Neodymium multilayer sintered magnet

Abstract

To provide a neodymium laminated sintered magnet that has high magnetic properties and excellent electrical resistance.SOLUTION: A plurality of unit neodymium sintered magnets are stacked together, and each of the unit neodymium sintered magnets is joined to each other via a bonding layer to be integrated, the bonding layer includes a rare earth-rich region, moreover, the rare earth-rich region has electrical insulation properties. The thickness of this bonding layer is preferably 1.0 μm or more and 200 μm or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a neodymium sintered magnet, and more particularly to a neodymium sintered magnet used in large motors such as the main motor of an electric vehicle or a generator.

Background Art

[0002] The neodymium sintered magnet was invented by the inventors of the present application in 1982 (Japanese Patent Publication No. 61-34242). It not only far exceeds the magnetic properties of the samarium cobalt magnet, which was the representative of high-performance permanent magnet materials until then, but is also inexpensive because it is mainly composed of resources-rich raw materials such as neodymium (Nd: a kind of rare earth element), iron, and boron. It has steadily expanded its market as an ideal permanent magnet material. Its applications cover a wide range, including computer HDDs (hard disk drives), motors for driving magnetic heads (VCM: voice coil motors), high-end speakers, headphones, electric-assist bicycles, golf carts, permanent magnet magnetic resonance diagnostic devices (MRI), etc. Furthermore, in recent years, it has been rapidly put into practical use in the main motors of hybrid electric vehicles (HEV) and electric vehicles (EV), energy-saving and low-noise large home appliances (coolers and refrigerators), elevators, and other industrial motors.

[0003] Generally, neodymium sintered magnets have high magnetic properties but have the drawback of poor temperature characteristics. In particular, the temperature characteristics of the coercive force are important. When used in home appliances and industrial motors, it is impossible to avoid the temperature rise caused by the coil current. In addition, since a demagnetizing field acts from the motor armature, if the temperature rises and the coercive force decreases, irreversible demagnetization occurs in the permanent magnet. In order to prevent irreversible demagnetization, it is necessary to increase the coercive force in advance.

[0004] After the discovery of neodymium sintered magnets, various methods have been used to improve properties such as coercivity, including the addition of alloying elements (Patent No. 1606420, etc.), heat treatment (Patent No. 1818977, etc.), and crystal grain size control (Patent No. 1662257, etc.). However, the most effective method for improving coercivity was the addition of heavy rare earth elements (Dy, Tb) (Patent No. 1802487). While using large amounts of heavy rare earth elements certainly increases coercivity, it also reduces saturation magnetization and decreases the maximum energy product. Furthermore, Dy and Tb are scarce and expensive resources, making it difficult to meet the large and expected demand for HEVs, EVs, and industrial and household motors.

[0005] Subsequently, as described in Patent Document 1 below, a method was discovered to increase coercivity without causing a significant decrease in saturation magnetization or maximum energy product. This method involves creating a sintered body, coating the outside with metals or compounds such as Dy / Tb, and then diffusing the heavy rare earth elements of Dy / Tb into the grain boundaries within the sintered body through heat treatment.

[0006] On the other hand, for magnets used in HEVs and EVs, neodymium sintered magnets have been proposed in which the magnet is divided and stacked to reduce losses due to eddy current generation during operation, suppress heat generation in the magnet, and reduce the temperature rise of the magnet. In one example of this proposal, as described in Patent Document 2 below, a neodymium sintered magnet with a thickness of 5 mm is coated with Dy fluoride powder, heated in Ar at 900°C for 1 hour to perform grain boundary diffusion treatment (GBD treatment), and then 18 of these divided magnets are stacked and inserted into the magnet insertion holes of the motor rotor and solidified with epoxy resin. An IPM motor equipped with a rotor manufactured in this way has been proposed.

[0007] As the electrification of automobiles continues to advance, there is a need for neodymium sintered magnets used in main motors to maintain the best magnetic properties while reducing costs to the absolute minimum, and to reduce the amount of Dy and Tb contained in the magnets to the extent that resource-wise permissible. However, currently, neodymium magnets used in EVs and HEVs contain a certain amount of Dy and Tb, and the reduction in manufacturing costs is not sufficient. One reason for this is that the reduction of Dy and Tb in neodymium sintered magnets for EVs and HEVs is not sufficient, as mentioned above.

[0008] Furthermore, the silicon steel sheets used as the core material in motors are punched out into a predetermined shape with a thickness of 0.5 mm or less and laminated to reduce overcurrent losses that occur during motor operation. It would be desirable if the Nd-Fe-B sintered magnets used in the same motor could also be punched out thinly to minimize eddy current losses, but this is not possible. Therefore, neodymium sintered magnets used in the main motors of electric vehicles are used as segmented magnets with a thickness of approximately 5 mm, as in the aforementioned known example (Patent Document 2). Since 5 mm thick magnets are cut from large blocks of magnets (called block magnets) by machining, processing costs and costs due to reduced material yield are incurred. Moreover, the eddy current loss reduction effect is insufficient with laminates of approximately 5 mm thickness. While a thickness of 5 mm for a unit neodymium sintered magnet reduces eddy current losses compared to a single block of magnet, it is still not sufficiently reduced. It is expected that using magnets made by thinning unit neodymium sintered magnets to less than half the thickness of 5 mm, i.e., 2.5 mm or less, and then laminating them, would significantly reduce the eddy current losses generated in the motor. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 4450239 [Patent Document 2] Japanese Patent Publication No. 2011-78268 [Overview of the project] [Problems that the invention aims to solve]

[0010] This invention has been made in view of the above circumstances, and aims to provide a neodymium multilayer sintered magnet having high magnetic properties and excellent high electrical resistance. [Means for solving the problem]

[0011] The neodymium stacked sintered magnet of the present invention is formed by stacking multiple unit neodymium sintered magnets, and by joining each unit neodymium sintered magnet to each other via a bonding layer, The bonding layer of the neodymium multilayer sintered magnet is characterized in that it includes a rare earth-rich region, and that the rare earth-rich region has electrical insulating properties. In this specification, each individual thin-plate magnet forming a laminated magnet will be referred to as a unit neodymium sintered magnet. Furthermore, joining here does not mean bonding the individual unit neodymium magnets together using adhesives or the like, but rather joining and integrating them metallurgically by methods such as hot pressing. Therefore, in the joining method according to the present invention, no organic substances such as adhesives are present in the joining layer. Furthermore, in this specification, "rare earth-rich region" refers to a region where rare earth elements such as Nd, Pr, Dy, and Tb are present in high concentrations (50% by weight or more), and is an electrically insulating junction region composed of compounds such as oxides, fluorides, oxyfluorides, and carbides. Here, electrical insulation refers to the specific electrical resistivity of a neodymium sintered magnet, which is 1.4 x 10⁻⁶. -6 The value was set to be at least two orders of magnitude larger than Ω·m. This is because a difference of two orders of magnitude or more in resistivity reduces the current flowing through it by two orders of magnitude or more, thereby significantly reducing eddy current losses. In the neodymium multilayer magnet of the present invention, an insulating effect is achieved if the proportion (volume) of the "rare earth-rich region" in the bonding layer is 20% or more. However, the proportion of this rare earth-rich region is preferably 50% or more, more preferably 70% or more, even more preferably 80% or more, and ideally 90% or more. This proportion can be confirmed by elemental analysis of the bonding layer cross-section using EDS (energy-dispersive X-ray spectroscopy).

[0012] Furthermore, the present invention relates to a neodymium multilayer sintered magnet having the above-mentioned features, characterized in that the thickness of the bonding layer is 1.0 μm or more and 200 μm or less, preferably 1.0 μm or more and 100 μm or less, more preferably 1.0 μm or more and 50 μm or less, and particularly preferably an average thickness of 2.0 μm or more and 30 μm or less. The upper limit of the bonding layer thickness is set at 200 μm because exceeding this limit reduces the effective volume ratio of the sintered magnet portion, and the residual magnetic flux density Br decreases significantly.

[0013] Furthermore, the present invention is characterized in that, in a neodymium multilayer sintered magnet having the above features, the rare earth-rich region is continuously and / or intermittently present in the bonding layer. That is, the bonding layer of the neodymium multilayer sintered magnet of the present invention is a phase that includes continuous and / or intermittent rare earth-rich regions.

[0014] Furthermore, the present invention is characterized in that, in a neodymium multilayer sintered magnet having the above features, there is an Fe-rich metal region containing 50% by weight or more of Fe in the bonding layer, and the Fe-rich metal region connects adjacent unit neodymium sintered magnets via the bonding layer. In this specification, the "Fe-rich metal region" refers to a phase in which Fe is present in large quantities (50% by weight or more) and compounds of rare earth elements such as Nd, Pr, Dy, and Tb are substantially absent.

[0015] Furthermore, the present invention is characterized in that, in a neodymium multilayer sintered magnet having the above features, the difference between the weight percentage of the total amount of rare earth elements in the surface region less than 50 μm from the surface of the neodymium multilayer sintered magnet and the weight percentage of the total amount of rare earth elements in the internal region 50 μm or more from the surface is less than 1 weight percent. This weight percentage difference is preferably 0.8 weight percent or less, more preferably 0.5 weight percent or less, and particularly preferably 0.3 weight percent or less.

[0016] Furthermore, in the neodymium laminated sintered magnet having the above characteristics, at least one of Dy element and Tb element as heavy rare earth elements is present in the rare earth-rich region existing in the bonding layer, and the heavy rare earth element is Nd2Fe in each unit neodymium sintered magnet. 14 It is also characterized in that it diffuses (GBD diffusion) from each bonding layer toward the adjacent bonding layer side along the grain boundary phase of the Nd2FeB crystal.

[0017] Furthermore, in the neodymium laminated sintered magnet having the above characteristics, the sum of the maximum magnetic energy product (BH)max (MGOe) and the coercive force iHc (kOe) represented in the CGS unit system (hereinafter, this may be referred to as the magic number "MN") is 70 or more, and it is characterized by having magnetic properties.

[0018] Furthermore, the present invention is a neodymium laminated sintered magnet having the above characteristics, and the outer surface of the neodymium laminated sintered magnet orthogonal to the bonding layer has a flatness of 0.1 mm or less on at least one pair of opposing outer surfaces or all outer surfaces.

Effects of the Invention

[0019] The neodymium laminated sintered magnet according to the present invention has high magnetic properties and excellent high electrical resistivity, can be manufactured at a cost comparable to the conventional one, has the world's highest magnetic properties and sufficient electrical insulation for automotive applications, and is applicable to magnets for EVs and HEVs that are expected to experience explosive growth in the future.

Brief Description of the Drawings

[0020] [Figure 1] It is a diagram showing a manufacturing method of the neodymium laminated sintered magnet according to the present invention. [Figure 2] It is a diagram showing a state when a laminate of unit neodymium sintered magnets is set inside the apparatus together with a mold and a punch. [Figure 3] It is a photograph showing the appearance of a cylindrical sample after a deformation test. [Figure 4]This is a photograph showing the appearance of the hot press machine used for the joining test. [Figure 5] This is a cross-sectional view of the apparatus used for the bonding test. [Figure 6] These are photographs showing the condition of a unit neodymium sintered magnet before and after the bonding test. [Figure 7] This is an SEM image of the bonding surface of the neodymium multilayer sintered magnet according to Example 1. [Figure 8] This is an EDS image of the region (rare earth-rich area) on the bonding surface of the neodymium multilayer sintered magnet according to Example 1, where rare earth compounds are continuously distributed. [Figure 9] This is an EDS image of the area where the Fe-rich metal region is distributed across the bonding surface of the neodymium multilayer sintered magnet according to Example 1. [Figure 10] Figure 10a shows a jig for applying GBD paste to the surface of a unit neodymium sintered magnet, Figure 10b shows the process of applying the GBD paste, and Figure 10c shows a cassette with the unit neodymium sintered magnets stacked after the GBD paste has been applied. [Figure 11] These are photographs of neodymium multilayer sintered magnets according to Example 1 and Example 2. [Figure 12] This is a photograph of a damaged neodymium multilayer sintered magnet according to Comparative Example 1. [Figure 13] This figure shows an overview of a C-type yoke and measurement system for eddy current loss evaluation. [Figure 14] This is a photograph of an eddy current evaluation sample, in which the surface is covered with insulating tape and a search coil is wound on a surface perpendicular to the direction of magnetic field generation, for the purpose of evaluating eddy currents. [Figure 15] This is a photograph of a C-type coil with an evaluation sample set inside. [Figure 16] This graph shows the eddy current evaluation results up to a frequency of 200 Hz when excited with a 5 Arms current. [Figure 17] This graph shows the eddy current evaluation results up to a frequency of 500 Hz when excited with a 2 Arms current. [Figure 18]This is a photograph showing the appearance of a neodymium multilayer sintered magnet fabricated using a unit neodymium sintered magnet produced by the NPLP method (New-PressLess Process) according to Example 3. [Figure 19] This is a photograph showing the appearance of the convex lens-shaped neodymium multilayer sintered magnet fabricated in Example 7. [Modes for carrying out the invention]

[0021] First, the inventors considered the ideal method for manufacturing neodymium multilayer sintered magnets in order to obtain neodymium multilayer sintered magnets that have high magnetic properties and excellent high electrical resistance. The model considered is shown in Figure 1. First, a laminate of unit neodymium sintered magnets made of neodymium sintered bodies is prepared. Here, there are methods for manufacturing the unit neodymium sintered magnets, such as cutting them from a larger sintered body or manufacturing them so that the shape of the unit neodymium sintered magnet itself after sintering. This laminate is placed inside a mold with a limited volume space, specifically a space that matches the shape of the product, and by heating to a high temperature and applying pressure, the unit neodymium sintered magnets are joined together, and at the same time, the laminate is deformed to the inner wall of the mold in the direction of pressure and in two directions perpendicular to that direction. As a result, the neodymium laminated sintered magnet body can be made to occupy virtually 100% of the space limited to the shape of the product, so there is no remaining space and the magnetic properties of this neodymium laminated sintered magnet can achieve the same high residual magnetic flux density as a normal one-piece magnet of the same shape. Whether or not deformation is possible, whether or not joining is possible, and whether or not the low melting point Nd-rich phase dissolves and welds to the mold will determine the feasibility of this study model, but the inventors have established this manufacturing method and, in performance evaluation, have made it possible to deliver a completely net-shape, processing-free, low-cost product to the world that has the best magnetic properties.

[0022] The first key point to consider is whether a unit neodymium sintered magnet can be deformed under high temperature and high pressure. This is because the size of the main phase crystal grains constituting a unit neodymium sintered magnet is generally 10 μm or less, averaging around 5 μm, and its abundance in the magnet structure is 90% or more, and its main phase crystal is tetragonal Nd2Fe. 14 Since it is a type B intermetallic compound, the main phase itself is unlikely to undergo much plastic deformation. However, there is a possibility of crystal slippage of the main phase via the second phase, a low-melting-point Nd-rich phase, but there have been no reports of plastic deformation being possible at grain sizes of 1 μm or larger. Figure 2 shows the details of the experiment conducted to determine this. Table 1 shows the composition of the unit neodymium sintered magnet used.

[0023] [Table 1]

[0024] The preparation of the unit neodymium sintered magnet was carried out as follows. First, an SC alloy having the components shown in Table 1 was prepared, and after hydrogen pulverization, it was jet-milled in a high-pressure nitrogen gas atmosphere of 0.6 MPa. The average particle size (D50) can be adjusted by adjusting the raw material supply rate to the jet mill grinding device and changing the classification conditions of the fine powder after grinding, and in this experiment, raw material powder with an average particle size of 3 μm was used. This powder was packed into a carbon mold at a packing density of 3.6 g / cc, and after aligning the c-axis direction of the powder crystal grains in one direction by applying a 4 T pulsed magnetic field, it was set in a sintering furnace and 10 -4 A block-shaped magnet with dimensions of approximately 50 mm x 70 mm x 20 mm thickness (orientation direction) was obtained by sintering at a temperature of 985°C for 4 hours under a vacuum atmosphere of less than Pa. The resulting sintered body was processed into a cubic shape of 7 mm x 7 mm x 7 mm (orientation direction), and its magnetic properties were evaluated.

[0025] The results showed Br=14.1kG, iHc=16.1kOe, (BH)max=47.7MGOe, and magnetic orientation degree Br / 4πMs=0.965. From this block magnet, a cylindrical neodymium sintered magnet with a diameter of φ15mm and a thickness of 6mm (in the orientation direction) was prepared by processing. This was placed inside a cylindrical carbon mold with an inner diameter of φ20mm, and carbon punches slightly smaller than the mold's internal dimensions were attached to the top and bottom. The entire assembly was then loaded into a Spark Plasma Sintering (SPS) device LABOX-325R manufactured by Sinterland Co., Ltd. (maximum pressing force 30kN, maximum pulse current output 2.5kA). A carbon sheet approximately 0.2mm thick was placed between the inner wall of the mold and the contact surface between the punch and the sample. The atmosphere during the SPS process was a vacuum of approximately 20Pa, which could be reached by a rotary pump. In an SPS (Scaling Process) device, a large current is passed through the mold and workpiece via upper and lower punches, heating them with Joule heating while applying pressure through the upper and lower punches to perform processing. In this experiment, a thermocouple was inserted into a hole in the upper punch, and the temperature of the upper part of the sample was measured and controlled.

[0026] Table 2 shows the experimental conditions and results obtained, and Figure 3 shows a photograph of the sample after the experiment. In this experiment, the heating rate from room temperature to the SPS processing temperature was fixed at 25°C / min, and the displacement in the thickness direction (pressure application direction) of the sample was measured when the applied pressure was varied from the initial pressure at the start of the experiment (the same as the maximum applied pressure) to 40 MPa. In addition, the presence or absence of elution of the low-melting-point Nd-rich phase from the side of the sample after the experiment was also shown. As can be seen from Figure 3, no defects such as cracks or fissures were observed in the sintered body itself in any case of this experiment.

[0027] [Table 2]

[0028] This experiment revealed the following: First, neodymium sintered bodies can be deformed without cracking or chipping. Second, at a maximum temperature of 800°C, a displacement of over 7% can be achieved even at a pressure of 10 MPa. On the other hand, at 700°C, 100°C lower, only a displacement of 3.81% was obtained even with a pressure of 20 MPa. Further increasing the pressure to 30 MPa and 40 MPa increased the displacement to 4.54% and 5.13%, respectively, but it did not reach the displacement of over 7% achieved at 800°C and 10 MPa. In other words, it was found that the magnitude of displacement is more strongly influenced by the temperature factor than by the pressure. Furthermore, regarding the elution of the low-melting-point Nd-rich phase from the sample side, the liquid phase eluted in experiments No. 1 and No. 2 (see Figure 3), but no elution was observed in experiments No. 3 and No. 4, where the temperature was the same as 700°C but the applied pressure was 30 MPa or higher. In other words, the presence or absence of elution of the low-melting-point Nd-rich phase is determined not only by temperature but also by pressure conditions, and we were able to find that the higher the pressure, the less likely the liquid phase is to elute. Generally, it is thought that the liquid phase component is more likely to escape at higher pressures, so this result was unexpected, and we are currently investigating the mechanism behind it. In any case, the fact that elution of the low-melting-point Nd-rich phase does not occur during the deformation of the sintered body means that the risk of damage to the sample and mold due to a reaction between the eluted low-melting-point Nd-rich liquid phase and the mold when the sintered body reaches the inner wall of the mold during deformation can be reduced. From the above, we have found the possibility of deforming neodymium sintered bodies without causing elution of the low-melting-point Nd-rich phase by appropriately selecting the temperature and pressure conditions.

[0029] Next, we investigated the feasibility of joining the unit sintered magnet stack, which is the second key point of evaluation. Figure 4 is a photograph showing the appearance of the hot press experimental apparatus used, and Figure 5 is a diagram showing the cross-sectional configuration of the apparatus. The sample space provided in the center of the apparatus in Figure 5 is φ15mm x length 10mm. The unit neodymium sintered magnets used in this experiment were cut from the same sintered body block as those used in the previous experiment, to a size of φ15mm x 1.5mm (orientation direction). Eight of these were stacked, and the feasibility of joining them was investigated. The experiment was conducted in an Ar atmosphere, heated to a temperature of 850°C before applying pressure, and then subjected to pressure up to 40MPa using a hydraulic press (Figure 6). As a result of this experiment, the stack of eight unit neodymium sintered magnets was firmly joined and integrated. When the joint strength was evaluated, it was found to be comparable to that of a block-shaped neodymium sintered magnet.

[0030] These experimental results showed that we were able to clear both the first and second criteria mentioned above, and we are now on track to realize the initially envisioned processing-free neodymium multilayer sintered magnet. Based on the findings obtained from the above experiments, the inventors completed the present invention and were able to manufacture the neodymium multilayer sintered magnet of the present invention.

[0031] The following describes the neodymium laminated sintered magnet of the present invention, which is formed by stacking multiple unit neodymium sintered magnets and bonding, integrating, and deforming them at a high temperature (for example, 700 to 950°C). First, a unit neodymium sintered magnet is prepared. This is done by coarse grinding, fine grinding, molding, orientation, and sintering an SC alloy prepared according to a conventional method to have a predetermined composition. However, in this invention, both methods can be employed: one is to cut a neodymium sintered magnet body larger than the unit neodymium sintered magnet shape to the predetermined size; the other is to obtain a unit neodymium sintered magnet of that shape and size in a sintered state using the NPLP method (Patent No. 6280137) developed by the inventors of this application. Various thicknesses of the unit neodymium sintered magnet are possible; the thinner the magnet, the more effective it is in reducing eddy current loss. However, from the standpoint of productivity and cost, considering material loss and processing costs when using processing, and from the standpoint of powder packing and productivity when using the NPLP method, a thickness of 0.5 mm to 3 mm is appropriate for the unit neodymium sintered magnet, and more preferably, 1 mm to 2.5 mm. This is because a thickness of less than 0.5 mm makes manufacturing difficult, and a thickness exceeding 3 mm results in insufficient insulation. Furthermore, when obtaining a unit neodymium sintered magnet by processing such as cutting from a block-shaped neodymium sintered magnet body, it is preferable to use the same thickness as described above.

[0032] The average particle size (D50) of the fine powder is preferably 10 μm or less from the viewpoint of coercivity properties, and preferably 1 μm or more from the viewpoint of pulverability, but more preferably 5 μm or less, and 2 μm or more is appropriate. Tetragonal Nd2Fe is formed by applying a magnetic field after powder molding. 14 The magnetic properties of the sintered body are improved by aligning the c-axis orientation of fine powder, mainly B-type crystals, but the degree of c-axis crystal orientation must be 90% or higher. If it is less than 90%, the magnetic properties, especially the residual magnetic flux density Br, will decrease, and it will not be able to supply sufficient magnetic force when applied to EV motors, etc. Furthermore, this crystal orientation direction must be parallel and unidirectional to the main surface of the unit neodymium sintered magnet, that is, the plane perpendicular to the stacking direction of the neodymium multilayer sintered magnet. This is because eddy currents generated in the direction that crosses the stacking plane of the neodymium multilayer sintered magnet are interrupted by a high electrical resistance layer made of an electrically insulating compound (oxide, fluoride, carbide, etc.) formed on the stacking plane, and eddy current losses can be greatly reduced.

[0033] Next, a laminate is constructed by stacking multiple units of the prepared unit neodymium sintered magnets. The number of layers in the laminate is determined by the balance between the thickness of the unit neodymium sintered magnets and the final product dimensions. For example, if the thickness of the unit neodymium sintered magnets is 2 mm and the size of the final product in the stacking direction is 30 mm, a simple calculation shows that at least 15 unit neodymium sintered magnets need to be stacked. However, considering deformation in two directions perpendicular to the direction of pressure, 16, 17, or even more layers may be required. The number of layers can be determined by dividing the product volume calculated from the final product shape by the volume of the unit neodymium sintered magnets.

[0034] When stacking a predetermined number of unit neodymium sintered magnets, inserting adhesives such as silicone grease, fluorides containing heavy rare earth elements such as Dy and Tb, oxides and carbides, fluorides such as LiF and CaF, oxides such as SiO2 and Al2O3, carbonates such as CaCO3, hydroxides such as Ca(OH)2, or porcelain powder of BaTiO3 between the stacks as needed is effective in improving production efficiency and electrical insulation. In particular, inserting compounds containing heavy rare earth elements such as Dy and Tb is extremely effective in improving the magnetic properties of neodymium stacked sintered magnets, especially their coercivity.

[0035] The prepared neodymium sintered magnet stack is then set into a mold designed to match the shape and size of the product. The mold material can be selected from carbon, heat-resistant alloys, etc., and coating its surface with an anti-welding material such as BN is also effective in improving the mold's lifespan. This stack, mold, and punch that applies pressure to the stack from above and below are set in a device that allows for atmosphere control, such as a hot press (HP) device or a spark plasma sintering (SPS) device, which can heat to a high temperature of up to about 1000°C while applying high pressure. In this invention, studies were mainly conducted using an SPS device, but it goes without saying that the essence of the results obtained is the same with an HP device. The optimal bonding deformation conditions need to be adjusted according to the raw material alloy composition, but it is preferable that the heating temperature be 700°C or higher and below the sintering temperature of the sintered body, and the applied pressure be 25 MPa or higher and less than 100 MPa. At temperatures below 700°C, the deformation rate of the sintered body is slow, and cracks occur in the bonded body, resulting in low productivity. Furthermore, above the sintering temperature, the dissolution of the low-melting-point Nd-rich phase is significant, leading to severe welding between the laminate and the mold, resulting in heavy wear of both the laminate and the mold, as well as significant changes in the magnetic component and deterioration of magnetic properties. Below a pressure of 25 MPa, the dissolution of the low-melting-point Nd-rich phase, as revealed in the possibility verification stage above, occurs, causing welding with the mold. At pressures of 100 MPa or higher, the equipment capable of applying such high pressures becomes enormous and unsuitable for mass production.

[0036] The neodymium multilayer sintered magnets fabricated in this way have oxide or fluoride films formed on the bonding surfaces, blocking the flow of current across the bonding layers. When fluorides or oxides made of heavy rare earth elements such as Dy or Tb are sandwiched between the layers, additional heat treatment after the bonding deformation process induces grain boundary diffusion (GBD), significantly improving the coercivity. Similar to GBD from the surface to the interior of a normal sintered body, the GBD treatment temperature is generally between 800°C and 950°C, and the GBD treatment time is between 5 and 20 hours, as is well known. The atmosphere during GBD treatment can be a vacuum. If the GBD treatment temperature is below 800°C, the improvement in coercivity is insufficient, and if it is above 950°C, it causes a decrease in Br. Also, if the heat treatment time is less than 5 hours, the improvement in coercivity is insufficient, and if it exceeds 20 hours, productivity deteriorates.

[0037] The neodymium laminated sintered magnet according to the present invention has a characteristic structure resulting from its special manufacturing process. Here, the following tests were conducted to clarify its structural characteristics. First, a sintered body having the components listed in Table 1 was prepared, and from this, circular sintered bodies with a diameter of φ14.5 mm and a thickness of 1.3 mm (in the orientation direction) were prepared. Four sets of laminates with a total thickness of 3.9 mm were constructed by stacking three of these. A mixed paste of oxides and fluorides shown in Table 3 was divided into two equal parts and applied between two layers of the laminate. At that time, an appropriate amount of liquid paraffin was mixed in to form a slurry to facilitate the application of these powders between layers. The amount applied was set so that the amount of Tb was 0.5 wt% of the weight of the sintered body. In Test 3, the elemental ratio of Tb:Nd was set to 1:1, and in Test 4, the amount of Tb from TbF3 and Tb4O7 was weighed so that the amount of Tb was equal and their total was 0.5 wt%.

[0038] [Table 3]

[0039] This three-layer laminate was loaded into the cylindrical mold shown in Figure 2, and bonded and deformed using the same SPS apparatus LABOX-325R as above, under conditions of a maximum temperature of 700°C and a pressure of 50 MPa. The three laminates were completely bonded, no elution of the Nd-rich phase from the surroundings was observed, and the thickness dimension was reduced from 3.9 mm to 3.6 mm or 3.7 mm, resulting in a deformation rate of approximately 6%. After this integrated laminated sintered body underwent GBD heat treatment at 890°C for 20 hours in a vacuum, it was cut perpendicular to the thickness direction of the sample, and the cross-sectional structure including the bonding surface (cross-section of the bonding layer) was observed. The measuring instruments used were a HITACHI SU3500 scanning electron microscope (SEM) and a HORIBA EMAX ENERGY EX-250 energy-dispersive X-ray analyzer (EDS).

[0040] Figure 7 shows SEM compositional images taken at magnifications of 100x, 500x, 1000x, and 2000x. Here, regions with a high concentration of heavier elements appear white, while regions with a high concentration of lighter elements appear black. Therefore, whitish regions indicate areas with a high concentration of rare earth elements, and blackish regions indicate areas with a relatively low concentration of rare earth elements. In Figure 7, whitish regions can be seen forming along the bonding surface, indicating the presence of a high concentration of rare earth elements. The average thickness measured at 15 points (3 points in each of the 5 fields of view) is shown in Table 3. As shown in Table 3, the average thickness of the bonding surface (bonding layer) was slightly less than 4 μm for fluoride-coated materials such as DyF3 and TbF3, and 18.5 μm for oxide mixtures of Tb4O7 and Nd2O3, which were the thickest.

[0041] Next, Figures 8 and 9 show the elemental mapping results obtained by EDS measurement at a magnification of 1000x on the cross-section of the neodymium multilayer sintered magnet fabricated in Experiment 4. In most of the junction surface (rare earth-rich region), as shown in Figure 8, Nd, Pr, O, and F are continuously present along the junction surface, and Fe is absent here. On the other hand, in the region shown in Figure 9 (Fe-rich metal region present in part of the junction surface), Fe is abundant, while Nd, Pr, O, and F are almost absent. In other words, although the junction surface appears to have a continuous structure with a thickness of 8.60 μm morphologically, it was found that it is actually composed of a region of rare earth oxides and rare earth fluorides (rare earth-rich region) and an Fe-rich region. The above observations were observed not only in Test 4 but also in Tests 1 through 3, indicating a structure unique to neodymium multilayer sintered magnets. In other words, rare earth oxides and fluorides have high electrical resistance, thus improving the total electrical resistivity of the bonding surface. At the same time, the Fe-rich regions strongly connect adjacent unit neodymium sintered magnets in a bridge-like manner across the bonding layer, thus ensuring the high strength of the neodymium multilayer sintered magnets. [Examples]

[0042] The following describes specific embodiments of the neodymium multilayer sintered magnet of the present invention in detail with reference to examples and comparative examples, but the content of the present invention is not limited thereto. [Examples 1 and 2]

[0043] SC alloys with the compositions shown in Table 4 below were prepared. After hydrogenation, 0.05 wt% methyl caprylate was mixed in, and the mixture was kneaded and coarsely ground using a Piccolo agitator. The coarsely ground raw material was jet-milled under a nitrogen atmosphere using a Hosokawa Micron Corporation MJT-LAB jet mill. The grinding pressure was 0.6 MPa, and the average particle size D50 was set to approximately 3 μm by adjusting the rotation speed of the classification rotor. 0.07 wt% methyl laurate was mixed with the resulting fine powder raw material and kneaded in a Piccolo agitator to obtain the raw material before sintering. This was packed into a carbon container at a packing density of 3.6 g / cc, and the powder particles were oriented by applying a pulsed magnetic field of up to 4 T multiple times. This oriented body, along with the carbon mold, was placed in a sintering furnace and sintered in a vacuum at 1030°C for 4 hours to obtain a block-shaped sintered body. The following results were obtained using a 7mm cube-shaped evaluation sample cut from this material and evaluated with a pulse BH tracer PBH-1000 manufactured by Nippon Denji Sokki Co., Ltd.: the residual magnetic flux density was Br = 13.8 kG and the coercivity iHc = 19.9 kOe.

[0044] [Table 4]

[0045] Furthermore, a predetermined number of unit neodymium sintered magnets with a target size of 15.24 mm x 4.24 mm (orientation direction) x 1.8 mm were prepared by processing from the aforementioned block-shaped sintered body. Separately, a material to be coated between the layers was prepared. Its components are those shown in Test 1 and Test 4 in Table 3. In this case, the number of layers of unit neodymium laminated sintered magnets was set to 24. The internal shape of the mold into which this laminate was loaded was set to the target product size, specifically 15.5 (±0.1) mm x 5.0 (±0.05) mm (orientation direction), and the size in the pressing direction was made longer than the target product size to allow the upper and lower punches to be set.

[0046] Subsequently, two types of 24-layer laminates were prepared by applying coating materials for Test 1 and Test 4 between the layers. These will be designated as Example 1 and Example 2, respectively. An overview of the preparation method is shown in Figure 10. First, prepare the coating jig shown in 10a). This jig has through holes slightly smaller than the dimensions of the unit neodymium sintered magnet. The thickness of these through holes is designed so that a predetermined coating amount is achieved when the coating material is uniformly filled into the holes. As shown in 10b), fill the through holes with the coating material and scrape the top surface with a spatula to adjust the coating amount to the calculated value. After that, remove the jig and take out the unit neodymium sintered magnets coated with the coating material and stack them. At this time, preparing a laminate holding jig as shown in 10c) is convenient for transporting the laminates. Of course, the coating jig is not limited to this. The point is that it should be able to apply a predetermined amount of coating material, be easily removed, and be stacked.

[0047] Next, the 24-piece laminates from each of Example 1 and Example 2, prepared in this manner, were set into carbon molds for SPS manufactured to match the above product dimensions. The molds were split to facilitate sample removal after SPS processing. The laminates were set into these split molds while aligning the upper and lower punches. Since a split mold is used, a die is also required to hold the split mold. The assembled unit neodymium sintered magnet laminates, upper and lower punches, split mold, and die were set in an SPS apparatus (LABOX-325R) manufactured by Sinterland Co., Ltd., and a K-type thermocouple for temperature measurement was attached to the upper punch. After evacuating the SPS apparatus to a vacuum of approximately 20 Pa using a rotary pump, a large current of several hundred amperes was passed through the upper and lower punches and the apparatus was heated until the thermocouple reading reached 850°C. After the temperature reached 850°C, a pressure of 65 MPa was applied to the unit neodymium sintered magnet laminates using the upper and lower punches to perform bonding deformation. In this experiment, the laminates used in Example 1 and Example 2 were different in terms of coating material, but the SPS heating and pressurizing conditions were the same. The results are shown in Figure 11, and the final dimensions are shown in Table 5.

[0048] [Table 5]

[0049] Thus, the dimensions and shapes of these two neodymium multilayer sintered magnets are almost identical, and they are indistinguishable in appearance. When the flatness of the two opposing pairs of outer surfaces parallel to the direction of pressure (i.e., the two pairs of outer surfaces from which the layering state can be seen) was evaluated with a surface roughness meter with a sensor head diameter of 0.05 mm, it was confirmed that the value fell within a range of plus or minus 0.05 mm from the center value. This is because the layering misalignment that inevitably occurs during the layering stage is corrected by the deformation of each unit neodymium sintered magnet during the SPS process, which presses it against the inner surface of the mold. Therefore, in this invention, the outer surfaces are flat even without machining such as polishing or grinding. As a result, the size of the neodymium multilayer sintered magnet is the same as the dimensions defined on the inner surface of the mold, and its density can be estimated to be almost 100%. This is how the above-mentioned high Br characteristics are obtained.

[0050] The neodymium multilayer sintered magnets after the SPS process were subjected to a GBD treatment in a vacuum at 890°C for 20 hours as a post-processing step. The magnetic properties of the neodymium multilayer sintered magnets obtained from Example 1 and Example 2 were evaluated without further processing and at their original size, as shown in Table 6. The measuring device used in this study was the TM-VSM70100-SMS high-sensitivity vibration sample type magnetometer manufactured by Tamagawa Seisakusho Co., Ltd., and its characteristics were evaluated by applying a magnetic field of up to 8T using a superconducting magnet coil.

[0051] [Table 6]

[0052] From the above results, it was found that the neodymium multilayer sintered magnet according to this embodiment has very high magnetic properties, and although the type of Tb compound laminated between layers differs between Example 1 and Example 2, equivalent coercivity can be obtained by adjusting the Tb content to be the same.

[0053] [Comparative Example 1] Figure 12 shows the results of SPS treatment performed under the same conditions as in Example 1, except that the temperature was changed to 945°C and the applied pressure to 30 MPa. The liquid phase eluted and reacted with the split mold, and cracks occurred on the upper and central parts of the sample, spanning the joint surface. This is because the temperature range was exceeded and the applied pressure was low, causing the low-melting-point Nd-rich phase to elute, come into contact with the split mold, react, and cracks to occur during cooling.

[0054] In Comparative Example 1, the heating temperature and pressure conditions were inappropriate, resulting in significant elution of the low-melting-point Nd-rich grain boundary phase, suggesting a possible alteration of the magnetic composition. Since this elution occurred from within the neodymium multilayer sintered magnet toward the inner wall of the mold, it is possible that there are differences in composition depending on the location within the neodymium multilayer sintered magnet. Therefore, we investigated whether there were differences in composition between the surface region (less than 50 μm from the magnet surface near the inner wall) and the central part of the magnet (a region more than 50 μm from the surface) using SEM-EDX at a low magnification of approximately 50x. Table 7 shows the results regarding the main elements compared to Example 1.

[0055] [Table 7]

[0056] From these results, it can be seen that in Comparative Example 1, the total amount of rare earth elements in the components near the surface was about 34 wt%, while the amount in the interior was less, about 30 wt%, a difference of about 4 wt%. For confirmation, a similar analysis was performed on Example 1, but no large difference in the total amount of rare earth elements between the surface and the interior, as in Comparative Example 1, was observed (the difference was 0.11 wt%). This can be said to be a characteristic of the neodymium multilayer sintered magnet according to the present invention, in which the dissolution of the Nd-rich grain boundary phase is controlled.

[0057] For the neodymium multilayer sintered magnet used in Comparative Example 1, the largest extractable sample size (15.5 mm x 5.0 mm x 20.0 mm) was subjected to an additional heat treatment at 890°C for 20 hours in a vacuum to obtain its magnetic properties. The results are shown in Table 8.

[0058] [Table 8]

[0059] Table 6 shows that the neodymium multilayer sintered magnets in the examples obtained extremely high magnetic properties, and the Magic Number (hereinafter referred to as MN), which is the sum of coercivity iHc and maximum magnetic energy product (BH)max, was 77 or higher, achieving the world's best performance. On the other hand, in the sample of Comparative Example 1, in which the low-melting-point Nd-rich phase dissolved and was damaged, although the Br properties were high, the iHc properties and prismatic properties deteriorated, which is thought to be due to the decrease in rare earth element content, resulting in an MN of just under 69 (Table 8), and only ordinary magnetic properties obtainable by ordinary methods.

[0060] [Examples 3 and 4] Using SC raw materials of composition A shown in Table 1, and using the coating materials from Test 1 and Test 4 in Table 3, a laminate of unit neodymium sintered magnets was prepared to have a Tb content of 0.5 wt%. Neodymium laminate sintered magnets were then fabricated under the same conditions as in Examples 1 and 3, except that the SPS conditions were a heating temperature of 700°C and an SPS pressure of 65 MPa. Their magnetic properties are shown in Table 9.

[0061] [Table 9]

[0062] Similar to Examples 1 and 2, the magnetic properties were extremely high. Cross-sectional observation of the bonding surface revealed the characteristics of the bonding surface shown in Figures 8 and 9, respectively. In Example 3, the average thickness of the bonding layer was 3.71 μm, and in Example 4, it was 8.60 μm. In both examples, the Fe-rich metal phase region shown in Figure 9 and the rare earth oxide and rare earth fluoride phases shown in Figures 8 and 9 were observed. Elemental analysis of the bonding layer cross-section by EDS showed that the volume ratio of the rare earth-rich region in the bonding layer of the neodymium multilayer sintered magnet in Example 3 was approximately 20%, and the volume ratio of the rare earth-rich region in the bonding layer of the neodymium multilayer sintered magnet in Example 4 was approximately 80%. Eddy current losses were measured for neodymium laminated sintered magnets according to Examples 3 and 4, which have these different rare earth compound layers. As shown in Table 10, continuous neodymium sintered magnets, fabricated from block magnets to nearly identical shapes, and resin-bonded magnets, fabricated by laminating individual neodymium sintered magnets with resin bonding to achieve nearly identical shapes, were also measured simultaneously. This allowed for a comparison of the degree of eddy current loss in the neodymium laminated sintered magnets according to the present invention between the case without lamination and the case with complete insulation and resin bonding.

[0063] [Table 10]

[0064] The measurement system used for this measurement is shown in Figure 13. First, a C-shaped yoke made of a laminated iron core with a circular cross-section was prepared, and a 9 mm gap was provided in it for setting the laminated sample. A coil for exciting this C-shaped yoke was attached around the yoke on the opposite side of the gap, and a search coil with 50 turns was attached to the sample to be measured, as shown in Figure 14, perpendicular to the direction of the magnetic field generated by the C-shaped yoke, i.e., the orientation direction. A large alternating current was passed through the C-shaped yoke, so it was fixed in place as shown in Figure 15 for the measurement. The measurement results are shown in Figures 16 and 17. Figure 16 shows the frequency dependence of eddy current loss up to 200 Hz at a current of 5 A. From this, the expected result was obtained that the resin-bonded sample had the smallest eddy current loss and the continuum sample had the largest. Furthermore, it was found that the eddy current loss for a high-resistivity bonding layer with an average thickness of 3.71 μm was almost the same as that of the continuum, while the loss level was similar for an average thickness of 8.60 μm. On the other hand, Figure 17 shows the results for a smaller current value of 2A and a higher frequency of 500Hz. Here, the overall trend is the same as in Figure 16, but it can be seen that a reduction in loss is observed even with an average thickness of 3.71μm for the continuous sample. Generally, as the frequency increases, the distance to which the magnetic field penetrates, the so-called skin depth, decreases, so it is expected that the magnetic field division effect will increase even with a small high-resistivity film thickness, and the reduction in eddy current loss will become apparent. In other words, it is thought that as the excitation frequency increases, even a high electrical resistance film thickness of 3.71μm will be effective in reducing eddy current loss.

[0065] [Examples 5 and 6] In this example, ten unit neodymium sintered magnets having composition C as shown in Table 11 were fabricated using the NPLP method (New-PressLess Process, Japanese Patent No. 6280137, WO2016 / 047593) previously devised by the inventors, and bonding and deforming tests were performed using an SPS apparatus. The weight, size, etc. of the unit neodymium sintered magnets fabricated by the NPLP method are shown in Table 12.

[0066] [Table 11]

[0067] [Table 12]

[0068] The weight and size of each individual neodymium sintered magnet are not identical; there are variations in both weight and size. However, by laminating these magnets while applying the coating material from Test 2 in Table 3 between layers, and performing SPS treatment at a heating temperature of 850°C and a pressure of 60 MPa, a rectangular parallelepiped neodymium laminated sintered magnet with a width of 15.91 mm, an orientation direction thickness of 7.61 mm, and a pressure direction thickness of 17.05 mm was fabricated. Furthermore, the magnetic properties were evaluated by performing GBD treatment at 875°C for 16 hours in a vacuum (Example 5). Figure 18 shows a photograph of the neodymium laminated sintered magnet from Example 5, and Table 13 shows the obtained magnetic properties. Also, Table 13 shows the magnetic properties when the coating material from Test 5 in Table 3 was used when laminating 10 unit sintered magnets with similar weight and dimensional variations as in Table 12 (Example 6). The flatness of the opposing surfaces pressed against the mold in each embodiment was evaluated using the same measurement method as described above, and the maximum was 0.1 mm, which was almost the same as when a single neodymium stacked magnet was prepared by cutting.

[0069] [Table 13]

[0070] Thus, we obtained neodymium multilayer sintered magnets that possess high Br (Br) while also exhibiting world-class magnetic properties, including a coercivity of approximately 20 kOe or higher and MN (Magnitude) values ​​of 77.8 and 74.0, regardless of whether a Tb compound or a Dy compound was used as the coating material. This demonstrates that the method for preparing the unit neodymium sintered magnets, whether by NPLP (Non-Particle Printing) or processing, can be any cost-effective method, and that the magnetic properties themselves can be improved by appropriately selecting the GBD (Magnetic Band Decay) coating material sandwiched between the layers.

[0071] In previous examples, we have shown that high coercivity can be obtained by performing GBD treatment after SPS processing, and the reason for this is as follows: Although the GBD coating material is sandwiched between the layers when stacking unit neodymium sintered magnets, no disadvantages to the GBD treatment process due to the subsequent SPS treatment have been observed. Rather, the diffusion distance of heavy rare earth elements such as Dy and Tb associated with the GBD treatment is short, at most about 2.5 mm, i.e., about the thickness of the unit neodymium sintered magnet, so uniform magnetic properties can be obtained regardless of the size of the product shape.

[0072] [Example 7] A lens-shaped neodymium sintered body with a thickness of 2 mm having the above composition B was prepared, 22 of these were stacked, and the coating material from Test 3 was applied between the stacks. A photograph of the neodymium stacked sintered magnet obtained by the SPS test is shown in Figure 19. The flatness of the neodymium multilayer sintered magnets fabricated in this way was ±0.34 mm on the upper and lower convex long surfaces of the lens shape, and ±0.09 mm on the left and right short surfaces of the lens. This suggests that the left and right short surfaces (the ends where the lens thickness is thinner) deformed first and hit the inside of the mold, where the deformation stopped. When the properties of these neodymium multilayer sintered magnets were measured, it was confirmed that even with a complex shape like a lens, it is possible to obtain neodymium multilayer magnets with no problems in other dimensions, including magnetic properties.

[0073] Furthermore, in this invention, when a laminate of unit neodymium sintered magnets (the laminate before pressurization) is placed in the space inside the mold, the gap that forms around the laminate, i.e., the gap between the laminate and the inner wall of the mold, is manufactured by considering the deformation rate in each direction so that when the laminate deforms due to pressurization, the entire surface of the laminate comes into contact with the inner wall of the mold simultaneously. This makes it effective to manufacture neodymium laminated sintered magnets using a mold with an internal space of the same shape as the product size. [Industrial applicability]

[0074] The neodymium multilayer sintered magnet of the present invention possesses high magnetic properties and excellent high electrical resistance, making it suitable for use in various home appliance motors, industrial motors, and especially as a magnet for EVs (electric vehicles) and HEVs (hybrid electric vehicles).

Claims

1. A method for manufacturing a neodymium laminated sintered magnet, wherein multiple unit neodymium sintered magnets are stacked, and each unit neodymium sintered magnet is joined to each other via a bonding layer, the bonding layer includes a rare earth-rich region, the rare earth-rich region has electrical insulating properties, and the sum of the maximum magnetic energy product (BH) max (MGOe) and coercivity iHc (kOe) expressed in the CGS unit system is 70 or more. Multiple unit neodymium sintered magnets are prepared, each having a thickness of 0.5 mm or more and 3 mm or less, and oriented parallel to a plane perpendicular to the thickness direction. A laminate is prepared by inserting Dy and / or Tb compounds between the unit neodymium sintered magnets. The laminate is heated and pressurized at 700°C or higher, below the sintering temperature, 25 MPa or higher, and less than 100 MPa, to bond, integrate, and deform within a range where the elution of the low-melting-point Nd-rich phase does not occur. A method for manufacturing neodymium laminated sintered magnets, characterized by subsequently performing a grain boundary diffusion treatment in a vacuum at a temperature of 800°C or higher and less than 950°C for 5 to 20 hours.

2. The method for manufacturing a neodymium laminated sintered magnet according to claim 1, characterized in that the thickness of the bonding layer is 1.0 μm or more and 200 μm or less.

3. The aforementioned unit neodymium sintered magnet is the actual form after sintering. A method for manufacturing a neodymium multilayer sintered magnet according to claim 1 or 2.

4. The electrical resistivity of the aforementioned bonding layer is 140 μΩm or more. A method for manufacturing a neodymium multilayer sintered magnet according to claim 3.

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