Conductive paste, method for producing the same, and conductive film

Abstract

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

Description

Technical Field

[0001] This disclosure relates to conductive pastes, methods for manufacturing the same, and conductive films. Background Technology

[0002] With the miniaturization of electronic devices, there is a growing demand for finer wiring. As a conductive paste for forming fine wiring, Patent Document 1 discloses a conductive paste containing a binder resin and cellulose nanofibers.

[0003] Prior art literature

[0004] Patent documents

[0005] Patent Document 1: Japanese Patent Application Publication No. 2019-175675 Summary of the Invention

[0006] One aspect of the conductive paste disclosed herein contains a conductor, a binder resin, and cellulose nanofibers. When the content of the conductor is set to 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.

[0007] In addition, one method for manufacturing a conductive paste according to the present disclosure includes a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser. Attached Figure Description

[0008] Figure 1 This is a schematic diagram showing a conductive film of one embodiment of the present disclosure formed on a substrate.

[0009] Figure 2 yes Figure 1 The conductive film shown is a cross-sectional view along line II.

[0010] Figure 3 This is a schematic cross-sectional view illustrating an example of a process for dispersing a conductor, binder resin, and cellulose nanofibers in an organic solvent using a high-pressure disperser. Detailed Implementation

[0011] Previously, efforts have been made to improve the productivity and conductivity of conductive films using conductive pastes. According to a solution disclosed herein, the productivity and conductivity of conductive films can be improved.

[0012] Hereinafter, one embodiment of the present disclosure will be described in detail. However, the present disclosure is not limited thereto, and various modifications can be made within the scope described. 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" indicating a numerical range means "A or more and B or less".

[0013] (1. Conductive paste)

[0014] The conductive paste disclosed herein contains a conductor, a binder resin, and cellulose nanofibers. When the content of the conductor is set to 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.

[0015] Compared with conventional conductive pastes, the conductive paste of this disclosure contains less binder resin, which is effectively removed during sintering, thus facilitating the efficient production of conductive films with low resistivity.

[0016] Furthermore, although the conductive paste disclosed herein contains a low amount of binder resin, it instead contains cellulose nanofibers in the aforementioned amount. Therefore, it exhibits moderate viscosity, excellent sedimentation stability, and excellent printability.

[0017] (1) Conductor

[0018] The conductor imparts conductivity to the conductive film. The conductor can be any metal, or it can be a conductor commonly used in conductive pastes. For example, copper, silver, nickel, and gold can be used. However, due to its excellent conductivity, copper is preferred as the conductor.

[0019] The shape of the conductor is not particularly limited; for example, it can be spherical, sheet-like, or various other shapes. The average particle size of the conductor, based on the 50% average particle size (D50), can be 0.1–20 μm, 1–10 μm, or 2–5 μm. The conductor is a powder that is an aggregate of particles having such an average particle size.

[0020] D50 can be determined by laser diffraction. For example, 0.3g of conductor can be weighed into a 50ml beaker, and 30ml of IPA can be added. The mixture is then treated with an ultrasonic cleaner for 5 minutes to disperse the conductor in the IPA. The D50 can then be determined using a laser diffraction particle size distribution measuring device. Here, IPA is isopropanol. For example, a Hielscher UP200S ultrasonic cleaner can be used, and a Horiba LA-950 laser diffraction particle size distribution measuring device can be used.

[0021] (2) Adhesive resin

[0022] The adhesive resin has the function of dispersing the conductor. Furthermore, the adhesive resin has the function of bringing the conductor into contact with each other through curing, thereby enabling conductivity.

[0023] There are no particular limitations on the type of adhesive resin used; epoxy resin, phenolic resin, polyester resin, acrylic resin, polyurethane resin, urethane acrylate resin, and capped isocyanate compounds can be used. Examples of epoxy resins include bisphenol A type, bisphenol F type, linear phenolic resin, and polyol type. One type of adhesive resin can be used, or a combination of two or more can be used.

[0024] Among them, acrylic resin is preferred due to its good thermal decomposition properties.

[0025] (3) Cellulose nanofibers

[0026] Cellulose nanofibers refer to cellulose fibers with a number-average fiber diameter in the nanometer range. "Nanometer range" means greater than 0 nm and less than 1000 nm. The number-average fiber diameter can be, for example, 3–800 nm, 3–100 nm, or 3–30 nm.

[0027] The number-average fiber diameter can be determined, for example, as follows. First, a dispersion of cellulose nanofibers with a solid content of 0.05 to 0.1% by mass is prepared and dispersed in an organic solvent. This preparation method is based on the method described in (7) of the <Evaluation Method> in the examples described later. The dispersion is cast onto a grid covered with a hydrophilically treated carbon film as a sample for observation using a transmission electron microscope (TEM). The sample is observed through the TEM at magnifications of 5000x, 10000x, or 50000x. At this time, the image width is assumed to be an arbitrary axis, and the observation conditions, such as the sample and magnification, are adjusted such that at least 20 fibers intersect the axis. For the images used for observation that meet the adjusted observation conditions, two random axes are set for each image, and the diameter of the fibers intersecting the axis is read visually. As described above, at least three non-repeating images of the surface are taken with the TEM, and the diameter of the fibers intersecting the two axes is read for each image. The arithmetic mean of the obtained diameters is taken as the number-average fiber diameter of the cellulose nanofibers.

[0028] The cellulose constituting the cellulose nanofibers can have a type I crystalline structure. This type I crystalline structure can be determined, for example, based on the fact that the cellulose exhibits typical peaks in the diffraction pattern obtained by wide-angle X-ray diffraction measurements near 2θ = 14°–17° and 2θ = 22°–23°.

[0029] Cellulose nanofibers can be anionicly modified cellulose nanofibers formed by introducing anionic groups into the glucose units of cellulose molecules. The anionic groups can be, for example, one or more selected from the group consisting of carboxyl groups, phosphate groups, sulfonic acid groups, and sulfate groups. The anionic groups can be acidic or salt-type.

[0030] Cellulose nanofibers can be oxidized cellulose nanofibers, which are formed by selectively oxidizing the hydroxyl groups at the C6 position of the glucose units contained in cellulose molecules to obtain carboxyl groups.

[0031] Oxidized cellulose nanofibers can be obtained, for example, by oxidizing natural cellulose using sodium hypochlorite as a co-oxidant and 2,2,6,6-tetramethylpiperidinyl oxygen radicals, which are N-oxygen compounds, as a catalyst. As a specific method, for example, oxidized cellulose nanofibers can be obtained using the method described in (7) of the <Evaluation Method> in the examples described later.

[0032] The amount of anionic groups in the cellulose nanofibers can be, for example, 0.05–3.0 mmol or 0.5–2.5 mmol. The amount of anionic groups can be determined, for example, using the method described in Patent Document 1.

[0033] Cellulose nanofibers can also be dispersed in organic solvents. This promotes the dispersion of cellulose nanofibers into the conductive paste, thereby facilitating the preparation of the conductive paste. The organic solvent can be a terpene solvent such as terpineol, or a glycol ether solvent such as butylcarbidol or butylcarbidol acetate. One or more of the organic solvents can be used.

[0034] (4) Component content in conductive paste

[0035] The component content in the conductive paste disclosed herein refers to the content of each component as a solid component. In the conductive paste of this disclosure, when the content of the conductor is set to 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. Furthermore, the percentage of the mass of the binder resin relative to the total mass of the conductor and the binder resin is preferably less than 2.

[0036] The binder resin has the aforementioned functions. On the other hand, if carbon or other substances derived from the binder resin remain in the conductive film formed by curing the conductive paste, it will adversely affect the conductivity of the conductive film. Therefore, it is sought to decompose and remove the binder resin contained in the conductive paste so that carbon or other substances do not remain in the conductive film.

[0037] If the conductive paste contains a high amount of binder resin, even when the conductive paste is sintered at high temperatures, removing the binder resin requires a significant amount of labor, thus reducing the productivity of the conductive film. On the other hand, if the content is too low, the viscosity of the conductive paste decreases, thereby reducing its printability.

[0038] Therefore, in this disclosure, in-depth research was conducted with the aim of ensuring the printability of the conductive paste and obtaining a conductive film with high conductivity through high productivity. As a result, compared with conventional conductive pastes, the content of binder resin is reduced, and cellulose nanofibers are included, with the content of binder resin and cellulose nanofibers set within the aforementioned range.

[0039] At this point, the binder resin is contained in an amount sufficient to ensure the stated function. On the other hand, a portion of the binder resin is replaced by cellulose nanofibers; therefore, the cellulose nanofibers can compensate for the decrease in viscosity of the conductive paste accompanying the reduction in the binder resin content.

[0040] Therefore, while ensuring the aforementioned functions of the adhesive resin, the viscosity of the conductive paste can be appropriately maintained, thereby improving printability.

[0041] In addition, compared with conventional conductive pastes, the content of binder resin is reduced, so the binder resin can be easily removed by sintering, thereby improving the productivity and conductivity of the conductive film.

[0042] The conductive paste disclosed herein may contain an organic solvent. The organic solvent may be an organic solvent that does not chemically react with the binder resin but dissolves the binder resin. The organic solvent acts as a dispersion medium relative to the conductor and the cellulose nanofibers.

[0043] Examples of organic solvents include terpineol, methanol, ethanol, isopropanol, isobutyl alcohol, 1-butanol, diacetone alcohol, ethylene glycol, and glycerol; 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 or more of these organic solvents may be used.

[0044] Given the ease of preparing conductive pastes, the organic solvent can be an alcohol-based solvent, including terpineol.

[0045] Regarding the content of organic solvent in the conductive paste of this disclosure, including cases where cellulose nanofibers are dispersed in the organic solvent, when the content of the conductor is set to 100 parts by mass, it can be 15.0 to 25.0 parts by mass or 19.0 to 22.5 parts by mass.

[0046] When the conductive paste disclosed herein contains 100 parts by mass, the content of the conductor 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.

[0047] In this case, the content of binder resin and cellulose nanofibers ensures the function of the binder resin and compensates for the decrease in viscosity of the conductive paste that accompanies the reduction in binder resin content, thus providing a better conductive paste with superior printability. Furthermore, it enables the production of conductive films with superior productivity and conductivity.

[0048] (5) Physical properties of conductive paste, etc.

[0049] (5-1) Dispersibility and sedimentation stability of conductive pastes

[0050] The dispersibility of the conductive paste disclosed herein refers to the degree of sedimentation of the conductor immediately after the conductive paste is manufactured, and the sedimentation stability refers to the degree of sedimentation of the conductor after standing overnight at 25°C. These evaluation methods are described later in the examples.

[0051] In the conductive paste disclosed herein, the contents of the binder resin and cellulose nanofibers are determined to be within the aforementioned ranges. Therefore, as shown in the examples described later, it exhibits excellent dispersibility and sedimentation stability.

[0052] (5-2) Viscosity

[0053] The viscosity of the conductive paste disclosed herein at a shear rate of 4 / s can be 120–230 Pa·s or 140–210 Pa·s. Furthermore, the viscosity of the conductive paste disclosed herein at a shear rate of 40 / s can be 30–90 Pa·s or 50–70 Pa·s. The method for determining the viscosity is described later in the examples.

[0054] The conductive paste disclosed herein has a moderate viscosity and, as previously mentioned, excellent dispersibility and sedimentation stability, thus enabling the production of wiring patterns with excellent printability.

[0055] The printing method for printing the conductive paste onto the substrate disclosed herein can include, for example, screen printing, spin coating, rod coating, die coating, blade coating, gravure coating, roll coating, spray coating, and dip coating. Among these, screen printing is preferred. The substrate can be alumina substrates or other ceramic substrates, glass substrates, printed wiring boards, PET films, or other flexible substrates.

[0056] (2. Conductive film)

[0057] The conductive film disclosed herein is a conductive film formed by curing the conductive paste disclosed herein, with 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.

[0058] As previously stated, the conductive paste of this disclosure contains less binder resin than conventional conductive pastes and correspondingly contains cellulose nanofibers, thus exhibiting excellent printability. Therefore, it is possible to provide high-density wiring having the aforementioned thickness and length in the short side direction.

[0059] Furthermore, the conductive film disclosed herein has a low amount of carbon residue caused by the binder resin, resulting in a very low resistivity of the conductive film obtained from the conductive paste, enabling both thick film production and fine wire production.

[0060] The curing of the conductive paste can be carried out, for example, by printing the conductive paste onto a substrate and drying it in a hot air dryer at 80–120°C for 30–120 minutes.

[0061] Figure 1 This is a schematic diagram showing a conductive film of one embodiment of the present disclosure formed on a substrate. Figure 2 yes Figure 1 The conductive film shown is a cross-sectional view along line II. Figure 1 and Figure 2 This is merely a schematic diagram and does not accurately depict the conductive film of one embodiment of this disclosure. 1 is the conductive film, and 2 is the substrate. The cross-section along line II of the conductive film 1 is cuboid, but it is not limited to this, and the shape of the cross-section along line II is not limited.

[0062] The term "thickness" refers to the length of the longest perpendicular line drawn from the junction formed by the conductive film 1 and the substrate 2 to the surface of the conductive film 1 opposite to the substrate 2 in the II-line cross-section of the conductive film 1. For example, the thickness of the conductive film 1 is... Figure 1 and Figure 2 The height A shown.

[0063] The "length in the short side direction" refers to the length of the connection formed by the conductive film 1 and the substrate 2 in the cross-section along line II of the conductive film 1. For example, the length in the short side direction of the conductive film 1 is... Figure 1 and Figure 2 The length B shown is referred to as the "line width" in this specification.

[0064] The thickness and the length in the short side direction can be determined by the method described in the embodiments described later.

[0065] For the conductive film of this disclosure, a plurality of the conductive films are spaced apart and opposite each other, and the distance between the opposing conductive films can be less than 20 μm.

[0066] As previously stated, the conductive paste of this disclosure contains less binder resin than conventional conductive pastes and correspondingly contains cellulose nanofibers, resulting in excellent printability. Therefore, it is possible to provide higher density wiring, not only in terms of thickness and length in the short side direction, but also in the distance between opposing conductive films being less than 20 μm.

[0067] "Multiple conductive films separated by space and facing each other" can also be, for example, as follows Figure 1 As shown, multiple independent conductive films are spaced apart and opposed to each other. Alternatively, for example, a conductive film formed by bending on a substrate may have multiple spaced apart and opposed to each other.

[0068] The "distance between opposing conductive films" (hereinafter also referred to as "split") refers to the length of the shortest straight line when a straight line is drawn parallel to the substrate from the face of one of the opposing films facing each other, separated by a space. For example, in Figure 2 In this context, the spacing is distance C.

[0069] The lower limit of the spacing is not particularly limited; for example, it can be 15 μm or 10 μm. The spacing can be measured, for example, using a Keyence VHX series microscope.

[0070] The resistivity of the conductive film disclosed herein can be 2.00 × 10⁻⁶. -8 Below Ω·m. As mentioned above, compared with conventional conductive pastes, the conductive paste of this disclosure contains less binder resin, thus making it easier to remove the binder resin through sintering. Therefore, the conductive film of this disclosure has a very low resistivity value, achieving excellent conductivity.

[0071] The resistivity value was measured after sintering the conductive film of this disclosure. The measurement method can be the method described in the embodiments described later. Sintering is performed, for example, by treating the conductive film at 800°C for 10 minutes under 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.

[0072] (3. Manufacturing method of conductive paste)

[0073] The conductive paste disclosed herein can be manufactured by mixing the following method: when the mass of the conductor is set to 100 parts by mass, the binder resin is set to 0.5 parts by mass and 2 parts by mass and the cellulose nanofibers are set to 0.175 parts by mass and 0.35 parts by mass and then mixed with an organic solvent.

[0074] The content of the adhesive resin can be more than 0.5 parts by weight and less than 1 part by weight, and the content of the cellulose nanofiber can be more than 0.175 parts by weight and less than 0.25 parts by weight.

[0075] Regarding the content of the organic solvent in the conductive paste, when the content of the conductor is set to 100 parts by mass, including the organic solvent in the case where the aforementioned cellulose nanofibers are dispersed in an organic solvent, it can be 15.0 to 25.0 parts by mass or 19.0 to 22.5 parts by mass.

[0076] The mixing can be carried out, for example, by placing the conductor, binder resin, cellulose nanofibers and organic solvent into a container such as a beaker and stirring it with a mixer.

[0077] The method for manufacturing the conductive paste disclosed herein may include a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

[0078] According to the described structure, since it is supplied to a high-pressure disperser, the dispersibility of the conductor and cellulose nanofibers in the organic solvent can be further improved. Therefore, a conductive paste with a suitable viscosity can be efficiently obtained while reducing the content of the binder resin compared to conventional conductive pastes. Hereinafter, the conductor, binder resin, cellulose nanofibers, and organic solvent will also be referred to as the "sample". When using a high-pressure disperser, the amount of sample used can also be the same as in the manufacturing method based on the aforementioned mixing.

[0079] The high-pressure disperser can apply high pressure while feeding the sample into the nozzle, and apply shear force to the sample inside the nozzle to micronize the cellulose nanofibers in the sample and uniformly mix the conductor and cellulose nanofibers. For example, the Yoshida Machinery NanoVater NVL-AS200-D14 can be used as the high-pressure disperser.

[0080] Figure 3 This is a schematic cross-sectional view illustrating an example of a process for dispersing a conductive material and cellulose nanofibers in an organic solvent using a high-pressure disperser. The conductive material, binder resin, cellulose nanofibers, and organic solvent are stored in a sample accumulation tank 10. The sample may be in the form of a slurry. The sample is fed from the sample accumulation tank 10 into a working cylinder 11, and pressure is applied to the slurry via a plunger 12 and fed into nozzles 13. The method of pressurization is not limited to that based on the plunger 12. The number of nozzles 13 may be one or more.

[0081] Nozzle 13 applies shear force to the sample by generating turbulence within it. This shear force micronizes the cellulose nanofibers in the sample, dispersing them along with the conductor into the organic solvent, ejecting them from nozzle 13, and recovering them into the ejected liquid collection tank 14. As a result, a conductive paste in which the conductor and cellulose nanofibers are sufficiently dispersed in the organic solvent can be obtained. As shown in the examples described later, the conductive paste obtained by the method for manufacturing the conductive paste of this disclosure exhibits good dispersibility and sedimentation stability.

[0082] The process includes simultaneously pressurizing and feeding the conductor, binder resin, cellulose nanofibers, and organic solvent into the nozzle of the high-pressure disperser. This process may have the following conditions (i) or (ii):

[0083] (i) The nozzle has a diameter of more than 100 μm and less than 200 μm, and the pressure applied to the conductor, the adhesive resin, the cellulose nanofiber and the organic solvent before being injected into the nozzle is more than 100 MPa and less than 150 MPa;

[0084] (ii) The nozzle has a diameter of 100 μm or more and less than 200 μm, and the pressure applied to the conductor, the adhesive resin, the cellulose nanofiber and the organic solvent before being fed into the nozzle is 50 MPa or more and less than 100 MPa.

[0085] By adjusting the nozzle diameter and the pressure, the shear force applied to the cellulose nanofibers becomes stronger compared to cases other than (i) or (ii), thereby improving the dispersibility of the conductor and cellulose nanofibers in the organic solvent. Consequently, a conductive paste with appropriate viscosity can be efficiently obtained while reducing the binder resin content compared to conventional conductive pastes.

[0086] "Putting the sample into the nozzle of the high-pressure disperser while pressurizing it" means putting the sample into the nozzle under a pressure higher than that before it was put into the high-pressure disperser. This pressure can be the pressure described in (i) or (ii). For example, the sample exists... Figure 3 The pressure applied when the sample is in the working cylinder 11 is greater than the pressure applied when the sample is in the sample storage tank 10.

[0087] The nozzle diameter refers to the diameter of the nozzle orifice of the sample, which is the diameter of the largest inscribed circle relative to the two-dimensional shape of the orifice. For example, if the two-dimensional shape of the orifice is circular, the diameter is the diameter of that circle; if the two-dimensional shape of the orifice is elliptical, the minor axis of the ellipse corresponds to the diameter. Figure 3 The illustration shows an example where nozzle 13 has a circular inlet with a diameter of D. The nozzle diameter can be the diameter described in (i) or (ii). A nozzle can have one or more inlets.

[0088] (Summarize)

[0089] This disclosure may include the following (1) to (8).

[0090] (1) A conductive paste comprising a conductor, a binder resin, and cellulose nanofibers,

[0091] When the content of the conductor is set to 100 parts by mass, the content of the adhesive 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.

[0092] (2) The conductive paste according to (1), wherein when the content of the conductor is set to 100 parts by mass, 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.

[0093] (3) The conductive paste according to (1) or (2), wherein the conductor is copper.

[0094] (4) A conductive film, which is formed by curing the conductive paste according to any one of (1) to (3), wherein the thickness of the conductive film is 14 μm or more and 17.5 μm or less, and the length in the short side direction is 20 μm or more and 25 μm or less.

[0095] (5) The conductive film according to (4), wherein the resistivity of the conductive film is 2.00 × 10⁻⁶. -8 Below Ω·m.

[0096] (6) The conductive film according to (4) or (5), wherein a plurality of the conductive films are spaced apart and opposed to each other, and the distance between the opposing conductive films is less than 20 μm.

[0097] (7) A method for manufacturing a conductive paste, comprising a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

[0098] (8) The method for manufacturing the conductive paste according to (7), wherein,

[0099] The process includes simultaneously pressurizing and feeding the conductor, binder resin, cellulose nanofibers, and organic solvent into a nozzle of a high-pressure disperser, wherein the process has the following (i) or (ii) conditions:

[0100] (i) The nozzle has a diameter of more than 100 μm and less than 200 μm, and the pressure applied to the conductor, the adhesive resin, the cellulose nanofiber and the organic solvent before being injected into the nozzle is more than 100 MPa and less than 150 MPa;

[0101] (ii) The nozzle has a diameter of 100 μm or more and less than 200 μm, and the pressure applied to the conductor, the adhesive resin, the cellulose nanofiber and the organic solvent before being fed into the nozzle is 50 MPa or more and less than 100 MPa.

[0102] The invention described above is based on the accompanying drawings and embodiments. However, the invention is not limited to the embodiments described above. That is, various modifications can be made to the invention within the scope shown in this disclosure, and embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included within the technical scope of this invention. It should be noted that those skilled in the art can easily make various modifications or alterations based on this disclosure. Furthermore, it should be noted that the aforementioned modifications or alterations are included within the scope of this disclosure.

[0103] Example

[0104] The present disclosure will be described in more detail with reference to the embodiments and comparative examples shown below. The present disclosure is not limited to these embodiments and comparative examples. Embodiments obtained by appropriately combining the technical means disclosed in the embodiments are also included in the scope of the present disclosure.

[0105] <Evaluation Methods>

[0106] The physical properties of the conductive pastes and conductive films obtained in the examples and comparative examples were evaluated using the methods shown below.

[0107] (1) Dispersion

[0108] The conductive pastes prepared in the examples and comparative examples were allowed to stand immediately after preparation, and the state of the copper powder was observed visually. For the results, cases where no copper powder sedimentation was observed were rated as "○", and cases where copper powder sedimentation was observed were rated as "×".

[0109] (2) Settlement stability

[0110] The conductive pastes prepared in the examples and comparative examples were left to stand overnight at 25°C, and the conductive pastes after standing were observed visually. For the results, no copper powder sedimentation was evaluated as "○", and observed copper powder sedimentation was evaluated as "×".

[0111] (3) Viscosity and viscosity ratio

[0112] The viscosity of the conductive paste with a dispersibility rating of "○" evaluated in (1) above was measured at 25°C using an E-type viscometer (Anton Paar, MCR303). Additionally, the viscosity at a shear rate of 4 / s and at a shear rate of 40 / s was measured, and the viscosity ratio was calculated by dividing the former by the latter.

[0113] (4) Printability

[0114] An alumina substrate was used as the substrate. A rectangular wiring pattern with a length of 100 μm in the short side and a length of 1 cm in the long side was screen-printed on the surface of the alumina substrate using the conductive paste prepared in Examples 1-7 and Comparative Examples 1-11.

[0115] The wiring pattern was observed using a microscope (Keyence VHX7000) at 100x magnification. Well-printed patterns were rated "○", unclear patterns were rated "×", and patterns with bleeding were rated "△".

[0116] (5) Line width and film thickness

[0117] An alumina substrate with a wiring pattern marked "○" for printability as described in (4) was placed in a hot air dryer at 150°C for 10 minutes to cure the conductive paste into a conductive film. The linewidth of the conductive film was measured using a microscope (Keyence VHX7000). The film thickness was measured using a white interferometer (AMETEK Zygo newview9000).

[0118] (6) Resistivity of the conductive film after sintering

[0119] An alumina substrate was used as the substrate. A rectangular wiring pattern with a short side length of 75 μm and a long side length of 500 μm was screen-printed onto the surface of the alumina substrate using the conductive paste prepared in Examples 1-7 and Comparative Example 1. Next, the alumina substrate with the wiring pattern was placed in a hot air dryer at 150°C for 10 minutes to allow the conductive paste to cure into a conductive film. The conductive pastes used in Comparative Examples 2-11 had poor printability as described later, therefore resistivity measurements were not performed.

[0120] Subsequently, the conductive film was sintered at 800°C for 10 minutes in an atmosphere sintering furnace (JTEKT KBF542N1-S). The film thickness was measured using a surface roughness meter (Tokyo Seimitsu Corporation Surfcom480A), and the resistance at room temperature (25°C) was measured using a digital multimeter (ADC Corporation 7541A). The resistivity (volume resistivity) was calculated based on the measured film thickness, resistance, and aspect ratio of the sintered conductive film.

[0121] (7) Raw materials

[0122] In the examples and comparative examples, copper powder (Mitsui Metal Mining, trade name 1030Y), binder resin (Mitsubishi Chemical, trade name DIANAL) and terpineol (Japan Fragrance) were used as organic solvents.

[0123] Cellulose nanofibers are dispersed in an organic solvent in the following manner.

[0124] 0.025 g of 2,2,6,6-tetramethylpiperidinyl oxygen radical, 0.25 g of sodium bromide, and 150 ml of water were added to 2 g of coniferous pulp and stirred thoroughly to disperse the coniferous pulp in the water. Hereinafter, the 2,2,6,6-tetramethylpiperidinyl oxygen radical will be referred to as "TEMPO".

[0125] Next, 1.0 g of the coniferous pulp was added to a 13% by mass aqueous solution of sodium hypochlorite as a co-oxidant to achieve a sodium hypochlorite concentration of 12.0 mmol / g, and the reaction was initiated. A 0.5 N sodium hydroxide aqueous solution was added dropwise to the reaction solution to adjust the pH to 10-11, and the reaction was continued until no change in pH was observed. The reaction time was 120 minutes. After the reaction was completed, the reaction solution was centrifuged, and the precipitate was recovered. Pure water was added to the obtained precipitate to adjust the solid component concentration to 4% by mass, thereby obtaining the pulp. Then, a 24% by mass aqueous solution of sodium hydroxide was added to set the pH of the pulp to 10. The pulp was derived from the cellulose fibers of the coniferous pulp.

[0126] The slurry was heated to 30°C, and sodium borohydride was added at a rate of 0.2 mmol / g relative to 1g of solid component in the slurry. The mixture was allowed to react for 2 hours to reduce the cellulose fibers. After the reaction, 0.1N hydrochloric acid was added to the reaction solution for neutralization, followed by repeated filtration and washing with water to obtain modified cellulose fibers. Methanol was added to the modified cellulose fibers and the mixture was filtered. The fibers were repeatedly washed with methanol to replace the water in the modified cellulose fibers with methanol.

[0127] Next, methanol and a polyetheramine in an amount equal to the carboxyl group content of the modified cellulose fibers were added to the modified cellulose fibers to obtain a dilution with a cellulose fiber concentration of 2.5% by mass. The polyetheramine was JEFFAMINEM-2070 manufactured by HUNTSMAN. The dilution was then subjected to a single treatment at 100 MPa using a high-pressure homogenizer (Yoshida Machinery's NanoVater) to obtain a gel-like composition.

[0128] The gel-like composition was obtained by dispersing cellulose nanofibers oxidized with 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 crystalline structure.

[0129] (Example 1)

[0130] The copper powder, binder resin, gel composition, and terpineol described in section (7) Raw Materials were weighed into a beaker in the amounts specified in Table 1 to obtain a mixture. The mixture was stirred using a mixer (MIZHAI KUK LR-1A) to obtain a slurry. The stirring was performed at 2000 rpm for 15 minutes. This slurry was used as a conductive paste.

[0131] The dispersibility, sedimentation stability, viscosity, and viscosity ratio of the conductive paste were evaluated using the methods described above. The results are shown in Table 1.

[0132] The "ratio of binder resin to gel composition" shown in Table 1 represents the total mass parts of the binder resin and the mass parts of the solid components in the gel composition. Furthermore, "dried solid components" is the percentage of the mass of copper powder, binder resin, and the solid components of the gel composition relative to the mass of copper powder, binder resin, gel composition, and terpineol. "Cu content of the dried film" is the percentage of the mass of copper powder relative to the mass of copper powder, binder resin, and the solid components in the gel composition.

[0133] Furthermore, using the aforementioned conductive paste, the wiring pattern and conductive film were modulated using the methods shown in (4) to (6) of the aforementioned "Evaluation Methods," and 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 Table 1.

[0134] (Example 2) to (Example 7)

[0135] Except for changes to the amounts of copper powder, binder resin, gel composition, and terpineol as described in Table 1, the same experiments as in Example 1 were conducted to evaluate the dispersibility, sedimentation stability, viscosity, and viscosity ratio of the conductive paste. Furthermore, the printability of the wiring pattern, the linewidth and thickness of the conductive film, and the resistivity of the conductive film after sintering were evaluated. The results are shown in Table 1.

[0136] (Comparative Example 1) to (Comparative Example 13)

[0137] Except for changes to the amounts of copper powder, binder resin, gel composition, and terpineol as described in Table 1, the same experiments as in Example 1 were conducted to evaluate the dispersibility, sedimentation stability, viscosity, and viscosity ratio of the conductive paste. Furthermore, 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. "-" in the tables indicates that no measurement was performed. Due to poor printability in Comparative Examples 2-11, film thickness, line width, and resistivity after sintering were not measured.

[0138]

[0139]

[0140]

[0141] The conductive pastes described in Examples 1 to 6 of this disclosure have a low content of binder resin, which is easily removed during sintering. Therefore, this facilitates the efficient production of conductive films.

[0142] In addition, as shown in Table 1, although the conductive pastes described in Examples 1 to 6 have a low content of binder resin, a portion of the binder resin is replaced by cellulose nanofibers, thus exhibiting moderate viscosity, excellent sedimentation stability, and excellent printability.

[0143] As shown in Table 1, for the conductive pastes described in Examples 1-6, the amount of carbon residue caused by the binder resin is low, resulting in a very low resistivity of the conductive film obtained from the conductive paste, enabling both thick film production and fine wire production. In other words, based on the results shown in Table 1, it can be understood that the conductive paste exhibits excellent conductivity and can provide a highly fine conductive film.

[0144] On the other hand, as shown in Tables 2 and 3, the conductive pastes of this disclosure that lack the necessary conditions do not possess the excellent properties of the conductive pastes described in Examples 1 to 6. That is, even with excellent printability, the conductive paste described in Comparative Example 1 can only provide a conductive film with high resistivity. Furthermore, the conductive paste described in Comparative Example 10 has poor settling stability, making it impossible to print wiring patterns. Moreover, the conductive pastes described in Comparative Examples 2 to 9 and 11 have poor printability, resulting in the inability to form a sufficiently conductive film.

[0145] The results above show that, under the necessary conditions of the conductive paste of this disclosure, satisfactory productivity, printability, and conductivity can be obtained.

[0146] (Example 8)

[0147] The copper powder, binder resin, gel composition, and terpineol described in item (7) Raw Materials were weighed into a beaker in the amounts specified in Table 4, and acetone was added to obtain a mixture with a viscosity of less than 1000 cP. The mixture was stirred using a mixer (MIZHAI KUK LR-1A) to obtain a slurry. Stirring was performed at 2000 rpm for 15 minutes.

[0148] Next, the slurry is fed into a high-pressure disperser (Yoshida Machinery NanoVater NVL-AS200-D14). The slurry is ejected through a nozzle while the pressure shown in Table 4 is applied to the slurry via the plunger of the high-pressure disperser. The nozzle diameter of the high-pressure disperser and the pressure applied to the slurry before being fed into the nozzle are 100 μm and 50 MPa, respectively, as shown in Table 4 under "Nozzle Diameter" and "Pressure".

[0149] Inside the nozzle, shear forces generated by turbulence are applied to the slurry. The liquid ejected from the nozzle is recovered, and acetone is evaporated at 40°C using an evaporator to obtain a conductive paste.

[0150] The sedimentation stability of the aforementioned conductive paste was evaluated as described in (2) of the <Evaluation Method>. Furthermore, the presence or absence of aggregates larger than 10 μm was confirmed using a microscope (Keyence VK7000). The results are shown in Table 4.

[0151] (Example 9) to (Example 16)

[0152] Except for changes to the nozzle diameter of the high-pressure disperser and the pressure applied to the slurry before it is fed into the nozzle, as described in Table 4, the same experiments as in Example 8 were conducted. The results are shown in Table 4.

[0153]

[0154] As shown in Table 4, the conductive paste manufactured using a method comprising dispersing the conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser exhibits excellent sedimentation stability. Furthermore, in Examples 8-11, microscopic examination revealed no agglomerates. These results demonstrate that the method can yield a conductive paste with excellent dispersibility, and that by employing the nozzle diameter and pressure used in Examples 8-11, an even more excellent dispersibility conductive paste can be obtained. While agglomerates were observed under a microscope in the conductive pastes obtained in Examples 12-16, their excellent sedimentation stability allows them to be used as the conductive paste of this disclosure.

[0155] Industrial availability

[0156] This disclosure can be used for the formation of fine wiring in electronic devices, etc.

[0157] Explanation of reference numerals in the attached figures

[0158] 1. Conductive film

[0159] 2. Substrate

[0160] A. Height of the conductive film

[0161] B ··· Length of the short side of the conductive film

[0162] C ... the distance between the opposing conductive films

[0163] D ··· Nozzle diameter

[0164] 10··· Sample accumulation tank

[0165] 11··· Working cylinder

[0166] 12···plunger

[0167] 13··· Nozzle

[0168] 14···Ejected liquid accumulation tank.

Claims

1. A conductive paste, wherein, The conductive paste contains a conductor, a binder resin, and cellulose nanofibers. When the content of the conductor is set to 100 parts by mass, the content of the adhesive 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, When the content of the conductor is set to 100 parts by mass, the content of the adhesive 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.

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

4. A conductive film, which is formed by curing the conductive paste according to any one of claims 1 to 3, wherein, The conductive film has a thickness of 14 μm or more and 17.5 μm or less, and a length of 20 μm or more and 25 μm or less in the short side direction.

5. The conductive film according to claim 4, wherein, The resistivity of the conductive film is 2.00 × 10⁻⁶. -8 Below Ω·m.

6. The conductive film according to claim 4 or 5, wherein, The conductive films are spaced apart and opposite each other, with the distance between the opposing conductive films being less than 20 μm.

7. A method for manufacturing a conductive paste, wherein, The method for manufacturing the conductive paste includes a step of dispersing a conductor and cellulose nanofibers in an organic solvent using a high-pressure disperser.

8. The method for manufacturing the conductive paste according to claim 7, wherein, The process includes simultaneously pressurizing and feeding the conductor, binder resin, cellulose nanofibers, and organic solvent into a nozzle of a high-pressure disperser, wherein the process has the following (i) or (ii) conditions: (i) The nozzle has a diameter of more than 100 μm and less than 200 μm, and the pressure applied to the conductor, the adhesive resin, the cellulose nanofiber and the organic solvent before being injected into the nozzle is more than 100 MPa and less than 150 MPa; (ii) The nozzle has a diameter of 100 μm or more and less than 200 μm, and the pressure applied to the conductor, the adhesive resin, the cellulose nanofiber and the organic solvent before being fed into the nozzle is 50 MPa or more and less than 100 MPa.

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