A high-performance RTB rare earth permanent magnet for electric motor and its preparation method

Abstract

The invention discloses a high-performance R-T-B rare earth permanent magnet for a motor and a preparation method thereof, wherein the method comprises: covering a heavy rare earth diffusion source on the surface of a magnet substrate, then performing surface oxidation, heating to 250-450°C in an atmosphere with an oxygen content of 10 vol.%-70 vol.%, and keeping the temperature for 10-50 min; performing grain boundary diffusion treatment on the surface-oxidized magnet to obtain the high-performance R-T-B rare earth permanent magnet for a motor. The present invention oxidizes the grain boundary rich R phase within a certain depth range of the magnet surface layer to a rare earth oxide R-O phase, increases the resistivity within a certain depth range from the magnet surface without affecting other regions of the magnet, realizes reducing the generation of Joule heat of the magnet when the motor is working without reducing the effect of the magnet grain boundary diffusion, reduces the temperature rise when the magnet is working, thereby reducing the deterioration of the magnetic properties of the magnet, combines the improvement of the coercive force of the magnet, the improvement of the magnetic property temperature coefficient and the reduction of the heat generation when the magnet is working, thereby preparing a high-performance R-T-B rare earth permanent magnet for a motor and improving the working efficiency of the motor.

Description

Technical Field

[0001] The invention relates to a high-performance RTB rare earth permanent magnet for a motor and a preparation method thereof, and belongs to the field of rare earth magnets. Background Art

[0002] RTB rare earth permanent magnet is a permanent magnet material with excellent magnetic properties. Compared with other permanent magnet materials, it has the highest magnetic energy product and is widely used in modern industry. The high magnetic energy product of RTB rare earth magnet can effectively realize the miniaturization of equipment, so it has gradually replaced other permanent magnet materials in the field of electric motors.

[0003] The alternating magnetic field will induce eddy currents in the conductor, and the eddy currents will generate Joule heat when doing work. The resistivity of RTB rare earth permanent magnets is about 150μΩ·cm, which is relatively low and about two orders of magnitude lower than that of ferrite. Therefore, when working in a motor, the Joule heat generated by the eddy currents under the action of the alternating magnetic field is greater, causing the magnet to heat up severely. In addition, since the temperature coefficient of the magnetic properties of RTB magnets is negative, the magnetic properties will gradually decrease as the temperature of the magnet increases, resulting in a decrease in the working efficiency of the motor. At present, there are two main solutions to the loss of magnetic properties caused by heat during the use of magnets: one is to increase the coercive force of the magnet and improve the temperature coefficient of the magnetic properties by adding heavy rare earth elements; the other is to increase the resistivity of the magnet and reduce the heat generated during operation, thereby reducing the temperature of the magnet during operation.

[0004] Adding heavy rare earth elements to the magnet composition can increase the coercivity of the magnet and improve the temperature coefficient of the RTB magnet. However, since heavy rare earth elements and iron elements are antiferromagnetic, adding heavy rare earth elements during the smelting stage will significantly reduce the remanence of the magnet. Grain boundary diffusion is a new technology developed in recent years. The use of grain boundary diffusion to diffuse heavy rare earth elements along the grain boundaries into the interior of the magnet can significantly increase the coercivity of the magnet with a slight decrease in remanence.

[0005] By increasing the resistivity of the magnet, the Joule heat generated by the eddy current can be reduced, the temperature of the magnet when working in the motor can be reduced, and the working efficiency of the motor can be improved. CN1983471B provides a method of adding high-resistivity particles to the inside of the magnet to increase the resistivity of the RTB magnet. However, due to the addition of such particles, the magnet is difficult to sinter densely, and a sintering aid needs to be added to assist sintering. At the same time, the presence of the particles and the sintering aid increases the volume fraction of the non-magnetic phase, affecting the remanence of the magnet.

[0006] Studies have shown that although eddy currents in an alternating magnetic field are distributed throughout the entire magnet, the distribution density in different areas is not uniform. The current density at the edge of the magnet is large, and the Joule heat generated is relatively high, while the current density in the core of the magnet is small, and the Joule heat generated is low. Therefore, a certain method is used to increase the resistivity of a certain thickness of the magnet surface layer, that is, to increase the resistivity of the area with a larger eddy current density in the magnet without affecting other areas of the magnet, so as to suppress the Joule heat generated by the eddy current without affecting the magnetic properties of the magnet. Combined with the grain boundary diffusion treatment of heavy rare earth elements, the coercive force of the magnet is increased, and it is expected to prepare high-performance RTB rare earth permanent magnets for electric motors. Summary of the invention

[0007] In view of the fact that the resistivity of RTB rare earth permanent magnet is low, the Joule heat generated by eddy current under the action of alternating magnetic field is large, which leads to serious heating of the magnet; in addition, the temperature coefficient of magnetic properties of RTB magnet is negative, and the magnetic properties will gradually decrease with the increase of magnet temperature, resulting in the deterioration of the working efficiency of the motor. The present invention provides a high-performance RTB rare earth permanent magnet for motors and a preparation method thereof.

[0008] The technical solution adopted by the present invention is:

[0009] A method for preparing a high-performance RTB rare earth permanent magnet for a motor, the method comprising the following steps:

[0010] (1) Covering a heavy rare earth diffusion source on the surface of a magnet substrate, wherein the magnet substrate is a RTB sintered magnet;

[0011] (2) oxidizing the surface of the magnet covered with the diffusion source, heating it to 250-450° C. in an atmosphere with an oxygen content of 10 vol.% to 70 vol.%, and keeping it at this temperature for 10 min to 50 min;

[0012] (3) The surface oxidized magnet is subjected to grain boundary diffusion treatment at a diffusion temperature of 800 to 1000° C. for a holding time of 5 to 25 hours. After the holding time is completed, the magnet is cooled to below 200° C. and then heated up for tempering treatment to obtain the high-performance RTB rare earth permanent magnet for the motor.

[0013] In step (1), the heavy rare earth diffusion source is pure heavy rare earth element metal, heavy rare earth element hydride or alloy of heavy rare earth element and other metal elements; the heavy rare earth element is at least one of Dy and Tb, and the mass content of heavy rare earth element in the heavy rare earth diffusion source is ≥20wt.%, and the oxygen content is ≤5wt.%.

[0014] Furthermore, in the step (1), the RTB sintered magnet is prepared by the following method: vacuum induction melting strip spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing, vacuum sintering, primary high temperature heat treatment and secondary low temperature heat treatment, and then machined into the desired shape and size. This is a method for preparing RTB sintered magnets known in the art.

[0015] RTB sintered magnet, that is, NdFeB magnet, mainly consists of Nd, Fe, and B. It may also contain various other elements such as one or more of the rare earth elements Pr, Ho, Gd, Dy, and Tb, or other metal or non-metal elements such as Al, Cu, Zn, Sn, In, Ti, V, Co, Mn, Ni, Ca, Zr, Ga, Nb, Mo, and Si. The element composition and component ratio of the RTB sintered magnet can adopt the general formula in the art, and the formula has no effect on the process of this application.

[0016] The first-level high-temperature heat treatment temperature is 850-950°C, and the insulation time is 2-12h. Preferably, the first-level high-temperature heat treatment temperature is 880-950°C, and the insulation time is 3-6h. The second-level low-temperature heat treatment temperature is 400-650°C, and the insulation time is 2-12h. Preferably, the second-level low-temperature heat treatment temperature is 480-520°C, and the insulation time is 3-6h.

[0017] In the step (1), the magnet substrate is generally subjected to surface treatment before being covered with the heavy rare earth diffusion source. The surface treatment is performed by grinding, pickling and other methods to remove oil stains and rust spots on the magnet surface.

[0018] In the step (1), the heavy rare earth diffusion source can be covered by evaporation, magnetron sputtering or multi-arc ion plating, etc., and the diffusion source is deposited on the surface of the magnet substrate, preferably multi-arc ion plating. The thickness of the heavy rare earth diffusion source is between 3 μm and 1 mm, preferably between 5 μm and 25 μm.

[0019] In the step (1), the surface of the magnet substrate perpendicular to the orientation direction must be covered with heavy rare earth diffusion sources, and other surfaces of the magnet may or may not be covered with heavy rare earth diffusion sources. Preferably, the surface of the magnet substrate not perpendicular to the orientation direction is not covered with heavy rare earth diffusion sources.

[0020] In the step (2), the magnet is preferably placed in an air atmosphere and heated to perform surface oxidation.

[0021] The preferred heating method is tunnel furnace heating.

[0022] In the step (2), the surface oxidation temperature is 300 to 400° C. and the insulation time is preferably 15 to 30 minutes.

[0023] In the step (3), the diffusion temperature is preferably 880 to 1000°C;

[0024] The insulation time is preferably 8 to 20 hours;

[0025] After reaching the diffusion temperature, the absolute vacuum degree in the furnace should be 10 -2 ~10 -5 Pa.

[0026] In the step (3), after the heat preservation is completed, the magnet is preferably cooled at a cooling rate of not less than 80°C / min until the temperature of the magnet is less than 200°C.

[0027] In the step (3), the tempering treatment is to heat the magnet to 400-650°C, keep the temperature for 2-12 hours, and then cool it to below 200°C at a cooling rate of not less than 80°C / min.

[0028] The present invention also provides a high-performance RTB rare earth permanent magnet for a motor prepared by the above method, wherein the magnet is composed of R 2 T 14 The main phase B is composed of a grain boundary R-rich phase, wherein more than 95 vol.% (volume ratio) of the R-rich phase at the grain boundary of the magnet within a depth range of 0 to 30 μm from the surface of the magnet coated with a diffusion source is composed of a rare earth oxide RO phase; more than 95 vol.% (volume ratio) of the R-rich phase at the grain boundary of the magnet within a depth range of 0 to 40 μm from the surface of the magnet coated with a non-diffusion source is composed of a rare earth oxide RO phase; within a depth range greater than 60 μm from the magnet surface, the proportion of the rare earth oxide RO phase in the magnet grain boundary R-rich phase is less than 5 vol.% (volume ratio).

[0029] The magnet has a resistivity ρ>250μΩ·cm within a range of less than 30μm from the magnet surface at room temperature (20°C), and a resistivity ρ<170μΩ·cm within a range of more than 60μm from the magnet surface.

[0030] The oxygen atomic ratio in the rare earth oxide RO phase within the depth range of 0 to 40 μm from the surface of the magnet is between 40 and 80 at.%.

[0031] The present invention adopts the methods of vacuum induction melting strip throwing, hydrogen cracking, air flow grinding, orientation molding, isostatic pressing, vacuum sintering, primary high temperature heat treatment and secondary low temperature heat treatment to prepare RTB sintered magnet. The RTB sintered magnet is processed into a finished magnet of required shape and size by a machining method, and a heavy rare earth element grain boundary diffusion source is coated on the surface of the finished magnet after surface treatment. The magnet is then surface oxidized to form an oxide layer of a certain thickness on the surface of the grain boundary diffusion source. In the subsequent high temperature grain boundary diffusion process, the heavy rare earth element will first diffuse into the interior of the magnet along the magnet grain boundary R-rich phase, and form a shell layer with a higher anisotropy field on the surface of the main phase grains, thereby improving the coercive force of the magnet. As the grain boundary diffusion proceeds, the heavy rare earth diffusion source layer on the surface of the magnet is continuously consumed. In the late stage of grain boundary diffusion, the oxygen element on the surface of the heavy rare earth grain boundary diffusion source will diffuse into the interior of the magnet and react with the grain boundary R-rich phase of the magnet, so that the grain boundary R-rich phase within a certain depth range of the magnet surface layer is oxidized to a rare earth oxide RO phase. For magnet surfaces that are not coated with grain boundary diffusion sources, during the magnet surface oxidation treatment, the grain boundary R-rich phase within a certain depth from the magnet surface will also react with oxygen to form a rare earth oxide RO phase. Since the resistivity of rare earth oxides is relatively high, the resistivity within a certain depth from the magnet surface can be significantly increased without affecting other areas of the magnet.

[0032] The present invention has found through research that although the eddy currents of the RTB magnets are distributed inside the magnets when the motor is working, the distribution density of the eddy currents in different parts of the magnets is different. The current density of the surface layer of the magnet is large, while the current density of the core of the magnet is small. Therefore, the present invention can prepare high-performance RTB rare earth permanent magnets for motors by increasing the resistivity of the high current density area on the surface of the magnet, reducing the heat generated when the magnet is working, and combining the grain boundary diffusion of heavy rare earth elements to enhance the coercive force of the magnet and improve the temperature coefficient of the magnet. It can reduce the generation of Joule heat when the magnet is working when the motor is working without reducing the effect of the grain boundary diffusion of the magnet, reduce the temperature rise when the magnet is working, thereby reducing the deterioration of the magnetic properties of the magnet, and improve the efficiency of the motor when it works for a long time.

[0033] During surface oxidation, the present invention allows a portion of the heavy rare earth layer to react with oxygen to generate heavy rare earth element oxides. During high-temperature grain boundary diffusion, the unoxidized heavy rare earth elements in contact with the magnet surface will diffuse along the grain boundaries into the interior of the magnet, and replace part of the rare earth elements in the surface layer of the main phase grains, generating a layer of heavy rare earth element shell layer on the surface layer of the main phase grains. Since the shell layer has a high anisotropy field, the coercive force of the magnet can be effectively improved and the temperature coefficient of the magnetic properties of the magnet can be improved. As the grain boundary diffusion proceeds, the heavy rare earth layer on the surface layer of the magnet will be continuously consumed, and the heavy rare earth layer that is finally oxidized will contact the magnet surface. In the later stage of grain boundary diffusion, in addition to the grain boundary diffusion of the heavy rare earth elements, oxygen atoms will also diffuse along the grain boundaries into the interior of the magnet, and react with the grain boundary R-rich phase of the magnet to generate oxides with high melting point and high resistivity. The resistivity of the rare earth oxide RO phase is significantly higher than that of the metallic grain boundary R-rich phase, and the main phase grains are also wrapped by the high-resistivity rare earth oxide RO phase. Therefore, the resistivity of the surface layer of the magnet can be significantly improved, and the Joule heat generated when the magnet is used in an alternating magnetic field can be reduced, thereby ensuring the working efficiency of the motor.

[0034] The thickness of the oxide layer on the surface of the final magnet is closely related to the surface oxidation temperature and time. For the magnet surface coated with grain boundary diffusion sources, the oxygen element cannot directly oxidize the magnet matrix due to the isolation effect of the diffusion source. Therefore, the final oxide layer thickness of the surface of the magnet coated with grain boundary diffusion sources is determined by the oxygen content of the grain boundary diffusion source layer. For the magnet surface not coated with grain boundary diffusion sources, due to the absence of the isolation effect of the intermediate layer, the oxygen element will directly oxidize the grain boundary R-rich phase of the magnet matrix, resulting in the oxide layer depth of the uncoated grain boundary diffusion magnet surface being deeper than the surface coated with the grain boundary diffusion source. In order to prevent the oxide layer of the magnet from being too thick and affecting the magnetic properties of the magnet, it is necessary to reasonably control the oxidation temperature and time. The oxidation temperature determines the oxidation rate of the magnet surface. When the temperature is low, the oxidation rate is slow and the production efficiency is low. When the temperature is too high, the oxidation rate is too fast, and the controllability of the oxide layer thickness of the magnet matrix and the grain boundary diffusion source becomes poor. In addition, the thickness of the oxide layer is also closely related to the oxidation time. As the oxidation time increases, the thickness of the oxide layer of the grain boundary diffusion source increases. After high-temperature grain boundary diffusion and secondary low-temperature heat treatment, the thickness of the oxide layer on the surface of the magnet will also increase. The thickness of the oxide layer on the surface of the magnet is not the thicker the better. When the oxide layer is too thick, the volume ratio of the surface oxide layer to the magnet increases, and it will affect the coercive force increment during the grain boundary diffusion and secondary aging treatment of the magnet. Therefore, in order to ensure that the resistivity of the magnet in the high-density eddy current zone in the alternating magnetic field is high enough, it is necessary to ensure that the rare earth oxide RO phase in the magnet grain boundary R-rich phase within a depth range of at least 30μm from the magnet surface accounts for more than 95vol.%. At the same time, in order to prevent the excessively thick oxide layer from having a significant impact on the magnetic properties of the magnet, the rare earth oxide RO phase accounts for less than 5vol.% (volume ratio) in the depth range greater than 60μm from the magnet surface. For the magnet surface that is not coated with a grain boundary diffusion source, the grain boundary R-rich phase will also produce a certain oxidation effect during the oxidation stage of the magnet surface. Since there is no isolation effect of the surface grain boundary diffusion source layer, the diffusion rate of oxygen atoms is faster, so the thickness of the oxide layer is thicker than that of the diffusion coating surface. In order to ensure the thickness requirement of the oxide layer of the final magnet, the magnet surface oxidation temperature used in the present invention is 250°C to 450°C, the insulation time is 10min to 50min, preferably 300 to 400°C, and the insulation time is 15min to 30min.

[0035] In addition, the magnet surface oxidation process requires the participation of oxygen elements, so there are certain requirements for the oxygen content in the atmosphere. In order to achieve a suitable oxidation effect, the present invention has found through a large number of experiments that the oxygen content in the atmosphere needs to be 10 vol.% to 70 vol.%, and air is preferably used as the surface oxidation atmosphere.

[0036] In the early stage of grain boundary diffusion, in order to effectively diffuse the heavy rare earth elements into the magnet, it is necessary to ensure that the magnet has sufficient molten grain boundary phase. Therefore, it is necessary to ensure that the grain boundary R-rich phase on the surface of the magnet is not over-oxidized in the early stage of diffusion. Therefore, in the present invention, it is necessary to control the oxygen content of the grain boundary diffusion source so that the oxygen content in the heavy rare earth element diffusion source is ≤5wt.%.

[0037] There is anisotropy in the grain boundary diffusion process of the magnet, that is, the diffusion rate along the orientation direction of the magnet is high, while the grain boundary diffusion rate perpendicular to the orientation direction of the magnet is low. Therefore, in the present invention, in order to ensure the coercive force increment of the magnet after grain boundary diffusion, the surface perpendicular to the orientation direction of the magnet must be coated with a grain boundary diffusion source. In addition, coating the grain boundary diffusion layer on other surfaces of the magnet will increase the coercive force of the magnet, and will also improve the uniformity of the thickness of the oxide layer on the surface of the final magnet. However, due to the anisotropy of grain boundary diffusion, coating the grain boundary diffusion source on other surfaces is likely to induce body diffusion during the grain boundary diffusion process, resulting in an increase in the amount of reduction in the remanent magnetization of the magnet, and at the same time, the material cost is also significantly increased. Therefore, in the present invention, it is preferred that the grain boundary diffusion source layer is not deposited on other surfaces of the magnet.

[0038] The beneficial effects of the present invention are: the resistivity of a certain thickness of the surface layer of the magnet is increased by the surface oxidation method, that is, the resistivity of the region with a larger eddy current density in the magnet is increased without affecting other regions of the magnet, so as to suppress the Joule heat generated by the eddy current without affecting the magnetic properties of the magnet. At the same time, the grain boundary diffusion treatment of heavy rare earth elements is adopted to increase the coercive force of the magnet. By combining the increase of the coercive force of the magnet, the improvement of the temperature coefficient of magnetic properties and the reduction of the heat generated by the magnet during operation, a high-performance RTB rare earth permanent magnet for motors is prepared to improve the working efficiency of the motors. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The eddy current and loss distribution diagram of different parts of the traditional RTB magnet in the alternating magnetic field.

[0040] Figure 2 Schematic diagram of the four-probe method for testing the surface resistivity of a magnet.

[0041] Figure 3 (a), (b) and (c) are SEM microstructure images of the magnets in Experiment No. 2, Experiment No. 5 and Experiment No. 8 within a certain depth range from the diffusion source coating surface, respectively.

[0042] Figure 4 Curve diagram of the proportion of rare earth oxide RO phase to grain boundary R-rich phase at different depths from the surface of the coated diffusion source for the magnets of Experiment No. 1 to Experiment No. 8.

[0043] Figure 5 Temperature increment curves of the magnets in Experiment No. 1 to Experiment No. 8 at different times in the alternating magnetic field.

[0044] Figure 6 (a), (b) and (c) are SEM microstructure images of the magnets of Experiment No. 9, Experiment No. 10 and Experiment No. 13 within a certain depth range from the diffusion source coating surface.

[0045] Figure 7 Curve diagram of the proportion of rare earth oxide RO phase to grain boundary R-rich phase at different depths from the surface of the coated diffusion source for the magnets of Experiment No. 9 to Experiment No. 13.

[0046] Figure 8 Temperature increment curves of the magnets in Experiment No. 9 to Experiment No. 13 at different times in the alternating magnetic field.

[0047] Fig. 9 (a) and (b) are SEM microstructure images of the magnets of Experiment No. 14 and Experiment No. 15, respectively, within a certain depth range from the magnet surface without high temperature grain boundary diffusion treatment.

[0048] Fig.10 This is a curve diagram of the proportion of rare earth oxide RO phase to grain boundary R-rich phase at different depths from the surface of the coated diffusion source of the magnet in Experiment No. 16 to Experiment No. 22. DETAILED DESCRIPTION

[0049] The technical solution of the present invention is further described below in conjunction with specific embodiments, but the protection scope of the present invention is not limited thereto.

[0050] The raw materials are mixed in a certain proportion and then vacuum induction melting, belt throwing, hydrogen crushing, air flow grinding, orientation forming, isostatic pressing and vacuum sintering are used to prepare sintered magnets.

[0051] The sintered magnet is subjected to a primary high temperature heat treatment at 850-950°C for 2-12 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 400-650°C for 2-12 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0052] The magnet after the secondary low-temperature heat treatment is processed into a finished magnet of the required shape and size by machining. After removing the oil and rust on the surface of the finished magnet by surface grinding, pickling and other methods, a heavy rare earth element diffusion source layer with a thickness of 3μm to 1mm is deposited on the surface of the magnet by evaporation, magnetron sputtering or multi-arc ion plating. The thickness of the diffusion source layer is preferably between 5 and 25μm. The surface of the magnet perpendicular to the orientation direction must be deposited with a heavy rare earth element grain boundary diffusion source, and the other surfaces of the magnet are preferably not deposited with a heavy rare earth element grain boundary diffusion source.

[0053] The magnet with a heavy rare earth element diffusion source layer deposited on the surface is heated to 250-450° C. in air and kept at this temperature for 10-50 minutes. Preferably, a tunnel furnace is used for heating.

[0054] The surface oxidized magnet is placed in a vacuum sintering furnace and heated to 800-1000°C for high temperature grain boundary diffusion. After reaching the highest temperature, the vacuum degree in the furnace should be higher than 10 -2 Pa, keep warm for 5 to 25 hours. After the heat preservation is completed, cool down at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 400-650℃, keep warm for 2 to 12 hours. After the heat preservation is completed, cool down at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0055] The final magnet was surface treated by sandblasting to remove the residual diffusion source layer on the magnet surface. The magnetic properties of the magnet were measured by NIM magnetic testing equipment, the magnet microstructure was observed by SEM, the magnet composition was analyzed by ICP, and the magnet microscopic composition was analyzed by EPMA. The surface resistivity of the magnet was tested by the four-probe method, and then the magnet was polished with sandpaper. After the magnet surface was polished by 60μm, the surface resistivity of the magnet was tested again. After the magnet was saturated with magnetization, the surface of the magnet was wrapped with insulation material and placed in a coil, and an alternating magnetic field of 8.5kA / m was applied to the magnet at a frequency of 2000Hz. The temperature rise of the magnet per unit time was measured using a thermocouple attached to the magnet.

[0056] Embodiment 1:

[0057] The composition is Nd 22.5 Pr 7.5 B 0.95 Ga 0.1 Co 0.5 Cu 0.1 Zr 0.2 Fe 68.15 After the ingredients are prepared in a mass ratio, vacuum induction melting, belt spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing and vacuum sintering are adopted to prepare the sintered magnet.

[0058] The sintered magnet is subjected to a primary high temperature heat treatment at 880°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 520°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0059] The magnets after secondary low temperature heat treatment were machined into finished magnets with a length × width × thickness of 60 mm × 50 mm × 10 mm, where the thickness direction is the magnet orientation direction. After surface grinding to remove oil and rust spots on the surface of the finished magnets, a 10 μm thick pure Tb diffusion source layer was deposited on the two surfaces of the magnets perpendicular to the orientation direction by multi-arc ion plating, and no heavy rare earth element diffusion source was deposited on other surfaces.

[0060] The magnet with a heavy rare earth element diffusion source layer deposited on the surface was heated to different temperatures in a tunnel furnace in an air atmosphere, kept at this temperature for 30 minutes, and then air-cooled.

[0061] The surface oxidized magnet is placed in a vacuum sintering furnace and heated to 910°C for high temperature grain boundary diffusion. When the highest temperature is reached, the absolute vacuum degree in the furnace is 10 -2 Pa~10 -4 Pa, keep warm for 10 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 520℃ and keep warm for 3 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0062] The final magnet is treated with sandblasting to remove the remaining diffusion source layer on the magnet surface. The magnetic properties of the magnet are measured with NIM magnetic testing equipment, the magnet microstructure is observed with SEM, the magnet composition is analyzed with ICP, and the magnet microstructure is analyzed with EPMA. The surface resistivity of the magnet is tested with the four-probe method. The schematic diagram of the four-probe method for testing the surface resistivity of the magnet is shown in the figure. Figure 2 As shown, the magnet is then polished with sandpaper, and the surface resistivity of the magnet is tested again after 60μm of the magnet surface is removed. After the magnet is saturated with magnets, the surface of the magnet is wrapped with insulation material and placed in a coil, and an alternating magnetic field of 8.5kA / m is applied to the magnet at a frequency of 2000Hz. The temperature rise of the magnet is measured every 10min within 1h using a thermocouple attached to the magnet.

[0063] Maxwell software is used to simulate the eddy current and loss distribution of traditional NdFeB magnets in alternating magnets, such as Figure 1 It can be found that in the alternating magnetic field, the current is mainly concentrated on the surface layer of the magnet, which leads to increased loss of the surface layer of the magnet and increased temperature rise.

[0064] The different surface oxidation temperatures, final Tb content and room temperature (20°C) magnetic properties of the magnets of Experiment No. 1 to Experiment No. 8 are shown in Table 1:

[0065] Table 1

[0066] Experiment No. Surface oxidation temperature / ℃ Tb content (wt.%) Br(kGs) Hcj(kOe) 1 No oxidation 0.23 14.31 20.59 2 100 0.23 14.31 20.58 3 200 0.22 14.30 20.54 4 300 0.22 14.31 20.52 5 350 0.21 14.31 20.53 6 400 0.21 14.29 20.51 7 500 0.18 14.32 18.38 8 700 0.14 14.34 17.96

[0067] From the data in Table 1, it can be seen that when the magnet is not subjected to surface oxidation treatment or the oxidation temperature is low (below 400°C), the magnet Br is basically the same, and the coercive force of Experiment 1 is slightly higher than that of the magnet of the present invention. However, when the surface oxidation temperature is too high, the heavy rare earth element Tb is difficult to diffuse due to the excessive thickness of the oxide layer of the heavy rare earth diffusion source, so the Tb element content in the final magnet is low, and the coercive force increment is also reduced.

[0068] The SEM microstructures of the magnets in Experiment No. 2, Experiment No. 5 and Experiment No. 8 within a certain depth range from the diffusion source coating surface are shown in Figure 1. Figure 3 As shown in (a), (b) and (c), it can be seen that when the oxidation temperature of the magnet surface is low, there are fewer rare earth oxide RO phases in the grain boundary R-rich phase within a depth range of 30μm from the magnet surface layer. When the oxidation temperature is within the recommended range of the present invention (250-450℃), it can be seen from the microstructure of the magnet of Experiment No. 5 that there are more rare earth oxide RO phases in the grain boundary R-rich phase within a depth range of 30μm from the magnet surface layer. When the oxidation temperature is too high (such as Experiment No. 8), the amount of rare earth oxide RO phase generated in the magnet increases, and a large amount of rare earth oxide RO phase can still be observed at a depth of 60μm from the magnet diffusion source coating surface.

[0069] The ratio curve of rare earth oxide RO phase to grain boundary R-rich phase at different depths from the surface of the coated diffusion source in Experiment No. 1 to Experiment No. 8 is shown in the figure. Figure 4 As shown, Figure 4 It can be seen that when the magnet is not oxidized or the oxidation temperature is low, there is only a small amount of rare earth oxide RO phase within 0 to 5 μm from the surface of the magnet coated with the diffusion source. When the oxidation temperature is between 250 and 450 ° C, the rare earth oxide RO phase is mainly concentrated within a depth of 30 μm from the magnet surface. When the distance exceeds 60 μm, the proportion of rare earth oxide RO phase to grain boundary R-rich phase is less than 5 vol.%.

[0070] The same method was used to test the proportion of rare earth oxide RO phase within a certain depth range from the non-diffusion source coated surface of the magnet and the oxygen content of the magnet. It was found that the distribution pattern of the rare earth oxide RO phase was the same as that of the surface of the magnet coated with diffusion source. When the oxidation temperature was between 250 and 450 ° C, the rare earth oxide RO phase was mainly concentrated within a depth range of 40 μm from the magnet surface. When the distance exceeded 60 μm, the proportion of rare earth oxide RO phase to grain boundary R-rich phase was less than 5 vol.%.

[0071] From the proportion of rare earth oxide RO phase in the grain boundary R-rich phase within a certain depth range from the surface of the coated diffusion source of the magnets in different experimental groups, it can be seen that when the oxidation temperature is too high, the thickness of the oxide layer on the surface of the magnet increases significantly. From the proportion of RO rare earth oxide in the grain boundary R-rich phase of the magnet, it can be seen that after the magnet of Experiment No. 8 was oxidized at 700℃ for 30min, the proportion of rare earth oxide RO phase in the grain boundary R-rich phase within 60-70μm from the surface of the magnet coated diffusion source still exceeded 80vol.%. Therefore, in the present invention, the oxidation temperature should be controlled in the range of 250-450℃.

[0072] EPMA analysis of the rare earth oxide RO phase of the magnet shows that its composition is rare earth elements and oxygen elements, and the atomic ratio of oxygen atoms is between 40at.% and 80at.%, which shows that the R-rich phase at the grain boundary of the oxide layer on the surface of the magnet is mainly oxides of rare earth elements.

[0073] The surface resistivity of the magnet was measured by a four-probe method. Then, each surface of the magnet was abraded by sandpaper to remove 60 μm, and the surface resistivity of the magnet was measured again. The results are shown in Table 2.

[0074] Table 2

[0075] Experiment No. Magnet surface resistance / μΩ·cm Surface resistivity after 60μm surface grinding / μΩ·cm 1 171 162 2 182 163 3 194 161 4 264 162 5 268 163 6 267 161 7 268 246 8 269 252

[0076] As can be seen from Table 2, for magnets that have not undergone surface oxidation or have a low surface oxidation temperature, the surface resistivity of the magnet is basically the same as the surface resistivity of the magnet after 60μm of the magnet surface has been ground off. When the surface oxidation temperature is within the range of the present invention (experiments No. 4, 5, and 6), the surface resistivity of the magnet increases significantly, and the resistivity values ​​are all greater than 250μΩ·cm. In addition, the surface resistivity of the magnet cannot be further increased with the increase in oxidation temperature. The increase in the surface resistance of the magnet mainly comes from the reaction of the R-rich phase at the grain boundary of the magnet surface with oxygen to form the rare earth oxide RO phase. When the R-rich phase at the grain boundary of the magnet surface layer is completely oxidized, further increasing the temperature will not change the resistivity of the surface layer, but will only increase the depth of the high resistivity interval. From the surface resistivity data after 60μm of the magnet surface has been ground off, it can be seen that when the oxidation temperature is within the recommended range of the present invention (250-450°C), the resistivity of the magnet at a depth of 60μm from the magnet surface is basically the same as the resistivity of the magnet that has not undergone surface oxidation. This indicates that when the distance from the magnet surface exceeds 60 μm, there is basically no rare earth oxide RO phase in the R-rich phase at the magnet grain boundary, and the resistivity is therefore significantly reduced.

[0077] When the oxidation temperature of the magnet surface is high (such as Experiment No. 7 and Experiment No. 8), the surface resistivity is still at a high level after 60 μm of the magnet surface is worn away, indicating that the thickness of the oxide layer of the magnet increases significantly after high-temperature oxidation.

[0078] The temperature increment of the magnet in the alternating magnetic field at different times in Experiment No. 1 to Experiment No. 8 is as follows Figure 5 As shown, from Figure 5 It can be seen that for magnets that have not undergone surface oxidation or have a lower surface oxidation temperature, their surface resistivity is lower, and the induced eddy current in the alternating magnetic field is larger, so the heat generated is higher. When the oxidation temperature is within the recommended range of the present invention (250-450°C), since the surface layer of the magnet has a thicker oxide layer, the resistivity increases, so the Joule heat generated in the high current density area on the surface of the magnet is lower, and the temperature increment of the magnet after a period of time is also lower. Since the thickness of the high current density area on the surface of the magnet is limited, the thickness of the oxide layer on the surface of the magnet does not need to be too thick. From the temperature increment of the magnets in Experiment No. 7 and Experiment No. 8, it can be seen that when the oxide layer is too thick, the temperature increment of the magnet does not change significantly compared with the magnet of the present invention, but the magnetic properties of the magnet are seriously deteriorated. Therefore, it is necessary to control the thickness of the oxide layer of the magnet in the present invention.

[0079] The magnetic loss of the magnet after the alternating magnetic field test is shown in Table 3. It can be seen that the magnetic loss of the magnet of the present invention after treatment in the alternating magnetic field is significantly reduced due to the combined effect of increasing the resistivity of the magnet surface layer to suppress the increase in magnet temperature and the grain boundary diffusion of heavy rare earth elements.

[0080] Table 3

[0081] Experiment No. 1 2 3 4 5 6 7 8 Magnetic loss(%) 5.81 5.71 5.68 0.85 0.83 0.82 0.82 0.81

[0082] Due to the low resistivity of NdFeB magnets, the eddy current generated on the magnet surface in the alternating magnetic field is larger, thus generating more Joule heat. In addition, since the temperature coefficient of magnetic properties of RTB rare earth permanent magnets is negative, the magnetic properties of the magnets will deteriorate when the temperature rises, resulting in reduced working efficiency of the motor.

[0083] The present invention improves the coercive force of the magnet and the temperature coefficient of the magnetic properties of the magnet by the grain boundary diffusion of heavy rare earth elements. The thickness of the oxide layer on the surface of the magnet can be controlled by regulating the oxidation process of the magnet. An oxide layer of a certain thickness is generated in the high current density area on the surface of the magnet while ensuring that the magnetic properties of the magnet are not affected. The Joule heat generated by the magnet in the alternating magnetic field is reduced by increasing the resistivity of the high current density area of ​​the magnet, thereby ensuring the working efficiency of the magnet in the motor. However, the thickness of the oxide layer is not the thicker the better. When the thickness of the oxide layer on the surface of the magnet is too thick, not only is the effect of suppressing the temperature rise of the magnet in the alternating magnetic field limited, but the magnetic properties of the magnet will also be degraded. Therefore, in the present invention, it is necessary to control the surface oxidation temperature of the magnet between 250 and 450°C.

[0084] Embodiment 2:

[0085] The composition is Nd 22.5 Pr7.5 B 0.95 Ga 0.1 Co 0.5 Cu 0.1 Zr 0.2 Fe 68.15 After the ingredients are prepared in a mass ratio, vacuum induction melting, belt spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing and vacuum sintering are adopted to prepare the sintered magnet.

[0086] The sintered magnet is subjected to a primary high temperature heat treatment at 880°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 520°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0087] The magnets after secondary low temperature heat treatment were machined into finished magnets with a length × width × thickness of 60 mm × 50 mm × 10 mm, where the thickness direction is the magnet orientation direction. After surface grinding to remove oil and rust spots on the surface of the finished magnets, a 10 μm thick pure Tb diffusion source layer was deposited on the two surfaces of the magnets perpendicular to the orientation direction by multi-arc ion plating, and no heavy rare earth element diffusion source was deposited on other surfaces.

[0088] The magnet with a heavy rare earth element diffusion source layer deposited on the surface was heated to 350°C in a tunnel furnace in an air atmosphere, kept at this temperature for different times, and then air-cooled.

[0089] The surface oxidized magnet is placed in a vacuum sintering furnace and heated to 910°C for high temperature grain boundary diffusion. When the highest temperature is reached, the absolute vacuum degree in the furnace is 10 -2 Pa~10 -5 Pa, keep warm for 10 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 520℃ and keep warm for 3 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0090] The final magnet was surface treated by sandblasting to remove the remaining diffusion source layer on the magnet surface. The magnetic properties of the magnet were measured by NIM magnetic testing equipment, the magnet microstructure was observed by SEM, the magnet composition was analyzed by ICP, and the magnet microscopic composition was analyzed by EPMA. The surface resistivity of the magnet was tested by the four-probe method, and then the magnet was polished with sandpaper. After the magnet surface was polished by 60μm, the surface resistivity of the magnet was tested again. After the magnet was saturated with magnetization, the surface of the magnet was wrapped with insulation material and placed in a coil, and an alternating magnetic field of 8.5kA / m was applied to the magnet at a frequency of 2000Hz. The temperature rise of the magnet was measured every 10min within 1h using a thermocouple attached to the magnet.

[0091] The different surface oxidation times, final Tb content and room temperature (20°C) magnetic properties of the magnets of Experiment No. 9 to Experiment No. 13 are shown in Table 4:

[0092] Table 4

[0093] Experiment No. Surface oxidation time / min Tb content (wt.%) Br(kGs) Hcj(kOe) 9 5 0.22 14.32 20.59 10 15 0.22 14.32 20.58 11 30 0.21 14.30 20.54 12 60 0.17 14.33 18.75 13 120 0.15 14.35 17.28

[0094] From the data in Table 4, it can be seen that when the surface oxidation time of the magnet is within the recommended range of the present invention (10min to 50min), the magnetic properties of the magnet are basically the same as those of the magnet with a shorter surface oxidation time. However, when the surface oxidation time is too long, the heavy rare earth element Tb is difficult to diffuse due to the thick thickness of the heavy rare earth diffusion source oxide layer, so the Tb element content in the final magnet is low, and the coercive force increment is correspondingly reduced.

[0095] The SEM microstructures of the magnets in Experiment No. 9, Experiment No. 10 and Experiment No. 13 within a certain depth range from the surface of the coated diffusion source are shown in Figure 1. Figure 6 As shown in (a), (b) and (c), it can be seen that when the surface oxidation time is short, there are fewer rare earth oxide RO phases in the grain boundary R-rich phase within a depth range of 30 μm from the magnet surface layer. When the oxidation time is within the recommended range of the present invention, it can be seen from the microstructure of the magnet of Experiment No. 10 that there are more rare earth oxide RO phases in the grain boundary R-rich phase within a depth range of 30 μm from the magnet surface layer. When the oxidation time is too long, such as the magnet of Experiment No. 13, the amount of rare earth oxide RO phase generated increases, and a large amount of rare earth oxide RO phase can still be observed at a depth of 60 μm from the surface of the magnet coated with the diffusion source.

[0096] The ratio curve of rare earth oxide RO phase to grain boundary R-rich phase at different depths from the surface of the coated diffusion source in Experiment No. 9 to Experiment No. 13 is shown in the figure. Figure 7 As shown, it can be seen that when the magnet oxidation time is short, there is only a small amount of rare earth oxide RO phase within the range of 0 to 5 μm from the surface of the magnet coated diffusion source. When the oxidation time is within the recommended range of the present invention (10min to 50min), the rare earth oxide RO phase is mainly concentrated within a depth range of 30 μm from the magnet surface, and when the distance exceeds 60 μm, the proportion of the rare earth oxide RO phase to the grain boundary R-rich phase is less than 5 vol.%. The same method was used to test the proportion of the rare earth oxide RO phase within a certain depth range from the magnet non-diffusion source coated surface and the oxygen content of the magnet, and it was found that the distribution law of the rare earth oxide RO phase is the same as that of the surface of the magnet coated diffusion source. When the oxidation time is within the recommended range of the present invention (10min to 50min), the rare earth oxide RO phase is mainly concentrated within a depth range of 40 μm from the magnet surface, and when the distance exceeds 60 μm, the rare earth oxide RO phase accounts for less than 5 vol.% of the grain boundary R-rich phase.

[0097] When the oxidation time is too long, the thickness of the oxide layer on the magnet surface increases significantly. From the proportion of RO rare earth oxide in the R-rich phase at the grain boundary of the magnet, it can be seen that after the magnet of Experiment No. 13 was oxidized at 350°C for 120 minutes, the proportion of the rare earth oxide RO phase in the grain boundary R-rich phase within 60 to 70 μm from the surface of the magnet coated with the diffusion source still exceeded 80 vol.%. Therefore, in the present invention, when the surface oxidation temperature is between 250 and 450°C, the oxidation time should be controlled within the range of 10 to 50 minutes.

[0098] EPMA analysis of the rare earth oxide RO phase of the magnet shows that its composition is rare earth elements and oxygen elements, and the atomic ratio of oxygen atoms is between 40 and 80 at.%, which shows that the R-rich phase at the grain boundary of the oxide layer on the surface of the magnet is mainly oxides of rare earth elements.

[0099] The surface resistivity of the magnet was measured by a four-probe method. Then, each surface of the magnet was abraded by sandpaper to remove 60 μm, and the surface resistivity of the magnet was measured again. The results are shown in Table 5.

[0100] Table 5

[0101] Experiment No. Magnet surface resistance / μΩ·cm Surface resistivity after 60μm surface grinding / μΩ·cm 9 168 165 10 265 164 11 267 165 12 265 238 13 267 249

[0102] As can be seen from Table 5, for magnets with too short surface oxidation time, the surface resistivity of the magnet is basically the same as the surface resistivity after 60μm of the magnet surface is worn off. When the surface oxidation time is within the recommended range of the present invention (10min~50min), the surface resistivity of the magnet increases significantly, and the resistivity values ​​are all greater than 250μΩ·cm. In addition, the surface resistivity of the magnet cannot be further improved with the extension of oxidation time. The increase in the surface resistance of the magnet mainly comes from the reaction of the R-rich phase at the grain boundary of the magnet surface with oxygen to form the rare earth oxide RO phase. When the R-rich phase at the grain boundary of the magnet surface layer is completely oxidized, further extending the time will not change the resistivity of the surface layer, but will only increase the depth of the high resistivity interval. It can be seen from the surface resistivity data after 60μm of the magnet surface is worn off that when the oxidation time is within the recommended range of the present invention (10min~50min), the resistivity of the magnet at a depth of 60μm from the magnet surface is basically the same as the resistivity of the magnet with a shorter oxidation time. This indicates that when the distance from the magnet surface exceeds 60 μm, there is basically no rare earth oxide RO phase in the R-rich phase at the magnet grain boundary, and the resistivity is therefore significantly reduced.

[0103] When the magnet surface oxidation time is long (such as Experiment No. 12 and Experiment No. 13), the surface resistivity is still at a high level after 60 μm of the magnet surface is worn away, indicating that the thickness of the magnet oxide layer increases significantly after a long period of oxidation.

[0104] The temperature increment curves of the magnets in the alternating magnetic field at different times in Experiment No. 9 to Experiment No. 13 are shown in the figure below. Figure 8 As shown, it can be seen that for magnets with shorter surface oxidation time, their surface resistivity is lower, and the induced eddy current in the alternating magnetic field is larger, so the heat generated is higher. When the oxidation time is within the recommended range of the present invention (10min to 50min), since the surface layer of the magnet has a thicker oxide layer, the resistivity increases, so the Joule heat generated in the high current density area on the surface of the magnet is lower, and the temperature increment of the magnet after a period of time is also lower. Since the thickness of the high current density area on the surface of the magnet is limited, the thickness of the oxide layer on the surface of the magnet does not need to be too thick. From the temperature increment of the magnets in Experiment No. 12 and Experiment No. 13, it can be seen that when the oxide layer is too thick, the temperature increment of the magnet does not change significantly compared with the magnet of the present invention, but the magnetic properties of the magnet are seriously deteriorated. Therefore, in the present invention, it is necessary to control the thickness of the oxide layer of the magnet by controlling the oxidation time.

[0105] The magnetic loss of the magnet after the alternating magnetic field test is shown in Table 6. It can be seen that the magnetic loss of the magnet of the present invention after treatment in the alternating magnetic field is significantly reduced due to the combined effect of increasing the resistivity of the magnet surface layer to suppress the increase in magnet temperature and the grain boundary diffusion of heavy rare earth elements.

[0106] Table 6

[0107] Experiment No. 9 10 11 12 13 Magnetic loss(%) 5.85 0.85 0.83 0.81 0.81

[0108] In order to ensure that the high current area of ​​the magnet has a higher resistivity during use in an alternating magnetic field, the present invention adopts a magnet surface oxidation method to form a high resistivity area on the magnet surface layer to reduce the Joule heat generated when used in an alternating magnetic field. At that time, the thickness of the oxide layer is not the thicker the better. As the thickness of the oxide layer increases, the grain boundary diffusion effect of the heavy rare earth elements will be affected. Therefore, in the present invention, when the surface oxidation temperature is between 250 and 450°C, the surface oxidation time needs to be controlled within the range of 10 to 50 minutes.

[0109] Embodiment three:

[0110] The composition is Nd 22.5 Pr 7.5 B 0.95 Ga 0.1 Co 0.5 Cu 0.1 Zr 0.2 Fe 68.15 After the ingredients are prepared in a mass ratio, vacuum induction melting, belt spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing and vacuum sintering are adopted to prepare the sintered magnet.

[0111] The sintered magnet is subjected to a primary high temperature heat treatment at 880°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 520°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0112] The magnets after the secondary low temperature heat treatment are processed into finished magnets with a length×width×thickness of 60mm×50mm×10mm by machining, wherein the thickness direction is the magnet orientation direction. The surface oil and rust spots of the finished magnets are removed by surface grinding.

[0113] In experiment No. 14, the magnet was first heated to 350°C in a tunnel furnace in air atmosphere, kept warm for 30 minutes, and after cooling, a pure Tb diffusion source layer with a thickness of 10 μm was deposited on two surfaces of the magnet perpendicular to the orientation direction by multi-arc ion plating, and no heavy rare earth element diffusion source was deposited on other surfaces. In experiment No. 15, a pure Tb diffusion source layer with a thickness of 10 μm was deposited on two surfaces of the magnet perpendicular to the orientation direction by multi-arc ion plating, and no heavy rare earth element diffusion source was deposited on other surfaces, and then heated to 350°C in a tunnel furnace in air atmosphere, kept warm for 30 minutes, and then air-cooled.

[0114] The surface oxidized magnet is placed in a vacuum sintering furnace and heated to 910°C for high temperature grain boundary diffusion. When the highest temperature is reached, the absolute vacuum degree in the furnace is 10 -2 Pa~10 -4 Pa, keep warm for 10 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 520℃ and keep warm for 3 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0115] The final magnet is surface treated by sandblasting to remove the remaining diffusion source layer on the magnet surface. The magnetic properties of the magnet are measured by NIM magnetic testing equipment, the magnet microstructure is observed by SEM, and the magnet composition is analyzed by ICP.

[0116] The final Tb content and magnetic properties of the magnets of Experiment No. 14 and Experiment No. 15 at room temperature (20°C) are shown in Table 7:

[0117] Table 7

[0118] Experiment No. Tb content (wt.%) Br(kGs) Hcj(kOe) 14 0.11 14.35 15.52 15 0.22 14.30 20.63

[0119] From the experimental data in Table 7, it can be seen that the magnetic properties of the magnets are significantly different when the order of magnet surface oxidation and heavy rare earth element diffusion source deposition is changed. For magnets that first deposit heavy rare earth element diffusion and then surface oxidation, the final Tb content and magnetic properties of the magnets are both high. When the magnet surface is first oxidized and then the heavy rare earth element diffusion source is deposited, the final Tb content and magnetic properties of the magnets are both low.

[0120] The SEM microstructures of the magnets of Experiment No. 14 and Experiment No. 15 that have not been treated with high temperature grain boundary diffusion within a certain depth from the magnet surface are shown in Figure 1. Fig. 9 As shown in (a) and (b), it can be seen that when a heavy rare earth element diffusion source layer is deposited on the magnet surface and then surface oxidation is performed, the oxygen element is mainly concentrated in the outer layer of the heavy rare earth diffusion source layer before high-temperature grain boundary diffusion.

[0121] When the magnet is first subjected to surface oxidation and then the diffusion source is deposited, the oxygen element is mainly concentrated in the magnet surface layer. In the subsequent high temperature grain boundary diffusion process, the oxygen element on the magnet surface will first oxidize the grain boundary R-rich phase on the magnet surface layer, preventing the diffusion of heavy rare earth elements. Therefore, when the magnet is first subjected to surface oxidation, the Tb content of the final magnet will be significantly reduced, and the magnetic properties of the magnet will also be significantly reduced. Therefore, in the present invention, the magnet needs to first deposit the heavy rare earth element diffusion source, and then conduct surface oxidation.

[0122] Embodiment 4:

[0123] The composition is Nd 22.5 Pr 7.5 B 0.95 Ga 0.1 Co 0.5 Cu 0.1 Zr 0.2 Fe 68.15 After the ingredients are prepared in a mass ratio, vacuum induction melting, belt spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing and vacuum sintering are adopted to prepare the sintered magnet.

[0124] The sintered magnet is subjected to a primary high temperature heat treatment at 880°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 520°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0125] The magnets after secondary low temperature heat treatment were machined into finished magnets with a length × width × thickness of 60 mm × 50 mm × 10 mm, where the thickness direction is the magnet orientation direction. After surface grinding to remove oil and rust spots on the surface of the finished magnets, a 10 μm thick pure Tb diffusion source layer was deposited on the two surfaces of the magnets perpendicular to the orientation direction by multi-arc ion plating, and no heavy rare earth element diffusion source was deposited on other surfaces.

[0126] The magnet with a heavy rare earth element diffusion source layer deposited on the surface was heated to 350°C in a tunnel furnace with different atmospheres, kept at this temperature for 30 minutes, and then air-cooled.

[0127] The surface oxidized magnet is placed in a vacuum sintering furnace and heated to 910°C for high temperature grain boundary diffusion. When the highest temperature is reached, the absolute vacuum degree in the furnace is 10 -2 Pa~10 -4 Pa, keep warm for 10 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 520℃ and keep warm for 3 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0128] The final magnet is surface treated by sandblasting to remove the remaining diffusion source layer on the magnet surface. The magnetic properties of the magnet are measured by NIM magnetic testing equipment, the magnet microstructure is observed by SEM, the magnet composition is analyzed by ICP, and the magnet microscopic composition is analyzed by EPMA.

[0129] The different surface oxidation atmospheres, final magnet Tb content, and room temperature (20°C) magnetic properties of magnets for Experiment No. 16 to Experiment No. 22 are shown in Table 8:

[0130] Table 8

[0131] Experiment No. Oxidizing atmosphere Tb content (wt.%) Br(kGs) Hcj(kOe) 16 Air 0.21 14.30 20.54 17 Argon (Ar) 0.22 14.31 20.65 18 5vol.%O2+95vol.%Ar 0.22 14.32 20.63 19 10vol.%O2+90vol.%Ar 0.21 14.31 20.54 20 40vol.%O2+60vol.%Ar 0.21 14.30 20.44 21 70vol.%O2+30vol.%Ar 0.20 14.29 20.31 22 80vol.%O2+20vol.%Ar 0.16 14.34 17.65

[0132] It can be seen from the experimental data in Table 8 that when the oxygen content of the oxidizing atmosphere on the magnet surface is greater than 80 vol.%, the Tb content of the magnet after high-temperature grain boundary diffusion is low, and the final magnetic properties are also low.

[0133] The ratio curve of rare earth oxide RO phase to grain boundary R-rich phase at different depths from the surface of the coated diffusion source in Experiment No. 16 to Experiment No. 22 is shown in the figure. Fig.10As shown, it can be seen that when there is no oxygen element in the surface oxidation atmosphere, there is basically no oxide layer on the surface of the final magnet. When the oxygen content of the oxidation atmosphere on the surface of the magnet is less than 10vol.%, there is only a small amount of rare earth oxide RO phase within 0 to 5μm from the surface of the magnet coated with the diffusion source. When the oxygen content of the surface oxidation atmosphere is within the recommended range of the present invention (10vol.% to 70vol.%), the rare earth oxide RO phase is mainly concentrated within a depth range of 30μm on the surface of the magnet, and when the distance exceeds 60μm, the proportion of the rare earth oxide RO phase to the grain boundary R-rich phase is less than 5vol.%. The same method was used to test the proportion of the rare earth oxide RO phase within a certain depth range from the non-diffusion source coated surface of the magnet and the oxygen content of the magnet, and it was found that the distribution pattern of the rare earth oxide RO phase is similar to that of the surface of the above-mentioned magnet coated with the diffusion source. When the oxygen content of the surface oxidation atmosphere is within the recommended range of the present invention (10 vol.% to 70 vol.%), the rare earth oxide RO phase is mainly concentrated within a depth of 40 μm from the magnet surface. When the distance exceeds 60 μm, the proportion of the rare earth oxide RO phase in the grain boundary R-rich phase is less than 5 vol.%.

[0134] When the oxygen content in the oxidation atmosphere on the magnet surface exceeds 70 vol.%, the thickness of the oxide layer on the magnet surface increases significantly. From the proportion of RO rare earth oxide in the R-rich phase at the grain boundary of the magnet, it can be seen that after the magnet of Experiment No. 22 was oxidized at 350°C for 30 minutes in an 80 vol.% O2 atmosphere, the proportion of the rare earth oxide RO phase in the grain boundary R-rich phase within 60 to 70 μm from the surface of the magnet coated with the diffusion source still exceeded 80 vol.%. Therefore, in the present invention, the oxygen content of the oxidation atmosphere should be controlled in the range of 10 vol.% to 70 vol.%.

[0135] Embodiment five:

[0136] The composition is Nd 22.5 Pr 7.5 B 0.95 Ga 0.1 Co 0.5 Cu 0.1 Zr 0.2 Fe 68.15 After the ingredients are prepared in a mass ratio, vacuum induction melting, belt spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing and vacuum sintering are adopted to prepare the sintered magnet.

[0137] The sintered magnet is subjected to a primary high temperature heat treatment at 880°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 520°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0138] The magnets after secondary low temperature heat treatment were machined into finished magnets with a length×width×thickness of 60mm×50mm×10mm, where the thickness direction is the magnet orientation direction. After surface grinding to remove oil and rust spots on the surface of the finished magnets, multi-arc ion plating was used to deposit different heavy rare earth diffusion source layers with a thickness of 10μm on the two surfaces of the magnets perpendicular to the orientation direction, and no heavy rare earth element diffusion source was deposited on other surfaces.

[0139] The magnet with a heavy rare earth element diffusion source layer deposited on the surface was heated to 350°C in a tunnel furnace in an air atmosphere, kept at this temperature for 30 minutes, and then air-cooled.

[0140] The surface oxidized magnet is placed in a vacuum sintering furnace and heated to 910°C for high temperature grain boundary diffusion. When the highest temperature is reached, the absolute vacuum degree in the furnace is 10 -2 Pa~10 -4 Pa, keep warm for 10 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 520℃ and keep warm for 3 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0141] The final magnet is surface treated by sandblasting to remove the remaining diffusion source layer on the magnet surface. The magnetic properties of the magnet are measured by NIM magnetic testing equipment, the magnet microstructure is observed by SEM, and the magnet composition is analyzed by ICP.

[0142] The different diffusion source components, Tb content of the final magnets, and magnetic properties of the magnets at room temperature (20°C) for the experiments No. 23 to No. 26 are shown in Table 9:

[0143] Table 9

[0144] Experiment No. Diffusion source components Tb content (wt.%) Br(kGs) Hcj(kOe) 23 Pure Tb 0.22 14.30 20.60 24 Tb with oxygen content of 1wt.% 0.22 14.29 20.58 25 Tb with oxygen content of 5wt.% 0.21 14.31 20.54 26 Tb with an oxygen content of 20 wt.% 0.16 14.34 17.52

[0145] It can be seen from the data in Table 9 that when the oxygen content in the diffusion source exceeds 5 wt.%, the Tb content in the final magnet is significantly reduced and the final magnetic properties are also lower.

[0146] In order to allow the heavy rare earth elements to effectively diffuse into the interior of the magnet during grain boundary diffusion, it is necessary to ensure that the magnet has sufficient molten grain boundary phase. Therefore, in the early stage of grain boundary diffusion, it is necessary to ensure that the grain boundary R-rich phase on the surface of the magnet is not over-oxidized. When the oxygen content in the grain boundary diffusion source is high, the oxygen element in the diffusion source will react with the grain boundary R-rich phase of the magnet in the early stage of grain boundary diffusion to form a rare earth oxide RO phase, which hinders the grain boundary diffusion efficiency of the heavy rare earth elements and reduces the coercive force increment of the magnet. Therefore, in the present invention, it is necessary to control the oxygen content in the grain boundary diffusion source to ensure that the oxygen content in the heavy rare earth element diffusion source is ≤5wt.%.

[0147] Embodiment six:

[0148] The composition is Nd 23 Pr 7 Dy 0.5 B 0.95 Al 0.45 Co 0.6 Cu 0.15 Ga 0.1 Zr 0.1 Fe 67.15 After the ingredients are prepared in a mass ratio, vacuum induction melting, belt spinning, hydrogen crushing, air flow milling, orientation molding, isostatic pressing and vacuum sintering are adopted to prepare the sintered magnet.

[0149] The sintered magnet is subjected to a primary high temperature heat treatment at 900°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min. Then, a secondary low temperature heat treatment is carried out at 500°C for 3 hours, and after the heat preservation is completed, it is cooled to below 200°C at a cooling rate of not less than 80°C / min.

[0150] The magnets after secondary low temperature heat treatment were machined into finished magnets with a length × width × thickness of 60 mm × 50 mm × 10 mm, where the thickness direction is the magnet orientation direction. After surface grinding to remove oil and rust spots on the surface of the finished magnets, a 10 μm thick pure Tb diffusion source layer was deposited on the two surfaces of the magnets perpendicular to the orientation direction by multi-arc ion plating, and no heavy rare earth element diffusion source was deposited on other surfaces.

[0151] Experiment No. 27 Magnet: The magnet with a heavy rare earth element diffusion source layer deposited on the surface was heated to 350° C. in a tunnel furnace in an air atmosphere, kept at this temperature for 30 minutes, and then air-cooled.

[0152] The magnet of Experiment No. 28 was directly subjected to grain boundary diffusion without going through the surface oxidation step.

[0153] The magnet is placed in a vacuum sintering furnace and heated to 910°C for high temperature grain boundary diffusion. When the highest temperature is reached, the absolute vacuum degree in the furnace is 10 -2 Pa~10 -4 Pa, keep warm for 10 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃. Then heat the magnet to 520℃ and keep warm for 3 hours. After the heat preservation is completed, cool at a cooling rate of not less than 80℃ / min until the magnet temperature is below 200℃.

[0154] The final magnet was treated with sandblasting to remove the remaining diffusion source layer on the magnet surface. The magnetic properties of the magnet were measured using NIM magnetic testing equipment. After the magnet was saturated, the magnet surface was wrapped with insulation material and placed in a coil. An alternating magnetic field of 8.5 kA / m was applied to the magnet at a frequency of 2000 Hz. The magnetic loss of the magnet was measured after 1 hour, and the results are shown in Table 10:

[0155] Table 10

[0156] Experiment No. 27 28 Magnetic loss(%) 0.72 4.51

[0157] The present invention forms a high resistivity region on the surface layer of the magnet by oxidizing the surface of the magnet, thereby reducing the Joule heat generated when the magnet is used in an alternating magnetic field. Therefore, the magnetic loss of the magnet after being used under high-frequency conditions can be significantly reduced, thereby ensuring the working efficiency of the magnet in the motor.

Claims

1. A high-performance RTB rare earth permanent magnet for a motor is prepared by the following method: (1) Covering a heavy rare earth diffusion source on the surface of a magnet substrate, wherein the magnet substrate is a RTB sintered magnet; (2) oxidizing the surface of the magnet covered with the diffusion source, heating it to 250-450°C in an atmosphere with an oxygen content of 10 vol.%-70 vol.%, and keeping it at this temperature for 10 min-50 min; (3) subjecting the magnet with surface oxidation to grain boundary diffusion treatment, the diffusion temperature is 800-1000° C., the holding time is 5-25 hours, cooling to below 200° C. after the holding is completed, and then heating up for tempering treatment to obtain the high-performance RTB rare earth permanent magnet for the motor; The magnet is made by R2T 14 It is composed of B main phase and grain boundary R-rich phase. More than 95 vol.% of the R-rich phase at the grain boundary of the magnet within the depth range of 0~30μm from the surface of the magnet coated with diffusion source is composed of rare earth oxide RO phase; more than 95 vol.% of the R-rich phase at the grain boundary of the magnet within the depth range of 0~40μm from the surface of the magnet coated with non-diffusion source is composed of rare earth oxide RO phase; within the depth range greater than 60μm from the magnet surface, the proportion of rare earth oxide RO phase in the R-rich phase at the grain boundary of the magnet is less than 5 vol.%.

2. The high-performance RTB rare earth permanent magnet for electric motor according to claim 1, characterized in that The resistivity of the magnet at room temperature is greater than 250 μΩ·cm within a range of less than 30 μm from the magnet surface, and less than 170 μΩ·cm within a range of greater than 60 μm from the magnet surface.

3. The high performance RTB rare earth permanent magnet for electric motor according to claim 1, characterized in that The oxygen atomic ratio in the rare earth oxide RO phase within the depth range of 0 to 40 μm from the surface of the magnet is between 40 and 80 at.%.

4. The high performance RTB rare earth permanent magnet for motor according to claim 1, characterized in that In step (1), the heavy rare earth diffusion source is pure heavy rare earth element metal, heavy rare earth element hydride or alloy of heavy rare earth element and other metal elements; the heavy rare earth element is at least one of Dy and Tb, and the mass content of heavy rare earth element in the heavy rare earth diffusion source is ≥20wt.%, and the oxygen content is ≤5wt.%.

5. The high-performance RTB rare earth permanent magnet for motor according to claim 1, characterized in that In the step (1), the heavy rare earth diffusion source is covered by evaporation, magnetron sputtering or multi-arc ion plating to deposit the diffusion source on the surface of the magnet substrate; the thickness of the heavy rare earth diffusion source is between 3 μm and 1 mm.

6. The high performance RTB rare earth permanent magnet for motor according to claim 1, characterized in that In the step (1), the surface of the magnet substrate perpendicular to the orientation direction is covered with the heavy rare earth diffusion source, and the surface of the magnet substrate not perpendicular to the orientation direction is not covered with the heavy rare earth diffusion source.

7. The high performance RTB rare earth permanent magnet for motor according to claim 1, characterized in that In the step (2), the magnet is placed in an air atmosphere and heated to perform surface oxidation.

8. The high performance RTB rare earth permanent magnet for electric motor according to claim 1, characterized in that In the step (2), the surface oxidation temperature is 300-400° C. and the insulation time is 15 min-30 min.

9. The high performance RTB rare earth permanent magnet for motor according to claim 1, characterized in that In the step (3), after the diffusion temperature is reached, the absolute vacuum degree in the furnace is 10 -2 ~10 -5 Pa.

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