JP6460505B2 - Manufacturing method of dust core - Google Patents

Description

  The present invention relates to a method for manufacturing a dust core.

  Conventionally, coil parts such as inductors, transformers and chokes have been used in a wide variety of applications such as home appliances, industrial equipment, and vehicles. The coil component includes a magnetic core and a coil wound around the magnetic core. For such a magnetic core, ferrite having excellent magnetic properties, flexibility in shape and price is widely used.

  In recent years, as power supply devices such as electronic devices have been downsized, the demand for coil parts that are small and low in profile and can be used for large currents has become stronger, and the saturation magnetic flux density is higher than that of ferrite. Adoption of powder magnetic cores using metallic magnetic powder is progressing. As the metal-based magnetic powder, for example, magnetic alloy powders such as Fe-Si, Fe-Ni, and Fe-Si-Al are used.

On the other hand, a dust core obtained by compacting a magnetic alloy powder such as Fe—Si or Fe—Ni has a higher saturation magnetic flux density than ferrite, but has a low electrical resistivity because it is an alloy powder. Therefore, a method for increasing the insulation between the magnetic alloy powders, such as forming after forming an insulating coating on the surface of the alloy powder, is applied. In Patent Document 1, _a heat-resistant insulating oxide is attached to a soft magnetic metal powder having a composition of Fe _100-ab Si _a Cr _b (by weight, 0 ≦ a ≦ 8, 0 <b ≦ 3). A soft magnetic metal powder in which a Cr-rich insulating film is formed on the powder surface by heat treatment in a non-oxidizing atmosphere and a green compact using the same are disclosed. In Patent Document 1, an object is to obtain a green compact that can be heat-treated at high temperature and has low hysteresis loss and high electrical resistance.

JP 2008-195986 A

  Since the dust core disclosed in Patent Document 1 contains Cr and an insulating oxide is formed, the dust core is superior in corrosion resistance and insulation as compared to a Fe-Si based dust core. Is expected to be obtained. However, in a dust core, high strength is required to ensure not only corrosion resistance and the like, but also stability during production and reliability during use. The need for higher strength increases as the coil components become smaller. In this regard, as described above, the dust core disclosed in Patent Document 1 is not necessarily sufficient for increasing the strength, although improvement in corrosion resistance and the like is expected.

  Then, in view of the said problem, an object of this invention is to provide the manufacturing method of the powder magnetic core excellent in intensity | strength.

  The present invention relates to a method for producing a dust core using Fe-Si-Cr soft magnetic alloy powder containing Fe, Si and Cr, wherein the surface is provided with a Si oxide film. A first step of mixing the soft magnetic alloy powder and the binder, a second step of press-molding the mixture obtained through the first step, and a molded body obtained through the second step. A dust core obtained through the third step, in which the alloy phases of the Fe—Si—Cr soft magnetic alloy powder are Fe, Si and Having a structure bonded through an oxide phase containing Cr, the oxide phase has a Cr content and a Si content larger than the alloy phase in mass ratio on the alloy phase side, and It has a Cr-Si enriched part in which the total content of Cr and Si is larger than the content of Fe. A method for producing a dust core according to claim Rukoto.

  In the method for producing a dust core according to the present invention, the Si oxide film is prepared by mixing Fe-Si-Cr soft magnetic alloy powder with a mixed solution of TEOS (tetraethoxysilane), ethanol, and ammonia water, and then drying. Preferably formed.

  In the method for manufacturing a dust core according to the present invention, the Si oxide film preferably has a thickness of 20 to 500 nm.

  In the method for producing a dust core according to the present invention, the binder is preferably an organic binder of polyethylene, polyvinyl alcohol, or acrylic resin.

  In the method for producing a dust core according to the present invention, it is preferable that the heat treatment is performed in the atmosphere at a temperature of 500 to 900 ° C.

  In the method for producing a dust core according to the present invention, the Fe—Si—Cr soft magnetic alloy powder has a Si content of 1.0 mass% to 10.0 mass% and a Cr content of 1.0 mass% to 7.0 mass%. It is preferable that the composition be non-magnetic elements of less than or equal to 1% by mass and less than 1.0% by mass (including 0), the remainder being inevitable impurities, and Fe.

  In the method for producing a dust core according to the present invention, the Fe—Si—Cr soft magnetic alloy powder is preferably a granular powder having an average particle diameter d50 of 1 μm to 30 μm.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the powder magnetic core excellent in intensity | strength can be provided.

It is a cross-sectional TEM photograph of the powder magnetic core which is one Embodiment of this invention.

The dust core obtained by the present invention is a dust core using Fe-Si-Cr soft magnetic alloy powder containing Fe, Si, and Cr, and has an alloy homogeneity with the Fe-Si-Cr soft magnetic alloy powder. Has a structure bonded through an oxide phase containing Fe, Si and Cr, and the oxide phase is closer to the alloy phase than the alloy phase in terms of mass ratio and Si content. One of its features is that it has a Cr—Si concentrated portion (Cr—Si rich portion) having a large content and a total content of Cr and Si larger than the content of Fe. With such an oxide structure, it is possible to increase the strength of the powder magnetic core using the Fe—Si—Cr soft magnetic alloy powder.
Hereinafter, although embodiment of the powder magnetic core which concerns on this invention is described concretely, this invention is not limited to this.

  The Fe—Si—Cr based soft magnetic alloy powder contains Fe, Si and Cr as three main elements having a high content ratio. Among these, since Cr is an element that contributes to the improvement of corrosion resistance, the Fe—Si—Cr-based soft magnetic alloy powder is superior in corrosion resistance compared to Fe—Si-based alloy powder. A powder magnetic core having excellent corrosion resistance and strength can be obtained by using such soft magnetic alloy powder and combining the alloy phases of Fe-Si-Cr soft magnetic alloy powder through the above-mentioned oxide phase. .

  The structure of the oxide phase will be described in detail together with the method for producing a dust core. A dust core according to the present invention includes a first step of mixing a soft magnetic alloy powder and a binder, a second step of pressure-molding the mixture obtained through the first step, and the second step. It can obtain by the manufacturing method which has the 3rd process of heat-processing the molded object obtained through the process. By performing the third step and the like under specific conditions described later, a structure in which the alloy phases of the Fe-Si-Cr soft magnetic alloy powder are bonded via an oxide phase containing Fe, Si, and Cr is provided. And the powder magnetic core which the said oxide phase has a Cr-Si concentration part in the said alloy phase side can be obtained.

  The Cr—Si concentrating portion is formed by concentrating Cr and Si on the surface of the Fe—Si—Cr soft magnetic alloy powder by the heat treatment in the third step. Since Cr is an easily oxidizable element, if oxygen is present on the surface of the Fe—Si—Cr soft magnetic alloy powder, it diffuses and concentrates on the surface to form an oxide. During the formation of such an oxide, Fe—Si—Cr soft magnetic alloy powders are bonded together. At this time, it is particularly effective to improve the strength to form a Cr—Si concentrated portion in which not only Cr but also Si is concentrated. Moreover, this Cr-Si concentration part is a structure advantageous also from a viewpoint of the improvement of an electrical resistivity, and reduction of a core loss. Since the Cr-Si concentrated portion has relatively less Fe, it is also an Fe-deficient portion (Fe poor portion).

  Cr and Si forming the Cr-Si concentrated portion are supplied from the Fe-Si-Cr soft magnetic alloy powder itself, but in addition, Si is supplied from other than the Fe-Si-Cr soft magnetic alloy powder itself. It is preferable. Such a configuration can be obtained, for example, by performing the heat treatment in the third step in an oxidizing atmosphere in a state where the Si compound is arranged on the surface of the Fe—Si—Cr soft magnetic alloy powder. These configurations relating to the Cr-Si concentrated portion contrast with the configuration shown in Patent Document 1 in which Si or Cr is not concentrated on the surface of the soft magnetic metal powder in order to heat-treat the green compact in nitrogen.

  The state in which the Si compound is arranged on the surface of the Fe—Si—Cr soft magnetic alloy powder is, for example, (a) TEOS (on the surface of the Fe—Si—Cr soft magnetic alloy powder in advance before being subjected to the first step. (Si) (Si) coating using tetraethoxysilane), (b) In the first step, Fe-Si-Cr soft magnetic alloy powder and colloidal silica are mixed, and Fe-Si This can be realized by placing silica on the surface of the Cr-based soft magnetic alloy powder. The state in which the Si compound is disposed on the surface of the Fe—Si—Cr soft magnetic alloy powder is not limited to the above configurations (a) and (b), and a Si supply source other than those described above may be used. Good. In addition, the Si compound is not limited to silica, and may include elements other than Si or may include compounds other than Si compounds.

  In the case of the method (a), the thickness of the Si oxide (silica) film is not particularly limited. From the viewpoint of ensuring a sufficient thickness as the coating and preventing the distance between the soft magnetic alloy powders from becoming unnecessarily large, the thickness may be, for example, in the range of 20 to 500 nm. For example, the surface of the Fe-Si-Cr soft magnetic alloy powder is impregnated with a mixed solution of TEOS (tetraethoxysilane), ethanol, and ammonia water with Fe-Si-Cr soft magnetic alloy powder, stirred and then dried. In addition, a Si oxide (silica) film can be formed.

When the method (a) is used, the Fe-enriched part (Fe having the highest Fe content among Fe, Si and Cr) in mass ratio between the Cr-Si enriched parts in the oxide phase. A dust core having a rich portion is obtained. Such a configuration is suitable for improving the strength of the dust core. When the method (b) is used, a Cr-enriched part having the largest Cr content among Fe, Si and Cr in a mass ratio between the Cr-Si-enriched parts in the oxide phase. A dust core having (Cr-rich part) is obtained. Such a configuration is particularly suitable for improving the strength of the dust core. In addition, these phases with much content of Fe or Cr have less Si content than a Cr-Si concentrated part, and can be distinguished from this. The oxide phase having a Cr—Si enriched portion or the like contains Fe, Si and Cr as a whole, but may contain other elements. However, the oxide phase is preferably an oxide of an element constituting the Fe—Si—Cr soft magnetic alloy powder, and the oxide phase is composed of only Fe, Si and Cr except for inevitable impurities. More preferred.
Even when the surface of the Fe—Si—Cr soft magnetic alloy powder subjected to pressure forming is covered with a Si compound as in the case of using the method (a) or the like, the formed body is heat-treated in an oxidizing atmosphere. Thus, an oxide is further formed between the Fe—Si—Cr soft magnetic alloy powders, and the Fe—Si—Cr soft magnetic alloy powders are bonded to each other.

The Fe—Si—Cr soft magnetic alloy powder will be further described in detail. The composition of the Fe-Si-Cr soft magnetic alloy powder is not particularly limited as long as the above-described configuration of the powder magnetic core can be realized. For example, the following composition can be used. .
Si is an element that increases electrical resistivity and magnetic permeability. From this viewpoint, for example, Si is preferably 1.0% by mass or more. More preferably, it is 2.0 mass% or more. On the other hand, if the amount of Si is excessively increased, the saturation magnetic flux density is greatly decreased. More preferably, it is 6.0 mass% or less, More preferably, it is 4.0 mass% or less.
As described above, Cr is an element that improves corrosion resistance and the like. From this viewpoint, for example, Cr is preferably 1.0% by mass or more. More preferably, Cr is 2.0 mass% or more. On the other hand, if the amount of Cr is too much, the saturation magnetic flux density is lowered, so 7.0% by mass or less is preferable. More preferably, Cr is 5.0 mass% or less.

  The balance other than Si and Cr is mainly composed of Fe, but may contain other elements as long as the advantage of using the Fe—Si—Cr alloy powder is exhibited. However, the content of other elements is less than the content of Fe, Si and Cr. Further, since the saturation magnetic flux density and the like of the nonmagnetic element is decreased, it is more preferably less than 1.0% by mass excluding inevitable impurities. The Fe—Si—Cr-based alloy powder is more preferably composed of Fe, Si and Cr excluding inevitable impurities.

  The average particle diameter of the soft magnetic alloy powder (here, the median diameter d50 in the cumulative particle size distribution is used) is not limited to this, but for example, a powder having an average particle diameter of 1 μm or more and 100 μm or less should be used. Can do. By reducing the average particle size, the strength, core loss, and high frequency characteristics of the powder magnetic core are improved. Therefore, the median diameter d50 is more preferably 30 μm or less, and even more preferably 15 μm or less. On the other hand, in order to obtain a high magnetic permeability, it is effective that the average particle diameter is large, so the median diameter d50 is more preferably 5 μm or more.

  Further, the form of the soft magnetic alloy powder is not particularly limited. For example, from the viewpoint of fluidity and the like, it is preferable to use granular powder represented by atomized powder. Atomization methods such as gas atomization and water atomization are suitable for producing powders of alloys that have high malleability and ductility and are difficult to grind. The atomization method is also suitable for obtaining a substantially spherical soft magnetic alloy powder.

  Next, the binder used in the first step will be described. When the binder is pressure-molded, the binder binds the powders together and gives the molded body the strength to withstand handling after molding. Although the kind of binder does not limit this, For example, various organic binders, such as polyethylene, polyvinyl alcohol, an acrylic resin, can be used. The organic binder is thermally decomposed by heat treatment after molding. Therefore, an inorganic binder such as a silicone resin that solidifies and remains after the heat treatment and binds the powders may be used in combination. However, in the method of manufacturing a powder magnetic core according to the present invention, the oxide layer formed in the third step has an action of binding soft magnetic alloy powders, and thus the use of the above inorganic binder is omitted. Thus, the process can be simplified.

  The amount of the binder added may be an amount that can be sufficiently distributed between the soft magnetic alloy powders or that can secure a sufficient compact strength. On the other hand, if this is too much, the density will decrease. For example, the amount is preferably 0.5 to 3.0 parts by weight with respect to 100 parts by weight of the soft magnetic alloy powder.

  The mixing method of the soft magnetic alloy powder and the binder in the first step is not particularly limited. Conventionally known mixing methods and mixers can be used. In a state where the binder is mixed, the mixed powder is an agglomerated powder having a wide particle size distribution due to its binding action. By passing the mixed powder through a sieve using, for example, a vibration sieve or the like, a granulated powder having a desired secondary particle size suitable for molding can be obtained. Further, in order to reduce the friction between the powder and the mold during pressure molding, it is preferable to add a lubricant such as stearic acid or stearate. The addition amount of the lubricant is preferably 0.1 to 2.0 parts by weight with respect to 100 parts by weight of the soft magnetic alloy powder. On the other hand, the lubricant can be applied to the mold.

  Next, the 2nd process of press-molding the mixture obtained through the 1st process is explained. The mixture obtained in the first step is preferably granulated as described above and subjected to the second step. The granulated mixture is pressure-molded into a predetermined shape such as a toroidal shape or a rectangular parallelepiped shape using a molding die.

  Next, the 3rd process of heat-processing the molded object obtained through the said 2nd process is demonstrated. Heat treatment is performed on the molded body that has undergone the second step for the purpose of alleviating stress strain introduced by molding or the like and obtaining good magnetic properties. As described above, by performing the heat treatment in an oxidizing atmosphere in a state where the Si compound is disposed on the surface of the Fe—Si—Cr soft magnetic alloy powder, the Fe—Si—Cr soft magnetic alloy is further obtained. A structure is formed in which the alloy phases of the powder are bonded together through an oxide phase containing Fe, Si and Cr. That is, an oxide phase containing Fe, Si, and Cr is formed in layers at the grain boundaries between the Fe—Si—Cr soft magnetic alloy powders. The above-described Cr—Si enriched portions are formed on both end sides in the thickness direction of the oxide phase, that is, on the alloy phase side.

  As the oxidizing atmosphere, an atmosphere in which oxygen gas exists, such as the air, a mixed gas of oxygen gas and inert gas, can be applied. In addition, an atmosphere in which water vapor exists, such as in a mixed gas of water vapor and an inert gas, can also be applied. Of these, heat treatment in the air is simple and preferable. Further, the temperature of the heat treatment is not particularly limited as long as it is a temperature at which the Cr-Si concentrated portion is formed, but it is preferably performed at a temperature at which the soft magnetic alloy powder is not significantly sintered. For example, the heat treatment temperature is preferably in the range of 500 to 900 ° C.

  A coil component is provided using the above-described dust core and a coil wound around the dust core. The coil may be configured by winding a conductive wire around a powder magnetic core, or may be configured by winding it around a bobbin. A coil component having the dust core and the coil is used as, for example, a choke, an inductor, a reactor, a transformer, or the like.

  The powder magnetic core may be manufactured in the form of a powder magnetic core obtained by pressing only the soft magnetic material powder mixed with the binder as described above, or the soft magnetic material powder and the coil may be integrated. You may press-mold and manufacture with the form of the powder magnetic core of a coil enclosure structure.

A powder magnetic core was produced using Fe—Si—Cr soft magnetic alloy powder as the soft magnetic material powder as follows. Such Fe—Si—Cr-based soft magnetic alloy powder was a granular atomized powder, and its composition was Fe-3.5% Si-4.0% Cr in mass percentage. The average particle size (median diameter d50) measured with a laser diffraction / scattering particle size distribution analyzer (LA-920, manufactured by Horiba, Ltd.) was 9.8 μm.
Using the Fe-Si-Cr-based soft magnetic alloy powder and having a Si compound disposed on the surface thereof, a silicon oxide film formed as follows (powder a) and colloidal silica were adhered. A thing (powder b) was produced.

The Fe—Si—Cr soft magnetic alloy powder, TEOS (tetraethoxysilane, Si (OC ₂ H ₅ ) ₄ ), an aqueous ammonia solution and ethanol are mixed and stirred, and then Fe—Si—Cr soft The magnetic alloy powder was filtered and separated, and dried in an oven to obtain powder a. A silicon oxide film having a thickness of about 30 nm was formed on the surface of the dried Fe-Si-Cr soft magnetic alloy powder (powder a).

  Further, 1.2 parts by weight of colloidal silica (solid content concentration: 60% by mass) was added to and mixed with 100 parts by weight of the Fe—Si—Cr soft magnetic alloy powder. After this mixed powder was dried at 120 ° C., powder b was obtained through a sieve having an opening of 425 μm.

  An acrylic resin binder of emulsion (Polysol AP-604 solid content 40% by Showa Polymer Co., Ltd.) was added and mixed at a ratio of 2.0 parts by weight with respect to 100 parts by weight of each of the powders a and b. . The mixed powder was dried at 120 ° C. for 10 hours, and the dried mixed powder was passed through a sieve to obtain granulated powder. To this granulated powder, zinc stearate was added and mixed at a ratio of 0.4 parts by weight with respect to 100 parts by weight of the soft magnetic alloy powder to obtain a mixture for molding.

  The resulting mixture was pressure molded at room temperature with a molding pressure of 1 GPa using a press. The obtained toroidal shaped molded body was heat-treated in the atmosphere at a heat treatment temperature of 700 ° C. for 1 hour to obtain a powder magnetic core (No 1) using powder a and a powder magnetic core (No 2) using powder b. It was. Moreover, the powder magnetic core corresponding to the structure of patent document 1 was prepared for the comparison. That is, a powder magnetic core was prepared in the same manner as in No. 1 except that powder a was used and the heat treatment atmosphere was changed to nitrogen (No. 3). Each powder magnetic core had an outer diameter of 14 mm, an inner diameter of 8 mm, and a height of 5 mm.

The conductive wire is wound on the primary side and the secondary side for 15 turns with respect to the dust core produced by the above process, and the maximum magnetic flux density is 35 mT and the frequency is measured by a BH analyzer (SY-8232 manufactured by Iwatatsu Measurement Co., Ltd.). Core loss Pcv was measured under the condition of 100 kHz. The initial permeability μi was measured at a frequency of 100 kHz by winding a conducting wire 30 turns around the toroidal powder magnetic core and using 4284A manufactured by Hewlett-Packard Company.
Further, a load was applied in the radial direction of the toroidal powder magnetic core, the maximum load P (N) at the time of fracture was measured, and the crushing strength σr (MPa) was obtained from the following equation.
σr = P (D−d) / (Id ² )
(Here, D: outer diameter of the core (mm), d: thickness of the core (mm), I: height of the core (mm))
The results of the above measurements are shown in Table 1.

  Furthermore, using a transmission electron microscope (TEM / EDX), the cross section of the powder magnetic core of Examples (No. 1 and 2) was observed and analyzed. FIG. 1 is a TEM photograph of a grain boundary portion between Fe-Si-Cr soft magnetic alloy powders of a No. 2 dust core. Table 2 shows the analysis results of the intragranular 1 and the grain boundary phase 2 of the Fe—Si—Cr soft magnetic alloy powder in FIG.

  As shown in FIG. 1, the grain boundary phase 2 has a multilayer structure, and both layers of the grain boundary phase in contact with the alloy phase of the soft magnetic alloy powder are both thinner than the central layer. The total thickness of the grain boundary phase was about 70 nm. In Table 2, the term “inside grain” refers to the alloy phase portion (corresponding to analysis point A in FIG. 1) of the soft magnetic alloy powder, and the grain boundary phase (edge) refers to the alloy phase of the grain boundary phase 2. The portion of the layer at both ends (corresponding to analysis points B and B ′ in FIG. 1) and the grain boundary phase (center) are the portion of the center layer sandwiched between the two layers at both ends (analysis point C in FIG. 1). Analysis value).

  As is clear from the analysis results of the grain boundary phases (edges) and the like in Table 2 and the structure observation of FIG. 1, oxides containing Fe, Si, and Cr are formed in both the No. 1 and No. 2 dust cores, The alloy phases of the Fe—Si—Cr soft magnetic alloy powder were bonded to each other through the oxide. In addition, the oxide phase has a mass ratio on the alloy phase side (grain boundary phase (end portion)), a Cr content and a Si content larger than the alloy phase, and a Cr and Si content. It can be seen that a Cr—Si concentrated portion having a total amount larger than the Fe content is formed. Moreover, as shown in Table 1, the dust cores of No. 1 and No. 2 having such a configuration showed higher crushing strength than the dust core of the comparative example (No. 3).

Moreover, from the analysis result of the grain boundary phase (center) in Table 2, the No. 1 dust core is located above the oxide phase of the grain boundary phase (end), that is, between the Cr-Si concentrated portions. It can be seen that the oxide phase has the largest Fe content among Fe, Si and Cr by mass ratio. On the other hand, it can be seen that the No. 2 dust core has an oxide phase having the largest Cr content among Fe, Si and Cr in terms of mass ratio between the Cr-Si concentrated portions. In addition, it can be seen from Table 2 that the Fe content in the Fe enriched portion and the Cr enriched portion sandwiched between the Cr—Si enriched portions has the smallest Si content among Fe, Cr, and Si. As shown in Table 1, it was confirmed that the No. 2 dust core having such a configuration has particularly high strength while having good magnetic properties.

1: Intragranular 2: Grain boundary phase

Claims (7)

A method for producing a dust core using Fe-Si-Cr soft magnetic alloy powder containing Fe, Si and Cr,
A first step of mixing Fe-Si-Cr-based soft magnetic alloy powder having a Si oxide film on its surface and a binder;
A second step of pressure-molding the mixture obtained through the first step;
And a third step of heat-treating the molded body obtained through the second step in an oxidizing atmosphere,
The dust core obtained through the third step has a structure in which the alloy phases of the Fe-Si-Cr soft magnetic alloy powder are bonded via an oxide phase containing Fe, Si, and Cr. ,
The oxide phase has a Cr content and a Si content larger than the alloy phase on the alloy phase side, and the total content of Cr and Si is larger than the Fe content. A method for producing a powder magnetic core comprising a Si-enriched part. It is a manufacturing method of the dust core according to claim 1,
The Si oxide film is formed by mixing Fe-Si-Cr soft magnetic alloy powder with a mixed solution of TEOS (tetraethoxysilane), ethanol, and ammonia water and then drying the powder. Production method. It is a manufacturing method of the dust core according to claim 1 or 2,
The method of manufacturing a dust core, wherein the Si oxide film has a thickness of 20 to 500 nm. It is a manufacturing method of the dust core according to any one of claims 1 to 3,
The method for producing a dust core, wherein the binder is an organic binder of polyethylene, polyvinyl alcohol, or acrylic resin. A method for producing a dust core according to any one of claims 1 to 4,
A method for producing a dust core, wherein the heat treatment is performed at 500 to 900 ° C. in the atmosphere. A method for producing a dust core according to any one of claims 1 to 5,
In the Fe—Si—Cr soft magnetic alloy powder, Si is 1.0% by mass or more and 10.0% by mass or less, Cr is 1.0% by mass or more and 7.0% by mass or less, and less than 1.0% by mass ( A non-magnetic element (including 0), the balance is inevitable impurities, and the composition is Fe. It is a manufacturing method of the dust core according to any one of claims 1 to 6,
The method for producing a dust core, wherein the Fe—Si—Cr soft magnetic alloy powder is a granular powder having an average particle diameter d50 of 1 μm to 30 μm.