A method for preparing multi-principal-phase sintered NdFeB permanent magnets with core-shell structure

Abstract

This application relates to the field of rare earth permanent magnet materials and discloses a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure, comprising the following steps: S1 Mixing: Under a protective atmosphere, the main phase matrix powder and the kinetically modified composite powder are mixed uniformly to obtain a mixed powder; S2 Shaping: The mixed powder is oriented and pressed into shape under an orientation magnetic field to obtain a green blank; S3 Sintering: The green blank is placed in a vacuum sintering furnace for sintering, the sintering process including a first stage, a second stage, and a third stage; S4 Heat Treatment: The sintered magnet is subjected to aging treatment. By introducing nano-titanium powder or nano-zirconium powder as a thermal trigger in the kinetically modified composite powder, its hydrogen absorption and exothermic properties provide reaction activation energy at the microscopic interface. Combined with a heavy rare earth-iron ratio close to the eutectic point, the modifier is promoted to generate a heavy rare earth-rich liquid phase in situ at a lower temperature.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet materials technology, specifically to a method for preparing a multi-principal-phase sintered NdFeB permanent magnet with a core-shell structure. Background Technology

[0002] Sintered NdFeB permanent magnets, due to their extremely high energy product, are widely used in high-end fields such as drive motors for new energy vehicles, wind turbines, and industrial servo motors. To meet the requirements of high-temperature applications, the intrinsic coercivity of the magnets must be significantly improved. Traditional processes typically involve directly adding heavy rare earth elements such as dysprosium or terbium during the alloy smelting stage. However, this results in the uniform incorporation of these heavy rare earth elements into the main phase grains, creating antiferromagnetic coupling with iron atoms. While this increases coercivity, it also significantly reduces the remanent magnetization of the magnet. Furthermore, the high cost of heavy rare earth elements substantially increases production costs.

[0003] To balance high coercivity and high remanence, constructing a hard magnetic shell structure rich in heavy rare earth elements on the surface of the main phase grains using grain boundary modulation technology has become a key research focus in the industry. While current dual-alloy methods or mixed powder sintering techniques aim to achieve grain boundary modification, in actual preparation, the slow diffusion rate between solid-phase powders often necessitates high sintering temperatures or long holding times to achieve densification. This high-temperature, long-duration process can easily lead to excessive diffusion of heavy rare earth elements into the grain core, weakening the magnetic performance advantages of the core-shell structure. It may also induce abnormal growth of the main phase grains, damaging the microstructure uniformity of the magnet.

[0004] As another mainstream technology, grain boundary diffusion technology can form a good core-shell structure by coating the surface of sintered magnets with heavy rare earth sources and performing diffusion heat treatment. However, this technology relies on the concentration gradient of heavy rare earth elements from the outer surface to the interior, and its penetration depth is strictly limited by the physical diffusion mechanism, usually only applicable to thin-walled magnets. For thick magnets, the diffusing agent is difficult to effectively penetrate to the central region of the magnet, resulting in insufficient coercivity in the core, which in turn causes a deterioration in the squareness of the overall demagnetization curve of the magnet. In addition, existing powder mixing and addition processes often face the problem of insufficient wettability of modifiers during sintering, making it difficult to form a continuous and uniform liquid phase coating channel before the main phase particles close the pores, which easily causes local component segregation and pore residue, affecting the final density and overall performance of the magnet. Summary of the Invention

[0005] The technical problem solved by this invention is that in the existing sintered NdFeB preparation process, the dual alloy method is prone to dilution of the rare earth concentration in the main phase due to the uncontrollable diffusion of heavy rare earth elements, making it difficult to form a clear core-shell structure; while conventional grain boundary penetration technology is limited by the physical diffusion distance, which cannot effectively improve the coercivity of the central region of large-size magnets, and has the problem of low production efficiency.

[0006] To address the above problems, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides a kinetically modified composite powder, the kinetically modified composite powder comprising the following components: heavy rare earth hydride powder, nano iron powder, and nano thermal triggering agent powder; wherein, the heavy rare earth hydride powder is selected from one or a combination of terbium hydride powder and dysprosium hydride powder; the nano thermal triggering agent powder is selected from one or a combination of nano titanium powder and nano zirconium powder; the molar ratio of the heavy rare earth hydride powder to the nano iron powder is 68:32 to 72:28; and the mass of the nano thermal triggering agent powder is 3.0wt%-5.0wt% of the total mass of the heavy rare earth hydride powder, the nano iron powder, and the nano thermal triggering agent powder.

[0008] By adopting the above technical solution, the kinetically modified composite powder utilizes the thermodynamic properties of the reactions between its components to construct a reaction system capable of generating a low-melting-point liquid phase in situ and spontaneously releasing heat during sintering. The specific mechanism of action is as follows:

[0009] Eutectic composition design: The molar ratio of heavy rare earth hydrides to nano-iron powder is controlled between 68:32 and 72:28, which corresponds to the eutectic region in the phase diagram of heavy rare earth-iron binary alloys. Compared with elemental heavy rare earth metals, this mixed powder has a lower melting temperature and can form a liquid phase at a lower sintering temperature stage.

[0010] Hydrogen absorption and exothermic triggering mechanism: Nano-titanium powder or nano-zirconium powder is introduced as a thermal trigger. During the heating process, heavy rare earth hydrides decompose and release hydrogen gas. The adjacent nano-titanium powder or nano-zirconium powder captures the hydrogen gas and undergoes a hydrogen absorption reaction. The above hydrogen absorption reaction is an exothermic reaction. The released reaction heat forms a local high-temperature region at the particle contact interface, thus providing activation energy for the reaction before the overall furnace temperature reaches the liquidus temperature. This induces the surrounding heavy rare earth components and iron components to undergo a eutectic reaction in advance, generating a heavy rare earth-rich liquid phase.

[0011] Preferably, the average particle size D50 of the heavy rare earth hydride powder is 0.5μm-0.9μm; the average particle size D50 of the nano iron powder is 40nm-80nm; and the average particle size D50 of the nano thermal trigger powder is 40nm-100nm.

[0012] By adopting the above technical solution, the combination of micron-sized heavy rare earth hydrides with nano-sized iron powder and thermal trigger powder increases the reaction contact surface area, which helps to reduce the onset temperature of the solid-phase reaction and ensures that the reaction proceeds more uniformly and thoroughly.

[0013] Secondly, the present invention provides a method for preparing kinetic modified composite powder, comprising the following steps: under a protective atmosphere with an oxygen content of less than 50 ppm, weighing the heavy rare earth hydride powder, nano iron powder and nano thermal trigger powder in proportion, adding an antioxidant dispersant, and mixing at a speed of 15 rpm to 25 rpm for 4 to 8 hours.

[0014] By adopting the above technical solution, under low-oxygen environment and low-speed mixing conditions, van der Waals forces and electrostatic adsorption are used to make nano-iron powder and nano-thermal trigger powder uniformly adhere to the surface of micron-sized heavy rare earth hydride particles, forming a coating structure. This prevents the nano-sized powder from oxidizing or agglomerating in subsequent processing, ensures close contact of each component at the microscale, and provides a structural basis for the in-situ reaction in the subsequent sintering process.

[0015] Thirdly, the present invention provides a method for preparing a multi-principal-phase sintered NdFeB permanent magnet with a core-shell structure, comprising the following steps:

[0016] S1 Mixing: Under a protective atmosphere, the main phase matrix powder and the kinetically modified composite powder are mixed evenly to obtain a mixed powder;

[0017] S2 molding: The mixed powder is oriented and pressed into shape under an orientation magnetic field to obtain a green body;

[0018] S3 Sintering: The green body is placed in a vacuum sintering furnace for sintering. The sintering process includes a first stage, a second stage and a third stage. The first stage includes raising the temperature to 350℃-650℃ and performing pulse charge-discharge cycle operation within this temperature range.

[0019] S4 heat treatment: Aging treatment is performed on the sintered magnet.

[0020] By employing the above technical solution, this preparation method solves the problems of uneven distribution of heavy rare earth elements at the main phase grain boundaries and difficulty in penetrating into the interior of large-size magnets through the coupling mechanism of thermodynamics and gas dynamics. The specific process and principle are as follows:

[0021] Pulsed dehydrogenation and thermal coupling triggering (first stage): In the temperature range of 350℃-650℃, heavy rare earth hydrides decompose and release hydrogen gas. At this time, a pulsed charge-discharge cycle is executed, using pressure fluctuations to disrupt the gas balance between powder particles and promote the release of hydrogen gas from deep within the green body. Simultaneously, the released hydrogen gas comes into contact with the nano-thermal triggering agent distributed at the grain boundaries and undergoes an endothermic and exothermic reaction. The released heat generates a local temperature rise at the micro-interface, activating heavy rare earth atoms and iron atoms, and promoting the eutectic reaction.

[0022] Pressure differential-driven liquid phase infiltration (second stage): As the temperature rises, the activated modified powder generates a low-viscosity heavy rare earth-iron eutectic liquid phase in situ. The pressure gradient established by the pulse operation and the channels formed by gas discharge provide the physical driving force for the liquid phase flow, causing the heavy rare earth-rich liquid phase to spread along the grain boundary gaps and penetrate into the magnet, encapsulating the main phase matrix powder particles.

[0023] In-situ shell formation (third stage): During the high-temperature densification process, the heavy rare earth-rich liquid phase distributed at the grain boundaries undergoes a diffusion replacement reaction with the surface of the main phase matrix powder. Heavy rare earth atoms replace neodymium atoms on the surface of the main phase, forming a shell structure with a high anisotropic field, which restricts the entry of heavy rare earth elements into the grain core, thereby improving coercivity without reducing remanence.

[0024] Preferably, in step S1, the amount of the kinetic modified composite powder added is 1.0wt%-3.0wt% of the mass of the main phase matrix powder; the main phase matrix powder is a PrNd-Fe-B alloy powder, the average particle size D50 of the main phase matrix powder is 3.0μm-4.0μm, and the main phase matrix powder does not contain heavy rare earth elements Dy and Tb.

[0025] By adopting the above technical solution, using a main phase without heavy rare earth elements in combination with a modifier containing high concentrations of heavy rare earth elements, a hard magnetic shell is constructed only at the grain boundaries while ensuring that the main phase has a high saturation magnetization intensity, thus achieving efficient utilization of heavy rare earth elements.

[0026] Preferably, in the first stage of step S3, the specific method of the pulse charging and discharging gas cycle operation is as follows: inert gas is charged into the furnace to raise the pressure to 300Pa-800Pa, and then vacuum is drawn to below 20Pa, with the cycle period being once every 5 minutes to 20 minutes.

[0027] By adopting the above technical solution, the mechanical force generated by the pressure fluctuation of 300Pa-800Pa can destroy the oxide film on the particle surface and loosen the powder accumulation. Combined with the vacuuming process, the hydrogen gas and adsorbed water vapor generated by the reaction can be carried out, reducing the final oxygen content of the magnet and reducing porosity defects.

[0028] Preferably, the second stage described in step S3 specifically includes: stopping the pulse operation and maintaining a vacuum level better than... The temperature is raised to 700℃-950℃ and held for 60-120 minutes; the third stage described in step S3 specifically includes: continuing to raise the temperature to 1040℃-1080℃ and holding for 3-5 hours.

[0029] By adopting the above technical solution, a high vacuum and heat preservation are maintained in the liquid phase formation temperature zone (700℃-950℃) to ensure that the eutectic liquid phase fully wets and covers the surface of the main phase matrix powder; in the densification temperature zone (1040℃-1080℃), the magnet is induced to shrink and densify, and the final structural evolution of the shell is completed.

[0030] Preferably, the heat treatment in step S4 specifically includes: first-stage aging: holding at 880℃-920℃ for 2-3 hours; second-stage aging: holding at 490℃-530℃ for 3-5 hours.

[0031] By adopting the above technical solution, the two-stage aging treatment optimizes the distribution and composition of the grain boundary phase, eliminates the internal stress during the sintering process, makes the grain boundary layer more continuous and straight, and fully utilizes the magnetic hardening effect of the core-shell structure.

[0032] This invention provides a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure. It has the following beneficial effects:

[0033] 1. This invention introduces nano-titanium powder or nano-zirconium powder as a thermal trigger into a kinetically modified composite powder. Utilizing their hydrogen absorption and exothermic properties, they provide activation energy at the microscopic interface. Combined with a heavy rare earth-iron ratio close to the eutectic point, this promotes the in-situ generation of a heavy rare earth-rich liquid phase at a lower temperature. This in-situ liquid phase generation mechanism improves the wettability and uniformity of distribution of heavy rare earth elements on the surface of the main phase particles, promoting the formation of a continuous core-shell structure, thereby enhancing the coercivity of the magnet while reducing the total amount of heavy rare earth added.

[0034] 2. This invention employs a pulsed charge-discharge cyclic sintering process, utilizing periodic pressure fluctuations to disrupt the gas equilibrium state within the powder pack and establish a pressure gradient capable of driving liquid phase flow. This pressure gradient assists the in-situ generated low-viscosity eutectic liquid phase in penetrating into the magnet and spreading along grain boundaries, overcoming the limitations of traditional grain boundary diffusion technology on magnet thickness. This allows even the central region of large-size sintered magnets to achieve effective heavy rare earth modification, improving the overall performance consistency of the magnet.

[0035] 3. This invention utilizes active hydrogen generated from hydride decomposition to in-situ reduce the oxide layer on the surface of the main phase particles, and combines this with pulsed negative pressure operation to forcibly expel the water vapor and residual hydrogen generated in the reaction. This in-situ purification and forced exhaust mechanism reduces the oxygen content and porosity of the sintered magnet, reduces the contamination of the grain boundary phase by non-magnetic oxides, and while ensuring the high density of the magnet, reduces the obstruction of impurities to the movement of magnetic domain walls, which is beneficial to the optimization of magnetic properties. Attached Figure Description

[0036] Figure 1 This is a flowchart of the preparation process of the present invention. Detailed Implementation

[0037] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0039] Praseodymium-neodymium-iron-boron (PrFeB) rapid-solidification thin strips are custom-produced by professional rare-earth permanent magnet alloy manufacturers. Their chemical composition, by mass percentage, includes 30.5% PrFeB, 1.0% boron, 0.2% aluminum, 1.0% cobalt, 0.1% copper, 0.1% gallium, and the balance iron. The alloy strip thickness ranges from 0.25mm to 0.35mm, and the microstructure is a uniformly distributed columnar crystal structure rich in neodymium, free of heavy rare-earth elements. Terbium hydride and dysprosium hydride powders are commercially available high-purity rare-earth compounds with a purity ≥99.9% and an average particle size D50 of 0.5μm-0.9μm. Nano-iron powder has a purity ≥99.9%, an average particle size D50 of 40nm-80nm, and an oxygen content controlled below 3000ppm. Nano-titanium powder and nano-zirconium powder both have a purity ≥99.9% and an average particle size D50 of 40nm-100nm. Oleic acid and zinc stearate are both commercially available analytical grade products.

[0040] Preparation Example 1:

[0041] This preparation example provides a kinetically modified composite powder, including the following steps:

[0042] In a high-purity nitrogen protective glove box with an oxygen content of less than 50 ppm, terbium hydride powder and nano iron powder were accurately weighed, and the molar ratio of the two was controlled to be 70:30 (terbium hydride: iron). Then, nano titanium powder was weighed, and its addition amount was 4.0 wt% of the total mass of terbium hydride powder, nano iron powder and nano titanium powder. The three powders weighed above were placed in a three-dimensional motion mixer, and 0.1% of the total mass of the powder was added as an antioxidant dispersant. The mixture was mixed at 20 rpm for 6 hours to obtain the kinetically modified composite powder.

[0043] Preparation Example 2:

[0044] This preparation example provides a kinetically modified composite powder, including the following steps:

[0045] In a high-purity nitrogen protective glove box with an oxygen content of less than 50 ppm, dysprosium hydride powder and nano iron powder were accurately weighed, and the molar ratio of the two was controlled to be 68:32 (dysprosium hydride: iron). Then, nano zirconium powder was weighed, and its addition amount was 3.0 wt% of the total mass of dysprosium hydride powder, nano iron powder and nano zirconium powder. The three powders weighed above were placed in a three-dimensional motion mixer, and 0.1% of zinc stearate as an antioxidant dispersant was added to the powder. The mixture was mixed at 15 rpm for 8 hours to obtain the kinetically modified composite powder.

[0046] Preparation Example 3:

[0047] This preparation example provides a kinetically modified composite powder, including the following steps:

[0048] In a high-purity nitrogen protective glove box with an oxygen content of less than 50 ppm, terbium hydride powder and nano iron powder were accurately weighed, and the molar ratio of the two was controlled to be 72:28 (terbium hydride: iron). Then, nano titanium powder was weighed, and its addition amount was 5.0 wt% of the total mass of terbium hydride powder, nano iron powder and nano titanium powder. The three powders weighed above were placed in a three-dimensional motion mixer, and 0.05% of the total mass of the powder was added as an antioxidant dispersant. The mixture was mixed at 25 rpm for 4 hours to obtain the kinetically modified composite powder.

[0049] Example 1:

[0050] This embodiment provides a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure, including the following steps:

[0051] (1) Mixing: In a protective atmosphere of high-purity nitrogen with an oxygen content of less than 50 ppm, the main phase matrix powder obtained by the above-mentioned praseodymium-neodymium iron boron rapid solidification thin strip was taken and 2.0 wt% of the kinetic modified composite powder obtained in Preparation Example 1 was added. The mixture was mixed in a three-dimensional motion mixer at a speed of 20 rpm for 6 hours to obtain the mixed powder.

[0052] (2) Molding: The mixed powder is placed in a magnetic field orientation molding machine, oriented and pressed under an orientation magnetic field of 2.0T, and then subjected to cold isostatic pressing under a pressure of 200MPa to obtain a green body;

[0053] (3) Sintering: The green billets are loaded into a vacuum sintering furnace and evacuated until a vacuum is reached. The sintering process is as follows:

[0054] Phase 1 (Pulse Dehydrogenation and Thermal Triggering): The temperature is increased from room temperature to 650°C at a rate of 3°C / min; within the temperature range of 350°C to 650°C, pulse charge-discharge cycles are performed. Each cycle includes charging the furnace with argon gas to 600Pa (charging time 30 seconds), followed by rapid evacuation to below 10Pa (evacuation time 90 seconds). The cycle is repeated every 15 minutes.

[0055] Second stage (liquid phase wetting): Stop pulse operation and maintain the vacuum level inside the furnace better than The temperature was increased to 900℃ at a rate of 5℃ / min and held for 90 minutes.

[0056] Third stage (densification): Continue heating to 1060℃, maintaining a vacuum level better than... Keep warm for 4 hours, then fill with argon gas and cool rapidly to below 100°C;

[0057] (4) Heat treatment: The sintered magnet was kept at 900℃ for 2 hours for a first-stage aging treatment and then air-cooled; then it was kept at 510℃ for 4 hours for a second-stage aging treatment and then rapidly cooled to room temperature to obtain the multi-phase sintered NdFeB permanent magnet with core-shell structure.

[0058] Example 2:

[0059] This embodiment provides a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure, including the following steps:

[0060] (1) Mixing: In a protective atmosphere of high-purity nitrogen with an oxygen content of less than 50 ppm, the above main phase matrix powder was taken and 1.0 wt% of the kinetic modified composite powder obtained in Preparation Example 2 was added. The powder was mixed in a three-dimensional motion mixer at a speed of 15 rpm for 8 hours to obtain the mixed powder.

[0061] (2) Molding: The mixed powder is placed in a magnetic field orientation molding machine, oriented and pressed under an orientation magnetic field of 1.8T, and then subjected to cold isostatic pressing under a pressure of 180MPa to obtain a green blank.

[0062] (3) Sintering: The green billets are loaded into a vacuum sintering furnace and evacuated until a vacuum is reached. The sintering process is as follows:

[0063] Phase 1 (Pulse Dehydrogenation and Thermal Triggering): The temperature is increased from room temperature to 600°C at a rate of 4°C / min; within the temperature range of 350°C to 600°C, pulse charge-discharge cycles are performed. Each cycle includes charging the furnace with argon gas to 500Pa (charging time 30 seconds), followed by rapid evacuation to below 10Pa (evacuation time 60 seconds). The cycle is repeated every 10 minutes.

[0064] Second stage (liquid phase wetting): Stop pulse operation and maintain the vacuum level inside the furnace better than The temperature was increased to 850℃ at a rate of 6℃ / min and held for 120 minutes.

[0065] Third stage (densification): Continue heating to 1050℃, maintaining a vacuum level better than... Keep warm for 5 hours, then fill with argon gas and cool rapidly to below 100°C;

[0066] (4) Heat treatment: The sintered magnet was kept at 880℃ for 3 hours for first-stage aging treatment and then air-cooled; then kept at 490℃ for 5 hours for second-stage aging treatment and then rapidly cooled to room temperature to obtain the multi-phase sintered NdFeB permanent magnet with core-shell structure.

[0067] Example 3:

[0068] This embodiment provides a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure, including the following steps:

[0069] (1) Mixing: In a protective atmosphere of high-purity nitrogen with an oxygen content of less than 50 ppm, the above main phase matrix powder was taken and 3.0 wt% of the kinetic modified composite powder obtained in Preparation Example 3 was added. The powder was mixed in a three-dimensional motion mixer at a speed of 25 rpm for 4 hours to obtain the mixed powder.

[0070] (2) Molding: The mixed powder is placed in a magnetic field orientation molding machine, oriented and pressed under an orientation magnetic field of 2.2T, and then subjected to cold isostatic pressing under a pressure of 220MPa to obtain a green body;

[0071] (3) Sintering: The green billets are loaded into a vacuum sintering furnace and evacuated until a vacuum is reached. The sintering process is as follows:

[0072] Phase 1 (Pulse Dehydrogenation and Thermal Triggering): The temperature is increased from room temperature to 650°C at a rate of 5°C / min; within the temperature range of 400°C to 650°C, pulse charge-discharge cycles are performed. Each cycle includes charging the furnace with argon gas to 800Pa (charge time 40 seconds), followed by rapid evacuation to below 10Pa (evacuation time 80 seconds). The cycle is once every 12 minutes.

[0073] Second stage (liquid phase wetting): Stop pulse operation and maintain the vacuum level inside the furnace better than The temperature was increased to 950℃ at a rate of 8℃ / min and held for 60 minutes.

[0074] Third stage (densification): Continue heating to 1070℃, maintaining a vacuum level better than... Keep warm for 3 hours, then fill with argon gas and cool rapidly to below 100°C;

[0075] (4) Heat treatment: The sintered magnet was kept at 920℃ for 2 hours for first-stage aging treatment and then air-cooled; then kept at 530℃ for 3 hours for second-stage aging treatment and then rapidly cooled to room temperature to obtain the multi-phase sintered NdFeB permanent magnet with core-shell structure.

[0076] Example 4:

[0077] This embodiment provides a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure. The difference from Embodiment 1 lies only in adjusting the pulse parameters during the sintering process to verify the effect of high-frequency pressure fluctuations. The specific steps are as follows:

[0078] (1) The mixing and (2) molding steps are exactly the same as in Example 1;

[0079] (3) Sintering: In the first stage (pulse dehydrogenation and thermal triggering), the temperature range is 350°C to 650°C. The pulse cycle is adjusted to once every 5 minutes. Gas is charged to 500Pa (20 seconds) and vacuumed to below 10Pa (40 seconds). The parameters of the other second and third stages are the same as those in Example 1.

[0080] (4) The heat treatment steps are exactly the same as those in Example 1.

[0081] Example 5:

[0082] This embodiment provides a method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure. The difference from Embodiment 1 is only in the adjustment of the pulse parameters during the sintering process to verify the effect of low pressure difference and long cycle. The specific steps are as follows:

[0083] (1) The mixing and (2) molding steps are exactly the same as in Example 1;

[0084] (3) Sintering: In the first stage (pulse dehydrogenation and thermal triggering), the temperature range is 350°C to 650°C. The pulse cycle is adjusted to once every 20 minutes. Gas is charged to 300Pa (60 seconds) and vacuumed to below 20Pa (120 seconds). The parameters for the other second and third stages are the same as in Example 1.

[0085] (4) The heat treatment steps are exactly the same as those in Example 1.

[0086] Comparative Example 1:

[0087] Compared with Example 1, the difference is that no kinetic modified composite powder was added, and only the main phase matrix powder was used for molding and sintering. The remaining steps and parameters are the same.

[0088] (Purpose: To serve as a blank control and demonstrate the basic improvement effect of adding modifiers.)

[0089] Comparative Example 2:

[0090] Compared with Example 1, the difference is that the kinetic modified composite powder does not contain nano-titanium powder, but is only made of terbium hydride powder and nano-iron powder mixed in a molar ratio of 70:30. The other raw material ratios and preparation process parameters are the same.

[0091] (Objective: To verify whether the eutectic component alone can achieve the desired wetting effect without the "hydrogen absorption and exothermic triggering agent", and to prove the necessity of "thermal triggering".)

[0092] Comparative Example 3:

[0093] Compared with Example 1, the difference is that in the first stage of the sintering step (350℃-650℃), the pulse charge and discharge cycle operation is not performed, but a constant dynamic vacuum mode is adopted (maintaining a vacuum degree better than 10Pa). The remaining steps and parameters are the same.

[0094] (Objective: To verify the difference in penetration depth and uniformity of the liquid phase at grain boundaries without "pulse pneumatic assistance," and to demonstrate the necessity of "pneumatic coupling.")

[0095] Comparative Example 4:

[0096] Compared with Example 1, the difference is that the kinetic modified composite powder does not contain nano iron powder and nano titanium powder, but only adds an equimolar amount of terbium hydride powder, while the other steps and parameters are the same.

[0097] (Objective: To simulate the traditional technique of simply adding heavy rare earth hydrides and compare the advantages of the "in-situ eutectic" strategy of this invention.)

[0098] Comparative Example 5:

[0099] Compared with Example 1, the difference is that the amount of nano-titanium powder added in the kinetic modified composite powder is adjusted to 15.0 wt% (severely excessive) of the total mass of terbium hydride powder, nano-iron powder and nano-titanium powder, while the other steps and parameters are the same.

[0100] (Objective: To verify the negative impact of impurities on magnetic properties when the content of the thermal triggering agent exceeds the preferred range (3-5%) of this invention, thus supporting the rationality of the numerical range.)

[0101] Comparative Example 6:

[0102] Compared with Example 1, the difference is that in the kinetic modified composite powder, the molar ratio of terbium hydride powder to nano iron powder is adjusted to 90:10 (deviating from the low eutectic point), while the other steps and parameters are the same.

[0103] (Objective: To verify the problem of decreased wettability due to increased liquid melting point when the eutectic ratio is not at the eutectic point, thus demonstrating the crucial role of the "eutectic formulation".)

[0104] Comparative Example 7:

[0105] Compared with Example 3, the difference is that in the first stage of the sintering step (pulse dehydrogenation and thermal triggering), only the gas filling operation is performed and the gas extraction operation is not performed (to maintain a slightly positive pressure atmosphere), while the remaining steps and parameters are the same.

[0106] (Objective: To demonstrate that "negative pressure suction" during the pulse process is essential for removing the airbag and dragging the liquid phase, and that atmosphere protection alone is insufficient to achieve deep penetration.)

[0107] Test Example 1: Analysis of Sintering Densification Behavior and Impurity Content

[0108] Experimental steps

[0109] Sintered magnet samples prepared in Examples 1 to 5 and Comparative Examples 1 to 7 were selected as test objects. First, the bulk density of the samples was determined using the Archimedes displacement method. Before testing, the surface of the sintered magnet samples was polished to completely remove the oxide scale. The mass of the samples in air and the suspended mass in distilled water were weighed using an electronic balance with an accuracy of 0.0001 g. The bulk density was calculated based on the mass difference and the density of water. Five samples were randomly selected from each experimental group for testing, and the arithmetic mean was taken. Subsequently, the dimensions of the green blank and the sintered sample were measured using a digital micrometer with an accuracy of 0.01 mm. The linear shrinkage rate of the samples in the orientation direction and perpendicular to the orientation direction was calculated. Finally, the oxygen content was determined using an inert gas melting-infrared absorption method. Small pieces of 0.1 g to 0.2 g were cut from the center of the sintered magnet sample, polished, and then placed in an oxygen, nitrogen, and hydrogen analyzer for measurement.

[0110] Test Results

[0111] The specific test data for each experimental group of samples are shown in the table below:

[0112] Table 1 shows the test results of sintering density and oxygen content of samples in each experimental group.

[0113] Results Analysis and Conclusions

[0114]

[0115] The test data and preparation process mechanism in Table 1 were analyzed. The sample densities of Examples 1 to 5 were all higher than 7.57 g / cm³, and the oxygen content was controlled below 950 ppm, indicating that the thermo-gas dynamic coupling process promotes the sintering densification process.

[0116] Comparing the data from Example 1 with Comparative Examples 2 and 4, under the same sintering temperature and time conditions, the density of Example 1 was higher than that of the sample containing only terbium hydride or without nano-titanium. During the hydrogen absorption reaction, nano-titanium releases a chemical enthalpy change, generating a localized heating effect in the micro-region, causing the terbium-iron eutectic liquid phase to form before the main phase particles close their pores. The early appearance of the liquid phase reduces the resistance to particle rearrangement and fills the interparticle gaps, improving densification efficiency. In Comparative Example 5, because the titanium powder content reached 15%, the refractory titanium-boron compound generated in the reaction hindered grain boundary migration and volume shrinkage, resulting in a decrease in density.

[0117] Comparing the data of Example 1 with Comparative Examples 3 and 7, the residual oxygen content of Example 1 was lower than that of Comparative Examples 3 and 7. Comparative Example 3 used a constant vacuum mode, making it difficult for the water vapor and hydrogen generated in the reaction to escape from deep within the green body. Comparative Example 7 only performed a gas filling operation without a gas extraction operation, failing to establish a pressure gradient capable of driving gas expulsion. The pulsed negative pressure operation used in Example 1 disrupted the gas balance between powders through periodic pressure fluctuations, forcibly expelling the reactant gases from the pores, reducing the oxygen content and minimizing residual porosity.

[0118] Comparing the data from Example 1 and Comparative Example 6, the density of the sample in Example 1 is higher than that in Comparative Example 6. At a molar ratio of 70:30, the modifier is in the low-melting-point eutectic region and exhibits good fluidity. When the ratio deviates from the eutectic point, the liquid phase formation temperature increases, and the liquid phase volume fraction decreases under the same process conditions, leading to a decrease in sintering shrinkage. By inducing in-situ eutectic liquid phase formation through hydrogen absorption and exothermic reaction of nano-titanium, coupled with pulsed pressure differential to remove pore gases, a material transport channel is established during sintering, reducing oxygen content and increasing density.

[0119] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure, characterized in that, The method employs a main phase matrix powder and a kinetically modified composite powder, wherein the kinetically modified composite powder includes heavy rare earth hydride powder, nano iron powder, and nano thermal trigger powder. The heavy rare earth hydride powder is selected from one or a combination of terbium hydride powder and dysprosium hydride powder. The nano thermal trigger powder is selected from one or a combination of two of nano titanium powder or nano zirconium powder; The molar ratio of the heavy rare earth hydride powder to the nano iron powder is 68:32 to 72:

28. The mass of the nano-thermal trigger powder is 3.0wt%-5.0wt% of the total mass of the heavy rare earth hydride powder, nano-iron powder, and nano-thermal trigger powder. Includes the following steps: S1 Mixing: Under a protective atmosphere, the main phase matrix powder and the kinetic modified composite powder are mixed evenly to obtain a mixed powder; S2 molding: The mixed powder is oriented and pressed into shape under an orientation magnetic field to obtain a green body; S3 sintering: The green blank is placed in a vacuum sintering furnace for sintering, and the sintering process includes a first stage, a second stage and a third stage; The first stage includes raising the temperature to 350℃-650℃ and performing pulse charge-discharge cycle operation within this temperature range; In step S3, the specific method of the pulse charging and discharging cycle operation is as follows: inert gas is charged into the furnace to raise the pressure to 300Pa-800Pa, and then the vacuum is evacuated to below 20Pa. The cycle is once every 5 minutes to 20 minutes. S4 heat treatment: Aging treatment is performed on the sintered magnet.

2. The method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that, The average particle size D50 of the heavy rare earth hydride powder is 0.5μm-0.9μm; the average particle size D50 of the nano iron powder is 40nm-80nm; and the average particle size D50 of the nano thermal trigger powder is 40nm-100nm.

3. The method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that: Under a protective atmosphere with an oxygen content of less than 50 ppm, weigh out heavy rare earth hydride powder, nano iron powder and nano thermal trigger powder in proportion, add antioxidant dispersant, and mix at a speed of 15 rpm-25 rpm for 4-8 hours.

4. The method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that, The main phase matrix powder mentioned in step S1 is a PrNd-Fe-B alloy powder with an average particle size D50 of 3.0μm-4.0μm, and the main phase matrix powder does not contain heavy rare earth elements Dy and Tb.

5. The method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that, In step S1, the amount of the kinetic modified composite powder added is 1.0wt%-3.0wt% of the mass of the main phase matrix powder.

6. The method for preparing a multi-principal-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that, The second stage described in step S3 specifically includes: stopping pulse operation and maintaining a vacuum level better than 1×10⁻⁶. -3 Pa, heat to 700℃-950℃ and hold for 60-120 minutes.

7. The method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that, The third stage described in step S3 specifically includes: continuing to heat to 1040℃-1080℃ and holding at that temperature for 3-5 hours.

8. The method for preparing a multi-phase sintered NdFeB permanent magnet with a core-shell structure according to claim 1, characterized in that, The heat treatment described in step S4 specifically includes: Level 1 aging: Keep at 880℃-920℃ for 2-3 hours; Level 2 aging: Keep warm at 490℃-530℃ for 3-5 hours.

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