A high coercive force Sm-Fe-N all-metal block permanent magnet and its preparation method

Abstract

This invention discloses a high-coercivity Sm-Fe-N all-metal bulk permanent magnet and its preparation method. The invention involves directly coating Zn onto the surface of Sm-Fe-N magnetic powder, followed by orientation pressing, hot pressing, and heat treatment. This ensures sufficient reaction and diffusion of Zn with Sm-Fe-N or its surface iron oxides and α-Fe, guaranteeing uniform Zn distribution throughout the permanent magnet bulk from surface to interior during heat treatment. Furthermore, the preparation process does not impose any limitations on the thickness of the permanent magnet bulk. The prepared Sm-Fe-N all-metal bulk permanent magnet comprises multiple Sm-Fe-N grains; the outer layer of each grain is sequentially coated with an Sm-(FeZn)-N phase and a FeZn phase; wherein the diffusion thickness of the Sm-(FeZn)-N phase is less than 3 nm. By precisely controlling the time, temperature, and Zn content during heat treatment, and rationally regulating the phase structure changes at the interface before and after heat treatment, the coercivity is maximized.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet material preparation technology, and relates to a high coercivity Sm-Fe-N magnet material with a "core-shell structure" and a preparation method of obtaining a high coercivity magnet by hot pressing and annealing using low coercivity Sm-Fe-N magnetic powder as raw material. Background Technology

[0002] High-performance permanent magnets play a crucial role in the traction motors of hybrid vehicles (electric vehicles). In particular, Nd-Dy-Fe-B sintered magnets, as a key material, have attracted widespread attention due to their excellent magnetic properties. However, given the scarcity and high cost of heavy rare earth metals such as Dy, and the continued growth in demand for high-performance permanent magnet materials, researchers have begun to explore alternatives to Nd-Dy-Fe-B magnets. In this regard, Sm-Fe-N magnets exhibit highly attractive magnetic properties, becoming a promising candidate material. Their anisotropic field μ0H... a It reaches an astonishing 13.8T, 50% higher than Nd-Fe-B, and its high saturation magnetization μ0M s The Curie temperature is approximately 1.54 °C, similar to that of Nd-Fe-B. Furthermore, Sm-Fe-N has a Curie temperature as high as 476 °C, 34% higher than Nd-Fe-B. However, Sm-Fe-N also has some drawbacks, such as decomposition into α-Fe and SmN at temperatures above 600 °C, which limits the possibility of manufacturing fully dense magnets using conventional sintering techniques.

[0003] Currently, there are two main methods for preparing bulk Sm-Fe-N magnets. The first method involves mixing Sm-Fe-N powder with a polymer binder to form a resin-bonded magnet. While this method is simple to operate, the addition of the polymer inevitably sacrifices the magnetic properties of the magnet. The second method uses low-melting-point metals (such as Zn, Bi, Sn, and Al) to prepare metal-bonded Sm-Fe-N magnets. For sintered NdFeB magnets, the coercivity of the magnet does not depend on the magnetic powder, but rather on the neodymium-rich phase grain boundary microstructure obtained by the subsequent tempering treatment after sintering. However, the coercivity of Sm-Fe-N bonded magnets ultimately depends on the coercivity of the Sm-Fe-N magnetic powder. A common method to improve the coercivity of the magnetic powder is to refine the particle size through methods such as ball milling, gradually bringing the magnetic powder closer to the single-domain size. However, refined Sm-Fe-N magnetic powder is more prone to oxidation, especially for industrial production. Highly active ultrafine powders often pose production safety risks, and the refining of magnetic powder also places extremely high demands on experimental equipment. Therefore, using low-coercivity coarse Sm-Fe-N powder as a raw material to obtain high-coercivity magnets is significant for reducing production safety risks and simplifying production processes.

[0004] Currently, the maximum energy product of Sm-Fe-N bulk magnets can reach over 25 MGOe, demonstrating excellent magnetic properties. However, their intrinsic coercivity is far from the theoretically expected value. To address this issue, Chinese patent CN202011320872.8 proposes a method for preparing high-coercivity, high-energy-product diffused Sm-Fe-N magnets. This method involves coating a diffusion layer inside and on the surface of the magnet using laser perforation technology, and introducing dopant elements using three-dimensional diffusion technology to improve the anisotropy and overall magnetic properties of the Sm-Fe-N magnet. Chinese patent CN202110821453.0 also proposes covering a cold-pressed magnet blank with a metal element or alloy in sheet or powder form with a melting point below 800℃ onto the blank, followed by thermal diffusion densification treatment at a temperature below 600℃. This process generates only the grain boundary phase SmFeXN through the reaction of a low-melting-point metal X with Sm-Fe-N. The methods described above are all based on the preparation of Sm-Fe-N bulk materials, followed by surface coating with low-melting-point metals for diffusion. This technique effectively increases the contact between the dopant element and Sm-Fe-N, resulting in a significant improvement in the magnetic properties of the sintered Sm-Fe-N magnet. However, due to the limited diffusion rate, this technique inevitably leads to uneven distribution of diffused elements between the surface and interior of the Sm-Fe-N bulk material. Furthermore, it cannot achieve internal penetration for thicker bulk materials. Therefore, Chinese patent CN202110821453.0 explicitly states that "the distance between the two opposing surfaces of the cold-formed magnet is ≤20mm," indicating a requirement for the thickness of the Sm-Fe-N bulk material. Because of this uneven diffusion, the coercivity obtained by these methods is still lower than expected, and the cost is high, failing to meet practical application requirements. This reveals potential challenges in controlling the material structure and microstructure during the preparation process, necessitating further optimization and research to achieve higher coercivity and meet the stringent requirements of industrial applications. As is well known, the magnetic properties of bonded bulk Sm-Fe-N magnets largely depend on the properties of the powder. Therefore, how to prepare low-coercivity magnetic powder into high-coercivity magnets is a key issue.

[0005] To overcome these problems, this invention proposes a high-coercivity Sm-Fe-N all-metal bulk permanent magnet and its preparation method. This method not only minimizes the impact on remanence and energy product, but also effectively prepares high-coercivity magnets from low-coercivity magnetic powder, overcoming the current limitations of Sm-Fe-N magnets in high-performance applications. This invention is of great significance to the future development of magnet technology. Summary of the Invention

[0006] This invention addresses the limitations of existing Sm-Fe-N magnets in terms of coercivity by proposing a high-coercivity Sm-Fe-N all-metal bulk permanent magnet and its preparation method. The core of this invention lies in precisely controlling the time, temperature, and Zn content during the heat treatment (annealing) process to rationally regulate the phase structure changes at the interface of the Sm-Fe-N / Zn composite magnet before and after heat treatment, thereby maximizing the coercivity enhancement. This method significantly transforms low-coercivity magnetic powder into high-coercivity magnets, effectively overcoming the shortcomings of existing technologies and providing a reliable solution for the application of high-performance magnets.

[0007] The technical solution adopted by this invention to achieve its objective is as follows:

[0008] In a first aspect, the present invention provides a high coercivity Sm-Fe-N all-metal bulk permanent magnet, comprising: a plurality of Sm-Fe-N grains; the outer layer of the grains is sequentially wrapped with Sm-(FeZn)-N phase and FeZn phase (like a core-shell structure of "sandwich structure"), that is, from the core to the outer shell, the phases are Sm-Fe-N, Sm-(FeZn)-N phase and FeZn phase respectively; wherein the diffusion thickness of Sm-(FeZn)-N phase is less than 3nm, and the FeZn phase is continuously distributed along the Sm-Fe-N grains coated with Sm-(FeZn)-N phase, and the coating thickness is greater than 1nm and less than 50nm.

[0009] Preferably, the crystal structure of both the Sm-Fe-N grains and the Sm-(FeZn)-N phase is Th2Zn. 17 type.

[0010] Secondly, this invention provides a method for preparing a high-coercivity Sm-Fe-N all-metal bulk permanent magnet, comprising the following steps:

[0011] (1) Preparation of composite magnetic powder: Under a protective atmosphere, a Zn coating was uniformly deposited on the surface of Sm-Fe-N magnetic powder by vapor deposition or liquid deposition to obtain composite magnetic powder. This step ensured the uniformity and integrity of the coating, laying the foundation for subsequent experiments.

[0012] (2) Low-temperature hot pressing to prepare composite magnets: The composite magnetic powder in step (1) is first pressed by magnetic field orientation, and then cold blank magnets are obtained by cold isostatic pressing technology; then these cold blank magnets are placed in hot pressing molds and rapidly hot pressed to obtain composite magnets.

[0013] (3) Heat treatment (annealing): After the composite magnet in step (2) is encapsulated, it is placed in a heat treatment furnace for heat treatment. Zn reacts and diffuses with Sm-Fe-N and the iron oxide and α-Fe on its surface to form Sm-(FeZn)-N phase and FeZn phase respectively, and elemental Zn gradually disappears. Finally, the outer layer of Sm-Fe-N magnetic powder grains is wrapped with Sm-(FeZn)-N phase and FeZn phase in sequence, forming a core-shell structure similar to a "sandwich structure", which significantly improves the coercivity of the magnet.

[0014] Preferably, the average particle size of the Sm-Fe-N magnetic powder in step (1) is ≤5μm;

[0015] Preferably, the Zn content deposited on the surface of the Sm-Fe-N magnetic powder in step (1) is 3-25 wt.%.

[0016] Preferably, the packaging process of the composite magnet in step (3) is as follows: the composite magnet is placed in a quartz test tube with a volume not exceeding 5 times the volume of the compacted magnet, and the quartz test tube is filled with argon gas for protection or to ensure that the vacuum degree is not higher than 1×10⁻⁶. - 5 Pa.

[0017] Preferably, the heat treatment temperature in step (3) is 400-500℃ and the annealing time is 0.5-3h. More preferably, the heat treatment temperature is 430℃ and the annealing time is 1h.

[0018] The beneficial effects of this invention are mainly reflected in:

[0019] (1) This invention achieves full reaction and diffusion of Zn with Sm-Fe-N or its surface iron oxide and α-Fe by directly coating Zn onto the surface of Sm-Fe-N magnetic powder, and then sequentially through orientation pressing, hot pressing and heat treatment. This ensures that the Zn element is evenly distributed from the surface to the interior of the permanent magnet block during the heat treatment process, and the preparation process does not impose any restrictions on the thickness of the permanent magnet block.

[0020] (2) This invention provides a method for precise control of the coercivity and energy product of a magnet. Compared with traditional preparation methods, this method uses simple equipment, is easy to operate, and has low cost, making it very suitable for large-scale production. These characteristics give this technology high economic value and indicate broad application prospects in the field of permanent magnet materials.

[0021] (3) In this invention, the composite magnet prepared by hot pressing is encapsulated in a quartz test tube with a volume not exceeding five times that of the densified magnet and then subjected to heat treatment to protect the magnet from oxidation and other environmental factors, while preventing the volatilization of samarium during the heat treatment process. This method plays an important role in improving the coercivity of the magnet.

[0022] (4) During the heat treatment process, Zn reacts with the iron oxide and α-Fe phase on the surface of Sm-Fe-N particles to generate a large amount of continuous non-magnetic FeZn phase, which surrounds the magnetic powder to form a "core-shell structure". This structure not only enhances the coercivity of the magnet, but also lays a solid foundation for the industrialization and application of high-performance Sm-Fe-N magnets.

[0023] (5) By precisely adjusting the heat treatment time, temperature, and Zn content, the thickness of the FeZn phase and the Sm-(FeZn)-N diffusion layer can be controlled. With minimal sacrificial magnetic energy product and remanence, low-coercivity magnetic powder can be significantly transformed into high-coercivity magnets. This precise control strategy provides an important technical means for preparing high-performance magnets. Attached Figure Description

[0024] Figure 1 The microstructure evolution diagram of the Sm-Fe-N / Zn composite magnet before and after heat treatment (annealing) in a preferred embodiment of the present invention. Detailed Implementation

[0025] As mentioned above, in view of the shortcomings of the prior art, the inventors of this case, through long-term research and extensive practice, proposed the technical solution of this invention, which is mainly based on at least the following: This invention first coats Zn with Sm-Fe-N magnetic powder, and then performs magnetic field orientation pressing to prepare a permanent magnet block; This invention has no limitation on the thickness of the permanent magnet block, and then performs heat treatment for 0.5-3 hours; The grain boundary phase of this invention includes the Sm-(FeZn)-N phase generated by Zn and Sm-Fe-N and the FeZn phase generated by the reaction of Zn with iron oxide and α-Fe, wherein the diffusion thickness of the Sm-(FeZn)-N phase is less than 3 nm.

[0026] This invention provides a high coercivity Sm-Fe-N all-metal bulk permanent magnet, comprising: multiple Sm-Fe-N grains; each Sm-Fe-N grain is sequentially wrapped with an Sm-(FeZn)-N phase and a FeZn phase, forming a "core-shell structure".

[0027] The diffusion thickness of the Sm-(FeZn)-N phase is less than 3 nm.

[0028] The FeZn phase is continuously distributed along the Sm-(FeZn)-N phase coated on the outer layer, with a coating thickness of 1-50 nm.

[0029] The weight percentage of Zn element in the Sm-Fe-N all-metal bulk permanent magnet is 3-25%.

[0030] The crystal structures of the Sm-Fe-N grains and the Sm-(FeZn)-N phase are both Th2Zn.17 type.

[0031] The density of the Sm-Fe-N permanent magnet is 6.8-7.5 g / cm³. 3 Intrinsic coercivity ≥15kOe, maximum energy product ≥13MGOe.

[0032] The method for preparing the aforementioned permanent magnet is also provided, including the following steps:

[0033] Under a protective atmosphere, a Zn coating is applied to the surface of Sm-Fe-N magnetic powder by vapor deposition or liquid deposition to obtain composite magnetic powder.

[0034] The composite magnetic powder is oriented and pressed in a magnetic field, and then cold isostatically pressed to form a cold blank magnet.

[0035] The cold blank magnet is placed in a hot pressing mold and hot-pressed to obtain a composite magnet;

[0036] After the composite magnet is encapsulated, it is placed in a heat treatment furnace for heat treatment. Zn reacts and diffuses with Sm-Fe-N and the iron oxide and α-Fe on its surface to form Sm-(FeZn)-N phase and FeZn phase respectively, and elemental Zn gradually disappears. Finally, the outer layer of Sm-Fe-N magnetic powder grains is successively wrapped with Sm-(FeZn)-N phase and FeZn phase, forming a "core-shell structure" Sm-Fe-N all-metal block permanent magnet.

[0037] In this embodiment of the invention, the average particle size of the Sm-Fe-N magnetic powder is ≤5μm.

[0038] In this embodiment of the invention, the Zn mass content deposited on the surface of the Sm-Fe-N magnetic powder in the composite magnetic powder is 3-25 wt.%. When the Zn content is low, the Zn distribution is relatively uneven, and the FeZn on the "core-shell structure" surface is relatively less, failing to completely encapsulate the magnetic powder surface. In other words, unreacted α-Fe or iron oxides exist on the magnetic powder surface at the discontinuous Γ-FeZn phase locations. These α-Fe or iron oxides have a demagnetizing nucleation effect, thereby affecting H cj The remanence of the magnet is greatly reduced when the Zn content is high. During the heat treatment process, some Zn does not participate in the reaction but diffuses into the interior of the magnetic powder or exists between the magnetic powder. The presence of non-magnetic Zn phase will greatly reduce the remanence of the magnet, thus causing a decrease in magnetic properties.

[0039] In this embodiment of the invention, the entire hot pressing process is carried out in a vacuum or a glove box filled with an inert gas (such as argon). The hot pressing conditions for the magnet are: hot pressing temperature of 300-600℃, hot pressing pressure greater than 0.5 GPa, hot pressing time of 0.5-10 min, holding time of 0.5-10 min, and hot-pressed magnet density ≥ 6.8 g / cm³. 3Although hot pressing has the advantages of short preparation time and high speed, and is widely used, the short heating time may lead to insufficient diffusion between Zn and Sm-Fe-N. Therefore, the hot-pressed magnets need further heat treatment to optimize and improve their magnetic properties.

[0040] In this embodiment of the invention, the heat treatment temperature is 400-500℃, more preferably 430℃; the heat treatment time is 0.5-3h, more preferably 1h. Figure 1 The microstructure evolution of the Sm-Fe-N / Zn composite magnet before and after heat treatment (annealing) in a preferred embodiment of the present invention is shown in (1), (2), and (3). During the heat treatment process, the possible chemical reactions inside the composite magnet are shown in (1), (2), and (3):

[0041] Zn + Sm-Fe-N → Sm-(FeZn)-N (1)

[0042] Zn + Fe₂O₃(FeO) → FeZn (2)

[0043] Zn + α-Fe → FeZn (3)

[0044] Of the above reactions, reaction (1) leads to a decrease in remanence and should be avoided as much as possible, while reactions (2) and (3) increase coercivity and are valuable reactions. Specifically, during the heat treatment process, the Zn coating on the surface of Sm-Fe-N magnetic powder reacts and diffuses with Sm-Fe-N and the iron oxide on its surface. The outer layer of the grains is successively wrapped with Sm-(FeZn)-N phase and FeZn phase, wherein the diffusion thickness of Sm-(FeZn)-N phase is less than 3 nm, and the FeZn phase is continuously distributed along the Sm-Fe-N grains with a coating thickness greater than 1 nm and less than 50 nm. The density of the Sm-Fe-N permanent magnet after heat treatment is 6.8-7.5 g / cm³. 3 The intrinsic coercivity is ≥15kOe, and the maximum magnetic energy product is ≥13MGOe. The Zn coating on the surface of Sm-Fe-N magnetic powder reacts and diffuses with Sm-Fe-N to form FeZn and Sm-(FeZn)-N phases, and the elemental Zn gradually disappears; the coercivity of the permanent magnet is increased by more than 1.5 times before and after heat treatment.

[0045] In this embodiment of the invention, the composite magnet encapsulation process specifically involves encapsulating the composite magnet within a quartz test tube with a volume not exceeding five times the volume of the compacted magnet. The quartz test tube is filled with argon gas for protection or to ensure a vacuum level not exceeding 1×10⁻⁶. -5To protect the magnet from oxidation and other environmental factors, a dynamic sealing technology using a quartz tube vacuum sealing machine is employed, where the opening of the quartz tube is sealed with a flame. The purpose of sealing is to prevent oxidation, while maintaining a relatively low vacuum level prevents samarium volatilization during heat treatment. Furthermore, limiting the volume of the quartz tube also helps prevent samarium volatilization.

[0046] During the experiments of this invention, it was found that heat treatment temperature and time have a significant impact on the magnetic properties of the magnet. When the heat treatment temperature and time are optimal, Zn element diffuses through grain boundaries and reacts with the iron oxide layer and α-Fe on the surface of the magnetic powder to form a large number of continuously distributed FeZn phases. Although this FeZn phase itself does not directly contribute to the magnetic properties, its "core-shell structure" surrounding the magnetic powder effectively prevents exchange coupling between adjacent Sm-Fe-N magnetic powders during the magnetization process. In addition, the generated FeZn phase also smooths the surface of the magnetic powder, further eliminating potential reverse magnetic domain nucleation sites. This significantly improves the coercivity of the Sm-Fe-N / Zn composite magnet while having a relatively small impact on remanence and energy product. Excessive increases in heat treatment temperature and time can lead to excessive diffusion of Zn into the magnetic powder, forming an intermediate Sm-(FeZn)-N diffusion layer. This Sm-(FeZn)-N layer reduces anisotropy at the powder grain boundaries. During demagnetization, reverse magnetic domains tend to preferentially nucleate on the particle surface in regions of reduced magnetic anisotropy, thus decreasing the coercivity of the composite magnet. Therefore, it is necessary to avoid the formation of this Sm-(FeZn)-N layer during heat treatment, limiting the Sm-(FeZn)-N phase diffusion thickness to less than 3 nm to maintain the anisotropy of the magnetic powder and thus preserve the high coercivity of the composite magnet. Furthermore, further increases in heat treatment temperature will increase the thermodynamic tendency for high-temperature decomposition of Sm-Fe-N. When the heat treatment temperature is too low or the time is too short, Zn fails to become liquid and cannot diffuse sufficiently along the interface. This results in Zn failing to fully react with the iron oxide layer (Fe2O3) and α-Fe on the magnetic powder surface. The residual α-Fe can still serve as nucleation sites for reverse magnetic domains, thereby reducing the coercivity of the magnet. Therefore, it is crucial to precisely adjust the heat treatment temperature, time, and Zn content to ensure a continuous distribution of the FeZn phase while avoiding excessive Zn diffusion.

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0048] The present invention will be further described below with reference to specific embodiments, but the methods and technical parameters involved in the solution should not be construed as limitations on the present invention.

[0049] Example 1:

[0050] Step (1) Preparation of composite magnetic powder:

[0051] Under a protective atmosphere (such as nitrogen), a Zn coating is applied to the surface of Sm-Fe-N magnetic powder (particle size: ≤5μm) by vapor deposition or liquid deposition to obtain composite magnetic powder containing 20wt.%Zn.

[0052] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0053] The magnetic powder coated with Zn in step (1) was oriented and pressed into a blank under a 3T magnetic field. The blank was then subjected to cold isostatic pressing under a pressure of 300 MPa to obtain a cold-formed magnet. Next, these cold-formed magnets were placed in a hot-pressing mold and rapidly hot-pressed to obtain a composite magnet. The entire pressing process was carried out in a glove box filled with inert gas to prevent oxidation of the magnetic powder. Hot-pressing experimental parameters: hot-pressing temperature 430℃, hot-pressing pressure 3 GPa, heating time 3 min, holding time 2 min.

[0054] The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.84 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.87KGs, H cj =5.19KOe, (BH) max = 25.61 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0055] Step (3) Composite magnet encapsulation:

[0056] The composite magnet prepared by hot pressing in step (2) is placed inside a quartz tube, the volume of which does not exceed 5 times the volume of the compacted magnet. The tube is then filled with argon gas or a vacuum level not exceeding 1×10⁻⁶. -5 Under Pa conditions, a quartz test tube vacuum sealing machine is used with dynamic sealing technology to seal the opening of the quartz test tube with a flame.

[0057] Step (4) Heat treatment (annealing):

[0058] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment at 400℃ for 1 hour, and then cooled and removed from the furnace. The density of the composite magnet was tested to be 6.91 g / cm³. 3The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.13KGs, H cj =17.13KOe, (BH) max =20.63MGOe.

[0059] Example 2:

[0060] Step (1) Preparation of composite magnetic powder:

[0061] The composite magnetic powder was prepared as described in Example 1.

[0062] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0063] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 30 mm and a density of 6.84 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.87KGs, H cj =5.19KOe, (BH) max = 25.61 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0064] Step (3) Composite magnet encapsulation:

[0065] The specific steps for packaging the composite magnet are as described in Example 1.

[0066] Step (4) Heat treatment (annealing):

[0067] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment at 430℃ for 1 hour, and then cooled and removed from the furnace. The density of the composite magnet was tested to be 6.97 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 9.93KGs, H cj =24.36KOe, (BH) max =20.68MGOe.

[0068] Example 3:

[0069] Step (1) Preparation of composite magnetic powder:

[0070] The composite magnetic powder was prepared as described in Example 1.

[0071] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0072] The low-temperature hot pressing process for preparing the composite magnet is as described in Example 1. The composite magnet was tested and found to have a thickness of 20 mm and a density of 6.84 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.87KGs, H cj =5.19KOe, (BH) max = 25.61 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0073] Step (3) Composite magnet encapsulation:

[0074] The specific steps for packaging composite magnets are as described in Example 1.

[0075] Step (4) Heat treatment (annealing):

[0076] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment at 450℃ for 1 hour, and then cooled and removed from the furnace. The density of the composite magnet was tested to be 6.99 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 9.36KGs, H cj =19.69KOe, (BH) max =16.63MGOe.

[0077] Comparative Example 1: Heat treatment temperature different from Example 1

[0078] Step (1) Preparation of composite magnetic powder:

[0079] The composite magnetic powder was prepared as described in Example 1.

[0080] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0081] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.84 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r=10.87KGs, H cj =5.19KOe, (BH) max = 25.61 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0082] Step (3) Composite magnet encapsulation:

[0083] The specific steps for packaging composite magnets are as described in Example 1.

[0084] Step (4) Heat treatment (annealing):

[0085] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 300℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.96 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.76KGs, H cj = 4.65 KOe, (BH) max =23.65MGOe.

[0086] Comparative Example 2: Heat treatment temperature different from Example 1

[0087] Step (1) Preparation of composite magnetic powder:

[0088] The composite magnetic powder was prepared as described in Example 1.

[0089] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0090] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.84 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.87KGs, H cj =5.19KOe, (BH) max = 25.61 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0091] Step (3) Composite magnet encapsulation:

[0092] The specific steps for packaging composite magnets are as described in Example 1.

[0093] Step (4) Heat treatment (annealing):

[0094] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 350℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.88 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.63KGs, H cj =4.98KOe, (BH) max =21.79MGOe.

[0095] Comparative Example 3: Heat treatment temperature different from Example 1

[0096] Step (1) Preparation of composite magnetic powder:

[0097] The composite magnetic powder was prepared as described in Example 1.

[0098] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0099] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.84 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.87KGs, H cj =5.19KOe, (BH) max = 25.61 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0100] Step (3) Composite magnet encapsulation:

[0101] The specific steps for packaging composite magnets are as described in Example 1.

[0102] Step (4) Heat treatment (annealing):

[0103] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 550℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.98 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =7.56KGs, H cj =12.69KOe, (BH) max =10.69MGOe.

[0104] Comparative Example 4:

[0105] Step (1) Preparation of composite magnetic powder:

[0106] A glass bottle containing 3mm agate beads was pre-filled with the mixture. Commercially available 1000-mesh Zn powder and Sm-Fe-N magnetic powder with a particle size ≤5μm from Example 1 were then added to the glass bottle and mixed. The powder and agate beads were then separated using a sieve. The ratio of agate beads to powder was 1:10. The entire mechanical mixing process was carried out in a glove box with an oxygen content below 10ppm. The final product was a composite magnetic powder containing 20wt.% Zn.

[0107] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0108] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.64 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 9.78KGs, H cj =4.83KOe, (BH) max =20.76 MGOe. The Zn content in the composite powder was determined to be 20 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0109] Step (3) Composite magnet encapsulation:

[0110] The specific steps for packaging composite magnets are as described in Example 1.

[0111] Step (4) Heat treatment (annealing):

[0112] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.69 g / cm³.3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =8.76KGs, H cj =12.01 KOe, (BH) max = 12.12 MGOe. The magnetic properties of the Sm-Fe-N / Zn composite magnets before and after heat treatment in Examples 1-3 and Comparative Examples 1-4 were tested, and the results are shown in Table 1.

[0113] Table 1: Magnetic performance test results of composite magnets prepared by low-temperature hot pressing before and after heat treatment.

[0114]

[0115] Table 1 shows the magnetic properties of the Sm-Fe-N / Zn composite magnets before and after heat treatment. It can be seen that the untreated magnet exhibits lower H... cj ~5KOe, (BH) max ~26MGOe, B r ~11KGs. When the heat treatment temperature is 300-400℃, the coercivity after heat treatment is lower than before heat treatment. When the heat treatment temperature reaches 400-450℃ (the melting point of Zn is 419.53℃), the coercivity of the magnet shows a significant increase. When the heat treatment temperature reaches 550℃, the increase in coercivity decreases, while the decrease in remanence and maximum energy product also increases significantly. This is because when the heat treatment temperature is too low, Zn fails to become liquid and cannot diffuse sufficiently along the interface, which results in Zn failing to react with the iron oxide layer (Fe2O3) on the surface of the magnetic powder. 3、 The reaction between FeO and α-Fe is complete. Residual α-Fe may still act as nucleation sites for reverse magnetic domains, thus reducing the coercivity of the magnet. When the heat treatment temperature is too high, it will cause excessive diffusion of Zn into the magnetic powder, forming too many Sm-(FeZn)-N intermediate diffusion layers. The appearance of the Sm-(FeZn)-N intermediate diffusion layers reduces the anisotropy at the powder grain boundaries. During demagnetization, reverse magnetic domains tend to preferentially nucleate on the particle surface in regions of reduced magnetic anisotropy, thereby reducing the coercivity of the composite magnet. Of course, further increases in heat treatment temperature will also increase the thermodynamic tendency of Sm-Fe-N high-temperature decomposition. A comparison of the magnetic properties of hot-pressed magnets with mechanically mixed magnetic powder before and after heat treatment reveals that, with the same Zn content, hot-pressed magnets coated with Zn magnetic powder exhibit higher coercivity after heat treatment. This indicates that improving the dispersion uniformity of Zn and magnetic powder is also one of the conditions for forming a complete "sandwich structure" core-shell structure after heat treatment.

[0116] Example 4:

[0117] Step (1) Preparation of composite magnetic powder:

[0118] The composite magnetic powder was prepared as described in Example 1, except that the Zn mass content was adjusted, as shown in Table 2.

[0119] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0120] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.61 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 11.36KGs, H cj = 4.69 KOe, (BH) max = 25.96 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 5 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0121] Step (3) Composite magnet encapsulation:

[0122] The specific steps for packaging composite magnets are as described in Example 1.

[0123] Step (4) Heat treatment (annealing):

[0124] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.65 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.65KGs, H cj =15.36KOe, (BH) max =22.85MGOe.

[0125] Example 5:

[0126] Step (1) Preparation of composite magnetic powder:

[0127] The composite magnetic powder was prepared as described in Example 1, except that the Zn mass content was adjusted, as shown in Table 2.

[0128] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0129] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.68 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: Br = 11.02KGs, H cj =4.83KOe, (BH) max = 25.87 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 10 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0130] Step (3) Composite magnet encapsulation:

[0131] The specific steps for packaging composite magnets are as described in Example 1.

[0132] Step (4) Heat treatment (annealing):

[0133] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.71 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.61KGs, H cj =17.85KOe, (BH) max =21.96MGOe.

[0134] Example 6:

[0135] Step (1) Preparation of composite magnetic powder:

[0136] The composite magnetic powder was prepared as described in Example 1, except that the Zn mass content was adjusted, as shown in Table 2.

[0137] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0138] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.78 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.88KGs, H cj =4.96KOe, (BH) max = 25.63 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 15 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0139] Step (3) Composite magnet encapsulation:

[0140] The specific steps for packaging composite magnets are as described in Example 1.

[0141] Step (4) Heat treatment (annealing):

[0142] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.79 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.54KGs, H cj =19.74 KOe, (BH) max =20.98MGOe.

[0143] Example 7:

[0144] Step (1) Preparation of composite magnetic powder:

[0145] The composite magnetic powder was prepared as described in Example 1, except that the Zn mass content was adjusted, as shown in Table 2.

[0146] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0147] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.89 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.21KGs, H cj =5.26KOe, (BH) max = 24.63 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 25 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0148] Step (3) Composite magnet encapsulation:

[0149] The specific steps for packaging composite magnets are as described in Example 1.

[0150] Step (4) Heat treatment (annealing):

[0151] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.92 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =10.03KGs, H cj =26.03 KOe, (BH) max =18.63MGOe.

[0152] Comparative Example 5: Zn-free compared to Example 2

[0153] Step (1) Preparation of composite magnetic powder:

[0154] The Sm-Fe-N magnetic powder (particle size: ≤5μm) without Zn coating from Example 1 was selected.

[0155] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0156] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.42 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r =11.56KGs, H cj =4.43KOe, (BH) max = 26.02 MGOe. The Zn content of the magnetic powder was 0 wt.% as determined by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). Microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0157] Step (3) Composite magnet encapsulation:

[0158] The specific steps for packaging composite magnets are as described in Example 1.

[0159] Step (4) Heat treatment (annealing):

[0160] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 6.56 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 9.85KGs, H cj =3.16KOe, (BH) max=12.96MGOe.

[0161] Comparative Example 6: The Zn content is higher compared to Example 2.

[0162] Step (1) Preparation of composite magnetic powder:

[0163] The composite magnetic powder was prepared as described in Example 1, except that the Zn mass content was adjusted, as shown in Table 2.

[0164] Step (2) Low-temperature hot pressing to prepare composite magnets:

[0165] The steps for preparing the composite magnet by low-temperature hot pressing are as described in Example 1. The composite magnet was tested and found to have a thickness of 25 mm and a density of 6.96 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 9.68KGs, H cj =5.36KOe, (BH) max = 23.41 MGOe. The Zn content of the Zn-coated magnetic powder was determined to be 35 wt.% by energy-dispersive X-ray fluorescence spectrometry (XRF, ZSX Primus II). The microstructure and morphology were studied using X-ray diffraction (XRD, X'Pert Pro) and a benchtop scanning electron microscope (Phenom ProX).

[0166] Step (3) Composite magnet encapsulation:

[0167] The specific steps for packaging composite magnets are as described in Example 1.

[0168] Step (4) Heat treatment (annealing):

[0169] The quartz tube-encapsulated sample obtained in step (3) was placed in a heat treatment furnace for heat treatment, and then cooled and removed from the furnace. The heat treatment temperature was 430℃, and the heat treatment time was 1 hour. The density of the composite magnet was tested to be 7.16 g / cm³. 3 The magnetic properties of the magnet were measured using an AMT-4 permanent magnet parameter measuring instrument: B r = 8.63KGs, H cj =26.01 KOe, (BH) max = 9.36 MGOe. Magnetic properties of the composite magnets prepared by low-temperature hot pressing in Examples 2, 4-7, and Comparative Examples 5 and 6 were tested before and after heat treatment, and the results are shown in Table 2.

[0170] Table 2: Magnetic property test results of composite magnets prepared by low-temperature hot pressing before and after heat treatment

[0171]

[0172] Table 2 shows the magnetic properties of hot-pressed magnets coated with magnetic powder of different Zn contents before and after heat treatment. It can be seen that for magnets with different Zn contents at the same heat treatment temperature (430℃), the initial H of the composite magnet before heat treatment is... cj ~5KOe, (BH) max ~26MGOe, B r The remanence is approximately 11 kgs. After heat treatment, when the Zn content is between 3-25 wt.%, the coercivity increases by up to 4.94 times while ensuring a small decrease in remanence and maximum energy product. However, when the Zn content is 35 wt.%, although the increase in coercivity is significant, the remanence and maximum energy product decrease substantially. This is because with the increase in Zn content, the "core-shell structure" of the FeZn phase-coated magnetic powder after heat treatment becomes more complete. This structure prevents exchange coupling between adjacent magnetic particles, thereby improving the coercivity of the magnet. When the Zn content is excessive, some Zn does not participate in the reaction during heat treatment but diffuses into the interior of the magnetic powder or exists between the magnetic particles. The presence of non-magnetic Zn phase greatly reduces the remanence of the magnet, thus causing a decrease in magnetic properties.

[0173] The above embodiments are used to explain and illustrate the present invention, but not to limit the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims fall within the protection scope of the present invention.

Claims

1. A high coercivity Sm-Fe-N all-metal bulk permanent magnet, characterized in that, include: Multiple Sm-Fe-N grains; each Sm-Fe-N grain is successively wrapped with Sm-(FeZn)-N phase and FeZn phase, forming a "core-shell structure"; The permanent magnet is obtained using the following preparation method: Under a protective atmosphere, a Zn coating is applied to the surface of Sm-Fe-N magnetic powder by vapor deposition or liquid deposition to obtain composite magnetic powder. The composite magnetic powder is oriented and pressed in a magnetic field, and then cold isostatically pressed to form a cold blank magnet. The cold blank magnet is placed in a hot pressing mold and hot-pressed to obtain a composite magnet; The composite magnet is encapsulated and then placed in a heat treatment furnace for heat treatment. Zn reacts and diffuses with Sm-Fe-N and its surface iron oxides and α-Fe, forming Sm-(FeZn)-N phase and FeZn phase respectively, while elemental Zn gradually disappears. Finally, the outer layer of Sm-Fe-N magnetic powder grains is successively wrapped with Sm-(FeZn)-N phase and FeZn phase, forming a core-shell structured Sm-Fe-N all-metal bulk permanent magnet. The heat treatment temperature is 400-500℃, and the heat treatment time is 0.5-3h. The weight percentage of Zn element in the Sm-Fe-N all-metal bulk permanent magnet is 3-25%.

2. The permanent magnet according to claim 1, characterized in that, The diffusion thickness of the Sm-(FeZn)-N phase is less than 3 nm.

3. The permanent magnet according to claim 1 or 2, characterized in that, The FeZn phase is continuously distributed along the Sm-Fe-N grains coated with the Sm-(FeZn)-N phase on the outer layer, with a coating thickness of 1-50 nm.

4. The Sm-Fe-N all-metal bulk permanent magnet according to claim 1, characterized in that, The density of the Sm-Fe-N all-metal bulk permanent magnet is 6.8-7.5 g / cm³. 3 The intrinsic coercivity is ≥15 kOe, the maximum magnetic energy product is ≥13 MGOe, and the crystal structure of both Sm-Fe-N grains and Sm-(FeZn)-N phase is Th2Zn. 17 type.

5. The permanent magnet according to claim 1, characterized in that, The average particle size of the Sm-Fe-N magnetic powder is ≤5μm.

6. The permanent magnet according to claim 1, characterized in that, The heat treatment temperature is 430℃ and the heat treatment time is 1 hour.

7. The permanent magnet according to claim 1, characterized in that, The encapsulation process of the composite magnet is as follows: the composite magnet is placed in a quartz test tube with a volume not exceeding 5 times the volume of the compacted magnet, and the quartz test tube is filled with argon gas for protection or to ensure that the vacuum degree does not exceed 1×10⁻⁶. -5 Pa.

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