In-situ reduction diffusion method for improving coercive force of neodymium-iron-boron magnet at low cost

Abstract

The invention provides a low-cost in-situ reduction diffusion method for improving the coercive force of a neodymium-iron-boron magnet, which comprises the following steps: (1) mixing an in-situ reducing agent, a diffusant and a rare earth compound to obtain mixed powder; (2) covering the orientation surface of the neodymium-iron-boron magnet with the mixed powder obtained in the step (1) to obtain a covered magnet; and (3) the magnet covered in the step (2) is sequentially subjected to two-stage in-situ reduction-grain boundary diffusion heat treatment and tempering treatment, and the diffused neodymium-iron-boron magnet is obtained. The in-situ reduction diffusion method is easy to operate and low in raw material cost, the diffusion source cost of grain boundary diffusion is greatly reduced while it is guaranteed that the coercive force of the neodymium-iron-boron magnet is improved, and the in-situ reduction diffusion method has wide-range application and popularization prospects.

Description

Technical Field

[0001] The invention relates to the technical field of rare earth permanent magnet materials, and in particular to an in-situ reduction diffusion method for improving the coercive force of a neodymium iron boron magnet at a low cost. Background Art

[0002] As the third generation of rare earth permanent magnet materials, NdFeB permanent magnet materials have excellent magnetic properties and are the core key materials of the modern motor industry. They play an important role in wind power, new energy vehicles, energy-saving motors, rail transportation, aerospace and other industries. The emergence of NdFeB permanent magnet materials has greatly promoted the progress of modern science and technology and the information industry. At present, the relevant energy industries such as wind power generation and industrial permanent magnet motors are developing rapidly, and the demand for NdFeB magnets is also increasing.

[0003] CN109003799A discloses a method for preparing a high coercive force NdFeB magnet, including: 1) slicing and surface treatment: sintering the NdFeB magnet, and then pre-treating the magnet surface with dilute acid and alcohol respectively, and drying; 2) preparing a diffusion source: dissolving a dysprosium and / or terbium compound in anhydrous ethanol, and then adding an inorganic acid salt of dysprosium and / or terbium with a mass not greater than 2.5% of the total amount of the dysprosium and / or terbium compound, ultrasonically dispersing to obtain a mixed solution, attaching the mixed solution to the magnet surface, drying, and forming a diffusion coating layer; 3) magnet diffusion aging treatment: keeping the magnet warm in a reducing atmosphere. Under the premise of not introducing too much impurity additives and not affecting the structure of the magnet itself (corrosion, oxidation, etc.), a specific content of dysprosium and / or terbium inorganic acid salt is added to the diffusion source to improve the rare earth adhesion, so as to achieve the purpose of promoting the grain boundary diffusion effect.

[0004] CN109935462A discloses a preparation method of a grain boundary diffusion heavy rare earth NdFeB magnet and the NdFeB magnet. The preparation method comprises the following steps: pressing NdFeB powder into a NdFeB rough billet and then immersing the billet into a heavy rare earth source melt, and actively immersing the heavy rare earth source melt into the NdFeB rough billet, so that solid solution and penetration occur between the heavy rare earth source melt and the NdFeB rough billet to fill gaps and defects in the NdFeB rough billet, and at the same time, the heavy rare earth element diffuses at the grain boundary in the NdFeB rough billet to improve the grain boundary diffusion efficiency of the heavy rare earth element, so that the grain boundary diffusion heavy rare earth element process becomes feasible for enhancing the coercive force of thicker NdFeB magnets, and a NdFeB magnet with a thickness greater than 5 mm and a coercive force enhancement effect higher than that of the existing grain boundary diffusion heavy rare earth element process is obtained.

[0005] CN108962579A discloses a method for preparing a high coercive force NdFeB magnet, comprising the following steps: 1) preparing NdFeB coarse powder; 2) mixing the NdFeB coarse powder with a 1 mol / L silane aqueous solution and ball milling for 2 to 4 hours; 3) adding a Dy or Tb compound to the ball milled product in step 2) and continuing ball milling for 2 to 4 hours; 4) washing and drying the ball milled product, and calcining the dried product for 1 to 2 hours; 5) grinding the calcined product in step 4) into fine powder, and then performing magnetic field orientation molding, isostatic pressing, vacuum sintering and tempering heat treatment to obtain NdFeB permanent magnet material. The process steps of attaching heavy rare earth elements through silane branching are simple and easy to operate; the obtained NdFeB magnet has fine grains, and the heavy rare earth elements are evenly distributed at the grain boundaries, and the obtained magnet has high coercive force.

[0006] However, the heavy rare earth metals such as dysprosium and terbium required for the above-mentioned grain boundary diffusion process are expensive. At the same time, the grain boundary diffusion also requires the heavy rare earth metals such as dysprosium and terbium to be prepared into diffusion source powders through processes such as hydrogen crushing and air flow grinding, resulting in high grain boundary diffusion costs. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets. The in-situ reducing agent, diffusing agent and rare earth compound are used as raw materials, which are covered on the orientation surface of the NdFeB magnet, and two stages of in-situ reduction-grain boundary diffusion heat treatment and tempering treatment are carried out in sequence, so that the coercive force of the grain boundary diffused NdFeB magnet is significantly improved, and the cost required for the grain boundary diffusion process is greatly reduced.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] The present invention provides an in-situ reduction diffusion method for improving the coercive force of NdFeB magnets at low cost, and the in-situ reduction diffusion method comprises the following steps:

[0010] (1) mixing an in-situ reducing agent, a diffusing agent and a rare earth compound to obtain a mixed powder;

[0011] (2) covering the mixed powder described in step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet;

[0012] (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and tempering treatment in sequence to obtain a diffused NdFeB magnet;

[0013] The temperature of the first stage in-situ reduction-grain boundary diffusion heat treatment is 800-880°C; the temperature of the second stage in-situ reduction-grain boundary diffusion heat treatment is 900-920°C.

[0014] The low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets of the present invention covers the mixed powder composed of the in-situ reducing agent, the diffusing agent and the rare earth compound on the orientation surface of the NdFeB magnet for two-stage in-situ reduction-grain boundary diffusion heat treatment. The in-situ reducing agent reduces the rare earth compounds such as rare earth oxides and rare earth chlorides into rare earth metals in situ while performing grain boundary diffusion, which effectively solves the problem of poor grain boundary diffusion effect of rare earth compounds such as rare earth oxides and rare earth chlorides in the prior art. In addition, the use of rare earth compounds as raw materials for grain boundary diffusion reduces the hydrogen crushing and air flow grinding processes in the metallurgical preparation process of rare earth metals and the preparation process of traditional diffusion sources, greatly reduces the cost of grain boundary diffusion diffusion sources while ensuring the improvement of the coercivity of NdFeB magnets, and can also reduce the discharge of waste liquid generated in the metallurgical preparation process of rare earth metals.

[0015] The present invention optimizes the microstructure of the NdFeB magnet by attaching a mixed powder consisting of an in-situ reducing agent, a diffusing agent and a rare earth compound to the surface of the magnet, combining two stages of in-situ reduction-grain boundary diffusion heat treatment and tempering treatment, and can effectively improve the anisotropy field of the grain epitaxial layer, inhibit the nucleation of the reverse magnetization domain, and thus improve the coercive force of the magnet.

[0016] The present invention adopts two-stage in-situ reduction-grain boundary diffusion heat treatment, the temperature of the first stage in-situ reduction-grain boundary diffusion heat treatment is 800-880°C; the temperature of the second stage in-situ reduction-grain boundary diffusion heat treatment is 900-920°C, which can reduce low-cost rare earth oxides and rare earth chlorides to metals first, and then enter the interior of the magnet along the grain boundary, greatly improving the diffusion depth of the heavy rare earth of the magnet, thereby improving the coercive force of the magnet. If only one stage in-situ reduction-grain boundary diffusion heat treatment with a temperature of 800-920°C is used, heavy rare earth will accumulate on the surface of the magnet, the diffusion depth of heavy rare earth will be insufficient, and the coercive force will be limited. If the temperature of the first stage in-situ reduction-grain boundary diffusion heat treatment is lower than 800°C, the rare earth compound will be insufficiently reduced, and the heavy rare earth will not be able to smoothly enter the magnet to form a uniform core-shell structure; if the temperature of the second stage in-situ reduction-grain boundary diffusion heat treatment is higher than 920°C, the heavy rare earth element lattice will diffuse, and too much heavy rare earth will enter the interior of the main phase, resulting in a significant decrease in magnetic properties.

[0017] The temperature of the first stage in-situ reduction-grain boundary diffusion heat treatment of the present invention is 800-880°C, for example, it can be 800°C, 810°C, 830°C, 850°C, 870°C or 880°C.

[0018] The temperature of the second stage in-situ reduction-grain boundary diffusion heat treatment is 900-920°C, for example, it can be 900°C, 905°C, 910°C, 912°C, 917°C or 920°C.

[0019] Preferably, the in-situ reducing agent in step (1) comprises CaH 2Powder, Ca powder, CaO powder or CaC 2 Any one or a combination of at least two of the powders, wherein a typical but non-limiting combination includes CaH 2 Combination of Ca powder and Ca powder, CaO powder and CaC 2 A combination of powder or CaH 2 Combination of powder and CaO powder.

[0020] Preferably, the particle size of the in-situ reducing agent is ≤20 μm, for example, it may be 5 μm, 7 μm, 9 μm, 11 μm, 12 μm, 14 μm, 16 μm, 18 μm or 20 μm, etc.

[0021] In the present invention, the particle size of the in-situ reducing agent is preferably ≤20 μm. When the particle size of the in-situ reducing agent is larger, it will lead to insufficient bonding between powders, resulting in insufficient reduction reaction.

[0022] Preferably, the melting point of the diffusant in step (1) is below 800°C, for example, it may be 800°C, 790°C, 750°C, 730°C or 700°C.

[0023] In the present invention, the melting point of the diffusion agent is preferably below 800° C., which can effectively improve the effect of the reduction reaction. When a diffusion agent with a higher melting point is used, the effect of the reduction reaction will decrease.

[0024] Preferably, the diffusing agent comprises CaCl 2 and / or KCl.

[0025] Preferably, the rare earth compound in step (1) includes Tb 4 O 7 , Tb 2 O 3 、Dy 2 O 3 、Nd 2 O 3 , Pr 6 O 11 、Nd 2 O 6 Pr 2 、TbCl 3 , DyCl 3 、NdCl 3 、PrCl 3 Any one or a combination of at least two of the following, wherein a typical but non-limiting combination includes Tb 4 O 7 and Tb 2 O 3 The combination of Dy 2 O 3 and Nd 2 O3 The combination of Pr 6 O 11 and Nd 2 O 6 Pr 2 Combination of TbCl 3 and DyCl 3 Combination or NdCl 3 and PrCl 3 combination.

[0026] Preferably, the particle size of the rare earth compound is ≤10 μm, for example, it may be 1 μm, 2 μm, 5 μm, 7 μm, 9 μm or 10 μm.

[0027] In the present invention, the particle size of the rare earth compound is preferably ≤10 μm. When the particle size of the rare earth compound is larger, it will lead to insufficient bonding between powders, resulting in insufficient reduction reaction.

[0028] Preferably, in terms of mass percentage, the mixed powder of step (1) comprises: 22-35wt.% in-situ reducing agent, 22wt.%, 24wt.%, 25wt.%, 27wt.%, 29wt.%, 30wt.% or 35wt.%, etc.;

[0029] 1-15wt.% of a diffusant, for example, 1wt.%, 3wt.%, 5wt.%, 7wt.%, 10wt.%, 12wt.%, 14wt.% or 15wt.%, etc.;

[0030] and 50-89wt.% of rare earth compounds, for example, 50wt.%, 56wt.%, 60wt.%, 65wt.%, 70wt.%, 75wt.%, 80wt.% or 89wt.%, etc.

[0031] The mixed powder of the present invention preferably includes 22-35wt.% of an in-situ reducing agent, which can realize grain boundary diffusion in the subsequent two-stage in-situ reduction-grain boundary diffusion heat treatment while reducing rare earth compounds such as rare earth oxides and rare earth chlorides to rare earth metals in situ, effectively solving the problem of poor grain boundary diffusion effect of rare earth oxides and rare earth chlorides in the prior art, and significantly improving the coercive force of the NdFeB magnet. When the mass percentage of the in-situ reducing agent is low, it will lead to insufficient bonding between the powders, resulting in insufficient reduction reaction. When the mass percentage of the in-situ reducing agent is high, it will lead to insufficient bonding between the heavy rare earth and the matrix, affecting the increase in coercive force.

[0032] Preferably, the thickness of the mixed powder covered on the orientation surface of the NdFeB magnet in step (2) is 0.02-2 mm, for example, 0.02 mm, 0.05 mm, 0.1 mm, 0.5 mm, 1 mm or 2 mm.

[0033] The present invention preferably has a thickness of 0.02-2 mm for the mixed powder covering the orientation plane of the NdFeB magnet. When the thickness of the mixed powder is thin, the amount of heavy rare earth matrix entering is reduced, and the coercive force is not significantly improved; when the thickness of the mixed powder is thick, the amount of heavy rare earth matrix entering is too much, affecting the magnetic properties.

[0034] Before the mixed powder in step (1) is covered on the orientation surface of the NdFeB magnet, the NdFeB magnet is subjected to conventional surface grinding, degreasing, pickling, water washing, alcohol washing, ultrasonic cleaning and drying treatments in sequence.

[0035] Preferably, the time of the first in-situ reduction-grain boundary diffusion heat treatment in step (3) is 5-12 h, for example, 5 h, 7 h, 10 h, 11 h, 11.5 h or 12 h.

[0036] Preferably, the vacuum degree of the first stage of in-situ reduction-grain boundary diffusion heat treatment is 1×10 -3 -1×10 -2 Pa, for example, can be 1×10 -3 Pa, 2×10 -3 Pa, 3×10 -3 Pa, 4×10 -3 Pa, 5×10 -3 Pa, 6×10 -3 Pa, 7×10 -3 Pa, 8×10 -3 Pa, 9×10 -3 Pa or 1×10 -2 Pa et al.

[0037] Preferably, the time of the second in-situ reduction-grain boundary diffusion heat treatment in step (3) is 7-14 h, for example, 7 h, 8 h, 10 h, 12 h or 14 h.

[0038] Preferably, the vacuum degree of the second stage of in-situ reduction-grain boundary diffusion heat treatment is 1×10 -3 -1×10 -2 Pa, for example, can be 1×10 -3 Pa, 2×10 -3 Pa, 3×10 -3 Pa, 4×10 -3 Pa, 5×10 -3 Pa, 6×10 -3 Pa, 7×10 -3 Pa, 8×10 -3 Pa, 9×10 -3 Pa or 1×10 -2 Pa et al.

[0039] Preferably, the temperature of the tempering treatment in step (3) is 460-530°C, for example, it can be 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C or 530°C.

[0040] Preferably, the tempering treatment time is 2-6 hours, for example, it can be 2 hours, 3 hours, 4 hours, 5 hours or 6 hours.

[0041] Preferably, the vacuum degree of the tempering treatment is 1×10 -3 -1×10 -2 Pa, for example, can be 1×10 -3 Pa, 2×10 -3 Pa, 3×10 -3 Pa, 4×10 -3 Pa, 5×10 -3 Pa, 6×10 -3 Pa, 7×10 -3 Pa, 8×10 -3 Pa, 9×10 -3 Pa or 1×10 -2 Pa et al.

[0042] As a preferred technical solution of the present invention, the in-situ reduction diffusion method comprises the following steps:

[0043] (1) mixing 22-35 wt.% of an in-situ reducing agent, 1-15 wt.% of a diffusing agent and 50-89 wt.% of a rare earth compound to obtain a mixed powder;

[0044] The in situ reducing agent includes CaH 2 Powder, Ca powder, CaO powder or CaC 2 Any one or a combination of at least two of the powders; the particle size of the in-situ reducing agent is ≤20 μm;

[0045] The melting point of the diffusant is below 800°C; the diffusant includes CaCl 2 and / or KCl;

[0046] The rare earth compound includes Tb 4 O 7 , Tb 2 O 3 、Dy 2 O 3 、Nd 2 O 3 , Pr 6 O 11 、Nd 2 O 6 Pr 2 、TbCl3 , DyCl 3 、NdCl 3 、PrCl 3 Any one or a combination of at least two of the following; the particle size of the rare earth compound is ≤10 μm;

[0047] (2) covering the mixed powder of step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet; the thickness of the mixed powder covered on the orientation surface of the NdFeB magnet is 0.02-2 mm;

[0048] (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and temperature of 460-530°C and vacuum of 1×10 -3 -1×10 -2 Pa is tempered for 2-6 hours to obtain diffused NdFeB magnets;

[0049] The first stage of in-situ reduction-grain boundary diffusion heat treatment has a temperature of 800-880°C, a time of 5-12h, and a vacuum degree of 1×10 -3 -1×10 -2 Pa;

[0050] The second stage of in-situ reduction-grain boundary diffusion heat treatment is performed at a temperature of 900-920°C, a time of 7-14 h, and a vacuum degree of 1×10 -3 -1×10 -2 Pa.

[0051] Compared with the prior art, the present invention has at least the following beneficial effects:

[0052] (1) The low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets provided by the present invention uses low-cost rare earth oxides and rare earth chlorides as diffusion sources to significantly reduce the cost of grain boundary diffusion, and does not require traditional metal diffusion source preparation processes such as traditional hydrogen crushing and air flow grinding; and the low-cost rare earth compound is mixed with an in-situ reducing agent and a diffusing agent to perform a two-stage in-situ reduction-grain boundary diffusion heat treatment, and the rare earth compound is in-situ reduced to a rare earth metal while the rare earth metal diffuses along the grain boundary phase, thereby significantly improving the coercivity of the NdFeB magnet;

[0053] (2) The low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets provided by the present invention uses low-cost rare earth oxides as raw materials. Compared with the prior art that uses rare earth metals as raw materials, it avoids the discharge of waste liquid generated in the rare earth metal metallurgical preparation process. DETAILED DESCRIPTION

[0054] For the convenience of understanding the present invention, the present invention lists the following embodiments. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0055] The present invention is further described in detail below. However, the following examples are only simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

[0056] Example 1

[0057] This embodiment provides a low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets. The in-situ reduction diffusion method comprises the following steps:

[0058] (1) mixing 27 wt.% of an in-situ reducing agent, 4 wt.% of a diffusing agent, and 69 wt.% of a rare earth compound to obtain a mixed powder;

[0059] The in-situ reducing agent is CaH 2 powder; the particle size of the in-situ reducing agent is 16 μm;

[0060] The melting point of the diffusant is below 800°C; the diffusant is KCl;

[0061] The rare earth compound is Tb 4 O 7 ; The particle size of the rare earth compound is 2 μm;

[0062] (2) covering the mixed powder of step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet; the thickness of the mixed powder covered on the orientation surface of the NdFeB magnet is 0.5 mm;

[0063] (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and a temperature of 500°C and a vacuum of 6×10 -3 Pa was tempered for 4 h to obtain diffused NdFeB magnets;

[0064] The first stage of in-situ reduction-grain boundary diffusion heat treatment was performed at a temperature of 845 °C, a time of 8 h, and a vacuum degree of 5 × 10 -3 Pa;

[0065] The second stage of in-situ reduction-grain boundary diffusion heat treatment was carried out at a temperature of 900 °C, a time of 10 h, and a vacuum degree of 5 × 10 - 3 Pa.

[0066] Example 2

[0067] This embodiment provides a low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets. The in-situ reduction diffusion method comprises the following steps:

[0068] (1) mixing 22 wt.% of an in-situ reducing agent, 15 wt.% of a diffusing agent, and 75 wt.% of a rare earth compound to obtain a mixed powder;

[0069] The in-situ reducing agent is Ca powder; the particle size of the in-situ reducing agent is 20 μm;

[0070] The melting point of the diffusant is below 800°C; the diffusant is CaCl 2 ;

[0071] The rare earth compound is TbCl 3 ; The particle size of the rare earth compound is 5 μm;

[0072] (2) covering the mixed powder of step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet; the thickness of the mixed powder covered on the orientation surface of the NdFeB magnet is 0.6 mm;

[0073] (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and temperature of 480°C and vacuum of 1×10 -2 Pa was tempered for 6 hours to obtain diffused NdFeB magnets;

[0074] The first stage of in-situ reduction-grain boundary diffusion heat treatment was performed at a temperature of 880 °C, a time of 5 h, and a vacuum degree of 1 × 10 -2 Pa;

[0075] The second stage of in-situ reduction-grain boundary diffusion heat treatment was performed at a temperature of 920 °C, a time of 7 h, and a vacuum degree of 1 × 10 -2 Pa.

[0076] Example 3

[0077] This embodiment provides a low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets. The in-situ reduction diffusion method comprises the following steps:

[0078] (1) mixing 35 wt.% of an in-situ reducing agent, 15 wt.% of a diffusing agent, and 50 wt.% of a rare earth compound to obtain a mixed powder;

[0079] The in-situ reducing agent is CaC 2 powder; the particle size of the in-situ reducing agent is 5 μm;

[0080] The melting point of the diffusant is below 800°C; the diffusant is CaCl 2 ;

[0081] The rare earth compound is Tb 2 O 3 ; The particle size of the rare earth compound is 1 μm;

[0082] (2) covering the mixed powder of step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet; the thickness of the mixed powder covered on the orientation surface of the NdFeB magnet is 0.4 mm;

[0083] (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and temperature of 520°C and vacuum of 1×10 -3 Pa was tempered for 3 h to obtain diffused NdFeB magnets;

[0084] The first stage of in-situ reduction-grain boundary diffusion heat treatment was carried out at a temperature of 800 °C, a time of 12 h, and a vacuum degree of 1×10 - 3 Pa;

[0085] The second stage of in-situ reduction-grain boundary diffusion heat treatment was performed at a temperature of 910 °C, a time of 14 h, and a vacuum degree of 1×10 - 3 Pa.

[0086] Example 4

[0087] This embodiment provides a low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets. The in-situ reduction diffusion method is the same as that of Embodiment 1 except that the particle size of the rare earth compound in step (1) is 18 μm.

[0088] Example 5

[0089] This embodiment provides a low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets. The in-situ reduction diffusion method is the same as that of Embodiment 1 except that the in-situ reducing agent in the mixed powder in step (1) is 18 wt.%.

[0090] Example 6

[0091] This embodiment provides a low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets. The in-situ reduction diffusion method is the same as that of Embodiment 1 except that the in-situ reducing agent in the mixed powder in step (1) is 45 wt.%.

[0092] Comparative Example 1

[0093] This comparative example provides a low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets. The in-situ reduction diffusion method does not add the in-situ reducing agent CaH 2Except for the powder, the rest is the same as in Example 1.

[0094] Comparative Example 2

[0095] This comparative example provides a low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets. The in-situ reduction diffusion method, except for step (3), is only carried out at a temperature of 900°C, a time of 10 h, and a vacuum degree of 5×10 -3 Except for the in-situ reduction-grain boundary diffusion heat treatment of Pa, the rest is the same as Example 1.

[0096] Comparative Example 3

[0097] This comparative example provides a low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets. The in-situ reduction diffusion method is the same as Example 1 except that the temperature of the first in-situ reduction-grain boundary diffusion heat treatment in step (3) is 680°C.

[0098] Comparative Example 4

[0099] This comparative example provides a low-cost in-situ reduction diffusion method for improving the coercive force of NdFeB magnets. The in-situ reduction diffusion method is the same as Example 1 except that the temperature of the second in-situ reduction-grain boundary diffusion heat treatment in step (3) is 970°C.

[0100] The preparation method of the original magnet includes: using the processes of medium-speed solidification casting, hydrogen crushing, air flow grinding, molding and sintering in the field of sintered NdFeB processing; the magnet contains the following components: 29.3wt.% PrNd, 0.96wt.% B, and the remainder Fe and other trace elements.

[0101] The coercive force of the original magnets and the diffused NdFeB magnets obtained in the examples and comparative examples was tested using GB / T 3217-2013 “Magnetic Test Methods for Permanent Magnet (Hard Magnetic) Materials”. The results are shown in Table 1.

[0102] Table 1

[0103] Coercivity(kOe) Original magnet 11.8 Example 1 23.8 Example 2 22.6 Example 3 23.1 Example 4 22.5 Example 5 21.8 Example 6 22.1 Comparative Example 1 10.9 Comparative Example 2 21.1 Comparative Example 3 20.7 Comparative Example 4 21.5

[0104] From Table 1 we can see that:

[0105] (1) It can be seen from Examples 1 to 3 that the coercive force of the NdFeB magnet obtained by the low-cost in-situ reduction diffusion method for improving the coercive force of the NdFeB magnet provided by the present invention is greatly improved compared with the original magnet, and the coercive force can reach more than 22.6 kOe;

[0106] (2) Combining Example 1 and Example 4, it can be seen that the particle size of the rare earth compound in Example 4 is relatively large, which will lead to insufficient bonding between the powders and insufficient reduction reaction. The coercive force of the NdFeB magnet finally obtained is slightly lower than that of Example 1, which is 22.5 kOe.

[0107] (3) Combining Example 1 with Examples 5 to 6, it can be seen that the proportion of the in-situ reducing agent in the mixed powder of Example 5 is relatively low, while the proportion of the in-situ reducing agent in the mixed powder of Example 6 is relatively high, both of which will affect the effect of grain boundary diffusion in the subsequent two-stage in-situ reduction-grain boundary diffusion heat treatment, and the coercive force of the NdFeB magnet finally obtained is smaller than that of Example 1;

[0108] (4) From Example 1 and Comparative Example 1, it can be seen that in Comparative Example 1, no in-situ reducing agent CaH 2 Powder, rare earth compounds such as rare earth oxides and rare earth chlorides cannot be reduced in situ to rare earth metals, resulting in poor grain boundary diffusion effect. The coercive force of the resulting NdFeB magnet is greatly reduced to only 10.9kOe;

[0109] (5) It can be seen from the combination of Example 1 and Comparative Examples 2 to 4 that Comparative Example 2 only performs one stage of in-situ reduction-grain boundary diffusion heat treatment, resulting in the accumulation of heavy rare earth on the surface of the magnet, insufficient diffusion depth of heavy rare earth, limited improvement in coercivity, and the coercivity of the NdFeB magnet finally obtained is 21.1 kOe; the temperature of the first stage of in-situ reduction-grain boundary diffusion heat treatment in Comparative Example 3 is relatively low, resulting in insufficient reduction of rare earth compounds, and the heavy rare earth cannot smoothly enter the magnet to form a uniform core-shell structure, and the coercivity of the NdFeB magnet finally obtained is 20.7 kOe; the temperature of the second stage of in-situ reduction-grain boundary diffusion heat treatment in Comparative Example 4 is relatively high, which will cause the heavy rare earth element lattice diffusion, and too much heavy rare earth will enter the main phase, and the coercivity of the NdFeB magnet finally obtained is 21.5 kOe, and the magnetic properties are greatly reduced.

[0110] The applicant declares that the above is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention shall fall within the protection scope and disclosure scope of the present invention.

Claims

1. A low-cost in-situ reduction diffusion method for improving the coercivity of NdFeB magnets, characterized in that: The in-situ reduction diffusion method comprises the following steps: (1) mixing an in-situ reducing agent, a diffusing agent and a rare earth compound to obtain a mixed powder; (2) covering the mixed powder described in step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet; (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and tempering treatment in sequence to obtain a diffused NdFeB magnet; The temperature of the first stage in-situ reduction-grain boundary diffusion heat treatment is 800-880°C; the temperature of the second stage in-situ reduction-grain boundary diffusion heat treatment is 900-920°C.

2. The in-situ reduction diffusion method according to claim 1, characterized in that: The in-situ reducing agent in step (1) comprises any one of CaH2 powder, Ca powder, CaO powder or CaC2 powder, or a combination of at least two thereof; Preferably, the particle size of the in-situ reducing agent is ≤20 μm.

3. The in-situ reduction diffusion method according to claim 1 or 2, characterized in that: The melting point of the diffusing agent in step (1) is below 800°C; Preferably, the diffusing agent comprises CaCl2 and / or KCl.

4. The in-situ reduction diffusion method according to any one of claims 1 to 3, characterized in that: The rare earth compound in step (1) includes Tb4O7, Tb2O3, Dy2O3, Nd2O3, Pr6O 11 , any one of Nd2O6Pr2, TbCl3, DyCl3, NdCl3, PrCl3 or a combination of at least two thereof; Preferably, the particle size of the rare earth compound is ≤10 μm.

5. The in-situ reduction diffusion method according to any one of claims 1 to 4, characterized in that: In terms of mass percentage, the mixed powder in step (1) comprises: 22-35wt.% in-situ reducing agent, 1-15wt.% diffusing agent and 50-89wt.% rare earth compound.

6. The in-situ reduction diffusion method according to any one of claims 1 to 5, characterized in that: In step (2), the thickness of the mixed powder covering the orientation surface of the NdFeB magnet is 0.02-2 mm.

7. The in-situ reduction diffusion method according to any one of claims 1 to 6, characterized in that: Step (3) The time of the first in-situ reduction-grain boundary diffusion heat treatment is 5-12 hours; Preferably, the vacuum degree of the first stage of in-situ reduction-grain boundary diffusion heat treatment is 1×10 -3 -1×10 -2 Pa.

8. The in-situ reduction diffusion method according to any one of claims 1 to 7, characterized in that: Step (3) The time of the second in-situ reduction-grain boundary diffusion heat treatment is 7-14 hours; Preferably, the vacuum degree of the second stage of in-situ reduction-grain boundary diffusion heat treatment is 1×10 -3 -1×10 -2 Pa.

9. The in-situ reduction diffusion method according to any one of claims 1 to 8, characterized in that: The temperature of the tempering treatment in step (3) is 460-530°C; Preferably, the tempering treatment time is 2-6h; Preferably, the vacuum degree of the tempering treatment is 1×10 -3 -1×10 -2 Pa.

10. The in-situ reduction diffusion method according to any one of claims 1 to 9, characterized in that: The in-situ reduction diffusion method comprises the following steps: (1) mixing 22-35 wt.% of an in-situ reducing agent, 1-15 wt.% of a diffusing agent and 50-89 wt.% of a rare earth compound to obtain a mixed powder; The in-situ reducing agent comprises any one of CaH2 powder, Ca powder, CaO powder or CaC2 powder or a combination of at least two thereof; the particle size of the in-situ reducing agent is ≤20 μm; The melting point of the diffusant is below 800°C; the diffusant includes CaCl2 and / or KCl; The rare earth compounds include Tb4O7, Tb2O3, Dy2O3, Nd2O3, Pr6O 11 , any one of Nd2O6Pr2, TbCl3, DyCl3, NdCl3, PrCl3 or a combination of at least two thereof; the particle size of the rare earth compound is ≤10 μm; (2) covering the mixed powder of step (1) on the orientation surface of the NdFeB magnet to obtain a covered magnet; the thickness of the mixed powder covered on the orientation surface of the NdFeB magnet is 0.02-2 mm; (3) The covered magnet in step (2) is subjected to two stages of in-situ reduction-grain boundary diffusion heat treatment and temperature of 460-530°C and vacuum of 1×10 -3 -1×10 -2 Pa is tempered for 2-6 hours to obtain diffused NdFeB magnets; The first stage of in-situ reduction-grain boundary diffusion heat treatment has a temperature of 800-880°C, a time of 5-12h, and a vacuum degree of 1×10 -3 -1×10 -2 Pa; The second stage of in-situ reduction-grain boundary diffusion heat treatment is performed at a temperature of 900-920°C, a time of 7-14 h, and a vacuum degree of 1×10 -3 -1×10 -2 Pa.