Neodymium laminated sintered magnet

Abstract

Provided is a neodymium laminated sintered magnet having high magnetic characteristics and superior high electrical resistance.　The neodymium laminated sintered magnet is obtained by laminating a plurality of unit neodymium sintered magnets, and bonding and integrating the unit neodymium sintered magnets together via a bonding layer. The bonding layer includes a rare earth-rich region, and the rare earth-rich region has electrical insulation properties. The thickness of the bonding layer is preferably 1.0-200 µm.

Description

Neodymium laminated sintered magnet

[0001] The present invention relates to a neodymium sintered laminated magnet, and more particularly to a neodymium sintered laminated magnet used in large motors such as the main motors of electric vehicles and in generators.

[0002] Neodymium sintered magnets, invented by the present inventors in 1982 (JP Patent Publication No. 61-34242), not only far surpass the magnetic properties of samarium-cobalt magnets, which had previously been the leading high-performance permanent magnet material, but also boast low cost due to their primary components being abundantly sourced, such as neodymium (Nd, a rare earth element), iron, and boron, making them an ideal permanent magnet material. Their applications are diverse, including computer hard disk drives (HDDs), magnetic head drive motors (VCMs, or voice coil motors), high-end speakers, headphones, electrically assisted bicycles, golf carts, and permanent magnet magnetic resonance imaging (MRI) systems. Furthermore, their practical application has been rapidly expanding in recent years, including in the main motors of hybrid electric vehicles (HEVs) and electric vehicles (EVs), energy-saving, low-noise large home appliances (air conditioners and refrigerators), elevators, and other industrial motors.

[0003] Generally, neodymium sintered magnets have high magnetic properties, but one drawback is their poor temperature characteristics. The temperature characteristics of coercivity are particularly important. When used in home appliance or industrial motors, it is unavoidable that temperatures will rise due to the coil current. Furthermore, because an opposing magnetic field acts from the motor armature, if the temperature rises and the coercivity decreases, irreversible demagnetization will occur in the permanent magnet. The only way to prevent irreversible demagnetization is to increase the coercivity in advance.

[0004] After the discovery of neodymium sintered magnets, the effects of adding elements (Patent No. 1606420, etc.), heat treatment (Patent No. 1818977, etc.), and controlling crystal grain size (Patent No. 1662257, etc.) to improve properties such as coercivity were demonstrated, but the most effective way to improve coercivity was to add heavy rare earth elements (Dy, Tb) (Patent No. 1802487). Using large amounts of heavy rare earth elements certainly increases coercivity, but it also reduces saturation magnetization and the maximum energy product. Furthermore, Dy and Tb are scarce and expensive resources, making it difficult to meet the demand for HEVs, EVs, and industrial and residential motors, which are expected to continue to see significant growth in demand.

[0005] Subsequently, in Patent Document 1 and the like, a method was discovered in which, rather than adding Dy / Tb or the like to the alloy composition, a sintered body was prepared, a metal or compound such as Dy / Tb was applied to the outside, and the heavy rare earth elements of Dy / Tb were diffused into the grain boundaries within the sintered body by heat treatment, thereby increasing only the coercive force with almost no decrease in saturation magnetization or maximum energy product.

[0006] Meanwhile, proposed magnets for HEVs and EVs are neodymium sintered magnets that are divided and stacked to reduce losses due to eddy currents during operation, suppress heat generation within the magnet, and reduce temperature rise in the magnet. One example of this proposal, for example, in Patent Document 2 listed below, involves coating a 5 mm thick neodymium sintered magnet with Dy fluoride powder, heating it in Ar at 900°C for one hour for grain boundary diffusion (GBD) treatment, and then stacking 18 of these divided magnets and inserting them into magnet insertion holes in the motor rotor and solidifying them with epoxy resin. An IPM motor equipped with a rotor manufactured in this way was proposed.

[0007] As the electrification of automobiles continues to advance, it is desirable to minimize the cost of sintered neodymium magnets used in main motors while maintaining the best possible magnetic properties, and to reduce the amount of Dy and Tb contained in the magnets to the lowest possible limit allowed by resource availability. However, the neodymium magnets currently used in EVs and HEVs contain a certain amount of Dy and Tb, and reductions in manufacturing costs are not sufficient. One reason for this is that the Dy- and Tb-reducing neodymium magnets for EVs and HEVs mentioned above are not sufficiently low.

[0008] Furthermore, silicon steel sheets, the iron core material used in motors, are stamped into a predetermined shape at a thickness of 0.5 mm or less and laminated to reduce eddy current loss during motor operation. It would be desirable to use Nd—Fe—B sintered magnets in the same motors by stamping them thinly, as this would minimize eddy current loss, but this is not possible. Therefore, as in the aforementioned known example (Patent Document 2), neodymium sintered magnets used in electric vehicle main motors are used as divided magnets approximately 5 mm thick. 5 mm thick magnets are machined from large block magnets (called block magnets), which increases processing costs and costs due to reduced material yield. Furthermore, laminates approximately 5 mm thick are insufficient in reducing eddy current loss. While increasing the thickness of unit neodymium sintered magnets to 5 mm reduces eddy current loss compared to block magnets without lamination, it is still not sufficient. Using magnets laminated by processing unit neodymium sintered magnets to a thickness of less than half of 5 mm, i.e., 2.5 mm or less, is expected to significantly reduce eddy current loss in motors.

[0009] Patent No. 4450239 JP 2011-78268 A

[0010] The present invention has been made in view of the above circumstances, and has as its object to provide a neodymium sintered laminated magnet having high magnetic properties and excellent high electrical resistivity.

[0011] The neodymium sintered laminate magnet of the present invention is formed by stacking multiple unit neodymium sintered magnets, each of which is bonded and integrated via a bonding layer. The bonding layer of the neodymium sintered laminate magnet includes a rare-earth-rich region, and the rare-earth-rich region is electrically insulating. In this specification, each thin plate-shaped magnet forming the laminate magnet is referred to as a "unit neodymium sintered magnet." Bonding does not refer to the bonding of the individual unit neodymium magnets using an adhesive or other means, but rather refers to metallurgical bonding and integration using a hot press or other method. The "bonding layer" refers to a flat layer formed along the bonding surface between adjacent unit neodymium sintered magnets by applying pressure while heating the unit neodymium sintered magnets to a high temperature. This bonding layer can be visualized by observing the cross section of the bonding surface between the unit neodymium sintered magnets using a scanning electron microscope (SEM) or energy dispersive X-ray spectroscopy (EDS). Therefore, the bonding layer of the present invention does not contain any organic substances, such as adhesives. In this specification, the term "rare earth-rich region" refers to a region containing a large amount (50% by weight or more) of rare earth elements such as Nd, Pr, Dy, and Tb, and is a bonding region having electrical insulation made of compounds such as oxides, fluorides, oxyfluorides, and carbides. Here, electrical insulation is defined as a region having a specific electrical resistance of 1.4x10, which is the specific electrical resistance of a neodymium sintered magnet. -6 The value is set to be at least two orders of magnitude larger than Ω·m. A difference of two orders of magnitude or more in resistivity reduces the current flowing therethrough by two orders of magnitude or more, significantly reducing eddy current loss. While an insulating effect is achieved when the proportion (volume percentage) of the "rare earth-rich region" in the bonding layer of the neodymium laminate magnet of the present invention is 20% or more, 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, in the neodymium laminate sintered magnet of the present invention having the above characteristics, the thickness of the bonding layer is 1.0 μm to 200 μm, and the thickness of the bonding layer is preferably 1.0 μm to 100 μm, more preferably 1.0 μm to 50 μm, and particularly preferably an average thickness of 2.0 μm to 30 μm. The reason for setting the upper limit of the bonding layer thickness to 200 μm is that if the thickness exceeds this limit, the effective volume ratio of the sintered magnet portion will decrease, resulting in a significant decrease in remanence Br.

[0013] The present invention also provides a neodymium sintered laminate magnet having the above-mentioned characteristics, characterized in that the rare-earth-rich regions are present continuously and / or intermittently in the joining layer. That is, the joining layer of the neodymium sintered laminate magnet of the present invention is a phase including planar rare-earth-rich regions that are present continuously and / or intermittently along the joining surface. In particular, the rare-earth-rich regions are in contact with the unit neodymium sintered magnet. In this case, the thickness of the joining layer can be measured essentially as the thickness of the rare-earth-rich regions.

[0014] Furthermore, the present invention provides a neodymium sintered laminate magnet having the above characteristics, characterized in that the bonding layer contains an Fe-rich metal region containing 50% or more by weight of Fe, and the Fe-rich metal region connects adjacent neodymium sintered magnet units via the bonding layer. In other words, the Fe-rich metal region exists so as to penetrate a portion of the planar rare-earth-rich region. In this specification, the term "Fe-rich metal region" refers to a phase containing a large amount of Fe (50% or more by weight) and substantially no compounds of rare earth elements such as Nd, Pr, Dy, or Tb.

[0015] Furthermore, the present invention is characterized in that in the neodymium sintered laminate magnet having the above characteristics, the difference in weight percent between the total amount (total quantity) of rare earth elements in a surface region less than 50 μm from the surface of the neodymium sintered laminate magnet and the total amount (total quantity) of rare earth elements in an internal region 50 μm or more from the surface is less than 1 weight percent, 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, the rare earth-rich region in the joining layer contains at least one of Dy and Tb as a heavy rare earth element, and the heavy rare earth element is present in the Nd of each unit neodymium sintered magnet. ２ Fe １４ It is also characterized by diffusion from each bonding layer toward the adjacent bonding layer (GBD diffusion) along the grain boundary phase of the B crystal.

[0017] Furthermore, the present invention is characterized in that the neodymium laminated sintered magnet having the above-mentioned characteristics has magnetic properties in which the sum of the maximum magnetic energy product (BH)max (MGOe) and the coercive force iHc (kOe) expressed in the CGS unit system (hereinafter, this may be referred to as the magic number "MN") is 70 or greater.

[0018] Furthermore, the present invention is also characterized in that the neodymium sintered laminate magnet has the above-mentioned characteristics, and that the outer surfaces of the neodymium sintered laminate magnet perpendicular to the joining layer have a flatness of 0.1 mm or less on at least one pair of opposing outer surfaces or on all outer surfaces.

[0019] The neodymium sintered laminated magnet of the present invention has high magnetic properties and excellent electrical resistivity, can be manufactured at a cost comparable to that of conventional magnets, and has the world's best magnetic properties and sufficient electrical insulation for automotive applications, making it suitable for use as a magnet in EVs and HEVs, which are expected to see explosive growth in the future.

[0020] 10 is a diagram showing a method for manufacturing a neodymium sintered laminate magnet according to the present invention. FIG. 10 is a diagram showing the state when a laminate of unit neodymium sintered magnets is set inside the apparatus together with a mold and a punch. FIG. 10 is a photograph showing the appearance of a cylindrical sample after a deforming test. FIG. 10 is a photograph showing the appearance of a hot pressing apparatus used in a joining test. FIG. 10 is a cross-sectional view of the apparatus used in the joining test. FIG. 10 is a photograph showing the state of unit neodymium sintered magnets before and after a joining test. FIG. 10 is an SEM photograph of the joining surface of the neodymium sintered laminate magnet according to Example 1. FIG. 10 is an EDS photograph of a portion (rare earth-rich region) where rare earth compounds and the like are continuously distributed on the joining surface of the neodymium sintered laminate magnet according to Example 1. FIG. 10 is an EDS photograph of a portion (rare earth-rich region) where an Fe-rich metal region is distributed across the joining surface on the joining surface of the neodymium sintered laminate magnet according to Example 1. FIG. 10 is a photograph of a jig for applying GBD paste to the surface of a unit neodymium sintered magnet ( FIG. 10a ), the state during application of the GBD paste ( FIG. 10b ), and a cassette in which unit neodymium sintered magnets are stacked after application of the GBD paste ( FIG. 10c ). 1 is a photograph of a neodymium sintered laminate magnet according to Examples 1 and 2. FIG. 2 is a photograph of a damaged neodymium sintered laminate magnet according to Comparative Example 1. FIG. 3 is a diagram showing an outline of a C-shaped yoke for eddy current loss evaluation and a measurement system. FIG. 4 is a photograph of an eddy current evaluation sample whose surface is covered with insulating tape and whose search coil is wound on a surface perpendicular to the magnetic field generation direction for eddy current evaluation. FIG. 5 is a photograph of an evaluation sample set in a C-shaped coil. FIG. 6 is a graph showing eddy current evaluation results up to a frequency of 200 Hz when excited with a 5 Arms current. FIG. 7 is a graph showing eddy current evaluation results up to a frequency of 500 Hz when excited with a 2 Arms current. FIG. 8 is a photograph showing the appearance of a neodymium sintered laminate magnet produced using a unit neodymium sintered magnet produced by the NPLP (New-PressLess Process) according to Example 3. FIG. 9 is a photograph showing the appearance of a convex lens-shaped neodymium sintered laminate magnet produced in Example 7.

[0021] The inventors first considered the ideal manufacturing method for neodymium sintered laminate magnets in order to obtain neodymium sintered laminate magnets with high magnetic properties and excellent electrical resistivity. The model they investigated is shown in Figure 1. First, a laminate of unit neodymium sintered magnets made of neodymium sintered compacts is prepared. Here, methods for manufacturing unit neodymium sintered magnets include cutting them from a larger sintered compact or manufacturing them so that the sintered unit neodymium sintered magnet shape itself is the unit neodymium sintered magnet. This laminate is placed in a space with a limited volume—specifically, a mold with a space that matches the product shape—and heated to a high temperature while applying pressure. This simultaneously bonds the unit neodymium sintered magnets and deforms the laminate to the mold inner wall in two directions, the pressure direction and perpendicular to that direction. This allows the neodymium sintered laminate magnets to occupy essentially 100% of the space limited by the product shape, leaving no residual space. The magnetic properties of this neodymium sintered laminate magnet are therefore comparable to those of a conventional monolithic magnet of the same shape, achieving a high residual magnetic flux density. Whether or not this model can be realized depends on factors such as whether it can be deformed, whether or not it can be joined, and whether or not the low-melting-point Nd-rich phase will dissolve and cause welding to the mold. However, the inventors have established this manufacturing method, and have made it possible to bring to market a completely net-shaped, process-free, low-cost product that not only has the best magnetic properties in performance evaluations, but also has the potential to produce a product with excellent magnetic properties.

[0022] The first point to consider is whether or not a unit neodymium sintered magnet can be deformed under high temperature and pressure. The reason is that the size of the main phase crystal grains that make up a unit neodymium sintered magnet is generally 10 μm or less, with an average of around 5 μm, and the proportion of these crystal grains in the magnet structure is 90% or more. Furthermore, the main phase crystals are tetragonal Nd ２ Fe １４ This is because, as a B-type intermetallic compound, the main phase itself is unlikely to undergo plastic deformation. However, while there is a possibility of crystal slippage of the main phase via a low-melting-point Nd-rich phase, which acts as a second phase, there have been no reports to date of plastic deformation being possible with a grain size of 1 μm or more. Figure 2 shows the outline of an experiment conducted to ascertain this. The composition of the unit neodymium sintered magnet used is also shown in Table 1.

[0023]

[0024] Here, the preparation of unit neodymium sintered magnets was carried out as follows. First, an SC alloy having the components shown in Table 1 was prepared. After hydrogen decomposition, 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 feed rate of the raw material to the jet-milling device and changing the classification conditions of the fine powder after grinding. In this experiment, raw material powder with an average particle size of 3 μm was used. This powder was filled into a carbon mold at a packing density of 3.6 g / cc, and a 4 T pulsed magnetic field was applied to align the c-axis direction of the powder crystal grains in one direction. After that, it was placed in a sintering furnace and sintered for 10 minutes. －４ The magnet was sintered at 985°C for 4 hours in a vacuum atmosphere of 100 Pa or less to obtain a block magnet measuring approximately 50 mm x 70 mm x 20 mm thick (in the orientation direction). The sintered body thus obtained was machined into a cube measuring 7 mm x 7 mm x 7 mm (in the orientation direction), and its magnetic properties were evaluated.

[0025] The results were Br = 14.1 kG, iHc = 16.1 kOe, (BH)max = 47.7 MGOe, and magnetic orientation Br / 4πMs = 0.965. A cylindrical neodymium sintered magnet measuring φ15 mm and 6 mm thick (orientation direction) was prepared from this block magnet by machining. This was placed inside a carbon cylindrical mold with an inner diameter of φ20 mm, and carbon punches slightly smaller than the mold's internal dimensions were attached to the top and bottom. The entire magnet was then loaded into a Spark Plasma Sintering (SPS) device, LABOX-325R (maximum pressure 30 kN, maximum pulse current output 2.5 kA), manufactured by Sinterland Co., Ltd. A carbon sheet approximately 0.2 mm thick was sandwiched 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 20 Pa, achievable with a rotary pump. In the SPS device, a large current is passed through the upper and lower punches to the mold and workpiece, and the workpiece is heated by the Joule heat generated while pressure is applied through the upper and lower punches to perform processing. In this experiment, a thermocouple was inserted into a hole in the upper punch to measure the temperature above the sample and control the temperature.

[0026] Table 2 shows the experimental conditions and results, 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 initial pressure at the start of the experiment was varied from 10 MPa, the same as the maximum pressure, to 40 MPa, and the amount of displacement in the sample thickness direction (pressure application direction) was measured. The presence or absence of elution of a 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 fractures were observed in the sintered body itself in any of the cases in this experiment.

[0027]

[0028] This experiment revealed the following: First, neodymium sintered compacts can be deformed without cracking or chipping. Furthermore, 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, a displacement of only 3.81% was achieved even at 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 did not reach the 7% displacement achieved at 800°C and 10 MPa. This indicates that temperature, rather than pressure, has a stronger influence on the magnitude of the displacement. Furthermore, regarding the dissolution of the low-melting-point Nd-rich phase from the sample side, a liquid phase was observed in Experiments No. 1 and No. 2 (see Figure 3), but no dissolution was observed in Experiments No. 3 and No. 4, where the applied pressure was 30 MPa or higher and the temperature was the same as 700°C. In other words, we discovered that not only temperature but also pressure conditions play an important role in determining whether or not the low-melting-point Nd-rich phase elutes, and that the higher the pressure, the less likely the liquid phase elutes. This result was unexpected, since it is generally thought that the higher the pressure, the more likely the liquid phase components are to escape, and we are currently working to clarify the underlying mechanism. In any case, the absence of elution of the low-melting-point Nd-rich phase during deformation of the sintered compact means that the risk of the eluted low-melting-point Nd-rich liquid phase reacting with the mold when the sintered compact reaches the inner wall of the mold due to deformation can be reduced, thereby reducing the risk of damaging the sample and the mold. From the above, we discovered the possibility of deforming a neodymium sintered compact without eluting the low-melting-point Nd-rich phase from the sintered compact by appropriately selecting the temperature and pressure conditions.

[0029] Next, we investigated the second key point: whether the unit sintered magnet laminate could be bonded. Figure 4 is a photograph showing the appearance of the hot press experimental apparatus used, and Figure 5 is a diagram showing a cross-sectional view of the apparatus. The sample space provided in the center of the apparatus in Figure 5 measured φ15 mm x 10 mm long. The unit sintered neodymium magnets used in this experiment were cut from the same sintered compact block as the sintered compact used in the previous experiment, measuring φ15 mm x 1.5 mm thick (in the orientation direction). Eight of these magnets were stacked to examine whether they could be bonded. The experiment was conducted in an Ar atmosphere, where the magnets were heated to 850°C before applying pressure, and then hydraulically pressed to 40 MPa (Figure 6). As a result of this experiment, the laminate of eight unit sintered neodymium magnets was firmly bonded and integrated. Evaluation of the bond strength revealed that it was comparable to that of a block-shaped sintered neodymium magnet.

[0030] The results of these experiments cleared both the first and second hurdles, bringing the realization of the machining-free neodymium sintered laminated magnet initially envisioned into sight. Based on the findings obtained through these experiments, the inventors have completed the present invention and have been able to manufacture the neodymium sintered laminated magnet of the present invention.

[0031] Below, we will explain the neodymium sintered laminate magnet of the present invention, which is formed by stacking multiple unit neodymium sintered magnets and then bonding and integrating them and deforming them at high temperatures (e.g., 700 to 950°C). First, the unit neodymium sintered magnets are prepared. According to conventional methods, an SC alloy prepared to have a predetermined composition is coarsely crushed, finely crushed, molded, oriented, and sintered. However, in the present invention, either a neodymium sintered magnet body larger than the unit neodymium sintered magnet shape is cut or otherwise processed to the desired shape and size, or, more preferably, a method invented by the present inventors to obtain unit neodymium sintered magnets of the desired shape and size in the as-sintered state using the NPLP method (Patent No. 6280137) developed by the present inventors, etc., can be used. The thickness of the unit neodymium sintered magnet can be selected from a variety of options, with the thinner the magnet, the more effective it is in reducing eddy current loss. However, from the standpoints of productivity and cost, in terms of material loss and processing costs when processing, and in terms of powder packing and productivity when using the NPLP method, it is appropriate for the thickness of the unit neodymium sintered magnet to be between 0.5 mm and 3 mm, and more preferably between 1 mm and 2.5 mm. This is because a thickness less than 0.5 mm makes manufacturability difficult, and a thickness greater than 3 mm results in insufficient insulation. Furthermore, when obtaining unit neodymium sintered magnets by cutting or other processing from a block-shaped neodymium sintered magnet body, the same thickness as above is also preferred.

[0032] The average particle size (D50) of the fine powder is preferably 10 μm or less from the viewpoint of coercive force characteristics, and 1 μm or more from the viewpoint of pulverizability, but more preferably 5 μm or less, and 2 μm or more. ２ Fe １４The magnetic properties of the sintered compact are improved by aligning the c-axis orientation of the fine powder, which is primarily composed of B-type crystals. However, the degree of c-axis crystal orientation must be 90% or higher. If it is less than 90%, the magnetic properties, particularly the residual magnetic flux density (Br), will decrease, and sufficient magnetic force will not be supplied when used in applications such as EV motors. Furthermore, the direction of this crystal orientation must be parallel to and unidirectional with the main surfaces of the unit neodymium sintered magnets, i.e., the plane perpendicular to the lamination direction of the neodymium sintered laminate magnet. This is because eddy currents that occur across the lamination surfaces of the neodymium sintered laminate magnet are interrupted by high-electrical resistance layers made of electrically insulating compounds (oxides, fluorides, carbides, etc.) formed on the lamination surfaces, significantly reducing eddy current loss.

[0033] Next, a laminate is constructed by stacking multiple unit neodymium sintered magnets prepared in this way. The number of layers in the laminate is determined by balancing 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 must be stacked, but when considering deformation in two directions perpendicular to the pressure direction, 16, 17, or more layers are 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, 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, SiO ２ and Al ２ O ３ oxides such as CaCO ３ Carbonates such as Ca(OH) ２ hydroxides such as BaTiO ３ The incorporation of ceramic powders such as those mentioned above is effective in improving production efficiency and electrical insulation. In particular, the incorporation of compounds containing heavy rare earth elements such as Dy and Tb is extremely effective in improving the magnetic properties, particularly the coercive force, of neodymium sintered laminate magnets.

[0035] The resulting unit neodymium sintered magnet laminate is placed in a mold whose interior is designed to fit the product's shape and size. The mold can be made of carbon or a heat-resistant alloy, and its surface can be coated with a material such as BN to prevent adhesion, effectively extending the mold's lifespan. The laminate, mold, and punch that applies pressure from above and below the laminate are placed in a hot press (HP) or spark plasma sintering (SPS) device, which allows for atmospheric control and can apply high pressure while heating to temperatures up to approximately 1000°C. While the present study primarily used an SPS device, the results obtained are essentially the same for an HP device. The optimal bonding and deformation conditions must be adjusted depending on the raw material alloy composition. However, a heating temperature of 700°C or higher and below the sintering temperature of the sintered magnet, and a pressure of 25 MPa or higher but less than 100 MPa are preferred. Temperatures below 700°C result in slow deformation of the sintered magnet, leading to cracks in the bonded product and reducing productivity. Furthermore, above the sintering temperature, the low-melting-point Nd-rich phase elutes significantly, causing significant welding between the laminate and the mold, resulting in severe wear of the laminate and the mold, as well as significant changes in the magnetic components and deterioration of the magnetic properties.A pressure of less than 25 MPa causes the low-melting-point Nd-rich phase to elute, as was revealed in the feasibility verification stage, and welding to the mold occurs, and at 100 MPa or more, the equipment capable of applying such a large pressure would be enormous and unsuitable for mass production.

[0036] The neodymium sintered laminate magnets thus fabricated have oxide or fluoride films formed on the bonding surfaces, blocking the flow of current across the bonding layers. When a fluoride or oxide of a heavy rare earth element such as Dy or Tb is sandwiched between the layers, additional heat treatment after the bonding and deforming process can induce grain boundary diffusion (GBD) and significantly improve coercivity. As with conventional sintered compacts, the GBD treatment temperature must be between 800°C and 950°C, and the GBD treatment time must be between 5 and 20 hours. The GBD treatment can be performed in a vacuum. A GBD treatment temperature below 800°C does not sufficiently improve coercivity, while a treatment temperature above 950°C causes a decrease in Br. Furthermore, a heat treatment time of less than 5 hours results in insufficient improvement in coercivity, while a treatment time of more than 20 hours reduces productivity.

[0037] The neodymium sintered laminate magnets of the present invention have a unique structure resulting from their unique manufacturing process. The following tests were conducted to clarify their structural characteristics. First, a sintered body having the composition listed in Table 1 was prepared. From this, circular sintered bodies measuring 14.5 mm in diameter and 1.3 mm thick (in the orientation direction) were prepared. Three of these were stacked to form four laminates with a total thickness of 3.9 mm. A mixed paste of the oxides and fluorides listed in Table 3 was divided into two equal portions and applied between each pair of layers. To facilitate application of the powder between the layers, an appropriate amount of liquid paraffin was mixed to form a slurry. The amount of application was set so that the Tb content was 0.5 wt% relative to the weight of the sintered body. In Test 3, the element ratio was set to Tb:Nd = 1:1, and in Test 4, TbF ３ and Tb ４ O ７ The amounts of Tb from the above were weighed out so that they were equal and the total amount was 0.5 wt %.

[0038]

[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, at a maximum temperature of 700°C and a pressure of 50 MPa. The three laminates were completely bonded, and no elution of Nd-rich phase from the surroundings was observed. The thickness dimension was reduced from 3.9 mm to 3.6 mm or 3.7 mm, representing a deformation rate of approximately 6%. This integrated laminated sintered body was subjected to a GBD heat treatment in vacuum at 890°C for 20 hours, after which the sample was cut perpendicular to the thickness direction and the cross-sectional structure, including the bonded surface (cross-section of the bonded layer), was observed. The measuring equipment used was 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 composition images taken at magnifications of 100x, 500x, 1000x, and 2000x. Here, regions where elements with heavy atomic numbers are abundant appear white, while regions where lighter elements are abundant appear black. It can be seen that the whitish regions are regions containing a large amount of rare earth elements, while the darker regions are regions where these elements are relatively scarce. In Figure 7, whitish regions (bonding layers) are visible along the bonding surface, indicating the presence of a large amount of rare earth elements. The average thickness measured at a total of 15 points, five visual fields, three points per visual field, is shown in Table 3. As shown in Table 3, the average thickness of the bonding surface (bonding layer) is DyF ３ and TbF ３ The thinnest fluoride coating material is just under 4 μm, Tb ４ O ７ and Nd ２ O ３ The oxide mixture was the thickest at 18.5 μm.

[0041] Next, the elemental mapping results obtained by EDS measurement at 1000x magnification on the cut surface of the neodymium sintered laminate magnet fabricated in Test 4 are shown in Figures 8 and 9. In most of the region (rare-earth-rich region) of the bonding surface (bonding layer), as shown in Figure 8, Nd, Pr, O, and F are continuously present along the bonding surface, with no Fe present. This rare-earth-rich region is in contact with the unit neodymium sintered magnet. Meanwhile, in the region shown in Figure 9 (the Fe-rich metal region present in part of the bonding surface (bonding layer)), Fe is abundant, with almost no Nd, Pr, O, or F present. In other words, the bonding surface (bonding layer) appears to have a continuous structure with a thickness (thickness of the rare-earth-rich region) of 8.60 μm. However, it was found that the bonding layer is composed of a region (rare-earth-rich region) consisting of rare-earth oxides and rare-earth fluorides, and an Fe-rich region. In particular, it was found that the Fe-rich region exists so as to penetrate a portion of the planar rare-earth-rich region. This revealed that the thickness of the bonding layer can be essentially expressed by the thickness of the rare-earth-rich region. The above observation results were observed not only in Test 4 but also in Tests 1 to 3, and are a structure unique to neodymium sintered laminated magnets. In other words, rare earth oxides and fluorides have high electrical resistance, so they improve the total electrical resistivity of the bonding surface, and at the same time, the Fe-rich region can be considered to play a role in ensuring the high strength of the neodymium sintered laminated magnet by spanning the adjacent unit neodymium sintered magnets through the bonding layer, forming a strong bridge-like connection.

[0042] Specific embodiments of the neodymium sintered laminate magnet of the present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these. [Examples 1 and 2]

[0043] An SC alloy with the composition shown in Table 4 below was prepared. After hydrogen decomposition, the alloy was mixed with 0.05 wt% methyl caprylate and kneaded and coarsely pulverized using a Piccolo mixer. The coarsely pulverized raw material was jet milled under a nitrogen atmosphere using a Hosokawa Micron Corporation MJT-LAB jet mill. The milling 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. The resulting fine powder raw material was mixed with 0.07 wt% methyl laurate and kneaded using a Piccolo mixer to prepare the pre-sintered raw material. This was then packed into a carbon container at a packing density of 3.6 g / cc, and a pulsed magnetic field of up to 4 T was applied multiple times to orient the powder particles. This oriented body, along with the carbon mold, was placed in a sintering furnace and sintered in vacuum at 1030°C for 4 hours to obtain a block-shaped sintered body. A 7 mm cube-shaped evaluation sample was cut out from this and evaluated using a pulse BH tracer PBH-1000 manufactured by Nippon Denji Sokki Co., Ltd. The residual magnetic flux density Br was 13.8 kG and the coercive force iHc was 19.9 kOe.

[0044]

[0045] Furthermore, a predetermined number of unit neodymium sintered magnets with a target size of 15.24 mm x 4.24 mm (in the orientation direction) x 1.8 mm were prepared by machining the block-shaped sintered body. Separately, a material to be applied between the laminations was prepared. Its components are shown in Table 3 for Tests 1 and 4. This time, the number of laminations in the unit neodymium sintered magnet laminate was set to 24. The internal dimensions of the mold into which this laminate was loaded were set to the target product size, specifically, 15.5 (±0.1) mm x 5.0 (±0.05) mm (in the orientation direction), and the length in the pressure direction was set to be greater than the target product size so that the upper and lower punches could be set.

[0046] The coating materials for Tests 1 and 4 were then applied between the layers to produce two types of 24-ply laminates. These are designated Examples 1 and 2, respectively. An overview of the production method is shown in Figure 10. First, a coating jig, as shown in 10a), is prepared. This jig has through-holes slightly smaller than the dimensions of the unit sintered neodymium magnets. The thickness of these through-holes is designed so that the desired coating amount can be achieved by uniformly filling the holes with the coating material. As shown in 10b), the coating material is filled along the through-holes, and the top surface is scraped with a spatula to adjust the coating amount to the calculated value. This jig is then removed, and the coated unit sintered neodymium magnets are removed and stacked. For this purpose, a laminate holding jig, as shown in 10c), may be prepared for convenience in transporting the laminate. Of course, the coating jig is not limited to this. Any tool that can apply the desired amount of coating material and easily remove and stack the magnets is sufficient.

[0047] Next, a set of 24 laminates for each of Examples 1 and 2 prepared in this manner was placed in a carbon mold for SPS, which was made to match the above-mentioned product dimensions. The mold had a split-mold structure to facilitate sample removal after SPS processing. The laminate was set by fitting upper and lower punches to this split mold. Since a split mold was used, a die was also required to hold the split mold. The assembled unit neodymium sintered magnet laminate, upper and lower punches, split mold, and die were placed in an Sinterland SPS apparatus (LABOX-325R) manufactured by Sinterland Co., Ltd., and a K-type thermocouple was attached to the upper punch for temperature measurement. The SPS apparatus was evacuated to a vacuum of approximately 20 Pa using a rotary pump, and then heated until the thermocouple reading reached 850°C while passing a large current of several hundred amperes through the upper and lower punches. After the temperature reached 850°C, a pressure of 65 MPa was applied to the unit neodymium sintered magnet laminate by the upper and lower punches, performing a bonding and deforming process. In this experiment, the laminates in Examples 1 and 2, which had different coating materials, were subjected to the SPS treatment, but the SPS heating and pressing conditions were the same. The results are shown in FIG. 11, and the final dimensions are shown in Table 5.

[0048]

[0049] As shown, these two neodymium sintered laminate magnets have nearly identical dimensions and are completely indistinguishable in appearance. The flatness of two pairs of opposing outer surfaces parallel to the pressure direction (i.e., the two pairs of outer surfaces where the stacking state can be visually confirmed) was evaluated using a surface roughness tester with a sensor head diameter of 0.05 mm, and it was confirmed to be within a range of ±0.05 mm from the center value. This is because the lamination misalignment that inevitably occurs during the stacking stage is repaired by the SPS process, where each unit neodymium sintered magnet is deformed and pressed against the inner surface of the mold. Therefore, in this invention, the outer surface is flat even without mechanical processing such as polishing or grinding. As a result, the size of the neodymium sintered laminate magnet is the same as the dimensions specified by the inner surface of the mold, and its density can be estimated to be nearly 100%. This is why the high Br characteristics described above are obtained.

[0050] After the SPS process, the neodymium sintered laminate magnets were then subjected to a post-processing GBD treatment in a vacuum at 890°C for 20 hours. The magnetic properties of the resulting neodymium sintered laminate magnets according to Examples 1 and 2 were evaluated in their original size without any processing, and the results are shown in Table 6. The measuring device used was a high-sensitivity vibrating sample magnetometer TM-VSM70100-SMS manufactured by Tamagawa Seisakusho Co., Ltd., and the properties were evaluated by applying a magnetic field of up to 8 T using a superconducting magnet coil.

[0051]

[0052] The above results demonstrate that the neodymium sintered laminate magnet according to this example has extremely high magnetic properties, and that although the type of Tb compound laminated between the layers in Example 1 and Example 2 is different, adjusting the Tb content to the same value will result in equivalent coercive force.

[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 conditions were changed to 945°C and the applied pressure to 30 MPa. The liquid phase dissolved and reacted with the split mold, and cracks occurred across the joining surface on the top and center of the sample. This was because the temperature was above the appropriate range and the applied pressure was low, causing the low-melting-point Nd-rich phase to dissolve, come into contact with the split mold, react, and cause cracks 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, which may have resulted in changes in the magnet's composition. Because this elution occurred from the inside of the neodymium sintered laminate magnet toward the mold inner wall, it is possible that differences in composition existed depending on the location of the neodymium sintered laminate magnet. Therefore, SEM-EDX was used at a low magnification of approximately 50x to investigate whether there were differences in composition between the surface region less than 50 μm from the magnet surface near the inner wall and the center of the magnet (a region more than 50 μm away from the surface). Table 7 lists the results for the main elements compared to Example 1.

[0055]

[0056] These results show that in the case of Comparative Example 1, the total amount of rare earth elements in the components near the surface was approximately 34 wt%, while in the interior it was reduced to approximately 30 wt%, a difference of approximately 4 wt%. Just to be sure, a similar analysis was performed on Example 1, but no significant difference in the total amount of rare earth elements between the surface and 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 sintered laminate magnet of the present invention, in which the elution of the Nd-rich grain boundary phase is controlled.

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

[0058]

[0059] As can be seen from Table 6, the neodymium laminate sintered magnets according to the examples exhibited extremely high magnetic properties, with the Magic Number (hereafter referred to as MN), the sum of the coercive force iHc and the maximum magnetic energy product (BH)max, exceeding 77, the highest performance in the world. On the other hand, the sample of Comparative Example 1, which was damaged by the elution of the low-melting-point Nd-rich phase, exhibited high Br properties, but the iHc properties and squareness deteriorated, likely due to the reduced rare earth element content, resulting in an MN of just under 69 (Table 8), and the magnet exhibited mediocre magnetic properties comparable to those obtained by conventional techniques.

[0060] Examples 3 and 4 Using the SC raw material of composition A shown in Table 1 and the coating materials of Tests 1 and 4 in Table 3, a laminate of unit neodymium sintered magnets was prepared so that the Tb content was 0.5 wt %, and neodymium sintered laminate magnets were produced 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. The magnetic properties are shown in Table 9.

[0061]

[0062] Like Examples 1 and 2, they exhibited extremely high magnetic properties. Cross-sectional observation of the bonding surface revealed the characteristics of the bonding surface shown in Figures 8 and 9, respectively. Specifically, the average thickness of the bonding layer in Example 3 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 also observed. Elemental analysis of the bonding layer cross-sections using EDS revealed that the volume fraction of the rare earth-rich region in the bonding layer in the NdFeB sintered laminate magnet of Example 3 was approximately 20%, while the volume fraction of the rare earth-rich region in the bonding layer in the NdFeB sintered laminate magnet of Example 4 was approximately 80%. Eddy current loss measurements were performed on the NdFeB sintered laminate magnets of Examples 3 and 4, which have different rare earth compound layers. In this measurement, measurements were also performed on a continuous NdFeB sintered magnet machined to approximately the same shape from a block magnet and a resin-bonded magnet manufactured to approximately the same shape by laminating unit NdFeB sintered magnets with resin bonding, as shown in Table 10. This allowed the degree of eddy current loss in the neodymium laminated sintered magnet according to the present invention to be compared between a non-laminated case and a completely insulated resin-bonded case.

[0063]

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

[0065] In these examples, ten unit neodymium sintered magnets having composition C shown in Table 11 below were fabricated using the NPLP method (New-Pressless Process, Patent No. 6280137, WO2016 / 047593) previously devised by the inventors, and a joining and deforming test was carried out using an SPS apparatus. The weights, sizes, etc. of the unit neodymium sintered magnets fabricated using the NPLP method are shown in Table 12.

[0066]

[0067]

[0068] The weights and sizes of the individual neodymium sintered magnets are not identical, and there is some variation in weight and size. However, these magnets were laminated by applying the coating material shown in Test 2 in Table 3 between the layers and then subjected to SPS treatment at a heating temperature of 850°C and a pressure of 60 MPa to produce rectangular neodymium sintered magnet laminates measuring 15.91 mm wide, 7.61 mm in the orientation direction, and 17.05 mm thick in the pressure direction. This was then subjected to GBD treatment in a vacuum at 875°C for 16 hours to evaluate its magnetic properties (Example 5). Figure 18 shows a photograph of the neodymium sintered magnet laminate according to Example 5, and Table 13 shows the resulting magnetic properties. Table 13 also shows the magnetic properties of a laminate of 10 neodymium sintered magnets with similar weight and dimensional variations as those shown in Table 12, when the coating material shown in Test 5 in Table 3 was used (Example 6). The flatness of the opposing surfaces pressed against the mold in each example was evaluated using the same measurement method as above, and was found to be a maximum of 0.1 mm, which was approximately the same as when unit neodymium laminated magnets were prepared by cutting.

[0069]

[0070] In this way, whether a Tb compound or a Dy compound was used as the coating material, we obtained a neodymium sintered laminate magnet that has high Br, coercivity of approximately 20 kOe or more, and MN of 77.8 and 74.0, the world's highest magnetic properties. This shows that the neodymium sintered magnet unit can be prepared using a cost-effective method, whether it is the NPLP method or processing, and the magnetic properties themselves can be determined by appropriately selecting the GBD coating material sandwiched between the layers.

[0071] The examples so far have shown that performing GBD treatment after SPS processing results in high coercivity, and the reason for this is as follows. That is, although a GBD coating material is sandwiched between the layers of unit sintered neodymium magnets when they are stacked, no disadvantages to the GBD treatment process due to the subsequent SPS treatment have been observed. Rather, this is because the diffusion distance of heavy rare earth elements such as Dy and Tb during GBD treatment is short, at a maximum of approximately 2.5 mm, i.e., about the thickness of the unit sintered neodymium magnet, and uniform magnetic properties can be obtained regardless of the size of the product shape.

[0072] [Example 7] 2 mm-thick lens-shaped neodymium sintered compact units having the composition B were prepared. Twenty-two of these were stacked, and the coating material from Test 3 was applied between the layers. A photograph of the resulting neodymium sintered laminate magnet is shown in Figure 19. The flatness of the lens-shaped, convex long faces of the neodymium sintered laminate magnet produced in this way was ±0.34 mm, and the flatness of the left and right short faces of the lens was ±0.09 mm. This suggests that the left and right short faces (thinner edges of the lens) deformed first, hitting the mold interior and then halting deformation. Measurement of the properties of this neodymium sintered laminate magnet confirmed that, even with a complex lens-shaped product, it was possible to produce a neodymium sintered laminate magnet with satisfactory magnetic properties and other dimensions.

[0073] In the present invention, it is effective to manufacture neodymium sintered magnet laminates using a mold having an internal space of the same shape as the product size by manufacturing unit neodymium sintered magnets in consideration of the deformation rate in each direction so that when the laminate (before pressure is applied) of unit neodymium sintered magnets is placed in the space inside a mold, gaps that form around the laminate, i.e., gaps between the laminate and the mold inner wall, are formed so that when the laminate is deformed by pressure, the entire surface of the laminate comes into contact with the mold inner wall at the same time.

[0074] The neodymium laminated sintered magnet of the present invention has high magnetic properties and excellent electrical resistivity, and can be used in motors for various home appliances and industrial motors, as well as magnets for EVs (electric vehicles) and HEVs (hybrid electric vehicles).

Claims

1. A neodymium sintered magnet in which a plurality of unit neodymium sintered magnets are laminated, and the unit neodymium sintered magnets are joined to each other through a joining layer to be integrated, wherein the joining layer of the neodymium laminated sintered magnet contains a rare earth-rich region, and the rare earth-rich region has electrical insulation properties.

2. The neodymium laminated sintered magnet according to claim 1, wherein the thickness of the joining layer is 1.0 µm or more and 200 µm or less.

3. The neodymium laminated sintered magnet according to claim 1 or 2, wherein the rare earth-rich region exists continuously and / or intermittently in the joining layer.

4. The neodymium laminated sintered magnet according to claim 1 or 2, wherein an Fe-rich metal region containing 50% by weight or more of Fe exists in the joining layer, and the Fe-rich metal region connects between adjacent unit neodymium sintered magnets through the joining layer.

5. The neodymium laminated sintered magnet according to claim 1 or 2, wherein the difference between the weight percentage of the total amount of rare earth elements in the surface layer region less than 50 µm from the surface of the neodymium laminated sintered magnet and the weight percentage of the total amount of rare earth elements in the internal region more than 50 µm away from the surface is less than 1% by weight.

6. 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 Nd of each unit neodymium sintered magnet ２ Fe １４ The neodymium laminated sintered magnet according to claim 1 or 2, characterized in that it diffuses from each bonding layer toward the adjacent bonding layer side along the grain boundary phase of the NdFeB crystal.

7. The neodymium laminated sintered magnet according to claim 1 or 2, having magnetic properties such that the sum of the maximum magnetic energy product (BH)max (MGOe) and the coercive force iHc (kOe) represented in the CGS unit system is 70 or more.

8. The neodymium laminated sintered magnet according to claim 1 or 2, wherein the outer surface of the neodymium laminated sintered magnet orthogonal to the joining layer has a flatness of 0.1 mm or less on at least a pair of opposing outer surfaces or all outer surfaces.

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