Metal powder and production method

Abstract

To provide metal powder having high filling properties.SOLUTION: Metal powder is configured so that, in a volume particle size distribution in a laser diffraction scattering particle size distribution measurement method, a peak A is observed in a range of a particle size of 0.10 μm or more and less than 3.0 μm, and a peak B is observed in a range of a particle size of 3.0 μm or more and 30 μm or less. In the volume particle size distribution, a ratio IA / IB of a frequency IA of the peak A to a frequency IB of the peak B is 0.50 or more and 99 or less, preferably. In the metal powder, preferably, a circularity factor is 0.70 or more. In the metal powder, a crystallite size is preferably 50 nm or more and 130 nm or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a metal powder and a method for producing the same. [Background technology]

[0002] As small portable devices become more sophisticated and multifunctional, the inductors used in these devices are required to be able to handle higher frequency currents. When high frequency currents are passed through an inductor, it is desirable to use metal powder with a small particle size as the raw material for the inductor core. However, metal powder with a small particle size tends to have a low green density, making it difficult to sufficiently increase the magnetic flux density of the inductor.

[0003] As a metal powder with a high green density, for example, Patent Document 1 proposes an iron-based soft magnetic powder for dust cores, in which a phosphate conversion coating layer having hydroxyl groups is formed on the surface of the iron-based soft magnetic powder. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-196101 Summary of the Invention [Problem to be solved by the invention]

[0005] However, although the iron-based soft magnetic powder for dust cores described in Patent Document 1 has a high compact density, its particle size is large, making it unsuitable for use as a material for inductors compatible with high-frequency currents. Although powders made of small particles are suitable for use in inductors compatible with high-frequency currents, it has traditionally been difficult to improve the packing properties of powders made of small particles.

[0006] Therefore, an object of the present invention is to provide a metal powder with high packing properties. [Means for solving the problem]

[0007] The present invention relates to a volume particle size distribution measured by a laser diffraction / scattering particle size distribution measurement method, Peak A is observed in the particle size range of 0.10 μm or more and less than 3.0 μm. The present invention provides a metal powder in which peak B is observed in a particle size range of 3.0 μm or more and 30 μm or less.

[0008] The present invention also provides a method for producing a metal powder, comprising the steps of supplying base powder containing a metal element into a plasma flame in a laminar flow state generated in a chamber, gasifying the base powder in the plasma flame, and cooling the gasified base powder to generate metal particles, The volume cumulative particle size D of the base powder at 50% cumulative volume by laser diffraction scattering particle size distribution measurement method 50 is 5 μm or more and 15 μm or less, The plasma output for generating the plasma flame is E (kW), When the supply rate of the base powder into the plasma flame is S (g / min), The present invention provides a method for producing metal powder in which E / S is 0.05 kW·min / g or more and 1.40 kW·min / g or less. [Effects of the Invention]

[0009] According to the present invention, a metal powder with high packing properties is provided. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing an example of a DC plasma device for producing the magnetic metal powder of the present invention. [Figure 2] FIG. 2 shows the volume particle size distribution of the magnetic metal powder obtained in Example 1. [Figure 3] FIG. 3 shows the volume particle size distribution of the magnetic metal powder obtained in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described below based on its preferred embodiments. The metal powder of the present invention is composed of an aggregate of metal particles. The metal particles contain a metal element. The metal particles may be composed of a single metal element, an alloy of two or more metal elements, or an alloy of one or more metal elements and one or more non-metal elements.

[0012] There is no particular limitation on the type of metal element that can be used in the metal powder of the present invention, and any metal element may be used as long as it can be produced into particles by the production method described below. When the metal powder of the present invention is used as a material for a power inductor, for example, the metal element is preferably a magnetic metal element. In the present invention, "magnetism" refers to ferromagnetism. Therefore, the magnetic metal element that can be used in the present invention has ferromagnetism. When the metal powder of the present invention contains a ferromagnetic metal element, it is preferable to use at least one metal element selected from iron (Fe), cobalt (Co), and nickel (Ni) as the metal element from the viewpoints of increasing the saturation magnetic flux density and economy. In particular, it is preferable that the metal powder consists of Fe or an Fe-based alloy. When the metal powder of the present invention contains Fe element, not only Fe alone but also alloys using Fe element such as FeSi alloys (including FeSi, FeSiCr, FeSiAl, and FeSiB), FeNi alloys (including FeNi, FeNiMo, FeNiCr, FeNiCu, and FeNiNb), FeCo alloys (including FeCo, FeCoV, and FeCoNi), and FeCr alloys (including FeCr and FeCrSi) are preferably used as the metal powder.

[0013] One of the features of the metal powder of the present invention is its high packing ability. In the technical field of powders, a common method for improving the packing ability of a powder has been to add a small amount of fine particles to a powder composed mainly of coarse particles, so that the fine particles fill the voids formed between the coarse particles. However, the present inventors have discovered that by adjusting the ratio of coarse particles to fine particles produced in a single manufacturing process, the packing ability of the powder can be improved compared to conventional methods. The particle ratio here is based on the peak area in the particle size distribution.

[0014] Specifically, in the volumetric particle size distribution measured by a laser diffraction / scattering particle size distribution measurement method, it is preferable that Peak A is observed in the particle size range of 0.10 μm or more and less than 3.0 μm, and Peak B is observed in the particle size range of 3.0 μm or more and 30 μm or less, since this improves the packing ability of the metal powder of the present invention. To make this advantage even more pronounced, it is more preferable that Peak A be observed in the range of 0.44 μm or more and 0.85 μm or less, more preferably in the range of 0.51 μm or more and 0.78 μm or less, and even more preferably in the range of 0.58 μm or more and 0.71 μm or less. From the same viewpoint, it is more preferable that Peak B is observed in the range of 3.5 μm or more and 17.3 μm or less, even more preferable that Peak B is observed in the range of 5.8 μm or more and 15.0 μm or less, and even more preferable that Peak B is observed in the range of 8.1 μm or more and 12.7 μm or less.

[0015] In the metal powder of the present invention, the number of peaks A observed in the range of 0.10 μm or more and less than 3.0 μm may be one or two or more. The number of peaks B observed in the range of 3.5 μm or more and less than 17.3 μm may be one or two or more. From the viewpoint of further improving the packing ability of the metal powder, it is preferable that only one peak A is observed in the range of 0.10 μm or more and less than 3.0 μm. From the same viewpoint, it is preferable that only one peak B is observed in the range of 3.5 μm or more and less than 17.3 μm.

[0016] In the metal powder of the present invention, in addition to Peak A observed in the range of 0.10 μm or more and less than 3.0 μm and Peak B observed in the range of 3.5 μm or more and 17.3 μm or less, one or more further peaks may be observed in the range less than 0.10 μm and / or in the range greater than 17.3 μm. From the viewpoint of further improving packing ability, it is preferable that only Peak A and Peak B are observed in the volume particle size distribution of the metal powder of the present invention.

[0017] From the viewpoint of further improving the packing property, in the metal powder of the present invention, the frequency of peak A in the volumetric particle size distribution is set to I A and the frequency of peak B is I B When the ratio I A / I B It is preferable that the value of is 0.50 or more and 99 or less. In particular, in order to manufacture microchip electronic components such as microinductors, it is necessary to use a powder mainly composed of fine particles as the material, and the inventors have found through their investigations that it is advantageous from the viewpoint of improving the packing property to compose the metal powder of the present invention with a large amount of fine particles and a small amount of coarse particles. From this viewpoint, the ratio I A / I B The value is more preferably 0.8 or more, and even more preferably 0.9 or more. Also, the ratio I A / I B The value is more preferably 10 or less, and even more preferably 7 or less. Frequency of Peak A I A and frequency of peak B I B correspond to the areas of Peak A and Peak B in the particle size distribution of the metal powder of the present invention. As will be apparent from Figures 2 and 3 described below, the particle size distribution of a metal powder is a graph in which the horizontal axis represents the particle diameter of the particles and the vertical axis represents the volumetric frequency of particles having that diameter. The area of ​​Peak A is determined by calculating the area of ​​the peak in the particle diameter range of 0.10 μm or more and less than 3.0 μm. The area of ​​Peak B is determined by calculating the area of ​​the peak in the particle diameter range of 3.0 μm or more and less than 30 μm.

[0018] In the prior art in the field of powder technology, a method of mixing two types of powder with different average particle sizes has been adopted to make the particle size distribution of the powder have two peaks. Unlike this technique, in the present invention, as will be clear from the description of the manufacturing method described below, it is possible to obtain a powder with a particle size distribution having two peaks by a single manufacturing method without mixing two types of powder with different average particle sizes.

[0019] In the present invention, the volumetric particle size distribution of the metal powder can be measured, for example, by the following method. Specifically, 0.1 g of the measurement sample was mixed with 50 mL of water and dispersed for 1 minute using an ultrasonic homogenizer (US-300T, manufactured by Nippon Seiki Seisakusho Co., Ltd.). Thereafter, the volumetric particle size distribution was measured using a laser diffraction / scattering particle size distribution analyzer, such as the MT3300 EXII manufactured by Microtrackbell. D described below 10 , D 50 and D 90 is also measured in this way.

[0020] The inventors have found through their investigations that the metal powder of the present invention has improved fluidity and thereby further improved packing properties by making the average particle size of the particles relatively small and by using fine particles as the major component and coarse particles as the minor component. 50 The value of D is preferably 0.28 μm or more, more preferably 0.29 μm or more, and even more preferably 0.30 μm or more. 50 The value is preferably 2.4 μm or less, more preferably 1.9 μm or less, and even more preferably 1.6 μm or less.

[0021] D 50 In relation to the above, the metal powder of the present invention has a volume cumulative particle size D at 90% by volume of cumulative volume measured by a laser diffraction scattering particle size distribution measurement method. 90 When metal powder is used as a material for a power inductor, for example, the value of D 50Provided that the thickness is greater than 1 μm, it is preferably 32.6 μm or less, more preferably 26.5 μm or less, and even more preferably 20.5 μm or less. 90 The value of D is determined from the viewpoint of improving the packing property of the powder. 50 Provided that the thickness is greater than 1.0 μm, it is preferably 2.2 μm or more, more preferably 4.2 μm or more, and even more preferably 8.3 μm or more.

[0022] Similarly, D 50 In relation to the above, the metal powder of the present invention has a volume cumulative particle size D at 10% by volume of cumulative volume measured by a laser diffraction scattering particle size distribution measurement method. 10 From the viewpoint of suppressing aggregation of primary particles and improving packing properties, the value of D 50 Provided that the thickness is smaller than , it is preferably 0.30 μm or more, more preferably 0.34 μm or more, and even more preferably 0.37 μm or more. 10 The value of D is determined from the viewpoint of improving the packing property of the powder. 50 Provided that the thickness is smaller than 0.53 μm or less, it is preferably 0.49 μm or less, and more preferably 0.46 μm or less.

[0023] The inventors have found through their investigations that in order to improve the packing property of the metal powder of the present invention, in which fine particles are the major component and coarse particles are the minor component, the closer the particles constituting the powder are to being spherical, the more advantageous it is. From this perspective, in the metal powder of the present invention, the circularity coefficient of the metal particles constituting the powder is preferably 0.70 or more, more preferably 0.85 or more, and even more preferably 0.90 or more. Metal particles having such a sphericity coefficient are preferably produced by the method described below.

[0024] The circularity coefficient is calculated as follows: A scanning electron microscope image of metal particles is taken, and 1,000 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 using 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.

[0025] As described above, the shape of the metal particles of the present invention is preferably spherical with a high circularity coefficient, but may also include some flake-like or polyhedral shapes as long as the effects of the present invention are not impaired.

[0026] From the viewpoint of suppressing aggregation of primary particles and improving packing properties, the metal powder of the present invention has a BET specific surface area (hereinafter also referred to as "SSA") of 9.1 m 2 / g or less, and 2 / g or less is more preferable, and 2 It is more preferable that the saturation coefficient is 1 / g or less. Furthermore, when the metal powder of the present invention is used as a material for a power inductor, for example, the SSA value is set to 0.8 m 2 / g or more, and 1.0m 2 / g or more is more preferable, and 1.3m 2 It is more preferable that the saturation coefficient is 1 / g or more. In this specification, a primary particle refers to an object that can be recognized as the smallest unit of a particle, judging from its apparent geometric shape.

[0027] The SSA of the metal powder of the present invention is measured by the following method. The adsorption gas is a nitrogen-helium mixture containing 30% by volume of nitrogen gas and 70% by volume of helium gas as the carrier gas. The BET specific surface area measurement device used is the HM model-1210 manufactured by Mountec Co., Ltd. Measurements are performed according to "(3.5) Single-point method" under "6.2 Flow method" in JIS R1626 "Method for measuring the specific surface area of ​​fine ceramic powders by gas adsorption BET method."

[0028] The metal powder of the present invention preferably has minimal agglomeration between primary particles. Minimal agglomeration between primary particles contributes to improved particle dispersibility and packing. As a result, when the metal powder of the present invention is used as a material for, for example, a power inductor, the performance of the power inductor is improved. The degree of particle aggregation can be measured, for example, by SSA (m 2 / g) versus volume cumulative particle size D 50 (μm) ratio D 50 It can be evaluated by the value of / SSA. In the metal powder of the present invention, D 50 From the viewpoint of improving the dispersibility of particles, the value of / SSA is preferably 13.5 or less, more preferably 10.3 or less, and even more preferably 8.0 or less. From a similar perspective, D 50 The value of / SSA is preferably 0.59 or more, more preferably 0.65 or more, and even more preferably 0.72 or more.

[0029] The metal powder of the present invention preferably has a large crystallite size of the material constituting the metal powder, because this improves the saturation magnetic flux density when the metal powder is used, for example, as a material for a power inductor. This is because metal particles with a large crystallite size have fewer grain boundaries, resulting in higher purity. From this perspective, when the metal powder of the present invention is Fe or an alloy containing Fe, the crystallite size is preferably 50 nm or more, more preferably 70 nm or more, and even more preferably 82 nm or less. Furthermore, when the metal powder of the present invention is Fe or an alloy containing Fe, the crystallite size is preferably 130 nm or less, more preferably 115 nm or less, and even more preferably 104 nm or less.

[0030] The measurement method for the crystallite size varies depending on the type of metal particle, but it is generally calculated using the Scherrer formula below from the peak width (half width) of the X-ray diffraction peak of the crystal plane with the highest intensity within the measurement range (20° to 150°). For example, when an FeCo alloy is used as the metal particle, the crystallite size is calculated using CuKα radiation, and the X-ray diffraction intensity of the metal particle is measured in the measurement range (2θ: 20° to 150°), with a sampling width of 0.01°, and a scan speed of 2° / min. <110> It is calculated from the peak width (half width) of the X-ray diffraction peak at Scherrer formula: D=Kλ / βcosθ D: Crystallite size (unit: nm) K: Scherrer constant (0.9400) λ: wavelength of X-rays (unit: Kα1 1.54056Å) β: Half-width (unit: rad) θ: Diffraction angle (unit: rad)

[0031] As described above, the metal powder of the present invention has high packing property. The packing property of a powder can be evaluated by its green density. The higher the packing property of a powder, the higher the green density. When the metal powder of the present invention is Fe or an alloy containing Fe, the green density of the metal powder is 5.0 g / cm 3 It is preferable that the concentration is 5.1 g / cm or more. 3 More preferably, it is 5.2 g / cm or more. 3 More preferably, it is more than this. Furthermore, when the metal powder of the present invention is Fe or an alloy containing Fe, the green density thereof is 7.5 g / cm 3 Preferably, it is 7.45 g / cm or less. 3 More preferably, it is 7.4 g / cm or less. 3 More preferably, it is more than this.

[0032] When the metal powder of the present invention is used, for example, as a material for a power inductor, the metal powder may contain a magnetic metal element and a non-magnetic metal element. In particular, it is advantageous from the viewpoint of reducing eddy current loss due to improved electrical insulation if the non-magnetic metal element is contained in a shell portion disposed on the outermost surface of a core portion containing the magnetic metal element. From the viewpoint of reducing eddy current loss, it is particularly preferable that the metal particles constituting the metal powder of the present invention have 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 a non-magnetic metal element. This shell portion containing an oxide of a non-magnetic metal element acts as an electrical insulating layer for the core portion, thereby further reducing eddy current loss. The shell portion may be crystalline or amorphous as long as it has electrical insulation properties.

[0033] The non-magnetic metal element is preferably at least one of Mg, Al, Si, Ca, Zr, Ti, Hf, Zn, Mn, rare earth elements (excluding those having magnetism), Ba, and Sr, from the viewpoint of improving the electrical insulation of the shell portion, and more preferably any of Al, Si, and Zr. These non-magnetic metal elements can be used alone or in combination of two or more. In particular, it is preferable that alumina, which is an oxide of Al, or silica, which is an oxide of Si, is present in the shell portion from the viewpoint of further improving the electrical insulation. In this case, the metal powder preferably has a powder resistance of 1.0 × 10 when compressed at 20 kN. 1 Ω·cm or more, more preferably 1.0×10 2 Ω·cm or more, more preferably 1.0×10 7 It is preferable that the material exhibits high insulation properties of Ω·cm or more.

[0034] In particular, when producing metal particles having a core portion and a shell portion using the method described below, it is preferable, from the standpoint of ease of production, to use a non-magnetic metal element constituting the shell portion whose oxide has a boiling point lower than the boiling point of the metal constituting the core portion.

[0035] The core may be composed solely of magnetic metal elements or may be formed from an alloy containing both magnetic and non-magnetic metal elements, while the shell is preferably composed solely of an oxide of a non-magnetic metal element and does not contain any magnetic metal elements.

[0036] In metal particles having a core and a shell, the shell preferably covers the entire core evenly from the viewpoint of electrical insulation. However, an increased amount of shell coating leads to a decrease in the magnetic force of the metal particle as a whole. Taking these factors into consideration, the proportion of the shell in the entire metal particle is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.05% by mass or more and 8% by mass or less, and even more preferably 0.1% by mass or more and 6% by mass or less. The proportion of the shell in the metal particle can be measured by converting the Si concentration obtained by ICP analysis into the SiO2 concentration if the shell is made of SiO2, for example. Similar measurements can be made when the shell is made of other materials.

[0037] If the shell thickness is too large, particles tend to aggregate, affecting packing properties, so it is desirable for the thickness to be as small as possible. On the other hand, if the shell thickness is too small, the electrical insulation effect of the shell is less likely to be realized, and eddy current loss tends to increase. Therefore, from the perspective of balancing these two, it is preferable to form a shell portion that is thin and has a dense structure. Having a dense shell structure contributes to improving the heat resistance of the metal powder of the present invention. The heat resistance of the metal powder of the present invention can be evaluated by the resistance of the particles to oxidation. Specifically, when the metal powder of the present invention is Fe or an alloy containing Fe, the metal powder is heated from 25°C at a heating rate of 10°C / min under an air flow of 150 mL / min, and the mass increase ΔW when the temperature reaches 600°C is 600 is preferably 5.3% by mass or less, more preferably 4.5% by mass or less, and even more preferably 3.8% by mass or less. 600There is no particular limit to the lower limit of ΔW, and the closer it is to zero, the better. 600 The value of is obtained by thermogravimetry.

[0038] The heat resistance of the metal powder of the present invention is ΔW 600 The mass increase ΔW when the temperature is raised from 25°C to 700°C under the same conditions 700 , and the mass increase ΔW when the temperature is raised from 25°C to 800°C 800 When the metal powder of the present invention is Fe or an alloy containing Fe, its ΔW 700 is preferably 7.0 mass % or less, more preferably 6.1 mass % or less, and even more preferably 5.2 mass % or less. Furthermore, when the metal powder of the present invention is Fe or an alloy containing Fe, its ΔW 800 is preferably 11.0% by mass or less, more preferably 9.4% by mass or less, and even more preferably 7.9% by mass or less.

[0039] As long as the shell has the above-mentioned heat resistance, it is desirable that the thickness of the shell be small. Specifically, regardless of whether the shell covers the entire surface of the core, the thickness of the shell is preferably 0.5 nm to 50 nm, more preferably 0.5 nm to 30 nm, and even more preferably 1.0 nm to 10 nm. The thickness of the shell can be determined by identifying the shell by cross-sectional observation using a transmission electron microscope (TEM) and measurement using energy dispersive X-ray analysis (EDS) and measuring the thickness.

[0040] The metal powder of the present invention may be composed entirely of metal particles having a core portion and a shell portion disposed on the surface of the core portion, or a portion of the metal particles constituting the metal powder may be composed of metal particles having a core portion and a shell portion disposed on the surface of the core portion. In the latter case, from the viewpoint of improving packing properties and electrical insulation, it is preferable that the metal powder of the present invention contains at least metal particles having a particle size of 0.10 μm or more and less than 3.0 μm and having a core portion and a shell portion disposed on the surface of the core portion. Most preferably, the metal powder of the present invention is composed entirely of metal particles having a core portion and a shell portion disposed on the surface of the core portion.

[0041] Next, a preferred method for producing metal particles of the present invention will be described. In this method, base powder containing a metal element is subjected to a direct current thermal plasma (hereinafter also referred to as "DC plasma") process to generate metal particles from the base powder. Specifically, this method includes the steps of supplying the base powder to a laminar plasma flame generated in a chamber to gasify the base powder, and cooling the gasified base powder to generate metal particles. When the metal particles are composed of a single metal element, base powder made of that single metal element can be used. When the metal particles are composed of an alloy of two or more metal elements, base powder made of that alloy can be used. For example, when the desired metal particles are composed of an FeCr alloy, base powder made of an FeCr alloy can be used. There are no particular limitations on the method for producing the base powder, and for example, atomized powder, wet-reduced powder, electrolytic powder, etc. can be used.

[0042] The particle size of the mother powder is one of the factors for adjusting the particle size distribution of the produced metal powder. By adjusting the particle size of the mother powder, it becomes easier to obtain a metal powder in which peaks A and B are observed in the particle size distribution described above. From this perspective, the particle size D of the mother powder 50 is preferably 5 μm or more, more preferably 6 μm or more, and even more preferably 7 μm or more. In addition, the particle size of the base powder D 50is preferably 15 μm or less, more preferably 14 μm or less, and even more preferably 12 μm or less.

[0043] 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, in the DC plasma device 1 of this embodiment, as shown in FIG. 1, a cylindrical body 9 surrounding the plasma flame may be disposed within the chamber 3 and directly below the plasma torch 4. The cylindrical body 9 has a longitudinal direction and a width direction perpendicular to the longitudinal direction. The interior of the cylindrical body 9 is hollow. The cylindrical body 9 is installed as needed. From the viewpoint of improving the packing property of the target metal powder, the cylindrical body 9 does not have to be installed. The outer peripheral surface of the cylindrical body 9 and the inner wall of the chamber 3 are connected by a partition plate 10. The partition plate 10 is located below the lower end of the plasma torch 4 and extends horizontally radially from the outer surface of the cylindrical body 9. Therefore, the interior of the chamber 3 is divided into an upper section and a lower section by the partition plate 10. In this apparatus, the base powder passes through the inside of the plasma torch 4 from the powder supply device 2 through the powder supply nozzle 6. 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, and a plasma flame is generated. The base powder is gasified in the plasma flame generated by the plasma torch 4 and released into the chamber 3. The gasified base powder is cooled and becomes fine particles of metal particles 11, which are accumulated and collected in the collection pot 5.

[0044] From the viewpoint of supplying sufficient energy to the base powder in the plasma flame and successfully forming particles of the desired diameter, 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, a frame aspect ratio of 3 or more can be determined to be a laminar flow state, and a frame aspect ratio of less than 3 can be determined to be a turbulent flow state.

[0045] In particular, from the viewpoint of suppressing aggregation of metal particles, it is preferable to adjust the length of the plasma flame so that it is in a laminar flow state. Specifically, it is preferable to maintain the laminar flow state of the plasma flame for 1.0 cm or more, more preferably 2.0 cm or more, and even more preferably 4.0 cm or more. Furthermore, it is preferable to maintain the laminar flow state for 20 cm or less, more preferably 15 cm or less, and even more preferably 12 cm or less. Maintaining the laminar flow state of the plasma flame for 20 cm or less prevents the temperature of the plasma flame from decreasing and facilitates a uniform temperature, thereby promoting the generation of metal particles. Furthermore, the generated metal particles are less likely to return, which reduces the risk of coarsening of the metal particles due to collisions between the particles. Furthermore, maintaining the laminar flow state of the plasma flame for 15 cm or less achieves the effect of making the temperature of the plasma flame more uniform. The return of the generated metal particles is further suppressed by the partition plate 10 dividing the chamber 3 into upper and lower spaces.

[0046] In order to make the plasma flame thick and long in a laminar flow state, it is advantageous to adjust various plasma conditions, such as plasma power, etc. Adjusting the plasma power is also advantageous from the viewpoint of making it easier to obtain metal powder in which Peak A and Peak B are observed in the particle size distribution described above. In particular, the plasma output of the DC plasma device is preferably 2.0 kW or more and 100 kW or less, more preferably 10 kW or more and 40 kW or less, and even more preferably 15 kW or more and 30 kW or less. The plasma gas is preferably a reducing gas such as hydrogen gas or an inert gas such as nitrogen gas or argon gas.

[0047] From the viewpoint of supplying sufficient thermal energy to the base powder in the plasma flame to successfully form particles of the desired particle size, as well as from the viewpoint of facilitating the production of a metal powder in which Peak A and Peak B are observed in the particle size distribution described above, when the plasma output for generating the plasma flame is E (kW) and the supply rate of the base powder into the plasma flame is S (g / min), the value of E / S, which is the ratio of the plasma output to the supply rate of the base powder, is preferably 0.05 kW·min / g or more, more preferably 0.1 kW·min / g or more, and even more preferably 0.3 kW·min / g or more. Furthermore, the E / S value is preferably 1.40 kW·min / g or less, more preferably 1.20 kW·min / g or less, even more preferably 1.00 kW·min / g or less, and even more preferably 0.90 kW·min / g or less.

[0048] In relation to the above-mentioned E / S value, the supply speed S of the mother powder into the plasma flame is preferably 5 g / min or more and 80 g / min or less, more preferably 10 g / min or more and 70 g / min or less, and even more preferably 20 g / min or more and 55 g / min or less.

[0049] 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 base powder, the ratio F / E (unit: L / (min·kW)) of the plasma gas flow rate F to the plasma output E is set to preferably 0.15 or more and 30 or less, more preferably 0.20 or more and 25 or less, and even more preferably 0.30 or more and 20 or less, while maintaining the plasma output within the above-mentioned range.

[0050] In this production method, as shown in FIG. 1, the plasma flame may be generated in a state where a cylindrical body 9 surrounding the plasma flame is placed directly below the plasma torch 4. The cylindrical body 9 is a separate part from the plasma torch 4 that generates the plasma. In this production method, it is preferable to generate at least a portion of the plasma flame within the cylindrical body 9. The cylindrical body 9 suppresses a temperature drop in the plasma flame, thereby promoting grain growth, making it possible to successfully obtain metal particles with a desired particle size distribution. In addition, it is possible to successfully obtain metal particles with a desired crystallite size.

[0051] There are no particular limitations on the cross-sectional shape of the cylindrical body 9. For example, the cross section may be a polygon such as a circle or a rectangle. From the viewpoint of effectively suppressing a temperature drop in the plasma flame, it is preferable that the cylindrical body 9 be a cylinder. In this case, from the viewpoint of effectively suppressing a temperature drop in the plasma flame, it is preferable that the center position of the cross section of the cylinder roughly coincides with the position where the plasma flame is generated. In addition, from the viewpoint of temperature durability of the plasma flame, the material of the cylindrical body 9 is preferably zirconia, carbon, alumina, quartz, silicon carbide, or the like.

[0052] The laminar flow state of the plasma flame described above is preferably generated inside the cylindrical body 9, from the viewpoint of further suppressing the temperature drop of the plasma flame. In order to make this advantage more prominent, the longitudinal length of the part of the plasma flame that is in a laminar flow state is set to length L P The length of the cylindrical body 9 is L S When L S L for P The ratio of L P / L S The value is preferably 0.10 or more and 0.95 or less, more preferably 0.15 or more and 0.90 or less, and even more preferably 0.20 or more and 0.85 or less.

[0053] From a similar perspective, when the cylindrical body 9 is a cylinder, where D (mm) is the inner diameter of the cylinder and d (mm) is the maximum diameter of the plasma flame generated within the cylinder, it is preferable to maintain the laminar flow state of the plasma flame while maintaining the value of d / D at 0.003 or more, particularly 0.005 or more, and especially 0.010 or more, since this suppresses heat dissipation and makes it easier to obtain metal particles with a uniform particle size. From this perspective, the closer the value of d / D is to 1, the better. However, since a value of d / D of 1 affects the stability of the plasma flame, the value of d / D is preferably 0.20 or less, more preferably 0.15 or less, and even more preferably 0.10 or less.

[0054] When obtaining metal particles by a DC plasma method, it is preferable to set the chamber to a low vacuum rather than a high vacuum, as this makes it easier to obtain metal powder in which peaks A and B are observed in the particle size distribution described above. Specifically, the pressure in the chamber, expressed as gauge pressure, is preferably set to between −0.01 kPa and −90 kPa, more preferably between −1.0 kPa and −80 kPa, and even more preferably between −5.0 kPa and −65 kPa.

[0055] 1 to produce metal particles having a core and an electrically insulating shell, the base powder may be a mixture of a first base powder containing a magnetic metal element and a second base powder made of an oxide of a non-magnetic metal element, such as alumina or silica. Both base powders are gasified in a plasma flame and then cooled to produce metal particles having a core containing a magnetic metal element and a shell that is disposed on the surface of the core and contains an oxide of a non-magnetic metal element, such as aluminum or silicon. In this case, it is advantageous to use a first mother powder whose boiling point is higher than that of the second mother powder. By using a mixed mother powder consisting of first and second mother powders having such a temperature relationship, in the particle generation process, first, the evaporated first mother powder is 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.

[0056] In particular, when using the first and second mother powders to obtain metal particles having a core and a shell, it is preferable to generate a plasma flame inside the cylindrical body 9 and set the dimensional relationship between the cylindrical body 9 and the plasma flame within the above-mentioned range. This suppresses a decrease in the temperature of the plasma flame and ensures that the core is densely covered by the shell. As a result, it becomes easier to obtain metal powder in which Peak A and Peak B are observed in the particle size distribution described above.

[0057] From the viewpoint of production efficiency of the obtained metal particles, the supply rate of the mother powders, 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, and even more preferably 5 g / min to 60 g / min. Furthermore, with regard to the ratio of the first mother powder to the second mother powder, it is preferable that the second mother powder accounts for 1 to 5 mass% of the total amount of the first mother powder and the second mother powder.

[0058] 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.

[0059] In the preferred method for producing metal particles of the present invention, the shell portion made of silicon oxide is preferably formed using SiO rather than SiO2. The reason for this is as follows: 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 over which SiO2 can maintain its liquid phase is 580°C (=2230°C-1650°C), while the temperature range over which SiO2 can maintain its liquid phase is 172°C (=1880°C-1702°C). In other words, the temperature range over which SiO2 can maintain its 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.

[0060] 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.

[0061] The metal particles obtained in this manner tend to be spherical in shape. These metal particles, as an aggregate, are suitable for use as a raw material for a sintered body in the form of a metal powder. When the sintered body is Fe or an alloy containing Fe, it is suitable for use, for example, as a core material for a power inductor. In this case, from the viewpoint of reducing eddy current loss, it is advantageous for the metal particles to be composed of a core portion and an electrically insulating shell portion. On the other hand, when the metal particles are made of Fe or an alloy containing Fe, the metal powder as an aggregate of the 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, the paste can be molded into a predetermined shape, dried, and solidified to form a solidified compact, which can be used as a core material for a power inductor.

[0062] In relation to the above-described embodiment, the present invention further discloses the following metal powder and a method for producing the same. [1] In the volume particle size distribution measured by the laser diffraction scattering particle size distribution measurement method, Peak A is observed in the particle size range of 0.10 μm or more and less than 3.0 μm. A metal powder in which peak B is observed in the particle size range of 3.0 μm or more and 30 μm or less. [2] In the volume particle size distribution, the peak height I of peak B B Peak height I of peak A relative to A Ratio of I A / I B The metal powder according to [1], wherein the value of [σ] is 0.50 or more and 99 or less. [3] The metal powder according to claim [1] or [2], having a circularity coefficient of 0.70 or more. [4] [4] The metal powder according to any one of [1] to [3], having a crystallite size of 50 nm or more and 130 nm or less. [5] The metal powder according to any one of [1] to [4], which is a magnetic metal powder. [6] [6] The metal powder according to any one of [1] to [5], comprising metal particles having a particle size of 0.10 μm or more and less than 3.0 μm, and having a core portion and a shell portion disposed on the surface of the core portion. [7] 7. The metal powder according to claim 6, wherein the shell portion comprises an oxide of silicon or an oxide of aluminum. [8] Compacted density is 5.0g / cm 3 More than 7.5g / cm 3 [7] The metal powder according to any one of [1] to [7], which is: [9] A method for producing a metal powder, comprising the steps of supplying base powder containing a metal element into a plasma flame in a laminar flow state generated in a chamber, gasifying the base powder in the plasma flame, and cooling the gasified base powder to generate metal particles, The volume cumulative particle size D of the base powder at 50% cumulative volume by laser diffraction scattering particle size distribution measurement method 50 is 5 μm or more and 15 μm or less, The plasma output for generating the plasma flame is E (kW), When the supply rate of the base powder into the plasma flame is S (g / min), A method for producing metal powder in which E / S is 0.05 kW·min / g or more and 1.40 kW·min / g or less. [Example]

[0063] 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."

[0064] Example 1 Using the DC plasma device 1 shown in FIG. 1 (but not equipped with the cylindrical body 9), magnetic metal particles composed of a core portion made of FeCrSi and a shell portion made of SiO 2 were produced as follows. The first mother powder was an atomized powder (particle size D50 SiO powder (particle size 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 4.5%. The mixed mother powder obtained by mixing the two mother powders was supplied to the apparatus 1. The supply rate was set to 30 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 21.6 kW. The generated plasma flame was photographed from the side where the flame 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 3, confirming that the plasma flame was a laminar flow. The length of the laminar flow portion of the plasma flame was 10 cm. The gauge pressure in the chamber was set to -50 kPa. These conditions are summarized in Table 1.

[0065] Example 2 In Example 1, the plasma output was set to 22.1 kW. Other than this, magnetic metal particles were obtained in the same manner as in Example 1.

[0066] Example 3 In Example 1, the plasma output was set to 23.4 kW. Other than this, magnetic metal particles were obtained in the same manner as in Example 1.

[0067] Example 4 In Example 1, the plasma output was set to 24.1 kW. Other than this, magnetic metal particles were obtained in the same manner as in Example 1.

[0068] Example 5 Using the DC plasma device 1 (equipped with a cylindrical body 9) shown in FIG. 1, magnetic metal particles composed of a core portion made of FeSi and a shell portion made of SiO2 were produced as follows. In Example 1, the first mother powder was an atomized powder (particle size D 50 The ratio of (amount of second mother powder used) / (amount of first mother powder used + amount of second mother powder used) was 0.5%, and the gauge pressure in the chamber was set to -10 kPa. A quartz cylinder was used as the cylindrical body 9. The length L of the cylindrical body 9 S The flame length L of the laminar flow portion in the plasma flame P The ratio of L P / L S A plasma flame was generated so that the ratio d / D of the diameter d of the plasma flame to the inner diameter D of the cylindrical body 9 was 0.80 and the ratio d / D of the diameter d of the plasma flame to the inner diameter D of the cylindrical body 9 was 0.050. The plasma output was set to 18.2 kW. The supply rate of the base powder was 45 g / min. Other than these, magnetic metal particles were obtained in the same manner as in Example 1.

[0069] Example 6 In Example 5, the plasma output was set to 18.8 kW, and the mother powder supply rate was set to 45 g / min. Other than these, magnetic metal particles were obtained in the same manner as in Example 5.

[0070] Example 7 In Example 5, the plasma output was set to 17.9 kW, and the mother powder supply rate was set to 45 g / min. Other than these, magnetic metal particles were obtained in the same manner as in Example 5.

[0071] Comparative Example 1 In Example 1, the plasma output was set to 28.3 kW. The mother powder supply rate was set to 20 g / min. Other than these, magnetic metal particles were obtained in the same manner as in Example 1.

[0072] 〔evaluation〕 The metal particles obtained in the examples and comparative examples were subjected to the above-mentioned method. 50 , D 10 , D 90 , SSA, circularity coefficient, core crystallite size, and mass increase W 600 , W 700 and W 800The magnetic properties, green density, and green resistance were measured using the following methods. The flame retardancy was also evaluated using the following method. The results are shown in Table 2 below. The volume particle size distributions of the magnetic metal powders obtained in Example 1 and Comparative Example 1 are shown in Figures 2 and 3, respectively.

[0073] [Method for measuring magnetic properties] 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.

[0074] [Method for measuring green density and green resistance] The pressed powder resistance was measured using a pressed powder resistance measurement system (Mitsubishi Chemical Analytech PD-51) and a resistivity meter (Mitsubishi Chemical Analytech 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 (pressed powder resistance) and pressed powder density were calculated from the measured resistance value and sample thickness.

[0075] [Table 1]

[0076] [Table 2]

[0077] As is clear from the results shown in Table 2, the metal particles obtained in each Example have lower coercive force than the metal particles obtained in Comparative Example 1, and are superior in magnetization properties. In addition, the metal particles obtained in each example have a D 50 / SSA and D 50The small size and high green density indicate that the powder has excellent packing properties. Furthermore, the metal particles obtained in each Example have a smaller mass increase and higher heat resistance than the metal particles in each Comparative Example. They also have higher powder resistance and better insulation properties. [Explanation of symbols]

[0078] 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 Cylindrical body

Claims

1. In the volume particle size distribution measured by the laser diffraction scattering particle size distribution measurement method, Peak A is observed in the particle size range of 0.10 μm or more and less than 3.0 μm, A metal powder in which peak B is observed in a particle size range of 3.0 μm or more and 30 μm or less.

2. In the volume particle size distribution, the frequency I of peak B Ｂ Frequency of peak A relative to I Ａ Ratio I Ａ / I Ｂ The metal powder according to claim 1, wherein the value of the σ is 0.50 or more and 99 or less.

3. 3. The metal powder according to claim 1, having a circularity coefficient of 0.70 or more.

4. The metal powder according to claim 1 or 2, having a crystallite size of 50 nm or more and 130 nm or less.

5. The metal powder according to claim 1 or 2, which is a magnetic metal powder.

6. The metal powder according to claim 1 or 2, comprising metal particles having a particle size of 0.10 μm or more and less than 3.0 μm, and having a core portion and a shell portion disposed on the surface of the core portion.

7. The metal powder of claim 6 , wherein the shell portion comprises an oxide of silicon or an oxide of aluminum.

8. The powder density is 5.0 g / cm ３ 7.5g / cm or more ３ 3. The metal powder according to claim 1, wherein:

9. A method for producing a metal powder, comprising the steps of supplying base powder containing a metal element into a plasma flame in a laminar flow state generated in a chamber, gasifying the base powder in the plasma flame, and cooling the gasified base powder to generate metal particles, The volume cumulative particle size D of the base powder at 50% by volume of cumulative volume measured by a laser diffraction scattering particle size distribution measurement method ５０ is 5 μm or more and 15 μm or less, The plasma output for generating the plasma flame is E (kW), When the supply rate of the base powder into the plasma flame is S (g / min), A method for producing a metal powder, wherein E / S is 0.05 kW·min / g or more and 1.40 kW·min / g or less.

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