Conductive paste, method for manufacturing same, and conductive film

Abstract

This conductive paste contains a conductor, a binder resin, and cellulose nanofibers. When the content of the conductor is 100 parts by mass, the content of the binder resin is 0.5-2 parts by mass, and the content of the cellulose nanofibers is 0.175-0.35 parts by mass.

Description

Conductive paste, manufacturing method thereof, and conductive film

[0001] The present disclosure relates to a conductive paste, a method for producing the same, and a conductive film.

[0002] As electronic devices become smaller, there is a demand for finer wiring. As a conductive paste for forming finer wiring, Patent Document 1 discloses a conductive paste containing a binder resin and cellulose nanofibers.

[0003] Japanese Patent Application Publication No. 2019-175675

[0004] A conductive paste according to one embodiment of the present disclosure contains a conductor, a binder resin, and cellulose nanofibers, and when the content of the conductor is 100 parts by mass, the content of the binder resin is 0.5 parts by mass or more and 2 parts by mass or less, and the content of the cellulose nanofibers is 0.175 parts by mass or more and 0.35 parts by mass or less.

[0005] Furthermore, a method for producing a conductive paste according to one embodiment of the present disclosure includes a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

[0006] Fig. 1 is a schematic diagram showing a conductive film according to one embodiment of the present disclosure formed on a substrate. Fig. 2 is a cross-sectional view taken along line II of the conductive film shown in Fig. 1. Fig. 3 is a schematic cross-sectional view showing an example of a process for dispersing a conductor, a binder resin, and cellulose nanofibers in an organic solvent using a high-pressure disperser.

[0007] According to one aspect of the present disclosure, it is possible to improve the productivity and conductivity of conductive films that use conductive pastes.

[0008] An embodiment of the present disclosure will be described in detail below. However, the present disclosure is not limited thereto, and various modifications are possible within the described scope. For example, embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present disclosure. Unless otherwise specified in this specification, "A to B" representing a numerical range means "greater than or equal to A and less than or equal to B."

[0009] [1. Conductive Paste] The conductive paste of the present disclosure contains a conductor, a binder resin, and cellulose nanofibers, and when the content of the conductor is taken as 100 parts by mass, the content of the binder resin is 0.5 parts by mass or more and 2 parts by mass or less, and the content of the cellulose nanofibers is 0.175 parts by mass or more and 0.35 parts by mass or less.

[0010] The conductive paste of the present disclosure contains less binder resin than conventional conductive pastes, and the binder resin is efficiently removed during sintering, which can contribute to the efficient production of conductive films with low resistivity.

[0011] Furthermore, the conductive paste of the present disclosure contains a small amount of binder resin, but instead contains cellulose nanofibers in the above-mentioned amount, which results in an appropriate viscosity, excellent sedimentation stability, and excellent printability.

[0012] (1) Conductor The conductor has the function of imparting conductivity to the conductive film. The conductor may be any metal, and may be a conductor commonly used in conductive pastes. For example, copper, silver, nickel, gold, etc. may be used. Among these, copper may be used as the conductor because of its excellent conductivity.

[0013] The shape of the conductor is not particularly limited and may be various shapes such as spherical, flake, etc. The average particle size of the conductor may be 0.1 to 20 μm, 1 to 10 μm, or 2 to 5 μm in terms of 50% average particle size (D50). The conductor may be a powder that is an aggregate of particles having such an average particle size.

[0014] D50 can be measured by laser diffraction. For example, 0.3 g of the conductor is weighed into a 50 ml beaker, 30 ml of IPA is added, and the conductor is dispersed in the IPA by treating with an ultrasonic cleaner for 5 minutes. D50 can then be measured using a laser diffraction particle size distribution analyzer. Here, IPA is isopropyl alcohol. As the ultrasonic cleaner, for example, a UP200S manufactured by Hielscher can be used, and as the laser diffraction particle size distribution analyzer, for example, an LA-950 manufactured by Horiba, Ltd. can be used.

[0015] (2) Binder Resin The binder resin has the function of dispersing the conductors and also has the function of bringing the conductors into contact with each other when cured, thereby providing electrical conductivity.

[0016] The binder resin is not particularly limited, and examples thereof include epoxy resin, phenol resin, polyester resin, acrylic resin, polyurethane resin, acrylic urethane resin, blocked isocyanate compound, etc. Examples of epoxy resins include bisphenol A type, bisphenol F type, phenol novolac type, polyhydric alcohol type, etc. One type of binder resin may be used, or two or more types may be used in combination.

[0017] Among these, acrylic resins may be used because of their good thermal decomposition properties.

[0018] (3) Cellulose nanofibers Cellulose nanofibers are cellulose fibers with a number-average fiber diameter on the nanometer order. "Nanometer order" refers to a diameter greater than 0 nm and less than 1,000 nm. The number-average fiber diameter may be, for example, 3 to 800 nm, 3 to 100 nm, or 3 to 30 nm.

[0019] The number-average fiber diameter can be measured, for example, as follows. First, a dispersion of cellulose nanofibers in an organic solvent with a solid content of 0.05 to 0.1% by mass is prepared. The preparation method is based on the method described in (7) of the Evaluation Methods in the Examples below. The dispersion is cast onto a grid coated with a hydrophilic carbon film to prepare a sample for observation with a transmission electron microscope (TEM). The sample is observed with a TEM at a magnification of 5,000x, 10,000x, or 50,000x. In this case, an axis with arbitrary vertical and horizontal image widths is assumed within the image, and the observation conditions, such as the sample and magnification, are adjusted so that 20 or more fibers intersect with the axis. For observation images that satisfy the adjusted observation conditions, two random vertical and two random horizontal axes are set per image, and the diameters of the fibers intersecting the axes are visually determined. As described above, at least three non-overlapping surface images are captured with the TEM, and the diameters of the fibers intersecting the two axes are determined for each image. The arithmetic mean of the obtained diameters is defined as the number average fiber diameter of the cellulose nanofibers.

[0020] The cellulose constituting the cellulose nanofibers may have a crystalline structure of type I. Whether the cellulose has a crystalline structure of type I can be determined, for example, based on the presence of typical peaks in the vicinity of 2θ = 14° to 17° and 2θ = 22° to 23° in the diffraction profile obtained by measuring a wide-angle X-ray diffraction image.

[0021] The cellulose nanofibers may be anion-modified cellulose nanofibers in which anionic groups have been introduced into glucose units in cellulose molecules. The anionic groups may be, for example, one or more groups selected from the group consisting of carboxyl groups, phosphate groups, sulfonate groups, and sulfate groups. The anionic groups may be in either an acid form or a salt form.

[0022] The cellulose nanofibers may be oxidized cellulose nanofibers in which the hydroxyl group at the C6 position of the glucose unit contained in the cellulose molecule is selectively oxidized to a carboxyl group.

[0023] Oxidized cellulose nanofibers can be obtained, for example, by oxidizing native cellulose using sodium hypochlorite as a co-oxidant and 2,2,6,6-tetramethylpiperidinooxy radical, an N-oxyl compound, as a catalyst. Specific methods for obtaining oxidized cellulose nanofibers include the method described in (7) of the "Evaluation Methods" section of the Examples below.

[0024] The amount of anionic groups in the cellulose nanofibers may be, for example, 0.05 to 3.0 mmol, or 0.5 to 2.5 mmol. The amount of anionic groups can be measured, for example, by the method described in Patent Document 1.

[0025] The cellulose nanofibers may be dispersed in an organic solvent. This promotes dispersion of the cellulose nanofibers in the conductive paste, facilitating preparation of the conductive paste. The organic solvent may be a terpene-based solvent such as terpineol, or a glycol ether-based solvent such as butyl carbitol or butyl carbitol acetate. One type of organic solvent may be used, or two or more types may be used in combination.

[0026] (4) Component Content in Conductive Paste The component content in the conductive paste of the present disclosure is the content of each component as a solid content. In the conductive paste of the present disclosure, when the content of the conductor is 100 parts by mass, the content of the binder resin is 0.5 parts by mass or more and 2 parts by mass or less, and the content of the cellulose nanofiber is 0.175 parts by mass or more and 0.35 parts by mass or less. Furthermore, the percentage of the mass of the binder resin relative to the total mass of the conductor and the mass of the binder resin may be less than 2.

[0027] The binder resin has the above-mentioned functions. However, if carbon or other substances derived from the binder resin remain in the conductive film formed by curing the conductive paste, this adversely affects the conductivity of the conductive film. Therefore, it is necessary to decompose and remove the binder resin contained in the conductive paste so that no carbon or other substances remain in the conductive film.

[0028] If the binder resin content in the conductive paste is high, removing the binder resin requires a great deal of effort even when the conductive paste is sintered at high temperatures, which reduces the productivity of the conductive film.On the other hand, if the content is too low, the viscosity of the conductive paste decreases, which reduces the printability of the conductive paste.

[0029] Therefore, in this disclosure, we have conducted extensive research with the aim of ensuring the printability of the conductive paste and obtaining a conductive film with high conductivity with high productivity.As a result, we have reduced the binder resin content compared to conventional conductive pastes and added cellulose nanofibers, so that the binder resin and cellulose nanofiber contents are within the above-mentioned ranges.

[0030] In this case, the binder resin is contained in an amount sufficient to ensure the above-mentioned functions. Meanwhile, because part of the binder resin is replaced by cellulose nanofiber, the cellulose nanofiber can compensate for the decrease in viscosity of the conductive paste that accompanies the reduction in the binder resin content.

[0031] This ensures the above-mentioned functions of the binder resin while maintaining an appropriate viscosity of the conductive paste, thereby improving printability.

[0032] Furthermore, since the binder resin content is reduced compared to conventional conductive pastes, the binder resin can be easily removed by sintering, thereby improving the productivity and conductivity of the conductive film.

[0033] The conductive paste of the present disclosure may contain an organic solvent. The organic solvent may dissolve the binder resin without chemically reacting with the binder resin. The organic solvent serves as a dispersion medium for the conductor and the cellulose nanofibers.

[0034] Examples of the organic solvent include alcohol solvents such as terpineol, methanol, ethanol, isopropyl alcohol, isobutyl alcohol, 1-butanol, diacetone alcohol, ethylene glycol, and glycerin; hydrocarbon solvents such as toluene and xylene; ester solvents such as ethyl acetate and butyl acetate; ketone solvents such as acetone and methyl ethyl ketone; ether solvents such as ethylene glycol dimethyl ether and tetrahydrofuran; and amide solvents such as N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. One of the organic solvents may be used, or two or more of them may be used in combination.

[0035] In consideration of the ease of preparing the conductive paste, the organic solvent may be an alcohol-based solvent, and in particular, terpineol.

[0036] The content of the organic solvent in the conductive paste of the present disclosure may be 15.0 to 25.0 parts by mass, or 19.0 to 22.5 parts by mass, when the content of the conductor is 100 parts by mass, including when the cellulose nanofibers are dispersed in the organic solvent.

[0037] In the conductive paste of the present disclosure, when the content of the conductor is 100 parts by mass, the content of the binder resin may be 0.5 parts by mass or more and 1 part by mass or less, and the content of the cellulose nanofiber may be 0.175 parts by mass or more and 0.25 parts by mass or less.

[0038] In this case, the binder resin content and the cellulose nanofiber content are more favorable for ensuring the functionality of the binder resin and compensating for the decrease in viscosity of the conductive paste that accompanies a decrease in the binder resin content, thereby providing a conductive paste with superior printability and a conductive film with superior productivity and conductivity.

[0039] (5) Physical Properties of Conductive Paste (5-1) Dispersibility and Sedimentation Stability of Conductive Paste The dispersibility of the conductive paste of the present disclosure refers to the degree of sedimentation of the conductor immediately after the production of the conductive paste, and the sedimentation stability refers to the degree of sedimentation of the conductor after leaving the paste to stand overnight in an environment of 25° C. The evaluation methods for these will be described later in the examples.

[0040] The conductive paste of the present disclosure has binder resin and cellulose nanofiber contents specified within the aforementioned ranges, and therefore has excellent dispersibility and sedimentation stability, as will be shown in the examples described below.

[0041] (5-2) Viscosity The viscosity of the conductive paste of the present disclosure at a shear rate of 4 / S may be 120 to 230 Pa s or 140 to 210 Pa s. The viscosity of the conductive paste of the present disclosure at a shear rate of 40 / S may be 30 to 90 Pa s or 50 to 70 Pa s. The method for measuring viscosity will be described later in the examples.

[0042] The conductive paste of the present disclosure has an appropriate viscosity and, as described above, also has excellent dispersibility and sedimentation stability, and therefore has excellent printability and can produce a good wiring pattern.

[0043] The conductive paste of the present disclosure can be printed on a substrate by, for example, screen printing, spin coating, bar coating, die coating, blade coating, gravure coating, roll coating, spray coating, dipping, etc. Among these, screen printing is preferred. The substrate can be a ceramic substrate such as an alumina substrate, a glass substrate, a printed wiring board, or a flexible substrate such as a PET film.

[0044] [2. Conductive Film] The conductive film of the present disclosure is a conductive film obtained by curing the conductive paste of the present disclosure, and has a thickness of 14 μm or more and 17.5 μm or less and a length in the short direction of 20 μm or more and 25 μm or less.

[0045] As described above, the conductive paste of the present disclosure has a lower binder resin content than conventional conductive pastes and contains cellulose nanofibers to compensate, which results in excellent printability, making it possible to provide high-density wiring having the above-mentioned thickness and short-side length.

[0046] Furthermore, since the conductive film of the present disclosure has a small amount of residual carbon resulting from the binder resin, the conductive film obtained from the conductive paste has very low resistivity and can be made thick and thin.

[0047] The conductive paste can be cured, for example, by printing the conductive paste on a substrate and drying it in a hot air dryer at 80 to 120° C. for 30 to 120 minutes.

[0048] Fig. 1 is a schematic diagram showing a conductive film according to one embodiment of the present disclosure formed on a substrate. Fig. 2 is a cross-sectional view of the conductive film shown in Fig. 1 taken along line II. Figs. 1 and 2 are merely schematic diagrams and do not accurately depict the conductive film according to one embodiment of the present disclosure. Reference numeral 1 denotes the conductive film, and 2 denotes the substrate. The cross section of the conductive film 1 taken along line II is a rectangular parallelepiped, but of course this is not a limitation, and the shape of the cross section taken along line II is not limited.

[0049] The "thickness" is the length of the maximum perpendicular line when a perpendicular line is drawn from a tangent line between the conductive film 1 and the substrate 2 to the surface of the conductive film 1 facing the substrate 2 in the cross section of the conductive film 1 taken along line II. For example, the thickness of the conductive film 1 is height A shown in FIGS. 1 and 2.

[0050] The "length in the short side direction" is the length of the tangent line formed by the conductive film 1 and the substrate 2 in the cross section of the conductive film 1 taken along line II. For example, the length in the short side direction of the conductive film 1 is length B shown in Figures 1 and 2. In this specification, the length in the short side direction is also referred to as "line width."

[0051] The thickness and the length in the short side direction can be measured by the method described in the examples below.

[0052] In the conductive film of the present disclosure, a plurality of the conductive films may be arranged opposite each other with a space therebetween, and the distance between the opposing conductive films may be 20 μm or less.

[0053] As described above, the conductive paste of the present disclosure has a lower binder resin content than conventional conductive pastes and contains cellulose nanofibers to compensate for this, resulting in superior printability. As a result, it is possible to provide higher density wiring, with not only the thickness and short-side length but also the distance between opposing conductive films being 20 μm or less.

[0054] The phrase "plurality of conductive films facing each other across a space" may mean that a plurality of independent conductive films face each other across a space, as shown in Fig. 1. Also, for example, one conductive film formed in a curved shape on a substrate may have a plurality of surfaces facing each other across a space.

[0055] The "distance between opposing conductive films" (hereinafter also referred to as "pitch") is the length of the shortest straight line when a straight line is drawn parallel to the substrate from one surface to the other surface of the opposing conductive films separated by a space. For example, in Figure 2, the pitch is distance C.

[0056] The lower limit of the pitch is not particularly limited, but may be, for example, 15 μm or 10 μm. The pitch can be measured using, for example, a microscope VHX series manufactured by Keyence Corporation.

[0057] The conductive film of the present disclosure has a specific resistance of 2.00×10 －８ The specific resistance may be Ω·m or less. As described above, the conductive paste of the present disclosure contains a smaller amount of binder resin than conventional conductive pastes, and therefore the binder resin can be easily removed by sintering. Therefore, the conductive film of the present disclosure has an extremely low specific resistance and exhibits excellent conductivity.

[0058] The resistivity is measured after sintering the conductive film of the present disclosure. The measurement method may be the method described in the Examples below. Sintering can be performed, for example, by treating the conductive film at 800°C for 10 minutes in a nitrogen atmosphere. By treating the conductive film under these conditions, the binder resin and cellulose nanofibers contained in the conductive paste are removed from the conductive film.

[0059] 3. Method for producing conductive paste The conductive paste of the present disclosure can be produced by mixing 0.5 parts by mass or more and 2 parts by mass or less of binder resin and 0.175 parts by mass or more and 0.35 parts by mass or less of cellulose nanofibers with an organic solvent, where the mass of the conductor is 100 parts by mass.

[0060] The content of the binder resin may be 0.5 parts by mass or more and 1 part by mass or less, and the content of the cellulose nanofibers may be 0.175 parts by mass or more and 0.25 parts by mass or less.

[0061] The content of the organic solvent in the conductive paste, including the organic solvent in the case where the cellulose nanofibers are dispersed in the organic solvent, may be 15.0 to 25.0 parts by mass or 19.0 to 22.5 parts by mass, relative to 100 parts by mass of the conductor content.

[0062] The mixing can be carried out, for example, by placing the conductor, binder resin, cellulose nanofibers, and organic solvent in a container such as a beaker and stirring them with a mixer.

[0063] The method for producing a conductive paste according to the present disclosure may include a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

[0064] According to the above configuration, the high-pressure disperser further enhances the dispersibility of the conductor and cellulose nanofibers in the organic solvent. Therefore, a conductive paste with a suitable viscosity can be efficiently obtained while reducing the binder resin content compared to conventional conductive pastes. Hereinafter, the conductor, binder resin, cellulose nanofiber, and organic solvent are also referred to as the "sample." When using a high-pressure disperser, the amount of sample used can be the same as in the aforementioned manufacturing method using mixing.

[0065] The high-pressure disperser may be one that feeds a sample into a nozzle while applying high pressure, applies shear force to the sample inside the nozzle, and atomizes the cellulose nanofibers in the sample while uniformly mixing the conductor and the cellulose nanofibers. For example, the Nanovater NVL-AS200-D14 manufactured by Yoshida Kikai can be used as the high-pressure disperser.

[0066] 3 is a cross-sectional schematic diagram showing an example of a process for dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser. A conductor, a binder resin, cellulose nanofibers, and an organic solvent are stored in a sample reservoir 10. The sample may be in the form of a slurry. The sample is introduced from the sample reservoir 10 into a cylinder 11, and pressure is applied to the slurry by a plunger 12, which then introduces the sample into a nozzle 13. The manner of pressurization is not limited to that using the plunger 12. The number of nozzles 13 may be one or more.

[0067] The nozzle 13 applies shear force to the sample by, for example, generating turbulence therein. The shear force atomizes the cellulose nanofibers in the sample, dispersing them together with the conductor into the organic solvent, which is then ejected from the nozzle 13 and collected in the ejection liquid storage tank 14. As a result, a conductive paste can be obtained in which the conductor and cellulose nanofibers are sufficiently dispersed in the organic solvent. As shown in the examples described below, the conductive paste obtained by the conductive paste manufacturing method of the present disclosure exhibits good dispersibility and sedimentation stability.

[0068] The process includes a step of injecting the conductor, binder resin, cellulose nanofibers, and organic solvent into a nozzle of the high-pressure disperser under pressure, and the process may satisfy the following condition (i) or (ii): (i) the nozzle diameter is greater than 100 μm and less than 200 μm, and the pressure applied to the conductor, binder resin, cellulose nanofibers, and organic solvent before injection into the nozzle is 100 MPa or more and 150 MPa or less; (ii) the nozzle diameter is 100 μm or more and less than 200 μm, and the pressure applied to the conductor, binder resin, cellulose nanofibers, and organic solvent before injection into the nozzle is 50 MPa or more and 100 MPa or less.

[0069] By using the nozzle with the above diameter and pressure, the shear force applied to the cellulose nanofibers is stronger than in cases other than (i) or (ii), which improves the dispersibility of the conductor and cellulose nanofibers in the organic solvent. This makes it possible to more efficiently obtain a conductive paste with an appropriate viscosity while reducing the binder resin content compared to conventional conductive pastes.

[0070] "The sample is introduced into the nozzle of the high-pressure disperser while being pressurized" means that the sample is introduced into the nozzle while a higher pressure is being applied to the sample than before it was introduced into the high-pressure disperser. The pressure may be the pressure described in (i) or (ii) above. For example, the pressure applied when the sample is present in the cylinder 11 in Figure 3 is greater than the pressure applied when the sample is present in the sample reservoir 10.

[0071] The nozzle diameter refers to the diameter of the sample inlet in the nozzle, and is the diameter of the largest inscribed circle relative to the two-dimensional shape of the inlet. For example, if the two-dimensional shape of the inlet is circular, the diameter is the diameter of the circle, and if it is elliptical, the diameter is the minor axis of the ellipse. Figure 3 shows an example in which nozzle 13 has one circular inlet, and the diameter of the inlet is D. The diameter of the nozzle may be the diameter described in (i) or (ii) above. The number of inlets per nozzle may be one or more.

[0072] (Summary) The present disclosure may include the following [1] to [8].

[0073] [1] A conductive paste containing a conductor, a binder resin, and a cellulose nanofiber, wherein, when the content of the conductor is 100 parts by mass, the content of the binder resin is 0.5 parts by mass or more and 2 parts by mass or less, and the content of the cellulose nanofiber is 0.175 parts by mass or more and 0.35 parts by mass or less.

[0074] [2] The conductive paste according to [1], wherein the content of the binder resin is 0.5 parts by mass or more and 1 part by mass or less, and the content of the cellulose nanofiber is 0.175 parts by mass or more and 0.25 parts by mass or less, when the content of the conductor is 100 parts by mass.

[0075] [3] The conductive paste according to [1] or [2], wherein the conductor is copper.

[0076] [4] A conductive film obtained by hardening the conductive paste according to any one of [1] to [3], wherein the conductive film has a thickness of 14 μm or more and 17.5 μm or less and a length in the short side direction of 20 μm or more and 25 μm or less.

[0077] [5] Resistivity is 2.00 x 10 －８ The conductive film according to [4], having a resistivity of Ω·m or less.

[0078] [6] The conductive film according to [4] or [5], wherein a plurality of the conductive films are opposed to each other with a space therebetween, and the distance between the opposed conductive films is 20 μm or less.

[0079] [7] A method for producing a conductive paste, comprising a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

[0080] [8] The method for producing a conductive paste according to [7], wherein the step includes a step of injecting the conductor, binder resin, cellulose nanofibers, and organic solvent into a nozzle of the high-pressure disperser while applying pressure, and the step satisfies the following condition (i) or (ii): (i) the nozzle diameter is greater than 100 μm and less than 200 μm, and the pressure applied to the conductor, binder resin, cellulose nanofibers, and organic solvent before being injected into the nozzle is 100 MPa or more and 150 MPa or less; (ii) the nozzle diameter is 100 μm or more and less than 200 μm, and the pressure applied to the conductor, binder resin, cellulose nanofibers, and organic solvent before being injected into the nozzle is 50 MPa or more and 100 MPa or less.

[0081] The invention according to the present disclosure has been described above based on the drawings and examples. However, the invention according to the present disclosure is not limited to the above-described embodiments. In other words, the invention according to the present disclosure can be modified in various ways within the scope of the present disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the invention according to the present disclosure. In other words, it should be noted that a person skilled in the art can easily make various modifications or corrections based on the present disclosure. It should also be noted that these modifications or corrections are included in the scope of the present disclosure.

[0082] The present disclosure will be explained in more detail with reference to the following examples and comparative examples, but the present disclosure should not be interpreted as being limited to these examples. Examples obtained by appropriately combining the technical means disclosed in each example are also included in the scope of the present disclosure.

[0083] <Evaluation Method> The physical properties of the conductive pastes and conductive films obtained in the examples and comparative examples were evaluated by the methods described below.

[0084] (1) Dispersibility The conductive pastes prepared in the examples and comparative examples were left to stand immediately after preparation, and the state of the copper powder was visually observed. The results were evaluated as "Good" when no sedimentation of the copper powder was observed, and as "Poor" when sedimentation of the copper powder was observed.

[0085] (2) Sedimentation Stability The conductive pastes prepared in the examples and comparative examples were left to stand overnight in an environment of 25°C, and the conductive pastes after standing were visually observed. The results were evaluated as "Good" when no sedimentation of copper powder was observed, and as "Poor" when sedimentation of copper powder was observed.

[0086] (3) Viscosity and Viscosity Ratio The viscosity of the conductive pastes whose dispersibility was evaluated as "good" in (1) above was measured at 25° C. using an E-type viscometer (MCR303, manufactured by Anton Paar). In addition, the viscosity was measured at a shear rate of 4 / S and at a shear rate of 40 / S, and the viscosity ratio was calculated by dividing the former by the latter.

[0087] (4) Printability An alumina substrate was used as the base material. A rectangular wiring pattern measuring 100 μm in the short direction and 1 cm in the long direction was screen-printed on the surface of this alumina substrate using the conductive pastes prepared in Examples 1 to 7 and Comparative Examples 1 to 11.

[0088] The wiring pattern was observed at a magnification of 100 times using a microscope (Keyence VHX7000). The results were evaluated as follows: good printing was indicated by "○", faint wiring patterns were indicated by "×", and bleeding was indicated by "Δ".

[0089] (5) Line Width and Film Thickness The alumina substrate on which the wiring pattern, which was evaluated as "good" in printability in (4) above, was formed was placed in a hot air dryer at 150°C for 10 minutes to harden the conductive paste and form a conductive film. The line width of the conductive film was measured using a microscope (Keyence, VHX7000). The film thickness of the conductive film was also measured using a white light interferometer (AMETEK, Zygo New View 9000).

[0090] (6) Resistivity of Conductive Film After Sintering An alumina substrate was used as the base material. A rectangular wiring pattern measuring 75 μm in the short direction and 500 μm in the long direction was screen-printed on the surface of this alumina substrate using the conductive pastes prepared in Examples 1 to 7 and Comparative Example 1. Next, the alumina substrate on which the wiring pattern was formed was left to stand in a hot air dryer at 150°C for 10 minutes to harden the conductive paste, forming a conductive film. The conductive pastes of Comparative Examples 2 to 11 had poor printability as described below, so their resistivity was not measured.

[0091] The conductive film was then sintered at 800°C for 10 minutes in an atmospheric firing furnace (KBF542N1-S, manufactured by JTEKT THERMO CORPORATION). The film thickness of the resulting conductive film was measured using a surface roughness meter (SURFCOM 480A, manufactured by Tokyo Seimitsu Co., Ltd.), and the electrical resistance at room temperature (25°C) was measured using a digital multimeter (7541A, manufactured by ADC Corporation). The specific resistance (volume resistivity) was calculated based on the measured film thickness, electrical resistance, and aspect ratio of the sintered conductive film.

[0092] (7) Raw Materials In the examples and comparative examples, copper powder (manufactured by Mitsui Mining & Smelting Co., Ltd., product name 1030Y), binder resin (manufactured by Mitsubishi Chemical Corporation, product name Dianale), and terpineol (manufactured by Nippon Fragrance Co., Ltd.) were used as an organic solvent.

[0093] The cellulose nanofibers were dispersed in an organic solvent as follows.

[0094] 0.025 g of 2,2,6,6-tetramethylpiperidinoxy radical, 0.25 g of sodium bromide, and 150 ml of water were added to 2 g of softwood pulp, and the mixture was thoroughly stirred to disperse the softwood pulp in the water. Hereinafter, the 2,2,6,6-tetramethylpiperidinoxy radical will be referred to as "TEMPO."

[0095] Next, a 13% by mass aqueous solution of sodium hypochlorite, serving as a co-oxidant, was added to 1.0 g of the softwood pulp so that the amount of sodium hypochlorite was 12.0 mmol / g, and the reaction was initiated. A 0.5 N aqueous solution of sodium hydroxide was added dropwise to the reaction solution so that the pH of the reaction solution was 10 to 11, and the reaction was continued until no change in the pH was observed. The reaction time was 120 minutes. After completion of the reaction, the reaction solution was centrifuged to recover the precipitate. Pure water was added to the obtained precipitate, and the solids concentration was adjusted to 4% by mass, to obtain a slurry. Subsequently, a 24% by mass aqueous solution of sodium hydroxide was added to adjust the pH of the slurry to 10. The slurry was a slurry of cellulose fibers derived from the softwood pulp.

[0096] The temperature of the slurry was adjusted to 30°C, and sodium borohydride was added to the slurry at a concentration of 0.2 mmol / g per 1 g of solids, followed by a reaction for 2 hours to reduce the cellulose fibers. After the reaction, the reaction solution was neutralized by adding 0.1 N hydrochloric acid, and then purified by repeated filtration and washing with water to obtain modified cellulose fibers. The modified cellulose fibers were added with methanol, filtered, and washed with methanol repeatedly to replace the water contained in the modified cellulose fibers with methanol.

[0097] Next, methanol and an amount of polyetheramine equal to the amount of carboxyl groups in the modified cellulose fibers were added to the modified cellulose fibers to obtain a diluted solution with a cellulose fiber concentration of 2.5% by mass. The polyetheramine was JEFFAMINE M-2070 manufactured by HUNTSMAN. The diluted solution was subjected to a single treatment in a high-pressure homogenizer (NanoVata manufactured by Yoshida Kikai) at a pressure of 100 MPa to obtain a gel composition.

[0098] The gel composition was prepared by dispersing cellulose nanofibers oxidized using TEMPO as a catalyst in methanol to a concentration of 2.5% by mass. The cellulose nanofibers had a carboxyl group content of 2.1 mol / g, a number-average fiber diameter of 4 nm, and a type I crystal structure.

[0099] Example 1 The copper powder, binder resin, gel composition, and terpineol described in the above section "(7) Raw Materials" were weighed into a beaker in the parts by mass shown in Table 1 to obtain a mixture. The mixture was stirred using a mixer (LR-1A manufactured by Mizuho Industries) to obtain a slurry. Stirring was carried out at 2000 rpm for 15 minutes. The slurry was used as a conductive paste.

[0100] The conductive paste was evaluated for dispersibility, sedimentation stability, viscosity, and viscosity ratio by the methods described above. The results are shown in Table 1.

[0101] The "binder resin + gel composition ratio" shown in Table 1 represents the sum of the parts by mass of the binder resin shown in Table 1 and the parts by mass of the solids in the gel composition. Furthermore, the "dry solids content" is the percentage of the mass of the copper powder, the mass of the binder resin, and the mass of the solids in the gel composition relative to the mass of the copper powder, the mass of the binder resin, the mass of the gel composition, and the mass of terpineol. The "Cu content of the dry film" is the percentage of the mass of the copper powder relative to the mass of the copper powder, the mass of the binder resin, and the mass of the solids in the gel composition.

[0102] Furthermore, using the conductive paste, wiring patterns and conductive films were prepared by the methods shown in (4) to (6) of the "Evaluation Methods" above, and the printability of the wiring patterns, the line width and film thickness of the conductive films, and the resistivity of the conductive films after sintering were evaluated. The results are shown in Table 1.

[0103] Examples 2 to 7 The same experiments as in Example 1 were conducted, except that the amounts of copper powder, binder resin, gel composition, and terpineol used were changed as shown in Table 1, and the dispersibility, sedimentation stability, viscosity, and viscosity ratio of the conductive paste were evaluated. The printability of the wiring pattern, the line width and film thickness of the conductive film, and the resistivity of the conductive film after sintering were also evaluated. The results are shown in Table 1.

[0104] Comparative Examples 1 to 13 The same experiment as in Example 1 was carried out, except that the amounts of copper powder, binder resin, gel composition, and terpineol used were changed as shown in Table 1, and the dispersibility, sedimentation stability, viscosity, and viscosity ratio of the conductive paste were evaluated. In addition, the printability of the wiring pattern, the line width and film thickness of the conductive film, and the resistivity of the conductive film after sintering were evaluated. The results are shown in Tables 2 and 3. A "-" in the tables indicates that no measurement was made. In Comparative Examples 2 to 11, the printability was poor, so the film thickness, line width, and resistivity after sintering were not measured. The conductive pastes described in Examples 1 to 6, which correspond to the conductive pastes of the present disclosure, have a low binder resin content, and the binder resin is easily removed during sintering, which can contribute to the efficient production of conductive films.

[0105] Furthermore, as shown in Table 1, although the conductive pastes described in Examples 1 to 6 have a low binder resin content, part of the binder resin is replaced with cellulose nanofiber, and therefore the pastes have a moderate viscosity, excellent sedimentation stability, and excellent printability.

[0106] As shown in Table 1, the conductive pastes described in Examples 1 to 6 have a small amount of residual carbon due to the binder resin, so the conductive films obtained from these conductive pastes have very low resistivity and can be made thick and thin. In other words, from the results shown in Table 1, it can be seen that the conductive pastes have excellent conductivity and can provide high-definition conductive films.

[0107] On the other hand, as shown in Tables 2 and 3, conductive pastes that do not satisfy the requirements of the conductive paste of the present disclosure do not have the excellent properties of the conductive pastes described in Examples 1 to 6. In other words, the conductive paste described in Comparative Example 1 had excellent printability but could only provide a conductive film with high resistivity. Furthermore, the conductive paste described in Comparative Example 10 had poor sedimentation stability, and it was not possible to print a wiring pattern. Furthermore, the conductive pastes described in Comparative Examples 2 to 9 and 11 had poor printability, and as a result, it was not possible to form a sufficient conductive film.

[0108] From the above results, it can be seen that when the requirements of the conductive paste of the present disclosure are met, satisfactory productivity, printability, and conductivity are achieved.

[0109] Example 8 The copper powder, binder resin, gel composition, and terpineol described in the section "(7) Raw Materials" above were weighed into a beaker in the amounts shown in Table 4, and acetone was added to obtain a mixture with a viscosity of 1000 cP or less. The mixture was stirred using a mixer (LR-1A manufactured by Mizuho Industries) to obtain a slurry. Stirring was carried out at 2000 rpm for 15 minutes.

[0110] Next, the slurry was subjected to a high-pressure disperser (Nanovaita NVL-AS200-D14 manufactured by Yoshida Kikai). The slurry was injected into a nozzle and discharged while being subjected to the pressure shown in Table 4 using a plunger provided in the high-pressure disperser. The diameter of the nozzle provided in the high-pressure disperser and the pressure applied to the slurry before being injected into the nozzle were 100 μm and 50 MPa, respectively, as shown in the "nozzle diameter" and "pressure" columns in Table 4.

[0111] Inside the nozzle, shear force generated by turbulence was applied to the slurry. The liquid discharged from the nozzle was collected, and the acetone was evaporated at 40° C. using an evaporator to obtain a conductive paste.

[0112] The sedimentation stability of the conductive paste was evaluated as described in (2) of the <Evaluation Method> above. Furthermore, the presence or absence of aggregates of 10 μm or more was confirmed using a microscope (Keyence VK7000). The results are shown in Table 4.

[0113] [Example 9] to [Example 16] The same experiments as in Example 8 were carried out, except that the diameter of the nozzle provided in the high-pressure disperser and the pressure applied to the slurry before feeding into the nozzle were changed as shown in Table 4. The results are shown in Table 4. As shown in Table 4, the conductive pastes produced by a method including a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser exhibited excellent sedimentation stability. Furthermore, in Examples 8 to 11, no aggregates were observed when examined under a microscope. These results demonstrate that the method yields conductive pastes with excellent dispersibility, and that employing the nozzle diameter and pressure used in Examples 8 to 11 results in conductive pastes with even better dispersibility. Although aggregates were observed under a microscope in the conductive pastes obtained in Examples 12 to 16, they exhibited excellent sedimentation stability and can be used as the conductive pastes of the present disclosure.

[0114] The present disclosure can be used for forming fine wiring in electronic devices, etc.

[0115] DESCRIPTION OF SYMBOLS 1 Conductive film 2 Substrate A Height of conductive film B Length of conductive film in the short direction C Distance between opposing conductive films D Nozzle diameter 10 Sample reservoir 11 Cylinder 12 Plunger 13 Nozzle 14 Discharge liquid reservoir

Claims

1. A conductive paste comprising a conductor, a binder resin, and cellulose nanofiber, wherein when the content of the conductor is 100 parts by mass, the content of the binder resin is 0.5 parts by mass or more and 2 parts by mass or less, and the content of the cellulose nanofiber is 0.175 parts by mass or more and 0.35 parts by mass or less.

2. The conductive paste according to claim 1, wherein the content of the binder resin is 0.5 parts by mass or more and 1 part by mass or less, and the content of the cellulose nanofiber is 0.175 parts by mass or more and 0.25 parts by mass or less, when the content of the conductor is 100 parts by mass.

3. The conductive paste according to claim 1 or 2, wherein the conductor is copper.

4. A conductive film formed by hardening the conductive paste according to any one of claims 1 to 3, the conductive film having a thickness of 14 μm or more and 17.5 μm or less and a length in the short direction of 20 μm or more and 25 μm or less.

5. The resistivity is 2.00 x 10 －８ The conductive film according to claim 4 , which has a resistance of Ω·m or less.

6. The conductive film according to claim 4 or 5, wherein a plurality of said conductive films face each other with a space therebetween, and the distance between the opposing conductive films is 20 μm or less.

7. A method for producing a conductive paste, comprising a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

8. A method for producing a conductive paste according to claim 7, wherein the process includes a step of feeding the conductor, the binder resin, the cellulose nanofibers, and the organic solvent into a nozzle of the high-pressure disperser under pressure, and the process satisfies the following condition (i) or (ii): (i) the nozzle has a diameter of more than 100 μm and not more than 200 μm, and the pressure applied to the conductor, the binder resin, the cellulose nanofibers, and the organic solvent before feeding into the nozzle is 100 MPa or more and 150 MPa or less; (ii) the nozzle has a diameter of 100 μm or more and less than 200 μm, and the pressure applied to the conductor, the binder resin, the cellulose nanofibers, and the organic solvent before feeding into the nozzle is 50 MPa or more and 100 MPa or less.

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