JP2008159498A - Conductive paste and manufacturing method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of conductive paste that does not generate any aggregated blocks easily as compared with before and is capable of suppressing the mixture of unneeded constituents. <P>SOLUTION: The manufacturing method of conductive paste comprises: a process for heating solution containing metal salt and a first organic solvent, reducing the metal salt for generating a metal ultra-fine particle, and obtaining a composite solution containing the metal ultra-fine particle; a process for removing a first organic solvent and a reaction residue from the composite solution and obtaining a metal ultra-fine particle in a non-dried and non-hardened state; and a process for mixing the metal ultra-fine particle in a non-dried and non-hardened state with a second organic solvent for pasting. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a conductive paste and a conductive paste, and more particularly to a method for producing a conductive paste using ultrafine metal particles, and a conductive paste obtained by this production method.

  Conventionally, conductive paste using fine metal particles of μm size (hereinafter sometimes referred to as “micron size”) is used as, for example, a conductive material used for forming circuits such as electrodes and wiring on printed circuit boards and interlayer bonding. Widely known.

  In recent years, electronic devices in which printed boards are incorporated have become highly functional and miniaturized, and accordingly, wiring circuits have been narrowed. For this reason, general-purpose conductive pastes using micron-sized metal fine particles cannot cope with the narrow pitch sufficiently.

  Therefore, recently, an attempt has been made to replace micron-sized metal fine particles with nano-sized metal ultrafine particles (hereinafter sometimes referred to as “nano-size”).

  As a method for synthesizing this kind of ultrafine metal particles, for example, in Patent Document 1 by the present applicant, a fatty acid silver salt such as capric acid silver salt is dissolved or dispersed in an organic solvent such as hexanol, and in this state, a microwave is used. Has been disclosed to produce ultrafine silver particles.

  Up to now, the present applicant has synthesized metal ultrafine particles in the production of conductive paste using metal ultrafine particles such as silver ultrafine particles, and then dried the synthesized liquid to recover the metal ultrafine particles. The method of mixing this with an appropriate organic solvent to form a paste has been studied.

JP 2004-353038 A

  However, it has been found that the conventional method for producing a conductive paste has the following problems.

  That is, in the conventional method for producing a conductive paste, the synthesized ultrafine metal particles are collected by drying. Therefore, it is difficult to sufficiently pulverize the agglomerates of ultrafine metal particles at the time of forming a paste, and as a result, there is a problem that a large number of agglomerates remain in the conductive paste. If the agglomerates are present in the conductive paste, for example, when a circuit is formed by screen printing, a problem such as clogging of the plate occurs.

  In order to eliminate the agglomerates, for example, when forming a paste, it may be possible to add a process for crushing or removing the agglomerates separately. Will fall. Moreover, this does not necessarily eliminate the agglomerates. It is also difficult to redisperse the particles once agglomerated cleanly.

  Furthermore, in addition to the synthesized ultrafine metal particles, many unnecessary components such as synthesis solvents and reaction residues (in the above case, free fatty acids, etc.) remain in the synthesis solution.

  When this synthetic solution is dried and the ultrafine metal particles are collected, unnecessary components are also taken into the paste. In particular, the reaction residue causes the quality of the conductive paste to deteriorate, such as reducing the sinterability of the conductive paste and impairing the conductive properties.

  Therefore, the problem to be solved by the present invention is to provide a method for producing a conductive paste that is less likely to cause an agglomerate and can suppress the mixing of unnecessary components as compared with the conventional art.

  In order to solve the above problems, a step of heating a solution containing a metal salt and a first organic solvent, reducing the metal salt to produce ultrafine metal particles, and obtaining a synthetic solution containing the ultrafine metal particles; The step of removing the first organic solvent and the reaction residue from the synthesis solution to obtain undried metal ultrafine particles, the undried metal ultrafine particles, and the second organic solvent. And a step of mixing and pasting.

  Here, the heating is preferably performed by an external heat source or microwave irradiation.

Further, the metal salt is preferably a metal salt represented by the following chemical formula 1.
(Chemical formula 1)
(R-A) _n -M
(However, R is a hydrocarbon group, A is COO, OSO ₃ , SO ₃ or OPO ₃ , M is a metal, and n is a valence of M.)

  In addition, in order to obtain the undried metal ultrafine particles, the first organic solvent and the reaction residue are removed by substitution with a third organic solvent, and the third organic solvent is further removed. Good to be removed.

  On the other hand, the conductive paste according to the present invention is obtained by the above-described method for producing a conductive paste.

  In the method for producing a conductive paste according to the present invention, the organic solvent used during the synthesis and the reaction residue generated from the metal salt of the raw material are removed from the synthesis liquid containing the ultrafine metal particles, and the undried solid ultrafine metal is obtained. Get fine particles. Then, the undried metal ultrafine particles and an organic solvent for pasting are mixed and pasted to produce a conductive paste.

  Therefore, an agglomerate hardly occurs. In addition, a conductive paste with few unnecessary components such as organic solvents for synthesis and reaction residues can be produced. Therefore, the use of this conductive paste has advantages such as the formation of highly reliable microcircuits and interlayer bonding. Also, the viscosity stability of the conductive paste is improved.

  Here, in particular, when the solution is heated by microwave irradiation, the solution can be heated uniformly, so that the metal ultrafine particles can be synthesized in a relatively short time, and the productivity of the conductive paste is excellent.

  Moreover, when the said metal salt is a specific metal salt represented by the said Chemical formula 1, since it is excellent in the decomposability | decomposability in low-temperature baking, the electrically conductive paste which can aim at a lower resistance more easily is obtained.

  Further, the first organic solvent and the reaction residue are removed by substitution with a third organic solvent, and the third organic solvent is further removed to remove the undried metal ultrafine particles. When obtained, there is an advantage that the first organic solvent and the reaction residue are more easily removed.

  Hereinafter, a method for producing a conductive paste according to the present embodiment (hereinafter sometimes referred to as “the present production method”) will be described in detail.

  This production method has at least a synthetic solution preparation step for preparing a synthetic solution containing ultrafine metal particles, a recovery step for collecting undried metallic ultrafine particles, and a pasting step. Hereinafter, each process is demonstrated in order.

1. Synthetic liquid preparation process In this manufacturing method, the synthetic liquid preparation process heats the solution containing the metal salt and the first organic solvent, reduces the metal salt to produce ultrafine metal particles, and contains the ultrafine metal particles. It is the process of obtaining.

(solution)
Specific examples of the metal salt include, for example, general formula (RA) _n -M (where R is a hydrocarbon group, A is COO, OSO ₃ , SO ₃ or OPO ₃ , M is a metal, n Is the same as the valence that the metal M can take and is an integer of 1 or more), metal alkoxide (metal isopropoxide, metal ethoxide, etc.), metal acetylacetone complex (metal acetylacetonate, etc.) ) And the like. These may be contained alone or in combination of two or more.

Among the above metal salts, those represented by the general formula (R-A) _n -M can be preferably used. Since this metal salt is relatively inexpensive, there is an advantage that a conductive paste advantageous in cost can be obtained. In addition, since the organic component derived from the metal salt is easily decomposed at a relatively low temperature, there is an advantage that a conductive paste that can easily reduce resistance by low-temperature firing can be obtained.

In the general formula (RA) _n -M, the hydrocarbon group R may be a saturated hydrocarbon group such as an alkyl group or an unsaturated hydrocarbon group such as an alkenyl group. The molecular structure may be linear or branched. Further, a part of hydrogen in the hydrocarbon group may be substituted with another substituent such as a halogen element as long as it does not adversely affect the properties of the obtained conductive paste.

  The number of carbon atoms of the hydrocarbon group is not particularly limited, but as the number of carbon atoms becomes relatively small, the effects of the present production method are easily exhibited.

  That is, in this step, as will be described later, basically, ultrafine metal particles whose surface is modified with a fatty acid group or the like are generated. According to the conventional method, as the number of carbon atoms decreases, the steric hindrance decreases, and adjacent particles are likely to approach each other, and agglomerates are more likely to occur. According to this production method, even when the number of carbon atoms is relatively small, it becomes easy to obtain a conductive paste having almost no aggregate. Therefore, even if carbon number is comparatively small, the effect by this manufacturing method can be exhibited.

  On the other hand, when carbon number becomes comparatively large, the low-temperature sinterability of an electrically conductive paste falls and the tendency for conduction | electrical_connection reliability to fall easily is seen.

  Taking these into account, the lower limit of the carbon number of the hydrocarbon group is preferably 1 or more, more preferably 3 or more, and even more preferably 5 or more. The upper limit of the carbon number of the hydrocarbon group is preferably 17 or less, more preferably 15 or less, and even more preferably 11 or less.

In the general formula (R-A) _n -M, CO can be particularly preferably used as A.

In the general formula (RA) _n -M, M may basically be any kind of metal, and can be appropriately selected according to the use of the conductive paste. Specific examples of the metal M include, for example, silver, gold, white metal (platinum, palladium, iridium, rhodium, ruthenium, osmium), copper, nickel, zirconium, niobium, molybdenum, calcium, strontium, barium, indium, Examples include cobalt, zinc, cadmium, aluminum, gallium, iron, chromium, manganese, yttrium, and combinations of two or more thereof.

  Among these, as the metal M, silver, gold, white metal, copper, nickel, a combination of two or more of these, etc. are particularly preferable from the viewpoint of low resistance, safety, reducibility, etc. Can do.

  Specific examples of such metal salts include fatty acid metal salts and alkylsulfonic acid metal salts.

  The first organic solvent is an organic solvent for synthesis. As such an organic solvent, any organic solvent can be used as long as it can dissolve or disperse the metal salt.

  As the first organic solvent, a reducing organic solvent exhibiting reducibility with respect to the metal salt can be suitably used. Further, the reducing organic solvent preferably has a relatively low solubility in water.

  Specific examples of such a reducing organic solvent include monohydric alcohols having 3 or more carbon atoms such as propanol, butanol, pentanol, hexanol, heptanol, and octanol. In particular, a monohydric alcohol having 3 to 30 carbon atoms, preferably 3 to 20 carbon atoms, more preferably 3 to 10 carbon atoms, and most preferably 4 to 8 carbon atoms, and the like can be exemplified as suitable ones. These may be contained alone or in combination of two or more.

  This is because when the carbon number is within the above range, the metal salt is not easily reduced rapidly, and the metal salt can be easily reduced with an appropriate reducing power.

  Note that whether the metal salt is dissolved or dispersed in the first organic solvent depends on the combination of the selected metal salt and the first organic solvent, the amount of the metal salt relative to the first organic solvent, and the like. In selecting the first organic solvent, it is preferable to select a solvent that has a boiling point higher than the heating temperature and hardly volatilizes during heating in consideration of the heating temperature described later.

  In the solution, in addition to the metal salt and the first organic solvent, for example, one or two additives such as a catalyst and a reducing agent may be used within a range that does not adversely affect the formation of metal ultrafine particles. These may be added as appropriate.

(Heating the solution)
In this step, the metal salt is reduced by heating the solution. Here, the heating method is not particularly limited as long as it is given heat that can reduce the metal salt in the solution. Specific examples of the heating method include, for example, electric heating using a heater, heated oil, a heat medium such as water, a method of heating a solution by an external heat source such as a burner flame, hot air, etc., microwave, etc. Examples thereof include a method of heating a solution by irradiating an electromagnetic wave, a high frequency, a laser beam, an electron beam or the like. These heating methods may be used alone or in combination of two or more methods.

  At this time, the heating temperature of the solution varies depending on the type of metal salt used. In addition, the heating is preferably performed in a state where the solution is present in an inert gas atmosphere such as nitrogen, helium, or argon in order not to oxidize the generated ultrafine metal particles.

  Of the above heating methods, a method of heating the solution with an external heat source or a method of heating the solution by irradiating microwaves is preferably used. More preferably, the latter is used. This is because there is an advantage that the solution can be heated uniformly and the metal ultrafine particles can be synthesized in a short time.

  In order to heat the solution using these, specifically, for example, the following may be performed.

  In the former case, a heater in which the reaction vessel containing the solution is brought into contact with or in proximity to a heating medium such as a liquid (eg, oil, water, etc.) heated to a temperature capable of reducing the metal salt in the solution. The reaction vessel may be heated by a burner flame or the like.

  On the other hand, in the latter case, the microwave used is not particularly limited. Specifically, for example, a microwave with a frequency of 2.45 GHz, which is usually used frequently in Japan, may be used. Hereinafter, the microwave irradiation conditions are based on the assumption that a microwave with a frequency of 2.45 GHz is selected. However, when other microwaves are selected, the irradiation conditions are appropriately set according to this. Change it.

  In general, the irradiation intensity of microwaves varies depending on the metal salt in the solution, the type of the first organic solvent, and the like. When the irradiation intensity of the microwave is excessively decreased, a tendency such as a longer heating time is observed. On the other hand, when the irradiation intensity of the microwave is excessively increased, the heating time is extremely shortened, and it tends to be difficult to control the particle size distribution of the generated ultrafine metal particles. Therefore, these should be taken into account when selecting the microwave irradiation intensity.

The upper limit value of the microwave irradiation intensity is preferably 24 W / cm ³ or less, more preferably 18 W / cm ³ or less, and even more preferably 12 W / cm ³ or less.

On the other hand, the lower limit value of the microwave irradiation intensity is preferably 1 W / cm ³ or more, more preferably 2 W / cm ³ or more, and even more preferably 3 W / cm ³ or more. The irradiation intensity of these microwaves is a value represented by microwave output (W) / volume of reaction solution (cm ³ ).

  In any of the heating methods described above, the heating time generally varies depending on the metal salt in the solution, the type of the first organic solvent, the reaction temperature, and the like. When the heating time is excessively short, there is a tendency that metal ultrafine particles are not sufficiently generated. On the other hand, when the heating time is excessively long, productivity tends to decrease, and the purity of the metal ultrafine particles decreases due to the formation of by-products. Therefore, these should be noted in selecting the heating time.

  The upper limit of the heating time is preferably 2 hours or less, more preferably 1.5 hours or less, and even more preferably 1 hour or less.

  On the other hand, the lower limit of the heating time is preferably 30 seconds or longer, more preferably 1 minute or longer, and even more preferably 2 minutes or longer. However, in the case of microwave heating, the temperature raising time to the reaction temperature can be performed in a shorter time than external heating.

  In any of the heating methods described above, the temperature of the solution during the reaction (reaction temperature) is preferably controlled so as to be substantially constant.

  The upper limit of the reaction temperature is preferably 300 ° C. or lower, more preferably 275 ° C. or lower, and even more preferably 250 ° C. or lower. On the other hand, the lower limit of the reaction temperature is preferably 80 ° C. or higher, more preferably 100 ° C. or higher, and even more preferably 120 ° C. or higher.

  In the case of performing microwave heating, the reaction temperature is controlled by, for example, immersing a temperature sensor in the solution and repeating on / off of microwave irradiation so that the temperature of the solution becomes constant. be able to. Further, microwave irradiation may be performed using a known microwave irradiation apparatus.

(Ultrafine metal particles)
In this step, the metal ultrafine particles produced by reducing the metal salt in the solution by heating the solution basically have the following structure.

That is, the metal ultrafine particles include a metal core mainly composed of a metal component derived from the metal salt, and an organic component derived from the metal salt and covering the periphery of the metal core (hereinafter referred to as “coating organic component”). There is.) For example, when a metal salt represented by the general formula (RA) _n -M is used, the metal core is a metal M such as silver, and the covering organic component is an RA group such as a fatty acid group. is there.

  The metal core may be composed of one or more metal components derived from one or more metal salts. Moreover, the said covering organic component may be comprised from 1 type, or 2 or more types of organic components originating in 1 type or 2 or more types of metal salt.

  Among the ultrafine metal particles, the type of the metal core can be confirmed by, for example, an X-ray diffraction method. Moreover, about the kind of coating | coated organic component, it can confirm by NMR (nuclear magnetic resonance method), GC / MS (gas chromatography / mass spectrometry) etc., for example.

  The average particle diameter of the metal core can be controlled, for example, by appropriately selecting the heating temperature and the heating time. The upper limit value of the average particle diameter of the metal core is preferably 100 nm or less, more preferably 75 nm or less, and even more preferably 50 nm or less.

  On the other hand, the lower limit of the average particle diameter of the metal core is preferably 1 nm or more, more preferably 3 nm or more, and even more preferably 5 nm or more.

  In addition, the average particle diameter is obtained by arbitrarily extracting 100 ultrafine metal particles (although only a metal core can be observed with a TEM) from a transmission electron microscope (TEM) photograph of ultrafine metal particles, and measuring the particle diameter. This is the value of the particle size (D50) when the number of particles is 50% when counted in order from the smallest diameter.

2. Recovery Step This step is a step in which the first organic solvent and reaction residue are removed from the prepared synthetic solution to recover the undried metal ultrafine particles.

  Here, in the prepared synthetic solution, in addition to the generated ultrafine metal particles, the organic solvent used at the time of synthesis and the reaction residue derived from the metal salt as the raw material (derived from the metal salt, Residual organic components such as fatty acid groups remaining after the synthesis of the fine particles).

  It is preferable that these organic solvents and reaction residues do not substantially remain in the conductive paste from the viewpoint of the quality of the conductive paste and the stability of properties.

  In particular, when a large amount of reaction residue is contained in the paste, the sinterability of the conductive paste is lowered and the conduction performance is likely to be lowered.

  In this regard, when the synthetic solution is heated to dryness and the metal ultrafine particles are recovered as in the conventional production method, most of the unnecessary components such as reaction residues are recovered together with the metal ultrafine particles. On the other hand, in the recovery step of this production method, unnecessary components such as reaction residues are removed from the ultrafine metal particles, and the ultrafine metal particles are recovered.

  In this step, it is sufficient that unnecessary components such as reaction residues can be substantially removed from the ultrafine metal particles. This is because complete removal is difficult.

  Further, in this step, the ultrafine metal particles are recovered in an undried state. This is to make it difficult for agglomerates to occur during pasting in the pasting step and improve the dispersibility of the ultrafine metal particles.

  Specifically, the recovery method includes, for example, a method in which the synthetic solution is allowed to stand or centrifuged to precipitate the ultrafine metal particles in the synthetic solution, and then the supernatant is removed. It can be illustrated.

  At this time, a third organic solvent of a type different from the first organic solvent may be added to the precipitate and stirred again to reprecipitate the ultrafine metal particles, and then the supernatant may be removed. This operation may be repeated a plurality of times.

  As described above, the first organic solvent and the reaction residue are removed by substitution with the third organic solvent. Further, by removing the third organic solvent, the undried state of the metal When the fine particles are collected, the first organic solvent and the reaction residue are more easily removed, which is preferable.

  In this case, the third organic solvent mainly functions as an organic solvent for cleaning. As the third organic solvent, a solvent having a boiling point lower than that of the second organic solvent used in the pasting step described later may be used. This is because if a vacuum concentrator such as an evaporator is used in the pasting step, the third organic solvent that could not be removed in the recovery step can be removed relatively easily using the difference in boiling points. . Further, the third organic solvent is preferably a poor solvent having low compatibility with the ultrafine metal particles.

  Specific examples of the third organic solvent include alcohols such as methanol, ethanol, propanol and 2-propanol, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran (THF) and diethyl ether, Examples thereof include hydrocarbons such as hexane, pentane and heptane. These may be used alone or in combination.

3. Pasting Step In this step, the undried metal ultrafine particles collected in the collecting step and the second organic solvent are mixed and pasted to obtain a conductive paste.

  Here, the second organic solvent is an organic solvent for pasting. It is not particularly limited as long as it can easily disperse the undried metal ultrafine particles and can maintain the paste state. It can be selected in consideration of the use of the obtained conductive paste. Specifically, an organic solvent having a boiling point higher than the use temperature can be suitably selected from the viewpoint that it is difficult to volatilize easily at the use temperature of the conductive paste and the viscosity of the paste is easily maintained stably.

  Specific examples of the second organic solvent include terpineol, dihydroterpineol, decanol, hexanol, methanol, ethanol, ethyl carbitol, butyl carbitol, diols, glycols, polyols and other alcohols, dimethyl Amines such as formamide (DMF) and N-methyl-2-pyrrolidone (NMP), hydrocarbons such as hexane, toluene, xylene, octane, decane, undecane and tetradecane, ketones such as methyl ethyl ketone (MEK) and acetone, Examples include ethers such as tetrahydrofuran (THF) and dipropylene glycol monomethyl ether, and esters such as ethyl acetate, ethyl carbitol acetate, and butyl carbitol acetate. These may be used alone or in combination.

  The second organic solvent may be a mixed organic solvent in which two or more organic solvents having different boiling points are combined. In this case, when the conductive paste coating film is dried, the organic solvent having a relatively low boiling point volatilizes first, but the organic solvent having a relatively high boiling point remains. For this reason, the stress at the time of volume shrinkage due to the volatilization of the organic solvent is relieved, and there are advantages such as easy generation of cracks.

  The means for mixing and pasting the metal ultrafine particles is not particularly limited. Specific examples of the above-mentioned means include, for example, ultrasonic treatment, bead mill, dispersion processing in a supercritical state, propeller type, grinding type such as a mortar, rotary type (rotation by rotation / revolution), vibration type, etc. Various means such as three rolls can be exemplified. These may be used alone or in combination of two or more.

  In addition, when a third organic solvent is used in the recovery step, the third organic solvent is substantially removed in this step using, for example, an evaporator or an apparatus having a function equivalent to this. For example, the conductive paste may be subjected to vacuum concentration treatment. Moreover, you may perform a defoaming process etc. as needed using various defoaming apparatuses.

  In addition, the content of the metal component in the conductive paste is not particularly limited, and can be appropriately adjusted in consideration of the application.

  The upper limit of the content of the metal component is preferably 95% by weight or less, and more preferably 90% by weight or less. On the other hand, the lower limit of the content of the metal component is preferably 5% by weight or more, more preferably 10% by weight or more, and still more preferably 30% by weight or more.

  In addition, you may add 1 type, or 2 or more types of various additives, such as a dispersing agent which improves the dispersion stability of a metal ultrafine particle, at the time of the said mixing.

  Hereinafter, the present invention will be described in detail using examples.

1. Production of conductive paste according to examples and comparative examples (Example 1)
<Preparation of synthetic solution containing ultrafine silver particles>
Hexane silver _{(C 5 H 11 COOAg) 5mmol} , were mixed in 1-hexanol 30 ml, then subjected to ultrasonic treatment, to prepare a dispersion solution.

Next, using a microwave irradiation apparatus (manufactured by Micro Electronics Co., Ltd., 2,450 MHz microwave heating apparatus “MMG-213VP”), a microwave (frequency 2.) is applied at an irradiation intensity of 6 W / cm ^{3 in} a nitrogen atmosphere. (45 GHz) was irradiated to the dispersion solution and heated for 11 minutes while controlling the temperature of the solution at 157 ° C. (reaction temperature) to obtain a synthetic solution.

  The reaction temperature was controlled by immersing a temperature sensor in the solution and repeating on / off of microwave irradiation so that the temperature of the solution became constant.

  Here, in order to identify the product produced | generated in the synthetic | combination liquid, the following confirmation tests were done. In addition, the following confirmation tests are only for specifying a product, and do not constitute a part of the production method according to the present invention.

  First, a sample obtained by dispersing the above synthetic solution in hexane and dropping and drying it on a carbon mesh was obtained with a transmission electron microscope (manufactured by Hitachi High-Technologies Corporation, “Hitachi Transmission Electron Microscope H-9000”). Observed. As a result, nano-sized ultrafine particles were generated.

  Next, the synthesis solution was centrifuged, the separated product was washed with methanol, and X-ray diffraction was performed using the collected ultrafine particles. As a result, silver was produced. Therefore, it was found that the metal constituting the ultrafine particle core was silver.

  Next, GC / MS (gas chromatography / mass spectrometry) analysis was performed on the collected silver ultrafine particles. As a result, hexanoic acid groups were detected. Therefore, it was found that the periphery of the silver core was covered with hexanoic acid groups.

  From the results of the above confirmation test, it was confirmed that the above-mentioned synthetic solution contains silver ultrafine particles (silver core average particle diameter: 35 nm) whose surface is modified with hexanoic acid groups. .

<Undried collection of ultra-fine silver particles>
Next, the synthetic solution was allowed to stand for 6 hours to precipitate silver ultrafine particles, and then the supernatant was removed.

  Furthermore, 30 ml of methanol as an organic solvent for washing was added to the precipitate after the supernatant was removed, and the mixture was stirred and allowed to stand for 1 hour to precipitate ultrafine silver particles, and then the supernatant was removed. Thereafter, the same operation was repeated twice.

  As a result, hexanol used for the synthesis and hexanoic acid groups as the reaction residue were substantially removed to obtain an undried silver ultrafine particle.

<Paste>
Next, 9.00 g of the undried silver ultrafine particles, 5.10 g of terpineol as an organic solvent for pasting, 0.54 g of a dispersant (“Octylamine” manufactured by Wako Pure Chemical Industries, Ltd.), Was placed in an eggplant-shaped flask of a rotary evaporator. In addition, the water temperature of the water bath of the said evaporator was set to 30 to 40 degreeC.

  Next, methanol attached to the undried silver ultrafine particles was removed by vacuum concentration using the evaporator.

  Next, the obtained mixture (silver concentration is 60 wt%) was sonicated for 30 minutes using an ultrasonic cleaner (40 ° C.) in a glass tube to form a paste. As a result, a conductive paste according to Example 1 was obtained.

(Example 2)
When the conductive paste according to Example 1 was prepared, the same procedure was performed except that silver laurate (C ₁₁ H ₂₃ COOAg) was used instead of silver hexanoate (C ₅ H ₁₁ COOAg). A conductive paste according to Example 2 was obtained.

(Example 3)
When the conductive paste according to Example 1 was prepared, the same procedure was carried out except that silver oleate (C ₁₇ H ₃₃ COOAg) was used instead of silver hexanoate (C ₅ H ₁₁ COOAg). A conductive paste according to Example 3 was obtained.

Example 4
When the conductive paste according to Example 1 was prepared, the same procedure was performed except that silver propanoate (C ₂ H ₅ COOAg) was used instead of silver hexanoate (C ₅ H ₁₁ COOAg). A conductive paste according to Example 4 was obtained.

(Comparative Example 1)
A synthetic solution was obtained in the same manner as in the preparation of the conductive paste according to Example 1. Next, methanol was added to this synthesis solution, and centrifugation was performed at 3000 rpm for 10 minutes to precipitate silver ultrafine particles, and then the supernatant was removed. This operation was repeated twice.

  Subsequently, the obtained precipitate was dried under reduced pressure (at room temperature for 180 minutes) to obtain dry silver ultrafine particles.

  Next, 9.00 g of the dried silver ultrafine particles, 5.10 g of terpineol as an organic solvent for pasting, and 0.54 g of a dispersant were mixed while grinding on a mortar for 1 hour, and then the mixture. (Silver concentration is 60 wt%) was subjected to ultrasonic treatment under the same conditions as in Example 1 to form a paste. This obtained the electrically conductive paste which concerns on the comparative example 1.

(Comparative Example 2)
A synthetic solution was obtained in the same manner as in the preparation of the conductive paste according to Example 1. Next, methanol was added to this synthesis solution, and centrifugation was performed at 3000 rpm for 10 minutes to precipitate silver ultrafine particles, and then the supernatant was removed. This operation was repeated twice.

  Subsequently, the obtained precipitate was dried under reduced pressure (at room temperature for 180 minutes) to obtain dry silver ultrafine particles.

  Next, 9.00 g of the dried silver ultrafine particles, 5.10 g of terpineol as an organic solvent for pasting, 0.54 g of the dispersant, and 10.4 g of methanol as an organic solvent for dilution. Then, ultrasonic treatment was performed under the same conditions as in Example 1 to obtain a paste. Dilution with methanol is not possible because the dispersion effect by ultrasonic waves cannot be obtained when the concentration of the ultrafine silver particles is high, so that the above effect can be obtained by adding methanol to lower the viscosity. is there. Here, the dilution concentration with methanol is 35% by weight.

  Next, this paste (silver concentration 60 wt%) was placed in the eggplant-shaped flask of the rotary evaporator.

  Subsequently, the methanol contained in the paste was removed by concentration under reduced pressure using the evaporator.

  As a result, a conductive paste according to Comparative Example 2 was obtained.

2. Evaluation of each conductive paste Each conductive paste was applied onto a slide glass by a bar coating method (application thickness 50 μm, application dimension 76 mm × 20 mm). Thereafter, each coating film was dried at 100 ° C. for 60 minutes to volatilize terpineol, and then baked at a predetermined temperature in an air atmosphere for a predetermined time to obtain each baked film.

  In addition, the firing temperature and time of each conductive paste are 200 ° C. and 30 minutes.

  Next, using an optical microscope (450 times magnification), the surface of the obtained fired film was observed to confirm the presence or absence of aggregates.

  Here, each conductive paste was evaluated by counting the number of agglomerates of 10 μm or more contained in one arbitrarily selected visual field (543 μm × 677 μm) and comparing the numbers mainly.

  Further, after measuring the surface resistivity of each fired film using a low resistance measuring device (4-terminal 4-probe method, manufactured by Dia Instruments Co., Ltd., “Loresta GP MCP-T610 type”), a stylus type surface The film thickness difference was determined by measuring the film thickness step with a shape measuring instrument ("Dektak" manufactured by ULVAC, Inc.).

The volume resistivity was calculated from the obtained surface resistivity and film thickness. As a result, the volume resistivity of the fired film of Example 1 was 3.0 × 10 ⁻⁶ Ω · cm, and the volume resistivity of the fired film of Example 2 was 3.0 × 10 ⁻⁶ Ω · cm. The volume resistivity of the fired film of Example 3 is 3.0 × 10 ⁻⁶ Ω · cm, and the volume resistivity of the fired film of Example 4 is 3.0 × 10 ⁻⁶ Ω · cm. The volume resistivity of the film was 8.0 × 10 ⁻⁶ Ω · cm, and the volume resistivity of the fired film of Comparative Example 2 was 5.0 × 10 ⁻⁶ Ω · cm.

  The measurement of the surface resistivity and the calculation of the volume resistivity were performed in accordance with JIS K7194 “Resistivity test method for conductive plastics by 4-probe method”. However, the area is 20 mm × 76 mm, and the measuring devices are (X, Y) = (15 mm, 5 mm), (15 mm, 15 mm), (38 mm, 10 mm), (60 mm, 5 mm), (60 mm, 15 mm). After calculating the correction coefficient at 5 points, the surface resistivity was measured and the volume resistivity was calculated.

  1 to 3 show optical micrographs (450 magnifications) of fired films produced from the conductive pastes according to Examples 1 to 3. FIG. In FIG. 4, the optical microscope (450 times magnification) photograph of the baking film produced from the electrically conductive paste which concerns on the comparative example 1 is shown. In FIG. 5, the optical microscope (450 times magnification) photograph of the baking film produced from the electrically conductive paste which concerns on the comparative example 2 is shown.

  As can be seen from FIG. 4, in the fired film produced from the electric paste according to Comparative Example 1, a large number of aggregates of 50 μm or less were confirmed. This is presumably because the silver ultrafine particles could not be sufficiently pulverized at the time of making a paste because the paste was prepared using dry silver ultrafine particles.

  Next, in the manufacturing method of Comparative Example 2, the viscosity is lowered using a dilution solvent to make a paste. Therefore, compared with the manufacturing method of the comparative example 1, it seems that the grindability of the silver ultrafine particle at the time of ultrasonic treatment becomes good. However, as can be seen from FIG. 5, about 4 aggregates of 10 μm or more were still confirmed. Aggregates of less than 10 μm were also confirmed.

  From this, it can be seen that although the production method of Comparative Example 2 can reduce large agglomerates as compared with the production method of Comparative Example 1, it is not a drastic countermeasure against agglomerates.

  On the other hand, as can be seen from FIGS. 1 to 3 and the like, according to the conductive paste produced by the manufacturing method according to Examples 1 to 4, it can be seen that aggregates of 10 μm or more are not seen at all. This is presumably because the silver ultrafine particles were collected in an undried state and made into a paste, and the dispersibility of the silver ultrafine particles at the time of pasting was extremely high.

  Moreover, in the manufacturing method which concerns on Examples 1-4, the hexanoic acid group of the reaction residue and the hexanol used at the time of a synthesis | combination are removed at the time of collection | recovery of silver ultrafine particles. Therefore, it can be said that mixing into the paste can also be suppressed.

  Although the embodiments and examples have been described above, the present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the spirit of the present invention.

2 is an optical micrograph of a fired film produced from the conductive paste according to Example 1. 4 is an optical micrograph of a fired film produced from a conductive paste according to Example 2. 4 is an optical micrograph of a fired film produced from a conductive paste according to Example 3. 4 is an optical micrograph of a fired film produced from a conductive paste according to Comparative Example 1. 5 is an optical micrograph of a fired film produced from a conductive paste according to Comparative Example 2.

Claims (5)

Heating a solution containing a metal salt and a first organic solvent, reducing the metal salt to produce ultrafine metal particles, and obtaining a synthetic liquid containing the ultrafine metal particles;
Removing the first organic solvent and reaction residue from the synthesis solution to obtain undried metal ultrafine particles;
A method for producing a conductive paste, comprising: mixing the undried metal ultrafine particles with a second organic solvent to form a paste.   The method for producing a conductive paste according to claim 1, wherein the heating is performed by an external heat source or microwave irradiation. The method for producing a conductive paste according to claim 1 or 2, wherein the metal salt is represented by the following chemical formula (1).
(Chemical formula 1)
(R-A) _n -M
(However, R is a hydrocarbon group, A is COO, OSO ₃ , SO ₃ or OPO ₃ , M is a metal, and n is a valence of M.)   The first organic solvent and the reaction residue are removed by substitution with a third organic solvent, and the third organic solvent is removed to obtain the undried metal ultrafine particles. The method for producing a conductive paste according to any one of claims 1 to 3, wherein:   The electrically conductive paste obtained by the manufacturing method of the electrically conductive paste in any one of Claim 1 to 4.