WO2025197747A1 - Silver fine particles and method for producing silver fine particles - Google Patents

Abstract

Provided are silver fine particles having a small particle diameter, a granular particle shape, and a uniform particle size, and a method for producing silver fine particles. The silver fine particles have a BET specific surface area of 10 m²/g or more and a regular reflected light value of 10 or more. The exothermic peak temperature in differential thermal analysis is 250°C or lower, and the weight loss per unit area is 0.5×10^-3 g/m² or more.

Description

Silver microparticles and method for producing silver microparticles

The present invention relates to silver microparticles and a method for manufacturing silver microparticles that can be used in conductive pastes, electrodes for electronic components such as multilayer ceramic capacitors, wiring for printed circuit boards, wiring for touch panels, flexible electronic paper, and various devices such as solar cells and light-emitting elements, and in particular to silver microparticles and a method for manufacturing silver microparticles that can be used to form conductive wiring, etc.

Currently, various types of fine particles, such as metal fine particles, oxide fine particles, nitride fine particles, and carbide fine particles, are used for a variety of purposes. Among these fine particles, silver fine particles can be used in conductive pastes, electrodes for electronic components such as multilayer ceramic capacitors, wiring for printed circuit boards, wiring for touch panels, flexible electronic paper, and various devices such as solar cells and light-emitting elements. Silver electrodes and silver wiring can be obtained by firing the silver fine particles. Silver fine particles and their manufacturing method are described, for example, in Patent Document 1.

Patent Document 1 describes silver particles having a surface coating, the surface coating containing at least a carboxyl group.
Furthermore, Patent Document 1 describes a method for producing silver fine particles by a vapor phase method using silver powder, which method includes a step of supplying an organic acid to the silver fine particles.

International Publication No. 2019/146412

As mentioned above, Patent Document 1 describes silver microparticles and a method for producing silver microparticles. However, current requirements require that the particle size be small, granular, and uniform in size.

The object of the present invention is to resolve the problems associated with the prior art described above and to provide silver microparticles that are small in particle size, have a granular particle shape, and are uniform in particle size, as well as a method for producing silver microparticles.

The above-mentioned object can be achieved by the following configuration.
The invention [1] is fine silver particles having a BET specific surface area of 10 m ² /g or more and a specular reflection value of 10 or more.
Invention [2] is the fine silver particles according to invention [1], which have an exothermic peak temperature of 250°C or less in differential thermal analysis.
Invention [3] is the fine silver particles according to invention [1] or [2], in which the weight loss per unit area is 0.5×10 ⁻³ g/m ² or more.
Invention [4] is a method for producing silver microparticles by a thermal plasma method using silver powder, which includes the steps of supplying the silver powder into a thermal plasma flame and supplying a cooling gas to the thermal plasma flame, wherein the cooling gas contains methane gas and oxygen gas, and the ratio of oxygen gas to methane gas, oxygen gas/methane gas, is 1 to 50 volume %.

The present invention provides silver microparticles with small particle diameters, granular particle shapes, and uniform particle diameters, as well as a method for producing silver microparticles.

1 is a schematic diagram showing an example of a fine particle manufacturing apparatus used in a method for manufacturing fine silver particles according to an embodiment of the present invention. FIG. 1 is a schematic diagram showing a silver fine particle according to an embodiment of the present invention. FIG. FIG. 10 is a schematic diagram illustrating a specular reflection value. FIG. 10 is a schematic diagram illustrating a specular reflection value. FIG. 1 is a schematic diagram showing an SEM image of the silver fine particles of Example 5. FIG. 10 is a schematic diagram showing an SEM image of silver fine particles of Comparative Example 3.

Hereinafter, the silver fine particles and the method for producing the silver fine particles of the present invention will be described in detail based on preferred embodiments shown in the accompanying drawings.
It should be noted that the drawings described below are illustrative for explaining the present invention, and the present invention is not limited to the drawings shown below.
In the following, the term "to" indicating a range of values includes the values written on both sides. For example, if ε is a value _εα to a value _εβ , the range of ε includes the values _εα and _εβ , and expressed in mathematical notation as _εα ≦ε≦ _εβ .
Furthermore, unless otherwise specified, various numerical values include error ranges generally accepted in the relevant technical field.

[Method of manufacturing silver fine particles]
An example of the method for producing fine silver particles of the present invention will be described below.
FIG. 1 is a schematic diagram showing an example of a fine particle manufacturing apparatus used in a method for manufacturing fine silver particles according to an embodiment of the present invention.
1 (hereinafter simply referred to as the manufacturing apparatus 10) is used for manufacturing silver fine particles. The manufacturing apparatus 10 is used in a manufacturing method for manufacturing silver fine particles by a thermal plasma method.

The manufacturing apparatus 10 includes a plasma torch 12 that generates a thermal plasma flame, a material supply device 14 that supplies raw material powder of silver microparticles into the plasma torch 12, a chamber 16 that functions as a cooling tank for generating primary silver microparticles 15, a cyclone 19 that removes coarse silver particles having a particle size equal to or larger than an arbitrarily specified particle size from the primary silver microparticles 15, and a recovery section 20 that recovers secondary silver microparticles 18 having a desired particle size that have been classified by the cyclone 19.
Furthermore, the manufacturing apparatus 10 has a plasma gas supply unit 22 , a plasma generation unit 24 , and a gas supply unit 26 .

The primary silver particles 15 and the secondary silver particles 18 both correspond to the silver particles of the present invention. The secondary silver particles are obtained by classifying the primary silver particles 15.
For example, various devices disclosed in Japanese Patent Application Laid-Open No. 2007-138287 can be used for the material supply device 14, chamber 16, cyclone 19, and recovery section 20. The primary fine particles 15 of silver are also simply referred to as primary fine particles 15.

In this embodiment, silver powder is used to produce the silver microparticles. The average particle size of the silver powder is appropriately set so that it evaporates easily in the thermal plasma flame. The average particle size is, for example, 100 μm or less, preferably 10 μm or less, and more preferably 5 μm or less.

The plasma torch 12 is composed of, for example, a quartz tube 12a and a high-frequency oscillation coil 12b surrounding the outside of the quartz tube 12a. A supply pipe 14a (described below) is provided in the center of the upper part of the plasma torch 12 for supplying raw material powder of silver microparticles into the plasma torch 12. A plasma gas supply port 12c is formed around the periphery of the supply pipe 14a (on the same circumference), and the plasma gas supply port 12c is ring-shaped.

The plasma gas supply unit 22 supplies plasma gas into the plasma torch 12 and is connected to the plasma gas supply port 12c via a pipe 22c. The plasma gas is supplied from the plasma gas supply unit 22 through the ring-shaped plasma gas supply port 12c into the plasma torch 12, for example, in the directions indicated by arrows P and S.
The plasma gas supply unit 22 has a gas supply unit corresponding to the gas species that constitute the plasma. When the plasma gas is, for example, a mixed gas of hydrogen gas and argon gas, hydrogen gas is stored in a first gas supply unit (not shown), and argon gas is stored in a second gas supply unit (not shown). The first gas supply unit and the second gas supply unit are each provided with a regulator (pressure adjuster) and an adjustment valve for controlling the gas supply amount, although not shown.

Hydrogen gas is supplied from the first gas supply section of the plasma gas supply unit 22, and argon gas is supplied from the second gas supply section via pipe 22c and plasma gas supply port 12c, and then into the plasma torch 12 in the directions indicated by arrows P and S. Note that only argon gas may be supplied in the direction indicated by arrow P.

The plasma generating unit 24 has a high-frequency power supply (not shown), which is connected to the high-frequency oscillation coil 12 b. The plasma generating unit 24 has a function of supplying a high-frequency voltage to the high-frequency oscillation coil 12 b. When the high-frequency voltage is applied from the plasma generating unit 24 to the high-frequency oscillation coil 12 b, a thermal plasma flame 28 is generated inside the plasma torch 12.
The temperature of the thermal plasma flame 28 must be higher than the boiling point of the raw material powder. On the other hand, a higher temperature of the thermal plasma flame 28 is preferable because it makes it easier for the raw material powder to become gaseous, but the temperature is not particularly limited. For example, the temperature of the thermal plasma flame 28 can be set to 6000°C, and theoretically it is thought to reach approximately 10000°C.
The pressure atmosphere inside the plasma torch 12 is preferably equal to or lower than atmospheric pressure. The pressure atmosphere is not particularly limited, but is, for example, 0.5 to 100 kPa.

The outside of the quartz tube 12a is surrounded by a concentric tube (not shown), and cooling water is circulated between this tube and the quartz tube 12a to water-cool the quartz tube 12a and prevent the quartz tube 12a from becoming too hot due to the thermal plasma flame 28 generated within the plasma torch 12.

The material supply device 14 is connected to the upper part of the plasma torch 12 via a supply pipe 14a. The material supply device 14 supplies raw material powder, for example, in the form of powder, into the thermal plasma flame 28 in the plasma torch 12.
As described above, for example, the material supply device 14 that supplies silver powder in powder form can be that disclosed in Japanese Patent Application Laid-Open No. 2007-138287. In this case, the material supply device 14 has, for example, a storage tank (not shown) that stores the silver powder, a screw feeder (not shown) that transports a fixed amount of the silver powder, a dispersing section (not shown) that disperses the silver powder transported by the screw feeder into primary particles before it is finally dispersed, and a carrier gas supply source (not shown).

The silver powder is supplied together with carrier gas under extrusion pressure from a carrier gas supply source through the supply pipe 14a into the thermal plasma flame 28 in the plasma torch 12.
The configuration of the material supply device 14 is not particularly limited as long as it can prevent aggregation of the silver powder and spray the silver powder into the plasma torch 12 while maintaining the dispersion state. An inert gas such as argon gas is used as the carrier gas. The flow rate of the carrier gas can be controlled using a flow meter such as a float flow meter. The flow rate value of the carrier gas refers to the scale value of the flow meter.

The chamber 16 is located adjacent to and below the plasma torch 12, and is connected to a gas supply unit 26. Primary silver particles 15 are generated within the chamber 16. The chamber 16 also functions as a cooling tank.

The gas supply unit 26 supplies cooling gas into the chamber 16. The gas supply unit 26 has a pipe 26c. The gas supply unit 26 further has a gas supply source (not shown) in which gas is stored, and a pressure applying unit (not shown) such as a compressor or blower that applies extrusion pressure to the quenching gas supplied into the chamber 16. The gas supply unit 26 is also provided with an adjustment valve (not shown) that controls the amount of gas supplied from the gas supply source. A gas supply source appropriate for the components of the cooling gas is used, and the cooling gas is a mixed gas of argon gas, methane gas, and oxygen gas.
In the gas supply unit 26, for example, argon gas is stored in a first gas supply source (not shown), methane gas ( _CH4 gas) is stored in a second gas supply source (not shown), and oxygen gas ( _O2 gas) is stored in a third gas supply source (not shown).

The gas supply unit 26 supplies a mixture of argon gas, methane gas, and oxygen gas as cooling gas in the direction of arrow Q, at an angle of, for example, 45°, toward the tail 28b of the thermal plasma flame 28, i.e., the end of the thermal plasma flame 28 opposite the plasma gas supply port 12c, i.e., the terminal end (tail 28b) of the thermal plasma flame 28, and also supplies the above-mentioned cooling gas from top to bottom along the inner wall 16a of the chamber 16, i.e., in the direction of arrow R shown in Figure 1.

The silver powder that has been put into a gaseous state by the thermal plasma flame 28 is rapidly cooled by the cooling gas supplied into the chamber 16 from the gas supply unit 26, thereby obtaining the silver primary particles 15. In addition to this, the cooling gas has an additional function of contributing to the classification of the primary particles 15 in the cyclone 19. The cooling gas is, for example, a mixed gas of argon gas, methane gas, and oxygen gas.
The cooling gas contains methane gas and oxygen gas, and the ratio (volume ratio) of oxygen gas to methane gas, oxygen gas/methane gas, is 1 to 50% by volume, preferably 10 to 40% by volume.
When the oxygen gas/methane gas ratio is 1 to 50% by volume, fine silver particles having a BET specific surface area of 10 m ² /g or more and a specular reflection value of 10 or more can be obtained.
Furthermore, when the oxygen gas/methane gas ratio is 10 to 40% by volume, the specular reflection value of the silver microparticles produced will be higher, i.e., the granularity of the silver microparticles will be higher, the particle size will be more uniform, and the particle size variation will be smaller, which is preferable.
If the oxygen gas/methane gas ratio exceeds 50% by volume, the mixture will not form particles but will instead form a foil, and silver microparticles will not be obtained. This is because, as the amount of oxygen gas increases, methane gas will burn rather than be thermally decomposed, and the amount of hydrocarbons that cover the surfaces of the silver microparticles produced by the thermal decomposition of methane gas will decrease, resulting in insufficient surface coverage of the silver microparticles and promoting fusion.
The cooling gas is a mixed gas containing argon gas, methane gas, and oxygen gas, but may be configured to contain only two types of gases, methane gas and oxygen gas.
The oxygen gas/methane gas ratio (volume ratio) of oxygen gas to methane gas uses the flow rate of oxygen gas under standard conditions (temperature 0°C, 1 atmosphere) and the flow rate of methane gas under standard conditions. The flow rates of oxygen gas and methane gas are measured using a flow meter. For example, a float type flow meter is used as the flow meter.

If the primary fine particles 15 of silver collide with each other immediately after generation and form aggregates, which cause uneven particle size, this will be a cause of quality degradation. However, the mixed gas supplied as a cooling gas in the direction of arrow Q toward the tail 28b (terminal end) of the thermal plasma flame 28 dilutes the primary fine particles 15, thereby preventing the silver fine particles from colliding with each other and agglomerating.
In addition, the mixed gas supplied as a cooling gas in the direction of arrow R prevents the primary particles 15 from adhering to the inner wall 16a of the chamber 16 during the recovery process of the primary particles 15, thereby improving the yield of the generated primary particles 15.

As shown in Figure 1, the chamber 16 is connected to a cyclone 19 for classifying the primary silver particles 15 into particles of the desired particle size. The cyclone 19 is equipped with an inlet pipe 19a that supplies the primary silver particles 15 from the chamber 16, a cylindrical outer tube 19b connected to the inlet pipe 19a and located at the top of the cyclone 19, a truncated cone section 19c that continues downward from the bottom of the outer tube 19b and has a gradually decreasing diameter, a coarse particle recovery chamber 19d connected to the bottom of the truncated cone section 19c and recovering coarse particles with particle sizes equal to or larger than the desired particle size, and an inner tube 19e that protrudes from the outer tube 19b and is connected to the recovery section 20, which will be described in detail later.

An airflow containing primary fine particles 15 is blown from the inlet pipe 19a of the cyclone 19 along the inner peripheral wall of the outer cylinder 19b, and as a result, this airflow flows from the inner peripheral wall of the outer cylinder 19b toward the truncated cone portion 19c as shown by the arrow T in Figure 1, forming a downward swirling flow.
When the downward swirling flow reverses and becomes an upward flow, the balance between centrifugal force and drag prevents the coarse particles from joining the upward flow, and they descend along the side surface of the truncated cone portion 19c and are collected in the coarse particle collection chamber 19d. Furthermore, fine particles, which are more affected by drag than centrifugal force, are discharged from the inner pipe 19e to the outside of the system along with the upward flow on the inner wall of the truncated cone portion 19c.

In addition, negative pressure (suction force) is generated through the inner tube 19e from the recovery section 20, which will be described in detail later. This negative pressure (suction force) causes the silver microparticles separated from the swirling airflow to be sucked in as indicated by the symbol U, and are sent to the recovery section 20 through the inner tube 19e.

A collection unit 20 for collecting secondary fine particles (fine silver particles) 18 having a desired nanometer-order particle size is provided on the extension of inner tube 19e, which is the outlet of the airflow within cyclone 19. Collection unit 20 includes a collection chamber 20a, a filter 20b provided within collection chamber 20a, and a vacuum pump 30 connected via a tube provided below collection chamber 20a. The fine particles sent from cyclone 19 are sucked by vacuum pump 30 and drawn into collection chamber 20a, where they remain on the surface of filter 20b and are collected.
In the above-described manufacturing apparatus 10, the number of cyclones used is not limited to one, but may be two or more.

Next, an example of a method for producing silver fine particles using the above-described production apparatus 10 will be described.
The method for producing silver particles is to use silver powder and produce silver particles by a thermal plasma method.
In the method for producing silver microparticles, first, as raw material powder for the silver microparticles, for example, silver powder having an average particle size of 5 μm or less is fed into the material supply device 14. The material supply device 14 supplies the silver powder to the thermal plasma flame 28 as described below.
A high frequency voltage is applied from the plasma generating unit 24 to the high frequency oscillation coil 12b to generate a thermal plasma flame 28 within the plasma torch 12. For example, argon gas and hydrogen gas are used as the plasma gas.
Furthermore, a cooling gas, for example, a mixed gas of argon gas, methane gas, and oxygen gas, is supplied from the gas supply unit 26 to the tail 28b of the thermal plasma flame 28, i.e., the terminal end of the thermal plasma flame 28, in the direction of arrow Q (a step of supplying a cooling gas). The oxygen/methane ratio in the cooling gas is 1 to 50% by volume. At this time, a mixed gas of argon gas, methane gas, and oxygen gas is also supplied as a cooling gas in the direction of arrow R.
Next, in the material supply device 14, the silver powder is gaseously transported using, for example, argon gas as a carrier gas, and the silver powder is supplied via the supply pipe 14a into the thermal plasma flame 28 in the plasma torch 12. The supplied silver powder evaporates into a gaseous state in the thermal plasma flame 28 and is rapidly cooled by a cooling gas to generate primary silver particles 15 (silver particles). Thus, the method for producing silver particles includes the steps of supplying silver powder into the thermal plasma flame and supplying a cooling gas to the thermal plasma flame.

The primary silver particles 15 obtained in the chamber 16 are then blown together with the airflow from the inlet pipe 19a of the cyclone 19 along the inner wall of the outer cylinder 19b. As a result, this airflow flows along the inner wall of the outer cylinder 19b as indicated by arrow T in Figure 1, forming a swirling flow and descending. When the descending swirling flow reverses and becomes an ascending flow, the balance between centrifugal force and drag forces prevents coarse particles from joining the ascending flow, and they descend along the side of the truncated cone portion 19c and are collected in the coarse particle collection chamber 19d. Furthermore, fine particles that are more affected by drag than centrifugal force are expelled from the system along with the ascending flow on the inner wall of the truncated cone portion 19c.

The discharged secondary fine particles (silver fine particles) 18 are sucked in the direction indicated by the symbol U in Figure 1 by the negative pressure (suction force) from the recovery section 20 by the vacuum pump 30, sent to the recovery section 20 through the inner tube 19e, and recovered by the filter 20b of the recovery section 20. At this time, the internal pressure within the cyclone 19 is preferably below atmospheric pressure. Furthermore, the particle size of the secondary fine particles (silver fine particles) 18 is specified to any particle size on the order of nanometers depending on the purpose.

Next, the silver particles will be described.
The silver fine particles of the present invention are produced, for example, by the above-mentioned production method and obtained in a particle state. As such, the silver fine particles of the present invention are not dispersed in a solvent or the like, but exist as silver fine particles alone. Therefore, there are no particular limitations on the combination with a solvent, and there is a high degree of freedom in the selection of the solvent.
Here, FIG. 2 is a schematic diagram showing silver fine particles according to an embodiment of the present invention.
2, silver fine particle 40 has surface coating 41 on its surface 40a. When the surface condition of silver fine particle 40, including surface coating 41 on surface 40a, was examined, it was found that hydrocarbons (CnHm) were present on the surface, and therefore surface coating 41 was composed of hydrocarbons (CnHm).
The hydrocarbons (CnHm) are produced by the thermal decomposition of methane gas contained in the cooling gas by the thermal plasma flame.
The surface state of the silver particles 40 can be examined using, for example, an FT-IR (Fourier transform infrared spectrophotometer).

The silver fine particles of the present invention have a BET specific surface area of 10 m ² /g or more and a specular reflection value of 10 or more. If the BET specific surface area is 10 m ² /g or more and the specular reflection value is 10 or more, the silver fine particles have a small and uniform particle size.
The BET specific surface area is a value measured using the BET method. If the BET specific surface area is 10 m ² /g or more, the particle size of the silver particles is small. The BET specific surface area is preferably 10 to 25 m ² /g.
The fine silver particles of the present invention are called nanoparticles, and have a particle size of 1 to 57 nm. The particle size is a value converted from the BET specific surface area and represents the average particle size.
The specular reflection value is an index showing the granularity of particle shape and the variation in particle size, with the upper limit of the specular reflection value being 100. The larger the specular reflection value, the more granular the particle shape and the more uniform the particle size. If the specular reflection value is 10 or more, the silver microparticles have a granular particle shape with little variation in particle size and a uniform particle size. It is preferable that the silver microparticles have a specular reflection value of 15 to 40.

As described above, the silver fine particles have a specular reflection value of 10 or more, preferably 15 to 40.
The specular reflection value can be used to evaluate whether the silver microparticles have a granular shape and uniform particle size. This is based on the fact that the specular reflection value can evaluate the uniformity of a film. Specifically, if the silver microparticles have a granular shape and uniform particle size, the film will have high uniformity, resulting in more specular reflection from the film. On the other hand, if the silver microparticles have a non-granular shape and irregular particle size, the film will have low uniformity, resulting in more diffuse reflection from the film. This can be used to evaluate whether the silver microparticles have a granular shape and uniform particle size.
3 and 4 are schematic diagrams for explaining the specular reflection value. The incident light incident on an object is expressed as follows:
Incident light = specularly reflected light + diffusely reflected light + internally scattered light + transmitted light + absorbed light If the silver microparticles 40 have a granular particle shape and a uniform particle size, the film will be uniform, dense, and smooth, and the specular reflected light value will be high. Therefore, the specular reflected light value is an indicator of whether the silver microparticles have a granular particle shape and a uniform particle size, and the uniformity, density, and smoothness of the film.
The film 50 shown in Fig. 3 is formed on a substrate 52 and has a plurality of silver microparticles 40 disposed therein. The plurality of silver microparticles 40 have a granular particle shape, uniform particle size, and a good dispersion state. The film 51 shown in Fig. 4 is formed on a substrate 52 and has a plurality of silver microparticles 40 disposed therein, but the plurality of silver microparticles 40 have a non-granular particle shape, non-uniform particle size, and the dispersion state of the silver microparticles 40 within the film 51 is poor.

Incident light Li incident on the surface 50a of the film 50 shown in Fig. 3 generates specularly reflected light Lr. Incident light Li incident on the surface 51a of the film 51 shown in Fig. 4 also generates specularly reflected light Lr.
3, the silver microparticles 40 have a granular particle shape, a uniform particle size, a good dispersion state, and a uniform film with few voids, so the internal scattered light Ls and diffuse reflected light Ld are small, and the amount of specular reflected light Lr is large. In other words, when the silver microparticles 40 have a granular particle shape and a uniform particle size, the specular reflected light value is large.
4, the silver microparticles 40 have a non-granular particle shape and irregular particle sizes, are poorly dispersed, and are non-uniform with many voids, resulting in an increase in internal scattered light Ls and diffuse reflected light Ld and a decrease in the amount of specular reflected light Lr. In other words, when the silver microparticles 40 have a non-granular particle shape and irregular particle sizes, the specular reflected light value is small.
In this way, the specular reflection value can be used to evaluate the granularity of the particle shape of the silver fine particles and the variation in particle size. The specular reflection value can be measured using, for example, a spectrophotometer.

The silver particles preferably have an exothermic peak temperature of 250°C or less, more preferably 170 to 200°C, in differential thermal analysis.
If the exothermic peak temperature in differential thermal analysis is 250°C or less, sintering of the silver particles occurs at the exothermic peak temperature of 250°C or less, and electrical conductivity can be exhibited.
When the silver microparticles of the present invention are heated in the atmosphere, the hydrocarbons (CnHm) that constitute the surface coating that coats the surfaces of the silver microparticles react with oxygen in the atmosphere, combusting and decomposing with heat. The exothermic peak temperature (°C) in differential thermal analysis is measured using a TG-DTA (thermogravimetric differential thermal analyzer) to measure the degree of heat generation, and indicates the temperature at which the heat generation is greatest. In other words, the lower this exothermic peak temperature, the more easily the hydrocarbon compounds in the thin film coating the surface are decomposed, and the more easily the silver microparticles that no longer have the thin film come into contact with each other, indicating that the silver microparticles can be fired at a lower temperature.
The exothermic peak temperature in the differential thermal analysis is measured in air using a TG-DTA (thermogravimetric and differential thermal analyzer), such as the STA7200 (trade name) manufactured by Hitachi High-Tech Science Corporation.

The weight loss of the fine silver particles per unit area is preferably 0.5×10 ⁻³ g/m ² or more, and more preferably 0.9 to 2×10 ⁻³ g/m ² .
The weight loss per unit area (g/m ² ) is calculated by measuring the weight loss rate (mass %) in air using a TG-DTA (thermogravimetric differential thermal analyzer) and dividing this weight loss rate (mass %) by the BET specific surface area (m ² /g). That is, weight loss per unit area (g/m ² ) = (weight loss rate (mass %)) / (BET specific surface area (m ² /g)).
When the weight loss per unit area is 0.5×10 ⁻³ g/m ² or more, the granularity of the resulting fine silver particles is high and the variation in particle size is small.

The present invention is basically configured as described above. The silver microparticles and method for producing silver microparticles of the present invention have been described in detail above, but the present invention is not limited to the above-described embodiments, and various improvements and modifications may of course be made without departing from the spirit of the present invention.

The features of the present invention will be explained in more detail below with reference to examples. The process contents, process procedures, etc. shown in the following examples can be appropriately modified without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be construed as being limited by the specific examples shown below.
The manufacturing conditions for the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8 in this example are shown in Table 1 below. Furthermore, the BET specific surface area, specular reflection value, exothermic peak temperature, and weight loss per unit area were measured for the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 3. The results are shown in Table 2 below. For Comparative Examples 4 to 8, the specular reflection value, exothermic peak temperature, weight loss rate, and weight loss per unit area that were not measured are marked with "-" in the corresponding columns for specular reflection value, exothermic peak temperature, weight loss rate, and weight loss per unit area in Table 2 below.
Table 1 below shows the production conditions, ie, the plasma gas flow rate and the cooling gas flow rate when producing the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8.
The BET specific surface area, the specular reflection value, the exothermic peak temperature in differential thermal analysis, and the weight loss per unit area will be explained below.

<BET specific surface area>
The BET specific surface area (m ² /g) of the prepared silver particles was measured using Macsorb HM-1208 manufactured by Mountech Co., Ltd.
The BET diameters shown in Table 2 below are particle sizes of the silver particles converted from the measured BET specific surface area (m ² /g).

<Specular reflection value>
The specular reflection value was measured as follows.
First, silver particles were dispersed in a solution consisting of a solvent and a dispersant. This dispersion was then applied to a glass slide to form a coating film, and the specular reflection value of the coating film was measured using a spectrophotometer. The specular reflection value of the coating film was taken as the specular reflection value of the silver particles.
The dispersant used was BYK-112 (manufactured by BYK Japan KK).

<Exothermic peak temperature in differential thermal analysis>
The exothermic peak temperature in the differential thermal analysis was measured using STA7200 (trade name) manufactured by Hitachi High-Tech Science Corporation.

<Weight reduction per unit area>
The weight loss rate (mass %) was measured using STA7200 (trade name) manufactured by Hitachi High-Tech Science Corporation.
Using the BET specific surface area and the weight loss rate measured as described above, the weight loss amount per unit area (g/m ² ) was calculated according to the following formula.
Weight reduction amount per unit area (g/m ² )=(weight reduction rate (mass %))/(BET specific surface area (m ² /g))

The silver fine particles of Examples 1 to 6 and Comparative Examples 1 to 8 will be described below.
The silver fine particles of Examples 1 to 6 and Comparative Examples 1 to 8 were produced using the fine particle production apparatus 10 described above.
Silver powder having an average particle size of 15 μm was used as the raw material powder to produce the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8. The conditions for producing the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8 were as follows: the input power to the plasma was constant at 15 kW, and the pressure inside the plasma torch (internal pressure of the device) was 40 kPa.
Argon gas was used as the carrier gas, and a mixed gas of argon gas and hydrogen gas was used as the plasma gas. In Examples 1 to 6, a mixed gas of argon gas, methane gas, and oxygen gas was used as the cooling gas. In Comparative Examples 1 to 4, a mixed gas of argon gas and methane gas was used as the cooling gas. In Comparative Examples 5 and 6, a mixed gas of argon gas, methane gas, and hydrogen gas was used as the cooling gas. In Comparative Examples 7 and 8, a mixed gas of argon gas, methane gas, and oxygen gas was used as the cooling gas.
In Examples 1 to 6 and Comparative Examples 1 to 8, the flow rate of the plasma gas and the components and flow rate of the cooling gas were changed. The flow rate of the cooling gas was measured under standard conditions (temperature of 0°C, 1 atmosphere). The flow rate of the cooling gas was measured using a float-type flow meter for each component of the cooling gas, such as argon gas.

Examples 1 to 6
In Example 1, the flow rates of the argon gas and hydrogen gas were set to 170 L/min and 5 L/min, respectively, for the plasma gas.
The cooling gases were argon gas at a flow rate of 50 L/min, methane gas at a flow rate of 15 L/min, and oxygen gas at a flow rate of 0.15 L/min. In Example 1, the oxygen gas/methane gas ratio was 1% by volume.
Example 2 was the same as Example 1 except that the cooling gases were argon gas at a flow rate of 100 L/min, methane gas at a flow rate of 15 L/min, and oxygen gas at a flow rate of 1.5 L/min. Example 2 had an oxygen gas/methane gas ratio of 10% by volume.
Example 3 was the same as Example 1 except that the cooling gas flow rates of argon gas, methane gas, and oxygen gas were set to 450 L/min, 15 L/min, and 1.5 L/min, respectively. Example 3 had an oxygen/methane ratio of 10% by volume.

Example 4 was the same as Example 1 except that the plasma gas flow rates of argon gas were 200 L/min and hydrogen gas were 5 L/min. The cooling gas flow rates of argon gas were 900 L/min, methane gas 10 L/min, and oxygen gas 5 L/min. Example 4 had an oxygen/methane gas ratio of 50% by volume.
Example 5 was the same as Example 1 except that the plasma gas flow rates of argon gas were 200 L/min and hydrogen gas were 5 L/min. The cooling gas flow rates of argon gas were 900 L/min, methane gas 15 L/min, and oxygen gas 5 L/min. Example 5 had an oxygen/methane ratio of 33.3% by volume.
Example 6 was the same as Example 1 except that the plasma gas flow rates of argon gas were 200 L/min and hydrogen gas were 5 L/min, and the cooling gas flow rates were 900 L/min, 25 L/min, and 5 L/min, respectively. Example 6 had an oxygen/methane gas ratio of 20% by volume.

(Comparative Examples 1 to 8)
In Comparative Example 1, the flow rates of the argon gas and the hydrogen gas were set to 200 L/min and 5 L/min, respectively, compared to Example 1. The cooling gas was a mixed gas of argon gas and methane gas, and the flow rates of the argon gas and the methane gas were set to 150 L/min and 15 L/min, respectively.
In Comparative Example 2, the flow rates of the argon gas and the hydrogen gas were set to 200 L/min and 5 L/min, respectively, compared to Example 1. The cooling gas was a mixed gas of argon gas and methane gas, and the flow rates of the argon gas and the methane gas were set to 450 L/min and 15 L/min, respectively.
In Comparative Example 3, the flow rates of the argon gas and the hydrogen gas were set to 160 L/min and 5 L/min, respectively, compared to Example 1. The cooling gas was a mixed gas of argon gas and methane gas, and the flow rates of the argon gas and the methane gas were set to 900 L/min and 6 L/min, respectively.

In Comparative Example 4, the flow rates of the argon gas and the hydrogen gas were set to 200 L/min and 5 L/min, respectively, compared to Example 1. The cooling gas was a mixed gas of argon gas and methane gas, and the flow rates of the argon gas and the methane gas were set to 900 L/min and 10 L/min, respectively.
In Comparative Example 5, the plasma gas flow rates were set to 210 L/min for argon gas and 5 L/min for hydrogen gas, as compared with Example 1. The cooling gas was a mixed gas of argon gas, methane gas, and hydrogen gas, and the flow rates of argon gas, methane gas, and hydrogen gas were set to 600 L/min, 10 L/min, and 5 L/min, respectively, except for the other conditions.
In Comparative Example 6, the plasma gas flow rates were set to 210 L/min for argon gas and 5 L/min for hydrogen gas, as compared with Example 1. The cooling gas was a mixed gas of argon gas, methane gas, and hydrogen gas, and the flow rates of argon gas, methane gas, and hydrogen gas were set to 600 L/min, 10 L/min, and 10 L/min, respectively, except for the other conditions.

In Comparative Example 7, the plasma gas flow rates were the same as in Example 1 except that the argon gas flow rate was 200 L/min and the hydrogen gas flow rate was 5 L/min. The cooling gas flow rates were the same as in Example 1 except that the argon gas flow rate was 900 L/min, the methane gas flow rate was 1 L/min, and the oxygen gas flow rate was 5 L/min. In Comparative Example 7, the oxygen gas/methane gas ratio was 500% by volume.
In Comparative Example 8, the plasma gas flow rates were the same as in Example 1 except that the argon gas flow rate was 200 L/min and the hydrogen gas flow rate was 5 L/min. The cooling gas flow rates were the same as in Example 1 except that the argon gas flow rate was 900 L/min, the methane gas flow rate was 1 L/min, and the oxygen gas flow rate was 1 L/min. In Comparative Example 8, the oxygen gas/methane gas ratio was 100% by volume.
In Comparative Examples 7 and 8, the powders were not in a particulate form but in a foil form, and therefore the BET specific surface area and BET diameter could not be measured. Therefore, "-" is entered in the BET specific surface area and BET diameter columns in Table 2 below. Since Comparative Examples 7 and 8 were in a foil form, the specular reflection value, exothermic peak temperature, weight loss rate, and weight loss amount per unit area were not measured. Therefore, "-" is entered in the columns for specular reflection value, exothermic peak temperature, weight loss rate, and weight loss amount per unit area in Table 2 below.

As shown in Table 2, in comparison with Comparative Examples 1 to 8, Examples 1 to 6 produced fine silver particles having a BET specific surface area of 10 m ² /g or more and a specular reflection value of 10 or more.
In Examples 1 to 6, the BET specific surface area was 10 m ² /g or more, indicating that the particle size was small. In addition, in Examples 1 to 6, the specular reflection value was 10 or more, indicating that the particle shape was granular and the particle size was uniform.
On the other hand, in Comparative Examples 1 to 6, none of the particles obtained had a BET specific surface area of 10 m ² /g or more and a specular reflection value of 10 or more. This is thought to be because, if the cooling gas did not contain oxygen gas, the thermal decomposition of methane gas did not occur to a level that would generate hydrocarbons sufficient to coat the surface of the silver particles, and as a result, the growth of the silver particles could not be suppressed, resulting in the generation of non-granular particles and coarse particles.
In addition, in Comparative Examples 7 and 8, the particles were not in a granular form but in a foil form. This is because the amount of oxygen gas was large in Comparative Examples 7 and 8, and methane gas was burned rather than thermally decomposed, resulting in a small amount of hydrocarbons covering the surface of the silver particles produced by thermal decomposition of methane gas. As a result, the surface coating of the silver particles was insufficient, and fusion was promoted. Since Comparative Examples 7 and 8 were in a foil form, it was not possible to measure the BET specific surface area and BET diameter.
Here, Fig. 5 is a schematic diagram showing an SEM (Scanning Electron Microscope) image of the silver fine particles of Example 5, and Fig. 6 is a schematic diagram showing an SEM image of the silver fine particles of Comparative Example 3. The SEM images were obtained using a JSM-6700F manufactured by JEOL Ltd.
As shown in Figure 5, the silver microparticles of Example 5 had a granular particle shape, small particle diameters, and were uniform in particle size with no coarse particles. On the other hand, as shown in Figure 6, the silver microparticles of Comparative Example 3 had a non-granular particle shape, contained 100 coarse particles, and were non-uniform in particle size.
Furthermore, from Examples 1 to 6, when the oxygen gas/methane gas (volume ratio) is 10 to 40% by volume, the specular reflection value is high.

10 Fine particle manufacturing equipment (manufacturing equipment)
12 Plasma torch 12a Quartz tube 12b High frequency oscillation coil 12c Plasma gas supply port 14 Material supply device 14a Supply pipe 15 Primary particles 16 Chamber 16a Inner wall 18 Secondary particles 19 Cyclone 19a Inlet pipe 19b Outer cylinder 19c Circular truncated cone portion 19d Coarse particle collection chamber 19e Inner pipe 20 Collection portion 20a Collection chamber 20b Filter 22 Plasma gas supply portion 22c Piping 24 Plasma generation portion 26 Gas supply portion 26c Piping 28 Thermal plasma flame 28b Tail portion 30 Vacuum pump 40 Silver particles 40a, 51a Surface 41 Surface coating 50, 51 Film 52 Substrate 100 Coarse particles Ld Diffuse reflected light Li Incident light Lr Specular reflected light Ls Internally scattered light

Claims (4)

The BET specific surface area is 10 m ² /g or more,
Silver particles having a specular reflection value of 10 or more. The silver microparticles described in claim 1, having an exothermic peak temperature of 250°C or less in differential thermal analysis. 3. The fine silver particles according to claim 1, wherein the weight loss per unit area is 0.5×10 ⁻³ g/m ² or more. A method for producing silver microparticles by a thermal plasma method using silver powder, comprising the steps of:
supplying the silver powder into a thermal plasma flame and supplying a cooling gas to the thermal plasma flame;
A method for producing silver microparticles, wherein the cooling gas contains methane gas and oxygen gas, and the ratio of the oxygen gas to the methane gas, oxygen gas/methane gas, is 1 to 50 volume %.