Powder magnetic core, inductor, and method for manufacturing powder magnetic core

Abstract

The object of the present invention is to provide a powder magnetic core that can achieve miniaturization while also achieving low loss in the high frequency range. [Solution] One aspect of the present invention provides a powder magnetic core in which magnetic powder is bound via a binder layer. The powder magnetic core contains 88 volume % or more of magnetic powder, and the proportion of binder layers present between the magnetic powder particles that are 20 nm or less in thickness is 6% or less (excluding 0).

Description

[Technical Field]

[0001] The present invention relates to a powder magnetic core, an inductor, and a method for manufacturing a powder magnetic core. [Background technology]

[0002] In recent years, inductors have been used in a variety of electronic devices. In particular, inductors used in electronic devices such as personal computers are required to be compact and to exhibit high inductance characteristics even when a large current is passed through them. Patent Document 1 discloses a method for manufacturing a powder compact of an amorphous soft magnetic alloy that exhibits little decrease in magnetic permeability in the high frequency range. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-212503 Summary of the Invention [Problem to be solved by the invention]

[0004] As mentioned above, inductors are required to be small in size and also to exhibit high inductance characteristics even when a large current flows through them. In particular, inductors used in electronic devices such as personal computers are used in the high frequency range (e.g., 750 kHz to 2 MHz), so inductors with low loss in the high frequency range are required.

[0005] In view of the above problems, an object of the present invention is to provide a powder magnetic core, an inductor, and a method for manufacturing a powder magnetic core that can achieve miniaturization while also achieving low loss in the high frequency range. [Means for solving the problem]

[0006] A powder magnetic core according to one embodiment of the present invention is a powder magnetic core in which magnetic powder is bound via a binder layer, and the powder magnetic core contains 88 volume % or more of magnetic powder, and the proportion of binder layers present between the magnetic powder particles that have a thickness of 20 nm or less is 6% or less (excluding 0).

[0007] A method for producing a powder magnetic core according to one embodiment of the present invention includes the steps of coating a magnetic powder with low-melting point glass, coating the low-melting point glass-coated magnetic powder with a resin material and granulating the resulting powder, and hot-compacting the granulated magnetic powder. The resulting compact contains 88 volume % or more of magnetic powder, binder layers containing the low-melting point glass and the resin material are formed between the magnetic powder particles, and the proportion of binder layers between the magnetic powder particles that are 20 nm or less in thickness is 6% or less (excluding 0). [Effects of the Invention]

[0008] The present invention can provide a powder magnetic core, an inductor, and a method for manufacturing a powder magnetic core that can achieve miniaturization and low loss in the high frequency range. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a perspective view illustrating an example of an inductor according to an embodiment. [Figure 2] 1 is an electron microscope photograph of a powder magnetic core according to a conventional technique and a powder magnetic core according to the present invention. [Figure 3] 1A and 1B are schematic diagrams illustrating the microstructure of a powder magnetic core according to a prior art and the microstructure of a powder magnetic core according to the present invention. [Figure 4] 1 is an electron microscope photograph showing the microstructure of a powder magnetic core according to an embodiment. [Figure 5] 1 is a flowchart illustrating a method for manufacturing a powder magnetic core according to an embodiment. [Figure 6] 1A to 1C are schematic diagrams illustrating a method for manufacturing a powder magnetic core according to an embodiment. [Figure 7]1 is a horizontal cross-sectional view of a powder magnetic core according to an embodiment. [Figure 8] 1 is a horizontal cross-sectional view of a powder magnetic core according to an embodiment. [Figure 9] 1 is a horizontal cross-sectional view of a powder magnetic core according to an embodiment. [Figure 10] 1 is a horizontal cross-sectional view of a powder magnetic core according to an embodiment. [Figure 11] 1 is a graph plotting the iron loss and the proportion of the binder layer of 20 nm or less for samples with the same binder amount and magnetic powder particle size. DETAILED DESCRIPTION OF THE INVENTION

[0010] <Inductor> Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a perspective view showing an example of an inductor according to this embodiment. As shown in Fig. 1, the inductor 1 according to this embodiment includes powder magnetic cores 10_1 and 10_2 and a coil 13. The powder magnetic core 10_1 has a cavity that passes vertically through the center and is disposed so as to surround the outside of the coil 13. The powder magnetic core 10_2 is provided inside the coil 13 and is disposed in a recess of the coil 13 that has a U-shaped cross section.

[0011] For example, the inductor 1 shown in FIG. 1 can be formed by placing a powder magnetic core 10_2 in the recess of a coil 13 and then press-fitting a powder magnetic core 10_1 from above. This allows for the formation of an inductor 1 in which the coil 13 is surrounded by powder magnetic cores 10_1 and 10_2. Note that in this specification, the powder magnetic cores 10_1 and 10_2 are also collectively referred to as a powder magnetic core 10. The configuration of the inductor 1 shown in FIG. 1 is just one example, and the powder magnetic core 10 according to this embodiment may be used for an inductor having a configuration other than that shown in FIG. 1. The powder magnetic core according to this embodiment is characterized by achieving miniaturization while achieving low loss in the high-frequency range. The powder magnetic core according to this embodiment will be described in detail below.

[0012] <Powder magnetic core> The powder magnetic core according to this embodiment is a powder magnetic core in which magnetic powder is bound via a binder layer. The powder magnetic core contains 88% or more by volume of magnetic powder, and the proportion of binder layers present between the magnetic powder particles that are 20 nm or less in thickness is 6% or less (excluding 0). This configuration makes it possible to provide a powder magnetic core that can achieve miniaturization while also achieving low loss in the high frequency range. The proportion of binder layers present between the magnetic powder particles that are 20 nm or less in thickness may preferably be 3.3% or less.

[0013] The magnetic powder used in the dust core according to this embodiment is a soft magnetic powder containing iron. For example, the particle size of the magnetic powder is 2 μm or more and 25 μm or less, preferably 5 μm or more and 15 μm or less. In the present invention, the particle size is the median diameter D50, which is a value measured using a laser diffraction / scattering method.

[0014] In this embodiment, metallic glass can be used as the magnetic powder. For example, amorphous metallic glass produced by atomization can be used as the metallic glass. For example, Fe-PB alloy, Fe-BP-Nb-Cr alloy, Fe-Si-B alloy, Fe-Si-BP alloy, Fe-Si-BP-Cr alloy, or Fe-Si-BPC alloy can be used, and by powdering them by atomization, metallic glass with a glass transition point can be formed. In particular, in the present invention, it is preferable to use Fe-BP-Nb-Cr-based materials. However, metallic glasses obtained by atomization are not limited to these, and metallic glasses without a glass transition point can also be used.

[0015] In this embodiment, for example, nanocrystalline powder may be used as the magnetic powder. For example, nanocrystalline powder produced by atomization may be used as the nanocrystalline powder. For example, by powdering Fe-Si-BPC-Cu, Fe-Si-B-Cu-Cr, Fe-Si-BP-Cu-Cr, Fe-BPC-Cu, Fe-Si-BP-Cu, Fe-BP-Cu, or Fe-Si-B-Nb-Cu materials by atomization, nanocrystalline powder having at least two exothermic peaks indicating crystallization during the heat treatment process of the magnetic powder can be formed. The nanocrystalline powder used is not particularly limited, but it is preferable to use, for example, an Fe-Si-BP-Cu-Cr material.

[0016] In this embodiment, the closer the particle shape of the magnetic powder is to a spherical shape, the better. If the particle sphericity is low, protrusions will form on the particle surface, and when compaction pressure is applied, stress from surrounding particles will concentrate on these protrusions, destroying the coating and resulting in insufficient insulation, which may result in poor magnetic properties (particularly loss) of the resulting dust core. The particle sphericity can be controlled within a suitable range by adjusting the manufacturing conditions of the magnetic powder, for example, in the case of water atomization, the amount of water and water pressure of the high-pressure water jet used for atomization, the temperature and feed rate of the molten raw material, etc. Specific manufacturing conditions vary depending on the composition of the magnetic powder to be manufactured and the desired productivity.

[0017] In the powder magnetic core according to this embodiment, the binder layer functions to bind the magnetic powder together. The binder layer includes a low-melting-point glass and a resin material. In this embodiment, the total amount of the low-melting-point glass and the resin material is less than 10% by volume relative to the magnetic powder in the powder magnetic core. The low-melting-point glass may be phosphate-based, tin phosphate-based, borate-based, silicate-based, borosilicate-based, barium silicate-based, bismuth oxide-based, germanate-based, vanadate-based, aluminophosphate-based, arsenate-based, or telluride-based. In particular, in the present invention, it is preferable to use a phosphate-based or tin phosphate-based low-melting-point glass. The volume ratio of the low-melting-point glass to the magnetic powder is 0.5% by volume or more and 6% by volume or less, preferably 1.25% by volume or more and 3% by volume or less.

[0018] The resin material contained in the binder layer may be at least one selected from the group consisting of phenolic resin, polyimide resin, epoxy resin, and acrylic resin, and the volume ratio of the resin material to the magnetic powder is 0.5% to 9% by volume, preferably 1% to 5% by volume.

[0019] The powder magnetic core according to this embodiment having the above configuration contains 88% or more by volume of magnetic powder, and the proportion of binder layers present between the magnetic powder particles that are 20 nm or less in thickness is 6% or less (excluding 0). Therefore, it is possible to thin the binder layers to increase the packing density of the magnetic powder while maintaining sufficient insulation between the magnetic powder particles. Therefore, the powder magnetic core according to this embodiment can reduce inductor loss in the high frequency range while achieving miniaturization.

[0020] Figure 2 shows electron microscope photographs of a conventional powder magnetic core and a powder magnetic core of the present invention. The conventional technology shown in Figure 2 has a low magnetic powder packing rate. In contrast, the powder magnetic core of the present invention has a higher magnetic powder packing rate than the conventional powder magnetic core. Therefore, it exhibits high inductance characteristics even when a large current is passed through it.

[0021] FIG. 3 is a schematic diagram illustrating the microstructure of a conventional powder magnetic core and the microstructure of a powder magnetic core of the present invention. In the conventional technology shown in FIG. 3, the thickness of the binder layer 122 between the magnetic powder particles 121 is non-uniform. For example, the binder layer 122 is thick in region 131, but is thin in regions 132 and 133. In other words, in this case, the proportion of the binder layer 122 between the magnetic powder particles 121 that is 20 nm or less thick (i.e., the proportion of areas where the binder layer is thin, such as regions 132 and 133) is high. As a result, the proportion of areas where the binder layer 122 is thick is high.

[0022] In contrast, in the powder magnetic core of the present invention, the thickness of the binder layer 22 between the magnetic powder particles 21 is uniform. In other words, the proportion of the binder layer 22 between the magnetic powder particles 21 that is 20 nm or less in thickness (i.e., the proportion of areas where the binder layer is thin) is small. As a result, the proportion of thick areas of the binder layer 22 is low, and the binder layer 22 becomes uniform overall. As an example, the median thickness of the binder layer 22 in the powder magnetic core of the present invention is 31 to 68 nm.

[0023] FIG. 4 is an electron microscope photograph showing the microstructure of a powder magnetic core according to this embodiment, illustrating a method for determining the "proportion of binder layers with a thickness of 20 nm or less among the binder layers present between magnetic powder particles." To measure the thickness of the binder layer, an electron microscope photograph (SEM image) of the powder magnetic core is used to identify regions where the binder is filled between the magnetic powder particles and where the spacing between the magnetic powder particles is 200 nm or less over a length of 100 nm or more. Then, in the identified regions, the thickness of the binder layer is measured every 100 nm. An example of measurement is shown on the right side of FIG. 4. Whether or not a binder is present between magnetic powder particles can be determined using the contrast of the SEM image or the results of elemental analysis using EDX (Energy Dispersive X-ray spectroscopy). For example, it is preferable to measure the thickness of the binder layer at 400 or more points. The spacing between the magnetic powder particles can be determined by imagining a normal line at a point on the surface of one of the magnetic powder particles and measuring the distance between the two powder particles in the direction of that normal line.

[0024] For example, if there are 400 measurement points and 20 of those measurement points have binder layer thicknesses of 20 nm or less, then the "percentage of binder layers between magnetic powders that have a thickness of 20 nm or less" = (20 / 400) × 100 = 5%.

[0025] As shown in the lower left diagram of FIG. 4, even if the spacing between magnetic powder particles is 200 nm or less (90 nm in the illustrated location), if no binder is filled, the particle is excluded from the measurement.

[0026] <Method of manufacturing powder magnetic cores> Next, a method for manufacturing a powder magnetic core according to this embodiment will be described. Fig. 5 is a flowchart for explaining the method for manufacturing a powder magnetic core according to this embodiment. Fig. 6 is a schematic diagram for explaining the method for manufacturing a powder magnetic core according to this embodiment.

[0027] As shown in FIG. 5, when manufacturing a powder magnetic core, magnetic powder is first prepared (step S1). The magnetic powders described above can be used. It is preferable to use a magnetic material that softens at 400°C or higher (a material that easily deforms during hot compaction) for the magnetic powder. For example, amorphous magnetic powder can be obtained by vacuum melting the raw materials for the magnetic powder, followed by simultaneously pulverizing and quenching them using a water atomization method. The magnetic powder obtained in this manner can be classified as needed to remove abnormally coarse powder.

[0028] Next, the magnetic powder is coated with low-melting-point glass (step S2). It is preferable to use a material that softens at 400°C or higher for the low-melting-point glass, that is, a material that softens during hot-forming and acts as an insulating material and binder after hot-forming. For example, phosphate-based glass can be used as the low-melting-point glass. When coating the magnetic powder with low-melting-point glass, a wet thin-film production method such as mechanofusion or the sol-gel method, or a dry thin-film production method such as sputtering can be used. For example, the mechanofusion method can form a low-melting-point glass layer on the surface of the magnetic powder by mixing the magnetic powder and low-melting-point glass powder while applying strong mechanical energy.

[0029] For example, 1000 g of magnetic powder is mixed with 10 g of low-melting-point glass powder, and the magnetic powder is coated with the low-melting-point glass using the mechanofusion method, so that the volume ratio of the coated low-melting-point glass to the magnetic powder can be set to 0.5% to 6% by volume.

[0030] Next, the magnetic powder coated with low-melting-point glass is coated with a resin material and granulated (step S3). The resin material can be any of the resin materials described above. It is preferable to use a material that softens at about 100°C and acts as an insulating material and binder after hot forming. It is also preferable to use a material that does not easily decompose during hot forming (at high temperatures). When coating (granulating) the resin material, methods such as rolling granulation and spray drying can be used. Specifically, a resin layer can be formed on the low-melting-point glass of the magnetic powder by mixing the resin material dissolved in an organic solvent with the magnetic powder coated with low-melting-point glass and drying it.

[0031] The left diagram in Figure 6 shows magnetic powder 20 after granulation. As shown in Figure 6, in magnetic powder 20 after granulation, low-melting point glass 31 is coated on magnetic powder 21, and resin material 32 is further coated on low-melting point glass 31. For example, the diameter of magnetic powder 21 is 9 μm, the thickness of low-melting point glass 31 is 20 nm, and the thickness of resin material is 20 nm.

[0032] Next, the granulated magnetic powder is preformed (step S4). For example, the preformed magnetic powder is placed in a mold and pressurized (for example, 500 kgf / cm at room temperature). 2 ), and then the green compact is heated at a predetermined temperature (for example, 100°C to 150°C) without applying pressure to harden it. If the resin material used is a thermosetting resin, the intermediate molded body is formed by hardening the resin during heating. If the resin material used is a thermoplastic resin, the intermediate molded body is formed by softening the resin during heating and solidifying it during cooling.

[0033] That is, as shown in the center diagram of Figure 6, when preforming is performed, the magnetic powder 21 (coated with low-melting point glass 31) is bound via the outermost resin material 32 to form an intermediate molded body 25. Note that low-melting point glass does not soften at the preforming temperature (e.g., 150°C), and therefore does not exhibit binding properties or fluidity. Note that the preforming step (step S4) may be omitted.

[0034] Next, the preformed intermediate compact (or the granulated magnetic powder if step S4 is omitted) is hot-formed (step S5). Hot-forming is carried out by heating the preformed intermediate compact (or the granulated magnetic powder) placed in a mold while applying pressure. The heating temperature at this time is set, for example, as follows:

[0035] When the magnetic powder used is metallic glass, the hot-molding temperature is set to the higher of the softening temperature of the low-melting glass and the glass transition temperature of the magnetic powder, but below the crystallization temperature of the magnetic powder. By setting the hot-molding temperature above the glass transition temperature of the magnetic powder, plastic deformation of the magnetic powder is more likely to occur, resulting in a high filling rate of the magnetic powder. For example, the hot-molding temperature is set to 450°C or higher and 500°C or lower.

[0036] When the magnetic powder used is a nanocrystalline powder, the hot-forming temperature is set to the higher of the softening temperature of the low-melting glass and the first crystallization temperature of the magnetic powder, but lower than the second crystallization temperature of the magnetic powder. Setting the hot-forming temperature around the first crystallization temperature facilitates the crystallization of the α-Fe phase and the plastic deformation of the magnetic powder, resulting in a high magnetic powder packing density. For example, the hot-forming temperature is 400°C or higher and 500°C or lower. In the present invention, the hot-forming temperature is preferably higher than the softening temperature of the low-melting glass or the first crystallization temperature of the magnetic powder + 40°C, whichever is higher. Here, the first crystallization temperature and the second crystallization temperature are as follows: When an amorphous magnetic material is heat-treated, crystallization occurs two or more times. The temperature at which crystallization first begins is the first crystallization temperature, and the temperature at which crystallization subsequently begins is the second crystallization temperature. More specifically, the magnetic powder has at least two exothermic peaks indicating crystallization during the heating process of a DSC curve obtained by differential scanning calorimetry (DSC). The lowest exothermic peak is the first crystallization temperature at which the α-Fe phase crystallizes, and the next exothermic peak is the second crystallization temperature at which borides and the like crystallize.

[0037] In this embodiment, it is preferable that the heating temperature is set within the above-mentioned temperature range and that the temperature conditions are such that the iron loss value of the powder magnetic core is low.

[0038] The pressure during hot forming is, for example, 5 to 10 ton f / cm 2 If the pressure is too low, the packing density of the compact (dust core) will be low, and the iron loss of the dust core will be high. Conversely, if the pressure is too high, the die will wear out rapidly, which is undesirable from a cost perspective. Therefore, it is preferable to set the pressure within the above range.

[0039] The hot compaction time is preferably in the range of 5 to 60 seconds, and more preferably 30 seconds or less. If the compaction time is too short, heat is not sufficiently transmitted to the interior of the compact, and deformation due to softening of the magnetic powder is not sufficiently achieved, resulting in a low packing density of the compact and increased iron loss of the powder magnetic core. Conversely, if the compaction time is too long, thermal decomposition of the resin material used in the binder layer progresses, reducing the effect of suppressing the fluidity of the low-melting-point glass and increasing iron loss of the powder magnetic core. Therefore, the hot compaction time should be set within a cost-effective range that allows heat to be sufficiently transmitted to the interior of the compact, completes deformation due to softening of the magnetic powder, and suppresses thermal decomposition of the resin material used in the binder layer. It is preferable to set the compaction time within the above-mentioned range.

[0040] For example, the hot forming conditions are: hot forming temperature: 480°C, hot forming pressure: 8 ton·f / cm 2 , hot forming time: can be 10 seconds.

[0041] As shown in the right diagram of FIG. 6 , in the compact (powder magnetic core) 10 after hot compaction, magnetic powder particles 21 are bound together via binder layers 22 containing low-melting-point glass and a resin material. In this embodiment, the volume fraction of magnetic powder contained in the powder magnetic core 10 is set to 88 volume % or more. Furthermore, the proportion of binder layers with a thickness of 20 nm or less among the binder layers present between the magnetic powder particles is set to 6% or less. This makes it possible to increase the packing density of the magnetic powder and maintain sufficient insulation between the magnetic powder particles. Therefore, the method for manufacturing a powder magnetic core according to this embodiment makes it possible to manufacture a powder magnetic core that can be miniaturized while achieving low loss in the high-frequency range.

[0042] As explained in the background art, inductors are required to be compact and to exhibit high inductance characteristics even when a large current is passed through them. There is also a demand for inductors with low loss in the high frequency range. To achieve such inductors, it is necessary to increase the magnetic powder packing rate in the powder magnetic core used in the inductor while maintaining sufficient insulation between the magnetic powder particles. However, with conventional technology, it has been difficult to achieve both an increase in the magnetic powder packing rate and sufficient insulation between the magnetic powder particles.

[0043] In contrast, in the manufacturing method of the powder magnetic core according to the present embodiment, the binder layer is formed using low-melting-point glass and a resin material. By using low-melting-point glass and a resin material as the binder in this manner, a thin and uniform binder layer (insulating layer) can be formed even with a small amount of binder added. In other words, by mixing a binder component (low-melting-point glass) that flows easily at the hot-forming temperature with a binder component (resin material) that does not flow easily, insulation between magnetic powder particles can be maintained even with a small amount of binder added. In other words, in this embodiment, intentionally leaving the resin behind during hot-forming can somewhat suppress the flow of the low-melting-point glass, which is relatively softer than the magnetic powder, thereby preventing contact between magnetic powder particles without the binder layer (insulating layer) interposed therebetween.

[0044] Furthermore, in the method for producing a powder magnetic core according to this embodiment, the amount of resin material used as a binder is small, which reduces the amount of gas generated by decomposition of the resin material during hot compaction, thereby suppressing cracks in the compact (powder magnetic core) caused by the generated gas.

[0045] In this embodiment, the iron loss of the powder magnetic core is 2500 kW / m 3 It is preferable that the power consumption is 1500 kW / m or less. 3 More preferably, it is:

[0046] <Dust core dimensions> Next, the dimensions of the powder magnetic core according to this embodiment will be described. In this embodiment, when the vertical length of the powder magnetic core (distance h in the example shown in FIG. 1) is longer than 3.5 mm, the distance between the molding dies that sandwich the powder magnetic core in the horizontal cross section of the powder magnetic core, in a direction approximately perpendicular to the direction in which the portion that takes the longest time for heat to transfer into the powder magnetic core when hot compacted extends, is set to 3.5 mm or less. A specific example will be explained below.

[0047] For example, if the shape of the horizontal cross section of a powder magnetic core is like that of powder magnetic core 10_1 shown in FIG. 7 (powder magnetic core 10_1 shown in FIG. 7 corresponds to powder magnetic core 10_1 shown in FIG. 1), powder magnetic core 10_1 is sandwiched between molding dies 61 during hot compaction. At this time, heat is transferred from molding dies 61 to powder magnetic core 10_1, but the portion inside powder magnetic core 10_1 to which heat is transferred least is the portion indicated by reference numeral 71. In this embodiment, the distance b between the molding dies in a direction substantially perpendicular to the extension direction of portion 71, which takes the longest time for heat to transfer inside powder magnetic core 10_1, is set to 3.5 mm or less. By using such dimensions, heat can be transferred quickly throughout powder magnetic core 10_1 during hot compaction.

[0048] Furthermore, for example, when the shape of the horizontal cross section of a powder magnetic core is like that of powder magnetic core 52 shown in FIG. 8 (i.e., a shape without a cavity in the center), powder magnetic core 52 is sandwiched between molding dies 62 during hot compacting. At this time, heat is transferred from molding dies 62 to powder magnetic core 52, but the portion of powder magnetic core 52 to which heat is transferred least is the portion indicated by reference numeral 72. In this embodiment, the distance b2 between the molding dies in a direction approximately perpendicular to the extension direction of portion 72, which takes the longest time for heat to transfer into powder magnetic core 52, is set to 3.5 mm or less. By using such dimensions, heat can be transferred quickly throughout powder magnetic core 52 during hot compacting.

[0049] Furthermore, for example, when the shape of the horizontal cross section of a powder magnetic core is like that of powder magnetic core 53 shown in FIG. 9 (i.e., a shape with two cavities in the center), powder magnetic core 53 is sandwiched between molding dies 63 during hot compacting. At this time, heat is transferred from molding dies 63 to powder magnetic core 53, but the portion inside powder magnetic core 53 to which heat is transferred least is the portion indicated by reference numeral 73. In this embodiment, the distance b3 between the molding dies in a direction approximately perpendicular to the extension direction of portion 73, which takes the longest time for heat to transfer inside powder magnetic core 53, is set to 3.5 mm or less. By using such dimensions, heat can be transferred quickly throughout powder magnetic core 53 during hot compacting.

[0050] Furthermore, for example, when the shape of the horizontal cross section of a powder core is a shape like powder core 54 shown in FIG. 10 (i.e., an E-shaped core), powder core 54 is sandwiched between molding dies 64 during hot compacting. At this time, heat is transferred from molding dies 64 to powder core 54, but the portion inside powder core 54 to which heat is transferred least is the portion indicated by reference numeral 74. In this embodiment, the distance b4 between the molding dies in a direction approximately perpendicular to the extension direction of portion 74, which takes the longest time for heat to transfer inside powder core 54, is set to 3.5 mm or less. By using such dimensions, heat can be transferred quickly throughout powder core 54 during hot compacting.

[0051] 7 to 10 are merely examples, and the dimensions of the powder magnetic core according to this embodiment can also be applied to powder magnetic cores having other configurations. Furthermore, for example, if the horizontal cross section of the powder magnetic core is circular, the point where it takes the longest time for heat to transfer into the powder magnetic core 54 is a point. In this case, the diameter of the circle passing through this point is set to 3.5 mm or less. Furthermore, in this embodiment, the vertical length of the powder magnetic core may be set to 3.5 mm or less. In this way, if the vertical length of the powder magnetic core is set to 3.5 mm or less, the distance between the molding dies in the horizontal cross section of the powder magnetic core can be set as desired.

[0052] As explained above, by setting the dimensions of the powder magnetic core according to this embodiment as described above, heat can be easily transferred to the powder magnetic core during hot compaction. This makes it possible to shorten the hot compaction time and suppress thermal decomposition of the resin material. This enhances the effect of suppressing the fluidity of the low-melting-point glass, thereby reducing the iron loss of the powder magnetic core. [Example]

[0053] Next, an embodiment of the present invention will be described.

[0054] <Experiment 1> Samples for Experiment 1 were produced using the powder magnetic core manufacturing method described above (see Figure 5). The powder magnetic core for Experiment 1 had a toroidal shape with an outer diameter of 13 mm, an inner diameter of 8 mm, and a length of 5 mm. Specifically, magnetic powder was first prepared. The magnetic powder was an Fe-BP-Nb-Cr metallic glass powder with a particle size of 9 μm (median diameter D50). Next, the magnetic powder was mixed with low-melting-point glass powder, and the magnetic powder was coated with the low-melting-point glass using the mechanofusion method. Phosphate-based glass was used as the low-melting-point glass. 2.5% by volume of the low-melting-point glass was mixed with the magnetic powder.

[0055] The magnetic powder coated with low-melting-point glass was then coated with a resin material and granulated. The resins used were those shown in Table 1. 2.5% by volume of each resin material was mixed with the magnetic powder. Note that the "resin weight loss on heating at 500°C" in Table 1 is the result of thermogravimetric analysis of the resin (measurement conditions: air atmosphere, heating rate 100°C / min), and the smaller the weight loss on heating, the higher the heat resistance of the resin.

[0056] Next, the granulated magnetic powder is poured into a mold and pressurized to 500 kgf / cm 2 After pressing under these conditions, the green compact was preformed by heating it to a temperature of 150°C without applying pressure and hardening it. The preformed intermediate compact was then placed in a mold and hot formed. The hot forming conditions were a forming temperature of 490°C and a pressure of 8 tonf / cm. 2 The pressure was applied for 30 seconds.

[0057] For each sample prepared as described above, the powder filling rate of the magnetic core, magnetic permeability, iron loss, the proportion of binder layers with a thickness of 20 nm or less among the binder layers present between the magnetic powders, and the median thickness of the binder layers were measured. Note that the binder layer thickness was measured at 1000 points.

[0058] The powder filling rate of the magnetic core was determined by comparing the volume of the magnetic powder contained in the magnetic core with the volume of the entire magnetic core measured using the Archimedes method. The volume of the magnetic powder contained in the magnetic core was determined by subtracting the weight of the low-melting-point glass added as a binder and the weight of the remaining resin material from the weight of the entire magnetic core to determine the weight of the magnetic powder contained in the magnetic core, and then dividing the weight of the magnetic powder by the true density of the magnetic powder.

[0059] The magnetic permeability was measured using an impedance analyzer at a frequency of 1 MHz, and the iron loss was measured by preparing a toroidal powder magnetic core and measuring the powder magnetic core using a BH analyzer (manufactured by Iwasaki Electric Co., Ltd.) with the two-coil method. The measurement conditions were a 1 MHz, 50 mT sinusoidal wave excitation condition.

[0060] The proportion of binder layers between the magnetic powder particles that were 20 nm or less thick (hereinafter referred to as "the proportion of binder layers 20 nm or less") was measured using the above-mentioned method using electron micrographs. The median thickness of the binder layers was also measured using electron micrographs.

[0061] Table 1 shows the type of resin used in each sample and the measurement results for each sample. As shown in Table 1, Example 1-1, which used a phenolic resin as the binder resin, Example 1-2, which used a polyimide resin, Example 1-3, which used an epoxy resin, and Example 1-4, which used an acrylic resin, all showed good iron loss values ​​of 1100 or less. Furthermore, in Examples 1-1 to 1-4, the proportion of binder layers of 20 nm or less was 2.2% or less, which was good. In particular, in Examples 1-1 to 1-3, the proportion of binder layers of 20 nm or less was lower than 1%, and the iron loss value was also lower than 1000.

[0062] On the other hand, in Comparative Example 1-1, which used silicone resin as the binder resin, Comparative Example 1-2, which used PVB (polyvinyl butyral) resin, and Comparative Example 1-3, which did not use resin, the iron loss value was 5500 or more, which was a large value.

[0063] From the above results, it can be said that it is preferable to use phenol resin, polyimide resin, epoxy resin, and acrylic resin as the resin used for the binder layer.

[0064] [Table 1]

[0065] <Experiment 2> In Experiment 2, powder magnetic cores were produced by varying the particle size (median diameter D50) of the metallic glass powder used as the magnetic powder. In Experiment 2, phosphate-based glass and phenolic resin were used as binder materials. The same methods as in Experiment 1 were used to produce the powder magnetic cores and measure the samples. In Comparative Example 2-1 and Example 2-1, the volume ratio of the phosphate-based glass to the magnetic powder was 5 volume %, and the volume ratio of the phenolic resin to the magnetic powder was 2.5 volume %. In Example 2-2, the volume ratio of the phosphate-based glass to the magnetic powder was 2.5 volume %, and the volume ratio of the phenolic resin to the magnetic powder was 2.5 volume %. Furthermore, as shown in Table 2, the softening temperature of the phosphate-based glass is 400°C, the glass transition temperature of the magnetic powder is 480°C, and the crystallization temperature of the magnetic powder is 510°C, so the molding temperature was set to 490°C.

[0066] As shown in Table 2, in Comparative Example 2-1, in which the particle size of the metallic glass powder was 4 μm, the iron loss value was 12,000 and the proportion of the binder layer of 20 nm or less was 13.5%, both of which were large values. On the other hand, in Example 2-1, in which the particle size of the metallic glass powder was 7 μm, and Example 2-2, in which the particle size of the metallic glass powder was 9 μm, the iron loss values ​​were 1,100 and 900, respectively, which were good values. Furthermore, in Examples 2-1 and 2-2, the proportions of the binder layer of 20 nm or less were 1.7% and 0.92%, respectively, which were also good values. Therefore, in Experiment 2, when the particle size of the metallic glass powder was 7 μm or more, the iron loss and the proportion of the binder layer of 20 nm or less were good values.

[0067] While phosphate glass and phenolic resin were used as binder materials in Experiment 2, the inventors also conducted an experiment using 5% by volume of phosphate glass and 2.5% by volume of polyimide resin relative to the magnetic powder as binders. In this case, it was confirmed that even when the particle size of the metallic glass (magnetic powder) was 2 μm, the packing rate of the powder core was 88% by volume or more, the proportion of binder layers of 20 nm or less was 6% or less, and the iron loss was 2500 or less.

[0068] [Table 2]

[0069] <Experiment 3> In Experiment 3, powder magnetic cores were fabricated using nanocrystalline powder with various particle sizes (median diameter D50) of Fe-Si-BP-Cu-Cr magnetic powder. Phosphate-based glass and phenolic resin were used as binder materials in Experiment 3. The same methods as in Experiment 1 were used to fabricate the powder magnetic cores and measure the samples. In Experiment 3, the volume ratio of the phosphate-based glass to the magnetic powder was 2.5% by volume, and the volume ratio of the phenolic resin to the magnetic powder was 2.5% by volume. As shown in Table 3, the molding temperature was set to a value between the higher of the softening temperature of the low-melting-point glass (400°C) and the first crystallization temperature of the magnetic powder, and the second crystallization temperature of the magnetic powder.

[0070] As shown in Table 3, in Example 3-1, in which the particle size of the nanocrystalline powder was 11 μm, Example 3-2, in which the particle size of the nanocrystalline powder was 14 μm, and Example 3-3, in which the particle size of the nanocrystalline powder was 23 μm, the iron loss value was 2500 or less and the proportion of the binder layer of 20 nm or less was 1% or less, which were good values. In particular, in Example 3-1, in which the particle size of the nanocrystalline powder was 11 μm, the iron loss value was 860, which was an extremely good value. On the other hand, in Comparative Example 3-1, in which the particle size of the nanocrystalline powder was 41 μm, the iron loss value was large at 5300, and the proportion of the binder layer of 20 nm or less was 0%.

[0071] The results of Experiments 2 and 3 showed that if the particle size is too small, the median binder layer thickness becomes too thin, which means that the insulation between the magnetic powder particles is not sufficiently maintained, and the iron loss of the powder core increases due to eddy current loss between the magnetic powder particles. On the other hand, if the particle size is too large, the median binder layer thickness becomes too thick, which ensures sufficient insulation between the magnetic powder particles, but the iron loss of the powder core increases due to eddy current loss within the magnetic powder particles. Based on the above, it can be said that the particle size of the magnetic powder is preferably 2 μm or more and 25 μm or less, and more preferably 5 μm or more and 15 μm or less.

[0072] [Table 3]

[0073] <Experiment 4> In Experiment 4, powder magnetic cores were produced by varying the compounding ratio of phosphate glass and phenolic resin, which are used as binder materials. In Experiment 4, metallic glass powder with a particle size of 9 μm (median diameter D50) was used as the magnetic powder. The same methods as in Experiment 1 were used to produce the powder magnetic cores and measure the samples. Table 4 shows the compounding ratio of phosphate glass and phenolic resin for each sample.

[0074] As shown in Table 4, in Comparative Example 4-1, where the blending ratio (volume %) of phosphate-based glass to phenolic resin was 2.5:0 (i.e., no phenolic resin was added), the iron loss value was 17,000 and the proportion of binder layers of 20 nm or less was 13.3%, both of which were large values. Furthermore, in Example 4-1, where the blending ratio (volume %) of phosphate-based glass to phenolic resin was 2.5:2.5, the iron loss value was 900 and the proportion of binder layers of 20 nm or less was 0.92%, both of which were good values. In Example 4-2, where the blending ratio (volume %) of phosphate-based glass to phenolic resin was 2.5:5, the iron loss value was 1,100 and the proportion of binder layers of 20 nm or less was 0.57%, both of which were good values. On the other hand, in Comparative Example 4-2, in which the compounding ratio (volume %) of phosphate glass and phenolic resin was 2.5:10, the iron loss value was 2100, but the proportion of binder layers of 20 nm or less was 0%, and the powder filling rate was a low value of 84.2%.

[0075] [Table 4]

[0076] <Experiment 5> In Experiment 5, powder magnetic cores were produced by varying the compounding ratio of phosphate glass and phenolic resin, which are used as binder materials. In Experiment 5, nanocrystalline powder with a particle size of 11 μm (median diameter D50) was used as the magnetic powder. The same methods as in Experiment 1 were used to produce the powder magnetic cores and measure the samples. Table 5 shows the compounding ratio of phosphate glass and phenolic resin for each sample.

[0077] As shown in Table 5, in Examples 5-1 to 5-5, the iron loss was 2500 or less and the proportion of the binder layer of 20 nm or less was 6% or less (excluding 0), which were good values. In particular, in Example 5-3, in which the compounding ratio (volume %) of phosphate glass and phenolic resin was 2.5:2.5, the iron loss value was 860, which was a very good value. On the other hand, in Comparative Examples 5-1 to 5-3, the iron loss was 2500 or less, but the packing factor of the powder magnetic core was lower than 88 volume %, and the magnetic permeability was also low, at 78 or less. From the results of Experiments 4 and 5, it can be said that the total amount of low-melting-point glass and resin material relative to the magnetic powder is preferably less than 10% by volume.

[0078] [Table 5]

[0079] <Experiment 6> For Experiment 6, cylindrical samples with an outer diameter of 40 mm and varying vertical lengths (thickness h) were prepared. In Experiment 6, nanocrystalline powder with a particle size of 11 μm (median diameter D50) was used as the magnetic powder. Phosphate-based glass and phenolic resin were used as the binder materials. The volume ratio of the phosphate-based glass to the magnetic powder was 2.5 volume %, and the volume ratio of the phenolic resin to the magnetic powder was 2.5 volume %. The same method as in Experiment 1 was used to prepare the powder cores. In Experiment 6, the prepared powder cores were machined into the same shape as in Experiment 1 (a toroidal shape with an outer diameter of 13 mm, an inner diameter of 8 mm, and a length of 5 mm) to prepare measurement samples. The samples were then measured using the same method as in Experiment 1.

[0080] As shown in Table 6, the compaction time for each sample was changed depending on the thickness of the thinnest part. In other words, the thicker the thickness h, the longer the compaction time for the sample, so that heat would be transferred to the part of the powder core that takes the longest time to transfer heat, and so that heat would be transferred throughout the powder core. More specifically, the compaction time was set so that heat would be transferred to the middle part of the powder core's vertical length (thickness h), and sufficient deformation due to softening of the magnetic powder throughout the powder core would be achieved.

[0081] As shown in Table 6, in Example 6-1 with a thickness h of 1.7 mm, Example 6-2 with a thickness h of 2.5 mm, Example 6-3 with a thickness h of 3.0 mm, and Example 6-4 with a thickness h of 3.5 mm, the iron loss value was 2500 or less, and the proportion of binder layers of 20 nm or less was 6% or less (excluding 0). In particular, in Example 6-1 with a thickness h of 1.7 mm, the iron loss value was 860, which was a very good value.

[0082] On the other hand, in Comparative Example 6-1 with a thickness h of 4.5 mm, Comparative Example 6-2 with a thickness h of 7 mm, and Comparative Example 6-3 with a thickness h of 14 mm, the iron loss value was greater than 2500, and the proportion of binder layers of 20 nm or less was greater than 6%.

[0083] Based on these results, it can be concluded that the vertical length (thickness h) of the powder core, which is the portion of the powder core that takes the longest time for heat to transfer into the powder core when hot-molded, is preferably 3.5 mm or less. In other words, by rapidly transferring heat throughout the powder core during hot-molding, it is possible to prevent a decrease in the effect of suppressing the thermal decomposition of the binder resin and the fluidity of the low-melting-point glass, thereby achieving a good iron loss value. Furthermore, because heat is transferred quickly throughout the powder core, the hot-molding time can be shortened, thereby reducing manufacturing time and costs. Note that in Experiment 6, the vertical length of the powder core was varied. However, for the same reasons, it can also be concluded that it is preferable to set the distance between the molding dies in the direction approximately perpendicular to the direction of extension of the portion of the powder core that takes the longest time for heat to transfer into the powder core to 3.5 mm or less.

[0084] [Table 6]

[0085] <Experiment 7> In Experiment 7, samples were prepared by changing the type of low-melting-point glass used as the binder material. In Experiment 7, metallic glass powder with a particle size of 9 μm (median diameter D50), a first crystallization temperature (Tg) of 480°C, and a second crystallization temperature (Tx) of 510°C was used as the magnetic powder. Phenolic resin was used as the binder resin. The volume ratio of each low-melting-point glass to the magnetic powder was 2.5% by volume, and the volume ratio of the phenolic resin to the magnetic powder was 2.5% by volume. The same methods as in Experiment 1 were used to prepare the powder cores and measure the samples.

[0086] As shown in Table 7, in Example 7-1, which used phosphate-based glass as the low-melting point glass, and Example 7-2, which used tin-phosphate-based glass, the iron loss values ​​were 900 and 1600, respectively, and the proportions of the binder layer of 20 nm or less were 0.92% and 3.6%, respectively, which were good values.

[0087] On the other hand, in Comparative Example 7-1, which used bismuth oxide glass as the low-melting point glass, Comparative Example 7-2, which used borosilicate glass, and Comparative Example 7-3, which used barium silicate glass, the iron loss value was greater than 2500, and the proportion of binder layers of 20 nm or less was greater than 6%.

[0088] [Table 7]

[0089] FIG. 11 is a graph plotting the iron loss and the proportion of binder layers of 20 nm or less for samples in Experiments 1 to 7, where the binder amount and magnetic powder particle size were the same. In the graph shown in FIG. 11, the binder amount for the sample was 2.5 volume % low-melting-point glass and 2.5 volume % resin material relative to the magnetic powder, and the particle size of the magnetic powder was 9 μm. As shown in the graph in FIG. 11, the iron loss tended to increase as the proportion of binder layers of 20 nm or less increased. In the present invention, by setting the proportion of binder layers of 20 nm or less to 6% or less (excluding 0), the iron loss can be set to 2500 or less, and this range is the range of the embodiment.

[0090] The present invention has been described above in accordance with the above-mentioned embodiment, but the present invention is not limited to the configuration of the above-mentioned embodiment, and naturally includes various modifications, alterations, and combinations that a person skilled in the art can make within the scope of the invention as defined in the claims of this application. [Explanation of symbols]

[0091] 1 inductor 10_1, 10_2 Powder magnetic core 13 Coil 20 Granulated magnetic powder 21 Magnetic powder 22 Binder layer 25 Intermediate molding 31 Low-melting glass 32 Resin materials

Claims

1. A powder magnetic core in which magnetic powder is bound via a binder layer, The proportion of binder layers having a thickness of 20 nm or less among the binder layers present between the magnetic powder particles is 6% or less (excluding 0), a region in which the spacing between the magnetic powder particles is 200 nm or less exists over a length of 100 nm or more; The magnetic powder is a soft magnetic powder containing iron element, The iron loss of the powder magnetic core is 2500 kW / m ３ is as follows: The particle size of the magnetic powder is 25 μm or less, the binder layer has a median thickness of 68 nm or less; the binder layer contains a low-melting-point glass and a resin material, the total amount of the low-melting-point glass and the resin material is less than 10% by volume with respect to the magnetic powder; Powder magnetic core.

2. 2. The powder magnetic core according to claim 1, wherein the proportion of binder layers present between the magnetic powder particles that have a thickness of 20 nm or less is 3.3% or less.

3. 3. The powder magnetic core according to claim 1, wherein the powder magnetic core contains 88% by volume or more of magnetic powder.

4. The powder core according to claim 1 , wherein the magnetic powder is a metallic glass or a nanocrystalline powder.

5. 2. The powder magnetic core according to claim 1, wherein a volume ratio of the low-melting glass to the magnetic powder is 0.5% by volume or more and 6% by volume or less.

6. The powder magnetic core according to any one of claims 1 to 5, wherein a volume ratio of the resin material to the magnetic powder is 0.5 volume % or more and 9 volume % or less.

7. The powder magnetic core according to any one of claims 1 to 6, wherein the low-melting-point glass is a phosphate-based or tin-phosphate-based glass.

8. The powder magnetic core according to any one of claims 1 to 7, wherein the resin material is at least one selected from the group consisting of a phenolic resin, a polyimide resin, an epoxy resin, and an acrylic resin.

9. 9. The powder magnetic core according to claim 1, wherein, when the length of the powder magnetic core in the vertical direction is longer than 3.5 mm, the distance between the molding dies that sandwich the powder magnetic core in the horizontal cross section of the powder magnetic core when the powder magnetic core is hot-molded is 3.5 mm or less.

10. The powder magnetic core according to any one of claims 1 to 8, wherein the powder magnetic core has a length in the vertical direction of 3.5 mm or less.

11. An inductor comprising the powder magnetic core according to any one of claims 1 to 10 and a coil.

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