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

Abstract

The purpose of the present invention is to provide a spherical silver powder that can impart exceptional low-temperature sinterability to an electrically conductive paste. The present invention is a spherical silver powder having, in an XRD analysis, a cubic Ag peak and a hexagonal Ag peak, the ratio of the intensity of the hexagonal Ag peak to the intensity of the cubic Ag peak being 0.5% or greater.

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 having a cubic Ag peak and a hexagonal Ag peak in XRD analysis, wherein the ratio of the intensity of the hexagonal Ag peak to the intensity of the cubic Ag peak is 0.5% or more.

[0010] [2] The spherical silver powder according to [1], having a crystallite diameter of 28 nm or less.

[0011] [3] BET specific surface area is 0.1 m ２ / g or more 1.8m ２ / g or less.

[0012] [4] 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 ５０ The spherical silver powder according to any one of [1] to [3], which satisfies the relationship: ≦2.5 (1).

[0013] [5] Volume-based cumulative 50% particle diameter D by laser diffraction method ５０ The spherical silver powder according to any one of [1] to [4], wherein the particle size is 0.5 μm or more and 6 μm or less.

[0014] [6] Volume-based cumulative 100% particle diameter D by laser diffraction method ＭＡＸ The spherical silver powder according to any one of [1] to [5], wherein the diameter of the spherical silver powder is 15 μm or less.

[0015] [7] A method for producing spherical silver powder according to any one of [1] to [6], comprising: a silver complex formation step of adding ammonia and a first chelating agent consisting of ethylenediaminetetraacetic acid 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, 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.

[0016] [8] The method for producing spherical silver powder according to [7], further comprising adding a second chelating agent made of a polymer to the silver-containing aqueous solution or the silver complex solution.

[0017] [9] The method for producing spherical silver powder according to [8], 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.

[0018]

[10] The method for producing spherical silver powder according to any one of [7] to [9], wherein a surface treatment agent is added to a slurry containing the precipitated silver particles after the reduction step.

[0019]

[11] A conductive paste containing the spherical silver powder according to any one of [1] to [6] as a conductive filler.

[0020] 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.

[0021] FIG. 1 is a magnified graph of an XRD analysis of a spherical silver powder according to Example 1. FIG. 2 is a magnified graph of an XRD analysis of a spherical silver powder according to Example 2. FIG. 3 is a magnified graph of an XRD analysis of a spherical silver powder according to Example 3. FIG. 4 is a magnified graph of an XRD analysis of a spherical silver powder according to Example 5. FIG. 5 is a magnified graph of an XRD analysis of a spherical silver powder according to Comparative Example 1. FIG. 6 is a magnified graph of an XRD analysis of a spherical silver powder according to Comparative Example 2. FIG. 7 is a magnified graph of an XRD analysis of a spherical silver powder according to Comparative Example 3. FIG. 8 is a graph of an XRD analysis of the spherical silver powder according to Examples 1 to 5 and Comparative Examples 1 to 3. FIG. 9 is a 50,000x SEM image of the spherical silver powder after crushing in the separation step of Example 1. FIG. 10 is a 50,000x SEM image of the spherical silver powder after crushing in the separation step of Comparative Example 1. FIG. 11 is a 50,000x SEM image of the spherical silver powder after crushing in the separation step of Comparative Example 2.

[0022] 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.

[0023] (Terminology and Measurement Methods) First, prior to describing the embodiments, the terminology and measurement methods used in this specification will be described.

[0024] <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.

[0025] <X-ray Diffraction (XRD) Analysis> In this specification, X-ray diffraction (XRD) analysis was performed using an X-ray diffractometer (SmartLab manufactured by Rigaku Corporation). The measurement conditions were as follows: Target material: Cu Target voltage: 45 kV Target current: 200 mA Measurement method: θ-2θ Scan speed: 1 deg / min Scan step: 0.02 deg Scan range: 30°≦2θ≦50°

[0026] <Cubic Ag Peak> In this specification, the "cubic Ag peak" refers to an X-ray diffraction peak derived from the 3C(111) plane, and is a peak that usually has a peak top (maximum point in the X-ray diffraction intensity profile) around 2θ = 38.14°. In addition, in this specification, the "intensity of the cubic Ag peak" refers to the numerical value (count number) of the intensity of the peak top of the cubic Ag peak. In addition, in this specification, the numerical value of the intensity at 38.14° is used.

[0027] <Hexagonal Ag Peak> In this specification, the term "hexagonal Ag peak" refers to an X-ray diffraction peak derived from the 4H (0004) plane, which typically has a peak top near 2θ = 36.00°. In this specification, taking into account the influence of the base of the cubic Ag peak, a point where the difference between the baseline and the X-ray diffraction intensity profile is maximized when a straight line is drawn between the intensity at 35.60° and the intensity at 36.40° and the straight line is used as the baseline is also considered to be a hexagonal Ag peak. In addition, in this specification, the term "hexagonal Ag peak intensity" refers to the difference between the numerical value (count number) of the hexagonal Ag peak intensity and the count number of the baseline at the 2θ position of the peak top of the hexagonal Ag peak when a straight line is drawn between the intensity at 35.60° and the intensity at 36.40° and the straight line is used as the baseline. Here, the intensity of the hexagonal Ag peak will be specifically explained using an enlarged graph ( FIG. 1 ) of the XRD analysis of the spherical silver powder according to Example 1, which will be described later. The dotted line in FIG. 1 is the straight line (baseline) connecting the intensities at 35.60° and 36.40°, and the intensity of the hexagonal Ag peak is in the range indicated by the arrow in FIG. 1 where the 2θ value is 36.00°. In this specification, the intensities of the hexagonal Ag peaks in Examples 1 to 4 were calculated using a 2θ position of 36.00°. The intensity of the hexagonal Ag peak in Example 5 was calculated using a 2θ position of 35.92°, and the intensity of the hexagonal Ag peak in Comparative Example 3 was calculated using a 2θ position of 36.04°.

[0028] <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.

[0029] <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 ＭＡＸ was 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 ".

[0030] <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 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".

[0031] <Crystallite diameter Dx> The crystallite diameter Dx was determined using an X-ray diffractometer (SmartLab manufactured by Rigaku Corporation) from the half-width of the (111) peak appearing around 2θ = 38.14° according to the Scherrer formula (D ｈｋｌ = Kλ / β cos θ) where D ｈｋｌ means the crystallite diameter (the size of the crystallite in the direction perpendicular to hkl) (unit: nm), λ means the wavelength of the measured X-ray (0.15405 nm when a Cu target is used), β means the broadening (rad) of the diffraction line due to the size of the crystallite (expressed using the half-width), θ means the Bragg angle (rad) of the diffraction angle (the angle when the angle of incidence and the angle of reflection are equal, and the angle at the peak top is used), and K means the Scherrer constant (K=0.94).

[0032] (Spherical Silver Powder) In XRD analysis, the spherical silver powder of the present invention has a cubic Ag peak at 2θ of approximately 38.14° and a hexagonal Ag peak at 2θ of approximately 36.00°, and the ratio of the intensity of the hexagonal Ag peak to the intensity of the cubic Ag peak (hexagonal Ag peak intensity / cubic Ag peak intensity × 100 [%]) is 0.5% or more. The spherical silver powder described above can impart excellent low-temperature sinterability to a conductive paste. This is presumably due to the possibility of a transition from a hexagonal structure to a cubic structure occurring during sintering at low temperatures, but the reason for this is not entirely clear. However, the results of the following examples and comparative examples clearly demonstrate that a spherical silver powder satisfying the above requirements can impart excellent low-temperature sinterability to a conductive paste. The spherical silver powder of the present invention can be obtained by the method for producing the spherical silver powder of the present invention described below.

[0033] The ratio of the hexagonal Ag peak intensity to the cubic Ag peak intensity is 0.5% or more, preferably 0.8% or more, and more preferably 1% or more, and may be, for example, 30% or less, 15% or less, or 9% or less.

[0034] The BET specific surface area of ​​the spherical silver powder is 0.1 m ２ / g or more, and ２ / 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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 ５０is 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.

[0041] 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.

[0042] The crystallite diameter Dx of the spherical silver powder is preferably 28 nm or less, and more preferably 20 nm or less. If the crystallite diameter Dx of the spherical silver powder is 28 nm or less, low-temperature sintering properties can be improved. The crystallite diameter Dx of the spherical silver powder may be, for example, 1 nm or more, or 10 nm or more.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] (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 ammonia and a first chelating agent composed of ethylenediaminetetraacetic acid 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, 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 (Ag) in the silver-containing aqueous solution. The above-described production method can produce the spherical silver powder of the present invention, which 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 sometimes 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 from the mixed solution containing 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.

[0047] <Silver Complex Formation Step> In the silver complex formation step, ammonia and a first chelating agent consisting of ethylenediaminetetraacetic acid are added to a silver-containing aqueous solution to obtain a silver complex aqueous solution. The order of adding ammonia and the first chelating agent to the silver-containing aqueous solution may be ammonia first, or the first chelating agent may be added first, or ammonia and the first chelating agent may be added simultaneously to the silver-containing aqueous solution. Both ammonia and the first chelating agent can form a complex with silver, but it is preferable to stir and mix them until they are complexed.

[0048] In the silver complex formation step, a second chelating agent consisting of a polymer may be further added to the silver-containing aqueous solution or the silver complex aqueous solution. The timing of adding the second chelating agent is not particularly limited, and the second chelating agent may be added before the addition of ammonia, before the addition of the first chelating agent, after the addition of ammonia, or after the addition of the first chelating agent, or may be added simultaneously with the ammonia and the first chelating agent.

[0049] 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.

[0050] Examples of the ammonia to be added to the silver-containing aqueous solution include aqueous ammonia and ammonium salts.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] The amount of the first chelating agent added is 3 parts by mass or more, preferably 3.5 parts by mass or more, and 40 parts by mass or less, preferably 36 parts by mass or less, per 100 parts by mass of silver in the silver-containing aqueous solution. If the amount of the first chelating agent added is less than 3 parts by mass, the resulting silver powder may have a very low proportion of hexagonal Ag, which may result in failure to achieve the effects of the present invention. However, if the amount is within the above range, it is easy to effectively obtain a silver powder in which the ratio of the hexagonal Ag peak intensity to the cubic Ag peak intensity falls within the range of the present invention. Although the reason for this is unclear, it is believed that the coexistence of the first chelating agent and ammonia, which can form a complex with silver, affects the anisotropy of silver precipitation. 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, per 100 parts by mass of ammonia in the silver-containing aqueous solution. It is preferable that the amount be 70 parts by mass or less, and more preferably 65 parts by mass or less.

[0055] The second chelating agent added to the silver-containing aqueous solution is composed of a polymer. The use of a second chelating agent composed of a polymer can effectively suppress aggregation of silver particles obtained in the reduction step.

[0056] 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.

[0057] 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.

[0058] The amount of the second chelating agent added is preferably 0.1 parts by mass or more, more preferably 0.3 parts by mass or more, relative to 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 relative to 100 parts by mass of silver in the silver-containing aqueous solution, silver particles will grow uniformly and spherically, and aggregation can be effectively suppressed. There is no particular upper limit on the amount added, but if an excessive amount is added, the growth of silver will be significantly inhibited, so it is, for example, 10 parts by mass or less, and preferably 3 parts by mass or less.

[0059] 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 the variation in 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.

[0060] <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.

[0061] <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.

[0062] 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.

[0063] 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.

[0064] <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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] <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.

[0071] 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.

[0072] 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.

[0073] 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 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 temperature of the spherical silver powder is 120° C. or lower, the transition from a hexagonal crystal structure to a cubic crystal structure in the silver particles or silver particles coated with a surface treatment agent can be effectively suppressed.

[0074] 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.

[0075] 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.

[0076] (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.

[0077] The present invention will be described in more detail below using examples, but the present invention is not limited to the following examples. Note that the XRD analysis, cubic Ag peak intensity, hexagonal Ag peak intensity, BET specific surface area, particle size distribution, ignition loss (Ig-loss), and crystallite diameter Dx were measured or calculated by the methods described above.

[0078] 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 a silver complex aqueous solution. 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, and the liquid temperature was adjusted to 35 ° C.

[0079] <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.

[0080] <Surface Treatment Agent Addition Step> Next, the resulting slurry containing silver particles was stirred for 5 seconds after the addition of hydrazine, and then the stirring was stopped once. 130 seconds after the addition of hydrazine, the stirring was restarted. 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 the surface treatment agent, and the mixture was stirred 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.

[0081] <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.

[0082] Using the obtained spherical silver powder, XRD analysis, cubic Ag peak intensity, hexagonal Ag peak intensity, ratio of hexagonal Ag peak intensity to cubic Ag peak intensity, BET specific surface area, particle size distribution, ignition loss (Ig-loss), and crystallite diameter Dx were measured or calculated. The results are shown in Table 2. Furthermore, an enlarged graph of the XRD analysis of the spherical silver powder according to Example 1 in the 2θ range of 35.00° to 37.00° is shown in FIG. 1, a graph of the XRD analysis in the 2θ range of 35.00 to 40.00° is shown in FIG. 9, and a 50,000x SEM image of the spherical silver powder after crushing in the separation step is shown in FIG. 10. The cubic Ag peak intensity, hexagonal Ag peak intensity, and ratio of the hexagonal Ag peak intensity to the cubic Ag peak intensity of the spherical silver powder after vacuum drying (before crushing) in the separation step are shown in Table 2 as a reference example.

[0083] <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.

[0084] 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, XRD analysis, BET specific surface area, particle size distribution, ignition loss (Ig-loss), and crystallite diameter Dx were measured or calculated. The results are shown in Table 2. Resistivity measurements were also performed using the obtained spherical silver powders. The results are shown in Table 3. Furthermore, enlarged graphs of XRD analysis of the spherical silver powders according to Examples 2 to 4 in the 2θ range of 35.00° to 37.00° are shown in Figures 2 to 4, and a graph of XRD analysis in the 2θ range of 35.00° to 40.00° is shown in Figure 9.

[0085] Example 5 A spherical silver powder according to Example 5 was obtained in the same manner as in Example 1, except that the dried silver powder obtained was milled using a coffee mill (manufactured by Melitta Japan Co., Ltd.) containing 50 g of silver and crushed twice for 30 seconds. Using the obtained spherical silver powder, XRD analysis, BET specific surface area, particle size distribution, ignition loss (Ig-loss), and crystallite diameter Dx were measured or calculated. The results are shown in Table 2. Resistivity measurements were also performed using the obtained spherical silver powder. The results are shown in Table 3. Furthermore, an enlarged graph of XRD analysis of the spherical silver powder according to Example 5 in the 2θ range of 35.00° to 37.00° is shown in FIG. 5, and a graph of XRD analysis in the 2θ range of 35.00° to 40.00° is shown in FIG. 9.

[0086] 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 serving as a pH adjuster was added to the obtained aqueous silver complex solution, and the liquid temperature was adjusted to 28°C.

[0087] <Reduction Step> 180 seconds after the addition of the ammonia water to the pH-adjusted solution, 251.3 g 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 the solution, thereby obtaining a slurry containing silver particles.

[0088] <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.

[0089] <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.

[0090] The obtained spherical silver powder was used to measure or calculate XRD analysis, BET specific surface area, particle size distribution, loss on ignition (Ig-loss), and crystallite diameter Dx. The results are shown in Table 2. Furthermore, the obtained spherical silver powder was used to measure electrical resistance. The results are shown in Table 3. Furthermore, an enlarged graph of the XRD analysis of the spherical silver powder according to Comparative Example 1 in the 2θ range of 35.00° to 37.00° is shown in FIG. 6 , a graph of the XRD analysis in the 2θ range of 35.00° to 40.00° is shown in FIG. 9 , and a 50,000x SEM image of the spherical silver powder after crushing in the separation step is shown in FIG. 11 .

[0091] Comparative Example 2 <Silver Complex Formation Step> First, 3,448 g of a silver nitrate aqueous solution containing 50.84 g of silver was stirred at 174 rpm while 103.2 g of 28% by mass aqueous ammonia (manufactured by Junsei Chemical Co., Ltd.) was added to obtain an aqueous silver complex solution. Next, with continued stirring, 1.02 g of a 5% by mass aqueous sodium carbonate solution serving as a pH adjuster was added to the obtained aqueous silver complex solution, and 60 seconds after the addition of the aqueous ammonia, 0.508 g of a 5% by mass aqueous PEI solution (manufactured by Nippon Shokubai Co., Ltd., weight-average molecular weight 600) (amount of PEI added per 100 parts by mass of silver: 0.05 parts by mass) was added, and the liquid temperature was adjusted to 35°C.

[0092] <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.

[0093] <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 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.

[0094] <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.

[0095] The obtained spherical silver powder was used to measure or calculate XRD analysis, BET specific surface area, particle size distribution, loss on ignition (Ig-loss), and crystallite diameter Dx. The results are shown in Table 2. Furthermore, the obtained spherical silver powder was used to measure electrical resistance. The results are shown in Table 3. Furthermore, an enlarged graph of the XRD analysis of the spherical silver powder according to Comparative Example 2 in the 2θ range of 35.00° to 37.00° is shown in FIG. 7 , a graph of the XRD analysis in the 2θ range of 35.00° to 40.00° is shown in FIG. 9 , and a 50,000x SEM image of the spherical silver powder after crushing in the separation step is shown in FIG. 12 .

[0096] Comparative Example 3 A spherical silver powder according to Comparative Example 3 was 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 powder, XRD analysis, BET specific surface area, particle size distribution, loss on ignition (Ig-loss), and crystallite diameter Dx were measured or calculated. The results are shown in Table 2. Resistivity measurements were also carried out using the obtained spherical silver powder. The results are shown in Table 3. Furthermore, an enlarged graph of XRD analysis of the spherical silver powder according to Comparative Example 3 in the 2θ range of 35.00° to 37.00° is shown in FIG. 8, and a graph of XRD analysis in the 2θ range of 35.00° to 40.00° is shown in FIG. 9.

[0097]

[0098]

[0099]

[0100] As is clear from the results in Table 3, the spherical silver powders of Examples 1 to 5 can impart excellent low-temperature sintering properties to conductive pastes. It can be seen that conductive pastes using spherical silver powder in which the ratio of hexagonal Ag peak intensity to cubic Ag peak intensity is less than 0.5%, as in Comparative Examples 1 to 3, have poor low-temperature sintering properties. It can also be seen from the results in Table 3 that the spherical silver powders obtained by the manufacturing methods of Examples 1 to 5 can impart excellent low-temperature sintering properties to conductive pastes. It can be seen that conductive pastes using spherical silver powder obtained without adding a predetermined amount of the first chelating agent, as in Comparative Examples 1 to 3, have poor low-temperature sintering properties.

[0101] 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 having a cubic Ag peak and a hexagonal Ag peak in XRD analysis, wherein the ratio of the intensity of the hexagonal Ag peak to the intensity of the cubic Ag peak is 0.5% or more.

2. The spherical silver powder according to claim 1, having a crystallite diameter of 28 nm or less.

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

4. 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 2. The spherical silver powder according to claim 1, which satisfies the relationship: ≦2.5 (1).

5. Cumulative 50% particle diameter D on a volume basis measured by laser diffraction method ５０ The spherical silver powder according to claim 1, wherein the particle size is 0.5 μm or more and 6 μm or less.

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

7. A method for producing spherical silver powder as described in claim 1, comprising: a silver complex formation step of adding ammonia and a first chelating agent consisting of ethylenediaminetetraacetic acid 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, 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.

8. The method for producing spherical silver powder according to claim 7, further comprising adding a second chelating agent made of a polymer to the silver-containing aqueous solution or the silver complex aqueous solution.

9. A method for producing spherical silver powder according to claim 8, 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.

10. A method for producing spherical silver powder according to any one of claims 7 to 9, wherein a surface treatment agent is added to the slurry containing the precipitated silver particles after the reduction step.

11. A conductive paste containing the spherical silver powder according to any one of claims 1 to 6 as a conductive filler.

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