JP4897624B2 - Low-temperature sinterable silver fine powder and silver paint and method for producing them - Google Patents

Description

  The present invention relates to a silver fine powder composed of silver nanoparticles coated with an organic substance, a silver paint using the same, and particularly excellent in low-temperature sinterability, and a method for producing them. In the present specification, “nanoparticles” broadly refer to particles having a particle diameter of about 20 nm or less.

  Since metal fine powder has high activity and sintering proceeds even at low temperatures, it has long been noted as a patterning material for materials with low heat resistance. In recent years, in particular, due to advances in nanotechnology, it has become possible to manufacture single nanoclass particles relatively easily.

  Patent Document 1 discloses a method for synthesizing a large amount of silver nanoparticles using an amine compound using silver oxide as a starting material. Patent Document 2 discloses a method of synthesizing silver nanoparticles by mixing and melting an amine and a silver compound raw material. Non-Patent Document 1 describes making a paste using silver nanoparticles. Patent Document 4 discloses a technique for producing silver nanoparticles having extremely good dispersibility in a liquid. On the other hand, Patent Document 3 discloses a polar solvent in which an organic protective material B having a functional group such as a mercapto group having a good affinity for metal particles is dissolved in a nonpolar solvent in which metal nanoparticles protected by the organic protective material A are present. Is added, and a method of exchanging the protective material for metal nanoparticles from A to B by stirring and mixing is disclosed.

JP 2006-219893 A International Publication No. 04/012884 Pamphlet JP 2006-89786 A JP 2007-39718 A Nakami Masami et al., “Application of Silver Nanoparticles to Conductive Pastes”, Chemical Industry, Chemical Industry, October 2005, p.749-754

  In general, the surface of the metal fine powder is generally coated with an organic protective material. This protective material has a role of separating the particles from each other during the silver particle synthesis reaction. Therefore, it is advantageous to select one having a molecular weight that is somewhat large. If the molecular weight is small, the distance between particles becomes narrow, and in a wet synthesis reaction, sintering may progress during the reaction. If it becomes so, particle | grains will become coarse and manufacture of a fine powder will become difficult.

  On the other hand, when forming fine wiring on a substrate using fine metal powder protected with an organic protective material, it is necessary to sinter the metal fine particles after drawing the wiring. At the time of sintering, the organic protective material present between the particles must be removed by volatilization or the like. Although some carbon content may be allowed to remain in the sintered body (wiring), it causes an increase in electrical resistance, so it is desirable that it be completely removed.

  However, organic protective materials having a large molecular weight are generally difficult to volatilize and remove even when heated. For example, in the case of silver fine powder, a highly conductive sintered body (wiring) is constructed unless exposed to a high temperature of 250 ° C. or higher. It is difficult. For this reason, the types of applicable substrates are limited to some materials having a high heat-resistant temperature, such as polyimide, glass, and aramid.

  The present applicant has found that it is possible to synthesize highly dispersible silver nanoparticles by reducing a silver salt with an alcohol in the presence of a primary amine having an unsaturated bond such as oleylamine, This is disclosed in Patent Document 4 and the like. The silver particles synthesized by this method are coated with an organic protective material made of a primary amine present during the reduction reaction. Since this organic protective material has a relatively large molecular weight of 200 or more, it adheres to the periphery of metallic silver and plays a role of so-called “buoyancy ring (or float bag)”, and has excellent dispersibility in a liquid organic medium. Bear. In addition, the organic protective material has an effect of facilitating sintering of metallic silver particles in a thin film drawn with ink containing the silver particles, although the molecular weight is relatively large. This is thought to be because the organic protective material itself tends to oxidize and decompose during firing due to having an unsaturated bond in the molecule of the organic protective material, and detachment from the metallic silver particles occurs relatively easily. In the case of oleylamine, the conductive film can be formed even by low-temperature baking at about 180 ° C.

  However, even if the firing temperature can be lowered to about 180 ° C., the restriction on the substrate is still large. If a metal fine powder that can be sintered at a low temperature of about 100 to 180 ° C., preferably about 100 to 150 ° C. can be produced by a simple technique, its use will inevitably expand. For example, when transparent polycarbonate is used for the substrate, it becomes possible to draw fine wiring directly on the surface of a medium such as a CD or DVD or a lens, and various functions can be imparted. An inexpensive antenna in which fine wiring is drawn on a PET (polyethylene terephthalate) substrate, an IC tag using paper as a material, and the like are also feasible. Furthermore, it is considered that it is possible to draw a metal wiring directly on a conductive polymer, and it is expected that applications of various electrode materials will be expanded. If silver is used as the metal fine powder, its antibacterial action can be utilized. There are an unlimited number of other applications.

  Patent Document 3 discloses a technique for replacing a protective material covering the surface of metal particles with another protective material. However, this technique employs a method of obtaining metal particles covered with a protective material by dropping a reducing agent into a solvent in which the metal supply substance and the protective material are dissolved at the stage of synthesizing the metal nanoparticles. Is. In the case of the reaction in which the reducing agent is dropped into the solvent in this way, it is necessary to use a reducing agent having strong reducing properties because the reducing agent itself is diluted with the solvent, and even if the liquid is stirred, it is completely uniform. It is not easy to deposit metal nanoparticles with a good reducing power. In addition, the reducing agent component is likely to be mixed into the particles. For this reason, it is difficult to control the quality control in order to make the particle size distribution uniform and to reduce impurities in the metal particles. The invention of Patent Document 3 shows an example in which an organic compound having a molecular weight as small as about 100, such as naphthenic acid or octylamine, is used as a protective material formed at the particle synthesis stage. No specific technique for synthesizing protected metal nanoparticles is shown. The metal nanoparticles having a small molecular weight of the protective material tend to aggregate and settle in the liquid medium. Actually, in the invention of Patent Document 3, a process of precipitating and collecting metal nanoparticle aggregates at the synthesis stage is essential. Such agglomerated and settled particles are difficult to maintain in a dispersed state in a liquid medium, are troublesome to handle in intermediate processes including washing, and maintain uniform quality in the process of replacing protective materials. Therefore, it is considered that strong stirring and mixing are essential. As described above, the technique of Patent Document 3 is further applied to industrial implementation in that it is difficult to control a uniform reduction reaction and the particles are likely to aggregate and settle (dispersibility is not so good). Improvement is desired.

  The present invention is intended to provide a silver fine powder coated with a protective material capable of significantly reducing the sintering temperature as compared with the prior art and a silver paint using the same by a simple method.

In order to achieve the above object, in the present invention, an average particle diameter coated with a composite organic protective material composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms. D _TEM: 3 to 20 nm or X-ray crystal particle diameter D _X,: silver fine powder and silver paint using the same consists of silver particles 1~20nm is provided. However, the amount of primary amine B in the composite organic protective material is determined when the coating film coated with the silver paint (obtained by mixing the silver fine powder with an organic medium) is baked at 120 ° C. in the atmosphere. A sufficient amount is ensured so as to exhibit the property of forming a conductive film having a specific resistance of 25 μΩ · cm or less. Specifically, the molar ratio of amine B to amine A and amine B; B / (A + B) is 0.40 to 0.99 is provided. Preferred examples of the primary amine A include oleylamine (C ₉ H ₁₈ = C ₉ H ₁₇ —NH ₂ , molecular weight of about 267).

As a method for producing this low-temperature sinterable silver fine powder, silver particles having an average particle diameter D _{TEM of 3} to 20 nm coated with a primary amine A having an unsaturated bond and a molecular weight of 200 or more were monodispersed in an organic medium. A step of mixing the silver particle dispersion and the primary amine B having 6 to 12 carbon atoms (mixing step), and maintaining the mixed solution at 5 to 30 ° C. in a standing state or stirring state to generate precipitated particles. There is provided a production method including a step (sedimentation step) and a step (solid-liquid separation step) of recovering the precipitated particles as a solid content by solid-liquid separation operation. The recovered solid content is composed of low-temperature sinterable silver fine powder.

  The low-temperature sinterable silver paint of the present invention can be applied by mixing the solid content (silver fine powder) collected as described above (cleaning step), and mixing the solid content after cleaning with an organic medium. It can be manufactured by a method having a process of forming properties (painting process).

  By the way, there is no particular limitation on the method of baking the coating film coated with the silver paint at 120 ° C. in the atmosphere and measuring the specific resistance of the fired film, but it is desirable to adopt a conventional general method. Here, the condition that the specific resistance of the fired film is evaluated to be 20 μΩ · cm or less when the sample to be measured is fired at 200 ° C. in the atmosphere is applied to 120 ° C. baking, and the conductivity of the 120 ° C. fired film is evaluated. . That is, the conditions for the preparation, application, firing and measurement of the paint are the same as those when the specific resistance is 20 μΩ · cm or less after firing at 200 ° C. (however, only the firing temperature is changed to 120 ° C.). Measure the specific resistance. If it is a technique (a publicly known general technique) that can confirm that sintering has occurred at 200 ° C. firing, the presence or absence of sintering can be determined even when applied to 120 ° C. firing. In addition, the silver fine powder or silver paint which cannot find the conditions under which a fired film having a specific resistance of 20 μΩ · cm or less is originally formed by firing at 200 ° C. in the atmosphere is out of the scope of the present invention.

  In this specification, the term “coated with a protective material” means that a sufficient amount of the protective material that can exist independently is attached to the surface of the metallic silver without the metallic silver of the individual particles being bonded to each other. The state that is.

  According to the present invention, a silver fine powder that can be sintered at a temperature as low as 120 ° C. and a silver paint using the same are realized. The silver fine powder and the silver paint can be manufactured relatively easily using silver nanoparticles synthesized by the method disclosed by the present applicant in Patent Document 4 and the like, and industrial implementation is sufficiently possible. It is believed that there is.

  The silver fine powder excellent in low temperature sinterability of the present invention is composed of a primary amine A having an unsaturated bond and a primary amine A having a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms. It is characterized by being covered with a “composite organic protective material”.

In general, an organic compound having a function as a surfactant has a structure of R—X having a hydrophobic group R and a hydrophilic group X. The hydrophobic group R is typically an alkyl group in which hydrogen is bonded to the carbon skeleton, and there are various hydrophilic groups X, but “—COOH” for fatty acids and “—NH ₂ ” for amines. Such a surfactant can also be used as an organic protective material for protecting the active outermost surface of the metal silver particles. In this case, it is considered that the hydrophilic group X is bonded to the surface of the metallic silver, and the hydrophobic group R is oriented toward the outside of the particles covered with the organic protective material. Since metal nanoparticles are extremely active, normally, the surface of the particles cannot be stably present unless they are covered with a protective material. However, in order to impart conductivity to the thin film drawn with the silver nanoparticle paint, it is necessary that the silver particles of the silver particles sinter at as low a temperature as possible. In addition to being extremely fine such as 20 nm or less, the particle surface protective material must be easily detached from the particle surface and volatilized and removed during low-temperature firing.

  In order to facilitate desorption from the particles and volatilization during low-temperature firing, it is advantageous to use an organic compound having a molecular weight as small as possible as the protective material if the hydrophilic groups are the same. On the other hand, if the molecular weights are approximately equal, the ease of desorption and volatilization varies depending on the structure of the hydrophilic group X. According to the study by the inventors, it has been found that amines are more advantageous for low-temperature sinterability when comparing fatty acids and amines. If metal nanoparticles whose surface of metallic silver is coated with an amine having a low molecular weight can be obtained, it is considered that paints (inks, pastes, etc.) excellent in low-temperature sinterability can be produced.

  However, when silver nanoparticles are synthesized by a “wet process”, which is advantageous for mass production compared to synthesis from the gas phase, if silver particles coated directly with a low molecular weight amine are produced during synthesis, they are dispersed by aggregation or the like. It is difficult to obtain silver fine powder having good properties, and it tends to hinder the operation of preparing a paint through a process such as washing after the synthesis reaction. Therefore, in the present invention, silver nanoparticles having good dispersibility coated with amine A having a molecular weight of 200 or more are obtained in advance, and then a part or most of amine A is replaced with amine B having a low molecular weight. And silver nanoparticles coated with a “composite organic protective material” consisting of amine B are obtained. Since the silver fine powder composed of silver particles is composed of a part or most of the protective material with amine B, the protective material can be easily detached even when firing at a low temperature of around 120 ° C. A conductive film having a small specific resistance can be formed. In addition, the presence of amine A having a relatively large molecular weight present in the composite organic protective material is considered to play a role of increasing the role of the composite organic protective material as a “buoy ring”, and the silver is uniformly dispersed. This is advantageous in obtaining a paint.

  As the amine A, a primary amine having an unsaturated bond and having a molecular weight of 200 or more is employed. It has been found by the inventors that primary amines having unsaturated bonds are easily desorbed from metal silver and volatilized and removed by heating during firing even if the molecular weight is somewhat large. However, if the molecular weight is too small, the function of imparting the function of “floating ring” to the composite organic protective material becomes small. As a result of various studies, it is effective to select an amine having a molecular weight of 200 or more. When the molecular weight is excessively large, it becomes difficult to desorb and volatilize when the coating film is baked at a low temperature. As a result of various studies, oleylamine is very suitable in combination with the ease of silver particle synthesis described below.

  As the amine B, a primary amine having 6 to 12 carbon atoms can be used. A remarkable reduction effect of the sintering temperature is recognized at least in this range.

  Regarding the ratio of amine A and amine B in the composite organic protective material, the molar ratio of amine A to the total of amine A and amine B (when amine A is composed of two or more amines, the total molar ratio thereof) If it is about 1% or more, it is considered sufficient to give the action of “floating ring”. On the other hand, the presence of amine B is indispensable for greatly improving the low temperature sinterability. Considering the formation method of the conductive film for various uses, it is extremely effective to impart the property of causing sintering at a firing temperature of about 120 ° C. Further, if a fired film having a specific resistance reduced to 25 μΩ · cm or less can be formed, it can be considered that the film can be sufficiently used as a conductive film. Therefore, the blending amount of amine B (the total amount when amine A is composed of two or more amines) is determined when the coating film coated with the silver nanoparticles is baked at 120 ° C. in the atmosphere. Is specified as an amount sufficient to show the property of forming a conductive film having a specific resistance of 25 μΩ · cm or less. In terms of the molar ratio of amine B to the total of amine A and amine B, it seems that approximately 40% or more of amine B is necessary.

The particle size of the silver particles coated with a composite organic protective material can be represented by the average particle diameter D _TEM or X-ray crystal particle diameter D _X is measured from the image of the TEM (transmission electron microscope). Silver particles is preferred subject silver particles D _TEM is 3~20nm or X-ray crystal particle diameter D _X is a 1~20nm the present invention. Silver fine powder having such a particle size range is advantageous for producing a paint having good characteristics. Among, D _TEM is 6 to 20 nm, D _X silver particles having a particle size of about 4~20nm is easily synthesized by the technique disclosed such as Patent Document 4. Also, very fine silver particles of about D _TEM is 3 to 7 nm, D _X is 1~5nm, for example oleylamine can be synthesized by such method of reducing the direct silver compound as the solvent. In addition, impurities are likely to be mixed into the crystal grain boundaries of the synthesized metallic silver, and if the amount of impurities increases, pores are generated when firing fine wiring, and good conductivity cannot be secured, or migration resistance Inconvenience that is inferior to that. As a result of various studies, silver particles having a single crystallinity represented by D _TEM / D _X of 2.5 or less are desirable, and 2.0 or less is even more preferable.

  Since silver particles coated with such a composite organic protective material are more likely to settle in a liquid medium than those coated only with an organic protective material having a high molecular weight, they are suitable for paste-like paints. However, in the case where the average particle size is relatively small, it is considered that liquid ink can be produced by appropriately selecting the medium.

This silver fine powder excellent in low-temperature sinterability can be obtained as follows.
[Synthesis of silver particles]
It is important that the silver nanoparticle raw material used in the present invention has a stable particle property such as a particle size distribution and has a property of being difficult to aggregate and settle in a liquid medium. Here, as a method for synthesizing such silver particles, the synthesis method disclosed in Patent Document 4 will be briefly described. That is, in this synthesis method, silver particles are precipitated by reducing the silver compound in alcohol or polyol using alcohol or polyol as a reducing agent. In this case, alcohol or polyol is a solvent and a reducing agent. The reduction reaction can proceed by raising the temperature of the solvent solution, preferably by bringing it to a reflux state. By adopting such a method, it is possible to prevent the entry of impurities and to reduce the resistance value when used as a wiring material, for example.

  However, when the reduction reaction proceeds, it is important that an organic compound that functions as a protective material coexists in the solvent. This organic compound will constitute a protective material for silver particles, which will be a raw material later. Here, primary amine A having an unsaturated bond is used as the organic compound. According to the study by the inventors, when an amine having no unsaturated bond is used in a method of directly depositing silver from a highly uniform solvent in which a silver compound is dissolved as in this reduction step, silver nano-particles are presently used. No particles have been synthesized. On the other hand, it has been found that when an amine having an unsaturated bond is used, silver nanoparticles whose surface is protected with the amine are synthesized. There are many unclear points about the reason, but so far, the surface of precipitated silver is surrounded by the amine molecules due to the influence of unsaturated bonds, and the amine prevents the silver reduction from proceeding to some extent. As a result, it is presumed that the growth of silver particles is suppressed and silver nanoparticles having a relatively uniform particle size can be formed.

  According to the inventors' knowledge, it is sufficient that the number of unsaturated bonds at this time be at least one in one molecule of amine A. Two or more amines may be used. By increasing the number of unsaturated bonds, the number of carbons in the protective material present on the surface of the silver particles can be adjusted. Therefore, organic compounds having different numbers of unsaturated bonds may be added as required.

  However, in the present invention, amine A having a molecular weight of 200 or more is used. The thing of about 200-400 is more preferable. When the molecular weight is small, aggregation and precipitation are likely to occur in the liquid medium, which may hinder a uniform reduction reaction. In this case, it becomes difficult to control quality control such as making the particle size distribution uniform. In addition, it becomes difficult to create a situation where silver particles are monodispersed in a liquid organic medium. On the other hand, when an organic compound having an excessively large molecular weight is used, the aggregation suppressing effect is enhanced, but there is a concern that it is difficult to replace part or most of amine A covering the surface of silver particles with amine B in a later step. The Specific examples of amine A include oleylamine.

  The amount of the organic compound (primary amine A) that is allowed to coexist in the solvent during the reduction reaction can be 0.1 to 20 equivalents relative to silver, more preferably 1.0 to 15 equivalents. 0-10 equivalent is still more preferable. Here, in the primary amine, 1 mol of amine corresponds to 1 equivalent per 1 mol of silver. If the amount of primary amine used is too small, the amount of protective material on the surface of the silver particles is insufficient, and monodispersion in the liquid cannot be realized. If the amount is too large, there is a possibility that a reaction for replacing a part or most of amine A with amine B in the subsequent step cannot be efficiently performed.

  As the reducing agent, alcohol or polyol as a solvent is used. As a result, silver nanoparticles with less impurities can be obtained. In the reaction, it is efficient to perform a reflux operation. For this reason, it is preferable that the alcohol or polyol has a low boiling point, specifically 80 to 300 ° C, preferably 80 to 200 ° C, more preferably 80 to 150 ° C. Further, it is preferable from the viewpoint of reducing ability that the alcohol has as long a carbon chain as possible.

  As alcohol, propyl alcohol, n-butanol, isobutanol, sec-butyl alcohol, hexyl alcohol, heptyl alcohol, 1-octyl alcohol, 2-octyl alcohol, allyl alcohol, crotyl alcohol, cyclopentanol and the like can be used. As the polyol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, or the like can be used. Of these, isobutanol and n-butanol are preferred.

  In order to promote the reduction reaction, a reduction auxiliary agent may be added. Although the specific example of a reduction adjuvant should just select 1 or more types from what is indicated by patent documents 4, it is especially preferred to use diethanolamine and triethanolamine among these.

  As the silver compound which is a supply source of silver, various compounds can be applied as long as they can be dissolved in the above solvent, and examples thereof include silver chloride, silver nitrate, silver oxide, and silver carbonate. preferable. In the method of the present invention, the Ag ion concentration in the liquid during the reaction can be 0.05 mol / L or more, preferably 0.05 to 5.0 mol / L. The amine A / Ag molar ratio can be in the range of 0.05 to 5.0. The molar ratio of reducing aid / Ag can be in the range of 0.1-20.

  Silver particles covered with amine A (synthesized by the above reduction) have a proportion of amine A present to the total of silver particles and amine A (hereinafter simply referred to as “amine A ratio”) of 0.05 to 25 mass. It is desirable to adjust to%. If the amine A ratio is too low, particles are likely to aggregate. On the other hand, when the ratio of amine A is high, there is a possibility that the reaction for replacing part or most of amine A with amine B in the subsequent step cannot be performed efficiently.

The temperature of the reduction reaction is preferably in the range of 50 to 200 ° C. If the reaction temperature is too low, the reducing action of the alcohols is hardly exerted, the reaction is difficult to proceed, and at the same time there is a risk of causing a reduction failure. If the reaction temperature is too high, excessive reduction or sintering in the liquid tends to occur, and there is a possibility that the coarsening of the particles and the variation in the particle diameter become large. In applications where fine wiring is formed as ink or paste, it is desirable to obtain silver fine particles having an average particle diameter D _TEM (described later) of 20 nm or less. The reaction temperature is more preferably 50 to 150 ° C, and further preferably 60 to 140 ° C. Specifically, good results can be obtained, for example, by controlling in the range of 80 to 130 ° C.

  In some cases, the reduction can be performed in multiple stages. That is, when the reduction proceeds rapidly, the particle growth may become excessive. In order to effectively control the particle size, it is preferable to perform the reduction at a low temperature first and then switch the temperature to a high temperature or proceed with the reduction while gradually increasing the temperature. At this time, if the temperature difference is large, there is a concern that the particle size distribution may change significantly. Therefore, it is desirable that the difference between the lowest temperature and the highest temperature be within 20 ° C. It is more preferable to strictly control within 15 ° C or even within 10 ° C.

[Preparation of silver particle dispersion]
The silver particles covered with amine A are synthesized, for example, by a reduction reaction in a wet process as described above, and then subjected to solid-liquid separation and washing. Then, it mixes with a liquid organic medium and makes a dispersion liquid. As the liquid organic medium, a substance in which silver particles covered with amine A are well dispersed is selected. For example, a hydrocarbon system can be suitably used. In particular, aliphatic hydrocarbons such as isooctane, n-decane, isododecane, isohexane, n-undecane, n-tetradecane, n-dodecane, tridecane, hexane, heptane, fragrances such as benzene, toluene, xylene, ethylbenzene, decalin, tetralin, etc. Group hydrocarbons can be used. One or more of these substances may be used to form a liquid organic medium.

  However, in the present invention, it is important to prepare a silver particle dispersion in which silver particles coated with amine A are monodispersed. Here, “monodispersed” means that individual silver particles exist in a liquid medium in a state where they can move independently without aggregating with each other. Specifically, when a liquid containing silver particles is subjected to a solid-liquid separation operation by centrifugation, a liquid (supernatant) in which the particles remain dispersed can be adopted as the silver particle dispersion here. it can.

[Coating with composite organic protective material]
When a liquid organic medium in which silver particles coated with amine A are monodispersed and primary amine B having 6 to 12 carbon atoms are mixed, a state in which amine B exists around each particle, that is, the particles A state of being surrounded by amine B in the liquid (hereinafter referred to as an “enclosed state by amine B”) can be realized. The inventors have discovered that if this state is maintained for a while, a phenomenon occurs in which a part of the amine A is detached from the silver particles and replaced with the amine B. Stirring is not always necessary, and this replacement (hereinafter referred to as “replacement reaction”) proceeds even if left standing. As a result, silver nanoparticles coated with a composite organic protective material composed of amine A and amine B can be obtained. Even if all of amine A is replaced with amine B, it is considered that good results can be obtained with respect to low-temperature sinterability, but this method can obtain a composite state. Rather, since the amine A remaining in the protective film exhibits the action of a “floating ring” as described above, it is not inconvenient to be a composite protective material.

  There are many unclear parts about the mechanism of this replacement reaction at present, but the difference in affinity between metallic silver and amine due to the difference in the size of the hydrophobic groups of amine A and amine B is the progress of this reaction. It is thought that this is the main factor. In addition, the fact that amine A having an unsaturated bond is employed also facilitates elimination of amine A from metallic silver, and is thought to contribute to the progress of the substitution reaction with amine B.

  The replacement reaction proceeds at room temperature, but if the temperature is increased too much, inadvertent sintering may occur. Therefore, the replacement reaction is preferably performed at a temperature of about 5 to 30 ° C, preferably about 10 to 20 ° C. Moreover, when the surrounding state by amine B is made, the amount of substitution by amine B increases with time, but it is desirable to secure a replacement reaction time of 30 minutes or more in both the stationary state and the stirring state. However, since the replacement reaction beyond that does not proceed much beyond 24 hours, it is practical to terminate the replacement reaction within 24 hours.

  The amount of amine B to be mixed ensures an amount sufficient to realize the “enclosed state by amine B”. It is desirable to add a much larger molar ratio to the amount of amine A present before mixing. Specifically, it is desirable to mix amine B at a molar ratio of at least 2 times with respect to amine A present before mixing.

  The particles that have undergone the replacement reaction settle in the liquid. This is because when the amine A is replaced with the amine B to some extent, the ability of the protective material as a “floating ring” is lowered. The amount of substitution when sedimentation occurs depends on the particle size. It is considered that further replacement reaction can proceed even in the settled particles. In order to maintain the replacement reaction in the precipitated particles, it is effective to maintain the surrounding state of the precipitated particles with the amine B by stirring.

  It is advantageous to improve the yield if the alcohol is added and the mixture is stirred for a while after the replacement reaction is sufficiently advanced.

(Solid-liquid separation)
As described above, the particles that have undergone the replacement reaction settle, so after completion of the reaction (including the case where the progress of the reaction is completed in a state where the replacement reaction can still proceed), the liquid is separated into solid and liquid. The particles that have undergone the replacement reaction can be recovered as a solid content. Centrifugation is desirable as the solid-liquid separation. The obtained solid content is mainly composed of silver nanoparticles coated with a composite organic protective film composed of amine A and amine B. Thus, the silver fine powder of the present invention is obtained.

〔Washing〕
The solid content is desirably washed using a solvent such as alcohol. The solid content obtained after the solid-liquid separation after one or more washing operations is used for the paint.

[Preparation of paint]
The silver paint of the present invention can be obtained by mixing the solid content after washing (washed silver fine powder) with an organic medium so as to be capable of being applied. It is important that the organic medium to be mixed here is one that is easy to volatilize and remove at a temperature of about 120 ° C.

<< Comparative Example 1 >>
As a reference, a silver paint was prepared using silver fine powder synthesized by the alcohol reduction method disclosed in Patent Document 4 and the like, and the specific resistance of the fired film fired at firing temperatures of 200 ° C. and 120 ° C. was examined. The silver fine powder is such that the surface of each particle is covered with an organic protective material made of amine A (here, oleylamine). Specifically, the experiment was conducted as follows.

[Synthesis of silver particles]
200 mL of isobutanol (special grade manufactured by Wako Pure Chemical Industries, Ltd.) as a reaction medium and reducing agent, 27 mL of oleylamine (manufactured by Wako Pure Chemical Industries, Ltd., molecular weight = 267) as amine A, silver nitrate crystals as a silver compound (manufactured by Kanto Chemical Co., Inc.) ) 13.7 g were prepared, mixed and stirred with a magnetic stirrer to dissolve silver nitrate. This solution is transferred to a container equipped with a refluxer and placed in an oil bath. While nitrogen gas is blown as an inert gas at a flow rate of 400 mL / min into the container, the solution is raised to 100 ° C. while stirring with a magnetic stirrer. Warm up. After refluxing for 3 hours at a temperature of 100 ° C., 8.5 g of a secondary amine diethanolamine (manufactured by Wako Pure Chemical Industries, Ltd., molecular weight = 106) as a reducing auxiliary agent to an Ag molar ratio of 1.0. Added. Thereafter, after holding for 1 hour, the reaction was terminated. The slurry after completion of the reaction was subjected to solid-liquid separation with a centrifuge, and the separated liquid was discarded to recover the solid component. Thereafter, a washing operation of “the solid component was mixed with methanol and then solid-liquid separated with a centrifuge, and the separated liquid was discarded to recover the solid component” was performed twice.

[Preparation of silver particle dispersion]
Tetradecane was prepared as a liquid organic medium. The solid component after washing was mixed and dispersed therein, and solid-liquid separation was performed with a centrifuge for 30 minutes, and the separated liquid was recovered. In this liquid, silver particles covered with amine A are monodispersed.

This silver particle dispersion was observed with a transmission electron microscope (TEM) to obtain an average particle diameter _DTEM . That is, among the particles observed at a magnification of 600,000 by TEM (JEM-2010 manufactured by JEOL Ltd.), the particle diameters of 300 independent silver particles that did not overlap were measured, and the average particle diameter was calculated. . As a result, D _TEM was 8.5 nm. In this example, as will be described later, since this silver particle dispersion is used for the silver paint, Table 1 shows the D _TEM values.

Using an TG-DTA apparatus, the ratio of amine A to the total of silver particles and amine A (oleylamine) covering the surface thereof (hereinafter referred to as “amine A coating amount”) was determined. In order to calculate the amine A coating amount, the heat pattern shown in FIG. 1 is adopted. Specifically, first, the temperature is raised from room temperature to 200 ° C. at 10 ° C./min (Stage I) and maintained at 200 ° C. for 60 minutes (Stage II), and the organic medium contained in the silver particle dispersion ( Here, tetradecane) is volatilized. Next, the temperature is raised from 200 ° C. to 700 ° C. at 10 ° C./min (Stage III), and maintained again at 700 ° C. for 60 minutes (Stage IV). It can be considered that all of the organic medium is volatilized in stages I to II and amine A as a protective material remains, and in stages III to IV, all of amine A is volatilized. The weight change measured by the TG-DTA apparatus is monitored with the heat pattern of FIG. 1, and the weight change becomes almost zero by the end of stage II. Therefore, the weight W ₁ reduced by this time is reduced to the organic medium ( The weight of the dispersion medium. Then, after the start of stage III, the weight decrease occurs again, and the change in weight becomes almost zero by the end of stage IV. Therefore, the newly reduced weight W ₂ between stages III to IV is used as the weight of amine A. To do. Let the remaining weight W ₃ be the net weight of silver. The amine A coating amount (%) is calculated by W ₂ / (W ₂ + W ₃ ) × 100. As a result, the amine A coating amount of the particles in this silver particle dispersion was 8.0% by mass.

[TG-DTA measurement of organic protective material]
The above TG-DTA measurement was carried out using a special heat pattern in order to examine the amine A coating amount. Here, a normal temperature increase (temperature increase rate of 10 ° C./min.) Was separately performed for the silver particle dispersion. ) -TG-DTA measurement was performed. The DTA curve is shown in FIG. In FIG. 2, the large peak between 200 and 300 ° C. and the peak between 300 and 330 ° C. are considered to be attributed to oleylamine, which is amine A.

[FT-IR measurement of organic protective material]
Using FT-IR (Fourier transform infrared spectrophotometer), the spectrum of the organic compound was measured for the reagent oleylamine and the particles in the silver particle dispersion. As a result, it was confirmed that the organic protective film was oleylamine alone.

[Measurement of X-ray crystal grain size D _X ]
For the X-ray crystal grain size D _X , solid-liquid separation after particle synthesis before washing into a silver particle dispersion → wet precipitate after washing is applied to a glass cell, set in an X-ray diffractometer, and Ag Using the diffraction peak of the (111) plane, the X-ray crystal grain size D _X was determined by the Scherrer equation shown in the following equation (1). Cu-Kα was used for X-rays.
D _X = K · λ / (β · cos θ) (1)
However, K is a Scherrer constant and 0.94 is adopted. λ is the X-ray wavelength of the Cu—Kα ray, β is the half width of the diffraction peak, and θ is the Bragg angle of the diffraction line.
The results are shown in Table 1 (same in the following examples).

[Preparation of silver paint]
Here, a silver paint using silver particles coated with an organic protective material made of amine A was prepared. When the viscosity of the silver paint dispersion was measured with a rotary viscometer (RE550L manufactured by Toki Sangyo Co., Ltd.), the viscosity was 5.8 mPa · s. Moreover, the silver concentration in this silver particle dispersion liquid was 60 mass% by the measurement using the TG-DTA apparatus mentioned above. Since this silver particle dispersion was judged to have properties that can be applied as an ink, it was decided to use this as it is as a silver paint.

[Formation of coating film]
A coating film was formed by coating the silver paint on a glass substrate by spin coating.

[Formation of fired film]
The substrate on which the coating film has been formed is first pre-baked on a hot plate at 60 ° C. for 30 minutes in the air, and then held at 200 ° C. for 1 hour in the air on the hot plate to obtain a “200 ° C. baking film”. It was. Similarly, after pre-baking at 60 ° C., it was held on a 120 ° C. hot plate for 1 hour to obtain a “120 ° C. baking film”.

[Measurement of specific resistance (volume resistance) of fired film]
From the surface resistance measured with a surface resistance measuring device (Mitsubishi Chemical; Loresta HP) and the film thickness of the fired film measured with a fluorescent X-ray film thickness measuring device (SII; STF9200), a volume resistance value is obtained by calculation. This was adopted as the specific resistance of the fired film.
The results are shown in Table 1 (same in the following examples).

  As can be seen from Table 1, according to the silver fine powder of this example in which the protective material is composed of amine A alone, the specific resistance of the 200 ° C. fired film is very low. It can be said that sintering occurs. However, the 120 ° C. fired film was not recognized as having conductivity. Therefore, it can be said that the sintering of silver particles sufficient to impart conductivity is not occurring under the condition of 120 ° C. × 1 hour.

<< Examples 1-7 >>
Silver nanoparticles were synthesized in the same manner as in Comparative Example 1 to obtain a silver particle dispersion in which silver particles coated with amine A (oleylamine) were monodispersed in tetradecane.

[Mixing with amine B and generation of precipitated particles]
As amine B, a reagent octylamine (C ₈ H ₁₇ —NH ₂ , a special grade manufactured by Wako Pure Chemical Industries, Ltd.) was prepared. Octylamine was added to the silver particle dispersion obtained by the method of Comparative Example 1 (amine A coating amount was 8.0% by mass, silver concentration was about 60% by mass). The liquid temperature was 15 ° C., and the amount of octylamine added was equivalent to 10 equivalents with respect to silver. Thereafter, the liquid temperature was kept at 15 ° C., and the stirring state shown in Table 1 was maintained for the time shown in Table 1. In the meantime, it was observed that precipitated particles were formed.

[Solid-liquid separation and washing]
The liquid in which the precipitated particles were generated was solid-liquid separated by centrifugation for 5 minutes. The obtained solid content is recovered, methanol is further added to this solid content, ultrasonic dispersion is performed for 30 minutes, and then a washing operation is performed once to recover the solid content by solid-liquid separation by centrifugation for 5 minutes. went.

[TG-DTA measurement of organic protective material]
About the solid content after washing | cleaning, the TG-DTA measurement by the normal temperature rising similar to the comparative example 1 was performed. As a result, peaks were observed between 100 and 200 ° C. and between 300 and 330 ° C. in any of Examples 1 to 7. The former peak is considered to be attributed to octylamine, which is amine B, and the latter peak is considered to be attributed to oleylamine, which is amine A. For reference, the DTA curves of Example 3, Example 4, and Example 6 are shown in FIGS. 3, 4, and 5, respectively.

[FT-IR measurement of organic protective material]
As in Comparative Example 1, FT-IR measurement was performed on the solid content after washing obtained in each example. As a result, it was confirmed that in each example, the organic protective material was a “composite organic protective material” composed of oleylamine (amine A) and octylamine (amine B).

[Measurement of average particle diameter D _TEM ]
Of the silver particles observed at a magnification of 600,000 times by TEM (JEM-2010 manufactured by JEOL Ltd.) for the sample powder (solid content in the wet state after being coated with a composite organic protective material with amine B) Then, 300 independent silver particles that did not overlap were randomly selected, and the particle size (major axis on the image) was measured. The average particle diameter D _TEM was obtained by arithmetically averaging the particle diameters of the individual particles.

[Measurement of X-ray crystal grain size D _X ]
A sample powder (wet solid content after washing coated with a composite organic protective material with amine B) is applied to a glass cell, set in an X-ray diffractometer, and subjected to X-ray crystals under the same conditions as in Comparative Example 1. The particle size D _X was determined.

[Preparation of silver paint]
A small amount of tetradecane was added to the solid content after the washing, and the mixture was applied to a kneading deaerator to obtain a pasty silver paint.

[Formation of coating film]
The coating film was formed by apply | coating a silver coating material on the board | substrate similar to the comparative example 1 using an applicator.

[Formation of fired film]
It carried out by the method similar to the comparative example 1.

[Measurement of specific resistance (volume resistance) of fired film]
It carried out by the method similar to the comparative example 1.
In addition, the “yield” shown in Table 1 represents the treatment liquid after coating with the composite organic protective material with amine B with respect to the mass of silver present in the silver particle dispersion before mixing with amine B. This is the mass of silver recovered as a precipitate when subjected to centrifugation at 3000 rpm for 5 minutes. The remaining silver could not be recovered while floating in the supernatant.

  As can be seen from Table 1, in these examples, by obtaining silver nanoparticles coated with a composite organic protective material composed of amine A (oleylamine) and low molecular weight amine B (octylamine), it was 120 ° C. Sintering at low temperature was possible.

<< Examples 8 and 9 >>
In Example 1 and Example 4, amine B was mixed to produce precipitated particles, and the mixture was allowed to stand without stirring. Otherwise, the same operations as in Example 1 and Example 4 were performed to obtain Example 8 and Example 9, respectively.
Even when allowed to stand, a “composite organic protective film” composed of amine A and amine B was formed. In addition, as in the other examples, sintering occurred at 120 ° C. For reference, the DTA curves of Example 8 and Example 9 are shown in FIGS. 6 and 7, respectively.

Example 10
In Example 1, after completing the reaction for generating the precipitated particles, before performing solid-liquid separation, alcohol (here, methanol) is added to the liquid at a ratio of 1 L with respect to 1 L (liter) of liquid, Thereafter, the mixture was stirred at 400 rpm for 5 minutes. Otherwise, the same operation as in Example 1 was performed.
As can be seen from Table 1, the yield was significantly improved by adding alcohol (compared with Example 1). However, the specific resistance of the 120 ° C. fired film was slightly higher.

Example 11
In Example 10, was changed amine B hexylamine reagent _{(C 6 H 13 -NH 2)} . Otherwise, the same operation as in Example 10 was performed.
It was confirmed that a “composite organic protective film” composed of amine A (oleylamine) and amine B (hexylamine) was formed. In this example as well, sintering at a low temperature of 120 ° C. was possible.

The figure which showed typically the heat pattern by the TG-DTA apparatus at the time of measuring the amine A coating amount of the synthetic | combination silver particle. The figure which illustrated the DTA curve about the protective material of the particle | grains used for the silver coating material in the comparative example 1. FIG. The figure which illustrated the DTA curve about the protective material of the particle | grains used for the silver coating material in Example 3. FIG. The figure which illustrated the DTA curve about the protective material of the particle | grains used for the silver coating material in Example 4. FIG. The figure which illustrated the DTA curve about the protective material of the particle | grains used for the silver coating material in Example 6. FIG. The figure which illustrated the DTA curve about the protective material of the particle | grains used for the silver coating material in Example 8. FIG. The figure which illustrated the DTA curve about the protective material of the particle | grains used for the silver coating material in Example 9. FIG.

Claims (12)

It consists of silver particles having an average particle diameter D _{TEM of 3} to 20 nm coated with a composite organic protective material composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms. It is a silver fine powder, and the silver fine powder is mixed with an organic medium to form a silver paint. When a coating film coated with the silver fine powder is baked at 120 ° C. in the atmosphere, it becomes a conductive film having a specific resistance of 25 μΩ · cm or less. Silver fine powder. It is composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms, and a molar ratio of amine A to amine B in amine B; B / (A + B) is 0.40. Silver fine powder composed of silver particles having an average particle diameter D _{TEM of 3} to 20 nm coated with a composite organic protective material of ˜0.99. Silver particles having an X-ray crystal grain size D _X : 1 to 20 nm coated with a composite organic protective material composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms A silver fine powder comprising: a silver paint mixed with an organic medium to form a silver paint, and a film having a specific resistance of 25 μΩ · cm or less when the coated film is baked at 120 ° C. in the atmosphere. Silver powder with It is composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms, and a molar ratio of amine A to amine B in amine B; B / (A + B) is 0.40. X-ray crystal grain size D _X coated with a composite organic protective material of ˜0.99: Silver fine powder composed of silver particles of 1 to 20 nm.   The silver fine powder according to any one of claims 1 to 4, wherein the primary amine A is oleylamine. Silver particles having an average particle diameter D _TEM : 3-20 nm coated with a composite organic protective material composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms A silver paint having the property of forming a conductive film having a specific resistance of 25 μΩ · cm or less when the coating film coated with the silver paint is baked at 120 ° C. in the air. It is composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms, and a molar ratio of amine A to amine B in amine B; B / (A + B) is 0.40. A silver paint containing silver particles having an average particle diameter D _{TEM of 3} to 20 nm coated with a composite organic protective material of ˜0.99 as a component. Silver particles having an X-ray crystal grain size D _X : 1 to 20 nm coated with a composite organic protective material composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms A silver paint having a property of forming a conductive film having a specific resistance of 25 μΩ · cm or less when a coating film coated with the silver paint is baked at 120 ° C. in the air. It is composed of a primary amine A having an unsaturated bond and a molecular weight of 200 or more and a primary amine B having 6 to 12 carbon atoms, and a molar ratio of amine A to amine B in amine B; B / (A + B) is 0.40. X-ray crystal grain size D _x coated with a composite organic protective material of ˜0.99: silver paint containing silver particles of 1-20 nm as a component.   The silver paint according to claims 6 to 9, wherein the primary amine A is oleylamine. An average particle diameter D _TEM coated with a primary amine A having an unsaturated bond and a molecular weight of 200 or more: _TEM : a silver particle dispersion in which silver particles of 3 to 20 nm are monodispersed in an organic medium, and 1 having 6 to 12 carbon atoms A step of mixing with the secondary amine B, a step of generating the precipitated particles by maintaining the mixed solution at 5 to 30 ° C. in a standing state or a stirring state, and a step of recovering the precipitated particles as a solid content by solid-liquid separation operation The manufacturing method of the silver fine powder in any one of Claims 1-5 which have these. An average particle diameter D _TEM coated with a primary amine A having an unsaturated bond and a molecular weight of 200 or more: _TEM : a silver particle dispersion in which silver particles of 3 to 20 nm are monodispersed in an organic medium, and 1 having 6 to 12 carbon atoms A step of mixing with the secondary amine B, a step of generating the precipitated particles by maintaining the mixed solution at 5 to 30 ° C. in a standing state or a stirring state, and a step of recovering the precipitated particles as a solid content by solid-liquid separation operation The method for producing a silver paint according to any one of claims 6 to 10, further comprising: a step of washing the obtained solid content, and a step of mixing the washed solid content and an organic medium so as to allow application.

Images