JP7417038B2 - Manufacturing method of rare earth magnetic powder - Google Patents

Description

The present invention relates to a method for producing rare earth magnetic powder.

Since rare earth magnets used in rare earth bonded magnets are easily oxidized, a method is known in which a thin silica film is formed on the surface of a rare earth magnet to improve its oxidation resistance. For example, Patent Document 1 proposes a method of hydrolyzing and condensing a mixture of rare earth magnetic powder and alkyl silicate under basic conditions. Patent Document 1 shows that when hydrolyzed and condensed under acidic conditions, residual magnetization and coercive force were insufficient.

Incidentally, there is a demand for rare earth bonded magnets with excellent oxidation resistance so that they can withstand use under severe operating conditions. One possible way to improve oxidation resistance is to improve the density of the silica coating, but it has not been easy to improve the density when hydrolyzed under basic or acidic conditions.

Japanese Patent Application Publication No. 2000-309802

An object of the present invention is to provide a method for producing rare earth magnetic powder that can maintain residual magnetization and coercive force by increasing the density of a silica coating.

In order to increase the density of the silica thin film in SmFeN magnetic powder, the present inventor investigated various hydrolytic condensation conditions and found that alkyl silicate is hydrolyzed under acidic conditions and dehydrated and condensed under basic conditions. discovered that a highly dense silica thin film could be formed on the surface of rare earth magnetic powder without impairing its magnetic properties, and completed the present invention.

That is, the present invention
(1) A step of mixing an alkyl silicate and an acidic solution,
(2) a step of mixing the obtained alkyl silicate mixture and rare earth magnetic powder, and
(3) A method for producing rare earth magnetic powder, including a step of mixing the obtained mixture of rare earth magnetic powder and an alkaline solution.

In the method for producing rare earth magnetic powder of the present invention, alkyl silicate is hydrolyzed under acidic conditions, and then dehydration condensation is performed under basic conditions in the presence of rare earth magnetic powder, so that a highly dense silica thin film is formed on the surface of the rare earth magnetic powder. It is possible to form a rare earth magnetic powder with excellent magnetic properties.

Embodiments of the present invention will be described in detail below. However, the embodiment shown below is an example for embodying the technical idea of the present invention, and the present invention is not limited to the following. Note that in this specification, the term "process" is used not only to refer to an independent process, but also to include a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. included.

The method for producing rare earth magnetic powder of this embodiment is as follows:
(1) A step of mixing an alkyl silicate and an acidic solution,
(2) a step of mixing the obtained alkyl silicate mixture and rare earth magnetic powder, and
(3) It is characterized by including a step of mixing the obtained mixture of rare earth magnetic powder and an alkaline solution.

[Step (1)]
Step (1) is a step of mixing an alkyl silicate and an acidic solution. Since the alkyl silicate is hydrolyzed under acidic conditions, the hydrolysis of the alkyl silicate progresses sufficiently. Here, the acidic condition may be any acidity, but the pH is preferably 2 or more and 6 or less, more preferably 2.5 or more and 5.0 or less. If the pH is less than 2, the magnetic powder will dissolve, resulting in a decrease in magnetic properties and oxidation resistance, and if the pH is more than 6, the hydrolysis of the alkyl silicate will tend not to proceed sufficiently.

Alkyl silicates have the general formula:
Si _n O _(n-1) (OR) _(2n+2)
(R is an alkyl group, n is an integer from 1 to 10.)
It is a silicate ester shown by Here, examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, and the like. As a specific alkyl silicate, ethyl silicate is preferred because it is inexpensive, non-toxic, and easy to handle. Further, the value of n is related to the molecular weight of the alkyl silicate, and preferably n=1 or more and 10 or less. When n is larger than 10, it becomes difficult to obtain a dense silica thin film.

The acidic solution is a solution having a function as an acid catalyst that promotes hydrolysis of alkyl silicate, and is a solution in which an acid is dissolved in a solvent. Examples of the acid include acetic acid, hydrochloric acid, phosphoric acid, nitric acid, and sulfuric acid. Among these, acetic acid, hydrochloric acid, and phosphoric acid are preferred, and acetic acid is particularly preferred because it can be easily removed during drying. Examples of the solvent include water, ethanol, etc. Among these, water is preferred. The pH of the acidic solution may be as long as it is acidic, but preferably 3 or more and 4 or less. If the pH is less than 3, the magnetic properties of the rare earth magnetic powder will tend to deteriorate when mixed with the rare earth magnetic powder in step (2), and if it exceeds 4, the hydrolysis of the alkyl silicate will tend not to proceed sufficiently. The amount of the acidic solution may be 5 parts by weight or more and 100 parts by weight or less, and preferably 10 parts by weight or more and 80 parts by weight or less, based on 100 parts by weight of the alkyl silicate. If it is less than 5 parts by weight, hydrolysis will be insufficient, and if it exceeds 100 parts by weight, the mixability with magnetic powder tends to be poor.

Along with the acidic solution, mix the alcohol. By mixing alcohol, hydrolysis of the alkyl silicate and acidic solution proceeds. Moreover, the compatibility with the rare earth magnetic powder used in step (2) can be improved. Examples of the alcohol include ethanol, methanol, and the like. The amount of alcohol added may be in the range of 30 parts by weight or more and 200 parts by weight or less, preferably 40 parts by weight or more and 80 parts by weight or less, and 50 parts by weight or more and 60 parts by weight or less, based on 100 parts by weight of the alkyl silicate. More preferred. If it is less than 30 parts by weight, hydrolysis will be insufficient, and if it exceeds 200 parts by weight, the mixability with magnetic powder tends to be poor.
An indication that hydrolysis is almost complete is that when the alkyl silicate, acidic solution, and alcohol are mixed, the mixture changes from cloudy to transparent.

The alcohol added for hydrolysis is preferably 0.1 to 3 times the theoretical amount required for hydrolysis of alkyl silicate, more preferably 0.5 to 2 times the theoretical amount, and more preferably 0.5 to 2 times the theoretical amount. Most preferred is the addition of an amount of If it is less than 0.1 times, a silica thin film with dense hydrolysis cannot be obtained, and if it exceeds 3 times, the rare earth magnetic powder tends to be oxidized.

[Step (2)]
Step (2) is a step of mixing the alkyl silicate mixture obtained in step (1) and rare earth magnetic powder.

It is preferable that the alkyl silicate be coated on the surface of the magnetic powder in a dry manner in a high-speed shear mixer. This coating does not depend solely on the wettability of the alkyl silicate, but also utilizes the shearing force of the mixer to disperse the magnetic powder with strong agitation, while uniformly coating the surface of the magnetic powder particles with silica sol. Dispersing the silica sol as uniformly and uniformly as possible at this stage greatly influences the oxidation resistance of the silica film later.

Examples of the rare earth magnetic powder include Sm-Fe-N-based, Nd-Fe-B-based, and Sm-Co-based rare earth magnetic powder. Among these, Sm--Fe--N is preferred because it has excellent heat resistance and does not contain rare metals. The Sm-Fe-N magnetic powder has a Th ₂ Zn ₁₇ type crystal structure and is composed of the rare earth metal Sm, iron Fe, and nitrogen N whose general formula is Sm _x Fe _100-xy N _y . Nitride is preferred. Here, it is preferable that x is 8.1 atomic % or more and 10 atomic % or less, y is 13.5 atomic % or more and 13.9 atomic % or less, and the balance is mainly Fe.

The Sm--Fe--N magnetic powder can be produced by the method disclosed in JP-A-11-189811. The Nd-Fe-B magnetic powder can be manufactured by the HDDR method disclosed in International Publication No. 2003/85147. The Sm--Co magnetic powder can be produced by the method disclosed in JP-A-08-260083.

The mixing amount of the alkyl silicate is preferably 1 part by weight or more and 4 parts by weight or less, more preferably 1.5 parts by weight or more and 2.5 parts by weight or less, based on 100 parts by weight of the rare earth magnetic powder. If the amount of alkyl silicate mixed is less than 1 part by weight per 100 parts by weight of rare earth magnetic powder, the alkyl silicate will be insufficient and the magnetic powder will not be sufficiently coated. Furthermore, if the amount of alkyl silicate mixed exceeds 4 parts by weight per 100 parts by weight of rare earth magnetic powder, silica tends to aggregate during dehydration condensation and the magnetic properties tend to deteriorate.

[Step (3)]
Step (3) is a step of mixing the rare earth magnetic powder mixture obtained in step (2) with an alkaline solution. In step (3), the alkyl silicate hydrolyzate is dehydrated and condensed under basic conditions, so that the dehydration condensation reaction sufficiently proceeds. At the end of step (3), rare earth magnetic powder with a thin silica film formed on the surface is obtained. Here, the basic conditions may be as long as they are basic, but the pH is preferably 9 or more and 13 or less, more preferably 10 or more and 13 or less. If the pH is less than 9, dehydration condensation tends not to proceed sufficiently, and if it exceeds 13, the magnetic properties of the rare earth magnetic powder tend to deteriorate.

The alkaline solution is a solution having a function as a basic catalyst that promotes dehydration condensation of a hydrolyzate of an alkyl silicate, and is a solution in which an alkaline component is dissolved in a solvent. Examples of the alkali component include ammonia, hydroxides of alkali metals or alkaline earth metals, and metal hydroxides other than these hydroxides. Among these, ammonia is particularly preferred since it easily evaporates upon heating. Examples of the solvent include water, ethanol, etc., of which water is preferred. The pH of the alkaline solution may be basic as long as it is basic, but preferably 9 or higher. If the pH is less than 9, hydrolysis tends to be insufficient.

In step (3), a silica thin film having a three-dimensional network structure is formed on the surface of the rare earth magnetic powder particles. After step (3), heating may be performed in order to cause the remaining SiOH to undergo a polycondensation reaction to stabilize it and form a stronger silica thin film. The heating temperature is not particularly limited, but is preferably 60°C or higher and 250°C or lower, and preferably 100°C or higher and 250°C or lower.

When the silica thin film thus obtained is coated with a thickness in the range of 0.001 μm or more and 0.5 μm or less, oxidation resistance can be improved without impairing magnetic performance. The thickness of the silica thin film is more preferably 0.001 μm or more and 0.2 μm or less. Here, the thickness of the silica thin film can be measured using a TEM photograph of a particle cross section.

Furthermore, in the rare earth magnetic powder obtained by the production method of the present invention, the silica content is preferably 0.1% by weight or more and 0.5% by weight or less, and preferably 0.2% by weight or more and 0.35% by weight or less. . If it is less than 0.1% by weight, the rare earth magnetic powder may not be sufficiently coated with silica, while if it exceeds 0.5% by weight, the silica tends to aggregate and the magnetic properties tend to deteriorate. Here, the Si content can be measured by ICP-AES method.

Further, in the rare earth magnetic powder obtained by the production method of the present invention, the total carbon content (TC) is preferably 1500 ppm or less, and preferably 1000 ppm or less. If it exceeds 1500 ppm, unreacted alkyl silicate remains and aggregates, which tends to deteriorate magnetic properties. Here, the total carbon amount can be measured by the TOC method.

The rare earth magnetic powder obtained by the production method of the present invention maintains magnetic properties, especially high residual magnetization and coercivity, and has excellent oxidation resistance compared to powder obtained by conventional methods. Become something.

[Phosphoric acid treatment process]
The present invention may include a step of treating the rare earth magnetic powder with phosphoric acid before step (2). By treating the rare earth magnetic powder with phosphoric acid, a passive film having PO bonds is formed on the surface of the rare earth magnetic powder.

In the phosphoric acid treatment step, a phosphating agent and rare earth magnetic powder are reacted. Examples of phosphating agents include phosphates such as orthophosphoric acid, sodium dihydrogen phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, zinc phosphate, calcium phosphate, and hypochlorite. Examples include inorganic phosphoric acids and organic phosphoric acids such as phosphoric acid, hypophosphite, pyrophosphoric acid, and polyphosphoric acid. Basically, these phosphoric acid sources are dissolved in water or an organic solvent such as IPN, and if necessary, a reaction accelerator such as nitrate ion, a crystal refiner such as V ion, Cr ion, Mo ion, etc. are added. Magnetic powder is put into the added phosphoric acid bath to form a passive film having PO bonds on the surface of the rare earth magnetic powder.

[Addition process of tungstate or vanadate]
The present invention may include a step of adding tungstate or vanadate together with the alkaline solution in step (3) or after addition of the alkaline solution in step (3). By adding tungstate or vanadate, agglomeration of silica, which is a hydrolysis condensate of alkyl silicate, can be prevented and residual magnetization can be improved. If added before step (3) of adding an alkaline solution, silica tends to aggregate.

The cation of tungstate or vanadate is not particularly limited, and examples thereof include ammonium, sodium, potassium, and the like. Among these, ammonium is preferred because it evaporates during the process and does not remain in the material.

The amount of tungstate or vanadate to be added is preferably 0.01 part by weight or more and 0.5 part by weight or less as tungsten or vanadine per 100 parts by weight of rare earth magnetic powder, and 0.05 part by weight. The amount added is more preferably 0.3 parts by weight or less. If it is less than 0.01 part by weight, the effect of preventing powder agglomeration due to the formation of a complex is small because the amount added is small, and if it exceeds 0.5 part by weight, the magnetic properties tend to deteriorate.

Although tungstate or vanadate may be added in a solid state, it is preferably added in an aqueous solution state to ensure uniform mixing.

[Silane coupling treatment process]
The present invention may include a step of treating with a silane coupling agent after step (3). By subjecting the rare earth magnetic powder on which a silica thin film has been formed to a silane coupling treatment, a coupling agent film is formed on the silica thin film, improving the magnetic properties of the rare earth magnetic powder, improving its wettability with resin, and the strength of the magnet. can be improved. The silane coupling agent is not particularly limited as long as it can be selected according to the type of resin, but examples include γ-(2-aminoethyl)aminopropyltrimethoxysilane, γ-(2-aminoethyl)aminopropylmethyldimethoxysilane , γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropylmethyldimethoxysilane, N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltrimethoxysilane hydrochloride, γ-glycidoxypropyltri Methoxysilane, γ-mercaptopropyltrimethoxysilane, methyltrimethoxysilane, methyltriethoxysilane, vinyltriacetoxysilane, γ-chloropropyltrimethoxysilane, hexamethylenedisilazane, γ-anilinopropyltrimethoxysilane, vinyl trimethoxysilane Methoxysilane, octadecyl[3-(trimethoxysilyl)propyl]ammonium chloride, γ-chloropropylmethyldimethoxysilane, γ-mercaptopropylmethyldimethoxysilane, methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, vinyltrichlorosilane, vinyltris( β-methoxyethoxy)silane, vinyltriethoxysilane, β-(3,4epoxycyclohexyl)ethyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, N-β(aminoethyl)γ-aminopropyltrimethoxysilane , N-β(aminoethyl)γ-aminopropylmethyldimethoxysilane, γ-aminopropyltriethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, oleidopropyltriethoxysilane, γ-isocyanatepropyltriethoxysilane , polyethoxydimethylsiloxane, polyethoxymethylsiloxane, bis(trimethoxysilylpropyl)amine, bis(3-triethoxysilylpropyl)tetrasulfane, γ-isocyanatepropyltrimethoxysilane, vinylmethyldimethoxysilane, 1,3, 5-N-tris(3-trimethoxysilylpropyl)isocyanurate, t-butylcarbamatetrialkoxysilane, N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine, etc. Examples include silane coupling agents. These silane coupling agents may be used alone or in combination of two or more. The amount of the silane coupling agent added is preferably 0.2 parts by weight or more and 0.4 parts by weight or less, more preferably 0.25 parts by weight or more and 0.35 parts by weight or less, per 100 parts by weight of the rare earth magnetic powder. If it is less than 0.2 parts by weight, the effect of the silane coupling agent is small, and if it exceeds 0.4 parts by weight, the magnetic properties of the rare earth magnetic powders and rare earth magnets tend to deteriorate due to agglomeration of the rare earth magnetic powders.

By mixing the rare earth magnetic powder obtained by the method of the present invention with a resin, a composition for a bonded magnet can be obtained.

The resin used for the bonded magnet is not particularly limited, and examples thereof include polypropylene, polyethylene, polyvinyl chloride, polyester, polyamide, polycarbonate, polyphenylene sulfide, and acrylic resin. Among these, polyamide is preferred, and nylon 12 is preferred because it has a relatively low melting point, low water absorption, and good moldability because it is a crystalline resin. It is also possible to use a mixture of these as appropriate.

Examples will be described below. Note that unless otherwise specified, "%" is based on mass.

Production example 1 (phosphorylation treatment of rare earth magnetic powder)
In a treatment tank, 100 parts by weight of Sm ₂ Fe ₁₇ N ₃ magnetic powder with an average particle size of 3 μm was stirred in 1% by weight diluted hydrochloric acid for 1 minute, and then the supernatant liquid was drained and an aqueous solvent at room temperature was poured. . Next, 5 parts by weight of a phosphoric acid treatment solution whose concentration was adjusted to a weight ratio of orthophosphoric acid: sodium dihydrogen phosphate = 1:35 was added to 100 parts by weight of Sm ₂ Fe ₁₇ N ₃ based magnetic powder into a treatment tank. Added. The concentration of the phosphoric acid treatment solution was pH 3.5 and 20% by weight as phosphate ions. Next, after stirring for 10 minutes, the mixture was suction-filtered and vacuum-dried to obtain a magnetic powder with a fine phosphate film of about 10 nm formed on the surface.

Examples 1 to 24
To a mixer, 2.8 g of ethyl silicate (Si ₅ O ₄ (OR) ₁₂ ), an acidic solution, and ethanol were added with the type, pH, and amount of acid solution shown in Table 1, and mixed for 1 minute under a nitrogen atmosphere. . 150 g of the magnetic powder obtained in Production Example 1 was added to the obtained ethyl silicate mixture and further mixed for 1 minute. Aqueous ammonia (concentration 10%) with a pH of 12 in the amount shown in Table 1 was added to the resulting mixture of magnetic powders and mixed for 1 minute. The mixture was taken out from the mixer and heat treated at 180° C. for 30 minutes under reduced pressure to obtain Sm ₂ Fe ₁₇ N ₃ based magnetic powder with a silica thin film formed on the surface.

To 300 g of the obtained Sm ₂ Fe ₁₇ N ₃ magnetic powder, 1.2 g of a silane coupling agent (γ-aminopropyltriethoxysilane), 0.6 g of ammonia water (ammonia content 10% by weight) with a pH of 11.7, and ethanol were added. 3.6 g of the mixed solution was added and mixed for 1 minute under a nitrogen atmosphere. The mixture was taken out and heat treated at 90° C. for 30 minutes under reduced pressure to obtain magnetic powder in which a coupling agent film was formed on a silica film.

Comparative example 1
1590 g of the magnetic powder obtained in Production Example 1 and 2.8 g of ethyl silicate (Si ₅ O ₄ (OR) ₁₂ ) were added to a mixer and mixed for 1 minute under a nitrogen atmosphere. Aqueous ammonia with a pH of 12 in the amount shown in Table 1 was added to the resulting mixture of magnetic powders and mixed for 1 minute. Only the magnetic powder was taken out from the mixer and heat-treated at 180° C. for 30 minutes under reduced pressure to obtain Sm ₂ Fe ₁₇ N ₃ -based magnetic powder with a silica thin film formed on the surface, followed by the same treatment as in Example 1. A magnetic powder in which a coupling agent film was formed on a silica film was obtained.

The Si content (Si) and total carbon content (TC) of the Sm ₂ Fe ₁₇ N ₃ based magnetic powder with a silica thin film formed on the surface were measured by the following method, and it was determined that a coupling agent film was formed. Regarding the magnetic powder, residual magnetization (σr), coercive force (iHc), and squareness ratio (Hk) were measured. The measurement results are shown in Table 1.

<Si content>
After surface melting of the Sm ₂ Fe ₁₇ N ₃ magnetic powder with a silica thin film formed on its surface, the Si content was measured by ICP-AES method.

<Total carbon amount (TC)>
The total carbon content (TC) of the Sm ₂ Fe ₁₇ N ₃ magnetic powder having a silica thin film formed on its surface was measured by the TOC method.

<Residual magnetization, coercive force, squareness ratio>
The magnetic powder was packed into a sample container together with paraffin wax, and after melting the paraffin wax with a dryer, the axis of easy magnetization was aligned with an orienting magnetic field of 16 kA/m. This magnetically oriented sample was pulse magnetized with a magnetizing magnetic field of 32 kA/m, and using a VSM (vibrating sample magnetometer) with a maximum magnetic field of 16 kA/m, residual magnetization (σr), coercive force (iHc), square shape, etc. The ratio (Hk) was measured.

In Comparative Example 1, the hydrolysis and dehydration condensation of the alkyl silicate were performed under basic conditions, so the Si content remained at 0.10% and the residual magnetization (σr) was also low. On the other hand, in Examples 1 to 24, the alkyl silicate was hydrolyzed under acidic conditions and the dehydration condensation was performed under basic conditions, so the Si content increased significantly and no deterioration in magnetic properties was observed.

Example 25
To a mixer, 2.8 g of ethyl silicate (Si ₅ O ₄ (OR) ₁₂ ), an acidic solution, and ethanol were added with the type, pH, and amount of acid solution shown in Table 2, and mixed for 1 minute under a nitrogen atmosphere. . 150 g of the magnetic powder obtained in Production Example 1 was added to the obtained ethyl silicate mixture and further mixed for 1 minute. To the resulting mixture of magnetic powders, ammonium tungstate aqueous solution (tungsten content 16%) was added together with aqueous ammonia at pH 12 (ammonia content 10%) in the amounts shown in Table 2, and mixed for 1 minute. The mixture was taken out from the mixer and heat treated at 180° C. for 30 minutes under reduced pressure to obtain Sm ₂ Fe ₁₇ N ₃ based magnetic powder with a silica thin film formed on the surface.

To 300 g of the obtained Sm ₂ Fe ₁₇ N ₃ magnetic powder, 1.2 g of a silane coupling agent (γ-aminopropyltriethoxysilane), 0.6 g of ammonia water (ammonia content 10% by weight) with a pH of 11.7, and ethanol were added. 3.6 g of the mixed solution was added and mixed for 1 minute under a nitrogen atmosphere. The mixture was taken out and heat treated at 90° C. for 30 minutes under reduced pressure to obtain magnetic powder in which a coupling agent film was formed on a silica film.

The Si content (Si) and total carbon content (TC) of the Sm ₂ Fe ₁₇ N ₃ based magnetic powder with a silica thin film formed on its surface by the method described above were measured, and it was determined that a coupling agent film had been formed. Regarding the magnetic powder, residual magnetization (σr), coercive force (iHc), and squareness ratio (Hk) were measured. The measurement results are shown in Table 2.

In Example 25, residual magnetization was increased by adding ammonium tungstate together with an alkaline solution.

Since the rare earth magnetic powder obtained by the method of the present invention has superior magnetic properties compared to conventional ones, it can be suitably applied to applications such as bonded magnets.

Claims (7)

(1) A step of mixing an alkyl silicate and an acidic solution,
(2) a step of mixing the obtained alkyl silicate mixture and rare earth magnetic powder, and
(3) A method for producing rare earth magnetic powder, which includes a step of mixing the obtained mixture of rare earth magnetic powder and an alkaline solution. The method for producing rare earth magnetic powder according to claim 1, wherein the acidic solution has a pH of 3 to 4. The method for producing rare earth magnetic powder according to claim 1, wherein alcohol is mixed with the acidic solution in step (1) . The method for producing rare earth magnetic powder according to any one of claims 1 to 3, further comprising a step of treating the rare earth magnetic powder with phosphoric acid before step (2). The method for producing rare earth magnetic powder according to any one of claims 1 to 4, further comprising a step of treating with a silane coupling agent after step (3). 6. The method according to claim 1, further comprising a step of adding tungstate or vanadate together with the alkaline solution in step (3) or after addition of the alkaline solution in step (3). A method for producing rare earth magnetic powder. The method for producing rare earth magnetic powder according to any one of claims 1 to 6, wherein the rare earth magnetic powder is an Sm-Fe-N magnetic powder.