Dust core and method for producing same

Abstract

A dust core according to the present invention is characterized by comprising a granulated powder in which 2.0 to 4.0 mass% of a binder containing an acrylic resin as a main component is added to a magnetic alloy powder. The pH (hydrogen ion index) of the binder is preferably 6.0 to 8.0. The glass transition point of the binder is preferably from -5°C to lower than 25°C. The magnetic alloy powder is preferably any one of sendust powder, an amorphous alloy powder, or a nanocrystalline alloy powder.

Description

Powder magnetic core and manufacturing method thereof

[0001] The present invention relates to a powder magnetic core for miniaturizing and increasing the efficiency of magnetic devices, and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2024-054328, filed on March 28, 2024, the contents of which are incorporated herein by reference.

[0002] When constructing a magnetic device using a powder magnetic core, increasing the magnetic flux density of the magnetic powder used can result in a smaller, more efficient device. Known magnetic powders capable of achieving high magnetic flux density include magnetic alloy powders such as Sendust powder, amorphous metal alloy powder, and nanocrystalline material powder. However, these magnetic alloy powders all have high hardness, making it difficult to achieve high density when used to form a powder magnetic core. Furthermore, these magnetic alloy powders exhibit poor intertwining when compacted, making it difficult to ensure the strength of the compact and to form complex shapes.

[0003] Powder magnetic cores using the aforementioned magnetic alloy powders have low hysteresis loss, and the use of fine particles can also reduce eddy current loss. However, reducing the particle size of the magnetic alloy powder reduces fluidity, and the powder can also get into the clearance of the mold during compaction, causing problems such as galling. Furthermore, because these magnetic alloy powders are hard and do not entangle with each other much during compaction, attempts have been made to granulate them with the addition of a resin binder such as PVA, as described in the following patent documents, in order to avoid the problem of reduced strength.

[0004] Japanese Patent Publication No. 2016-180154 (A) Japanese Patent Publication No. 2014-120678 (A) Japanese Patent No. 7049752 (B) Japanese Patent No. 7096220 (B)

[0005] When powder magnetic cores made with magnetic alloy powder are press-molded, the magnetic alloy powder is hard and resistant to plastic deformation, resulting in little entanglement between the magnetic alloy powder particles. This means that the compact is brittle, prone to chipping and cracking, and the more complex the shape, the more difficult it is to mold. While fine-grained magnetic alloy powder can be used to reduce eddy current loss, powder with a particle size that is too small can get into the die clearance and cause galling. Furthermore, when press-molding, if the powder has poor fluidity and poor mold filling, the density of the compact varies from part to part, which can also cause cracks.

[0006] The present invention has been made in view of the above circumstances, and aims to provide a powder magnetic core that can achieve a high magnetic flux density, has excellent flowability even when using a magnetic powder with high hardness, and can also achieve high strength when formed into a compact, and a method for manufacturing the same.

[0007] (1) In order to solve the above problems, the powder magnetic core of the present invention is characterized in that it comprises a granulated powder obtained by adding 2.0 mass % to 4.0 mass % of a binder containing an acrylic resin as a main component to a magnetic alloy powder. (2) In the powder magnetic core of the present invention described in (1), it is preferable that the pH (hydrogen ion exponent) of the binder is 6.0 to 8.0.

[0008] (3) In the dust core according to the present invention as described in (1) or (2), the glass transition point of the binder is preferably −5° C. or higher and lower than 25° C. (4) In the dust core according to any one of (1) to (3), the magnetic alloy powder is preferably a mixed powder of one or more of sendust powder, amorphous alloy powder, and nanocrystalline alloy powder.

[0009] (5) A method for producing a powder magnetic core according to the present invention is characterized in that a granulated powder obtained by adding 2.0 mass % to 4.0 mass % of a binder having a glass transition temperature of −5° C. or more and less than 25° C. and containing an acrylic resin as a main component to a magnetic alloy powder is compacted at a die temperature during die press compaction of 80° C. or less. (6) In the method for producing a powder magnetic core according to (5) according to the present invention, it is preferable to add a binder to the magnetic alloy powder having an insulating coating in a wet state, vacuum dry the resulting granulated powder, add a lubricant powder in a dry state, mix the granulated powder, and anneal the resulting powder to form a powder magnetic core.

[0010] (7) In the method for producing a powder magnetic core according to the present invention as set forth in (5) or (6), it is preferable to use a binder having a pH of 6.0 or more and 8.0 or less. (8) In the method for producing a powder magnetic core according to any one of (5) to (7), it is preferable to use a mixed powder of one or more of sendust powder, amorphous alloy powder, and nanocrystalline alloy powder as the magnetic alloy powder.

[0011] The powder magnetic core of the present invention provides a powder magnetic core with a structure having high radial crushing strength, in which an acrylic resin-based binder is sufficiently spread to every corner of the gaps between magnetic alloy powders. This powder magnetic core can use FeSiAl alloy powder (Sendust powder), amorphous alloy powder, nanocrystalline alloy powder, amorphous metal powder, and other powders with excellent soft magnetic properties, resulting in high magnetic permeability and high saturation magnetic flux density, making it possible to provide a magnetic core that contributes to the miniaturization of magnetic devices. Furthermore, the use of these magnetic alloy powders allows for the provision of a magnetic core that exhibits low hysteresis loss and reduced eddy current loss.

[0012] According to the method for producing a powder magnetic core of the present invention, the frictional heat generated during warm compaction causes the binder, which is primarily composed of acrylic resin, to soften at a temperature moderately above the glass transition point, which is between −5° C. and 25° C. Then, the magnetic alloy powder is compacted while the acrylic resin is in a softened state, so that the soft magnetic alloy powder coated with the acrylic resin is more densely bound, thereby providing a powder magnetic core with improved radial crushing strength.

[0013] 1 is a perspective view showing a first example of a magnetic core formed from a powder magnetic core according to the present invention; 2 is a perspective view showing a second example of a magnetic core formed from a powder magnetic core according to the present invention; 3 is a perspective view showing a third example of a magnetic core formed from a powder magnetic core according to the present invention;

[0014] An embodiment of the present invention will now be described with reference to the drawings. Fig. 1 shows a first embodiment of an inductor made from a powder magnetic core according to the present invention. In this example, the inductor 1 is formed in a ring shape with a uniform thickness and height around its entire circumference. This inductor 1 is a powder magnetic core obtained by placing an insulating coating powder (described below) together with a binder into a mold for forming the desired shape, warm compacting at 80°C or less, and annealing the compacted powder. The insulating coating powder is formed by forming an insulating coating layer on a magnetic alloy powder using an insulating material such as resin.

[0015] The magnetic alloy powder is a soft magnetic alloy powder primarily composed of Fe. It can be a mixture of one or more of FeSi alloy powder, FeNi alloy powder, FeSiAl alloy powder (Sendust powder), amorphous alloy powder, nanocrystalline alloy powder, and amorphous metal powder. These magnetic alloy powders are all highly hard and virtually undeformable. Therefore, annealing the magnetic alloy powder as is or as an insulating powder coated with an insulating film does not produce a compact with the required strength. For example, Sendust powder can be Sendust powder with a composition of Fe-9.5%Si-5.5%Al by mass. For amorphous metal powder, powder of an Fe-Cr-Si-B-C amorphous alloy can be used. For example, the Fe-Si-Al alloy powder may contain 7 to 11 mass % Si and 3.0 to 11.0 mass % Al, and the Fe-Si alloy powder may contain 4.5 mass % to 7.0 mass % Si. In addition, various compositions of FeSi alloy powder, FeNi alloy powder, FeSiAl alloy powder (Sendust powder), amorphous alloy powder, nanocrystalline alloy powder, and amorphous metal powder are known, and therefore, the compositions are not limited to those described above, and any known compositions may be used.

[0016] 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. Methyl-based, methylphenyl-based, propylphenyl-based, epoxy resin-modified, alkyd resin-modified, polyester resin-modified, rubber-based, etc. can be used as the silicone resin. Among these, it is preferable to use a silicone resin composed of methyl groups and phenyl groups. A mixture of these silicone resins with a solvent can be used. The amount of silicone resin added can be, for example, about 1.0% by mass relative to the magnetic alloy powder.

[0017] The inductor 1 of this embodiment is obtained by wet-adding 2.0% by mass or more and 4.0% by mass or less of an acrylic resin binder to the insulating coating powder on which the insulating coating is formed, granulating the powder, and then placing the granulated powder in a mold together with a lubricant powder, warm-forming (powder compacting) the powder in a temperature range of 40°C to 80°C to obtain a shape close to the desired shape, followed by annealing at a high temperature that does not destroy the insulating coating. The mold temperature during mold press molding is preferably in the range described above.

[0018] When a silicone resin insulating coating is used on the magnetic alloy powder, the silicone resin is water-repellent. Here, with a water-repellent binder that is compatible with water, such as an anionic or cationic binder, its affinity with water increases during granulation during the process of forming liquid bridges that bind the powder together, resulting in poor compatibility with the powder. For this reason, a nonionic binder, such as a nonionic binder, is desirable. Therefore, the binder used for granulation is preferably one primarily composed of acrylic resin, a nonionic binder with a pH (hydrogen ion exponent) of 6.0 to 8.0. The lower limit of the range may be 5.8 or more, or 6.5 or more. The upper limit of the range may be 8.1 or less, or 6.9 or less. Here, a binder primarily composed of acrylic resin contains 22% by mass or more of resin, or 40% by mass or more and 50% by mass or less.

[0019] As an example, the glass transition temperature of a binder primarily composed of acrylic resin is preferably −5°C or higher and lower than 25°C. The lower limit of the range may be −5°C or higher, or 10°C or higher. The upper limit of the range may be lower than 25°C or lower than 20°C. The amount of binder primarily composed of acrylic resin is preferably 2.0% by mass or higher and 4.0% by mass or lower. By using a binder primarily composed of acrylic resin with a low glass transition temperature, the magnetic alloy powder coated with the acrylic resin is more densely bound during molding, improving shape retention. If the binder has a glass transition temperature higher than the aforementioned range (−5°C or higher and lower than 25°C), the strength of the molded body after warm molding will not improve. If the glass transition temperature is too low below the aforementioned range (−5°C or higher and lower than 25°C), the binder will soften due to frictional heat generated during molding, and the molded body may deform when gripped immediately after molding.

[0020] By warm molding using an acrylic resin binder having the aforementioned glass transition point, further compaction is achieved and the binder spreads between the particles of the magnetic alloy powder, resulting in improved strength of the molded body. When using a binder having the aforementioned glass transition point, if the warm molding temperature exceeds 80°C, the fluidity of the powder decreases. For this reason, the warm molding temperature is preferably in the range of room temperature (25°C ± 10°C) to 80°C, and more preferably 40°C or higher and 80°C or lower.

[0021] The frictional heat generated during warm molding softens the acrylic resin at a temperature moderately above its glass transition point (Tg). Because the acrylic resin is pressure molded in a softened state, the soft magnetic alloy powder coated with the acrylic resin is more densely bound, improving shape retention. Furthermore, heating the mold to 40°C to 80°C reliably softens the binder and binds it more densely, improving the strength of the bound body after warm molding.

[0022] The inductor 1 produced by the above-mentioned warm compaction can use FeSiAl alloy powder (Sendust powder), amorphous alloy powder, nanocrystalline alloy powder, amorphous metal powder, and the like, which have excellent soft magnetic properties, and therefore can provide a magnetic core that exhibits high magnetic permeability and high saturation magnetic flux density, contributing to the miniaturization of magnetic devices. Furthermore, the use of these magnetic alloy powders can provide a magnetic core that can reduce hysteresis loss and eddy current loss.

[0023] (Manufacturing Method) The manufacturing method of the inductor 1 includes (1) a resin coating step for forming an insulating coating, (2) a granulation step, (3) a lubricant addition step, (4) a press molding step, and (5) 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, magnetic alloy powder with an insulating coating can be obtained, the surface of which is covered with an insulating coating layer (resin coating layer) of the required thickness.

[0024] In the granulation step, the acrylic resin solvent can be dissolved in pure water and used for granulation. A known granulation method, such as agitation granulation, can be used as the granulation method. A required amount of binder (main component: acrylic resin) is gradually added dropwise to the powder being mixed by agitation granulation. After the binder is added, it is preferable to dry the mixture in a vacuum. The amount of acrylic resin added can be 2.0% by mass or more and 4.0% by mass or less. The average particle size (D ５０ The dried powder can be granulated to a size of about 50 μm to 200 μm. The required amount of lubricant (lubricating powder) (for example, 1 mass % or less) is added dry to the dried powder, which is then placed in the cavity of a die and warm-molded at room temperature to 80°C to obtain a green body. The molding pressure is 10 to 16 ton / cm. ２The obtained compact is then heat-treated to remove distortion by annealing at a high temperature (650 to 750°C for Sendust, or -50 to 0°C below the glass transition temperature for amorphous alloys) that does not destroy the insulating coating layer. Through the above manufacturing process, a powder core can be obtained in which the magnetic alloy powder with the insulating coating is bound with an acrylic resin binder and compacted.

[0025] The binder used for granulation is a nonionic (pH 6.0-8.0) binder mainly composed of acrylic resin. Because acrylic resin has good compatibility with magnetic alloy powder, it easily spreads into the gaps between the magnetic alloy powder during warm compaction, resulting in a compact with high strength and excellent shape retention.

[0026] In the previous embodiment, the case of manufacturing a ring-shaped inductor 1 was described, but the shape of the inductor is not limited to a ring shape. For example, the inductor may be an inductor 2 made of an E-shaped core as shown in FIG. 2, or an inductor 3 made of an EER core as shown in FIG. 3. In the case of an inductor 2 made of an E-shaped core, a transformer can be formed by combining and winding two inductors 2 of the same shape. A transformer can also be formed by combining and winding an inductor 2 made of an E-shaped core with an I-shaped core. Similarly, in the case of an inductor 3 made of an EER core, a transformer can also be formed by combining and winding two inductors 3 of the same shape. In addition, since there are various types of core shapes, any conventionally known core shape may be formed using the powder magnetic core of this embodiment.

[0027] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Sendust powder (particle size D50 = 50 μm) and amorphous alloy powder (composition: (Fe, Cr, Mn)-Si-P-B-C-S, particle size D50 = 10 μm) were used as magnetic alloy powders. Xylene was mixed with a silicone resin solvent and sprayed to form an insulating coating, with the amount added being 1% by mass.

[0028] To magnetic alloy powder having an insulating coating, binders A to E having pH and glass transition temperatures shown in Table 1 below were added in the amounts shown in Table 2. This mixture was then added with 1 mass % or less of a lubricant and placed in a mold. It was then warm-molded at room temperature or 40 to 80°C as shown in Table 2, yielding a ring-shaped compact having an inner diameter of 25 mm, an outer diameter of 35 mm, and a height of 5 mm, as shown in Figure 1. This ring-shaped compact was heated to 700°C in an inert gas atmosphere for 0.5 hours, and then subjected to an annealing process in which it was slowly cooled, yielding an annealed compact. The sample using amorphous alloy powder was annealed at 470°C.

[0029]

[0030] The radial crushing strength, Rattler value, and fluidity of each of the obtained ring-shaped annealed samples were measured by the following methods. [Radial Crushing Strength] The radial crushing strength of the compact was measured by measuring the breaking load with a universal testing machine using a ring-shaped test piece of ψ35 (outer diameter) × ψ25 (inner diameter) × 5H (height). The radial crushing strength was calculated from the compact dimensions (ring dimensions) according to the following relational formula: 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)

[0031] [Rattler Value] A ψ20 x 10 test piece was prepared using the same method as described above and placed in a mesh box at 87 rpm for a total rotation speed of 1,000 revolutions. The rate of decrease after charging was measured. The Rattler Value can be calculated using the formula: Rattler Value = 1 - (weight before charging / weight after charging), with a smaller value indicating better moldability. [Flow Test] The test sample was placed in the funnel of a bulk density measuring instrument based on JIS K6720, with the mechanical shutter properly closed and the funnel properly sealed. The shutter was opened, and the timer was started as the powder fell. The timer was stopped the moment the last powder left the orifice, and the passage time was measured. The passage time was measured and recorded to the nearest 0.1 second. The results are summarized in Table 2 below.

[0032]

[0033] As shown in Table 2, the samples of Examples 1 to 9 were produced by warm molding at room temperature or 40°C, 60°C, or 80°C using an acrylic resin binder with a pH of 6.5 to 6.9 and a glass transition temperature of -5°C to 20°C, with the binder content ranging from 2.0% to 4.0% by mass. In Table 2, the standard for excellent fluidity is an FR of 13 seconds or less, which was judged to be pass (G), and an FR of more than 13 seconds was judged to be insufficient fluidity (NG). Regarding radial crushing strength, if it was 1 MPa or more, it was judged to have high strength. Regarding the Rattler value, if it was 10% or less, it was judged to have good moldability.

[0034] The samples of Examples 1 to 7, which used sendust powder as the raw material, exhibited excellent fluidity, high radial crushing strength, and small Rattler values, resulting in excellent moldability. Excellent fluidity allows the material to flow smoothly into the mold cavity when inserted into the mold, and is therefore a criterion for judging excellent moldability. In particular, the samples of Examples 5 to 7 were warm-molded products in which the mold was heated to 40°C to 80°C during molding. These samples are believed to have significantly improved strength as powder magnetic cores due to the softening of the acrylic resin binder, which has a glass transition temperature of -5°C to less than 25°C, allowing the binder to sufficiently spread between the magnetic alloy powder particles. Examples 8 and 9 were samples in which the raw material was changed to amorphous alloy powder. Similar to the samples using sendust powder, they exhibited excellent fluidity, high radial crushing strength, small Rattler values, and excellent moldability. In Examples 8 and 9, the improved strength as powder magnetic cores is believed to be due to the softening of the binder.

[0035] The sample of Comparative Example 1 was a sample using a binder with a low pH and a high glass transition point, but had low radial crushing strength and a large Rattler value. The sample of Comparative Example 2 was a sample using a binder with a high pH, ​​so had low radial crushing strength and a large Rattler value. The sample of Comparative Example 3 was a sample using a binder with a high pH and a glass transition point of 25°C, so had low radial crushing strength and a large Rattler value. The sample of Comparative Example 4 used a binder with a desirable pH and glass transition point, but the amount of binder added was too small, resulting in poor fluidity and a large Rattler value. The sample of Comparative Example 5 used a binder with a desirable pH and glass transition point, but the amount of binder added was too large. This sample had high radial crushing strength and a low Rattler value, but cracks occurred in the sample after annealing due to the influence of gas generated during the annealing process.

[0036] From the above test results, it was found that a powder magnetic core with high radial crushing strength and excellent formability can be obtained by warm-compacting and annealing a granulated powder in which 2.0 mass % to 4.0 mass % of an acrylic resin binder has been added to a magnetic alloy powder. Furthermore, it was found that when manufacturing a powder magnetic core with high radial crushing strength and excellent formability, a powder magnetic core with high radial crushing strength and excellent formability can be manufactured by using a hard magnetic alloy powder, a pH of 6.0 to 8.0, and an acrylic resin binder with a glass transition point of -5°C to less than 25°C, and warm-compacting the powder magnetic core into the desired shape, followed by annealing.

[0037] 1 Inductor 2 Inductor 3 Inductor

Claims

1. A dust core made of granulated powder in which 2.0% by mass or more and 4.0% by mass or less of a binder whose main component is acrylic resin is added to a magnetic alloy powder.

2. The dust core according to claim 1, wherein the pH (hydrogen ion exponent) of the binder is 6.0 or more and 8.0 or less.

3. The dust core according to claim 1 or 2, wherein the glass transition point of the binder is -5°C or higher and lower than 25°C.

4. A dust core according to claim 1 or 2, wherein the magnetic alloy powder is a mixed powder of one or more of sendust powder, amorphous alloy powder, and nanocrystalline alloy powder.

5. A method for producing a powder magnetic core, characterized in that a granulated powder to which a magnetic alloy powder having a glass transition temperature of -5°C or higher but lower than 25°C and a binder primarily composed of acrylic resin is added in an amount of 2.0% by mass or higher but 4.0% by mass or lower is compacted at a mold temperature of 80°C or lower, which is the mold temperature used for mold press molding.

6. A method for producing a powder magnetic core according to claim 5, characterized in that a binder is added in a wet process to magnetic alloy powder having an insulating coating, the resulting granulated powder is vacuum dried, and a lubricant powder is added in a dry process to the resulting granulated powder, which is then mixed and annealed to form a powder magnetic core.

7. A method for producing a dust core according to claim 5 or 6, characterized in that a binder having a pH of 6.0 or more and 8.0 or less is used.

8. A method for producing a dust core according to claim 5 or 6, characterized in that the magnetic alloy powder is one or a mixture of two or more of sendust powder, amorphous alloy powder, and nanocrystalline alloy powder.

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