Sintered neodymium-iron-boron magnet and preparation method thereof

Abstract

The invention provides a sintered neodymium-iron-boron magnet and a preparation method thereof. The preparation method comprises the steps that S1, raw materials including neodymium iron boron powder and cerium oxide powder are sequentially mixed and subjected to compression molding, and a pressed blank is obtained; s2, the pressed blank is subjected to cold isostatic pressing treatment, sintering treatment and tempering treatment in sequence, and a sintered neodymium-iron-boron magnet is obtained; wherein the cerium oxide powder is composed of first cerium oxide powder and second cerium oxide powder, the average particle size of the first cerium oxide powder is 20-500 nm, and the average particle size of the second cerium oxide powder is 0.5-2 [mu] m. Through the addition of cerium oxide powder with different particle sizes and the synergistic effect of the preparation process, the oxygen content and the carbon content in the magnet are greatly reduced, the reduction of the performance of the magnet caused by impurity elements is reduced, and meanwhile, compared with a traditional process, the preparation method has the advantages that the process time is shortened, the production efficiency is improved, and the production cost is saved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of magnets, in particular to a sintered neodymium-iron-boron magnet and a preparation method thereof. BACKGROUND

[0002] As the most important branch of rare earth materials and the largest industry scale, sintered neodymium-iron-boron rare earth permanent magnet is widely used in medical devices, aerospace, information communication and other high-tech industries, especially in the development of strategic emerging industries such as energy-saving home appliances, intelligent networking, rail transit, new energy vehicles and wind power. Developing high-temperature-resistant and high-coercivity sintered neodymium-iron-boron permanent magnet can effectively promote the development of emerging industries and meet the development requirements of China's "double carbon" target. The development of application market towards lightweight and miniaturization puts forward more stringent requirements on the consistency of sintered neodymium-iron-boron permanent magnet in the preparation process of bulk blanks and the degree of cracking in the cutting and grinding process. The existing bulk blank preparation is prone to produce dark cracks during sintering heating process, and the inconsistent magnetic properties of different parts will also lead to poor product performance, which seriously affects the yield of blanks.

[0003] In addition, antioxidants and lubricants are indispensable in the production and preparation process of sintered neodymium-iron-boron. The carbon and oxygen in such organic compounds will react with the main phase and neodymium-rich phase to form new phases with low anisotropy constant, which seriously affects the coercivity of the magnet.

[0004] In the prior art, a patent application with the patent application publication number CN109935463A discloses a method for reducing the oxygen content of rare earth neodymium iron boron, which reduces the oxygen content in neodymium iron boron by adding alloy additives to the neodymium iron boron raw material. This method can effectively reduce the use amount of heavy rare earth elements and reduce production costs. However, it is not clear whether the content of impurity elements (oxygen) in the final blank and whether the processing performance can be improved. A patent application with the patent application publication number CN115458268A discloses a method for reducing the carbon content of neodymium iron boron magnets, which controls the time and flow of discontinuous inert gas by placing the neodymium iron boron magnet green body into multiple dewaxing vacuum chambers for pretreatment at different temperatures before the final sintering temperature (350℃~400℃@1~2.5h, 550℃~620℃@2~4h) to reduce the carbon content of the product and improve the magnetic properties of the product. However, the heat treatment of this method, especially the sintering section temperature, is still relatively long, and the production efficiency is low. A patent application with the patent application publication number CN109161819A discloses a preparation method of a low-carbon sintered neodymium iron boron magnet, which adds Si to the raw material and performs electric discharge treatment on the prepared neodymium iron boron green body at a temperature of 100℃, which helps to remove carbon. However, since Si also has an adverse effect on the performance of the magnet, it is necessary to seek a method for reducing the carbon content without adding Si. A patent application with the patent application publication number CN106601459A discloses a sintering method for reducing the carbon content of neodymium iron boron magnets, which places the neodymium iron boron green body in a sintering furnace for vacuum sintering, and when the sintering furnace is heated to a preset value, hydrogen is introduced and the hydrogen content is controlled outside the explosion point, the preset value range is 200~800℃, this method can effectively reduce the carbon content of the magnet, improve the magnetic properties and corrosion resistance of the neodymium iron boron magnet. However, hydrogen has a certain explosion risk. In summary, the existing solutions to the problem of high impurity element content in magnets have the problems of complex preparation process (sintering process), introduction of elements affecting the magnetic properties of the matrix, safety risks, etc. SUMMARY

[0005] The main purpose of the present application is to provide a sintered neodymium iron boron magnet and a preparation method thereof, to solve the problem of high carbon content and oxygen content in the sintered neodymium iron boron magnet in the prior art.

[0006] In order to achieve the above object, according to one aspect of the present application, there is provided a preparation method of a sintered neodymium-iron-boron magnet, comprising: step S1, mixing and press forming raw materials comprising neodymium-iron-boron powder and cerium oxide powder in sequence to obtain a green compact; step S2, sequentially performing cold isostatic pressing treatment, sintering treatment and tempering treatment on the green compact to obtain the sintered neodymium-iron-boron magnet; wherein the cerium oxide powder is composed of first cerium oxide powder and second cerium oxide powder, the average particle size of the first cerium oxide powder is 20-500 nm, and the average particle size of the second cerium oxide powder is 0.5-2 μm.

[0007] Further, the mass ratio of the above-mentioned cerium oxide powder to the neodymium-iron-boron powder is (0.01-2.5):100.

[0008] Further, the mass ratio of the above-mentioned first cerium oxide powder to the second cerium oxide powder is (0.1-1.8):1; and / or, the ratio of the average particle size of the first cerium oxide powder to the average particle size of the second cerium oxide powder is 1:(10-40).

[0009] Further, the average particle size of the above-mentioned neodymium-iron-boron powder is 2.0-3.5 μm; and / or, the X90 / X10 of the neodymium-iron-boron powder is ≤5.

[0010] Further, the density of the above-mentioned green compact is 3.2-4.0 g / cm 3 ; and / or, the pressure of the cold isostatic pressing treatment is 180-250 MPa; and / or, the time of the cold isostatic pressing treatment is 20-120 s; and / or, the temperature of the sintering treatment is 1010-1100 ℃; and / or, the time of the sintering treatment is 2-8 h; and / or, the density of the sintered neodymium-iron-boron magnet is 7.40-7.65 g / cm 3 .

[0011] Further, the temperature of the above-mentioned tempering treatment is 480-950 ℃; and / or, the time of the tempering treatment is 7-15 h; preferably, the tempering treatment comprises sequentially performing first tempering treatment and second tempering treatment, the temperature of the first tempering treatment is 900-950 ℃, the time of the first tempering treatment is 2-6 h; the temperature of the second tempering treatment is 480-600 ℃, and the time of the second tempering treatment is 3-10 h.

[0012] Further, the preparation method of the neodymium-iron-boron powder comprises: sequentially performing melt casting, hydrogen decrepitation coarse crushing and jet mill fine crushing on the metal raw material of the neodymium-iron-boron powder according to a proportion to obtain the neodymium-iron-boron powder; preferably, the hydrogen decrepitation coarse crushing comprises sequentially performing hydrogen absorption treatment, hydrogen desorption treatment and cooling treatment, the pressure of the hydrogen absorption treatment is 110-220 kPa, the temperature of the hydrogen desorption treatment is 450-620 ℃, and the time of the hydrogen desorption treatment is 2-6 h; and / or, the oxygen content in the environment of the jet mill fine crushing is not more than 30 ppm; and / or, the pressure of the jet mill fine crushing is 0.4-0.6 MPa; and / or, the neodymium-iron-boron powder comprises the following elements in percentage by mass: the content of Re element is 20-35%, the content of M element is 0.1-2.5%, the content of B element is 0.93-1.2%, the total content of inevitable impurities is ≤0.1%, the content of a single impurity is less than 0.05%, and the balance is Fe element; wherein, the Re element is selected from any one or more of La element, Ce element, Pr element, Nd element, Dy element and Tb element, and the M element is selected from any one or more of Cu element, Al element, Ga element, Co element, Ni element, Zr element and Nb element.

[0013] According to another aspect of the present application, a sintered neodymium-iron-boron magnet is provided, which is prepared by the aforementioned preparation method.

[0014] Further, the sintered neodymium-iron-boron magnet comprises main phase grains and grain boundaries, and the volume fraction of cerium oxide phase in the grain boundaries is 1.5-4.5%.

[0015] Further, the carbon content in the sintered neodymium-iron-boron magnet is 400-600 ppm; and / or, the oxygen content in the sintered neodymium-iron-boron magnet is 400-600 ppm; and / or, the residual magnetism intensity of the sintered neodymium-iron-boron magnet is 14.29-14.60 kGs; and / or, the coercive force of the sintered neodymium-iron-boron magnet is 13.82-15.00 kOe.

[0016] The technical scheme of the present application is applied, and the present application mixes cerium oxide powder with different particle sizes and neodymium-iron-boron powder. On the one hand, it helps to control the penetration of impurity elements in the sintering process of the magnet, thereby helping to greatly reduce the oxygen content and carbon content in the magnet, and further helping to reduce the performance decline of the magnet caused by impurity elements, and avoiding the adverse effects caused by the addition of antioxidants and lubricants. On the other hand, the addition of cerium oxide powder with different particle sizes helps to form more stable grain boundary phases. The presence of these phases can effectively inhibit the abnormal growth of the grains, avoid the appearance of uneven microstructure in the sintered neodymium-iron-boron magnet, and thereby improve the overall density and magnetic performance consistency of the magnet. The first cerium oxide powder helps to optimize the grain boundary structure, and the second cerium oxide powder helps to control the excessive penetration of impurity elements in the three-phase region. The synergistic effect of the two ensures the consistency and excellence of the microstructure of the magnet. In addition, through the synergistic effect of the addition of cerium oxide powder with different particle sizes and the preparation process, the oxygen content and carbon content in the magnet can be greatly reduced, the performance decline of the magnet caused by impurity elements can be reduced, the process time can be shortened compared with the traditional process, the production efficiency can be improved, and the production cost can be saved. BRIEF DESCRIPTION OF DRAWINGS

[0017] The drawings accompanying the specification of the present application form a part thereof, serve to provide further understanding of the present application, and together with the description of the exemplary embodiments of the present application and the explanation thereof serve to explain the present application, and do not constitute improper limitations on the present application. In the drawings:

[0018] Figure 1 A scanning electron microscope image of the sintered neodymium-iron-boron magnet in Example 1 of the present application is shown. DETAILED DESCRIPTION

[0019] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0020] Explanation of terms: X90 of neodymium-iron-boron powder means that 90% of all particles have a particle size less than a certain value; X10 of neodymium-iron-boron powder means that 10% of all particles have a particle size less than a certain value.

[0021] As analyzed in the background art of the present application, the problem of excessive carbon content and oxygen content in the sintered neodymium-iron-boron magnet in the prior art, in order to solve the above problems, the present application provides a sintered neodymium-iron-boron magnet and a preparation method thereof.

[0022] In an embodiment of the present application, a preparation method of a sintered Nd-Fe-B magnet is provided, which comprises: S1, mixing and molding raw materials comprising Nd-Fe-B powder and cerium oxide powder to obtain a compact; S2, performing cold isostatic pressing, sintering and tempering on the compact to obtain the sintered Nd-Fe-B magnet; wherein the cerium oxide powder is composed of a first cerium oxide powder and a second cerium oxide powder, the average particle size of the first cerium oxide powder is 20-500 nm, and the average particle size of the second cerium oxide powder is 0.5-2 μm.

[0023] In the present application, the cerium oxide powder with different particle sizes is mixed with the Nd-Fe-B powder, which helps to control the penetration of impurity elements in the sintering process of the magnet, thereby greatly reducing the oxygen content and carbon content in the magnet, and further reducing the performance decline of the magnet caused by impurity elements, and avoiding the adverse effects caused by the addition of antioxidants and lubricants. On the other hand, the addition of cerium oxide powder with different particle sizes helps to form more stable grain boundary phases, which can effectively inhibit the abnormal growth of the grains, avoid the uneven microstructure in the sintered Nd-Fe-B magnet, and thus improve the overall density and magnetic performance consistency of the magnet. The first cerium oxide powder helps to optimize the grain boundary structure, and the second cerium oxide powder helps to control the excessive penetration of impurity elements in the three-phase region, and the synergistic effect of the two ensures the consistency and excellence of the microstructure of the magnet. In addition, through the synergistic effect of the addition of cerium oxide powder with different particle sizes and the preparation process, the oxygen content and carbon content in the magnet can be greatly reduced, the performance decline of the magnet caused by impurity elements can be reduced, and the process time can be shortened compared with the traditional process, the production efficiency can be improved, and the production cost can be saved.

[0024] In an embodiment of the present application, the mass ratio of the above-mentioned cerium oxide powder to the Nd-Fe-B powder is (0.01-2.5):100.

[0025] Controlling the mass ratio of the cerium oxide powder to the Nd-Fe-B powder within the above-mentioned range helps to reduce the carbon content and oxygen content of the magnet while improving the remanence of the magnet.

[0026] In order to further reduce the carbon content and oxygen content of the magnet, thereby improving the remanence of the magnet, in an embodiment of the present application, the mass ratio of the above-mentioned cerium oxide powder to the Nd-Fe-B powder is (0.05-0.56):100.

[0027] In an embodiment of the present application, the mass ratio of the above-mentioned first cerium oxide powder to the second cerium oxide powder is (0.1-1.8):1; and / or, the ratio of the average particle size of the first cerium oxide powder to the average particle size of the second cerium oxide powder is 1:(10-40).

[0028] Controlling the mass ratio of the first cerium oxide powder and the second cerium oxide powder and the ratio of the average particle size of the first cerium oxide powder to the average particle size of the second cerium oxide powder within the above range helps to form a more uniformly distributed cerium oxide phase in the microstructure of the sintered neodymium-iron-boron magnet. The first cerium oxide powder can more easily penetrate between the magnetic powder particles, effectively reducing the grain boundary oxygen content and carbon content, and inhibiting abnormal grain growth; while the second cerium oxide powder forms a stable grain boundary phase on a larger scale, preventing excessive defects in the transition zone between the grains, thereby optimizing the microstructure and performance of the magnet.

[0029] In an embodiment of the present application, the average particle size of the neodymium-iron-boron powder described above is 2.0-3.5 μm; and / or, the X90 / X10 of the neodymium-iron-boron powder is ≤ 5.

[0030] Controlling the average particle size of the neodymium-iron-boron powder within the above range helps to make the powder have good flowability during the oriented press forming process, thereby helping to form a magnet with high density and uniform grain distribution. Controlling the X90 / X10 of the neodymium-iron-boron powder within the above range helps to form smaller grains during high-temperature sintering, avoiding excessive grain growth, thereby maintaining a high value of the coercive force of the magnet, while the remanence (Br) can also be well maintained, overall improving the magnetic performance of the magnet.

[0031] In an embodiment of the present application, the density of the green compact described above is 3.2-4.0 g / cm 3 ; and / or, the pressure of the cold isostatic pressing process is 180-250 MPa; and / or, the time of the cold isostatic pressing process is 20-120 s; and / or, the temperature of the sintering process is 1010-1100 °C; and / or, the time of the sintering process is 2-8 h; and / or, the density of the sintered neodymium-iron-boron magnet is 7.40-7.65 g / cm 3 .

[0032] Controlling the density of the green compact within the above range helps to form a more dense structure, thereby facilitating the improvement of the performance of the magnet. Controlling the pressure and time of the cold isostatic pressing process within the above range helps to make the contact between the magnetic powder particles more close, reducing internal voids, and enhancing the structural stability of the magnet, thereby improving the yield and performance consistency of the sintered neodymium-iron-boron magnet. Controlling the temperature and time of the sintering process within the above range helps to promote the optimization of the microstructure inside the material. The sintered neodymium-iron-boron magnet with the above density has higher remanence and coercive force.

[0033] In an embodiment of the present application, the temperature of the tempering treatment is 480-950℃; and / or, the time of the tempering treatment is 7-15h; preferably, the tempering treatment comprises a first tempering treatment and a second tempering treatment performed in sequence, the temperature of the first tempering treatment is 900-950℃, the time of the first tempering treatment is 2-6h; the temperature of the second tempering treatment is 480-600℃, the time of the second tempering treatment is 3-10h.

[0034] Controlling the temperature and time of the tempering treatment within the above ranges helps to promote the stress relaxation inside the magnet, making the grain boundary phase more stable, thereby helping to improve the coercivity and remanence performance of the magnet. Controlling the temperature and time of the first tempering treatment and the second tempering treatment within the above ranges helps to more fully adjust the microstructure of the magnet, reduce defects, and increase the density of the magnet.

[0035] In an embodiment of the present application, the preparation method of the neodymium-iron-boron powder comprises: sequentially performing melt casting, hydrogen breaking coarse crushing and jet mill fine crushing on the metal raw materials of the neodymium-iron-boron powder according to the proportions to obtain the neodymium-iron-boron powder; preferably, the hydrogen breaking coarse crushing comprises sequentially performing hydrogen absorption treatment, hydrogen desorption treatment and cooling treatment, the pressure of the hydrogen absorption treatment is 110-220kPa, the temperature of the hydrogen desorption treatment is 450-620℃, and the time of the hydrogen desorption treatment is 2-6h; and / or, the oxygen content in the environment of the jet mill fine crushing is not more than 30ppm; and / or, the pressure of the jet mill fine crushing is 0.4-0.6MPa; and / or, the neodymium-iron-boron powder comprises the following elements in mass percentage: the content of Re element is 20-35%, the content of M element is 0.1-2.5%, the content of B element is 0.93-1.2%, the total content of unavoidable impurities is ≤0.1%, the content of a single impurity is less than 0.05%, and the balance is Fe element; wherein, the Re element is selected from any one or more of La element, Ce element, Pr element, Nd element, Dy element and Tb element, and the M element is selected from any one or more of Cu element, Al element, Ga element, Co element, Ni element, Zr element and Nb element.

[0036] Controlling the conditions of the hydrogen breaking coarse crushing and the jet mill fine crushing within the above ranges helps to prepare the neodymium-iron-boron powder with appropriate size, thereby being conducive to improving the density and magnetic performance of the subsequent sintered magnet. Controlling the element content in the neodymium-iron-boron powder within the above ranges helps to improve the magnetic performance and chemical stability of the sintered magnet.

[0037] In another typical embodiment of the present application, a sintered neodymium-iron-boron magnet is provided, which is prepared by the aforementioned preparation method.

[0038] Since the sintered Nd-Fe-B magnet is prepared by the preparation method of the present application, the sintered Nd-Fe-B magnet has low carbon content and oxygen content, and has high remanence and coercivity.

[0039] In an embodiment of the present application, the sintered Nd-Fe-B magnet includes main phase grains and grain boundaries, and the volume fraction of cerium oxide phase in the grain boundaries is 1.5-4.5%.

[0040] Controlling the volume fraction of cerium oxide phase in the grain boundaries within the above range helps to optimize the grain boundary phase composition, improve the microstructure stability, reduce the impurity element content, and improve the magnetic performance.

[0041] In an embodiment of the present application, the carbon content in the sintered Nd-Fe-B magnet is 400-600 ppm; and / or, the oxygen content in the sintered Nd-Fe-B magnet is 400-600 ppm; and / or, the remanence of the sintered Nd-Fe-B magnet is 14.29-14.60 kGs; and / or, the coercivity of the sintered Nd-Fe-B magnet is 13.82-15.00 kOe.

[0042] The sintered Nd-Fe-B magnet with the above carbon content, oxygen content, remanence and coercivity is more suitable for application in the fields of medical devices, aerospace, information communication, etc.

[0043] The beneficial effects of the present application will be further illustrated below with examples.

[0044] Example 1

[0045] The Nd-Fe-B powder includes the following elements in mass percentage: the content of Nd element is 30%, the content of Al element is 0.02%, the content of Cu element is 0.02%, the content of Zr element is 0.02%, the content of Co element is 0.08%, the content of B element is 0.98%, the total content of unavoidable impurities is ≤0.1%, the content of single impurity is lower than 0.05%, and the balance is Fe element.

[0046] 1. Put the metal raw materials of Nd-Fe-B powder into an alumina crucible according to the above ratio, vacuum smelt in a high-frequency vacuum smelting furnace at 1450℃, then pour the steel liquid onto a rotating copper roller to obtain an alloy thin strip with an average thickness of 0.20mm;

[0047] 2. Hydrogen crushing process: place the alloy thin strip obtained in step 1 into a hydrogen crushing furnace, introduce hydrogen gas with a purity of more than 99.99%, and hydrogen absorption pressure is 150kPa, after the thin sheet is fully hydrogenated, the dehydrogenation temperature is 540℃, the dehydrogenation time is 4h, and the hydrogenated powder with an average particle size of 100μm is prepared after fully dehydrogenated and cooled;

[0048] 3. Jet mill pulverizing process: the hydrogen pulverized powder obtained in step 2 is placed in the grinding chamber of a jet mill, and is prepared into neodymium-iron-boron powder with an average particle size of 3.2 μm and X90 / X10 = 4.9 under the condition of a nitrogen atmosphere (oxygen content controlled below 20 ppm) and a pulverizing chamber pressure of 0.55 MPa;

[0049] 4. Mixed powder forming process: the neodymium-iron-boron powder obtained in step 3 is mixed with cerium oxide powder, the mass ratio of the cerium oxide powder to the neodymium-iron-boron powder being 0.01:100, the cerium oxide powder including first cerium oxide powder and second cerium oxide powder, the average particle size of the first cerium oxide powder being 50 nm, the average particle size of the second cerium oxide powder being 1 μm, and the mass ratio of the first cerium oxide powder to the second cerium oxide powder being 0.47:1, the obtained mixed fine powder is subjected to oriented compression molding under a magnetic field of 1.8 T, the magnetic powder is always in a completely sealed oxygen-isolated press during the compression process, and is always protected by nitrogen gas, thereby obtaining a green compact with a density of 3.6 g / cm 3 ;

[0050] 5. Cold isostatic pressing and sintering process: the green compact obtained in step 4 is subjected to cold isostatic pressing at an oil pressure of 200 MPa and for 60 s, and then the cold isostatic pressed compact is subjected to high-temperature sintering and cooling under vacuum argon protection, the sintering temperature being 1030 °C and the sintering time being 6 h;

[0051] 6. Tempering process: the magnet obtained in step 5 is subjected to first tempering at 950 °C for 4 h and second tempering at 500 °C for 6 h, thereby obtaining a sintered neodymium-iron-boron magnet with a density of 7.543 g / cm 3 .

[0052] Example 2

[0053] The difference from example 1 is that the mass ratio of the cerium oxide powder to the neodymium-iron-boron powder is 0.05:100, and the mass ratio of the first cerium oxide powder to the second cerium oxide powder is 0.32:1, and finally a sintered neodymium-iron-boron magnet is obtained.

[0054] Example 3

[0055] The difference from example 2 is that the mass ratio of the first cerium oxide powder to the second cerium oxide powder is 0.98:1, and finally a sintered neodymium-iron-boron magnet is obtained.

[0056] Example 4

[0057] The difference from example 1 is that the mass ratio of the cerium oxide powder to the neodymium-iron-boron powder is 0.56:100, and the mass ratio of the first cerium oxide powder to the second cerium oxide powder is 0.33:1, and finally a sintered neodymium-iron-boron magnet is obtained.

[0058] Example 5

[0059] The difference from Example 4 is that the mass ratio of the first ceria powder to the second ceria powder is 1.46:1, and a sintered neodymium-iron-boron magnet is finally obtained.

[0060] Example 6

[0061] The difference from Example 1 is that the mass ratio of the ceria powder to the neodymium-iron-boron powder is 1.52:100, the mass ratio of the first ceria powder to the second ceria powder is 0.34:1, and a sintered neodymium-iron-boron magnet is finally obtained.

[0062] Example 7

[0063] The difference from Example 6 is that the mass ratio of the first ceria powder to the second ceria powder is 1.23:1, and a sintered neodymium-iron-boron magnet is finally obtained.

[0064] Example 8

[0065] The difference from Example 1 is that the mass ratio of the ceria powder to the neodymium-iron-boron powder is 2.49:100, the mass ratio of the first ceria powder to the second ceria powder is 1.47:1, and a sintered neodymium-iron-boron magnet is finally obtained.

[0066] Example 9

[0067] The difference from Example 4 is that the mass ratio of the first ceria powder to the second ceria powder is 2:1, and a sintered neodymium-iron-boron magnet is finally obtained.

[0068] Example 10

[0069] The difference from Example 1 is that the average particle size of the first ceria powder is 50 nm, the average particle size of the second ceria powder is 0.5 μm, the ratio of the average particle size of the first ceria powder to the average particle size of the second ceria powder is 1:10, and a sintered neodymium-iron-boron magnet is finally obtained.

[0070] Example 11

[0071] The difference from Example 1 is that the average particle size of the first ceria powder is 50 nm, the average particle size of the second ceria powder is 2 μm, the ratio of the average particle size of the first ceria powder to the average particle size of the second ceria powder is 1:40, and a sintered neodymium-iron-boron magnet is finally obtained.

[0072] Example 12

[0073] The difference from Example 1 is that the average particle size of the first ceria powder is 30 nm, the average particle size of the second ceria powder is 1.8 μm, the ratio of the average particle size of the first ceria powder to the average particle size of the second ceria powder is 1:60, and a sintered neodymium-iron-boron magnet is finally obtained.

[0074] Example 13

[0075] The difference from Example 1 is that the temperature of the first tempering treatment is 900℃, and the time of the first tempering treatment is 6h; the temperature of the second tempering treatment is 480℃, and the time of the second tempering treatment is 6h, finally obtaining sintered Nd-Fe-B magnets.

[0076] Example 14

[0077] The difference from Example 1 is that the temperature of the first tempering treatment is 950℃, and the time of the first tempering treatment is 3h; the temperature of the second tempering treatment is 540℃, and the time of the second tempering treatment is 6h, finally obtaining sintered Nd-Fe-B magnets.

[0078] Example 15

[0079] The difference from Example 1 is that the temperature of the first tempering treatment is 960℃, and the time of the first tempering treatment is 1h; the temperature of the second tempering treatment is 650℃, and the time of the second tempering treatment is 6h, finally obtaining sintered Nd-Fe-B magnets.

[0080] Example 16

[0081] The difference from Example 1 is that,

[0082] 1. Put the metal raw materials of Nd-Fe-B powder into an alumina crucible according to the proportion of Example 1, vacuum smelt in a high-frequency vacuum smelting furnace at 1450℃, then pour the steel liquid onto a rotating copper roller to obtain an alloy thin strip with an average thickness of 0.20mm;

[0083] 2. Hydrogen crushing process: Put the alloy thin strip obtained in step 1 into a hydrogen crushing furnace, introduce hydrogen gas with a purity of more than 99.99%, and make the thin sheet fully absorb hydrogen at an absorption pressure of 220kPa. After fully absorbing hydrogen, the dehydrogenation temperature is 580℃, and the dehydrogenation time is 6h. After dehydrogenation, cool to obtain hydrogen crushed powder with an average particle size of 107μm;

[0084] 3. Airflow mill crushing process: Put the hydrogen crushed powder obtained in step 2 into the airflow mill grinding chamber, place it in a nitrogen atmosphere (oxygen content controlled below 30ppm), and prepare Nd-Fe-B powder with an average particle size of 3.3μm and X90 / X10=4.5 under the condition of a powder crushing chamber pressure of 0.6MPa;

[0085] 4. Powder mixing forming process: the neodymium-iron-boron powder obtained in step 3 is mixed with cerium oxide powder, the mass ratio of cerium oxide powder to neodymium-iron-boron powder is 0.01:100, the cerium oxide powder includes first cerium oxide powder and second cerium oxide powder, the average particle size of the first cerium oxide powder is 50 nm, the average particle size of the second cerium oxide powder is 1 μm, the mass ratio of the first cerium oxide powder to the second cerium oxide powder is 0.47:1, the obtained mixed fine powder is subjected to orientation compression molding under a magnetic field of 1.8 T, the magnetic powder is always in a completely sealed oxygen isolation press during the compression process, and is always protected by nitrogen gas, to obtain a green compact with a density of 3.6 g / cm 3 ;

[0086] 5. Cold isostatic pressing and sintering process: the green compact obtained in step 4 is subjected to cold isostatic pressing, the oil pressure is 250 MPa, and the oil pressure time is 120 s, and then the cold isostatic pressed compact is subjected to high temperature sintering and cooling under vacuum argon protection, the sintering temperature is 1080℃, and the sintering time is 6 h;

[0087] 6. Tempering process: the magnet obtained in step 5 is subjected to first tempering at 950℃ for 4 h and second tempering at 500℃ for 6 h, to obtain a sintered neodymium-iron-boron magnet with a density of 7.53 g / cm 3 .

[0088] Comparative Example 1

[0089] The difference from Example 1 is that the second cerium oxide powder is completely replaced by the first cerium oxide powder, and finally a sintered neodymium-iron-boron magnet is obtained.

[0090] Comparative Example 2

[0091] The difference from Example 1 is that the first cerium oxide powder is completely replaced by the second cerium oxide powder, and finally a sintered neodymium-iron-boron magnet is obtained.

[0092] Comparative Example 3

[0093] The difference from Example 1 is that the first cerium oxide powder and the second cerium oxide powder are not added, and finally a sintered neodymium-iron-boron magnet is obtained.

[0094] Performance test

[0095] The sintered neodymium-iron-boron magnets obtained in the examples and comparative examples are measured by scanning electron microscopy, the volume fraction of cerium oxide in the grain boundary is calculated, the carbon content and oxygen content in the sintered neodymium-iron-boron magnet are tested by carbon and sulfur analyzer and oxygen and nitrogen analyzer, and the remanence Br (kGs) and coercive force Hcj (kOe) are tested by NIM-62000 type high temperature permanent magnet measuring instrument, and the test results are shown in Table 1.

[0096] Table 1

[0097]

[0098]

[0099] Figure 1 From the scanning electron microscope image of the sintered neodymium-iron-boron magnet in Example 1 of the present application, it can be seen that there are two sizes of cerium oxide particles distributed.

[0100] From the above description, it can be seen that the above-mentioned embodiments of the present application achieve the following technical effects:

[0101] The present application mixes cerium oxide powder of different particle sizes with neodymium-iron-boron powder, which on the one hand helps to control the penetration of impurity elements during sintering of the magnet, thereby greatly reducing the oxygen content and carbon content in the magnet, and further helping to reduce the decline in magnet performance caused by impurity elements, and avoiding the adverse effects caused by the addition of antioxidants and lubricants; on the other hand, the addition of cerium oxide powder of different particle sizes helps to form more stable grain boundary phases, the presence of these phases can effectively inhibit the abnormal growth of grains, avoid the appearance of uneven microstructure in the sintered neodymium-iron-boron magnet, and thus improve the overall density and magnetic performance consistency of the magnet. The first cerium oxide powder helps to optimize the grain boundary structure, while the second cerium oxide powder helps to control the excessive penetration of impurity elements in the three-phase region, and the synergistic effect of the two ensures the consistency and excellence of the microstructure of the magnet. In addition, through the synergistic effect of the addition of cerium oxide powder of different particle sizes and the preparation process, the present application greatly reduces the oxygen content and carbon content in the magnet, reduces the decline in magnet performance caused by impurity elements, at the same time compared with the traditional process, the process time is shortened, the production efficiency is improved, and the production cost is saved.

[0102] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a sintered NdFeB magnet, characterized in that, The preparation method includes: Step S1: The raw materials, including neodymium iron boron powder and cerium oxide powder, are mixed and pressed sequentially to obtain a pressed blank; Step S2: The pressed blank is subjected to cold isostatic pressing, sintering and tempering in sequence to obtain the sintered NdFeB magnet; The cerium oxide powder is composed of a first cerium oxide powder and a second cerium oxide powder, wherein the average particle size of the first cerium oxide powder is 20-500 nm and the average particle size of the second cerium oxide powder is 0.5-2 μm.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the cerium oxide powder to the neodymium iron boron powder is (0.01-2.5):

100.

3. The preparation method according to claim 1 or 2, characterized in that, The mass ratio of the first cerium oxide powder to the second cerium oxide powder is (0.1 to 1.8):1; and / or, the ratio of the average particle size of the first cerium oxide powder to the average particle size of the second cerium oxide powder is 1:(10 to 40).

4. The preparation method according to any one of claims 1 to 3, characterized in that, The average particle size of the neodymium iron boron powder is 2.0 to 3.5 μm; and / or, the X90 / X10 ratio of the neodymium iron boron powder is ≤5.

5. The preparation method according to any one of claims 1 to 4, characterized in that, The density of the pressed blank is 3.2–4.0 g / cm³. 3 ; And / or, the pressure of the cold isostatic pressing treatment is 180-250 MPa; and / or, the time of the cold isostatic pressing treatment is 20-120 s; And / or, the sintering temperature is 1010–1100°C; and / or, the sintering time is 2–8 hours; And / or, the density of the sintered NdFeB magnet is 7.40–7.65 g / cm³. 3 .

6. The preparation method according to any one of claims 1 to 5, characterized in that, The tempering temperature is 480–950°C; and / or the tempering time is 7–15 hours. Preferably, the tempering process includes a first-stage tempering process and a second-stage tempering process performed sequentially. The temperature of the first-stage tempering process is 900–950°C, and the time of the first-stage tempering process is 2–6 hours. The temperature of the second-stage tempering process is 480–600°C, and the time of the second-stage tempering process is 3–10 hours.

7. The preparation method according to any one of claims 1 to 6, characterized in that, The method for preparing the neodymium iron boron powder includes: The metal raw materials of the neodymium iron boron powder are sequentially smelted and cast, hydrogen-induced coarse crushing and air jet milling according to the formula to obtain the neodymium iron boron powder. Preferably, the hydrogen-based coarse crushing includes sequential hydrogen absorption treatment, dehydrogenation treatment, and cooling treatment. The pressure of the hydrogen absorption treatment is 110–220 kPa, the temperature of the dehydrogenation treatment is 450–620 °C, and the time of the dehydrogenation treatment is 2–6 h. And / or, the oxygen content in the environment of the air jet milling process does not exceed 30 ppm; and / or, the pressure of the air jet milling process is 0.4 to 0.6 MPa; And / or, by mass percentage, the NdFeB powder comprises the following elements: 20-35% Re, 0.1-2.5% M, 0.93-1.2% B, ≤0.1% total unavoidable impurities, less than 0.05% of a single impurity, and the balance being Fe; wherein the Re element is selected from any one or more of La, Ce, Pr, Nd, Dy, and Tb, and the M element is selected from any one or more of Cu, Al, Ga, Co, Ni, Zr, and Nb.

8. A sintered NdFeB magnet, characterized in that, The sintered NdFeB magnet is prepared by the preparation method according to any one of claims 1 to 7.

9. The sintered NdFeB magnet according to claim 8, characterized in that, The sintered NdFeB magnet comprises main phase grains and grain boundaries, wherein the volume fraction of cerium oxide phase in the grain boundaries is 1.5–4.5%.

10. The sintered NdFeB magnet according to claim 8 or 9, characterized in that, The carbon content in the sintered NdFeB magnet is 400–600 ppm; and / or, the oxygen content in the sintered NdFeB magnet is 400–600 ppm; and / or, the remanence of the sintered NdFeB magnet is 14.29–14.60 kGs; and / or, the coercivity of the sintered NdFeB magnet is 13.82–15.00 kOe.

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