Sintered cerium iron boron composition and method of making same

Abstract

The application relates to the field of rare earth permanent magnet material preparation, and discloses a sintered cerium-iron-boron composition and a preparation method thereof. The composition comprises praseodymium, neodymium, cerium, boron, cobalt, gallium, copper, zirconium, titanium and iron, wherein the zirconium and titanium elements are used for inhibiting abnormal grain growth. The preparation method covers rapid solidification casting, hydrogen crushing, airflow milling, magnetic field forming, temperature control segmented variable pressure sintering and aging heat treatment. The temperature control segmented variable pressure sintering process comprises sequentially performed vacuum exhaust, low-pressure inhibition and high-pressure sintering stages; the low-pressure inhibition stage is operated at a first partial pressure to inhibit cerium element volatilization and regulate liquid phase distribution, and the high-pressure sintering stage is operated at a second partial pressure higher than the first partial pressure to promote densification. Through the synergistic effect of composition regulation and variable pressure sintering, the application effectively solves the problems of surface rare earth depletion and microstructure unevenness of the cerium-containing magnet, and significantly improves the density, coercivity and squareness of the magnet.

Description

Technical Field

[0001] This invention relates to the field of rare earth permanent magnet material preparation technology, specifically to a sintered cerium-iron-boron composition and its preparation method. Background Technology

[0002] Sintered NdFeB permanent magnets are widely used in new energy vehicles, wind power generation, energy-saving home appliances, and industrial motors due to their excellent magnetic properties. However, with the expansion of applications, the supply and demand imbalance of light rare earth elements such as praseodymium and neodymium, which are core raw materials, has become increasingly prominent, and their prices fluctuate wildly, severely restricting the cost control capabilities of downstream industries. In contrast, cerium is extremely abundant in rare earth ores and is often stockpiled as a by-product, resulting in low prices. Therefore, developing high-abundance rare earth cerium to partially replace praseodymium and neodymium in the preparation of sintered magnets has become an important technical approach to reduce raw material costs and balance the utilization of rare earth resources.

[0003] However, introducing cerium into the NdFeB system faces multiple technical challenges. First, the intrinsic magnetocrystalline anisotropy field of the Ce₂Fe₁₄B phase is significantly lower than that of the Nd₂Fe₁₄B phase, and the addition of cerium inevitably leads to a decrease in the macroscopic coercivity and remanence of the magnet. More importantly, the introduction of cerium significantly alters the sintering behavior and microstructure evolution of the alloy. Cerium is chemically extremely reactive and has a low melting point, easily inducing abnormal grain growth during sintering, leading to microstructure coarsening and severely deteriorating the squareness and coercivity of the magnet. To suppress grain growth, existing techniques often attempt to add high-melting-point elements such as zirconium and titanium for grain boundary pinning, but these refractory elements have limited solubility in cerium-containing low-melting-point alloy liquids, easily resulting in incomplete dissolution or local agglomeration, which in turn forms inclusion phases that damage magnetic properties.

[0004] Furthermore, cerium has a high saturated vapor pressure, which presents a dilemma for traditional vacuum sintering processes. If high-vacuum sintering is used, cerium readily volatilizes from the magnet surface, leading to a rare-earth-depleted layer or a soft magnetic phase, resulting in deterioration of magnetic properties and reduced processing performance. If high-pressure sintering is used throughout to suppress volatilization, while reducing element loss, excessively high gas pressure prematurely closes pores, hindering the discharge of impurities from the green body and restricting the proper flow and distribution of the liquid phase under capillary action, ultimately resulting in insufficient magnet density or uneven grain boundary phase distribution. Therefore, how to synergistically address cerium volatilization, solid solution of refractory elements, and optimized liquid phase distribution while ensuring chemical composition homogeneity is a crucial technical challenge for the preparation of high-performance sintered cerium-iron-boron magnets. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a sintered cerium-iron-boron composition and its preparation method, which solves the problems of surface component loss, abnormal grain growth, and uneven microstructure caused by the easy volatilization of cerium and the difficulty in solidifying refractory elements during the preparation of existing cerium-containing sintered magnets, thus severely restricting the improvement of magnet density and magnetic properties.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of the present invention provides a sintered cerium-iron-boron composition, which is composed of specific elemental components in a specific weight ratio, with the aim of optimizing the microstructure through component control.

[0007] The sintered cerium-iron-boron composition comprises the following components in parts by weight:

[0008] Praseodymium-neodymium: 25.5 parts to 27.5 parts

[0009] Cerium: 3.0 parts to 4.0 parts

[0010] Boron: 0.88 parts to 0.95 parts

[0011] Cobalt: 0.4 to 0.6 parts

[0012] Gallium: 0.08 parts to 0.15 parts

[0013] Copper: 0.08 parts to 0.15 parts

[0014] Zirconium: 0.08 parts to 0.15 parts

[0015] Titanium: 0.08 parts to 0.15 parts

[0016] The balance consists of iron and unavoidable impurities.

[0017] In this composition system, cerium is introduced to replace part of the praseodymium and neodymium. To address the potential for abnormal grain growth and changes in intrinsic magnetic properties caused by the introduction of cerium, the composition is regulated through the synergistic addition of zirconium and titanium. During sintering, zirconium and titanium form high-melting-point precipitates that are distributed at or within grain boundaries, acting as grain boundary pinning agents and physically limiting the excessive growth of the main phase grains. Simultaneously, gallium and copper, as low-melting-point auxiliary elements, form a liquid phase during sintering and aging. This liquid phase improves the wettability of the rare-earth-rich phase on the surface of the main phase grains, promotes the continuous distribution of the grain boundary phase, and thus weakens the magnetic coupling between the main phase grains. The addition of cobalt is used to increase the Curie temperature of the material and improve its corrosion resistance.

[0018] The second aspect of the present invention provides a method for preparing the above-mentioned sintered cerium-iron-boron composition. The method combines rapid solidification casting, hydrogen crushing, air jet milling, and magnetic field forming processes, and adopts a temperature-controlled segmented pressure-switching strategy to adapt to the sintering characteristics of formulations containing cerium and low-melting-point elements.

[0019] The preparation method includes the following steps:

[0020] S1: Melt the raw materials and prepare them into quick-solidifying casting sheets;

[0021] S2: The rapidly solidified casting sheet is subjected to hydrogen explosion crushing and dehydrogenation treatment to obtain coarse crushed powder;

[0022] S3: The coarsely crushed powder is subjected to air jet milling to obtain fine powder;

[0023] S4: The fine powder is oriented and shaped under a magnetic field, and then subjected to isostatic pressing to obtain a green body;

[0024] S5: The green blank is subjected to temperature-controlled segmented pressure sintering;

[0025] S6: Perform a two-stage aging treatment on the sintered magnet.

[0026] In S5, the temperature-controlled segmented variable pressure sintering process is configured to include a vacuum exhaust stage, a low-pressure suppression stage, and a high-pressure sintering stage performed sequentially.

[0027] Specifically, the low-pressure suppression stage operates as follows: when the temperature rises to the first temperature zone, inert gas is introduced to maintain the furnace at the first partial pressure. This stage is designed based on the high vapor pressure characteristics of cerium and the flow characteristics of the low-melting-point phases of gallium and copper.

[0028] By introducing a first partial pressure, the gas back pressure is used to suppress the surface volatilization of cerium during the heating process, while also slowing down the excessively rapid migration of the liquid phase.

[0029] The operation of the high-pressure sintering stage is as follows: when the temperature continues to rise to the second temperature zone and the final sintering temperature, the content of inert gas in the furnace is adjusted to keep the furnace at the second partial pressure and to maintain the temperature at the second partial pressure, wherein the second partial pressure is higher than the first partial pressure.

[0030] This stage utilizes higher gas pressure to assist heat conduction and provide isostatic pressure, promoting the densification process of the sintered body.

[0031] As a specific implementation method, in S1, to address the problem of uneven melting of high-melting-point elements in low-melting-point alloy liquid, zirconium and titanium in the raw materials are added in the form of iron-boron-zirconium master alloys and iron-boron-titanium master alloys, respectively. The content of zirconium or titanium in the master alloy is 15wt% to 20wt%.

[0032] In one specific implementation, in S1, the process parameters for preparing the rapid solidification casting sheet are set as follows: the alloy liquid casting temperature is 1420℃ to 1480℃, the copper roller linear speed is 2.5m / s to 3.5m / s, and the thickness of the prepared rapid solidification casting sheet is 0.25mm to 0.35mm.

[0033] In one specific implementation, in S3, the process parameters for air jet milling are set as follows: air source pressure of 0.7 MPa to 0.8 MPa, milling pressure of 0.6 MPa to 0.7 MPa, and classifier rotation speed of 3500 rpm to 4500 rpm, thereby obtaining fine powder with an average particle size D50 of 2.8 μm to 3.2 μm. Furthermore, an organic antioxidant lubricant is added online during the powdering process at an amount of 0.08 wt% to 0.12 wt% of the total powder weight to reduce the oxidative activity of the powder surface.

[0034] As a specific implementation method, in the temperature-controlled segmented pressure-variable sintering of S5, the vacuum exhaust stage raises the temperature from room temperature to T1 at a heating rate of 5°C / min to 8°C / min, while maintaining a vacuum level of ≤5×10⁻⁶ within the furnace during this process. - 2 Pa, wherein T1 is selected from a temperature in the range of 750°C to 800°C;

[0035] The low-pressure suppression stage raises the temperature from T1 to T2 at a heating rate of 3°C / min to 5°C / min, and during this process, the partial pressure inside the furnace is always maintained at the first partial pressure P1. T2 is selected from the temperature range of 980°C to 1020°C, and the first partial pressure P1 is selected from the pressure value range of 600Pa to 800Pa.

[0036] The high-pressure sintering stage raises the temperature from T2 to the final sintering temperature Tsinter at a heating rate of 2°C / min to 3°C / min. During this process, the partial pressure inside the furnace is maintained at the second partial pressure P2, and the furnace is held at Tsinter and P2 for 3.0h to 4.0h. Tsinter is selected from a temperature range of 1090°C to 1110°C, and the second partial pressure P2 is selected from a pressure value range of 2500Pa to 3500Pa.

[0037] As a specific implementation method, the two-stage aging process in S6 includes:

[0038] First-stage aging heating to 890℃ to 910℃, holding for 2.5h to 3.0h, followed by air cooling;

[0039] The temperature is increased to 500℃ to 520℃ by secondary aging and held for 3.0h to 4.0h, followed by rapid air cooling to room temperature at a cooling rate of ≥30℃ / min.

[0040] In summary, this application includes at least one of the following beneficial technical effects:

[0041] 1. This invention reduces raw material costs by introducing cerium to replace part of the praseodymium and neodymium in the composition. At the same time, the fine precipitates formed by zirconium and titanium pin the relative grain boundaries, physically limiting the abnormal grain growth caused by the tendency of cerium introduction. In combination with gallium and copper, the wettability of rare earth-rich phases is improved, so that the sintered body can still maintain a fine grain size and optimized grain boundary structure while reducing the amount of precious rare earths, thereby maintaining high coercivity and magnetic energy product.

[0042] 2. This invention employs a temperature-controlled segmented pressure-variable sintering process. In particular, a first partial pressure is introduced during the low-pressure suppression stage. A gas back pressure mechanism is established to address the characteristics of cerium's easy volatility and the high fluidity of its low-melting-point phase. This technical feature effectively suppresses the surface volatilization of cerium during the heating process, prevents the formation of a rare-earth-depleted layer on the magnet surface, and also avoids the excessively rapid migration and local segregation of the liquid phase under capillary action, ensuring the uniformity of chemical composition and microstructure of the final sintered body from the surface to the core.

[0043] 3. This invention introduces high-melting-point zirconium and titanium elements in the form of intermediate alloys, and combines this with densification assisted by the second partial pressure during high-pressure sintering. This solves the problem of high-melting-point elements being difficult to dissolve and easy to agglomerate in low-melting-point cerium-containing systems, promotes the formation of high-density sintered bodies, and induces the grain boundary phase to form a continuous and clear thin-layer network distribution around the main phase grains. This effectively removes the magnetic exchange coupling effect between grains, thereby improving the squareness and overall magnetic properties of the magnet. Detailed Implementation

[0044] Example 1:

[0045] This embodiment provides a sintered cerium-iron-boron composition, comprising:

[0046] Praseodymium and neodymium: 26.5 parts; cerium: 3.5 parts; boron: 0.91 parts; cobalt: 0.5 parts; gallium: 0.11 parts; copper: 0.11 parts; zirconium: 0.11 parts; titanium: 0.11 parts; balance iron.

[0047] Preparation steps:

[0048] S1: Weigh the raw materials according to the above proportions, with zirconium and titanium added as Fe-B-Zr and Fe-B-Ti master alloys, respectively. Melt uniformly in a vacuum induction melting furnace, controlling the alloy liquid casting temperature at 1450℃. Using a rapid solidification casting process, with the copper roller linear speed set at 3.0 m / s, a rapid solidification casting sheet with a thickness of approximately 0.30 mm is obtained.

[0049] S2: The rapidly solidified casting sheet is placed in a hydrogen crushing furnace and saturated with hydrogen at room temperature. Then, it is dehydrogenated under vacuum at 580°C for 5.0 hours. After cooling, it is taken out to obtain coarse crushed powder.

[0050] S3: The coarse powder was crushed using an air jet mill with an air source pressure of 0.75 MPa and a classifying wheel speed of 4000 rpm. Stearamide, accounting for 0.10% of the powder weight, was added as an organic antioxidant lubricant during the powder preparation process to obtain fine powder with an average particle size D50 of 3.0 μm.

[0051] S4: The fine powder is oriented and shaped under a 2.0T magnetic field, and then subjected to cold isostatic pressing at 190MPa for 60 seconds to obtain a green body. S5: The green body is placed in a vacuum sintering furnace for temperature-controlled segmented pressure sintering.

[0052] Vacuum exhaust stage: The temperature is increased from room temperature to T1 (780℃) at a rate of 6℃ / min, and the vacuum degree inside the furnace is maintained at 3×10−23×10−2Pa.

[0053] Low-pressure suppression stage: Argon gas is introduced, and the temperature is increased from T1 to T2 (1000℃) at a rate of 4℃ / min. During this process, the first partial pressure P1 in the furnace is maintained at 700Pa.

[0054] High-pressure sintering stage: Continue to adjust the argon flow rate, raising the temperature from T2 to the final sintering temperature Tsinter (1100℃) at a rate of 2.5℃ / min, maintaining the second partial pressure P2 in the furnace at 3000Pa, and holding for 3.5 hours. S6: After sintering, perform two-stage aging: first-stage aging at 900℃ for 2.8 hours, followed by air cooling; second-stage aging at 510℃ for 3.5 hours, followed by rapid air cooling to room temperature.

[0055] Example 2:

[0056] This embodiment provides a sintered cerium-iron-boron composition, comprising:

[0057] Praseodymium and neodymium: 25.5 parts; cerium: 3.0 parts; boron: 0.88 parts; cobalt: 0.4 parts; gallium: 0.08 parts; copper: 0.08 parts; zirconium: 0.08 parts; titanium: 0.08 parts; balance iron.

[0058] Preparation steps:

[0059] S1: Weigh the raw materials according to the above proportions, with zirconium and titanium added as intermediate alloys. Control the casting temperature of the alloy liquid at 1420℃. Set the linear speed of the copper roller in the rapid solidification casting strip to 2.5m / s to obtain a rapid solidification casting sheet with a thickness of approximately 0.35mm.

[0060] S2: The rapidly solidified cast sheet is subjected to a hydrogen crushing process with a dehydrogenation temperature of 550℃ and a dehydrogenation time of 4.5 hours to obtain coarsely crushed powder.

[0061] S3: Air jet milling, air source pressure 0.7MPa, classifier speed 4500rpm (to obtain finer powder). Add zinc stearate at 0.08% of powder weight as an organic antioxidant lubricant to obtain fine powder with an average particle size D50 of 2.8μm.

[0062] S4: Oriented under a 1.8T magnetic field, cold isostatic pressing at 180MPa for 60 seconds.

[0063] S5: Temperature-controlled segmented variable voltage sintering process is as follows:

[0064] Vacuum exhaust stage: Heat to T1 at a rate of 5℃ / min and maintain high vacuum.

[0065] Low-pressure suppression stage: Argon gas is introduced and the temperature is increased to T2 at a rate of 3℃ / min, while the first partial pressure P1 in the furnace is maintained at 600Pa.

[0066] High-pressure sintering stage: Adjust the argon gas and heat to Tsinter at a rate of 2℃ / min, control the second partial pressure P2 in the furnace to maintain at 2500Pa, and hold for 3.0 hours. S6: Two-stage aging: First-stage aging at 890℃ for 2.5 hours; second-stage aging at 500℃ for 3.0 hours, followed by rapid air cooling.

[0067] Example 3:

[0068] This embodiment provides a sintered cerium-iron-boron composition, comprising:

[0069] Praseodymium and neodymium: 27.5 parts; Cerium: 4.0 parts; Boron: 0.95 parts; Cobalt: 0.6 parts; Gallium: 0.15 parts;

[0070] Copper: 0.15 parts; Zirconium: 0.15 parts; Titanium: 0.15 parts; Balance: Iron.

[0071] Preparation steps:

[0072] S1: Weigh the raw materials according to the above proportions, with zirconium and titanium added as intermediate alloys. Control the casting temperature of the alloy liquid at 1480℃. Set the linear speed of the copper roller in the rapid solidification casting strip to 3.5m / s to obtain a rapid solidification casting sheet with a thickness of approximately 0.25mm.

[0073] S2: The rapidly solidified cast sheet is subjected to a hydrogen crushing process with a dehydrogenation temperature of 600℃ and a dehydrogenation time of 6.0 hours to obtain coarsely crushed powder.

[0074] S3: Air jet milling, air source pressure 0.8MPa, classifier speed 3500rpm. Polyethylene glycol (0.12% by weight of powder) was added as an organic antioxidant lubricant to obtain fine powder with an average particle size D50 of 3.2μm.

[0075] S4: Oriented under a 2.2T magnetic field, cold isostatic pressing at 200MPa for 90 seconds.

[0076] S5: Temperature-controlled segmented variable voltage sintering process is as follows:

[0077] Vacuum exhaust stage: Heat to T1 at a rate of 8℃ / min and maintain high vacuum.

[0078] Low-pressure suppression stage: Argon gas is introduced and the temperature is increased to T2 at a rate of 5℃ / min, while the first partial pressure P1 in the furnace is maintained at 800Pa.

[0079] High-pressure sintering stage: Adjust the argon gas and heat to Tsinter at a rate of 3℃ / min, control the second partial pressure P2 in the furnace to maintain at 3500Pa, and hold for 4.0 hours.

[0080] S6: Two-level time limit:

[0081] Level 1 aging: 910℃ for 3.0 hours;

[0082] The temperature is maintained at 520℃ for 4.0 hours under secondary aging conditions, followed by rapid air cooling.

[0083] Comparative Example 1

[0084] Compared to Example 1, the difference lies in that S5 does not employ a temperature-controlled segmented pressure-variable sintering strategy, but instead uses a traditional vacuum sintering process. Specifically, after the vacuum degassing stage (when the temperature reaches T1, i.e., 780°C), the vacuum level inside the furnace is maintained at a level better than 5 × 10⁻⁶. -2 Pa, directly heat to the final sintering temperature Tsinter and hold at that temperature, the remaining steps and parameters are the same as in Example 1.

[0085] Comparative Example 2

[0086] Compared with Example 1, the difference is that the pressure difference between the low-pressure suppression stage and the high-pressure sintering stage is eliminated in S5, and single-pressure sintering is adopted. Specifically, after the temperature rises to T1, argon gas is directly introduced to make the partial pressure in the furnace reach 3000Pa and then held at that temperature. The remaining steps and parameters are the same as in Example 1.

[0087] Comparative Example 3

[0088] Compared with Example 1, the difference is that zirconium and titanium were not added to the sintered cerium-iron-boron composition. The weight of both elements was 0, and the reduced weight was made up by iron. The remaining components and preparation steps were the same as in Example 1.

[0089] Comparative Example 4

[0090] Compared with Example 1, the difference is that gallium and copper were not added to the sintered cerium-iron-boron composition. The weight of both elements was 0, and the reduced weight was made up by iron. The remaining components and preparation steps were the same as in Example 1.

[0091] Comparative Example 5

[0092] Compared with Example 1, the difference lies in the form of introduction of zirconium and titanium elements in the preparation process of S1: Fe-B-Zr and Fe-B-Ti master alloys are no longer used, but pure metallic zirconium particles and pure metallic titanium particles are directly added. The remaining proportions and subsequent preparation steps are the same as in Example 1.

[0093] Test Example 1:

[0094] In order to obtain physical and magnetic property data of the sintered magnets prepared in Examples 1 to 3 and Comparative Examples 1 to 5, the following test steps were performed in this test example.

[0095] First, the sintered magnet blanks prepared in the above embodiments and comparative examples were machined into cubic samples with dimensions of 10mm × 10mm × 10mm by wire cutting. The surface of the samples was mechanically ground and polished to remove the oxide scale and the processed altered layer.

[0096] Subsequently, the actual density of each sample was determined using an electronic density balance based on Archimedes' principle of water displacement, with a measurement accuracy of ±0.01 g / cm³. 3 .

[0097] Next, the magnetic properties of the sample were measured using a NIM-10000H type BH bulk rare earth permanent magnet material tester. The sample was magnetized to saturation in a pulsed magnetic field at a constant temperature of 20℃, and the demagnetization curve was then recorded. Based on the demagnetization curve, the remanence, intrinsic coercivity, and maximum energy product of the sample were calculated and output.

[0098] The specific data obtained from the test are recorded in Table 1.

[0099] Table 1. Density and magnetic property test data of the examples and comparative examples.

[0100] Sample number Density (g / cm³) remanence (kGs) Innate resistance (kOe) Maximum magnetic energy product (MGOe) Example 1 7.61 13.42 14.53 43.15 Example 2 7.59 13.35 14.28 42.66 Example 3 7.62 13.48 14.61 43.52 Comparative Example 1 7.43 12.51 12.14 37.89 Comparative Example 2 7.54 13.04 13.45 40.72 Comparative Example 3 7.58 13.29 11.23 41.05 Comparative Example 4 7.55 12.87 12.66 39.41 Comparative Example 5 7.57 13.12 13.25 41.53

[0101] The data in Table 1 show that the samples prepared using the temperature-controlled segmented pressure-sintering process have higher density and magnetic properties than those prepared using constant vacuum or single-pressure sintering. In the comparative examples without a low-pressure suppression stage, a decrease in density was observed, which is related to the high vapor pressure of cerium. During the heating process, the lack of a specific partial pressure in the gas back pressure causes cerium to volatilize from the magnet surface, resulting in pores or defects due to component loss. The first partial pressure setting in the technical solution of this invention provides the physical environment required to suppress atomic escape, while the introduction of the second partial pressure provides isostatic pressure in the high-temperature section, promoting pore closure and densification of the main phase, thereby achieving high material density while maintaining the cerium content.

[0102] Regarding the introduction of zirconium and titanium, samples containing these two elements and added via intermediate alloys exhibited higher intrinsic coercivity values ​​than comparative samples without these elements or added in pure metal form. Samples without zirconium and titanium showed significantly reduced coercivity, consistent with the mechanism of grain growth induced by the lack of precipitate pinning. Furthermore, compared to directly adding pure metal particles, the introduction via iron-boron-zirconium and iron-boron-titanium intermediate alloys reduced the melting point difference between the additives and the matrix alloy liquid. This treatment promoted the dispersed distribution of zirconium and titanium atoms in the grain boundary phase, avoiding local agglomerations formed due to incomplete melting of high-melting-point metal particles, thereby reducing nucleation centers for antimagnetic domains and maintaining a high level of coercivity.

[0103] The addition of gallium and copper positively impacted the remanence and coercivity of the magnet. The data primarily reflect the liquid phase effect formed during sintering and heat treatment by the low-melting-point elements. This liquid phase improved the wettability of the cerium-rich grain boundary phase on the surface of the main phase grains, resulting in a more continuous and smooth grain boundary layer distribution. This structural change physically isolates adjacent ferromagnetic main phase grains, weakens the magnetic exchange coupling between grains, and makes the magnetic moment reversal within the magnet more independent, macroscopically manifesting as a numerical improvement in the overall magnetic properties of the magnet.

[0104] Test Example 2:

[0105] To evaluate the microstructure uniformity of the magnets prepared in the examples and comparative examples, especially the influence of the consistency of composition distribution on magnetization reversal behavior, this test example further extracted and calculated parameters based on the demagnetization data obtained in Test Example 1.

[0106] The specific analysis steps are as follows: First, retrieve the demagnetization curve data of each sample recorded by the BH bulk rare earth permanent magnet material tester in Test Example 1. In the second quadrant of the demagnetization curve, determine the reverse magnetic field strength value corresponding to the decrease of magnetic polarization intensity to 90% of the remanence value, and define this value as the knee magnetic field strength.

[0107] Then, using the formula Calculate the squareness factor for each sample, where This is the intrinsic coercivity measured in Test Example 1. This ratio is a dimensionless value used to quantitatively characterize the concentration of coercivity distribution in magnet grains and the uniformity of the microstructure.

[0108] The specific data obtained from the calculations are recorded in Table 2.

[0109] Table 2. Knee-point magnetic field and squareness factor data for the embodiments and comparative examples.

[0110] Sample number Innate resistance (kOe) Knee magnetic field (kOe) Squareness factor SQ Example 1 14.53 13.92 0.958 Example 2 14.28 13.59 0.952 Example 3 14.61 14.05 0.962 Comparative Example 2 13.45 12.21 0.908 Comparative Example 5 13.25 11.54 0.871

[0111] Table 2 shows that the squareness factor of the example group is generally higher than that of the comparative example group, with the samples using the intermediate alloy addition method and segmented variable pressure sintering process exhibiting even higher ratios. Comparing the data of Example 1 and Comparative Example 5, it can be seen that changing the form of introduction of high-melting-point elements has a significant impact on the squareness of the magnet. In Comparative Example 5, zirconium and titanium were added in pure metallic form. Due to their melting points being much higher than the matrix alloy liquid, incompletely diffused high-melting-point enriched regions or inclusions were formed during melting and subsequent sintering. This local compositional inhomogeneity leads to differences in the pinning field of grains in different regions, causing asynchronous magnetic moment reversal of the magnet under the action of a reverse magnetic field, thereby reducing the knee magnetic field strength and squareness. In contrast, Example 1, through the introduction of iron-boron-zirconium and iron-boron-titanium intermediate alloys, reduced the melting point difference, promoted the dispersed precipitation of zirconium and titanium elements at the grain boundaries, and improved the uniformity of the microstructure.

[0112] Comparing the data from Example 1 and Comparative Example 2 reflects the impact of sintering pressure strategies on liquid phase distribution. Comparative Example 2 used a single high-pressure sintering method, and the data shows that its squareness factor was lower than that of Example 1. In systems containing cerium and low-melting-point gallium and copper, the fluidity of the liquid phase is highly sensitive to pressure. While a single high-pressure environment is conducive to densification, in the critical temperature range for liquid phase formation, excessively high gas pressure may drive the liquid phase to migrate excessively or accumulate in specific areas under capillary action, resulting in poor continuity of the grain boundary phase distribution. The low-pressure suppression stage used in Example 1 provides a moderate gas back pressure environment, which suppresses cerium volatilization and avoids excessive interference with the liquid phase flow, ensuring that the grain boundary phase forms a uniform coating layer around the main phase grains.

[0113] The improved squareness factor confirms the effectiveness of this invention in optimizing grain magnetic isolation. High squareness means that the coercivity values ​​of most grains within the magnet are distributed within a narrow range. This is because the uniformly distributed cerium-rich and gallium- and copper-rich grain boundary phases effectively remove the ferromagnetic exchange coupling between the main phase grains. When each grain is uniformly isolated by non-magnetic or weakly magnetic grain boundary phases, their flipping behavior in a reverse magnetic field becomes more independent and consistent, resulting in a flat demagnetization curve near the knee. This decoupling effect in the microstructure is achieved through the synergistic effect of temperature-controlled segmented transformer sintering and compositional regulation, thereby ensuring the magnet's operational stability under high-temperature or complex magnetic field conditions.

[0114] Test Example 3:

[0115] To verify the effectiveness of temperature-controlled segmented pressure-sintering process in suppressing the loss of volatile elements, this test case quantitatively analyzed the chemical composition of the sintered body at different depths.

[0116] The specific testing procedures are as follows: Samples were taken from the sintered magnet blanks prepared in Example 1, Comparative Example 1, and Comparative Example 2, before grinding. First, a thin slice with a thickness of 0.5 mm was cut from one outer surface of the magnet using a precision cutter, labeled as the "surface sample"; subsequently, a block with a volume of approximately 5×5×5 mm³ was cut from the geometric center of the same magnet, labeled as the "core sample". All samples were mechanically crushed and ground to ensure they all passed through a 200-mesh sieve. 0.1000 g of each powder sample was accurately weighed and digested using a mixed acid solution of nitric acid and hydrochloric acid under heating conditions until the sample was completely dissolved. The solution was then diluted to a 100 mL volumetric flask.

[0117] The characteristic spectral intensities of cerium in each solution were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES), and the weight percentage of cerium in the sample was calculated based on the standard curve. Finally, the formula was used... Calculate the cerium content retention rate ,in The cerium content of the surface sample, This refers to the cerium content in the heart sample.

[0118] The specific data obtained from the test are recorded in Table 3.

[0119] Table 3. Test data of cerium content in the surface and core of the examples and comparative examples.

[0120] Sample number Cerium content in the heart (wt%) Surface cerium content (wt%) Cerium content retention rate (%) Example 1 3.49 3.47 99.43 Comparative Example 1 3.46 2.92 84.39 Comparative Example 2 3.48 3.43 98.56

[0121] Table 3 shows the test results, which intuitively reflect the effect of sintering atmosphere pressure on the content of volatile rare earth elements. Comparative Example 1, sintered under full-process high-vacuum conditions, exhibited a significantly lower cerium content on the surface compared to the core, with a retention rate of less than 85%. This data confirms that under high-temperature conditions and without gas pressure protection, cerium, with its high saturated vapor pressure, undergoes severe surface volatilization. This volatilization-induced rare earth depletion disrupts the stoichiometry of the magnet's surface, causing the loss of material that should have formed cerium-rich grain boundary phases, thus leaving pores or creating soft magnetic properties in the surface region. This phase has an adverse effect on the overall magnetic properties of the magnet.

[0122] Example 1 exhibits excellent stability in cerium content retention, with minimal compositional difference between the surface and core. This is attributed to the low-pressure suppression stage in the temperature-controlled segmented pressure-variable sintering process. In this stage, where cerium begins to volatilize significantly but the pores of the sintered body are not yet closed, the system is filled with inert gas and maintained at a first partial pressure. This partial pressure establishes a gas back pressure on the magnet surface and within the interconnected pores. When the ambient gas pressure is greater than or close to the saturated vapor pressure of cerium at this temperature, it physically inhibits the escape of cerium atoms from the solid or liquid phase surface to the gas phase, thereby ensuring the spatial uniformity of the chemical composition.

[0123] Although Comparative Example 2 also achieved a high cerium retention rate through a single high-pressure method, the combined results of Test Examples 1 and 2 show that the single high-pressure strategy is inferior to the segmented pressure-variable strategy in terms of overall performance. Pressure control in the segmented pressure-variable process is not only for suppressing volatilization but also involves precise management of liquid phase flow behavior. By applying matched first and second partial pressures within a specific temperature range, this process effectively curbs volatilization while avoiding premature closure of the exhaust channels or uneven liquid phase distribution caused by applying extremely high pressure too early. Therefore, the technical solution of Example 1 achieves the dual goals of zero component loss and structural densification.

Claims

1. A sintered cerium-iron-boron composition, characterized in that, The composition comprises the following components in parts by weight: Praseodymium-neodymium (PrNd): 25.5–27.5 parts; Cerium (Ce): 3.0–4.0 parts; Boron B: 0.88–0.95 parts; Cobalt (Co): 0.4–0.6 parts; Gallium (Ga): 0.08–0.15 parts; Copper (Cu): 0.08–0.15 parts; Zirconium (Zr): 0.08–0.15 parts; Titanium (Ti): 0.08–0.15 parts; The balance consists of iron (Fe) and unavoidable impurities.

2. A method for preparing a sintered cerium-iron-boron composition as described in claim 1, characterized in that, Includes the following steps: S1: Melt the raw materials and prepare them into quick-solidifying casting sheets; S2: The rapidly solidified casting sheet is subjected to hydrogen explosion crushing and dehydrogenation treatment to obtain coarse crushed powder; S3: The coarsely crushed powder is subjected to air jet milling to obtain fine powder; S4: The fine powder is oriented and shaped under a magnetic field, and then subjected to isostatic pressing to obtain a green body; S5: The green body is subjected to temperature-controlled segmented pressure sintering; the temperature-controlled segmented pressure sintering includes a vacuum degassing stage, a low-pressure suppression stage and a high-pressure sintering stage performed sequentially. The low-pressure suppression stage is as follows: when the temperature rises to the first temperature zone, inert gas is introduced to maintain the furnace at the first partial pressure. The high-pressure sintering stage is as follows: when the temperature continues to rise to the second temperature zone and the final sintering temperature, the content of inert gas in the furnace is adjusted to keep the furnace at the second partial pressure and maintain the temperature at the second partial pressure. The second partial voltage is higher than the first partial voltage.

3. The method for preparing a sintered cerium-iron-boron composition according to claim 2, characterized in that, In S1, the zirconium (Zr) and titanium (Ti) in the raw materials are added in the form of Fe-B-Zr master alloy and Fe-B-Ti master alloy, respectively; In the Fe-B-Zr master alloy and Fe-B-Ti master alloy, the content of Zr or Ti is 15wt% to 20wt%.

4. The method for preparing a sintered cerium-iron-boron composition according to claim 2, characterized in that, In S1, the process parameters for preparing the rapidly solidifying cast sheet are as follows: The alloy liquid casting temperature is 1420℃~1480℃, the copper roller linear speed is 2.5m / s~3.5m / s, and the thickness of the prepared rapid solidification casting sheet is 0.25mm~0.35mm.

5. The method for preparing a sintered cerium-iron-boron composition according to claim 2, characterized in that, In S3, the process parameters for the air jet mill crushing are: The air source pressure is 0.7MPa~0.8MPa, the grinding pressure is 0.6MPa~0.7MPa, the classifier speed is 3500rpm~4500rpm, and the average particle size D50 of the fine powder is 2.8μm~3.2μm. An organic antioxidant lubricant is added online during the powdering process. The organic antioxidant lubricant is selected from zinc stearate, stearamide, oleic acid, polyethylene glycol or a combination thereof, and the amount of the organic antioxidant lubricant added is 0.08wt% to 0.12wt% of the total weight of the powder.

6. The method for preparing a sintered cerium-iron-boron composition according to claim 2, characterized in that, In S4, the process parameters for the orientation forming and isostatic pressing treatment are as follows: The orientation magnetic field strength is 1.8T~2.2T, and the compression pressure is 15MPa~20MPa; The medium pressure for cold isostatic pressing is 180MPa~200MPa, and the holding time is 60s~90s.

7. The method for preparing a sintered cerium-iron-boron composition according to claim 2, characterized in that, In S5, the specific process of the vacuum exhaust stage is as follows: The temperature is increased from room temperature to T1 at a heating rate of 5℃ / min to 8℃ / min, while maintaining a vacuum level of ≤5×10⁻⁶ inside the furnace. -2 Pa; T1 is selected from the temperature range of 750℃ to 800℃.

8. The method for preparing a sintered cerium-iron-boron composition according to claim 7, characterized in that, In S5, the specific process of the low-voltage suppression stage is as follows: Inert gas is introduced into the furnace, and the furnace pressure is adjusted and maintained at the first partial pressure P1. At the same time, the temperature is raised from T1 to T2 at a heating rate of 3℃ / min to 5℃ / min. Wherein, T2 is selected from a temperature in the range of 980℃ to 1020℃, and the first partial pressure P1 is selected from a pressure value in the range of 600Pa to 800Pa.

9. The method for preparing a sintered cerium-iron-boron composition according to claim 7, characterized in that, In S5, the specific process of the high-pressure sintering stage is as follows: The temperature is increased from T2 to the final sintering temperature Tsinter at a heating rate of 2℃ / min to 3℃ / min, during which the partial pressure inside the furnace is maintained at the second partial pressure P2, and the temperature is held at Tsinter and P2 for 3.0h to 4.0h. The Tsinter is selected from a temperature range of 1090℃ to 1110℃, and the second partial pressure P2 is selected from a pressure value range of 2500Pa to 3500Pa.

10. The method for preparing a sintered cerium-iron-boron composition according to claim 2, characterized in that, It also includes S6, which involves performing a two-stage aging treatment on the sintered magnet: Level 1 aging: Heat to 890℃~910℃, hold for 2.5h~3.0h, then air cool; Secondary aging: Heat to 500℃~520℃, hold for 3.0h~4.0h, then rapidly air-cool to room temperature, with a cooling rate ≥30℃ / min.