A catalyst-free, small-particle-size na2bacop(o4)2 material synthesis method for magnetic refrigeration

Abstract

The application discloses a catalyst-free small-particle-size Na2BaCo(PO4)2 material synthesis method for magnetic refrigeration, and the method comprises the following steps: firstly, sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate are weighed and added into a dilute nitric acid solution and stirred until dissolved; secondly, citric acid is added for dissolution, then ammonia water is added dropwise, and then ethylene glycol is added dropwise and stirred uniformly; thirdly, continuous stirring is conducted until a sol is formed, and then the sol is dried to obtain a gel; fourthly, the gel is calcined in stages and then high-temperature calcined; and fifthly, the gel is sintered into a phase. The sol-gel process is adopted, highly uniform dispersion and mixing of reactants are realized at a molecular level, the highly uniformity and small grain size of the synthesized material are ensured, high reactivity is obtained, a catalyst is not needed to participate in the reaction, the Na2BaCo(PO4)2 material with smaller particle size, more concentrated size distribution and more consistent microstructure is obtained, the magnetic heat performance and stability of the material are improved, and the material is suitable for the fields of magnetic refrigeration and related functional materials.

Description

Technical Field

[0001] This invention belongs to the field of magnetic functional materials and refrigerant preparation technology, specifically relating to a catalyst-free, small-particle-size Na2BaCo(PO4)2 material synthesis method for magnetic refrigeration. Background Technology

[0002] Ultra-low temperature technology is a key cornerstone for advancing cutting-edge exploration and technological development in modern basic science. It suppresses atomic thermal motion, allowing the quantum properties of matter to manifest, thus spurring major breakthroughs such as superconductivity. From large particle colliders and medical imaging to quantum computing, ultra-low temperature technology has become a core component supporting the development of large scientific facilities and advanced technologies. With the rapid development of quantum computing, deep space exploration, and other fields, higher demands are being placed on ultra-low temperature environments at the millikelvin and even sub-millikelvin levels. Against this backdrop, solid-state magnetic refrigeration technology based on the principle of adiabatic demagnetization has attracted significant attention due to its advantages such as no dependence on liquid helium and compact structure. This technology achieves cooling through the magnetic entropy change of magnetic materials under varying magnetic fields; its core performance lies in the intrinsic properties of the magnetic working material. Therefore, developing high-performance magnetocaloric materials that combine high magnetocaloric effect, low heat dissipation, and excellent cycle stability has become crucial for overcoming technological bottlenecks and realizing the practical application of solid-state magnetic refrigeration.

[0003] In recent years, condensed matter physics research has shown that frustrated quantum magnets exhibit strong quantum fluctuations, rich order parameter competition, and evolved quantum states such as quantum spin liquids and spin supersolids in extremely low-temperature environments. This has not only opened up a new paradigm for basic physics research, but its huge potential for magnetic entropy change has also made it an international research frontier for the next generation of extremely low-temperature magnetic refrigerants.

[0004] Among them, Na₂BaCo(PO₄)₂ (NBCP), as a quasi-two-dimensional triangular lattice antiferromagnet with an effective spin 1 / 2, exhibits strong geometric frustration characteristics and is a typical geometrically frustrated quantum magnet. In 2024, a Chinese research team made a breakthrough in this system, successfully achieving an extremely low temperature of 94 mK using its single-crystal sample through adiabatic demagnetization. Nature (2024, 625: 270–275.), whose Grinnellsen parameter characterizing the efficiency of magnetic field-driven cooling is up to four times that of the commercial benchmark material gadolinium gallium garnet, laying an important foundation for the application and transformation of this type of material.

[0005] Despite its promising prospects, research on the application of frustrated quantum magnets in cryogenic magnetic refrigeration is still in its early stages. Moving from laboratory "proof of principle" to engineered "practical devices" faces technological bottlenecks, with significant challenges in the large-scale, high-purity preparation of high-performance materials. In magnetic refrigeration applications, to achieve even lower limiting temperatures, sufficient refrigerant is often required to fill the vacuum chamber of a sufficiently large refrigerator. However, current synthesis of geometrically frustrated magnets primarily relies on traditional solid-state reaction methods. This method requires mixing raw materials in stoichiometric proportions and, under high temperature and catalysis, forming the target phase through solid-state diffusion reactions and bonding. However, during mass production scaling, uneven mixing between raw materials and between raw materials and catalysts easily leads to uneven mass and heat transfer, introducing impurities and lattice defects, potentially degrading the intrinsic quantum frustration properties and magnetocaloric performance of the material. Therefore, developing large-scale, reproducible preparation techniques capable of achieving high chemical purity, high phase homogeneity, and fine particle size is a crucial prerequisite for unlocking its application potential and reducing engineering costs. Summary of the Invention

[0006] The technical problem this invention aims to solve is to address the shortcomings of the prior art by providing a catalyst-free, small-particle-size Na2BaCo(PO4)2 material synthesis method for magnetic refrigeration. This method utilizes a sol-gel process, achieving highly uniform dispersion and mixing of reactants at the molecular level, ensuring high uniformity and fine grain size of the synthesized material while simultaneously achieving high reactivity. It eliminates the need for a catalyst, ensuring high uniformity and fine grain size, and realizes catalyst-free, high-purity, small-particle-size, and large-scale preparation of Na2BaCo(PO4)2 materials. This solves the problems of uneven metal ion mixing, easy introduction of defects leading to decreased purity and performance degradation, as well as the complexity and high cost of traditional solid-state reaction methods for preparing magnetic refrigeration materials.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for synthesizing catalyst-free, small-particle-size Na2BaCo(PO4)2 material for magnetic refrigeration, characterized in that the method includes the following steps: Step 1: Weigh out sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate according to the stoichiometric ratio of the target product, add them to dilute nitric acid solution, and stir continuously until completely dissolved to obtain a purple-red mixed solution of metal ions. Step 2: Add citric acid as a complexing agent to the purple-red mixed solution of metal ions from Step 1. After it is completely dissolved, add ammonia water to adjust the pH of the solution, then add ethylene glycol and stir until homogeneous to obtain the reaction solution. Step 3: Stir the reaction solution from Step 2 continuously at 80℃~100℃ until a purple-red sol is formed. Then dry the sol in air to obtain a gel. Step 4: Place the gel obtained in Step 3 in an air atmosphere for segmented calcination, and then perform high-temperature calcination in an air atmosphere to obtain precursor powder. Step 5: The precursor powder from Step 4 is sintered in an air atmosphere to form a phase, and then cooled to room temperature in the furnace to obtain Na2BaCo(PO4)2 material with a particle size of less than 400nm.

[0008] This invention uses sodium carbonate, barium carbonate, cobalt powder, and diammonium hydrogen phosphate as raw materials, citric acid as a complexing agent, and ethylene glycol as a solvent. Through a sol-gel process, a homogeneous and stable sol system is first formed, followed by gelation treatment. Then, through segmented calcination and high-temperature calcination, organic matter in the system is thoroughly removed. Finally, sintering is performed to obtain a structurally stable Na₂BaCo(PO₄)₂ material with high phase purity. The synthesis method of this invention achieves uniform mixing and reaction of sodium, barium, and cobalt ions at the molecular scale, ensuring not only the high homogeneity and fine grain size of the synthesized material but also effectively avoiding the introduction of impurities due to the catalyst-free process, thus guaranteeing the high phase purity of the Na₂BaCo(PO₄)₂ material. Furthermore, this synthesis method has the advantages of simple process, low cost, and ease of large-scale production and promotion, providing a new and efficient preparation route for magnetic refrigeration materials.

[0009] The above-mentioned method for synthesizing catalyst-free, small-particle-size Na2BaCo(PO4)2 material for magnetic refrigeration is characterized in that the concentration of the dilute nitric acid solution in step one is 1.5 mol / L to 3 mol / L.

[0010] The above-mentioned method for synthesizing a catalyst-free, small-particle-size Na2BaCo(PO4)2 material for magnetic refrigeration is characterized in that, in step two, the amount of citric acid used is 1.5 to 2 times the total molar number of metal ions in the purple-red metal ion mixed solution, and the amount of ethylene glycol used is 2 to 3 times the molar number of citric acid.

[0011] The above-mentioned method for synthesizing catalyst-free, small-particle-size Na2BaCo(PO4)2 material for magnetic refrigeration is characterized in that, in step two, ammonia water is added dropwise to adjust the pH of the solution to 2.0~3.0.

[0012] The above-mentioned method for synthesizing catalyst-free, small-particle-size Na2BaCo(PO4)2 material for magnetic refrigeration is characterized by the following step: the segmented calcination process in step four involves sequentially holding the material at 130℃~160℃, 200℃~250℃, and 300℃~350℃ for 6h~12h, with grinding performed after each calcination stage; the high-temperature calcination is performed at 600℃ for 6h~12h. This invention removes residual moisture from the gel through a segmented calcination process, followed by high-temperature calcination to remove organic components and nitrate ions, thus obtaining a pure precursor powder.

[0013] The above-mentioned method for synthesizing catalyst-free, small-particle-size Na2BaCo(PO4)2 material for magnetic refrigeration is characterized in that the heating rate for sintering in step five is 5℃ / min~10℃ / min, the sintering temperature is 800℃, and the holding time is 24h. Compared with the prior art, the present invention has the following advantages: 1. Advantages of material synthesis uniformity and microstructure Compared to the problems of component segregation and coarse particles caused by uneven physical mixing and limited diffusion at high temperatures in traditional solid-state reactions, this invention uses a sol-gel method to prepare Na2BaCo(PO4)2 materials. By complexing the metal cations with citric acid, a stable and homogeneous sol system is formed under the action of ethylene glycol. This allows for highly uniform dispersion and mixing of the metal ions at the molecular level, which not only significantly improves the uniformity of ion mixing and reaction efficiency, but also yields Na2BaCo(PO4)2 materials with smaller particle size, more concentrated size distribution, and more consistent microstructure. This fine microstructure is beneficial to improving the magnetocaloric properties and stability of Na2BaCo(PO4)2 materials, showing great application potential in the fields of magnetic refrigeration and related functional materials.

[0014] 2. Advantages of catalyst-free synthesis process Unlike traditional solid-phase synthesis, which relies on external catalysts (ammonium chloride) to promote high-temperature solid-state diffusion and reaction, the sol-gel process of this invention achieves highly uniform mixing and pre-assembly of reactants at the molecular level. The resulting precursor gel itself has high reactivity, thus enabling spontaneous and efficient crystal phase formation and structural crystallization during subsequent calcination. Therefore, no catalyst needs to be added during the entire sintering stage. This not only significantly simplifies the process and reduces raw material and operating costs, but also fundamentally avoids the adverse effects that catalyst residues may have on the final purity, crystal integrity, and performance stability of the material. This is more conducive to obtaining high-purity, high-performance Na2BaCo(PO4)2 materials.

[0015] 3. Process versatility and scalability Compared to traditional solid-phase reaction methods that rely on high-temperature, long-term solid-state diffusion, have high requirements for the uniformity of raw material particles, and are prone to introducing impurities, the sol-gel synthesis route of this invention achieves uniform mixing and reaction at the molecular level through a liquid-phase system at a lower temperature. The process is simpler, the conditions are milder, and the reproducibility is better. It is not only suitable for the high-quality synthesis of Na2BaCo(PO4)2 materials, but can also be easily extended to other magnetic refrigeration material systems with similar crystal structures or chemical compositions by adjusting the types and ratios of metal ions. It has good process adaptability and potential for large-scale production.

[0016] 4. Raw material costs are significantly reduced. Compared to traditional methods that require the use of expensive cobalt oxide (market price approximately 1130 yuan / 5g) as the cobalt source, this invention uses a sol-gel process, which only requires the use of metallic cobalt powder (market price approximately 50 yuan / 5g) to achieve uniform dispersion and reaction of cobalt ions, thereby reducing the raw material cost by approximately 95%. Therefore, this method demonstrates outstanding economic advantages while ensuring that the product Na2BaCo(PO4)2 material has similar phase purity and crystal quality.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 The X-ray diffraction (XRD) spectra of Na2BaCo(PO4)2 materials prepared by the sol-gel method in Example 1 of the present invention and those prepared by the solid-phase reaction method in Comparative Example 1 are shown.

[0019] Figure 2 This is a comparison of the surface morphology of Na2BaCo(PO4)2 materials prepared by the sol-gel method in Example 1 of the present invention and by the solid-phase reaction method in Comparative Example 1, obtained by atomic force microscopy (AFM). Detailed Implementation

[0020] Example 1 The sol-gel method used in this embodiment includes the following steps: Step 1: Dissolving and mixing raw materials: Weigh out sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate according to the stoichiometric ratio Na:Ba:Co:P=1:1:1:2, and add them to a 3mol / L dilute nitric acid solution. Stir continuously until completely dissolved to obtain a purple-red mixed solution of metal ions. Step 2, Preparation of reaction solution: Weigh citric acid according to 1.5 times the total molar amount of metal ions in the purple-red metal ion mixed solution in Step 1, dissolve it in deionized water to prepare citric acid solution, and then add the citric acid solution to the purple-red metal ion mixed solution in Step 1 under continuous stirring. After complete dissolution, add ammonia water dropwise to adjust the pH to about 3.0, mix well to obtain a clear solution, and then add ethylene glycol in 3 times the molar amount of citric acid, stir until uniform to obtain the reaction solution; Step 3, Sol and Gel Formation: The reaction solution from Step 2 is continuously stirred at 80℃~100℃ until a purple-red sol is formed in the middle. Then the sol is dried in air to obtain a gel. Step 4, sintering: The gel obtained in step 3 is placed in an air atmosphere and kept at 160℃, 200℃ and 350℃ for 6 hours in sequence. After each calcination, it is ground and then calcined in an air atmosphere at 600℃ for 6 hours to obtain the precursor powder. Step 5, Phase Formation: The precursor powder from Step 4 is placed in an air atmosphere and heated to 800℃ at a heating rate of 5℃ / min~10℃ / min, held for 24 hours to sinter and form a phase. The powder is then cooled to room temperature in the furnace to obtain Na2BaCo(PO4)2 material.

[0021] Comparative Example 1 The solid-phase reaction method used in this comparative example is based on the preparation method of Ruidan Zhong et al. ( Proc. Natl Acad. Sci. USA (, 2019, 116: 14505.), including the following steps: Step 1: Weigh and mix the dried raw materials sodium carbonate, barium carbonate, cobalt oxide and diammonium hydrogen phosphate according to the stoichiometric ratio of the target product, and then grind and mix them thoroughly with the catalyst NH4Cl at a molar ratio of 2:1 to obtain a mixed powder. Step 2: Press the mixed powder from Step 1 into thin sheets and place them in an alumina crucible. Sinter the sheets in air at 800°C for 24 hours and then cool them to room temperature. Repeat the above sintering and cooling process three times to minimize the formation of impurity phases and obtain Na2BaCo(PO4)2 material.

[0022] Figure 1 The figures show a comparison of X-ray diffraction (XRD) spectra of Na₂BaCo(PO₄)₂ materials prepared by the sol-gel method in Example 1 of this invention and by the solid-state reaction method in Comparative Example 1. Figure (a) represents Comparative Example 1, and Figure (b) represents Example 1. Figure 1 The center dot represents the observed intensity, the solid line is the calculated fitted curve, the difference between the observed intensity and the calculated intensity is the bottom curve of the spectrum, and the vertical line indicates the position of the Na2BaCo(PO4)2 diffraction peak. Figure 1The XRD fitting results showed that both methods successfully prepared pure-phase Na2BaCo(PO4)2 materials, and no impurity phases were observed; according to the goodness-of-fit factor ( R p) Compared with the previous example, the Na2BaCo(PO4)2 material prepared by the sol-gel method in Example 1 has higher purity.

[0023] Figure 2 These are atomic force microscopy (AFM) surface morphology comparison images of Na2BaCo(PO4)2 materials prepared by the sol-gel method in Example 1 of this invention and those prepared by the solid-phase reaction method in Comparative Example 1. In Figure (a), the left image represents the morphology of the Na2BaCo(PO4)2 material in Comparative Example 1 at a scale of 1 μm × 1 μm, and the right image is the height distribution curve of the Na2BaCo(PO4)2 material in Comparative Example 1. In Figure (b), the left image represents the morphology of the Na2BaCo(PO4)2 material in Example 1 at a scale of 1 μm × 1 μm, and the right image is the height distribution curve of the Na2BaCo(PO4)2 material in Example 1. Figure 2 It can be seen that the Na2BaCo(PO4)2 material prepared by the sol-gel method in Example 1 is superior to the Na2BaCo(PO4)2 material prepared by the solid-state reaction method in Comparative Example 1 in terms of particle morphology, dispersibility and size uniformity. The average particle size of the Na2BaCo(PO4)2 material prepared in Example 1 is about 200 nm, with regular shape and clear boundary, while the average particle size of the Na2BaCo(PO4)2 material prepared in Comparative Example 1 is about 400 nm, with poor particle size and shape uniformity.

[0024] In summary, the sol-gel method of this invention demonstrates unique advantages in controlling the microstructure, surface quality, and particle uniformity of Na2BaCo(PO4)2 materials, laying a more refined and controllable structural foundation for subsequent optimization of material properties.

[0025] Example 2 The sol-gel method used in this embodiment includes the following steps: Step 1: Dissolving and mixing raw materials: Weigh out sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate according to the stoichiometric ratio Na:Ba:Co:P=1:1:1:2, and add them to 2.5mol / L dilute nitric acid solution. Stir continuously until completely dissolved to obtain a purple-red mixed solution of metal ions. Step 2, Preparation of reaction solution: Weigh out citric acid at twice the total molar amount of metal ions in the purple-red metal ion mixed solution in Step 1, dissolve it in deionized water to prepare a citric acid solution, and then add the citric acid solution to the purple-red metal ion mixed solution in Step 1 under continuous stirring. After complete dissolution, add ammonia water dropwise to adjust the pH to about 2.2, mix well to obtain a clear solution, and then add ethylene glycol at three times the molar amount of citric acid, and stir until homogeneous to obtain the reaction solution. Step 3, Sol and Gel Formation: The reaction solution from Step 2 is continuously stirred at 80℃~100℃ until a purple-red sol is formed in the middle. Then the sol is dried in air to obtain a gel. Step 4, sintering: The gel obtained in step 3 is placed in an air atmosphere and kept at 160℃, 200℃ and 350℃ for 6 hours in sequence. After each calcination, it is ground and then calcined in an air atmosphere at 600℃ for 6 hours to obtain the precursor powder. Step 5, Phase Formation: The precursor powder from Step 4 is placed in an air atmosphere and heated to 800℃ at a heating rate of 5℃ / min~10℃ / min, held for 24 hours to sinter and form a phase. The powder is then cooled to room temperature in the furnace to obtain Na2BaCo(PO4)2 material.

[0026] Upon testing, the properties of the Na2BaCo(PO4)2 material obtained in this embodiment are basically the same as those in Example 1.

[0027] Example 3 The sol-gel method used in this embodiment includes the following steps: Step 1: Dissolving and mixing raw materials: Weigh out sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate according to the stoichiometric ratio Na:Ba:Co:P=1:1:1:2, and add them to a 3mol / L dilute nitric acid solution. Stir continuously until completely dissolved to obtain a purple-red mixed solution of metal ions. Step 2, Preparation of reaction solution: Weigh citric acid according to 1.5 times the total molar amount of metal ions in the purple-red metal ion mixed solution in Step 1, dissolve it in deionized water to prepare citric acid solution, and then add the citric acid solution to the purple-red metal ion mixed solution in Step 1 under continuous stirring. After complete dissolution, add ammonia water dropwise to adjust the pH to about 3.0, mix well to obtain a clear solution, and then add ethylene glycol in 3 times the molar amount of citric acid, stir until uniform to obtain the reaction solution; Step 3, Sol and Gel Formation: The reaction solution from Step 2 is continuously stirred at 80℃~100℃ until a purple-red sol is formed in the middle. Then the sol is dried in air to obtain a gel. Step 4, sintering: The gel obtained in step 3 is placed in an air atmosphere and kept at 130℃, 250℃ and 300℃ for 12 hours in sequence. After each calcination, it is ground and then calcined in an air atmosphere at 600℃ for 12 hours to obtain the precursor powder. Step 5, Phase Formation: The precursor powder from Step 4 is placed in an air atmosphere and heated to 800℃ at a heating rate of 5℃ / min~10℃ / min, held for 24 hours to sinter and form a phase. The powder is then cooled to room temperature in the furnace to obtain Na2BaCo(PO4)2 material.

[0028] Upon testing, the properties of the Na2BaCo(PO4)2 material obtained in this embodiment are basically the same as those in Example 1.

[0029] Example 4 The sol-gel method used in this embodiment includes the following steps: Step 1: Dissolving and mixing raw materials: Weigh out sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate according to the stoichiometric ratio Na:Ba:Co:P=1:1:1:2, and add them to 2mol / L dilute nitric acid solution. Stir continuously until completely dissolved to obtain a purple-red mixed solution of metal ions. Step 2: Preparation of reaction solution: Weigh out citric acid at twice the total molar amount of metal ions in the purple-red metal ion mixed solution in Step 1, dissolve it in deionized water to prepare a citric acid solution, and then add the citric acid solution to the purple-red metal ion mixed solution in Step 1 under continuous stirring. After complete dissolution, add ammonia water dropwise to adjust the pH to about 2.0, mix well to obtain a clear solution, and then add ethylene glycol at three times the molar amount of citric acid, and stir until homogeneous to obtain the reaction solution. Step 3, Sol and Gel Formation: The reaction solution from Step 2 is continuously stirred at 80℃~100℃ until a purple-red sol is formed in the middle. Then the sol is dried in air to obtain a gel. Step 4, sintering: The gel obtained in step 3 is placed in an air atmosphere and kept at 150℃, 250℃ and 350℃ for 6 hours in sequence. After each calcination, it is ground and then calcined in an air atmosphere at 600℃ for 12 hours to obtain the precursor powder. Step 5, Phase Formation: The precursor powder from Step 4 is placed in an air atmosphere and heated to 800℃ at a heating rate of 5℃ / min~10℃ / min, held for 24 hours to sinter and form a phase. The powder is then cooled to room temperature in the furnace to obtain Na2BaCo(PO4)2 material.

[0030] Upon testing, the properties of the Na2BaCo(PO4)2 material obtained in this embodiment are basically the same as those in Example 1.

[0031] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for synthesizing catalyst-free, small-particle-size Na₂BaCo(PO₄)₂ material for magnetic refrigeration, characterized in that, The method includes the following steps: Step 1: Weigh out sodium carbonate, barium carbonate, cobalt powder and diammonium hydrogen phosphate according to the stoichiometric ratio of the target product, add them to dilute nitric acid solution, and stir continuously until completely dissolved to obtain a purple-red mixed solution of metal ions. Step 2: Add citric acid as a complexing agent to the purple-red mixed solution of metal ions from Step 1. After it is completely dissolved, add ammonia water to adjust the pH of the solution, then add ethylene glycol and stir until homogeneous to obtain the reaction solution. Step 3: Stir the reaction solution from Step 2 continuously at 80℃~100℃ until a purple-red sol is formed. Then dry the sol in air to obtain a gel. Step 4: Place the gel obtained in Step 3 in an air atmosphere for segmented calcination, and then perform high-temperature calcination in an air atmosphere to obtain precursor powder. Step 5: The precursor powder from Step 4 is sintered in an air atmosphere to form a phase, and then cooled to room temperature in the furnace to obtain Na2BaCo(PO4)2 material with a particle size of less than 400nm.

2. The method for synthesizing catalyst-free, small-particle-size Na₂BaCo(PO₄)₂ material for magnetic refrigeration according to claim 1, characterized in that, The concentration of the dilute nitric acid solution mentioned in step one is 1.5 mol / L to 3 mol / L.

3. The method for synthesizing catalyst-free, small-particle-size Na₂BaCo(PO₄)₂ material for magnetic refrigeration according to claim 1, characterized in that, In step two, the amount of citric acid used is 1.5 to 2 times the total number of moles of metal ions in the purple-red metal ion mixed solution, and the amount of ethylene glycol used is 2 to 3 times the number of moles of citric acid.

4. The method for synthesizing catalyst-free, small-particle-size Na₂BaCo(PO₄)₂ material for magnetic refrigeration according to claim 1, characterized in that, In step two, ammonia water is added dropwise to adjust the pH of the solution to 2.0~3.

0.

5. The method for synthesizing catalyst-free, small-particle-size Na₂BaCo(PO₄)₂ material for magnetic refrigeration according to claim 1, characterized in that, The segmented calcination procedure in step four is as follows: the temperature is kept at 130℃~160℃, 200℃~250℃ and 300℃~350℃ for 6h~12h respectively, and each segment is ground after calcination; the high-temperature calcination is: calcination at 600℃ for 6h~12h.

6. The method for synthesizing catalyst-free, small-particle-size Na₂BaCo(PO₄)₂ material for magnetic refrigeration according to claim 1, characterized in that, The heating rate for sintering phase formation in step five is 5℃ / min~10℃ / min, the sintering temperature is 800℃, and the holding time is 24h.

Images