Preparation method of low-carbon sintered neodymium-iron-boron permanent magnet material

Abstract

This invention belongs to the field of rare earth permanent magnet materials technology, and relates to a method for preparing sintered NdFeB permanent magnet materials with low carbon content. The preparation method includes the following steps: (1) placing a sintered NdFeB powder compact containing organic additives into the extraction medium of a supercritical extraction device for supercritical extraction, so as to dissolve the organic additive molecules originally adsorbed on the surface of the NdFeB powder particles in the extraction medium; (2) subjecting the supercritically extracted sintered NdFeB powder compact to vacuum sintering and tempering treatment. By using the method for preparing sintered NdFeB permanent magnet materials with low carbon content of this invention, it is possible to significantly reduce carbon impurities in sintered NdFeB materials while ensuring the normal addition of powdered organic additives to NdFeB magnetic powder, thereby comprehensively improving the overall magnetic properties of sintered NdFeB permanent magnet materials.

Description

Technical Field

[0001] This invention belongs to the field of rare earth permanent magnet materials technology, and relates to a method for preparing a sintered NdFeB permanent magnet material with low carbon content. Background Technology

[0002] With the rapid expansion of applications for sintered NdFeB magnets and the increasing demand for precision in various fields, the requirements for the comprehensive magnetic properties of NdFeB permanent magnet materials are also rising. NdFeB permanent magnets are rare-earth functional materials, and their comprehensive magnetic properties are closely related to the impurity content of the alloy, especially the carbon impurity content, which has a significant impact on the overall magnetic properties. Therefore, effectively reducing the carbon content in sintered NdFeB permanent magnet products has become a key technical challenge.

[0003] Currently, sintered NdFeB permanent magnet materials are generally produced using powder metallurgy. The preparation process is as follows: alloy melting and strip casting—hydrogenation crushing—air jet milling—magnetic field orientation forming—sintering—tempering. In the air jet milling and magnetic field orientation forming stages, 0.1% to 0.3% of organic surface modifiers are typically added to the NdFeB powder. These organic modifiers are generally composed of single or composite components such as esters, low-chain aliphatic hydrocarbons, and organic dispersants. The functional groups of these compounds interact with the surface atoms of the NdFeB powder particles, forming an organic film. This organic film can prevent oxidation of the material and thus protect its performance. Furthermore, it can act as a "lubricant" by acting as a barrier, reducing the displacement friction resistance between powder particles. During the magnetic field orientation process of the compact, the easy magnetization axis (c-axis) of the particles is arranged as orderly as possible along the direction of the applied magnetic field, thereby improving the orientation degree of the material and the remanence (Br) of the sintered magnet. Simultaneously, it effectively reduces the internal stress of the compact and lowers the rate of defects such as cracks and chipped corners. Generally, the higher the performance grade of the magnet, the finer the grain size of the material is required, the lower the required powder particle size, the higher the requirement for the orientation degree of the compact, and the more difficult the compact forming process. Therefore, the amount of organic additives added is also greater.

[0004] However, during subsequent vacuum sintering, these organic additives are difficult to completely remove from the magnet compact due to the electronic coordination of their functional groups with the surface atoms of the powder particles. Ultimately, during high-temperature sintering, the organic compounds decompose, and the resulting carbon atoms form harmful impurity phases with the matrix material, severely affecting the overall magnetic properties of the material. In fact, the carbon content in the original NdFeB alloy raw materials is relatively low, generally 50-100 ppm. Due to the influence of these organic additives, the carbon content of sintered NdFeB magnets is generally between 700-1500 ppm. Even for companies with excellent technical control that manage to maintain remanence by minimizing the amount of organic additives, the carbon content of their products is often above 600 ppm, and they inevitably incur costs related to surface defects such as cracking and chipping in the compact.

[0005] Therefore, how to significantly reduce carbon impurities in sintered NdFeB materials while ensuring the normal addition of powdered organic additives to NdFeB magnetic powder is a key technical problem that the sintered NdFeB industry urgently needs to solve. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing sintered NdFeB permanent magnet materials with low carbon content, which can significantly reduce carbon impurities in sintered NdFeB materials while ensuring the normal addition of powdered organic additives to NdFeB magnetic powder, thereby comprehensively improving the overall magnetic properties of sintered NdFeB permanent magnet materials.

[0007] To achieve this objective, in a basic implementation, the present invention provides a method for preparing a low-carbon content sintered NdFeB permanent magnet material, the method comprising the following steps: (1) The sintered NdFeB powder compact containing organic additives is placed into the extraction medium of a supercritical extraction device for supercritical extraction, so as to dissolve the organic additive molecules originally adsorbed on the surface of the NdFeB powder particles in the extraction medium. (2) The sintered NdFeB powder compact after supercritical extraction is subjected to vacuum sintering and tempering treatment.

[0008] The relevant principles of this invention are as follows: The present invention preferably includes the following steps: Preparation steps: Use sintered NdFeB powder compacts containing organic additives as raw materials.

[0009] Extraction Steps: The sintered NdFeB powder compact containing organic additives is placed in the extraction vessel of a supercritical fluid extractor. Trifluoromethane gas is rapidly introduced into the extraction system. The high-pressure plunger pump of the extractor pressurizes the trifluoromethane gas in the extraction vessel, bringing it to and well above its supercritical pressure P. c= 4.84 MPa. The system's automatic temperature control device maintains the temperature inside the extraction vessel consistently above its supercritical temperature T. c = 33℃. At this temperature, the trifluoromethane medium in the extraction vessel is in a supercritical state, possessing both the high permeability of a gas and the high solubility of a liquid. Therefore, it can easily penetrate into the interior of the sintered NdFeB powder compact, mix with the organic additive molecules in the compact, and dissolve the thin film of organic additive molecules originally adsorbed on the surface of the NdFeB powder particles in the supercritical trifluoromethane medium.

[0010] Adding an entrainer: To increase the solubility of organic additives in the supercritical trifluoromethane medium, entrainers such as ethyl acetate need to be added to the extraction vessel to balance the polarity difference between the extraction medium and the dissolved organic additives, thereby improving extraction efficiency. Ethyl acetate entrainers are chemically stable, have a high saturated vapor pressure, and a low boiling point (77°C), making them easy to remove through vacuum heat treatment.

[0011] Ultrasonic extraction step: An ultrasonic generator is attached to the extraction vessel to generate ultrasonic waves of a certain frequency and intensity in supercritical trifluoromethane. As the extraction medium penetrates into the sintered NdFeB powder compact, a cavitation effect is formed on the surface of the powder particles inside the compact, which promotes the organic additive molecules to accelerate their detachment from the particle surface and quickly dissolve in the supercritical extraction medium, thereby improving the extraction efficiency.

[0012] The extraction vessel pressurization-depressurization step involves repeatedly and intermittently pressurizing and depressurizing the supercritical trifluoromethane medium inside the extraction vessel. This causes the supercritical extraction medium to repeatedly flow in and out of the compact (during pressurization, pure trifluoromethane supercritical medium is "pressed" into the NdFeB compact, and during depressurization, supercritical trifluoromethane containing dissolved organic additives is released from the compact). This process "washes" the supercritical trifluoromethane containing dissolved organic additive molecules out of the compact, improving the efficiency of the organic additive molecules being discharged from the compact.

[0013] Organic additive separation step: The supercritical trifluoromethane extraction medium containing dissolved organic additives is circulated into the separation vessel of the extractor, and the pressure in the separation vessel is reduced to the critical pressure P. c Below 4.84 MPa, the trifluoromethane medium reverts to its gaseous physicochemical properties and loses its ability to dissolve organic additives. The organic additives precipitate from the trifluoromethane and other gaseous media, condense into droplets, and are discharged from the separator outlet.

[0014] Residual extraction medium and entrainer removal steps: The extracted sintered NdFeB powder compact is placed in a vacuum sintering furnace and heated to 100-150℃ to remove trace amounts of trifluoromethane and ethyl acetate molecules from the compact.

[0015] Vacuum sintering and tempering process: The sintered NdFeB powder compact is subjected to vacuum sintering and tempering in a vacuum sintering furnace.

[0016] In a preferred embodiment, the present invention provides a method for preparing a sintered NdFeB permanent magnet material with low carbon content, wherein in step (1), the organic additive is an ester hydrocarbon agent, and the amount added in the sintered NdFeB powder compact is 0.1-0.3 wt%.

[0017] In a preferred embodiment, the present invention provides a method for preparing a sintered NdFeB permanent magnet material with low carbon content, wherein in step (1), the extraction medium is trifluoromethane.

[0018] In a preferred embodiment, the present invention provides a method for preparing a sintered NdFeB permanent magnet material with low carbon content, wherein in step (1), the temperature of the supercritical extraction is 33-90℃ and the pressure is 4.84-50 MPa.

[0019] In a preferred embodiment, the present invention provides a method for preparing a sintered NdFeB permanent magnet material with low carbon content, wherein in step (1), the supercritical extraction is performed by first holding the material under a pressure of 10-50 MPa for 30-120 minutes, and then holding it under a pressure of 5-9 MPa for 10-30 minutes, and repeating this operation 10-30 times.

[0020] In a preferred embodiment, the present invention provides a method for preparing a sintered NdFeB permanent magnet material with low carbon content, wherein in step (1), an entrainer is added during the supercritical extraction process, the mass of the entrainer being added is 5-15% of the mass of the extraction medium, and the entrainer is ethyl acetate.

[0021] In a preferred embodiment, the present invention provides a method for preparing sintered NdFeB permanent magnet material with low carbon content, wherein in step (1), the supercritical extraction process is also carried out under the assistance of ultrasound, the power of the ultrasound is 1-3kW, and the frequency of the ultrasound is 20-100kHz.

[0022] In a preferred embodiment, the present invention provides a method for preparing a sintered NdFeB permanent magnet material with low carbon content, wherein after the supercritical extraction in step (1) is completed, the extraction medium containing dissolved organic additives, which is separated from the sintered NdFeB powder compact, is depressurized to below the critical pressure of the extraction medium, so that the organic additives are precipitated and released.

[0023] In a preferred embodiment, the present invention provides a method for preparing a low-carbon content sintered NdFeB permanent magnet material, wherein in step (2), The vacuum sintering process is carried out at a temperature of 1050-1090℃ for 5-10 hours, with an absolute pressure of 1×10⁻⁶. -3 -1×10 -1 Pa; and / or The tempering process is a two-stage tempering process. The first stage tempering process is carried out at a temperature of 900-930℃ for 2-3 hours; the second stage tempering process is carried out at a temperature of 490-630℃ for 4-6 hours.

[0024] In a preferred embodiment, the present invention provides a method for preparing sintered NdFeB permanent magnet material with low carbon content, wherein in step (2), before the vacuum sintering treatment, the supercritical extracted sintered NdFeB powder compact is heat-treated at a temperature of 100-150°C and a system absolute pressure of 0.1-10Pa for 60-120 minutes.

[0025] In a preferred embodiment, the present invention provides a method for preparing a low-carbon sintered NdFeB permanent magnet material, wherein the preparation method is performed by a supercritical extraction machine, the supercritical extraction machine comprising a plunger pump, an extraction vessel, an ultrasonic generator, a separation vessel, a trifluoromethane cylinder, an entrainer vessel, a temperature control system, and necessary pipelines and valves.

[0026] The beneficial effect of the present invention is that, by using the preparation method of the low-carbon content sintered NdFeB permanent magnet material of the present invention, it is possible to significantly reduce carbon impurities in the sintered NdFeB material while ensuring the normal addition of powdered organic additives to the NdFeB magnetic powder, thereby comprehensively improving the overall magnetic properties of the sintered NdFeB permanent magnet material.

[0027] Carbon impurities in sintered NdFeB permanent magnet materials have a significant destructive effect on their overall magnetic properties. These impurities are mainly introduced by organic surface additives (i.e., "organic additives") added to the NdFeB powder, consisting of single or compound additives such as esters, low-chain aliphatic hydrocarbons, and organic dispersants. This invention involves placing a sintered NdFeB powder compact containing organic additives in an extraction vessel using supercritical trifluoromethane as the extraction medium. An appropriate amount of ethyl acetate entrainer is added to the extraction medium to improve extraction efficiency. An ultrasonic generator is installed on the extraction vessel to accelerate the dissolution of organic molecules within the powder compact in the supercritical trifluoromethane medium. Repeated, intermittent pressurization and depressurization of the supercritical trifluoromethane are used to improve the efficiency of organic additive molecules being expelled from the compact. The supercritical trifluoromethane extraction medium is circulated into a separation vessel and depressurized to the critical pressure P. cBelow 4.84 MPa, the organic additives precipitate and are removed from the system; the extracted NdFeB compact is placed in a vacuum sintering furnace and heated to 100-150°C to remove trace amounts of trifluoromethane and ethyl acetate molecules from the compact. This invention can significantly reduce the content of harmful impurities—carbon—in sintered NdFeB permanent magnet materials and improve the overall magnetic properties of sintered NdFeB materials. Attached Figure Description

[0028] Figure 1 A flowchart illustrating the preparation method of the low-carbon content sintered NdFeB permanent magnet material of the present invention; Figure 2 The supercritical extraction machine for exemplarily implementing the preparation method of low-carbon content sintered NdFeB permanent magnet material of the present invention is shown in the following structural diagram: it includes an extraction vessel 1, an ultrasonic generator 2, a back pressure valve 3, a first separation vessel 4, a second separation vessel 5, a trifluoromethane gas cylinder 6, a filter 7, a condenser 8, a trifluoromethane gas pump 9, an entrainer gas pump 10, an entrainer vessel 11, and a mixer 12. Detailed Implementation

[0029] To better understand the technical solutions and advantages of the present invention, the present invention will be further described below through embodiments and accompanying drawings.

[0030] An exemplary method for preparing the low-carbon content sintered NdFeB permanent magnet material of the present invention is as follows: Figure 1 As shown, the exemplary supercritical fluid extractor used to implement this preparation method has the following structure: Figure 2 As shown. The preparation method includes the following steps: Preparation step P1: Use sintered NdFeB powder compact containing 0.1-0.3wt% organic additives as raw material.

[0031] Supercritical extraction step P2: Trifluoromethane gas is introduced into the extraction system. The preform is placed in extraction vessel 1 of the supercritical extractor. The high-pressure plunger pump of the supercritical extractor pressurizes the trifluoromethane gas in extraction vessel 1 to 10-50 MPa. The temperature inside the extraction vessel is controlled at 40-90℃ using the system's automatic temperature control device. At this time, the trifluoromethane medium in extraction vessel 1 is in a supercritical state, possessing both the high permeability of a gas and the high solubility of a liquid. Therefore, it easily penetrates into the interior of the high-density sintered NdFeB powder preform, mixes with the organic additive molecules inside the preform, and dissolves the organic additive molecules originally adsorbed on the surface of the NdFeB powder particles in the supercritical trifluoromethane medium.

[0032] Step P3: To increase the solubility of organic additives in the supercritical trifluoromethane medium, entrainers such as ethyl acetate need to be added to extraction vessel 1 to balance the polarity difference between the extraction medium and the dissolved organic additives, thereby improving extraction efficiency. The entrainer should possess physical and chemical properties such as chemical stability, non-reaction with the NdFeB matrix material at 200℃, and a low boiling point.

[0033] Ultrasonic extraction step P4: An ultrasonic generator 2 is attached to the extraction vessel 1. The ultrasonic generator 2 and the extraction vessel 1 should be firmly pressed together. Coupling agent is applied to the contact surface to generate ultrasonic waves of a certain frequency and intensity in the supercritical trifluoromethane medium. As the trifluoromethane penetrates into the NdFeB powder compact, a cavitation effect is formed on the surface of the powder particles inside the compact, which promotes the organic additive molecules to accelerate their detachment from the particle surface and quickly dissolve in the supercritical trifluoromethane, thereby improving the extraction efficiency.

[0034] Extraction vessel pressurization-depressurization step P5: Repeatedly adjust the pressure of supercritical trifluoromethane in extraction vessel 1: First, increase the pressure of extraction vessel 1 to 10-50 MPa and maintain this pressure for 30-120 minutes; then, depressurize the pressure of extraction vessel 1 to 5-9 MPa and maintain this pressure for 10-30 minutes. This causes supercritical trifluoromethane to repeatedly flow in and out of the compact: during pressurization, pure supercritical trifluoromethane is "pressed" into the NdFeB compact; during depressurization, supercritical trifluoromethane containing dissolved organic additives is released from the compact. Repeat the above operation 10-30 times to improve the efficiency of organic additive molecules being discharged from the compact.

[0035] Organic additive separation step P6: The supercritical trifluoromethane extraction medium containing dissolved organic additives is successively circulated into the first separation vessel 4 and the second separation vessel 5 connected in series in the extraction machine, and the pressure is reduced to below 4.84 MPa. The trifluoromethane medium reverts to its gaseous properties and loses its ability to dissolve organic additives. The organic surface additives precipitate from the trifluoromethane and other gaseous media, condense into droplets, and are discharged from the exhaust ports of the first separation vessel 4 and the second separation vessel 5.

[0036] Residual extractant and entrainer removal step P7: The extracted NdFeB compact is placed in a vacuum sintering furnace for heating treatment to remove trace amounts of trifluoromethane and ethyl acetate molecules from the compact. Treatment temperature: 100-150℃; holding time: 60-120 minutes; system absolute pressure: 0.1-10 Pa.

[0037] The core principle of the above method is to utilize the high permeability and high solubility of supercritical trifluoromethane for organic additives, combined with the polarity adjustment effect of ethyl acetate entrainer and the cavitation effect of ultrasound, to dissolve and extract the organic additives in the sintered NdFeB permanent magnet compact. Before high-temperature sintering, the original organic additives in the compact are cleaned out to prevent the residual carbon atoms from the decomposition of these organic compounds during high-temperature sintering from reacting with the matrix components of the sintered NdFeB permanent magnet material to generate CR impurity phases. This achieves the technical objective of significantly improving the overall magnetic properties of the sintered NdFeB permanent magnet material.

[0038] The carbon content of the NdFeB powder compacts processed by P1 to P7 above, after high-temperature sintering and tempering, can be reduced to 200-300 ppm, preferably below 200 ppm.

[0039] The following are examples of applications of the above-described exemplary method for preparing the low-carbon content sintered NdFeB permanent magnet material of the present invention.

[0040] Example 1: Preparation of low-carbon sintered NdFeB permanent magnet materials (Part 1) (1) Five sintered NdFeB magnet powder compacts of grade 52H were selected (alloy composition: Pr 5.9wt%; Nd 23.6wt%; Dy 0.2wt%; Gd 0.3wt%; B 0.93wt%; Co 0.4wt%; Cu 0.35wt%; Ga 0.25wt%; Zr 0.35wt%; Fe 67.72wt%, with an average particle size of 2.5 micrometers). 0.1wt% of organic additives (i.e., powder modifier, glyceryl stearate 37wt%, petroleum ether 55wt%, polyacrylate dispersant 8wt%) purchased from Tianjin Yuesheng Magnetoelectric Technology Co., Ltd. were added to the powder compacts. The density of the powder compacts was 4.2 g / cm³. 3 The compact dimensions are 73×65×49 mm. 3 .

[0041] (2) Fill the extraction vessel 1 of the supercritical extraction machine system with trifluoromethane gas with a purity of 99.99% (at atmospheric pressure 0.1MPa), open the sealing cover of the extraction vessel 1, put the basket containing the above 5 sintered NdFeB magnet powder compacts into the extraction vessel 1 of the supercritical extraction machine, and press the sealing cover tightly.

[0042] (3) Turn on the plunger pump of the supercritical extractor to pressurize the extraction vessel 1 to 50MPa and set the temperature of the extraction vessel 1 to 40℃.

[0043] (4) Inject ethyl acetate entrainer into extraction vessel 1 at a rate of 5% (relative to the mass ratio of the total amount of trifluoromethane in the supercritical extraction system).

[0044] (5) Press the ultrasonic generator 2 (Kunshan Shumei Ultrasonic Instrument Co., Ltd., KBC-3000D type) tightly against the top cover of the extraction vessel 1, apply Xinmeida CG-88 coupling agent to the contact surface, and adjust the power of the ultrasonic generator 2 to 1kw and the frequency to 100kHz.

[0045] (6) Repeatedly adjust the pressure inside extraction vessel 1: First, maintain the pressure of extraction vessel 1 at 50 MPa for 120 minutes, then depressurize to 9 MPa and maintain for 30 minutes. Repeat the above operation 10 times. The trifluoromethane after supercritical extraction enters the first separation vessel 4 and the second separation vessel 5 of the supercritical extractor.

[0046] (7) Set the pressure in the first separation vessel 4 and the second separation vessel 5 of the supercritical extraction machine to 2.5 MPa. Every 20 minutes, discharge the organic additive that has condensed into droplets from the discharge ports of the first separation vessel 4 and the second separation vessel 5.

[0047] (8) The sintered NdFeB powder compact after supercritical extraction is placed in a vacuum sintering furnace for heating treatment. The temperature is set at 100℃ and held for 60 minutes. The absolute pressure of the system is maintained between 0.1-10 Pa.

[0048] (9) Five powder compacts after supercritical extraction were placed in a vacuum sintering furnace for high-temperature sintering at a temperature of 1090℃ and an absolute pressure of 1×10⁻⁶. -3 -1×10 -1 Pa, heat for 10 hours, then cool and remove from the oven.

[0049] (10) The sintered magnets undergo two-stage tempering in a vacuum sintering furnace. The tempering temperatures are 900℃ and 490℃, and the holding times are 3 hours and 6 hours, respectively. After that, the magnets are cooled and taken out of the furnace.

[0050] (11) Take a D10×10 mm sample cylinder from each magnet and test the magnetic properties of the above magnet samples using the NIM2000 magnetic measuring instrument of the National Institute of Metrology of China. The average value of the results of the 5 test samples is recorded. The results are shown in Table 1.

[0051] (12) Remove the outer skin of 5 samples, and crush the other parts of the samples into small particles and mix them. Randomly select 3 samples and test the carbon content using the Steel Research Nake CS4600 carbon and sulfur analyzer. The average value of the 3 test results is recorded. The results are shown in Table 1.

[0052] Example 2: Preparation of low-carbon sintered NdFeB permanent magnet materials (Part 2) (1) Five N52 sintered NdFeB magnet powder compacts were selected (alloy composition: Pr 5.3wt%; Nd 21.2wt%; Ce 4wt%; B 0.95wt%; Co 0.4wt%; Cu 0.35wt%; Ga 0.1wt%; Zr 0.25wt%; Fe 67.45wt%, with an average particle size of 3.1 micrometers). 0.3wt% of organic additives (i.e., powder modifier, dioctyl adipate 50wt%, acetone 45wt%, and polyacrylate dispersant 5wt%) purchased from Beijing Xudong Chemical Co., Ltd. were added to the powder compacts. The density of the powder compacts was 4.2 g / cm³. 3 The blank dimensions are 83×77×46 mm. 3 .

[0053] (2) Fill the extraction vessel 1 of the supercritical extraction machine system with trifluoromethane gas with a purity of 99.99% (at atmospheric pressure 0.1MPa), open the sealing cover of the extraction vessel 1, put the basket containing the above 5 sintered NdFeB magnet powder compacts into the extraction vessel 1 of the supercritical extraction machine, and press the sealing cover tightly.

[0054] (3) Turn on the plunger pump of the supercritical extractor to pressurize the extraction vessel 1 to 10MPa and set the temperature of the extraction vessel 1 to 90℃.

[0055] (4) Inject ethyl acetate entrainer into extraction vessel 1 at a rate of 15% (relative to the mass ratio of the total amount of trifluoromethane in the supercritical extraction system).

[0056] (5) Press the ultrasonic generator 2 (Kunshan Shumei Ultrasonic Instrument Co., Ltd., KBC-3000D type) tightly against the top cover of the extraction vessel 1, apply Xinmeida CG-88 coupling agent to the contact surface, and adjust the power of the ultrasonic generator 2 to 3kw and the frequency to 20kHz.

[0057] (6) Repeatedly adjust the pressure inside extraction vessel 1: First, maintain the pressure of extraction vessel 1 at 10 MPa for 30 minutes, then depressurize to 5 MPa and maintain for 10 minutes. Repeat the above operation 30 times. The trifluoromethane after supercritical extraction enters the separation vessel of the supercritical extractor.

[0058] (7) Set the pressure in the first separation vessel 4 and the second separation vessel 5 of the supercritical extraction machine to 2.5 MPa. Every 20 minutes, discharge the organic additive that has condensed into droplets from the discharge ports of the first separation vessel 4 and the second separation vessel 5.

[0059] (8) The sintered NdFeB powder compact after supercritical extraction is placed in a vacuum sintering furnace for heating treatment. The temperature is set at 150℃ and held for 120 minutes. The absolute pressure of the system is maintained between 0.1-10 Pa.

[0060] (9) Five powder compacts after supercritical extraction were placed in a vacuum sintering furnace for high-temperature sintering at a temperature of 1050℃ and an absolute pressure of 1×10⁻⁶. -3 -1×10 -1 Pa, heat for 5 hours, then cool and remove from the oven.

[0061] (10) The sintered magnets undergo two-stage tempering in a vacuum sintering furnace. The tempering temperatures are 930°C and 630°C, and the holding times are 2 hours and 4 hours, respectively. After that, the magnets are cooled and taken out of the furnace.

[0062] (11) Take a D10×10 mm sample cylinder from each magnet and test the magnetic properties of the above magnet samples using the NIM2000 magnetic measuring instrument of the National Institute of Metrology of China. The average value of the results of the 5 test samples is recorded. The results are shown in Table 1.

[0063] (12) Remove the outer skin of 5 samples, and crush the other parts of the samples into small particles and mix them. Randomly select 3 samples and test the carbon content using the Steel Research Nake CS4600 carbon and sulfur analyzer. The average value of the 3 test results is recorded. The results are shown in Table 1.

[0064] Comparative Example 1: Preparation of Ordinary Sintered NdFeB Permanent Magnet Materials (Part 1) Five sintered NdFeB magnet powder blanks of grade 52H selected in step (1) of Example 1 were processed directly in the same way as steps (9) to (12) of Example 1 without undergoing the processing in steps (2) to (8) of Example 1. That is, the sintered NdFeB magnet powder blanks were directly sintered, tempered and sampled without undergoing supercritical extraction.

[0065] Comparative Example 2: Preparation of Ordinary Sintered NdFeB Permanent Magnet Materials (Part 2) Five N52 sintered NdFeB magnet powder blanks selected in step (1) of Example 2 were processed directly in the same way as steps (9) to (12) of Example 2 without undergoing the processing in steps (2) to (8) of Example 2. That is, the sintered NdFeB magnet powder blanks were directly sintered, tempered and sampled without undergoing supercritical extraction.

[0066] Table 1. Test results of carbon content and magnetic properties of sintered NdFeB permanent magnet materials.

[0067] As shown in Table 1, compared with Comparative Examples 1 and 2, the carbon content of the sintered NdFeB permanent magnet materials prepared using Examples 1 and 2 of this invention is significantly reduced, while the magnetic performance parameters such as remanence (Br), intrinsic coercivity (Hcj), and demagnetization curve squareness (Hk / Hci) are significantly improved. This indicates that the supercritical trifluoromethane extraction technology of this invention can significantly reduce the carbon content of sintered NdFeB permanent magnet materials and can significantly improve the comprehensive magnetic properties of sintered magnets, including remanence, intrinsic coercivity, and demagnetization curve squareness.

[0068] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims and their equivalents, this invention is also intended to include these modifications and variations. The above embodiments or implementations are merely illustrative examples of this invention, and it can also be implemented in other specific ways or forms without departing from its gist or essential characteristics. Therefore, the described embodiments should be considered illustrative rather than limiting in any respect. The scope of this invention should be defined by the appended claims, and any changes equivalent to the intent and scope of the claims should also be included within the scope of this invention.

Claims

1. A method for preparing a low-carbon content sintered NdFeB permanent magnet material, characterized in that, The preparation method includes the following steps: (1) The sintered NdFeB powder compact containing organic additives is placed into the extraction medium of a supercritical extraction device for supercritical extraction, so as to dissolve the organic additive molecules originally adsorbed on the surface of the NdFeB powder particles in the extraction medium. (2) The sintered NdFeB powder compact after supercritical extraction is subjected to vacuum sintering and tempering treatment.

2. The preparation method according to claim 1, characterized in that: In step (1), the organic additive is an ester hydrocarbon agent, and its addition amount in the sintered NdFeB powder compact is 0.1-0.3wt%.

3. The preparation method according to claim 1, characterized in that: In step (1), the extraction medium is trifluoromethane.

4. The preparation method according to claim 1, characterized in that: In step (1), the temperature of the supercritical extraction is 33-90℃ and the pressure is 4.84-50 MPa.

5. The preparation method according to claim 1, characterized in that: In step (1), the supercritical extraction is performed by first holding the pressure at 10-50 MPa for 30-120 minutes, then holding the pressure at 5-9 MPa for 10-30 minutes, and repeating this operation 10-30 times.

6. The preparation method according to claim 1, characterized in that: In step (1), an entrainer is added during the supercritical extraction process. The mass of the entrainer added is 5-15% of the mass of the extraction medium, and the entrainer is ethyl acetate.

7. The preparation method according to claim 1, characterized in that: In step (1), the supercritical extraction process is also carried out with the assistance of ultrasound, with the power of the ultrasound being 1-3kW and the frequency of the ultrasound being 20-100kHz.

8. The preparation method according to claim 1, characterized in that: After the supercritical extraction in step (1) is completed, the organic additives are released from the extraction medium containing the dissolved organic additives, which is separated from the sintered NdFeB powder compact, under reduced pressure.

9. The preparation method according to any one of claims 1-8, characterized in that: In step (2), The vacuum sintering process is carried out at a temperature of 1050-1090℃ for 5-10 hours, with an absolute pressure of 1×10⁻⁶. -3 -1×10 -1 Pa; and / or The tempering process is a two-stage tempering process. The first stage tempering process is carried out at a temperature of 900-930℃ for 2-3 hours; the second stage tempering process is carried out at a temperature of 490-630℃ for 4-6 hours.

10. The preparation method according to claim 9, characterized in that: In step (2), before the vacuum sintering treatment, the supercritical extracted sintered NdFeB powder compact is heat-treated at a temperature of 100-150℃ and a system absolute pressure of 0.1-10Pa for 60-120 minutes.

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