A method for improving the remanence of NdFeB magnets by magnetic field heat treatment

Abstract

The present invention discloses a method for improving the remanence of a NdFeB magnet by magnetic field heat treatment: a pulsed magnetic field is applied after a sintered NdFeB magnet matrix is ​​heated to a temperature higher than the melting temperature of a grain boundary phase, thereby obtaining a NdFeB magnet. The present invention heats the NdFeB magnet blank to a temperature higher than the grain boundary phase temperature and then applies a pulsed magnetic field to the magnet, thereby improving the orientation degree and remanence. The present invention does not introduce a large amount of carbon elements, the process is simple and easy to implement, and the remanence of the magnet can be increased by more than 0.1 kG, and the magnet performance is significantly improved, so it has great value for promotion and application.

Description

Technical Field

[0001] The invention belongs to the field of sintered NdFeB magnets, and in particular relates to a method for improving the remanence of NdFeB magnets through magnetic field heat treatment. Background Art

[0002] Neodymium iron boron magnets are known as the third generation of permanent magnet materials. They have excellent magnetic properties and are widely used in wind power generation, new energy vehicles, consumer electronics and other fields, playing an important role in economic and social development.

[0003] In industrial production, the residual magnetization intensity of sintered NdFeB can reach up to about 15-15.1 kGs. The residual magnetism is affected by the orientation of the magnet powder, and a good orientation can improve the residual magnetism of the magnet.

[0004] However, in the actual process, the powder particles produced by jet milling are irregular in shape and have many edges and corners, which makes orientation difficult, reduces the degree of orientation and thus reduces the remanence. The commonly used methods to improve the degree of orientation in the industry include: increasing the mixing time after jet milling, increasing the forming orientation field, and increasing the amount of lubricant.

[0005] Increasing the mixing time of the jet mill allows the powder to be mixed more evenly and reduces the sharp corners of the fine powder surface after the NdFeB jet mill. It helps the powder particles to be better arranged in the magnetic field, thereby improving the orientation of the magnet. However, long mixing time will slow down the production progress.

[0006] Increasing the molding orientation field is subject to equipment limitations. Usually, the electromagnet orientation field cannot be further increased after it is increased to more than 2.0T.

[0007] Increasing the amount of lubricant can improve the degree of orientation, but this will bring a large amount of carbon into the magnet, affecting the coercive force and squareness of the magnet.

[0008] Therefore, it is necessary to develop a new process, a simpler production process, to improve the orientation degree, and thus improve the remanence of the magnet. Summary of the invention

[0009] In order to solve the above technical problems, the present invention provides a method for magnetic field heat treatment, which can significantly improve the remanence of NdFeB magnets without significantly affecting other magnetic properties.

[0010] The technical solution adopted by the present invention is: A method for improving the remanence of a neodymium iron boron magnet by magnetic field heat treatment, the method comprising: The sintered NdFeB magnet matrix is ​​heated to a temperature higher than the melting temperature of the grain boundary phase and then a pulse magnetic field is applied to obtain the NdFeB magnet.

[0011] The melting temperature of the grain boundary phase is generally between 500-650℃.

[0012] Therefore, the heating temperature is generally higher than 650°C, preferably the heating temperature is 700-1100°C, more preferably the heating temperature is 900-1100°C, and more preferably 1000-1100°C.

[0013] A higher heating temperature means that the rare earth-rich phase in the grain boundary phase has better fluidity after melting, which is more conducive to improving the degree of orientation.

[0014] The heating is generally carried out under the protection of an inert gas or under vacuum to prevent the introduction of carbon and oxygen impurities.

[0015] Inert gases are generally argon, helium, etc.

[0016] In the present invention, the heating and heat preservation time is 3 to 5 hours.

[0017] Furthermore, the sintered NdFeB magnet matrix is ​​preferably heated to 700-1100° C., kept warm for 2-4 hours (preferably 2-3 hours), then a pulsed magnetic field is applied, and the keeping warm time is continued to be 3-5 hours to obtain a NdFeB magnet.

[0018] Generally speaking, the sintered NdFeB magnet matrix needs to be heated to a temperature higher than the melting temperature of the grain boundary phase to make the grain boundary phase fluid. It is required that the inside of the magnet matrix must reach above this temperature. Therefore, after heating to the set temperature, the insulation time is generally 2~4h. After insulation at the above temperature for more than 2h, the temperature inside the magnet can generally be guaranteed to be uniform, and the rare earth-rich phase can fully flow. At this time, the pulse magnetic field is applied.

[0019] The pulse magnetic field is 2T-5T, with a frequency of 20-50Hz. The intensity of the pulse magnetic field can be higher than that of the conventional constant magnetic field, and can reach more than 2T.

[0020] More specifically, the method of the present invention is preferably carried out according to the following steps: The sintered NdFeB magnet matrix is ​​heated to 1000-1100°C under argon protection or vacuum conditions, and after being kept warm for 2-4 hours, a pulse magnetic field is applied with an intensity of 2T-5T and a frequency of 20-50Hz, and the heat preservation is continued to a total time of 3-5 hours to obtain a NdFeB magnet.

[0021] The sintered NdFeB magnet matrix refers to a NdFeB magnet blank obtained by high-temperature sintering and tempering treatment, and is generally obtained by the following method: raw materials of various elements of the magnet matrix are taken according to the composition ratio, and flakes are obtained after smelting and casting; the flakes are hydrogen-crushed and air-flow-milled to obtain powder, the powder is pressed into shape, vacuum-high-temperature sintered, and tempered to obtain a magnet blank, which is used as a matrix for standby use.

[0022] The invention heats the NdFeB magnet blank to a temperature above the grain boundary phase temperature and then applies a pulse magnetic field to the magnet to improve the orientation degree.

[0023] Since the magnet blank undergoes a dissolution and recrystallization process during the sintering stage, the crystal grains of the magnet become rounded compared to the magnetic powder produced by airflow grinding. Therefore, when performing the heat treatment of the present invention, the magnet is heated to a state where the rare earth-rich phase fully flows, and a pulsed magnetic field is applied in this state, so that the rounded crystal grains are easier to rotate, so that the magnetic moment of the main phase is further oriented along the direction of the external magnetic field, thereby improving the orientation degree and remanence.

[0024] The beneficial effects of the present invention are: a large amount of carbon elements will not be introduced into the magnet blank by performing magnetic field heat treatment, the process is simple and easy to implement, and the remanence of the magnet can be increased by more than 0.1 kG, and the magnet performance is significantly improved, so it has great promotion and application value. DETAILED DESCRIPTION

[0025] The technical solution of the present invention is further described below in conjunction with specific embodiments. The embodiments described below are exemplary and are only used to better understand the present invention, and should not be construed as limiting the present invention.

[0026] Example 1 According to the nominal composition of the main phase alloy Nd 29.3 Fe bal Co 0.5 Ga 0.1 Cu 0.05 Ti 0.1 Zr 0.08 B 0.96 (wt. %) Prepare 40 kg of tablets.

[0027] The flakes were hydrogen-destroyed to prepare coarse powder, the hydrogen absorption time was 15 min, the dehydrogenation temperature was 550°C, the dehydrogenation time was 180 min, 0.12 wt.% of Dongyang antioxidant was added to the coarse powder and mixed for 2 hours.

[0028] The bottom spray pressure and grinding pressure of the coarse powder jet mill were set to 0.26MPa and 0.52MPa respectively. The speed of the classifying wheel was 2900rpm, and the weight of the grinding chamber was 30kg. The fine powder was added with 0.15wt.% of Tianjin lubricant and mixed for 3 hours.

[0029] Under an inert gas protective atmosphere, the green body is oriented and molded to obtain a green body, which is then vacuum packaged and cold isostatically pressed. The orientation center magnetic field is 1.7T and the isostatic pressure is 220MPa.

[0030] The block NdFeB magnet A is prepared by the sintering process of 1095℃*4h+1100*4h+900℃*4h+500℃*4h.

[0031] The prepared magnet A was subjected to magnetic property testing, and the results were as follows: Br(kG) Hci(kOe) (BH)max(MGOe) Hk / Hci 14.84 10.81 53.5 0.97 Example 2 The magnet was heated to 700℃ and kept for 4 hours. This temperature was slightly higher than the melting point of the rare earth-rich phase. The heating was carried out under argon protection. Then a pulse magnetic field of 2T (measured at the sample) and a frequency of 20Hz were applied to the magnet. The specific steps were: after heating to 700℃ and keeping it for 2 hours, the pulse magnetic field was magnetized 3 times with a magnetic field strength of 2T and a frequency of 50Hz. Each magnetization lasted for 1 second, and then magnetized again after an interval of 10 minutes. A total of 3 magnetizations were performed, and the temperature was kept for a total of 4 hours. The sample was tested and the following properties were obtained.

[0032] Br(kG) Hci(kOe) (BH)max(MGOe) Hk / Hci 14.89 10.72 53.8 0.96 Example 3 The magnet is heated to 900℃ and kept for 4 hours. The rare earth-rich phase melts and has a certain fluidity. The heating is carried out under vacuum. Then a pulse magnetic field of 2T (measured at the sample) and a frequency of 20Hz are applied to the magnet. The specific steps are: after heating to 900℃ and keeping for 2 hours, the pulse magnetic field is magnetized for 3 times with a magnetic field strength of 2T and a frequency of 50Hz. Each magnetization lasts for 1 second, and then magnetizes again after an interval of 10 minutes. A total of 3 magnetizations are performed, and the total insulation time is continued to be 4 hours. The sample is tested and the following properties are obtained.

[0033] Br(kG) Hci(kOe) (BH)max(MGOe) Hk / Hci 14.93 10.52 54.3 0.98 Example 4 The magnet was heated to 1090℃ and kept for 4 hours. The rare earth-rich phase melted and had good fluidity. The heating was carried out under argon protection. Then a pulse magnetic field of 2T (measured at the sample) and a frequency of 20Hz were applied to the magnet. The specific steps were: after heating to 1090 degrees and keeping warm for 2 hours, the pulse magnetic field was magnetized 3 times with a magnetic field strength of 2T and a frequency of 50Hz. Each magnetization lasted for 1 second, and then magnetized again after an interval of 10 minutes. A total of 3 magnetizations were performed, and the total insulation time was continued to be 4 hours. The sample was tested and the following properties were obtained.

[0034] Br(kG) Hci(kOe) (BH)max(MGOe) Hk / Hci 14.96 10.33 55.1 0.96 From the comparison of the magnet performances of Examples 1 to 4, it can be seen that the present invention can increase the remanence by up to 0.12 kGs by adopting the method of magnetic field heat treatment, and the effect of improving the remanence is very obvious.

[0035] Moreover, the higher the heating temperature, the better the fluidity of the rare earth-rich phase after melting, and the more conducive it is to improving the orientation degree, that is, improving the remanence.

[0036] The coercivity of Example 4 is slightly lower than that of Example 1, but the coercivity can be greatly improved by subsequent grain boundary diffusion, and grain boundary diffusion does not have a significant impact on remanence. However, the prior art is very rare in terms of processes for improving remanence, and the method of improving remanence in the present invention is simple and has a very significant effect.

[0037] Comparative Example 1 The magnetic field heat treatment was carried out according to the steps of Example 2, except that the temperature was heated to 700° C. and the total holding time was 8 h. The remaining steps were the same as those of Example 2.

[0038] However, the Br of the obtained magnet was 14.88 kG, which was not significantly improved compared with Example 2. It can be seen that extending the holding time does not lead to an increase in fluidity.

[0039] Comparative Example 2 The magnetic field heat treatment is performed according to the steps of Example 4, except that the pulse magnetic field is 1T.

[0040] The magnetic properties of the prepared magnets are as follows: Br(kG) Hci(kOe) (BH)max(MGOe) Hk / Hci 14.92 10.49 54.4 0.97 It can be seen that if the magnetic field intensity is low, the effect of improving the residual magnetism is limited.

[0041] The greater the magnetic field intensity, the better the effect of magnetic domain reversal and grain reversal, the better the orientation, and the more significant the improvement of remanence. Therefore, the present invention requires that the intensity of the pulsed magnetic field reaches 2T or above.

Claims

1. A method for improving the remanence of NdFeB magnets by magnetic field heat treatment, characterized in that The method is: The sintered NdFeB magnet matrix is ​​heated to a temperature higher than the melting temperature of the grain boundary phase and then a pulse magnetic field is applied to obtain the NdFeB magnet.

2. The method according to claim 1, characterized in that The heating temperature is 700~1100℃.

3. The method according to claim 1, characterized in that The heating temperature is 900-1100°C.

4. The method according to claim 1, characterized in that The heating and insulation time is 3~5h.

5. The method according to claim 4, characterized in that The sintered NdFeB magnet matrix is ​​heated to 700-1100°C, kept warm for 2-4 hours, and then a pulse magnetic field is applied, and the keeping warm time is continued to be 3-5 hours to obtain a NdFeB magnet.

6. The method according to claim 1, characterized in that The heating is carried out under inert gas protection or under vacuum.

7. The method according to claim 1, characterized in that The intensity of the pulse magnetic field is 2T-5T, and the frequency is 20-50Hz.

8. The method according to claim 1, characterized in that The method comprises the following steps: heating a sintered NdFeB magnet matrix to 1000-1100°C under argon protection or vacuum conditions, keeping the temperature for 2-4 hours, applying a pulse magnetic field with an intensity of 2T-5T and a frequency of 20-50Hz, and continuing to keep the temperature for a total time of 3-5 hours to obtain a NdFeB magnet.