Method for producing metal particles

Abstract

To provide fine metal particles having high insulation properties and excellent in magnetization properties.SOLUTION: Each of metal particles comprises: a core part containing a magnetic metal element; and a shell part arranged on a surface of the core part and containing an oxide of silicon. When a volume cumulative particle diameter at accumulation volume of 50 vol.% of the metal particles, measured by scanning electron microscope observation, is DSEM50, and a volume cumulative particle diameter at accumulation volume of 50 vol.% thereof, measured by a laser diffraction / scattering particle diameter distribution-measuring method, is DLDS50, the following relations are satisfied: DSEM50 is 0.1 μm or more and 1 μm or less; and a value of DLDS50 / DSEM50 is 10 or less. A coverage of the core part by the shell part, measured by elemental mapping for the cross-section of the metal atom, is 70% or more. Suitably, the magnetic metal element contains at least one of Fe, Co, and Ni.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to metal particles, a magnetic paste using the same, a dust core and an inductor, and a method for producing the metal particles. [Background technology]

[0002] As small portable devices become more sophisticated and multifunctional, the inductors used in these devices are required to be capable of handling large currents, i.e., power inductors. To pass large currents through an inductor, it is advantageous to use a magnetic material with a high saturation magnetic flux density (saturation magnetization) as the material that constitutes the inductor. While ferrite has traditionally been the primary magnetic material used for inductors, metallic materials with higher saturation magnetization have recently been attracting attention. When using metallic materials, core-shell particles with an insulating coating are usually required to reduce eddy current loss in the inductor.

[0003] As an example of the core-shell particles, Patent Document 1 discloses silicon oxide-coated iron powder in which the surfaces of iron particles having an average particle diameter of 0.25 μm or more and 0.80 μm or less and an average axial ratio of 1.5 or less are coated with silicon oxide. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-123933 Summary of the Invention [Problem to be solved by the invention]

[0005] Although the iron powder described in Patent Document 1 has insulating properties, it is produced by a wet process and has a particle size of 0.25 μm or more and 0.80 μm or less, which causes it to aggregate. As a result, when attempting to disperse this iron powder in a resin, poor dispersion occurs, which can lead to problems such as a decrease in the fluidity of the resin. Due to the aggregation of particles, it is difficult to increase the compact density of this iron powder. As a result, it is difficult to obtain excellent magnetic properties. Furthermore, particles obtained by a wet method tend to contain impurities derived from the solvent, resulting in low crystallinity and purity, which can lead to deterioration in magnetic properties. Therefore, an object of the present invention is to provide fine metal particles that have high insulating properties, high dispersibility, and excellent magnetization properties. [Means for solving the problem]

[0006] The present invention provides a magnetic material comprising: a core portion containing a magnetic metal element; A metal particle having a shell portion disposed on a surface of the core portion and containing an oxide of silicon, The volume cumulative particle size at 50% of the cumulative volume as determined by scanning electron microscopy of the metal particles is D SEM50 The volume cumulative particle size at 50% cumulative volume measured by the laser diffraction scattering particle size distribution measurement method is D LDS50 When D SEM50 is 0.1 μm or more and 1 μm or less, D LDS50 / D SEM50 The value of satisfies the relationship of 10 or less, The present invention provides metal particles in which the coverage of the core portion by the shell portion is 70% or more, as measured by elemental mapping of a cross section of the metal atom.

[0007] Furthermore, the present invention provides a method for producing metal particles, which comprises the steps of supplying first mother powder containing a magnetic metal element and second mother powder containing SiO into a plasma flame in a laminar flow state generated in a chamber, gasifying both mother powders in the plasma flame, and cooling both gasified mother powders to produce metal particles having a core portion containing a magnetic metal element and a shell portion disposed on the surface of the core portion and containing an oxide of silicon. [Effects of the Invention]

[0008] The present invention provides fine metal particles with high insulating properties, high dispersibility, and excellent magnetization characteristics. By using these metal particles, it is possible to easily obtain inductors with high saturation magnetization, low coercive force, and excellent soft magnetic properties. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing an apparatus that is preferably used for producing the metal particles of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described below based on preferred embodiments. The metal particles of the present invention have a core portion containing a magnetic metal element. In other words, the core portion is magnetic. As a result, the metal particles of the present invention exhibit magnetism. In other words, the metal particles of the present invention are magnetic metal particles.

[0011] The core may be composed of a single magnetic metal element, an alloy of two or more magnetic metal elements, or an alloy of one or more magnetic metal elements and one or more non-magnetic metal elements. In the present invention, "magnetism" refers to ferromagnetism. Therefore, the magnetic metal elements used in the present invention have ferromagnetism. In the present invention, it is preferable to use at least one of Fe, Co, and Ni as the ferromagnetic metal element from the viewpoints of increasing saturation magnetization and economic efficiency. Among these, when Fe is used, not only Fe alone but also alloys using Fe such as FeSi alloys (including FeSi, FeSiCr, FeSiAl, and FeSiB), FeNi alloys (including FeNi, FeNiMo, FeNiCr, FeNiCu, and FeNiNb), and FeCo alloys (including FeCo, FeCoV, and FeCoNi) are preferably used.

[0012] The core portion constitutes the main part of the metal particle. The shell portion is disposed on the surface of the core portion. The shell portion may be disposed over the entire surface of the core portion. Alternatively, the shell portion may be partitioned by the surface of one or more core portions. Alternatively, the shell portion may have a so-called "sea-island structure" in which "islands" consisting of one or more powder coating portions exist in a "sea portion" consisting of the surface of the core portion. It is also desirable that the boundary between the core and shell portions is clear, but it is permissible for the boundary to be partially unclear as long as the core and shell portions can be distinguished.

[0013] The shell portion preferably contains an oxide of silicon. It is also preferable that the shell portion does not contain a magnetic material. It is particularly preferable that the shell portion contains an oxide of silicon and is non-magnetic. It is advantageous from the viewpoint of reducing eddy current loss that the non-magnetic shell portion is disposed on the surface of the magnetic core portion. The non-magnetic shell portion acts as an electrical insulating layer for the core portion, thereby reducing eddy current loss. The shell portion may be crystalline or amorphous as long as it has electrical insulation properties.

[0014] As mentioned above, it is preferable that the shell portion is nonmagnetic. However, the shell portion may be composed solely of silicon oxide, or may be composed of silicon oxide and other nonmagnetic metal element materials. From the viewpoint of improving the electrical insulation of the shell portion, the nonmagnetic metal element is preferably at least one of Mg, Al, Ca, Zr, Ti, Hf, Zn, Mn, rare earth elements (excluding magnetic elements), Ba, and Sr, and more preferably either Al or Zr. These nonmagnetic metal elements may be used alone or in combination of two or more. From the viewpoint of improving the electrical insulation of the shell portion, it is particularly preferable that the shell portion be composed solely of silicon oxide.

[0015] The oxide of silicon is SiO X (wherein x is a number greater than 0 and equal to or less than 2) is preferably used. From the viewpoint of improving the electrical insulation of the shell part, SiO X In this formula, x is preferably 1 or more and 2 or less, and more preferably 1.5 or more and 2 or less. The value of x can be estimated by TEM-EELS and AES.

[0016] Regardless of the arrangement of the shell portion on the surface of the core portion, it is preferable from the viewpoint of insulation that the shell portion cover the surface of the core portion with a high coverage. Furthermore, from the viewpoint of achieving the desired magnetic properties of the metal particles of the present invention, particularly from the viewpoint of increasing saturation magnetization and decreasing coercive force to improve the magnetic properties as a soft magnetic material, it is preferable that the shell portion cover the surface of the core portion with a high coverage. From these viewpoints, the coverage of the core portion with the shell portion is preferably 70% or more, more preferably 73% or more, even more preferably 80% or more, and even more preferably 90% or more. The higher the coverage value, the higher the saturation magnetization and lower the coercive force of the metal particles, which is preferable. In other words, it is preferable because the metal particles become soft magnetic. On the other hand, while the upper limit of the coverage is theoretically 100%, 99.9% is more realistic. Metal particles having such a coverage can be suitably produced, for example, by the production method described below. The coverage of the shell can be measured by elemental mapping of the cross section of the metal particle. Details of the measurement method will be described in the examples described below.

[0017] Wet coating is a known technique, as described in, for example, Patent Document 1. This technique makes it relatively easy to obtain metal particles with a high coverage. However, metal particles with a high coverage and small particle size have not been known, and no method for producing such metal particles has been known.

[0018] With regard to the coverage rate of the core portion by the shell portion, the proportion of silicon in the metal particles is preferably 0.01% by mass or more and 20% by mass or less, more preferably 0.03% by mass or more and 15% by mass or less, and even more preferably 0.05% by mass or more and 10% by mass or less, from the viewpoint of improving the electrical insulation of the metal particles.

[0019] As described above, it is preferable to increase the coverage of the shell part from the viewpoint of improving insulation properties. In addition, it is also preferable to increase the coverage of the shell part from the viewpoint of facilitating the production of soft magnetic metal particles having high saturation magnetization and low coercive force. However, even if the coverage is high, if the shell thickness is excessively large, the magnetic properties of the core, particularly the saturation magnetization, are likely to be impaired. From this viewpoint, the average thickness of the shell is preferably 0.5 nm to 40 nm, more preferably 0.5 nm to 30 nm, and even more preferably 0.5 nm to 20 nm. Metal particles having a shell portion with such a thickness are preferably produced by the production method described below. The method for measuring the shell portion thickness is described in detail in the examples described below.

[0020] The metal particles of the present invention have a small particle size. Until now, particles containing magnetic metal elements have mainly been produced by atomization, but there has been a limit to how small the particle size of particles produced by this method can be reduced. It has been particularly difficult to reduce the particle size of magnetic metal particles made of alloys. In contrast, the present invention employs the manufacturing method described below, making it possible to reduce the particle size of magnetic metal particles, particularly magnetic metal particles made of alloys, more than ever before.

[0021] Specifically, the metal particles of the present invention have a volume cumulative particle diameter D at 50% by volume of the cumulative volume as determined by scanning electron microscope observation. SEM50 However, the particles are preferably fine particles of 0.1 μm or more and 1 μm or less. SEM50 represents the particle size of the primary particle. Such fine metal particles have good packing properties, high saturation magnetization, and facilitate the enhancement of magnetic properties. Furthermore, by combining such fine metal particles with hitherto known relatively large metal particles (for example, the particles described in Patent Document 1), the packing properties of the particles are also improved, and the saturation magnetization can be easily enhanced. In the present invention, a primary particle refers to an object that can be recognized as the smallest unit of a particle, judging from its apparent geometric shape. From the viewpoint of further improving the packing property, the metal particles of the present invention have the D SEM50 It is more preferable that the thickness is 0.1 μm or more and 0.75 μm or less, and even more preferable that the thickness is 0.1 μm or more and 0.5 μm or less.

[0022] D SEM50 is defined as the volume cumulative particle diameter at 50% by volume of the volume obtained by randomly selecting 1,000 particles that are not overlapping from a scanning electron microscope image of metal particles and measuring their particle diameter (Heywood diameter).Then, the volume of the particles is calculated from the obtained particle diameter assuming that the particles are spherical.

[0023] The metal particles of the present invention are preferably not only fine but also highly dispersible. Fine and highly dispersible metal particles exhibit good packing properties, high saturation magnetization, and facilitate improved magnetic properties. Furthermore, by combining such fine metal particles with previously known relatively large metal particles, the packing properties of the particles are further improved, the saturation magnetization is further increased, and the magnetic properties are improved. From these viewpoints, the particle size distribution is D LDS50 / D SEM50 When using this value, it is preferable that this value is 10 or less, more preferably 8 or less, and even more preferably 6 or less. LDS50 / D SEM50 Theoretically, the lower limit is 1, but 1.5 is more realistic. The metal particles of the present invention having such a particle size distribution have a low degree of particle aggregation and a high proportion of primary particles. Metal particles having such a particle size distribution can be suitably produced, for example, by the production method described below.

[0024] D, a measure of particle size distribution LDS50 / D SEM50 In D LDS50 represents the volume cumulative particle size at 50% cumulative volume measured by the laser diffraction scattering particle size distribution measurement method. LDS50 is preferably 0.2 μm or more and 1.7 μm or less, more preferably 0.2 μm or more and 1.5 μm or less, and even more preferably 0.2 μm or more and 1.3 μm or less. LDS50 By setting the value of in this range, it is possible to easily increase the saturation magnetization while maintaining good handleability of the metal particles, which is preferable.

[0025] D LDS50 Measurement of the particle size distribution can be performed, for example, by the following method. 0.1 g of a measurement sample is mixed with 50 mL of water and dispersed for 1 minute using an ultrasonic homogenizer (US-300T, manufactured by Nippon Seiki Seisakusho). The particle size distribution is then measured using a laser diffraction / scattering particle size distribution analyzer, such as the MT3300 EXII manufactured by Microtrackbell.

[0026] The BET specific surface area of ​​the metal particles is set to 12 m from the viewpoint of improving the green density and achieving excellent magnetic properties. 2 / g or less is preferable, and 10m 2 / g or less is more preferable, and 8m 2 / g or less is more preferable, and 6m 2 / g or less is particularly preferred. The BET specific surface area can be measured, for example, using a nitrogen-helium mixed gas containing 30% by volume of nitrogen as an adsorption gas and 70% by volume of helium as a carrier gas, and a BET specific surface area measuring device (HM model-1210, manufactured by Mountec Co., Ltd.), in accordance with "(3.5) Single-point method" in "6.2 Flow method" of JIS R 1626 "Method for measuring the specific surface area of ​​fine ceramic powders by gas adsorption BET method."

[0027] The metal particles of the present invention having the above-mentioned configuration have a submicron-sized particle size and little aggregation between particles, regardless of the composition of the metal particles. Furthermore, since the metal particles of the present invention have a submicron-sized particle size and little aggregation between particles, they can improve packing properties, which in turn can improve saturation magnetization and result in excellent magnetic properties. Furthermore, in order to improve the packing properties of the particles, the metal particles of the present invention can be mixed with other metal particles having a particle size larger than the metal particles.

[0028] The shape of the metal particles of the present invention is not particularly limited, and various shapes can be adopted, for example, spherical, flake, polyhedral, etc. From the viewpoint of improving the packing property of the metal particles and increasing the saturation magnetization, the shape of the metal particles is preferably spherical. The term "spherical" means that the circularity coefficient measured by the following method is preferably 0.85 or more, more preferably 0.90 or more. The circularity coefficient is calculated as follows: A scanning electron microscope image of metal particles is taken, and 1000 particles that do not overlap are randomly selected. When the area of ​​the two-dimensional projection image of the particle is S and the perimeter is L, the circularity coefficient of the particle is calculated as 4πS / L. 2The circularity coefficient is calculated from the formula: The arithmetic mean value of the circularity coefficients of each particle is the circularity coefficient mentioned above. If the two-dimensional projected image of a particle is a perfect circle, the circularity coefficient of the particle is 1.

[0029] Next, a preferred method for producing the magnetic particles of the present invention will be described. In this method, first mother powder containing a magnetic metal element and second mother powder containing SiO are subjected to a direct current thermal plasma (hereinafter also referred to as "DC plasma") process to produce metal particles from the mother powders, each having a core containing a magnetic metal element and a shell containing an oxide of silicon disposed on the core's surface. In detail, this method includes the steps of supplying the first mother powder and the second mother powder to a laminar plasma flame generated in a chamber to gasify the mother powders, and cooling the gasified mother powders to produce the desired metal particles.

[0030] When the core portion is composed of a single magnetic metal element, the first mother powder may be composed of that single magnetic metal element. When the core portion is composed of an alloy of two or more magnetic metal elements, the first mother powder may be composed of that alloy. For example, when the target core portion is composed of an FeCo alloy, the first mother powder may be composed of an FeCo alloy. On the other hand, as for the second mother powder, one containing SiO is used as described above, and preferably one consisting of SiO is used. To form a shell portion consisting of silicon oxide, it is conceivable to use a second mother powder consisting of SiO2, but from the viewpoint of obtaining metal particles with a high shell coverage and a sharp particle size distribution, it is advantageous to use a second mother powder containing SiO. The reason for this will be described later. There are no particular limitations on the method for producing the first and second mother powders, and for example, atomized powder, wet reduced powder, electrolytic powder, etc. can be used.

[0031] A DC plasma apparatus suitable for use in this manufacturing method is shown in Figure 1. As shown in the figure, the DC plasma apparatus 1 includes a powder supply device 2, a chamber 3, a DC plasma torch 4, a collection pot 5, a powder supply nozzle 6, a gas supply device 7, and a pressure adjustment device 8. Furthermore, the DC plasma apparatus 1 includes an annular wall 9 surrounding the plasma flame, located outside the chamber 3 and directly below the plasma torch 4. In this apparatus, the first and second mother powders pass from the powder supply device 2 through the powder supply nozzle 6 and into the DC plasma torch 4. A gas for generating thermal plasma (hereinafter also referred to as "plasma gas") is supplied to the plasma torch 4 from the gas supply device 7, generating a plasma flame. The first and second mother powders are gasified in the plasma flame generated by the DC plasma torch 4 and released into the chamber 3. The gasified first and second mother powders are then cooled and converted into the desired metal particle powder, which is then accumulated and collected in the collection pot 5. The interior of the chamber 3 is controlled by a pressure regulator 8 to maintain a negative pressure relative to the powder feeding nozzle 6, facilitating the feeding of the first and second mother powders to the DC plasma torch 4 and providing a structure that stably generates a plasma flame. Note that the apparatus shown in Fig. 1 is an example of a DC plasma apparatus, and the production of metal particles of the present invention is not limited to this apparatus.

[0032] From the viewpoint of supplying sufficient energy to the first and second mother powders in the plasma flame to successfully form submicron-order fine particles, it is preferable to adjust the plasma flame so that it is thick and long in a laminar flow state. Whether the plasma flame is in a laminar flow state can be determined by whether the aspect ratio of the frame length to the frame width (hereinafter referred to as the frame aspect ratio) is 3 or more when the plasma flame is observed from the side where the frame width is widest. Specifically, if the frame aspect ratio is 3 or more, it can be determined to be in a laminar flow state, and if it is less than 3, it can be determined to be in a turbulent flow state.

[0033] In order to make the plasma flame thick and long in a laminar flow state, it is advantageous to adjust the plasma output and plasma gas flow rate. Specifically, the plasma output of the DC plasma device is preferably 2 kW or more and 100 kW or less, and more preferably 2 kW or more and 40 kW or less. The flow rate of the plasma gas is preferably 0.1 L / min or more and 25 L / min or less, and more preferably 0.5 L / min or more and 21 L / min or less. As the plasma gas, reducing gases such as hydrogen gas and inert gases such as nitrogen gas and argon gas are preferred.

[0034] From the viewpoint of supplying sufficient thermal energy to the first and second mother powders in the plasma flame to successfully form submicron-order fine particles, and from the viewpoint of making it difficult for the coarse-grained first and second mother powders to remain, the ratio of plasma power to the supply amount of the first and second mother powders is preferably 0.01 kW·min / g or more and 20 kW·min / g or less, and more preferably 0.05 kW·min / g or more and 15 kW·min / g or less.

[0035] Furthermore, from the viewpoint of reliably maintaining the plasma flame in a laminar flow state and reliably obtaining the flow rate required for gasifying the first and second mother powders, the ratio of the plasma gas flow rate to the plasma output (unit: L / (min·kW)) is set to preferably 0.50 or more and 2.00 or less, more preferably 0.70 or more and 1.70 or less, and even more preferably 0.75 or more and 1.50 or less, while maintaining the plasma output and gas flow rate within the above-mentioned ranges.

[0036] In particular, in this manufacturing method, as shown in Figure 1, the plasma flame is generated under the condition that an annular wall 9 that surrounds the plasma flame is placed directly below the plasma torch 4. The annular wall 9 is a part that is different from the plasma torch 4 that generates the plasma, and this annular wall 9 suppresses the temperature drop of the plasma flame, thereby promoting grain growth. It also makes it possible to make the grain size distribution sharper.

[0037] From the viewpoint of further suppressing the temperature drop of the plasma flame by using the annular wall portion 9, it is preferable to generate the plasma flame so that the ratio of the length of the annular wall portion 9 (the length indicated by the symbol L in FIG. 1) to the flame length of the plasma flame is 0.05 or more and 0.90 or less, particularly 0.10 or more and 0.90 or less, and especially 0.30 or more and 0.90 or less.

[0038] From a similar viewpoint, when the annular wall portion 9 is a torus, it is preferable to generate the plasma flame so that the ratio of the diameter of the plasma flame to the inner diameter of the annular wall portion 9 is 0.03 or more and 0.55 or less, particularly 0.06 or more and 0.55 or less, and especially 0.15 or more and 0.55 or less.

[0039] The annular wall portion 9 may be an annular body, and there are no particular restrictions on its cross-sectional shape. For example, an annular body having a circular or rectangular cross section can be used as the annular wall portion 9. From the viewpoint of effectively suppressing a decrease in the temperature of the plasma flame, it is preferable that the annular wall portion 9 be an annular body, i.e., a cylinder. In this case, from the viewpoint of effectively suppressing a decrease in the temperature of the plasma flame, it is preferable that the center position of the cross section of the annular body roughly coincides with the position where the plasma flame is generated.

[0040] When producing metal particles having a core portion and a shell portion using the DC plasma device 1 shown in FIG. 1, the particle diameter D of the base powder is set to 1 / 200 of the base powder from the viewpoint of plasma sprayability and cost. LDS50 In both the first and second mother powders, the D of the mother powder is preferably 3.0 μm or more and 50 μm or less, and more preferably 5.0 μm or more and 30 μm or less. LDS50 The measurement of D of the metal particles mentioned above LDS50 This can be done in the same way as measuring

[0041] From the viewpoint of the production efficiency of the obtained metal particles, the supply rate of the mother powder, expressed as the total amount of the first mother powder and the second mother powder, is preferably 5 g / min to 200 g / min, more preferably 5 g / min to 100 g / min. The ratio of the first mother powder to the second mother powder should be such that the surface of the core portion is sufficiently covered with the shell portion, and for example, it is preferable that the total amount of the first mother powder and the second mother powder contains the second mother powder in an amount of 1 mass % to 5 mass %.

[0042] The proportion of the second mother powder in the first mother powder and second mother powder supplied to the DC plasma device 1 can be determined depending on the coverage of the shell portion of the target metal particles and the magnetization properties of the metal particles. For example, the proportion of the second mother powder to the total amount of the first mother powder and second mother powder can be set to preferably 0.1% by mass to 40% by mass, more preferably 0.1% by mass to 20% by mass, and even more preferably 0.1% by mass to 15% by mass.

[0043] The shape of the mother powder is not particularly limited for either the first mother powder or the second mother powder, and examples thereof include dendritic, rod-like, flake-like, cubic, spherical, etc. From the viewpoint of stabilizing the efficiency of supply to the plasma torch, it is preferable to use spherical mother powder.

[0044] The first and second mother powders preferably have different boiling points and a specific relationship between them. Specifically, it is advantageous that the boiling point of the first mother powder containing a magnetic metal element is higher than the boiling point (1880°C) of the second mother powder containing SiO. By using a mixed mother powder consisting of the first and second mother powders having such a temperature relationship, in the particle production process, the evaporated first mother powder is first cooled, causing nucleation, aggregation, and condensation of the core portion, thereby forming the core portion. Next, the evaporated second mother powder (the second mother powder has a lower boiling point than the first mother powder) is cooled, causing nucleation, aggregation, and condensation on the surface of the core portion, thereby forming the shell portion. The metal particles thus formed may have an unclear boundary between the shell and core portions due to the fact that the shell portion is gradually formed simultaneously during the core formation process, and in this portion, the concentration of the elements constituting the second mother powder increases stepwise or continuously from the center to the surface of the metal particle.

[0045] In addition to the above-mentioned methods, it is also possible to form a core containing a magnetic metal element by, for example, an atomization method, and then form an oxide layer of a non-magnetic metal element on its surface by a wet method. However, in this case, the surface of the core is likely to be oxidized during the formation of the oxide layer, which may result in a decrease in the magnetic force of the particles. It is also possible to form the core using an RF plasma method instead of a DC plasma method, but the particles obtained by the RF plasma method have a particle size that is too small and prone to agglomeration, making it difficult to improve the particle packing. Another drawback is that the magnetic force decreases as the particle size decreases. In contrast to these methods, the manufacturing method using the DC plasma method described above does not cause problems such as a decrease in magnetic force or packing.

[0046] In the preferred method for producing metal particles of the present invention, the use of SiO rather than SiO2 for forming the shell portion made of silicon oxide is preferable for the following reasons. In the DC plasma method described above, the second mother powder is evaporated and then cooled, causing nucleation, aggregation, and condensation. Comparing the boiling point and melting point, which are physical properties related to this process, between SiO2 and SiO, the melting point of SiO2 is 1650°C and the boiling point is 2230°C, while the melting point of SiO2 is 1702°C and the boiling point is 1880°C. Therefore, the temperature range in which SiO2 can maintain a liquid phase is 580°C (=2230°C-1650°C), while the temperature range in which SiO2 can maintain a liquid phase is 172°C (=1880°C-1702°C). In other words, the temperature range in which SiO2 can maintain a liquid phase is narrower than that of SiO2. This means that SiO2 is less likely to aggregate due to surface tension in the liquid phase than SiO2. The fact that aggregation due to surface tension is unlikely to occur is advantageous in that it can increase the coverage of the core with the shell and can prevent aggregation due to necking between particles. For these reasons, SiO is preferable to SiO2 as the second mother powder for forming the shell containing an oxide of silicon.

[0047] When SiO2 is used as the second mother powder, a shell part made of SiO2 is generally formed. On the other hand, when SiO2 is used as the second mother powder, a shell part made of SiO2 is generally formed. X (x is a number greater than 0 and equal to or less than 2) is formed. The value of x depends on the oxygen concentration in the chamber in the above-mentioned manufacturing method, etc.

[0048] The metal particles obtained in this manner tend to be spherical in shape. These metal particles are preferably used as a metal powder, which is an aggregate of these metal particles, as a raw material for a sintered body. This sintered body is preferably used, for example, as a core material for a power inductor. Meanwhile, the metal powder, which is an aggregate of metal particles, can be used in the form of a magnetic paste by adding a binder or vehicle. The above-mentioned metal powder or magnetic paste can also be compression-molded to form a powder magnetic core. Furthermore, this paste can be molded into a predetermined shape, dried, and solidified to form a solidified body, which can be used as a core material for a power inductor. [Example]

[0049] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to such examples. Unless otherwise specified, "%" means "% by mass."

[0050] Example 1 Using the DC plasma device 1 shown in FIG. 1, metal particles each having a core made of an FeCo alloy and a shell made of SiO 2 were produced as follows. The first mother powder was atomized powder (D 50 SiO powder (D: 10 μm, spherical) was used as the second mother powder. 50: 5 μm, irregular shape) was used. The ratio (amount of second mother powder used) / (amount of first mother powder used + amount of second mother powder used) was 1.1%. The mixed mother powder obtained by mixing the two mother powders was supplied to the apparatus 1. The supply rate was set to 15 g / min, and the mixed mother powder was supplied to the DC plasma torch 4 through the powder supply nozzle 6. A mixed gas of nitrogen gas and argon gas was used as the plasma gas. The flow rate of nitrogen gas was set to 4 L / min, and the flow rate of argon gas was set to 15 L / min. The plasma output was set to 24 kW. The generated plasma flame was photographed from the side where the frame width was observed to be the widest, and the image was binarized to measure the frame aspect ratio, which is the ratio of the frame length to the frame width. As a result, the frame aspect ratio was 4, confirming that the plasma flame was a laminar flow. In addition, a circular torus was used as the annular wall portion 9. A plasma flame was generated so that the ratio of the length of the annular wall 9 to the length of the plasma flame was 0.8, and the ratio of the diameter of the plasma flame to the inner diameter of the annular wall 9 was 0.5.

[0051] Examples 2 and 3 Metal particles were obtained in the same manner as in Example 1, except that in Example 1, (amount of second mother powder used) / (amount of first mother powder used + amount of second mother powder used) was set to 3.3% in Example 2 and 11.4% in Example 3.

[0052] Example 4 Using the DC plasma device 1 shown in FIG. 1, metal particles each having a core made of an FeNi alloy and a shell made of SiO 2 were produced as follows. That is, in Example 1, the first mother powder was an atomized powder (D 50 Metal particles were obtained in the same manner as in Example 1, except that a second mother powder (9 μm, spherical) was used and the ratio (amount of second mother powder used) / (amount of first mother powder used+amount of second mother powder used) was set to 4.5%.

[0053] Example 5 Using the DC plasma device 1 shown in FIG. 1, metal particles each having a core made of an FeSiCr alloy and a shell made of SiO 2 were produced as follows. That is, in Example 1, the first mother powder was an atomized powder (D 50 Metal particles were obtained in the same manner as in Example 1, except that a second mother powder (5 μm, spherical) was used and the ratio (amount of second mother powder used) / (amount of first mother powder used+amount of second mother powder used) was set to 9.1%.

[0054] Comparative Example 1 In Example 1, SiO powder (BET specific surface area: 50 m) was used as the second mother powder. 2 / g, spherical). In addition, (amount of second mother powder used) / (amount of first mother powder used + amount of second mother powder used) was set to 1.5%. Furthermore, the annular wall portion 9 was not installed in the chamber 3 of the plasma device 1. Other than these, metal particles were obtained in the same manner as in Example 1.

[0055] 〔evaluation〕 The metal particles obtained in the examples and comparative examples were subjected to the above-mentioned method. SEM50 , D LDS50 The BET specific surface area and the BET specific surface area were measured. The proportion of silicon in the metal particles, as well as the saturation magnetization and coercive force were also measured by the following methods. Furthermore, the coverage rate of the shell portion, the average thickness of the shell portion, the pressed powder resistance of the metal particles, and the pressed powder density of the metal particles were measured by the following methods. The results are shown in Table 1 below. Note that the pressed powder resistance in Table 1 is ">1.0 x 10" 7 " indicates that the upper limit of the detection value of the measuring device described below has been exceeded.

[0056] [Ratio of silicon to metal particles] The proportion of silicon in the metal particles was measured using an ICP optical emission spectrometer (ICP-SPS-3000 manufactured by SSI NanoTechnology Inc.).

[0057] [Shell coverage] The coverage rate of the shell portion was calculated by accumulating the outer circumferential length of the core portion (hereinafter also referred to as "core particle") and the coverage length of the shell portion obtained from multiple two-dimensional images, respectively, and regarding these values ​​as the surface area of ​​the core particle and the coverage area of ​​the shell portion of a three-dimensional core-shell structured metal particle, and dividing the surface area of ​​the core particle by the coverage area of ​​the shell portion. Specifically, elemental analysis was performed on the cross sections of the metal particles obtained in Examples 1 to 4 and Comparative Example 1 by mapping using a scanning transmission electron microscope (hereinafter referred to as "STEM-EDS") equipped with an energy dispersive X-ray analyzer. The elements targeted for STEM-EDS analysis were the elements constituting the core particle and the elements constituting the shell portion. The observation magnification was set to a range of 100,000 to 1,000,000 times so that the entire area of ​​the particle to be measured was included in one image. Under these conditions, multiple image data were acquired until 100 metal particles could be observed. Here, the perimeter length of the particle shape consisting of the elements constituting the core particle was defined as the perimeter length of the core particle, and the length of the curve of the elements constituting the shell portion was defined as the coating length of the shell portion. Note that when there were multiple curves of the elements constituting the shell portion, the sum of these curves was defined as the coating length of the shell portion. 100 of these were extracted, and the perimeter lengths of the extracted 100 core particles and the coating lengths of the 100 shell portions were respectively added up to determine the pseudo-surface area of ​​the core particle and the coating area of ​​the shell portion. The perimeter of the core particle was determined using "Image-Pro," an image analysis software from Media Cybernetics, using the "Perimeter / Contour Polygon" option in the "Best Fit Circle Tool (12 points)" measurement command. The coating length of the shell was also determined using the "Length" option in the "Curve Tool" measurement command. On the other hand, when the Si element is contained not only in the shell portion but also in the core particle as in Example 5, the following method (i) or (ii) can be used to distinguish between the core particle and the shell portion. (i) By performing STEM-EDS analysis on metal particles and performing multivariate analysis of the obtained spectra, map images of the core particle and shell regions, each with different ratios of constituent elements, including Si, can be obtained, and the core particle and shell region can be distinguished based on these map images. (ii) A method can be used in which the mapping region of the core particle elements containing Fe, Ni, etc. is removed from the mapping region of the Si element. Since the shell portion does not contain Fe, Ni, etc., by removing the mapping region of the core particle elements from the mapping region of the Si element, the remaining region becomes the region of the shell portion. In Example 5, the coverage of the shell portion was determined by distinguishing between the Si element of the core particle and the Si element of the shell portion through the multivariate analysis of (i). The method for determining the coverage will be described in detail later.

[0058] The surface of a core particle is defined as the surface portion of a core particle made of FeCo, FeNi, or FeSiCr in a STEM image. For example, when a particle made of FeCo, FeNi, or FeSiCr is covered with an oxide, the boundary between the particle and the oxide layer is defined as the surface of the core particle. This portion takes on an intermediate value of the brightness signal in the STEM image. Furthermore, when a material that may become a background in the measurement, such as an embedding resin, a resin grid, or a vacuum, is present together with the particle made of FeCo, FeNi, or FeSiCr, the brightness signal takes on an intermediate value between the particle and the background. On the other hand, the surface of the shell part was defined as the position where the detected count of Si K was 10 or more in the part that could be distinguished from the SiOx film based on the X-ray image obtained from the Si Kα mapping data by STEM-EDS analysis.

[0059] For STEM-EDS, a scanning transmission electron microscope (JEM-ARM200F manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray analyzer (JED-2300T Dry SD100GV detector manufactured by JEOL Ltd., controlled by NORAN System 7 manufactured by Thermo Fisher Scientific) was used. The measurement conditions were as follows: [Conditions for STEM-EDS] STEM accelerating voltage: 200 keV EDS detector: Silicon drift detector EDS detection area: 100mm² x 2 EDS solid angle of acceptance: 2.1 sr

[0060] The calculation of the coverage rate of the shell portion based on the multivariate analysis in (i) of Example 5 was performed as follows. That is, an EDS spectrum was obtained using a STEM-EDS analyzer. Next, spectral mapping data containing EDS spectra for each pixel was collected. The collected spectral mapping data was subjected to multivariate analysis using the COMPASS (PCA) mode in the measurement command of the NORAN System 7 mentioned above, and map images of the core particle and shell regions were extracted. The settings were as follows: Kernel size: 1×1 Quantitative map setting: High (slow) Filter Fit Type: High Precision (Slow) Thereafter, as in Examples 1 to 4 and Comparative Example 1, 100 map images of each core particle and shell portion were extracted, and the surface area of ​​the core particle and the coverage area of ​​the shell portion were calculated in a pseudo manner to determine the coverage rate of the shell portion.

[0061] [Average thickness of shell] The average thickness of the shell portion was measured using Image-Pro, an image analysis software from Media Cybernetics. Using the "curve tool" in the software's measurement command, the perimeter of the cross section of the core particle where the shell portion contacts and the perimeter of the shell portion (excluding the portion contacting the core portion) were set, and the shell portion thickness was calculated using "continuous interval measurement" in the "relative measurement" menu. This operation was performed for all pixels on the same particle to calculate the shell portion thickness, and the average value was calculated. The same operation was performed for another nine particles. The average values ​​of the 10 particles were then further averaged to obtain the average shell portion thickness.

[0062] [Magnetization characteristics] The saturation magnetization Ms and coercive force Hc, which are parameters of the magnetic properties, were measured using a vibrating sample magnetometer (VSM-5 manufactured by Toei Kogyo Co., Ltd.) in an external magnetic field of 10 kOe. Particles with high saturation magnetization Ms and low coercive force Hc can be said to have good soft magnetic properties and excellent magnetization characteristics.

[0063] [Compression Resistance and Compression Density] The green resistance value was measured using a green resistance measurement system (Mitsubishi Chemical Analytech Co., Ltd. PD-51) and a resistivity meter (Mitsubishi Chemical Analytech Co., Ltd. MCP-T600). Five grams of metal particles (sample) were placed in a probe cylinder (Φ20 mm), and the probe unit was attached to the PD-51. The resistance value was measured using the resistivity meter when a load of 20 kN was applied using a hydraulic jack. The volume resistivity (green resistance) and green density were calculated from the measured resistance value and sample thickness. When calculating the green density, the load of the hydraulic jack was set to 5 kN or 20 kN.

[0064] [Table 1]

[0065] As is clear from the results shown in Table 1, the metal particles obtained in each Example have a low coercive force while maintaining a high saturation magnetization, and are also high in resistance, compared to the metal particles obtained in the Comparative Examples. Furthermore, when comparing the metal particles obtained in Example 1 with the metal particles obtained in Comparative Example 1, it is found that although both have similar Si contents, the metal particles obtained in Example 1 have a higher green density and better packing properties. [Explanation of symbols]

[0066] 1 DC plasma device 2 Powder feeding device 3 chambers 4 DC plasma torches 5. Collection Pot 6 Powder feeding nozzle 7 Gas supply equipment 8 Pressure Regulating Device 9 Annular wall

Claims

1. A method for producing metal particles, comprising the steps of supplying a first mother powder containing a magnetic metal element and a second mother powder containing SiO into a plasma flame in a laminar flow state generated in a chamber, gasifying both mother powders in the plasma flame, and cooling both gasified mother powders to produce metal particles having a core portion containing a magnetic metal element and a shell portion disposed on the surface of the core portion and containing an oxide of silicon, A method for producing metal particles, wherein the plasma flame is generated outside the chamber and directly below the plasma torch, with an annular wall surrounding the plasma flame being disposed.

2. The manufacturing method according to claim 1 , wherein the plasma flame is generated so that the ratio of the length of the annular wall portion to the length of the plasma flame is 0.05 or more and 0.90 or less.

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