JP7555871B2 - R-T-B based permanent magnet and its manufacturing method - Google Patents

Description

The present invention relates to a method for producing an R-T-B system permanent magnet through a grain boundary diffusion process, and to an R-T-B system permanent magnet produced by this method.

Rare earth magnets with an R-T-B system composition have superior magnetic properties to other permanent magnets, and in recent years, various studies have been conducted with the aim of further improving these magnetic properties. For example, Patent Document 1 discloses a method (grain boundary diffusion method) in which, after producing an R-T-B system permanent magnet, a heavy rare earth element is attached to the surface and heated to diffuse the heavy rare earth element through the grain boundaries. By using this method, it is possible to improve the magnetic properties, particularly the coercive force.

However, today, there is a demand for many magnetic components (such as motors) to be even smaller, lighter, and more powerful and efficient. To meet these demands, it is necessary to further improve both the residual magnetic flux density and coercive force of R-T-B permanent magnets.

International Publication No. 2016/121790

The present invention was made in consideration of the above-mentioned circumstances, and its purpose is to provide an R-T-B system permanent magnet that combines high residual magnetic flux density and high coercive force, and a method for manufacturing said R-T-B system permanent magnet.

In order to achieve the above object, the method for producing an R-T-B system permanent magnet according to the present invention comprises the steps of:
preparing a magnet substrate containing an R-T-B based alloy;
a first diffusion step of attaching a first diffusion material containing at least one light rare earth element to at least a part of a surface of the magnet base material, and heating the magnet base material to which the first diffusion material is attached;
After the first diffusion step, a second diffusion material containing at least one heavy rare earth element is adhered to at least a portion of the surface of the magnet base material, and the magnet base material to which the second diffusion material is adhered is heated.

As described above, by carrying out a two-stage grain boundary diffusion process in which the light rare earth elements are diffused first, followed by the heavy rare earth elements, an R-T-B permanent magnet that combines high residual magnetic flux density and high coercivity can be obtained.

Preferably, the first diffusion material contains one or more elements selected from the group consisting of Nd and Pr as the light rare earth element.

Also, preferably, the first diffusion material includes an RL-M alloy containing the light rare earth element and M, and in the RL-M alloy, RL is the light rare earth element, and M is an element whose eutectic temperature with RL is 800°C or less.

In addition, preferably, the content of heavy rare earth elements in the first diffusion material is 10 mass % or less, and more preferably, the first diffusion material is substantially free of heavy rare earth elements.

On the other hand, the second diffusion material preferably contains one or more elements selected from the group consisting of Tb and Dy as the heavy rare earth element.

Preferably, the R-T-B based alloy contained in the magnet base material contains one or more rare earth elements, with Nd being essential as R, one or more iron group elements, with Fe being essential as T, boron, and Cu. And, preferably, in the R-T-B based alloy, the total content of R is 27.5 mass% or more and 30.8 mass% or less, the content of Cu is 0.05 mass% or more and 0.5 mass% or less, and the content of B is 0.92 mass% or more and 1.03 mass% or less.
When the magnet base material satisfies the above composition, the effects of improving the residual magnetic flux density and the coercive force become more pronounced.

The R-T-B system permanent magnet according to the present invention can be obtained, for example, by the above-mentioned manufacturing method,
An R-T-B system permanent magnet in which R is one or more light rare earth elements essentially consisting of Nd and one or more heavy rare earth elements, T is one or more iron group elements essentially consisting of Fe, and B is boron,
The R-T-B based permanent magnet includes a plurality of core-shell particles,
Each of the core-shell particles has a core portion containing an R ₂ T ₁₄ B crystal and a shell portion covering the core portion and having a higher content of the heavy rare earth element than the core portion,
In a cross section at a depth of 100 μm from the surface of the R-T-B system permanent magnet, the maximum thickness of the shell portion is, on average, 0.5 μm or less,
An R-T-B system permanent magnet having a concentration distribution in which the content of the light rare earth element and the content of the heavy rare earth element both decrease in the depth direction from the surface of the R-T-B system permanent magnet.

The above characteristics of R-T-B permanent magnets enable both residual magnetic flux density and coercive force to be improved.

Preferably, the R-T-B system permanent magnet of the present invention contains an M element having a eutectic temperature with the light rare earth element of 800° C. or lower,
The RTB system permanent magnet has a concentration distribution in which the content of the M element decreases from the surface toward the depth direction.

Also, preferably, the R-T-B system permanent magnet further contains Cu, the total content of R being 28.4 mass% or more and 32.0 mass% or less, the Cu content being 0.2 mass% or more and 0.7 mass% or less, and the B content being 0.92 mass% or more and 1.03 mass% or less.
When the RTB based permanent magnet satisfies the above composition, the magnetic properties (particularly the coercive force) are further improved.

Preferably, the heavy rare earth element contained in the R-T-B system permanent magnet is Tb and/or Dy. Also, preferably, the R-T-B system permanent magnet further contains Pr as the light rare earth element.

Furthermore, it is preferable that the R-T-B system permanent magnet has a residual magnetic flux density of 1450 mT or more and a coercive force of 1800 kA/m or more.

The R-T-B system permanent magnet of the present invention can be used as a component of motors, generators, compressors, actuators, magnetic sensors, speakers, etc., and is particularly suitable as a component of motors. Furthermore, motors containing the R-T-B system permanent magnet of the present invention can be mounted in various electronic devices, industrial equipment, etc., and are particularly suitable for use as automobile motors (EV, HV, PHV, etc.).

FIG. 1 is a perspective view showing an example of a magnet substrate. FIG. 2 is a schematic diagram illustrating the flow of the first and second diffusion steps. FIG. 3A is an enlarged schematic cross-sectional view of a cross section of a magnet substrate. FIG. 3B is an enlarged schematic cross-sectional view of the RTB system permanent magnet after the second diffusion step. FIG. 4A is a perspective view showing an example of an RTB system permanent magnet. FIG. 4B is a cross-sectional view taken along line VB-VB shown in FIG. 4A. FIG. 5 is a schematic diagram showing a cross section of a core-shell particle. FIG. 6 is a SEM cross-sectional photograph of an RTB system permanent magnet according to an example (sample 1). FIG. 7 is a SEM cross-sectional photograph of the RTB system permanent magnet according to Comparative Example 1. FIG. 8 is a schematic diagram of an IPM motor using the RTB system permanent magnet of the present invention. FIG. 9 is a schematic diagram showing an automobile utilizing the IPM motor shown in FIG.

The present invention will now be described in detail with reference to the embodiments shown in the drawings.

In this embodiment, we will first explain the manufacturing method of an R-T-B system permanent magnet, and then explain the characteristics of the R-T-B system permanent magnet obtained by this manufacturing method. The manufacturing method of an R-T-B system permanent magnet in this embodiment includes a step of preparing a magnet base material, a first diffusion step of diffusing a light rare earth element into the magnet base material, and a second diffusion step of further diffusing a heavy rare earth element into the magnet base material after the first diffusion step. Each step will be explained in order below.

(Magnetic substrate preparation process)
In this process, the magnet base material 20 shown in FIG. 1 is manufactured. First, the features of the magnet base material 20 will be described in detail. The magnet base material 20 is a sintered body of an R-T-B alloy, and is a material before the diffusion process described later is performed. In FIG. 1, the magnet base material 20 is a rectangular parallelepiped having two main surfaces 20a perpendicular to the Z axis and four side surfaces 20b perpendicular to the X axis or the Y axis. However, the shape of the magnet base material 20 is not particularly limited, and may be, for example, a polygonal shape, a cylindrical shape, a hollow cylindrical shape, or an arc segment shape in which the main surfaces are curved in an arc shape. In the drawings, the X axis, the Y axis, and the Z axis are approximately perpendicular to each other.

In this embodiment, the two main surfaces 20a of the magnet base material 20 are magnetic pole surfaces. A magnetic pole surface is a surface through which magnetic flux mainly passes, and is the positive pole (N pole) or negative pole (S pole) of a permanent magnet. When the R-T-B system permanent magnet to be manufactured is an anisotropic magnet, the magnetic pole surface can be determined by the direction of the magnetic field applied in the molding process described below. There is no particular limit to which surface of the magnet base material 20 is to be the magnetic pole surface, and one main surface 20a and one side surface 20b may be the magnetic pole surface, or the two opposing side surfaces 20b may be the magnetic pole surfaces.

In the R-T-B alloy constituting the magnet base material 20, R is one or more rare earth elements, of which Nd (neodymium) is essential, T is one or more iron group elements, of which Fe (iron) is essential, and B is boron. Here, the rare earth elements refer to Sc, Y, and lanthanoid elements, which belong to Group 3 of the long-form periodic table. Among the rare earth elements, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu are referred to as heavy rare earth elements (RH), and rare earth elements other than RH are referred to as light rare earth elements (RL). In addition, the iron group elements refer to Fe, Co, and Ni.

The total content of R contained in the magnet base material 20 can be 25% by mass to 35% by mass, and is preferably 27.5% by mass or more and 30.8% by mass or less. When the total content of R satisfies the above composition range, the precipitation of α-Fe and the like exhibiting soft magnetism is suppressed, and R ₂ T ₁₄ B crystals are more likely to be generated. Here, in this embodiment, the "content of the predetermined element contained in the magnet base material 20" is the content per unit mass of the magnet base material 20, and means the ratio of the predetermined element to 100% by mass of the magnet base material. That is, the total content of R is calculated as the ratio of the total mass of the rare earth elements to 100% by mass of the magnet base material. The content of the predetermined element in the permanent magnet 2 described later will also be expressed in the same manner as above.

When multiple rare earth elements are included as R, the content of Nd included in the magnet base material 20 is not particularly limited, but is preferably 22 mass% or more. In this case, it is preferable that Pr, Tb, and Dy are included in the magnet base material 20 as rare earth elements other than Nd. When heavy rare earth elements (RH) such as Tb and Dy are added as R other than Nd, it is preferable that the total content of RH included in the magnet base material 20 is 2.5 mass% or less.

The B content of the magnet base material 20 can be 0.5% to 1.5% by mass, and is preferably 0.92% to 1.03% by mass.

On the other hand, the content of T contained in the magnet base material 20 is expressed as the balance and may be appropriately determined according to the content of other elements. T may be Fe alone, or a portion of Fe may be substituted with Co. In this case, the content of Co is not particularly limited, and may be, for example, 4 mass% or less, and preferably 1 mass% or less.

In addition to the above-mentioned R, T, and B, the magnet base material 20 preferably contains one or more m elements selected from the group consisting of Cu, Ga, Al, and Zr. It is more preferable that the m element contains Cu among the above.

The content of Cu contained in the magnet base material 20 is preferably 0.05% by mass or more and 0.50% by mass or less. In addition, when Ga, Al, and Zr are contained as the m elements, the contents of these elements are not particularly limited, and for example, it is preferable that the Ga content be 0.08% by mass or more and 0.30% by mass or less, the Al content be 0.10% by mass or more and 0.30% by mass or less, and the Zr content be 0.10% by mass or more and 0.30% by mass or less.

The magnet base material 20 may also contain carbon (C), nitrogen (N), and oxygen (O). The carbon content in the magnet base material 20 is preferably 1100 ppm by mass or less, and more preferably 900 ppm by mass or less. The nitrogen content in the magnet base material 20 is preferably 1000 ppm by mass or less, and more preferably 600 ppm by mass or less. The oxygen content in the magnet base material 20 is preferably 1200 ppm by mass or less, and more preferably 800 ppm by mass or less.

Furthermore, in addition to the above-mentioned elements, the magnet base material 20 may contain inevitable impurities such as Mn, Ca, Cl, S, and F, and the total content of the inevitable impurities is, for example, approximately 0.001% to 1.0% by mass.

The composition of the magnet substrate 20 can be analyzed by methods that are generally known in the art. For example, the content of various elements such as R, T, and B can be measured by X-ray fluorescence analysis (XRF) or inductively coupled plasma emission spectrometry (ICP). For example, the oxygen content can be measured by inert gas fusion-non-dispersive infrared absorption method, the carbon content can be measured by combustion in an oxygen stream-infrared absorption method, and the nitrogen content can be measured by inert gas fusion-thermal conductivity method.

As shown in FIG. 3A , the magnet base material 20 includes main phase grains 4 made of R ₂ T ₁₄ B crystals and grain boundaries 6 located between the main phase grains 4, and may also include subphases to the extent that they do not impair the magnetic properties.

The average particle size of the main phase particles 4 is not particularly limited, and can be, for example, 1.0 μm to 10 μm, preferably 2.5 μm to 6.0 μm, in terms of the circle-equivalent diameter. The average particle size of the main phase particles 4 can be measured by observing a cross section of the magnet substrate 20 as shown in FIG. 3A with various electron microscopes (SEM, STEM, TEM) and measuring the circle-equivalent diameters of at least 20 main phase particles 4 contained in the cross section.

In the R ₂ T ₁₄ B crystal constituting the main phase grains 4, part of the iron group element T may be substituted with a transition metal element such as an m element.

The grain boundaries 6 include two-particle boundaries 6a located between two adjacent main phase particles 4, and grain boundary multiple points 6b surrounded by three or more main phase particles 4. The grain boundaries 6 are composed of phases other than the main phase particles 4 (i.e., phases other than the R ₂ T ₁₄ B crystal phase), and the types and proportions of the phases constituting the grain boundaries 6 are not particularly limited. Examples of the other phases constituting the grain boundaries 6 include a phase (R-rich layer) in which the concentration of R is higher than that of the main phase particles 4, a phase in which the concentration of transition metal elements such as T (iron group elements) and Ga is high, an oxide layer of a rare earth element, and an R-O-C phase.

The magnet base material 20 having the above-mentioned characteristics can be manufactured through a process of producing a raw alloy, a process of crushing the raw alloy (crushing process), a process of molding the raw alloy powder (molding process), and a process of sintering the molded body (sintering process).

First, a raw metal corresponding to the composition of the R-T-B alloy described above is prepared, and the prepared raw metal is melted in a vacuum or in an inert gas atmosphere such as Ar gas. The molten raw metal is then cast to obtain the raw alloy.

In this case, there is no particular restriction on the type of raw metal, and for example, rare earth metals or rare earth alloys, pure iron, pure cobalt, ferroboron, and further alloys and compounds thereof can be used. In addition, there is no particular restriction on the method for casting the raw metal, and examples include ingot casting, strip casting, book molding, and centrifugal casting. In addition, if the raw alloy after casting has solidification segregation, it may be subjected to homogenization treatment (solution treatment) as necessary.

Next, the raw alloy obtained in the above process is crushed. In order to obtain high magnetic properties, it is preferable that the atmosphere in each process from the crushing process to the sintering process be an atmosphere with a low oxygen concentration. An atmosphere with a low oxygen concentration is, for example, an atmosphere with an oxygen concentration of 200 ppm or less. By controlling the oxygen concentration in each process, the amount of oxygen contained in the magnet base material 20 and the final permanent magnet 2 can be controlled.

The crushing process is carried out in two stages: coarse crushing and fine crushing. However, the raw alloy may be crushed in a single stage, the fine crushing process alone.

In the coarse pulverization process, the material is coarsely pulverized until the particle size is on the order of several hundred μm to several mm, to obtain a coarsely pulverized powder. The method of coarse pulverization is not particularly limited, and for example, a method of performing hydrogen absorption pulverization or a method using a coarse pulverizer can be adopted. When performing hydrogen absorption pulverization, the amount of nitrogen contained in the magnet base material 20 and the final permanent magnet 2 can be controlled by controlling the nitrogen gas concentration in the atmosphere during the dehydrogenation process.

Next, the obtained coarsely pulverized powder is pulverized until the average particle size is about several μm to obtain a pulverized powder (raw alloy powder). The average particle size of the pulverized powder is not particularly limited, but can be, for example, 1 μm to 10 μm, and preferably 2 μm to 6 μm. In the pulverization process, the amount of nitrogen contained in the magnet base material 20 and the final permanent magnet 2 can be controlled by controlling the nitrogen gas concentration in the atmosphere.

The method of fine grinding is not particularly limited, and various fine grinding machines can be used. In addition, it is preferable to add a grinding aid during fine grinding, and examples of the grinding aid include lauric acid amide and oleic acid amide. By using the grinding aid, it is possible to obtain a finely ground powder with high orientation during molding. Note that the amount of carbon contained in the magnet base material 20 and the final permanent magnet 2 can be controlled by changing the amount of grinding aid added.

In the above steps (from the raw alloy preparation step to the crushing step), the raw alloy powder was obtained using the one-alloy method, but the two-alloy method may also be used to prepare the raw alloy powder by mixing two alloys, a first alloy and a second alloy.

Next, the obtained raw alloy powder is molded into a predetermined shape. The molding method is not particularly limited, and may be dry molding or wet molding. In this embodiment, the raw alloy powder is filled into a die and pressurized in a magnetic field (dry molding).

The pressure during compaction can be, for example, 20 MPa to 300 MPa, and the applied magnetic field can be 950 kA/m to 1600 kA/m. The applied magnetic field is not limited to a static magnetic field, but can also be a pulsed magnetic field, or a combination of a static magnetic field and a pulsed magnetic field. When wet compaction is used, the raw alloy powder is dispersed in a solvent such as oil to form a slurry, and the slurry is used for press compaction in the same manner as above.

Next, the above-mentioned compact is sintered in a vacuum or in an inert gas to obtain a sintered body. The sintering conditions may be appropriately determined depending on various conditions such as the composition, the crushing method, and the particle size of the raw alloy powder. For example, the compact can be sintered by holding it in a vacuum or in an inert gas at a temperature of 1000°C to 1200°C for 1 to 10 hours.

After the sintering process, the sintered body may be decarbonized as necessary. The method of decarbonization is not particularly limited, and any known method may be used. For example, the carbon content in the sintered body can be reduced by attaching a specific metal S to the sintered body and heat treating it. In the above, the specific metal S is a metal whose standard Gibbs energy of formation for producing metal carbide from the metal is lower than that of the rare earth elements (especially Nd) in the sintered body. By attaching the metal S to the sintered body and heat treating it, the carbon in the sintered body reacts with the metal S to become carbide. The attached metal S hardly penetrates into the sintered body and remains on the surface of the sintered body. By performing the decarbonization process as described above, the amount of carbon contained in the magnet base material 20 and the final permanent magnet 2 can be reduced.

Furthermore, the sintered body may be subjected to shaping processes such as cutting and grinding, or chamfering processes such as barrel polishing, as necessary. These processes are carried out before the diffusion process described below.

The above method results in the magnet substrate 20 shown in Figures 1 and 3A.

(Grain boundary diffusion process)
Next, the grain boundary diffusion process (first diffusion process and second diffusion process) will be described. As described above, in this embodiment, the first diffusion process in which mainly light rare earth elements are diffused into the magnet base material 20 and the second diffusion process in which mainly heavy rare earth elements are diffused into the magnet base material 20 are carried out (see FIG. 2 ).

Preparation of Diffusion Material First, the first diffusion material 11 used in the first diffusion step and the second diffusion material 12 used in the second diffusion step are prepared.

The first diffusion material 11 contains at least one type of light rare earth element (RL), and the added RL is preferably one or more elements selected from the group consisting of Nd and Pr. The first diffusion material 11 can be a pure metal, an alloy, or a compound containing RL, such as a hydride, and is preferably an RL-M alloy containing RL and a specified M element.

In the above, the M element is an element whose eutectic temperature with M and RL is 800°C or less, and specifically, one or more elements selected from the group consisting of Al, Mg, Fe, Mn, Co, Ni, Cu, Zn, Ga, Ge, Ru, Rh, Pd, Ag, Sn, Sb, Pt, Au, Hg, Bi, Si, and Cl. Preferably, the M element is an element whose eutectic temperature with M and RL is 700°C or less, and specifically, one or more elements selected from the group consisting of Al, Mg, Fe, Mn, Co, Ni, Cu, Zn, Ga, Ag, Au, Hg, Si, and Cl. More preferably, the M element is an element whose eutectic temperature with M and RL is 600°C or less, and specifically, Mg, Co, Ni, Cu, and Cl. More preferably, the M element is Cu or Co.

If the sum of the contents of the rare earth element R and the M element contained in the first diffusion material 11 (i.e., R+M or RH+RL+M) is 100% by mass, the total content of the M element in the first diffusion material 11 (RL-M alloy) is preferably, for example, 5% to 20% by mass, depending on the type of M element selected. In particular, when Cu is added as the M element, the content of Cu in the first diffusion material 11 (RL-M alloy) is preferably 10% to 15% by mass. Note that the above content is the ratio of the M element to 100% by mass of the first diffusion material. By including the above M element in the first diffusion material 11, the melting point of the first diffusion material 11 is lowered, and RL is more easily diffused into the magnet base material 20.

The first diffusion material 11 may contain a heavy rare earth element (RH), but the RH content is preferably 10 mass% or less relative to 100 mass% of the sum of the contents of R and M elements in the first diffusion material. However, it is more preferable that the first diffusion material 11 does not substantially contain RH. "Substantially does not contain RH" more specifically means that the RH content is less than 0.5 mass%.

On the other hand, the second diffusion material 12 contains at least one heavy rare earth element RH, and the added RH is preferably one or more elements selected from the group consisting of Tb and Dy. The second diffusion material 12 can be a pure metal, an alloy, or a compound containing RH such as _TbH2 , and is preferably an RH-M alloy containing RH and an M element. The M element in the second diffusion material 12 is the same as the M element in the first diffusion material 11, and the amount of the M element added can be the same as that of the first diffusion material 11. The types of M elements selected for the first diffusion material 11 and the second diffusion material 12 may be different or the same.

The second diffusing material 12 may contain RL in addition to the elements RH and M. In this case, it is preferable that the content of RL is 22.5 mass% or less relative to 100 mass% of the sum of the contents of R and M elements contained in the second diffusing material (i.e., R+M or RH+RL+M). It is also preferable to adjust the amounts of RL and M elements added to the second diffusing material 12 so that the content of RH is 60 mass% or more.

First diffusion process In the first diffusion process, the above-mentioned first diffusion material 11 is attached to at least a portion of the surface of the magnet base material 20, and then the magnet base material 20 is heat-treated under predetermined conditions, thereby diffusing the RL and M elements in the first diffusion material 11 into the interior of the magnet base material 20.

The surface of the magnet base material 20 to which the first diffusion material 11 is attached is preferably at least a part of the main surface 20a that is the magnetic pole surface. In this case, the first diffusion material 11 may be attached to only one of the pair of main surfaces 20a, or the first diffusion material 11 may be attached to both main surfaces 20a. When the thickness of the magnet base material 20 in the Z-axis direction (the distance between the magnetic pole surfaces) is as thin as 2 mm or less, it is preferable to attach the first diffusion material 11 only to one of the main surfaces 20a. The first diffusion material 11 may also be attached to at least a part of the side surface 20b that is the non-magnetic pole surface. The number of surfaces to which the first diffusion material 11 is attached and the attachment area thereof may be appropriately determined depending on the dimensions of the magnet base material 20 and the application of the R-T-B system permanent magnet 2 that is the target. In this embodiment, as an example, the first diffusion material 11 is attached to the entire surface of the main surface 20a above the Z axis.

The method for attaching the first diffusion material 11 to the surface of the magnet substrate 20 is not particularly limited, and for example, a vapor deposition method, a sputtering method, an electrodeposition method, a coating method, a printing method (such as screen printing or squeegee printing), a sheet method, etc. can be used.

When using the sheet method, the powdered first diffusion material 11 is mixed with a binder to produce a sheet 11a of the first diffusion material as shown in FIG. 2, and the sheet 11a is adhered to the surface of the magnetic substrate 20. When adhering the sheet 11a to the magnetic substrate 20, an organic solvent such as alcohol, aldehyde, or ketone may be applied to the surface of the sheet 11a or the surface of the magnetic substrate 20.

When the coating method is used, a slurry is prepared by dispersing the powdered first diffusion material 11 in an organic solvent such as alcohol, aldehyde, or ketone. The slurry is then applied to the surface of the magnet substrate 20 using a spray, a brush, a jet dispenser, a nozzle, or the like. A binder may be added to the slurry so that the first diffusion material 11 can easily adhere to the surface of the magnet substrate 20. A paste having a higher viscosity than the slurry may be prepared using the first diffusion material 11, and the paste may be applied to the surface of the magnet substrate 20. In various printing methods, the paste may be printed on the surface of the magnet substrate 20.

As described above, the first diffusion material 11 can be attached to the magnet substrate 20 by a number of methods, but whichever method is used, it is preferable to control the amount of the first diffusion material 11 attached to satisfy a predetermined condition. In other words, it is preferable to control the amount of the first diffusion material 11 attached so that the total amount of RL to be attached is 0.1 to 2.0 parts by mass (more preferably 0.4 to 1.5 parts by mass) per 100 parts by mass of the magnet substrate.

The conditions for the heat treatment in the first diffusion step are that the heat treatment atmosphere is in a vacuum or in an inert gas, the holding temperature is preferably above 700°C and below 1000°C, and the temperature holding time is preferably 1 hour to 24 hours. After the specified holding time has elapsed, it is preferable to rapidly cool the magnet base material 20, and for example, the cooling rate during rapid cooling is preferably 50°C/min or more. By performing heat treatment under the above conditions, the RL (RL and M element if M element is included) contained in the first diffusion material 11 diffuses into the grain boundaries 6 of the magnet base material 20.

It is believed that by diffusing RL into the magnet base material 20 in the first diffusion step, the grain boundaries 6 (particularly the two-particle grain boundaries 6a) expand, and RH is more likely to diffuse to the vicinity of the center of the interior of the magnet base material 20 in the second diffusion step described below. When the magnet base material 20 contains only Nd as R and the first diffusion material 11 contains RL other than Nd, such as Pr, a shell containing RL other than Nd may be formed on the outer periphery of the main phase grains 4 by the first diffusion step. It is believed that the shell is generated by replacing a part of R (Nd) in _{the R2T14B crystal} with RL other than Nd.

After the first diffusion step, residues such as an oxide film may remain on the surface of the magnet base material 20. Therefore, after the first diffusion step, it is preferable to carry out a processing step to remove the residues. There are no particular limitations on the method for removing the residues, and for example, chemical removal such as etching, physical cutting, shaping such as grinding, or chamfering such as barrel polishing may be carried out.

Second Diffusion Step Next, the second diffusion step is carried out using the above-mentioned second diffusion material 12. In the second diffusion step, the second diffusion material 12 is attached to at least a part of the surface of the magnet base material 20, and then the magnet base material 20 is heat-treated under predetermined conditions, thereby diffusing the RH and M elements in the second diffusion material 12 into the inside of the magnet base material 20.

The second diffusion material 12 is preferably attached to the surface (principal surface 20a and side surface 20b) of the magnet base material 20 to which the first diffusion material 11 was attached in the first diffusion step. For example, as shown in FIG. 2, when the first diffusion material 11 is attached to the principal surface 20a (pole surface) above the Z axis, the second diffusion material 12 is preferably attached to the principal surface 20a above the Z axis in the same manner as the first diffusion material 11. The attachment area of the second diffusion material 12 may be larger or smaller than the attachment area of the first diffusion material 11, but is preferably about the same. When the attachment area of the second diffusion material 12 is reduced, the second diffusion material 12 is preferably attached to a location in the resulting permanent magnet where it is desired to improve the coercive force HcJ in particular.

The method for attaching the second diffusion material 12 is not particularly limited, and similar to the first diffusion process, vapor deposition, sputtering, electrodeposition, coating, printing (screen printing, squeegee printing, etc.), sheeting, etc. can be used. FIG. 2 shows, as an example, the use of a sheet 12a of the second diffusion material, which is obtained by mixing the powdered second diffusion material 12 with a binder and forming the mixture into a sheet.

The amount of the second diffusion material attached is preferably controlled to satisfy a predetermined condition. In other words, it is preferable to control the amount of the second diffusion material 12 attached so that the total amount of RH attached is 0.3 parts by mass to 2.0 parts by mass (more preferably 0.4 parts by mass to 1.0 parts by mass) per 100 parts by mass of the magnet base material.

The conditions of the heat treatment in the second diffusion step are that the heat treatment atmosphere is in a vacuum or in an inert gas, the holding temperature is preferably more than 700°C and less than 1000°C, and the temperature holding time is preferably 1 hour to 24 hours. After the predetermined holding time has elapsed, it is preferable to rapidly cool the magnet base material 20, and for example, the cooling rate during rapid cooling is preferably 50°C/min or more. By performing heat treatment under the above conditions, the RH contained in the second diffusion material 12 (RH and M element if M element is included) diffuses to the grain boundary 6 of the magnet base material 20. The second diffusion step may include an aging step. The aging step is preferably performed by holding the magnet base material 20 at a temperature of 450°C to 700°C for 0.2 hours to 3 hours in a vacuum or in an inert gas after the above rapid cooling, and it is preferable to rapidly cool it after the holding time has elapsed.

The RH diffused to the grain boundaries 6 in the second diffusion step reacts with a part of the R ₂ T ₁₄ B crystals and becomes concentrated at the outer periphery of the main phase grains 4. That is, after the second diffusion step, as shown in Fig. 3B, a shell portion 42 with a high RH content is formed in the main phase grain 4 in the region where RH has diffused, and the main phase grain 4 becomes a core-shell grain 4a. The core-shell grains 4a will be described in detail later in the explanation of the R-T-B system permanent magnet 2.

After the second diffusion step, it is preferable to carry out a process to remove residues, as in the case of the first diffusion step. The method for removing residues is not particularly limited, and may be, for example, chemical removal such as etching, physical cutting, shaping such as grinding, or chamfering such as barrel polishing.

By using the above method, an R-T-B system permanent magnet 2 (Figure 4A) that combines high residual magnetic flux density and high coercivity is obtained. Below, we will explain the characteristics of the R-T-B system permanent magnet 2 obtained by the manufacturing method of this embodiment.

(RTB system permanent magnet 2)
In the R-T-B system permanent magnet 2 (hereinafter referred to as permanent magnet 2) of this embodiment shown in FIG. 4A, a pair of main surfaces 2a are magnetic pole surfaces, and of the pair of main surfaces 2a, The main surface 2a above the axis is the surface to which the first diffusion material 11 and the second diffusion material 12 are attached. In FIG. 4A, the permanent magnet 2 is arranged in such a manner as to match the shape of the magnet base material 20 in FIG. However, the shape and dimensions of the permanent magnet 2 are not particularly limited, and may be, for example, a polygonal shape, a cylindrical shape, a hollow cylindrical shape, or an arc segment shape whose main surface is curved in an arc shape. It may be a shape.

The permanent magnet 2 has a composition that is slightly different from the composition of the magnet base material 20 used during manufacture, due to the grain boundary diffusion of RL and RH in that order during the manufacturing process.

Specifically, in the permanent magnet 2, R includes one or more light rare earth elements (RL), of which Nd is an essential element, and one or more heavy rare earth elements (RH). RL other than Nd preferably includes Pr, and RH preferably includes Tb and/or Dy.

The total content of R relative to 100% by mass of the permanent magnet can be 25% to 35% by mass, and is preferably 28.4% by mass or more and 32.0% by mass or less. Furthermore, the total content of RL relative to 100% by mass of the permanent magnet is preferably 26.0% by mass or more and 31.5% by mass or less, and the total content of RH is preferably 0.3% by mass or more and 3.0% by mass or less. In addition, the content of Nd relative to 100% by mass of the permanent magnet is preferably 22.4% by mass or more and 31.5% by mass or less.

As described above, in the permanent magnet 2, the R content increases compared to the R content in the magnet base material 20 depending on the amount of RL and RH diffused by the diffusion materials (11, 12). On the other hand, the B content in the permanent magnet 2 can be made approximately the same as the B content in the magnet base material 20. In other words, the B content relative to 100% by mass of the permanent magnet can be 0.5% to 1.5% by mass, and is preferably 0.92% to 1.03% by mass. When the B content satisfies the above composition range, the magnetic properties (Br, HcJ, squareness ratio Hk/HcJ, etc.) of the permanent magnet 2 tend to be further improved.

In addition, the permanent magnet 2 preferably contains the M element added to the first diffusion material 11 and/or the second diffusion material 12. The content C _M of the M element resulting from the diffusion material (11, 12) is preferably 0.5 mass% or less, and more preferably 0.05 mass% or more and 0.2 mass% or less, relative to 100 mass% of the permanent magnet. For example, when an element M _NO (Ag, Au, etc.) that is not substantially contained in the magnet base material 20 is selected as the M element to be added to the diffusion material, the above content C _M becomes the content of the element M _NO in the permanent magnet 2. On the other hand, when an element M _IN originally contained in the magnet base material 20 is selected as the M element to be added to the diffusion material, the content of the element M _IN in the permanent magnet 2 is in the range obtained by adding the above content C _M to the content in the magnet base material 20.

For example, when Fe, Co, or Ni is added to the first diffusion material 11 and/or the second diffusion material 12, the content of T in the permanent magnet 2 increases slightly compared to that of the magnet base material 20 in the manufacturing process (the above-mentioned content C _M is added). The content of T in the permanent magnet 2 is expressed as the remainder. T in the permanent magnet 2 may be Fe alone, or a part of Fe may be substituted with Co. In this case, the content of Co is not particularly limited, and may be, for example, 4 mass% or less, and preferably 1 mass% or less. The inclusion of Co improves the corrosion resistance of the permanent magnet 2.

The permanent magnet 2 preferably contains one or more m elements selected from the group consisting of Cu, Ga, Al, and Zr, and more preferably contains Cu as the m element. The m elements are added to the permanent magnet 2 by the magnet base material 20 and/or the diffusion material (11, 12). The total content of the m elements relative to 100% by mass of the permanent magnet is preferably 0.05% by mass to 1.5% by mass, and more preferably 0.5% by mass to 1.0% by mass.

More specifically, the content of Cu relative to 100% by mass of the permanent magnet can be 0.05% by mass or more and 0.70% by mass or less, and preferably 0.20% by mass or more and 0.70% by mass or less. By containing Cu in the above ratio, the magnetic properties and corrosion resistance of the permanent magnet 2 are further improved. In addition, when Ga, Al, and Zr are included as the m element, the contents of these elements are not particularly limited. For example, the content of Ga can be 0.08% by mass or more and 0.50% by mass or less, and preferably 0.08% by mass or more and 0.3% by mass or less. The content of Al and the content of Zr can both be 0.10% by mass or more and 0.50% by mass or less, and preferably 0.10% by mass or more and 0.30% by mass or less. By containing Ga, Al, Zr, etc. in the above-mentioned predetermined amounts, it is possible to further improve the magnetic properties (Br, HcJ, Hk/HcJ, etc.), reduce the variation in properties, and improve the manufacturing stability.

The permanent magnet 2 may also contain carbon (C), nitrogen (N), and oxygen (O). The carbon content in the permanent magnet 2 is preferably 1100 ppm by mass or less, and more preferably 900 ppm by mass or less. The nitrogen content in the permanent magnet 2 is preferably 1000 ppm by mass or less, and more preferably 600 ppm by mass or less. The lower the carbon content and nitrogen content, the more the coercive force HcJ tends to improve. The oxygen content in the permanent magnet 2 is preferably 1200 ppm by mass or less, and more preferably 800 ppm by mass or less. The lower the oxygen content, the more the corrosion resistance tends to improve.

Furthermore, in addition to the above elements, the permanent magnet 2 may contain unavoidable impurities such as Mn, Ca, Cl, S, and F, and the total content of the unavoidable impurities is, for example, approximately 0.001% to 1.0% by mass.

The composition of the permanent magnet 2 can be measured by XRF, ICP, inert gas fusion - non-dispersive infrared absorption method, combustion in oxygen flow - infrared absorption method, inert gas fusion - thermal conductivity method, etc., in the same manner as the composition analysis of the magnet substrate 20.

In this embodiment, a concentration distribution of a specific element occurs inside the permanent magnet 2. Specifically, the total content of RL decreases continuously from the main surface 2a (pole surface) toward the depth direction (i.e., from the surface of the permanent magnet 2 toward the inside). This concentration distribution of RL occurs due to grain boundary diffusion of RL in the first diffusion process. Also, in the permanent magnet 2, the total content of RH decreases continuously from the main surface 2a (pole surface) toward the depth direction. This concentration distribution of RH occurs due to grain boundary diffusion of RH in the second diffusion process.

Furthermore, in the permanent magnet 2, it is preferable that the content of the M element (preferably one or more selected from Cu, Al, Fe, and Co) continuously decreases from the main surface 2a (pole surface) toward the depth direction. Such a concentration distribution of the M element is generated by the grain boundary diffusion of the M element in the diffusion material (11, 12) in the first diffusion step and/or the second diffusion step. Note that, when an iron group element (Fe, Co, or Ni) is added as the M element in the diffusion material (11, 12), the above-mentioned concentration distribution of the M element becomes the concentration distribution of T. Therefore, when an iron group element is selected as the M element of the diffusion material, it is preferable that the content of T continuously decreases from the main surface 2a (pole surface) toward the depth direction.

The concentration distribution of each of the above-mentioned elements (RL, RH, M elements) can be confirmed using multiple samples β shown in FIG. 4B. Specifically, samples β of a predetermined width Tβ (width in the Z-axis direction) are continuously collected from the main surface 2a along the depth direction (toward the inside of the permanent magnet 2), and each of the collected multiple samples β is subjected to component analysis using ICP. In this case, it is preferable that the predetermined width Tβ is 1 mm or less, or that Tβ<(1/10)×T0.

In the permanent magnet 2 of this embodiment, the results of the above analysis show that the total content of RL, the total content of RH, and the content of M tend to decrease continuously and gradually as the location where sample β is taken increases in depth. In particular, when the difference (β1-β center ) in the content of a given element is measured between the outermost sample β1 and the central sample β _center (sample β taken at a depth of 1/2T0 from the main _surface 2a), the difference in the total content of RL is preferably about 0.1% to 0.5% by mass, the difference in the total content of RH is preferably about 0.1% to 0.5% by mass, and the difference in the content of M element is preferably about 0.1% to 0.3% by mass.

In addition to the above measurement methods, the concentration distribution of each element (RL, RH, M elements) can also be confirmed by performing line analysis or mapping analysis using EDX or EPMA, or laser ablation-ICP mass spectrometry (LA-ICP-MS) on a cross section such as that shown in Figure 4B.

3B is a schematic diagram showing an enlarged cross section of the permanent magnet 2 in this embodiment. As described above, in the permanent magnet 2, the main phase particles 4 are core-shell particles 4a in the region where RH is diffused. The core-shell particles 4a have a core portion 41 made of an R ₂ T ₁₄ B crystal and a shell portion 42 that covers the core portion 41 and has a higher content of heavy rare earth elements than the core portion 41. The average particle size of the core-shell particles 4a is not particularly limited, and can be, for example, 1.0 μm to 10 μm, and preferably 2.5 μm to 6.0 μm, in terms of the circle-equivalent diameter.

The shell portion 42 does not need to cover the entire circumference of the core portion 41, but only needs to cover at least a portion of the core portion 41. The coverage of the core portion 41 by the shell portion 42 is not particularly limited, but for example, it is preferable that the shell portion 42 covers 50% or more of the outer periphery of the core portion 41. In addition, in the cross section shown in FIG. 3B, it is not necessary for all main phase particles 4 to be core-shell particles 4a, and there may be main phase particles 4 that do not have a core-shell structure.

As described above, the formation of a shell portion 42 with concentrated RH on the outer periphery of the main phase grains 4 suppresses the generation of nuclei of magnetization reversal near the grain boundaries, thereby improving the coercive force HcJ.

In addition, in the permanent magnet 2 of this embodiment, the shell thickness of the core-shell particles 4a near the surface where RH is diffused (in this embodiment, near the main surface 2a) can be made thinner than in a conventional permanent magnet in which RH is diffused by only one stage of grain boundary diffusion treatment. In fact, FIG. 6 is a cross-sectional photograph of the permanent magnet 2 of this embodiment in which two stages of grain boundary diffusion treatment have been performed, and FIG. 7 is a cross-sectional photograph of a conventional permanent magnet in which only one stage of grain boundary diffusion treatment has been performed. Note that both FIG. 6 and FIG. 7 are cross sections perpendicular to the surface (main surface 2a) to which the diffusion material is attached, and are cross sections observed in a range from the surface to a predetermined depth. In addition, in FIG. 6 and FIG. 7, the area with the brightest contrast is the grain boundary phase, the area with the darkest contrast is the core portion 41 (main phase particles 4), and the gray contrast surrounding the core portion 41 is the shell portion 42.

In the conventional permanent magnet shown in Figure 7, the presence of a gray shell portion around the core portion can be clearly seen, and it can be confirmed that a thick shell portion on the order of submicrons to microns is formed near the surface (up to a depth of 100 μm). On the other hand, in the cross section of the permanent magnet 2 of this embodiment shown in Figure 6, it can be seen that the thickness of the shell portion 42 near the surface is clearly thinner than that of the conventional permanent magnet in Figure 7.

More specifically, in the permanent magnet 2 of this embodiment, in the cross section A at a depth of 100 μm from the surface (main surface 2a), the maximum thickness t1 of the shell portion 42 is, on average, 0.5 μm or less, preferably 0.2 μm or less, and more preferably 0.1 μm or less. By controlling the maximum thickness t1 of the shell portion 42 within the above range near the surface of the permanent magnet 2, a higher residual magnetic flux density Br and a higher coercive force HcJ can be obtained than in the conventional permanent magnet as shown in FIG. 7. The reason why the magnetic properties are improved in this way is not necessarily clear, but it is thought to be related to the RH concentration in the shell portion 42. Specifically, it is thought that by reducing the thickness of the shell portion 42, the RH concentration in the shell portion 42 becomes higher than that of the conventional permanent magnet (FIG. 7). As a result, it is thought that the permanent magnet 2 of this embodiment improves HcJ more efficiently without reducing Br.

In addition, in the permanent magnet 2 of this embodiment, as described above, the thickness of the shell portion 42 is thinner near the surface, and RH is diffused deeper into the interior of the permanent magnet 2. Specifically, even when the diffusion material (11, 12) is attached only to one of the main surfaces 2a of the permanent magnet 2, it can be confirmed that the core-shell particles 4a are present from the main surface 2a to a depth of 3.5 mm, and an improvement in HcJ can be expected up to a depth of 3.5 mm.

The maximum thickness t1 of the shell portion 42 is calculated, for example, based on an analysis using STEM-EDS. First, cross section A (for example, the XY plane shown by the dashed line in FIG. 4B) is exposed at a depth of 100 μm from the main surface 2a to which the diffusion material (11, 12) is attached, and an observation sample for STEM is taken from the cross section. The method of taking the sample may be a microsampling method using FIB processing.

Then, the main phase grains 4 (core-shell grains 4a) contained in the collected observation sample (observation cross section) are analyzed by STEM-EDS. In this embodiment, the core portion 41 and the shell portion 42 are distinguished based on the concentration difference Dc of the heavy rare earth element and the concentration degree E of the heavy rare earth element. Specifically, in the cross section analysis, a virtual rectangle VR (see FIG. 5) is prepared that contains the main phase grain 4 to be measured and has the smallest area relative to the main phase grain 4, and the intersection of the diagonals of the rectangle VR is set as the grain center 41c. Then, the RH concentration distribution and the RL concentration distribution in the main phase grain 4 are obtained by EDS, and the difference (Ch _n -Ch _center ) between the RH concentration at a predetermined location (Ch _n ) and the RH concentration at the grain _center 41c is set as the RH concentration difference Dc (unit: wt%). The molar ratio of RH to RL at a given location is defined as M _H/L ⁿ , the molar ratio of RH to RL at the particle center 41c is defined as M _H/L ^center , and the ratio of M _H/L ⁿ to M _H/L ^center is defined as the concentration degree E of RH (no unit).

In this embodiment, the shell portion 42 has the concentration difference Dc of 0.7 wt% or more and the enrichment degree E of 1.25 or more. That is, in the cross-sectional analysis, the region satisfying these two requirements is recognized as the shell portion 42, and the region of the main phase particle 4 other than the shell portion 42 is recognized as the core portion 41. However, there may be cases where the RH concentration detected at the particle center 41c is less than 0.2 wt%. In this case, in consideration of the measurement noise, it is assumed that RH is not substantially detected at the particle center 41c (i.e., Ch _center ≒0), and the region with the RH concentration of 0.7 wt% or more (i.e., the concentration difference Dc is 0.7 wt% or more) may be identified as the shell portion 42. In addition, when the cross-sectional sample is observed using a HAADF image or the like, it is also possible to simply distinguish the core portion 41 from the shell portion 42 based on the difference in contrast.

After defining the boundary between the core portion 41 and the shell portion 42 by the above-mentioned method, the thickness of the thickest part of the shell portion 42 within the unit particle is measured as shown in FIG. 5. This measurement is performed on at least 20 core-shell particles 4a, and the average value is taken as the maximum thickness t1 mentioned above. The shell thickness can also be analyzed using a three-dimensional atom probe (3DAP).

The diffusion depth of RH can be measured by analyzing samples β (FIG. 4B) taken continuously from the surface (principal surface 2a) of the permanent magnet 2 along the depth direction. For example, the cross sections of multiple samples β can be observed with an STEM to confirm the presence or absence of core-shell particles 4a, thereby measuring the depth to which RH has diffused. Furthermore, the magnetic properties of multiple samples β taken can be measured with various BH tracers, etc., to measure the depth to which an increase in HcJ can be observed.

Specifically, the magnetic properties of the permanent magnet 2 are preferably Br of 1450 mT or more and HcJ of 1800 kA/m or more. By making the permanent magnet 2 satisfy these magnetic properties, the motor having the permanent magnet 2 can be made to have high performance and high efficiency.

(Field of application of R-T-B system permanent magnet 2)
The RTB system permanent magnet 2 of this embodiment can be used as a component of motors, generators, compressors, actuators, magnetic sensors, speakers, etc., and is particularly suitable as a component of motors.

An example of a motor that uses a permanent magnet 2 is an IPM motor 50 as shown in FIG. 8. The IPM motor 50 has a rotor 51 and a stator core 52, and the permanent magnet 2 of this embodiment is embedded in the rotor 51. The permanent magnet 2 in the rotor 51 faces a coil 54 in the stator via a gap 53. Note that the permanent magnet 2 of this embodiment is not limited to the IPM motor of FIG. 8, and can be applied to various motors such as an SPM motor.

Motors including the permanent magnet 2 of this embodiment can be mounted on various electronic devices and industrial equipment, and are particularly suitable for use as automobile motors 50 (EV, HV, PHV, etc.) as shown in FIG. 9. In FIG. 9, a typical EV vehicle 101 and HV 102 are shown in a simple schematic diagram, but the use of motors including the permanent magnet 2 is not limited to the form shown in FIG. 9. In FIG. 9, reference numeral 60 denotes an inverter, reference numeral 70 denotes a battery, reference numeral 80 denotes an engine, and reference numeral 90 denotes a generator.

(Summary of the embodiment)
In the manufacturing method of the R-T-B system permanent magnet 2 in this embodiment, a first diffusion step is carried out in a first diffusion material 11 containing a light rare earth element (RL), and then a second diffusion step is carried out in a second diffusion material 12 containing a heavy rare earth element (RH). In other words, a two-stage grain boundary diffusion is carried out in which RL is diffused and then RH is diffused.

In the R-T-B system permanent magnet 2 obtained by this method, the maximum thickness t1 of the shell portion 42 at a depth of 100 μm from the surface of the magnet is thin, t1≦0.5 μm, preferably t1≦0.2, and more preferably t1≦0.1. In this way, by reducing the thickness of the shell portion 42 near the surface, it is possible to further increase the RH concentration in the shell portion 42, and it is believed that RH is favorably diffused to the inside of the magnet. As a result, in the R-T-B system permanent magnet 2, Br and HcJ are both improved more than in the past. In other words, the two-stage grain boundary diffusion process of this embodiment produces a permanent magnet 2 that has both a higher residual magnetic flux density Br and a higher coercive force HcJ compared to the case of performing the conventional one-stage grain boundary diffusion process.

The reason why the above effects are achieved by the two-stage grain boundary diffusion treatment is not entirely clear, but the following reasons are considered, for example:

First, it is believed that the suppression of decomposition of _{R2T14B crystals} (main phase) by RH is related. In the grain boundary diffusion treatment, when RH penetrates along the grain boundaries and contacts the _{R2T14B crystals} in a heated state _, the outer periphery of the _R2T14B crystals (particularly _{Nd2Fe14B crystals} ) reacts with RH and is decomposed into a crystal phase or liquid phase (e.g., _RFe2 phase or _RFe4B4 phase ₎ different from the main phase. Then, it is believed that the different crystal phases and liquid phases generated by this decomposition are recrystallized to form a shell portion in which RH is concentrated.

When RH is diffused through grain boundaries in a single diffusion step as in the past, it is believed that a high concentration of RH comes into contact with the R ₂ T ₁₄ B crystals near the surface of the magnet, and the decomposition reaction of the R ₂ T ₁₄ B crystals described above occurs actively. As a result, in the conventional grain boundary diffusion treatment, a thick shell is formed near the surface of the magnet. Although the shell part where RH is concentrated has the effect of improving HcJ, the thickness of the shell required to improve HcJ is only a few nanometers, and it is believed that an excessively thick shell will warp and cause a decrease in Br. Furthermore, in the conventional each step process, a lot of the attached RH is consumed near the surface of the magnet, making it difficult to sufficiently diffuse into the inside of the magnet, and the effect of improving HcJ is weakened.

On the other hand, in the two-stage grain boundary diffusion process in this embodiment, RL is diffused before RH is diffused. Since no metal compound phase other than R ₂ T ₁₄ B occurs between RL and R ₂ T ₁₄ B crystal (specifically, between Nd ₂ Fe ₁₄ B crystal and Nd or Pr), it is considered that the decomposition reaction of the main phase that occurs between RH and R ₂ T ₁₄ B crystal does not occur in the first diffusion step of this embodiment. Therefore, it is considered that after the first diffusion step, RL is concentrated in the grain boundary 6, and the concentration of RL is higher than that before the first diffusion step. It is considered that if RH is diffused in this state (second diffusion step), the ratio of RH to all rare earth elements in the grain boundary 6 can be relatively reduced.

That is, in the two-stage grain boundary diffusion process of this embodiment, RH comes into contact with the _R2T14B crystal at a lower concentration than in the conventional one-stage diffusion, and it is believed that the decomposition reaction of the main phase is suppressed. As a result, in this embodiment, the thickness of the shell portion ₄₂ can be made thinner than in the conventional one-stage diffusion, and the RH concentration in the shell portion 42 can be increased by the reduced thickness, and a high Br can be maintained. In addition, in this embodiment, the suppression of the decomposition reaction of the main phase is believed to suppress the consumption of RH near the surface of the magnet, and RH is believed to be more easily diffused to the inside of the magnet. As a result, the effect of improving HcJ by the diffusion of RH is enhanced, and it is believed that both high HcJ and high Br can be achieved.

The reason why RH is more likely to diffuse into the magnet is thought to be that the diffusion of RL in the first diffusion step expands the two-particle grain boundary 6a. The expansion of the grain boundary 6 by RL makes it easier for RH to penetrate into the magnet in the second diffusion step, and it is thought that a higher HcJ can be obtained than with conventional methods. Furthermore, in the manufacturing method of this embodiment, RL is diffused before the diffusion of RH, which is thought to suppress the grain growth of the main phase particles 4 in the second diffusion step, and it is thought that the effect of suppressing this grain growth contributes to improving the magnetic properties.

The above-mentioned principle regarding the modification of the grain boundary 6 (suppression and expansion of the decomposition reaction) is merely speculation, and the effects of the present invention are not limited to the above. Furthermore, the manufacturing method of this embodiment (two-stage grain boundary diffusion treatment) can reduce the amount of RH required to obtain magnetic properties (particularly HcJ) equivalent to those of conventional permanent magnets. As described above, the two-stage grain boundary diffusion treatment of this embodiment can reduce the shell thickness near the surface and can diffuse RH deeper inside the magnet.

When the magnet base material 20 used during manufacturing has a specified composition, the above-mentioned effect of improving Br and HcJ becomes more pronounced. That is, in the magnet base material 20 (R-T-B alloy), the total content of R is 27.5 mass% or more and 30.8 mass% or less, the content of Cu is 0.05 mass% or more and 0.5 mass% or less, and the content of B is 0.92 mass% or more and 1.03 mass% or less. In this composition, the total content of R is lower than the composition range of a general R-T-B alloy. In this way, by using a magnet base material 20 with a low content of R, the effect of improving Br and HcJ by the two-stage grain boundary diffusion treatment becomes even greater.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-mentioned embodiments and can be modified in various ways within the scope of the present invention.

The present invention will be described in more detail below using examples and comparative examples. However, the present invention is not limited to the following examples.

Experiment 1
In experiment 1, the composition of the magnet base material 20 was varied and a two-stage grain boundary diffusion treatment was carried out to produce RTB system permanent magnets according to samples 1 to 21, and the magnetic properties of these samples were evaluated.

(Samples 1 to 21)
First, Nd, Pr, Tb, DyFe, electrolytic iron, and a low carbon ferroboron alloy were prepared as raw materials. Furthermore, Al, Ga, Cu, Co, and Zr were prepared in the form of pure metal or alloy with Fe. Then, the above raw materials were weighed so that the composition of the magnet substrate 20 would be as shown in Table 2, and a raw material alloy was produced by a strip casting method. The thickness of the raw material alloy obtained here was 0.2 mm to 0.6 mm.

Next, hydrogen gas was flowed through the raw alloy at room temperature for one hour to allow hydrogen to be absorbed. After that, the atmosphere was switched from hydrogen gas to Ar gas, and dehydrogenation was performed at 450°C for one hour, and the raw alloy was hydrogen-pulverized. After the pulverization, the raw alloy was cooled and classified using a sieve to obtain a powder (coarsely pulverized material) with a particle size of 400 μm or less. In the manufacture of the samples other than sample 20, a low-oxygen atmosphere with an oxygen concentration of 200 ppm or less was always used from this hydrogen absorption and pulverization to the sintering process described below. Meanwhile, for sample 20, the oxygen concentration was adjusted in each process so that the oxygen content of the magnet base material 20 was a predetermined amount.

Next, oleic acid amide was added as a grinding aid to the coarsely ground material and mixed. This coarsely ground material was then finely ground in a nitrogen gas flow using a collision plate type jet mill device to obtain fine powder (raw alloy powder) with an average particle size of approximately 3.9 μm to 4.2 μm. In this fine grinding process, the amount of oleic acid amide added was adjusted so that the carbon content in the magnet base material 20 was within the desired range. The above average particle size is the average particle size D50 measured using a laser diffraction type particle size distribution analyzer.

Next, the raw alloy powder was molded in a magnetic field to produce a compact. At this time, the applied magnetic field was a static magnetic field of 1200 kA/m, the pressing force during molding was 120 MPa, and the magnetic field application direction and the pressing direction were perpendicular to each other. When the density of the compacts obtained in this molding process was measured, the density of all the compacts was in the range of 4.10 Mg/ ^m3 to 4.25 Mg/ ^m3 .

Next, the compact was sintered to obtain a sintered body made of an R-T-B alloy. The sintering conditions, which differed slightly depending on the composition, were maintained in a vacuum atmosphere at a temperature of 1040°C to 1100°C for 4 hours. The sintered density obtained in this sintering process was in the range of 7.45 Mg/ ^m3 to 7.55 Mg/ ^m3 for all samples. After that, the sintered body was vertically processed into a size of 15 mm x 10 mm x 5 mm (thickness in the direction of easy magnetization 5 mm) to obtain a magnet base material 20.

Here, the average composition of the magnet base material 20 in each sample was measured. Samples for analysis were obtained by crushing each sample to a size of about 1 mm average particle diameter using a stamp mill and randomly selecting the crushed material. The contents of various metal elements were measured using XRF and ICP. The oxygen content was measured by inert gas fusion-non-dispersive infrared absorption method, the carbon content was measured by combustion in oxygen gas flow-infrared absorption method, and the nitrogen content was measured by inert gas fusion-thermal conductivity method. The measurement results of each sample are shown in Table 2. The Fe content is the remainder, meaning that the contents of elements not listed in Table 2 are included in the Fe content, and the sum of R, T, B, and m is 100 mass%.

Then, the magnet base material 20 was subjected to a two-stage grain boundary diffusion treatment in the following manner. First, a first diffusion material 1A containing Nd (RL) and a second diffusion material 2K containing Tb (RH) were prepared with the compositions shown in Table 1. Both of these diffusion materials were alloy powders.

Next, as a pretreatment for grain boundary diffusion, the magnet substrate 20 was etched. Specifically, a mixed solution of 100% ethanol and 3% nitric acid by mass was prepared, and the magnet substrate 20 was immersed in the mixed solution for 3 minutes for etching, and then immersed in ethanol for 1 minute for cleaning. The above etching was performed twice for each sample.

Next, 75 parts by mass of the first diffusion material 1A containing Nd, 23 parts by mass of alcohol, and 2 parts by mass of acrylic resin were kneaded to obtain a first paste. The alcohol used here was a solvent, and the acrylic resin was a binder. The first paste was then applied to both of the pair of magnetic pole faces of the magnet base material 20. The amount of the first paste applied was adjusted so that the total amount of Nd attached to the magnet base material 20 was 1.0 part by mass relative to 100 parts by mass of the magnet base material. After application of the first diffusion material 1A, the magnet base material 20 was held at 900°C for 10 hours in an Ar atmosphere at atmospheric pressure, and then rapidly cooled (first diffusion process). After the first diffusion process, the magnet base material 20 was polished with 1200 grit abrasive paper until the residue present on the surface of the magnet base material 20 was removed.

Next, 75 parts by mass of the second diffusion material 2K containing Tb, 23 parts by mass of alcohol (solvent), and 2 parts by mass of acrylic resin (binder) were kneaded to obtain a second paste. The second paste was then applied to both of the pair of magnetic pole faces of the magnet base material 20. At this time, the amount of the second paste applied was adjusted so that the total amount of Tb attached to the magnet base material 20 was 0.5 parts by mass relative to 100 parts by mass of the magnet base material. After application of the second diffusion material 2K, the magnet base material 20 was held at 900°C for 10 hours in an Ar atmosphere at atmospheric pressure, and then quenched (second diffusion process). After the above heat treatment, the magnet base material 20 was heat treated at 600°C for 1 hour in an Ar atmosphere at atmospheric pressure, and then quenched (aging process). After the aging process, the magnet base material 20 was polished using 1200 grit abrasive paper until the residue present on the surface of the magnet base material 20 was removed.

The above steps resulted in the R-T-B system permanent magnets 2 relating to samples 1 to 21. The average composition of the R-T-B system permanent magnets 2 was measured in the same manner as the average composition of the magnet substrate 20 described above. The measurement results are shown in Table 4.

In addition, the maximum thickness t1 (average value of the results of measuring 20 particles) of the shell portion 42 at a cross section 100 μm deep from the magnetic pole surface was measured using the method described in the embodiment. The measurement results are shown in Table 4.

The obtained R-T-B system permanent magnet 2 was further magnetized in a pulsed magnetic field of 4000 kA/m, and then the residual magnetic flux density Br (mT), coercive force HcJ (kA/m), and squareness ratio Hk/HcJ (%) of each sample were measured using a BH tracer. In this example, Hk/HcJ was measured by taking the magnitude of the magnetic field Hk (kA/m) when the magnetization J was 90% of Br. Br was judged to be particularly good when it was 1450 mT or more, HcJ was judged to be particularly good when it was 1800 or more, and Hk/HcJ was judged to be particularly good when it was 95% or more. The measurement results for each of samples 1 to 21 are shown in Table 4.

Although not shown in Table 4, the presence or absence of concentration distribution of RH (Tb), RL (Nd), and M (Cu) inside each sample was investigated. As a result, it was confirmed that in all samples 1 to 21, the Nd content, Tb content, and Cu content gradually decreased from the pole surface toward the depth direction.

Comparative Example
In experiment 1, as shown in Tables 3 and 4, R-T-B system permanent magnets according to comparative examples 1, 2, 6, 7, 10, 11, 14, 16, 17, 20, 21, and 22 were manufactured. Specifically, in comparative examples 1, 2, 6, 7, 10, 11, 14, 16, 17, 20, and 21, magnet base materials were manufactured with the same compositions as the above-mentioned samples 1, 2, 6, 7, 10, 11, 14, 16, 17, 20, and 21, respectively, and the magnet base materials were subjected to only one stage of grain boundary diffusion treatment using only the diffusion material 2K containing Tb. Note that numbers correspond to comparative examples 1 to 21 and samples 1 to 21, and the compositions of the magnet base materials of the samples and comparative examples with the same numbers are approximately the same.

In addition, in Comparative Example 22, a magnet base material with the same composition A as Sample 1 was produced, and then a diffusion material 2K containing Tb (RH) was diffused in the first diffusion step, and a diffusion material 1A containing Nd (RL) was diffused in the second diffusion step. That is, in Comparative Example 22, the order of diffusion of RH and RL is reversed from that of Sample 1.

In the above comparative example, the experimental conditions other than those mentioned above were the same as those of samples 1 to 21, and evaluations were performed in the same manner as samples 1 to 21. The evaluation results of the comparative example are shown in Tables 3 and 5.

As shown in the results of Tables 4 and 5, when comparing Samples 1 to 21 with Comparative Examples 1 to 21 of the corresponding numbers, it can be confirmed that Samples 1 to 21, which underwent a two-stage grain boundary diffusion treatment, have improved magnetic properties compared to the Comparative Examples of the corresponding numbers, and in particular that HcJ is improved while maintaining Br. From these results, it can be confirmed that a high Br and a high HcJ can be obtained by the two-stage grain boundary diffusion treatment.

In addition, it was confirmed that in Comparative Examples 1 to 21, in which only one stage of grain boundary diffusion treatment was performed, the maximum thickness t1 of the shell portion near the surface exceeded 0.5 μm, while in Samples 1 to 21, in which two stages of grain boundary diffusion treatment were performed, t1 was 0.5 μm or less. In fact, Figure 6 is an SEM photograph of the R-T-B system permanent magnet 2 obtained in Sample 1, and Figure 7 is an SEM photograph of the R-T-B system permanent magnet obtained in Comparative Example 1.

In the SEM photograph of Comparative Example 1 in FIG. 7, it can be seen that a thicker shell is formed than in the SEM photograph of FIG. 6, and that the shell is particularly thick on the surface side. In contrast, in the SEM photograph of Sample 1 shown in FIG. 6, it can be seen that the thickness of shell portion 42 is clearly thinner than in Comparative Example 1. From this result, it was found that the thickness of shell portion 42, where RH is concentrated, can be reduced by the two-stage grain boundary diffusion treatment, and that by setting the thickness of shell portion 42 near the surface to a predetermined thickness or less (t1≦0.5 μm), it is possible to achieve both high Br and high HcJ.

Comparing the evaluation results of samples 1 to 21, when the magnet base material 20 and the R-T-B permanent magnet 2 were within a specified composition range, 1450 mT≦Br and 1800 kA/m≦HcJ were satisfied, and Br and HcJ tended to improve.

That is, it was found that the average composition of the magnet base material 20 is preferably such that the total content of R is 27.5 mass% or more and 30.8 mass% or less, the content of Cu is preferably 0.05 mass% or more and 0.5 mass% or less, and the content of B is preferably 0.92 mass% or more and 1.03 mass% or less.

In addition, it was found that the average composition of the R-T-B system permanent magnet 2 after grain boundary diffusion is preferably such that the total R content is 28.4 mass% or more and 31.9 mass% or less, the Cu content is preferably 0.2 mass% or more and 0.7 mass% or less, and the B content is preferably 0.92 mass% or more and 1.03 mass% or less.

In Comparative Example 22, the maximum thickness of the shell portion near the surface exceeded 0.5 μm, resulting in insufficient improvement in magnetic properties. From these results, it was found that even if RH and RL are subjected to grain boundary diffusion in two stages, if RL is subjected to grain boundary diffusion after RH, the generation of a thick shell portion cannot be suppressed, and sufficient magnetic properties cannot be obtained.

The reason why the magnetic properties were not sufficiently improved when RH was diffused before RL as in Comparative Example 22 is not necessarily clear, but the following reasons are considered. First, as in the case of the grain boundary diffusion with only one stage (Comparative Example 1), when RH was diffused before the diffusion of RL, it is considered that the high concentration of RH came into contact with the R ₂ T ₁₄ B crystal, and the decomposition reaction of the R ₂ T ₁₄ B crystal actively occurred. This is considered to have formed a thick shell portion near the surface, leading to a decrease in Br. In addition, it is considered that the shell portion including RH formed in the first diffusion step was partially destroyed by diffusing RL after the diffusion of RH, and the improvement effect of HcJ was weakened. Furthermore, it is considered that the thickness of the shell portion including RH increased and the RH concentration in the shell portion decreased due to the diffusion of RL, thereby weakening the improvement effect of HcJ.

As described above, in the two-stage diffusion (Comparative Example 22) where RH is diffused first, it is believed that as the thickness of the shell increases, the RH concentration in the shell decreases. As a result, it is believed that Br decreases and sufficient improvement in HcJ cannot be achieved.

(Experiment 2)
In experiment 2, several types of diffusion materials shown in Table 6 were prepared, and a two-stage grain boundary diffusion process was carried out using the combinations of diffusion materials shown in Table 7. In experiment 2, the composition of magnet base material 20 was the same as composition A of sample 1 in experiment 1, and R-T-B system permanent magnet 2 was prepared under the same conditions as in experiment 1, except that the composition of the diffusion material was changed. The evaluation results of samples 25 to 36 in experiment 2 are shown in Table 7.

As shown in Table 7, in all of samples 25 to 36 in experiment 2, the maximum thickness t1 of the shell portion 42 was thin, at 0.2 μm or less, and 1450 mT≦Br and 1800 kA/m≦HcJ. From these results, it was found that when the diffusion material (11, 12) is within the composition range shown in Table 6, it is possible to improve both Br and HcJ. Although not shown in Table 7, it was confirmed that in all of samples 25 to 36 in experiment 2, the RL content, RH content, and M element content gradually decreased from the pole face toward the depth direction.

In addition, comparing sample 35 and sample 36, it was found that Tb was more effective than Dy as the RH to be diffused. It was also confirmed that sample 36, in which Dy was diffused, had a lower HcJ than the other samples (25-35) in which Tb was diffused, but that a higher HcJ was obtained than when Dy was diffused in only one grain boundary diffusion process.

2... RTB system permanent magnet 2a... Principal surface (magnetic pole surface)
2b ... side surface 4 ... main phase grain 4a ... core-shell grain 41 ... core portion 42 ... shell portion 6 ... grain boundary 6a ... two-particle grain boundary 6b ... grain boundary multiple point 20 ... magnet substrate 20a ... main surface 20b ... side surface 50 ... DESCRIPTION OF REFERENCE NUMERALS IPM motor 51 rotor 52 stator core 53 gap 54 coil 101 EV vehicle 102 HV vehicle 60 inverter 70 battery 80 engine 90 generator

Claims (12)

preparing a magnet substrate containing an R-T-B based alloy;
a first diffusion step of attaching a first diffusion material containing at least one light rare earth element to at least a part of a surface of the magnet base material, and heating the magnet base material to which the first diffusion material is attached;
a second diffusion step of adhering a second diffusion material containing at least one heavy rare earth element to at least a part of the surface of the magnet base material after the first diffusion step, and heating the magnet base material to which the second diffusion material is adhered,
The R-T-B based alloy contained in the magnet base material contains one or more rare earth elements as R, with Nd being essential, one or more iron group elements as T, with Fe being essential, boron, and Cu;
In the R-T-B based alloy,
The total content of R is 27.5 mass% or more and 30.8 mass% or less,
The Cu content is 0.05 mass% or more and 0.5 mass% or less,
The content of B is 0.92 mass% or more and 1.03 mass% or less,
The first diffusing material contains one or more elements selected from the group consisting of Nd and Pr as the light rare earth element,
the second diffusible material is an RH-M alloy containing the heavy rare earth element and M,
The M is an element having a eutectic temperature with the light rare earth element of 800° C. or less,
the second diffusive material includes one or more elements having a eutectic temperature with the light rare earth element of 800° C. or less selected from the group consisting of Mg, Co, Ni, Cu, and Cl;
The method for producing an RTB system permanent magnet, in which the content of the light rare earth element contained in the RH-M alloy is 0 mass % or more and 22.5 mass % or less. The method for manufacturing an R-T-B system permanent magnet according to claim 1, wherein the second diffusion material contains one or more elements selected from the group consisting of Tb and Dy as the heavy rare earth element. the first diffusible material includes an RL-M alloy containing the light rare earth element and the M,
In the RL-M alloy,
3. The method for producing an RTB system permanent magnet according to claim 1, wherein RL is the light rare earth element. preparing a magnet substrate containing an R-T-B based alloy;
a first diffusion step of attaching a first diffusion material containing at least one light rare earth element to at least a part of a surface of the magnet base material, and heating the magnet base material to which the first diffusion material is attached;
a second diffusion step of adhering a second diffusion material containing at least one heavy rare earth element to at least a part of the surface of the magnet base material after the first diffusion step, and heating the magnet base material to which the second diffusion material is adhered,
The R-T-B based alloy contained in the magnet base material contains one or more rare earth elements as R, with Nd being essential, one or more iron group elements as T, with Fe being essential, boron, and Cu;
In the R-T-B based alloy,
The total content of R is 27.5 mass% or more and 30.8 mass% or less,
The Cu content is 0.05 mass% or more and 0.5 mass% or less,
The content of B is 0.92 mass% or more and 1.03 mass% or less,
the first diffusible material includes an RL-M alloy containing the light rare earth element and M,
In the RL-M alloy,
R is the light rare earth element,
The M is an element having a eutectic temperature with the RL of 800° C. or less,
The first diffusing material contains one or more elements selected from the group consisting of Nd and Pr as the light rare earth element,
A method for producing an RTB system permanent magnet, wherein the second diffusible material is TbH ₂ , pure Tb metal, or pure Dy metal. 5. The method for producing an RTB system permanent magnet according to claim 1, wherein the first diffusion material does not substantially contain a heavy rare earth element. An R-T-B system permanent magnet in which R is one or more light rare earth elements essentially consisting of Nd and one or more heavy rare earth elements, T is one or more iron group elements essentially consisting of Fe, and B is boron,
The R-T-B based permanent magnet includes a plurality of core-shell particles,
Each of the core-shell particles has a core portion containing an _{R2T14B crystal} and a shell portion covering the core portion and having a higher content of the heavy rare earth element than the core portion,
In a cross section at a depth of 100 μm from the surface of the R-T-B system permanent magnet, the maximum thickness of the shell portion is, on average, 0.5 μm or less,
the R-T-B system permanent magnet has a concentration distribution in which the content of the light rare earth element and the content of the heavy rare earth element both decrease from the surface toward the depth direction,
An RTB system permanent magnet having a residual magnetic flux density of 1450 mT or more and a coercive force of 1800 kA/m or more. M having a eutectic temperature with the light rare earth element of 800° C. or less is included,
7. The R-T-B system permanent magnet according to claim 6 , having a concentration distribution in which the M content decreases from the surface toward the depth direction of the R-T-B system permanent magnet. The R-T-B system permanent magnet further contains Cu,
The total content of R is 28.4 mass% or more and 32.0 mass% or less,
The content of Cu is 0.2% by mass or more and 0.7% by mass or less,
8. The R-T-B system permanent magnet according to claim 6 , wherein the B content is 0.92 mass % or more and 1.03 mass % or less. 9. The RTB system permanent magnet according to claim 6 , wherein the heavy rare earth element is Tb and/or Dy. 10. The RTB system permanent magnet according to claim 6 , further comprising Pr as the light rare earth element. A motor having the RTB system permanent magnet according to any one of claims 6 to 10 . A motor vehicle comprising the motor according to claim 11 .

Images