Cleaning solution

Abstract

To improve sinterability while suppressing agglomeration of metal particles when used as a metal ink, to obtain a sintered compact that appropriately transmits visible light, and to improve cleanability and suppress or eliminate clogging when used as a cleaning fluid.SOLUTION: Metal ink 10 comprises: metal particles 12; a solvent 16; a polyhydric alcohol 14 containing two or more OH groups and soluble in water and low-grade alcohols; and an additive 17 containing a water-soluble phosphoric acid compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a cleaning solution. [Background technology]

[0002] As an example of forming a metal layer on a component, Patent Document 1 describes forming a solder layer on a component. Furthermore, Patent Document 2 describes using silver paste to form a metal layer. Silver paste can be sintered under relatively low temperature conditions, and the melting point of the bonded layer formed after sintering is equivalent to that of silver. Therefore, the metal layer made from this sintered silver paste has excellent heat resistance and can be used stably even in high-temperature environments and high-current applications. On the other hand, from the viewpoint of material cost, copper paste may be used, as shown in Patent Document 3, for example.

[0003] Furthermore, when forming a metal layer in this manner, a metal ink, in which metal particles are dispersed in a liquid, may be used instead of a metal paste such as copper paste. Metal inks can offer manufacturing advantages, for example, because they can be sprayed from a nozzle.

[0004] Furthermore, in inkjet devices used to form metal layers, prolonged storage or exposure to high-temperature, low-humidity environments can cause problems such as drying, sedimentation, aggregation, and solidification of ink in the internal piping, flow paths, and discharge ports of the device. To address this, cleaning methods have been proposed, such as those using cleaning solutions containing organic solvents or surfactants, as shown in Patent Document 4, or cleaning solutions using highly alkaline solutions or surfactant solutions, as shown in Patent Document 5. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2004-172378 [Patent Document 2] Patent No. 6531547 [Patent Document 3] Japanese Patent Publication No. 2019-67515 [Patent Document 4] Japanese Patent Application Publication No. 4-115954 [Patent Document 5] Japanese Patent Application Publication No. 6-8471 [Overview of the project] [Problems that the invention aims to solve]

[0006] Such metallic inks may cause a decrease in the density of the sintered body and other degraded properties of the manufactured product due to the aggregation of metal particles. Furthermore, as with the cleaning solutions and methods described above, if the cleaning solution contains high-viscosity materials, there is a problem that the residue after cleaning will aggregate and become highly viscous, leading to malfunctions. In addition, if the cleaning solution is a highly alkaline solution, there is a problem that the alkali will corrode the equipment components. Therefore, metal inks are required to suppress the aggregation of metal particles. Furthermore, improved sinterability of the metal ink is also required. Additionally, the resulting sintered body is required to transmit visible light appropriately. Moreover, a cleaning solution that improves cleanability and can suppress or eliminate clogging is also needed.

[0007] The present invention has been made in view of the above, and aims to provide a sintered body that, when used as a metal ink, suppresses the aggregation of metal particles, improves sinterability, and appropriately transmits visible light, and when used as a cleaning solution, improves cleaning performance and suppresses or eliminates clogging. [Means for solving the problem]

[0008] The metal ink of this disclosure comprises metal particles, a solvent, a polyhydric alcohol containing two or more OH groups and soluble in water and lower alcohols, and an additive containing a water-soluble phosphoric acid compound.

[0009] The metal ink of this disclosure preferably further contains an organic solvent that has a boiling point of 150°C or higher at atmospheric pressure and is miscible with water.

[0010] Preferably, the additive is contained in a mass ratio of 0.01% or more and 10% or less based on the total amount of the metal ink.

[0011] Preferably, the molecular weight of the phosphoric acid compound is 90 or more and 200 or less.

[0012] Preferably, the additive contains at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, trimethyl phosphate, triethyl phosphate, diisopropyl phosphate, monoisopropyl phosphate, 2-aminoethyl dihydrogen phosphate, diisopropyl phosphite, and dimethyl phosphite.

[0013] Preferably, the polyhydric alcohol is contained in a mass ratio of 0.01% or more and 20% or less based on the total amount of the metal ink.

[0014] Preferably, the polyhydric alcohol has a melting point of 30°C or higher.

[0015] Preferably, the organic solvent is contained in a mass ratio of 0.01% or more and 30% or less based on the total amount of the metal ink.

[0016] Preferably, the organic solvent contains at least one of glycol ether and aprotic polar solvent.

[0017] Preferably, the metal particles are contained in a mass ratio of 1% or more and 50% or less based on the total amount of the metal ink.

[0018] Preferably, the metal particles are copper.

[0019] Preferably, the solvent contains water.

[0020] Preferably, it further contains a phosphate ester compound having a higher molecular weight than the phosphoric acid compound contained in the additive as a dispersant.

[0021] The solvent preferably contains a lower alcohol.

[0022] The solvent preferably includes a high-boiling-point solvent that contains one or more OH groups, has a boiling point of 150°C or higher, and is a liquid that is sparingly soluble or insoluble in water.

[0023] The metal particles are copper, the solvent contains water, the polyhydric alcohol contains at least one polyhydric alcohol that has two or more OH groups, is soluble in water and lower alcohols, and has a melting point of 30°C or higher, and the additive preferably has a molecular weight of 90 to 200 of the phosphoric acid compound.

[0024] The present disclosure is a method for producing a metal ink, which involves mixing metal particles, a solvent, a polyhydric alcohol containing two or more OH groups and soluble in water and lower alcohols, and an additive containing a water-soluble phosphoric acid compound to produce a metal ink comprising the metal particles, the solvent, the polyhydric alcohol, and the additive.

[0025] In the method for producing a metal ink according to the present disclosure, it is preferable to mix the metal particles, water as a solvent, the polyhydric alcohol, and the additive to produce a first metal ink which is a metal ink containing the metal particles, water, the polyhydric alcohol, and the additive.

[0026] In the method for producing the metal ink of this disclosure, it is preferable to mix an organic solvent, which has a boiling point of 150°C or higher at atmospheric pressure and is miscible with water, in order to produce the first metal ink which also contains the organic solvent.

[0027] The present disclosure is a method for producing a metal ink, and it is preferable to mix the first metal ink with a lower alcohol as a solvent to produce a second metal ink which is a metal ink containing the metal particles, water, the lower alcohol, the polyhydric alcohol, and the additive.

[0028] The present disclosure is a method for producing a metal ink, and it is preferable to mix the second metal ink with a high-boiling-point solvent as the solvent, which is a liquid containing one or more OH groups, having a boiling point of 150°C or higher, and being sparingly soluble or insoluble in water, to produce a third metal ink which is a metal ink containing the metal particles, water, the lower alcohol, the high-boiling-point solvent, the polyhydric alcohol, and the additive.

[0029] In the method for manufacturing a sintered body according to the present disclosure, it is preferable to heat the metal ink to form the sintered body.

[0030] The sintered body of this disclosure is preferably made using the metal ink.

[0031] The cleaning solution of this disclosure comprises metal particles, a solvent, a polyhydric alcohol containing two or more OH groups and soluble in water and lower alcohols, and an additive containing a water-soluble phosphoric acid compound. [Effects of the Invention]

[0032] According to the present invention, when used as a metal ink, it is possible to obtain a sintered body that suppresses the aggregation of metal particles, improves sinterability, and appropriately transmits visible light. When used as a cleaning solution, it is possible to improve cleaning performance and suppress or eliminate clogging. [Brief explanation of the drawing]

[0033] [Figure 1] Figure 1 is a schematic diagram of the metallic ink according to this embodiment. [Figure 2] Figure 2 is a flowchart illustrating the method for manufacturing the metal ink according to this embodiment. [Figure 3] Figure 3 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 4] Figure 4 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 5] Figure 5 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 6] Figure 6 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 7] Figure 7 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 8] Figure 8 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 9] Figure 9 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 10] Figure 10 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 11] Figure 11 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 12] Figure 12 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 13] Figure 13 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 14] Figure 14 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 15] Figure 15 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 16] Figure 16 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 17] Figure 17 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 18] Figure 18 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 19] Figure 19 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 20] Figure 20 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 21]Figure 21 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 22] Figure 22 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 23] Figure 23 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 24] Figure 24 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 25] Figure 25 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 26] Figure 26 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 27] Figure 27 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 28] Figure 28 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 29] Figure 29 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 30] Figure 30 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 31] Figure 31 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 32] Figure 32 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 33] Figure 33 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 34] Figure 34 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 35] Figure 35 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 36] Figure 36 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 37] Figure 37 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 38] Figure 38 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 39] Figure 39 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 40] Figure 40 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 41] Figure 41 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 42] Figure 42 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 43] Figure 43 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 44] Figure 44 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 45] Figure 45 is a table showing the content of the metal ink components in each example and the evaluation results. [Figure 46] Figure 46 is a table showing the component content of the cleaning solution in each example and the evaluation results. [Figure 47] Figure 47 is a table showing the component content of the cleaning solution in each example and the evaluation results. [Figure 48] Figure 48 is a table showing the component content of the cleaning solution in each example and the evaluation results. [Figure 49] Figure 49 is a table showing the component content of the cleaning solution in each example and the evaluation results. [Figure 50] Figure 50 is a table showing the component content of the cleaning solution in each example and the evaluation results. [Modes for carrying out the invention]

[0034] The present invention will now be described in detail with reference to the drawings. However, the present invention is not limited to the embodiments described below. Furthermore, the components in the embodiments below include those easily conceivable by those skilled in the art, those substantially identical, and those within the so-called equivalent range. Moreover, the components disclosed in the embodiments below can be combined as appropriate. Numerical values ​​are rounded to the nearest whole number.

[0035] Figure 1 is a schematic diagram of the metal ink according to this embodiment. As shown in Figure 1, the metal ink 10 according to this embodiment contains metal particles 12, a polyhydric alcohol 14, a solvent 16, an additive 17, and an organic solvent 18. The metal ink 10 refers to an ink-like substance in which the metal particles 12 are not dissolved in the liquid solvent 16, but are present in the solvent 16 as solid metal particles 12. In the metal ink 10, the metal particles 12 may be settled in the solvent 16 or dispersed. Note that the metal ink 10 does not necessarily contain the organic solvent 18.

[0036] The metallic ink 10 is used to form a sintered body (metal layer) (for example, to form wiring). For example, after spraying and drying the metallic ink 10 from a nozzle onto a substrate (a film of resin, metal, etc., or a substrate made of resin, metal, ceramic, etc., or a composite thereof), the metal particles 12 are sintered or melted while other components are removed by heating under an inert gas atmosphere or a reducing gas atmosphere, and then cooled to form a sintered body (metal layer) on the substrate made of the metallic component of the metal particles 12. Alternatively, after spraying and drying the metallic ink 10 from a nozzle onto a substrate (a film of resin, metal, etc., or a substrate made of resin, metal, ceramic, etc., or a composite thereof), the metal particles 12 are oxidized and sintered while other components are removed by heating under an oxidizing gas atmosphere, and then cooled to form a sintered body (metal oxide layer) on the substrate made of the metal oxide component of the oxidized metal particles 12. The applications of the sintered body formed by the metallic ink 10 are arbitrary, but examples include wiring formed from the metallic component of the metal particles 12, or layers made from the metal oxide component of the metal particles 12, such as cuprous oxide. However, the uses of the metallic ink 10 are not limited to these and may be arbitrary.

[0037] (metal particles) The metal particles 12 are particles of a metal. In this embodiment, it is preferable that the metal particles 12 are copper particles.

[0038] The metal particles 12 preferably have a particle size (Peak value of particle size distribution (number of particles)) of 10 nm or more and 1000 nm or less. The particle size of the metal particles 12 in the metal ink 10 can be determined using a particle size analyzer (Malvern Zetasizer Nano Series ZSP) by setting the refractive index of the metal particles 12 and the refractive index and viscosity of the solvent in the ink, and measuring at 20°C or 25°C, matching the temperature conditions of the physical properties, to obtain the Peak value of the particle size distribution (number of particles) of the metal particles 12. If sufficient measurement quality cannot be obtained in the measurement of the particle size distribution due to the high concentration of metal particles 12 in the metal ink 10, the metal particles may be diluted and dispersed 10 to 1000 times with the main solvent in the metal ink 10 (water, lower alcohol, or high-boiling point solvent) before measurement.

[0039] If the particle size is 10 nm or less, the specific surface area increases inversely proportional to the particle size, which can lead to a greater effect of surface oxidation and a decrease in the sinterability of the coating film obtained using the metal particles 12. On the other hand, if the particle size of the metal particles 12 is 1000 nm or more, the particle size becomes too large, which can lead to the metal particles 12 being more likely to settle and separate in the ink dispersed in the solvent. The particle size of the metal particles 12 is preferably in the range of 30 nm to 500 nm, and particularly preferably in the range of 30 nm to 300 nm.

[0040] The BET specific surface area of ​​the metal particle 12 can be determined by measuring the amount of gas adsorbed by the metal particle 12 using a specific surface area measuring device (QUANTACHROME AUTOSORB-iQ2, manufactured by Quantachrome Instruments) with nitrogen or krypton gas as the measuring gas. The BET specific surface area of ​​the metal particle 12 is 2.0 m². 2 / g or more 8.0m 2 It is preferable that the range be less than or equal to / g, and 3.5m 2 / g or more 8.0m 2 It is more preferable that the range be less than or equal to / g, and 4.0m 2 / g or more 8.0m 2 It is particularly preferable that the amount is within the range of / g or less. Furthermore, the shape of the metal particles 12 is not limited to spherical, but may also be needle-shaped or flattened plate-shaped.

[0041] Preferably, the surface of the metal particles 12 is partially or entirely coated with an organic substance. This coating suppresses oxidation of the metal particles 12, further reducing the likelihood of a decrease in sinterability due to oxidation. Furthermore, the organic substance coating the metal particles 12 is not formed by or derived from the polyhydric alcohol 14 or solvent 16. Also, the organic substance coating the metal particles 12 is not metal oxide (copper oxide) formed by the oxidation of the metal.

[0042] That the metal particles 12 are coated with an organic substance can be confirmed by analyzing the surface of the metal particles 12 using time-of-flight secondary ion mass spectrometry (TOF-SIMS). For example, when the metal particles 12 are copper, the metal particles 12 are detected as Cu by analyzing the surface using time-of-flight secondary ion mass spectrometry. + The ratio of the detected amount of C3H3O3 - ions to the detected amount of - Cu (C3H3O3 + / Cu ratio) is preferably 0.001 or more. The C3H3O3 - / Cu ratio is more preferably in the range of 0.05 or more and 0.2 or less. In addition, the surface of the metal particles 12 in this analysis refers to the surface of the metal particles 12 including the coating organic substance (that is, the surface of the organic substance), rather than the surface of the metal particles 12 when the organic substance is removed from the metal particles 12.

[0043] When the metal particles 12 are copper, C3H4O2 - ions and ions having 5 or more carbon atoms may be detected by analyzing the surface using time-of-flight secondary ion mass spectrometry. The ratio of the detected amount of C3H4O2 + ions to the detected amount of Cu (C3H4O2 - / Cu ratio) is preferably 0.001 or more. Also, the ratio of the detected amount of ions having 5 or more carbon atoms to the detected amount of Cu ions (ions having 5 or more carbon atoms / Cu - ratio) is preferably less than 0.005.

[0044] The C3H3O3 - ions, C3H4O2 - ions, and ions having 5 or more carbon atoms detected by time-of-flight secondary ion mass spectrometry are derived from the organic substance coating the surface of the metal particles 12. Therefore, the C3H3O3 - / Cu ratio and the C3H4O2 + / Cu ratio and <​​​​​​​​​​If each of the ratios is 0.001 or higher, the surface of the metal particles 12 becomes less susceptible to oxidation, and the metal particles 12 become less susceptible to aggregation. Also, C3H3O3 - / Cu + Ratio and C3H4O2 - / Cu + When the ratio is 0.2 or less, oxidation and aggregation of the metal particles 12 can be suppressed without excessively reducing the sinterability of the metal particles 12, and the generation of organic decomposition gases during heating can be suppressed, so that a bonding layer with fewer voids can be formed. To further improve the oxidation resistance of the metal particles 12 during storage and to further improve the sinterability at low temperatures, C3H3O3 - / Cu + Ratio and C3H4O2 - / Cu + The ratio is preferably within the range of 0.08 to 0.16. Also, C5 or higher ions / Cu + If the ratio is 0.005 or higher, a large amount of organic matter with a relatively high desorption temperature is present on the particle surface, resulting in insufficient sinterability and difficulty in obtaining a strong bonding layer. C5 or higher ions / Cu + The ratio is preferably less than 0.003 times.

[0045] The organic material coating the metal particles 12 is preferably a carboxylic acid derived from the carboxylic acid metal used in the production of the metal particles 12. The method for producing metal particles 12 coated with a carboxylic acid-derived organic material will be described later. The amount of organic material coating the metal particles 12 is preferably in the range of 0.5% to 2.0% by mass, more preferably in the range of 0.8% to 1.8% by mass, and even more preferably in the range of 0.8% to 1.5% by mass, based on 100% by mass of the metal particles. By having an organic material coating amount of 0.5% by mass or more, the metal particles 12 can be uniformly coated with the organic material, and the oxidation of the metal particles 12 can be more reliably suppressed. Furthermore, by having an organic material coating amount of 2.0% by mass or less, the generation of voids in the sintered body (bonding layer) of the metal particles due to gases generated by the decomposition of the organic material by heating can be suppressed. The amount of organic material coating can be measured using commercially available equipment. For example, the amount of coating can be measured using a differential thermal balance TG8120-SL (manufactured by RIGAKU Corporation). In this case, for example, the sample used is metal particles from which moisture has been removed by freeze-drying. To suppress oxidation of the metal particles, the measurement is performed in nitrogen (G2 grade) gas, and the heating rate is set to 10°C / min. The weight loss rate when heated from 250°C to 300°C can be defined as the amount of organic coating. That is, coating amount = (weight of sample after measurement) / (weight of sample before measurement) × 100 (wt%). The measurement is performed three times for each metal particle from the same lot, and the arithmetic mean may be used as the coating amount.

[0046] When the metal particles 12 are heated at 300°C for 30 minutes under an inert gas atmosphere such as argon gas, it is preferable that 50% or more by mass of the organic matter decomposes. The organic matter derived from carboxylic acid generates carbon dioxide gas, nitrogen gas, acetone vapor, and water vapor during decomposition.

[0047] (Polyhydric alcohol) Polyhydric alcohol 14 is an alcohol containing two or more OH groups and soluble in water and lower alcohols. Preferably, polyhydric alcohol 14 has a melting point of 30°C or higher. Lower alcohols refer to alcohols with five or fewer carbon atoms.

[0048] The polyhydric alcohol 14 may be at least one of the following, for example: 2,2-dimethyl-1,3-propanediol, 2,5-dimethyl-2,5-hexanediol, 2-hydroxymethyl-2-methyl-1,3-propanediol, 1-phenyl-1,2-ethanediol, 1,1,1-tris(hydroxymethyl)propane, erythritol, pentaerythritol, ribitol, resorcinol, (pyro)catechol, 5-methylresorcinol, pyrogallol, 1,2,3-cyclohexanetriol, and 1,3,5-cyclohexanetriol.

[0049] The polyhydric alcohol 14 is a non-electrolyte and exists in the metal ink 10 dissolved in the solvent 16 (with the molecules of the polyhydric alcohol 14 dispersed in the solvent 16). However, the form in which the polyhydric alcohol 14 exists in the metal ink 10 is arbitrary, and it may also be in an insoluble state in the solvent 16.

[0050] The inclusion of polyhydric alcohol 14 in the metal ink 10 allows the polyhydric alcohol 14 to coordinate around the metal particles 12, thereby effectively suppressing the aggregation of the metal particles 12. In other words, in this embodiment, it is preferable that the polyhydric alcohol 14 is coordinated around the metal particles 12.

[0051] (solvent) Solvent 16 is a liquid (medium) for dispersing the metal particles 12. Details of solvent 16 will be described later.

[0052] (Additives) Additive 17 contains a phosphate compound. The phosphate compound used as additive 17 is water-soluble and is present in the metal ink 10 in a dissolved state in solvent 16 (the molecules of the phosphate compound are dispersed in solvent 16). By including additive 17 containing the phosphate compound in the metal ink 10, a sintered body with appropriate transmittance of visible light can be obtained while appropriately sintering the metal particles 12.

[0053] The phosphoric acid compound used as additive 17 preferably has a molecular weight of 90 to 200, more preferably 100 to 200, and even more preferably 140 to 190. Having the molecular weight of the phosphoric acid compound within this range allows for proper dissolution in the solvent 16 without causing aggregation of the metal particles 12, and reduces or eliminates the residue of organic components during firing, thus allowing for more favorable improvement without hindering the sinterability of the metal particles. The molecular weight can be measured by methods such as vapor pressure lowering, boiling point elevation, freezing point (melting point) depression (Rust method), isothermal distillation (Buerger method), and mass spectrometry. Furthermore, the molecular weight here refers to the molecular weight excluding the molecular weight of water, even in the case of hydrates, and the same applies hereafter.

[0054] The phosphoric acid compound used as additive 17 preferably has a solubility of 10 g or more, more preferably 20 g or more, and even more preferably 50 g or more, in water at 1 atmosphere and 20°C. Having a solubility within this range allows for proper dissolution when water is used as solvent 16, thereby more favorably improving sinterability.

[0055] The phosphate compound used in additive 17 can be any compound, but examples of phosphate compounds that can be used in additive 17 include phosphoric acid, monosodium phosphate (sodium dihydrogen phosphate), disodium phosphate (disodium hydrogen phosphate), trisodium phosphate, monopotassium phosphate (potassium dihydrogen phosphate), dipotassium phosphate (dipodium hydrogen phosphate), trimethyl phosphate, triethyl phosphate, diisopropyl phosphate, monoisopropyl phosphate, 2-aminoethyl dihydrogen phosphate (O-phosphorylethanolamine), diisopropyl phosphite, and dimethyl phosphite. Additive 17 may be one of these, or two or more may be used.

[0056] (organic solvent) The organic solvent 18 is an organic solvent with components different from the polyhydric alcohol 14 and the solvent 16. The organic solvent 18 has a boiling point of 150°C or higher at atmospheric pressure and is miscible with water. It is more preferable that the organic solvent 18 has a boiling point of 200°C or higher. Here, miscible means that the organic solvent 18 can be mixed with water in any ratio (i.e., they can completely dissolve each other at any concentration). In this embodiment, it is preferable that the organic solvent 18 is miscible with the solvent 16.

[0057] The organic solvent 18 is preferably a glycol ether or an aprotic polar solvent. More specifically, the organic solvent 18 may contain both a glycol ether and an aprotic polar solvent; in other words, it is preferable that it contains at least one of a glycol ether and an aprotic polar solvent. Examples of glycol ethers contained in the organic solvent 18 include diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, polyethylene glycol monomethyl ether, diethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, ethylene glycol isobutyl ether, diethylene glycol monoisobutyl ether, ethylene glycol monoallyl ether, diethylene glycol monobenzyl ether, dipropylene glycol monomethyl ether, tripropylene glycol monomethyl ether, propylene glycol monopropyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, diethylene glycol methyl ethyl ether, and diethylene glycol diethyl ether. If the organic solvent 18 contains glycol ethers, it may contain at least one selected from the enumerated list. Examples of aprotic polar solvents included in organic solvent 18 include N-methylpyrrolidone, dimethylformamide, 2-pyrrolidone, and propylene carbonate. If organic solvent 18 contains aprotic polar solvents, it may contain at least one selected from the enumerated list.

[0058] (impurities) The metallic ink 10 may contain unavoidable impurities in addition to the components listed above. Examples of unavoidable impurities include substances produced when the components listed above undergo reactions such as decomposition, polymerization, addition, oxidation, or reduction with themselves, other components, or oxygen due to light, heat, etc.

[0059] (Metallic ink) The metallic ink 10 preferably contains metal particles 12 in a mass ratio of 1% to 50% of the total metallic ink 10, more preferably 5% to 50%, and even more preferably 5% to 30%. Having the metal particles 12 within this range allows for sufficient concentration of the metal particles 12 while suppressing a decrease in the fluidity of the metallic ink 10, thus offering advantages in manufacturing, such as improved nozzle ejection performance.

[0060] Preferably, the metallic ink 10 contains polyhydric alcohol 14 in a mass ratio of 0.01% to 20% of the total metallic ink 10. This range of polyhydric alcohol 14 content allows for proper dispersion of the metal particles 12 while preventing the concentration of the metal particles 12 from becoming too low.

[0061] The metal ink 10 preferably contains 50% to 99% by mass of the solvent 16 relative to the total metal ink 10, more preferably 50% to 95%, and even more preferably 60% to 95%. Having the solvent 16 content within this range allows for sufficient concentration of the metal particles 12 while suppressing a decrease in the fluidity of the metal ink 10, thus offering advantages in manufacturing, such as improved nozzle ejection performance.

[0062] The metallic ink 10 preferably contains additive 17 in a mass ratio of 0.01% to 10% of the total metallic ink 10, more preferably 0.1% to 10%, and even more preferably 0.1% to 5%. By having the additive 17 content within this range, the concentration of the metallic particles 12 can be sufficiently maintained while the metallic particles 12 can be more suitably sintered.

[0063] The metallic ink 10 preferably contains organic solvent 18 in a mass ratio of 0.01% to 30% of the total metallic ink 10, and more preferably 0.1% to 30%. This range of organic solvent 18 content ensures sufficient mold resistance even when the metallic ink 10 is left unattended for extended periods, allowing for proper long-term storage.

[0064] The metallic ink 10 may contain ionized metal particles 12 (ions of the metals that make up the metallic particles 12). That is, the liquid component of the metallic ink 10 may contain ionized metal particles 12. The ionized metal particles 12 may be copper ions.

[0065] The metal ink 10 described above can have variations in the components of the solvent 16. Below, we will describe each metal ink 10 with different solvent components.

[0066] (First Metal Ink) One of the metal inks 10, each with a different solvent 16 component, is designated as the first metal ink 10A. In the first metal ink 10A, the solvent 16 is water. The first metal ink 10A is formed by dissolving a polyhydric alcohol 14, an additive 17, and an organic solvent 18 in water (solvent 16) while mixing in metal particles 12. That is, the first metal ink 10A is an aqueous solution of polyhydric alcohol 14, an additive 17, and an organic solvent 18, with metal particles 12 included. Note that the first metal ink 10A does not necessarily need to contain the organic solvent 18, and in this case, the organic solvent 18 does not need to be added during the manufacturing process of the first metal ink 10A.

[0067] The first metal ink 10A preferably contains metal particles 12 in a mass ratio of 1% to 50% of the total first metal ink 10A, more preferably 5% to 50%, and even more preferably 5% to 30%. Having the metal particle 12 content within this range allows for sufficient concentration of metal particles 12 while suppressing a decrease in the fluidity of the first metal ink 10A, thus offering advantages in manufacturing, such as improved nozzle ejection performance.

[0068] The first metal ink 10A preferably contains polyhydric alcohol 14 in a mass ratio of 0.1% to 20% of the total first metal ink 10A, more preferably 0.5% to 20%, and even more preferably 1% to 20%. By having the polyhydric alcohol 14 content within this range, it is possible to appropriately disperse the metal particles 12 while preventing the concentration of the metal particles 12 from becoming too low.

[0069] The first metal ink 10A preferably contains additive 17 in a mass ratio of 0.1% to 10% of the total first metal ink 10A, more preferably 0.5% to 10%, and even more preferably 1% to 5%. By having the additive 17 content within this range, the metal particles 12 can be sintered more effectively while maintaining a sufficient concentration of the metal particles 12.

[0070] The first metal ink 10A preferably contains organic solvent 18 in a mass ratio of 0.5% to 30% of the total first metal ink 10A, more preferably 1% to 30%, and even more preferably 2% to 30%. This range of organic solvent 18 content allows for proper storage over a long period.

[0071] In this embodiment, it is preferable that the first metal ink 10A does not contain any substances other than metal particles 12, polyhydric alcohol 14, solvent 16 which is water, additives 17 and organic solvent 18, excluding unavoidable impurities. However, it is not limited to this, and the first metal ink 10A may contain additives other than metal particles 12, polyhydric alcohol 14, solvent 16 which is water, additives 17 and organic solvent 18 (dispersants, adhesion enhancers, rheology modifiers, rust inhibitors, sedimentation inhibitors, etc.).

[0072] For example, the first metal ink 10A may contain a dispersant other than the metal particles 12, polyhydric alcohol 14, solvent 16 which is water, additive 17, and organic solvent 18. Examples of dispersants include cationic dispersants, anionic dispersants, nonionic dispersants, and amphoteric dispersants. Among these, examples of anionic dispersants include carboxylic acid dispersants, sulfonic acid dispersants, and phosphoric acid dispersants, and phosphoric acid ester compounds are particularly preferred as phosphoric acid dispersants. The phosphoric acid ester compound used as a dispersant preferably has a higher molecular weight than the phosphoric acid compound used as additive 17. The molecular weight of the phosphoric acid ester compound used as a dispersant is preferably 200 to 3500, more preferably 300 to 3000, and even more preferably 400 to 2500. A molecular weight of 200 or higher ensures sufficient hydrophobic groups, resulting in good dispersibility of metal particles in water. A molecular weight of 3500 or lower allows for decomposition and reaction at the target heating temperature (approximately 200-350°C), thus avoiding interference with sintering of metal particles. Furthermore, the phosphate ester compound used as a dispersant may be any type, such as a mono-di-tryester or a neutralized salt, as long as it has good solubility in water. In the case of a mono-di-tryester, it is preferable that the HLB (Hydrophilic-Lipophilic Balance) value is 6 or more and 20 or less, more preferably 8 or more and 20 or less, and even more preferably 10 or more and 20 or less. The phosphate ester compound used as a dispersant can be any compound, but examples include polyoxyethylene alkyl (aliphatic) ether phosphate esters, polyoxyethylene phenyl (aromatic) ether phosphate esters, alkyl (aliphatic) phosphate esters, phenyl (aromatic) phosphate esters, and their neutralized salts. One of these may be used as a dispersant, or two or more may be used.

[0073] When the first metal ink 10A contains a phosphate ester compound as a dispersant, the content of the phosphate ester compound in the first metal ink 10A is preferably 0.001% to 2% by mass ratio of the total first metal ink 10A, more preferably 0.001% to 1%, and even more preferably 0.01% to 1%. By having the content of the phosphate ester compound within this range, aggregation of the metal particles 12 can be more effectively suppressed.

[0074] (Second Metal Ink) One of the metal inks 10, each with a different solvent 16 component, is designated as the second metal ink 10B. The second metal ink 10B contains a lower alcohol as the solvent 16, and moreover, the solvent 16 may contain something other than a lower alcohol; in this embodiment, it may contain water. The second metal ink 10B is obtained by dissolving a polyhydric alcohol 14, an additive 17, and an organic solvent 18 in the solvent 16 and mixing in metal particles 12. That is, for example, the second metal ink 10B is obtained by containing metal particles 12 in an aqueous solution of polyhydric alcohol 14, an additive 17, an organic solvent 18, and a lower alcohol. Note that the second metal ink 10B does not necessarily contain the organic solvent 18, and in this case, the organic solvent 18 does not need to be added during the manufacturing process of the second metal ink 10B.

[0075] Here, "lower alcohol" refers to an alcohol with five or fewer carbon atoms. More specifically, the lower alcohol used in solvent 16 is preferably an alcohol with one OH group, and preferably an alcohol with one to four carbon atoms. Examples of lower alcohols include methanol, ethanol, propyl alcohol, isopropyl alcohol, butyl alcohol, and sec-butyl alcohol. Note that one or more of these lower alcohols may be used.

[0076] The second metal ink 10B preferably contains metal particles 12 in a mass ratio of 1% to 50% of the total second metal ink 10B, more preferably 5% to 50%, and even more preferably 5% to 30%. Having the metal particle 12 content within this range allows for sufficient concentration of metal particles 12 while suppressing a decrease in the fluidity of the second metal ink 10B, thus offering advantages in manufacturing, such as improved nozzle ejection performance.

[0077] The second metal ink 10B preferably contains polyhydric alcohol 14 in a mass ratio of 0.01% to 10% of the total second metal ink 10B, more preferably 0.1% to 10%, and even more preferably 0.1% to 5%. By having a polyhydric alcohol 14 content within this range, it is possible to appropriately disperse the metal particles 12 while preventing the concentration of the metal particles 12 from becoming too low.

[0078] The lower alcohol content of the second metal ink 10B is preferably 5% to 99% by mass, more preferably 5% to 95%, and even more preferably 10% to 95%. Having the lower alcohol content within this range allows for sufficient concentration of the metal particles 12 while suppressing a decrease in the fluidity of the second metal ink 10B, thus offering advantages in manufacturing, such as improved nozzle spraying performance. Note that if multiple types of lower alcohols are included, the lower alcohol content here refers to the total content of each lower alcohol. The same applies to other components.

[0079] The second metal ink 10B preferably contains additive 17 in a mass ratio of 0.05% to 5% of the total second metal ink 10B, more preferably 0.1% to 5%, and even more preferably 0.1% to 2%. By having the additive 17 content within this range, the metal particles 12 can be sintered more favorably while maintaining a sufficient concentration of the metal particles 12.

[0080] The second metal ink 10B preferably contains organic solvent 18 in a mass ratio of 0.01% to 30% of the total second metal ink 10B, more preferably 0.1% to 20%, and even more preferably 0.5% to 20%. This range of organic solvent 18 content allows for proper storage over a long period.

[0081] In this embodiment, the second metal ink 10B does not need to contain any substances other than metal particles 12, polyhydric alcohol 14, solvent 16 (in this case, water and lower alcohol), additives 17 and organic solvent 18, excluding unavoidable impurities. However, it is not limited to this, and the second metal ink 10B may contain additives other than metal particles 12, polyhydric alcohol 14, solvent 16, additives 17 and organic solvent 18 (dispersants, adhesion enhancers, rheology modifiers, rust inhibitors, anti-settling agents, etc.).

[0082] For example, the second metal ink 10B may contain a dispersant other than the metal particles 12, polyhydric alcohol 14, solvent 16, additive 17, and organic solvent 18. Examples of dispersants include cationic dispersants, anionic dispersants, nonionic dispersants, and amphoteric dispersants. Among these, carboxylic acid dispersants, sulfonic acid dispersants, and phosphate dispersants are examples of anionic dispersants, and phosphate ester compounds are particularly preferred as phosphate dispersants. The phosphate ester compound used as a dispersant preferably has a higher molecular weight than the phosphate compound used as additive 17. The molecular weight of the phosphate ester compound used as a dispersant is preferably 200 to 3500, more preferably 300 to 3000, and even more preferably 400 to 2500. A molecular weight of 200 or higher provides sufficient hydrophobic groups, resulting in good dispersibility of metal particles in lower alcohols. A molecular weight of 3500 or lower allows for decomposition and reaction at the target heating temperature (around 200-350°C), thus avoiding interference with sintering of metal particles. Furthermore, the phosphate ester compound used as a dispersant may be any type, such as a mono-di-tryester or a neutralized salt, as long as it has good solubility in lower alcohols. In the case of a mono-di-tryester, it is preferable that the HLB value is 4 to 15, more preferably 5 to 15, and even more preferably 5 to 13. The phosphate ester compound used as a dispersant can be any compound, but examples include polyoxyethylene alkyl (aliphatic) ether phosphate esters, polyoxyethylene phenyl (aromatic) ether phosphate esters, alkyl (aliphatic) phosphate esters, phenyl (aromatic) phosphate esters, and their neutralized salts. One of these may be used as a dispersant, or two or more may be used.

[0083] When the second metal ink 10B contains a phosphate ester compound as a dispersant, the content of the phosphate ester compound in the second metal ink 10B is preferably 0.001% to 2% by mass ratio of the total second metal ink 10B, more preferably 0.001% to 1%, and even more preferably 0.01% to 1%. By having the phosphate ester compound content within this range, aggregation of the metal particles 12 can be more effectively suppressed.

[0084] Metal inks that use lower alcohols as the main solvent may experience aggregation of metal particles due to the lower alcohols. In contrast, the second metal ink 10B, by mixing in polyhydric alcohol 14, allows the polyhydric alcohol 14 to coordinate around the metal particles 12, for example, thereby suppressing aggregation of the metal particles 12 themselves.

[0085] (Third metal ink) One of the metal inks 10, each with a different solvent 16 component, is designated as the third metal ink 10C. The third metal ink 10C contains a high-boiling point solvent as the solvent 16, and more specifically, the main solvent, which is the primary component of the solvent 16, is a high-boiling point solvent. For example, the third metal ink 10C contains metal particles 12 while a polyhydric alcohol 14, additive 17, and organic solvent 18 are dissolved in the solvent 16. The third metal ink 10C may also contain a solvent other than the main high-boiling point solvent as the solvent 16. The third metal ink 10C may contain at least one of water and a lower alcohol, and in this embodiment, it contains both water and a lower alcohol. The third metal ink 10C does not necessarily contain the organic solvent 18, and in this case, the organic solvent 18 does not need to be added during the manufacturing process of the third metal ink 10C.

[0086] A high-boiling point solvent is a liquid containing one or more OH groups, with a boiling point of 150°C or higher, and sparingly soluble or insoluble in water. A high-boiling point solvent that is sparingly soluble or insoluble in water is preferably a solvent classified as a non-water-soluble liquid in Appendix 3 of the Cabinet Order Concerning the Regulation of Hazardous Materials under the Fire Service Act. The high-boiling point solvent is preferably a so-called organic solvent, and may be, for example, at least one of α-terpineol and 2-ethyl-1,3-hexanediol. Note that any of these solvents may contain isomers.

[0087] The third metal ink 10C preferably contains metal particles 12 in a mass ratio of 1% to 50% of the total third metal ink 10C, more preferably 5% to 50%, and even more preferably 5% to 30%. Having the metal particle 12 content within this range allows for sufficient concentration of the metal particles 12 while suppressing a decrease in the fluidity of the second metal ink 10B, thus offering advantages in manufacturing, such as improved nozzle ejection performance.

[0088] The third metal ink 10C preferably contains polyhydric alcohol 14 in a mass ratio of 0.01% to 5% of the total third metal ink 10C, more preferably 0.01% to 5%, and even more preferably 0.01% to 3%. By having the polyhydric alcohol 14 content within this range, it is possible to appropriately disperse the metal particles 12 while preventing the concentration of the metal particles 12 from becoming too low.

[0089] The third metal ink 10C preferably contains a high-boiling-point solvent in a mass ratio of 50% to 99% of the total third metal ink 10C, more preferably 50% to 95%, and even more preferably 60% to 95%. Having the high-boiling-point solvent content within this range allows for sufficient concentration of the metal particles 12 while suppressing a decrease in the fluidity of the third metal ink 10C, thus offering advantages in manufacturing, such as improved nozzle ejection performance.

[0090] The third metal ink 10C preferably contains additive 17 in a mass ratio of 0.01% to 2% of the total third metal ink 10C, more preferably 0.1% to 2%, and even more preferably 0.1% to 1%. By having the additive 17 content within this range, the metal particles 12 can be sintered more favorably while maintaining a sufficient concentration of the metal particles 12.

[0091] The third metal ink 10C preferably contains organic solvent 18 in a mass ratio of 0.01% to 30% of the total third metal ink 10C, more preferably 0.01% to 10%, and even more preferably 0.1% to 10%. This range of organic solvent 18 content allows for proper storage over a long period.

[0092] The third metal ink 10C preferably contains a dispersant, which is a component other than the metal particles 12, polyhydric alcohol 14, solvent 16, additive 17, and organic solvent 18. Examples of dispersants include cationic dispersants, anionic dispersants, nonionic dispersants, and amphoteric dispersants. Among these, carboxylic acid dispersants, sulfonic acid dispersants, and phosphate dispersants are examples of anionic dispersants, and phosphate ester compounds are particularly preferred as phosphate dispersants. The phosphate ester compound used as a dispersant preferably has a higher molecular weight than the phosphate compound used as additive 17. The molecular weight of the phosphate ester compound used as a dispersant is preferably 200 to 3500, more preferably 300 to 3000, and even more preferably 400 to 2500. A molecular weight of 200 or higher ensures sufficient hydrophobic groups, resulting in good dispersibility of metal particles in high-boiling point solvents. A molecular weight of 3500 or lower allows for decomposition and reaction at the target heating temperature (approximately 200-350°C), thus avoiding interference with sintering of metal particles. Furthermore, the phosphate ester compound used as a dispersant may be any type, such as a mono-di-tryester or a neutralized salt, as long as it has good solubility in lower alcohols. In the case of a mono-di-tryester, it is preferable that the HLB value is 2 to 15, more preferably 3 to 15, and even more preferably 4 to 13. The phosphate ester compound used as a dispersant can be any type, but examples include polyoxyethylene alkyl (aliphatic) ether phosphate esters, polyoxyethylene phenyl (aromatic) ether phosphate esters, alkyl (aliphatic) phosphate esters, phenyl (aromatic) phosphate esters, and polyoxyethylene alkyl ether phosphate esters such as laureth-n phosphate, oleth-n phosphate, steareth-n phosphate (n=2~10), and alkyl phosphate esters. One of these may be used as a dispersant, or two or more may be used.

[0093] The third metal ink 10C preferably contains a dispersant in a mass ratio of 0.01% to 5% of the total third metal ink 10C, more preferably 0.1% to 5%, and even more preferably 0.1% to 3%. This range of dispersant content effectively suppresses the aggregation of metal particles 12.

[0094] In this embodiment, it is preferable that the third metal ink 10C, excluding unavoidable impurities, does not contain any substances other than metal particles 12, polyhydric alcohol 14, solvent 16 (here, water, lower alcohol, and high-boiling point solvent), additives 17, organic solvent 18, and dispersants. However, it is not limited to this, and the third metal ink 10C may not contain a dispersant, or it may contain additives other than metal particles 12, polyhydric alcohol 14, solvent 16, additives 17, organic solvent 18, and dispersants (adhesion enhancers, rheology modifiers, rust inhibitors, sedimentation inhibitors, etc.).

[0095] Metal inks that use a high-boiling point solvent as the main solvent may experience aggregation of metal particles 12 due to the high-boiling point solvent. In contrast, the third metal ink 10C, by mixing in a polyhydric alcohol 14, can suppress aggregation of metal particles 12 by, for example, coordinating the polyhydric alcohol 14 around the metal particles 12.

[0096] (Method of manufacturing metal ink) Next, the method for manufacturing the metal ink 10 described above will be explained. Figure 2 is a flowchart illustrating the method for manufacturing the metal ink according to this embodiment.

[0097] (Manufacturing of metal particles) As shown in Figure 2, in this manufacturing method, metal particles 12 are produced by mixing a carboxylic acid metal aqueous dispersion with a reducing agent (step S10). Specifically, first, an aqueous dispersion of a carboxylic acid metal (e.g., copper carboxylic acid) is prepared, and a pH adjusting agent is added to this carboxylic acid metal aqueous dispersion to adjust the pH to 2.0 or higher and 7.5 or lower. Next, under an inert gas atmosphere, a hydrazine compound capable of reducing metal ions is added to this pH-adjusted carboxylic acid metal aqueous dispersion in an amount of 1.0 to 1.2 equivalents as a reducing agent and mixed. The resulting mixture is heated to a temperature of 60°C to 80°C under an inert gas atmosphere and held for 1.5 to 2.5 hours. This reduces the metal ions eluted from the carboxylic acid metal to produce metal particles 12, and also causes organic matter derived from the metal acid to form on the surface of these metal particles 12. The carboxylic acids used here include glycolic acid, citric acid, malic acid, maleic acid, malonic acid, fumaric acid, succinic acid, tartaric acid, oxalic acid, phthalic acid, benzoic acid, and their salts. While hydrazine compounds were used as reducing agents, the formula is not limited to these; hydrazine, ascorbic acid, oxalic acid, formic acid, and their salts may also be used.

[0098] (Metal particles: Manufacturing of copper particles) The following describes a method for producing metal particles 12 when the metal particles 12 are copper particles. An aqueous dispersion of copper carboxylate can be prepared by adding powdered metal carboxylate to pure water such as distilled water or deionized water at a concentration of 25% to 40% by mass, stirring with a stirring blade, and dispersing it uniformly. Examples of pH adjusting agents include triammonium citrate, ammonium hydrogen citrate, and citric acid. Among these, triammonium citrate is preferred because it is easy to adjust the pH mildly. The pH of the aqueous dispersion of copper carboxylate is set to 2.0 or higher in order to increase the elution rate of copper ions eluted from copper carboxylate, thereby rapidly promoting the formation of copper particles and obtaining the target fine copper particles. The pH is set to 7.5 or lower in order to suppress the formation of copper(II) hydroxide from the eluted metal ions and to increase the yield of copper particles. In addition, by setting the pH to 7.5 or lower, it is possible to suppress the reducing power of the hydrazine compound from becoming excessively high, making it easier to obtain the target copper particles. It is preferable to adjust the pH of the copper carboxylate aqueous dispersion to a range of 4 to 6.

[0099] The reduction of copper carboxylate with hydrazine compounds is carried out under an inert gas atmosphere. This is to prevent the oxidation of copper ions dissolved in the solution. Examples of inert gases include nitrogen gas and argon gas. Hydrazine compounds have advantages such as not producing residue after the reduction reaction when reducing copper carboxylate under acidic conditions, being relatively safe, and being easy to handle. Examples of hydrazine compounds include hydrazine monohydrate, anhydrous hydrazine, hydrazine hydrochloride, and hydrazine sulfate. Among these hydrazine compounds, hydrazine monohydrate and anhydrous hydrazine, which do not contain components that can become impurities such as sulfur and chlorine, are preferred.

[0100] Generally, copper generated in acidic solutions with a pH below 7 dissolves. However, in this embodiment, a hydrazine compound, which is a reducing agent, is added to an acidic solution with a pH below 7 and mixed, generating copper particles in the resulting mixture. As a result, the carboxylic acid-derived components generated from copper carboxylate quickly coat the surface of the copper particles, suppressing the dissolution of the copper particles. It is preferable to raise the aqueous dispersion of copper carboxylate, after adjusting the pH, to a temperature of 50°C to 70°C to facilitate the reduction reaction.

[0101] The mixture of copper particles and hydrazine compounds, heated to a temperature of 60°C to 80°C under an inert gas atmosphere and held for 1.5 hours to 2.5 hours, is intended to generate copper particles and to form and coat the surface of the generated copper particles with organic matter. Heating and holding under an inert gas atmosphere is intended to prevent oxidation of the generated copper particles. The starting material, copper carboxylate, typically contains about 35% by mass of copper. By adding a hydrazine compound, which is a reducing agent, to an aqueous dispersion of carboxylic acid containing this amount of copper, heating it to the above temperature, and holding it for the above time, the generation of copper particles and the generation of organic matter on the surface of the copper particles proceed in a balanced manner, so that copper particles can be obtained in which the amount of organic matter coating is in the range of 0.5% by mass to 2.0% by mass per 100% by mass of copper particles. If the heating temperature is below 60°C and the holding time is less than 1.5 hours, the carboxylic acid metal may not be completely reduced, the rate of copper particle generation may become too slow, and there is a risk that the amount of organic matter coating the copper particles will be excessive. Furthermore, if the heating temperature exceeds 80°C and the holding time exceeds 2.5 hours, the rate of copper particle formation may become too fast, potentially resulting in insufficient organic matter coating the copper particles. The preferred heating temperature is 65°C to 75°C, and the preferred holding time is 2 hours to 2.5 hours.

[0102] The copper particles produced in the mixture may be washed and desalted using pure water or the like under an inert gas atmosphere. Furthermore, by dewatering using a centrifuge, for example, a water slurry containing metal particles 12 with a fixed solid-liquid ratio (e.g., solid-liquid ratio: 50 / 50 [mass%]) can be obtained. In some cases, solid-liquid separation can be performed, and copper particles coated with organic matter can be obtained by drying using freeze-drying or vacuum drying methods. Because the surface of these copper particles is coated with organic matter, they are less susceptible to oxidation even when stored in the atmosphere.

[0103] (Manufacturing of the first metal ink) Next, the metal particles 12, water as the solvent 16, polyhydric alcohol 14, additive 17, and organic solvent 18 are mixed to produce the first metal ink 10A (step S12). Here, it is preferable to produce the first metal ink 10A by mixing the metal particles 12, polyhydric alcohol 14, additive 17, organic solvent 18, and water so that the content of each of the metal particles 12, polyhydric alcohol 14, additive 17, and organic solvent 18 falls within the numerical range described above. The method of mixing the metal particles 12, polyhydric alcohol 14, additive 17, organic solvent 18, and water is arbitrary. For example, an aqueous solution of polyhydric alcohol 14 and organic solvent 18, which includes water, may be mixed into a water slurry containing metal particles 12, or an aqueous solution of polyhydric alcohol 14, additive 17, and organic solvent 18 may be mixed into metal particles 12 that do not contain water. Furthermore, if necessary, the mixture may be filtered using a filter with a predetermined mesh size to remove foreign matter or aggregated metal particles 12. Such filtration may be performed at any stage in the subsequent metal ink manufacturing process.

[0104] (Manufacturing of second-metal ink) Next, the first metal ink 10A is mixed with a lower alcohol as a solvent 16 to produce the second metal ink 10B (step S14). Here, it is preferable to produce the second metal ink 10B by mixing the first metal ink 10A with the lower alcohol so that the content of metal particles 12, polyhydric alcohol 14, additive 17, lower alcohol, and organic solvent 18 falls within the numerical range described above. The method of mixing the first metal ink 10A with the lower alcohol is arbitrary. For example, the first metal ink 10A obtained in step S12 may be left to stand for a predetermined time (e.g., about one day) or centrifuged under predetermined conditions, and then a portion of the supernatant may be removed, and the lower alcohol may be added to the first metal ink 10A from which the supernatant has been removed. Alternatively, the second metal ink 10B may be produced by mixing the metal particles 12, water and lower alcohol as solvents 16, polyhydric alcohol 14, additive 17, and organic solvent 18 without going through step S12.

[0105] (Manufacturing of third-metal ink) Next, the second metal ink 10B is mixed with a high-boiling point solvent as solvent 16 and a dispersant to produce the third metal ink 10C (step S16). Here, it is preferable to produce the third metal ink 10C by mixing the second metal ink 10B with the high-boiling point solvent and dispersant so that the content of metal particles 12, polyhydric alcohol 14, additive 17, high-boiling point solvent, dispersant, and organic solvent 18 is within the numerical range described above. The method of mixing the second metal ink 10B with the high-boiling point solvent and dispersant is optional. For example, the second metal ink 10B obtained in step S14 may be left to stand for a predetermined time (e.g., about one day) or centrifuged under predetermined conditions, and then a portion of the supernatant may be removed, and the high-boiling point solvent may be added to the second metal ink 10B from which the supernatant has been removed. The addition of a dispersant is not essential. Furthermore, solvents (water, lower alcohols, high-boiling point solvents, etc.) may be removed from or added to the third metal ink 10C so that the numerical range described above is maintained.

[0106] The third metal ink 10C produced in this manner is used as the metal ink 10. In the above explanation, the second metal ink 10B was produced using the first metal ink 10A, and the third metal ink 10C was produced using the second metal ink 10B. In other words, the first metal ink 10A and the second metal ink 10B were intermediate materials for producing the third metal ink 10C. However, the first metal ink 10A and the second metal ink 10B are not limited to being intermediate materials, and the first metal ink 10A and the second metal ink 10B themselves may be used as the metal ink 10.

[0107] The manufacturing methods for the metal particles 12 and metal ink 10 described above are merely examples, and the metal particles 12 and metal ink 10 may be manufactured by any method.

[0108] (Cleaning solution) The metal ink 10 described above is used as a raw material for sintered bodies. For example, the metal ink 10 is discharged or sprayed from a nozzle through a flow path inside various coating and patterning devices to obtain coating films, coating patterns, and molded bodies of the metal ink 10. By heating these, the metal ink 10 is sintered to obtain sintered films, sintered patterns, and sintered bodies. In such cases, if the discharge or spraying of the metal ink 10 is stopped for a long time, the flow of the flow path or nozzle inside the device may become poor, or in the worst case, become clogged, due to the sedimentation of metal particles 12 contained in the metal ink 10 or the drying due to the evaporation of solvent components contained in the metal ink 10. To suppress or resolve such clogging of flow path or nozzle, it is effective to redisperse or dissolve the settled and dried components in the flow path or nozzle using a cleaning solution. In this case, it is desirable to redisperse the settled and dried solid components with a cleaning solution.

[0109] As a result of diligent research, the inventors have found that by using the metal ink 10 described above as a cleaning solution, clogging of the flow path and nozzle can be effectively suppressed and resolved. In other words, the cleaning solution in this embodiment comprises metal particles 12, a solvent 14, a polyhydric alcohol 16 containing two or more OH groups and soluble in water and lower alcohols, and an additive 17 containing a water-soluble phosphoric acid compound. The preferred components of the cleaning solution are the same as those of the metal ink 10 described above, so their description is omitted. For example, the first metal ink 10A, the second metal ink 10B, or the third metal ink 10C described above may be used as the cleaning solution.

[0110] However, the content of metal particles 12 in the cleaning solution is preferably less than the content of metal particles 12 in the metal ink 10 described above. The content of metal particles 12 in the cleaning solution is preferably 0.5% by mass or more and 5% by mass or less, more preferably 1% by mass or more and 5% by mass or less, and even more preferably 1% by mass or more and 3% by mass or less, relative to the total amount of the cleaning solution.

[0111] In this case, the cleaning solution described above is supplied to the flow path or nozzle in which the metal ink 10 described above is stored, and the cleaning solution is ejected from the nozzle. By cleaning the flow path or nozzle with such a cleaning solution, clogging of the flow path or nozzle by metal particles 12 remaining in the nozzle can be effectively suppressed and resolved. Therefore, by using the metal ink 10 according to this embodiment as a cleaning solution, clogging can be suppressed or resolved, and a sintered body can be properly obtained.

[0112] (Other examples of cleaning solutions) As mentioned above, it is preferable to use a cleaning solution with the same components as the metal ink 10, but it is not limited to this. For example, if metal ink 10A is used, a cleaning solution containing only water may be used; if metal ink 10B is used, a cleaning solution containing water and a lower alcohol may be used; and if metal ink 10C is used, a cleaning solution containing a lower alcohol and a high-boiling point solvent that is poorly soluble or insoluble in water may be used. In this case, the cleaning solution is supplied to the flow path or nozzle where the metal inks 10A to C are stored, and sprayed from the nozzle. As a result, the metal particles 12 remaining in the flow path or nozzle are redispersed together with the cleaning solution and sprayed from the nozzle, effectively suppressing and resolving clogging of the flow path or nozzle. Furthermore, if metal ink 10A is used, a cleaning solution containing only water may be used; if metal ink 10B is used, a cleaning solution containing water and a lower alcohol may be used; and if metal ink 10C is used, a cleaning solution containing a lower alcohol and a high-boiling point solvent that is poorly soluble or insoluble in water may be used, as this does not leave metal particles 12 in the flow path or nozzle, which is preferable.

[0113] (effect) As described above, the metal ink 10 according to this embodiment comprises metal particles 12, a solvent 16, a polyhydric alcohol 14 containing two or more OH groups and soluble in water and lower alcohols, and an additive 17 containing a water-soluble phosphoric acid compound. In metal inks in which metal particles are dispersed in a solvent, there is a risk of the metal particles agglomerating. If the metal particles agglomerate, it may lead to a decrease in the properties of the manufactured product, such as a decrease in the density of the sintered body. In contrast, since the metal ink 10 according to this embodiment contains a polyhydric alcohol 14, the aggregation of the metal particles 12 can be suppressed by the polyhydric alcohol 14. According to the metal ink 10 according to this embodiment, since the aggregation of metal particles 12 can be suppressed, a decrease in the properties of the manufactured product can be suppressed and a suitable sintered body can be obtained. Furthermore, for example, when the metal ink 10 is sprayed with a nozzle, suppressing the aggregation of metal particles 12 can also suppress manufacturing defects such as nozzle clogging. Furthermore, since the metal ink 10 according to this embodiment contains an additive 17 containing a phosphoric acid compound, the sinterability of the sintered body can be improved when the metal ink 10 is heated to produce a sintered body of metal particles 12 in the metal ink 10. Moreover, since the metal ink 10 according to this embodiment contains an additive 17 containing a phosphoric acid compound, a sintered body with appropriate transmittance of visible light can be obtained. Therefore, a sintered body sintered using the metal ink 10 according to this embodiment can be appropriately applied to a tandem solar cell combined with a crystalline Si solar cell. However, the application of the sintered body sintered using the metal ink 10 may be arbitrary.

[0114] The metal ink 10 preferably further contains an organic solvent 18 that has a boiling point of 150°C or higher at atmospheric pressure and is miscible with water. The inclusion of the organic solvent 18 provides sufficient mold resistance even when left for a long period of time, allowing the metal ink 10 to be stored properly for an extended period.

[0115] Furthermore, it is preferable that the additive 17 is present in an amount of 0.01% to 10% by mass relative to the total amount of the metal ink 10. By setting the content of additive 17 within this range, it is possible to suppress the aggregation of metal particles 12 while improving sinterability, and to obtain a sintered body that appropriately transmits visible light.

[0116] Furthermore, the molecular weight of the phosphoric acid compound used as additive 17 is preferably between 90 and 200. By setting the molecular weight of the phosphoric acid compound within this range, it is possible to appropriately dissolve it in solvent 16 while more favorably improving sinterability.

[0117] Furthermore, it is preferable that additive 17 contains at least one of the following: phosphoric acid, monosodium phosphate (sodium dihydrogen phosphate), disodium phosphate (disodium hydrogen phosphate), trisodium phosphate, monopotassium phosphate (potassium dihydrogen phosphate), dipotassium phosphate (dipodium hydrogen phosphate), trimethyl phosphate, triethyl phosphate, diisopropyl phosphate, monoisopropyl phosphate, 2-aminoethyl dihydrogen phosphate (O-phosphorylethanolamine), diisopropyl phosphite, and dimethyl phosphite. By using such additive 17, sinterability can be more favorably improved, and a sintered body that appropriately transmits visible light can be obtained.

[0118] Furthermore, it is preferable that the polyhydric alcohol 14 is present in an amount of 0.01% to 20% by mass relative to the total amount of the metal ink 10. This allows for proper dispersion of the metal particles 12 while preventing the concentration of the metal particles 12 from becoming too low.

[0119] Furthermore, the polyhydric alcohol 14 preferably has a melting point of 30°C or higher. This allows for proper dispersion of the metal particles 12 while preventing the concentration of the metal particles 12 from becoming too low.

[0120] Furthermore, it is preferable that the organic solvent 18 is present in an amount of 0.01% to 30% by mass relative to the total amount of the metal ink 10. This ensures sufficient mold resistance even when the metal ink 10 is left unattended for a long period of time, allowing for proper storage over extended periods.

[0121] Furthermore, it is preferable that the organic solvent 18 contains at least one of a glycol ether and an aprotic polar solvent. This ensures sufficient mold resistance even when the metal ink 10 is left for a long period of time, allowing for proper storage over extended periods.

[0122] Furthermore, it is preferable that the metal particles 12 are present in an amount of 1% to 50% by mass relative to the total amount of metal ink 10. This allows for maintaining a sufficient concentration of metal particles 12 while suppressing a decrease in the fluidity of the metal ink 10.

[0123] Furthermore, the metal particles 12 are preferably copper. According to this embodiment, a copper sintered body can be manufactured appropriately.

[0124] Furthermore, the solvent 16 preferably contains water. According to this embodiment, it is possible to suppress the aggregation of metal particles 12 while improving sinterability, and to obtain a sintered body that appropriately transmits visible light.

[0125] Furthermore, it is preferable that the solvent 16 contains a lower alcohol. According to this embodiment, it is possible to suppress the aggregation of metal particles 12 while improving sinterability, and to obtain a sintered body that appropriately transmits visible light.

[0126] Furthermore, it is preferable that the metal ink 10 further contains a phosphate ester compound with a higher molecular weight than the phosphate compound contained in the additive 17 as a dispersant. According to this embodiment, aggregation of metal particles 12 can be suitably suppressed. More specifically, in metal inks 10A and 10B, if a water-soluble polyhydric alcohol 14 is present during drying, the solvent 16 such as water or lower alcohol will evaporate, increasing the content ratio of the polyhydric alcohol 14, which may reduce the dispersion of metal particles 12 in the film. In contrast, by adding a dispersant phosphate compound with a high molecular weight, it is possible to maintain dispersibility even just before drying.

[0127] Furthermore, it is preferable that the solvent 16 contains one or more OH groups, has a boiling point of 150°C or higher, and is a high-boiling point solvent that is sparingly soluble or insoluble in water. According to this embodiment, sinterability can be improved more appropriately.

[0128] Furthermore, the metal particles 12 are copper, the solvent 16 contains water, the polyhydric alcohol 14 contains at least one polyhydric alcohol that has two or more OH groups, is soluble in water and lower alcohols, and has a melting point of 30°C or higher, and the additive 17 preferably has a molecular weight of 90 to 200. According to this embodiment, it is possible to improve sinterability while suppressing aggregation of the metal particles 12, and to obtain a sintered body that appropriately transmits visible light.

[0129] Furthermore, the method for producing the metal ink 10 according to this disclosure involves mixing metal particles 12, a solvent 16, a polyhydric alcohol 14 containing two or more OH groups and soluble in water and lower alcohols, and an additive 17 containing a water-soluble phosphoric acid compound to produce the metal ink 10 containing the metal particles 12, the solvent 16, the polyhydric alcohol 14, and the additive 17. According to this disclosure, it is possible to produce a metal ink 10 that can suitably improve sinterability while suppressing aggregation of the metal particles 12, and it is also possible to obtain a sintered body that has appropriate transmittance of visible light.

[0130] Furthermore, in the method for producing the metal ink 10 according to this disclosure, it is preferable to mix metal particles 12, water (which is a solvent 16), a polyhydric alcohol 14, and an additive 17 to produce a first metal ink 10A which is a metal ink containing metal particles 12, water, polyhydric alcohol 14, and additive 17. According to this disclosure, it is possible to produce a first metal ink 10A that can suitably improve sinterability while suppressing aggregation of metal particles 12.

[0131] Furthermore, in the method for producing the metal ink 10 according to this disclosure, it is preferable to also mix in an organic solvent 18 having a boiling point of 150°C or higher at atmospheric pressure and being miscible with water, in order to produce a first metal ink 10A that also contains the organic solvent 18. According to this disclosure, it is possible to produce a first metal ink 10A that can suitably improve sinterability while suppressing aggregation of metal particles 12. In addition, even when left for a long period of time, sufficient mold resistance is achieved, and the metal ink 10 can be stored appropriately for a long period of time.

[0132] Furthermore, in the method for producing the metal ink 10 according to this disclosure, it is preferable to mix the first metal ink 10A with a lower alcohol which is a solvent 16 to produce a second metal ink 10B which is a metal ink containing metal particles 12, water, a lower alcohol, a polyhydric alcohol 14, and an additive 17. According to this disclosure, it is possible to produce a second metal ink 10B that can suitably improve sinterability while suppressing aggregation of the metal particles 12.

[0133] Furthermore, a method for producing the metal ink 10 according to this disclosure preferably involves mixing a second metal ink 10B with a high-boiling-point solvent 16, which is a liquid containing one or more OH groups, having a boiling point of 150°C or higher, and being sparingly soluble or insoluble in water, to produce a third metal ink 10C containing metal particles 12, water, a lower alcohol, a high-boiling-point solvent, a polyhydric alcohol 14, and an additive 17. According to this disclosure, a third metal ink 10C can be produced that can suitably improve sinterability while suppressing aggregation of the metal particles 12.

[0134] Furthermore, the method for manufacturing a sintered body according to this disclosure involves heating a metal ink 10 to form a sintered body. According to this disclosure, the sinterability can be suitably improved while suppressing the aggregation of metal particles 12. The manufacturing conditions for the sintered body using the metal ink 10 may be arbitrary, but it is preferable to heat the metal ink 10 in an oxidizing gas atmosphere, an inert gas atmosphere, or a reducing gas atmosphere.

[0135] Furthermore, the cleaning solution according to this disclosure comprises metal particles 12, a solvent 16, a polyhydric alcohol 14 containing two or more OH groups and soluble in water and lower alcohols, and an additive 17 containing a water-soluble phosphoric acid compound. By using such a cleaning solution, for example, the flow path and nozzle in which the metal ink 10 according to this embodiment has accumulated can be properly cleaned.

[0136] (Examples) Next, we will describe the examples. Figures 3 to 45 are tables showing the content of the metal ink components and the evaluation results for each example. Figures 46 to 50 are tables showing the content of the cleaning solution components and the evaluation results for each example. In the "Dispersant (Phosphate Ester Compound)" column of each table, steareth-2-phosphate has an HLB value of 4 to 13 and a molecular weight range of 400 to 2500. Oleth-4-phosphate has an HLB value of 6 to 20 and a molecular weight range of 500 to 3000. (C12-C15)pareth-6-phosphate has an HLB value of 8 to 20 and a molecular weight range of 500 to 3000. (C12-C15)pareth-10-phosphate has an HLB value of 10 to 20 and a molecular weight range of 700 to 3000. Polyoxyethylene (18EO) alkyl (C13) ether phosphate has an HLB value of 13 to 20 and a molecular weight range of 1000 to 3500. Furthermore, polyoxyethylene alkyl (C8) ether phosphate monoethanolamine salt has an HLB value of 10-20 and a molecular weight range of 500-1500. Polyoxyethylene (C18 unsaturated) ether phosphate sodium salt has an HLB value of 10-20 and a molecular weight range of 700-1500. CRODAFOS O3A has an HLB value of 6-20 and a molecular weight range of 400-2500.

[0137] (First experimental example) As the first experimental example, we manufactured and evaluated a metallic ink.

[0138] (Example 1) In Example 1, copper phthalate was prepared as the starting material, copper carboxylate. The copper phthalate was placed in deionized water at room temperature and stirred with a stirring blade to prepare an aqueous dispersion of copper phthalate with a concentration of 30% by mass. Next, an aqueous solution of ammonium phthalate was added to this aqueous dispersion of copper phthalate as a pH adjusting agent to adjust the pH of the aqueous dispersion to 3. Then, the pH-adjusted solution was heated to 50°C and, under a nitrogen gas atmosphere, an aqueous solution of hydrazine monohydrate with an oxidation-reduction potential of -0.5V (diluted twice) was added all at once as a reducing agent, equivalent to 1.2 times the amount that can reduce copper ions, and the mixture was uniformly mixed using a stirring blade. Furthermore, in order to synthesize the target copper particles (metal particles), the mixture of the aqueous dispersion and the reducing agent was heated to the holding temperature of 70°C under a nitrogen gas atmosphere and held at 70°C for 2 hours. Finally, an aqueous slurry of copper particles (copper powder concentration: 50% by mass) was obtained by dehydrating and desalting using a centrifuge.

[0139] 18 g of the obtained copper particle (metal particle) aqueous slurry (copper powder concentration: 50% by mass), 36 g of an aqueous solution of 2,5-dimethyl-2,5-hexanediol as a polyhydric alcohol (concentration: 5% by mass), 1.8 g of trimethyl phosphate (molecular weight 140.08) as an additive, and 34.2 g of water were mixed to obtain 90 g of copper ink (metal ink) with water as the solvent. The mass and content ratio of each component of the copper ink in Example 1 are shown in Figure 3. The copper ink in Example 1 is an example of the first metal ink 10A of this embodiment.

[0140] (Examples 2-337, Comparative Examples 1-48) Examples 2-337 and Comparative Examples 1-48 were manufactured using the same method as in Example 1, except that the raw materials for the metal ink and their addition amounts were as shown in Figure 3 and later. Comparative Examples 1-24 did not contain additives containing phosphoric acid compounds. Comparative Examples 25-48 did not contain polyhydric alcohols (polyhydric alcohols containing two or more OH groups and soluble in water and lower alcohols).

[0141] (Evaluation method: variance) Regarding the dispersibility of the copper ink (metallic ink) obtained in the examples and comparative examples, after thoroughly dispersing the obtained metallic ink using an ultrasonic cleaner or the like, 10 g of the metallic ink was taken into a 20 ml glass sample container and left overnight in a refrigerator. After leaving it overnight, with the height from the bottom of the sample container to the liquid surface of the metallic ink settling and separation interface of metal particles in the metallic ink settling and separation interface was judged as "○" if the height from the bottom of the container was 50 or more, and as "×" if it was less than 50. (Evaluation method: Sinterability) Furthermore, for the copper ink (metal ink) whose dispersibility was rated "○" in the examples, a 10mm x 10mm area was applied to the center of a 100μm thick, 50mm x 50mm polyimide film using an inkjet device and dried. Subsequently, it was heated in a nitrogen atmosphere at 200°C for 30 seconds to obtain a fired metal ink film with a thickness of approximately 1-3μm. The sinterability of the obtained fired film was evaluated by observing the cross-section with an SEM (Scanning Electron Microscope; Hitachi High-Tech Corporation, observation magnification 10,000x). In the cross-sectional SEM image, sinterability was rated "◎" if the percentage of voids in the film was 15% or less, "○" if it was between 15% and 20%, "△" if it was between 20% and 30%, and "×" if it was above 30%. (Evaluation method: light transmittance) Furthermore, for the copper ink (metallic ink) whose dispersibility was marked as "○" in the examples, a 20mm x 20mm area was applied and dried on the center of a 0.7mm thick, 50mm x 50mm soda glass using an inkjet device. Subsequently, using a high-temperature observation microscope SMT Scope SK-8000 (manufactured by Sanyo Seikou), the copper particles were oxidized and sintered, and a sintered body with a thickness of approximately 0.5 to 1 μm was obtained by heating at 300°C for 30 seconds (heating rate to peak temperature of 1°C / second) in an air atmosphere (oxygen concentration of approximately 20%) using a halogen lamp heater manufactured by Ushio Inc. (which mainly emits light with a spectral distribution peaking in the visible to infrared region, more specifically light with a spectral distribution peaking at a wavelength of 1 μm). The degree of transmission of the obtained sintered body to visible to infrared light was evaluated. For the evaluation of transmittance, the transmittance of the sintered body was measured using a Hitachi High-Tech Science Co., Ltd. UV-Vis-Near-Infrared Spectrophotometer (UH4150) as a visible-infrared spectrophotometer. The data mode was set to transmittance measurement (%T), with a starting wavelength of 1200 nm, an ending wavelength of 300 nm, a scan speed of 600 nm / min, and a sampling interval of 1.00 nm. At this time, an average transmittance of 50% or more in the wavelength range of 600 nm to 1200 nm was judged as "○" for light transmittance, and a transmittance of less than 50% was judged as "×".

[0142] (Evaluation results) The evaluation focused on dispersibility, sinterability, and light transmittance. In Examples 1-337, which included additives containing polyhydric alcohols and phosphate compounds, the evaluations for dispersibility, sinterability, and light transmittance were all "○" or "◎". This indicates that excellent sinterability is ensured while suppressing the aggregation of metal particles, and that light transmittance is obtained in the sintered bodies obtained by heating in an atmospheric environment. Therefore, it can be concluded that copper oxide sintered bodies (films) with excellent light transmittance can be applied to tandem solar cells combined with crystalline silicon solar cells. On the other hand, in Comparative Examples 1-24, which contained polyhydric alcohols but did not include additives consisting of phosphate compounds, dispersibility and sinterability were "○", but light transmittance was "×". Furthermore, in Comparative Examples 25-48, which did not contain polyhydric alcohols, it was found that the aggregation of metal particles could not be suppressed, and the metal particles settled. In addition, because the metal particles in the ink aggregated, settled, and separated, ink application by an inkjet device was not possible, and subsequent evaluations of sinterability and light transmittance could not be performed, so the evaluations for sinterability and light transmittance were marked "-". Furthermore, for Examples 52 to 337, which contain at least one of a lower alcohol, an organic solvent with a boiling point of 150°C or higher at atmospheric pressure and miscible with water, or a high-boiling point solvent, although not shown in the table results, the long-term storage properties of the metal inks were also evaluated according to the method of "6. Testing of Paints" described in the Japanese Industrial Standard (JIS Z 2911) (mold resistance test: instead of a test piece, the metal ink was used as a sample and tested by placing it in a culture medium with a diameter of 30 mm in the center). No fungal growth was observed in the metal inks, indicating good long-term storage properties.

[0143] (Second experimental example) As a second experimental example, we manufactured and evaluated metallic inks and cleaning solutions corresponding to each metallic ink.

[0144] (Examples 1-18, Comparative Examples 1-18) In Examples 1-18 and Comparative Examples 1-18, the metal inks and cleaning solutions used for evaluation were prepared in the same manner as in the first experimental example. The masses and content ratios of each component in Examples 1-18 and Comparative Examples 1-18 are shown in Figures 46-50.

[0145] (Evaluation method: Cleanability) The cleanability of the first metal ink A1 was evaluated using the cleaning solutions of Examples 1-3 and Comparative Examples 1 and 2; the cleanability of the first metal ink A2 was evaluated using the cleaning solutions of Examples 4-6 and Comparative Examples 3 and 4; the cleanability of the first metal ink A3 was evaluated using the cleaning solutions of Examples 7-9 and Comparative Examples 5 and 6; the cleanability of the second metal ink B1 was evaluated using the cleaning solutions of Examples 10-12 and Comparative Examples 7-10; the cleanability of the second metal ink B2 was evaluated using the cleaning solutions of Examples 13-15 and Comparative Examples 11-14; and the cleanability of the third metal ink was evaluated using the cleaning solutions of Examples 16-18 and Comparative Examples 15-18. The cleaning performance of the cleaning solutions obtained in the examples and comparative examples was evaluated using the following procedure. A glass bottle (9 ml) containing 1 ml of cleaning solution, weighed beforehand using a micropipette, was into which 20 μl of metal ink was added dropwise using a micropipette. Immediately after addition, the dispersion state of the metal ink and the powder components within the metal ink was visually observed. Solutions where no precipitate was observed in the metal ink or the powder components within the metal ink immediately after addition were rated "○: Good," and solutions where precipitate was observed were rated "×: Poor."

[0146] (Evaluation results) Examples 1 to 18, which included metal particles, a solvent, a polyhydric alcohol, and a phosphate compound (additive), were evaluated as "good" in terms of cleaning performance. In contrast, Comparative Examples 1 to 18, which did not contain at least one of the metal particles, solvent, polyhydric alcohol, and phosphate compound (additive), were all evaluated as "poor" in terms of cleaning performance, indicating that they were unsuitable as cleaning solutions.

[0147] Although embodiments of the present invention have been described above, the embodiments are not limited to those described herein. Furthermore, the aforementioned components include those that can be easily conceived by those skilled in the art, those that are substantially the same, and those that fall within the so-called equivalent range. Moreover, the aforementioned components can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the components can be made without departing from the spirit of the embodiments described above. [Explanation of Symbols]

[0148] 10 Metallic Ink 12 Metal particles 14 Polyhydric alcohols 16 Solvents 17 Additives 18 Organic solvents

Claims

1. Metal particles, which are copper particles, Solvent and, A polyhydric alcohol containing two or more OH groups and soluble in water and lower alcohols, An additive containing a water-soluble phosphate compound, including, Cleaning solution.

2. The washing solution according to claim 1, further comprising an organic solvent having a boiling point of 150°C or higher at atmospheric pressure and being miscible with water.

3. The cleaning solution according to claim 1 or claim 2, wherein the additive is contained in an amount of 0.01% or more and 10% or less by mass relative to the total amount of the cleaning solution.

4. The washing solution according to claim 1 or claim 2, wherein the molecular weight of the phosphate compound is 90 or more and 200 or less.

5. The washing solution according to claim 4, wherein the additive comprises at least one of phosphoric acid, monosodium phosphate, disodium phosphate, trisodium phosphate, monopotassium phosphate, dipotassium phosphate, trimethyl phosphate, triethyl phosphate, diisopropyl phosphate, monoisopropyl phosphate, 2-aminoethyl dihydrogen phosphate, diisopropyl phosphate, and dimethyl phosphate.

6. The cleaning solution according to claim 1 or claim 2, wherein the polyhydric alcohol is contained in an amount of 0.01% or more and 20% or less by mass relative to the total amount of the cleaning solution.

7. The cleaning solution according to claim 1 or claim 2, wherein the polyhydric alcohol has a melting point of 30°C or higher.

8. The cleaning solution according to claim 2, wherein the organic solvent is contained in an amount of 0.01% or more and 30% or less by mass relative to the total amount of the cleaning solution.

9. The washing solution according to claim 2 or claim 8, wherein the organic solvent comprises at least one of a glycol ether and an aprotic polar solvent.

10. The cleaning solution according to claim 1 or claim 2, wherein the metal particles are contained in an amount of 1% or more and 50% or less by mass relative to the total amount of the cleaning solution.

11. The solvent comprises water, or water and a lower alcohol, or water, a lower alcohol, and a high-boiling point solvent. The washing solution according to claim 1 or claim 2, wherein the high-boiling point solvent contains one or more OH groups, has a boiling point of 150°C or higher, and is a liquid that is sparingly soluble or insoluble in water.

12. The washing solution according to claim 11, further comprising a phosphate ester compound having a molecular weight higher than the phosphate compound contained in the additive as a dispersant.

13. The cleaning solution according to claim 1 or claim 2, wherein the solvent contains water, the polyhydric alcohol contains at least one polyhydric alcohol having two or more OH groups, being soluble in water and lower alcohols, and having a melting point of 30°C or higher, and the additive has a molecular weight of 90 or more and 200 or less of the phosphoric acid compound.

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