Linear Halbach array based on non-uniform permanent magnets and preparation method thereof

Abstract

The present invention provides a linear Halbach array based on non-uniform permanent magnets and a method for preparing the same, belonging to the field of permanent magnets. The method comprises: dividing a mold into two symmetrical, arc-shaped, concave regions A1 and A2, with a region B located between the two regions; loosely packing low-grade magnetic powder in region B, and loosely packing high-grade magnetic powder in regions A1 and A2; molding and magnetizing the NdFeB compacts to produce them; heat-treating the NdFeB compacts after demolding to produce non-uniform permanent magnets, including preheating, gradient sintering (first increasing the temperature and then decreasing the temperature), and tempering; and bonding (2n+3) groups of non-uniform permanent magnets using magnetic glue to obtain a linear Halbach array. The present invention utilizes non-uniform permanent magnets produced by zoned molding of different grades of magnetic powder, regionally strengthening the edges to reduce demagnetization at corners under extreme conditions, increase the local intrinsic coercivity, and enhance the service performance of the magnets, resulting in superior performance of the assembled Halbach array.

Description

Technical Field

[0001] The present invention belongs to the technical field of permanent magnets, and in particular relates to a linear Halbach array based on non-uniform permanent magnets and a preparation method thereof. Background Art

[0002] Maglev trains are a new type of transportation system consisting of contactless magnetic support, magnetic guidance, and a linear drive system. Maglev utilizes permanent magnets to repel the track and maintain suspension at the centerline of the slot. Electromagnetic guidance enables frictionless operation, offering energy-saving advantages. Neodymium iron boron (Nd-Fe-B) is a rare earth permanent magnet material with extremely high magnetic energy product, intrinsic coercivity, and energy density. Due to its excellent magnetic properties, it is known as the "King of Magnets." It has become a key material in maglev rail transit systems, a core component that generates levitation force and ensures stable operation.

[0003] Maglev rail transit systems require not only excellent magnet stability but also a specific array combination to produce the levitation effect. The Halbach array is a magnet structure that arranges permanent magnets with different magnetization directions in a certain pattern. This allows the magnetic lines of force to converge on one side of the magnet while weakening them on the other side, thereby enhancing the field strength in a specific direction. The goal is to generate the strongest magnetic field with the least number of magnets, which is why it is often used as a permanent magnet track for magnetic levitation.

[0004] Placing NdFeB magnetic powder into a mold for orientation and compaction is a critical step in manufacturing high-performance NdFeB magnets. The mold, composed of multiple magnetic conductive and non-magnetic blocks, enables multi-pole orientation. During orientation compaction, the magnetic powder is aligned in a specific direction under the influence of a magnetic field, forming a pre-pressed part with a specific density and orientation.

[0005] However, the distribution of magnetic flux lines within the unit magnet exhibits significant localized enhancement, making demagnetization more likely to occur at the edges. This, in turn, reduces the magnetic performance of the Halbach array structure composed of NdFeB magnets, limiting its application. While the use of high-performance magnets can reduce the impact of edge demagnetization on the overall magnet structure, this significantly increases costs. Summary of the Invention

[0006] In view of this, the present invention aims to provide a linear Halbach array based on non-uniform permanent magnets and a preparation method thereof, aiming to solve at least one technical problem in the background technology.

[0007] The present invention is achieved in that:

[0008] A first aspect of the present invention provides a linear Halbach array based on non-uniform permanent magnets, the linear Halbach array comprising (2n+3) groups of non-uniform permanent magnets, where n≥1 and is rounded to an integer;

[0009] Each set of non-uniform permanent magnets is made of two different grades of NdFeB magnetic powder. During compression molding, the low-grade magnetic powder is located in the middle area of ​​the mold, and the high-grade magnetic powder is located in the symmetrical arc-shaped concave areas on both sides of the mold.

[0010] Among them, the magnetic field direction is set to NS, the regional connection direction of the two groups of high-grade magnetic powder is set to A1-A2, and the NS direction is perpendicular or parallel to the A1-A2 direction;

[0011] The intrinsic coercive force of the low-grade magnetic powder is lower than the intrinsic coercive force of the high-grade magnetic powder.

[0012] A second aspect of the present invention provides a method for preparing a linear Halbach array based on a non-uniform permanent magnet, comprising the following steps:

[0013] The cavity of the mold is divided into two symmetrical and arc-shaped concave areas A1 and A2, and an area B located between the two areas;

[0014] Area B is loosely packed with low-grade magnetic powder, and areas A1 and A2 are loosely packed with high-grade magnetic powder;

[0015] The NdFeB compact is obtained by compression molding and magnetization orientation;

[0016] After demolding, the NdFeB compact is heat treated to obtain a non-uniform permanent magnet; the heat treatment includes preheating, high-temperature sintering and tempering, wherein the high-temperature sintering adopts a gradient sintering process of first increasing the temperature and then decreasing the temperature;

[0017] According to the topological structure of the Halbach array, (2n+3) groups of the non-uniform permanent magnets are adhered and fixed using magnetic glue to obtain a linear Halbach array, where n≥1 and is rounded to an integer.

[0018] Preferably, the preparation method further comprises: pre-treating the low-grade magnetic powder and the high-grade magnetic powder before loosely loading them into the mold, specifically: adding 1wt% to 2wt% of copper nitride nanoparticles to the low-grade magnetic powder and the high-grade magnetic powder, respectively, then mixing them evenly with a high shear mixer, and finally adding 0.2wt% to 0.5wt% of solid lubricant powder to each of them.

[0019] Preferably, the preparation method further comprises: performing surface treatment on the non-uniform permanent magnet, specifically: depositing a diamond film on the surface of the non-uniform permanent magnet.

[0020] Preferably, the NdFeB compact is obtained by compression molding and magnetization orientation, and the specific operations include:

[0021] First, mechanical vibration preloading or ultrasonic preloading treatment;

[0022] Bidirectional pressing is performed by a magnetic field press, with the magnetic field direction NS being perpendicular or parallel to the A1-A2 direction;

[0023] Finally, the NdFeB green body is produced through cold isostatic pressing.

[0024] Preferably, the preheating temperature is 600°C to 800°C, and the time is 0.5h to 1h;

[0025] The specific operation of the gradient sintering is:

[0026] First, heat the vacuum sintering furnace to 1000℃~1050℃ and keep it warm for 1h~2h;

[0027] Then heat to 1050℃~1100℃ and keep warm for 2h~3h;

[0028] Finally, cool down to 900℃~1000℃ and keep warm for 1h~2h.

[0029] Preferably, the particle size of the low-grade magnetic powder and the high-grade magnetic powder is 2.5 μm to 5 μm; and the particle size of the copper nitride nanoparticles is 80 nm to 120 nm.

[0030] Preferably, the diamond film is produced by vapor deposition.

[0031] Preferably, the diamond film has a thickness of 2 μm to 5 μm.

[0032] Preferably, in the centerline area of ​​the mold cavity, the sum of the widths of zone A1 and zone A2 is not greater than the width of zone B and not less than 1 / 2 of the width of zone B.

[0033] Preferably, the magnetic field press uses an elastic pressing head during bidirectional pressing.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. The present invention uses two different brands of magnetic powder to form a non-uniform permanent magnet through zone molding, and regionally strengthens the edges to reduce the demagnetization effect at the corners of the magnet under extreme conditions, increase the local intrinsic coercive force and enhance the service performance of the magnet, making the performance of the assembled Halbach array even better.

[0036] 2. The present invention improves the magnetic powder raw materials and optimizes the molding process and heat treatment process to overcome the technical defects in the molding and sintering processes of magnetic powders of different brands and suppress the adverse effects caused by the raw materials.

[0037] 3. The present invention adds nano-scale copper nitride (Cu3N) to the magnetic powder and uses a high-shear mixer to adsorb it on the surface of the magnetic powder, promoting the formation of particles with a more regular shape, increasing the fluidity of the magnetic powder, reducing its friction, and reducing the mechanical resistance during molding, thereby improving the orientation of the green compact and the magnet, and further enhancing the remanence, intrinsic coercive force and magnetic energy product of the magnet.

[0038] 3. The present invention enriches copper elements on the surface of the magnetic powder, reduces the melting point of the grain boundary, and indirectly improves the magnetic properties of the magnet; and a small amount of Fe-N compound and Nd-N compound can be formed at the interface between the copper nitride and the magnetic powder after heat treatment as a protective layer, which can improve the high temperature resistance and corrosion resistance of the material.

[0039] 4. Before applying magnetic field orientation, the present invention uses mechanical vibration or ultrasonic treatment for rapid pre-compression to increase the packing density of magnetic powder, inhibit the free rotation of particles, and avoid the orientation degree of the corner area being lower than that of the center area due to the rotation of magnetic powder particles.

[0040] 5. The high temperature sintering in the present invention adopts gradient sintering of first increasing the temperature and then decreasing the temperature, which further suppresses technical defects such as poor internal fusion effect and cracks on the bonding interface.

[0041] 6. The present invention deposits a diamond film on the surface of the magnet to form a protective layer, which can improve the mechanical properties, high temperature resistance and corrosion resistance of the material while promoting magnetic uniformity. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 This is a schematic diagram of the mold after internal partitioning of the present invention;

[0043] Figure 2 Schematic diagram of the structures of existing NdFeB magnetic powder particles and the NdFeB magnetic powder particles pretreated according to the present invention;

[0044] Figure 3 A schematic diagram of the topological structure of a linear Halbach array assembled in the present invention;

[0045] Figure 4 Schematic diagram of the internal partition of the mold of Comparative Example 3 and Comparative Example 4. DETAILED DESCRIPTION

[0046] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention and are not intended to limit the present invention.

[0047] A method for preparing a linear Halbach array based on a non-uniform permanent magnet comprises the following steps:

[0048] S1. Modified mold

[0049] like Figure 1 As shown, two sets of arc plates are used to divide the cavity of the mold into two symmetrical and arc-shaped concave areas A1 and A2, and an area B located between the two areas;

[0050] The low-grade magnetic powder is located in the middle area of ​​the mold (i.e., area B), and the high-grade magnetic powder is located in the arc-shaped concave areas symmetrically on both sides of the mold (i.e., areas A1 and A2). In a specific implementation, the intrinsic coercivity of the low-grade magnetic powder is lower than that of the high-grade magnetic powder.

[0051] At the same time, in the centerline area of ​​the mold cavity, the sum of the widths of area A1 and area A2 is not greater than the width of area B and not less than 1 / 2 of the width of area B, 1 / 2W B ≤W A1 +W A2 ≤W B The regional size distribution restrictions of the A1 and A2 regions not only ensure the overall magnetic properties but also effectively improve the local enhancement and edge demagnetization.

[0052] The preparation of sintered NdFeB magnets mainly includes the following steps: batching, smelting, rapid solidification casting, powder making, magnetic powder pressing, sintering and tempering. Powder making includes hydrogen crushing to produce coarse powder and air flow grinding to produce fine powder. Hydrogen crushing uses the phase change and volume change during the hydrogen absorption and release process of the rapid solidification sheet to generate internal stress, causing it to break spontaneously. Air flow grinding uses high-pressure gas to accelerate the coarse powder to supersonic speed, achieving the purpose of particle comminution through high-speed collisions between particles. Magnetic powder forming refers to the orientation and dense formation of fine powder under an external magnetic field. Compression molding is the most widely used forming method.

[0053] During compression molding, the present invention divides the mold into areas and utilizes the mutual diffusion of magnetic powders of different brands and different magnetic properties to improve the overall performance of the magnetic block, especially the intrinsic coercive force of the edge area. In addition, compared with planar contact, the contact surface between area B, area A1 and area A2 in the present invention is a concave arc area, and the mutual diffusion effect is better.

[0054] S2. Raw material pretreatment

[0055] Since different grades of magnetic powder have different green shrinkage during subsequent pressing and heat treatment, and the working temperature is inconsistent, in order to solve this problem, the present invention applies pretreatment to low-grade magnetic powder and high-grade magnetic powder. Specifically, 1wt% to 2wt% of copper nitride nanoparticles are added to the low-grade magnetic powder and the high-grade magnetic powder respectively, and then mixed uniformly by a high shear mixer, and finally 0.2wt% to 0.5wt% of solid lubricant powder is added to each;

[0056] In a specific implementation, the particle size of the low-grade magnetic powder and the high-grade magnetic powder is 2 μm to 5 μm; the particle size of the copper nitride nanoparticles is 80 nm to 120 nm.

[0057] The B area contains low-grade magnetic powder that has been loosely packed and pre-treated, and the A1 and A2 areas contain high-grade magnetic powder that has been loosely packed and pre-treated. The low-grade and high-grade magnetic powders can be made of two grades with small or large differences in magnetic properties. The specific grade can be selected according to actual needs.

[0058] The compression molding process of NdFeB powder is that the powder particles are rearranged and squeezed under the pressure of the mold, the gap between the particles is continuously reduced, and finally a green body with a certain density is formed. This process is achieved through contact, collision and compression between particles. In this process, the interaction and movement behavior of the particles will directly affect the orientation of the magnet, and thus affect the performance of the grade. The orientation of magnetic powder particles is divided into the magnetization stage and the movement stage of the particles. The important stage is the overall movement of the particles during the compression molding. In the overall movement stage, the orientation is affected by four forces: (1) static magnetostatic torque; (2) agglomeration force caused by static magnetostatic interaction and adsorption force between particles, which is related to the surface field strength and particle size of the particles; (3) friction generated by particle contact; (4) mechanical resistance caused by irregular particle shape. During compression molding, factors such as particle interaction, contact friction and shape effect will have an obstructive effect. Sintered NdFeB magnets are polycrystalline. Although the magnetic powder particles produced by hydrogen crushing + air flow grinding are small in size, their microstructure is actually irregular in shape, rather than regular spherical or three-dimensional. In fact, the edges of magnetic powder particles are irregular and have obvious edges and corners, such as Figure 2 As shown in (a), during the forming process, mechanical interlocking and meshing will occur between the particles to form an arch bridge phenomenon, and the friction coefficient will increase, which will lead to the agglomeration of magnetic powder particles.

[0059] In view of this, the present invention adds nano-scale copper nitride (Cu3N) to the magnetic powder, and its functions include the following aspects: (1) using a high shear mixer to adsorb it on the surface of the magnetic powder to promote its formation of a more regular particle shape, such as Figure 2 As shown in (b), the fluidity of the magnetic powder is increased, its friction is reduced, and the mechanical resistance during molding is reduced, thereby improving the orientation of the compact and the magnet, and further strengthening the remanence and magnetic energy product of the magnet; (2) copper elements are enriched on the surface of the magnetic powder, which reduces the melting point of the grain boundary and indirectly improves the magnetism of the magnet; (3) During the subsequent heat treatment process, under high temperature conditions, a small amount of Fe-N compounds and Nd-N compounds can be formed at the interface between copper nitride and magnetic powder as a protective layer, which can improve the high temperature resistance and corrosion resistance of the material.

[0060] Compared to adding copper nitride directly during the batching stage, the present invention adds copper nitride particles after smelting and powdering, which prevents the complete decomposition of copper nitride during the smelting stage to produce copper and nitrogen. Nitrogen then reacts with iron, neodymium, and other elements to further produce iron nitride and neodymium nitride. Excessive levels of iron nitride and neodymium nitride can reduce the intrinsic coercivity of the magnetic powder. Furthermore, during heat treatment and curing after compression molding, the degree of thermal decomposition of copper nitride is significantly reduced due to the compactness of the magnet and the influence of the copper-iron-neodymium alloy phase. This maximizes the positive impact of copper nitride on the performance of the magnetic powder and reduces its negative impact on its magnetic properties.

[0061] In a specific implementation, in order to further increase the fluidity of the magnetic powder particles during molding, a solid lubricant, such as stearic acid, is added to the magnetic powder to improve the orientation and permanent magnetism.

[0062] In a specific implementation, other auxiliary agents permitted in the art may be added to the magnetic powder, such as antioxidants, release agents, etc., which will not be elaborated here.

[0063] S3, Non-uniform permanent magnet forming

[0064] 1. Molding and magnetization orientation to obtain NdFeB green compacts; specifically:

[0065] (1) Mechanical vibration preloading or ultrasonic preloading treatment first;

[0066] In the initial stage, the magnetic powder particles are in a loosely packed state and the particle orientation is disordered. If the relative movement between the particles is directly suppressed, the particles will rotate. Studies have found that the rotation of magnetic powder particles is one of the important factors that causes the orientation degree of the corner area to be lower than that of the center area, destroying the orientation of the magnetic powder and exacerbating the magnetic inhomogeneity in the edge area of ​​the magnet.

[0067] The present invention uses mechanical vibration or ultrasonic treatment to quickly pre-press the magnetic powder before applying a magnetic field for orientation, increasing the packing density of the magnetic powder and inhibiting the free rotation of the particles. Furthermore, compared to conventional pressing with a pressing head, mechanical vibration or ultrasonic pre-pressing is faster and more evenly distributes the particles within the mold.

[0068] In addition, since the present invention applies to the regional pressing of two different brands of magnetic powder, mechanical vibration or ultrasonic treatment for rapid pre-pressing can make the magnetic powders at the contact interface of the two mix with each other, avoiding sudden changes in the magnetic properties of the contact interface due to different magnetic powders.

[0069] (2) Bidirectional pressing by a magnetic field press, with the magnetic field direction NS perpendicular or parallel to the A1-A2 direction;

[0070] In a specific implementation, the pressing molding adopts a molding method permitted in the art, such as firstly pressing the green compact by vertical and horizontal bidirectional pressing at a pressure of 15MPa to 25MPa, and then performing orientation treatment by using a magnetic field with a magnetic field strength of 1.5T to 5T during the pressing process, and then further compacting the green compact by cold isostatic pressing at 170MPa to 200MPa for 20s to 120s to obtain the NdFeB green compact. The above molding parameters can be adjusted according to actual conditions and are not specifically limited here;

[0071] In order to further reduce the phenomenon of local stress concentration, which leads to uneven orientation of the magnetic powder, an elastic pressing head is used during pressing. For example, elastic materials such as high-temperature resistant rubber or silicone are used. The elastic pressing head can reduce the excessive gaps and uneven density during the pressing process of the magnetic powder, thereby making the force on the magnetic powder more uniform during the pressing process. In addition, during the pressing process, the elastic pressing head can fit the inner surface of the mold, ensuring that the magnetic powder is subjected to uniform pressure in all parts, thereby improving the quality and magnetic properties of the magnet.

[0072] 2. Heat treatment of NdFeB green body after demoulding;

[0073] In the field of sintered NdFeB preparation, heat treatment uses preheating, high-temperature sintering and at least two-stage tempering. In specific implementations, since the present invention uses different brands of magnetic powder for compression molding, due to the differences in their physical and chemical properties, conventional high-temperature sintering is prone to poor internal fusion effect and even cracks at the bonding interface. To solve this technical problem, the present invention adopts gradient sintering with first increasing the temperature and then decreasing the temperature for high-temperature sintering, and the tempering treatment can adopt conventional operations in this field.

[0074] During the heat treatment process, the preheating temperature is 600℃~800℃ and the time is 0.5h~1h;

[0075] The specific operation of gradient sintering is as follows: first, heat the vacuum sintering furnace to the range of 1000℃~1050℃ and keep it for 1h~2h; then heat it to 1050℃~1100℃ and keep it for 2h~3h; finally, cool it to 900℃~1000℃ and keep it for 1h~2h; the above sintering temperature, time, and number of stages can be adjusted according to actual needs; the heating and cooling rate in this process is not specifically limited and is generally 1℃ / s~5℃ / s;

[0076] The tempering treatment is as follows: a first tempering heat treatment at 800°C to 920°C for 1 hour to 5 hours, and a second tempering heat treatment at 450°C to 600°C for 1 hour to 4 hours to obtain a non-uniform permanent magnet; the above tempering treatment parameters can be adjusted according to actual conditions and are not specifically limited here;

[0077] 3. Deposit diamond film on the outer surface of non-uniform permanent magnet to achieve surface treatment;

[0078] In a specific implementation, the diamond film is prepared by microwave plasma chemical vapor deposition or magnetron sputtering physical vapor deposition and covers the outer surface of the non-uniform permanent magnet, with a thickness of about 2μm to 5μm;

[0079] The effects of diamond film are reflected in the following aspects: (1) Diamond film can produce a weak magnetoresistance effect, thereby regulating the overall magnetic field intensity distribution of the magnet, making the magnet performance more uniform and indirectly improving the local strengthening phenomenon; (2) enhancing the mechanical properties of the magnet, such as strength and hardness; (3) Diamond film can play a role in corrosion resistance;

[0080] In a specific implementation, the thickness of the diamond film is limited to 2μm~5μm, for example, it can be 2μm, 3μm, or 5μm; but it is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable. The reason for the thickness limitation is to ensure the effective protection of the diamond film on the magnet; at the same time, avoid excessive thickness leading to excessive carbon content in the magnet and excessive magnetoresistance effect, which has a negative impact on the magnetic properties.

[0081] S4. Assemble Halbach array

[0082] like Figure 3 As shown, according to the topological structure of the Halbach array, 2n+3 groups of the non-uniform permanent magnets are adhered and fixed using magnetic glue to obtain a linear Halbach array, where n≥1 and is rounded to an integer.

[0083] Example 1

[0084] The preparation method of the linear Halbach array based on non-uniform permanent magnets is as follows:

[0085] Step 1: Weigh NdFeB magnetic powders of N33EH and N38UH, add 1wt% of copper nitride nanoparticles (D 50 =80nm), and then mixed evenly by a high shear mixer, and finally added 0.2wt% of stearic acid powder to prepare low-grade magnetic powder and high-grade magnetic powder; Figure 1 As shown in the figure, two sets of arc plates are used to divide the cavity of the mold into two symmetrical and arc-shaped concave areas A1 and A2, and area B located between the two areas; in the centerline area of ​​the mold cavity, the sum of the widths of areas A1 and A2 is basically equal to the width of area B, that is, W A1 +W A2 =W B The low-grade magnetic powder N38UH is loosely packed in area B, and the high-grade magnetic powder N33EH is loosely packed in areas A1 and A2. That is, high-performance (high intrinsic coercivity) magnetic powder is distributed at the edge, and low-performance (low intrinsic coercivity) magnetic powder is distributed in the middle area.

[0086] Step 2: Pre-press the mold with mechanical vibration to make the filling density in the mold about 3.6g / cm 3 The green compact was obtained by vertical and horizontal bidirectional pressing at 10 MPa using an elastic pressing head, and an orientation treatment was performed using a magnetic field with a magnetic field strength of 2.5 T during the pressing process. The green compact was further compacted by cold isostatic pressing at 100 MPa for 60 seconds to obtain a NdFeB green compact.

[0087] Step 3: placing the NdFeB magnet green body in a vacuum sintering furnace, first rapidly heating it to 600° C. for preheating for 1 hour; then rapidly heating it to 1000° C. and holding it for 1.5 hours; then rapidly heating it to 1095° C. and holding it for 3 hours; then rapidly cooling it to 950° C. and holding it for 1.5 hours; then performing a primary tempering heat treatment at 900° C. for 3 hours and a secondary tempering heat treatment at 480° C. for 3 hours, and cooling it to obtain a non-uniform permanent magnet; and coating the outer surface of the non-uniform permanent magnet with a diamond film having a thickness of approximately 2 μm by using a microwave plasma chemical vapor deposition method;

[0088] Step 4: Take five groups of non-uniform permanent magnets obtained in the above steps and Figure 3 Arrangement shown, using magnet glue to assemble non-uniform permanent magnets into a linear Halbach array.

[0089] Example 2

[0090] In Example 2, based on Example 1, the amount of copper nitride nanoparticles was adjusted to 1.5 wt %, and other reaction conditions were the same as those in Example 1.

[0091] Example 3

[0092] In Example 3, based on Example 1, the amount of copper nitride nanoparticles was adjusted to 2 wt %, and other reaction conditions were the same as those in Example 1.

[0093] Example 4

[0094] In Example 4, based on Example 1, the particle size of the copper nitride nanoparticles was adjusted to 100 nm, and other reaction conditions were the same as those in Example 1.

[0095] Example 5

[0096] In Example 5, based on Example 1, the particle size of the copper nitride nanoparticles was adjusted to 120 nm, and other reaction conditions were the same as those in Example 1.

[0097] Example 6

[0098] In Example 6, based on Example 1, the thickness of the diamond film was adjusted to 3.5 μm, and other reaction conditions were consistent with those in Example 1.

[0099] Example 7

[0100] In this Example 7, based on Example 1, the thickness of the diamond film was adjusted to 5 μm, and the other reaction conditions were the same as those in Example 1.

[0101] Example 8

[0102] In Example 8, based on Example 1, the pre-pressing treatment in step 2 was adjusted to ultrasonic pre-pressing, and the other reaction conditions were the same as those in Example 1.

[0103] Example 9

[0104] In Example 9, based on Example 1, the sum of the widths of the A1 and A2 areas on the centerline of the mold cavity is adjusted to half the width of the B area, that is, W A1 +W A2 =1 / 2W B ; Other reaction conditions are consistent with those in Example 1.

[0105] Example 10

[0106] In this embodiment 10, based on the embodiment 1, the sum of the widths of the A1 area and the A2 area on the centerline of the mold cavity is adjusted to 2 / 3 of the width of the B area, that is, W A1 +W A2 =2 / 3W B ; Other reaction conditions are consistent with those in Example 1.

[0107] Example 11

[0108] In this Example 11, based on Example 1, the brands of magnetic powders in areas B, A1, and A2 are modified as follows: area B uses NdFeB magnetic powder with a brand of N40M, and areas A1 and A2 use NdFeB magnetic powder with a brand of N50M; other reaction conditions are the same as in Example 1.

[0109] Comparative Example 1

[0110] The difference between this comparative example and Example 1 is that the mold is not partitioned, and high-performance magnetic powder (NdFeB magnetic powder with the brand name N38UH) is used entirely. Other reaction conditions are the same as those in Example 1.

[0111] Comparative Example 2

[0112] The difference between this comparative example and Example 1 is that the mold is not partitioned, and low-performance magnetic powder (NdFeB magnetic powder with the brand name N33EH) is used entirely. Other reaction conditions are the same as those in Example 1.

[0113] Comparative Example 3

[0114] The difference between this comparative example and Example 1 is that the mold partition is based on Figure 4As shown in (a), the interior of the mold is divided into three areas by a straight plate, and other reaction conditions are the same as those in Example 1.

[0115] Comparative Example 4

[0116] The difference between this comparative example and Example 1 is that the mold partition is based on Figure 4 As shown in (b), the interior of the mold is divided into three areas by an outwardly convex arc plate, and other reaction conditions are the same as those in Example 1.

[0117] Comparative Example 5

[0118] The difference between this comparative example and Example 1 is that copper nitride particles are not added to the low-performance magnetic powder and the high-performance magnetic powder, and other reaction conditions are the same as those in Example 1.

[0119] Comparative Example 6

[0120] The difference between this comparative example and Example 1 is that the amount of copper nitride particles in the low-performance magnetic powder and the high-performance magnetic powder is adjusted to 2.5%, and the other reaction conditions are the same as those in Example 1.

[0121] Comparative Example 7

[0122] The difference between this comparative example and Example 1 is that the outer surface of the non-uniform permanent magnet is not covered with a diamond film, and other reaction conditions are the same as those in Example 1.

[0123] Comparative Example 8

[0124] The difference between this comparative example and Example 1 is that the thickness of the diamond film on the outer surface of the non-uniform permanent magnet is adjusted to 7 μm, and the other reaction conditions are the same as those in Example 1.

[0125] Comparative Example 9

[0126] The difference between this comparative example and Example 1 is that no pre-pressing treatment is performed before compression molding, and other reaction conditions are the same as those in Example 1.

[0127] Comparative Example 10

[0128] The difference between this comparative example and Example 1 is that the sum of the widths of the A1 and A2 areas on the centerline of the mold cavity is adjusted to 1 / 3 of the width of the B area, that is, W A1 +W A2 =1 / 3W B ; Other reaction conditions are consistent with those in Example 1.

[0129] Comparative Example 11

[0130] The difference between this comparative example and Example 1 is that the sum of the widths of the A1 and A2 areas on the centerline of the mold cavity is adjusted to 4 / 3 of the width of the B area, that is, W A1 +W A2=4 / 3W B ; Other reaction conditions are consistent with those in Example 1.

[0131] Comparative Example 12

[0132] The difference between this comparative example and Example 1 is that the high-temperature sintering adopts a traditional sintering operation, specifically: the preheated NdFeB magnet green body is placed in a vacuum sintering furnace and rapidly heated to 1050° C. and sintered for 6 hours. Other reaction conditions are the same as those in Example 1.

[0133] Comparative Example 13

[0134] The difference between this comparative example and Example 1 is that the high-temperature sintering adopts gradient temperature sintering, specifically: the preheated NdFeB magnet green body is placed in a vacuum sintering furnace, first rapidly heated to 1000°C and sintered for 1.5 hours; then rapidly heated to 1050°C and kept warm for 3 hours; then rapidly heated to 1095°C and kept warm for 1.5 hours. Other reaction conditions are the same as those in Example 1.

[0135] A high temperature permanent magnet measuring instrument was used to test the magnetic properties of the permanent magnets obtained in Examples 1 to 11 and Comparative Examples 1 to 13. The magnetic properties of the permanent magnets included the remanence B r (unit: kGs), maximum magnetic energy product (BH) max (unit: MGOe), intrinsic coercivity H cj (unit: kOe), and the permanent magnet was placed under extreme conditions (high temperature of 80°C) for 1 hour, the demagnetization rate at the corners of the permanent magnet was detected, and then its demagnetization situation was analyzed. The results are shown in Table 1.

[0136] The magnet utilization rate and maximum operating temperature of the Halbach arrays assembled in Examples 1 to 11 and Comparative Examples 1 to 13 were tested, and the results are shown in Table 2.

[0137] Table 1

[0138]

[0139] Table 2

[0140]

[0141] As can be seen from the data in Tables 1 and 2, the embodiments of the present invention use two different brands of magnetic powder to produce non-uniform permanent magnets by partitioned molding, which reduces the demagnetization effect at the corners of the magnet under extreme conditions (the demagnetization rate at the corners is less than 4%), increases the local intrinsic coercivity, and enhances the service performance of the magnet; the theoretical maximum operating temperatures of NdFeB magnets N33EH and N38UH are 200°C and 180°C, respectively, and the theoretical maximum operating temperatures of NdFeB magnets N40M and N50M are both 100°C. After improvements in raw materials, pressing, and heat treatment, the present invention has increased the maximum operating temperature to a certain extent.

[0142] A comparison of Example 1 and Comparative Examples 1 to 4 shows that the magnetic properties of the permanent magnet units obtained are as follows: Example 1 ≈ Comparative Example 1 > Comparative Example 3 > Comparative Example 4 > Comparative Example 2. Example 1 uses a partitioning arrangement, placing low-performance magnetic powder in the middle and high-performance magnetic powder on both sides. Ultimately, compared to magnets made entirely with a single brand of magnetic powder (Comparative Examples 1 and 2), the magnetic properties of the high- and low-grade regions in its magnet units do not decrease, but rather improve slightly. Because Example 1 uses two types of high- and low-performance magnetic powders, its cost is lower than that of Comparative Example 1. Regarding the demagnetization rate at corners, Example 1 < Comparative Example 3 < Comparative Example 4 < Comparative Example 1 < Comparative Example 2. This shows that, with respect to demagnetization, magnets made by partitioning with different grades of magnetic powder are superior to magnets made with a single brand. Furthermore, in terms of the partitioning arrangement, the concave arc partitioning arrangement is superior to both flat and convex arc partitioning. In the Halbach array structure assembled by multiple groups of permanent magnet units, in terms of magnet utilization, Example 1 > Comparative Example 3 ≈ Comparative Example 4 > Comparative Example 2 ≈ Comparative Example 1, and in terms of maximum operating temperature, Comparative Example 1 > Example 1 > Comparative Example 3 > Comparative Example 4 > Comparative Example 2. The reason is that Comparative Example 1 uses only high-grade magnetic powder, and its own maximum operating temperature is the highest.

[0143] From the comparison of Examples 1 to 3 and Comparative Examples 5 and 6, it can be seen that as the amount of copper nitride particles increases, the remanence and magnetic energy product of the non-uniform magnet are increased, and the magnetic utilization rate and the maximum operating temperature of the assembled Halbach array are improved; however, when its amount is increased to a certain value, the performance enhancement effect is not significant. The reason is that the addition of an appropriate amount of copper nitride particles reduces the mechanical resistance and friction between the magnetic powder particles, increases the fluidity, and improves the orientation of the compact and the magnet, thereby enhancing the remanence and magnetic energy product of the magnet. However, when its amount is excessive, the iron nitride and neodymium nitride content inside the magnet is too high, which leads to a decrease in the intrinsic coercive force of the magnetic powder, a decrease in the maximum operating temperature, and a decrease in the magnet utilization rate.

[0144] From the comparison of Example 1, Example 4 and Example 5, it can be seen that as the particle size of the copper nitride particles increases, the magnetic properties of the non-uniform magnet first increase and then slightly decrease, and the magnetism and maximum operating temperature of the assembled Halbach array first increase and then decrease. The reason is that one of the functions of the copper nitride particles is to adhere to the irregular surface of the magnetic powder, making its surface more regular and reducing the mechanical interlocking between the particles during subsequent molding. However, when the particle size is too large, a new meshing contact interface is formed, which leads to a decrease in the magnetic properties of the magnet.

[0145] From the comparison of Example 1, Example 6 to Example 7, Comparative Example 7 and Comparative Example 8, it can be seen that as the thickness of the diamond film increases, the remanence and magnetic energy product of the non-uniform magnet increase. However, when its thickness increases to a certain value, although the demagnetization is further improved, the magnetic properties decrease. The reason is that the diamond film can produce a weak magnetoresistance effect, which in turn regulates the overall magnetic field intensity distribution of the magnet, making the magnet performance more uniform, indirectly improving the local enhancement phenomenon, and improving the magnet utilization rate and maximum operating temperature of the assembled Halbach array. However, when its thickness is excessive, the carbon content in the magnet increases and the magnetoresistance effect intensifies, which in turn causes the magnetism to decrease.

[0146] From the comparison among Example 1, Example 8 and Comparative Example 9, it can be seen that when mechanical vibration or ultrasonic pre-pressing is used before molding, the remanence and magnetic energy product of the non-uniform magnet are increased, and the effect of ultrasonic pre-pressing is slightly better than that of mechanical vibration. The reason is that pre-pressing increases the packing density of magnetic powder, suppresses the free rotation of particles, and avoids the orientation degree of the corner area being lower than that of the center area due to the rotation of magnetic powder particles. As a result, the magnet utilization rate and the maximum operating temperature of the assembled Halbach array are improved.

[0147] From the comparison of Example 1, Example 9 and Example 10, Comparative Example 10 and Comparative Example 11, it can be seen that as the width of the A1 area and the A2 area on the centerline area of ​​the mold cavity increases, the magnetism of the edge area of ​​the final magnet increases. However, after increasing to a certain value, the demagnetization effect at the corners is not significantly improved or even decreases. In addition, due to the increase in the area of ​​the A1 area and the A2 area, the cost of high-performance magnetic powder increases. Based on cost considerations, the present invention sets 1 / 2W B ≤W A1 +W A2 ≤W B .

[0148] From the comparison of Example 1, Comparative Example 12 and Comparative Example 13, it can be seen that the performance of the magnet finally obtained by traditional sintering or gradient temperature sintering alone is slightly reduced, and the demagnetization rate at the corners is significantly reduced. The reason is that after conventional sintering heat treatment, the fusion effect of the two magnetic powders inside the magnet is poor, which is prone to quality problems; and the magnet utilization rate and maximum operating temperature of the obtained Halbach array are inferior to those in Example 1.

[0149] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A linear Halbach array based on non-uniform permanent magnets, characterized in that: The linear Halbach array includes (2n+3) groups of non-uniform permanent magnets, where n≥1 and is rounded to an integer; Each set of non-uniform permanent magnets is made of two different grades of NdFeB magnetic powder. During compression molding, the low-grade magnetic powder is located in the middle area B of the mold, and the high-grade magnetic powder is located in the symmetrical arc-shaped concave areas A1 and A2 on both sides of the mold. Among them, the magnetic field direction is set to NS, the regional connection direction of the two groups of high-grade magnetic powder is set to A1-A2, and the NS direction is perpendicular or parallel to the A1-A2 direction; The intrinsic coercivity of the low-grade magnetic powder is lower than the intrinsic coercivity of the high-grade magnetic powder; 1 wt% to 2 wt% of copper nitride nanoparticles are added to low-grade magnetic powder and high-grade magnetic powder, respectively, and then mixed evenly using a high-shear mixer; the particle size of the copper nitride nanoparticles is 80 nm to 120 nm; In the centerline area of ​​the mold cavity, the sum of the widths of zone A1 and zone A2 is not less than 1 / 2 of the width of zone B.

2. A method for preparing a linear Halbach array based on a non-uniform permanent magnet, characterized in that: The preparation method is used to prepare the linear Halbach array based on non-uniform permanent magnets according to claim 1, and the preparation method comprises the following steps: The mold cavity is divided into two symmetrical and arc-shaped concave areas A1 and A2, and a B area located between the two areas. In the centerline area of ​​the mold cavity, the sum of the widths of areas A1 and A2 is not less than 1 / 2 of the width of area B. Area B is loosely packed with low-grade magnetic powder, and areas A1 and A2 are loosely packed with high-grade magnetic powder; The NdFeB compact is obtained by compression molding and magnetization orientation; After demolding, the NdFeB compact is heat treated to obtain a non-uniform permanent magnet; the heat treatment includes preheating, high-temperature sintering and tempering, wherein the high-temperature sintering adopts a gradient sintering process of first increasing the temperature and then decreasing the temperature; According to the topological structure of the Halbach array, (2n+3) groups of the non-uniform permanent magnets are bonded and fixed using magnetic glue to obtain a linear Halbach array, where n is greater than or equal to 1 and is rounded up; Before being loosely packed into the mold, the low-grade magnetic powder and the high-grade magnetic powder are pretreated. Specifically, 1wt%~2wt% of copper nitride nanoparticles are added to the low-grade magnetic powder and the high-grade magnetic powder respectively, and then mixed evenly by a high shear mixer.

3. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 2, characterized in that: The pretreatment further comprises: finally adding 0.2 wt% to 0.5 wt% of solid lubricant powder to the low-grade magnetic powder and the high-grade magnetic powder respectively.

4. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 2, characterized in that: The preparation method further comprises: performing surface treatment on the non-uniform permanent magnet, specifically: depositing a diamond film on the surface of the non-uniform permanent magnet.

5. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 2, characterized in that: The NdFeB compact is obtained by compression molding and magnetization orientation, and the specific operations include: First, mechanical vibration preloading or ultrasonic preloading treatment; Bidirectional pressing is performed by a magnetic field press, with the magnetic field direction NS being perpendicular or parallel to the A1-A2 direction; Finally, the NdFeB green body is produced through cold isostatic pressing.

6. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 2, characterized in that: The preheating temperature is 600℃~800℃ and the time is 0.5h~1h; The specific operation of the gradient sintering is: First, heat the vacuum sintering furnace to 1000℃~1050℃ and keep it warm for 1h~2h; Then heat to 1050℃~1100℃ and keep warm for 2h~3h; Finally, cool down to 900℃~1000℃ and keep warm for 1h~2h.

7. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 3, characterized in that: The particle size of the low-grade magnetic powder and the high-grade magnetic powder is 2 μm to 5 μm.

8. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 4, characterized in that: The diamond film is prepared by a vapor deposition method; the thickness of the diamond film is 2 μm to 5 μm.

9. The method for preparing a linear Halbach array based on a non-uniform permanent magnet according to claim 4, characterized in that: In the centerline area of ​​the mold cavity, the sum of the widths of zone A1 and zone A2 is no greater than the width of zone B.

10. The method for preparing a linear Halbach array based on non-uniform permanent magnets according to claim 5, characterized in that: The magnetic field press uses an elastic pressing head during bidirectional pressing.

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