Coated SmFeN permanent magnet material and preparation method thereof

Abstract

This invention provides a coated SmFeN permanent magnet material and its preparation method. The coated SmFeN permanent magnet material includes SmFeN powder and a coating film on its surface; the coating film material includes ferric hydroxide. This application coats the surface of the SmFeN powder. During the coating process, HClO not only reacts with Ca(OH)2 to remove impurities but also forms a dense coating film with anionic surfactants. This not only improves the performance of the permanent magnet material but also ensures safety during the production process. Hypochlorite ions react with ferrous ions, reducing hypochlorite ions to chloride ions and oxidizing ferrous ions to ferric ions. The ferric ions then precipitate as ferric hydroxide under alkaline conditions and coat the powder surface. Furthermore, the coating film of the coated SmFeN permanent magnet material of this application does not affect subsequent applications.

Description

Technical Field

[0001] This invention relates to the field of magnetic materials technology, and more specifically, to a coated SmFeN permanent magnet material and its preparation method. Background Technology

[0002] Rare earth permanent magnet materials are special materials with permanent magnetic properties formed by alloys of rare earth elements such as Nd, Sm, and Pr and some transition metal elements, and prepared through specific processes. They are currently widely used in electric motors, generators, nuclear magnetic resonance imaging instruments, microwave communication technology, instruments and other devices and equipment that require permanent magnetic fields.

[0003] Among rare-earth permanent magnet materials, SmFeN stands out as the most valuable permanent magnet material after NdFeB due to its high Curie temperature, good temperature and chemical properties, and low price. Recent research on the preparation of rare-earth permanent magnet alloys, especially SmFeN, has focused on powder metallurgy, mechanized alloying, rapid quenching, and reduction-diffusion methods. Melting and rapid quenching methods for preparing metal alloys are prone to producing impurity α-iron phases and require prolonged ball milling, resulting in high costs and irregular particle morphology after milling, affecting performance. The reduction-diffusion method uses metals such as Ca, CaH2, and CaCl2 with inexpensive metal oxides or chlorides and Fe, and reacts them under a high-temperature Ar or N2 atmosphere. Utilizing the strong reducing properties of Ca or CaH2, a redox reaction occurs with the metal oxides to generate the desired alloy. However, this process also generates Ca compounds and leaves Ca residues. If these impurities are mixed in the alloy, they can significantly impact product performance. High-energy ball milling utilizes frequent collisions between the ball mill, the milling jar, and the powder phase to cause intense plastic deformation and cold welding of powder particles, forming composite powders with a lamellar structure. This process of repeated cold welding, cracking, and re-welding achieves alloying. However, the powders obtained by high-energy ball milling are too reactive and are prone to violent reactions during drying, which can not only affect material properties but also pose a danger during production. Summary of the Invention

[0004] The main objective of this invention is to provide a coated SmFeN permanent magnet material and its preparation method, so as to solve the problem of excessive impurities in the SmFeN powder prepared in the prior art.

[0005] To achieve the above objectives, according to one aspect of the present invention, a coated SmFeN permanent magnet material is provided, the coated SmFeN permanent magnet material comprising SmFeN powder and a coating film located on its surface; the material of the coating film comprises iron hydroxide.

[0006] Furthermore, the D50 of the coated SmFeN permanent magnet material is 3.9–4.2 μm, the average thickness of the coating film is preferably 0.2–0.6 μm, and the intrinsic coercivity of the coated SmFeN permanent magnet material is more preferably 7.5–14 KOe.

[0007] To achieve the above objectives, according to one aspect of the present invention, a method for preparing the above-mentioned coated SmFeN permanent magnet material is provided. The method includes: step S1, mixing SmFeN powder, a dispersant, an anionic surfactant, and HClO in a solvent to obtain a dispersion; step S2, ball milling the dispersion to obtain a mixture; step S3, sedimenting and separating the mixture to obtain a liquid and a slurry; and step S4, drying the slurry to obtain the coated SmFeN permanent magnet material.

[0008] Furthermore, the mass content of the dispersant is 5-20% of the mass of the SmFeN powder, preferably the mass content of the anionic surfactant is 5-15% of the mass of the SmFeN powder, and preferably the mass content of HClO is 10-40% of the mass of the SmFeN powder.

[0009] Furthermore, the dispersant is stearyl alcohol polyether-20 and / or sodium sulfonate, the anionic surfactant is preferably anionic polyacrylamide, and the solvent is preferably water.

[0010] Furthermore, the composition of the SmFeN powder, by mass percentage, includes 22-25% Sm, 4-5% N, 0.1-0.5% Ca, and the remainder is Fe.

[0011] Further, step S2 includes: subjecting the dispersion to a first ball mill to obtain a first mixture; and subjecting the first mixture to a second ball mill to obtain a mixture.

[0012] Furthermore, the frequency of the first ball mill is 5–30 Hz, and the time of the first ball mill is 1–20 h.

[0013] Furthermore, the frequency of the second ball mill is 1–10 Hz, and the time of the second ball mill is 3–30 min.

[0014] Furthermore, in step S1, the liquid level of the dispersion is 3-4 cm higher than the steel ball in the ball mill.

[0015] Furthermore, the drying includes vacuum drying, preferably at a temperature of 10–100°C, more preferably 30–60°C; and preferably for a drying time of 1–2 hours.

[0016] By applying the technical solution of this invention, during the coating process, HClO can not only react with Ca(OH)2 to remove impurities, but also form a dense coating film with anionic surfactants. This not only improves the performance of the permanent magnet material but also ensures safety during the production process. Hypochlorite ions react with ferrous ions, reducing hypochlorite ions to chloride ions and oxidizing ferrous ions to ferric ions. Ferric ions then precipitate as ferric hydroxide under alkaline conditions and coat the powder surface. Furthermore, the coating film of the coated SmFeN permanent magnet material of this application will not affect subsequent applications. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0018] As analyzed in the background section of this application, among the methods for preparing rare earth permanent magnet alloys, the rapid quenching method, powder metallurgy method, and hydrogen crushing method are complex, time-consuming, energy-intensive, and difficult to produce industrially. The powder obtained by high-energy ball milling has a disordered structure, containing many impurity phases in addition to the SmFeN phase. The reduction-diffusion method yields fewer impurity phases in SmFeN, but due to the production process, the resulting products are all blocky and contain Ca compounds and Ca residues, requiring ball milling for crushing. During ball milling, the material releases a large amount of heat, making the powder easily oxidized or even burned, which is detrimental to subsequent drying and also generates Ca(OH)2, significantly affecting the powder's properties. To solve these problems, this application provides a coated SmFeN permanent magnet material and its preparation method.

[0019] In one typical embodiment of this application, a coated SmFeN permanent magnet material is provided, which includes SmFeN powder and a coating film on its surface; the material of the coating film includes iron hydroxide.

[0020] In the coating process of this application, HClO not only reacts with Ca(OH)₂ to remove impurities, but also forms a dense coating film with anionic surfactants. This not only improves the performance of the permanent magnet material but also ensures safety during the production process. The hypochlorite ions in HClO react with the ferrous ions in SmFeN, reducing the hypochlorite ions to chloride ions and oxidizing the ferrous ions to ferric ions. The ferric ions then precipitate as ferric hydroxide under alkaline conditions and coat the powder surface. Furthermore, the coating film of the coated SmFeN permanent magnet material of this application will not affect subsequent applications.

[0021] In some embodiments, since the components are mixed with the solvent to form a dispersion, the oxygen in the solvent will cause oxidation of the powder during ball milling. Therefore, the final coated SmFeN permanent magnet material contains a certain amount of oxygen.

[0022] In some embodiments, to ensure suitable activity of the SmFeN powder and prevent violent reactions between the SmFeN powder and air, the D50 of the aforementioned coated SmFeN permanent magnet material is 3.9–4.2 μm, and preferably the average thickness of the coating film is 0.2–0.6 μm. The average thickness of the coating film can be obtained by SEM electron microscopy. The coated SmFeN permanent magnet material of this application has good performance, and preferably the intrinsic coercivity of the coated SmFeN permanent magnet material is 7.5–14 Koe.

[0023] In another typical embodiment of this application, a method for preparing a coated SmFeN permanent magnet material is provided. The preparation method includes: step S1, mixing SmFeN powder, dispersant, anionic surfactant and HClO in a solvent to obtain a dispersion; step S2, ball milling the dispersion to obtain a mixture; step S3, sedimentation separation of the mixture to obtain a liquid and a slurry; and step S4, drying the slurry to obtain the coated SmFeN permanent magnet material.

[0024] This application removes impurities by adding HClO to the raw materials and utilizing the reaction between HClO and Ca(OH)2. Simultaneously, the anionic surfactant generates hydrophobic anions in water, which negatively charge the particle surface, forming a stable system with an electric double layer structure. The cations ionized from HClO dissolved in water neutralize the negative charge of the anionic surfactant, reducing the charge density, lowering the surface potential, and compressing the electric double layer thickness, thereby lowering the potential barrier. As more positively charged ions are adsorbed, the potential barrier gradually disappears. When the barrier disappears, the surface potential approaches zero. At this point, the anionic surfactant carries the ionized cations from HClO and coats the SmFeN powder surface, reducing powder activity, preventing violent reactions upon contact with air, and ensuring safety during production. During the coating process, hypochlorite ions in HClO react with ferrous ions in SmFeN, reducing hypochlorite ions to chloride ions and oxidizing ferrous ions to ferric ions. The ferric ions precipitate as ferric hydroxide under alkaline conditions and coat the powder surface. Furthermore, considering powder oxidation prevention, safety issues, and affinity with the ball milling media, this application also adds a dispersant to prevent the raw material powder from agglomerating during ball milling. After ball milling, the dispersant dissolves in the solvent, and the anionic surfactant and HClO coat the surface of the SmFeN powder. In addition, during ball milling, water in the solvent oxidizes the raw material powder, resulting in a certain oxygen content in the final coated SmFeN permanent magnet material. The preparation method of this application is simple, conducive to large-scale production, and has high safety during the production process.

[0025] Dispersants can improve the particle size distribution curve during subsequent ball milling, thereby increasing the coercivity of the material without affecting its D50. To remove impurities from SmFeN powder and achieve coating, in some embodiments, the mass content of the dispersant is controlled at 5-20% of the SmFeN powder mass. Excessive dispersant usage makes it difficult to remove residues from the coated SmFeN permanent magnet material, and a high oxygen content in the dispersant leads to lower material density, affecting material performance. To form a uniform, dense coating film with suitable thickness on the SmFeN powder surface, the mass content of the anionic surfactant is preferably 5-15% of the SmFeN powder mass. To neutralize the charge in the anionic surfactant, the mass content of HClO is preferably 10-40% of the SmFeN powder mass. HClO needs to react not only with ferrous ions but also with Ca(OH)2 in the SmFeN powder; therefore, the HClO content is excessive compared to the anionic surfactant. Insufficient HClO leads to incomplete impurity removal and a thinner coating film, affecting material performance. However, considering economic costs, the HClO content should not be too high, otherwise it will be wasteful.

[0026] This application does not impose any particular limitation on the types of dispersants and anionic surfactants; commonly used dispersants and anionic surfactants in the art can be applied to this application. In some embodiments, the dispersant is stearyl alcohol polyether-20 and / or sodium sulfonate, and the preferred anionic surfactant is anionic polyacrylamide. To ensure sufficient ionization of HClO, water is preferred as the solvent.

[0027] This application does not impose any particular limitation on the quality of SmFeN powder; the content of SmFeN powder can be selected based on the quality of the steel balls used in the ball mill. In some embodiments, to prevent excessive SmFeN powder from causing uneven ball milling, the mass m of the SmFeN powder is in the range of 10 ≤ m ≤ 500 g, preferably 50 ≤ m ≤ 200 g. Preferably, the composition of the SmFeN powder, by mass percentage, includes Sm 22–25%, N 4–5%, Ca 0.1–0.5%, with the remainder being Fe. The calcium content in the untreated SmFeN powder is at a relatively high level of 0.05–0.5%, and the nitrogen content characterizes the nitriding effect of the SmFeN powder itself.

[0028] In some embodiments, step S2 includes: subjecting the dispersion to a first ball mill to obtain a first mixture; and subjecting the first mixture to a second ball mill to obtain a final mixture. The first ball milling homogenizes the components, resulting in finer particle sizes and improved coercivity of the material. To achieve coating on the surface of the SmFeN powder, slow stirring is required; therefore, the frequency of the second ball milling is reduced.

[0029] To refine the particles of each component, in some embodiments, the first ball milling frequency is 5–30 Hz, and the first ball milling time is 1–20 h. During the first ball milling process, water in the solvent oxidizes the raw material powder, resulting in oxygen content in the final coated SmFeN permanent magnet material.

[0030] During the second ball milling process, HClO and anionic surfactants form a coating film on the surface of the SmFeN powder. In order to uniformly coat the SmFeN powder, in some embodiments, the frequency of the second ball milling is 1 to 10 Hz, and the time of the second ball milling is 3 to 30 min.

[0031] In step S1, steel balls and SmFeN powder are poured into a ball mill jar, then a dispersant, anionic surfactant, and HClO are added, followed by solvent. The liquid level of the dispersion is maintained 3-4 cm higher than the steel balls to ensure uniform dispersion of the components, which facilitates the formation of a dense coating film on the SmFeN powder surface. This application does not impose any particular limitation on the drying method; commonly used drying methods in the art can be applied, as long as the solvent in the coated SmFeN permanent magnet material can be removed. In some embodiments, drying includes vacuum drying, preferably at a temperature of 10-100°C, more preferably 30-60°C; and preferably for 1-2 hours.

[0032] In step S3, there are no particular restrictions on the sedimentation separation method. Commonly used separation methods in the art can be applied to this application, such as centrifugal separation.

[0033] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0034] Example 1

[0035] (1) Feeding: Pour the steel ball and 100g SmFeN powder into the ball mill jar, then add 10g anionic polyacrylamide, 20g HClO and 10g stearyl alcohol polyether-20, and finally add water and ensure that the liquid level is 3-4cm higher than the steel ball.

[0036] (2) Ball milling: After the SmFeN powder is initially mixed with the ball milling media, it is ball milled for 5 hours at a frequency of 20Hz in a ball mill. After the first ball milling is completed, the second ball milling is continued for 10 minutes at a frequency of 5Hz.

[0037] (3) Discharge: After ball milling, water and slurry are separated by centrifugation, and then the slurry is washed with ethanol;

[0038] (4) Drying: Place the container containing the slurry into a vacuum drying oven with an absolute pressure of 0.1 Pa, a time of 1 h, and a temperature of 40 °C.

[0039] (5) Storage: Collect the dried powder for later use.

[0040] Example 2

[0041] (1) Feeding: Pour the steel ball and 100g SmFeN powder into the ball mill jar, then add 10g anionic polyacrylamide, 20g HClO and 10g sodium sulfonate, and finally add water and ensure that the liquid level is 3-4cm higher than the steel ball.

[0042] (2) Ball milling: After the SmFeN powder is initially mixed with the ball milling media, it is ball milled for 5 hours at a frequency of 20 Hz. After the first ball milling is completed, the second ball milling is continued for 10 minutes at a frequency of 5 Hz.

[0043] (3) Discharge: After ball milling, water and slurry are separated;

[0044] (4) Drying: Place the container containing the slurry into a vacuum drying oven with an absolute pressure of 0.1 Pa, a time of 1 h, and a temperature of 40 °C.

[0045] (5) Storage: Collect the dried powder for later use.

[0046] Example 3

[0047] The method is basically the same as that in Example 1, except that in step (1), 10g stearyl alcohol polyether-20 is replaced with 3g stearyl alcohol polyether-20.

[0048] Example 4

[0049] The method is basically the same as in Example 1, except that 10g of stearyl alcohol polyether-20 is replaced with 30g of stearyl alcohol polyether-20.

[0050] Example 5

[0051] The method is basically the same as that in Example 1, except that in step (1), 10g of anionic polyacrylamide is replaced with 3g of anionic polyacrylamide.

[0052] Example 6

[0053] The method is basically the same as that in Example 1, except that in step (1), 10g of anionic polyacrylamide is replaced with 18g of anionic polyacrylamide.

[0054] Example 7

[0055] The method is basically the same as that in Example 1, except that in step (1), 20g HClO is replaced with 5g HClO.

[0056] Example 8

[0057] The method is basically the same as that in Example 1, except that in step (1), 20g HClO is replaced with 50g HClO.

[0058] Example 9

[0059] The method is basically the same as in Example 1, except that ball milling is performed only once; specifically, it includes:

[0060] In step (2), after the SmFeN powder is initially mixed with the ball milling media, it is ball milled in a ball mill at a frequency of 20 Hz for 5 hours.

[0061] Example 10

[0062] The method is basically the same as that in Example 1, except that in step (1), the liquid level of the medium is only 0.5 cm higher than the steel ball.

[0063] Example 11

[0064] The method is basically the same as that in Example 1, except that in step (1), the liquid level of the medium is 6 cm higher than that of the steel ball.

[0065] Comparative Example 1

[0066] The method is basically the same as in Example 1, except that no dispersant is added. Specifically, it includes:

[0067] (1) Feeding: Pour the steel ball and 100g SmFeN powder into the ball mill jar, then add 10g anionic polyacrylamide and 20g HClO, and finally add water and ensure that the liquid level is 3-4cm higher than the steel ball.

[0068] Comparative Example 2

[0069] The method is basically the same as in Example 1, except that anionic polyacrylamide is not added; specifically, it includes:

[0070] In step (1), steel balls and 100g of SmFeN powder are poured into a ball mill jar, then 20g of HClO and 10g of stearyl alcohol polyether-20 are added, and finally water is added to ensure that the liquid level is 3-4cm higher than the steel balls.

[0071] Comparative Example 3

[0072] The method is basically the same as in Example 1, except that HClO is not added; specifically, it includes:

[0073] In step (1), steel balls and 100g of SmFeN powder are poured into a ball mill jar, then 10g of anionic polyacrylamide and 10g of stearyl alcohol polyether-20 are added, and finally water is added to ensure that the liquid level is 3-4cm higher than the steel balls.

[0074] test:

[0075] The coated SmFeN permanent magnet materials in the above embodiments and comparative examples were tested, and the test results are shown in Table 1. The powder D50 was measured using a laser particle size analyzer. The N and O element contents in the powder were obtained through CONH gas analysis. The thickness of the phosphating film on the surface was measured using a scanning electron microscope (SEM). The content of metal elements (calcium in this invention) and Cl elements in the powder was obtained through fluorescence testing. Magnetic properties were obtained using a NIM-2000 analyzer. Oxygen content characterizes the degree of oxidation, chlorine content characterizes whether a coating film has formed, calcium content characterizes the effect of impurity removal, nitrogen content characterizes the nitriding effect of the SmFeN powder itself, D50 characterizes the particle size of the powder after ball milling, the average thickness of the coating film can be obtained using SEM, and intrinsic coercivity characterizes the magnetic powder performance after ball milling.

[0076] Table 1

[0077]

[0078]

[0079]

[0080] From the above description, it can be seen that the embodiments of the present invention achieve the following technical effects: In the coating process, HClO can not only react with Ca(OH)2 to remove impurities, but also form a dense coating film with anionic surfactants, which can not only improve the performance of permanent magnet materials, but also ensure safety in the production process. Hypochlorite ions react with ferrous ions, reducing hypochlorite ions to chloride ions and oxidizing ferrous ions to ferric ions. Ferric ions will precipitate as ferric hydroxide under alkaline conditions and coat the powder surface. Furthermore, the coating film of the coated SmFeN permanent magnet material of this application will not affect subsequent applications.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a coated SmFeN permanent magnetic material, characterized in that The preparation method includes: Step S1: Mix SmFeN powder, dispersant, anionic surfactant and HClO in a solvent to obtain a dispersion; Step S2: The dispersion is ball-milled to obtain a mixture; Step S3: Sedimentation separation of the mixture to obtain liquid and slurry; Step S4: Dry the slurry to obtain the coated SmFeN permanent magnet material; The coated SmFeN permanent magnet material includes SmFeN powder and a coating film on its surface; the material of the coating film includes iron hydroxide.

2. The production method according to claim 1, characterized by, The mass content of the dispersant is 5-20% of the mass of the SmFeN powder.

3. The preparation method according to claim 1, characterized in that, The mass content of the anionic surfactant is 5-15% of the mass of the SmFeN powder.

4. The method of claim 1, wherein, The mass content of HClO is 10-40% of the mass of the SmFeN powder.

5. The preparation method according to claim 1, characterized in that, The dispersant is stearyl alcohol polyether-20 and / or sodium sulfonate.

6. The method of claim 1, wherein, The anionic surfactant is anionic polyacrylamide.

7. The preparation method according to claim 1, characterized in that, The solvent is water.

8. The method of claim 1, wherein, The composition of the SmFeN powder, by mass percentage, includes Sm 22-25%, N 4-5%, Ca 0.1-0.5%, and the remainder is Fe.

9. The method of claim 1, wherein, Step S2 includes: The dispersion was subjected to a first ball milling to obtain a first mixture; The first mixture is subjected to a second ball milling to obtain the mixture.

10. The method of claim 9, wherein, The frequency of the first ball mill is 5~30Hz, and the time of the first ball mill is 1~20h.

11. The preparation method according to claim 9, characterized in that, The frequency of the second ball mill is 1~10Hz, and the time of the second ball mill is 3~30min.

12. The method of claim 9, wherein, In step S1, the liquid level of the dispersion is 3-4 cm higher than the steel ball of the ball mill.

13. The production method according to any one of claims 1 to 12, characterized by, The drying process includes vacuum drying.

14. The method of claim 13, wherein, The drying temperature is 10~100℃.

15. The method of claim 14, wherein, The drying temperature is 30~60℃.

16. The method of claim 13, wherein, The drying time is 1 to 2 hours.

17. The method of claim 1, wherein, The D50 of the coated SmFeN permanent magnet material is 3.9~4.2μm.

18. The method of claim 1, wherein, The average thickness of the coating film is 0.2~0.6μm.

19. The method of claim 1, wherein, The intrinsic coercivity of the coated SmFeN permanent magnet material is 7.5~14 KOe.

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