Method for producing insulating magnetic powder

Abstract

To provide a method for efficiently producing insulating coated magnetic powder having an insulating coating with high coverage and thinness. [Solution] A method for producing insulating coated magnetic powder, comprising: a raw material preparation step of preparing a raw material solution containing magnetic powder, a nonionic dispersant, water, and alcohol; an ammonia addition step of adding ammonia to the raw material solution; and a film formation step of adding silicon alkoxide to the ammonia-containing raw material solution and then performing a stirring treatment to form an insulating film on the surface of the magnetic powder particles, wherein the nonionic dispersant is an ether compound having an ether bond and a number average molecular weight of 250,000 to 3,000,000, an ester compound having an ester bond and a number average molecular weight of 1,000 to 150,000, or a cyclic amide compound having a cyclic amide structure and a number average molecular weight of 8,000 to 1,000,000.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing insulated magnetic powder.

Background Art

[0002] In magnetic powder used for inductors and the like, it is necessary to perform an insulation treatment on the particle surface to suppress eddy currents flowing between particles or to insulate between terminals. For this reason, methods of forming an insulating film on the particle surface of magnetic powder using various film-forming methods have been studied.

[0003] For example, Patent Document 1 discloses a method for forming a metal oxide film on the surface of substrate particles by hydrolyzing a metal alkoxide in an organic solvent in which the substrate particles are dispersed, wherein a polymer compound having a hydroxy group is present in the organic solvent.

[0004] Patent Document 1 discloses that, by the above-described forming method, even when forming a thick metal oxide film in the production of powder having a metal oxide film, aggregation of the powders does not occur, the dispersibility is good, and a uniform film can be obtained.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In the forming method described in Patent Document 1, while a thick film can be formed on the surface of the substrate particles, it is difficult to form a thin and uniform-thickness film. There is also room for study on increasing the coating rate of the film. Therefore, there is a need to realize an efficient method for producing insulating coated magnetic powder having an insulating coating with high coverage and thin film thickness. [Means for solving the problem]

[0007] The method for producing insulating coated magnetic powder according to an application example of the present invention is as follows: A raw material preparation step involves preparing a raw material solution containing magnetic powder, a nonionic dispersant, water, and alcohol. An ammonia addition step is performed by adding ammonia to the raw material liquid, A film formation step is performed in which silicon alkoxide is added to the raw material liquid to which ammonia has been added, and then a stirring treatment is carried out to form an insulating film on the surface of the magnetic powder particles. It has, The aforementioned nonionic dispersant is Ether compounds having ether bonds and having a number-average molecular weight of 250,000 or more and 3,000,000 or less. An ester compound having an ester bond and a number-average molecular weight of 1,000 or more and 150,000 or less, A cyclic amide compound having a cyclic amide structure and a number-average molecular weight of 8,000 to 1,000,000. That is the case. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic cross-sectional view showing one particle of the insulating coated magnetic powder according to the embodiment. [Figure 2] This is a process diagram showing the configuration of a method for manufacturing insulating coated magnetic powder according to an embodiment. [Figure 3] This is a cross-sectional view showing how the raw material liquid contained in the stirring tank is stirred by the stirring blades. [Figure 4] Table 1 shows the configuration and evaluation results of the methods for producing insulating coated magnetic powder in Examples 1 to 8. [Figure 5] Table 2 shows the configuration and evaluation results of the methods for producing insulating coated magnetic powder in Examples 9 to 15. [Figure 6]Table 3 shows the configuration and evaluation results of the methods for producing insulating coated magnetic powders in Comparative Examples 1 to 7. [Modes for carrying out the invention]

[0009] The method for producing the insulating coated magnetic powder of the present invention will be described in detail below based on preferred embodiments shown in the accompanying drawings.

[0010] 1. Insulating magnetic powder First, the insulating coated magnetic powder produced by the method for producing insulating coated magnetic powder according to the embodiment will be described. Figure 1 is a schematic cross-sectional view showing one particle of insulating coated magnetic powder 1 according to the embodiment. In the following description, one particle of insulating coated magnetic powder 1 will also be referred to as "insulating coated magnetic particle 4".

[0011] The insulating coated magnetic particles 4 shown in Figure 1 comprise magnetic particles 2 and an insulating coating 3 provided on the surface of the magnetic particles 2. Of these, the magnetic particles 2 contain a magnetic material, which will be described later. The insulating coating 3 is provided to cover the surface of the magnetic particles 2 and has insulating properties. In this specification, "coating" is a concept that includes not only covering the entire surface of the magnetic particles 2 but also covering a part of the surface. Furthermore, in the following description, the aggregate of magnetic particles 2 is also referred to as "magnetic powder".

[0012] The insulating coated magnetic particles 4 have an insulating coating 3, thus providing insulation from each other. As a result, in a compact formed by compacting the insulating coated magnetic powder 1, it is possible to suppress eddy currents flowing between particles and to provide insulation between terminals provided on the compact.

[0013] 1.1.Magnetic particles The magnetic particles 2 are composed of a soft magnetic material. Examples of the soft magnetic material include materials mainly composed of at least one of Fe, Ni, and Co, that is, materials containing these elements at 50% or more in terms of atomic ratio. Further, the soft magnetic material may contain at least one selected from the group consisting of Cr, Nb, Cu, Al, Mn, Mo, Si, Sn, B, C, P, Ti, and Zr, in addition to the elements serving as the main components, according to the target characteristics. Also, the soft magnetic material may contain inevitable impurities as long as the effects of the present embodiment are not impaired. Inevitable impurities are impurities that are unintentionally mixed during raw material or manufacturing. Inevitable impurities include all elements other than the above-mentioned elements, and examples thereof include O, N, S, Na, Mg, K, etc.

[0014] Specific examples of the soft magnetic material include Fe-Si-based alloys such as silicon steel, Fe-Si-Al-based alloys such as Sendust, and various alloys such as Fe-Ni-based, Fe-Co-based, Fe-Ni-Co-based, Fe-Si-B-based, Fe-Si-B-C-based, Fe-Si-B-Cr-C-based, Fe-Si-Cr-based, Fe-B-based, Fe-P-C-based, Fe-Co-Si-B-based, Fe-Si-B-Nb-based, Fe-Si-B-Nb-Cu-based, Fe-Zr-B-based, Fe-Cr-based, Fe-Cr-Al-based Fe-based alloys, Ni-Si-B-based, Ni-P-B-based Ni-based alloys, Co-Si-B-based Co-based alloys, etc.

[0015] By using a soft magnetic material having such a composition, an insulating-coated magnetic particle 4 having high magnetic properties such as permeability and magnetic flux density and low coercive force, that is, excellent soft magnetic properties, can be obtained.

[0016] In the soft magnetic material, the content rate of the main component described above is preferably 50% or more, more preferably 70% or more in terms of atomic ratio. Thereby, the magnetic properties such as the permeability and magnetic flux density of the insulating-coated magnetic particle 4 can be particularly enhanced.

[0017] The structure constituting the soft magnetic material is not particularly limited and may be any of a crystalline structure, an amorphous structure, or a microcrystalline (nanocrystalline) structure. Among these, the soft magnetic material preferably contains an amorphous or microcrystalline material. By including these, the coercive force is reduced, contributing to the reduction of the hysteresis loss of the magnetic element. Note that in the soft magnetic material, structures with different crystallinities may coexist.

[0018] Examples of the amorphous material and the microcrystalline material include Fe-based alloys such as Fe-Si-B-based, Fe-Si-B-C-based, Fe-Si-B-Cr-C-based, Fe-Si-Cr-based, Fe-B-based, Fe-P-C-based, Fe-Co-Si-B-based, Fe-Si-B-Nb-based, Fe-Si-B-Nb-Cu-based, Fe-Zr-B-based; Ni-based alloys such as Ni-Si-B-based, Ni-P-B-based; Co-based alloys such as Co-Si-B-based, etc.

[0019] The average particle diameter of the magnetic particles 2 is preferably 1 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less, and even more preferably 3 μm or more and 9 μm or less.

[0020] When the average particle diameter of the magnetic particles 2 is within the above range, the eddy current loss of the magnetic element in the high-frequency range can be sufficiently reduced. Also, when the average particle diameter of the magnetic particles 2 is within the above range, the filling property during powder pressing becomes high, so that magnetic properties such as the magnetic permeability and saturation magnetic flux density of the compacted powder can be enhanced.

[0021] Note that when the average particle diameter of the magnetic particles 2 is below the lower limit value, aggregation is likely to occur, making it difficult to form the insulating film 3 and possibly reducing the filling property during powder pressing. On the other hand, when the average particle diameter of the magnetic particles 2 exceeds the upper limit value, the path of the eddy current within the particles becomes long, so there is a possibility that the eddy current loss increases. The average particle diameter of the magnetic particles 2 refers to the particle diameter D50 at which the cumulative frequency is 50% from the smaller diameter side in the volume-based cumulative particle size distribution of the magnetic powder obtained using a laser diffraction type particle size distribution measuring device.

[0022] 1.2. Insulating Film The insulating film 3 covers the surface of the magnetic particles 2. The insulating film 3 is composed of a compound derived from silicon alkoxide. Specifically, as described later, by using silicon alkoxide as a raw material (precursor), hydrolyzing the hydrolyzable groups contained in the silicon alkoxide, and further performing dehydration condensation, silicon oxide can be generated. Silicon oxide is represented by SiOx (1 < x ≦ 2). And an insulating film 3 mainly composed of silicon oxide is obtained.

[0023] The insulating film 3 may contain components other than silicon oxide. Examples of components other than silicon oxide include organic components derived from hydrocarbon groups such as alkyl groups contained in silicon alkoxide.

[0024] The content rate of silicon oxide in the insulating film 3 is preferably 50% by mass or more, more preferably 70% by mass or more, and even more preferably 90% by mass or more. Thereby, the insulating film 3 has good insulation even at high temperatures.

[0025] The average thickness of the insulating film 3 is preferably 5 nm or more, more preferably 7 nm or more and 100 nm or less, and even more preferably 10 nm or more and 70 nm or less. If the average thickness of the insulating film 3 is within the above range, while sufficiently ensuring the insulation of the insulating film 3, the occupancy rate of the insulating film 3 can be reduced and the occupancy rate of the magnetic particles 2 can be increased. Also, if the average thickness of the insulating film 3 is within the above range, even when there are irregularities on the surface of the magnetic particles 2, the insulating film 3 also contributes to flattening the irregularities. Thereby, the amount of the binder used to bind the insulating-coated magnetic particles 4 to each other can be suppressed.

[0026] The average thickness of the insulating film 3 is measured, for example, by magnifying and observing the cross-section of the insulating magnetic particle 4. Specifically, the insulating magnetic particle 4 is cut with a focused ion beam to prepare a cross-sectional thin section sample. Next, the obtained cross-sectional thin section sample is observed with a scanning transmission electron microscope, and the thickness of the insulating film 3 is measured at five or more locations for each insulating magnetic particle 4. The measured values ​​are then averaged, and the calculated result is taken as the average thickness of the insulating film 3. The distribution range of the insulating film 3 in the observed image can be more clearly confirmed by using, for example, EDX analysis (energy-dispersive X-ray analysis) or Auger electron spectroscopy.

[0027] 2. Method for producing insulating magnetic powder Next, a method for producing insulating coated magnetic powder according to an embodiment will be described. Figure 2 is a process diagram showing the configuration of a method for manufacturing insulating coated magnetic powder according to an embodiment.

[0028] The method for producing the insulating coated magnetic powder shown in Figure 2 comprises a raw material liquid preparation step S102, an ammonia addition step S104, a film formation step S106, a washing step S108, and a calcination step S110. In the film formation step S106, an insulating material containing silicon oxide is generated on the surface of the magnetic particles 2 by a sol-gel method using silicon alkoxide, which is the raw material for the insulating material, thereby forming an insulating coating 3.

[0029] 2.1. Raw material liquid preparation process In the raw material preparation step S102, a raw material solution containing magnetic powder, a nonionic dispersant, water, and alcohol is prepared. This raw material solution contains magnetic powder that will be used to form the insulating film 3 by the sol-gel method described later.

[0030] 2.1.1.Magnetic powder The magnetic powder may be produced by any method. Examples of production methods include various atomization methods such as water atomization, gas atomization, and rotary water flow atomization, as well as reduction, carbonylation, and pulverization methods. Of these, atomization is preferred. In other words, the magnetic powder is preferably atomized powder. Atomized powder is fine, highly spherical, and has high production efficiency. In particular, water atomized powder or rotary water flow atomized powder has a thin oxide film on its surface because it is produced by contact between molten metal and water. This oxide film can serve as a base for the insulating film 3. Therefore, the adhesion between the magnetic particles 2 and the insulating film 3 is excellent, and ultimately, insulating coated magnetic particles 4 with high inter-particle insulation are obtained. Furthermore, because the cooling rate is fast, it is also possible to produce magnetic powder containing amorphous or microcrystalline structures.

[0031] 2.1.2. Nonionic Dispersants Nonionic dispersants are ether compounds, ester compounds, or cyclic amide compounds that satisfy the molecular weights described later. These compounds adsorb onto the surface of magnetic powder particles, preventing them from coming together. Furthermore, nonionic dispersants do not contain metal elements or other contaminants that could cause contamination, unlike ionic dispersants. For this reason, nonionic dispersants are useful because they do not easily impair the insulating properties even when incorporated into the insulating coating 3.

[0032] Among these, the ether compound is a nonionic dispersant having an ether bond and a number-average molecular weight of 250,000 to 3,000,000. Furthermore, ester compounds are nonionic dispersants having ester bonds and a number-average molecular weight of 1,000 to 150,000. Furthermore, cyclic amide compounds are nonionic dispersants having a cyclic amide structure and a number-average molecular weight of 8,000 to 1,000,000.

[0033] By having the molecular weights of each compound within the above range, the repulsive stability of the magnetic particles 2 due to steric hindrance of the nonionic dispersant can be enhanced. This suppresses aggregation of the magnetic particles 2 during the formation of the insulating film 3, thereby increasing the coverage of the insulating film 3. Furthermore, the sol-gel method allows for the formation of an insulating film 3 with excellent adhesion through the reaction between the surface of the magnetic particles 2 and the silicon alkoxide, while also suppressing excessive film thickness. Therefore, according to this embodiment, an insulating coated magnetic powder 1 having an insulating film 3 with high coverage and thin film thickness can be efficiently produced. In addition, the highly covering insulating film 3 enhances the acid resistance (chemical resistance) of the insulating coated magnetic powder 1. Moreover, the deposition of the insulating film 3 by the sol-gel method offers superior mass production (scalability) compared to other deposition methods, such as mechanochemical methods represented by the mechanofusion method.

[0034] 2.1.2.1. Ether Compounds As mentioned above, ether compounds are nonionic dispersants that have ether bonds and a number-average molecular weight of 250,000 to 3,000,000. Specific examples of ether compounds include nonionic cellulose ethers. Examples of nonionic cellulose ethers include cellulose-based ether compounds such as hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose. Hydroxypropylcellulose is preferably used as the cellulose-based ether compound. Such cellulose-based ether compounds exhibit particular effectiveness in the dispersion of magnetic powders due to their unique molecular structure.

[0035] The number-average molecular weight of the ether compound is between 250,000 and 3,000,000, but preferably between 400,000 and 2,000,000. If the number-average molecular weight of the ether compound is within the above range, the steric hindrance of the ether compound can enhance the repulsive stability between the magnetic particles 2. This allows for an increase in the coverage of the insulating film 3.

[0036] Furthermore, if the number-average molecular weight of the ether compound falls below the lower limit, the repulsive stability between the magnetic particles 2 decreases, resulting in a decrease in the coverage of the insulating film 3. Additionally, the amount of dispersant required may increase. On the other hand, if the number-average molecular weight of the ether compound exceeds the upper limit, the viscosity of the raw material liquid increases, potentially requiring more time and energy for stirring and other processes described later.

[0037] Gel permeation chromatography (GPC) is used to measure the number-average molecular weight of ether compounds. Specifically, N-methylpyrrolidone is used as the solvent, polystyrene gel is used, and the molecular weight is determined using a converted molecular weight calibration curve that has been previously obtained from the constituent curve of standard monodisperse polystyrene. Examples of GPC instruments include the HLC-8220GPC manufactured by Tosoh Corporation.

[0038] 2.1.2.2. Ester Compounds As mentioned above, ester compounds are nonionic dispersants that have ester bonds and a number-average molecular weight of 1,000 to 150,000.

[0039] Specific examples of ester compounds include glycerol fatty acid esters, polyoxyethylene fatty acid esters, polyoxyethylene sorbitol fatty acids, polyoxyethylene hydrogenated castor oil, and alkyl alkanolamides.

[0040] The number-average molecular weight of the ester compound is 1,000 to 150,000, preferably 1,500 to 70,000, and more preferably 2,000 to 50,000. If the number-average molecular weight of the ester compound is within the above range, the steric hindrance of the ester compound can enhance the repulsive stability between the magnetic particles 2. This allows for an increase in the coverage of the insulating film 3.

[0041] Furthermore, if the number-average molecular weight of the ester compound falls below the lower limit, the repulsive stability between the magnetic particles 2 decreases, resulting in a decrease in the coverage of the insulating film 3. Additionally, the amount of dispersant required may increase. On the other hand, if the number-average molecular weight of the ester compound exceeds the upper limit, the viscosity of the raw material liquid increases, potentially requiring more time and energy for stirring and other processes described later. The number-average molecular weight of ester compounds is measured in the same way as the number-average molecular weight of ether compounds described above.

[0042] 2.1.2.3. Cyclic Amide Compounds As mentioned above, cyclic amide compounds are nonionic dispersants having a cyclic amide structure and a number-average molecular weight of 8,000 to 1,000,000.

[0043] Polylactams are a specific example of cyclic amide compounds. Examples of polylactams include polyvinylpyrrolidone and polyvinylcaprolactam. Of these, polyvinylpyrrolidone is preferably used as the cyclic amide compound. Due to its unique molecular structure, polyvinylpyrrolidone is particularly effective in the dispersion of magnetic powders.

[0044] The number-average molecular weight of the cyclic amide compound is 8,000 to 1,000,000, preferably 10,000 to 500,000, and more preferably 10,000 to 400,000. If the number-average molecular weight of the cyclic amide compound is within the above range, the steric hindrance of the cyclic amide compound can enhance the repulsive stability between the magnetic particles 2. This allows for an increase in the coverage of the insulating film 3.

[0045] Furthermore, if the number-average molecular weight of the cyclic amide compound falls below the lower limit, the repulsive stability between the magnetic particles 2 decreases, resulting in a decrease in the coverage of the insulating film 3. Additionally, the amount of dispersant required may increase. On the other hand, if the number-average molecular weight of the cyclic amide compound exceeds the upper limit, the viscosity of the raw material liquid increases, potentially requiring more time and energy for stirring and other processes described later. The number-average molecular weight of cyclic amide compounds is measured using the same method as described above for measuring the number-average molecular weight of ether compounds.

[0046] 2.1.3. Solvent The raw material liquid contains water and alcohol as solvents. Examples of water include pure water, distilled water, purified water, and ion-exchanged water. Examples of alcohols include lower alcohols with 1 to 3 carbon atoms. Specifically, these include methanol, ethanol, and isopropyl alcohol.

[0047] The mixing ratio of water to alcohol in the solvent is not particularly limited, but it is preferable that the amount of water is 2 parts by mass or more and 20 parts by mass or less per 100 parts by mass of alcohol, and more preferably 3 parts by mass or more and 10 parts by mass or less.

[0048] The solvent may include organic solvents other than water and alcohol, as needed. Examples of organic solvents include amide solvents, alkyl sulfoxides, polyhydric alcohols, amino alcohols, and cellosolves.

[0049] Examples of amide solvents include N-methyl-2-pyrrolidone, N,N-dimethylacetamide, and N,N-dimethylformamide. Examples of alkyl sulfoxides include dimethyl sulfoxide. Examples of polyhydric alcohols include glycerin, ethylene glycol, and diethylene glycol. Examples of amino alcohols include ethanolamine and 2-diethylaminoethanol. Examples of cellosolves include 2-methoxyethanol, 2-ethoxyethanol, and propylene glycol monomethyl ether.

[0050] 2.1.4. Stirring Method In the raw material liquid preparation step S102, the raw material liquid may be stirred. By stirring, the magnetic powder can be monodispersed. Examples of stirring methods include using a stirring blade, using a stirring rod, or shaking. Of these, the method using a stirring blade is preferred. The method using a stirring blade makes it easy to form a laminar flow in the tank. This laminar flow makes it easy to control the turbulence generated by the shape of the tank. As a result, the magnetic powder can be dispersed stably in a short time.

[0051] In the raw material liquid preparation step S102, the rotation speed of the stirring blade is preferably 500 rpm to 3000 rpm, and more preferably 1000 rpm to 2000 rpm. This allows for the formation of laminar and turbulent flows of appropriate speeds in the raw material liquid. The stirring time in the raw material liquid preparation step S102 is not particularly limited, but is preferably 10 minutes or more and 120 minutes or less, and more preferably 20 minutes or more and 60 minutes or less.

[0052] 2.2. Ammonia addition process In the ammonia addition step S104, ammonia is added to the raw material liquid. The ammonia functions as a catalyst in the production of silicon dioxide by the sol-gel method.

[0053] The method of adding ammonia is not particularly limited, and a method of blowing in ammonia gas can be used, but a method of adding aqueous ammonia is preferred. The ammonia concentration in the raw material solution after ammonia addition is not particularly limited, but is preferably 0.03 mol / L or more and 0.50 mol / L or less, and more preferably 0.05 mol / L or more and 0.40 mol / L or less.

[0054] In the ammonia addition step S104, the raw material solution after ammonia addition may also be stirred. By stirring, the ammonia, which acts as a catalyst, can be uniformly dissolved. This allows for the uniform generation of silicon dioxide in the film formation step S106, which will be described later. The stirring method in the ammonia addition step S104 is the same as in the raw material liquid preparation step S102.

[0055] The rotation speed of the stirring blade in the ammonia addition step S104 is preferably 500 rpm to 3000 rpm, and more preferably 1000 rpm to 2000 rpm. This allows for more uniform dissolution of ammonia.

[0056] The stirring time in the ammonia addition step S104 is not particularly limited, but is preferably 10 minutes or more and 120 minutes or less, and more preferably 20 minutes or more and 60 minutes or less.

[0057] 2.3. Film formation process In the film formation process S106, silicon alkoxide is first added to the raw material solution to which ammonia has been added. The added silicon alkoxide is hydrolyzed using ammonia as a catalyst, and then undergoes dehydration condensation. This yields an inorganic polymer containing siloxane bonds. The liquid containing this inorganic polymer is also called a sol.

[0058] Examples of silicon alkoxides include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane. At least one of the alkoxy groups of the added silicon alkoxide may be hydrolyzed.

[0059] The amount of silicon alkoxide added to the entire raw material solution after addition is preferably 0.1 mol / L to 4.0 mol / L, and more preferably 0.3 mol / L to 2.0 mol / L. This optimizes the silicon alkoxide concentration, suppressing aggregation of the resulting inorganic polymer and significantly increasing the film formation rate of the final insulating film 3.

[0060] In the film formation step S106, the raw material solution after the addition of silicon alkoxide is stirred. By stirring, the magnetic powder, ammonia, and silicon alkoxide can be well dispersed. This causes the inorganic polymer to adhere to the surface of the magnetic particles 2, and a coating film is obtained. In other words, stirring evenly coats the surface of the magnetic particles 2 and results in a coating film that is sufficiently thin.

[0061] Figure 3 is a cross-sectional view showing the stirring of the raw material liquid 6 contained in the stirring tank 50 by the stirring blade 58. The stirring device 5 shown in Figure 3 comprises a stirring tank 50 containing the raw material liquid 6 and having a baffle plate 51, legs 52 supporting the stirring tank 50, and a stirring unit 54 having stirring blades 58 and a shaft 56.

[0062] The baffle plate 51 shown in Figure 3 extends vertically along the inner wall of the stirring tank 50. By providing such a baffle plate 51, turbulence can be created in the raw material liquid 6, which has a laminar flow. This allows the raw material liquid 6 to be sufficiently stirred in a short time. As a result, a state in which the magnetic particles 2 are well dispersed can be formed. Furthermore, even when the film deposition rate of the insulating coating 3 is fast, the monodisperse state of the magnetic particles 2 can be maintained, so an insulating coating 3 with high coverage and sufficient thinness can be obtained.

[0063] The rotational speed of the stirring blade 58 in the film formation process S106 is preferably 500 rpm to 3000 rpm, and more preferably 1000 rpm to 2000 rpm. This allows for the formation of laminar and turbulent flows of appropriate speeds in the raw material liquid 6.

[0064] The stirring blade 58 may be positioned in a location that overlaps with the extension of the baffle plate 51, but preferably it is positioned below the lower end of the baffle plate 51 as shown in Figure 3. This allows laminar and turbulent flow to work effectively, enabling particularly good dispersion of the magnetic powder and suppressing foaming and air entrapment. In other words, laminar flow can be formed below the baffle plate 51 while turbulent flow can be formed within the extension of the baffle plate 51, thereby effectively suppressing aggregation of the magnetic powder and preventing foaming. As a result, an insulating coating 3 with particularly high coverage and sufficient thinness can be formed.

[0065] The stirring time in the film formation step S106 is not particularly limited, but is preferably 10 minutes or more and 120 minutes or less, and more preferably 20 minutes or more and 60 minutes or less.

[0066] In the resulting coating, gelation progresses as it dries, generating silicon dioxide (an insulator). Ultimately, an insulating film 3 composed of silicon dioxide (an insulator) is obtained. If the gelation and desolvation of the insulating film 3 are insufficient, or if organic components need to be removed or densified, the firing process S110 described later may be performed.

[0067] Furthermore, applying ultrasound to the raw material liquid 6 may promote the dispersion of the magnetic powder. However, applying ultrasound may damage the coating due to the impact of the ultrasound. Also, since cavitation decreases with distance from the oscillation tip, it is difficult to achieve uniform dispersion and to accommodate mass production by expanding the stirring tank 50. For this reason, it is preferable not to apply ultrasound in this process.

[0068] The concentration of the nonionic dispersant in the raw material liquid 6 is preferably 0.01% by mass or more and 3.0% by mass or less, more preferably 0.1% by mass or more and 2.0% by mass or less, and even more preferably 0.3% by mass or more and 1.0% by mass or less. By setting the concentration of the nonionic dispersant within the above range, it is possible to sufficiently ensure the function of the nonionic dispersant to repel the magnetic particles 2 from each other while suppressing the inhibition of the formation of the insulating film 3 by the nonionic dispersant. As a result, an insulating coated magnetic powder 1 having an insulating film 3 with high coverage and thin film thickness can be produced.

[0069] Furthermore, if the concentration of the nonionic dispersant falls below the lower limit, there will be insufficient nonionic dispersant, which may prevent the magnetic particles 2 from repelling each other sufficiently, potentially reducing the coverage of the insulating film 3. On the other hand, if the concentration of the nonionic dispersant exceeds the upper limit, there will be an excess of nonionic dispersant, which may lead to an increase in viscosity. In this case, the formation of the insulating film 3 may be inhibited by the nonionic dispersant.

[0070] 2.4. Washing Process In the cleaning step S108, the magnetic powder on which a silicon oxide (insulator) film has been formed is cleaned. This removes alcohol, ammonia, and by-products from various reactions. A cleaning solution such as water or alcohol is used for cleaning. The cleaning method is not particularly limited as long as it is a process that brings the magnetic powder on which the silicon oxide film has been formed into contact with the cleaning solution, but examples include immersion treatment and spray treatment. The number of times such a cleaning treatment is performed may be once, but it is preferable to perform it multiple times. This enhances the cleaning effect.

[0071] 2.5. Firing Process In the firing process S110, the magnetic powder with a silicon oxide (insulator) film, which has undergone the cleaning process S108, is fired. This removes the solvent, cleaning solution, organic components, etc. contained in the generated insulator. Depending on the firing conditions, crystallization of silicon oxide may also proceed. This leads to densification and improved insulation of the insulating film 3.

[0072] In the firing process S110, a drying treatment is performed first. The drying treatment may be performed as needed and may be omitted. Examples of drying conditions include a heating temperature of 50°C to 150°C and a heating time of 10 minutes to 120 minutes.

[0073] In the firing process S110, the firing treatment is performed. Examples of firing conditions include a heating temperature of 100°C to 300°C, a heating time of 30 minutes to 240 minutes, and an oxygen-containing gas atmosphere. After firing in an oxygen-containing gas atmosphere, firing may be performed under reduced pressure if necessary.

[0074] The difference between the average particle size of the magnetic powder before the formation of the insulating film 3 and the average particle size of the magnetic powder (insulating coated magnetic powder 1) after the formation of the insulating film 3 (the difference in average particle size before and after film formation) is preferably 0.1 μm or less. If the difference in average particle size is within the above range, it is considered that aggregation is sufficiently suppressed in the magnetic powder before film formation and in the insulating coated magnetic powder 1. In other words, it is considered that monodispersion is sufficiently achieved. Therefore, if the difference in average particle size is within the above range, insulating coated magnetic powder 1 having an insulating film 3 with high coverage and thin film formation can be efficiently manufactured.

[0075] Furthermore, the magnetic powder after drying may contain organic components derived from the nonionic dispersant. These organic components are thought to contribute to suppressing the aggregation of the magnetic powder in the raw material liquid 6 and improving the insulating properties of the final insulating film 3.

[0076] The quantitative evaluation of organic components can be performed as follows. First, the dried magnetic powder is dispersed in pure water to prepare a suspension with a concentration of 50% by mass. Next, the resulting suspension is heated at 70°C for 3 hours. Then, the heated suspension is washed with ethanol, and the washed magnetic powder is dried. Finally, the dried magnetic powder is subjected to a calcination treatment at a temperature of 200°C to 350°C for 2 hours.

[0077] Next, 50 mg of the calcined powder is evaluated by thermal desorption gas analysis (TDS). The controlled temperature is raised from 80°C to 700°C at a rate of 1°C / second. The ionic strengths are then measured for mass-to-charge ratios (m / z) of 1, 2, 14, 15, 16, 17, 18, 19, 28, 32, 40, and 44.

[0078] Next, the sum of the ionic intensities of specific ions with mass-to-charge ratios (m / z) of 14, 15, 28, and 44 is calculated. These specific ions are considered to be organic components derived from the nonionic dispersant. Next, the ratio of the sum of the ionic intensities of the specific ions to the ionic intensity of the total desorbed gas (ionic intensity ratio) is calculated.

[0079] In the magnetic powder after drying, the ionic intensity ratio is preferably 0.51% or higher, more preferably 0.53% or higher, and even more preferably 0.55% or higher. This allows for the formation of an insulating film 3 with high coverage, sufficient thinness, and excellent insulating properties. The upper limit is not particularly limited, but considering that stable formation is possible, it is set to, for example, 2.0% or less.

[0080] It should be noted that the above ionic intensity ratio is due to the organic components and can be adjusted according to the molecular weight of the nonionic dispersant added to the raw material solution. For example, increasing the molecular weight of the nonionic dispersant tends to increase the ionic intensity ratio.

[0081] 3. Effects of the Embodiment As described above, the method for producing insulating coated magnetic powder according to the embodiment comprises a raw material liquid preparation step S102, an ammonia addition step S104, and a film formation step S106. In the raw material liquid preparation step S102, a raw material liquid containing magnetic powder, a nonionic dispersant, water, and alcohol is prepared. In the ammonia addition step S104, ammonia is added to the raw material liquid. In the film formation step S106, silicon alkoxide is added to the raw material liquid to which ammonia has been added, and then a stirring treatment is performed to form an insulating film on the surface of the magnetic powder particles. Furthermore, nonionic dispersants are ether compounds, ester compounds, or cyclic amide compounds. Ether compounds have ether bonds and a number-average molecular weight of 250,000 to 3,000,000. Ester compounds have ester bonds and a number-average molecular weight of 1,000 to 150,000. Cyclic amide compounds have a cyclic amide structure and a number-average molecular weight of 8,000 to 1,000,000.

[0082] With this configuration, insulating coated magnetic powder 1 having an insulating coating 3 with high coverage and thin film thickness can be efficiently manufactured.

[0083] In the method for producing insulating coated magnetic powder according to the embodiment, it is preferable that the difference between the average particle size of the magnetic powder before film formation and the average particle size of the magnetic powder after film formation is 0.1 μm or less.

[0084] With this configuration, aggregation is considered to be sufficiently suppressed in the magnetic powder before film formation and in the magnetic powder after film formation (insulating coated magnetic powder 1). In other words, monodispersion is considered to be sufficiently achieved. Therefore, if the difference in average particle size is within the aforementioned range, insulating coated magnetic powder 1 having an insulating coating 3 with high coverage and thin film formation can be efficiently manufactured.

[0085] The method for producing insulating coated magnetic powder according to the embodiment may include a washing step S108 and a firing step S110. In the washing step S108, the magnetic powder on which the insulating film has been formed is washed. In the firing step S110, the insulating material that has gone through the washing step S108 is fired.

[0086] With this configuration, alcohol, ammonia, and by-products from various reactions can be removed in the washing step S108. Furthermore, the washing solution and organic components can be removed in the calcination step S110. In addition, the crystallization of silicon dioxide can be promoted, resulting in densification and improved insulation of the insulating film 3.

[0087] In the method for producing insulating coated magnetic powder according to this embodiment, the stirring process may involve placing the raw material liquid 6 to which silicon alkoxide has been added into a stirring tank 50 (container) equipped with a baffle plate 51 and stirring it using a stirring blade 58.

[0088] With this configuration, turbulence can be created in the raw material liquid 6, where laminar flow has been formed by the stirring blade 58. This allows the raw material liquid 6 to be sufficiently stirred in a short time. As a result, a state in which the magnetic particles 2 are well dispersed can be formed. Furthermore, even when the deposition rate of the insulating film 3 is fast, the monodisperse state of the magnetic particles 2 can be maintained, so an insulating film 3 with high coverage and sufficient thinness can be obtained.

[0089] In the method for producing insulating coated magnetic powder according to the embodiment, the concentration of the nonionic dispersant in the raw material liquid 6 is preferably 0.01% by mass or more and 3.0% by mass or less.

[0090] With this configuration, it is possible to sufficiently ensure the function of repelling magnetic particles 2 from each other by the nonionic dispersant while suppressing the inhibition of the formation of the insulating film 3 by the nonionic dispersant. As a result, it is possible to manufacture insulating coated magnetic powder 1 having an insulating film 3 with high coverage and thinness.

[0091] In the method for producing insulating coated magnetic powder according to the embodiment, the ether compound is preferably hydroxypropyl cellulose.

[0092] With this configuration, the unique molecular structure of hydroxypropyl cellulose makes the dispersion of magnetic powder particularly pronounced.

[0093] In the method for producing insulating coated magnetic powder according to the embodiment, the cyclic amide compound is preferably polyvinylpyrrolidone.

[0094] With this configuration, the unique molecular structure of polyvinylpyrrolidone makes the dispersion of magnetic powder particularly pronounced.

[0095] The method for producing insulating coated magnetic powder of the present invention has been described above based on preferred embodiments, but the present invention is not limited thereto. For example, the present invention may be modified by adding any desired steps to the above embodiments. [Examples]

[0096] Next, specific embodiments of the present invention will be described. 4. Preparation of insulating coated magnetic powder Figure 4 is Table 1, showing the configuration and evaluation results of the methods for producing insulating coated magnetic powder in Examples 1 to 8. Figure 5 is Table 2, showing the configuration and evaluation results of the methods for producing insulating coated magnetic powder in Examples 9 to 15. Figure 6 is Table 3, showing the configuration and evaluation results of the methods for producing insulating coated magnetic powder in Comparative Examples 1 to 7.

[0097] 4.1. Example 1 First, as the magnetic powder, an Fe-Si-B-Cr-C amorphous alloy soft magnetic powder prepared by the water atomization method was prepared. The average particle size of the magnetic powder is shown in Table 1 (Figure 4). In addition, hydroxypropyl cellulose (HPC), an ether compound, was prepared as a nonionic dispersant. The molecular weight (number average molecular weight) of HPC and the concentration of the dispersant are shown in Table 1. Furthermore, ethanol was prepared as the alcohol. The magnetic powder, nonionic dispersant, water, and alcohol were then mixed to prepare the raw material solution. The amount of water added per 100 parts by mass of alcohol was 4 parts by mass.

[0098] Next, the raw material liquid was stirred. The stirring conditions were a rotor speed of 1500 rpm and a stirring time of 30 minutes.

[0099] Next, ammonia water was added to the raw material liquid. Then, the raw material liquid was stirred again. The stirring conditions were a rotor speed of 1500 rpm and a stirring time of 30 minutes.

[0100] Next, tetraethoxysilane (TEOS), a silicon alkoxide, was added to the raw material solution. The amount of silicon alkoxide added to the raw material solution was 0.61 mol / L. The raw material solution was then stirred again. The stirring conditions were a rotor speed of 1500 rpm and a stirring time of 90 minutes. This allowed for the formation of silicon oxide (insulator) by the sol-gel method, and a magnetic powder with an insulating film was obtained. Table 1 shows other stirring conditions, including the presence or absence of ultrasonic irradiation, the presence or absence of a baffle plate attached to the stirring tank, and the position of the stirring blade relative to the lower end of the baffle plate.

[0101] Next, the magnetic powder was removed from the raw material liquid and subjected to washing, drying, and calcination treatments. The washing treatment consisted of three alcohol washes. The drying treatment consisted of a heating temperature of 80°C for 60 minutes. In the calcination treatment, the heating temperature was gradually increased from 120°C to 200°C, with a total heating time of 120 minutes.

[0102] 4.2. Examples 2-15 and Comparative Examples 1-7 An insulating coated magnetic powder was manufactured in the same manner as in Example 1, except that the configuration of the method for manufacturing the insulating coated magnetic powder was changed as shown in Table 1 (Figure 4), Table 2 (Figure 5), and Table 3 (Figure 6).

[0103] The symbols shown in Tables 1, 2, and 3 correspond to the following compounds. SG: Sol-gel method MC: Mechanochemical method (Mechanofusion method) HPC: Hydroxypropylcellulose POE: Polyoxyethylene hydrogenated castor oil PVP: Polyvinylpyrrolidone

[0104] 5. Evaluation of the method for producing insulating magnetic powder 5.1. Average thickness of the insulating coating The cross-sections of the insulating coated magnetic powder particles produced in each example and comparative example were observed, and the average thickness of the insulating coating was calculated. The calculation results are shown in Tables 1, 2, and 3.

[0105] 5.2. Difference in average particle size before and after film formation For each example and comparative example, the difference in average particle size ΔD50 before and after film formation was calculated. The calculated ΔD50 was then evaluated against the following evaluation criteria to assess the difference in average particle size before and after film formation. The evaluation results are shown in Tables 1, 2, and 3.

[0106] A: ΔD50 is 0.1 μm or less. B: ΔD50 is greater than 0.1 μm and less than or equal to 0.3 μm. C:ΔD50 is greater than 0.3 μm and less than or equal to 1.0 μm. D:ΔD50 is greater than 1.0 μm

[0107] 5.3.Acid resistance First, 50 mg of the insulating magnetic powder produced in each example and comparative example was placed in a 1.5 mL tube, and then dispersed in 40 mg of pure water to prepare a dispersion.

[0108] Next, 160 μL of 5 mM hydrochloric acid was added to the tube, and the contents were mixed in a vortex mixer for 30 minutes. Next, standard solutions were prepared using the following method. First, 10 mL of 60 mM hydroxylamine solution was placed in a 110 mL screw-cap tube. Next, 10 mL of 1 M sodium acetate solution was added to the screw-cap tube. Then, 10 mL of 1,10-phenanthroline solution with a concentration of 0.25% by mass was added to the screw-cap tube. Finally, pure water was added to bring the total volume to 100 mL.

[0109] Next, 600 μL of a standard solution was added to the supernatant of the magnetic powder in the tube. Then, the tube was placed in a spectrophotometer, and the iron ion concentration was quantified from the absorbance at a wavelength of 510 nm. The acid resistance of the insulating coated magnetic powder was then evaluated against the following evaluation criteria based on the quantification results of the iron ion concentration. The evaluation results are shown in Tables 1, 2, and 3.

[0110] A: The iron ion concentration is 0.2 ppm or less. B: The iron ion concentration is greater than 0.2 ppm and less than or equal to 0.5 ppm. C: The iron ion concentration is greater than 0.5 ppm and less than or equal to 1.0 ppm. D: The iron ion concentration is greater than 1.0 ppm.

[0111] 5.4. Mass Production Capability (Scalability) In each example and comparative example of the method for producing insulating coated magnetic powder, the time required to produce 5 kg of insulating coated magnetic powder was calculated. The scalability was then evaluated based on the following evaluation criteria. The evaluation results are shown in Tables 1, 2, and 3.

[0112] A: The time required for manufacturing is 8 hours or less. B: The time required for manufacturing is more than 8 hours but within 24 hours. C: The time required for manufacturing is between 24 and 48 hours. D: The time required for manufacturing exceeds 48 hours.

[0113] 5.5. Determination of organic components by thermal desorption gas analysis (TDS) In the methods for producing insulating coated magnetic powder in each example and comparative example, the powder after drying was removed and dispersed in pure water to prepare a suspension with a concentration of 50% by mass. Next, the obtained suspension was heated at 70°C for 3 hours. Then, the heated suspension was washed with ethanol, and the washed powder was dried. The dried powder was then subjected to a calcination treatment at a temperature of 850°C for 2 hours.

[0114] Next, 50 mg of the powder subjected to the calcination process was evaluated by thermal desorption gas analysis (TDS). The controlled temperature was raised from 80°C to 700°C at a heating rate of 1°C / second. The ionic intensities for mass-to-charge ratios (m / z) of 1, 2, 14, 15, 16, 17, 18, 19, 28, 32, 40, and 44 were then measured. The TDS instrument used was an EMD-WA1000S / W manufactured by Denshi Kagaku Co., Ltd.

[0115] Next, the sum of the ionic intensities of specific ions with mass-to-charge ratios (m / z) of 14, 15, 28, and 44 was calculated. Then, the ratio of the sum of the ionic intensities of the specific ions to the ionic intensity of the total desorbed gas (ionic intensity ratio) was calculated. Finally, the calculated ionic intensity ratio was used to quantitatively evaluate the organic components contained in the insulating film according to the following evaluation criteria.

[0116] A: The above ionic intensity ratio is 0.55% or higher. B: The above ionic intensity ratio is 0.53% or more and less than 0.55%. C: The above ionic intensity ratio is 0.51% or more and less than 0.53%. D: The above ionic intensity ratio is less than 0.51%.

[0117] As shown in Tables 1, 2, and 3, it was found that the manufacturing methods in each example efficiently formed insulating coatings with a sufficiently thin average thickness and a sufficiently small difference in average particle size before and after film formation. The small difference in average particle size before and after film formation suggests that the insulating coating has a high coverage rate and that aggregation is suppressed. Therefore, these results confirm that the present invention can efficiently produce insulating coated magnetic powders having insulating coatings with high coverage rates and thin films.

[0118] Furthermore, quantitative evaluation of organic components using TDS revealed that optimizing the molecular weight of the nonionic dispersant allows for the inclusion of a sufficient amount of organic components in the insulating film. These organic components are thought to contribute to suppressing aggregation and improving insulation properties. [Explanation of Symbols]

[0119] 1...Insulating coated magnetic powder, 2...Magnetic particles, 3...Insulating coating, 4...Insulating coated magnetic particles, 5...Agitator, 6...Raw material liquid, 50...Agitation tank, 51...Baffle plate, 52...Legs, 54...Agitation unit, 56...Shaft, 58...Agitation blade, S102...Raw material liquid preparation process, S104...Ammonia addition process, S106...Film formation process, S108...Washing process, S110...Firing process

Claims

1. A raw material preparation step involves preparing a raw material solution containing magnetic powder, a nonionic dispersant, water, and alcohol. An ammonia addition step is performed by adding ammonia to the raw material liquid, A film formation step is performed in which silicon alkoxide is added to the raw material liquid to which ammonia has been added, and then a stirring treatment is carried out to form an insulating film on the surface of the magnetic powder particles. It has, The aforementioned nonionic dispersant is An ether compound having an ether bond and a number-average molecular weight of 250,000 or more and 3,000,000 or less. An ester compound having an ester bond and a number-average molecular weight of 1,000 or more and 150,000 or less, A cyclic amide compound having a cyclic amide structure and a number-average molecular weight of 8,000 to 1,000,000. A method for producing insulating coated magnetic powder, characterized by the following:

2. A method for producing an insulating coated magnetic powder according to claim 1, wherein the difference between the average particle size of the magnetic powder before film formation and the average particle size of the magnetic powder after film formation is 0.1 μm or less.

3. A cleaning step for cleaning the magnetic powder on which the insulating film has been formed, A firing process for firing the insulating material that has undergone the cleaning process, A method for producing an insulating coated magnetic powder according to claim 1 or 2.

4. The method for producing insulating coated magnetic powder according to claim 1 or 2, wherein the stirring process involves placing the raw material liquid to which the silicon alkoxide has been added into a container with a baffle plate and stirring it using a stirring blade.

5. The method for producing insulating coated magnetic powder according to claim 1 or 2, wherein the concentration of the nonionic dispersant in the raw material liquid is 0.01% by mass or more and 3.0% by mass or less.

6. The method for producing an insulating coated magnetic powder according to claim 1 or 2, wherein the ether compound is hydroxypropylcellulose.

7. The method for producing an insulating coated magnetic powder according to claim 1 or 2, wherein the cyclic amide compound is polyvinylpyrrolidone.

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