WO2023119612A1 - Rare earth magnet powder and production method therefor - Google Patents

Abstract

The present invention provides a production method for a rare earth magnet powder that is capable of stably exhibiting high magnetic properties even in a corrosive environment. The present invention is a production method for obtaining a rare earth magnet powder constituted by magnet particles that have a coating film including P, O, and Fe, said production method comprising a treatment step for bringing magnet particles containing Nd, Fe, and B, into contact with a treatment solution containing phosphoric acid ions, and a firing step for heating the magnet particles after the treatment step at 250-350°C. The content ratio (Nd/Fe), which is the atomic ratio of Nd with respect to Fe in the coating film, is, for example, not more than 0.5 in the range (outermost region) from the outermost surface of the coating film to a depth of 10 nm. The coating film may be dense, with no prominent voids etc. near the surface thereof. A bonded magnet using such a rare earth magnet powder is suitable, for example, as a field source for a pump motor to be used in corrosive environments (in water, oil, etc.).

Description

Rare earth magnet powder and method for producing the same

The present invention relates to a method for producing rare earth magnet powder and the like.

In order to improve performance and save energy, many electromagnetic devices (electric motors, etc.) using rare earth magnets are used. Rare earth magnets include sintered magnets obtained by sintering rare earth magnet particles and bonded magnets obtained by binding rare earth magnet particles with a binder resin.

Bonded magnets are made by injecting a mixture of magnet particles and binder resin (mainly thermoplastic resin) into a cavity and molding them. There is a compression bond magnet molded by compression and solidification (including hardening) with.

　Bonded magnets have better formability than sintered magnets and have a greater degree of freedom in shape, so their applications are expanding. Along with this, there is a strong demand for bond magnets to have high reliability (also called durability, corrosion resistance, etc.) so that magnetic properties can be stably exhibited. Descriptions related to this can be found, for example, in the following patent documents.

Patent No. 3882490 Patent No. 3882545 Patent No. 4650593 Patent No. 5499738

Patent Documents 1 to 4 propose to subject magnetic powder to phosphoric acid treatment for the purpose of improving rust prevention and weather resistance in order to suppress the deterioration of magnetic properties. At that time, the coating treatment is promoted by heat drying (baking drying). However, the heating temperature is limited to 200° C. or less at most in order to suppress deterioration of magnetic properties (see, for example, Patent Document 3 [0065]). Note that Patent Documents 1 and 2 only describe examples in which SmFeN magnet alloy powder containing a large amount of fine particles, which is generally obtained by nitriding, is treated with phosphoric acid.

The present invention has been made in view of such circumstances, and aims to provide a new method for producing rare earth magnet powder.

As a result of intensive research to solve this problem, the present inventors newly found that the magnetic properties of NdFeB magnet powder that has been heated in a predetermined high temperature range after phosphate treatment are less likely to deteriorate even in a corrosive environment. rice field. Developing this result led to the completion of the present invention described below.

<<Method for producing rare earth magnet powder>>
The present invention comprises a treatment step of bringing magnet particles containing Nd, Fe and B into contact with a treatment solution containing phosphate ions, and a sintering step of heating the magnet particles after the treatment step at a sintering temperature of 250 to 350°C. and a method for producing a rare earth magnet powder comprising magnet particles having a coating containing P, O and Fe.

According to the production method of the present invention, for example, rare earth magnet powder whose magnetic properties (eg, coercive force) are less likely to deteriorate even when exposed to a corrosive environment in which water, oil, etc. are present can be obtained. Although the reason for this is not clear, it is thought that a highly corrosion-resistant coating that is difficult to oxidize is formed near the surface of the magnet particles, which greatly affects the deterioration of the magnetic properties.

《Rare earth magnet powder》
(1) The present invention can also be understood as a rare earth magnet powder. For example, the present invention is a rare earth magnet powder consisting of magnet particles having a coating, the magnet particles comprising Nd, Fe and B, the coating comprising P, O and Fe contained in the coating The rare earth magnet powder may have a content ratio (Nd/Fe), which is the atomic ratio of Nd to Fe, of 0.5 or less in the outermost surface region from the outermost surface to a depth of 10 nm.

The film formed on the outermost surface area of the magnet particles, for example, has a significantly lower content of base Nd, which is easily oxidized, than conventional phosphate films. A rare earth magnet powder composed of magnet particles coated with such a film is less likely to deteriorate in magnetic properties even in a corrosive environment, and can exhibit high reliability.

(2) The present invention provides, for example, a rare earth magnet powder composed of magnet particles having coatings, the magnet particles containing Nd, Fe and B, the coatings containing P, O and Fe, and the coatings The maximum size of voids contained in the rare earth magnet powder may be 50 nm or less.

Although the mechanism is not clear, rare earth magnet powder consisting of magnet particles coated with a dense phosphate film without large voids (defects) does not easily deteriorate in magnetic properties even in a corrosive environment and has high reliability. can demonstrate.

A film with less Nd present in the outermost surface area and a dense film without conspicuous voids can be produced, for example, by the production method of the present invention described above.

《Bond magnet and its manufacturing method》
The present invention can also be grasped as a bonded magnet using rare earth magnet powder and a manufacturing method thereof. A bonded magnet is composed of magnet particles and a binder resin. Such a bonded magnet can be produced, for example, by an injection molding method in which a cavity (magnet hole, etc.) is filled with a molten mixture of rare earth magnet powder and a thermoplastic resin and solidified; These compositions (compounds) are obtained by a compression molding method or the like in which they are compressed, melted and solidified. When the rare earth magnet powder is an anisotropic magnet powder, it is preferable to compact while applying an aligning magnetic field.

"others"
(1) In this specification, magnet particles containing Nd, Fe, and B are simply referred to as "magnet particles" as appropriate, regardless of whether they are before or after coating treatment (treatment step or firing step) or with or without a coating. A rare earth magnet powder composed of such magnet particles is also simply referred to as "magnet powder".

When daring to distinguish, 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 after the firing process. The magnet particles (magnet powder) are referred to as "fired particles (fired powder)".

(2) Coatings containing P, O and Fe are also simply referred to as "phosphate (based) coatings" regardless of their component composition, structure or structure. The phosphate coating may contain elements other than P, O and Fe (for example, Nd, B, etc.).

The film according to the present invention only needs to have at least one layer on the surface of the magnet particles, and may be laminated (combined) with other types of films (films). The film according to the present invention should normally have at least one layer on the outermost layer of the raw material particles.

(3) "Corrosion resistance" as used in this specification means the degree of deterioration (decrease rate) of the magnetic properties of the magnet particles, regardless of the deterioration (deterioration, etc.) of the film itself. For example, a "highly corrosion-resistant film" means a film that can suppress the deterioration of the magnetic properties of magnet particles. As an index of corrosion resistance, unless otherwise specified, the reduction rate (demagnetization rate) of coercive force (iHc) of magnet particles, which is practically important, is used.

(4) Unless otherwise specified, "x to y" as used herein includes the lower limit value x and the upper limit value y. A new range such as “a to b” can be established as a new lower or upper limit of any numerical value included in the various numerical values or numerical ranges described herein. Further, unless otherwise specified, "x to ykA/m" as used herein means xkA/m to ykA/m. The same applies to other unit systems.

FIG. 4 is a scatter diagram showing the relationship between demagnetization rate and firing temperature before and after coating treatment. It is a scatter diagram showing the relationship between the demagnetization rate and the firing temperature before and after the corrosion resistance test. 4 is a scatter diagram showing the relationship between the demagnetization rate after the corrosion resistance test and the firing temperature with respect to the initial state (before coating treatment). 4 is a bar graph comparing the demagnetization rate after the corrosion resistance test with respect to the initial state for samples with different coating treatment methods. It is the STEM image which observed the film|membrane with which baking temperature differs. It is an elemental mapping image by EDS concerning those films. 4 is a table summarizing the number and size of voids contained in films with different firing temperatures. 4 is a graph showing elemental distributions obtained by analyzing films with different firing temperatures by AES. It is a graph which shows the content ratio (Nd/Fe) of Nd with respect to Fe calculated|required from the element distribution. FIG. 4 is a schematic diagram for explaining a conjecture relating to the relationship between the morphology of the phosphate coating and the firing temperature. 4 is a bar graph comparing the demagnetization rate after the corrosion resistance test with respect to the initial state for samples with different coating treatment conditions.

One or more components arbitrarily selected from the matters described in this specification can be added to the configuration of the present invention described above. A component related to a manufacturing method can also be a component related to an object. Which embodiment is the best depends on the target, required performance, and the like.

《Process》
The treatment step is carried out by bringing magnet particles (powder) into contact with a treatment liquid containing phosphate ions. In the treatment process, as long as a film containing P, O and Fe (phosphate film) can be formed on the surface of the magnet particles, the raw material, preparation method, concentration, solvent, etc. of the treatment liquid are not limited. Phosphate ions are not limited to typical orthophosphate ions (PO ₄ ^3- ), but may be phosphite ions (PO ₃ ^3- ), hypophosphite ions (PO ₂ ^3- ), and the like. The phosphate ions contained in the treatment liquid may have different compositions and different valences.

Specifically, the treatment liquid may be, for example, phosphoric acid alone, or phosphoric acid or a phosphorous compound (including phosphate) prepared in a solvent. The solvent may be water or an organic solvent (particularly a volatile solvent). The coating treatment (film formation treatment) can be promoted by using a highly volatile treatment liquid.

In addition to orthophosphoric acid, phosphoric acid may be inorganic phosphoric acid such as phosphorous acid, hypophosphorous acid, pyrophosphoric acid, metaphosphoric acid, polyphosphoric acid, organic phosphoric acid, and the like. In particular, orthophosphoric acid has high reactivity with iron and tends to form a film on the surface of magnet particles. Phosphates, which are a type of phosphorus compound, include zinc phosphate, manganese phosphate, magnesium phosphate, and the like. Organic solvents include, for example, 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.).

When orthophosphoric acid (H ₃ PO ₄ ) is used for the treatment liquid, the orthophosphoric acid is, for example, 0.3 to 1.0% by mass, further 0.4 to 0.8% by mass with respect to the entire magnet powder. Good to have.

The raw material powder and the treatment liquid are brought into contact with each other by, for example, an immersion method, a spray method, or the like. In order to uniformly form a phosphate film on the surface of the magnet particles, the two may be brought into contact (mixed) while stirring or the like. Alternatively, at least one of the raw material powder and the treatment liquid may be heated and brought into contact with each other. The heating vaporizes (evaporates) the solvent and promotes the formation of a phosphate film on the magnet particle surface. The heating temperature is, for example, 40-110°C, further 60-90°C. The treatment process is performed, for example, in an anti-oxidation atmosphere (eg, inert gas (N ₂ , Ar, etc.) atmosphere).

《Baking process》
The firing step is performed by heating the magnet particles (treated particles) after the treatment step. The heating temperature (referred to as "firing temperature") is, for example, 250 to 350.degree. C., 270 to 330.degree. C., 275 to 325.degree. If the sintering temperature is too low, it is difficult to form a film with excellent corrosion resistance. If the sintering temperature is too high, the magnetic properties of the magnet particles themselves may deteriorate.

The firing process may be performed in an anti-oxidizing atmosphere (eg, vacuum atmosphere, inert gas (N ₂ , Ar, etc.) atmosphere, etc.) or in an oxidizing atmosphere (eg, atmospheric atmosphere, semi-atmospheric atmosphere, etc.). If it is carried out in an oxidizing atmosphere, it is possible to reduce the burden on equipment and processes involved in the firing process.

The firing process is performed, for example, for 1 to 180 minutes, 3 to 150 minutes, or even 10 to 50 minutes. If the vicinity of the surface of the magnet particles (for example, about 1 μm in depth) reaches the desired firing temperature, the firing process may be short. By shortening the time of the sintering process of heating to a high temperature, not only the production efficiency can be improved, but also the deterioration of the magnetic properties can be suppressed.

《Preheating process》
The magnet powder after the treatment process may be heated at a temperature lower than the firing temperature described above before the firing process (preheating process). The preheating step is not essential, and its purpose does not matter. The preheating process is performed for the purpose of, for example, drying, baking, preheating before the firing process, and calcination before firing (the firing process).

The temperature of the preheating process is, for example, 80-220°C or 100-150°C. The duration of the preheating step is, for example, 0.1 to 4 hours, or 0.5 to 3 hours. The atmosphere of the preheating step may be either an anti-oxidizing atmosphere or an oxidizing atmosphere, as in the firing step. The transition from the preheating step to the firing step may be continuous or intermittent. If the transfer is continuous, residual heat can be used, and if the transfer is intermittent, batch processing becomes possible. When a large amount of magnet powder is processed, the preheating step can further stabilize the formation of the desired phosphate-based film.

《Magnet Particles》
NdFeB magnet particles containing Nd, Fe, and B as essential components (basic components) are treated with phosphoric acid. The magnet particles may contain elements (heavy rare earth elements such as Dy and Tb, Cu, Al, Co, Nb, etc.) that enhance coercive force and heat resistance. However, the total amount of the essential elements is usually 80 atomic % or more, preferably 90 atomic % or more, relative to the entire magnet particles.

The magnet particles may be anisotropic magnet particles or isotropic magnet particles. A bonded magnet with high magnetic properties can be obtained by molding anisotropic magnet particles (powder) in an oriented magnetic field.

Magnet particles are obtained, for example, by hydrogen-treating a magnet alloy. Hydrogenation usually includes a disproportionation reaction due to hydrogen absorption (Hydrogenation-Disproportionation/simply referred to as "HD reaction") and a recombination reaction due to dehydrogenation (Desorption-Recombination/simply referred to as "DR reaction"). Accompany. The HD reaction and the DR reaction are collectively referred to simply as the "HDDR reaction", and the hydrogen treatment resulting in the HDDR reaction is simply referred to as "HDDR". The HDDR also includes an improved d-HDDR (dynamic-Hydrogenation-Disproportionation-Desorption-Recombination). The d-HDDR is described in detail in, for example, International Publication (WO2004/064085). Furthermore, the magnet alloy before HDDR may be subjected to hydrogen cracking treatment in which it is exposed to a high temperature (425 to 550° C.) hydrogen atmosphere (see International Publication (WO2020/017529)).

Incidentally, the NdFeB magnet alloy usually contains more Nd than the theoretical composition (Nd: 11.8 at%, B: 5.9 at%, Fe: balance) constituting the main phase (Nd ₂ Fe ₁₄ B). It consists of an Nd-rich composition. Some of the Nd richer than the theoretical composition also appears on the surface of the magnet particles obtained by pulverizing the magnet alloy, forms an oxide (referred to as "Nd-rich oxide"), and is a factor in the deterioration of magnetic properties. Become. Such a tendency is more pronounced when the magnet alloy consists of an ingot (or its pulverized powder) than when it consists of a rapidly solidified amorphous ribbon (or its pulverized powder). According to the present invention, the corrosion resistance of magnet particles can be improved not only in the former case but also in the latter case. This point is particularly significant.

The magnet particles may be subjected to one or more types of antirust treatment in addition to the phosphoric acid treatment described above. Examples of such antirust treatment include metal alkoxy oligomer treatment for forming an organometallic compound layer, coupling treatment for forming a coupling agent layer, and the like. A phosphate film directly formed on the surface of the magnet particles usually serves as a base layer for antirust treatment.

The size of the magnet particles (the particle size (particle size) of the magnet powder) does not matter. NdFeB magnet particles obtained by HDDR have, for example, an average particle size of 40 to 250 μm. Unless otherwise specified, the average particle size referred to in this specification is a volume median diameter (VMD) determined by measurement with a laser diffraction particle size distribution analyzer (HELOS manufactured by Japan Laser Co., Ltd.).

The magnet particles may include fine particles with a small average particle size as well as coarse particles with a relatively large average particle size. The average particle size of the fine particles is, for example, 1 to 10 μm, further 2 to 6 μm. Composite magnet powder obtained by mixing the magnet powder of the present invention with another magnet powder having different composition, anisotropy, isotropy, average particle size, etc., may be used for manufacturing bonded magnets according to the required specifications. good too.

《Film》
(1) Composition The film contains at least P, O and Fe, and may have various specific compositions, textures, and structures. For example, the lower the Nd content in the outermost surface area of the film, the better. Specifically, the content ratio (Nd/Fe), which is the atomic ratio of Nd to Fe contained in the coating, is preferably 0.5 or less, 0.4 or less, or 0.3 or less in the outermost region. . It should be noted that Nd/Fe can be 0.1 or more, or even 0.2 or more.

The outermost surface area referred to in this specification is, for example, the range from the outermost surface of the film to a depth of 10 nm. The content ratio (Nd/Fe) can be obtained from the atomic ratio of Nd and Fe obtained by analyzing the vicinity of the outermost surface of the film by Auger Electron Spectroscopy (AES), for example. Specifically, the arithmetic mean value of the atomic ratio (Nd/Fe) at a depth of 0 nm (outermost surface), a depth of 5 nm, and a depth of 10 nm is compared with a predetermined value (threshold) as the content ratio (Nd/Fe). do it. In addition, if the content ratio (Nd/Fe) is calculated based on the atomic ratio (Nd/Fe) of at least three points (excluding the singular point) in the outermost surface area, the measurement position is not necessarily the above-mentioned depth It doesn't have to be.

(2) Structure The coating preferably has a dense structure with few defects near the surface where corrosion resistance is greatly affected. For example, the maximum size of voids contained in the film is preferably 50 nm or less, 40 nm or less, or even 30 nm or less. In terms of the average size of voids, for example, it is preferably 30 nm or less, 20 nm or less, or even 15 nm.

The size of the void is the maximum length of the line segment that has both endpoints on the outline of the extracted void on the observation image of the film cross section. The measurement of the maximum length may be performed visually and manually by setting a scale bar on the observation image, or may be performed using image processing software. The maximum size of a void is the maximum of its maximum length. The average size of voids is the arithmetic mean of their maximum lengths (Σ(maximum length)/number).

The observed image of the cross section of the film is obtained by observing the vicinity of the surface of particles randomly extracted from the coated magnet powder with a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM). Extraction of voids and measurement of their sizes may be performed, for example, on the basis of a cross section of the film within a predetermined field of view (14 μm×14 μm) with respect to a secondary electron image at a magnification of about 8000 times. Since the thickness of the coating is not constant, the cross-sectional range of the coating is defined as the region where P is detected in the elemental mapping image obtained by energy dispersive X-ray spectroscopy (EDS). In addition, even for very small voids (for example, 5 nm or less) that are difficult to detect (visually recognize) based on secondary electron images, when the size measurement is performed, Nd, O and P are not detected on the elemental mapping image. Consider the airspace to be a void and size it according to the method described above.

《Bond Magnet》
The rare earth magnet powder of the present invention can be used for any application, but for example, it is used for bonded magnets that are exposed to water, oil, their mists, and the like. Bonded magnets may be injection molded or compression molded.

Bonded magnets, for example, serve as the magnetic field source for the magnetic field elements of electric motors and solenoids. A field element of an electric motor is, for example, a rotor or a stator. The rotor may be an internal magnet type in which bond magnets are integrally formed in slots (cavities) of the rotor core, or a surface magnet type in which bond magnets are arranged on the surface side of the rotor core. The electric motor may be a generator as well as a motor. The motor may be a DC motor or an AC motor. The electric motor according to the present invention can be used for any application, but for example, it is used as a drive source for water pumps and oil pumps exposed to corrosive environments.

We produced a large number of rare earth magnet powders (samples) with different temperatures during the coating treatment, and evaluated their corrosion resistance (demagnetization rate after exposure to a corrosive environment). The present invention will be described in detail below based on such specific examples.

[First embodiment]
《Production of samples》
(1) Raw material powder As the raw material powder, commercially available NdFeB-based anisotropic magnet powder manufactured by hydrogen treatment (d-HDDR) (manufactured by Aichi Steel Co., Ltd. Magfine: MF-P15 (Br: 1.3 T, iHc : 1116 kA/m, (BH) max: 312 kJ/m ³ , average particle diameter (VMD): 119 μm).

(2) Treatment Process 6 mL of orthophosphoric acid (manufactured by Kanto Chemical Co., Ltd.) and 40 mL of solvent (isopropyl alcohol: IPA, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed and stirred to prepare a treatment liquid. The mass ratio of orthophosphoric acid to the raw material powder was set to 0.5%.

　Into the mixing tank of a small Henschel mixer (FM mixer FM-3C manufactured by Nippon Coke Kogyo Co., Ltd.), 2000 g of the raw material powder and the above-described treatment solution were added and mixed and stirred (treatment process). At this time, atmosphere: nitrogen gas flow (0.4 mL/min), temperature of mixing tank: 80° C., blade rotation speed: 300 rpm, treatment time: 60 minutes. A treated powder was thus obtained.

(3) Preheating Process The treated powder recovered from the Henschel mixer was placed in a dry oven and heated at 120° C. for 2.5 hours in an air atmosphere.

(4) Firing Step The treated powder after the preheating step was placed in a Labo Plastomill (manufactured by Toyo Seiki Seisakusho Co., Ltd.) and heated in an air atmosphere. The heating temperature (firing temperature) was adjusted between 200 and 400°C. The firing temperature shown in this example is the temperature of the sample actually measured. The heating time was 0.5 hours (30 minutes). Samples 3 to 9 (fired powder) shown in Table 1 were thus obtained.

(5) Reference samples (samples 1, 2 and sample C)
The following reference samples were also prepared.

Sample 1 is a powder produced by performing only the above-described treatment process without performing the preheating process and the firing process. Sample 2 is a powder produced by performing the treatment and preheating steps described above without performing the firing step. Therefore, sample 1 has a powder maximum heating temperature of 80°C, and sample 2 has a powder maximum heating temperature of 120°C.

Sample C is a powder produced by subjecting the raw material powder described above to the following three stages of treatment in order according to the description of Patent No. 5499738 (paragraph [0153]). In the first stage, the treatment and preheating steps described above were applied. In the second step, the powder after the first step, the metal alkoxy oligomer (X-40-9246 manufactured by Shin-Etsu Chemical Co., Ltd.) and the above-described orthophosphoric acid solution are stirred and mixed (in the air, 80 ° C. x 1 hour). and further dried by heating (120° C.×2.5 hours in air). In the third step, the powder after the second step and a solution of a coupling agent (KBE-903 manufactured by Shin-Etsu Chemical Co., Ltd.) are stirred and mixed (in the atmosphere, 80 ° C. x 1 hour), and further dried by heating (inert 120° C.×2.5 hours in atmosphere). The thus-obtained magnet particles of Sample C are coated with three layers: a phosphate compound layer (first layer) and a composite coating containing a silicon compound and a phosphate compound (second and third layers). .

《Corrosion resistance》
(1) Corrosion Resistance Test Powders of each sample (fired powder and raw material powder) were exposed to the following corrosive environment. 10 g of each powder and 50 mL of test solution were placed in a pressure test container and left in an oven at 150° C. for 100 hours. As the test solution, an aqueous solution obtained by diluting SLLC (genuine Toyota Super Long Life Coolant manufactured by Toyota Corporation) with deionized water twice (volume ratio) (water: SLLC = 1:1) was used.

(2) Measurement For each sample before the corrosion resistance test, from the BH curve obtained using a pulse BH tracer (manufactured by OP Electronics Industry Co., Ltd.), the maximum energy product: (BH) max, residual magnetic flux density: Br and Coercive force: iHc was obtained. Also, the coercive force of each sample after the corrosion resistance test was measured in the same manner. All measurements were performed at room temperature. Table 1 summarizes the magnetic properties of each sample thus obtained. The iHc before the corrosion resistance test is denoted as iHc ₀ , and the iHc after the corrosion resistance test is denoted as iHc ₁ . Note that the iHc ₀ of the raw material powder before the corrosion resistance test (Sample S, later described Sample T), which is not surface-treated and serves as a reference, was taken as iHc _S.

Further, for each sample, demagnetization rate (decrease rate of coercive force) before and after coating treatment: ΔiHc _S0 = (iHc ₀ - iHc _S )/iHc _S , demagnetization rate before and after corrosion resistance test: ΔiHc ₀₁ = (iHc ₁ -iHc ₀ )/iHc ₀ and total demagnetization rate: ΔiHc _S1 =(iHc ₁ -iHc _S )/iHc _S were obtained. Their percentages are also shown in Table 1. Also, the relationship between each demagnetization rate (%) and the maximum heating temperature (referred to as "firing temperature" regardless of the presence or absence of the firing process) is shown in FIGS. shown respectively.

Table 1 also shows the magnetic properties and demagnetization ratios of Samples 1 and 2, which were not subjected to the firing process, and Sample S, which was not subjected to coating treatment (as raw material powder). The demagnetization ratios of samples S, 1 and 2 are plotted in order from the firing temperatures of 20° C., 80° C. and 120° C. shown in FIG.

Further, Table 1 also shows the magnetic properties and demagnetization rate of Sample C, which is coated with a different method. FIG. 2 shows a bar graph comparing the demagnetization rate: ΔiHc _S1 (%) of sample 6 and sample C. As shown in FIG.

"structure"
(1) Observation For particles randomly extracted from each powder of samples 2, 3, and 6 (before the corrosion resistance test), which have different maximum temperatures during coating treatment, a cross section near the surface is examined with a scanning transmission electron microscope (STEM). / HD-2700 manufactured by Hitachi High-Tech Co., Ltd.) and an energy dispersive X-ray analyzer (EDAX Octane T Ultra W manufactured by Ametech Co., Ltd.). An electron beam processing observation device (FIB-SEM/NB5000 manufactured by Hitachi High-Tech Co., Ltd.) was used for processing the cross-sectional sample. Its dark field image and secondary electron image are shown in FIG. 3A, and elemental mapping (P, O, Nd) based on energy dispersive X-ray spectroscopy (EDS) is shown in FIG. 3B. Note that the magnification of the observation image shown in FIGS. 3A and 3B (both figures are simply referred to as "FIG. 3") is 80,000 times.

(2) Measurement Sample Based on the observation image shown in FIG. According to the method, each size (maximum width) was measured for all voids (holes) observed in the film cross section. The results are also shown in FIG. All the voids in sample 6 were extremely minute and could not be observed at a low magnification of 8000 times. Therefore, based on an STEM image (80,000 times) magnified 10 times more than others, the measurement was performed while referring to the EDS image (see FIG. 3).

"analysis"
For each particle randomly extracted from each powder of samples 2, 3, 6, and S (before the corrosion resistance test), the cross section near the surface was analyzed with a scanning Auger electron spectrometer (AES/PHI700Xi manufactured by ULVAC-Phi, Inc.). bottom. Elements existing near the surface of each particle (for example, at a depth of about 400 nm) were quantitatively analyzed. The results are shown in FIG.

Based on the analysis results shown in FIG. 5, the atomic ratio (Nd/Fe) of Nd to Fe in the near-surface region (0 to 50 nm depth) was calculated and shown in FIG.

"evaluation"
(1) Corrosion resistance As is clear from Table 1 and Fig. 1 (especially Fig. 1C), magnet powder coated at a specific firing temperature has a remarkably small demagnetization rate even in a corrosive environment, and maintains high magnetic properties stably. It was found that

As is clear from FIG. 2, although the magnet powder according to the present invention is coated with only one phosphate film, the demagnetization rate is much higher than that of the conventional three-layer-coated magnet powder. was small.

(2) Composition As can be seen from FIG. 6, the content ratio (Nd/Fe) contained in the outermost surface region of the film was significantly different between Samples 2 and 3 and Sample 6. That is, samples 2 and 3, which had large demagnetization rates (ΔiHc ₀₁ and ΔiHc _S1 ) before and after the corrosion resistance test, contained a large amount of Nd in the outermost surface region of the film. On the other hand, in sample 6, whose demagnetization rate was remarkably small, the amount of Nd contained in the outermost surface region of the film was considerably small. Therefore, it was found that the composition distribution of the phosphate coating in the outermost surface region greatly affects the corrosion resistance of the magnet particles.

(3) Structure As is clear from FIG. 3, Sample 6, which was fired at a temperature of 300° C., did not show conspicuous large voids or Nd-enriched portions in the film, unlike the other samples.

Specifically, only four very small voids were observed in the film of sample 6. Moreover, the voids had a maximum size of 20 nm and an average size of 13 nm or less, which is extremely small. Furthermore, it was found that the film of Sample 6 had a very dense structure with almost no voids.

On the other hand, many voids with a maximum size of more than 50 nm and even more than 100 nm were observed in the films of samples 2 and 3, which had poor corrosion resistance. Although the detailed mechanism is not clear, it is conceivable that there is a correlation between the corrosion resistance of the coated particles and the film structure.

《Consideration》
Although the mechanism by which the rare-earth magnet powder of the present invention exhibits high corrosion resistance is not clear, one example of the mechanism inferred from the above results is schematically shown in FIG. First, it is thought that the surface of the untreated raw material particles (Sample S) is dotted with rich Nd or its oxide (referred to as "Nd oxide"), which was contained in a large amount relative to the theoretical composition. be done. Such Nd and Nd oxides are considered to be a major factor in degrading (that is, corroding) the magnetic properties of magnet particles (Sample S).

Corrosion resistance is improved by forming a film (referred to as a "phosphate film") on the surface of such raw material particles by phosphating. However, when the firing temperature is low, a large amount of Nd oxide remains near the outermost surface, and the improvement in corrosion resistance is insufficient (Samples 2 and 3).

However, when fired at a predetermined temperature or higher, the Nd oxide migrates to the lower layer side of the film for unknown reasons, and the Nd oxide remaining near the outermost surface is significantly reduced, greatly improving corrosion resistance. (Sample 6). It can be seen from the Nd concentration and O concentration shown in FIG. 5 that the Nd that has migrated to the lower layer side can be stably retained as Nd oxide at a deep position. It should be noted that the deterioration of the magnetic properties due to heating is slight because the magnetic properties are sintered at a predetermined temperature or lower. It is believed that, in this way, a highly corrosion-resistant magnet powder capable of remarkably suppressing deterioration of magnetic properties was obtained.

[Second embodiment]
(1) Fabrication and Measurement of Samples Samples were fabricated by variously changing the heating conditions after the treatment process shown in the first embodiment. The raw material powder used was a commercially available NdFeB-based anisotropic magnet powder (manufactured by Aichi Steel Co., Ltd.: MF-P15 (Br: 1.3 T, iHc: 1093 kA/m, (BH) max: 297 kJ/m ³ , average particle size (VMD): 119 μm).

The magnetic properties of these samples were measured before and after the corrosion resistance test, and the demagnetization factors (ΔiHc ₀₁ and ΔiHc _S1 ) were calculated based on the measurement results. Table 2 summarizes the heating conditions, magnetic properties and demagnetization rate of each sample. FIG. 8 shows the demagnetization rate (ΔiHc _S1 ) of each sample for comparison. Unless otherwise specified, samples were manufactured and measured in the same manner as in the first example (sample 6).

The details of each sample shown in Table 2 are as follows. Sample 20 was not preheated. Sample 21 is identical to Sample 6. For Samples 22 to 24, the heating time in the firing process was changed. In sample 25, the atmosphere in the firing process was an anti-oxidation atmosphere (Ar gas atmosphere).

(2) Evaluation As is clear from Table 2 and Fig. 8, if the firing temperature is within a specific range, the presence or absence of a preheating process, the time and atmosphere of the firing process, etc. have almost no effect on the corrosion resistance (demagnetization rate) of the magnet powder. I found out not to.

From the above, it was found that if the powder is heated in the temperature range specified in the present invention after the phosphoric acid treatment, it is possible to obtain a rare earth magnet powder that is excellent in corrosion resistance while suppressing deterioration in magnetic properties.

Claims (7)

a treatment step of contacting magnet particles containing Nd, Fe and B with a treatment liquid containing phosphate ions;
a firing step of heating the magnet particles after the treatment step at a firing temperature of 250 to 350 ° C.,
A method for producing rare earth magnet powder comprising magnet particles having a film containing P, O and Fe. The method for producing rare earth magnet powder according to claim 1, wherein the firing step is performed in an oxidizing atmosphere. The method for producing rare earth magnet powder according to claim 1 or 2, further comprising a preheating step of heating at a temperature lower than the firing temperature before the firing step. A rare earth magnet powder comprising magnet particles having a coating,
the magnet particles comprise Nd, Fe and B;
The coating contains P, O and Fe,
A rare earth magnet powder, wherein the content ratio (Nd/Fe), which is the atomic ratio of Nd to Fe contained in the coating, is 0.5 or less in the outermost region of the coating from the outermost surface to a depth of 10 nm. A rare earth magnet powder comprising magnet particles having a coating,
the magnet particles comprise Nd, Fe and B;
The coating contains P, O and Fe,
A rare earth magnet powder, wherein the maximum size of voids contained in the film is 50 nm or less. The rare earth magnet powder according to claim 4 or 5, wherein the magnet particles are anisotropic magnet particles obtained by hydrogen-treating a magnet alloy. The rare earth magnet powder according to any one of claims 4 to 6, which is used for bonded magnets exposed to a wet environment or an oil environment.

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