JP2013159805A - Method for producing silver microparticle, silver microparticle produced by the method for producing silver microparticle, and conductive paste containing the silver microparticle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide silver microparticles of average particle diameters 30-100 nm excellent not only in heat shrinkability and low-temperature sinterability but also infilling properties into an electrode and a circuit pattern formed on a substrate; a method for producing the silver microparticles; and a conductive paste containing the silver microparticles.SOLUTION: A method for producing silver microparticles includes: preparing an aqueous solution (solution A) using silver nitrate and a high-molecular-weight protecting agent; preparing another aqueous solution (solution B) different from the liquid A by dissolving a reductant and a low-molecular-weight protecting agent; and separating, washing, and drying the silver microparticles gained by dropping the liquid B into the liquid A for reduction precipitation. The silver microparticles are obtained not only by controlling the temperature of the mixed solution to 40°C or below when the liquid B is dropped into the liquid A but also by performing a drying step in vacuum freeze-drying; and thereby have 3.0 g/cmor higher tapped density although the average diameters of the fine particles are 30-100 nm, so that the filling properties in an electrode or a circuit pattern formed on the substrate are excellent.

Description

  The present invention provides silver fine particles having an average particle size of 30 to 100 nm, excellent in heat shrinkability and low-temperature sinterability, and excellent in filling properties in electrodes and circuit patterns formed on a substrate, and a method for producing the same, and the silver fine particles The present invention relates to a conductive paste containing

  The electrodes and circuit patterns of electronic devices are formed by printing electrodes and circuit patterns on a substrate using a conductive paste containing metal particles, and then baking by heating and sintering the metal particles contained in the conductive paste. However, in recent years, the heating and baking temperature tends to be lowered.

  For example, as a mounting substrate for an electronic device, a polyimide flexible substrate is generally used because it can be heated up to about 300 ° C. and has excellent heat resistance. Inexpensive PET (polyethylene terephthalate) substrates and PEN (polyethylene naphthalate) substrates are being investigated as alternative materials. However, PET substrates and PEN substrates have lower heat resistance than polyimide flexible substrates, and in particular, PET film substrates used for membrane wiring boards need to be heated and fired at 150 ° C. or lower.

  Moreover, if heating and baking can be performed at a temperature lower than 200 ° C., it becomes possible to form electrodes and circuit patterns on a substrate such as polycarbonate and paper, and it is expected that applications of various electrode materials and the like will be expanded.

  Nanometer-order silver fine particles are expected as metal particles that can be used as a raw material for conductive paste that can be fired at such a low temperature. The reason for this is that when the size of the metal particles is on the order of nanometers, the surface activity becomes high and the melting point is much lower than that of the bulk metal, so that it can be sintered at a low temperature. . In addition, silver fine particles are expensive compared to other conductive particles such as copper, and there is a defect that migration is likely to occur among metal particles, but it is difficult to oxidize compared to copper having the same specific resistance. It is easy to handle.

  In addition, nanometer-order silver fine particles can be sintered at low temperatures, and heat resistance is maintained once sintered, which is also expected as a lead-free solder replacement material using a property not found in conventional solder. Has been.

  On the other hand, since silver fine particles generally tend to have a smaller tap density as the particle size becomes smaller, fine wiring formed with a conductive paste containing the silver fine particles is difficult to increase the filling rate of the silver fine particles, This is disadvantageous for reducing the electric resistance value.

In addition, silver fine particles having low crystallinity of the particles tend to have a high thermal shrinkage rate upon heating and firing. For this reason, for example, in the case of electronic components, fine wiring formed from a conductive paste containing the silver fine particles is the basis. There are problems such as peeling from the material and high resistance due to thinning of the wiring. In powder metallurgy, problems such as deterioration of the dimensional accuracy of the sintered body occur.

  So far, metal colloidal particles having a distribution in particle size including metal nanoparticles and a dispersing agent have been proposed as metal colloidal particles including metal nanoparticles that can form a hard film with excellent low-temperature sintering properties. (Patent Document 1). Further, as fine silver particles that can exist stably even in a polar solvent containing water, silver particles bonded with a straight chain fatty acid having 6 or less carbon atoms as a protective agent (Patent Document 2) have been proposed.

JP 2010-229544 A JP 2009-120949 A

In the aforementioned Patent Document 1, metal colloidal particles having a distribution in particle size including metal nanoparticles and a dispersant are disclosed. However, the production method described in Patent Document 1 is a mixture of a silver nitrate solution and a reducing agent solution. No particular consideration is given to the temperature control at that time, and the silver colloidal particle aggregates are only recovered by a pressure filter in the drying process. For this reason, the colloidal silver particles obtained by the production method described in Patent Document 1 have a tap density of 3 g / cm ³ or less as shown in a comparative example, and the fine wiring formed by the conductive paste containing the silver colloid particles is It is difficult to increase the filling rate of the silver fine particles, which is disadvantageous for reducing the electric resistance value. Further, since the residual amount of organic matter on the surface of the silver fine particles was 2.5% by weight or more, it is difficult to say that the low-temperature sinterability is excellent.

In addition, Patent Document 2 discloses silver particles bonded with a straight chain fatty acid having 6 or less carbon atoms as a protective agent, but was obtained by the production method described in Patent Document 2 as shown in a comparative example below. Since the silver particles have a tap density of 3 g / cm ³ or less and a BET specific surface area value of 7 m ² / g or more, the fine wiring formed by the conductive paste containing the silver particles is filled with silver particles. It is difficult to increase the rate, which is disadvantageous for reducing the electric resistance value.

Therefore, the present invention has a high tap density of 3.0 g / cm ³ or more while having an average particle size (D _SEM ) of 30 to 100 nm and fine particles, and is excellent in heat shrinkability and low temperature sinterability, An object of the present invention is to provide silver fine particles having excellent filling properties in electrodes and circuit patterns formed on a substrate and a method for producing the same.

  The technical problem can be achieved by the present invention as follows.

  That is, the present invention prepares an aqueous solution using silver nitrate and a polymer protective agent (liquid A), and prepares an aqueous solution in which a reducing agent and a low molecular weight protective agent are dissolved separately from the liquid A (liquid B). In the method for producing silver fine particles, in which the silver fine particles obtained by dropping the B liquid into the A liquid and reducing and depositing the B liquid are separated, washed, and dried, the mixed solution when the B liquid is dropped into the A liquid. A method for producing silver fine particles, characterized in that the temperature is controlled to 40 ° C. or lower and the drying step is performed by vacuum freeze-drying (Invention 1).

  Further, the present invention is the method for producing silver fine particles of the present invention 1 wherein the water content of the hydrated product before freeze-drying is 30% or more (Invention 2).

Further, the present invention has an average particle diameter _{(D SEM)} is 30 to 100 nm, a silver fine particle tap density is obtained by the present invention 1 or the present invention 2 of the production method is 3.0 g / cm ³ or more ( Invention 3).

Further, the present invention is the silver fine particles of the present invention 3 having a BET specific surface area value of 7.0 m ² / g or less (the present invention 4).

Further, the present invention is the silver fine particles of the present invention 3 or the present invention 4 having a crystallite diameter (D _x ) of 30 nm or more (the present invention 5).

  Further, the present invention is the silver fine particles according to any one of the present invention 3 to the present invention 5 wherein the remaining amount of organic matter on the surface of the silver fine particles is 0.5 to 2.0% by weight (Invention 6).

  Further, the present invention is the silver fine particle according to any one of the present invention 3 to the present invention 6 having a heat shrinkage rate at 2.0 ° C. of 2.0% or more (Invention 7).

  Moreover, this invention is the electrically conductive paste containing the silver fine particles in any one of this invention 3 to this invention 7 (this invention 8).

In general, the tap density tends to decrease as the particle size of the silver fine particles decreases. However, the method for producing silver fine particles according to the present invention has a mean particle size (D _SEM ) of 30 to 100 nm and fine particles. Therefore, since silver fine particles having a high tap density of 3.0 g / cm ³ or more can be obtained, it is suitable as a method for producing silver fine particles having excellent low-temperature sintering properties and filling properties in electrodes and circuit patterns.

Further, the silver fine particles according to the present invention have a high tap density of 3.0 g / cm ³ or more despite the average particle diameter (D _SEM ) of 30 to 100 nm and fine particles by the above production method. Therefore, it is suitable as a raw material for conductive paste and the like having excellent filling properties in electrodes and circuit patterns formed on a substrate.

  The configuration of the present invention will be described in more detail as follows.

  First, a method for producing silver fine particles in the present invention will be described.

  For the silver fine particles according to the present invention, an aqueous solution is prepared using silver nitrate and a polymer protective agent (liquid A), and separately from the liquid A, an aqueous solution in which a reducing agent and a low molecular weight protective agent are dissolved is prepared (B Liquid), in the method for producing silver fine particles, in which the silver fine particles obtained by dropping the B liquid into the A liquid and reducing and depositing the liquid B are separated, washed and dried, the mixing when the B liquid is dropped into the A liquid While controlling the temperature of a solution to 40 degrees C or less, it can obtain by performing a drying process by vacuum freeze-drying.

  First, an aqueous solution is prepared using silver nitrate and a polymer protective agent (A liquid). The polymer protective agent in the present invention is preferably water-soluble or water-soluble. The acid value is preferably 1 mgKOH / g or more, more preferably 10 mgKOH / g or more. The upper limit of the acid value can be used without any particular limitation. However, when the acid value is 0 mgKOH / g, silver particles having a large particle size are produced, so that it is difficult to obtain fine silver fine particles of 100 nm or less. . The amine value is preferably 0 mgKOH / g. When a polymer protective agent having an amine value is used, a silver amine complex is formed when mixed with silver nitrate, and the reduction reaction is not completed or requires a very long time. The fine particles are not preferable because they have poor distribution. In addition, a polymeric protective agent may be used independently or may use 2 or more types together.

  The addition amount of the polymer protective agent is preferably 1 to 10% by weight, more preferably 1.5 to 8% by weight, based on the silver fine particles. When the addition amount of the polymer protective agent is less than 1% by weight, the particle size of the obtained silver fine particles is increased, which is not preferable. If it exceeds 10% by weight, the remaining organic matter on the surface of the obtained silver fine particles exceeds 2.0% by weight, which is not preferable because the low-temperature sinterability is impaired.

  As the reducing agent in the present invention, water-soluble or water-soluble ones can be used, but hydrazine, alkali borohydride, lithium aluminum hydride, ascorbic acid, erythorbic acid and the like are not preferable because the reducing power is too strong. . From the viewpoint of reducing power, amino alcohols can be preferably used in the present invention, and more preferably N, N-dimethylethanolamine, N, N-diethylethanolamine, N, N-diethylisopropanolamine, N- An amino alcohol having a tertiary amine such as methyldiethanolamine, N-ethyldiethanolamine, Nn-butyldiethanolamine, or Nt-butyldiethanolamine.

  The amount of the reducing agent added is preferably 2.0 to 5.0 moles, more preferably 2.2 to 4.0 moles per mole of silver nitrate. When the amount of the reducing agent added is less than 2.0 moles with respect to 1 mole of silver nitrate, the reduction reaction does not proceed sufficiently, which is not preferable.

  Next, an aqueous solution in which a reducing agent and a low molecular weight protective agent are dissolved is prepared separately from the liquid A (liquid B). When an aqueous solution is prepared by adding a low molecular weight protective agent to an aqueous solution (A solution) containing silver nitrate, the low molecular weight protective agent and silver nitrate react to produce silver carboxylate, which lowers the yield of silver fine particles. At the same time, the silver fine particles obtained by this are not preferable because of poor distribution.

  As the low molecular weight protective agent in the present invention, a carboxylic acid having 3 to 7 carbon atoms can be used. Propionic acid, hexanoic acid and heptanoic acid are preferable, and hexanoic acid and heptanoic acid are more preferable. The longer the carbon chain of the low molecular protective agent, the easier it is to obtain silver fine particles with a higher tap density. A low molecular protective agent may be used independently or may use 2 or more types together.

  The addition amount of the low molecular weight protective agent is preferably 0.05 to 0.4 mole, and more preferably 0.1 to 0.35 mole relative to 1 mole of silver nitrate. When the addition amount of the low molecular protective agent exceeds 0.4 mol with respect to 1 mol of silver nitrate, the generated silver fine particles tend to aggregate, which is not preferable.

  An aqueous solution (liquid B) in which a reducing agent and a low molecular weight protective agent are dissolved is dropped into an aqueous solution (liquid A) prepared using silver nitrate and a polymer protective agent, and a mixing reaction is performed. The temperature during the mixing reaction usually rises to 50 ° C. or higher unless temperature control is performed, but in the present invention, it is preferably controlled in the range of 25 to 40 ° C., more preferably in the range of 30 to 35 ° C. is there. When the temperature during the mixing reaction exceeds 40 ° C., the distribution of the generated silver fine particles tends to be non-uniform, which is not preferable.

  After dropping the solution B, the reaction solution is heated to 60 to 80 ° C. and stirred to complete the reduction reaction. The heating temperature of the reaction solution is preferably 65 to 75 ° C. When the heating temperature of the reaction solution is less than 60 ° C., it takes an extremely long time to complete the reduction reaction, which is industrially disadvantageous. Moreover, when heating temperature exceeds 80 degreeC, since there exists a tendency for the produced | generated silver fine particles to aggregate, it is unpreferable. The reduction reaction ends when the pH value of the reaction solution becomes constant. The reduction reaction preferably proceeds as slowly as possible, and from this viewpoint, it is preferable to use a long chain fatty acid as much as possible for the low molecular weight protective agent.

  The reaction solution after the reduction reaction is repeatedly decanted and washed with water until the supernatant has an electric conductivity of 50 μS / cm or less, and the resulting hydrated product containing silver fine particles is lyophilized in a vacuum and then pulverized by a conventional method. As a result, the silver fine particles of the present invention can be obtained. When drying using a normal dryer without vacuum freeze-drying, the silver fine particles can only be taken out as a large lump, and the subsequent pulverization process becomes very complicated, and the silver fine particles share more than necessary. Will take. For this reason, the obtained particles are coarsened and the tap density is lowered, so that the target silver fine particle powder of the present invention cannot be obtained.

  In vacuum freeze-drying, a water-containing material containing silver fine particles is placed in a dryer, and then the pressure is reduced until the product temperature reaches −40 to −10 ° C., and then the temperature is raised to about 40 ° C. and held for 2 hours or more. By doing.

  The water content of the water-containing material containing silver fine particles during vacuum freeze-drying is preferably 30% or more, more preferably 35 to 80%, and even more preferably 40 to 70%. When the water content is less than 30%, even if vacuum freeze drying is performed, the silver fine particles can be taken out only as a large lump, as in the case of using the above-mentioned ordinary dryer, and the obtained particles are Since it becomes coarse and the tap density decreases, the silver fine particle powder targeted by the present invention cannot be obtained.

  Next, the silver fine particles according to the present invention will be described.

The silver fine particles according to the present invention have an average particle diameter (D _SEM ) of 30 to 100 nm and a tap density of 3.0 g / cm ³ or more.

The average particle diameter (D _SEM ) of the silver fine particles according to the present invention is 30 to 100 nm, preferably 35 to 95 nm, and more preferably 40 to 90 nm. When the average particle diameter (D _SEM ) is in the above range, it is easy to refine an electronic device obtained using the average particle diameter (D _SEM ). When the average particle size (D _SEM ) is less than 30 nm, the surface activity of the silver fine particles increases, and it is not preferable because a large amount of organic matter or the like needs to be adhered in order to stably maintain the fine particle size. .

The tap density of the silver fine particles according to the present invention is 3.0 g / cm ³ or more, preferably 3.5 g / cm ³ or more, more preferably 4.0 g / cm ³ or more. When the tap density is less than 3.0 g / cm ^3, the fine wiring formed by the conductive paste containing the silver fine particles is difficult to increase the filling rate of the silver fine particles, which is disadvantageous for reducing the electric resistance value. is there. The upper limit of the tap density of the silver fine particles is about 6.0 g / cm ³ , and more preferably about 5.5 g / cm ³ .

The BET specific surface area value of the silver fine particles according to the present invention is preferably 7 m ² / g or less, more preferably 6 m ² / g or less. When the BET specific surface area value exceeds 7 m ² / g, the viscosity of the conductive paste obtained by using this is not preferable. The lower limit of the BET specific surface area value of the silver fine particles is about 1.5 m ² / g, more preferably about 2.0 m ² / g.

The crystallite diameter (D _X ) of the silver fine particles according to the present invention is preferably 30 nm or more, more preferably 35 to 95 nm, and more preferably 40 to 90 nm. When the crystallite diameter (D _X ) is less than 30 nm, the silver fine particles become unstable and partial sintering and fusion start even at room temperature, which is not preferable.

The crystallinity of the silver fine particles according to the present invention [ratio of average particle diameter (D _SEM ) to crystallite diameter (D _X ) (D _SEM / D _X )] is preferably 2.7 or less, more preferably 2.5 or less, still more preferably 2.3 or less. The closer the degree of crystallinity is to 1, the more it is a single crystal. When the degree of crystallinity exceeds 2.7, the heat shrinkage rate of the silver fine particles is high, so that the fine wiring formed from the conductive paste obtained by using this is peeled off from the base material, or the wiring becomes thin. This is not preferable because of the problem of high resistance.

  The amount of organic matter remaining on the surface of the silver fine particles according to the present invention is preferably 0.5 to 2.0% by weight, more preferably 0.6 to 1.9% by weight, still more preferably 0.7 to 1.8% by weight. When the residual amount of organic matter on the surface of the silver fine particles exceeds 2.0% by weight, the amount of organic matter present on the surface of the silver fine particles is too much, so that the low temperature sinterability is impaired. On the other hand, if it is less than 0.5% by weight, the wettability to the solvent and the resin is lowered, and the uniform dispersibility of the conductive paste obtained using this is impaired, which is not preferable.

  The heat shrinkage rate of the silver fine particles according to the present invention is preferably 2.0% or more, more preferably 2.1% or more at 240 ° C. In general, a silver coating film formed from a conductive paste containing silver fine particles has fine gaps between the silver fine particles, and a silver coating having a lower resistance can be obtained by eliminating this gap. The silver fine particles according to the present invention have a high heat shrinkage rate at 240 ° C. of 2.0% or more, so that the gaps between the silver fine particles of the coating film formed from the conductive paste obtained by using this are easily filled. For this reason, the electric resistance value can be further reduced.

  The particle shape of the silver fine particles according to the present invention is preferably spherical or granular.

  Next, the conductive paste containing silver fine particles according to the present invention will be described.

  The conductive paste according to the present invention may be in any form of a fired paste and a polymer paste. In the case of a fired paste, the conductive paste is composed of the silver fine particles and the glass frit according to the present invention. Other components may be blended. Moreover, in the case of a polymer type paste, it consists of silver fine particles and a solvent according to the present invention, and if necessary, other components such as a binder resin, a curing agent, a dispersant, and a rheology modifier may be blended.

  As the binder resin, those known in the art can be used. For example, cellulose resins such as ethyl cellulose and nitrocellulose, various modified materials such as polyester resins, urethane modified polyester resins, epoxy modified polyester resins, and acrylic modified polyesters. Examples include polyester resin, polyurethane resin, vinyl chloride / vinyl acetate copolymer, acrylic resin, epoxy resin, phenol resin, alkyd resin, butyral resin, polyvinyl alcohol, polyimide, and polyamideimide. These binder resins can be used alone or in combination of two or more.

  As the solvent, those known in the art can be used, for example, tetradecane, toluene, xylene, ethylbenzene, diethylbenzene, isopropylbenzene, amylbenzene, p-cymene, tetralin and petroleum aromatic hydrocarbon mixtures. Hydrocarbon solvents: ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-n-butyl ether, propylene glycol mono-t-butyl ether, diethylene glycol monoethyl ether, diethylene glyco- Monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripro Ether or glycol ether solvents such as pyrene glycol monomethyl ether; glycol ester solvents such as ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, propylene glycol monomethyl ether acetate, propylene glycol monoethyl ether acetate Ketone solvents such as methyl isobutyl ketone and cyclohexanone; terpene alcohols such as terpineol, linalool, geraniol and citronellol; alcohol solvents such as n-butanol, s-butanol and t-butanol; glycol solvents such as ethylene glycol and diethylene glycol; Γ-butyrolactone and water. Solvents can be used alone or in combination of two or more.

  Although the content of silver fine particles in the conductive paste varies depending on the application, it is preferably as close to 100% by weight as possible, for example, in the case of wiring formation.

  The conductive paste according to the present invention is obtained by mixing and dispersing each component using various kneaders and dispersers such as a laika machine, a pot mill, a three roll mill, a rotary mixer, a twin screw mixer, and the like. Can do.

  The conductive paste according to the present invention can be applied to various coating methods such as screen printing, inkjet method, gravure printing, transfer printing, roll coating, flow coating, spray coating, spin coating, dipping, blade coating, and plating.

  In addition, the conductive paste according to the present invention is used for forming electrodes such as FPD (flat panel display), solar cell, organic EL, wiring for LSI substrates, and wiring for filling fine trenches, via holes, contact holes, etc. It can be used as a material. In addition to firing applications at high temperatures, such as for the formation of internal electrodes for multilayer ceramic capacitors and multilayer inductors, as well as low temperature firing, it is possible to form wiring and materials for wiring on flexible substrates, IC cards, and other substrates. It is suitable as. Moreover, it can also be used for an electromagnetic wave shielding film, an infrared reflection shield, etc. as a conductive film. In electronics mounting, it can also be used as a bonding material for component mounting.

Examples of the present invention are shown below, and the present invention will be specifically described.

The average particle diameter of the silver fine particles was obtained by taking a photograph of the particles using a scanning electron micrograph “S-4800” (manufactured by HITACHI), measuring the particle diameter of 100 or more particles using the photograph, The value was calculated and taken as the average particle size (D _SEM ).

  The specific surface area of the silver fine particles was shown as a value measured by BET method using “Monosorb MS-11” (manufactured by Kantachrome Co., Ltd.).

  The tap density (ρt) of the silver fine particles is measured by dropping the powder into a 25 ml tapping cell using a shaking specific gravity measuring instrument (Kurachi Kagaku Kikai Seisakusho Co., Ltd.). Was measured 600 times.

  The amount of organic matter remaining in the silver fine particles was 10 ° C./min from room temperature (30 ° C.) to 550 ° C. using a thermal analyzer (EXSTAR 6000 TG / DTA6300 manufactured by Seiko Instruments Inc.) under the condition of flowing dry air at 300 ml / min. The amount of the sample is subtracted from the sample amount at the beginning of heating (30 ° C) to the point when the weight reduction is completed (at the start of oxidation of silver fine particles (depending on the sample, 250 to 300 ° C)). It was.

The crystallite size (D _X ) of the silver fine particles is the peak of the plane index (1,1,1) plane using the Kα ray of Cu as the radiation source using an X-ray diffractometer “RINT 2500” (manufactured by Rigaku Corporation). And the crystallite diameter was calculated from the Scherrer equation.

The degree of crystallinity of the silver fine particles was indicated by the ratio of the average particle diameter (D _SEM ) to the crystallite diameter (D _X ) (D _SEM / D _X ).

  The thermal contraction rate of the silver fine particles was determined by using a pellet-shaped silver fine particle sample prepared by applying a load of 125.8 N to silver fine particles placed in a 4 mm diameter mold so as to form a pellet having a height of 5 mm. The length of the sample heated to 30 to 300 ° C. at a heating rate of 10 ° C./min using an analyzer “Thermo Plus2 TMA8310” (manufactured by Rigaku Corporation) is a value calculated according to the following formula 1.

<Equation 1>
Thermal contraction rate at 240 ° C. (%) = {(sample length at 30 ° C.−sample length at 240 ° C.) / Sample length at 30 ° C.} × 100

  The specific resistance of the conductive coating was applied to the polyimide film described later on the polyimide film, preliminarily dried at 120 ° C., heated and cured at 150 ° C. for 10 minutes, and then immersed in 1 mol of HCl aqueous solution for 20 seconds. After washing with water, each conductive film obtained by heating and drying again at 150 ° C. for 1 minute was measured using a four-terminal electrical resistance measuring device “Loresta GP / MCP-T600” (manufactured by Mitsubishi Chemical Analytech Co., Ltd.). The specific resistance was calculated from the sheet resistance and the film thickness.

<Example 1-1: Production of silver fine particles>
To a 60 L container, 2.8 kg of silver nitrate, 25.2 L of water, and a polymer protective agent “DISPERBYK-190” (trade name: manufactured by Big Chemie Japan Co., Ltd.) (acid value 10 mgKOH / kg, amine value 0 mgKOH / kg) 92 g are added. A liquid A was prepared by mixing and stirring. Separately, 4.41 kg of N, N-dimethylethanolamine and 214.5 g of heptanoic acid as a low molecular weight protective agent were added to a 50 L container and mixed and stirred, and then 18.8 L of water was added and mixed and stirred. , B liquid was prepared.

  Next, while controlling the temperature of the mixed solution to be 35 ° C. or lower, the B solution was added dropwise to the A solution. After the temperature was raised to 70 ° C., the mixture was stirred for 3 hours and allowed to stand for 30 minutes to precipitate the solid. After removing the supernatant, it was washed with pure water, and decantation and water washing were repeated until the conductivity of the supernatant was 50 μS / cm or less.

  The obtained water-containing material containing silver fine particles and having a water content of 55% was put in a vacuum freeze dryer, and the degree of vacuum was raised to about 10 Pa, and the product temperature was −30 ° C. to self-freeze. Then, it heated up to 40 degreeC, maintaining a vacuum degree at 10 Pa (article temperature is about 30 degreeC), after hold | maintaining the state for 2 hours, it grind | pulverized and the silver fine particle of Example 1-1 was obtained.

The obtained silver fine particles are granular, the average particle diameter (D _SEM ) is 72 nm, the crystallite diameter (D _X ) is 41.8 nm, the crystallinity (D _SEM / D _X ) is 1.7, and the tap density. (Ρt) was 4.59 g / cm ³ , the BET specific surface area value was 3.5 m ² / g, the residual amount of organic matter was 1.37% by weight, and the heat shrinkage rate was 2.63%.

<Example 2-1: Production of conductive paste>
Diethylene glycol monoethyl so that the content of the silver fine particles in the conductive paste is 70 wt% with respect to 100 parts by weight of the silver fine particles of Example 1-1, 11.0 parts by weight of the polyester resin and 1.4 parts by weight of the curing agent. Add ether, premix using autorotation / revolution mixer “Awatori Nertaro ARE-310” (registered trademark, manufactured by Sinky Co., Ltd.), and then uniformly knead and disperse using 3 rolls. Conductive paste was obtained.

  The conductive paste obtained above was applied onto a polyimide film having a thickness of 50 μm, pre-dried at 120 ° C., heat-cured at 150 ° C. for 10 minutes, then immersed in 1 mol HCl aqueous solution for 20 seconds and washed with water. Then, the electroconductive coating film was obtained by heat-drying again at 150 degreeC for 1 minute again.

  The specific resistance of the obtained conductive coating film was 7.5 μΩ · cm.

Examples 1-2 to 1-3 and Comparative Examples 1-1 to 1-2:
Silver fine particles were obtained by variously changing the production conditions of the silver fine particles.

  The production conditions at this time are shown in Table 1, and the characteristics of the obtained silver fine particles are shown in Table 2.

Comparative Example 1-3: JP 2010-229544 A (Further examination of Example 1)
66.8 g of silver nitrate, 10 g of acetic acid as an agglomeration aid having a carboxyl group, 2.0 g of a polymer dispersant “DISPERBYK-190” having a carboxyl group as a polymer dispersant (trade name: manufactured by Big Chemie Japan Co., Ltd.) The mixture was put into 100 g of ion-exchanged water and stirred vigorously. When 100 g of N, N-dimethylethanolamine was gradually added thereto, the reaction solution rose to 60 ° C. When the liquid temperature dropped to 50 ° C., the mixture was heated and stirred in a water bath set at 70 ° C. for 2 hours. After 1 hour, silver colloidal particle aggregates were obtained as gray precipitates.

  Next, the supernatant of the reaction solution in which the silver colloid particle aggregates were precipitated was removed and diluted with ion-exchanged water. After standing, the supernatant was removed and further diluted with methanol. After standing again, the supernatant was removed and diluted with methanol. Thereafter, the silver colloidal particle aggregates were collected with a pressure filter equipped with a membrane filter (manufactured by Advantech, pore size 0.5 μm). Various properties of the obtained silver colloidal particle aggregate are shown in Table 2.

Comparative Example 1-4: JP-A-2009-120949 (Additional test of Example 1)
In order to remove residual oxygen, 273 g of water was put into a 1 L beaker reaction tank, and after flowing nitrogen at a flow rate of 500 mL / min for 600 seconds from the bottom of the reaction tank, it was supplied at a flow rate of 500 mL / min from the top of the reaction tank. Was a nitrogen atmosphere. The rotating speed of the stirring bar was adjusted to 280 to 320 rpm, and the temperature was adjusted so that the solution temperature in the reaction vessel was 60 ° C.

  After adding 7.5 g of aqueous ammonia (containing 30% by mass as ammonia) to the reaction vessel, the mixture was stirred for 1 minute to make the solution uniform, and then 7.5 g of hexanoic acid (2% with respect to silver) as a protective agent. (Corresponding to 0.01 mol) and stirred for 10 minutes to dissolve the protective agent. Thereafter, 20.9 g of 50% by weight hydrazine hydrate aqueous solution was added as a reducing agent.

  A silver nitrate aqueous solution in which 36 g of silver nitrate crystals were dissolved in 175 g of water was prepared in another container, and this was used as a raw material liquid. The temperature of the aqueous silver nitrate solution was adjusted to 60 ° C., the same as the solution in the reaction vessel.

  Thereafter, the raw material solution was added to the reducing solution at once and a reduction reaction was performed. Stirring was performed continuously and aged for 10 minutes in that state. Thereafter, stirring was stopped, and a fine silver particle mass was obtained through a filtration / washing step and a drying step. Table 2 shows various characteristics of the obtained fine silver particle block.

<Manufacture of conductive paint>
Examples 2-2 to 2-3 and comparative examples 2-1 to 2-4:
A conductive paint and a conductive film were produced according to the method for producing a conductive paint of Example 2-1 except that the type of silver fine particles was variously changed.

  Table 3 shows the production conditions at this time and various characteristics of the obtained conductive coating film.

In general, the tap density tends to decrease as the particle size of the silver fine particles decreases. However, the method for producing silver fine particles according to the present invention has a mean particle size (D _SEM ) of 30 to 100 nm and fine particles. Therefore, since silver fine particles having a high tap density of 3.0 g / cm ³ or more can be obtained, it is suitable as a method for producing silver fine particles having excellent low-temperature sintering properties and filling properties in electrodes and circuit patterns.

Further, the silver fine particles according to the present invention have a high tap density of 3.0 g / cm ³ or more despite the average particle diameter (D _SEM ) of 30 to 100 nm and fine particles by the above production method. Therefore, it is suitable as a raw material for conductive paste and the like having excellent filling properties in electrodes and circuit patterns formed on a substrate.

Claims (8)

An aqueous solution is prepared using silver nitrate and a polymer protective agent (liquid A), and an aqueous solution in which a reducing agent and a low molecular weight protective agent are dissolved separately from liquid A (liquid B). In the method for producing silver fine particles in which the silver fine particles obtained by dropping into the A liquid and reducing and depositing are separated, washed and dried, the temperature of the mixed solution when the B liquid is dropped into the A liquid is 40 ° C. or lower. A method for producing silver fine particles, characterized in that the drying step is performed by vacuum freeze-drying while controlling. The method for producing silver fine particles according to claim 1, wherein the water content of the water-containing material before vacuum freeze-drying is 30% or more. Silver fine particles obtained by the production method according to claim 1 or 2, wherein the average particle diameter (D _SEM ) is 30 to 100 nm and the tap density is 3.0 g / cm ³ or more. The silver fine particles according to claim 3, which have a BET specific surface area value of 7.0 m ² / g or less. The silver fine particles according to claim 3 or 4, wherein a crystallite diameter ( _Dx ) is 30 nm or more. The silver fine particles according to any one of claims 3 to 5, wherein the remaining amount of organic matter on the surface of the silver fine particles is 0.5 to 2.0% by weight. The silver fine particles according to any one of claims 3 to 6, wherein a heat shrinkage rate at 240 ° C is 2.0% or more. A conductive paste comprising the silver fine particles according to claim 3.