METHOD FOR PRODUCING ELECTRO-CONDUCTIVE PASTE

Abstract

Provided is an electro-conductive paste suitable to yield a sintered metal fine particulate layer having excellent adhesion to an ITO substrate. Powdery silver oxide is dispersed in a non-polar solvent. An excess amount of formic acid is added to allow the formic acid to react with the powdery silver oxide to thereby convert the powdery silver oxide into powdery silver formate (HCOOAg). A primary amine is allowed to react with the powdery silver formate to provide a primary amine addition salt of the silver formate, and the primary amine addition salt of the silver formate is subjected to a decompositional reduction reaction at a liquid temperature of around 70 C. to generate silver nanoparticles having a coating layer including the primary amine. To the resulting silver nanoparticle dispersion liquid, more than 0 parts by mass and 2.0 parts by mass or less of a titanium compound or manganese compound is added per 100 parts by mass of silver.

Claims

1. A method for producing an electro-conductive paste comprising: A) a step of preparing silver nanoparticles having an average particle size of 5 nm to 20 nm, the silver nanoparticles having a coating layer comprising coating agent molecules on the surface, the step of preparing silver nanoparticles being a step in which powdery silver(I) oxide is used as a raw material, in a liquid phase, formic acid is allowed to react with the powdery silver(I) oxide to convert the powdery silver(I) oxide into silver(I) formate, silver cations contained in the silver(I) formate are reduced to silver atoms, and the silver nanoparticles are prepared from the silver atoms; and the step of preparing silver nanoparticles comprising Step i: a step of preparing a dispersion liquid of the powdery silver(I) oxide using a hydrocarbon solvent; Step ii: a step of adding formic acid to the dispersion liquid of the powdery silver(I) oxide to allow formic acid to react with the powdery silver(I) oxide to thereby convert the powdery silver(I) oxide into silver(I) formate, and preparing a dispersion liquid of powdery silver(I) formate containing powder of silver(I) formate formed in the hydrocarbon solvent; and Step iii: a step of adding a primary amine to the dispersion liquid of the powdery silver(I) formate to allow the primary amine to react with the powdery silver(I) formate to thereby form a primary amine complex of the silver(I) formate, dissolving the formed primary amine complex of the silver(I) formate in the hydrocarbon solvent, and then, forming silver nanoparticles having an average particle size of 5 nm to 20 nm comprising silver atoms by a decompositional reduction reaction of the primary amine complex of the silver(I) formate, wherein the silver nanoparticles having an average particle size of 5 nm to 20 nm formed in the step iii have a structure in which silver atoms on the surface of the silver nanoparticles are coated with the primary amine via coordination bonding by means of the lone electron pair present on the amino nitrogen atom of the primary amine; and B) a step of adding one or more metal compounds selected from the group consisting of a titanium compound and a manganese compound to the dispersion liquid of the silver nanoparticles obtained from the step A, metal contained in the metal compound being in an amount of more than 0 part by mass and 2.0 parts by mass or less based on 100 parts by mass of silver contained in the dispersion of the silver nanoparticles obtained from the step A.

2. The method according to claim 1, wherein the titanium compound is one or more selected from the group consisting of alkoxy titanium, carboxy titanium, and titanium acetyl acetonate and the manganese compound is one or more selected from the group consisting of carboxy manganese and manganese acetyl acetonate.

3. The method according to claim 1, wherein the amount of the metal contained in the metal compound added in the step B is 0.5 to 2 parts by mass based on 100 parts by mass of silver contained in the dispersion liquid of the silver nanoparticles obtained from the step A.

4. The method according to claim 1, wherein the hydrocarbon solvent used in the step i is a hydrocarbon solvent having a boiling point in the range of 65 C. to 155 C. in an amount selected in the range of 350 parts by mass to 550 parts by mass per 100 parts by mass of the powdery silver(I) oxide as the raw material.

5. The method according to claim 1, wherein the hydrocarbon solvent used in the step i is a hydrocarbon having 6 to 9 carbon atoms.

6. The method according to claim 1, wherein the amount of the formic acid used in step ii is selected in the range of 1.1 molar amount to 1.4 molar amount per 1 molar amount of the silver cation contained in the powdery silver(I) oxide as the raw material.

7. The method according to claim 1, wherein a monocarboxylic acid having 8 to 11 carbon atoms is added in the step iii.

8. The method according to claim 1, wherein a primary amine having 9 to 11 carbon atoms is used as the primary amine and a secondary amine is added in the step iii.

9. The method according to claim 1, wherein the primary amine used in the step iii is a primary amine (RNH.sub.2) comprising an amino group and an atomic group R having an aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent, the amount of the primary amine being selected in the range of 1.2 molar amount to 1.8 molar amount per 1 molar amount of the silver cation contained in the powdery silver(I) oxide as the raw material.

10. The method according to claim 9, wherein, in the primary amine (RNH.sub.2) comprising the amino group and the atomic group R having the aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent, the atomic group R having an aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent is selected from an (alkyloxy)alkyl group, an (alkylamino)alkyl group, a (dialkylamino)alkyl group and an alkyl group having 7 to 12 carbon atoms in total.

11. The method according to claim 9, wherein the primary amine (RNH.sub.2) comprising the amino group and the atomic group R having the aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent is an amine compound having a boiling point more than 170 C.

12. The method according to claim 11, wherein the primary amine (RNH.sub.2) comprising the amino group and the atomic group R having the aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent is an amine compound having a boiling point in the range of 200 C. to 270 C.

13. The method according to claim 9, wherein the primary amine (RNH.sub.2) comprising the amino group and the atomic group R having the aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent is 3-alkyloxypropylamine (ROCH.sub.2CH.sub.2CH.sub.2NH.sub.2), and the alkyl group (R) constituting the alkyloxy atomic group (RO) is an alkyl group having 4 to 9 carbon atoms.

14. The method according to claim 1, wherein in the step iii, a primary amine (RNH.sub.2) comprising an amino group and an atomic group R having an aliphatic hydrocarbon chain having compatibility with the hydrocarbon solvent is diluted using the hydrocarbon solvent to provide a diluted solution, which is then added to the dispersion liquid of the powdery silver(I) formate, and the diluted solution is subjected to the dilution by adding the hydrocarbon solvent in an amount in the range of 20 parts by mass to 45 parts by mass per 100 parts by mass of the primary amine.

15. The method according to claim 1, wherein in the step iii, in parallel with the reaction in which the primary amine reacts with the powdery silver(I) formate to form the primary amine complex of the silver(I) formate, there proceeds a reaction in which the added primary amine reacts with residual formic acid that has not been consumed in the reaction with the powdery silver(I) oxide in the step ii, to form a primary amine addition salt of the formic acid, and the liquid temperature is raised by reaction heat ascribed to the reaction of forming the primary amine addition salt of the formic acid.

16. The method according to claim 1, wherein the step A further comprises the following step iv to step vi after the step iii: Step iv: a step of distilling the hydrocarbon solvent off under reduced pressure after the step iii is finished, the hydrocarbon solvent being contained in the reaction liquid containing the silver nanoparticles having an average particle size of 5 nm to 20 nm, the surface of the silver nanoparticles being coated with the primary amine, and recovering a residue containing residual primary amine, a primary amine addition salt of the formic acid, and the silver nanoparticles having an average particle size of 5 nm to 20 nm, the surface of the silver nanoparticles being coated with the primary amine; Step v: a step of adding to the residue recovered in the step iv methanol in an amount selected in the range of 200 parts by mass to 300 parts by mass and distilled water in an amount selected in the range of 50 parts by mass to 300 parts by mass per 100 parts by mass of the powdery silver(I) oxide as the raw material, dissolving the primary amine addition salt of the formic acid and the residual primary amine contained in the residue in the mixed solvent of methanol and distilled water, separating a resulting mixture into a precipitate layer containing the silver nanoparticles having an average particle size of 5 nm to 20 nm, the surface of the silver nanoparticles being coated with the primary amine, and a liquid phase layer containing the primary amine addition salt of the formic acid and primary amine dissolved in the mixed solvent, and removing the liquid phase layer containing the primary amine addition salt of the formic acid and the primary amine dissolved in the mixed solvent to recover a precipitate layer containing the silver nanoparticles having an average particle size of 5 nm to 20 nm, the surface of the silver nanoparticles being coated with the primary amine; Step vi: a step of adding a hydrocarbon solvent having a boiling point in the range of 65 C. to 155 C. in an amount selected in the range of 100 parts by mass to 200 parts by mass per 100 parts by mass of the powdery silver(I) oxide as the raw material to the precipitate layer recovered in the step v, homogeneously dispersing the silver nanoparticles having an average particle size of 5 nm to 20 nm, the surface of the silver nanoparticles being coated with the primary amine, contained in the precipitate layer in the hydrocarbon solvent having a boiling point in the range of 65 C. to 155 C. to obtain a dispersion liquid, separating this dispersion liquid into a layer of a small amount of the mixed solvent of methanol and distilled water with which the precipitate layer has been impregnated and a layer of a dispersion liquid containing the hydrocarbon solvent having a boiling point in the range of 65 C. to 155 C. as a dispersion solvent, and removing the layer of the small amount of the mixed solvent of methanol and distilled water to recover the layer of the dispersion liquid containing the hydrocarbon solvent having a boiling point in the range of 65 C. to 155 C. as the dispersion solvent.

Description

EXAMPLES

[0152] The present invention will be described in more detail hereinbelow based on Examples, but the present invention is not restricted thereby.

Example 1

Step A

[0153] First, a silver nanoparticle dispersion liquid was prepared in the step A.

Step i

[0154] Dispersed was 100 parts by mass (0.43 parts by mole) of powdery silver(I) oxide (Ag.sub.2O, formula weight: 231.735) in 550 parts by mass of methylcyclohexane (boiling point: 100.9 C., density: 0.7737).

Step ii

[0155] To the obtained dispersion liquid, 50 parts by mass (1.09 parts by mole) of formic acid (HCOOH, formula weight: 46.03, boiling point: 100.75 C.) was added dropwise over 3 to 5 minutes under stirring at room temperature (25 C.). Due to the addition of the formic acid, an exothermic reaction proceeded, and the liquid temperature rose to around 45 C. When the powdery silver oxide was converted to silver formate, the temperature of reaction liquid dropped thereafter.

Step iii

[0156] When the temperature of the obtained reaction liquid dropped to 27 C. or less, a solution of 230 parts by mass of 2-ethylhexyloxypropylamine (C.sub.11H.sub.25NO, formula weight: 187.32, boiling point: 235 C.) dissolved in 50 parts by mass of methylcyclohexane was added to the reaction liquid.

[0157] Due to an acid-base neutralization reaction which was caused by the addition of the amine, the liquid temperature rose to around 65 C. As the liquid temperature rises, a decompositional reduction reaction of silver formate via an amine complex of silver formate occurs. Silver nanoparticles which are precipitated by the reduction reaction are protected by the primary amine (2-ethylhexyloxypropylamine) in the system. After the liquid temperature rose to around 65 C., the stirring of the reaction liquid was continued, and when the liquid temperature dropped to 45 C., the stirring was stopped.

Step iv

[0158] The obtained navy-blue dispersion liquid was transferred to an eggplant flask, and then diisopropylamine and methylcyclohexane as the reaction solvent were distilled off under reduced pressure. The contents containing the silver nanoparticles in the eggplant flask turned into the form of slurry form because the solvent and the like were removed.

Step v

[0159] To the residue, after removing the solvent, 280 parts by mass of methanol (boiling point: 64.7 C.) and 50 parts by mass of distilled water were added.

[0160] In the mixed solvent containing methanol and distilled water, a diisoropylamine addition salt of formic acid or neodecanoic acid, a 2-ethylhexyloxypropylamine addition salt of formic acid or neodecanoic acid, and methylcyclohexane are dissolved. On the other hand, silver nanoparticles settle down without dispersing in the hydrous methanol.

[0161] The supernatant phase of the mixed solvent (hydrous methanol) was removed by decantation.

[0162] In order to improve the efficiency of removing the remaining components, 280 parts by mass of methanol was further added to the settled phase which was obtained from the decantation, and the resulting mixture was stirred. Then, the supernatant phase was removed by decantation.

Step vi

[0163] To the settled phase obtained from the decantation, 120 parts by mass of heptane was added. The settled silver nanoparticles were dispersed in methylcyclohexane. Methanol which had remained in the settled silver particles was phase-separated due to its poor compatibility with heptane. The phase-separated methanol phase (hydrous methanol) portion was removed.

Purification Step

[0164] In the heptane layer in which the silver nanoparticles were dispersed, a slight amount of methanol was contained as an impure ingredient. The contained methanol was distilled off under reduced pressure. The difference in the boiling point between methanol and heptane was used to selectively distill methanol off. Specifically, methanol was removed at 45 C. (bath temperature) and 150 hPa for 5 minutes. Then, the degree of the pressure reduction was raised to 120 hPa, and methanol was further removed for 3 minutes.

[0165] The heptane liquid in which the obtained silver nanoparticles were dispersed was filtered with a 0.2 m membrane filter to remove aggregates. As the filtrate obtained from the filtration, a silver nanoparticle dispersion liquid was obtained.

Evaluation on Silver Nanoparticle Dispersion Liquid

[0166] The total amount of metal silver which was contained in the obtained silver nanoparticle dispersion liquid was measured, and the yield was calculated based on the content of silver which was contained silver(I) oxide as the starting material. The calculated yield was 98%.

[0167] A method for measuring the total amount of metal silver is as follows. The obtained silver nanoparticle dispersion liquid was weighed into a crucible, and methylcyclohexane which was contained in the liquid was dried off to obtain a solid, by using a hot-air dryer. Then, the crucible was placed in a muffle furnace and fired at 700 C. for 30 minutes. After the firing, only metal remained, and thus, the metal amount was weighed. From the concentration of the dispersion liquid, the total amount of metal silver was calculated.

[0168] Additionally, the obtained silver nanoparticle dispersion liquid was left to stand at room temperature for a week, and then the settlement of particles was visually checked. Settlement of particles was not observed.

[0169] The particle diameter of the silver nanoparticles dispersed in the obtained silver nanoparticle dispersion liquid was measured by use of a light scattering particle size analyzer (manufactured by MicrotracBEL Corp., product name: Nanotrac UPA150). From the measurement results, it was found that the average particle size of the silver nanoparticles which were homogeneously dispersed in the filtrate was 9 nm.

[0170] In the obtained silver nanoparticle dispersion liquid, the surface of the silver nanoparticles was coated with 25.0 parts by mass of 2-ethylhexyloxypropylamine per 100 parts by mass of the silver nanoparticles which were coated with 2-ethylhexyloxypropylamine (100 parts by mass as the mass of silver only, not containing the coating agent).

[0171] A method for measuring the amount of the coating agent with which the silver nanoparticles were coated is as follows. That is, about 0.1 g of the dispersion liquid in which silver nanoparticles were dispersed in heptane was weighed in a glass bottle, and the solvent content was dried by means of a dryer (cold air) into a powder form. About 10 mg of the dry powder was heated to 500 C. in a thermal analyzer (product name: TG/DTA6200, manufactured by SII NanoTechnology Inc.) to be analyzed. The amount of the coating agent was calculated from the weight reduction ratio.

Step B

[0172] The silver nanoparticle dispersion liquid which was obtained in the step A was used in such an amount that the amount of silver which was contained in the dispersion liquid reached 60 parts by mass. Into this dispersion liquid, 38.2 parts by mass of tetradecane (boiling point: 253.6 C., density: 0.7624 g/cm.sup.3) and 1.8 parts by mass of titanium tetraisopropoxide (manufactured by Wako Pure Chemical Industries, Ltd.) were mixed.

[0173] Heptane which was contained in the obtained liquid mixture was distilled off under reduced pressure to prepare ink (electro-conductive paste for printing) including tetradecane as the dispersion solvent. The amount of metal titanium contained in the ink was 0.5 parts by mass with respect to 100 parts by mass of silver.

[0174] The viscosity of the produced ink was 11 m.Math.Pas (20 C.), and the metal content was 55.2% by mass. The prepared electro-conductive ink was applied by spin coating to ITO-coated glass having a width of 25 mm and a length of 75 mm. The average film thickness of the applied coating film was 6 m. The silver nanoparticles were sintered by heat-treating the obtained coating film in the atmosphere at 200 C. for 60 minutes using an air drying furnace. The resistivity of the produced low-temperature fired film of the silver nanoparticles was measured. The film thickness after the firing was 0.9 m, and the resistivity of the low-temperature fired film was 13 .Math.cm. With respect to adhesion of the fired film to the ITO-coated glass, a cross cut test was carried out to check the presence of peel-off for 81 squares, each square having a size of 1 mm1 mm. No peel-off was observed in all squares.

Examples 2 to 9, and Comparative Examples 1 to 3

[0175] Preparation and evaluation of an electro-conductive paste were carried out in the same manner as in Example 1 except that electro-conductive paste formulations shown in Table 1 were each employed in the step B. The results are shown in Table 1. Note that, for instance, titanium(IV) 2-ethylhexanolate was used as the metal compound in Example 4. In Examples 5 to 7, for instance, manganese 2-ethylhexanoate solution in mineral spirits (manufactured by Wako Pure Chemical Industries, Ltd., Mn: 8% by mass) is used in order to add a metal compound.

[0176] In Comparative Example 1, no metal compound was added to the electro-conductive paste. In Comparative Example 1, peeling-off occurred in 40 squares out of a total 81 squares, each square having a size of 1 mm1 mm, in the cross cut test.

[0177] In each of Comparative Examples 2 and 3, when the electro-conductive paste was fired, cracks occurred. Thus, it was not possible to measure the conductivity and the film thickness after the firing.

TABLE-US-00001 TABLE 1 Comparative Example Example 1 2 3 4 5 6 7 8 9 1 2 3 Paste formulation Silver 60 60 60 60 60 60 60 60 60 60 60 60 nanoparticles (in the silver nanoparticle dispersion obtained from step A) Tetradecane 38.2 36.4 32.8 36.46 36.2 32.4 24.8 38.93 37.75 40 31.1 21.25 Titanium Molecular weight: 1.8 3.6 7.2 1.07 8.9 tetraisopropoxide 284.22 (Ti content: 16.84% by mass) Titanium(IV) 2- Molecular weight: 3.54 2.25 18.75 ethylhexanolate 564.75 (Ti content: 8.47% by mass) Manganese(II) 2- Mn content: 3.8 7.6 15.2 ethylhexanoate 8% by mass Solution in mineral spirits Proportion of 0.5% 1.0% 2.0% 0.5% 0.3% 2.5% Ti to Ag Proportion of 0.5% 1.0% 2.0% 0.3% 2.5% Mn to Ag Paste evaluation Metal content 700 C., 30 min 55.2 55.8 56 55.4 56.2 57 56.8 54.8 57.1 55.7 56 57.2 (% by mass) Viscosity (mPa .Math. s) 20 C., 60 rpm 11 13 19 14 10 12 18 11 9 10 24 20 Conductivity Air drying furnace, 13 25 37 17 24 38 56 12 16 3.6 *1 *1 ( .Math. cm) 200 C. 1 hr Film thickness 0.9 1 1.3 1.0 0.8 0.9 1.1 0.9 0.8 1.0 after firing (m) Cross cut test Proportion of the 0 0 0 0 0 0 0 30 20 50 0 0 (peeled-off %) number of peeled-off Squares out of all the 81 Squares *1: Not measurable due to occurrence of cracks

INDUSTRIAL APPLICABILITY

[0178] The silver nanoparticles prepared by the present invention can be suitably used for mounting electronic components on an ITO film or glass or for forming wiring on an ITO film or glass, for example.