-temperature magnetic refrigeration.

[0004] In view of the above problems, the present invention provides a magnetic card material which can be driven by a small magnetic field and obtain a large magnetic entropy change. The magnetic card material is a potassium-based rare earth fluoride polycrystalline powder. Summary of the invention

[0005] The purpose of the present invention is to disclose the preparation of a potassium-based rare earth fluoride polycrystalline powder and its application in magnetic refrigeration. The rare earth fluoride is potassium-based gadolinium fluoride polycrystalline. The orthorhombic phase and the rhombohedral phase are sintered twice at 500-550 and 630-680°C in a muffle furnace, and the cubic phase is obtained by sintering at 720-770°C and then quenching. In addition, a large magnetic entropy change can be obtained by applying a changing magnetic field of 0-5kOe, 0-10kOe or 0-20kOe to the material in the low temperature range. The discovery of this material provides the possibility for the simplification and wide application of low-temperature magnetic refrigeration devices.

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

[0007] The invention discloses a preparation method of potassium-based rare earth fluoride polycrystalline powder.

[0008] The sample is prepared by solid-phase sintering. The powder system with two or more components is sintered at a temperature below the melting point of the low-melting component. The mixed powder or sample diffuses mutually at a specific temperature, so that the microscopic discrete particles gradually form a continuous solid structure. In this process, the overall free energy of the sample is reduced, but the strength is improved. The solid-phase sintering process includes the effects of sintering mechanisms such as surface diffusion, grain boundary diffusion, grain boundary migration, and particle rearrangement.

[0009] As the preferred technical solution:

[0010] The solid phase sintering method is carried out in a muffle furnace, and the mixed powder sample is sealed in a tube, and the atmosphere in the tube is selected to be high-purity argon.

[0011] The rare earth element in the potassium-based rare earth fluoride is gadolinium (Gd), and the chemical formula is KGdF4.

[0012] Among them, KF and GdF3 powders were selected as raw materials. They were mixed in a mortar in a 1:1 equimolar ratio in a glove box, fully ground and pressed into sheets (thickness 1-3mm) under a pressure of 15MPa. Pressing the mixed powder into sheets is beneficial to the uniform heating of the sample powder on the one hand, and promotes the diffusion and interaction between the powders on the other hand. The sheet sample was then sealed in a high-purity Ar gas atmosphere.

[0013] The single phase of the orthorhombic phase structure is obtained by sintering at 500-550℃ for 6-12 hours, taking out and grinding it thoroughly, sealing it with Ar gas again, and sintering it for the second time under the same conditions to ensure that the sample is fully reacted. The final sample is subjected to an XRD diffraction experiment, and the obtained spectrum is compared with the structure spectrum calculated from the standard KGdF4 orthorhombic phase cif file. It is found that there are no other impurity peaks in the experimental spectrum, and the sample purity is very high.

[0014] The single phase of the rhombohedral phase structure is obtained by sintering at 630-680℃ for 6-12 hours, taking out and grinding it thoroughly, and then sealing it with Ar gas again, and sintering it for the second time under the same conditions to ensure that the sample is fully reacted. The final sample is subjected to XRD diffraction experiment, and the obtained spectrum is compared with the structure spectrum calculated from the standard KYF4 rhombohedral phase cif file (since the cif file of KGdF4 rhombohedral phase is not reported, the cif file of similar compounds can be used for comparison). It is found that there are no other impurity peaks in the spectrum obtained by the experiment, and the sample purity is very high.

[0015] During the solid-state sintering process, high temperature makes the atoms react more rapidly, accelerates diffusion and atomic rearrangement, and produces a specific structural phase. The orthorhombic phase represents the low-temperature phase produced at 500-550°C, which is relatively stable and easy to obtain. The trigonal phase represents the medium-temperature phase produced at 630-680°C, which can be successfully obtained after cooling with the furnace after the insulation is completed. The cubic phase is synthesized at 720-770°C and belongs to the high-temperature phase. As the temperature slowly decreases, it will continue to transform into a low-temperature phase, so it is necessary to use ice water quenching to obtain the cubic phase structure at high temperature through the principle of rapid cooling. The specific steps are to sinter the sealed sample at 720-770°C for 6-12 hours, then keep the temperature in the furnace unchanged, take out the sample directly and put it into water for quenching. After sufficient grinding, press the tablet again and seal the tube with Ar gas, and sinter for the second time under the same conditions to ensure that the sample reacts fully. The sample obtained after the final water quenching was subjected to an XRD diffraction experiment, and the obtained spectrum was compared with the structural spectrum calculated from the standard NaYF4 cubic phase cif file (since the cif file of the KGdF4 trigonal phase has not been reported, the cif file of a similar compound can be used for comparison). It was found that there were no other impurity peaks in the spectrum obtained from the experiment, and the sample purity was very high.

[0016] Application of potassium-based rare earth fluoride polycrystalline powder in magnetic refrigeration.

[0017] Under the action of an external magnetic field, the magnetic moment of a potassium-based rare earth fluoride polycrystalline powder sample changes from disorder to order, the interaction energy between the atomic magnetic moments and the external magnetic field decreases, its magnetic entropy decreases and it releases heat to the outside world. Conversely, in the process of removing the magnetic field, the magnetic moment of the potassium-based rare earth fluoride polycrystalline changes from order to disorder, the magnetic entropy increases, and it absorbs energy from the outside world. In the case of adiabatic sample, it manifests as a temperature change of the potassium-based rare earth fluoride polycrystalline sample itself, that is, adiabatic temperature change.

[0018] As the preferred technical solution:

[0019] The rare earth element in the potassium-based rare earth fluoride is Gd element, and the chemical formula is KGdF4.

[0020] The potassium-based rare earth fluoride is KGdF4 polycrystalline powder with an orthorhombic, trigonal or cubic crystal structure.

[0021] Among them, the three phase structures of KGdF4 all show paramagnetism in the temperature range above 2K, the orthorhombic and trigonal phases of KGdF4 show ferromagnetic order at the Curie temperature below 2K, and the cubic phase of KGdF4 shows antiferromagnetic order at the Curie temperature below 2K. The application temperature range of KGdF4 polycrystalline powder is the low temperature range below 20K.

[0022] Maximum isothermal magnetic entropy change ΔS m It is an important parameter to measure the performance of a magnetic card material. Therefore, the present invention uses a superconducting quantum interference device to measure the magnetization curves of KGdF4 polycrystalline samples with different phase structures at different temperatures. Then the Maxwell relationship is used to process the magnetization curve data obtained at different temperatures, and the relationship between the magnetic entropy change and the temperature change is calculated. By applying a small variable magnetic field of 0-20kOe to the single-phase polycrystalline sample, the sample shows a large isothermal magnetic entropy change, among which the KGdF4 polycrystalline powder with orthorhombic phase, rhombohedral phase and cubic phase respectively obtains 29.18Jkg at 2K. -1 K -1 、27.92Jkg -1 K -1 and 31.99Jkg -1 K -1 Isothermal magnetic entropy change of magnitude.

[0023] In the field of solid-state magnetic refrigeration, the change of specific heat of magnetic card materials with temperature under different magnetic fields will indirectly reflect the magnetocaloric effect of the material. By calculating the total entropy of the measured data, the change of total entropy of the material under different magnetic fields can be known. The test of specific heat of potassium-based rare earth fluoride polycrystalline is carried out by the specific heat option of the comprehensive physical property measurement system. Since the specific heat of potassium-based rare earth fluoride polycrystalline samples is sensitive to the measurement environment, it is measured in a high vacuum environment to reduce errors, and the samples are prepared in the form of polycrystalline powder compacts. In the absence of an external magnetic field, the peak specific heat of cubic phase KGdF4 at a low temperature of 2K reaches 19.20 J·kg -1 ·K -1 When the external magnetic field increases to 2T, it can be seen that its low-temperature specific heat reaches 72.15J·kg -1 ·K -1 .

[0024] A magnetic refrigeration method based on potassium-based rare earth fluoride polycrystals, characterized in that potassium-based rare earth fluoride polycrystals are used as magnetic working fluids, and by applying and removing a magnetic field to the magnetic working fluids, the temperature of the magnetic working fluids changes and heat is transferred with a heat exchanger, thereby achieving the purpose of refrigeration.

[0025] As the preferred technical solution:

[0026] The magnetic working fluid potassium-based rare earth fluoride polycrystal can be one or more of the KGdF4 polycrystal samples with orthorhombic, rhombohedral or cubic phase structures; when there are multiple types, the low-temperature refrigeration range can be expanded by mixing and arranging KGdF4 polycrystal pressed blocks with different phase structures.

[0027] The magnetic working medium may also be a potassium-based rare earth fluoride single crystal. When the magnetic working medium is a single crystal, the magnetic field direction needs to be placed parallel to the easy magnetization direction of the potassium-based rare earth fluoride single crystal to achieve higher low-temperature refrigeration performance.

[0028] Different rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) can also be used to replace potassium-based rare earth fluorides in the process of preparing them to achieve the expansion of the low-temperature refrigeration range.

[0029] A magnetic refrigeration device driven by a small magnetic field comprises potassium-based rare earth fluoride polycrystals as magnetic working fluid.

[0030] As the preferred technical solution:

[0031] The potassium-based rare earth fluoride polycrystal of the magnetic working fluid can be one or more of the KGdF4 polycrystal samples with orthorhombic, rhombohedral or cubic phase structures; when there are multiple types, the low-temperature refrigeration range can be expanded by mixing and arranging KGdF4 polycrystal powder compacts with different phase structures.

[0032] Different rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) can also be used to replace them in the process of preparing potassium-based rare earth fluoride polycrystals, thereby expanding the low-temperature refrigeration range.

[0033] The refrigeration device also includes a magnetic field applying component, a thermal switch, a heat sink, and a load. The magnetic field applying component can be a permanent magnet, an electromagnet or a superconducting magnet, preferably a permanent magnet, which can effectively simplify the size of the device 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, preferably a superconducting thermal switch.

[0034] The technical effects of the present invention are as follows:

[0035] 1. Potassium-based rare earth fluorides can be successfully prepared by solid phase sintering. By controlling the working temperature in the furnace to 500-550°C, 630-680°C and 720-770°C, polycrystalline powders with orthorhombic, trigonal and cubic phase structures can be obtained respectively.

[0036] 2. Potassium-based rare earth fluoride polycrystals undergo phase transition at low temperatures. Therefore, when they are used as magnetic working fluids in magnetic refrigeration devices, the refrigeration temperature zone of the magnetic refrigeration device can be kept within a low temperature range of less than 20K.

[0037] 2. Potassium-based rare earth fluoride polycrystals 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 cheap and small in size. They can not only greatly reduce the cost of refrigeration devices, but also optimize the design of refrigeration devices, and expand the application scope of magnetic refrigeration devices to large scientific experimental devices and any devices and operations that require low and ultra-low temperature environments.

[0038] 3. The present invention can effectively expand the refrigeration range by mixing and arranging KGdF4 polycrystalline powder compacts with different phase structures, meet the needs of different refrigeration ranges, and further expand the scope of application. Alternatively, by replacing other rare earth elements (Gd, Tb, Dy, Ho, Er, Tm and Yb) in the process of preparing potassium-based rare earth fluoride polycrystalline, the refrigeration range can also be widened.

[0039] 4. Compared with the currently used gadolinium gallium garnet Gd3Ga5O 12 (GGG) single crystal material has 14.5 J·kg at 0-20 kOe at 2K -1 ·K -1 The isothermal magnetic entropy change of KGdF4 polycrystalline powder with orthorhombic, rhombohedral and cubic phases was 29.18 J kg at 2K. -1 K -1 , 27.92J kg -1 K -1 and 31.99 J kg -1 K -1 Isothermal magnetic entropy change of magnitude.

[0040] Among them, the excellent performance of cubic phase KGdF4 polycrystalline powder enables it to completely replace these mainstream materials and is widely used in low-temperature magnetic refrigeration fields such as gas liquefaction, ultra-low temperature scientific equipment and aerospace, achieving low-cost, high-efficiency and pollution-free low-temperature magnetic refrigeration. In addition, when there is no external magnetic field, the peak specific heat of cubic phase KGdF4 at low temperature 2K reaches 19.20J·kg -1 ·K -1 When the external magnetic field increases to 2T, it can be seen that its low-temperature specific heat reaches 72.15J·kg -1 ·K -1 .

[0041] 5. The preparation process of potassium-based rare earth fluoride polycrystals is simple and mature, and can be used for large-scale industrial production, which greatly reduces the use cost of the material and the manufacturing cost of the refrigeration system, and can be quickly and widely promoted and applied. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 These are three crystal structures of potassium-based rare earth fluoride (KGdF4).

[0043] Figure 2 It is the X-ray diffraction diagram of potassium-based rare earth fluoride (KGdF4), and the bottom curve is the calculation result of KGdF4 (Example 1).

[0044] Figure 3 This is the principle diagram of magnetic refrigeration.

[0045] Figure 4 Gadolinium gallium garnet Gd3Ga5O 12 Magnetic entropy change curves of (GGG) single crystal material and potassium-based rare earth fluoride polycrystal under a 0-10kOe changing magnetic field (Example 2-3).

[0046] Figure 5 Gadolinium gallium garnet Gd3Ga5O 12 Magnetic entropy change curves of (GGG) single crystal material and potassium-based rare earth fluoride polycrystal under a 0-20kOe changing magnetic field (Example 2-3).

[0047] Figure 6 Gadolinium gallium garnet Gd3Ga5O 12 Magnetic entropy change curves of (GGG) single crystal material and KGdF4 polycrystal under 0-10kOe and 0-20kOe changing magnetic fields (Example 4).

[0048] Figure 7 The specific heat variation with temperature when a constant magnetic field of 10kOe, 20kOe is applied and when there is no electric field.

[0049] Figure 8 Schematic diagram of the structure of the magnetic refrigeration device. DETAILED DESCRIPTION

[0050] A potassium-based rare earth fluoride polycrystal application device is specifically a magnetic refrigeration device driven by a magnetic field, wherein the magnetic working fluid is potassium-based rare earth fluoride polycrystal, specifically one or more of KGdF4 polycrystal samples with orthorhombic, trigonal or cubic phase structures.

[0051] The device also includes a magnetic field applying component, a thermal switch, a heat sink (a miniature heat sink), a load (the end to be cooled, for example, a copper refrigeration box), etc. The magnetic field applying component can be selected from one or more of permanent magnets, electromagnets or superconducting magnets; the thermal switch can be selected from one or more of mechanical contact, superconducting, gas or magnetoresistive thermal switches; the function of the load is to achieve cooling; and the heat sink plays a role in heat dissipation. In the initial state, the amino rare earth fluoride polycrystal has the same temperature state as the heat sink under the zero magnetic field state, and then the magnetic field is increased while the thermal switch remains closed. During the isothermal magnetization process, heat flows from the magnetic fluid to the heat sink, and the entropy decreases. When the magnetic field rises to the maximum, the thermal switch is turned on, and then the magnetic field is reduced. When the temperature of the magnetic fluid is reduced to the temperature required by the load, some magnetic field is still retained. As the magnetic field continues to decrease, the magnetic fluid will absorb heat from the load to cool the load, thereby achieving the purpose of cooling and realizing the refrigeration function of the entire device.

[0052] The following examples describe the preparation methods of potassium-based rare earth fluoride (KGdF4) with different phase structures and the magnetic card performance test of KGdF4 polycrystalline powders with orthorhombic, trigonal and cubic phase structures. The test results show that in the low temperature range of less than 20K, the three different phase structures of KGdF4 polycrystalline materials show better performance than gadolinium gallium garnet Gd3Ga5O under the change of 0-20kOe magnetic field. 12 The isothermal magnetic entropy change of (GGG) single crystal material. The specific heat performance of the cubic phase KGdF4 sample with good performance in the low temperature range of 2-10K under different magnetic fields was tested, and it was found that the peak specific heat reached 72.15J·kg under a magnetic field of 2T. -1 ·K -1 .

[0053] Example 1

[0054] KGdF4 (orthorhombic, trigonal and cubic phase) sample preparation method (solid-state sintering method): First, the raw materials anhydrous KF (> 99.99%) and GdF3 powder (> 99.99%) are mixed in a glove box under the protection of Ar gas atmosphere in an equimolar ratio of 1:1 (± 0.05) (1:1 in this embodiment), and put into a mortar for full grinding to mix the two powders evenly. Then the ground powder is placed in a disc mold with a diameter of 12.7 mm, and a pressure of 10-18 MPa (15 MPa in this embodiment) is applied on a tablet press to compact, and finally a solid disc sample (thickness 1-3 mm) is obtained. Then the tube sealing operation is performed, first the obtained disc sample is placed in an alumina crucible (to prevent the gas precipitated by KF at high temperature from corroding the glass tube and causing the tube to explode), and then it is placed in a quartz tube and sealed under Ar gas atmosphere (sealing the quartz tube mouth).

[0055] Finally, the three samples with sealed tubes were placed in the muffle furnace and the following operations were performed:

[0056] For the first sample, the heating rate is 1-3°C / min, and the sample is kept at 500-550°C for 6-12h (in this embodiment, the heating rate is 2°C / min, and the sample is kept at 520°C for 8h), then the sample is cooled to room temperature with the furnace, and after grinding, the sample is sealed and sintered for the second time under the same conditions. After grinding, KGdF4 polycrystalline powder with an orthorhombic phase structure is finally obtained.

[0057] For the second sample, the heating rate is 1-3°C / min, and the sample is kept at a holding temperature of 630-680°C for 6-12h (in this embodiment, the heating rate is 2°C / min, and the sample is kept at a holding temperature of 650°C for 8h), then cooled to room temperature with the furnace, and after grinding, secondary sealing and sintering are performed under the same conditions. After grinding, KGdF4 polycrystalline powder with a rhombohedral phase can be obtained.

[0058] For the third sample, the heating rate was 1-3°C / min, and the sample was kept at 720°C for 6-12h (in this embodiment, the heating rate was 2°C / min and the sample was kept at 720°C for 8h), then ice-water quenched, ground and pressed into tablets again (thickness 1-3mm) and sealed, sintered under the same temperature conditions (the heating rate was 1-3°C / min, and the sample was kept at 720-770°C for 6-12h (in this embodiment, the heating rate was 2°C / min and the sample was kept at 750°C for 8h)) and water quenched to finally obtain KGdF4 polycrystalline powder with a cubic phase structure.

[0059] The crystal structure of this series of materials was characterized by an X-ray diffractometer. The Cal-orthorhombic phase / trigonal phase / cubic phase curve in the figure was calculated using VESTA software from the officially published crystal structure KGdF4 (orthorhombic phase) / KYF4 (trigonal phase) / NaYF4 (cubic phase) CIF file. Comparison with the experimentally obtained XRD spectrum shows that the obtained KGdF4 powder is a single-phase orthorhombic, trigonal and cubic structure, and no extra impurity peaks are found, indicating that the sample has a high purity (see Appendix). Figure 2 ).

[0060] Example 2

[0061] Magnetic card performance test of orthorhombic phase KGdF4 polycrystalline powder

[0062] In the magnetic test, first take out a part of the orthorhombic phase KGdF4 powder sample and weigh it with an electronic balance. The sample mass is 2.24mg. Then put the sample into a transparent plastic capsule with an upper cover (cylindrical shape with an open top, 1.5cm long; inner diameter is 0.56cm; can hold about 0.15g sample). Use tweezers to clamp the absorbent cotton and put it on the sample. Fill the capsule with absorbent cotton and press it with your hands to fix the sample at the bottom of the plastic capsule. Finally, cover the capsule lid and press it tightly. Then fix the capsule with the sample in a plastic straw about 20cm long and 6mm in inner diameter. The bottom of the sample is 9cm away from the tube mouth, and two holes are pierced on the sample tube near the capsule (about 1cm) (because vacuum will be performed during the test, and the holes are pierced to prevent the plastic sample tube from shrinking and squeezing the sample during the vacuum process, thereby affecting the test results). Next, we used superconducting quantum interference magnetometry (SQUID; Quantum Design) to perform a magnetization curve MH test (M is the magnetic moment, H is the magnetic field). In the temperature range of 2-20K, we measured an MH curve every 1K (see Appendix Figure 4 Inset). In each tested MH curve, a data point was measured every 2000 Oe under 0-20 kOe magnetic field.

[0063] According to the MH curves at different temperatures obtained from the above magnetic test, the relationship between the magnetic entropy change and temperature after applying a 0-5kOe, 0-10kOe or 0-20kOe changing magnetic field to the orthorhombic phase KGdF4 sample was obtained by the Maxwell equation (reference: PhahulZhemas Zul Nehan et al., The magnetocaloric effect properties for potential applications of magnetic refrigerator technology: a review, PhysicalChemistry Chemical Physics, 2024-04-29, DOI: 10.1039 / d4cp01077a). 4.35 J·kg were obtained under 0-5kOe, 0-10kOe and 0-20kOe changing magnetic fields. -1 ·K -1 , 13.32 J·kg -1 ·K -1 and 29.18 J·kg -1 ·K -1 The magnetic entropy change of size (see Appendix Figure 4-6 ).

[0064] Example 3

[0065] Magnetic card performance test of rhombohedral phase KGdF4 polycrystalline powder

[0066] In the magnetic test, first take out a part of the rhombohedral KGdF4 powder sample and weigh it with an electronic balance. The sample mass is 2.54mg. Then put the sample into a transparent plastic capsule (cylindrical with an open top, 1.5cm long and 0.56cm in diameter; about 0.15g of sample). Use tweezers to pick up cotton wool and put it on the sample. Fill the capsule with cotton wool and press it with your hands. Fix the sample at the bottom of the plastic capsule. Finally, cover the capsule lid and press it tightly. Then fix the capsule with the sample in a plastic straw about 20cm long and 6mm in inner diameter. The bottom of the sample is 9cm away from the tube mouth, and two holes are pierced on the sample tube near the capsule (about 1cm) (because vacuum will be performed during the test, and the holes are pierced to prevent the plastic sample tube from shrinking and squeezing the sample during the vacuum process, thereby affecting the test results). Next, we used superconducting quantum interference magnetometry (SQUID; Quantum Design) to perform a magnetization curve MH test (M is the magnetic moment, H is the magnetic field). In the temperature range of 2-20K, we measured an MH curve every 1K (see Appendix Figure 5 Inset). In each tested MH curve, a data point was measured every 2000 Oe under 0-20 kOe magnetic field.

[0067] According to the MH curves at different temperatures obtained from the above magnetic test, the relationship between the magnetic entropy change and temperature after applying a 0-5kOe, 0-10kOe or 0-20kOe changing magnetic field to the orthorhombic phase KGdF4 sample was obtained by the Maxwell equation (reference: PhahulZhemas Zul Nehan et al., The magnetocaloric effect properties for potential applications of magnetic refrigerator technology: a review, PhysicalChemistry Chemical Physics, 2024-04-29, DOI: 10.1039 / d4cp01077a). 3.32 J·kg were obtained under 0-5kOe, 0-10kOe and 0-20kOe changing magnetic fields. -1 ·K -1 , 11.20 J·kg -1 ·K -1 and 27.93 J·kg -1 ·K -1 The magnetic entropy change of size (see Appendix Figure 4-6 ).

[0068] Example 4

[0069] Magnetic card performance test of cubic phase KGdF4 polycrystalline powder

[0070] In the magnetic test, first take out a part of the cubic phase KGdF4 powder sample and weigh it with an electronic balance. The sample mass is 2.43mg. Then put the sample into a transparent plastic capsule (cylindrical with an open top, 1.5cm long and 0.56cm in diameter; about 0.15g of sample). Use tweezers to pick up cotton wool and put it on the sample. Fill the capsule with cotton wool and press it with your hands to fix the sample at the bottom of the plastic capsule. Finally, cover the capsule lid and press it tightly. Then fix the capsule with the sample in a plastic straw about 20cm long and 6mm in inner diameter. The bottom of the sample is 9cm away from the tube mouth, and two holes are pierced on the sample tube near the capsule (about 1cm) (because vacuum will be performed during the test, and the holes are pierced to prevent the plastic sample tube from shrinking and squeezing the sample during the vacuum process, thereby affecting the test results). Next, we used superconducting quantum interference magnetometry (SQUID; Quantum Design) to perform a magnetization curve MH test (M is the magnetic moment, H is the magnetic field). In the temperature range of 2-10K, we measured an MH curve every 1K (see Appendix Figure 6 Inset). In each tested MH curve, a data point was measured every 2000 Oe under 0-20 kOe magnetic field.

[0071] According to the MH curves at different temperatures obtained from the above magnetic test, the relationship between the magnetic entropy change and temperature after applying a 0-5kOe, 0-10kOe or 0-20kOe changing magnetic field to the orthorhombic KGdF4 sample was obtained by the Maxwell equation (reference: PhahulZhemas Zul Nehan et al., The magnetocaloric effect properties for potential applications of magnetic refrigerator technology: a review, PhysicalChemistry Chemical Physics, 2024-04-29, DOI: 10.1039 / d4cp01077a). 4.43 J·kg were obtained under 0-5kOe, 0-10kOe and 0-20kOe changing magnetic fields. -1 ·K -1 , 13.78 J·kg -1 ·K -1 and 31.99 J·kg -1 ·K -1 The magnetic entropy change of size (see Appendix Figure 4-6 ).

[0072] Using the comprehensive material property measurement system ( DynaCool TM ; Quantum Design) conducted a test on the specific heat of cubic phase KGdF4 polycrystal under different magnetic fields. A portion of the KGdF4 powder sample was taken out and pressed into a block (the applied pressure was 4-6MPa), and then a small piece of sample (length * width * height is 1mm * 1mm * 0.2mm) was cut out. After weighing on an electronic balance, the sample mass was 2.6mg. The sample was fixed to the center of the sample stage with 0.1ml of low-temperature vacuum grease (ApiezonN grease; the data on the change of specific heat of low-temperature vacuum grease with temperature has been determined and will be used as the background). Subsequently, the measurement environment was evacuated to a high vacuum state (vacuum degree of 1×10 - 5 Torr) at different temperatures, and then the specific heat was measured with the temperature change when the external constant magnetic field was 10kOe and 20kOe (such as Figure 7 ). Each curve has a logarithmic point sampling mode for 2-10K, with a total of 20 points, and a linear point sampling mode for 10-300K, with a total of 30 points. For example, when there is no external magnetic field, the peak specific heat of cubic phase KGdF4 at low temperature 2K reaches 19.20 J·kg -1 ·K -1 When the external magnetic field increases to 2T, it can be seen that its low-temperature specific heat reaches 72.15J·kg -1 ·K -1 .

[0073] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

[0074] Additionally, descriptions of well-known structures and techniques are omitted herein to avoid unnecessarily obscuring the concepts of the present invention.

Claims

1. Application of potassium-based rare earth fluoride as magnetic material in magnetic refrigeration.

2. The use according to claim 1, characterized in that: The chemical formula of the potassium-based rare earth fluoride is KGdF4, the rare earth element is Gd, and the material has a single-phase structure; The single-phase structure in the material is one or more of orthorhombic phase, trigonal phase or cubic phase.

3. The use according to claim 1 or 2, characterized in that: Preparation methods of three single-phase structure samples: First, the raw materials KF and GdF3 powders were mixed in an equimolar ratio of 1:1 (±0.05), pressed into sheet samples (thickness 1-3 mm) under a pressure of 10-18 MPa on a tablet press, and placed in a sealed container under the protection of an inert atmosphere; Then perform any of the following operations: The first method is to heat the sample at a rate of 1-3°C / min, keep the sample at 500-550°C for 6-12h, and cool the sample to room temperature to obtain KGdF4 polycrystalline powder with an orthorhombic phase structure. Or, the second method, the heating rate is 1-3°C / min, the temperature is kept at 630-680°C for 6-12 hours, and then cooled to room temperature to obtain KGdF4 polycrystalline powder with a rhombohedral phase; Or, the third method is to heat the pellets at a rate of 1-3°C / min, keep the pellets at 720-770°C for 6-12h, quench with ice water, grind the pellets, seal the pellets, sinter at a rate of 1-3°C / min, keep the pellets at 720-770°C for 6-12h, and quench with water to obtain a cubic KGdF4 polycrystalline powder.

4. The use according to claim 1 or 2, characterized in that: The potassium-based rare earth fluoride is a polycrystalline powder having an orthorhombic, trigonal or cubic crystal structure; The three phase structures of KGdF4 exhibit paramagnetism in the temperature range above 2K, the orthorhombic and trigonal phases of KGdF4 exhibit ferromagnetic order at the Curie temperature below 2K, and the cubic phase of KGdF4 exhibits antiferromagnetic order at the Curie temperature below 2K.

5. The use according to any one of claims 1 to 4, characterized in that: The application temperature range of potassium-based rare earth fluoride polycrystalline powder is a low temperature range below 20K, which can produce magnetocaloric effect.

6. The use according to any one of claims 1 to 4, characterized in that: When a 0-20 kOe magnetic field is applied to a potassium-based rare earth fluoride polycrystalline powder sample, the potassium-based rare earth fluoride exhibits a large isothermal magnetic entropy change, among which KGdF4 with orthorhombic, rhombohedral and cubic phases obtains 29.18 J·kg at 2K, respectively. -1 ·K -1 , 27.92 J·kg -1 ·K -1 and 31.99 J·kg -1 ·K -1 Isothermal magnetic entropy change of magnitude; The peak specific heat of cubic phase KGdF4 at low temperature 2K reached 19.20 J·kg without external magnetic field. -1 ·K -1 When the external magnetic field increases to 2T, it can be seen that its low-temperature specific heat reaches 72.15J·kg -1 ·K -1 .

7. The use according to any one of claims 1 to 4, characterized in that: The magnetic refrigeration method based on potassium-based rare earth fluoride polycrystalline powder is as follows: potassium-based rare earth fluoride polycrystalline in the form of a pressed block (2-20MPa, preferably 10-18MPa pressure pressing sample) is used as a magnetic working medium, and by applying and removing a magnetic field to the magnetic working medium, the temperature of the magnetic working medium changes and heat is transferred to a heat exchanger, thereby achieving the purpose of refrigeration.

8. The use according to claim 7, characterized in that: The potassium-based rare earth fluoride polycrystal is one or more of KGdF4 having different single-phase structures (one or more than two of orthorhombic, trigonal and cubic phases); When there are multiple types, the low-temperature refrigeration range can be expanded by pressing and mixing KGdF4 polycrystalline powders with different single-phase structures respectively. Or, the phase structure of the potassium-based rare earth fluoride is a single phase, which is an orthorhombic phase, a trigonal phase or a cubic phase, and at least two or more phase structure powders are mixed to achieve the expansion of the low temperature refrigeration range; Alternatively, by changing the atomic ratio (changing the molar ratio of the raw materials KF and GdF3, such as 1:1, 1:2, 2:1) in the process of preparing the polycrystalline potassium-based rare earth fluoride (here, KGdF4 polycrystalline powder with a single-phase structure), the low-temperature refrigeration range can be expanded when preparing potassium-based rare earth fluoride with different element contents; Alternatively, by replacing other rare earth elements (other rare earth elements refer to one or more of Gd, Tb, Dy, Ho, Er, Tm and Yb) during the preparation of potassium-based rare earth fluoride (here rare earth refers to Gd) polycrystals, potassium-based rare earth fluoride with different rare earth elements can be prepared, and the low-temperature refrigeration range can also be expanded.

9. A magnetic refrigeration device driven by a small magnetic field of 0.5-3T, characterized in that: The device comprises the potassium-based rare earth fluoride polycrystal according to any one of claims 1 to 8 as a magnetic working medium; The potassium-based rare earth fluoride polycrystal of the magnetic working fluid is one or more of KGdF4 having an orthorhombic phase, a rhombohedral phase or a cubic phase structure; When there are multiple types, the low-temperature refrigeration range can be expanded by mixing and arranging different single-phase KGdF4 polycrystalline compacts; Or, the phase structure of the potassium-based rare earth fluoride (KGdF4) is a single phase, namely, an orthorhombic phase, a trigonal phase and a cubic phase, and at least two or more phase structure powders are mixed; Alternatively, by changing the atomic ratio (changing the molar ratio of the raw materials KF and GdF3, such as 1:1, 1:2, 2:1) in the process of preparing the polycrystalline potassium-based rare earth fluoride (here, KGdF4 polycrystalline powder with a single-phase structure), the low-temperature refrigeration range can be expanded when preparing potassium-based rare earth fluoride with different element contents; Alternatively, by replacing other rare earth elements (other rare earth elements refer to one or more of Gd, Tb, Dy, Ho, Er, Tm and Yb) during the preparation of potassium-based rare earth fluoride (here rare earth refers to Gd) polycrystals, potassium-based rare earth fluoride with different rare earth elements can be prepared, and the low-temperature refrigeration range can also be expanded.

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

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