Carbon-coated metal powder, paste, and method for producing carbon-coated metal powder

Abstract

A carbon-coated metal powder having metal particles and a coating layer that covers at least a part of a surface of the metal particles and contains carbon, wherein the metal particles consist of (1) tin, copper or silver, and inevitable impurities, or (2) an alloy of tin, copper or silver having a tin, copper or silver content of 50 mass% or more, and inevitable impurities, the average particle diameter calculated from a BET specific surface area of the carbon-coated metal powder is 150 nm or less, and the contact angle with respect to a surface of water is less than 90°.

Description

Carbon-coated metal powder, paste, and method for producing carbon-coated metal powder

[0001] This specification describes carbon-coated metal powder, paste, and a method for producing carbon-coated metal powder.

[0002] Metal powders consisting of a specified pure metal or alloy may be mixed with a liquid containing organic substances such as an organic binder or organic solvent to form a paste. Such pastes can be used, for example, as conductive pastes for forming wiring by printing, filling holes or joining, or as pastes for electronic equipment or energy devices, or as solder pastes used for soldering. In some fields, depending on the compatibility with the substrate to which the paste is applied, water, alcohol, or polar organic solvents may be used as solvents to form the paste.

[0003] As a technology related to carbon-coated metal powders, for example, Patent Document 1 proposes a carbon-coated metal powder suitable for additive manufacturing, under the objective of "providing a carbon-coated metal powder that has excellent fluidity and can efficiently absorb energy rays." The carbon-coated metal powder is characterized by "a carbon coating on the surface of metal particles having an average particle diameter of 1 μm or more and 100 μm or less, which is chemically coated with carbon derived from an organic compound, wherein the amount of carbon coating in the carbon-coated metal powder is 0.001% by mass or more and 2.0% by mass or less."

[0004] Japanese Patent Publication No. 2018-199862

[0005] As mentioned above, carbon-coated metal powders are sometimes used dispersed in polar solvents. However, due to the properties of carbon-coated metal powders, it can be difficult to disperse them well in polar solvents. Poor dispersion of carbon-coated metal powders in polar solvents can lead to a decrease in the quality of the manufactured material.

[0006] In view of these problems, embodiments of the present invention provide a carbon-coated metal powder, a paste, and a method for producing a carbon-coated metal powder that can be well dispersed in a polar solvent.

[0007] Embodiments of the present invention that solve the above problems are defined below. 1. A carbon-coated metal powder comprising metal particles and a coating layer containing carbon that covers at least a part of the surface of the metal particles, wherein the metal particles consist of (1) tin, copper or silver and unavoidable impurities, or (2) an alloy of tin, copper or silver with a tin, copper or silver content of 50% by mass or more and unavoidable impurities, the average particle size calculated from the BET specific surface area of ​​the carbon-coated metal powder is 150 nm or less, and the contact angle with the surface of water is less than 90°. 2. The carbon-coated metal powder according to 1, wherein the carbon content is 0.5 to 3.0% by mass. 3. The carbon-coated metal powder according to 1 or 2, wherein the hydrogen content is 0.01 to 0.25% by mass. 4. The carbon-coated metal powder according to any one of 1 to 3, wherein the coating layer containing carbon is composed of a mixture of graphite-based carbon and hydrocarbons. 5. 1. A carbon-coated metal powder according to any one of 1 to 4, wherein the oxygen content is 3.0% by mass or less. 6. A carbon-coated metal powder according to any one of 1 to 5, used in a paste. 7. A paste comprising the carbon-coated metal powder according to any one of 1 to 6 and a liquid containing organic matter. 8. A method for producing a carbon-coated metal powder, comprising the step of treating the surface of the metal particles of a carbon-coated metal powder with a polycarboxylic acid organic compound solution, wherein the metal particles consist of (1) tin, copper or silver and unavoidable impurities, or (2) an alloy of tin, copper or silver with a tin, copper or silver content of 50% by mass or more and unavoidable impurities.

[0008] According to embodiments of the present invention, it is possible to provide carbon-coated metal powder, paste, and a method for producing carbon-coated metal powder that can be well dispersed in a polar solvent.

[0009] This is a schematic cross-sectional view of the surface-coated particles. This is a photograph of the appearance of a test tube showing the results of a dispersibility evaluation test in NMP for the carbon-coated metal powder according to the example. This is a graph of the particle size distribution for Example 1 and Comparative Example 1. This is the Raman spectrum for Example 1.

[0010] Preferred embodiments of the present invention will be described below, but the present invention should not be construed as being limited thereto, and various modifications and improvements can be made based on the knowledge of those skilled in the art, without departing from the spirit of the invention. The multiple components disclosed in this embodiment can be combined in appropriate ways to form various inventions. For example, some components may be removed from all the components shown in this embodiment.

[0011] <Carbon-Coated Metal Powder> The carbon-coated metal powder according to an embodiment of the present invention comprises metal particles and a coating layer containing carbon that covers at least a portion of the surface of the metal particles, wherein the metal particles consist of (1) tin, copper, or silver and unavoidable impurities, or (2) an alloy of tin, copper, or silver with a tin, copper, or silver content of 50% by mass or more and unavoidable impurities, and have a contact angle with respect to the surface of water of less than 90°. When a coating layer containing carbon is formed by a normal process, the wettability of the carbon coating itself to water is poor, resulting in a contact angle of 90° or more for the carbon-coated metal powder. In that case, if it is a powder, it will not disperse in a polar solvent such as water, or the agglomeration of the powder cannot be broken down. For this reason, it is necessary to make the contact angle with respect to the surface of water less than 90°. The contact angle should be less than 90°, but since dispersion takes time, it is preferable that the contact angle be as low as possible. Preferably, the contact angle is less than 85°, more preferably less than 80°, and even more preferably less than 75°.

[0012] More specifically, the carbon-coated metal powder 4 according to the embodiment of the present invention is an aggregate of particles including surface-coated particles 3, in which metal particles 1 are covered with a coating layer 2, as illustrated in Figure 1, and is composed mostly or almost entirely of surface-coated particles 3. In Figure 1, the entire surface of the metal particle 1 is covered with the coating layer 2, but it is sufficient if the coating layer 2 adheres to the surface of the metal particle 1 such that it covers at least a portion of the surface of the metal particle 1. In other words, a portion of the surface of the metal particle 1 may be exposed and not covered with the coating layer 2.

[0013] (Composition) The metal particles contained in the carbon-coated metal powder according to the embodiment of the present invention consist of (1) tin, copper or silver and unavoidable impurities, or (2) an alloy of tin, copper or silver with a tin, copper or silver content of 50% by mass or more and unavoidable impurities.

[0014] When the metal particles of the carbon-coated metal powder according to the embodiment of the present invention contain metal, the content of tin, copper, or silver in the metal portion of the carbon-coated metal powder may be, for example, 99.0% by mass or more, typically 99.9% by mass or more, and preferably 99.99% by mass or more.

[0015] When the metal particles of the carbon-coated metal powder according to the embodiment of the present invention include an alloy, the content of tin, copper, or silver in the carbon-coated metal powder is 50% by mass or more. More preferably, the content of tin, copper, or silver in the carbon-coated metal powder is 60% by mass or more. For tin, copper, or silver alloys, the content of tin, copper, or silver in the carbon-coated metal powder is preferably controlled according to the application and other factors.

[0016] Examples of unavoidable impurities contained in the metal particles of the carbon-coated metal powder according to the embodiment of the present invention include Si, Cu, Ag, Fe, Na, Mg, etc. The content of unavoidable impurities contained in the metal particles of the carbon-coated metal powder is preferably 1.0% by mass or less, and more preferably 0.1% by mass or less. Note that the content of unavoidable impurities is the total content of multiple types of unavoidable impurities if they are present.

[0017] The tin, copper, or silver content in carbon-coated metal powder, as well as the content of alloying elements and impurities when the metal particles are alloys, can be measured by ICP-MS if the sample is liquid, and by GD-MS if it is solid. For example, a solid sample of carbon-coated metal powder can be measured by GD-MS after being compressed in a measuring container. The impurity content can be analyzed, for example, as a percentage (mass ppm) of tin, copper, silver, or their alloys.

[0018] The carbon-coated metal powder according to the embodiment of the present invention contains a coating layer containing carbon, and therefore contains carbon. The carbon content of the carbon-coated metal powder is preferably 0.5 to 3.0% by mass, more preferably 0.5 to 2.0% by mass. If the carbon content is too low, there is a concern that oxidation will not be sufficiently suppressed. On the other hand, if the carbon content is too high, the conductivity between particles may decrease, or the surface may become inert, which may hinder the fusion of particles when heated. In the carbon-coated metal powder according to the embodiment of the present invention, the coating layer containing carbon may be composed of a mixture of graphite-based carbon and hydrocarbons. Here, graphite-based carbon is produced by superheating hydrocarbons in an inert atmosphere (for example, a gas atmosphere such as nitrogen or argon) or in a sealed environment with air. The coating on the metal powder can be formed by spraying hydrocarbons such as methane onto the superheated metal powder. Graphite-based carbon has a Raman shift of 1350 cm⁻¹ by Raman analysis. -1 Nearby is the "D band," 1600 cm -1 Nearby is the "G band," at 2680 cm. -1 A peak in the vicinity of the "2D band" or "G' band" is detected. The hydrogen content is preferably 0.25% by mass or less, more preferably 0.1% by mass or less. As the hydrogen content increases, the compatibility with polar solvents improves and the dispersibility improves. The hydrogen content may be 0.01 to 0.25% by mass. The hydrogen contained in this coating layer can be imparted by dissolving a compound containing hydroxyl groups or carboxylic acids, such as citric acid, in water or alcohol, surface treating by spraying or immersion, and then drying. The carbon content or hydrogen content of the carbon-coated metal powder can be analyzed by non-dispersive infrared absorption spectroscopy and measured as a percentage (mass ppm) of the total mass of the carbon-coated metal powder.

[0019] The carbon-coated metal powder according to the embodiment of the present invention has a low oxygen content because the metal particles are covered with a carbon-containing coating layer, which suppresses the oxidation of the metal particles. The oxygen content of the carbon-coated metal powder may be, for example, 3.0% by mass or less, typically 2.5% by mass or less, and even 1.5% by mass or less. It is believed that a low oxygen content makes the metal more likely to melt when heated. The oxygen content can be analyzed by non-dispersive infrared absorption spectroscopy and measured as a percentage (mass ppm) of the total mass of the carbon-coated metal powder.

[0020] (Particle Size) The carbon-coated metal powder according to the embodiment of the present invention has a relatively small particle size. Specifically, the average particle size (so-called BET diameter) calculated from the BET specific surface area of ​​the carbon-coated metal powder is 150 nm or less, typically 100 nm or less, and more typically 50 to 80 nm. If the average particle size is too large, the dispersibility in polar solvents may decrease.

[0021] The BET specific surface area of ​​the carbon-coated metal powder according to the embodiment of the present invention can be measured in accordance with JIS Z8830:2013, for example, using a Macsorb / HM model-1208 manufactured by MOUNTEC Corporation, with helium as the carrier gas and nitrogen as the adsorbent gas (mixing concentration 30.1%, flow rate 25 ml / m). The average particle size d (μm) calculated from the BET specific surface area can be calculated from this BET specific surface area using the formula: d = 6 / (ρ × s). In this formula, ρ is the density (g / cm³) of tin, copper, or silver, which is determined according to the composition of the metal particles. 3 ) or the density (g / cm³) of an alloy of tin, copper, or silver 3 ) and s is the BET specific surface area (m 2 It is / g).

[0022] The carbon-coated metal powder according to the embodiment of the present invention can be well dispersed in a polar solvent. Examples of such polar solvents include water, alcohols (methanol, ethanol, propanol, butanol), organic solvents (NMP (N-methyl-2-pyrrolidone), DMA (Di-methyl-Acetamide), acetonitrile, DMSO (Di-methyl-Sulfoxide), DMF (Di-methyl-Formamide)), etc.

[0023] The carbon-coated metal powder according to the embodiment of the present invention has a contact angle of less than 90° with respect to the surface of water. With this configuration, it can be well dispersed in a polar solvent.

[0024] The contact angle of water with respect to the surface can be measured by the following method: A compact of carbon-coated metal powder (a flattened mass of powder) is prepared, and the contact angle is measured when water is dropped onto it. For this contact angle measurement, the CA-DT.A contact angle meter from Kyowa Interface Science Co., Ltd. can be used. As stated in the manual, it is important to create droplets that are not affected by gravity. Furthermore, since water penetrates the compact over time, the angle measurement should be performed within 15 seconds after the water droplet is dropped.

[0025] <Method for producing carbon-coated metal powder> A method for producing carbon-coated metal powder according to an embodiment of the present invention comprises metal particles and a coating layer containing carbon that covers at least a part of the surface of the metal particles, wherein the metal particles consist of (1) tin, copper or silver and unavoidable impurities, or (2) an alloy of tin, copper or silver with a tin, copper or silver content of 50% by mass or more and unavoidable impurities, and comprises the step of treating the surface of the metal particles of the carbon-coated metal powder with a polycarboxylic acid organic compound solution.

[0026] In a method for producing carbon-coated metal powder according to an embodiment of the present invention, it is preferable to first prepare a metal powder with a particle size of several micrometers to several tens of micrometers, produced by gas atomization or the like, and then to refine the metal powder using RF plasma. In RF plasma, the metal on the surface of the metal particles of the metal powder evaporates, and the diameter of the metal particles decreases.

[0027] Then, while the metal powder is still at a high temperature after the RF plasma treatment, a hydrocarbon gas containing methane or the like can be blown onto it. At this time, the blowing of the hydrocarbon gas causes decomposition and carbonization reactions on the surface of the metal particles, forming a carbon-containing coating layer. If necessary, after the RF plasma treatment, airflow classification or other classification is performed to remove coarse particles. In this way, a carbon-coated metal powder is obtained that has a predetermined small average particle size and contains surface-coated particles in which at least a portion of the surface of the metal particles is covered with a coating layer.

[0028] Next, the carbon-coated metal powder is treated by spraying it with a polycarboxylic acid organic compound solution, specifically, water or ethanol (or other organic solvent) in which the polycarboxylic acid organic compound solution is dissolved, thereby treating the surface of the surface-coated particles of the carbon-coated metal powder. This allows the carbon-coated metal powder to be well dispersed in the polar solvent. Examples of polycarboxylic acid organic compounds include citric acid, malic acid, and tartaric acid.

[0029] The concentration of the polycarboxylic acid organic compound in the ethanol in which the polycarboxylic acid organic compound is dissolved is preferably 0.3 to 0.8 mol%.

[0030] <Paste> The carbon-coated metal powder according to the embodiment of the present invention may be used in conductive pastes, solder pastes, or other pastes. Such a paste contains a liquid containing organic matter in addition to the carbon-coated metal powder according to the embodiment of the present invention, and can be prepared, for example, by mixing and stirring the carbon-coated metal powder according to the embodiment of the present invention with the liquid containing organic matter.

[0031] Examples of liquids containing organic matter in the paste include organic binders and organic solvents. Commonly used organic binders and solvents can be used.

[0032] Next, we fabricated the carbon-coated metal powder described above and confirmed its effects, which are explained below. However, this explanation is for illustrative purposes only and is not intended to be limiting.

[0033] <Example 1> As Example 1, atomized tin powder (average particle size D50: 5.5 μm) manufactured by Nippon Atomize Processing Co., Ltd. was prepared as the raw material. The raw material for Example 1 was atomized using RF plasma output, and then coated with carbon by spraying methane after the RF plasma treatment. Next, the surface of the carbon-coated metal powder surface coating particles was treated by spraying ethanol (citric acid concentration: 0.47 mol%) in which citric acid was dissolved onto the carbon-coated powder. In this way, a sample of carbon-coated metal powder according to Example 1 was prepared.

[0034] <Comparative Example 1> As Comparative Example 1, atomized tin powder (average particle size D50: 5.5 μm) manufactured by Nippon Atomize Processing Co., Ltd. was prepared as the raw material. The raw material of Comparative Example 1 was micronized by the output of an RF plasma, and then coated with carbon by blowing methane after the RF plasma treatment. In this way, a sample of carbon-coated metal powder according to Comparative Example 1 was prepared.

[0035] (BET diameter) The BET diameter (average particle size calculated from the BET specific surface area) of the carbon-coated metal powder sample according to Example 1 was measured as follows. The BET specific surface area was measured in accordance with JIS Z8830:2013 using the BET method with a Macsorb / HM model-1208 manufactured by MOUNTEC Corporation, with helium as the carrier gas and nitrogen as the adsorbent gas (mixing concentration 30.1%, flow rate 25 ml / m). The average particle size d calculated from the BET specific surface area was calculated using the formula: d = 6 / (ρ × s). In this formula, ρ is the density of tin (g / cm³) which is determined according to the composition of the metal particles. 3), or the density of the tin alloy (g / cm 3 ), and s is the BET specific surface area (m 2 / g). The obtained BET diameter (average particle diameter calculated from the BET specific surface area) was 80 nm.

[0036] (XRD measurement) For the sample of the carbon-coated metal powder according to Example 1, measurement was carried out using an XRD diffractometer SmartLab manufactured by Rigaku Corporation. As a result, it was found that the sample of the carbon-coated metal powder according to Example 1 was pure tin.

[0037] (SEM observation) For the sample of the carbon-coated metal powder according to Example 1, using a scanning electron microscope (SEM): JXA-8500F (5CH) manufactured by JEOL Ltd., observations were made at an acceleration voltage of 15.0 kV and magnifications of 2000 times and 10000 times. As a result, it was confirmed that the carbon-coated metal powder according to Example 1 consisted of particles of several tens of nm.

[0038] (Structural analysis of the carbon-containing coating layer) For the sample of the carbon-coated metal particles according to Example 1, using a microscopic Raman apparatus manufactured by Renishaw plc, measurements were carried out at a laser wavelength of 532 nm, a Raman shift range of 100 to 3500 cm -1 , and an integration number of 100 times, and structural analysis was performed using the Raman spectrum. The results are shown in FIG. 4. For Example 1, peaks of the "D band" were detected around a Raman shift of 1350 cm -1 , the "G band" around 1600 cm -1 , the "2D band" or the "G' band" around 2680 cm -1 , and it was confirmed that it contained graphite-based carbon. Also, peaks due to CH bonds were obtained at 2931 cm -1 and 3221 cm -1 , and it was confirmed that it contained hydrocarbons. Thus, it was found that the carbon-coated metal particles according to Example 1 were a mixture of graphite-based carbon and hydrocarbons.

[0039] For the samples of carbon-coated metal powders according to Example 1 and Comparative Example 1, the contents of carbon (C), hydrogen (H), nitrogen (N), oxygen (O), and sulfur (S) were analyzed by non-dispersive infrared absorption method (LECO Japan Co., Ltd., model numbers: CS600 (C, S), TC600 (O, N), TCH600 (H)), and measured as the ratio (mass ppm) to the total mass of the carbon-coated metal powder. Similarly, the total content of impurities excluding CHNOS was analyzed and measured as the ratio (mass ppm) to the total mass of the carbon-coated metal powder. The measurement results are shown in Tables 1 and 2.

[0040]

[0041]

[0042] (Wettability) As the contact angle with respect to the surface of water, compacts of samples of carbon-coated metal powders according to Example 1 and Comparative Example 1 were prepared, and the contact angle when water was dropped was measured. For this contact angle measurement, a contact angle meter CA-DT.A of Kyowa Interface Science Co., Ltd. was used. At that time, as described in the manual, it is important to make a droplet that is not affected by gravity. Also, since water penetrates into the compact over time, the angle was measured within 15 seconds after the water droplet was dropped. The contact angle with respect to the surface of water in Comparative Example 1 was 91°, while the contact angle with respect to the surface of water in Example 1 was 74°, and an improvement in wettability was confirmed.

[0043] (Dispersibility) To confirm dispersibility, 0.05 g of a sample of carbon-coated metal powder according to Example 1 was added to NMP in a test tube at room temperature. Figure 2 shows a photograph of the appearance 30 seconds after addition. As can be seen from Figure 2, it was confirmed by visual inspection that the carbon-coated metal powder was uniformly mixed in the NMP. Specifically, when 0.05 g of the carbon-coated metal powder sample was added, the powder spread well and remained in the NMP without settling after 30 seconds. On the other hand, when the carbon-coated metal powder sample of Comparative Example 1 was added, it floated on the surface of the NMP or settled immediately, and there was almost no retention of the powder in the NMP. Furthermore, to confirm uniform mixing, particle size distribution measurement may be performed as follows. That is, NMP was used as the dispersion medium for the carbon-coated metal powder samples according to Example 1 and Comparative Example 1, and only a stirrer was used for dispersion, stirring for 90 minutes. For this particle size distribution measurement, the MASTERSIZER 3000 of Malvern Panalytical Division, Spectris Corporation was used. The particle size distribution is shown in Figure 3. In Example 1, citric acid treatment resulted in good dispersion even with only stirring by a stirrer, and the peak of aggregates observed in the measurement of Comparative Example 1 was reduced. This indicates that the mixture was uniform.

[0044] (Potential contribution to the SDGs) According to the embodiment described above, it is possible to provide a carbon-coated metal powder, paste, and a method for producing the carbon-coated metal powder that can be well dispersed in a polar solvent. For this reason, this embodiment has the potential to contribute to Goal 9, "Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation," and Goal 12, "Ensure sustainable consumption and production patterns," of the United Nations-led Sustainable Development Goals (SDGs).

[0045] 1 metal particles 2 coating layer 3 surface coating particles 4 carbon-coated metal powder

Claims

1. A carbon-coated metal powder comprising metal particles and a coating layer containing carbon that covers at least a portion of the surface of the metal particles, wherein the metal particles consist of (1) tin, copper, or silver and unavoidable impurities, or (2) an alloy of tin, copper, or silver with a tin, copper, or silver content of 50% by mass or more and unavoidable impurities, the average particle size calculated from the BET specific surface area of ​​the carbon-coated metal powder is 150 nm or less, and the contact angle with the surface of water is less than 90°.

2. The carbon-coated metal powder according to claim 1, wherein the carbon content is 0.5 to 3.0% by mass.

3. The carbon-coated metal powder according to claim 1, wherein the hydrogen content is 0.01 to 0.25% by mass.

4. The carbon-coated metal powder according to claim 1, wherein the carbon-containing coating layer is composed of a mixture of graphite-based carbon and hydrocarbons.

5. The carbon-coated metal powder according to claim 1, wherein the oxygen content is 3.0% by mass or less.

6. The carbon-coated metal powder according to claim 1, used in a paste.

7. A paste comprising a carbon-coated metal powder according to any one of claims 1 to 6 and a liquid containing an organic substance.

8. A method for producing carbon-coated metal powder, comprising: metal particles; a coating layer containing carbon covering at least a portion of the surface of the metal particles, wherein the metal particles consist of (1) tin, copper, or silver and unavoidable impurities, or (2) an alloy of tin, copper, or silver with a tin, copper, or silver content of 50% by mass or more and unavoidable impurities, and comprising the step of treating the surface of the metal particles of the carbon-coated metal powder with a polycarboxylic acid organic compound solution.

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