Sintered r-fe-b permanent magnet, preparation method therefor and use thereof

Abstract

The present disclosure provides a sintered R-Fe-B permanent magnet, a preparation method therefor and use thereof. Main phase grains of a surface layer of the sintered R-Fe-B permanent magnet of the present disclosure are denoted as surface layer grains, the size of the surface layer grain is D1, main phase grains of a core along an magnetization direction of the sintered R-Fe-B permanent magnet are denoted as core grains, the size of the core grain is Dn, and D1-Dn ≤ 0.34. The present disclosure improves the consistency of the size of main phase grains in the sintered R-Fe-B permanent magnet, thereby improving the magnetic property of the sintered R-Fe-B permanent magnet.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of rare earth permanent magnet material manufacture, and particularly, to a sintered R-Fe-B permanent magnet manufactured through grain boundary diffusion, a preparation method thereof and use thereof.BACKGROUND

[0002] Improving the efficiency of energy utilization is a key way to effectively solve the energy problem. According to statistics, in the industrial sector, the consumption of electric energy by motors has always accounted for a relatively high proportion. Therefore, improving the efficiency of the motor is of great significance for saving power resources. Compared with the conventional motor, the permanent magnet motor has higher motor efficiency. Permanent magnet motors have been widely used in many industrial fields such as electric vehicles and elevators. The sintered neodymium-iron-boron permanent magnet motor has the advantages of high efficiency, low noise, small size and the like, and thus becomes the preferred material for manufacturing high-efficiency permanent magnet motors. However, the Nd-Fe-B magnet will be subjected to high temperature caused by eddy currents during the operation of the motor, so that the performance of the magnet is reduced with the increase of the temperature, and finally the efficiency of the motor is reduced, and the energy consumption is increased. By improving the intrinsic coercivity of the neodymium-iron-boron magnet, the high-temperature-resistant property of the magnet can be improved, the power density and the torque density of the motor can be improved, and it is ensured that the efficiency of the motor is not reduced.

[0003] There are various ways to improve the intrinsic coercivity of the neodymium-iron-boron magnet, among which grain refinement and heavy rare earth grain boundary diffusion technologies are the most effective and easily-achieved methods at present. The coercivity of the magnet can be significantly improved by using the magnet with refined grains as the substrate for further diffusion treatment. However, the grain refinement causes an increase in the number of grains per unit volume, resulting in an increase in the permeation inhibition of heavy rare earth elements. In order to obtain a better diffusion effect and ensure that the surface layer and the center of the magnet have the same heavy rare earth elements, a longer diffusion time is required to achieve the diffusion of the diffusion source elements into the interior of the magnet. At a high temperature in the diffusion process, the boundary layer atoms of the main phase grains of the magnet are in a high-energy state and are in contact with elements such as high-concentration rare earth in a thermally activated state in the grain boundary phase for a long time, and the main phase grains will grow spontaneously and inevitably. In particular, the abnormal growth of the surface layer grain of the magnet firstly in contact with the heavy rare earth diffusion source can greatly reduce the diffusion channels, which hinders the diffusion of the diffusion source to a deeper part of the magnet, so that the main phase grains of the core are less in contact with the diffusion source elements, and the size and the performance of the original grains are still maintained, resulting in a large difference in the size of the main phase grains of the surface layer and core of the magnet, and poor magnetic property, squareness and mechanical performance of the magnet.SUMMARY

[0004] In order to solve the defects described above, the technical solutions of the present disclosure are as follows: A sintered R-Fe-B permanent magnet, wherein main phase grains of a surface layer of the sintered R-Fe-B permanent magnet are denoted as surface layer grains, the size of the surface layer grain is D1, main phase grains of a core along an magnetization direction of the sintered R-Fe-B permanent magnet are denoted as core grains, the size of the core grain is Dn, and 0 < D1-Dn ≤ 0.34, such as 0.3, 0.2, or 0.1; n is an integer greater than 1, such as 2, 3, 4, 5, ...

[0005] In the present disclosure, the surface layer of the sintered R-Fe-B permanent magnet refers to a region with a depth of 0-0.3 mm from the outer surface of the permanent magnet in an magnetization direction of the sintered R-Fe-B permanent magnet, and D1, the size of the surface layer grains, refers to the average size of the main phase grains in the surface layer; the core of the sintered R-Fe-B permanent magnet refers to a region from a position 0.3 mm from the outer surface of the permanent magnet to the geometric center point of the permanent magnet in the magnetization direction of the sintered R-Fe-B permanent magnet, and Dn, the size of the core grains, refers to the average size of the main phase grains in the core. The regions corresponding to the surface layer and the core are shown in FIG. 2. In the present disclosure, the magnetization direction of the sintered R-Fe-B permanent magnet refers to a thickness direction, and the average size of the main phase grains refers to an average size of a plurality of (at least 6 or more) main phase grains in a corresponding region.

[0006] According to a preferred embodiment of the present disclosure, in the core, the average size of the core grains close to the surface layer of the permanent magnet is larger, and the average size of the core grains close to the geometric center point of the permanent magnet is smaller.

[0007] According to an embodiment of the present disclosure, the core may also be divided into a plurality of portions in the magnetization direction of the sintered R-Fe-B permanent magnet, for example, into a first core, a second core, and a third core equally, wherein the first core, the second core, and the third core have the same thickness. The first core is close to the surface layer of the permanent magnet, and the third core is close to the geometric center point of the permanent magnet. Preferably, the average size of the first core grains in the first core is D2, the average size of the second core grains in the second core is D3, and the average size of the third core grains in the third core is D4. Preferably, D4 ≤ D3 ≤ D2. Further, 0 < D1-D2 ≤ 0.34, 0 < D1-D3 ≤ 0.34, and 0 < D1-D4 ≤ 0.34.

[0008] According to an embodiment of the present disclosure, the thickness of the sintered R-Fe-B permanent magnet is not particularly limited, and can be selected from thicknesses known in the art, such as 3.0 mm and 3.1 mm.

[0009] According to an exemplary embodiment of the present disclosure, the sintered R-Fe-B permanent magnet has a thickness of 3 mm, and the core comprises sequentially a first core, a second core, and a third core in the magnetization direction of the sintered R-Fe-B permanent magnet. Preferably, the first core refers to a region from 0.3 mm away from the surface of the permanent magnet to 0.7 mm away from the surface of the permanent magnet, and an average size of first core grains corresponding to the region is D2; the second core refers to a region from 0.7 mm away from the surface of the permanent magnet to 1.1 mm away from the surface of the permanent magnet, and an average size of second core grains corresponding to the region is D3; the third core is a region from 1.1 mm away from the surface of the permanent magnet to 1.5 mm away from the surface of the permanent magnet, and an average size of third core grains corresponding to the region is D4. Further, D1-D2 ≤ 0.34, and D1-D3 ≤ 0.34, and D1-D4 ≤ 0.34.

[0010] According to an embodiment of the present disclosure, the sintered R-Fe-B permanent magnet comprises a main phase and a grain boundary phase, wherein the grain boundary phase comprises a Zr-B phase, and the Zr-B phase is in a needle-shaped structure, as shown in FIG. 1. Preferably, the length of the needle-shaped Zr-B phase is 20 nm to 1000 nm, such as 100 nm and 500 nm. Preferably, the needle-shaped Zr-B phase is uniformly dispersed in the grain boundary phase, changing the state of the grain boundary.

[0011] According to the present disclosure, the Zr-B phase in a needle-shaped structure can be a needle-shaped Zr-B phase.

[0012] According to the present disclosure, the needle-shaped structure is known in the art. For example, it can be a structure with a length greater than the maximum width (such as diameter) of its transverse section. The shape of the transverse section is not specifically limited and can include, but is not limited to, circular, elliptical, rectangular, and polygonal shapes. Preferably, the length of the needle-shaped structure is more than 10 times the maximum width of its transverse section, for example, 10 times or 50 times or 100 times or more.

[0013] According to an embodiment of the present disclosure, the sintered R-Fe-B permanent magnet has the following magnetic properties: (1) Br is not less than 1.41 T, e.g., 1.419 T, 1.420 T, and 1.421 T; (2) Hcj is not less than 1900 KA / m, e.g., 1910 KA / m, 1920 KA / m, 1930 KA / m, 1940 KA / m, 1950 KA / m, 1960 KA / m, 1970 KA / m, 1980 KA / m, and 1990 KA / m.

[0014] According to an embodiment of the present disclosure, the starting material of the sintered R-Fe-B permanent magnet comprises a Zr element and a B element. Preferably, the mass content of the Zr element and the B element in the starting materials are denoted as [Zr] and [B], respectively, and satisfy 0.13 ≤ [Zr] / [B] ≤ 0.55, and the mass content of the Zr element in the starting materials is not less than 0.15 wt%, thereby ensuring that the needle-shaped Zr-B phase can be formed in the grain boundary phase and achieving to change the grain boundary phase.

[0015] According to an embodiment of the present disclosure, the starting material of the sintered R-Fe-B permanent magnet comprises: R is selected from at least one or more of Sm, La, Ce, Y, Nd, Pr, Ho, Gd, Dy, and Tb, and the mass content of Re is not less than 28 wt% and not more than 35 wt%; M1 is selected from at least one or more of Co, Ga, Cu, Al, Nb, Zr, and Ti, and the mass content of M1 is not less than 0.5 wt% and not more than 5 wt%; the mass content of B is not less than 0.95 wt% and not more than 1.3 wt%; the mass content of Zr is not less than 0.15 wt%, preferably not less than 0.15 wt% and not more than 0.5 wt%, e.g., 0.45 wt%; the balance is Fe and inevitable impurity elements.

[0016] The present disclosure further provides a preparation method of the sintered R-Fe-B permanent magnet described above, which comprises: (1) formulating starting materials according to a corresponding mass ratio to prepare a neodymium-iron-boron strip-casting alloys, and performing crushing to obtain a magnetic powder with a target granularity; (2) subjecting the magnetic powder obtained to press molding and cold isostatic pressing to obtain a green body; (3) subjecting the green body to sintering treatment and aging treatment to obtain a sintered neodymium-iron-boron matrix; (4) subjecting the sintered neodymium-iron-boron matrix to diffusion treatment to obtain the sintered R-Fe-B permanent magnet.

[0017] According to an embodiment of the present disclosure, in step (1), the starting material refers to the starting material of the sintered R-Fe-B permanent magnet described above.

[0018] According to an embodiment of the present disclosure, in step (1), the neodymium-iron-boron strip-casting alloys is prepared using a method known in the art, such as a rapid hardening and strip casting technique. Illustratively, the preparation method of the neodymium-iron-boron strip-casting alloys is as follows: heating the starting materials for preparation to 1300-1600 °C by electromagnetic induction for melting, then pouring the alloy melt on a quenching roller (preferably a ground quenching roller), and dropping into a water-cooling bucket for further cooling to obtain the neodymium-iron-boron strip-casting alloys.

[0019] According to an embodiment of the present disclosure, in step (1), the step of crushing specifically comprises: subjecting the neodymium-iron-boron strip-casting alloys to hydrogen decrepitation to obtain a coarsely crushed powder, and then performing a high-energy ball milling process or a jet milling process jet milling crushing to obtain the magnetic powder. Preferably, a lubricant may also be added during crushing, and the lubricant may be selected from lubricants known in the art and is not specifically limited in the present disclosure. Further, the lubricant is added in an amount of 0.1-1 wt% of the magnetic powder, such as 0.1-0.5 wt%.

[0020] Illustratively, in step (1), the crushing is performed as follows: subjecting the neodymium-iron-boron strip-casting alloys to hydrogen decrepitation to obtain a coarsely crushed powder, and then performing grinding and crushing by jet milling to obtain the magnetic powder with the target granularity; optionally, after the magnetic powder with the target particle size is obtained, 0.1-0.5 wt% lubricant mixture may be added for blending. For example, the mixture is blended for 0.1-5 h to uniformly mix the magnetic powder.

[0021] In the present disclosure, the hydrogen decrepitation, the high-energy ball milling process, and the jet milling process can all be performed by using process conditions known in the art, as long as the magnetic powder can be obtained.

[0022] According to an embodiment of the present disclosure, in step (1), the magnetic powder has a target granularity of 1-8 µm, such as 5 µm.

[0023] According to an embodiment of the present disclosure, in step (2), the press molding comprises: subjecting the magnetic powder to orientated press molding in a magnetic field. Preferably, the magnetic field may be a magnetic field known in the art, for example, a magnetic field with a magnetic field intensity of 2 T.

[0024] According to an embodiment of the present disclosure, in step (2), the cold isostatic pressing can be performed under conditions of cold isostatic pressing known in the art, which is not specifically limited in the present disclosure.

[0025] According to an embodiment of the present disclosure, in step (3), the sintering treatment comprises: placing the green body at a high temperature condition of 1055-1090 °C in a vacuum environment and performing sintering.

[0026] According to an embodiment of the present disclosure, in step (3), the aging treatment refers to performing tempering treatment at 1000 °C or lower after the sintering, which may be performed under conditions known in the art, such as tempering at 910 °C and 520 °C for 4 h.

[0027] According to an embodiment of the present disclosure, in step (4), the sintered neodymium-iron-boron matrix may also be machined first to process the sintered neodymium-iron-boron matrix into an R-Fe-B thin sheetwith a target size.

[0028] According to an embodiment of the present disclosure, in step (4), the sintered neodymium-iron-boron matrix (e.g., the R-Fe-B thin sheet) may also be cleaned first, and may be cleaned by a method known in the art, for example, by acid cleaning at a volume concentration of 1%-5% to remove oxide impurities and oily stains on the surface of the sintered neodymium-iron-boron matrix (e.g., the R-Fe-B thin sheet).

[0029] According to an embodiment of the present disclosure, in step (4), the diffusion treatment specifically comprises: coating the surface of the sintered neodymium-iron-boron matrix (e.g., R-Fe-B thin sheet) with heavy rare earth elements to uniformly distribute the heavy rare earth elements on the surface of the sintered neodymium-iron-boron matrix (e.g., R-Fe-B thin sheet); and performing a two-stage heat treatment diffusion in a vacuum atmosphere, wherein a primary heat treatment has a temperature range of 790-930 °C (preferably not higher than 900 °C, e.g., 800 °C, 850 °C, and 900 °C), a secondary heat treatment has a temperature range of 440-530 °C (preferably not higher than 520 °C, e.g., 450 °C, 500 °C, and 520 °C), and the vacuum degree is 10 -2< -10 -4< Pa. The inventors have found that when the two-stage heat treatment diffusion of the present disclosure is used, a lower vacuum degree is used to reduce the temperature during diffusion, so that the Zr-B phase is not dissolved in the diffusion stage.

[0030] According to an embodiment of the present disclosure, the heavy rare earth element is provided by a diffusion source. Preferably, the diffusion source comprises heavy rare earth powder particles, an antioxidant, and an organic solvent. Preferably, the diffusion source coats the surface of the sintered neodymium-iron-boron matrix (e.g., R-Fe-B thin sheet) in the form of a slurry. Preferably, the heavy rare earth powder particles comprise at least one of or a mixture of more of dysprosium, a hydride of dysprosium, an oxide of dysprosium, a fluoride of dysprosium, terbium, a hydride of terbium, an oxide of terbium, and a fluoride of terbium.

[0031] According to an embodiment of the present disclosure, the heavy rare earth powder particles have an average particle size of 2-100 µm (e.g., 10 µm and 50 µm) and a D90 / D10 of 1.2-5.1 (e.g., 2, 3, and 4).

[0032] The inventors found that in order to prevent the Zr element from dissolving in a neodymium-rich phase at a relatively high temperature and alloying with other trace elements so that the magnetic isolation cannot be achieved and the growth of the main phase grains cannot be inhibited, the diffusion needs to be performed at a relatively low diffusion temperature to ensure that the Zr-B phase in the grain boundary phase of the permanent magnet is not dissolved after the diffusion and forms an elongated tubular shape with the main phase grains, thereby enhancing the capillary force and promoting the permeation of the diffusion source into the interior of the permanent magnet. For this purpose, a high vacuum degree (≥ 10 -2< Pa) is adopted in the diffusion process to achieve a relatively low diffusion temperature; also, the average particle size range of the powder particles in the diffusion source satisfies 2-100 µm and the D90 / D10 range is 1.2-5.1, so that the diffusion source is ensured to synchronously melt during the diffusion process, the concentration difference between the surface and the internal diffusion source of the magnet is maximized, the diffusion driving force is increased, the diffusion source is promoted to penetrate into the interior of the permanent magnet, the diffusion time is shortened, and the abnormal growth of surface layer grains is inhibited while the magnetic property is ensured.

[0033] The present disclosure further provides a sintered R-Fe-B permanent magnet prepared by the preparation method described above, wherein the sintered R-Fe-B permanent magnet has the meaning as described above.

[0034] The present disclosure further provides use of the sintered R-Fe-B permanent magnet described above, preferably in a motor.Advantageous Effects:

[0035] Compared with a conventional fine-granularity diffusion magnet, the distribution of the needle-shaped Zr-B phase in the grain boundary phase, which formed by Zr and B in the starting material of the permanent magnet of the present disclosure, changes the grain boundary state , and a relatively elongated channel is formed between the needle-shaped Zr-B phase and the main phase particles, so that the diffusion hindrance is reduced, the capillary force is enhanced, the transfer of the heavy rare earth elements in the diffusion source into the interior of the permanent magnet is accelerated, the accumulation of the heavy rare earth elements in the grain boundary phase is reduced, and the growth of the main phase grains is inhibited. The present disclosure improves the consistency of the size of main phase grains in the sintered R-Fe-B permanent magnet, thereby improving the magnetic property of the sintered R-Fe-B permanent magnet. In addition, when a low-temperature diffusion treatment condition with a high vacuum degree is adopted, the melting point of the grain boundary phase is reduced; the large-sized diffusion source particles with concentrated particle size distribution synchronously melt at a relatively low temperature, which promotes to form a greater concentration difference between the diffusion source elements on the surface and inside of the magnet, further accelerates the diffusion of the heavy rare earth elements into the interior of the magnet, reduces the accumulation of the diffusion source in the grain boundary phase, inhibits the grain growth, and ensures the consistency of the size of the main phase grains in the permanent magnet after fine-grain magnet diffusion.BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1 shows a TEM of the sintered Nd-Fe-B permanent magnet obtained in Example 1. FIG. 2 shows a schematic diagram of corresponding regions of the surface layer and the core of the sintered R-Fe-B permanent magnet. FIG. 3 shows a schematic diagram of main phase grains corresponding to different regions along the thickness direction in a cross section of the sintered R-Fe-B permanent magnet. FIG. 4 shows a schematic diagram of sampling positions of surface layer grains. FIG. 5 shows a TEM of the sintered Nd-Fe-B permanent magnet obtained in Comparative Example 1-6. DETAILED DESCRIPTION

[0037] The technical solutions of the present disclosure are further described in detail below with reference to specific examples. It will be appreciated that the following examples are merely exemplary illustrations and explanations of the present disclosure and should not be construed as limiting the claimed scope of the present disclosure. All techniques implemented on the basis of the above content of the present disclosure fall within the claimed scope of the present disclosure.

[0038] Unless otherwise stated, the starting materials and reagents used in the following examples are all commercially available products or can be prepared by using known methods.Example 1

[0039] A preparation method of a high-performance sintered Nd-Fe-B permanent magnet, which comprises the following steps: (1) the starting materials required for the Nd-Fe-B alloy according to the target proportion melted under argon atmosphere at 1400 °C for 15 min to ensure that the starting materials were homogenized, and then the mixture was cast onto the surface of a rotating quenching roller to give a quick-setting sheet of the Nd-Fe-B alloy with a thickness of about 0.3 mm, wherein in the Nd-Fe-B alloy, Nd accounted for 31.3 wt%, B accounted for 1.0 wt%, Zr accounted for 0.45 wt%, Al accounted for 0.10 wt%, Ga accounted for 0.2 wt%, and Co accounted for 1.0 wt%; the balance was Fe and inevitable impurities; (2) the Nd-Fe-B strip-casting alloys of step (1) was subjected to coarse crushing in a hydrogen decrepitation furnace and then discharged under argon atmosphere to reduce coarse powder oxidation. The coarse powder obtained after hydrogen decrepitation was directly subjected to jet milling grinding to obtain a magnetic powder with a granularity of 1-8 µm and an average particle size of 2.8 µm; (3) an antioxidant fatty acid ester of 0.15 wt% was added into the magnetic powder obtained in step (2), and the mixture was continuously and mechanically stirred for 4 h until the mixture was uniformly dispersed; (4) the magnetic powder of step (3) was subjected to press molding in vacuum in a magnetic field with an intensity of 2 T in the direction of the magnetic field, and a green body was obtained after cold isostatic pressing at 170 MPa; (5) the green body of step (4) was placed in a vacuum sintering furnace, subjected to sintering treatment at 1065 °C for 3 h, and then subjected to tempering treatment at 910 °C and 520 °C for 4 h to prepare a sintered Nd-Fe-B substrate; (6) the sintered Nd-Fe-B permanent magnet prepared in step (5) was machined into a thin sheet with a thickness of 3.1 mm, a length of 35 mm, and a width of 21 mm, wherein the thickness direction was an magnetization direction of the magnet, and the thin sheet was subjected to degreasing and stain removal with 3% dilute nitric acid and then dried; (7) the cleaned thin sheet obtained in step (6) was coated with a heavy rare earth element in the atmosphere, and two large surfaces of the thin sheet were coated with the slurry containing a Dy diffusion source with the coating amount being 0.7% of the weight of the thin sheet. The weight ratio of Dy powder particles to the fatty acid ester to the organic solvent in the slurry was 80:15:5, the average particle size of the Dy powder was 4.7 µm, and D90 / D10 = 3.1; and (8) the thin sheet coated with heavy rare earth obtained in step (7) was subjected to two-stage heat treatment diffusion in a vacuum atmosphere of 2×10 -3< Pa, wherein the primary heat treatment was performed at a temperature of 870 °C for a period of 6 h, and the secondary heat treatment was performed at a temperature of 460 °C for a period of 4 h to obtain the sintered Nd-Fe-B permanent magnet.

[0040] In this example, the heat treatment temperature was lower than that of the conventional two-stage heat treatment diffusion (900 °C and 520 °C), so that the Zr element was inhibited from dissolving in the neodymium-rich phase and alloying with other trace elements, and the formation of the Zr-B phase in the grain boundary phase was ensured.

[0041] Performance test: A specimen with a thickness of 3 mm, a length of 7 mm and a width of 7 mm was machined from the obtained thin sheet after diffusion from the center, and after saturation magnetization with a 5 T pulse magnetic field, the demagnetization curve of the obtained magnet at 20 °C was tested by using a NIM-10000 measuring instrument.

[0042] The grain (i.e., the main phase grains of the permanent magnet) size test for the magnet: 0.05 mm of the magnet after diffusion was ground uniformly in all directions to remove the Dy diffusion source that did not penetrate into the magnet on the surface layer. Positions 0.3 mm, 0.7 mm, 1.1 mm, and 1.4 mm away from the outer surface of the permanent magnet were selected, respectively, and cut along a direction perpendicular to the magnetization direction (the thickness direction of the permanent magnet, also referred to as the orientation direction). The cutting positions are shown by dashed lines in FIG. 3. The main phase grains in different cross sections corresponding to the dashed lines were tested. In the present disclosure, D1 is the size of the magnet grain in a region 0.3 mm away from the outer surface in the cross section, D2 is the size of the magnet grain in a region (denoted as the second core) 0.7 mm away from the outer surface of the permanent magnet, D3 is the size of the magnet grain in a region (denoted as the third core) 1.1 mm away from the surface of the permanent magnet, and D4 is the size of the magnet grain in a region (denoted as the fourth core) 1.5 mm away from the surface of the permanent magnet. The D1, D2, D3, and D4 described above all refer to the mean value of the magnet grains measured at any 6 points in the corresponding cross sections. FIG. 4 shows 6 sampling points of D1.Comparative Example 1-1

[0043] Comparative Example 1-1 was substantially the same as the preparation method of Example 1, except that: the Zr content in the Nd-Fe-B alloy in step (1) of Example 1 was 0.10% to obtain an Nd-Fe-B strip-casting alloys with a thickness of about 0.3 mm.

[0044] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0045] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Comparative Example 1-2

[0046] Comparative Example 1-2 was substantially the same as the preparation method of Example 1, except that Zr element was not included in the formula of the quick-setting sheet prepared in step (1) of Example 1.

[0047] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0048] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Comparative Example 1-3

[0049] Comparative Example 1-3 was substantially the same as the preparation method of Example 1, except that: the thin sheet coated with the heavy rare earth in step (8) of Example 1 was subjected to two-stage heat treatment diffusion in an atmosphere at a vacuum degree of 5×10 -2< Pa.

[0050] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0051] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Comparative Example 1-4

[0052] Comparative Example 1-4 was substantially the same as the preparation method of Example 1, except that: the average particle size of the Dy powder used in the slurry containing heavy rare earth Dy in step (7) of Example 1 was 1.8 µm, and D90 / D10 in the granularity distribution was 5.5.

[0053] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0054] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Comparative Example 1-5

[0055] Comparative Example 1-5 was substantially the same as the preparation method of Example 1, except that: the Zr content in the Nd-Fe-B alloy in step (1) of Example 1 was 0.1%, and the thin sheet coated with the heavy rare earth in step (8) of Example 1 was subjected to two-stage heat treatment diffusion in an atmosphere at a vacuum degree of 2×10 -2< Pa; the average particle size of the diffusion source Dy powder used was still 4.7 µm.

[0056] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0057] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Comparative Example 1-6

[0058] Comparative Example 1-6 was substantially the same as the preparation method of Example 1, except that: the Zr content in the Nd-Fe-B alloy in step (1) of Example 1 was 0.1%, and the thin sheet coated with the heavy rare earth in step (8) of Example 1 was subjected to two-stage heat treatment diffusion in an atmosphere at a vacuum degree of 2×10 -2< Pa. The primary heat treatment was performed at a temperature of 900 °C for a period of 6 h, and the secondary heat treatment was performed at a temperature of 520 °C for a period of 4 h. The average particle size of the diffusion source Dy powder used was still 1.8 µm.

[0059] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0060] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Comparative Example 1-7

[0061] Comparative Example 1-7 was substantially the same as the preparation method of Example 1, except that: the coating process of the heavy rare earth element in step (7) of Example 1 was not performed, the diffusion source heavy rare earth element diffusion was not performed, and the thin sheet obtained by the processing in step (6) was directly subjected to two-stage heat treatment diffusion in a vacuum atmosphere of 2×10 -3< Pa. The primary heat treatment was performed at a temperature of 860 °C for a period of 6 h, and the secondary heat treatment was performed at a temperature of 460 °C for a period of 4 h.

[0062] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0063] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C and the grain size at each position were tested according to the method described in Example 1.Example 2

[0064] Example 2 was substantially the same as the preparation method of Example 1, except that: the Zr content in the Nd-Fe-B alloy in step (1) of Example 1 was 0.25%, and the thin sheet coated with the heavy rare earth in step (8) of Example 1 was subjected to two-stage heat treatment diffusion in an atmosphere at a vacuum degree of 2×10 -3< Pa. The average particle size of the diffusion source Dy powder used was 11 µm, wherein D90 / D10 = 3.4.

[0065] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0066] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C was tested according to the method described in Example 1.Example 3

[0067] Example 3 was substantially the same as the preparation method of Example 1, except that: the Zr content in the Nd-Fe-B alloy in step (1) of Example 1 was 0.25%, and the thin sheet coated with the heavy rare earth in step (8) of Example 1 was subjected to two-stage heat treatment diffusion in an atmosphere at a vacuum degree of 2×10 -4< Pa. The average particle size of the diffusion source Dy powder used was still 54 µm, wherein D90 / D10 = 3.2.

[0068] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0069] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C was tested according to the method described in Example 1.Example 4

[0070] Example 4 was substantially the same as the preparation method of Example 1, except that: the Zr content in the Nd-Fe-B alloy in step (1) of Example 1 was 0.55%, and the thin sheet coated with the heavy rare earth in step (8) of Example 1 was subjected to two-stage heat treatment diffusion in an atmosphere at a vacuum degree of 2×10 -4< Pa. The average particle size of the diffusion source Dy powder used was still 54 µm, wherein D90 / D10 = 3.1.

[0071] The rest was the same as that in Example 1, and a sintered Nd-Fe-B permanent magnet was obtained.

[0072] The magnetic property of the sintered Nd-Fe-B permanent magnet obtained in this example at 20 °C was tested according to the method described in Example 1.

[0073] In Example 1 and Comparative Example 1-6, the grain boundary phase was analyzed by TEM, and it can be observed that: FIG. 1 shows a TEM of the sintered Nd-Fe-B permanent magnet obtained in Example 1, in which the white region present in the grain boundary in the image is a Zr-B phase. It can be seen that a needle-shaped Zr-B phase is formed in the grain boundary phase of the sintered Nd-Fe-B permanent magnet prepared in this example, and the needle-shaped Zr-B phase is uniformly and completely distributed in the grain boundary phase.

[0074] FIG. 5 shows a TEM of the sintered Nd-Fe-B permanent magnet obtained in Comparative Example 1-6. It can be seen from FIG. 5 that Zr-B is discretely distributed in the grain boundary phase, and no elongated needle-shaped structure is formed for the Zr-B phase.

[0075] The magnetic property at 20 °C and the grain size at each position of the sintered Nd-Fe-B permanent magnets of the examples and comparative examples described above are recorded in Table 1. Table 1SubstrateBr@20 °C (T)Hcj@20 °C (KA / m)D1 (µm)D2 (µm)D3 (µm)D4 (µm)Example 11.42019724.664.614.594.58Comparative Example 1-11.42218745.064.744.554.42Comparative Example 1-21.42318295.194.694.504.43Comparative Example 1-31.42218504.884.704.564.47Comparative Example 1-41.42418385.034.684.494.41Comparative Example 1-51.42217475.174.934.504.39Comparative Example 1-61.42317105.234.904.444.39Comparative Example 1-71.43513694.374.364.374.37Example 21.42019804.684.644.604.58Example 31.41919754.674.634.594.57Example 41.42119614.704.644.604.55

[0076] As can be seen from the above table, in the sintered Nd-Fe-B permanent magnets prepared in Examples 1-4 of the present disclosure, D1-Dn ≤ 0.34, and Dn refers to D2, D3, and D4; while in the sintered Nd-Fe-B permanent magnets prepared in Comparative Example 1-6, D1-Dn are all greater than 0.34. The examples and comparative examples show that when D1-Dn of the diffusion products are less than 0.34 and greater than 0, the products have high grain size consistency and more excellent magnetic properties.

[0077] The inventors believe that although the conventional diffusion process can greatly improve the magnetic properties of the permanent magnet during diffusion, at a higher diffusion temperature, the atoms in the boundary layer of the main phase grains in the permanent magnet are in a high-energy state, and after long-term contact with elements such as high-concentration rare earth elments in a thermally activated state in the grain boundary phase, the main phase grains, especially the surface layer grains, will spontaneously and inevitably grow, and the growth of the surface layer grains will affect the further improvement of the magnetic properties.

[0078] The sintered R-Fe-B permanent magnet prepared by the present disclosure can control excessive growth of surface layer grains, so that the grains corresponding to different positions in the sintered R-Fe-B permanent magnet have a uniform size, i.e., 0 < D1-Dn ≤ 0.34. This is because: (1) the Zr element content is the basis for forming a relatively large needle-shaped Zr-B phase; the needle-shaped Zr-B phase changes the grain boundary phase state to ensure that the diffusion source element rapidly enters the interior of the magnet during diffusion, and reduce the contact time between the diffusion source element and the surface layer grains in the magnet to inhibit grain growth; (2) the high diffusion vacuum degree allows the elements in the diffusion source to complete diffusion at a relatively low temperature to ensure that the Zr-B phase in the grain boundary phase is not dissolved; (3) the granularity size and distribution uniformity of the diffusion source powder allow the diffusion source powder to melt at a relatively low diffusion temperature and realize synchronous diffusion of the diffusion source, so that the concentration difference between the interior and exterior of the magnet is maximized, and the diffusion time is shortened. Therefore, the Zr element content, the diffusion vacuum degree, and the granularity size and distribution of the diffusion source have a synergistic effect, so as to prevent the diffusion of the diffusion source element into the interior of the magnet from being blocked, which causes the surface layer grains of the magnet to be in contact with the diffusion source at a high temperature for a long time, resulting in the abnormal growth of the magnet grains. Details are as follows.

[0079] As can be seen from the comparison of Comparative Examples 1-1, 1-2, 1-3, and 1-4, if a needle-shaped Zr-B phase can not be formed, the low diffusion vacuum degree and the abnormal granularity size and non-concentrated distribution of the diffusion source will lead to the inhibition of the diffusion of the diffusion source element into the interior of the magnet, making it difficult for the diffusion source element to penetrate into the magnet; and the main phase grains at the surface layer of the magnet occurred a long-term process of mass transfer and heat transfer with the diffusion source element and the element in the grain boundary phase at high temperature, finally leading to different degrees of grain growth at different positions of the magnet, and the Hcj@20 °C of the magnet is significantly reduced.

[0080] Comparing Comparative Example 1-5 with Example 1, it can be seen that: after any two of the Zr element content, the diffusion vacuum degree, and the granularity size of the diffusion source are changed, the performance of the magnet is reduced more significantly, the grains at different positions of the magnet grow larger, and the differences in size are greater.

[0081] Therefore, the inventors believe that the synergistic effect of the Zr element content, the diffusion vacuum degree and the grain size of the diffusion source is the key to the preparation of a magnet with high performance and uniform grain size.

[0082] As can be seen from the comparison between Comparative Example 1-6 and Example 1, the Zr element content and the two-stage low-temperature diffusion process affect the formation of a needle-shaped Zr-B phase, thereby affecting the diffusion rate of the diffusion source element into the interior of the magnet, resulting in consistent magnet grain size.

[0083] As can be seen from Examples 2, 3, and 4, when the Zr element content, the diffusion vacuum degree, and the granularity size and distribution of the diffusion source are changed within the range of the present disclosure, no significant influence on the formation of the needle-shaped Zr-B phase in the grain boundary is observed, which effectively ensures the rapid diffusion of the diffusion source into the interior of the magnet, inhibits the abnormal growth of the magnet grains, and has little influence on the magnet performance and the grain size, which have a relatively small fluctuation range.

[0084] The exemplary embodiments of the present disclosure have been described above. However, the protection scope of the present application is not limited to the above embodiments. Any modification, equivalent, improvement, and the like made by those skilled in the art without departing from the spirit and principle of the present disclosure shall fall within the protection scope of the present disclosure.

Claims

1. A sintered R-Fe-B permanent magnet, wherein main phase grains of a surface layer of the sintered R-Fe-B permanent magnet are denoted as surface layer grains, the size of the surface layer grain is D1, main phase grains of a core along an magnetization direction of the sintered R-Fe-B permanent magnet are denoted as core grains, the size of the core grain is Dn, and 0 < D1-Dn ≤ 0.34, wherein n is an integer greater than 1; the magnetization direction of the sintered R-Fe-B permanent magnet refers to a thickness direction.

2. The sintered R-Fe-B permanent magnet as claimed in claim 1, comprising a main phase and a grain boundary phase, wherein the grain boundary phase comprises a Zr-B phase, and the Zr-B phase is in a needle-shaped structure.

3. The sintered R-Fe-B permanent magnet as claimed in claim 1, having the following magnetic properties, wherein, (1) Br is not less than 1.41 T; (2) Hcj is not less than 1900 KA / m.

4. The sintered R-Fe-B permanent magnet as claimed in claim 1, wherein starting materials of the sintered R-Fe-B permanent magnet comprise a Zr element and a B element; the mass content of the Zr element and the B element in the starting materials are denoted as [Zr] and [B], respectively, and satisfy 0.13 ≤ [Zr] / [B] ≤ 0.55, and the mass content of the Zr element in the starting materials is not less than 0.15 wt%.

5. A preparation method for the sintered R-Fe-B permanent magnet as claimed in any one of claims 1-4, wherein the preparation method comprises: (1) formulating starting materials according to a corresponding mass ratio to prepare a neodymium-iron-boron strip-casting alloys, and performing crushing to obtain a magnetic powder with a target granularity; (2) subjecting the magnetic powder obtained to press molding and cold isostatic pressing to obtain a green body; (3) subjecting the green body to sintering treatment and aging treatment to obtain a sintered neodymium-iron-boron matrix; (4) subjecting the sintered neodymium-iron-boron matrix to diffusion treatment to obtain the sintered R-Fe-B permanent magnet.

6. The preparation method as claimed in claim 5, wherein in step (1), the step of crushing specifically comprises: subjecting the neodymium-iron-boron strip-casting alloys to hydrogen decrepitation to obtain a coarsely crushed powder, and then performing a high-energy ball milling process or a jet milling process jet milling crushing to obtain the magnetic powder; and / or, in step (1), the magnetic powder has a target granularity of 1-8 µm.

7. The preparation method as claimed in claim 5, wherein in step (2), the press molding comprises: subjecting the magnetic powder to orientated press molding in a magnetic field; and / or, in step (3), the sintering treatment comprises: placing the green body at a high temperature condition of 1055-1090 °C in a vacuum environment and performing sintering; and / or, in step (3), the aging treatment refers to performing tempering treatment at 1000 °C or lower after the sintering.

8. The preparation method as claimed in claim 5, wherein in step (4), the diffusion treatment specifically comprises: coating the surface of the sintered neodymium-iron-boron matrix with heavy rare earth elements to uniformly distribute the heavy rare earth elements on the surface of the sintered neodymium-iron-boron matrix; and performing a two-stage heat treatment diffusion in a vacuum atmosphere, wherein a primary heat treatment has a temperature range of 790-930 °C, a secondary heat treatment has a temperature range of 440-530 °C, and the vacuum degree is 10-2-10-4 Pa.

9. The preparation method as claimed in claim 8, wherein the heavy rare earth element is provided by a diffusion source; the diffusion source comprises heavy rare earth powder particles, an antioxidant, and an organic solvent; the heavy rare earth powder particles comprise at least one or more of dysprosium, a hydride of dysprosium, an oxide of dysprosium, a fluoride of dysprosium, terbium, a hydride of terbium, an oxide of terbium, and a fluoride of terbium; the heavy rare earth powder particles have an average particle size of 2-100 µm and a D90 / D10 of 1.2-5.1.

10. Use of the sintered R-Fe-B permanent magnet as claimed in any one of claims 1-4 in a motor.

Images