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

Abstract

The invention provides a linear Halbach array based on a non-uniform permanent magnet and a preparation method of the linear Halbach array, and belongs to the field of permanent magnets. The method comprises the steps that the interior of a mold is divided into an area A1, an area A2 and an area B, wherein the area A1 and the area A2 are symmetrical on the two sides and concave in an arc shape; the B area is loosely filled with low-grade magnetic powder, and the A1 area and the A2 area are loosely filled with high-grade magnetic powder; carrying out compression molding and magnetization orientation to obtain a neodymium iron boron pressed blank; the neodymium-iron-boron pressed blank is subjected to heat treatment after being demolded to obtain the non-uniform permanent magnet, and the heat treatment comprises preheating, gradient sintering of first heating and then cooling, and tempering treatment; and (2n + 3) groups of non-uniform permanent magnets are adhered and fixed by using magnet glue to obtain the linear Halbach array. According to the non-uniform permanent magnet, the non-uniform permanent magnet is manufactured by adopting different marks of magnetic powder partition mold pressing, region strengthening is carried out on the edge, the demagnetization effect of the magnet at the corner angle under the extreme condition is reduced, the local intrinsic coercive force is improved, the service performance of the magnet is enhanced, and the performance of a Halbach array assembled by the non-uniform permanent magnet is promoted to be more excellent.

Description

Technical Field

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

[0002] A maglev train is a new type of transportation vehicle composed of a contactless magnetic force support, magnetic guidance, and linear drive system; maglev uses permanent magnets to repel the track and maintain a suspended state at the midline of the slot, and electromagnetic guidance can achieve zero-friction operation, having the advantage of energy conservation. Neodymium iron boron (Nd-Fe-B) is a rare earth permanent magnet material with extremely high magnetic energy product, intrinsic coercivity, and energy density, and is known as the "magnetic king" due to its excellent magnetic properties, and thus has become one of the key materials for maglev rail transit systems, and is the core component for generating levitation force and stable operation.

[0003] In a maglev rail transit system, it is required that the magnet not only has good stability, but also needs to be combined in a specific array to generate a levitation effect. The Halbach array is a magnet structure that can converge magnetic field lines on one side of the magnet and weaken magnetic field lines on the other side by arranging permanent magnets with different magnetization directions according to a certain rule, enhancing the field strength in the unit direction. The goal is to generate the strongest magnetic field with the least amount of magnets, so it is commonly used as a permanent magnet track for maglev.

[0004] Putting neodymium iron boron magnetic powder into a mold for orientation molding and pressing is an important step in manufacturing high-performance neodymium iron boron magnets. The mold consists of multiple magnetic conductors and non-magnetic bodies, and can achieve multi-pole positioning and orientation. During orientation pressing, the magnetic powder is arranged in a specific direction under the action of a magnetic field to form a pre-pressed part with a certain density and orientation degree.

[0005] However, there is an obvious local strengthening phenomenon in the distribution of magnetic field lines in the space of the unit magnet, and demagnetization is likely to occur in the edge region, which in turn leads to a decrease in the magnetic performance of the Halbach array structure composed of neodymium iron boron magnets, restricting its application. Although using high-performance magnets can reduce the influence of demagnetization in the edge region on the overall magnet, it greatly increases the cost. Summary of the Invention

[0006] In view of this, the purpose of the present invention is 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 art.

[0007] The present invention is implemented as follows: The first aspect of the present invention provides a linear Halbach array based on non-uniform permanent magnets, and the linear Halbach array includes (2n + 3) groups of non-uniform permanent magnets, where n ≥ 1 and is an integer; Each group of non-uniform permanent magnets is made of two different grades of neodymium-iron-boron magnetic powder; when molded, the low-grade magnetic powder is located in the middle area of the mold, and the high-grade magnetic powder is located in the symmetrically-arranged arc-shaped concave areas on both sides of the mold; Among them, the magnetic field direction is set as N-S, and the connection direction of the areas of the two groups of high-grade magnetic powder is set as A1-A2, and the N-S direction is perpendicular or parallel to the A1-A2 direction; The intrinsic coercivity of the low-grade magnetic powder is lower than that of the high-grade magnetic powder.

[0008] The second aspect of the present invention provides a preparation method of a linear Halbach array based on non-uniform permanent magnets, which includes the following steps: The cavity of the mold is divided into symmetrically-arranged arc-shaped concave A1 and A2 areas on both sides and a B area located between the two areas; Low-grade magnetic powder is loosely filled in the B area, and high-grade magnetic powder is loosely filled in the A1 and A2 areas; Molded and magnetized to obtain a neodymium-iron-boron green compact; After the neodymium-iron-boron green compact is demolded, it is heat-treated to obtain a non-uniform permanent magnet; the heat treatment includes preheating, high-temperature sintering and tempering treatment, and the high-temperature sintering adopts gradient sintering of first heating up and then cooling down; According to the topological structure of the Halbach array, (2n + 3) groups of the non-uniform permanent magnets are adhesively fixed using magnet adhesive to obtain a linear Halbach array, where n≥1 and is an integer.

[0009] Preferably, the preparation method further includes: before loosely filling into the mold, pre-treating the low-grade magnetic powder and the high-grade magnetic powder, specifically: adding 1wt% - 2wt% of copper nitride nanoparticles to the low-grade magnetic powder and the high-grade magnetic powder respectively, then mixing evenly through a high-shear mixer, and finally adding 0.2wt% - 0.5wt% of solid lubricant powder respectively.

[0010] Preferably, the preparation method further includes: surface-treating the non-uniform permanent magnet, specifically: depositing a diamond film on the surface of the non-uniform permanent magnet.

[0011] Preferably, the operation of molding and magnetizing to obtain a neodymium-iron-boron green compact specifically includes: First, perform mechanical vibration pre-pressing or ultrasonic pre-pressing treatment; Bidirectionally press through a magnetic field press, and the magnetic field direction N-S is perpendicular or parallel to the A1-A2 direction; Finally, cold isostatic pressing is performed to obtain a neodymium-iron-boron green body.

[0012] Preferably, the temperature of the preheating is 600°C - 800°C, and the time is 0.5h - 1h; The specific operation of the gradient sintering is: First, heat the vacuum sintering furnace to the temperature range of 1000°C to 1050°C and hold for 1 h to 2 h; Then heat it to 1050°C to 1100°C and hold for 2 h to 3 h; Finally, cool it to 900°C to 1000°C and hold for 1 h to 2 h.

[0013] Preferably, the particle sizes of the low-grade magnetic powder and the high-grade magnetic powder are 2.5 μm to 5 μm; the particle size of the copper nitride nanoparticles is 80 nm to 120 nm.

[0014] Preferably, the diamond film is prepared by chemical vapor deposition.

[0015] Preferably, the thickness of the diamond film is 2 μm to 5 μm.

[0016] Preferably, in the central line area of the inner cavity of the mold, 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.

[0017] Preferably, an elastic indenter is used during the two-way pressing of the magnetic field press.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. The non-uniform permanent magnet prepared by the present invention using two different grades of magnetic powder by zone die pressing strengthens the edge region, reduces the demagnetization effect at the edges of the magnet under extreme conditions, improves the local intrinsic coercivity and enhances the service performance of the magnet, and promotes the performance of the Halbach array assembled by it to be more excellent.

[0019] 2. The present invention improves the magnetic powder raw materials and optimizes the die pressing process and heat treatment process, overcomes the technical defects in the die pressing and sintering processes of different grades of magnetic powder, and suppresses the adverse effects caused by the raw materials.

[0020] 3. The present invention adds nanoscale copper nitride (Cu3N) to the magnetic powder, adsorbs it on the surface of the magnetic powder by using a high-shear mixer, promotes it to form a more regular particle shape, increases the fluidity of the magnetic powder, reduces its friction, reduces the mechanical resistance during die pressing, thereby improving the orientation degree of the green compact and the magnet, and further strengthening the remanence, intrinsic coercivity and magnetic energy product of the magnet.

[0021] 4. 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 magnetism of the magnet; and a small amount of Fe-N compounds and Nd-N compounds can be formed at the interface where the copper nitride contacts the magnetic powder after heat treatment as a protective layer, which can improve the high-temperature resistance and anti-corrosion performance of the material.

[0022] 5. Before applying the magnetic field orientation, the present invention rapidly pre-presses through mechanical vibration or ultrasonic treatment to increase the packing density of the magnetic powder, inhibit the free rotation of particles, and avoid the orientation degree in the corner area being lower than that in the central area due to the rotation of magnetic powder particles.

[0023] 6. In the present invention, high-temperature sintering adopts gradient sintering of first heating and then cooling to further inhibit technical defects such as poor internal fusion effect and cracks appearing at the bonding interface.

[0024] 7. 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 anti-corrosion properties of the material while promoting magnetic uniformity. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic diagram of the internal partition of the mold of the present invention; Figure 2 is a schematic structural diagram of existing NdFeB magnetic powder particles and the pretreated NdFeB magnetic powder particles of the present invention; Figure 3 is a schematic topological structure diagram of the linear Halbach array assembled by the present invention; Figure 4 is a schematic diagram of the internal partition of the molds of Comparative Example 3 and Comparative Example 4. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] In order to make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the following further details the present invention in combination with embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] A preparation method of a linear Halbach array based on a non-uniform permanent magnet includes the following steps: S1. Modify the mold As Figure 1 shown, use two groups of arc-shaped plates to divide the cavity of the mold into symmetric A1 and A2 regions with arc-shaped concave on both sides and a B region located between the two regions; Low-grade magnetic powder is located in the middle area of the mold (i.e., the B region), and high-grade magnetic powder is located in the symmetric arc-shaped concave regions on both sides of the mold (i.e., the A1 and A2 regions); In specific implementation, the intrinsic coercivity of the low-grade magnetic powder is lower than that of the high-grade magnetic powder; At the same time, in the middle line area of the inner cavity of the mold, the sum of the widths of the A1 and A2 regions is not greater than the width of the B region and not less than 1 / 2 of the width of the B region, 1 / 2W B ≤W A1 +W A2 ≤W BThe regional size distribution limits of Area A1 and Area A2 ensure both the overall magnetic properties and effectively improve the local strengthening and edge demagnetization conditions; The preparation of sintered NdFeB magnets mainly includes: batching, melting, rapid solidification casting, powder making, magnetic powder pressing and forming, sintering + tempering, etc. Among them, powder making includes making coarse powder by hydrogen breaking method + making fine powder by jet mill. Hydrogen breaking utilizes the phase change and volume change during the hydrogen absorption - hydrogen desorption process of the rapid solidification sheet to generate internal stress, causing it to spontaneously break. The jet mill uses high - pressure gas to accelerate the coarse powder to supersonic speed and achieves the purpose of crushing particles through high - speed collision between particles; Magnetic powder forming refers to orienting and densifying the fine powder under an external magnetic field. Among them, die pressing is a widely used forming method.

[0028] When the present invention is die - pressed and formed, the mold is divided into regions, and the overall performance of the magnetic block, especially the intrinsic coercivity of the edge region, is improved by the mutual diffusion of magnetic powders with different grades and different magnetic properties; In addition, compared with plane contact, the contact surfaces between Area B, Area A1, and Area A2 in the present invention are concave arc regions, and the mutual diffusion effect is better.

[0029] S2. Raw material pretreatment Due to the differences in the green body shrinkage of magnetic powders with different grades during the subsequent pressing and heat treatment processes and the inconsistent working temperatures, to solve this problem, the present invention applies pretreatment to low - grade magnetic powder and high - grade magnetic powder. Specifically: add 1wt% - 2wt% of copper nitride nanoparticles to the low - grade magnetic powder and high - grade magnetic powder respectively, then mix evenly through a high - shear mixer, and finally add 0.2wt% - 0.5wt% of solid lubricant powder respectively; In specific implementation, the particle size of the low - grade magnetic powder and high - grade magnetic powder is 2μm - 5μm; the particle size of the copper nitride nanoparticles is 80nm - 120nm.

[0030] Loosely load the pretreated low - grade magnetic powder in Area B, and loosely load the pretreated high - grade magnetic powder in Area A1 and Area A2; The low - grade magnetic powder and high - grade magnetic powder can use two kinds of magnetic powders with relatively small magnetic property differences or relatively large differences. The specific grade selection can be made according to actual needs; The die pressing process of NdFeB powder is a process in which powder particles are rearranged, extruded and deformed under the pressing action of a die, continuously reducing the particle gap, and finally forming a green body with a certain density. This process is achieved through the contact, collision and extrusion between particles. During this process, the interaction and movement behavior of particles will directly affect the orientation degree of the magnet, and thus affect the grade performance. The orientation of magnetic powder particles is divided into the magnetization stage and the movement stage of particles. An important stage among them is the overall movement of particles during die pressing. During the overall movement stage, the orientation is affected by four forces: (1) static magnetic torque; (2) agglomeration force caused by the static magnetic interaction and adsorption force between particles, which is related to the surface field strength and particle size of particles; (3) frictional force generated by particle contact; (4) mechanical resistance caused by the irregular shape of particles. During die pressing, factors such as the interaction between particles, contact friction and shape effect will produce a hindering effect. Sintered NdFeB magnets have a polycrystalline structure. Although the particle size of the magnetic powder obtained by hydrogen breaking + jet milling is small, its microstructure is actually an irregular shape, rather than a regular spherical or three-dimensional shape; because the edges of the magnetic powder particles are actually irregular and have obvious edges and corners, as shown in (a) of Figure 2 During the forming process, mechanical interlocking and arching phenomena will occur between particles, and the friction coefficient increases, which will lead to the agglomeration of magnetic powder particles.

[0031] 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, promoting the formation of a more regular particle shape, as shown in (b) of Figure 2 Increasing the fluidity of the magnetic powder, reducing its friction, reducing the mechanical resistance during die pressing, thereby improving the orientation degree of the green body and the magnet, and further strengthening the remanence and magnetic energy product of the magnet; (2) Enriching copper elements on the surface of the magnetic powder, reducing the melting point of the grain boundary, and indirectly improving the magnetism of the magnet; (3) During the subsequent heat treatment process, at high temperature, a small amount of Fe-N compounds and Nd-N compounds can be formed at the interface between copper nitride and the magnetic powder as a protective layer, which can improve the high-temperature resistance and anti-corrosion performance of the material.

[0032] Compared with directly adding copper nitride in the batching stage, the present invention adds copper nitride particles after melting and powder making, which can avoid all the copper nitride cracking into copper and nitrogen during the melting stage. Nitrogen reacts with iron, neodymium, etc. to further form iron nitride and neodymium nitride. Excessive content of iron nitride and neodymium nitride will lead to a decrease in the intrinsic coercivity of the magnetic powder. When the present invention performs heat treatment and curing after die pressing, due to the density of the magnet and the influence of the copper-iron-neodymium alloy phase, the degree of thermal decomposition of copper nitride is greatly reduced, and the positive effect of copper nitride on the performance of the magnetic powder is retained to the greatest extent, and its negative impact on the magnetic properties of the magnetic powder is reduced.

[0033] 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.

[0034] In a specific implementation, other auxiliary agents permitted in the art, such as antioxidants, release agents, etc., may be added to the magnetic powder, which will not be elaborated in detail here.

[0035] S3, Non-uniform permanent magnet forming 1. Molding and magnetization orientation to obtain NdFeB green billet; specifically: (1) Pre-press by mechanical vibration or ultrasonic pre-pressing; In the initial stage, the magnetic powder particles are in a loose stacking 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 cause the orientation degree of the corner area to be lower than that of the center area, destroying the orientation of the magnetic powder and aggravating the magnetic inhomogeneity of the edge area of ​​the magnet. The present invention uses mechanical vibration or ultrasonic treatment to quickly pre-press before applying magnetic field orientation, thereby increasing the packing density of magnetic powder and inhibiting the free rotation of particles. In addition, compared with conventional pressure head pressing, the mechanical vibration or ultrasonic pre-pressing is faster and the distribution of particles in the mold is more uniform; 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 mixed, avoiding sudden changes in the magnetic properties of the contact interface due to different magnetic powders.

[0036] (2) Bidirectional pressing by a magnetic field press, with the magnetic field direction NS being perpendicular or parallel to the A1-A2 direction; In the specific implementation, the pressing molding adopts the molding method allowed in the art, such as firstly pressing the green compact by vertical and horizontal bidirectional pressing at a pressure of 15MPa~25MPa, and using a magnetic field with a magnetic field strength of 1.5T~5T for orientation treatment during the pressing process, and then further compacting the green compact by cold isostatic pressing at 170MPa~200MPa for 20s~120s to obtain the NdFeB green compact. The above molding parameters can be adjusted according to the actual situation and are not specifically limited here; In order to further reduce the phenomenon of local stress concentration, which leads to uneven orientation of 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 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 to ensure that the magnetic powder is subjected to uniform pressure in all parts, thereby improving the quality and magnetic properties of the magnet.

[0037] 2. Heat treatment of NdFeB green body after demoulding; In the field of preparation of sintered neodymium iron boron, heat treatment adopts preheating, high-temperature sintering and at least two-stage tempering. In specific implementation, since different grades of magnetic powder are used for molding in the present invention, due to the differences in their physical and chemical properties, the conventional high-temperature sintering is prone to poor internal fusion effect, and even cracks appear at the bonding interface. To solve this technical problem, gradient sintering of heating first and then cooling is adopted in high-temperature sintering in the present invention, and the tempering treatment can adopt the conventional operation in this field; During the heat treatment process, the preheating temperature is 600°C to 800°C, and the time is 0.5h to 1h; The specific operation of gradient sintering is as follows: First, heat the vacuum sintering furnace to the interval of 1000°C to 1050°C and keep it warm for 1h to 2h; then heat it to 1050°C to 1100°C and keep it warm for 2h to 3h; finally, cool it to 900°C to 1000°C and keep it warm for 1h to 2h; the above sintering temperature, time, and number of segmentation times can be adjusted according to actual needs; the heating and cooling rates during this process can not be specifically limited, generally 1°C / s to 5°C / s; The tempering treatment is: After the first-stage tempering heat treatment at 800°C to 920°C for 1h - 5h and the second-stage tempering heat treatment at 450°C to 600°C for 1h - 4h, a non-uniform permanent magnet is obtained; the above tempering treatment parameters can be adjusted according to the actual situation and are not specifically limited here; 3. Deposit a diamond film on the outer surface of the non-uniform permanent magnet for surface treatment to achieve surface treatment; In specific implementation, the diamond film is prepared by methods such as microwave plasma chemical vapor deposition or magnetron sputtering physical vapor deposition, and covers the outer surface of the non-uniform permanent magnet, and its thickness is about 2μm to 5μm; The functions of the diamond film are reflected in the following aspects: (1) The diamond film can produce a weak magnetoresistance effect, and then play a role in regulating the magnetic field strength distribution of the whole magnet, making the magnet performance more uniform and indirectly improving the local strengthening phenomenon; (2) Enhance the mechanical properties of the magnet, such as strength, hardness, etc.; (3) The diamond film can play a corrosion-resistant role; In specific implementation, the thickness of the diamond film is limited to 2μm to 5μm, for example, it can be 2μm, 3μm, 5μm; but it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable. The reason for the thickness limitation is: to ensure the effective protection of the magnet by the diamond film; at the same time, to avoid too high carbon content in the magnet and too large magnetoresistance effect due to too large thickness, which will have a negative impact on the magnetic properties.

[0038] S4. Assemble the Halbach array Such as Figure 3As shown, according to the topological structure of the Halbach array, 2n + 3 groups of the non-uniform permanent magnets are adhesively fixed using magnet glue to obtain a linear Halbach array, where n ≥ 1 and is an integer.

[0039] Example 1 A preparation method of a linear Halbach array based on non-uniform permanent magnets is as follows: Step 1: Weigh neodymium iron boron magnetic powders of N33EH and N38UH, respectively add 1 wt% of copper nitride nanoparticles (D 50 = 80 nm), then mix them evenly through a high-shear mixer, and finally add 0.2 wt% of stearic acid powder respectively to obtain low-grade magnetic powder and high-grade magnetic powder; As Figure 1 shown, use two groups of arc-shaped plates to divide the cavity of the mold into symmetric A1 and A2 areas with concave arcs on both sides and a B area located between the two areas; in the middle line area of the inner cavity of the mold, the sum of the widths of the A1 and A2 areas is basically equal to the width of the B area, that is, W A1 + W A2 = W B ; Loosely fill the low-grade magnetic powder N38UH in the B area, and loosely fill the high-grade magnetic powder N33EH in the A1 and A2 areas, that is, high-performance (high intrinsic coercivity) magnetic powder is distributed at the edges, and low-performance (low intrinsic coercivity) magnetic powder is distributed in the middle area; Step 2: First, perform mechanical vibration pre-pressing treatment to make the filling density in the mold about 3.6 g / cm 3 ; Use an elastic indenter to obtain a green compact through vertical and horizontal double pressing at 10 MPa, and perform orientation treatment using a magnetic field with a magnetic field intensity of 2.5 T during the pressing process, and then further compact the green compact through a cold isostatic pressing method at 100 MPa for 60 s to obtain a neodymium iron boron green compact; Step 3: Place the neodymium iron boron magnet green compact in a vacuum sintering furnace, first quickly heat up to 600 °C and preheat for 1 h; then quickly heat up to 1000 °C and hold for 1.5 h; then quickly heat up to 1095 °C and hold for 3 h; then quickly cool down to 950 °C and hold for 1.5 h; then successively perform a first-stage tempering heat treatment at 900 °C for 3 h and a second-stage tempering heat treatment at 480 °C for 3 h, and obtain non-uniform permanent magnets after cooling; Use microwave plasma chemical vapor deposition method to cover a diamond film with a thickness of about 2 μm on the outer surface of the non-uniform permanent magnet; Step 4: Take five groups of non-uniform permanent magnets prepared in the above steps, and according to the Figure 3 arrangement shown, use magnet glue to assemble the non-uniform permanent magnets into a linear Halbach array.

[0040] Example 2 In this Example 2, based on Example 1, the dosage of copper nitride nanoparticles is adjusted to 1.5 wt%, and other reaction conditions are the same as those in Example 1.

[0041] Example 3 Based on Example 1, in this Example 3, the dosage of copper nitride nanoparticles is adjusted to 2 wt%, and other reaction conditions are the same as those in Example 1.

[0042] Example 4 Based on Example 1, in this Example 4, the particle size of copper nitride nanoparticles is adjusted to 100 nm, and other reaction conditions are the same as those in Example 1.

[0043] Example 5 Based on Example 1, in this Example 5, the particle size of copper nitride nanoparticles is adjusted to 120 nm, and other reaction conditions are the same as those in Example 1.

[0044] Example 6 Based on Example 1, in this Example 6, the thickness of the diamond film is adjusted to 3.5 μm, and other reaction conditions are the same as those in Example 1.

[0045] Example 7 Based on Example 1, in this Example 7, the thickness of the diamond film is adjusted to 5 μm, and other reaction conditions are the same as those in Example 1.

[0046] Example 8 Based on Example 1, in this Example 8, the pre-pressing treatment in Step 2 is adjusted to ultrasonic pre-pressing, and other reaction conditions are the same as those in Example 1.

[0047] Example 9 Based on Example 1, in this Example 9, the sum of the widths of Area A1 and Area A2 in the midline region of the inner cavity of the mold is adjusted to half of the width of Area B, that is, W A1 +W A2 =1 / 2W B ; other reaction conditions are the same as those in Example 1.

[0048] Example 10 Based on Example 1, in this Example 10, the sum of the widths of Area A1 and Area A2 in the midline region of the inner cavity of the mold is adjusted to 2 / 3 of the width of Area B, that is, W A1 +W A2 =2 / 3W B ; other reaction conditions are the same as those in Example 1.

[0049] Example 11 Based on Example 1, in this Example 11, the grades of the magnetic powder in Area B, Area A1, and Area A2 are modified as follows: Area B uses neodymium iron boron magnetic powder with grade N40M, and Area A1 and Area A2 use neodymium iron boron magnetic powder with grade N50M; other reaction conditions are the same as those in Example 1.

[0050] Comparative Example 1 The difference between this comparative example and Example 1 is that the mold is not partitioned, and all high-performance magnetic powder (neodymium iron boron magnetic powder with the grade of N38UH) is used, and other reaction conditions are the same as those in Example 1.

[0051] Comparative Example 2 The difference between this comparative example and Example 1 is that the mold is not partitioned, and all low-performance magnetic powder (neodymium iron boron magnetic powder with the grade of N33EH) is used, and other reaction conditions are the same as those in Example 1.

[0052] Comparative Example 3 The difference between this comparative example and Example 1 is that: the mold is partitioned as shown in (a) of Figure 4 , and the interior of the mold is divided into three regions by a straight plate, and other reaction conditions are the same as those in Example 1.

[0053] Comparative Example 4 The difference between this comparative example and Example 1 is that: the mold is partitioned as shown in (b) of Figure 4 , and the interior of the mold is divided into three regions by a convex arc plate, and other reaction conditions are the same as those in Example 1.

[0054] Comparative Example 5 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.

[0055] Comparative Example 6 The difference between this comparative example and Example 1 is that the dosage of copper nitride particles in the low-performance magnetic powder and the high-performance magnetic powder is adjusted to 2.5 wt%, and other reaction conditions are the same as those in Example 1.

[0056] Comparative Example 7 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.

[0057] Comparative Example 8 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 other reaction conditions are the same as those in Example 1.

[0058] Comparative Example 9 The difference between this comparative example and Example 1 is that: pre-pressing treatment is not carried out before molding by pressing, and other reaction conditions are the same as those in Example 1.

[0059] Comparative Example 10 The difference between this comparative example and Example 1 is that: the sum of the widths of Region A1 and Region A2 in the midline region of the inner cavity of the mold is adjusted to 1 / 3 of the width of Region B, that is, W A1 +W A2 =1 / 3WA ; Other reaction conditions are the same as those in Example 1.

[0060] Comparative Example 11 The difference between this comparative example and Example 1 is that the sum of the widths of Area A1 and Area A2 in the midline area of the inner cavity of the mold is adjusted to 4 / 3 of the width of Area B, i.e., W A1 +W A2 = 4 / 3W A ; Other reaction conditions are the same as those in Example 1.

[0061] Comparative Example 12 The difference between this comparative example and Example 1 is that traditional sintering operation is adopted for high-temperature sintering. Specifically, the preheated NdFeB magnet green body is placed in a vacuum sintering furnace and rapidly heated to 1050 °C for sintering for 6 h. Other reaction conditions are the same as those in Example 1.

[0062] Comparative Example 13 The difference between this comparative example and Example 1 is that gradient heating sintering is adopted for high-temperature sintering. Specifically, the preheated NdFeB magnet green body is placed in a vacuum sintering furnace. First, it is rapidly heated to 1000 °C for sintering for 1.5 h; then it is rapidly heated to 1050 °C and held for 3 h; subsequently, it is rapidly heated to 1095 °C and held for 1.5 h. Other reaction conditions are the same as those in Example 1.

[0063] The permanent magnets prepared in Examples 1 to 11 and Comparative Examples 1 to 13 were subjected to magnetic property tests using a high-temperature permanent magnet measuring instrument. The magnetic property indexes of the permanent magnets include remanence B r (unit: kGs), maximum magnetic energy product (BH) max (unit: MGOe), intrinsic coercivity H cj (unit: kOe). And the permanent magnet was treated under extreme conditions (80 °C high temperature) for 1 h, and the demagnetization rate at the corners of the permanent magnet was detected to analyze its demagnetization situation. The results are shown in Table 1.

[0064] The magnet utilization rate and the highest working temperature of the Halbach arrays assembled with Examples 1 to 11 and Comparative Examples 1 to 13 were tested. The results are shown in Table 2.

[0065] Table 1

[0066] Table 2

[0067] As can be seen from the data in Table 1 and Table 2, the non-uniform permanent magnets prepared by the embodiment of the present invention using two different grades of magnetic powder for partition molding can reduce the demagnetization effect at the edges and corners under extreme conditions (the demagnetization rate at the edges and corners is lower than 4%), improve the local intrinsic coercivity and enhance the service performance of the magnets; the theoretical maximum working temperatures of the NdFeB magnets of N33EH and N38UH are 200°C and 180°C respectively, and the theoretical maximum working temperatures of the NdFeB magnets of N40M and N50M are both 100°C. After the improvement of raw materials, pressing and heat treatment in the present invention, the maximum working temperature is increased to a certain extent.

[0068] As can be seen from the comparison of Example 1, Comparative Example 1 to Comparative Example 4, in terms of the magnetic properties of the prepared permanent magnet units, Example 1≈Comparative Example 1>Comparative Example 3>Comparative Example 4>Comparative Example 2; Example 1 adopts a partition setting, with low-performance magnetic powder placed in the middle and high-performance magnetic powder placed on both sides. Finally, compared with the magnets prepared using a single grade of magnetic powder (Comparative Example 1 and Comparative Example 2), the magnetic properties of the high and low grade regions in the magnet units do not decrease but increase slightly. Since Example 1 uses two kinds of magnetic powder with high and low performance, its cost is lower than that of Comparative Example 1. In terms of the demagnetization rate at the edges and corners, Example 1<Comparative Example 3<Comparative Example 4<Comparative Example 1<Comparative Example 2. It can be seen from this that for the demagnetization situation, the magnet prepared by partitioning and filling different grades of magnetic powder is superior to the magnet of a single grade. In terms of the partition method, the concave arc partition setting is superior to the plane partition and the convex arc partition. In the Halbach array structure assembled by multiple permanent magnet units, in terms of the magnet utilization rate, Example 1>Comparative Example 3≈Comparative Example 4>Comparative Example 2≈Comparative Example 1. In terms of the highest working temperature, Comparative Example 1>Example 1>Comparative Example 3>Comparative Example 4>Comparative Example 2. The reason is that Comparative Example 1 uses all high-grade magnetic powder, and its own highest working temperature is the highest.

[0069] As can be seen from the comparison of Example 1 to Example 3, Comparative Example 5 and Comparative Example 6, as the amount of copper nitride particles increases, the properties such as the remanence and magnetic energy product of the non-uniform magnet increase, and the magnetic utilization rate and the highest working temperature of the assembled Halbach array are both improved; however, when the amount increases to a certain value, the increase effect of the performance is not significant. The reason is that adding an appropriate amount of copper nitride particles reduces the mechanical resistance and friction between the magnetic powder particles, increases the fluidity, improves the orientation degree of the green compact and the magnet, and then strengthens the remanence and magnetic energy product of the magnet. However, when the amount is excessive, the content of iron nitride and neodymium nitride in the magnet is too high, which leads to a decrease in the intrinsic coercivity of the magnetic powder, a decrease in the highest working temperature and a decrease in the magnet utilization rate.

[0070] From the comparison of Example 1 and Example 4 to Example 5, it can be seen that as the particle size of 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 copper nitride particles is to adhere to the irregular surface of magnetic powder, making its surface more regular and reducing the mechanical interlaced meshing between particles during subsequent molding, but 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.

[0071] 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.

[0072] By comparing Example 1, Example 8 and Comparative Example 9, it can be seen that by using mechanical vibration or ultrasonic pre-pressing before molding, the remanence and magnetic energy product of the non-uniform magnet are increased, and the ultrasonic pre-pressing effect is slightly better than mechanical vibration. The reason is that pre-pressing increases the packing density of the magnetic powder, inhibits the free rotation of the particles, and avoids the orientation degree of the corner area being lower than that of the center area due to the rotation of the magnetic powder particles. The magnet utilization rate and the maximum operating temperature of the assembled Halbach array are improved.

[0073] It can be seen from the comparison between Example 1, Example 9 and Example 10, and Comparative Example 10 and Comparative Example 11 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, but when it increases to a certain value, the demagnetization effect at the corners is not significantly improved or even decreases, and due to the increase in the area of ​​the A1 area and the A2 area, the cost of the high-performance magnetic powder increases. Based on cost considerations, the present invention sets 1 / 2W B ≤W A1 +W A2 ≤W B .

[0074] From the comparison between 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 only is slightly reduced, and the demagnetization rate at the edges and corners is significantly reduced. The reason is that after conventional sintering heat treatment, the fusion effect of the two kinds of magnetic powder 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.

[0075] The above-described embodiments merely represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the invention patent of the present invention shall be subject to the appended claims.

Claims

1. 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 an integer; each group of non-uniform permanent magnets is made of two different grades of neodymium iron boron magnetic powder; when molded, the low-grade magnetic powder is located in the middle area of the mold, and the high-grade magnetic powder is located in the symmetrically concave arc areas on both sides of the mold; wherein, the magnetic field direction is set as N-S, and the connection direction of the areas of the two groups of high-grade magnetic powder is set as A1-A2, and the N-S direction is perpendicular or parallel to the A1-A2 direction; the intrinsic coercivity of the low-grade magnetic powder is lower than that of the high-grade magnetic powder.

2. Preparation method of a linear Halbach array based on non-uniform permanent magnets, 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 includes the following steps: The cavity of the mold is divided into symmetrically concave arc-shaped A1 area and A2 area on both sides and B area located between the two areas; Loosely load low-grade magnetic powder in area B, and loosely load high-grade magnetic powder in areas A1 and A2; Mold and magnetize and orient to obtain a neodymium iron boron green compact; After the neodymium iron boron green compact is demolded, it is heat-treated to obtain non-uniform permanent magnets; the heat treatment includes preheating, high-temperature sintering and tempering treatment, and the high-temperature sintering adopts gradient sintering of first heating and then cooling; According to the topological structure of the Halbach array, use magnet adhesive to bond and fix (2n + 3) groups of the non-uniform permanent magnets to obtain a linear Halbach array, where n ≥ 1 and is an integer.

3. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 2, wherein The preparation method also includes: before loosely loading into the mold, pre-treat the low-grade magnetic powder and the high-grade magnetic powder, specifically: add 1wt% - 2wt% of copper nitride nanoparticles to the low-grade magnetic powder and the high-grade magnetic powder respectively, then mix evenly by a high-shear mixer, and finally add 0.2wt% - 0.5wt% of solid lubricant powder respectively.

4. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 2, wherein, The preparation method also includes: perform surface treatment on the non-uniform permanent magnets, specifically: deposit diamond film on the surface of the non-uniform permanent magnets.

5. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 2, wherein The operation of molding and magnetizing and orienting to obtain a neodymium iron boron green compact specifically includes: First, perform mechanical vibration pre-pressing or ultrasonic pre-pressing treatment; Bidirectionally press by a magnetic field press, and the magnetic field direction N-S is perpendicular or parallel to the A1-A2 direction; Finally, obtain a neodymium iron boron green compact through cold isostatic pressing.

6. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 2, wherein The temperature of the preheating is 600°C - 800°C, and the time is 0.5h - 1h; The specific operation of the gradient sintering is: First, heat the vacuum sintering furnace to 1000°C - 1050°C and keep it warm for 1h - 2h; Then heat it to 1050°C - 1100°C and keep it warm for 2h - 3h; Finally, cool it to 900°C - 1000°C and keep it warm for 1h - 2h.

7. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 3, wherein The particle size of the low-grade magnetic powder and the high-grade magnetic powder is 2μm - 5μm; The particle size of the copper nitride nanoparticles is 80nm - 120nm.

8. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 4, wherein, The diamond film is prepared by chemical vapor deposition; the thickness of the diamond film is 2μm - 5μm.

9. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 4, wherein, In the middle line 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.

10. The preparation method of the linear Halbach array based on non-uniform permanent magnets according to claim 5, characterized in that Elastic heads are used during the bidirectional pressing by the magnetic field press.

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