Method for preparing rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by molten salt method

Abstract

The present invention discloses a method for preparing a rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by a molten salt method, which relates to the technical field of magnetic functional materials. The steps are as follows: Weigh manganese dioxide powder and rare earth oxide powder according to the stoichiometric ratio of the rare earth-based multivalent manganese oxide, mix them with sodium chloride, and grind to obtain a mixed powder; sinter the mixed powder at a high temperature and cool it to room temperature with the furnace to obtain a sintered powder; wash the sintered powder with water to remove sodium chloride and dry it to obtain the product. The process of the present invention is simple and low in cost. The prepared rare earth-based multivalent manganese oxide has a large magnetocaloric effect near the liquid helium temperature range. Among them, the REMn2O5-type rare earth-based multivalent manganese oxide has an isothermal magnetic entropy change of up to 12.61 J kg ‑1 K ‑1 , and the relative cooling power RCP is up to 338 J kg ‑1 . It can be used as a low-temperature magnetic refrigeration material in low-temperature engineering, aerospace, medical devices and other fields.

Description

Technical Field

[0001] The invention relates to the technical field of magnetic functional materials, and in particular to a method for preparing rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration materials by a molten salt method. Background Art

[0002] As the greenhouse effect caused by global warming becomes increasingly serious, the traditional air compression refrigeration technology, which uses Freon refrigerants, is prone to cause the ozone hole to expand, exacerbating the greenhouse effect, thus hindering the sustainable development of the world. Magnetic refrigeration technology uses solid magnetic materials as refrigerants, and uses the magnetocaloric effect, the essential property of magnetic materials, to achieve the purpose of refrigeration by applying or removing a magnetic field to cause the temperature of the magnetic material to change. Due to its advantages such as high efficiency, energy saving, and green environmental protection, it is expected to replace traditional air compression refrigeration technology and be applied to civil, military and aerospace fields. In particular, near low-temperature areas such as liquid helium and liquid nitrogen, magnetic refrigeration technology has also received widespread attention due to the needs of aerospace and large scientific equipment research and development. Rare earth-based multivalent manganese oxide REMn2O5 (RE = rare earth element) is a type of multiferroic material with strong magnetoelectric coupling, which can be used as a magnetoelectric functional material in electronic devices. In 2013, Ge Heng et al. reported that REMn2O5 has both reversible magnetic entropy change and large magnetic refrigeration capacity in the near-liquid helium temperature zone, and is an excellent low-temperature magnetic refrigeration material. REMn2O5 material has rare earth ions, multivalent manganese ions (Mn 3+ ,Mn 4+ )The complex crystal environment in which they coexist places certain requirements on the preparation conditions.

[0003] At present, the mainstream methods for preparing powder samples of this series of materials are hydrothermal method, sol-gel method and solid phase reaction method, all of which require high pressure or high temperature reaction conditions. The preparation process is relatively cumbersome and costly, and improper reaction conditions can easily lead to insufficient reaction or the appearance of impurities such as REMnO3. The molten salt method is to select one or several salts with low melting points as reaction media (flux) and fully mix them with the reaction raw materials, so that the salts are in a molten state under certain high temperature conditions, and the reaction raw materials undergo chemical reactions in a molten salt environment. After the reaction is completed, the reaction mixture is cooled, the salts are dissolved with a suitable solvent, and the synthetic material is obtained after filtering and washing. If the appropriate reaction temperature and reaction time are adjusted during the reaction process, the molten salt method can prepare high-quality single crystal or polycrystalline target materials. However, there is no report on the preparation of rare earth-based manganese oxide materials by the molten salt method. Summary of the invention

[0004] Based on the technical problems existing in the background technology, the present invention proposes a method for preparing rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration materials by a molten salt method, which has a simple process and is easy to realize industrial production.

[0005] A method for preparing a rare-earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by a molten salt method is proposed as follows:

[0006] S1. Weigh manganese dioxide powder and rare-earth oxide powder according to the stoichiometric ratio of the rare-earth-based multivalent manganese oxide, mix them with sodium chloride, and grind to obtain a mixed powder;

[0007] S2. Sinter the mixed powder at a high temperature and cool it to room temperature with the furnace to obtain a sintered powder;

[0008] S3. Wash the sintered powder with water to remove sodium chloride, and dry it to obtain the product.

[0009] Preferably, the chemical formula of the rare-earth-based multivalent manganese oxide is REMn2O5; where RE is one or any combination of Gd, Tb, Dy, and Ho.

[0010] Preferably, in S1, the rare-earth-based multivalent manganese oxide and sodium chloride are mixed at a molar ratio of 1:2-4.

[0011] Preferably, in S2, the temperature is raised to 900-1100 °C at a heating rate of 3.5-4.5 °C / min and sintered for 9-11 h.

[0012] Preferably, in S3, the specific operation of washing away sodium chloride with water is as follows: ultrasonically disperse the sintered product in deionized water, centrifuge, and discard the supernatant; repeat the above operation 2-3 times.

[0013] Preferably, the centrifugation speed is 4500-5500 rpm and the centrifugation time is 2-5 min.

[0014] Preferably, in S3, it is dried at 60-80 °C for 8-12 h.

[0015] The present invention also proposes a rare-earth-based multivalent manganese oxide low-temperature magnetic refrigeration material prepared by the above method.

[0016] Compared with the prior art, the beneficial effects of the present invention are reflected in the following aspects:

[0017] 1. The present invention uses easily available and inexpensive NaCl as a molten salt to mix and sinter with the reactants, without the need for pressurized oxygen and extremely high temperature conditions. Compared with the traditional solid-phase reaction method, the reaction time is shorter, the removal of NaCl impurities from the synthesized product is convenient and fast, the process is simple, easy to implement, and is conducive to industrial production.

[0018] 2. The present invention successfully synthesizes a rare-earth-based multivalent manganese oxide by the molten salt method, which has a large magnetocaloric effect near the liquid helium temperature range. Among them, the REMn2O5-type rare-earth manganese oxide has an isothermal magnetic entropy change of up to 12.61 J kg under a magnetic field change of 0-70 kOe-1 K -1 , the relative magnetic refrigeration capacity RCP can reach up to 338 J kg -1 , and can be used as a cryogenic magnetic refrigeration material in cryogenic engineering, aerospace, medical devices and other fields. Description of the Drawings

[0019] Figure 1 XRD and Rietveld refinement results of polycrystalline REMn2O5 (RE = Gd, Tb, Dy, Ho) prepared in Examples 1-4 of the present invention;

[0020] Figure 2 Isothermal magnetic entropy change (magnetic field changes are 70, 60, 50, 40, 30, 20, 10, 2 kOe) of polycrystalline REMn2O5 (RE = Gd, Tb, Dy, Ho) prepared in Examples 1-4 of the present invention as a function of temperature;

[0021] Figure 3 Relative magnetic refrigeration capacity of polycrystalline REMn2O5 (RE = Gd, Tb, Dy, Ho) prepared in Examples 1-4 of the present invention as a function of magnetic field. Detailed Description of the Invention

[0022] Next, the technical solutions of the present invention will be described in detail through specific examples.

[0023] Example 1

[0024] In this example, a magnetic refrigeration material with the chemical formula GdMn2O5 was prepared by the molten salt method, and its preparation method was carried out according to the following specific steps:

[0025] (1) Weighing: According to the chemical reaction equation 8MnO2 + 2Gd2O3 → 4GdMn2O5 + O2↑, high-purity manganese dioxide, gadolinium oxide and sodium chloride powders with a molar ratio of 4:1:6 were weighed and placed in an agate mortar for thorough grinding and mixing to obtain a mixed powder.

[0026] (2) Sintering: The uniformly mixed powder was placed in a corundum crucible and heated in a high-temperature sintering furnace at a heating rate of 4 °C / min to 1000 °C, then sintered at this temperature for 10 hours, and finally cooled with the furnace.

[0027] (3) Washing the salt: After the furnace temperature dropped to room temperature, the corundum crucible was taken out, the sintered product was transferred to a centrifuge tube, deionized water was added, shaken vigorously, and placed in an ultrasonic cleaner for auxiliary oscillation to dissolve NaCl. The centrifuge tube was placed in a high-speed centrifuge for centrifugation. The rotation speed was 5000 revolutions / min and the time was 3 min. After centrifugation, the centrifuge tube was taken out and the supernatant was poured out. The above process was repeated 3 times.

[0028] (4) Drying: Place the centrifuge tube after washing away NaCl in a drying oven and bake at 70°C for 10 hours until completely dry to obtain a GdMn2O5 polycrystalline product.

[0029] The XRD pattern of the GdMn2O5 polycrystalline sample obtained by the molten salt method is shown in Figure 1 The XRD experimental data were refined by Rietveld crystal structure refinement. The refinement results showed that the synthesized GdMn2O5 had an orthorhombic crystal structure with space group Pbam.

[0030] The magnetic entropy change and magnetic refrigeration capacity of the material were tested, and the results are shown in Figures 2 - 3 According to the magnetic measurement, the isothermal magnetic entropy change of 0-70 kOe is 9.2 J kg -1 K -1 , the relative magnetic refrigeration capacity is 197.0J kg -1 .

[0031] Example 2

[0032] In this embodiment, a molten salt method is used to prepare a magnetic refrigeration material having a chemical formula of TbMn2O5, and the preparation method is carried out according to the following specific steps:

[0033] (1) Ingredients: According to the chemical reaction equation 16MnO2+2Tb4O7→8TbMn2O5+3O2↑, high-purity manganese dioxide, terbium oxide and sodium chloride powders with a molar ratio of 8:1:12 were weighed, placed in an agate mortar, fully ground, and mixed to obtain a mixed powder.

[0034] (2) Sintering: The uniformly mixed powder is placed in a corundum crucible, and the temperature is raised to 1000°C at a heating rate of 4°C / min in a high-temperature sintering furnace, and then sintered at this temperature for 10 hours, and finally cooled in the furnace.

[0035] (3) Salt washing: After the furnace temperature drops to room temperature, remove the corundum crucible, transfer the sintered material to a centrifuge tube, add deionized water, shake vigorously, and place in an ultrasonic cleaner to assist in shaking to dissolve the NaCl. Place the centrifuge tube in a high-speed centrifuge and centrifuge at a speed of 5000 rpm for 3 minutes. After centrifugation, remove the centrifuge tube and pour out the supernatant. Repeat the above process 3 times.

[0036] (4) Drying: Place the centrifuge tube after washing away NaCl in a drying oven and bake at 70°C for 10 hours until completely dry to obtain TbMn2O5 polycrystalline product.

[0037] The XRD pattern of TbMn2O5 polycrystalline sample obtained by molten salt method is shown in Figure 1, and the Rietveld crystal structure refinement was performed on the XRD experimental data. The refinement results show that the synthesized TbMn2O5 has an orthorhombic crystal structure with the space group Pbam.

[0038] The magnetic entropy change and magnetic refrigeration capacity of the material were tested, and the results are shown in Figures 2 - 3 . According to magnetic measurements, the isothermal magnetic entropy change with a magnetic field change of 0 - 70 kOe was calculated to be 12.6 J kg -1 K -1 , and the relative magnetic refrigeration capacity was 284.6 J kg -1 .

[0039] Example 3

[0040] In this example, a magnetic refrigeration material with the chemical formula DyMn2O5 was prepared by the molten salt method. The preparation method is carried out according to the following specific steps:

[0041] (1) Ingredient preparation: According to the chemical reaction equation 8MnO2 + 2Dy2O3 → 4DyMn2O5 + O2↑, high-purity manganese dioxide, dysprosium oxide and sodium chloride powders with a molar ratio of 4:1:6 were weighed and placed in an agate mortar for thorough grinding and mixing to obtain a mixed powder.

[0042] (2) Sintering: The uniformly mixed powder was placed in a corundum crucible and heated in a high-temperature sintering furnace at a heating rate of 4 °C / min to 1000 °C, then sintered at this temperature for 10 hours, and finally cooled with the furnace.

[0043] (3) Salt washing: After the furnace temperature dropped to room temperature, the corundum crucible was taken out, and the sintered product was transferred to a centrifuge tube. Deionized water was added, shaken vigorously, and placed in an ultrasonic cleaner for assisted oscillation to dissolve NaCl. The centrifuge tube was placed in a high-speed centrifuge for centrifugation. The rotation speed was 5000 rpm and the time was 3 min. After centrifugation, the centrifuge tube was taken out and the supernatant was poured out. The above process was repeated 3 times.

[0044] (4) Drying: The centrifuge tube washed with NaCl was placed in a drying oven and baked at 70 °C for 10 hours until completely dry to obtain the polycrystalline product of DyMn2O5.

[0045] By measuring the XRD pattern of the polycrystalline sample of DyMn2O5 obtained by the molten salt method, see Figure 1 , and the Rietveld crystal structure refinement was performed on the XRD experimental data. The refinement results show that the synthesized DyMn2O5 has an orthorhombic crystal structure with the space group Pbam.

[0046] The magnetic entropy change and magnetic refrigeration capacity of the material were tested, and the results are shown in Figures 2 - 3According to the magnetic measurement, the isothermal magnetic entropy change of 0-70 kOe is 11.9 J kg -1 K -1 , the relative magnetic refrigeration capacity is 338.4J kg -1 .

[0047] Example 4

[0048] In this embodiment, a molten salt method is used to prepare a magnetic refrigeration material having a chemical formula of HoMn2O5, and the preparation method is carried out according to the following specific steps:

[0049] (1) Ingredients: According to the chemical reaction equation 8MnO2+2Ho2O3→4HoMn2O5+O2↑, high-purity manganese dioxide, holmium oxide and sodium chloride powders with a molar ratio of 4:1:6 were weighed, placed in an agate mortar, fully ground, and mixed to obtain a mixed powder.

[0050] (2) Sintering: The uniformly mixed powder is placed in a corundum crucible, and the temperature is raised to 1000°C at a heating rate of 4°C / min in a high-temperature sintering furnace, and then sintered at this temperature for 10 hours, and finally cooled in the furnace.

[0051] (3) Salt washing: After the furnace temperature drops to room temperature, remove the corundum crucible, transfer the sintered material to a centrifuge tube, add deionized water, shake vigorously, and place in an ultrasonic cleaner to assist in shaking to dissolve the NaCl. Place the centrifuge tube in a high-speed centrifuge and centrifuge at a speed of 5000 rpm for 3 minutes. After centrifugation, remove the centrifuge tube and pour out the supernatant. Repeat the above process 3 times.

[0052] (4) Drying: The centrifuge tube after washing away NaCl was placed in a drying oven and baked at 70°C for 10 hours until completely dry to obtain a HoMn2O5 polycrystalline product.

[0053] The XRD pattern of HoMn2O5 polycrystalline sample obtained by molten salt method is shown in Figure 1 The XRD experimental data were refined by Rietveld crystal structure refinement. The refinement results showed that the synthesized HoMn2O5 had an orthorhombic crystal structure with space group Pbam.

[0054] The magnetic entropy change and magnetic refrigeration capacity of the material were tested, and the results are shown in Figures 2 - 3 According to the magnetic measurement, the isothermal magnetic entropy change of 0-70 kOe is 8.7 J kg -1 K -1 , the relative magnetic refrigeration capacity is 295.4J kg -1 .

[0055] Example 5

[0056] In this embodiment, a molten salt method is used to prepare a magnetic refrigeration material with a chemical formula of GdMn2O5, and the preparation method is carried out according to the following specific steps:

[0057] (1) Ingredients: According to the chemical reaction equation 8MnO2+2Gd2O3→4GdMn2O5+O2↑, high-purity manganese dioxide, gadolinium oxide and sodium chloride powders with a molar ratio of 4:1:4 were weighed, placed in an agate mortar, fully ground, and mixed to obtain a mixed powder.

[0058] (2) Sintering: The uniformly mixed powder is placed in a corundum crucible, and the temperature is raised to 900°C in a high-temperature sintering furnace at a heating rate of 3.5°C / min, and then sintered at this temperature for 11 hours, and finally cooled in the furnace.

[0059] (3) Salt washing: After the furnace temperature drops to room temperature, remove the corundum crucible, transfer the sintered material to a centrifuge tube, add deionized water, shake vigorously, and place in an ultrasonic cleaner to assist in shaking to dissolve the NaCl. Place the centrifuge tube in a high-speed centrifuge and centrifuge at a speed of 4500 rpm for 4 minutes. After centrifugation, remove the centrifuge tube and pour out the supernatant. Repeat the above process 3 times.

[0060] (4) Drying: Place the centrifuge tube after washing away NaCl in a drying oven and bake at 60°C for 12 hours until it is completely dry to obtain a GdMn2O5 polycrystalline product.

[0061] The XRD spectrum and Rietveld crystal structure refinement of GdMn2O5 polycrystalline samples showed that the synthesized GdMn2O5 had an orthorhombic crystal structure with space group Pbam.

[0062] Example 6

[0063] In this embodiment, a molten salt method is used to prepare a magnetic refrigeration material with a chemical formula of GdMn2O5, and the preparation method is carried out according to the following specific steps:

[0064] (1) Ingredients: According to the chemical reaction equation 8MnO2+2Gd2O3→4GdMn2O5+O2↑, high-purity manganese dioxide, gadolinium oxide and sodium chloride powders with a molar ratio of 4:1:8 were weighed, placed in an agate mortar, fully ground, and mixed to obtain a mixed powder.

[0065] (2) Sintering: The uniformly mixed powder is placed in a corundum crucible, and the temperature is raised to 1100°C in a high-temperature sintering furnace at a heating rate of 4.5°C / min, and then sintered at this temperature for 9 hours, and finally cooled in the furnace.

[0066] (3) Washing the salt: After the furnace temperature drops to room temperature, take out the corundum crucible, transfer the sintered product to a centrifuge tube, add deionized water, shake vigorously, and place it in an ultrasonic cleaner for auxiliary oscillation to dissolve NaCl. Place the centrifuge tube in a high-speed centrifuge and centrifuge. The rotation speed is 5500 revolutions per minute and the time is 2 minutes. After centrifugation, take out the centrifuge tube and pour out the supernatant. Repeat the above process 3 times.

[0067] (4) Drying: Place the centrifuge tube washed with NaCl in an oven and bake at 80 °C for 8 hours until completely dry to obtain the GdMn2O5 polycrystalline product.

[0068] Through the XRD pattern and Rietveld crystal structure refinement characterization of the GdMn2O5 polycrystalline sample, the results show that the synthesized GdMn2O5 is an orthorhombic crystal structure with the space group Pbam.

[0069] As mentioned above, it is only the preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A method for preparing a rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by a molten salt method, characterized in that, The steps are as follows: S1. Weigh manganese dioxide powder and rare earth oxide powder according to the stoichiometric ratio of rare earth-based multivalent manganese oxides, mix them with sodium chloride, and grind to obtain a mixed powder; S2. Sinter the mixed powder at high temperature and cool it to room temperature with the furnace to obtain a sintered powder; S3. Wash the sintered powder with water to remove sodium chloride, and dry it to obtain the product; The chemical formula of the rare earth-based multivalent manganese oxide is REMn2O5; wherein, RE is one or any several elements of Gd, Tb, Dy, and Ho; In S1, the rare earth-based multivalent manganese oxide and sodium chloride are mixed in a molar ratio of 1:2-4; The isothermal magnetic entropy change of the rare earth-based multivalent manganese oxide can reach up to 12.61 J kg -1 K -1 under a magnetic field change of 0 - 70 kOe, and the relative cooling power (RCP) can reach up to 338 J kg -1 .

2. The method for preparing a rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by a molten salt method according to claim 1, characterized in that, In S2, heat it up to 900-1100 °C at a heating rate of 3.5-4.5 °C / min and sinter for 9-11 h.

3. The method for preparing a rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by a molten salt method according to claim 1, characterized in that, In S3, the specific operation of washing and removing sodium chloride with water is as follows: ultrasonically disperse the sintered product into deionized water, centrifuge, and discard the supernatant; repeat the above operation 2-3 times.

4. The method for preparing a rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by the molten salt method according to claim 3, characterized in that, The centrifugation speed is 4500-5500 revolutions per minute, and the centrifugation time is 2-5 minutes.

5. The method for preparing a rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material by a molten salt method according to claim 1, characterized in that, In S3, dry it at 60-80 °C for 8-12 h.

6. A rare earth-based multivalent manganese oxide low-temperature magnetic refrigeration material prepared by the method according to any one of claims 1-5.

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