JP7480947B2 - Method for producing silver nanoparticles having a wide particle size distribution and silver nanoparticles - Google Patents

Description

The present invention relates to a method for producing silver nanoparticles that have a large particle size, a wide particle size distribution, and are easily adjusted to have viscosity behavior suitable for screen printing, and to silver nanoparticles.

Silver nanoparticles are used as electrical wiring on substrates and as a bonding material for power device semiconductors because they can be sintered at low temperatures due to the melting point lowering properties of metal nanoparticles. However, because silver nanoparticles are so fine, they tend to aggregate and are also easily fused due to the lowering of their melting point, so an organic layer called a protective agent is made to exist on the surface of the silver nanoparticles. These organic layers are often made of fatty acids or alkylamines, and silver nanoparticles coated with alkylamines or alkyldiamines in particular are known as silver nanoparticles that can be sintered at low temperatures because the protective agent is released at a relatively low temperature (Patent Documents 1 and 2).

It is expected that such a fired coating film using silver nanoparticles that can be fired at low temperatures will be used as a conductive material for wiring, etc., but in order to ensure the reliability and conductivity of the conductive material for wiring, etc., a method of making the wiring into a thick film is used. As silver nanoparticles designed for the purpose of making such thick films, Patent Document 3 reports a silver paint composition (ink) that uses silver nanoparticles with a primary particle diameter of several nm to several tens of nm to obtain a conductive silver coating film of 5 to 20 μm. In addition, Patent Document 4 describes that by including 30% or more silver particles with a particle diameter of 100 to 200 nm on a particle number basis, it is possible to reduce the resistance when made into a sintered body, and that such silver particles can be obtained by including 5 to 100 parts by weight of water in the reaction system per 100 parts by weight of the silver compound.

In these known documents, silver nanoparticles are created by utilizing a complex formation reaction between a silver compound and an amine compound. Specifically, a complex is formed without a solvent using silver oxalate and an alkylamine or alkyldiamine. However, when a complex is formed without a solvent, the resulting solid has no fluidity and is difficult to stir, the system lacks uniformity, and localized exothermic reactions occur, which creates quality and safety problems and makes industrial practical use difficult. Therefore, methods for producing silver nanoparticles that improve quality and safety by performing a complex formation reaction in an alcohol solvent to promote and assist the complex reaction, improve the stirring ability in the system, and suppress the sudden generation of carbon dioxide gas due to thermal decomposition have been reported (Patent Documents 5 to 7).

In addition, in the process of thermally decomposing a silver oxalate-alkylamine complex, when by-product gases (mainly carbon dioxide gas) are generated and discharged, the easily volatile alkylamine is also discharged from the system. In order to avoid this problem, a manufacturing method has been reported in which the complex is continuously introduced into a reaction vessel and the amount of by-product gas generated during the thermal decomposition reaction is controlled (Patent Document 8).

Patent No. 5574761 JP 2012-162767 A Patent No. 6001861 Japanese Patent No. 5795096 Patent No. 5975440 Patent No. 6026565 JP 2016-132825 A JP 2015-40319 A

However, in the examples of Patent Document 3, only coating films with thicknesses of less than 8 μm are created. Even if a conductive coating film of 10 μm or more is created, the silver particles are mainly composed of particles with a primary particle diameter of several tens of nm, so the amount of organic protective agent is large and volume shrinkage occurs due to the removal of the protective agent, resulting in low dimensional stability and a high possibility of disconnection due to cracks.

Even if the amount of by-product gas is controlled as in Patent Documents 5 to 8, the fact remains that carbon dioxide gas containing alkylamines is ultimately discharged outside the system.
Furthermore, although the silver nanoparticle dispersion and the coating composition (ink) are used by applying them to a substrate, the conditions for silver nanoparticles to have a viscosity behavior suitable for application to a substrate have not been fully investigated, and therefore the coating film actually obtained sometimes does not have sufficient performance.
In particular, in screen printing, precise viscosity characteristics are required in order to pass through the screen, but it has been found that merely using the particles described in Patent Document 4 is not sufficient.

The present invention aims to solve the above problems and provide a method for producing silver nanoparticles that takes into consideration the scale-up of workability, safety, and environmental aspects, as well as silver nanoparticles with a wide particle size distribution range that can be applied to various printing methods, particularly screen printing, and can form thick conductive films, and a silver nanoparticle coating composition.

In order to solve these problems, the present inventors conducted extensive research. As a result, they discovered that by forming a complex between a silver compound and an amine compound in an organic solvent and by having a certain amount of water present during the complex formation, the resulting silver particles have excellent particle size and distribution, and the resulting sintered coating film also has excellent performance, leading to the present invention. Furthermore, by bonding an amine compound with a specific molecular length to the silver particle surface, it is possible to produce silver particles with a good balance between dispersibility and low-temperature sintering properties, and it was found that a dispersion of these silver particles in an organic solvent has viscosity characteristics particularly suitable for screen printing, and a silver paint composition in which the generation of aggregates can be suppressed can be obtained.

That is, the present invention includes the following inventions.
(1) A method for producing silver nanoparticles, comprising reacting a thermally decomposable silver compound (a) with an amine compound (b) capable of forming a complex with (a) in an organic solvent (c) to form a complex, and then heating the complex to thermally decompose it, thereby forming silver nanoparticles, wherein 5 to 20 parts by weight of water is present per 100 parts by weight of the silver compound (a) during the formation of the complex, and the molar ratio of the amine compound (b) to the silver atoms in the silver compound (a) is 0.7 or more;
(2) The method for producing silver nanoparticles according to (1) above, wherein (b) is 2-amino-2-methyl-1-propanol.
(3) The method for producing silver nanoparticles according to (1) or (2) above, wherein (a) is silver oxalate.
(4) The method for producing silver nanoparticles according to (3) above, wherein the weight ratio of (c)/(a) is 0.8 to 1.3.
(5) A method for producing a silver nanoparticle dispersion, comprising preparing silver nanoparticles by the method according to any one of (1) to (4) above, and dispersing the obtained silver nanoparticles in an organic solvent.
(6) A method for producing a silver coating composition, comprising the steps of: preparing silver nanoparticles by the method according to any one of (1) to (4) above; dispersing the obtained silver nanoparticles in an organic solvent; and adding an organic binder to the silver nanoparticles.

(7) A method for producing a silver conductive material, comprising a step of applying the silver nanoparticle dispersion obtained by the method described in (5) above or the silver coating composition obtained by the method described in (6) above onto a substrate and baking the applied composition to form a silver conductive layer;
(8) Silver nanoparticles having an amine compound bonded to the particle surface, the viscosity ratio (v1/v2) of the viscosity (v1) at an increasing shear rate and the viscosity (v2) at a decreasing shear rate being 2.0 or ^less in a shear rate range of 2 to 30 s-1 when the silver nanoparticles are contained in Texanol at 78.5% by weight, the average particle diameter being 70 to 350 nm and the coefficient of variation being 40% to 80%.
(9) The silver nanoparticles according to the above (8), wherein the length of the molecule of the amine compound bonded to the particle surface is 7 to 8 Å.
(10) A silver nanoparticle dispersion, comprising the silver particles according to (8) or (9) above dispersed in an organic solvent.
(11) A silver nanoparticle dispersion containing 70 to 95 wt % of the silver nanoparticles according to (8) or (9) above, characterized in that the viscosity ratio (v1/v2) of the viscosity when the shear rate is increasing (v1) and the viscosity when the shear rate is decreasing (v2) is 2.0 or less in a shear rate range of ² to 30 s-1.
(12) A silver coating composition comprising the silver nanoparticles according to (8) or (9) above dispersed in an organic solvent and further comprising an organic binder.
(13) A method for producing a silver conductive material, comprising a step of applying the silver nanoparticle dispersion according to (10) or (11) above onto a substrate and baking the applied coating to form a silver conductive layer.
(14) A method for producing a silver conductive material, comprising the steps of applying the silver coating composition according to (12) above onto a substrate and baking the composition to form a silver conductive layer.

The present invention makes it easy to obtain silver particles that are large and controlled to have a wide particle size distribution. A silver coating composition containing such silver particles can be sintered even at low temperatures below 150°C, and the resulting sintered body exhibits a low resistance value close to that of bulk silver. The present invention is expected to be used as a material that can be used to form thick silver wiring of several to several tens of μm on plastic substrates with relatively low heat resistance, such as PET or polypropylene, by printing methods such as screen printing, or as a conductive bonding material or a bonding material for electrical equipment that handles large currents, such as power devices.

In addition, the synthesis of silver particles in the present invention can obtain large-sized and widely distributed silver particles even when using a smaller amount of amine compound than in conventional synthesis methods, and can further reduce the use of alkylamines, which are harmful to the human body and the environment, providing a highly safe production method for scaled-up industrial production.

FIG. 1 is a SEM photograph of the particles obtained in Example 1. FIG. 2 is a diagram showing SEM photographs of the particles obtained in Examples 2 and 3. FIG. 3 is a SEM photograph of the particles obtained in Example 4. FIG. 4 is a SEM photograph of the particles obtained in Example 5. FIG. 5 is a SEM photograph of the particles obtained in Example 6. FIG. 6 is a SEM photograph of the particles obtained in Example 7. FIG. 7 is a diagram showing SEM photographs of the particles obtained in Examples 8 and 9. FIG. 8 is a SEM photograph of the particles obtained in Comparative Example 1. FIG. 9 is a diagram showing an STEM photograph of the particles obtained in Comparative Example 2. FIG. 10 is a SEM photograph of the particles obtained in Comparative Example 3. FIG. 11 is a view showing an SEM photograph of the particles obtained in Comparative Example 4.

FIG. 12 is a SEM photograph of the particles obtained in Comparative Example 5. FIG. 13 is a diagram showing an STEM photograph of the particles obtained in Comparative Example 6. FIG. 14 is a diagram showing an STEM photograph of the particles obtained in Comparative Example 7. FIG. 15 is a diagram showing a particle size distribution histogram of the particles obtained in Example 1. FIG. 16 is a diagram showing particle size distribution histograms of the particles obtained in Examples 2 and 3. FIG. 17 is a diagram showing a particle size distribution histogram of the particles obtained in Example 4. FIG. 18 is a diagram showing a particle size distribution histogram of the particles obtained in Example 5. FIG. 19 is a diagram showing a particle size distribution histogram of the particles obtained in Example 6. FIG. 20 is a diagram showing a particle size distribution histogram of the particles obtained in Example 7. FIG. 21 is a diagram showing particle size distribution histograms of the particles obtained in Examples 8 and 9. FIG. 22 is a diagram showing a particle size distribution histogram of the particles obtained in Comparative Example 1. FIG. 23 is a diagram showing a particle size distribution histogram of the particles obtained in Comparative Example 2. FIG. 24 is a diagram showing a particle size distribution histogram of the particles obtained in Comparative Example 6. FIG. 25 is a diagram showing a particle size distribution histogram of the particles obtained in Comparative Example 7. FIG. 26 is a diagram showing an SEM photograph of the coating surface of the paste prepared from the particles obtained in Example 8. FIG. 27 is a diagram showing an SEM photograph of the coating surface of the paste prepared from the particles obtained in Example 9. FIG. 28 is a diagram showing an SEM photograph of the coating surface of the paste made from the particles obtained in Comparative Example 1. FIG. 29 is a diagram showing comparative viscosity data of pastes made from the particles obtained in Examples 8 and 9. FIG. 30 is a diagram showing viscosity data of a paste made from the particles obtained in Comparative Example 1.

The present invention is characterized in that, in a method for producing silver nanoparticles by reacting a thermally decomposable silver compound (a) with an amine compound (b) that forms a complex, the complex is formed in an organic solvent and a certain amount of water is present during the complex formation. This makes it easy to synthesize silver nanoparticles in the large particle size range of 200 to 500 nm when producing silver nanoparticles with a wide particle size distribution, and further, by using the method in combination with a polar solvent, the total amount of the amine compound used can be reduced.
Furthermore, by bonding a specific protective agent to the surface of silver particles having a specific particle size and particle size distribution, it is possible to suppress agglomeration and produce a silver coating composition with viscosity behavior that is particularly suitable for screen printing.
The details will be explained below.

<Description of materials used in silver nanoparticle synthesis>
[1. Silver compound (a)
In the method for producing silver particles of the present invention, a thermally decomposable silver compound is used as a starting material. The thermally decomposable silver compound refers to a silver compound that is complexed with the component (b) described below and thermally decomposed under heating conditions possible with normal equipment. Specifically, silver oxalate, silver nitrate, silver acetate, silver carbonate, silver oxide, silver nitrite, silver benzoate, silver cyanate, silver citrate, silver lactate, etc. can be applied. Among these silver compounds, silver carbonate or silver oxalate (Ag ₂ C ₂ O ₄ ) is particularly preferred. More preferably, silver oxalate is used. Silver oxalate can be decomposed at a relatively low temperature to produce silver particles without the need for a reducing agent. In addition, carbon dioxide generated by decomposition is released as a gas, so impurities do not remain in the solution.

[2. Complex-forming amine compound (b)]
Next, in the present invention, an amine compound capable of forming a complex with a silver compound is used as component (b). This compound has the function of forming a complex with the silver compound to lower the thermal decomposition temperature of the silver compound and enable the production of silver particles at a low temperature. At the same time, the organic group of the amine compound functions as a protective agent that provides the effect of dispersion stability of the silver particles.
Such an amine compound is not particularly limited as long as it can form a complex with a silver compound. In particular, the number of hydrogen atoms bonded to the amino group of the amine compound is preferably one or two, i.e., a primary amine ( _RNH2 ) or a secondary amine ( _R2NH ).

As explained in the background art, the formation of silver particles from a silver compound and an amine compound capable of forming a complex with the silver compound is itself publicly known. In the present invention, it is also acceptable to use aliphatic hydrocarbon amine compounds such as "aliphatic hydrocarbon monoamines" and "aliphatic hydrocarbon diamines" that have been used in publicly known techniques as amine compounds. Specific examples of "aliphatic hydrocarbon monoamines" and "aliphatic hydrocarbon diamines" include alkylamines, alkoxyamines, and alkyl ether amines. Among these, examples of alkylamines include n-butylamine, n-pentylamine, n-hexylamine, n-heptylamine, n-octylamine, and 2-ethylhexylamine.

However, among these amine compounds, alkylamines have a strong irritating odor and there is a risk that they will be emitted at high temperatures together with carbon dioxide gas during the decomposition reaction, so if it is desired to suppress the irritating odor, it is preferable to use amine compounds containing oxygen atoms, specifically alkoxyamines, alkyl ether amines, and amino alcohols.
Through the inventors' research, it has been found that not only do these amine compounds containing oxygen atoms have a reduced irritating odor, but also that the specific amino alcohols described below in particular produce silver particles with excellent physical properties.

(2-1. Amino alcohol (b1))
As the amine compound containing an oxygen atom, an amino alcohol having 3 to 4 carbon atoms is particularly preferred because it can further enhance the effect of increasing the particle size and widening the particle size distribution. More preferred are (i) a branched primary amino alcohol having 3 to 4 carbon atoms, (ii) one each of an amino group and a hydroxyl group, and (iii) one in which the amino group and the hydroxyl group are bonded via an alkyl chain having 2 carbon atoms, or iv) a linear secondary amino alcohol having 3 carbon atoms. By using such a specific amino alcohol, it is possible to easily make the particle size of the obtained silver particles large and have a wide distribution.
Here, (i) "branched" means that the skeleton consisting of carbon atoms and heteroatoms is not linear but branched. (ii) Preferably, the skeleton has one or more amino groups and one or more hydroxyl groups, and more preferably has one of each. (iii) "An amino group and a hydroxyl group are bonded via an alkyl chain having two carbon atoms" means that an amino group and a hydroxyl group are bonded to each of two adjacent carbon atoms in the skeleton consisting of carbon atoms and heteroatoms.
Furthermore, by using an amino alcohol that satisfies such conditions (i) to (iii) or (iv), it is possible to promote the formation of a complex with a silver compound and to obtain particles having a large particle size and a wide distribution.

Examples of amino alcohols with 4 or less carbon atoms that satisfy the above conditions (i) to (iii) include 1-amino-2-butanol, DL-1-amino-2-propanol, 2-amino-2-methyl-1-propanol (hereinafter, AMP), and DL-2-amino-1-propanol. An example of an amino alcohol that satisfies (iv) is N-methylethanolamine.

Among these, AMP, 1-amino-2-butanol, DL-1-amino-2-propanol, and DL-2-amino-1-propanol are particularly preferred because they are easy to handle, complex formation occurs easily in the presence of a polar solvent such as an alcohol solvent, and silver particles having a large particle size and a wide particle size distribution can be easily obtained. Among these, AMP is particularly excellent in terms of the above effects. Furthermore, by bonding to the silver surface, it is also excellent in the effect of increasing the viscosity of the dispersion.
The above component (b) may be used alone or in combination of two or more.

(2-2. Amine compound (b2) with a molecular length of 5 Å or more)
In the present invention, in order to prevent fusion and aggregation of particles and obtain a stable dispersion, it is preferable to contain the following amine compounds: Namely, amine compounds capable of forming a complex with a silver compound, having a molecular length of 5 Å or more, and containing N, C and H atoms or N, C, H and O atoms constituting the amine compound.

Here, the length of the molecule is the longest distance between two atoms that do not include hydrogen atoms. This length of the molecule can be calculated. The calculation conditions are density functional theory, function ωB97X-D, basis function 6-31+G*, environment in vacuum, energy state ground state, and various molecular calculation software such as SPARTAN`16V1,1,0 can be used. In addition, if the length of the molecule is 5 Å or more, more preferably 7 Å or more, it has excellent function as a protective agent that provides dispersion stability due to the steric hindrance effect. Among them, those with a molecular length of 5 to 8 Å are preferable, and those with a molecular length of 7 to 8 Å are even more preferable. Among these amine compounds, amine compounds with a main chain (main skeleton) consisting of 7 atoms or more including the amino group are particularly preferable. In addition, if it is branched, the longest chain is the main chain. The number of hydrocarbon groups bonded to the amino group of the amine compound is not limited, but primary amines or secondary amines with one or two are particularly preferable because they are easy to form coordinate bonds with silver.

Specific examples of such amine compounds with a molecular length of 5 Å or more include alkylamines such as n-butylamine, n-pentylamine, n-hexylamine, n-heptylamine, n-octylamine, and 2-ethylhexylamine. Alkoxyamines include 3-methoxypropylamine and 3-ethoxypropylamine. Alkyletheramines include the M series, M-600, M-1000, M-2005, and M-2070 of JEFFAMINE manufactured by HUNTSMAN. Aminoalcohols include 4-amino-1-butanol, 5-amino-1-pentanol, and 6-amino-1-hexanol. Aminoethoxys include diglycolamine.

More preferred examples of amines having 7 or more atoms and a molecular length of 7 Å or more include alkylamines such as n-pentylamine, n-hexylamine, n-heptylamine, n-octylamine, and 2-ethylhexylamine. Alkoxyamines include 3-methoxypropylamine and 3-ethoxypropylamine. Aminoalcohols include 5-amino-1-pentanol and 6-amino-1-hexanol. Aminoethoxys include diglycolamine. More preferred are n-hexylamine and 3-ethoxypropylamine. The above (b2) components can be used alone or in combination of two or more.

(2-3. Ratio of (b1) and (b2))
Among the amine compounds (component (b)) capable of forming a complex, when a specific aminoalcohol (b1) having 4 or less carbon atoms and an amine compound (b2) having a molecular length of 5 Å or more are used, the molar ratio represented by (b1)/[(b1)+(b2)] is preferably 0.3 to 0.8 (molar ratio), and more preferably 0.4 to 0.8.
This range is most suitable for producing large silver nanoparticles with a wide distribution. If the molar ratio is smaller than 0.3, the amount of amine compound with a long molecular length is increased, the particles tend to become smaller overall, and the particle size distribution tends to become narrower, making it difficult to obtain particles with a large particle size and a wide distribution. If the molar ratio is larger than 0.8, the steric hindrance effect as a protective agent is weakened, and the risk of fusion of silver particles during synthesis increases.

[3. Ratio and amount of amine compound (b) and silver compound (a)]
Regarding the mixed amount of silver atoms of the silver compound and amine compound (b), the molar ratio (amine compound/silver atoms of the silver compound) is 0.7 or more. The amount of amine compound (b) + (d) is adjusted so that it is preferably 0.7 to 2.0, more preferably 0.8 to 1.5, and most preferably 0.8 to 1.3. By doing so, the particle size is varied, and it is easy to obtain silver particles in the desired particle size range.

In the various conventional synthesis methods described above (Patent Documents 1 to 9), the molar ratio of the amine compound to the silver atom of the silver compound is 2.0 or more. In contrast, in the present invention, silver particles can be obtained with a smaller amount of amine, so that the amount of amine discharged can be reduced. Therefore, the risk of human body and environmental burden due to the release of amine outside the system can be reduced. At the same time, such conventional methods tend to synthesize particles with small particle diameters and narrow distributions, whereas in the present invention, silver particles with a wide distribution and large particle diameters can be obtained, and finally, a thick-film conductive sintered body with excellent low-temperature sintering properties and low resistance can be obtained.
On the other hand, if the molar ratio is less than 0.7, flat particles are likely to be produced and are likely to aggregate, resulting in a decrease in the dispersion stability of the silver coating composition.
In addition, under conditions that tend to produce flat particles, hysteresis in shear rate is likely to occur. This is thought to be because, when particles have a non-uniform shape, the adsorption of the dispersant on the particle surface becomes non-uniform, and the inter-particle interaction due to the increase in the contact area between particles is also large. For this reason, in the shear rate range of 2 to 30 s ^-1 described below, it becomes difficult to obtain particles with a viscosity ratio (v1/v2) of 1.2 or less between the viscosity at the time of increasing the shear rate (v1) and the viscosity at the time of decreasing the shear rate (v2).
The molar ratio of the b component, particularly (b2), to the silver compound (b2)/(silver atom of the silver compound) is preferably 0.2 or more. It is particularly preferably 0.2 to 1.4, and even more preferably 0.4 to 0.9. If this ratio is below 0.2, fusion of silver particles tends to occur easily. This is presumably because the steric hindrance effect of the b2 component as a protective material decreases. On the other hand, if it exceeds 1.4, it becomes difficult to satisfy the above-mentioned molar ratio of b/silver compound and the preferred ratio of b1/b2.

4. Organic Solvent (c)
In the present invention, the above-described complex-forming reaction between the silver compound and the amine compound is carried out in the presence of an organic solvent.
In the present invention, a solvent having a polar functional group is preferred, and specifically, an alcohol-based solvent, a ketone-based solvent, an aldehyde-based solvent, an amide-based solvent, an ester-based solvent, a nitrile-based solvent, an ether-based solvent, a glycol-based solvent, a glycol ether-based solvent, a glycol ester-based solvent, or a glyme-based solvent is preferred.
In particular, alcohol solvents or glyme-based solvents are preferred, and among these, alcohols having 3 to 12 carbon atoms are preferred. For example, n-propanol (boiling point bp: 97°C), isopropanol (bp: 82°C), n-butanol (bp: 117°C), isobutanol (bp: 107.89°C), sec-butanol (bp: 99.5°C), tert-butanol (bp: 82.45°C), n-pentanol (bp: 136°C), n-hexanol (bp: 156°C), n-octanol (bp: 194°C), 2-octanol (bp: 174°C), n-nonanol (bp: 215°C), 5-nonanol (bp: 195°C), n-decanol (bp: 232.9°C), n-undecanol (bp: 243°C), 2-undecanol (bp: 131°C), n-dodecanol (bp: 259°C), 2-dodecanol (bp: 250°C), and the like.
Examples of glyme-based solvents include monoglyme (bp: 85° C.), ethylglyme (bp: 121° C.), diglyme (bp: 162° C.), dipropylene glycol dimethyl ether (bp: 171° C.), ethyl diglyme (bp: 188° C.), triglyme (bp: 216° C.), butyl diglyme (bp: 256° C.), tetraglyme (bp: 275° C.), etc. Among these, those with a boiling point of 150° C. or higher are easy to handle.
Among these, n-butanol, n-hexanol, n-decanol, and diglyme are preferred in view of the ability to increase the temperature of the subsequent thermal decomposition step of the complex compound and the convenience of post-treatment after the formation of silver nanoparticles. These may be used alone or in combination of two or more kinds.

(4-1. Amount of organic solvent added)
In order to thoroughly stir the components, the organic solvent is preferably a mixture of 80 to 130 parts by weight (i.e., an amount such that the weight ratio of (c)/(a) is 0.8 to 1.3) per 100 parts by weight of the silver compound (a), and more preferably 80 to 125 parts by weight per 100 parts by weight of (a).

(4-2. Method of adding organic solvent)
In the present invention, the reaction of forming a complex between the amine compound (b) and the silver compound (a) in the presence of an organic solvent can take several forms.
For example, a solid silver compound may be mixed with an organic solvent, particularly an alcohol solvent, to obtain a silver compound-alcohol slurry, and then the amine compound (b) may be added to the obtained silver compound-alcohol slurry. In the present invention, the term "slurry" refers to a mixture in which a solid silver compound is dispersed in an organic solvent or a mixture of an organic solvent and an amine compound. To obtain the slurry, a solid silver compound may be charged into a reaction vessel, and an organic solvent or a mixture of an organic solvent and an amine compound may be added thereto to obtain the slurry.

Alternatively, a mixture of an organic solvent and an amine compound may be charged in a reaction vessel, and the silver compound may be added thereto.
It has been reported that silver oxalate is explosive in a dry state. Therefore, when silver oxalate is used as a silver compound, it is preferable to use it in a wet state. This is because the explosiveness is significantly reduced by making it wet, making it easier to handle. Therefore, it is sufficient to mix it with water or the above-mentioned organic solvent to make it wet before use.

[5. Aliphatic Carboxylic Acids]
In addition, in order to adjust the particle size and particle size distribution, an aliphatic carboxylic acid may be used during complex formation. By adding an aliphatic carboxylic acid, the particle size tends to become small and the particle size distribution tends to become narrow. It is preferable to use the aliphatic carboxylic acid after appropriately adjusting the water content. The aliphatic carboxylic acid is preferably used together with the amines, and can also be added when the silver compound and the amine are mixed.
The aliphatic carboxylic acid may be a saturated or unsaturated aliphatic carboxylic acid. Examples of the aliphatic carboxylic acid include saturated aliphatic monocarboxylic acids having 4 or more carbon atoms, such as butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, dodecanoic acid, tridecanoic acid, tetradecanoic acid, pentadecanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, nonadecanoic acid, icosanoic acid, and eicosenoic acid; and unsaturated aliphatic monocarboxylic acids having 8 or more carbon atoms, such as oleic acid, elaidic acid, linoleic acid, and palmitoleic acid.

Among these, saturated or unsaturated aliphatic monocarboxylic acids having 8 to 18 carbon atoms are preferred. By making the number of carbon atoms 8 or more, it is possible to ensure a distance between the silver particles when the carboxylic acid group is adsorbed onto the surface of the silver particles, improving the effect of preventing aggregation between the silver particles. Considering ease of availability, ease of removal during firing, etc., saturated or unsaturated aliphatic monocarboxylic acid compounds having up to 18 carbon atoms are usually preferred. In particular, octanoic acid, oleic acid, etc. are preferably used. Of the above aliphatic carboxylic acids, only one type may be used, or two or more types may be used in combination.

(5-1. Amount of Aliphatic Carboxylic Acid Added)
When the aliphatic carboxylic acid is used, it is preferable to use, for example, about 0.05 to 10 moles, preferably 0.1 to 5 moles, and more preferably 0.5 to 2 moles, per mole of silver atoms of the raw silver compound. If the amount of the aliphatic carboxylic acid is less than 0.05 moles per mole of silver atoms, the effect of controlling the particle size by adding the aliphatic carboxylic acid is weak. On the other hand, if the amount of the aliphatic carboxylic acid reaches 10 moles, the particle size may be too small and uniform, and the aliphatic carboxylic acid may remain in the washing or surface protective agent replacement process, making it difficult to remove the aliphatic carboxylic acid by low-temperature baking. However, it is not necessary to use the aliphatic carboxylic acid.

[6. Explanation of water and moisture content]
The present invention is characterized in that a certain amount of water is present in the presence of an organic solvent during the formation of the complex between the silver compound and the amine compound described above. That is, 5 to 20 parts by weight of water is present per 100 parts by weight of the silver compound (a). The inventors have found that the silver nanoparticles obtained by this method have a relatively large particle size and a wide particle size distribution. These silver nanoparticles with a large particle size and a wide distribution have excellent effects, as described below.

Particularly preferably, it is from 12 to 18 parts by weight, and further preferably, it is from 14 to 16 parts by weight.
If the water content is less than 5 parts by weight, the particle size distribution of the obtained silver particles will be too uniform, resulting in voids in the sintered body, making it difficult to obtain silver particles with a wide distribution. On the other hand, if the water content exceeds 20 parts by weight, the silver particles will become too coarse, resulting in portions where the particles fuse and coalesce, which is not preferable. As for the water used, ion-exchanged water with reduced metal ion impurities is preferable. The timing of adding water is before the heating step, and it may be added at any stage, either before or after the formation of the silver-amine complex.
The ratio of the organic solvent (c) to water is preferably 0.03 to 0.3 by weight, more preferably 0.1 to 0.25 by weight. Within this range, it is particularly easy to obtain silver nanoparticles having the particle size and particle size distribution of the present invention described below.

<Method of manufacturing silver nanoparticles>
[7. Mixing of liquid raw materials]
In the present invention, the amine compound that forms a complex is usually placed in the polar solvent (c) and mixed. If necessary, an aliphatic carboxylic acid and water can be added and mixed to adjust the liquid raw materials required for the reaction.
If the liquid raw material contains a solid substance at room temperature, it can be mixed by heating appropriately. The heating temperature is preferably 100° C. or less, more preferably 80° C. or less, and more preferably 60° C. or less to liquefy the liquid raw material. If the temperature is higher than the above range, when the material is mixed with a silver compound to form a slurry, a partial complexation/oxalic acid decomposition reaction may start first, and silver nanoparticles may be generated without ensuring uniformity in the system.

8. Preparation of silver compound slurry
The silver compound (a) and the liquid raw material are mixed to prepare a silver compound slurry. Alternatively, the polar solvent and the silver compound (a) alone may be mixed first, and the amine compound may be added later.
A silver compound is mixed with a predetermined amount of an amine mixture, or, if necessary, an aliphatic carboxylic acid and water. The mixture is preferably stirred at room temperature, or cooled to room temperature or lower as appropriate, since the coordination reaction (complexation reaction) of the silver compound with amines is exothermic, and then stirred. The mixture of the silver compound and the amine compound is mixed in the presence of an organic solvent, so stirring and cooling can be performed well. The organic solvent and the excess of the amine compound serve as the reaction medium.

In addition, the odor of highly volatile alkylamines has a significant adverse effect on the working environment. In the present invention, the amount of highly volatile alkylamines used in the synthesis of silver nanoparticles can be reduced or eliminated, reducing odors and worker exposure when preparing raw materials.

[9. Silver Amine Complex]
Since the complex compound produced generally exhibits a color according to its constituent components, the progress of the reaction of producing the complex compound can be detected from the change in color of the reaction mixture. In addition, when it is difficult to confirm the progress by the change in color, the production state can be detected by the change in viscosity or temperature of the reaction mixture. In this way, a silver-amine complex is obtained in a medium mainly composed of a polar solvent and an amine compound.

[10. Explanation of heating rate conditions from complexation to decomposition reaction]
In the heating step of the reaction system, the heating rate affects the particle size of the precipitated silver particles, so the particle size of the silver particles can be controlled by adjusting the heating rate in the heating step. Here, the heating rate is desirably adjusted within the range of 3.0 to 50°C/min up to the set decomposition temperature. The slower the heating time, the more likely the particles will grow and the larger the particle size will be. However, if the heating rate is slower than 3.0°C/min, the particle growth is likely to be promoted and the particles will become the same as adjacent particles, which is not preferable.

[11. Mechanism of broadly distributed silver particle generation by adding water]
By adding the amount of water specified in the present invention to the reaction mechanism of silver particle formation by thermal decomposition as described above, the particle size of the formed silver nanoparticles varies, and silver particles with a high distribution are obtained. Although some parts of the mechanism are unclear, it is thought that water approaches the silver compound, especially silver oxalate, and inhibits the amine compound from adsorbing to the silver atoms when the silver amine complex is formed or decomposed by heating, and the inhibited part grows into particles. Furthermore, since the amount of water molecules adsorbed to the silver oxalate is also biased (localized), it is thought that a moderate variation in particle size occurs. In contrast, if water is added in an amount greater than 20 wt% to the silver compound, it is thought that the adsorption of the amine to the silver atoms is too inhibited, causing the silver particles themselves to swell and sinter and coalesce with adjacent particles.

[12. Silver particle washing process]
The thermal decomposition of the silver compound produces a suspension with a color ranging from dark brown to gray, although the color varies depending on the particle size of the resulting particles. By removing the polar solvent and excess amine compound from this suspension, for example by settling the silver nanoparticles and performing decantation and washing with an appropriate solvent (water or organic solvent), silver nanoparticles to which the amine compound is bonded can be obtained.

13. Description of cleaning solvent
The silver particles are preferably washed using an alcohol solvent with a boiling point of 150° C. or less, such as methanol, ethanol, or propanol. As a detailed washing method, it is preferable to add a solvent to the solution after the synthesis of the silver particles, stir until the solution is suspended, and then remove the supernatant liquid by decantation. The amount of amine removed can be controlled by the volume of the solvent added and the number of washings. When the above-mentioned series of operations is performed once, it is preferable to wash the silver particles 1 to 5 times using a solvent with a volume of 1/20 to 3 times that of the solution after the synthesis of the silver particles.

[14. Protective agent replacement step]
Furthermore, the organic group introduced into the silver nanoparticles by the amine compound in the above complex formation process can be replaced with another amine compound. For example, it can be replaced with an amine compound having a functional group with a larger steric hindrance and a higher dispersion stabilization effect. In addition, it may be replaced with an amine compound suitable for the use of the silver particles. The amine compound used for replacement here may be the above-mentioned (b) component, particularly (b2) component or (b1), or other compounds may be used.
In particular, alkylamines having 4 to 8 carbon atoms or amine compounds containing oxygen atoms (alkoxyamines, alkyl ether amines, amino alcohols, aminoethoxy) are preferred. The alkylamines and amine compounds containing oxygen atoms can be used alone or in combination of two or more kinds, and the viscosity of the paste can be adjusted depending on the composition.

The replacement method involves stirring and suspending the washed silver particles in the amine compound that is ultimately to be replaced for a certain period of time, thereby replacing the surface protective agent on the silver particles. At this time, 50-100 wt % of the amine compound that is ultimately to be replaced is added to the pure silver content, and the mixture is usually stirred and suspended at room temperature for about 1 hour. The difference between before and after the surface protective agent replacement process can be confirmed by checking the difference in sintering properties in DTA measurements, headspace GC/MS, etc. After the above-mentioned surface protective agent replacement process, the desired silver particles are obtained by going through a washing process again.

[15. State of generated silver particles (protective agent, particle size distribution, viscosity hysteresis)]
In this way, silver nanoparticles to which the amine compound is bound are formed. The silver nanoparticles refer to fine particles that can be produced by the above method and are mainly composed of silver and usually have a particle size of about 1 nm to 1000 nm.
The amine compound bound to the surface of the silver nanoparticles functions as a protective agent that prevents the particles from fusing and agglomerating together. Even when checking the STEM image of the silver nanoparticles of the present invention, a layer of 1 nm to several nm is confirmed on the particle surface, and this is considered to be the amine compound bound to the particle surface. Here, "bound to the particle surface" refers to a state in which the amine compound is coordinate-bonded to the silver atoms present on the silver particle surface. Because of the coordinate bond, the amine compound remains even after a washing step to remove excess amine compound is performed. The difference between the free amine compound and the amine compound bound to the particle surface can also be confirmed by thermal analysis such as TG/DTA. The thermal decomposition temperature of the bound amine compound tends to be higher than that of the free amine compound. When the amine compound bound as a protective agent is thermally decomposed, the sintering of the silver particles begins at the same time, and an exothermic peak appears in the DTA chart. The amine compound that decomposes near that temperature is the so-called amine compound "bound to the particle surface".
Silver nanoparticles contain the amine compounds used in the complex formation process and the substitution process, and also contain the aliphatic carboxylic acid if used. The content ratio of each of these components in the silver particles is basically determined by taking into account the use ratio in the particle preparation process, as well as the increase or decrease in the washing process and the protective agent substitution process. Therefore, it is possible to adjust the type and total amount of the amine compound by the blending ratio and conditions of these processes.

The molecular length of the amine compound that is ultimately bonded to the silver particles is preferably 2 to 8 Å, more preferably 5 to 8 Å, and most preferably 7 to 8 Å. Specific examples of amine compounds that satisfy these conditions are as described above as component (b2). The total amount of the amine compound is preferably 0.3 to 2.0 parts by weight per 100 parts by weight of pure silver, and even more preferably 0.5 to 1.0 part by weight.

The silver nanoparticles of the present invention generally have a particle size of 1000 nm or less.
The average particle size is from 70 to 350 nm, preferably from 70 to 300 nm, and more preferably from 80 to 200 nm.
The coefficient of variation, which indicates the variation (distribution) of particle diameter, is comprised between 40 and 80%, preferably between 40 and 70%, and more preferably between 50 and 60%.

The average particle size and coefficient of variation are determined as follows. The particle shape of the obtained silver nanoparticles is observed using an FE-SEM. Then, using image analysis software SCANDIUM (manufactured by Olympus), the particle sizes (major axes) of 400 or more particles are measured to determine the average particle size and standard deviation. Using these values, the coefficient of variation is calculated based on the following formula.
Coefficient of variation (%) = {standard deviation (nm) / average particle size (nm)} x 100
There are no limitations on the equipment used to measure particle size, so long as it can provide results equivalent to those of the above method.

By having the above average particle size and coefficient of variation, the thickness of the coating film obtained by applying the silver paint can be increased. Specifically, a thick film of 10 to 30 μm can be obtained. Furthermore, not only is the thickness large, but the volume resistivity of the obtained film can also be reduced. Specifically, a film with a thickness of 20 μm or more and a low volume resistivity of about 6 to 7 μΩ·cm can be obtained. This is presumably because the particle size distribution is wide and small particles are packed close to close packing between large particles, resulting in a film with a high silver particle content due to high packing of silver particles.
If the average particle size is less than 70 nm, the amount of the amine compound protecting the surface of the silver particles increases, and the volume resistivity of the resulting coating film cannot be reduced.On the other hand, if the average particle size exceeds 350 nm, the phenomenon of the melting point depression of the silver nanoparticles becomes weak, making it difficult to sinter at low temperatures, and in this case too, the volume resistivity of the coating film cannot be reduced.
If the coefficient of variation is less than 30%, the particles will be uniform, making it impossible to fill the gaps between the particles and to reduce the volume resistivity of the coating film. On the other hand, if the coefficient of variation is more than 80%, even if there is some variation in the particles, the particle sizes will be too different, making it difficult to fill the gaps between the particles and also making it impossible to reduce the volume resistivity of the coating film.

By using the silver particles of the present invention having the above average particle size and variation, it is possible to adjust the viscosity of the silver coating composition to a suitable value.
The viscosity of the screen printing ink is preferably in the range of 0.1 to 500 Pa·s (at a shear rate of 5 s ⁻¹ ). If it is too high, there is no fluidity and printing defects are likely to occur, and if it is too low, the printed ink will sag and the line width will widen. Therefore, to increase the viscosity, an organic binder is usually added, but the organic binder increases the resistance value of the resulting coating film. In contrast, the silver particles of the present invention can have a relatively high viscosity even in a paste with a silver content of about 70 wt %, and even in a state where 1 wt % of Ethocel 45 (manufactured by Nisshin Chemical Industry) is added as an organic binder relative to the pure silver content, and the viscosity can be adjusted to about 30 to 40 Pa·s by adjusting the particle size to an average particle diameter of about 80 nm and a coefficient of variation of about 35%. Therefore, even if the amount of organic binder added is 1 wt % or less relative to the pure silver content, the viscosity can be made suitable for the above screen printing. In this way, since the viscosity can be controlled by adjusting the particle size, the degree of freedom in the amount of organic binder added increases and it can be reduced, which is very excellent.

Furthermore, in the present invention, small particles of 100 nm or less and large particles of 200 nm or more are mixed in an appropriate range, so that when applied, the coating is smooth and tends to give a glossy appearance.
In particular, it is preferred that the silver nanoparticles of the present invention have a particle size other than 100 to 200 nm (the total of (i) particles having a diameter of 100 nm or less and (ii) particles having a diameter of 200 nm or more) account for 70% or more by number.
The particle size here is the size of each particle determined by measuring the length of an SEM image using image analysis software according to the method described above.
More preferably, the total of particles having a particle size of (i) 100 nm or less and (ii) 200 to 500 nm or less is 70% or more by number. Still more preferably, the total of particles having a particle size of 100 nm or less is 60% or more by number.

The inclusion of both small particles of 100 nm or less and large particles of 200 nm or more has a particularly high effect of the small particles penetrating and filling in between the large particles, and is excellent in the effect of obtaining a thick coating film with a low volume resistivity due to the high content of silver particles. In particular, the inclusion of a high content of 60% of particles of 100 nm or less also has the effect of improving adhesion.
The silver nanoparticles of the present invention described above ensure low-temperature sintering properties and electrical conductivity, and it is easy to give a sintered coating film (a coating film obtained by applying to a substrate and sintering) a thickness of about 20 to 30 μm. This is presumably because the silver particles are densely packed when the coating film is formed due to the coexistence of large particle diameter regions and small particle diameter regions, resulting in fewer voids.
Furthermore, when the silver nanoparticles of the present invention are contained in Texanol at 78.5% by weight, the viscosity ratio (v1/v2) of the viscosity at increasing shear rate (v1) to the viscosity at decreasing shear rate (v2) is 2.0 or less in the shear rate range of 2 to 30 s ^-1 , preferably 1.5 or less, and more preferably 1.2 or less.

This means that the difference in viscosity between when the shear rate increases and decreases is small, that is, the hysteresis of the viscosity when the shear rate is changed is small. For example, comparing FIG. 29 showing the viscosity change in Examples 8 and 9 using the silver nanoparticles of the present invention with FIG. 30 showing the viscosity change in Comparative Example 1, in FIG. 29, the difference in viscosity between when the shear rate increases and when it decreases is small, and (v1/v2) is 2.0 or less in any range of shear rates from 2 to 30 s ^-1 , whereas in FIG. 30, there are some parts that exceed 2.0 (the viscosity (v1) when the shear rate increases and the viscosity (v2) when the shear rate decreases at shear rates of 1, 2, 3, and 4 s ^-1 , as well as their viscosity ratios (v1/v2) are as shown in Table 9). A large (v1/v2) means that viscosity hysteresis occurs, and that the interaction between particles is large and a pseudo-crosslinked state is formed. From this, it is considered that aggregation is likely to occur, and this is supported by the observation of aggregates in FIG. 28, which is a photograph of the coating surface.
In contrast, the silver nanoparticles of the present invention have a small (v1/v2) ratio and are less likely to aggregate, and therefore have excellent surface smoothness when formed into a coating film.
In addition, because there are few aggregates, it has excellent printing accuracy and continuous printability, especially in screen printing, and is less likely to clog the screen mesh.

As described above, the production method of the present invention makes it possible to control the particle size by the type of amine used, the type of organic solvent, the amount of water added, etc. Therefore, it is also suitable for industrial production, since it is possible to synthesize silver particles in the large particle size range of 200 to 500 nm and silver particles in the small particle size range of 50 to 200 nm in one batch.
The silver nanoparticles of the present invention thus obtained can contain many silver particles in the large particle size range of 200 nm or more, so they are less likely to aggregate (sinter) during processes such as washing off excess protective agent from the silver nanoparticles, protective agent replacement treatment, and pasting, and can easily produce silver nanoparticle dispersions and silver paint compositions without impairing the inherent properties of the silver particles. This is also effective when considering scale-up.

<Applications>
[16. Silver nanoparticle dispersion and silver coating composition and their manufacturing methods]
A silver nanoparticle dispersion can be prepared using the silver nanoparticles obtained by the method described above. Here, the silver nanoparticle dispersion refers to a composition containing at least silver nanoparticles and a dispersion medium. Such a silver nanoparticle dispersion can take various forms without limitation. The silver nanoparticle dispersion of the present invention can be obtained by dispersing the silver nanoparticles in a suspended state in a suitable organic solvent (dispersion medium).
As described above, the silver nanoparticles of the present invention have excellent dispersibility, and can be stably present in a dispersion medium at a high concentration. For example, the silver nanoparticles can be contained in the composition at a high concentration of 70 to 95 wt %, more preferably 75 to 80 wt %, and can be in a so-called paste state.
Furthermore, the silver coating composition may contain a so-called binder component in addition to the silver nanoparticles and the dispersion medium. By making the silver coating composition contain silver nanoparticles at a high concentration of 70 to 95 wt %, more preferably 75 to 80 wt %, it is possible to obtain a silver coating composition that has good printability and can be used to easily produce a thick conductive film.
That is, it is possible to obtain a silver nanoparticle dispersion or a silver paint composition which contains 70 to 95 wt % of the silver nanoparticles of the present invention described above, and is characterized in that in the shear rate range of 2 to 30 s ^-1 , the viscosity ratio (v1/v2) of the viscosity when the shear rate is increasing (v1) to the viscosity when the shear rate is decreasing (v2) is 1.2 or less.

(16-1. Dispersion medium of dispersion or coating composition)
Examples of the dispersion medium for obtaining the silver nanoparticle dispersion or the silver coating composition include various organic solvents, for example, aliphatic hydrocarbon solvents such as pentane, hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane, and tetradecane; alicyclic hydrocarbon solvents such as cyclohexane and methylcyclohexane; aromatic hydrocarbon solvents such as toluene, xylene, and mesitylene; and alcohol solvents such as methanol, ethanol, propanol, n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-nonanol, n-decanol, and n-dodecanol.

Among these, the organic solvent is preferably an organic solvent having 8 to 16 carbon atoms, an oxygen atom in the structure, and a boiling point of 280°C or less. This is because, when the target sintering temperature of silver particles is 150°C or less, it is difficult to volatilize and remove a solvent having a boiling point of more than 280°C. Preferred examples of this solvent include terpineol (C10, boiling point 219°C), dihydroterpineol (C10, boiling point 220°C), texanol (C12, boiling point 260°C), ethyl carbitol acetate (C8, boiling point 219°C), butyl carbitol acetate (C10, boiling point 247°C), 2,4-dimethyl-1,5-pentanediol (C9, boiling point 150°C), and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (C16, boiling point 280°C). A mixture of multiple types of solvents may be used, or a single solvent may be used.
The type and amount of the organic solvent may be appropriately determined depending on the concentration and viscosity of the desired silver coating composition or silver nanoparticle dispersion.

(16-2. Description of the binder of the coating composition)
By adding a binder to the silver coating composition, it is possible to aid in the dispersibility of silver particles or to impart adhesion to the substrate. The amount of organic binder added is preferably 0.1 to 10 wt % based on the amount of silver contained.
The binder resin may be dissolved in a solvent, or may be in the form of an emulsion or suspension in the coating composition.The binder is not particularly limited, but may be, for example, polyester resin, polyurethane resin, polyamide resin, polyvinyl chloride resin, polyacrylamide resin, polyether resin, acrylic resin, melamine resin, vinyl resin, phenol resin, epoxy resin, urea resin, vinyl acetate resin, polybutadiene resin, vinyl chloride-vinyl acetate copolymer resin, fluororesin, silicone resin, rosin, rosin ester, chlorinated polyolefin resin, modified chlorinated polyolefin resin, chlorinated polyurethane resin, cellulose resin, polyethylene glycol, polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyvinyl butyral, polyvinyl pyrrolidone, etc.
The binder may be used alone or in combination of two or more kinds.

[17. Printing method and usage with silver coating composition]
The silver coating composition is generally applied onto a substrate and then baked. The application can be performed by known methods such as spin coating, inkjet printing, screen printing, dispenser printing, letterpress printing (flexographic printing), dye-sublimation printing, offset printing, laser printer printing (toner printing), intaglio printing (gravure printing), contact printing, and microcontact printing. When a printing technique is used, a patterned silver coating composition layer is obtained, and when baked, a patterned silver conductive layer is obtained. In addition, this silver conductive layer can be used as a bonding material with excellent electrical conductivity and thermal conductivity, and is also useful as a bonding material for electrical equipment that handles large currents, such as power devices.

The firing can be carried out at a temperature of 200° C. or less, for example, from room temperature (25° C.) to 150° C., preferably from room temperature (25° C.) to 120° C. However, in order to complete the sintering of silver by firing in a short time, the firing can be carried out at a temperature of from 60° C. to 200° C., for example, from 80° C. to 150° C., preferably from 90° C. to 120° C. The firing time can be appropriately determined taking into consideration the amount of silver ink applied, the firing temperature, and the like, and can be, for example, within several hours (for example, 3 hours or 2 hours), preferably within 1 hour, and more preferably within 30 minutes.
As described above, the silver nanoparticles obtained in the present invention have a large particle size and a wide particle size distribution, so that the sintering of the silver particles proceeds sufficiently even in such a low-temperature, short-time firing process. As a result, excellent conductivity (low resistance value) is exhibited, and a silver conductive layer having a low resistance value (for example, 15 μΩcm or less, in the range of 7 to 15 μΩcm) can be formed. The resistance value of bulk silver is 1.6 μΩcm.

18. Uses of silver nanoparticle dispersions and silver coating compositions
Since baking at a low temperature is possible, in addition to heat-resistant plastic substrates such as glass substrates and polyimide-based films, general-purpose plastic substrates with low heat resistance such as polyester-based films such as polyethylene terephthalate (PET) films and polyethylene naphthalate (PEN) films, and polyolefin-based films such as polypropylene can be suitably used as substrates. In addition, short baking times reduce the load on general-purpose plastic substrates with low heat resistance and improve production efficiency.

The thickness of the silver conductive layer may be appropriately determined depending on the intended application, and in particular, by using the silver nanoparticles according to the present invention, high conductivity can be exhibited even when a silver conductive layer with a relatively large thickness is formed. The thickness of the silver conductive layer may be selected, for example, from the range of 100 nm to 30 μm, preferably 1 μm to 20 μm, and more preferably 10 μm to 20 μm.

The silver nanoparticle dispersion and silver coating composition of the present invention can be used in electromagnetic wave control materials, circuit boards, antennas, heat sinks, liquid crystal displays, organic EL displays, field emission displays (FEDs), IC cards, IC tags, solar cells, LED elements, organic transistors, capacitors, electronic paper, flexible batteries, flexible sensors, membrane switches, touch panels, EMI shields, etc.

The present invention will now be described more specifically with reference to examples and comparative examples.
The names, structural formulas, and other characteristics of the amine compounds used in the examples and comparative examples are shown in Tables 1 and 2.

[Example 1]
(Production of Silver Particles)
In a 2L flask, 420.00g (1.38mol) of dried silver oxalate as the raw silver compound and 339.87g (3.33mol) of n-hexanol as the polar solvent were stirred to wet the silver oxalate. Then, 128.22g (1.75mol) of n-butylamine and 55.83g (0.55mol) of n-hexylamine were added, and 4.71g (0.017mol) of oleic acid (NOF Corp.: NAA-35) and 21.00g of ion-exchanged water (15.0wt% relative to silver oxalate) were added as additives for particle size control. Then, the mixture was stirred for 1 hour to produce a silver-amine complex. Then, the mixture was heated at a temperature rise rate of 3°C/min, and the generation of carbon dioxide, which is thought to be the decomposition reaction of silver oxalate, was confirmed at 100°C. Heating was continued until the generation of carbon dioxide stopped, and a liquid in which silver particles were suspended was obtained. After the silver particles were precipitated, the reaction solution was washed with 900 cc of methanol and centrifuged. This washing and centrifugation was repeated three times to obtain silver nanoparticles.

(Protective agent replacement treatment)
In order to replace the n-butylamine in the obtained silver nanoparticles with n-hexylamine, 71.8 wt % of n-hexylamine based on the pure silver content of the obtained silver nanoparticles and the silver nanoparticles were stirred at room temperature for 1 hour, and washing and centrifugation were repeated three times in the same manner as above to obtain silver nanoparticles with hexylamine as a protective agent.

(Checking particle size)
The obtained silver nanoparticles wet with methanol were suspended in n-hexanol using a vortex mixer, the liquid was dropped onto a support such as a collodion film, and the solvent was dried to obtain a sample. The sample was observed and photographed at a magnification of 20,000 to 70,000 times using FE-SEM observation, and an image with a magnification in which 400 or more particles were present was selected. Then, the number of particles was counted to 400 or more using image analysis software SCANDIUM (manufactured by OLYMPUS). When measuring the particle size, the major axis of the particle was measured, and the average particle size, particle size distribution, etc. were calculated from the data.
The percentages (%) of particles with sizes of 100 nm or less, 200 to 500 nm, and over 500 nm, the average particle size (nm), and the coefficient of variation (%) are shown in Table 3. An FE-SEM photograph is shown in Figure 1. A particle size distribution histogram is shown in Figure 15.

(Preparation and firing of silver nanoparticle paste and ink)
Next, Texanol was added as a solvent to the collected silver nanoparticles so that the silver content was 75 wt%, and mixed. Ethocel 45 (manufactured by Nisshin Kasei) was added as an organic binder so that the amount added to the silver particles was 1 wt%, and finally a silver nanoparticle paste ink with a silver content of about 70 wt% was produced. This paste was cast on a slide glass and heated at 150°C for 1 h in an air dryer. The coating thickness after drying was adjusted to be 10 to 30 μm.
The surface resistance of the resulting coating film was measured by a four-terminal method, and the surface resistance was multiplied by the thickness of the resulting coating film to obtain the volume resistivity.
The volume resistivity values are shown in Table 3.

[Examples 2 to 9, Comparative Examples 1 to 7]
(Production of Silver Particles)
Silver particles were produced in the same manner as in Example 1 (Production of silver particles) except that the materials used and the blending ratios were changed to those shown in Tables 3 to 7, the heating rate after production of the silver-amine complex compound was changed to those shown in Tables 3 to 7, and the reaction vessel/heating device was changed to those shown in Tables 3 to 7.
As shown in Tables 3 to 4, for Examples 3, 4, 7, and 8, silver particles were also prepared by carrying out (protective agent replacement treatment) in the same manner as in Example 1. For Example 9, silver particles were prepared in the same manner as in Example 1, except that n-butylamine was used instead of n-hexylamine in the (protective agent replacement treatment).

The obtained silver particles were subjected to confirmation of particle size in the same manner as in Example 1. Note that, for Comparative Examples 2, 6, and 7, the particle size was confirmed using STEM images.
In addition, for Examples 2 to 7 and Comparative Examples 1 and 2, the obtained particles were used in the same manner as in Example 1 (preparation and firing of silver nanoparticle paste and ink). Note that, in Example 3, Terpineol C (a mixture of α-, β-, and γ-isomers manufactured by Nippon Terpene Co., Ltd.) was used as the ink solvent.
In addition, for Examples 3 and 4, the preparation and firing of the silver nanoparticle paste and ink were performed using the particles before and after the protective agent replacement treatment, respectively.
In Comparative Examples 6 and 7, as in Patent Documents 1 and 2, a silver nanoparticle dispersion was dispersed in a mixed solvent of isooctane/n-butanol = 4/1 (volume ratio) so that the silver content was 55 wt %, and the dispersion was applied onto glass by spin coating.

For Examples 2-9 and Comparative Examples 1-7, the average particle size (nm), coefficient of variation (%), and particle ratio (%) in each particle size range of the obtained particles are shown in Tables 3-7. SEM or STEM images are shown in Figures 2-14. Particle size distribution histograms for Examples 2-9 and Comparative Examples 1, 2, 6, and 7 are shown in Figures 16-25. For Examples 2-9 and Comparative Examples 1, 2, 6, and 7, the volume resistivity and film thickness of the sintered coating film are shown in Tables 3-7. In the tables, "(b)/silver particle molar ratio" indicates the molar ratio of silver particles in (b)/(a).

(Observation of coating film)
The Texanol pastes with a silver content of 78.5% were prepared using the particles described in Example 8, Comparative Example 1, and Example 9, and the pastes were passed through a nylon mesh with an opening of 20 μm, and then coated on an aluminum foil with a spatula. The coated film was dried with a dryer, and the surface of the coated film was observed with an SEM. The coated films observed with an SEM are shown in Figures 26 to 28.

(Measuring the viscosity of silver paste)
Furthermore, the viscosity of the 78.5% silver content Texanol pastes prepared using the particles of Example 8, Comparative Example 1, and Example 9 was measured using a rheometer (HAAKE MARSIII manufactured by Thermo Fisher Scientific). The measurement conditions were as follows: measurement mode: hysteresis loop, shear rate: 0.1 s ^-1 → 30 s ^-1 (90 s), 30 s ^-1 → 0.1 s ^-1 (90 s), measurement jig: cone plate (Cone C35/1° TiL, Lower plate TMP35), gap: 0.052 mm, measurement temperature: 25° C. The measured viscosity data for Examples 8 and 9 are as shown in FIG. 29, and for Comparative Example 1 are as shown in FIG. 30.
In addition, based on the measured viscosity data, the viscosity value at each shear rate was confirmed using analysis software (HAAKE RheoWin Data Manager manufactured by Thermo Fisher Scientific). Table 9 shows the viscosity (v1) at the time of shear rate increase and the viscosity (v2) at the time of shear rate decrease at shear rates of 1, 2, 3, and 4 s ^-1 in Example 8, Example 9, and Comparative Example 1. In Examples 8 and 9 using the silver nanoparticles of the present invention, (v1/v2) is 2.0 or less at shear rates of 1 to 4 s ^-1 . As can be seen from the graph, in the range of 4 to 30 s ^-1 , it is smaller and there is no point exceeding 2.0. In contrast, in Comparative Example 1, the viscosity ratio (v1/v2) exceeds 1.2 at points of 2 s ^-1 and 3 s ^-1 , indicating that the viscosity hysteresis is large.

As described above, it has been confirmed that, according to the present invention, by allowing water in the range of 5 to 20 parts by weight per 100 parts by weight of the silver compound during complex formation and reacting the amine compound with the silver compound at a molar ratio of silver atoms in the silver compound in the range of 0.7 to 2.0, the amine compound bonds to the particle surfaces, and silver particles having an average particle size of 70 to 350 nm and a coefficient of variation of 40% to 80% can be obtained.
It was also confirmed that the silver particles of the present invention can form a sintered coating film of 20 μm or more, and that the coating film has a volume resistivity of 50 μΩ·cm or less and is conductive when baked at 150° C. Among them, in Examples 2 and 4 in which AMP is used as the amine compound and Texanol is used as the ink solvent, particles having a small particle diameter of about 80 nm or a large particle diameter of 260 nm and a coefficient of variation of 40% to 80% can be obtained, and that these particles can have a coating film with a volume resistivity of 50 μΩ·cm or less when baked at 100° C., and therefore it was confirmed that silver nanoparticles with excellent low-temperature sinterability can be obtained.

Furthermore, in Example 3, the same particles as in Example 2 were used, but the ink solvent was changed to terpineol C. It was also confirmed that by replacing the protective group with n-hexylamine, which has a molecular length of 7 to 8 Å, the volume resistivity of the coating film was significantly reduced under baking conditions at 100°C compared to the state without protective group replacement. Thus, it can be seen that with the silver nanoparticles of the present invention, an appropriate silver paint composition and sintered coating film can be obtained by adjusting the type of protective group on the silver particle surface, the ink solvent, the sintering conditions, etc.

In addition, regarding the increase in the particle size of silver nanoparticles by using the (b1) component, in Examples 5 and 6, the same number of moles of N,N-dimethylpropyldiamine and 1-amino-2-butanol were added as activators to promote complex formation, and the other raw materials had the same composition and were synthesized under the same conditions to compare the sizes of silver nanoparticles. Comparing Examples 5 and 6, it was confirmed that Example 6, which used the (b1) component, was easier to synthesize particles with a diameter of 200 to 500 nm than Example 5. In addition, as in Examples 2 and 3, it can be seen that even if the (b1) component is used, the particle diameter can also be reduced by increasing the proportion of the (b2) component.
Furthermore, in Example 7, diglyme is used as the synthesis solvent instead of n-hexanol, and silver nanoparticles having a wide particle size distribution are produced, making it possible to obtain a thick conductive film of 20 to 30 μm.

In contrast, in Comparative Examples 1 and 2, no water was added, and in this case, not only was the average particle size small at less than 65 nm, but the variation coefficient was less than 30%, with small variation, and 99% or more had a small particle size of 100 nm or less. It can be seen that with such small particles with little variation, sufficient conductivity cannot be obtained at each firing temperature unless the film thickness is 10 μm or less. In particular, when the film thickness is 20 μm or more and the firing condition is in the low temperature range of 120° C. or less, the volume resistivity increases significantly.
Furthermore, even when component (b1) or water is used, if too much water is added as in Comparative Example 3, or if too little component (b2) is added as in Comparative Example 4, or if the heating rate is too slow as in Comparative Example 5, the particles fuse together, making it impossible to obtain independent silver particles, and it is clear that control of the particle size is difficult.

In the particles of Comparative Examples 6 and 7, which were produced by methods equivalent to those of Patent Documents 1 and 2 in which the molar ratio of silver atoms in (b) and (a) falls outside the range of the present invention, the coefficient of variation is small and falls outside the range of the present invention, so that when the ink is applied, a sintered coating film of about 0.5 μm is formed, making it difficult to produce a thick film.
As can be seen from the above results, in the complex formation reaction of the amine compound and the silver compound in an organic solvent, the method of the present invention, in which the ratio of water to the amine compound is within the range specified in the present invention, and the ratio of the amine compound to the silver compound is within the range specified in the present invention, can obtain the silver nanoparticles of the present invention having a moderate variation in particle size distribution, and it is possible to prepare a silver coating composition that is easy to obtain a low-resistance thick-film conductive film. In addition, it can be seen that the silver nanoparticles obtained by using the component (b1) as the amine compound in particular have the above effects remarkably excellent.

The silver pastes of Example 8 and Example 9, which show a comparison of viscosity behavior in Figure 29, differ only in the length of the amine molecule introduced in the protective agent replacement treatment in the particles as described above (n-hexylamine in Example 8: 7.559 Å, whereas n-butylamine in Example 9: 5.004 Å). As shown in Table 9, the dispersion of Example 8 is a pseudoplastic fluid in which the viscosity (v1) when the shear rate increases and the viscosity (v2) when the shear rate decreases within the shear rate range of 2 to 30 s ^-1 are almost the same, that is, the viscosity ratio (v1/v2) is about 1.0, whereas the dispersion of Example 9 is a pseudoplastic fluid in which the viscosity (v2) when the shear rate decreases is lower than the viscosity (v1) when the shear rate increases within the shear rate range of 2 to 4 s ^-1 , that is, the viscosity ratio (v1/v2) is 1.1 to 1.2, and it tends to behave like a slightly thixotropic fluid.

Comparing the photographs of the results of (film observation) of the silver particle dispersions of Example 8 and Example 9, a smooth coating film is obtained with almost no agglomerates in Example 8, but the presence of agglomerates of several tens of μm can be confirmed in Example 9. It is speculated that the phenomenon of a large amount of agglomerates causes the paste to behave like a thixotropic fluid. In Example 9, the length of the molecules of the amine compound on the particle surface is short, so the degree of steric hindrance is relatively low, and it can be speculated that the silver particles create a pseudo-bridged state, which causes agglomeration and increases the viscosity of the paste when left stationary. Once shear is applied, the pseudo-aggregated state is eliminated, the primary particles become dispersed, and the viscosity of the paste increases when the viscosity increases. However, it is thought that if it is left stationary for a while, a pseudo-bridged state is created again, and secondary agglomerates and high viscosity of the paste are reconstructed. For this reason, it can be speculated that the dispersion of Example 8 has better continuous printability. In addition, the agglomerates may be a cause of mesh clogging.

Therefore, in screen printing applications, the dispersion of Example 8, in which the molecular length of the amine compound is in the range of 7 to 8 Å, is superior. On the other hand, the dispersion of Example 9 also has an average particle size and a coefficient of variation in the preferred range, as in the case of the dispersion of Example 8, and therefore has excellent electrical properties in a 20 μm thick film as shown in Table 8, and can be suitably used as a bonding material for wiring with a line width of 500 μm to several mm order or for power semiconductors.
In contrast, the silver particles of Comparative Example 1 are outside the range of the average particle size and coefficient of variation of the present invention. As can be seen from Figure 29, even when compared with the particles of Example 9, which have the same length of amine compound on the surface, as shown in Table 9, the viscosity (v2) when the shear rate is decreasing is lower than the viscosity (v1) when the shear rate is increasing within the range of ^shear rates of 2 to 4 s -1, that is, the viscosity ratio (v1/v2) is 1.1 to 2.2, and the particles tend to behave like a thixotropic fluid. The reason for this is presumed to be the large amount of aggregates, as can be seen from Figure 28, and the particles are prone to cracking even when fired, resulting in poor conductivity when made into a thick film.

From the above results, it can be seen that the silver particles of the present invention, which have an appropriate particle size and variation, can easily produce thick films that exhibit excellent electrical properties. In particular, those in which a protective agent with an appropriate molecular length (7 to 8 Å) is bonded to the silver surface have particularly good dispersibility, and it is possible to produce a silver paint composition with viscosity behavior suitable for screen printing, and the printed silver paint composition can provide a thick conductive film with good conductivity when fired at a low temperature.

The present invention makes it possible to obtain silver nanoparticles with large particle size and wide distribution that can easily form a thick, highly conductive silver layer, and a silver coating composition that is particularly suitable for screen printing, using a method that reduces the amount of amines that have a strong irritating odor.

Claims (4)

A method for producing silver nanoparticles, comprising reacting a thermally decomposable silver compound (a) with an amine compound (b) capable of forming a complex with (a) in an organic solvent (c) to form a complex, and then heating the complex to thermally decompose it, thereby forming silver nanoparticles. The method for producing silver nanoparticles is characterized in that, during the formation of the complex, 5 to 20 parts by weight of water is present per 100 parts by weight of the silver compound (a), and the molar ratio of the amine compound (b) to the silver atoms in the silver compound (a) is 0.7 or more. The amine compound (b) includes an aminoalcohol (b1) having 3 to 4 carbon atoms and an amine compound (b2) having a molecular length of 5 Å or more and containing N, C, and H atoms, or N, C, H, and O atoms, and the molar ratio represented by (b1)/[(b1)+(b2)] is 0.3 to 0.8 (molar ratio), and the heating rate is 3.0° C./min or more . A method for producing silver nanoparticles, comprising reacting a thermally decomposable silver compound (a) with an amine compound (b) capable of forming a complex with (a) in an organic solvent (c) to form a complex, and then heating the complex to thermally decompose it, thereby forming silver nanoparticles, wherein during the formation of the complex, 5 to 20 parts by weight of water is present per 100 parts by weight of the silver compound (a), the molar ratio of the amine compound (b) to the silver atoms in the silver compound (a) is 0.7 or more, and the heating rate is 3.0°C/min or more , and wherein an organic group introduced into the silver nanoparticles by the amine compound during the complex formation process is replaced by another amine compound. A method for producing silver nanoparticles, comprising reacting a thermally decomposable silver compound (a) with an amine compound (b) capable of forming a complex with (a) in an organic solvent (c) to form a complex, and then heating the complex to thermally decompose it, thereby forming silver nanoparticles, wherein during the formation of the complex, 5 to 20 parts by weight of water is present per 100 parts by weight of the silver compound (a), and the molar ratio of the amine compound (b) to the silver atoms in the silver compound (a) is 0.8 or more and 1.5 or less, wherein the amine compound (b) contains an amine compound having a molecular length of 7 Å or more , and the heating rate is 3.0° C./min or more . A method for producing silver nanoparticles, comprising reacting a thermally decomposable silver compound (a) with an amine compound (b) capable of forming a complex with (a) in an organic solvent (c) to form a complex, and then heating the complex to thermally decompose it, thereby forming silver nanoparticles, wherein the method is characterized in that, during the formation of the complex, 5 to 20 parts by weight of water is present per 100 parts by weight of the silver compound (a), the molar ratio of the amine compound (b) to the silver atoms in the silver compound (a) is 0.7 or more, and the heating rate is 3.0°C/min or more , and the amine compound (b) contains an amine compound having a molecular length of 7 Å or more, excluding those containing only amine compounds having a molecular length of 7 Å or more.

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