JP2007095525A - Conductive paste - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide conductive paste capable of suppressing occurrence of a crack due to increase of film thickness as compared with a conventional one while reducing resistance. <P>SOLUTION: This conductive paste is prepared by dispersing metal ultrafine particles in a mixture organic solvent prepared by mixing two or more kinds of organic solvents each having a distinct boiling point with one another. In this case, the mixture organic solvent is preferably prepared by mixing a low-boiling-point-side organic solvent evaporating in drying the paste with a high-boiling-point-side organic solvent evaporating in sintering the paste. It is preferable that the content of the low-boiling-point-side organic solvent is in the range of 50-95 wt.% and the content of the high-boiling-point-side organic solvent is in the range of 5-50 wt.%. Each metal ultrafine particle preferably has a metal core and an organic constituent both of which are derived from a metal salt represented by (R-A)<SB>n</SB>-M (wherein R is a hydrocarbon group; A is COO, OSO<SB>3</SB>, SO<SB>3</SB>or OPO<SB>3</SB>; M is a metal; and n is the valence of M). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a conductive paste, and more particularly to a conductive paste using ultrafine metal particles.

  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, various conductive pastes using ultrafine metal particles of nm size (hereinafter sometimes referred to as “nanosize”) have been proposed. Nano-sized ultrafine metal particles generally have properties different from bulk, such as sintering at a temperature lower than the original melting point of the metal. For this reason, this type of conductive paste is expected to reduce organic components contained in the sintered body by, for example, low-temperature firing, and to exhibit good conductive performance.

  For example, in Patent Document 1, the surface is coated with a protective agent, metal ultrafine particles having an average particle diameter of 1 to 100 nm, metal fillers having an average particle diameter of 0.5 to 20 μm, a binder resin, A conductive paste is disclosed in which a compound that reacts with a functional group of a protective agent during heating is dispersed in an organic solvent.

WO2002 / 035554

  However, conductive pastes in which ultrafine metal particles are dispersed in an organic solvent generally have the following problems. That is, when a film thickness of several microns or more is formed using a conventional conductive paste, for example, the volume shrinkage becomes severe due to the volatilization of the organic solvent during the drying and baking of the film, and the stress generated in the film surface direction. For example, there is a problem that cracks are likely to occur in the fired film.

  For such a problem, the conductive paste of Patent Document 1 uses a nano-sized metal ultrafine particle and a micron-sized metal filler in combination, and further adds a binder resin to increase the film thickness. It is decided to suppress the generation of cracks.

  However, this conductive paste contains a binder resin as an essential component. Therefore, there has been a problem that the volume resistance of the sintered film tends to increase and it is difficult to reduce the resistance by low-temperature firing.

  Therefore, the problem to be solved by the present invention is to provide a conductive paste capable of suppressing the occurrence of cracks due to thickening of the film as compared with the prior art while reducing resistance.

  In order to solve the above-mentioned problems, the conductive paste according to the present invention is characterized in that ultrafine metal particles are dispersed in a mixed organic solvent in which two or more organic solvents having different boiling points are mixed.

  At this time, the mixed organic solvent is preferably a mixture of a low-boiling organic solvent that volatilizes mainly when the paste is dried and a high-boiling organic solvent that volatilizes mainly when the paste is fired.

  The difference between the boiling point of the low-boiling side organic solvent and the boiling point of the high-boiling side organic solvent is preferably 50 ° C. or more.

  The mixed organic solvent preferably has a content of the low boiling point organic solvent of 50 to 95% by weight and a content of the high boiling point organic solvent of 50 to 5% by weight.

Further, the ultrafine metal particles have a metal core composed of a metal component derived from the metal salt represented by the following chemical formula 1, and an organic component derived from the metal salt and covering the periphery of the metal core. Good to be.
(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 ultrafine metal particles may be synthesized by heating a solution obtained by dissolving or dispersing the metal salt in a synthesis organic solvent.

  Further, the heating of the solution may be performed by an external heat source or microwave irradiation.

  The conductive paste according to the present invention is obtained by dispersing ultrafine metal particles in a mixed organic solvent in which two or more organic solvents having different boiling points are mixed.

  Therefore, since it does not contain a binder resin, it is easy to reduce the resistance by low-temperature firing as compared with the conventional conductive paste.

  In addition, even when the fired film is formed thick, cracks can be suppressed as compared with the conventional case.

  This is because when the 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, so that even if the volume shrinkage due to the volatilization of the organic solvent occurs. It is presumed that the stress during volume shrinkage is relieved.

  On the other hand, when the film is baked, the organic solvent having a relatively high boiling point volatilizes, but the amount of volatilization decreases compared to the case where the organic solvent volatilizes at one time. This is presumed to be because the stress is reduced and the generated stress is also reduced.

  Moreover, according to the said conductive paste, when obtaining a comparatively thick fired film, it is not necessary to laminate | stack a thin film many times. Therefore, the conductive paste is excellent in the formability of fine circuits.

  At this time, when the mixed organic solvent is a mixture of a low boiling point organic solvent that volatilizes mainly when the paste is dried and a high boiling point organic solvent that volatilizes mainly when the paste is baked, the low boiling point organic solvent is dried when the paste is dried. Volatilizes first, and the organic solvent on the high boiling point side volatilizes during subsequent baking of the paste. Therefore, the above phenomenon is likely to occur, and there is an advantage that the occurrence of cracks can be more effectively suppressed.

  When the difference between the boiling point of the low boiling point organic solvent and the boiling point of the high boiling point organic solvent is 50 ° C. or more, the content of the low boiling point organic solvent is 50 to 95% by weight, and the high boiling point organic solvent is When the content of the solvent is in the range of 50 to 5% by weight, the above effects are excellent.

  Further, when the metal ultrafine particles have a metal core composed of a metal component derived from the specific metal salt and an organic component derived from the metal salt and covering the periphery of the metal core. Since it is excellent in decomposability by low-temperature firing, it is easy to reduce resistance at a lower temperature.

  Hereinafter, the conductive paste according to the present embodiment (hereinafter, also referred to as “main paste”) will be described in detail.

1. This paste consists of metal ultrafine particles dispersed in a mixed organic solvent. Hereinafter, each structure of this paste is demonstrated in order.

1.1 Ultrafine metal particles In this paste, as the ultrafine metal particles, those having a metal core and an organic component covering the periphery of the metal core can be suitably used.

  In this case, the metal core may be mainly composed of one kind of metal component, or may be mainly composed of two or more kinds of metal components. Moreover, the said organic component may consist of 1 type of organic components, and may consist of 2 or more types of organic components.

  In particular, the metal ultrafine particles preferably have a metal core mainly composed of a metal component derived from a metal salt and an organic component derived from the metal salt and covering the periphery of the metal core.

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.

Among the above metal salts, those represented by the general formula (R-A) _n -M can be preferably used. This is because this metal salt is relatively inexpensive and advantageous in terms of cost. Moreover, since the organic component derived from this metal salt is easily decomposed at a relatively low temperature, there is an advantage that resistance can be easily reduced by low-temperature firing. In addition, when this metal salt is used, it is estimated that the said organic component is mainly RA group.

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 conductive paste.

  Moreover, the carbon number of the said hydrocarbon group is not specifically limited. However, when the carbon number is relatively large, there is a tendency that the low-temperature sinterability of the conductive paste decreases. On the other hand, when the carbon number is relatively small, there is a tendency that the dispersion stability is lowered. Therefore, the carbon number of the hydrocarbon group is preferably selected in consideration of these.

  Generally, as a preferable upper limit of the number of carbon atoms of the hydrocarbon group, specifically, for example, 40, 35, 30, 25, 20, 18 and the like can be exemplified. On the other hand, specific examples of preferable lower limit values that can be combined with these upper limit values include 1, 2, 3, 4, and the like.

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 type 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.

  Among the ultrafine metal particles, the type of the metal core can be confirmed by, for example, an X-ray diffraction method. The type of the organic component can be confirmed by, for example, NMR (nuclear magnetic resonance method), GC / MS (gas chromatography / mass spectrometry) or the like.

  Moreover, the average particle diameter of the metal core can be appropriately adjusted according to the use of the conductive paste. Specifically, for example, 500 nm, 400 nm, 300 nm, 200 nm, 100 nm, 50 nm and the like can be exemplified as the preferable upper limit value. On the other hand, specific examples of preferable lower limit values that can be combined with these preferable upper limit values include 1 nm, 2 nm, and 3 nm.

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

  Further, the particle size distribution of the ultrafine metal particles is not particularly limited, but is preferably relatively sharp. Specific examples of the sharpness ε = (D90−D10) / D50 of the particle size distribution include, for example, 2, 1.5, 1.3 and the like as preferable upper limit values. On the other hand, the lower limit value of ε is not particularly illustrated because ε is preferably as close to 0 as possible.

  D90 and D10 are values calculated in the same manner as D50, and are the particle size at which the number of particles is 90% and the particle size at which the number of particles is 10%.

  Further, the content of the organic component in the metal ultrafine particles is not particularly limited. Generally, when the content of the organic component is excessively increased, the low temperature sintering property tends to decrease. On the other hand, when the content of the organic component is excessively reduced, aggregation tends to occur, and the dispersion stability in the conductive paste tends to decrease. Therefore, it is good to pay attention to these when selecting the content of the organic component.

  Specific examples of the content of the organic component include 50, 40, 35, and 30% by weight as preferable upper limit values. On the other hand, examples of preferable lower limit values that can be combined with these preferable upper limit values include 0.5 and 1% by weight.

  The content of the organic component is determined by performing thermogravimetric analysis on dried metal ultrafine particles in accordance with JIS K0129 “General Rules for Thermal Analysis” and JIS K7120 “Method for Thermogravimetric Measurement of Plastics”, and from room temperature to 600 ° C. The content of the organic component may be calculated from the weight loss rate.

  In addition, the content of the metal component in the paste is not particularly limited, and can be appropriately adjusted in consideration of the application. Generally, as a preferable upper limit, for example, 90% by weight, 85% by weight, 80% by weight and the like can be exemplified. On the other hand, examples of preferable lower limit values that can be combined with these preferable upper limit values include 1% by weight, 2% by weight, and 5% by weight.

  The ultrafine metal particles can be synthesized using, for example, a solution reduction method. A suitable method for synthesizing the ultrafine metal particles will be described later in the section of the preferred method for producing the paste.

1.2 Mixed organic solvent In this paste, a mixed organic solvent is used. Here, the mixed organic solvent is a mixture of two or more organic solvents having different boiling points.

  Specific examples of the organic solvent constituting the mixed organic solvent include hexane, toluene, xylene, octane, decane, undecane, tetradecane, terpineol, decanol, methyl ethyl ketone (MEK), acetone, and tetrahydrofuran (THF). , Hexanol, dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dipropylene glycol monomethyl ether, methanol, ethanol, ethyl acetate, ethyl carbitol, ethyl carbitol acetate, butyl carbitol, butyl carbitol acetate Etc. can be illustrated.

  At this time, the mixed organic solvent is preferably a mixture of a low-boiling organic solvent that volatilizes mainly when the paste is dried and a high-boiling organic solvent that volatilizes mainly when the paste is fired.

  Each of the low boiling point side organic solvent and the high boiling point side organic solvent may be composed of one kind or two or more kinds of organic solvents. Further, a part of the high-boiling side organic solvent may be volatilized during the drying of the paste, while a part of the low-boiling side organic solvent remaining during the baking of the paste may be volatilized.

  In this paste, the boiling range of each of the low-boiling-side organic solvent and the high-boiling-side organic solvent is basically adjusted as appropriate in consideration of the type of ultrafine metal particles and the drying / firing temperature of the paste. Can do.

  Specific examples of the boiling point of the low-boiling side organic solvent include a range of 50 ° C. or higher and lower than 150 ° C. On the other hand, specific examples of the boiling point of the high-boiling side organic solvent include a range of 150 ° C. or higher and 250 ° C. or lower.

  Moreover, the low boiling point side organic solvent and the high boiling point side organic solvent preferably have a boiling point difference of 50 ° C. or more. This is because if the boiling points of both are too close, rapid volume shrinkage of the paste is likely to occur at a certain temperature.

  In selecting an organic solvent constituting the mixed organic solvent, in addition to the boiling point, the similarity of polarities between the organic solvents may be considered from the viewpoint of obtaining a uniform mixture.

  In the present paste, the mixed organic solvent may have a content of the low-boiling side organic solvent in the range of 50 to 95% by weight and a content of the high-boiling side organic solvent in the range of 50 to 5% by weight. This is because if the content of the low-boiling side organic solvent and the high-boiling side organic solvent is within the above range, the paste is excellent in volume shrinkage balance at the time of drying and baking of the paste, and thus cracking is more easily suppressed. .

1.3 Others In addition to the above, the paste includes, for example, one or more various additives such as a dispersant that improves the dispersion stability of the ultrafine metal particles, as long as it does not depart from the spirit of the present invention. Two or more kinds may be added. Moreover, the reducing agent utilized at the time of the synthesis | combination of a metal ultrafine particle, an inevitable impurity, etc. may be contained.

  The manufacturing method of this paste demonstrated above is not specifically limited. As a preferred method for producing this paste, specifically, for example, a metal ultrafine particle is produced from a metal salt using a solution reduction method, and the obtained metal ultrafine particle is dispersed in the above mixed organic solvent to obtain a paste. The method etc. which make a state can be illustrated. Hereinafter, this will be referred to as “the present manufacturing method” and the content thereof will be described in detail.

2. Production Method 2.1 Solution In this production method, ultrafine metal particles are basically synthesized using a solution obtained by dissolving or dispersing a metal salt in an organic solvent for synthesis.

  The metal salt is the same as that exemplified in the section 1.1 Ultrafine metal particles, and detailed description thereof is omitted.

  As the organic solvent for synthesis, any kind of organic solvent can be used as long as it can dissolve or disperse the metal salt. Specific examples include alcohols such as diols, glycols, and polyols, amines, hydrocarbons, ketones, ethers, esters, and the like. The above may be mixed.

  Of these organic solvents for synthesis, it is preferable to use a reducing organic solvent that exhibits reducing properties with respect to the metal salt. 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.

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

  Whether the metal salt is dissolved or dispersed in the synthesis organic solvent depends on the combination of the selected metal salt and the synthesis organic solvent, the amount of the metal salt relative to the synthesis organic solvent, and the like. Further, the amount of the metal salt may be adjusted as appropriate in consideration of the amount of ultrafine metal particles to be contained in the conductive paste.

  In addition, for example, one or two or more additives such as a catalyst and a reducing agent may be appropriately added to the above solution within a range that does not adversely affect the formation of ultrafine metal particles.

2.2 Heating of the solution In this production method, 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, the reaction vessel containing the solution is brought into contact with or close to a heat medium such as a liquid (for example, oil, water, etc.) heated to a temperature capable of reducing the metal salt in the solution, The reaction vessel may be heated by a heater or a burner flame.

  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.

  The irradiation intensity of microwaves generally varies depending on the metal salt in the solution, the type of organic solvent for synthesis, and the like. When the irradiation intensity of the microwave is excessively reduced, there is a tendency that the heating time becomes longer. 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.

Usually, the irradiation intensity of the microwave, specifically, for example, as a preferable upper limit ^value, and the like can be exemplified ^{24W / cm 3, 18W / cm} 3, 12W / cm 3.

On the other hand, specific examples of preferable lower limit values that can be combined with these preferable upper limit values include 1 W / cm ³ , 2 W / cm ³ , 3 W / cm ³ , 4 W / cm ^{3, and the} like. 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 organic solvent for synthesis, 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, there is a tendency that productivity is lowered or purity of the metal ultrafine particles is lowered due to generation of a side reaction product. Therefore, these should be noted in selecting the heating time.

  Usually, as a heating time, specifically, as its preferable upper limit, for example, 2 hours or less, 1.5 hours or less, 1 hour or less can be exemplified.

  On the other hand, specific examples of preferable lower limit values that can be combined with these preferable upper limit values include 1 minute or more, 2 minutes or more, 3 minutes or more, 5 minutes or more, and the like. 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 reaction temperature is preferably set to a temperature near the boiling point of the organic solvent for synthesis to be used.

  Specific examples of the reaction temperature include 300 ° C., 275 ° C., and 250 ° C. as preferred upper limits. On the other hand, specific examples of preferable lower limit values that can be combined with these preferable upper limit values include 80 ° C., 100 ° C., 120 ° C., and the like.

  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.

2.3 Recovery of metal fine particles In this production method, the solution is heated to synthesize metal ultrafine particles, and then the generated metal ultrafine particles are recovered from the synthesis solution. A general method may be used as the recovery method. Specifically, for example, methods such as centrifugation, filtration, and solvent extraction can be exemplified, and these may be used alone or in combination of two or more. These may be performed once or a plurality of times.

2.4 Pasting In this production method, the obtained ultrafine metal particles are dispersed in a mixed organic solvent, thereby obtaining a conductive paste.

  About the said mixed organic solvent, since it is the same as that of what was illustrated by the said 1.2 term, detailed description is abbreviate | omitted.

  Further, in this production method, the mixing of the metal ultrafine particles and the mixed organic solvent may be performed by mixing the prepared mixed organic solvent and the metal ultrafine particles, or individual organic solvents constituting the mixed organic solvent. It is also possible to carry out by mixing the metal ultrafine particles. Further, the dispersion of the metal ultrafine particles may be performed after mixing the metal ultrafine particles and the mixed organic solvent, or may be performed during the mixing of the metal ultrafine particles and the mixed organic solvent.

  That is, the order of the mixing and dispersion treatment is not particularly limited as long as the ultra-fine metal particles contained in the mixed organic solvent are finally dispersed in a paste state.

  Moreover, the said mixing means is not specifically limited. Specific examples of the mixing means include a propeller type, a crushing type such as a mortar, a rotating type (rotation by rotation / revolution), a vibration type, etc., a three-roller, a bead mill, and the like. .

  On the other hand, as the dispersing means, any kind of dispersing means may be used as long as the ultrafine metal particles can be uniformly dispersed. Specific examples of the dispersing means include ultrasonic treatment, bead mill, and dispersion treatment in a supercritical state.

  In addition, the content of the metal component in the obtained conductive paste is not particularly limited, and can be appropriately adjusted in consideration of the application. In general, examples of preferable upper limit values include 90% by weight, 85% by weight, and 80% by weight. On the other hand, examples of preferable lower limit values that can be combined with these preferable upper limit values include 1% by weight, 2% by weight, and 5% by weight.

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

(Conductive paste according to Example 1)
5 mmol of lauric acid silver salt (C ₁₁ H ₂₃ COOAg) was mixed in 25 ml of 1-hexanol as an organic solvent for synthesis, and 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) with 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). 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.

  Next, a sample obtained by dispersing the synthetic solution after these operations in hexane and dripping and drying it on a carbon mesh was used as a transmission electron microscope (manufactured by Hitachi High-Technologies Corporation, “Hitachi Transmission Electron Microscope H- 9000 "). As a result, it was confirmed that nano-sized ultrafine particles were generated.

  Next, methanol was added to the synthetic solution after the above operation, ultrafine particles were precipitated by centrifugation, and the supernatant was removed. Subsequently, methanol was added again, and precipitation of ultrafine particles and removal of the supernatant were repeated. Next, the collected precipitate was vacuum-dried to dry-collect ultrafine particles.

  Next, X-ray diffraction was performed using the collected ultrafine particles. According to the result, it was confirmed that only silver was produced. Therefore, it was confirmed 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, lauric acid was detected. Therefore, it was confirmed that the kind of the organic component covering the periphery of the silver core is a lauric acid group.

  Next, the collected silver ultrafine particles are subjected to thermogravimetric analysis in accordance with JIS K0129 “General Rules for Thermal Analysis” and JIS K7120 “Thermogravimetric Methods for Plastics”. From the weight loss rate from room temperature to 600 ° C., the ultrafine silver particles The content of organic components contained therein was determined. As a result, the content of the organic component was 14% by weight.

  Next, 100 silver ultrafine particles were arbitrarily extracted from the transmission electron microscope (TEM) photograph, and the average particle size (D50) of the silver core and the sharpness ε of the particle size distribution were analyzed. As a result, the average particle diameter of the silver core was 5 nm. Further, the sharpness ε of the particle size distribution was 0.33, and it was confirmed that the particle size distribution had a narrow particle size distribution.

  Next, 58 g of the dry ultrafine silver particles, 21 g of xylene (boiling point 140 ° C.) as a low boiling point side organic solvent, and 21 g of terpineol (boiling point 219 ° C.) as a high boiling point side organic solvent are blended and ground on a mortar. The mixture was mixed while being put in a glass tube, and the silver ultrafine particles were ultrasonically dispersed for 15 minutes by an ultrasonic cleaner. As a result, a conductive paste according to Example 1 was obtained.

  Next, the conductive paste according to Example 1 was applied on a glass substrate by a bar coating method (application thickness 25 μm, application size 76 mm × 20 mm), and dried in air at 60 ° C. for 60 minutes to give xylene. After volatilization, it was baked at 200 ° C. for 30 minutes in an air atmosphere to obtain a baked film.

  Subsequently, using an optical microscope (450 times magnification), the surface of the obtained fired film was observed to confirm the presence or absence of cracks. As a result, no cracks occurred in the fired film.

Next, after measuring the surface resistivity of the fired film using a low resistance measuring instrument (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 was 4.0 × 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.

  Next, the scraped fired film was subjected to thermogravimetric analysis (temperature range of room temperature to 600 ° C.), and the content of residual organic components contained in the fired film was determined from the obtained weight loss rate. As a result, the content of residual organic components was 0.12% by weight.

(Conductive paste according to Example 2)
A conductive paste according to Example 2 was obtained in the same manner as in the production of the conductive paste according to Example 1 except that 37 g of xylene and 5 g of terpineol were used.

  Subsequently, as a result of performing the same test as Example 1 about the obtained electrically conductive paste, the result almost equivalent to Example 1 was obtained.

(Conductive paste according to Comparative Example 1)
In the production of the conductive paste according to Example 1, the conductive paste according to Comparative Example 1 was obtained in the same manner except that 42 g of xylene was used alone without using the mixed organic solvent.

Next, the obtained conductive paste was tested in the same manner as in Example 1. As a result, cracks occurred in the fired film, and the volume resistivity of the fired film exceeded 3.5 × 10 ³ Ω · cm. It was greatly different from the examples in that.

(Conductive paste according to Comparative Example 2)
In the production of the conductive paste according to Example 1, the conductive paste according to Comparative Example 2 was obtained in the same manner except that 42 g of terpineol was used alone without using the mixed organic solvent.

Next, the obtained conductive paste was subjected to the same test as in Example 1. As a result, cracks occurred in the fired film, and the volume resistivity of the fired film exceeded 3.0 × 10 ³ Ω · cm. It was greatly different from the examples in that.

  Table 1 summarizes the above examples and comparative examples.

  From the above results, the following can be understood. That is, in Comparative Examples 1-2, the mixed organic solvent is not used. For this reason, when a film is dried and baked, one organic solvent volatilizes rapidly in the vicinity of its boiling point, resulting in rapid volume shrinkage, and stress generated by the volume shrinkage cannot be relieved and cracks occur. Inferred.

  In addition, the film thickness of the fired film in the comparative example is thicker than the film thickness of the fired film in the example because many cracks are generated, and the volume is apparently increased by the amount of cracks. The film thickness was measured to be thick.

  Further, in Comparative Examples 1 and 2, the volume resistance of the fired film became extremely high although the residual organic component in the fired film was relatively small. This is presumably because the resistance is remarkably increased because the conductive path in the fired film is blocked by the crack.

  On the other hand, in Examples 1-2, since mixed organic solvent is used, it turns out that generation | occurrence | production of a crack can be suppressed effectively. Moreover, it can be seen that the content of the residual organic component in the fired film is relatively low, and the resistance is reduced by low-temperature firing.

  Furthermore, when a fired film having a relatively large thickness is obtained, it is not necessary to stack a thin film over and over, so that it can be said that the fine circuit formability is excellent.

  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.

Claims (7)

  A conductive paste, wherein ultrafine metal particles are dispersed in a mixed organic solvent in which two or more organic solvents having different boiling points are mixed.   2. The conductive paste according to claim 1, wherein the mixed organic solvent is a mixture of a low-boiling organic solvent that volatilizes mainly when the paste is dried and a high-boiling organic solvent that volatilizes mainly when the paste is fired. .   The conductive paste according to claim 2, wherein the difference between the boiling point of the low-boiling side organic solvent and the boiling point of the high-boiling side organic solvent is 50 ° C. or more.   The mixed organic solvent has a content of the low-boiling side organic solvent in the range of 50 to 95% by weight and a content of the high-boiling side organic solvent in the range of 50 to 5% by weight. Or the electrically conductive paste of 3. The ultrafine metal particles have a metal core composed of a metal component derived from a metal salt represented by the following chemical formula 1, and an organic component derived from the metal salt and covering the periphery of the metal core. The conductive paste according to claim 1, wherein the conductive paste is a paste.
(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 conductive paste according to claim 5, wherein the metal ultrafine particles are synthesized by heating a solution obtained by dissolving or dispersing the metal salt in an organic solvent for synthesis.   The conductive paste according to claim 6, wherein the solution is heated by an external heat source or microwave irradiation.