Spherical silver powder, method for producing spherical silver powder, and conductive paste

Abstract

The purpose of the present invention is to provide a spherical silver powder that can impart an excellent low-temperature sinterability to conductive pastes. A first embodiment of the present invention is a spherical silver powder that contains silver particles that have closed voids in the particle interior, wherein: when at least 10 fields of view of a silver particle cross section are observed at 100,000X, the number per the cross-sectional area of voids A having a Heywood diameter of at least 4 nm is at least 70 / μm2 and not more than 500 / μm2, and the ratio, to the number of voids A per the cross-sectional area, of the number per the cross-sectional area of voids a having a Heywood diameter of at least 4 nm and less than 30 nm is at least 90%.

Description

Spherical silver powder, method for producing spherical silver powder, and conductive paste

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

[0002] A method of forming a conductive film such as an electrode or electrical wiring by applying or printing a conductive paste containing a conductive metal powder onto a substrate such as a film, a board, or an electronic component, and then heating it to dry, harden, or bake it has been widely used. However, with the recent increase in the performance of electronic devices, conductive films formed using conductive pastes are required to have lower resistance, and this requirement is becoming stricter every year.

[0003] In response to the above requirements, for example, Patent Document 1 discloses a silver powder having a volume resistivity lower than that of conventional silver powders when used as a conductive paste, and an apparent density of 8.2 g / cm ３ 9.2g / cm or more ３ and a silver powder has been proposed in which the ratio of the length of the outer peripheral line in the cross section of a silver particle to the length of the line circumscribing the periphery of the cross section of the particle is 1.1 or more and 1.4 or less.

[0004] International Publication No. 2023 / 054405

[0005] In recent years, in addition to lowering the resistance value of the conductive film, it has become desirable to sinter the conductive paste at a low temperature (for example, 200° C. or lower) in order to reduce damage to the substrate and the like.

[0006] Therefore, an object of the present invention is to provide a spherical silver powder that can impart excellent low-temperature sintering properties to a conductive paste. Another object of the present invention is to provide a method for producing a spherical silver powder that can impart excellent low-temperature sintering properties to a conductive paste. Another object of the present invention is to provide a conductive paste that has excellent low-temperature sintering properties.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and as a result have completed the present invention described below.

[0008] That is, the gist and configuration of the present invention for solving the above-mentioned problems is as follows.

[0009] [1] A spherical silver powder containing silver particles having closed voids therein, wherein when a cross section of the silver particle is observed at 100,000 magnifications in 10 or more fields of view, the number of voids A having a Heywood diameter of 4 nm or more per cross-sectional area is 70 / μm ２ More than 500 pieces / μm ２ or less, and the ratio of the number of voids a having a Heywood diameter of 4 nm or more and less than 30 nm per cross-sectional area to the number of voids A per cross-sectional area is 90% or more.

[0010] [2] A spherical silver powder containing silver particles having closed voids therein, wherein when a cross section of the silver particle is observed at 100,000 magnifications in 10 or more fields of view, the number of voids B having a Heywood diameter of 10 nm or more per cross-sectional area is 30 / μm ２ More than 450 pieces / μm ２ or less, and the ratio of the number of voids b having a Heywood diameter of 10 nm or more and less than 30 nm per cross-sectional area to the number of voids B per cross-sectional area is 90% or more.

[0011] [3] The spherical silver powder according to [1] or [2], wherein the maximum void diameter is 50 nm or less.

[0012] [4] The spherical silver powder according to any one of [1] to [3], wherein the average void diameter is 4 nm or more and less than 30 nm.

[0013] [5] The spherical silver powder according to any one of [1] to [4], which has a thermal shrinkage rate of 15% or more from room temperature to 900°C.

[0014] [6] BET specific surface area is 0.1 m ２ / g or more 1.8m ２ / g or less.

[0015] [7] Volume-based cumulative 10% particle diameter D by laser diffraction method １０ , cumulative 50% particle diameter D ５０ , and cumulative 90% particle diameter D ９０ is satisfied by the following formula (1): 0.5≦(D ９０ -D １０ ) / D ５０The spherical silver powder according to any one of [1] to [6], which satisfies the relationship: ≦2.5 (1).

[0016] [8] Volume-based cumulative 50% particle diameter D by laser diffraction method ５０ The spherical silver powder according to any one of [1] to [7], wherein the particle size is 0.5 μm or more and 6 μm or less.

[0017] [9] Volume-based cumulative 100% particle diameter D by laser diffraction method ＭＡＸ The spherical silver powder according to any one of [1] to [8], wherein the diameter of the spherical silver powder is 15 μm or less.

[0018]

[10] A method for producing spherical silver powder, comprising: a silver complex formation step of adding ammonia, a first chelating agent comprising ethylenediaminetetraacetic acid, and a second chelating agent comprising a polymer to a silver-containing aqueous solution to obtain a silver complex aqueous solution; and a reduction step of adding a reducing agent to the silver complex aqueous solution to reduce and precipitate silver particles.

[0019]

[11] The method for producing spherical silver powder according to

[10] , wherein the amount of the second chelating agent added is 0.1 parts by mass or more per 100 parts by mass of silver in the silver-containing aqueous solution.

[0020]

[12] The method for producing spherical silver powder according to

[10] or

[11] , wherein the amount of the first chelating agent added is 3 parts by mass or more and 40 parts by mass or less per 100 parts by mass of silver in the silver-containing aqueous solution.

[0021]

[13] The method for producing spherical silver powder according to any one of

[10] to

[12] , wherein a surface treatment agent is added to the precipitated silver particles after the reduction step.

[0022]

[14] A conductive paste containing the spherical silver powder according to any one of [1] to [9] as a conductive filler.

[0023] According to the present invention, a spherical silver powder capable of imparting excellent low-temperature sintering properties to a conductive paste can be provided. Furthermore, according to the present invention, a method for producing a spherical silver powder capable of imparting excellent low-temperature sintering properties to a conductive paste can be provided. Furthermore, according to the present invention, a conductive paste with excellent low-temperature sintering properties can be provided.

[0024] FIG. 1 is a graph showing thermomechanical analysis results for spherical silver powders according to Examples 1 to 4 and Comparative Examples 1 to 3. FIG. 2 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Example 1. FIG. 3 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Example 2. FIG. 4 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Example 3. FIG. 5 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Example 4. FIG. 6 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Comparative Example 1. FIG. 7 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Comparative Example 2. FIG. 8 is an example of an SEM image, at 100,000x magnification, of a cross section of a silver particle in a spherical silver powder according to Comparative Example 3.

[0025] The spherical silver powder of the present invention is suitable as a conductive filler for conductive pastes. Conductive pastes using the spherical silver powder of the present invention can be used for forming conductive patterns on substrates, or for forming or joining electrodes. Conductive pastes using the spherical silver powder of the present invention can be printed on substrates by, for example, screen printing, offset printing, photolithography, or the like, to form conductive films such as conductive patterns and electrodes. Furthermore, components can be joined via the paste printed on the substrate.

[0026] (Terminology and Measurement Methods) First, prior to describing the embodiments, the terminology and measurement methods used in this specification will be described.

[0027] <Confirmation of spherical silver powder (particle shape)> In this specification, spherical silver powder means silver powder in which the average shape factor of 100 or more particles observed by image analysis based on scanning electron microscope (SEM) images is in the range of 1.0 or more but less than 1.7. Note that the shape factor in this specification is the ratio of the area of ​​a virtual circle whose diameter is the average maximum length of 100 or more particles observed by the image analysis to the average particle area of ​​the silver particles obtained by tracing the outer shapes of the particles, and is the value obtained by dividing the area of ​​the virtual circle by the average particle area. The shape factor is calculated by π(average maximum length / 2) ２ / average particle area.

[0028] <Cross-section observation of silver particles and voids> The cross-sections of silver particles and voids contained in the spherical silver powder were observed by embedding the silver particles in a resin, polishing them with a cross-section polisher or the like to expose the cross-sections of the silver particles, and then observing the cross-sections of the particles at 100,000 magnifications using a field emission scanning electron microscope (FE-SEM) or the like.

[0029] In the present invention, the term "closed voids" or "voids" present inside silver particles means that when a cross section of a silver particle is observed, the voids observed inside the particle do not have any connecting portion from the outer periphery of the particle to the outside of the particle, and are voids that are closed inside the particle.

[0030] <Method for photographing silver particle cross sections and cross-sectional area> In the present invention, the cross sections of a large number of exposed silver particles are first observed at 5000x magnification. Then, cross sections of silver particles that are thought to have cross-sectional areas above average are selected, and for any silver particle among them, the magnification is increased so that the particle center is at the center of the field of view, and an SEM image at 100,000x magnification is obtained. In the 100,000x SEM image of the silver particle cross sections thus obtained, if the cross section of one particle falls entirely within a single field of view, the cross section of that one particle is measured. At this time, even if fragments of the cross sections of other particles fall within the field of view, they are not included in the measurement. If the cross section of one particle does not fit within a single field of view, only the area within that field of view is measured for the cross section of that particle. In other words, the outer diameter of the silver particle is traced with a pointer, excluding parts other than the silver particle. If the outer diameter of the silver particle extends beyond the field of view, the cross-sectional area is calculated by treating the entire field of view as the cross section of that silver particle. At this time, voids outside the field of view are not counted. The area of ​​the entire field of view of an SEM image at 100,000 magnifications is calculated from the scale bar to be 1.2 μm ２ is.

[0031] <Void Area and Heywood Diameter (Void Diameter)> Using image analysis software (for example, Mac-View, an image analysis particle size distribution measurement software manufactured by Mountec Co., Ltd.), the periphery of a void (closed void not connected to the periphery of the silver particle) observed in the photographed cross section of the silver particle was traced with a pointer on the screen displaying the image, and the area of ​​the void within the closed area traced in one stroke was calculated, and the Heywood diameter of the void (hereinafter, the Heywood diameter of the void may be referred to as the "void diameter") was also calculated. Note that, when a void is partially cut off by the field of view frame of the SEM image, the Heywood diameter is unknown and therefore is not considered to be a void.

[0032] <Average void diameter> The average void diameter is the average value of void diameters, and is the ratio of the total void diameter to the total number of voids (total void diameter / total number of voids) in cross-sectional observation of 10 or more fields of view of SEM images of silver particles at 100,000x magnification.

[0033] <Maximum pore diameter> The maximum pore diameter is the largest pore diameter among pore diameters observed in cross-sections of silver particles in 10 or more fields of view of SEM images at 100,000 times magnification.

[0034] <Porosity> The porosity (%) is expressed as the average value of the ratio of the total area of ​​voids to the area of ​​the cross-section of the silver particle that was the subject of cross-sectional observation in 10 or more fields of view of SEM images of the silver particle at 100,000 times magnification (total area of ​​voids / cross-section area of ​​silver particle × 100 [%]).

[0035] <Thermal shrinkage from room temperature to 900°C> First, 0.3 g of spherical silver powder was weighed. Next, the spherical silver powder was placed in a predetermined mold with a diameter of 5 mm and pressed for 1 minute using a press under a load of 50 kg to prepare a cylindrical measurement sample. This measurement sample was set in the sample holder of a thermomechanical analysis (TMA) device (Thermo plus EVO 2 series TMA8311), and a measurement load of 98 mN was applied using the measurement probe. The temperature was raised from room temperature (25°C ± 5°C) to 900°C at a heating rate of 10°C / min to perform thermomechanical analysis (TMA) of the measurement sample. The thermal shrinkage from room temperature to 900°C (hereinafter sometimes simply referred to as "thermal shrinkage") was calculated using the following formula (2): Thermal shrinkage from room temperature to 900°C (%) = (L ＲＴ -L ９００ ) / L ＲＴ × 100... (2) where L ＲＴ is the length (mm) of the cylindrical sample in the axial direction at room temperature (25°C ± 5°C), and L ９００ is the length (mm) of the cylindrical measurement sample in the axial direction at a temperature of 900°C.

[0036] <BET Specific Surface Area> The "BET specific surface area" was measured using a specific surface area measuring device employing the BET method (Macsorb HM-model 1210, manufactured by MOUNTECH Corp.) by placing 3 g of spherical silver powder in a measurement cell, passing a carrier gas mixture of 70 vol% He gas and 30 vol% nitrogen gas through the measurement cell at 25 mL / min, and degassing the cell at 60°C for 10 minutes, followed by measurement by the BET single-point method.

[0037] <Particle size distribution> In this specification, the volume-based cumulative 10% particle diameter D １０ , cumulative 50% particle diameter D ５０ , cumulative 90% particle diameter D ９０ , and cumulative 100% particle diameter D ＭＡＸwas measured using a laser diffraction / scattering particle size distribution analyzer (Microtrac MT-3300 EXII, manufactured by Microtrac Bell Corporation). For the measurement, 0.1 g of sample (spherical silver powder) was added to 40 mL of isopropyl alcohol (IPA) and dispersed. An ultrasonic homogenizer (manufactured by Nippon Seiki Seisakusho, device name: US-150T; 19.5 kHz, tip diameter 18 mm) was used for dispersion. The dispersion time was 2 minutes. The dispersed sample was subjected to the above-mentioned device, and the particle size distribution was determined using the attached analysis software. Note that an SDC device was used as the circulator of the laser diffraction / scattering particle size distribution analyzer during measurement, and the setting value of the "flow rate (%)" of the circulator was 60. Furthermore, the post-measurement calculation mode of the MT-3300 EXII was HRA mode. Note that, hereinafter, the cumulative 10% particle diameter D on a volume basis determined by laser diffraction method is １０ , cumulative 50% particle diameter D ５０ , cumulative 90% particle diameter D ９０ , and cumulative 100% particle diameter D ＭＡＸ , respectively, simply "D １０ "," "D ５０ "," "D ９０ " and "D ＭＡＸ "It is sometimes referred to as ".

[0038] <Ignition loss (Ig-loss)> In this specification, the term "ignition loss (Ig-loss) value" refers to the amount of change in mass when heated from room temperature to 800°C, and specifically serves as an index of the amount of components other than silver contained in the spherical silver powder, and is an index of the amount of components remaining in the spherical silver powder, such as processing agents and additives used in the manufacturing process of the spherical silver powder. In this specification, the "ignition loss (Ig-loss) value" is determined by precisely weighing a spherical silver powder sample (weighing value: w １ ) into a porcelain crucible, heated to 800°C, and held at 800°C for 30 minutes, sufficient time to reach a constant weight, after which it was cooled and reweighed (weight: w ２ ) and "Ignition loss (Ig-loss) value (mass%) = (w １ -w ２ ) / w １ × 100".

[0039] (Spherical silver powder) The spherical silver powder of the first embodiment of the present invention comprises silver particles having closed voids inside the particles. The spherical silver powder of the first embodiment has voids A (hereinafter sometimes simply referred to as "voids A") having a Heywood diameter of 4 nm or more and a cross-sectional area of ​​1 μm or more when the cross section of the silver particle is observed at 100,000 magnifications in 10 or more fields of view. ２ The number of hits (number per cross-sectional area) is 70 / μm ２ More than 500 pieces / μm ２ The ratio of the number of voids a having a Heywood diameter of 4 nm or more and less than 30 nm (hereinafter simply referred to as "voids a") per cross-sectional area of ​​1 μm ２ The ratio of the number of voids a (number per cross-sectional area) (number of voids a / number of voids A × 100 [%]) is 90% or more. In the spherical silver powder of the first form, the content of the above-mentioned specified silver particles is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably substantially 100% by mass, i.e., it is even more preferable that the spherical silver powder of the first form consists of the above-mentioned specified silver particles.

[0040] The spherical silver powder of the second embodiment of the present invention contains silver particles having closed voids inside the particles. When the cross section of the silver particle is observed at 100,000 magnifications in 10 or more fields of view, the spherical silver powder of the second embodiment has voids B (hereinafter sometimes simply referred to as "voids B") having a Heywood diameter of 10 nm or more and a cross-sectional area of ​​1 μm ２ The number of hits (number per cross-sectional area) is 30 / μm ２ More than 450 pieces / μm ２ The ratio of the number of voids b having a Heywood diameter of 10 nm or more and less than 30 nm (hereinafter simply referred to as "voids b") per cross-sectional area to the number of voids B per cross-sectional area is 1 μm ２ The ratio of the number of voids b (number per cross-sectional area) (number of voids b / number of voids B × 100 [%]) is 90% or more. In the spherical silver powder of the second form, the content of the specified silver particles is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably substantially 100% by mass, i.e., it is even more preferable that the spherical silver powder of the second form consists of the specified silver particles.

[0041] The spherical silver powder of the first or second form can impart excellent low-temperature sinterability to a conductive paste. The reason for this is presumably that, when heated at low temperatures, the silver particles contain a large number of voids of a predetermined size (voids a and / or voids b), which causes the gas in the voids to expand, thereby increasing the driving force for sintering. However, the reason for this is not entirely clear. However, based on the results of the following examples and comparative examples, it is clear that spherical silver powders that satisfy the above requirements can impart excellent low-temperature sinterability to a conductive paste. The spherical silver powder of the first form and the spherical silver powder of the second form can be obtained by the method for producing spherical silver powder of the present invention, which will be described later.

[0042] Here, the spherical silver powder of the present invention may be a combination of the first and second embodiments. That is, the spherical silver powder of the present invention contains silver particles having closed voids inside the particles, and when the cross section of the silver particles is observed at 100,000 magnifications in 10 or more fields of view, the number of voids A per cross-sectional area is 70 / μm ２ More than 500 pieces / μm ２ The ratio of the number of voids a per cross-sectional area to the number of voids A per cross-sectional area is 90% or more, and the number of voids B per cross-sectional area having a Heywood diameter of 10 nm or more is 30 / μm ２ More than 450 pieces / μm ２ or less, and the ratio of the number of voids b having a Heywood diameter of 10 nm or more and less than 30 nm per cross-sectional area to the number of voids B per cross-sectional area may be 90% or more.

[0043] In the spherical silver powder of the first embodiment, the number of voids A per cross-sectional area is 70 / μm ２ or more, and 100 pieces / μm ２ It is preferable that the number of particles is 500 / μm or more. ２ less than 400 particles / μm ２ In the spherical silver powder of the second embodiment, the number of voids A per cross-sectional area is more preferably 70 / μm ２ It is preferable that the number of particles is 100 or more per μm. ２ More preferably, the number is 500 / μm or more. ２Preferably, the number is 400 or less per μm. ２ It is more preferable that the number of voids A per cross-sectional area is 70 / μm or less. ２ If the number of voids A per cross-sectional area is 500 / μm or more, the silver particles will have well-dispersed voids therein, thereby improving the low-temperature sintering property. ２ If the content is equal to or less than this, an excessive decrease in the density of the particles can be prevented, and low-temperature sintering properties can be improved.

[0044] The ratio of the number of voids a per cross-sectional area to the number of voids A per cross-sectional area (number of voids a / number of voids A × 100 [%]) is preferably 90% or more, and more preferably 95% or more. If the ratio of the number of voids a per cross-sectional area to the number of voids A per cross-sectional area is less than 90%, the number of large voids will increase, and the density of the silver conductive film obtained by firing a paste made using that silver powder may be reduced.

[0045] In the spherical silver powder of the first embodiment, the number of voids B per cross-sectional area is 30 / μm ２ It is preferable that the number of particles is 60 or more per μm. ２ More preferably, the number is 450 / μm or more. ２ In the spherical silver powder of the second embodiment, the number of voids B per cross-sectional area is preferably 30 / μm or less, and more preferably 350 or less. ２ or more, 60 pieces / μm ２ It is preferable that the number of particles is 450 / μm or more. ２ The number of voids B per cross-sectional area is 30 / μm or less, and preferably 350 or less. ２ On the other hand, if the number of voids B per cross-sectional area is 450 / μm or more, the low-temperature sintering property can be improved. ２ If the content is equal to or less than this, an excessive decrease in the density of the particles can be prevented, and low-temperature sintering properties can be improved.

[0046] The ratio of the number of voids b per cross-sectional area to the number of voids B per cross-sectional area (number of voids b / number of voids B × 100 [%]) is preferably 90% or more, and more preferably 95% or more. If the ratio of the number of voids b per cross-sectional area to the number of voids B per cross-sectional area is less than 90%, the number of large voids will increase, and the density of the silver conductive film obtained by firing a paste made using that silver powder may be reduced.

[0047] In the spherical silver powder, the maximum void diameter is preferably 50 nm or less, and more preferably 45 nm or less. If the maximum void diameter is 50 nm or less in the number of voids A or voids B, there are no large voids, and a dense silver conductive film can be obtained by firing a paste made using that silver powder.

[0048] In the spherical silver powder, the average void diameter is preferably 4 nm or more, more preferably 10 nm or more, and is preferably less than 30 nm, more preferably 20 nm or less. When the average void diameter is 4 nm or more and less than 30 nm, silver particles having a large number of predetermined voids well dispersed inside the particles can be obtained.

[0049] In the spherical silver powder, the porosity of the silver particles is preferably 1.5% or more, more preferably 2.0% or more, and is preferably 8.0% or less, more preferably 6.0% or less. When the porosity of the silver particles is 1.5% or more and 8.0% or less, excessive density reduction of the particles can be prevented, and low-temperature sintering properties can be improved.

[0050] The thermal shrinkage of the spherical silver powder is preferably 15% or more, more preferably 18% or more, and is preferably 40% or less, and more preferably 30% or less. If the thermal shrinkage is 15% or more, a good conductive film can be obtained when a conductive paste containing the spherical silver powder is used to obtain a conductive film. On the other hand, if the thermal shrinkage is 40% or less, excessive shrinkage can be effectively suppressed.

[0051] The BET specific surface area of ​​the spherical silver powder is 0.1 m ２ / g or more, and２ / g or more, and ２ / g or less, and 1.5m ２ It is more preferable that the BET specific surface area of ​​the spherical silver powder is 0.1 m / g or less. ２ On the other hand, if the BET specific surface area of ​​the spherical silver powder is 1.8 m / g or more, the low-temperature sintering property can be improved. ２ / g or less, when the spherical silver powder is used in a conductive paste or the like, the viscosity of the resulting conductive paste or the like can be effectively reduced.

[0052] Spherical silver powder D １０ is preferably 0.2 μm or more, more preferably 0.5 μm or more, and is preferably 3 μm or less, more preferably 2 μm or less.

[0053] Spherical silver powder D ５０ is preferably 0.5 μm or more, more preferably 0.8 μm or more, and is preferably 6 μm or less, more preferably 3 μm or less.

[0054] Spherical silver powder D ９０ is preferably 1 μm or more, more preferably 2 μm or more, and is preferably 8 μm or less, more preferably 5 μm or less.

[0055] Spherical silver powder D ＭＡＸ is preferably 2 μm or more, more preferably 3 μm or more, and is preferably 15 μm or less, more preferably 10 μm or less.

[0056] Spherical silver powder D １０、 D ５０、 D ９０、 D ＭＡＸ When the spherical silver powder is used in a conductive paste or the like, if the D of the spherical silver powder is equal to or greater than the above lower limit, the viscosity of the resulting conductive paste or the like can be effectively reduced. １０ D ５０、 D ９０、 D ＭＡＸ When each of these is equal to or less than the above upper limit, the particle size of the spherical silver powder becomes favorable, and low-temperature sintering properties can be improved.

[0057] For spherical silver powder, the cumulative 10% particle diameter D on a volume basis measured by laser diffraction method １０ , cumulative 50% particle diameter D ５０ , and cumulative 90% particle diameter D ９０ is expressed by the following formula (1): 0.5≦(D ９０ -D １０ ) / D ５０ It is preferable that the relationship: ≦2.5 (1) is satisfied. ９０ -D １０ ) / D ５０ If the spherical silver powder is in the above range, when the spherical silver powder is used in a conductive paste or the like, the stability of the resulting conductive paste can be improved. ９０ -D １０ ) / D ５０ is more preferably 2.0 or less, and even more preferably 1.5 or less. (D ９０ -D １０ ) / D ５０ If the value is 2.5 or less, the uniformity of the silver particles is high, and when a conductive paste is prepared using the spherical silver powder, the conductive paste can be easily prepared.

[0058] The ignition loss (Ig-loss) value of the spherical silver powder is preferably 0.05% by mass or more, more preferably 0.2% by mass or more. It is also preferably 10% by mass or less, more preferably 5% by mass or less. If the ignition loss value of the spherical silver powder is 0.05% by mass or more, when the spherical silver powder is used in a conductive paste or the like, aggregation of the spherical silver powder in the conductive paste can be effectively suppressed. On the other hand, if the ignition loss value of the spherical silver powder is 10% by mass or less, the impurities are low, and therefore, when a conductive film is obtained using a conductive paste containing the spherical silver powder, an increase in the resistance value of the obtained conductive film can be effectively suppressed.

[0059] The shape of the silver particles is not particularly limited and can be appropriately selected depending on the purpose.

[0060] In one embodiment, the spherical silver powder of the present invention preferably contains a surface treatment agent. The surface treatment agent is not particularly limited as long as the spherical silver powder satisfies the above-mentioned specific requirements, but examples thereof include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, protective colloids, etc. Here, the surface treatment agent is preferably one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts, from the viewpoint of being able to adhere uniformly to the surface of the spherical silver powder and achieving high dispersibility.

[0061] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. These may be used alone or in combination of two or more. Examples of fatty acid salts include salts of the fatty acids listed above. Examples of salts include sodium salts and potassium salts.

[0062] Examples of the compound having an azole structure include benzotriazole, sodium salt of benzotriazole, potassium salt of benzotriazole, etc. These may be used alone or in combination of two or more.

[0063] (Method for Producing Spherical Silver Powder) The method for producing spherical silver powder of the present invention (hereinafter sometimes simply referred to as the "production method") includes a silver complex formation step in which a first chelating agent composed of ammonia, ethylenediaminetetraacetic acid, and a second chelating agent composed of a polymer are added to a silver-containing aqueous solution to obtain a silver complex aqueous solution, and a reduction step in which a reducing agent is added to the silver complex aqueous solution to reduce and precipitate silver particles. This production method can produce spherical silver powder that can impart excellent low-temperature sinterability to conductive pastes. The production method of the present invention may optionally include steps other than the silver complex formation step and the reduction step (hereinafter sometimes referred to as "other steps"). Examples of other steps include a pH adjuster addition step in which a pH adjuster is added to the silver complex aqueous solution before reduction, a surface treatment agent addition step in which a surface treatment agent is added to a mixed solution containing precipitated silver particles, and a separation step in which the silver particles or the surface treatment agent-coated silver particles (silver particles coated with a surface treatment agent) obtained in the surface treatment agent addition step are separated and dried.

[0064] <Silver Complex Formation Step> In the silver complex formation step, ammonia, a first chelating agent comprising ethylenediaminetetraacetic acid, and a second chelating agent comprising a polymer are added to a silver-containing aqueous solution to obtain a silver complex aqueous solution. The order in which the ammonia, the first chelating agent, and the second chelating agent are added to the silver-containing aqueous solution may be ammonia first, the first chelating agent first, or the second chelating agent first, or the ammonia, the first chelating agent, and the second chelating agent may be added simultaneously to the silver-containing aqueous solution. Although the ammonia, the first chelating agent, and the second chelating agent can all form a complex with silver, it is preferable to stir and mix them until they are complexed.

[0065] The silver-containing aqueous solution is not particularly limited, but may be a silver nitrate aqueous solution, a silver oxide-containing aqueous solution, etc. Among these, a silver nitrate aqueous solution is preferred.

[0066] Examples of the ammonia to be added to the silver-containing aqueous solution include aqueous ammonia and ammonium salts.

[0067] The amount of ammonia added is not particularly limited as long as a silver ammine complex is obtained as a complex constituting the silver complex aqueous solution, but it is preferably 38 parts by mass or more, more preferably 47 parts by mass or more, and preferably 78 parts by mass or less, more preferably 62 parts by mass or less, per 100 parts by mass of silver in the silver-containing aqueous solution.

[0068] The first chelating agent added to the silver-containing aqueous solution is ethylenediaminetetraacetic acid. Ethylenediaminetetraacetic acid may be in the form of an alkali metal salt. That is, ethylenediaminetetraacetic acid may have some or all of its four carboxylic acids in the form of alkali metal salts. From the viewpoint of solubility in the silver-containing aqueous solution, it is preferable to use ethylenediaminetetraacetic acid in the form of an alkali metal salt. Ethylenediaminetetraacetic acid in the form of an alkali metal salt may also be in the form of a hydrate.

[0069] Examples of ethylenediaminetetraacetic acid in the form of an alkali metal salt include disodium ethylenediaminetetraacetic acid, trisodium ethylenediaminetetraacetic acid, tetrasodium ethylenediaminetetraacetic acid, dipotassium ethylenediaminetetraacetic acid, tripotassium ethylenediaminetetraacetic acid, and tetrapotassium ethylenediaminetetraacetic acid. Among these, disodium ethylenediaminetetraacetic acid is preferred. The ethylenediaminetetraacetic acid may be used alone or in combination of two or more thereof.

[0070] The amount of the first chelating agent added is preferably 3 parts by mass or more, more preferably 3.5 parts by mass or more, and preferably 40 parts by mass or less, more preferably 36 parts by mass or less, per 100 parts by mass of silver in the silver-containing aqueous solution. When the amount of the first chelating agent added is within the above range, and the ammonia capable of complexing and the first chelating agent coexist in the silver complex aqueous solution, it becomes easier to suppress variations in the void distribution, and silver powder having a predetermined void density can be easily obtained without the voids concentrating in specific locations. In the present invention, the amount of the first chelating agent added is preferably 5 parts by mass or more, more preferably 6 parts by mass or more, and preferably 70 parts by mass or less, more preferably 65 parts by mass or less, per 100 parts by mass of ammonia in the silver-containing aqueous solution.

[0071] The second chelating agent added to the silver-containing aqueous solution is made of a polymer. Use of the second chelating agent made of a polymer can control the rate of particle aggregation and particle growth after the addition of a reducing agent, thereby suppressing variation in the pore size of pores generated inside the silver particles.

[0072] Specific examples of preferred second chelating agents include amino compounds and imine compounds. Among these, polyethyleneimine (PEI) is preferred. In particular, PEI, which is an imine compound, has a structure containing a primary amine (-NH ２ It is a network structure having both a hydroxyl group (=NH) and a secondary amine (=NH), which gives the preferred results in the present invention.

[0073] The second chelating agent preferably has a weight-average molecular weight of 600 or less, and more preferably 145 or more and 600 or less. This is because a weight-average molecular weight of 145 or more of the second chelating agent has the effect of producing highly dispersible silver particles. On the other hand, a weight-average molecular weight of 600 or less of the polymeric amine ensures the water solubility of the polymeric amine, and it is believed that the polymeric amine hardly remains on the surface or inside of the produced silver particles. The weight-average molecular weight of the second chelating agent can be measured by GPC-MALS.

[0074] The amount of the second chelating agent added is preferably 0.1 parts by mass or more, and more preferably 0.3 parts by mass or more, per 100 parts by mass of silver in the silver-containing aqueous solution. It is presumed that if the amount of the second chelating agent added is 0.1 parts by mass or more per 100 parts by mass of silver in the silver-containing aqueous solution, the silver particles are grown uniformly and spherically, effectively suppressing aggregation, thereby suppressing variation in the pore size of pores formed within the silver particles and preventing the occurrence of coarse pores. In the present invention, when the amount of the second chelating agent added is less than 0.1 parts by mass, uniformly dispersed spherical silver powder could not be obtained, and it was difficult to obtain silver powder with the desired pore size. The upper limit of the amount added is not particularly limited, but is, for example, 10 parts by mass or less, and preferably 3 parts by mass or less.

[0075] The temperature of the aqueous silver complex solution is preferably 5°C or higher, more preferably 20°C or higher, and preferably 50°C or lower, more preferably 40°C or lower. When the temperature of the aqueous silver complex solution is 5°C or higher, the reduction reaction can proceed effectively. On the other hand, when the temperature of the aqueous silver complex solution is 50°C or lower, the reaction rate of the reduction reaction described below can be effectively prevented from becoming excessive, and variation in the particle size of the silver particles can be effectively suppressed. Furthermore, when the temperature of the aqueous silver complex solution is within the above range, an increase in energy costs can be effectively suppressed.

[0076] <pH Adjuster Addition Step> In the pH adjuster addition step, a pH adjuster may be added to the aqueous silver complex solution before the addition of the reducing agent. Adding a pH adjuster to the aqueous silver complex solution before reduction can easily adjust the particle size of the resulting silver powder. Common acids or bases may be used as the pH adjuster, such as nitric acid and sodium hydroxide. The amount of pH adjuster added can be adjusted appropriately depending on the amount of aqueous silver nitrate solution used and the particle size of the silver powder to be obtained. Examples of methods for this adjustment include conducting a level test on the particle size of the silver powder depending on the amount of pH adjuster added and adjusting the amount added.

[0077] <Reduction Step> In the reduction step, a reducing agent is added to the stirred aqueous silver complex solution to reduce and precipitate silver particles, thereby obtaining a slurry in which silver particles are dispersed, which is a mixture containing silver particles.

[0078] The reducing agent to be added to the aqueous silver complex solution is not particularly limited, but examples thereof include hydrazine, formalin, sodium borohydride, glucose, hypophosphorous acid, etc. Among these, hydrazine is preferred.

[0079] In order to increase the reaction yield of silver, the amount of reducing agent added is preferably 1 equivalent or more relative to the silver of the reducing agent to be reacted. The term "equivalent" here refers to the molar equivalent, which represents the quantitative relationship in the chemical reaction between silver and the reducing agent. For example, in the case of hydrazine reduction, 1 equivalent of hydrazine per mole of silver is 0.25 moles. When using a reducing agent with weak reducing power, such as formalin or glucose, 2 equivalents or more relative to the silver are preferred, and 10 to 20 equivalents are more preferred.

[0080] <Surface Treatment Agent Addition Step> When the surface treatment agent addition step is performed, a surface treatment agent is added to a mixed solution containing silver particles precipitated in the reduction step. This results in silver particles coated with the surface treatment agent. The mixed solution containing silver particles coated with the surface treatment agent is usually a slurry in which silver particles coated with the surface treatment agent are dispersed.

[0081] Examples of surface treatment agents to be added to the mixture containing precipitated silver particles include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, protective colloids, etc. Here, from the viewpoint of being easily and uniformly attached to the silver powder surface, the surface treatment agent is preferably one or more surface treatment agents selected from the group consisting of fatty acids, compounds having an azole structure, and fatty acid salts.

[0082] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, and linolenic acid. These may be used alone or in combination of two or more. Examples of fatty acid salts include salts of the fatty acids listed above. Examples of salts include sodium salts and potassium salts.

[0083] Examples of the compound having an azole structure include benzotriazole, sodium salt of benzotriazole, potassium salt of benzotriazole, etc. These may be used alone or in combination of two or more.

[0084] The amount of surface treatment agent added to the mixed solution containing silver particles is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and preferably 3 parts by mass or less, more preferably 1 part by mass or less, relative to 100 parts by mass of silver in the silver-containing aqueous solution. If the amount of surface treatment agent added is 0.05 parts by mass or more relative to 100 parts by mass of silver in the silver-containing aqueous solution, the dispersibility of the silver particles can be improved. On the other hand, if the amount of surface treatment agent added is 3 parts by mass or less relative to 100 parts by mass of silver in the silver-containing aqueous solution, the risk of a decrease in low-temperature sinterability can be effectively suppressed.

[0085] The surface treatment agent is preferably added to the mixed solution containing silver particles after a specific time has elapsed since the addition of the reducing agent to the aqueous silver complex solution. In the present invention, the addition of the surface treatment agent stops the particle growth of the silver particles, so the timing of addition can be determined as long as it allows the silver in the mixed solution to be sufficiently precipitated as silver particles.

[0086] <Separation step> In the separation step, silver particles are separated from a mixed solution containing silver particles or silver particles coated with a surface treatment agent. After the separation step, a washing and recovery step can be performed, or a drying step can be performed without going through the washing and recovery step.

[0087] In the washing and recovery step, for example, a cake-like aggregate of separated silver particles or silver particles coated with a surface treatment agent is washed. Washing in the washing and recovery step may be performed using, for example, pure water. Dehydration in the washing and recovery step may be performed by, for example, decantation or a filter press. The end point of washing may be determined using the electrical conductivity of the washing water. Specifically, the end of washing may be determined when the electrical conductivity of the washing water becomes a predetermined value or less. After washing, the silver particles or silver particles coated with a surface treatment agent may be subjected to a drying step in an aggregated state such as a cake.

[0088] In the drying step, aggregates of silver particles containing moisture and in an agglomerated state or silver particles coated with a surface treatment agent are dried. The drying step may be performed by vacuum drying or using an airflow dryer. In the drying step, a high-pressure air flow may be blown onto the aggregates of silver particles or silver particles coated with a surface treatment agent, or the cake or spherical silver powder in the drying process may be placed in a mixer having a stirring rotor and stirred, thereby applying a dispersing force to the cake or spherical silver powder in the drying process and promoting dispersion and drying.

[0089] In the drying step, the temperature of the spherical silver powder is not particularly limited as long as it satisfies the conditions for sufficiently drying the aggregates of silver particles, but is preferably 40° C. or higher, more preferably 70° C. or higher, and preferably 120° C. or lower, and more preferably 100° C. or lower. If the temperature of the spherical silver powder is 40° C. or higher, the drying efficiency can be improved. On the other hand, if the drying temperature of the spherical silver powder is 120° C. or lower, there is little risk of impairing the thermal properties (such as thermal shrinkage) of the silver powder of the present invention having voids.

[0090] Since the spherical silver powder after drying may be in the form of lumps, it is preferable to carry out a dry crushing treatment or classification operation simultaneously with or after the drying step in order to improve the handleability of the spherical silver powder, etc. Here, improving the handleability of the spherical silver powder means, for example, ensuring fluidity to the extent that it does not interfere with the supply operation into the apparatus, or loosening the spherical silver powder to an appropriate extent so that processing in the apparatus proceeds efficiently.

[0091] The method for the dry crushing treatment is not particularly limited and can be appropriately selected depending on the purpose. However, it is preferable to use a crusher that rotates a stirring blade to crush the particles and fluidize the spherical silver powder, and for example, a sample mill, a blender, a coffee mill, or the like can be used.

[0092] (Conductive Paste) The conductive paste of the present invention contains the spherical silver powder of the present invention described above as a conductive filler. In addition to the spherical silver powder, the conductive paste preferably contains a solvent and a binder, and may further contain other components as necessary. The solvent, binder, etc. may be selected appropriately depending on the usage mode.

[0093] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples. The pore size (Heywood diameter of pores), porosity, average pore size, maximum pore size, heat shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were measured or calculated by the methods described above.

[0094] Example 1 <Silver Complex Formation Step> First, 3,436 g of a silver nitrate aqueous solution containing 50.84 g of silver was stirred at 174 rpm, and 103.2 g of 28% by weight aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added. Then, while continuing stirring, 30 seconds after the addition of the aqueous ammonia, 4.17 g of a 43% by weight aqueous EDTA solution (manufactured by Chelest, Chelest OD-50) as a first chelating agent (addition amount of EDTA per 100 parts by weight of silver: 3.53 parts by weight) was added to obtain an aqueous silver complex. Subsequently, 60 seconds after the addition of the aqueous ammonia, 10.17 g of a 5% by weight aqueous PEI solution (manufactured by Nippon Shokubai Co., Ltd., weight average molecular weight 600) as a second chelating agent (addition amount of PEI per 100 parts by weight of silver: 1.00 parts by weight) was added to obtain an aqueous silver complex, and the liquid temperature was adjusted to 35°C.

[0095] <Reduction Step> 180 seconds after the addition of the ammonia water, 302.8 g of an aqueous solution of hydrazine (manufactured by Mitsubishi Gas Chemical Company, Inc.) with a concentration of 2.43 mass % (amount of hydrazine added per 100 mass parts of silver: 14.47 mass parts) was added all at once to the obtained aqueous silver complex solution, thereby obtaining a slurry containing first silver particles.

[0096] <Surface Treatment Agent Addition Step> Next, stirring of the obtained slurry containing silver particles was stopped 5 seconds after the addition of hydrazine, and stirring was resumed 130 seconds after the addition of hydrazine. At the same time, 5.12 g of a 1.55% stearic acid emulsion (amount of stearic acid added per 100 parts by mass of silver: 0.16 parts by mass) was added as a surface treatment agent, and stirring was continued for an additional 75 seconds to obtain a slurry containing silver particles coated with the surface treatment agent. Note that a baffled reaction chamber and a two-stage turbine blade were used for the reaction.

[0097] <Separation Step> Thereafter, stirring was stopped to allow the silver particles coated with the surface treatment agent to settle, and the liquid in which the silver particles coated with the surface treatment agent had precipitated was filtered and washed with water until the electrical conductivity of the liquid after passing through the water was 0.5 mS / m or less, and then vacuum dried at 73° C. The dried silver powder obtained was milled using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) to which 50 g of silver had been added, and the mill was crushed twice for 30 seconds using a dial scale of 100, to obtain the spherical silver powder according to Example 1.

[0098] Using the obtained spherical silver powder, cross-sections of the silver particles were observed by the method described above, and the porosity, average void diameter, number of voids per size, and maximum void diameter were calculated. Furthermore, the thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were measured or calculated. The results are shown in Table 2. Furthermore, a graph of the thermomechanical analysis of the spherical silver powder according to Example 1 is shown in Figure 1, and a 100,000-magnification SEM image of the cross-section of a silver particle in the spherical silver powder according to Example 1 is shown in Figure 2. The cross-sectional area of ​​the silver particle illustrated in the center of Figure 2 is 0.54 μm ２ It was.

[0099] <Resistance Measurement> 93.01 parts by weight of the spherical silver powder obtained above, 0.25 parts by weight of ethyl cellulose, 1.59 parts by weight of Texanol, 3.86 parts by weight of butyl carbitol acetate, 0.26 parts by weight of tributyl citrate, 0.25 parts by weight of oleic acid, 0.26 parts by weight of triacetin, and 0.51 parts by weight of methylphenylpolysiloxane (KF96-100) were mixed to obtain a mixture. The resulting mixture was then premixed using a rotary / revolutionary mixer (revolution 1000 rpm) and kneaded using a three-roll mill (manufactured by EXAKT) with a roll gap ranging from 100 μm to 20 μm to obtain a conductive paste. A linear pattern was printed using the conductive paste obtained above by screen printing. The linear pattern had a design line width of 500 μm and a linear length of 128 mm. For printing, a Microtec printer was used, and printing was performed at a squeegee speed of 80 mm / sec. For printing, a silicon substrate with a thickness of about 170 μm (for solar cell applications, texture formation, SiN ｘ After printing, the conductive paste was dried for 10 minutes in a dryer set at 100°C to form a conductive film. The formed conductive film was measured for its resistance (unit: Ω) over a temperature rise rate of 10°C / min from room temperature to 300°C using a high-temperature microscope (manufactured by Yonekura Seisakusho Co., Ltd.). The resistance values ​​at 100°C, 140°C, 160°C, 180°C, 190°C, 195°C, and 200°C are shown in Table 3. The lower the resistance value shown in Table 3, the better the conductive paste's low-temperature sintering properties.

[0100] Examples 2 to 4 Spherical silver powders according to Examples 2 to 4 were obtained in the same manner as in Example 1, except that the amounts of the first chelating agent and the second chelating agent added were as shown in Table 1. Using the obtained spherical silver powders, cross-sectional observation of the silver particles was performed by the method described above, and the porosity, average void diameter, number of voids per size, and maximum void diameter were calculated. Furthermore, the thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were measured or calculated. The results are shown in Table 2. Furthermore, resistance measurements were performed using the obtained spherical silver powders. The results are shown in Table 3. Furthermore, a graph of the thermomechanical analysis of the spherical silver powders according to Examples 2 to 4 is shown in FIG. 1, and SEM images of the cross sections of silver particles in the spherical silver powders according to Examples 2 to 4 at 100,000 magnifications are shown in FIGS. 3 to 5, respectively. The cross-sectional area of ​​the silver particle illustrated in the center of FIGS. 3 to 5 is 0.76 μm. ２ , 0.53 μm ２ , 0.65 μm ２ It was.

[0101] Comparative Example 1 <Silver Complex Formation Step> First, 113.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to 3,489 g of an aqueous silver nitrate solution containing 53.71 g of silver while stirring at 332 rpm to obtain an aqueous silver complex solution. Next, with continued stirring, 4.98 g of a 20% by mass aqueous sodium hydroxide solution was added as a pH adjuster to the obtained aqueous silver complex solution, and the liquid temperature was adjusted to 18.5°C.

[0102] <Reduction Step> 180 seconds after the addition of the ammonia water to the solution with the adjusted pH, 251.3 g of a 25.9 mass% aqueous formaldehyde solution (formalin) (amount of formaldehyde added per 100 mass parts of silver: 121.11 mass parts) was added all at once to obtain a slurry containing first silver particles.

[0103] <Surface Treatment Agent Addition Step> Next, 15 seconds after the addition of the reducing agent, 6.13 g of a 1.55% stearic acid emulsion (amount of stearic acid added per 100 parts by mass of silver: 0.18 parts by mass) was added as a surface treatment agent to the obtained slurry containing silver particles, and the mixture was stirred for an additional 180 seconds to obtain a slurry containing silver particles coated with the surface treatment agent. Note that a baffled reaction chamber and a two-stage turbine blade were used for the reaction.

[0104] <Separation Step> Thereafter, stirring was stopped to allow the silver particles coated with the surface treatment agent to settle, and the liquid in which the silver particles coated with the surface treatment agent had precipitated was filtered and washed with water until the electrical conductivity of the liquid after passing through the water was 0.5 mS / m or less, and then vacuum dried at 73° C. The dried silver powder obtained was milled using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) to which 50 g of silver had been added, and the mill was crushed twice for 30 seconds using a dial scale of 100, to obtain spherical silver powder according to Comparative Example 1.

[0105] Using the obtained spherical silver powder, cross-sections of the silver particles were observed by the method described above, and the porosity, average pore diameter, number of pores by size, and maximum pore diameter were calculated. Furthermore, the thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were measured or calculated. The results are shown in Table 2. Furthermore, resistance measurements were performed using the obtained spherical silver powder. The results are shown in Table 3. Furthermore, a graph of the thermomechanical analysis of the spherical silver powder according to Comparative Example 1 is shown in FIG. 1, and a 100,000-magnification SEM image of the cross-section of a silver particle in the spherical silver powder according to Comparative Example 1 is shown in FIG. 6. The cross-section of one particle does not fit completely within one field of view in the silver particles illustrated in FIG. 6, and the cross-sectional area of ​​the field of view is 1.2 μm. ２ It was.

[0106] Comparative Example 2 <Silver Complex Formation Step> First, 113.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to 3,489 g of an aqueous silver nitrate solution containing 53.71 g of silver while stirring at 332 rpm to obtain an aqueous silver complex solution. Next, with continued stirring, 4.98 g of a 20% by mass aqueous sodium hydroxide solution was added as a pH adjuster to the obtained aqueous silver complex solution, and the liquid temperature was adjusted to 28°C.

[0107] <Reduction Step> 180 seconds after the addition of the ammonia water to the solution with the adjusted pH, 251.3 parts of a 26% by mass aqueous formaldehyde solution (formalin) (amount of formaldehyde added per 100 parts by mass of silver: 121.11 parts by mass) was added all at once to obtain a slurry containing first silver particles.

[0108] <Surface Treatment Agent Addition Step> Next, 15 seconds after the addition of the reducing agent, 6.13 g of a 1.55% stearic acid emulsion (amount of stearic acid added per 100 parts by mass of silver: 0.18 parts by mass) was added as a surface treatment agent to the obtained slurry containing silver particles, and the mixture was stirred for an additional 180 seconds to obtain a slurry containing silver particles coated with the surface treatment agent. Note that a baffled reaction chamber and a two-stage turbine blade were used for the reaction.

[0109] <Separation Step> Thereafter, stirring was stopped to allow the silver particles coated with the surface treatment agent to settle, and the liquid in which the silver particles coated with the surface treatment agent had precipitated was filtered and washed with water until the electrical conductivity of the liquid after passing through the water was 0.5 mS / m or less, and then vacuum dried at 73° C. The dried silver powder obtained was milled using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) together with 50 g of silver, and crushed twice for 30 seconds using a dial scale of 100, to obtain a spherical silver powder according to Comparative Example 2.

[0110] Using the obtained spherical silver powder, cross-sections of the silver particles were observed by the method described above, and the porosity, average void diameter, number of voids by size, and maximum void diameter were calculated. Furthermore, the thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were measured or calculated. The results are shown in Table 2. Furthermore, resistance measurements were performed using the obtained spherical silver powder. The results are shown in Table 3. Furthermore, a graph of the thermomechanical analysis of the spherical silver powder according to Comparative Example 2 is shown in FIG. 1, and a 100,000-magnification SEM image of the cross-section of a silver particle in the spherical silver powder according to Comparative Example 2 is shown in FIG. 7. The cross-section of a single particle does not fit completely within a single field of view, and the cross-sectional area of ​​the field of view is 1.2 μm. ２ It was.

[0111] Comparative Example 3 <Silver Complex Formation Step> First, 3,448 g of an aqueous silver nitrate solution containing 50.84 g of silver was stirred at 174 rpm, and 103.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to obtain an aqueous silver complex solution.

[0112] Next, while continuing stirring, 1.02 g of a 5 mass % aqueous sodium carbonate solution was added to the obtained aqueous complex solution as a pH adjuster, and 60 seconds after the addition of the ammonia water, 0.508 g (amount of PEI added per 100 mass parts of silver: 0.05 mass parts) of a 5 mass % aqueous PEI solution (manufactured by Nippon Shokubai Co., Ltd., weight-average molecular weight: 600) was added, and the liquid temperature was adjusted to 35°C.

[0113] <Reduction Step> 180 seconds after the addition of the ammonia water, 302.8 g of an aqueous solution of hydrazine (manufactured by Mitsubishi Gas Chemical Company, Inc.) with a concentration of 2.43 mass % (amount of hydrazine added per 100 mass parts of silver: 14.47 mass parts) was added all at once to the obtained aqueous solution of the silver complex, thereby obtaining a slurry containing silver particles.

[0114] <Surface Treatment Agent Addition Step> Next, 15 seconds after the addition of the reducing agent, 5.12 g of a 1.55% stearic acid emulsion (amount of stearic acid added per 100 parts by mass of silver: 0.16 parts by mass) was added as a surface treatment agent to the obtained slurry containing the first silver particles, and the mixture was stirred for an additional 180 seconds to obtain a slurry containing silver particles coated with the surface treatment agent. Note that a baffled reaction chamber and a two-stage turbine blade were used for the reaction.

[0115] <Separation Step> Thereafter, stirring was stopped to allow the silver particles coated with the surface treatment agent to settle, and the liquid in which the silver particles coated with the surface treatment agent had precipitated was filtered and washed with water until the electrical conductivity of the liquid after passing through the water reached 0.5 mS / m or less, and then vacuum dried at 73° C. The dried silver powder obtained was milled using a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) to which 50 g of silver had been added, and the mill was crushed twice for 30 seconds using a dial scale of 100, to obtain a spherical silver powder according to Comparative Example 3.

[0116] The cross-sections of the silver particles were observed using the obtained spherical silver powder by the method described above. Furthermore, the thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were measured or calculated. The results are shown in Table 2. Since no voids were observed in the cross-section of the spherical silver powder according to Comparative Example 3, the porosity, average void diameter, number of voids by size, and maximum void diameter were not calculated. Furthermore, the resistance value was measured using the obtained spherical silver powder. The results are shown in Table 3. Furthermore, a graph of the thermomechanical analysis of the spherical silver powder according to Comparative Example 3 is shown in FIG. 1, and a 100,000x SEM image of the cross-section of a silver particle in the spherical silver powder according to Comparative Example 3 is shown in FIG. 8.

[0117]

[0118]

[0119]

[0120] As is clear from the results in Table 3, the spherical silver powders of Examples 1 to 4 can impart excellent low-temperature sintering properties to the conductive paste.

[0121] According to the present invention, a spherical silver powder capable of imparting excellent low-temperature sintering properties to a conductive paste can be provided. Furthermore, according to the present invention, a method for producing a spherical silver powder capable of imparting excellent low-temperature sintering properties to a conductive paste can be provided. Furthermore, according to the present invention, a conductive paste with excellent low-temperature sintering properties can be provided.

Claims

1. A spherical silver powder containing silver particles having closed voids therein, wherein when the cross section of the silver particles is observed at 100,000x magnification in 10 or more fields of view, the number of voids A having a Heywood diameter of 4 nm or more per cross-sectional area is 70 / μm ２ More than 500 pieces / μm ２ or less, and the ratio of the number of voids a having a Heywood diameter of 4 nm or more and less than 30 nm per cross-sectional area to the number of voids A per cross-sectional area is 90% or more.

2. A spherical silver powder containing silver particles having closed voids inside the particles, wherein when the cross section of the silver particles is observed at 100,000x magnification in 10 or more fields of view, the number of voids B having a Heywood diameter of 10 nm or more per cross-sectional area is 30 / μm ２ More than 450 pieces / μm ２ or less, and the ratio of the number of voids b having a Heywood diameter of 10 nm or more and less than 30 nm per cross-sectional area to the number of voids B per cross-sectional area is 90% or more.

3. The spherical silver powder according to claim 1 or 2, wherein the maximum void diameter is 50 nm or less.

4. The spherical silver powder according to claim 1 or 2, wherein the average void diameter is 4 nm or more and less than 30 nm.

5. The spherical silver powder according to claim 1 or 2, which has a thermal shrinkage rate of 15% or more from room temperature to 900°C.

6. BET specific surface area is 0.1m ２ / g or more 1.8m ２ 3. The spherical silver powder according to claim 1, wherein the surface roughness is 0.1 μm or less.

7. Volume-based cumulative 10% particle diameter D measured by laser diffraction method １０ , cumulative 50% particle diameter D ５０ , and cumulative 90% particle diameter D ９０ is satisfied by the following formula (1): 0.5≦(D ９０ -D １０ ) / D ５０ 3. The spherical silver powder according to claim 1, which satisfies the relationship: ≦2.5 (1).

8. Volume-based cumulative 50% particle diameter D measured by laser diffraction method ５０ The spherical silver powder according to claim 1 or 2, wherein the particle size is 0.5 μm or more and 6 μm or less.

9. Volume-based cumulative 100% particle diameter D measured by laser diffraction method ＭＡＸ The spherical silver powder according to claim 1 or 2, wherein the particle size is 15 μm or less.

10. A method for producing spherical silver powder, comprising: a silver complex formation step of adding ammonia, a first chelating agent comprising ethylenediaminetetraacetic acid, and a second chelating agent comprising a polymer to a silver-containing aqueous solution to obtain a silver complex aqueous solution; and a reduction step of adding a reducing agent to the silver complex aqueous solution to reduce and precipitate silver particles.

11. A method for producing spherical silver powder as described in claim 10, wherein the amount of the second chelating agent added is 0.1 parts by mass or more per 100 parts by mass of silver in the silver-containing aqueous solution.

12. A method for producing spherical silver powder as described in claim 10, wherein the amount of the first chelating agent added is 3 parts by mass or more and 40 parts by mass or less per 100 parts by mass of silver in the silver-containing aqueous solution.

13. A method for producing spherical silver powder according to any one of claims 10 to 12, wherein a surface treatment agent is added to the precipitated silver particles after the reduction step.

14. A conductive paste containing the spherical silver powder according to claim 1 or 2 as a conductive filler.

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