Silver powder, production method for silver powder, and conductive paste

Abstract

To provide a silver powder and a production method for the silver powder, making it possible to prepare a conductive paste that can produce a wiring pattern having a comparatively large ratio (aspect ratio) of height to line width.SOLUTION: A production method for a silver powder includes a crushing step for crushing an agglomerated silver powder with an airflow-type pulverizer and a classification step for classifying the silver powder that has undergone the crushing step with a pneumatic classifier, the agglomerated silver powder having a water content of at least 5.0 wt.% and at most 30.0 wt.%, wherein, at the crushing step, compressed air that is at a temperature of at least 80°C and at most 180°C is supplied to the airflow-type pulverizer as supplied air, and at the classification step, exhaust air from the airflow-type pulverizer and the silver powder that has undergone the crushing step are supplied to the pneumatic classifier, the exhaust air being at a temperature of at least 30°C and having a volume absolute humidity of at least 20 g / m3.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a silver powder, a method for producing the silver powder, and a conductive paste. [Background technology]

[0002] Patent Document 1 describes silver powder and a method for producing the same. In this method, a reducing agent is added to an aqueous reaction system containing silver ions to precipitate silver particles, and then the aqueous reaction system is filtered to obtain a cake, which is then dried in an airflow dryer to obtain the silver powder.

[0003] Patent Document 2 describes a method for producing silver particles. In this method, alcohol is added to a slurry of silver particles having an average particle size of submicrons, and the mixture is stirred, then filtered, dehydrated, dried, and crushed. In this method, the drying process may be either hot air drying or vacuum drying, and in the case of hot air drying, the filtrate may be placed under hot air at 30 to 100°C.

[0004] Patent Document 3 describes a silver powder and a method for producing the same. In this method, a reducing agent is added to an aqueous reaction system containing silver ions to reduce and precipitate silver particles, and the resulting silver-containing slurry is filtered and washed with water to obtain a cake, which is dehydrated at room temperature, crushed at room temperature to form crushed powder, and classified at room temperature to obtain the silver powder. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2008-1974 A [Patent Document 2] JP 2010-229481 A [Patent Document 3] JP 2016-216824 A Summary of the Invention [Problem to be solved by the invention]

[0006] The wiring and contacts of electronic components manufactured by applying a conductive paste (hereinafter sometimes simply referred to as "paste") are obtained by applying the paste by printing or the like and then heating it. It is required that a wiring pattern with the desired line width and height is obtained when the paste is applied and heated. In electronic components, thinning of the wiring is desired in order to maximize the area other than the wiring portion and to miniaturize the electronic components, and a wiring pattern with a high wiring height is desired so that the resistivity does not become too high even if the wiring is made thin.

[0007] The present invention aims to provide a silver powder capable of preparing a conductive paste capable of obtaining a wiring pattern with a relatively large ratio of height to line width (hereinafter sometimes referred to as "aspect ratio"), a method for producing the silver powder, and a conductive paste containing the silver powder. [Means for solving the problem]

[0008] In order to achieve the above object, the silver powder and method for producing the silver powder according to the present invention, as well as a conductive paste containing the silver powder, are as follows.

[0009] <1> The silver particles have closed voids within them. The arithmetic average roughness Sa (nm) in the surface roughness measurement of a 500 nm × 500 nm range of the surface of the silver particles is Specific surface area (m2) measured by BET single point method 2 The value of Sa / BET diameter, calculated by dividing the value of Sa (absolute particle diameter / g) and true density by the BET diameter calculated by the following formula (1), is 0.0070 or more. BET diameter = 6 / (specific surface area x true density)...Equation (1)

[0010] <2> The arithmetic mean roughness Sa is 5 nm or more and 15 nm or less. <1> The silver powder described in

[0011] <3> In the particle size distribution determined on a volume basis in a laser diffraction particle size distribution measurement, the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values ​​accumulated from the smaller particle diameter side are defined as D10, D50, and D90, respectively. The above, wherein the difference obtained by subtracting the D10 from the D90 and dividing the difference by the D50 is 1.0 or less. <1> or <2> The silver powder described in

[0012] <4> a crushing step of crushing the agglomerated silver powder using an airflow crusher; A classification step of classifying the silver powder after the crushing step by an air classifier, The agglomerated silver powder has a moisture content of 5.0 wt% or more and 30.0 wt% or less, In the disintegration step, compressed air having a temperature of 80° C. or more and 180° C. or less is supplied to the airflow type pulverizer as supply air, and the silver powder concentration in the airflow is set to 0.10 kg / m 3 More than 0.50kg / m 3 The agglomerated silver powder is supplied so as to satisfy the following: In the classification step, the exhaust gas from the air flow mill and the silver powder obtained after the crushing step are supplied to the air classifier, The exhaust gas has a temperature of 30° C. or higher and a volume absolute humidity of 20 g / m 3 The above is the method for producing silver powder.

[0013] <5> The air classifier and the air flow pulverizer are connected by a connecting pipe, and the exhaust gas from the air flow pulverizer and the silver powder after the crushing step are supplied from the air flow pulverizer to the air classifier through the connecting pipe. <4> A method for producing silver powder according to the present invention.

[0014] <6> The method further includes a collection step of collecting the silver powder after the classification step with a collector, In the collecting step, the exhaust gas from the air classifier and the silver powder after the classification step are supplied to the collector. <4> or <5> A method for producing silver powder according to the present invention.

[0015] <7> In the classification step, the air is classified while being sucked into the air classifier. <4> ~ <6> 13. A method for producing silver powder according to any one of the preceding claims.

[0016] <8> the above <1> ~ <3> A conductive paste comprising the silver powder according to any one of claims 1 to 4. Effect of the Invention

[0017] It is possible to provide a method for producing silver powder that can prepare a conductive paste that can obtain a wiring pattern with a relatively large ratio of height to line width (aspect ratio), and the silver powder. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a flow diagram of a plant for implementing the silver powder manufacturing method according to this embodiment. [Diagram 2] 1 is an SEM image of the silver powder of Example 1. [Diagram 3] 1 is an SEM image of the silver powder of Example 2. [Figure 4] 1 is an SEM image of the silver powder of Example 3. [Diagram 5] 1 is an SEM image of the silver powder of Example 4. [Figure 6] 1 is an SEM image of the silver powder of Comparative Example 1. [Figure 7] 1 is an SEM image of the silver powder of Comparative Example 2. [Figure 8] 1 is an SEM image of the silver powder of Comparative Example 3. [Figure 9] 1 is an SEM image of the silver powder of Comparative Example 4. [Figure 10] FIG. 13 is a diagram showing the shape of a wiring pattern for performing thin line evaluation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The silver powder according to the present invention will be described below.

[0020] The silver powder according to the present invention has closed voids within the silver particles, and the arithmetic mean roughness Sa (nm) in the surface roughness measurement of the silver particle surface in a range of 500 nm x 500 nm is calculated by multiplying the specific surface area (m2) measured by the BET one-point method. 2The silver powder has a value of Sa / BET diameter, calculated by dividing the value of Sa (mol / g) and true density by the BET diameter using the following formula (1), of 0.0070 or more. BET diameter = 6 / (specific surface area x true density)...Equation (1)

[0021] First, the silver particles constituting the silver powder according to the present invention have closed voids within the silver particles, which makes it possible to lower the firing temperature for obtaining a wiring pattern. Since the silver particles have closed voids within them, the true density of the silver powder according to the present invention is 10.49 g / cm 3 ) and is 9.0-10.0g / cm 3 For example, 9.7 g / cm 3 In this specification, the true density is a value measured by a true density measuring device, and can be measured using, for example, AccuPycII1340 manufactured by Micromeritics. The density is measured excluding the voids open to the outside of the particle surface, and the density is measured including the closed voids inside the particle.

[0022] The value of Sa / BET diameter obtained by dividing the arithmetic mean roughness Sa (nm) by the BET diameter in a surface roughness measurement of a 500 nm × 500 nm range of the silver particles constituting the silver powder is 0.0070 or more, preferably 0.009 or more, and preferably 0.015 or less. By setting the value in this range, when the silver powder is made into a paste by the method of the examples described later to obtain a conductive paste having the same degree of viscosity (within about ±20 Pa·s at a rotation speed of 1 rpm), regardless of the size of the BET diameter, the aspect ratio of the wiring pattern obtained by firing the conductive paste by the method of the examples described later can be made higher than that of the conventional technology. The smaller the BET diameter (the larger the specific surface area), the more the number of collisions between particles in the crushing process, so that the surface is more easily smoothed, and the arithmetic mean roughness Sa tends to be smaller. In the manufacturing method of the present invention, the smoothing and re-adhesion are different from those of the conventional method, so that silver powder having unevenness in the above range can be obtained regardless of the size of the BET diameter. For example, in the method for producing silver powder of the present invention described later, the raw silver powder to be subjected to the drying and crushing process is a silver powder (agglomerated silver powder) that contains moisture and is agglomerated in a wet state. In the case of producing a silver powder with less agglomeration using this agglomerated silver powder, for example, compressed air at a temperature of 80°C to 180°C is supplied to an airflow type crusher as the supply air to perform the crushing process (i.e., drying and crushing are performed simultaneously), and then, classification is performed in a humid air stream to obtain silver powder, thereby making it possible to make the Sa / BET diameter value of the obtained silver powder 0.0070 or more. On the other hand, it is difficult to make the Sa / BET diameter value of the silver powder obtained by drying the agglomerated silver powder to a dry state, followed by crushing and classification, 0.0070 or more. Therefore, usually, when the same agglomerated silver powder is used as a raw material, the silver powder that can be obtained through the method for producing silver powder of the present invention described later can make the Sa / BET diameter value 0.0070 or more, and it is possible to increase the ratio of height to line width (aspect ratio) of the wiring pattern obtained by pasting and firing.

[0023] The surface roughness of the silver particles constituting the silver powder in a range of 500 nm x 500 nm can be measured using a scanning probe microscope (e.g., Nano Cute manufactured by SII Nano Technology), and the arithmetic mean roughness Sa (nm) is the arithmetic mean roughness specified in ISO25178. The arithmetic mean roughness Sa (nm) is preferably 5 nm or more, more preferably 7 nm or more, and preferably 15 nm or less. The arithmetic mean roughness Sa (nm) of the silver powder crushed and classified in a humid atmosphere in the present invention is larger than that of the silver powder crushed and classified in a conventional dry atmosphere.

[0024] The BET diameter (μm) of the silver powder is preferably 0.30 μm or more, more preferably 0.50 μm or more, and is preferably 3.00 μm or less, more preferably 1.50 μm or less, even more preferably 1.00 μm or less, and even more preferably 0.90 μm or less.

[0025] Specific surface area of ​​silver powder (m 2The BET specific surface area (μm / g) is the BET specific surface area determined by the BET method. The BET specific surface area can be measured, for example, by using a BET specific surface area measuring device (Macsorb HM-model 1210 manufactured by Mountech Co., Ltd.) and flowing a He-N2 mixed gas (30% nitrogen) into the measuring device at 60° C. for 10 minutes to degas the device, followed by the BET one-point method. The specific surface area measured by the BET one-point method of the silver powder crushed and classified in a humid atmosphere in the present invention is larger than that of the silver powder crushed and classified in a conventional dry atmosphere, and is 0.3 m 2 / g or more, and 0.55m 2 / g or more is more preferable, and 0.70m 2 More preferably, it is 1.0m 2 / g or less. The BET specific surface area is preferably 1.00 m 2 / g or less is preferable.

[0026] In the silver powder of the present invention, the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values ​​accumulated from the smaller particle diameter side in the particle size distribution determined on a volume basis in a laser diffraction particle size distribution measurement are designated as D10, D50, and D90, respectively, and the difference obtained by subtracting D10 from D90 and dividing the difference by D50 is preferably 1.0 or less, more preferably 0.85 or less, and even more preferably 0.75 or less. If this value exceeds 1.0, it may be difficult to obtain a wiring pattern of the desired line width and height when the silver powder is made into a paste and fired to obtain a wiring pattern. It is also preferable that the difference obtained by subtracting the D10 from the D90 and dividing it by the D50 be 1.00 or less.

[0027] The D50 of the silver powder is preferably 0.4 μm or more, more preferably 0.8 μm or more, and is preferably 4.0 μm or less, more preferably 2.5 μm or less, even more preferably 1.6 μm or less, and even more preferably 1.35 μm or less. If the D50 of the silver powder exceeds 4.0 μm, it may be difficult to obtain a wiring pattern with a narrow width. If the D50 of the silver powder is less than 0.4 μm, the viscosity of the paste may become too high when it is made into a paste, making it difficult to handle. The D50 of the silver powder is preferably 0.40 μm or more, more preferably 0.80 μm or more, and is preferably 4.00 μm or less, more preferably 2.50 μm or less, and even more preferably 1.60 μm or less.

[0028] The D10 of the silver powder is preferably 0.1 μm or more, more preferably 0.3 μm or more, and is preferably 1.5 μm or less, more preferably 0.90 μm or less, and even more preferably 0.86 μm or less. If the D10 of the silver powder exceeds 1.5 μm, it may be difficult to obtain a wiring pattern with a narrow width. If the D10 of the silver powder is less than 0.1 μm, the viscosity of the paste may become too high when it is made into a paste, making it difficult to handle. The D10 of the silver powder is preferably 0.10 μm or more, more preferably 0.30 μm or more, and is preferably 1.50 μm or less.

[0029] The D90 of the silver powder is preferably 0.9 μm or more, more preferably 1.3 μm or more, preferably 8.0 μm or less, more preferably 5.0 μm or less, even more preferably 2.5 μm or less, and even more preferably 1.9 μm or less. If the D90 of the silver powder exceeds 8.0 μm, it may be difficult to obtain a wiring pattern with a narrow width. If the D90 of the silver powder is less than 0.9 μm, the viscosity of the paste may become too high when it is made into a paste, making it difficult to handle.

[0030] The TAP (g / mL) of the silver powder according to the present invention is the tap density. The tap density of the silver powder can be measured, for example, using a tap density measuring device (SS-DA-2 bulk specific gravity measuring device manufactured by Shibayama Scientific Co., Ltd.). The TAP density is preferably 3.0 g / mL or more, more preferably 4.5 g / mL or more, even more preferably 4.8 g / mL or more, and preferably 6.5 g / mL or less. If the TAP density is less than 3.0 g / mL, the silver powder is in an agglomerated state, and when it is made into a paste, fine line printing may not be possible. If the TAP density is 4.8 g / mL or more, the wiring pattern becomes dense, which is preferable.

[0031] In addition, the ignition loss (Ig-loss) (%) of the silver powder according to the present invention is a value calculated by the following formula (2) based on the mass (w) of the silver powder after heating. The Ig-loss of the silver powder according to the present invention indicates the amount of dispersant, and is preferably 0.40% or more, more preferably 0.70% or more, even more preferably 0.80% or more, preferably 1.50% or less, and more preferably 1.10% or less. If the Ig-loss is less than 0.40%, the effect of dispersing the silver powder may not be sufficiently obtained. Also, if the Ig-loss is more than 1.50%, impurities may remain in the electrode after the silver powder is fired, causing deterioration of the electrical resistance. Ignition loss (%)=(3-w) / 3×100...Equation (2)

[0032] The moisture content of the silver powder (%) indicates the moisture content of the silver powder obtained after the classification process, and is a value obtained by dividing the weight of the sample after drying by the weight of the sample before drying, and multiplying the obtained value by 100. The silver powder obtained after the classification process is dry, and the moisture content of the silver powder is preferably 0.1 mass% or less, preferably 0.05 mass% or less, and more preferably 0.01 mass% or less. If the silver powder is not dried and the moisture content exceeds 0.1 mass%, the silver powder may aggregate during storage or in the paste, causing an increase in viscosity, or the silver powder may aggregate to produce coarse particles that may cause clogging of the printing plate and lead to breakage.

[0033] Next, a manufacturing method for producing the above-mentioned silver powder of the present invention will be described with reference to the flow of a plant 100 for implementing the method for producing silver powder according to this embodiment shown in FIG.

[0034] The method for producing silver powder according to an embodiment of the present invention includes a crushing step of crushing agglomerated silver powder with an airflow crusher, and a classification step of classifying the silver powder after the crushing step with an air classifier. Here, the agglomerated silver powder has a moisture content of 5.0 wt% or more and 30.0 wt% or less. In the crushing step, compressed air at a temperature of 80°C or more and 180°C or less is supplied to the airflow crusher as supply air, and the silver powder concentration in the airflow is adjusted to 0.10 kg / m or less. 3 More than 0.50kg / m 3 In the classification process, the exhaust gas from the airflow mill and the silver powder after the crushing process are fed to a wind classifier. The exhaust gas is kept at a temperature of 30°C or higher and a volumetric absolute humidity of 20 g / m 3 That's all.

[0035] In the plant 100, silver powder containing moisture and agglomerated in a wet state, in which the surfaces of the silver particles constituting the silver powder are uneven (hereinafter referred to as agglomerated silver powder), is provided as a raw material. This agglomerated silver powder can be produced, for example, by a wet reduction method. In the plant 100, this agglomerated silver powder is crushed, dried, classified, and collected to produce silver powder suitable for preparing a conductive paste. The silver powder produced in the plant 100 retains to some extent the surface shape of the silver particles in the agglomerated silver powder. According to the present invention, the silver powder produced in the plant 100 can be made smoother than the surfaces of the silver particles in the agglomerated silver powder, while still having the desired unevenness on the surfaces of the silver particles.

[0036] The raw material agglomerated silver powder can be produced, for example, by the following wet reduction method. The wet reduction method can be a method in which an alkali or a complexing agent is added to a silver salt-containing aqueous solution to produce a silver oxide-containing slurry or a silver complex salt-containing aqueous solution, and then a reducing agent such as formalin is added to reduce and precipitate the silver powder. It can also be produced by a method including a process of adding sodium hydroxide to the silver oxide-containing slurry or the silver complex salt-containing aqueous solution to adjust the pH. Hereinafter, these methods will be simply referred to as wet reduction methods. Also, silver particles may be simply referred to as particles. Note that silver powder refers to silver powder, which is an aggregate of silver particles.

[0037] In the wet reduction method, it is preferable to prevent the silver particles from bonding to obtain a monodispersed silver powder. In order to obtain a monodispersed silver powder, the wet reduction method may include a process of adding a dispersant to the silver slurry precipitated by reduction, or a process of adding a dispersant to an aqueous reaction system containing at least one of a silver salt and silver oxide before the silver particles are precipitated by reduction. As the dispersant, one or more of fatty acids, fatty acid salts, surfactants, organic acids such as amino acids, organic metals, chelating agents, and protective colloids may be selected and used. In order to obtain a dried silver powder from the silver slurry, it is necessary to go through a solid-liquid separation process such as a filter press and a drying process. Even if the silver particles are monodispersed in the silver slurry precipitated by reduction using the wet reduction method, the silver powder immediately after the silver slurry is subjected to a solid-liquid separation process such as a filter press, or the silver powder dried without crushing the cake after the solid-liquid separation process, is agglomerated, and is referred to as agglomerated silver powder in this specification.

[0038] The plant 100 includes a feeder 1, an air flow type pulverizer 2 (hereinafter referred to as the pulverizer 2), an air classifier 3 (hereinafter referred to as the classifier 3), a cyclone 4 as a collector, a dust collector 5, and a blower 6. The method for producing silver powder according to this embodiment includes a crushing step in which the agglomerated silver powder is crushed by the pulverizer 2, and a classification step in which the silver powder after the crushing step is classified by the classifier 3.

[0039] In the plant 100, a pulverizer 2, a classifier 3, a cyclone 4, a dust collector 5 and a blower 6 are connected in series in this order, and an airflow that has passed through the pulverizer 2, the classifier 3, the cyclone 4 and the dust collector 5 is sucked in by the blower 6. Between each of the devices, pulverizer 2 and classifier 3, classifier 3 and cyclone 4, and cyclone 4 and dust collector 5, the silver powder is preferably transported by the airflow.

[0040] The feeder 1 is a device that feeds the agglomerated silver powder to the pulverizer 2. For example, a screw feeder can be used as the feeder 1. Agglomerated silver powder that is not completely dried after a solid-liquid separation process using a filter press or the like in the wet reduction method is fed into the feeder 1. The moisture content of the agglomerated silver powder is 5.0 wt% (mass%) or more, preferably 12.0 wt% or more, and 30.0 wt% or less, preferably 20.0 wt% or less, and more preferably 15.0 wt% or less.

[0041] The pulverizer 2 is a device that realizes a crushing process for crushing agglomerated silver powder. In the pulverizer 2, the agglomerated silver powder is crushed by supplying the agglomerated silver powder and compressed air to the internal space of the pulverizer 2. In the crushing process, the pulverizer 2 supplies compressed air at a temperature of 80°C or higher and 180°C or lower to the pulverizer 2 as supply air, and the silver powder concentration in the airflow in the pulverizer 2 is 0.10 kg / m 3 More than 0.50kg / m 3 The silver powder concentration in the airflow of the airflow mill must be 0.10 kg / m 3 By setting the amount to 0.50 kg / m or more, the silver powder particles collide with each other in the airflow a sufficient number of times, which increases the dispersion efficiency of the silver powder. 3 By setting the moisture content of the aggregated silver powder to 12.0 wt. % or more, the silver powder concentration in the airflow in the airflow mill should be set to 0.30 kg / m or less to prevent clogging. 3The silver powder concentration in the airflow in the airflow type mill is the value obtained by dividing the supply speed of the agglomerated silver powder from the feeder 1 by the total amount of supplied air (the sum of the supply air A and the supply air B), and is controlled by adjusting the supply speed of the agglomerated silver powder and the total amount of supplied air. Although it depends on the capacity of the mill 2, the supply speed of the agglomerated silver powder from the feeder 1 can be, for example, 0.50 kg / min to 2.00 kg / min, and the total amount of supplied air can be, for example, 1 to 20 m 3 / min.

[0042] In the pulverizer 2, compressed air for disintegrating the agglomerated silver powder is supplied so as to be sprayed into the internal space through a spray nozzle communicating with the internal space of the pulverizer 2. The spray nozzle is provided, for example, in the lower part of the pulverizer 2 and arranged so as to spray into the lower region of the internal space. The spray nozzle may be provided at a plurality of positions in the pulverizer 2. In FIG. 1, the spray nozzles are provided at the upper and lower parts of the pulverizer 2.

[0043] In the disintegration step, the compressed air supplied to the injection nozzle of the pulverizer 2 (hereinafter, the compressed air supplied to the injection nozzle is referred to as supply air or disintegration air) is heated in the supply path of the supply air by a heater 22 so that the temperature of the compressed air entering the pulverizer 2 is 80°C or higher and 180°C or lower. The temperature of the compressed air entering the pulverizer 2 is preferably 150°C or lower. The pressure of the compressed air supplied to the injection nozzle of the pulverizer 2 can be, for example, 0.1 MPa or higher, preferably 0.4 MPa or higher, and 0.8 MPa or lower, preferably 0.6 MPa or lower, as the supply pressure value of the air pump 24 (at room temperature before heating).

[0044] In the pulverizer 2, the agglomerated silver powder is supplied to the internal space of the pulverizer 2 from a route separate from the compressed air. For supply, a mechanism capable of sending the agglomerated silver powder to the inside of the pulverizer 2 may be provided, and a jet nozzle may be provided at the pulverizer supply port 21 to be used as a force for supplying the disintegrating air and the agglomerated silver powder into the pulverizer. As another configuration, a supply mechanism such as a venturi or ejector provided at the pulverizer supply port 21 may be used. The supply air used when supplying the agglomerated silver powder to the pulverizer 2 may be heated to 80° C. or higher and 180° C. or lower by a heater 22 in the supply path of the supply air. The pressure of the supply air may be set to 0.1 MPa or higher and 0.8 MPa or lower as the supply pressure value of the air pump 24 (at room temperature before heating).

[0045] In the internal space of the pulverizer 2, the disintegration of the agglomerated silver powder proceeds due to the shear force of the disintegrating air ejected from the injection nozzle, collisions between agglomerated particles in the agglomerated silver powder accelerated by the disintegrating air, and collisions between the agglomerated particles accelerated by the disintegrating air and the wall surface inside the tank of the pulverizer 2. In this embodiment, the agglomerated particles in the agglomerated silver powder refer to secondary particles formed by agglomeration of monodispersed silver particles as explained in the wet reduction method described above.

[0046] In the crushing process, the surface irregularities of the silver particles are reduced at the same time as the silver particles are crushed by the collision described above, and fine particles are generated and the surfaces of the silver particles are smoothed. In this embodiment, the atmosphere in the pulverizer becomes a moist gas as the moisture in the aggregated silver powder evaporates with the high-temperature crushing air. The progress of crushing and the smoothing of the surface in the crushing process are carried out in the moist gas. In general, particles are more likely to aggregate when there is moisture in the gas. Therefore, the progress of crushing and the smoothing of the surface are carried out in a state where aggregation is more likely to occur than when dry silver powder is crushed with dry air. As a result, it is expected that collisions between aggregated particles in the pulverizer and re-adhesion of the scraped fine particles are more likely to occur. The silver particles obtained in this embodiment have larger irregularities on the silver particle surface than silver particles that have been subjected to the crushing process in a conventional atmosphere with a low moisture content.

[0047] Examples of the crusher 2 include a Current Jet Mill (manufactured by Nisshin Engineering Inc.), an Sk Jet-O-Mill (manufactured by Seishin Enterprise Co., Ltd.), a Super Jet Mill (manufactured by Nisshin Engineering Inc.), and a Spiral Jet Mill (manufactured by Hosokawa Micron Corporation), which continuously supply compressed air into the internal space and achieve crushing in the swirling flow that is thereby generated, as well as a Counter Jet Mill (manufactured by Hosokawa Micron Corporation) and a Cross Jet Mill (manufactured by Kurimoto Iron Works Co., Ltd.), which have a built-in classification rotor and achieve crushing by supplying compressed air into the fluidized bed formed in the internal space.

[0048] In the following, an example will be described in which the pulverizer 2 is an airflow type pulverizer in which disintegrating air is continuously supplied to the internal space, thereby generating a swirling flow in which disintegration is achieved, and in which agglomerated silver powder is supplied to the internal space of the pulverizer 2 from the pulverizer supply port 21 via a route separate from the supply air.

[0049] The exhaust gas from the pulverizer 2 and the silver powder after the crushing process are supplied to a classifier 3, where the classification process is carried out. The exhaust gas has a temperature of 30°C or higher and a volume absolute humidity of 20 g / m 3 Adjust the volumetric absolute humidity to 30g / m 3 It is also preferable to set the volumetric absolute humidity to 100% or more. Hereinafter, the volumetric absolute humidity will be simply referred to as absolute humidity. The temperature and absolute humidity of the exhaust gas from the pulverizer 2 can be obtained by measuring the temperature and humidity (relative humidity) in the connecting pipe 23 using, for example, a thermo-hygrometer 26.

[0050] The classifier 3 is an air classifier that realizes a classification process in which coarse particles (coarse powder) or fine powder is separated and removed (so-called classification) from the silver powder after the crushing process. The following describes an example in which the classifier 3 separates and removes coarse powder.

[0051] The classifier 3 may be, for example, one having a classification mechanism that classifies the air by balancing the centrifugal force caused by the swirling flow generated by supplying or suctioning the airflow and the force of the airflow flowing in a direction against the centrifugal force. Another example may be one having a classification mechanism that classifies the air by balancing the centrifugal force generated by a rotating rotor and the force of the airflow flowing in a direction against the centrifugal force. Specific examples include the Aero Fine Classifier (manufactured by Nisshin Engineering Inc.) that realizes classification using the centrifugal force caused by the swirling flow generated by supplying a high-speed airflow, and the Turbo Classifier (manufactured by Nisshin Engineering Inc.) that uses the centrifugal force caused by the swirling flow generated by a rotating rotor.

[0052] The following description will be given taking as an example a case where the classifier 3 is an air classifier having a classification mechanism that performs classification by balancing the centrifugal force caused by the swirling flow generated by the rotating rotor with the force of the airflow when the exhaust air of the pulverizer 2 or an airflow separate from the exhaust air (for example, outside air) is sucked in against the centrifugal force and heads toward the airflow classifier exhaust port 39. Here, the exhaust air of the pulverizer 2 is supplied to the classifier supply port 31 together with the silver powder that has been through the crushing process, and the airflow separate from the exhaust air can be generated by gas sucked into the classifier 3 via a route separate from the exhaust air of the pulverizer 2. The outside air sucked into the classifier 3 has an absolute humidity of 30 g / m 3 less than 15g / m 3 Absolute humidity may be less than 12 g / m 3 The temperature of the outside air sucked into the classifier 3 may be less than 50°C, less than 40°C, or less than 30°C, and the relative humidity may be 80% or less, 70% or less, less than 50%, or 40% or less.

[0053] The classifier 3 is supplied with silver powder after the crushing process (silver powder crushed by the crusher 2) and the exhaust air from the crusher 2. The classifier supply port 31 of the classifier 3 is preferably connected to the crusher exhaust port 29 of the crusher 2 via the connecting pipe 23. This allows all of the silver powder after the crushing process and all of the exhaust air from the crusher 2 to be supplied to the classifier 3 via the connecting pipe 23 and the classifier supply port 31. In other words, in the connecting pipe 23, pneumatic transport of the silver powder after the crushing process is achieved by the airflow of the exhaust air from the crusher 2.

[0054] In the plant 100, the exhaust gas from the pulverizer 2 flowing through the connecting pipe 23 has a temperature of 30° C. or more and an absolute humidity of 20 g / m 3 This makes it possible to simplify the process by integrating the drying process and the crushing process, and to realize a method for producing silver powder that can prepare a conductive paste that can provide a wiring pattern with a desired line width and height.

[0055] In addition, the exhaust gas from the pulverizer 2 flowing through the connecting pipe 23 has a temperature of 30° C. or more and an absolute humidity of 20 g / m 3 By controlling as described above, the gas at the classifier supply port 31 is moist, but as the classification process progresses, outside air is mixed in and the moisture content of the gas transporting the silver powder gradually decreases. Classification is performed along with this change in the moisture content of the transporting gas. Generally, particles tend to agglomerate more easily when there is moisture in the gas, so silver particles that tend to agglomerate are more likely to be removed as coarse powder in the early stages of classification. In other words, compared to classifying dry silver powder with dry air, it is possible to select silver particles based on their ease of agglomeration.

[0056] Of the silver powder after the crushing process supplied to the classifier 3, coarse powder is discharged from a coarse powder discharge port 35. Of the silver powder after the crushing process supplied to the classifier 3, the rest other than the coarse powder is discharged from a classifier exhaust port 39 of the classifier 3 together with the exhaust air from the classifier 3, and is supplied (sucked) into the cyclone 4 from a cyclone inlet 41 by pneumatic transport.

[0057] In the cyclone 4, a collection process is realized in which the silver powder supplied from the classifier 3 is collected by a collector. At this time, fine particles are removed from the silver powder. In the cyclone 4, the silver powder is collected, for example, in a collection pot 49 at the bottom. The silver powder collected in the cyclone 4 is the silver powder according to this embodiment.

[0058] The agglomerated silver powder is crushed and dried due to solid-gas contact during crushing in the pulverizer 2, classification in the classifier 3, and collection in the cyclone 4. As a result, the silver powder recovered in the cyclone 4 is drier than the agglomerated silver powder. In the plant 100, compressed air heated to 80° C. or more and 180° C. or less is supplied to the internal space of the pulverizer 2, whereby the silver powder is dried in the pulverizer 2, the classifier 3, and the cyclone 4, and the silver powder recovered in the cyclone 4 is sufficiently dried so that further finish drying is not required (for example, the moisture content of the silver powder is 0.1% by mass or less).

[0059] The exhaust gas from the cyclone 4 is sucked into a blower 6 via a dust collector 5 and discharged outside the system of the plant 100. The dust collector 5 filters the exhaust gas from the cyclone 4, and fine silver powder that has not been collected by the cyclone 4 is recovered.

[0060] The above describes the method for producing silver powder according to the present embodiment. The above-mentioned devices and the like are merely examples, and various modifications and combinations with other devices are possible within the scope of the effects of the invention.

[0061] The conductive paste of the present invention contains at least the silver powder of the present invention, and may optionally contain an organic binder and a solvent.

[0062] The organic binder is not particularly limited, and examples thereof include silicone resin, epoxy resin, acrylic resin, polyester resin, polyimide resin, polyurethane resin, phenoxy resin, cellulose-based resin (ethyl cellulose, hydroxypropyl cellulose, etc.), etc. These may be used alone or in any combination of two or more kinds in any ratio.

[0063] The solvent is not particularly limited, and examples thereof include alcohol-based solvents such as terpineol, butyl carbitol, texanol, ethylene glycol, and diethylene glycol; ester-based solvents such as butyl carbitol acetate and ethyl acetate; hydrocarbon-based solvents such as toluene, xylene, and cyclohexane; and glycerin. These may be used alone or in any combination of two or more kinds in any ratio.

[0064] The content of the silver powder in the conductive paste can be 80% by mass or more, preferably 85% by mass or more, and can be 95% by mass or less, preferably 90% by mass or less. Of the silver powder contained in the conductive paste, it is preferable that the silver powder of the present invention accounts for more than half.

[0065] The content of the organic binder in the conductive paste can be 0.1 mass % or more, and preferably 0.2 mass % or more, and can be 0.4 mass % or less, and preferably 0.3 mass % or less.

[0066] The content of the solvent in the conductive paste can be 6% by mass or more, and preferably 7% by mass or more, and can be 20% by mass or less, and preferably 15% by mass or less.

[0067] The conductive paste may contain, as optional components, glass frit, dispersant, surfactant, viscosity modifier, slip agent, etc. The conductive paste used in the manufacture of solar cell electrodes preferably contains glass frit, such as Pb-Te-Bi-based and Pb-Si-B-based glass frits.

[0068] The method for producing the conductive paste is not particularly limited, and includes a method of mixing the spherical silver powder of the present invention, an organic binder, a solvent, and optionally optional components. The mixing method is not particularly limited, and for example, a planetary stirrer, ultrasonic dispersion, a disper, a three-roll mill, a ball mill, a bead mill, a two-axis kneader, etc. can be used.

[0069] The conductive paste of the present invention can be applied to a substrate to form a coating film by, for example, screen printing, offset printing, printing such as photolithography, dipping, etc. The coating film may be formed into a predetermined pattern shape by photolithography using a resist.

[0070] The coating film can be fired to form a conductive film. The firing may be performed in an air atmosphere or in a non-oxidizing atmosphere such as nitrogen. The firing temperature of the coating film can be 600°C or higher, preferably 690°C or higher, and 800°C or lower, preferably 740°C or lower. The firing time can be 20 seconds or higher, preferably 40 seconds or higher, and 1 hour or lower, preferably 2 minutes or lower. EXAMPLES

[0071] Below, a method for producing silver powder according to this embodiment and examples of silver powder produced by this method will be described.

[0072] Example 1 The powder according to Example 1 was produced as follows.

[0073] In a 5L beaker, 3.3L of silver nitrate aqueous solution containing 50.8g of silver was stirred by rotating the stirring blade at 332 rpm, and 153.0g of 28.0 mass% ammonia aqueous solution was added to generate a silver ammine complex aqueous solution. The obtained silver ammine complex solution was adjusted to 26.5°C, and 339.1g of 25.9 mass% formalin was added at once as a reducing agent to obtain a silver-containing slurry. Note that, in order to make the silver in the slurry have a predetermined particle size, 19.9g of 20 mass% sodium hydroxide aqueous solution was added before the addition of the reducing agent, and the particle size was adjusted by adjusting the amount added.

[0074] Then, 13.238 g of an aqueous emulsion solution containing 0.382 mass% stearic acid relative to the silver in the slurry was added, and stirring was continued until 3 minutes after the reduction was completed, to obtain a slurry of silver particles. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate reached 0.5 mS / m, and then suction filtered to obtain silver powder (agglomerated silver powder). When the cross-section of the silver fine particles (raw material particles) of the obtained silver powder was observed with a scanning electron microscope, it was found to have closed voids inside. The moisture content (loss on drying) of the agglomerated silver powder was 12.8 wt%. The above steps were repeated to prepare 4,000 g of agglomerated silver powder.

[0075] The agglomerated silver powder was further fed into the pulverizer 2 at a feed rate of 1 kg / min in the above-mentioned plant 100 for crushing and drying, and the silver powder was transported together with moist hot air to the classifier 3 for classification, after which the silver powder collected in the cyclone 4 was further sieved through a sieve with an opening of 40 μm, and the silver powder that passed through the sieve was the silver powder according to Example 1. When the particle cross-section of the silver powder according to Example 1 was observed with a scanning electron microscope, it was found to have closed voids inside. Furthermore, the true density was measured with a true density measuring device (AccuPycII1340 manufactured by Micromeritics), and was found to be 9.7 g / cm. 3 It was.

[0076] Table 1 shows the operating conditions of the plant 100 when the silver powder of Example 1 was produced. Table 1 also shows the operating conditions of Examples 2 to 4 and Comparative Examples 1 to 4 described below. Here, supplied air B is air supplied to the injection nozzle at the bottom of the pulverizer, and supplied air A is air supplied to the injection nozzle at the top of the pulverizer.

[0077] [Table 1]

[0078] In Table 1, the "environment" section lists the temperature (℃), relative humidity (%), and absolute humidity (g / m 3 ) These are the temperature and relative humidity of the outside air taken in by the classifier 3, which will be described later.

[0079] In addition, in Table 1, the item "moisture content of agglomerated silver powder" shows the moisture content (wt%) of the above-mentioned agglomerated silver powder.

[0080] In Table 1, the "feeder" column shows the feed rate (kg / min) when the aggregated silver powder was fed from the feeder 1 to the crusher 2. In addition, the "crusher" column shows the total amount of air (m 3 / min), the amount of air supplied to each nozzle (m 3 / min) and its pressure (MPa), temperature (℃), silver powder concentration (kg / m 3 ) The total air volume here is the total volume of supply air supplied to the upper and lower injection nozzles of the pulverizer. The total air volume and the volume of supply air supplied to each injection nozzle are values ​​before heating, values ​​measured by a flowmeter 27 installed in the supply path, and values ​​at 1 atmosphere and 0°C (normal conversion values) are shown. In Table 1, the value in the pulverizer temperature column is the outlet temperature of the heater 22, and indicates the temperature of the supply air after heating and before it is supplied to the pulverizer.

[0081] In the following description, the case where the temperature of the supply air A and the supply air B is set to 80° C. or higher will be treated as the case where the drying process is performed in the plant 100. In the first embodiment, the temperature of the supply air A and the supply air B is set to 160° C. as shown in Table 1, so the drying process is performed in the plant 100. Then, the crushing and classification are performed in a humid air flow.

[0082] In addition, in Table 1, the item "crusher exhaust" includes the temperature (°C) of the exhaust from the crusher 2, the relative humidity (%) of the exhaust, and the absolute humidity (g / m 3 ) is shown.

[0083] In addition, in the "Classifier" section of Table 1, the condition of the classifier 3 is the intake air volume (m 2 ) of the outside air that is drawn into the classifier 3 separately from the exhaust air of the pulverizer 2. 3 / min). Note that the suction air volume indicates the normal flow rate.

[0084] In addition, in Table 1, the exhaust air volume in the "Blower" section is the exhaust air volume (m 3 / min) setting value. Note that the exhaust air volume indicates the normal flow rate.

[0085] Here, the intake air volume of the outside air sucked into the classifier 3 in Table 1 is not an actual measured value, but a calculated value obtained by subtracting the total amount of air supplied to the pulverizer 2 from the exhaust air volume of the blower 6.

[0086] Example 2 The silver powder according to Example 2 was produced as follows.

[0087] In a 5L beaker, 3.4L of silver nitrate aqueous solution containing 53.7g of silver was stirred by rotating the stirring blade at 332 rpm, and 113.2g of 28.0% by mass ammonia aqueous solution was added to generate a silver ammine complex aqueous solution. The silver ammine complex solution was adjusted to 26.5°C, and 251.4g of 25.9% by mass formalin was added at once as a reducing agent. In order to obtain a predetermined particle size, 12.9g of 20% by mass sodium hydroxide aqueous solution was added before the addition of the reducing agent, and the particle size was adjusted by adjusting the amount added.

[0088] Then, 7.970 g of an aqueous emulsion solution containing 0.230% by mass of stearic acid relative to the silver was added, and stirring was continued until 3 minutes after the reduction was completed, to obtain a slurry of silver particles. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate reached 0.5 mS / m, and then suction filtered to obtain silver powder (agglomerated silver powder). When the cross-sections of the silver particles (raw material particles) of the obtained silver powder were observed with a scanning electron microscope, they had closed voids inside. The moisture content (loss on drying) of the agglomerated silver powder was 17.0 wt%. The above steps were repeated to prepare 4,000 g of agglomerated silver powder.

[0089] The agglomerated silver powder was further fed into pulverizer 2 at a feed rate of 1 kg / min in plant 100 described above for crushing and drying, and the silver powder was transported together with moist hot air to classifier 3 for classification, after which the silver powder collected in cyclone 4 was further sieved through a sieve with 40 μm openings, and the silver powder that passed through the sieve was the silver powder according to Example 2. When the particle cross-section of the silver powder according to Example 2 was observed with a scanning electron microscope, it was found to have closed voids inside. The true density was measured with a true density measuring device, and was found to be 9.7 g / cm. 3 It was.

[0090] Table 1 shows the operating conditions of the plant 100 when the silver powder of Example 2 was produced. In Example 2, as shown in Table 1, the temperatures of the supply air A and the supply air B were set to 135° C., and therefore a drying process was performed in the plant 100. Then, crushing and classification were performed in a humid air stream.

[0091] Example 3 The silver powder according to Example 3 was produced as follows.

[0092] In a 5L beaker, 3.8L of silver nitrate aqueous solution containing 50.8g of silver was stirred by rotating the stirring blade at 332 rpm, while 153.0g of 28.0 mass% ammonia aqueous solution was added to generate a silver ammine complex aqueous solution. The obtained silver ammine complex solution was adjusted to 20.0°C, and 339.1g of 25.9 mass% formalin was added at once as a reducing agent to obtain a silver-containing slurry. Note that, in order to make the silver in the slurry have a predetermined particle size, 14.61g of 20 mass% sodium hydroxide aqueous solution was added before the addition of the reducing agent, and the particle size was adjusted by adjusting the amount of the added agent.

[0093] Then, 18.577 g of an aqueous emulsion solution containing 0.566% by mass of stearic acid relative to the silver in the slurry was added, and stirring was continued until 3 minutes after the reduction was completed, to obtain a slurry of silver particles. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate reached 0.5 mS / m, and then suction filtered to obtain silver powder (agglomerated silver powder). When the cross-sections of the silver particles (raw material particles) of the obtained silver powder were observed with a scanning electron microscope, they had closed voids inside. The moisture content (loss on drying) of the agglomerated silver powder was 12.6 wt%. The above steps were repeated to prepare 4,000 g of agglomerated silver powder.

[0094] The agglomerated silver powder was further fed into the pulverizer 2 at a feed rate of 1 kg / min in the above-mentioned plant 100 for crushing and drying, and the silver powder was transported together with moist hot air to the classifier 3 for classification, after which the silver powder collected in the cyclone 4 was further sieved through a sieve with an opening of 40 μm, and the silver powder that passed through the sieve was the silver powder according to Example 3. When the particle cross-section of the silver powder according to Example 3 was observed with a scanning electron microscope, it was found to have closed voids inside. Furthermore, the true density was measured with a true density measuring device (AccuPycII1340 manufactured by Micromeritics), and was found to be 9.7 g / cm. 3 It was.

[0095] Table 1 shows the operating conditions of the plant 100 when the silver powder of Example 3 was produced. In Example 3, as shown in Table 1, the temperatures of the supply air A and the supply air B were set to 127° C., and therefore a drying process was performed in the plant 100. Then, crushing and classification were performed in a humid air stream.

[0096] Example 4 The silver powder according to Example 4 was produced as follows.

[0097] In a 5L beaker, 3.8L of silver nitrate aqueous solution containing 50.8g of silver was stirred by rotating the stirring blade at 332 rpm, while 153.0g of 28.0 mass% ammonia aqueous solution was added to generate a silver ammine complex aqueous solution. The obtained silver ammine complex solution was adjusted to 20.0°C, and 339.1g of 25.9 mass% formalin was added at once as a reducing agent to obtain a silver-containing slurry. In addition, 9.14g of 20 mass% sodium hydroxide aqueous solution was added before the addition of the reducing agent so that the silver in the slurry had a predetermined particle size, and the particle size was adjusted by adjusting the amount added.

[0098] Then, 11,300 g of an aqueous emulsion solution containing 0.344% by mass of stearic acid relative to the silver in the slurry was added, and stirring was continued until 3 minutes after the reduction was completed, to obtain a slurry of silver particles. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate reached 0.5 mS / m, and then suction filtered to obtain silver powder (agglomerated silver powder). When the cross-section of the silver fine particles (raw material particles) of the obtained silver powder was observed with a scanning electron microscope, it was found that they had closed voids inside. The moisture content (loss on drying) of the agglomerated silver powder was 12.2 wt%. The above steps were repeated to prepare 4,000 g of agglomerated silver powder.

[0099] The agglomerated silver powder was further fed into the pulverizer 2 at a feed rate of 1 kg / min in the above-mentioned plant 100 for crushing and drying, and the silver powder was transported together with moist hot air to the classifier 3 for classification, after which the silver powder collected in the cyclone 4 was further sieved through a sieve with an opening of 40 μm, and the silver powder that passed through the sieve was the silver powder according to Example 4. When the particle cross section of the silver powder according to Example 4 was observed with a scanning electron microscope, it was found to have closed voids inside. Furthermore, the true density was measured with a true density measuring device (AccuPycII1340 manufactured by Micromeritics), and was found to be 9.7 g / cm. 3 It was.

[0100] Table 1 shows the operating conditions of the plant 100 when the silver powder of Example 4 was produced. In Example 4, as shown in Table 1, the temperatures of the supply air A and the supply air B were set to 120° C., and therefore a drying process was carried out in the plant 100. Then, crushing and classification were carried out in a humid air stream.

[0101] Comparative Example 1 The silver powder according to Comparative Example 1 was produced without drying in the plant 100, unlike the above-mentioned Examples. That is, the silver powder according to Comparative Example 1 was produced by drying the agglomerated silver powder produced in the same manner as in Example 1 in a vacuum rotary dryer, and then roughly crushing the dried silver powder (moisture content: 0.01 wt%) by stirring in a Henschel mixer, and then feeding the dried silver powder to the plant 100 in the same manner as in Example 1. The silver powder according to Comparative Example 1 was produced by crushing the silver powder under different operating conditions of the plant 100 from those in Example 1 and collecting the silver powder in the cyclone 4 under the operating conditions of the plant 100. Table 1 shows the operating conditions of the plant 100 when the silver powder according to Comparative Example 1 was produced. In Comparative Example 1, the temperatures of the supply air A and the supply air B are set to 16° C. as shown in Table 1, and therefore the drying process in the plant 100 was not carried out as described above. In addition, the absolute humidity of the air exhausted from the pulverizer supplied to the classifier is low, and crushing and classification in a humid air stream as in the Examples was not carried out.

[0102] Comparative Example 2 The silver powder according to Comparative Example 2 was produced without drying in the plant 100, unlike the above-mentioned Examples. That is, the silver powder according to Comparative Example 2 was produced by drying the aggregated silver powder produced in the same manner as in Example 2 in a vacuum rotary dryer, and then roughly crushing the dried silver powder (moisture content: 0.01 wt%) by stirring in a Henschel mixer, and then feeding the dried silver powder to the plant 100 in the same manner as in Example 2. The silver powder according to Comparative Example 2 was produced by crushing the silver powder under different operating conditions of the plant 100 from those in Example 2 and collecting the silver powder in the cyclone 4 under the operating conditions of the plant 100. Table 1 shows the operating conditions of the plant 100 when the silver powder according to Comparative Example 2 was produced. In Comparative Example 2, the temperatures of the supply air A and the supply air B are set to 16° C. as shown in Table 1, and therefore the drying process in the plant 100 was not carried out as described above. In addition, the absolute humidity of the air exhausted from the pulverizer supplied to the classifier is low, and crushing and classification in a humid air stream as in the Examples was not carried out.

[0103] Comparative Example 3 The silver powder according to Comparative Example 3 was produced without drying in the plant 100, unlike the above-mentioned Examples. That is, the silver powder according to Comparative Example 3 was produced by drying the aggregated silver powder produced in the same manner as in Example 3 in a vacuum rotary dryer, and then roughly crushing the dried silver powder (moisture content: 0.01 wt%) by stirring in a Henschel type mixer, and then feeding the dried silver powder to the plant 100 in the same manner as in Example 3. The silver powder according to Comparative Example 3 was produced by crushing the silver powder under different operating conditions of the plant 100 from those in Example 3 and collecting the silver powder in the cyclone 4. Table 1 shows the operating conditions of the plant 100 when the silver powder according to Comparative Example 3 was produced. In Comparative Example 3, the temperatures of the supply air A and the supply air B are set to 25° C. as shown in Table 1, and therefore the drying process in the plant 100 was not carried out as described above. In addition, the absolute humidity of the air exhausted from the pulverizer supplied to the classifier is low, and crushing and classification in a humid air stream as in the Examples was not carried out.

[0104] Comparative Example 4 The silver powder according to Comparative Example 4 was produced without drying in the plant 100, unlike the above-mentioned Examples. That is, the silver powder according to Comparative Example 4 was produced by drying the aggregated silver powder produced in the same manner as in Example 4 in a vacuum rotary dryer, and then roughly crushing the dried silver powder (moisture content: 0.01 wt%) by stirring in a Henschel mixer, and then feeding the dried silver powder to the plant 100 in the same manner as in Example 4. The silver powder according to Comparative Example 4 was produced by crushing the silver powder under different operating conditions of the plant 100 from those in Example 4 and collecting the silver powder in the cyclone 4. Table 1 shows the operating conditions of the plant 100 when the silver powder according to Comparative Example 3 was produced. In Comparative Example 4, the temperatures of the supply air A and the supply air B are set to 25° C. as shown in Table 1, and therefore the drying process in the plant 100 was not carried out as described above. In addition, the absolute humidity of the air exhausted from the pulverizer supplied to the classifier is low, and crushing and classification in a humid air stream as in the Examples was not carried out.

[0105] (Reference example) When the feed rate of the agglomerated silver powder to the pulverizer in Examples 1 and 2 was increased to 1.5 kg / min, clogging occurred inside the pulverizer, and operation had to be stopped.

[0106] Table 2 shows the evaluation values ​​of the silver powders according to the examples and comparative examples, and the evaluation values ​​of the aspect ratios of the wiring patterns obtained by making the silver powders according to the examples and comparative examples into pastes and firing them, as described below. Each evaluation value shown in Table 2 is explained below.

[0107] [Table 2]

[0108] In Table 2, “specific surface area (SSA)” (m 2 / g) is the specific surface area of ​​the silver powder. The BET specific surface area obtained by the BET method was used as the specific surface area of ​​the silver powder. The BET specific surface area was measured by the BET single-point method using a BET specific surface area measuring device (Macsorb HM-model 1210 manufactured by Mountec Co., Ltd.) after degassing by flowing a He-N2 mixed gas (30% nitrogen) into the measuring device at 60°C for 10 minutes.

[0109] In Table 2, "BET diameter" (μm) is the BET specific surface area (m 2 The specific surface area diameter was calculated from the specific surface area (μm / g) and the true density of the silver powder by the following formula (1). BET diameter (μm) = 6 / (specific surface area x true density)...Equation (1) In the above formula (1), the true density measured above is 9.7 g / cm 3 was used.

[0110] In Table 2, "D10" (μm), "D50" (μm), and "D90" (μm) refer to the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values ​​accumulated from the smaller particle diameter side in the particle size distribution of silver powder determined on a volume basis in laser diffraction particle size distribution measurement. Note that the cumulative 50% diameter on a volume basis refers to the median diameter. Below, the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values ​​will be referred to as D10, D50, and D90, respectively.

[0111] Specifically, the particle size distribution of the silver powder was measured using a laser diffraction / scattering particle size distribution measuring device (Microtrac MT-3300 EXII, manufactured by Microtrack Bell Co., Ltd.) that realizes laser diffraction particle size distribution measurement. The measurement of particle size distribution using the laser diffraction / scattering particle size distribution measuring device was performed as follows. First, 0.1 g of silver powder was added to 40 mL of isopropyl alcohol (IPA) and dispersed. An ultrasonic homogenizer (manufactured by Nippon Seiki Seisakusho Co., Ltd., device name: US-150T; 19.5 kHz, tip diameter 20 mm) was used for dispersion. The dispersion time was 2 minutes. Then, the dispersed sample was subjected to the above-mentioned laser diffraction / scattering particle size distribution measuring device, and the particle size distribution was obtained using the attached analysis software.

[0112] In Table 2, "(D90-D10) / D50" indicates the value obtained by subtracting D10 from D90 to obtain the difference value and dividing the difference value by D50. This value indicates the degree of spread of the particle size distribution (sharpness of the particle size distribution), and the larger the value, the broader the particle size distribution, and the smaller the value, the sharper the particle size distribution. Hereinafter, "(D90-D10) / D50" may be referred to as the sharpness value.

[0113] "Moisture content of silver powder" in Table 2 refers to the moisture content of silver powder obtained through the classification process. "Moisture content of agglomerated silver powder" (%) in Table 1 and "Moisture content of silver powder" (%) in Table 2 were calculated as follows. 10 g of agglomerated silver powder or silver powder was placed in a weighing bottle as a sample, dried at 90°C for 3 hours without a lid, and the weight loss of the sample after cooling for 40 minutes or more was divided by the weight of the sample before drying (10 g) and multiplied by 100 to obtain the moisture content.

[0114] In Table 2, the "loss on ignition" (%) is a value calculated as follows. First, 3 g of silver powder is weighed out and placed in a magnetic crucible and heated to 800°C. It is then heated at 800°C for 30 minutes to achieve a constant weight. The silver powder is then cooled and weighed to determine the mass (w) after heating. The loss on ignition (%) is a value calculated based on this mass (w) using the following formula (2). Ignition loss (%)=(3-w) / 3×100...Equation (2)

[0115] In Table 2, "TAP" (g / mL) is the tap density of the silver powder. The tap density of the silver powder was determined using a tap density measuring device (SS-DA-2 bulk specific gravity measuring device manufactured by Shibayama Scientific Co., Ltd.). The tap density was measured as follows. 30 g of a silver powder sample was weighed and placed in a 20 mL test tube, and tapped 1000 times with a drop of 20 mm. The sample volume (cm3) after tapping was then measured. 3 The tap density (g / cm 3 ) is the sample volume after tapping (cm 3 ) based on the following formula (3): Tap density (g / cm 3) = 30 (g) / sample volume after tapping (cm 3 )...Equation (3)

[0116] In Table 2, "Sa" (nm) is the arithmetic mean roughness as specified in ISO25178 in measuring the surface roughness of silver particle surfaces. The arithmetic mean roughness Sa was obtained based on a shape image obtained by a scanning probe microscope (SPM). Specifically, an SPM (Nano Cute) manufactured by SII Nano Technology Inc. was used, and the cantilever was an SI-DF40P2 manufactured by Hitachi High-Tech Fielding Co., Ltd. The measurement mode was selected as tapping mode (DFM). In detail, first, a Q curve measurement was performed to adjust the cantilever. At this time, it was confirmed that the resonance frequency was in the range of 200Hz to 500Hz and the Q value was in the range of 100 to 1000. The target vibration amplitude of the cantilever was set to 1V. Next, a shape image and an error signal image of the silver particle with a field of view of 5μm were obtained using the SPM. At this time, the amplitude attenuation rate was automatically set to the range of -0.1 to -0.2. In addition, the scanning frequency was set to the range of 0.6Hz to 1Hz. The feedback control parameters were set automatically. The number of pixels when acquiring the shape image was 256 x 256. Then, the range in which roughness was to be analyzed was specified in the shape image, and the arithmetic mean roughness Sa of the particle surface defined in ISO25178 was automatically calculated by performing third-order tilt correction and flattening to remove components derived from the curved surface of the particle. At this time, no cutoff processing was performed. The range to be analyzed was a square range with one side of 500 nm ("500 nm x 500 nm range" in this specification). When analyzing, 10 particles were randomly selected and analyzed, and their average value was calculated. The value of Sa / BET diameter was calculated from the values ​​of the arithmetic mean roughness Sa and the above-mentioned BET diameter.

[0117] A conductive paste was prepared from the obtained silver powder, and in order to determine the aspect ratio of the wiring pattern of this conductive paste, a conductive paste was prepared as follows using the silver powders of Examples 1 to 4 and Comparative Examples 1 to 4. Specifically, the conductive paste was obtained by carrying out the following treatment on the substances having the composition ratios shown in Table 3. After stirring and mixing for 30 seconds at 1400 rpm using a propeller-less self-revolving stirring and degassing device (AR310 manufactured by Thinky Corporation), the mixture was kneaded by passing it through a three-roll machine (80S manufactured by EXAKT) with a roll gap from 100 μm to 20 μm.

[0118] [Table 3]

[0119] The viscosity of the paste was measured using a viscometer 5XHBDV-IIIUC manufactured by BROOKFIELD. The measurement conditions were as follows: A cone spindle CP-52 was used. The paste temperature was 25°C. The rotation speed and measurement time were 1 rpm (shear rate 2 sec -1 ) for 5 minutes and 5 rpm (shear rate 10 sec -1 ) for one minute.

[0120] The fine line evaluation was performed by forming a wiring pattern. The wiring pattern was formed as follows. First, a solid pattern of 154 mm square was formed on the back surface of a silicon substrate for solar cells (100 Ω / □) using an aluminum paste (Rutech 28D22G-2) using a screen printer (Microtec, MT-320TV). Next, the above paste was filtered with a 500 mesh, and then electrodes (finger electrodes) with a design line width (screen opening width) of 14 to 26 μm and electrodes (busbar electrodes) with a design line width of 1 mm were printed (applied) on the front surface of the substrate in the pattern shown in FIG. 10 at a squeegee speed of 350 mm / sec. The printed paste was dried with hot air at 200°C for 10 minutes, and then fired at a peak temperature of 750°C and an in-out time of 41 seconds using a high-speed firing furnace IR furnace (NGK Insulators, Ltd., high-speed firing test 4-chamber furnace) to obtain a wiring pattern.

[0121] The line width and height of the wiring pattern were measured using a laser microscope (Keyence Corporation VKX-1000) for electrodes printed with a design line width of 24 μm out of the design line widths in Figure 10. Images were taken with a 20x objective lens (electrode length per field of view is approximately 500 μm), and 601 cross-sectional shapes were measured at intervals of 0.687 μm in one field of view (corresponding to an electrode length of approximately 400 μm), and their average cross-sectional shape was calculated. For the average cross-sectional shape, the height of the substrate was set as the baseline, and the distance from the baseline to the rise was measured as the line width, and the difference between the height of the highest point and the baseline was measured as the height, to obtain the line width and height in one field of view. Of the 10 electrodes with a design line width of 24 μm, the fourth, sixth, and eighth electrodes from the left were photographed at two locations, 5 cm and 10 cm from the top of the electrode, for a total of six locations, and the average line width and height of the six fields of view was calculated. From these results, the "aspect ratio" in Table 2 was calculated.

[0122] 2 to 9 show SEM images (20,000 times magnification) of the silver powders of Examples 1 to 4 and Comparative Examples 1 to 4, respectively. In the SEM images of the Examples, it can be seen that there are irregularities on the silver particle surface. It is presumed that the irregularities on the silver particle surface observed in the SEM images of the Examples are relatively large because the irregularities on the silver particle surface generated by the wet reaction are not sufficiently removed in the crushing step and the classification step, and remain relatively large on the surface of the silver particles constituting the silver powder of the present invention. The irregularities on the silver particle surface observed in the SEM images of Comparative Examples 1 to 4 are smaller than those of Examples 1 to 4. It is presumed that this is because the moisture content of the atmosphere surrounding the silver particles in the crushing step and the classification step is low, so that the irregularities on the silver particle surface are sufficiently removed in the crushing step and the classification step, resulting in smaller irregularities on the silver particle surface compared to the Examples. The degree of irregularities on the silver particle surface of the Examples and Comparative Examples is reflected in their Sa values.

[0123] As shown in Table 2, in the wiring patterns obtained by making a paste from the silver powder in Comparative Example 1 and Example 1, Comparative Example 2 and Example 2, Comparative Example 3 and Example 3, and Comparative Example 4 and Example 4, which were all made from the same agglomerated silver powder, and then firing them, the ratio of height to line width (aspect ratio) is relatively larger in each comparison for the Examples, indicating that they have preferable characteristics. Note that the aspect ratio value is also affected by the viscosity of the paste, so a comparison was made between pastes with similar viscosities (within approximately ±20 Pa·S at 1 rpm).

[0124] From the results of the above thin line evaluation, it can be evaluated that the silver powder according to the embodiment is a silver powder that can be used to prepare a paste that can obtain a wiring pattern of the desired line width and height.

[0125] In addition, from Table 2, in comparison with Comparative Examples 1 to 4, in Examples 1 to 4, the exhaust gas from the pulverizer 2 flowing through the connecting pipe 23 has a temperature of 30° C. or more and an absolute humidity of 20 g / m 3 By controlling as described above, the classification performance of the classifier is improved, and it can be seen that (D90-D10) / D50 in Examples 1-4 is smaller than that in Comparative Examples 1-4.

[0126] Thus, in order to prepare a paste that can obtain a wiring pattern with a desired line width and height, it is necessary to use silver powder having a moderate unevenness on the surface of the silver particles. This is also clear from the SEM images of the silver powders of the examples and comparative examples shown in Figures 2 to 9. That is, moderate unevenness can be observed on the surface of the spherical silver particles of the silver powder of the examples. In contrast, the silver powder of the comparative example has smaller unevenness on the surface of the silver particles than the silver powder of the examples, and the surface of the silver particles is generally smooth. Thus, according to the manufacturing method of this embodiment, it is possible to obtain silver particles suitable for preparing a paste that can obtain a wiring pattern with a desired line width and height by suppressing smoothing of the silver particle surface.

[0127] Looking at the operating conditions of plant 100 shown in Table 1, it can be seen that the method of manufacturing silver powder capable of preparing a paste that can obtain a wiring pattern of the desired line width and height is not a manufacturing method in which agglomerated silver powder is dried in advance and then processed other than drying in plant 100 as in the comparative example, but a manufacturing method in which agglomerated silver powder is crushed, classified and dried in plant 100 to obtain silver powder as in the example. And, when drying agglomerated silver powder in plant 100 to obtain dried silver powder, it is necessary that compressed air at a temperature of 80°C or more and 180°C or less is supplied to pulverizer 2 in the crushing step, and further, the exhaust gas from pulverizer 2 is supplied to classifier 3, and it is considered that the exhaust gas needs to have a temperature of 30°C or more and a relative humidity of 30%.

[0128] In this manner, it is possible to provide a method for producing silver powder that can prepare a conductive paste that can provide a wiring pattern with a desired line width and height, and the silver powder.

[0129] In addition, the configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, provided no contradiction arises. Furthermore, the embodiments disclosed in this specification are illustrative, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope that does not deviate from the purpose of the present invention. [Industrial Applicability]

[0130] The present invention is applicable to silver powder, a method for producing silver powder, and a conductive paste. [Explanation of symbols]

[0131] 1: Feeding machine 100:Plant 2: Crusher (airflow crusher) 21: Crusher supply port 22: Heater 23: Connecting pipe 24: Air pump 26: Temperature and humidity meter 27:Flow meter 29: Crusher exhaust port 3: Classifier (wind classifier) 31: Classifier supply port 35: Coarse powder outlet 39: Classifier exhaust port 4: Cyclone 41: Cyclone entrance 49: Collection pot 5: Dust collector 6: Blower

Claims

1. The silver particles have closed voids within them. The arithmetic average roughness Sa (nm) in surface roughness measurement of a 500 nm × 500 nm area of ​​the surface of the silver particles is Specific surface area (m2) measured by BET single point method ２ The silver powder has a value of Sa / BET diameter of 0.0070 or more, calculated by dividing the value of Sa (absolute particle diameter) by the BET diameter calculated by the following formula (1) using the value of the particle diameter (absolute particle diameter) and the true density: BET diameter = 6 / (specific surface area x true density)...Formula (1)

2. The silver powder according to claim 1 , wherein the arithmetic mean roughness Sa is 5 nm or more and 15 nm or less.

3. In the particle size distribution determined on a volume basis in a laser diffraction particle size distribution measurement, the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values ​​accumulated from the smaller particle diameter side are defined as D10, D50, and D90, respectively.

2. The silver powder according to claim 1, wherein the difference obtained by subtracting the D10 from the D90 and dividing the difference by the D50 is 1.0 or less.

4. a crushing step of crushing the agglomerated silver powder using an airflow crusher; A classification step of classifying the silver powder after the crushing step by an air classifier, The agglomerated silver powder has a moisture content of 5.0 wt % or more and 30.0 wt % or less, In the disintegration step, compressed air having a temperature of 80° C. or more and 180° C. or less is supplied to the airflow type pulverizer as supply air, and the silver powder concentration in the airflow is set to 0.10 kg / m ３ More than 0.50kg / m ３ The agglomerated silver powder is supplied so as to satisfy the following: In the classification step, the exhaust gas from the air flow mill and the silver powder obtained after the crushing step are supplied to the air classifier, The exhaust gas has a temperature of 30° C. or higher and a volume absolute humidity of 20 g / m 3 The above is the method for producing silver powder.

5. A method for producing silver powder as described in claim 4, wherein the air classifier and the air flow pulverizer are connected by a connecting pipe, and the exhaust gas from the air flow pulverizer and the silver powder after the crushing process are supplied from the air flow pulverizer to the air classifier through the connecting pipe.

6. The method further includes a collection step of collecting the silver powder after the classification step with a collector, The method for producing silver powder according to claim 4 , wherein in the collecting step, the exhaust air from the air classifier and the silver powder after the classification step are supplied to the collector.

7. A method for producing silver powder according to any one of claims 4 to 6, wherein in the classification step, classification is carried out while sucking in outside air into the air classifier.

8. A conductive paste comprising the silver powder according to any one of claims 1 to 3.

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