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] This invention relates to silver powder, a method for producing silver powder, and a conductive paste. [Background technology]

[0002] Patent Document 1 describes silver powder and a method for producing silver powder. In this method for producing silver powder, 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. The cake is then dried in an air-flow drying apparatus to obtain 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 with an average particle size of submicrons, stirred, then filtered, dehydrated, dried, and crushed. In this method for producing silver particles, the drying process can be either hot air drying or vacuum drying, and in the case of hot air drying, the filtered material should be placed under hot air at 30 to 100°C.

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

[0005] [Patent Document 1] Japanese Patent Publication No. 2008-1974 [Patent Document 2] Japanese Patent Publication No. 2010-229481 [Patent Document 3] Japanese Patent Publication No. 2016-216824 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] 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 the paste. When the paste is applied and heated, it is required that a wiring pattern with a desired line width and height can be obtained. In electronic components, thinning of wiring is desired in order to maximize the area other than the wiring portion and to reduce the size of electronic components. A wiring pattern with high wiring height is desired so that the specific resistance does not become excessively large even when the wiring is thinned.

[0007] An object of the present invention is to provide silver powder capable of preparing a conductive paste that can obtain a wiring pattern having 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 according to the present invention, the method for producing silver powder, and the conductive paste containing the silver powder are as follows.

[0009] <1> having closed voids in silver particles, the arithmetic average roughness Sa (nm) in a surface roughness measurement of a 500 nm × 500 nm range on the surface of the silver particles, specific surface area (m 2 / g) measured by the BET one-point method and the true density value, wherein a value of Sa / BET diameter obtained by dividing by the BET diameter calculated by the following formula (1) is 0.0070 or more. BET diameter = 6 / (specific surface area × true density) ··· Formula (1)

[0010] <2> The silver powder according to <1> above, wherein the arithmetic average roughness Sa is 5 nm or more and 15 nm or less.

[0011] <3> In the particle size distribution obtained using the volume-based method in laser diffraction particle size distribution measurement, the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values, accumulated from the smallest particle size, are defined as D10, D50, and D90, respectively. The above is the case where the difference obtained by subtracting D10 from D90 is divided by D50 and the result is 1.0 or less. <1> or <2> Silver powder as described.

[0012] <4> The crushing process involves crushing the aggregated silver powder using an air-jet crusher, The process includes a classification step in which the silver powder after the crushing step is classified using an air-powered classifier, The aforementioned agglomerated silver powder has a moisture content of 5.0 wt% or more and 30.0 wt% or less. In the crushing process, compressed air with a temperature of 80°C to 180°C is supplied to the airflow crusher as the supply air, and the silver powder concentration in the airflow is set to 0.10 kg / m³. 3 More than 0.50kg / m 3 The aggregated silver powder is supplied as follows: In the classification process, the exhaust from the air-flow crusher and the silver powder after the crushing process are supplied to the wind classifier. The exhaust gas has a temperature of 30°C or higher and a volumetric absolute humidity of 20 g / m³. 3 The above describes the method for manufacturing silver powder.

[0013] <5> The wind classifier and the airflow crusher are connected by a connecting pipe, and the exhaust from the airflow crusher and the silver powder after the crushing process are supplied from the airflow crusher to the wind classifier via the connecting pipe. <4> The method for producing silver powder as described above.

[0014] <6> The process further includes a collection step in which the silver powder after the classification step is collected by a collection machine, In the collection process, the exhaust from the wind classifier and the silver powder after the classification process are supplied to the collection machine. <4> or <5> The method for producing silver powder as described above.

[0015] <7> In the classification process, the above classification is performed while drawing outside air into the wind classifier. <4> ~ <6> A method for producing silver powder as described in any of the following.

[0016] <8> the above <1> ~ <3> A conductive paste containing silver powder as described in any of the following. [Effects of the Invention]

[0017] A method for producing silver powder and a silver powder can be provided that can prepare a conductive paste capable of obtaining a wiring pattern with a relatively large aspect ratio (ratio to line width). [Brief explanation of the drawing]

[0018] [Figure 1] This is a flow diagram of the plant that implements the silver powder manufacturing method according to this embodiment. [Figure 2] This is an SEM image of the silver powder from Example 1. [Figure 3] This is an SEM image of the silver powder from Example 2. [Figure 4] This is an SEM image of the silver powder from Example 3. [Figure 5] This is an SEM image of the silver powder from Example 4. [Figure 6] This is an SEM image of the silver powder in Comparative Example 1. [Figure 7] This is an SEM image of the silver powder in Comparative Example 2. [Figure 8] This is an SEM image of the silver powder in Comparative Example 3. [Figure 9] This is an SEM image of the silver powder in Comparative Example 4. [Figure 10] This figure shows the shape of the wiring pattern used for evaluating thin wires. [Modes for carrying out the invention]

[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 the 500 nm × 500 nm range is measured by the specific surface area (m²) measured by the BET 1-point method. 2The silver powder is one in which the value of Sa / BET diameter, calculated by dividing the value of Sa / g by the BET diameter calculated using the following formula (1) with respect to the true density, is 0.0070 or greater. 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 required to obtain the wiring pattern. Because the silver particles have closed voids, the true density of the silver powder according to the present invention is equal to the density of silver (10.49 g / cm³). 3 It is smaller than ) and contains 9.0-10.0 g / cm³. 3 It is within the range of, for example, 9.7 g / cm³. 3 In this specification, true density refers to the value measured by a true density measuring device, which can be measured using, for example, the AccuPycII1340 manufactured by Micromeritix. The density is measured excluding open voids on the outside of the particle surface, and the density is measured including closed voids within the particle.

[0022] Furthermore, the value of Sa / BET diameter, obtained by dividing the arithmetic mean roughness Sa (nm) in the surface roughness measurement of the silver particles constituting the silver powder in a 500nm × 500nm range by the BET diameter, is preferably 0.0070 or more, preferably 0.009 or more, and preferably 0.015 or less. By keeping it within this range, regardless of the size of the BET diameter, when the silver powder is pasteified using the method of the examples described later to obtain a conductive paste having a viscosity of a certain degree (within approximately ±20 Pa·s at a rotation speed of 1 rpm), the aspect ratio of the wiring pattern obtained by firing this conductive paste using the method of the examples described later can be made higher compared to the conventional technology. When the BET diameter is small (specific surface area is large), the number of collisions between particles is large in the crushing process, so surface smoothing is more likely to progress, and the arithmetic mean roughness Sa tends to be small. In the manufacturing method of the present invention, since the smoothing and re-adhesion process is different from the conventional method, it is possible to obtain silver powder having irregularities within the above range regardless of the size of the BET diameter. Furthermore, for example, in the method for producing silver powder of the present invention described later, the raw material silver powder subjected to the drying and crushing step is silver powder (aggregated silver powder) that contains moisture and aggregates in a wet state. When producing silver powder with less aggregation using this aggregated silver powder, for example, compressed air at a temperature of 80°C or higher and 180°C or lower is supplied as supply air to an airflow crusher to perform the crushing step (i.e., drying and crushing are performed simultaneously), and thereafter classification is performed in a humid airflow to obtain silver powder, whereby the value of Sa / BET diameter of the obtained silver powder can be 0.0070 or more. On the other hand, for silver powder obtained by drying the aggregated silver powder to a dry state and then performing crushing and classification, it is difficult to make the value of Sa / BET diameter 0.0070 or more. Therefore, normally, when the same aggregated silver powder is used as a raw material, the silver powder obtainable through the method for producing silver powder of the present invention described later can have a Sa / BET diameter value of 0.0070 or more, which makes it possible to increase the ratio of the height to the line width (aspect ratio) of a wiring pattern obtained by forming into a paste and firing the same.

[0023] The measurement of surface roughness in a 500 nm×500 nm range on the surface of silver particles constituting silver powder can be performed using a scanning probe microscope (for example, Nano Cute manufactured by SII NanoTechnology), and the arithmetic average roughness Sa (nm) is the arithmetic average roughness specified in ISO 25178. The arithmetic average roughness Sa (nm) is preferably 5 nm or more, more preferably 7 nm or more, and preferably 15 nm or less. The arithmetic average roughness Sa (nm) of silver powder subjected to crushing and classification in a humid atmosphere according to the present invention is larger than that obtained when crushing and classification are performed in a conventional dry atmosphere.

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

[0025] The specific surface area of silver powder (m 2The value per g is the BET specific surface area determined by the BET method. The BET specific surface area can be measured, for example, 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) through the measuring device at 60°C for 10 minutes, and then measuring by the BET single-point method. The specific surface area measured by the BET single-point method for silver powder crushed and classified in a humid atmosphere in this invention is larger than that obtained when crushing and classification is performed in a dry atmosphere as in the conventional method, at 0.3 m². 2 It is preferable that it be 0.55m or more. 2 It is more preferable that it be 0.70m or more. 2 It is even more preferable that it be 1.0m or more per gram. 2 It is preferable that the amount is less than or equal to / g. Furthermore, the BET specific surface area is 1.00 m². 2 Preferably, it is less than or equal to / g.

[0026] In the present invention, the silver powder is preferably such that the cumulative 10% diameter (μm), cumulative 50% diameter (μm), and cumulative 90% diameter (μm) values, accumulated from the smallest particle size in the volume-based particle size distribution measurement by laser diffraction, are D10, D50, and D90, respectively, and the difference between D90 and D10 divided by D50 is 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 with the desired line width and height when the silver powder is pasteurized and fired. Furthermore, it is preferable that the difference obtained by subtracting D10 from D90 and dividing it by D50 is 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, 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 become difficult to obtain a narrow wiring pattern. 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. Furthermore, the D50 of the silver powder is preferably 0.40 μm or larger, more preferably 0.80 μm or larger, preferably 4.00 μm or smaller, more preferably 2.50 μm or smaller, and even more preferably 1.60 μm or smaller.

[0028] The D10 of the silver powder is preferably 0.1 μm or more, more preferably 0.3 μm or more, 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 become difficult to obtain a narrow wiring pattern. 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. Furthermore, the D10 of the silver powder is preferably 0.10 μm or larger, more preferably 0.30 μm or larger, and preferably 1.50 μm or smaller.

[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 become difficult to obtain a narrow wiring pattern. 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] Furthermore, 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 (a bulk density measuring device SS-DA-2 manufactured by Shibayama Kagaku Co., Ltd.). The TAP density is preferably 3.0 g / mL or higher, more preferably 4.5 g / mL or higher, even more preferably 4.8 g / mL or higher, and preferably 6.5 g / mL or lower. If the TAP density is less than 3.0 g / mL, the silver powder is in an aggregated state, and fine line printing may not be possible when it is pasteurized. Also, a TAP density of 4.8 g / mL or higher is preferable because it results in a denser wiring pattern.

[0031] Furthermore, the ignition loss (Ig-loss) (%) of the silver powder according to the present invention is a value obtained 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 greater than 1.50%, impurities may remain in the electrode after the silver powder firing, worsening the electrical resistance. Ignition loss (%)=(3-w) / 3×100...Equation (2)

[0032] Furthermore, the silver powder moisture content (%) indicates the moisture content of the silver powder obtained after the classification process, and is calculated by dividing the weight of the dried sample by the weight of the sample before drying and multiplying the result by 100. The silver powder obtained after the classification process is dry, and its moisture content is preferably 0.1% by mass or less, preferably 0.05% by mass or less, and even more preferably 0.01% by mass or less. If the silver powder is not dry and its moisture content exceeds 0.1% by mass, the silver powder may aggregate during storage or in the paste, increasing its viscosity, or the silver powder may aggregate into coarse particles, potentially clogging the printing plate and causing disconnections.

[0033] Next, the manufacturing method for producing the silver powder of the present invention described above will be explained with reference to the flow diagram of the plant 100 that realizes the silver powder manufacturing method according to this embodiment shown in Figure 1.

[0034] A method for producing silver powder according to an embodiment of the present invention includes a crushing step of crushing agglomerated silver powder with an air-flow crusher, and a classification step of classifying the silver powder after the crushing step with an air-powered classifier. Here, the agglomerated silver powder has a moisture content of 5.0 wt% to 30.0 wt%. In the crushing step, compressed air with a temperature of 80°C to 180°C is supplied to the air-flow crusher as supply air, and the silver powder concentration in the airflow is 0.10 kg / m³. 3 More than 0.50kg / m 3 The aggregated silver powder is supplied as follows: In the classification process, the exhaust from the air-flow pulverizer and the silver powder after the crushing process are supplied to the air-powered classifier. The exhaust has a temperature of 30°C or higher and a volumetric absolute humidity of 20 g / m³. 3 That's all.

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

[0036] The aggregated silver powder used as a raw material can be manufactured by, for example, the following wet reduction method. The wet reduction method involves adding an alkali or complexing agent to a silver salt-containing aqueous solution to produce a silver oxide-containing slurry or a silver complex salt-containing aqueous solution, and then adding a reducing agent such as formalin to reduce and precipitate the silver powder. Alternatively, it can be manufactured by a method that includes adjusting the pH by adding sodium hydroxide to the silver oxide-containing slurry or silver complex salt-containing aqueous solution. Hereafter, these methods will be simply referred to as the wet reduction method. Also, silver particles may sometimes be simply referred to as particles. Note that silver powder is a powder of silver, and is an aggregate of silver particles.

[0037] In the wet reduction method, it is preferable to prevent the bonding of silver particles and obtain monodisperse silver powder. The wet reduction method may include adding a dispersant to the reduced and precipitated silver slurry, or adding a dispersant to an aqueous reaction system containing at least one of a silver salt and silver oxide before the reduction and precipitate of silver particles. As the dispersant, one or more of the following can be selected and used: fatty acids, fatty acid salts, surfactants, organic acids such as amino acids, organometallic compounds, chelating agents, and protective colloids. To obtain 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 monodisperse in the reduced and precipitated silver slurry by the wet reduction method, the silver powder immediately after performing a solid-liquid separation process such as a filter press on the silver slurry, or the silver powder dried without crushing the cake after the solid-liquid separation process, will be agglomerated, and will be referred to as agglomerated silver powder in this specification.

[0038] The plant 100 includes a feeder 1, an air-flow pulverizer 2 (hereinafter referred to as pulverizer 2), a wind-powered classifier 3 (hereinafter referred to as 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 in pulverizer 2, and a classification step in which the silver powder after the crushing step is classified in classifier 3.

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

[0040] The feeder 1 is a device that supplies agglomerated silver powder to the pulverizer 2. The feeder 1 can be, for example, a screw feeder. Agglomerated silver powder that has undergone a solid-liquid separation process such as a filter press using a wet reduction method and is not completely dried 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, 30.0 wt% or less, preferably 20.0 wt% or less, and more preferably 15.0 wt% or less.

[0041] The crusher 2 is a device that performs a crushing process to break down agglomerated silver powder. In the crusher 2, agglomerated silver powder is crushed by supplying compressed air and agglomerated silver powder into the internal space of the crusher 2. In the crushing process, compressed air with a temperature of 80°C to 180°C is supplied to the crusher 2 as supply air, and the silver powder concentration in the airflow inside the crusher 2 is 0.10 kg / m³. 3 More than 0.50kg / m 3 The following conditions should be met: The concentration of silver powder in the airflow inside the air-jet pulverizer should be 0.10 kg / m³. 3 By doing so, the silver powders collide with each other a sufficient number of times in the airflow, improving the dispersion efficiency of the silver powders, and the concentration of silver powders in the airflow inside the airflow crusher can be reduced to 0.50 kg / m³. 3 By doing the following, the airflow can impart sufficient kinetic energy to the silver powder, thereby improving its dispersibility. If the moisture content of the aggregated silver powder is high, exceeding 12.0 wt%, the concentration of silver powder in the airflow within the airflow crusher should be set to 0.30 kg / m³ to prevent blockage. 3The following is preferable. The concentration of silver powder in the airflow within the airflow crusher is the value obtained by dividing the supply rate of agglomerated silver powder from the feeder 1 by the total amount of supplied air (sum of supply air A and supply air B), and is controlled by adjusting the supply rate of agglomerated silver powder and the total amount of supplied air. Depending on the capacity of the crusher 2, the supply rate of agglomerated silver powder from the feeder 1 can be, for example, 0.50 kg / min or more and 2.00 kg / min or less, and the total amount of supplied air can be, for example, 1 to 20 m³. 3 It can be set to / min.

[0042] In the crusher 2, compressed air for crushing the agglomerated silver powder is supplied to be injected into the internal space of the crusher 2 via an injection nozzle that communicates with the internal space of the crusher 2. The injection nozzle is, for example, located at the bottom of the crusher 2 and is positioned to inject into the lower region of the internal space. Multiple injection nozzles may be provided at different locations on the crusher 2. In Figure 1, injection nozzles are provided at the top and bottom of the crusher 2.

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

[0044] In the crusher 2, the agglomerated silver powder is supplied to the internal space of the crusher 2 from a separate path from the compressed air. The supply only requires a mechanism capable of sending the agglomerated silver powder into the crusher 2. An injection nozzle can be provided at the crusher supply port 21 and used as a force to supply the crushing air and agglomerated silver powder into the crusher. Alternatively, a supply mechanism such as a venturi or ejector provided at the crusher supply port 21 can be used. The supply air used to supply the agglomerated silver powder to the crusher 2 should be heated to between 80°C and 180°C by a heater 22 in the supply air path. The supply air pressure should be between 0.1 MPa and 0.8 MPa (at room temperature before heating).

[0045] In the internal space of the crusher 2, the crushing of the agglomerated silver powder proceeds due to the shear force of the crushing air ejected from the injection nozzle, collisions between agglomerated particles in the agglomerated silver powder accelerated by the crushing air, and collisions between the agglomerated particles accelerated by the crushing air and the walls inside the crusher 2's chamber. In this embodiment, the agglomerated particles in the agglomerated silver powder are secondary particles formed by the aggregation of monodisperse silver particles as described in the wet reduction method above.

[0046] In the crushing process, the collisions described above cause crushing, simultaneously reducing the surface irregularities of the silver particles, generating fine particles that are scraped off, and smoothing the surface of the silver particles. In this embodiment, the atmosphere inside the crusher becomes a humid gas as the moisture in the agglomerated silver powder evaporates due to the high-temperature crushing air. The crushing process and surface smoothing in the crushing process take place in a humid gas environment. Generally, particles tend to agglomerate more easily when there is moisture in the gas. Therefore, the crushing process and surface smoothing proceed in a state where agglomeration is more likely to occur compared to when dry silver powder is crushed with dry air. As a result, collisions between agglomerated particles and re-adhesion of scraped-off fine particles are expected to occur more easily inside the crusher. The silver particles obtained in this embodiment have larger irregularities on their surface compared to silver particles crushed in a conventional atmosphere with low moisture content.

[0047] Examples of crushing machines 2 include the current jet mill (manufactured by Nisshin Engineering Co., Ltd.), Sk Jet-O-Mill (manufactured by Seishin Corporation), Super Jet Mill (manufactured by Nisshin Engineering Co., Ltd.), and Spiral Jet Mill (manufactured by Hosokawa Micron Corporation), which continuously supply compressed air to the internal space and achieve crushing in the resulting swirling flow, as well as the counter jet mill (manufactured by Hosokawa Micron Corporation) and cross jet mill (manufactured by Kurimoto Iron Works Co., Ltd.), which have a built-in classification rotor and achieve crushing by supplying compressed air to the fluidized bed formed in the internal space.

[0048] In the following, we will explain using an example where the pulverizer 2 is an airflow type pulverizer in which crushing air is continuously supplied to its internal space, and crushing is achieved in the swirling flow generated by this airflow, and agglomerated silver powder is supplied to the internal space of the pulverizer 2 from a separate path from the supply air from the pulverizer supply port 21.

[0049] The classification process is carried out by supplying the exhaust from the crusher 2 and the silver powder after the crushing process to the classifier 3. The exhaust from this machine has a temperature of 30°C or higher and a volumetric absolute humidity of 20 g / m³. 3 Adjust to the above. Volumetric absolute humidity should be 30 g / m³ 3 It is also preferable to use the above values. Hereafter, volumetric absolute humidity will be simply referred to as absolute humidity. The temperature and absolute humidity of the exhaust from the pulverizer 2 can be determined, for example, by measuring the temperature and humidity (relative humidity) inside the connecting pipe 23 using a thermometer and hygrometer 26.

[0050] Classifier 3 is a wind-powered classifier that performs a classification process in which coarse particles (coarse powder) or fine powder are separated and removed (so-called classification) from the silver powder after the crushing process. The following explanation will illustrate the case in which Classifier 3 separates and removes coarse powder.

[0051] Examples of classifier 3 include those having a classification mechanism that classifies based on the balance between the centrifugal force generated by a swirling flow created by the supply or suction of airflow and the force of airflow flowing in the opposite direction to the centrifugal force. Alternatively, examples include those having a classification mechanism that classifies based on the balance between the centrifugal force generated by a rotating rotor and the force of airflow flowing in the opposite direction to the centrifugal force. Specific examples include the AeroFine Classifier (manufactured by Nisshin Engineering Co., Ltd.), which achieves classification using the centrifugal force generated by a swirling flow created by the supply of high-speed airflow, and the Turbo Classifier (manufactured by Nisshin Engineering Co., Ltd.), which utilizes the centrifugal force generated by a swirling flow created by a rotating rotor.

[0052] The following explanation will illustrate the case where the classifier 3 is a wind-powered classifier having a classification mechanism that classifies materials based on the balance between the centrifugal force generated by the swirling flow produced by the rotating rotor and the force of the airflow when the exhaust from the crusher 2 or another airflow (for example, outside air) is drawn in against the centrifugal force and heads toward the exhaust port 39 of the airflow classifier. In this case, the exhaust from the crusher 2 is supplied to the classifier supply port 31 together with the silver powder that has undergone the crushing process, and the aforementioned airflow other than the exhaust can be generated by gas drawn into the classifier 3 via a different path from the exhaust from the crusher 2. The outside air drawn in by the classifier 3 has an absolute humidity of 30 g / m³. 3 It can be less than 15 g / m², with an absolute humidity of 15 g / m². 3 It can be less than 12 g / m², and the absolute humidity is 12 g / m². 3 The following is also acceptable: The outside air drawn in by classifier 3 may have a temperature of less than 50°C, less than 40°C, or less than 30°C, and a relative humidity of 80% or less, 70% or less, less than 50%, or 40% or less.

[0053] The classifier 3 is supplied with the silver powder after the crushing process (silver powder crushed in the crusher 2) and the exhaust 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 by a connecting pipe 23. This ensures that all of the silver powder after the crushing process and all of the exhaust from the crusher 2 are supplied to the classifier 3 via the connecting pipe 23 and the classifier supply port 31. In other words, the connecting pipe 23 enables pneumatic transport of the silver powder after the crushing process using the airflow from the crusher 2's exhaust.

[0054] In plant 100, the exhaust from the crusher 2, which passes through the connecting pipe 23, has a temperature of 30°C or higher and an absolute humidity of 20 g / m³. 3 The process is controlled to achieve the above. This simplifies the process by integrating the drying and crushing steps, and enables the creation of a silver powder manufacturing method that can prepare a conductive paste capable of producing wiring patterns with desired line widths and heights.

[0055] Furthermore, the exhaust from the pulverizer 2, through which the connecting pipe 23 flows, has a temperature of 30°C or higher and an absolute humidity of 20 g / m³. 3 By controlling the process as described above, the gas at the classifier supply port 31 is initially humid, but as the classification process progresses, outside air is mixed in, gradually reducing the moisture content of the gas transporting the silver powder. Classification is performed in accordance with this change in the moisture content of the transported gas. Generally, particles tend to aggregate more easily when there is moisture in the gas, so silver particles that tend to aggregate easily can be easily removed as coarse powder in the initial stages of classification. In other words, compared to classifying dry silver powder with dry air, it is possible to separate silver particles based on their tendency to aggregate.

[0056] Of the silver powder after the crushing process that is supplied to the classifier 3, the coarse powder is discharged from the coarse powder discharge port 35. The silver powder other than the coarse powder supplied to the classifier 3 is discharged from the classifier exhaust port 39 of the classifier 3 along with the exhaust of the classifier 3, and is supplied (suctioned) to the cyclone 4 from the cyclone inlet 41 by pneumatic transport.

[0057] Cyclone 4 implements a collection process in which silver powder supplied from classifier 3 is collected by a collector. During this process, fine particles are removed from the silver powder. In Cyclone 4, the silver powder is collected, for example, in a lower collection pot 49. The silver powder collected in Cyclone 4 is the silver powder according to this embodiment.

[0058] The agglomerated silver powder undergoes decomposition and drying through solid-gas contact during the processes of decomposition 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 plant 100, compressed air heated to between 80°C and 180°C is supplied to the internal space of the pulverizer 2, causing the silver powder to dry in the pulverizer 2, classifier 3, and cyclone 4. The silver powder recovered in the cyclone 4 is sufficiently dried to the point where further finishing drying is unnecessary (for example, the moisture content of the silver powder is 0.1% by mass or less).

[0059] The exhaust from cyclone 4 is drawn into blower 6 via dust collector 5 and exhausted outside the plant 100 system. In dust collector 5, the exhaust from cyclone 4 is filtered, and fine silver powder that was not captured by cyclone 4 is recovered.

[0060] The method for producing silver powder according to this embodiment has been described above. The above-described apparatus and other equipment are illustrative examples, and various modifications and combinations of other equipment are possible within the scope of achieving 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 includes, for example, silicone resin, epoxy resin, acrylic resin, polyester resin, polyimide resin, polyurethane resin, phenoxy resin, cellulosic resin (ethylcellulose, hydroxypropylcellulose, etc.). These may be used individually or in any combination of two or more in any ratio.

[0063] The solvent is not particularly limited and includes, for example, 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 individually or in any combination of two or more in any ratio.

[0064] The silver powder content 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% by mass or more, preferably 0.2% by mass or more, and can also be 0.4% by mass or less, preferably 0.3% by mass or less.

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

[0067] The conductive paste may contain optional components such as glass frit, dispersants, surfactants, viscosity modifiers, and slip agents. Conductive pastes used in the manufacture of solar cell electrodes preferably contain glass frit, such as Pb-Te-Bi and Pb-Si-B glass frits.

[0068] The method for producing the conductive paste is not particularly limited, and includes a method of mixing the spherical silver powder, organic binder, solvent, and optionally optional components of the present invention. The mixing method is not particularly limited, and for example, a self-rotating agitator, ultrasonic dispersion, disperser, three-roll mill, ball mill, bead mill, twin-screw 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 printing methods such as screen printing, offset printing, or photolithography, or by dipping. The coating film may be shaped into a predetermined pattern using photolithography with a resist.

[0070] A conductive film can be formed by firing the coating film. The firing may be carried out in an atmospheric environment 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 can be 800°C or lower, preferably 740°C or lower. The firing time can be 20 seconds or more, preferably 40 seconds or more, and can be 1 hour or less, preferably 2 minutes or less. [Examples]

[0071] The following describes a method for producing silver powder according to this embodiment and examples of silver powder produced by this method.

[0072] (Example 1) The powder used in Example 1 was manufactured as follows.

[0073] In a 5L beaker, 3.3L of a silver nitrate aqueous solution containing 50.8g of silver was stirred with a stirring blade rotating at 332 rpm, and 153.0g of a 28.0% by mass ammonia aqueous solution was added to produce a silver ammine complex aqueous solution. The resulting silver ammine complex solution was then prepared at 26.5°C, and 339.1g of 25.9% by mass formalin was added all at once as a reducing agent to obtain a silver-containing slurry. To ensure that the silver particles in the slurry reached the desired particle size, 19.9g of a 20% by mass sodium hydroxide aqueous solution was added before the addition of the reducing agent, and the amount of added sodium hydroxide was adjusted to achieve the desired particle size.

[0074] Subsequently, 13.238 g of an emulsion aqueous solution containing 0.382% by mass of stearic acid relative to the silver in the slurry was added, and stirring was continued for 3 minutes until the reduction was complete, to obtain a silver particle slurry. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate was 0.5 mS / m, and then filtered by suction to obtain silver powder (aggregated silver powder). When the cross-section of the obtained silver powder (raw material particles) 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 process was repeated to prepare 4000 g of agglomerated silver powder.

[0075] Furthermore, the aggregated silver powder was supplied to the pulverizer 2 at a supply rate of 1 kg / min in the aforementioned plant 100 for crushing and drying. The silver powder was then transported to the classifier 3 along with moist hot air for classification, and the silver powder collected by the cyclone 4 was further sieved using a sieve with a mesh size of 40 μm. The silver powder that passed through this sieve was designated as the silver powder for Example 1. When the particle cross-section of the silver powder for Example 1 was observed with a scanning electron microscope, it was found to have closed voids inside. In addition, the true density was measured using a true density analyzer (AccuPycII1340 manufactured by Micromeritix) and was found to be 9.7 g / cm³. 3 That was the case.

[0076] Table 1 shows the operating conditions of Plant 100 when producing the silver powder according to Example 1. Examples 2 to 4 and Comparative Examples 1 to 4, described later, are also shown. Here, supply air B is the air supplied to the injection nozzle at the bottom of the pulverizer, and supply air A is the air supplied to the injection nozzle at the top of the pulverizer.

[0077] [Table 1]

[0078] In Table 1, the "Environment" column includes the ambient temperature (°C), relative humidity (%), and absolute humidity (g / m³) of the surrounding atmosphere of the plant during manufacturing. 3 This indicates the temperature and relative humidity of the outside air drawn in by classifier 3, which will be described later.

[0079] Furthermore, in Table 1, the "Moisture Content of Agglomerated Silver Powder" column shows the moisture content (wt%) of the agglomerated silver powder mentioned above.

[0080] Furthermore, in Table 1, the "Supplier" column shows the supply rate (kg / min) when the agglomerated silver powder was supplied from supplier 1 to crusher 2. The "Crusher" column also shows the total amount of air supplied to crusher 2 (m³) as the crushing conditions in crusher 2. 3 ( / min), the amount of supply air supplied to each injection nozzle (m³) 3 The minimum flow rate ( / min) and its pressure (MPa), as well as the temperature (°C) and silver powder concentration (kg / m³). 3 Table 1 shows the total amount of air supplied to the upper and lower injection nozzles in the pulverizer. The total amount of air and the amount of air supplied to each injection nozzle are values ​​before heating, measured by a flow meter 27 installed in the supply path, and are shown as values ​​at 1 atmosphere and 0°C (normal conversion values). 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 explanation, the case where the temperature of supply air A and supply air B is 80°C or higher will be treated as the case where drying treatment is performed at plant 100. In Example 1, as shown in Table 1, the temperature of supply air A and supply air B is 160°C, so drying treatment is performed at plant 100. Then, crushing and classification are performed in a humid airflow.

[0082] Furthermore, in Table 1, the "Crusher Exhaust" column includes the temperature (°C) of the exhaust from crusher 2, the relative humidity (%) of the exhaust, and the absolute humidity (g / m³) of the exhaust. 3 This indicates that.

[0083] Furthermore, in Table 1, under the "Classifier" column, the conditions for Classifier 3 include the intake volume of outside air drawn into Classifier 3 separately from the exhaust of Crusher 2 (m³ 3 The value shown is the flow rate ( / min). Note that the suction airflow rate represents the normal flow rate.

[0084] Also, in Table 1, the exhaust airflow rate for the "Blower" item is the exhaust airflow rate (m³) for blower 6. 3 The setting value ( / min) is shown. Note that the exhaust airflow rate represents the normal flow rate.

[0085] Here, the intake volume of outside air drawn 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 crusher 2 from the exhaust volume of the blower 6.

[0086] (Example 2) The silver powder used in Example 2 was manufactured as follows.

[0087] In a 5L beaker, 3.4L of silver nitrate aqueous solution containing 53.7g of silver was stirred with a stirring blade rotating at 332 rpm, and 113.2g of 28.0% by mass ammonia aqueous solution was added to produce a silver ammine complex aqueous solution. The silver ammine complex solution was prepared at 26.5°C, and then 251.4g of 25.9% by mass formalin was added all at once as a reducing agent. To achieve the desired particle size, 12.9g of 20% by mass sodium hydroxide aqueous solution was added before adding the reducing agent, and the amount added was adjusted to match the particle size.

[0088] Subsequently, 7.970 g of an emulsion aqueous solution containing 0.230 mass% stearic acid relative to the silver was added, and stirring was continued for 3 minutes after the reduction was complete to obtain a silver particle slurry. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate was 0.5 mS / m, and then filtered by suction to obtain silver powder (aggregated silver powder). When the cross-section of the obtained silver powder (raw material particles) 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 17.0 wt%. The above process was repeated to prepare 4000 g of agglomerated silver powder.

[0089] Furthermore, the aggregated silver powder was supplied to the pulverizer 2 at a supply rate of 1 kg / min in the aforementioned plant 100 for crushing and drying. The silver powder was then transported to the classifier 3 along with moist hot air for classification. The silver powder collected by the cyclone 4 was then sieved using a sieve with a mesh size of 40 μm, and the silver powder that passed through the sieve was designated as the silver powder for Example 2. When the particle cross-section of the silver powder for Example 2 was observed with a scanning electron microscope, it was found to have closed voids inside. The true density was measured using a true density analyzer and was found to be 9.7 g / cm³. 3 That was the case.

[0090] Table 1 shows the operating conditions of plant 100 when producing the silver powder according to Example 2. In Example 2, as shown in Table 1, the temperatures of supply air A and supply air B were set to 135°C, so drying treatment was performed in plant 100. Then, crushing and classification were carried out in a humid airflow.

[0091] (Example 3) The silver powder used in Example 3 was manufactured as follows.

[0092] In a 5L beaker, 3.8L of a silver nitrate aqueous solution containing 50.8g of silver was stirred with a stirring blade rotating at 332 rpm, and 153.0g of a 28.0% by mass ammonia aqueous solution was added to produce a silver ammine complex aqueous solution. The resulting silver ammine complex solution was then prepared at 20.0°C, and 339.1g of 25.9% by mass formalin was added all at once as a reducing agent to obtain a silver-containing slurry. To ensure that the silver particles in the slurry reached the desired particle size, 14.61g of a 20% by mass sodium hydroxide aqueous solution was added before the addition of the reducing agent, and the amount of added sodium hydroxide was adjusted to achieve the desired particle size.

[0093] Subsequently, 18.577 g of an emulsion aqueous solution containing 0.566% by mass of stearic acid relative to the silver in the slurry was added, and stirring was continued for 3 minutes after the reduction was complete to obtain a silver particle slurry. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate was 0.5 mS / m, and then filtered by suction to obtain silver powder (aggregated silver powder). When the cross-section of the obtained silver powder (raw material particles) 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 12.6 wt%. The above process was repeated to prepare 4000 g of agglomerated silver powder.

[0094] Furthermore, the aggregated silver powder was supplied to the pulverizer 2 at a supply rate of 1 kg / min in the aforementioned plant 100 for crushing and drying. The silver powder was then transported to the classifier 3 along with moist hot air for classification. The silver powder collected by the cyclone 4 was then sieved using a sieve with a mesh size of 40 μm, and the silver powder that passed through the sieve was designated as the silver powder for Example 3. When the particle cross-section of the silver powder for Example 3 was observed with a scanning electron microscope, it was found to have closed voids inside. In addition, the true density was measured using a true density analyzer (AccuPycII1340 manufactured by Micromeritix) and was found to be 9.7 g / cm³. 3 That was the case.

[0095] Table 1 shows the operating conditions of plant 100 when producing the silver powder according to Example 3. In Example 3, as shown in Table 1, the temperatures of supply air A and supply air B were set to 127°C, so drying treatment was performed in plant 100. Then, crushing and classification were carried out in a humid airflow.

[0096] (Example 4) The silver powder used in Example 4 was manufactured as follows.

[0097] In a 5L beaker, 3.8L of a silver nitrate aqueous solution containing 50.8g of silver was stirred with a stirring blade rotating at 332 rpm, and 153.0g of a 28.0% by mass ammonia aqueous solution was added to produce a silver ammine complex aqueous solution. The resulting silver ammine complex solution was then prepared at 20.0°C, and 339.1g of 25.9% by mass formalin was added all at once as a reducing agent to obtain a silver-containing slurry. To ensure that the silver particles in the slurry reached the desired particle size, 9.14g of a 20% by mass sodium hydroxide aqueous solution was added before the addition of the reducing agent, and the amount of added sodium hydroxide was adjusted to achieve the desired particle size.

[0098] Subsequently, 11.300 g of an emulsion aqueous solution containing 0.344% by mass of stearic acid relative to the silver in the slurry was added, and stirring was continued for 3 minutes after the reduction was complete to obtain a silver particle slurry. This silver slurry was filtered, washed with pure water until the conductivity of the filtrate was 0.5 mS / m, and then filtered by suction to obtain silver powder (aggregated silver powder). When the cross-section of the obtained silver powder (raw material particles) 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 12.2 wt%. The above process was repeated to prepare 4000 g of agglomerated silver powder.

[0099] Furthermore, the aggregated silver powder was supplied to the pulverizer 2 at a supply rate of 1 kg / min in the aforementioned plant 100 for crushing and drying. The silver powder was then transported to the classifier 3 along with moist hot air for classification. The silver powder collected by the cyclone 4 was then sieved using a sieve with a mesh size of 40 μm, and the silver powder that passed through the sieve was designated as the silver powder for Example 4. When the particle cross-section of the silver powder for Example 4 was observed with a scanning electron microscope, it was found to have closed voids inside. In addition, the true density was measured using a true density analyzer (AccuPycII1340 manufactured by Micromeritix) and was found to be 9.7 g / cm³. 3 That was the case.

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

[0101] (Comparative Example 1) Unlike the above examples, the silver powder for Comparative Example 1 was manufactured without drying treatment at Plant 100. Specifically, the agglomerated silver powder for Comparative Example 1 was manufactured in the same manner as in Example 1, but unlike in Example 1, it was dried in a vacuum rotary dryer and then roughly crushed by stirring in a Henschel mixer. This already dried silver powder (moisture content: 0.01 wt%) was then fed to Plant 100 in the same manner as in Example 1, but the operating conditions of Plant 100 were changed from those in Example 1 to crush the silver powder, and the silver powder collected by Cyclone 4 was designated as the silver powder for Comparative Example 1. Table 1 shows the operating conditions of Plant 100 when the silver powder for Comparative Example 1 was manufactured. In Comparative Example 1, as shown in Table 1, the temperatures of supply air A and supply air B are 16°C, so drying treatment at Plant 100 is not performed as described above. In addition, the absolute humidity of the exhaust air from the pulverizer supplied to the classifier is low, so crushing and classification in a humid airflow as in the examples is not performed.

[0102] (Comparative Example 2) Unlike the above example, the silver powder for Comparative Example 2 was manufactured without drying treatment in Plant 100. Specifically, the agglomerated silver powder for Comparative Example 2 was manufactured in the same manner as in Example 2, but unlike in Example 2, it was dried in a vacuum rotary dryer and then roughly crushed by stirring in a Henschel mixer. This already dried silver powder (moisture content: 0.01 wt%) was then fed to Plant 100 in the same manner as in Example 2, but the operating conditions of Plant 100 were changed from those of Example 2 to crush the silver powder, and the silver powder collected by Cyclone 4 was used as the silver powder for Comparative Example 2. Table 1 shows the operating conditions of Plant 100 when the silver powder for Comparative Example 2 was manufactured. In Comparative Example 2, as shown in Table 1, the temperature of supply air A and supply air B is 16°C, so drying treatment in Plant 100 is not performed as described above. In addition, the absolute humidity of the exhaust air from the pulverizer supplied to the classifier is low, so crushing and classification in a humid airflow as in the example is not performed.

[0103] (Comparative Example 3) Unlike the above examples, the silver powder for Comparative Example 3 was manufactured without drying treatment in Plant 100. Specifically, the agglomerated silver powder for Comparative Example 3 was manufactured in the same manner as in Example 3, but unlike in Example 3, it was dried in a vacuum rotary dryer and then roughly crushed by stirring in a Henschel mixer. This already dried silver powder (moisture content: 0.01 wt%) was then fed to Plant 100 in the same manner as in Example 3, but the operating conditions of Plant 100 were changed from those in Example 3 to crush the silver powder, and the silver powder collected by Cyclone 4 was used as the silver powder for Comparative Example 3. Table 1 shows the operating conditions of Plant 100 when the silver powder for Comparative Example 3 was manufactured. In Comparative Example 3, as shown in Table 1, the temperature of supply air A and supply air B is 25°C, so drying treatment in Plant 100 is not performed as described above. In addition, the absolute humidity of the exhaust air from the pulverizer supplied to the classifier is low, so crushing and classification in a humid airflow as in the examples is not performed.

[0104] (Comparative Example 4) Unlike the above examples, the silver powder for Comparative Example 4 was produced without drying treatment in Plant 100. Specifically, the agglomerated silver powder produced in the same manner as in Example 4 was dried in a vacuum rotary dryer, then stirred in a Henschel mixer to roughly crush it, resulting in already dried silver powder (moisture content: 0.01 wt%). This was then fed into Plant 100 in the same manner as in Example 4, but the operating conditions of Plant 100 were changed from those in Example 4 to crush the powder, and the silver powder collected in Cyclone 4 was used as the silver powder for Comparative Example 4. Table 1 shows the operating conditions of Plant 100 when the silver powder for Comparative Example 3 was produced. In Comparative Example 4, as shown in Table 1, the temperatures of supply air A and supply air B are set to 25°C, so drying treatment in Plant 100 is not performed as described above. In addition, the absolute humidity of the exhaust air from the pulverizer supplied to the classifier is low, so crushing and classification in a humid airflow as in the examples is not performed.

[0105] (Reference example) When the supply rate of agglomerated silver powder to the pulverizer in Examples 1 and 2 was increased to 1.5 kg / min, blockage occurred inside the pulverizer, causing it to shut down.

[0106] Table 2 shows the evaluation values ​​of silver powder for the examples and comparative examples, and the evaluation values ​​of the aspect ratio in the wiring patterns obtained by pasteuring and firing the silver powder for the examples and comparative examples described later. Each evaluation value shown in Table 2 will be explained below.

[0107] [Table 2]

[0108] In Table 2, “specific surface area (SSA)” (m 2 The value per g is the specific surface area of ​​the silver powder. The specific surface area of ​​the silver powder was determined using the BET specific surface area method. The BET specific surface area was measured 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) through the measuring device at 60°C for 10 minutes, and then measured using the BET single-point method.

[0109] In Table 2, "BET diameter" (μm) represents the BET specific surface area (m²) of the silver powder. 2 This is the specific surface area diameter calculated by the following equation (1) based on the (g) and true density of the silver powder. BET diameter (μm) = 6 / (specific surface area x true density)...Equation (1) Furthermore, in formula (1) above, the aforementioned measurement result of true density is 9.7 g / cm³. 3 I used it.

[0110] In Table 2, "D10" (μm), "D50" (μm), and "D90" (μm) represent the cumulative 10% diameter (μm), 50% diameter (μm), and 90% diameter (μm) values ​​for the volume-based particle size distribution of silver powder obtained from laser diffraction particle size distribution measurement, starting from the smallest particle size. Note that the cumulative 50% diameter in volume-based measurement refers to the median diameter. Hereafter, the values ​​for the cumulative 10% diameter (μm), 50% diameter (μm), and 90% diameter (μm) 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 analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Co., Ltd.), which enables laser diffraction particle size distribution measurement. The particle size distribution was measured using the laser diffraction / scattering particle size distribution analyzer as follows: First, 0.1 g of silver powder was dispersed in 40 mL of isopropyl alcohol (IPA). 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 analyzer, and the particle size distribution was determined using the accompanying analysis software.

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

[0113] In Table 2, "Moisture Content of Silver Powder" refers to the moisture content of the silver powder obtained after the classification process. In Table 1, "Moisture Content of Agglomerated Silver Powder" (%) and in Table 2, "Moisture Content of Silver Powder" (%) were determined as follows: 10g of agglomerated silver powder or silver powder was placed in a weighing bottle, dried at 90°C for 3 hours without a lid, and then cooled for 40 minutes or more. The weight loss of the sample was divided by the weight of the sample before drying (10g), and the resulting value was multiplied by 100.

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

[0115] In Table 2, "TAP" (g / mL) represents 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 density measuring device manufactured by Shibayama Kagaku Co., Ltd.). The tap density was measured as follows: 30 g of 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 after tapping (cm³) was then measured. 3 The tap density (g / cm³) was calculated. 3 ) is the sample volume (cm³) after this tapping. 3 Based on this, the value is obtained by the following equation (3). Tap density (g / cm³) 3) = 30(g) / sample volume after tapping (cm³) 3 )...Equation (3)

[0116] In Table 2, "Sa" (nm) represents the arithmetic mean roughness of the silver particle surface as defined in ISO 25178. The arithmetic mean roughness Sa was determined based on the shape image obtained by a scanning probe microscope (SPM). Specifically, an SPM (Nano Cute) manufactured by SII Nanotechnology Co., Ltd. was used, and a cantilever SI-DF40P2 manufactured by Hitachi High-Tech Fielding Co., Ltd. was used. The measurement mode was tapping mode (DFM). In detail, first, a Q curve measurement was performed and the cantilever was adjusted. At this time, it was confirmed that the resonance frequency was in the range of 200 Hz to 500 Hz and the Q value was in the range of 100 to 1000. The target vibration amplitude of the cantilever was set to 1 V. Next, the shape image and error signal image of silver nanoparticles with a field of view of 5 μm were acquired using the SPM. At this time, the amplitude attenuation rate was automatically set to the range of -0.1 to -0.2. The scanning frequency was set to the range of 0.6 Hz to 1 Hz. The parameters for feedback control were set to automatic. The number of pixels used when acquiring the shape image was set to 256 × 256. Then, after specifying the range for which roughness analysis was to be performed on the shape image, the arithmetic mean roughness Sa of the particle surface, as defined in ISO 25178, was automatically calculated by performing third-order tilt correction and flattening to remove components originating from the curved surface of the particles. No cutoff processing was performed at this time. The range to be analyzed was a square area with sides of 500 nm (the "500 nm × 500 nm range" in this specification). When performing the analysis, 10 particles were randomly selected and analyzed, and their average value was calculated. The value of Sa / BET diameter was calculated from the arithmetic mean roughness Sa and the BET diameter mentioned above.

[0117] To prepare a conductive paste from the obtained silver powder and to determine the aspect ratio of the wiring pattern of this conductive paste, conductive pastes were prepared using the silver powders from Examples 1-4 and Comparative Examples 1-4 as follows. Specifically, conductive pastes were obtained by performing the following treatment on a substance with the composition ratios shown in Table 3. The mixture was stirred and mixed at 1400 rpm for 30 seconds using a propellerless self-rotating stirring and defoaming device (AR310 manufactured by Shinky Co., Ltd.), and then kneaded using a three-roll machine (80S manufactured by EXAKT Corporation), passing the roll gap from 100 μm to 20 μm.

[0118] [Table 3]

[0119] The viscosity of the paste was measured using a Brookfield 5XHBDV-IIIUC viscometer. The measurement conditions were as follows: A CP-52 cone spindle 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 ) was set to 1 minute.

[0120] Fine wire evaluation was performed by forming a wiring pattern. The wiring pattern was formed as follows: First, a 154 mm square solid pattern was formed on the back surface of a silicon substrate for solar cells (100 Ω / □) using an aluminum paste (Rutech 28D22G-2) with a screen printing machine (Microtech MT-320TV). Next, the paste was filtered through a 500 mesh, and then electrodes with a design line width (screen aperture width) of 14 to 26 μm (finger electrodes) and electrodes with a design line width of 1 mm (busbar electrodes) were printed (coated) on the surface side of the substrate at a squeegee speed of 350 mm / sec, according to the pattern shown in Figure 10. After hot-air drying of the printed paste at 200°C for 10 minutes, the wiring pattern was obtained by firing in a high-speed firing furnace (IR furnace, NGK Insulators, Ltd., High-Speed ​​Firing Test 4-chamber furnace) at a peak temperature of 750°C and an in / out time of 41 seconds.

[0121] The line width and height of the wiring pattern were measured using a laser microscope (KEYENCE VKX-1000) for electrodes printed with a design line width of 24 μm, as shown in Figure 10. Images were taken with a 20x objective lens (electrode length per field of view: approximately 500 μm), and the cross-sectional shapes of 601 electrodes were measured at intervals of 0.687 μm in one field of view (corresponding to an electrode length of approximately 400 μm), and the average cross-sectional shape was calculated. For the average cross-sectional shape, the height of the substrate was used as the baseline, the distance from the baseline to the rise was measured as the line width, and the difference between the highest point and the baseline height was measured as the height, thereby obtaining the line width and height in one field of view. Of the 10 electrodes with a design line width of 24 μm, the 4th, 6th, and 8th electrodes from the left were photographed at two locations, 5 cm and 10 cm from the top of each electrode, for a total of 6 locations, and the average line width and height for the 6 fields of view were calculated. Based on these results, the "aspect ratio" in Table 2 was calculated.

[0122] Figures 2 to 9 show SEM images (20,000x magnification) of silver powder from Examples 1-4 and Comparative Examples 1-4, respectively. The SEM images of the Examples show that the silver particle surface has irregularities. The irregularities observed on the silver particle surface in the Examples' SEM images are presumed to be because the irregularities on the silver particle surface generated by the wet reaction were not sufficiently removed in the crushing and classification processes, and remained relatively large on the surface of the silver particles constituting the silver powder of the present invention, resulting in relatively large irregularities on the silver particle surface. The irregularities on the surface of the silver particles observed in the SEM images of Comparative Examples 1-4 are smaller compared to those of Examples 1-4. This is presumed to be because the amount of moisture in the atmosphere surrounding the silver particles during the crushing and classification processes was low, resulting in sufficient removal of the irregularities on the silver particle surface during these processes, thus reducing the irregularities on the silver particle surface compared to the Examples. The degree of irregularity on the silver particle surface of the Examples and Comparative Examples is reflected in their Sa values.

[0123] As shown in Table 2, in all comparisons, the wiring patterns obtained by pasteuring and firing the silver powder from 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 manufactured using the same aggregated silver powder, show that the Examples have a relatively larger ratio of height to line width (aspect ratio), indicating that they possess more favorable characteristics. Since the viscosity of the paste also affects the aspect ratio, comparisons were made between pastes with similar viscosities (within approximately ±20 Pa·s at 1 rpm).

[0124] Based on the results of the fine-line evaluation described above, the silver powder used in the example can be evaluated as a silver powder that can be used to prepare a paste that yields a wiring pattern with a desired line width and height.

[0125] Furthermore, as shown in Table 2, compared to Comparative Examples 1 to 4, the exhaust from the pulverizer 2 passing through the connecting pipe 23 in Examples 1 to 4 had a temperature of 30°C or higher and an absolute humidity of 20 g / m³. 3 By controlling the system in this manner, the classification performance of the classifier is improved, and it can be seen that the (D90-D10) / D50 ratio in Examples 1-4 is smaller than that in Comparative Examples 1-4.

[0126] Thus, in order to prepare a paste that yields a wiring pattern with a desired line width and height, it is necessary to use silver powder with a moderate amount of surface irregularities on the silver particles. This is evident from the SEM images of the silver powders of the examples and comparative examples shown in Figures 2 to 9. Specifically, the silver powder in the examples shows moderate irregularities on the surface of its spherical silver particles. In contrast, the silver powder in the comparative examples has less surface irregularities on its silver particles compared to the silver powder in the examples, and the surface of the silver particles is generally smoother. Thus, according to the manufacturing method of this embodiment, it is possible to obtain silver particles suitable for preparing a paste that yields a wiring pattern with a desired line width and height by suppressing the 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 for producing silver powder that can prepare a paste capable of obtaining a wiring pattern of the desired line width and height is not the method in which agglomerated silver powder is dried in advance and then subjected to processes other than drying in Plant 100, as in the comparative example, but rather the method in which agglomerated silver powder is crushed, classified, and dried in Plant 100 to obtain silver powder, as in the example. Furthermore, in order to obtain dried silver powder by drying agglomerated silver powder in Plant 100, it is necessary that compressed air with a temperature of 80°C to 180°C is supplied to the pulverizer 2 in the crushing process, and that the exhaust from the pulverizer 2 is supplied to the classifier 3, and that this exhaust has a temperature of 30°C or higher and a relative humidity of 30%.

[0128] As described above, we can provide a method for manufacturing silver powder and silver powder that can prepare a conductive paste that yields a wiring pattern with a desired line width and height.

[0129] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with configurations disclosed in other embodiments, as long as no inconsistencies arise. Moreover, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto, and can be modified as appropriate without departing from the object of the present invention. [Industrial applicability]

[0130] This invention can be applied to silver powder, a method for producing silver powder, and conductive pastes. [Explanation of Symbols]

[0131] 1: Feeding machine 100: Plant 2: Crusher (air-jet type crusher) 21: Crusher supply port 22: Heater 23: Connecting pipe 24: Air pump 26: Temperature and humidity meter 27:Flowmeter 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. having closed voids inside silver particles, wherein, the arithmetic average roughness Sa (nm) obtained by measuring surface roughness in a 500 nm×500 nm range on a surface of said silver particles, Specific surface area (m²) measured by the BET one-point method ２ The value of Sa / BET diameter, obtained by dividing the value of ( / g) and true density by the BET diameter calculated using the following formula (1), is 0.0070 or greater. said arithmetic average roughness Sa is 5 nm or more and 12.0 nm or less. The silver powder. BET diameter = 6 / (specific surface area × true density)... Formula (1)

2. in a particle size distribution determined on a volume basis in laser diffraction particle size distribution measurement, let the values of cumulative 10% diameter (μm), cumulative 50% diameter (μm) and cumulative 90% diameter (μm) accumulated from the smaller particle diameter side be D10, D50 and D90 respectively, a value obtained by dividing a difference value obtained by subtracting D10 from D90 by D50 is 1.0 or less. The silver powder according to claim 1.

3. A conductive paste comprising the silver powder according to claim 1 or 2.

Images