A heavy rare earth grain boundary diffusion method based on hydrothermal synthesis and a neodymium-iron-boron permanent magnet prepared by the method

Abstract

The application discloses a heavy rare earth grain boundary diffusion method based on hydrothermal synthesis and a prepared neodymium-iron-boron permanent magnet, and comprises the following steps: S1, adding magnetic material, heavy rare earth salt, a surfactant and deionized water into a hydrothermal container, controlling the reaction temperature to be 100-220 DEG C, controlling the reaction time to be 4-48 h, cooling to room temperature after the reaction is completed, taking out the magnetic material and vacuum drying or low-temperature inert atmosphere heat treatment to prepare a coated magnet; S2, pretreatment of the coated magnet, under an inert atmosphere, the treatment temperature is 60-300 DEG C, and the treatment time is 0.5-4 h; S3, grain boundary diffusion heat treatment, the atmosphere is high-purity inert gas or vacuum, first keeping at 300-500 DEG C for 0.5-2 h, then rising to 650-900 DEG C for 0.5-10 h, cooling to room temperature, cleaning and drying. Through the above technical scheme, the purposes of reducing the heavy rare earth consumption, improving the utilization rate, reducing the cost and process difficulty, improving the heavy rare earth diffusion uniformity and depth, and reducing the remanence and magnetic energy product loss of the neodymium-iron-boron permanent magnet are achieved.

Description

Technical Field

[0001] This invention relates to the field of permanent magnet material preparation technology, specifically to a method for heavy rare earth grain boundary diffusion based on hydrothermal synthesis and the resulting neodymium iron boron permanent magnet. Background Technology

[0002] Currently, a major technical approach to improving the coercivity (Hcj) of Nd-Fe-B permanent magnets is through grain boundary diffusion (GBD) using heavy rare earth elements (such as Dy and Tb). This method introduces a diffusion source onto the surface of the sintered magnet, which diffuses along the grain boundaries into the interior during heat treatment, forming a rare earth-rich shell of (Nd,HRE)₂Fe₁₄B. This significantly enhances the magnet's coercivity, while requiring far less heavy rare earth elements than those directly incorporated into the traditional monolithic alloy. This technology is widely used, and the fabrication of most SH-level (Hcj > 1600 kA / m) high-performance magnets relies on it.

[0003] While existing technologies for heavy rare earth grain boundary diffusion are effective, they still have several shortcomings, particularly in improving diffusion efficiency, reducing costs, and maintaining remanence (Br):

[0004] Heavy rare earth resources are scarce and expensive: the supply of heavy rare earth elements such as Dy and Tb is insufficient, their prices fluctuate greatly, and they are almost entirely used in the Nd-Fe-B industry, resulting in rising material costs and resource sustainability issues.

[0005] Limited diffusion depth: The diffusion effect is usually limited to a part of the magnet surface, and the internal rare earth doping is still insufficient, which cannot uniformly improve the coercivity of magnets with a large overall thickness.

[0006] During diffusion, remanence and energy product may be lost: some diffusion source components (such as Tb) penetrate into the main phase, causing a decrease in Br or (BH)max; moreover, excessive diffusion may induce the formation of rare earth oxides, further consuming effective elements and affecting magnetic properties. Summary of the Invention

[0007] This application provides a hydrothermal synthesis-based method for heavy rare earth grain boundary diffusion and the resulting neodymium iron boron permanent magnet, which achieves the effects of reducing the amount of heavy rare earth, improving utilization rate, improving diffusion uniformity and depth, reducing remanence and magnetic energy product loss, and reducing cost and process difficulty.

[0008] This application provides a method for heavy rare earth grain boundary diffusion based on hydrothermal synthesis, comprising the following steps:

[0009] S1. Add magnetic material, heavy rare earth salt, surfactant and deionized water into a hydrothermal container, control the reaction temperature at 100-220 ℃, and the reaction time at 4-48 h. After the reaction is completed, cool to room temperature, take out the magnetic material and vacuum dry or heat treat it in a low temperature inert atmosphere to obtain a coated magnet.

[0010] S2 coated magnet pretreatment: under an inert atmosphere, the treatment temperature is 60-300 ℃ and the treatment time is 0.5-4 h.

[0011] S3 grain boundary diffusion heat treatment, in a high-purity inert gas or vacuum atmosphere, first hold at 300-500℃ for 0.5-2h, then raise to 650-900℃ and hold for 0.5-10h, cool to room temperature, clean and dry.

[0012] Nanoscale heavy rare earth salt particles obtained by hydrothermal synthesis can be uniformly distributed on the magnet surface and grain boundaries, effectively improving the infiltration efficiency of heavy rare earth elements. This significantly reduces the total amount of heavy rare earth elements used while maintaining a substantial increase in coercivity. The nanoscale heavy rare earth salt particles enhance reactivity, forming a continuous and uniform rare earth-rich phase shell during diffusion, significantly improving the diffusion depth in thick magnets. The uniform and highly active diffusion source avoids excessive infiltration of heavy rare earth elements into the main phase, effectively reducing the loss of remanence (Br) and maximum energy product (BH)max, thus improving the overall balance of magnetic properties. Hydrothermal synthesis offers mild conditions and a simple process, with lower equipment investment and energy consumption than traditional melt-based preparation processes, demonstrating better economic viability and industrialization potential.

[0013] Preferably, the reaction temperature in step S1 is 120-200 °C and the reaction time is 6-24 h.

[0014] Preferably, the concentration of the heavy rare earth salt in step S1 is 0.01-0.5 mol·L⁻¹. -1 .

[0015] Preferably, in step S1, the surfactant has a mass percentage of 0.1-5 wt% in the solution, and the solution also includes a complexing agent with a concentration of 0-0.2 mol·L⁻¹. -1 .

[0016] Preferably, in step S1, the heavy rare earth salt is one or a combination of two of DyCl3·6H2O and TbCl3·6H2O, the surfactant is one or a combination of two of PVP and CTAB, and the complexing agent is citric acid.

[0017] Preferably, in step S1, the vacuum drying temperature is 60-120 ℃ and the time is 1-12 h, and the low-temperature inert atmosphere heat treatment is performed with N2 or Ar as the inert gas at a temperature of 150-300 ℃ for 1-4 h.

[0018] Preferably, the magnetic material in step S1 is ultrasonically cleaned with acetone or ethanol for 5-15 minutes and then dried with air or N2.

[0019] Preferably, the temperature maintained for the second time in step S3 is 700-850 ℃, and the time is 0.5-6 h.

[0020] Preferably, after cleaning and drying in step S3, the product undergoes short-term low-temperature annealing in an inert atmosphere or vacuum, with an annealing temperature of 150-350°C and a time of 0.5-2 h.

[0021] A neodymium iron boron permanent magnet is prepared by the above-mentioned heavy rare earth grain boundary diffusion method based on hydrothermal synthesis.

[0022] Compared with related technologies, the present invention has the following advantages:

[0023] Reduce the amount of heavy rare earth elements used and improve their utilization rate.

[0024] Nanoscale heavy rare earth particles obtained by hydrothermal method can be uniformly distributed on the magnet surface and at grain boundaries, effectively improving the infiltration efficiency of heavy rare earth elements, thereby significantly reducing the total amount of heavy rare earth used while maintaining a significant improvement in coercivity.

[0025] Improve diffusion uniformity and depth

[0026] Compared with traditional molten or powdered diffusion sources, hydrothermally synthesized diffusion sources have controllable particle size and good dispersibility, which enhances reactivity and can form a continuous and uniform rare earth-rich phase shell during diffusion, significantly improving the diffusion depth problem in thick magnets.

[0027] Reduce remanence and energy product loss

[0028] Because the diffusion source is uniformly distributed and highly active, it can prevent excessive entry of heavy rare earth elements into the main phase, thereby effectively reducing the loss of remanence (Br) and maximum magnetic energy product (BH)max, and improving the overall balance of magnetic properties.

[0029] Reduce costs and process difficulty

[0030] Hydrothermal synthesis offers mild conditions and a simple process, with lower equipment investment and energy consumption compared to traditional melt preparation processes, making it more economical and promising for industrial application. Attached Figure Description

[0031] Figure 1 This is a scanning electron microscope image of the magnet before the hydrothermal reaction in Example 1;

[0032] Figure 2 This is a scanning electron microscope image of the magnet after the thermal reaction in Example 1;

[0033] Figure 3 This is a scanning electron microscope image of the magnet after grain boundary diffusion heat treatment in Example 1;

[0034] Figure 4 These are transmission electron microscopy (TEM) images of magnet grains after grain boundary diffusion in Example 1 (a. Magnet without hydrothermal reaction; b. Magnet after hydrothermal reaction coating). Detailed Implementation

[0035] First, those skilled in the art should understand that these embodiments are merely used to explain the technical principles of the embodiments of this application and are not intended to limit the scope of protection of the embodiments of this application. Those skilled in the art can make adjustments as needed to adapt to specific application scenarios.

[0036] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0037] Example 1: Low dosage of 0.2 wt% Dy equivalent

[0038] Solution preparation (taking a single 10 g magnet as an example)

[0039] Weigh out 0.0464 g of DyCl3·6H2O;

[0040] PVP(10k) 0.05 g;

[0041] 20 mL of deionized water (used to dissolve / disperse the salt and PVP to form a homogeneous solution).

[0042] Operating steps

[0043] Magnet surface cleaning: Ultrasonic cleaning with acetone / ethanol for 5-15 minutes, air or N2 drying, and slight mechanical roughening (sandblasting, grinding) if necessary to improve coating adhesion.

[0044] Size and weight record: Record the initial mass (m0) and size of each magnet to facilitate subsequent calculation of diffusion addition amount (wt%).

[0045] Place the cleaned and weighed magnet into a PTFE-lined hydrothermal reactor.

[0046] Pour the prepared solution (20 mL) into the lining of the PTFE-lined hydrothermal reactor to cover the magnet surface with the solution (if necessary, tilt / rotate to ensure uniform contact of the solution with the surface). To enhance the effect, sonicate the solution for 15-30 min first.

[0047] Set up the hydrothermal reactor, set the hydrothermal temperature to 140 ℃, and incubate for 12 hours (self-pressurized) (the temperature can be adjusted within a small range of 120-160 ℃).

[0048] After the reaction is complete, allow the sample to cool naturally to room temperature and remove it; gently wipe off any excess liquid from the surface with filter paper.

[0049] Vacuum drying at 100 ℃ for 2 h (or vacuum drying at 60 ℃ for 4 h), record the mass m1 after drying (based on m1). m0 verifies the surface solid coverage.

[0050] Low-temperature pretreatment: 200 °C (N2) for 1 h to remove some organic residues and promote the formation of passivation nuclei.

[0051] Grain boundary diffusion annealing (using a tube furnace, atmosphere is Ar): heating rate 10 ℃ / min, heating to 300-500℃, holding for 1 h (degassing), heating to 750 ℃, holding for 1 h (main diffusion), then rapid cooling (or slow cooling as needed), the gas flow rate is stable throughout the process (e.g. 200 sccm Ar).

[0052] Post-treatment: Clean the surface of residual chloride salts with deionized water, dry quickly, and then anneal the stabilized phase for 1 hour at 200°C under an Ar atmosphere. If passivation is required, perform a thin-layer epoxy or nickel plating.

[0053] Characterization: VSM measurements (20 ℃) ​​were performed to record Br, Hc, and (BH)max; SEM / EDS was used for surface / cross-sectional elemental distribution analysis; ICP-OES was used to determine the residual Dy concentration in the washing solution to estimate the utilization rate.

[0054] Example 2: Medium dosage of 0.5 wt% Dy equivalent

[0055] Solution preparation (taking a single 10 g magnet as an example)

[0056] Weigh out 0.1160 g of DyCl3·6H2O;

[0057] PVP (10k) 0.100 g (For better wetting, add a small amount of citric acid 0.01 mol·L⁻¹) -1 (equal volume solutions);

[0058] 30-40 mL of deionized water (used to dissolve / disperse the salt and PVP to form a homogeneous solution).

[0059] Operating steps

[0060] Magnet surface cleaning: Ultrasonic cleaning with acetone / ethanol for 5-15 minutes, air or N2 drying, and slight mechanical roughening (sandblasting, grinding) if necessary to improve coating adhesion.

[0061] Size and weight record: Record the initial mass (m0) and size of each magnet to facilitate subsequent calculation of diffusion addition amount (wt%).

[0062] Place the cleaned and weighed magnet into a PTFE-lined hydrothermal reactor.

[0063] Pour the prepared solution (20 mL) into the lining of the PTFE-lined hydrothermal reactor to cover the magnet surface with the solution (if necessary, tilt / rotate to ensure uniform contact of the solution with the surface). To enhance the effect, sonicate the solution for 15-30 min first.

[0064] Set up the hydrothermal reactor and sonicate it for 20-30 minutes to improve dispersibility. Set the hydrothermal temperature to 160 ℃ and heat it for 12 hours (self-pressurization) (the time can be adjusted within the range of 6-18 hours).

[0065] After the reaction is complete, allow the sample to cool naturally to room temperature and remove it; gently wipe off any excess liquid from the surface with filter paper.

[0066] Vacuum drying at 100 ℃ for 2 h (or vacuum drying at 60 ℃ for 4 h), and record the mass m1 after drying (the surface solid coverage can be verified by m1-m0).

[0067] Low-temperature pretreatment: 200 °C (N2) for 1 h to remove some organic residues and promote the formation of passivation nuclei.

[0068] Grain boundary diffusion annealing (using a tube furnace, atmosphere is Ar): heating rate 10 ℃ / min, heating to 300-500℃, holding for 0.5-1 h (degassing), heating to 780 ℃, holding for 2 h (main diffusion), then rapid cooling (or slow cooling as needed), the gas flow rate is stable throughout the process (e.g. 200 sccm Ar).

[0069] Post-treatment: Clean the surface of residual chloride salts with deionized water, dry quickly, and then anneal the stabilized phase for 0.5-1 h at a temperature of 200-300 ℃ under an Ar atmosphere. If passivation is required, perform a thin-layer epoxy or nickel plating.

[0070] Characterization: VSM measurements (20 ℃) ​​were performed to record Br, Hc, and (BH)max; SEM / EDS was used for surface / cross-sectional elemental distribution analysis; ICP-OES was used to determine the residual Dy concentration in the washing solution to estimate the utilization rate.

[0071] Example 3: Add 1.0 wt% Dy to medium-high temperature feed.

[0072] Solution preparation (taking a single 10 g magnet as an example)

[0073] Weigh out 0.2320 g of DyCl3·6H2O;

[0074] PVP (10k) 0.15-0.2 g (For better wetting, add a small amount of citric acid 0.01 mol·L⁻¹) -1 (equal volume solutions);

[0075] 40 mL of deionized water (used to dissolve / disperse the salt and PVP to form a homogeneous solution).

[0076] Operating steps

[0077] Magnet surface cleaning: Ultrasonic cleaning with acetone / ethanol for 5-15 minutes, air or N2 drying, and slight mechanical roughening (sandblasting, grinding) if necessary to improve coating adhesion.

[0078] Size and weight record: Record the initial mass (m0) and size of each magnet to facilitate subsequent calculation of diffusion addition amount (wt%).

[0079] Place the cleaned and weighed magnet into a PTFE-lined hydrothermal reactor.

[0080] Pour the prepared solution (20 mL) into the lining of the PTFE-lined hydrothermal reactor to cover the magnet surface with the solution (if necessary, tilt / rotate to ensure uniform contact of the solution with the surface). To enhance the effect, sonicate the solution for 30 min first.

[0081] Set up the hydrothermal reactor, set the hydrothermal temperature to 180 ℃, and heat for 12-18 h (self-pressurized) (the temperature range can be 160-200 ℃ depending on the particle size / morphology requirements).

[0082] After the reaction is complete, allow the sample to cool naturally to room temperature and remove it; gently wipe off any excess liquid from the surface with filter paper.

[0083] Vacuum drying at 100-120 ℃ for 3 h, and record the mass m1 after drying (the surface solid coverage can be verified by m1-m0).

[0084] Low-temperature pretreatment: 250 °C (Ar), 1-2 h, to remove some organic residues and promote the formation of passivation nuclei.

[0085] Grain boundary diffusion annealing (using a tube furnace, atmosphere is Ar): heating rate 10 ℃ / min, heating to 300-500℃, holding for 1 h (degassing), heating to 800 ℃, holding for 2-4 h (main diffusion), then rapid cooling (or slow cooling as needed), the gas flow rate is stable throughout the process (e.g. 200 sccm Ar).

[0086] Post-treatment: Clean the surface of residual chloride salts with deionized water, dry quickly, and then anneal the stabilized phase for 0.5-1 h at a temperature of 200-300 ℃ under an Ar atmosphere. If passivation is required, perform a thin-layer epoxy or nickel plating.

[0087] Characterization: VSM measurements (20 ℃) ​​were performed to record Br, Hc, and (BH)max; SEM / EDS was used for surface / cross-sectional elemental distribution analysis; ICP-OES was used to determine the residual Dy concentration in the washing solution to estimate the utilization rate.

[0088] Example 4: High-volume addition of 2.0 wt% Dy equivalent

[0089] Solution preparation (10 g per block)

[0090] Weigh out 0.4640 g of DyCl3·6H2O;

[0091] PVP (10k) 0.3-0.5 g;

[0092] 50 mL of deionized water.

[0093] Operating steps

[0094] Magnet surface cleaning: Ultrasonic cleaning with acetone / ethanol for 5-15 minutes, air or N2 drying, and slight mechanical roughening (sandblasting, grinding) if necessary to improve coating adhesion.

[0095] Size and weight record: Record the initial mass (m0) and size of each magnet to facilitate subsequent calculation of diffusion addition amount (wt%).

[0096] Place the cleaned and weighed magnet into a PTFE-lined hydrothermal reactor.

[0097] Pour the prepared solution (20 mL) into the lining of the PTFE-lined hydrothermal reactor to cover the magnet surface with the solution (if necessary, tilt / rotate to ensure uniform contact of the solution with the surface). To enhance the effect, sonicate the solution for 30 min first.

[0098] Set up the hydrothermal reactor, set the hydrothermal temperature to 180-200 ℃, and heat for 18-24 hours (self-pressurized) (the temperature range can be 160-200 ℃ depending on the particle size / morphology requirements).

[0099] After the reaction is complete, allow the sample to cool naturally to room temperature and remove it; gently wipe off any excess liquid from the surface with filter paper.

[0100] Vacuum drying at 120 ℃ for 4 h, record the mass m1 after drying (m1 can be used as a reference). m0 verifies the surface solid coverage.

[0101] Low-temperature pretreatment: 250-300 ℃ (Ar), 2 h, to remove some organic residues and promote the formation of passivation nuclei.

[0102] Grain boundary diffusion annealing (using a tube furnace, atmosphere is Ar): heating rate 10 ℃ / min, heating to 300-500℃, holding for 1-2 h (degassing), heating to 820 ℃, holding for 3-6 h (main diffusion), then rapid cooling (or slow cooling as needed), the gas flow rate is stable throughout the process (e.g. 200 sccm Ar).

[0103] Post-treatment: Clean the surface of residual chloride salts with deionized water, dry quickly, and then anneal the stabilized phase for 0.5-1 h at a temperature of 200-300 ℃ under an Ar atmosphere. If passivation is required, perform a thin-layer epoxy or nickel plating.

[0104] Characterization: VSM measurements (20 ℃) ​​were performed to record Br, Hc, and (BH)max; SEM / EDS was used for surface / cross-sectional elemental distribution analysis; ICP-OES was used to determine the residual Dy concentration in the washing solution to estimate the utilization rate.

[0105] Comparative Example 1

[0106] Traditional DyCl3 suspension coatings have an equivalent Dy dosage of approximately 1.5 wt% (for a 10 g magnet, Dy ≈ 0.15 g, and DyCl3·6H2O approximately 0.348 g).

[0107] step

[0108] Weigh the required Dy salt (calculate as above, take 0.348 g of solid DyCl3·6H2O or an equimolar amount of DyF3), and grind it into a fine powder (if using the powder method, spread the powder directly).

[0109] If using the suspension spraying method: Dissolve 0.348 g of DyCl3·6H2O in 20 mL of deionized water, add 0.5 wt% PVP (relative to the solution), and sonicate for 20 min to obtain a homogenate. Apply the homogenate to the magnet surface using a micro-spray or brush to ensure uniform coverage (or spread the powder in a thin layer on the magnet).

[0110] Drying: After natural drying at room temperature, vacuum dry at 100℃ for 2 hours.

[0111] Diffusion annealing (tube furnace, Ar atmosphere, O2 < 50 ppm): heating rate 10℃ / min to 700℃, hold for 2 h, then rapid cooling (remove to room temperature or cool in the furnace, as specified in the instructions).

[0112] Post-treatment: Quickly rinse the surface with deionized water to remove residual salt, anneal at 200℃ for 1 hour under inert gas for a short time, dry and weigh.

[0113] Characterization: VSM was used to measure Br, Hc, and (BH)max; SEM / EDS was used to examine the Dy enrichment layer on the surface and cross-section.

[0114] Table 1 shows the Br, Hc, and (BH)max of the neodymium iron boron permanent magnets prepared by the method described in Examples 1-4 and Comparative Example 1.

[0115]

[0116] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for heavy rare earth grain boundary diffusion based on hydrothermal synthesis, characterized in that, Includes the following steps: S1 involves adding magnetic materials, heavy rare earth salts, surfactants, complexing agents, and deionized water into a hydrothermal container. The heavy rare earth salts are one or a combination of DyCl3·6H2O and TbCl3·6H2O; the surfactants are one or a combination of PVP and CTAB; the complexing agent is citric acid; and the concentration of the heavy rare earth salts is 0.01-0.5 mol·L⁻¹. -1 The concentration of the complexing agent is 0-0.2 mol·L⁻¹. -1 The surfactant in the solution is 0.1-5 wt% by mass. The reaction temperature is controlled at 120-200℃ and the reaction time is 6-24 h. After the reaction is completed, the material is cooled to room temperature, and the magnetic material is removed and vacuum dried or heat-treated in a low-temperature inert atmosphere to obtain a coated magnet. The vacuum drying temperature is 60-120 ℃ and the time is 1-12 h. The low-temperature inert atmosphere heat treatment is carried out with N2 or Ar as the inert gas at a temperature of 150-300 ℃ for 1-4 h. S2 coated magnet pretreatment: under an inert atmosphere, the treatment temperature is 60-300℃ and the treatment time is 0.5-4h. S3 grain boundary diffusion heat treatment: The atmosphere is high-purity inert gas or vacuum. First, maintain at 300-500℃ for 0.5-2h, then raise to 650-900℃ and maintain for 0.5-10h. Cool to room temperature, clean and dry.

2. The method for heavy rare earth grain boundary diffusion based on hydrothermal synthesis according to claim 1, characterized in that: After ultrasonic cleaning with acetone or ethanol for 5-15 minutes, the magnetic material in step S1 is dried with air or N2.

3. The method for heavy rare earth grain boundary diffusion based on hydrothermal synthesis according to claim 1, characterized in that: In step S3, the temperature is maintained at 700-850 ℃ for the second time, and the time is 0.5-6 h.

4. The method for heavy rare earth grain boundary diffusion based on hydrothermal synthesis according to claim 1, characterized in that: After cleaning and drying in step S3, perform short-time low-temperature annealing in an inert atmosphere or vacuum at a temperature of 150-350℃ for 0.5-2 hours.

5. A neodymium iron boron permanent magnet, characterized in that, It is prepared by the heavy rare earth grain boundary diffusion method based on hydrothermal synthesis as described in any one of claims 1-4.

Images