Powder magnetic core, and inductor

Abstract

A powder magnetic core capable of achieving low loss in the high-frequency range while reducing its size is provided. The powder magnetic core of the present invention is formed by bonding magnetic powder through an adhesive layer. The aforementioned powder magnetic core contains 88% by volume or more of magnetic powder, and the percentage of the portion of the aforementioned adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the particles of the aforementioned magnetic powder is equal to or less than 6% (excluding 0%).

Description

Technical Field

[0001] This invention relates to powder magnetic cores, inductors, and methods for manufacturing powder magnetic cores. Prior Technology

[0002] In recent years, inductors have been used in a variety of electronic devices. Specifically, there is a requirement for inductors used in electronic devices such as personal computers to be small in size and to exhibit high inductance characteristics even when large currents flow through them. Japanese Unexamined Patent Application Publication No. H10-212503 discloses a method for manufacturing a pressed powder of amorphous magnetic soft alloy having a low permeability reduction in the high-frequency range. Summary of the Invention

[0003] As mentioned above, inductors are required to have a small size and to exhibit high inductance even when large currents flow through them. Specifically, because inductors used in electronic devices such as personal computers operate in the high-frequency range (e.g., 750 kHz to 2 MHz), inductors with low losses in the high-frequency range are required.

[0004] In view of the aforementioned problems, the present invention aims to provide powder magnetic cores, inductors, and methods for manufacturing powder magnetic cores, all of which can achieve low loss in the high-frequency range while reducing the individual dimensions of powder magnetic cores and inductors.

[0005] According to one embodiment of the present invention, a powder core is formed by bonding magnetic powder through an adhesive layer, wherein the powder core contains 88% by volume or more of magnetic powder, and the percentage of the portion of the adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the particles of the magnetic powder is equal to or less than 6% (excluding 0%).

[0006] A method for manufacturing a powder magnetic core according to one embodiment of the present invention includes: a process of coating magnetic powder with low-melting-point glass; a process of coating the low-melting-point glass-coated magnetic powder with a resin material for granulation; and a process of thermoforming the granulated magnetic powder. The thermoformed body contains 88% by volume or more of magnetic powder, including a binder layer of low-melting-point glass and resin material formed between the particles of magnetic powder, and the percentage of the binder layer with a thickness of 20 nm or less in the binder layer present between the particles of magnetic powder is equal to or less than 6% (excluding 0%).

[0007] According to the present invention, a powder core, an inductor, and a method for manufacturing a powder core can be provided, the foregoing of which can achieve low loss in the high frequency range while reducing the individual dimensions of the powder core and the inductor.

[0008] The above and other objects, features and advantages of the present invention will be more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only, and therefore should not be construed as limiting the invention. Simple Explanation of the Diagram

[0009] [Figure 1] is a perspective view showing an example of an inductor according to an embodiment. [Figure 2] shows an electron micrograph of a powder magnetic core according to the relevant art and a powder magnetic core according to the present invention. [Figure 3] is a schematic diagram illustrating the microstructure of a powder magnetic core according to the relevant field and the microstructure of a powder magnetic core according to the present invention. [Figure 4] shows an electron micrograph of the microstructure of the powder magnetic core according to an embodiment. [Figure 5] is a flowchart describing a method for manufacturing a powder magnetic core according to an embodiment. [Figure 6] is a schematic diagram illustrating a method for manufacturing a powder magnetic core according to an embodiment. [Figure 7] is a horizontal cross-sectional view of the powder magnetic core according to an embodiment. [Figure 8] is a horizontal cross-sectional view of the powder magnetic core according to an embodiment. [Figure 9] is a horizontal cross-sectional view of the powder magnetic core according to an embodiment. [Figure 10] is a horizontal cross-sectional view of the powder magnetic core according to an embodiment. [Figure 11] is a graph showing the iron loss of the sample and the percentage of the binder layer at 20 nm or less, plotted with the same amount of binder and the same particle size of magnetic powder. Implementation

[0010] <Inductor>

[0011] In the following description, embodiments of the present invention will be illustrated with reference to the accompanying drawings.

[0012] Figure 1 is a perspective view showing an example of an inductor according to this embodiment. As shown in Figure 1, the inductor 1 according to this embodiment includes powder cores 10_1 and 10_2 and a coil 13. The powder core 10_1 is disposed around the outside of the coil 13, and the powder core 10_1 includes a cavity penetrating its center in a vertical direction. The powder core 10_2 disposed inside the coil 13 is disposed in a recessed portion of the coil 13 having a U-shaped cross-section.

[0013] For example, the inductor 1 shown in FIG1 is formed by the following steps: a powder core 10_2 is disposed in a recessed portion of a coil 13, and a mating powder core 10_1 is pressed in from above. Thus, an inductor 1 comprising a coil 13 surrounded by powder cores 10_1 and 10_2 can be formed. Powder cores 10_1 and 10_2 may also be collectively referred to as powder core 10 in this description. Furthermore, the structure of the inductor 1 shown in FIG1 is only one example, and the powder core 10 according to this embodiment can be used in inductors with structures different from those shown in FIG1. ​​The powder core according to this embodiment achieves low loss in the high-frequency range while reducing its size. The powder core according to this embodiment will be described in detail below. <Powder Core>

[0014] According to this embodiment, the powder core is a powder core in which magnetic powder is bonded together via an adhesive layer. The powder core contains 88% by volume or more of magnetic powder, and the adhesive layer, with a thickness of 20 nm or less, constitutes a percentage of 6% (excluding 0%) of the adhesive layer present between the magnetic powder particles. With this structure, it is possible to provide a powder core that achieves low loss in the high-frequency range while reducing its size. Preferably, the percentage of the adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the magnetic powder particles may be equal to or less than 3.3%.

[0015] The magnetic powder used in the powder core according to this embodiment is a soft magnetic powder containing iron. For example, the particle size of the magnetic powder is equal to or greater than 2 μm but equal to or less than 25 μm, preferably equal to or greater than 5 μm but equal to or less than 15 μm. In this invention, the particle size is the median diameter D50. This is a value measured using a laser diffraction scattering method.

[0016] In this embodiment, metallic glass can be used as the magnetic powder. The metallic glass can be, for example, an amorphous metallic glass prepared by atomization. It can be, for example, an 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. By pulverizing it using atomization, a metallic glass with a glass transition temperature can be formed. In this invention, specifically, Fe-BP-Nb-Cr based materials are preferred. The metallic glass obtained by atomization is not limited to this and can also be a metallic glass without a glass transition temperature.

[0017] Furthermore, in this embodiment, nanocrystalline powder can be used as the magnetic powder, for example. The nanocrystalline powder can be a powder prepared by an atomization method. For example, by pulverizing Fe-Si-BPC-Cu-based materials, Fe-Si-B-Cu-Cr-based materials, Fe-Si-BP-Cu-Cr-based materials, Fe-BPC-Cu-based materials, Fe-Si-BP-Cu-based materials, Fe-BP-Cu-based materials, or Fe-Si-B-Nb-Cu-based materials using an atomization method, nanocrystalline powder comprising at least two exothermic peaks can be formed, the exothermic peaks indicating crystallization during the heat treatment process of the magnetic powder. The nanocrystalline powder to be used (without specific limitation) is preferably, for example, an Fe-Si-BP-Cu-Cr-based material.

[0018] In this embodiment, the closer the magnetic powder particles are to a spherical shape, the better. When the sphericity of the particles is low, protrusions form on the particle surface. When molding pressure is applied, stress from surrounding particles concentrates on these protrusions, causing the coating to crack and failing to maintain sufficiently high insulation. This can lead to a deterioration (specifically, loss) in the magnetic properties of the resulting powder core. By adjusting the manufacturing conditions of the magnetic powder, such as the volume and pressure of the high-pressure water jet used for atomization in the case of a water atomization method, the temperature and supply rate of the molten material, the sphericity of the particles can be controlled within a suitable range. Specific manufacturing conditions vary depending on the composition of the magnetic powder to be manufactured or the desired productivity.

[0019] In the powder core according to this embodiment, the binder layer functions to bind the particles of magnetic powder together. The binder layer comprises low-melting-point glass and resin material. In this embodiment, the total amount of low-melting-point glass and resin material is less than 10% by volume relative to the amount of magnetic powder in the powder core. The low-melting-point glass may be phosphate-based glass, tin phosphate-based glass, borate-based glass, silicate-based glass, borosilicate-based glass, barium silicate-based glass, bismuth oxide-based glass, germanate-based glass, vanadate-based glass, aluminum phosphate-based glass, arsenate-based glass, telluride-based glass, or the like. Specifically, in this invention, phosphate-based or tin phosphate-based low-melting-point glass is preferred. Furthermore, the volume percentage of the low-melting-point glass relative to the volume of the magnetic powder is equal to or greater than 0.5% by volume but equal to or less than 6% by volume, preferably equal to or greater than 1.25% by volume but equal to or less than 3% by volume.

[0020] Furthermore, the resin material included in the adhesive layer may be at least one type of resin material selected from the group consisting of phenolic resin, polyimide resin, epoxy resin, and acrylic resin. Additionally, the volume percentage of the resin material relative to the volume of the magnetic powder is equal to or greater than 0.5% by volume but equal to or less than 9% by volume, preferably equal to or greater than 1% by volume but equal to or less than 5% by volume.

[0021] The powder core according to this embodiment, having the aforementioned configuration, comprises 88% by volume or more magnetic powder, and the percentage of the binder layer with a thickness of 20 nm or less in the binder layer present between the magnetic powder particles is equal to or less than 6% (excluding 0%). Therefore, it is possible to reduce the thickness of the binder layer and thus increase the filling percentage of the magnetic powder while maintaining sufficiently high insulation between the magnetic powder particles. Therefore, with the powder core according to this embodiment, it is possible to reduce the inductor size while also reducing losses in the inductor in the high-frequency range.

[0022] Figure 2 shows electron micrographs of powder magnetic cores according to the relevant art and powder magnetic cores according to the present invention. In the relevant art shown in Figure 2, the filling percentage of magnetic powder is low. On the other hand, the filling percentage of magnetic powder in the powder magnetic core according to the present invention is higher than that in the powder magnetic core according to the relevant art. Therefore, the inductor still exhibits high inductance characteristics even when a large current flows through it.

[0023] Figure 3 is a schematic diagram illustrating the microstructure of a powder core according to the relevant art and the microstructure of a powder core according to the present invention. In the relevant art shown in Figure 3, the thickness of the binder layer 122 existing between the particles of the magnetic powder 121 is uneven. For example, although the thickness of the binder layer 122 is larger in region 131, the thickness of the binder layer 122 is smaller in regions 132 and 133. That is, in this case, the percentage of the portion of the binder layer with a thickness of 20 nm or less in the binder layer 122 existing between the particles of the magnetic powder 121 (i.e., the percentage of the thinner portion of the binder layer (such as regions 132 and 133)) is higher. Therefore, the percentage of the thicker portion of the binder layer 122 becomes higher.

[0024] On the other hand, in the powder core according to the invention, the thickness of the binder layer 22 existing between the particles of the magnetic powder 21 is uniform. That is, the percentage of the portion of the binder layer with a thickness of 20 nm or less in the binder layer 22 existing between the particles of the magnetic powder 21 (i.e., the percentage of the thinner portion of the binder layer) is small. Therefore, as a result, the percentage of the thicker portion of the binder layer 22 becomes smaller, and the thickness of the binder layer 22 becomes uniform overall. As an example, the median thickness of the binder layer 22 in the powder core according to the invention is 31 to 68 nm.

[0025] Figure 4, showing an electron micrograph of the microstructure of the powder core according to this embodiment, is a diagram illustrating a method for obtaining the percentage of the binder layer with a thickness of 20 nm or less present between the particles of the magnetic powder. When measuring the thickness of the binder layer, an electron micrograph (SEM image of the powder core) is used to designate an area in which the spaces between the magnetic powder particles are filled with binder, and for a length of 100 nm or greater, the gap between the magnetic powder particles is 200 nm or less. Then, within the designated area, the thickness of the binder layer is measured for every 100 nm. Figure 4 shows a measurement example in the right-hand side. The presence of binder between the particles of the magnetic powder can be determined using the contrast of the SEM image or the results of elemental analysis in energy dispersive X-ray spectroscopy (EDX). For example, the number of measurement points for the thickness of the adhesive layer is preferably 400 or greater. Note that the gap between magnetic powder particles can be measured by the following steps: assuming a normal at a point on the surface of a magnetic powder, and measuring the distance between two magnetic powder particles in the direction of the normal.

[0026] When, for example, the number of measurement points is 400 and the number of measurement points with an adhesive layer thickness equal to or less than 20 nm is 20, "the percentage of the portion of the adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the particles of the magnetic powder" = (20 / 400) × 100 = 5%

[0027] Please note that, as shown in the lower left of Figure 4, powder magnetic cores where the gaps between magnetic powder particles are not filled by the binder (even when the gaps are 200 nm or smaller (90 nm in the diagram)) are excluded from the measurement target. <Manufacturing Method of Powder Magnetic Cores>

[0028] Next, a method for manufacturing a powder magnetic core according to this embodiment will be described. Figure 5 is a flowchart illustrating the method for manufacturing a powder magnetic core according to this embodiment. Figure 6 is a schematic diagram illustrating the method for manufacturing a powder magnetic core according to this embodiment.

[0029] As shown in Figure 5, when preparing a powder magnetic core, magnetic powder is first prepared (step S1). The magnetic powder can be the aforementioned magnetic powder. Preferably, the magnetic powder is made from a magnetic material that softens at 400 °C or higher (a material that is easily deformable during heat forming). For example, amorphous magnetic powder can be obtained by vacuum melting the raw material of the magnetic powder and then simultaneously performing pulverization and quenching using a water atomization method. The magnetic powder thus obtained can be sorted as needed to remove abnormally coarse powder.

[0030] Next, magnetic powder is coated with low-melting-point glass (step S2). The low-melting-point glass is preferably made of a material that softens at 400 °C or higher (i.e., a material that softens during thermoforming and acts as an insulating or bonding material after thermoforming). The low-melting-point glass can be, for example, phosphate-based glass. When coating magnetic powder with low-melting-point glass, wet film formation methods such as mechanical fusion or sol-gel methods, or dry film formation methods such as sputtering, can be used. For example, according to the mechanical fusion method, a low-melting-point glass layer can be formed on the surface of the magnetic powder by mixing the magnetic powder with the low-melting-point glass powder while applying strong mechanical energy.

[0031] As an example, 1000 g of magnetic powder is mixed with 10 g of low-melting-point glass powder, and the magnetic powder is coated onto the low-melting-point glass using a mechanical fusion method. Therefore, the volume percentage of the low-melting-point glass coated with magnetic powder relative to the volume of the magnetic powder can be equal to or greater than 0.5% by volume but equal to or less than 6% by volume.

[0032] Next, the magnetic powder coated with low-melting-point glass is granulated by coating it with a resin material (step S3). This resin material can be any of the aforementioned resin materials. Preferably, the resin material is made of a material that softens at approximately 100°C and acts as an insulating or bonding material after heat forming. Furthermore, the resin material is preferably a material that does not easily decompose during heat forming (at high temperatures). When coating the magnetic powder with the resin material (granulation), a rolling granulation method, a spray drying method, or similar methods can be used. Specifically, by mixing and drying the article obtained by dissolving the resin material in an organic solvent with the magnetic powder coated with low-melting-point glass, a resin layer can be formed on the low-melting-point glass of the magnetic powder.

[0033] Figure 6 shows the granulated magnetic powder 20 on the left. As shown in Figure 6, in the granulated magnetic powder 20, the magnetic powder 21 is coated with a low-melting-point glass 31, and further, the low-melting-point glass 31 is coated with a resin material 32. As an example, the diameter of the magnetic powder 21 is 9 μm, the thickness of the low-melting-point glass 31 is 20 nm, and the thickness of the resin material is 20 nm.

[0034] Next, the magnetic powder after preforming and granulation is processed (step S4). For example, preforming can be performed by the following steps: placing the granulated magnetic powder into a mold for pressurization (e.g., 500 kgf / cm² at room temperature); heating the pressed powder body (i.e., the green body) to a predetermined temperature (e.g., 100°C to 150°C); and curing the pressed powder body without pressurization. When the resin material used is a thermosetting resin, the intermediate molded body is formed by the curing of the resin during heating. When the resin material used is a thermoplastic resin, the intermediate molded body is formed by the softening of the resin during heating and its solidification during cooling.

[0035] That is, as shown in the central diagram of Figure 6, when the magnetic powder is pre-formed and granulated, the particles of the magnetic powder 21 (coated with low-melting-point glass 31) are bonded together by the outermost resin material 32, forming an intermediate molded body 25. Because the low-melting-point glass does not soften at the pre-forming temperature (e.g., 150 °C), it does not exhibit bonding and flow properties. Note that the pre-forming process (step S4) can be omitted.

[0036] Next, the pre-formed intermediate molded body (or granulated magnetic powder if step S4 is omitted) undergoes heat forming (step S5). While placed in a mold, heat forming is performed under pressure by heating the pre-formed intermediate molded body (or granulated magnetic powder). For example, the heating temperature is set as follows.

[0037] When the magnetic powder used is metallic glass, the temperature at which the magnetic powder is heated and formed is set to be equal to or higher than the higher of the softening temperature of the low-melting-point glass and the glass transition temperature of the magnetic powder, but equal to or lower than the crystallization temperature of the magnetic powder. By setting the heating and forming temperature to be equal to or higher than the glass transition temperature of the magnetic powder, plastic deformation of the magnetic powder is more likely to occur, thereby achieving a high filling percentage of the magnetic powder. As an example, the heating and forming temperature is equal to or higher than 450 °C but equal to or lower than 500 °C.

[0038] When the magnetic powder used is a nanocrystalline powder, the temperature at which the magnetic powder undergoes heat forming is set to a temperature that is equal to or higher than the higher of the softening temperature of the low-melting-point glass and the first crystallization temperature of the magnetic powder, but equal to or lower than the second crystallization temperature of the magnetic powder. By setting the heat forming temperature to approximately the first crystallization temperature, the α-Fe phase crystallizes, and at the same time, plastic deformation of the magnetic powder becomes easier to occur, thereby obtaining a high filling percentage of the magnetic powder. As an example, the heat forming temperature is set to be equal to or higher than 400 °C but equal to or lower than 500 °C. Furthermore, in this invention, the heat forming temperature is preferably equal to or higher than the higher of the softening temperature of the low-melting-point glass and the first crystallization temperature of the magnetic powder + 40 °C. The first crystallization temperature and the second crystallization temperature are defined as follows: that is, the heat treatment of the magnetic powder having an amorphous structure results in crystallization occurring more than once. The temperature at which crystallization first begins is the first crystallization temperature, and the temperature at which subsequent crystallization begins is the second crystallization temperature. More specifically, the magnetic powder includes at least two exothermic peaks, which exhibit crystallization during heating as shown in the DSC curve obtained by differential scanning calorimetry (DSC). Among the exothermic peaks, the peak on the lowest temperature side indicates the first crystallization temperature during the crystallization of the α-Fe phase, and the next exothermic peak indicates the second crystallization temperature during the crystallization of the boride or similar.

[0039] In this embodiment, the heating temperature is preferably set to a temperature within the aforementioned temperature range, and the temperature conditions preferably reduce the iron loss of the powder core.

[0040] Furthermore, the pressure during thermoforming is, for example, 5 to 10 ton-f / cm². If the pressure is too low, the filling percentage of the formed body (powder core) becomes low and the iron loss of the powder core increases. On the other hand, if the pressure is too high, the mold will wear severely, which is undesirable in terms of cost. Therefore, the pressure is preferably set within the aforementioned range.

[0041] Furthermore, the heating and forming process is preferably performed within the range of 5 to 60 seconds, and more preferably, equal to or less than 30 seconds. If the forming time is too short, the heat does not sufficiently reach the interior of the molded body, and sufficient deformation caused by the softening of the magnetic powder cannot be achieved. Consequently, the fill percentage of the molded body decreases, and the iron loss of the powder core increases. On the other hand, if the forming time is too long, the thermal decomposition of the resin material used for the adhesive layer occurs prematurely, thereby reducing the effectiveness of suppressing the flow properties of low-melting-point glass and increasing the iron loss of the powder core. Therefore, the heating and forming time can be set within a range in which: heat is sufficiently transferred to the interior of the molded body, deformation caused by the softening of the magnetic core is completed, the thermal decomposition of the resin material used for the adhesive layer does not occur prematurely, and the cost is not high. The forming time is preferably set within the aforementioned range.

[0042] As an example, heat forming can be performed at a heat forming temperature of 480 °C, a heat forming pressure of 8 ton∙f / cm², and a heat forming time of 10 seconds.

[0043] As shown in the right-hand view of Figure 6, in the formed body (powder core) 10 after heat forming, the particles of magnetic powder 21 are bonded together through an adhesive layer 22 comprising a low-melting-point glass and a resin material. In this embodiment, the volume percentage of the magnetic powder particles contained in the powder core 10 is set to 88% by volume or higher. Furthermore, the percentage of the adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the magnetic powder particles is set to be equal to or less than 6%. Therefore, it is possible to increase the filling percentage of magnetic powder while maintaining a sufficiently high level of insulation between the magnetic powder particles. Thus, by the powder core manufacturing method according to this embodiment, it is possible to prepare a powder core that achieves low loss in the high-frequency range while reducing its size.

[0044] As described in the prior art, inductors are required to have small dimensions and exhibit high inductance even when large currents flow through them. Furthermore, inductors with low losses in the high-frequency range are also required. To provide an inductor that meets these conditions, the powder core used in the inductor must have a high percentage of magnetic powder filling while maintaining sufficiently high insulation between the magnetic powder particles. However, according to relevant art, it is difficult to increase the percentage of magnetic powder filling while maintaining sufficiently high insulation between the magnetic powder particles.

[0045] On the other hand, in the method for manufacturing the powder core according to this embodiment, the binder layer is formed using low-melting-point glass and resin material. In this way, by using low-melting-point glass and resin material as binders, a thin binder layer (insulating layer) with uniform thickness can be formed even when the amount of binder added is small. That is, by using a binder component (low-melting-point glass) with a tendency to flow and a binder component (resin material) with a tendency to not flow in a mixed manner at the heating forming temperature, a sufficiently high level of insulation between the magnetic powder particles can be maintained even when the amount of binder added is small. In other words, according to this embodiment, by intentionally leaving resin during heating forming, the flow of the low-melting-point glass, which is relatively softer than the magnetic powder, can be suppressed to some extent, preventing the magnetic powder particles from contacting each other without using the binder layer (insulating layer).

[0046] Furthermore, in the method for manufacturing the powder core according to this embodiment, the amount of resin material used as a binder is set to be relatively small, thereby reducing the amount of gas generated due to the decomposition of the resin material during heating and molding. Therefore, it is possible to prevent cracks from occurring in the molded body (powder core) due to the generated gas.

[0047] In this embodiment, the iron loss of the powder core is preferably 2500 kW / m³ or less, more preferably 1500 kW / m³ or less. <Dimensions of the powder core>

[0048] Next, the dimensions of the powder core according to this embodiment will be described.

[0049] In this embodiment, when the length of the powder core in the vertical direction (distance h in the example shown in Figure 1) is greater than 3.5 mm, the distance between the inner walls of the molds in one direction within the horizontal cross-section of the powder core when the powder core is held by the mold is set to be equal to or less than 3.5 mm. This direction is generally perpendicular to the direction in which the portion of the powder core with the longest heat transfer time required during the heating and forming of the powder core extends. The dimensions of the powder core will be described below with specific examples.

[0050] When, for example, the shape of the horizontal cross-section of the powder core is as shown in the powder core 10_1 in FIG. 7 (the powder core 10_1 shown in FIG. 7 corresponds to the powder core 10_1 shown in FIG. 1), the powder core 10_1 is formed in a state held by the molding die 61 during heat forming. At this time, heat is transferred from the molding die 61 to the powder core 10_1, and the part inside the powder core 10_1 that is least likely to transfer heat is the part indicated by reference numeral 71. In this embodiment, the distance b between the inner walls of the molding die in one direction is set to be equal to or less than 3.5 mm, and the aforementioned direction is generally perpendicular to the direction in which the part 71 inside the powder core 10_1 with the longest required heat transfer time extends. By giving the powder core the aforementioned dimensions, heat can be rapidly transferred to the entire powder core 10_1 during heat forming.

[0051] Furthermore, when the shape of the horizontal cross-section of the powder core is, for example, as shown in the powder core 52 in FIG. 8 (i.e., a shape without a cavity at the center), the powder core 52 is formed during heat forming while it is held by the molding die 62. In this case, heat is transferred from the molding die 62 to the powder core 52, and the part inside the powder core 52 that is least likely to transfer heat is indicated by reference numeral 72. In this embodiment, the distance b2 between the inner walls of the molding die in one direction is set to be equal to or less than 3.5 mm, and this direction is generally perpendicular to the direction in which the part 72 inside the powder core 52 with the longest required heat transfer time extends. By giving the powder core the aforementioned dimensions, heat can be rapidly transferred to the entire powder core 52 during heat forming.

[0052] Furthermore, when the shape of the horizontal cross-section of the powder core is, for example, as shown in Figure 9 (i.e., a shape with two cavities at the center), the powder core 53 is formed in a state held by the molding die 63 during thermoforming. In this case, heat is transferred from the molding die 63 to the powder core 53, and the part inside the powder core 53 that is least likely to transfer heat is indicated by reference numeral 73. In this embodiment, the distance b3 between the inner walls of the molding die in one direction is set to be equal to or less than 3.5 mm, and this direction is generally perpendicular to the direction in which the part 73 inside the powder core 53 with the longest required heat transfer time extends. By giving the powder core the aforementioned dimensions, heat can be rapidly transferred to the entire powder core 53 during thermoforming.

[0053] Furthermore, when the shape of the horizontal cross-section of the powder core is, for example, the shape shown in the powder core 54 as in FIG. 10 (i.e., E-type core), the powder core 54 is formed in a state held by the molding die 64 during thermoforming. In this case, heat is transferred from the molding die 64 to the powder core 54, and the part inside the powder core 54 that is least likely to transfer heat is indicated by reference numeral 74. In this embodiment, the distance b4 between the inner walls of the molding die in one direction is set to be equal to or less than 3.5 mm, and this direction is generally perpendicular to the direction in which the part 74 inside the powder core 54 with the longest required heat transfer time extends. By giving the powder core the aforementioned dimensions, heat can be rapidly transferred to the entire powder core 54 during thermoforming.

[0054] Please note that the configuration examples shown in Figures 7 to 10 are merely examples, and the dimensions of the powder core according to this embodiment can also be applied to powder cores with other structures. Furthermore, when, for example, the horizontal cross-sectional shape of the powder core is circular, the portion inside the powder core 54 with the longest required heat transfer time is a single point. In this case, the diameter of the circle passing through this point is set to 3.5 mm or less. Additionally, in this embodiment, the length of the powder core in the vertical direction can be equal to or less than 3.5 mm. In this way, when the length of the powder core in the vertical direction is set to be equal to or less than 3.5 mm, the distance between the inner walls of the molding die in the horizontal cross-section of the powder core can be set to the desired value.

[0055] As described above, by giving the powder core according to this embodiment the aforementioned dimensions, heat can be easily transferred to the powder core during thermoforming. Therefore, it is possible to reduce thermoforming time and prevent thermal decomposition of the resin material. Consequently, the effect of suppressing the flow properties of low-melting-point glass is improved, and the iron loss of the powder core can be reduced. [Example]

[0056] Next, an example according to the present invention will be described. <Experiment 1>

[0057] The sample from Experiment 1 was prepared using the aforementioned powder core manufacturing method (see Figure 5). The powder core from Experiment 1 was formed as a ring 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. Fe-BP-Nb-Cr based powder (a metallic glass powder with a particle size of 9 μm (median diameter D50)) was used as the magnetic powder. Next, the magnetic powder and low-melting-point glass powder were mixed, and the magnetic powder was coated with low-melting-point glass using a mechanical fusion method. Phosphate-based glass was used as the low-melting-point glass. At this point, 2.5% by volume of the low-melting-point glass was mixed with the magnetic powder.

[0058] Subsequently, magnetic powder coated with low-melting-point glass was granulated using resin materials. Each of the resins listed in Table 1 was used as the resin material. At this time, 2.5% by volume of each resin material was mixed with the magnetic powder. The "Loss of Resin When Heated at 500 °C" in Table 1 indicates the results of thermogravimetric analysis of the resin (measurement conditions: air atmosphere, heating rate 100 °C / min), which shows that the smaller the loss during heating, the higher the heat resistance of the resin.

[0059] Next, the granulated magnetic powder is placed in a mold and pressurized at 500 kgf / cm², then heated and pressed, and solidified at 150 °C without pressure to form a pre-formed intermediate body. Subsequently, the pre-formed intermediate body is subjected to heat forming while still placed in the mold. The heat forming is performed at a forming temperature of 490 °C, a pressing pressure of 8 tonf / cm², and a pressing time of 30 seconds.

[0060] For each of the samples prepared as described above, the following measurements were taken: powder filling percentage of the magnetic core, permeability, iron loss, percentage of the binder layer with a thickness of 20 nm or less in the binder layer present between the magnetic powder particles, and median thickness of the binder layer. The number of measurement points for the thickness of the binder layer was 1000.

[0061] The powder filling percentage of the magnetic core is obtained by comparing the volume of the magnetic powder included in the core with the volume of the entire core measured by Archimedes' method. The volume of the magnetic powder included in the core is obtained by the following steps: first, by subtracting the weight of the low-melting-point glass added as a binder and the remaining resin material from the weight of the entire core to obtain the weight of the magnetic powder included in the core; then, the weight of the magnetic powder is divided by the true density of the magnetic powder.

[0062] The permeability was obtained using an impedance analyzer at a frequency of 1 MHz, and the iron loss was obtained by fabricating a toroidal powder core and measuring the fabricated powder core using a BH analyzer (manufactured by IWATSU ELECTRIC Ltd.) via the double-coil method. The measurements were performed under sinusoidal excitation at 1 MHz and 50 mT.

[0063] The percentage of the adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the particles of the magnetic powder (hereinafter, this percentage will be referred to as "the percentage of the 20 nm or less portion of the adhesive layer") was measured using electron microscopy using the aforementioned method. Furthermore, the median thickness of the adhesive layer was also measured using electron microscopy.

[0064] Table 1 shows the type of resin used in each sample and the measurement results for each sample. As shown in Table 1, in Examples 1-1 (using phenolic resin as the adhesive resin), 1-2 (using polyimide resin as the adhesive resin), 1-3 (using epoxy resin as the adhesive resin), and 1-4 (using acrylic resin as the adhesive resin), the iron loss value became equal to or less than 1100, which is considered good. Furthermore, in Examples 1-1 to 1-4, the percentage of the adhesive layer in the 20 nm or smaller portion was equal to or less than 2.2%, which is considered good. Specifically, in Examples 1-1 to 1-3, the percentage of the adhesive layer in the 20 nm or smaller portion was less than 1% and the iron loss value was less than 1000.

[0065] On the other hand, in Comparative Example 1-1 using polysiloxane as the adhesive resin, Comparative Example 1-2 using polyvinyl butyral (PVB) resin as the adhesive resin, and Comparative Example 1-3 without using resin, the iron loss value is equal to or greater than 5500, which is a relatively large value.

[0066] Based on the above results, it can be said that phenolic resin, polyimide resin, epoxy resin, and acrylic resin are preferred as the resins to be used in the adhesive layer. [Table 1] Experiment 1 Resin type Resin loss when heated to 500°C Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Example 1-1 phenol 14% 93.7% 121 900 0.92% 40 Example 1-2 Polyimide 3% 91.9% 118 780 0.83% 37 Example 1-3 Epoxy resin 65% 92.3% 157 970 0.95% 42 Examples 1-4 acrylic acid 73% 92.8% 182 1,100 2.2% 34 Comparison Example 1-1 Polysiloxane 19% 91.4% 108 10,000 10.2% 33 Comparison Examples 1-2 PVB 83% 93.1% 188 5,500 8.3% 31 Comparison Examples 1-3 Resin-free - 95.9% 192 17,000 13.3% 27 <Experiment 2>

[0067] In Experiment 2, powder magnetic cores were prepared by altering the particle size (median diameter D50) of the metallic glass powder (magnetic powder). In Experiment 2, phosphate-based glass and phenolic resin were used as binders. Powder magnetic cores were prepared using a method similar to that in Experiment 1, and the samples were measured. In Comparative Example 2-1 and Example 2-2, the volume percentage of phosphate-based glass relative to the volume of magnetic powder was set to 5% by volume, and the volume percentage of phenolic resin relative to the volume of magnetic powder was set to 2.5% by volume. In Example 2-2, the volume percentage of phosphate-based glass relative to the volume of magnetic powder was set to 2.5% by volume, and the volume percentage of phenolic resin relative to the volume of magnetic powder was set to 2.5% by volume. Furthermore, as shown in Table 2, because the softening temperature of 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, the forming temperature was set to 490 °C.

[0068] As shown in Table 2, in Comparative Example 2-1 with a metallic glass powder particle size of 4 μm, the iron loss value was 12000 and the percentage of the adhesive layer with a particle size of 20 nm or smaller was 1.3%, both of which are relatively high. On the other hand, in Example 2-1 with a metallic glass powder particle size of 7 μm and Example 2-2 with a metallic glass powder particle size of 9 μm, the iron loss values ​​were 1100 and 900, respectively, which are good values. Furthermore, the percentage of the adhesive layer with a particle size of 20 nm or smaller was 1.7% in Example 2-1 and 0.92% in Example 2-2, which are also good values. Therefore, in Experiment 2, when the metallic glass powder particle size was 7 μm or larger, both the iron loss and the percentage of the adhesive layer with a particle size of 20 nm or smaller were good.

[0069] Although phosphate-based glass and phenolic resin were used as adhesive materials in Experiment 2, the inventors also conducted experiments using 5% by volume of phosphate-based glass and 2.5% by volume of polyimide resin relative to the volume of the magnetic powder as adhesives. It was confirmed that, under these conditions, even when the particle size of the metallic glass (magnetic powder) was 2 μm, the filling percentage of the powder core became equal to or greater than 88% by volume, the percentage of the adhesive layer at 20 nm or smaller was equal to or less than 6%, and the iron loss was equal to or less than 2500. [Table 2] Experiment 2 Types of magnetic powder Particle size D50 of magnetic powder Glass transition temperature (°C) Crystallization temperature (°C) Molding temperature (°C) Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Comparison Example 2-1 Metallic glass 4 480 510 490 89.3 75 12000 13.50% 27 Example 2-1 Metallic glass 7 480 510 490 90.6 103 1100 1.7% 43 Example 2-2 Metallic glass 9 480 510 490 93.7 120 900 0.92% 40 -Including the manufacturing conditions of the magnetic powder, such as in Experiment 3 at high frequencies using water atomization methods.

[0070] In Experiment 3, powder magnetic cores were prepared, with the particle size (median diameter D50) of the nanocrystalline powder (Fe-Si-BP-Cu-Cr based magnetic powder) modified. In Experiment 3, phosphate-based glass and phenolic resin were used as binders. Powder magnetic cores were prepared using a method similar to that in Experiment 1, and the samples were measured. In Example 3, the volume percentage of phosphate-based glass relative to the volume of the magnetic powder was set to 2.5% by volume, and the volume percentage of phenolic resin relative to the volume of the magnetic powder was also set to 2.5% by volume. Furthermore, as shown in Table 3, the forming temperature was set to 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.

[0071] As shown in Table 3, in Example 3-1 with a nanocrystalline powder particle size of 11 μm, Example 3-2 with a nanocrystalline powder particle size of 14 μm, and Example 3-3 with a nanocrystalline powder particle size of 23 μm, the iron loss value is equal to or less than 2500, and the percentage of the adhesive layer with a particle size of 20 nm or less is 1% or less, which are considered good values. Specifically, in Example 3-1 with a nanocrystalline powder particle size of 11 μm, the iron loss value is 860, which is excellent. On the other hand, in Comparative Example 3-1 with a nanocrystalline powder particle size of 41 μm, the iron loss value is 5300, which is relatively large, and the percentage of the adhesive layer with a particle size of 20 nm or less becomes 0%.

[0072] The results of Experiments 2 and 3 show that if the particle size is too small, the median thickness of the binder layer becomes too thin, thereby failing to ensure sufficient insulation between the magnetic powder particles, and the iron loss of the powder core increases due to eddy current losses between the magnetic powder particles. On the other hand, if the particle size is too large, the median thickness of the binder layer increases, thereby ensuring sufficient insulation between the magnetic powder particles, but at the same time, the iron loss of the powder core increases due to eddy current losses within the magnetic powder particles. Based on the above experiments, the preferred particle size of the magnetic powder is equal to or greater than 2 μm but equal to or less than 25 μm, more preferably equal to or greater than 5 μm but equal to or less than 15 μm. [Table 3] Experiment 3 Types of magnetic powder Particle size D50 of magnetic powder First crystallization temperature (°C) Second crystallization temperature (°C) Molding temperature (°C) Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Example 3-1 Nano crystals 11 420 510 470 92.9 115 860 0.62% 46 Example 3-2 Nano crystals 14 400 490 460 92 118 1300 0.27% 58 Example 3-3 Nano crystals twenty three 350 470 440 94.3 114 2500 0.15% 85 Comparative Example 3-1 Nano crystals 41 400 510 480 93.6 100 5300 0% (220) Experiment 4

[0073] In Experiment 4, powder magnetic cores were prepared by altering the blending ratio of phosphate-based glass (the binder material) to phenolic resin. In Experiment 4, metallic glass powder with a particle size of 9 μm (median diameter D50) was used as the magnetic powder. Powder magnetic cores were prepared using a method similar to that in Experiment 1, and the samples were measured. Table 4 shows the blending ratio of phosphate-based glass and phenolic resin in 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 added), the iron loss was 17000 and the percentage of the adhesive layer in the 20 nm or smaller portion was 13.3%, both of which were relatively high. 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 was 900 and the percentage of the adhesive layer in the 20 nm or smaller portion was 0.92%, both of which are favorable. In Example 4-2, where the blending ratio (volume %) of phosphate-based glass to phenolic resin was 2.5:5, the iron loss was 1100 and the percentage of the adhesive layer in the 20 nm or smaller portion was 0.57%, both of which are favorable. On the other hand, in Comparative Example 4-2, where the blending ratio (volume %) of phosphate-based glass and phenolic resin was 2.5:10, the iron loss value was 2100, but at the same time, the percentage of the 20 nm or smaller portion of the adhesive layer was 0% and the powder filling percentage was 84.2%, which are relatively small. [Table 4] Experiment 4 Types of magnetic powder The ratio of glass to magnetic powder is 100. The ratio of resin to magnetic powder is 100. Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Comparison Example 4-1 Metallic glass 2.5 0 95.9 180 17000 13.3% 27 Example 4-1 Metallic glass 2.5 2.5 93.3 122 900 0.92% 40 Example 4-2 Metallic glass 2.5 5 89.0 84 1100 0.57% 68 Comparative Example 4-2 Metallic glass 2.5 10 84.2 52 2100 0% 131 <Experiment 5>

[0075] In Experiment 5, powder magnetic cores were prepared in a modified ratio of phosphate-based glass (the binder material) to phenolic resin. In Experiment 5, nanocrystalline powder with a particle size of 11 μm (median diameter D50) was used as the magnetic powder. Powder magnetic cores were prepared using a method similar to that in Experiment 1, and the samples were measured. Table 5 shows the phosphate-based glass to phenolic resin ratio for each component in the samples.

[0076] As shown in Table 5, in Examples 5-1 to 5-5, the iron loss is equal to or less than 2500, and the percentage of the adhesive layer at 20 nm or less is equal to or less than 6% (excluding 0%), which are considered good values. Specifically, in Example 5-3, where the mixing ratio (volume %) of phosphate-based glass and phenolic resin is 2.5:2.5, the iron loss is 860, which is excellent. On the other hand, in Comparative Examples 5-1 to 5-3, the iron loss is equal to or less than 2500, but the filling percentage of the powder core is less than 88% by volume and the permeability is also equal to or less than 78, which are considered low values.

[0077] Based on the results of experiments 4 and 5, it can be said that the total amount of low-melting-point glass and resin materials relative to the amount of magnetic powder is preferably less than 10% by volume. [Table 5] Experiment 5 Types of magnetic powder The ratio of glass to magnetic powder is 100. The ratio of resin to magnetic powder is 100. Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Example 5-1 Nano crystals 0.63 2.5 91.1 134 1600 4.1% 32 Example 5-2 Nano crystals 1.25 2.5 92.2 128 900 1.1% 35 Example 5-3 Nano crystals 2.5 2.5 92.9 115 860 0.62% 46 Example 5-4 Nano crystals 5 2.5 88.9 95 1600 0.83% 58 Comparative Example 5-1 Nano crystals 7.5 2.5 86.9 78 2000 0.18% 70 Example 5-5 Nano crystals 1.25 5 90.1 109 1400 0.78% 63 Comparative Example 5-2 Nano crystals 5 5 86.3 69 2200 0% 88 Comparative Example 5-3 Nano crystals 0.63 10 85.9 65 2500 0% 105 <Experiment 6>

[0078] In Experiment 6, a cylindrical sample with an outer diameter of 40 mm was prepared, and the length (thickness h) of the sample in the vertical direction was varied. In Experiment 6, nanocrystalline powder with a particle size of 11 μm (median diameter D50) was used as the magnetic powder. Furthermore, phosphate-based glass and phenolic resin were used as binder materials. The volume percentage of phosphate-based glass relative to the volume of the magnetic powder was set to 2.5% by volume, and the volume percentage of phenolic resin relative to the volume of the magnetic powder was also set to 2.5% by volume. A powder core was prepared using a method similar to that in Experiment 1. In Experiment 6, the prepared powder core was cut into a shape similar to that in Experiment 1 (a ring with an outer diameter of 13 mm, an inner diameter of 8 mm, and a length of 5 mm) and a sample for measurement was prepared. The sample was then measured using a method similar to that in Experiment 1.

[0079] As shown in Table 6, the forming time for each sample varies depending on the thickness of the smallest portion. That is, as the thickness h increases, the forming time of the sample becomes longer, resulting in the longest heat transfer time required to transfer heat to the interior of the powder core and to the entire powder core. More specifically, the forming time is set such that heat is transferred to the middle portion of the vertical length (thickness h) of the powder core and sufficient deformation caused by the softening of the magnetic powder is achieved throughout the powder core.

[0080] 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 is equal to or less than 2500, and the percentage of the adhesive layer at 20 nm or less is equal to or less than 6% (excluding 0%). Specifically, in Example 6-1 with a thickness h of 1.7 mm, the iron loss value is 860, which is excellent.

[0081] On the other hand, in Comparative Example 6-1 with a thickness of 4.5 mm, Comparative Example 6-2 with a thickness of 7 mm and Comparative Example 6-3 with a thickness of 14 mm, the value of iron loss becomes greater than 2500 and the percentage of the adhesive layer of 20 nm or less becomes greater than 6%.

[0082] Based on the above results, it can be said that the length (thickness h) of the powder core in the vertical direction, i.e., the part inside the powder core where the heat transfer time is longest during the heating and forming of the powder core, is preferably equal to or less than 3.5 mm. That is, heat is rapidly transferred to the entire powder core during heating and forming, thereby suppressing the thermal deformation of the binder resin and preventing a decrease in the effect of suppressing the flow properties of low-melting-point glass, and obtaining a good value for iron loss. In addition, because heat is rapidly transferred to the entire powder core, the heating and forming time can be shortened, thereby reducing production time and cost. Although Experiment 6 was conducted with variations in the length of the powder core in the vertical direction, for reasons similar to those stated above, it is also preferable to set the distance between the forming molds in a certain direction to be equal to or less than 3.5 mm, the aforementioned direction being generally perpendicular to the direction in which the part inside the powder core where the heat transfer time is longest extends. [Table 6] Experiment 6 The thickness h (mm) of the powder core Forming time Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Example 6-1 1.7 10 seconds 92.9 115 860 0.62% 46 Example 6-2 2.5 30 seconds 93.1 120 1500 3.2% 41 Example 6-3 3.0 45 seconds 92.8 123 1800 5.2% 36 Example 6-4 3.5 1 minute 92.5 118 2300 6.0% 35 Comparison Example 6-1 4.5 1.5 minutes 91.7 103 2800 7.5% 31 Comparative Example 6-2 7 4 minutes 92.2 113 5200 8.6% 30 Comparative Example 6-3 14 15 minutes 91.1 107 13000 12.2% 29 <Experiment 7>

[0083] In Experiment 7, the type of low-melting-point glass (the binder material) was changed in the sample preparation. In Experiment 7, a metallic glass powder with a particle size of 9 μm (median diameter D50), a first crystallization temperature (Tg) of 480 ℃, and a second crystallization temperature (Tx) of 510 ℃ was used as the magnetic powder. Phenolic resin was used as the binder resin. The volume percentage of each low-melting-point glass relative to the magnetic powder was set to 2.5 vol%, and the volume percentage of the phenolic resin relative to the magnetic powder was also set to 2.5 vol%. Powdered magnetic cores were prepared using a method similar to that in Experiment 1, and the samples were measured.

[0084] As shown in Table 7, in Example 7-1 where phosphate-based glass was used as the low-melting-point glass and in Example 7-2 where tin phosphate-based glass was used as the low-melting-point glass, the iron loss values ​​were 900 and 1600, respectively, and the percentages of the adhesive layer of 20 nm or smaller were 0.92% and 3.6%, respectively, which are good values.

[0085] On the other hand, in comparative examples 7-1 (using bismuth oxide-based glass as a low-melting-point glass), 7-2 (using borosilicate-based glass as a low-melting-point glass), and 7-3 (using barium silicate-based glass as a low-melting-point glass), the iron loss value was greater than 2500 and the percentage of the adhesive layer at 20 nm or less became greater than 6%. [Table 7] Experiment 7 Glass composition Softening temperature of glass (°C) Powder filling percentage (by volume) of the magnetic core Permeability at 1MHz Iron loss (kW / m³) at 1MHz and 50mT Percentage of the adhesive layer at 20 nm or smaller Median thickness of adhesive layer (nm) Example 7-1 Phosphate 400 93.7 121 900 0.92% 40 Example 7-2 tin phosphate 350 93.6 112 1600 3.6% 31 Comparison Example 7-1 Bismuth oxide 410 92.6 117 3300 7.1% 42 Comparative Example 7-2 borosilicate group 520 91.6 132 5300 8.6% 33 Comparative Example 7-3 Barium silicate 800 90.1 122 7100 9.4% 45

[0086] Figure 11 is a graph showing the iron loss of the sample and the percentage of the 20 nm or smaller portion of the adhesive layer, plotted with the same amount of adhesive and particle size of magnetic powder as in Examples 1-7 above. In the graph shown in Figure 11, the amount of adhesive in the sample is 2.5 vol% low-melting-point glass and 2.5 vol% resin material relative to the amount of magnetic powder, and the particle size of the magnetic powder is 9 μm. As shown in the graph in Figure 11, the iron loss tends to increase as the percentage of the 20 nm or smaller portion of the adhesive layer increases. According to the present invention, by setting the percentage of the 20 nm or smaller portion of the adhesive layer to 6% or less (excluding 0%), the iron loss can be 2500 or less, and this range is within the range of the examples.

[0087] As will be apparent from the invention described herein, embodiments of the invention can be modified in various ways. Such modifications should not be considered as departing from the spirit and scope of the invention, and all such modifications that are obvious to those skilled in the art are intended to be included within the scope of the following claims.

[0088] 1: Inductor 10,10_1,10_2,52,53,54: Powder magnetic core 13: Coil 20: Magnetic powder after granulation 21,121: Magnetic powder 22,122: Adhesive layer 25: Intermediate Forming Material 61, 62, 63, 64: Molding mold 71, 72, 73, 74: Partial 131, 132, 133: District h: distance b, b2, b3, b4: Distance S1, S2, S3, S4, S5: Steps

Claims

1. A powder core, which is formed by bonding magnetic powder through an adhesive layer, wherein the percentage of the portion of the adhesive layer with a thickness of 20 nm or less in the adhesive layer present between the particles of the magnetic powder is equal to or less than 6%, but not including 0%; and the iron loss of the powder core is equal to or less than 2500 kW / m3.

2. The powder core as described in claim 1, wherein the percentage of the portion of the aforementioned adhesive layer with a thickness of 20 nm or less in the aforementioned adhesive layer present between the particles of the aforementioned magnetic powder is equal to or less than 3.3%.

3. The powder core as described in claim 1, wherein the aforementioned magnetic powder is a soft magnetic powder containing iron; and the particle size of the aforementioned magnetic powder is equal to or greater than 2 μm but equal to or less than 25 μm.

4. The powder core as described in claim 3, wherein the aforementioned magnetic powder is metallic glass or nanocrystalline powder.

5. The powder core as described in any of claims 1 to 4, wherein the aforementioned adhesive layer comprises low-melting-point glass and resin material.

6. The powder core as described in claim 5, wherein the total amount of the aforementioned low-melting-point glass and the aforementioned resin material is less than 10% of the volume of the aforementioned magnetic powder.

7. The powder core as described in claim 5, wherein the volume percentage of the aforementioned low-melting-point glass relative to the volume of the aforementioned magnetic powder is equal to or greater than 0.5% by volume but equal to or less than 6% by volume.

8. The powder core as described in claim 5, wherein the volume percentage of the aforementioned resin material relative to the volume of the aforementioned magnetic powder is equal to or greater than 0.5% by volume but equal to or less than 9% by volume.

9. The powder core as described in claim 5, wherein the aforementioned low-melting-point glass is phosphate-based glass or tin phosphate-based glass.

10. The powder core as described in claim 5, wherein the aforementioned resin material is at least one type of resin material selected from the group consisting of phenolic resin, polyimide resin, epoxy resin and acrylic resin.

11. The powder core as described in any one of claims 1 to 3, wherein when the length of the powder core in the vertical direction is longer than 3.5 mm, in the distance between the inner walls of the mold in the horizontal cross section of the powder core when the powder core is held by the mold, the distance between the inner walls of the mold in one direction is set to be equal to or less than 3.5 mm, the direction being substantially perpendicular to the direction in which the portion of the powder core inside extends for the longest heat transfer time required during the heating and forming of the powder core.

12. The powder core described in any of claims 1 to 3, wherein the length of the powder core in the vertical direction is equal to or less than 3.5 mm.

13. The powder core as described in any of claims 1 to 3, wherein the median thickness of the aforementioned adhesive layer is equal to or less than 85 nm.

14. An inductor comprising a powder core and a coil as described in any one of claims 1 to 3.

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