JP7609669B2 - R-T-B magnet powder and its manufacturing method - Google Patents

Description

This disclosure relates to R-T-B magnet powder and its manufacturing method.

Among R-T-B permanent magnets, Nd-Fe-B sintered magnets have excellent magnetic properties and are therefore mounted, for example, in motors, generators or actuators. Nd-Fe-B sintered magnets used in motors and the like are required to have high coercive force even in high temperature environments. However, the coercive force of Nd-Fe-B sintered magnets decreases with increasing temperature. The coercive force of Nd-Fe-B permanent magnets at high temperatures can be increased by substituting a part of the Nd element constituting the main phase, Nd ₂ Fe ₁₄ B phase, with a heavy rare earth element (Dy or Tb, etc.). However, heavy rare earth elements are expensive, and the price and supply of heavy rare earth elements are unstable. Therefore, in order to reduce manufacturing costs and management risks, it is desirable to improve the magnetic properties of Nd-Fe-B permanent magnets without using heavy rare earth elements.

It is generally known that the smaller the particle size of the main phase particles in a sintered magnet, the easier it is to improve its magnetic properties, especially its coercive force. In order to reduce the particle size of the main phase particles, it is necessary to reduce the average particle size of the magnet powder, which is the raw material for the sintered magnet. Conventionally, Nd-Fe-B alloy powder has been produced by crushing an alloy produced by the strip casting method in a jet mill using argon gas or nitrogen gas. However, due to the crushing limit, it is usually not possible to obtain magnet powder with an average particle size of 1 μm or less.

Patent documents 1 and 2 disclose a method for producing magnet powder with an average particle size of 1.0 μm or less by a jet mill pulverization method using helium gas.

International Publication No. 2017/022684 International Publication No. 2014/142137

However, the magnet powder produced by the methods disclosed in Patent Documents 1 and 2 has a wide particle size distribution. As a result, the particle size of the main phase particles in the resulting sintered magnet becomes coarse due to abnormal grain growth after sintering caused by the variation in particle size of the magnet powder.

One aspect of the present invention was made in consideration of the above circumstances, and aims to provide a method for producing a magnetic powder with a small average particle size and a narrow particle size distribution, and a new magnetic powder.

One aspect of the present invention includes a heat treatment step of obtaining an R-T-B system alloy powder by heating a composition containing a rare earth metal element R, a transition metal element T, boron B, and at least one metal halide selected from the group consisting of halides of alkali metal elements, halides of alkaline earth metal elements, and halides of rare earth metals, at a temperature equal to or higher than the melting point of the metal halide,
The rare earth metal element R is at least one selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu;
This is a method for producing an R-T-B system magnet powder, in which the transition metal element T is at least one selected from the group consisting of Fe, Ni, Co, Cr and Mn.

In one embodiment, the above manufacturing method may further include a cleaning step of cleaning the R-T-B alloy powder with a cleaning solution that dissolves metal halides.

In one embodiment, the cleaning solution may contain a solvent that does not have a hydroxy group.

In one embodiment, the cleaning solution may contain at least one selected from the group consisting of formamide and dimethylformamide.

Another aspect of the present invention is a method for producing a semiconductor device comprising:
The average particle size is 0.1 to 1.0 μm,
The particle size distribution width ( _D90 - _D10 )/ _D50 is 0.85 or less;
The rare earth metal element R is at least one selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu;
The R-T-B system magnet powder is one in which the transition metal element T is at least one element selected from the group consisting of Fe, Ni, Co, Cr and Mn.
(Note that _D10 , _D50 and _D90 are the particle sizes at which the cumulative frequency is 10%, 50% and 90%, respectively, in an area-based cumulative particle size distribution curve based on image analysis of scanning electron microscope (SEM) images of the R-T-B magnet powder.)

In one embodiment, the magnet powder may include R ₂ T ₁₄ B single crystal particles.

In one embodiment, the magnetic powder may have a circularity of more than 70%.

In one embodiment, the magnet powder may have a Ca content of 100 mass ppm or less.

In one embodiment, the Cl content of the magnet powder may be 1 ppm by mass or more and 50,000 ppm by mass or less.

According to one aspect of the present invention, a method for producing a magnetic powder having a small average particle size and a narrow particle size distribution and a novel magnetic powder are provided.

The following describes an embodiment of the present invention. However, the present invention is not limited to the following embodiment.

[Method of manufacturing R-T-B based magnet powder]
The method for producing the R-T-B based magnet powder according to this embodiment (hereinafter simply referred to as the "production method according to this embodiment") will be described below.

In this specification, powder refers to an aggregate of particles.

(Heat treatment process)
The manufacturing method according to this embodiment includes a heat treatment step of obtaining an R-T-B based alloy powder by heating a composition containing a rare earth metal element R, a transition metal element T, boron B, and at least one metal halide (hereinafter simply referred to as a "metal halide") selected from the group consisting of halides of alkali metal elements, halides of alkaline earth metal elements, and halides of rare earth metals, at a temperature equal to or higher than the melting point of the metal halide.

R is at least one selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

R may be a metal or an alloy. Examples of the form of R include powder, granules, and ingots.

T is at least one selected from the group consisting of Fe, Ni, Co, Cr and Mn.

T may be a metal, alloy, or compound. Examples of the form of T include powder, granules, and lumps (ingots). Since the average particle size of the resulting magnet powder tends to be small, powder is preferred, and fine powder with an average particle size of 1 μm or less is particularly preferred.

B may be a metal, an alloy, or a compound. Examples of the form of B include powder, granules, and lumps (ingots).

Examples of metal halides include fluorides, chlorides, bromides, and iodides. Examples of alkali metal element halides include LiCl, KCl, NaCl, and LiF. Examples of alkaline earth metal element halides include CaCl ₂ , MgCl ₂ , BaCl ₂ , and SrCl _2. Examples of rare earth metal element halides include NdCl ₃ , SmCl ₃ , and CeCl _3. Examples of the form of metal halides include powder.

In the manufacturing method according to this embodiment, when R and T are Nd and Fe, respectively, Nd-Fe-B alloy powder can be produced.

In this specification, when the metal halide is a mixture, a temperature equal to or higher than the melting point of the metal halide means a temperature equal to or higher than the eutectic point of the mixture as shown in the phase diagram.

The heat treatment temperature is not particularly limited as long as it is equal to or higher than the melting point of the metal halide, but may be, for example, 300°C or higher and 1200°C or lower.

The heat treatment time may be from 0.5 hours to 6 hours.

The concentration of R based on the total amount of metal halide at the heat treatment temperature may be, for example, 3.2 mol/L or more and 8.2 mol/L or less.

The manufacturing method according to this embodiment may or may not include steps other than the heat treatment step described above. If the manufacturing method according to this embodiment does not include steps other than the heat treatment step described above, the alloy powder obtained in the heat treatment step can be used as magnet powder.

(Washing process)
The manufacturing method according to this embodiment may further include a washing step of washing the R-T-B based alloy powder with a washing liquid in which the metal halide is dissolved.

The cleaning method in the cleaning step is not particularly limited as long as it can remove metal halides, but an example is a method in which a cleaning solution is added to the R-T-B alloy powder, the mixture is stirred, and then the mixture is decanted repeatedly.

The cleaning liquid is not particularly limited as long as it is a solvent in which the metal halide dissolves, and may be a solvent having a hydroxyl group or a solvent having no hydroxyl group. The cleaning liquid is preferably a solvent having no hydroxyl group, since it can suppress oxidation of the resulting R-T-B alloy powder. Examples of the solvent having a hydroxyl group include water, methanol, ethanol, and glycol-based solvents such as ethylene glycol. Examples of the solvent having no hydroxyl group include formamide, dimethylformamide, dimethylacetamide, acetonitrile, dimethylsulfoxide, acetone, acetylacetone, and ethylene carbonate. Formamide and dimethylformamide are preferable because they have high solubility of metal halides and can efficiently remove metal halides. The cleaning liquid may use one type of solvent alone or two or more types of solvents in combination. The content of the solvent having no hydroxyl group in the cleaning liquid may be 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, 95 mass% or more, or 100 mass%.

(Dehydrogenation process)
When a solvent having a hydroxyl group is used as the cleaning liquid, hydrogen may penetrate between the crystal lattices of the R-T-B system alloy powder. In this case, the manufacturing method according to the present embodiment may further include a dehydrogenation step of dehydrogenating the R-T-B system alloy powder.

Methods for dehydrogenating R-T-B alloy powder include, but are not limited to, heat treating the R-T-B alloy powder in a vacuum or in an inert gas atmosphere.

The temperature of the heat treatment in the dehydrogenation process may be 150 to 250°C. The time of the heat treatment may be 1 to 3 hours.

(Drying process)
When the manufacturing method according to this embodiment includes a cleaning step, it may further include a drying step of removing the cleaning liquid from the R-T-B based alloy powder by drying the R-T-B based alloy powder.

The temperature at which the R-T-B alloy powder is dried is preferably between room temperature (20°C) and 200°C, since this can prevent oxidation of the R-T-B alloy powder. The atmosphere during drying may be, for example, an inert gas such as argon or nitrogen, or a vacuum.

(Crushing process)
The manufacturing method according to the present embodiment may further include a pulverization step of pulverizing the R-T-B based alloy powder. The pulverization method is not particularly limited, but examples thereof include pulverization using a jet mill, a dry or wet ball mill, a vibration mill, and a media stirring mill.

[Action and Effect]
The manufacturing method according to the embodiment includes a heat treatment step of heating the composition at a temperature equal to or higher than the melting point of the metal halide. Unlike R contained in the composition, B is generally considered to be difficult to dissolve in a metal halide melt due to its high electronegativity. However, the inventors have found through their research that the composition dissolves in a metal halide melt even when it contains B by combining R and a metal halide with B. Then, the composition is heated at a temperature equal to or higher than the melting point of the metal halide, and R and B dissolved in the metal halide melt diffuse into T, thereby obtaining an R-T-B alloy at a lower temperature than before, and a magnet powder having a small average particle size and a narrow particle size distribution can be manufactured.

[RTB magnet powder]
The R-T-B based magnet powder according to this embodiment (hereinafter simply referred to as "the magnet powder according to this embodiment") will be described below.

The magnet powder according to this embodiment contains a rare earth metal element R, a transition metal element T, and boron B, has an average particle size of 0.1 to 1.0 μm, and has a particle size distribution width (D ₉₀ -D ₁₀ )/D ₅₀ of 0.85 or less, where R is at least one element selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and T is at least one element selected from the group consisting of Fe, Ni, Co, Cr, and Mn.

The average particle size of the magnetic powder according to this embodiment is 1.0 μm or less, and from the viewpoint of improving the magnetic properties, it is preferably 0.8 μm or less, and more preferably 0.6 μm or less. The method for measuring the average particle size will be described later.

The particle size distribution width (D ₉₀ -D ₁₀ )/D ₅₀ of the magnet powder according to this embodiment is 0.85 or less, and from the viewpoint of improving the magnetic properties, it is preferably 0.80 or less, and more preferably 0.75 or less. The method for measuring the particle size distribution width will be described later.

The circularity of the magnetic powder according to this embodiment is preferably greater than 70%, and more preferably greater than 72%, because this improves the fluidity of the magnetic powder when it is molded. The method for measuring the circularity will be described later.

The average particle size, particle size distribution width, and circularity of the magnet powder according to this embodiment are measured as follows. That is, the R-T-B magnet powder and the thermosetting resin are mixed in approximately equal amounts by volume to obtain a mixture. The obtained mixture is applied onto a sample stage (pin stub) for FIB (Focused Ion Beam), vacuum degassed, and then heated at 120°C for 1 hour using a hot plate to harden the mixture, thereby obtaining a hardened product. The surface of the hardened product is then dry-polished with abrasive paper. The hardened product is first roughly polished with rough abrasive paper (#600), then further polished with abrasive paper with medium roughness (#1200), and finally finish-polished with fine abrasive paper (#3000), to obtain a measurement sample with a mirror-finished polished surface. The obtained measurement sample is observed with a SEM (Scanning Electron Microscope) to obtain an SEM image. The observation magnification is 10,000 times. 300 magnet powder particles are randomly selected from the magnet powder particles that can be confirmed in the SEM image. The area S and perimeter L of the selected magnet powder particles are measured using image analysis and image measurement software. Then, the particle diameter D, particle size distribution width, and circularity coefficient defined by the following formulas (1) to (3) are calculated. Here, D ₁₀ , D ₅₀ , and D ₉₀ are the particle diameters with a cumulative frequency of 10%, 50%, and 90%, respectively, in the area-based cumulative particle diameter distribution curve of 300 magnet powder particles. Then, the area-based average value of the particle diameters D of the 300 magnet powder particles is defined as the average particle diameter of the magnet powder, and the number-based average value of the circularity coefficient of the 300 magnet powder particles is defined as the circularity of the magnet powder.

Particle size distribution width = (D ₉₀ - D ₁₀ )/D ₅₀ ...Formula (2)
Circularity coefficient=4πS/(L ² ) (3)

The magnet powder according to the present embodiment preferably contains single crystal particles of R ₂ T ₁₄ B because the residual magnetization is improved. The fact that the magnet powder according to the present embodiment contains single crystal particles of R ₂ T ₁₄ B can be confirmed as follows. That is, a measurement sample with a mirror-polished surface is obtained in the same manner as in the measurement of the average particle size of the magnet powder described above. The measurement sample is sliced by focused ion beam processing. The obtained slice is observed with a TEM (Transmission Electron Microscope). The observation magnification is 50,000 times. If the selected-area diffraction image of the TEM observation is spot-shaped and coincides with P4 ₂ /mmm, which is a characteristic of the crystal structure of R ₂ T ₁₄ B, the magnet powder is judged to have single crystal particles of R ₂ T ₁₄ B.

In this specification, a single crystal particle refers to an isolated particle that does not contain grain boundaries, has a uniform crystal orientation, and is not aggregated with other particles.

In the magnet powder according to this embodiment, the Ca content is preferably 100 ppm by mass or less, 10 ppm by mass or less, 1 ppm by mass or less, or 0.1 ppm by mass or less, since this improves remanent magnetization. The Ca content can be measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy).

In the magnet powder according to this embodiment, the Cl content is preferably 1 ppm by mass or more and 50,000 ppm by mass or less, since this can suppress oxidation of the powder. The Cl content can be measured using ICP-AES.

In the magnet powder according to this embodiment, the C content is preferably 1000 mass ppm or less, since this further improves the magnetic properties of the resulting magnet. The C content of the magnet powder can be measured by combustion in an oxygen stream - infrared absorption method.

The magnetic powder according to this embodiment may be manufactured by the manufacturing method for the R-T-B magnetic powder according to the above embodiment.

The applications of the magnetic powder according to this embodiment are not particularly limited, but it can be suitably used, for example, in the manufacture of sintered magnets and bonded magnets.

The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these.

Example 1
[Preparation of iron powder]
101.8 g of iron nitrate and 14.9 g of calcium nitrate were dissolved in 819 mL of water to obtain an aqueous solution. 441 ml of 1 mol potassium hydroxide aqueous solution was added dropwise to the obtained aqueous solution while stirring to obtain a suspension. The suspension was collected by filtration, washed, and then dried overnight in air at 120°C in a hot air drying oven to produce iron oxide powder. The Fe oxide powder was reduced in a hydrogen stream at 500°C for 6 hours to produce iron powder.

[Preparation of magnet powder]
(Heat treatment process)
0.48 g of iron powder, 0.60 g of neodymium powder, 2.07 g of lithium chloride powder, and 0.10 g of boron powder were placed in an iron crucible and heat-treated in an Ar atmosphere at 800° C. for 3 hours to obtain an Nd—Fe—B based alloy powder.

(Washing process)
The obtained Nd-Fe-B alloy powder was washed with water as a washing liquid to remove lithium chloride.

(Vacuum drying process)
The Nd-Fe-B alloy powder from which the lithium chloride had been removed had water replaced with acetone, and was then vacuum dried at room temperature (20° C.) to obtain magnet powder.

[Measurement of average particle size, particle size distribution range and circularity of magnet powder]
The obtained Nd-Fe-B magnet powder and a thermosetting resin (epoxy resin) were blended in approximately equal amounts by volume, and mixed well to obtain a mixture. The obtained mixture was applied onto a sample stage (pin stub) for FIB (Focused Ion Beam) and vacuum degassed. Then, the mixture was heated at 120°C for 1 hour using a hot plate to harden the mixture and obtain a hardened product. The surface of the hardened product was then dry-polished with abrasive paper. The hardened product was first roughly polished with rough abrasive paper (#600), then further polished with abrasive paper (#1200) having a medium roughness, and finally polished with fine abrasive paper (#3000) to obtain a measurement sample with a mirror-finished polished surface. The obtained measurement sample was observed with a SEM (Scanning Electron Microscope) to obtain an SEM image. The observation magnification was 10,000 times.

Next, 300 particles were randomly selected from the magnetic powder particles that could be seen in the SEM images. The area S and perimeter L of the magnetic powder particles selected were measured using image analysis and image measurement software. Then, the particle diameter D, particle size distribution width, and circularity coefficient, which are respectively defined by the following formulas (1) to (3), were calculated.

Particle size distribution width = (D ₉₀ - D ₁₀ )/D ₅₀ ...Formula (2)
Circularity coefficient=4πS/(L ² ) (3)

_D10 , _D50 and _D90 are the particle diameters at which the cumulative frequency is 10%, 50% and 90%, respectively, in the area-based cumulative particle diameter distribution curve of 300 magnet powder particles. The area-based average value of the particle diameters D of the 300 magnet powder particles was taken as the average particle diameter of the magnet powder, and the number-based average value of the circularity coefficients of the 300 magnet powder particles was taken as the circularity of the magnet powder. The results are shown in Table 1.

[Confirmation of the presence or absence of Nd ₂ Fe ₁₄ B single crystal particles]
A measurement sample with a mirror-finished polished surface was obtained in the same manner as in the measurement of the average particle size, particle size distribution width, and circularity of the magnet powder. The measurement sample was sliced by focused ion beam processing. The obtained slice was observed with a TEM (Transmission Electron Microscope). The observation magnification was 50,000 times. When the selected-area diffraction image of the TEM observation was spot-shaped and matched P4 ₂ /mmm, which is a characteristic of the crystal structure of Nd ₂ Fe ₁₄ B, it was determined that the magnet powder had single crystal particles of Nd ₂ Fe ₁₄ B. The results are shown in Table 1.

[Measurement of Ca content]
The Ca content in the obtained magnet powder was measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy), and the results are shown in Table 1.

<Examples 2 to 4, 6>
Magnet powder was obtained in the same manner as in Example 1, except that the cleaning solution shown in Table 1 was used instead of water. Moreover, the average particle size, particle size distribution width, and circularity of the magnet powder were measured, the presence or absence of _Nd2Fe14B single _crystal particles was confirmed, and the Ca content was measured in the same manner as in Example 1. The results are shown in Table 1.

In Table 1, EG stands for ethylene glycol, FA stands for formamide, and DMF stands for dimethylformamide.

Example 5
Magnet powder was obtained in the same manner as in Example 1, except that the cleaning solution shown in Table 1 was used instead of water. Moreover, the average particle size, particle size distribution width, and circularity of the magnet powder were measured, the presence or absence of _Nd2Fe14B single _crystal particles was confirmed, and the Ca content was measured in the same manner as in Example 1. The results are shown in Table 1.

[Measurement of Cl content]
The Cl content in the obtained magnet powder was measured using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy), and was found to be 12,900 ppm by mass.

[Evaluation of magnetic properties]
The mass magnetization and coercivity of the obtained magnet powder were evaluated using a VSM (Vibrating Sample Magnetometer) as follows. That is, the magnetization of the magnet powder was measured by applying an external magnetic field of 1592 kA/m (20 kOe) to -1592 kA/m (-20 kOe), and each value was obtained from the obtained demagnetization curve. The mass magnetization (σ _20kOe ) in an external magnetic field of 20 kOe was 102 emu/g. The coercivity was 1.1 kOe.

According to a method for producing R-T-B based magnet powder according to one aspect of the present invention, it is possible to produce magnet powder having a small average particle size and a narrow particle size distribution.

Claims (4)

A heat treatment step is provided for obtaining an R-T-B type alloy powder by heating a composition containing a rare earth metal element R, a transition metal element T, boron B, and lithium chloride at a temperature equal to or higher than the melting point of the lithium chloride ,
The rare earth metal element R is at least one selected from the group consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu;
A method for producing an R-T-B system magnet powder, wherein the transition metal element T is at least one selected from the group consisting of Fe, Ni, Co, Cr and Mn. The method for producing R-T-B system magnet powder according to claim 1 , further comprising a washing step of washing the R-T-B system alloy powder with a washing liquid in which the lithium chloride is dissolved. The method for producing R-T-B magnet powder according to claim 2, wherein the cleaning solution contains a solvent that does not have a hydroxyl group. The method for producing R-T-B magnet powder according to claim 2, wherein the cleaning solution contains at least one selected from the group consisting of formamide and dimethylformamide.