Dust core

Abstract

A dust core according to the present invention is characterized by having: a plurality of soft magnetic particles each having an insulating coating formed on the surface thereof; and an inter-grain layer that is located between the soft magnetic particles and is made of a low-melting-point glass having an Na content of 3.0 at% or less. The soft magnetic powder is preferably pure iron powder or an iron-based soft magnetic alloy powder. The insulating coating preferably comprises a silicone resin or SiO2 derived from a silicone resin.

Description

powder magnetic core

[0001] The present invention relates to a powder magnetic core that can achieve both high strength and low iron loss. This application claims priority based on Japanese Patent Application No. 2024-055352, filed on March 29, 2024, the contents of which are incorporated herein by reference.

[0002] Powder magnetic cores used in reactors and motors are required to have low iron loss, and alloy materials such as sendust are used as low-iron-loss powder magnetic core materials. These alloy materials are hard and difficult to plastically deform, so even if the alloy powder is compacted under pressure, sufficient strength cannot be obtained. To compensate for the lack of strength, a method has been proposed in which low-melting-point glass is added to the alloy powder and then compacted.

[0003] For example, Patent Document 1 below describes a technique for forming a first coating layer of aluminum nitride on the surface of soft magnetic particles and forming a second coating layer made of low-melting-point glass having a softening point lower than the annealing temperature of the soft magnetic particles. Patent Document 2 below also describes a technique for blending soft magnetic powder having an insulating coating and glass frit having a softening point 100°C or more lower than the magnetic annealing temperature together with a granulation binder of polyvinyl alcohol in a specific ratio.

[0004] Japanese Unexamined Patent Publication No. 2016-058732 (A) Japanese Unexamined Patent Application No. 2017-073447 (A)

[0005] Soft magnetic particles for powder magnetic cores generally require an insulating coating to suppress eddy current loss. One known method for forming an insulating coating is forming aluminum nitride on the surface of soft magnetic particles, as described in Patent Document 1. The aluminum nitride-coated insulating layer subsequently suppresses component diffusion between the low-melting-point glass and the soft magnetic particles, thereby exhibiting high resistivity and strength. However, forming the insulating layer requires a long heat treatment at high temperatures of 1000 to 1200°C for 500 hours. Another known insulating method involves coating soft magnetic particles with silicone resin. However, due to the low heat resistance of silicone resin-based insulating coatings, the annealing temperature must be set low, and the low-melting-point glass used must have a low softening point. However, when powder magnetic cores are formed using low-melting-point glass with a softening point below 600°C, the resulting powder magnetic cores experience increased (worsened) eddy current loss in high-frequency ranges, such as 10 kHz to 100 kHz.

[0006] The present invention was made in consideration of the above circumstances, and aims to provide a powder magnetic core that can achieve both high strength and low iron loss. The inventors conducted various studies on the cause of increased eddy current loss in the high frequency range when a powder magnetic core is formed using low-melting-point glass, and found that Na contained in the low-melting-point glass is related to eddy current loss in the high frequency range. Based on this finding, the present invention was completed.

[0007] (1) The powder magnetic core of the present invention is characterized in that it has a plurality of soft magnetic particles with an insulating coating formed on the surface thereof, and an intergranular layer of low-melting point glass having a Na content of 3.0 at% or less between the soft magnetic particles. (2) In the powder magnetic core of the present invention described in (1), it is preferable that the soft magnetic particles are made of pure iron powder or iron-based soft magnetic powder. (3) In the powder magnetic core of the present invention described in (1) or (2), it is preferable that the insulating coating is made of silicone resin or SiO derived from silicone resin. ２ (4) In the powder magnetic core according to (1) or (2) of the present invention, the low-melting glass preferably accounts for 1.0% by mass or more and 6.0% by mass or less of the amount of the soft magnetic particles.

[0008] According to the powder magnetic core of the present invention, it is possible to obtain a powder magnetic core that has high strength and low eddy current loss in the high frequency range.

[0009] 1 is a schematic cross-sectional view showing the configuration of a powder magnetic core according to the present invention.

[0010] One embodiment of the present invention will now be described. The dust core according to the present invention is obtained by mixing a small amount of low-melting glass with an insulating-coated iron-based soft magnetic powder to prepare a mixed powder, molding this mixed powder into a desired shape, and annealing it at 600 to 700°C in an inert gas atmosphere. Low-melting glass refers to glass that softens, deforms, and flows at temperatures of 600°C or less.

[0011] The soft magnetic powder can be a mixture of one or more of pure iron powder, FeSi alloy powder, FeNi alloy powder, FeSiAl alloy powder (sendust powder), and iron-based amorphous metal powder. Most soft magnetic powders, except for pure iron powder, are hard and difficult to deform. Therefore, it is difficult to obtain a compact with the required strength by compacting the soft magnetic powder as is or by compacting the insulating powder covered with an insulating film. For example, the sendust powder can be sendust powder having a composition of Fe-9.5%Si-5.5%Al by mass. The iron-based amorphous metal powder can be a powder of an Fe-Cr-Si-B-C amorphous alloy. The Fe-Si-Al alloy powder may contain, for example, 7.0 to 11.0 mass% Si and 3.0 to 11.0 mass% Al, and the Fe-Si alloy powder may contain, for example, 4.5 mass% to 7.0 mass% Si. In addition, various compositions of FeSi alloy powder, FeNi alloy powder, FeSiAl alloy powder (Sendust powder), and iron-based amorphous metal powder are known, and therefore, the compositions are not limited to those described above, and any known compositions may be used.

[0012] Silicone resin can be used for the insulating coating. Silicone resin is a resin with a siloxane bond (Si-O-Si) as its main skeleton. Silicone resins that can be used include methyl-based, methylphenyl-based, propylphenyl-based, epoxy resin-modified, alkyd resin-modified, polyester resin-modified, and rubber-based. Among these, silicone resins composed of methyl and phenyl groups are preferable. These silicone resins can be mixed with a solvent. The amount of silicone resin added can be 3.0% by mass or less, for example, about 2.0% by mass, relative to the magnetic alloy powder.

[0013] The powder core is made by adding 1.0 mass % to 6.0 mass % of low-melting glass with a Na content limited to 3.0 at % or less to soft magnetic powder and uniformly mixing them to form a mixed powder. The mixed powder is then placed in a mold and compacted under a molding pressure of 10 to 14 t / cm. ２ The powder magnetic core is obtained by molding the powder magnetic material into a shape close to the desired shape under a pressure of about 100° C., and then annealing it for about 30 minutes at 600 to 700° C. The resulting powder magnetic core has a plurality of soft magnetic particles formed by sintering soft magnetic powder having an insulating coating, and an intergranular layer formed by sintering low-melting point glass and filling the voids between the soft magnetic particles.

[0014] As the low-melting glass, glass that can be softened and deformed at a low temperature range of about 500 to 610°C can be used. ２ O ３ , Al ２ O ３ , SiO ２ Glasses containing the following as main components can be used. ２ O ３ and SiO ２ Glass mainly composed of B ２ O ３ Glass mainly composed of SiO ２ , Al ２ O ３ , B ２ O ３ Glass mainly composed of SiO ２ , B ２ O ３ Glass containing the above as a main component can be used.

[0015] If the low-melting-point glass used in this embodiment contains Na, the Na content must be 3.0 at% or less. If the amount of Na contained in the low-melting-point glass exceeds 3.0 at%, the eddy current loss of the powder magnetic core increases. The Na content is preferably 3.0 at% or less. Although not particularly limited, the lower limit of the Na content may be 0.0 at%. The amount of low-melting-point glass blended is preferably 1.0 mass% or more and 6.0 mass% or less with respect to the soft magnetic powder. If the amount of low-melting-point glass blended exceeds 6.0 mass%, the proportion of soft magnetic particles in the powder magnetic core decreases, and the powder magnetic core cannot achieve the desired magnetic flux density.

[0016] A predetermined amount of low-melting-point glass is added to the insulating coating powder, and then the powder is inserted into a mold or the like, and a molding pressure of 10 to 14 t / cm is applied. ２ The molded product is then pressed to a shape close to the desired shape using a pressure of about 1000 kJ / cm. ２ By annealing the powder magnetic core at 600 to 700°C for several tens of minutes to an hour (for example, about 30 minutes) in an inert gas atmosphere such as a gas atmosphere, it is possible to obtain a powder magnetic core with the desired strength and low eddy current loss in the high frequency range. When soft magnetic powder is produced by a rapid cooling method from a melt, such as atomization, the soft magnetic powder generally becomes a spherical powder. In terms of the ease of inserting the soft magnetic powder into a mold when producing a powder compact, and compactibility, it is desirable to regulate the average particle size of the soft magnetic powder to a range of about 10 μm to 100 μm.

[0017] According to the manufacturing method described above, heating to the annealing temperature allows the low-melting-point glass to sufficiently wet and spread between the soft magnetic particles, improving the strength of the final powder magnetic core. Furthermore, the low-melting-point glass used in this embodiment contains 3.0 at% or less of Na, which can suppress the increase in eddy current loss due to the influence of Na in the powder magnetic core obtained after annealing. Therefore, the eddy current loss in the high-frequency range can be reduced in the resulting powder magnetic core.

[0018] FIG. 1 is an enlarged view of a main portion of a powder magnetic core 1 obtained by the manufacturing method described in this embodiment. An insulating coating 3 is formed around the outer periphery of soft magnetic particles 2 made of soft magnetic powder, forming insulating-coated soft magnetic particles 4. A plurality of insulating-coated soft magnetic particles 4 are aggregated and consolidated, forming intergranular layers 5 made of a sintered product of low-melting-point glass that fill the spaces between adjacent insulating-coated soft magnetic particles 4. The intergranular layers 5 are made of a sintered product of low-melting-point glass with a Na content of 3.0 at% or less, and, together with the presence of the insulating coating 3, provide excellent insulation for the soft magnetic particles 2. As a result, the resulting powder magnetic core 1 has high strength and low eddy current loss in the high-frequency range. This allows for a high saturation magnetic flux density, contributing to the miniaturization of magnetic devices. Furthermore, the powder magnetic core 1 of this embodiment allows for a powder magnetic core with low eddy current loss in the high-frequency range.

[0019] (Manufacturing Method) The manufacturing method of the powder magnetic core 1 includes (1) a resin coating step for forming an insulating coating, (2) a step of adding low-melting-point glass, (3) a press molding step, and (4) an annealing step. In the resin coating step, silicone resin is dissolved in an organic solvent, and this organic solvent is sprayed onto the magnetic alloy powder to apply the required thickness to the surface of the magnetic alloy powder. The organic solvent is then volatilized by a drying process such as vacuum drying to form an insulating coating. By vacuum drying, an insulating-coated soft magnetic powder can be obtained whose surface is covered with an insulating coating layer (resin coating layer) of the required thickness.

[0020] In the low-melting-point glass addition step, about 1.0 to 6.0 mass % of low-melting-point glass is mixed with the insulating coated soft magnetic powder, and a lubricant is further added and mixed as necessary, and the mixed powder is dried as necessary. The dried powder is charged into the cavity of a mold and applied at a rate of 10 to 14 ton / cm. ２ The powder is then pressed into the desired shape at a pressure of about 1000 to 1500°C (press molding process), and then subjected to an annealing process in which the powder is heated to a high temperature of about 600 to 700°C for about 30 minutes in an inert gas atmosphere such as a nitrogen gas atmosphere. Through the above manufacturing process, a powder core can be obtained in which the insulating-coated soft magnetic powder is bound and compacted with low-melting-point glass.

[0021] This dust core has a structure in which there are intergranular layers 5 made of a sintered product of low-melting-point glass between a plurality of insulating-coated soft magnetic particles 4 obtained by pressurizing and sintering a plurality of insulating-coated soft magnetic powders. The insulating coating 3 formed around the insulating-coated soft magnetic particles 4 is made of silicone resin or SiO 2 derived from silicone resin. ２ The intergranular layer 5 is an insulating coating made of the above. The intergranular layer 5 is obtained by firing a low-melting glass in which the Na content is suppressed to 3.0 at % or less.

[0022] In the powder magnetic core 1 having the structure shown in FIG. 1 , the outer periphery of the soft magnetic particles 2 made of soft magnetic powder is covered with an insulating coating 3, and the insulating-coated soft magnetic particles 4 having the insulating coating 3 are consolidated via an intergranular layer 5. This makes it possible to provide a powder magnetic core that is high in strength, has little eddy current loss in the high frequency range, and has low iron loss.

[0023] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. A mixed powder obtained by mixing pure iron powder (average particle size D50 = 50 μm) and sendust powder (average particle size D50 = 50 μm) in a 1:1 ratio was used as the soft magnetic powder. Xylene was mixed with a silicone resin solvent and sprayed onto the mixed powder to form an insulating coating with a loading of 2.0 mass %, yielding an insulating-coated soft magnetic powder. For comparison, a mixed powder without an insulating coating was also prepared.

[0024] Low-melting point glass was added to the insulating coated soft magnetic powder in the amount shown in Table 1 below, and the mixture was uniformly mixed to obtain a mixed powder, which was then used to produce a dust core as described below. Low-melting point glass was also added to the mixed powder not forming an insulating coating as shown in Table 1, and the mixture was uniformly mixed to produce a dust core as described below. The above mixed powder was used to produce a dust core at 14 t / cm using a mold. ２ The ring-shaped compact was heated to 640°C in an inert gas atmosphere for 0.5 hours, and then slowly cooled to obtain a powder magnetic core.

[0025] The obtained ring-shaped powder magnetic core was subjected to the measurement of radial crushing strength and magnetic flux density by the following method. Iron loss (0.1 T, 50 kHz) was separated into hysteresis loss (0.1 T, 50 kHz) and eddy current loss (0.1 T, 50 kHz) and determined using the following formulas: Pc = Ph + Pe Pc = af + bf ２ Pc / f = a + bf, where Pc is iron loss, Ph is hysteresis loss, Pe is eddy current loss, f is frequency, and a and b are proportionality constants. The eddy current loss (0.1 T, 50 kHz) of each sample is shown relative to the eddy current loss of Comparative Sample No. 1 shown in Table 1, which is set to 1.00. Regarding eddy current loss, a sample is judged as passing (G) if it is up to 1.4 times that of Comparative Sample No. 1, and is judged as failing (NG) if it exceeds this limit. Regarding specific magnetic flux density, the magnetic flux density of Comparative Sample No. 1 shown in Table 1 is set to 1.00, and is shown relative to that. Regarding magnetic flux density, a sample is judged as passing (G) if it is up to 50% lower than that of Comparative Sample No. 1, and is judged as failing (NG) if the magnetic flux density reduction exceeds 50%.

[0026] "Radial crushing strength" The radial crushing strength of the powder magnetic core was measured by measuring the breaking load with a universal testing machine using a ring-shaped test piece with dimensions of φ35 (outer diameter) × φ25 (inner diameter) × 5H (height). The radial crushing strength was calculated from the dimensions of the compact (ring dimensions) according to the following relational expression: K = (F × (D - e)) / (L × e ２ ) K = radial crushing strength (MPa), F = maximum load at break (N), D = outer diameter of sample (mm), e = wall thickness of sample (mm), L = length and thickness of sample (mm) The above results are summarized in Table 1 below. The specific radial crushing strength is shown as a relative value to the radial crushing strength shown by Comparative Example No. 1 in Table 1. If the radial crushing strength is increased compared to Comparative Example No. 1, it is judged as pass (G), and if it is decreased, it is judged as fail (NG).

[0027]

[0028] As shown in Table 1, Example Samples No. 3 to No. 9 (dust cores) are dust cores obtained by mixing low-melting-point glass with a softening point of 515 to 603°C with insulating-coated soft magnetic powder and then compacting the mixture. Comparative Sample No. 1 is a dust core made using only insulating-coated soft magnetic powder without using low-melting-point glass, while Comparative Sample No. 12 is a dust core made by directly compacting soft magnetic powder without using low-melting-point glass. Compared to Comparative Sample No. 12, Comparative Sample No. 1 has significantly improved eddy current loss and superior radial crushing strength. For this reason, Table 1 shows the eddy current loss and radial crushing strength of each sample relative to Comparative Sample No. 1.

[0029] The example samples Nos. 3 to 9 exhibited eddy current loss values ​​that varied slightly but were generally similar to those of the comparative example sample No. 1. The example samples Nos. 3 to 9 exhibited radial crushing strengths (0.95 to 1.30) that were equivalent to or superior to that of the comparative example sample No. 1. The example samples Nos. 3 to 9 exhibited magnetic flux densities (0.56 to 0.98) that were equivalent to that of the comparative example sample No. 1 or were usable as magnetic cores.

[0030] Comparative Example Sample No. 2 is a sample to which 0.5 mass % of low-melting point glass was added, but the radial crushing strength was lower than that of Comparative Example Sample No. 1. Example Sample No. 3, to which 1.0 mass % of low-melting point glass was added, showed improved radial crushing strength, and it is believed that the amount of low-melting point glass added should be 1.0 mass % or more.

[0031] The comparative sample No. 7 contained 7.0 mass% low-melting-point glass, but its magnetic flux density was more than 50% lower than that of the comparative sample No. 1. From the perspective of magnetic flux density, it can be determined that the upper limit of the amount of low-melting-point glass to be added is 6.0 mass%. The comparative sample No. 10 contained 4.9 at% sodium in the low-melting-point glass. This sample used soft magnetic powder with an insulating coating, but exhibited increased eddy current loss and a decrease in radial crushing strength. The example sample No. 9 contained 3.0 at% sodium in the low-melting-point glass, but exhibited low eddy current loss. Therefore, it was found necessary to limit the amount of sodium contained in the low-melting-point glass to 3.0 at% or less.

[0032] Comparative Example Sample No. 11 is a sample in which the amount of Na contained in the low-melting-point glass was 4.9 at%, and this sample uses soft magnetic powder that is not formed with an insulating coating, but it has significantly increased eddy current loss and a large decrease in radial crushing strength. From the above comparison, it was found that in order to suppress eddy current loss, have excellent radial crushing strength, and obtain the magnetic flux density required for a magnetic core, it is desirable to produce a powder core using insulating-coated soft magnetic powder and adding 1.0 to 6.0 mass% of low-melting-point glass with an Na content of 3.0 at% or less.

[0033] REFERENCE SIGNS LIST 1 Powder magnetic core 2 Soft magnetic particles 3 Insulating coating 4 Insulating coated soft magnetic particles 5 Intergranular layer

Claims

1. A powder magnetic core characterized by having a plurality of soft magnetic particles with an insulating coating formed on the surface thereof, and an intergranular layer of low-melting glass having a Na content of 3.0 at % or less between the soft magnetic particles.

2. The dust core according to claim 1, wherein the soft magnetic particles are made of pure iron powder or iron-based soft magnetic powder.

3. The insulating coating is made of silicone resin or SiO derived from silicone resin. ２ 3. The powder magnetic core according to claim 1, comprising:

4. A powder magnetic core according to claim 1 or 2, characterized in that the amount of the low-melting glass is 1.0 mass % or more and 6.0 mass % or less relative to the amount of the soft magnetic particles.

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