JP7335768B2 - Silver-coated metal powder, method for producing the same, and conductive paint - Google Patents

Description

The present invention relates to a silver-coated metal powder composed of metal particles in which the surfaces of copper particles or copper alloy particles are coated with metallic silver, and a method for producing the same. The present invention also relates to a conductive paint using the above silver-coated metal powder as a conductive filler.

Conventionally, in order to form electrodes and wiring of electronic parts by printing methods, etc., conductive paints made by mixing conductive metal powder and non-conductive substances such as solvents, resins, and dispersants are used. ing. Conductive paints include those in paste form (conductive paste) and those in liquid form (conductive ink). The metal powder contained in the conductive paint functions as a "conductive filler" responsible for conductivity. Typical metal powders used for the conductive filler include silver powder and copper powder. Silver powder is excellent in conductivity and oxidation resistance, but has problems such as high cost and susceptibility to migration. On the other hand, copper powder is excellent in conductivity and cost, and hardly causes migration, but has a problem of poor oxidation resistance.

Therefore, techniques for producing silver-coated copper powder have been developed in order to enjoy the merits of silver powder and copper powder.
For example, in Patent Document 1, copper powder is dispersed in an acidic solution (pH 2 to 5), a chelating agent is added to the copper powder dispersion to prepare a copper powder slurry, and then a buffer is added to adjust the pH to about 4 and continuously adding a silver ion solution to the copper powder slurry to form a silver layer on the surface of the copper powder through a substitution reaction.

Further, in Patent Document 2, after coating a copper or copper alloy powder with a silver-containing layer, a silver-coated copper powder that is subjected to surface modification by heating at 60 to 160 ° C. for 0.5 to 50 hours in a reducing atmosphere. is disclosed.

Japanese Unexamined Patent Application Publication No. 2004-52044 JP 2016-176093 A

According to the technique of Patent Document 1, it is said that a silver-coated copper powder whose electrical conductivity changes little over time even when left in an air atmosphere can be obtained. However, as shown in Patent Document 1, when a silver ion solution is added to a copper powder slurry having a pH of about 4 to carry out a silver coating reaction, copper oxide is produced and adversely affects the conductivity of the silver-coated copper powder. There is a risk. This is because copper oxide can be generated at pH 4 in the pH-potential diagram of copper.

According to the technique of Patent Document 2, aggregation and sintering of particles are unlikely to occur, and when used in a conductive paste, the silver coating that can suppress the increase in the "viscosity" of the conductive paste over time. Metal powder is now available. However, no effort has been made to improve the temporal change (reliability) of the "conductivity" of the conductive film. Due to recent demands for miniaturization and high performance of electronic components, further improvement in reliability of conductivity is desired in order to cope with thinning and thinning conductive films.

The present invention provides a silver-coated metal powder that can form a highly conductive conductive film when used as a conductive filler and has a high performance of suppressing changes in the conductivity of the conductive film over time, and a conductive paint using the same. is intended to provide

In order to achieve the above objects, the following inventions are disclosed in this specification.
[1] A powder of metal particles having a metal core particle with a copper content of 80% by mass or more and a silver coating layer coating the metal core particle,
L*, a* values in CIE 1976 L*a*b* color space measured by a color difference meter in specular-removed mode on the surface flattened by tapping of the powder, and laser diffraction/scattering of the powder. A silver-coated metal powder having an A value of 4.0 or more in the following formula (1) expressed as a cumulative 50% particle size D ₅₀ (μm) in a volume-based particle size distribution according to the law.
A=L*/a*/( _D50 ^1.45) (1)
[2] The silver-coated metal powder according to [1] above, wherein D ₅₀ is 0.1 to 10.0 μm.
[3] The silver-coated metal powder according to [1] or [2] above, which has a particle size distribution in which the B value defined by the following formula (2) is from 0.5 to 2.5.
B=( _D90 - _D10 )/ _D50 (2)
Here, D ₁₀ , D ₅₀ and D ₉₀ are respectively the cumulative 10% particle size D ₁₀ (μm), the cumulative 50% particle size D ₅₀ (μm) and the cumulative 90% particle size distribution in the volume-based particle size distribution by laser diffraction/scattering method. % particle diameter D ₉₀ (μm).
[4] The silver-coated metal powder according to any one of [1] to [3] above, wherein the mass ratio of silver in the powder is 2.0 to 30.0%.
[5] The silver-coated metal powder according to any one of [1] to [3] above, wherein the mass ratio of silver in the powder is 5.0 to 15.0%.
[6] A step of mixing an aqueous solution containing a chelating agent and a powder of metal particles having a copper content of 80% by mass or more to prepare a slurry of the metal particles having a pH of 2.3 to 3.2 ( slurrying process),
a step of introducing silver ions into the slurry to form a coating layer of metallic silver on the surface of the metal particles while maintaining the pH of the slurry liquid at 2.3 to 3.2 (silver coating step);
The method for producing a silver-coated metal powder according to any one of [1] to [5] above, having
[7] A step of mixing an aqueous solution containing a chelating agent and a powder of metal particles having a copper content of 80% by mass or more to prepare a slurry of the metal particles having a pH of 2.3 to 3.2 ( slurrying process),
a step of washing the surface of the metal particles in the slurry by holding the slurry at 15 to 70° C. for 10 minutes or longer (washing step);
A step of introducing silver ions into the slurry after the washing step to form a coating layer of metallic silver on the surface of the metal particles while maintaining the pH of the slurry liquid at 2.3 to 3.2 (silver coating step). ,
The method for producing a silver-coated metal powder according to any one of [1] to [5] above, having
[8] A conductive paint using the silver-coated metal powder according to any one of [1] to [5] above as a conductive filler.

In the above formula (1), the symbol "^" means exponentiation.

According to the present invention, it has become possible to provide a silver-coated metal powder having a highly uniform silver coating layer with little exposure of the copper or copper alloy of the metal core. When this silver-coated metal powder is used as a conductive filler in a conductive paint, it is possible to form a conductive film that is excellent in conductivity and that changes little over time.

[Metal core particles]
The silver-coated metal powder of the present invention is an aggregate of silver-coated metal particles formed by forming a silver coating layer on the surface of metal particles made of copper or a copper alloy. In this specification, the portion of the silver-coated metal particles derived from the "metal particles made of copper or copper alloy" is referred to as "metal core". In addition, the above-mentioned "metal particles made of copper or copper alloy" may be particularly referred to as "metal core particles" in the sense of distinguishing them from silver-coated metal particles, and the powder that is an aggregate of the metal core particles is referred to as " It is sometimes called "metal core powder".

As the metal core powder, a metal powder having a copper content of 80% by mass or more is used. If the copper content is too low, the volume resistivity increases and the conductivity decreases, which is disadvantageous in forming a conductive film with excellent conductivity. More preferably, the copper content is 90% by mass or more. A "copper powder" having an impurity element content of, for example, 0.1% by mass or less may be applied. Examples of the inevitable impurity elements include iron, sodium, potassium, calcium, palladium, magnesium, oxygen, carbon, nitrogen, phosphorus, silicon and chlorine. When "copper alloy powder" is applied, typical alloy systems include copper-zinc alloys, copper-nickel alloys, and copper-nickel-zinc alloys. However, copper alloy systems other than these can be employed as long as they do not inhibit the effects of the present invention.

In the case of a copper-zinc alloy, for example, copper: 80.0 to 95.5% by mass, preferably 88.0 to 98.5% by mass, and the balance being zinc and unavoidable impurities. . In the case of a copper-nickel alloy, for example, a composition of copper: 80.0 to 95.5% by mass, preferably 88.0 to 98.5% by mass, and the balance being nickel and unavoidable impurities can be adopted. . In the case of a copper-nickel-zinc alloy, for example, copper: 80.0 to 95.5% by mass, more preferably 88.0 to 97.5% by mass, nickel: 0.1 to 10.0% by mass, the balance being Compositions that are zinc and incidental impurities can be employed. In the case of any of these copper alloy systems, elements other than nickel or zinc can be contained within a range that does not impair the effects of the present invention.

The shape of the metal core particles can be selected depending on the application, such as substantially spherical particles, scale-like (flake-like) particles, and dendritic particles. As for the average particle size of the metal core particles, those having a cumulative 50% particle size D ₅₀ of 0.05 to 15.0 μm in the volume-based particle size distribution according to the laser diffraction/scattering method may be used. When it is desired to adjust the average particle size of the silver-coated metal powder to a predetermined range (for example, D ₅₀ is in the range of 0.1 to 10.0 μm), an appropriate average particle size (for example, D ₅₀ in the range of 0.08-9.9 μm).

[Silver coating layer]
In the silver-coated metal powder of the present invention, a silver coating layer is formed on the surface of the metal core particles. Although this silver coating layer is substantially made of silver, it may contain unavoidable impurities derived from the manufacturing process and raw materials. In addition, the constituent elements of the metal core particles may diffuse into the silver coating layer, resulting in the silver coating layer containing these elements.

The silver coating layer does not have to be formed on the entire surface of the metal core particles, and may cover the surfaces of the metal core particles so that the A value is at least a predetermined value as described later. Such a coating can be easily achieved when the mass ratio of silver in the silver-coated metal powder is in the range of 1.0% or more by following the manufacturing method described below. From the viewpoint of ensuring more stable and good conductivity, the mass ratio of silver in the silver-coated metal powder is preferably 2.0% or more, more preferably 5.0% or more. 0% or more is more preferable. Excessive silver coating causes cost increase. The mass ratio of silver in the silver-coated metal powder is preferably 30.0% or less, more preferably 20.0% or less, and may be controlled to 15.0% or less.

[A value]
As an index representing the degree of exposure of the metal core of the particles (silver-coated metal particles) constituting the silver-coated metal powder, the A value determined by the following formula (1) is used in the present invention.
A=L*/a*/( _D50 ^1.45) (1)
Here, L* and a* denote lightness L* and green-red chromaticity a*, respectively, in the CIE 1976 L*a*b* color space. D ₅₀ is the cumulative 50% particle size D ₅₀ (μm) in the volume-based particle size distribution of the silver-coated metal powder measured by the laser diffraction/scattering method.
L* and a* are measured by a color difference meter in specular reflection removal mode (SCE mode) on the surface of a sample powder placed in a cylindrical container and flattened by tapping.

The greater the exposure of the metal core of the silver-coated metal particles, the smaller the lightness L*. However, according to the study of the inventors, it is difficult to compare the degree of exposure of the metal core with high accuracy only by L* in the case of silver-coated metal particles in which the metal core is relatively little exposed. Therefore, as a result of detailed investigation, it was found that the value of "L*/a*", which is obtained by dividing the lightness L* by the chromaticity a* in the green-red direction, was used as a parameter. Or it was found to be extremely effective in evaluating the degree of exposure of the metal core of the copper alloy. Furthermore, the effect of particle size on L*/a* values should also be considered. The inventors have found that various silver-coated metal powders with similar silver coating amounts (% by mass) and different average particle sizes are obtained by dividing the L*/a* value by the power of _D50 (μm). The coefficient of variation CV value of was examined. In the power of D ₅₀ (μm) "D ₅₀ ^X", when the value of X is increased from 1, the coefficient of variation CV value of the value of "L*/a*/(D ₅₀ ^X)" is , X tends to be the minimum around 1.45. Therefore, for silver-coated metal powders with different average particle sizes, as an index that can accurately compare the exposure degree of the metal core based on the same standard, "L*/a*" normalized by _D50 , "L*/ a*/(D ₅₀ ^1.45)” was adopted. The value determined by this is the A value of the formula (1). A silver-coated metal powder having a high A value is evaluated to have a small exposure of the metal core and a high uniformity of the silver coating layer existing on the surface of the metal core particles.

According to the research of the inventors, when copper or a copper alloy having a copper content of 80% by mass or more is applied as the metal core particles, the silver-coated metal powder having the above A value of 4.0 or more has a metal core , and the uniformity of the silver coating layer is very low. Therefore, in a conductive film using the powder as a conductive filler, the initial resistance is significantly reduced and the "change in conductivity over time" is highly effective. demonstrate. From the viewpoint of the conductivity of the conductive film, it is more effective that the A value is 4.1 or more. Although the upper limit of the A value is not particularly defined, a range of, for example, 8.0 or less is sufficient, and it is advantageous to suppress it to a range of, for example, 6.5 or less in terms of cost.

[D ₅₀ of silver-coated metal powder]
Regarding the particle size of the silver-coated metal powder of the present invention, the cumulative 50% particle size in the volume-based particle size distribution by the laser diffraction/scattering method is determined from the viewpoint of fine line drawing and the formation of a thin conductive film when used in a conductive paint. D ₅₀ is preferably in the range of 0.1 to 10.0 μm. D ₅₀ is more preferably 0.8 to 8.0 μm, even more preferably 1.2 to 6.0 μm.

[B value]
The B value defined by the following formula (2) is an index representing the sharpness of the particle size distribution of the silver-coated metal powder.
B=( _D90 - _D10 )/ _D50 (2)
Here, D ₁₀ , D ₅₀ and D ₉₀ are respectively the cumulative 10% particle size D ₁₀ (μm), the cumulative 50% particle size D ₅₀ (μm) and the cumulative 90% particle size distribution in the volume-based particle size distribution by laser diffraction/scattering method. % particle diameter D ₉₀ (μm).
It is judged that the smaller the B value, the sharper the particle size distribution. From the viewpoint of powder filling properties and ease of handling, it is advantageous to have a moderately sharp particle size distribution according to the average particle size. As a result of various investigations, it is more preferable that the B value is in the range of 0.5 to 2.5.

[BET specific surface area]
The specific surface area of the silver-coated metal powder of the present invention measured by the BET single-point method is preferably 0.08 to 1.50 m ² /g, more preferably 0.10 to 1.5 m 2 /g, from the viewpoint of exhibiting good conductivity. 00 m ² /g, more preferably 0.15 to 0.80 m ² /g.

[Tap density]
The tap density of the silver-coated metal powder of the present invention is preferably 3.0 to 8.5 g/cm ³ , more preferably 4.0 to 7, from the viewpoint of increasing the packing density of the powder and exhibiting good electrical conductivity. More preferably 0.0 g/cm ³ .

[Oxygen content]
From the viewpoint of conductivity, the oxygen content of the silver-coated metal powder of the present invention is preferably 0.40% by mass or less, more preferably 0.05 to 0.20% by mass.

[Carbon content]
The carbon content of the silver-coated metal powder of the present invention is preferably 0.40 mass % or less, more preferably 0.005 to 0.10 mass %.

[Oxidation resistance]
As an index showing the oxidation resistance of the silver-coated metal powder, the TG change rate (%) represented by the following formula (3) can be used.
TG change rate (%)=100×(W ₁ −W ₀ )/W ₀ (3)
Here, W ₀ is the weight (g) of the sample powder before the heating test, and W ₁ is the weight (g) of the sample powder when heated to a predetermined temperature. In the heating test, the weight of the sample powder is monitored, and the temperature of the sample powder is raised from room temperature (25°C) to 650°C in an air atmosphere at a heating rate of 5°C/min while air is passed through. . In the weight change curve at this time, the weight when reaching a predetermined temperature (for example, 200 ° C. or 300 ° C.) is read as W ₁ (g), and the TG change rate (%) up to that temperature is calculated by the above equation (3). demand.
A silver-coated metal powder having a metal core with a copper content of 80% by mass or more, with a TG change rate of 0.7% or less up to 200 ° C. and a TG change rate of 5.0% or less up to 300 ° C. is conductive It is judged that it has sufficient oxidation resistance as a filler, and the TG change rate up to 200 ° C. is 0.5% or less and the TG change rate up to 300 ° C. is 3.5% or less. is determined to have

[Production method]
The silver-coated metal powder of the present invention can be produced, for example, by a production method including the following steps.

[Slurry process]
An aqueous solution containing a chelating agent and a powder of metal particles having a copper content of 80% by mass or more are mixed, and the pH of the liquid is 2.3 to 3.2, more preferably 2.3 to 2.7. Make a slurry of metal particles. By setting the pH of the liquid within the above range, the metal particle powder is well dispersed in the slurry, and the silver coating reaction (displacement plating reaction) proceeds smoothly in the subsequent silver coating step. As a result, even with the same silver coating amount, it is possible to form a silver coating layer with higher uniformity. A stirrer can be used for mixing.

The pH referred to in this specification is the pH at 25° C. measured using a glass electrode after performing zero calibration and span calibration with a pH standard solution based on JIS Z8802:2011.

The chelating agent has the function of removing the oxide film on the surface of the metal core particles to ensure good silver coating. In addition, the chelating agent captures the copper ions eluted from the metal powder in the silver coating reaction (displacement plating reaction) by complexing, thereby promoting the progress of the silver coating reaction, such as reprecipitation of copper ions on the metal core particles. Suppress the interfering reaction of A chelating agent that dissolves in an acidic aqueous solution is used here. Specific examples of chelating agents include ethylenediaminetetraacetic acid, hydroxyethylethylenediaminetriacetic acid, hydroxyethyliminodiacetic acid, nitrilotriacetic acid and salts thereof. These may be used individually by 1 type, or may be used in combination of 2 or more type. Examples of the salts include alkali metal salts and alkaline earth metal salts, and specific examples thereof include sodium salts, potassium salts, magnesium salts and calcium salts.

The content of the chelating agent in the liquid medium (the portion of the slurry excluding the metal core particles, which are solid substances in the slurry) may be adjusted, for example, within the range of 0.02 to 0.5 mol/L. The mass ratio of the metal core particles in the slurry may be adjusted, for example, within the range of 5 to 30%. The slurry may contain a substance that functions as a pH adjuster/buffer. Such substances include, for example, malonic acid, succinic acid, glycolic acid, lactic acid, malic acid, tartaric acid, phthalic acid and salts thereof. These may be used individually by 1 type, or may be used in combination of 2 or more type. These also function as buffers to prevent pH changes (especially to the alkaline side) during the silver coating reaction. Examples of the salts include alkali metal salts and alkaline earth metal salts, and specific examples thereof include sodium salts, potassium salts, magnesium salts and calcium salts.

[Washing process]
If necessary, the slurry is kept at 15 to 70° C. (that is, the temperature of the slurry itself is set to 15 to 70° C.) for 10 minutes or more to wash the surfaces of the metal particles in the slurry in an acidic environment. can be done. It is desirable to blow non-oxidizing gas (nitrogen, argon, etc.) into the slurry during holding. This gas blowing has the effect of reducing the amount of oxygen in the liquid and the effect of ensuring the state of dispersion of the particles with an appropriate stirring force. The pH of the liquid during retention is desirably maintained between 2.3 and 3.2, more desirably between 2.3 and 2.7. By holding the metal core particles in an acidic solution containing a chelating agent, the effect of removing oxides on the surface of the metal core particles is improved, the active metal is exposed on the particle surface, and the silver coating is performed more uniformly. is effective in In order to obtain a silver-coated metal powder having a high A value and excellent conductivity, it is more preferable to set the holding time to 90 minutes or longer. Since holding for too long is uneconomical, it is preferable to set the holding time within the range of, for example, 180 minutes or less when performing the washing step.

[Silver coating process]
Silver ions are introduced into the slurry (the slurry after the washing step when the washing step is performed), and the pH of the slurry is adjusted to 2.3 to 3.2, more preferably 2.3 to 2.3. A coating layer of metallic silver is formed on the surface of the metal particles while maintaining the temperature at 7. If the pH of the solution is less than 2.3, the progress of the silver coating reaction (displacement plating reaction) slows down. Even if the pH drops below 2.3, the reaction rate can be recovered by increasing the pH by adding a pH adjuster or the like. However, pH variation during the reaction is a negative factor in forming a highly uniform silver coating layer. On the other hand, if the pH is too low, the stability of the chelating agent-copper ion complex may be lowered, and reprecipitation of copper ions may occur, which is a negative factor in obtaining a highly conductive silver-coated metal powder. On the other hand, when the pH of the liquid exceeds 3.2, the dispersibility of the metal core particles in the reaction liquid decreases, and the rate of progress of the substitution reaction increases, thereby stably forming a highly uniform silver coating layer. becomes difficult. Therefore, here, a technique is adopted in which the reaction proceeds while maintaining the pH of the liquid at 2.3 to 3.2. It is more preferable to proceed the reaction while maintaining the pH of the liquid at 2.3 to 2.7. In this silver coating reaction, the potential of the liquid is not particularly limited, but is usually 0 to 1.0 V (versus the standard electrode potential).

The introduction of silver ions into the slurry is desirably carried out by preparing a silver ion-containing liquid in which a silver compound is dissolved in advance and then adding it to the slurry. The silver ion-containing liquid may be added all at once, or may be added continuously or intermittently. Silver salts (such as silver nitrate, silver chloride, silver iodide, silver oxalate, and silver carbonate) can be mentioned as a silver ion source for preparing a silver ion-containing liquid, and silver nitrate is preferred because of its high water solubility and cost. preferable. The amount of silver ions introduced into the slurry is adjusted according to the desired amount of silver coated on the metal powder. Generally, it can be adjusted in the range of 0.1 to 100 parts by mass, for example, in the range of 8 to 50 parts by mass, based on 100 parts by mass of the metal core powder, in terms of mass as silver.
A chelating agent may also be added to the silver ion-containing liquid. The chelating agent forms a silver complex, lowers the reaction rate of the silver coating reaction, and exerts the action of making the silver coating reaction proceed smoothly. As this chelating agent, the same kind as those used in the slurrying step can be used. The amount of the chelating agent to be used may be set, for example, within the range of 0.5 to 4 mol per 1 mol of silver.

It is preferable to maintain the liquid temperature at 15 to 70°C during the reaction. Moreover, it is preferable to proceed the reaction while blowing a non-oxidizing gas (nitrogen, argon, etc.) into the slurry. This gas blowing has the effect of reducing the amount of oxygen in the liquid and the effect of ensuring the state of dispersion of the particles with an appropriate stirring force. As for grasping the final stage of the silver coating reaction, the liquid near the surface of the slurry is fractionated with a dropper, and the filtrate obtained by filtering the fractionated liquid with a filter is added with a halide such as potassium iodide. It can be considered that the silver coating reaction is completed at the point when the silver halide formation reaction does not occur anymore.

The silver-coated metal powder of the present invention is obtained by performing solid-liquid separation on the slurry after the silver-coating step to recover the solid content, washing the recovered solid content with water, drying it, and subjecting it to a pulverization treatment if necessary. can be obtained.

[Conductive paint]
The silver-coated metal powder obtained as described above is suitable as a conductive filler for constructing a "conductive paint" such as a conductive paste or conductive ink. A known method can be used to prepare the conductive paint. For example, in the case of a conductive paste, it is fired at a relatively high temperature to decompose and volatilize the solvent and resin components. There is a "resin-curing conductive paste" in which the resin component is cured by heating at a relatively low temperature, and conductive filler particles are brought into contact with each other by utilizing shrinkage of the resin during curing to obtain conduction. The silver-coated metal powder of the present invention can be used in any of the above types of conductive pastes.

In the conductive paint using the silver-coated metal powder of the present invention as a conductive filler, metal powders other than the silver-coated metal powder of the present invention can be included as necessary within a range that does not impair the effects of the present invention. Such metal powders include copper powder, silver powder, aluminum powder, nickel powder, zinc powder, tin powder, bismuth powder and the like. Phosphorus flour can also be added. Assuming that phosphorus powder is also treated as metal powder for convenience, the total content of the silver-coated metal powder of the present invention and other metal powders in the conductive paint is in the range of 50 to 98% by mass, for example, in the case of a conductive paste. can be

Conductive coatings generally contain solvents and resins. Various known solvents and resins can be used singly or in combination of two or more, as long as the effects of the present invention are not impaired.

Examples of solvents include texanol (2,2,4-trimethyl-1,3-pentanediol 2-methylpropanoate), terpineol, carbitol acetate (diethylene glycol monoethyl ether acetate), ethylene glycol, dibutyl acetate and diethylene glycol. Polar solvents such as monobutyl ether acetate can be mentioned. The total content of the solvent in the conductive paint is preferably 0.5 to 49 mass %, more preferably 1 to 28 mass %, for example, in the case of a conductive paste.

Examples of resins used in the sintered conductive paste include thermoplastic resins such as ethylcellulose, ethylhydroxyethylcellulose, hydroxypropylmethylcellulose, hydroxypropylmethylcellulose phthalate, rosin, phenoxy resins and polyacetal resins. The total content of these resins in the conductive paint is preferably 0.5 to 40% by mass, more preferably 1 to 25% by mass.

Examples of the resin used for the resin-curable conductive paste include thermosetting resins such as phenol resin, urea resin, epoxy resin, polyurethane resin, polyvinyl butyral resin, polyimide resin, and (meth)acrylic resin, and (meth)acrylic resin. Photocurable resins such as acrylic resins, maleic acid resins, polybutadiene resins, epoxy resins, and phenol resins can be used. The total content of these curable resins in the conductive paint is preferably 0.5 to 40% by mass, more preferably 1 to 25% by mass. When curing the resin by heating, in order to accelerate curing, a curing agent such as a thermal polymerization initiator, polyamine, acid anhydride, boron trihalide compound, or amine complex salt of boron trihalide compound is added. good too. In the case of photopolymerization, a photopolymerization initiator is contained. As the photopolymerization initiator, a photoradical generator or a photocationic polymerization initiator is usually used. A photosensitizer can also be added.

The conductive paint may contain substances other than those mentioned above as long as the effects of the present invention are not impaired. For example, additives such as surfactants, dispersants, stabilizers, plasticizers, and metal oxide powders can be added as necessary.

A method for producing the conductive paint is not particularly limited, and known methods can be applied. For example, it can be prepared by preliminarily kneading the constituent components (raw materials) of the paint using a kneading/defoaming machine, a squeegee machine, a universal stirrer, a kneader, etc., and then kneading the main kneading with three rolls. If necessary, a solvent may be added thereafter to adjust the viscosity.

[Example 1]
(Preparation of metal core powder)
The copper balls are heated to 1500°C in the air atmosphere, and the melted metal is dropped from the bottom of the tundish, and is rapidly cooled and solidified by spraying high-pressure water in the air atmosphere from a water atomizer, and the obtained slurry is separated into solid and liquid. Then, the solid matter was washed with water, dried, pulverized and classified to obtain copper powder. Here, this copper powder is used as the metal core powder. When the particle size distribution of this copper powder was examined by the measurement method described later, the volume-based cumulative 50% particle diameter _D50 was 2.5 μm.

(Production of silver-coated metal powder)
170.2 g of hydroxyethylethylenediaminetriacetic acid as a chelating agent and 386.3 g of tartaric acid as a pH adjuster/buffer were dissolved in 1939 g of pure water. The liquid temperature of this solution was adjusted to 25° C., and 350 g of copper powder (the above metal core powder) was added while stirring to prepare a copper powder slurry. When the pH of this slurry was measured with a pH meter (Toa DKK portable pH meter HM-31P), the pH was 2.5. The pH at this point is shown in Table 1 as "pre-reaction pH" (same for each example below).

The slurry was kept at 25° C. for 120 minutes while nitrogen gas was blown thereinto, thereby washing the surfaces of the copper powder particles in an acidic environment of pH 2.5.

As a silver ion-containing liquid, a silver nitrate aqueous solution was prepared by dissolving 61.2 g of silver nitrate in 618 g of pure water. This silver ion-containing liquid was continuously added to the washed slurry over 30 minutes to allow the substitution reaction to proceed. During the reaction, the above nitrogen gas blowing was continued. The liquid temperature during the reaction was maintained at approximately 25°C. The completion of the substitution reaction is carried out by fractionating the vicinity of the surface of the reaction solution with a dropper, adding potassium iodide to the filtrate obtained by filtering the fractionated liquid with a filter, and ensuring that silver iodide (AgI) does not precipitate. I decided based on that. The pH of the reaction solution after completion of the substitution reaction was 2.4. The pH at this point is shown in Table 1 as "post-reaction pH" (same for each example below). After completion of the substitution reaction, the reaction solution was filtered, and the collected solid matter was washed with water, dried and pulverized to obtain a silver-coated metal powder.

The following investigations were conducted on the obtained silver-coated metal powder.
(mass ratio of silver)
After dissolving a silver-coated metal powder (sample powder) with a known mass in nitric acid, hydrochloric acid is added to generate a precipitate of silver chloride (AgCl), the precipitate is dried, and the mass of the dried product obtained is measured. Then, the mass of silver in the dried product (AgCl) was calculated. The mass ratio (%) of silver was defined as the ratio of the mass of the silver to the mass of the sample powder.

(BET specific surface area)
Using a BET specific surface area measuring device (4Sorb US manufactured by Yuasa Ionics Co., Ltd.), nitrogen gas was flowed into the measuring device at 105 ° C. for 20 minutes to deaerate, and then a mixed gas of nitrogen and helium (N ₂ : 30% by volume, He: 70% by volume) was measured by the BET one-point method.

(tap density)
In the same manner as the method described in JP-A-2007-263860, a silver-coated metal powder (sample powder) was filled into a bottomed cylindrical die with an inner diameter of 6 mm and a height of 11.9 mm up to 80% of the volume of the sample. A powder layer is formed, a pressure of 0.160 N/m ² is uniformly applied to the upper surface of the sample powder layer, and the sample powder is compressed with this pressure until the sample powder is no longer densely packed. The height of the layer was measured, and the density of the sample powder was obtained from the measured value of the height of the sample powder layer and the mass of the packed sample powder, and this was taken as the tap density.

(oxygen content)
Measured with an oxygen/nitrogen analyzer (Model TC-436 manufactured by LECO).
(carbon content)
Measured with a carbon/sulfur analyzer (EMIA-22V manufactured by Horiba, Ltd.).

(particle size distribution)
Using a laser diffraction/scattering particle size distribution measuring device (HELOS particle size distribution measuring device manufactured by SYMPATEC, HELOS & RODOS (airflow dispersion module)), the volume-based cumulative 10% particle diameter D was measured at a dispersion pressure of 5 bar. ₁₀ (μm), cumulative 50% particle size D ₅₀ (μm) and cumulative 90% particle size D ₉₀ (μm) were determined.
By substituting these measured values into the following formula (2), the B value, which is an index representing the sharpness of the particle size distribution, was calculated.
B=( _D90 - _D10 )/ _D50 (2)

(L*, a*, b*)
5 g of silver-coated metal powder (sample powder) is weighed and placed in a round cell with a diameter of 30 mm, and the surface of the powder is flattened by tapping 10 times. The values of L*, a*, and b* in the CIE 1976 L*a*b* color space were measured in specular light removal mode (SCE mode) using a Spectro Color Meter SQ2000 manufactured by the company.
_A value was calculated.
A=L*/a*/( _D50 ^1.45) (1)

(TG change rate)
The weight increase rate during heating was determined by a simultaneous differential thermal thermogravimetric measurement device (EXATERTG/DTA6300 model from SII Nano Technology Co., Ltd.). Specifically, 20 mg of silver-coated metal powder (sample powder) is placed in a sample container (alumina open type sample container φ 5.2 mm, height 2.5 mm), the container is set in the holder of the measuring device, and the measurement The temperature was raised from room temperature (25° C.) to 650° C. at a rate of 5° C./min while air flowed through the apparatus at a flow rate of 200 mL/min to obtain a weight change curve in the atmosphere. In this weight change curve, the weight when reaching a predetermined temperature (200° C. or 300° C.) was read as W ₁ (g), and the TG change rate (%) up to that temperature was determined by the following formula (3). Here, the TG change rate up to 200°C and the TG change rate up to 300°C were obtained.
TG change rate (%)=100×(W ₁ −W ₀ )/W ₀ (3)
Here, W ₀ is the weight (g) of the sample powder before the heating test, and W ₁ is the weight (g) of the sample powder when heated to a predetermined temperature.

(Preparation of conductive paint)
93 g of silver-coated metal powder (sample powder), 8.2 g of bisphenol F type epoxy resin (ADEKA Co., Ltd. ADEKA RESIN EP-4901E) which is a thermosetting resin, 0.41 g of boron trifluoride monoethylamine, and a solvent After mixing 2.5 g of butyl carbitol acetate and 0.1 g of oleic acid with a kneader and deaerator, the paste-like conductive paint (conductive A viscous paste) was obtained.

(Preparation of conductive film)
After printing the above conductive paint (conductive paste) on an alumina substrate with a 20 μm thick screen plate having a pattern with a line width of 500 μm and a line length of 37.5 mm by screen printing, it was dried at 200° C. for 40 minutes in the atmosphere. A conductive film was obtained by baking for a minute and curing.

(Evaluation of conductivity of conductive film)
Regarding the obtained conductive film, the volume resistivity immediately after forming the conductive film (hereinafter referred to as “initial resistance”) and the volume resistivity after holding the conductive film at 150 ° C. for 7 days in an air atmosphere (hereinafter referred to as “heating and holding test later resistance”). The volume resistivity of the conductive film is measured by measuring the line resistance of the conductive film with a digital multimeter (AD7451A manufactured by ADC), and measuring the film thickness with a surface roughness profiler (Surfcom 1500DX manufactured by Tokyo Seimitsu Co., Ltd.). It was measured and calculated by the following formula (4).
Volume resistivity (Ω cm) = line resistance (Ω) x film thickness (cm) x line width (cm)/line length (cm) (4)
Also, the resistance increase rate of the conductive film was obtained by the following formula (4).
Resistance increase rate (%)=100×(R ₁ −R ₀ )/R ₀ (4)
Here, R ₀ is the initial resistance (Ω·cm) and R ₁ is the resistance (Ω·cm) after the heating and holding test.
The above results are shown in Table 1 (same for each example below).

[Example 2]
In the preparation of the silver-coated metal powder, 150.7 g of hydroxyethylethylenediaminetriacetic acid and 1101 g of pure water were added to the silver nitrate aqueous solution prepared in Example 1 as the silver ion-containing liquid added during the substitution reaction. Other than that, the experiment was conducted under the same conditions as in Example 1.

[Example 3]
An experiment was conducted under the same conditions as in Example 1 except that the time for washing the surface of the copper powder particles was changed from 120 minutes to 30 minutes in the preparation of the silver-coated metal powder.

[Comparative Example 1]
(Production of silver-coated metal powder)
112.6 g of a 50% by mass solution of tetrasodium ethylenediaminetetraacetate as a chelating agent and 129.2 g of ammonium carbonate as a pH buffer were dissolved in 1939 g of pure water. The liquid temperature of this solution was adjusted to 25° C., and 350 g of copper powder (the above metal core powder) was added while stirring to prepare a copper powder slurry. The pH of this slurry was 8.6.

The surface of the copper powder particles was washed in an alkaline environment of pH 8.6 by keeping the slurry at 25° C. for 30 minutes while blowing nitrogen gas into the slurry.

As a silver ion-containing liquid, an aqueous solution was prepared by dissolving 735 g of a 50% by weight tetrasodium ethylenediaminetetraacetate aqueous solution and 175 g of ammonium carbonate in 1317 g of pure water and adding and dissolving 61.2 g of silver nitrate. This silver ion-containing liquid was continuously added to the washed slurry over 10 minutes to allow the substitution reaction to proceed. During the reaction, the above nitrogen gas blowing was continued. The liquid temperature during the reaction was maintained at approximately 25°C. The method for judging the completion time of the substitution reaction is the same as in Example 1. The pH of the reaction solution after completion of the substitution reaction was 8.8. After completion of the substitution reaction, the reaction solution was filtered, and the collected solid matter was washed with water, dried and pulverized to obtain a silver-coated metal powder.

An experiment was conducted under the same conditions as in Example 1 using the silver-coated metal powder thus obtained.

[Comparative Example 2]
An experiment was conducted under the same conditions as in Example 1, except that the amount of hydroxyethylethylenediaminetriacetic acid added when preparing the slurry of the copper powder in the preparation of the silver-coated metal powder was changed from 170.2 g to 85.1 g. went.

[Comparative Example 3]
An experiment was conducted under the same conditions as in Example 1 except that the amount of tartaric acid added in preparing the copper powder slurry was changed from 386.3 g to 772.5 g in preparing the silver-coated metal powder.

For reference, Table 1 also shows the results of some measurement items for the metal core powder.
All the silver-coated metal powders of the examples according to the present invention have high A values, and the conductive films using these silver-coated metal powders as conductive fillers are compared with the silver-coated metal powders obtained by the conventional method. Compared with the conductive film of Example 1, the initial resistance and the resistance increase rate before and after the heating and holding test were significantly reduced.

Comparative Example 1 is a conventional example in which the silver coating layer was formed in a pH range as high as pH 8.6 to 8.8, and a silver-coated metal powder with a low A value was obtained. As a result, the initial resistance of the conductive film and the resistance increase rate before and after the heating and holding test were significantly inferior to those of the above examples.
In Comparative Examples 2 and 3, the A value was lowered by changing the production conditions of the silver-coated metal powder. Compared to Comparative Example 1 (conventional example), the effect of improving the initial resistance of the conductive film and the rate of increase in resistance was small compared to the above-described examples in which the A value was within the range defined by the present invention.

Claims (8)

A powder of metal particles having a metal core particle having a copper content of 80% by mass or more and a silver coating layer coating the metal core particle,
L*, a* values in CIE 1976 L*a*b* color space measured in specular-removed mode with a color difference meter and laser diffraction/scattering of the powder on the surface flattened by tapping of the powder. The A value in the following formula (1), which is represented by the cumulative 50% particle diameter D ₅₀ (μm) in the volume-based particle size distribution according to the method, is 4.0 or more , and the carbon content is 0.10% by mass or less. A silver-coated metal powder that is
A=L*/a*/( _D50 ^1.45) (1) The silver-coated metal powder according to claim 1, wherein said _D50 is 0.1-10.0 µm. 3. The silver-coated metal powder according to claim 1, which has a particle size distribution in which the B value defined by the following formula (2) is from 0.5 to 2.5.
B=( _D90 - _D10 )/ _D50 (2)
Here, D ₁₀ , D ₅₀ and D ₉₀ are respectively the cumulative 10% particle size D ₁₀ (μm), the cumulative 50% particle size D ₅₀ (μm) and the cumulative 90 in the volume-based particle size distribution according to the laser diffraction/scattering method. % particle diameter D ₉₀ (μm). The silver-coated metal powder according to any one of claims 1 to 3, wherein the mass ratio of silver in the powder is 2.0 to 30.0%. The silver-coated metal powder according to any one of claims 1 to 3, wherein the mass ratio of silver in the powder is 5.0 to 15.0%. A step of mixing an aqueous solution containing a chelating agent and a metal particle powder having a copper content of 80% by mass or more to prepare a slurry of the metal particles having a pH of 2.3 to 3.2 (slurrying step) ),
a step of introducing silver ions into the slurry to form a coating layer of metallic silver on the surface of the metal particles while maintaining the pH of the slurry liquid at 2.3 to 3.2 (silver coating step);
A method for producing a silver-coated metal powder defined in (X) below .
(X) A metal particle powder having a metal core particle with a copper content of 80% by mass or more and a silver coating layer covering the metal core particle, the surface of the powder being flattened by tapping , the values of L* and a* in the CIE 1976 L*a*b* color space measured by a color difference meter in the specular light removal mode, and the cumulative 50 in the volume-based particle size distribution of the powder by the laser diffraction/scattering method. A silver-coated metal powder having an A value of 4.0 or more in the following formula (1) expressed by % particle diameter D ₅₀ (μm).
A=L*/a*/(D50 _^ 1.45) (1) A step of mixing an aqueous solution containing a chelating agent and a metal particle powder having a copper content of 80% by mass or more to prepare a slurry of the metal particles having a pH of 2.3 to 3.2 (slurrying step) ),
a step of washing the surface of the metal particles in the slurry by holding the slurry at 15 to 70° C. for 10 minutes or longer (washing step);
A step of introducing silver ions into the slurry after the washing step to form a coating layer of metallic silver on the surface of the metal particles while maintaining the pH of the slurry liquid at 2.3 to 3.2 (silver coating step). ,
A method for producing a silver-coated metal powder defined in (X) below .
(X) A metal particle powder having a metal core particle with a copper content of 80% by mass or more and a silver coating layer covering the metal core particle, the surface of the powder being flattened by tapping , the values of L* and a* in the CIE 1976 L*a*b* color space measured by a color difference meter in the specular light removal mode, and the cumulative 50 in the volume-based particle size distribution of the powder by the laser diffraction/scattering method. A silver-coated metal powder having an A value of 4.0 or more in the following formula (1) expressed by % particle diameter D ₅₀ (μm).
A=L*/a*/(D50 _^ 1.45) (1) A conductive paint using the silver-coated metal powder according to any one of claims 1 to 5 as a conductive filler.