JP7460904B2 - Manufacturing method of rare earth magnet powder - Google Patents

Description

The present invention relates to a method for producing rare earth magnet powder used in the manufacture of bonded magnets that are exposed to high oil temperatures.

Rare earth magnets are widely used in electromagnetic devices (electric motors, etc.) to improve performance and save energy. There are two types of rare earth magnets: sintered magnets, which are made by sintering rare earth magnet particles, and bonded magnets, which are made by binding rare earth magnet particles with a binder resin.

Bonded magnets include injection bonded magnets that are formed by injecting a mixture of magnet particles and binder resin (mainly thermoplastic resin) into a cavity, and injection bonded magnets that are molded by injecting a mixture of magnet particles and binder resin (mainly thermosetting resin) into a cavity. There is a compression bonded magnet that is formed by compression solidification (including hardening) within a cavity.

Both types of bonded magnets have a greater degree of freedom in shape than sintered magnets and are superior in formability, so their uses are expanding. Along with this, bonded magnets are required to have durability (corrosion resistance) that allows stable maintenance of high magnetic properties. Descriptions related to this can be found, for example, in the following patent documents.

Patent No. 3882490 Japanese Patent No. 4650593 Patent No. 5499738

Patent Documents 1 to 3 propose the use of magnet particles coated with a phosphorus compound (phosphate, etc.) in a bonded magnet. However, all of these methods merely focus on the oxidation resistance of magnetic particles in a high-temperature, high-humidity atmospheric atmosphere.

The inventors' research has revealed that the magnetic properties of the magnetic particles that make up bonded magnets can deteriorate even in oil environments, where oxidation due to oxygen or moisture in the air was previously thought to be unlikely to occur.

The present invention was made in consideration of these circumstances, and aims to provide a manufacturing method and the like that can obtain rare earth magnet powder that can contribute to improving the durability of bonded magnets exposed to oil environments.

The inventors conducted extensive research to solve this problem and discovered that magnetic particles that were heated to a specific temperature after phosphoric acid treatment could maintain high magnetic properties (e.g., squareness: Hk) even after being exposed to a high-temperature oil environment. By expanding on this finding, the inventors were able to complete the present invention, which is described below.

《Method for manufacturing rare earth magnet powder》
(1) The present invention comprises a treatment step of bringing rare earth magnet particles into contact with a treatment liquid containing phosphoric acid, and a firing step of heating the rare earth magnet particles at 170 to 215° C. after the treatment step, and contains P. This is a method for producing rare earth magnet powder used in the production of bonded magnets that are made of rare earth magnet particles coated with a compound and exposed to an oil environment of 90° C. or higher.

(2) According to the manufacturing method of the present invention, rare earth magnet powder can be obtained that exhibits little deterioration in magnetic properties (e.g., squareness: Hk) even after exposure to a high-temperature oil environment. The reason for this is unclear, but it is thought to be because a stable coating made of a compound containing P (referred to as a "P-based compound") is formed only on the surface region of the rare earth magnet particles. The coating of the P-based compound (simply referred to as a "P-based coating") is preferably present on the entire surface of the rare earth magnet particles, but may be present on only a portion of the surface.

<Rare earth magnetic powder/bonded magnet>
The present invention can be understood as a rare earth magnetic powder obtained by the above-mentioned manufacturing method, and can also be understood as a bonded magnet using the rare earth magnetic powder or a manufacturing method thereof.

Manufacturing methods for bonded magnets include injection molding, in which a molten mixture of rare earth magnet powder and thermoplastic resin is filled into a cavity such as a slot and solidified, and compression molding, in which rare earth magnet powder and thermosetting resin are poured into a cavity and compressed, melted, and solidified. When the rare earth magnet powder is anisotropic magnet powder, it is recommended that it be molded with an aligning magnetic field applied.

"others"
(1) In this specification, rare earth magnet particles are simply referred to as "magnet particles" and rare earth magnet powder is simply referred to as "magnet powder". As appropriate, magnet particles (magnet powder) before the treatment process are referred to as "raw material particles (raw material powder)", magnet particles (magnet powder) after the treatment process are referred to as "treated particles (treated powder)", and magnet particles (magnet powder) after the sintering process are referred to as "sintered particles (sintered powder)". In other words, the present invention provides a method for producing sintered powder obtained by subjecting raw material powder made of a rare earth magnet alloy to a treatment process and a sintering process.

(2) The degree to which the magnetic properties of magnet particles are maintained after exposure to an oil environment (the degree to which deterioration of the magnetic properties is suppressed) is called "oil resistance". As an example of an index of oil resistance, the squareness (Hk) of magnet particles after exposure to a high-temperature (e.g., 150°C) oil environment, or its rate of change, can be used. Hk is an index of the resistance of magnet particles to reverse magnetic fields. Specifically, Hk represents the magnitude of the reverse magnetic field that reduces the residual magnetic flux density (Br), an index of magnetization potential, by 10%, and is also an index of the effective magnetic flux density B against the reverse magnetic field.

The rate of decrease in Hk of the sintered particles before and after exposure to an oil environment may be, for example, 5% or less. In comparison with the raw material particles (initial state) and the sintered particles after exposure to an oil environment, the rate of decrease in Hk may be, for example, 25% or less, or even 22% or less. In this specification, the state before exposure to an oil environment is referred to as the "initial state."

The Hk of the fired particles in the initial state (referred to as "initial Hk") is preferably, for example, 565 kA/m or more, and further 580 kA/m or more. The Br of the fired particles in the initial state (referred to as "initial Br") may be, for example, 1.2 T or more, and further preferably 1.25 T or more.

(3) Unless otherwise specified, "x to y" in this specification includes a lower limit of x and an upper limit of y. Any numerical value included in the various numerical values or numerical ranges described in this specification may be used as a new lower limit or upper limit to create a new range such as "a to b." Additionally, unless otherwise specified, "x to ykA/m" in this specification means xkA/m to ykA/m. The same applies to other units.

1 is a graph illustrating the effect of firing temperature on initial Hk. It is a graph illustrating the influence of firing temperature on Hk after a durability test with respect to an initial state. It is a graph illustrating the influence of firing temperature on Hk after a durability test with respect to the raw material state. It is a graph illustrating the influence that oil temperature and firing temperature have on Hk in a durability test.

One or more components selected from the items described in this specification may be added to the above-mentioned configuration of the present invention. The components may be related to the manufacturing method or to the product. Which embodiment is best depends on the target, required performance, etc.

《Processing process》
The treatment step is performed by bringing the magnet particles (powder) into contact with a treatment liquid containing phosphoric acid. In the treatment step, the type of phosphoric acid, the concentration of the treatment liquid, the solvent, etc. are not limited as long as a P-based compound (for example, phosphate) can be formed on the surface of the magnet particles.

Types of phosphoric acid include orthophosphoric acid, as well as inorganic and organic phosphoric acids such as phosphates, phosphorous acid, hypophosphorous acid, pyrophosphoric acid, metaphosphoric acid, and polyphosphoric acid. Of these, it is recommended to use orthophosphoric acid, which is highly reactive with rare earth metals and iron and easily forms P-based compounds on the surface of magnet particles.

The treatment liquid is obtained, for example, by preparing phosphoric acid or a phosphorus compound (including a phosphate) in a solvent. The solvent may be water, but it is preferable to use an organic solvent (particularly a volatile solvent). Examples of organic solvents include alcohols (isopropyl alcohol (IPA), ethanol, methanol, 2-methoxyethanol, etc.), formamide, N,N-dimethylformamide, and the like. The treatment liquid may further contain a surfactant (for example, a silane coupling agent, etc.).

For example, when using orthophosphoric acid (H ₃ PO ₄ ), the mixing ratio (mass %) of the treatment liquid to the magnet powder is 0.5 to 1.0 mass %, and more preferably 0.7 to 0.9 mass %. It would be good if it were.

The raw powder and the treatment liquid are brought into contact with each other by, for example, immersion or spraying. In order to uniformly form the P-based compound on the surface of the magnet particles, it is advisable to bring the two into contact (mix) while stirring or the like. Alternatively, the raw powder and the treatment liquid may be brought into contact with each other while at least one of them is heated. Heating vaporizes (evaporates) the solvent and promotes the formation of a P-based coating on the surface of the magnet particles. The heating temperature is, for example, 40 to 110°C, or even 60 to 90°C.

Firing process
The sintering step is performed by heating the rare earth magnet particles (treated particles) after the treatment step. The heating temperature (referred to as the "sintering temperature") is preferably, for example, 170 to 215°C, 175 to 210°C, or even 185 to 205°C. If the sintering temperature is too low, the formation of stable P-based compounds is not promoted, and the oil resistance of the sintered particles may decrease. If the sintering temperature is too high, the formation of P-based compounds may progress deep into the magnet particles, and the initial magnetic properties (for example, initial Hk) may decrease.

The firing process is preferably carried out in an oxidation-preventing atmosphere (e.g., in a vacuum, in an inert gas, in nitrogen gas, etc.). The firing process is preferably carried out for, for example, 0.5 to 5 hours, or even 1 to 3 hours.

《Magnetic particles》
The magnet particles (powder) are made of a magnet alloy containing a rare earth element (R). There are various compositions of magnet alloys. For example, NdFeB magnet particles whose basic components are Nd, Fe, and B (essential components), SmFeN magnet particles whose basic components are Sm, Fe, and N, SmCo magnet particles whose basic components are Sm and Co, etc. be. Note that the total amount of elements serving as base components (essential components or main components) is usually 80 atomic % or more, further 90 atomic % or more, based on the entire magnet particle. The rare earth magnet particles may contain elements (heavy rare earth elements such as Dy and Tb, Cu, Al, Co, Nb, etc.) that enhance their coercive force, heat resistance, and the like.

The magnet particles may be anisotropic or isotropic magnet particles. By using anisotropic magnet particles (powder) and molding in an aligning magnetic field, a bonded magnet with high magnetic properties can be obtained.

The magnet particles are obtained, for example, by subjecting a magnet alloy to hydrogen treatment. Hydrogen treatment usually involves a disproportionation reaction due to hydrogen absorption (Hydrogenation-Disproportionation, also simply called the "HD reaction") and a recombination reaction due to dehydrogenation (Desorption-Recombination, also simply called the "DR reaction"). The HD reaction and the DR reaction are collectively referred to simply as the "HDDR reaction", and the hydrogen treatment that produces the HDDR reaction is simply referred to as "HDDR (treatment)". Note that HDDR also includes an improved version, d-HDDR (dynamic-Hydrogenation-Disproportionation-Desorption-Recombination). d-HDDR is described in detail, for example, in International Publication WO2004/064085.

The magnet particles may be subjected to one or more types of rust prevention treatment in addition to the above-mentioned phosphoric acid treatment. Such rust prevention treatments include, for example, metal alkoxy oligomer treatment to form an organometallic compound layer, coupling treatment to form a coupling agent layer, gradual oxidation treatment to form an oxide film, and the like.

The size of the magnet particles (particle size of the magnet powder) is not important. For NdFeB magnet particles, for example, the average particle size should be 40 to 200 μm, or even 80 to 160 μm. Unless otherwise specified, the average particle size referred to in this specification is the volume median diameter (VMD) determined by measurement using a laser diffraction particle size distribution measuring device (HELOS, manufactured by Nippon Laser Co., Ltd.).

The magnetic particles may be coarse particles with a relatively large average particle size, or fine particles with a small average particle size. The fine particles have an average particle size of, for example, 1 to 10 μm or even 2 to 6 μm. Such fine particles include SmFeN magnetic particles. Note that the magnetic powder referred to in this specification may be a mixture of two or more types of magnetic particles with different compositions, anisotropy/isotropy, average particle size, etc. By using such composite magnetic powder, the filling rate of the magnetic powder is improved, and a bonded magnet with high magnetic properties is obtained.

Oil Environment
The oil environment to which the magnet particles (or bonded magnets) are exposed is an environment in which the temperature (oil temperature) of the oil in contact with the magnet particles is, for example, 90°C or higher, 100°C or higher, 115°C or higher, or even 130°C or higher. The oil environment is in oil (immersed state), in an oil mist atmosphere, etc. The oil may be lubricating oil, hydraulic oil, cooling oil, or an oil that serves both purposes. Specific examples include automatic transmission fluid (ATF), continuously variable transmission fluid (CVTF), transmission oil, engine oil, etc. The oil may be mineral oil or chemically synthetic oil. Mineral oil is mainly used for ATF and CVTF.

The temperature of the ATF and CVTF used in typical transmissions is usually around 40 to 80°C. According to the inventor's research, in this oil temperature range, even magnet particle powder with a sintering temperature of 160°C or less, or even 150°C or less, can exhibit sufficient oil resistance.

To give a specific example, in an ATF environment of 80°C, the rate of decrease in Hk was approximately the same for magnet particles with sintering temperatures of 150°C, 180°C, and 200°C. However, in an ATF environment of 120°C and even 150°C, the rate of decrease in Hk differed greatly depending on the sintering temperature of the magnet particles. In other words, for magnet particles with a sintering temperature of 180°C or 200°C, there was almost no change in the rate of decrease in Hk (oil resistance) in either ATF environment. On the other hand, for magnet particles with a sintering temperature of 150°C, Hk decreased significantly when the ATF was 120°C or 150°C (see Figure 2).

《Bond magnet》
The rare earth magnet powder according to the present invention is used for manufacturing bonded magnets. A bonded magnet (permanent magnet) used in an oil environment serves as a field source for a field element of an electric motor or a solenoid, for example. The bonded magnet may be an injection molded product or a compression molded product.

The field element for an electric motor may be a rotor or a stator. Electric motors include not only motors but also generators. The electric motor may be a DC motor or an AC motor. When the field element is a rotor, the bonded magnet is, for example, integrally molded into a slot (cavity) in the rotor core.

As a bonded magnet exposed to a high-temperature oil environment, there is a field source used in an electric motor (particularly a drive motor) of an electric vehicle. The electric vehicle may be an electric vehicle or various hybrid vehicles.

The oil resistance of rare earth magnet powder (samples) coated with various firing temperatures was evaluated. The present invention will be described in detail below based on such specific examples.

《Sample production》
(1) Raw material powder As raw material magnet powder, commercially available NdFeB-based anisotropic magnet powder manufactured by hydrogen treatment (d-HDDR) (Magfine: MF18P (Br: 1.29T, manufactured by Aichi Steel Co., Ltd.), (iHc: 1321 kA/m, (BH) max: 311.2 kJ/m ³ , average particle diameter (VMD): 121 μm) was prepared. This magnet powder is referred to as raw material powder (sample C0).

(2) Treatment Process The raw material powder was subjected to the following treatment process to produce a treated powder.

A treatment solution was prepared by mixing and stirring 4.8 mL of orthophosphoric acid (manufactured by Kanto Kagaku Co., Ltd.) and 300 mL of a solvent (isopropyl alcohol: IPA, manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.).

1000 g of the raw material powder and the above treatment liquid were put into a small Henschel mixer (FM mixer FM3/I manufactured by Nippon Coke Industry Co., Ltd.), and heated to 80°C using an oil temperature controller in a nitrogen gas flow atmosphere (0.1 L/min). The raw material powder and the treatment liquid were mixed and stirred for 60 minutes at a blade rotation speed of 300 rpm while heating the mixing tank (treatment step).

(3) Firing process The treated powder was subjected to the following firing process to produce a fired powder.

The treated powder recovered from the Henschel mixer was placed in a dry oven and heated in a vacuum atmosphere of 10 ^-1 Pa. The heating temperature (sintering temperature) was adjusted in the range of 120 to 250°C. The heating time was 2 hours in all cases. In this way, the samples shown in Table 1 (sintered powder: coated rare earth anisotropic magnet powder) were obtained.

"An endurance test"
The powder of each sample (calcined powder and raw material powder) was exposed to the following oil environment. 30 g of each powder and 30 mL of ATF were placed in a beaker, and the mixture was left at 150° C. for 1000 hours in the air. As the ATF, Dexron-VI manufactured by ACDelco, which uses mineral oil as a base oil, was used.

"observation"
The cross sections of particles (sintered particles) randomly extracted from the sintered powder before the durability test were observed with a scanning electron microscope (SEM).

Magnetic properties
The magnetic properties of each sample before and after the durability test were measured at room temperature using a pulse BH tracer (manufactured by OP Electronics Co., Ltd.). From the obtained BH curves, the maximum energy product: (BH)max, residual magnetic flux density: Br, coercive force: iHc, and squareness: Hk were determined and shown in Table 1. Note that for (BH)max, Br, and iHc, only the measured values before the durability test (referred to as the "initial state") are shown in Table 1.

"evaluation"
The rate of change (ΔHk) was calculated from the Hk before and after the durability test. The results are also shown in Table 1. ΔHk _s0 shown in Table 1 indicates the rate of change in Hk ₀ of each fired powder with respect to Hk _s of the raw material powder (sample C0) before the durability test (initial state). ΔHk ₀₁ indicates the rate of change in Hk ₁ after the durability test relative to Hk ₀ before the durability test for the fired powder. ΔHk _s1 represents the rate of change in Hk ₁ of the fired powder after the durability test with respect to Hk _s of the raw material powder.

Further, the relationships between ΔHk _s0 , ΔHk ₀₁ and ΔHk _s1 and each firing temperature are shown in FIGS. 1A to 1C (collectively referred to as "FIG. 1").

(1) Particle Surface From the SEM images of the sintered particles observed before the durability test, it was found that a substantially uniform coating layer (film) was present on the surface.

(2) Initial magnetic properties As is clear from Table 1, the fired powder in the initial state after the coating treatment (treatment process and firing process) has (BH)max, Br and Hk slightly lower than the raw powder. However, there was almost no change in iHc. Further, (BH)max, Br and iHc of the fired powder hardly changed depending on the firing temperature. However, as is clear from FIG. 1A, the Hk of the fired powder clearly began to decrease when the firing temperature reached 220° C. or higher.

(3) Magnetic properties after durability test (oil resistance)
1B, the Hk of the sintered powder decreased due to the durability test. However, in the case of the sintered particles sintered at a temperature of 170° C. or higher, the decrease in Hk (ΔHk ₀₁ ) was only 5% or less.

Furthermore, as is clear from Table 1 and Figure 1C, the particles sintered at temperatures between 170 and 215°C showed significantly less degradation in magnetic properties (especially Hk) compared to the raw material powder.

From the above, it was found that by heating at a specified temperature range after phosphoric acid treatment, rare earth magnet powder can be obtained that has little deterioration in magnetic properties compared to the raw material powder and also suppresses deterioration of magnetic properties in an oil environment.

Claims (5)

a treatment step of bringing rare earth magnet particles into contact with a treatment solution containing phosphoric acid;
a firing step of heating the rare earth magnet particles at 170 to 215° C. after the treatment step,
Consists of rare earth magnet particles coated with a compound containing P,
The rare earth magnet particles include NdFeB-based magnet particles containing Nd, Fe, and B as essential components,
A method for producing rare earth magnet powder used in the production of bonded magnets that are exposed to an oil environment of 90°C or higher. 2. The method for producing rare earth magnet powder according to claim 1, wherein the NdFeB magnet particles are obtained by hydrogen treating a magnet alloy . The method for producing rare earth magnet powder according to claim 1 or 2, wherein the oil is mineral oil. The method for producing rare earth magnet powder according to any one of claims 1 to 3, wherein the oil is an automatic transmission fluid (ATF). The method for producing rare earth magnet powder according to any one of claims 1 to 4, wherein the bonded magnet is a field source for a motor of an electric vehicle.

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