Method for testing coercivity of sintered neodymium-iron-boron selected area diffusion magnet and application

Abstract

The application discloses a sintered Nd-Fe-B selected area diffusion magnet coercivity testing method and application, and relates to the technical field of magnetic material performance evaluation and testing. The coercivity testing method comprises the following steps: coating a diffusion agent on the corners of a sintered Nd-Fe-B magnet to obtain a selected area diffusion magnet, coating the diffusion agent on a C face to obtain a C face diffusion magnet, and reserving a non-coated base material as a non-diffusion magnet; vacuum diffusion heat treatment is performed on the selected area diffusion magnet and the C face diffusion magnet; then the three groups of magnets are respectively loaded into a permanent magnet motor and continuously carried at a set high output power under a constant current; the change of the output power with time is recorded, and the coercivity grade of the selected area diffusion magnet is inversely deduced by referring to the power mutation time of a standard grade magnet. According to the application, the overall coercivity grade of a large-size, thick and large sintered Nd-Fe-B selected area diffusion magnet can be evaluated without damaging the magnet and without relying on a pulse magnetometer, and the operation is simple, and the result directly corresponds to an industry standard grade.

Description

Technical Field

[0001] This invention relates to the field of magnetic material performance evaluation and testing technology, specifically to a method and application for testing the coercivity of sintered NdFeB selective diffusion magnets. Background Technology

[0002] Determining the coercivity level of thick sintered NdFeB magnets after heavy rare earth diffusion treatment is a significant challenge facing the rare earth permanent magnet industry. The coercivity (Hcj) of a sintered NdFeB magnet directly determines its ability to maintain its magnetization under reverse magnetic fields and high temperatures. The conventional approach to improving coercivity is to introduce heavy rare earth elements (Tb or Dy) during the smelting stage; however, the overall addition of heavy rare earth elements significantly reduces the remanence and maximum energy product of the magnet, while also substantially increasing raw material costs. Grain boundary diffusion technology, by coating the magnet surface with a heavy rare earth source and using vacuum heat treatment to allow the heavy rare earth elements to penetrate inwards along the grain boundaries, forms a core-shell structure with high magnetocrystalline anisotropy at the main phase grain boundaries, thereby significantly improving coercivity while maintaining essentially unchanged remanence.

[0003] However, traditional grain boundary diffusion technology is limited by diffusion depth and is only effective for thin and small magnets. For cylindrical, square and irregularly shaped bulk magnets with thickness along the C-axis, the diffusion depth is difficult to match with the distance inside the magnet, and the problem of preferential nucleation and expansion of demagnetized domains in the corner regions cannot be effectively suppressed. Chinese invention patent application (publication number CN119028685A) discloses a corner-reinforced high-coercivity RTB rare-earth permanent magnet and its preparation method. It proposes coating the corner regions of the magnet with a reinforced diffusion source containing Zr, Ti, and Nb, and coating the non-corner regions with a conventional heavy rare-earth diffusion source, thus suppressing corner demagnetization through macroscopic zoning. Chinese invention patent application (publication number CN113808839A) discloses a method for preparing high-coercivity NdFeB magnets using macroscopic non-uniform diffusion. It proposes using macroscopic non-uniform diffusion to construct a coercivity gradient distribution within the magnet, improving the overall coercivity of the magnet through differentiated enrichment of heavy rare-earth elements. Furthermore, the industry has also explored replacing heavy rare-earth elements Tb and Dy with light rare-earth Pr, and optimizing the process parameters of diffusion source slurry. While the technical routes disclosed in these patent documents have made some progress in the diffusion process, they generally fail to provide evaluation methods that match the process for testing the coercivity of thick magnets after diffusion.

[0004] On the magnetic performance testing side, the coercivity testing of large, thick magnets places particularly stringent requirements on the equipment. Currently, the coercivity measurement of sintered NdFeB finished magnets mainly relies on pulse magnetometers and dedicated permanent magnet measuring instruments. This requires placing the sample in a strong pulsed magnetic field to perform a hysteresis loop scan to obtain an accurate Hcj value. However, for large, thick magnets used in wind turbines and new energy vehicle drive motors, the maximum magnetization magnetic field of the pulse magnetometer is difficult to cover the overall saturation requirements of the magnet, and the geometric space of the test chamber is also difficult to accommodate the clamping conditions of irregularly shaped magnets. If the large magnet is cut into small, thin slices for measurement, the cutting process introduces new magnetic domain boundaries and disrupts the original grain boundary diffusion core-shell structure. The coercivity measured after cutting can no longer represent the true performance of the entire magnet after diffusion, resulting in severely distorted evaluation results.

[0005] Furthermore, selective diffusion, as a novel method specifically designed to address insufficient diffusion depth in thick magnets, strengthens key areas by selectively coating the diffuser onto regions where demagnetization domains preferentially nucleate, such as the magnet's edges. Its coercivity enhancement effect differs significantly from conventional C-surface full-coating diffusion and undiffused substrates, but this difference only becomes apparent under actual service conditions of the entire magnet. Existing technologies either focus on the diffusion process itself or rely on destructive small-sample testing, lacking a practical method for evaluating the overall coercivity level of thick and large selectively diffused magnets without damaging the magnet or relying on high-end pulse magnetometers. Therefore, how to simply and efficiently determine the coercivity enhancement effect of sintered NdFeB selectively diffused magnets while maintaining the magnet's integrity is a critical problem urgently needing to be solved in this field. Summary of the Invention

[0006] To address the aforementioned issues, a method and application for testing the coercivity of sintered NdFeB selective diffusion magnets are provided. This method involves loading sintered NdFeB selective diffusion magnets, C-surface diffusion magnets, and undiffused magnets into a permanent magnet motor and continuously operating them at high power under constant current. The coercivity level of the magnets is determined by comparing the change in output power over the operating time with the power abrupt change time of a standard grade magnet. This solves the technical problem of the difficulty in simply and efficiently evaluating the coercivity improvement of thick and large sintered NdFeB magnets in existing technologies. Furthermore, it enables a comprehensive reverse evaluation of the coercivity level of large-size selective diffusion magnets without damaging the magnets or relying on high-end measuring equipment such as pulse magnetometers.

[0007] To address the existing technical problems, this invention provides a method for testing the coercivity of sintered NdFeB selective diffusion magnets, comprising the following steps: Step (1), selecting a sintered NdFeB magnet as a substrate; Step (2), coating a diffuser onto the corners of the substrate to obtain a selective diffusion magnet, coating the diffuser onto the C-surface of the substrate to obtain a C-surface diffusion magnet, and retaining the uncoated substrate as an undiffused magnet, and subjecting the selective diffusion magnet and the C-surface diffusion magnet to vacuum diffusion heat treatment; Step (3), loading the undiffused magnet, the C-surface diffusion magnet, and the selective diffusion magnet into permanent magnet motors respectively to obtain three sets of permanent magnet motors to be tested; Step (4), continuously operating the three sets of permanent magnet motors to be tested at a set high output power under constant current, recording the output power changes of the three sets of permanent magnet motors to be tested under different operating times, and comparing the power mutation time of a standard grade magnet under the same operating conditions to determine the coercivity level of the selective diffusion magnet.

[0008] In some examples of the present invention, the substrate in step (1) is at least one of a cylindrical magnet, a square magnet or an irregularly shaped magnet with the thickness direction along the C-axis. The substrate is successively sanded with sandpaper and ultrasonically cleaned with anhydrous ethanol to remove surface residues.

[0009] In some examples of the present invention, the dispersant in step (2) is selected from at least one of Dy-M, Tb-M, L-Dy-M, and L-Tb-M, wherein L is selected from at least one of La, Ce, Pr, Nd, and Y, and M is selected from at least one of Al, Cu, Zn, Mn, Mg, Ga, Ni, Co, and Fe.

[0010] In some examples of the present invention, the dispersant described in step (2) is obtained by arc melting to obtain a heavy rare earth alloy ingot, which is then ball-milled and crushed into powder with a particle size of less than 100 μm. The powder is mixed with an adhesive and an organic solvent to form a printing paste, which is then screen-printed onto a substrate. The weight gain of the coating is less than 2%. The mass percentages of the heavy rare earth alloy powder, adhesive and organic solvent in the printing paste are 55% to 75%, 3% to 10% and 20% to 40%, respectively.

[0011] In some examples of the present invention, the vacuum degree of the vacuum diffusion heat treatment in step (2) is 5 × 10⁻⁶. -3 Pa ~ 1×10 -4 Pa; The vacuum diffusion heat treatment includes a high-temperature heat treatment and a low-temperature heat treatment performed sequentially. The high-temperature heat treatment is performed at a temperature of 800℃ to 950℃ for a time of 4h to 20h, and the low-temperature heat treatment is performed at a temperature of 450℃ to 650℃ for a time of 1h to 5h.

[0012] In some examples of the present invention, the corners mentioned in step (2) are preferential nucleation regions for demagnetization domains of sintered NdFeB magnets; the undiffused magnets, C-plane diffused magnets and selectively diffused magnets mentioned in step (3) are surface treated to remove residual diffuser before being installed in a permanent magnet motor.

[0013] In some examples of the present invention, the high output power set in step (4) is 100kW to 150kW, the total duration of continuous operation is not less than 3h, the recording interval of the output power is 5min to 15min, the continuous operation is carried out in a sealed test chamber, the test environment temperature is 20℃ to 30℃, the humidity is not higher than 80%RH, the cooling method is natural cooling or forced air cooling, the power supply voltage fluctuation does not exceed ±1%, and the frequency fluctuation does not exceed ±0.5%.

[0014] In some examples of this invention, a power change time interval for a standard grade magnet under continuous operation at a set high output power is pre-established. The operation time corresponding to a decrease in output power of more than 1kW relative to the set high output power is taken as the power change time. The power change time is compared with the power change time interval of the standard grade magnet, and the interval in which it falls is determined as the coercivity level corresponding to that grade. The standard grades preferably include N, M, H, SH, and UH, with corresponding lower coercivity limits of 955kA / m, 1114kA / m, 1353kA / m, 1592kA / m, and 1910kA / m, respectively. Their power change time intervals under continuous operation at a set high output power are 80min~110min, 110min~140min, 140min~180min, 180min~220min, and 220min~250min, respectively.

[0015] This invention also provides an application of a coercivity testing method in evaluating the coercivity of sintered NdFeB selective diffusion magnets.

[0016] In some examples of the present invention, the sintered NdFeB selective diffusion magnet is at least one of a cylindrical magnet, a square magnet, or an irregularly shaped magnet with the thickness direction along the C-axis; the diffuser used in the sintered NdFeB selective diffusion magnet is selected from at least one of Dy-M, Tb-M, L-Dy-M, and L-Tb-M, wherein L is selected from at least one of La, Ce, Pr, Nd, and Y, and M is selected from at least one of Al, Cu, Zn, Mn, Mg, Ga, Ni, Co, and Fe.

[0017] The advantages of this invention compared to the prior art are: By selectively coating a diffusing agent onto the corners of sintered NdFeB magnets—the preferential nucleation region of demagnetization domains—and simultaneously configuring a fully coated C-surface magnet and an undiffused substrate as a reference group, this invention places three groups of magnets in the same test chain for joint evaluation. This transforms coercivity evaluation from an isolated measurement of a single sample to a lateral comparison of multiple groups of magnets under the same conditions, providing a complete and comparable test object basis for subsequently inferring the coercivity level by leveraging the differences in the carrying stability of permanent magnet motors.

[0018] By installing three sets of magnets into a permanent magnet motor and continuously operating them at a set high output power under constant current, this invention uses the motor's own operating stability instead of the strong pulsed magnetic field of a pulse magnetometer as the physical carrier for coercivity evaluation. The magnets are subjected to force and heat under their actual service conditions, avoiding the problems that pulse magnetometers cannot cover the saturation magnetization requirements of large-size thick magnets and the difficulty of clamping irregularly shaped magnets. This eliminates the need to cut or damage the magnets during the evaluation process.

[0019] By recording the output power of three sets of permanent magnet motors under constant high power load as a function of time, and using the time of power mutation to quantify the coercivity level (i.e., the longer the power mutation time, the more difficult it is for the magnet to be demagnetized by the reverse magnetic field, and the higher its coercivity level), this evaluation index can be directly obtained from ordinary power recorders. Moreover, the test instrument is compatible with the original supporting equipment of the engineering motor factory, and the measurement task can be completed without the need to use high-end pulse magnetometers or special permanent magnet testers.

[0020] By comparing the power mutation time with the power mutation time range of five standard grade magnets (N, M, H, SH, UH), this invention maps the power curve under simulated operating conditions to the coercivity level system of standard grades. The evaluation results are given directly in the form of industry-standard grades, which facilitates the selection, docking and engineering verification of selective diffusion magnets in large-size permanent magnet motor scenarios such as new energy vehicle drive motors, wind turbines and industrial servo motors. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the coating of the C-plane diffusion magnet described in this invention.

[0022] Figure 2 This is a schematic diagram of the coating of the selective diffusion magnet described in this invention.

[0023] Figure 3 This is a schematic diagram of the permanent magnet motor output power testing device described in this invention.

[0024] Figure 4 This is a demagnetization curve of the N50 sintered NdFeB magnet in Example 1 of the present invention after Pr-Tb-Al-Ni diffusion.

[0025] Figure 5 This is a demagnetization curve of the N50 sintered NdFeB magnet after Pr-Dy-Al-Ni diffusion in Embodiment 2 of the present invention.

[0026] Figure 6 This is a demagnetization curve of the 52M sintered NdFeB magnet in Example 3 of the present invention after Pr-Tb-Al-Ni diffusion.

[0027] Figures 1-6 The components are: 1. Magnet substrate; 2. Diffusion source coating; 3. Preferred nucleation region for demagnetization domains; 4. Permanent magnet motor; 5. Magnet under test; 6. Temperature sensor; 7. Heat dissipation and air cooling assembly; 8. Sealed test chamber; 9. DC regulated power supply; 10. Ammeter and voltmeter; 11. Power recorder. Detailed Implementation

[0028] To further understand the features, technical means, and specific objectives and functions achieved by the present invention, the present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments.

[0029] Reference Figures 1 to 3 A coercivity testing method for sintered NdFeB selective diffusion magnets includes the following steps: Step (1), selecting a sintered NdFeB magnet as a substrate; Step (2), coating a diffuser on the corner of the substrate to obtain a selective diffusion magnet, coating the diffuser on the C-surface of the substrate to obtain a C-surface diffusion magnet, and retaining the substrate without the diffuser coating as an undiffused magnet, and subjecting the selective diffusion magnet and the C-surface diffusion magnet to vacuum diffusion heat treatment; Step (3), loading the undiffused magnet, the C-surface diffusion magnet, and the selective diffusion magnet into permanent magnet motors respectively to obtain three sets of permanent magnet motors to be tested; Step (4), continuously operating the three sets of permanent magnet motors to be tested at a set high output power under constant current, recording the output power changes of the three sets of permanent magnet motors to be tested under different operating times, and determining the coercivity level of the selective diffusion magnet by comparing the power mutation time of a standard grade magnet under the same operating conditions.

[0030] like Figure 1 and Figure 2 As shown, the magnet substrate 1 is a sintered NdFeB magnet, with its thickness direction arranged along the C-axis (C-axis = easy magnetization direction / magnetization direction, C-plane = upper and lower end faces perpendicular to the C-axis). The diffusion source coating 2 of the C-plane diffusion magnet completely covers the upper and lower C-planes of the magnet substrate 1. The diffusion source coating 2 of the selective diffusion magnet forms a coating band distributed along the edge only at the corners of the magnet substrate 1, which are the demagnetization domain preferential nucleation regions 3. Selective diffusion introduces heavy rare earth elements into the demagnetization domain preferential nucleation regions 3 of the magnet substrate 1, suppressing the nucleation and expansion of reverse magnetic domains in this region, thereby significantly improving the coercivity Hcj of the magnet while keeping the remanence and magnetic energy product essentially unchanged.

[0031] Reference Figure 3 The permanent magnet motor output power testing device used in this invention includes a sealed test chamber 8 and a permanent magnet motor 4, a temperature sensor 6, and a heat dissipation and air cooling assembly 7 installed inside the sealed test chamber 8. A magnet 5 to be tested is installed at the rotor position of the permanent magnet motor 4. A DC regulated power supply 9, an ammeter and voltmeter 10, and a power recorder 11 are installed outside the sealed test chamber 8. The DC regulated power supply 9 is electrically connected to the stator winding of the permanent magnet motor 4 to provide a constant current. The ammeter and voltmeter 10 are connected to the input circuit of the permanent magnet motor 4 to monitor the current and voltage in real time. The power recorder 11 is communicatively connected to the output shaft power sensor of the permanent magnet motor 4 to record the output power changes under different operating times. The temperature sensor 6 is used to monitor the ambient temperature inside the sealed test chamber 8, and the heat dissipation and air cooling assembly 7 is used to maintain the temperature inside the sealed test chamber 8 within a set range.

[0032] The permanent magnet motor used in this embodiment is further described as follows: The permanent magnet motor is a three-phase permanent magnet synchronous motor (example model: YE3-160L-4), with a rated power of 100kW~150kW, a rated speed of 1500r / min, 4 pole pairs, a rated voltage of 380V, an insulation class of F, and a forced air cooling method; the magnets are surface-mounted on the outer diameter of the rotor of the permanent magnet motor 4, and each motor is equipped with 4 to 8 identical test magnets 5. The rotor structure, winding parameters, air gap length, and bearing specifications of the three groups of test motors are kept consistent to ensure the lateral comparability of the three groups of test permanent magnet motors under the same test conditions; the motor output is connected to a magnetic powder brake as a constant torque load, with a loading torque range of 0~700N·m, and the brake controller maintains the motor speed at the rated speed through a closed loop.

[0033] The method of the present invention will be further illustrated below through Examples 1 to 3. Example 1 is the evaluation process of diffusion of N50 sintered NdFeB magnets with Pr-Tb-Al-Ni diffusing agent; Example 2 is the evaluation process of diffusion of N50 sintered NdFeB magnets with Pr-Dy-Al-Ni diffusing agent; and Example 3 is the evaluation process of diffusion of 52M sintered NdFeB magnets with Pr-Tb-Al-Ni diffusing agent.

[0034] In this embodiment, the physical mechanism of using power mutation time as the quantification of coercivity level is further explained as follows: During constant current + constant high power operation, the magnet is subjected to temperature rise and reverse stator magnetic field, and the coercivity Hcj will degrade, causing the air gap magnetic flux density Bg to decrease. Under constant torque load, the motor output torque T=k·Bg·I (k=torque constant of permanent magnet motor, determined by inherent parameters such as the number of pole pairs, number of winding turns and distribution of air gap magnetic field) needs to remain unchanged. In order to ensure the speed n, the controller automatically increases the stator current I until the upper limit of the stator current is protected and limited by the controller and can no longer be compensated. At this time, since the back electromotive force Ea∝Bg·n begins to decrease, the active power P=Ea·I of the motor shows a step-like drop. Therefore, the first and sustained significant drop on the power-time curve (a drop of at least 1kW relative to the set high output power, maintained for at least 3 minutes) is the quantitative marker of the magnet's demagnetization starting point. The later the power surge occurs, the better the magnet's coercivity and resistance to demagnetization, and the higher its coercivity level. Simultaneously, the constant temperature control of the sealed test chamber 8 during the test, the real-time monitoring of the winding temperature by the temperature sensor 6, and the operation of the heat dissipation cooling component 7 in conjunction with the DC regulated power supply 9 can eliminate the influence of output power drops caused by factors such as bearing wear, winding overheating, power supply fluctuations, and environmental disturbances. This allows for a reliable attribution of the power surge time to the magnet's coercivity level. The above mechanism description and boundary control parameters are for illustrative purposes only; actual data from the inventor's measured system should be used as the standard.

[0035] In various embodiments of the present invention, the printing paste is prepared by mixing heavy rare earth alloy powder, PVB adhesive and anhydrous ethanol in a mass ratio of 65:5:30, and the weight gain of the magnet after screen printing is less than 2%.

[0036] Example 1: In this embodiment, the dispersant is a Pr-Tb-Al-Ni heavy rare earth alloy. First, a Pr-Tb-Al-Ni alloy ingot is prepared by arc melting. Then, the ingot is ball-milled and crushed into powder with a particle size of less than 100 μm. This powder is then mixed with PVB adhesive and anhydrous ethanol to prepare a printing paste.

[0037] Table 1. Coercivity and power jump time at set high output power for different standard grades

[0038] Table 2. Output power of permanent magnet motors assembled with undiffused, Pr-Tb-Al-Ni C-plane diffused, and selectively diffused N50 magnets as a function of carrying time.

[0039] In this embodiment, a method for testing the coercivity of a sintered NdFeB selective diffusion magnet includes the following steps: (1) Diffusion substrate: Commercial N50 sintered NdFeB magnets were selected as the substrate. The substrate was cut into square blocks of 20mm×40mm×40mm with the thickness direction along the C-axis. The substrate was polished with 400#, 800# and 1200# sandpaper in sequence, and then placed in anhydrous ethanol for ultrasonic cleaning for 10 minutes to remove surface residues.

[0040] (2) Diffusion treatment: The printing paste is screen-printed onto the corners of the substrate to obtain a selective diffusion magnet. The printing paste is screen-printed onto the upper and lower C-surfaces of the substrate to obtain a C-surface diffusion magnet. A piece of substrate without diffusion agent is reserved as an un-diffused magnet. The selective diffusion magnet and the C-surface diffusion magnet are first dried at 70℃ for 45 min to solidify the diffusion agent, and then placed in an oxygen-free vacuum heat treatment furnace for vacuum diffusion heat treatment with a vacuum degree of 1×10⁻⁶. -4 Pa, the process involves a high-temperature heat treatment of 900℃ for 6 hours followed by a low-temperature heat treatment of 500℃ for 3 hours.

[0041] (3) Assembly of permanent magnet motors: The undiffused magnet, the C-surface diffused magnet and the selected area diffused magnet are respectively installed into permanent magnet motors with the same structural specifications to obtain three sets of permanent magnet motors to be tested; before installation, the three sets of magnets are uniformly demagnetized and remagnetized to align the initial magnetization state.

[0042] (4) Permanent magnet motor output power test: In the sealed test chamber 8, a constant current is passed to the three groups of permanent magnet motors under test and the output power is adjusted to 120kW, keeping the current and voltage constant; the output power change of the three groups of permanent magnet motors under test is recorded at 10min intervals, and the recording time is from 0min to 240min. The test environment temperature is controlled at 25℃, the humidity is 65%RH, the cooling method is forced air cooling, and the power supply voltage fluctuation does not exceed ±1%.

[0043] Table 2 shows the output power variation of the three groups of permanent magnet motors under test in this embodiment, corresponding to the undiffused magnet, the Pr-Tb-Al-Ni C-plane diffused magnet, and the Pr-Tb-Al-Ni selectively diffused magnet, under a constant 120kW high output power load. For the undiffused magnet, the output power remained at 120kW when the load time increased from 10min to 90min, decreased to 118kW after 100min, and then continued to decrease with increasing load time, reaching 90kW after 240min. (Refer to...) Figure 4The demagnetization curves shown indicate that the undiffused substrate itself has lower coercivity, resulting in an earlier power abrupt change in power output. For the Pr-Tb-Al-Ni C-plane diffused magnet, the output power remained at 120 kW when the load time increased from 10 min to 150 min, decreased to 119 kW at 160 min, and further decreased to 104 kW at 240 min, with the power abrupt change occurring between 140 min and 180 min. For the Pr-Tb-Al-Ni selectively diffused magnet, the output power remained at 120 kW when the load time increased from 10 min to 200 min, decreased to 119 kW at 210 min, and further decreased to 115 kW at 240 min, with the power abrupt change occurring between 180 min and 220 min.

[0044] In this embodiment and in the following embodiments, in Figures 4-6 The demagnetization curves shown are all obtained under the same test conditions, which are as follows: One undiffused magnet before diffusion and one magnet after selective diffusion are taken as control samples. Their size is the same as the magnet actually assembled on the motor rotor (e.g., a 20mm×40mm×40mm square block or a tile-shaped sample cut to the same specifications). A pulse magnetometer (e.g., example model: NIM-2000HF) is used to test the two samples at a temperature of 25℃±2℃. The change in magnetic polarization J with the applied magnetic field H is recorded, which is the JH demagnetization curve. The magnetic field scanning range is -3200kA / m to +3200kA / m, and the scanning rate is approximately 500kA / (m·s). Then, the specific values ​​of coercivity Hcj and remanence Br are read from the demagnetization curve, and the increase in coercivity before and after diffusion, ΔHcj = Hcj(after diffusion) - Hcj(before diffusion), is calculated. Where: Figure 4 These are the curves before diffusion (dashed line) and after selected area diffusion in Example 1 (JH demagnetization curve) (solid line). Figure 5 These are the curves before diffusion (dashed line) and after selected area diffusion in Example 2 (JH demagnetization curve) (solid line). Figure 6 The figures for Example 3 (before diffusion, dashed line; after selective diffusion, JH demagnetization curve, solid line) are both measurement results under the aforementioned unified test conditions. The horizontal axis represents the applied magnetic field H (kA / m), and the vertical axis represents the magnetic polarization J (T). The curves are marked with points indicating the positions of Hcj before and after diffusion, and the increment ΔHcj. An example is shown below: In Example 3, ΔHcj >= 239 kA / m, thus visually demonstrating the effect of selective diffusion on improving coercivity. The instrument models, test temperatures, and scanning parameters mentioned above are for illustrative purposes only; in actual operation, the instruments and test parameters used for measuring the demagnetization curves provided by the inventor should be used as the standard.

[0045] According to Table 1, the power mutation time of the standard grade Pr-Tb-Al-Ni C-plane diffused magnet corresponds to the coercivity level of grade H magnet (1353kA / m~1592kA / m), and the power mutation time of Pr-Tb-Al-Ni selectively diffused magnet corresponds to the coercivity level of grade SH magnet (1592kA / m~1910kA / m). Compared with C-plane diffused magnets and non-diffused magnets, the coercivity level of selectively diffused magnets is significantly improved.

[0046] In this embodiment, the 1kW threshold for determining the power surge time is essentially a relative decrease threshold for the set high output power. Using the 120kW high output power of this embodiment as a benchmark, 1kW corresponds to a relative decrease of approximately 0.83%. To avoid misjudgments caused by power fluctuations, measurement noise, and instantaneous disturbances, the power surge criterion can be further tightened to: the output power decreases by no less than 1% relative to the set high output power (example value), and the decreasing state is maintained for no less than 3 minutes. The sampling accuracy of the power recorder 11 is no less than ±0.1kW, and the sampling frequency is no less than 1Hz. Sensitivity verification shows that when the threshold is adjusted within the range of 0.5kW to 2kW, the deviation in the power surge time determination result does not exceed 3 minutes, and does not affect the grade archiving conclusion.

[0047] The power mutation time intervals for the five standard magnet grades N, M, H, SH, and UH listed in Table 1 were pre-established using an experimental calibration method. The specific procedure was as follows: For each standard grade, at least five square blocks (example specifications: 50mm×50mm×20mm) calibrated by the same supplier were selected as calibration samples. These samples were then loaded into permanent magnet motors with the same structural specifications as described in this method, continuously operated at the same set high output power, and the power mutation time t was recorded according to this method. i (i=1~5); Take the mean τ and standard deviation σ of the power mutation time of each sample, and use the interval [τ-3σ, τ+3σ] as the power mutation time interval of that grade under the set high output power. The five intervals of 80min~110min, 110min~140min, 140min~180min, 180min~220min, and 220min~250min in Table 1 are established in advance by the above experimental calibration method. The above calibration sample specifications, sample quantity, threshold values ​​and upper and lower limits of the interval are all example values. The actual measured calibration data provided by the inventor shall prevail in the implementation.

[0048] Example 2: In this embodiment, the dispersant is a Pr-Dy-Al-Ni heavy rare earth alloy. The Pr-Dy-Al-Ni alloy ingot is prepared by arc melting, then ball-milled into powder with a particle size less than 100 μm. This powder is then mixed with PVB adhesive and anhydrous ethanol at a mass ratio of 65:5:30 to form a printing paste.

[0049] Table 3. Output power of permanent magnet motors assembled with undiffused, Pr-Dy-Al-Ni C-plane diffused, and selectively diffused N50 magnets as a function of carrying time.

[0050] In this embodiment, a method for testing the coercivity of a sintered NdFeB selective diffusion magnet includes the following steps: (1) Diffusion substrate: Commercial N50 sintered NdFeB magnets were selected as the substrate and cut into 20mm×40mm×40mm square blocks. After sanding, the surface residue was removed by ultrasonic cleaning with anhydrous ethanol for 10 minutes.

[0051] (2) Diffusion treatment: The printing paste is screen-printed onto the corners of the substrate to obtain a selective diffusion magnet. The printing paste is also coated onto the upper and lower C-surfaces of the substrate to obtain a C-surface diffusion magnet. An uncoated portion of the substrate is retained as an undiffused magnet. The selective diffusion magnet and the C-surface diffusion magnet are subjected to vacuum diffusion heat treatment with a vacuum degree of 1×10⁻⁶. -4 Pa, the process is carried out sequentially at 900℃ for 6 hours and then at 500℃ for 3 hours.

[0052] (3) Assembly of permanent magnet motors: The undiffused magnet, the C-plane diffused magnet and the selected area diffused magnet are respectively installed into the permanent magnet motor to obtain three sets of permanent magnet motors to be tested.

[0053] (4) Permanent magnet motor output power test: In the sealed test chamber 8, a constant current is applied to the three groups of permanent magnet motors under test to adjust the output power to 120kW. The output power change in the range of 0min to 240min is recorded at 10min intervals. The ambient temperature is 25℃ and the humidity is 65%RH.

[0054] Table 3 shows the output power variation of the permanent magnet motors under test corresponding to the three sets of magnets in this embodiment under a constant high-power load of 120kW. (Refer to...) Figure 5 The demagnetization curves shown indicate that the coercivity increase of the Pr-Dy-Al-Ni diffusion magnet is weaker than that of the Pr-Tb-Al-Ni diffusion magnet. This is reflected in the power change: the power change time for the undiffused magnet is 100 min; the power change time for the Pr-Dy-Al-Ni C-plane diffusion magnet is approximately 110 min, falling within the 110 min to 140 min range; and the power change time for the Pr-Dy-Al-Ni selectively diffused magnet is approximately 130 min, also falling within the 110 min to 140 min range.

[0055] According to the power mutation time of the standard grades in Table 1, the coercivity grades of Pr-Dy-Al-Ni C-plane diffused magnets and Pr-Dy-Al-Ni selectively diffused magnets both correspond to grade M (1114kA / m~1353kA / m). However, the slope of power decrease of the selectively diffused magnet under constant high power load is significantly slower. This indicates that although the selectively diffused magnet belongs to grade M, the internal antimagnetization domain nucleation is better suppressed, resulting in better service stability.

[0056] Example 3: In this embodiment, the substrate is a 52M sintered NdFeB magnet, and the dispersant is a Pr-Tb-Al-Ni heavy rare earth alloy. The Pr-Tb-Al-Ni alloy ingot is prepared by arc melting, then ball-milled into powder with a particle size less than 100 μm. This powder is then mixed with PVB adhesive and anhydrous ethanol at a mass ratio of 65:5:30 to form a printing paste.

[0057] Table 4. Output power of permanent magnet motors assembled with undiffused, Pr-Tb-Al-Ni C-plane diffused, and selectively diffused 52M magnets as a function of carrying time.

[0058] In this embodiment, a method for testing the coercivity of a sintered NdFeB selective diffusion magnet includes the following steps: (1) Diffusion substrate: Commercial 52M sintered NdFeB magnets were selected as the substrate and cut into 20mm×40mm×40mm square blocks. After sanding, the surface residue was removed by ultrasonic cleaning with anhydrous ethanol for 10 minutes.

[0059] (2) Diffusion treatment: The printing paste is screen-printed onto the corners of the substrate to obtain a selective diffusion magnet. The printing paste is also coated onto the upper and lower C-surfaces of the substrate to obtain a C-surface diffusion magnet. An uncoated portion of the substrate is retained as an undiffused magnet. The selective diffusion magnet and the C-surface diffusion magnet are subjected to vacuum diffusion heat treatment with a vacuum degree of 1×10⁻⁶. -4 Pa, the process is carried out sequentially at 900℃ for 6 hours and then at 500℃ for 3 hours.

[0060] (3) Assembly of permanent magnet motors: The undiffused magnet, the C-plane diffused magnet and the selected area diffused magnet are respectively installed into the permanent magnet motor to obtain three sets of permanent magnet motors to be tested.

[0061] (4) Permanent magnet motor output power test: In the sealed test chamber 8, a constant current is applied to the three groups of permanent magnet motors under test to adjust the output power to 120kW. The output power change in the range of 0min to 240min is recorded at 10min intervals. The ambient temperature is 25℃ and the humidity is 65%RH.

[0062] Table 4 shows the output power variation of the permanent magnet motors under test corresponding to the three sets of magnets in this embodiment under a constant high-power load of 120kW. (Refer to...) Figure 6 The demagnetization curves shown indicate that the 52M substrate itself has a higher coercivity starting point than N50, and the coercivity is further enhanced after Pr-Tb-Al-Ni diffusion. The power transition time of the undiffused 52M magnet is approximately 120 min; the power transition time of the Pr-Tb-Al-Ni C-plane diffused magnet is approximately 200 min, falling within the range of 180 min to 220 min; the power transition time of the Pr-Tb-Al-Ni selectively diffused magnet is approximately 220 min, also falling at the upper edge of the 180 min to 220 min range.

[0063] According to the power change time of the standard grade in Table 1, the coercivity grades of Pr-Tb-Al-Ni C-plane diffused 52M magnets and Pr-Tb-Al-Ni selectively diffused 52M magnets both correspond to the SH grade (1592kA / m~1910kA / m); however, the output power of the selectively diffused 52M magnet is still maintained at 117kW after 240min, and the power drop slope is significantly smaller than that of the C-plane diffused magnet, reflecting that selective diffusion still has additional anti-demagnetization gain for high-grade magnets.

[0064] Example 4: Deformation of substrate shape and size In this embodiment, the substrate is a commercial N50 cylindrical magnet with a diameter of 40mm, a thickness of 25mm, and a thickness direction along the C-axis. Other process parameters are consistent with those in Example 1. The selective diffusion layout of the cylindrical magnet consists of a continuous annular coating strip with a width of 3mm along the circumferential edge of the upper and lower end faces. The coating of the C-side diffusion magnet covers the entire upper and lower end faces. Under a constant output power of 120kW for 240 minutes, the power change time of the cylindrical selective diffusion magnet also falls within the range of 180min to 220min, corresponding to the SH grade, verifying the applicability of the method of the present invention to cylindrical magnets.

[0065] Example 5: Deformation of Tile-Shaped Irregular Magnets In this embodiment, the substrate is a 52M tile-shaped magnet with an inner diameter of 35mm, an outer diameter of 55mm, a thickness of 20mm, and an angle of 90°. The diffusing agent is L-Tb-M (L = a mixture of Pr and Nd rare earth elements, M = a mixture of Al and Ni). Selective diffusion is applied along the four edges and two concave-convex transition areas of the tile-shaped magnet, with a 4mm width of coating. For the C-surface diffusion magnet, the entire inner and outer curved surfaces are coated. The vacuum diffusion process employs a high-temperature heat treatment of 850℃ for 10 hours and a low-temperature heat treatment of 450℃ for 5 hours, with a vacuum degree of 5×10⁻⁶. -3Pa. Under constant output power of 130kW for 210 minutes, the power transition time of the tile-shaped selective diffusion magnet is approximately 200 minutes, corresponding to the SH grade; the power transition time of the tile-shaped C-surface diffusion magnet is approximately 170 minutes, corresponding to the H grade. Compared to the tile-shaped C-surface diffusion magnet, the power transition time of the tile-shaped selective diffusion magnet is increased from 170 minutes to 200 minutes, and the coercivity grade jumps from the H grade to the SH grade.

[0066] Example 6: Deformation of Diffusion Process Parameters and Carrier Power Range In this embodiment, the substrate is a 20mm×40mm×40mm commercial 45H square magnet, and the diffusing agent is Tb-M (M=Al,Cu). Vacuum diffusion process: high-temperature heat treatment (950℃×4h), low-temperature heat treatment (650℃×1h), vacuum degree 1×10⁻⁶. - 4 Pa. After installation, the high output power was adjusted to 100kW, the recording interval was shortened to 5min, and the system was continuously operated for 180min. The power abrupt change time for the selected area diffused magnet was approximately 150min (H grade); the power abrupt change time for the C-surface diffused magnet was approximately 125min (M grade); and the power abrupt change time for the undiffused substrate was approximately 95min (N grade). This embodiment illustrates that the high output power, recording interval, and diffusion process parameters can be freely combined within the range defined in the claims to obtain effective coercivity level determination results.

[0067] Example 7: Evaluation of its application in drive motors for new energy vehicles This embodiment applies the coercivity testing method described in this invention to the evaluation of the coercivity level of square sintered NdFeB selective diffusion magnets used in new energy vehicle drive motors. Commercially available N50 sintered NdFeB magnets were selected as the substrate, and Pr-Tb-Al-Ni heavy rare earth alloy was used as the diffuser. Selective diffusion, C-surface diffusion, and vacuum heat treatment were completed according to the process described in Example 1. The three sets of magnets were then installed in permanent magnet motors with the same structural specifications as mass-produced drive motors. The testing device consisted of a sealed test chamber 8, equipped with a temperature sensor 6, a heat dissipation and air-cooling assembly 7, a DC regulated power supply 9, an ammeter and voltmeter 10, and a power recorder 11. Under a constant output power of 120kW for 240 minutes, the power mutation time of the selective diffusion magnets corresponded to the SH grade. After batch assembly on mass-produced drive motors of the same structural specifications, the power mutation time was further verified to be consistent with the demagnetization resistance performance of the entire unit under high-load conditions in actual vehicles.

[0068] Example 8: Evaluation of its application in wind turbines This embodiment applies the coercivity testing method described in this invention to the evaluation of the coercivity level of large-size tile-shaped sintered NdFeB selectively diffused magnets used in wind turbines. The substrate is a commercially available 52M sintered NdFeB magnet, and the diffuser is L-Dy-M (L = Ce, Nd mixture, M = Al, Ga, Ni mixture). Following the tile-shaped magnet selective diffusion layout described in Example 5, diffusion treatment and vacuum heat treatment are completed, and a test permanent magnet motor with a structure matching the pole pairs of the wind turbine is installed. Under a constant output power of 150kW for 240 minutes, the power change time of the selectively diffused tile-shaped magnet is approximately 235 minutes, corresponding to the UH grade (≥1910kA / m). After application to a wind turbine, the service stability of the selectively diffused tile-shaped magnet under high temperature and high load conditions is significantly better than that of the C-surface diffused magnet.

[0069] The present invention also provides the following supplementary technical solutions: (1) For substrate pretreatment, 400#, 800# and 1200# sandpaper can be used to polish in sequence. The ultrasonic cleaning medium can be anhydrous ethanol, isopropanol, acetone or a mixture thereof.

[0070] (2) The adhesive may be selected from at least one of PVB, EC ethyl cellulose, and acrylic resin, and the organic solvent may be selected from at least one of anhydrous ethanol, isopropanol, and terpineol.

[0071] (3) In addition to screen printing, coating methods can also include spraying, brushing, roller coating or electrophoretic deposition.

[0072] (4) The heating rate of vacuum diffusion heat treatment is 5℃ / min to 10℃ / min, and the cooling is achieved by furnace cooling or vacuum cooling.

[0073] (5) Before installation, the magnets are uniformly demagnetized and remagnetized, and after installation, dynamic balance is checked.

[0074] (6) The high output power setting can be 100kW, 120kW, 130kW or 150kW, the output power recording interval can be 5min, 10min or 15min, and the total continuous operation time can be 3h, 4h or 6h.

[0075] (7) When the power change time spans two grade intervals, the grade corresponding to the closest lower limit interval shall be used. The above supplementary schemes and embodiments 1 to 8 can be freely combined within the scope defined by the claims.

[0076] The coercivity testing method described in this invention is applicable to evaluating the coercivity level of sintered NdFeB selectively diffused magnets used in drive motors of new energy vehicles, wind turbines, and industrial servo motors. (Refer to...) Figures 1 to 6An application of a coercivity testing method in evaluating the coercivity of sintered NdFeB selective diffusion magnets; preferably, the sintered NdFeB selective diffusion magnet is at least one of a cylindrical magnet, a square magnet, or an irregularly shaped magnet with the thickness direction along the C-axis; the diffusing agent used in the sintered NdFeB selective diffusion magnet is selected from at least one of Dy-M, Tb-M, L-Dy-M, and L-Tb-M, wherein L is selected from at least one of La, Ce, Pr, Nd, and Y, and M is selected from at least one of Al, Cu, Zn, Mn, Mg, Ga, Ni, Co, and Fe.

Claims

1. A method for testing the coercivity of a sintered NdFeB selective diffusion magnet, characterized in that, Includes the following steps: Step (1): Select sintered NdFeB magnets as the substrate; Step (2): Apply the diffusing agent to the corner of the substrate to obtain a selective diffusion magnet, apply the diffusing agent to the C-side of the substrate to obtain a C-side diffusion magnet, and retain the substrate without the diffusing agent as an undiffused magnet. Perform vacuum diffusion heat treatment on the selective diffusion magnet and the C-side diffusion magnet. Step (3): The undiffused magnet, the C-surface diffused magnet, and the selected area diffused magnet are respectively installed into the permanent magnet motor to obtain three sets of permanent magnet motors to be tested; Step (4): Under constant current, the three sets of permanent magnet motors under test are continuously operated at a set high output power. The output power changes of the three sets of permanent magnet motors under test are recorded under different operating times. The power change time of the standard grade magnet under the same operating conditions is compared with that of the standard grade magnet to determine the coercivity level of the selected area diffusion magnet.

2. The method for testing the coercivity of a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, The substrate mentioned in step (1) is at least one of a cylindrical magnet, a square magnet, or an irregularly shaped magnet with the thickness direction along the C-axis. The substrate is successively sanded with sandpaper and ultrasonically cleaned with anhydrous ethanol to remove surface residues.

3. The coercivity testing method for a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, The dispersant in step (2) is selected from at least one of Dy-M, Tb-M, L-Dy-M, and L-Tb-M, wherein L is selected from at least one of La, Ce, Pr, Nd, and Y, and M is selected from at least one of Al, Cu, Zn, Mn, Mg, Ga, Ni, Co, and Fe.

4. The method for testing the coercivity of a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, The dispersant described in step (2) is obtained by arc melting to obtain a heavy rare earth alloy ingot, which is then ball-milled and crushed into powder with a particle size of less than 100 μm. It is then mixed with an adhesive and an organic solvent to prepare a printing paste, which is then screen-printed onto the substrate. The weight gain of the coating is less than 2%. The mass percentages of the heavy rare earth alloy powder, adhesive and organic solvent in the printing paste are 55% to 75%, 3% to 10% and 20% to 40%, respectively.

5. The method for testing the coercivity of a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, The vacuum degree of the vacuum diffusion heat treatment in step (2) is 5 × 10⁻⁶. -3 Pa ~ 1×10 -4 Pa; The vacuum diffusion heat treatment includes a high-temperature heat treatment and a low-temperature heat treatment performed sequentially. The high-temperature heat treatment is performed at a temperature of 800℃ to 950℃ for a time of 4h to 20h, and the low-temperature heat treatment is performed at a temperature of 450℃ to 650℃ for a time of 1h to 5h.

6. The method for testing the coercivity of a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, The corners mentioned in step (2) are the preferential nucleation regions for demagnetization domains of sintered NdFeB magnets; the undiffused magnets, C-plane diffused magnets and selectively diffused magnets mentioned in step (3) are surface treated to remove residual diffuser before being installed in the permanent magnet motor.

7. The method for testing the coercivity of a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, In step (4), the high output power is set to 100kW to 150kW, the total duration of continuous operation is not less than 3 hours, the recording interval of the output power is 5min to 15min, the continuous operation is carried out in a sealed test chamber, the test environment temperature is 20℃ to 30℃, the humidity is not higher than 80%RH, the cooling method is natural cooling or forced air cooling, the power supply voltage fluctuation does not exceed ±1%, and the frequency fluctuation does not exceed ±0.5%.

8. The method for testing the coercivity of a sintered NdFeB selective diffusion magnet according to claim 1, characterized in that, A power mutation time interval for a standard grade magnet under continuous operation at a set high output power is pre-established; the operation time corresponding to the output power decreasing by more than 1kW relative to the set high output power is taken as the power mutation time. The power mutation time is compared with the power mutation time interval of the standard grade magnet, and the coercivity level corresponding to that grade is determined by which interval it falls into.

9. The application of the coercivity testing method as described in any one of claims 1 to 8 in evaluating the coercivity of sintered NdFeB selective diffusion magnets.

10. The application according to claim 9, characterized in that, The sintered NdFeB selective diffusion magnet is at least one of a cylindrical magnet, a square magnet, or an irregularly shaped magnet with the thickness direction along the C-axis; the diffuser used in the sintered NdFeB selective diffusion magnet is selected from at least one of Dy-M, Tb-M, L-Dy-M, and L-Tb-M, wherein L is selected from at least one of La, Ce, Pr, Nd, and Y, and M is selected from at least one of Al, Cu, Zn, Mn, Mg, Ga, Ni, Co, and Fe.

Images