CN112423912A

CN112423912A - Metal powder, method for producing same, and method for predicting sintering temperature

Info

Publication number: CN112423912A
Application number: CN201980043273.5A
Authority: CN
Inventors: 西岛一元; 小林谅太; 六角广介; 浅井刚
Original assignee: Toho Titanium Co Ltd
Current assignee: Toho Titanium Co Ltd
Priority date: 2018-06-28
Filing date: 2019-06-17
Publication date: 2021-02-26
Anticipated expiration: 2039-06-17
Also published as: JPWO2020004105A1; CN112423912B; JP7193534B2; KR20210019547A; TW202000344A; WO2020004105A1; KR102484793B1; TWI720523B

Abstract

One object of the present invention is to provide a metal powder containing metal particles in which the concentration of sulfur or the distribution thereof is controlled, and a method for producing the same. The present invention provides a method of manufacturing metal powder. The method comprises the following steps: the metal chloride gas is produced by chlorinating a metal with chlorine, and the metal particles are produced by reducing the metal chloride as a gas in the presence of a sulfur-containing gas. The reduction is performed so that the total concentration of sulfur in the metal particles is 0.01 to 1.0 wt%, and the local concentration of sulfur at a position 4nm from the surface of the metal particles is 2 atomic% or more. The total concentration and the local concentration were estimated by an inductively coupled plasma emission spectrometer and an energy dispersive X-ray spectrometer disposed in a scanning transmission electron microscope, respectively.

Description

Metal powder, method for producing same, and method for predicting sintering temperature

Technical Field

One embodiment of the present invention relates to a metal powder and a method for producing the same. Alternatively, one embodiment of the present invention relates to a method for quality control of metal powder, a method for estimating characteristics of metal powder, or a method for predicting sintering temperature.

Background

An aggregate containing fine metal particles (hereinafter referred to as metal powder) is used in various fields, and powders of metals having high conductivity such as copper, nickel and silver are widely used as raw materials of electronic components such as internal electrodes of multilayer ceramic capacitors (MLCCs). The MLCC has a laminate of ceramic layers including a dielectric material and internal electrodes including a metal as a basic structure. The laminate is formed by alternately coating a dispersion containing a dielectric material and a dispersion containing a metal powder, and then heating to sinter the dielectric material and the metal powder. For example, patent documents 1 and 2 disclose methods for controlling the sintering characteristics of metal powder when heating.

[ Prior art documents ]

[ patent document ]

[ patent document 1] Japanese patent application laid-open No. Hei 11-80816

[ patent document 2] Japanese patent application laid-open No. 2014-189820

Disclosure of Invention

Technical problem to be solved by the invention

An object of an embodiment of the present invention is to provide a metal powder containing metal particles in which the concentration of sulfur or the distribution thereof is controlled, and a method for producing the same. Alternatively, it is an object of an embodiment of the present invention to provide a metal powder having a high sintering initiation temperature and a method for producing the same. Alternatively, it is an object of an embodiment of the present invention to provide a metal powder with a small variation in sintering initiation temperature and a method for producing the same. Alternatively, it is an object of an embodiment of the present invention to provide a method for quality control of a metal powder, a method for estimating characteristics of a metal powder, or a method for predicting a sintering temperature.

Means for solving the problems

One embodiment of the present invention is a metal powder. The metal powder includes a metal and sulfur-containing metal particles. The total concentration of sulfur in the metal particles is 0.01 to 1.0 wt%, and the local concentration of sulfur in a position 4nm from the surface of the metal particles is 2 atomic% or more. The total concentration and the local concentration were estimated by an inductively coupled plasma emission spectrometer and an energy dispersive X-ray spectrometer disposed in a scanning transmission electron microscope, respectively.

One embodiment of the present invention is a method of manufacturing a metal powder. The method includes producing a metal chloride gas by chlorinating a metal with chlorine, and producing metal particles by reducing the metal chloride as a gas in the presence of a sulfur-containing gas. The reduction is performed so that the total sulfur concentration of the metal particles is 0.01 to 1.0 wt%, and the local sulfur concentration at a position 4nm from the surface of the metal particles is 2 atomic% or more. The total concentration and the local concentration were estimated by an inductively coupled plasma emission spectrometer and an energy dispersive X-ray spectrometer disposed in a scanning transmission electron microscope, respectively.

One embodiment of the present invention is a method of predicting a sintering temperature of a metal powder. The method comprises measuring the local concentration of sulphur at a location 4nm from the surface of metal particles selected from the group of metal powders. The local concentration of sulfur was measured with a scanning transmission electron microscope equipped with an energy dispersive X-ray spectrometer.

Drawings

Fig. 1 is a schematic sectional view of a reduction furnace of a metal powder manufacturing apparatus according to an embodiment of the present invention.

Fig. 2 is a graph showing the sulfur concentration distribution of metal particles in the metal powders containing examples and comparative examples.

Fig. 3 is a graph showing the relationship between the sintering initiation temperature and the total concentration of sulfur for the metal powders of the examples and comparative examples.

Detailed Description

Embodiments of the present invention will be described below with reference to the drawings and the like. However, the present invention can be implemented in various ways within a scope not departing from the gist thereof, and is not limited to the description of the embodiments shown below for explanation.

Although the drawings may schematically show the width, thickness, shape, etc. of each component compared to the actual embodiment for clearer description, the drawings are only examples and do not limit the explanation of the present invention. In the present specification and each drawing, elements having the same function as that of the elements described in the already-mentioned drawings may be denoted by the same reference numerals, and overlapping description may be omitted.

< embodiment 1 >

In this embodiment, the structure and characteristics of the metal powder 100, which is one of the embodiments of the present invention, are described.

1. Structure of the device

The metal powder 100 is an aggregate of a plurality of metal particles 102, and the metal particles 102 contain a metal and sulfur. The metal is selected from nickel, copper, silver, etc., typically nickel. The number average particle diameter of metal powder 100 may be 50nm or more and 400nm or less, 100nm or more and 300nm or less, or 100nm or more and 250nm or less. In other words, the average particle diameter of a plurality (e.g., 600) of metal particles 102 selected from the metal powder 100 may fall within the above range as the number average particle diameter of the metal powder 100. As the number average particle diameter, for example, the metal particles 102 contained in the metal powder 100 are observed by a scanning electron microscope, the particle diameters of a plurality of particles (for example, 600 particles) are measured, and the average value thereof is adopted. The particle diameter is the diameter of the smallest circle that inscribes the particle.

The metal powder 100 contains sulfur. Specifically, the total sulfur concentration of the metal powder 100 is 0.01 wt% or more and 1.0 wt% or less, or higher than 0.01 wt% and 0.6 wt% or less, or 0.15 wt% or more and 0.6 wt% or less, or 0.16 wt% or more and 0.6 wt% or less. In other words, the average value of the particle concentration of sulfur of a plurality (e.g., the number corresponding to 0.5 g) of metal particles 102 selected from the metal powder 100 falls within the above range. The overall concentration of sulfur is the ratio of the weight of sulfur to the weight of metal particles 102. The overall concentration of the metal powder 100 is calculated as an average of the overall concentration of sulfur of one metal particle 102 or the plurality of metal particles 102 selected from the metal powder 100.

The overall concentration of sulfur can be measured by inductively coupled plasma emission spectroscopy. For example, the measurement can be performed using an inductively coupled plasma emission spectrometer (SPS3100) manufactured by SII nanotechnology co. The specific measurement method is that after the metal powder 100 is dissolved by acid, ICP emission spectrum analysis is carried out at the measurement wavelength of 182.036nm, and the total concentration of sulfur can be obtained.

The metal particles 102 contain sulfur not only near the surface but also in the interior relatively far from the surface toward the interior of the particle. Specifically, the concentration of sulfur decreases as it approaches from the surface toward the inside of the metal particle 102, but the concentration of sulfur at a position 4nm from the surface (hereinafter, the concentration of sulfur at a specific position of the metal particle 102 is referred to as a local concentration) is 2 atomic% or more. The sulfur concentration at a position 4nm from the surface may be 4 atomic% or less. The average value of the local concentration of sulfur at the above-described positions of the plurality (e.g., 10) of metal particles 102 selected from the metal powder 100 falls within the above-described range.

In addition, a position having a local concentration of 1/2 of the local concentration of sulfur on the surface of the metal particle 102 (hereinafter referred to as a half-life depth) may exist in a range of 2nm or more and 4nm or less from the surface. That is, the average value of the half-decay depths of a plurality (e.g., 10) of metal particles 102 selected from the metal powder 100 may fall within the above-described range.

The local concentration of sulfur can be estimated, for example, by an Energy Dispersive X-ray spectrometer (STEM-EDS: Scanning Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy) provided in a Scanning Transmission Electron Microscope. In a specific measurement method, first, the metal powder 100 is dispersed in a resin, and the resin is cured. Then, the cross section was exposed using a cross section polisher (CP), and a thin film sample was prepared by planar sampling using a Focused Ion Beam (FIB). By setting the thickness of the sample to about 100nm, the metal particles 102 were formed into a thin film having the thickness. Then, by performing EDS measurement on the obtained thin film on a straight line passing through the center of the metal particle 102, the local concentration can be obtained. As the conditions for the EDS measurement, for example, conditions of an acceleration voltage of 200kV, a probe diameter of 1nm, a pitch width of 3nm, and a measurement time per point of 15 seconds may be selected.

2. Characteristics of

Since the metal powder 100 containing the metal particles 102 has a high overall concentration and sulfur is widely distributed in the surface layer portion of the metal particles 102, it has a high sintering start temperature, for example, in the range of 600 ℃ or more, showing the sintering start temperature. The sintering initiation temperature may be 700 ℃ or less. Based on the above characteristics, the local concentration of sulfur in the surface layer portion of the metal particles is measured, and when the local concentration satisfies the above conditions, it can be determined that the metal powder as the aggregate of the metal particles has a high sintering initiation temperature. Thus, the present embodiment provides an efficient method for predicting the properties of metal powders.

As shown in this embodiment, it is suggested that the broad distribution and high overall concentration of sulfur is associated with a high sintering initiation temperature of the metal powder 100. If the total concentration of sulfur is the same, the distribution of sulfur is broad (sulfur exists deep in the surface layer), which is advantageous from the viewpoint of increasing the sintering initiation temperature. Using this fact, the sintering initiation temperature of the metal powder can be estimated or estimated by measuring the distribution and the overall concentration of sulfur in the surface layer. For example, when metal particles arbitrarily selected from metal powders are analyzed by STEM-EDS and the condition that the local concentration of sulfur is 2 atomic% or more at a position 4nm from the surface of the metal particles is satisfied, it is estimated that the sintering temperature of the metal powder containing the metal particles is 600 ℃. In other words, according to the present embodiment, the sintering behavior of the metal powder can be estimated by measuring the sulfur concentration in the surface layer even without sintering the metal powder, and thus an effective method of managing the quality of the metal powder is provided by the embodiments of the present invention.

For example, when metal powder is used as a raw material of an internal electrode of an MLCC, a dispersion liquid containing a dielectric and a dispersion liquid containing metal powder are alternately applied and then fired. The dispersion liquid containing a dielectric contains Ba or Ti-based oxide powder, a polymer material used as a binder, a solvent, a dispersant, and the like, and the dispersion liquid containing a metal powder contains not only the metal powder but also the binder, the solvent, the dispersant, and the like. During firing, the binder, solvent and dispersant are evaporated or decomposed, and the oxide powder and metal powder are sintered to provide a dielectric film and an internal electrode, respectively. Since the sintering start temperature of the dielectric is generally higher than that of the metal powder, sintering of the metal powder starts first at the time of sintering. Therefore, during the baking process, a gap is generated between the dielectric and the internal electrode, and sometimes peeling occurs between the internal electrode and the dielectric film due to the gap, thereby reducing the characteristics and yield of the MLCC.

On the other hand, since the metal powder 100 exhibits a high sintering start temperature, sintering is started at a temperature closer to the sintering start temperature of the oxide powder or the like. As a result, at the time of baking, high adhesion between the internal electrode and the dielectric can be secured, and peeling can be suppressed. Therefore, the metal powder 100 can be used as a raw material for providing various electronic components having excellent characteristics.

As described above, since the sintering behavior of the metal powder can be estimated without sintering, a quality management method for manufacturing a metal powder having high reliability as an electrode material of an MLCC can be provided by the present embodiment.

< embodiment 2 >

In the present embodiment, an example of a manufacturing method of the metal powder 100 is described.

The metal powder 100 is manufactured by a vapor phase method. That is, a metal chloride is produced by reducing a vapor of a metal chloride (hereinafter, simply referred to as a chloride) obtained by chlorinating a metal or a vapor obtained by heating a metal chloride in the presence of a sulfur-containing gas. However, since a high-purity chloride vapor can be obtained and the supply amount of the chloride vapor can be stabilized, it is more preferable to generate the chloride vapor by chlorinating the metal. Since a known apparatus (chlorination furnace) for chlorinating a metal can be used, the description thereof is omitted.

A schematic cross-sectional view of a reduction apparatus 110 as an apparatus for reducing chloride is shown in fig. 1. The reduction device 110 has a function of generating the metal powder 100 by reducing chloride and introducing sulfur into the metal particles 102 at the same time. The reduction apparatus 110 includes a reduction furnace 112 and a heater 114 for heating the reduction furnace 112 as a basic structure. The 1 st transport pipe 116 is connected to the reduction furnace 112, and the metal chloride gas is introduced into the reduction furnace 112 through this. The reduction furnace 112 is further provided with a 1 st gas introduction pipe 118 for supplying a reducing gas such as hydrazine, ammonia, methane, or the like. A reducing gas supply source, not shown, is connected to the 1 st gas introduction pipe 118. The valve 120 is attached to the 1 st gas introduction pipe 118, and thus the supply amount of the reducing gas can be controlled.

The 1 st transport pipe 116 is provided with a 2 nd gas introduction pipe 122 for supplying a sulfur-containing gas. A sulfur-containing gas source, not shown, is connected to the 2 nd gas introduction pipe 122 via a valve 124, and the supply amount thereof can be adjusted by the valve 124. With this structure, the reducing gas can be brought into contact with the mixed gas of the chloride gas and the sulfur-containing gas. The 1 st gas introduction pipe 118 and the 2 nd gas introduction pipe 122 may be further connected to an inert gas supply source, whereby an inert gas as a carrier gas can be mixed into the reducing gas or the sulfur-containing gas and supplied into the reduction furnace 112. With this structure, the mixed gas of the chloride gas and the sulfur-containing gas is supplied into the reduction furnace 112. Although not shown in the drawings, the 2 nd gas introduction pipe 122 may be connected to the reduction furnace 112 instead of being connected to the 1 st transport pipe 116, or the chloride gas and the sulfur-containing gas may be supplied to the reduction furnace 112 separately.

The chloride is reduced by the reducing gas in the reducing furnace 112 heated by the heater 114 to produce the metal particles 102, and sulfur derived from the sulfur-containing gas is introduced into the metal particles 102. Note that, preferably, not the separated chloride gas, but the chloride gas generated in the chlorination furnace not shown in the drawings is introduced. By adopting such a form, chlorination and reduction can be continuously performed, and metal powder can be efficiently produced.

The reduction furnace 112 is further provided with a 3 rd gas introduction pipe 126 for supplying a cooling gas to the reduction furnace 112. The 3 rd gas introduction pipe 126 is preferably provided at a position distant from the 1 st delivery pipe 116. For example, in the case where the 1 st duct 116 is provided at the upper portion of the reduction furnace 112, the 3 rd gas introduction pipe 126 is provided at the lower portion of the reduction furnace 112. As the cooling gas, inert gas such as nitrogen gas or argon gas can be used, and a supply source (not shown) of these gases is connected to the 3 rd gas introduction pipe 126. The flow of cooling gas is controlled by valve 128. By supplying the cooling gas, the growth of the metal particles 102 formed in the reduction furnace 112 can be controlled. The metal powder 100 is fed to a separation apparatus and a recovery apparatus through a 2 nd feed pipe 130 by a cooling gas, and separated and purified.

In the reduction, the reduction furnace 112 is heated by the heater 114, and the metal chloride gas and the sulfur-containing gas are introduced into the reduction furnace 112 through the 1 st transport pipe 116 and the 2 nd gas introduction pipe 122, while the reducing gas is supplied into the reduction furnace 112 through the 1 st gas introduction pipe 118. The heating temperature of the reduction furnace 112 is preferably lower than the melting point of the metal, and is selected from, for example, the range of 800 ℃ to 1100 ℃. Therefore, the metal generated in the reduction furnace 112 can be taken out as the solid metal particles 102. The amount of the reducing gas supplied into the reduction furnace 112 is adjusted by using the valve 120 so that the amount is stoichiometrically equivalent to or slightly excessive from the metal chloride supplied.

As the sulfur-containing gas, a gas containing a component selected from the group consisting of hydrogen sulfide, sulfur dioxide and a sulfur halide is used. Examples of the sulfur halide include SnCl₂(n is a whole number of 2 or more)Number), SF₆，SF₅Cl，SF₅Br, and the like. Among them, sulfur dioxide which is easy to handle is preferable. The flow rate of the sulfur-containing gas is adjusted to 0.01 wt% to 1.0 wt% by using a valve 124 for the metal powder produced from the chloride per unit time supplied from the reduction furnace 112.

By adopting the above method, the total concentration and the local concentration of sulfur can be controlled within the ranges described in embodiment 1, and the metal particles 102 containing sulfur at a high concentration and the metal powder 100 containing the metal particles 102 can be produced not only in the vicinity of the surface but also in the interior far from the surface.

[ examples ]

1. Example 1

In this example, an example of manufacturing the metal powder 100 by applying the manufacturing method described in embodiment 2 is shown.

A chlorine gas and nickel are reacted in a chlorination furnace to generate a nickel chloride gas, the reduction furnace 112 is heated to 1100 ℃, and a mixed gas of nickel chloride, sulfur dioxide gas as a sulfur-containing gas, and nitrogen gas is introduced into the reduction furnace 112 from a 1 st transport pipe 116 connected to the chlorination furnace at a flow rate of 2.8 m/sec (in terms of 1100 ℃). Meanwhile, hydrogen gas was introduced into the reduction furnace 112 from the 1 st gas introduction pipe 118 at a flow rate of 2.2 m/sec (in terms of 1100 ℃. As the cooling gas, nitrogen gas was used and supplied from the 3 rd gas inlet pipe 126. The obtained nickel powder (number average particle diameter of 190nm) was purified by using a production apparatus and the like not shown. The overall sulfur concentration of the resulting nickel powder was 0.15 wt%.

As comparative example 1 of example 1, a nickel powder prepared by subjecting a nickel powder obtained by reducing nickel chloride in the absence of a sulfur-containing gas to a sulfur treatment was used, and the sulfur concentration thereof was measured. The nickel powder in comparative example 1 was produced by producing a nickel powder without introducing a sulfur-containing gas into the reduction furnace 112 in the above-described example, and then performing post-treatment as described below.

That is, a thiourea aqueous solution having a sulfur content of 0.15 wt% with respect to the nickel powder was added to the slurry obtained in the step of purifying the nickel powder (number average particle diameter 190nm) produced in the absence of the sulfur-containing gas, and stirred for 30 minutes. Then, the slurry was dried by an air flow dryer to obtain nickel powder of comparative example 1.

Regarding the nickel powders of example 1 and comparative example 1, the local concentration of sulfur from the surface to the depth direction was measured using STEM-EDS. The measurement was carried out using a scanning transmission electron microscope (JEM-2100F, manufactured by JEOL Ltd.) equipped with an energy-dispersive X-ray spectrometer (JED-2300T, manufactured by JEOL Ltd.). The results obtained are shown in table 1 and fig. 2.

TABLE 1 local concentration of sulfur in nickel powder

As shown in table 1 and fig. 2, the local concentration of sulfur on the surface of the nickel powder in comparative example 1 is higher than that in example 1, but rapidly decreases as the depth from the surface increases, i.e., as the inside is approached. On the other hand, in the nickel powder in example 1, although the local concentration of sulfur on the surface was low, the reduction rate in the depth direction was small, and sulfur was also distributed in the nickel powder. In this example 1, the depth of half-decay is 3.2 nm.

The results show that by using the manufacturing method according to the embodiment of the present invention, it is possible to obtain metal powder in which sulfur is distributed to a deeper position.

2. Example 2

In example 2, the effect of the overall concentration of sulfur on the sintering initiation temperature was investigated. Nickel powders having various total concentrations of sulfur were prepared by changing the flow rate of the sulfur-containing gas in the range from 1.7 m/sec to 2.2 m/sec (in terms of 1100 c) using the same method as in example 1. Similarly, the concentration and the addition amount of the aqueous thiourea solution were changed to prepare nickel powders having various overall concentrations of sulfur as comparative example 2, using the same method as comparative example 1 described in example 1. The total concentration of sulfur was measured by the same method as in example 1.

The sintering initiation temperature was measured by a scanning electron microscope (SU-5000, manufactured by Hitachi Kagaku K.K.) equipped with a heating stage (Murano 525heating stage, manufactured by Gatan Co.). As a specific method, first, 100 metal powders were molded into particles having a diameter of 5mn × 1mm, adhered to a heating stage, and then introduced into a scanning electron microscope. The heating stage was observed with a scanning electron microscope while gradually raising the temperature of the heating stage from room temperature to 800 ℃. The sintering of the metal particles 102 is started as the temperature rises, but the sintering temperature is set to a temperature at which half or more of the nickel powder in the visual field is sintered. The results are shown in FIG. 3.

In comparative example 2, it can be seen that the sintering initiation temperature increases with the increase in the overall concentration of sulfur. However, as described in example 1, in the metal powder of comparative example 2, sulfur is not distributed to the inside of the metal particles at a high concentration, and therefore the overall concentration of sulfur has an upper limit. This may result in a total sulfur concentration of up to about 0.2 wt.% and a sintering initiation temperature of from about 500 to about 600 c.

In contrast, the nickel powder of example 2 had a sintering initiation temperature higher than that of comparative example 2. In example 2, since sulfur is distributed inside the nickel particles, a higher overall concentration of sulfur can be achieved as compared with the nickel powder in comparative example 2. For example, in this example 2, the total concentration of sulfur exceeded 0.2 wt%, and even metal powder having an overall concentration of sulfur of 0.3 wt% or more was obtained. As a result, the sintering initiation temperature of the nickel powder in example 2 can reach over 600 c, and even about 700 c. When the total concentration of sulfur is the same, by applying the manufacturing method of the present embodiment, nickel powder having a higher sintering initiation temperature can be manufactured.

It is noteworthy here that in example 2, when the overall concentration of sulfur is above 0.15 wt.%, a sintering onset temperature above 600 ℃ can be achieved, even a sintering onset temperature above 600 ℃ can be achieved with high probability. Therefore, by setting the total concentration of sulfur in the metal powder 100 to 0.15 wt% or more, the sintering initiation temperature is not affected even if the total concentration of sulfur is largely changed, and the fluctuation of the sintering initiation temperature can be effectively suppressed. In other words, by the manufacturing method of the present embodiment, metal powder with small change in sintering initiation temperature can be provided.

Those skilled in the art can add or delete structural elements, change the design of the structural elements, or add, omit, or change the conditions of the processes as appropriate according to the embodiments of the present invention, and the embodiments are also included in the scope of the present invention as long as the gist of the present invention is included.

Even other operational effects different from the operational effects brought by the aspects of the above-described embodiments will be apparent from the description of the present specification or can be easily predicted by those skilled in the art, and it is needless to say that the effects of the present invention can be understood.

Description of the reference numerals

100 metal powder

102 metal particles

110 reduction device

112 reduction furnace

114 heater

116 delivery pipe 1

118 the 1 st gas introduction tube

120: valve

122 No. 2 gas inlet pipe

124: valve

126 No. 3 gas introduction pipe

128: valve

130: 2 delivery pipe.

Claims

1. A metal powder comprising a metal and metal particles containing sulfur having an overall concentration of 0.01 wt% or more and 1.0 wt% or less; wherein,

a local concentration of sulfur at a position 4nm from a surface of the metal particle is 2 atomic% or more; and

the total concentration and the local concentration are estimated by an inductively coupled plasma emission spectrometer and an energy dispersive X-ray spectrometer disposed in a scanning transmission electron microscope, respectively.

2. The metal powder according to claim 1, wherein the number average particle diameter is 50nm or more and 400nm or less.

3. The metal powder according to claim 1, wherein a sintering initiation temperature of the metal powder is 600 ℃ or higher.

4. The metal powder according to claim 1, wherein the metal is nickel, copper or silver.

5. A method of making a metal powder comprising:

generating a metal chloride gas by chlorinating a metal with chlorine; and

producing metal particles by reducing the metal chloride as a gas in the presence of a sulfur-containing gas; wherein

The reduction is performed so that the total concentration of sulfur in the metal particles is 0.01 wt% or more and 1.0 wt% or less, and the local concentration of sulfur at a position of 4nm from the surface of the metal particles is 2 atomic% or more;

6. The method of claim 5, wherein the reduction is performed without isolating the metal chloride.

7. The method of claim 5, wherein the sulfur-containing gas is a sulfur dioxide-containing gas.

8. A method of predicting a sintering temperature of a metal powder, comprising:

measuring a local concentration of sulfur at a location 4nm from a surface of a metal particle selected from the group consisting of metal powders; wherein

The local concentration of sulfur is measured using a scanning transmission electron microscope equipped with an energy dispersive X-ray spectrometer.

9. The prediction method of claim 8, further comprising:

measuring the overall concentration of sulfur of the metal powder; wherein

The total concentration of sulfur is measured using the scanning transmission electron microscope provided with the energy-dispersive X-ray spectrometer.