High-density bonded magnet and preparation method thereof

Abstract

The invention relates to the field of magnetic materials, and particularly discloses a high-density bonded magnet and a preparation method thereof. The high-density bonded magnet comprises the following raw materials in percentage by mass: 70-85% of neodymium-iron-boron alloy and 15-30% of a binder, wherein the binder comprises epoxy resin, polyphenylene sulfide and nano silicon dioxide; the preparation method comprises the following steps: mixing the epoxy resin, the polyphenylene sulfide and the nano silicon dioxide to form a binder; the method comprises the following steps: heating and mixing a binder and a neodymium-iron-boron alloy, putting a mixed material into a mold after mixing is completed, pre-pressing for 3-5 minutes at the pressure of 40-50 MPa, then carrying out hot press molding at the pressure of 800-1200 MPa, and maintaining the pressure for 5-10 minutes; and heating and curing the hot-press molded product in a 0.5-1.5 T axial magnetic field to obtain the high-density bonded magnet after curing. The high-density bonded magnet has the advantages that the density of the magnet is improved, and meanwhile the mechanical property and durability of the magnet are guaranteed.

Description

Technical Field

[0001] The present application relates to the field of magnetic materials, and more specifically, to a high-density bonded magnet and a preparation method thereof. Background Art

[0002] At present, with the rapid development of new energy, intelligent equipment and other fields, magnetic materials are widely used in new energy vehicle drive motors, micro servo actuators, precision medical sensors and other equipment. Such application scenarios require that magnetic components not only need to be miniaturized, lightweight and highly durable, but also need to generate high-intensity magnetic fields in a limited space, which puts higher requirements on magnetic materials.

[0003] While traditional sintered magnets offer excellent magnetic properties, their brittleness and limited processing flexibility make them difficult to manufacture for complex structural devices. Currently, the mainstream bonded magnet manufacturing processes include extrusion, tape casting, and compression molding. While these technologies offer advantages in shape customization, they still face significant bottlenecks in balancing magnetic performance and overall reliability.

[0004] First, existing bonding processes require excessive amounts of binder to maintain formability, which directly reduces the volume density of the magnet and severely limits the potential for increasing the magnetic energy product. Second, the poor interfacial compatibility common in thermosetting resin systems makes the magnets susceptible to microcrack propagation under temperature and humidity cycling conditions, resulting in a magnetic performance degradation rate of up to 20%-30%. This technical dilemma severely restricts the application of bonded magnets in high-end applications such as precision motors and medical sensors. Summary of the Invention

[0005] In order to improve the density of the magnet while ensuring its mechanical properties and durability, the present application provides a high-density bonded magnet and a preparation method thereof.

[0006] The high-density bonded magnet provided in this application adopts the following technical solution: A high-density bonded magnet comprises raw materials of 70-85% neodymium iron boron alloy and 15-30% binder by mass percentage. The binder comprises epoxy resin, polyphenylene sulfide and nano-silicon dioxide mixed in a mass ratio of (5-6):(3-4):2.

[0007] By adopting this technical solution, a balance of high magnetic performance and mechanical stability is achieved by combining 70-85% NdFeB alloy with 15-30% composite binder. Epoxy resin provides low-temperature curing initial strength, polyphenylene sulfide (PPS) imparts high-temperature dimensional stability, and nano-silica acts as a rigid filler to inhibit shrinkage and crack propagation. This allows the magnet to achieve both high magnetic energy product and flexural strength, while the high proportion of magnetic powder ensures performance levels close to those of sintered magnets.

[0008] Optionally, the NdFeB alloy is (Nd, Pr)2(Fe, Co, Ga) 14 B. The grain boundary diffusion Dy content of the NdFeB alloy is 0.5-1.2 wt%.

[0009] By adopting the above technical solution, (Nd, Pr)2(Fe, Co, Ga) 14 B alloy composition and control of grain boundary Dy diffusion content, Pr to increase the Curie temperature and reduce temperature sensitivity, Co / Ga to inhibit the formation of soft magnetic phase to improve coercivity, and Dy to form enriched layers at grain boundaries to enhance the pinning effect, so that the coercivity is significantly improved, while avoiding the remanence loss caused by excessive Dy, and achieving stable magnetic properties at high temperatures.

[0010] Optionally, the nano-silicon dioxide is treated with plasma activation, and the surface hydroxyl density of the nano-silicon dioxide after the plasma activation treatment is ≥3 / nm 2 .

[0011] By adopting this technical solution, the chemical bonding ability of nano-silica and epoxy resin is significantly enhanced. The activated silica bridges the interface between the resin and the magnetic powder through covalent bonds, reducing porosity while improving dispersion uniformity, enhancing interfacial bonding strength, and effectively suppressing interfacial debonding caused by stress concentration.

[0012] Optionally, the surface of the NdFeB alloy is coated with an amorphous silicon carbide layer with a thickness of 5-10 nm.

[0013] By employing this technical solution, a 5-10nm amorphous silicon carbide layer is coated on the surface of the NdFeB alloy, forming a dense anti-oxidation barrier that reduces the oxidation weight gain of the magnetic powder in hot and humid environments. The amorphous structure does not interfere with the magnetic domain orientation, minimizing remanence loss and significantly improving the long-term reliability of the magnet.

[0014] Optionally, the coercive force of the NdFeB alloy is ≥20 kOe, and the NdFeB alloy is made of metal powder with a particle size of 3-5 μm.

[0015] By adopting the above technical solution, the coercive force of NdFeB alloy is limited to ≥20kOe and the particle size is 3-5μm, which not only ensures the anti-demagnetization ability of the magnet at high temperature, but also optimizes the powder fluidity through moderate particle size, thereby increasing the filling density during molding and ensuring the efficiency and consistency of industrial production.

[0016] In a second aspect, the present application provides a method for preparing a high-density bonded magnet, which adopts the following technical solution: A method for preparing a high-density bonded magnet comprises the following steps: Mixing epoxy resin, polyphenylene sulfide and nano-silicon dioxide to form a binder; Heat and mix the binder and NdFeB alloy at 80-100°C. After mixing, place the mixture in a mold, pre-press at 40-50 MPa for 3-5 minutes, and then hot-press at 800-1200 MPa for 5-10 minutes. The hot-pressed product is heated and solidified in an axial magnetic field of 0.5-1.5 T to obtain a high-density bonded magnet.

[0017] By adopting this technical solution, a combination of pre-pressing and degassing, high-pressure forming, and magnetic field-oriented heating and curing processes achieves high-density magnetic powder stacking and precise orientation. The pre-pressing stage eliminates air gaps, the high-pressure stage induces plastic deformation of the magnetic powder, and the magnetic field curing process orients the easy magnetization axis. The resulting magnet has both high remanence and low porosity.

[0018] Optionally, the specific method of heating and curing the product is: first heating the product to 120-130° C., keeping it warm for 1-2 hours, and then heating it to 280-290° C., and keeping it warm for 2-3 hours.

[0019] By adopting the above technical solution and a two-stage heating and curing process, the epoxy resin is first fully cross-linked to form a three-dimensional network, and then the PPS crystallization is promoted. The temperature is stepped to avoid sudden thermal stress changes, making the resin phase change process smooth, thereby improving the crystallinity and creep resistance temperature, while reducing residual stress and ensuring the dimensional accuracy of the magnet.

[0020] Optionally, the surface of the high-density bonded magnet obtained by heating and curing is further coated with an aluminum oxide / titanium dioxide composite film layer with a thickness of 5-10 nm.

[0021] By adopting the above technical solution, a 5-10nm Al2O3 / TiO2 composite film layer is deposited on the surface of the magnet. The Al2O3 layer blocks the penetration of water and oxygen, and the TiO2 layer improves the surface hardness and wear resistance, taking into account both protective performance and magnetic efficiency.

[0022] Optionally, during the hot pressing process, ultrasonic vibration of 10-50 kHz and an amplitude of 2-5 μm is simultaneously applied.

[0023] By employing this technical solution, 10-50kHz ultrasonic vibrations are simultaneously applied during hot pressing, using high-frequency mechanical waves to break up magnetic powder agglomerates and promote particle rearrangement and densification. The ultrasonic cavitation effect reduces the friction coefficient between powders, improving magnetic powder stacking efficiency and molding density. Simultaneously, vibration assists the resin in filling micron-level gaps, reducing interfacial defects and significantly enhancing the mechanical integrity of the magnet.

[0024] In summary, this application has the following beneficial effects: 1. Because the present application uses a system of NdFeB alloy and composite binder, while maintaining a high magnetic energy product, it utilizes the low-temperature cross-linking property of epoxy resin, the high-temperature stability of PPS and the interface strengthening effect of nano-SiO2 to significantly improve the bending strength and dimensional accuracy, thereby solving the performance-reliability imbalance problem caused by low density and weak interface of traditional bonded magnets.

[0025] 2. In this application, (Nd, Pr)2(Fe, Co, Ga) is preferred. 14 The grain boundary diffusion restriction of the B alloy and Dy improves coercivity while optimizing the Curie temperature and temperature stability to meet the requirements of high-temperature operating conditions. The introduction of an amorphous silicon carbide coating reduces the oxidation rate of the magnetic powder. The amorphous structure does not interfere with the magnetic domain orientation, resulting in minimal remanence loss.

[0026] 3. The method of the present application combines pre-compression degassing with high-pressure forming technology, supplemented by magnetic field directional solidification. This preparation method achieves high-density filling and precise orientation of magnetic powder, and has both magnetic properties close to those of sintered magnets and complex forming capabilities of bonded magnets, breaking through the density and orientation bottlenecks of traditional processes. DETAILED DESCRIPTION

[0027] The present application is further described in detail below with reference to the examples. It is particularly noted that if no specific conditions are specified in the following examples, the reactions are carried out according to conventional conditions or the conditions recommended by the manufacturer. Unless otherwise specified, the raw materials used in the following examples can be obtained from common commercial sources.

[0028] Preparation example of plasma activated nano-silicon dioxide Preparation Example 1 A method for plasma activation treatment of nano-silicon dioxide: A radio frequency glow discharge plasma treatment system is used, equipped with a vacuum reaction chamber, a gas flow controller and a power adjustable radio frequency source (13.56 MHz).

[0029] The nano-silica with a particle size of 20-40 nm was placed in a vacuum drying oven and dehydrated at 120° C. for 2 hours to remove physically adsorbed water.

[0030] The reaction chamber was introduced with an Ar / O2 mixed gas with an Ar / O2 volume ratio of 4:1 and a total gas flow rate of 50 sccm, maintaining an operating pressure of 10 Pa. The temperature in the reaction chamber was 80°C, and the nano-silica was plasma treated with a radio frequency power of 200 W. The surface hydroxyl density of the nano-silica was detected to be ≥3 / nm. 2 The activated nano-silicon dioxide was obtained.

[0031] Preparation example of NdFeB alloy surface coated with amorphous silicon carbide layer Preparation Example 2 A method for coating the surface of a NdFeB alloy with an amorphous silicon carbide layer: NdFeB alloy powder with a particle size of 3-5 μm was immersed in 5% by mass diluted nitric acid for ultrasonic cleaning for 5 minutes to remove the surface oxide layer, rinsed with deionized water until neutral, and then vacuum dried.

[0032] A hot-wall low-pressure CVD system was used, equipped with silane (SiH4) and methane (CH4) gas lines, and the deposition temperature was controlled at 400°C. The SiH4:CH4 ratio was set to 1:3 (volume ratio), the total gas flow rate was 200 sccm, the deposition pressure was 50 Pa, and the deposition time was 30 minutes.

[0033] After deposition, argon gas is introduced to rapidly cool the NdFeB alloy powder at a cooling rate of 100°C / s to inhibit the crystallization of silicon carbide and obtain an amorphous silicon carbide layer with a thickness of 5-10nm. Example

[0034] Example 1 A method for preparing a high-density bonded magnet: The raw material specifications of this embodiment are as follows: NdFeB alloy powder: composition is (Nd, Pr)2(Fe, Co, Ga) 14 B, average particle size in the range of 3-5 μm, coercivity Hcj ≥ 20 kOe, grain boundary diffused Dy content 0.8 wt%; Epoxy resin: bisphenol A epoxy resin E-51, epoxy value 0.51; Polyphenylene sulfide (PPS): linear structure, melt index 200g / 10min; Nano-silicon dioxide: particle size 20-40nm.

[0035] Preparation process: The raw materials were weighed according to the mass ratio of epoxy resin:PPS:nano-SiO2=5.5:3.5:2, and the epoxy resin and PPS were pre-melted and mixed at 80°C for 10 minutes. Then SiO2 was added and stirred at 500 rpm for 30 minutes to form a homogeneous adhesive slurry.

[0036] 78 kg of NdFeB alloy powder and 22 kg of binder were added to a twin-screw internal mixer. The mixing temperature was set to 90°C, the screw speed was 60 rpm, the vacuum degree was ≤10 Pa, and the mixing time was 40 minutes. The mixture was cooled to 60°C and then discharged. Agglomerates were removed through a 100-mesh sieve.

[0037] The mixture is loaded into the mold and pre-pressed at a pressure of 45 MPa for 4 minutes to eliminate internal air gaps; the temperature is raised to 100°C and hot-pressed at a pressure of 1000 MPa for 8 minutes; the mold is demoulded after the pressure is released to obtain a semi-finished product.

[0038] The semi-finished product was placed in a magnetic field curing furnace and a 1.0T axial magnetic field was applied; the temperature was kept at 120°C for 1 hour, and then the temperature was increased to 280°C and kept for 2 hours; the product was taken out after cooling to 80°C in the furnace to obtain a high-density bonded magnet.

[0039] Example 2 A method for preparing a high-density bonded magnet: The difference from Example 1 is that 70 kg of NdFeB alloy powder and 30 kg of binder are added to a twin-screw internal mixer.

[0040] Example 3 A method for preparing a high-density bonded magnet: The difference from Example 1 is that 85 kg of NdFeB alloy powder and 15 kg of binder are added to a twin-screw internal mixer.

[0041] Example 4 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the coercive force Hcj of the NdFeB alloy powder is less than 20 kOe.

[0042] Example 5 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the grain boundary diffusion Dy content of the NdFeB alloy powder is 0.5 wt%.

[0043] Example 6 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the grain boundary diffusion Dy content of the NdFeB alloy powder is 1.2 wt%.

[0044] Example 7 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the grain boundary diffusion Dy content of the NdFeB alloy powder is 0.2 wt%.

[0045] Example 8 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the grain boundary diffusion Dy content of the NdFeB alloy powder is 1.5 wt%.

[0046] Example 9 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the NdFeB alloy powder composition is Nd2Fe 14 B.

[0047] Example 10 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the binder is a mixture of epoxy resin, polyphenylene sulfide and nano-silicon dioxide in a mass ratio of 5:4:2.

[0048] Example 11 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the binder is a mixture of epoxy resin, polyphenylene sulfide and nano-silicon dioxide in a mass ratio of 6:3:2.

[0049] Example 12 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the surface of the NdFeB alloy is coated with an amorphous silicon carbide layer with a thickness of 5-10 nm, which is obtained by the method of Preparation Example 2.

[0050] Example 13 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the nano-silicon dioxide is treated with plasma activation and obtained by the method of Preparation Example 1.

[0051] Example 14 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the semi-finished product is placed in a magnetic field curing furnace and a 1.0T axial magnetic field is applied; the temperature is raised to 280°C and kept warm for 3 hours; and the semi-finished product is taken out after cooling to 80°C in the furnace to obtain a high-density bonded magnet.

[0052] Example 15 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the prepared high-density bonded magnet is further placed in an atomic layer deposition system (equipment is Beneq TFS 500) with trimethylaluminum (purity ≥99.999%), titanium tetraisopropoxide (purity ≥99.99%), high-purity water and ozone (concentration ≥200g / m 3 ) as a precursor, and the surface atomic layer deposition of high-density bonded magnet is performed to form an aluminum oxide / titanium dioxide composite film layer. The specific process is shown in Table 1 below.

[0053] Table 1 ALD process parameters Example 16 A method for preparing a high-density bonded magnet: The difference from Example 1 is that 30 kHz ultrasonic vibration with an amplitude of 4 μm is simultaneously applied during the 1000 MPa hot pressing process.

[0054] Comparative Example Comparative Example 1 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the binder does not contain polyphenylene sulfide.

[0055] Comparative Example 2 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the binder does not contain nano-silicon dioxide.

[0056] Comparative Example 3 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the binder is a mixture of epoxy resin, polyphenylene sulfide and nano-silicon dioxide in a mass ratio of 4:5:2.

[0057] Comparative Example 4 A method for preparing a high-density bonded magnet: The difference from Example 1 is that the binder is a mixture of epoxy resin, polyphenylene sulfide and nano-silicon dioxide in a mass ratio of 7:2:2.

[0058] Comparative Example 5 A method for preparing a high-density bonded magnet: The difference from Example 1 is that 65 kg of NdFeB alloy powder and 35 kg of binder are added to a twin-screw internal mixer.

[0059] Comparative Example 6 A method for preparing a high-density bonded magnet: The difference from Example 1 is that 90 kg of NdFeB alloy powder and 10 kg of binder are added to a twin-screw internal mixer.

[0060] Performance testing Density is measured by the Archimedes displacement method, referring to GB / T 3850-2015; The magnetic energy product (BH)max was measured by a vibrating sample magnetometer (model Lake Shore 7404); Flexural strength is measured by three-point bending method, referring to GB / T 6569-2006; 200℃ magnetic loss is calculated by measuring the change in magnetic flux before and after aging at 200℃ for 1000h, referring to IEC 60404-5; The salt spray life is measured by a neutral salt spray test with a salt solution of 5wt% NaCl, pH=6.5-7.2, and an ambient temperature of 35±2°C, referring to GB / T 10125-2021.

[0061] All test data are collected and summarized in Table 2.

[0062] Table 2 Test data Combining Example 1 and Comparative Example 1 with Table 2, it can be seen that polyphenylene sulfide is responsible for high-temperature dimensional stability and crystallization support. Its absence results in the resin network being unable to effectively resist high-temperature creep after hot pressing, resulting in a decrease in magnetic powder orientation and concentration of interfacial stress. At the same time, the epoxy resin has insufficient temperature resistance after curing alone, and the magnet is easily demagnetized at high temperatures.

[0063] Combining Example 1 and Comparative Example 2 with Table 2, it can be seen that nano-SiO2 enhances mechanical properties through interface bonding and pore filling, and its absence leads to weakening of the interface bonding force between the binder and the magnetic powder, increased porosity, decreased stress transfer efficiency, and reduced protective performance.

[0064] Combining Example 1 and Comparative Examples 3-4 with Table 2, it can be seen that a too high proportion of epoxy resin leads to insufficient high-temperature stability, and a too high proportion of PPS leads to insufficient low-temperature curing strength, both of which destroy the resin synergistic effect and affect the magnetic powder orientation and interface strength.

[0065] Combining Example 1 and Comparative Examples 5-6 with Table 2, it can be seen that when the magnetic powder content is too low, the binder cannot fully coat the particles, resulting in a decrease in density and magnetic properties; when the magnetic powder content is too high, insufficient binder leads to increased interface defects and a collapse of mechanical properties.

[0066] Combining Examples 1-3 with Table 2, it can be seen that a high magnetic powder content improves magnetic properties, but a reduced binder ratio leads to a weakened interface bonding force. The 78% magnetic powder ratio in Example 1 has the most balanced performance.

[0067] Combining Example 1 and Example 4 with Table 2, it can be seen that insufficient coercivity leads to an aggravated high-temperature demagnetization tendency, a decrease in magnetic domain stability, and insufficient pinning effect when Dy grain boundary diffusion does not reach a threshold.

[0068] Combining Example 1 and Examples 5-8 with Table 2, it can be seen that Dy enrichment at the grain boundary needs to be precisely controlled at 0.5-1.2 wt %. Excessive Dy enrichment will lead to remanence loss, while insufficient Dy enrichment will result in insufficient coercivity improvement.

[0069] Combining Example 1 and Example 9 with Table 2, it can be seen that the introduction of Pr increases the Curie temperature, Co / Ga suppresses the soft magnetic phase, and the magnetic temperature stability and oxidation resistance of the non-Pr system decrease. 14 The effect of B is more obvious than that of conventional NdFeB alloy powder.

[0070] From Example 1 and Examples 10-11 and Table 2, it can be seen that increasing the proportion of PPS enhances the high-temperature mechanical properties, and increasing the proportion of epoxy resin optimizes the low-temperature curing strength. The component ratio of the adhesive in Example 1 of the present application is the best in the present application.

[0071] It can be seen from Example 1 and Example 12 and Table 2 that the amorphous silicon carbide layer blocks water and oxygen penetration, while reducing surface friction and wear, thereby extending the life of the magnet.

[0072] Combining Example 1 and Example 13 with Table 2, it can be seen that after activation, the hydroxyl density on the SiO2 surface increases, the covalent bonding with the epoxy resin is enhanced, and the interfacial stress transfer efficiency is improved.

[0073] From Example 1 and Example 14 and Table 2, it can be seen that the lack of staged curing results in insufficient cross-linking of the epoxy resin and insufficient crystallinity of the PPS, and increases in residual interfacial thermal stress.

[0074] From Example 1 and Example 15 and Table 2, it can be seen that Al2O3 blocks the corrosive medium, TiO2 improves the surface hardness, and the composite film layer synergistically enhances environmental tolerance.

[0075] From Example 1 and Example 16 and Table 2, it can be seen that ultrasonic vibration promotes the compaction and arrangement of magnetic powder, reduces porosity, and assists the resin in filling micro gaps, thereby improving overall performance.

[0076] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are protected by the patent law.

Claims

1. A high-density bonded magnet, characterized in that: The raw materials include 70-85% of neodymium iron boron alloy and 15-30% of binder in terms of mass percentage. The binder includes epoxy resin, polyphenylene sulfide and nano silicon dioxide mixed in a mass ratio of (5-6):(3-4):

2.

2. The high-density bonded magnet according to claim 1, characterized in that: The NdFeB alloy is (Nd, Pr)2(Fe, Co, Ga) 14 B. The grain boundary diffusion Dy content of the NdFeB alloy is 0.5-1.2 wt%.

3. The high-density bonded magnet according to claim 1, characterized in that: The nano-silicon dioxide is subjected to plasma activation treatment, and the surface hydroxyl density of the nano-silicon dioxide after the plasma activation treatment is ≥3 / nm².

4. The high-density bonded magnet according to claim 1, characterized in that: The coercive force of the NdFeB alloy is ≥20kOe, and the NdFeB alloy is made of metal powder with a particle size of 3-5 μm.

5. The high-density bonded magnet according to claim 1, characterized in that: The surface of the NdFeB alloy is coated with an amorphous silicon carbide layer with a thickness of 5-10 nm.

6. The method for preparing a high-density bonded magnet according to any one of claims 1 to 5, characterized in that: The following steps are involved: Mixing epoxy resin, polyphenylene sulfide and nano-silicon dioxide to form a binder; Heat and mix the binder and NdFeB alloy at 80-100°C. After mixing, place the mixture in a mold, pre-press at 40-50 MPa for 3-5 minutes, and then hot-press at 800-1200 MPa for 5-10 minutes. The hot-pressed product is heated and solidified in an axial magnetic field of 0.5-1.5 T to obtain a high-density bonded magnet.

7. The method for preparing a high-density bonded magnet according to claim 5, wherein: The specific method of heating and curing the product is: first heating the product to 120-130° C., keeping it warm for 1-2 hours, then heating it to 280-290° C., keeping it warm for 2-3 hours.

8. The method for preparing a high-density bonded magnet according to claim 5, wherein: The surface of the high-density bonded magnet obtained by heating and curing is also coated with an aluminum oxide / titanium dioxide composite film layer with a thickness of 5-10 nm.

9. The method for preparing a high-density bonded magnet according to claim 5, wherein: During the hot pressing process, ultrasonic vibration of 10-50 kHz and an amplitude of 2-5 μm is simultaneously applied.

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