JP5070524B2 - Production method of conductive film - Google Patents

Description

  The present invention relates to a method for advantageously producing a transparent conductive film and wiring using conductive oxide powder.

Sn-containing In oxide (ITO), SnO _{2, and the} like are used as transparent oxide conductive films for various display devices, solar cells, and the like because they exhibit translucency for visible light and high conductivity. For the formation of a transparent conductive film using ITO, a physical method such as a sputtering method and a coating method in which a particle dispersion or an organic compound is applied are known, but the coating method is compared with a physical method such as a sputtering method. Therefore, it is possible to form a film with a large area or a complicated shape without using an expensive apparatus, which is advantageous in terms of cost. For this reason, it is widely used as an electromagnetic wave shielding film for cathode ray tubes, but in recent years it has been applied to display devices such as touch panels, liquid crystal displays (LCD), plasma displays (PDP), electroluminescence (EL), and transparent electrodes for solar cells. Has also been considered.

  In addition, the coating-type transparent electrode wiring is formed by linearly arranging a transparent conductive film by screen printing, an inkjet method, or the like, and is often provided for an electronic path between the terminal device and the main body power supply. For this reason, it is important that the volume resistance is lower than the surface resistance. If the wire diameter, film thickness, and structure are the same, the volume resistance is proportional to the surface resistance, and the shape is thin in the horizontal direction and long in the vertical direction. Since the characteristics required for wiring are the same, in an increasingly complex device, the distinction between the two is becoming unclear.

Patent Document 1 and Patent Document 2 propose a film forming method in which a conductive metal oxide fine particle coating film or a semiconductor fine particle dispersion liquid coating film is irradiated with microwaves to perform sintering between particles. . Microwaves do not heat anything other than those with a large dielectric loss, and the heated objects themselves generate heat directly at the same time. Since there is no alteration, the target material can be sintered without losing the film.
JP 2000-123658 A JP 2004-342319 A

A transparent conductive film or wiring using a conductive oxide is basically required to have good conductivity, that is, low resistance. However, due to various factors, there is a limit in reducing the resistance to a low frequency depending on the type of conductive oxide. The object of the present invention is to easily reduce the resistance of a transparent conductive film or wiring beyond this limit. Further, in order to sinter the ITO particles, generally heating at 400 ° C. or higher is necessary, and it was impossible to sinter the ITO particles coated on the film. The present invention also makes this possible.

  The present inventor formed a low-resistance transparent conductive film and wiring when a coating film using a conductive oxide mixed metal nanoparticles with the conductive oxide and sintered the conductive oxides together. I found what I could do. In particular, when sintering is performed using microwaves, a low-resistance transparent conductive film and wiring can be advantageously obtained.

  That is, according to the present invention, a coating liquid is prepared by applying a dispersion liquid in which a dispersion of metallic silver particles of 200 nm or less and conductive oxide powder is dispersed in a liquid medium to a substrate, and then the frequency is 1 GHz to 1 THz. The manufacturing method of the electrically conductive film which irradiates this microwave and sinters the conductive oxide powder particle of the said coating film is provided.

  As the conductive oxide powder, one having an average particle diameter calculated from the specific surface area by the BET method of 1 nm or more and 200 nm or less can be used, and the ratio of the metal silver particles to the conductive oxide powder is 0.1% or more by weight percentage. However, if it is too much, the original transparency of the transparent conductive film is impaired, so that it is 10 wt% or less, preferably 5 wt% or less. A typical conductive oxide powder is a tin-containing indium oxide. The material of the substrate is not particularly limited, but can be a polymer film in the present invention.

  According to the present invention, the sinterability of the conductive oxide powder dispersion or paste can be improved, the sintering can be performed at a low temperature, and the resistance of the transparent conductive film or wiring can be significantly reduced.

  In the present invention, when a dispersion or paste of conductive oxide powder is applied to a substrate and sintered, an appropriate amount of metal nanoparticles is blended in the dispersion or paste, and then a microwave or a temperature of 400 ° C. or lower. It is characterized in that the conductive oxide particles are sintered by heat treatment.

Examples of the conductive oxide to which the present invention can be applied include Sn-containing In ₂ O ₃ (ITO), Zn-containing In ₂ O ₃ (IZO), F-containing In ₂ O ₃ (FTO), Sb-containing SnO ₂ (ATO), ZnO, Al-containing ZnO (AZO), Ga-containing ZnO (GZO), CdSnO ₃ , Cd ₂ SnO ₄ , TiO ₂ , CdO, etc. may be mentioned, and these may be used alone or in combination of two or more. Good. Among these, metal oxides mainly composed of In or Sn are preferable in order to achieve both conductivity, transparency and safety. The particle size of these conductive oxide powders is 1 nm or more, preferably 10 nm or more and 200 nm or less as calculated from the specific surface area by the BET method. When the particle size is less than 10 nm, dispersion in the case of forming a paint becomes difficult, and in the case of particles exceeding 200 nm, the transparency of the film becomes insufficient.

  Such oxide particles can be produced by a known method, for example, a sputtering method or a vacuum evaporation method described in JP 2000-3618 A may be employed, or tin tetrachloride added. A vapor phase reaction using a solution of indium trichloride prepared, a mixed solution of indium trichloride and tin tetrachloride is dropped into an ammonium carbonate solution to form a coprecipitated hydroxide of indium and tin. It is possible to employ a reduction firing method or the like that is washed with water, dried, and further heat-reduced in a hydrogen atmosphere or vacuum atmosphere, and then pulverized. In the above production method, tin dichloride may be used instead of tin tetrachloride. In this case, lowering the resistance is more advantageous.

  The oxide particles are dispersed in a liquid medium to form a dispersion or a paste. A conventional method can be used as a method for forming a paint. As the liquid medium, an organic solvent such as alcohol, ketone, ether or ester or water can be used, and it is preferable to add a surfactant or a coupling agent as a dispersant and disperse using a dispersing device such as a bead mill. . Although a binder may or may not be used, when a binder is used, an epoxy resin, an acrylic resin, a vinyl resin, a polyurethane resin, a polyvinyl alcohol resin, or the like can be used, but is not limited thereto.

  In the present invention, 0.1 wt% or more of metal nanoparticles are mixed in such a dispersion or paste of conductive oxide powder, and the metal nanoparticles are interposed between the conductive oxide particles. As the metal nanoparticles, Au, Ag, Cu, Ni, or the like is suitable, but Ag is particularly desirable from the viewpoint of low resistivity, weather resistance, and cost.

The metal nanoparticles have an average particle size (D _TEM ) of 200 nm or less when sintering is performed by microwaves, and the average particle size (D _TEM ) is 50 nm or less when performed by heat treatment at 400 ° C. or less. It is better to use one. The average particle diameter D _TEM represents the average particle diameter measured by TEM (transmission electron microscope) observation. In the measurement, the average value is obtained by measuring the diameters of 300 independent particles not overlapping from the TEM image magnified 600,000 times.

  In the case of metal particles having a particle size exceeding 50 nm, heating at 300 ° C. or higher is usually required for sintering. Normally used plastic films cannot withstand temperatures exceeding 300 ° C. Even the most heat-resistant polyimide film has a heat-resistant temperature of 300 ° C. to 350 ° C. at most. Therefore, when sintering on a plastic film, if metal nanoparticles having a particle size of 50 nm or less are used, the sintering temperature is lowered, so that it is 400 ° C. or less, preferably 350 ° C. or less, more preferably 300 ° C. or less. It can be sintered even by heat treatment at a temperature.

  On the other hand, when microwaves are used as a heating source, metal nanoparticles up to 200 nm can be used because the conductive layer is directly heated. Since the plastic film is not heated by the microwave, it can be sintered on the plastic film by using the microwave as a heating source. However, when metal particles exceeding 200 nm are used, it is not desirable because the light transmittance of the film is lowered without contributing much to the improvement in sintering of the conductive oxides.

  As a method for producing metal nanoparticles, either a gas phase method or a liquid phase method may be used, but from the industrial viewpoint, the liquid phase method is desirable. In the liquid phase method, as a method for reducing a metal compound in an organic solvent, an organic solvent that functions as a reducing agent such as an alcohol or a polyol is used, and the reduction reaction proceeds in the presence of an organic protective agent. Dispersed metal nanoparticles can be obtained.

  In order to mix the metal nanoparticles into the conductive oxide powder dispersion or paste, it is preferable to disperse the metal nanoparticles in a liquid medium to form a paint, and then mix the metal nanoparticles into the dispersion or paste. For this coating, an organic solvent such as alcohol, ketone, ether or ester or water is used as a liquid medium, a surfactant, a coupling agent or the like is added as a dispersant, and the metal nanoparticles are dispersed with a dispersing device such as a bead mill. Can be dispersed in a liquid medium. When using a binder, an epoxy resin, an acrylic resin, a vinyl chloride resin, a polyurethane resin, a polyvinyl alcohol resin, or the like can be used.

  The metal nanoparticle coating obtained in this way is blended and mixed in the conductive oxide powder dispersion or paste described above so that the ratio of the metal nanoparticles to the conductive oxide is 0.1 wt% or more. Thus, a dispersion or paste of the low-temperature sinterable conductive oxide powder according to the present invention can be obtained. In order to apply this dispersion or paste to a substrate, it can be applied by a method such as screen printing, spin coating, dip coating, roll coating, brush coating, spray coating or the like. The thickness of the coating layer may be in the range of 0.1 to 50 μm. If it becomes thinner than this, sufficient conductivity cannot be obtained, and if it becomes thicker than this, the transmittance decreases. A preferable coating thickness is 0.2 μm or more and 20 μm or less.

  Examples of the substrate to which the dispersion or paste of the present invention is applied include organic polymers, plastics, glass films and the like. For a substrate that requires flexibility such as a touch panel, an organic polymer film is preferable, and a film such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyimide, aramid, polycarbonate, or the like can be used. .

  After the dispersion or paste is applied to the substrate, the particles in the dispersion or paste are sintered by microwave irradiation or heat irradiation. When microwaves are used as the heating source, the oxide particles in the coating can be directly heated. The metal nanoparticles in the coating film reflect microwaves, but when the thermal energy generated by the oxide particles is transferred to the metal nanoparticles and melted, it becomes a medium for bonding the oxide particles together.

  A microwave having a frequency of 1 GHz to 1 THz can be used. When irradiated with microwaves, materials with low dielectric loss such as plastic film simply transmit, but materials with high dielectric loss such as oxide particles absorb heat and generate heat. The oxide coating can be selectively heated. Further, since it has high-speed microwave response performance, it can be easily controlled to the required temperature by adjusting the heating time and output. The most common microwave frequency is 2.45 GHz (household microwave oven) or 28 GHz (industrial furnace, etc.). In the examples described later, a microwave of 2.45 GHz is used, but the same effect can be obtained when a frequency in the above range is used. The irradiation input power may be 500 to 1000 W and the irradiation time may be 1 to 10 minutes, but the optimum irradiation condition varies depending on the composition ratio of the conductive oxide and the metal nanoparticles.

  In the microwave irradiation, the coating film should be on the back side of the substrate with respect to the irradiation direction (the substrate with the coating film on the metal sheet installed in the applicator and the coating surface on the bottom side) It is better to irradiate microwaves (from above the substrate). At that time, it is preferable to use a porous metal sheet (foamed metal sheet) as the metal sheet. By doing in this way, the crack of the coating-film surface can be prevented. As the foam metal sheet, a foam Ni sheet was used in the examples described later, but any material having good electronic conductivity and excellent heat dissipation can be used instead.

  As a mechanism of sintering a conductive oxide coating containing metal nanoparticles by microwave irradiation, oxide particles with a large dielectric loss are first heated by microwaves, and the heat propagates to the metal nanoparticles. It is considered that a melt of metal nanoparticles having a low melting point is interposed between oxide particles, and the presence of this melt excites sintering between oxide particles.

  When a coated substrate is heated by heat irradiation using an electric furnace or the like to sinter the coated film, the substrate is also heated by being exposed to the same temperature as the ambient temperature. When using a molecular film, the heating temperature is limited to 400 ° C. For example, even a polyimide resin used as a heat resistant film has a limit of 310 ° C.

  The atmosphere during microwave irradiation and heat irradiation can be performed in the air, but can also be performed in an inert atmosphere or a weak reducing atmosphere.

  According to the inventor's experience, when a coating film is formed only with tin-containing indium oxide (ITO) without blending metal nanoparticles, interparticle sintering is observed at a heat treatment temperature of 400 ° C. or lower. In addition, a decrease in resistance value cannot be confirmed. However, in the coating film in which the low melting point metal nanoparticles were added to the tin-containing indium oxide, a decrease in the resistance value was confirmed even when heat-treated at 400 ° C. or lower. That is, sintering was confirmed. This sintering process is considered as follows. First, dissolution of metal nanoparticles starts at a temperature of 400 ° C. or lower. The dissolved metal forms a neck at the contact portion with the ITO particles. The distance between the ITO particles is reduced by the surface tension of the neck, and sintering of the ITO particles starts. It is unclear whether sintering between ITO particles occurs at a temperature of 400 ° C. or lower. However, since a decrease in resistance is observed after heat treatment, it is sintered, or at least the metal nanoparticles have a distance between the ITO particles. It can be considered that it is shrunk and bonded.

An overcoat layer can be formed on the transparent conductive film layer using silicate ink or the like. The overcoat layer is used according to the purpose for the purpose of improving the film strength, weather resistance, reducing reflection, and improving conductivity.

Examples will be given below, but the measurement conditions for various characteristics will be described first.
Specific surface area (g / m ² ): determined by the BET method (one-point method).
Average particle diameter (nm): 200 diameters in a TEM photograph were measured with a vernier caliper, and converted into a magnification to obtain an average value.
BET particle size (nm): Determined using the following formula.
BET particle size (nm) = 6 / (ρ × specific surface area by BET method) × 10 ⁹
Where ρ is the true specific gravity (g / m ³ ). For example, the true specific gravity ρ in the case of ITO was 7.13 × 10 ⁹ g / m ³ .
Sheet resistance: Measured by a four-point probe method using Loresta HP manufactured by Mitsubishi Chemical Corporation.
Light transmittance: The total light transmittance was measured by the method based on the standard of JIS7361-1 using NDH2000 made by Nippon Denshoku Industries Co., Ltd.

[Control Example 1]
5 g of ITO powder (BET particle size 30 nm) containing 15% of SnO ₂ , 20 g of a mixed solvent (ethanol: propanol = 7: 3), and 0.25 g of an anionic dispersing agent, a planetary ball mill (P-5 manufactured by Fritche) Mold, container capacity 80 mL, PSZ 0.3 mm ball) and rotated for 30 minutes at 300 rpm. Ethanol was added to the obtained dispersion to prepare a coating material containing ITO powder content = 2% and the balance consisting of ethanol and propanol.

  This paint was applied to a film substrate made of polyethylene terephthalate using an applicator. The obtained coating film had a resistance of 800Ω / □ and a transparency of 86%.

[Control Example 2]
The same procedure as in Control Example 1 was performed until the coating material was applied on the film substrate. The obtained film substrate with a coating film was put in an electric furnace and heat-treated at 200 ° C. for 1 hour. The resistance value of the heat-treated alloy coating film was 800Ω / □, and the transparency was 85%, which was almost the same as that of Control Example 2.

[Example 1]
First, a dispersion of silver nanoparticles was produced by the following method. That is, 140 mL of isobutanol (special grade manufactured by Wako Pure Chemical Industries, Ltd.) as a solvent and reducing agent, 186 mL of oleylamine (Mw = 267 manufactured by Wako Pure Chemical Industries, Ltd.) as an organic protective material, and silver nitrate crystals (Kanto) as a silver compound (Chemical Co., Ltd.) 19 g is added and stirred with a magnetic stirrer to dissolve silver nitrate. These solutions are transferred to a container equipped with a reflux device and placed in an oil bath. While N ₂ is blown into the container as an inert gas, both are refluxed at a temperature of 100 ° C., and then raised to 108 ° C. The mixture was refluxed again to complete the reaction.

  The slurry after the reaction was subjected to solid-liquid separation at 3000 rpm for 30 minutes using a centrifuge, and then the supernatant was discarded. Ethanol was added to the resulting precipitate and dispersed by ultrasonic dispersion. This operation was repeated three times to obtain a precipitate. 40 mL of kerosene was added to this precipitate, and it was applied to an ultrasonic disperser. The turbid solution of silver particles and kerosene was subjected to solid / liquid separation at 3000 rpm for 30 minutes using a centrifuge, and the supernatant was recovered. This supernatant was a dispersion of silver particles having an average particle diameter of 10 nm.

  The obtained dispersion of silver nanoparticles and the ITO powder used in Control Example 1 were dispersed in kerosene so that the silver content was 0.1 wt% (Example 1-1), 1 wt% (Example 1 2), 5 wt% (Example 1-3, or 10 wt% (Example 1-4)) was mixed to prepare a composite dispersion solution, and a paint was prepared from the composite dispersion solution. The coating was applied to a film substrate made of polyethylene terephthalate using an applicator.

  A foamed Ni sheet is placed on a tray of a household microwave oven (2.45 GHz), and the film substrate is set so that the coated film surface is in contact with the foamed Ni sheet. Irradiated for 2 seconds. The film resistance and light transmittance of each conductive film obtained were measured, and the results are shown in Table 1.

[Example 2]
In the same liquid phase method as in Example 1, silver particle dispersions having an average particle diameter of 10 nm, 50 nm, 100 nm, and 200 nm were produced, and these dispersion liquids and the ITO powder used in Control Example 1 were all used. Silver particles are dispersed in kerosene in an amount of 0.1 wt%, and the average particle size of silver particles is 10 nm (Example 2-1), 50 nm (Example 2-2), 100 nm (Example 2-3). And 4 types of composite dispersion liquid of 200 nm (Example 2-4) was produced, the coating material was produced from these composite dispersion liquids, and the microwave was irradiated on the conditions similar to Example 1. FIG. The film resistance and light transmittance of each conductive film obtained were measured, and the results are shown in Table 1.

Example 3
Example 1-2 was repeated except that the ITO powder used had a BET particle size of 8 nm (Example 3-1) and 84 nm (Example 3-2). The film resistance and light transmittance of each conductive film obtained were measured, and the results are shown in Table 1.

Example 4
Example 2-1 and Example 2-2 were repeated except that instead of microwave heating, electric furnace heating at 200 ° C. × 1 hour was performed. The film resistance and light transmittance of each conductive film obtained were measured, and the results are shown in Table 1.

  As seen in Table 1, it can be seen that the film resistance of the ITO coating containing silver nanoparticles is greatly reduced by microwave heating. It can also be seen that the ITO coating without silver nanoparticles does not decrease in membrane resistance when heated at 200 ° C, whereas the ITO coating with silver nanoparticles decreases in membrane resistance when heated at 200 ° C.

Claims (5)

A coating film is prepared by applying a dispersion liquid in which a dispersion of metallic silver particles of 200 nm or less and conductive oxide powder is dispersed in a liquid medium to a substrate, and then irradiating a microwave with a frequency of 1 GHz to 1 THz. The manufacturing method of the electrically conductive film which sinters the electroconductive oxide powder particle of the said coating film . 2. The method for producing a conductive film according to claim 1, wherein the conductive oxide powder has an average particle size of 1 nm to 200 nm calculated from a specific surface area by a BET method. The ratio of the metal silver particle with respect to electroconductive oxide powder is 0.1% or more by weight percentage, The manufacturing method of the electrically conductive film of Claim 1 or 2. The method for producing a conductive film according to claim 1, wherein the conductive oxide powder is a tin-containing indium oxide . The method for producing a conductive film according to claim 1, wherein the substrate is a polymer film.