WO2012169628A1 - Silver powder and process for manufacturing same - Google Patents

Abstract

The present invention pertains to a silver powder which, when preparing a silver paste, exhibits excellent dispersibility in a solvent and which minimizes the generation of coarse powder particles such as flakes during kneading. The present invention provides a silver powder which exhibits a cohesive force of -0.2 to 0.7N/cm², a compressibility of 20 to 50% as determined by powder-bed shear stress measurement, and a dibutyl phthalate absorption of 3.0 to 9.0ml/100g as determined according to JIS-K6217-4 method.

Description

Silver powder and method for producing the same

The present invention relates to a silver powder and a method for producing the same, and more particularly to a silver powder that is a main component of a silver paste used for forming a wiring layer, an electrode, and the like of an electronic device and a method for producing the same.
This application claims priority on the basis of Japanese Patent Application No. 2011-128015 filed in Japan on June 8, 2011. By reference to these applications, the present application Incorporated.

For the formation of wiring layers and electrodes for electronic devices, silver pastes such as resin-type silver paste and fired-type silver paste are widely used. Conductive films such as wiring layers and electrodes are formed by applying or printing a silver paste, followed by heat curing or heat baking.

For example, the resin-type silver paste is composed of silver powder, resin, curing agent, solvent, etc., and this resin-type silver paste is printed on a conductor circuit pattern or terminal and then heat-cured at 100 ° C. to 200 ° C. to form a conductive film. Thus, wiring and electrodes are formed. The fired silver paste is made of silver powder, glass, solvent, etc., and this sintered silver paste is printed on a conductor circuit pattern or terminal and then heated and fired at 600 ° C. to 800 ° C. to form a conductive film. Thus, wiring and electrodes are formed. Fillability and sinterability of silver powder are important for the conductivity of these wirings and electrodes formed by heating silver paste.

In general, silver powder having a particle size of 0.1 μm to several μm is used for the conductive silver paste. The particle size of the silver powder used is fine according to the thickness of the intended wiring and the thickness of the electrode. Selected. Further, high uniformity is required for the thickness of the formed wiring and the thickness of the electrode, and the dispersibility in the paste of silver powder is important for that purpose. Improved dispersibility also leads to improved fillability.

The characteristics required for the silver powder for conductive silver paste vary depending on the application and use conditions, but generally high dispersibility and sinterability in the paste. When silver powder with low dispersibility in the paste is used, not only the thickness of the wiring and the thickness of the electrode become non-uniform, but also the treatment of curing and baking becomes non-uniform, increasing the resistance of the conductive film and the conductive film Will lead to embrittlement. Moreover, the deterioration of sinterability is directly linked to an increase in the resistance of the conductive film. However, these three characteristics largely depend on the stability of the silver powder production process and the surface treatment of the silver powder.

By the way, when producing the silver paste, first, the silver powder is kneaded with other components such as a solvent and then kneaded with a three-roll mill or the like while applying a predetermined pressure. At this time, the silver powder is required to be efficiently kneaded with a roll, that is, to have good kneading properties.

However, if there is a large lump of silver powder in the paste, kneading with a roll causes the lump of silver powder in the paste to collapse, producing coarse powder such as flaky powder (flakes) in units of several mm. End up. Since it is not desirable to leave the generated flakes in the paste as they are, they are removed by sieving using a mesh or the like, but if too much flakes are formed, there is a problem such as coarse powder clogging between the meshes It is generated and cannot be removed efficiently, and productivity is significantly impaired.

In addition, when flakes are generated in the paste as described above, when screen printing is performed using the paste, coarse flakes are clogged on a fine screen, and accurate pattern printing becomes difficult.

Thus, the occurrence of flakes greatly affects the kneadability during paste production and the printability during screen printing. Therefore, it is desired that the silver powder has good dispersibility in a solvent at the time of preparing the paste and does not generate coarse powder such as flakes.

Japanese Patent Application Laid-Open No. 2004-197030 JP 2000-129318 A

Therefore, the present invention has been proposed in view of such circumstances, silver powder having good dispersibility in a solvent during paste preparation, and suppressing the generation of coarse powder such as flakes during kneading and It aims at providing the manufacturing method.

As a result of intensive studies in order to achieve the above-mentioned object, the present inventors have found that silver powder having an aggregate formed by connecting substantially spherical particles to a predetermined size has good dispersibility in the paste. And the generation of coarse powders such as flakes during kneading can be suppressed, and the present invention has been completed.

That is, silver powder according to the present invention, cohesion is at -0.2N / cm ² or more 0.7 N / cm ² or less, a compression ratio in the powder layer shear force measurements 20-50%, and JIS- The absorption amount of dibutyl phthalate measured by the K6217-4 method is 3.0 to 9.0 ml / 100 g.

In addition, the silver powder production method according to the present invention includes a silver complex containing solution obtained by dissolving silver chloride and a complexing agent and a reducing agent solution, and reducing the silver complex to produce silver powder. In the production method, 0.1 to 15% by mass of a water-soluble polymer based on silver is added to one or both of the silver complex-containing solution and the reducing agent solution.

According to the present invention, the paste is excellent in dispersibility in a solvent, and generation of coarse powder such as flakes can be effectively suppressed during kneading of the paste preparation.

It is a figure which shows typically about a silver particle form.

Hereinafter, specific embodiments of the silver powder and the production method thereof according to the present invention will be described in detail. Note that the present invention is not limited to the following embodiments, and can be appropriately changed without changing the gist of the present invention.

In the description, the name for the silver particle form is defined as shown in FIG. That is, as shown in FIG. 1 (A), silver particles are judged from the apparent geometric form, and those considered as unit particles are called primary particles. Further, as shown in FIG. 1B, particles in which two to three or more primary particles are connected by necking are called secondary particles. Further, as shown in FIG. 1C, an aggregate of primary particles and secondary particles is called an aggregate.

Conventionally, silver powder having an individual primary particle dispersed as much as possible and having an average particle size of 0.1 to 1.5 μm has been used in the production of silver paste. Fine silver in which such primary particles are dispersed has been used. Since the particles are densely packed, there are many contact points with other particles and the cohesive force increases, and silver particles easily aggregate in the paste to form a large lump. Then, for example, when kneading is performed by a three-roll mill generally used in paste production, the aggregated mass enters the roll as it is without breaking, and as a result, coarse powder of mm order such as flakes is formed. I found out.

On the other hand, in the case of silver powder having a large particle size distribution having an aggregate in which primary particles and secondary particles are loosely aggregated at a predetermined ratio, there are sufficient voids between the aggregates, and the number of contacts is small. It was confirmed that no large lump was formed in the paste and flakes were not generated. In such silver powder, primary particles and secondary particles are aggregated and connected in a predetermined size to form, for example, an aggregate as shown in FIG. The aggregate has a size of about 5 to 10 μm, and has a structure in which secondary particles formed by relatively strong bonds of several primary particles and a structure in which the secondary particles and the primary particles are connected by relatively weak bonds. It is assumed that it consists of From this, the present inventors have appropriately formed an aggregate in which primary particles or secondary particles are linked to a predetermined size, and the aggregate has a predetermined strength. In addition, the cohesive force between silver particles when viewed as one particle is reduced, the dispersibility is excellent, the generation of coarse powder such as flakes is effectively suppressed during kneading of paste preparation, and the kneadability is improved. It turns out that it can improve.

That is, silver powder according to the present embodiment, the cohesive force is at -0.2N / cm ² or more 0.7 N / cm ² or less, a compression ratio in the powder layer shear force measurements 20-50%, and The absorption of dibutyl phthalate measured by JIS-K6217-4 method is 3.0 to 9.0 ml / 100 g. Silver powder having such characteristics has good dispersibility in a solvent during paste preparation, and can effectively suppress generation of coarse powder such as flakes during kneading.

The silver particles used in the present embodiment preferably have a primary particle having an average particle size in the range of 0.1 to 1.5 μm. When the average particle size of the primary particles is 0.1 μm or more, when the conductive paste is made, a large resistance is not generated and the conductivity is good. In addition, when the average particle size of the primary particles is 1.5 μm or less, as described later, even when primary particles are connected to a predetermined size to form an aggregate, kneading is performed without deteriorating dispersibility. And printability are good.

The agglomeration force represents the ease of aggregation of the silver powder itself, and is an index for aggregation of silver particles in the paste. This cohesive force can be defined as a shear stress in a state where silver powder is not loaded. Therefore, for example, using a powder layer shear force measuring device, it can be obtained from a graph of shear stress and normal stress, and in the graph of shear stress against normal stress, the shear stress value of the Y-intercept becomes the cohesive force. That is, the higher the Y intercept, the greater the cohesive force. In addition, the inclination of the graph of the shear stress with respect to the normal stress is an internal friction force of the silver powder, and is an index of the slipperiness of the powder.

Silver powder according to the present embodiment, the cohesive force is -0.2N / cm ² or more 0.7 N / cm ² or less. When the cohesive force is within the above-described range, the generation of coarse flakes is suppressed without excessively aggregating silver particles in the paste.

The absorption amount of dibutyl phthalate can be measured based on JIS-K6217-4 method. The silver powder according to the present embodiment has an absorption amount of dibutyl phthalate of 3.0 to 9.0 ml / 100 g. Silver powder having an absorption of dibutyl phthalate of 3.0 to 9.0 ml / 100 g indicates that silver particles of a predetermined size are connected to form, for example, a bunch of grape-like aggregates. .

That is, in a silver powder having an aggregate formed by connecting silver particles of a predetermined size, the voids increase, and when dibutyl phthalate is dropped, phthalates are formed between the silver particles forming the aggregate. Dibutyl acid is absorbed (oil absorption). In silver powder with less aggregate formation, the amount of absorption decreases because there are few voids between silver particles. Therefore, by measuring the amount of dibutyl phthalate absorbed, it is possible to determine how much the aggregate is formed. Moreover, by having a predetermined absorption amount, components such as the solvent of the paste and the silver particles can be easily blended and can be kneaded well.

Also, based on the absorbed amount of dibutyl phthalate, the viscosity of the paste made using the silver powder can be judged. As described above, since the silver powder having an aggregate in which silver particles are connected takes in the solvent component of the paste between the particles constituting the aggregate, the amount of the solvent component in the paste outside the aggregate is relatively high. To reduce the viscosity of the paste. When the viscosity becomes high, the shearing force generated between the rolls during kneading is efficiently propagated to the paste as a dispersion force for dispersing the silver powder, and the silver powder does not aggregate and is easily dispersed.

In addition, when the absorbed amount of dibutyl phthalate is less than 3.0 ml / 100 g, it indicates that the number of the above-mentioned aggregates formed is small, and flakes are generated during paste production. On the other hand, when the amount of absorption is larger than 9.0 ml / 100 g, it indicates that the silver particles are excessively aggregated, the dispersibility is deteriorated, and flakes are generated.

Compressibility is a volume reduction rate of silver powder from a set load to a state where a set load is applied, and is an index representing the void amount between silver particles and the structural strength of the aggregate of silver powder. This compression rate is determined by using a powder layer shear force measuring device to fill a cell with a predetermined amount of silver powder and measure it under no load (static bulk height) and the volume when a set load (60 N) is applied ( It can be measured from (bulkness). When a load is applied to the silver powder by the powder layer shear force measuring device, the powder layer will be compressed, but if the silver particles are separated into primary particles, the amount of voids between the compressed particles will be reduced, The compression rate is large. On the other hand, when the silver particles form the above-described aggregate, voids after compression including the voids inside the aggregate increase, and the compression rate is small. However, even if aggregates are formed, if the compression ratio becomes too large in a state where a set load is applied, the aggregates do not have sufficient strength and are easily crushed into primary particles during kneading. It shows that it will be done.

As described above, the silver powder according to the present embodiment contains an aggregate formed by connecting silver particles in a predetermined size. Since the presence of this aggregate can effectively suppress the generation of flakes, the structure of the aggregate does not easily change, and preferably has structural strength.

For example, agglomerates contained in silver powder are not preferable if they are easily broken by the hands of paste workers. Moreover, when producing a paste using silver powder, pre-kneading by a self-revolving mixer or the like and main kneading by a three-roll mill or the like are generally performed. At this time, in the case of silver powder having an aggregate having a low structural strength, the aggregate breaks into primary particles or secondary particles in the initial stage of kneading, so that they are densely packed in the paste, Since the number of contact points with the particles increases and the cohesive force increases, the particles tend to aggregate in the paste, and flakes are generated. Thereby, kneadability is impaired remarkably.

Therefore, it is preferable that the formed aggregate has a structural strength and does not easily change in structure. Thereby, the state in which the aggregate is present in the silver powder is maintained, the viscosity is not reduced, the generation of coarse powder such as flakes can be suppressed, and good kneadability can be exhibited.

The silver powder according to the present embodiment has a compression rate of 20 to 50%. When the compression ratio is less than 20%, the above-mentioned aggregate structure is strong, indicating that the aggregate structure is not easily broken, and flakes are produced during paste production. On the other hand, if the compression ratio is greater than 50%, the mechanical strength of the agglomerate is weak, indicating that the agglomerated structure is easily broken, and the silver particles are densely packed during the preparation of the paste, and the contact with other particles is Since it increases and the cohesion force becomes large, it becomes easy to aggregate in a paste and will produce flakes.

Thus, the cohesive force is at -0.2N / cm ² or more 0.7 N / cm ² or less, the compression ratio in the powder layer shear force measurement is 20-50%, and in JIS-K6217-4 method Silver powder having a measured absorption of dibutyl phthalate of 3.0 to 9.0 ml / 100 g has agglomerates with many voids in which silver particles are linked to a predetermined size, and the aggregates are The predetermined strength is maintained. According to such silver powder, the dispersibility in a solvent is good at the time of paste preparation, and generation of coarse powder such as flakes during kneading can be effectively suppressed.

The presence of aggregates contained in the silver powder as described above can also be determined by comparing the average particle diameter as follows. Specifically, the determination is made by comparing the volume average particle size D50 measured using the laser diffraction scattering method with the average particle size DS obtained by image analysis of a scanning electron microscope (SEM). Can do.

The particle size measurement by laser diffraction confusion method is the particle size of the unit particles dispersed in the solvent, that is, when aggregated particles are included, not only the primary particles dispersed alone but also the aggregates and secondary particles The particle size containing On the other hand, the average particle diameter obtained by image analysis of SEM is an average value of the particle diameters of the primary particles. Therefore, as the ratio determined by D50 / DS is larger than 1, it indicates that secondary particles and aggregates in which primary particles are connected at a predetermined ratio are formed.

The silver powder according to the present embodiment is obtained by D50 / DS, where D50 is the volume average particle diameter measured using the laser diffraction scattering method and DS is the average particle diameter obtained by SEM image analysis. The resulting ratio is 1.5 to 5.0.

In addition, when the ratio calculated | required by D50 / DS is smaller than 1.5, there are few aggregates mentioned above and there exists a possibility of producing flakes at the time of paste preparation. On the other hand, when the ratio calculated by D50 / DS is larger than 5.0, the silver particles are excessively aggregated to form a large amount of large aggregates, and the dispersion stability of the paste in the solvent is deteriorated. May cause flakes.

The strength of the aggregate can also be determined by comparing the specific surface areas as follows. Specifically, the determination can be made by comparing the specific surface area obtained by the BET method with the specific surface area obtained from the average particle diameter obtained by SEM image analysis.

Here, the BET method is a method for measuring a surface area of a powder by a gas phase adsorption method, and is a method for obtaining a total surface area, that is, a specific surface area of a 1 g sample from an adsorption isotherm. Nitrogen gas is often used as the adsorbed gas, and a method of measuring the amount of adsorption from the change in pressure or volume of the gas to be adsorbed is often used. The specific surface area can be determined by determining the amount of adsorption based on the BET formula and multiplying the area occupied by one adsorbed molecule on the surface.

The strength of the aggregate is related to the strength of the connection between the silver particles. In the measurement by the BET method, when the connection between the particles is weak, for example, when the spherical primary particles are connected only at the contact points, the surface area is reduced only at the contact points where the particles are connected. The reduction in the specific surface area measured as a result is the sum of the specific surface areas in a state where the particles are completely dispersed, that is, the specific surface area of the primary particles is slightly smaller. On the other hand, when the connection between the particles is strong, in other words, when the primary particles are strongly connected so that the secondary particles are in the shape of a gourd or snowman, the specific surface area of the thick connection part decreases. As a result, the specific surface area measured by the BET method decreases more than the specific surface area of the primary particles. On the other hand, as described above, the average particle diameter obtained by image analysis of SEM is the average value of the particle diameter of the primary particles, and the specific surface area obtained from this average particle diameter is the sum of the surface areas of individual particles as spheres, The value approximates the specific surface area of the primary particles.

Therefore, the ratio (SSA ₁ / SSA ₂ ) between the specific surface area SSA ₁ determined by the BET method and the specific surface area SSA ₂ determined from the average particle diameter obtained by SEM image analysis is the aggregation index or spherical shape of the silver powder. It becomes an index, whereby it can be determined how strongly the above-mentioned connected particles are connected, and the strength of the aggregate can be determined.

Silver powder according to the present embodiment, when the specific surface area determined by the BET method and SSA _1, the specific surface area determined from the average particle diameter obtained by image analysis of SEM was SSA _2, SSA 1 / SSA ₂ The ratio determined by is less than 1.0. Thus, the silver powder having a ratio determined by SSA ₁ / SSA ₂ of less than 1.0 means that the formed aggregate has a predetermined strength, and the aggregate structure is maintained even by kneading, for example, The generation of flakes during paste production can be more effectively suppressed. On the other _hand, it is preferable ratio sought SSA 1 / SSA ₂ is 0.7 or more. When the ratio is less than 0.7, the aggregation proceeds, indicating that the aggregate is coarse and has a high strength. If such an aggregate is contained in the silver powder, it may cause clogging during screen printing, and may impair the uniformity of the wiring layer and electrodes formed with the silver paste.

In the case when the ratio sought SSA 1 _/ SSA ₂ is 1.0 or more, or aggregates are not formed, a case connection of the connecting particle is weak, for example, was kneaded in a predetermined or more pressure Therefore, the aggregate structure may be easily broken and flakes may be generated.

By the way, in general, when producing a baking paste using silver powder, each component is weighed and placed in a predetermined container, pre-kneaded using a self-revolving mixer, etc., and then main-kneaded with three rolls. To make. As described above, it is important to maintain the aggregate structure of the aggregate formed by connecting silver particles to a predetermined size. Even if the preliminary kneading and the main kneading are performed during paste preparation, It is desirable to maintain the aggregate structure at a high level. That is, it is desirable that the aggregate has an appropriate structural stability.

Therefore, a paste was prepared by experimentally kneading silver powder and an epoxy resin with a centrifugal force of 420 G, and the volume average particle diameter D1 measured by using a laser diffraction scattering method for the silver powder in the paste, and then further The stability of the aggregated structure can be determined by comparing the silver powder in the paste obtained by kneading with a three-roll mill with the volume average particle diameter D2 measured using the laser diffraction scattering method. That is, in general, the structure of the aggregates collapses with kneading, and the average particle diameter of silver powder shifts to become smaller. Therefore, the average particle diameter D1 after preliminary kneading and the average particle diameter D2 after main kneading are By comparing, the stability of the structure of the aggregate can be determined.

In the silver powder according to the present embodiment, as an evaluation of the structural stability of the above-described aggregate, the silver powder in the paste obtained by kneading the silver powder and the epoxy resin with a centrifugal force of 420 G is used by laser diffraction scattering method. When the average particle diameter of volume integration measured by the laser diffraction scattering method is D2 and the average particle diameter of volume integration measured by laser diffraction scattering method is D1 , The ratio obtained by D2 / D1 is 0.5 to 1.5.

When the ratio calculated by D2 / D1 is 0.5 to 1.5, it can be determined that the structure of the aggregate is stable also by the preliminary kneading and the main kneading. In addition, when the ratio calculated | required by D2 / D1 is smaller than 0.5, there is no stability of the structure of an aggregate, the structure breaks by kneading | mixing, and it is possible to generate | occur | produce a flake while causing a rapid viscosity fall There is sex.

In the case of obtaining D1, the apparatus for kneading silver powder and epoxy resin with a centrifugal force of 420 G (preliminary kneading) is not particularly limited as long as it can knead with the centrifugal force of 420 G. A mixer or the like can be used. Moreover, when calculating | requiring D2, kneading | mixing (main kneading | mixing) by a 3 roll mill is performed on conditions with a roll diameter of 150 mm and roll pressure of 10 bar, for example.

Further, the stability of the structure of the aggregate can be evaluated by measuring the viscosity of the paste after kneading, in addition to comparing the average particle diameter after kneading as described above.

That is, as described above, the silver powder according to the present embodiment has an aggregate with many voids in which silver particles (primary particles and secondary particles) are aggregated to a predetermined size. Therefore, as described above, the viscosity increases in the initial stage of paste production. However, when the strength of the aggregate is weak, the viscosity gradually shifts with kneading. Therefore, the stability of the structure of the agglomerates is judged by preparing a paste with silver powder and an epoxy resin on a trial basis and comparing the viscosity η1 of the paste after preliminary kneading with the viscosity η2 of the paste after main kneading. can do.

The silver powder according to the present embodiment is a shear rate obtained by measuring a paste obtained by kneading the silver powder and an epoxy resin with a centrifugal force of 420 G using a viscoelasticity measuring device as an evaluation of the structural stability of the aggregate. When the viscosity at 4 sec ⁻¹ is η1, and then the paste obtained by further kneading with a three-roll mill is measured by a viscoelasticity measuring device, the viscosity at shear rate 4 sec ⁻¹ is η2, and the ratio obtained by η2 / η1 Becomes 0.5 to 1.5.

When the ratio obtained by η2 / η1 is 0.5 to 1.5, it can be determined that the structure of the aggregate is stable also by the preliminary kneading and the main kneading. In addition, when the ratio calculated | required by (eta) 2 / (eta) 1 is smaller than 0.5, there is no stability of the structure of an aggregate, the structure breaks by kneading | mixing, and it is possible to generate | occur | produce a flake while causing a rapid viscosity fall There is sex.

As described above, as an apparatus for kneading (preliminary kneading) silver powder and epoxy resin with a centrifugal force of 420 G, for example, a self-revolving mixer or the like can be used. Further, the kneading (main kneading) by the three roll mill is performed under the conditions of a roll diameter of 150 mm and a roll pressure of 10 bar, for example. The viscoelasticity measuring device is not particularly limited as long as it can measure the viscosity at a desired shear rate.

The composition of the paste for evaluation prepared in this viscosity measurement is, for example, 80% by mass of silver powder, epoxy resin (100 to 200 P (10 to 20 Pa · s) / 25 ° C., preferably 120 to 150 P (12 to 15 Pa · s). s) / 25 ° C.) is preferably 20% by mass.

As described above, silver powder according to the present embodiment, cohesion -0.2N / cm ² or more 0.7 N / cm ² or less, at a compression rate of the powder layer shear force measurements 20-50% And the absorption amount of dibutyl phthalate measured by the JIS-K6217-4 method is 3.0 to 9.0 ml / 100 g. That is, this silver powder has an aggregate with many voids in which silver particles are connected in a predetermined size, and the aggregate has a predetermined strength. According to such silver powder, dispersibility in a solvent becomes good at the time of paste preparation, and silver powder is prevented from agglomerating and lumping in the paste, and coarse powder such as flakes is generated. Can be suppressed.

And according to the silver powder which can suppress generation | occurrence | production of such flakes, it can prevent clogging also when screen printing, without impairing kneadability at the time of paste kneading, and excellent printability Can be realized.

Next, a method for producing the above-described silver powder will be described. The method for producing silver powder according to the present embodiment uses, for example, silver chloride or silver nitrate as a starting material, and basically a solution containing a silver complex obtained by dissolving silver chloride or the like with a complexing agent. A silver particle slurry is obtained by mixing the (silver complex-containing solution) and the reducing agent solution, and reducing the silver complex to precipitate silver particles. When silver chloride is used as the starting material, there is no need to install a nitrite gas recovery device or a treatment system for nitrate nitrogen in wastewater, which is required in the method using silver nitrate as a starting material. Therefore, the manufacturing cost can be reduced.

In the method for producing silver powder according to the present embodiment, 0.1 to 15% by mass of a water-soluble polymer with respect to silver is added to either or both of the silver complex-containing solution and the reducing agent solution. To do. More preferably, more than 3.0 mass% and 10 mass% or less water-soluble polymer is added with respect to silver. In this way, by adding 0.1 to 15% by mass of a water-soluble polymer with respect to silver to either or both of the silver complex-containing solution and the reducing agent solution, the cohesion force is -0.2 N / cm ² or more and 0.7 N / cm ² or less, the compressibility in the measurement of the powder layer shear force is 20 to 50%, and the absorption of dibutyl phthalate measured by the JIS-K6217-4 method is 3.0. A silver powder of ˜9.0 ml / 100 g can be produced.

In the production of silver powder according to the present embodiment, it is important to select a water-soluble polymer as an anti-aggregation agent and to add it. Silver particles (primary particles) produced by reduction with a reducing agent solution have an active surface, and are easily connected to other silver particles to form secondary particles. Further, the secondary particles aggregate to form an aggregate. At this time, when an anti-aggregation agent having a high anti-aggregation effect, for example, a surfactant or a fatty acid is used, secondary particles and aggregates are not sufficiently formed, primary particles increase, and appropriate aggregates are not formed. On the other hand, when an anti-agglomeration agent having a low anti-agglomeration effect is used, the formation of secondary particles and aggregates becomes excessive, resulting in a silver powder containing excessively aggregated coarse aggregates. The water-soluble polymer has an appropriate anti-aggregation effect, so it is possible to easily control the formation of secondary particles and aggregates by adjusting the addition amount, and the silver complex containing after the addition of the reducing agent solution Aggregates of an appropriate size can be formed in the solution.

The water-soluble polymer to be added is not particularly limited, but is preferably at least one of polyethylene glycol, polyethylene oxide, polyvinyl alcohol, and polyvinyl pyrrolidone. According to these water-soluble polymers, in addition to preventing excessive aggregation, the aggregation of the grown nuclei is insufficient, preventing the silver particles (primary particles) from becoming fine, The silver powder which has can be formed easily.

Here, it is considered that the mechanism by which silver particles are connected to a predetermined size and aggregates are formed by adding a water-soluble polymer is as follows. That is, by adding a water-soluble polymer, the water-soluble polymer is adsorbed on the surface of the silver particles. At this time, when almost the entire surface of the silver particles is covered with the water-soluble polymer, the silver particles will be present alone, but 0.1 to 15% by mass of the water-soluble polymer is added to the silver. Thus, it is considered that a surface in which a part of the water-soluble polymer does not exist remains, and silver particles are connected through the surface to form an aggregate.

For this reason, the addition amount of the water-soluble polymer is 0.1 to 15% by mass with respect to silver. When the addition amount of the water-soluble polymer is less than 0.1% by mass with respect to silver, the dispersibility in the silver particle slurry is deteriorated, the silver powder is excessively aggregated, and many coarse aggregates Will cause flakes. On the other hand, when the addition amount with respect to silver is more than 15% by mass, almost all the silver particle surfaces are covered with the water-soluble polymer, the silver particles cannot be connected to each other, and an aggregate is formed. I can't. As a result, silver powder composed of primary particles is formed, and even in this case, flakes are generated during paste preparation. Therefore, by adding 0.1 to 15% by mass of a water-soluble polymer based on silver, silver particles can be connected with an appropriate cohesive force to form a structurally stable aggregate. The dispersibility in the inside can be improved, and the occurrence of flakes can be effectively suppressed.

Further, the water-soluble polymer is added to either or both of the silver complex-containing solution and the reducing agent solution. The addition of the water-soluble polymer to either or both of the silver complex-containing solution and the reducing agent solution may be added in advance to the solution to be added prior to the reduction treatment, and may contain a silver complex for the reduction treatment. You may make it add at the time of mixing of a solution and a reducing agent solution.

More preferably, a water-soluble polymer is added in advance to the reducing agent solution. In this way, by adding a water-soluble polymer to the reducing agent solution in advance, the water-soluble polymer exists in the nucleation or nucleation field, and the surface of the generated nuclei or silver particles is rapidly water-soluble. Polymers can be adsorbed and the aggregation of silver particles can be controlled efficiently. And more preferably, by adding the concentration so as to be more than 3.0% by mass and 10% by mass or less, the silver particles are more appropriately connected to a predetermined size to form a highly stable aggregate. And the occurrence of flakes can be more effectively suppressed.

In addition, when a water-soluble polymer is added to a silver complex-containing solution in advance, it is difficult to supply the water-soluble polymer to the site of nucleation or growth, and the water-soluble polymer is adsorbed to the surface of the silver particles appropriately. There is a possibility that it cannot be made. Therefore, when adding to a silver complex containing solution previously, it is preferable to make the addition amount of water-soluble polymer more than 3.0 mass%.

Next, the method for producing silver powder will be described more specifically for each process. First, in the reduction step, a starting material such as silver chloride is dissolved using a complexing agent to prepare a solution containing a silver complex. Although it does not specifically limit as a complexing agent, It is preferable to use the ammonia water which is easy to form a complex with silver chloride etc. and does not contain the component which remains as an impurity. Moreover, when using silver chloride, it is preferable to use a high purity thing.

As a method for dissolving silver chloride, for example, when ammonia water is used as a complexing agent, a slurry such as silver chloride may be prepared and ammonia water may be added, but in order to increase the complex concentration and increase productivity. Is preferably dissolved by adding silver chloride in ammonia water. The aqueous ammonia used for the dissolution may be a normal one used industrially, but preferably has a purity as high as possible in order to prevent contamination with impurities.

Next, a reducing agent solution to be mixed with the silver complex solution is prepared. As the reducing agent, it is preferable to use a strong reducing power such as ascorbic acid, hydrazine, formalin and the like. Ascorbic acid is particularly preferred because the crystal grains in the silver particles are easy to grow. Hydrazine or formalin can reduce the crystals in the silver particles. Moreover, in order to control the uniformity of reaction or reaction rate, it can also be used as an aqueous solution whose concentration is adjusted by dissolving or diluting a reducing agent with pure water or the like.

As described above, in this silver powder production method, 0.1 to 15% by mass of a water-soluble polymer with respect to silver is added to either or both of the silver complex-containing solution and the reducing agent. At this time, foaming may occur during the reduction reaction due to the addition of the water-soluble polymer, and therefore an antifoaming agent may be added to the silver complex solution or the reducing agent mixed solution. The antifoaming agent is not particularly limited and may be one usually used during reduction. However, in order not to inhibit the reduction reaction, the addition amount of the antifoaming agent is preferably set to a minimum level at which an antifoaming effect can be obtained.

In addition, about the water used when preparing a silver complex solution and a reducing agent solution, in order to prevent mixing of an impurity, it is preferable to use the water from which the impurity was removed, and it is especially preferable to use a pure water.

Next, the silver complex solution prepared as described above and the reducing agent solution are mixed, and the silver complex is reduced to precipitate silver particles. This reduction reaction may be performed by a batch method or a continuous reduction method such as a tube reactor method or an overflow method. The particle size of the silver particles can be controlled by the mixing rate of the silver complex solution and the reducing agent solution and the reduction rate of the silver complex, and can be easily controlled to the intended particle size.

A large amount of chloride ions and water-soluble polymers are adsorbed on the surface of the silver particles obtained in the reduction process. Therefore, in order to make the conductivity of the wiring layer and the electrode formed using the silver paste sufficient, the obtained silver particle slurry is washed in the next washing step, and the surface adsorbate is removed by washing. It is preferable. As will be described later, in order to suppress the occurrence of excessive aggregation by removing the water-soluble polymer adsorbed on the surface of the silver particles, the washing step may be performed after the surface treatment step on the silver particles. preferable.

The washing method is not particularly limited, but the silver particles solid-liquid separated from the slurry by a filter press or the like are put into the washing liquid, stirred using an agitator or an ultrasonic washing machine, and then again solid-liquid. A method of separating and collecting silver particles is generally used. Further, in order to sufficiently remove the surface adsorbate, it is preferable to repeat the operations consisting of charging into the cleaning liquid, stirring cleaning, and solid-liquid separation several times.

As the cleaning liquid, water may be used, but an alkaline aqueous solution may be used in order to efficiently remove chlorine. Although it does not specifically limit as an alkaline solution, It is preferable to use the sodium hydroxide aqueous solution with few remaining impurities and cheap. In the case of using a sodium hydroxide aqueous solution as the cleaning liquid, it is desirable to further wash the silver particles or the slurry thereof with water in order to remove sodium after washing with the sodium hydroxide aqueous solution.

Further, the concentration of the sodium hydroxide aqueous solution is preferably 0.01 to 1.00 mol / l. When the concentration is less than 0.01 mol / l, the cleaning effect is insufficient. On the other hand, when the concentration exceeds 1.00 mol / l, sodium may remain in silver particles more than allowable. The water used for the cleaning liquid is preferably water that does not contain an impurity element harmful to silver particles, and it is particularly preferable to use pure water.

In the production of the silver powder according to the present embodiment, before the aggregate formed by reduction in the silver complex-containing solution further aggregates to form a coarse aggregate, the surface of the formed aggregate is aggregated. The second important factor is to prevent excessive aggregation by surface treatment with a treatment agent having a high prevention effect. That is, after the above-described aggregates are formed and before excessive aggregation proceeds, the silver particles are treated with a surfactant, or more preferably a surface treatment on silver particles treated with a surfactant and a dispersant. Perform the process. Thereby, excessive aggregation can be prevented from occurring, the structural stability of the desired aggregate can be maintained, and the generation of flakes can be more effectively suppressed.

Since excessive agglomeration is particularly advanced by drying, the effect of surface treatment can be obtained at any stage before the silver particles are dried. For example, it can be performed after the reduction process and before the above-described cleaning process, simultaneously with the cleaning process, or after the cleaning process.

Among these, it is particularly preferable to carry out after the reduction step and before the washing step. Or it is preferable to carry out after one washing process. As a result, aggregates formed through a reduction treatment and aggregated to a predetermined size can be maintained, and the surface treatment is performed on the silver particles including the aggregates. Can be manufactured.

More specifically, in the method for producing silver powder according to the present embodiment, as described above, by adding 0.1 to 15% by mass of a water-soluble polymer with respect to silver, the surface of the silver particles A water-soluble polymer is adsorbed moderately to form an aggregate in which silver particles are linked to a predetermined size. However, the water-soluble polymer adsorbed on the surface of the silver particles is easily cleaned by the cleaning process. Therefore, when the washing process is performed prior to the surface treatment, the water-soluble polymer on the surface of the silver particles is washed, the silver particles start to excessively aggregate with each other, and a larger amount of agglomerates than the formed aggregates. Lumps may be formed. And this may cause flakes.

Therefore, from this, the surface treatment step is performed before the washing step or after one washing step, that is, in a state where an amount of water-soluble polymer that can suppress at least aggregation of silver particles remains on the surface of the silver particles. Preferably it is done. The surface treatment after the reduction process and before the cleaning process may be performed after solid-liquid separation of the slurry containing silver particles with a filter press or the like after the reduction process. By performing the surface treatment after the solid-liquid separation in this way, it is possible to cause the surfactant or dispersant, which is a surface treatment agent, to act directly on the generated silver particles. It is possible to effectively suppress the surface treatment agent from being adsorbed and the occurrence of flakes due to an excessive aggregation due to an increased aggregate structure.

In this surface treatment step, it is more preferable to treat the surface with both a surfactant and a dispersant. When the surface treatment is performed with both the surfactant and the dispersant as described above, a strong surface treatment layer can be formed on the surface of the silver particles due to the interaction. It is effective to maintain the aggregate. As a specific method of preferable surface treatment using a surfactant and a dispersant, the silver particles are put into water added with a surfactant and a dispersant and stirred, or put into water added with a surfactant. After stirring, a dispersant may be further added and stirred. Moreover, when performing surface treatment simultaneously with a washing | cleaning process, a surfactant and a dispersing agent should be added simultaneously to a washing | cleaning liquid, or a dispersing agent should just be added after addition of surfactant. In order to improve the adsorptivity of the surfactant and the dispersant to the silver particles, the silver particles are added to the water or the cleaning liquid to which the surfactant is added and stirred, and then the dispersant is further added and stirred. It is preferable.

Here, the surfactant is not particularly limited, but a cationic surfactant is preferably used. Since the cationic surfactant is ionized into positive ions without being affected by pH, for example, an effect of improving the adsorptivity to silver powder using silver chloride as a starting material can be obtained.

The cationic surfactant is not particularly limited, but is an alkyl monoamine salt type represented by a monoalkylamine salt, an alkyldiamine salt represented by an N-alkyl (C14 to C18) propylenediamine dioleate. Type, alkyltrimethylammonium salt type represented by alkyltrimethylammonium chloride, alkyldimethylbenzylammonium salt type represented by alkyldimethylbenzylammonium chloride, quaternary ammonium salt type represented by alkyldipolyoxyethylenemethylammonium chloride, Alkyl pyridinium salt type, tertiary amine type represented by dimethylstearylamine, polyoxyethylene alkylamine type represented by polyoxypropylene / polyoxyethylene alkylamine, N N ′, N′-tris (2-hydroxyethyl) -N-alkyl (C14-18) is preferably at least one selected from oxyethylene addition types of diamines represented by 1,3-diaminopropane Any of ammonium salt type, tertiary amine salt type or a mixture thereof is more preferable.

Further, the surfactant preferably has at least one alkyl group having a C4 to C36 carbon number represented by methyl group, butyl group, cetyl group, stearyl group, beef tallow, hard beef tallow, and plant stearyl. The alkyl group is preferably a group to which at least one selected from polyoxyethylene, polyoxypropylene, polyoxyethylene polyoxypropylene, polyacrylic acid, and polycarboxylic acid is added. Since these alkyl groups are strongly adsorbed with a fatty acid used as a dispersant described later, the fatty acid can be strongly adsorbed when the dispersant is adsorbed to the silver particles via the surfactant.

Further, the addition amount of the surfactant is preferably in the range of 0.002 to 1.000% by mass with respect to the silver particles. Since almost the entire amount of the surfactant is adsorbed on the silver particles, the addition amount of the surfactant and the adsorption amount are almost equal. When the addition amount of the surfactant is less than 0.002% by mass, the effect of suppressing aggregation of silver particles or improving the adsorptivity of the dispersant may not be obtained. On the other hand, when the addition amount exceeds 1.000% by mass, the conductivity of the wiring layer or electrode formed using the silver paste is not preferable.

As the dispersant, for example, protective colloids such as fatty acids, organometallics, and gelatins can be used. However, in view of adsorbability with a surfactant without the possibility of contamination with impurities, it is preferable to use fatty acids or salts thereof. . In addition, you may add a fatty acid or its salt as an emulsion.

The fatty acid used as the dispersant is not particularly limited, but is preferably at least one selected from stearic acid, oleic acid, myristic acid, palmitic acid, linoleic acid, lauric acid, and linolenic acid. This is because these fatty acids have a relatively low boiling point and thus have little adverse effect on the wiring layer and electrodes formed using the silver paste.

Further, the addition amount of the dispersant is preferably in the range of 0.01 to 3.00% by mass with respect to the silver particles, and more preferably in the range of 0.01 to 1.00% by mass with respect to the silver particles. The amount of adsorption on the silver particles varies depending on the type of the dispersant, but when the added amount is less than 0.01% by mass, the amount of the dispersant is sufficient to sufficiently suppress the aggregation of the silver particles or improve the adsorptivity of the dispersant. May not be adsorbed by silver powder. On the other hand, when the added amount of the dispersant exceeds 3.00% by mass, the dispersant adsorbed on the silver particles increases, and the conductivity of the wiring layer and the electrode formed using the silver paste cannot be sufficiently obtained. Sometimes.

After washing and surface treatment, solid-liquid separation is performed to collect silver particles. In addition, the apparatus used for washing | cleaning and surface treatment may be used normally, For example, the reaction tank with a stirrer etc. can be used. Moreover, the apparatus used for solid-liquid separation may also be a normally used apparatus, for example, a centrifuge, a suction filter, a filter press, etc. can be used.

The silver particles that have been washed and surface-treated are dried by evaporating moisture in the drying step. As a drying method, for example, silver powder recovered after completion of cleaning and surface treatment is placed on a stainless steel pad and heated at a temperature of 40 to 80 ° C. using a commercially available drying apparatus such as an atmospheric oven or a vacuum dryer. Good.

Furthermore, the dried silver powder is crushed and classified. Even if the silver powder after the surface treatment described above is further aggregated between the aggregates by subsequent drying or the like, since the binding force is weak, it is easily separated into aggregates of a predetermined size at the time of preparing the paste. However, in order to stabilize the paste, it is preferable to crush and classify. The crushing method is not particularly limited, but it is preferable to use an apparatus having a weak crushing force such as a jet mill or a high-speed stirrer. In an apparatus with a strong crushing force, even the above-mentioned aggregates are crushed or the silver powder is deformed, which is not preferable. Moreover, what is necessary is just to adjust to the grade which the formed aggregate is maintained as crushing conditions. The classifying device is not particularly limited, and an airflow classifier, a sieve, or the like can be used.

Hereinafter, specific examples of the present invention will be described. However, the present invention is not limited to the following examples.

[Example 1]
A silver complex solution was prepared by adding 2492 g of silver chloride (manufactured by Sumitomo Metal Mining Co., Ltd.) with stirring to 36 L of 25% aqueous ammonia maintained at a liquid temperature of 36 ° C. in a 38 ° C. bath. The complex solution was kept at 36 ° C. in a warm bath.

On the other hand, 1068 g of reducing agent ascorbic acid (manufactured by Kanto Chemical Co., Ltd., reagent) was dissolved in 14 L of pure water at 36 ° C. to obtain a reducing agent solution. Next, 64.1 g of polyvinyl alcohol which is a water-soluble polymer (manufactured by Kuraray Co., Ltd., PVA205, 3.5% by mass with respect to silver) is dissolved in 550 ml of pure water at 36 ° C. and then mixed with the reducing agent solution. did.

The prepared silver complex solution and the reducing agent solution are fed into the mixing tube at a silver complex solution of 2.7 L / min and a reducing agent solution of 0.9 L / min using a MONO pump (manufactured by Hyojin Equipment Co., Ltd.). The silver complex was reduced. The reduction rate at this time is 127 g / min in terms of silver. The ratio of the reducing agent supply rate to the silver supply rate was 1.4. A PVC pipe having an inner diameter of 25 mm and a length of 725 mm was used as the mixing tube. The slurry containing silver particles obtained by reduction of the silver complex was received in a receiving tank with stirring.

Thereafter, 0.88 g of a polyoxyethylene-added quaternary ammonium salt that is a commercially available cationic surfactant as a surface treatment agent is added to the silver particle slurry obtained by reduction (trade name: Silasol G-265, manufactured by Croda Japan Co., Ltd.). , 0.048% by mass with respect to silver particles) and 16.47 g of stearic acid emulsion as a dispersant (manufactured by Chukyo Yushi Co., Ltd., Cellosol 920, 0.90% by mass with respect to silver particles), 60 Surface treatment was performed by stirring for a minute. After the surface treatment, the silver particle slurry was filtered using a filter press, and the silver particles were solid-liquid separated.

Subsequently, before the collected silver particles are dried, the silver particles are put into 23 L of a 0.2 mass% sodium hydroxide (NaOH) aqueous solution maintained at 40 ° C., washed with stirring for 15 minutes, and then filtered. And silver particles were collected.

Next, the collected silver particles were put into 23 L of pure water kept at 40 ° C., stirred and filtered, and then the silver particles were transferred to a stainless steel pad and dried at 60 ° C. for 10 hours in a vacuum dryer. Subsequently, the dried silver particles were crushed at a peripheral speed of 22.7 m / sec using a 5 L high-speed stirrer (manufactured by Nippon Coke Industries, Ltd., FM5C). After the pulverization treatment, the silver particles were removed using a gas stream classifier (Nippon Mining Co., Ltd., EJ-3) to remove coarse particles with a classification point of 7 μm to obtain silver particles.

For the obtained silver particles, the cohesive force was measured using a powder layer shear force measuring device (NS-S300, manufactured by Nano Seeds Co., Ltd.). The measurement was performed by continuously using 18 g of silver powder in a measurement container having an inner diameter of 15 mm and setting the applied load to 20 N, 40 N, and 60 N, and putting the silver powder. At this time, the indentation speed to the silver powder is 0.2 mm / sec. When the set load is reached, the indentation is stopped, and after waiting for 100 sec from there, the side slide is performed at a speed of 10 μm / sec for measuring the shear force. Start and measure shear force. The shearing force sampling frequency was 10 Hz. From the maximum value of the shear force and the vertical load immediately before the start of the sideslip, obtain the shear stress and normal stress at each set applied load of 20N, 40N, and 60N, create a graph of the shear stress against the normal stress, A linear relational expression using multiplication was obtained. As a result, the cohesive force corresponding to the Y slice was 0.37 N / cm ² . The compressibility was a value when the applied load was 60 N, and was 30.1%. In addition, the amount of dibutyl phthalate absorbed measured by the JIS-K6217-4 method using an absorption measuring device (manufactured by Asahi Research Institute) was 6.9 ml / 100 g.

Moreover, the average particle diameter DS of the silver powder measured by SEM observation was 1.12 μm. Further, the volume average particle diameter D50 measured by using a laser diffraction scattering method in which silver powder was dispersed in isopropyl alcohol was 2.37 μm. Therefore, the ratio calculated | required by D50 / DS was 2.12. The specific surface area SSA ₁ measured by the BET method is 0.42 m ² / g, the specific surface area SSA ₂ obtained from the average particle diameter DS obtained by SEM observation is 0.51 m ² / g, and SSA _The ratio determined by ₁ / SSA ₂ was 0.82.

Next, 80% by mass of the obtained silver powder and 20% by mass of an epoxy resin (Mitsubishi Chemical Co., Ltd., 819) were weighed, and a self-revolving mixer (ARE-250, manufactured by Shinkey Co., Ltd.) was used. The mixture was kneaded with a centrifugal force of 420 G to form a paste, and further evaluated by kneading using a three-roll mill (Bueller Co., Ltd., three-roll mill SDY-300). During the kneading by the three roll mill, the occurrence of flakes by visual observation was not recognized, and the kneading property was good.

The viscosity of the obtained paste at a shear rate of 4 sec ⁻¹ was determined using a viscoelasticity measuring device (Anton Paar, MCR-301). The viscosity η1 after kneading by the self-revolving mixer is 62.7 (Pa · s), the viscosity η2 after kneading by the three roll mill is 56.3 (Pa · s), and the ratio obtained by η2 / η1 is 0.90.

Also, 2 g of the paste kneaded by a three roll mill was put into 40 ml of isopropyl alcohol and subjected to ultrasonic dispersion, followed by suction filtration using a sieve having an opening of 20 μm, and the particles on the sieve were collected to obtain 500 times The number was measured from the SEM image. As a result, there were two particles of 20 μm or more. Further, when the paste was dispersed in isopropyl alcohol and the average particle size of volume integration was measured using a laser diffraction scattering method, the average particle size D1 of the paste after kneading by the self-revolving mixer was 2.35 μm, 3 roll mill The later average particle diameter D2 was 2.10 μm, and the ratio determined by D2 / D1 was 0.89.

As described above, in Example 1, it was confirmed that almost no flakes were generated in the paste due to the formation of aggregates in which silver particles were linked to a predetermined size. Further, it was found that there was little change in viscosity and average particle size after kneading with the self-revolving mixer and after kneading with the three-roll mill, and the aggregate structure of the aggregate was maintained.

[Example 2]
In Example 2, silver powder was produced in the same manner as in Example 1 except that the amount of polyvinyl alcohol, which is a water-soluble polymer, was 183 g (10% by mass with respect to silver).

As a result of evaluating the obtained silver powder in the same manner as in Example 1, the cohesive force was 0.14 N / cm ² and the compression rate was 35.0%. The absorbed amount of butyl phthalate measured by JIS-K6217-4 method was 7.0 ml / 100 g.

Moreover, the average particle diameter DS of the silver powder measured by SEM observation was 1.05 μm. The volume average particle diameter D50 measured by using a laser diffraction scattering method in which silver powder was dispersed in isopropyl alcohol was 2.16 μm. Therefore, the ratio calculated | required by D50 / DS was 2.06. The specific surface area SSA ₁ as measured by the BET method was 0.46 m ² / g, average particle specific surface area SSA ₂ obtained from the diameter DS obtained by the SEM observation is 0.55 m ² / g, SSA _The ratio determined by ₁ / SSA ₂ was 0.84.

Next, in the same manner as in Example 1, a paste was prepared using the obtained silver powder. Also in Example 2, when flaking was made using a three-roll mill, visual flake generation was not recognized, and the kneadability was good.

When the viscosity of the obtained paste was determined at a shear rate of 4 sec ⁻¹ using a viscoelasticity measuring apparatus (Anton Paar, MCR-301), the viscosity η1 after kneading with a self-revolving mixer was 58.6 (Pa · s), the viscosity η2 after kneading with a three-roll mill was 46.4 (Pa · s), and the ratio determined by η2 / η1 was 0.79.

Further, it was confirmed that the number of flakes on the 20 μm-opening sieve was 6 after 2 g of paste kneaded by a three roll mill was put into 40 ml of isopropyl alcohol and subjected to ultrasonic dispersion. Further, when the paste was dispersed in isopropyl alcohol and the volume average particle size was measured using a laser diffraction scattering method, the average particle size D1 of the paste after kneading by the self-revolving mixer was 2.14 μm, after three roll mills The average particle diameter D2 was 2.13 μm, and the ratio determined by D2 / D1 was 0.99.

As described above, in Example 2, as in Example 1, it was confirmed that almost no flakes were generated in the paste due to the formation of aggregates in which silver particles were linked to a predetermined size. Further, it was found that there was little change in viscosity and average particle size after kneading with a self-revolving mixer and after kneading with a three-roll mill, and the aggregate structure of the aggregate was maintained.

[Comparative Example 1]
In Comparative Example 1, silver powder was produced in the same manner as in Example 1 except that the amount of polyvinyl alcohol, which is a water-soluble polymer, was 329.4 g (18% by mass with respect to silver).

The obtained silver powder was evaluated in the same manner as in Examples 1 and 2. As a result, the cohesive force was 0.80 N / cm ² and the compression rate was 38.1%. Further, the absorption amount of butyl phthalate measured by the JIS-K6217-4 method was 2.5 ml / 100 g.

Moreover, the average particle diameter DS of the silver powder measured by SEM observation was 1.04 μm. Further, the volume average particle diameter D50 measured by using a laser diffraction scattering method in which silver powder was dispersed in isopropyl alcohol was 1.51 μm. Therefore, the ratio calculated | required by D50 / DS was 1.45. The specific surface area SSA ₁ as measured by the BET method is 0.62 meters ² / g, average particle specific surface area SSA ₂ obtained from the diameter DS obtained by the SEM observation is 0.55 m ² / g, SSA _The ratio determined by ₁ / SSA ₂ was 1.13.

Next, in the same manner as in Examples 1 and 2, a paste was prepared using the obtained silver powder. Then, when it was made into a paste using a three-roll mill, visual flake generation was observed.

When the viscosity of the obtained paste was determined at a shear rate of 4 sec ⁻¹ using a viscoelasticity measuring apparatus (Anton Paar, MCR-301), the viscosity η1 after kneading with a self-revolving mixer was 42.8 (Pa · s), the viscosity η2 after kneading with a three-roll mill was 38.1 (Pa · s), the ratio obtained by η2 / η1 was 0.89, and the change in viscosity was small.

Also, after 2 g of paste kneaded by a three-roll mill was put into 40 ml of isopropyl alcohol and subjected to ultrasonic dispersion, the number of flakes on the opening 20 μm sieve was 36, and many flakes were generated. confirmed. Further, when the paste was dispersed in isopropyl alcohol and the volume average particle diameter was measured using a laser diffraction scattering method, the average particle diameter D1 after the revolution mixer was 1.62 μm, and the average particle diameter after three roll mills. D2 was 1.56 μm, the ratio calculated by D2 / D1 was 0.96, D1 and D2 were almost the same, and silver powder was dispersed by kneading with a revolving mixer.

As described above, in Comparative Example 1, although there was no sudden decrease in viscosity or dispersibility, a large amount of flakes were generated, resulting in a decrease in kneadability. This is probably because the formation of aggregates was not sufficient, and the strength of the aggregated structure was weak, and it was easily released by kneading, resulting in excessive aggregation.

[Comparative Example 2]
In Comparative Example 2, silver powder was produced in the same manner as in Example 1 except that the amount of polyvinyl alcohol, which is a water-soluble polymer, was 0.92 g (0.05% by mass with respect to silver).

The obtained silver powder was evaluated in the same manner as in Examples 1 and 2. As a result, the cohesive strength was −0.82 N / cm ² and the compression rate was 18.4%. The absorbed amount of butyl phthalate measured by JIS-K6217-4 method was 14.8 ml / 100 g.

Moreover, the average particle diameter DS of the silver powder measured by SEM observation was 1.02 μm. The volume average particle diameter D50 measured by using a laser diffraction scattering method in which silver powder was dispersed in isopropyl alcohol was 5.92 μm. Therefore, the ratio calculated | required by D50 / DS was 5.80. The specific surface area SSA ₁ measured by the BET method is 0.12 m ² / g, the specific surface area SSA ₂ determined from the average particle diameter DS obtained by SEM observation is 0.56 m ² / g, and SSA _The ratio determined by ₁ / SSA ₂ was 0.21.

Next, in the same manner as in Examples 1 and 2, a paste was prepared using the obtained silver powder. Then, it became a very hard paste by kneading with a self-revolving mixer. Furthermore, when kneading was performed using a three-roll mill, generation of flakes was confirmed during kneading.

When the viscosity of the obtained paste was determined at a shear rate of 4 sec ⁻¹ using a viscoelasticity measuring apparatus (Anton Paar, MCR-301), the viscosity η1 after kneading with a self-revolving mixer was 211.3 (Pa · s), the viscosity η2 after kneading with a three-roll mill was 95.1 (Pa · s), and the ratio determined by η2 / η1 was 0.45.

In addition, after 2 g of paste kneaded by a three-roll mill was put into 40 ml of isopropyl alcohol and subjected to ultrasonic dispersion, the number of flakes on the opening 20 μm sieve was 134, and particularly large flakes exceeding 50 μm were generated. It was confirmed that Further, when the paste was dispersed in isopropyl alcohol and the volume average particle size was measured using a laser diffraction scattering method, the average particle size D1 after the auto-revolving mixer was 5.94 μm, and the average particle size after three roll mills D2 was 2.49 μm, and the ratio determined by D2 / D1 was 0.42.

As described above, in Comparative Example 2, a large amount of flakes were generated in the paste, and the viscosity was suddenly lowered and the dispersibility was lowered. As a result, pasting was difficult and the kneadability was significantly lowered. It was. This is presumably because a large amount of excessively aggregated large agglomerates were formed, resulting in silver powder that was difficult to unravel.

Table 1 below summarizes the evaluation results in each example and comparative example.

The silver powders of Examples 1 and 2 and Comparative Example 1 were used to knead an epoxy resin, a curing agent (phenol resin) and a solvent (trimethylene glycol, diprolene glycol), and a commercially available general type A silver paste was prepared and kneadability was confirmed. For kneading, pre-kneading and main kneading were performed using a mass production type kneader and three rolls.

As a result, the pastes produced using the silver powders of Examples 1 and 2 showed almost no flakes and showed good kneading properties. On the other hand, the paste produced using the silver powder of Comparative Example 1 produced many flakes and poor kneadability.

As can be seen from the above results, the silver powder has a cohesive force of −0.2 or more and 0.7 N / cm ² or less, a compressibility in the measurement of the powder layer shear force of 20 to 50%, and JIS-K6217. The absorption of dibutyl phthalate measured by the -4 method is 3.0 to 9.0 ml / 100 g, so that the generation of flakes can be effectively suppressed during paste preparation, and good kneading properties are exhibited. I understood that I could do it. It was also found that an appropriate viscosity can be maintained and excellent printability can be realized.

Claims (11)

1. Cohesion is at -0.2N / cm ² or more 0.7 N / cm ² or less, the compression ratio in the powder layer shear force measurement is 20-50%, and phthalic acid as measured by JIS-K6217-4 method A silver powder characterized by having an absorption of dibutyl of 3.0 to 9.0 ml / 100 g.
2. When the average particle diameter of volume integration measured using the laser diffraction scattering method is D50, and the average particle diameter obtained by image analysis of SEM is DS, the ratio obtained by D50 / DS is 1.5 to 5. The silver powder according to claim 1, wherein the silver powder is zero.
3. When the specific surface area obtained by the BET method is SSA ₁ and the specific surface area obtained from the average particle diameter obtained by image analysis of SEM is SSA ₂ , the ratio obtained by SSA ₁ / SSA ₂ is less than 1.0. The silver powder according to claim 1 or 2, wherein:
4. The silver powder in the paste obtained by kneading the silver powder and the epoxy resin with a centrifugal force of 420 G is D1 as an average particle diameter of volume integration measured using a laser diffraction scattering method, and then kneaded with a three-roll mill. The ratio obtained by D2 / D1 is 0.5 to 1.5, where D2 is the average particle diameter of volume integration measured using a laser diffraction scattering method for the silver powder in the paste obtained. The silver powder according to any one of claims 1 to 3.
5. The paste obtained by kneading the silver powder and the epoxy resin with a centrifugal force of 420 G was obtained by kneading with a three-roll mill after setting the viscosity at a shear rate of 4 sec ⁻¹ measured with a viscoelasticity measuring device to η1. The ratio obtained by η2 / η1 is 0.5 to 1.5 when the viscosity at a shear rate of 4 sec ⁻¹ measured by a viscoelasticity measuring device is η2, and the ratio is 0.5 to 1.5. The silver powder of any one of Claims.
6. In a silver powder production method of producing a silver powder by mixing a silver complex-containing solution obtained by dissolution with silver chloride and a complexing agent and a reducing agent solution, and reducing the silver complex to produce silver powder,
   To the silver complex-containing solution and the reducing agent solution, or both of them, 0.1 to 15% by mass of a water-soluble polymer based on silver is added, and after the reduction, before the drying, A method for producing silver powder, characterized by surface-treating with a surfactant and a dispersant.
7. The surface treatment is performed in a state where at least an amount of a water-soluble polymer capable of suppressing aggregation of silver particles remains on the surface of the silver particles before or after one cleaning. A method for producing silver powder.
8. The method for producing silver powder according to claim 7, wherein the washing is performed using an aqueous sodium hydroxide solution of 0.01 to 1.00 mol / l.
9. The method for producing silver powder according to any one of claims 6 to 8, wherein the water-soluble polymer is at least one selected from polyethylene glycol, polyvinyl alcohol, polyethylene oxide and polyvinylpyrrolidone.
10. The method for producing silver powder according to any one of claims 6 to 9, wherein the water-soluble polymer is previously added to the reducing agent solution.
11. The method for producing silver powder according to any one of claims 6 to 10, wherein the reducing agent solution is a solution containing at least one selected from ascorbic acid, hydrazine and formalin.

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