Rare earth phosphate low-temperature magnetic refrigeration material, preparation method and application thereof

Abstract

This invention belongs to the field of magnetic refrigeration materials technology, specifically disclosing a rare-earth phosphate low-temperature magnetic refrigeration material, its preparation method, and its applications. The material has the chemical formula Eu3Gd(PO4)3, belongs to the cubic crystal system, and has space group I-43d. This material can be prepared by a stepwise solid-state reaction method under a hydrogen-argon mixed gas atmosphere. The preparation method of this invention produces a product with high purity and a large magnetic entropy change. The material provided by this invention exhibits excellent low-temperature magnetocaloric effects, meeting the application requirements of low-temperature magnetic refrigeration technology and is suitable for cutting-edge fields such as deep space exploration, quantum computing, and condensed matter physics.

Description

Technical Field

[0001] This invention relates to the field of rare earth magnetic functional materials technology, and in particular to a rare earth phosphate low-temperature magnetic refrigeration material, its preparation method and application. Background Technology

[0002] Since the late 20th century, liquid helium and lower cryogenic refrigeration technologies have been widely applied in numerous fields, including scientific research, industry, and medicine. The cryogenic environment is an indispensable and crucial component of cryogenic refrigeration technology, holding immense importance for cutting-edge scientific fields such as quantum computing, controlled nuclear fusion, high-energy physics, and advanced medicine. However, current methods for obtaining cryogenic environments face many limitations. 3 He adsorption refrigeration and 3 He- 4 He dilution refrigeration is one of the two main methods, and the refrigerant it uses is... 3 Helium is an extremely scarce resource. Its global reserves are very limited and it mainly comes from the byproducts of nuclear weapons decommissioning. Its supply is severely restricted and its price is extremely high. This fundamentally restricts the widespread application and large-scale promotion of cryogenic refrigeration technology.

[0003] Therefore, there is a need to research novel refrigeration technologies that do not rely on helium resources. Magnetic refrigeration, which utilizes the magnetocaloric effect (MCE) of magnetic materials, can achieve this without relying on rare helium resources. 3 The technology of achieving ultra-low temperature operation without relying on gravity is crucial. The core of magnetic refrigeration technology lies in the performance of the magnetic refrigeration materials. Developing giant magnetocaloric materials with large magnetic entropy changes, high cooling capacity, stability, reliability, and ease of mass production is key to promoting the application of low-temperature magnetic refrigeration technology. In recent years, rare earth non-metallic materials (such as rare earth oxides, rare earth sulfides, and rare earth borides) have attracted widespread attention due to their unique magnetic and magnetocaloric effects. Currently, the main magnetocaloric materials used in low-temperature environments are europium-based and gadolinium-based oxides. However, these magnetocaloric materials all have some unresolved problems, such as: CN119306746A discloses a gadolinium hydroxide magnetic refrigeration material and its preparation method, with the chemical formula {Na2[Gd6(mpcd)6(μ3:η]}. 2 :η 1 -AC)6(μ3-O)]}, mpcd is deprotonated (E)-N-[(2-hydroxy-3-methoxyphenyl)methylene]pyrazine-2-carboxylhydrazine), which can reach a maximum magnetic entropy of 29.6 J / kg·K at 2.6K under a magnetic field change of 0-7T, but its maximum magnetic entropy change under a magnetic field change of 0-2T is only about 17 J / kg·K. This defect directly leads to the inability of existing materials to meet the requirements of cryogenic cooling under low magnetic field conditions, and also limits the feasibility of using small superconducting magnets or permanent magnets with lower cost as magnetic field sources.

[0004] Divalent europium ions (Eu) 2+ Eu is an ideal magnetic center for cryogenic magnetic refrigeration due to its unique electronic structure. 2+ With 4f 7 It has a half-filled shell, a total angular momentum quantum number J = 7 / 2, a Landé factor g ≈ 2, a theoretical effective magnetic moment of approximately 7.94 μB, and exhibits pure spin magnetism with no orbital angular momentum contribution. Its ground state is... 8 S 7 / 2 Since the first-order perturbation of the crystal electric field to the ground state is zero, the magnetic anisotropy is extremely weak, and the hysteresis loss is negligible. 2+ Eu-based compounds (such as EuAl₂O₄, Eu₂SiO₄, EuTiO₃, EuCl₂, etc.) often exhibit large magnetocaloric effects in the low-temperature region, with some materials showing significant magnetic entropy changes even under low magnetic fields. For example, CN122233392A discloses a europium-based aluminosilicate low-temperature magnetic refrigeration material and its preparation method, with the chemical formula Eu₂Al₂SiO₇. Its magnetic entropy change is 19-46 J / kg·K in the temperature range of 0.45-1.7 K and a magnetic field change of 0-7 T. It exhibits a large magnetic entropy in the low-temperature range. However, Eu-based magnetocaloric materials require Eu... 2+ Ions, with their unique electronic structure serving as magnetic centers, Eu 3+ Ions do not possess this characteristic, while Eu 2+ It is extremely unstable in air and is readily oxidized to Eu. 3+ This leads to a sharp deterioration in the magnetic properties of the material. Therefore, the high purity and controllable synthesis of Eu-based magnetic refrigeration materials face enormous challenges. Impurities caused by low purity will dilute the effective magnetic refrigeration centers and severely hinder the release of magnetic entropy, resulting in the actual measured magnetocaloric effect being far lower than theoretical expectations.

[0005] Existing europium-based phosphate magnetic refrigeration materials are haloapatite type Eu5(PO4)3X, such as those disclosed in CN121823518A. Although these materials have a high magnetic entropy change, the halogen-containing components are unstable during long-term product cycling, which may pose risks of halogen gas release and corrosion. Summary of the Invention

[0006] Gd-based magnetocaloric materials exhibit good stability in air, but their magnetic entropy is low at low temperatures. Eu-based magnetocaloric materials, however, have lower magnetic entropy. 2+ It is extremely unstable in air and is readily oxidized to Eu. 3+ This leads to a sharp deterioration in the magnetic properties of the material, and it is extremely easy to generate substances such as or containing Eu. 3+ Impurities. How to overcome the low magnetic entropy of Gd-based magnetocaloric materials at low temperatures, Eu 2+The oxidation problem, thus hindering the preparation of materials with pure phase, high crystallinity, and intrinsic giant magnetocaloric effect, remains a long-standing technical challenge in this field. This invention addresses this need by providing a simple and reproducible method for preparing high-purity rare-earth-based phosphate low-temperature magnetic refrigeration materials, aiming to fill the gap in the practical application of these materials. It possesses advantages such as simple preparation process, good material stability, and significant magnetocaloric effect under low magnetic fields, making it applicable to low-temperature magnetic refrigeration. The purpose of this invention is to solve the problems of low magnetic entropy reduction under low fields, complex preparation processes, long preparation cycles, and poor stability in existing low-temperature magnetic refrigeration materials.

[0007] The technical solution provided by the present invention to solve the above problems is as follows: a rare earth phosphate low-temperature magnetic refrigeration material, wherein the magnetic refrigeration material is composed of a single phase, has the chemical formula Eu3Gd(PO4)3, belongs to the cubic crystal system, and has the space group I-43d.

[0008] Preferably, the lattice constant of the magnetic refrigeration material Eu3Gd(PO4)3 is a=b=c=10.03000(1)Å. α = β = γ =90 o The X-ray diffraction pattern of the magnetic refrigeration material contains only diffraction peaks corresponding to the cubic crystal system I-43d space group, and no diffraction peaks of impurity phases.

[0009] The preparation method of the rare earth phosphate low-temperature magnetic refrigeration material includes the following steps: (1) Obtain europium source, gadolinium source and phosphorus source as raw materials; wherein the europium source includes Eu2O3; the gadolinium source includes Gd2O3; the phosphorus source includes NH4H2PO4. (2) Weigh the raw materials according to the stoichiometric ratio of Eu, Gd and P elements in the chemical formula; mix the raw materials evenly and grind them thoroughly and press them into tablets; pre-calcine them at 300-600℃ for 3-6 hours in a flowing reducing mixed atmosphere. (3) After the pre-calcined product is thoroughly ground again, it is pressed into tablets and sintered at 1000-1200℃ for 10-14 hours in a flowing reducing mixed atmosphere. After the second sintering, a cooling program is set to cool to room temperature to obtain the high-purity rare earth-based phosphate low-temperature magnetic refrigeration material.

[0010] Preferably, the pressure applied during tablet compression in step (2) and step (3) is 500-1000 MPa, and the pressure holding time is 2-5 minutes.

[0011] Preferably, the reducing mixed atmosphere described in steps (2) and (3) is specifically a mixed atmosphere containing hydrogen and argon.

[0012] Preferably, the heating rate in both the pre-firing in step (2) and the sintering in step (3) is 2-5℃ / min.

[0013] Preferably, the cooling process in step (3) has a cooling rate of 2℃ / min.

[0014] Preferably, the volume fraction of hydrogen in the mixed atmosphere is 5-10%.

[0015] Preferably, the purity of Eu2O3, Gd2O3, and NH4H2PO4 is not less than 99.9%; the grinding time is 30-60 min.

[0016] The present invention also provides the application of the rare earth phosphate low-temperature magnetic refrigeration material. When the temperature is below 3K and the magnetic field varies from 0 to 2T, the maximum magnetic entropy of the magnetic refrigeration material Eu3Gd(PO4)3 becomes 36.2-40.5J / kg·K; when the temperature is below 3K and the magnetic field varies from 0 to 5T, the maximum magnetic entropy of Eu3Gd(PO4)3 becomes 59.1-66.0J / kg·K.

[0017] The magnetic refrigeration material provided by this invention is a two-stage magnetic phase transition material with a phase transition temperature in the low-temperature region.

[0018] The magnetic refrigeration material described in this invention exhibits a giant magnetocaloric effect: When the magnetic field changes from 0 to 2 T, the maximum magnetic entropy of the magnetic refrigeration material Eu3Gd(PO4)3 at temperatures of 3 K and below is 36.2-40.5 J / kg·K; when the magnetic field changes from 0 to 5 T, the maximum magnetic entropy of the magnetic refrigeration material Eu3Gd(PO4)3 at temperatures of 4 K and below is 59.1-66.0 J / kg·K.

[0019] The magnetic refrigeration material described in this invention exhibits magnetocaloric effects under both low and high magnetic fields, placing it among the top-tier magnetic refrigeration materials and making it an excellent magnetic refrigeration material.

[0020] This invention uses europium oxide, gadolinium oxide, and ammonium dihydrogen phosphate as raw materials, which are uniformly mixed in a certain stoichiometric ratio, followed by high-temperature solid-state sintering to obtain the desired magnetic refrigeration material. The preparation method described in this invention is simple, has a short preparation cycle, and uses inexpensive raw materials, making it suitable for large-scale industrial production. Specifically, it has the following beneficial effects: This invention is the first to prepare Eu3Gd(PO4)3, which exhibits good stability, is not easily deliquescent, and is not easily oxidized. Furthermore, it achieves the high-purity, single-phase, controllable synthesis of Eu3Gd(PO4)3.

[0021] The rare-earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in this invention has a magnetic phase transition temperature of 1.8K, and under applied magnetic fields of 0-2T and 0-5T, the maximum isothermal magnetic entropy change is 36.2-40.5 J / kg·K and 59.1-66.0 J / kg·K, respectively, both of which are superior to the commercially available low-temperature magnetic refrigeration material Gd3Ga5O. 12 (14.6 J / kg·K and 32.8 J / kg·K).

[0022] The rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in this invention has Eu 2+ +Gd 3+ Double 4f 7 The coexisting new coupling state gives it good environmental stability; after being placed in air at room temperature for 7 days, there are no significant changes in crystal structure and magnetocaloric properties.

[0023] It can be prepared by a stepwise solid-phase reaction method, which has the advantages of simple preparation process, short preparation cycle and good structural stability, and has excellent application prospects in the field of low temperature magnetic refrigeration.

[0024] This invention relates to rare earth magnetic functional materials, which can simultaneously promote the high-value-added utilization of rare earth resources and enhance the international competitiveness and technological strength of my country's rare earth industry.

[0025] The preparation method proposed in this invention has a wider applicable process window and greater process tolerance compared to the apatite system, which has a narrow process window and an optimal sintering temperature of 900 degrees Celsius, while impurity phases appear at 800 and 1000 degrees Celsius. It also reduces the processing difficulty. Attached Figure Description

[0026] Figure 1 X-ray powder diffraction pattern of Eu3Gd(PO4)3, a rare earth-based phosphate low-temperature magnetic refrigeration material prepared in Example 1 of this invention; Figure 2 The crystal structure diagram of Eu3Gd(PO4)3, a rare earth-based phosphate low-temperature magnetic refrigeration material prepared in Example 1 of this invention; Figure 3 X-ray photoelectron spectrum of Eu3Gd(PO4)3, a rare earth-based phosphate low-temperature magnetic refrigeration material prepared in Example 1 of this invention; Figure 4 The isothermal magnetic entropy change curve of the rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in Example 1 of this invention is shown as a function of temperature under different magnetic field strengths. Figure 5 The graph shows the magnetization intensity of the rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in Example 1 of this invention as a function of magnetic field at different temperatures.

[0027] Figure 6 The graph shows the magnetization intensity of the rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in Example 2 of this invention as a function of magnetic field at different temperatures.

[0028] Figure 7 The graph shows the magnetization intensity of the rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in Example 3 of this invention as a function of magnetic field at different temperatures. Detailed Implementation

[0029] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0030] Example 1 The gadolinium-europium phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 was prepared and tested using a stepwise solid-state method, including the following steps: (1) Weigh Eu2O3, Gd2O3, and NH4H2PO4 according to the stoichiometric ratio for 1g of Eu3Gd(PO4)3, mix them evenly, grind them thoroughly and press them into tablets, and pre-calcine them at 300°C for 5 hours in a 10% hydrogen-argon mixed gas. (2) After the pre-calcined powder is fully ground again, it is pressed into tablets and sintered for a second time at 1200°C for 12 hours in a 10% hydrogen-argon mixed gas. After the second sintering is completed, the cooling program is set to 2°C / min to cool to room temperature, and the high-purity rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 is obtained. Figure 1 The X-ray powder diffraction pattern of the rare-earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in this embodiment is shown. The crystal structure of the Eu3Gd(PO4)3 material obtained in this embodiment was characterized and refined using X-ray powder diffraction. The synthesized sample belongs to the cubic crystal system, space group I-43d. The lattice constants of Eu3Gd(PO4)3 were obtained as a=b=c=10.03000(1)Å. α = β = γ =90 o The crystal structure diagram of the rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 prepared in Example 1 of this invention is shown below. Figure 2 As shown.

[0031] The chemical valence state of the Eu3Gd(PO4)3 material obtained in this embodiment was characterized by X-ray photoelectron spectroscopy, and the results are as follows: Figure 3 As shown, it can be seen that Eu has a +2 valence, Gd has a +3 valence, P has a +5 valence, and O has a -2 valence in the measured sample.

[0032] The magnetization and specific heat of the Eu3Gd(PO4)3 material obtained in this embodiment under different temperatures and magnetic fields were measured using a vibrating sample magnetometer manufactured by Quantum Design, Inc., USA. The results are as follows: Figure 4-5 As shown, the measured phase transition temperature of the sample is in the low-temperature region, and it exhibits strong magnetism at low temperatures.

[0033] The magnetic entropy change of the Eu3Gd(PO4)3 material obtained in this embodiment under different magnetic field changes was calculated based on Maxwell's equations, and the results are as follows: Figure 6 As shown, it can be seen that under magnetic field changes of 0-2T and 0-5T, the isothermal magnetic entropy change of Eu3Gd(PO4)3 can reach 36.2J / kg·K and 59.1J / kg·K, respectively.

[0034] Example 2 The gadolinium-europium phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 was prepared and tested using a stepwise solid-state method, including the following steps: (1) Weigh Eu2O3, Gd2O3, and NH4H2PO4 according to the stoichiometric ratio for 1g of Eu3Gd(PO4)3, mix them evenly, grind them thoroughly and press them into tablets, and pre-calcine them at 450°C for 4 hours in an 8% hydrogen-argon mixed gas. (2) After the pre-calcined powder is fully ground again, it is pressed into tablets and sintered for 12 hours at 1050°C in a 10% hydrogen-argon mixed gas. After the second sintering, the cooling program is set to 2°C / min to cool to room temperature to obtain the high-purity rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3. The crystal structure of the Eu3Gd(PO4)3 material obtained in this embodiment was characterized by X-ray powder diffraction, which showed that the sample exhibited excellent single-phase properties. The magnetization of the Eu3Gd(PO4)3 material obtained in this embodiment under different temperatures and magnetic fields was measured using a vibrating sample magnetometer, and the results are as follows. Figure 6 As shown, according to Maxwell's equations, the isothermal magnetic entropy changes of the Eu3Gd(PO4)3 material obtained in this embodiment under magnetic field variations of 0-2T and 0-5T can reach 40.5 J / kg·K and 66.0 J / kg·K, respectively. This indicates that the Eu3Gd(PO4)3 material obtained in this embodiment has good application potential in the field of low-temperature magnetic refrigeration.

[0035] Example 3 (1) Weigh Eu2O3, Gd2O3, and NH4H2PO4 according to the stoichiometric ratio for 1.5g of Eu3Gd(PO4)3, mix them evenly, grind them thoroughly and press them into tablets, and pre-calcine them at 500°C for 5 hours in a 5% hydrogen-argon mixed gas. (2) After the pre-calcined powder is fully ground again, it is pressed into tablets and sintered for a second time at 1200°C for 14 hours in a 5% hydrogen-argon mixed gas. After the second sintering is completed, the cooling program is set to 2°C / min to cool to room temperature, and the high-purity rare earth-based phosphate low-temperature magnetic refrigeration material Eu3Gd(PO4)3 is obtained. The crystal structure of the Eu3Gd(PO4)3 material obtained in this embodiment was characterized by X-ray powder diffraction, which showed that the sample exhibited excellent single-phase properties. The magnetization of the Eu3Gd(PO4)3 material obtained in this embodiment under different temperatures and magnetic fields was measured using a vibrating sample magnetometer, and the results are as follows. Figure 7 As shown, according to Maxwell's equations, the isothermal magnetic entropy changes of the Eu3Gd(PO4)3 material obtained in this embodiment under magnetic field variations of 0-2T and 0-5T can reach 37.1 J / kg·K and 65.7 J / kg·K, respectively. This indicates that the Eu3Gd(PO4)3 material obtained in this embodiment has good application potential in the field of low-temperature magnetic refrigeration.

[0036] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A rare-earth phosphate low-temperature magnetic refrigeration material, characterized in that, The magnetic refrigeration material is composed of a single phase with the chemical formula Eu3Gd(PO4)3, belonging to the cubic crystal system and space group I-43d.

2. The rare earth-based phosphate low-temperature magnetic refrigeration material as described in claim 1, characterized in that, The lattice constant of the magnetic refrigeration material Eu3Gd(PO4)3 is a=b=c=10.03000 (1) Å. α = β = γ =90 o The X-ray diffraction pattern of the magnetic refrigeration material contains only diffraction peaks corresponding to the cubic crystal system I-43d space group, and no diffraction peaks of impurity phases.

3. The preparation method of a rare earth phosphate low-temperature magnetic refrigeration material as described in claim 1, characterized in that, Includes the following steps: (1) Obtain europium source, gadolinium source and phosphorus source as raw materials; wherein the europium source includes Eu2O3; the gadolinium source includes Gd2O3; the phosphorus source includes NH4H2PO4; (2) Weigh the raw materials according to the stoichiometric ratio of Eu, Gd and P elements in the chemical formula; mix the raw materials evenly and grind them thoroughly and press them into tablets; pre-calcine them at 300-600℃ for 3-6 hours in a flowing reducing mixed atmosphere. (3) After the pre-calcined product is thoroughly ground again, it is pressed into tablets and sintered at 1000-1200℃ for 10-14 hours in a flowing reducing mixed atmosphere. After the second sintering, a cooling program is set to cool to room temperature to obtain the high-purity rare earth-based phosphate low-temperature magnetic refrigeration material.

4. The preparation method according to claim 3, characterized in that, The pressure applied during tablet compression in step (2) and step (3) is 500-1000 MPa, and the pressure holding time is 2-5 minutes.

5. The preparation method according to claim 3, characterized in that, The reducing mixed atmosphere described in steps (2) and (3) is specifically a mixed atmosphere containing hydrogen and argon.

6. The preparation method according to claim 3, characterized in that, The heating rate for both the pre-firing in step (2) and the sintering in step (3) is 2-5℃ / min.

7. The preparation method according to claim 3, characterized in that, The cooling process in step (3) has a cooling rate of 2℃ / min.

8. The preparation method according to claim 3, characterized in that, The volume fraction of hydrogen in the mixed atmosphere is 5-10%.

9. The preparation method according to claim 3, characterized in that: The purity of Eu2O3, Gd2O3, and NH4H2PO4 is not less than 99.9%; the grinding time is 30-60 min.

10. The application of the rare earth-based phosphate low-temperature magnetic refrigeration material as described in claim 1, characterized in that: When the magnetic field changes from 0 to 2 T, the maximum magnetic entropy of the magnetic refrigeration material Eu3Gd(PO4)3 at temperatures of 3 K and below is 36.2-40.5 J / kg·K; when the magnetic field changes from 0 to 5 T, the maximum magnetic entropy of the magnetic refrigeration material Eu3Gd(PO4)3 at temperatures of 4 K and below is 59.1-66.0 J / kg·K.

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