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

Abstract

To provide a spherical silver powder that can impart an excellent low-temperature sinterability to conductive pastes.SOLUTION: A first embodiment of the present invention is a spherical silver powder that contains silver particles that have closed voids in the particle interior. When the cross sections of the silver particles are observed at a magnification of 100,000× in at least 10 fields of view, the number per the cross-sectional area of voids A having a Heywood diameter of at least 4 nm is greater than or equal to 70 / μm2 and less than or equal to 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 greater than or equal to 4 nm and less than 30 nm is at least 90%.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[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 an apparent density of 8.2 g / cm3, which has a lower volume resistivity than conventional silver powders when used as a conductive paste. 3 More than 9.2g / cm 3 and a silver powder has been proposed in which the ratio of the line length of the outer periphery of the cross section of a silver particle to the line length circumscribing the periphery of the cross section of the particle is 1.1 or more and 1.4 or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2023 / 054405 Summary of the Invention [Problem to be solved by the invention]

[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 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 is excellent in low-temperature sintering properties. [Means for solving the problem]

[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 inside the particles, When the cross section of the silver particle is observed at 100,000x magnification in 10 or more fields of view, The number of voids A with a Heywood diameter of 4 nm or more per cross-sectional area is 70 / μm 2 More than 500 pieces / μm 2 is as follows: 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 inside the particles, When the cross section of the silver particle is observed at 100,000x magnification in 10 or more fields of view, The number of voids B with a Heywood diameter of 10 nm or more per cross-sectional area is 30 / μm 2 More than 450 pieces / μm 2 is as follows: A spherical silver powder in which 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 0.1m 2 / g or more 1.8m 2 / g or less.

[0015] [7] Cumulative 10% particle size (D) based on volume using laser diffraction method 10 , Cumulative 50% particle size D 50 , and cumulative 90% particle diameter D 90 However, the following formula (1): 0.5≦(D 90 -D 10 ) / D 50 ≦2.5 (1) The spherical silver powder according to any one of [1] to [6], which satisfies the relationship:

[0016] [8] Cumulative 50% particle diameter D by volume using 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] Cumulative 100% particle size D by volume using 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 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; a reduction step of adding a reducing agent to the aqueous silver complex 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. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a spherical silver powder that can impart excellent low-temperature sintering properties to a conductive paste. Furthermore, the present invention can provide a method for producing spherical silver powder that can impart excellent low-temperature sintering properties to a conductive paste. Furthermore, according to the present invention, a conductive paste having excellent low-temperature sintering properties can be provided. [Brief explanation of the drawings]

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

[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 terms and measurement methods used in this specification will be explained.

[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. 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 using the formula π(average maximum length / 2) 2 / average particle area.

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

[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 of photographing silver particle cross section and cross-sectional area> In the present invention, the cross sections of a large number of exposed silver particles are first observed at a magnification of 5000. Then, cross sections of silver particles that are thought to have a cross-sectional area equal to or greater than the average are selected, and an SEM image of any silver particle from among them is obtained at a magnification of 100,000 by increasing the magnification so that the particle center is at the center of the field of view. In the 100,000x SEM image of the cross section of a silver particle obtained in this way, if the entire cross section of one particle falls within a single field of view, the cross section of that one particle is measured. In this case, 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 a particle does not fit within one field of view, the cross section of the particle is measured only within the frame of that field of view. In other words, the outer diameter of the silver particle is traced with the pointer, excluding parts other than the silver particle. If the outer diameter of the silver particle extends beyond the entire field of view, the cross-sectional area is calculated as if the entire field of view were the cross section of the silver particle. In this case, voids outside the field of view are not counted. The area of ​​one field of view of an SEM image at 100,000x magnification is calculated from the scale bar to be 1.2 μm 2 is.

[0031] <Gap area and Heywood diameter (gap 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 voids (closed voids not connected to the periphery of the silver particles) visible in the cross section of the photographed silver particles was traced with a pointer on the screen displaying the image, and the area of ​​the voids within the closed area traced in one stroke was calculated, as well as the Heywood diameter of the voids (hereinafter, the Heywood diameter of the voids may be referred to as the "void diameter"). Note that voids in which part of the void is cut off by the field of view frame of the SEM image were not counted as voids because the Heywood diameter was unknown.

[0032] <Void average diameter> The average void diameter is the average value of void diameters, and is the ratio of the total void diameters to the total number of voids (total void diameters / 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 void diameter is the largest void diameter among the void diameters observed in cross-sections of silver particles in 100,000x SEM images of 10 or more fields of view.

[0034] <Porosity> The porosity (%) is expressed as the average value of the ratio of the total area of ​​voids to the cross-sectional area of ​​silver particles observed in cross-sectional observations of 10 or more fields of view using SEM images of silver particles at 100,000x magnification (total area of ​​voids / cross-sectional area of ​​silver particles × 100[%]).

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

[0036] <BET specific surface area> The "BET specific surface area" was determined by the BET one-point method after putting 3 g of spherical silver powder into the measurement cell using a specific surface area measuring device (manufactured by MOUNTECH Co., Ltd., Macsorb HM-model 1210) adopting the BET method, passing a carrier gas in which 70 vol% of He gas and 30 vol% of nitrogen gas were mixed through the measurement cell at 25 mL / min, and performing degassing at 60 °C for 10 minutes.

[0037] <Particle size distribution> In this specification, the 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 placed in the above-mentioned device, and the particle size distribution was calculated using the attached analysis software. During the measurement, an SDC device was used as the circulator of the laser diffraction / scattering particle size distribution analyzer, and the "flow rate (%)" setting of the circulator was set to 60. Furthermore, the post-measurement calculation mode of the MT-3300 EXII was HRA mode. In the following, the cumulative 10% particle diameter D on a volume basis determined by laser diffraction method is calculated. 10 , Cumulative 50% particle size D 50 , Cumulative 90% particle size 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] <Ig-loss> In this specification, the term "loss on ignition (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 indicates 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 "loss on ignition (Ig-loss) value" was calculated by precisely weighing a spherical silver powder sample (weighing value: w1), placing it in a porcelain crucible, heating it to 800°C, and holding it at 800°C for 30 minutes, a time sufficient to reach constant weight, then cooling and reweighing it (weighing value: w2), and calculating the "loss on ignition (Ig-loss) value (mass%) = (w1 - w2) / w1 × 100."

[0039] (spherical silver powder) The spherical silver powder of the first 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 100,000x magnification in 10 or more fields of view, 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 2 Number of particles (number per cross-sectional area) is 70 particles / μm 2 More than 500 pieces / μm 2 The number of voids a (hereinafter simply referred to as "voids a") having a Heywood diameter of 4 nm or more and less than 30 nm per cross-sectional area of ​​1 μm 2 The ratio of the number of voids (number per cross-sectional area) (number of voids a / number of voids A × 100 [%]) is 90% or more. In addition, 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 particles (number per cross-sectional area) is 30 particles / μm 2 More than 450 pieces / μm 2 The number of voids B per cross-sectional area is 1 μm or less, and the number of voids b (hereinafter simply referred to as "voids b") having a Heywood diameter of 10 nm or more and less than 30 nm per cross-sectional area is 2 The ratio of the number of voids (number per cross-sectional area) (number of voids b / number of voids B × 100 [%]) is 90% or more. In addition, in the spherical silver powder of the second 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 second form consists of the above-mentioned specified silver particles.

[0041] The spherical silver powder of the first or second embodiment 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, the results of the following examples and comparative examples clearly demonstrate 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 type and the spherical silver powder of the second type can be obtained by the method for producing spherical silver powder of the present invention described below.

[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 particle is observed at 100,000 times magnification 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 type, the number of voids A per cross-sectional area is 70 / μm 2 More than 100 particles / μm 2 It is preferable that the number of particles is 500 / μm or more. 2 400 particles / μm or less 2 More preferably, it is: In the spherical silver powder of the second type, the number of voids A per cross-sectional area is 70 / μm 2It is preferable that the number of particles is 100 or more per μm. 2 More preferably, it is 500 particles / μm or more. 2 It is preferable that the number of particles is 400 or less per μm. 2 More preferably, it is: The number of voids A per cross-sectional area is 70 / μm 2 If the content is above this, the silver particles will have well-dispersed voids inside, thereby improving the low-temperature sintering property. On the other hand, the number of voids A per cross-sectional area is 500 / μm 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 poor.

[0045] In the spherical silver powder of the first type, 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, it is 450 particles / μm or more. 2 It is preferable that the number of the nuclei is 1 or less, and more preferable that the number of the nuclei is 350 or less. In the spherical silver powder of the second type, the number of voids B per cross-sectional area is 30 / μm 2 More than 60 particles / μm 2 It is preferable that the number of particles is 450 / μm or more. 2 or less, and preferably 350 or less. The number of voids B per cross-sectional area is 30 / μm 2 If it is equal to or greater than this, the low-temperature sintering property can be improved. On the other hand, the number of voids B per cross-sectional area is 450 / μm 2If 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 poor.

[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 in the number of voids A or voids B is 50 nm or less, there will be 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, it is possible to obtain silver particles having a large number of predetermined voids well dispersed inside the particles.

[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 rate 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 rate is 15% or more, a good conductive film can be obtained when a conductive paste containing spherical silver powder is used to obtain a conductive film. On the other hand, if the heat shrinkage rate is 40% or less, excessive shrinkage can be effectively suppressed.

[0051] The BET specific surface area of ​​spherical silver powder is 0.1m 2 / g or more, and 0.2m 2 / g or more is more preferable, and 1.8m 2 / g or less, and 1.5m 2 It is more preferable that the saturation coefficient is 1 / g or less. The BET specific surface area of ​​the spherical silver powder is 0.1m 2 If it is / g or more, the low-temperature sintering property can be improved. On the other hand, the BET specific surface area of ​​spherical silver powder is 1.8m 2 If the spherical silver powder has a viscosity of 1 / 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, the viscosity of the resulting conductive paste or the like can be effectively reduced if each of the above is equal to or greater than the above lower limit. On the other hand, spherical silver powder D 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 size D 50 , and cumulative 90% particle diameter D 90 is expressed by the following formula (1): 0.5≦(D 90 -D 10 ) / D 50 ≦2.5 (1) It is preferable that the following relationship is satisfied. (D 90 -D 10 ) / D 50 If the particle diameter is within 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. Here, the spherical silver powder (D 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, and is 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 mass% or less, the impurities are low, so 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 predetermined 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, linolenic acid, etc. 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, such as sodium salts and potassium salts.

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

[0063] (Method of manufacturing spherical silver powder) The method for producing spherical silver powder of the present invention (hereinafter sometimes simply referred to as the "production method") comprises a silver complex formation step in which a first chelating agent consisting of ammonia, ethylenediaminetetraacetic acid, and a second chelating agent consisting 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. The above-described manufacturing method makes it possible to obtain spherical silver powder that can impart excellent low-temperature sintering properties 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, these steps may be referred to as "other steps"). Examples of other steps include a pH adjuster addition step of adding a pH adjuster to the aqueous silver complex solution before reduction, a surface treatment agent addition step of adding a surface treatment agent to a mixed solution containing precipitated silver particles, and a separation step of separating and drying 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.

[0064] <Silver complex formation process> 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. Alternatively, 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, some or all of the four carboxylic acids in ethylenediaminetetraacetic acid may be in the form of an alkali metal salt. 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. The ethylenediaminetetraacetic acid in the form of an alkali metal salt may 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.

[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, per 100 parts by mass of silver in the silver-containing aqueous solution, and is preferably 40 parts by mass or less, more preferably 36 parts by mass or less. When the amount of the first chelating agent added is within the above range and ammonia capable of complexing and the first chelating agent coexist in the silver complex aqueous solution, it becomes easier to suppress variations in the distribution of voids, and silver powder having the desired voids 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 is 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. By using a second chelating agent made of a polymer, it is possible to control the rate of particle aggregation and particle growth after the addition of a reducing agent, thereby suppressing the variation in pore size of voids that occur inside silver particles.

[0072] Specific examples of preferred second chelating agents include amino compounds and imine compounds. Among these, polyethyleneimine (PEI) is preferred. In particular, the imine compound PEI has a network structure containing both a primary amine (-NH) and a secondary amine (=NH) in the molecule, and provides 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 the GPC-MALS method.

[0074] The amount of the second chelating agent added is preferably 0.1 parts by mass or more, and more preferably 1 part by mass or more, per 100 parts by mass of silver in the silver-containing aqueous solution. If the addition amount of the second chelating agent is 0.1 part by mass or more with respect to 100 parts by mass of silver in the silver-containing aqueous solution, the silver particles can grow uniformly and spherically, and by effectively suppressing aggregation, it is presumed that the variation in the pore diameter of the voids generated inside the silver particles can be suppressed, and the generation of coarse voids can be suppressed. In the present invention, when the addition amount of the second chelating agent is less than 0.1 part by mass, it is impossible to obtain uniformly dispersed spherical silver powder, and it was difficult to obtain silver powder having predetermined voids. The upper limit of the addition amount is not particularly limited, but for example, it is 10 parts by mass, and it is more preferable to be 0.3 to 3 parts by mass.

[0075] The temperature of the silver complex aqueous 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. If the temperature of the silver complex aqueous solution is 5°C or higher, the reduction reaction can proceed effectively. On the other hand, if the temperature of the silver complex aqueous solution is 50°C or lower, it is possible to effectively suppress the excessive reaction rate of the reduction reaction described later, and effectively suppress the variation in the particle diameter of the silver particles. Also, if the temperature of the silver complex aqueous solution is within the above range, it is possible to effectively suppress the increase in energy cost.

[0076] <pH Adjusting Agent Addition Step> As the pH adjusting agent addition step, a pH adjusting agent may be added to the silver complex aqueous solution before adding the reducing agent. When a pH adjusting agent is added to the silver complex aqueous solution before reduction, the particle diameter of the obtained silver powder can be easily adjusted. As the pH adjusting agent, general acids or bases may be used, and examples include nitric acid, sodium hydroxide, etc. The amount of the pH adjusting agent to be added can be appropriately adjusted according to the amount of the silver nitrate aqueous solution used and the particle diameter of the silver powder to be obtained. As the adjustment method, for example, a method of adjusting the addition amount by conducting a level test of the particle diameter of the silver powder according to the addition amount of the pH adjusting agent can be mentioned.

[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] To increase the silver reaction yield, the amount of reducing agent added is preferably at least one equivalent relative to the silver of the reducing agent being 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, an amount of at least 2 equivalents relative to the silver is preferred, with 10 to 20 equivalents being more preferred.

[0080] <Surface treatment agent addition process> When the surface treatment agent addition step is performed, the surface treatment agent is added to the mixture containing the silver particles precipitated in the reduction step, thereby obtaining silver particles coated with the surface treatment agent. The mixed liquid containing silver particles coated with a surface treatment agent is usually a slurry in which silver particles coated with a surface treatment agent are dispersed.

[0081] Examples of surface treatment agents that can be added to the mixture containing precipitated silver particles include fatty acids, compounds having an azole structure, fatty acid salts, surfactants, organometallic chelating agents, and protective colloids. 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 easily and uniformly attached to the silver powder surface.

[0082] Examples of fatty acids include behenic acid, stearic acid, palmitic acid, myristic acid, lauric acid, ricinoleic acid, oleic acid, linoleic acid, linolenic acid, etc. 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, such as sodium salts and potassium salts.

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

[0084] The amount of the 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, per 100 parts by mass of silver in the silver-containing aqueous solution, and is preferably 3 parts by mass or less, more preferably 1 part by mass or less. When the amount of the surface treatment agent added is 0.05 parts by mass or more per 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 the surface treatment agent added is 3 parts by mass or less per 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 process> In the separation step, the silver particles are separated from the mixed solution containing the silver particles or the silver particles coated with the surface treatment agent. After the separation step, a washing and recovery step can be carried out, or the drying step can be carried out 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 that contain moisture and are in an aggregated 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 the conditions are such that the aggregates of silver particles are sufficiently dried, but it is preferably 40°C or higher, more preferably 70°C or higher, and is 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 for the spherical silver powder is 120°C or lower, there is little risk of impairing the thermal properties (such as thermal shrinkage rate) 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 an 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 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 application mode. [Example]

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

[0094] Example 1 <Silver complex formation process> First, 3436 g of a silver nitrate solution containing 50.84 g of silver was stirred at 174 rpm, and 103.2 g of 28% by weight aqueous ammonia (Junsei Chemical Co., Ltd.) was added. Then, with continued stirring, 30 seconds after the addition of the aqueous ammonia, 4.17 g of a 43% by weight aqueous EDTA solution (Chelest OD-50, manufactured by Chelest) was added as a first chelating agent (3.53 parts by weight of EDTA per 100 parts by weight of silver), yielding an aqueous silver complex. 60 seconds after the addition of the aqueous ammonia, 10.17 g of a 5% by weight aqueous PEI solution (Nippon Shokubai Co., Ltd., weight-average molecular weight 600) was added as a second chelating agent (1.00 parts by weight of PEI per 100 parts by weight of silver), yielding an aqueous silver complex. The solution temperature was adjusted to 35°C.

[0095] <Reduction process> 180 seconds after the addition of the ammonia water, 302.8 g of a 2.43 mass% aqueous solution of hydrazine (manufactured by Mitsubishi Gas Chemical Company, Inc.) (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 silver complex, thereby obtaining a slurry containing first silver particles.

[0096] <Surface treatment agent addition process> Next, stirring of the resulting slurry containing silver particles was stopped 5 seconds after the addition of hydrazine, and then restarted 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 the mixture was stirred for an additional 75 seconds to obtain a slurry containing silver particles coated with a surface treatment agent. Note that a baffled reaction chamber and a two-stage turbine blade were used for the reaction.

[0097] <Separation process> 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 put into a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 30 seconds using a dial scale of 100, to obtain the spherical silver powder of Example 1.

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

[0099] <Resistance measurement> A mixture was obtained by mixing 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). The mixture was then premixed using a planetary mixer (revolution 1000 rpm) and kneaded using a three-roll mill (manufactured by EXAKT) with a roll gap varying from 100 μm to 20 μm to obtain a conductive paste. The conductive paste obtained above was used to print a linear shape by screen printing. The linear shape had a designed line width of 500 μm and a linear length of 128 mm. A Microtec printer was used for printing, and the printing was performed at a squeegee speed of 80 mm / sec. A silicon substrate (for solar cell applications, texture formation, SiN) with a thickness of approximately 170 μm was used for printing. x Pre-formed film) was used. After printing, the conductive paste was dried for 10 minutes in a dryer set at 100°C to form a conductive film. The conductive film was measured for its resistance (unit: Ω) over time from room temperature to 300°C at a heating rate of 10°C / min 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. The cross-sections of the obtained spherical silver powder were observed using the method described above, and the porosity, average pore diameter, number of pores by size, and maximum pore diameter were calculated. The thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were also measured or calculated. The results are shown in Table 2. The spherical silver powder was then subjected to resistance measurement, and 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 Figure 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 Figures 3 to 5, respectively. The cross-sectional area of ​​the silver particle exemplified in the center of Figures 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 process> First, 113.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to 3489 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, while continuing to stir, 4.98 g of a 20% by mass aqueous solution of sodium hydroxide as a pH adjuster was added to the obtained aqueous silver complex solution, and the liquid temperature was adjusted to 18.5°C.

[0102] <Reduction process> To the pH-adjusted solution, 180 seconds after adding the ammonia water, 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 the first silver particles.

[0103] <Surface treatment agent addition process> Next, 15 seconds after the addition of the reducing agent, 6.13 g of a 1.55% stearic acid emulsion (0.18 parts by mass of stearic acid per 100 parts by mass of silver) was added as a surface treatment agent to the resulting 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 process> 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 placed in a sample mill (SK-M10, manufactured by Kyoritsu Riko Co., Ltd.) and crushed twice for 30 seconds using a dial scale of 100, to obtain spherical silver powder according to Comparative Example 1.

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

[0106] (Comparative Example 2) <Silver complex formation process> First, 113.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to 3489 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, while continuing to stir, 4.98 g of a 20% by mass aqueous solution of sodium hydroxide as a pH adjuster was added to the obtained aqueous silver complex solution, and the liquid temperature was adjusted to 28°C.

[0107] <Reduction process> To the pH-adjusted solution, 180 seconds after adding the ammonia water, 251.3 parts of a 26% by mass formaldehyde aqueous 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 the first silver particles.

[0108] <Surface treatment agent addition process> Next, 15 seconds after the addition of the reducing agent, 6.13 g of a 1.55% stearic acid emulsion (0.18 parts by mass of stearic acid per 100 parts by mass of silver) was added as a surface treatment agent to the resulting 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 process> 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.) with 50 g of silver added, and crushed twice for 30 seconds using a dial scale of 100, to obtain spherical silver powder according to Comparative Example 2.

[0110] The cross-sections of the obtained spherical silver powder were observed using the method described above, and the porosity, average pore diameter, number of pores by size, and maximum pore diameter were calculated. The thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were also measured or calculated. The results are shown in Table 2. The spherical silver powder was then subjected to resistance measurement, and 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 Figure 1, and a 100,000x SEM image of the cross section of a silver particle in the spherical silver powder according to Comparative Example 2 is shown in Figure 7. The cross section of one silver particle does not fit within one 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 process> First, 103.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to 3448 g of an aqueous silver nitrate solution containing 50.84 g of silver while stirring at 174 rpm to obtain an aqueous silver complex solution.

[0112] Next, while continuing to stir, 1.02 g of a 5% by 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 parts by mass of silver: 0.05 parts by mass) of a 5% by 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 process> 180 seconds after adding the ammonia water, 302.8 g of a 2.43 mass% aqueous solution of hydrazine (manufactured by Mitsubishi Gas Chemical Company, Inc.) (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 silver complex, thereby obtaining a slurry containing silver particles.

[0114] <Surface treatment agent addition process> Next, 15 seconds after the addition of the reducing agent, 5.12 g of a 1.55% stearic acid emulsion (0.16 parts by mass of stearic acid per 100 parts by mass of silver) was added as a surface treatment agent to the resulting 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 process> 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.) with 50 g of silver added, and crushed twice for 30 seconds using a dial scale of 100, to obtain spherical silver powder according to Comparative Example 3.

[0116] The cross-sections of the obtained spherical silver powders were observed by the method described above. The thermal shrinkage, BET specific surface area, particle size distribution, and ignition loss (Ig-loss) were also measured or calculated. The results are shown in Table 2. Since no voids were observed in the cross-sections of the spherical silver powders of Comparative Example 3, the porosity, average void diameter, number of voids by size, and maximum void diameter were not calculated. The spherical silver powder was then subjected to resistance measurement, and 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 an SEM image of a cross section of a silver particle in the spherical silver powder according to Comparative Example 3 at 100,000 magnification is shown in FIG.

[0117] [Table 1]

[0118] [Table 2]

[0119] [Table 3]

[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. [Industrial Applicability]

[0121] According to the present invention, it is possible to provide a spherical silver powder that can impart excellent low-temperature sintering properties to a conductive paste. Furthermore, the present invention can provide a method for producing spherical silver powder that can impart excellent low-temperature sintering properties to a conductive paste. Furthermore, according to the present invention, a conductive paste having excellent low-temperature sintering properties can be provided.

Claims

1. A spherical silver powder containing silver particles having closed voids therein, When the 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 is as follows: A spherical silver powder in which 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 therein, When the 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 is as follows: A spherical silver powder in which 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. 3. The spherical silver powder according to claim 1, wherein the maximum void diameter is 50 nm or less.

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

5. 3. The spherical silver powder according to claim 1, 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 expressed by the following formula (1): 0.5≦(D ９０ -D １０ ) / D ５０ ≦2.5・・・(1) The spherical silver powder according to claim 1 or 2, which satisfies the relationship:

8. Cumulative 50% particle diameter D on a volume basis 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 silver complex forming 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; a reduction step of adding a reducing agent to the aqueous silver complex solution to reduce and precipitate silver particles; A method for producing spherical silver powder, comprising:

11. 11. The method for producing spherical silver powder according to 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. 11. The method for producing spherical silver powder according to 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. 13. The method for producing spherical silver powder according to claim 10, wherein a surface treatment agent is added to the precipitated silver particles after the reduction step.

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

Images