conductive paste

Abstract

This invention provides a conductive paste that has a high sintering start temperature and can suppress cracks and poor coverage of internal electrodes during firing. [Solution] The conductive paste comprises conductive metal particles, metal oxide particles, a dispersant, a binder resin, and an organic solvent, and the dispersant comprises at least a cationic group-containing acrylic polymer. The cationic group-containing acrylic polymer may be a cationic acrylic polymer, or it may be an amphoteric acrylic polymer containing anionic groups in addition to cationic groups.

Description

Technical Field

[0001] The present invention relates to a conductive paste suitably used for forming internal electrodes of multilayer ceramic capacitors.

Background Art

[0002] A multilayer ceramic capacitor (MLCC) has a structure in which dielectric layers and internal electrodes are alternately laminated, and miniaturization and high capacitance can be achieved by thinning the dielectric layers and internal electrodes.

[0003] In the manufacture of a multilayer ceramic capacitor, first, a conductive paste for internal electrodes is printed in a pattern on the surface of a dielectric sheet containing dielectric particles made of metal oxides such as barium titanate (BaTiO3) and a binder resin, and dried to form a dry film. Next, the dry film and the dielectric sheet are laminated so as to alternately overlap, heat-pressed and integrated, and a debinding treatment is performed in an oxidizing atmosphere or an inert atmosphere. Thereafter, by firing this laminate, an alternating laminate of dielectric layers and internal electrodes is obtained.

[0004] The conductive paste for internal electrodes generally contains conductive metal particles such as nickel, a dispersant, a binder resin, and an organic solvent. The sintering start temperature of the conductive metal particles is lower than that of the metal oxide particles of the dielectric sheet. Generally, the sintering start temperature of the metal oxide particles is 1000 °C or higher, whereas the sintering start temperature of metal particles such as nickel is about 600 to 800 °C. When firing an alternating laminate of the dry film of the conductive paste and the dielectric sheet, the sintering of the conductive paste proceeds first, and since the shrinkage behaviors of the dry film of the conductive paste and the dielectric sheet are different, cracks and a decrease in continuity (coverage failure) occur in the internal electrodes, which causes a decrease in capacitance. In particular, when the thickness of the internal electrode is small, cracks and coverage failures due to the difference in shrinkage behavior are likely to occur.

[0005] As a method to suppress the reduction in coverage caused by the difference in sintering start temperatures and shrinkage behavior during firing of the dried films of dielectric sheets and conductive paste, it has been proposed to incorporate metal oxides, which are components of the dielectric layer, as a "co-material" into the conductive paste for internal electrodes, thereby inhibiting the sintering of conductive metal particles.

[0006] Patent Document 1 discloses that by mixing a slurry of conductive metal particles with a slurry of metal oxide particles as a co-material, a heteroaggregate is formed in which the metal oxide particles adhere to the conductive metal particles and cover the surface of the conductive metal particles.

[0007] Patent Document 2 discloses that by incorporating an amine-based dispersant into a conductive paste, the increase in surface activity of conductive metal particles in the conductive paste is suppressed, the dispersion state is maintained, and the viscosity change of the conductive paste over time is suppressed. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2009-266716 [Patent Document 2] Japanese Patent Publication No. 2021-180073 [Overview of the project] [Problems that the invention aims to solve]

[0009] To further reduce the thickness of the internal electrodes, it is necessary to further reduce the particle size of the conductive metal particles and metal oxide particles contained in the conductive paste. However, reducing the particle size tends to decrease dispersibility, making it difficult to sufficiently raise the sintering start temperature of the conductive paste using conventional methods.

[0010] In view of the above, the present invention aims to provide a conductive paste that has a high sintering start temperature and can suppress cracks and poor coverage of internal electrodes during firing. [Means for solving the problem]

[0011] The conductive paste comprises conductive metal particles, metal oxide particles, a dispersant, a binder resin, and an organic solvent, and the dispersant comprises at least a cationic group-containing acrylic polymer.

[0012] The cationic group-containing acrylic polymer may be a cationic acrylic polymer, or it may be an amphoteric acrylic polymer containing anionic groups in addition to cationic groups. The amine value of the cationic group-containing acrylic polymer is preferably 0.1 to 100 mg KOH / g.

[0013] The conductive paste may contain an anionic group-containing compound as a dispersant. The anionic group-containing compound may be an anionic group-containing acrylic polymer. The anionic group-containing acrylic polymer may be an anionic acrylic polymer, or it may be an amphoteric acrylic polymer containing cationic groups in addition to anionic groups.

[0014] The amphoteric acrylic polymer can be either a cationic group-containing acrylic polymer or an anionic group-containing polymer. In other words, the conductive paste of one embodiment contains an amphoteric acrylic polymer as a dispersant, and this amphoteric acrylic polymer can act as both a cationic group-containing acrylic polymer and an anionic group-containing polymer.

[0015] In another embodiment, the conductive paste comprises a cationic acrylic polymer and an amphoteric acrylic polymer as dispersants. [Effects of the Invention]

[0016] By using a specific dispersant, the sintering start temperature of the conductive paste can be increased. This increased temperature reduces the difference between the sintering start temperature of the dry conductive paste film and the dielectric sheet, as well as the difference in shrinkage behavior during firing, thereby suppressing cracks and poor coverage of the internal electrodes. [Modes for carrying out the invention]

[0017] [Components of conductive paste] The conductive paste according to the embodiment of the present invention comprises conductive metal particles, metal oxide particles, a dispersant, a binder resin, and an organic solvent. Preferred forms of each of these constituent materials will be described in order below. Unless otherwise specified, each of the constituent materials and monomer components described below may be used individually or in combination of two or more.

[0018] <Conductive metal particles> Preferred metal materials for conductive metal particles include Ni, Ag, Cu, Pd, Au, Pt, and alloys thereof. Among these, Ni or Ni alloys are preferred from the viewpoint of conductivity and corrosion resistance. Examples of Ni alloys include alloys of Ni with one or more elements selected from the group consisting of Sn, Mn, Cr, Co, Al, Fe, Cu, Zn, Ag, Au, Pt, and Pd. The Ni content in the Ni alloy is, for example, 50% by mass or more, preferably 80% by mass or more, and may be 90% by mass or more or 95% by mass or more.

[0019] The particle size of the conductive metal particles is not particularly limited, but from the viewpoint of sintering start temperature, the average primary particle size is preferably 30 nm or more, and from the viewpoint of thinning the internal electrodes of the multilayer ceramic capacitor, the average primary particle size is preferably 1 μm or less. The average primary particle size of the conductive metal particles is more preferably 50 to 500 nm, even more preferably 100 to 300 nm, and may also be 120 to 200 nm.

[0020] From the viewpoint of ensuring the dispersibility of the conductive metal particles and the conductivity of the internal electrodes and the film continuity after firing, the content of the conductive metal particles in the entire conductive paste is preferably 30 to 70% by mass, more preferably 35 to 65% by mass, and still more preferably 40 to 60% by mass.

[0021] <Metal oxide particles> Metal oxide particles are "co-materials" used for the purpose of increasing the sintering start temperature of the conductive paste and making the shrinkage behavior of the dry film during firing closer to the shrinkage behavior of the dielectric sheet. Examples of the metal oxide constituting the metal oxide particles include BaTiO3, SrTiO3, CaZrO3, MgO, TiO2 - BaCO3, SiO2, TiO2, Al2O3, Cr2O3, MnO2, ZnO, BaO, Bi2O3, Y2O3, etc.

[0022] From the viewpoint of making the shrinkage behavior of the dry film of the conductive paste during firing closer to the shrinkage behavior of the dielectric sheet, it is preferable that the metal oxide particles are made of the same material as the dielectric layer of the MLCC. Generally, barium titanate (BaTiO3) is often used. As the metal oxide, those containing barium titanate as the main component and other oxides as sub-components may also be used. Examples of the oxides that can be contained in the metal oxide particles mainly composed of barium titanate include oxides of Mn, Cr, Si, Ca, Ba, Mg, V, W, Ta, Nb, and one or more rare earth elements.

[0023] The particle size of the metal oxide particles is not particularly limited. From the viewpoint of the dispersibility of the particles, the average primary particle size of the metal oxide particles is preferably 5 nm or more. From the viewpoint of increasing the sintering start temperature by covering the surface of the conductive metal particles with the metal oxide particles, the average primary particle size of the metal oxide particles is preferably 200 nm or less. The average primary particle size of the metal oxide particles is more preferably 7 to 100 nm, and may also be 8 to 70 nm or 10 to 50 nm.

[0024] From the perspective of increasing the sintering start temperature by covering the surface of the conductive metal particles with metal oxide particles, the average primary particle diameter of the metal oxide particles is preferably smaller than the average primary particle diameter of the conductive metal particles, and may be 0.7 times or less, 0.5 times or less, 0.3 times or less, or 0.2 times or less of the average primary particle diameter of the conductive metal particles. The average primary particle diameter of the metal oxide particles is preferably 0.01 times or more of the average primary particle diameter of the conductive metal particles, and may be 0.03 times or more, 0.05 times or more, 0.07 times or more, or 0.1 times or more.

[0025] From the perspective of ensuring the dispersibility of the metal oxide particles and the conductivity and film continuity of the internal electrode after firing, the content of the metal oxide particles in the entire conductive paste is preferably 1 to 20% by mass, more preferably 2 to 15% by mass, and even more preferably 3 to 10% by mass. The amount of the metal oxide particles is preferably 3 to 30 parts by mass, more preferably 5 to 20 parts by mass, and even more preferably 7 to 15 parts by mass with respect to 100 parts by mass of the conductive metal particles.

[0026] <Dispersant> The conductive paste contains the above-mentioned dispersant for the conductive metal particles and the metal oxide particles. In the present invention, at least an acrylic polymer having a cationic group is used as the dispersant.

[0027] (Acrylic polymer containing cationic group) The acrylic polymer having a cationic group may be a cationic acrylic polymer or an amphoteric ionic acrylic polymer. The acrylic polymer containing a cationic group adsorbs on the surface of the conductive metal particles, and the acrylic polymer containing a cationic group adsorbed on the surface has an effect of preventing the aggregation of the conductive metal particles.

[0028] The acrylic polymer containing a cationic group contains a nonionic monomer (meth)acrylic acid alkyl ester and a cationic group-containing monomer as constituent monomers. In this specification, "(meth)acrylic" means acrylic and / or methacrylic.

[0029] Alkyl (meth)acrylate is the main constituent monomer of acrylic polymers. The alkyl group of the alkyl (meth)acrylate is preferably a linear or branched linear alkyl group. The number of carbon atoms in the alkyl group is preferably 1 to 35, but may be 1 to 20, 1 to 12, 4 to 10, or 6 to 8.

[0030] Specific examples of alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, s-butyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, neopentyl (meth)acrylate, isoamyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, isooctyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. Examples include lylate, nonyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, 1-methyltridecyl (meth)acrylate, hexadecyl (meth)acrylate, octadecyl (meth)acrylate, stearyl (meth)acrylate, isostearyl (meth)acrylate, eicosyl (meth)acrylate, docosyl (meth)acrylate, behenyl (meth)acrylate, tetracosyl (meth)acrylate, triacontyl (meth)acrylate, etc.

[0031] The amount of alkyl (meth)acrylate per 100 parts by mass of the total constituent monomer components of the cationic group-containing acrylic polymer is preferably 50 to 99.9 parts by mass. The amount of alkyl (meth)acrylate is more preferably 60 to 99 parts by mass, and may be 70 to 98 parts by mass, or 80 to 97 parts by mass.

[0032] A cationic group-containing monomer is a vinyl monomer having a cationic group. A tertiary amino group or a quaternary ammonium base is preferred as the cationic group.

[0033] Examples of tertiary amino group-containing vinyl monomers include N,N-dialkylaminoalkyl(meth)acrylates such as N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-di-t-butylaminoethyl(meth)acrylate, and N,N-dimethylaminobutyl(meth)acrylate; and N,N-dialkylaminoalkyl(meth)acrylamides such as N,N-dimethylaminoethyl(meth)acrylamide, N,N-diethylaminoethyl(meth)acrylamide, and N,N-dimethylaminopropyl(meth)acrylamide.

[0034] Examples of quaternary ammonium group-containing vinyl monomers include those obtained by reacting the above-mentioned tertiary amino group-containing vinyl monomers with quaternizing agents such as epihalohydrins, benzyl halides, and alkyl halides. Specific examples of quaternary ammonium group-containing vinyl monomers include (meth)acryloyloxyalkyltrialkylammonium salts such as 2-((meth)acryloyloxy)ethyltrimethylammonium chloride, 2-((meth)acryloyloxy)ethyltrimethylammonium bromide, and 2-((meth)acryloyloxy)ethyltrimethylammonium dimethyl phosphate; (meth)acryloylaminopropyltrimethylammonium salts such as (meth)acryloylaminopropyltrialkylammonium chloride and (meth)acryloylaminopropyltrimethylammonium bromide; tetraalkyl(meth)acrylates such as tetrabutylammonium(meth)acrylate; and trialkylbenzylammonium(meth)acrylates such as trimethylbenzylammonium(meth)acrylate.

[0035] The amount of cationic group-containing monomer per 100 parts by mass of the total constituent monomer components of the cationic group-containing acrylic polymer is preferably 0.01 to 40 parts by mass, more preferably 0.1 to 30 parts by mass, even more preferably 0.5 to 20 parts by mass, and may also be 1 to 15 parts by mass or 2 to 10 parts by mass.

[0036] From the viewpoint of enhancing its dispersant effect, the amine value of the cationic group-containing acrylic polymer is preferably 0.1 to 100 mg KOH / g, more preferably 1 to 50 mg KOH / g, even more preferably 2 to 40 mg KOH / g, and may also be 3 to 30 mg KOH / g, 4 to 25 mg KOH / g, or 5 to 20 mg KOH / g. It is preferable to adjust the proportion of cationic group-containing monomers in the constituent monomers of the acrylic polymer so that the amine value falls within this range. The amine value is the amount of potassium hydroxide required to neutralize 1 g of the polymer's solid content and is measured in accordance with JIS K 7237.

[0037] As described above, the cationic group-containing acrylic polymer may also be an amphoteric acrylic polymer containing anionic groups in addition to cationic groups. An amphoteric acrylic polymer can be obtained by copolymerizing the above-mentioned alkyl (meth)acrylate and cationic group-containing vinyl monomer with anionic group-containing vinyl monomer. Examples of anionic groups include acidic groups such as carboxyl groups, phosphate groups, and sulfonic acid groups, or salts thereof. Examples of acidic group salts include sodium salts, potassium salts, and ammonium salts.

[0038] Examples of carboxyl group-containing vinyl monomers include α,β-unsaturated monocarboxylic acids such as (meth)acrylic acid and crotonic acid; α,β-unsaturated dicarboxylic acids such as itaconic acid, maleic acid, fumaric acid, mesaconic acid, and citraconic acid; and half-esterified compounds of hydroxyalkyl (meth)acrylates and acid anhydrides. The carboxyl group-containing vinyl monomer is preferably an α,β-unsaturated monocarboxylic acid, with (meth)acrylic acid being particularly preferred.

[0039] Examples of phosphate-containing vinyl monomers include phosphate-containing (meth)acrylates such as acid phosphooxyethyl (meth)acrylate, acid phosphooxypolyoxypropylene glycol mono(meth)acrylate, and mono(2-hydroxyethyl (meth)acrylate) phosphate.

[0040] Examples of vinyl monomers containing sulfonic acid groups include vinyl sulfonic acid, styrene sulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid.

[0041] When the cationic group-containing acrylic polymer is amphoteric, the amount of anionic group-containing monomer per 100 parts by mass of the total constituent monomer components is preferably 0.01 to 40 parts by mass, more preferably 0.1 to 30 parts by mass, even more preferably 0.5 to 20 parts by mass, and may also be 1 to 15 parts by mass or 2 to 10 parts by mass.

[0042] When the cationic group-containing acrylic polymer is amphoteric, the acid value may be 0.1-100 mg KOH / g, 1-50 mg KOH / g, 2-40 mg KOH / g, 3-30 mg KOH / g, 4-25 mg KOH / g, or 5-20 mg KOH / g. It is preferable to adjust the proportion of anionic group-containing monomers in the constituent monomers of the acrylic polymer so that the acid value falls within this range. The acid value is the amount of potassium hydroxide required to neutralize the acidic groups contained in 1 g of the solid content (or non-volatile content in the case of a liquid) of the sample, and is measured in accordance with JIS K 2501.

[0043] Cationic group-containing acrylic polymers may contain nonionic monomers other than alkyl (meth)acrylates as constituent monomers. Examples of nonionic monomers other than alkyl (meth)acrylates include hydroxyl group-containing vinyl monomers, nitrogen-containing vinyl monomers (excluding the above-mentioned tertiary amino group-containing vinyl monomers and quaternary ammonium base-containing vinyl monomers), alkane polyol poly(meth)acrylates, saturated alicyclic group-containing vinyl monomers, aromatic vinyl monomers, esters of α,β-unsaturated carboxylic acids other than (meth)acrylic acid, (meth)acrylonitrile, vinyl acetate, and the like.

[0044] Examples of hydroxyl group-containing vinyl monomers include hydroxyl group-containing (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, and polyethylene glycol (meth)acrylate.

[0045] Examples of nitrogen-containing vinyl monomers include N-vinylpyrrolidone, N-vinylcaprolactam, (meth)acryloylmorpholine, and N,N-dimethyl(meth)acrylamide.

[0046] Examples of alkane polyol poly(meth)acrylates include (meth)acrylic acid esters of polyhydric alcohols such as neopentyl glycol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, tetramethylolmethane tri(meth)acrylate, and dipentaerythritol hexa(meth)acrylate.

[0047] Examples of saturated alicyclic group-containing vinyl monomers include (meth)acrylic acid alicyclic alkyl esters such as cyclohexyl (meth)acrylate, bornyl (meth)acrylate, isobornyl (meth)acrylate, dicyclopentanyl (meth)acrylate, adamantyl (meth)acrylate, and dimethyladamantyl (meth)acrylate.

[0048] Examples of aromatic vinyl monomers include styrene, vinyltoluene, and α-methylstyrene.

[0049] An example of an ester of an α,β-unsaturated carboxylic acid is benzyl itaconate.

[0050] The amount of nonionic monomers other than alkyl (meth)acrylates, relative to 100 parts by mass of the total constituent monomer components of the cationic group-containing acrylic polymer, is preferably 45 parts by mass or less, more preferably 30 parts by mass or less, and may be 20 parts by mass or less, 10 parts by mass or less, 5 parts by mass or less, 1 part by mass or less, or 0 parts by mass.

[0051] The polymerization method for cationic group-containing acrylic polymers is not particularly limited, and known polymerization methods such as solution polymerization, bulk polymerization, and suspension polymerization can be applied. From the viewpoint of reactivity and cost, solution polymerization is preferred. In solution polymerization, polymerization is carried out by blending the above monomer components with a polymerization initiator in a solvent under an inert gas atmosphere such as nitrogen.

[0052] Examples of solvents include petroleum hydrocarbon solvents such as hexane and mineral spirits; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, and cyclohexanone; ester solvents such as methyl acetate, ethyl acetate, butyl acetate, γ-butyrolactone, and propylene glycol monomethyl ether acetate; aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, N-methylpyrrolidone, and pyridine; alcohol solvents such as methanol, ethanol, propanol, isopropanol, and butanol; glycol ether solvents such as ethylene glycol monoethyl ether and propylene glycol monomethyl ether; terpene solvents such as terpineol, dihydroterpineol, dihydroterpineol acetate, and dihydroterpineol propionate; and water.

[0053] As the polymerization initiator, a radical polymerization initiator is preferred, and various known ones such as azo-based and peroxide-based ones can be used. The amount of polymerization initiator used is about 0.1 to 20 parts by mass per 100 parts by mass of the total monomer components, and may be 0.5 to 15 parts by mass or 1 to 10 parts by mass. In order to adjust the molecular weight, a chain transfer agent may be used in addition to the polymerization initiator.

[0054] The polymerization temperature for solution polymerization is, for example, around 30 to 150°C, but may also be 50 to 120°C or 60 to 100°C. The reaction time is, for example, 1 to 20 hours, but may also be 2 to 10 hours or 3 to 8 hours.

[0055] The weight-average molecular weight of the cationic group-containing acrylic polymer is, for example, 1,000 to 1,000,000, and may also be 2,000 to 500,000, 3,000 to 200,000, or 5,000 to 100,000.

[0056] (Anionic group-containing compounds) The conductive paste may contain, in addition to the cationic group-containing acrylic polymer described above, an anionic group-containing compound as a dispersant. The anionic group-containing compound may be an anionic compound or an amphoteric compound. If the cationic group-containing acrylic polymer described above is an amphoteric acrylic polymer, the amphoteric acrylic polymer may be both a cationic group-containing acrylic polymer and an anionic group-containing compound.

[0057] Dispersants having anionic groups adsorb onto the surface of metal oxide particles, and the dispersant adsorbed on the surface prevents the metal oxide particles from aggregating.

[0058] Examples of anionic group-containing compounds include lower carboxylic acids such as acetic acid, propionic acid, and butanoic acid; fatty acids such as oleic acid, stearic acid, palmitic acid, oleoylsarcosine, and ω-carboxypolycaprolactone; and low molecular weight acidic group-containing compounds such as polycarboxylic acids such as adipic acid, phthalic acid, and citric acid. The anionic group-containing compound may also be a surfactant having a carboxyl group or a phosphate group. The anionic group-containing compound may also be a polymer containing an acidic group, preferably an acrylic polymer.

[0059] Anionic group-containing acrylic polymers include nonionic monomers such as alkyl (meth)acrylates and anionic group-containing monomers as constituent monomers. Specific examples of alkyl (meth)acrylates and anionic group-containing monomers are the same as those described above for cationic group-containing acrylic polymers.

[0060] From the viewpoint of enhancing its dispersing effect, the acid value of the anionic group-containing acrylic polymer is preferably 0.1 to 100 mg KOH / g, more preferably 1 to 50 mg KOH / g, even more preferably 2 to 40 mg KOH / g, and may also be 3 to 30 mg KOH / g, 4 to 25 mg KOH / g, or 5 to 20 mg KOH / g. It is preferable to adjust the proportion of anionic group-containing monomers in the constituent monomers of the acrylic polymer so that the acid value falls within this range.

[0061] When the anionic group-containing acrylic polymer is an amphoteric acrylic polymer containing cationic groups in addition to anionic groups, specific examples of cationic group-containing monomers are the same as those described above for cationic group-containing acrylic polymers. When the anionic group-containing acrylic polymer is amphoteric, the amine value may be 0.1 to 100 mg KOH / g, 1 to 50 mg KOH / g, 2 to 40 mg KOH / g, 3 to 30 mg KOH / g, 4 to 25 mg KOH / g, or 5 to 20 mg KOH / g. It is preferable to adjust the proportion of cationic group-containing monomers in the constituent monomers of the acrylic polymer so that the amine value falls within this range.

[0062] Anionic group-containing acrylic polymers may contain nonionic monomers other than alkyl (meth)acrylates as constituent monomers. Specific examples of nonionic monomers other than alkyl (meth)acrylates are the same as those described above for cationic group-containing acrylic polymers.

[0063] The polymerization method for anionic group-containing acrylic polymers is not particularly limited, and known polymerization methods such as solution polymerization, bulk polymerization, and suspension polymerization can be applied. As with cationic group-containing acrylic polymers, solution polymerization is preferred. Details of solution polymerization are the same as those described above for cationic group-containing acrylic polymers.

[0064] The weight-average molecular weight of the anionic group-containing acrylic polymer is, for example, 1,000 to 1,000,000, and may also be 2,000 to 500,000, 3,000 to 200,000, or 5,000 to 100,000.

[0065] When both the cationic group-containing acrylic polymer and the anionic group-containing compound are amphoteric acrylic polymers, these amphoteric acrylic polymers may have the same composition or may have different compositions.

[0066] (Dispersant formulation) The dispersant content relative to the entire conductive paste is preferably 0.5 to 12% by mass, more preferably 1 to 10% by mass, and even more preferably 2 to 8% by mass. The amount of dispersant is preferably 1 to 35 parts by mass, more preferably 2 to 25 parts by mass, even more preferably 3 to 20 parts by mass, and may also be 5 to 15 parts by mass per 100 parts by mass of conductive metal particles.

[0067] A diluted solution obtained by dissolving or dispersing the above-mentioned cationic group-containing acrylic polymer and / or anionic group-containing compound in a solvent may be used as a dispersant. When diluting the dispersant with a solvent, the solid content concentration is, for example, 10% by mass or more, and may be 20% by mass or more, or 30% by mass or more.

[0068] As mentioned above, cationic group-containing acrylic polymers primarily act as dispersants for conductive metal particles, and in conductive pastes, the cationic group-containing acrylic polymer is adsorbed onto the surface of the conductive metal particles. The adsorption of the dispersant onto the surface of the conductive metal particles suppresses aggregation of the conductive metal particles.

[0069] Because aggregation between conductive metal particles is suppressed, metal oxide particles tend to aggregate and bond to the surface of conductive metal particles to which the dispersant is adsorbed, forming heteroaggregates in which metal oxide particles cover the surface of conductive metal particles. The formation of heteroaggregates hinders the sintering of conductive metal particles, which tends to raise the sintering start temperature of the conductive paste.

[0070] When an anionic group-containing compound is included as a dispersant, the anionic group-containing compound is adsorbed onto the surface of the metal oxide particles, suppressing aggregation between the metal oxide particles. Because the dispersant is adsorbed onto the surfaces of both the conductive metal particles and the metal oxide particles, there is a tendency to promote the formation of heteroaggregates in which metal oxide particles adsorbed with anionic group-containing dispersant aggregate and bond to the surface of conductive metal particles on which a cationic group-containing dispersant is adsorbed.

[0071] As described above, the dispersant having a cationic group is a cationic group-containing acrylic polymer and is cationic or zwionic. The dispersant having anionic group is preferably an anionic group-containing acrylic polymer and is zwionic or anionic. The combinations of the cationic group-containing dispersant and the anionic group-containing dispersant are any of the following: (1) cationic / zwionic, (2) zwionic / zwionic, (3) cationic / anionic, and (4) zwionic / anionic.

[0072] Among the above combinations, (1) the cationic / amphoionic combination tends to be superior in improving the sintering start temperature of the conductive paste, and it is particularly preferable to use a cationic acrylic polymer and an amphoteric acrylic polymer together as a dispersant. In the cationic / amphoteric combination, the crosslinking cohesive force by the dispersant adsorbed on the surfaces of the conductive metal particles and metal oxide particles is high, and it is presumed that this contributes greatly to raising the sintering start temperature of the paste.

[0073] When a cationic group-containing acrylic polymer and an anionic group-containing compound are used in combination as a dispersant, the content of the cationic group-containing acrylic polymer relative to the total conductive paste is preferably 0.5 to 10% by mass, more preferably 1 to 8% by mass, and even more preferably 2 to 6% by mass. The amount of cationic group-containing acrylic polymer is preferably 1 to 30 parts by mass, more preferably 2 to 20 parts by mass, even more preferably 3 to 15 parts by mass, and may also be 5 to 10 parts by mass per 100 parts by mass of conductive metal particles. When a cationic acrylic polymer and an amphoteric acrylic polymer are used in combination as a dispersant, it is preferable that the amount of cationic acrylic polymer be within this range.

[0074] When a cationic group-containing acrylic polymer and an anionic group-containing compound are used in combination as a dispersant, the content of the anionic group-containing compound relative to the total conductive paste is preferably 0.01 to 2% by mass, more preferably 0.05 to 1.5% by mass, and even more preferably 0.1 to 0.5% by mass. The amount of the anionic group-containing compound is preferably 0.5 to 25 parts by mass, more preferably 1 to 20 parts by mass, even more preferably 1.5 to 15 parts by mass, and may also be 2 to 10 parts by mass or 2.5 to 8 parts by mass per 100 parts by mass of metal oxide particles. When a cationic acrylic polymer and an amphoteric acrylic polymer are used in combination as a dispersant, the amount of the amphoteric acrylic polymer is preferably within this range.

[0075] <Binder resin> The binder resin is not particularly limited, and examples include cellulosic resins such as methylcellulose, ethylcellulose, ethylhydroxyethylcellulose, and nitrocellulose, as well as polymers soluble in organic solvents such as acrylic resins and butyral resins. The binder resin is preferably one with a weight-average molecular weight of about 20,000 to 200,000.

[0076] The binder resin content relative to the entire conductive paste is preferably 0.5 to 10% by mass, more preferably 1 to 6% by mass, and even more preferably 2 to 5% by mass. The amount of binder resin is preferably 1 to 15 parts by mass, more preferably 2 to 10 parts by mass, and even more preferably 3 to 8 parts by mass per 100 parts by mass of conductive metal particles.

[0077] <organic solvents> The organic solvent is not particularly limited as long as it is capable of dissolving the binder resin described above. Examples of organic solvents include acetate solvents such as isobonyl acetate, isobonyl propinate, isobonyl butyrate and isobonyl isobutyrate, ethylene glycol monobutyl ether acetate and dipropylene glycol methyl ether acetate; terpene solvents such as terpineol, dihydroterpineol, dihydroterpineol acetate and dihydroterpineol propionate; and hydrocarbon solvents such as tridecane, nonane and cyclohexane.

[0078] The organic solvent content relative to the entire conductive paste is preferably 20 to 60% by mass, more preferably 25 to 55% by mass, and even more preferably 30 to 50% by mass. The amount of organic solvent is preferably 40 to 120 parts by mass, more preferably 50 to 110 parts by mass, even more preferably 60 to 100 parts by mass, and may also be 65 to 95 parts by mass or 70 to 90 parts by mass per 100 parts by mass of conductive metal particles.

[0079] <Other ingredients> The conductive paste may contain components other than those listed above. These other components include additives such as viscosity modifiers, plasticizers, flame retardants, and settling inhibitors. Furthermore, metal components capable of forming alloys with conductive metal particles during the paste's firing process may be incorporated. For example, if the conductive metal particles are Ni particles, incorporating metal components such as Sn, Au, or Cr will allow these metals to form alloys with Ni during firing, suppressing a decrease in the capacitance of the MLCC. These metal components can be incorporated, for example, in the form of particles with a primary particle diameter of approximately 10 nm or as metal salts.

[0080] [Preparation of conductive paste] A conductive paste can be obtained by mixing the above components. All components may be mixed at once, added sequentially, or some components may be mixed in advance. For example, a dispersion (slurry) may be prepared by mixing conductive metal particles and / or metal oxide particles with a dispersant and an organic solvent, and the binder resin may be added to the slurry. The organic solvent may also be added at the same time as the binder resin. The binder resin may be added in the form of a vehicle dissolved or dispersed in the organic solvent.

[0081] Mixing, stirring, and kneading of each component is performed using stirring and kneading equipment such as Ishikawa stirrers, Silverson stirrers, cavitation stirrers, planetary stirrers, ultra-thin film high-speed rotary dispersers, ultrasonic dispersers, Raikai machines, twin-shaft kneaders, bead mills, ball mills, three-roll mills, homogenizers, planetary mixers, ultra-high-pressure dispersers, thin-layer shear dispersers, wet ultra-fine atomizers, and supersonic jet mills.

[0082] When preparing a dispersion (slurry) by mixing conductive metal particles and / or metal oxide particles with a dispersant and an organic solvent, the conductive metal particles, metal oxide particles, dispersant and organic solvent may be mixed at once, or the slurry containing conductive metal particles and the slurry containing metal oxide particles may be prepared separately and then mixed. Alternatively, metal oxide particles may be added to the slurry containing conductive metal particles, or conductive metal particles may be added to the slurry containing metal oxide particles.

[0083] When preparing slurries containing conductive metal particles and slurries containing metal oxide particles separately, it is preferable to incorporate a cationic group-containing acrylic polymer as a dispersant into the slurry containing conductive metal particles. It is preferable to incorporate an anionic group-containing compound into the slurry containing metal oxide particles.

[0084] If it is necessary to disperse conductive metal particles and / or metal oxide particles, dispersers such as paint shakers, roll mills, ball mills, attritors, bead mills, and ultrasonic dispersers can be used. From the viewpoint of efficiently dispersing aggregated particles, in addition to conductive metal particles and / or metal oxide particles, dispersants, and organic solvents, a dispersion medium such as inert beads may be mixed and dispersed, and then the dispersion medium may be separated and removed by filtration or the like.

[0085] [Applications of conductive paste] Conductive paste is used to form metal layers (conductor films) and electrodes that make up electronic components such as multilayer ceramic capacitors. The above-mentioned conductive paste is preferably used to form the internal electrodes of multilayer ceramic capacitors.

[0086] In the manufacturing of multilayer ceramic capacitors, a conductive paste is printed at a predetermined position on an unfired dielectric sheet (green sheet), and then dried to form a dried film of the conductive paste. From the viewpoint of thinning the internal electrodes, it is preferable that the thickness of the dried film of the conductive paste be 1 μm or less.

[0087] Multiple sheets, each having a dried conductive paste film formed on a dielectric sheet, are stacked and then heat-pressed together to create an alternating laminate of dielectric sheets and dried conductive paste films. As mentioned above, it is preferable that the dielectric material of the dielectric sheet is the same material as the metal oxide particles of the conductive paste.

[0088] The laminate is cut to a predetermined size and subjected to a binder removal treatment in an oxidizing or inert atmosphere. In the binder removal treatment, the binder is volatilized by heating in a temperature range in which the conductive metal particles and metal oxide particles do not sinter (generally less than 500°C, preferably 200-400°C).

[0089] Subsequently, by firing the ceramic body in a reducing atmosphere to prevent oxidation of the internal electrodes, a ceramic substrate with the internal electrodes formed is obtained. By forming external electrodes on this ceramic substrate, a multilayer ceramic capacitor is obtained.

[0090] The firing temperature is, for example, 1000 to 1400°C. Firing completely removes the binder from the dielectric sheet, and the dielectric material sintersects, forming a dielectric layer. Simultaneously, the resin binder in the dried conductive paste film is completely removed, and the conductive metal particles are sintered and integrated, forming internal electrodes.

[0091] Generally, conductive metal particles in conductive paste have a lower sintering start temperature than the dielectric material in the dielectric sheet. The dry film of the conductive paste sintersects and shrinks before the dielectric sintersects. If the difference between the sintering start temperature of the conductive paste and the dielectric sheet is large, the difference in shrinkage behavior during firing is significant, leading to cracks and poor coverage in the internal electrodes, which causes a decrease in capacitance. In particular, when the thickness of the internal electrodes is small, cracks and poor coverage due to the difference in shrinkage behavior are more likely to occur, and the decrease in capacitance becomes more pronounced.

[0092] In this invention, a conductive paste with a high sintering start temperature can be obtained by using a predetermined dispersant. As the sintering start temperature of the conductive paste increases, the difference between the sintering start temperature of the conductive paste and the sintering start temperature of the dielectric sheet becomes smaller, and consequently the difference in shrinkage behavior decreases, thereby suppressing cracks and poor coverage of the internal electrodes.

[0093] When the conductive metal particles of the conductive paste are Ni particles, the sintering start temperature of the conductive paste is preferably 900°C or higher, more preferably 930°C or higher, even more preferably 950°C or higher, and may be 955°C or higher or 960°C or higher. The sintering start temperature of the conductive paste can be measured by thermomechanical analysis (TMA) of the dried film of the conductive paste, as described in the examples below.

[0094] As described above, the conductive paste of the present invention is suitably used for forming internal electrodes in multilayer ceramic capacitors, but its applications are not limited to multilayer ceramic capacitors; it can also be used for forming electrodes and conductive films in various electronic components. [Examples]

[0095] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.

[0096] [Preparation of dispersant (acrylic polymer)] <Manufacturing Example A1> In a flask equipped with a stirrer, condenser, thermometer, inert gas inlet tube, and dropping funnel, 150.0 parts by mass of terpineol were placed, nitrogen gas was introduced as the inert gas, and the temperature was raised to 95°C. Then, under stirring, 4.3 parts by mass of N,N-dimethylaminopropylacrylamide (DMAPAA) as a cationic group-containing monomer, 95.7 parts by mass of n-octyl acrylate (NOA) as a nonionic monomer, and 8.0 parts by mass of 2,2'-azobis(2-methylbutyronitrile) (AMBN) as a polymerization initiator were added dropwise through the dropping funnel over 3 hours. After 30 minutes, 0.5 parts by mass of t-hexyl=2-ethylhexaneperoxoate (t-HP2EH) was added as a polymerization initiator, and after another 30 minutes, 0.5 parts by mass of t-HP2EH was added, and the mixture was reacted for 2 hours to obtain a solution of cationic acrylic polymer A1 (solid content concentration 40%).

[0097] <Manufacturing examples A2~A4> Solutions of cationic acrylic polymers A2 to A4 were obtained in the same manner as in production example A1, except that the monomer content was changed as shown in Table 1.

[0098] <Manufacturing example B1> The reaction temperature was changed to 90°C, and the monomer composition was changed to 4.4 parts by mass of DMAPAA as a cationic group-containing monomer, 2.4 parts by mass of methacrylic acid (MA) as an anionic group-containing monomer, and 93.2 parts by mass of NOA as a nonionic monomer. Otherwise, the process was carried out in the same manner as in Production Example A1 to obtain a solution of the amphoteric acrylic polymer B1 (solid content concentration 40%).

[0099] <Manufacturing examples B2~B5> Solutions of the zwitterionic acrylic polymers B2 to B5 were obtained in the same manner as in production example B1, except that the monomer amounts were changed as shown in Table 1.

[0100] <Manufacturing example C1> A solution of anionic acrylic polymer C1 (solid content concentration 40%) was obtained in the same manner as in Production Example A1, except that the monomer composition was 2.4 parts by mass of MA as an anionic group-containing monomer and 97.6 parts by mass of NOA as a nonionic monomer.

[0101] Table 1 shows the monomer ratio of the polymers in the above manufacturing example and the measured amine value per unit mass of the polymer solids, measured in accordance with JIS K 7237.

[0102] [Table 1]

[0103] [Example 1] In the dispersant solution of the amphoteric acrylic polymer B1 obtained in Production Example B1 (solid content concentration 40% by mass): 2.4 parts by mass, Ni particles with an average primary particle size of 150 nm (manufactured by Guangbo Group Co., Ltd., product name "NiGB-0151"): 13.0 parts by mass, dihydroterpineol acetate as a solvent (manufactured by Nippon Terpene Chemical): 4.6 parts by mass, and zirconia beads with a particle size of 0.1 mm as a dispersion medium: 20.0 parts by mass were placed in a 70 mL glass bottle and processed at 60 Hz for 2.5 hours using a disperser (rocking shaker RS-05W, manufactured by Seiwa Giken Co., Ltd.) to disperse the Ni particles. After that, the zirconia beads were removed by filtration to obtain a dispersion (slurry) of conductive metal (Ni) particles. The solid content concentration of this dispersion was 70% by mass.

[0104] In the preparation of the metal slurry described above, 2.4 parts by mass of a dispersant solution (solid content concentration 40%) of the amphoteric acrylic polymer B1 obtained in production example B1, 8.0 parts by mass of BaTiO3 particles (Toda Kogyo Co., Ltd. "T-BTO-010RF") with an average primary particle diameter of 30 nm as metal oxide particles, 9.6 parts by mass of dihydroterpineol acetate (Nippon Terpene Chemical Co., Ltd.) as a solvent, and 150.0 parts by mass of zirconia beads with a particle diameter of 0.05 mm as a dispersion medium were placed in a 70 mL glass bottle. Dispersion using a disperser and filtration of the zirconia beads were carried out in the same manner as the preparation of the metal slurry described above to obtain a dispersion (slurry) of metal oxide (BaTiO3) particles. The solid content concentration of this dispersion was 45% by mass.

[0105] 74.7 parts by mass of the Ni particle slurry and 11.9 parts by mass of the BaTiO3 particle slurry obtained above were stirred for 3 minutes in a rotary-orbiting stirrer (Kurabo "MAZERUSTAR"). To the mixed slurry, 1.5 parts by mass of butyral resin (Sekisui Material Solutions "SV-12") and 1.5 parts by mass of ethylcellulose resin (Dow Chemical "Product Name: STD-4") as binder resins, and 10.4 parts by mass of dihydroterpineol acetate (Nippon Terpene Chemical) as a solvent were added, and the mixture was stirred for 3 minutes in a rotary-orbiting stirrer to obtain a conductive paste.

[0106] [Examples 2-5] A conductive paste was obtained in the same manner as in Example 1, except that the dispersants for the metal particle slurry and metal oxide particle slurry were changed to amphoteric acrylic polymers B2 to B5.

[0107] [Example 6] A conductive paste was obtained in the same manner as in Example 1, except that the dispersant of the metal particle slurry was changed to cationic acrylic polymer A1 obtained in Production Example A1.

[0108] [Examples 7-10] A conductive paste was obtained in the same manner as in Example 6, except that the dispersant for the metal particle slurry was changed to dispersant 1 shown in Table 2, and the dispersant for the metal oxide particle slurry was changed to dispersant 2 shown in Table 2. In Example 8, a mixture of polymer A1 and polymer A2 in a 1:1 mass ratio was used as the dispersant for the metal particle slurry. Note that in Table 2, the amine value of dispersant 1 in Example 8 is the amine value of the mixed polymer.

[0109] [Comparative Example 1] A conductive paste was obtained in the same manner as in Example 1, except that the dispersant for the metal particle slurry and the dispersant for the metal oxide particle slurry were changed to the anionic acrylic polymer C1 obtained in Production Example C1.

[0110] [Comparative Example 2] In the preparation of the metal oxide slurry in Example 1, 9.5 parts by mass of a dispersant solution of the anionic acrylic polymer C1 obtained in Production Example C1 (solid content concentration 40%), 8.0 parts by mass of BaTiO3 particles with an average primary particle diameter of 30 nm (Toda Industries Co., Ltd. "T-BTO-010RF") as metal oxide particles, 20.2 parts by mass of dihydroterpineol acetate (Nippon Terpene Chemical Co., Ltd.) as a solvent, and 150.0 parts by mass of zirconia beads with a particle size of 0.05 mm as a dispersion medium were placed in a 70 mL glass bottle. Dispersion using a disperser and filtration of the zirconia beads were carried out in the same manner as in the preparation of the metal oxide slurry in Example 1 to obtain a dispersion (slurry) of BaTiO3 particles. The solid content concentration of this dispersion was 31% by mass.

[0111] To 37.5 parts by mass of the obtained BaTiO3 particle slurry, 49.1 parts by mass of Ni particles (manufactured by Guangbo Group Co., Ltd., product name "NiGB-0151") with an average primary particle diameter of 150 nm were added and the mixture was stirred for 3 minutes using a rotary-orbiting stirrer. After mixing, a binder resin and solvent were added to the slurry, as in Example 1, and the mixture was stirred for 3 minutes using a rotary-orbiting stirrer to obtain a conductive paste.

[0112] [Comparative Example 3] A conductive paste was obtained in the same manner as in Example 10, except that N-lauryldiethanolamine (labeled "A11" in Table 2), a cationic compound, was used as a dispersant for the metal particle slurry instead of the cationic acrylic polymer A1.

[0113] [Comparative Example 4] A conductive paste was obtained in the same manner as in Example 1, except that the dispersant for the metal particle slurry and the metal oxide particle slurry was changed to 6-aminohexanoic acid (labeled "B11" in Table 2), which is an amphoteric compound.

[0114] [Comparative Example 5] A conductive paste was obtained in the same manner as in Example 1, except that the dispersant for the metal particle slurry and the metal oxide particle slurry was changed to an amphoteric polymer, which is an alkylammonium polyphosphate salt (BYK's "DISPER BYK-140", hereinafter referred to as "B12").

[0115] [evaluation] An appropriate amount of conductive paste was applied to a PET film, and the solvent was removed by heating at 150°C for 1 hour to obtain a dry coating of conductive paste. The obtained dry coating was peeled off the PET film and weighed out 0.2 g. The weighed sample was formed into a cylindrical shape with a diameter of 5 mm and a height of 2 mm by applying a pressure of 6 MPa using a compression molding machine (NPA Systems "Carbide Die DT3025A-0525") to obtain a sample for measurement. Thermomechanical analysis (TMA) of the sample was performed using a thermomechanical analyzer (Rigaku "TMA8311") under the following conditions. Atmosphere: Nitrogen / Hydrogen = 97% / 3% Temperature range: Room temperature to 1100℃ Heating rate: 5°C / min

[0116] In the obtained TMA curve, the sintering start temperature was defined as the intersection point of the tangent to the TMA curve in the temperature region where there is no shrinkage before the start of sintering, and the tangent to the TMA curve at the minimum point of the differential curve (DTMA) (the temperature at which the amount of shrinkage per unit temperature is maximum).

[0117] [Evaluation Results] Table 2 shows the formulations and sintering start temperatures of the conductive pastes in the examples and comparative examples. In Table 2, the formulation amounts are the amount (parts by mass) of each component relative to 100 parts by mass of the total conductive paste, and the amount of dispersant is the solid content. In examples where different types of dispersants were used for metal particle slurry and metal oxide particle slurry, the dispersant for the metal particle slurry is listed as "Dispersant 1" and the dispersant for the metal oxide particle slurry is listed as "Dispersant 2".

[0118] [Table 2]

[0119] In Comparative Example 1, where anionic acrylic polymer C1 was added as a dispersant to both the metal particle slurry and the metal oxide particle slurry, the sintering start temperature of the paste was less than 900°C. In Comparative Example 2, where anionic acrylic polymer C1 was added as a dispersant only to the metal oxide particle slurry, the sintering start temperature was also less than 900°C.

[0120] In contrast, in Examples 1-5, which used an amphoteric acrylic polymer as a dispersant, the sintering start temperature was above 900°C in all cases. In Examples 6-9, which used a combination of cationic acrylic polymer and amphoteric acrylic polymer as a dispersant, and in Example 10, which used a combination of cationic acrylic polymer and anionic acrylic polymer as a dispersant, the sintering start temperature was also above 900°C.

[0121] In Example 10, which used cationic acrylic polymer A1 as a cationic dispersant, the paste's sintering start temperature was 949°C, whereas in Comparative Example 3, which used cationic compound A11 (N-lauryldiethanolamine), the paste's sintering start temperature was less than 900°C. Similar trends were observed when comparing Example 1, which used amphoteric acrylic polymer B1 as an amphoteric dispersant, with Comparative Example 4, which used amphoteric compound B11 (6-aminohexanoic acid), and Comparative Example 5, which used amphoteric polyphosphate ester salt B12, as described above for Example 10 and Comparative Example 3.

[0122] These results show that using a cationic group-containing acrylic polymer (cationic acrylic polymer and / or zwionic acrylic polymer) as a dispersant significantly increases the sintering start temperature of the paste.

[0123] A comparison of Examples 1-5 and Examples 6-9 shows that the sintering start temperature of the paste can be further increased by adjusting the amine value of the cationic group-containing acrylic polymer used as a dispersant.

[0124] A comparison between Example 1 and Examples 6 and 8, and between Example 3 and Example 7, reveals that the effect of raising the sintering start temperature is particularly significant when a cationic acrylic polymer and an amphoteric acrylic polymer are used in combination as dispersants.

Claims

1. A conductive paste comprising conductive metal particles, metal oxide particles, a dispersant, a binder resin, and an organic solvent, The dispersant includes cationic acrylic polymers and anionic acrylic polymers. Conductive paste.

2. The conductive paste according to claim 1, wherein the amine value of the cationic acrylic polymer is 0.1 to 100 mg KOH / g.

3. The conductive paste according to claim 1 or 2, wherein the acid value of the anionic acrylic polymer is 0.1 to 100 mg KOH / g.

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