Europium-based halogenated phosphate as well as preparation method and application thereof

Abstract

The invention provides europium-based halogenated phosphate as well as a preparation method and application thereof. The general structural formula of the europium-based halogenated phosphate is Eu5 (PO4) 3X, wherein X is halogen. The europium-based halogenated phosphate is prepared through a simple solid-phase reaction, the high-purity europium-based halogenated phosphate can be prepared through the dosage of raw materials, a specific sintering process and secondary tabletting, the unit cell space structure can be optimized, the stability of the europium-based halogenated phosphate is further improved, and the europium-based halogenated phosphate has large magnetic entropy change. The magnetic phase transition temperature of the prepared europium-based halogenated phosphate is smaller than 1K, the maximum magnetic entropy change under the magnetic field change of 0-5T is larger than or equal to 60 J.kg <-1 >. K <-1 >, the europium-based halogenated phosphate is an extremely-low-temperature region magnetic refrigeration material with excellent performance, scientific and technological progress in the fields of low-temperature physics, deep space exploration, aerospace and the like in China can be promoted, and the europium-based halogenated phosphate can be widely applied to the fields of low-temperature physics, deep space exploration, aerospace and the like. And high value-added utilization of rare earth resources can be promoted.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnetic refrigeration materials, and more particularly to a europium-based halogenated phosphate and a preparation method and application thereof. BACKGROUND

[0002] Ultra-low temperature refrigeration technology generally refers to a refrigeration technology for obtaining a temperature below 1K and providing cold energy, and has very important applications in the fields of frontier scientific research such as quantum information science, dark matter exploration and condensed matter physics. At present, the mainstream technology for achieving ultra-low temperature is 3 He adsorption refrigeration and 3 He- 4 He dilution refrigeration, in which, 3 He is mainly produced as a byproduct of tritium radioactivity decay from nuclear reactors or nuclear weapon stockpiles. However, these technologies not only rely on rare and expensive helium resources, but also are limited by the deep space weightless environment [9] , which will have a huge impact on the application in the field of aerospace. Especially for the dilution refrigeration technology that can achieve ultra-low temperature (mK), it has the advantages of continuous refrigeration, large cold energy and no electromagnetic interference, but the dilution refrigerator needs to rely on gravity to realize phase separation, 3 He and 4 He interface can be stably placed in the correct refrigeration chamber.

[0003] Therefore, it is necessary to study a new type of refrigeration technology that does not rely on helium resources. Adiabatic demagnetization refrigeration (ADR) is a technology that uses the magnetocaloric effect (MCE) of magnetic materials to achieve ultra-low temperature in the absence of rare 3 He resources and without relying on gravity operation. Adiabatic demagnetization refrigeration technology is a solid-state refrigeration method based on the magnetocaloric effect (MCE), and the magnetocaloric effect is an intrinsic property of all magnetic materials. At present, the general definition of the magnetocaloric effect is the physical phenomenon that a magnetic material releases or absorbs heat when the magnetic field is enhanced or weakened. When there is no external magnetic field, the magnetic moment direction of the magnetic material is chaotic, and the magnetic entropy is large. When there is an external magnetic field, the magnetic moment tends to be uniform, and the magnetic entropy decreases. When the magnetic material is excited, the magnetic moment changes from disorder to order, the magnetic entropy decreases, and heat is released to the outside. When demagnetization, the magnetic moment changes from order to disorder, the magnetic entropy increases, and heat is absorbed from the outside. Under adiabatic conditions, the magnetic field does work on the material, changes the internal energy of the material, and changes the temperature, thereby achieving the refrigeration effect. This technology does not rely on the gravity environment, does not rely on helium resources, and is widely used in the fields of quantum computing and deep space exploration due to its high refrigeration efficiency, low vibration and high reliability.

[0004] High-performance magnetic heat materials play a key role in magnetic refrigeration technology, and refrigerants with large magnetic heat effect under low applied magnetic field are particularly desirable because they can simplify the design by using permanent magnets and greatly reduce the cost of magnetic refrigerators, so materials with large magnetic heat effect under low magnetic field have more application prospects. However, although traditional paramagnetic salts (such as Gd2(SO4)3·8H2O) are used for very low temperature, they are chemically unstable, have poor thermal conductivity and are difficult to scale up; some oxide materials (such as manganese-based perovskites) have too high magnetic phase transition temperature and cannot be applied to the very low temperature range; the benchmark material GGG (gadolinium gallium garnet) has good performance, but its maximum magnetic entropy change requires a strong magnetic field of more than 3T to drive, resulting in a complex system and high cost, and there are many difficulties in actual use.

[0005] Therefore, it is necessary to develop a very low temperature magnetic refrigeration material with large magnetic entropy under a lower magnetic field, and a simple preparation process, a short cycle and suitable for industrial production. SUMMARY

[0006] The purpose of the present application is to provide a new very low temperature magnetic refrigeration material, a europium-based halogenated phosphate salt, and its preparation method and application in magnetic refrigeration, in order to solve the problems of small magnetic entropy, unstable chemical structure and complex preparation process of existing magnetic refrigeration materials.

[0007] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: In a first aspect of the present application, a europium-based halogenated phosphate salt is provided, and the general structure of the europium-based halogenated phosphate salt is Eu5(PO4)3X, wherein X is a halogen. The europium-based halogenated phosphate salt is prepared by a method comprising the following steps: S1: A europium source, a phosphorus source and an excess amount of a halogen source are mixed and ground according to the stoichiometric ratio, and then a mixed raw material is obtained by one-time tabletting; S2: The mixed raw material obtained in step S1 is sintered in a reducing atmosphere, and the sintering process is as follows: First, sintering is carried out at 150-300℃ for 3-6h, and then the temperature is raised to 600-800℃ for sintering for 8-10h to obtain a precursor; S3: The precursor obtained in step S2 is tabletted for the second time, and then sintered at 860-900℃ for 18-30h in a reducing atmosphere to obtain the europium-based halogenated phosphate salt.

[0008] As an embodiment of the present application, the halogen X is selected from at least one of F or Cl.

[0009] As an embodiment of the present application, the europium source comprises EuO.

[0010] As an embodiment of the present application, the europium source is Eu2O3, which is mixed with graphite uniformly, ground, and sintered in a reducing atmosphere at 1400-1500°C for 10-12h to obtain EuO, which is then mixed and ground with the phosphorus source and the halogen source in step S1.

[0011] As an embodiment of the present application, the phosphorus source comprises NH4H2PO4.

[0012] As an embodiment of the present application, the halogen source comprises ammonium halide.

[0013] As an embodiment of the present application, in step S1, the molar ratio of Eu element in the europium source, P element in the phosphorus source, and halogen X in the halogen source is 5:3:(6-20).

[0014] As an embodiment of the present application, the halogen X is F, the sintering temperature of the first mixed raw material is 150°C, and the sintering time is 3h; the sintering temperature of the second mixed raw material is 300°C, and the sintering time is 8h.

[0015] As an embodiment of the present application, the halogen X is Cl, the sintering temperature of the first mixed raw material is 300°C, and the sintering time is 6h; the sintering temperature of the second mixed raw material is 600°C, and the sintering time is 10h.

[0016] As an embodiment of the present application, the reducing atmosphere comprises hydrogen, and further comprises at least one of argon, helium, and neon, and the mass proportion of hydrogen in the reducing atmosphere is 10-15%.

[0017] As an embodiment of the present application, the inert atmosphere comprises at least one of argon, helium, and neon.

[0018] As an embodiment of the present application, the europium-based halophosphate is hexagonal, and the space group is P63 / m.

[0019] As an embodiment of the present application, the europium-based halophosphate is Eu5(PO4)3F, and the unit cell parameters of Eu5(PO4)3F are: a = 9.72950 Å, b = 9.7295 Å, c = 7.28897 Å, α = β = 90°, gamma = 120° As an embodiment of the present application, the europium-based halophosphate is Eu5(PO4)3Cl, and the unit cell parameters of Eu5(PO4)3Cl are: a = 9.88553 Å, b= 9.88553 Å, c = 7.20585 Å, α = 7.20585 Å, β = 90°, gamma = 120°.

[0020] As an embodiment of the present application, the magnetic phase transition temperature of the europium-based halogenated phosphate salt is < 1 K.

[0021] As an embodiment of the present application, the europium-based halogenated phosphate salt is Eu5(PO4)3F, and the magnetic phase transition temperature of Eu5(PO4)3F is < 0.4 K.

[0022] As an embodiment of the present application, the europium-based halogenated phosphate salt is Eu5(PO4)3Cl, and the magnetic phase transition temperature of Eu5(PO4)3Cl is 0.5-0.7 K.

[0023] As an embodiment of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of magnetic field of 0-5 T is ≥ 60 J·kg -1 · K -1 .

[0024] As an embodiment of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of magnetic field of 0-1 T is ≤ 27.1 J·kg -1 · K -1 .

[0025] As an embodiment of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of magnetic field of 0-2 T is ≤ 46.2 J·kg -1 · K -1 .

[0026] As an embodiment of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of magnetic field of 0-3 T is ≤ 55.8 J·kg -1 · K -1 .

[0027] As an embodiment of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of magnetic field of 0-4 T is ≤ 61.1 J·kg -1 · K -1 .

[0028] As an embodiment of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of magnetic field of 0-5 T is ≤ 64.4 J·kg -1 · K -1 .

[0029] The second aspect of the present application provides a preparation method of the europium-based halogenated phosphate salt according to the first aspect of the present application, comprising the following steps: S1: Eu source, phosphorus source, according to the stoichiometric ratio of ingredients, add excess halogen source, mix and grind, then press once to obtain the mixed raw material; S2: The mixed raw material obtained in step S1 is sintered in a reducing atmosphere, and the sintering process is: First sintering at 150-300 DEG C for 3-6h, then heating to 600-800 DEG C for 8-10h to obtain the precursor; S3: The precursor obtained in step S2 is pressed twice, and then sintered at 860-900 DEG C in a reducing atmosphere for 18-30h to obtain the europium-based halogenated phosphate.

[0030] The third aspect of the application provides the application of the europium-based halogenated phosphate in the first aspect of the application in low-temperature magnetic refrigeration.

[0031] As an embodiment of the application, the europium-based halogenated phosphate is used as an adiabatic demagnetization refrigeration working medium.

[0032] Compared with the prior art, the application has the following beneficial effects: The europium-based halogenated phosphate is prepared by a simple solid phase reaction, the amount of raw materials, a specific sintering process, and secondary pressing, so that high-purity europium-based halogenated phosphate can be prepared, the crystal cell space structure can be optimized, and the stability and large magnetic entropy change can be further improved.

[0033] The magnetic phase transition temperature of the europium-based halogenated phosphate prepared by the application is <1K, and the maximum magnetic entropy change under the magnetic field change of 0-5T is ≥60J·kg -1 ·K -1 It is a kind of excellent low-temperature magnetic refrigeration material, which can not only promote the scientific and technological progress in the fields of low-temperature physics, deep space exploration and aerospace, but also promote the high value-added utilization of rare earth resources. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 It is the XRD test spectrum of the europium-based halogenated phosphate Eu5 (PO4) 3Cl of the embodiments 1-3 of the application.

[0035] Figure 2 It is the XRD test spectrum of the europium-based halogenated phosphate Eu5 (PO4) 3F of the embodiments 4-5 and the comparative example 4 of the application.

[0036] Figure 3 It is the XRD test spectrum of the europium-based halogenated phosphate of the comparative examples 1-3 of the application.

[0037] Figure 4The thermal magnetic curve of the Eu5(PO4)3Cl sample prepared in Embodiment 3 of the present application under field cooling (FC) at a magnetic field of 0.01T.

[0038] Figure 5 The isothermal magnetization curve of the Eu5(PO4)3Cl sample prepared in Embodiment 3 of the present application at 0.4-1.8K under a magnetic field change of 0-5T.

[0039] Figure 6 The curve of the magnetic entropy change versus temperature of the Eu5(PO4)3Cl sample prepared in Embodiment 3 of the present application under different magnetic field changes.

[0040] Figure 7 The thermal magnetic curve of the Eu5(PO4)3F sample prepared in Embodiment 4 of the present application under field cooling (FC) at a magnetic field of 0.01T.

[0041] Figure 8 The isothermal magnetization curve of the Eu5(PO4)3F sample prepared in Embodiment 4 of the present application at 0.4-1.8K under a magnetic field change of 0-5T.

[0042] Figure 9 The curve of the magnetic entropy change versus temperature of the Eu5(PO4)3F sample prepared in Embodiment 4 of the present application under different magnetic field changes. DETAILED DESCRIPTION

[0043] For better illustrating the purpose, technical scheme and advantages of the present application, the present application will be further explained in combination with specific embodiments, but the embodiments do not limit the present application in any form. Unless specifically stated, the reagents, methods and devices used in the present application are conventional reagents, methods and devices in the technical field. Unless specifically stated, the reagents and materials used in the present application are commercially available.

[0044] In the present application, the technical features described in an open form include a closed technical scheme consisting of listed features, and also include an open technical scheme containing listed features.

[0045] In the present application, if no specific description is provided, the numerical range is regarded as continuous and includes the minimum value and the maximum value of the range, and each value between the minimum value and the maximum value. Further, when a range is referred to as an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a property, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all sub-ranges subsumed therein.

[0046] The reagents or instruments used in the present application are not specified by the manufacturer, and are conventional products that can be obtained by commercial purchase.

[0047] In a first aspect of the present application, an europium-based halophosphate is provided in embodiments of the present application, and the general structure of the europium-based halophosphate is Eu5(PO4)3X, X is halogen; The europium-based halophosphate is prepared by a method comprising the following steps: S1: ingredients of europium source, phosphorus source, and excess halogen source are mixed and ground according to stoichiometric ratio, and then a mixed raw material is obtained by one-time tabletting; S2: the mixed raw material obtained in step S1 is sintered in a reducing atmosphere, and the sintering process is as follows: first sintering at 150-300℃ for 3-6h, and then sintering at 600-800℃ for 8-10h to obtain a precursor; S3: the precursor obtained in step S2 is secondly tabletted, and then sintered at 860-900℃ for 18-30h in a reducing atmosphere to obtain the europium-based halophosphate.

[0048] The europium-based halophosphate is prepared by a simple solid-phase reaction, and through the amount of raw materials, a specific sintering process, and second tabletting, not only a high-purity europium-based halophosphate can be prepared, but also the crystal cell space structure can be optimized, and the stability and large magnetic entropy change are further improved.

[0049] Specifically: First, the precise excess of halogen source (such as NH4Cl) not only acts as a reactant at high temperature, but also plays a role of mineralizer and dynamic atmosphere protection. It significantly accelerates the interfacial diffusion of Eu 2+ , PO4 3- and Cl⁻ / F⁻ by forming a local solid-phase environment, thereby driving the reaction to the target stoichiometric ratio Eu5(PO4)3X more completely, effectively inhibiting the generation of impurities such as Eu3(PO4)2, and ensuring high phase purity of the product.

[0050] Second, the specific sintering temperature and time are matched with the volatility of the halogen source, so that the reaction system can fully relax the crystal lattice after the crystal phase is formed. This promotes the more ordered stacking of the crystal along the c-axis direction under the framework of the P63 / m space group, and may induce a slight optimization of the crystal cell parameter c / a value. This optimization reduces the internal stress and defects of the crystal lattice, and enhances the intrinsic thermodynamic stability of the structure.

[0051] Third, the introduced two-step sintering process further improves the density and uniformity of the material on the basis of the good crystal grains formed in the first step. This not only reduces the residual pores in the sintered body, promotes the contact and mass transfer between the crystal grains, and makes the microstructure of the final product more dense and uniform.

[0052] Finally, the microstructure with high purity, low defect density and optimized crystal field obtained through synergistic regulation creates a perfect environment for the magnetic Eu 2+ The superexchange interaction through the Eu-O-X bridging path between ions creates an ideal environment. The optimized magnetic interaction on one hand consolidates the lattice stability, and on the other hand promotes the spins to produce a stronger collective response and greater magnetic moment change under a low applied magnetic field, thereby showing a significantly enhanced magnetic entropy change (-Δ S M ).

[0053] In some embodiments of the present application, the halogen X is selected from at least one of F or Cl. In the present application, Br element can also be theoretically used, but since Br element is highly toxic, in the scheme of the present application, F or Cl which is relatively less toxic is preferred.

[0054] In some embodiments of the present application, the europium source comprises EuO.

[0055] In some embodiments of the present application, other europium sources can also be used, for example, when the europium source is Eu2O3, Eu2O3 is first mixed with graphite uniformly and ground, then sintered in a reducing atmosphere at 1400-1500℃ for 10-12h to obtain EuO intermediate, and then mixed and ground with the phosphorus source and the halogen source in step S1. Eu element is much more expensive and is easily oxidized, with poor stability, greatly increasing the difficulty of preparation. Therefore, in the embodiments of the present application, EuO which can be prepared on a large scale is preferred as the europium source.

[0056] In some embodiments of the present application, the phosphorus source comprises NH4H2PO4.

[0057] In some embodiments of the present application, the halogen source comprises ammonium halide, for example: NH4F, NH4Cl.

[0058] In the present application, the purity of the above-mentioned europium source, phosphorus source and halogen source is not less than 99%.

[0059] In some embodiments of the present application, in step S1, the molar ratio of Eu element in the europium source, P element in the phosphorus source and halogen X in the halogen source is 5:3:(6-20). The halogen source is in excess, which is beneficial to forming a fluxing environment in the high-temperature solid-phase reaction, promoting the ion diffusion and mass transfer process of the reactants, thereby significantly improving the crystallinity, phase purity of the target product europium-based halogenated phosphate, and making the microcrystalline grains more complete and uniform in size. At the same time, ammonium salt has extremely strong decomposition volatility, and the excess ensures that the raw materials participate in the reaction.

[0060] In the present application, the first grinding and mixing is followed by tabletting, which increases the contact area between the reaction raw materials and reduces the volatilization of the raw materials; then the first sintering is performed to form a precursor; the grinding and tabletting are continuously performed to increase the uniformity and contact area between the substances and improve the activity of the solid-phase reaction, which is beneficial to improving the purity of the product.

[0061] In the present application, the grinding and tabletting process is performed in an air atmosphere.

[0062] In some embodiments of the present application, the halogen X is F, the sintering temperature of the first mixed raw material is 150 DEG C, and the sintering time is 3h; the sintering temperature of the second mixed raw material is 300 DEG C, and the sintering time is 8h.

[0063] In some embodiments of the present application, the halogen X is Cl, the sintering temperature of the first mixed raw material is 300 DEG C, and the sintering time is 6h; the sintering temperature of the second mixed raw material is 600 DEG C, and the sintering time is 10h.

[0064] In some embodiments of the present application, the reducing atmosphere includes hydrogen, and further includes at least one of argon, helium, and neon, and the mass proportion of hydrogen in the reducing atmosphere is 10-15%.

[0065] In some embodiments of the present application, the inert atmosphere includes at least one of argon, helium, and neon.

[0066] In some embodiments of the present application, the europium-based halophosphate is hexagonal crystal system, and the space group is P63 / m.

[0067] In some embodiments of the present application, the europium-based halophosphate is Eu5(PO4)3F, and the unit cell parameters of Eu5(PO4)3F are: a = 9.72950 Å, b = 9.7295 Å, c = 7.28897 Å, α = β = 90°, gamma = 120°.

[0068] In some embodiments of the present application, the europium-based halophosphate is Eu5(PO4)3Cl, and the unit cell parameters of Eu5(PO4)3Cl are: a = 9.88553 Å, b = 9.88553 Å, c = 7.20585 Å, α = β = 90°, gamma = 120°.

[0069] In some embodiments of the present application, the magnetic phase transition temperature of the europium-based halogenated phosphate salt is < 1 K.

[0070] In some embodiments of the present application, the europium-based halogenated phosphate salt is Eu5(PO4)3F, and the magnetic phase transition temperature of Eu5(PO4)3F is < 0.4 K.

[0071] In some embodiments of the present application, the europium-based halogenated phosphate salt is Eu5(PO4)3Cl, and the magnetic phase transition temperature of Eu5(PO4)3Cl is 0.5-0.7 K.

[0072] In some embodiments of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of a magnetic field of 0-5 T is ≥ 60 J·kg -1 ·K -1 More specifically: In some embodiments of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of a magnetic field of 0-1 T is ≤ 27.1 J·kg -1 ·K -1 .

[0073] In some embodiments of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of a magnetic field of 0-2 T is ≤ 46.2 J·kg -1 ·K -1 .

[0074] In some embodiments of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of a magnetic field of 0-3 T is ≤ 55.8 J·kg -1 ·K -1 .

[0075] In some embodiments of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of a magnetic field of 0-4 T is ≤ 61.1 J·kg -1 ·K -1 .

[0076] In some embodiments of the present application, the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the change of a magnetic field of 0-5 T is ≤ 64.4 J·kg -1 ·K -1 .

[0077] It can be seen that the europium-based halogenated phosphate salt of the present application has an extremely low magnetic phase transition temperature, and the magnetic phase transition temperature is < 1 K; at the same time, it also has a relatively high magnetic entropy change under a low magnetic field, and the maximum magnetic entropy change under the change of a magnetic field of 0-5 T is ≥ 60 J·kg -1 ·K -1It is a kind of superior performance extremely low temperature region magnetic refrigeration material, not only can promote our country in low temperature physics, deep space exploration and aerospace etc. The scientific and technological progress of field, can also promote the high value-added utilization of rare earth resources.

[0078] It should be noted that in the present application, the magnetic phase transition temperature refers to the temperature at which the material changes from paramagnetic state to antiferromagnetic state.

[0079] The present application can calculate the magnetic entropy change under different magnetic field intensity according to the isothermal magnetization curve of the europium-based halogenated phosphate extremely low temperature magnetic refrigeration material by using Maxwell relation, and the maximum magnetic entropy change under the magnetic field intensity can be obtained. The maximum magnetic entropy change of the europium-based halogenated phosphate extremely low temperature magnetic refrigeration material is obtained near 1.3K temperature.

[0080] Maxwell relation: Wherein, Δ S M ( T , H ) is the magnetic entropy change, M is the magnetization, T is the temperature, H is the applied magnetic field.

[0081] The second aspect of the present application provides a preparation method of the europium-based halogenated phosphate according to the first aspect of the present application in some embodiments of the present application, comprising the following steps: S1: the europium source, the phosphorus source, and the excess halogen source are mixed and ground according to the stoichiometric ratio, and then one-time tabletting is carried out to obtain the mixed raw material; S2: the mixed raw material obtained in step S1 is sintered in a reducing atmosphere, and the sintering process is as follows: First, sintering at 150~300℃ for 3~6h, and then sintering at 600~800℃ for 8~10h to obtain the precursor; S3: the precursor obtained in step S2 is second-time tabletting, and then sintering at 860~900℃ for 18~30h in a reducing atmosphere to obtain the europium-based halogenated phosphate.

[0082] The preparation method of the europium-based halogenated phosphate according to the present application is a solid phase reaction method, which is simple in preparation method, low in energy consumption, and suitable for large-scale industrial production.

[0083] The third aspect of the present application provides the application of the europium-based halogenated phosphate according to the first aspect of the present application in low temperature magnetic refrigeration in some embodiments of the present application.

[0084] In some embodiments of the present application, the europium-based halogenated phosphate salt is used as a heat-insulating demagnetization refrigerant. The europium-based halogenated phosphate salt of the present application has a very low magnetic phase transition temperature, i.e., the magnetic phase transition temperature is less than 1K, and also has a relatively high magnetic entropy change, i.e., the maximum magnetic entropy change under a magnetic field change of 0-5T is greater than or equal to 60J·kg -1 ·K -1 The europium-based halogenated phosphate salt is a high-performance magnetic refrigerant in the extremely low temperature range, which can not only promote the scientific and technological progress in the fields of low-temperature physics, deep space exploration and aerospace, but also promote the high-value utilization of rare earth resources.

[0085] The following are specific embodiments of the present application.

[0086] Embodiment 1 The present embodiment provides a europium-based halogenated phosphate salt-Eu5(PO4)3Cl, and the preparation method comprises the following steps: S1: EuO, NH4H2PO4 and NH4Cl are mixed according to a molar ratio of 5:3:6, placed in a mortar for grinding, and after the mixture is uniformly ground, the first tablet is obtained after tabletting (also referred to as mixed raw material in the present application); S2: the mixed raw material obtained in step S1 is sintered in a reducing atmosphere (H2 and Ar in a mass ratio of 10:90), and the sintering process is as follows: first sintering at 300℃ for 6h; then sintering at 600℃ for 10h to obtain a precursor; S3: the precursor obtained in step S2 is tabletted to obtain a second tablet, which is sintered at 900℃ for 24h in an argon atmosphere to obtain the europium-based halogenated phosphate salt-Eu5(PO4)3Cl.

[0087] Embodiment 2 The present embodiment provides a europium-based halogenated phosphate salt-Eu5(PO4)3Cl, and the preparation method comprises the following steps: S1: EuO, NH4H2PO4 and NH4Cl are mixed according to a molar ratio of 5:3:8, placed in a mortar for grinding, and after the mixture is uniformly ground, the first tablet is obtained after tabletting (also referred to as mixed raw material in the present application); S2: the mixed raw material obtained in step S1 is sintered in a reducing atmosphere (H2 and Ar in a mass ratio of 10:90), and the sintering process is as follows: first sintering at 300℃ for 6h, then sintering at 600℃ for 10h to obtain a precursor; S3: the precursor obtained in step S2 is tabletted to obtain a second tablet, which is sintered at 900℃ for 24h in an argon atmosphere to obtain the europium-based halogenated phosphate salt-Eu5(PO4)3Cl.

[0088] Embodiment 3 The embodiment provides a europium-based halogenated phosphate Eu5(PO4)3Cl, and a preparation method thereof comprises the following steps: S1: EuO, NH4H2PO4 and NH4Cl are mixed according to a molar ratio of 5:3:10, and are placed in a mortar for grinding; after uniform grinding and mixing, a first tablet (also referred to as a mixed raw material in the embodiment) is obtained after tabletting; S2: the mixed raw material obtained in the step S1 is sintered in a reducing atmosphere (H2 and Ar have a mass ratio of 10:90); the sintering process is as follows: sintering at 300 DEG C for 6h, and then sintering at 600 DEG C for 10h to obtain a precursor; S3: the precursor obtained in the step S2 is tabletted to obtain a second tablet; the second tablet is sintered at 900 DEG C for 24h in an argon atmosphere to obtain the europium-based halogenated phosphate Eu5(PO4)3Cl.

[0089] Embodiment 4 The embodiment provides a europium-based halogenated phosphate Eu5(PO4)3F, and a preparation method thereof comprises the following steps: S1: EuO, NH4H2PO4 and NH4F are mixed according to a molar ratio of 5:3:10, and are placed in a mortar for grinding; after uniform grinding and mixing, a first tablet (also referred to as a mixed raw material in the embodiment) is obtained after tabletting; S2: the mixed raw material obtained in the step S1 is sintered in a reducing atmosphere (H2 and Ar have a mass ratio of 10:90); the sintering process is as follows: sintering at 300 DEG C for 6h, and then sintering at 600 DEG C for 10h to obtain a precursor; S3: the precursor obtained in the step S2 is tabletted to obtain a second tablet; the second tablet is sintered at 900 DEG C for 24h in an argon atmosphere to obtain the europium-based halogenated phosphate Eu5(PO4)3Cl.

[0090] Embodiment 5 The embodiment provides a europium-based halogenated phosphate Eu5(PO4)3F, and a preparation method thereof comprises the following steps: S1: EuO, NH4H2PO4 and NH4F are mixed according to a molar ratio of 5:3:20, and are placed in a mortar for grinding; after uniform grinding and mixing, a first tablet (also referred to as a mixed raw material in the embodiment) is obtained after tabletting; S2: the mixed raw material obtained in the step S1 is sintered in a reducing atmosphere (H2 and Ar have a mass ratio of 10:90); the sintering process is as follows: sintering at 300 DEG C for 6h, and then sintering at 600 DEG C for 10h to obtain a precursor; S3: The precursor obtained in step S2 is pressed into a tablet, and the second tablet is sintered at 900°C for 24 h in an argon atmosphere to obtain the europium-based halophosphate Eu5(PO4)3F.

[0091] Example 6 This example provides a europium-based halophosphate Eu5(PO4)3F, which is prepared by referring to the steps of Example 3, except that the sintering temperature in step S3 is 860°C, and the sintering time is 30 h.

[0092] Example 7 This example provides a europium-based halophosphate Eu5(PO4)3F, which is prepared by referring to the steps of Example 3, except that the sintering temperature in step S3 is 900°C, and the sintering time is 18 h.

[0093] Example 8 This example provides a europium-based halophosphate Eu5(PO4)3F, which is prepared by referring to the steps of Example 3, except that in step S1, the europium source is Eu2O3, Eu2O3 is uniformly mixed with graphite at a mass ratio of 1:2.2 and ground, and then sintered at 1450°C in a reducing atmosphere (H2 and Ar at a mass ratio of 10:90) for 12 h to obtain an EuO intermediate, which is then mixed and ground with NH4H2PO4 and NH4Cl.

[0094] Comparative Example 1 This comparative example provides a europium-based halophosphate Eu5(PO4)3F, which is prepared by referring to the steps of Example 3, except that no excess halide salt NH4Cl is added, and the molar ratio of EuO, NH4H2PO4 and NH4Cl in the reaction raw material of step S1 is 5:3:1.

[0095] Comparative Example 2 This comparative example provides a europium-based halophosphate Eu5(PO4)3F, which is prepared by referring to the steps of Example 3, except that the sintering temperature in step S3 is 1000°C, and the sintering time is 18 h.

[0096] Comparative Example 3 This comparative example provides a europium-based halophosphate Eu5(PO4)3F, which is prepared by referring to the steps of Example 3, except that the sintering temperature in step S3 is 800°C, and the sintering time is 30 h.

[0097] Comparative Example 4 The comparative example 1 provides a europium-based halophosphate Eu5(PO4)3F, and the preparation method comprises the following steps: S1: EuO, NH4H2PO4 and NH4F are mixed in a molar ratio of 5:3:1, placed in a mortar for grinding, and after the mixture is uniformly ground, the first tablet is obtained after tabletting (also referred to as a mixed raw material in the present application); S2: The mixed raw material obtained in step S1 is sintered in a reducing atmosphere (mass ratio of H2 and Ar is 10:90), and the sintering process is as follows: first sintering at 150°C for 3h, and then sintering at 600°C for 8h to obtain a precursor; S3: The precursor obtained in step S2 is tabletted to obtain a second tablet, which is sintered at 1000°C for 18h in an argon atmosphere to obtain the europium-based halophosphate Eu5(PO4)3F.

[0098] Comparative example 5 The comparative example 1 provides a europium-based halophosphate Eu5(PO4)3F, and the preparation method comprises the following steps:

[0099] Performance test The materials prepared in the above examples and comparative examples are subjected to performance tests, and the test items and test results are as follows: 1. XRD identification analysis X-ray powder diffraction phase analysis of the Eu5(PO4)3Cl and Eu5(PO4)3F samples prepared in each example is carried out by using a D8A A25 type X-ray diffractometer (XRD) of Brucker company, and the test pattern is shown in Figure 1 It can be seen that the XRD patterns of the europium-based halophosphates of examples 1-3 are well matched with the theoretical patterns, indicating that examples 1-3 all prepared Eu5(PO4)3Cl magnetic refrigeration materials, wherein the XRD pattern of the sample of example 3 is well matched with the standard pattern, indicating that the sample has high purity and is composed of a single Eu5(PO4)3Cl phase, which belongs to hexagonal system and has a space group of P63 / m, and the cell parameters are as follows: a = 9.88553, b = 9.88553, c = 7.20585, α = β = 90°, gamma = 120°; Figure 2As can be seen, the XRD patterns of the europium-based halogenated phosphate salts of Example 4 and Example 5 are both well matched with the theoretical pattern, indicating that both Example 4 and Example 5 have prepared the Eu5(PO4)3F magnetic refrigeration material. The XRD pattern of the sample of Example 4 is well matched with the standard pattern, indicating that the sample has high purity and is composed of a single Eu5(PO4)3F phase. The sample has a hexagonal crystal system, a space group of P63 / m, and the following cell parameters: a = 9.72950, b = 9.7295, c = 7.28897, α = β = 90°, γ = 120°. The XRD pattern of the europium-based halogenated phosphate salt in Comparative Example 4 has obvious impurity peaks, indicating that there are obvious impurity phases in the material, and even the main phase of Eu5(PO4)3F is not obtained. It can be seen that the ratio of NH4F in the reactant and the sintering temperature both result in the failure to successfully synthesize the pure-phase Eu5(PO4)3F magnetic refrigeration material.

[0100] Figure 3 The XRD patterns of the europium-based halogenated phosphate salts of Comparative Examples 1-3 have obvious impurity peaks, indicating that there are obvious impurity phases in the material, and pure-phase Eu5(PO4)3Cl crystals are not obtained. Among them, the NH4Cl raw material of the comparative example is insufficient; the sintering temperature of Comparative Examples 2-3 is too low or too high, which results in the failure to successfully synthesize the Eu5(PO4)3Cl magnetic refrigeration material with high purity. This indicates that during preparation, the reactant ratio, sintering temperature and other parameters need to be adjusted in order to obtain Eu5(PO4)3Cl with better purity.

[0101] 2. Magnetic phase transition The magnetic entropy change (Δ S M ) can be determined from the magnetization-temperature curve by the following Maxwell relation: S max , wherein the magnetization-temperature curve is obtained from the isothermal magnetization curve: Maxwell relation: , wherein Δ S M (Δ T , H ) is the magnetic entropy change, M is the magnetization, T is the temperature, H is the applied magnetic field.

[0102] Figure 4 is the thermomagnetic curve of the Eu5(PO4)3Cl sample prepared in Example 3 under field cooling (FC) at a magnetic field of 0.01 T; Figure 5The isothermal magnetization curve of the Eu5(PO4)3Cl sample prepared in Embodiment 3 of the present application under the magnetic field change of 0-5T at 0.4-1.8K is shown in the figure. As can be seen from the figure, the magnetization of the material increases rapidly with the increase of the external magnetic field under the external magnetic field of 1.5T, and tends to be saturated when the magnetic field is 5T, and the magnetic phase transition temperature is 0.5-0.7K.

[0103] The magnetic entropy change under different magnetic field changes can be calculated according to the isothermal magnetization curve at different temperatures by using the Maxwell relationship. For example: Figure 6 The relationship curve of the magnetic entropy change of the Eu5(PO4)3Cl sample prepared in Embodiment 3 of the present application and the temperature under different magnetic field changes is shown in the figure. Figure 6 As can be seen from the figure, the maximum magnetic entropy change of the material appears at 1.2K-1.4K, and the maximum magnetic entropy change values are 27.1J·kg -1 ·K -1 , 46.2J·kg -1 ·K -1 and 62.2J·kg -1 ·K -1 respectively when the magnetic field changes are 0-1T, 0-2T and 0-5T, and the material has a large magneto-caloric effect under a low external magnetic field (especially ≤3T).

[0104] Figure 7 The thermomagnetic curve of the Eu5(PO4)3F sample prepared in Embodiment 4 of the present application under field cooling (FC) at 0.01T magnetic field is shown in the figure. Figure 8 The isothermal magnetization curve of the Eu5(PO4)3F sample prepared in Embodiment 4 of the present application under the magnetic field change of 0-5T at 0.4-1.8K is shown in the figure. As can be seen from the figure, the magnetization of the material increases rapidly with the increase of the external magnetic field under the external magnetic field of 1.5T, and tends to be saturated when the magnetic field is 5T, and the magnetic phase transition temperature is <0.4K.

[0105] Figure 9 The relationship curve of the magnetic entropy change of the Eu5(PO4)3F sample prepared in Embodiment 4 of the present application and the temperature under different magnetic field changes is shown in the figure. As can be seen from the figure, the maximum magnetic entropy change of the material appears at 1.2K-1.4K, and the maximum magnetic entropy change values are 15.2J·kg -1 ·K -1 , 39.8J·kg -1 ·K -1 and 64.4J·kg -1 ·K -1 respectively when the magnetic field changes are 0-11T, 0-2T and 0-5T, and the material has a large magneto-caloric effect under a high external magnetic field (especially ≥2T).

[0106] The XRD test results of other embodiments are similar to those of Example 3; the magnetic phase transition test results of each embodiment and the comparative example are shown in Table 1.

[0107] Table 1 The above results of the above embodiments and the comparative example show that: The present application can not only prepare high-purity europium-based halogenated phosphate by the simple solid-phase reaction, but also optimize the crystal cell space structure, further improve the stability and have a large magnetic entropy change by the amount of raw materials, specific sintering process and secondary tabletting.

[0108] The magnetic phase transition temperature of the europium-based halogenated phosphate prepared by the present application is <1K, and the maximum magnetic entropy change under the magnetic field change of 0~5T is ≥60J·kg -1 ·K -1 , which is a superior low-temperature magnetic refrigeration material, can not only promote the scientific and technological progress of China in the fields of low-temperature physics, deep space exploration and aerospace, etc., but also promote the high-value-added utilization of rare earth resources.

[0109] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit the protection scope of the present application, although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. An europium-based halophosphate characterized in that, The structure general formula of the europium-based halogenated phosphate is Eu5(PO4)3X, X is halogen; The europium-based halogenated phosphate is prepared by a method comprising the following steps: S1: Eu source, phosphorus source, and excess halogen source are mixed and ground according to the stoichiometric ratio, and then a mixed raw material is obtained after one-time tabletting; S2: The mixed raw material obtained in step S1 is sintered in a reducing atmosphere, and the sintering process is as follows: First, sintering at 150-300 DEG C for 3-6 h, and then sintering at 600-800 DEG C for 8-10 h to obtain a precursor; S3: The precursor obtained in step S2 is secondly tabletted, and then sintered at 860-900 DEG C in an inert atmosphere for 18-30 h to obtain the europium-based halogenated phosphate.

2. The europium-based halophosphate salt according to claim 1, characterized in that, The halogen X is selected from at least one of F or Cl.

3. The europium-based halophosphate salt according to claim 1, characterized in that, At least one of the following characteristics is satisfied: (1) The Eu source is Eu2O3, which is uniformly mixed with graphite and ground, and then sintered in a reducing atmosphere at 1400-1500 DEG C for 10-12 h to obtain EuO, which is then mixed and ground with the phosphorus source and the halogen source in step S1; (2) The Eu source comprises EuO; (3) The phosphorus source comprises NH4H2PO4; (4) The halogen source comprises ammonium halide; (5) In step S1, the molar ratio of Eu element in the Eu source, P element in the phosphorus source, and halogen X in the halogen source is 5:3:(6-20); (7) The halogen X is F, the sintering temperature of the first mixed raw material is 150-300 DEG C, and the sintering time is 3 h; the sintering temperature of the second mixed raw material is 300-600 DEG C, and the sintering time is 8 h; (8) The halogen X is Cl, the sintering temperature of the first mixed raw material is 300 DEG C, and the sintering time is 6 h; the sintering temperature of the second mixed raw material is 600 DEG C, and the sintering time is 10 h; (9) The reducing atmosphere comprises hydrogen, and further comprises at least one of argon, helium, and neon, and the mass proportion of hydrogen in the reducing atmosphere is 10-15%; (10) The inert atmosphere comprises at least one of argon, helium, and neon.

4. The europium-based halophosphate salt according to claim 1, characterized in that, At least one of the following characteristics is satisfied for the europium-based halogenated phosphate: (1) The europium-based halogenated phosphate is hexagonal crystal system, and the space group is P63 / m; (2) The europium-based halogenated phosphate is Eu5(PO4)3F, and the unit cell parameters of Eu5(PO4)3F are: a ＝9.72950Å， b ＝9.7295Å， c ＝7.28897Å， α ＝ β ＝90°， γ ＝120°； (3) The europium-based halogenated phosphate is Eu5(PO4)3Cl, and the unit cell parameters of Eu5(PO4)3Cl are: a ＝9.88553Å， b ＝9.88553Å， c ＝7.20585Å， α ＝ β ＝90°， γ ＝120°。 5. The europium-based halophosphate salt of claim 1, wherein, The europium-based halogenated phosphate has a magnetic phase transition temperature < 1K; the europium-based halogenated phosphate has a maximum magnetic entropy change ≥ 60J·kg under a magnetic field change of 0-5T -1 ·K -1 .

6. The europium-based halophosphate salt according to claim 1, characterized in that, The europium-based halogenated phosphate is Eu5(PO4)3F, and the magnetic phase transition temperature of Eu5(PO4)3F is <0.4 K.

7. The europium-based halophosphate salt according to claim 1, characterized in that, The europium-based halogenated phosphate is Eu5(PO4)3Cl, and the magnetic phase transition temperature of Eu5(PO4)3Cl is 0.5-0.7 K.

8. The europium-based halophosphate salt of claim 1, wherein, The maximum magnetic entropy change of the europium-based halogenated phosphate satisfies at least one of the following characteristics: (1) the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the magnetic field change of 0-1T is ≤27.1 J·kg -1 ·K -1 ; (2) the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the magnetic field change of 0-2T is ≤46.2 J·kg -1 ·K -1 ; (3) the maximum magnetic entropy change of the europium-based halogenated phosphate salt under a magnetic field change of 0-3T is ≤55.8 J·kg -1 ·K -1 ; (4) the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the magnetic field change of 0-4T is ≤61.1 J·kg -1 ·K -1 ; (5) the maximum magnetic entropy change of the europium-based halogenated phosphate salt under the magnetic field change of 0-5T is ≤64.4 J·kg -1 ·K -1 .

9. The method of producing an europium-based halophosphate salt according to any one of claims 1 to 8, characterized in that, Comprising the following steps: S1: Eu source, phosphorus source, according to the stoichiometric ratio of ingredients, add excess halogen source, mixed grinding, after one tablet, get mixed raw materials; S2: the mixed raw materials obtained in step S1 are sintered in a reducing atmosphere, and the sintering process is as follows: first sintering at 150-300 ℃ for 3-6 h, then heating to 600-800 ℃ for 8-10 h to obtain a precursor; S3: the precursor obtained in step S2 is secondly tabletted, sintered at 860-900 ℃ for 18-30 h in a reducing atmosphere, to obtain the Eu-based halogenated phosphate.

10. Use of the europium-based halophosphate salt according to any one of claims 1 to 8 in a magnetic refrigeration at low temperature, characterized in that, The Eu-based halogenated phosphate is used as a working substance for adiabatic demagnetization refrigeration.

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