JP2013153172A - Manufacturing method of neodymium-iron-boron sintered magnet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a NdFeB sintered magnet with high coercive force and high square-shape property for the magnetization curve.SOLUTION: The manufacturing method of the NdFeB sintered magnet comprises a grain boundary diffusion treatment. In the grain boundary diffusion treatment, Dy and/or Tb in a layer comprising Dy and/or Tb are diffused to an inner part of a magnet base through a crystal grain boundary of the magnet base, by forming the layer on a surface of the NdFeB sintered magnet base, and heating it to a temperature of not higher than the sintering temperature of the magnet base; a rare earth amount in a metal state, which is included in the magnet base, is not less than 12.9 atom%; and the layer comprises fluoride of Dy and/or fluoride of Tb of not lower than 50 mass%. Even if the magnet has a thickness of 3-6 mm which is difficult for improvement of a characteristic in conventional cases, the NdFeB sintered magnet having high coercive force of not lower than 1.4 mA/m and high square-shape property of the magnetization curve can be manufactured.

Description

  The present invention relates to a method for producing a sintered NdFeB magnet having a high coercive force.

NdFeB sintered magnets are expected to increase in demand in the future as magnets for motors of hybrid cars. For motors for automobiles, further reduction in weight is desired, and for that purpose, it is desired to further increase the coercive force _HcJ of the NdFeB sintered magnet. As one of the methods for increasing the coercive force _HcJ of the NdFeB sintered magnet, a method of replacing a part of Nd with Dy and / or Tb is known. However, this method has problems that Dy and Tb resources are scarce and unevenly distributed worldwide, and that the residual magnetic flux density _Br and the maximum energy product (BH) _max are reduced.

  In Patent Literature 1, at least one of Nd, Pr, Dy, Ho, and Tb is deposited on the surface of the NdFeB sintered magnet in order to prevent a decrease in coercive force that occurs when the surface of the NdFeB sintered magnet is processed. Is described. Patent Document 2 describes that irreversible demagnetization that occurs at high temperatures is suppressed by diffusing at least one of Tb, Dy, Al, and Ga on the surface of the NdFeB sintered magnet.

Also, recently, a method called a grain boundary diffusion method, it has been found that can increase the coercive force H _cJ with little lowering the residual magnetic flux density B _r of the magnet (Non-Patent Documents 1 to 3). The principle of the grain boundary diffusion method is as follows.
When Dy and / or Tb is deposited on the surface of a NdFeB sintered magnet by sputtering and heated at 700 to 1000 ° C., the Dy and / or Tb on the magnet surface enters the inside of the sintered body through the grain boundary of the sintered body. . A grain boundary phase called an Nd-rich phase rich in rare earth exists at the grain boundary in the NdFeB sintered magnet. The Nd-rich phase has a melting point lower than that of the magnet particles and is melted at the heating temperature. Therefore, the Dy and / or Tb dissolves in the liquid at the grain boundary and diffuses from the sintered body surface into the sintered body. Since the diffusion of the material is much faster in the liquid than in the solid, it diffuses into the sintered body through the molten grain boundary rather than diffusing into the grain from the Dy and / or Tb grain boundary. The speed to go is much greater. By utilizing the difference in diffusion rate and setting the heat treatment temperature and time to appropriate values, the entire sintered body is in a region (surface region) very close to the grain boundary of the main phase particles in the sintered body. Only a high concentration of Dy and / or Tb can be realized. When the concentration of Dy and / or Tb increases, the residual magnetic flux density B _{r of the} magnet decreases. However, since such a region is only the surface region of each main phase particle, the residual magnetic flux density B _r as the main phase particle as a whole. Is hardly reduced. Thus, a large coercive force H _cJ, remanence B _r can performance magnet production unchanged so the NdFeB sintered magnet is not replaced with Dy or Tb.

  As an industrial manufacturing method for NdFeB sintered magnets by the grain boundary diffusion method, a powder layer of fluoride or oxide of Dy and / or Tb is used as a base material for NdFeB sintered magnet (hereinafter referred to as “magnet base material”). A method of heating by forming on the surface of a metal (Patent Document 3) and a method of heating by embedding a magnet base material in a mixed powder of Dy and / or Tb fluoride powder and hydrogenated Ca powder. It has been announced (Non-Patent Documents 4 and 5).

  More recently, an alloy powder of Dy and / or Tb and other metals is deposited on the surface of the NdFeB sintered magnet body, and then heat treatment is performed (Patent Document 4), fluoride powder of Dy and / or Tb and Al A method for realizing a high coercive force has been found by performing heat treatment after depositing a mixed powder of one or more kinds selected from Cu, Zn and Zn (Patent Document 5).

JP 62-074048 Japanese Unexamined Patent Publication No. 01-117303 International Publication W02006 / 043348A1 Pamphlet JP2007-287875 JP 2007-287874

KT Park et al., "Effects of metal coating and heating on the coercivity of Nd-Fe-B thin film sintered magnets", Proceedings of the 16th International Conference on Rare Earth Magnets and their Applications, Japan Institute of Metals, 2000 257-264 (KT Park et al., "Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd-Fe-B Sintered Magnets", Proceeding of the Sixteenth International Workshop on Rare-Earth Magnets and their Appiications (2000), pp. 257-264.) Naoyuki Ishigaki et al., “Surface modification and improvement of properties of neodymium-based sintered magnets”, published by NEOMAX Technical Report, NEOMAX Co., Ltd., 2005, Volume 15, Pages 15-19 Kenichi Machida et al., "Grain boundary modification and magnetic properties of Nd-Fe-B sintered magnets", Summary of the 2004 Spring Meeting of the Powder and Powder Metallurgy Association, published by the Powder and Powder Metallurgy Association, 1-47A Junichi Hamada et al., “High coercivity of Nd-Fe-B sintered magnets by grain boundary diffusion method”, Powder and Powder Metallurgy Association 2005 Spring Meeting Abstracts, Issued by Powder and Powder Metallurgy Association, page 143 Kenichi Machida et al., “Magnetic Properties of Grain Boundary Modified Nd-Fe-B System Sintered Magnets”, Summary of Presentations of the 2005 Spring Meeting of the Powder Powder Metallurgy Association, Issued by the Powder Powder Metallurgy Association, page 144

The above prior art has the following problems.
(1) The methods described in Patent Documents 1 and 2 have a low effect of improving the coercive force.
(2) A method of attaching a component containing Dy or Tb to the magnet surface by sputtering or ion plating (Non-Patent Documents 1 to 3) is not practical because of high processing costs.
(3) The method of applying a DyF ₃ or Dy ₂ O ₃ or TbF ₃ or Tb ₂ O ₃ powder to the surface of a magnetic substrate (Patent Document 3) is a process for attaching a component containing Dy or Tb. It is advantageous in that the cost is low, and in particular, the method using DyF ₃ or TbF ₃ is expected as an industrially valuable technique because the effect of improving the coercive force by the grain boundary diffusion method is remarkable. However, this method can be applied only when the thickness of the magnet substrate is 3 mm or less. Therefore, this method has a big problem that a thick magnet used for a relatively large motor or generator that requires a high coercive force cannot be manufactured.

Patent Documents 4 and 5 and Non-Patent Document 4 describe examples (experimental results) using a magnet base material having a thickness of 3 mm or more. However, these examples also show that when the thickness is 3 mm or more, the value of the coercive force H _cJ decreases as the thickness of the magnet base increases. For example, in Non-Patent Document 4, the coercive force is 1.2 T when the thickness of the magnet substrate is 3 mm by applying a grain boundary diffusion method with Tb powder to a magnet substrate that does not contain Dy or Tb. It is shown. In general, the coercive force of only 1.2T can be obtained even when Tb, which has a higher effect by grain boundary diffusion treatment than Dy, is used. Indicates that there is a limit. Non-Patent Document 5 does not describe experimental results for a magnet base material having a thickness of 3 mm or more.

  Non-Patent Document 5 describes the experimental results of a 5 mm square cubic NdFeB sintered body sample, but such a cubic sample has a magnetic pole area that is too small for a motor magnet. The magnet for a motor has a plate shape such as a flat plate shape or a straw shape, and the length of one side of the magnetic pole is usually three times larger than the thickness. Since the amount of diffusion of Dy and / or Tb from each side surface other than the magnetic pole surface is almost the same as the amount of diffusion from the magnetic pole surface, the experimental results for the cubic sample are in comparison with a normal plate magnet for motors. Is not applicable. For example, if Dy and / or Tb can diffuse from the surface by 1 mm, a cube with a side of 5 mm would have Dy and / or Tb diffused to 80% of the total volume. Only 40% of the sample. Therefore, it can be said that the experimental result for a cube sample having a side of 5 mm is equivalent to the experimental result for a plate sample having a thickness of 2.5 mm. Non-Patent Document 5 does not describe the squareness of the magnetization curve of the magnet after grain boundary diffusion. A high squareness of the magnetization curve is an essential requirement for a high performance motor magnet. Therefore, it can be said that it is not shown whether the magnet shown in Non-Patent Document 5 is applicable as a general motor magnet to a thickness of 3 mm or more.

When applying the NdFeB sintered magnet to a motor or generator, the inventor of the present application not only has a sufficiently high coercive force H _cJ, but also has a squareness of a magnetization curve that has not been sufficiently studied in the above document. I think that raising it is extremely important. Hereinafter, the importance of the squareness of the magnetization curve will be described.
In motors and generators, magnets are often exposed to temperatures as high as 200 ° C. Under such high temperature conditions, irreversible demagnetization occurs if the squareness of the magnetization curve is low. Therefore, even if the coercive force is high, if the squareness of the magnetization curve is low, such a magnet cannot be used for high-tech products such as hybrid cars. In the grain boundary diffusion method using the powder of DyF ₃ and TbF ₃ shown in the above document, if the thickness of the magnet substrate exceeds 3 mm, the effect of grain boundary diffusion cannot be maximized, As a result, an NdFeB sintered magnet having high coercive force and sufficiently high squareness of the magnetization curve could not be produced.

Here, the definition of the squareness of the magnetization curve will be described. The absolute value of the magnetic field when the magnetization in the magnetization curve has dropped by 10% from the value of the residual magnetization and H _K value, the value H which magnetization is obtained by dividing the H _K value in coercivity H _cJ is the absolute value of the magnetic field becomes 0 _{Let K} / H _cJ be the SQ value. In the present application, this SQ value is used as an index of the squareness of the magnetization curve. Here, high squareness means that the SQ value measured by the pulse magnetometer shown in the examples is 85% or more, preferably 90% or more.

By the way, characteristics such as coercive force and squareness of the magnetization curve of NdFeB magnets that have undergone grain boundary diffusion treatment are the substances (including Dy and Tb) that are attached to the surface of the magnet substrate (hereinafter referred to as “attachments”). ")" And also depends on the composition of the magnet substrate. Among these compositions, the amount of Dy or Tb has a great influence on the characteristics. In the known literatures so far, since characteristics are compared between various combinations of magnet base materials and deposits, it is not easy to compare superiority and inferiority with respect to the effect of grain boundary diffusion. Therefore, in order to appropriately compare the effects of grain boundary diffusion, the inventor of the present application considered that it is necessary to perform comparison under strict conditions for simple and improved characteristics as follows.
Condition (1): A magnet base material containing neither Dy nor Tb is used.
Condition (2): Use deposits containing Dy, not Tb.
The condition (1) aims to eliminate the influence of Dy and Tb contained in the magnet base material because the characteristics of the magnet itself improve. Condition (2) is intended to highlight the effect of grain boundary diffusion by making a comparison using Dy, which is more stringent, since the effect of improving magnet properties is generally smaller than Tb. . In addition, Tb is a much rarer substance than Dy, and it is becoming increasingly difficult to use it industrially. Therefore, it can be said that the limitation in (2) is in line with the conditions in practical use. If a method capable of obtaining a property higher than a predetermined value under the conditions of (1) and (2) is found, the method is used to satisfy the conditions other than (1) and (2) (the magnetic substrate is Dy and / or When Tb is included, an NdFeB sintered magnet having a property higher than a predetermined value can be obtained even when the deposit contains Tb).

  From this viewpoint, the matters described in the above documents are arranged.In the conventional grain boundary diffusion method, under the conditions (1) and (2) above, (i) has a thickness of 3 mm or more, and (ii) 1.4 MA / An NdFeB sintered magnet with a coercive force of m or more and (iii) a squareness of a magnetization curve of 85% or more could not be produced.

  The problem to be solved by the present invention is that NdFeB sintering can be applied even to a thick magnet of 3 mm or more with high coercivity and squareness of the magnetization curve even under the severe conditions of (1) and (2) above. It is to provide a method for manufacturing a magnetized magnet. Note that the method provided by the present invention can improve characteristics over the conventional method even under conditions other than the above (1) and (2). Therefore, the above conditions (1) and (2) do not limit the scope of the present invention.

In order to solve the above problems, a method for producing a sintered NdFeB magnet according to the present invention comprises forming a layer containing Dy and / or Tb on the surface of a NdFeB sintered magnet substrate and then firing the magnet substrate. In a method of performing a grain boundary diffusion treatment in which Dy and / or Tb in the layer is diffused into the inside of the magnet base material through crystal grain boundaries of the magnet base material by heating to a temperature equal to or lower than the sintering temperature.
a) The amount of the rare earth element contained in the magnet base material is 12.9 atomic% or more,
b) the layer contains 50% by weight or more of Dy fluoride and / or Tb fluoride;
It is characterized by that.

In the present invention, “a rare earth in a metallic state” means a rare earth constituting a substance existing in a metallic state in a sintered NdFeB magnet. Here, the metal refers to an intermetallic compound including a pure metal, an alloy, and an Nd ₂ Fe ₁₄ B phase which is a parent phase. Ionic or covalent compounds such as rare earth oxides, fluorides, carbides and nitrides are not included in the “metallic rare earth”.

First, the technical significance of a) will be explained.
The main phase of the NdFeB magnet is an Nd ₂ Fe ₁₄ B compound, and in the stoichiometric composition, that is, the composition of Nd: Fe: B = 2: 14: 1, the atomic ratio occupied by the rare earth is 2/17 = 11.76 atomic%. . In the NdFeB sintered magnet, in addition to the main phase, there are an Nd-rich phase with a larger amount of Nd than a stoichiometric composition and a B-rich phase with a larger amount of B. The inventor of the present application has found that a sufficient amount of Nd-rich phase needs to be present at the grain boundaries in order for the grain boundary diffusion method of the NdFeB sintered magnet to work effectively. The reason is that when Dy and Tb are sent from the layer formed on the surface of the magnet base material to the inside of the magnetic base material through the grain boundary, the diffusion speed of Dy and Tb is increased to promote the diffusion to the deep part of the base material. In order to achieve this, a thick passage formed by the melted Nd-rich phase is formed at the grain boundary, and Dy and Tb are diffused quickly in the deep part of the substrate. As a result, the present invention makes it possible to increase the coercive force while maintaining high squareness for a thick substrate, which is impossible with the conventional grain boundary diffusion method using Dy fluoride powder. The inventor of the present application has shown by experiment that when the thickness of the base material exceeds 3 mm, the magnet base material has a metal state rare earth of 12.9 atomic% or more, which is 1.14 atomic% in excess of the above stoichiometric amount of 11.76 atomic%. I found it necessary to have it.

There are usually three types of rare earth compounds as impurity phases produced when producing a magnet base material: oxides, carbides and nitrides. Therefore, the amount of the rare earth element in the metallic state in the magnet base material is defined as an amount obtained by subtracting the amount of the rare earth forming the rare earth oxide, carbide and nitride from the total rare earth amount contained in the magnetic base material. be able to. Here, for example, (1) first, the composition ratio of the elements in the magnet base material is obtained by a known analysis method, and (2) oxygen, carbon, nitrogen is determined based on the amounts of oxygen, carbon, nitrogen obtained by the analysis. R ₂ O ₃ , RC and RN (where R is a rare earth element) are formed, and the amount of R constituting these R ₂ O ₃ , RC and RN is obtained. (3) From the amount of the rare earth element obtained in the analysis By subtracting the amounts of R constituting R ₂ O ₃ , RC and RN determined in (2), the amount of the rare earth in the metallic state in the magnet substrate can be determined. Some types of R are known to have a composition ratio other than R ₂ O ₃ , RC and RN. However, the inventor of the present application defines the scope of the present invention by calculating the amount of rare earth in the metal state, assuming that the compound having the above composition ratio is generated in most industrially produced NdFeB sintered magnets. Confirmed that you can.

  It is well known that the coercive force of the magnet base material itself is increased by reducing the oxygen content of the NdFeB sintered magnet or increasing the amount of rare earth in the metallic state. It is not a combination of high coercivity due to grain boundary diffusion. Even if the grain boundary diffusion method is applied to a magnet base material that has a high coercivity with a low oxygen or metallic rare earth content, if the effect of grain boundary diffusion does not penetrate deep inside the magnet base material, the coercive force is It is large only in the vicinity of the surface of the base material, and has an original coercive force inside. As a result, such a magnet has a stepwise demagnetization curve and has a low squareness. On the other hand, the feature of the present invention is that Dy and Tb can reach the deep part of the substrate from the Dy fluoride and Tb fluoride applied to the substrate surface by increasing the amount of rare earth metal. As a result, a magnet having a high coercive force and a high squareness can be produced even with a thick magnet.

  The condition of b) relates to the deposit that is a supply source for supplying Dy and Tb to the substrate. As proposed in Patent Document 3 and Non-Patent Document 4, Dy fluoride and Tb fluoride contain (1) high concentrations of Dy and Tb, and (2) the effect of grain boundary diffusion is an oxide. Compared to the above, there is an advantage that (3) it is not chemically active and easy to handle, and (4) it is easily available. These advantages have been known for some time, but for the NdFeB sintered magnet base material with a thickness of more than 3 mm, the condition of a) above is necessary to diffuse Dy and Tb to the deepest part of the base material. . In order to fully exhibit the effect of grain boundary diffusion, it is necessary that at least 50% by mass or more of these fluorides are contained in the powder layer to be applied.

  In the present invention, the layer may contain 30% by mass or less of Al. Thereby, the coercive force of the NdFeB sintered magnet can be further increased. Also in this case, high squareness is maintained under the condition a).

  In order to industrially implement the grain boundary diffusion method for NdFeB sintered magnets, the above layer is preferably a layer composed of powder (powder layer). Conventionally known sputtering methods have low productivity and are too expensive to process, resulting in no industrial value. The barrel painting method (see Japanese Patent Application Laid-Open No. 2004-359873) is optimal as a method for forming the powder layer on the substrate surface. In addition, it is also possible to apply using a solvent such as a spray method. Alternatively, a Dy or Tb fluoride solution may be applied to the surface of the substrate, and moisture may be evaporated by heating to form a Dy or Tb powder layer.

  By the method for producing a sintered NdFeB magnet according to the present invention, the grain boundary diffusion method using Dy fluoride and / or Tb fluoride can work effectively even for a NdFeB sintered magnet having a thickness exceeding 3 mm. Become. As a result, a high-performance NdFeB sintered magnet with a thickness of 3 mm or more having high coercive force and high squareness can be produced.

The table | surface which shows the composition of the magnet base material used in Example 1 of the manufacturing method of the NdFeB sintered magnet which concerns on this invention. The table | surface which shows the composition of the powder which is the deposit used in Example 1. FIG. The _{table |} surface which shows the measurement result of the parameter _| index SQ value of the coercive force _HcJ of the NdFeB sintered magnet produced in Example 1, and the _squareness of a magnetization curve. The _{table |} surface which shows the measurement result of the parameter _| index SQ value of the coercive force _HcJ of the NdFeB sintered magnet produced by the comparative example, and the _squareness of a magnetization curve. The table | surface which shows the composition of the magnet base material used in Example 2. FIG. The table _| surface which shows the measurement result of the coercive force _HcJ and SQ value of the NdFeB sintered magnet produced in Example 2. FIG.

An embodiment of a method for producing a NdFeB sintered magnet according to the present invention will be described with reference to FIGS.
The magnet base material used in this example is produced by the same method as the conventional method for producing a NdFeB sintered magnet, that is, the steps of melting, coarsely pulverizing, finely pulverizing, orientation in a magnetic field, molding, and sintering. did.

  However, in this preparation, adjustment of the alloy composition and prevention of preferential reduction of rare earths generated during the process so that the amount of rare earths in the metallic state in the sintered body after sintering is 12.9 atomic% or more. And measures to prevent contamination by impurities. Here, “preferential reduction of rare earth” means that the degree of reduction of rare earth is larger than that of elements other than rare earth, thereby causing the composition ratio to change. The preferential reduction of the rare earth is due to evaporation of the rare earth component in the metallic state that occurs when melting the alloy, reduction due to oxidation or reaction with the crucible, or the Nd-rich phase being crushed too finely during grinding. For example, a decrease caused by not being collected in the collection container. Further, the amount of rare earth in the metallic state is also reduced by the rare earth in the powder chemically reacting with impurities after the alloy is pulverized. Here, impurities mainly refer to oxygen, carbon, and nitrogen. Oxygen is mainly due to the oxidation of the alloy powder during and after the alloy grinding, carbon remains in the alloy powder to provide lubricity to the alloy powder, and nitrogen is the same as the nitrogen in the alloy powder. By reacting, it is incorporated into the product. In order to produce the sintered magnet base material used in the present invention, it is necessary to suppress the reduction of the rare earth amount in the metallic state during the process as much as possible and to suppress the contamination by the impurity element as much as possible. If this is not possible, the amount of rare earth in the alloy must be increased in advance. The base material of No. 4 in Example 1 described later is an example prepared by suppressing contamination by oxygen and carbon as much as possible because the rare earth amount is low, and the base material of No. 5 cannot be lowered by carbon during the process, so it is in the alloy. This is an example in which the amount of rare earth in the metallic state is adjusted within the scope of the present invention by increasing the amount of rare earth.

  The lower limit of the amount of rare earth in the NdFeB alloy that is the starting material for producing the magnetic base material is 12.9 atomic%, which is essential when performing grain boundary diffusion treatment in the present invention, and the amount of rare earth expected to decrease during grinding. The value was obtained by adding the rare earth amount expected to be consumed by oxygen, carbon, and nitrogen during or after pulverization. If the amount of rare earth in the alloy is large, the present invention can be carried out even if there is a certain amount of contamination by oxygen, carbon, and nitrogen, but if the amount of rare earth is too large, the magnetization and maximum energy product will decrease, so that the NdFeB sintered magnet The value will decline. Practically, the upper limit of the amount of rare earth in the alloy is 16 atomic%. Further, as a kind of rare earth in the alloy, Nd is a main component, but Nd may be substituted with Pr depending on the circumstances of the raw material. Depending on the required coercivity of the final product, part of Nd can be replaced by Dy or Tb.

  The NdFeB sintered body produced in this way is processed into the shape and dimensions required for the final product by machining. Thereafter, the surface is chemically or mechanically cleaned before the grain boundary diffusion treatment. The NdFeB sintered magnet thus produced is the magnet base material that is finally used in the present invention.

Next, the powder applied (attached) to the substrate surface for grain boundary diffusion will be described. The powder used in this example is a powder containing 50% by mass or more of fluoride of Dy or Tb. The average particle diameter of the powder is 100 μm or less, preferably 10 μm or less. The lower limit is not particularly limited, but is preferably 10 nm or more. A powder containing Al powder together with fluoride of Dy or Tb is also used. The Al powder used is 20 μm or less, preferably 10 μm or less. As constituents of coating powders other than Al powder, powders of compounds other than fluorides of Dy and Tb, fluoride powders such as Nd and Pr, alloy powders containing Dy and Tb, metals such as Zn and Cu There is powder. The coating amount was about 5 mg or more per 1 cm ² of the substrate surface.

As shown in Patent Document 3, the method of applying powder to the surface of the substrate is to blow hot air after immersing the magnet substrate in a slurry in which fluoride powder of Dy or Tb is dispersed in an organic solvent. A method of drying by putting in a vacuum apparatus or a barrel painting method developed by the inventors of the present application (see Japanese Patent Application Laid-Open No. 2004-359873) is suitable.
In the barrel painting method, powder coating is performed as follows. First, an adhesive layer is formed on a NdFeB sintered magnet substrate having a clean surface. The optimum thickness of the adhesive layer is 1 to 5 μm. The adhesive layer forming material may be an adhesive material that does not corrode the substrate surface. Most commonly, liquid organic substances such as epoxy and paraffin are used. When using epoxy or the like, no curing agent is required. In this adhesive layer coating method, a small amount of a liquid organic substance is added and stirred in a container filled with ceramic or metallic spheres (called impact media) having a diameter of 0.5 to 1 mm, and then the above-described base material is added. The adhesive layer is formed on the substrate surface by vibrating the entire container. Next, after adding and stirring the powder to be applied to a container filled with similar impact media, the base material on which the adhesive layer is formed is put into the container, and the whole container is vibrated, so that the surface of the base material is A powder layer is formed. The amount of powder applied in this way is about ² mg to about 30 mg per 1 cm ^{2 of the} substrate surface. In the present invention, the preferred range of the amount of the liquid substance added to the impact media when forming the adhesive layer and the amount of the powder added to the impact media when applying the powder is 3 mg to 25 mg per 1 cm ² of the substrate surface.

  Next, the magnetic base material coated with Dy or Tb fluoride powder is placed in a heating furnace and heated. The atmosphere of the heating furnace is a vacuum or a high purity inert gas atmosphere. The temperature at which high coercivity is effectively achieved by grain boundary diffusion is 700 ° C to 1000 ° C. Typical heating conditions are 800 ° C for 10 hours or 900 ° C for 3 hours. After heating under such conditions, a normal post-sintering heat treatment is performed.

  The NdFeB sintered magnet produced by the above-mentioned process has high coercive force and high remanent magnetization, exceeding the limits of the conventional NdFeB sintered magnet produced by the grain boundary diffusion method, even when the thickness exceeds 3 mm. In addition, it has characteristics as a high-quality magnet with high squareness of the magnetization curve. In the conventional method, for a thick magnet, only the vicinity of the surface of the base material has a high coercive force, and the inside does not have the effect of grain boundary diffusion, so the squareness of the magnetization curve is poor. This is a typical example of a magnet in which a high coercive force portion and a low coercive force portion are mixed, and can be said to be a product with low quality. On the other hand, according to the present invention, even if the NdFeB sintered magnet is a relatively thick product, the squareness of the magnetization curve is high, and a high-quality product can be produced.

An NdFeB sintered magnet powder is prepared by alloy preparation using the strip casting method, hydrogen cracking, lubricant mixing, and fine pulverization using a jet mill using nitrogen gas, and the powder is mixed with the lubricant. Ten steps of NdFeB sintered magnet blocks (base materials) with different compositions were prepared by performing each step of orientation in the magnetic field, molding and sintering (FIG. 1). The base material numbers 1 to 5 in FIG. 1 are base materials used in this example, and those with “(ratio)” in the “base number” column are base materials of comparative examples. The composition shown in FIG. 1 is a chemical analysis value of the sintered body after sintering. In this experiment, the composition of the strip cast alloy, the purity of the nitrogen gas used during jet mill grinding or the amount of oxygen to be added, the type and amount of lubricant added before and after jet mill grinding, and the atmosphere during orientation molding in a magnetic field. By changing, the composition of the sintered body was changed. The particle size of the fine powder after jet milling was adjusted so that the median value (D ₅₀ ) of the particle size distribution measured by the laser diffraction method was 5 μm. All of these 10 types of sintered magnets have a composition close to that of NdFeB sintered magnets that are produced in large quantities by each magnet manufacturer as a material having the largest maximum magnetic energy product because the rare earth is composed only of Nd. However, among these magnets, those with substrate numbers 1 to 5 are contaminated with impurities by setting the atmosphere during orientation molding in a magnetic field to a 99.9999% high purity Ar atmosphere or minimizing the amount of lubricant added. It was made with a contrivance to minimize. On the other hand, the substrate numbers “(ratio) 1 to (ratio) 5” have compositions close to those of commercially available products. These include powders that are manufactured and sold as low-oxygen sintered products having high magnetic properties by handling powders in a high-purity argon or nitrogen atmosphere. In FIG. 1, the MR value indicates the amount of rare earth in the metallic state, and is calculated from the chemical analysis value of the sintered magnet. That is, the MR value is a value obtained by subtracting the amount of rare earth consumed (non-metalized) by oxygen, carbon, and nitrogen from the total amount of rare earth in the analysis value. Here, these impurity elements were calculated as those for forming rare earth and R ₂ O ₃ , RC, and RN compounds, respectively.

The powders (adhesives) applied to the surface of the NdFeB sintered magnet base material in order to carry out the grain boundary diffusion method have the powder numbers 1 to 9 and “ratio 1” shown in the table shown in FIG. Was used. Powder No. 1 is 100% DyF ₃ powder with an average particle size of 3 μm. In addition, powder Nos. 2, 4 to 9 have an average particle diameter of one or more of NdF ₃ powder having an average particle diameter of 2 μm, Al powder having a diameter of about 3 μm, and Cu powder having an average particle diameter of 4 μm. 1 μm DyF ₃ was mixed at the ratio shown in the table. The powder of powder number 3 is a mixture of TbF ₃ having an average particle diameter of 3 μm and the above Al powder at a ratio shown in the table.

A rectangular parallelepiped base material is cut out from the 10 types of sintered body blocks shown in FIG. 1 so that the length direction is 7 mm × width 7 mm × thickness 5 mm, and the thickness direction is the magnetization direction. The powder shown in FIG. 2 was applied to the surface.
A plastic beaker having a capacity of 200 cm ³ was charged with 100 cm ³ of zirconia small balls having a diameter of 1 mm, and 0.1 to 0.5 g of liquid paraffin was added and stirred. A rectangular parallelepiped substrate was put into this, and the beaker was brought into contact with the vibrator. Thereby, liquid paraffin is apply | coated to the surface of a rectangular parallelepiped base material. Next, masking with a plastic plate was performed on the side surface (surface other than the magnetic pole surface) of the rectangular parallelepiped base material so that the powder did not adhere to the magnet side surface (the reason will be described later). Thereafter, 8 cm ³ of stainless steel spheres having a diameter of 1 mm were put into a 10 cm ³ glass bottle, 1 to 5 g of the powder shown in FIG. 2 was added, and a rectangular parallelepiped substrate coated with an adhesive layer was added. The glass bottle containing the rectangular parallelepiped substrate was brought into contact with the vibrator. Thereby, the powder containing Dy was applied only to the magnetic pole surface.

The material coated with the powder in this way was placed on a molybdenum plate with one of its side faces down and heated in a vacuum of 10 ⁻⁴ Pa. The heating temperature was 800 ° C. for 3 hours. Then, it rapidly cooled to near room temperature, heated at 500-550 degreeC for 2 hours, and rapidly cooled to room temperature again. Thereby, the target sample was obtained.

  The reason why the powder coating is limited to the magnetic pole surface is as follows. Since the present invention aims at application to a relatively large motor, it must be effective for a magnet having a somewhat large magnetic pole area. However, the magnetic pole area is limited due to the convenience of the magnetization curve measuring instrument. Therefore, after using a sample with a relatively small magnetic pole area of 7 mm square, by not applying powder on the side surface, it approached the usage state of the motor magnet that the area of the magnetic pole surface is sufficiently larger than the side surface. .

For the sample of this example produced in this way, the coercive force H _cJ and the squareness index SQ value of the magnetization curve were measured. Here, a method for measuring magnetic properties will be described. As described above, the sample is a rectangular parallelepiped having a square magnetic pole surface with a side of 7 mm and a thickness of 2 mm to 6 mm. The magnetization direction is parallel to the thickness direction. The magnetic characteristics were measured using a pulse magnetization characteristic measuring device (manufactured by Nippon Electromagnetic Sokki Co., Ltd., trade name: pulse BH curve tracer BHP-1000). With this measuring apparatus, a magnetic field measurement was performed by applying a pulse magnetic field with a pulse width of 17.7 mssec and a peak value of 10 T in the thickness direction of the sample. In this measurement method, with appropriate calibration, the residual magnetic flux density B _r , maximum energy product (BH) _max , and coercive force H _cJ are measured almost the same as a DC BH curve tracer measured by a normal DC magnetic field application device. Although the value is obtained, the squareness SQ value of the magnetization curve tends to be lower than the value obtained with the direct current BH curve tracer. Since the pulse magnetization characteristic measuring device is often used for evaluating the magnetic characteristics of the high coercive force magnet, in this embodiment, the magnetic characteristics were evaluated using the pulse magnetization characteristic measuring device. As a result of measuring the SQ value using a DC BH curve tracer for some samples, the SQ value obtained by measuring with a pulse magnetometer is 5-7% lower than the SQ value obtained by the DC BH curve tracer. I understood that. Although it is better to evaluate the SQ value with a direct current BH curve tracer from the viewpoint of strictness, data with sufficiently high reproducibility can be obtained even with a pulse magnetization measuring device, and there is no problem as a quality evaluation means of the magnet. In the following examples, only the coercive force H _cJ and the SQ value are shown in the magnetic characteristic results. It is known that the residual magnetic flux density and the maximum magnetic energy product do not change much before and after the grain boundary diffusion treatment, and this was confirmed for all samples in this example.

FIG. 3 shows the measurement results of the coercive force H _cJ and the SQ value for the samples of this example (sample numbers 1 to 21). In addition, FIG. 4 shows the results of measuring the coercive force H _cJ and SQ value for the sample of the comparative example. Here, the sample of the comparative example uses one of the base materials of “ratio 1” to “ratio 5” (sample numbers “ratio 1” to “ratio 10”), and powder of “ratio 1” is used. (Sample numbers “ratio 11”, “ratio 12”) or those not subjected to grain boundary diffusion treatment (sample numbers “ratio 13” to “ratio 22”).

The following can be understood from the results of FIGS.
(1) All the samples of this example have a high coercive force of 1.4 MA / m or more and a high squareness of SQ value of 85% or more. Conventionally, it is possible to produce an NdFeB sintered magnet having such a high coercive force and squareness by using a base material having a thickness of 5 mm that does not contain either Dy or Tb as in this example. There wasn't. Among these samples, samples 1 to 11, 13 to 18, 20 and 21 are industrial in that such high coercive force and squareness can be obtained without using Tb which is a very rare resource. Is very valuable. Samples 12 and 19 have a feature that by using TbF ₃ powder, a higher coercive force can be obtained as compared with other samples.
(2) When the amount of rare earth metal contained in the substrate is less than 12.9 atomic% (sample “ratio 1” to “ratio 10”), the coercive force suddenly decreases and the SQ value becomes 85% or less. (For SQ values, except for “Ratio 5”, which has a very small grain boundary diffusion effect).
(3) Samples 14 and 15 of this example (DyF ₃ amounts of 60 and 50% by mass) and comparative sample “Ratio 11” (DyF ₃ amount of 40% by mass) are both based on the base material number 1 Although the material is used, only the sample of this example has a coercive force of 1.4 MA / m or more.
(4) Samples 1 to 5 using powder 1 (DyF ₃ : 100%) and the base material is the same as any of samples 1 to 5 and powder 4 (DyF ₃ : 90%, Al: 10%) When samples 6 to 10 using the above are compared, it can be seen that samples 6 to 10 have higher coercivity. Therefore, it can be said that the coercive force is improved by adding Al to the powder. The effect of improving the coercive force due to Al is observed even when the Al concentration in the powder is 30% by mass (Sample 16), but when it exceeds 40% by mass, the effect is not observed (Sample No. 17).
(5) Of the ten types of base materials used in this example and the comparative example, the base material number 1 with the largest amount of rare earth in the metal state is compared with the base material number “ratio 5” with the minimum base material number. The difference in coercive force between the two when the grain boundary diffusion treatment is performed using the powder 1 (Sample No. 1 and “ratio 5”) is 0.56 MA / m, both when the grain boundary diffusion treatment is not performed. It becomes larger than the difference (0.41 MA / m) of the coercive force of (sample number “ratio 13” and “ratio 22”). This is because when the grain boundary diffusion treatment is performed, a liquid phase is more easily formed at the grain boundary during heating for the grain boundary diffusion treatment as the amount of the rare earth in the metal state in the base material increases. Thus, the diffusion of Dy and Tb is promoted, so that the coercive force increasing effect becomes more prominent.

  An NdFeB sintered magnet base material having the composition shown in FIG. 5 was produced by the same method as in Example 1. These substrates contain about 1 atomic percent of Dy. The base material No. 6 has a metal state rare earth content of 13.29 atomic% and is within the scope of the present invention, and the base material number “Ratio 6” has a rare earth content of 12.81 atomic% and is outside the scope of the present invention. It is in. From these sintered bodies, five types of rectangular parallelepiped substrates having a magnetic pole surface of a square having a side of 7 mm and a thickness of 2 mm to 6 mm were cut out by machining. These rectangular parallelepiped substrates were subjected to the grain boundary diffusion treatment by applying the powder of powder No. 4 only on the magnetic pole face in the same manner as in Example 1 and heating at 800 ° C. for 10 hours. After the grain boundary diffusion treatment, it was quenched and further heat treated at 520 ° C. for 2 hours and then quenched.

FIG. 6 shows the magnetic measurement results of these samples. From this result, the following can be said.
(1) When a base material (base number 6) with a high amount of rare earth in the metal state is used, the coercive force after diffusion of the grain boundary is the same even when the base thickness is 6 mm (sample number 26). It shows a high value that does not change much compared to the case of 2 mm (Sample No. 22).
(2) When a grain boundary diffusion treatment is applied to a substrate to which Dy is added as in this example, the effect of Dy originally contained in the substrate is diffused from the surface by grain boundary diffusion. With the added Dy effect, the coercive force obtained after grain boundary diffusion is a large value of around 2 MA / m.
(3) If the amount of rare earth metal in the substrate is outside the scope of the present invention (Sample No. “Ratio 23” to “Ratio 27”), the increase in coercive force after grain boundary diffusion is 2 mm. It is not so large, and it becomes even smaller when the thickness is 3 mm or more.

Claims (1)

After forming a layer containing Dy and / or Tb on the surface of the NdFeB sintered magnet base, heating to a temperature below the sintering temperature of the magnet base allows the Dy and / or Tb in the layer to be converted into the magnet In the manufacturing method of the NdFeB sintered magnet that performs the grain boundary diffusion treatment for diffusing into the magnet base material through the crystal grain boundary of the base material,
a) The amount of the rare earth element contained in the magnet base material is 12.9 atomic% or more,
b) the layer contains 50% by weight or more of Dy fluoride and / or Tb fluoride;
A method for producing a sintered NdFeB magnet.

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