JP7649142B2 - Magnetic cores, magnetic components and electronic devices - Google Patents

Description

The present invention relates to a magnetic core, a magnetic component, and an electronic device.

Patent Document 1 describes an inductor in which a dust core obtained by compressing metal magnetic powder is integrally molded with a coil section. However, when metal magnetic powder is used, core loss tends to increase. Here, core loss can be improved by using amorphous alloy powder as the metal magnetic powder. However, in this case, it is known that it becomes difficult to improve the density of the dust core during molding.

Patent documents 2 and 3 propose using a mixture of crystalline and amorphous magnetic alloy powders.

Patent document 4 describes how the use of amorphous soft magnetic powder with a high average practical sphericity makes it possible to provide inductors and the like with lower losses than conventional products.

JP 2003-309024 A JP 2004-197218 A JP 2004-363466 A Patent No. 5110660

The present invention aims to obtain a magnetic core etc. that has high relative permeability and improved voltage resistance characteristics.

In order to achieve the above object, the magnetic core according to the present invention comprises:
In a cross section, the large particles are observed as soft magnetic particles having a Heywood diameter of 5 μm or more and 25 μm or less, and the small particles are observed as soft magnetic particles having a Heywood diameter of 0.5 μm or more and 3 μm or less,
The average aspect ratio of the large particles is A1, and the average aspect ratio of the small particles is A2,
1.00≦A1≦1.50
1.30≦A2≦2.50
A1<A2
Meet the following.

It has been discovered that the magnetic core of the present invention uses large and small particles having a specific average aspect ratio, and by making the average aspect ratio of the small particles larger than the average aspect ratio of the large particles, it is possible to improve the voltage resistance characteristics while maintaining a high relative permeability.

In the cross section, the maximum Heywood diameter of soft magnetic particles that do not fall into the large particles or small particles may be 40 μm or less.

The average ellipticity of the large particles in the cross section may be 0.93 or more.

The large particles may include nanocrystals.

The small particles may contain Fe as a main component.

The small particles may contain Fe and at least one selected from Si and Ni as the main components.

The magnetic component of the present invention includes the magnetic core described above.

The electronic device of the present invention includes the magnetic core described above.

1 is an example of a chart obtained by X-ray crystal structure analysis. 2 is an example of a pattern obtained by profile fitting the chart of FIG. 1.

An embodiment of the present invention is described below.

The magnetic core of this embodiment includes large particles that are observed in cross section as soft magnetic particles with a Heywood diameter of 5 μm or more and 25 μm or less, and small particles that are observed as soft magnetic particles with a Heywood diameter of 0.5 μm or more and 3 μm or less.

The total area ratio of the large particles to the entire cross section may be 10% or more, or may be 30% or more. The total area ratio of the small particles may be 10% or more, or may be 15% or more. In addition, the total area ratio of the large particles may be S1, and the total area ratio of the small particles may be S2, and S1:S2 may be 1:9 to 9:1.

The Heywood diameter is the diameter of a circle equivalent to a projected area. The Heywood diameter of the soft magnetic particle in this embodiment is (4S/π) ^1/2 , where S is the area of the soft magnetic particle in cross section.

In the magnetic core according to this embodiment, the average aspect ratio of the large particles is A1, the average aspect ratio of the small particles is A2, and
1.00≦A1≦1.50
1.30≦A2≦2.50
A1<A2
It is a magnetic core that satisfies the above.

In addition, the large particles and small particles according to this embodiment may have a coating on the surface. The coating may be an insulating coating. There is no particular limitation on the type of coating, and the coating may be formed by a coating that is commonly used in this technical field. For example, iron oxides, phosphates, silicates (water glass), soda-lime glass, borosilicate glass, lead glass, aluminosilicate glass, borate glass, sulfate glass, etc. are included. Examples of phosphates include magnesium phosphate, calcium phosphate, zinc phosphate, manganese phosphate, and cadmium phosphate . Examples of silicates include sodium silicate. There is also no particular limitation on the thickness of the coating. If the coating is formed on a large particle, it may be 5 nm or more and 100 nm or less on average. If the coating is formed on a small particle, it may be 5 nm or more and 100 nm or less on average. Note that the thickness of the above coating is significantly smaller than the particle diameter of each particle, so it does not substantially affect the Heywood diameter and aspect ratio of each particle.

The magnetic core according to this embodiment may also have a resin used as a binder. There are no particular limitations on the type of resin. Examples include silicone resin and epoxy resin. There are also no particular limitations on the amount of resin contained in the magnetic core. For example, it may be 1.5 parts by mass or more and 5.0 parts by mass or less per 100 parts by mass of soft magnetic particles.

Below, we explain how to observe the cross section of a magnetic core.

First, the magnetic core is cut and the resulting cross section is polished to create an observation surface. Next, the observation surface is observed using an SEM and an SEM image is taken.

There is no particular limit to the size of the observation range using SEM, as long as 100 or more, preferably 1000 or more, of any large particles can be observed. Alternatively, different observation ranges may be set and SEM images of each observation range may be taken, and the above number of particles may be observed in the total of the multiple SEM images.

There is no particular limit to the magnification of the SEM image, as long as it allows the measurement of various parameters related to this embodiment. For example, it may be 200 times or more and 5000 times or less.

The aspect ratio of an individual soft magnetic particle is calculated by dividing the length of the major axis by the length of the minor axis.

The magnetic core according to this embodiment is capable of improving the voltage resistance characteristics while maintaining the relative permeability because the average aspect ratio A1 of the large particles, the average aspect ratio A2 of the small particles, and the relationship between A1 and A2 are all as described above. Note that A2-A1 may be ≧ 0.01.

The reasons why the magnetic core according to this embodiment has improved relative permeability and voltage resistance are as follows:

The voltage endurance characteristic of the magnetic core is more likely to be improved when the soft magnetic particles contained in the magnetic core are in surface contact than when they are in point contact. When the soft magnetic particles are in point contact with each other and when they are in surface contact, the contact area between the soft magnetic particles is larger in the surface contact. Here, the larger the contact area between the soft magnetic particles is, the smaller the voltage applied per unit area of the part where the soft magnetic particles are in contact with each other when a voltage is applied . In other words, when the soft magnetic particles are in surface contact with each other, there are fewer places where the electric field is concentrated when a voltage is applied . From the above, when the soft magnetic particles are in surface contact with each other, the voltage endurance characteristic of the magnetic core is increased. In addition, the greater the degree to which the soft magnetic particles contained in the magnetic core are deformed from a spherical shape, the more likely the contact between the soft magnetic particles is in surface contact. In other words, it is considered that the greater the aspect ratio of the soft magnetic particles is, the more likely the voltage endurance characteristic of the magnetic core is improved.

However, at the same time, if the aspect ratio of the soft magnetic particles is too large, the electric field tends to concentrate in the long axis direction of the soft magnetic particles when a voltage is applied. This concentration of the electric field reduces the voltage resistance characteristics of the magnetic core. Therefore, simply increasing the aspect ratio of all soft magnetic particles is unlikely to improve the voltage resistance characteristics of the magnetic core, and in some cases may even reduce them.

The inventors have found that the phenomenon in which the larger the aspect ratio of the soft magnetic particles, the lower the voltage resistance of the magnetic core is, is more likely to occur in soft magnetic particles with a large particle diameter. They have also found that the voltage resistance is improved by deforming small particles more than large particles to increase the aspect ratio of the small particles. The details of why the voltage resistance is improved by increasing the aspect ratio of the small particles are unclear. However, when the aspect ratio of the small particles is increased to increase the proportion of surface contact, it is believed that the number of places where the electric field concentrates when a voltage is applied, especially in large particles, is reduced compared to when the aspect ratio of the large particles is increased to increase the proportion of surface contact. It is believed that the reduction in the number of places where the electric field concentrates in large particles suppresses the deterioration of the voltage resistance.

Furthermore, generally, increasing the filling rate of soft magnetic particles tends to increase the relative permeability. However, increasing the filling rate of soft magnetic particles narrows the spacing between the soft magnetic particles. Therefore, increasing the filling rate of soft magnetic particles increases the number of locations where the electric field concentrates when a voltage is applied. For these reasons, generally, increasing the filling rate of soft magnetic particles deteriorates the voltage resistance characteristics.

In other words, in the past, the relative permeability was improved by increasing the filling rate, but increasing the filling rate deteriorates the voltage resistance characteristics, making it difficult to improve the voltage resistance characteristics when the relative permeability is high. The inventors have succeeded in improving the voltage resistance characteristics when the relative permeability is high by setting both the aspect ratio of the large particles and the aspect ratio of the small particles within a specific range and further making the aspect ratio of the small particles larger than the aspect ratio of the large particles.

There are no particular limitations on the method for calculating the filling rate of the magnetic core. For example, the above observation surface is observed using an SEM. Then, the area ratio of the particles to the entire area of the observation surface is calculated. In this embodiment, the area ratio is considered to be equal to the filling rate, and the area ratio is taken as the filling rate.

In the cross section, the maximum Heywood diameter of the soft magnetic particles that do not fall into the category of the large particles or the small particles may be 40 μm or less. In other words, the Heywood diameter of the soft magnetic particle with the largest Heywood diameter in the cross section may be 40 μm or less.

The Heywood diameter of the soft magnetic particle with the largest Heywood diameter in the cross section may be 25 μm or less. In other words, soft magnetic particles with a Heywood diameter larger than that of the large particles may not be observed in the cross section.

When the maximum Heywood diameter of soft magnetic particles that do not fall into the category of large particles or small particles exceeds 40 μm, it becomes difficult to improve the voltage resistance characteristics of the magnetic core. This is because when soft magnetic particles with a large Heywood diameter are contained in a magnetic core, there are areas where the number of particles per unit length in the direction of voltage application decreases. In particular, when resin is filled between the particles, the difference in voltage resistance characteristics depending on whether or not soft magnetic particles with a large Heywood diameter are contained tends to be large.

The average ellipticity of the large particles in the cross section may be 0.930 or more. More preferably, it is 0.950 or more.

Although the circularity calculated by 2×(π×cross-sectional area) ^1/2 /(perimeter of cross section) is often used to evaluate the shape of a particle, in this embodiment, the particle shape is evaluated by the elliptical circularity, which is 4×cross-sectional area/(major axis×minor axis×π).

In general, when a particle is flat, the circularity is low. However, even when the particle is flat, the elliptical circularity is high. On the other hand, even when a particle has a concave or distorted shape, the circularity may not be low. However, when a particle has a concave or distorted shape, the elliptical circularity is low. Note that when a particle has a shape with significant irregularities, both the circularity and the elliptical circularity are low. In other words, to evaluate whether a particle has a deformation other than flattening when viewed from a perfect circle, for example, whether a particle has a depression, distortion, or unevenness, it may be preferable to use the elliptical circularity.

Here, whether or not the large particles contained in the magnetic core are flattened does not affect the voltage resistance characteristics. In contrast, whether or not the large particles are deformed in a way other than being flattened, for example, whether or not the large particles contained in the magnetic core are concave, distorted, or have large irregularities, is likely to affect the voltage resistance characteristics. This is because the voltage resistance characteristics of a magnetic core improve the fewer the points at which the electric field concentrates when a voltage is applied, and the number of points at which the electric field concentrates is less dependent on whether or not the large particles are flattened and more dependent on whether or not the large particles are deformed in a way other than being flattened.

There is no particular restriction on the microstructure inside the soft magnetic particles. For example, the soft magnetic particles may have a structure including amorphous matter, or may have a structure made of crystals. It is preferable that the large particles have a nanoheterostructure containing initial microcrystals with an average crystal grain size of 0.3 nm to 10 nm in the amorphous matter. In addition, when the soft magnetic particles have a structure made only of amorphous matter or a nanoheterostructure, the amorphization rate X described later is 85% or more. When the soft magnetic particles have a structure made of crystals, the amorphization rate X described later is less than 85%. When the filling rate is approximately the same, when the large particles have a nanoheterostructure, the relative permeability is improved compared to when the large particles have a structure made only of amorphous matter. Furthermore, it is preferable that the large particles have a structure made of crystals, an average crystal grain size is 1 nm to 30 nm, and a maximum crystal grain size is 100 nm or less (nanocrystalline structure). When the filling rate is approximately the same, when the large particles have a nanocrystalline structure, the relative permeability of the magnetic core is further improved. In addition, soft magnetic particles containing crystals, especially nanocrystals, usually contain many crystals in one particle. In other words, the particle size of the soft magnetic particles is different from the crystal grain size. There are no particular limitations on the method for calculating the crystal grain size. For example, there is a method of calculating the crystal grain size by analyzing the half-width by XRD measurement and evaluating the crystallite size, and a method of calculating the crystal grain size by observing the crystals using a TEM.

When the soft magnetic particles have an amorphous structure, the soft magnetic particles may have _a main component represented by the composition formula (Fe _{(1-(α+β)) X1αX2β} ) _{(1-(a+b+c+d+e+f))} M _{aB bP cSi dC eS f} ,
X1 is one or more selected from the group consisting of Co and Ni;
X2 is one or more selected from the group consisting of Al, Mn, Ag, Zn, Sn, As, Sb, Cu, Cr, Bi, N, O and rare earth elements;
M is one or more selected from the group consisting of Nb, Hf, Zr, Ta, Mo, W, Ti and V;
0≦a≦0.14
0≦b≦0.20
0≦c≦0.20
0≦d≦0.14
0≦e≦0.20
0≦f≦0.02
0.70≦1-(a+b+c+d+e+f)≦0.93
α≧0
β≧0
0≦α+β≦0.50
The above composition formula is expressed by the atomic ratio.

Furthermore, the nanocrystals contained in the soft magnetic particles (especially large particles) may be Fe-based nanocrystals. Fe-based nanocrystals are crystals with an average crystal grain size of the nano order (specifically, 0.1 nm or more and 100 nm or less) and a Fe crystal structure of bcc (body-centered cubic lattice structure). There are no particular limitations on the method of calculating the average crystal grain size of Fe-based nanocrystals. For example, there are a method of calculating the crystal grain size by analyzing the half-width by XRD measurement and evaluating the crystallite size, and a method of calculating the crystal grain size by observation using a TEM. There are also no particular limitations on the method of confirming that the crystal structure is bcc. For example, there are a method of confirming using XRD and a method of confirming by analyzing the electron beam diffraction pattern obtained using a TEM.

In this embodiment, the Fe-based nanocrystals may have an average crystal grain size of 1 to 30 nm. Soft magnetic particles having a structure made of such Fe-based nanocrystals tend to have a high saturation magnetic flux density and a low coercive force. In other words, the soft magnetic properties are likely to be improved. In other words, the inclusion of these soft magnetic particles makes it easier to achieve a magnetic core with low coercive force and high relative permeability. Furthermore, the saturation magnetic flux density of the magnetic core containing these soft magnetic particles increases, improving the DC superposition properties of the magnetic core. As described above, the use of soft magnetic particles having a structure made of Fe-based nanocrystals makes it easier to improve the properties of the magnetic core.

When the soft magnetic particles have a structure made of Fe-based nanocrystals, the soft magnetic particles may have a main component made of, for example, the composition formula (Fe _(1-(α+β)) X1 _α X2 _β ) _{(1-(a+b+c+d+e+f))} M _a B _b P _c Si _d C _e S _f ;
X1 is one or more selected from the group consisting of Co and Ni;
X2 is one or more selected from the group consisting of Al, Mn, Ag, Zn, Sn, As, Sb, Cu, Cr, Bi, N, O and rare earth elements;
M is one or more selected from the group consisting of Nb, Hf, Zr, Ta, Mo, W, Ti and V;
0≦a≦0.14
0≦b≦0.20
0≦c≦0.20
0≦d≦0.14
0≦e≦0.20
0≦f≦0.02
0.70≦1-(a+b+c+d+e+f)≦0.93
α≧0
β≧0
0≦α+β≦0.50
The above composition formula is expressed by the atomic ratio.

In the manufacturing method of the magnetic core described later, when the soft magnetic metal powder containing the soft magnetic particles having the above composition is heat treated, Fe-based nanocrystals are easily precipitated in the soft magnetic particles. In other words, the soft magnetic particles having the above composition are easily used as the starting material for the soft magnetic metal powder containing the soft magnetic particles having the precipitated Fe-based nanocrystals.

When Fe-based nanocrystals are precipitated in soft magnetic particles by heat treatment, the soft magnetic particles before heat treatment may have a structure consisting of only amorphous material, or may have a nanoheterostructure in which initial microcrystals exist in the amorphous material. The initial microcrystals may have an average crystal grain size of 0.3 nm or more and 10 nm or less. In addition, when the soft magnetic particles have a structure consisting of only amorphous material or a nanoheterostructure, the amorphization rate X described below is 85% or more.

Furthermore, it is preferable that the small particles contain Fe as the main component. It is also preferable that the small particles contain Fe and at least one selected from Si and Ni as the main components.

Incidentally, containing Fe as the main component means that the Fe content in the entire soft magnetic particle is 85 at% or more and 100 at% or less.

The phrase "containing Fe and at least one selected from Si and Ni as the main components" means that the soft magnetic particles contain at least Fe and at least one selected from Si and Ni, and the total content of Fe, Si and Ni in the entire soft magnetic particle is 85 at% or more and 100 at% or less. In this case, the Fe content may be 15 at% or more and 95 at% or less. The total content of Si and Ni may be 3 at% or more and 90 at% or less. There are no particular restrictions on the type of elements other than the main components. Examples include Co, Cr, Al, etc.

When the small particles have the above composition, it is possible to produce a magnetic core with high magnetic properties. That is, when the magnetic core has small particles containing Fe as the main component, it is possible to increase the saturation magnetic flux density of the small particles. Therefore, it is possible to improve the saturation magnetic flux density of the entire magnetic core, and it is possible to improve the DC superposition characteristics of the magnetic core. Also, when the magnetic core has small particles containing Fe and at least one selected from Si and Ni as the main components, it is possible to increase the relative magnetic permeability of the small particles. Therefore, it is possible to improve the relative magnetic permeability of the entire magnetic core.

The manufacturing method of the magnetic core according to this embodiment is described below, but the manufacturing method of the magnetic core is not limited to the method described below.

First, a soft magnetic metal powder containing the soft magnetic particles according to the present embodiment is prepared. The soft magnetic metal powder according to the present embodiment can be obtained by mixing a soft magnetic metal powder that will ultimately be mostly large particles with a soft magnetic metal powder that will ultimately be mostly small particles.

Soft magnetic metal powder, which ultimately consists mostly of large particles, can be produced, for example, by gas atomization.

In the gas atomization method, raw metal is melted and the molten metal is powdered by gas atomization to produce soft magnetic metal powder. The composition of the molten metal is the same as that of the large particles to be ultimately obtained. At this time, the molten metal is dripped from a container with an outlet toward a cooling section. The temperature of the molten metal is the spray temperature. There is no particular limit to the spray temperature. For example, it is between 1200°C and 1600°C. The higher the spray temperature, the easier it is for the average aspect ratio to approach 1, and the easier it is for the average particle size to become smaller.

A gas injection nozzle equipped with a gas injection port is arranged so as to surround the discharge port. From the gas injection port, high-pressure gas (gas with an injection pressure (gas pressure) of 2.0 MPa or more and 10 MPa or less) is injected toward the molten metal dropped from the discharge port. As a result, the molten metal becomes a large number of droplets. By controlling the pressure of the high-pressure gas at this time, the particle size and shape of the soft magnetic metal powder finally obtained can be changed. Specifically, when the amount of molten metal injected is the same, the higher the pressure of the high-pressure gas, the smaller the particle size of the soft magnetic metal powder finally obtained. In other words, the particle size and shape of the soft magnetic metal powder can be changed by the ratio of the pressure of the high-pressure gas to the amount of molten metal injected.

The gas to be injected from the gas injection port is preferably an inert gas such as nitrogen gas, argon gas, or helium gas, or a reducing gas such as an ammonia decomposition gas, etc. If the molten metal is not easily oxidized, air may be used.

There is no particular restriction on the shape of the cooling section onto which the molten metal is dripped, but it may be, for example, a cylinder formed with a cooling liquid flow inside to collide with the molten metal. In this case, by controlling the amount of molten metal sprayed and the water pressure of the cooling liquid flow in addition to the high-pressure gas pressure described above, the particle diameter and average aspect ratio of the large particles in the final magnetic core can be changed. In other words, the particle diameter and average aspect ratio of the large particles are controlled by controlling the balance between the amount of molten metal sprayed, the high-pressure gas pressure, and the water pressure of the cooling liquid flow. The amount of molten metal sprayed may be 0.5 kg/min or more and 4.0 kg/min or less, and the water pressure may be 5.0 MPa or more and 20.0 MPa or less. Specifically, the larger the amount of sprayed, the larger the particle diameter of the large particles is likely to be. Also, the smaller the water pressure, the more likely the average aspect ratio of the large particles is to approach 1. In other words, when changing the aspect ratio of the particles without changing the particle diameter, it is necessary to appropriately adjust the amount of sprayed, the water pressure of the high-pressure gas, and the water pressure of the cooling liquid flow.

The molten metal discharged into the cooling liquid flow collides with the cooling liquid flow and is further divided and refined. As the molten metal is refined, the refined molten metal changes shape as it is cooled and solidified, becoming solid soft magnetic metal powder. The soft magnetic metal powder discharged together with the cooling liquid is separated from the cooling liquid in an external storage tank or the like and taken out. There are no particular limitations on the type of cooling liquid. For example, cooling water can be used. If no cooling liquid is used, the soft magnetic metal powder, which will ultimately consist mostly of large particles, is more likely to contain coarse crystals with a crystal grain size exceeding 100 nm.

In addition, the shorter the time between when the molten metal is dropped and when it collides with the cooling liquid flow, the less likely the molten metal will be oxidized. This will promote the rapid cooling effect and make it more likely to become amorphous.

The obtained soft magnetic metal powder may be subjected to a heat treatment. There are no particular restrictions on the conditions of the heat treatment. For example, the heat treatment may be performed at 400 to 700°C for 0.1 to 10 hours. By performing the heat treatment, when the microstructure of the particles is a structure having only amorphous matter or a nanoheterostructure in which primary crystallites exist in the amorphous matter, the microstructure of the particles is more likely to become a structure containing nanocrystals. As a result, the coercive force of the soft magnetic metal powder tends to decrease. Note that if the temperature of the heat treatment is too high, the coercive force of the soft magnetic metal powder tends to increase.

There are no particular limitations on the method for confirming the microstructure of soft magnetic metal powder. For example, it can be confirmed by XRD. Furthermore, the microstructure of the soft magnetic metal powder before compaction and the microstructure of the particles contained in the magnetic core after compaction are usually the same.

In this embodiment, a soft magnetic metal powder having an amorphization rate X of 85% or more as shown in the following formula (1) has a structure consisting only of amorphous matter or a nanoheterostructure, and a soft magnetic metal powder having an amorphization rate X of less than 85% has a structure consisting of crystals.
X=100-(Ic/(Ic+Ia)×100)…(1)
Ic: Crystalline scattering integrated intensity Ia: Amorphous scattering integrated intensity

The amorphous ratio X is calculated by performing X-ray crystal structure analysis on the soft magnetic metal powder by XRD, identifying the phase, reading the peaks of the crystallized Fe or compound (Ic: crystalline scattering integrated intensity, Ia: amorphous scattering integrated intensity), determining the crystallization ratio from the peak intensity, and calculating it using the above formula (1). The calculation method will be explained in more detail below.

The soft magnetic metal powder according to this embodiment is subjected to X-ray crystal structure analysis by XRD to obtain a chart as shown in FIG. 1. This is subjected to profile fitting using the Lorentz function of the following formula (2), and a crystalline component pattern α _c showing crystalline scattering integral intensity, an amorphous component pattern α _a showing amorphous scattering integral intensity, and a combined pattern α _{c + a} are obtained as shown in FIG. 2. From the crystalline scattering integral intensity and amorphous scattering integral intensity of the obtained pattern, the amorphization rate X is obtained by the above formula (1). The measurement range is the range of diffraction angle 2θ = 30 ° to 60 ° where an amorphous-derived halo can be confirmed. In this range, the error between the integrated intensity actually measured by XRD and the integrated intensity calculated using the Lorentz function is set to within 1%.

Soft magnetic metal powder, which ultimately consists mostly of small particles, is produced by various powdering methods, such as the liquid phase method, spray pyrolysis method, and melting method.

The aspect ratio can be changed by further treating the obtained soft magnetic metal powder with a ball mill. Specifically, the soft magnetic metal powder can be deformed by treatment with a ball mill, and the average aspect ratio of the small particles contained in the final magnetic core can be increased. The longer the treatment time with the ball mill, the more deformed the soft magnetic metal powder becomes. The average aspect ratio of the small particles contained in the final magnetic core can also be controlled by mixing small particles that have been treated with a ball mill with small particles that have not been treated with the ball mill.

The average particle size of the soft magnetic metal powder, which ultimately consists mostly of small particles, can be controlled by appropriately removing coarse and/or fine powder using an air classifier.

When making a magnetic core from the soft magnetic metal powder according to this embodiment, first mix the soft magnetic metal powder that will eventually become mostly large particles with the soft magnetic metal powder that will eventually become mostly small particles. There are no particular limitations on the mixing method. The mixing ratio may be, for example, 9:1 to 1:9 by mass of the soft magnetic metal powder that will eventually become mostly large particles: the soft magnetic metal powder that will eventually become mostly small particles.

Next, resin may be added to the mixed soft magnetic metal powder as necessary. The ratio of resin to the entire magnetic core may be set to 1.5% by mass or more and 5.0% by mass or less. Furthermore, the powder mixed with the resin may be granulated.

The obtained powder is then filled into a mold and compression molded to obtain a magnetic core. There are no particular limitations on the conditions for compression molding. Compression molding may be performed at a molding pressure of, for example, 1 t/cm2 or ^more and 8 t/cm2 or ^less . The filling rate can be controlled by changing the molding pressure.

The magnetic core according to this embodiment has been described above, but the magnetic core of the present invention is not limited to the above embodiment.

The magnetic core of the present invention can be used in any application, including coil components (magnetic components) such as inductors, choke coils, and transformers. Furthermore, electronic devices using the magnetic core of the present invention, such as DC-DC converters, can also be used.

The present invention will be described below in more detail with reference to examples, but the present invention is not limited to these examples.

Experimental Example 1
In Samples No. 1 to 28, soft magnetic metal powders that ultimately consisted mostly of large particles were prepared by gas atomization _and had a _{composition of Fe0.800Nb0.070B0.098P0.030S0.002 .}

At this time, the amount of molten metal sprayed, the temperature of the molten metal (spray temperature), the spray gas pressure, and the water pressure of the cooling liquid flow were set to the values shown in Table 2, and soft magnetic metal powders were produced in which the average aspect ratios A1 of the large particles in the final magnetic core were 1.01, 1.30, 1.50, and 2.00. The gas type used was Ar. Other conditions were also appropriately controlled so that the average particle diameter (D50) of the resulting soft magnetic metal powder on a number basis was 10.3 μm.

The resulting soft magnetic metal powder was then subjected to heat treatment. The heat treatment conditions were 600°C for 1 hour, and the heat treatment was performed in an Ar atmosphere.

The average particle size (D50) of the obtained soft magnetic metal powder based on the number was confirmed to be 10.3 μm. The average particle size based on the number was measured using Morphologi G3. In addition, it was confirmed using XRD and STEM that each soft magnetic metal powder had a nanocrystalline structure. In addition, after producing a magnetic core using the obtained soft magnetic metal powder by the method described below, the cross section of the magnetic core was observed using SEM at a size where at least 10 soft magnetic particles were observed. The observation using SEM was repeated so that a total of 100 or more soft magnetic particles were observed. Then, the Heywood diameter of 100 or more soft magnetic particles was measured. As a result, the average particle size (D50) of the soft magnetic particles in the Heywood diameter was 7.5 μm. The reason why the average particle size of the soft magnetic particles measured by observing the cross section of the magnetic core is smaller than the average particle size of the actual soft magnetic metal powder is because the soft magnetic metal powder is cut at an arbitrary location when the cross section of the magnetic core is cut.

In samples No. 1 to 28, soft magnetic metal powders that ultimately consisted mostly of small particles were prepared by spray pyrolysis. The composition was 100 at% Fe. In addition, the average particle diameter (D50) of the small particle powder was controlled to be 1.5 μm by appropriately removing coarse and/or fine powders using an air classifier. In addition, at this point, the soft magnetic metal powder had a shape close to a perfect sphere, and the average aspect ratio of the soft magnetic particles when a magnetic core was prepared using this soft magnetic metal powder was 1.00. In addition, after preparing a magnetic core using the obtained soft magnetic metal powder by the method described below, the cross section of the magnetic core was observed using an SEM at a size where at least 10 or more soft magnetic particles were observed. The observation using the SEM was repeated so that a total of 100 or more soft magnetic particles were observed. Then, the Heywood diameter of 100 or more soft magnetic particles was measured. As a result, the average particle size (D50) of the soft magnetic particles in Heywood diameter was 1.1 μm.

Next, by processing using a ball mill, soft magnetic metal powders were produced in which the average aspect ratios A2 of the small particles in the final magnetic core were 1.00, 1.30, 1.50, 1.70, 2.00, 2.50, and 3.00. The processing times are shown in Table 3.

For the soft magnetic metal powder that will eventually become mostly large particles, a coating with an average thickness of 20 nm was formed using zinc phosphate. For the soft magnetic metal powder that will eventually become mostly small particles, a coating with an average thickness of 10 nm was formed using zinc phosphate.

Then, the soft magnetic metal powder that will eventually become mostly large particles and the soft magnetic metal powder that will eventually become mostly small particles were mixed in a mass ratio of 8:2. If the powder densities are the same, the above mass ratio and area ratio (S1:S2) will roughly match. It is also possible to calculate the mass ratio of the large particles to the small particles from the area ratio of the large particles to the small particles and the density of each particle. Furthermore, epoxy resin was diluted with acetone and added to 100 parts by mass of the soft magnetic metal powder obtained by mixing to make 3 parts by mass. The mixture was then kneaded in a kneader, dried, and the resulting aggregate was sized to have an average particle diameter of 355 μm or less in the volume particle size distribution to obtain granules. This was filled into a mold. The shape of the mold was such that the shape of the magnetic core finally obtained was toroidal.

Next, the soft magnetic metal powder was pressure-molded. The molding pressure was controlled so that the packing rate of the resulting magnetic core would be the value shown in Table 1. Specifically, the molding pressure was controlled within the range of 1 to 8 ton/ ^cm2 .

For each experimental example, a cross section cut parallel to the molding direction (height direction) was observed. Specifically, an SEM was used to set the observation range so that 10 or more large particles were visible. The magnification was 1000x. It was also confirmed that for each experimental example, the area ratio of large particles to the total soft magnetic particles contained in the magnetic core was 10% or more, and the area ratio of small particles was 10% or more.

The average aspect ratio A1 of the large particles, the average ellipticity of the large particles, the average aspect ratio A2 of the small particles, the maximum Heywood diameter of the soft magnetic particles, the filling rate, the relative magnetic permeability, and the voltage resistance characteristics of the magnetic cores of each experimental example were measured. A1, A2, the average ellipticity of the large particles, the maximum Heywood diameter, and the filling rate of each magnetic core were calculated from SEM images. The relative magnetic permeability was measured using an impedance/gain-phase analyzer (4194A, manufactured by Yokogawa Hewlett-Packard Co., Ltd.). A relative magnetic permeability higher than 40 was considered good.

The withstand voltage characteristics of each magnetic core were evaluated by forming a pair of In-Ga electrodes on the magnetic core, sandwiching the pair of In-Ga electrodes between a pair of copper plates, and applying a voltage to the pair of copper plates to evaluate the voltage when a current of 1 mA flows. In this experimental example, a value of 50 V/mm or more was considered good. The results are shown in Table 1.

From Table 1, the examples that satisfied all of 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2 had significantly improved voltage resistance characteristics compared to the comparative examples that did not satisfy any of 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2. In addition, the filling factor was the same in each example and each comparative example, and the relative permeability was good in each example.

Experimental Example 2
In Experimental Example 2, the experiment was carried out under the same conditions as in Experimental Example 1, except for the points described below. In Experimental Example 2, the gas pressure was changed when preparing the soft magnetic metal powder that would eventually become mostly large particles. The average particle size of the soft magnetic metal powder and the average aspect ratio A1 of the large particles in the finally obtained magnetic core were set to the values shown in Table 5. In addition, the average particle size of the soft magnetic metal powder that would eventually become mostly small particles was controlled to the values shown in Table 4 by appropriately removing coarse powder and/or fine powder using an air classifier. In addition, for each example, it was confirmed that the area ratio of the large particles to the total soft magnetic particles contained in the magnetic core was 10% or more, the area ratio of the small particles was 10% or more, the average ellipticity of the large particles was 0.95 or more, and the soft magnetic metal powder that would eventually become mostly large particles had a nanocrystalline structure.

As can be seen from Table 4, even when the average particle size of the soft magnetic metal powder was changed, each of the examples that satisfied all of the conditions 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2 had good voltage resistance characteristics despite having a high relative magnetic permeability. In addition, compared to sample No. 31, in which the maximum Heywood diameter of the soft magnetic particles exceeded 40 μm, the other examples had even better voltage resistance characteristics.

Experimental Example 3
In Experimental Example 3, the average aspect ratio A1 and average ellipticity of the large particles were controlled by changing the atomization conditions from those in Experimental Example 1. The atomization conditions are shown in Table 7. The atomization conditions for Samples No. 38 to 42 were the same as those for Sample No. 9.

In Experimental Example 3, the soft magnetic metal powders that would eventually become mostly large particles were heat-treated in the same manner as in Experimental Examples 1 and 2 to produce soft magnetic metal powders with a nanocrystalline structure, and soft magnetic metal powders without heat treatment to produce soft magnetic metal powders with an amorphous structure or nanoheterostructure. When heat treatment was not performed, the average particle size (D50) of the soft magnetic metal powders that would eventually become mostly large particles was 10.4 μm based on the number of particles.

Furthermore, in samples No. 39 and 41, the soft magnetic metal powder that will eventually become mostly small particles has a composition mainly composed of Fe and Ni, that is, the atomic ratio is Ni ₈₀ Fe _20. The soft magnetic metal powder was produced in the same manner as in Experimental Examples 1 and 2. Furthermore, in samples No. 40 and 42, the soft magnetic metal powder that will eventually become mostly small particles has a composition mainly composed of Fe and Si and further contains Cr, that is, the atomic ratio is Fe _57.1 Si _28.6 Cr _14.3. The soft magnetic metal powder was produced in the same manner as in Experimental Examples 1 and 2.

The above soft magnetic metal powders were appropriately combined to produce magnetic cores. The results are shown in Table 6.

As can be seen from Tables 6 and 7, the lower the average ellipticity of the large particles, the lower the withstand voltage. Furthermore, changing the composition of the small particles does not significantly change the withstand voltage characteristics, but when the microstructure of the large particles is an amorphous structure or a nanoheterostructure, the relative permeability is lower than when it is a nanocrystalline structure.

Experimental Example 4
In Experimental Example 4, the same conditions were used as for Sample No. 9, except that the amount of zinc phosphate added was changed to change the coating thickness of each particle. The results are shown in Table 8.

As can be seen from Table 8, the same trend was observed even when the coating thickness was changed. In addition, the greater the coating thickness, the better the voltage resistance characteristics were, but the relative permeability tended to decrease.

Experimental Example 5
In Experimental Example 5, the crystal structure of the soft magnetic metal powder, the majority of which eventually became large particles, was changed by controlling the composition and atomization conditions. The composition and crystal structure are shown in Table 9. Specifically, the atomization conditions for Samples No. 51 to 56 were the same as those for Sample No. 1 shown in Table 2. The atomization conditions for Samples No. 57 to 62 were the same as those for Sample No. 1, except that cooling water was not sprayed. No heat treatment was performed on Samples No. 51 to 62. The other points were the same as those in Experimental Example 1. The composition of the large particles was described in terms of atomic ratio. The results are shown in Table 9.

From Table 9, the examples that satisfied all of 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2 had significantly improved voltage resistance characteristics compared to the comparative examples that did not satisfy any of 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2. In addition, the filling factor was the same in each example and each comparative example, and the relative permeability was good in each example. In addition, in the examples (samples No. 52 to 55) in which the microstructure of the large particles was made only of amorphous matter, and in the examples (samples No. 58 to 61) in which the microstructure of the large particles was made of crystals and contained coarse crystals with a crystal grain size exceeding 100 nm, the relative permeability was lower than in the examples (samples No. 5, 13, 17, and 21) in which the microstructure of the large particles was made of nanocrystals.

Experimental Example 6
In Experimental Example 6, for Samples No. 1 and 5, the mixing ratio of the soft magnetic metal powder that will eventually become mostly large particles and the soft magnetic metal powder that will eventually become mostly small particles was changed to obtain S1:S2 as shown in Table 10. Also, unlike Experimental Examples 1 to 5, the molding pressure was unified to 4 t/ ^cm2 . This is to evaluate the change in the filling rate when S1:S2 is changed. The other points were carried out under the same conditions. The results are shown in Table 10.

Table 10 compares the examples and comparative examples in which S1:S2 are the same. The examples that satisfy all of 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2 have improved voltage resistance characteristics compared to the comparative examples that do not satisfy any of 1.00≦A1≦1.50, 1.30≦A2≦2.50, and A1<A2.

From Table 10, if the molding pressure was the same, the filling rate was higher when S1:S2 was 8:2. The relative permeability was highest when S1:S2 was 8:2. The higher the ratio of S2 to S1, the more the voltage resistance characteristics tended to improve. However, the higher the ratio of S2 to S1, the lower the filling rate and the lower the relative permeability.

For each example in Table 10, the molding pressure was changed between 1 and 8 t/ ^cm2 to confirm the change in the filling rate. When comparing examples made at the same molding pressure, the filling rate and relative permeability were higher when S1:S2 was 8:2 regardless of the molding pressure. The higher the ratio of S2 to S1, the more the withstand voltage characteristics tended to improve. However, the higher the ratio of S2 to S1, the more the filling rate and the relative permeability tended to decrease. Therefore, it was confirmed that the filling property was best when S1:S2 was 8:2.

Claims (8)

In a cross section, the large particles are observed as soft magnetic particles having a Heywood diameter of 5 μm or more and 25 μm or less, and the small particles are observed as soft magnetic particles having a Heywood diameter of 0.5 μm or more and 3 μm or less,
The average aspect ratio of the large particles is A1, and the average aspect ratio of the small particles is A2,
1.00≦A1≦1.50
1.30≦A2≦2.50
A1<A2
Meet the magnetic core. The magnetic core according to claim 1, wherein the maximum Heywood diameter of the soft magnetic particles that do not fall into the large particles or small particles in the cross section is 40 μm or less. The magnetic core according to claim 1 or 2, wherein the average ellipticity of the large particles in the cross section is 0.93 or more. A magnetic core according to any one of claims 1 to 3, wherein the large grains contain nanocrystals. A magnetic core according to any one of claims 1 to 4, in which the small particles contain Fe as a main component. The magnetic core according to any one of claims 1 to 4, wherein the small particles contain, as main components, Fe and at least one selected from Si and Ni. A magnetic component including a magnetic core according to any one of claims 1 to 6. An electronic device including a magnetic core according to any one of claims 1 to 6.

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