High-density anisotropic manganese-bismuth sintered permanent magnet for motor and preparation method of high-density anisotropic manganese-bismuth sintered permanent magnet
By preparing high-purity MnBi alloy and performing multi-step processing, the problem of high densification of MnBi sintered magnets was solved, and high-density, high-coercivity and high-energy-product MnBi sintered permanent magnets were realized, which are suitable for high-temperature motors.
Patent Information
- Application Number
- CN202511798654.8
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-02
- Publication Date
- 2026-02-27
AI Technical Summary
Existing technologies cannot achieve high densification of MnBi sintered magnets while avoiding the decomposition of the low-temperature MnBi magnetic phase, resulting in problems such as low density, insufficient coercivity, and low magnetic energy product.
High-purity manganese and bismuth raw materials are smelted under vacuum or protective atmosphere to prepare rapidly solidified sheets with uniform composition. After two-stage heat treatment, the sheets are crushed and ball-milled into magnetic powder with a particle size of 1-5μm. The powder is then subjected to magnetic field orientation pressing and isostatic pressing treatment, followed by two-stage sintering under an inert atmosphere, and finally coated with an epoxy resin coating.
A highly dense, anisotropic, and magnetically excellent MnBi sintered permanent magnet was prepared, with a density ≥8.25 g/cm³ and a coercivity ≥6 kOe. It is suitable for high-temperature motors and possesses high-temperature stability and mechanical properties.
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Figure CN121583675A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application relates to the technical field of sintered permanent magnets for electric machines, and in particular to a high-density anisotropic manganese bismuth sintered permanent magnet for electric machines and a preparation method thereof. BACKGROUND
[0002] NdFeB permanent magnets have dominated the modern industry due to their extremely high magnetic energy product, but their Curie temperature is relatively low, and their magnetic properties deteriorate significantly with increasing temperature, which severely limits their application in high-temperature environments (such as some high-performance electric machines). Therefore, developing new permanent magnet materials that can work stably at high temperatures has become an important research direction in the field. MnBi alloy, especially its low-temperature phase (LTP-MnBi), as a rare-earth-free permanent magnet material, shows great application potential. Unlike NdFeB, MnBi permanent magnets have a unique advantage: their coercivity increases with increasing temperature, and they can maintain excellent magnetic properties at high temperatures. In addition, MnBi is also a non-rare earth permanent magnet material with good comprehensive performance of magnetic properties and cost, which further enhances its economic competitiveness in industrial electric machines.
[0003] In recent years, some progress has been made in the research of MnBi materials, such as through rapid solidification or high-pressure synthesis methods, researchers have successfully improved the purity of the LTP phase and optimized the nanocrystalline structure. However, these methods are mostly limited to thin films or powders, and it is difficult to extend to bulk magnets, and there are still problems of high-temperature oxidation and mechanical stability in actual electric machine applications, which leads to the overall performance not meeting the industrial requirements. The preparation of high-performance MnBi sintered permanent magnets faces great challenges, which restricts its practical application. First, the melting process of MnBi alloy is prone to segregation, making it difficult to obtain a compositionally uniform initial alloy ingot. Second, and most importantly, the low-temperature phase MnBi (LTP-MnBi) with strong magnetic crystal anisotropy is unstable at high temperatures, and when the temperature exceeds a certain range (about 355℃), it will transform into the high-temperature phase (HTP-MnBi), and the high-temperature phase is paramagnetic or weakly magnetic. This characteristic makes it difficult to prepare MnBi sintered permanent magnets: the traditional powder metallurgy sintering process needs to be carried out at a high temperature much higher than the phase transition temperature to drive the diffusion and densification between powder particles; but at this high temperature, the LTP-MnBi phase, which is the source of magnetic properties, has already decomposed, resulting in the magnet failing to obtain the expected magnetic properties.
[0004] Therefore, in the prior art, although there are studies on the preparation of MnBi magnetic powder, how to make the magnetic powder into a full-dense anisotropic bulk sintered magnet with high density, high orientation and excellent magnetic properties through a sintering process is still a technical bottleneck. The commonly used sintering method is difficult to realize the complete densification of the magnet without avoiding the decomposition of the LTP phase, resulting in the problems of low density, insufficient coercivity and low magnetic energy product of the prepared MnBi sintered magnet. SUMMARY
[0005] The purpose of the present application is to provide a high-density anisotropic MnBi sintered permanent magnet for electric machines and a preparation method thereof, which solves the problem of difficulty in realizing high densification of bulk magnets without avoiding the decomposition of the LTP-MnBi magnetic phase in the prior art.
[0006] To achieve the above-mentioned purpose, the present application provides a preparation method of a high-density anisotropic MnBi sintered permanent magnet for electric machines, comprising the following steps: High-purity manganese and bismuth raw materials are melted under vacuum or protective atmosphere to obtain a rapid solidification sheet with uniform alloy composition; The rapid solidification sheet is subjected to two-stage heat treatment under inert atmosphere protection; The heat-treated rapid solidification sheet is crushed, then subjected to ball milling under organic solvent protection, and antioxidant is added to obtain a magnetic powder with a particle size of 1-5 μm; The magnetic powder is subjected to orientation pressing in a magnetic field, and then the green compact formed by pressing is subjected to isostatic pressing to obtain a billet; The billet is subjected to two-stage sintering under inert atmosphere protection; The sintered billet is subjected to wire cutting and grinding, and after cleaning and drying, an epoxy resin coating is sprayed on the surface to obtain a magnet for electric machines.
[0007] Among them, high-purity manganese and bismuth raw materials are melted under vacuum or protective atmosphere to obtain a rapid solidification sheet with uniform alloy composition, and The melting is carried out under argon protective atmosphere.
[0008] Among them, the rapid solidification sheet is subjected to two-stage heat treatment under inert atmosphere protection, specifically including: The first stage is kept at 300-320℃ for 5-24 hours, and the second stage is kept at 250-290℃ for 10-100 hours.
[0009] Among them, the heat-treated rapid solidification sheet is crushed, then subjected to ball milling under organic solvent protection, and antioxidant is added to obtain a magnetic powder with a particle size of 1-5 μm, and Crushed to 100-400 mesh.
[0010] In the method, the magnetic powder is oriented and pressed in a magnetic field, and then the green compact formed by the pressing is subjected to isostatic pressing to obtain a blank. The isostatic pressing pressure is 180-300 MPa.
[0011] In the method, the blank is subjected to two-stage sintering under the protection of inert atmosphere, and the two-stage sintering specifically comprises: The first stage is at 250-280 DEG C for 0.5-5 hours, and the second stage is at 290-320 DEG C for 5-30 minutes.
[0012] The application discloses a high-density anisotropic MnBi sintered permanent magnet for a motor, and an alloy composition of the high-density anisotropic MnBi sintered permanent magnet for the motor is MnxBi100-x-yRy, wherein x=45-55, y=0-3, and x and y are atomic ratios; and R is one of Ga, Cu, Sn, Sb, Gd and Zn.
[0013] The application discloses a high-density anisotropic MnBi sintered permanent magnet for a motor and a preparation method thereof. High-purity manganese and bismuth raw materials are used to prepare a MnBi alloy ingot with uniform composition through induction melting, so that segregation problems are avoided. The alloy ingot is processed into magnetic powder with high-purity LTP-MnBi phase through low-energy ball milling, so that the powder particles are ensured to be small and the phase is stable. The magnetic powder is oriented and formed in a magnetic field to form an oriented green compact, so that preliminary orientation and densification of grains are realized. Warm-pressing sintering is adopted to sinter under the condition of temperature and atmosphere control, so that decomposition of the LTP phase is avoided, and meanwhile, diffusion between particles is promoted, and high-density sintered body is realized. In view of the problem that the MnBi material is extremely easy to be oxidized and corroded, black epoxy resin is sprayed on the sintered body, so that corrosion resistance and mechanical properties of the magnet are enhanced, and the magnet is suitable for a motor environment. The obtained MnBi sintered permanent magnet has a density of greater than or equal to 8.25 g / cm3 and a coercive force of greater than or equal to 6 kOe, and is stable at high temperature. The high-density anisotropic MnBi sintered permanent magnet for the motor prepared by the above method mainly comprises LTP-MnBi phase, has high density and good orientation degree, and is suitable for a high-temperature motor rotor or stator component. The high-density anisotropic MnBi sintered permanent magnet for the motor is realized to be dense, oriented and optimized in performance under the premise of avoiding high-temperature decomposition of the LTP-MnBi phase, so that the obtained permanent magnet has high density, high coercive force, high magnetic energy product and good high-temperature stability, and is suitable for high-temperature motor applications. BRIEF DESCRIPTION OF DRAWINGS
[0014] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced.
[0015] Figure 1 The X-ray diffraction pattern of the sample of Example 1 of the present application.
[0016] Figure 2is a SEM image of a magnet section of Example 1 of the present application.
[0017] Figure 3 is a sample image of a large magnet - small magnet with coating of Example 1 of the present application.
[0018] Figure 4 is a magnet performance curve graph of Example 1 of the present application.
[0019] Figure 5 is an X-ray diffraction pattern of a sintered sample of Example 2 of the present application.
[0020] Figure 6 is a magnet performance curve graph of Example 2 of the present application.
[0021] Figure 7 is a flow chart of a method for preparing a high-density anisotropic manganese-bismuth sintered permanent magnet for a motor according to the present application. DETAILED DESCRIPTION
[0022] Embodiments of the present application are described in detail below, examples of which are shown in the accompanying drawings, which are described below by referring to the drawings, are exemplary, and are intended to explain the present application, and cannot be understood as a limitation of the present application.
[0023] Referring to Figures 1 to 7 The present application provides a method for preparing a high-density anisotropic manganese-bismuth sintered permanent magnet for a motor, comprising the following steps: S101: Using high-purity manganese and bismuth raw materials, melting under vacuum or protective atmosphere to obtain a rapid solidification sheet with uniform alloy composition; S102: Two-stage heat treatment of the rapid solidification sheet under inert atmosphere protection; S103: Crushing the heat-treated rapid solidification sheet, then ball milling under organic solvent protection, and adding an antioxidant to obtain a magnetic powder with a particle size of 1-5 μm; S104: Oriented pressing of the magnetic powder in a magnetic field, followed by isostatic pressing of the green compact to obtain a blank; S105: Two-stage sintering of the blank under inert atmosphere protection; S106: Linear cutting and grinding of the sintered blank, cleaning and drying, then spraying an epoxy resin coating on the surface to obtain a magnet for a motor.
[0024] Specifically, the alloy composition is MnxBi100-x-yRy, x = 45-55, y = 0-3, x and y are atomic ratios; R can be one of Ga, Cu, Sn, Sb, Gd, Zn. The material according to the composition is melted in an argon protective atmosphere using a vacuum rapid solidification furnace, and a rapid solidification piece with uniform alloy composition is obtained; the rapid solidification piece is placed in an inert atmosphere furnace for heat treatment, which is divided into two stages: first, rise to 300-320℃, and keep for 5-24h; then, reduce to 250-290℃, and keep for 10-100h. The rapid solidification piece after heat treatment is crushed to 100-400 mesh, then ball milled under the protection of an organic solvent, and an appropriate amount of antioxidant is added to obtain a powder with a particle size of 1-5μm. The powder is placed in a magnetic field forming press for orientation pressing, and then the formed blank is further subjected to isostatic pressing process, and the isostatic pressing pressure is 180-300MPa, so as to obtain a pre-sintering blank; the blank is placed in a sintering furnace in an inert atmosphere environment, and the sintering temperature is divided into two stages: first, rise to 250-280℃ and keep for 0.5-5h to obtain a higher content of low-temperature MnBi magnetic phase; then, rise to 290-320℃ and keep for 5-30min to further promote the densification of the blank. The sintered blank is subjected to wire cutting and plane grinding according to the size requirement, then the sample is cleaned and dried, and an epoxy resin coating is sprayed on the surface of the sample to better prevent the magnet from oxidizing and corroding, so as to obtain a magnetic steel for motor.
[0025] Example one: The alloy composition is Mn52Bi48, and 5kg of raw materials are vacuum melted and rapidly solidified by tape casting, and the rapid solidification furnace tape casting speed is 10r / min, so as to obtain MnBi ingot piece. Figure 1 The X-ray diffraction pattern of the sample shows that the ingot contains LTP-MnBi, Bi and Mn phases. The obtained MnBi ingot piece is placed in a heat treatment furnace for argon protection heat treatment, the heat treatment temperature is 320℃, and the heat preservation time is 5h; then the temperature is reduced to 255℃, and the heat preservation time is further increased to 100h. The two-step high and low temperature heat treatment and long time heat preservation are helpful for the occurrence of peritectic reaction in the alloy, so as to obtain more ferromagnetic LTP-MnBi phase.
[0026] The MnBi ingot piece after heat treatment is further mechanically crushed under nitrogen protection, and sieved to 400 mesh, then the sieved powder is placed in a ball mill for ball milling, the protection solvent is n-heptane, 0.5% antioxidant is added to the powder, the ball milling speed is 120r / min, and the ball milling time is 90min. From Figure 1X-ray diffraction patterns of the samples show that the content of LTP-MnBi phase increases significantly after heat treatment, and decreases after ball milling. Adding a small amount of antioxidant during ball milling helps to inhibit the oxidation of the powder and the decomposition of the main phase. After the powder is dried, 400g of magnetic powder is placed in a magnetic field orientation compression molding mold, the inner cavity size of the mold is 52x100mm, the magnetic field strength is 2T, and the pressure is 600MPa. After the compression molded sample is taken out, it is further compressed using a cold isostatic pressing machine, the pressure is 300MPa, and then a relatively dense blank is obtained. After the compression of the blank, it is placed in a sintering furnace, protected by argon atmosphere, heated to 250℃ for 5h, and then further heated to 320℃ for 5min. Low temperature and long time heat preservation helps to form more LTP-MnBi phase, while high temperature and short time heat preservation helps to densify the magnet. After cooling to room temperature in the furnace, the sample is taken out, and the density is measured by Figure 1 X-ray diffraction patterns of the samples show that the sintered magnet shows typical anisotropy, and the diffraction intensity of LTP-MnBi phase (00l) direction increases significantly. By Figure 2 The sintered magnet section SEM diagram shows that part of the low melting point Bi phase is distributed around the MnBi matrix particles after sintering, which helps to densify the magnet and decouple the hard magnetic particles. Then the sample is wire cut according to the drawing size, and the surface is ground using a surface grinder. Then the sample is cleaned and dried, and an epoxy resin coating is sprayed on the surface of the magnet using an electrostatic spraying system, so as to obtain a magnetic steel for motor, as shown in Figure 3 The size of the magnetic steel is 20x9x12mm. The remanence of the magnet is 5985Gs, the coercivity is 5880Oe, and the maximum magnetic energy product reaches 7.7MGOe. The magnetic properties are shown in Figure 4
[0027] Example two: The alloy composition is Mn47Bi53, and 5kg of raw materials are vacuum melted and rapidly quenched by tape casting. The tape casting speed of the rapid quenching furnace is 10 revolutions per minute, so as to obtain MnBi cast sheet. The obtained MnBi cast ingot sheet is placed in a heat treatment furnace for argon protection heat treatment, the heat treatment temperature is 300℃, and the heat preservation time is 24h; then the temperature is lowered to 280℃, and the heat preservation time is further increased to 10h.
[0028] The heat-treated MnBi ingot sheet was further mechanically broken under nitrogen protection, sieved to 100 mesh, and then the sieved powder was put into a ball mill with n-heptane as the protection solvent, 0.5% antioxidant was added by mass of the powder, the ball milling speed was 120 rpm, and the ball milling time was 90 min. After ball milling, 400 g of the magnetic powder was weighed after drying and placed in a magnetic field orientation pressing mold, the inner cavity size of the mold was 52x100 mm, the magnetic field strength was 2T, and the pressure was 600 MPa. The pressed sample was further pressed using a cold isostatic press, the pressure was 300 MPa, and then a relatively dense blank was obtained. The pressed blank was placed in a sintering furnace under argon atmosphere protection, heated to 280°C for 0.5 h, then further heated to 290°C for 30 min. The furnace was cooled to room temperature, and the sample was taken out, Figure 5 The sample X-ray diffraction pattern showed that the sintered magnet had obvious anisotropy, and the LTP-MnBi phase (00l) crystal face direction diffraction peak intensity increased significantly. Then the sample was linearly cut according to the drawing size, and then ground using a surface grinder, and then the sample was cleaned and dried, and an epoxy resin coating was sprayed on the surface of the magnet using an electrostatic spraying system, thereby obtaining a magnetic steel for a motor, the size of the magnetic steel was 20x9x12 mm. The residual magnetism of the magnet was 5188Gs, the coercive force was 6180Oe, the maximum magnetic energy product reached 5.6 MGOe, and the magnetic properties were as shown in Figure 6
[0029] Example Three: The alloy composition was Mn52Bi47.5Ga0.5, 5 kg of raw materials were vacuum melted and rapidly quenched by tape casting, and the rapid quenching furnace tape casting speed was 10 rpm, thereby obtaining an MnBi cast sheet. The obtained MnBi ingot sheet was placed in a heat treatment furnace for argon protection heat treatment, the heat treatment temperature was 300°C, and the heat preservation time was 24 h; then the temperature was lowered to 280°C, and further heat preservation was carried out for 10 h.
[0030] The heat-treated MnBi ingot sheet is further mechanically broken under nitrogen protection, sieved to 100 meshes, and then the sieved powder is put into a ball mill, the protection solvent is n-heptane, 0.5% antioxidant is added by mass of the powder, the ball milling speed is 120 r / min, and the ball milling time is 90 min. After ball milling, 400 g of the powder is weighed after drying and put into a magnetic field orientation compression molding mold, the inner cavity size of the mold is 52*100 mm, the magnetic field strength is 2 T, and the pressure is 600 MPa. The compression molded sample is taken out and further compressed by using a cold isostatic pressing machine, the pressure is 180 MPa, and then a relatively dense blank is obtained. The compressed blank is put into a sintering furnace, protected by an argon atmosphere, heated to 280°C for 0.5 h, and then further heated to 290°C for 30 min. The furnace is cooled to room temperature, the sample is taken out, linear cutting is performed according to the drawing size, and grinding is performed by using a surface grinder. Then the sample is cleaned and dried, and an epoxy resin coating is sprayed on the surface of the magnet by using an electrostatic spraying system, so that a magnetic steel for a motor is obtained. The remanence of the magnet is 5.23 kGs, the coercivity is 5.96 kOe, and the density of the magnet is 8.26 g / cm 3 .
[0031] The experimental process of Example Three is adopted, and the contents of Ga, Cu, Sb, Sn, Zn and Gd are 1% by changing different component additions. The respective performances of the magnets are shown in Table 1.
[0032] Table 1
[0033] Comparative Example 1: The alloy composition is Mn52Bi48, and the process of Example One is adopted to obtain MnBi cast sheet by rapid solidification and tape casting. The difference is that the heat treatment process of the ingot only has a single temperature holding, that is, the temperature is 255°C, and the holding time is 100 h. The subsequent ball milling, powder preparation and compression sintering processes are completely the same as those of Example One. The density of the prepared MnBi magnet is 8.27 g / cm 3 , but the remanence of the magnet is only 4.9 kGs due to the small content of LTP-MnBi phase.
[0034] Comparative Example 2: The alloy composition is Mn52Bi48, and the process of Example One is adopted to obtain MnBi cast sheet by rapid solidification and tape casting, and MnBi blank is obtained by the same heat treatment, ball milling and compression process. The difference is that the sintering process of the compressed blank adopts single-stage sintering, that is, 250°C for 5 h. Due to the low sintering temperature, the density of the prepared MnBi magnet is only 7.5 g / cm 3 , which seriously affects the comprehensive performance of the magnet.
[0035] Comparative Example 3: The alloy composition is Mn52Bi48, a MnBi ingot is obtained by vacuum induction melting, and then the ingot is heat treated, crushed, ball milled, pressed and sintered, and the process is the same as that of Example 1. The density of the prepared MnBi magnet is 8.26 g / cm 3 , and the residual magnetism of the magnet is 5.05 kGs.
[0036] The present application can obtain high-density manganese bismuth permanent magnet, and the coercive force increases greatly with the increase of temperature, when the temperature increases to 120℃, the intrinsic coercive force of the magnet is generally greater than 10kOe (1T). Vacuum rapid solidification and two-stage long-time heat treatment promote the generation of LTP-MnBi phase, and make the magnet obtain excellent residual magnetism; and the addition of alloying elements helps to refine the grains, and further improves the coercive force of the magnet.
[0037] The above only discloses one or more preferred embodiments of the present application, and cannot limit the scope of the rights of the present application. Those skilled in the art can understand that all or part of the above-mentioned embodiments can be implemented, and equivalent changes made according to the claims of the present application still belong to the scope covered by the present application.
Claims
1. A method for preparing a high-density anisotropic manganese bismuth sintered permanent magnet for electric motors, characterized in that, Includes the following steps: High-purity manganese and bismuth raw materials are used to melt the material under vacuum or a protective atmosphere to obtain rapidly solidified sheets with homogeneous alloy composition. The quick-setting tablets were subjected to a two-stage heat treatment under an inert atmosphere. The heat-treated quick-setting flakes were crushed, then ball-milled under the protection of organic solvents, and antioxidants were added to obtain magnetic powder with a particle size of 1-5 μm. The magnetic powder is oriented and pressed in a magnetic field, and then the pressed green blank is subjected to isostatic pressing to obtain a blank. The blank is sintered in two stages under an inert atmosphere. The sintered blank is wire-cut and ground, cleaned and dried, and then an epoxy resin coating is sprayed onto the surface to obtain a magnet for motors.
2. The method for preparing high-density anisotropic manganese bismuth sintered permanent magnets for motors as described in claim 1, characterized in that, High-purity manganese and bismuth raw materials are used for melting under vacuum or a protective atmosphere to obtain rapidly solidified flakes with homogeneous alloy composition. Melting is carried out under an argon protective atmosphere.
3. The method for preparing high-density anisotropic manganese bismuth sintered permanent magnets for motors as described in claim 1, characterized in that, The quick-setting tablets are subjected to a two-stage heat treatment under an inert atmosphere, specifically including: The first stage involves keeping the temperature at 300-320℃ for 5-24 hours, and the second stage involves keeping the temperature at 250-290℃ for 10-100 hours.
4. The method for preparing high-density anisotropic manganese bismuth sintered permanent magnets for motors as described in claim 1, characterized in that, The heat-treated, rapidly solidified flakes were crushed, then ball-milled under organic solvent protection, with antioxidants added, to obtain magnetic powder with a particle size of 1-5 μm. Break it to 100-400 mesh.
5. The method for preparing a high-density anisotropic manganese bismuth sintered permanent magnet for an electric motor as described in claim 1, characterized in that, The magnetic powder is oriented and pressed in a magnetic field, and then the pressed green blank is subjected to isostatic pressing to obtain a blank. The isostatic pressure is 180-300 MPa.
6. The method for preparing a high-density anisotropic manganese bismuth sintered permanent magnet for an electric motor as described in claim 1, characterized in that, The billet is sintered in two stages under an inert atmosphere, specifically including: The first stage involves holding the temperature at 250-280℃ for 0.5-5 hours, and the second stage involves holding the temperature at 290-320℃ for 5-30 minutes.
7. A high-density anisotropic manganese bismuth sintered permanent magnet for motors, prepared by the method for preparing high-density anisotropic manganese bismuth sintered permanent magnets for motors as described in claim 1, characterized in that... The alloy composition of the high-density anisotropic manganese bismuth sintered permanent magnet for the motor is MnxBi100-x-yRy, where x=45-55, y=0-3, and x and y are atomic ratios; R is one of Ga, Cu, Sn, Sb, Gd, and Zn.