Silver fine particles and method for producing silver fine particles

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

[0001] The present invention relates to silver microparticles and a method for producing silver microparticles that can be used in various devices such as conductive pastes, electrodes of electronic components such as multilayer ceramic capacitors, wiring for printed wiring boards, wiring for touch panels, flexible electronic paper, and solar cells and light-emitting elements, and in particular to silver microparticles and a method for producing silver microparticles that can be used to form conductive wiring, etc.

[0002] Currently, various types of fine particles, such as metal fine particles, oxide fine particles, nitride fine particles, and carbide fine particles, are used for various applications. Among these fine particles, silver fine particles can be used in conductive pastes, electrodes of 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 a method for producing them are described, for example, in Patent Document 1.

[0003] Patent Document 1 describes silver microparticles having a surface coating containing at least a carboxyl group. Patent Document 1 also describes a method for producing silver microparticles by a vapor phase method using silver powder, the method including a step of supplying an organic acid to the silver microparticles.

[0004] International Publication No. 2019 / 146412

[0005] As mentioned above, silver fine particles and a method for producing silver fine particles are described in Patent Document 1. However, currently, there is a demand for the particles to have a small particle size, a granular shape, and a uniform particle size.

[0006] An object of the present invention is to solve the problems associated with the prior art described above and to provide silver microparticles having a small particle size, a granular particle shape, and a uniform particle size, and a method for producing the silver microparticles.

[0007] The above object can be achieved by the following configuration. Invention [1] is a method for producing a granular material 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 silver fine particles according to invention [1], which have an exothermic peak temperature of 250°C or less in differential thermal analysis. Invention [3] is the silver fine particles according to invention [1], which have a weight loss per unit area of ​​0.5 × 10 －３ g / m ２ The above are the fine silver particles according to invention [1] or [2]. Invention [4] is a method for producing fine silver particles by a thermal plasma method using silver powder, the method comprising the steps of supplying the silver powder into a thermal plasma flame and supplying a cooling gas to the thermal plasma flame, the cooling gas containing methane gas and oxygen gas, and the ratio of oxygen gas to methane gas, oxygen gas / methane gas, is 1 to 50 volume %.

[0008] According to the present invention, it is possible to provide silver microparticles having a small particle size, a granular particle shape, and a uniform particle size, and a method for producing silver microparticles.

[0009] Fig. 1 is a schematic diagram showing an example of a fine particle manufacturing apparatus used in a method for manufacturing silver fine particles according to an embodiment of the present invention. Fig. 2 is a schematic diagram showing silver fine particles according to an embodiment of the present invention. Fig. 3 is a schematic diagram illustrating specular reflection light values. Fig. 4 is a schematic diagram illustrating specular reflection light values. Fig. 5 is a schematic diagram showing an SEM image of silver fine particles of Example 5. Fig. 6 is a schematic diagram showing an SEM image of silver fine particles of Comparative Example 3.

[0010] The silver microparticles and the method for producing silver microparticles of the present invention will be described in detail below based on the preferred embodiments shown in the accompanying drawings. Note that the drawings described below are merely illustrative for explaining the present invention, and the present invention is not limited to the drawings shown below. Note that in the following, the "to" symbol indicating a numerical range includes the numerical values ​​written on both sides. For example, when ε is a numerical value ε α ~Number ε β That is, the range of ε is the number ε α and the number ε β The range includes ε α ≦ε≦ε β In addition, unless otherwise specified, various numerical values ​​include an error range generally accepted in the relevant technical field.

[0011] [Method for producing silver fine particles] An example of a method for producing silver fine particles of the present invention will now be described. Fig. 1 is a schematic diagram showing an example of a fine particle production apparatus used in a method for producing silver fine particles according to an embodiment of the present invention. A fine particle production apparatus 10 shown in Fig. 1 (hereinafter simply referred to as production apparatus 10) is used for producing silver fine particles. The production apparatus 10 is used in a production method for producing silver fine particles by a thermal plasma method.

[0012] The manufacturing apparatus 10 comprises 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 unit 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 comprises a plasma gas supply unit 22, a plasma generation unit 24, and a gas supply unit 26.

[0013] The primary silver particles 15 and the secondary silver particles 18 both correspond to silver particles of the present invention. The secondary silver particles are obtained by classifying the primary silver particles 15. For the material supply device 14, chamber 16, cyclone 19, and recovery section 20, various devices disclosed in JP 2007-138287 A can be used, for example. The primary silver particles 15 can also be simply referred to as primary particles 15.

[0014] 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 can be easily evaporated 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.

[0015] The plasma torch 12 is composed of, for example, a quartz tube 12a and a high-frequency oscillation coil 12b surrounding the quartz tube 12a. A supply pipe 14a (described later) is provided in the center of the upper part of the plasma torch 12 for supplying raw material powder of fine silver particles 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.

[0016] 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 gas supply units corresponding to the gas species constituting the plasma. For example, if the plasma gas is a mixture of hydrogen gas and argon gas, the hydrogen gas is stored in a first gas supply unit (not shown), and the 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 regulator) and an adjustment valve for controlling the gas supply amount, both of which are not shown.

[0017] Hydrogen gas is supplied from the first gas supply unit of plasma gas supply unit 22, and argon gas is supplied from the second gas supply unit via pipe 22c and plasma gas supply port 12c, and then supplied into 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.

[0018] The plasma generating unit 24 has a high-frequency power supply (not shown), which is connected to the high-frequency oscillation coil 12b. The plasma generating unit 24 has the function of supplying a high-frequency voltage to the high-frequency oscillation coil 12b. When the high-frequency voltage is applied from the plasma generating unit 24 to the high-frequency oscillation coil 12b, 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. A higher temperature of the thermal plasma flame 28 is preferable because it facilitates the transition of the raw material powder to a gaseous state, 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 believed to reach approximately 10,000°C. The pressure atmosphere inside the plasma torch 12 is preferably subatmospheric pressure. The subatmospheric pressure atmosphere is not particularly limited, but may be, for example, 0.5 to 100 kPa.

[0019] 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.

[0020] The material supply device 14 is connected to the top 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, the material supply device 14 that supplies silver powder in the form of powder can be, for example, the one disclosed in Japanese Patent Application Laid-Open No. 2007-138287. In this case, the material supply device 14 includes, for example, a storage tank (not shown) for storing the silver powder, a screw feeder (not shown) for transporting a fixed amount of the silver powder, a dispersing unit (not shown) for dispersing the silver powder transported by the screw feeder into primary particles before final dispersion, and a carrier gas supply source (not shown).

[0021] The silver powder is supplied into the thermal plasma flame 28 in the plasma torch 12 via the supply pipe 14a together with carrier gas under extrusion pressure from a carrier gas supply source. The material supply device 14 is not particularly limited in configuration as long as it can prevent the silver powder from agglomerating and can disperse the silver powder into the plasma torch 12 while maintaining the silver powder in a dispersed state. An inert gas such as argon gas is used as the carrier gas. The carrier gas flow rate 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.

[0022] The chamber 16 is provided adjacent to and below the plasma torch 12, and is connected to a gas supply unit 26. Primary silver particles 15 are generated in the chamber 16. The chamber 16 also functions as a cooling tank.

[0023] 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. An adjustment valve (not shown) is also provided to control 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), and methane gas (CH ４ gas) is stored in a third gas source (not shown) and oxygen gas (O ２ gas) is stored.

[0024] The gas supply unit 26 supplies a mixed gas of argon gas, methane gas, and oxygen gas as a cooling gas in the direction of arrow Q, at an angle of, for example, 45°, toward the tail 28 b of the thermal plasma flame 28, i.e., the end of the thermal plasma flame 28 opposite the plasma gas supply port 12 c, i.e., the terminal end (tail 28 b) of the thermal plasma flame 28, and also supplies the above-mentioned cooling gas from above to below along the inner wall 16 a of the chamber 16, i.e., in the direction of arrow R shown in FIG. 1.

[0025] 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, and primary silver particles 15 are obtained. In addition to this, the above-mentioned cooling gas has additional functions such as 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, that is, oxygen gas / methane gas, is 1 to 50% by volume, and preferably 10 to 40% by volume. When the oxygen gas / methane gas ratio is 1 to 50% by volume, the BET specific surface area is 10 m ２ / g or more and a specular reflection value of 10 or more can be obtained. Furthermore, an oxygen gas / methane gas ratio of 10 to 40% by volume is preferable because the specular reflection value of the produced silver microparticles is higher, i.e., the granularity of the silver microparticles is higher, the particle size is more uniform, and the particle size variation is smaller. If the oxygen gas / methane gas ratio exceeds 50% by volume, the resulting silver microparticles are foil-like rather than granular, and no silver microparticles can be obtained. This is because, as the amount of oxygen gas increases, methane gas burns rather than pyrolyzes, reducing the amount of hydrocarbons that coat the surfaces of the silver microparticles produced by pyrolysis of methane gas. As a result, the surface coating of the silver microparticles is insufficient, promoting fusion. The cooling gas is a mixed gas containing argon gas, methane gas, and oxygen gas, but it may also be composed of only two gases: methane gas and oxygen gas. The oxygen gas / methane gas ratio (volume ratio) of oxygen gas to methane gas described above uses the flow rates of oxygen gas and methane gas at standard conditions (0°C, 1 atmosphere). The flow rates of the oxygen gas and the methane gas are measured using flow meters, such as float flow meters.

[0026] If the primary silver particles 15 collide with each other immediately after generation and form aggregates, resulting in non-uniform particle sizes, this will cause a decrease in quality. 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 particles 15, preventing the silver particles from colliding with each other and agglomerating. Furthermore, 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 process of collecting the primary particles 15, improving the yield of the generated primary particles 15.

[0027] 1, the chamber 16 is connected to a cyclone 19 for classifying the silver primary particles 15 into particles of a desired particle size. The cyclone 19 includes an inlet pipe 19a for supplying the primary particles 15 from the chamber 16, a cylindrical outer cylinder 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 cylinder 19b and has a gradually decreasing diameter, a coarse particle recovery chamber 19d connected to the bottom of the truncated cone section 19c and that recovers coarse particles having particle sizes equal to or larger than the desired particle size, and an inner pipe 19e connected to a recovery section 20, which will be described in detail later, and protruding from the outer cylinder 19b.

[0028] 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 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 forces prevents 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 that are more affected by drag than centrifugal force are discharged from the inner pipe 19e to the outside of the system together with the upward flow on the inner wall of the truncated cone portion 19c.

[0029] Furthermore, a negative pressure (suction force) is generated through the inner tube 19e from the recovery unit 20, which will be described in detail later. This negative pressure (suction force) causes the silver particles separated from the swirling airflow to be sucked in as indicated by the symbol U, and sent to the recovery unit 20 through the inner tube 19e.

[0030] A recovery unit 20 for recovering 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. Recovery unit 20 includes recovery chamber 20a, filter 20b provided within recovery chamber 20a, and vacuum pump 30 connected via a tube provided below recovery chamber 20a. Fine particles sent from cyclone 19 are sucked by vacuum pump 30 and drawn into recovery chamber 20a, where they are collected and remain on the surface of filter 20b. Note that the number of cyclones used in the above-described manufacturing apparatus 10 is not limited to one, and may be two or more.

[0031] Next, an example of a method for manufacturing silver microparticles using the above-described manufacturing apparatus 10 will be described. In this method, silver microparticles are manufactured by a thermal plasma method using silver powder. In this method, a raw material powder for the silver microparticles, such as silver powder having an average particle size of 5 μm or less, is first introduced 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 generator 24 to the high-frequency oscillation coil 12b, generating the 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, such as a mixed gas of argon gas, methane gas, and oxygen gas, is supplied from the gas supply device 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 gas-transported using, for example, argon gas as a carrier gas, and the silver powder is supplied into the thermal plasma flame 28 in the plasma torch 12 via the supply pipe 14a. The supplied silver powder evaporates into a gaseous state in the thermal plasma flame 28 and is quenched by the 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.

[0032] 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 peripheral wall of the outer cylinder 19b, causing this airflow to flow along the inner peripheral 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 surface of the truncated cone portion 19c and are recovered in the coarse particle recovery chamber 19d. Furthermore, fine particles that are more affected by drag than centrifugal force are discharged from the inner wall of the truncated cone portion 19c together with the ascending flow and discharged to the outside of the system.

[0033] The discharged secondary fine particles (silver fine particles) 18 are sucked in the direction indicated by the symbol U in FIG. 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 pipe 19e, and recovered by the filter 20b of the recovery section 20. At this time, the internal pressure in the cyclone 19 is preferably equal to or lower than atmospheric pressure. 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.

[0034] Next, we will explain silver microparticles. The silver microparticles of the present invention are produced, for example, by the above-mentioned production method and obtained in a particle state. As such, the silver microparticles of the present invention are not dispersed in a solvent or the like, but exist as silver microparticles alone. Therefore, the combination with a solvent is not particularly limited, and the solvent selection is highly flexible. FIG. 2 is a schematic diagram showing silver microparticles according to an embodiment of the present invention. As shown in FIG. 2, silver microparticles 40 have a surface coating 41 on their surface 40a. When the surface condition of silver microparticles 40, including the surface coating 41 on surface 40a, was examined, it was found that hydrocarbons (CnHm) were present on the surface, and therefore the surface coating 41 was composed of hydrocarbons (CnHm). The hydrocarbons (CnHm) were produced by the thermal decomposition of methane gas contained in the cooling gas by a thermal plasma flame. The surface condition of silver microparticles 40 can be examined, for example, using a Fourier transform infrared spectrophotometer (FT-IR).

[0035] The silver particles of the present invention have a BET specific surface area of ​​10 m ２ / g or more, and the specular reflection value is 10 or more. ２ / g or more and the specular reflection value is 10 or more, the silver particles have a small particle size and a uniform particle size. The BET specific surface area is a value measured using the BET method. ２ If the BET specific surface area is 10 to 25 m / g or more, the particle size of the silver particles is small. ２ / g. The silver fine 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 the 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 fine particles have a granular particle shape, small variation in particle size, and uniform particle size. It is preferable that the silver fine particles have a specular reflection value of 15 to 40.

[0036] As described above, the silver microparticles 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 particle shape and uniform particle size. This is based on the fact that the specular reflection value can evaluate the uniformity of a film. Specifically, when the silver microparticles have a granular particle shape and uniform particle size, the film uniformity is high, resulting in more specular reflection from the film. On the other hand, when the silver microparticles have a non-granular particle shape and non-uniform particle size, the film uniformity is low, resulting in more diffuse reflection from the film. This is used to evaluate whether the silver microparticles have a granular particle shape and uniform particle size. Here, Figures 3 and 4 are schematic diagrams explaining the specular reflection value. Incident light incident on an object is expressed as follows: Incident light = specular reflection light + diffuse reflection light + internally scattered light + transmitted light + absorbed light. When the silver microparticles 40 have a granular particle shape and uniform particle size, the film is uniform, dense, and smooth, resulting in a high specular reflection value. Therefore, the specular reflection value is an index of whether the silver microparticles have a granular particle shape and a uniform particle size, and of the uniformity, density, and smoothness of the film. Film 50 shown in Fig. 3 is formed on substrate 52, and has a plurality of silver microparticles 40 disposed therein. The plurality of silver microparticles 40 have a granular particle shape and a uniform particle size, and are well dispersed. Film 51 shown in Fig. 4 is formed on 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 and a non-uniform particle size, and the dispersion of the silver microparticles 40 within film 51 is poor.

[0037] 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. The film 50 shown in FIG. 3 is a film in which the silver microparticles 40 have a granular particle shape, uniform particle size, good dispersion, and a uniform film with few voids, resulting in less internally scattered light Ls and diffusely reflected light Ld and a greater amount of specularly reflected light Lr. In other words, when the silver microparticles 40 have a granular particle shape and uniform particle size, the specular reflected light value is large. The film 51 shown in FIG. 4 is a film in which the silver microparticles 40 have a non-granular particle shape, irregular particle size, poor dispersion, and a non-uniform film with many voids, resulting in increased internally scattered light Ls and diffusely reflected light Ld and a smaller amount of specularly reflected light Lr. In other words, when the silver microparticles 40 have a non-granular particle shape and irregular particle size, the film is in a non-uniform state with many voids. 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.

[0038] The silver microparticles preferably have an exothermic peak temperature of 250°C or less in differential thermal analysis, more preferably 170 to 200°C. If the exothermic peak temperature in differential thermal analysis is 250°C or less, sintering of the silver microparticles occurs at the exothermic peak temperature below 250°C, thereby enabling electrical conductivity to be exhibited. When the silver microparticles of the present invention are heated in the atmosphere, the hydrocarbons (CnHm) constituting the surface coating of the silver microparticles react with oxygen in the atmosphere, burning 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 highest heat generation occurs. In other words, the lower the 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 with the thin film removed 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.

[0039] The weight loss per unit area of ​​silver particles is 0.5 × 10 －３ g / m ２ It is preferable that the ratio is 0.9 to 2×10 or more. －３ g / m ２ The weight loss per unit area (g / m ２ The weight loss rate (mass%) in air was measured using a TG-DTA (thermogravimetric differential thermal analyzer), and this weight loss rate (mass%) was calculated based on the BET specific surface area (m ２ / g) to calculate the weight loss per unit area (g / m ２ ) = (weight reduction rate (mass%)) / (BET specific surface area (m ２ / g). The weight loss per unit area is 0.5 × 10 －３ g / m ２ If the amount is above this, the resulting fine silver particles will have a high granularity and small particle size variation.

[0040] The present invention is basically configured as described above. While the silver fine particles and the method for producing silver fine particles of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various improvements and modifications may be made without departing from the spirit and scope of the present invention.

[0041] The features of the present invention will be explained in more detail below with reference to examples. The process details, process procedures, etc. shown in the following examples can be modified as appropriate 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. Table 1 below shows the production conditions for the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8 in this example. 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. Furthermore, Table 1 below shows the production conditions, including the plasma gas flow rate and cooling gas flow rate, used to produce 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.

[0042] <BET specific surface area> The BET specific surface area (m ２ / g). The BET diameters shown in Table 2 below are the measured BET specific surface areas (m ２ / g) is the particle size of the silver particles.

[0043] <Specular Reflection Value> The specular reflection value was measured as follows. First, silver fine particles were dispersed in a solution consisting of a solvent and a dispersant. This dispersion was then applied to a slide glass 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 fine particles. BYK-112 (manufactured by BYK Japan KK) was used as the dispersant.

[0044] <Exothermic Peak Temperature in Differential Thermal Analysis> The exothermic peak temperature in differential thermal analysis was measured using STA7200 (trade name) manufactured by Hitachi High-Tech Science Corporation.

[0045] <Weight loss per unit area> The weight loss rate (mass%) was measured using STA7200 (product name) manufactured by Hitachi High-Tech Science Corporation. Using the BET specific surface area and weight loss rate measured as described above, the weight loss rate per unit area (g / m) was calculated according to the following formula: ２ The weight loss per unit area (g / m) was calculated. ２ ) = (weight reduction rate (mass%)) / (BET specific surface area (m ２ / g)

[0046] The silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8 will be described below. The silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8 were produced using the microparticle production apparatus 10 described above. To produce the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8, silver powder having an average particle size of 15 μm was used as the raw material powder. The production conditions for the silver microparticles of Examples 1 to 6 and Comparative Examples 1 to 8 were a constant 15 kW input power to the plasma and a pressure inside the plasma torch (apparatus internal pressure) of 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.

[0047] (Examples 1 to 6) In Example 1, the plasma gas flow rate was 170 L / min for argon gas and 5 L / min for hydrogen gas. The cooling gas flow rate was 50 L / min for argon gas, 15 L / min for methane gas, and 0.15 L / min for oxygen gas. Example 1 had an oxygen / methane ratio of 1% by volume. Example 2 was the same as Example 1, except that the cooling gas flow rate was 100 L / min for argon gas, 15 L / min for methane gas, and 1.5 L / min for oxygen gas. Example 2 had an oxygen / methane ratio of 10% by volume. Example 3 was the same as Example 1, except that the cooling gas flow rate was 450 L / min for argon gas, 15 L / min for methane gas, and 1.5 L / min for oxygen gas. In Example 3, the oxygen gas / methane gas ratio is 10% by volume.

[0048] Example 4 was the same as Example 1 except that the plasma gas flow rate was 200 L / min for argon gas and 5 L / min for hydrogen gas. Example 4 was the same as Example 1 except that the cooling gas flow rate was 900 L / min for argon gas, 10 L / min for methane gas, and 5 L / min for oxygen gas. Example 4 had an oxygen / methane ratio of 50% by volume. Example 5 was the same as Example 1 except that the plasma gas flow rate was 200 L / min for argon gas and 5 L / min for hydrogen gas. Example 5 was the same as Example 1 except that the cooling gas flow rate was 900 L / min for argon gas, 15 L / min for methane gas, and 5 L / min for oxygen gas. 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.

[0049] Comparative Examples 1 to 8: In Comparative Example 1, 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 was a mixed gas of argon gas and methane gas, and the argon gas flow rate was 150 L / min and the methane gas flow rate was 15 L / min. Comparative Example 2 was the same as in Example 1 except that the plasma gas flow rate was 200 L / min and the hydrogen gas flow rate was 5 L / min. The cooling gas was a mixed gas of argon gas and methane gas, and the argon gas flow rate was 450 L / min and the methane gas flow rate was 15 L / min. Comparative Example 3 was the same as in Example 1 except that the plasma gas flow rate was 160 L / min and the hydrogen gas flow rate was 5 L / min. The same procedures as in Example 1 were carried out except that the cooling gas was a mixed gas of argon gas and methane gas, the flow rate of the argon gas was 900 L / min, and the flow rate of the methane gas was 6 L / min.

[0050] In Comparative Example 4, 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 was a mixed gas of argon gas and methane gas, and the argon gas flow rate was 900 L / min and the methane gas flow rate was 10 L / min. In Comparative Example 5, the plasma gas flow rates were the same as in Example 1 except that the argon gas flow rate was 210 L / min and the hydrogen gas flow rate was 5 L / min. The cooling gas was a mixed gas of argon gas, methane gas, and hydrogen gas, and the argon gas flow rate was 600 L / min, the methane gas flow rate was 10 L / min, and the hydrogen gas flow rate was 5 L / min. In Comparative Example 6, the plasma gas flow rates were the same as in Example 1 except that the argon gas flow rate was 210 L / min and the hydrogen gas flow rate was 5 L / min. The cooling gas was a mixture of argon gas, methane gas, and hydrogen gas, and the flow rates of argon gas, methane gas, and hydrogen gas were 600 L / min, 10 L / min, and 10 L / min, respectively, except that the flow rates of hydrogen gas and argon gas were 10 L / min, 10 L / min, and 10 L / min, respectively.

[0051] In Comparative Example 7, the plasma gas flow rate was 200 L / min for argon gas and 5 L / min for hydrogen gas. The cooling gas flow rate was 900 L / min for argon gas, 1 L / min for methane gas, and 5 L / min for oxygen gas, but the other factors were the same as in Example 1. Comparative Example 7 had an oxygen / methane gas ratio of 500 vol%. Comparative Example 8 had the plasma gas flow rate of 200 L / min for argon gas and 5 L / min for hydrogen gas. The cooling gas flow rate was 900 L / min for argon gas, 1 L / min for methane gas, and 1 L / min for oxygen gas, but the other factors were the same as in Example 1. Comparative Example 8 had an oxygen / methane gas ratio of 100 vol%. In Comparative Examples 7 and 8, the particles were not in a granular form but in a foil form, so the BET specific surface area and BET diameter could not be measured. For this reason, "-" is entered in the BET specific surface area and BET diameter columns in Table 2 below. Since Comparative Examples 7 and 8 are foil-shaped, the specular reflection value, exothermic peak temperature, weight loss rate, and weight loss amount per unit area were not measured. For this reason, "-" 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.

[0052]

[0053]

[0054] As shown in Table 2, Examples 1 to 6 have a BET specific surface area of ​​10 m compared to Comparative Examples 1 to 8. ２ / g or more and a specular reflection value of 10 or more. ２ / g or more, the particle size is small. In addition, in Examples 1 to 6, the specular reflection value is 10 or more, so the particle shape is granular and the particle size is uniform. On the other hand, in Comparative Examples 1 to 6, all particles have a BET specific surface area of ​​10 m ２ / g or more and a specular reflection value of 10 or more were not obtained. This is thought to be because, without oxygen gas in the cooling gas, the thermal decomposition of methane gas did not occur to a level sufficient to generate hydrocarbons to coat the surfaces of the silver particles. As a result, the growth of the silver particles could not be suppressed, resulting in the generation of non-granular particles or coarse particles. Furthermore, both Comparative Examples 7 and 8 were in a foil-like state rather than a granular state. This is because the amount of oxygen gas in Comparative Examples 7 and 8 was high, causing methane gas to combust rather than pyrolyze, resulting in a reduced amount of hydrocarbons coating the surfaces of the silver particles generated by the thermal decomposition of methane gas. As a result, the surface coating of the silver particles was insufficient, promoting fusion. Because Comparative Examples 7 and 8 were in a foil-like state, it was not possible to measure the BET specific surface area and BET diameter. Figure 5 is a schematic diagram showing an SEM (Scanning Electron Microscope) image of the silver particles of Example 5, and Figure 6 is a schematic diagram showing an SEM image of the silver particles of Comparative Example 3. The SEM images were obtained using a JEOL JSM-6700F microscope. As shown in Figure 5, the silver microparticles of Example 5 had a granular particle shape, small particle size, and uniform 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 had non-uniform particle size. Furthermore, from Examples 1 to 6, it was found that when the oxygen gas / methane gas (volume ratio) was 10 to 40 volume %, the specular reflection value was high.

[0055] DESCRIPTION OF SYMBOLS 10 Fine particle manufacturing apparatus (manufacturing apparatus) 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 fine particles 16 Chamber 16a Inner wall 18 Secondary fine particles 19 Cyclone 19a Inlet pipe 19b Outer cylinder 19c Circular truncated cone portion 19d Coarse particle recovery chamber 19e Inner pipe 20 Recovery section 20a Recovery chamber 20b Filter 22 Plasma gas supply section 22c Piping 24 Plasma generation section 26 Gas supply section 26c Piping 28 Thermal plasma flame 28b Tail section 30 Vacuum pump 40 Silver fine 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 Internal scattered light

Claims

1. BET specific surface area is 10m ２ / g or more and having a specular reflection value of 10 or more.

2. The fine silver particles according to claim 1, which have an exothermic peak temperature of 250°C or less in differential thermal analysis.

3. Weight loss per unit area is 0.5 x 10 －３ g / m ２ The fine silver particles according to claim 1 or 2, wherein 4. 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, 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 %.

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