JP7633974B2 - Manufacturing method of powder for dust core - Google Patents

Description

The present invention relates to a method for producing powder for dust cores, which is made by mixing soft magnetic powder with inorganic insulating powder, and the powder for dust cores.

Coil components such as reactors are used in a variety of applications, including office equipment, solar power generation systems, and automobiles. Coil components have a coil attached to a core. This core is made of a powder magnetic core.

Powder cores are produced by press-molding powder for powder cores at high pressures of several tons/ ^cm2 to several tens of tons/ ^cm2 to produce a powder compact, and then subjecting this powder compact to a heat treatment known as annealing. Annealing is carried out to remove distortion that occurs during press molding.

The powder for dust cores contains soft magnetic powder and inorganic insulating powder. The inorganic insulating powder is attached to the periphery of the soft magnetic powder. By attaching the inorganic insulating powder to the periphery of the soft magnetic powder, it is possible to insulate the soft magnetic powder particles from each other, making it possible to increase the heat treatment temperature during annealing.

JP 2021-145065 A

On the other hand, there are cases where the inorganic insulating powder is not uniformly attached around the soft magnetic powder. In other words, the inorganic insulating powder may clump together and adhere to one place, or conversely, there may be cases where the inorganic insulating powder does not adhere at all, leaving the surface of the soft magnetic powder exposed.

If the inorganic insulating powder is not uniformly adhered, the fluidity of the soft magnetic powder deteriorates and the mold is not filled to every corner, affecting the dimensional accuracy and weight of the powder core. In addition, when an insulating coating is formed using silicone resin or the like, the adhesion of the insulating coating also becomes unstable, affecting the magnetic properties. Furthermore, when the soft magnetic powder with the inorganic insulating powder adhered thereto is heat treated, the soft magnetic powder solidifies and must be broken down by hitting with a hammer or the like, resulting in poor productivity.

The present invention has been made to solve the above problems, and its purpose is to provide a method for producing powder for dust cores in which inorganic insulating powder adheres uniformly around soft magnetic powder, and a powder for dust cores.

The method for producing a powder for dust cores of the present invention includes a mixing step of adding a soft magnetic powder and an inorganic insulating powder to a mixing container and adhering the inorganic insulating powder around the soft magnetic powder, and the mixing step is performed so that the soft magnetic powder and the inorganic insulating powder are dispersed and scattered in multiple directions in the mixing container rather than in one direction, and after the mixing step, the soft magnetic powder to which the inorganic insulating powder is attached has a coefficient of variation CV value of 0.0169 or more and 0.0229 or less, where X is the average oxygen content of the soft magnetic powder to which the inorganic insulating powder is attached, the standard deviation σ is the average value X of the soft magnetic powder to which the inorganic insulating powder is attached, the coefficient of variation CV value being 0.0169 or more and 0.0229 or less, where σ is the standard deviation.

The present invention provides a method for producing powder for dust cores in which inorganic insulating powder is uniformly attached around soft magnetic powder, and a powder for dust cores can be obtained.

FIG. 2 is a schematic diagram showing a place where powder for dust cores was collected from a mixer. 1 is a graph showing the coefficient of variation CV values of Examples 1 to 4 and Comparative Example 1. 1A is an SEM image of Example 2, and FIG. 1B is an SEM image of Comparative Example 1 at a magnification of 5000. 1A is an SEM image of Example 2, and FIG. 1B is an SEM image of Comparative Example 1 at a magnification of 10,000. 1 is a graph showing the density of Examples 1 to 4 and Comparative Example 1. 1 is a graph showing hysteresis loss, eddy current loss, and iron loss for Examples 1 to 4 and Comparative Example 1. 1 is a graph showing the initial permeability μ0 and the permeability μ10k at 10 kA/m of Examples 1 to 4 and Comparative Example 1. 1 is a graph showing the resistivity values of Examples 1 to 4 and Comparative Example 1. 1 is a graph showing the rattler values of Examples 1A to 4A and Comparative Example 1A.

(Embodiment)
The powder for dust core and the dust core according to the present embodiment will be described in detail below. Note that the present invention is not limited to the embodiment described below.

Powder cores are magnetic materials used in the cores of coil components installed in office equipment, solar power generation systems, automobiles, etc. Powder cores are made by compressing powder for powder cores and annealing it. Powder for powder cores includes soft magnetic powder and inorganic insulating powder. Powder for powder cores is made by mixing soft magnetic powder and inorganic insulating powder, with the inorganic insulating powder uniformly attached around the soft magnetic powder.

An insulating coating made of an insulating material is formed around the powder for powder cores. The powder for powder cores covered with this insulating coating is pressed into a green compact, which is then annealed to produce the powder core.

The soft magnetic powder is mainly composed of iron. As the soft magnetic powder, pure iron powder, permalloy (Fe-Ni alloy) mainly composed of iron, Si-containing iron alloy (Fe-Si alloy), sendust alloy (Fe-Si-Al alloy), or a mixture of two or more of these powders can be used. Amorphous alloy and nanocrystalline alloy powder can also be used as the soft magnetic powder. The particle size (median size D50) of the soft magnetic powder is preferably 1 μm or more and 200 μm or less.

The Fe-Si-Al alloy powder contains, for example, about 7 wt% to 11 wt% Si and about 4 wt% to 8 wt% Al relative to Fe. The Fe-Si-Al alloy powder may contain, for example, about 1 wt% to 3 wt% Ni relative to Fe. Furthermore, the Fe-Si-Al alloy powder may contain Co, Cr, or Mn.

The Si-containing iron alloy may contain Co, Al, Cr, or Mn. When using permalloy (Fe-Ni alloy), the ratio of Ni to Fe is preferably 50:50 or 25:75, but other ratios are also acceptable. For example, Fe-80Ni, Fe-36Ni, Fe-78Ni, or Fe-47Ni may be used. In addition to Fe and Ni, Si, Cr, Mo, Cu, Nb, Ta, etc. may also be contained. Examples of Fe-Si alloy powder include Fe-3.5% Si alloy powder and Fe-6.5% Si alloy powder, but the ratio of Si to Fe may be other than 3.5% or 6.5%. Pure iron powder contains 99% or more Fe.

The inorganic insulating powder adheres to the periphery of the soft magnetic powder. By adhering the inorganic insulating powder to the periphery of the soft magnetic powder, the soft magnetic powder can be insulated from each other. The inorganic insulating powder adheres uniformly to the periphery of the soft magnetic powder. "Uniform" here means that the inorganic insulating powder is adhered to the periphery of the soft magnetic powder evenly and in approximately the same amount. In other words, it means that the inorganic insulating powder is not aggregated and clumped together, or that the inorganic insulating powder is not attached and the surface of the soft magnetic powder is not exposed. Whether or not the powder is adhered uniformly can be confirmed by observing the powder at 5,000 or 10,000 times magnification under an electron microscope, or by checking whether the coefficient of variation CV value described below is 0.023 or less.

By the inorganic insulating powder being uniformly attached around the soft magnetic powder, the fluidity of the powder for the dust core is improved, the density is increased, and hysteresis loss can be reduced. In addition, the inorganic insulating powder ensures the distance between the soft magnetic powder particles, and the binder effect of the insulating materials such as silicone resin and silane coupling agent that make up the insulating coating is better obtained, resulting in extremely high strength.

If the inorganic insulating powder aggregates and adheres unevenly around the soft magnetic powder, there is a risk that the insulating material will not adhere to the aggregated areas or that the amount of adhesion will be small. If the amount of adhesion of the insulating material is small, the binding force between the soft magnetic powder particles in that area will be weak, making the core more susceptible to chipping and cracking.

On the other hand, when the inorganic insulating powder is uniformly attached around the soft magnetic powder as in this embodiment, it is possible to prevent the occurrence of areas where the insulating material is not attached or where the amount of attached insulating material is small around the soft magnetic powder (strictly speaking, around the inorganic insulating powder attached around the soft magnetic powder). This increases the binding force between the soft magnetic powder and increases the strength. This makes it possible to prevent chipping and cracking of the core.

Alumina powder, magnesia powder, silica powder, titania powder, zirconia powder, etc. can be used as the inorganic insulating powder. The particle size of the inorganic insulating powder is smaller than that of the soft magnetic powder. The particle size (median diameter D50) of the inorganic insulating powder is preferably 7 nm or more and 200 nm or less.

The coefficient of variation CV value of a powder in which inorganic insulating powder is attached around soft magnetic powder (hereinafter also referred to as "mixed powder") is 0.023 or less. The coefficient of variation CV value is determined by sampling the mixed powder from multiple locations in a mixing container and measuring the amount of oxygen at each location. The average value X and standard deviation σ of the oxygen amount are calculated from each measurement result. The coefficient of variation CV value is then calculated by dividing the standard deviation σ by the average value X.

The locations from which the samples are taken are not limited to the above, but are five locations on the top surface of the mixing vessel. For example, as shown in Figure 1, the five locations are the center of the circular mixer ((1) in Figure 1) and four equally spaced locations along the periphery ((2) to (5) in Figure 1, which would be the 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock positions on a clock). In this way, by measuring the amount of oxygen in the mixed powder at multiple dispersed locations, rather than just one location, an accurate coefficient of variation CV value can be calculated.

If the mixer is rectangular, a total of five locations may be collected, including the center and each vertex. Also, instead of only collecting samples from the top surface of the mixing container, samples may be collected from the bottom surface of the mixing container or the center area between the top and bottom surfaces, or samples may be collected from multiple locations each from the top surface, bottom surface, and center area.

An insulating coating is formed around the powder for the dust core. The insulating coating is made of an insulating material. As the insulating material, a silane compound, a silicone resin, or a silicone oligomer can be used. The insulating material used as the insulating coating may be one type or two or more types. When two or more types are used, each type of insulating coating layer may be laminated, or a single layer of two or more types of insulating materials may be mixed. That is, for example, when a silane compound and a silicone resin are used, an insulating coating layer made of the silane compound may be formed around the powder for the dust core, and an insulating coating layer made of silicone resin may be formed around this insulating coating layer, or a single layer insulating coating layer made of a mixture of the silane compound and silicone resin may be formed.

The silane compounds include silane compounds without functional groups and silane coupling agents. As the silane compounds without functional groups, for example, alkoxysilanes such as ethoxy and methoxy types can be used, with tetraethoxysilane being particularly preferred. As the silane coupling agents, aminosilane, epoxysilane, and isocyanurate type silane coupling agents can be used, with 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, and tris-(3-trimethoxysilylpropyl)isocyanurate being particularly preferred.

The amount of silane compound added is preferably 0.05 wt% or more and 1.0 wt% or less of the soft magnetic powder. By adding the amount of silane compound within this range, the fluidity of the soft magnetic powder can be improved, and the density, magnetic properties, and strength properties of the molded powder core can be improved.

Silicone resin is a resin that has a siloxane bond (Si-O-Si) in its main skeleton. By using silicone resin, it is possible to form a coating with excellent flexibility. Silicone resins that can be used include methyl-based, methylphenyl-based, propylphenyl-based, epoxy resin-modified, alkyd resin-modified, polyester resin-modified, and rubber-based silicone resins. Among these, when using methylphenyl-based silicone resins in particular, it is possible to form an insulating coating with little loss in weight when heated and excellent heat resistance.

The amount of silicone resin added is preferably 0.6 wt% or more and 2.5 wt% or less relative to the soft magnetic powder. If the amount added is less than 0.6 wt%, it will not function as an insulating coating, and eddy current loss will increase, resulting in reduced magnetic properties. If the amount added is more than 2.5 wt%, it will result in a reduction in the density of the powder core.

Silicone oligomers that can be used include methyl-based and methylphenyl-based ones that have an alkoxysilyl group but no reactive functional group, epoxy-based, epoxymethyl-based, mercapto-based, mercaptomethyl-based, acrylic methyl-based, methacrylic methyl-based, and vinylphenyl-based ones that have an alkoxysilyl group and a reactive functional group, and alicyclic epoxy-based ones that have a reactive functional group instead of an alkoxysilyl group. In particular, by using methyl-based or methylphenyl-based silicone oligomers, a thick and hard insulating coating can be formed. Also, taking into consideration the ease of forming an insulating coating, methyl-based and methylphenyl-based ones with relatively low viscosity may be used.

The amount of silicone oligomer added is preferably 0.1 wt% or more and 2.0 wt% or less of the soft magnetic powder. If the amount added is less than 0.1 wt%, it will not function as an insulating coating, and eddy current loss will increase, resulting in reduced magnetic properties. If the amount added is more than 2.0 wt%, it will result in a decrease in the density of the powder core.

When forming an insulating coating around the powder for dust cores, pure water may be added in addition to the insulating material. Adding pure water promotes the reaction of the insulating material.

The powder for the dust core with the insulating coating is filled into a die and compressed under a specified pressure to form a dust compact. This dust compact is then annealed to form a dust core.

Next, we will explain the powder for powder cores and the manufacturing method for powder cores. The manufacturing method for powder for powder cores includes a mixing process. The manufacturing method for powder cores includes a powder heat treatment process, an insulating coating formation process, a lubricant addition process, a pressure molding process, and an annealing process.

The mixing process is a process in which the soft magnetic powder and the inorganic insulating powder are added to a mixing container and mixed. In the mixing process, the soft magnetic powder and the inorganic insulating powder are mixed so that they scatter in all directions. In other words, the soft magnetic powder and the inorganic insulating powder move around in the mixing container not only in a certain direction but in many directions. For example, a semispherical elastic member (e.g., a rubber ball) having flexibility may be provided on the bottom of the mixing container, and the mixing container may be rocked while the elastic member is rotated, and the soft magnetic powder and the inorganic insulating powder may be scattered in all directions by utilizing the expansion and contraction of the elastic member. The mixing container may also be rotated eccentrically in an irregular manner to scatter the soft magnetic powder and the inorganic insulating powder in all directions. Through the mixing process, the inorganic insulating powder adheres uniformly to the periphery of the soft magnetic powder.

The mixing time is preferably 2 minutes or more and 20 minutes or less, and more preferably 5 minutes or more and 20 minutes or less. By keeping the time within this range, the inorganic insulating powder adheres uniformly around the soft magnetic powder, and the coefficient of variation CV value is 0.023 or less. In general, it seems that mixing for a long time will result in more uniform adhesion, but if mixing is done for a long time, further aggregations of the inorganic insulating powder will occur around the uniformly adhered inorganic insulating powder, which may result in uneven adhesion.

The present invention is more effective when the particle diameter of the soft magnetic powder is 1 μm or more and 200 μm or less, and the particle diameter of the inorganic insulating powder is 7 nm or more and 200 nm or less. In particular, the effect is more pronounced when the particle diameter of the soft magnetic powder is a small powder of 1 μm or more and 5 μm or less, and the particle diameter of the inorganic insulating powder is 7 nm or more and 200 nm or less.

For example, when mixing soft magnetic powder with a size between 5 μm and 200 μm with inorganic insulating powder with a size between 7 nm and 200 nm, if the powder cannot be dispersed in multiple directions as in a conventional V-type mixer, the particle size difference is large and only the inorganic insulating powder will aggregate. On the other hand, as in this embodiment, the powder can be dispersed in all directions, that is, in multiple directions instead of one direction, to prevent the inorganic insulating powder from agglomerating.

In addition, when mixing small soft magnetic powder with a particle size of 1 μm to 5 μm with inorganic insulating powder with a particle size of 7 nm to 200 nm, the nano-sized inorganic insulating powder will undergo primary and secondary aggregation due to van der Waals forces between the soft magnetic powder particles and between the inorganic insulating powder particles. However, as in this embodiment, primary and secondary aggregation can be suppressed by dispersing the powder in all directions, that is, in multiple directions rather than in one direction.

After the mixing step, the powder undergoes a powder heat treatment step. The powder heat treatment step is a step of heat treating the powder for dust core. In other words, it is a step of heat treating the soft magnetic powder with inorganic insulating powder attached to the periphery. In the powder heat treatment step, the powder is heated in a non-oxidizing atmosphere for 1 to 6 hours. The non-oxidizing atmosphere includes a low oxygen atmosphere such as 0.01% in the atmosphere, an inert gas atmosphere, or a reducing gas atmosphere. Examples of the inert gas include noble gases such as Ar and _N2 . Examples of the reducing gas include _H2 . The heat treatment temperature is 400°C or higher and 1200°C or lower.

The insulating coating process is a process for forming an insulating coating around the powder for the powder core. In the insulating coating process, an insulating material is added to and mixed with the powder for the powder core. After mixing the insulating material, the powder for the powder core is heated and dried to form an insulating coating around the powder for the powder core. The heating and drying conditions are not limited to the following, but are typically dried at a temperature of 25°C or higher and 350°C or lower for approximately 2 hours.

The lubricant addition process is a process in which a lubricant is added to the powder for the powder core on which the insulating coating has been formed. Examples of lubricants include, but are not limited to, stearic acid and its metal salts, as well as ethylene bisstearamide, ethylene bisstearamide, and ethylene bisstearamide. The amount of lubricant added is preferably about 0.2 wt% to 0.8 wt% of the powder for the powder core. By setting it in this range, it is possible to further improve the sliding between the powder for the powder core.

The pressure molding process is a process for producing a powder compact by pressure molding the powder for powder cores to which a lubricant has been added. First, the powder for powder cores is filled into a die, and then pressure is applied at 5 to 20 tons/ ^cm2 . In this manner, the powder compact is produced.

The annealing process is a process in which the powder compact produced through the pressure molding process is annealed to remove distortion within the soft magnetic powder. In the annealing process, the powder compact is heat-treated in a non-oxidizing atmosphere such as nitrogen gas, hydrogen gas, a mixture of nitrogen and hydrogen, or a low-oxygen atmosphere of about 0.01%, at a temperature of 600°C or higher and lower than the temperature at which the insulating coating formed around the soft magnetic powder is destroyed (for example, 900°C). Through this annealing process, a powder magnetic core is produced.

(Example)
The present invention will be described in more detail with reference to examples. Note that the present invention is not limited to the following examples. Powders for dust cores and dust cores of Examples 1 to 4 and Comparative Example 1 were produced.

First, the powder for the dust core of Example 1 was prepared. Pure iron powder with an average particle size of 43 μm was used as the soft magnetic powder, and Al ₂ O ₃ powder (alumina powder) with an average particle size of 13 nm was used as the inorganic insulating powder. The _{Al 2} O ₃ powder was used in an amount of 1.0 wt % relative to the pure iron powder.

The pure iron powder and Al ₂ O ₃ powder were added to a oscillating mixer (Chiyoda Machinery Co., Ltd., OM30SA) and mixed. In this oscillating mixer, the pure iron powder and Al ₂ O ₃ powder were mixed while scattering in all directions in the mixing container. The mixing time was 5 minutes. In this way, the powder for the dust core of Example 1 was produced.

The powders for the dust cores in Examples 2 to 4 differ from Example 1 only in the mixing time, and the other materials, manufacturing procedures, and manufacturing conditions are the same as those in Example 1. The mixing times are 10 minutes for Example 2, 15 minutes for Example 3, and 20 minutes for Example 4.

In Comparative Example 1, as in Example 1, pure iron powder with an average particle size of 43 μm was used as the soft magnetic powder, and _{Al 2} O ₃ powder with _an average particle size of 13 nm was used as the inorganic insulating powder. The Al 2 O 3 powder was used at 1.0 wt% relative to the pure iron powder. Then, the pure iron powder and Al ₂ O ₃ powder were added to a V-type mixer (Tokuju Manufacturing Co., Ltd., V-60) and mixed. Since it is a V-type mixer, the pure iron powder and Al ₂ O ₃ powder are mixed by moving only in a certain direction inside the mixer. In other words, in Comparative Example 1, the pure iron powder and Al ₂ O ₃ powder are not scattered in all directions. The mixing time is 5 minutes. In this way, the powder for the dust core of Comparative Example 1 was produced.

The powders for dust cores produced in Examples 1 to 4 and Comparative Example 1 were sampled from the mixer and the oxygen content was measured. In each of Examples 1 to 4 and Comparative Example 1, the powders for dust cores were sampled from five locations on the top surface of the mixer. As shown in Figure 1, samples were taken from the center of the circular mixer ((1) in Figure 1) and from four equally spaced locations along the circumferential direction ((2) to (5) in Figure 1). The amount sampled was 50 g from each location. In Comparative Example 1, a V-type mixer was used, so the powder was transferred to a circular container and sampled from locations (1) to (5) shown in Figure 1.

The amount of oxygen was measured using an oxygen and nitrogen analyzer (TC500, manufactured by LECO). For the measurement, 0.1 g of the collected powder for powder core was placed in a carbon crucible and heated at 2000 ° C., and the amount of oxygen was calculated for the generated CO or _CO2 gas by infrared absorption method. The amount of oxygen was measured for each of the powder for powder core collected at five locations, and the average value X and standard deviation σ were calculated. Furthermore, the coefficient of variation CV value was calculated by dividing the standard deviation σ by the average value X. In Examples 1 to 4 and Comparative Example 1, the amount of oxygen was measured at five locations, and the average value X, standard deviation σ, and coefficient of variation CV were calculated.

The calculated results are shown in Table 1. Moreover, a graph of the coefficient of variation CV values of Examples 1 to 4 and Comparative Example 1 is shown in FIG.

As shown in Table 1 and FIG. 2, the coefficient of variation CV values of Examples 1 to 4 in which the pure iron powder and Al ₂ O ₃ powder were scattered in all directions were 0.023 or less, which is about 0.005 smaller than the CV of Comparative Example 1, which was 0.0278.

Furthermore, in order to confirm the adhesion of the Al ₂ O ₃ powder to the pure iron powder in the collected dust core powders in Example 2 and Comparative Example 1, the dust core powders were observed using a field emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.). A predetermined amount of the powder was dispersed and attached to a carbon tape, and a secondary electron image was observed and photographed at 10 kV in a vacuum atmosphere.

The photographing results are shown in Figures 3 and 4. Figure 3 is an SEM image at a magnification of 5000, (a) is an SEM image of Example 2, and (b) is an SEM image of Comparative Example 1. Figure 4 is an SEM image at a magnification of 10000, (a) is an SEM image of Example 2, and (b) is an SEM image of Comparative Example 1. In each SEM image, the white portion indicates Al ₂ O ₃ powder.

As shown in Figures 3(a) and 4(a), in Example 2, the white parts are not concentrated in one place, and the Al ₂ O ₃ powder is uniformly dispersed around the pure iron powder. On the other hand, in Comparative Example 1, as shown by the black circles in Figures 3(b) and 4(b), white lumps are scattered, and the Al ₂ O ₃ powder is aggregated. Therefore, when the pure iron powder and the Al ₂ O ₃ powder are mixed so that they scatter in all directions, the coefficient of variation CV value becomes 0.023 or less, and it was confirmed that the Al ₂ O ₃ powder adheres uniformly.

In addition, as shown in Table 1 and Figure 2, the coefficient of variation CV value reaches its lower limit peak around Examples 2 and 3, and the coefficient of variation CV value of Example 4, where the mixing time is 20 minutes, increases. From this, it was confirmed that it is better to set the mixing time to 5 minutes or more and 20 minutes or less, rather than to perform the mixing for a long period of time.

Powder cores were then produced using the powders for powder cores in Examples 1 to 4 and Comparative Example 1. The powder cores in Examples 1 to 4 and Comparative Example 1 were produced using the same method, as follows.

After mixing the pure iron powder and the Al ₂ O ₃ powder, the powder for the dust core was subjected to a powder heat treatment. The powder heat treatment was performed at 1000° C. in a non-oxidizing atmosphere in which a mixed gas of hydrogen and nitrogen was injected. The heat treatment time was 2 hours.

After the powder was heat-treated, silicone resin and a silane coupling agent were added to form an insulating coating around the powder for the dust core. 1.8 wt% of silicone resin was added to the pure iron powder, and 0.5 wt% of silane coupling agent was added to the pure iron powder, and mixed for 1 minute. 0.5 wt% of pure water was then added to the pure iron powder, and mixed for another minute. The mixture was then exposed to a dryer maintained at 180°C for 2 hours, heated and dried, and an insulating coating was formed.

After the insulating coating was formed, the powder for the dust core was cooled to room temperature and passed through a sieve with 850 μm openings for crushing purposes. A lubricant was then added and mixed for 1 minute. Zinc stearate was used as the lubricant. 0.5 wt% of the lubricant was added to the pure iron powder.

After mixing the lubricant, the powder for the dust core to which the lubricant was added was filled into a die and press-molded to produce a toroidal powder compact having an outer diameter of 20.97 mm, an inner diameter of 12.48 mm, and a height of 4.8 mm. The pressure for the press molding was 9.5 ton/ ^cm2 .

After press molding, the densities of the powder compacts of Examples 1 to 4 and Comparative Example 1 were measured. The density (kg/m ³ ) is the apparent density. The outer diameter, inner diameter, and height of the powder core were measured, and the volume (m ³ ) of the powder compact was calculated from these values based on π × (outer diameter ² − inner diameter ² ) × height. The weight of the powder core was then measured, and the measured weight was divided by the calculated volume to calculate the density.

The calculation results are shown in Table 2. Moreover, a graph of the densities of Examples 1 to 4 and Comparative Example 1 is shown in FIG.

As shown in Table 2 and Figure 5, the densities of Examples 1 to 4 are equal to or greater than those of Comparative Example 1. Specifically, only Example 3 had the same density as Comparative Example 1, but it was confirmed that Examples 1, 2, and 4 had improved density compared to Comparative Example 1.

Furthermore, each of the powder compacts of Examples 1 to 4 and Comparative Example 1 was annealed to remove distortion caused by press molding. The annealing was performed at 620°C in a non-oxidizing atmosphere injected with a mixed gas of hydrogen and nitrogen. The heat treatment time was 2 hours. In this manner, each of the powder magnetic cores of Examples 1 to 4 and Comparative Example 1 was produced.

The iron loss, magnetic permeability, resistivity, and rattle value were measured for the powder magnetic cores of Examples 1 to 4 and the comparative example.

When measuring the iron loss, a copper wire having a diameter of 0.5 mm was wound around the powder magnetic core as a primary winding with 30 turns, and as a secondary winding with 30 turns. Then, using a magnetic measuring device, a BH analyzer (Iwatsu Measuring Instruments Co., Ltd.: SY-8219), the iron loss Pcv (kW/m ³ ) was measured under the measurement conditions of a frequency of 20 kHz and a maximum magnetic flux density Bm of 200 mT. From the measurement results of the iron loss Pcv, the hysteresis loss Phv (kW/m ³ ) and the eddy current loss Pev (kW/m ³ ) were calculated. The hysteresis loss Phv (kW/m ³ ) and the eddy current loss Pev (kW/m ³ ) were calculated by calculating the hysteresis loss coefficient (Kh) and the eddy current loss coefficient (Ke) from the frequency curve of the iron loss Pcv using the following formulas (1) to (3) by the least squares method.

Pcv =Kh×f+Ke×f ² ...(1)
Phv = Kh×f...(2)
Pev = Ke×f ² ...(3)
Pcv: Iron loss Kh: Hysteresis loss coefficient Ke: Eddy current loss coefficient f: Frequency Phv: Hysteresis loss Pev: Eddy current loss

The measurement results are shown in Table 3. Also, Fig. 6 shows graphs of the hysteresis loss, eddy current loss, and iron loss of Examples 1 to 4 and Comparative Example 1. In the bar graph of Fig. 6, the black bars indicate hysteresis loss.

As shown in Table 3 and Figure 6, the eddy current loss of Examples 1 to 4, in which the Al ₂ O ₃ powder is uniformly adhered, is reduced by about 100 (kW/m ³ ) compared to Comparative Example 1. Also, the hysteresis loss of Examples 1 to 4 is reduced by about 150 to 200 (kW/m ³ ) compared to Comparative Example 1. In this way, in Examples 1 to 4, the eddy current loss and hysteresis loss are both greatly reduced by 100 (kW/m ³ ), and as a result, the iron loss is greatly reduced by more than 200 (kW/m ³ ). In other words, the effect of uniformly adhering the Al ₂ O ₃ powder is clearly evident.

The magnetic permeability was measured by winding two φ0.5 mm copper wires in parallel around the powder magnetic core for 30 turns. Then, using an LCR meter (Hewlett Packard, 4284A), the initial magnetic permeability μ0 at 0 A/m and the magnetic permeability μ10k at 10 kA/m were measured from the inductance of the magnetic field strength at 20 kHz and 1.0 V.

The measurement results are shown in Table 4. Also, Fig. 7 shows a graph of the initial permeability μ0 and the permeability μ10k at 10 kA/m of Examples 1 to 4 and Comparative Example 1. In the bar graph of Fig. 7, the black bars indicate the initial permeability μ0.

As shown in Table 4 and Fig. 7, in Examples 1 to 4 in which the Al ₂ O ₃ powder is uniformly attached, both the initial magnetic permeability μ0 and the magnetic permeability μ10k at 10 kA/m are improved compared to Comparative Example 1. In particular, in Examples 1 to 4, the distance between the pure iron powder particles is uniformly ensured by the Al ₂ O ₃ powder, and minute gaps are formed, so that the magnetic permeability μ10k at 10 kA/m when superimposed is 32.5 or more, which is significantly improved compared to Comparative Example 1.

The resistivity was measured using a resistivity meter (Loresta-GX MCP-T700, manufactured by Nitto Seiko Analytech Co., Ltd.) at four equally spaced locations on the circumference of the circular surface of the toroidal core using a four-point probe method, and the average of the four measurement results was calculated.

The calculation results are shown in Table 5. Moreover, a graph of the resistivity values of Examples 1 to 4 and Comparative Example 1 is shown in FIG.

As shown in Table 5 and Fig. 8, Examples 1 to 4 have significantly higher resistivity values than Comparative Example 1. As described above, the eddy current loss of Examples 1 to 4 is reduced by about 100 (kW/ ^m3 ) compared to Comparative Example 1, and it can be inferred that the uniform adhesion of the _{Al2O3 powder} results in improved resistivity and reduced eddy current loss.

Next, the powder compacts of Examples 1A to 4A and Comparative Example 1A were produced. The materials, production processes, and production conditions used for Examples 1A to 4A and Comparative Example 1A were the same as those of Examples 1 to 4 and Comparative Example 1, respectively, up until the process of adding the lubricant. For example, for Example 1A, the same materials, processes, and conditions were used as in Example 1 up until the process of adding the lubricant.

For each sample of Examples 1A to 4A and Comparative Example 1A, a lubricant was mixed, then the mixture was filled into a metal mold and press molded to produce a cylindrical powder compact having an outer diameter of 11.3 mm and a height of 10 mm. The pressure for press molding was 9.0 ton/ ^cm2 . In this manner, each powder compact of Examples 1A to 4A and Comparative Example 1A was produced.

Then, the rattler value was measured. A rattler device (manufactured by INTESUKOH) was used to measure the rattler value. The rattler value was measured based on the Japan Powder Metallurgy Association (JPMA) standard method for measuring the rattler value of metal powder compacts (JPMA P11 1992). That is, the core was placed in a cylindrical cage covered with a stainless steel wire mesh with a mesh size of 1180 μm, and rotated 1000 times at a rotation speed of 87 rpm. After that, the weight of the powder compact was measured, and the weight was subtracted from the weight measured before rotation to obtain the mass reduction rate, thereby calculating the rattler value.

The calculation results are shown in Table 6. Also, a graph of the rattler values of Examples 1A to 4A and Comparative Example 1A is shown in FIG.

As shown in Table 6 and Figure 9, the rattler values of Examples 1A to 4A were about 1/5 of that of Comparative Example 1A, and it was confirmed that the strength was significantly increased. Therefore, by uniformly attaching Al ₂ O ₃ powder to pure iron powder, a dust core with improved strength can be manufactured, and it can be used for coil parts such as reactors that require high vibration resistance for use in vehicles.

As described above, Examples 1 to 4 have results at least equal to or better than Comparative Example 1 in each measurement item. In particular, the rattler value is significantly reduced to about 1/5 of Comparative Example 1A, and strength is improved. It is believed that the pure iron powder and Al ₂ O ₃ powder are dispersed and scattered in all directions, and the Al ₂ O ₃ powder is uniformly attached around the pure iron powder, improving the flowability of the powder and allowing the powder to reach every corner of the mold, thereby improving strength.

Other Embodiments
In this specification, an embodiment of the present invention has been described, but this embodiment is presented as an example and is not intended to limit the scope of the invention. The above-mentioned embodiment can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the scope of the invention. The embodiment and its modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and gist of the invention.

Claims (7)

A mixing step of adding a soft magnetic powder and an inorganic insulating powder to a mixing vessel and adhering the inorganic insulating powder to the periphery of the soft magnetic powder,
The mixing step includes mixing the soft magnetic powder and the inorganic insulating powder so that the soft magnetic powder and the inorganic insulating powder are dispersed and scattered in all directions within a mixing container,
After the mixing step, the soft magnetic powder to which the inorganic insulating powder is attached has a coefficient of variation CV value obtained by dividing the standard deviation σ by the average value X, where X is an average value of the oxygen content of the soft magnetic powder to which the inorganic insulating powder is attached, the average value being taken from a plurality of points on the top surface of a mixing container, and σ is a standard deviation of the oxygen content of the soft magnetic powder to which the inorganic insulating powder is attached, of 0.0169 or more and 0.0229 or less.
A method for producing powder for dust cores, comprising the steps of: The mixing vessel is provided with a semi-spherical elastic member having flexibility on a bottom surface,
the mixing step includes rocking the mixing container and rotating the elastic member, and utilizing the expansion and contraction of the elastic member, the soft magnetic powder and the inorganic insulating powder are dispersed and scattered in all directions within the mixing container;
The method for producing a powder for dust core according to claim 1, The mixing step includes: rotating the mixing vessel irregularly and eccentrically;
The method for producing a powder for dust core according to claim 1 or 2, In the mixing step, the mixing time is 5 minutes or more and 20 minutes or less.
The method for producing a powder for dust core according to claim 1 or 2, In the mixing step, the mixing time is 5 minutes or more and 20 minutes or less.
The method for producing a powder for dust core according to claim 3, The particle size of the soft magnetic powder is 1 μm or more and 200 μm or less,
The particle size of the inorganic insulating powder is 7 nm or more and 200 nm or less;
The method for producing a powder for dust core according to claim 1 or 2, The particle size of the soft magnetic powder is 1 μm or more and 200 μm or less,
The particle size of the inorganic insulating powder is 7 nm or more and 200 nm or less;
The method for producing a powder for dust core according to claim 3,

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